MXPA98004905A - Thermoplastic foam and process to produce it using carb dioxide - Google Patents

Abstract

The present invention offers a process for the production of a thermoplastic foam, said process includes: (a) the melting of a thermoplastic polymer to produce a melted polymer material, (b) the introduction of a carbon dioxide expansion agent into melted polymer material; (c) addition to the melted polymer material of one or more selected additives within the group consisting of (1) polysiloxane, and (2) mineral oil, and (d) extrusion and foaming of melted polymer melt material, expanding agent, and one or more additives to produce a thermoplastic foam. The resulting thermoplastic foam can be thermoformed into various molded articles, such as foamed packaging trays. The quality of the foams made with a carbon dioxide expansion agent, and particularly 100% carbon dioxide is substantially improved by the addition of one or more of the aforementioned additives, ie, polysiloxane, mineral oil and optionally polyolefin to the melted polymer material during the process of extrusion and foaming. Such foams have less fractured cells, less surface cracks and corrugations than comparable foams made with carbon dioxide but without the additives of the present invention.

Description

THERMOPLASTIC FOAM AND PROCESS TO PRODUCE IT USING CARBON DIOXIDE BACKGROUND OF THE INVENTION The present invention relates to thermoplastic foams and a process for producing thermoplastic foams using a carbon dioxide blowing agent. More particularly, the invention relates to a process for producing a foam using a carbon dioxide blowing agent "by which certain additives are used to produce foams of improved quality. Thermoplastic foams are generally produced in a tandem extrusion process (two extruders in series). The first extruder melts the polymer terpoly to produce a polymer melt. A high pressure metering pump delivers a blowing agent to the polymer melt near the end of the first extruder where mixing is initiated before entering the second extruder where additional mixing and cooling of the polymer melt blowing agent occurs. . After leaving the second extruder, the polymer melt passes through and becomes a foam structure in a die, generally an annular die. The foam structure. generally in the configuration of an annular tube, it is then stretched over a forming mandrel. The annular tube that emerges from the mandrel is cut and opened to form a sheet which is then gathered into one or more rolls. The rolled foam sheet is typically aged for a predetermined period of time and then thermoformed into a foamed article, eg, a packaging tray. The blowing agents that are most commonly used today include aliphatic hydrocarbons, .gr., C3-C6 alkanes, and wholly or partially halogenated hydrocarbons, e.g., chlorinated and / or fluorinated hydrocarbons- These conventional blowing agents possess or cause one or more of the following undesirable characteristics: contamination, potential damage to the ozone layer, flammability, low thermoforming characteristics, fragility, high cost, and the need for a period of "prolonged aging to allow part or all of the blowing agent to diffuse from the foam sheet before thermoforming Due to these reasons, alternatives to previous blowing agents have been sought, one of which is carbon dioxide, which prevents the most or all of the above undesirable characteristics of conventional blowing agents. However, due to the extreme volatility and high vapor pressure of carbon dioxide, it is a difficult to use blowing agent, frequently resulting in a sheet of broken cell foam, surface defects (eg, cracks), and corrugation, that is, variations of thickness within the foam sheet that produce a wrinkled or corrugated surface appearance. Each of the above occurrences detrimentally affects the physical properties of the foam (e.g., resistance) as well as the aesthetic qualities thereof. Accordingly, there is a need in the art for an improved process for producing thermoplastic foam from a carbon dioxide blowing agent that produces a high quality foam, i.e. one with fewer broken cells, surface defects and corrugation.

COMPENDIUM OF THE INVENTION That need is filled by the present invention which provides a process for producing thermoplastic foam, comprising: (a) melting a thermoplastic polymer to produce a polymer melt; (b) introducing a blowing agent comprising carbon dioxide towards the polymer melt; (c) adding to the polymer melt one or more additives selected from the group consisting of: (1) polysiloxane, and (2) mineral oil; and (d) extruding and foaming the polymer melt, blowing agent and one or more additives to produce thermoplastic foam. Preferably, the blowing agent consists essentially of carbon dioxide, that is, 100% © carbon dioxide. It is also preferred that both the polysiloxane and the mineral oil are added to the polymorphic melt. A polyolefin can also be added to the polymer melt as an additional additive. In accordance with another aspect of the present invention, there is provided a thermoplastic foam comprising: (a) a polymeric matrix comprising a thermoplastic polymer and one or more materials selected from the group consisting of (1) polysiloxane, and (2) ) mineral oil; and (b) a plurality of cells dispersed through the polymer matrix and containing a blowing agent comprising carbon dioxide, preferably 100% carbon dioxide. The plastic-plastic foam can be formed into a thermoformed article such as a tray or packing bowl. If desired, an oxygen barrier film may be adhered to the thermoplastic foam to form a packing tray or bowl with oxygen barrier functionality. The inventors have found that the quality of foams made from a blowing agent - carbon dioxide, and particularly 100% carbon dioxide, can be substantially improved by adding one or more of the above additives, ie, polysiloxane, oily mineral and, optionally, polyolefin, to the polymer melt during extrusion and foaming process. These foams have fewer broken cells, surface cracks and corrugation than comparable foams made with a carbon dioxide blowing agent, but lacking the additives of the present invention. The improvements of the present invention are particularly significant when 100% carbon dioxide is used, as opposed to a mixture of carbon dioxide and a conventional aliphatic or halogenated blowing agent.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic drawing of a preferred extrusion and foaming system in accordance with the present invention; and Figure 2 is a schematic drawing of the thermoplastic foam produced in the system of Figure 1 which is cut into two sheets and rolled into rolls for subsequent thermoforming.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES With reference to Figures 1 and 2, a preferred extrusion / foaming system for carrying out the process of the present invention will be described. A thermoplastic polymer, usually in the form of a granule, is introduced to the first extruder 10 through the hopper 12. The polymer can be any thermoplastic polymer capable of foaming, such as polystyrene, polypropylene, or polyethylene terephthalate (e.g. , APET, CPET, or PETG). The first extruder 10 is a screw type extruder that heats, mixes and melts the thermoplastic polymer to form a polymer melt. A nucleating agent, such as citric acid, sodium bicarbonate or talc, is optionally added to the first extruder 10 in the hopper 12 together with the polymer. The polymer melt moves through the first extruder 10, crossover 16, and second extruspr 20 in the direction indicated by the arrows in Figure 1. A carbon dioxide blowing agent, e.g., a blowing agent which comprises carbon dioxide, is introduced to the polymer melt in the first extruder 10 in the injection port 14. Preferably, the blowing agent consists essentially of carbon dioxide. That is, it is preferred that the blowing agent be 100% carbon dioxide, although any lower amounts of impurities or other substances may also be present in the carbon dioxide. 100% carbon dioxide is preferred in that it avoids the undesirable effects of conventional aliphatic or halogenated hydrocarbon blowing agents as discussed above. If desired, however, m, less or greater amounts of other blowing agents can be mixed with carbon dioxide. These other blowing agents can include nitrogen or conventional aliphatic or halogenated hydrocarbons, e.g., difluoroethane (commercially known as FreonCR) 152A). Preferably, the carbon dioxide blowing agent is present in the polymer melt in an amount ranging from about 0.1 to about 15 weight percent, and more preferably from about 1 to about 3 weight percent, the percentages by weight being based on the total weight of the polymer melt. The carbon dioxide blowing agent is preferably transported to the injection port 14 by means of a pumping system that generates a pressure of between about 351.50 to about 386.65 kg / cm.2. The polymer melt is mixed with the carbon dioxide blowing agent in the first extruder 10 and then moved through the crossing 16 and into the second extruder 20 where the polymer melt / blowing agent mixture is further mixed and cooled . Even when the exact temperatures and pressures within the first and second extruders will vary depending on, e.g., the type of polymer, the rate at which the polymer is forced through the extruders, etc., the typical pressures within the first extruder 10 will vary from 126.54 to about 253.08 kg / cm2 while the pressures within the second extruder 20 will vary from about 126.54 to about 217.93 kg / cm.2. The temperature in the first extruder 10 will usually vary from about 121 to about 227 ° C while the temperature of the second extruder 20 will vary from about 66 to about 149 ° C.

The second extruder 20 forces the polymer melt / blowing agent mixture through the annular die. The temperature of the annular die 30 typically ranges from about 121 to about 177 ° C while the pressure therein ranges from about 56.24 to 140.60 kg / cm2. During the exit of the die, the temperature and pressure of the polymorph melting / blowing agent mixture suddenly decreases at ambient temperature and pressure. The decrease in temperature and pressure causes the polymer melt to begin to harden into a polymer matrix. At the same time, the solubility of the carbon dioxide blowing agent in the polymer greatly decreases. As a result of these two phenomena occurring simultaneously, the polymer melt / blowing agent mixture is transformed into an annular foam tube 40 in view of the expansion of the carbon dioxide blowing agent into a plurality of cells which are dispersed to through the polymer matrix. After leaving the die 30, the annular foam tube 40 is stretched over the forming and cooling mandrel 50 (shown in shadows). The ratio of the diameter of the foam tube 40 in the mandrel 50 to the diameter of the tube as it leaves the die 30, commonly referred to as the blowing ratio, can vary from about 2: 1 to about 10: 1, but preferably it varies from around 3.5: 1 to approximately 5: 1. As shown more clearly in Figure 2, as the annular foam tube 40 exits the mandrel 50, it is cut to two foam sheets 780 and 80 by the cutter 60. The rollers 72, 74 and 82 82 conduct the sheets 70 and 80 respectively, respectively to roll rolls 76 and 86, from rolls 76 and 80, foam sheets 70 and 80 will be thermoformed and cut to foamed articles such as, for example, packaging trays, product containers etc. Advantageously, using a blowing agent of 100% carbon dioxide, very little or no aging time is required to allow the blowing agent to diffuse from the cells prior to thermoforming. conventional aliphatic or halogenated hydrocarbon, the aging time, eg, three to seven days, is required before the foam sheet can be thermoformed.In the thermoforming process, a roll of foam sheet is Feeds through an oven to heat it to near its softening point. The hot sheet is then forced into molds (.gr., Tray molds) by vacuum, air pressure, and mechanical pressure. The molded sheet is then fed through a cutting press wherein a punch and die mechanism cuts the individual thermoformed articles from the sheet. As noted above, carbon dioxide is a blowing agent which is very difficult to use, mainly due to its extreme volatility and high vapor pressure (at a certain temperature). When used in commercial tandem extrusion / foaming systems, such as the one described above, carbon dioxide blowing agents often result in foam sheets having broken cells as well as corrugation, surface cracks and other defects. The inventors have found that when one or more additives selected from the group consisting of polysiloxane, mineral oil, and optionally polyolefin is added to the polymer melt during the extrusion process, the above problems associated with the dioxide blowing agents of carbon are eliminated or at least substantially reduced. In this way, high quality foams can be produced from carbon dioxide blowing agent. Even though the reasons for the improvement in foam quality through the use of the above additives are not fully understood, the theory is made that the additives serve to minimize the heat buildup in the extruders by reducing the shear energy of the melt polymeric The shear energy is caused by the frictional forces generated as the carbon dioxide moves past the polymer molecules when the carbon dioxide is mixing and dissolving in the polymer melt. In general, the vapor pressure of the blowing agent is directly proportional, and the viscosity of the polymer melt is inversely proportional, at the temperature of the polymer melt. If the vapor pressure of the carbon dioxide blowing agent is too high and the viscosity of the polymer melt is too low, the carbon dioxide will have a tendency to rapidly diffuse out of the foam upon exiting the die, leading from this way to broken cells, corrugation and a broken or cracked surface appearance. Due to the extreme volatility of carbon dioxide, it has been a tendency to generate very high vapor pressures even at moderate temperatures. as a result, the inventors have found that it is difficult to sufficiently cool the polymer / blowing agent melt mixture in the second extruder 20 to prevent rapid diffusion of the carbon dioxide from the foam. However, the use of one or more of the above additives according to the present invention appears to "lubricate" the polymer so that the carbon dioxide can be mixed with and dissolved in the polymer melt with less heat generation due to a decrease in frictional forces (shear energy) within the polymer melt. In this way, the viscosity of the polymer melt and the vapor pressure of the Blowing agent can be better controlled. In this way, the carbon dioxide expands into more uniform, smaller cells as the foam tube leaves the die without breaking the cells and producing greeks and / or corrugation on the surface of the foam. As a result, the foams produced in accordance with the present invention are of high quality, that is, they have a high degree of cell integrity and a uniform, aesthetically pleasing surface appearance. Unexpectedly, the additives also allow a thicker, lower density foam to be produced from a non-corrugated carbon dioxide blowing agent. It has been generally found that since foams made with carbon dioxide become less dense and thicker, the severity of corrugation increases. The use of one or more of the above additives according to the present invention, however, has been found to prevent this from occurring so that thick foams can be made. { v.gr., 3.05 to 3.81 millimeters) with a carbon dioxide blowing agent without corrugation. In one embodiment of the present invention, the additive that is added to the polymer melt is polysiloxane. A preferred polysiloxane is polydimethylsiloxane. A particularly preferred polysiloxane is an organomodified polydimethylsiloxane which is available from the Union Carbide C emicals and Plastics Company, Inc. under the trade name UCARS L * 01 PA-1. PA-1 can be obtained in a linear low density polyethylene carrier, together with diatomaceous earth and silica, from Specialty Polymer Corporation, PO-Box 249, Dunbar, West Virginia, under the trade name SPC-D55, SPC-D55 has been found that acts as a nuCleador (as well as improving the quality of the foam due to its content of PA-1). In this manner, additional nucleating agents, such as citric acid or sodium bicarbonate, are not needed when using SPC-D55 (organomodified polydimethylsiloxane, together with diatomaceous earth and silica) as an additive in the process of the present invention. The polysiloxane may be present in the polymer melt in any effective amount that achieves a desired level of foam quality. Preferably, the polysiloxane is present in the polymer melt at a low percentage of 1.0 or less and, more preferably, ranges from about 2 x 10"s to about 0.01 weight percent, based on the weight of the polymer melt. In another embodiment of the present invention, the mineral acfeite, that is, a liquid petroleum derivative or extract, is used as an additive in the production of foams from a carbon dioxide blowing agent. The mineral oil may be present in the polymer melt in any effective amount that achieves the desired level of foam quality., but preference varies from about 0.03 to about 0.5 weight percent, based on the weight of the polymer melt. More preferably, the mineral oil is present in a weight percentage of about 0.2. In addition to improving the quality of foams made from carbon dioxide blowing agent, the mineral oil has been advantageously found to improve the flexibility of the foam so that the foam is less brittle. In a more preferred embodiment, both the polysiloxane and the mineral oil are added to the polymer melt within scales of respective weight percentages determined on top of each additive.

In yet another embodiment of the present invention, one or more polyolefins are included with the polymer melt together with polysiloxanes and / or mineral oil. The polyolefins may be selected from the group consisting of ethylene / alpha-olefin copolymer, polypropylene, propylene / ethylene copolymer, ethylene / vinyl acetate copolymer, and ethylene / vinyl alcohol copolymer. The polyolefins may be present in the polymer melt in any effective amount that achieves a desired level of foam quality, but preferably ranges from about 1 to about 15 weight percent, based on the weight of the polymer melt. More preferably, the polyolefin is added to a weight percentage of about 8.5. Referring again to Figure 1, the introduction of the additives of the present invention to the polymer melt will be described. In general, additives can be added to the polymer melt at any convenient point in the process. More preferably, however, the additives are added to the polymer melt either at the same time with or before the addition of the carbon dioxide blowing agent in the injection port 14. In the case of polysiloxane, it is preferred that this additive be added to the polymer melt before adding the carbon dioxide blowing agent. This is most conveniently achieved by adding the polysiloxane to the extruder primer 10 in the hopper 12 together with the thermoplastic polymer. This can be done either by mixing the polysiloxane (or the polysiloxane mixture in SPC-D55) with the polymer granules before adding the polymer to the hopper, or by adding the polysiloxane to the hopper at the same time, but separately from the polymer. In the case of mineral oil, it is preferred that this additive be added to the polymer mixture at the same time with the addition of the carbon dioxide blowing agent in the injection port 14. To this end, a "T" fitting (not shown) can be attached to the injection port 14 whereby the carbon dioxide enters the injection port through a 1"branch of the T while the mineral oil enters. to the injection port through another branch of the T. Both the carbon dioxide and the mineral oil are preferably transported to the injection port 14 by means of separate pumping systems generating pressures of between approximately 351.50 to 386.65 kg / cm2. These pressures, the desired volumes of carbon dioxide and mineral oil can be supplied to the polymer melt.

The polyolefin is most conveniently provided to the polymer melt in the form of "barrier foam tray" waste granules being recycled and added to the hopper 12, either alone or as a mixture with thermoplastic polymer granules. virgin ", for processing into a foam in accordance with the method of the present invention. A barrier foam tray is a foam packing tray to which a flexible oxygen barrier film adheres, thereby providing a packaging tray with oxygen barrier functionality. Barrier foam trays are described, e.g., in US Patents. Do not-? . 4,847,148 and 4,935,089, and in the copending application of E.U.A. Serial No. 08 / 326,176, the disclosures of which are incorporated herein by reference. The oxygen barrier film typically has one or more polyolefins, including polyolefin-based copolymers, such as ethylene / alpha-olefin copolymer, polypropylene, propylene / ethylene copolymer, ethylene / vinyl acetate copolymer, and copolymer ethylene / vinyl alcohol (a preferred oxygen barrier material). Preferred oxygen barrier films for adhesion to a foam packaging tray include a bonding layer (the layer that is in contact with and provides adhesion to the foam tray) of styrene / butadiene copolymer and / or ethylene copolymer / acetate © vinyl; an oxygen barrier layer of ethylene / vinyl alcohol copolymer, vinylidene chloride and copolymers thereof, acrylonitrile, and / or polyamide; and an outer layer of polyethylene, polypropylene, propylene-ethylene copolymer, and ethylene / alpha-olefin copolymer, including both homogeneous and heterogeneous ethylene / alpha-olefin copolymers, such as linear low density polyethylene. Other internal layers, e.g., of polyolefins including olefin copolymers such as linear low density ppl-ethylene, or ethylene / vinyl acetate copolymer, can be included in the oxygen barrier film to impart the desired processing benefits or end use characteristics. An oxygen barrier film in particular. preferred for adhesion to a polystyrene foam tray includes a styrene / butadiene copolymer bond layer, an ethylene / vinyl alcohol copolymer oxygen barrier layer, and an outer layer of linear low density polyethylene or copolymer styrene / bütadiene. In the production of barrier foam trays, the foam sheets are produced as described above. A flexible oxygen barrier film is then adhered to the foam sheet, generally by laminating the film of the foam sheet under heat and pressure either with corona treatment or with an appropriate adhesive. The resulting foam sheet / film composite is then thermoformed into trays or the like and the trays are cut from the sheet. The remaining portion of the sheet, ie, after the trays have been cut and separated from it, becomes waste that is desirably recycled. Another recyclable waste comes from trays that are rejected due to quality control reasons and extruders as a result of changes in size and color. The waste is milled and granulated, creating "recovery granules" in this way. The recovery granules comprise the material from which the foam sheet was formed, e.g., polystyrene, as well as the polyolefins and other materials from which the oxygen barrier film was made. These recovery granules are then placed in the hopper 12, with or without virgin polymer granules, and are extruded and foamed into a foam sheet containing polyolefins as well as the thermoplastic polymer from which the foam is mainly formed. The above recycling process is described in greater detail in the U.S. Patents. Nos. 5,118, 561 and 65,330,596, the teachings of which are incorporated herein by reference. It has surprisingly and unexpectedly been found that foams made from recycled barrier foam trays as described immediately above by using a carbon dioxide blowing agent, and specifically a blowing agent of 100% dioxide carbon, are superior to carbon dioxide foams that are made from virgin polymer (ie, without recycled foam material from barrels and, therefore, without polyolefin). That is, the polyolefin that is added to the polymer melt through the recovery granules has been found to eliminate or reduce the incidence of surface cracks, corrugation and cell rupture. This improvement is particularly pronounced when the polysiloxane and / or mineral oil are also present in the polymer melt. If desired, pure polyolefin, that is, not in the form of recovery granules, can be added to the polymer melt. Regardless of which of the above additives or combination of additives is used, the process of the present invention results in a thermoplastic foam, preferably a foam sheet, comprising a polymorphic matrix containing a thermoplastic polymer and one or more selected materials from the group consisting of polysiloxane, mineral oil and, optionally, polyolefin. A plurality of cells (not broken) are uniformly dispersed through the polymer matrix that, at least immediately after foaming, they contain a carbon dioxide blowing agent that is preferably 100% carbon dioxide. The cells of preference have a size ranging from about 0.25 to about 0.45 millimeters. However, larger or smaller cells may be formed, depending on the particular thermoplastic polymer, the blowing agent, the additive, the processing conditions, etc. The thickness of the foam sheet can vary from about 40 to about 5.08 millimeters, but preferably ranges from about 2.54 to about 3.56 millimeters. The foam sheet can be thermoformed into a variety of foamed articles such as packaging trays or bowls for, eg, license, poultry, products, cheese, etc. Depending on the oxygen sensitivity of the product being packaged, it may be desirable for the foamed article to have an oxygen barrier film adhered thereto as described in lp above.the invention will now be described more specifically in the following examples, which are intended to be illustrative only and not limitative in scope.

EXAMPLES In each of the following examples, foam sheets were made in accordance with the preferred extrusion / foaming method described above and illustrated in Figures 1 and 2. In the examples, "quality" of the foamed sheet or foil product Thermoformed article refers to a numerical, qualitative scale, which varies from 1 to 5, where 1 indicates low quality and 5 indicates excellent quality.

EXAMPLE 1 (COMPARATIVE) Polymer a. Composition: Polystyrene b. Flow rate: 315.25 kg / hour Blowing agent a. Composition: 100% of C02 b. Quantity: 2.8 percent by weight (based on the polystyrene flow rate) Additive None Nucleation Agents a. Citric acid - 0.32 kg / hour b. Sodium bicarbonate - 0.22 kg / hour Process Conditions a. Primary extruder 1) Temperature scale: 279-2l3 ° C 2) Pressure: 253.08 kg / cm2 b. Secondary extruder 1) Temperature scale: 81-132 ° C 2) Pressure: 260.11 kg / cm.2 c. Die 1) Temperature: 132 ° C 2) Pressure: 275.75 kg / cm2 Foamed Sheet Product a. Average cell size: 0.25 mm b. Average sheet thickness: 2.54 mm c. Quality 1) Corrugation: 2 2) Open cells: 1 S) Surface cracks: 2 Thermoforming Not attempted EXAMPLE 2 Polymer a. Composition: Polystyrene b. Flow rate: 353.35 kg / hour Blowing agent a. Composition: 100% COz b. Quantity: 3.3 percent by weight (based on the polystyrene flow rate) Additive a. Composition: Mineral oil b. Quantity: 9.07-11.34 kg / hour Nucleation Agent a. Composition: Citric acid and sodium bicarbonate b. Quantity: 0.025 percent by weight (based on the polystyrene flow rate) Process Conditions a. Primary extruder 1) Temperature scale 18G-229 ° C 2) Pressure: 205.49 kg / cm2 b. Secondary extruder 1) Temperature scale: 66-132 ° C 2) Pressure: 153.46 kg / cm2 c. Die 1) Temperature: 127 ° C 2) Pressure: 94.76 kg / cm2 Foamed Sheet Product a. Average cell size: 0.1225 mm b. Average sheet thickness: 1.88 mm c. Quality 1) Corrugation: 3 2) Open cells: 3 3) Surface cracks: 3 Thermoforming Not attempted EXAMPLE 3 Polymer a. Composition: Polystyrene b. Flow rate: 107.96 kg / hour Blowing agent a. Composition: 100% of C02 b. Quantity: 1.5 percent by weight (based on the polystyrene flow rate) Additive a. Composition: organomodified polydimethylsiloxane (UCARSIL ™ PA-1 from Union Carbide) mixed with LLDPE, diatomaceous earth and silica (from Specialty Polymer Corp. under the trade name SPC-D55) b. Quantity: 8 x 10 ~ 4 percent by weight (based on the polystyrene flow rate) Nucleation Agents None a. Primary Extruder 1) Temperature scale 146-179 ° C 2) Pressure: 210.80 kg / cm2 b. Secondary extruder 1) Temperature scale: 83-100 ° C 2) Pressure: 203.87 kg / cm2 c. Die 1) Temperature: 154 ° C Pressure: 140.60 kg / cm2 Foamed Sheet Product a. Average cell size: 0.35 - 0.41 mm b. Average sheet thickness: 3.56 mm c. Quality 1) Corrugation: 3 2) Open cells: 3 3) Surface cracks: 4 Thermoforming a. Thermoformed article: Tray b. Quality: 4 EXAMPLE 4 Polymer a. Composition: Polystyrene b. Flow regime; 108.86 kg / hour Blowing Agent a. Composition 100% of C02 b. Quantity: 1.5 percent by weight (based on the polystyrene flow rate) Additive - Mixture of: a. Polydimethylsiloxane organomodifiCa o. (UCARSIL ™ PA-1 from Union Carbide), mixed with LLDPE, diatomaceous earth, and silica (from Specialty Polymer Corp. under the trade name SPC-D55); present in the polymer melt at 0.0025 weight percent (based on the polystyrene flow rate) b. mineral oil: present in the polymer melt at 0.2 percent by weight (based on the flow rate of polystyrene). Nucleation Agents None Process Conditions a. Primary extruder 1) Temperature scale 147-176 ° C 2) Pressure: 217.93 kg / cm2 b. Secondary extruder 1) Temperature scale: 73-91 ° C 2) Pressure: 203.87 kg / cm.2 c. Die 1) Temperature scale: 137-150 ° C Foamed Sheet Product a. Average cell size: 0, 37-0.47 mm b. Average sheet thickness: 3.56 mm c-. Quality 1) Corrugation: 5 2) Open cells: 5 3) Surface cracks: 5 Thermoforination a. Thermoformed article: Bande a b. Quality: 5 EXAMPLE 5 Polymer a. Composition: Poiiestireno b. Flow rate: 113.40 kg / hour Blowing agent a. Composition: 100% CO2 b. Quantity: 2.4 percent by weight (based on the polystyrene flow rate) Additive - Mixture of: a. Polyolefin; Present in 15 percent by weight polymer melt (based on the polystyrene flow rate) b. Mineral oil; Present in the polymer melt at 0.4 percent by weight / based on the polystyrene flow rate) Nucleation Agent a. composition: talc b. Quantity: 0.19 percent by weight (based on the polystyrene flow rate) Process Conditions a. Primary extruder 1) Temperature scale: 121-175 ° C 2) Pressure: 1 $ 1.69 kg / cm2 b. Secondary extruder 1) Temperature scale: 79-103 ° C 2) Pressure: 161.69 kg / cm2 c. Die 1) Temperature scale: 152-157 ° C 2) Pressure: 133.57 kg / cm2 Product Laminated Foam a. Average sheet thickness: 3.18 mm b. Quality 1) Corrugation: 4 2) Surface cracks: 4 Thermoforming a. Thermoformed article: Tray b. Quality: 3 The above examples demonstrate that the C02 foams made with an additive according to the present invention are superior to the C02 foams made without said additives. Although the invention has been described with reference to illustrative examples, those skilled in the art will understand that various modifications to the invention can be made as described without departing from the scope of the claims that follow.

Claims (28)

1. - A process for producing thermoplastic foam, comprising: (a) melting a thermoplastic polymer to produce a polymer melt; (b) introducing a blowing agent comprising carbon dioxide towards the polymer melt; (c) adding to the polymer melt one or more additives selected from the group consisting of (1) polysiloxane, and (2) mineral oil; and (d) extruding and foaming the polymer melt, blowing agent, and one or more additives to produce thermoplastic foam.

2. The process of claim 1, wherein the blowing agent consists essentially of carbon dioxide.

3. The process of claim 1, further including the step of thermoforming the thermoplastic foam towards a foamed article.

4. The process of claim 1, wherein the thermoplastic foam contains a plurality of cells having a size ranging from about 0.25 to about 0.45 millimeters.

5. The process of claim 1, wherein the blowing agent is present in the polymer melt in an amount ranging from about 0.1 to about 15 weight percent, based on the weight of the polymer melt.

6. The process of claim 1, wherein the thermoplastic polymer is selected from the group consisting of polystyrene, polypropylene and polyethylene terephthalate.

7. The process of claim 1, wherein the polysiloxane is a polydimethyl siloxane modified with an organic compound.

8. The process of claim 1, wherein the polysiloxane is present in the polymer melt in an amount not greater than about 1.0 weight percent, based on the weight of the polymer melt.

9. - The process of claim 1, wherein the one or more additives comprise both polysiloxane and mineral oil.

10. The process of claim 1, wherein the mineral oil is present in the polymer melt on a scale ranging from about 0.03 to about 0.5 weight percent, based on the weight of the polymer melt. .

11. The process of claim 1, further including the addition of polyolefin to the polymer melt.

12. The process of claim 11, wherein the polyolefin is selected from the group consisting of ethylene / alpha-olefin copolymer, polypropylene, propylene / ethylene copolymer, ethylene / vinyl acetate copolymer, and copolymer of ethylene / vinyl alcohol.

13. The process of claim 11, wherein the polyolefin is present in the polymer melt in an amount ranging from about 1 to about 15 weight percent, based on the weight of the polymer melt.

14. The process of claim 1, wherein the one or more additives are added to the polymer melt before introducing the blowing agent into the polymer melt.

15. The process of claim 1, wherein the one or more additives are added to the polymer melt at the same time with the introduction of the blowing agent to the polymer melt.

16. The process of claim 1, further including the step of adhering an oxygen barrier film to the foam optic.

17, - The process of claim 16, wherein the oxygen barrier film comprises: (a) a binding layer comprising a material selected from the group consisting of styrene / butadiene copolymer, ethylene / acetate copolymer of vinyl, and mixtures of the above materials; (b) An oxygen barrier layer comprising a material selected from the group consisting of ethylene / vinyl alcohol copolymer, vinylidene chloride and copolymers thereof, acrylonitrile, polyamide and mixtures of the above materials; and (c) an outer layer comprising a material selected from the group consisting of polyethylene, polypropylene, propylene / ethylene copolymer, ethylene / alpha-α-olefin copolymer, and mixtures of the above materials.

18. A thermoplastic foam, comprising: (a) a polymeric matrix comprising a thermoplastic polymer and one or more materials selected from the group consisting of (1) polysiloxane, and (2) mineral oil; and (b) a plurality of cells dispersed through the polymer matrix and containing a blowing agent comprising carbon dioxide.

19. The thermoplastic foam of claim 18, wherein the blowing agent consists essentially of carbon dioxide.

20. The thermoplastic foam of claim 18, wherein the thermoplastic foam is a thermoformed article.

21. The thermoplastic foam of claim 18, wherein the thermoplastic foam contains a plurality of cells having a size ranging from about 0.25 to about 0. ~ 45 millimeters.

22. The thermoplastic foam of claim 18, wherein the thermoplastic polymer is selected from the group consisting of polystyrene, polypropylene and polyethylene terephthalate.

23. The thermoplastic foam of claim 18, wherein the polysiloxane is a polydimethylsiloxane modified with an organic compound.

24. The oplastic foam of claim 18, wherein both the polysiloxane and the mineral oil are present in the polymer matrix.

25. The thermoplastic foam of claim 18, wherein the polymer matrix further includes polyolefin.

26. - The thermoplastic foam of claim 25, wherein the polyolefin is selected from the group consisting of ethylene / alpha-olefin copolymer, polypropylene, propylene / ethylene copolymer, ethylene / vinyl acetate copolymer, and ethylene copolymer / vinyl alcohol.

27. The thermoplastic foam of claim 18, further including an oxygen barrier film that adheres to the thermoplastic foam.

28. The thermoplastic foam of claim 26, wherein the oxygen barrier film comprises: (a) a tie layer comprising a material selected from the group consisting of styrene / butadiene copolymer, ethylene copolymer / vinyl acetate, and mixtures of the above materials; (b) an oxygen barrier layer comprising a material selected from the group consisting of ethylene / vinyl alcohol copolymer, vinylidene chloride and copolymers thereof, acrylonitrile, polyamide and mixtures of the above materials; and (c) an outer layer comprising a material selected from the group consisting of polyethylene, polypropylene, propylene / ethylene copolymer, ethylene / alpha-olefin copolymer, and mixtures of the above materials. SUMMARY OF THE INVENTION The present invention provides a process for the production of a thermoplastic foam, said process includes: (a) melting a thermoplastic polymer to produce a melted polymer material; (b) the introduction of a carbon dioxide blowing agent into the melted polymer material; (c) adding to the melted polymer material one or more additives selected from the group consisting of (1) polysiloxane, and (2) mineral oil; and (d) extruding and foaming the molten material of melt polymers, blowing agent, and one or more additives to produce a thermoplastic foam. The resulting thermoplastic foam can be thermoformed into a number of molded articles, such as foamed packaging trays. The quality of the foams made with a carbon dioxide expansion agent, and particularly 100% carbon dioxide is substantially improved by the addition of one or more of the aforementioned additives, ie, polysiloxane, mineral oil and optionally polyolefin to the melted polymer material during the process of extrusion and foaming. Such foams have less fractured cells, fewer surface cracks and corrugations than comparable foams made with carbon dioxide but without the additives of the present invention.