Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Reexamination Certificate
2001-05-15
2002-04-09
Cameron, Erma (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
C427S536000, C427S307000, C427S309000, C427S340000, C427S412300, C427S419700
Reexamination Certificate
active
06368677
ABSTRACT:
BACKGROUND OF THE INVENTION
The coating of thermoplastic polymer substrates with moisture resistant barrier coating compositions to provide impermeability to gases such as oxygen, and liquids, is known. See, e.g., U.S. Pat. No. 3,282,729, which describes applying a water solution of poly(vinyl alcohol) (PVOH) and trimethylolphenol to a thermoplastic polymer substrate at an elevated temperature. Also, for example, U.S. Pat. No. 5,073,419 refers to a film composite comprising a linear low density polyethylene film having a PVOH coat of a thickness of about 0.1 to about 3 mils. U.S. Pat. No. 5,487,940 refers to a metallized polymeric film structure including an oxygen barrier and a moisture barrier. The oxygen barrier includes cross-linked PVOH and the moisture barrier is preferably metallized oriented polypropylene or polyethylene.
Similarly, U.S. Pat. No. 4,254,170 refers to bioriented polyester hollow bodies wherein one wall of a preform is coated with an aqueous composition consisting of at least two incompatible polymers, one of which is a water soluble PVOH and the other, a polymer latex with low water sensitivity.
U.S. Pat. No. 5,384,192 refers to a structure comprising an inorganic oxide substrate having an organic polymer layer. There is an adhesion promoting layer between the substrate and organic polymer layer. One component of the adhesion layer is poly(vinyl phenol), also known as poly(hydroxystyrene).
U.S. Pat. No. 5,192,620 refers to a metallized film composition comprising a polymer substrate having a surface which is modified by an adhesion promoting agent, in which the modified surface is provided with a skin layer of PVOH. The PVOH skin layer is applied by a solution coating process. The skin layer has a metal layer directly thereon.
U.S. Pat. No. 5,491,023 refers to a metallized film composition comprising a polymer substrate having a surface which is modified by an adhesion promoting agent; the modified surface is provided with a skin layer of PVOH. The PVOH skin layer is applied by an extrusion process. The skin layer has a metal layer directly thereon.
Despite the wealth of art in barrier coatings, currently available polymeric films do not accommodate the need in the market for longer shelf life of packaged food. Further, many of these products (e.g., films coated with aluminum) are not microwave-safe, or are not readily disposable (e.g., films coated with poly(vinylidene chloride), and thus fail to satisfy environmental concerns.
There exists a need in the art for additional compositions and methods which provide improved barrier coatings to polymeric products, e.g., films, bottles, etc.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides a method for providing a barrier coating to a polyolefin substrate, preferably a film. The method involves the steps of applying to the polymeric substrate a primer composition containing a random vinyl copolymer comprised of repeat units, wherein at least 75% of said copolymer repeat units contain side groups having hydroxyl moieties. The primer layer is then dried. The third step of the method involves subsequently applying a waterborne barrier coating solution which forms a dry inorganic barrier layer over the dried primer layer.
In another aspect, the present invention provides a barrier-coated polyolefin article.
Other aspects and advantages of the present invention are described further in the following detailed description of the preferred embodiments thereof.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a method of promoting the wetting of waterborne barrier coating solutions on polyolefin substrates and adhesion of resulting inorganic barrier layers to polyolefin substrates by applying a selected primer composition to the substrate before application of the barrier coating solution. The improved adhesion of dried inorganic barrier layers is manifested in improved vapor barrier performance.
I. The Primer Compositions
A primer composition according to the present invention contains a random vinyl polymer comprised of repeat units, (i.e., derived from “vinyl” monomers like ethylene, propylene, vinyl acetate, vinyl phenol, etc.) wherein at least 75% of said polymer repeat units contain side groups having hydroxyl moieties. Desirably, vinyl polymers include poly(vinyl alcohol) and poly(para-hydroxystyrene).
In one embodiment, the vinyl polymer useful in the primer is a poly(vinyl alcohol) (PVOH). There are numerous PVOH variants available commercially and/or known in the art. For example, such PVOH variants vary in characteristics such as molecular weight and percentage hydrolyzed. Poly(vinyl alcohol) is derived from poly(vinyl acetate) by hydrolysis of the acetate function. Typically a fraction of the acetate functions are left intact to impart different properties. The percent hydrolysis refers to the fraction of acetate groups which have been hydrolyzed to hydroxyl moieties. Desirably, the average molecular weight distribution of the PVOH polymers useful in the primer composition is between about 50,000 and about 185,000 and the PVOH is more than about 75% hydrolyzed.
The PVOH useful in this invention may be modified by randomly replacing some of the vinyl alcohol groups with vinyl butyral groups as depicted by the formula:
wherein n, p, and r denote the mole fractions of the polymer repeat units and the sum of n, p and r is 1. To achieve satisfactory wetting of barrier coating solutions on these primer layers it is preferable that r is at least 0.75. Desirably, the average molecular weight of such a modified PVOH is between about 40,000 and 100,000.
The PVOH variants may be dissolved in a suitable solvent (e.g. water, isopropanol, or mixtures thereof), so that the primer composition is characterized by a solids level of between 0.1% and 10% by weight. When the polymer in the primer composition is PVOH, preferably, the solvent useful in the primer composition is water. However, other suitable solvents for PVOH may be readily selected by one of skill in the art.
It is well known that PVOH solutions support the growth of microorganisms. To prevent this, it is conventional practice to optionally add at least one biocidal agent to the coating solution. Desirable biocides include those with the active ingredients 5-chloro-2-methyl-4-isothiazolin-3-one, 2-methyl-4-isothiazolin-3-one, or 1-(3-chloroallyl)-3,5,7-triaza-1-azoniaadamantane chloride. A representative biocide that was found not to interfere with the performance of dried primer layers in coated articles of the present invention is Kathon® LX (Rohm and Haas) biocide.
Another embodiment of a vinyl polymer useful in the primer composition is a linear or branched poly(para-hydroxystyrene) (PHS) or a variant thereof. There are a number of PHS variants available commercially and/or known to the art. For example, such PHS variants vary in characteristics such as molecular weight, impurities and degree of branching. While the examples below employ several variants of poly(para-hydroxystyrene), (branched, linear, and partially converted to phenoxide salts) it is anticipated that other PHS variants, for example copolymers in which the PHS mole fraction is at least 0.75, will work similarly in the primer composition and method of this invention.
Desirably, the primer composition containing PHS is characterized by a solids level of between 0.1% and 10% by weight. Preferably where the polymer in the primer composition is a PHS variant, the solvent is a dilute aqueous solution of alkaline metal hydroxide. For example, a lithium (or sodium or potassium) hydroxide concentration of 0.1 N may be used to prepare a 1 weight percent solution of PHS. In this solvent PHS is partially converted to an alkali metal (e.g., lithium) phenoxide salt. The resulting linear or branched random copolymer has the formula:
where M
+
is a monovalent cation (e.g., Li
+
)and the sum of the mole fractions, n and p, is 1. The mole fraction, p, of the phenoxide salt can be controlled by adjusting the concentration of alkaline metal hydroxide.
Dyn
Holcomb Douglas R.
Hubbard Michael A.
McCaulley James A.
Cameron Erma
Hoechst Trespaphan GmbH
Howson and Howson
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