MONOLITHIC EXPANDABLE STRUCTURES, METHODS OF MANUFACTURE AND COMPOSITE STRUCTURES

Abstract: An article of manufacture comprising an expandable sag-resistant nucleus-forming monolithic composite capable of being located within a hollow interior portion of a structural material and being expanded therein. Also, articles of manufacture comprising open-cellular structural material containing within the open-cell or cell thereof, at least one expandable sag-resistant nucleus-forming monolithic composite. The composite is desirably in the shape of a plug that is similar or close to similar to the shaped of the hollow interior. In addition, there is described a process that comprises forming a pre-shaped expandable sag-resistant nucleus-forming monolithic composite for use in reinforcing and stiffening a normally open-cellular structural material. Also described is a process for reinforcing or stiffening or normally open-cellular structure, any tubular structure, or any channel structure, by putting at least one expandable sag-resistant nucleus-forming monolithic composite within a hollow interior portion of said structure. The invention is particularly desirable for stiffening and/or reinforcing honeycomb structures.

Description: MONOLITHIC EXPANDABLE STRUCTURES, METHODS OF MANUFACTURE AND COMPOSITE STRUCTURES BRIEF DESCRIPTION OF THE INVENTION Monoliths of essentially uniform density containing in s/zta-expandable ther ¬ moplastic particles essentially uniformly dispersed in a thermosettable or thermoplastic matrix resin system that is incompatible with the particles, are described. Composites of the monolith and core-holding structures are made by incorporating the monolith into the core-holding structure, thermally treating the matrix resin of the monolith whereby expansion of the in szforexpandable thermoplastic particles causes the thermally treated monoliths to faultlessly interface with the wall(s) of the core-holding structures. The invention relates to core supported composite structures, fasteners, adhesives, paneling, insulation, and other structures that employ the invention as a component. BACKGROUND TO THE INVENTION Sandwich construction forms a major part of aerospace manufacturing, ' it is employed to some degree in almost every type of flight vehicle. Lightweight structural panels and panel systems for a wide variety of applications are industrially offered. They utilizing various laminating techniques to adhere thin, stiff "skins", such as aluminum, steel, hardboard and fiberglass onto lightweight core materials, like honeycombs, urethane foams and balsa. The resulting panel is lightweight and strong. Honeycomb sandwiches, i.e., honeycombs, are preferred structures in the fabrication of lightweight structures typically used in the aerospace and other commercial markets. The core material is usually "sandwiched" between skins of aluminum or other high strength composite material. A bonding adhesive is used to attach the "skin" material to the honeycomb core. The resultant honeycomb panel offers one of the highest strength to weight constructions available. For instance, the floor panels of most airliners use the lightweight/high strength construction of honeycomb. Air ¬ craft engine nacelles, flaps, overhead bins and galleys all are constructed from honey ¬ comb core. A honeycomb sandwich, or simply "honeycomb," may be called a multicellular structure, and it may be made of paper, plastic, fabric or metal, and other materials. The core of the sandwich is the honeycomb, a structure composed of row upon row of framed cells, or holes or wells, resembhng the honey-storage facility of a beehive and characterized by a hexagonal or rectangular shape. To each side of the core are bonded extremely thin sheets of metal, creating the sandwich, which is far hghter yet has greater resistance to bending than a comparable thickness of metal plate. Alumi ¬ num is the most extensively used metal, in both the core and the facing sheets, but the technique is applicable to a large number of metallic and nonmetallic materials. For example, M. C. Gill Corporation offers the following honeycomb products to Boeing's specifications: 2 ' M=Meets the requirements of specification To illustrate some of the physical characteristics of commercial honeycombs, the following is taken from Plascore Inc. (615 North Fairview Ave. Zeeland, MI 49464) PAMG-XRl 5052 Aluminum Honeycomb literature: Typical Property Values PAMG-XRl 5052 Aluminum Hone comb Other Plascore honeycomb products include'- PAMG-XRl 5056 aluminum honeycomb is a hghtweight core material which offers superior strength and corrosion resistance over PAMG-XRl 5052 and PCGA Commercial grade aluminum core. PAMG-XRl 5056 core is made from 5056 aluminum alloy foil and meets all the requirements of MIL-C-7438. PCGA-XRl honeycomb is a hghtweight core material offering excellent strength and corrosion resistance for industrial applications at low cost. PCGA-XRl core is made from 3003 aluminum alloy foil. PN Aramid Honeycomb is a hghtweight, high strength, non-metallic honeycomb manufactured with aramid fiber paper (DUPONT Nomex(tm) or equivalent). The aramid paper is treated with a heat resistant phenolic resin. This core material exhibits excellent resiliency, small cell size, low density and outstanding flame properties. • Plascore polycarbonate honeycomb core exhibits a unique cell structure : The core has 3 orientations vs. the 2 orientations common with other cores, making its properties more uniform. Each cell has a tubular form and inherently stable. • Plascore polypropylene honeycomb core exhibits a unique cell structure: The core has 3 orientations vs. the 2 orientations common with other cores, making its properties more uniform. Each cell has a tubular form and inherently stable. • Plascore polypropylene honeycomb is supplied with or without a non-woven polyester veil for better bonding. It is also supplied with or without a film barrier under the polyester veil to limit the amount of resin consumption. Euro-Composites® sell honeycombs with cell sizes ranging from 3.2 to 19.2mm and a density of between 24 and 200 kg/m 3 , in hexagonal and rectangular-celled cores. Aerospace manufacturers started the use of honeycomb products in airplanes and spacecraft because, pound-for-pound, it's the strongest, most rigid product known. Some of its special uses were the heat shield on John Glenn's space capsule, interior structures for America's first Skylab, and shock-resisting hulls of hydroplanes. There are many ways to fasten one structure to another. Fasteners are objects that attach one item to another, and create methods by which those attachments are effected. In industrial applications, fastening may be accomplished through needle stitching, anchoring, connecting, locking, welding, riveting, nailing, screwing, adhesive bonding, chemical reaction bonding, magnetic bonding, and the like. The concept of fastening started with the earliest concepts of interweaving of dissimilar materials, clamps, nailing, screwing and the like. Eventually, man learned to bond with metals, and this led to riveting, bolting and welding. Each of these techniques led to advances in the art of fastening. An adhesive is a substance used to bond two or more surfaces together. Most adhesives have the advantage of forming a bond by filling in the minute pits and fis ¬ sures normally present even in very smooth surfaces. Adhesive bonds are economical, distribute the stress at the bonding point, resist moisture and corrosion, and elimi ¬ nate the need for rivets and bolts. The effectiveness of an adhesive depends on sev ¬ eral factors, including resistance to slippage and shrinkage, malleability, cohesive strength, and surface tension, which determines how far the adhesive penetrates the tiny depressions in the bonding surfaces. Adhesives vary with the purpose for which they are intended. Such purposes now include the increasing use of adhesives in aerospace applications. Synthetic adhesives used both alone or as modifiers of natu ¬ ral adhesives, perform better and have a greater range of application than the natu ¬ ral products. Most of them form polymers, huge molecules incorporating large num ¬ bers of simple molecules to form strong chains and nets that hnk surfaces in a firm bond. Thermosetting adhesives, which are transformed into tough, heat-resistant solids by the addition of a catalyst or the application of heat, are used in such structural functions as bonding metallic parts of aircraft and space vehicles. Thermoplastic resins, which can be softened by heating, are used for bonding wood, glass, rubber, metal, and paper products. Elastomeric adhesives, such as synthetic or natural rubber cements, are used for bonding flexible materials to rigid materials. Many aerospace structures are adhesively bonded through the use of thin adhesive films, typically made from a filled thermosetting resin such as an epoxy resin. These films are easier to apply and cleaner to use, and therefore find wide acceptance in applications where neat utilization of the adhesive is a plus factor. SynCore ® , sold by The Dexter Corporation, Aerospace Materials Division, 2850 Willow Pass Road, Bay Point, CA 94565, is a syntactic foam film that takes the place of more expensive prepreg plies in stiffening critical structures. This isotropic foam is a composite material containing preformed microballoons in a thermosetting matrix resin. A wide variety of preformed microballoons and matrices can be combined to make SynCore® materials. Glass is the most common microballoon material of construction, but quartz, phenohc, carbon, thermoplastic and metal-coated preformed microbaUoons have been used. Epoxies curing at 350°F. (177°C) and 250°F. (121°C.) are the most common thermosetting matrix resins, but matrices of bismaleimide (BMI), phenohc, polyester, PMR-15 polyimide and acetylene or acrylic or vinyl- terminated resins have been used to produce SynCore ® syntactic foams. As a result of the variety of materials that successfully make SynCore ® , they are tailorable to a variety of applications. There is a version of SynCore ® available that will co-cure with all known available heat-cured composite-laminating resins. SynCore ® allows sandwich core concepts to be used in a thinner dimension than previously possible. The thickness Hmit on honeycomb cores is approximately 0.125 inch. SynCore ® is available in 0.007 to 0.125 inch (0.18 mm to 3.2 mm) thickness but can be made in thinner or thicker sheet forms. Other core materials such as wood and sheet foam can be made thin, but are not drapable and generally require an expensive/heavy ad ¬ hesive film to bond to the partner composite components. In addition, SynCore ® possesses excellent uniformity in thickness which provides the ability to assure quality for the composite in which it is used as a component. SynCore ® is typically used to replace prepreg plies where the intent is to increase stiffness by increasing thickness. Designing with SynCore ® is straightforward because all of the analysis methods that apply to other core materials such as honeycomb apply to it. Flexural stiffness of flat plates and beams increases as a cubic function of thickness allowing a hghter, stiffer lamination than could be made from prepreg plies alone. Since SynCore ® , on a per volume basis, typically costs less than half of a comparable carbon prepreg, it also leads to a lower cost lamination. This is illustrated by the following: 1. Adding one ply of .020 inch SynCore ® and eliminating one ply of prepreg does not change the weight or cost significantly, but nearly doubles the flexural rigidity. 2. Adding one ply of .020 inch SynCore ® and eliminating three phes of prepreg sharply decreases the cost and weight with a small decrease in rigidity. 3. Adding one ply of .040 inch SynCore ® and eliminating three phes of prepreg pro ¬ vides lower weight, cost and sharply increases rigidity. 4. The introduction of unidirectional tape allows a further increase in performance at lower cost and weight at nearly the same thickness. 5. A hybrid tape/fabric/SynCore ® construction gives a very attractive set of weight and cost savings coupled with a 3.4 times increase in flexural rigidity. SynCore ® has been recommended for thin composite structures in any applica ¬ tion where flexural stiffness, buckling, or minimum gauge construction is used. It has been shown to save weight and material cost in carbon fiber composites. It has been offered to save weight at approximately the same cost in the case of glass fiber composites. Illustrative apphcations are covered in U.S. Patents No. 4,861,649, pat ¬ ented August 28, 1989, U.S. Patent No. 4,968,545, patented November 6^ 1990, and U.S. Patent No. 4,994,316, patented February 19, 1991. The manufacturing methods for employing SynCore ® are very similar to those used for prepregs. Because it is not cured, it is tacky and very drapable when warmed to room temperature and is easier to lay-up than a comparable prepreg ply. It can be supplied in supported forms with a hghtweight scrim to prevent handling damage when it is frozen. It requires cold storage like prepregs, usually 0°F. (-17.7°C.) or below. The various SynCore ® materials typically have a room temperature out-time that is much longer than their companion prepregs. SynCore ® is less sensitive to cure cycle variations than prepreg making the controlling factor the composite cure cycle selection. It will cure void free under full vacuum or low (e.g. about 10 p.s.i.) autoclave pressure. It has been cured at up to about 150 p.s.i. without exhibiting balloon crushing. In a typical application, a sandwich of SynCore ® and prepreg, such as a thicker layer of SynCore ® between two thinner layers of prepreg, are held together under heat and pressure to cure the structure into a strong panel. Typical sandwich constructions of this nature are shown in U.S. patents 4,013,810, 4,433,068 and 3,996,654. Such composite structures typically are produced in flat sheets and in separable molds to obtain various desired shapes. Though SynCore® will cure void free under significantly reduced pressure or when put under pressure, it would be desirable to avoid those costly conditions to achieve void reduction. It would be desirable to have a material that has the proper ¬ ties of SynCore ® but achieves void free construction without costly full vacuum opera ¬ tions or low autoclave pressure systems. These methods are typically batch type op ¬ erations that materially add to the cost of making the composite. There are certain applications in which it is desirable to have the properties of a uniform thin drapable syntactic foam film in processing the formation of a laminated composite, yet have the capacity to autogenously expand so as to fill any void space existing in the composite's structure so as to minimize the effects of macro and micro void defects at interlaminate interfaces. These interlaminar interfacial micro or macro void spaces are magnified by the irregularity of the reinforcing layer of the composite structure. For example, if the composite is of a layer of prepreg-derived carbon fiber reinforced thermosetting resin material, bonded to a syntactic foam, such as a SynCore ® thin uniform film, the layer containing the prepreg-derived material will have an irregularly shaped surface and the SynCore ® layer will have a relatively smooth uniform surface. Though the SynCore ® is tacky and drapable, it is incapable of filhng in all of the irregularities of the prepreg-derived layer. Application of a full vacuum or the use of a low-pressure autoclave can be used to significantly reduce the void space, but complete avoidance of micro voids is not readily achievable. Also, conforming SynCore ® to the irregular surface causes transfer of the irregularity to the opposite surface of the SynCore® film. Such surface irregularity transfer may be avoided by sandwiching the SynCore® film using heat and pressure, ' such repositions the film's matrix resin and the micro- spheres so that the film within the sandwiched structure loses its original uniformity. It would be desirable to be able to adequately bond a syntactic foam thin film to an irregular surface 1 and fill the defects in the surface without transferring the shape of the defects to the unbonded side of the film. It would also be desirable to be able to adequately bond a syntactic foam thin film to a surface and, without the use of vacuum or low-pressure autoclaves, fill the micro voids with the syntactic foam with ¬ out repositioning the film's matrix resin and microspheres. Such advantages are achieved by the use of SynSpand ® , described below. There is a body of technology directed to fabricating expandable thermoplastic resinous material. For example, U.S. Patent No. 2,958,905, patented November 8, 1960, is directed to a method of making foam structures from particulate expandable granular thermoplastic resinous material containing in the particles a blowing agent for further expansion of the particles. A considerable number of thermoplastic resins are described as suitable for this purpose. The blowing agents are the conventional ones recommended for that apphcation. The expandable granular thermoplastic res ¬ inous material may be admixed with a thermosetting resin to generate on curing the exotherm needed to expand the expandable granular thermoplastic resinous material. The resulting mass can be poured into a mold to make a number of products. The patentees indicates that the expandable granular thermoplastic resinous material can be formed in the presence of non-expandable filler materials such as staple fibers from a variety of sources, and the mixture fed to a mold for forming an expanded product. The resulting foamed product may be designed to adhesively bond to a fabric layer for reinforcement of the foamed product. The density of the foamed product can be controlled by the amount of the expandable material fed to the mold. According to the patentees, starting at column 12, hnes 5 et seq., molded products are formed by charging the mold "with the expandable material in any desired manner including manual filling or pneumatic conveyance thereof." According to the description at column 12 relating to Figures 3 and 4 (see column 12, hnes 16-32): 1 Such a surface is one that may contain undulations, cracks, large pores, warpage, and the like defects. ..."a considerable occurrence of void and hollow spaces occurs between the charged expandable beads 21 in the mass to be fabricated, each of which (in the case of pre-expanded material) is a foam structure containing a plurality of in ¬ ternal cells or open spaces. When the liquid exothermus [sic] substance is added between such interparticle voids, the heat from its spontaneous self reaction causes the beads to expand whereby, as illustrated in Figure 4, the expanded and fabricated particles 22 force out a substantial portion (and frequently most) of the exothermus [sic] substance excepting for a minor quantity of reacted material 23 which remains, frequently as an interlaced and interlinking network between the expanded particles to assist in holding the expanded, cellular foam particles together." (Emphasis added) U.S. Patent No. 2,959,508, patented November 8, 1960, describes another variation of using expandable thermoplastic particles. In this patent, the unex- panded particles and the exothermus substance, such as an epoxy resin, are first mixed and then poured into the mold to form a composite foam of the two when the exothermus substance heats up the mixture and causes the blowing agent to volatilize. Thermosetting resins have had blowing agents incorporated in them (see U.S. Patent No. 3,322,700, patented May 30, 1967) to form expanded molded products and recently, such types of resin systems have included preformed microspheres in the formation of partial syntactic foam films. These expanded thermosets comprise a more open cellular structure unlike that of syntactic foams, and the inclusion of preformed microspheres does not alter that condition. There are commercial molding processes that utilize tacky sheets of thermosetting resins and reinforcing material. One such process involves the compression molding of sheet molding compounds ("SMC"). In that process, a thermosetting polyester resin filled with staple glass fiber and low profile thermoplastics, are sheeted out and thickened into a pourable paste retained between release surfaces such as polyethylene film. Chunks of the thickened paste are casually deposited around the surface of the mold by hand, and on closing the mold with heating, the paste is lique ¬ fied and it, and its fiber loading, are redistributed around the mold to fill it up and form the desired molded article. In other word, the chunks of sheets of SMC repre ¬ sent a convenient way in which to add a hquefiable moldable material to the mold. This process is presently commercially practiced in a number of industries. Advan ¬ tages of the process are the convenience of storing moldable mixture and the ease of loading a mold with the molding composition. An advantage of SynCore ® for many applications resides in its uniformity of distribution of the microsphere throughout the matrix resin. Such microspheres re ¬ main essentially intact throughout the cure cycle. As a result, it is not possible to have the microspheres concentrate at one or more surfaces, or one or more other loca ¬ tions in the final composite. It would be desirable to have a drapable thin film, having the handhng quahties of SynCore ® , but which would allow the production of a syntactic foam having a controllable density gradient that accommodates specific end use applications. There are a number of applications in which thin film syntactic foam could serve as a seal to preclude the passage of gases and liquids. In some applications, the seal could be subjected to abrasion forces. It would be desirable to have a thin film syntactic foam that can be apphed in a manner that allows it to be a sealant to gas or liquid flow in a confined space and be able to withstand abrasive forces. Such advantages are derived from the use of SynSpand ® , described below. U.S. Patents, Nos. 5,234,757; 5,397,611; 5,540,963; and 5,783,272 describe a thin film technology that can be used in forming syntactic foam. The thin film is commercially sold as SynSpand ® by the Dexter Corporation, Dexter Adhesive & Coating Systems, Bay Point, CA 94565. It combines expansion with syntactic technology to produce an efficient core filler and reinforcement material. It comprises a thin, tacky film of incompatible in sz ' fø-expandable thermoplastic particles in a thermo- settable matrix resin that contains an essentially uniform density and thickness across the breadth of the film. In its optimum form it provides closed ceU expansion, see U.S. 5,783,272. The thin unexpanded films can be laid up in a honeycomb core and each film that is laid up is homogeneously expanded therein to reinforce and stiffen the honeycomb structure at that portion of the structure, without unduly add ¬ ing weight to the structure. It is sold in 1 foot by 2-feet sheets and rolls stock (18 inches by 25 lineal feet) in thicknesses of 50 and 100 mils. SynSpand ® may contain preformed microspheres as well, such as those that are used in making SynCore ® . SynSpand ® is an excellent product and has gained wide commercial accep ¬ tance. However, as the utilizations of such forms of syntactic foams are expanded, there are uses where the thin film quahties of SynSpand ® do not conveniently meet the needs of the user. For example, in honeycomb apphcations, a sheet or sheets of SynSpand ® are applied over a plurality of honeycomb open spaces in the core, and with heat and pressure, generally under vacuum, but preferably by use of positive pressure from a heated platen, the SynSpand ® is forced into open spaces to which it is contiguous. This requires the SynSpand ® to flow into the honeycomb structure and assume some dimensional structural similarity to that of the open space of the honeycomb into which it flows. This technique insures that the SynSpand ® is in contact with the honeycomb surface over which it is originally laid, either the top or bottom surface of the honeycomb structure, and the walls of the open cells of the honeycomb structure. It is inevitable that some amount of the SynSpand ® film is left behind on that surface. Further heating cause the matrix resin to flow and sag into the core of the honeycomb structure, and the in situ expandable particles to expand, preferably before gelation of the matrix resin, primarily in the "z" direction (see the discussion in the bridging paragraph at columns 9 and 10 of U.S. 5,397,611), and allows for curing the expanded SynSpand® in the honeycomb. This serves to reinforce and stiffen the honeycomb structure without unduly adding weight to the structure. However, the small amounts of SynSpand ® left on the top or bottom surface of the honeycomb are going to be expanded as well. In addition, sagging of the resin causes it to contact the walls of the honeycomb core before expansion, thus expansion not only is in the "z" direction, but also starts from the wall to fill the center of the core,' which is opposite to the type of expansion that one would desire. The small amount of SynSpand ® left on the top or bottom surface can be ameliorated to some degree by applying an adhe ¬ sive film over that surface into which the residual SynSpand ® can be solubilized. A deficiency in thin film SynSpand® technology resides in the fact that the top of the film is the primary exposed surface area for expansion. Because the edges of the film possess relatively httle of the films surface area, and the molding method of choice forces the edges into contact with the walls of the open cells of the honeycomb structure followed by sagging, expansion must of necessity occur essentially from and in the "z" direction and also from the core walls. This concentration of expansion to essentially a single surface fails to optimize the strength/density/rigidity relationship in such honeycomb structures and does not readily allow reinforcement within the open cells of the honeycomb structure by continuous filament fibers aligned normal to the surface of the honeycomb structure. Under the sagging scenario, expansion in the "z" direction causes the resin to intersect at some part of the open space being filled. Unless the resin contact is seamless by virtue of molten mixing of the different resin film, there is an opportunity for the resin to form an interface within the open space that possesses a lower structural integrity than the remainder of the expanded polymer structure. In addition, thin film SynSpand ® technology does not readily allow structural combinations with continuous filament structures in a way that maximizes rigidity and strength to syntactic foam structures incorporated into a traditional honeycomb structure. The Invention This invention relates to solid adhesive structures, core fillers and reinforcement materials, and fasteners that rely on syntactic foam technology and are suitable for use in many different industries, such as aerospace, automotive, building construction, and the like. More specifically, this invention relates to reinforcement or stiffening of a normally open-cellular structural material (such as any honeycomb structures (such as those described above), any tubular structures, any channel structures, and the like). The invention is an expandable sag-resistant nucleus-forming forming monohthic composite that can be located within a hollow interior portion of the aforementioned structural material. This invention involves forms of a pre- shaped expandable sag-resistant nucleus-forming monohthic composite that can be used more conveniently and function more effectively in reinforcing and stiffening a normally open-cellular structural material. The invention relates to structural mate ¬ rial containing at least one of the expandable sag-resistant nucleus-forming mono ¬ hthic composites located within a hollow interior portion of the structural material. The invention relates as well to processing for making and using the pre-shaped ex ¬ pandable sag-resistant nucleus -forming monohthic composite of the invention. The pre-shaped expandable sag-resistant nucleus -forming monohthic composite comprises incompatible in situ isotropically expandable thermoplastic particles containing expansion agent therein, essentially uniformly distributed in a thermo- settable or thermoplastic matrix resin with which they are incompatible when the thermoplastic particles are in the thermo-expandable state. Where the monohthic composite contains a thermosettable matrix resin, the resin is not fully cured or at such a state of cure that it inhibits the desired degree of expansion of the composite. Preferably, the resin is devoid of a degree of crosslinking that exhibits viscosity increase in the composite. The composite contains an essentially uniform density and thickness across its breadth and it possesses an external shape and size that is dr mensionally similar to a hoUow interior component of the structural material. "Pre-shaped" means that the monohthic composite does not undergo a shaping process when introduced into the hollow interior component of the structural material. It is already shaped for introduction into the hollow interior component. The term "dimensionally similar" means that when the nucleus-forming composite is placed in the hollow interior component of the structural material and is heated sufficiently to cause isotropic expansion of the incompatible in s/forexpandable thermoplastic particles, the nucleus-forming composite uniformly expands to cause the formation of a syntactic foam, especially a closed ceU syntactic foam, that has an essentially faultless interface with the wall(s) of the hoUow interior component and pos- sesses an essentially uniform density throughout. As used herein, the term "mono ¬ hthic" means something that is essentially uniform and substantial or inflexible in quahty or character. The term "thermo-expandable state" means the in situ- expandable thermoplastic particles heated to a condition where the expansion agent in the particles are expanding; this means that the thermoplastic resin in the parti ¬ cles are sufficiently softened that the particles expand in size. The term "sag- resistant" means to resist sinking or bending, by or as if by weight or pressure when the composite is placed in the hollow interior component of the structural material, sufficiently to avoid snagging of the composite on the hollow interior. The term "faultless" means that the flaws inherently present in the wall are filled and sealed by the expanded composite. The term "essentially uniform" means that the product has a prescribed uniformity according to a predetermined standard. This invention relates to a sag-resistant nucleus-forming monohthic composite of incompatible in sizta-expandable thermoplastic particles that form closed-microcells upon isotropic expansion, essentially uniformly distributed in a thermosettable or thermoplastic matrix resin. The monohthic composite contains an essentially uniform density and thickness across its breadth. Also the monolithic composite possesses an external shape and size that is dimensionaUy similar to a hoUow interior component of a structural material that is about 1.01 to about 4 times greater in vol ¬ ume than the volume of monohthic composite. In other words, the sag-resistant nucleus-forming monohthic composite of the invention has a shape and has dimensions that, in a preferred embodiment, essentially correlates the shape and is at least close to proportional to the dimensions of the hollow interior component of a structural material into which it can be readily inserted and isotropically expanded to form a syntactic foam with closed microcells. The conformance of the shape and dimension of the sag-resistant nucleus-forming monohthic composite of the invention to the hollow interior component of a structural material is significant to achieve the advantages of the preferred invention. The proportionality of the dimension of the sag-resistant nucleus-forming monohthic composite to the hollow interior component is such that the composite isotropically expands about 1.01 to about 4 times in volume to fully fill the previously hoUow interior component. In the preferred embodiment, the end surface of the monohthic composite is appropriately shaped such that it can stand within the hoUow interior component without touching any of the sidewalls of the hollow interior component. Sag-resistance is an important feature of the sag-resistant nucleus-forming monohthic composite. Sag-resistance must exist at the time the monohthic composite is inserted in the hollow interior component of the structural material. Sag- resistance is commensurate with the dimension similarity of the monohthic composite to that of the hollow interior component. The closer the dimension similarity, the greater must be the sag-resistance. In other words, the monohthic composite must be capable of insertion into the hollow interior without snagging of the composite on any surface of the hollow interior during insertion. Preferably, the monohthic composite should have enough sag resistance that the composite does not sag once it is placed in the hollow interior component of the structural material. The term "isotropic expansion" as use herein means the expansion along all axes or directions up until a surface portion(s) of the monohthic composite during expansion makes contact with and forms an essentially faultless interface with the waU(s) of the hoUow interior component. For example, the hollow interior component may have an irregular surface such that a portion of the monohthic composite's outer surface may be dimensionally closer to a juxtaposed portion of the irregular surface. During expansion such closer outer surface may make faultless interface with the closer waU portions before other portions of the outer surfaces make faultless interface with other juxtaposed waU portions of the hoUow interior component. In many instances, the monohthic composite may be placed in contact with a segment of the waU surface of the hollow interior component before expansion is initiated. In those instances, "isotropic expansion" continues but the axis or directions may be reoriented such that multiple isotropic expansions occur within the expanding monohthic composite. The overall effect of such expansion is that the expanded monohthic com ¬ posite possesses an essentially uniform density throughout. The uniform density of the syntactic foam of the invention derived from iso ¬ tropic expansion of the monohthic composite means that the density in any direction is essentially uniform, that is, the density does not differ by more than about ± 5 per ¬ cent (%) in weight in any direction. However, density differences can be intentionally built into the resulting syntactic foam. For example, one may include reinforcing fi ¬ bers or fillers that are located at preferred segments of the resulting syntactic foam so that the resulting syntactic foam composite has increased reinforcement at those segments. An important alternative embodiment of the invention is a fiber reinforced sag-resistant nucleus-forming monohthic composite of incompatible in situ- expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin that contains an essentially uniform density and thickness across the breadth of the composite and which possesses an external shape and size that is dimensionaUy similar to a hoUow interior component of a structural material. In this embodiment, the fiber may be present as continuous filament, continuous tow, chopped or staple fibers, spunbonded fibers, fibrous mat or fibrous webs, or any combination of them. The fibers may be aligned essentially with the aspect orientation of the monohthic composite, assuming that the monohthic composite possesses an aspect orientation. In many instances, the fibers may be ahgned in a single direction or more than one direction. For example, a filamentary tow may be used that has a twist, thus imposing an ahgnment in several directions all within the same fibrous component of the composite. The fiber reinforcement may be of organic or inorganic fibers. In the preferred invention, at least a portion of the fiber reinforcement is made of a fiber that conducts heat better than the thermosettable matrix resin of the composite. Most preferably, the fiber reinforcement is made of a metal or carbon-based material, such as steel, aluminum, graphite, non-graphitic carbon (including pitch based fibers), and can include high performance fibers such fibers from polyarylamides and polyarylimides, polyaromatic ether ketones such as PEEK, PEK and/or PEKE, and the like. In a pre ¬ ferred embodiment, the fiber is a filamentary tow that is align centrally of the inte ¬ rior of the monohthic composite and extends from the bottom to the top of the mono ¬ hthic composite. The monohthic composite may also contain reinforcing fillers that are materi ¬ als, which enhance the toughness or tensile strength of the resulting cured, expanded product. The fillers may be of any of the known types that are used in thermosetting resins for the enhancement of toughness and/or tensile strength in cured thermoset ¬ ting resins. Fillers may be included solely or complimentarily for the purpose of al ¬ tering the viscosity of the monohthic composite. The invention relates to molding structures which function as sohd adhesive structures, core fillers and reinforcement materials, and/or fasteners that rely on syntactic technology and are suitable for use in many different industries, such as aero ¬ space, automobiles, building construction, and the like. A preferred apphcation of the sag-resistant nucleus-forming monohthic composite of incompatible in situ- expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin that contains an essentially uniform density and thickness across the breadth of the composite and which possesses an external shape and size that is dimensionally similar to a hollow interior component of a structural material, is to formulate the matrix resin to be an adhesive such that it adhesively bonds to the waU(s) of the hollow interior component and forms an essentially faultless adhesive interface with the wall(s) of the hollow interior component. Because of the shape similarity of the sag-resistant nucleus-forming monohthic composite of incompatible in szftrexpandable thermoplastic particles in a thermosettable or thermoplastic matrix resin of the invention, it is possible to produce sohd adhesive structures, core fillers and reinforcement materials, and/or fasteners that are relatively easier to employ and which provide surprisingly better functionahty. For example, it is described in U.S. Patents, Nos. 5,234,757; 5,397,611; 5,540,963; and 5,783,272 that the syntactic foam films that are now commercially sold as SynSpand ® , may be multi-plied into larger structures that can fill up, e.g., an open space (or hollow core or open cell) of a honeycomb aerospace structure. However, because of the sagging characteristics of the SynSpand®, it is not possible to readily form a moldable structure that conforms to the dimensions of the open space or core that does not snag when introduced to the open space or hol ¬ low core of the honeycomb structure. Secondly, multi-plied structures are incapable of having variability in the reinforcement built into the phed structure without mak ¬ ing different thin layers of SynSpand ® with different reinforcement properties and plying such layers. Moreover, each of the layers must be laid up prior to inserting the multi-plied structure into the hoUow core. That is a time consuming activity and the resulting phed structure must be caused to lose its phability in order to avoid snag ¬ ging on insertion into the hollow interior. In addition, SynSpand ® layers that are ex ¬ truded into a honeycomb take extensive time from lay-up of the layers to the extru ¬ sion and curing of the SynSpand ® within the honeycomb. As pointed out above, Syn ¬ Spand ® when shaped within a open cell of a honeycomb, will effect most of its expansion in the "z" direction because the SynSpand ® is drawn into the open cell and is seized by the walls of the open ceU. Expansions in the "x" and "y" directions are inhibit by the walls. This invention effects reinforcement and/or stiffening of a structural member that contains a hollow interior portion. This is accomplished by forming a sag- resistant nucleus -forming monohthic composite of incompatible in sz ' tru-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin that contains an essentially uniform density and thickness across the breadth of the composite and which possesses an external shape and size that is dimensionally similar to a hollow interior component of the structural member, for example, a honeycomb structure. This allows the monohthic composite to be inserted into the hollow interior portion of the structural member, such as a hoUow cell of a honeycomb structure, without filhng up the hollow interior portion. If the hollow interior portion contains more than one opening for inserting the composite, then that opening is closed off such as to allow the monohthic composite to reside wholly in the hoUow interior portion. Then the hoUow interior portion is heated sufficiently to cause the matrix resin of the monohthic composite to flow and the in situ- expandable thermoplastic particles to simultaneously isotropically expand whereby the composite uniformly expands in es ¬ sentially all directions (i.e., the "x", "y" and "z" directions) or essentially in the "x" and "y" directions, to cause the formation of a syntactic foam, especially a closed cell syn ¬ tactic foam, that has an essentially uniform microcellular structure and a more fault ¬ less interface with the waU(s) of the hollow interior portion of the structural member, e.g., the honeycomb structure. In other words, it is the viscosity reduction of the ma ¬ trix resin that allows the expansion agent in the in sz ' ftrexpandable thermoplastic particles to force expansion of the thermoplastic resin in which it is embedded. With the matrix resin phase changes, there is a commensurate expansion of the expansion agent in the expandable particles, to where expansion is finally constrained by the matrix resin reaching a state of gelation that puts a constraint on expansion agent expansion. When the matrix resin is a thermosetting resin, it is allowed to cure to form a degree of stiffness that is sought for the syntactic foam. In the case where the matrix resin is a thermoplastic resin, expansion is constrained by the pressure of the thermoplastic resin and the collision of the expanding particles. An adhesive layer can be placed over the opening to the hollow interior portion during such expansion whereby the expanded monohthic syntactic foam composite material adhesively and integrally bonds to the adhesive layer. If desired, a metal or plastic layer can be placed over the opening to the hollow interior portion during such expansion and the expanded monohthic syntactic foam composite material adhesively and integrally bonds to the metal or plastic layer. If desired, a seahng plate that does not bond to the expanded monohthic syntactic foam composite material can be used instead. It can be removed after expansion is complete. Another embodiment of this invention is a sag-resistant nucleus-forming monohthic composite of incompatible in sz ' fø-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin that is filament fiber reinforced and can be adhesively anchored in a hollow interior of structural member. In particular, this embodiment contemplates creating at least one hoUow interior that extends through a plurality of layers of a composite and fills such hoUow interior with the fiber rein ¬ forced sag-resistant nucleus-forming monohthic composite of incompatible in situ- expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin, of the invention, and through thermal expansion of the monohthic composite, forming adhesive anchoring in each layer of the composite through which the hoUow interior extends, thereby reinforcing and fastening the composite into a reinforced structure. This embodiment of the invention is especially apphcable to use in reinforcing honeycomb structures by including in the monohthic composite the filament fiber component such that it is aligned from surface to surface of the honeycomb structure, and through adhesive layers provided onto such surfaces prior to the curing and expansion of the monohthic composite, the filament fibers become bonded to the adhesive layers, thereby greatly reinforcing the strength of the honeycomb and any film that is bonded to the adhesive layers on the side thereof oriented away from the honeycomb structure. Sandwich structures having great strength and utility can be made comprising outer layer (such as films of metal, plastic, paper, fiber reinforced layers, and the like, followed by an uncured adhesive layer, the honeycomb filled (in whole or part) with the uncured and unexpanded monohthic composites of the invention, another adhesive layer, another similarly filled honeycomb layer, an additional adhesive layer, and lastly, another outer layer. This sandwich structure can be subjected to heat and pressure so as to facilitate the isotropic expansion and curing of the monohthic composites and the curing of the adhesive layers. This will produce a multifaceted composite having great utility in a variety of area, such as in aerospace, automotive structures, building structures and the like. A further embodiment of the invention relates to advancing the reinforcing and fastening of a composite as set forth in the preceding paragraph. In this further embodiment, the sag-resistant nucleus-forming monohthic composite of incompatible in s zto-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin that is introduced into the common hollow interior of the composite to be joined, has at least one end portion that extends outside of the composite's external surface, and during heat expansion of the monohthic composite, the end portion is al ¬ lowed to expand greater than the confinement space hmiting expansion that occurs within the hoUow interior. That end portion can be pressed during the curing process to force the resin and reinforcement fibers thereat to expand beyond the size of the diameter of the opening of the hollow interior at the composite's external surface. By applying a closed pressure on the end portion, it is flattened to some degree to form an expanded syntactic foam fiber-reinforced fastener head terminating the syntactic foam derived by heat expansion of the sag-resistant nucleus-forming monohthic composite of incompatible in s/trzrexpandable thermoplastic particles in a thermosettable or thermoplastic matrix resin. Fastening pressures can be increased in fiber-reinforced syntactic foam formed by the heat expansion and curing of the sag-resistant nucleus-forming monohthic composite of incompatible in situ- expandable thermoplastic particles in a thermosettable matrix resin by utilizing a matrix resin formulation that shrinks during cure, thereby causing the fiber reinforcement to become more taut within the syntactic foam. The advantage of such a fastener structure is the fact that only one end of the hollow interior need extend through the surface of the composites outer surface. Also, anchoring of the syntactic foam of the invention into the hollow interior can be enhanced by varying the diameter of the hollow interior such that the syntactic foam is allowed to be more expanded in portions of the hoUow interior, especiaUy at a distance from the open end of the hollow interior exiting the outer surface of the composite being fastener-reinforced. In a further embodiment of the invention, it is desired that during the process of converting the sag-resistant nucleus -forming monohthic composite of incompatible in sΛu-expandable thermoplastic particles in a thermosettable or thermoplastic matrix resin into a syntactic foam that the temperature applied be sufficiently high enough to cause expansion of the thermoplastic particles prior to the gelation of the resin matrix. This allows the in situ expandable particles to expand in a matrix that normally before gelation will have lost some viscosity thus favoring the expansion process, and which upon gelation will impede the expansion of the in situ expandable particles by the increase in viscosity. This invention relates to a process for reinforcing and/or stiffening a honey ¬ comb structure which comprises inserting into a select hoUow interior portion (i.e., hollow core, cell, holes or weUs) of a honeycomb structure, a preformed sag-resistant nucleus-forming monohthic composite of incompatible in s/tru- expandable thermoplas ¬ tic particles, which form closed-microceUs upon isotropic expansion, in a thermo ¬ settable or thermoplastic matrix resin, which composite possesses an external shape and size that is dimensionally similar to the hollow interior portion of the honeycomb, heating the composite to a temperature which causes isotropic expansion of the in fi/te-expandable thermoplastic particles in the composite and expansion of the composite within the interior portion honeycomb structure to achieve a faultless interface with the wall of the interior portion honeycomb structure. Another embodiment of the invention involves the processes for forming the preformed sag-resistant nucleus-forming monohthic composites of incompatible in s tzrexpandable thermoplastic particles, which form closed-microcells upon isotropic expansion, in a thermosettable or thermoplastic matrix resin, of the invention. These monohthic composites may be made by extrusion, pultrusion or casting processes. In these processes, all of the components for making the monohthic composite are blended in conventional mixing equipment, with the exception of any continuous fibrous component that becomes a component of the monohthic composite. In the absence of any continuous fibrous component the blend of components may be extruded or casted to the desired monohthic composite shape. When continuous fibrous components are incorporated into the monohthic composite, it is desirable to make the monohthic composite by pultrusion processes. The desired length of the monohthic composite in the case of extrusion and pultrusion may be achieved by cutting the exudates with a knife or other cutting device. The length ot the composite in the case of casting is determined by the size and shape of the mold used in casting. Brief Description Of The Drawings Figure 1 is perspective view of a section of prior art honeycomb structure. Figure 2 is a perspective view of a prior art honeycomb core. Figure 3 comprising Figures 3a, 3b and 3c, which depict different shaped monohthic composite inserts for honeycomb core, such as depicted perspectively from the top in Figure 3d. Figure 4 comprises a sectional view of an extrusion/pultrusion apparatus for making monohthic composites of the invention. Figure 5 is a cross -sectional view of monohthic composite taken along hnes 5- 5'. Figure 6 is a sectional schematic view of a segment of pultrusion apparatus for making the monohthic composites of the invention. Figure 7 is a cross-sectional schematic view of Figure 6 taken along hnes 7-7'. Figure 8 is a coUection of figures. Figure 8a is a sectional schematic view of a segment of a multi-orifice pultrusion apparatus for making the monohthic composites of the invention. Figure 8b is the cross-sectional view along fine 8-8'. Figure 9 comprises Figure 9a showing a casting tray for making a monohthic composites of the invention, as shown in Figure 9b. Detailed Description Of The Invention SynSpand ® has made major advances to the employment of syntactic foam films, especially in aerospace apphcations. As note previously in U.S. Patents, Nos. 5,234,757; 5,397,611; 5,540,963; and 5,783,272, syntactic foam films, such as SynCore ® , are typically used in stiffness critical apphcations. The bending stiffness of a structure is proportional to the third power of thickness (T 3 ). A relatively shght flue- tuation in thickness will result in a significantly large variation in stiffness. That art teaches us that controlling the film thickness is a crucial manufacturing parameter in making a molded composite structure from syntactic foams. Space volume (foam ceUs) in thin uniform syntactic foam films such as SynCore ® is achieved by substitut ¬ ing hght hollow microspheres for the relatively dense matrix resin on a volume to volume basis. A physical barrier and a process obstacle hmit the amount of space volume that can be achieved. The physical barrier occurs at maximum packing fraction. Recognizing that the objective involves packing spheres into a fixed volume, maximum packing occurs when point-to-point contacts are developed between adjacent/nearest packing spheres. Beyond this point, it is physically impossible to pack any additional spheres into this fixed volume. For mono-dispersed spheres, this represents about 64% of the packing volume. In commerciaUy available multi-dispersed sphere systems, the weight reduction is limited by the particular system's packing fraction. From a processing standpoint, adding glass microspheres to a matrix resin increases resin viscosity, similar to that of adding fillers or thixotropic agents to the resin. This viscosity increase is due to an interaction between the flowing resin and the surface of the sphere. In simphstic terms, as the resin moves past a sphere, it imparts an equal and opposite drag force on the surface of the sphere and develops a shear stress through the center of the sphere. Typically, the shear strength of hollow spheres is low and during processing the resin's viscosity increases proportionally to the volume of hollow spheres added. As a result of the proportional increase in shear stress caused by the resin's increased viscosity, sphere damage/fracture occurs. This shear stress is the primary cause of sphere damage/fracture during processing. Once the spheres are damaged, the weight saving advantage from the use of hollow spheres is negated. This invention estabhshes that these sphere related hmitations are avoided by the use of expandable thermoplastic particles to in situ generate microspheres from a thin film to produce a thin (albeit thicker) syntactic foam film. These in situ- expandable thermoplastic particles possess behavior and processing characteristics similar to pigments and fillers. The average particle size of the thermoplastic in situ- expandable particles is typically an order of magnitude, or more, smaUer than the pre-expanded hollow sphere used in commercial thin film syntactic foams. For ex ¬ ample, for a given volume, a significantly greater number of unexpanded particles may be added to a given volume of resin compared to the pre-expanded spheres. Since expansion takes place in situ during the cure step, the shear sensitivity of pre- expanded hollow spheres does not become a problem. The packing of the expanded spheres is also more efficient because in situ- expansion. During cure, the matrix resin's viscosity, even though processable at normal handling temperatures, such as at about room temperature (about 15-37°C.) or conventional handhng temperatures (which can vary from apphcation to application), decreases with increasing temperature and since the unexpanded spheres are not in a point-to-point contact configuration, their environment is mobile and this allows each particle to expand within the film. This mobihty results in a more densely microsphere-packed lattice. However, in the general practice of the invention, the film will be cut to the size of the mold. As a result, because the expansion forces are three-dimensionally directed, this mobility is initially primarily in the upwardly "z" direction rather than in the lateraUy "x" and "y" directions. As expansion takes place, the film's edge surfaces enlarge, so they expose more particles to heat through the edge surfaces. As a result, more of the particles at the edge surfaces initiate expansion so that a greater proportion of the particles continue to expand in the "z" direction. Having the film placed in the mold such that the edges of the film abut or essentially abut the waU or walls of the mold further magnifies the upward expansion. The confining walls inhibit the free expansion of the particles into microspheres so that the internal expansion forces in the particles at the walls are directed more in the z direction initially. One might expect that the edges of the film would rise to greater heights in an otherwise vertical free expansion, but this is not the typical case. Free vertical expansion of the him results m a generally uniform rise of the film. This means that the surfaces of the expanded film, which eventually contact the walls of mold, are essentiaUy uniform in thickness resulting in a molded product of exceptional uniformity both in density but also in surface skin thickness at the edges and surfaces. However, in the case of the SynSpand® film, when it is apphed across the open mouth of an open-ceh of a structure, the apphcation of heat will cause the resin therein to lose viscosity and cause the film to sag. The sagging film drapes into the opening of the cell in the shape of a concave film, and with expansion occurring in the "z" direction, opposite portions of the same film surface are facing or essentiaUy fac ¬ ing each other and expansion from those portions will meet at some intermediate space. Consequently, a resin interface will be formed and the extent of the interface representing a seamless fault hne within the expanded film wiU depend on the viscosity of the resin and the ability of the resin to repair a fault hne in the film. Even should a seamless fault hne be created, there is the possibility that the fault hne represents a weakness in the syntactic foam. In addition, with expansion of the sagging film occurring at the interior of the concave interior of the film, there is the possibility that the density at the interior of the expanded and cured film wiU be lower than at the waU of the open-ceU structure. Should one use multi-layers of SynSpand® to fill the openings of an open _ ceU structure, then the same issue relating to fault hnes extends to the joining of the oppositely facing surfaces of the films during their expansions. This invention encompasses sag-resistant monohthic composites that are essentiaUy uniform and substantial in size relative to the open ceU in which they are placed or are inflexible in quahty or character. This means that composite is firm enough to be placed in the open-ceU and large enough to be reachly expanded to the size of and conform to the dimensions of the open-ceU. The composite of the invention wiU typicaUy have one dimension greater than the other, and typicaUy, the greater dimension configures with the greater dimension of the open-ceU into which the com- posite is placed. As a result, it is expected that the composite of the invention will possess an aspect ratio conforming to or simUar to the aspect ratio of the open-ceU. In most of the cases, the monohthic composite of the invention wiU have the appear ¬ ance of a plug that conforms generaUy with the geometry of the open ceU of the struc ¬ ture into which they are placed. General conformity does not require that the plug rephcate the geometry of the open ceU, merely that the plug is capable of expanding uniformly within the open ceU and seamlessly bond to the waU(s) of the open ceU. However, the plug can rephcate the geometry of the open ceU. In addition, the plug is typicaUy sufficient to expand and seal the open ceU into which it is placed. In the usual case, one plug is sufficient for sealing a single open ceU. Some may seek to avoid this invention by using the advantages of the invention and contend non-infringement by virtue of using more than one plug per open ceU. Though such use would not be advantageous over a single plug use, such use would be equivalent in performance so long as any interface between the plugs during expansion is totaUy seamless. Such is contemplated as being within the claims of this invention. The monohthic composites or plugs of the invention are easUy placed in the open ceU of the structure. For example, a plug may be physicaUy, by hand, dropped into an open ceU of a honeycomb. In most cases, that will be the preferred manner of employing the plugs. However, more automatic means may be used to dehver the plug to the open ceU. For example, plugs may be fed to a stream of gaseous liquid nitrogen temperature moderated with another gas at ambient temperature so that if the plugs are made with a thermosettable matrix resin that is curable with heat, the temperature can be controUed to reduce, if not ehminate, plug cure advancement. The stream acts as a pneumatic force for dehvering the plug through a flexible tube to a dehvery system at the end of the tube for insertion into the open ceU. The delivery system may be a device that functions like a leaf blade shutter type aperture that opens and seals a flexible tube through which the plug is dehvered. The leaf blade shutter comprises overlapping sliding blades that can be simultaneously opened or closed to either open up or seal an opening as a leaf shutter opens and closes a lens aperture. The dehvery system can be opened or closed depending on use. For exam ¬ ple, the shutter blades, located over an open ceU, can be opened to aUow the gas stream to force a suspended plug into an open ceU. The shutter blades can be put into the closed position with the dehvery of a single plug to a single open ceU. This can be repeated without injuring any of the plugs or aUowing any of the plugs to ad ¬ vance in cure. The temperature of the gas stream can be kept below room tempera ¬ ture to about -40°C, if desired. The shutter blades may be controUed mechanicaUy or electronicaUy, just as they are in a camera lens operation. The pneumaticaUy em ¬ ployed vaporized hquid nitrogen may be aUowed to exhale from a tank into a gas jet hne to which is added a stream of air. Its rate of vaporization is controUed to assure that the temperature of the combined stream of stream of air/nitrogen. The tempera ¬ ture of the combined stream of air/nitrogen is monitored by one or more thermocouples to assure that the temperature of the combined stream is that which sought. That combined steam can be fed to a plug insertion tube that contains an opening through which a plug is aUowed to deposit into the cold stream. The plugs are kept in a plug repository tube, one end of which connects with the stream and contains a similar shutter leaf blade shutter that opens and closes in concert with the leaf shutter aperture that opens and seals the device dehvery system. The other end of the plug repository tube is open to the operator and aUows for the introduction of plugs to the tube. When a plug is dehvered to the open ceU, another plug is automaticaUy swept into the combined stream, and the process is repeated at another open ceU. Other devices for dehvering the plug will become apparent to the user of the invention. The composite of the invention can be made in a number of ways and with a variety of resin systems to achieve the advantages herein set forth. The formulation of the film will be dictated by the specific end-use apphcation of the plug and the resultant molded syntactic foam, as weU as the method employed in making the plugs. Therefore, it is not intended that this description should De hmited to any specific apphcation and to any specific formulation and process of manufacture. The plugs of the invention may be made by a number of processes. If the plug does not contain filament fibrous material extending from surface to surface (i.e., unidirectionaUy ahgned), then the preferred method of making the plugs is by extru ¬ sion or cast molding. If the plug is to contain a monofilament fibrous matter that ex ¬ tends from surface to surface (i.e., unidirectionaUy ahgned), then the preferred method of manufacture is by pultrusion and cast molding. In any case and process, the first stage requires the mixing to uniformly distributed the incompatible in situ isotropicaUy expandable thermoplastic particles containing expansion agent therein with either a thermosettable or thermoplastic matrix resin or a mixture of both types of matrix resins with which in situ isotropicaUy expandable thermoplastic particles are incompatible when the thermoplastic particles are in the thermo-expandable state. Such mixing may be effected in a standard extruder or dough mixer of the type that is used in mixing such types of mixtures to effect uniform distribution. Where the matrix resin is a thermosettable resin, the selected resin is sufficiently hquid that it could be mixed with and wet out the incompatible in situ isotropicaUy expandable thermoplastic particles containing expansion agent therein. Other materials are typicaUy included in the matrix resin/ώ situ isotropicaUy expandable thermoplastic particles mixture ("matrix resin/particle mixture") that affect some of the matrix resin/particle mixture's flow properties as weU as physical properties of the ultimate expanded plug made therefrom. Pultrusion is a weU-known continuous, automated closed-molding process that is cost-effective for high volume production of constant cross section parts. The process rehes on reciprocating or caterpillar-type puUer/clamping systems to puU fiber and resin continuously through a heated steel die. Roving (either of spun staple fiber or continuous filament fiber) is puUed from material racks and is then wet out, typicaUy in an open resin bath. In the case of this invention, the matrix resin/particle mixture typicaUy has a higher viscosity than the typical resin system used in pultru- sion. To compensate for this, it is desirable to extrude the matrix resin/particle mixture as a flowing viscous mixture, frequently possessing the consistency of a paste, around the fiber component. Using the compositions of the invention, the die is sized to accommodate the fiber component and the matrix resin/particle mixture extruded around and about the fiber component. The compacted package then enters the die, where the part is cooled to effect the elongated shape of the ultimate plug.. The shaped part is puUed out of the die and, finaUy, into a saw system at the end of the machine. The saw travels downstream w Ue it cuts the part to a pre-set length desired for the plug. PuUer and saw motions and speed are synchronized, usuaUy * through computer controls. Multiple streams can be pultruded in a single die with several cavities. To form hoUow or multiple-ceU parts, the material typicaUy wraps around heated mandrels that extend through the die. If off-axis structural strength is required by the part apphcation, mat and/or stitched fabrics may be folded into the material package before it enters the die. This can add substantial loads to the machine frame and pulling system. HydrauhcaUy driven systems capable of pulling up to 100,000 lbs. and 103-inch wide by 15 inch deep structural profiles are in use today. Heat control is a recognized significant variable, and commercial pultrusion apparatus utilize controUers that monitor and maintain a pre-set temperature in various zones throughout the die and mandrels. For aU-roving pultrusion, where a radio frequency (RF) generator may be used in the typical pultrusion to accelerate cure by pre-heating the wet material package (where the RF energy initiates cure before the material enters the die, thereby reducing cure time in the die and increasing running speeds), in the practice of this invention however, RF generators are typicaUy replaced with coohng chambers before entering the die, thus slowing down any cure so that the resulting extrudate can be cut into plugs that are curable when deposited into open ceUs, aUowing expansion of the plugs within the open ceUs. In the typical case, the matrix resin will be a thermosettable resin, e.g., polyester, vinyl ester, epoxy and phenohc. However, thermoplastic pultrusion technology brings the strength, toughness, reformabihty and repairability of thermoplastic com ¬ posites to pultruded parts and structural shapes. The process essentiaUy reverses thermoset pultrusion, in that the thermoplastic is heated to soften it, and cooled to harden it. In the case with this invention, using a matrix resin/particle mixture where the matrix resin is a thermoplastic, heating is preferred before entry to the die because heating does not permanently set the plug that is generated from the pul- truder. Extrusion and casting are weU known industrial techniques. In the practice of this invention, the combination of the matrix resin/particle mixture, with or without the inclusion of reinforcing staple fiber, are extruding as a Uowable mass through dies containing orifices of the appropriate size for making an elongated rod having the diameter (or its equivalent for shapes other than a cylinder) that may be sawed to the size desired for the plug. In casting, dough mixed or extruded masses of the matrix resin/particle mixture may be poured into casting forms sized to conform to the shape of the desired plug. Cooling in extrusion and casting is desirable when the matrix resin is thermosettable. The drawings illustrate schematicaUy some of the structure and processes for effecting this invention. Figure 1 is perspective view of a section of prior art honeycomb structure 1, which comprises honeycomb core 2, honeycomb bottom surface panel 3 and honeycomb top surface panel 4. In this depiction, the depth of core 2 appears relatively shaUow. In most cases, the depth of core is greater than its diameter. Figure 2 is an overhead perspective view of a prior art honeycomb core, which comprises honeycomb core opening (i.e., core well) 5 and honeycomb core waU and waU edge 6. Figure 3 comprises Figures 3a, 3b and 3c, which depict different shaped expandable monohthic composite inserts (or plugs) of the invention for honeycomb core, such as depicted perspectively from the top in Figure 3d. Figure 3a is a perspective view of a hexagonal shaped plug of the invention that corresponds to the shape of honeycomb core 15 of Figure 3d. Matrix resin/particle mixture 9, which is the resin component of composite containing in situ-expandable thermoplastic particles essen ¬ tiaUy uniformly dispersed in a thermosettable or thermoplastic matrix resin system that is incompatible with the particles, has a conventional hexagonal shape. In this embodiment of the plug, matrix resin/particle mixture 9 embraces in this case a rein ¬ forcing fibrous core 8. The reinforcing fibrous core 8 may be made of continuous fila ¬ mentary tow, yarn, and the like. It also may be made of spun continuous filament or staple yarn. If the yarn is made from staple fiber, it is preferred that the yarn has a twist or false twist. In many embodiments of this invention, a plug suitably contain ¬ ing matrix resin/particle mixture can be free of a fibrous core within the matrix resin/particle mixture. Instead, the matrix resin/particle mixture may contain reinforcing staple fiber uniformly dispersed therein. In the case of Figure 3b, matrix resin/particle mixture 11 corresponds to matrix resin/particle mixture 9 in descrip ¬ tion. In Figure 3b, matrix resin/particle mixture 11 has a cylindrical shape, and embraces an optional fibrous core 10, which corresponds to fibrous core 8 in description. The plug characterized in Figure 3b is suitable for insertion into any shaped core of a honeycomb, such as honeycomb cores that are cylindrical, rectangular or hexagonal (viz, Figure 3d). Figure 3c depicts a perspective view of a rectangular shaped plug containing matrix resin/particle mixture 13, which corresponds to matrix resin/particle mixture 9 in description, and an optional fibrous core 12, which corresponds to fibrous core 8 in description. Any of the shaped plugs of Figures 3a, b and c can be inserted into hexagonal core 15, so long as their shape is smaUer in dimension to that of core 15. The size of plug is determined by the degree of expansion that is designed into the formulation of the matrix resin/particle mixture and the density of the expanded plug that is desired. Figure 4 comprises a sectional view of a simplified extrusion/pultrusion apparatus for making monohthic composites of the invention. The apparatus comprises a pultrusion chamber 20, typicaUy cylindrical, rectangular or hexagonal in shape, a matrix resin/particle mixture flow hne 21 and an upper zone where fibrous compo ¬ nents 22 are transported into contact with the matrix resin/particle mixture, and an outlet section 28. The matrix resin/particle mixture 26 [resin component flow of com ¬ posite containing in situ-expandable thermoplastic particles essentiaUy uniformly dispersed in a thermosettable or thermoplastic matrix resin system that is incom ¬ patible with the particles having a cylindrical shape] is transported through flow hne 21 from a standard screw extruder (not shown) into chamber 20. A suitable extruder is a Theyson (Theyson America, Inc., Charlotte, NC) twin screw extruder. Fibrous components 22 are puUed though chamber 20, and the number of fibrous components 22 can range from 1 to hundreds in number depending on their size and weight and the degree of reinforcement sought for the plugs of the invention. The matrix resin/particle mixture 26 is forced around fibrous component 22 in chamber 20 and the flow of the composite is passed from section 28 through the orifice dies opening and saw resulting in formation of the plug. A conventional knife cutter (not shown) is downstream of composite 28 serving to cut the composite to any convenient size for use, such as the depth of the honeycomb core into which the composite is to be inserted. The resulting monohthic composite contains in sz ' zta-expandable thermoplastic particles essentiaUy uniformly dispersed in a thermosettable or thermoplastic matrix resin system that is incompatible with the particles, in this illustration, having a cylindrical shape, embracing the fibrous strands 22 and suitable for insertion into any shaped core of a honeycomb, such as honeycomb cores that are cylindrical, rectangular or hexagonal. Figure 5 is a schematic cross-sectional view of monohthic composite system in which the matrix resin/particle mixture is in the state of flow before plug formation, taken along hnes 5-5'. The number of component 22 may vary as indicated above. Figure 6 is a schematic sectional view of segment 40 of a pultrusion apparatus for making the monohthic composites of the invention. It comprises a cylindrical feed section 41, a matrix resin/particle mixture cylindrical dehvery section 41 for flow of matrix resin/particle mixture from an extruder (not shown) in the direction of flow 46, a conical throat channel 42 for forcing the matrix resin/particle mixture into the fibrous component feed 47, a fibrous feed section 44 sealed from section 41 for trans ¬ porting fibrous components 47 to blending channel 43 wherein the matrix resin/particle mixture wets the fibrous components 47, and an open bottom 45 that feeds to the orifice of the die. The orifice of the die may have a diameter, or its equivalent in the case of other shapes, that is equal to that of blending channel 43. Figure 7 is a schematic cross-sectional view of Figure 6 taken along hnes 7-7'. In this figure, the fibrous components 48 exceed the number shown in Figure 6, illus ¬ trating the fact that fiber reinforcement may vary considerably pursuant to this in ¬ vention. Figure 8a is a schematic sectional view of a segment 50 of a multi-orifice pultrusion apparatus for making the monohthic composites of the invention. Figure 8b is the cross-sectional view along hne 8-8'. It comprises upper cyhnder cover and outside waU of fibrous feed section 51 of the apparatus above blending channels 56; central cylindrical feed hne 52 for dehvering matrix resin particle mixture from an extruder (not shown); fibrous component feeds 53 are transported to blending channels 58 through sealing sleeves 55; matrix resin particle mixture feed channels 54 connecting matrix resin/particle mixture feed section 52 and blending channels 58; matrix resin/particle mixture fed to hne 52 is dehvered to channels 58 to impregnate fibrous components 53 and the composite is transported through channel openings 57 to dies (not shown) and cutting mechanisms (not shown) to make the plugs of the invention. Figure 9 comprises Figure 9a showing a casting tray 70 for making a monohthic composite 73 of the invention, as shown in Figure 9b. Tray 70 is made of any of a variety of materials 72, such as metal (e.g., steel, aluminum, metal aUoys, etc.), ceramics, plastic composites, and the like, contains casting weUs that rephcate the dimensions of monohthic composite 73. The top of the plug 74 (which rephcates the bottom of casting weU 71) optionaUy contains a centraUy fitted core indentation into which in casting weU 71 fibrous filamentary material 76 are frictionaUy engaged and held during the casting process. The presence of the fibrous filamentary material 76 is optional to the practice of the invention. In the embodiment of Figure 9, plug 74 contains a rim 75 which provides physical stability when the plug is placed into the core of a honeycomb. Rim 75 may be the base of the plug when deposited into a hon ¬ eycomb core, or it may be the top of the plug in the honeycomb core. If desired, the plug may be made free of a rim. The thermosetting matrix resin suitable for use in the invention comprise those typicaUy used in the manufacture of SynSpand® and SynCore® syntactic foams. For example, epoxies curing at 350°F. (177°C.) and 250°F. (121°C.) are the most common matrix resins, but matrices of bismaleimide (BMI), phenohc, polyester, PMR-15 polyimide and acetylene terminated resins that have been used to produce SynCore ® products, are usable in the practice of the invention. However, the inven ¬ tion includes as weU, other thermosetting resins,' indeed, the invention includes the famUy of thermosetting resins. For example, thermosetting resins from acryhcs, polyurethanes, free-radicaUy induced thermosetting resin, and the like, may also be used in the practice of the invention. As a result of such considerable choices in thermosetting resins, the thin in sz ' zta-expandable films of the invention are ta orable to making syntactic foams for a wide variety of apphcations. In addition, the matrix resin may be thermoplastic. Suitable thermoplastic resins are those that have a melting point below the melting point of the in situ- expandable thermoplastic particles. Suitable thermoplastic resins include the nylon resins, the polyphenylene oxide resin (e.g., the phenoxy resins such as Bakehte ® phenoxy resin PKHC, the GE PPO resins, the GE NORYL GTX® is a PPE/PA blend - a polyamide (PA) product which is reinforced with modified polyphenylene ether polymer (PPE), the GE NORYL modified PPE/PS blend, BPAmoco Chemicals' Radel®, Udel® and Mindel® sulfone polymers, BASF's Ultradur for partly crystalline, thermoplastic saturated polyester based on polybutylene terephthalate (PBT), Polycarbonate resins (e.g., GE's Lexan resins), polyethylene terephthalate and co- polymers with isophthalate, and.the like. Many thermosetting resins are sohds at about 23°C. and many of them are liquids at this temperature. Both kinds of resins can be made processable or ex- trudable. For example, a resin that is sohd and a resin that is hquid can be combined to form a mixed resin system that is processable or extrudable. In addition, a sohd or hquid thermosetting resin can have incorporated in it a variety of diverse materials which will render the resin more or less process at conventional handhng tempera ¬ ture conditions and pourable at room temperature (about 15-37°O). Conventional handhng temperatures are defined as a temperature of between about -20°C. to about 43°O 2 Though the in sΛu-expandable thermoplastic particles will render a hquid thermosetting resin more viscous, they alone are not effective for making rendering the resin processable. If the thermosetting resin is sohd, it can be calendared into a film by melting the resin with heat under conditions that avoid condensation or addi ¬ tion of the resin to a thermoset condition (C-stage). If the resin is a hquid, it can be blended with thixotropic agents, other sohd resins and/or hquid or thermoplastic elastomeric modifiers to convert the resin from a hquid to a processable material. The typical thermosetting resin is an A-stage resin. In some cases, it may be desirable to utilize a B-stage resin but in the typical case, such is done in combination with an A-stage resin. Such B-stage resin wiU affect the viscosity of the resin formulation but they are not relied on to achieve the level of proeessability for the most effective operation of the invention. A preferred class of thermosetting resin in the practice of the invention are the epoxy resins. They are frequently based, inter alia, on one or more of diglycidyl ethers of bisphenol A (2,2-bis(4"hydroxyphenyl)propane) or syπrtris(4- hydroxyphenyl)propane, tris(4-hydroxyphenyl)methane, their polyepoxide condensa- 2 This range reflects the fact that material handling can require low temperature storage to preclude premature reaction of the thermosetting resin system and relatively high temperatures because the film may be used on a hot factory floor. tion products, cycloahphatic epoxides, epoxy-modified novolacs (phenol-formaldehyde resins) and the epoxides derived from the reaction of epichlorohydrin with anahne, o-, m- or p-aminophenol, and methylene dianaline. The epoxy resins suitable in the practice of the invention include the various established thermosetting epoxy resins conventionaUy employed in making prepregs, especiaUy carbon and graphite fiber reinforced prepregs. It is desirable that the ep ¬ oxy resin be a low or lower viscosity version to facilitate film formation. lUustrations of suitable epoxy resins include, e.g., one or more of diglycidyl ethers of bisphenol A (2,2-bis(4-hydroxyphenyl)propane), such a those of the foUowing formula: or sym-tris(4-hydroxyphenyl)propane or tris(4"hydroxyphenyl)methane, their poly- epoxide condensation products, cycloahphatic epoxides, epoxy-modified novolacs (phenol-formaldehyde resins) of the formula: wherein n is 0-1.8, preferably 0.1-0.5. Other epoxy resins may be combined with the above epoxy resins or used alone. They include, e.g., 3,4-epoxy cyclohexyl methyl-3,4-epoxy cyclohexane carboxy- late, vinyl cyclohexene dioxide, 2-(3,4-epoxy cyclohexyl -5,5-spiro - 3,4-epoxy) cyclohexane - meta - dioxane, bis (3,4-epoxy cyclohexyl) adipate, and the like. The epoxy resins of the invention are combined with hardeners which cure the resin to a thermoset condition. The preferred hardeners are amine com ¬ pounds, ranging from dicyandiamide, to ureas, to ahphatic and aromatic amines. A preferred class of hardeners are the aromatic amines encompassed by the formula: Q is one or more of a divalent group such as — SO2— , -O— , — RR'C-, — NH— , —CO— — CONH-, -OCONH-, and the hke, R and R' may each independently be one or more of hydrogen, phenyl, alkyl of 1 to about 4 carbon atoms, alkenyl of 2 to about 4 carbon atoms, fluorine, cycloalkyl of 3 to about 8 carbon atoms, and the like, x may be 0 or 1, y may be 0 or 1 and is 1 when x is 1, and z may be 0 or a positive integer, typicaUy not greater than about 5. Preferred hardeners are diamines of the formula: Another preferred class of hardeners are the ahphatic amines such as the al- kyleneamines. IUustrative of suitable alkyleneamines are the foUowing: monoethanolamine ethylenediamine N- (2-aminoethyl)ethanolamine Diethylenetriamine Piperazine N-(2-aminoethyl)piperazine Triethylenetetramine Tetraethylenepentamine Pentaethylenehexamine Diaminoethylpiperazine Piperazinoethylethylenediamine 4-Aminoethyltriethylenetetramine Tetraethylenepentamine Aminoethylpiperazinoethylethylenediamine Piperazinoethyldiethylenetriamine Another class of hardeners, but which can also be used as extender of the epoxy resin, are the higher molecular weight poly(oxyalkylene)polyamines such as those of the foUowing formulas: y is b is 8-45 where x, y and z range from about 2-40 where m + t is about 82-86 The hardener may be a monoamine such as aniline, para-aminophenol, and alkylated versions of them. A further class of desirable hardeners are the reaction products of dial- kylamines, such as dimethylamine, diethylamine, methylethylamine, drn- propylamine, and the hke, with a variety of mono and dusocyanates to form mono and diureas. Any of the polyisocyanates hsted below may be so reacted for use as a hardener. Specific iUustration of useful hardeners are those encompassed by the following formulas and descriptions: Preferred urea hardeners are those that are the reaction products of di ¬ me thylamine with mixtures of 80% 2,4-tolylene diisocyanate and 20% 2,6-tolylene diisocyanate, polymeric isocyanate, p-chlorophenyhsocyanate, 3,4- dichlorophenylisocyanate or phenyhsocyanate. The amount of the hardener employed is usuaUy stoichiometricaUy equivalent on the basis of one amine group per epoxy group in the resin. If the epoxide is a triepoxide and the hardener is a diamine, then the molar ratio of hardener to epoxide would typicaUy be about 2.5/3 or 0.83. A typical formulation would have a weight ra ¬ tio of epoxy resin to hardener of about 3/2 to about 4/1. Where any of the hardeners serve primarily as extenders of the epoxide resin, then the amount of the hardener in the typical case wiU be less than that generaUy employed for hardening the epoxide. Mixtures the above hardeners and with other hardeners are within the contemplation of this invention. Other reactive resin systems include the various thermosetting or thermosettable resins include the bismaleimide (BMI), phenohc, polyester (especiaUy the unsaturated polyester resins typicaUy used in SMC production), PMR-15 polyimide and acetylene terminated resins are also suitable. The urethane systems represent a typical non-engineering polymer for apphcations demanding less performance than, for example, the epoxy resin systems. They typicaUy comprise the reaction of a polyisocyanate, a polyol, alone or with another active hydrogen compound, typicaUy in the presence of a catalyst, such as an amine catalyst. However, in the practice of this invention, the polyurethane is a mixture of a blocked polyisocyanate, such as the reaction product of a mono-ol or mono- hydroxy phenohc compound with a polyisocyanate that is an organic polyisocyanate. This includes an organic compounds that contain at least two isocyanato groups and include the hydrocarbon dusocyanates (e.g., the alkylene dusocyanates and the ary- lene dusocyanates), as weU as known trusocyanates and polymethylene polyCphenylene isocyanates). The blocked isocyanates are compounds of the formula: where R is a monovalent organic group; R' is alkyl, halo, alkoxy, and the like; and a is 0-4. IUustrative polyisocyanates for use in making the blocked isocyanates are: 2,4'- diisocyanatotoluene 2, 6- diisocyanatotoluene methylene bis(4-cyclohexyl isocyanate) 1, 2-dusocyanatoethane 1,3-diisocyanatopropane 1, 2-diisocyanatopropane 1,4-diisocyanatobutane 1,5-dusocyanatopentane 1,6-diisocyanatohexane bis(3-isocyanatopropyl)ether bis(3"isocyanatopropyl) sulfide 1, 7-diisocyanatoheptane l,5-diisocyanato-2,2-dimethylpentane l,6-dusocyanato-3-methoxyhexane 1, 8-diisocyanatooctane l,5-dusocyanato-2,2,4-trimethypentane 1, 9-diisocyanatononane l,10-disocyanatopropyl)ether of 1,4- butylene glycol 1, 11-dusocyanatoundecane 1, 12-diisocyanatododecane bis(isocyanatohexyl) sulfide 1,4- diisocyanatobenzene 2, 4-dusocyanatotolylene 2, 6-diisocyanatotolylene 1,3-dusocyanato-o-χylene 1,3-diisocyanato-m-χylene 1,3-dusocyanato-p-χylene 2,4-diisocyanato-l-chlorobenzene 2,4-dusocyanato-l-nitrobenzene 2,5-diisocyanato-l-nitrobenzene 4,4-diphenylmethylene diisocyanate 3,3-diphenyl-methylene diisocyanate polymethylene poly (phenyleneisocyanates) isophorone diisocyanate and mixtures thereof. The preferred polyisocyanates are mixture of 80% 2,4-tolylene diisocyanate and 20% 2,6-tolylene dusocyanate and polymeric isocyanate. The blocked isocyanates comprise any of the above polyisocyanates reacted with a monofunctional hydroxy containing compound. The resultant blocked polyisocyanate is unreactive towards hydroxyl compounds at room temperature but, at elevated temperatures, will func ¬ tion as an isocyanate to crosshnk the hydroxyl compounds to form the thermoset resin. For example, an adduct of tolylene diisocyanate and trimethylolpropane is first prepared in solution, foUowed by the addition of phenol to block the remaining isocyanate groups. lUustrative of such a blocked polyisocyanate is a phenol blocked toluene dusocyanate in ceUosolve acetate sold by Mobay Chemical Co., as Mondur S. Such blocked isocyanates, when mixed with the polyols, provide a thermosetting polyurethane matrix resin that can form a tacky in sΛirexpandable film that is stor- able and curable on demand, in accordance with the invention. The polyols used in forming the polyurethane may be an organic diol, triol, tetraol, pentaol, and the like. lUustrative are the foUowing compounds: ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propylene glycol, dr 1,2-propylene glycol, tri- 1,2-propylene glycol, tetra- 1,2-propylene glycol, 1,4- butanediol, 1,3-propanediol, and the like; or formed from by the alkoxylation of a starter polyol, such as the class of polyols characterized; or formed from reaction of the above diols, triols, etc., with caprolactone. The resulting ester polyols ("Tone's") are widely used in reactions with isocyanate. Desirable alkoxylated polyols are al- kylene oxide adducts of a hydroxylated alcohols of the formula: A(OH) >ι and preferably a "starter" diol, triol, tetraol and higher hydroxylated alcohols, of the formula: wherein A is a polyvalent organic moiety, the free valence of which is 2-6 or greater, or an average value equal thereto, as the case may be. lUustrative of suitable compounds embraced by the "starter" A(OH)-.2-β alcohol are the foUowing: ethylene glycol, diethylene glycol, 1,2-propylene glycol, polyethylene glycol, polypropylene glycol, glycerine, pentaerythritol, sorbitol, diether of sorbitol, mannitol, diether of mannitol, arabitol, diether or arabitol, sucrose, mixtures thereof, and the like. The starter A(OH)--2-6 is first reacted with 1,2-alkylene oxide in an amount and under conditions sufficient to convert its hydroxyl groups to hydroxyalkyl groups. The amount of 1,2-alkylene oxide reacted is sufficient to achieve the ultimate molecu ¬ lar weight of the alkoxylated polyol adduct. The molecular weight of the alkoxylated polyol adduct should be relatively high, preferably above about 4000 (number aver ¬ age) and, more preferably, above about 5000. The minimum molecular weight of the alkoxylated polyol adduct may be about 2000. The preferred 1,2-alkylene oxides are lower 1,2-alkylene oxides, such as ethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, and the like. The resulting polyol may be hydroxyethyl capped by reaction with 1,2-ethylene oxide to provide assurance of primary hydroxyl content in the polyol especiaUy if the alkoxylated polyol adduct is subsequently coupled, not polymerized, with an organic polyisocyanate. Such alkoxylation reactions, with consequent adduct formation, is weU known in the art. Adduct reactions may be base or acid catalyzed, with base catalyzation preferred. The organic polyol may be a polyester polyol, such as a polyester of a dicarbox- yhc acid, acid hahde or anhydride and a polyol, such as those characterized above. In this case, it is desirable to aUow the polymer to be hydroxyl terminated, and conventional procedures in the art are useful for this purpose. A polyol is also employed to produce the polyester. Such polyols include ethylene glycol, diethylene glycol, propyl- ene glycol, dipropylene glycol, butylene glycols, neopentyl glycol, glycerol and 1,1,1- trimethylolprop ane . Polyester resins usable as the thermosetting matrix resin, are typicaUy reaction products of a dicarboxyhc acid, acid hahde or anhydride, with a polyhydric alcohol. The dicarboxyhc acids or anhydrides that are employed to produce the polyester, either singly or in combination, include those that contain olefinic unsaturation, preferably wherein the olefinic unsaturation is alpha, beta- to at least one of the carboxylic acid groups, saturated ahphatic, heteroahphatic and aromatic polycarboxyhc ac- ids, and the hke. Such acids include maleic acid or anhydride, fumaric acid, methyl maleic acid, and itaconic acid (maleic acid or anhydride and fumaric acid are the most widely used commercially), saturated and/or aromatic dicarboxyhc acids or anhy ¬ drides such as phthahc acid or anhydride, terephthalic acid, hexahydrophthahc acid or anhydride, adipic acid, isophthalic acid, and "dimer" acid (i.e., dimerized fatty ac ¬ ids). They may be cured by providing a polymerization initiator and low viscosity crosshnking monomers in the formulation. Where the resin is a unsaturated polyes ¬ ter or vinyl ester, it is preferred that the monomers contain ethylenic unsaturation such that the monomer is copolymerizable with the polyester and terminaUy unsatu ¬ rated vinyl ester resins. Useful monomers include monostyrene, alkyl acrylates and methacrylates such as C1-12 alkyl acrylates and methacrylates, substituted styrenes such as α-methyl styrene, α-chlorostyrene, 4-methylstyrene, and the like, divinylben- zene, acrylonitrile, methacrylonitrile, and the like. Styrene is the preferred monomer in commercial practice today, although others can be used. Suitable polymerization initiators include t-butyl hydroperoxide, t-butyl perbenzoate, benzoyl peroxide, cu- mene hydroperoxide, methyl ethyl ketone peroxide, and others known to the art. The polymerization initiator is employed in a catalyticaUy effective amount, such as from about 0.3 to about 2 to 3 weight percent, based on the weight of polyester and the crosshnking monomer. When desired, a thickening agent can also be employed in the polyester thermosetting compositions. Such materials are known in the art, and include the oxides and hydroxides of the metals of Group I, II and III of the Periodic Table . lUustrative examples of thickening agents include magnesium oxide, calcium oxide, calcium hydroxide, zinc oxide, barium oxide, magnesium hydroxide and the hke, including mixtures of the same. Thickening agents are normaUy employed in proportions of from about 0.1 to about 6 weight percent, based upon weight of the polyester resin and crosshnking monomer. Particularly desirable materials for rendering the thermosetting resin processable are thixotropic agents and or elastomer-type polymers that provide discrete elastomer phases (second phases) in the thermosetting resin matrix. Certain of these material may reduce, to some finite degree, the crosshnking density of the thermoset resin (C-stage). Many of these materials introduce very favorable properties to the resulting thermoset resin. For example, a particularly desirable material for this purpose, is an elastomeric polymer containing soft and hard segments, the hard seg ¬ ments acting hke or forming on processing, crosshnking of the elastomeric type. Some of these elastomeric types contain functional end groups which allow it to cou ¬ ple with complementary functional monomers or polymers to form the desired elastomer in situ of the thermosetting resin and render it processable , while toughening the cured resin. As a class, these elastomeric polymers act or are crosshnked yet are thermoprocessable, which when discretely provided in the matrix resin renders the resin processable, and also toughens it. One class of suitable elastomer-type thermosplastic ABS (acrylonitrile-1,4- butadiene-styrene) block copolymers that are typicaUy used as modifiers of other resin systems. They are characterized as having a wide range of properties though the preferred systems of the invention utilize copolymers that are high rubber types that, when compared to other copolymers of this type, have a relatively low tensUe strength, low tensUe modulus, higher impact resistance, low hardness and heat deflection temperature. Another elastomer that is found desirable are the carboxyl and amine terminated hquid butadiene acrylonitrUe copolymers. Such copolymers may contain pendant carboxyl groups in the interior of the polymer structure through the inclusion of methacrylic or acrylic acid in the polymerization or through the hydrolysis of some of the pendant nitrile units. Such polymers react with the epoxy resin and as a result, the epoxy forms the hard segment generating the elastomer properties. Another class of block thermoplastic elastomers are Kraton™, avaUable from SheU Chemical Company. These thermoplastic rubber polymers possess usable thermoplastic properties. They can be softened and they flow under heat and pressure. They then recover their structures on coohng. The chemical make-up are of three discrete blocks of the hnear or A-B-A type. They are avaUable as styrene- butadiene-styrene (S-B-S) block copolymers, styrene-isoprene-styrene (S-B-S) block copolymers and styrene-ethylene/butylene-styrene (S-EB-S) block copolymers. They are characterized by styrene polymer endblocks and an elastomeric midblock. After processing, the polystyrene endblocks physicaUy crosshnk, locking the rubber net ¬ work in place. This physical crosshnking is reversible on heating. Another series of the Kraton™ thermoplastic rubbers are the diblock polymers in which one block is a hard thermoplastic and the other is a saturated soft elas ¬ tomer. lUustrative of this series is Kraton™ G 1701, a diblock polymer of a hard polystyrene block and a saturated, soft poly (ethylene -propylene) block. Other rubbers or elastomers include: (a) homopolymers or copolymers of conjugated dienes having a weight average molecular weight of 30,000 to 400,000 or higher as described in U.S. Pat. No. 4,020,036, in which the conjugated dienes con ¬ tain from 4-12 carbon atoms per molecule such as 1,3-butadiene, isoprene, and the like; (b) epihalohydrin homopolymers, a copolymer of two or more epihalohydrin monomer, or a copolymer of an epihalohydrin monomer(s) with an oxide monomer(s) having a number average molecular weight (MΛ) which varies from about 800 to about 50,000, as described in U.S. Pat. No. 4,101,604; (c) chloroprene polymers including homopolymers of chloroprene and copolymers of chloroprene with sulfur and/or with at least one copolymerizable organic monomer wherein chloroprene constitutes at least 50 weight percent of the organic monomer make-up of the copolymer as described in U.S. Pat. No. 4,161,471; (d) hydrocarbon polymers including ethyl- ene/propylene dipolymers and copolymers of ethylene/propylene and at least one non- conjugated diene, such as ethylene/propylene/hexadiene/norbornadiene, as described in U.S. Pat. No. 4,161,471; (e) conjugated diene butyl elastomers, such as copolymers consisting of from 85 to 99.5% by weight of a C4-C7 isolefin combined with 15 to 0.5% by weight of a conjugated multi-olefin having 4 to 14 carbon atoms, copolymers of iso- butylene and isoprene where a major portion of the isoprene units combined therein have conjugated diene unsaturation as described in U.S. Pat. No. 4,160,759. Specific Ulustrations of suitable elastomeric polymers are the foUowing: 1. Hycar™ CTBN hquid reactive rubbers, carboxyl terminated butadiene- acrylonitrile copolymers sold by B. F. Goodrich. 2. Hycar™ CTBNX, simUar to CTBN except that they contain internal pendant carboxyl groups, also supphed by B. F. Goodrich. 3. Hycar™ ATBN, amine terminated butadiene-acrylonitrUe copolymers sold by B. F. Goodrich. 4. K 1102-28:72 styrene butadiene hnear SBS polymer, avaUable from SheU Chemical Company under the registered trademark "Kraton" 1102. 5. KDX 1118-30:70 styrene :butadiene copolymer containing 20% SBS triblock and 80% SB diblock, avaUable from SheU Chemical Company under the registered trademark "Kraton" DX 1118. 6. KG 1657-14:86 stryene:ethylene-butylene:styrene copolymer avaUable from SheU Chemical Company under the registered trademark "Kraton" G1657. 7. S 840 A-Stereospecific 43:57 styrene -butadiene SB rubber avaUable from Firestone Synthetic Rubber & Latex Company under the registered trademark "Stereon" 840A. 8. SBR 1006-random 23.5:76.5 styrene butadiene SB block copolymer rubber avaUable from Goodrich Chemical Company under the registered trademark "Ameripol" 1006. 9. SBR 1502-Random 23.5:77.5 styrene butadiene rubber avaUable from Hules Mexicanos, or from Goodrich Rubber Company as "Ameripol" 1502. 10. Cycolac™ Blendex modifier resins (e.g., 305, 310, 336 and 405) - ABS polymers sold by Borg- Warner Chemicals, Inc. Different varieties are avaUable and their suitability depends on the properties sought. lUustrative of thixotropic agents that can render a thermosettable or thermoplastic resin processable are high surface area fumed silicas and organosUyl blocked fumed silicas, various microspheres, and the hke. The most common of the microspheres are made of glass, but quartz, phenohc, carbon, thermoplastic and metal-coated microspheres are usable. The microspheres in those syntactic foam plugs are synthetic hoUow microspheres that comprise individual round spheres or bubbles having diameters which range from about 1 to about 500 microns, preferably about 1 to about 200 microns, with waU thicknesses of about 0.1 to about 20 microns. They typicaUy possess densities ranging from about 0.1 to about 0.5 g./cc. The syn ¬ tactic foam comprising the rigid microspheres in a resin matrix as a result have rela ¬ tively low densities such as densities ranging from about 0.5 to about 0.7 g./cm 3 . Glass is the most common microsphere material in these types of materials, but quartz, phenohc, carbon, thermoplastic and metal-coated microspheres are suitably employable. The thermoplastic polymer used in forming the in szto-expandable thermoplastic particles is readily prepared from a wide choice of materials. A number of patents refer to their manufacture. For example, U.S. Patent No. 3,615,972 describes their preparation by polymerizing the monomer of an aqueous dispersion of (l) organic monomeric materials suitable for polymerization to a thermoplastic resinous material having the desired physical properties, (2) a hquid blowing or raising agent which exerts a httle solvent action on the resulting polymer, and in a quantity in excess of that which is soluble in the polymer, and (3) a dispersion stabilizing material which is utilized to maintain the dispersion. The resulting sohd spherical particles have a quantity of the liquid-blowing agent encapsulated in them as a distinct and separate phase. The thermoplastic polymers are formed by the polymerization of one or more of a variety of different types of alkenyl monomers, such as those of the formula: R° CH 2 =C-X to form homopolymers or copolymers, such as random or ordered (including block) co ¬ polymers. In the above formula, R° may be hydrogen, alkyl, such as methyl, ethyl and the hke, or halogen, such as chlorine, fluorine, bromine or iodine, and X may be an aromatic containing moiety bonded via an aromatic carbon atom, a carbonyl oxy ester moiety, halogen, cyano, oxycarbonyl ester, carboxyl, and the like. lUustrative of these monomers are those in which X is aromatic containing, such as styrene, o- methylstyrene, πrmethylstyrene, p-methylstyrene, ethylstyrene, ar-vinyl-χylene, ar- chlorostyrene, ar-bromostyrene, vinylbenzylchlori.de, p-tert.-butylstyrene, and the like. Also lustrative of these monomers are those in which X is a carbonyl oxy ester moiety to form acrylate monomers alone or in combination with the alkenyl aromatic monomers may also be utilized. Such acrylate-type monomers include methyl methacrylate, ethyl acrylate, propyl acrylate, butyl acrylate, butyl methacrylate, pro- pyl methacrylate, butyl methacrylate, lauryl acrylate, 2-ethyl hexyl acrylate, ethyl methacrylate, and the like. X and R° may be a halogen, such as chlorine, fluorine, bromine and iodine, thereby to encompass the formation of copolymers of vinyl chloride and vinylidene chloride, acrylonitrUe with vinyl chloride, vinyl bromide, and simUar halogenated vinyl compounds. X may be a cyano group and this includes polymers of acrylonitrUe and methacrylonitrile. When X is an oxycarbonyl esters, such as the vinyl esters, such as, vinyl acetate, vinyl butyrate, vinyl stearate, vinyl laurate, vinyl myristate, vinyl propionate, and the hke, are suitable polymeric components. One may also employ for specific purposes ethylenicaUy unsaturated co- polymerizable acids such as acrylic acid, methacryhc acid, itaconic acid, citraconic acid, maleic acid, fumaric acid, vinylbenzoic acid, and the hke. The thermoplastic polymers may also include copolymers (of the random or ordered varieties, especiaUy blocked copolymers) of the monomers described above with a variety of hydrocarbon monomers, such as propylene, butene, and one or more dienes, such as: straight chain acyclic dienes such as: 1,4-hexadiene, 1,6-octadiene, and the hke, ' branched chain acyclic dienes such as: 5-methyl- 1,4-hexadiene, 3,7- dimethyl- 1,6-octadiene, 3,7-dimethyM,7-octadiene and the mixed isomers of dihy- dromyrcene, dihydroocinene, and the like; single ring alicyclic dienes such as: 1,4- cyclohexadiene, 1,5-cyclooctadiene, 1,5-cyclododecadiene, and the hke, ' multi-ring ali ¬ cyclic fused and bridged ring dienes such as: tetrahydroindene, methyltetrahydroin- dene, dicyclopentadiene, bicyckr(2,2,l)-hepta-2,5-diene, aU enyl, alkyhdene, cycloal- kenyl and cycloalkylidene norbornenes such as 5-methylene-2-norbornene (MNB), 5- ethyhdene-2-norbornene (ENB), 5-propyl-2-norbornene, 5-isopropyhdene-2- norbornene, 5-(4-cyclopentenyl)-2-norbornene, 5-cyclohexyhdene-2-norbornene, and the hke. The thermoplastic polymer used in forming the in szfø-expandable thermo ¬ plastic particles may also be made from condensation type polymers, such as nylon- 6,6; nylon-6; nylon-4,6; polyester from polyethylene terephthalate! Kevlar™ polyara- mide; polycarbonates (viz., poly (2,2-bis (l.4-oxyphenyl) propane carbonate)); polyary- lates (viz., poly (2,2-bis(l.4-oxyphenyl) propane terephthalate); polyimides,' poly etherimides, such as Ultem™ 3 ; polysulfones (see U.S. Patents No. 4,175,175 and 4,108,837), such as Udel™ and Radel™ A-400 4 ; the polyethersulfones (see U.S. Patents Nos. 4,008,203, 4,175,175 and 4,108,837), such as Victrex™ PES; polyarylsul- fones; polyarylamideimides, such as Torlon™ 5 ,' and the like. A wide variety of blowing or raising agents may be incorporated within the polymerization system. They can be volatUe fluid-forming agents such as ahphatic hydrocarbons including ethane, ethylene, propane, propylene, butene, isobutylene, neopentane, acetylene, hexane, heptane, or mixtures of one or more such ahphatic hydrocarbons having a molecular weight of a least 26 and a boiling point below the range of the softening point of the resinous material when saturated with the particular blowing agent utilized. 3 General Electric Plastics 4 BP Amoco. 5 BP Amoco. Other suitable fluid-forming agents are the chlorofluorocarbons such as those described in U.S. 3,615,972 (column 4, hnes 21-30) and tetraalkyl sUanes such as tetramethyl sUane, trimethylethyl sUane, trimethylisopropyl sUane and trimethyl-n- propyl sUane. As pointed out in this patent, the boUing point of such foaming agents at atmospheric pressure should be about the same temperature range or lower than the softening point of the resinous material employed. As pointed out in U.S t Patent No. 4,397,799, patented August 9, 1983, the par ¬ ticle size of the unexpanded particles, as weU as the expanded microspheres can vary widely. Particle sizes for the unexpanded particles can range, for example, from about 1 μm to about 1 mm, preferably from about 2 μm to about 0.5 mm. One version of in sifrrexpandable particles are sold under the name Expancel ® , by Casco Prod ¬ ucts, (http://www.expancel.com/) a business unit of Akzo Nobel, a Dutch company, (http://www.akzonobel.com/companv/) located in SundsvaU, Sweden (with a subsidiary in Duluth, Georgia.). EXPANCEL ® microspheres are smaU spherical plastic particles. The microspheres consist of a polymer sheU encapsulating a gas. Expancel® range in unexpanded particle size from about 5μm to about 50μm. When the gas inside the sheU is heated, it increases its pressure and the thermoplastic sheU softens, resulting in an increase in the volume of the microspheres. The particle diameters expand 2 to 5 times. In some instancews, when fuUy expanded, the volume of the microspheres may increase more than 40 times. "The — expansion when heated and the other — properties of EXPANCEL ® microspheres are due to a smaU amount of a hydrocarbon encapsulated by a gas tight thermoplastic sheU. When the microspheres are heated the thermoplastic sheU softens and the hydrocarbon inside the sheU increases its pressure. This results in a[n] — expansion of the spheres (typical diameter values: from 10 to 40 μm), with a corresponding — decrease of the density (typical values: from 1000 to 30 g/liter). The different EXPANCEL ® microsphere grades vary according to expansion factor, heat resistance, particle size and chemical and solvent resistance." There are grades of EXPANCEL® microspheres avaUable with expansion temperatures in the range of 80 -190°C (176 - 374°F). The temperature at which ex ¬ pansion starts as weU as the temperature at which the maximum expansion and the lowest density is obtained depends to some degree on the heating rate. At tempera ¬ tures above the temperature at which the highest expansion is obtained the micro- spheres graduaUy coUapse. The unexpanded versions of EXPANCEL ® include EXPANCEL ® WU, repre ¬ senting wet unexpanded microspheres, EXPANCEL ® DU, representing dry unex ¬ panded microspheres, EXPANCEL ® slurry representing a dispersion of unexpanded microspheres, and EXPANCEL ® MB representing a master batch of unexpanded mi ¬ crospheres in a matrix. Preferred Expancel ® micropheres are the DU variety de ¬ scribed in the foUowing table. (1) Expancel Technical Bulletin no 3 (2) Expancel Technical Bulletin no 5 (3) Expancel Technical Bulletin no 7 Technical Bulletins obtainable from Expancel, Box 13000, S-850 13, Sundsvall, Sweden. Preferably, the particles used have a mixed particle size of wide spread to achieve the best packing, on expansion, in the syntactic molded foam. A particularly preferred in sΛu-expandable particle are the closed-ceU generating Expancel ® 092 DU 120, 091 DU, 091 DU 80, 091 DU 140, 093 DU 120, which are beheved to be terpoly- mers of vinylidene chloride, acrylonitrUe and methacrylonitrUe containing 10-18 weight % isopentane, and possessing the properties set forth in the foregoing table. Expancel® 092 DU 120 is the most preferred of this type of in ≤Λtrexpandable parti ¬ cle. Blowing agents such as the Freons ® , such as trichlorofluorome thane, hydro ¬ carbons such as n-pentane, i-pentane, neo-pentane, butane, i-butane, azodicarbona- mide are commonly suggested blowing agents found in these types of in situ- expandable particles. TypicaUy, the unexpanded particles contain from about 3 to about 40 weight % blowing agent. As noted above, the matrix resin/particle mixture may contain fibers. Such fibers provide toughness properties to the resulting molded syntactic foam product. Fibers that may be used in the practice of the invention may be any organic and inorganic fiber that has a melting temperature (T m ) greater than the cure temperature of the matrix resin in making the expanded molded syntactic foam. Also usable in the practice of this invention are fibrous type of structures, having a length greater than diameter, that are made of amorphous polymers. For example, certain polysulfone fibers having a high T g may be employed. In such a case, the polymer's T g should be greater than the cure temperature of the matrix resin. Suitable fibers may be made from any of the performance and engineering plastics or glass fibers, graphite fibers, carbon fibers, asbestos fibers, and the hke. For example, the fibers may be made from nylon-6,6, " nylon-6; nylon-4,6; polyester from polyethylene terephthalate,' polypropylene, ' ceUulose acetates, rayons, Kevlar ® polyaramide! polycarbonates (viz., poly (2,2-bis (1.4-oxyphenyl) propane carbonate)); polyarylates (viz., poly (2,2-bis(l.4- oxyphenyl) propane terephthalate); polysulfides (see U. S. Patent No. 3,862,095); polyimides; polyetherimides, such as Ultem ®6 ; polyetheretherketones, such as Vic- 6 General Electric Plastics trex® PEEK 7 and polyetherketone or polyetherketoneketone, such StUan® PEK or PEKK. 8 ; polysulfones (see U.S. Patents No. 4,175,175 and 4,108,837), such as Udel ® and Radel® A-400 9 ; the polyethersulfones (see U.S. Patents Nos. 4,008,203, 4,175,175 and 4,108,837), such as Victrex® PES,' polyarylsulfones; polyarylamideimides, such as Torlon ® 10 ; and the hke. The preferred fibers are the glass fibers, graphite fibers, carbon fibers, asbes ¬ tos fibers, and those made from the engineering plastics, such as the polyar lethers which include the polyetherimides, the polyetheretherketones, the polyetherketones, the polyetherketoneketone, the polysulfones, the polyethersulfones, the polyarylsul ¬ fones, the polyarylamideimides, and the like. Particularly preferred fibers are the glass fibers, graphite fibers, carbon fibers, asbestos fibers, and those made from poly ¬ etheretherketones, polyetherimides, polyarylamides, polyarylamideimides, polysul ¬ fones, polyethersulfones and polycarbonates. As indicated previously, there are two ways in which fibers can be incorpo ¬ rated into the plugs of the invention. The fibers can be uniformly distributed throughout the plug as cut or staple fibers or the fibers can be in the form of spun yarn or filaments (continuous filaments or spun twisted yarn) and strategicaUy lo ¬ cated within the plug. The staple or cut fibers are typicaUy in the form of short cut fibers, i.e., staple fibers, ranging from about 2.5 mUhmeters to about 13 mUhmeters. Longer fibers may be used and when they are used, they typicaUy concentrate at or near the syntactic foam's surfaces, as a result of migration during expansion. The diameter of the fibers may range considerably. Preferably, the fiber diameter ranges from about 20 μm to 7 Victrex pic < www.ampersandesign.net/victrex main.html> 8 Raychem Corporation 9 BP Amoco and Boedeker Plastics, Inc. Rt. 2 • Box 5 • 904 West 6th St., Shiner, Texas 77984 10 BP Amoco about 70 μm, preferably from about 30 μm to about 60 μm. The spun yarn or fila ¬ ments are placed within the casted or pultrusion extruded plug components, as Ulus- trated in the drawings and described above. Couphng agents may be used in formulations where bonding between fibrous and filler components of the formulation can be enhanced. Suitable couphng agents are the organofunctional sUane and sUicone coupling agents, where the organofunc- tionahty includes amino groups (e.g., gamma-aminopropyltriethoxy sUane, beta- aminoethyl-gamma-aminopropyl triethoxy sUane, and the hke), epoxy groups (e.g., glycidyloxypropyl trimethoxy sUane, epoxycyclohexyl trimethoxy sUane, and the hke), and acryhc groups (e.g., gamma-methacrylyloxypropyl trimethoxysUane). They can be coated on the fibrous and fiUer components and/or added directly into the resin formulation and effect in situ couphng. A typical resin formulation comprises the foUowing: Preferred Percent Percent Formulation By Weight By Weight Bisphenol A epoxy resin. A preferred resin comprises a mixture of (a) a sohd resin and a hquid resin that yield a proc40 to 80 60 to 75 essable, resin; or (b) a mixture of hquid resins that yield a processable resin! or (c) a mixture of a hquid Bisphenol A epoxy resin and low molecular weight novolak epoxy resin. Incompatible in situ isotropicaUy expandable thermoplastic particles containing expansion 5 to 45 6 to 15 agent therein An elastomer toughening agent. A desirable toughening agent may be a car- boxylated butadiene acrylonitrUe copolymer 0 to l2 0 to 9 elastomer," an ABS block copolymer elastomer; and SBS block copolymer elastomer. Hydroxyl extender for the epoxy resin(s). 0 to 20 4 to 12 The preferred extender is bisphenol A. Amine curing agent. Preferred amine curing agents include ah ¬ phatic amines, alkylene oxide amines, aro- 4 to 12 5 to 10 matic amines and aromatic ureas. Fibers 0 to 20 6 to 14 Glass microspheres 0 to 10 2 to 6 These resin formulations are made by conventional mixing of the components in standard mixing equipment for viscous compositions. Good results have been obtained using a Ross ® Double Planetary Mixer, provided with vacuum construction and jacketing to control temperature and deaerate the mixture. Mixing is typicaUy effected by blending the resin, unexpanded particles, elastomer components, extenders, dUuents, curing agent and fibers (these being added last), and vacuum pumping to remove entrained air. The temperature chosen is variable depending on the viscosity of the formulation. It may be desirable to separately mix the resin and the curing agent. In such a case, the formulation may be divided up to mix the resin with some portion of the formulation to effect a weU dispersed condition and do the same with the curing agent, and then combine the weU dispersed mixes with the fiber component and the unexpanded particles, so as to mix them aU under conditions avoiding premature reaction. Such procedures are weU within the skill of the art. The foUowing describes the use of a laboratory extrusion system for making the matrix resin/particle mixture of the invention that can be extruded, pultruded or casted into the plugs of the invention. The extruder contains the foUowing key units: 1. An intermeshing Theysohn TSK-TT 020 twin screw extruder, which has a screw diameter of 20.6 mm and an L/D ratio of 361 (i.e., the total barrel/extrusion length is 36D) was used for extrusion of the matrix resin/particle mixtures describe below. It is driven by a 3HP electrical motor and is capable of operating from 0 to 500 rpm. It comprises 6 temperature controUed zones, each of length 6D, and a die heating zone at the extruder outlet. Zone temperature is controUed automaticaUy through electrical heating and water-cooling. Various feed ports are located along the extruder barrel length for hquid injection, sohd feeding, venting and vacuum degassing. The extruder is also equipped with screws of various types for different op- eration modes such as melting and plastication, dispersion, distribution, devolatUiza- tion and degassing, and processing of high viscosity materials such as thermoplastic molding and highly fiUed composite processing. Extruder throughput rate ranges from less than 5 pounds per hour to about 30 pounds per hour. 2. The extrusion system contains 2 hquid injection carts with heating capabihty for the injection of hot melt and low viscosity hquids. A third injection cart without heating capabihty is also used for injecting smaU feed rate of low viscosity and/or heat sensitive hquids. 3. The extrusion system contains a twin screw side stuffer that conveys sohd materials through the side port located at 18D. The side stuffer is designed for effective feeding a large volume of sohds (especiaUy low density solids) with reduced suscepti ¬ bility of fluidization because of direct gravity feeding. 4. A cylindrical die with heating and coohng capabilities is directly connected to the outlet of the extruder. A custom designed cylindrical die can also be attached to the die to aUow transportation of extrudate through a hose for downstream processing such as pultrusion. 5. Formulation: Epoxies DEN 431 (Dow Chemical) 16.02 wt. % DEN 438 (Dow Chemical) 16.02 wt. % ECN 1280 (Ciba Products) 16.02 wt. % Curatives (Part B) Epon 826 (SheU Chemical) 11.31 wt. % 4, 4'-DDS (Araldite® epoxy cresol novolac (ECN) 2.6 wt. % Ciba Specialty ChemicalsO Amicure CG1400 (Air Products and Chemical 3.30 wt. % Inc.) MY-24 (Curing Agent, Ajinomoto U.S.A., Inc., 3.30 wt. % Teaneck, NJ) Epon 828/tinted green (SheU Chemical) (green 0.21 wt. % tint is Dupont Monastral ® Green G, Code GT- 751-D) Thixotropic agent TS-720 0.77 wt. % SI 310/SI 311 HoUow glass microspheres (Emer- 21.81 wt. % son & Cuming, Canton, MA) In situ expandable particles 092 DU 120 8.59 wt. % 6. The thixotropic agent is a high surface area inorganic oxide. It is fed into the extruder as a sohd powder through the main feed port at 1.5D. Epoxies are pre-blended as a hquid at elevated temperature (~200°F) in a batch vessel before it is transported to the holding tank of the hquid injection cart. The heated hquid can be pumped at a temperature below 250°F into the extruder, usuaUy through the ports a 4.5D or 6D. Dispersive mixing of the sohd with the hquid occurs in the first three zones of the extruder up to 18D at a temperature from 160°F to 250°F. A large number of kneading elements is used to provide high shear stress to facilitate the mixing. The hoUow glass microspheres, which are shear stress sensitive and hght in density, are introduced into the extruder through the side stuffer at 18D. Deep flighted conveying elements are used to aUow more effective downstream material transport due to the large increase in extrusion material volume after the microspheres addition. Low shear gear type mixing elements are used for low shear stress distributive mixing to avoid breaking and cracking of microspheres. In situ expandable particles which exhibit initiation of expansion at temperature above 220°F to 230°F and could be shear sensitive at temperature above 180°F can be introduced into the extruder either through direct gravity feeding at 12D or through the side stuffer. In the former case, extrusion temperature from zone 3 downstream is reduced to less than 180°F. Fluidization is not obvious at the above hsted loading range. Feeding through the side stuffer together with the glass microspheres (with an independent sohd feeder) is more commonly employed. The curatives are muled into a low viscosity epoxy to form a Part B intermediate. The intermediate is introduced into the extruder as a hquid through another hquid injection cart at around 110°F, usuaUy through the port at 27D. Injection ports at 12D, 15D, and 2 ID can also be used, but downstream injection is recommended to minimize the residence time of the curatives at elevated tern- perature in the extruder. Since maximum residence time in the extruder for this op ¬ eration is no longer than 5 minutes, premature material advancement is minimal at temperature below 180°F. A vacuum has been apphed at the vent port at 24D for de ¬ gassing. To aUow more effective degassing, upstream atmospheric venting at 12D or 15D is also recommended to evacuate the air introduced into the extruder through the side stuffer. The extruder usuaUy runs at a high rpm (250 rpm to 500 rpm) to maximize the throughput rate. 7. Extruded material can be directly discharged to a die for making plugs as defined herein or extruded to a pultrusion device or a cast mold for making plugs as defined. 8. A typical thermoplastic formulation for making plugs-' i. Epoxy novolac DEN 438 38.2 wt. % ii. Phenoxy resin PKHC 9.57 wt. % hi. glass microspheres 22.1 wt. % iv. Part B from above formulation 17.90 wt. % v. 1/64 inch glass fiber 3 wt. % vi. 92 DU 120 8.55 wt. % 9. An extruded non-sagging plug made from this formulation achieved about 30% expansion in a smaU rectangular mold, by increasing heating of the mold at a rate of 5°F/minute to an oven temperature of 260°F and hold the mold at that temperature for 2 hours. The cured plug rephcated the interior of the mold.

Claims: CLAIMS: 1. An article of manufacture comprising an expandable sag-resistant nu ¬ cleus-forming monohthic composite capable of being located within a hoUow interior portion of a structural material and being expanded therein. 2. A process that comprises forming a pre-shaped expandable sag-resistant nucleus-forming monohthic composite for use in reinforcing and stiffening a normaUy open-ceUular structural material. 3. A process for reinforcing or stiffening a normaUy open-ceUular structure, any tubular structure, or any channel structure, by putting at least one expandable sag-resistant nucleus -forming monohthic composite within a hoUow interior portion of said structure. 4. The process of claim 3 wherein the expandable sag-resistant nucleus- forming monohthic composite is pre-shaped 5. A pre-shaped expandable sag-resistant nucleus-forming monohthic com ¬ posite comprising incompatible in situ isotropicaUy expandable thermoplastic parti ¬ cles containing expansion agent therein, essentiaUy uniformly distributed in a thermosettable or thermoplastic matrix resin with which they are incompatible when the thermoplastic particles are in the thermo-expandable state. 6. The monohthic composite of claim 5 wherein the composite contains a thermosettable matrix resin, and the resin is not fuUy cured or at such a state of cure that it inhibits the desired degree of expansion of the composite. 7. The monohthic composite of claim 6 wherein the resin is devoid of a degree of crosshnking that exhibits viscosity increase in the composite. 8. The monohthic composite of claim 7 wherein the composite contains an essentiaUy uniform density and thickness across its breadth and it possesses an external shape and size that is dimensionaUy simUar to a hoUow interior component of a structural material for which it is designed to be introduced for subsequent thermal expansion. 9. The pre-shaped expandable sag-resistant nucleus -forming monohthic com ¬ posite of claim 5 wherein the monohthic composite does not undergo a shaping proc ¬ ess when introduced into a hoUow interior component of a structural material. 10. The pre-shaped expandable sag-resistant nucleus-forming monohthic com ¬ posite of claim 9 wherein the composite is already shaped for introduction into a hol ¬ low interior component. 11. The monohthic composite of claim 10 wherein when the nucleus -forming composite is placed in the hoUow interior component of the structural material and is heated sufficiently to cause isotropic expansion of the incompatible in situ- expandable thermoplastic particles, the nucleus-forming composite uniformly ex ¬ pands to cause the formation of a syntactic foam, that has an essentiaUy faultless interface with the waU(s) of the hoUow interior component and possesses an essentiaUy uniform density throughout. 12. The monohthic composite of claim 11 wherein syntactic foam is closed ceU syntactic foam. 13. The monohthic composite of claim 11 wherein the composite is essentiaUy uniform and substantial or inflexible in quahty or character. 14. The pre-shaped expandable sag-resistant nucleus -forming monohthic composite of claim 5 wherein in the thermo-expandable state the in s/tø-expandable thermoplastic particles are heated to a condition where the expansion agent in the particles are expanding; and the thermoplastic resin in the particles are sufficiently softened that the particles expand in size. 15. The article of manufacture of claim 1 wherein the sag-resistant composite resists sinking or bending, by or as if by weight or pressure when the composite is placed in a hoUow interior component of a structural material, sufficiently to avoid snagging of the composite on the hoUow interior. 16. The monohthic composite of claim 11 wherein the flaws inherently present in the waU of the hoUow interior component are fiUed and sealed by the expanded composite. 17. The monohthic composite of claim 16 wherein the essentiaUy uniform density means that the product has a prescribed uniformity according to a predeter ¬ mined standard. 18. A sag-resistant nucleus -forming monohthic composite of incompatible in sizrirexpandable thermoplastic particles that form closed-microceUs upon isotropic expansion, essentiaUy uniformly distributed in a thermosettable or thermoplastic matrix resin, the monohthic composite a. contains an essentiaUy uniform density and thickness across its breadth b. possesses an external shape and size that is dimensionaUy simUar to a hoUow interior component of a structural material that is about 1.01 to about 4 times greater in volume than the volume of monohthic composite. 19. The sag-resistant nucleus-forming monohthic composite of claim 18 wherein the composite has a shape and has dimensions that essentially correlates the shape and is at least close to proportional to the dimensions of the hoUow interior component of a structural material into which it can be readUy inserted and isotropicaUy expanded to form a syntactic foam with closed microceUs. 20. The sag-resistant nucleus-forming monohthic composite of claim 19 wherein the conformance of the shape and dimension of the sag-resistant nucleus- forming monohthic composite to the hoUow interior component of a structural material is proportional in the dimension of the sag-resistant nucleus -forming monohthic composite to the hoUow interior component such that the composite isotropicaUy expands about 1.01 to about 4 times in volume to fuUy fiU the previously hoUow interior component. 21. The sag-resistant nucleus -forming monohthic composite of claim 20 wherein an end surface of the monohthic composite is appropriately shaped such that it can stand within the hoUow interior component without touching any of the side- waUs of the hoUow interior component. 22. A fiber reinforced sag-resistant nucleus-forming monohthic composite of incompatible in sΛtrexpandable thermoplastic particles in a thermosettable or ther ¬ moplastic matrix resin that contains an essentiaUy uniform density and thickness across the breadth of the composite and which possesses an external shape and size that is dimensionaUy simUar to a hoUow interior component of a structural material, wherein the fiber reinforcement is one or more of continuous filament, continuous tow, chopped or staple fibers, spunbonded fibers, fibrous mat, fibrous webs, or any combination of them. 23. The fiber reinforced sag-resistant nucleus-forming monohthic composite of claim 22 wherein the monohthic composite possesses an aspect orientation and the fibers are ahgned essentiaUy with the aspect orientation of the monohthic composite. 24. The fiber reinforced sag-resistant nucleus -forming monohthic composite of claim 22 wherein the fibers are ahgned in a single direction. 25. The fiber reinforced sag-resistant nucleus-forming monohthic composite of claim 22 wherein the fibers are ahgned in more than one direction. 26. The fiber reinforced sag-resistant nucleus-forming monohthic composite of claim 25 wherein the fiber is a filamentary tow that has a twist. 27. The fiber reinforced sag-resistant nucleus-forming monohthic composite of claim 22 wherein the fiber reinforcement is one or more of organic and inorganic fibers. 28. The fiber reinforced sag-resistant nucleus-forming monohthic composite of claim 22, wherein at least a portion of the fiber reinforcement is made of a fiber that conducts heat better than the thermosettable matrix resin of the composite. 29. The fiber reinforced sag-resistant nucleus -forming monohthic composite of claim 22 wherein the fiber reinforcement is made of a metal or carbon-based mate ¬ rial. 30. The fiber reinforced sag-resistant nucleus-forming monohthic composite of claim 29 wherein the fiber reinforcement is one or more of steel, aluminum, graphite, non-graphitic carbon, and optionaUy contains high performance fibers. 31. The fiber reinforced sag-resistant nucleus -forming monohthic composite of claim 22 wherein the fiber is a filamentary tow that is ahgn centraUy of the interior of the monohthic composite and extends from the bottom to the top of the monohthic composite. 32. The monohthic composite of claim 1 in the shape of a plug. 33. The monohthic composite of claim 2 in the shape of a plug. 34. The monohthic composite of claim 3 in the shape of a plug. 35. The monohthic composite of claim 4 in the shape of a plug. 36. The monohthic composite of claim 5 in the shape of a plug. 37. The monohthic composite of claim 6 in the shape of a plug. 38. The monohthic composite of claim 7 in the shape of a plug. 39. The monohthic composite of claim 8 in the shape of a plug. 40. The monohthic composite of claim 9 in the shape of a plug. 41. The monohthic composite of claim 10 in the shape of a plug. 42. The monohthic composite of claim 11 in the shape of a plug. 43. The monohthic composite of claim 12 in the shape of a plug. 44. The monohthic composite of claim 13 in the shape of a plug. 45. The monohthic composite of claim 14 in the shape of a plug. 46. The monohthic composite of claim 15 in the shape of a plug. 47. The monohthic composite of claim 16 in tne snape of a plug. 48. The monohthic composite of claim 17 in the shape of a plug. 49. The monohthic composite of claim 18 in the shape of a plug. 50. The monohthic composite of claim 19 in the shape of a plug. 51. The monohthic composite of claim 20 in the shape of a plug. 52. The monohthic composite of claim 21 in the shape of a plug. 53. The monohthic composite of claim 22 in the shape of a plug. 54. The monohthic composite of claim 23 in the shape of a plug. 55. The monohthic composite of claim 28 in the shape of a plug. 56. The monohthic composite of claim 31 in the shape of a plug. 57. The monohthic composite plugs of any one of claims 32-56 containing microspheres. 58. The monohthic composite plugs of any one of claims 32-56 containing reinforcing fillers. 59. The process of claim 2 wherein a matrix resin/particle mixture is formed by essentiaUy uniformly distributing in situ isotropicaUy expandable thermoplastic particles containing expansion agent therein in a thermosettable or thermoplastic matrix resin. 60. The process of claim 59 wherein the distribution is effected by mixing in an extruder. 61. The process of claim 59 wherein the matrix resin/particle mixture is shaped into plugs by extrusion, pultrusion or casting. 62. An article of manufacture comprising an open-ceUular structural material containing within the open-ceU or ceUs thereof, at least one expandable sag-resistant nucleus-forming monohthic composite. 63. The article of manufacture of claim &l w erem tne open-ceUular structural material is a honeycomb structure. 64. The article of manufacture of claim 63 wherein the expandable sag- resistant nucleus-forming monohthic composite is a pre-shaped expandable sag- resistant nucleus-forming monohthic composite comprising incompatible in situ isot ¬ ropicaUy expandable thermoplastic particles containing expansion agent therein, es ¬ sentiaUy uniformly distributed in a thermosettable or thermoplastic matrix resin with which they are incompatible when the thermoplastic particles are in the thermo- expandable state. 65. The article of manufacture of claim 64 wherein the pre-shaped expandable sag-resistant nucleus -forming monohthic composite is in the shape of a plug that comphments the shape of the open ceUs of the honeycomb structure. 66. The article of manufacture of claim 65 wherein there is a single plug de ¬ posited for each single open ceU of the honeycomb structure that is so filled. 67. The article of manufacture of claim 66 wherein the plug is a composite con ¬ tains an essentiaUy uniform density and thickness across its breadth and it possesses an external shape and size that is dimensionaUy simUar to the hoUow interior component of the honeycomb structure for which it is designed to be introduced for subsequent thermal expansion. 68. The pre-shaped expandable sag-resistant nucleus-forming monohthic composite of claim 5 wherein the monohthic composite does not undergo a shaping process when introduced into a hoUow interior component of a structural material. 69. The article of manufacture of claim 64 wherein the matrix resin comprises an epoxy resin. 70. The article of manufacture of claim 64 wherein the matrix resin comprises a phenoxy resin. 71. The article of manufacture of claim 62 wherein the composite contains fi ¬ ber reinforcement. 72. The article of manufacture of claim 63 wherein the composite contains fi ¬ ber reinforcement. 73. The article of manufacture of claim 64 wherein the composite contains fi ¬ ber reinforcement. 74. The article of manufacture of claim 65 wherein the composite contains fiber reinforcement. 75. The article of manufacture of claim 66 wherein the composite contains fi ¬ ber reinforcement. 76. The article of manufacture of claim 67 wherein the composite contains fiber reinforcement. 77. The article of manufacture of claim 68 wherein the composite contains fiber reinforcement. 78. The article of manufacture of claim 69 wherein the composite contains fiber reinforcement. 79. The article of manufacture of claim 70 wherein the composite contains fiber reinforcement. 80. The article of manufacture of claim 62 wherein the composite contains microspheres. 81. The article of manufacture of claim 63 wherein the composite contains microspheres. 82. The article of manufacture of claim 64 wherein the composite contains microspheres. 83. The article of manufacture of claim 65 wherein the composite contains microspheres. 84. The article of manufacture ot claim 66 wnerem tne composite contains mi ¬ crospheres. 85. The article of manufacture of claim 67 wherein the composite contains mi ¬ crospheres. 86. The article of manufacture of claim 68 wherein the composite contains mi ¬ crospheres. 87. The article of manufacture of claim 69 wherein the composite contains mi ¬ crospheres. 88. The article of manufacture of claim 70 wherein the composite contains microspheres. 89. The article of manufacture of claim 71 wherein the composite contains microspheres. 90. The article of manufacture of claim 72 wherein the composite contains microspheres. 91. The article of manufacture of claim 73 wherein the composite contains microspheres. 92. The article of manufacture of claim 74 wherein the composite contains microspheres. 93. The article of manufacture of claim 75 wherein the composite contains microspheres. 94. The article of manufacture of claim 76 wherein the composite contains microspheres. 95. The article of manufacture of claim 77 wherein the composite contains microspheres. 96. The article of manufacture of claim 78 wherein the composite contains microspheres. 97. The article of manufacture of claim 79 wherein the composite contains mi ¬ crospheres. 98. A process for reinforcing and/or stiffening a honeycomb structure which comprises inserting into a select hoUow interior portion of a honeycomb structure, a preformed sag-resistant nucleus-forming monohthic composite of incompatible in ■sΛu-expandable thermoplastic particles, which form closed-microceUs upon isotropic expansion, in a thermosettable or thermoplastic matrix resin, which composite possesses an external shape and size that is dimensionaUy simUar to the hoUow interior portion of the honeycomb, heating the composite to a temperature which causes iso ¬ tropic expansion of the in sΛzrexpandable thermoplastic particles in the composite and expansion of the composite within the interior portion honeycomb structure to achieve a faultless interface with the waU of the interior portion honeycomb structure. 99. The process of claim 98 wherein the matrix resin is thermoplastic and the composite is maintained at temperature that reduces the cure rate of the resin. 100. The article of manufacture of claim 69 wherein the epoxy resin is at temperature that reduces the cure rate of the resin.

Inventors: WONG RAYMOND S, WONG, RAYMOND, S.
Applicants: DEXTER CORP, DEXTER CORPORATION
Application year: 2001
Main classification: B32B3
IPC codes: C08J9