FIELD OF THE INVENTION

This invention describes a process for physically treating polyester materials in order to increase the rate of crystallization and transparency of the material upon crystallization. The invention also includes the treated polyester material after processing and articles formed by thermoforming of such processed material.

BACKGROUND OF THE INVENTION

Polyesters have found wide-spread utility in a large number of diverse applications because of their low price, excellent chemical and barrier properties and high temperature stability upon crystallization.

Polyesters are resins consisting essentially of a linear saturation condensation product of at least one glycol or diol and at least one dicarboxylic acid. A widely used polyester is polyethylene terephthalate ("PET"). PET has been used for the production of fibers, textiles and films for many years, and has lately found application in carbonated soft drink beverage bottles and microwaveable and ovenable food containers. PET is a good material for these applications because of its excellent chemical resistance, low gas permeability and high temperature stability upon crystallization.

Two important physical parameters of PET are the extent of crystallinity and the extent of orientation. PET is a crystallizable thermoplastic, meaning that it can exist in an amorphous form or crystallized to varying extents. The extent of crystallization is a direct function of the density of the material, and can be easily measured by measuring the density of the material. PET is also an orientable thermoplastic, meaning that the random intermingled chains of molecules may be oriented by physically stretching the material. Orientation can be

either unidirectional or bidirectional, with biaxially oriented PET exhibiting the advantageous effects of orientation in all directions. Orientated

*"» thermoplastics have increased tensile strength and 5 elastic modulus. Other than by analyzing the physical characteristics of the thermoplastic or the operations performed on the material, it is difficult to quantify the extent of orientation.

PET and certain other polyesters can exist, 10 therefore, in various stages of crystallinity and in various stages of orientation. For many uses, these materials are shaped, via conventional means, in the amorphous state, then crystallized while being held in the new shape. This process of crystallizing after 15 shaping is referred to as heat setting, and it enables the shape of the formed article to be retained at higher temperatures.

Articles may be created out of polyesters by a variety of processes. For example, polyesters may be 20 shaped by injection molding, extruding and thermoforming. Thermoforming is the preferred process for forming many container-shaped articles. In thermoforming, a film or sheet of polyester (usually formed by extrusion processes) is preheated to a 25 temperature sufficient to allow the deformation of the sheet. The sheet is then made to conform to the contours of a mold by such means as vacuum assist, air pressure assist, plug assist or any combination of these processes. Thermoforming is a particularly good 30 means for producing relatively thin wall containers. Thermoforming polyesters is an excellent technique for producing heat set articles because the article can be formed and heat set in the same mold. However, there are a number of problems that are 5 associated with the thermoforming of pure PET. In order to heat set the product formed, the product must be exposed to an elevated temperature for a time

sufficient for crystallization to occur. For pure amorphous PET the time required to achieve an acceptable level of crystallization (e.g., greater than 25%) will take anywhere from 20 to 600 seconds depending on the temperature. Such a long heat set time means that cycle times are far too long to be commercially viable. In addition to the slow cycle times, articles heat set from pure PET are often difficult to remove from the mold surface at higher mold temperatures.

It has been known for many years that oriented polymers will exhibit increased rates of crystallization and improved clarity and light transmission upon crystallization relative to unoriented materials. See, for example, United States Patent No, 2,823,421 of Scarlett, which shows the effects of orientation on the rate of crystallization of PET. In the various examples given in that patent, sheets of PET are stretched — usually only in one direction — to at least 200% of their original length and then crystallized. Since the sheet is held in its stretched condition during crystallization, the product is highly oriented. The physical appearance of the crystallized polyester is also affected by the extent of orientation of the starting material. Non-oriented PET will appear opaque and milky-white after crystallization, while highly oriented and crystallized PET will be clear and transparent.

A common means for increasing the rate of crystallization of amorphous oriented or unoriented PET is by doping the PET with nucleating agents. In United States Patent No. 3,960,807 of McTaggert, nucleating agents are described as including "talc, gypsum, silica, calcium carbonate, alumina, titanium dioxide, alumina and calcium silicate, pyrophylite, finely divided metals, powdered glass, carbon black, graphite etc., individually or as mixtures of one or more." In

McTaggert, the optimum PET formula was also taught to include a "crack stopping agent", preferably polyethylen .

In United States Patent No. 4,463,121 of Gartland et al., a PET formulation containing just a polyolefin as an additive was also found to have a significantly increased rate of crystallization. Another doped PET formulation includes crystallization promoters which are derived from: (1) hydrocarbon acids containing between 7 and 54 carbon atoms or organic polymers having at least one carboxyl group attached thereto, and 2) sodium or potassium ion source capable of reacting with carboxyl groups of the acids or polymers of (1) . The preferred formulation with this crystallization promoter is PET having about 110 ppm sodium end groups and an intrinsic viscosity of about 0.85. See, United States Patent No. 4,753,980 of Deyrup.

The use of nucleating agents is generally successful in increasing the rate of crystallization of PET. However, for any given extent of crystallization (including no crystallization at all) , PET containing nucleating agents will almost always have reduced clarity or transparency. This can be extremely important in many applications. For example, for many food containing utilities it is desireable to be able to view the contained food during heating.

An additional problem encountered with PET containing nucleating agents is in the extrusion of relatively thick amorphous sheets of material. Under normal sheet extrusion procedures, the extruded sheet is cooled as quickly as possible so that the hot amorphous material (which is necessarily at a high temperature during extrusion) does not crystallize after being formed. Because the nucleating agent greatly increases the rate of crystallization, with thick extruded sheets the sheet cannot be cooled

rapidly enough, and the interior of the sheet will crystallize to a significant amount. For many applications the use of partially crystalline sheet is unacceptable. Any means for increasing the rate of crystallization that involves a modification of the PET composition will be subject to this problem.

SUMMARY OF THE INVENTION

This invention includes a means for processing polyesters, the processed polyester and articles made from the processed polyester. In the preferred embodiment the polyester material is PET, and PET is in extruded sheet prior to processing.

Sheet polyester is 1) heated to a temperature at or just above the glass transition temperature of the material; 2) unidirectionally or bidirectionally (serially or simultaneously) stretched so that the stretched length of the sheet (in the direction of stretching) is at least 120% of the length of the sheet prior to stretching; and 3) allowing—with or without assist—the sheet to return to a fully relaxed state.

The polyester sheet processed as described herein will have essentially the same thickness, length, density (degree of crystallization) , and appearance (extent of light transmission and clarity) as the sheet used prior to stretching. The processed sheet prepared generally as described above can be used in any application that PET sheets can be utilized. Specifically, the processed PET of the present invention can be used in thermoforming applications to particular advantage.

The processed PET of the present invention exhibits the following characteristics during thermoforming: 1) the rate of crystallization is dramatically increased relative to the rate of crystallization of the unprocessed material; 2) the optical transmission and clarity characteristics of the

product produced are increased—at a given degree of crystallization—relative to non-processed starting material; and 3) mold sticking problems are substantially reduced. In addition, because the processing of the present invention occurs after sheet extrusion/formation, it is possible to produce much thicker sheets of fast crystallizing polyester material.

The physical characteristics of thermoformed products made from the processed polyester of this invention may be altered by changing various parameters of the processing steps or by the use of different starting materials. For example, the light transmission and clarity characteristics of the product are effected by both the temperature of the sheet during stretching and the amount of stretching.

The processed polyester of the present invention may be shaped by any of the methods known in the art for creating useful articles out of polyester sheet. The processed sheet of the present invention is particularly well suited for the process for creating container-shaped articles described in United States Patent Application Serial No. 07/674,761. The '761 Application is commonly owned with the present invention and has the same inventor.

Heat set products created out of the processed sheet of the present invention will have improved optical transmission and clarity characteristics relative to products made either from pure PET or PET containing nucleating agents.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts an embodiment of the apparatus utilized for processing sheet polyester according to this invention. The apparatus is shown with the oven sidewall removed.

Figure 2 shows a diagrammatic cross-sectional

depiction of a device used in the thermoforming process for the determination of crystallization rates. The device shown is used to preheat the processed sheet of polyester. Figure 3 shows a diagrammatic cross-sectional view of the thermoforming apparatus used for the determination of crystallization rates.

Figure 4 shows a diagrammatic cross-sectional view of the mold and platen area of the thermoforming apparatus used for the determination of crystallization rates.

Figure 5 shows a cross-sectional view of an embodiment of a thermoformed product of the present invention. Figure 6 is a graph depicting percent of crystallinity versus time for: Δ = pure PET; 0 = DuPont SELAR 262;® = processed pure PET stretched 158%; x = processed pure PET stretched 166%; and O ⁼ processed pure PET stretched 176%. Figure 7 is a graph depicting distortion upon crystallization of processed pure PET into a shallow circular mold. The graph represents the extent of distortion versus the amount of stretching of the sheet during processing. The PET used was Eastman 9902 from Eastman Chemical Co.

Figure 8 is a graph similar to that shown in Figure 7, wherein the sheet PET was first unidirectionally stretched 175%, and then variably stretched in the perpendicular direction and that degree of stretching is plotted versus the amount of distortion of the crystallized processed sheet.

Figure 9 is a graph identical to Figure 7 but where the polyester used is A-150 from Eastman Chemical Co. Figure 10 is a graph depicting the light transmission of pure PET that has been processed and crystallized versus the amount of stretching of the

sheet during processing. The PET used was Eastman 9902 from Eastman Chemical Co.

Figure 11 is a graph identical to Figure 10 but where the polyester used was A-150 from Eastman Chemical Co.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention describes a method for processing thermoplastics that gives them unique properties without substantially effecting the physical dimensions, appearance or crystallinity of the material.

In many embodiments of the present invention, the thermoplastic material is configured as a sheet, and is typically comprised of essentially pure material (i.e., not including any nucleating agent utilized to increase the crystallization rate of the thermoplastic) . However, in certain embodiments the thermoplastic may contain nucleating agents or crack stopping agents, or may be comprised of a blend of polymeric materials.

The scope of the present invention extends to all thermoplastic materials (generally long chain polymers) that will be effected by the processing described herein in generally the same manner and to same extent as are polyesters. In the preferred embodiment, the thermoplastic material utilized is a polyester.

Polyesters are resins consisting essentially of a linear saturated condensation product of at least one glycol selected from the group consisting of neopentyl glycol, cyclohexane dimethanol and aliphatic glycols of the formula HO(CH ₂ )_OH where n is an integer of at least 2, and at least one dicarboxylic acid having 8 to 14 carbon atoms, or reactive derivatives thereof.

Examples of polyesters include homo and copolymers of polyethylene terephthalate ("PET"); poly 1,4-

cyclohexane dimethanol terepthalate; polyethylene-2, 6- napthalenedicarboxylic acid; polyethylene isophthalate; and polypropylene terephthalate. Representative comonomers that can be used up to a total of about 16 mole percent to reduce the crystallinity and melting point are: diethylene glycol, aliphatic dicarboxylic acids (including azelaic, sebacic, dodecanedioic acid) . Furthermore, the following comonomers can be used where not present in the base polyester: 1-4- cyclohexanedicarboxylic acid; 1,3-trimethylene glycol; 1-4-tetramethylene glycol;

1,6-hexanemethylene glycol; 1-8-octamethylene glycol; 1,10-decamethylene glycol; ethylene glycol; isophthalic acid; terephthalica acid; 2,6-naphthalene dicarboxylic acid; butylene glycol; cyclohexanedimethanols; and propylene glycol. The polyesters will generally have an intrinsic viscosity of at least 0.3, preferably at least 0.5 and more preferably at least 0.65.

In addition, monomers that result in branch points can be used for improved melt strength and processability. They would include trimethylpropane, pentaerythritol, trimellitic acid, and trimesic acid. These monomers, if used, should be added at less than about 1 mole percent. The processed polyester of the present invention exhibits a greatly increased rate of crystallization. In addition, the processed polyester also has improved light transmission and clarity characteristics after crystallization. In certain embodiments, polyesters containing nucleating agents may be used in the present invention even though an increase in crystallization rate is not required, but an improvement in the optical characteristics of the thermoformed product is desireable. Nucleated PETs for use in the present invention include PET having about

110 ppm sodium end groups and an intrinsic viscosity of about 0.85 measured as described in United States

Patent No. 4,753,980 of Deyup (col. 3, lines 4-13), specifically incorporated herein by this reference. Such nucleated PET was used in the experiments depicted in Figure 6, and is described as SELAR, which is a federally registered trademark of E.I. DuPont de

NeMours, Inc. Additional polyesters that have utility in this invention are Eastman A-150 (a copolymer formed by the condensation of terephthalic acid and isophthalic acid with cyclohexane dimethanol) and Eastman 9902 (essentially pure PET) which can be obtained from Eastman Chemical Co.

In the preferred embodiment of the invention, the starting material is pure PET. The pure PET is configured as a sheet—formed by extrusion processes— that is amorphous and has a thickness from .001 to .500 inches. According to the method described herein, the polyester sheet is held by processing means for stretching the sheet uniaxially or biaxially. If biaxially, the stretching may occur simultaneously or serially. The temperature of the sheet of polyester is raised to at or just above the glass-transition temperature of the material, which is often referred to as being within the orientation temperature regime of the material. For PET, the temperature for preferred processing is between 176 and 200 °F.

It is to be understood that temperatures of thermoplastics are inherently difficult if not impossible to measure when physical manipulations of the thermoplastic are undergone, due to the internal generation of heat caused by molecular rearrangements in the material. All temperatures referred to herein refer to surface temperatures before physical treatment, and are accurate within about + 3°F. The heated and moldable thermoplastic is stretched so that the length of the thermoplastic upon ultimate extension, in the direction of stretching, is at least about 120% of the length of the material in

that direction prior to stretching. Preferably, such stretching is greater than 140%, and in the most preferred embodiments for processing PET, such stretching is greater than 150% and less than 200%. After stretching, the sheet is held at full extension for about .1 to 20 seconds, and then allowed to return to a fully relaxed state. With increased times at the stretched position, the material will increasingly lose its ability to relax back to near its original dimensions.

After stretching, the polyester resumes its preprocessing dimensions almost exactly as long as it is allowed to return to a fully relaxed state. In the preferred embodiments of this invention, the physical dimensions of the processed sheet (length in direction(s) of stretching, thickness, and density) of polyester will deviate from that of the starting material by less than 10%, and in the most preferred embodiments by less than 5%. The extent of stretching during processing is, therefore, limited by the elastic limit of the material used at any given temperature.

In one embodiment of the invention the clamping means are released in order to allow the processed sheet to resume a fully relaxed state without assist. In a preferred embodiment, the clamping means remain secured to the sheet, and actively return with the sheet to its starting position. It is believed that actively returning the clamping means to their starting positions does not exert significant conformational forces on the thermoplastic as it relaxes back to its original configuration.

The processing as described herein may also be made a part of the extrusion process using conventional machine direction orientation equipment adapted so that the stretched polyester would be allowed to shrink back to a relaxed state rather than going into transverse direction orientation.

The density of the processed sheet is substantially the same as that of the original material. The thickness of processed sheet does not deviate from that of the original material by more than 5%, and typically less than 1%.

In one preferred embodiment for biaxially processing the polyester sheet, the sheet is first stretched and then allowed to relax in one direction, and then stretched and relaxed in a direction perpendicular to the direction of the first process step.

The processed polyester of the present invention may be characterized by the processing steps that are used to form it, by the properties with which it exhibits during crystallization, and by its properties as processed. The processed material has substantially the same physical dimensions as the original material, and does not substantially shrink parallel to any axis of orientation upon heating. Polyester sheet processed according to the methods described herein, using cross polarized filters, can be shown to have less orientation than a sheet conventionally oriented (stretched then cooled in a elongated position) as little as 5%. In addition, a sheet conventionally oriented as much as 25% does not exhibit any of the advantageous characteristics of the processed polyester described herein and has the disadvantages of significant shrinkage and anisotropic forming characteristics. Upon crystallization, either in conjunction with a thermoforming process or by simply raising the temperature of an unrestrained processed sheet of polyester, the processed sheet of the present invention will not substantially shrink. In certain cases, as described in conjunction with Figures 7, 8 and 9 the processed sheet will actually grow in a direction parallel to the direction of stretching.

As described in Example 2 below, the processed polyester of the present invention will have an increased rate of crystallization relative to the unprocessed material. This effect is most notable in the embodiment of the invention where the polyester utilized is pure PET.

And further upon crystallization, again either in conjunction with a thermoforming process or by simply raising the temperature of the processed sheet of polyester, equally crystallized products of processed and unprocessed material will have different optical characteristics. The products formed from the processed sheet will exhibit greater light transmission and clarity relative to the unprocessed sheet. See Figures 10 and 11.

As described in Example 4 below, under certain processing conditions the processed material will grow or become distorted upon crystallization. Beyond a certain point, the extent or tendency to distort increases with the extent of stretching of the sheet during processing. It is also seen that the optical characteristics of crystallized processed sheet improves (greater light transmission and clarity) as the extent of stretching during processing increases. Under a given set of conditions, therefore, there may be instances where the optical characteristics of the crystallized product may not be increased further without introducing dimensional distortion in the crystallized product. However, by biaxially stretching the sheet during processing, the distortion effects can be eliminated. As can be seen in the Example 6, the particular polyester utilized may also effect these characteristics.

The present invention also includes all products formed by thermoforming or crystallization of the processed polyester. The variety of shapes and dimensions of products that can be formed from the

processed material is almost unlimited. The ability to form a specific shape or size of product is commensurate with the ability now known in the art for the thermoforming of thermoplastic materials. In a preferred embodiment, the processed PET of the present invention is used in the process for forming clear, biaxially oriented, and heat set container-shaped articles with unoriented, crystallized rims as described in United States Patent Application Serial No. 07/674,761. This application, with common ownership and inventorship as the present application, is incorporated herein by this reference.

The scope and breadth of the present invention is further elaborated by reference to the drawings and the specific examples presented below.

Reference should be made to Figure 1, in which an apparatus is depicted for unidirectionally stretching heated polyester according to the present invention. The apparatus consists of an oven 10 and air cylinder driving means 12 for the stretching of the polyester. The oven 10 consists of an oven cover 14 and a base 16. The oven cover 14 includes heating elements 18 and a fan 20 for assuring uniform heat distribution within the oven.

The oven base 16 also includes two sets of upper 22 and lower 24 clamps for securing a sheet of thermoplastic 26. In the embodiment depicted in Figure 1, one side of sheet 26 is held by fixed clamping means 28, while the other side of the sheet 26 is held by moveable clamping means 30. In an alternate embodiment, not shown, both sets of clamping means may be moveable.

The moveable clamping means 30 are associated with cylinder rod 32 and air cylinder 12. Cylinder rod 32 is encircled by a stopping sleeve 34, which defines the furthest movement of the cylinder rod. Adjustment

of the length of the stopping sleeve defines the extent of stretching of the thermoplastic material being processed. Control means 36 are associated with the air cylinder 12 for controlling the rate of stretching, the time period the material is held at its furthest stretched point, and the rate of return of the cylinder rod 32 back into its original position.

Sheet thermoplastic is processed in the device shown in Figure 1 by clamping opposing edges of the sheet 26 between the clamps 22 and 24 of the fixed 28 and moveable 30 clamping means. The oven cover 14 is placed over the base 16, and the heating elements heat the oven to the desired temperature and the fan is turned on. When the temperature within the oven is stabilized for a period of time sufficient to assure that the sheet 26 temperature is at the desired temperature, the cylinder rod 32 is drawn by the air cylinder 12 to unidirectionally stretch the sheet at a predetermined rate. The extent of draw is determined by the length of the stopping sleeve 34, which prevents further withdrawal of the cylinder rod 32 into the air cylinder.

After a preset period of time, the cylinder rod 32 is pushed back into the oven 10 by the air cylinder 12 until it is at the same point as it was prior to stretching. The oven cover 14 may then be removed and the processed sheet removed from the clamping means. To biaxially process the sheet, the sheet may then be rotated 90° and resecured in the clamping means and the process repeated.

Figures 2-4 show portions of a thermoforming process. In Figure 2, a sheet preheating device is shown. Prior to thermoforming, the processed polyester sheet 100 of the invention is placed in this apparatus 102 which consists of heated platen 104 and a layer of insulating foam 106. The platen is equipped with

heating elements 108. To preheat the sheet 100, the sheet is held tightly between the heated platen 104 and the foam 106 until the temperature of the sheet is uniformly raised to a desired temperature. Figure 3 shows a thermoforming apparatus 200.

This device consists of a frame 202, a fixed lower platen 206, a moveable upper platen 208, and control elements 210. Details of the platen and mold elements are more clearly shown in Figure 4. Figure 4 shows in cross section the platens and their associated elements. Upper platen 208 has heating elements 212 on its upper surface. Also seen is a recessed area 214 on the undersurface of platen 208. The recessed area 214 is in fluid communication with a gas introduction channel 216, that can allow either the introduction of air or the creation of a vacuum.

The lower platen 206 consists of a base 218, and a mold 220 with a recessed area 222 that defines the contours of the desired thermoformed product. The base 218 also contains heating elements 209. The control elements 210 (Figure 3) control the raising and lowering of the upper platen 208, the introduction or removal of air via channel 216, and the temperature of the upper platen 208, and the lower platen 206.

The preheated processed sheet 230 is placed between the upper platen 208 and the lower platen 206 of the thermoforming apparatus. The first step of thermoforming in this embodiment is to close the mold and create a vacuum in the recessed area 214 of the upper platen 208. This draws the thermoplastic sheet into contact with the heated upper platen 208, to raise the sheet temperature to the desired forming temperatures and to assure accurate temperature consistency prior to thermoforming.

After a preset time, the vacuum is released, and air is introduced into channel 216 to force the

thermoplastic to conform to the shape of the mold 220. After a desired period of time, the upper platen 208 is raised and the thermoformed material removed from the mold. Figure 5 shows a cross section of a thermoformed product produced according to the method of the present invention.

EXAMPLE 1: PREPARATION OF PROCESSED POLYESTER

The apparatus shown in Figure 1 was used to process a sheet of PET according to the present invention. The PET used was a 4.5 inch by 6 inch portion of .040 inch thick roll stock PET, from Eastman Chemical Co. as Eastman 9902.

The oven was preheated to 186-188°F. After preheating the oven, the PET sample was secured by the clamping means and the oven cover replaced. After about 5 minutes the sheet temperature was stabilized prior to processing. In most cases, the stretching occurs to full length in less than .5 seconds, is allowed to remain at its full extension for about 1 second and the air cylinder is returned to its original position (again in less than about .5 seconds). The extent of stretching is controlled by the length of the stopping arm. The oven cover was then removed and the processed sheet removed from the clamping means.

For biaxially processed experiments, 6 inch square sheets of PET were used, and after the first processing the sheet was rotated 90° and reprocessed.

EXAMPLE 2: THERMOFORMING PROCESSED POLYESTER

The devices shown in Figures 2-4 were used to produce thermoformed products from the PET sheets processed according to Example 1.

The processed PET was first preheated in the apparatus shown in Figure 2. The platen was maintained at 200°F and the sheet was held between the platen and the insulating foam for about 2 minutes. The preheated

sheet was quickly transferred to the thermoforming apparatus as shown in Figure 3.

The upper platen.was lowered by an air cylinder to clamp the preheated sheet between the upper platen and the bottom mold. The upper platen was held at

240°F and the bottom mold was at 385°F. A vacuum was applied to draw the sheet against the upper platen for about 6 seconds. Air pressure was then introduced against the top of the preheated sheet to force its conformation to the bottom mold (air pressure about 10 p.s.i.) .

After the desired period of time the upper platen was raised, and the thermoformed product was removed from the bottom mold. The thermoformed product was quickly cooled by immersing in room temperature water.

EXAMPLE 3: DETERMINATION OF CRYSTALLIZATION RATE OF

PROCESSED POLYESTER Processed PET material was processed as described in Example 1, and thermoformed and crystallized according to the process described in

Example 2.

Crystallinity of all samples was determined by the products density. The products density was determined by weighing in and out of water and applying the following equation:

Dw.t

1 - W w_

where: D _s = Density of Sample D _wt = Density of water at test temperature

W _H = Weight of sample in water W _d = Weight of sample dry

The crystallinity of the sample was then determined from the density with the following equation: _χ = D _s ____I_ _a D _c - D _a where: W _χ = Weight % crystallinity of sample D _s = Density of sample D _a = Density of amorphous PET (1.333) D _c = Density of crystalline PET (1.455)

Processed PET sheets were prepared as described in Example 1, where the material was unidirectionally stretched 158%, 166% and 176% the original length of the material. These processed sheets—as well as unprocessed PET sheets and unprocessed DuPont Selar 262—were crystallized for various time periods as described in Example 2. The results of percent crystallinity versus time for the various samples are shown in Figure 6.

As can be seen, the rate of crystallization for all processed sheets was significantly greater than for unprocessed PET, and at least as great as that for the PET containing nucleating agents. It was also shown that the greater the amount of stretching, the greater the rate of crystallization.

EXAMPLE 4: MEASUREMENT OF SHAPE DISTORTION OF PROCESSED POLYESTER

To measure the dimensional distortion seen in the crystallization of certain samples of processed PET sheets, an analysis of the amount of distortion versus the degree of stretching was undertaken. As described above, the significant distortion occurs—if at all—by "growing" in the direction of the original stretching. Pure PET sheet material was processed as described in Example 1, and the processed sheet was thermoformed according to the procedure described in Example 2 to form an article as shown in Figure 5. Dimensional distortion upon thermoforming was evaluated by measuring the diameter of the thermoformed article in the direction parallel to processing and transverse to the direction of processing. The difference in diameter (amount of distortion) versus the degree of processing is shown in Figure 7.

Example 5: Measurement of Shape Distortion of Biaxially Processed Polyester.

Experiments were performed to determine if the distortion effects seen upon thermoforming uniaxially processed sheet PET could be minimized by biaxially processing the material.

Distortion of processed sheets drawn to varying extents was measured by scribing a 2.6 inch circle on the surface of the sheet, and then placing the sample in a 325°F oven for 10 minutes. The sheet was then removed from the oven, and the dimensions of the scribed circle measured in the direction of drawing and in the transverse direction.

Samples were run wherein sheets of PET were stretched as described in Example 1 to 175% of the original length of the sheet. These sheets were then rotated 90% and processed biaxially to varying degrees of stretching. The distortion of these samples was

measured after crystallizing as described above, and the results are shown in Figure 8. As can be seen, by biaxially stretching the PET in perpendicular direction by approximately the same amount, the distortion can be eliminated.

EXAMPLE 6: PROCESSING A-150 PET

The method for processing polyester described in Example 1 was repeated except that the polyester used was A-150 from Eastman Chemical Co. , and the processing temperature was about 218°F. The processed polyester which had been stretched to varying degrees was then thermoformed as in Example 2 and the degree of dimensional distortion measured as in Example 4. The results of the dimensional distortion experiments are shown in Figure 9.

EXAMPLE 7: LIGHT TRANSMISSION OF THERMOFORMED ARTICLES OF PROCESSED POLYESTERS. Variously processed and thermoformed articles as described in Example 2 (pure PET) and Example 6 (A- 150 polyester) were tested for visible light transmission. These results are shown in Figures 10 and 11 respectively.