Elevated temperature dimensionally stable, impact modified...

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...

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C524S401000, C524S503000, C524S507000, C524S513000, C525S067000, C525S123000, C525S166000, C525S176000, C525S177000, C528S503000, C264S237000

Utility Patent

active

06169143

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a thermoplastic crystallizable impact modified polyester composition having improved gas barrier and dimensional stability characteristics, method of making and articles made of this composition.
2. Technology Review
Polyesters have heretofore been widely used in the food package industry, including blister packs for meats, containers for frozen foods, ovenable and microwavable (“dual ovenable”) trays and carbonated beverage bottles. A major effort in such packaging applications has been directed toward reducing the gas permeability of the package, since a decrease in such permeability will lead to a longer shelf life of the food product, be it at frozen, refrigerated or ambient temperature storage. Another focus of great effort in the food packaging industry is the dimensional stability of the package over long periods of time at ambient temperature or upon heating of the contents in either microwave or conventional ovens. Of particular concern in this regard are the breakage of seals and warping due to excessive shrinkage of the container during retort or any of the thermal sterilization processes. Still another effort of great focus is the impact strength of the package at low temperatures. Improved impact strength of the package allows for greater durability at low temperature applications which translates to greater versatility of the packaging product.
A number of prior patents have addressed the above-mentioned concerns, for the most part individually. As a result, for example, polyester compositions that are particularly suitable as carbonated beverage containers due to there low gas permeability, are generally unsuitable for applications and processes requiring elevated temperatures, as these materials generally exhibit excessive shrinkage and warping at these higher temperatures. Examples of such prior activity include:
U.S. Pat. No. 4,560,741 discloses a polyester resin derived from a C
(2-8)
diol, oxydiacetic acid and naphthalene dicarboxylic acid having improved resistance to gas permeability as compared to polyethylene terephthalate homopolymers.
U.S. Pat. No. 3,960,807 discloses a heat-set article comprising a polymeric crack-stopping agent and a nucleant as having a good dimensional stability and impact resistance.
U.S. Pat. No. 4,463,121 discloses thermoformed articles of partially crystallized polyethylene terephthalate and a polyolefin, as having improved impact resistance and high temperature dimensional stability.
U.S. Pat. No. 4,572,852 discloses a crystalline polyethylene terephthalate/polyolefin article as having high dimensional stability.
U.S. Pat. No. 4,618,515 discloses a polyethylene terephthalate wide mouth bottle wherein the neck portion has higher thermal and strain crystallinity than the rest of the bottle, such that the neck is more resistant to shrinkage during a hot-fill process.
U.S. Pat. No. 4,535,025 discloses a biaxially-oriented, heat-set polyethylene terephthalate material with a density of over 1.4050 g/cc at 25° C. as having improved gas barrier properties.
U.S. Pat. No. 4,282,277 discloses a biaxially-oriented thermoset polymer seamless conduit as having good low temperature tensile impact strength.
U.S. Pat. No. 5,003,041 discloses blends of polyethylene terephthalate and trans-4,4′-stilbenedicarboxylic acid as having improved gas barrier properties as compared to polyethylene terephthalate homopolymer.
U.S. Pat. No. 4,764,403 discloses a biaxially-oriented, heat-set, multi-layer article with an inner layer of polyethylene terephthalate, as having high barrier properties and improved thermal stability.
U.S. Pat. No. 4,874,647 discloses a composition of polyethylene terephthalate and bisphenol-A polycarbonate for use in a polyester laminate. The composition is disclosed as providing for improved mechanical strength for a heat resistant polyester.
U.S. Pat. No. 4,061,706 discloses a continuous melt process for thermoforming thermoplastic polymers, preferably polyamides.
U.S. Pat. No. 4,261,473 discloses a container made of thermoplastic resin, oriented in at least one direction, as having an oxygen permeability of lower than 5×10
−11
cc-cm/cm
2
-sec-cm Hg.
U.S. Pat. No. 4,469,270 discloses a shaped container of polyalkylene terephthalate with a crystallinity of at least 20 percent as determined by density measurement.
U.S. Pat. No. 4,996,269 discloses a thermoplastic resin of polyethylene naphthalate and a polyester elastomer and having a crystallinity from 10 to 40 percent as measured by density, as having high dimensional stability.
U.S. Pat. No. 4,022,748 discloses a polyester which is impact modified by a rubber elastic graft copolymer and is melt extruded at between 230-280° C. with improved impact properties.
U.S. Pat. No. 4,244,859 discloses a polyester which is impact modified by an acrylic rubber and is melt mixed with improved impact properties.
U.S. Pat. No. 4,525,529 discloses a polyester which is impact modified by an ethylene copolymer and an acrylic rubber which is melt kneaded at 250° C. with improved impact properties.
U.S. Pat. No. 4,564,658 discloses a polyester which is impact modified by a linear low density polyethylene and is melt extruded at between 230-300° C. with improved impact properties.
U.S. Pat. No. 4,948,842 discloses a polyester which is impact modified by a copolymer of ethylene, propylene, and diene rubber which is melted extruded at 230° C. with improved impact properties.
U.S. Pat. No. 4,977,217 discloses a polyester which is impact modified by a copolymer of ethylene, propylene, polyene grafted with an epoxide functional ester which is melt extruded at between 180-260° C. with improved impact properties.
U.S. Pat. No. 5,086,118 discloses a polyester which is impact modified by a copolymer of ethylene and which is melt extruded as having improved impact properties.
The conventional process for manufacturing polyester containers, herein referred to as a “glass-to-mold” process, has at least two heating steps; the first during production of the polyester source material by the supplier, and the second during shaping of the polyester into a container by the manufacturer. In the first step of the conventional process, the source polyester material is cooled as it is formed into pellets, rolls, sheets or other shapes suitable for shipping, storage and subsequent processing into articles of manufacture. In many processes, such as that for producing amorphous polyethylene terephthalate (A-PET), the cooling of the material from the molten state is at a sufficiently rapid rate so as to thermally quench most of the dynamic crystallization of the polymer and thus produce an undercrystallized material. In addition, thermal gradients may arise in the polyester during heating and cooling. The material stresses due to these thermal gradients then become frozen in the ambient temperature material. Such stresses due to thermal gradients are referred to herein as thermally induced stresses.
In the second step of conventionally producing articles made of polyesters, the pellets, sheets, etc. of polyester made in the first step are reheated until the material reaches a recrystallization onset temperature. At this point recrystallization of the material begins. Increased crystallinity is desirable in a product as it increases the melting temperature of the polyester so as to allow it to be used in a conventional oven for reconstitution.
Recrystallization upon reheating of a crystallizable polyester may be due to the further growth of existing crystals in the material or to the formation of new crystals, or both. The recrystallization onset temperature of a polyester may be easily detected by heating differential scanning calorimetry as that temperature at which the exothermic recrystallization reaction begins. The recrystallization onset temperature of a polyester material as determined in this way is localized to a temperature between the glass transition temperature and the melting temperature of the material and is

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