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
2001-07-18
2004-03-23
Egwim, Kelechi C. (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...
C526S073000, C526S080000, C526S087000, C526S331000
Reexamination Certificate
active
06710113
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for preparing two-phase polymers based on vinyl esters and ethylene, in the form of their aqueous polymer dispersions or water-redispersible polymer powders, by means of free-radical initiated emulsion or suspension polymerization of one or more vinyl esters of carboxylic acids having 1 to 12 carbon atoms and ethylene as monomers, optionally with further copolymerizable monomers, in the presence of one or more protective colloids and/or emulsifiers, followed when desired, by drying the resultant aqueous polymer dispersion.
2. Background Art
Protective-colloid-stabilized polymers are generally employed, in the form of their aqueous dispersions or as water-redispersible polymer powders, in a variety of applications. Examples include coating compositions or adhesives for use with a very wide variety of substrates, one example being their use in cementitious tile adhesives. Protective colloids used generally include polyvinyl alcohols. The use of polyvinyl alcohol is desirable since, in comparison to systems stabilized by low molecular weight compounds (emulsifiers), the protective colloid adds its own contribution to the adhesive strength, for example improved tensile bonding strength in tile adhesives. For applications in the coatings field, however, such as in interior and exterior paints and plasters, emulsifier- or cellulose-stabilized dispersions are often preferred on Theological grounds.
Monomers used to prepare redispersible powders have preferably included vinyl esters and ethylene, since stabilization of acrylic copolymers, styrene-acrylic copolymers, and styrene-butadiene copolymers by polyvinyl alcohol is difficult. In particular, stabilization of acrylate dispersions using polyvinyl alcohol alone such that the resulting powders are stable to blocking and also storage stable, is very difficult. However, because of their hydrophobic nature, these monomer combinations result in good strengths within the tile adhesive following wet storage and have little susceptibility to saponification. Thus, such polymer dispersions are quite desirable.
When using vinyl esters in combination with ethylene, formulating for stability to saponification necessitates a high ethylene content, which lowers the Tg. This may detract from the tensile adhesive strength of the tile adhesives, especially following storage at elevated temperatures. A constant and high level (at least 0.5 N/mm
2
) of tensile adhesive strength is desirable for tile adhesives after all forms of storage, whether under dry, wet, elevated temperature, or frost conditions. Also desirable are high flexibilities coupled with high strengths, which are inherently contradictory properties. High flexibility requires a resin of low Tg, while high strength necessitates a resin of high Tg. Further contradictory requirements occur in the coatings sector, where for interior paints, good wet abrasion resistance coupled with low minimum film formation temperature are desired. For glossy paints, both high gloss and low blocking tendency are required.
Difficulties may arise not only during product preparation, but also during application of the products. Especially when polymers are used in the form of their redispersible powders to improve mortar properties, a principal use of redispersible powders, the formulations are required to remain stable over a certain period and must not undergo any significant alteration in their processing consistency (viscosity or cement stability). In the concrete and mortar industry, moreover, the mechanical properties, such as the compressive strength, the porosity and thus the air pore content are important factors. 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 lacks sufficient freeze/thaw stability. Hydraulically setting systems enhanced with dispersion powders are also formulated to increase adhesion relative to the unenhanced systems.
In an attempt to resolve some of the previously mentioned problems, DE 4431343 employs heterogeneous polyvinyl ester dispersions and polyvinyl ester powders comprising a mixture of a hard vinyl ester polymer and a soft vinyl ester-ethylene copolymer. The heterogeneous dispersions are prepared by mixing the corresponding polymer dispersions, which, if desired, can be subsequently dried to form powders.
DE 19739936 discloses a process wherein a soft vinyl ester-ethylene copolymer is first prepared as an aqueous dispersion. This vinyl ester-ethylene copolymer dispersion is then used as a seed latex in the polymerization of a hard vinyl ester polymer.
In DE 19528380, two-phase vinyl acetate-ethylene copolymers are produced by employing ethylene-rich copolymer phases at the beginning of the polymerization and low ethylene-content copolymer phases in subsequent courses of the polymerization. The aim of the process of DE 1952830 is to copolymerize as much ethylene as possible without developing crystalline polymer phases. However, these copolymers lack the desired high polymer film strengths, and are extremely soft.
DE 19853461 describes a process for preparing vinyl acetate-ethylene copolymers with a core/shell morphology in which, following the first stage, the process is interrupted by cooling, and the second stage is started anew. This process has disadvantages both from energy standpoints and because of the relatively long process time.
SUMMARY OF THE INVENTION
It was an object of the present invention to provide a stable copolymer dispersion and the corresponding water-redispersible dispersion powders thereof, the copolymers being based on vinyl esters and ethylene monomers which, especially when used in cementitious applications, possess both fully satisfactory viscosity stability and cement stability, which do not hinder setting of the cement, and which at the same time provide high elasticity and high strength. The polymers are obtainable by a process which allows two-phase polymers to be obtained without interruption of the polymerization. A yet further object was to provide powders which have improved and more uniform tensile bond strengths after all forms of storage. A further object was to provide dispersions which have improved wet abrasion resistance at low film formation temperatures.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The invention provides a process for preparing two-phase polymers based on vinyl ester and ethylene in the form of their aqueous polymer dispersions or water-redispersible polymer powders by means of free-radical initiated emulsion or suspension polymerization of one or more vinyl esters of carboxylic acids having 1 to 12 carbon atoms and ethylene as monomers and optionally further monomers copolymerizable therewith, in the presence of one or more protective colloids and/or emulsifiers, and if desired, drying the resultant aqueous polymer dispersion, the process comprising polymerizing in a first polymerization phase at a low ethylene pressure of P less than or equal to 20 bar, followed by polymerizing in a second polymerization phase wherein the ethylene pressure is in the range of greater than 20 bar to less than 60 bar, resulting in an ethylene distribution in the polymer of less than 50% by weight in the first polymerization phase and of greater than 50% by weight in the second polymerization phase, based in each case on the total amount of ethylene used.
Suitable vinyl esters are those of carboxylic acids having 1 to 12 carbon atoms. 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, examples being VeoVa9® or VeoVa10® (trade names of Shell). Vinyl acetate is particularly preferred. The vinyl esters are generally copolymerized with other monomers in an amount from 30 to 97% by weight, preferably from 70 to 95% by weight, based in each case o
Brooks & Kushman P.C.
Egwim Kelechi C.
Wacker Polymers Systems GmbH & Co. KG
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