Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
1999-12-02
2003-06-10
Short, Patricia A. (Department: 1712)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C428S035700, C428S036920, C525S064000, C525S166000, C525S176000
Reexamination Certificate
active
06576309
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to polymeric materials and, more particularly, to thermoplastic polyester compositions having high dimensional stability at elevated temperatures with improved toughness, which especially are useful in food-grade applications such as dual-ovenable containers.
BACKGROUND OF THE INVENTION
Polyesters are polymeric materials typically made by a condensation reaction of dibasic acids and dihydric alcohols. Common examples of polyesters include alkylene terephthalate and naphthalate polymers such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polycycloterephthate (PCT), polycycloterephthatlic acid (PCTA), (poly)ethylene-co-1,4-cyclohexanedimethylene terephthalate (PETG), and polytrimethylene terephthalate (PTT). Polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) are examples of polyesters having excellent barrier properties, excellent chemical resistance, good dimensional stability at room temperature, and high abrasion resistance. However, polyalkylene terephthalates tend to become brittle upon crystallization, especially upon thermal crystallization, and are not dimensionally stable at temperatures above their glass transition temperature (T
g
). As a consequence, non-oriented, thermally crystallized polyalkylene terephthalates have poor ductility, poor impact resistance, and poor heat resistance, which limit the utility of the polymer in many applications.
Several attempts have been made to improve the impact properties of polyalkylene terephthalates, including the addition of various impact modifiers as described in U.S. Pat. Nos. 4,172,859, 4,284,540, and 4,753,980. U.S. Pat. No. 4,753,980 discloses the use of an ethylene
-butylene acrylate/glycidyl methacrylate ter-polymer to produce toughened polyester.
Another class of polymer having widespread utility is polyethylenes, which are ethylene-based polyolefin polymers. Polyethylenes most often are linear but also can be branched. Linear polyethylenes typically are classified by density, e.g., low-density polyethylene (LDPE), medium-density polyethylene (MDPE), high-density polyethylene (HDPE), and the like. Polyethylenes exhibit good toughness, low moisture absorption, high chemical resistance, excellent electrical insulating properties, low coefficient of friction, and ease of processing. However, polyethylenes have poor load-bearing and gas barrier characteristics, and relatively poor heat resistance properties.
Numerous attempts have been made to combine polyalkylene terephthalates and polyethylene polymers in efforts to realize the above-described useful properties of each of the two types of polymer. However, polyalkylene terephthalates and polyethylenes are highly incompatible because of their significantly different physical and chemical properties, such as solubility, surface tension, and polarity. Combining the two polymers typically results in a phase-separated blend exhibiting poor mechanical properties, especially impact properties. In addition, each of the polymers retains its own thermal properties. Therefore, the phase-separated blend thermally degrades at the lower degradation temperature of the two polymers. Such a blend is entirely unsatisfactory.
Various thermoplastic elastomers have been proposed to improve the compatibility of polyalkylene terephthalates and polyethylenes. Traugott et at.,
J. Appl. Poly. Sci.,
28, 2947 (1983), describes blends of polyethylene terephthalate and high density polyethylene (PET/HDPE) which are said to exhibit high ductility. The blend compositions utilize up to 20 weight percent of a styrene/ethylene-butadiene/styrene tri-block co-polymer (SEBS) or an ethylene-propylene co-polymer as a compatibilizing thermoplastic elastomer.
The relatively large concentrations of the thermoplastic elastomers which are required to improve compatibility, however, can diminish other desirable properties of the polymer blends, most notably impact properties and thermal stabilities. U.S. Pat. No. 5,436,296 describes a co-polymer of a C
2
-C
10
alpha-olefin and a glycidyl or isocyanate group-containing functional compound which is said to compatibilize thermoplastic blends of polyalkylene terephthalates and polyethylenes while improving impact properties and heat resistance. The thermoplastic blend is described as a continuous matrix of polyalkylene terephthalate with polyethylene domains dispersed therein.
Food grade containers which can be used for cooking or reconstituting foodstuffs are of particular interest. Such containers, whether disposable or intended for re-use, typically are heated to temperatures exceeding 250° F. and must be capable of being heated to at least about 350-400° F. without significant distortion of the rigid package if the container is to be considered ovenable. Food containers made of polymeric materials are used in a wide variety of applications. For example, foamed polystyrene is widely used in making hot drink cups. It is also used in making “clam shells” which are used by the fast food industry as packages for hamburgers and other types of sandwiches. One drawback associated with the use of polystyrene is the possible migration of residual styrene into food products, especially when the container is reheated, e.g., by a microwave oven. There are strict limitations on the quantities of styrene and various other plastics components which may be liberated from a plastic container into food in the container.
The wide spread popularity of microwave ovens for home use has initiated interest in food trays which can be used in either microwave ovens or convection ovens. Such trays are of particular value as containers for frozen prepared foods. It is important for such trays to have good impact strength and dimensional stability at both freezer and oven temperatures. Of course, it also is important for such trays to be capable of withstanding rapid heating from freezer temperatures of about −22° F. to oven temperatures exceeding about 250° F.
Containers which are capable of being heated in either convection ovens or microwave ovens are sometimes described as being dual-ovenable. Polyesters are highly suitable for use in making such dual-ovenable containers. However, it is important for the polyester to be in the crystalline state rather than the amorphous state in order to achieve satisfactory high temperature stability. Normally, polyesters will undergo crystallization by heat treatment at elevated temperatures and the crystallites formed will remain substantially stable up to near the melting point of the polyester. As a general rule, dual-ovenable containers which are comprised of polyester will be heat-treated to attain a crystallinity of higher than about 20%.
Injection molding and thermoforming are widely known methods for forming thermoplastic polyester articles. In injection molding, the polyester is heated above its melting point and injected under sufficient pressure to force the molten polyester to fill the mold cavity. The molten polyester is cooled in the mold until it is rigid enough to be removed. Injection molding of a polyester composition containing 0.5% to 10% by weight isotactic polybutene-1 is described in U.S. Pat. No. 3,839,499. This injection molding method, however, generally is not satisfactory for the production of thin walled articles, such as dual-ovenable trays, due to flow lines and layering which develop during the filling of the mold which lead to non-uniform properties, surface irregularities, and warping of the finished article.
Thermoforming is another process which is used commercially in the production of polyester articles. It is a particularly valuable technique for use in producing thin walled articles, such as dual-ovenable food trays, on a commercial basis. In thermoforming, a sheet of preformed polyester is preheated to a temperature sufficient to allow 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, or
Bond Adriana
Bond John
Dalgewicz, III Edward J.
Associated Packaging Enterprises
Banner & Witcoff , Ltd.
Bond Adriana
Short Patricia A.
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