POLYMER FOAM, THERMOFORMED SHAPES THEREOF AND METHODS OF FORMING SAME

Abstract: Foam polymer structure having a density of less than 320.4 kg/m<3>. A composition comprising polymer particles imbibed with a solvent is also provided. Methods of forming the polymer foam (for example Fig. 1 nos. 10, 12, and 14) and thermoformed shapes thereof are also disclosed.

Description: POLYMER FOAM, THERMOFORMED SHAPES THEREOF AND METHODS OF FORMING SAME The present invention relates to a polymer foam, thermoformed shapes thereof and methods of obtaining the same. In Modern Plastics Encyclopedia, 1982-83, Volume 59, No. 10A, pages 275-278 there are disclosed certain processes for preparing what is known as "structural foam". This reference states that structural foam molding processes are similar to conventional injection molding except that a blowing agent is required in the melted thermoplastic resin and the mold is not completely filled during injection. The material adjacent to the surface of the mold forms a non-foamed skin and the blowing agent expands the remaining material to fill the mold and form a cellular core. In all cases described in this reference, a thermoplastic material and a chemical blowing agent are melt blended under pressure outside of the mold. Thereafter, if it is a high pressure process, the mold is filled completely with the combination of a decomposed chemical blowing ' agent held in suspension in the melt by the pressure of the system. The melt in contact with the mold forms a solid skin with no cell development. Thereafter, a portion of the mold cavity is expanded to permit the hot core to foam and fill out the mold. This process produces parts with a smooth surface finish. If it is a low pressure process, the molten resin and blowing agent is injected in a volume less than the volume required to fill the mold cavity. Because the mold is not packed, the pressure developed in the mold cavity, as a result of the injection and heat, rarely exceeds 35 kg/cm (500 psi). When the pressure is released and the foam expands to fill the mold, the result is a foamed resin part characterized by a swirl pattern of nonuniform color which requires post-finishing for appearance purposes. Important parameters for this process include maintaining the melted material under pressure at all times prior to injection, control of injection velocity and the design of special gates, runners and mold venting. Foamed polymers are used extensively for insulation and structural purposes, e.g., aircraft and electrical industries. It is essential that the foamed polymer be comparatively resistant to heat. The art is ever on the alert for a foam which is fire resistant and which gives off low level of smoke or toxic fumes as it is heated to degradation temperatures. The art would be advanced by a process which could provide molded foam polymer by a simplified process involving, ease of control and ease of variation from structural foam to substantially uniform low density foam. The art would be further advanced by these techniques as applied to the preparation of fire resistant polymer foam structures. The present invention provides a polymer in foam form having a density of from 16.0 kg/m 3 (1 lb/ft 3 ) to 320.4 kg/m 3 (20 lb/ft ). The polymers which can be foamed include those which can imbibe a solvent. Examples include polyetherimide, polycarbonate and a blend of polyphenylene oxide and polystyrene. Examples of the solvent are methylene chloride, chloroform, 1,1,2-trichloroethane and mixtures thereof. The present invention also provides a composition comprising discrete particles of a polymer imbibed with a solvent, the imbibed particles being in at least substantially free flowing form. The present invention further provides a method for preparing a low density foam structure comprising: (a) imbibing a polymer in particle form with a solvent; (b) homogeneously melt blending the particles and solvent in an extrusion system under pressure; and (c) extruding the blend into a lower pressure atmosphere to cause the solvent to vaporize to yield a foam structure. The present invention further provides a method of preparing a thermoformed polymer foam structure comprising: (a) subjecting a sheet of the polymer foam having a density of less than 320.4 kg/m 3 (20 lb/ft 3 ) to a first temperature sufficient to permit deformation thereof; (b) effecting a shape in the sheet while at this temperature; and (c) reducing the temperature of the sheet to permit permanent retention of the shape at or below the second temperature. The present invention also provides a method for preparing a shaped polymer foam structure having a density of less than 320.4 kg/m (20 lb/ft ) comprising: (a) imbibing particles of a thermoplastic polymer with a solvent; (b) heating the imbibed particles to a temperature sufficient to cause expansion of the particles to a density significantly less than that of the imbibed particles; and (c) filling a mold with the expanded particles and heating the particles to fuse the particles together. The present invention further provides a method for preparing a molded thermoplastic foam structure having a density of less than 320.4 kg/m (20 lb/ft ) comprising: (a) impregnating a thermoplastic polymer in particle form, the polymer being capable of imbibing a solvent or blowing agent, with an amount of blowing agent sufficient to foam the polymer to a density of less than 320.4 kg/m 3 to yield at least a substantially free flowing particulate combination; (b) partially filling a mold with the free flowing particulate combination; (c) pressurizing the mold to prevent any substantial vaporization of the blowing agent during subsequent polymer melting; (d) heating the polymer to at least a flowable molten state; (e) releasing the mold pressure to permit foaming of the melt and complete filling of the mold wth the foamed polymer; and (f) removing the molded foam structure. Fig. 1 is a side view of a schematic of a resin solvent imbibition system described in Example 3. Fig. 2 shows a side view of a schematic of a pre-expansion system described in Example 3. Fig. 3 is a side view of a schematic of a foam mold system described in Example 3. Fig. 4 is a side view of a schematic of a resin imbibition system described in Example 4. Fig. 5 shows a side view of a schematic of a foam mold system described in Example 4. The present invention relates to a polymer in foam form having a density of less than 320.4 kg/m 3 (20 lbs/ft 3 ). Any thermoplastic polymer in particulate form -which can be comparatively easily imbibed or impregnated with a suitable blowing agent or solvent to yield the low density foam is contemplated. Examples of the polymer are polyetheri ide, polycarbonate and a blend of polyphenylene oxide and polystyrene. Examples of the solvent include ethylene chloride, chloroform, 1,1,2-trichloroethane and mixtures thereof. Since the solvent acts as a blowing agent, these terms are used interchangeably herein. Preferred polyetheri ides imbibable with the solvent are those within the following chemical structure: wherein Ar is a divalent organic radical containing from 6-20 carbon atoms, R is a bivalent radical selected from the group consisting of aliphatic, cycloaliphatic, aromatic and araliphatic, and n is a integer having a value greater than 1, for instance 2, 3, 4 or greater. Preferred polycarbonates imbibable with said solvent have the following chemical structure 0 see Text Book of Polymer Science, 2nd Edition, Fred W. Billmeyer, Jr., 1971, Wiley-Interscience, N.Y., N.Y., page 456. Suitable commercially available polycarbonates are the LEXAN ® polycarbonates from General Electric Company. Preferred thermoplastic blends of polyphenylene oxide and polystyrene imbibable with the solvent are blends of poly 2,6-dimethyl-l,4-phenylene oxide and a high impact polystyrene. The blend can be in a 20 to 80 weight percent ratio of either component. The term "high impact polystyrene" as used herein is intended to be generic to both the high impact polystyrene and the high impact copolymers derived from the iso eric methyl ethenyl benzenes mixtures and rubbery backbone polymers disclosed in U.S. Patent No. 4,284,733. In a preferred form the foam of the present invention has a density of less than about 80.1 kg/m (5 lbs/ft ). The invention is illustrated mainly with reference to the polyetherimide. One skilled in the art will appreciate that it is equally applicable to the appropriate polycarbonates and poly- phenyleneoxide-polystyrene blends. Polyetherimides of the type contemplated by the present invention have been known for some time but their preparation in foam form such that they have a density of less than about 320.4 kg/m (20 lbs. per cubic foot) is believed to be hitherto unknown. The contemplated polyetherimides are those which can be foamed according to the present process by means which, comprise the use of a solvent member selected from methylene chloride, chloroform, 1,1,2-trichloroethane and mixtures thereof. U.S. Patent Nos. 3,787,364 and 4,024,110 disclose polyetherimides which can be solvent imbibed and foamed according to the present process. Preferred polyetherimides are those having the chemical structure shown above. Polyetherimides within the scope of this structure can be prepared by procedures outlined in the article by D. M. White et al entitled "Polyetherimides Via Nitro-Displacement Polymerization...", etc. Journal of Polymer Science: Polymer Chemistry Edition, Vol. 19, 1635-1658 (1981), copyright 1981, John Wiley and Sons, Inc.. Particular reference is made to the preparation of polymer "(18 ip)", having a molecular weight (Mw) of about 21,000, on page 1653 thereof. A commercially available polyetherimide resin which corresponds to the above recited chemical formula is Ultem ® 1000 available from General Electric Company, Plastics Operations, One Plastics Avenue, Pittsfield, MA. This material has a T of 216°C g (421°F). It is available in particle form having a size roughly 0.16-0.32 cm (1/16-1/8 inch) in diameter by 0.16-0.48 cm (1/16-3/16 inch) in length. The following table details certain characteristics of the resin. TABLE ASTM ULTEM MECHANICAL TEST UNITS 1000 Tensile strength, yield 0638 psi 15,200 Tensile modulus, 1% secant D638 psi 430,000 Tensile elongation, yield D638 % 7-8 Tensile elongation, ultimate D638 % 60 Flexural strength D790 psi 21,000 Flexural modulus, tangent D790 psi 480,000 Compressive strength D695 psi 20,300 Compressive modulus D695 psi 420,000 Gardner impact -.__ in-lb 320 Izod impact D256 notched (1/8") ft-lb/in 1.0 unnotched (1/8") ft-lb/in 25 Shear strength, ultimate psi 15,000 Rockwell hardness D785 _.-__. M109 Taber abrasion (CS 17, 1 kg) D1044 mg wt. loss/ 10 1000 cycles THERMAL Deflection temperature, unannealed D648 U264 psi(l/4") °F 392 ®S6 psi(l/4") °F 410 Vicat softening point, method B D1525 °F ■ 426 Continuous service temperature index (UL Bulletin 746B) °F 338 Coefficient of thermal expansion (0 to 300°F). mold direction D696 iπ/in- °F 3.1x10-5 Thermal conductivity C177 Btu-in/h- •ft2- °F 1.5 FLAMMABILITY Oxygen index (0.060") D2863 % 47 Vertical burn (UL Bulletin 94) —— —— V-0 ® 0.1 5V (ϋ 0.Q- NBS smoke, flaming mode (0.060") E662 D_ 4 min 0.7 D MAX® 20 min 30 To prepare the present polymer foam, the polymer in particle form is first imbibed with the solvent or blowing agent. Thereafter, the imbibed particles which are free flowing particles are melt blended under pressure and then extruded into a lower pressure atmosphere. It is preferred to employ a polyetherimide that is anhydrous so as not to introduce the likelihood of forming acidic components through the combination of H D and the solvent. Any such acid products would be corrosive to the extrusion equipment and possibly degradative to the polyetherimide or its foam structure. Subjecting the Ultem ® 1000 particles to a temperature of approximately 149°C (300°F) for a period of about 4 hours will assure at least the substantial absence of H 2 0 in the resin. In preparing a homogeneous melt blend of the solvent or blowing agent and the polyetherimide, if the two materials are combined by bringing them together in the barrel of an extruder, as is done i ' n the case of polystyrene and isopentane for example, high processing temperatures and additional cooling equipment would be necessary in order to extrude a polyetherimide foam. In such a case the processing temperatures would have to be in the range of from about 343-371°C (650-700°F) and a separate high pressure control system would be necessary for the introduction of the above-defined blowing agent. Thereafter, a separate cooling means or zone would need to be employed in the system before extrusion could take place otherwise the foamed system would collapse and an inferior foam structure would result. In order to avoid this, it has been found that the polyetherimide particles can be readily imbibed or impregnated with the above-identified class of blowing agents and the imbibed resin particles can be melt processed at a temperature below about 260°C (500°F) and as low as from about 218°-232°C (425°-450°F). To impregnate the Ultem ® 1000 particles they merely need be subjected to a concentrated environment of the blowing agent vapors, for example, at room temperature for a period of up to about 48 hours. This will yield free flowing particles containing sufficient blowing agent to form a low density foam. The Ultem ® 1000 resin particles can contain up to about 30 parts of blowing agent per 100 parts of resin without any problem of inter particle adhesion. The particles are preferably impregnated with 3-20 parts of blowing agent per 100 parts by weight of resin. By employment of the blowing agent-impregnated Ultem ® 1000 particles this permits the use of significantly lower processing temperatures and simpler processing equipment since the blowing agent is already effectively dispersed throughout the resin matrix. Example 2 illustrates the production of the polymer foam by an extrusion process. - As indicated above the employment of chloroform, 1,1,2-trichloroethane or mixtures thereof and with methylene chloride will result in an equivalent foam. It is to be understood that conventional additives, such as nucleating agents, may be added to the starting material or melt before extrusion. The foam resins of the present invention can be thermofor ed into any desired shape. Generally this involves preheating a sheet of the foam structure so as to gradually bring the temperature of the foam up to molding temperature and thereafter shaping the foam by means of either, a female mold assisted by some force to draw the softened foam into conformation with the mold, or by the use of matching male and female dies. By way of example, a polyetherimide foam sheet of a density less than about 320.4 kg/cm (20 lbs/ft ) can be formed employing the resin of Example 2 in the system described in Example 2, modified by the employment of a circular slit die which will yield a sheet of about 0.23 cm (90 mils) thickness. The foam sheet can be preheated to a temperature of about 246°-274°C (475°-525°F) and incrementally advanced to male and female dies which will conform the resin sheet into a plurality of semi-circular sheaths two of which may accommodate the insulation of a conduit having an outside diameter of about 2.54 cm (1 inch). After the structures are thermoformed in the foam sheet, the molds are cooled, the thermofor ed sheet removed and the impressed structures are separated from the selvage of the sheet. The polymer foam may be formed into shaped structures. One method of preparing a shaped polymer foam structure according to the present invention comprises: (a) imbibing particles of a polyetherimide, a polycarbonate or a polymer blend of polyphenylene oxide and polystyrene with a solvent or blowing agent selected from methylene chloride, chloroform, 1,1,2-trichloroethane and mixtures thereof, (b) heating the imbibed particles to a temperature sufficient to cause expansion of the particles to a density significantly less than that of the imbibed particles; and (c) filling a mold with the expanded particles and subjecting the particles to sufficient heat to fuse the particles together on cooling to form a shaped coherent foam structure. In carrying out this process, it is also preferred to employ a polymer that is anhydrous so as not to introduce the likelihood of forming acidic components through the combination of H2O and the solvent blowing agent. Any such acid products would be corrosive to the equipment and possibly degradative to the polymer or its foam structure. Subjecting the Ultem ® 1000 particles, for example, to a temperature of approximately 149°C (300°F) for a period of about 4 hours will assure at least the substantial absence of HJ3 in the resin. Equivalent drying conditions can be employed for the other polymers. To prepare the low density shaped foam articles of the present invention, the selected polyetherimide resin is first imbibed with the above-identified solvent member. This solvent, for example, methylene chloride, should be imbibed or absorbed or otherwise taken up by the resin particles at a temperature less than about 37.8°C (100°F), preferably room temperature, to an extent which will subsequently permit expansion of said particles on heating the same above 37.8°C (100°F). The polyetherimide particles contemplated have the ability to readily absorb the subject solvent under relatively mild conditions, such as exposing the particles at approximately room temperature at standard pressure over a time period of up to 48 hours. Full exposure of the surface of the particles during this process enhances the absorption of the solvent. Imbibition of the solvent under these mild conditions will yield a composition which is still free flowing. This composition can be packaged for transport to a remote site for further processing according to the present process. The particle size of the resin is not critical so long as the subdivided resin is conveniently imbibed with the selected solvent. Obviously if the particles are too large they cannot be easily thoroughly imbibed with the solvent except by the use of extraordinary conditions. Conversely, if the particles are too small, this will increase the danger of premature agglomeration of the particles. A convenient particle size for the resin pellets would be from about-less than 0.08 cm-0.32 cm (1/32-1/8 inch), or larger, in diameter and in length. Any convenient system which will permit the particles to be exposed to the selected solvent can be used. The imbibing process can be well controlled by uniformly exposing all surfaces of the particles to the solvent in vapor form for whatever time is necessary to yield still free flowing particles which have been imbibed with sufficient solvent to subsequently yield a pre-expanded particle of the ultimately desired density. Particles which are imbibed to the maximum, yet are still free flowing, will produce foamed pre-expanded particles of extremely low density, for example, as low as less than 16 kg/m (1 lb. per cubic foot). On the other hand, particles which have been imbibed with a significantly lesser amount will yield ultimately a pre-expanded particle with a density approaching that of approximately 320.4 kg/m (20 lbs. per ft. ). When imbibing the particles, they may be supported on a suitable size mesh screen and the solvent vapors passed through the layer of the particles at approximately room temperature until the degree of imbibition is reached. Alternatively, the particles may be slowly fed into a vessel equipped with one or more agitation means to permit thorough exposure of the particles to the vapor form of the solvent. By these techniques, the pellets can be easily impregnated with from about 5-15 parts, preferably 10 parts, of the selected solvent per 100 parts of the resin particles. The imbibed particles are now in condition to be pre-expanded so that ultimately they may be incorporated into a suitable mold for formation into a comparatively low density part. These imbibed pellets are pre-expanded by placing the same into a system where the particles can be uniformly exposed to a heating system which will cause each particle to foam as the temperature of the particle and the imbibed solvent increases. While the particles will foam by the application of heat alone, it is preferred that the particles be exposed to a heated mixture of an inert gas and the selected solvent in vapor form. For example, a mixture of methylene chloride and carbon dioxide in a ratio of from about 30:70 to 70:30 by volume can be employed. Preferably, a 50/50 volume mixture is passed through the imbibed particles. The temperature of the system can range from about 163° to 260°C (325°-500°F). As the individual pellets increase in volume they become entrained in the gas mixture and pass from the heating system to a point of collection remote from the heating system. The degree of pre-expansion can be controlled by monitoring the residence time of the particles so that the density of the expanded particles can range from approximately 16 kg/m 3 to 320.4 kg/m3 (1 lb. per cubic foot to about 20 lbs/ft ). This control will also largely dictate the final density of the finished foamed parts. After the imbibed particles are pre-expanded to the required degree, they can be then put through a molding cycle. The molding cycle includes the filling of the mold, fusion of the individual pre-expanded particles one to the other, cooling the mold, and removing the molded part. Any convenient prior art molding system can be employed utilizing known techniques for enhancing heat transfer and means for facilitating the separation of the part from the mold. By way of example, Teflon coated aluminum molds can be employed for high heat transfer rate and ease of separation of the part from the mold. As in the case of pre-expandiπg the particles, heat and positive pressure alone can be employed in effecting a suitably fused foam particle part. It is preferred, however, to employ heat and vaporized solvent, e.g. methylene chloride, to effect fusion of the particles. For better control, it is preferred to employ a gaseous mixture of the solvent and an inert gas, such as carbon dioxide, to fuse the particles. Following this type of particle heating and, while a positive pressure of anywhere from about 0.141 to 2.11 kg/cm (2-30 lbs per square inch) is maintained, the part is cooled and subsequently ejected from the mold. During fusion of the particles the temperature can be maintained at from about 163° to 260°C (325°-500°F). This process is further illustrated in Example 3. The subject process can be carried out with equivalent results by employing chloroform or trichloroethane or mixtures thereof with or without methylene chloride. The other disclosed polymers which are solvent absorbable to the extent of the limits stated herein can also be formed into molded parts having an excellent foam network. Another method for preparing a molded thermoplastic foam structure having a density of less than 320.4 kg/m (20 lb/ft ) comprises: (a) impregnating a thermoplastic polymer in particle form, the polymer being capable of imbibing a solvent or blowing agent, with an amount of blowing agent sufficient to foam the polymer to a density of less than 320.4 kg/m to yield at least a substantially free flowing particulate combination; (b) partially filling a mold with the free flowing particulate combination; (c) pressurizing the mold to prevent any substantial vaporization of the blowing agent during subsequent polymer melting; (d) heating the polymer to at least a flowable molten state; (e) releasing the mold pressure to permit foaming of the melt and complete filling of the mold with the foamed polymer; and (f) removing the molded foam structure. It has been found that wide processing latitude is available when the starting material is a thermoplastic polymer in particulate form which has been imbibed therein the requisite amount of blowing agent. When the polymer particles are processed into an ostensibly dry, particulate, free flowing form, the following process sequence can be carried out: (1) this blowing agent-imbibed particulate polymer can be incorporated into a mold of the desired shape, (2) the mold system pressurized to prevent any substantial premature volatization of the blowing agent, (3) the polymer, which already has the blowing agent substantially uniformly dispersed therein, is melted and (4) the pressure released to form an excellent polymer foam structure assuming the shape of the mold. By this technique the density of the polymer foam can be controlled so that denser structures can be obtained by controlling the concentration of blowing agent in the system in combination with the amount of particulate resin incorporated into the mold and the pressure applied to the closed mold system. By such control, foam structures which range from structural'foam having unfoamed skins of controllable thickness to foams of lesser density which have no appreciable unfoamed surface skin can be formed. In preparing the shaped foam articles of the present invention, the selected resin is first imbibed with the appropriate blowing agent. In the case of the polyetherimides, the polycarbonates and the polyphenylene oxide-polystyrene blends, those which are imbibable with methylene chloride, chloroform, 1, 1, 2-trichloroethane or mixtures thereof are employed. The selected solvent, for example, methylene chloride, should be imbibed or absorbed or otherwise taken up by the polyetherimide particles under the mild conditions of a temperature less than about 38°C (100°F), preferably at room temperature and standard pressure, within a period of about less than 48 hours, to an extent which will subsequently permit foaming of the polymer. Full exposure of the surface of the particles during this process enhances the absorption of the solvent. Imbibition of the solvent blowing agent under these mild conditions yields a composition which is still a free flowing powder or particles. This composition can be packaged for transport to a remote site for further processing according to the present process. The particle size of the thermoplastic resin is not critical so long as the subdivided resin is conveniently imbibed with the selected solvent. Obviously if the particles are too large they cannot be easily thoroughly imbibed with the solvent except by the use of extraordinary conditions. Conversely, if the particles are too small, this will increase the danger of premature solvent agglomeration of the particles. Any convenient system can be employed which will permit the particles to be exposed to the selected solvent. Good control of the imbibing process can be had by uniformly exposing all surfaces of the particles to the solvent in vapor form for whatever time is necessary to yield still free flowing particles which are imbibed with sufficient blowing agent to ultimately yield a thermoplastic foam of the deisre density. Particles which are imbibed to the maximum, yet are still free flowing, will produce foamed polymer of extremely low density, for example, as low as less than 16 kg/ (1 lb. per cubic foot). On the other hand, particles which have been imbibed with a significantly lesser amount will yield foams which' can have a density approaching that of 320.4 kg/m (20 lbs per ft ) or more. When imbibing the particles, they may be supported on a suitable size mesh screen and the solvent or blowing agent vapors permitted to permeate the layer of particles at approximately room temperature until the degree of imbibition is reached. Alternatively, the particles may be slowly fed into a vessel equipped with one or more agitation means to permit thorough exposure of the particles to the vapor form of the solvent or blowing agent. By these techniques, the pellets or particles can be easily impregnated with from about 5-15 parts, preferably 10 parts, of the selected solvent per 100 parts by weight of the resin particles. The imbibed particles are now in condition to be placed into a mold of the desired shape. The imbibed particles are placed in the selected mold so that they occupy only a portion of the volume of the mold. The extent to which the particles occupy the mold volume depends upon the character of the foam structure desired. Filling the mold to the maximum with the particulate imbibed resin will obviously occupy a significant portion of the mold when the resin is melted, thus not permitting a great deal of expansion thereby yielding a foam structure of comparatively high density. On the other hand, filling the mold to a lesser extent will result in the ultimate formation of a foam structure of correspondingly lower density. In heating the mold system of the present invention it is to be understood that some of the imbibed blowing agent will vaporize and contribute to the pressurization of the mold. In order to maintain sufficient blowing agent dissolved or dispersed in the molten resin to obtain the desired foam density it is necessary to take into consideration the amount of blowing agent which will be driven from the molten resin to the pressurized atmosphere above it during resin heating and before pressure release. Control of the foam density can be accomplished by adding vaporized blowing agent as part of the partial pressure of the system. One skilled in the art can readily calculate the amount of imbibed blowing agent which would be driven from the heated particles into the space available in any given mold. An equivalent amount or any fraction thereof can be employed during the pressurization. By employing a pressure system ranging from all inert gas to an appropriate mixture of inert gas and blowing agent vapor, foam densities can be varied from a high density to a low density foam. For instance, using Ultem ® 1000 imbibed with 15 parts by weight of resin particles in mold of cylindrical shape, 6.35 cm (2.5 inches) in diameter, 2.54 cm (1 inch) in length, containing about 30 grams of impregnated resin pressurized with 21 kg/cm (300 psi) of C0 9 , will yield a comparatively high density foam of about 320.4 3 3 kg/m (20 lbs/ft ). By employing the same resin-blowing agent combination in a weight amount of 25 grams but in a cylindrical mold 6.35 cm (2.5 inches) in diameter by 38.7 cm (6 inches) in length, pressurized to 42 kg/cm 2 (600 psi) at 204°C (400°F) with a 40:60 by weight ratio of methylene chloride vapor and C0 will yield a ^ 3 cylindrical foam shape of a density of about 48.06 kg/ (3 lbs/ft ). This process is illustrated in Example 4. The present invention is further illustrated in the following examples. Example 1 This example shows a process of producing a polyetherimide useful in the present invention. A mixture containing 2.855 parts of l,3-bis(4-phenoxyphthalimido) benzene, 1.180 parts of Bisphenol A, 0.081 part of o-phenylphenol sodium salt and 20 parts of N-methylpyrrolidone is heated to reflux under nitrogen atmosphere. The heating is continued for one hour during which time an approximate total of 10 parts of liquid is distilled off. The reaction mixture is cooled and poured into about 300 parts of methanol which is stirred in a blender. A white polymer is precipitated. The polymer is filtered, washed and dried under vacuum. The polymer is a polyetherimide within the structure defined above, wherein the precursor of Ar is Bisphenol A and R is pheπylene. Example 2 This example illustrates the formation of a polymer foam by an extrusion process. Anhydrous Ultem ® 1000 particles were exposed to an environment of methylene chloride at room temperature for a period of approximately 48 hours to yield free flowing particles containing about 15 parts by weight methylene chloride per 100 parts of resin. These particles were fed into the feed throat of a single screw extruder having an L/D ratio of 24:1. The screw had a diameter of 3.2 cm (11/4"). The impregnated particles were homogeneously melt blended at a temperature of 232.2°C (450°F) and thereafter transported directly to the die area of the system. The homogeneous melt was extruded through a capillary die having a diameter of 0.203 cm (0.080 inches) and a L/D of 16.1. The extruded product had a 3 3 density of approximately 40 kg/cm (2.5 lbs/ft ) with a fine closed cell structure. It has been determined that during and within a short period (about 2 hours) after the extrusion, the methylene chloride was virtually totally expelled from the foam cells to yield the excellent foam product. The foam resin retains all of the high flame resistant and low smoke generation characteristics of the virgin polymer. Example 3 This example illustrates a process of the invention wherein solvent imbibed polymer particles are pre-expanded and then molded. Polyetherimide resin pellets having a particle size of about 0.16 to 0.32 cm (1/16-1/8 inch) in diameter and length, (available as Ultem ® 1000 from General Electric Company, Plastics Operation, 1 Plastics Avenue, Pittsfield, MA), are impregnated with methylene chloride to an extent of 10 parts per 100 parts of resin. Referring to Fig. 1, this is accomplished by placing a 5.08 cm (2 inch) layer of the resin pellets 10 on a supporting screen 12 and exposing the pellets to methylene chloride vapors 14 at approximately room temperature. After a period of 24 hours the particles are imbibed to the extent indicated above. The particles remain free flowing and can be packaged at this point for shipment to a remote site for completion of the process. Referring to Fig. 2, the imbibed particles are pre-expanded by feeding the imbibed pellets into an upright vessel 16 equipped with a motor-driven vertical shaft 18 having horizontal aquitator bars 20 attached thereto. As the imbibed particles are agitated, a 50/50 volume mixture of carbon dioxide and methylene chloride 22 is passed up and around each particle at a temperature of about 204°C (400°F) and at a rate sufficient to entrain the expanded particles. This causes the volume of the individual pellets to increase and eventually become entrained in the rising gas mixture. The pre-expanded particles 24 pass out of the vessel where they are collected for subsequent use in the molding operation. Conditions are selected so as to yield a density of the pre-expanded particles of about 160.2 kg/m 3 (10 lbs/ft 3 ). Referring to Fig. 3, the pre-expanded particles 24 are then fed into a mold 26 having the dimensions of 30.5 x 30.5 x 1.3 cm (12 x 12 x 0.5 inches). A 50/50 volume mixture of carbon dioxide and methylene chloride 28 at a temperature of 232°C (450°F) and a pressure of 0.14 kg/cm 2 .g (2 psig) is passed through the confined expanded pellets. On cooling the part is removed from the mold and will have a density of approximately 80.1 kg/ (5 lbs. per ft. ) with excellent beam strength. This part will have all of the excellent flame resistant and low smoke characteristics of the virgin resin. Example 4 This example illustrates another process of the invention wherein solvent-imbibed polymer particles are placed in a mold, pressurized and heated, with the pressure subsequently being released to cause foaming. Polyetherimide resin pellets having a particle size of about 0.16 to 0.32 cm (1/16-1/8 inch) in diameter and length, available as Ultem ® 1000 from General Electric Company, Plastics Operation, 1 Plastics Avenue, Pittsfield, MA., are impregnated with methylene chloride to an extent of 15 parts per 100 parts by weight of resin. Referring to Fig. 4, this is accomplished by placing a 5.08 cm (2 inch) layer of the resin pellets 30 on a supporting screen 32 and exposing the pellets to methylene chloride vapors 34 at approximately room temperature. After a period of 48 hours, the particles are imbibed to the extent indicated above. The particles remain free flowing. Referring to Fig. 5, a mold 36 having the internal dimensions of a cylinder 6.35 cm (2.5 inches) in diameter and 15.24 cm (6 inches) long is filled with 30 grams of the imbibed resin 38. The mold is closed and pressurized to 42 kg/c (600 psi) with a heated mixture of methylene chloride and carbon dioxide in the ratio of 40 to 60 wt % by way of pressure means 40. The mold is then heated to a temperature of 204°C (400°F) by way of heating means 42. After the pellets are completely melted, the pneumatic pressure is rapidly released by way of release means 44 resulting in foaming inside the mold. The mold is then cooled and the foamed article removed therefrom. The resulting foam structure has the shape and dimensions of the mold with a fine substantially uniform closed cell structure throughout the article. Its density is about 64.08 kg/m (4 lbs/ft ). This foam structure has all of the excellent flame resistant and low smoke characteristics of the virgin resin.

Claims: CLAIMS: 1. A polymer in foam form having a density of less than 320.4 kg/m , the polymer being a solvent imbibable polyetherimide, polycarbonate or a blend of polyphenylene oxide and polystyrene, and the solvent being methylene chloride, chloroform, 1, 1, 2-trichloroethane or mixtures thereof. 2. The foam of claim 1 wherein the polyetherimide has the following structure: wherein n is a positive integer, Ar is a divalent radical containing 6 to 20 carbon atoms, R is a bivalent radical selected from aliphatic, cycloaliphatic, aromatic and araliphatic groups. 3. The foam of claim 2 wherein the precursor of Ar is Bisphenol A and R is phenylene. 4. A composition comprising discrete particles of a polymer imbibed with a solvent in an amount sufficient to foam the polymer to a density of less than 320.4 kh/m , the polymer being a polyetherimide, a polycarbonate or a polymer blend of polyphenylene oxide and polystyrene, and the solvent bein methylene chloride, chloroform, 1,1,2-trichloroethane or mixtures thereof. 5. The composition of claim 4 containing up to 30 parts of the solvent per 100 parts of the polymer. 6. A process of preparing a low density polymer foam comprising: (a) imbibing the polymer in particle form with a solvent (b) homogeneously melt blending the particles and solvent under pressure; and (c) extruding the melt blend into a lower pressure atmosphere to cause the solvent to vaporize, yielding a foam structure, the polymer being a polyetherimide, a polycarbonate or a blend of a pheπylene oxide and polystyrene and the solvent being methylene chloride, chloroform, 1,1,2-trichloroethane or mixtures thereof. 7. The process of claim 6 wherein the polymer is a polyetherimide having the following structure: wherein n is a positive integer, Ar is a divalent radical containing 6 to 20 carbon atoms, R is a bivalent radical selected from aliphatic, cycloaliphatic, aromatic and araliphatic groups. 8. The process of claim 7 wherein the polymer is imbibed with an amount of solvent sufficient to foam the polymer to a density of less than 320.4 kg/m . 9. A thermoformed structure of a polymer foam prepared by the method comprising: (a) subjecting a sheet of the foam having a density of less than 320.4 kh/m to a first temperature sufficient to permit a shape formation to be effective therein; (b) effecting a shape in the sheet while at the first temperature; (c) cooling the sheet to a second temperature to permit permanent retention of the shape in the sheet at or below the second temperature, the polymer foam being a polyetherimide foam, a polycarbonate foam or a blend of polyphenylene oxide and polystyrene foam. 10. The structure of claim 9 wherein the polymer is polyimide having the following structure: wherein n is a positive integer, Ar is a divalent radical containing 6 to 20 carbon atoms, R is a bivalent radical selected from aliphatic, cycloaliphatic, aromatic and araliphatic groups. 11. A process for forming a shaped polymer foam structure comprising: (a) imbibing particles of a polyetherimide, a polycarbonate or a polymer blend of polyphenylene oxide and polystyrene, with a solvent selected from methylene chloride, chloroform, 1,1,2-trichloethane or mixtures thereof; (b) heating the imbibed particles to a temperature sufficient to cause expansion of the particles to a density significantly less than that of the imbibed particles; and (c) filling a mold with the expanded particles and subjecting the particles to sufficient heat to fuse the particles together on cooling to form a shaped coherent foam structure. 12. The process of claim 11 wherein the polymer is a polyetherimide of the following chemical structure: wherein n is a positive number, Ar is a divalent radical containing from 6-20 carbon atoms, R is a bivalent radical selected from the group consisting of aliphatic, cycloaliphatic, aromatic and araliphatic groups. 13. The process of claim 12 wherein the particles are 2 subjected to a pressure of from 0.141 to 2.11 kg/cm during the molding thereof. 14. The process of claim 13 wherein the imbibed particles of step (a) are at least substantially free flowing and at least not substantially expanded. 15. The process of claim 14 wherein the solvent is present in up to 30 parts per 100 parts by weight of polymer. 16. The process of claim 15 wherein the solvent member is present in from 0.5 to 20 parts per 100 parts by weight of resin. 17. The process of claim 14 wherein the imbioed particles are expanded in a heated gas stream so as to entrain the same and subsequently collecting the expanded particles. 18. The process of claim 17 wherein the temperature of the heated gas stream ranges from that temperature sufficient to just cause some sensible particle expansion up to the highest processing temperature possible without causing particle agglomeraton. 19. The process of claim 18 wherein the temperature range if from about 163° to 260°C . 20. The process of claim 17 wherein the heated gas is a mixture of the solvent member and an inert gas. 21. The process of claim 11 wherein the temperature in (c) is provided by hot gas contacting the particles. 22. The process of claim 21 wherein the hot gas includes the solvent member in gaseous-form. 23. The process of claim 22 wherein the hot gas includes an inert gas. 24. A shaped polymer foam structure made by the process of any one of claims 11-23. 25. A process for preparing a molded thermoplastic foam structure comprising: (a) impregnating a thermoplastic polymer in particulate form, which polymer is capable of imbibing a solvent, with an amount of solvent sufficient to foam the polymer to a density of less than about 320.4 kg/m , to. yield at least a substantially free flowing particulate combination; (b) partially filling a mold with the free flowing particulate combination; (c) pressurizing the mold to prevent any substantial vaporization of the solvent during subsequent polymer melting; (d) heating polymer to at least a molten state; (e) releasing the mold pressure to permit foaming of the melt and expansion of the foamed polymer within the mold; and (f) removing the molded foam structure. 26. The process of claim 25 wherein the polymer is selected from a solvent imbibable polyetherimide, polycarbonate and polymer blend of polyphenylene oxide and polystyrene. 27. The process of claim 26 wherein the solvent is selected from methylene chloride, chloroform, 1,1,2-trichloroethane and mixtures thereof. 28. The process of claim 27 wherein the polyetherimide is a polymer of the following chemical structure: wherein n is a positive integer, Ar is a divalent radical containing from 6-20 carbon atoms, R is a bivalent radical selected from aliphatic, cycloaliphatic, aromatic and araliphatic groups. 29. The process of claim 28 wherein the solvent is present in the particles in up to 30 parts per 100 parts by weight of the polymer. 30. The process of claim 29 wherein the blowing agent is present in from 0.5 to 20 parts per 100 parts by weight of the polymer. 31. The process of claim 25 wherein the mold is 2 pressuriz_eedd ttoo ffrroomm 2211 ttιo 105 kg/m with a gaseous system comprising an inert gas. 32. The process of claim 31 wherein the gaseous system comprises a mixture of an inert gas and a partial pressure of separately added solvent in vapor form. 33. A molded polymer foam structure made by the process of any one of claims 25-32.

Inventors: KRUTCHEN CHARLES M, WU WEN PAO, KRUTCHEN, CHARLES, M., WU, WEN, PAO
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1985
Main classification: B29C39
IPC codes: B29B9, B29C44, B29K105, C08J9