Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
  • C08J9/04—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent
  • C08J9/12—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a physical blowing agent
  • C08J9/127—Mixtures of organic and inorganic blowing agents
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C44/00—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles
  • B29C44/34—Auxiliary operations
  • B29C44/3469—Cell or pore nucleation
  • B29C44/348—Cell or pore nucleation by regulating the temperature and/or the pressure, e.g. suppression of foaming until the pressure is rapidly decreased
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/25—Component parts, details or accessories; Auxiliary operations
  • B29C48/78—Thermal treatment of the extrusion moulding material or of preformed parts or layers, e.g. by heating or cooling
  • B29C48/80—Thermal treatment of the extrusion moulding material or of preformed parts or layers, e.g. by heating or cooling at the plasticising zone, e.g. by heating cylinders
  • B29C48/83—Heating or cooling the cylinders
  • B29C48/832—Heating
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/25—Component parts, details or accessories; Auxiliary operations
  • B29C48/78—Thermal treatment of the extrusion moulding material or of preformed parts or layers, e.g. by heating or cooling
  • B29C48/80—Thermal treatment of the extrusion moulding material or of preformed parts or layers, e.g. by heating or cooling at the plasticising zone, e.g. by heating cylinders
  • B29C48/83—Heating or cooling the cylinders
  • B29C48/834—Cooling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/25—Component parts, details or accessories; Auxiliary operations
  • B29C48/78—Thermal treatment of the extrusion moulding material or of preformed parts or layers, e.g. by heating or cooling
  • B29C48/86—Thermal treatment of the extrusion moulding material or of preformed parts or layers, e.g. by heating or cooling at the nozzle zone
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/25—Component parts, details or accessories; Auxiliary operations
  • B29C48/78—Thermal treatment of the extrusion moulding material or of preformed parts or layers, e.g. by heating or cooling
  • B29C48/86—Thermal treatment of the extrusion moulding material or of preformed parts or layers, e.g. by heating or cooling at the nozzle zone
  • B29C48/87—Cooling
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
  • C08J9/04—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent
  • C08J9/12—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a physical blowing agent
  • C08J9/122—Hydrogen, oxygen, CO2, nitrogen or noble gases
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C48/00—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor
  • B29C48/03—Extrusion moulding, i.e. expressing the moulding material through a die or nozzle which imparts the desired form; Apparatus therefor characterised by the shape of the extruded material at extrusion
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2203/00—Foams characterized by the expanding agent
  • C08J2203/14—Saturated hydrocarbons, e.g. butane; Unspecified hydrocarbons
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2203/00—Foams characterized by the expanding agent
  • C08J2203/14—Saturated hydrocarbons, e.g. butane; Unspecified hydrocarbons
  • C08J2203/142—Halogenated saturated hydrocarbons, e.g. H3C-CF3
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2325/00—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by an aromatic carbocyclic ring; Derivatives of such polymers
  • C08J2325/02—Homopolymers or copolymers of hydrocarbons
  • C08J2325/04—Homopolymers or copolymers of styrene
  • C08J2325/06—Polystyrene

Definitions

• This invention relates to the production of a plastics foam which is formed utilising a non-fluorocarbon blowing agent.
• the description of the invention hereinafter generally refers to the use of carbon dioxide alone as the blowing agent.
• C0 2 such as nitrogen, air and water.
• C0 2 As the sole blowing agent in the manufacture of polystyrene foam using molten polystyrene.
• Most commercial operations using C0 2 as a blowing agent use C0 2 in combination with a hydrocarbon blowing agent such as pentane or butane or with a fluorocarbon product such as Du Pont 152A.
• a hydrocarbon blowing agent such as pentane or butane or with a fluorocarbon product such as Du Pont 152A.
• the foam produced should have characteristics and properties at least as good as a fluorocarbon blown foam.
• the foam produced will have an average cell diameter below 22 microns, a cell wall thickness below 4 microns, a density between 1.5 to 3.0 lbs/ft 3 and cells which are substantially uniformly oriented in all three dimensions.
• plastic foams such as polystyrene foams when blown using a natural gas should incorporate a fairly low molar amount of the blowing agent entrained in the resin. It has been previously considered that generally as the proportion of blowing agent is increased the density of the foam is decreased and that the physical strength of the foam decreases with the density. For example polystyrene foams produced using above about 0.1 moles blowing agent per 100g 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 an end product such as a tray or other supporting substrate. It has also been previously considered that the control of the foaming process is more difficult as the proportion of blowing agent is increased.
• blowing agent if it incorporates C0 2 can act as a viscosity modifier of the resin (this has various ramifications as hereinafter described) and also that the temperature at which the plastic/blowing agent mix is extruded is critical to the strength of the resulting foam.
• the temperature of foaming is not particularly important.
• polystyrene a temperature range from 155°C to 135°C has been quoted in the literature, but actual examples of C0 2 blown foams have all been limited to above at least 140°C.
• the scant regard to the temperature of the material at the exit die in the prior art highlights the fact that it has not previously been appreciated how important this parameter is in the production of strong foams.
• the temperature of the stock varies considerably at or near the exit die but there has not previously been any emphasis on the manner of measurement of the temperature or exactly how it is to be measured.
• the only benefit of lowering the temperature previously reported has been to increase control over the forming process to avoid surface defects in the formed sheet.
• reduction of temperature has not been favoured because the output of the extruder normally reduces with reduction of temperature of the material being extruded.
• the applicants have now found that the temperature at which the material is extruded is particularly important.
• the applicants have ascertained that if the temperature of the material, when it is being extruded, is below a particular critical temperature this can enhance the strength of the resulting product.
• the applicants have found that whilst the physical strength of a blown foam generally decreases, at first, with a drop in the temperature of the material at the exit die, the relationship is not a direct one and in fact there is a temperature (hereinafter called “the critical temperature") below which the strength of the resulting foam sharply increases.
• the critical temperature a temperature below which the strength of the resulting foam sharply increases.
• a method of producing an extruded plastics foam of enhanced physical strength which comprises: (a) intimately mixing a blowing agent incorporating C0 2 and in which the major proportion is a natural gas, in a plastics resin melt to form an homogeneous resin mix; and
• the preferred resin is a styrene polymer.
• Other useful resins in the practice of the invention include other polymers which C0 2 will plasticize.
• the resin is a polymer or copoiymer having at least 90% styrene monomers.
• Other monomeric units present in suitable copolymers and interpolymers include acrylic acid, acrylonitrile and other equivalents known in the art.
• virgin polystyrene polymer 80%
• regrind polystyrene (20%) is mixed with regrind polystyrene (20%).
• a nucleating agent may be incorporated into the resin mix but this is usually not necessary. At concentrations of CO 2 blowing agent above about 6.0% by weight, the C0 2 will act as its own nucleating agent.
• Suitable nucleating agents include sodium bicarbonate, citric acid or mixtures thereof. If a nucleating agent is used it should make up no more than about 0.2% of the weight of the resin mix.
• the polystyrene may be blended with an impact modifier.
• the melt flow index of the resin is not narrowly critical. It is preferably between 1.5 to 16 and most preferably in the range 2.0-4.0. (Reference here and throughout this specification to the melt flow index of the resin is that as tested according to Australian Standard Test Method ASTM D1238-G)
• the blowing agent is 100% C0 2 although other natural gases such as nitrogen, air or water or mixtures of these gases can be utilised with C0 2 .
• a natural gas useful in the invention is any naturally occurring atmospheric material which is a vapour at the temperature and pressure at which the foam is produced.
• the blowing agent does not need to be introduced in a gaseous state - in fact it is preferred to introduce the substance in a liquid or super critical state.
• regrind a convenient method of addition of a hydrocarbon blowing agent is via 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% - 3% hydrocarbon weight per resin weight are quite usual.
• a convenient source for such regrind is packaging foam blown from polystyrene pellets using pentane or butane.
• a foam is produced in accordance with the aforementioned method an increased proportion of blowing agent can be used (so to form a foam of lower density) yet still produce a foam of enhanced strength.
• the utilisation of a greater amount of blowing agent brings with it important further advantages.
• 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 and in which the average thickness of the cell walls is reduced.
• the smaller cell size improves the appearance of the product as the smaller the cell size the smoother the surface of the end product.
• small cell size foams are of reduced brittleness as compared with foams with larger cells.
• the applicants have found that the critical temperature when measured by an infra-red probe beamed onto the material as it exits the extrusion die to be about 135°C.
• the temperature cited here is the temperature as measured on a Scotchtrak Heat Tracer made by the 3M company.
• the instrument was set for the emissivity of opaque white plastic (0.95).
• the IR probe was found to be consistent (within ⁇ 1°C) with a thermocouple placed in a probe extending to the mid point of the feed line immediately preceeding the die.
• the temperature of the extruded material varies significantly in various regions at or near the exit die. As the temperature of the material as it exits 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 outlined above is used. In this specification all reference to the temperature of the resin mix at the point of extrusion out of the exit die is the temperature as measured by an infra-red probe as described above, unless specifically stated otherwise.
• the temperature of the resin mix at the point of extrusion out of the exit 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 the higher the Melt Flow Index) of the resin, the lower the preferred temperature.
• the temperature of the resin mix at the exit die is preferably between about 126° to 132°C. Below 125°C the applicants have found that even with high levels of carbon dioxide addition the material is too pool to form in line.
• the lower limit for the temperature for polystyrene is about 120°C at which only very simple shapes can be formed in line.
• the temperature of the material at the exit die is preferably between 130° to 137°C.
• the temperature of the material at the exit die is preferably between 124° to 130°C.
• the content of the CO be above 5.5% and below 10% (0.125 - 0.23 moles/1 OOg polystyrene) by weight to the weight of resin.
• a stock at 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.
• the C0 2 concentration is between 6 to 8% (0.136 - 0.180 moles/1 OOg polystyrene). Similar molar amounts are preferred if using other natural gases.
• C0 2 concentration levels indicated above as preferments it has been found that the density of the foam increases leading to an extremely strong foam.
• a polystyrene foam formed using about 6% C0 2 has a density of between about 2 to 2.5 lbs/ft ⁇ and a cellular structure where the diameter of each of the cells is below about 0.002 inches.
• the foam has a cellular structure with a cell size below 0.001 inches.
• An important aspect of the method of this invention is the intimate mixing of the blowing agent in the resin melt. Unless the blowing agent is intimately entrained within the resin a satisfactory foam cannot be produced.
• Commercial equipment for extruding foam may be used in the process of the present invention although some modification may be required to meter the blowing agent into the extruder and to ensure adequate mixing especially at higher percentages of blowing agent addition.
• commercial equipment comprises either a single extruder or two extruders in series (tandem extrusion). In either system, there are access points provided in the apparatus through which materials required to make the foam can be introduced.
• resin granules In a single extruder system, resin granules, combined in most cases with a nucleating agent, are introduced into the extruder at or near its upstream end. The resin is melted and mixed in the extruder. A blowing agent is usually introduced into the extruder at some point downstream from the point at which the resin is introduced into the molten resin.
• a blowing agent is introduced after the thermoplastic melt has passed through the extruder at a point intermediate the extruder and the outlet die in which case a further mixer is incorporated into the line to ensure proper 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. Blowing agent is then introduced into the melt prior to being introduced into a second extruder where mixing and cooling takes place. In both systems, the foam is formed by controlled release of the melt with the blowing agent entrained therein, through an exit die into a region of lower pressure. The back pressure of the die is important in this regard.
• the back pressure at the die must be sufficiently high to prevent premature foaming of the mixture as it extrudes through the die. It is also known that if the die back pressure is too low that this will lead to surface imperfections.
• the die back pressure can be increased by reducing the flow through the die by altering the dimension of the die. It has been found that the die back pressure as measured immediately behind the die head should preferably be above 3,500 psi. For foams with a blowing agent content of 6% of more by weight the die back pressure is preferably above 4,000 psi.
• the geometry of the die and the treatment of the foam post die should be such that the foam is produced as a smooth uniform sheet at the die exit at the desired die exit temperature.
• foam extruded through a slit die and passed over a bar mandrel to flatten out any waves is easier to control than foam extruded through an annular die and passed over a conical mandrel.
• Control of the foam will also be effected by the thickness of the die slit and other modifications obvious to those skilled in the art. Concentrations of the C0 2 greater than about 8% by weight are difficult to control. Control is easier if the foam is extruded into a zone of pressure intermediate that of the mixture before extrusion and atmospheric.
• the take up of foam from the die is preferably such that the foam is not significantly stretched as such stretching will impart undesired mechanical distortion to the cell structure.
• a typical take up mechanism is a forming wheel rotating so that the velocity of the circumference of the wheel is substantially equal to the speed of the exit of the foam from the die.
• the apparatus includes a cooling device.
• This cooling device may be a cooled extruder, or part extruder, a dynamic cooler or other means known to those skilled in the art.
• dynamic cooler it is meant a cooler having a rotating shaft. If used, the dynamic cooler is located at a point in the extrusion line to receive the resin/blowing agent mix after the blowing agent has been intimately entrained within the resin.
• the dynamic cooler includes means to continue the admixture of the resin and blowing agent while simultaneously reducing its temperature.
• the temperature of the resin mix is preferably adjusted to a temperature below the critical temperature by means of a dynamic cooler.
• An unexpected advantage of the present invention is the suitability of existing apparatus to make the preferred foams.
• a C0 2 blown polystyrene foam is formed both conventionally and in the process of the present invention by incorporating carbon dioxide into the polystyrene melt at a temperature broadly in the range of between 170°C to 230°C. After the blowing agent has been intimately entrained within the resin melt it is cooled to the appropriate temperature for extrusion out of the exit die.
• the conventional apparatus will reduce the temperature of the melt through the dynamic cooler to about 155°C.
• the dynamic cooler When exercising the preferred embodiment of this invention (which involves the use of a higher percentage of carbon dioxide) the dynamic cooler will operate to bring about a greater drop in the temperature of the mix. This is because of the viscosity modifying effect of the carbon dioxide. The greater percentage addition of carbon dioxide, the less viscous the resin mix and this means that there is less shear heating when the mix is passed through the dynamic cooler.
• the applicants have found that on conventional apparatus the critical temperature for polystyrene/C0 2 foam of about 135°C can be achieved through unmodified dynamic cooling apparatus simply by increasing the CO 2 addition rate to about 5.5% by weight.
• an extruded polystyrene foam sheet incorporating a cellular structure in which the average cell diameter is less than 0.002 inches and which has a density of less than about 4.0 lbs/ft 3 .
• the foam sheet has a density of between 2.0 to 3.0 lbs/ft 3 .
• the average cell diameter is most preferably less than 0.001 inches.
• the cell structure is preferably closed.
• the average cell wall thickness is between 1 to 2 microns (0.00004 - 0.0008 inches) and most preferably between 1 to 1.5 microns.
• Figure 1 is a schematic diagram of a single extruder extrusion system for practicing the current invention
• Figure 2 is a schematic representation of the cross-sectional view of a foam sheet made in accordance with the invention.
• Figure 3 is a cross-sectional view of the mixing tip in the extruder shown in
• Polystyrene foam made substantially entirely by virgin and regrind polystyrene and C0 2 is prepared on a single extrusion line as shown in Figure 1.
• Resin granules are introduced into the upstream end of extruder 2 through hopper 3.
• the resin may be mixed with a nucleating agent such as sodium bicarbonate, citric acid, hydrocerol, talc or any other nucleating agent as known in the art.
• the addition rate of nucleator in the practice of the present invention is in the range of 0% to l% and preferably between 0% to 0.1%.
• Regrind material previously extruded with CO can be added to the resin granules in a ratio preferably between 10 to 40%.
• a screw extruder rotates within the barrel of extruder 2. The barrel is maintained at a temperature between 170° to 180°C to melt the polystyrene and enable it to move easily along the barrel.
• 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 barrel is maintained at a pressure of between 3500 to 4500 psi.
• the pressure in the extruder barrel 2 is checked by a transducer 4 fitted before a screen changer 5.
• CO is metered and introduced into the polystyrene melt at access point 6 at the end of zone C.
• the carbon dioxide is added to the polystyrene at a rate of above 5.5% consumption by weight compared with the weight of extruded polystyrene.
• the C0 2 is preferably injected as a liquid at a pressure higher than the pressure within the extruder barrel 2, most preferably at around 5000 psi.
• the resin and the C0 2 are intimately mixed in zone D of extruder barrel 2.
• a mixing tip 7 which is fitted at the end of the screw in zone D.
• Mixing tip 7 is shown in more detail in Figure 3. It will be noted that the mixing tip comprises a high density of fixed mixing pegs 8 and an expanded mixing tip head 9 such that the clearance between the outermost edge of the mixing tip head 9 and the inner wall of the extruder barrel 2 is about 1.4 mil or less.
• gear pump 10 is operated at an appropriate rate having regard to the rate of foam output at die II and the rate of resin production through extruder 2 to balance the pressures on either side of the gear pump.
• the pressure variation after gear pump 10 is preferably less than 300 psi. Greater variations can cause fluctuation of sheet at the die II.
• the well mixed material is then fed into the dynamic cooler 12 preferably at a pressure above 4000 psi.
• the dynamic cooler 12 can be of any type as known in the art. Preferably it is a cooler having a rotating shaft with gears and the geared teeth carry small amounts of material allowing the C0 2 /resin mix to cool down to the desired temperature.
• the heat exchanger 12a is used to cool the dynamic cooler body and shaft. Oil temperature cooling the cooler depends on the cooler output. This oil temperature can be in a range of between 45 to 100°C.
• the resin mix is cooled to a temperature of about 130°C.
• the sheet so formed can be used in a continuous process to form products immediately after it exits the die or the foam can be used at a later time to thermoform products.
• Foams made in accordance with the aforementioned method have low expansion on reheating. Therefore conventional foam re-heat thermoforming apparatus needs to be modified for use with such foams. It is preferred to form the foam so produced while it is still hot after exiting the die on a continuous vacuum assist or plug assist thermoformer of conventional design.
• foam produced in accordance with this invention may be made on any equipment capable of making conventional coarse foam sheet using 100% natural gas blowing agent at an exit die temperature of between 140 to 155°C provided that the equipment has adequate mixing capability so to intimately entrain the higher proportion of natural gas blowing agent used in the preferred embodiments of this invention.
• the temperature is decreased and preferably the natural gas concentration simultaneously increased keeping the viscosity of the cooled molten mixture roughly the same. Viscosity can be monitored from the torque on the cooling screw or dynamic cooler.
• the cell size reduces to a microfine cellular structure as detailed by reference to Figure 2.
• the foam 13 has a microfine cellular structure wherein each cell 14 has a maximum diameter of about 0.001 inches and the average cell wall thickness is between 1 to 2 microns.
• the temperature at the point of extrusion out of the exit die may be reduced to the lowest temperature that the foam sheet can be either thermoformed or otherwise manipulated. Depending on the nature of the post forming operation, this temperature will be somewhere in the region between 120 to 127°C for polystyrene with a melt flow index of between 2 to 4.
• the preferred foam produced by the process described by reference to Figures I and 3 above has a microfine cellular structure as can be seen in Figure 2.
• the physical characteristics of the foam are as follows: density: 2.0-2.2 lbs/ft 3 average cell size: less than 0.001 inch average cell wall thickness : 1 to 2 microns.
• the applicant conducted a number of trials using the same apparatus, polystyrene having the same melt index and using exclusively C0 2 as the blowing agent.
• the polystyrene grade used was AUSTREX 112 having a melt flow index of 3.5 and the C0 2 was food grade.
• the foam produced in each trial was formed into a meat tray (7 inch x 5 inch x 5/10 inch deep) using a continuous vacuum former and the side strength of the tray was tested.
• the strength test involved placing the formed foam tray flat on a jig and measuring the maximum force that the side wall of the tray could withstand before it would collapse.
• the results of this test for foams produced over a range of different exit die temperatures are set out in the table reproduced in Table 1 below.
• the resultant foam is of reduced strength with a decrease in the die temperature to about 135°C whereafter further reduction in the temperature of the material as it exits the die results in a quite significant increase in the physical strength of the foam produced.
• the side strength of the foams produced at temperatures between 131 to 134°C were all 20 newtons or greater which is substantially the same or better than the side strength of the trays produced from a foam having a much higher density such as those in trials 1 , 2 and 3.
• At lower exit die temperatures it is possible to incorporate higher levels of C0 2 and thus produce foams of lower density yet having enhanced side strength.
• the critical temperature is about 135 0 C. It will be noted that the side strength of the material extruded at 128°C is greater than the side strength of the material extruded at 150°C notwithstanding that the density of the material is some 30% less than the material which was extruded at 150°C.
• a third series of tests were conducted by the applicant using a polystyrene resin having a higher melt flow index.
• the applicant used Austrex 555 resin which has a melt flow index of 16.
• Line conditions for the production of a foam tray were set so to be substantially the same as in previous trials using Austrex 112 resin. CO 2 addition was at 6.8%.
• Good trays were produced at a stock at die temperature 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 lbs/ft 3 and the foam was found to have a microfine structure having a cell size of less than 0.001 inch.
• the present invention provides many flow on benefits. Any grade of polystyrene can be used because the viscosity of the material can be lowered by use of a higher percentage carbon dioxide. This reduces the stress on the equipment.
• a foam sheet can be made having lower density without sacrificing tensile properties. It is possible to achieve microfine cellular structure and this has advantages of smoothness, reduced brittleness and enhanced insulation properties.
• microfine cell foams allow increased extruder output due to reduced material viscosity as a result of the higher CO content. This enables equipment to run faster with low stress.
• microfine cell foams are flexible.

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Citations (7)

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• 1995
  • 1995-03-09 EP EP95912079A patent/EP0749455A4/en not_active Withdrawn
  • 1995-03-09 NZ NZ282337A patent/NZ282337A/en not_active IP Right Cessation
  • 1995-03-09 AU AU19413/95A patent/AU1941395A/en not_active Abandoned
  • 1995-03-09 JP JP7523123A patent/JPH09509975A/ja active Pending
  • 1995-03-09 WO PCT/AU1995/000127 patent/WO1995024440A1/en not_active Application Discontinuation
  • 1995-03-09 BR BR9507102A patent/BR9507102A/pt not_active Application Discontinuation
  • 1995-03-09 CA CA002185221A patent/CA2185221A1/en not_active Abandoned
  • 1995-03-10 CO CO95009620A patent/CO4410417A1/es unknown
  • 1995-03-10 IN IN287MA1995 patent/IN188233B/en unknown
  • 1995-03-13 PE PE1995263936A patent/PE22495A1/es not_active Application Discontinuation

Patent Citations (7)

Non-Patent Citations (1)

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