Heat seal coating and coated film

Details Heat seal coating and coated film Heat seal coating and coated film

2001-09-05

2003-08-19

Chen, Vivian (Department: 1773)

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C428S346000, C428S347000, C428S349000, C428S3550RA, C428S480000, C428S483000, C528S293000, C528S295000, C156S244110, C156S244230, C156S297000, C156S299000, C427S372200, C427S384000, C427S385500, C427S393500

Reexamination Certificate

active

06607823

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a coated polyester film having a heat seal capability. More specifically, it is related to a water dispersible copolyester coating suitable for application to polyester film that provides a durable, strong, moisture resistant heat seal and other desirable properties. The methods of coating and of forming a heat sealable film are also disclosed.
2. Description of Related Art
Heat sealable films are known in the art. Such films are useful for many applications including lamination and packaging of food materials. Lamination of important documents and other objects is well known in the art. Menus, ID cards and other objects can be protected by such lamination films. Lamination films are typically heat sealed to themselves or to another substrate to form a protective pouch. These heat seals typically are of a strength that the film is destroyed before the seal will release. This gives these seals a tamper-resistant quality that is useful in many applications. With regard to packaging, many prepared foods are packaged in paper or plastic-based trays, with a clear film heat sealed across the top of the tray to protect the food within while leaving it visible to the consumer and easily removable or perforated for cooking and serving. In both such applications, it is desirable for the film to have a number of properties. It should be safe for food contact, for those applications in which edible items will be in proximity to the coating. It is preferably clear or transparent. It must be stable even after heat seal, and preferably after cold storage and heating. It should form a good heat seal, preferably under typical conditions and using standard equipment. It should be able to form a heat seal with itself, the intended tray or other material to which it will be sealed, preferably without use of adjunct adhesives. It is preferably moisture resistant so as to avoid degradation by contact with moisture in or steam released by the food or other contents. It is preferably durable under normal packaging, transport and storage conditions. For many applications, it should also be releasable by the end user without excess force (a “weld seal”, which is not stronger than the film itself). For other applications, it is desired that the film rip before the heat seal releases (a “destruct seal”). The heat seal film should optimally be cost effective to manufacture, and should minimize the use of organic solvents or other potentially environmentally hazardous compounds in its manufacture and use, to the extent possible. Reclaimability is also beneficial. Coatings adapted for in-line application are also desirable. A film providing a combination of these benefits is desirable.
Water-dispersible copolyester coatings for use on polyester film are also known. See, e.g., U.S. Pat. No. 4,493,872 to Funderburk et al. In addition, films formed of polytrimethylene isophthalate have been represented to have heat seal capabilities, in the absence of adhesives or coatings.
BRIEF SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a heat seal coating suitable for application to a base polymer film.
It is a further object of the present invention to provide a heat seal coating that is highly moisture resistant.
It is another object of the present invention to provide a heat seal coating that is substantially free of streaks and smears, particularly when applied to a base polymer film.
It is yet another object of the present invention to provide a heat seal coating that is water dispersible.
It is another object of the present invention to provide a heat seal coating that also offers good adhesion to metals, inks and adhesives.
The present invention provides a heat seal coated film comprising:
a polyester film layer; and
a heat seal coating thereon,
wherein said heat seal coating comprises a copolyester formed from components including isophthalic acid, the sodium salt of 5-sulfoisophthalic acid and propylene diol.
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have surprisingly found that excellent heat seal properties are provided by a coating that includes a propylene diol-based copolyester binder. The selection of binder has been found to be critical to the optimal heat seal results achieved by various coatings of this invention.
The heat seal coating of the present invention preferably includes a propylene diol-based copolyester. Preferably, such copolyester is a water soluble copolyester. More preferably, the copolyester is formed from isophthalic acid and the sodium salt of 5-sulfoisophthalic acid in approximately a 9:1 ratio, and equal parts of 1,3-propane diol. This water soluble copolyester differs from the copolyester as disclosed in U.S. Pat. No. 4,493,872 to Funderburk et al. (the disclosure of which is incorporated herein by reference in its entirety) primarily in the use of a propylene glycol, preferably a propane diol, more preferably 1,3-propane diol. Copolyesters explicitly disclosed in the Funderburk et al. patent, namely ethylene glycol based copolyesters, were first evaluated for vacuum metalization applications and are shown in the examples to have inferior heat seal capability versus the object of this invention. They are also believed to have inadequate resistance to moisture, among other things. Nonetheless, the inventors surprisingly found that a propylene diol based copolyester did have excellent moisture resistance and other desirable properties for a heat seal coating. Without intending to be bound by theory, it is believed that propylene diols, having an odd number of carbon atoms, are conformationally more stable than glycols having even numbers of carbons. This may provide additional stability and moisture resistance to the resulting copolyester. Similarly, the increased “hydrocarbon” character imparted by the longer diol chain may also impart improved properties to a propylene diol-based copolyester.
Thus, the preferred copolyester is the condensation product of the following monomers or their polyester forming equivalents: isophthalic acid, a sulfomonomer containing an alkali metal sulfonate group attached to a dicarboxylic aromatic nucleus and a propylene diol. Optionally, an aliphatic dicarboxylic acid of the formula HOOC(CH
2
)—
n
COOH, where n is about 1 to about 11, can also be employed as a monomer therein. Substitutes for the isophthalic acid preferably include other aromatic difunctional monomers (e.g., DMT, DM-2,6-NDC, etc.). However, highly preferred DMI-based copolyesters are usually amorphous and crystallize very slowly, if at all, making them easy to disperse in water, unlike some DMT-based copolyesters which tend to crystallize rapidly. An optimal copolyester is made up of about 90 mol percent isophthalic acid, about 10 mol percent of the sodium salt of 5-sulfoisophthalic acid and about 100 mol percent 1,3-propane diol.
It is important to note, however, that the preferred percentages of sulfomonomer, isophthalic acid and aliphatic dicarboxylic acid that can be employed are far broader in the context of the present invention. For example, in the context of the present invention, isophthalic acid is preferably about 40 to about 98 mol percent (more preferably about 50 to about 95 mol percent), aliphatic dicarboxylic acid is preferably about 0 to about 50 mol percent (more preferably about 5 to about 20 mol percent), and the sulfomonomer is preferably about 2 to about 20 mol percent (more preferably about 5 to about 15 mol percent). In addition, the sulfomonomer group of the present invention is not limited to an alkali metal sulfonate group. Any sulfomonomer in which a sulfonate group is attached to a dicarboxylic nucleus is preferred for use herein. Propylene diol may be used either alone or in combination with other diols or mixtures thereof. The copolyesters of the present invention preferably include at least about 10 mol percent propylene glycol, alternately at least about 25 mol percent propylene glycol, and furthe

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