MXPA96003699A - Foam of plastic materials and method to manufacture my - Google Patents

Abstract

A method for producing a foam of extruded plastic materials, of improved physical strength, comprising: (a) intimately mixing a blowing agent containing CO2, in which the main proportion is a natural gas, in a resin melt of plastic materials for forming a homogeneous mixture of resin, and (b) extruding the resin mixture through an outlet die into a lower pressure region, wherein the temperature of the resin mixture is adjusted in such a way that it is less than critical temperature at the extrusion point outside the die

Description

FOAM OF PLASTIC MATERIALS AND METHOD TO MANUFACTURE THE SAME This invention relates to the production of a foam of plastics materials that is formed using a blowing agent without fluorocarbon. The description of the invention below, in general, refers to the use of -carbon dioxide alone as the blowing agent. This -this is only because it is the preferred agent. Other natural gases with C0_ can be used, such as nitrogen, air and water. Until recently, the favored agents for use in the generation of extruded foams of plastics have been fluorocarbons such as dichloro-fluoromethane and trichlorofluoromethane. In recent times, however, there has been a substantial movement in the opposite direction to the use of these compounds since some scientific studies have indicated that fluorocarbons, when released into the atmosphere, can have deleterious sequences, in particular to the zono Alternative technologies have been developed using natural gases as blowing agents, such as carbon dioxide, and an example can be found in the United States Patent 4,436,679.

However, there have been, if any, very few reported commercial operations that use CO-a blowing agent in the manufacture of a polystyrene foam using molten polystyrene. Most commercial operations using C0_ as a blowing agent use C0_ in combination with a hydrocarbon blowing agent such as pentane or butane or with a fluorocarbon product such as Du Pont 152A. An object of the present invention is to produce a foam using -C02 or a blowing agent containing C0_. Preferably, the foam produced should have characteristics and properties at least as good as a fluorocarbon blowing foam. More preferably, the produced foam will have an average cell diameter of less than 22 microns, a cell wall thickness of less than 4 microns, a density of 1.5 to 3.0 pounds / cubic foot, and cells that are substantially uniformly oriented in all three dimensions. To date, it has been considered that foams of plastic materials such as polystyrene foams when blown using a natural gas, should incorporate a fairly low molar amount of the blowing agent retained in the resin. It has previously been found that, as a rule, as the blowing agent is increased, the density of the foam decreases and the physical strength of the foam decreases with density. For example, polystyrene foams produced using more than about 0.1 mole of blowing agent per 100 g of polystyrene have generally been considered to be too weak to be of commercial value particularly when the foam is intended for use in making a final product such as a tray or other support substrate. Previously it has been considered that control of the sputtering process is more difficult as the proportion of blowing agent is increased. Most of the previously published examples suggest the use of a proportion of CO- (when making a polystyrene foam) on the scale of 1.5 to 3.0% by weight (0.034-0.068 moles of C0- / 100 g of polystyrene) . These previously made foams have been extruded at a die temperature of about 140 to 155 ° C. It has also been found with these foams that if they are extruded at the lower end of this temperature scale, they have the tendency to shrink immediately after manufacture. This is probably due to the rapid diffusion of the C0- outside the cells causing a partial vacuum in the cells. The shrinkage can be controlled by increasing the collection in the temperature of the die or by decreasing the concentration of the C0-. Notwithstanding the accepted judgment regarding the appropriate level of addition of blowing agent, the applicant through a series of tests using higher levels of addition of natural gas blowing agent, has discovered that the blowing agent, if it incorporates C0_, can act as a resin viscosity modifier (this has various ramifications, as hereinafter described herein) and also, that the temperature at which the plastic / blowing agent mixture is extruded is critical in terms of the strength of the resulting foam. In general it has been considered that the foaming temperature is not particularly important. For polystyrene, a temperature on the scale of 155 ° C to 135 ° C has been indicated in the literature, but current examples of foams blown with C0_ have all been limited to, at least, 140 ° C. In fact, the insufficient judgment regarding the temperature of the material in the output die in the prior art, highlights the fact that it has not been previously appreciated how important this parameter is in the production of strong foams. The temperature of the material varies considerably at or near the exit die, but there has been no prior emphasis on how to measure the temperature or how exactly it will be measured. The only benefit of reducing the temperature, previously - - reported, has been to increase the control over the formation process to avoid surface defects in the formed sheet. In general, the reduction of temperature has not been favored because the extruder outlet is usually reduced with the reduction of the temperature of the material being extruded. Applicants have now found that the temperature at which the material is extruded is particularly important. More specifically, according to the pre-10 invention, applicants have discovered that, if the temperature of the material, when being extruded, is below a particular critical temperature, this may increase the strength of the resulting product. . 5 Quite unexpectedly, applicants have found that while the physical strength of a blown foam generally decreases, first, with a drop in the temperature of the material in the outlet die, the ratio is not direct and, in fact, 0 there is a lower temperature (which is called "critical temperature") which sharply increases the resistance of the resulting foam. With an initial drop in the material at the die temperature, the viscosity of the mixture in the extruder increases and it is difficult to extrude the mixture using a normal apparatus. If the percentage of C0_ is increased at the same time, it has unexpectedly been found that the foam sheet can be produced. However, with the initial drop in core temperature, the foam has a lower strength and has a larger shrinkage. The shrinkage can be as high as 30 to 40% of the initial dimensions of the foam. This increased deterioration of the properties was a barrier to further experiments in this region. However, it has surprisingly been found that if the material at the die temperature is further reduced (and if the concentration of C0_ is simultaneously increased) below a certain temperature, the shrinkage is rapidly reduced and the strength increased. This increase in strength and reduction in shrinkage occurs even if a high proportion of blowing agent is used. The critical temperature can easily be determined by a person skilled in the art for any resin / blowing agent mixture by marking the physical strength and / or shrinkage of the resulting foam against the temperature of the material in the exit die through of a scale of different temperatures. Below a specific temperature, applicants have found that the resistance of the resulting foam is increased sharply and the shrinkage is rapidly reducedIn addition, applicants have found that for foams extruded at relatively low temperatures and using a relatively high proportion of a natural gas blowing agent, the density of the foam produced is not directly proportional to the molar concentration of the blowing agent. Applicants have also found that, contrary to conventional belief, procedural control using relatively large amounts of blowing agent is not required. Thus, according to a first aspect of this invention, there is provided a method for producing an extruded foam of plastic material of increased physical strength, which comprises: (a) intimately mixing a blowing agent that contains CO- and in the which the main proportion is a natural gas, in a plastic resin melt to form a homogenous resin mixture; and (b) extruding the resin mixture through an exit die in a lower pressure region; wherein the temperature of the resin mixture is adjusted such that it is lower than the critical temperature (as previously defined herein) at the extrusion point outside the exit die. The preferred resin is a styrene polymer. Other resins useful in the practice of the invention include other polymers that will phosphide with the C0-. More preferably, the resin is a polymer or copolymer having at least 90% of styrene monomers. Other monomeric units present in suitable interpolated copolymers include acrylic acid, acrylonitrile and other equivalents known in the art. In one embodiment, virgin polystyrene polymer (80%) was mixed with recycled polystyrene (20%). Under appropriate circumstances, a nucleating agent may be incorporated in the resin mixture, but this is usually not necessary. At C0 blowing agent concentrations greater than about 6.0% by weight, C0-ac will act as its own nucleating agent. Suitable nucleating agents include sodium bicarbonate, citric acid or mixtures thereof. If a nucleation agent is used, it must complete no more than about 0.2% of the weight of the resin mixture. Polystyrene styrene can be combined with an impact modifier. The melt flow rate of the resin is not narrowly reduced. It is preferred between 1.5 to 16 and more preferably,. on the scale of 2.0 to 4.0. (The reference here and through this specification to the index of molten resin flow, is that evaluated in accordance with Test Method of Australian Standard, ASTM D-1238-G) (Australian Standard Test Method). Preferably, the blowing agent is C0- at 100% even when other natural gases such as nitrogen, air or water, or mixtures of these gases, can be used with C0_. A natural gas useful in the invention is any atmospheric material that occurs naturally, that is a vapor at the temperature and pressure at which the foam is produced. Of course, in the process of the invention, the blowing agent does not need to be introduced in the gaseous state - in fact, it is preferred to introduce the substance in a liquid or super critical state. Applicants have also found that at least some of the benefits of applicants' invention are withheld if up to 50% of the amount of C0_ or C0_ / natural gas is replaced by an equivalent molar amount of an agent of blowing hydrocarbon such as butane, pentane or a hydrofluorocarbon. It is preferred that the hydrocarbon, if used, be present in a ratio of 0.01 to 0.06 moles / 100 g of polystyrene. Applicants have found that a convenient method of adding a hydrocarbon blowing agent is through a regrind. If the regrind is obtained from polystyrene foam previously blown with hydrocarbons, then the regrind will contain appreciable amounts of residual hydrocarbon. Levels of 2% to 3% by weight of hydrocarbon by weight of resin are quite common.

- - A convenient source for this trawl is to pack puma blown from polystyrene granules using pentane or butane. If a foam is produced according to the aforementioned method, an increased proportion of blowing agent can be used (to form a lower density foam) yet, an improved strength foam is still produced. In fact, the use of a greater amount of blowing agent brings with it important additional advantages. First, if a higher proportion of blowing agent is used in the production of the foam, it is possible to produce the product with a smaller cell size in which the average thickness of the cement walls is reduced. . The smaller cell size improves the appearance of the product since the smaller the cell size, the smoother the surface of the final product. In addition, small cell size foams are of reduced fragility compared to foams with larger cells. Secondly, the use of a higher proportion of blowing agent allows one to extrude a resin mixture through the exit die of an extrusion apparatus more easily than if a low percentage of the agent had been used. of blowing. For example, in the case of a polystyrene foam blown with C0_, the effect of reducing the viscosity of C02 as previously described, allows the invention to be worked using known commercial apparatus without substantial modification. This is because the viscosity modifying effect of C0_ is the opposite of the effect on the viscosity caused by the reduced extrusion temperature. In a polystyrene / blown foam with C0_, using 100% styrene polymer with a melt flow rate of 3.5, applicants have found that the critical temperature when measured. - mediated * •• * a radiation probe infra-red radiated on the material as it leaves the extrusion die, ie be approximately 135 ° C. The temperature here is the temperature measured on a Scotchtrak Heat Tracer made by the 3M company. The instrument was set for opaque white plastic age (0.95). The IR probe was found to be consistent (with t 1 ° C) with a thermocouple placed in a probe extending to the midpoint of the feed line immediately preceding the die. The temperature of the extruded material varies significantly in several regions at or near the exit die. Since the temperature of the material - as it comes out of the die has been found to be important - it is critical that a specific location be chosen for the measurement of the temperature of the material, and it is for this reason that the specific procedure described above is used. .

In this specification any reference to the temperature of the resin mixture at the extrusion point outside the exit die is the temperature measured by a infra-red radiation probe as described above, unless specifically indicated Preferably, the temperature of the resin mixture at the extrusion point outside the outlet die is between 125 ° to 140 ° C. The most preferred temperature will depend in part on the nature of the resin used. In general, the lower the glass transition point (or higher the Fusion Flow Index) of the resin, the lower the preferred temperature, for example, with a polystyrene having a melt flow index of 3.5 (for example, example AUSTREX 112 - a product of Huntsman Chemical Company Australia Limited) the temperature of the resin mixture in the outlet die is preferably between about 126 ° C to 132 ° C. Below 125 ° C the sun The researchers have found that even with high levels of carbon dioxide addition, the material is too cold to form. The lower limit for the temperature for polystyrene is about 120 ° C at which only very simple shapes can be formed in line. For a polystyrene having a melt flow index of 1.8 (for example AUSTREX 103) the temperature of the material in the outlet die is preferably between 130 ° C to 137 ° C.

For a polystyrene having a melt flow index of 16 (for example AUSTREN 555) the temperature of the material in the outlet die is preferably between 124 ° and 130 ° C. The pressure of the die required to prevent premature foaming for any particular degree of polystyrene increases as the concentration of C0_ increases and the material increases at the die temperature. In practice to avoid excessive pressures of the die which, in turn, lead to extremely difficult control of foam flows leaving the die, as the concentration of C0_ is increased, it is preferred to further reduce the material in the die. the die temperature below the critical temperature. Applicants have found that using a die-2 die that die pressure in excess of 350 Kg / cm, leads to a foam that is difficult to control. It is preferred, when using C0_, that the CO content be greater than 5.5% and less than 10% (0.125 to 0.23 -moles / 100 g of polystyrene) by weight to weight of resin.

At a level of 10% a material at the die temperature of about 120-125 ° C is required to avoid excessive die pressures for a polystyrene having a melt flow index of 3.5. More preferably, the concentration of C02 is between 6 to 8% (0.136 to 0.180 moles / 100 g of polystyrene). Similar molar amounts are preferred if other natural gases are used. With the higher end of C02 concentration levels as preferred, it has been found that the density of the foam increases, leading to an extremely strong foam. With the lower levels of addition of blowing agent, the resulting foam has a low density. A polystyrene foam formed using approximately 6% CO- has a density between about 2a "? 2.5 pounds per cubic foot (0.454 g / 30.5 cm) and a cellular structure where the diameter of each of the cells is less than about 0.002 inch (0.05 mm) With a C0_ addition rate of 6.5% or more, the foam has a cellular structure with a cell size less than 0.001 inch (0.025 mm) This aspect of the invention carries with it additional specific advantages apart from the improved physical strength of the foam produced due to the temperature at which the product is extruded.Previous foams formed using only natural gases as blowing agents had relatively high densities resulting in cell sizes and cell wall thicknesses In the past it had not been possible to make foams by extrusion using a natural gas blowing agent with a microfine cellular structure.

The present invention, when practiced with the preferred amounts of blowing agent addition referred to above, allows the production of foams of increased strength which do not have the brittleness associated with foams having cell sizes. bigger. The reduced thickness of the cell wall of the foams produced according to the invention, results in foams with reduced density. An important aspect of the method of this invention is the intimate mixing of the blowing agent in the resin fume. Unless the blowing agent is intimately integrated into the resin, satisfactory foam can not be produced. Commercial equipment may be used to extrude foam in the process of the present invention although some modification may be required to measure the blowing agent within the extruder, and to ensure adequate mixing, especially at higher rates of addition of blowing agent. Typically, a commercial equipment comprises either an individual extruder or two extruders in series (tandem extrusion). In any system, there are access points provided in the apparatus, through which the materials required to make the foam, they can be introduced. In a single extrusion system, resin granules, combined in most cases with a nucleating agent, are introduced into the extruder or near the upstream end. The resin is melted and mixed in the extruder. Usually a blowing agent is introduced into the extruder at some point downstream from the point at which the resin is introduced into the molten resin. In some systems, a blowing agent is introduced after the thermoplastic melt has passed through the extruder at an intermediate point of the extruder and the exit die in which case an additional mixer is incorporated into the line to ensure mixing of the blowing agent in the thermoplastic melt. Tandem extrusion is a variation of this process. In tandem extrusion, the resin is melted and mixed in the first extruder. The blowing agent is then introduced into the melt before being introduced to a second extruder where mixing and cooling take place. In both systems, the foam is formed by the controlled release of the melt with the blowing agent incorporated therein, through an outlet die to a lower pressure region. The back pressure of the die is important in this regard. As with all foam making processes, the back pressure in the die should be high enough to prevent premature foaming of the mixture as it is extruded through the die. It is also known that if the back pressure of the die is too low, this will lead to imperfections on the surface. The back pressure of the die can be increased by reducing the flow through the die by altering the dimension of the die. It has been found that the compression pressure of the die, measured immediately behind the head of the die, should preferably be about 245 Kg / cm_. For foams having a blowing agent content of 6% or more by weight, the back pressure of the die is preferably 280 kg / cm2. The geometry of the die and the treatment of the puma post die should be such that the foam is produced as a uniform smooth sheet at the exit of the die at the exit temperature of the die, desired. In general, Applicants have found that foam extruded through a slit die and passed over a bar mandrel to flatten any undulations is easier to control than extruded foam through an annular troquel and passed over a conical mandrel. The control of the foam will also be effected by the thickness of the die groove and other modifications obvious to those skilled in the art. Concentrations of C0_ of more than about 8% by weight are difficult to control. Control is facilitated if the foam is extruded within a zone of pressure intermediate that of the mixture before extrusion and atmospheric. The taking of the die foam is preferably such that the foam does not shrink significantly because this shrinkage will impart undesirable mechanical distortion to the cell structure. In general, the higher the pressure of the faster die will emerge the foam and the faster the pick mechanism will run. A typical pick-up mechanism is a forming wheel that rotates in such a way that the speed of the circumference of the wheel is substantially equal to the exit velocity of the die foam. In most conventional arrangements, the apparatus includes a cooling device. This cooling device may be a cooled extruder, or extruder part, a dynamic cooler or other means known to those skilled in the art. By "dynamic cooler" is meant a cooler having a rotary shaft. If used, the dynamic cooler will be positioned at a point in the extrusion line to receive the resin / blowing agent mixture after the blowing agent has been intimately incorporated into the resin. The dynamic cooler includes means to control the mixing of the resin and the blowing agent while, simultaneously, the temperature is reduced.

For the process of the present invention, the temperature of the resin mixture is adjusted, preferably to a temperature below the critical temperature by means of a dynamic cooler. An unexpected advantage of the present invention is the suitability of the existing apparatuses to make the preferred foams. A polystyrene foam blown with C0_ was formed both in the conventional manner and by the process of the present invention, incorporating carbon dioxide into the molten polystyrene at a temperature widely in the range of 170 ° C to 230 ° C. After the blowing agent has been intimately -incorporated into the molten resin, it is cooled to the appropriate temperature for the extrusion of the exit die. In a polystyrene foam containing, for example, 3.5% C0_, the conventional apparatus will reduce the temperature of the melt through the dynamic cooler to about 155 ° C. When the preferred embodiment of this invention is practiced (which involves the use of a higher percentage of carbon dioxide) the dynamic cooler will operate to produce a greater drop in the temperature of the mixture. This is due to the modifying effect of the viscosity of the carbon dioxide, the larger the percentage of less viscous carbon dioxide addition will be the resin mixture and this means that there is less tangential heating when the mixture is passed through the cooler dynamic. Applicants have found that in a conventional apparatus, the temperature for a polystyrene foam / C0_ of about 150 ° C can be achieved through a non-modified dynamic cooler, simply by increasing the rate of addition of C0_ to about 5.5% by weight. The foams produced by the abovementioned methods have characteristics of resistance / smoothness and lack of brittleness not previously known in a foam of extruded plastic materials. Thus, according to a further aspect of the invention, there is provided a non-extruded polystyrene foam sheet incorporating a cellular structure in which, the average cell diameter is less than 0.05 mm and has a density less than about 1.816 Kg / . 5 cm - More preferably, the foam sheet has a density of between 0.908 Kg / 30.5 cm 2 to 1362 Kg / 30.5 cm3. The average diameter of the cell is, more preferably, less than 0.025 m. The structure of the cell, preferably, is closed. The average wall thickness of the cell is between 1 to 2 microns (0.00004-0.0008 of an inch) and more preferably, between 1 to 1.5 microns. The invention is further described in the present by reference to the preferred embodiment, in which: Figure 1 is a schematic diagram of a single extrusion extrusion system for practicing the present invention; Figure 2 is a schematic representation of the sectional sectional view of a foam sheet made according to the invention; and Figure 3 is a sectional sectional view of the mixing tip in the extruder illustrated in Figure 1. The polystyrene foam made substantially entirely of virgin and recycled polystyrene and CO-, was prepared in an individual extrusion line, as illustrated in Figure 1. Resin granules were introduced into the upstream end of the extruder 2 through the hopper 3 The resin can be mixed with a nucleating agent such as sodium bicarbonate, citric acid, hydrocerol, talc or any other nucleating agent such as is known in the art. The addition rate of the nuker, in the practice of the present invention, is on a scale of 0% to 1% and, preferably, 0% to 0.1%. The pre-hardened material extruded with C0_ can be added to the resin granules in a proportion between 10 to 40%, preferably. A screw extruder rotates inside the extruder 2. The barrel or cylinder is maintained at a temperature between 170 ° and 180 ° C to melt the polystyrene and allow it to move easily along the cylinder. The extruder 2 has four separate zones designated A, B, C and D in Figure 1. The polystyrene resin is melted in zones A, B and C, and the cylinder 2 is maintained at a pressure of between 245 and 315 Kg / cm.

The pressure in the extruder cylinder 2 is checked by means of a transducer 4 adjusted in front of a tacho changer 5. The C0_ is measured and is introduced into the polystyrene melt at the access point 6 at the end of the zone C. Carbon dioxide is added to the poly styrene at a rate of about 5.5% consumption per weight compared to the weight of the extruded polystyrene. The CO- is preferably injected with a liquid at a higher pressure than the pressure inside the extruded cylinder 2 for 2, more preferably at approximately 350 Kg / cm. The resin and C0_ are intimately mixed in zone D of the extruder cylinder 2. To ensure adequate mixing in this region, applicants have developed a mixing tip 7 which is mounted on the end of the screw in zone D. Mixing tip 7 is illustrated in greater detail in Figure 3. It will be noted that the mixing tip comprises a high density of mixing tips 8 and a mixing tip head 9, extended in such a way that the space between the edge more outside of the mixing tip head 9 and the inner wall of the extruder cylinder 2 is about 1.4 mils-plus or less. These modifications to the mixing tip head or similar modifications are necessary to ensure adequate mixing of the highest proportion of the blowing agent within the resin. An alternative provision has previously been described in the United States Applicants' Patent 5,129,728. The applicants have found that it is important that there is no dead space in the line between where the gas is added to where the mixture is extruded through the die. The mixture must be worked continuously. The resin with C0_ intimately incorporated therein after being thoroughly mixed in the extruder passes through the screen changer 5 to the gear pump 10. The gear pump 10 operates at an appropriate speed which is related to the speed of the output of die foam and with the rate of resin production through the extruder 2 to balance the pressures on either side of the gear pump. The pressure variation after the gear pump 10, preferably, is less than 21 Kg / cm. Larger variations may cause fluctuation of the sheet in the die 11. The well mixed material is then fed to the dynamic cooler 12, preferably at a pressure greater than 280 Kg / cm. The dynamic cooler 12 can be of any type, as is known in the art. It is preferably a cooler that has a rotary shaft with in-teeth and the meshing teeth carry small amounts of material that allow the C02 / resin mixture to cool the body and the shaft of the dynamic cooler. The temperature of the oil that cools the cooler depends on the outlet of the cooler. This oil temperature can be found on the scale between 45 to 100 ° C. Preferably, the slurry mixture is cooled to a temperature of about 130 ° C. Once the C02 / resin mixture has been cooled, it is extruded out of the die 11 and allowed to spread into a foam by passing it into a lower pressure region. The pressure in the die, preferably 2, is approximately 280 Kg / cm. The sheet formed in this way can be used in a continuous process to form products immediately after it leaves the die, or the foam can be used a later time for thermoformed products. The -pumps made according to the method mentioned above-have low expansion after overheating. Therefore, a conventional thermoforming apparatus, to re-calenflate - the foam needs to be modified to be used with these foams. It is preferred to form the foam thus produced while it is still hot coming out of the die, in a vacuum-assisted thermoformer or aided by continuous obstruction of conventional design. Although the preferred method described above is detailed by reference to a specific type of extruder apparatus, the foam produced in accordance with this invention can be made in any equipment capable of making a conventional coarse foam sheet using 100% natural gas blowing agent at an output temperature of between 140 and 155 °. C with the proviso that the equipment has adequate mixing capacity to intimately incorporate the highest proportion of natural-gas slugging agent used in the preferred embodiments of this invention. When the equipment is working softly, for example at 145 ° C making good foam, to make the foams of the present invention, the temperature is lowered and, preferably, the natural gas concentration is simultaneously increased while maintaining the viscosity of the molten mixture. cooled approximately the same. The viscosity can be monitored from the torque on the screw of the cooling or dynamic cooler. If there is excessive fluctuation of the pressure that occurs as the concentration of blowing agent is increased (or if gas bubbles start to shoot out of the die), then mixing is not appropriate and the equipment must be modified (as described). illustrated) to increase the mixing capacity. Assuming that the proper mixing capacity reduces the cell size to a microfine cell structure, as detailed by reference to Figure 2. It will be seen that the foam 13 has a microfine cell structure where each cell 14 has a diameter maximum of approximately 0.001 inch and the average cell wall thickness is between 1 and 2 microns. The temperature at the extrusion point outside the outlet die can be reduced to the lowest temperature at which the foam sheet can be either thermoformed or otherwise manipulated. Depending on the nature of the post-formation operation, this temperature will be somewhere in the region between 120 and 127 ° C for the polystyrene with a melt flow index of between 2 and 4. The preferred foam produced by the The procedure described by reference to Figures 1 and 3, have a microfine cell structure as can be seen in Figure 2. The physical characteristics of the foam are as follows: density, 2.0-2.2 pounds / cubic foot, fca - average cell size, less than 0.001 inch, thickness per cell wall medium, from 1 to 2 microns. In order to determine the influence of the temperature of the mixture of resin at the extrusion point - outside the exit die, the applicants brought to the trial a number of tests using the same apparatus, polystyrene having the same index fusion and exclusively using C02 as a blowing agent. In each case, the grade of polystyrene used was AUSTREX 112 which had a melt flow rate of 3.5 and C02 was of food grade. The foam produced in each test was formed on a meat tray (17.7 cm x 12.7 cm x 0.12 cm depth) using a continuous vacuum former and the tray's lateral resistance was tested. The resistance test consisted in placing the flat part of the formed foam tray on a support and measuring the maximum force that the sidewall of the tray can withstand before it can be crushed. The results of this test for foams produced on a scale of different outlet die temperatures are shown in the table reproduced in Table 1 below. A second series of tests was carried out using AUXTRES 112 resin in which the average cell size was also calculated. The results of these tests are shown in Table 2.

«Temperature • of ezc of Size% adié. (N) / eesin in cel Density of C02 Prue troquel d < (pounds / day (see output (° <) foot cube) (0.001") Note 2) (see note 1), 1 155 * 4.03 18-15 3.4 * 20.7 6.35 3.3 2 150 * 3.39 12-16 3.7 * 18.6 5.84 3.2 3 M8 2.60 6-12 5.0 19 5.09 3.73 147 2.40 5-6 52 17 4.97 3.42 5 147 2.59 10-12 5.5 18 4.92 3.65 G 146 2.49 7-9 5.5 15 5.14 2.91 7 145 2.41 4-5 5.5 18 4.88 3.68 ü 144 * 2.70 8-10 4.6 * 13.6 4.52 3.01 9 142 2.34 3-4 6.6 18 4.60 3.91 IU 142 2.29 2-3 6.4 18 4.82 3.73 rs » 11 141 00 2.06 3-3 6.9 17 4.58 n 3.71 137 2.15 1-2 6.1 16 4.55 3.51 13 137 2.11 1-1 6.2 14. 4.01 3.49 14 136 2.29 1-1 6.5 18 4.25 4.23 113 135 * 2.24 1-2 5.8 * 13.7 3.90 3.51 IÜ 134 2.01 1-1 7.7 20 4.24 4.71 17 134 2.20 1-1 6.4 23 4.53 5.07 IU 132 2.13 1-1 6.5 20 4.05 4.93 l? 131 2.21 1-1 6.8 28 4.25 6.58 2U 128 * 2.39 1-1 6.6 * 19.5 3.92 4.97 21 127 2.23 1-1 7.4 15 3.03. 4.95 Note 1 Temperatures marked with an asterisk were measured using an IR Probe radiated onto the material as it exits the die. The other temperatures, all, are those measures adjacent to, and immediately before, the output die, with a thermocouple. Note 2 The percentage of addition of C02 for those fi-gutras marked with an asterisk, is measured using a single tank of C02 placed on a scale. The total weight of the bottle was recorded every hour for 3 hours during the operation of the extruder making trays for the specific specification of the test. The mass of the trays or trays produced per hour was compared to the consumption of C02-per mass per hour and averaged over a 3-hour test period. Those figures without any marking were measured by reference to the volume of gas injected into the extruder by means of the calibrator of the apparatus. The soliai tants believe that these figures are less reliable, with a variance of approximately + _ 1.0%. 1 124 13.4 1-1.5 6.8 4.9 12.3 2 126 12.0 1-1.5 6.8 4.9 13.0 3 122 1 1.5 1-1.5 0.2 5.05 1 1.1 4 127 13.9 1-2 6.8 4.5 or 13.5 i 130 15.0 1-2 6.8 4.3 '12 2 G 129 14.0 1-2 6.8 4.7 15.0 7 128 21.0 1-2 6.3 4.3 20.0 8 130 21.0 1-2 6.3 4.2 20.0 and 132 23.9 1-2 6.? 4.0 21.1 10 134 17.9 1-2 1 5.5 3.6 20.3 11 137 64.5 2-3 4.77 3.0 46.7 12 139 204.4 10-12 4.9 3.7 As can be seen from the results of the above tests for polystyrene / CO- blends, the resulting foam is of reduced strength with a decrease in die temperature to approximately 135 ° C after which an additional reduction In the temperature of the material as it leaves the die, it results in a fairly significant increase in the physical strength of the foam produced. In particular, it should be noted from the tests illustrated in Table 1, that the lateral resistance of the foams produced at temperatures between 131 and 134 ° C were all, of 20 Newtons or more, which is substantially equal or better. that the lateral resistance of the trays produced from a foam having a much higher density, for example those of tests 1, 2 and 3. At lower outlet die temperatures it is possible to incorporate higher levels of C0_ and thus produce foams of lower density that still have an increased lateral resistance. The sudden increase in weight-to-weight ratio for temperatures below 135 ° C indicates that for this resin / gas mixture, the critical temperature is about 135 ° C. It will be noted that the lateral resistance of extruded material at 128 ° C is greater than the lateral resistance of the extruded material at 150 ° C, regardless of whether the material density is 30% or less than that of the material that is extruded to 150 ° C. A third series of tests was carried out by the applicants using a polystyrene resin having a higher melt flow index. For this series of tests, the applicants used AUSTREX 555 resin having a melt flow index of 16. The line conditions for the production of a foam tray were set to be substantially the same as in the previous tests using AUSTREX 112 resin. The addition of C02 was of 6.8%. Good trays were produced at a temperature of t roquel below 130 ° C. The optimum temperature appeared to be about 128 ° C, at which the foam trays produced were found to have a density of 2.11 pounds / cubic foot, and the foam was found to have a microfine structure with a cell size less than 0.001 inch While it was found that the quality of the sheet was slightly deteriorated using a resin having this melt flow index, it was found that the trays formed had properties similar to those made with a styrene having a much lower melt flow rate, such as AUSTREX 112. The present invention provides a lot of follow-up on the benefits. Any degree of poly-styrene can be used because the viscosity of the material can be - decreased by the use of a higher percentage of carbon dioxide. This reduces the stress on the equipment. A foam sheet having low density can be made without sacrificing the tensile properties. It is possible to achieve a microfine cell structure and this has the advantages of smoothness, reduced brittleness and improved insulation properties. In addition, microfine cell foams allow a better exit of the extruder due to the reduced viscosity of the material, as a result of the higher C0_ content. This allows the equipment to work faster with less stress. Finally, microfine cell foams are flexible. In order to produce the foams of the present invention in a conventional apparatus, it was necessary for the applicants to recognize the effect of modifying the viscosity of the C0_ and to increase the blowing agent ratio and to make a low density foam to allow the ex. trusion below the critical temperature. The reduction of the density of the materials to increase the resistance was counter-intuitive, but it led to a superior product that had several advantages, as detailed above. It should be understood that the aforementioned preferred description of the process of the invention can be modified (for example by incorporating additional mixing by means of a tandem extrusion system) or by the incorporation of additional materials (such as agents of nucleation) without departing from the spirit and the scope of the invention.

Claims (27)

1. A method for producing a polystyrene foam article of improved physical strength in which the article is thermoformed from an extruded polystyrene foam, article that is produced by: (a) intimately mixing a blowing agent consisting of, substantially, of a natural gas or gases, in a polystyrene melt to form a homogeneous, blowing agent containing from 5.5% to 10% by weight of C02 to the weight of the resin; (b) extruding the resin mixture through the exit die into a lower pressure region while maintaining the temperature of the resin mixture below the critical temperature (as hereinbefore defined) in the extrusion point outside the exit die to form a sheet of polystyrene foam; and (c) thermoforming the polystyrene sheet without reheating the sheet to form an article immediately after the extrusion of the resin mixture through the sheet punch.

2. A method as claimed in clause 1, wherein the blowing agent includes C02 mixed with another natural gas (as hereinbefore defined).

3. A method as claimed in clause 2, wherein the blowing agent includes C02 mixed with nitrogen.

4. A method as claimed in clause 3, wherein the blowing agent consists exclusively of C0-, and nitrogen.

5. A method as rievindica in the clause 1, wherein the blowing agent includes between 0.01-0.06 -moles / 100 g of resin of any one or more of pentane, butane or a hydrofluorocarbon, and wherein the remainder of the blowing agent is C02-

6. claimed in the clause 1, wherein the blowing agent is exclusively carbon dioxide.

7. A method as claimed in any of Claims 1 to 6, wherein the polystyrene melt includes virgin polystyrene polymer and recycled styrene polymer.

8. A method as claimed in any of Claims 1 to 7, wherein the swirled polystyrene is blown foam with a hydrocarbon blowing agent.

9. A method as claimed in the clause 1, in which the resin of plastics materials is a styrene polymer having a melt flow index of between 2 to 4 and the blowing agent is exclusively c®2 'where the critical temperature of the mixture of The resin at the extrusion point outside the exit die is approximately 145 ° C, measured by means of an infra-red probe radiated onto the material as it exits the extrusion die.

10. A method as claimed in the clause 8, wherein the temperature of the resin mixture in the extrusion point outside the outlet die is between 125 to 135 ° C.

11. A method as claimed in clause 10, wherein the temperature of the resin mixture at the extrusion point outside the exit die is approximately 128 ° C.

12. A method as set forth in clause 1, in which the resins of plastic materials is a styrene pointer having a melt flow index of between 1.5 to 2 and the blowing agent is exclusively CO-. wherein the critical temperature of the resin mixture at the extrusion point outside the exit die is approximately 140 ° C, measured by means of an in-fra-red probe radiated on the material as it exits the extrusion die .

13. A method as claimed in any of clauses 1 to 12, wherein the content of blowing agent of carbon dioxide is between 6 to 8% by weight to the weight of the resin.

14. A method as claimed in the clause 1, wherein the resin mixture is maintained at a pressure 2 of between 245 to 350 Kg / cm immediately before extrusion through the exit die and is allowed to extend into a foam by passing it into a region held at atmospheric pressure.

15. A method as claimed in clause 1, wherein a nucleating agent is added to the resin, prior to incorporation with a blowing agent, nucleating agent that is present in an amount of not more than 0.2 % by weight of the resin mixture.

16. A method as claimed in clause 15, wherein the nucleating agent is sodium bicarbonate, citric acid, talc or mixtures thereof.

17. A method as claimed in the clause 1, wherein the blowing agent is intimately mixed with the resin melt of plastic materials in a screw extruder, and wherein the temperature of the resin mixture is reduced to less than the critical temperature by means of a dynamic cooler that it is adapted to reduce the temperature of the mixture while simultaneously maintaining a homogeneous mixture of the resin and blowing agent.

18. A method as claimed in any of Claims 1 to 17, wherein the resin mixture is extruded through a slot die and passed over a bar mandrel.

19. A method as claimed in the clause 1, wherein the foam of extruded plastic material is thermoformed in an article at atmospheric pressure, immediately after the extrusion of the foam out of the outlet die.

20. An extruded polystyrene foam, made in accordance with the method of any of Clauses 1 to 19, which incorporates a cellular structure in which the average diameter of the cell is less than 0.002 inches and which has a density less than, or equal to 4.0 pounds / cubic foot

21. A foam as claimed in clause 20, wherein the density of the foam is between 2.0 to 3.0 pounds / cubic foot.

22. A polystyrene foam as claimed in clause 21, wherein the density is between 2.2 to 2.4 pounds / cubic foot.

23. A sheet of polystyrene foam as claimed in any of clauses 20 to 22, wherein the average diameter of the cell is less than, or equal to, 0.001 inches.

24. A polystyrene foam as claimed in clause 20, wherein the average thickness of the walls of the foam cell is between 1 to 2 microns.

25. A polystyrene foam as claimed in clause 24, wherein the average thickness of the walls of the foam cell is between 1 to 1.5 microns.

26. A polystyrene foam as claimed in clause 20, wherein the foam is predominantly of a closed cell structure.

27. A foam tray formed of an extruded polystyrene foam, as claimed in clause 20. FOAM OF PLASTIC MATERIALS AND METHOD TO MANUFACTURE THE SAME SUMMARY A method for producing a foam of extruded plastics, of improved physical strength, comprising: (a) intimately mixing a blowing agent containing c0 ~, in which the main proportion is a natural gas, in a resin melt plastic materials to form a homogeneous resin mixture; Y (b) extruding the resin mixture through an outlet die into an inert pressure region, wherein the temperature of the resin mixture is adjusted in such a way that it is below the critical temperature (as defined herein) in the -extrusion point outside the exit die. = or =