MXPA99010731A - Method of recycling polyester foam - Google Patents

Abstract

PET foam is processed (24) into flakes. The flakes are densified (26) in a pellet mill at between about 300°F and about 350°F to produce pellets. The pellets are then dried (28) in a desiccant dryer at about 350°F for about 6 hours, so that the dew point of the pellets reaches about - 40°F. The dried pellets have an intrinsic viscosity about equal to virgin PET and a crystallinity greater than 20%. This material can be reused (30) as if it were virgin PET.

Description

METHOD FOR RECYCLING POLYESTER FOAM BACKGROUND OF THE INVENTION TECHNICAL FIELD The present invention relates generally to recycling. More particularly, the present invention relates to a method for recycling polyester in the form of foam.

BACKGROUND INFORMATION In the past, low density polystyrene foam has been discovered as useful for insulation, packaging, beverage cups and food containers. However, polystyrene foam and the extrusion process to make it have been associated with undesirable environmental problems, without considering whether those problems have their origin in facts. In addition, polystyrene products in general have a service temperature limit of approximately 93.3 ° C. Above the service temperature limits, the product will buckle and distort. Therefore, there is a general desire for other types of low density foam that are not associated with such problems. There are polyester resins, such as PET (poly (ethylene terephthalate)), which could be used without such associated problems. PET is widely used today to make many recyclable plastic items, such as soft drink bottles. In the production of polyester foam articles, such as, for example, food service containers, the first step is typically to produce a polyester foam sheet, for example, by extrusion. This sheet is then thermoformed into the desired article. By doing so, the excess material around the parts (the "skeleton"), sheet residues and waste parts are reprocessed and fed back into the extrusion process so that no starting materials are wasted. Initial attempts to incorporate the polystyrene foam waste directly back into the sheet extrusion process had limited success. Due to the very low volumetric density of the waste, the extruder had to be either very large in relation to the performance, or running at very high speeds. Both scenarios result in high cutting speeds that lead to excessive degradation of the polymer and unstable process conditions. This method of direct incorporation, if taken with PET foam swirled material, could be fatal. The high shear rates could degrade the polymer in such a way that the intrinsic viscosity would no longer be sufficient to withstand foam production.

To solve the problem in conventional foam processing, a separate repelletization line has been incorporated into the total procedure. The repelletization line is designed to produce a high density pellet from waste, and operates at relatively low yields and low shear rates to minimize product degradation. Unfortunately, this operation is not well suited for polyester foam regrind material. Polyester in general is more sensitive to cutting than other foam polymers, such as, for example, polystyrene. However, the biggest challenge is to dry the reeled material before processing it. In PET processing, for example, the polymer must be dried to a dew point of about -39.9 ° C. Even small amounts of water cause excessive degradation. The problem is that with the low volumetric density of the material being blown, the size of the dryer becomes cost prohibitive and in fact is so large that it is still difficult to ensure drying and / or even flow through the dryer. Therefore, there is a need for an improved method for recycling polyester foam.

BRIEF DESCRIPTION OF THE INVENTION Briefly, the present invention satisfies the need for an improved method for recycling polyester foam by increasing the density of the foam, for example, by compressing it mechanically. The increased density is accompanied by a larger molecular surface area than that of unfoamed polyester of equal weight. The increased surface area results in a significant increase in intrinsic viscosity during subsequent drying, before being reused in sheet manufacture. According to the above, it is an object of the present invention to provide a method for recycling polyester foam. It is another object of the present invention to provide a method for recycling polyester foam that requires little or no drying time of the foam before densification. It is still another object of the present invention to provide a method for recycling polyester foam that increases the intrinsic viscosity of the polyester foam. The present invention provides a method for recycling polyester foam. The coast method of the steps of densifying a given type of polyester foam such that when in a densified form, the polyester foam has a larger molecular surface area than that of unfoamed polyester of equal weight of the given type; and drying the densified polyester foam so that the intrinsic viscosity thereof increases. These, and other objects, features and advantages of this invention will be apparent from the following detailed description of the various aspects of the invention taken in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified diagram of a pellet mill useful with the present invention. Fig. 2 is a partial cross-sectional view of the pellet mill of Fig. 1. Fig. 3 is a block diagram of the recycling process of the present invention. Figure 4 is a block diagram of a production line embodying the present invention. Figure 5 is a block diagram of an extrusion equipment useful in the production line of Figure 4. Figure 6 is a flow diagram for the operation of the extrusion equipment of Figure 5.

BEST WAY TO CARRY OUT THE INVENTION Fig. 1 is a simplified diagram of a pellet mill 10 useful with the present invention, and Fig. 2 is a partial cross-sectional view thereof. Such pellet mills are commercially available in many shapes and sizes from various companies. Pelletizing machines are used for a number of different objects, such as, for example, producing animal feed and producing coal pellets from coal dust. Pellet mills have also been used in the carpet industry to press polyester fibers together without destroying them. The pellet mill 10 includes a feeder conveyor 12, an evacuation hose 13, given 14, rollers 16 and 18, and measuring knife 20. The pellet mill used in conjunction with experiments for the present invention had a die with holes of 0.31. cm in diameter by 3.81 cm in depth. However, it will be understood that other pellet mills, as well as other types of densifying machines could be used. However, the densifying machine used is preferably able to densify the polyester foam without melting it, so that the densified polyester foam has a larger molecular surface area than that of unweighted polyester of equal weight (eg virgin polyester) . The polyester foam used was PET foam waste from a thermoforming process, however, it will be appreciated that other types of polyester foam could be used, and the foam need not be residue. The foam to be recycled could also be, for example, used polyester foam containers that have been properly cleaned or imperfect polyester foam containers rejected during manufacture. A general description of the basic operation of the pellet mill 10 will be given below. The feeder conveyor 12 transports ground material (in this case foam) to an evacuation sleeve 13. In the evacuation sleeve, gravity causes the material to fall into the mill. The die 14 compresses the material by rotating the rollers 16 and 18. That is, the die 14 rotates about its axis as the rollers 16 and 18 rotate about their axes. No other movement happens. This action forces the material into the holes 22 in the die. The measuring knife 20 is used to cut the compressed material that is forced outwards, forming pellets. Prior to the pelletization investigation, it was expected that the density of the PET foam would increase, at the expense of some intrinsic viscosity. As one skilled in the art will know, "intrinsic viscosity" refers to an indirect measure of molecular weight. However, the investigation revealed that loss in intrinsic viscosity was not carried out due to densification. More surprising was the result that, when the densified PET foam dried, the intrinsic viscosity increased dramatically. It has been the experience of several PET manufacturers that while typical PET resins are unable to form a stable foam structure, high molecular weight PET, typified by high intrinsic viscosities, can produce medium and low density foam products stable In the manufacture of PETIt is known that the intrinsic viscosity can be increased by a process known as "solid state formation". In the solid state deformation process, the virgin PET pellets are maintained at temperatures of 204.4 ° C for a period of about 24 hours. Small amounts of unreacted sites in the PET react, producing an increase in intrinsic viscosity from, for example, about 0.7 to 0.8 dl / g at 1.1 to 1.3 dl / g. Solid state formation is a slow process and explains the additional cost of producing high intrinsic viscosity polyester. A disadvantage of the solid state forming process is that an increased intrinsic viscosity is related to the particle surface area, which is relatively small for commonly available solid PET pellets. As will be shown from the data of the examples below, the volumetric density of the PET foam residue increased from typical values of between 96.11 to 160.18 grs / lit to values of between 480.56 to 640.75 grs / lit. The intrinsic viscosity of the PET waste foam was between 0.8 to 0.85 dl / g, with the densified PET foam without showing loss in intrinsic viscosity. When the PET pellets were dried in a desiccant dryer at 176.6 ° C, the intrinsic viscosity was increased to 1.2 dl / g in only 6 hours of time. As one skilled in the art will know, a "desiccant dryer" removes moisture, allowing the material to dry to a dew point of -39.9 ° C or less.

A detailed description of the preferred embodiment of the present invention will be given with reference to the block diagram of figure 3. The following description will be given with respect to PET waste foam, and using a pellet mill, such as that of the figures 1 and 2. However, it will be appreciated that other types of polyester foam could be used, although it is not preferred to mix different types, and other densifying machines could be used. The PET waste foam is first processed into flakes, by, for example, using a foil mill. Step 24, "PROCESSING OF PET FOAM IN SCALES". At this point the scales could be stored until needed. The PET foam flakes are then densified in pellets, for example, using the pellet mill 10 of figures 1 and 2. Although the densification of PET foam can take place at temperatures between 93.3 ° C and 204.4 ° C, Larger densities with better cohesion of the pellets are achieved at temperatures between 135 ° C and 190.5 ° C, and preferably between 148.8 ° C and 176.6 ° C. In any case, the densification must take place at a temperature below the melting point of the polyester foam used. Step 26, "DENSIFICATION OF PET FOAM FLAKES IN PELLAS". At this point, the pellets could be sorted by size, and the smaller sized particles (called "powders") can be removed and run again through the densification procedure in such a way that no material is lost. One way to densify is to compress the flakes, preferably at a bulk density greater than 0.3 g / cm3, and more preferably larger than 0.5 g / cm3. The PET foam pellets are then dried, preferably in a desiccant dryer, at less than 190.5 ° C and preferably at approximately 176.6 ° C for less than 6 hours and preferably approximately 4 hours, so that the dew point of the pellets reach around -39.9 ° C. After drying, the pellets preferably have an intrinsic viscosity greater than 0.95 g / dl. Step 28, "PELAS DRY." Optionally, the dried pellets can then be reused in a thermoforming process, such as, for example, an extrusion process. Step 30, "REUTILIZATION OF PELAS IN EXTRUSION PROCEDURES". Figure 4 is a block diagram of an illustrative manufacturing line 32 in which the present invention is incorporated. The mills 34 and 36 are sheet mills used to convert rejected or degraded parts (mill 34) and foam waste sheet or skeletons (mill 36) into foam flakes. The flakes are then transferred to a lint bin 38. The flakes are then sent through the conveyor belt 40 to the pellet mill 42 for pelletizing. The pellets produced are then sorted by a size 44 sorter according to size, and dried by a desiccant dryer 46. After drying, the pellets are ready to be used in, for example, an extrusion process using machinery extruder 48 Sample data will be presented right away. Although specific polyester resins are noted, it will be understood that other polyester resins could be used with the present invention.

EXAMPLE 1 Foam sheet was produced using shell polyester resin "TRAYTUF 2928" with hydrocarbon blowing agent. The sheet was ground into flakes and pelletized by the procedure described, using a California pellet mill with a die of 30 centimeters in diameter. The die had holes of 0.31 centimeters and was 3.81 centimeters thick. The temperature of the die during the pelletizing operation was in the range of 160 ° C to 171.1 ° C. The pellets were subsequently dried at 176.6 ° C for 6 hours. The following data was collected: V.l. Crystallinity (%) density (g / cm3, bulk) Foam scale 0.834 11.0 approx. 0.12 Pellas 0.828 30.6 approx. 0.61 Pellets (after drying) 1.16 virgin resin 1.20 (typical) 0.7 (typical) In this example, the pellets produced had a volumetric density close to that of the virgin resin and did not require additional density increase for subsequent processing in foam sheet . V.l. (intrinsic viscosity) of the pellets produced (before drying) was within the experimental error of being identical to V.l. of the flake before processing. In addition, the crystallinity of the pellets was 30.6%, showing a dramatic increase on the scale and thus eliminating the additional need to recrystallize the pellets before drying. The drying operation yielded an increase in V.l. of more than 0.3 dl / g, which is not expected when solid pellets dry during this duration at the set temperature. The repel (ie, the repelletized polyester foam) produced was further processed back to sheet form without any apparent negative product effect.

EXAMPLE 2 Foam sheet was produced using shell polyester "TRAYTUF 2928" with a hydrocarbon blowing agent. The sheet was ground into flakes and pelletized by the procedure described, using a California pellet mill with a die of 40.64 cm in diameter. The die had 0.31 cm diameter holes and was 3.81 cm thick. The temperature of the die during the pelletizing operation was in the range of 115.5 ° C to 132.2 ° C.

V.l. Crystallinity (%) Density (g / cm3, (dl / g) bulk) Scale 0.894 9.7 Approx. 0.12 foam Pellas 0.900 29.2 Approx.0.32 0.898 foam sheet (100% virgin) 0.875 foam sheet (50% repel) This example again illustrates the increase in crystallinity achieved by the method of pelletization and again shows that V.l. of the pelllas was, in essence, identical to the starting foam flakes. In this example, a lower operating temperature produced less increase in density but the pellets could still be adequately extruded without adverse procedural effects. In addition, V.l. of the final sheet, produced of 48% of repeted polyester, was compared to the produced sheet of 96% virgin resin (the remainder being a blowing and nucleating agent). The results indicate that V.l. of both materials again it was identical, that is, the repelletized polyester showed no deterioration in yield due to the history of previous processing. EXAMPLE 3 Polyester foam flakes were dried using a desiccant dryer and processed on a twin screw extruder at a melting temperature of approximately 271.1 ° C. The following data was collected: V.l. Crystallinity (%) Density (g / cm3, (dl / g) bulk) Foam scale 0.834 11.0 Approx. 0.12 Pellets 0.738 Approx.7 Although the density by this method was increased to the virgin resin value, a substantial fall in V.I was observed. Although the V.l. it was not measured after subsequent drying, the material would not support a stable foam formation and a collapse was observed when it was extruded with 50% virgin polyester resin. This example is used to illustrate the failure of conventional technology to produce a usable pellet from foam flakes. It can be seen from the foregoing that the method of the present invention increased the intrinsic viscosity of the PET pellets much faster and at lower temperatures than the solid state formation. One possible explanation for this phenomenon is that although the PET residue foam (after scale formation) was compressed to a high volumetric density, the individual flakes, although their cellular structure was destroyed, still existed. As a result, the pellets at a molecular level have a very large surface area. This large surface area allows the intrinsic viscosity to increase quickly at a relatively low temperature. The practical value of this phenomenon is that the intrinsic viscosity of the PET waste foam can be increased to that of the virgin PET resin using the normal drying process. No additional equipment or processing was required and the final product will not deteriorate with performance due to degradation of the waste or the amount incorporated. An additional benefit is that the material that comes out of the pellet mill was found to be crystallized, whether or not the material fed to the pellet mill was crystallized. This eliminates the need to crystallize before drying. With reference to Figures 5 and 6, a general mass extrusion process will be described below which is useful with the present invention. It will be understood, however, that there are other extrusion methods that could also be used, and this is simply a given example in order to put the invention into context. Figure 5 is a block diagram of the main portions of extrusion machinery 48 used in a serial extrusion process beyond the dryer 46 in Figure 4. The main portions include a primary extruder 50, a secondary extruder 52 and a given 54. Someone with ordinary skill in the art will understand the operation of the main portions. In general, the melting of the solids from the dryer 46 to be extruded (a polymer) and the mixture with the blowing agent 56 are achieved in the primary extruder 50. The cooling of the mixture is carried out in the secondary extruder 52. Finally, the cooled mixture is fed to die 54 for foaming.

Figure 6 is a flowchart for the extrusion process of Figure 5. The dry starting materials, including any additives, are first fed to the primary extruder 50 (Step 58, "FEEDING STARTING MATERIALS"). The starting materials will generally consist of a mixture of virgin polymer, recovered polymer generated in manufacture, colorants, stabilizers, nucleators, flame retardants, plasticizers, and possibly other additives. Although the addictive relationships can vary greatly, generally the virgin polymer and the recovered polymer constitute approximately 90% by weight or more of the fed solid. The starting materials may be fed to the first extruder 50 by volumetric or gravimetric feeders and may or may not use a mixer to homogenize the mixture before being fed. Often, the primary extruder is flooded with food; that is, there is a constant supply of starting material directly over the intake of the extruder or feed throat, although other types of feeding are practiced. After the starting materials are fed to the primary extruder 50, they are compressed and heated to melt them (step 60"COMPRESSION AND HEAT"). After melting the starting materials, the casting is pressurized (Step 62"FUSED PRESSURE"). Typical pressures are in the range of 150 atm to 350 atm. After pressurizing the melt, a blowing agent or agents (eg, hydrocarbons, halohydrocarbons and / or inert gases) are injected into the primary extruder 50. The pressure may be temporarily reduced to aid in the injection. The molten starting materials and the blowing agent are then mixed to create a homogeneous mixture before leaving the primary extruder 50 (step 64, "MIXING WITH BLOWING AGENT"). The mixture can be distributive or dispersive, depending on the solubility of the selected blowing agent. After injecting the blowing agent and combining it 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 would expand the cells within the foam too quickly, leading to cell wall breakdown and foam collapse. If, on the other hand, the mixture was too cold, the blowing agent would have insufficient potential energy to expand the mixture into a foam. Precise control of the foaming temperature is therefore needed to ensure good foam quality. The cooling of the mixture is achieved in the secondary extruder 52 (Step 66, "MIXING ENFORCEMENT"). The secondary extruder is usually larger than the primary extruder to minimize the amount of surface area for heat transfer. The heating cut of the mixture is minimized through various designs for the screw of the secondary extruder, which provides a continuous surface renewal. Without this renovation, the mixture on the surface of the barrel of the extruder would freeze and isolate the rest of the mass, which would pass through the secondary extruder without cooling. Normally, the barrel of the extruder in the secondary extruder operates at much lower revolutions than those of the primary extruder, to reduce the cut by heating. The particular screw design used can affect the pressure of the mixture. The cooled mixture is then supplied to die 54 for foaming (Step 68, "FOAM MIX"). The main purpose of the die is to form the polymer in a form, while maintaining the pressure to ensure that the blowing agent does not separate from the mixture prematurely. Ideally, the blowing agent remains in the mixture until it leaves the die. The design of the die determines the shape / thickness of the extruded foam. After the foam is extruded, any number of finishing equipment technologies can be used to produce the final product. Although various aspects of the present invention have been described and detailed herein, alternative aspects may be effected by those skilled in the art to achieve the same objectives. Accordingly, it is designed by the appended claims to cover all alternative aspects as they fall within the spirit and actual scope of the invention.

Claims (17)

NOVELTY OF THE INVENTION CLAIMS

1. A method for recycling polyester foam, consisting of the steps of: densifying (26) a given type of polyester foam in such a way that when in a densified form, the polyester foam has a larger molecular surface area than that of densified non-foamed solid polyester of equal weight of the given type, and drying (28) the densified polyester foam in such a way that the intrinsic viscosity thereof increases.

2. The method according to claim 1, further comprising, before the densification step, the processing step (24) of the polyester foam to create a plurality of polyester foam flakes.

3. The method according to claim 2, wherein the plurality of polyester foam flakes and the densified polyester foam have approximately the same intrinsic viscosity.

4. The method according to claim 2, wherein the step of densifying comprises densifying the plurality of polyester foam flakes at a temperature between 93.3 ° C and 204.4 ° C.

5. The method according to claim 4, wherein the temperature is between 135 ° C and 190.5 ° C.

6. - The method according to claim 5, wherein the temperature is between 148.8 ° C and 176.6 ° C.

7. The method according to claim 2, wherein the step of densification comprises compressing the plurality of flakes of polyester foam in a plurality of pellets, and wherein the step of drying comprises drying the plurality of pellets.

8. The method according to claim 7, wherein the drying step comprises drying the plurality of pellets to a condensation point of about -39.9 ° C.

9. The method according to claim 8, wherein the step of drying comprises drying the plurality of pellets at a temperature of less than 190.5 ° C for less than 6 hours.

10. The method according to claim 9, wherein the step of drying comprises drying the plurality of pellets in a desiccant dryer.

11. The method according to claim 9, wherein the temperature is about 176.6 ° C.

12. The method according to claim 1, further comprising the step of extruding (30) polyester foam sheet from the dried densified polyester foam, whereby the polyester foam is recycled.

13. - The method according to claim 1, wherein the densification step comprises compressing the polyester foam to a bulk density greater than 0.3 g / cm3.

14. The method according to claim 13, wherein the volumetric density is greater than 0.5 g / cm3.

15. The method according to claim 1, wherein the polyester foam consists of PET, wherein the densification step comprises densifying the PET, and wherein the drying step comprises drying the densified PET.

16. The method according to claim 1, wherein the densified polyester foam has a crystallinity of at least 20%.

17. The method according to claim 1, wherein the dried densified polyester foam has an intrinsic viscosity greater than 0.95 g / dl.