Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2001-11-09
2003-10-28
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C524S803000, C524S820000, C524S823000, C524S825000, C524S513000, C524S514000, C524S503000, C524S460000, C525S057000, C526S202000, C526S209000, C526S210000, C526S303100, C526S317100
Reexamination Certificate
active
06639049
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for the preparation of polymers stabilized with protective colloids and water-redispersible powders or aqueous dispersions having a reduced content of volatile components prepared therefrom.
DESCRIPTION OF THE RELATED ART
A growing ecological consciousness has resulted in more strenuous requirements on the market with respect to freedom from residual monomers and solvents in many polymer products. These requirements together are characterized by the term “VOC-free” (VOC=volatile organic compound). While VOC-free polymer dispersions are now common, this is not yet the case with redispersion powders. Possible explanations for this are that the volatile components in solid products are more difficult to remove than in liquid products, and that the volatile components become chemically bound when redispersion powders are used in many aqueous formulations. For example, residual vinyl acetate is hydrolyzed directly to calcium acetate and acetaldehyde in applications in cement media the main field of use of redispersion powders, the acetaldehyde immediately undergoing aldol condensations. The same occurs with other esters, such as methyl or ethyl acetate, and with acetone (aldol condensation).
However, very recent investigations have shown that considerable contamination by volatile components can nevertheless occur; for example, acetaldehyde is liberated when vinyl acetate-based redispersion powders are used in levelling compounds or floor toppings. The acetaldehyde is present in the powder or is formed by hydrolysis from residual vinyl acetate present in the powder.
A number of processes for removing volatile components from polymers are known from the prior art. A distinction may be made between chemical and physical deodorizing processes. The chemical processes are distinguished by the addition of substances which react with the residual monomer and thus reduce their content. The physical processes are substantially based on distillation or stripping phenomena with steam or inert gases such as nitrogen as entraining agents. In addition, combinations of these two processes have been described.
DE-A 19741185 describes the reduction of residual monomer content of polymer dispersions by postpolymerization with a special redox system comprising carboxylic acid and peroxide compounds. A further chemical process for removing residual monomers from polyvinyl ester dispersions is disclosed in EP-B 505959. In the latter reference, the polyvinyl ester dispersion is subjected to hydrolysis at weakly alkaline pH followed by a subsequent oxidative treatment. DE-A 19741189 describes a chemical process for removing residual monomers, in which a nucleophilic agent is metered into a specially dimensioned reactor in a defined mixing time.
The physical removal of volatile residues from polymer dispersions by passing in steam is disclosed in DE-A 19745580, and removal by means of inert gas, for example air or nitrogen, is disclosed in DE-A 4118526. A combined process for removing volatile components, in which first a postpolymerization with redox initiator and then an inert gas treatment are carried out, is disclosed in DE-A 19828183. In the process from EP-A 650977, the residual monomers are first removed by postpolymerization and then by means of steam stripping. EP-A 465964 describes a process for removing volatile components from emulsion polymers, these first being spray-dried and the powder then being aftertreated with inert gas.
It is known that chemical measures reduce residual monomer content by introducing other volatile components, such as tert-butanol and acetone, into the dispersion. It is, however, senseless and uneconomical to carry out these measures to achieve extremely low residual monomer content, since at the same time the content of other volatile components increases disproportionately. On the other hand, it is known that physical deodorization can result in damage to the dispersion, ranging from the presence of specks to coagulum. Exclusive physical deodorization is therefore also not expedient since the products are likely to be damaged owing to high levels of contamination, even with processes according to the latest state of the art. The considerable amounts of condensate contaminated with organic components, about 10 to 20%, based on the dispersion used, which, depending on the composition, have to be disposed of by expensive procedures, constitute further considerable disadvantages of physical deodorization.
An additional problem in the preparation of redispersion powders is the use of methanol-containing polyvinyl alcohol as a protective colloid during spraying. This protective colloid is added to the dispersion only after chemical and physical deodorization, so that the methanol content of the dispersion is high as a result.
It would be desirable to provide an economical process for the preparation of polymers stabilized with protective colloids in the form of water-redispersible powders or aqueous dispersions which have a reduced content of volatile components. It would be further desirable to provide such a process which avoids the disadvantages of the prior art.
SUMMARY OF THE INVENTION
The invention relates to a process for the preparation of polymers stabilized with protective colloids in the form of water-redispersible powders or aqueous dispersions which have a reduced content of volatile components, this process comprising
a) polymerizing by emulsion or suspension polymerization, one or more monomers from the group consisting of vinyl esters, (meth)acrylates, vinyl aromatics, olefins, 1,3-dienes and vinyl halides and, if required, further monomers copolymerizable therewith,
b) aftertreating the polymer dispersion thus obtained by means of postpolymerization and/or distillation, and introduction of steam or inert gas to a residual content of volatile, nonaqueous components of <2000 ppm, and then
c) spray drying the aftertreated polymer dispersion to a residual content of volatile, nonaqueous components of <400 ppm, the dispersion being adjusted to a solids content of <45% by weight before spraying, the spray drying carried out with air at an inlet temperature of >120° C, and optionally,
d) redispersing the resulting powder in water.
Volatile components (VOC) are understood as meaning nonaqueous compounds, such as residual monomers, decomposition products of monomer and initiator, impurities in polymerization assistants and the like; for example, residual monomers such as vinyl acetate, alkyl (meth)acrylate and styrene; decomposition products or impurities such as acetaldehyde, methanol, ethanol, tert-butanol, acetone, methyl acetate, and ethyl acetate.
Preferred vinyl esters are those of carboxylic acids having 1 to 12 C atoms. 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 C atoms, for example VeoVa9
R
or VeoVa10
R
(trade names of Shell), are more preferred. Vinyl acetate is particularly preferred.
Suitable monomers from the group consisting of acrylates or methacrylates are esters of straight-chain or branched alcohols having 1 to 15 carbon atoms. Preferred methacrylates or acrylates are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate and 2-ethylhexyl acrylate. Methyl acrylate, methyl methacrylate, n-butyl acrylate, tert-butyl acrylate and 2-ethylhexyl acrylate are particularly preferred.
Preferred vinyl aromatics are styrene, methylstyrene and vinyltoluene. A preferred vinyl halide is vinyl chloride. The preferred olefins are ethylene and propylene and the preferred dienes are 1,3-butadiene and isoprene.
If required, 0.1 to 5% by weight, based on the total weight of the monomer mixture, of auxiliary monomers may also be copolymerized. Preferably, 0.5 to 2.5% by weight of auxiliary mon
Boykin Terressa M.
Brooks & Kushman P.C.
Wacker Polymer Systems & Co. KG
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