Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-04-13
2003-08-19
Yoon, Tae H. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S503000, C524S803000, C524S904000, C525S061000, C525S934000
Reexamination Certificate
active
06608128
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to self-crosslinking polyvinyl alcohol compositions in the form of aqueous solutions having an extended stability on storage, to a process for their preparation and to their use for preparing films that crosslink at room temperature. Specifically, the invention relates to methods for preparing sheets and adhesive bonds that are resistant to boiling water.
2. Description of Related Art
Adhesives based on polyvinyl alcohol are known in the prior art. Due to their adhesive force, they can be used in the form of aqueous solutions, and specifically as a raw material for bonding cellulosic substrates such as wood, paper or cards. Examples of such fields of application include the preparation of special paper laminates and spiral or parallel sleeves. Alongside a wide range of other applications, polyvinyl alcohol films also are used as packaging materials, in which context they are used on their own or else as a constituent of laminates in composite films.
A known disadvantage of these applications lies in the hydrophilicity of the polyvinyl alcohol. When polyvinyl alcohol is used as an adhesive film, there typically is poor water resistance of the adhesive bonds or, when it is used in films, it produces an inadequate barrier effect to oxygen under conditions of high gas humidity. By using fully hydrolyzed polyvinyl alcohols which are insoluble in cold water it is possible, for example, for use as a paper adhesive, to bring about a certain level of resistance to the action of cold water. Adhesive bonds resistant to boiling water, however, are not obtainable, nor are water-resistant adhesive films or cast films formed from the easy-to-prepare aqueous solutions of partially hydrolyzed polyvinyl alcohols which are soluble in cold water.
It is known in principle to reduce the sensitivity to water of polyvinyl alcohol by means of a large number of possible reagents, for example, using bifunctional aldehydes (see, Finch, C. A., POLYVINYL ALCOHOL, C. A. Finch (Ed); John Wiley and Sons, New York (1992), Chap. 9).
Polyvinyl alcohol films which are resistant to boiling water and are suitable as a flexible packaging material can be prepared, according to U.S. Pat. No. 4,376,183, from an aqueous composition which comprises in aqueous form dissolved polyvinyl alcohol, dialdehydes capable of crosslinking, and, dispersed in this solution, metal orthophosphates, preferably of aluminum or iron. The resistance to hot water arises after heat treatment.
JP-A 117991/77 (Chemical Abstracts 88:74939) describes the preparation of polyvinyl alcohol molding compounds from aqueous solutions by crosslinking with difunctional aldehydes having at least 3 carbon atoms in the presence of acidic catalysts, including salts, and subsequent heat treatment at 50° C. These polyvinyl alcohol molding compositions are resistant to boiling water.
The resistance to boiling water of wet-spun polyvinyl alcohol fibers can be improved according to JP-A 163609/93 (Chemical Abstracts 119:141112) by treating them with dialdehydes, such as glutaraldehyde, or acetals of dialdehydes, for example tetramethoxypropane, in a medium acidified with sulfuric acid. The aqueous polyvinyl alcohol solution includes boric acid added prior to spinning.
Water-resistant polyvinyl alcohol compositions suitable for adhesives and sizes are described by JP-A 157860/94 (Chemical Abstracts 121:257180). In addition to polyvinyl alcohol, these compositions comprise chitosan, aldehydes, for example glyoxal, reducing agents and free-radical scavengers, for example hydroquinone monomethyl ether. The group of reducing agents includes sulfur compounds, including sodium hydrogen sulfite.
Despite the fact that the prior art provides access in principle to polyvinyl alcohol films that are resistant to boiling water, in the case of crosslinking by way of bifunctional aldehydes having at least 3 carbon atoms in an acidic medium, comparison Example 1 below shows that such compositions have a tendency to undergo rapid gelling, and are thus of only limited stability on storage. This feature is particularly detrimental to their use as an adhesive.
SUMMARY OF THE INVENTION
Thus, there exists a need to provide polyvinyl alcohol compositions in the form of aqueous solutions which feature an extended stability on storage of at least four weeks and from which it is possible to prepare, on drying, preferably even at room temperature, films having a high level of resistance to boiling water. It is accordingly an object of the present invention to provide aqueous polyvinyl alcohol compositions that are storage stable and that are resistant to boiling water.
In accordance with this and other objects of the invention, there is provided polyvinyl alcohol compositions in the form of aqueous solutions that, in addition to polyvinyl alcohol, comprise at least bifunctional aldehydes that have at least 3 carbon atoms and that are completely masked in the form of a water-soluble hydrogen sulfite adduct from which, by means of acidic compounds, aldehyde groups capable of crosslinking can be released in a controlled manner. Accordingly, the present invention provides a polyvinyl alcohol composition in the form of an aqueous solution comprising polyvinyl alcohol, at least one polyaldehyde having at least 3 carbon atoms which is masked completely as a water-soluble hydrogen sulfite adduct, and at least one compound which is acidic in water.
In accordance with an additional object of the present invention, there is provided a method of making aqueous polyvinyl alcohol compositions comprising admixing a polyvinyl alcohol with at least one polyaldehyde having at least 3 carbon atoms which is masked completely as a water-soluble hydrogen sulfite adduct, and at least one compound which is acidic in water. In accordance with yet another object of the invention, there is provided a method of making a water-soluble polyvinyl premix comprising admixing an at least partially water-soluble polyvinyl alcohol powder or granule with at least one polyaldehyde having at least 3 carbon atoms which is masked completely as a water-soluble hydrogen sulfite adduct and then drying. In accordance with yet another object of the invention, there is provided an adhesive or a film comprising an aqueous polyvinyl alcohol composition including polyvinyl alcohol, at least one polyaldehyde having at least 3 carbon atoms which is masked completely as a water-soluble hydrogen sulfite adduct, and at least one compound which is acidic in water.
These and other objects of the present invention will be readily apparent to those skilled in the art upon review of the detailed description that follows.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Any polyvinyl alcohol may be used in the present invention. Preferably, polyvinyl alcohol that is useful in the present invention can be prepared in general by hydrolysis of polyvinyl acetate. Suitable polyvinyl alcohol preferably has a degree of hydrolysis of from 70 to 100 mol-% and a viscosity, as a 4% strength aqueous solution, of from 2 to 70 mPas. Polyvinyl alcohols having a degree of hydrolysis of from 80 to 98 mol-% preferably are used due to their solubility in water. Partially hydrolyzed polyvinyl alcohol soluble in cold water, with a degree of hydrolysis of about 88 mol-% and a viscosity, as 4% strength aqueous solutions, of 18-40 mPas are particularly preferred.
The concentration of polyvinyl alcohol in the aqueous solution, whose upper limit is set by the molecular weight of the polyvinyl alcohol, preferably is at least 0.1% by weight, based on the total weight of the composition. Depending on the degree of viscosity, it is preferred to operate in concentrations of from 2 to 30% by weight. Preference is given to operating with a 1 to 25% strength by weight solution, particularly preferably one having a concentration of from 5 to 15% by weight. The viscosity range of the finished composition, after the additives described below have been added, preferably is below 10,000 mPas, more preferably below 10
Gutte Richard
Jakob Martin
Foley & Lardner
Kuraray Specialities Europe GmbH
Yoon Tae H.
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