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
2001-06-26
2003-12-16
Pezzuto, Helen L. (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...
C526S065000, C526S317100, C526S319000, C526S330000, C526S348000
Reexamination Certificate
active
06664322
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for preparing protective-colloid-stabilized polymers based on vinyl esters and ethylene and, optionally other comonomers, in the form of aqueous polymer dispersions or water-redispersible polymer powders, by means of continuous emulsion polymerization and, when appropriate, drying the resultant polymer dispersion.
2. Background Art
Protective-colloid-stabilized polymers are used in a wide variety of applications, especially in the form of their aqueous dispersions or water-redispersible polymer powders prepared therefrom. Applications include, for example coating compositions and adhesives for use with a very great variety of substrates. The protective colloids employed are generally polyvinyl alcohols. The use of polyvinyl alcohol is desirable since this material contributes to the strength characteristics of the polymer (i.e. tensile bond strengths of tile adhesives), as opposed to polymer systems stabilized by low-molecular-weight compounds (emulsifiers). The monomers preferably used for preparing redispersible powders have hitherto been vinyl esters and ethylene, since it is difficult to effect stabilization of acrylate copolymers or styrene-acrylate copolymers when employing polyvinyl alcohol.
Dispersions have traditionally been prepared by batch emulsion polymerization. This procedure has high flexibility and is therefore preferred in manufacturing facilities producing a wide range of products. In addition, this procedure is much less technically demanding than continuous processes. However, a disadvantage of batch processes as compared with continuous processes is the much lower cost-effectiveness of the former.
A continuous process is therefore the preferred choice for plants producing large quantities of a narrow range of products. The principle problem with continuous emulsion polymerization is establishing the proper conditions for constant nucleation of new latex particles. The favored plant configuration is a stirred tank cascade, since reactors of this type are commonly used in batch operations and could therefore be used in the continuous processes with only low conversion costs. The product properties and the process parameters often exhibit considerable variation, and in extreme cases these can be the cause of unusable product or even damage to the plant.
U.S. Pat. No. 2,703,794 describes a process for the continuous emulsion polymerization of vinyl acetate and ethylene in the presence of an emulsifier and, where appropriate, water-miscible organic solvent, with continuous and simultaneous feed of water, emulsifier, monomers, and initiator. The process is carried out without any seed latex, in but a single pressurized reactor. Disadvantages of this process are heavy reactor wall deposits and oscillation in particle size.
U.S. Pat. No. 3,551,396 describes a process for preparing vinyl acetate homopolymer dispersions in a loop reactor, in which monomer and water are continuously circulated, with continuous feed of additional monomer and initiator, the polymer formed being continuously removed from the reactor. The polymerization is carried out without any seed latex, and again, in a single pressurized reactor.
DE-A 2555142 describes a continuous process for the preparation of polymers of ethylenically unsaturated monomers by emulsion polymerization in a plurality of polymerization reactors arranged in series. In this process, a first reactor is supplied with a “preemulsion” made from water, initiator, part of the monomers, and other auxiliaries, and the resultant dispersion is transferred into a second reaction vessel. The remainder of the monomers is then introduced, and the polymerization of the dispersion is completed in a final reaction vessel, by supplying additional initiator.
DE-A 2456576 describes a plant configuration composed of a tank reactor for the main polymerization and a tubular reactor for postpolymerization. The polymerization is carried out in the presence of a redox initiator system, using a large excess of reducing agent. The monomers feed rate is adjusted so that their concentration does not exceed a defined limit. A disadvantage of this process is the rapid formation of wall deposits, leading to frequent shutdown, due in part to marked variations in the properties of the products.
U.S. Pat. No. 4,164,489 describes a process for the continuous preparation of vinyl acetate-ethylene dispersions employing a seed latex which is also introduced continuously. A disadvantage is that this seed latex represents a separate product which must to be specifically prepared for this purpose due to its low particle size, about 200 nm. As a result, undesirable components employed in seed preparation, the mixing specification, for example emulsifiers, are carried over into the product.
A process employing a seed latex is described by Petrocelli et al.,
POLYM. SCI. ENG.,
80, 544. Here, too, use is made of a fine-particle size emulsifier-stabilized dispersion, one having a particle size of about 200 nm. The theoretical basis for this seed latex process involves the introduction of new, very small latex particles which then grow in the course of the polymerization to give the desired particle size. The seed latex is intended to suppress any new internal nucleation.
When redispersible polymer powders are employed to improve mortar properties, a principle market for redispersible powders, the mixing proportions must remain stable for extended periods of time, and there must also be no substantial change in the working consistency, for example viscosity and cement stability of the products when mixed. In the concrete and mortar industry, mechanical properties are especially important, for example compressive strength, porosity, and thus air-pore content. In addition, hydraulically setting systems which are to be improved by adding dispersion powders should provide better adhesion than systems without such addition. All of these product properties must be reliably provided when batch processes for preparing redispersible polymers are replaced by continuous processes, which has not heretofore been possible.
SUMMARY OF THE INVENTION
It was an object of the present invention to develop a cost-effective and technologically improved process for the continuous preparation of vinyl ester-ethylene copolymer dispersions. Problems addressed by the present invention include those which are involved with the end use of the desired products as well as those involved with their preparation. For example, the products are eminently suitable for use in tile mortar, hydraulically setting systems, and other cementitious products while maintaining or improving upon the properties of dispersible powders produced by batch processes.
It has now been surprisingly discovered that the use of a seed latex which, at least in terms of particle size, corresponds to the end product of the process, or else without using any seed latex, results in a stable continuous process for the preparation of emulsion polymers. The use of the product itself as the seed latex contradicts conventional wisdom that the stability of the process should be controlled via ordered particle growth, since the particles introduced already have the desired ultimate size, and would be expected to grow further under the reaction conditions. To the contrary, however, a stable condition is observed, without oscillation of particle size or of other product properties.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The invention provides a process for preparing polyvinyl-alcohol-stabilized polymers based on vinyl esters and ethylene and optionally other comonomers, in the form of aqueous polymer dispersions or water-redispersible polymer powders, by means of free radical initiated continuous emulsion polymerization and, where appropriate, drying the resultant polymer dispersion.
The process comprises the steps of:
a) conducting an emulsion polymerization in at least two pressurized reactors arranged in series, at a temperature of fr
Brooks & Kushman P.C.
Pezzuto Helen L.
Wacker-Chemie GmbH
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