as illustrated in fig1 , an extrusion apparatus 100 useful for practicing methods according to the invention may comprise a single or double ( not shown ) screw extruder including a barrel 102 surrounding a screw 104 on which are provided a spiral flight 106 configured to compress , and thereby , heat material introduced into the screw extruder . as illustrated in fig1 , the basic polymeric composition can be feed into the screw extruder as a flowable solid , such as beads , granules or pellets , or as a liquid or semiliquid melt , from one or more ( not shown ) feed hoppers 108 . as the basic polymeric composition advances through the screw extruder , the decreasing spacing of the flight 106 , define a successively smaller space through which the polymer composition is forced by the rotation of the screw . this decreasing volume acts to increase the temperature of the polymer composition to obtain a polymeric melt ( if solid starting material was used ) and / or to increase the temperature of the polymeric melt . as the polymer composition advances through the screw extruder 100 , one or more ports may be provided through the barrel 102 with associated apparatus 110 configured for injecting one or more polymer processing aids into the polymer composition . similarly , one or more ports may be provided through the barrel 102 with associated apparatus 112 for injecting one or more blowing agents into the polymer composition . once the polymer processing aid ( s ) and blowing agent ( s ) have been introduced into the polymer composition , the resulting mixture is subjected to some additional blending sufficient to distribute each of the additives generally uniformly throughout the polymer composition to obtain an extrusion composition . this extrusion composition is then forced through an extrusion die 114 and exits the die into a region of reduced pressure ( which may be below atmospheric pressure ), thereby allowing the blowing agent to expand and produce a polymeric foam layer or slab . the polymeric foam may be subjected to additional processing such as calendaring , water immersion , cooling sprays or other operations to control the thickness and other properties of the resulting polymeric foam product . as illustrated in fig2 , an extrusion apparatus 200 useful for practicing methods according to the invention may comprise a single or double ( not shown ) screw extruder including a barrel 202 surrounding a screw 204 on which are provided a spiral flight 206 configured to compress , and thereby , heat material introduced into the screw extruder . as illustrated in fig2 , the basic polymeric composition , optionally compounded with one or more polymer processing aids , can be feed into the screw extruder as a flowable solid , such as beads , granules or pellets , or as a liquid or semiliquid melt , from one or more ( not shown ) feed hoppers 208 . as the basic polymeric composition advances through the screw extruder , the decreasing spacing of the flight 206 , define a successively smaller space through which the polymer composition is forced by the rotation of the screw . this decreasing volume acts to increase the temperature of the polymer composition to obtain a polymeric melt ( if solid starting material was used ) and / or to increase the temperature of the polymeric melt . as the polymer composition advances through the screw extruder 200 , one or more ports may be provided through the barrel 202 with associated apparatus 212 configured for injecting one or more blowing agents and , optionally one or more polymer processing aids , into the polymer composition . once the desired quantities of polymer , polymer processing aid ( s ) and blowing agent ( s ) have been introduced into the screw extruder , the resulting mixture is subjected to some additional blending sufficient to distribute each of the additives generally uniformly throughout the polymer composition to obtain an extrusion composition . this extrusion composition is then forced through an extrusion die 214 and exits the die into a region of reduced pressure ( which may be below atmospheric pressure ), thereby allowing the blowing agent to expand and produce a polymeric foam layer or slab . as illustrated in fig2 , this pressure reduction may be obtained gradually as the extruded polymeric mixture advances through successively larger openings provided in the die or through some suitable apparatus ( not shown ) provided downstream of the extrusion die for controlling to some degree the manner in which the pressure applied to the polymeric mixture is reduced . the polymeric foam may also be subjected to additional processing such as calendaring , water immersion , cooling sprays or other operations to control the thickness and other properties of the resulting polymeric foam product . exemplary methods according to the invention may utilize one or more of a variety of blowing agents to achieve the desired polymeric foam properties in the final product . in general , the polymeric composition will include at least a major portion of a high melt flow polystyrene ( e . g ., a polystyrene having a melt flow index of at least about 10 g / 10 minutes ( as measured according to astm d 1238 condition l ) using an atmospheric gas , preferably co 2 , as the primary blowing agent . in addition to the co 2 , one or more polymer processing aids ( ppa ) selected from a group of esters , particularly adipate esters , and more particularly bis ( n - r ) adipate esters , wherein r is selected from a group consisting of c 6 - c 16 , and preferably c 8 - c 13 , aliphatic ( linear , cyclic and branched , saturated and unsaturated ) and aromatic ( substituted and unsubstituted ) groups , particularly compounds such as bis ( n - decanyl ) adipate . the processing aid ( s ) will improve the stability of the extrusion pressure / temperature profile and thereby improve the uniformity in the production of different thicknesses of polystyrene foam board using an atmospheric gas such as air , n 2 or , preferably , co 2 as the primary blowing agent . the polymeric composition will preferably be a styrenic polymer and / or another polymer having a sufficiently high melt flow index ( mfi or melt flow number ), e . g ., a melt flow index of at least about 10 ( g / 10 minutes ), thereby increasing the co 2 solubility relative to that which can be achieved with polystyrenes having a mfi of less than 10 . this polymeric composition may then be combined with minor amount of a polymer processing aid , typically an ester and preferably an adipate ester according to the general formula i provided below : wherein x is nitrogen or oxygen , r 1 is selected from a group consisting of c 1 - c 20 alkyl , aryl or alkaryl and r 2 and r 3 are independently selected from a group consisting of hydrogen , c 1 - c 20 , preferably c 6 - c 16 , and more preferably c 8 - c 13 , aliphatic ( linear , cyclic and branched , saturated and unsaturated ) and aromatic ( substituted and unsubstituted ) groups ( and are generally identical ), alkaryl and alkoxylate , wherein r 2 and r 3 cannot both be h . representative compounds are represented by formulas ii - iv as provided below : in addition to the adipate esters detailed above , benzoates generally corresponding to the general formula v and reproduced below may be useful for increasing the solubility of co 2 in polymeric compositions , such as polystyrene . wherein x is nitrogen or oxygen , r 3 is selected from a group consisting of c 1 - c 20 alkyl , aryl , alkaryl and alkoxylate . suitable compounds corresponding to the general structure illustrated in formula v for use in this invention are illustrated below as formulas vi - x : in addition to the adipate esters and benzoates detailed above , dibenzoates generally corresponding to the general formula xi and reproduced below may be useful for increasing the solubility of co 2 in polymeric compositions , such as polystyrene . wherein x is nitrogen or oxygen , r 4 is selected from a group consisting of c 1 - c 20 alkyl , aryl , alkaryl and alkoxylate . suitable compounds corresponding to the general structure illustrated in formula v for use in this invention are illustrated below as formulas xii - xvi : a minor portion , typically less than about 5 wt %, preferably less than about 3 wt % or , perhaps , even less than about 2 wt % of a ppa , such as an adipate polymer processing aid , can be used in combination with a similar or greater concentration of the blowing agent ( s ). for example , bis ( n - decanyl ) adipate ( formula iii ) can be incorporated into a polymeric system or melt at a rates as low as about 0 . 5 wt % and still exhibit improvements to the co 2 solubility , extrusion process stability as reflected by temperature / pressure profiles of the process to produce foam board exhibiting improved dimensional stability . the esters , and particularly adipate esters , will tend to outperform smaller alcohol compounds , such as ethanol , for maintaining the properties of the resulting foam board products , particularly when co 2 is used as the only blowing agent . in those instances in which the ppa is available as a liquid at or near room temperature , such as bis ( n - decanyl ) adipate , the ppa may be pumped through an injector and into an intermediate point in the movement of the polymeric composition through the extrusion device or extruder . accordingly , for systems or apparatus that incorporated suitable liquid handling equipment , such as the assemblies that were utilized to inject one or more conventional ppa &# 39 ; s such as ethanol , these same assemblies can be utilized to inject one or more of the new ppas . other ppa additives such as bis ( 3 - ethylhexyl ) adipate ( formula ii ) and bis ( n - tridecanyl ) ( formula iv ) and other similar compounds tend to exhibit similar effects with regard to improving the processability of co 2 in the polymeric melt . other polymers and copolymers such as styrene methyl methacrylate ( smma ) copolymers can also be utilized as the polymeric composition in methods according to the present invention and can be processed on the same apparatus as general purpose crystalline polystyrene . with respect to copolymers in particular , utilizing bis ( n - decanyl ) adipate as a paa in a smma copolymer composition having a s : mma molar ratio of about 80 : 20 is able to support about 5 wt % co 2 in the extrusion composition . this polymeric composition and ppa additive scheme tends to increase blowing power and reduce the cooling demands associated with the resulting polystyrene foam board products . several of the paas , including the bis ( n - decanyl ) adipate , can be compounded with other polymers and copolymers such as ethylene methyl acrylate and added directly to the flowable particles , beads , pellets or other compounded forms and tend to exhibit similar effects on the foaming process in general and improving co 2 solubility within the polymeric composition . the paa compound ( s ) can be incorporated into the melt through direct injection into the extruder or through precompounding ( blending ) the paa compound ( s ) with one or more of the other compatible polymers or additives to achieve similar effective concentrations in the final extrusion composition and thereby produce similar effects . it was also noted by the inventors that the presence of bis ( n - decanyl ) adipate ( formula iii ) appears to increase the solubility of hfc - 134a in the polymeric composition . accordingly , presence of bis ( n - decanyl ) adipate helps support polymeric compositions using combinations of blowing agents such as hfc - 134a / co 2 as well as the addition of water as a co - blowing agent to produce polystyrene foam board with desired properties . a series of experiments were conducted in order to investigate the relative performance of the invention and conventional practices in the production of xps products . each of the trials used amorphous , general - purpose crystal polystyrene ( specifically ps nc0038 from nova chemical ) having a melt flow index of 5 - 30 ( g / 10 min ) ( astm d 1238 condition l ), 0 . 78 wt % of bis ( n - decanyl ) adipate , 1 . 0 wt % of hexabromocyclodecane as a flame retardant and 0 . 2 wt % talc as nucleating agent . this mixture was then fed into a twin screw extruder having a screw diameter of 132 mm . the solids are melted , and then mixed with 3 . 7 wt % of co 2 . from the extruder , the plastified foamable mixture was cooled to a second temperature ( generally referred to as die melt temperature ) and extruded into a region of reduced pressure to form a foam product . in this instance , the mixture was cooled to a die melt temperature of 116 ° f . ( about 47 ° c .) and was then ejected through a die opening ( 27 cm × 1 . 41 mm ) into a region of lower pressure . exemplary samples according to the invention were generated by foaming the mixture under both atmospheric pressure ( 101 . 3 kpa ) and subatmospheric pressure conditions , specifically a vacuum of 8 , 12 or 16 inches of hg ( about 74 . 3 , 60 . 8 and 47 . 3 kpa respectively ). comparative samples were prepared using hcfc - 142b ( 11 wt % based on the polymer content ) as the blowing agent and using a combination of co 2 ( 3 . 7 wt %) and ethanol ( 1 . 5 wt %) as a blowing agent system . additional exemplary samples were produced using co 2 ( 3 . 7 wt %) and bis ( n - decanyl ) adipate ( formula iii ) ( 0 . 5 wt %) as a blowing agent system . the low level of bis ( n - decanyl ) adipate helps foam surface characteristics and at the same time enhances polymer melt processability the die pressure fluctuation usually is an indication of how any of the polymeric blowing agent system processes on the pilot line . as one can see from fig1 the overall pressure changes is not significant , as comparing the above mentioned systems . fig3 shows a comparison between current hcfc - 142b 11 %, co 2 3 . 7 %/ ethanol 1 . 5 % and co 2 3 . 7 %/ bis ( n - decanyl ) adipate at 0 . 5 %, 1 . 0 % and 1 . 5 % levels with the respective extrusion out pressure , static cooler pressure and die pressure monitored on pilot line runs with these compositions . for the 142b system ( control ) general purposes ps ( nova 1220 ) with melt flow index of 1 . 6 ( g / 10 . 0 minutes ) was used . for co 2 / ethanol and all other runs ps high melt flow index 10 . 0 ( g / 10 minutes ) ( nova nn0038 ) was been used . attempts were made to produce foam board have a thickness of around 1 inch ( about 25 . 4 mm ) while keeping all other process parameters relatively constant . although fig3 shows the extrusion pressures at atmospheric condition , this relationship appears to hold true for 10 inches of hg ( about 67 . 5 kpa ) ( or any other vacuum levels ) as well too . these results were achieved on an owens corning pilot line facility at tallmadge , ohio . the extrusion pressure profile is an indication of ease of process ability and blowing agent solubility in the melt . although . the extrusion out , cooler and die pressures for co 2 / bis ( n - decanyl ) adipate system were higher relative to the conventional 142 b system , they were very comparable to the co 2 / ethanol system . in fact co 2 / bis ( n - decanyl ) adipate at 1 . 5 % level outperforms co 2 / ethanol system in terms of ease of process ability . later during the trials the polymer was switched to a s : mma 80 : 20 ( nova nc0044 ) copolymer . because the process appeared relatively stable , we increased the co 2 incrementally to 4 . 3 % to increase both the blowing potential and the cooling capabilities . this higher co 2 level provided acceptable products with a good surface under atmospheric conditions . apparently the presence of the ppa bis ( n - decanyl ) adipate combined with more polar and high melt index s : mma copolymer helps increase the solubility of co 2 in the polymer system . the ability to dissolve more co 2 has huge impact on cooling power of the ba system and ease of process ability . the results of the trials are reported below in table 1 . average cell size , compressive modulus , percentage of open cells and r - value were all measured . r - value , or total thermal resistance , is the measure of the resistance of heat transfer . the method of determining r - value is described as follows . thermal conductivity , k is defined as the ratio of the heat flow per unit cross - sectional to the temperature drop per unit thickness with the us unit : the heat transfer through an insulating material can occur through solid conductivity , gas conductivity , radiation , and convection . the total thermal resistance ( r - value ), r is the measure of the resistance to heat transfer , and is determined as : there were some notable physical properties differences between boards made using hcfc - 142b , co 2 / ethanol and co 2 / bis ( n - decanyl ) adipate , table 1 . the board becomes weaker as the level of bis ( n - decanyl ) adipate increases from 0 . 5 to 1 . 5 %, a results which suggests a higher degree of plasticization . also , apparently as a result of the increasing concentrations of bis ( n - decanyl ) adipate , the open cell content tended to increase accordingly . additional trials were run using ps nc0038 ( nova chemicals ) with both hfc - 134a 7 %/ co 2 0 . 5 %/ bis ( n - decanyl ) adipate 1 . 5 % and hfc - 134a / bis ( n - decanyl ) adipate 1 . 5 % as the blowing compositions on the same pilot line used to generate the hfc - 142b discussed above . these trials produced ps foams having 2 . 25 pcf densities with an open cell content of only about 2 . 29 %, a value which is well within the acceptable range . in both of these runs , an additional 0 . 2 % ethyl methyl acrylate polymer was used to improve foam board surface quality . as noted above , the disclosed apparatus and methods of making polystyrene based foam products using one or more atmospheric gases and / or a halohydrocarbon as the primary blowing agent ( s ) in combination with a polymer processing aid comprising one or more esters , particularly adipates , benzoates and dibenzoates . further , as the polystyrene foam is extruded , the melt can be foamed and cooled to a degree sufficient to maintain generally normal process levels and can , accordingly , be controlled with conventional practices for obtaining foam products adapted for particular final uses . these conventional practices may be adapted for use with the present invention to provide some degree of control over the foam density and cell size while utilizing conventional extrusion apparatus and post - extrusion processing . although the invention has been described in the context of particular polystyrene foam materials , the inventive method is also applicable to other polymeric compositions and various combinations of blending agents to obtain a variety of polymeric foam materials . example embodiments of the invention have been disclosed herein and , although specific terms are employed , they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation . accordingly , it will be understood by those of ordinary skill in the art that various changes in form and details of the disclosed apparatus and methods may be made without departing from the spirit and scope of the invention as set forth in the following claims .