Process for producing alkenyl aromatic foams using a combination of atmospheric and organic gases and foams produced thereby

There is disclosed a process for producing alkenyl aromatic foams utilizing a combination of atmospheric and organic gases as blowing agent, preferably using greater than 30% by weight of atmospheric gas, and preferably also using a predetermined about of a masterbatch mix comprising a styrenic polymer, a rubbery block copolymer, and a solid blowing agent. Also disclosed are alkenyl aromatic foams produced by the process which exhibit increased densities, increased thermoforming capabilities, increased post-expansion properties, and increased retainment of the atmospheric and organic gases.

FIELD OF THE INVENTION 
This invention relates to a process for producing alkenyl aromatic foams 
utilizing a combination of atmospheric and organic gases as blowing 
agents, preferably also using a predetermined amount of masterbatch mix. 
The invention also relates to alkenyl aromatic foams resulting from the 
process, and articles made therefrom. Preferably, the alkenyl aromatic is 
polystyrene. The process and resulting foams of the present invention are 
conferred with several benefits among which are an increase in the 
production rate of the process, a reduction in the amount of organic gas 
which must be used in the process in order to obtain a foamed product 
having a given density, an increase in the thermoforming output of the 
foams due to, inter alia, an increase in the post-expansion properties of 
the foams, and an increase in retainment of the atmospheric and organic 
gases in the foamed product. 
BACKGROUND OF THE INVENTION 
A variety of normally gaseous or liquid blowing agents have been proposed 
for olefinic or styrenic polymers, including virtually all of the common 
atmospheric gases and lower hydrocarbons. 
Alkenyl aromatic foams, particularly polystyrene foams in sheet form, are 
presently being made from a number of blowing agents which have many 
undesirable characteristics. Volatility, flammability, poor thermoforming 
qualities, brittle physical properties, high cost, or an adverse affect to 
the ozone layer are just a few. Examples of the blowing agents that 
produce these characteristics in the production of polystyrene foam would 
include the aliphatic hydrocarbons, and fully (or partially) halogenated 
hydrocarbons. 
For polystyrene, for example, the C.sub.4 -C.sub.6 alkanes have gathered 
widespread acceptance, especially pentane. Following a typical extrusion 
foaming step, the stock material is ordinarily aged before thermoforming 
into containers or the like. During aging, the foam cells and polymeric 
matrix become partially depleted of volatile hydrocarbons, which enter the 
atmosphere. However, potential atmospheric contamination by these 
by-products of foam manufacture has led workers to seek non-polluting 
alternative blowing agents, such as the usual atmospheric gases, e.g., 
nitrogen and carbon dioxide, and combinations of atmospheric gases with 
organic gases, e.g., the lower hydrocarbons or the freons. 
In the prior art, both atmospheric gases, per se, and combinations of 
atmospheric and organic gases have been disclosed as blowing agents for 
alkenyl aromatic polymers. 
Australian Patent Application No. 52724/79, published Canadian Patent 
Application No. 2,022,501 and published European Patent Application No. 
0,411,923 all disclose blowing agents consisting of carbon dioxide for 
alkenyl aromatic or styrenic polymers. The resulting foamed products are 
said to be flexible and/or have improved tensile elongation properties. 
However, the production rates of the processes are generally low, on the 
order of less than 200 lbs./hr., and also have generally low 
post-expansion properties, on the order of 50% or less. In addition, these 
processes require relatively high extrusion temperatures, on the order of 
130.degree. C. to 155.degree. C. Thus, these processes are not very 
economical. 
In co-pending patent application Serial No. 07/891,866, there are disclosed 
processes for producing polystyrene foams utilizing 100% of atmospheric 
gas, e.g. carbon dioxide and/or nitrogen, which can be effected at a much 
lower extrusion temperature, i.e. on the order of about 120.degree. C., 
utilizing in the melted polymer an additive comprised of a masterbatch mix 
containing alpha-methyl polystyrene, a rubbery block copolymer, a solid 
blowing agent comprised of an encapsulated combination of monosodium 
citrate and sodium bicarbonate, white mineral oil, and silica. 
U.S. Pat. Nos. 4,344,710 and 4,424,287 disclose blowing agents which are 
combinations of liquid carbon dioxide and liquid aliphatic, or fully (or 
partially) halogenated hydrocarbons. These patents state that the use of 
atmospheric gases, including 100% carbon dioxide or nitrogen as blowing 
agents has not been successfully employed, giving as a reason the extreme 
volatility of these gases, and further state that the use of these 
materials is said to produce corrugation and surface defects in the sheet 
product. These two patents disclose that a combination of atmospheric and 
organic gases, in an alkane: CO.sub.2 feed ratio in the range of 3:1 to 
1:1 by weight, can be used, with the total amount of blowing agent 
combination being in the range of 2.5 to 10 parts per 100 parts by weight 
of thermoplastic resin. As nucleating agents for the foamed products, the 
patents disclose the use of a mixture of sodium bicarbonate and citric 
acid. The process temperatures needed for extrusion of the foam are again 
quite high, on the order of 150.degree. C. 
U.S. Pat. No. 4,424,287 further discloses that the foams prepared with the 
combination of blowing agents exhibit the advantage of reduced atmospheric 
emissions upon aging without, however, any data to this effect, merely 
stating that the reduction in pollutant (i.e. the hydrocarbon blowing 
agents) is greater than the expected reduction due to the corresponding 
decrease in organic blowing agent use. The only rationale provided in U.S. 
Pat. No. 4,424,287 for the reduced hydrocarbon emissions is the ability of 
the foamed sheet product to be immediately thermoformed, thereby reducing 
the need for aging of the foamed sheet product. 
U.S. Pat. No. 4,419,309 discloses the use of two foaming agents; the first 
being introduced into a molten thermoplastic resin under higher pressure, 
with the first foaming agent being selected from a low molecular weight 
aliphatic hydrocarbon, a low molecular weight halocarbon and mixtures 
thereof, and the second foaming agent being introduced under lower 
pressure, with the second foaming agent being selected from carbon 
dioxide, water vapor and mixtures thereof, to cause foaming of the melted 
thermoplastic resin. Again, the extrusion rates are low, on the order of 
150 lbs./hr., and the extrusion temperatures are high, on the order of 
290.degree.-320.degree. F. 
U.S. Pat. Nos. 4,916,166 and 5,011,866 disclose alkenyl aromatic 
thermoplastic synthetic resinous elongated foam bodies having a machine 
direction, a transverse direction and closed, non-interconnecting 
gas-containing cells, which are prepared using, preferably at least 70% by 
weight of 1,1-difluoro-1-chloroethane (U.S. Pat. No. 4,916,166) and 
requiring the use of at least 70% by weight of 1,1,1,2-tetrafluoroethane 
or 1,1,1-trifluoroethane, based on the total weight of blowing agent 
mixture weight (U.S. Pat. No. 5,011,866), and using as a second blowing 
agent up to 30 weight percent (of the blowing agent in an amount of 
mixture) chemical or physical blowing agents, including water, 1-4 carbon 
aliphatic hydrocarbons, carbon dioxide, or other hydrogen-containing 
chlorofluorocarbons (HCFCs) such as chlorodifluoromethane (HCFC-22). 
U.S. Pat. No. 4,916,166 discloses that the amount of carbon dioxide is 
limited to no more than about 6% by weight and that extruded articles 
having densities between 2.4 and 5.0 pounds per cubic foot may be obtained 
only by extrusion at a die temperature of about 118.degree. C. or less. 
The extrusion rate at this temperature should necessarily be quite low, 
although the patent is silent on this point. The specific examples of U.S. 
Pat. No. 4,916,166 show that extruded foam articles having densities of 
less than 2.4 pounds per cubic foot are obtained only upon extrusion above 
118.degree. C., and these are obtained utilizing blowing agents which 
contain only about 2.7% by weight carbon dioxide based upon 100% by weight 
of the blowing agent mixture. 
U.S. Pat. No. 5,011,866 discloses alkenyl aromatic thermoplastic synthetic 
resinous elongated foamed products having densities of from about 1 to 
about 6 pounds per cubic foot which have a plurality of closed 
non-interconnecting gas-containing cells, with the limitation that the 
cells contain at least 70% by weight of either 1,1,1-trifluoroethane or 
1,1,1,2-tetrafluoroethane. U.S. Pat. No. 5,011,866 likewise prefers the 
use of less than 6% carbon dioxide as a component in a blowing agent 
mixture although some examples show the use of about 9% carbon dioxide. 
Thus, there still exists a need in the art for procedures for the 
production of alkenyl aromatic foams which utilize combinations of 
atmospheric and organic gases as blowing agents and having an increased 
amount of atmospheric gas, such as carbon dioxide or nitrogen. There also 
still exists a need in the art for such alkenyl aromatic foams which can 
be produced at increased temperatures and increased extrusion rates for a 
given density. Still further, there exists a need in the art for alkenyl 
aromatic foams having an increased percentage of closed, 
non-interconnected cell structure, increased post-expansion properties, 
and increased retainment of blowing agent within the cell structure of the 
alkenyl aromatic foam. 
These and other needs still remaining in the alkenyl aromatic foam art are 
met and satisfied by applicants' present invention, described below. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a process for 
the production of alkenyl aromatic foams, said process comprising: 
(a) heating an alkenyl aromatic resin to a temperature above its melting 
point to form a melted alkenyl aromatic resin; 
(b) adding to the melted alkenyl aromatic resin a masterbatch mix comprised 
of: 
(i) encapsulated monosodium citrate and sodium bicarbonate; 
(ii) styrene-ethylene/butylene-styrene block copolymer; 
(iii) alpha-methyl polystyrene; 
(iv) white mineral oil; and 
(v) silica, to form an alkenyl aromatic/masterbatch mix blend; 
(c) heating the alkenyl aromatic/masterbatch mix blend to a temperature 
sufficient to form a melted blend; 
(d) injecting into the melted blend a non-solid blowing agent comprised of 
a combination of atmospheric gas and organic gas to form an injected 
melted blend; 
(e) mixing the injected melted blend to form a mixed injected melted blend; 
and 
(f) cooling and extruding the mixed injected melted blend as an alkenyl 
aromatic foam. 
In another embodiment of the present invention, there is provided an 
alkenyl aromatic foam composition comprised of: 
(a) an alkenyl aromatic polymer; 
(b) alpha-methyl polystyrene; 
(c) styrene-ethylene/butylene-styrene block copolymer; 
(d) white mineral oil; 
(e) the decomposition products of an encapsulated monosodium citrate and 
sodium bicarbonate; and 
(f) silica, wherein the foam is comprised of closed cells containing 
therein a combination of atmospheric gas and organic gas. 1 In still a 
further embodiment of the present invention, there is provided an extruded 
alkenyl aromatic foam having a density greater than 2.5 pounds per cubic 
foot, having been extruded at a die temperature of 120.degree. C. or 
greater, and having a plurality of closed non-interconnected 
gas-containing cells therein, wherein the gas contained in the cells is 
comprised of a combination of atmospheric gas and organic gas and wherein 
the atmospheric gas is present in an amount of at least 30% by weight, 
based upon the total weight of the gas contained in the cells. 
In a still further embodiment, the present invention provides for a process 
for the production of alkenyl aromatic foams having a density of greater 
than about 2.5 pounds per cubic foot, said process comprising: 
(a) heating an alkenyl aromatic resin to a temperature above its melting 
point to form a melted alkenyl aromatic resin; 
(b) adding to the melted alkenyl aromatic resin a masterbatch mix comprised 
of: 
(i) a styrene resin; 
(ii) a rubbery block copolymer; and 
(iii)a solid blowing agent to form an alkenyl aromatic/masterbatch mix 
blend; 
(c) heating the alkenyl aromatic/masterbatch mix blend to a temperature 
sufficient to form a melted blend; 
(d) injecting into the melted blend a non-solid blowing agent comprised of 
a combination of atmospheric gas and organic gas to form an injected 
melted blend, wherein the atmospheric gas is present in an amount of at 
least about 30% by weight based upon the total weight of atmospheric gas 
and organic gas; 
(d) mixing the injected melted blend to form a mixed injected melted blend; 
(e) cooling the mixed injected melted blend; and 
(f) extruding the cooled blend at a temperature not below 120.degree. C. as 
an alkenyl aromatic foam. 
In yet a still further embodiment of the present invention, there is 
provided an alkenyl aromatic foam having a density of greater than 6.0 
pounds per cubic foot, having a plurality of closed, non-interconnecting 
gas-containing cells therein, wherein the gas contained in the cells is 
comprised of atmospheric gas and organic gas. 
In yet another further embodiment of the present invention, there is 
provided a process for the production of alkenyl aromatic foams having a 
density of greater than 6.0 pounds per cubic foot, said process 
comprising: 
(a) heating an alkenyl aromatic resin to a temperature above its melting 
point to form a melted alkenyl aromatic resin; 
(b) adding to the melted alkenyl aromatic resin a masterbatch mix comprised 
of: 
(i) encapsulated monosodium citrate and sodium bicarbonate; 
(ii) styrene-ethylene/butylene-styrene block copolymer; 
(iii) alpha-methyl polystyrene; 
(iv) white mineral oil; and 
(v) silica, to form an alkenyl aromatic/masterbatch mix blend; 
(c) heating the alkenyl aromatic/masterbatch mix blend to a temperature 
sufficient to form a melted blend; 
(d) injecting into the melted blend a non-solid blowing agent comprised of 
a combination of atmospheric gas and organic gas to form an injected 
melted blend; 
(e) mixing the injected melted blend to form a mixed injected melted blend; 
(f) cooling the mixed injected melted blend; and 
(g) extruding the cooled blend as an alkenyl aromatic foam.

DETAILED DESCRIPTION OF THE INVENTION 
The polyalkenyl aromatic polymers can be, for example, styrene polymers. 
The styrene polymers included in the compositions of the invention are 
homopolymers of styrene and copolymers and interpolymers of styrene 
containing a predominant proportion of styrene, e.g. greater than 50 
weight percent, and preferably greater than 75 weight percent, styrene. 
Examples of monomers that may be interpolymerized with the styrene include 
alpha, beta-unsaturated monocarboxylic acids and derivatives thereof, e.g. 
acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, 
2-ethylhexyl acrylate and the corresponding esters of methacrylic acid, 
acrylamide, methacrylamide, acrylonitrile, methacrylonitrile, maleic 
anhydride, etc. If desired, blends of the styrene polymer with other 
polymers may be employed, e.g. blends of the styrene polymer with grafted 
rubbery diene polymers, or the analogous compositions obtained by 
dispersing a rubber diene polymer in the styrene monomer and, optionally, 
other monomers, and subsequently polymerizing the mixture. In any of the 
above type resins, all or a portion of the styrene may be replaced with 
its closely related homologues such as alpha-methylstyrene, o-, m-, and 
p-methylstyrenes, o-, m-, and p-ethylstyrenes, 2,4-dimethylstyrene, 
bromostyrene, chlorostyrene, and the like. Copolymers of alkenyl aromatic, 
e.g. styrene, and alkenyl nitrile, e.g., acrylonitrile can also be used 
and can have a weight ratio of styrene to acrylonitrile of 95:5 to 5:95 
respectively. 
The rubber-containing blends can have the diene rubber moiety present in 
amounts of about 1 to 35% of grafted diene rubber particles dispersed in a 
matrix polymer or copolymer as a polyblend. Generally, the rubber 
particles are grafted with the polymers having the same composition as the 
matrix phase. The diene rubbers can be polybutadiene or copolymer rubbers 
having at least 50% by weight of a diene monomer, e.g. butadiene, 
chloroprene, isoprene or pentadiene. Comonomers copolymerizable with the 
diene monomers can be those disclosed above. The copolymer rubbers may be 
interpolymers or block copolymers. 
The masterbatch mix is a plasticizer which improves the flow 
characteristics of the foam. 
The masterbatch mix comprises in its broadest aspects: 
(a) a styrene resin; 
(b) a rubbery block copolymer; and 
(c) a solid blowing agent. 
An essential element of the masterbatch mix is the styrene resin. All 
commercially available styrene polymers can be used as the styrene resin. 
However, it is preferable that the Vicat softening temperature of the 
chosen styrene polymer be between 45 and 82 at 50.degree. C./hr. rise. 
Preferred as the styrene resin is alpha-methylstyrene. Commercially known 
alpha-methylstyrenes include Amoco's Resin 18-240, Resin 18-210 and Resin 
18-290, the preferred being the Resin 18-240 which has a Vicat softening 
temperature of 60.5 at 50.C/hr. rise and 62.9 at 120.degree. C./hr. rise. 
Another essential element of the masterbatch mix is the rubbery block 
copolymer. These are known in the art generally as having the formulae: 
A-B, A-B-A, A-B-A-B, and the like, including graft and radial block 
copolymers, as well as block copolymers containing other types of blocks, 
"C". These rubbery block copolymers of the above formulae generally 
contain a styrenic polymer as the "A" block, and generally contain a 
rubbery polymer, e.g. butadiene, ethylene/propylene, ethylene/butylene, 
isoprene, as the "B" block. Block "C", when present, may be either a 
second, different styrenic polymer from the "A" block or a second, 
different rubbery polymer from the "B" block, as the case may be. 
Preferred as the rubbery block copolymer in the masterbatch mix are those 
block copolymers available from Shell Chemical Company under the 
designations "Kraton G" and "Kraton D", such as Kraton D-1101, Kraton 
D-1102 Kraton D-1107, Kraton G-1650, Kraton G-1651, Kraton G-1652, Kraton 
G-1657X, Kraton G-1701X, and Kraton G-1726X. Especially preferred are 
Kraton G-1650 and Kraton G-1652. 
The solid blowing agents which can be used in the masterbatch mix are also 
known in the art and include mixtures of one or more solid organic acids, 
for example, oxalic acid, succinic acid, adipic acid, phthalic acid, and 
preferably citric acid; and an alkali metal carbonate or alkali metal 
bicarbonate, for example, sodium carbonate, potassium carbonate, and 
preferably sodium bicarbonate. The acid and carbonate and/or bicarbonate 
are generally used in alkali:acid equivalent ratios of from about 1:3 to 
about 3:1, acid to carbonate (and/or bicarbonate), and are preferably used 
in approximate stoichiometric amounts, i.e. about 0.7 to 1.3 alkali 
equivalents per acid equivalent, preferably about 0.9 to 1.1 alkali 
equivalents per acid equivalent. Especially preferred as the solid blowing 
agent of the masterbatch mix are combinations of monosodium citrate and 
sodium bicarbonate, preferably encapsulated in vegetable oil (i.e. a 
mixture of mono-, di-, and triglycerides), the amounts of monosodium 
citrate and sodium bicarbonate present preferably also as a stoichiometric 
mixture. The most preferred solid blowing agents are the SAFOAM P and 
SAFOAM FP powders, available from Reedy International Corporation, 
Keyport, NJ. 
The masterbatch mix also preferably contain a lubricant/plasticizer. 
Suitable lubricant/plasticizers are known to those in the art and include 
paraffin oil, silicone oil, medium to long chain alkyl esters of phthalic 
acid or isophthalic acid, propylene oxide and/or mineral oil. Preferred as 
the lubricant/plasticizer in the masterbatch mix is white mineral oil. 
Also preferably used in the masterbatch mix is a quantity of silica, which 
can either be incorporated into pellets of the masterbatch mix, or dusted 
over the surface thereof. 
The masterbatch mix preferably comprises essentially about 1 to 20 weight 
percent of stoichiometric amounts of monosodium citrate and sodium 
bicarbonate encapsulated in vegetable oil (preferably a mixture of mono-, 
di-, and triglycerides), about 3 to 50 weight percent of 
styrene-ethylene/butylene-styrene block copolymer, about 20 to 80 weight 
percent of alpha methyl styrene, about 1 to 20 weight percent of white 
mineral oil and about 0.2 weight percent of silica (which acts as a 
nucleating agent and aides in maintaining the free flow capability of the 
masterbatch mix under long term storage conditions). Among the preferred 
masterbatch mixes of the present invention are those available from Reedy 
International Corporation which are sold under the trademarks SAFOAM P-20, 
SAFOAM FP-20, SAFOAM FP-40, SAFOAM P-50 and SAFOAM FP-50. 
SAFOAM P-20 and SAFOAM FP-20 contain about 19.8% of an equimolar 
combination of monosodium citrate and sodium bicarbonate encapsulated in 
vegetable oil (SAFOAM P and SAFOAM FP, respectively) a combination of 14% 
mono-, 12% di-, and 72% triglycerides, 67.5% of alpha-methylstyrene (Amoco 
resin 18-240), about 10% of a combination of styrene-ethylene/propylene 
block copolymer (Shell Chemical Company, Kraton G-1726X) and 
styrene-ethylene/butylene-styrene block copolymer (Shell Chemical Company, 
Kraton G-1650), about 2.5% of white mineral oil, and about 0.2% of silica 
(predominantly present as a dusted coating on the outside of pellets made 
from the remaining ingredients). SAFOAM FP-40 contains about 38.8% of an 
equimolar combination of monosodium citrate and sodium bicarbonate 
encapsulated in vegetable oil (SAFOAM FP, available from Reedy 
International Corporation), 36.6% of alpha-methylstyrene (Amoco resin 
18-240), 14.4% of styrene-ethylene/butylene-styrene block copolymer (Shell 
Chemical Company, Kraton G-1652), about 9.% of white mineral oil and about 
0.2% silica. SAFOAM P-50 comprises about 54.8% of an equimolar 
combination of monosodium citrate and sodium bicarbonate encapsulated in 
vegetable oil (SAFOAM P, available from Reedy International Corporation), 
about 30.5% of alpha-methylstyrene (Amoco Resin 18-240), about 12% of 
styrene-ethylene/butylene-styrene block copolymer (Shell Chemical Company, 
Kraton G-1650), about 7.5% of white mineral oil, and about 0.2% of silica. 
The masterbatch mix, when used, is present in an amount of about 0.001 to 
about 1.0% by weight, based upon the weight of the polyalkenyl aromatic 
resin, preferably is present in an amount of about 0.01 to about 0.035 
weight percent, based upon the weight of the resin, and more preferably is 
present in an amount of about 0.02 to about 0.03% by weight, based upon 
the weight of the resin. 
The non-solid blowing agent combination of the present invention is 
comprised of atmospheric gas and organic gas. The atmospheric gas and 
organic gas may be added or injected into the melt either as a blend, or 
concurrently, or sequentially. The non-solid blowing agent can also be 
added to the melt in either gaseous or liquid forms, or combinations 
thereof. The amount of non-solid blowing agent combination which can be 
added in the process of the present invention ranges from about 2 to about 
20% by weight, based upon the weight of the resin. Preferably, non-solid 
blowing agent combination is added in an amount of about 3 to 10% by 
weight, based upon the weight of the resin and, more preferably, from 
about 4 to about 7% by weight, based upon the weight of the resin. 
As atmospheric gases, there can be used any of the gases normally present 
in the atmosphere, such as carbon dioxide, nitrogen, argon, helium, or 
neon, with carbon dioxide and nitrogen being preferred. As the organic 
gases, there can be used: the C.sub.4 -C.sub.6 alkanes, known to those 
skilled in the art, including butane, isobutane, pentane, neopentane, 
isopentane, and hexane; the chlorinated hydrocarbons (CHCs), such as 
methyl chloride, methylene chloride and methyl chloroform; chlorinated 
fluorocarbons (CFCs), such as CFC-11, -12, -113, -114, -115, Halon-1211, 
-1301, and -2402; the hydrogen-containing chlorofluoro carbons (HCFCs), 
such as chloro difluoromethane (HCFC-22), 1,1-difluoro-1,1-chloroethane 
(HCFC-142b); and the hydrogen-containing fluorocarbons (HFCs), such as 
1,1-difluoroethane (HFC-152a), 1,1,1,2-tetrafluoroethane (HFC-134a), 
1,1,1,-trifluoroethane (HFC-143a) and (HFC-123). The atmospheric gas and 
organic gas can be present in any relative amounts, such as a ratio from 1 
to 99 parts by weight atmospheric gas and 99 to 1 parts by weight organic 
gas. Preferably, the amount of atmospheric gas is present in an amount of 
greater than 30% by weight, more preferably present in an amount of 
greater than 40% by weight, and still more preferably present in an amount 
of about 50% by weight or more. Most preferably, the ratio of atmospheric 
gas to organic gas (based upon the weight of the combination) is from 
about 35/65 to about 65/35, preferably 40/60 to 60/40, and more preferably 
about 50/50. 
The use of an extrusion process for the manufacture of alkenyl aromatic 
foam is typical, but is not required. Such a process includes a primary 
extruder, a blowing agent addition system, a secondary extruder, an 
annular die, a sheet cutter or slitter and a sheet gathering device. 
However, the use of this exact equipment set up is not required in the 
process of this invention. 
In the preferred embodiments of the present invention, polystyrene foam is 
formed in a continuous process by delivering a well-mixed and uniform 
blend of styrenic polymer and masterbatch mix to the extruder throat. 
Masterbatch mix is preferably about 0.02 to 0.03% by weight of the 
styrenic polymer. Once in the screw, while being rotated at a controlled 
RPM, the blend or feed of styrenic polymer and masterbatch mix is heated 
to a temperature above the melting point of the blend, about 250.degree. 
to 500.degree. F. It is then delivered with the use of relatively stable 
pressure in the range of about 4000-6000 psi, to the point of injection. 
Here, an injection system delivers atmospheric gas, e.g. carbon dioxide in 
gas or liquid form, and/or nitrogen in gas form, or combinations thereof, 
into the melted feed. In combination with the atmospheric gas, and in a 
preferred embodiment sequentially with respect to the delivery of the 
atmospheric gas, the injection system delivers an organic gas, e.g. 
isopentane or HFC 152(a), into the melted feed. 
Next, the injected melted feed is passed into a second extruder. This 
extruder is designed for maximum cooling capability. It is of larger 
capacity than the first extruder. In this extruder, a minimum of shear is 
desired. Minimum shear is achieved by keeping the screw's roof diameter 
constant. The injected melted feed is mixed in this second extruder and 
cooled. 
The feed then exits this second extruder through a die at a temperature at 
or above 250.degree. F., preferably between about 250.degree.-290.degree. 
F., and more preferably at a temperature of between about 250.degree. 
F.-280.degree. F. and a pressure of about 2,500-3,750 psi. The extruded 
material is stretched out over a cooling drum and drawn to the desired 
thickness. The polystyrene foam sheet is then slit and can be wound into 
large rolls. 
The foams produced according to the present invention are characterized by 
having densities generally greater than about 2.5 pounds per cubic foot, 
more preferably greater than about 3.0 pounds per cubic foot, still more 
preferably greater than about 3.5 pounds per cubic foot, most preferably 
greater than about 4.5-5.0 lbs. per cubic foot, and especially preferably 
greater than about 6 pounds per cubic foot. On average, the density of the 
foams according to the present invention range between about 4 pounds per 
cubic foot and about 10 pounds per cubic foot, and more generally range 
from about 4 pounds per cubic foot to 6 pounds per cubic foot. 
The foams produced according to the present invention are also 
characterized by having a substantial plurality of closed, 
non-interconnecting gas-containing cells. Generally, the number of such 
closed cells in the foams according to the present invention is greater 
than 50% of all of the cells present, preferably greater than 60%, more 
preferably greater than 70%, still more preferably greater than 80%, 
especially preferably greater than 90%, and most especially preferably 
greater than 95%. The gas contained in the closed cells is comprised of a 
combination of atmospheric gas and organic gas and, preferably, contains 
combinations of atmospheric gas and organic gas, based on the weight of 
atmospheric gas an organic gas in the cells, having the components and, in 
the ratios, described above for the non-solid blowing agents which are 
utilized according to the present invention. For example, a foam resulting 
from the process of the present invention may have a percentage of closed 
cells greater than 70%, within which there may be a combination of gases 
comprised of carbon dioxide and isopentane in a weight ratio of about 
40/60 based on the total weight of carbon dioxide and isopentane. 
The foams of the present invention are further characterized in that they 
retain the injected gas, and particularly the organic gas, to a much 
greater degree than the foams of the prior art. The retainment of the 
injected gas is believed to be a function of several parameters, including 
the solubility of the organic gas in the foamed polymer and the percentage 
of closed cells in the foam. It is also theorized that the use of the 
rubbery block copolymer in the masterbatch mix of the preferred 
embodiments of the present invention aids in blocking and retaining within 
the foamed polymer the carbon dioxide and any water vapor which may be 
present in the polymer. The percentage of retained gas in the foams 
according to the present invention generally coincides with the percentage 
of closed cells. Thus, the percentage of gas retained in the foams 
according to the present invention is generally greater than 50%, 
preferably greater than 60%, more preferably greater than 70%, still more 
preferably greater than 80%, especially preferably greater than 90%, and 
most especially preferably greater than 95%. The percentage of closed 
cells can be measured by a Beckman Pycnometer. Due to the increased gas 
retention of the foams according to the present invention, there is a 
concomitant increase in the thermoforming output for the produced foams. 
The thermoforming output (or speed at which the foams may be thermoformed 
into finished product) is a function of a number of factors as well, 
including cell uniformity and percentage of closed cells. The foams 
according to the present invention attain greater than 25% improvement in 
thermoforming output rate as compared to prior art foams and can attain a 
35%, or 50%, or greater, improvement of thermoforming output rate. 
Still further, the foams obtained through the use of the masterbatch mix 
according to the preferred processes of the present invention provides for 
other significant economic improvements. Specifically, the production (or 
throughput) rate of foams according to the present invention when a 
masterbatch mix is used in the process, and particularly when the 
preferred SAFOAM masterbatch mixes are utilized, is generally greater than 
200 pounds of foamed product per hour, preferably is greater than 400 
pounds of foamed product per hour, more preferably greater than 500 pounds 
of foamed product per hour, and most preferably greater than 700 or 800 
pounds of foamed product per hour. 
In addition, the foams of the present invention, because of the improved 
gas retention characteristics of the foams when the masterbatch mix is 
used, exhibit post-expansion characteristics which are likewise greatly 
improved. The foams according to the present invention which are made by 
processes which utilize the masterbatch mix, and preferably the SAFOAM 
masterbatch mix, exhibit post-expansion properties of greater than 200%, 
preferably greater than 250%, more preferably greater than 300%, most 
preferably greater than 350%, especially preferably greater than 400%, and 
most especially preferably greater than about 450 to 500%. This 
post-expansion improvement provides for the ability to use a much thinner 
starting foam product in the post-extrusion thermoforming process. 
Another beneficial aspect of the use of the masterbatch mix according to 
the present process, and especially the SAFOAM masterbatch mix, is that it 
is possible to reduce by at least 5%, preferably at least 10%, and more 
preferably up to about 20% or more, the amount of organic gas utilized in 
the process, while maintaining the same density of the foamed product. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following examples are intended to merely illustrate the present 
invention, which is not limited thereby or thereto. 
EXAMPLE 1 
A virgin polystyrene resin having a weight average molecular weight of 
about 310,000 and SAFOAM P50 in an amount of 0.25 parts per 100, based on 
100 parts of resin weight were combined. 
These ingredients were uniformly blended and were added to an extruder. The 
mix was heated to 426.degree. F. and melted under an injection pressure 
ranging from 4250 to 4300 psi. At this point, isopentane at 3.65% and 
carbon dioxide at 2.43% (both based on 100 parts resin weight) were 
delivered into the melt. 
Next, the melt passed into a cooling extruder, cooled down to a die melt 
temperature of 274.degree. F. and a die pressure of 2830 psi (die diameter 
8") and extruded. The extruded material was stretched out over a cooling 
drum with a 24.9" diameter and drawn to the physical parameters listed 
below. 
Percent of closed cells =98.5% 
Post-expansion (1 hr. after extrusion) =285% 
The polystyrene foam was extruded with the following process and physical 
parameters: 
______________________________________ 
Total Output 825 lbs./hr. 
Sheet Cross Sectional 
.105" 
Thickness 
Sheet Density 3.8 lbs./ft..sup.3 
Blowing Agent 3.65 lbs. 
Isopentane 100 lbs. polymer 
CO.sub.2 2.43 lbs. 
100 lbs. polymer 
This represents a ratio of 60% isopentane to 40% CO.sub.2. 
Masterbatch P50 .25 lbs. 
100 lbs. polymer 
______________________________________ 
After aging the foamed sheet for three days, it was passed through a 
thermoformer at 32 cycles per minute; forming meat trays with a bottom 
cross sectional thickness ranging from 0.165" to 170". The percentage of 
isopentane retained in the formed trays was 3.32 lbs., based on 100 lbs. 
of resin, giving an isopentane retention of 91.1%. 
EXAMPLE 2 
A virgin polystyrene resin having a weight average molecular weight of 
about 310,000 of SAFOAM P50 in an amount of 0.25 parts per 100, based on 
100 parts of resin weight were combined. 
These ingredients were uniformly blended and were added to an extruder. The 
mix was heated to 435.degree. F. and melted under an injection pressure 
ranging from 4300 to 4370 psi. At this point, isopentane at 3.01% and 
carbon dioxide at 1.9% (both based on 100 parts resin weight) were 
delivered into the melt. 
Next, the melt was passed into a cooling extruder, cooled down to a die 
melt temperature of 26.degree. F. and a die pressure of 2490 psi (die 
diameter 8"). 
The extruded material was stretched out over a cooling drum with a 24.9" a 
diameter and drawn to the physical parameters listed below. 
Percent of closed cells =96.7% 
Post-expansion (1.5 hr. after extrusion) =295% 
Same extruded polystyrene foam with the following components: 
______________________________________ 
Total Output 825 lbs./hr. 
Sheet Cross Sectional 
.095" 
Thickness 
Sheet Density 4.8 lbs./ft..sup.3 
Blowing Agent 3.01 lbs. 
Isopentane 100 lbs. polymer 
CO.sub.2 Gas 1.90 lbs. 
100 lbs. polymer 
This represents a ratio of 60% isopentane to 40% CO.sub.2. 
Masterbatch P50 .25 lbs. 
100 lbs. polymer 
______________________________________ 
After aging the foamed sheet for three days, it was passed through a 
thermoformer at 30 cycles per minute, forming large meat trays with a 
bottom cross sectional thickness ranging from 0.170% to 0.173%. The 
percentage of isopentane retained in the formed trays was 2.79 lbs., based 
on 100 lbs. of resin, an isopentane retention of 93.0%. 
EXAMPLE 3 
A virgin polystyrene resin having a weight average molecular weight of 
about 310,000 and recycled polystyrene having a weight average molecular 
weight of about 290,999 at amounts ranging about 9:1 (virgin:recycled) 
based on weight and SAFOAM P-50 in an amount of 0.23 parts per hundred, 
based on 100 parts of resin weight, were combined. 
These ingredients were uniformly blended and added to an extruder. The mix 
was heated to 435.degree. F. and, melted under an injection pressure 
ranging from 4550 to 4600. At this point, HFC-152a at 2.6% and carbon 
dioxide at 2.4% (both based on 100 parts by weight virgin plus recycled) 
was delivered into the melt. 
Next, the melt was passed into a cooling extruder, cooled down to a die 
melt temperature of 284.degree. F. and a die pressure of 3250 psi (die 
diameter 8") and extruded. 
The extruded material was stretched out over a cooling drum with a 26.1" 
diameter and drawn to the physical parameters listed below. 
Percent of closed cells =71.2% 
Post-expansion (1.0 hr. after extrusion) =250% 
After aging the foamed sheet for two days, it was passed through a 
thermoformer at 27 cycles per minute, forming large hinged food containers 
with a bottom cross sectional thickness ranging from 0.105% to 0.110%. 
Same extruded polystyrene foam with the following components: 
______________________________________ 
Total Output 725 lbs./hr. 
Sheet Cross Sectional 
.08" 
Thickness 
Sheet Density 5.2 lbs./ft..sup.3 
Blowing Agent 2.6 lbs. 
HFCs 152A 100 lbs polymer 
CO.sub.2 Gas 2.4 lbs. 
100 lbs. polymer 
This represents a ratio of 52% HFC 152A to 48% CO.sub.2. 
Masterbatch P50 .23 lbs. 
100 lbs. polymer 
______________________________________ 
The present invention has been described with respect to the preferred 
embodiments. It is to be understood, however, that modifications and 
variations may be resorted to, without departing from the spirit and scope 
of the invention, as those skilled in the art would readily understand. 
These modifications and variations are considered to be within the scope 
of the appended claims. 
All of the above-mentioned patents and publications are incorporated herein 
by reference.