This invention relates to an improved polystyrene foam prepared with an endothermic agent and carbon dioxide as the blowing agent. This invention also includes a process for making this improved polystyrene foam utilizing the endothermic agent and carbon dioxide as the only blowing agent. This invention is particularly suited for making polystyrene foam sheet having a thickness of less then 0.5 inch.

10

Polystyrene foam, particularly in sheet form, is at the present time made from any number of blowing agents which may possess or cause one or more of the following undesirable characteristics: volatility, flammability, poorthermoforming characteristics, brittle foam sheet physical properties, high cost or an adverse effect to the ozone layer. Examples of

15 these blowing agents would include aliphatic hydrocarbons and fully or partially halogenated hydrocarbons.

Some have experimented with blends of carbon dioxide and other blowing agents such as aliphatic hydrocarbons or fully or partially halogenate hydrocarbons. An example of this can be seen in United States Patent Numbers 4,344,710 and 4,424,287. These

20 patents, however, state that the use of a 100 percent carbon dioxide blowing agent has not been successfully employed in continuous direct injection foam sheet manufacturing (Column 1 , lines 42-45 and lines 49-55 respectively) due to the extreme volatility. Use of these materials is said to produce corrugation and surface defects.

Others have developed methods and apparatuses for metering a gaseous blowing

25 agent, such as an atmospheric gas, into a molten resin charge, such as polystyrene, in an accurate and precise manner. One example of this can be seen in United States Patent Number 4,470,938. The apparatus of this patent meters discrete uniform volumetric charges of gaseous blowing agent into the molten resin using a free piston-cylinder combination which is given notice power by the supply gas source.

30 Still others, in United States Patent 4,455,272, have formed polystyrene foam by extruding polystyrene, a nucleating agent, a physical blowing agent, and water where the physical blowing agent and water are injected separately or together into extruder as a i blowing agent. The product produced has small cells at junction points between membranes of the foam. The patent states that extrusion processes for producing such foam are well known - _« . 35 in the art and are normally based on known prior art physical blowing agents such as lower hydrocarbons and halogenated hydrocarbons, including propane, normal and isobutane, all of the isometric pentanes and hexanes and halogenated hydrocarbons exemplified by Freon 1 1,

trϊchlorofluoromethane and Freon 12, dϊchlorodifluoromethane, but that also in some cases chemical blowing agents have been used. It is also noted in the patentthatthe water provides an extra amount of vapor pressure for the production of low density foams and also produces a large amount of evaporative cooling. The amount of water used ranges from 0.5 to 5 weight percentbased uponthe resin- The patentalso notesthattheamountoftalcshould be in the range from 0.01 to 0.3 weight percent based on the resin- More than 0.3 weight percent produces a product with very small cell structures resulting in lowcompressive strength.

Atypical current commercial polystyrene foam sheet extrusion process may use a tandem extrusion process (two extruders in series). The first extruder melts the polymer to produce a polymer melt. A high pressure metering pump would then deliver blowing agent 5to the polymer melt at the end of the first extruder where mixing is initiated prior to entering the second extruder where further mixing and cooling of the blowing agent and the poly = melt occurs. After exiting the second extruder the polymer melt then passes through and Obecomes a foam structure at an annular die. The foam structure, in the shape of an annul- tube, isthen stretched over a forming mandrel. The annular tube coming off the mandrel is then slit and opened to form a sheet which isthen gathered on one or more rolls. The sheet is then aged and then thermoformed into a foam sheet packaging article-

The use of an endothermic agent and carbon dioxide as the sole blowing agent avoids many of the problems of other blowing agents and blends of other blowing agents with carbon dioxide. The use of carbon dioxide in liquid form avoids the problems of handling a gas.

The process of the present invention can produce a thermoplastic foam having a greater density and less corrugation than a comparable thermoplastic foam produced using only carbon dioxide. It can also produce, at a increased die temperature, greater quantities of a thermoplastic foam having a lower density with an equivalent corrugation to a thermoplastic foam in an equivalent process for producing an equivalentiy corrugated thermoplastic foam produced using only carbon dioxide. Also, under equivalent process conditions, the thermoplastic foam of the present invention has less corrugation than a comparable thermoplastic foam produced using only carbon dioxide at the same density. Thus the invention is polystyrene foam blown solely with carbon dioxide and an endothermic agent, preferably liquid carbon dioxide, and the process necessary to make such polystyrene foam.

The invention is particularly suited for making polystyrene foam sheet having beneficial physical properties forthermoforming purposes.

The present invention is a thermoplastic foam of a styrenic polymer with a blowing agent consisting essentially of carbon dioxide, 1 percent to 30 percent of an endothermic agent based on the total weight of the carbon dioxide and the

endothermic agent and 0.005 to 1.0 weight percent based on styrenic polymer weight of a nucleating agent This thermoplastic foam has less corrugation than thermoplastic foam produced under the same process conditions and at the same density using only carbon dioxide as the blowing agent. The present invention is also a process for producing a thermoplastic foam. First, a styrenic polymer is melted, then a blowing agent consisting essentially of carbon dioxide is continuously directly introducing into the melted styrenic polymer. An endothermic agent in an amount of from about one percent to about thirty percent by total weight of the carbon dioxide and the endothermic agent is also continuously directly introducing into the melted o styrenic polymer as is 0.005 to 1.0 weight percent of a nucleating agent based on styrenic polymer weight. The styrenic polymer, the carbon dioxide and the endothermic agent are intimately mixed and then extruded and foamed at a die temperature no greater than about 1500 centigrade into a region of lower pressure to form a thermoplastic foam.

5 Figure 1 and Figure 2 are 50X machine-direction and cross-direction photomicrographs of the thermoplastic foam sheet of Example 2.

Polymers useful in the present invention are those polymers, copolymers and interpolymers having at least 70 percent by weight styrene monomer incorporated into the 0 final resin as the monomeric repeat units. This includes other polymers such as for example styrene acrylonitrile, styrene acrylic acid, other known equivalents and blends having the styrene monomer repeat unit requirement. For the purpose of this invention these useful polymers shall be referred to as styrenic polymers. The preferred materials are styrenic homopolymers. 5 The blowing agent to be used would be that carbon dioxide which is normally commercially available. For the purposes of this specification such carbon dioxide shall be defined as 100 percent carbon dioxide even though this carbon dioxide may contain minor impurities or amounts of other compounds.

The carbon dioxide is preferably added to a polymer melt in a liquid form, 0 although use of the carbon dioxide in the gaseous form would also be acceptable in the practice of the present invention.

The addition of a nucleating agent is required and is added in an amount of 0.005 to 1.0 percent of the total polystyrene by weight. Preferably the amount of nucleating agent is between 0.005 to 0.5 percent. Most preferably the amount of nucleating agent is 5 between 0.005 to 0.1. One preferred conventional nucleating agent is talc.

Other additives, such as pigments, conventionally known in the art may be added to the polystyrene foam of the present invention.

The endothermicagentare those endothermic agents which are not generally soluble in the polymer and have a heat absorption capability present upon the extrusion of the thermoplastic foam which is at least 50 percent of the heat absorption capability present in water. Water is the preferred endothermic agent. The use of a tandem extrusion process for the manufacture of polystyrene foam sheet istypical, but is not required- Such a process includes a primary extruder, a blowing agent addition system into the primary extruder, a secondary extruder, an annular die, a cooling mandrel, a sheet cutter or slitter and a sheet gathering device, such as for example rollers and winders. o However, the use of this exact equipment set-up is not required in the process of this invention.

Polystyrene foam sheet can be produced using carbon dioxide as the blowing agent and the endothermic agent provided certain modifications are made in the handling and the preparation of the polystyrene foam sheet. Preferably the carbon dioxide and the 5 endothermic agent are continuouslydirectly introduced or injected into the molten resin. Preferably the carbon dioxide is continuouslydirectly injected as a high pressure liquid and the endothermic agent is injected as a liquid, either separately or with the carbon dioxide.

To produce foam sheet having a density in the range of 2 to 10 pounds per cubic foot (PCF), preferably 3 to 6 PCF, 0.5 to 6 weight percent, preferably 1 to 4 weight percent of 0 carbon dioxide is required per weight of polymer. The endothermic agent is present in an amount of from 0.1 weight percent to 1-0 weight percent by styrenic polymer weight. A required limitation is that the endothermic agent be present in an amount of 1 weight percent to 30 weight percent, preferably from 3 percent to 18 percent and most preferably from 4 percentto 14 percent by total weight of the carbon dioxide and the endothermic agent. A 5 nucleating agent, preferably talc, is also required and is added in an amount of 0.005 toT.O percent, preferably 0.005 to 0.5 percent of the total polystyrene by weight most preferably 0.005 to 0.1.

Cell sizes in all directions are less than 1 millimeter (mm), and preferably less than 0.5 mm. The thickness of the foam sheet is preferably less than 0.5 inch and more preferably 0 lessthaπ 0.25 inch.

It is possible to have a blow up ratio, that is a stretching of the foam over a mandrel after it exits a die, greater than 4:1- Itis preferable to have a blow up ration of at least 2.0:1.

The foaming temperature at the die is no greater than 150°Cand preferably 5 between 135 and 150°C. The thermoplastic foam is extruded and foamed into a zone of lower pressure preferably air at atmospheric pressure-

While it may be possible to use a polystyrene foam sheet immediately it is preferred to age the polystyrene foam sheet for a period of time, usually at least about 20 hours prior to thermoforming the foam sheet. This will allow time for air to counter diffuse into the cells. It will also render the tensile elongation values substantially constant. This will provide uniform thermoforming characteristics and improved finished part physical properties for the polystyrene foam sheet to be thermoformed.

Substantially constant tensile elongation values over an extended period of time obtained with a minimum of aging of the foam sheet would yield more consistent results upon thermoforming the foam sheet, thus more consistent product with less scrap. The tensile o elongation values would preferably be in excess of five percent and would remain above five percent for an extended period of time, thus allowing the storage of foam sheet for an extended period of time before thermoforming without worrying about the changing values or the degradation below five percent of the tensile elongation values of the foam sheet.

The tensile elongation values are determined according to the method ASTM 5 D638 "Tensile Properties of Plastics" published by the American Society for Testing and

Materials (ASTM). The words machine or extrusion direction, transverse or cross direction and vertical direction are used in their normal and conventional manners.

Though the tensile elongation values of the foam sheet of the present invention are not perfectly constant, they are substantially constant and even more important for 0 thermoforming, the tensile elongation values remain above a value of five percent for an extended period of time. Generally when these values fall below five percent the foam sheet becomes too brittle for thermoforming.

The thermoforming temperature will be somewhat higher than that of a polystyrene foam sheet blown with fully or partially halogenated chlorofluorocarbon or 5 hydrocarbons since the C0 ₂ provides essentially no solvent effect which would normally lower the heat distortion temperature and therefore the melt strength of the foam walls upon reheating. The polystyrene foam sheet blown with C0 ₂ and an endothermic agent can be post expanded in a range of 20 to 50 percent during thermoforming. It is preferred that the polystyrene foam sheet be aged for 20 hours prior to the carefully controlled thermoforming of 0 the foam sheet.

It is also possible with the present invention to make 3-6 PCF foam sheet blown with C0 ₂ and an endothermic agent having less corrugation than a 3-6 PCF foam sheet blown with only C0 ₂ . 5 For the purpose of definition "less corrugation" means that there are less gauge bands, which are the corrugation peaks and valleys in the foam sheet, or that the Sheet Gauge Standard Deviation is less when compared to an equivalent foam sheet prepared under

equivalent conditions and blown with only carbon dioxide. Alternatively in the present invention, itis possibleto obtain a lower density foam sheet with equivalent corrugation. Example 1 and Comparative Example 1

Table 1 shows the process conditions and results for an 0 Example and a Comparative Example. The extruder had a 90 millimeter die with a 0.027 inch die gap. The polystyrene resin used had about a 300,000 Mw (molecular weight).

Table 1 100% CO2 vs . Cθ2/Water Blend

wt. % CO2 (based on resin) wt. % Water (based on resin)

% Water based on Cθ ₂ s Water wt. % Talc (based on resin)

Die Kelt Temperature, ^C C

Extruder Pressure, psi

Die Pressure, psi Output Rate , lbs/hr

Sheet Gauge Average, in.

Sheet Gauge Range, in.

Sheet Gauge Standard Deviation

Sheet Density, lb/cu. ft.

As can be seen in Table 1, theaddition ofa er small amountofwater in an equivalent process using 100 percent carbon dioxide produces a foam sheet having about the same density, shows a 38.6 percent decrease in the sheet Gauge Standard Deviation. Example 2 and Comparative Example 2

The extruder had a 2 inch die with a .025 inch die gap. The resin used had about a 300,000 Mw (molecular weight).

As can be seen in Table 2, the addition of water with a decrease in the amount of carbon dioxide used in an equivalent process produces a foam sheet having about the same density, with a smaller gauge (thickness) and no gauge bands. There is an 8 to 0 reduction in the number of gauge bands across the web (the annular thermoplastic foam output from the die).

Table 2 100% CO2 vs. Cθ2/Water Blend

wt. % CO2 (based on resin weight) wt. % Water (based on resin weight)

% Water based on CO2 & Water wt. % Talc (based on resin weight)

Die Melt Foam Temp. °C

Extruder Pressure, psi

Die Pressure, psi

Output Rate, lbs/hr

Sheet Gauge in.

Sheet Density, lbs/cu.ft.

Gauge Bands Across Web

As can be seen in Figures 1 and 2, which are 50 times blowup machine-direction

20 and photomicrographs of Example 2, there is no secondary foaming or small cells at junction points between membranes of the foam. The cells appear to be basically one size rather than a combination of large and small cells. Additional Examples and Comparative Examples

Additional examples and additional comparative examples are prepared

25 according to the parameters stated in Example 2 with various different water levels and carbon dioxide levels.

As can be seen in Table 3, the gauge bands across the web disappear at a water

Table 3 Gauge Bands vs . Weight Percent Water Added

30

Comparative

Amount of Water Example 2

100% CO?

< 0.1 wt . % water 8 0.1 to 1. 0 wt . % water 0

35 > 1.0 wt . % water 8

level of 0.1 to 1.0 wt.% water based on resin weight.

Interestingly the gauge bands across the web are the greatest (the most corrugated) when the amount of water is either too low or too high (greater than 30 weight percent based on the total weight of the water and carbon dioxide).

As is clear in these examples and comparative examples, corrugation can be minimized or even prevented when using carbon dioxide as a sole blowing agent by including the proper amount of an endothermic agent, such as water into the foamable mixture prior to forming the thermoplastic foam sheet.

The use of a proper amount of an endothermic agent allows the production of a thermoplastic foam having a greater density and less corrugation than a comparable thermoplastic foam produced using only carbon dioxide. Use of an endothermic agent will also produce, due to an increased die temperature, greater quantities of a thermoplastic foam having a lower density with an equivalent corrugation to a thermoplastic foam in an equivalent process for producing an equivalently corrugated thermoplastic foam produced using only carbon dioxide. Also, under equivalent process conditions, the use of an endothermic agent produces a thermoplastic foam with less corrugation than a comparable thermoplastic foam produced using only carbon dioxide at equivalent densities.

The present invention has been described with 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 will readily understand. These modifications and variations are considered to be within the scope and purview of the appended claims-