CA1266361A - Method of manufacturing partially crystalline polyester articles - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

METHOD OF MANUFACTURING PARTIALLY
CRYSTALLINE POLYESTER ARTICLES

A method of making a polyethylene terephthalate/polyolefin blend for sheeting and thermoformed articles which has improved color stability and can be recycled many times. A heat stabilizer is melt blended only with the polyolefin prior to its incorporation into the polyethylene terephthalate. The PET and polyolefin are then blended and extruded into sheeting and subsequently thermoformed into articles useful as microwave oven trays.

Description

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METHOD OF MANUFACTURING PARTIALLY
CRYSTALLINE POLYESTER ARTICLES
_ _ Background of the Inventio This invention relates to a method of making a polyethylene terephthalate/polyolefin blend sheeting for use in thermoforming partially crystalline heat set articles. In particular the invention describes a method for addin~ the antioxidant exclusively to the polyolefin prior to blendin~ with the polyethylene terephthalate.

nd Art The growing popularity of microwave ovens has created a general interest in production of low cost, microwave transparent, disposable containers for packaging food. The precooked prepared food may be placed in the container and subsequently frozen. The consumer will finish cooking the frozen food package in a microwave oven or a conventional convection oven prior to its use. The requirements which this type of dual ovenable tray application places on the container to be utilized are many and varied. Firstly, the container must be capable o wi~hstanding prolonged high temperature eY~posure without significant loss of impact strength or dimensional stability. Secondly, the container must maintain a uniform color and be resistant to any degradation which may alter the color during long term high temperature exposure in a microwave or conventional oven. United States Patent No 4,463,121 describes a method of manufacturing a partially crystalline polyester;article consisting of a major component of polyethylene tereph~halate (PET) and a minor component o~ a polyole~in to produce an article ::

which has a total crystallinity of about 10 to about 30%. These articles are usable as c~ntainers and exhibit stable impact strength and dimensional stabilitv due to the limitation on the degree of crystallinitv achieved during thermoforming. This patent also teaches the desirability of adding from about .05 to about 2 weight percent of a heat stabilizer to the PET/polvolefin blend for the purpose of stabilizing the intrinsic viscosity of the article.
United States Patent No 3,960,807 teaches a process for thermoforming articles from a composition having three essential components (l) a crystallizable polyester; (2) a crack stopping agent, preferably a polyolefin; (3~ a nucleating agent. The process disclosed in this patent improved the impact resistant of the article and the rate of crystallization during thermoforming.
In attempting to produce thin-walled articles such as microwave dual ovenable trays or containers made without antioxidant i~ was found that th~rmal-aging of the trays made by the methods of United States Patent 4,463,121 or 3,960,807 three distinct problems were eonsistently encountered, all related to high temperature e~posure, (1) a drop in intrinsic viscosity ~25 of the tray; (2) a tendency to discolor in brown or yellow hues; (3) the appearance of irregular yellow or brown patches on the tray surface particularly where the tray had been touched by someone's hand. This latter phenomena will be described herein as fingerprinting. The method of this invention effectively eliminates the three thermal degradation phenomena llsted above by eficiently incorporating into the polyolefin an effective heat stabilizer or antioxidant prior to the polyolefin being blended with .

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the PET resin. The method provides effective protection of the material with levels of antioxidant of one tenth to one hundredth of what is requlred when the antioxidant is added to the polyester or the blend of polyester/polyolefin.
An obJect of an aspect of this invention is to provide a method of manufacturing sheeting from polyethylene terephthalate and a polyolefin which is thermally stable during subsequent thermoforming operations on the sheeting. A derived benefit of the use of the method o this invention is the production of thin-walled articles or trays which resist discoloration or fingerprinting during high temperature thermal aging. An advantage of the invention is that a microwave or conventional oven tray manufactured from the method of this invention can withstand in excess of an hour at 200C with no discoloration, fingerprinting or substantial loss of intrinsic viscosity. A further advantage is that fractional levels of antioxidant will adequately protect the tray.

Rrief Description of the Invention The obJects and advantages of an aspect of this inventicn may be obtained by using a method of manufacturing an amorphous,thermallv stable polyolefin modified polyethylene terephthalate sheet comprising the steps of:
(a) melt blending an effective amount of a heat stabilizer with a polyolefin derived from olefin monomers containing two to six carbon atoms to form a stabilized polyolefin;
(b) heating a polyeth~llene ~erephthalate having an intrinsic viscosity of about .65 to abou~ 1.2 to at least 150C in a dry atmosphere for a time sufficient : ' ' ' , :
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to reduce the moisture level in the polyethylene terephthalate belo~ 0.02 weight percent to form a drv polvethylene terephthalate;
(c) mixing a minor amount of said stabiliæed.
polyolefin wlth a major amount of dry polyethylene terephthalate to form a homogeneous molten melt blend;
(d) forming a sheet from said homogeneous melt blend;
(e) cooling said sheet to form a substantially amorphous sheet;
Another aspect of the invention resides in a method of manufacturing a thermally stable, partially crystalline heat set, non-oriented article comprising the steps of:
(a) melt blending an effective amount of a heat stabilizer with a polyolefin derived from olefi.n monomers containing two to six carbon atoms to form a stabilized polyolefin;
(b) heating a polyethylene terephthalate having an intrinsic viscosity of about .65 to about 1.2 to at least 150C in a dry atmosphere for a time sufficient to reduce the moisture level in the polyethylene terephthalate below 0.02 weight percent to form a dry : polvethvlene terephthalate;
(c) simultaneously conveying said dry polyethylene terephthalate and said stabilized polyolefin to a means for melt blending;
(d) mixing a minor amount of said stabilized polyolefin with a major amount of dry polyethylene ter`ephthalate to form a homogeneous molten melt blend;
(e) forming a sheet from said homogeneous melt blend;
~(=) positioniDg said amorphous sheet over a mold;

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(g) thermoforming said sheet to form an article in a heated mold for a time sufficient to achieve crystallinit~ of between 15 and 35 percent;
th~ stripping said article out of said heated mold;
and (i) trimming said article out of said sheet.
Yet a~other aspect of the invention is a method of manufacturing a recyclable polvolefin modified polyethylene terephthalate sheet comprising the steps of:
(a) melt blending an effective amount of a heat stabilizer with a polvolefin ~erived from olefin monomers containing two to six carbon atoms to form a stabilized polyolefin;
(b) 'heating a polyethylene terephthalate having an intrinsic viscositY of about .65 to about l.Z to at least 150C in a dry atmosphere for a time sufficient to reduce the moisture level in the polyeth~vlene terephthalate below 0.02 weight percent to form a dry polyethylene terephthalate;
(c) simultaneously conveying said dry polyethylene terephthalate and said stabilized polyolefin to a means for melt blending;
(d) mixing a minor amount of said stabilized polyolefin with a major amount of dry polyethylene terephthalat,e to form a mixture;
(e) melt blending said mixture to form a homogeneous molten melt blend;
tf) forming a sheet from said homogeneous melt blend;
(g) cooling said sheet to form a substantially amorphous sheet;
th) positioning said amorphous sheet over a mold;

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(i) thermoforming said sheet in a heated mold for a time sufficlent to achieve partial crystallization of said amorphous sheet;
(~) stripping the partially crystalllne sheet out of said heated mold;
~ k) trim~ing out the portion of the sheet ~hich was in contact with the mold surface to leave a matrix of the amorphous sheet; and (1) grinding said matrix to form regrind and heating said regrind according to step (b) and then mixing said regrind with said mixture formed in step (d) and repeating steps (e) through (1) at least one additional time.

Detailed Description of the In~7ention In order to produce articles or containers usable in applications where high service temperatures are encountered, a polyester in the crystalline state rather than the amorphous state i9 necessary. Of the known thermoplastic, crystallizable polyesters, polyethylene terephthalate (hereinafter PET) offers the desirable properties of good high temperature dimensional stability, chemical, oil and solvent resistance and the ability to pass micr~wave radiation without absorbing or reflecting it. These properties make it the polymer of choice for use in high temperature food containers.
The polyethylene terephthalate polymer is obtained by known polymerization techniques ~rom either terephthalate acid or its lower alkyl ester (dimethyl terephthalate) and ethylene glycol. The terephthaIic acid or dimethyl terephthalate is esterified or transesterified and then polycondensed with ethylene glycol to a high molecular welght product. For use in " . ' . .. . ..
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this invention the polyester so produed should have an intrinsic viscosity ranging from about .65 to about 1.2 and preferably from about .~0 to about 1.05 as measured in a 60/40 bv volume mixed solvent of phenol/tetrachloroethane at 30C. Known methods of solid state polymerization may be employed to achieve the higher intrinsic viscosities.
In order to utilize polyethylene terephthalate in viable commercial forming processes such as thermoforming it is essential tha~ the desired level of crystallinity be achieved in a very short c~Tcle time.
An acceptable cycle time would be about 5 to 7 seconds.
Polyethvlene terephthalate polymer, completely unmodified, exhibits crystallization rates too slow to achieve the required cycle times. To overcome the slow crystalli~ation rate, nucleating agents mav be added in order to increase the number of crystallites formed.
Most known nucleating agents are inorganic materials having an average particle size of from 2 to 10 microns. Other known nucleating agents are-carbonaceous materials such as carbon black and graphite. Common nucleating agents may be ~alc, gypsum, silica, calcium carbonate, alumina, titanium dioxide, pryophylite silicates, finely divided metals, powdered glass, carbon black, and graphite. The common featurè shared by the foregoing list of known nucleating agents is that they exist in solid, form within the temperature range of 100C. to 300C. where polyesters are forming crystalline structures. Any of these particulate nucleating agents ma~ be used to good advantage, although a leveling off of degree of crystallinity occurs if these particulate nuclea~ing agents are reduced or eliminated.

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The second essential component of thls invention is a polyolefin, which must be present with the polyethylene terephthalate. Polyolefins as used herein are those produced from olefin monomers having from 2 to 6 carbon atoms. The resulting polymer contains repeat units derived from the original monomer units.
These repeat units differ from the monomers in that they no longer contain a carbon-carbon double bond.
Such polymers include low density polyethylene, high density polyethylene, linear low density polyethylene, polypropylene, polyisopropylene, polybutene, polypentene, polymethylpentene. The polyolefin should be present in levels of from 0.5 to 15 weight percent of the total composition. The preferred range was found to be from 1 to 5 weight percent. Most preferred is 2 to 4 weight percent. ~ preferred class of polyolefins is the polyethylenes with the most preferred type being linear low density polyethylene (LLDPE), as represented by products marketed by Dow Chemical under the tradenames DOWLEXTM 2045 and 2Q35.
When compared to unmodified PET, all the polyolefins provide improved impact strength in the finished article and improved mold release in the thermoforming process. The polyethylene and polypropylene have broader operating temperature ranges, faster rates of crystallization and lower temperatures for the onset of crystallinity. These improvements lead to faster cycle ti.mes, more parts per minute and a lower cost finished article.
The use of the polyolefins with the PET was found to give rates of crystallization at least as fast as PET compositions which contained both the polyolefin and an additional nucleating agent such as described in United States P~tent 3,g60,807.

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~L2$~i36 It is known that heat stabilizers or antioxldants .
may be added to polyethylene terephthalate, however, the problem of protecting the PET/pnlvolefin blend from thermal degradation in environments where the thermoplastic blend is subiected to heating at temperatures at near 200C for a period approaching one hour becomes particularl~ difficult. This is especiallv pronounced when the article which is to be made from the PET/polyolefin blend is to come in contact with food due to the desirability of minimizing the amount of stabilizers or antioY~idants present. It was ~,uite unexpectedly discovered that the PET/polyolefin blend may be optimumly protected bv adding a relatively low level of antioxidant or stabilizer directly to the polyolefin component before the PET/polyolefin blend is made. This method of incorporating the heat stabilizer prior to the bl.ending of the P~T and polyolefin provides a method for insuring (1) a minimal loss ln intrinsic viscosity during processing and subsequent heat aging; (2) ' eliminating discoloration of the blend during high temperature exposure; and (3) elimination of the development of fingerprints or blotchy areas of discoloration during high temperature aging. Heat stabilizers as used herein are compounds which demonstrate antioxidant properties, the most impo~tant of which is the capability of inhibiting oxida~ion. An effective heat stabilizer in the practice of this invention must be capable of protecting the thermoformed, hea~ set polyester article during exposure to elevated temperatures. U.S. Patent No.
3,987,004, U.S. Patent No. 3,904,578 and U.S. Patent No, 3,644,48~ disclose many examples of known heat stabilizers. The following compounds aFe - .
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representative of useful heat stabilizers in the practice of this invention: alkylated substituted phenols, bisphenols, substituted bis phenols, thiobisphenols, polyphenols, thiobisacrylates, aromatic 5 amines, organlc phosphites and polyphosphites. The particular aromatic amines which demonstrate specific heat stabilizing capabilities include: primary polyamines, diarylamines, bisdiarvlamines, alkylated diarylamines, ketone-diarylamines condensation products, aldehyde-amine condensation products, and aldehyde amines. Conditions which would be considered severe in the practice of this invention would be those in which the thermoformed, heat set article would be exposed to temperatures near 2dOC. or a period eY.ceeding 30 minutes. Preferred heat stabilizers for such severe high temperature applications particularly where any staining or discoloration from the heat stabilizer is undesirable are the polyphenols which have more than two phenol ring structures in the compound. Polyphenols which are useful include, but are not limited to: te~rakis(methylene 3-(3,5-ditertiarybutyl-4-hydroxyphenyl)-propionate) methane, and 1,3,5-trimethyl-2,4,6-tris(3,5-ditertiary butyl-4-hydroxybenzyl)benzene. The latter polyphenol is most preferred. The heat stabilizers may advantageouslv be added a~ levels up to 2 weight percent but more preferred are levels below .05 weight percent based on the total PET/polyolefin/stabilizer composition. The most preferred level is between .005 and .03 weight percent.
The antioxidant is added by melt blending directly into the polyolefin component of the polymer blend.
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antioxidant appropriate for the finished ratlo of PET
to polvolefin. The particular level used is determined by the degree of protection required, the identity of the particular stabilizer chosen, the severity of the hea~ exposure and any solubili~y limita~ion in the polyolefln. When food trays are the article being produced by the method of this invention appropriate regulatlons regarding materials in contact with food must be taken in~o consideration and generally form an upper bound on the amount of antioxidant which may be added to either component and to the finished blend of polymers.
The method of manufacturing a polyolefin modified polyethylene terephthalate sheet includes the steps o (1) melt blending a suitable heat stabilizer with a polyolefin derived from monoolefins having from 1 to 4 carbon atoms to ~orm a stabilized polyolefin. (2) the polvethylene terephthalate having an intrinsic viscosity of from .65 to 1.2 must be heated above its glass transition tempera~ure in a dry air or nitrogen atmosphere and maintained in that condition until a moisture level low enough to reduce hydrolytic degradation during subsequent method steps is attained.
(3) The stabilized polYolefin and the dried polyethylene tere~hthalate~resin is simultaneously conveyed in appropriate proportion to an extruder where the two components are melt blended to form a homogeneous molten blend thereof. (~) Forming a sheet from the homogeneous molten blend. (5) Quenching the sheet to form a substantially amorphous sheet.
The melt blending of the heat stabilizer maY be accomplished in the post-polymerization stage of the production of the polyolefin or it may be accompl~shed by the use of any conventional thermoplastic mixing .
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extruder which will adequately disperse the antioxidant throughout the polyethylene which has been rendered molten in the barrel of the extruder. The process step wherein the polyethylene terephthalate resin i5 heated in a dehumidified atmosphere of nitrogen or air is necessary ~o maintain the intrinsic viscosity level of the resin. Any moisture level is suitable which maintains a sufficiently high intrinsic viscosity level during the remaining process steps of the method.
Maintenance of the lowest practicable moisture level is advantageous. Generally levels below .02% are required. Moisture less than .005~ is most preferred for high intrinsic viscosity PET. Due to the requirement of good impact resistance and dimensional stability of containers to be made in this invention, it ls essential that the PET be carefully handled to maintain its high intrinsic viscosity. The mixing o~
the polyolefin containing the antioxidant with the dried PET may be accomplished in any conventionally known film extrusion technique where the polyolefin and PET are heated above their glass transition points and blended through shearing of the molten material to form a homogeneous blend of the ~wo dissimilar plastics. It is presumed that the polyolefin is dispersed throughout the PET but maintains its identity in a separate phase.
The forming of the sheet may be done by any conventional film forming technique. The sheets which are utilized in the examples which follow were produced on a ProdexT~ film extruder where the molten sheet was 3~ extruded onto a chilled casting roll and immediately cooled to minimize crystallinity buildup. Table I
shows the sheet extrusion conditions which were utilized to produce the amorphous sheeting used to make the trays in the examples of the specification.

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TABLE I
SHEET EXTRIJSION CONDITIONS

Extruder Size 1.75" Proclex E~truder Zones 288C
Throat Cooling Yes.
Screw Cooling No Extruder Speed 94 rpm Extruder Pressure 19 MPa Extruder Amps 10.5 Polymer Temp 296C
Adapter Zones 277C
Die Zone 1,2,3 All 2~8C
Casting Roll 1 75C
Casting Roll 2 63C
Take-up Speed 1.2mm/min Sheet Dimensions 0.76mm x .4m Material PET 97Z./LLDPE 3%

The manufacture of heat set thin-walled trays which are made from the sheeting produced rom stabilized polyolefin/PET can be made by using any of the kno~m thermoforming methods includlng vacuum assist, air assist, mechanical plug assist or matched mold. The mold should be preheated to a temperature sufficient to achieve the degree of crystallinity desired. Selection of optimum mold temperature is : dependent upon type of thermoforming equipment, con~iguration and wall thickness of articles being mQlded and other factors. The operable range of mold temperatures is 150-215C. The preferred range is 170~1g0C.

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Heatsetting is a term describing the process ofthermally inducing partial crystallization of a polyester article without appreciable orientation being present. In the practice of this invention, heatsettlng is achieved by maintaining intimate contact of the film or sheet with the heated mold surface for a sufficient time to achieve a level of crystallinity which gives adequate physical properties to the finished part. It has been found that desirable levels of crystallinity should be about lO to about 35 percent. For containers to be used in high temperature food application it was found that levels of crystallinity above 15 percent were necessary or adequate dimensional stabllitv during demolding and during oven exposure.
The heat set part can be stripped out o~ the mold cavity by known means for removal. One method, blow back, involves breaking the vacuum established between the mold and the formed sheet by the introduction of compressed air. Once the heat set part has been removed from the mold, the portion of the sheeting ~which remains in the original planar state is trimmed awa~ to leave the ~inished tray. Since most com~ercial thermoforming m~lds will contain a plurality of cavities for production of many trays from a slngle sheet, the dinking out of the trays will leave a flat matrix oL the original sheet which has the outline of the trays removed. Anywhere from 10 to 60~ of the original sheet remains in the matrix and must be recycled in order to make the thermoforming operation economically feasible. This recycling of the matrix means that a very substantial amount of thermal heat history is built into the sheeting. I~hen 40% of ~he sheeting is recvcled it is estimated that certain ~ , ~

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portions of the polyolefin PET blend will be subJected to five full recycle steps. These recycle steps include: (1) being ground; (2) heated int~o dry atmosphere; (3) melt blended with virgin material enterlng the system; (4) being formed into sheet; (5) quenched; (6) subsequent~y reheated prior to entry into the thermoforming mold; (7) drawing and heat setting in the thermoformer; (8) subsequently cooling the part;
and (9) stripping the part. All these steps are repeated five times. Thus, the resin is sub~ected to the very high temperatures of sheet manufacturing and thermoforming over a much longer time period than would first appear to be the case. This severe high temperature exposure is detrimental to the intrinsic viscosity of the PET and to the stability of the polyolefin component and it was not until this invention that it was recognized that the manner in which the PET polyolefin blend was protected was found to be critical, particularly in processes where recycle approaches 40~. In the following examples a 1.04 intrinsic viscosity polyethylene terephthalate was used either alone or with linear low density polyethylene (LLDPE~ available from Dow Chemical under the trade identification Dowlex 2045. The PET with the LLDPE was dr~ed and extruded according to the conditions of Ta~le I and the .76mm sheets were subsequently thermoformed on a Comet Labmaster thermoformer into a 13cm x 13cm square tray having a depth of 2.5cm. All percent expressions are weight percent based on the total weight of the composition, polymer sheet or tray.

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UNSTABILIZED FOOD TRAYS

Examples 1-2 The 1.04 intrinsic viscosity polyethylene terephthalate sheetlng was formed into trays. The intrinsic ~riscosit~ was determined after the thermoforming of the trays was complete and those trays were then aged at 200C in a circulating air oven for one hour. The intrinsic viscosity was then tested again. The results are sho~n in Table II under Example 1. Example 2 was a 97% PET/3~ LLDPE sheet with no antioxidant added. It is clear from the results shown in Table II that PET alone or PET blended with a polyolefin such as linear low density polvethylene undergoes a substantial drop in intrinsic viscosity during high temperature aging when unprotected with an antioxidant or stabilizer system. This loss of intrinsic viscosity would be totallv unacceptable in food trays for Example 3 would be characteristic of the articles produced having no antioxidant included according to the teachings of United States Patent 4,~63,121.

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~3 COMPARATIVE RESULTS

All samples in this group of examples were made from PET/LLDPE sheets which contained varying levels of the most preferred polyphenol antioxidant, specifically l, 3, 5-trimethyl-2,4,6-tris (3, 5-diter-tiarybutyl-4-hydroxybenzyl)benzene, available from Ethel Corporation under the trade designation Ethanox~ 330. The percent AO shown in Table II under the sheeting formulation is the percent of antioxidant based on the weight of the total sheet composition. In this series of experiments, the objective was to evaluate the effect of thermal heat aging at 200C on three properties which are important to trays usable for microwave and conventional ovens. These characteristics are: (1) Fingerprints - this term represents a phenomenon which manifests itsel as irregular and widely spaced discolorations on an aged tray sample. The discoloration occurs typically on surfaces of the tray which have been touched by human hands. This sporadic blotchiness in Table II has either a year or no value under the fingerprint category of aging effects. It is to be understood that if the entry is "yes", fingerprints appeared after aging. If the entry is "no", the tray retained its original uniform appearance; (2) Color - the term color designates the retention or lack of retention of the original color of the tray after one hour aging at 200C. The appearance . 30 of discoloration is a uniform change in the hue of the tray. If the entry under the column color is "stable", it denotes that the color did not change after aging.
If the entry is "discolors", it indicates that a discernible degree of discoloration resulted from the ~.,,,~

thermal aging; (3) Intrinsic Viscosity - intrinsic viscosity tends to decline when PET is subjected to high temperature. The original intrinsic viscosity is ~ested and compared to the intrinsic viscositv value after one hour at 200C. The variables in Examples 3-7 include the level of AO, the component to which the AO
is added and the method of adding the AO.

Example 3 In Example 3 0.1% of AO was added to the PET in the meLt phase during production of the polyethylene terephthalate polymer. The intrinsic viscosity was maintained satisfactorilv but the relatively high level of AO in the PET contributed to discoloration of the PET polymer utilized in the blend. This discoloration was a general yellowing to brown from the normal milky white of the virgin PET resin. It is speculated that the high temperatures e~cperienced during PET production and the prolonged high temperature maintenance during solid stating of the the base resin to a high intrinsic viscosity (1.04) all contribute to the overall discoloration. This discoloration is very objectionable in applications such as food trays. In addition, the aged samples showed fingerprinting which is also unsatisfactory.
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ExampLe 4 Example 4 is a blend identical in polymer composition to Example 3 but 0.19~ antioxidant was added. The antioxidant was added both to the PET in the reactor and to the linear low density polyethylene in the master batch. The column under AO addition method which is designated as "master ba~chl' indicates a procedure whereby an initial master batching step is ~. :
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taken in which a 77l23 weight percent PET/ILDPE were .
mechanicallv blended in particulate pellets to form a master batch.
This master batch blend of PET and linear low density polvethvlene was subsequently simultaneously fed to a Prodex film extruder along with PET resin in a ratio of 13 to 87 weight percent to yield a final LLDPE
percentage of 3 and a PET percentage of 97. This master batch method achieves an improved dispersion of polyethylene throughout the PET in a circumstance when the hoppers feeding the film extruder are not accurately calibrated to handle resin percentages as low as 37,. The intrinsic viscosity was maintained and no fingerprinting was evident, however, discoloration of the trav was evident with this higher level of antioxidant. The antioxidant has a distinct tendency to yello~? when relatively high le~Tels are added to the PET/polyolefin blend.

Examples 5-7 Film sheeting was prepared using a 97/3 weight percent ratio of PET/LLDPE along with varying ln percentages of antioxidant. This series utilized the method of this invention in which the antioxidant was added to the linear low density polyethylene prior to any incorporation with the PET. No antioxidant i6 added to the PET and this method is described in the antioxidant addition method columns of Table III as the direct LLDPE. In this method, the antioxidant was added to the polyolefin by remelting the particulate polyolefin and homogeneously bIending the desired level of antioxidant in ~he polyolefin and subsequently finishing ~he molten resin into the desired particulate form such as pellets, prilled beads or other desirable ;

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: .: ' forms. This blending was done in a Sterling Transfermix extruder with the barrel temperature maintained at approximately 195C and the die at 175C.
The screw speed ~as 84 rotations per minute. The linear lo~? density polyolefin ls then accurately blended with the dried PET at the throat entrance to the film extruder. The film extruder homogeneously blends the PET with the stabilized linear low density polvolefin to form a uniform melt blend. The result sho~ in Table II regarding the aging properties of the trays made with the polyolefin stabilized blends show that even at eY~tremely low levels of antioxidant the intrinsic viscosity is maintained and the color of the tray is stable during one hour aging at 200C. Example 5 which uses a .009 weight percent o, antioxidant shows evidence of fingerprinting while the Example 6 using a slightlv higher level of antioxidant shows only a barely discernible trace of fingerprinting. Example 7 which uses .024 weight percent of A0 shows no evidence of fingerprinting. This is in marked contrast to the Example 3 where nearly eight times the antioxidant level ~as required in order to eliminate fingerprin~ing and the resin displayed objectionable yellow color after aging.
RECYCLE TRIALS

When manufacturing trays by thermoforming from flat sheeting, a typical thermoforming process will yield approxlmately 40~ scrap sheeting after each forming and trim cycle. The sheeting must be reground and mixed with incoming virgin PET/polyolefin material 'or reuse. This recycle produces considerabie thermal , . , 6 ~3 heat history on the polymeric blend leading to degradation problems of discoloration, fingerprinting, intrinsic viscosity loss, and changes in crystallinity.
In order to produce acceptable ~hermoformed trays for food all of the foregoing properties must be stabilized or eliminated in a commercial process involving substantial percentages of regrind. A typical thermoforming process will regrind up to 40%.
Simulation of the steady state operation of such a thermoforming system assumes that the regrind will mean that the same resin must proceed through the sheet making and trav thermoforming system approximately five times. Accordingly, the following experimental scheme simulates the 40% rework, 5 cycle system for evaluating thermal stabilitv. The resin utilized was 97% PET
(1.04 intrinsic viscosity), 3~ linear low density polyethylene, Dowlex 2045 available from Dow Chemical Company and .015% Ethanox 330, available from Ethel Corporation. The antioxidant was melt blended into the LLDPE using a Sterling Transfermix extruder and then pelletized for s~bsequent blending into a 1.75 inch (4$mm) Prodex film extruder along with the PET. The steps of the process are the ~ollowing:
(1) The resin/regrind blends were dried our hours at 170C in a Conair dehumidifying hopper having a drv nitrogen atmosphere.

(2) arter drying, each sample was placed in a 100 vacuum oven to keep it dry and equilibrate the temperature at 100C.

(3) the 100 resin was placed in the extruder hopper and mixed with the LLDPE stabilized with antioxidant to achieve the correct percentage blend.

(4) the blended material was extruded into an amorphous sheet according to the specifications given previously on Table I.

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(5) trays for testing were thermoformed in a Comet Labmaster thermoformer under the following conditions:

Preheat Time in Oven 12 seconds Oven Temperatures 315C top 225C bottom Molding Time 8 seconds Molding Temperature 160C

The e~cess and unformed portions of the sheet were trimmed away from the trays which were thermoformed to yield a matri~ for reprocessing for recycle.

(6) The remaining nonformed matri~ sheet was crystallized at 150C, cooled and ground through a .6 mm screen and mixed with fresh 97/3 PET/LLDPE resin at 60/40 of new resin/regrind. This resin and regrind blend was dried according to step 1 and the experimental steps 1-6 were repeated for five cyc:Les.
The following properties of the trays were taken from each of the five cycles: (1) intrinsic viscosity; (2) Hunter color value "B"; and (3) percent crystallinity as calculated from the densities of the trays. All results are reported in Table III.
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o~ o ~ ~ ~ o~ I~ o o a~

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h -r~ u~ l C`J _ 1 o O ~ 1--l ~ 1` co O O

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r~ O ~ D U~~ a: ~ ~ co u~ o c~l oo co u~ CO O
0 o~00 00 00 00 00 oo a~ o~ o~ 0 00 ~0 o O

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The data shown in Table III illustrates that trays made from film having the antioxi.dant added to the polvolerin component had excellent retention of the intrinsic viscosity, good retention of color and excellent crystallinity control through five regrind cycles. This degree of stability indicates that the material utilizing this method of stabilizing the polyolefin prior to a~dition of PET can be recvcled a multitude of times without sacrificing strength and appearance properties of the finished tray. This ability to be recycled is of critical importance in com~ercial thermoformlng operations where matrix scrap may exceed 40~.

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Claims (21)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of manufacturing a recyclable polyolefin modified polyethylene terephthalate sheet comprising the steps of (a) melt blending an effective amount of a heat stabilizer with a polyolefin derived from olefin monomers containing two to six carbon atoms to form stabilized polyolefin;
(b) heating a polyethylene terephthalate having an intrinsic viscosity of about .65 to about 1.2 to at least 150°C in a dry atmosphere for a time sufficient to reduce the moisture level in the polyethylene terephthalate below 0.02 weight percent to form a dry polyethylene terephthalate;
(c) simultaneously conveying said dry polyethylene terephthalate and said stabilized polyolefin to a means for melt blending;
(d) mixing a minor amount of said stabilized polyolefin with a major amount of dry polyethylene terephthalate to form a mixture;
(e) melt blending said mixture to form a homogeneous molten melt blend;
(f) forming a sheet from said homogeneous melt blend;
(g) cooling said sheet to form a substantially amorphous sheet;
(h) positioning said amorphous sheet over a mold;
(i) thermoforming said sheet in a heated mold for a time sufficient to achieve partial crystallization of said amorphous sheet;

(j) stripping the partially crystalline sheet out of said heated mold;
(k) trimming out the portion of the sheet which was in contact with the mold surface to leave a matrix;
and (l) grinding said matrix to form regrind and heating said regrind according to step (b) and then mixing said regrind with said mixture formed in step (d) and repeating steps (e) through (l) at least one additional time.

2. The method as described in Claim 1 wherein said polyolefin is polyethylene.

3. The method as described in Claim 1 wherein said polyethylene is linear low density polyethylene,

4. The method as described in Claim 1 wherein said polyethylene is linear low density polyethylene, said heat stabilizer is a polyphenol selected from the group consisting of 1,3,5-Trimethyl-2,4,6-tris(3,5-di-t butyl-4-hydroxybenzyl)benzene, and tetrakis(methylene3-(3,5-ditertiary butyl-4-hydroxyphenyl)-propionate)methane.

5. The method of Claim 1 wherein the effective amount of heat stabilizer is less than .05 weight percent.

6. The method of Claim 1 wherein the effective amount of heat stabilizer is from about .005 to about .03 weight percent.

7. The method of Claim 1 wherein said heat stabilizer is 1,3,5-trimethyl-2,4,6-tris(3,5-di-t butyl-4-hydroxybenzyl)benzene.

8. A method of manufacturing an amorphous, thermally stable polyolefin modified polyethylene terephthalate sheet comprising the steps of:
(a) melt blending an effective amount of a heat stabilizer with a polyolefin derived from olefin monomers containing two to six carbon atoms to form a stabilized polyolefin;
(b) heating a polyethylene terephthalate having an intrinsic viscosity of about .65 to about 1.2 to at least 150°C in a dry atmosphere for a time sufficient to reduce the moisture level in the polyethylene terephthalate below 0.02 weight percent to form a dry polyethylene terephthalate;
(c) mixing a minor amount of said stabilized polyolefin with a major amount of dry polyethylene terephthalate to form a homogeneous molten melt blend;
(d) forming a sheet from said homogeneous melt blend;
(e) cooling said sheet point to form a substantially amorphous sheet;

9. The method as described in Claim 8 wherein said polyolefin is polyethylene.

10. The method as described in Claim 8 wherein said polyethylene is linear low density polyethylene.

11. The method as described in Claim 8 wherein said polyethylene is linear low density polyethylene, said heat stabilizer is a polyphenol selected from the group consisting of 1,3,5-Trimethyl-2,4,6-tris(3,5-di-t butyl-4-hydroxybenzyl)benzene, and tetrakis(methylene3-(3,5-ditertiary butyl-4-hydroxyphenyl)-propionate)methane.

12. The method of Claim 8 wherein the effective amount of heat stabilizer is less than .05 weight percent.

13. The method of Claim 8 wherein the effective amount of heat stabilizer is from about .005 to about .03 weight percent.

14. The method of Claim 8 wherein said heat stabilizer is 1,3,5-trimethyl-2,4,6-tris(3,5-di-t butyl-4-hydroxybenzyl)benzene.

15. A method of manufacturing a thermally stable, partially crystalline heat set, non-oriented article comprising the steps of:
(a) melt blending an effective amount of a heat stabilizer with a polyolefin derived from olefin monomers containing two to six carbon atoms to form a stabilized polyolefin;
(b) heating a polyethylene terephthalate having an intrinsic viscosity of about .65 to about 1.2 to at least 150°C in a dry atmosphere for a time sufficient to reduce the moisture level in the polyethylene terephthalate below 0.02 weight percent to form a dry polyethylene terephthalate;
(c) simultaneously conveying said dry polyethylene terephthalate and said stabilized polyolefin to a means for melt blending;

(d) mixing a minor amount of said stabilized polyolefin with a major amount of dry polyethylene terephthalate to form a homogeneous molten melt blend;
(e) forming a sheet from said homogeneous melt blend;
(f) positioning said amorphous sheet over a mold;
(g) thermoforming said sheet to form an article in a heated mold for a time sufficient to achieve crystallinity of between 15 and 35 percent;
(h) stripping said article out of said heated mold; and (i) trimming said article out of said sheet.

16. The method as described in Claim 15 wherein said polyolefin is polyethylene.

17. The method as described in Claim 15 wherein said polyethylene is linear low density polyethylene.

18. The method as described in Claim 15 wherein said polyethylene is linear low density polyethylene, said heat stabilizer is a polyphenol selected from the group consisting of 1,3,5-Trimethyl-2,4,6-tris(3,5-di-t butyl-4-hydroxybenzyl)benzene, and tetrakis(methylene3-(3,5-ditertiary butyl-4-hydroxyphenyl)-propionate)methane.

19. The method of Claim 15 wherein the effective amount of heat stabilizer is less than .05 weight percent.

20. The method of Claim 15 wherein the effective amount of heat stabilizer is from about .005 to about .03 weight percent.

21. The method of Claim 15 wherein said heat stabilizer is 1,3,5-trimethyl-2,4,6-tris(3,5-di-t butyl-4-hydroxybenzyl)benzene.