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
2000-05-31
2003-08-12
Shosho, Callie (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...
C524S560000, C524S832000, C526S078000, C526S079000, C526S080000, C526S081000
Reexamination Certificate
active
06605663
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for preparing polyvinyl alcohol-stabilized addition polymers based on (meth)acrylate monomers in the form of their aqueous dispersions and water-redispersible dispersion powders.
2. The Prior Art
Addition polymers stabilized by protective colloid are used in particular in the form of their aqueous dispersions or water-redispersible polymer powders in diverse applications. For example, they are useful as coating compositions or adhesives for a very wide variety of substrates. The protective colloids used are generally polyvinyl alcohols. The use of polyvinyl alcohol is worthwhile because, in comparison to systems stabilized by low molecular mass compounds (emulsifiers), it makes its own contribution to strength (e.g., tensile adhesion values in a tile adhesive). Monomers used to prepare redispersible powders are preferably vinyl esters and ethylene. This is because the stabilization of acrylate polymers or styrene-acrylate polymers by polyvinyl alcohol is not so easily accomplished. In particular, it is no trivial matter to stabilize an acrylate dispersion using polyvinyl alcohol alone in such a way that the resultant powders are stable to blocking and stable on storage. In general, for the purpose of obtaining a manageable dispersion viscosity, regulators are used in addition in order to reduce the molecular weight. This in turn greatly impairs the blocking stability of the powder.
Especially when addition polymers are used in the form of their redispersion powders to improve the properties of mortar, a major field of use for redispersion powders, the formulations are required to remain stable for a certain time. The formulations must not suffer any substantial alteration in their processing consistency (viscosity stability or cement stability). Indeed, a user cannot be expected to have to stir up a new mixture within a short time span. In the concrete and mortar industry, furthermore, an important part is played by the mechanical properties, such as the compressive strength, the porosity and thus the air pore content. If there are too many air pores the compressive strength falls sharply. If there are too few, or no, air pores in the mortar or concrete, the building material will lack sufficient freeze-thaw stability. The hydraulically setting systems enhanced with the dispersion powder are also intended, moreover, to give better adhesion than the unenhanced systems.
EP-B 718314 discloses a process for preparing polymer dispersions stabilized with protective colloid, in which at least 50% by weight of the monomers and up to 50% by weight of the initiator are introduced initially before the beginning of the polymerization and the remainder is metered in continuously. The initiator is used in the standard amounts of from 0.05% to 1.0% by weight, based on monomer.
EP-A 812863 describes a process for preparing polyvinyl alcohol-stabilized polymer dispersions based on acrylate monomer. This uses fully hydrolyzed polyvinyl alcohols having a defined low molecular weight for the purpose of stabilization and in which the polymerization is mandatorily conducted in the presence of regulators. Some of the monomers and initiator are included in the initial charge, and some is metered in.
For preparing polymer dispersions stabilized by protective colloid, EP-A 821016 recommends the copolymerization of relatively large amounts of hydroxyalkyl acrylates. As a result, the products obtained are markedly more susceptible to water than is desirable. For the polymerization, the oxidation component of the redox initiator is included in its entirety in the initial charge and the monomers and reducing agent are metered in continuously.
EP-A 671420 discloses aqueous, protective-colloid-stabilized dispersions of acrylate polymers having improved water resistance. They are obtained by polymerization in the presence of a mixture of polyvinyl alcohol and hydroxyethylcellulose. The preparation process it describes, with the metered addition of the total amount of monomer, leads to serviceable products only with this specific mixture.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a stable, low-viscosity, polyvinyl alcohol-stabilized dispersion and corresponding water-redispersible dispersion powders, based on acrylate monomers, which when used in cementitious applications possess a completely satisfactory viscosity stability or cement stability and do not hinder the setting of the cement.
Normally, emulsion polymerizations are conducted at high conversion, i.e. with little monomer, in order to avoid renewed nucleation of particles. Surprisingly it has now been found that only the opposite route leads to the objective. Thus polymerization must be conducted with very small amounts of initiator, with complete metering of the initiator using a metering gradient. That is, the first polymerization phase must be conducted with very large amounts of monomer, in order to obtain a dispersion which is free from gel specks and coagulum and has the desired properties, such as good cement stability and blocking stability of the resultant powders.
The present invention achieves this object by providing a process for preparing polyvinyl alcohol-stabilized addition polymers based on (meth)acrylate monomers in the form of their aqueous dispersions and water-redispersible dispersion powders by means of free-radically initiated emulsion polymerization or suspension polymerization of one or more monomers selected from the group consisting of the esters of acrylic acid and methacrylic acid and, if desired, further monomers copolymerizable therewith in the presence of one or more protective colloids from the group of the polyvinyl alcohols and, if appropriate, with drying of the resultant aqueous dispersion, which comprises
a) conducting polymerization in the presence of from 0.001% to 0.02% by weight of initiator and, if desired, from 0.001% to 0.03% by weight of reducing agent, based in each case on the total weight of the monomers, and
b) including from 10% to 50% by weight of the monomers, based on the total weight of the monomers, in the initial charge and metering in the rest, and
c) metering in the initiator and, if present, the reducing agent in toto, from 25% to 60% by weight of the total amount of initiator and, if present of reducing agent being metered in continuously until from 50% to 80% of the total polymerization time has elapsed, and
d) in the remaining polymerization time, adding the remaining amount of initiator and, if present, reducing agent at an accelerated metering rate which is at least twice as high as in step c).
Suitable monomers are selected from the group of the esters of acrylic acid and methacrylic acid and are esters of branched and unbranched alcohols having 1 to 15 carbon atoms. Preferred methacrylic esters and acrylic esters are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, and 2-ethylhexyl acrylate. Particular preference is given to methyl acrylate, methyl methacrylate, n-butyl acrylate, and 2-ethylhexyl acrylate.
If desired, the methacrylic esters and acrylic esters can also be copolymerized with further monomers; for example, with one or more monomers selected from the group of the vinyl esters of branched or unbranched carboxylic acids having 1 to 12 carbon atoms, vinylaromatic compounds, vinyl halides, olefins and dienes. If these monomers are copolymerized, then it is generally in an amount of from 10% to 70% by weight, based on the total weight of the monomers.
Preferred vinyl esters are vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate and vinyl esters of &agr;-branched monocarboxylic acids having 9 to 11 carbon atoms, an example being VeoVa9® or VeoVa10® (tradenames of Shell). Vinyl acetate is particularly preferred. Preferred vinylaromatic compounds are styrene, methylstyrene an
Collard & Roe P.C.
Shosho Callie
Wacker Polymer Systems GmbH & Co. KG
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