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
2000-11-24
2003-06-10
Wu, David W. (Department: 1713)
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...
C524S459000, C524S458000, C524S524000, C524S525000, C524S502000, C524S503000, C526S065000, C526S319000
Reexamination Certificate
active
06576698
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the Invention
The invention relates to a process for the preparation of polyvinyl alcohol-stabilized copolymers based on vinyl ester and (meth)acrylate comonomers in the form of their aqueous dispersions and dispersion powders redispersible in water, and their use.
2) Background Art
Protective colloid-stabilized polymers are used in particular in the form of their aqueous dispersions or polymer powders redispersible in water in various applications, for example as coating materials or adhesives for various substrates. The protective colloids used are as a rule polyvinyl alcohols. The use of polyvinyl alcohol is desirable because, in comparison with systems which are stabilized by low molecular weight compounds (emulsifiers), it itself contributes to the strength (e.g. adhesive strengths in tile adhesive).
The monomers preferably used to date for the preparation of redispersible powders are vinyl esters and ethylene, since the stabilization of acrylate copolymers or styrene/acrylate copolymers by polyvinyl alcohol is not so simple to realize. In particular, it is not a trivial matter to stabilize an acrylate dispersion with polyvinyl alcohol alone so that the powders resulting therefrom are stable to blocking and have a long shelf life. Furthermore, the combination of vinyl ester and ethylene with (meth)acrylates is for the same reason rarely described (for example in EP-A 765898). More frequently described on the other hand are aqueous dispersions based on these, e.g. vinyl acetate/butyl acrylate copolymers or vinyl acetate/acrylate/ethylene copolymers. However, these dispersions are emulsifier-stabilized and therefore cannot be sprayed to give redispersible powders.
With the use of the combination of vinyl ester with ethylene, the obtainable Tg range is limited to a range of from about +30° C. (vinyl acetate homopolymer) to about −25° C. Higher glass transition temperatures are not obtainable since there is no monomer which is suitable for this system and leads to higher glass transition temperatures than vinyl acetate; for ecological reasons, vinyl chloride is no longer contemporary and is scarcely used any more. On the other hand, lower glass transition temperatures than −25° C. can be obtained only with difficulty since amounts of ethylene of substantially more than 50 mol % are required for this purpose in the monomer mixture. Although these polymers having a high ethylene content can be prepared, they tend to crystallize so that a well defined glass transition temperature is no longer found; moreover, these polymers do not have the highly resilient properties expected for this Tg.
By combining vinyl ester and ethylene with (meth)acrylates, it would be possible to extend the Tg range to higher and lower Tg ranges. This may be contrasted with the initially described problems in the preparation of such systems with stabilization by polyvinyl alcohol alone, which systems are required for producing redispersible powders. The greatly differing copolymerization parameters and the associated poorer reactivity of the components are also problematic in the case of the combination of vinyl esters and ethylene with acrylates.
However, problems may occur not only in the preparation but also during the use of products based on vinyl ester copolymers containing ethylene and acrylate. Particularly when polymers are used in the form of their redispersion powders for improving the mortar properties, the main field of use of the redispersion powders, the formulation must remain stable over a certain time and may not substantially change their processing consistency (viscosity or cement stability); this is because the user cannot be expected to have to mix a new mixture within a short timespan. In the concrete and mortar industry, moreover, the mechanical properties, such as the compressive strength, the porosity and hence the air pore content, play a substantial role. If too many air pores are present, the compressive strength decreases considerably; if too few or no air pores are present in the mortar or concrete, the building material is not sufficiently stable to freezing and thawing. The hydraulically setting systems treated with the dispersion powder should moreover ensure better adhesion than the untreated systems.
DE-A 2214410 (U.S. Pat. No. 3,883,489) describes a process for the preparation of aqueous dispersions or dispersion powders redispersible in water and based on vinyl acetate/ethylene copolymers. The polymerization is carried out at a temperature of 60° C. and in the presence of emulsifiers and polyvinyl alcohol. The preparation of emulsifier-free, purely polyvinyl alcohol-stabilized vinyl ester/(meth)acrylate copolymers is not described.
EP-A 765898 (U.S. Pat. No. 5,763,508) discloses a dispersion or a dispersion powder based on a polyvinyl alcohol-stabilized vinyl acetate/vinyl versatate/butyl acrylate copolymer. No information is provided about the process for the preparation.
EP-A 432811 describes the preparation of emulsifier-stabilized vinyl ester/(meth)acrylate copolymer dispersions and dispersion powders, in which the monomer mixture is metered in continuously as a preliminary emulsion. The preparation of polyvinyl alcohol-stabilized vinyl ester/(meth)acrylate copolymers is not described.
EP-A 576844 describes, inter alia, the preparation of vinyl acetate/ethylene/acrylate copolymer dispersions and dispersion powders. The use of emulsifiers is expressly recommended, and the use of protective colloids other than polyvinyl alcohol is described as being advantageous.
WO-A 99/42504 describes a process for the preparation of vinyl ester/(meth)acrylate copolymer dispersions, in which the copolymer is prepared in the presence of a mixture of nonionic emulsifier and polyvinyl alcohol, and the comonomers are metered in continuously as a mixture.
EP-A 757065 (U.S. Pat. No. 5,447,578) describes the preparation of protective colloid-stabilized vinyl acetate/ethylene copolymers in the form of their aqueous dispersions and dispersion powders redispersible in water. Two-phase polymers having a first ethylene-rich, semicrystalline phase and a second low-ethylene phase are obtainable thereby. The copolymerization of (meth)acrylates is not discussed.
SUMMARY OF THE INVENTION
It was therefore the object of the invention to provide a stable, low-viscosity, polyvinyl alcohol-stabilized dispersion and corresponding dispersion powders redispersible in water, stable to blocking and based on monomers from the group consisting of the vinyl esters, ethylene and (meth)acrylates, in particular methacrylates, which, when used in cement-related applications, have completely satisfactory viscosity stability or cement stability and do not prevent the cement from setting.
The invention relates to a process for the preparation of polyvinyl alcohol-stabilized polymers based on vinyl ester, (meth)acrylate and optionally ethylene monomers in the form of their aqueous dispersions and dispersion powders redispersible in water, by means of free radical emulsion polymerization or suspension polymerization of one or more monomers from the group consisting of the esters of acrylic acid and methacrylic acid with vinyl esters, optionally ethylene and optionally further monomers copolymerizable therewith, in the presence of one or more polyvinyl alcohol protective colloids, and optionally drying of the aqueous dispersion obtained thereby, wherein
a) from 50 to 100% by weight of the total weight of vinyl ester monomers employed, from 0 to 40% by weight of the total weight of (meth)acrylate monomers employed and at least 80% by weight of the total weight of ethylene employed are initially introduced in the polymerization and
b) the polymerization is carried out at a temperature of ≧60° C.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Suitable monomers from the group consisting of the esters of acrylic acid or methacrylic acid are esters of straight-chain or branched alcohols having 1 to 15 carbon atoms. Preferred methacrylates or acrylates are methyl acrylate
Brooks & Kushman P.C.
Hu Henry S
Wacker Polymer Systems GmbH & Co. KG
Wu David W.
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