Modified polyvinylacetals with low solution viscosity

Details Modified polyvinylacetals with low solution viscosity Modified polyvinylacetals with low solution viscosity

1998-11-25

2001-04-03

Reddick, Judy M. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C523S400000, C523S437000, C524S503000, C524S557000, C525S058000, C525S060000

Reexamination Certificate

active

06211289

ABSTRACT:

BACKGROUND OF THE INVENTION
1) Field of the Invention
The invention relates to modified polvinyl acetals, a process for their preparation and their use.
2) Background Art
The preparation of polyvinyl acetals from the corresponding polyvinyl alcohols by polymer-analogous reaction with the corresponding aldehydes has been known since 1924, and many aldehydes have been used for preparing the corresponding polyvinyl acetals since then. Polyvinyl acetals are prepared in a three-stage process (polyvinyl acetate → polyvinyl alcohol → polyvinyl acetal), resulting in products which contain not only vinyl acetal groups but also vinyl alcohol and vinyl acetate units. In the following, modified polyvinyl acetals are those which contain further units in addition to the three units mentioned above. In particular, polyvinyl formal, polyvinyl acetal and polvinyl butyral have achieved commercial importance.
The largest application area for polyvinyl acetals is the production of safety glass for automobile construction and in architecture, where plasticized polyvinyl butyral films are used as intermediate layers in panes of glass. Modified polyvinyl butyrals have also been proposed for this application, for example the polyvinyl butyrals containing sulfonate groups described in EP-A-368832.
Polvinyl butyrals modified with amino groups are known from EP-A 461399. These are used as precipitants.
A further application area for polyvinyl butyrals is their use in anticorrosion coatings. For this purpose, EP-A 505939 (U.S. Pat. No. 5,236,999) proposes the use of modified polyvinyl butyrals which are crosslinkable due to the acetal functions derived from aldehydes containing keto groups.
Owing to, inter alia, their good pigment binding power, polyvinyl butyrals are also used as binders in surface coatings and specifically in printing inks. This application requires that the organic solutions of the polyvinyl butyrals should have a solution viscosity which is as low as possible in order to be able to produce inks having a high solids content at as high a binder content as possible.
In the prior art, the solution viscosity of polyvinyl butyral is regulated via the molecular weight by using polyvinyl acetates having a low molecular weight or degree of polymerization as starting materials for preparing the polyvinyl butyral. The use of low molecular weight polvinyl butyrals as carrier material in pigment preparations for printing inks is described, for example, in DE-B 2643076 (U.S. Pat. No. 4,130,539). A disadvantage here is that considerable problems arise in the preparation of low molecular weight polyvinyl butyrals by acetal formation from low molecular weight polvinyl alcohols, since solutions of low molecular weight, fully saponified polyvinyl alcohols tend, inter alia, to gel and therefore have to be handled at temperatures of at least 50° C.
SUMMARY OF THE INVENTION
It is an object of the invention to provide polyvinyl acetals which, at a comparable vinyl alcohol content in the polyvinyl acetal, generally from 10 to 28% by weight, and a comparable molecular weight compared with the polyvinyl alcohols used as starting materials, have a lower solution viscosity and higher storage stability than the polyvinyl acetals known hitherto.
It has surprisingly been found that polyvinyl acetals which are prepared by saponification and acetalization of copolymers containing vinyl acetate and 1-alkylvinyl acetate units have a significantly lower solution viscosity, for example in ethanol, than polyvinyl acetals which are prepared by acetalization of saponified polyvinyl acetate having a comparable vinyl alcohol content and molecular weight. In addition, aqueous solutions of such copolymeric polyvinyl alcohols have an unlimited shelf life at room temperature.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention provides modified polyvinyl acetals obtainable by saponification of 1-alkylvinyl acetate-vinyl acetate copolymers which comprise 1-alkylvinyl acetate/vinyl acetate in a weight ratio of from 1/99 to 40/60 and subsequent acetalization of the saponification products comprising at least 80% by weight of vinyl alcohol and 1-alkylvinyl alcohol units with one or more aliphatic aldehydes having from 1 to 6 carbon atoms in the presence of an acid catalyst in an aqueous and/or organic medium.
Examples of suitable aliphatic aldehydes having from 1 to 6 carbon atoms are formaldehyde, acetaldehyde, propionaldehyde and preferably butyraldehyde.
Suitable alkyl groups are C
1
-C
4
-radicals such as methyl, ethyl and n-propyl radicals; preference is given to the methyl group.
The modified polyvinyl acetals comprise
a) 50-89.5% by weight of vinyl acetal units
b) 10-30% by weight of vinyl alcohol units —[—CH
2
—CR(OH)—]—,
c) 0.5-20% by weight of vinyl acetate units —[—CH
2
—CR(OCOCH
3
)—]—,
where R is H or a C
1
-C
4
-alkyl radical and n is from 0 to 5 and the vinyl alcohol units b) and the vinyl acetate units c) comprise 1-alkylvinyl alcohol/vinyl alcohol and 1-alkylvinyl acetate/vinyl acetate in a weight ratio of from 1/99 to 40/60 in each case.
The most preferred polyvinyl acetals comprise from 55 to 89% by weight of vinyl acetal units a), from 10 to 25% by weight of vinyl alcohol units b) and from 1 to 20% by weight of vinyl acetate units c). The weight ratio of 1-alkylvinyl alcohol/vinyl alcohol and 1-alkylvinyl acetate/vinyl acetate is preferably from 10/90 to 20/80 in each case. Preferred vinyl acetal units are vinyl formal (n=0), vinyl acetal (n=1), vinyl propional (n=2) and, most preferably, vinyl butyral (n=3).
If desired, the polyvinyl acetals can further comprise from 1 to 5% by weight of additional monomer units. Examples of such units are carboxyl-functional comonomers such as acrylic acid, methacrylic acid and crotonic acid and amide-functional comonomer units such as acrylamide and methacrylamide.
The viscosity of the modified polyvinyl acetals (DIN 53015; Höppler method, 10% strength solution in ethanol) is from 6 to 600 mPas, preferably from 6 to 14 mPas.
The present invention further provides a process for preparing modified polyvinyl acetals by saponification of 1-alkylvinyl acetate-vinyl acetate copolymers which comprise 1-alkylvinyl acetate/vinyl acetate in a weight ratio of from 1/99 to 40/60, and subsequent acetalization of the saponification products comprising at least 80% by weight of vinyl alcohol and 1-alkylvinyl alcohol units with one or more aliphatic aldehydes having from 1 to 6 carbon atoms in the presence of an acid catalyst in an aqueous and/or organic medium.
To prepare the alkyl-modified polyvinyl acetals, use is made of partially or fully saponified vinyl acetate copolymers which contain at least 80% by weight of (1-alkyl)vinyl alcohol units. For the present purpose, fully saponified refers to polymers whose (1-alkyl)vinyl alcohol content is from 98 to 100% by weight. Suitable 1-alkylvinyl acetates are those containing saturated C
1
-C
4
-alkyl radicals. Preference is given to partially or fully saponified copolymers of 1-methylvinyl acetate (isopropenyl acetate) and vinyl acetate. Partially or fully saponified copolymers of 1-alkylvinyl acetate and vinyl acetate are known, for example, from DE-A 3932027 (Derwent Abstract AN 91-095470) as textile adhesives.
To prepare the partially or fully saponified vinyl acetate copolymers, vinyl acetate and 1-alkylvinyl acetate are copolymerized in the specified weight ratio of 1-alkylvinyl acetate/vinyl acetate of from 1/99 to 40/60, preferably from 10/90 to 20/80. If desired, from 1 to 5% by weight, based on the total weight of the comonomers, of further monomers such as comonomers containing carboxyl or amide groups can be copolymerized.
The polymerization is carried out in a known manner, preferably by bulk polymerization or by polymerization in organic solvents, particularly preferably in alcoholic solution. Suitable solvents and regulators are, for example, methanol, ethanol, propanol and isopropanol. The polymerization is carried out under reflux at a temperature of from 60 to 100°

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