LOW-DENSITY, DUCTILE, ALKENYL AROMATIC/UNSATURATED NITRILE/CONJUGATED DIENE MICROCELLULAR MATERIAL

Abstract: An alkenyl aromatic/unsaturated nitrile/conjugated diene microcellular foam, exemplified in a styrene/acrylonitrile/butadiene foam, has a void volume of at least about 50 % and a relative toughness of at least about 500 psi. The high toughness is achieved with little or no reinforcing agent. The foam is blown without chemical blowing agent and is highly thermoformable.

Description: LOW-DENSITY, DUCTILE, ALKENYL AROMATIC/UNSATURATED NITRILE/CONJUGATED DIENE MICROCELLULAR MATERIAL Field ofthe Invention The present invention relates generally to alkenyl aromatic/unsaturated nitrile/conjugated diene foams, and more particularly to microcellular acrylonitrile/butadiene/styrene foams of relatively low density and relatively high ductility. Background ofthe Invention Acrylonitrile/butadiene/styrene (ABS) polymers are well known, and can serve as structural plastics, including structural foam plastics for appliance, automotive, furniture, and other uses, providing relatively low cost, lightweight products having high utility. ABS polymer foams are known. US Patent Nos. 4,238,572 (Hoffman) and 4,330,635 (Tokas), both assigned to the Monsanto Company, each describe a foamable polymeric composition including a copolymer of alkenyl aromatic and alkenyl nitrile monomers and optionally diene rubber grafted with the monomers, and a chemical blowing agent. Tokas describes a polybasic acid blowing agent, while Hoffman describes a blowing agent that is an alkenyl half-ester of a styrene/maleic anhydride copolymer which can be chemically bonded or grafted to one ofthe copolymer components prior to blending and extrusion. Hoffman and Tokas describe formation of articles via injection molding. More particularly, Tokas states that it is known to foam alkenyl aromatic polymers with hydrocarbons or other volatile fluid foaming agents that boil below about 100° C, and that foaming has taken place using nucleating systems including compounds that decompose to form carbon dioxide or nitrogen but that such nucleating systems have not been found to be efficient as blowing systems for alkenyl aromatic polymers. Tokas reports that U.S. Patent No. 3,960,792 encourages the use of halogenated hydrocarbons as blowing systems in polystyrene, reporting that while carbon dioxide blowing systems will foam polystyrene, collapse of such foams occurs in thick sections resulting in loss of dimensional stability. Tokas also states that the prior art discloses that foaming agents for a particular polymeric composition are not readily adaptable to other polymer systems. Tokas' invention purportedly lies in the discovery that polybasic acids have the ability to foam alkenyl aromatic, alkenyl nitrile polymers without collapse of cells. Tokas and Hoffman thus each require the addition, to a copolymeric alkenyl aromatic/alkenyl nitrile/optionally diene rubber raw material to be foamed, of a particular chemical species that will react under foaming conditions to produce a blowing agent. US Patent No. 4,643,940 (Shaw) describes resin composite foams including from about 10%) to about 50% by weight randomly oriented reinforcing fibers to provide strength and flexural stiffness. Shaw reports that suitable resins can be selected from among a list that includes ABS. While alkenyl aromatic/unsaturated nitrile/conjugated diene alkenyl aromatic/unsaturated nitrile/conjugated diene structural materials are known, and foams of these materials are known along with techniques for formation of such foams and reinforcement of such foams, a need exists in the art for tough alkenyl aromatic/unsaturated nitrile/conjugated diene foams that can be produced simply and inexpensively. It is an object ofthe invention to provide such foams and techniques for forming such foams. Summary ofthe Invention The present invention provides a series of articles and methods associated with alkenyl aromatic/unsaturated nitrile/conjugated diene foams. In one aspect the invention provides a series of articles. One embodiment is defined by an article comprising an alkenyl aromatic/unsaturated nitrile/conjugated diene foam having at least about 50% void volume. The article includes residual chemical blowing agent or reaction by-product of chemical blowing agent in an amount less than that inherently found in articles blown with about 0.2% by weight chemical blowing agent or more. In another embodiment the invention provides an article comprising an alkenyl aromatic/unsaturated nitrile/conjugated diene microcellular foam. The foam has at least about 50%) void volume and a relative toughness of at least about 500 psi. The article includes less than about 10%> reinforcing agent and residual chemical blowing agent or reaction by-product of chemical blowing agent in an amount less than that inherently found in articles blown with about 0.1% by weight chemical blowing agent or more. In another aspect the invention provides a series of methods. One method involves extruding an alkenyl aromatic/unsaturated nitrile/conjugated diene material that includes less than 10%) reinforcing agent and less than about 0.1% by weight chemical blowing agent, and foaming the material and recovering a foam having at least about 50% void volume. In another embodiment a method is provided involving extruding an alkenyl aromatic/unsaturated nitrile/conjugated diene foam at a die melt temperature of less than about 180°C and recovering a foam having at least about 50% void volume. Also provided is a method that involves extruding an alkenyl aromatic/unsaturated nitrile/conjugated diene microcellular material including less than about 10% reinforcing agent and less than about 0.1% by weight chemical blowing agent. The material is foamed and a microcellular foam is recovered that has at least about 50% void volume and a relative toughness of about least about 500 psi. According to another embodiment, an alkenyl aromatic/unsaturated nitrile/conjugated diene terpolymer having at least about 50%> void volume is thermoformed. Other advantages, novel features, and objects ofthe invention will become apparent from the following detailed description ofthe invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment ofthe invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. Brief Description ofthe Drawings Fig. 1 is a schematic representation of a conventional foam article; Fig. 2 is a schematic representation ofthe article of claim 1 after rupture of a cell wall separating a cell from a surface ofthe article; Fig. 3 is a schematic representation of a microcellular article ofthe present invention; Fig. 4 is a schematic representation of a microcellular article ofthe present invention after rupture of a cell wall separating a cell from a surface ofthe .article; Fig. 5 is a photocopy of a scanning electron micrograph (SEM) image of a cross-section of a conventional acrylonitrile/butadiene/styrene (ABS) foam; Fig. 6 is a photocopy of an SEM image of a microcellular ABS foam ofthe present invention; Fig. 7 is a schematic representation of an extrusion system useful for producing foams for the invention; Fig. 8 is a schematic representation of another extrusion system useful for producing foams ofthe present invention; Fig. 9 is a photocopy of an SEM image of a cross-section of microcellular ABS foam of the invention; Fig. 10 is a photocopy of an SEM image of a cross-section of another microcellular ABS foam ofthe invention; Fig. 11 is a photocopy of an SEM image of a cross-section of another microcellular ABS foam ofthe invention; Fig. 12 shows stress/strain curves for a microcellular ABS foam ofthe invention; Fig. 13 shows stress/strain curves for another microcellular ABS foam ofthe invention; Fig. 14 shows stress/strain curves for another microcellular ABS foam ofthe invention; and Fig. 15 shows stress/strain curves for another microcellular ABS foam ofthe invention. Detailed Description ofthe Invention Commonly-owned, co-pending U.S. provisional patent application serial no. 60/024,623, entitled "Method and Apparatus for Microcellular Extrusion", filed August 27, 1996 by Burnham, et al., commonly-owned, co-pending U.S. provisional patent application serial no. 60/026,889 entitled "Method and Apparatus for Microcellular Extrusion", filed September 23, 1996 by Kim, et al., commonly-owned, co-pending U.S. patent application serial no.08/777,709 "Method and Apparatus for Microcellular Polymer Extrusion", filed December 20, 1996 and commonly-owned, co-pending International patent application serial no. PCT/US/97/ 15088, filed August 26, 1997 all are incorporated herein by reference. The various embodiments and aspects ofthe invention will be better understood from the following definitions. As used herein, "nucleation" defines a process by which a homogeneous, single-phase solution of polymeric material, in which is dissolved molecules of a species that is a gas under ambient conditions, undergoes formations of clusters of molecules ofthe species that define "nucleation sites", from which cells will grow. That is, "nucleation" means a change from a homogeneous, single-phase solution to a mixture in which sites of aggregation of at least several molecules of blowing agent are formed. Nucleation defines that transitory state when gas, in solution in a polymer melt, comes out of solution to form a suspension of bubbles within the polymer melt. Generally this transition state is forced to occur by changing the solubility ofthe polymer melt from a state of sufficient solubility to contain a certain quantity of gas in solution to a state of insufficient solubility to contain that same quantity of gas in solution. Nucleation can be effected by subjecting the homogeneous, single-phase solution to rapid thermodynamic instability, such as rapid temperature change, rapid pressure drop, or both. Rapid pressure drop can be created using a nucleating pathway, defined below. Rapid temperature change can be created using a heated portion of an extruder, a hot glycerine bath, or the like. A "nucleating agent" is a dispersed agent, such as talc or other filler particles, added to a polymer and able to promote formation of nucleation sites from a single-phase, homogeneous solution. Thus "nucleation sites" do not define locations, within a polymer, at which nucleating agent particles reside. "Nucleated" refers to a state of a fluid polymeric material that had contained a single-phase, homogeneous solution including a dissolved species that is a gas under ambient conditions, following an event (typically thermodynamic instability) leading to the formation of nucleation sites. "Non-nucleated" refers to a state defined by a homogeneous, single-phase solution of polymeric material and dissolved species that is a gas under ambient conditions, absent nucleation sites. A "non-nucleated" material can include nucleating agent such as talc. A "polymeric material/blowing agent mixture" can be a single-phase, non-nucleated solution of at least the two, a nucleated solution of at least the two, or a mixture in which blowing agent cells have grown. "Essentially closed-cell" microcellular material is meant to define material that, at a thickness of about 100 microns, contains no connected cell pathway through the material. "Nucleating pathway" is meant to define a pathway that forms part of microcellular polymeric foam extrusion apparatus and in which, under conditions in which the apparatus is designed to operate (typically at pressures of from about 1500 to about 30,000 psi upstream ofthe nucleator and at flow rates of greater than about 10 pounds polymeric material per hour), the pressure of a single-phase solution of polymeric material admixed with blowing agent in the system drops below the saturation pressure for the particular blowing agent concentration at a rate or rates facilitating rapid nucleation. A nucleating pathway defines, optionally with other nucleating pathways, a nucleation or nucleating region of a device ofthe invention. "Reinforcing agent", as used herein, refers to auxiliary, essentially solid material constructed and arranged to add dimensional stability, or strength or toughness, to material. Such agents are typified by fibrous material as described in U.S. Patent Nos. 4,643,940 and 4,426,470. "Reinforcing agent" does not, by definition, necessarily include filler or other additives that are not constructed and arranged to add dimensional stability. Those of ordinary skill in the art can test an additive to determine whether it is a reinforcing agent in connection with a particular material. The present invention provides an alkenyl aromatic/unsaturated nitrile/conjugated diene microcellular foam that exhibits good relative toughness at high void volume without the need for reinforcing agents. The foam can be blown with a physical blowing agent such as carbon dioxide and thus in one embodiment does not require the added expense and complication of formulating a polymeric precursor to include a species that will react under extrusion conditions to form a blowing agent, especially the expense and complication of providing a copolymer component having chemically attached or grafter thereto a chemical blowing agent. Since foams blown with chemical blowing agents inherently include residual, unreacted chemical blowing agent after a final foam product has been produced, as well as chemical by-products ofthe reaction that forms a blowing agent, material ofthe present invention in this set of embodiments includes residual chemical blowing agent or reaction by- product of chemical blowing agent, in an amount less than that inherently found in articles blown with 0.1 % by weight chemical blowing agent or more, preferably including residual chemical blowing agent or reaction by-product of chemical blowing agent in an amount less than that inherently found in articles blown with 0.05%> by weight chemical blowing agent or more. In particularly preferred embodiments, the material is characterized by being essentially free of residual chemical blowing agent or free of reaction by-products of chemical blowing agent. That is, they include less residual chemical blowing agent or by-product than is inherently found in articles blown with any chemical blowing agent. One advantage of embodiments in which a chemical blowing agent is not used or used in minute quantities is that recyclability of product is maximized. Use of a chemical blowing agent typically reduces the attractiveness of a polymer to recycling since residual chemical blowing agent and blowing agent by-products contribute to an overall non-uniform recyclable material pool. Another advantage of embodiments in which a chemical blowing agent is not used is that lower extrusion temperatures can be used, in many cases because higher temperatures associated with activation of certain chemical blowing agents need not be used. Thus, according to one aspect, the invention involves extrusion of alkenyl aromatic/unsaturated nitrile/conjugated diene foams of at least 50% void fraction at a die melt temperature of less than about 180°C, preferably less than about 160°C, more preferably less than about 150°C, more preferably still less than about 145°C. Lower temperatures are possible also because of the use of supercritical fluid blowing agent (described more fully below), which reduces viscosity and allows extrusion at lower temperatures. Specifically, the material ofthe invention can be extruded at melt temperatures of less than about 150° C, preferably from about 135° C to about 150° C, more preferably from about 140 to about 145° C. Melt temperature, in this context, is measured at the die just prior to extrusion, in particular within about 5 seconds of extrusion to ambient conditions, preferably within about 2 seconds of extrusion to ambient conditions. The articles ofthe invention have a relative toughness of at least about 500 psi, preferably at least about 600 psi, more preferably at least about 700 psi, and more preferably still at least about 800 psi. Toughness in this context is directly measured using a tensile stress machine. "Toughness", as used herein, is defined as the area below a stress/strain curve exhibited by material ofthe invention, as illustrated in Figs. 12-15. The articles have a tensile elongation of at least about 5% in preferred embodiments, or preferably at least about 10%>, more preferably still at least about 15%). The relative toughness ofthe foams ofthe present invention is surprising, because much ofthe prior art would lead those of ordinary skill in the art to expect that extra efforts such as the incorporation of reinforcing agents, or the use of a specialized chemical blowing agent would be required for the formation of tough alkenyl aromatic/unsaturated nitrile/conjugated diene foams. Without wishing to be bound by any theory, it is believed that the surprising toughness exhibited by the microcellular foams ofthe invention may be due to the fact that smaller cells might be less effective in propagating a surface crack in a polymeric article. Propagation of surface cracks in articles is characteristic of a failure mode associated with materials of low toughness. This concept is illustrated schematically in Figs. 1-4. Fig. 1 is a schematic representation of a conventional foam article 10, including a plurality of relatively large, closed cells 12, 14, etc. and a surface 16. If surface 16 is subject to an abrasive or otherwise disruptive force that causes rupture ofthe cell wall separating cell 14 from surface 16 (Fig. 2), then due to the relatively large size of cell 14 a channel of relatively large dimension X is formed in surface 16. Channel X defines a stress concentrator (point of significant structural weakness) that can reduce the amount of energy needed to break the material. If article 10 is subjected to further stress, massive failure ofthe article can occur via rupture of cell walls between cell 14 and adjacent cells. That is, the relatively large channel of dimension X defined by cell 14 and the point of rupture between cell 14 and wall 16 is relatively easily propagated throughout the article. Moreover, it is common in the art to reduce material density (increase void volume) in structural foams by increasing the amount of blowing agent in the foaming process. In most conventional foam processes, however, this can lead to uncontrolled cell growth and, with relatively larger cells, a relatively higher number cell interconnections that define stress concentrators (points of potential structural failure). In contrast, a microcellular article 18 ofthe present invention, represented schematically in Fig. 3, and including a plurality of cells 20, 22, etc. and a surface 24, is more resistant to fracture (is tougher) because any abrasion or other force experienced by surface 24 that would rupture a cell wall separating surface 24 from cell 20 (Fig. 4) results in a channel of only dimension Y, significantly less than dimension X. The channel of dimension Y of article 18 is much less effective at propagating fracture throughout article 18 than is the channel of dimension X of article 10. This phenomenon can be perhaps better understood with reference to actual polymer foams. Figs. 5 and 6 are photocopies of SEM images of ABS foams. Fig. 5 is a conventional foam, and Fig. 6 is a microcellular foam. The scales of Figs. 5 and 6 are almost identical. Fig. 5 is of approximately 40-45% void volume, and the material of Fig. 6 is of approximately 50% void volume. It can be seen that, in the material of Fig. 5, a large, interconnected, void 25 exists as defined by several independent cells that have ruptured walls or that are formed close enough together that walls are not defined between them, or a combination. This large void defines a very large stress concentrator that can easily cause material failure. In contrast, the material of Fig. 6, if interconnected cells due to cell wall rupture exist, includes stress concentrators of much, much smaller dimension. Described another way, as the density of foam material decreases (void volume increases), cell wall thickness (distance between cells) decreases. With smaller cells, rupture of a cell wall results in a smaller material defect that defines a stress concentrator. Thus, the present invention involves a microcellular, alkenyl aromatic/unsaturated nitrile/conjugated diene copolymeric species that has surprising toughness at relatively high void volume. A wide variety of alkenyl aromatic/unsaturated nitrile/conjugated diene copolymers can be used to form the microcellular foams ofthe present invention. For example, the alkenyl aromatic unit ofthe copolymer can be selected from styrene, alphamethyl styrene, halogenated alkenyl aromatics such as chlorostyrene, bromostyrene, etc. or mixtures thereof. The unsaturated nitrile monomer can be acrylonitrile, methacrylonitrile, chloroacrylonitrile, or the like, or mixtures thereof. The conjugated diene typically is a polymerized, conjugated diene such as polymerized 1,3 -butadiene, 2,3-dimethylbutadiene, isoprene, 1,3-pentadiene, or the like. The alkenyl aromatic/unsaturated nitrile/conjugated diene copolymers can includes an unsaturated nitrile/conjugated diene copolymer and an alkenyl aromatic species grafted to the unsaturated nitrile/conjugated diene copolymer. For example, an unsaturated nitrile/conjugated diene copolymer including from about 10 to about 60 wt %>, preferably from about 20 to about 50 wt %, bound unsaturated nitrile units can be used. Alternatively, an alkenyl aromatic/conjugated diene or unsaturated nitrile/conjugated diene copolymer can be used, with the third component grafted thereto, or all components can be copolymerized together. The alkenyl aromatic/unsaturated nitrile/conjugated diene copolymer preferably includes a conjugated diene component in an amount of at least about 5%> by weight, preferably from about 10%> to about 80%> by weight, more preferably from about 15% to about 75%) by weight. Suitable alkenyl aromatic/unsaturated nitrile/conjugated diene copolymers are those described in US Patent Nos. 4,238,572 and 4,330,635, incorporated herein and by reference, free of chemical blowing agent or including blowing agent in preferred low amounts as described above. In preferred embodiments, the polymer ofthe invention includes an alkenyl aromatic component in an amount of from about 15-40%, preferably about 20 to about 30%, and more preferably still about 25%> by weight, a conjugated diene component of from about 15 to about 40%) by weight, preferably from about 20 to about 30%> by weight, and more preferably about 25%) by weight, and an unsaturated nitrile component in an amount of from about 25 to about 75% by weight, preferably from about 45 to about 55% by weight, more preferably about 50% by weight. Preferably these components are styrene, butadiene, and acrylonitrile, respectively. The alkenyl aromatic/unsaturated nitrile/conjugated diene microcellular material ofthe invention can include co-blended auxiliary component in an amount of up to about 20% by weight, but preferably includes such component in an amount of less than about 10%> by weight or 5%> by weight, and preferably still is free of such a component. A co-blended component can be, for example, a compatible species such as a polycarbonate, polystyrene, butadiene, acrylonitrile, or a mixture thereof. One or more components ofthe copolymer can be halogenated, but generally non- halogenated components are used. Techniques for polymerization ofthe component include those well-known techniques such as free-radical polymerization or the like. The procedure by which the polymerization is effected is not particularly limited, and an appropriate procedure can be chosen from bulk polymerization, solution polymerization, suspension polymerization and emulsion polymerization. The copolymer can include a molecular weight modifier such as, for example, alkylthiol compounds, xanthogendisulfides, thiuram disulfides, halogenated hydrocarbons, hydrocarbons, and the like. The amount ofthe molecular weight modifier used for radical polymerization is usually 0.05 to 3 parts by weight, preferably 0.1 to 1 part by weight, based on 100 parts by weight ofthe monomer mixture for copolymerization. Adding a molecular weight modifier can increase processability ofthe copolymer. In general, it is preferable that 10 to 95%o by weight ofthe molecular weight modifier is incorporated in a monomer mixture before the commencement of polymerization and, when the conversion reaches 20 to 70%, the remainder is added to the polymerization mixture. The number of divided lots can be appropriately determined according to the need. In some cases it is desirable to add plasticizer. The usual monomeric and preferably oligomeric plasticizers known in the state ofthe technology can be used within the meaning of the invention, alone or mixed with the polymeric plasticizers. A preferred material ofthe invention is an acrylonitrile/butadiene/styrene terpolymer. In preferred embodiments, microcellular material ofthe invention is produced having average cell size of less than about 50 microns. In some embodiments particularly small cell size is desired, and in these embodiments material ofthe invention has average cell size of less than about 20 microns, more preferably less than about 10 microns, and more preferably still less than about 5 microns. The microcellular material preferably has a maximum cell size of about 100 microns. In embodiments where particularly small cell size is desired, the material can have maximum cell size of about 50 microns, more preferably about 25 microns, and more preferably still about 15 microns. A set of embodiments includes all combinations of these noted average cell sizes and maximum cell sizes. For example, one embodiment in this set of embodiments includes microcellular material having an average cell size of less than about 30 microns with a maximum cell size of about 50 microns, and as another example an average cell size of less than about 30 microns with a maximum cell size of about 35 microns, etc. That is, microcellular material designed for a variety of purposes can be produced having a particular combination of average cell size and a maximum cell size preferable for that purpose. In one set of particularly preferred embodiments cell size is less than about 10 microns, average, in material having a void volume of from about 50% to about 70%. In another set of particularly preferred embodiments cell size, average, is from about 15 to about 30 microns with a void volume of about 90% or more. In one embodiment, essentially closed-cell microcellular material is produced. As used herein, "essentially closed-cell" is meant to define material that, at a thickness of about 100 microns, contains no connected cell pathway through the material. The microcellular article ofthe invention can be produced according to a variety of batch or continuous processes, such as those described in U.S. Patent No. 5,158,986 of Cha, et al, U.S. Patent Application Serial No. 08/777,709, of Anderson, et al., filed 12/20/96 and entitled METHOD AND APPARATUS FOR MICROCELLULAR POLYMER EXTRUSION, or International Patent Application Serial No. PCT/US97/15088 of Anderson, et al., filed 08/26/97, ofthe same title, each of which is incorporated herein by reference. One exemplary continuous extrusion arrangement will be described with reference to Fig. 7. This arrangement involves multi-channel nucleation, followed by passage of a nucleated polymeric stream through a residence chamber, followed by shaping and extrusion into ambient. It is to understood that this is by way of example only, and that many other systems involving single- channel nucleation and/or nucleation immediately preceding shaping at the die (see Fig. 8 and related description), and the like can be used. It is noted that in a working example below a tandem extrusion system involving single-channel nucleation at the die, essentially immediately followed by shaping, is used. Referring to Fig. 7, an extrusion system 30 includes a barrel 32 having a first, upstream end 34 and a second, downstream end 36. Mounted for rotation within barrel 32 is an extrusion screw 38 operably connected, at its upstream end, to a drive motor 40. Although not shown in detail, extrusion screw 38 includes feed, transition, gas injection, mixing, and metering sections. Positioned along extrusion barrel 32, optionally, are temperature control units 42. Control units 42 can be electrical heaters, can include passageways for temperature control fluid, or the like. Units 42 can be used to heat a stream of pelletized or fluid polymeric material within the extrusion barrel to facilitate melting, and/or to cool the stream to control viscosity, skin formation and, in some cases, blowing agent solubility. The temperature control units can operate differently at different locations along the barrel, that is, to heat at one or more locations, and to cool at one or more different locations. Any number of temperature control units can be provided. Extrusion barrel 32 is constructed and arranged to receive a precursor of a fluid alkenyl aromatic/unsaturated nitrile/conjugated diene material. Typically, this involves a standard hopper 44 for containing pelletized alkenyl aromatic/unsaturated nitrile/conjugated diene to be fed into the extruder barrel through orifice 46. Although preferred embodiments do not use chemical blowing agents, when chemical blowing agents are used they typically are compounded in polymer pellets introduced into hopper 44. Immediately downstream ofthe downstream end 48 of screw 38 in Fig. 7 is a region 50 which can be a temperature adjustment and control region, auxiliary mixing region, auxiliary pumping region, or the like. For example, region 50 can include temperature control units to adjust the temperature of a fluid polymeric stream prior to nucleation, as described below. Region 50 can include instead, or in addition, standard mixing units (not shown), or a flow- control unit such as a gear pump (not shown). In another embodiment, region 50 is replaced by a second screw of a tandem extrusion apparatus, the second screw optionally including a cooling region. Any of a wide variety of blowing agents can be used in connection with the present invention. Preferably, a physical blowing agent (a blowing agent that is a gas under ambient conditions) or mixture of physical blowing agents is used and, in this case, along barrel 32 of system 30 is a port 54 in fluid communication with a source 56 of a physical blowing agent. Physical blowing agents known to those of ordinary skill in the art such as hydrocarbons, chlorofluorocarbons, nitrogen, carbon dioxide, and the like can be used in connection with this embodiment ofthe invention and, according to a preferred embodiment, source 56 provides an atmospheric blowing agent, most preferably carbon dioxide. A pressure and metering device 58 typically is provided between blowing agent source 56 and port 54. Supercritical fluid blowing agents are especially preferred, in particular supercritical carbon dioxide. Suitable chemical blowing agents include those typically relatively low molecular weight organic compounds that decompose at a critical temperature or another condition achievable in extrusion and release a gas or gases such as nitrogen, carbon dioxide, or carbon monoxide. Examples include azo compounds such as azo dicarbonamide. Where a chemical blowing agent is used, the blowing agents can be introduced into systems of a invention by being compounded within polymer pellets feed into the system, or other techniques available to those of ordinary skill in the art. Device 58 can be used to meter the blowing agent so as to control the amount ofthe blowing agent in the polymeric stream within the extruder to maintain a level of blowing agent at a level, according to one set of embodiments, between about 1%) and 15% by weight, preferably between about 3%o and 12%> by weight, more preferably between about 5%> and 10% by weight, more preferably still between about 7% and 9% by weight, based on the weight ofthe polymeric stream and blowing agent. In other embodiments it is preferred that lower levels of blowing agent be used. As described in PCT/US97/15088, referenced above, different levels of blowing agent are desirable under different conditions and/or for different purposes which can be selected in accordance with the invention. The pressure and metering device can be connected to a controller (not shown) that also is connected to drive motor 40 and/or a drive mechanism of a gear pump (not shown) to control metering of blowing agent in relationship to flow of polymeric material to very precisely control the weight percent blowing agent in the fluid polymeric mixture. Although port 54 can be located at any of a variety of locations along the extruder barrel, according to a preferred embodiment it is located just upstream from a mixing section 60 ofthe extrusion screw and at a location 62 ofthe screw where the screw includes unbroken flights. In a preferred embodiment ofthe blowing agent port system, two ports on opposing top and bottom sides ofthe barrel are provided. In this preferred embodiment, port 54 is located at a region upstream from mixing section of screw 38 (including highly-broken flights) at a distance upstream ofthe mixing section of no more than about 4 full flights, preferably no more than about 2 full flights, or no more than 1 full flight. Positioned as such, injected blowing agent is very rapidly and evenly mixed into a fluid polymeric stream to quickly produce a single-phase solution ofthe foamed material precursor and the blowing agent. Port 54, in the preferred embodiment is a multi-hole port including a plurality of orifices connecting the blowing agent source with the extruder barrel. In preferred embodiments a plurality of ports 54 are provided about the extruder barrel at various positions radially and can be in alignment longitudinally with each other. For example, a plurality of ports 54 can be placed at the 12 o'clock, 3 o'clock, 6 o'clock, and 9 o'clock positions about the extruder barrel, each including multiple orifices. In this manner, where each orifice is considered a blowing agent orifice, the invention includes extrusion apparatus having at least about 10, preferably at least about 40, more preferably at least about 100, more preferably at least about 300, more preferably at least about 500, and more preferably still at least about 700 blowing agent orifices in fluid communication with the extruder barrel, fluidly connecting the barrel with a source of blowing agent. Also in preferred embodiments is an arrangement in which the blowing agent orifice or orifices are positioned along the extruder barrel at a location where, when a preferred screw is mounted in the barrel, the orifice or orifices are adjacent full, unbroken flights. In this manner, as the screw rotates, each flight, passes, or "wipes" each orifice periodically. This wiping increases rapid mixing of blowing agent and fluid foamed material precursor by, in one embodiment, essentially rapidly opening and closing each orifice by periodically blocking each orifice, when the flight is large enough relative to the orifice to completely block the orifice when in alignment therewith. The result is a distribution of relatively finely-divided, isolated regions of blowing agent in the fluid polymeric material immediately upon injection and prior to any mixing. In this arrangement, at a standard screw revolution speed of about 30 rpm, each orifice is passed by a flight at a rate of at least about 0.5 passes per second, more preferably at least about 1 pass per second, more preferably at least about 1.5 passes per second, and more preferably still at least about 2 passes per second. In preferred embodiments, orifices are positioned at a distance of from about 15 to about 30 barrel diameters from the beginning ofthe screw (at upstream end 34). The described arrangement facilitates a method ofthe invention that is practiced according to one set of embodiments. The method involves introducing, into fluid polymeric material flowing at a rate of at least about 40 lbs/hr., a blowing agent that is a gas under ambient conditions and, in a period of less than about 1 minute, creating a single-phase solution ofthe blowing agent fluid in the polymer. The blowing agent fluid is present in the solution in an amount of at least about 2.5% by weight based on the weight ofthe solution in this arrangement. In preferred embodiments, the rate of flow ofthe fluid polymeric material is at least about 60 lbs/hr., more preferably at least about 80 lbs/hr., and in a particularly preferred embodiment greater than at least about 100 lbs/hr., and the blowing agent fluid is added and a single-phase solution formed within one minute with blowing agent present in the solution in an amount of at least about 3% by weight, more preferably at least about 5%> by weight, more preferably at least about 7%, and more preferably still at least about 10% (although, as mentioned, in a another set of preferred embodiments lower levels of blowing agent are used). In these arrangements, at least about 2.4 lbs per hour blowing agent, preferably C0 2 , is introduced into the fluid stream and admixed therein to form a single-phase solution. The rate of introduction of blowing agent is matched with the rate of flow of polymer to achieve the optimum blowing agent concentration. In the embodiment illustrated in Fig. 7, a system is provided having a multi-channel nucleator 66, including nucleating pathways, located substantially upstream of shaping die 68. As used herein, "nucleating pathway" is meant to define a pathway that forms part of microcellular polymer foam extrusion apparatus and in which, under conditions in which the apparatus is designed to operate (typically at pressures of from about 1500 to about 30,000 psi upstream ofthe nucleator and at flow rates of greater than about 10 lbs polymeric material per hour), the pressure of a single-phase solution of polymeric material admixed with blowing agent in the system drops below the saturation pressure for the particular blowing agent concentration at a rate or rates facilitating nucleation. A nucleating pathway defines, optionally with other nucleating pathways, a nucleation or nucleating region of an extruder. In a preferred embodiment, nucleator 66 has a polymer receiving end in fluid communication with the extrusion barrel, constructed and arranged to receive a fluid, non- nucleated, single-phase solution of polymeric material and blowing agent supplied by the barrel. The nucleator includes a nucleated polymer releasing end in communication with residence chamber 70 constructed and arranged to contain nucleated polymeric material under conditions controlling cell growth, and a fluid pathway connecting the receiving end to the releasing end. The arrangement allows for injecting blowing agent and maintaining the fluid stream, downstream of injection and upstream of nucleation, under pressure varying by no more than about 1,000 psi, preferably no more than about 750 psi, and more preferably still no more than about 500 psi. The fluid pathway ofthe nucleator has length and cross-sectional dimensions that subject the single-phase solution, as a flowing stream, to conditions of solubility change sufficient to create sites of nucleation at the microcellular scale in the absence of auxiliary nucleating agent. "At the microcellular scale" defines a cell density that, with controlled foaming, can lead to microcellular material. While nucleating agent can be used in some embodiments, in other embodiments no new nucleating agent is used. In either case, the pathway is constructed so as to be able to create sites of nucleation in the absence of nucleating agent whether or not nucleating agent is present. In particular, the fluid pathway has dimensions creating a desired pressure drop rate through the pathway. In one set of embodiments, the pressure drop rate is relatively high, and a wide range of pressure drop rates are achievable. A pressure drop rate can be created, through the pathway, of at least about 0.1 GPa/sec in molten polymeric material admixed homogeneously with about 6 wt % C0 2 passing through the pathway of a rate of about 40 pounds fluid per hour. Preferably, the dimensions create a pressure drop rate through the pathway of from about 0.2 GPa sec to about 1.5 GPa/sec, or from about 0.2 GPa/sec to about 1 GPa/sec. The nucleator is constructed and arranged to subject the flowing stream to a pressure drop at a rate sufficient to create sites of nucleation at a density of at least about 10 7 sights/cm 3 , preferably at least about 10 8 sights/cm 3 . The arrangement of Fig. 7, or a similar arrangement that involves a single-channel nucleator located immediately upstream of shaping in association with a die, is constructed and arranged to continuously nucleate a fluid stream of single-phase solution of polymeric material and flowing agent flowing at a rate of at least 20 lbs/hour, preferably at least about 40 lbs/hour, more preferably at least about 60 lbs/hour, more preferably at least about 80 lbs/hour, and more preferably still at least about 100 lbs/hour. Whether a multiple-channel or single-channel nucleator is used, it is preferred that nucleation take place separate from (upstream of) shaping. In Fig. 7 nucleation takes place significantly upstream of shaping. In the working examples below, nucleation takes place very closely upstream of final release and shaping. Any arrangement can serve as a nucleator that subjects a flowing stream of a single-phase solution of foamed material precursor and blowing agent to a solubility change sufficient to nucleate the blowing agent. This solubility change can involve a rapid temperature change, a rapid pressure change, or a combination, and those of ordinary skill in the art will recognize a variety of arrangements for achieving nucleation in this manner. A rapid pressure drop to cause nucleation is preferred. Where a rapid temperature change is selected to achieve nucleation, temperature control units can be provided about nucleator 66. Nucleation by temperature control is described in U.S. Patent No. 5,158,986 (Cha., et al.) incorporated herein by reference. Temperature control units can be used alone or in combination with a fluid pathway of nucleator 66 creating a high pressure drop rate in fluid polymeric material flowing therethrough. The described arrangement allows for creation of a single-phase solution at high flow rates. In particular, the arrangement allows for establishing the stream of fluid polymeric material flowing " in the extradite at a rate of at least 60 lbs/hour and introducing C0 2 blowing agent at a rate of at least 1 lb/hour into the stream at an injection location to create a fluid stream including at least about 2.5% C0 2 by weight. While creation of open-cell material is desirable for a variety of products, closed-cell microcellular alkenyl aromatic/unsaturated nitrile/conjugated diene material is preferred in the present invention. To achieve rapid pressure drop to create microcellular material, while foaming controllably to maintain closed-cell material, nucleating should be separated from shaping by a distance sufficient to achieve this control. Also illustrated in fig. 7 is an optional shaping element 69 downstream of shaping die 68. Shaping element 69 can provide further control over the thickness or shape of an extruded product by restricting expansion, further cooling the extradite (via, for example, fluid cooling channels or other temperature control units in element 69, not shown), or a combination. Without element 69, extradite is extruded into ambient conditions upon emergence from shaping die 68 (restricted only by polymeric extradite downstream ofthe exit ofthe shaping die). With element 69, the extradite generally emerges from shaping die into conditions of pressure slightly above ambient. With reference to Fig. 7, several arrangements ofthe invention are described. In one, polymeric extradite emerges from a nucleating pathway into ambient conditions and, where multi-channel nucleation is used, is recombined there. This would involve elimination of components downstream of nucleator 66. In another arrangement, only forming element 69 exists downstream ofthe nucleator. In another, the system includes nucleator 66, an enclosure downstream thereof (chamber 70) and a constriction at the end ofthe chamber (forming die 68). In still another, the system includes nucleator 66, chamber 70, forming die 68, and forming element 69, as illustrated in the complete system of Fig. 7. Described another way, the invention includes one or more constrictions constructed and arranged to define nucleating pathway(s) and one or more constrictions upstream and/or downstream ofthe nucleating pathway(s) that each optionally include temperature control and/or shaping capability. The system produces extruded article in the shape of a continuous extrusion. Referring now to Fig. 8, an alternate extrusion system 71 ofthe invention is illustrated schematically, representative ofthe system described in the working examples below. System 71 includes a die 73 similar to die 68 of Fig. 7, but including an exit 75 that is of dimension creating a nucleating pathway. That is, a homogeneous, single-phase solution is created by the extruder in region 50 and, when urged through nucleating pathway 75, the homogeneous, single-phase solution is nucleated to form a nucleated fluid polymeric material which then is foamed and shaped optionally with the assistance of forming element 69. Nery thin product, such as sheet, can be made by controlling cell growth such that very small cells result and the cells are well-contained within the sheet (the cells do not create holes across the sheet), and very thick material can be produced (especially with a multi-hole nucleator) because controlled-growth cells are evenly distributed within residence chamber 70 just upstream of shaping die 68. The microcellular material ofthe invention has a void volume of at least about 50%, more preferably at least about 60%o, more preferably at least about 70%>, more preferably still at least about 75%>. Increasing cell density while maintaining essentially closed-cell, microcellular material can be achieved by using high pressure drop rates as described in PCT/US97/15088, referenced above. These void volumes are achieved even at the high toughness values described above. Void volume, in this context, means initial void volume, i.e., typically void volume immediately after extrusion .and cooling to ambient conditions. That is, formation of microcellular material at a void volume of 50%, followed by compaction resulting in a void volume of 40%, is still embraced by the definition of material at 50% void volume in accordance with the invention. Good toughness in the articles ofthe invention is achieved without necessity of reinforcing agents. Preferably, the articles ofthe invention have less than about 10%> reinforcing agent by weight, more preferably less than about 5% reinforcing agent, more preferably still less than about 2% reinforcing agent, and in particularly preferred embodiments the articles ofthe invention are essentially free of reinforcing agent. "Reinforcing agent", as used herein, refers to auxiliary, essentially solid material constructed and arranged to add dimensional stability, or strength or toughness, to material. Such agents are typified by fibrous material as described in U.S. Patent Νos. 4,643,940 and 4,426,470. "Reinforcing agent" does not, by definition, include filler, colorant, or other additives that are not constructed and arranged to add dimensional stability. Since reinforcing agents are added to increase dimensional stability, they typically are rod-like in shape or otherwise shaped to have a ratio, of a maximum dimension to a minimum dimension (length to diameter in the case of a rod or fiber) of at least about 3, preferably at least about 5, more preferably at least about 10. It is a feature that the low-density, alkenyl aromatic/unsaturated nitrile/conjugated diene copolymeric microcellular material of this invention has sufficient relative toughness and ductility to be thermoformable. Thermoforming is a well-known process, and those of ordinary skill in the art understand the meaning ofthe terms "thermoformable" and "thermoforming", in the context ofthe present disclosure. The function and advantage of these and other embodiments ofthe present invention will be more fully understood from the examples below. The following examples are intended to illustrate the benefits ofthe present invention, but do not exemplify the full scope ofthe invention. Example 1 : Tandem Extrusion System for Alkenyl Aromatic/Unsaturated Nitrile/Conjugated diene Microcellular Material A tandem extrusion line (Akron Extruders, Canal Fulton, OH) was arranged including a 2" primary extruder with L/D of 32 and a 2.5" secondary extruder with L/D of 34. An injection system for injection of C0 2 into the primary was placed at a distance of approximately 20 diameters from the feed section. The injection system included 4 equally- spaced circumferentially, radially-positioned ports, each port including 176 orifices (of 0.02" diameter), for a total of 704 orifices. The primary extruder was equipped with a two-stage screw including conventional first- stage feed, transition, and metering sections, followed by a multi-flighted (four flights) mixing section for blowing agent dispersion. The screw was designed for high-pressure injection of blowing agent with minimized pressure drop between the first-stage metering section and point of blowing agent injection. The mixing section included four flights unbroken at the injection ports so that the orifices were wiped (open and closed) by the flights. At a screw speed of 40 rpm each orifice was wiped by a flight at a frequency of 2J times per second. The mixing section and injection system allowed for very rapid establishment of a single- phase solution of flowing agent and polymeric material. The injection system included air-actuated control valve to precisely meter a mass flow rate of flowing agent at rates from 0.2 to 12 lbs/hr at pressures up to 5500 psi. The secondary extruder was equipped with a deep channel, three-flighted cooling screw with broken flights, with provided the ability to maintain a pressure profile of Microcellular material precursor, between injection of blowing agent and entrance to the point of nucleation (the die, in this case) varying by no more than about 1500 psi, and in most cases considerably less. The system was equipped with a slit die at the exit ofthe secondary extruder. The slit die had a width of 1.75", a depth of 0.034", and a land length of 0.3". Melt temperature was measured using a thermocouple placed just before (immediately upstream of) the die exit. Examples 2-5: Extrusion of Microcellular ABS Low-medium Density Foam ABS pellets (GE Plastic, Cycolac GPP-4600, natural, with a density of 1.06 g/cc) were gravity-fed from the hopper ofthe primary screw into the extrusion system described above. The primary screw speed was controlled at 48 rpm and the secondary screw speed at 16 rpm. The throughput measured was 72 lb/hr. Process conditions specific to each of four samples (Examples 2-5) are listed in Table 1. The sample of Example 2 was solid (without foaming) and used as a control. Figs. 9, 10, and 11 are photocopies of SEM images of cross sections ofthe microcellular samples of examples 3, 4, and 5, respectively. Average cell sizes measured from the SEM images are included in Table 1. The samples were cut into standard sizes (3" long and 0.5" wide strips) for tensile tests. The instrument used for the tests was INSTRON-4444 with a load cell of 500 lb. The specimen grip length was set at 2.0". The crosshead speed was 0.1 inch/min. Figs. 12-15 show test results for the samples of Examples 2-5, respectively. The elongation (defined as percentage strain before break) and relative toughness (defined as the energy absorbed by the sample before break divided by relative density) test results are also shown in Table 1. Table 1 : Process Conditions and Test Results For Four Samples Selected Some ofthe above samples were further processed by compacting to reduce void volume. Upon compaction to about 35% void volume, these samples retained their superior toughness. Those skilled in the art would readily appreciate that all parameters listed herein are meant to be exemplary and that actual parameters will depend upon the specific application for which the methods and apparatus ofthe present invention are used. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope ofthe appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described. What is claimed is:

Claims: CLAIMS 1. An article comprising an alkenyl aromatic/unsaturated nitrile/conjugated diene microcellular foam having at least about 50% void volume and a relative toughness of at least about 500 psi, the article including less than 10% reinforcing agent and including residual chemical blowing agent or reaction by-product of chemical blowing agent in an amount less than that inherently found in .articles blown with about 0.1 %> by weight chemical blowing agent or more. 2. An article as in claim 1, wherein the alkenyl aromatic/unsaturated nitrile/conjugated diene microcellular foam is styrene/acrylonitrile/butadiene microcellular foam. 3. An article as in claim 2, including residual chemical blowing agent or reaction byproduct of chemical blowing agent in an amount less than that inherently found in articles blown with 0.05% by weight chemical blowing agent or more. 4. An article as in claim 2, being essentially free of residual chemical blowing agent or free of reaction by-products of chemical blowing agent. 5. An article as in claim 5, having a relative toughness of at least about 500 psi. 6. An article as in claim 2, having a relative toughness of at least about 600 psi. 7. An article as in claim 2, having a relative toughness of at least about 700 psi. 8. An article as in claim 2, having a relative toughness of at least about 800 psi. 9. An article as in claim 2, having an average cell size of less than about 50 microns. 10. An article as in claim 2, having an average cell size of less than about 20 microns. 11. An article as in claim 2, having an average cell size of less than about 10 microns. 12. An article as in claim 2, having a maximum cell size of less than about 50 microns. 13. An article as in claim 2, wherein the article is essentially closed cell. 14. An article as in claim 2, the article being in the shape of a continuous extrusion. 15. An article as in claim 2, having a tensile elongation of at least about 5%. 16. An article as in claim 3, having a tensile elongation of at least about 10%>. 17. An article as in claim 2, having a tensile elongation of at least about 15%. 18. A method comprising extruding an alkenyl aromatic/unsaturated nitrile/conjugated diene microcellular material including less than about 10% reinforcing agent and less than about 0.1% by weight chemical blowing agent and foaming the material and recovering a microcellular foam having at least about 50% void volume and a relative toughness of at least about 500 psi. 19. A method as in claim 18, involving establishing a stream of a fluid, single-phase solution of styrene/acrylonitrile/butadiene and a blowing agent and continuously nucleating the solution by continuously subjecting the stream to conditions of solubility change sufficient to create sites of nucleation at the microcellular scale in the solution in the absence of auxiliary nucleating agent. 20. A method as in claim 19, wherein the blowing agent is a supercritical fluid. 21. A method as in claim 20, where the supercritical fluid blowing agent is supercritical carbon dioxide. 22. A method as in claim 19, involving continuously subjecting the stream to a pressure drop at a pressure drop rate sufficient to create sites of nucleation. 23. A method as in claim 22, involving subjecting the stream to a pressure drop at a rate sufficient to create sites of nucleation at a density of at least about 10 8 sites/cm 3 . 24. A method as in claim 19, involving subjecting the fluid stream to a pressure drop at a rate of at least about 0.2 GPa/sec to create sites of nucleation. 25. A method as in claim 19, involving continuously nucleating the fluid stream flowing at a rate of at least about 20 lbs per hour. 26. A method as in claim 19, involving continuously nucleating the fluid stream flowing at a rate of at least about 40 lbs per hour. 27. A method as in claim 19, involving continuously nucleating the fluid stream flowing at a rate of at least about 60 lbs per hour. 28. A method as in claim 19, involving continuously nucleating the fluid stream flowing at a rate of at least about 80 lbs per hour. 29. A method as in claim 19, involving continuously nucleating the fluid stream flowing at a rate of at least about 100 lbs per hour. 30. A method as in claim 19, involving injecting the blowing agent into the fluid alkenyl aromatic/unsaturated nitrile/conjugated diene material at an injection location and nucleating the single-phase solution at a nucleation region downstream ofthe injection location, the method comprising maintaining the stream, downstream ofthe injection location and upstream ofthe nucleation region, under pressure varying by no more than about 1 ,000 psi. 31. A method as in claim 19, involving injecting the blowing agent into the fluid alkenyl aromatic/unsaturated nitrile/conjugated diene material at an injection location and nucleating the single-phase solution at a nucleation region downstream ofthe injection location, the method comprising maintaining the stream, downstream ofthe injection location and upstream ofthe nucleation region, under pressure varying by no more than about 750 psi. 32. A method as in claim 19, involving injecting the blowing agent into the fluid alkenyl aromatic/unsaturated nitrile/conjugated diene material at an injection location and nucleating the single-phase solution at a nucleation region downstream ofthe injection location, the method comprising maintaining the stream, downstream ofthe injection location and upstream ofthe nucleation region, under pressure varying by no more than about 500 psi. 33. A method as in claim 19, involving establishing the stream of fluid polymeric material flowing in the extruder at a rate of at least about 60 lbs per hour, and introducing C0 2 blowing agent at a rate of at least about 1 lb per hour into the stream at an injection location to create a fluid stream including at least about 2.5% C0 2 by weight. 34. A method comprising thermoforming an alkenyl aromatic/unsaturated nitrile/conjugated diene terpolymer microcellular foam having at least about 50%> void volume. 35. A method as in claim 34, wherein the alkenyl aromatic/unsaturated nitrile/conjugated diene is a styrene/acrylonitrile/butadiene terpolymer. 36. A method as in claim 35, the microcellular foam including residual chemical blowing agent or reaction by-product of chemical blowing agent in an amount less than that inherently found in articles blown with about 0.1 %> by weight chemical blowing agent or more. 37. A method as in claim 35, the microcellular foam including residual chemical blowing agent or reaction by-product of chemical blowing agent in an amount less than that inherently found in articles blown with 0.05% by weight chemical blowing agent or more. 38. A method as in claim 34, the microcellular foam being essentially free of residual chemical blowing agent or free of reaction by-products of chemical blowing agent. 39. A method as in claim 35, the microcellular foam having a relative toughness of at least about 500 psi. 40. A method as in claim 35, the microcellular foam having a relative toughness of at least about 600 psi. 41. A method as in claim 35, the microcellular foam having a relative toughness of at least about 700 psi. 42. A method as in claim 35, the microcellular foam having a relative toughness of at least about 800 psi. 43. An article comprising an alkenyl aromatic/unsaturated nitrile/conjugated diene foam having at least about 50% void volume and including residual chemical blowing agent or reaction by-product of chemical blowing agent in an amount less than that inherently found in articles blown with about 0.1% by weight chemical blowing agent or more. 44. A method comprising extruding an alkenyl aromatic/unsaturated nitrile/conjugated diene material including less than 10% reinforcing agent and less than about 0.1%) by weight chemical blowing agent and foaming the material and recovering a foam having at least about 50% void volume. 45. A method as in claim 44, comprising recovering a foam having a relative toughness of at least about 500 psi. 46. A method comprising: extruding an alkenyl aromatic/unsaturated nitrile/conjugated diene foam at a die melt temperature of less than about 180°C and recovering a foam having at least about 50%> void volume. 47. A method as in claim 46, comprising extruding the foam at a die melt temperature of less than about 160°C. 48. A method as in claim 46, comprising extruding the foam at a die melt temperature of less than about 150°C. 49. A method as in claim 46, comprising extruding the foam at a die melt temperature of less than about 145°C. 50. A method as in claim 46, comprising recovering a micro-cellular foam. 51. A method as in claim 46, comprising recovering a foam having less than about 10%> reinforcing agent and a relative toughness of at least 500 psi.

Inventors: CHEN LIQIN, ROBAR WALTER E, CHEN, LIQIN, ROBAR, WALTER, E.
Applicants: CHEN LIQIN, ROBAR WALTER E, TREXEL INC, CHEN, LIQIN, ROBAR, WALTER, E., TREXEL, INC.
Application year: 1998
Main classification: C08J9
IPC codes: B29C44, B29C47, C08J9