MXPA00010347A - Method and system for forming low-density polymer foam article - Google Patents

Abstract

A crystalline polymer resin, e.g. pet, is heated to melting. A blowing agent and resin are mixed to produce a theoretical sheet foam density of less than 0.4 g/cm3. The mixture is cooled to a temperature approaching the freezing point of the mixture, and then extruded into a substantially uniform closed cell polymer foam sheet of density less than 0.4 g/cm3. The extruded sheet is then cooled by direct contact with a cooling surface below the glass transition temperature such that the blowing agent(s) condenses and the sheet has a density of greater than 0.4 g/cm3 and a sheet crystallinity of less than 15%. An article is then formed from the cooled sheet by applying heat such that the condensed blowing agent(s) vaporizes and the article crystallizes at a density of less than 0.4 g/cm3 with a crystallinity of greater than 20%.

Description

METHOD AND SYSTEM FOR FORMING A FOAM ARTICLE LOW DENSITY POLYMER BACKGROUND OF THE INVENTION TECHNICAL FIELD The present invention relates generally to a method and system for producing polymer foam articles. In particular, the present invention relates to a method and system for producing articles of low density polymer foam, with high melting point.

BACKGROUND INFORMATION For some time, low density polystyrene foam (also known as "atactic" polystyrene foam) has been useful for insulation in the packaging of beverage containers and food containers. However, low density polystyrene products generally have a service temperature limit of about 93.3 ° C. Above the service temperature limit the product will deform and distort. Therefore, there is a general desire for other types of low density foam that do not have such drawbacks. There are polymer resins, for example, poly (ethylene terephthalate) (often referred to as "PET"), polyamides (such as nylon 6.6), and syndiotactic polystyrene, which could be used without presenting such drawbacks. These materials are currently available in the form of solid products, however, the high costs of starting material renders them useless in many applications for reasons of economy. For example, PET is currently used extensively to make many recyclable plastic items, such as bottles. However, attempts to produce low density foam articles from polymers with "high" melting points (ie, more than 232.2 ° C) have proven difficult, and the quality of such foam has been low. The foam articles made from such polymers have experienced crushing and / or severe imperfections in the foam cells. Therefore, the quality of polymer articles with high melting point, low density has not come close to polystyrene. The crushing problem is due to the high foaming temperature that such materials need, for example, PET foams at approximately 248.8 ° C. Using conventional blowing agents at said temperatures, a high rate of expansion is obtained, causing the cell walls to rupture and allowing the gas to escape. Without gas in the foam cells before cooling, the cells can not support themselves. In addition, many such polymer resins are crystalline in nature, and as such, have a lower melt strength compared to atactic polystyrene resins. A person skilled in the art will know that melt strength refers to the ability of the material to stretch at the melting temperature without breaking. The combination of a low melt strength polymer and a vapor pressure greater than the high foaming temperature also requires a reduction in the size of the extrusion die opening where the foam exits. Such small die apertures lead to a thin gauge foam sheet that undergoes severe corrugation at low densities. Corrugation is defined as a sheet with thick and thin alternating bands in the cross machine direction of the sheet. Corrugation is detrimental to the product formed due to the thin gauge areas in such product and partly irregular weight. Thus, there is a need for a method for making a low density, high temperature, high quality polymer foam article that achieves or achieves the quality of the existing polystyrene foam articles.

BRIEF DESCRIPTION OF THE INVENTION Briefly, the present invention satisfies the need for a way to make a quality low density polymer foam article by extruding a sheet of low density polymer foam, increasing the density of the foam sheet by direct contact cooling, and forming an article by, in part, the application of heat to reduce the density of the foam so that the article formed is of low density. In accordance with the above, it is an object of the present invention to provide a way to make a polymer foam article. Another object of the present invention is to provide a way to make a polymer foam article with high service temperature and low density. The present invention provides, in a first aspect, a method for forming a substantially uniform closed cell crystalline polymer foam article. The method comprises the steps of: heating a crystalline polymer resin to a melting temperature so that the resin melts; selecting one or more blowing agents, at least one of the blowing agents with a boiling point higher than the glass transition temperature of the polymer, but less than or equal to the forming temperature; combining the blowing agent (s) with the resin to create a mixture so that a concentration of the blowing agent in the mixture is sufficient to produce a theoretical sheet foam density of less than 0.4 g / cm 3; cooling the mixture to a temperature to achieve a freezing temperature for the mixture, extruding a sheet of substantially uniform closed cell polymer foam with a density of less than 0.4 g / cm 3; from the cooled mixture; cooling the extruded polymer foam sheet by direct contact with a cooling surface at a lower surface temperature than the glass transition temperature, so that the blowing agent (s) are condensed and the polymer foam sheet has a density of more than 0.4 g / cm3; and forming the cooled polymer foam sheet in an article, comprising the application of heat so that the condensed blowing agent vaporizes and the article crystallizes at a density of less than 0.4 g / cm3. The present invention also comprises, in a second aspect, a system for performing the first aspect method. These and other objects, features and utilities of the present invention will be apparent from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a block diagram of the main components of an extrusion system useful with the present invention. Figure 2 is a flow diagram for a general extrusion process with reference to the system of Figure 1. Figure 3 is a cross-sectional view of the extrusion die of Figure 1. Figure 4 illustrates an enlarged portion of the die. of Figure 3. Figure 5 is a flow chart for the method of the present invention.

Figure 6 is a block diagram of an exemplary thermoformer useful with the present invention. Figure 7 illustrates the forming station of Figure 6 in greater detail.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for producing a substantially uniform closed cell polymer foam article from a medium density polymer foam sheet. The term "low density" used in this context refers to a polymer foam of a density less than about 0.4 g / cm 3. The term "medium density" used in this context refers to a polymer foam of a density less than about 0.8 g / cm3 but greater than about 0.4 g / cm3. Figure 5 is a flow diagram for the steps of the basic method. A crystalline polymer resin, such as, for example, polyester resins, such as PET (melting point above 248.8 ° C), syndiotactic polystyrene (melting point above 261.6 ° C), or polyamides, for example, nylon 6.6. (melting point above 260 ° C), it is first heated to such a temperature that it melts, which depends on the resin used, but is above its crystalline melting point. Step 60, "HEATING POLYMER RESIN". Subsequently one or more blowing agents are selected, at least one of the blowing agents has a boiling point higher than the temperature of transition from glass to polymer resin (about 76.6 ° C for PET) and less than the forming temperature, for example, the temperature of a mold per forming tool (The forming temperature depends on the polymer being formed, the forming temperature must be high enough for the solid sheet to crystallize while in contact with the surface of the mold, but low enough so that the sheet does not melt, deform or adhere to the surface of the mold). For PET, the upper limit for the tool temperature apparently is about 204.4 ° C. At temperatures greater than about 204.4 ° C, the article will stick to the mold or be so hot and soft that it will deform after being removed from the mold. The practical lower limit for the mold temperature is about 148.8 ° C, since temperatures lower than this require excessive cycle times to induce crystallinity. Step 62, "SELECT BLOWING AGENT (S)". The subsequently selected blowing agent (s) is combined with the polymer resin to create a mixture. The concentration of the blowing agent in the mixture is such that a theoretical foamed sheet density of less than about 0.4 g / cm 3 will be obtained. Step 64"COMBINE BLOWER AND POLYMER RESIN (S) AGENT". The theoretical density is calculated using the ideal gas law, assuming that the solubility of the blowing agent in the polymer at room temperature is minimal and the internal pressure of gas in the foam reaches an atmosphere. Subsequently the mixture is cooled to a temperature that reaches at the freezing temperature of the blowing agent-polymer mixture to maximize the melt strength of the polymer during actual foaming of the mixture. The blowing agent can act as a plasticizer, depressing the melting point of the polymer. Therefore, this temperature depends on both the polymer and the selected blowing agent, and can be predicted using Flory-Huggins equations or determined experimentally. Typically, the term "arrive" that was used above refers to a temperature difference of about 11.2 ° C from the freezing point of the mixture (but not below the freezing point). However, since not all polymers show the same properties of viscoelasticity with respect to temperature, this temperature range will vary in some way. For PET, a temperature of less than about 265.5 ° C is required, depending on the blowing agent (s) used. Step 66, "COOL MIX". The subsequently cooled mixture is extruded to produce a substantially uniform closed cell polymer foam sheet with a density of less than 0.4 g / cm 3. Step 68, "EXTRUDE FOAM SHEET". To obtain a good quality foam, the blowing agent preferably remains in solution with the resin at least until it enters the die platform, and preferably until it comes out of the die platform. An evidence that this has happened is that a clear halo can be observed at the exit of the die, since the extruded material becomes opaque as soon as the bubbles begin to grow.

Then the extruded polymer foam sheet is cooled by direct contact with a cooling surface, such as a cold mandrel or cylinder, at a surface temperature lower than the glass transition temperature so that the blowing agent (s) are condensed and the polymer foam sheet collapses at a density greater than 0.4 g / cm3. Step 70, "COOL LAMINA BY DIRECT CONTACT". Finally, a useful article, for example, a food container, is then formed from the polymer foam sheet cooled by the application of heat, so that the condensed blowing agent (s) vaporizes and the article crystallizes to a density less than 0.4 g / cm3. Step 72, "FORM ARTICLE". The general method described above will now be analyzed in more detail. For the step of selecting the blowing agent (s) (step 62), the boiling point for at least one of the blowing agents must be greater than the glass transition temperature, so that when the blowing agent is condensed to a liquid, the foam sheet can still be folded and collapsed. If the boiling point is below the glass transition temperature, the foam sheet will freeze as a low density sheet before the blowing agent condenses to a liquid. If this happens, the sheet is self-insulating and will crystallize. Once crystallized, the production of a formed article that is not distorted will be difficult. The boiling point for at least one of the blowing agents must be less than the forming temperature of so that when the article is formed, the blowing agent or agents will vaporize. The vaporization of the blowing agent re-inflates the cells so that the extruded sheet again is a low density material, and the crystallization in the mold "freezes" the form, allowing the low density of the product to be maintained even after the product is cooled and the blowing agent is again condensed to a liquid form. With respect to the step of combining the blowing agent and resin (step 64), the actual concentration of blowing agent in the resin will depend on the blowing agent (s) used. The foam density of the theoretical sheet is calculated based on the ideal gas law (pV = nRT, where p = pressure V = volume, n = molar concentration and therefore depends on the molecular weight of the selected blowing agent , R is a constant and T = temperature). Again, the key assumptions of this calculation are that the concentration of the blowing agent residue in the polymer at room temperature is minimal and that the internal pressure of gas in the cell of the foam sheet before collapse reaches an atmosphere. Actually, most blowing agents apparently have an efficiency of 85% to 90%. That is, only 85% to 90% of the theoretical reduction in density based on the mole fraction of the added gas is realized. Although a theoretical foamed sheet density is less than 0.4 g / cm3 it allows a sufficient amount of gas to expand the foam to create a low density article, the theoretical density is preferably below of 0.3 g / cm3, and very much preferably below 0.2 g / cm3. The lower the foam density of the sheet, the better the material's strength to weight ratio will be, typically improving while the density is reduced. Because of this, low density products will usually weigh less and therefore have a more economical production. With respect to the first cooling step (step 66), a cooling temperature reaching the freezing point for the blowing agent-polymer mixture (less than 265.5 ° C for PET and less than 243.3 ° C for syndiotactic polystyrene) is It was chosen because it has been observed, at least for PET and syndiotactic polystyrene, that the melt strength of the extrudate is sufficient to produce a foam of substantially closed cells. It is believed that similar results have been observed for other crystalline polymers of high melting point. At higher temperatures, a low melting strength allows the cell walls to break, thus creating an open cell foam. An open cell foam will not retain the blowing agent and subsequently will not expand in the forming operation to produce a low density foam. With respect to the second cooling step (step 70), the extruded polymer foam sheet is "freezing" at the glass transition temperature due to direct contact with the cooling surface. However, the blowing agent condenses before reaching this temperature. When gas vapor pressure is lost from the cells of foam, a collapse is observed, which improves heat transfer and allows the sheet to cool in an amorphous state. In this way, cooling the extruded polymer foam sheet substantially increases its density and prevents crystallization. For PET, the surface temperature of the cooling surface is less than 76.6 ° C, the glass transition temperature of PET, and preferably less than 48.8 ° C. The step of forming an article (step 72) can be achieved, for example, by thermoforming. The heat necessary to vaporize the condensed blowing agent or agents can be provided, for example, by a mold or a furnace. Applied heat causes the blowing agent (s) to boil again, while at the same time softening the polymer foam sheet and allowing it to expand to a lower density sheet again. By boiling the blowing agent the foam sheet is expanded to a low density, and it keeps the material at a temperature that optimizes the polymer rate, the crystallization allows the article to crystallize and thus become a stable shape with a density of 0.4 g / cm3. Instead of trying to produce a sheet of low density polymer foam from which a useful article is made, the present invention increases the density of the foam sheet, but allows the heat of the forming process to decrease the density, producing a low density item. The second cooling step of the present invention (cooling the extruded polymer foam sheet) ensures that the foam sheet does not crystallize. The heat transfer effective from the extruded polymer foam sheet to the cooling surface allows the sheet to be cooled in an amorphous state, with crystallinity typically less than 10%, but always less than 15%. This level of crystallinity in the sheet can be formed as described above to produce a low density article. An article formed in this manner has a crystallinity of more than about 20%, as shown in the data of the examples below, and is structurally sound at high usage temperatures. It has been observed, at least with PET, that when the sheet has a crystallinity of more than 20% before being formed, some form of memory remains in the article and the article will deform under oven conditions, even when the crystallinity of the product it is such that such deformation would not be expected. Similar results are expected for other crystalline polymers with higher melting point. Now, a series extrusion process which is useful for the present invention will now be described with reference to figures 1 to 3. However, it will be understood that there are other extrusion processes that may also be used, and this is merely a given example to give context to the invention. Figure 1 is a block diagram of the main portions 10 of the machinery used in a series extraction process. The main portions 10 include a dryer with desiccant material 11, a primary extruder 12, a secondary extruder 14 and a die 16. One skilled in the art will understand the operation of the portions main. For example, polyesters are hydroscopic and should be dried to a dew point of -28.8 ° C or less, as they undergo severe degradation in the melting phase in the presence of water. Therefore, the first step for polyesters or other hydroscopic polymers is to dry the starting materials using the dryer with desiccant material 11. Other materials, for example, syndiotactic polystyrene, are not hydroscopic and do not need to be dried. Generally, heating of the solids 13 to be extruded (a polymer) and mixing with the blowing agent 15 is achieved in a primary extruder 12. The cooling of the mixture is carried out in a secondary extruder 14. Finally, the cooled mixture it is fed to die 16 for foaming. Fig. 2 is a flowchart for the extrusion process of Fig. 1. The starting materials, if they are hydroscopic, are first dried at a dew point of 28.8 ° C or less, and preferably at a point of condensation of less than -40 ° C (step 18, "DRY STARTING MATERIALS"). The starting materials are subsequently fed to the primary extruder 12 (step 20, "SUPPLY OF STARTING MATERIALS"). The starting materials will generally comprise a mixture of virgin polymer, recovered polymer generated in the manufacture of colorants, stabilizers, nucleators, flame retardants, plasticizers, stiffeners and possibly other additives. Although the ratios of the additives can vary greatly, in general the virgin polymer and the recovered polymer they constitute about 90% by weight or more of solid feed. The starting materials may be fed to a primary extruder 12 by volumetric or gravimetric feeders, and a mixer may or may not be used to homogenize the mixture before feeding. Frequently, the primary extruder is fed by overflow, that is, there is a constant supply of starting material directly at the inlet of the extruder or feed throat, although other types of feed are practiced. After feeding the starting materials to the primary extruder 12, they are compressed and heated to melt them (step 22, "COMPRESS AND HEAT"). After melting the starting materials, the molten material is pressurized (step 24, "PRESSURIZE THE MIXTURE"). Typical pressures vary from 150 atm to about 350 atm. After pressurizing the molten material, a blowing agent (s) (eg, hydrocarbons, haiohydrocarbons, and / or inert gases) are injected into a primary extruder 12. The pressure may temporarily be reduced to aid injection. The molten starting materials and the blowing agent are then mixed to create a homogeneous mixture before leaving the primary extruder 12 (step 26, "MIXING WITH BLOWING AGENT"). For example, the mixing can be distribution or dispersion, any method that is better suited to the selected blowing agent / polymer system. After injecting the blowing agent and combining with the molten starting materials, the mixture is generally too hot ^^ to foam. When the mixture is too hot, the viscosity is low, and if foaming is attempted, the blowing agent will expand the cells within the foam too fast, reaching a cell wall break and crushing the foam. On the other hand, if the mixture is too cold, the blowing agent will have insufficient potential energy to expand the mixture into a foam. In addition, there is a danger in the process that the mixture can be frozen in the process, which can especially occur with crystalline polymers. Therefore, it is necessary to keep precise control of the foaming temperature to ensure a good quality foam. The cooling of the mixture is achieved in the secondary extruder 14 (step 28, "COOL MIX"). The secondary extruder is usually larger than the primary extruder to maximize the amount of surface area for heat transfer. The heating under shear of the mixture is minimized through various designs for the screw of the secondary extruder, which provides a renewal of continuous surface to the cylinder. Without this renewal, the mixture on the surface of the extruder cylinder will freeze and isolate the rest of the mass, which could pass through the secondary extruder without cooling. Usually, the extruder screw in the secondary extruder operates at a rotation speed much lower than that of the primary extruder, to reduce heating under shear stress. The particular design of screw used can affect the pressure of the mixture.

The cooled mixture is then supplied to die 16 for foaming (step 30, "FOAM MIX"). The purpose of the die principle is to shape the polymer, while maintaining the pressure to ensure that the blowing agent will not prematurely separate from the mixture. Ideally, the blowing agent remains in the mixture until it leaves the die. The design of the die determines the width and thickness of the extruded foam. The combination of the separation speed of the sheet and the production determine the base weight of the produced sheet (g / 0.09m2). Figure 3 is a cross-sectional view of an exemplary embodiment of the die 16. The cooled mixture enters die 16 through openings 32 and 34. The temperature and pressure of the mixture while inside the die body 16 is verified by the instruments (not shown) connected in the intermediate connections 36 and 38. The female die flange 40 and the male die flange 42 define a die separation 43 (shown more clearly in Figure 4). The centering piece section or breaker plate 44 holds the male die flange 42 in place. Ideally the foaming is carried out when the polymer mixture exits through the die gap 43 and the blowing agent subsequently vaporizes. However, the foaming may begin in the arrival area 46 (called "pre-foaming"). When pre-foaming occurs, the result is often a less desirable surface finish and an irregular cell structure. To affect the die pressure, die separation can increase or decrease by various mechanisms. An example of such a mechanism is the screw 47 shown in Figure 3. Next, an exemplary thermoforming apparatus useful with the present invention will be described with reference to Figure 6. A foamed sheet roll is placed on a roll stand or unwinder 80 and fed into the forming oven 84 by a holding point 82. The holding point guides the foam roll and consists of two rubber rolls with hydraulic cylinders which move the rollers separating and joining them as to form a point of attachment. When the forming controller 85 attracts the sheet, the fastening points are joined and the sheet is fed to the oven in a base as required. The forming controller 85 is programmed to control the thermoforming process. The sheet enters the oven 84 on two chain rails mounted on either side of the oven. The rails separate the sheet through the oven towards the forming station 86. The sheet is heated in the oven by means of heating elements. Various types of elements are used in such procedures, but electric resistance type heaters are preferably used with the present invention. In the furnace, the temperature is controlled so that the sheet is heated to approximately the glass transition temperature of the polymer foam. If the sheet is colder than this, it may not be removed uniformly in the forming station and "cold cracks" may appear in the product. However, if the temperature of the sheet is in excess of the glass transition temperature, then the sheet can begin to crystallize before being formed, producing an undesirable memory to the part; that is, the part formed to some degree will deform during the use for which it was created trying to return to being a flat sheet. Once properly tempered, the sheet enters the forming station 86. Figure 7 is a more detailed drawing of that station. After the chain rails position the plate in position, the plates 90 and 92 close, joining the mold halves 94 and 96. In some cases it may be useful to close one plate slightly before the other, or to use closing speeds Differentials, to extract more uniformly the sheet in the shape of the mold. The mold consists of a male half 94 and a female half 96. These types of molds, also known as "coupling tools", are essentially identical, except that the male half is machined with misalignment with respect to the female half, creating a seperation. This separation is the cavity where the sheet is formed. The separation of the plates is adjusted to produce the desired thickness of the part as determined by this separation. For the polyester, the mold is heated by heating elements 98 to a crystallization temperature of about 176.6 ° C. A formation time or contact time with the mold of about 4 to 6 seconds is generally sufficient to induce a crystallinity of more than about 20% for PET. To help the formation of the part and to induce a higher heat transfer, a vacuum is applied to the mold just after the closure to help extract material from both surfaces. Just as the mold is opened, a slight amount of air is applied to release the vacuum and help eject the formed part of the mold without adhering. The formed part, still attached as part of the sheet, subsequently leaves the forming station. The formed part then enters the cutting press 88. A die is used to separate the part of the sheet. The formed parts, once cut, are collected for packing. The "skeleton" or portion of the sheet remaining after the parts have been removed is then milled into flakes and transported by air to a silo where it is collected for reprocessing. Typically, a cutting press will make adjustments for the record; that is, adjustments can be made to ensure that the cut close to the perimeter of the formed part is uniform. Although the given data of the examples are for the polyester, similar results are expected for other crystalline polymers with high melting point, for example, syndiotactic polystyrene and nylon 6.6.

EXAMPLE 1 Operating conditions for making a polyester sheet foam are illustrated below in table 1. As can be seen in the results, the material extruded immediately upon leaving the die formed a low density foam with a density of about 0.20 g / cm3, but with a matter of several seconds of contact time with the cooling mandrel collapsed to a density of about 0.49 g / cm3. However, during the formation, the blowing agent was again heated above its boiling point, resulting in a low density foam. The action of crystallization "freezes" the part in a low density geometry. In this case, a cake mold configuration with a diameter of 20.32 cm was formed with an average density of about 0.19 g / cm3. Upon cooling, this crystallized cake mold configuration did not collapse or subsequently distort. As a result of the collapse of the foam, the measured crystallinity of the foam sheet was only about 8%. During the training, the crystallinity increased to about 32%. The resulting cake mold configuration was dimensionally stable in an oven environment and performed its intended function well (baking a cake).

TABLE I Temperature Cooling extruder, 2.5"480 ° C Die temperature 494 ° C Die flange 501 ° C Fusion of crossing 562 ° C Die Fusion (estimated) 490 ° C Pressures Injection pressure 156 atm Pressure, 2"320 atm Input pressure of the gear pump 110 atm Cross pressure 250 atm Die pressure 73 atm AMP drive conditions, 2"30 amps Speed, 2"127 rpm Gear pump AMP 2.6 amps Gear pump speed 40 rpm AMP, 2.5"10 amps Speed, 2.5"32.1 rpm Formulation Type of polymer shell 2928 Polymer speed 61.74 kg / hr Type of shell nucleator 10480 Nucleator speed 1.04 kg / hr Type of blowing agent n-heptane Blowing agent speed 1.08 kg / hr Test data Thickness (when leaving the die) 2159 micras Thickness (final) 889 microns Base weight 40.8 g / 0.09 m2 Density (when leaving the dice) 0.20 g / cm3 Density of the sheet 0.49 g / cm3 Density of formed part (average) 0.19 g / cm3 Crystallinity of sheet 8% Crystallinity of the part 32% EXAMPLE 2 The following are operating conditions for the production of a foamed polyester sheet in table 2. As can be seen in the results, a sheet of low density foam was formed with a density of approximately 0.27 g / cm3. A slight collapse may occur during the cooling process, but the density of the sheet leaving the die is essentially the final density of the foam. During the formation, the foam expanded in a traditional sense, due to the infiltration of air, the expansion of the sheet being of the order of about 50% to 100% of the original thickness. The expansion resulted in a final part with a thickness of about 2184.40 microns and an average density of about 0.15 g / cm3. Again, a cake mold configuration of 20.32 cm in diameter was formed. However, in this case, the definition of part was scarce and the considerable dimensional distortion was measured in the finished part. While the measured crystallinity of the foam sheet was about 28%, the formed one only increased the crystallinity to about 32%. Apparently the part retains a memory of its geometry when it crystallized originally. The resulting part was not dimensionally stable in an oven environment. The flattened part during the oven test and bake test then failed completely.

TABLE II Temperature Cooling extruder, 2.5"490 ° C 550 ° C die temperature Die flange 496 ° C Fusion of crossing 500 ° C Fusion of die (estimated) 500 ° C Pressures Injection pressure 150 atm Pressure, 2"239 atm Input pressure of the gear pump 111 atm Cross pressure 225 atm Die pressure 43 atm AMP drive conditions, 2"26 amps Speed, 2"131 rpm Gear pump AMP 2 amps Gear pump speed 40 rpm AMP, 2.5"10 amps Speed, 2.5"31.7 rpm Formulation Type of polymer Shell 2928 Polymer speed 52.29 kg / hr Shell type 10480 nucleator Nucleator speed 1.36 kg / hr Type of blowing agent C-pentane Blowing agent speed 1.18 kg / hr Test data Thickness (final) 1397 microns Base weight 36.2 g / 0.09 m2 Density of the sheet 0.27 g / cm2 Density of formed part (average) 0.15 g / cm3 Crystallinity of film 28% Cstability of the part 32% While various aspects of the present invention have been described and illustrated in the present detailed description, alternative aspects can be realized by those skilled in the art to achieve the same objectives. Thus, in the appended claims it is intended to cover all the alternative aspects while they are within the spirit and real scope of the present invention.

Claims (30)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for forming a substantially uniform closed cell crystalline polymer foam article comprising the steps of: heating a crystalline polymer resin to a melting temperature so that the resin melts; selecting one or more blowing agents, at least one of which has a boiling point greater than a glass transition temperature for the resin and less than or equal to the forming temperature; combining the blowing agent (s) with the resin to create a mixture so that a concentration of the blowing agent (s) in the mixture is sufficient to produce a theoretical sheet foam density of less than 0.4 g / cm 3; cooling the mixture to a temperature that reaches the freezing temperature for the mixture; extruding a sheet of substantially uniform closed cell polymer foam with a density less than 0.4 g / cm3 of the cooled mixture; cooling the extruded polymer foam sheet by direct contact with a cooling surface at a surface temperature less than the glass transition temperature so that one or more blowing agents condense and the polymer foam sheet has a density of more than 0.4 g / cm3; and forming the cooled polymer foam sheet in an article comprising the application of heat so that the blowing agents condensates vaporize and the article crystallizes at a density of less than 0.4 g / cm3.

2. The method according to claim 1, wherein the crystalline polymer resin comprises a syndiotactic polystyrene resin.

3. The method according to claim 1, wherein the crystalline polymer resin comprises a polyester resin.

4. The method according to claim 3, wherein the polyester resin comprises poly (ethylene terephthalate).

5. The method according to claim 4, wherein the melting temperature is higher than about 249 ° C, wherein the glass transition temperature is about 77 ° C, and wherein the forming temperature It is about 177 ° C.

6. The method according to claim 4, wherein the forming step comprises thermoforming with a mold heated to a mold temperature approximately between 149 ° C and 204 ° C.

7. The method according to claim 6, wherein the mold temperature is between approximately 177 ° C and 191 ° C, and wherein the forming step further comprises allowing a contact time with the mold so that the The formed article has a crystallinity of more than about 20%.

8. The method according to claim 1, wherein the surface temperature of the cooling surface that makes contact with the extruded sheet is less than about 49 ° C.

9. The method according to claim 1, wherein the cooling step of the extruded polymer foam sheet comprises cooling with a mandrel.

10. The method according to claim 1, wherein the cooling step of the extruded polymer foam sheet comprises cooling with a cooling cylinder.

11. The method according to claim 1, wherein at least one of the blowing agents comprises heptane.

12. The method according to claim 11, wherein the concentration of heptane in the mixture is greater than about 1.2% by weight.

13. The method according to claim 12, wherein the concentration of heptane in the mixture is greater than about 1.6% by weight.

14. The method according to claim 1, wherein the extruded polymer foam sheet has a crystallinity of less than 15%.

15. The method according to claim 14, wherein the extruded polymer foam sheet has a crystallinity of less than 10%.

16. The method according to claim 1, wherein the formed article has a crystallinity of more than 20%.

17. - The method according to claim 1, wherein the polymer resin comprises one or more additives.

18. The method according to claim 17, wherein the additive or additives comprise one or more nucleation agents of cell size, a crystallization nucleation agent, a stiffener, a flame retardant, a dye and a plasticizer. .

19. An apparatus for forming a substantially uniform closed cell crystalline polymer foam article, comprising: means for heating a crystalline polymer resin to a melting temperature so that the resin melts; means for combining one or more selected blowing agents, together with the resin to create a mixture such that a concentration of the blowing agent (s) in the mixture is sufficient to produce a theoretical foam sheet density of less 0.4 g / cm3 , wherein at least one of the blowing agent (s) has a boiling point greater than the glass transition temperature of the resin and less than or equal to the forming temperature; means for cooling the mixture to a temperature that reaches a freezing temperature for the mixture; means for extruding a sheet of substantially closed closed cell polymer foam with a density of less than 0.4 g / cm 3 from the cooled mixture; means for cooling the extruded polymer foam sheet by direct contact with a cooling surface at a surface temperature less than the glass transition temperature so that the blowing agent (s) condenses and the polymer foam sheet has a density of more than 0.4 g / cm3; and means for forming the cooled polymer foam sheet in an article, comprising means for applying heat so that the condensed blowing agent (s) vaporizes and the article crystallizes at a density of less than 0.4 g / cm3.

20. The apparatus according to claim 19. further characterized in that the crystalline polymer resin comprises a syndiotactic polystyrene resin.

21. The apparatus according to claim 19. further characterized in that the crystalline polymer ream comprises a polyester resin.

22. The apparatus according to claim 21, wherein the polyester resin comprises poly (ethylene terephthalate).

23. The apparatus according to claim 22, wherein the melting temperature is higher than 249 ° C, wherein the glass transition temperature is about 77 ° C, and wherein the forming temperature is close to of 177 ° C.

24. The apparatus according to claim 19, wherein the cooling means comprise means for obtaining a cooling surface temperature of about 49 ° C.

25. The apparatus according to claim 19, wherein the means for cooling comprises a mandrel.

26. The apparatus according to claim 19, in wherein the means for cooling comprises a cooling cylinder.

27. - The apparatus according to claim 19, wherein the forming means comprises a tempered mold at a mold temperature of approximately 149 ° C to 204 ° C.

28. The apparatus according to claim 27, in mold temperature is approximately between 177 ° C and 191 ° C and wherein the forming means additionally comprise means for controlling the contact time of the mold, so that a Article can be formed with a crystallinity greater than 20%.

29. The apparatus according to claim 19, wherein the means for extruding comprise means for extruding a sheet of substantially uniform closed cell polymer foam having a crystallinity of less than 15%.

30. The apparatus according to claim 19, wherein the means for extruding comprise means for extruding a sheet of substantially uniform closed cell polymer foam having a crystallinity of less than 10%.