CROSS-LINKED FOAMABLE COMPOSITIONS OF SILANE-GRAFTED, ESSENTIALLY LINEAR POLYOLEFINS BLENDED WITH POLYPROPYLENE

Abstract: New cross-linked polymeric foam compositions, and methods for making the same, are provided. The new compositions utilize novel cross-linked polyolefin copolymers and show improvements in strength, toughness, flexibility, heat resistance and heat-sealing temperature ranges as compared to conventional low density polyethylene compositions. The new compositions also show processing improvements over linear low density polyethylene. The novel polyolefins, which are essentially linear, comprise ethylene polymerized with at least one alpha-unsaturated C3 to C20 olefinic comonomer, and optionally at least one C3 to C20 polyene, and exhibit, in an uncross-linked sense, a resin density in the range of about 0.86 g/cm<3> to about 0.96 g/cm<3>, a melt index in the range of about 0.5 dg/min to about 100 dg/min, a molecular weight distribution in the range of from about 1.5 to about 3.5, and a composition distribution breadth index greater than about 45 percent.

Description: CROSSLINKED FOAMABLE COMPOSITIONS OF SILANE-GRAFTED . ESSENTIALLY LINEAR POLYOLEFINS BLENDED WITH POLYPROPYLENE FIELD OF THE INVENTION This invention relates generally to the art of crosslmked polyolefins, and in particular to new cross- linked foam compositions made from blends of polyolefins and polypropylene. Background of the Invention Early attempts to produce physically blown foam compositions and articles from polypropylene resins melt- blended with a volatile expanding agent met with only limited success Tnis was due m part to the relatively low melt strengtn of polypropylene, which resulted in foam collapse and unacceptably high densities in the resulting products Another factor that proved problematic with early foams was the rapid crystallization of the expanding composition, which limited the extent of expansion, and thus the thickness and minimum density, of extruded foamable compositions. Attempts to increase the melt strength and reduce tne rate of crystallization of polypropylene were successfully commercialized in a series of resins from Kimont U.S. (now Montell U.S.A./ . These resins (described as HMS or High Melt Strengtn resins) are readily expanded into physically blown foams. The rigid and brittle (particularly at low temperatures) nature of polypropylene homopolymer and copolymers is a problem encountered m botn foamed and unfoamed polypropylene applications. One common solution to this problem is to blend polypropylene with other resins that possess much lower bulk moduli Polymeric compatibility is the primary factor considered in selecting a resin for softening a crystalline thermoplastic such as polypropylene Since tne resins will necessarily be melt-blended the various polymers must be melt -compatible, which generally requires that their individual solubility parameters be closely matched. Not only must the various polymers be mutually compatible, but the solubility of the expansion agent must be similar in each, otherwise they may segregate based upon differential solubility. Upon cooling, the polymers may not remain compatible, in which case phase-separation may occur. Phase-separation is exploited in order to impact- modify polymeric materials, such as in the case of Acrylonitrile/Butadiene/Styrene copolymers, wherein a separate rubber phase forms during the cooling of the material, thus creating micro-domains which arrest the propagation of a fracture front However, such phase - separating systems are rarely found m physically- expanded cellular plastics, due to the reduction of physical properties. Known methods for reducing the modulus and/or enhancing the impact strength of polypropylene resins include the incorporation into the resm of block styrene/butadiene copolymers, such as Shell's KRATON" resins, or poly (1-butene) homopolymer offered by Shell under the name DURAFLEX". Linear low-density polyethylenes are known to be at least partially compatible with polypropylene. Most recently, with the advent of new polyolefin catalyst technology, Exxon has publically, m brochures ("EXACT Plastomers Targeted Performance for Extrusion, Molding and Polymer Modification, " Brochure #119-0594- 0051-A, dated May, 1994) , suggested the use of their EXACT ™ metallocene-catalyzed linear low-density polyethylenes to improve the toughness and reduce the modulus of polypropylene homopolymer, random copolymer and impact copolymer (high-density and high-pressure low density polyethylenes are known to be incompatible with polypropylene) . Absent was mention of possible uses where the blended mixtures are physically-expanded (i.e., foam applications) . During the development of the present invention, extensive evaluations were made of EXACT " metallocene- catalyzed polyethylenes blended into Himont ' s homopolymer polypropylene (Himont #PF-814, 3.0 Melt Index, 0.900 g/cc.) on a laboratory foam extruder, using isobutane as a blowing agent. Not unexpectedly, as the level of EXACT" metallocene-catalyzed polyethylene was increased, the screw torque as evidenced by the motor draw (amperage) increased considerably. In addition, the minimum density obtained was greater, and the maximum thickness was less, than homopolymer HMS Since metallocene-catalyzed polyethylenes are linear low density polyethylenes by design and do not shear thin like high pressure low density polyethylenes, they possess greater melt or apparent viscosity. Higher melt viscosity leads to the detriment of processability for physically-blown foams due to shear heating Since shear stresses are dissipated as heat, a higher apparent melt -viscosity brings about a greater rise in temperature n the resin/blowing agent blend during melt processing, such as in an extruder. Consequently, materials wnich do not shear-thin as LDPE result m a greater cooling demand and limited output Furthermore, metallocene-catalyzed polyethylenes lack adequate melt strength to substantially expand bi- axially without collapsing so as to form c_.ose -cel_ structures. Melt strength is an attribute which is best observed by measurement of extensional viscosity, and physically-blown foams are best served by materials which shear thin extensionally . Such materials exhibit a rather high apparent viscosity at low shear rates (such as during cell formation, so the cells do not collapse) but low viscosity at high shear rates (such as those typically encountered m an extruder between the barrel and the screw, so as to limit shear heating) . In light of the above mentioned shortcomings of metallocene catalyzed polyolefins, these resins were not thought to be good candidates for producing quality foamed materials, either alone or in blends with other polymers. Surprisingly, however, it has been found that excellent foams can be made by blending silane-grafted metallocene catalyzed polyethylene resms with polypropylene. The cross-linkable physically blown foams produced from such blends have numerous properties not shared by foams produced from the ungrafted blends One of the most notable properties achieved by the new blends is a greater propensity to shear thin. Despite the fact that the melt index of the preferred grafted EXACT' resm of this invention is considerably less than either of the same EXACT ™ resin in ungrafted form or the preferred Himont HMS, lower motor current was observed when comparing HMS polypropylene blends of grafted EXACT ™ to the same ungrafted EXACT " A lower melt index (indicative of higher apparent viscosity at the rather low shear rates employed in the melt index test) coupled with lower motor amperage (indicative of lower apparent viscosity at the high shear rates employed in the extruder) suggests that the new grafted metallocene polyethylene blends have a greater tendency to shear- h n as compared to the non-grafted blends Along with this enhancement m capability to shear thin came a lower density and thicker gauge, as the attached experimental results demonstrate Other advantages were an improvement in toughness, strength and a reduction in modulus (lower compression-deflection stress) , as well as the higher temperature stability inherent to polypropylene-containing blends. In order to better highlight the differences between the present invention and the prior art, a detailed discussion of the closest related prior art follows . Polyolefm/polypropylene blends, including polyethylene/polypropylene blends in general and LLDPE/PP blends m particular, have been generally proposed as possible choices for resms used m foam extrusion and other applications. The silane grafting of such blends has also been suggested. Thus, U.S. 4,714,716 (Park) discloses a process for the production of a low density foam material having a substantially closed cell structure. Possible polymers suggested for making the material include linear olefinic polymers such as LLDPE, polypropylene, and blends thereof. Pursuant to the method, the polymeric materials are mixed with a blowing agent, which may be a volatile liquid or a solid that decomposes into gaseous materials at the extrusion temperature. A crosslmkmg agent, which may be a vinyl functional silane, is added to the olefinic polymer gel with the blowing agent, and serves to lightly crosslink the linear olefinic polymer with itself. Similarly, U.S. 5,026,736 (Pontiff) and U.S. 4,702,868 (Pontiff et al . ) disclose moldable polymer foam beads which are made from silane-modifled polyolefins. The silane-modifled polyolefin may be polyethylene, including linear low density polyethylene. The reference suggests that the polyethylene may possibly be blended with polypropylene and other compatible polymers. The blends are at least 50% by weight, and preferably 60% by weight, of the ethylene homopolymer or copolymer with the other compatible polymer. The polyolefins may be silane- grafted with vinyl tπmethoxysilane and similar agents, and may be cross -linked by exposure to moisture or radiation sources. U.S. 4,870,111 (Donuiff et al . ) discloses the production of moldable silane-crossl ked polyolefin foam beads. The beads are produced by mixing a silane grafted polyolefin with a silanol condensation catalyst in an extruder to form a melt. A blowing agent is then injected into the melt at a rate sufficient to produce a desired foam density the extrudate The beads are extruded and cut, and are then exposed to moisture to induce silane crossl k g in the polyolefin foam. The polyolefin may be low density polyethylene or linear low density polyethylene. The polyethylene may be blended with up to 50% by weight of another polymer that is compatible with it The reference suggests polypropylene as one such polymer. U.S. 4,591,606 (Bergstrom) discloses a silane crosslinked polyolefin foam and a method for making the same. The foam contains a polyolefin, a chemically bound hydrolysed silane, a condensation catalyst, and a foaming agent containing water and a water carrying substance. The reference notes that possible polyolefins used m the invention include LLDPE, polypropylene, and their mixtures . U.S. 5,053,446 (Salyer) discloses a composition useful in thermal energy storage. The composition may be formed from a crosslinked polyolefin having a phase change material, such as a crystalline alkyl hydrocarbon, incorporated therein. The polyolefin may be low density polyethylene or polypropylene U.S. 4,554,293 (Park) and U.S 4,581,383 (Park) disclose an expandable blend of a linear olefinic polymer and a crosslmkable polymer for the production of lightly crosslinked foam compositions The crosslmkable polymer serves to increase the melt strength of the linear olefin component, thereby allowing the use of conventional melt processing techniques for foam extrusion of the materials. The blend is about 5% to 95% by weight of a linear olefin, such as LLDPE, and from about 95% to 5%. by weight of a crosslmkable polymer. The preferred crosslmkmg agents include organofunctional silanes. The reference notes that, without crosslmkmg, the foam material produced by the method is totally collapsed. Col. 7, Lines 64-65. However, despite the general suggestions of the above noted references, the foams actually produced from polyethylene/ polypropylene resms to date have been unsatisfactory . Furthermore, the advantages afforded by blends of s lane-grafted LLDPE with polypropylene, particularly in foam applications, have heretofore gone unappreciated, so that these blends have not been used in practice. This is due part to the difficulties encountered producing satisfactory foams from the ungrafted blends and from LLDPE itself, as illustrated the Comparative Examples set forth in the present application. As illustrated there, the ungrafted materials tend to be difficult to process, and produce unacceptabiy high foam densities A further impediment developing foams from LLDPE/PP blends relates to the difficulties in processing LLDPE itself. These difficulties have been noted in the art Thus, U.S. 5,288,762 (Park et al . ) discloses a crosslinked- foam structure made from a substantially linear ethylemc polymer. The material is made by blending and heating a decomposible chemical blowing agent and an ethylemc polymer material Crosslmkmg is then induced in the material, and the foamable melt material is expanded by exposing it to an elevated temperature. The resulting material is substantially linear, and has better processibility and foamability than LLDPE. The reference notes that LLDPE is difficult to process into a crosslinked foam, Col. 1, Lines 28-31, and results m a relatively high density foam structure with poor processability . Yet another factor that has hampered the development of satisfactory LLDPE/PP foams is the high degree of unpredictability in the foam extrusion art. This is exemplified by U.S. 4,226,946 (Park et al . ) , which discloses foamed materials made from blends of low density branched polyethylene in admixture with intermediate density linear polyethylene. The reference notes that "Although a number of general principles are thought to be understood, much of the extrusion foaming technology is empirical, based on experience, and directed to very specific materials and details to produce saleable products of narrowly defined specification." Col 1, Lines 31-36. The use of metallocene catalysts in producing a variety of polymeric materials is known. Thus, U.S. 5,350,817 (Winter et al . ) discloses the use of a metallocene catalysts in producing polypropylenes (see Example 1) and other polyolefins having a broad molecular weight distribution U.S. 5, 278, 264 (Spaleck et al . ) and U.S 5, 329, 033 (Spaleck et al . ) describe the use of metallocene catalysts making polypropylene and other polyolefins U.S. 5,186,851 (Gutierrez et al . ) and U.S 5,151,204 (Struglmski) describe the use of metallocene catalysts in making lubricating oil additives U.S. 5,268,115 (Gutierrez et al . ) , U.S 5,275,747 (Gutierrez et al . ) , and U.S. 5,366,647 (Gutierrez et al ) describe the use of metallocene catalysts in making multifunctional viscosity index improver additives U.S. 5,277,833 (Song et al . ) , U.S. 5,382,698 (Song et al . ) , and U.S. 5,345,002 (Song et al . ) show the use of metallocene catalysts in making dispersant additives for lubricating oils. U.S. 5,391,629 (Turner et al . ) discloses the use of a catalyst system having a metallocene component and an electron donor cation component m making block copolymers of ethylene and an α-olefin such as propylene. The reference notes that the block copolymers are superior to blends in that the covalent bonding between the segments eliminates interface problems, and because the block copolymers can be used as emulsifiers to strengthen blends of immiscible polymers. U.S. 4,818,789 (Tomko et al . ) , U.S. 4,759,992 (Tomko et al . ) and U.S. 4,767,814 (Bae et al . ) disclose moisture curable low molecular weight polymers which have a silane grafted saturated carbon backbone. The backbone is preferably an ethylene/propylene copolymer which is prepared through the use of a metallocene catalyst. U.S. 5,385,972 (Yamamoto et al . ) describes a resin composition comprising an ethylene copolymer and a particulate inorganic filler. The ethylene copolymer is a copolymer of ethylene and an α-olefin, such as propylene, with a carbon number greater or equal to 3. The copolymer is formed through the use of a metallocene catalyst. The resm is used to make thin, gas permeable bodies. U.S. 5,376,428 (Palazzotto et al . ) describes an energy polymerizable composition containing at least one ethylenically unsaturated monomer, a polyurethane precursor, at least one epoxy monomer, a curing agent comprising an organometallic compound, and an onium salt as an oxidizing agent. The use of silane grafting agents in grafting polyethylene and similar materials is also well known, as noted some of the aforementioned references. Additional references include U.S. 4, 058, 583 (Glander et al . ) , which discloses the grafting of silane onto polyethylene. The grafting is accomplished by mixing the polymer in granulated form with a mixture of silane and peroxide. Grafting is then induced through extrusion or radiation. The grafted polymer may then be crosslinked through exposure to moisture. U.S. 4,873,042 (Topcik) discloses a process for extruding a thermoplastic copolymer, whereby the copolymer is coated with an organic peroxide. Under extrusion conditions, the peroxide decomposes to provide a silanol condensation catalyst. U.S. 5,047,476 (Keogh) discloses a process for crosslmkmg a copolymer containing a hydrolyzable silane moiety. The copolymer is mixed with dihydrocarbyltm oxide and a carboxylic acid or a carboxylate capable of forming a carboxylic acid through exposure to heat or moisture. The crosslmkmg is achieved by passing the mixture through a crosslmkmg zone where conditions are such that the carboxylic acid reacts with the dihydrocarbyltm oxide to form dihydrocarbyltm carboxylate . The crosslmkmg zone has a moisture content sufficient to crosslink the hydrolyzable copolymer in the presence of the dihydrocarbyltm carboxylate . U.S. 4,464,425 (Voigt et al . ) describes the use of a foamable, silane grafted polymer, such as polyethylene, in making shrink, wrap materials. U.S. 4,937,284 (Bergstrom) describes a method for manufacturing olefin/v yl alcohol block copolymers by chemically joining polyvinyl alcohol (PVA) to a polyolefin through the agency of silane The block copolymers obtained contain nonpolar polyolefin branches and polar polyvinyl branches. Various blends or copolymers of polyethylene with other polymers are also known, as described above. Further examples involving non-olef ic polymers include U.S. 4,181,762 (Benedyk) , which describes the formation of fibers from polymers having an elastic modulus of between 5,000 to 60,000 psi. The polymers are preferably copolymers of ethylene and vinyl acetate. U.S. 4,725,492 (Yazaki et al . ) discloses a composite heat insulating material comprising a urethane foam and a polyolefin based resin containing carboxyl groups or a polyolefin-based resin containing hydroxyl groups . The use of various agents to control the degree of crosslmkmg in foam extrusion applications is also known. Thus, U.S. 4,762,860 (Park) and U.S. 4,694,025 (Park) teach the use of alcohols to control the degree of crosslmkmg in a polymer prior to extrusion foaming. Summary of the Invention Detailed Description of the Invention Using conventional extrusion methods, a linear low-density polyethylene, either absent of, or possessing a low level of long-chain branching (such as metallocene- catalyzed polyolefins) is grafted with a mixture of multi-functional vinyl silane (such as vinyl trimetnoxy silane or vinyl triethoxy silane) and organic peroxide. Extrusion conditions are non-critical except that the choice of peroxide and temperatures should be such that the decomposition of the peroxide and the subsequent grafting reaction are accomplished within the residence time in said extruder. In a preferred embodiment, the grafting mixture is vinyl tπmethoxy silane (Huls #CV-4917) and alpha, alpha' -bis (t-butylperoxy) dusopropyl benzene (VULCUP-R) in a ratio of between 100:1 and 10:1. In a more preferred embodiment, the ratio of silane to peroxide is between 50:1 and 10:1. The most preferred ratio is 20:1. The rate of silane/peroxide mixture is most preferably 0.4% of the resm feed rate, although others may be advantageous. The objective is to provide for a subsequently crosslmkable resm which results in a (cross-linked) gel content of between 50% and 95% by weight. The feed blend to the extruder may optionally comprise other res s, including homopolymer or copolymer polypropylene, antioxidants, ultra-violet absorbers and other additives. The grafted resm or resin blend is then fed into any of the commonly utilized extrusion lines for the production of physically blown foams, including, but not limited to, single-screw, twin screw, single screw/single screw tandem line, and single screw/ accumulator tandem lines. Other additives may optionally be employed at this point, such as nucleants, cell-control additives, other grafted or ungrafted res s, colorants, antioxidants, ultra-violet absorbers, stabilizers and the like. Shaping and forming dies and mandrels may be any of those known to the art, such as sheet -producing or plank-produc g dies and forming equipment . In a preferred embodiment, the grafted metallocene-catalyzed polyolefin (grafted EXACT resin) is blended with homopolymer polypropylene (Himont PF-814) m a ratio of between 1199 and 50/50. In a more preferred embodiment, the ratio is between 5/95 and 40/60. The most preferred ratio is between 5/95 and 20/80. Most preferably, glycerol monostearate (GMS, Witco ATMOS-150 or Patco PATIONIC #1052) at a level of 1% Dy weight of fed resins is utilized to control the shrinkage of the foam, although many other cell control agents have been elucidated in the art which could be used herein. Any nucleant may be utilized, most preferably those which do not contain or liberate appreciable amounts of water. Suitable nucleants include talc (Luzenac MISTRON SUPER FROST or MISTRON ZΞC) , at levels from 0.25% to 2.5% by weight of combined feed. Any suitable volatile expansion agent as is known to the art may be employed in the present invention. However, since exclusion of moisture is necessary to prevent the premature crosslmkmg of the silane-grafted resms, this should be a consideration in choice or purity of expansion agent. Thus, the most preferred embodiment utilizes isobutane as a blowing agent. The rate of blowing agent is a function of the desired density, and is selected accordingly. A catalyst may also be included in the feed formulation to the foaming extruders in order to effect the subsequent silanolysis cross-linking reaction. Many catalysts suitable for this purpose are known to the art. However, the most preferred catalyst is dibutyl tin dilaurate, at a level of 0.015% of combined feed weight. Cross -linking of the extruded foam product is preferably accomplished at ambient conditions. However, where greatly reduced storage times are desirable, elevated temperatures or humidities may be used. The polyolefin resms used in this invention preferably possess a narrow molecular weight distribution and are "essentially linear, " although they contain a desired level of uniformly distributed, highly controlled "short chain branching". As a result of this combination, the resms exhibit a strength and toughness approaching that of linear low density polyethylenes, but have processability similar to high pressure, reactor produced low density polyethylene These "essentially linear, , polyolefin resins are characterized by a res density in the range of about 0.86 g/cm 3 to about 0.96 g/cm 1 , a melt index in the range of about 0.5 dg/mm to about 100 dg/mm, a molecular weight distribution in the range of from about 1.5 to about 3.5, and a composition distribution breadth index greater than about 45 percent. As used throughout this disclosure, the term "linear polyolefin" refers to an olefin polymer lacking "long chain branching," as exemplified by the conventionally produced linear low density polyethylene or linear high density polyethylene polymers made using Ziegler polymerization processes and disclosed, for example, U.S. 4,076,698 and U.S. 3,645,992. The term does not refer to high pressure, reactor produced branched polyethylenes, or to copolymers of ethylene and vinyl acetate, vinyl alcohol, ethyl acrylate, methyl acrylate, acrylic acid, or the like which are made using high-pressure technology and which are known to have numerous long-chain branches. As used throughout this disclosure, the term "essentially linear" refers to a "linear polymer" with a molecular backbone which is virtually absent of "long- chain branching," to the extent that less than about 0.01 "long-chain branches" per one-thousand carbon atoms are manifested thereof. Similarly, as used throughout this disclosure, the phrase "substantially free from long- chain branching" refers to a "linear polymer" with a molecular backbone having less than about 0.01 "long- chain branches" per one-thousand carbon atoms manifested thereof . As used throughout this disclosure, the term "long chain branching" refers to a molecular branch of a molecular backbone of at least 6 carbon atoms, above which the length cannot be distinguished using 13C nuclear magnetic resonance (NMR) spectroscopy. Tne long chain branch can be as long as about the same length as the molecular backbone Methods of quantifying long chain branching by use of 13C NMR spectroscopy were described by Randall (Rev. Macromol Chem. Phys . , C29 (2&3) , p. 285-297) . As used throughout this disclosure, the term "short-chain branching" is defined as a molecular branch of a molecular backbone of less than 6 carbon atoms which, as described above, would be distinguishable by 13C NMR spectroscopic methods. As used throughout this disclosure, the term "copolymer" refers to material resulting from the polymerization of two or more monomenc species, and specifically encompasses terpolymers (e.g., materials resulting from the polymerization of three or more monomenc species), sesquipolymers , and greater combinations of monomenc species thereof. The densities, or specific gravities, of the resms herein disclosed were measured using ASTM D-792 methods, except that they were additionally conditioned by holding them for 48 hours at ambient temperature (23 °C) prior to the density measurements. The essentially linear polyolefin res s disclosed in this invention are generally characterized by a resm density in the range of about 0.86 g/cm 3 to about 0.96 g/cnr preferably of about 0.86 g/cm 3 to about 0.91 g/cm 3 . The "Melt Index" (MI) is a measurement of processability under low shear rate conditions, accordance with ASTM D-1238 Condition E (190°C/2.16 kg) For the essentially linear polyolefins disclosed m this invention, the MI is generally in the range of about 0.2 dg/mm to about 100 dg/mm. Preferably, the MI is m the range of about 1 dg/mm to about 10 dg/mm, and most preferably in the range of about 2 dg/mm to about 8 dg/mm The molecular weight distribution (MWD or Mw/Mn) is a parameter determined by use of gel permeation chromatography with multiple mixed-porosity columns, comparing elution volumes of the unknown to those of narrow MWD polystyrene standards. The correspondence is accomplished by using the appropriate Mark-Houwink coefficients for the polystyrene standard and the polyethylene unknown, with procedures as described by Williams and Word in Journal of Polymer Science, Polymer Letters, Vol. 6, (621) 1968, incorporated herein by reference . The Composition Distribution Breadth Index (CDBI) is a measurement of the uniformity of distribution of comonomer to the copolymer molecules, and is determined by the technique of Temperature Rising Elution Fractionation (TREF) , as described , for example, Wild et. al . , J. Poly. Sci. , Poly. Phys. Phys. Ed., Vol. 20, p. 441 (1982). This attribute relates to polymer crystallizability, optical properties, toughness and many other important performance characteristics of compositions of the present art. For example, a polyolefin resin of high density with a high CDBI would crystallize less readily than another with a lower CDBI but equal comonomer content and other characteristics, enhancing toughness in objects of the present invention. The benefits to the discovery of the sub ect invention that accrue through the specific use of essentially linear polyolefin copolymers of narrow composition distribution are elucidated later the examples. As used herein, the CDBI is defined as the weight percent of the copolymer molecules having a comonomer content within 50% (i.e. +/- 50%) of the median total molar comonomer content. Unless otherwise indicated, terms such as "comonomer content," "average comonomer content" and the like refer to the bulk comonomer content of the indicated mterpolymer blend, blend component or fraction on a molar basis. For reference, the CDBI of linear polyethylene, which is absent of comonomer, is defined to be 100% CDBI determination clearly distinguishes the low density polyolefins of this art, which show narrow composition distribution as assessed by CDBI values generally above 70%, from very low density polyolefin copolymers produced by conventional linear catalyst technology, which have a broad composition distribution as assessed by CDBI values generally less than 55%. The CDBI of the essentially linear polyolefin copolymers disclosed in this invention is generally about 45% or higher, preferably about 50% or higher, more preferably about 60% or higher, and most preferably about 70% or higher. The "essentially linear" polyolefin copolymers of the present invention are preferably produced through the use of metallocene catalysts in accordance with any suitable polymerization process, including gas phase polymerization, slurry polymerization, and high pressure polymerization However, the methods of the present invention are not restricted to the use of metallocene catalysts. Preferably, the "essentially linear" polyolefins used in the foam compositions of the present invention are produced by gas-phase polymerization. Gas phase polymerization processes generally utilize super- atmospheric pressures and temperatures in the range of about 50°C to about 120°C. Such polymerization can be performed a stirred or fluidized bed of catalyst and product particles in a pressurized vessel adapted to facilitate tne separation of product particles form unreacted gases therein Maintenance of temperature may be accomplisned by circulation of ethylene, comonomer, hydrogen or inert gas such as nitrogen Tπethylaluminum may be added as needed as a scavenger of water, oxygen, ana other undesirable impurities The polymer produced thereof may be withdrawn continuously or semi- continuously at a rate necessary to maintain a constant product inventory in the reactor. The essentially linear polyolefin copolymers of the present invention may also be produced using a high pressure process by polymerizing ethylene combination with the other desired monomers in the presence of the metallocene alumoxane catalyst system. It is critical to this method is that the polymerization temperature be above 120°C, but below the decomposition temperature of the product, and that the polymerization pressure be above about 500 kg/cm 2 . In certain instances wherein the molecular weight of the product must be controlled, any of the suitable techniques known in the art for control of molecular weight, such as the use of hydrogen or reactor temperature, may be employed to effect such control therein. The essentially linear olefinic copolymers of the present invention are preferably derived from ethylene polymerized with at least one comonomer selected from the group consisting of at least one alpha-unsaturated C3 to C20 olefin comonomer, and optionally one or more C3 to C20 polyene . The types of comonomers selected to produce the essentially linear polymer utilized m the present invention will depend upon economics and the desired end- use of the resultant cross-linked foam structure. Generally, the alpha-unsaturated olefin comonomers suitable for use in the present invention contain m the range of about 3 to about 20 carbon atoms. Preferably, the alpha-unsaturated olefms contain in the range of about 3 to about 16 carbon atoms, and most preferably m the range of about 3 to about 8 carbon atoms . Illustrative, non-limitmg examples of such alpha- unsaturated olefin comonomers used as copolymers with ethylene include propylene, lsobutylene, 1-butene, 1- hexene, 3 -methyl - 1-pentene, 4 -methyl -1 -penten , 1-cctene, 1-decene, 1-dodecene, styrene, haloor alkyl-substituted styrene, tetrafluoroethylene, vinyl cyclohexene, vinyl - benzocyclobutane and the like. Generally, the polyenes used in the present invention contain about 3 to about 20 carbon atoms Preferably, the polyenes contain about 4 to about 20 carbon atoms, and most preferably about 4 to about 15 carbon atoms. Preferably, the polyene is a straight - chain, branched chain or cyclic hydrocarbon diene having from about 3 to about 20 carbon atoms, more preferably from about 4 to about 15 carbon atoms, and most preferably from about 6 to about 15 caroon atoms it is also preferred that the diene is non-conjugated Illustrative non-limitmg examples of such dienes include 1, 3 -butadiene, 1 , 4-hexadιene, 1 , 6-octadιene , 5 -methyl - 1, 4-hexadιene, 3 , 7 -dimethyl -1 , 6-octadιene, 3 , 7 -dimethyl - 1 , 7-octadιene, 5-ethylιdene-2-norbornene and dicyclopentadiene . Especially preferred is 1,4- hexadiene . Preferably, the polymeric foam composition of the present invention will comprise either ethylene/alpha- unsaturated olefin copolymers or ethylene/alpha- unsaturated olef /diene terpolymerε . Most preferably, the essentially linear copolymer will oe ethylene/l- butene or ethylene/1 -hexene . The comonomer content of the olefin copolymers utilized in the present invention is typically in tne range of about 1 percent to about 32 percent (baseα on the total moles of monomer) , preferably in the range of about 2 percent to about 26 percent, and most preferably the range of about 6 percent to about 25 percent The preferred essentially linear olefin copolymers used making the products of the present invention are produced commercially by Exxon Chemical Company, Baytown, Texas, under the tradename Exact " , and include Exact" 3022, Exact ™ 3024, Exact " 3025, Exact " 3027, Exact" 3028, Exact ™ 3031, Exact " 3034, Exact " 3035, Exact ™ 3037, Exact" 4003, Exact" 4024, 41, Exact " 4049, Exact" 4050, Exact" 4051, Exact " 5008, and Exact " 8002 Most preferably, the essentially linear olefin copolymers are selected from the group consisting of Exact " 3024, Exact " 4041, Exact ™ 5008. However, one skilled in the art will appreciate that other resms satisfying the requirements of an absence of long-chain branching, the range of molecular weight distributions, the range of composition distribution breadth indices, the range of resm densities, and the range of melt flow indices, are also available and may be used without departing from the scope of the invention. While the aforementioned essentially linear olefin copolymers are most preferable as the compositions of this invention, the addition of other polymers or res s to the composition, either prior or subsequent to grafting or cross-linking, can result in certain advantages m the economic, physical and handling characteristics of the cellular articles made in accordance with this invention. Examples of the polymers and res s which may be advantageously added include low density polyethylene, high density polyethylene, linear low density polyethylene, medium density polyethylene, polypropylene, ethylene propylene rubber, ethylene propylene diene monomer terpolymer, polystyrene, polyvinyl chloride, polyamides, polacrylics, cellulosics, polyesters, and polyhalocarbons Copolymers of ethylene with propylene, lsobutene, butene, nexene , octene, vinyl acetate, vinyl chloride, vinyl propionate, vinyl isooutyrate, vinyl alcohol, allyl alcohol, allyl acetate, allyl acetone, allyl benzene, allyl ether, ethyl acrylate, methyl acrylate, methyl methacrylate, acrylic acid, and methacrylic acid may also be used Various polymers and resms which find wide application in peroxide-cured or vulcanized rubber articles may also be added, such as polychloroprene, polybutadiene, polyisoprene, poly ( isobutylene) , nitrile-butadiene rubber, styrene-butadiene rubber, chlorinated polyethylene, chlorosulfonated polyethylene, epichlorohydrin rubber, polyacrylates, and butyl or halo- butyl rubbers. Other resins are also possible, as will be apparent to one skilled in the art, including blends of the above materials. Any or all of the additional polymers or resins may be advantageously grafted or cross-linked, in concert or separately, within the scope of the object of this invention. Preferred resms, to be added to the object copolymer of this invention, include polypropylene, polystyrene, low density polyethylene, linear low density polyethylene, ethylene/ethyl acrylate, and ethylene/methyl acrylate, and combinations of two or more of these materials. The preferred level of the essentially linear polyolefin copolymer, as a percentage of total polymeric resm, preferably ranges from about 5% to about 100%, more preferably from about 10% to about 60%, and most preferably from about 10% to about 40%. The cross-linking of the compositions useful in the practice of the present invention is preferably accomplished by the use of chemical cross -linking agents or high-energy radiation. Suitable methods of chemical cross-linking include the use of decomposable, free- radical generating species, or the use of silane- grafting, wherein the molecular backbone of the constituents of said composition are chemically reacted with a subsequently cross-linkable chemical species. In the latter case, the cross-link is appropriately effected by the use of warm, moist conditions subsequent to the grafting step, optionally with a suitable catalyst. Combinations of methods of cross-linking may be utilized to facilitate the degree of control and achieve the desired level of cross - linking . Representative chemical cross-link g agents which are usefully employed herein include the organic peroxides, azido and vinyl functional silanes, multifunctional vinyl monomers, organo- titanates , organo- zirconates and p-quinone dioximes . The chemical cross- link g agent may be advantageously selected by reference to the processing temperature and permissible time at the desired event of said cross-linking reaction. That is to say, by selecting a chemical cross-linking agent which exhibits a half-life of between one minute and 60 minutes at the preferred temperature of the cross -linking event, the rate of cross-linkmg may be expeditiously induced with the required degree of control The processing temperature and permissible time of the cross-linking event are often dictated by material handling requirements, for example proper conveyance of the composition through an extruder at reasonable rates thereof . Suitable chemical cross- linking agents for the compositions of this invention include, but are not limited to, organic peroxides, preferably alkyl and aralkyl peroxides. Examples of such peroxides include dicumylperoxide, 2, 5 -dimethyl -2 , 5-dι (t-butylperoxy) hexane, 1, 1-bιs (t-butylperoxy) -3,3,5- trimethylcyclohexane, 1,1-dι- (t-butylperoxy) -cyclohexane , 2 , 2 ' -bis (t-butylperoxy) dusopropylbenzene , 4,4' -bis (t- butylperoxy) butylvalerate , t -butyl -perbenzoate , t- butylperterephthalate , and t-butyl peroxide Most preferably, the cross-linkmg agent is dicumyl peroxide. Chemically-crosslmked compositions are improved upon with the addition of multi-functional monomerIC species, often referred to as "coagents.ll Illustrative, but non-limitmg, examples of coagents suitable for use in chemical crosslinking in accordance with the present invention include di- and tn -allyl cyanurates and isocyanurates, alkyl di- and tri-acrylates and methacrylates, zinc-based dimethacrylates and diacrylates, and 1 , 2-polybutadιene resins. Included in the cross-linking agents that may be used with the present invention are the azido-functional silanes of the general formula RR'SιY 2 wherein R represents an azido-functional radical attached to silicon through a silicon-to-carbon bond and composed of carbon, hydrogen, optionally sulfur and oxygen, wherein each Y represents a hydrolyzable organic radical, and wherein R' represents a monovalent hydrocarbon radical or a hydrolyzable organic radical. Azido-silane compounds graft onto an olefinic polymer through a nitrme insertion reaction. Crosslmkmg develops through hydrolysis of the silanes to silanols followed by condensation of silanols to siloxanes. The condensation of silanols to siloxanes is catalyzed by certain metal soap catalysts such as dioutyl tin dilaurate or butyl tin maleate and the like. Suitable azido-functional silanes include the trialkoxysilanes such as 2 - (tπmethoxylsilyl) ethyl phenyl sulfonyl azide and (tnethoxy silyl) hexyl sulfonyl azide. Other suitable silane cross-linking agents useful in the practice of the present invention include v yl functional alkoxy silanes such as vmyl trimethoxy silane and v yl tnethoxy silane. These silane cross-link g agents may be represented by the general formula RR'SιY 2 wherein R represents a v yl functional radical attached to silicon through a silicon-carbon bond and composed of carbon, hydrogen, and optionally oxygen or nitrogen, wherein each Y represents a hydrolyzable organic radical, and wherein R' represents a hydrocarbon radical or Y. Usually, free-radical initiating species, such as the organic peroxides described above, are incorporated along with the vinyl alkoxy silane to perform hydrogen extraction from the polymeric molecular backbone, whereupon the vmyl -functional silane may react and graft thereto. For reference, U.S. 3,646,155 presents further examples of such silanes. Subsequently, the grafted polymeric composition may be exposed to moisture to effect silanolysis condensation reactions therein, thereby cross-linking multiples of pendant silane grafts Preferably, the composition contains a suitable condensation catalyst. The composition is preferably shaped and formed into the desired profile or shape prior to contact with moisture. Most preferably, the silane cross- linking agent is vmyl trimethoxy silane, which is grafted on to the polymer backbone by the free-radical reaction initiated by 2 , 2 ' -bis (t-butylperoxy) dusopropylbenzene . The most preferred silanol condensation catalyst is dibutyl tin dilaurate, which greatly facilitates the cross-lmk g of pendent silane groups the presence of moisture, preferably hot water . As previously noted, methods of effecting the moisture induced cross-linking by condensation of silane grafts are widely disclosed in the art Aside from the obvious exposure to hot water, preferably at a temperature above the softening point of the composition, hydrated inorganic compounds such as gypsum or other water-solvable or water-absorbing species may be incorporated into the composition which, upon heating the composition above the hydration-liberation temperature, advantageously release moisture to effect the condensation or silane pendent groups Alternatively, moisture may be introduced directly into continuous rheit -processing equipment, such as an extruder, either alone or m combination with one of the components of the composition. The moisture is preferably introduced at a downstream feeding port, which may optionally be used m combination with a physically expanding foaming agent. For example, U.S. Pat. No. 4,058,583 (Glander) discloses the injection of moist inert gases, such as nitrogen, into a downstream port of a profile extruder, to both effect the expansion of silane-grafted compositions and the condensation of the silanes. For moisture-cured polyolefin systems wherein long-term moisture stability is essential, U.S. 4,837,272 (Kelley) discloses methods of subsequently reacting the silane-grafted compositions with organo titanates to result in relatively moisture-stable adducts which readily cross-link in the presence of atmospheric moisture, even in absence of silanol condensation catalysts, to form the cross-linked structures. Suitable methods for cross-lmkmg olefinic compositions with high-energy, ionizing radiation involve the use of equipment which generates electrons, X-rays, Beta-rays or Gamma-rays. "Ionizing radiation" denotes electromagnetic, waves or charged particles having the ability to interact directly or indirectly with a substance and consequently ionizing the substance "High energy" is used to denote the relatively high potential of such radiation, necessary to uniformly and sufficiently penetrate the articles of the composition of this invention. The most preferred method for cross-lmkmg olefinic compositions through exposure to ionizing radiation is through the use of an electron-beam radiation source The use of electron-beam radiation cross-lmkmg results in fine cell structure and good surface quality, due in large part to the completion of cross-linking prior to the initiation of the expansion process step. Disadvantages of this method include the high cost of the equipment and the infeasibility of utilizing this method in a continuous manufacture, since a single electron-beam source will only be economically supported by many continuous extrusion lines. Furthermore, certain polymers are susceptible to preferential chain scission or degradation instead of undergoing the desired cross-lmkmg reaction. Exposure of the compositions of the present invention to ionizing radiation may be accomplished at dosages the range of about 0.1 to 40 Megarads, and preferably, at about 1 to 20 Megarads. U. S. 4, 203, 815 (Noda) discloses suitable methods f or use with the present invention, including the exposure of compositions to both high and low-energy ionizing radiation so as to effect improvements in surface quality, strength and subsequent heat-sealing or embossing processes. The amount of cross-linking may be appropriately controlled by the dosage of ionizing radiation, with preference dictated by the requirements of the ultimate application of the composition of this invention. Optionally, coagents as described above may be incorporated into radiation-crosslinked compositions with advantageous results toward cure speed and uniformity of cross- link g . Regardless of the method of cross-lmkmg used, acceptable foamed articles may only be obtained by utilization of cross-linkmg over certain ranges of cross-linkmg density or level. Excessive cross-l kmg prior to foaming will render the foam composition coo inelastic, resulting m less than optimal expansion and greater than optimal density for a given level of foaming agent. For processes which invoke cross-linkmg subsequent to expansion, excessive cross-lmkmg would be economically inefficient. Less than optimal crosslmkmg may be detrimental to certain physical properties, such as compression set properties or thermal resistance. One parameter for quantifying the degree of cross -linking is the "gel content" of the composition. The term "gel content," as used in this disclosure, is intended to describe the weight percent of an insoluble portion of the cross-linked product (on a dried basis) remaining after about 50 mg of a sample of the cross- linked product has been immersed in 25 ml of molecular- sieve dried xylene for 24 hours at 120°C. Process conditions snould be utilized when providing for a cross- linked foam structure such that the resulting gel content is preferably in the range of about 50% to about 95%, more preferably the range of about 10% to about 40%, and most preferably in the range of about 12% to about 25%. The expanding medium, or foaming agents, useful in the practice of the present invention may be normally gaseous, liquid or solid compounds or elements, or mixtures thereof. In a general sense, these foaming agents may be characterized as either physically- expanding or chemically decomposing Of tne physically expanding foaming agents, the term "normally gaseous" is intended to mean that the expanding medium employed is a gas at the temperatures and pressures encountered during the preparation of the foamable compound, and that this medium may be introduced either in the gaseous or liquid state as convenience dictates. Included among the normally gaseous and liquid foaming agents are the halogen derivatives of methane and ethane, such as methyl fluoride, methyl chloride, difluoromethane, methylene chloride, perfluoromethane, trichloromethane, difluoro-chloromethane, dichlorofluoromethane, dichlorodifluoromethane (CFC-12), trifluorochloromethane, trichloromonofluoromethane (CFC- 11) , ethyl fluoride, ethyl chloride, 2 , 2 , 2-trιfluoro- 1, 1-dιchloroethane (HCFC-123), 1 , 1 , 1- trichloroethane, difluoro-tetrachloroethane, 1 , 1-dιchloro-l-fluoroethane (HCFC-141b) , 1, 1-difluorol-chloroethane (HCFC-142b) , dichloro- tetrafluoroethane (CFC-114) , chlorotrifluoroethane, trichlorotrifluoroethane (CFC- 113), l-chloro-l, 2, 2, 2 -tetrafluoroethane (HCFC-124), 1,1- difluoroethane (HFC-152a), 1 , 1 , 1-tπfluoroethane (HFC- 143a), 1, 1, 1, 2 -tetrafluoroethane (HFC-134a), perfluoroethane, pentafluoroethane, 2 , 2 -difluoropropane , 1,1, 1-trιfluoropropane, perfluoropropane , dichloropropane , difluoropropane, chloroheptafluoropropane, dichlorohexafluoropropane, perfluorobutane, perfluorocyclobutane , sulfur- hexafluoride, and mixtures thereof. Other normally gaseous and liquid foaming agents that may be employed are hydrocarbons and other organic compounds such as acetylene, ammonia, butadiene, butane, butene, isobutane, isobutylene, dimethylam e , propane, dimethylpropane, ethane ethylam e, methane, monomethylamme, tπmethylamme, pentane, cyclopentane , hexane, propane, propylene, alcohols, ethers, ketones, and the like. Inert gases and compounds, such as nitrogen, argon, neon or helium, can be used as foaming agents with satisfactory results. Solid, chemically decomposable foaming agents, which decompose at elevated temperatures to form gasses, can be used to expand the compositions of the invention. In general, the decomposable foaming agent will have a decomposition temperature (with the resulting lioeration of gaseous material) from 130°C to 350°C Representative chemical foaming agents include azodicarbonamide , p,pl- oxybis (benzene) sulfonyl hydrazide, p-toluene sulfonyl hydrazide, p- toluene sulfonyl semicarbazide , 5- phenyltetrazole, ethyl-5-phenyltetrazole , dinitroso pentamethylenetetram e, and other azo, N-nitroso, carbonate and sulfonyl hydrazides as well as various acid/bicarbonate compounds which decompose when heated. The preferred volatile liquid foaming agents include isobutane, difluoroethane or blends of the two. For decomposable solid foaming agents, azodicarbonamide is preferred, while for inert gases, carbon dioxide is preferred. The art of producing cross -linked foam structures is well known, especially for polyolefin compositions. The foam structure of the present invention may take any physical configuration known in the art, such as sheet, plank, other regular or irregular extruded profiles, and regular or irregular molded bun stock. Exemplary of other useful forms of foamed or foamable objects known in the art include expandable or foamable particles, moldable foam particles, or beads, and articles formed by expansion and/or consolidation and fusing of such particles. Such foamable article or particle compositions may be cross-linked prior to expansion, such as for the process of free-radical initiated chemical cross-lmkmg or ionizing radiation, or subsequent to expansion. Cross-lmkmg subsequent to expansion may be effected by exposure to chemical cross-lmkmg agents or radiation or, when silane-grafted polymers are used, exposure to moisture optionally with a suitable silanolysis catalyst. Illustrative, but non- limiting, of methods of combining the various ingredients of the foamable composition include melt -blending , diffusion-limited imbibition, liquid-mixing, and the like, optionally with prior pulverization or other particle-size reduction of any or all ingredients. Melt -blending may be accomplished in a batchwise or continuous process, and is preferably carried out with temperature control Furthermore, many suitable devices for mel -blending are known to the art, including those with single and multiple Archimedean-screw conveying barrels, high-shear "Banbury" type mixers, and other internal mixers The object of such blending or mixing, by means and conditions which are appropriate to the physical processing characteristics of the components, is to provide therein a uniform mixture One or more components may be introduced in a step-wise fashion, either later during an existing mixing operation, αurmg a subsequent mixing operation or, as would be the case with an extruder, at one or more downstream locations into the barrel . Expandable or foamable particles will have a foaming agent incorporated therein, such as a decomposable or physically expandable chemical blowing agent, so as to effect the expansion m a mold upon exposure of the composition to the appropriate conditions of heat and, optionally, the sudden release of pressure, one preferred method of providing a sheet object of this invention involves silane-grafting, subsequent extrusion of a melt-blended profile, moisture-induced cross-linking of the profile, and finally oven-expansion of the profile. In the first step, a portion of the polymeric resms of the foam composition, which contains at least a portion of the essentially linear olefin copolymer of this disclosure, is melt-blended with a 20:1 mixture of v yl trimethoxy silane (VTMOS) and dicumyl peroxide m an extruder to effect the grafting of VTMOS onto the polymers. This composition is extruded out of a multiple-strand die face, is chilled water, ana is then pelletized. In a subsequent step, the silane- grafted composition, along with ungrafted polymeric resms, chemically decomposable foaming agents, colorants, pigments, dibutyl t n dilaurate silanolysis catalyst, or, optionally, antioxidants and stabilizers, are melt -blended and extruded out of a sheet die and then passed through a three-roll stack to shape the profile to the correct gauge. The unexpanded sheet is then passed through a hot-water tank for sufficient time to effect the cross-l kmg, and is then passed through a gas- fired, hot-air oven to effect the decomposition of the foaming agent and expansion. In another preferred method, the extruded profile from the above method, prior to exposure to hot water, is multiple-stacked and consolidated in a press within a suitable mold at a temperature below the decomposition of the foaming agent Subsequently, it is exposed to hot water for sufficient time so as to effect the cross- linking via the silanolysis reaction. Optionally, at this point the resulting preform is again placed into a high-pressure press within a suitable mold to initiate the foaming agent decomposition Finally, the partially expanded preform is fully expanded within a hot-air forced-convection oven. In an alternate procedure, a "Banbury" type mixer is used to fuse a mixture of the grafted composition and other ungrafted resms and components. The fused mixture is then molded into a preform, cross -linked by exposure to hot water, and then expanded as described above In yet another preferred method, a silane-grafted composition is melt-blended with a physically-expanding foaming agent such as isobutane, additional ungrafted polymeric resms, dibutyl tin dilaurate silanolysis catalyst, nucleants such as talc, and optionally antioxidants and stabilizers in a single-screw extruder. Optionally, a twin-screw extruder may be utilized This composition is extruded out of a coat-hanger die whereupon the foaming agent expands and a fully-expanded foam sheet or plank results thereof . The net -shape sheet, plank, or board is the placed in humid storage for sufficient time to effect the cross-l kmg Several additives, as are known to the art, may be added to the compositions of the present invention without departing from the scope of the invention. Specifically contemplated is the addition of materials which are relevant toward cross -linked foam structure compositional development and production, such as particulate and fibrous fillers to reinforce, strengthen or modify the Theological properties of the foam composition Also contemplated is the addition of antioxidants (e.g., hindered phenolics such as Irganox 1010, phosphates such as Irgafos 168, or polymerized trimethyl-dihydroqumolme such as Agerite AK, Res D or Flectol H) , ultra-violet and thermal stabilizers, pigments or colorants, cell-growth nucleants such as talc and the like, cell-structure stabilizers such as fatty- acids, -esters (e.g. glycerol monostearate) or -amides, property-modifiers, processing aids, additives, catalysts to accelerate cross-lmkmg or other reactions, and blends of two or more of the aforementioned materials. Table IA is a non-limitmg tabulation of certain parametric characteristics of some essentially linear polyolefin copolymers which are suitable for use with the present invention. The materials in Table IA are commercially available and are produced by the Exxon Chemical Company at its facility Baytown, Texas TABLE IA Product Melt Index Density Comonomer Designation (dg/mm) (g/cc) Type Content CDBI(%) Mw/Mn Exact " 4041 3.0 0.878 1 -butene 23.5% NA 2.0+0 .2 Exact * 5008 10 0.865 1 -butene 31.6% NA 2 0+0 .2 Exact " 4028 10 0.680 1 -butene 23.1% NA 2 0+0. .2 Exact " 4017 4.0 0.885 1-butene 20.2% NA 2 0+ 0. .2 Exact " 3024 4.5 0.905 1-butene 11.2% 83 7% 2.0±0. .2 Exact * 3025 1.2 0.910 1 -butene 9.6% >83% 2.0+0 .2 Exact * 3012 4.5 0.910 1-hexene 10.4% 88.2% 2.0+0, .2 Exact" 3011 3.5 0.910 1-hexene 11.1% 92.0% 2.0+0. .2 Exact " 3030 2.25 0.905 1-hexene 12.9% 92.2% 2.0±0, .2 Exact " 3031 3.5 0.900 1-hexene 15.4% >88% 2.0+0. .2 Notes: NA Not Applicable, polymer is too soluble to determine by TREF The physical properties of the resms disclosed herein are measured by the appropriate ASTM test methods Further characterization and comparisons were drawn by use of the following definitions: Specific Tensile Strength = Tensile Strength, (psi) (i) (psi/pcf) Density, (pcf) Specific Secant Modulus = ( (Compression/Deflection © 50% strain) - (Compression/Deflection @ 25%-strain) /((0.25) * (Density, (pcf) ) (2) The use of specific secant modulus is a method intended to eliminate uncertainties the compression/deflection results at tne 25% strain level. Since compression/deflection is a measure of the stress at 25% or 50% strain, the differential of compression/deflection divided by the differential strain (0.25) results the secant modulus. As with specific tensile strength, the secant modulus has been normalized for (divided by) the foam density in order to obtain specific secant modulus. SILANE-GRAFTED RESINS In the following examples, samples were separately grafted at a rate of approximately 30 lb/hr using a 60 mm diameter, 24:1 L/D single-screw extruder maintained at approximately 200°C. A mixture of vmyl trimethoxy silane (Huls #CV-4917) and alpha, alpha' -bis (t-butylperoxy) dusopropyl benzene (VULCUP-R) in a ratio of 20:1 was metered directly into the feed throat of the extuder. The rate of silane/peroxide introduction was maintained at 0.4% of resm feedrate . The grafted composition was passed out of a multi-strand die head through a water-cooling trough, and chopped into pellets with a granulator. Graft designation L-68 was thereby produced from a metallocene-catalyed polyolefin (EXACT 4041, 3.0 Melt Index, 0.878 g/cc specific gravity, Exxon Chemical Co.) and used as a blending component for physically expanded polyolefin foams Graft designation L-416 was similarly prepared from EXACT 4049 (4.5 Melt Index, 0.873 g/cc specific gravity, Exxon Chemical Co.). RESINS Inclusive of res s utilized to produce physically blown foams were: TABLE IB Component Manuf cturer Melt Specific Comments Index Gravity EXACT 5008 Exxon Chemical 8.61 0.865 butene-based, Company metallocene- catalyzed VLD polyolefin EXACT 4049 Exxon Chemical 4.35 0.873 butene-based, Company metallocene- catalyzed VLD polyolefin EXACT 4041 Exxon Chemical 2.88 0.878 butene-based, Company metallocene- catalyzed VLD polyolefin DFDB-1085NT Union Carbide 1.0 0.885 FLEXOMER, Corporation octene-based VLD polyolefin, UNIPOL II process EG-8200 Dow Chemical 5.0 0.870 ENGAGE , Co. octene-based metallocene- catalyzed VLD polyolefin Graft L-68 Sentinel Products 1.70 from EXACT Corp 4041, VTMOS - graf ed Graft L-416 Sentinel Products 3.39 from EXACT Corp 4049, VTMOS- graf ed Profax PF-814 Himon , Inc . 3.0 0.900 propylene homopolymer, high melt- strength Profax SD-632 Himont , Inc . 2 - 4 0.900 ethylene/propyle ne copolymer, high melt- strength In the following examples, the indicated pellicular compositions were fed into a 2.51" diameter, 40:1 L/D single-screw extruder fitted with an in j ection port and high-pressure diaghram pump at mid-barrel for the introduction of volatile expansion agents. Isobutane was thus introduced into the melted blend and the composition further blended and cooled prior to being forced through a cross-head sheet die. The tooling for the die consisted of a 1.07" diameter p and bushing. The foam thus produced was stretched and formed over a 3.975" diameter, water-chilled mandrel. Further cooling was thereby provided by means of an annular air ring, thus cooling the exterior of the foamed annular profile. The profile was slit lengthwise, opened out and then rolled up. Process conditions for each of the examples are exhibited in Table IB. EXAMPLE I The following materials were tumbled in a 45 gallon drum and fed into the aforementioned foam extruder: 50 lb. Himont Profax PF-814; 0.5 lb. glycerol monostearate (Witco/Humko Div. ATMOS- 150) 2.5 grams citrate/bicarbonate nucleant (Reedy International SAFOAM FP) ,- and 21 grams naphthenic process oil (Stanlube-60) , used to adhere the powders to the pellets. The high melt strength propylene homopolymer was foamed to an apparent density of 2.72 pcf and a thickness of 0.173 in. As demonstrated in the attached Table IB, a motor current of 18-20 amps was observed at a screw speed of 34 rpm. The specific tensile strength (tensile strength divided by density) was 60.7 psi/pcf while the specific secant modulus (as defined above) between 25% and 50% compression was 19.5 psi/pcf. These responses may be taken as representative of a high melt strength polypropylene foam. EXAMPLE II The following materials were tumbled in a 45 gallon drum and fed into the aforementioned foam extruder: 50 lb. Himont Profax SD-632; 0.5 lb. glycerol monostearate (Witco/Humko Div. ATMOS- 150) ; 2.5 grams citrate/bicarbonate nucleant (Reedy International SAFOAM FP) ; and 21 grams naphthenic process oil (Stanlube-60 , used to adhere the powders to the pellets. The high melt strength ethylene/propylene copolymer was foamed to an apparent density of 3.2 pcf and a gauge of 0.100 in. As demonstrated in the attached Table IB, the motor current was 22-24 amps at a screw speed of 34 rpm The specific tensile strength was 54.7 psi/pcf, while the specific secant modulus was 8.03 psi/pcf. This may be considered a representative foam of a high melt strength copolymer polypropylene res , which is considerably softer than the homopolymer polypropylene. Thus, softer foams may be obtained by the use of copolymer polypropylene resms, but at a penalty of less density reduction, thinner gauge, lower specific tensile strength and higher motor draw EXAMPLE III, IV, V Herein, blends of various linear low density polyolefins with the homopolymer polypropylene were prepared to distinguish ease of processing and physical properties . EXAMPLE III EXAMPLE IV EXAMPLE V 40 lb 40 lb 40 lb Himont Profax SD-632 10 lb Union Carbide DFDB-1085NT 10 lb Dow Chemical Co. EG- 8200 10 lb Exxon EXACT-5008 0 5 lb 0 5 lb 0.5 lb Witco/Humko Div ATMOS-150 1 0 g 1 0 g 1.0 g SAFOAM FP 21 g 21 g 21 g Stanlube-60 In reference to Table IB, each candidate linear polyolefin exhibited a substantially reduced specific secant modulus compared with the homopolymer polypropylene res , due largely to the substantially reduced bulk moduli of the additional resm However, all cases the motor draw was higher tnan the homopolymer and, in the case of the metallocene-catalyzed LLDPE EXACT-5008, which is absent of long-chain brancnmg, higher than even the copolymer polypropylene res Furthermore, the specific tensile strengths were considerably lower for tne VLDPE Fiexomer DFDB-1085NT and the metallocene-catalyzed Engage EG-8200 blends, suggestive of borderline blend incompatibility. Visually, the extrudate of Example III and Example IV variants appeared streaky, almost milky The latter EG- 8200 metallocene-catalyzed linear low density polyolefin contains a low level of long chain branches, purported by the manufacturer to enhance processability , and of these three variants, exhibited the lowest motor current. In all cases, the ultimate tensile elongations were higher than either of the homopolymer or copolymer polypropyleneε, indicative of significantly greater toughness . EXAMPLE VI, VII A comparison of a 20/80 blend of a metallocene- catalyzed linear low density polyolefin, absent of long- chain branching, in homopolymer polypropylene, to that of a similar blend using the v yl trimethoxy silane graft of this same metallocene-catalyzed polyolefin, was accomplished using the following formulation. EXAMPLE VI EXAMPLE IV 40 lb 40 lb Himont Profax SD-632 10 lb Exxon EXACT-4049 10 lb Graft L-416 (from EXACT-4049) 0 5 lb 0 5 lb Witco/Humko Div ATMOS-150 1.25 g 1.0 g SAFOAM FP 21 g 21 g Stanlube-60 3.4 g dibutyl tin dilaurate In reference to Table IB, significantly higher motor draw accompanied the incorporation of 20% of the metallocene-catalyzed polyolefin EXACT-4049, but resulted softer product than the homopolymer polypropylene. Thermal stability, as evidenced by shrinkage of the foamed material after 7 days at 215°F, was essentially equivalent. Remarkably, utilization of the same blend ratio of a vinyl trimethoxy silane-graf ted EXACT-4049 provided for lower motor draw and greater density reduction than the ungrafted EXACT resm, despite cne fact that the grafted resm possesses a lower Melt Index than the virgin resm (3.39 versus 4.39) . Also, as would be expected, the foam of the silane-grafted variant exhibited less shrinkage after 7 days at 215°F, a consequence of the cross-linkmg of silane moeities. Furthermore, toughness of this grafted variant, as evidenced by the ultimate tensile elongation, was superior to either the homopolymer or copolymer polypropylene. By adjustments in the rate of silane incorporation, a balance between toughness, softness and tensile strength would be attained. EXAMPLE VIII An additional 20/80 blend was evaluated, herein with an alternate silane-grafted metallocene-catalyzed polyolefin resin, using the following formulation: 40 lb. Himont Profax SD-632 10 lb. L-68 (grafted Exxon EXACT-4041) 0.5 lb. Witco/Humko Div. ATMOS-150 1 g . SAFOAM FP 21 g. Stanlube-60 In reference to Table IB, the motor draw for this variant was less than Example VII (the EXACT-4049 grafted resin) despite the fact that tne melt index of the L-68 was considerably lower than L-416 (1.7 versus 3.6) Also, the specific secant modulus was lower, and the specific tensile strength and ultimate elongation were greater, tnan the Example VII grafted variant. In relation to the homopolymer polypropylene of Example I , the tensile elongation was dramatically higher and the specific secant modulus and shrinkage at 215°F considerably lower, but maximum thickness was compromised somewnat . Here as with Example VII, by the appropriate selection of silane grafting levels, a balance of properties should be possible. Wnat is claimed is:

Claims: 1. A foamable, crosslmkable composition, comprising a blend of polypropylene with silane-grafted, essentially linear polyolefin. 2. The composition of claim 1, wherein said polyolefin is LLDPE. 3. The composition of claim 1, wherein said polypropylene is homopolymer polypropylene. 4. The composition of claim 1, wherein said polypropylene is copolymer polypropylene. 5. The composition of claim 1, wherein said polypropylene is blended with said polyolefin at a ratio by weight of between 99/1 and 50/50. 6. The composition of claim 1, wherein said polypropylene is blended with said polyolefin at a ratio by weight of between 95/5 and 60/40. 7. The composition of claim 1, wherein said polypropylene is blended with said polyolefin at a ratio by weight of between 95/5 and 80/20. 8. The composition of claim 1, wherein said silane grafting agent is a multi-functional vinyl silane. 9. The composition of claim 8, wherein sa d silane grafting agent is selected from tne group consisting of v yl trimethoxy silane and v yl triethoxy silane 10 The composition of claim 1, wnerem said composition is physically expanded with a blowing agent 11. The composition of claim 1, wherein said polyolefin has a resin density in the range of about 0.86 g/cm 3 to about 0.96 g/cm 3 . 12. The composition of claim 1, wherein said polyolef in has a melt index in the range of about 0.5 dg/min to about 100 dg/min. 13. The composition of claim 1, wherein said polyolefin has a molecular weight distribution in the range of from about 1.5 to about 3.5. 14. The composition of claim 1, wherein said polyolefin has a composition distribution breadth index greater than about 45 percent. 15. The composition of claim 1, wherein said composition has a density within the range of about 1.2 to about 10 pcf. 16. The composition of claim 1, wherein said composition has a tensile strength within the range of about; 110 to about 170 psi. 17. The composition of claim 1, wherein said composition has a tensile elongation within the range of about 50% to about 80%. IE. The composition of claim 1, wherein said composition has a 25% compression deflection within the range of about 6 to about 8 psi. 12. The composition of claim 1, wherein said composition has a 50% compression deflection within the range of about 18 to about 20 psi. 20. The composition of claim 1, wherein said composition has a specific secant modulus withm the range of about 14 to about 19 psi/pcf. 21 The composition of claim 1 expanded into a foam, wherein said foam suffers essentially no lineal shrinkage across its thickness when held at 215°F over a 7 day period. 22. The composition of claim 1, wherein said olefin has a density withm the range of about 3.0 to about 3.3 pcf and a specific secant modulus withm the range of aoout 14 to about 15 psi/pcf, and wherein said composition has a density withm the range of about 2.5 to about 2.7 pcf and a specific secant modulus withm the range of about 17 to about 19 psi/pcf. 23. A foamable, crosslmkable composition, comprising a blend of substantially ungrafted homopolymer or copolymer polypropylene with silane-grafted, essentially linear, metallocene catalyzed LLDPE, wherein said polypropylene is blended with said LLDPE at a ratio by weight of between 99/1 and 50/50, and wherein said LLDPE is silanegraf ed with a multi-functional vmyl silane selected from the group consisting of vmyl trimethoxy silane and vmyl tnethoxy silane. 24 The composition of claim 23, wherein said polypropylene is blended with said LLDPE at a ratio by weight of oetween 95/5 and 60/40 25 The composition of claim 23, wherein said polypropylene is blended with said LLDPE at a ratio by weight of oetween 95/5 and 80/20 26. The composition of claim 23, wherein said composition has a density within the range of about 2.5 to about 3.2 pcf, a tensile strength withm the range of about 110 to about 170 psi, and a specific secant modulus with the range of about 14 to about 19 psi/pcf. 27. The composition of claim 23, wherein said composition has a tensile elongation with the range of about 50% to about 80%, a 25% compression deflection within the range of about 6 to about 8 psi, and a 50% compression deflection withm the range of about 18 to about 20 psi. 28. The composition of claim 23 expanded into a foam, wherein said foam suffers essentially no lineal shrinkage when held at 215°F over a 7 day period. 29. A crosslinked foam composition, comprising .- a blend of substantially ungrafted homopolymer or copolymer polypropylene with silane-grafted, essentially linear, metallocene catalyzed LLDPE, wherein said polypropylene is blended with said LLDPE at a ratio by weight of between 95/5 and 80/20, wherein said LLDPE is silane-grafted with a multi-functional vmyl silane selected from the group consisting of v yl trimethoxy silane and vmyl tnethoxy silane, and wherein said polyolefin has a res density the range of about 0.86 g/CM3 to about 0.96 g/CM3 , a melt index the range of about 0.5 dg/mm to about 100 dg/mm., a molecular weight distribution m the range of from about 1.5 to about 3.5, and a composition distribution breadth index greater than about 45 percent. 30. A method for producing a foam composition, comprising the steps of .- providing a foamable, crosslmkable composition comprising a blend of silane-grafted, essentially linear polyolefin with polypropylene; introducing a blowing agent into the composition,- and expanding the composition into a foam. 31. A method for making a foamed composition, comprising the steps of: obtaining a silane-grafted, essentially linear polyolefin; obtaining polypropylene; forming a blend with said polyolefin and said polypropylene; and expanding the blend with a blowing agent . 32. A method for making a foamed composition, comprising the steps of: copolymeriz g ethylene with an alpha- unsaturated ethylenic monomer containing about 3 to about 20 carbon atoms, wherein the copolymer is characterized by (i) a resm density m the range of about 0.86 g/cm 3 to about 0.96 g/cm 3 , (ii) a melt index the range of about 0.5 dg/mm to about 100 dg/mm, (ui) a molecular weight distribution in the range of from about 1.5 to about 3.5, and (iv) a composition distribution breadth index greater than about 45 percent; grafting the copolymer with a silane grafting agent ; blending the copolymer with polypropylene co form a resm,- inducing cross-lmkmg m the resm; and expanding the resm into a foamed composition. 33. The method of claim 32, wherein the ethylene is also copolymeπzed with at least one polyene containing about 3 to about 20 carbon atoms. 34. The method of claim 33, wherein the polyene contains 4 to 20 carbon atoms 35. The method of claim 34, wherein the polyene contains 4 to 15 carbon atoms. 36. The method of claim 33, wherein the polyene is a diene . 37. The method of claim 36, wherein the diene is non-conjugated. 38. The method of claim 36, wherein the diene is selected from the group consisting of: 1, 3 -butadiene, 1 , 4 -hexadiene , 1 , 6-octadιene, 5- methyl-1, 4 -hexadiene, 3 , 7-dιmethyl -1 , 6-octadιene, 3 , 7 -dimethyl -1 , 7-octadιene, 5-ethylιdene-2- norbornene, and dicyclopentadiene 39. The method of claim 38, wherein the diene is 1 , 4 -hexadiene . 40. The method of claim 32, wherein the copolymer is substantially free of long-cham branching. 41 The method of claim 32, wherein the copolymer is substantially free of oranch lengtns of more than about 20 carbon atoms. 42 The method of claim 32, wherein the resm comprises by weight about 1% to about 50% polypropylene. 43. The method of claim 42, wherein the copolymer is essentially linear. 44. The method of claim 43, wherein the resm comprises by weight about 5%. to about 40% of the copolymer. 45. The method of claim 43, wherein the resm comprises by weight about 5% to about 20% of the copolymer . 46. The method of claim 32, wherein the res is expanded through the use of a foaming agent . 47. The method of claim 46, wherein the foaming agent is added prior to inducing cross-lmkmg in the resm. 48. The method of claim 46, wherein the foaming agent is added subsequent to inducing cross-linking the resin. 49. The method of claim 32, wherein cross- linking is induced by exposing the grafted copolymer to moisture . 50. The method of claim 32, wherein crosslmkmg is induced by the addition of reactive monomers to the resm. 51. The method of claim 50, wherein additional polymeric resms are added to the res subsequent co the addition of the reactive monomers, but prior to induction of cross-lmkmg. 52 The method of claim 32, wherein the density of the foamed composition is withm the range of from about 0.7 pcf to about 22 pcf. 53. The method of claim 32, wherein at least 700i of the cells in the foamed composition are closed. 54. The method of claim 32, wherein the resm is extruded into foamed beads . 55. The method of claim 32, wherein the copolymer has a resm density in the range of about 0.86 g/cm 3 to about 0.91 g/cm 3 . 56. The method of claim 32, wherein the copolymer has a melt index in the range of about 1 dg/min to about 10 dg/mm. 57. The method of claim 32, wherein the copolymer has a melt index in the range of about 2 dg/min to about 8 dg/mm. 58. The method of claim 32, wherein the alpha- unsaturated ethylemc monomer contains about 3 to about 16 carbon atoms. 59. The method of claim 32, wherein the alpha- unsaturated ethylemc monomer contains about 3 to about 8 carbon atoms . 60. The method of claim 32, wherein the alpha- unsaturated ethylemc monomer s selected from tne group consisting of: propylene, isobutylene, 1 -butene, l-nexene, 3- methyl-1 -pentene, 4-methyi-l-pentene , 1-octene, 1-decene, 1-dodecene, styrene, halo-substituted styrene, alkyl- substituted styrene, tetrafluoroethylene , vmyl cyclohexene, and vmylbenzocyclobutane . 61. The method of claim 32, wherein the alpha- unsaturated ethylemc monomer s 1 -butene. 62. The method of claim 32, wherein the alpha- unsaturated ethylemc monomer is 1 -hexene 63. The method of claim 32, wherein the comonomer content of the copolymer is m the range of about 1 percent to about 32 percent. 64. The method of claim 63 , wherein the comonomer content of the copolymer is the range of about 2 percent to about 26 percent. 65. The method of claim 64, wherein the comonomer content of the copolymer is in the range of about 6 percent to about 25 percent by weight. 66. The method of claim 32, wherein the foamed composition has a gel content in the range of about 5% to about 95% . 67. The method of claim 32, wherein the foamed composition has a gel content in the range of about 10% to about 40%. 68. The method of claim 32, wherein the foamed composition has a gel content in the range of about 12% to about 25%. 69. The method of claim 32, wherein the resm is extruded prior to crosslmkmg, and wherein crosslmkmg is induced by exposure of the extruded resm to moisture. 70. A product formed in accordance with claim 32.

Inventors: FEICHTINGER KURT, FEICHTINGER, KURT
Applicants: SENTINEL PRODUCTS CORP, SENTINEL PRODUCTS CORP.
Application year: 1997
Main classification: C08J9
IPC codes: B82B1, C08L23, C08L51