Process for stripping polymer dispersions

Details Process for stripping polymer dispersions Process for stripping polymer dispersions

2000-08-17

2002-03-05

Michl, Paul R. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Treating polymer containing material or treating a solid...

C528S500000

Reexamination Certificate

active

06353087

ABSTRACT:

This invention relates to a process for removing volatile organic compounds (“VOCs”) and amines such as ammonia from reaction compositions of polymer dispersions (“stripping”). In particular, the invention relates to a process for stripping polymer dispersions which reduces gel in the polymer dispersion.
Polymers may be made by various processes, including solution, suspension, and emulsion polymerization. As used herein, by dispersion is meant an emulsion or suspension polymer. Although the polymerization processes are efficient, there is always some VOCs after the polymerization is complete. As used herein, by VOC is meant any volatile organic compound, such as residual monomers, impurities from monomers, and solvents from surfactants or monomers. Residual monomer is monomer that does not react during a polymerization and remains in the polymer dispersion. VOCs tend to have strong odors. Generally, people can smell VOCs at extremely low levels, such as in the part per million range. Some VOCs are toxic. Due to the toxicity of some VOCs and the fact that the smell of VOCs tends to be irritating, polymer dispersions generally are either chemically “chased” or stripped of VOCs.
Chemical chasing involves adding chemicals that react with unreacted monomers, such as tertiary butyl hydroperoxide, ammonium persulfate, potassium persulfate, or sodium persulfate which, for example may react with carbon-carbon double bonds of the unreacted monomers. Although chasing results in reduced residual monomers in the polymer latex, the level of residual monomers may still be unacceptably high. Also, any VOCs which do not have carbon-carbon double bonds can not be chased. Therefore, chemical chasing is sometimes followed by stripping to remove unreacted VOCs.
Stripping is a process wherein a dispersion is heated and a gas is sparged through the dispersion to remove VOCs. Stripping may also utilize vacuum to enable the stripping to be performed at lower temperatures. The advantage to stripping polymer dispersions is that one can achieve lower levels of VOCs by stripping than by chasing.
One problem associated with stripping polymer dispersions is that gel may form during the stripping process. Gel is an aggregation of polymer particles or molecules, resulting in small clumps of polymer in the polymer solution or dispersion. Gel may cause problems in down stream applications, therefore there is a need for a polymer dispersion stripping process which reduces gel formation.
A second problem associated with stripping polymer dispersions is that the polymer dispersions frequently contain surfactants. Polymers may also behave as surfactants due to carboxy end groups. The surfactants or surfactant behavior of a polymer may cause foaming during stripping operations. Foaming may be controlled through the addition of chemicals such as silicone, mineral and vegetable oils, ethoxylated fatty acids, and alcohols. Two disadvantages of chemical addition to control foaming are that the chemicals may be expensive and the chemicals may adversely effect the polymer properties, for example, the chemicals may cause surface imperfections such as fish eyes or holes in coatings.
Mechanical foam breakers may also be utilized to reduce foam during polymer dispersion stripping operations, however the foam breaker does not reduce the formation of gel in the polymer dispersion. Therefore, there is also a need for a polymer dispersion stripping process which prevents foam formation or destroys foam generated during stripping.
One approach to controlling foam generation during a polymer dispersion stripping process was taught in German unexamined patent application DE 19517680. The process disclosed in that patent required pH adjustment to the range of from 5 to 6.8 during stripping to reduce foam generation. After stripping, the pH was adjusted to from 8 to 10. The process did not utilize a mechanical foam breaker and did not deal with the issue of gel formation during stripping.
A process for foam destruction in the manufacturing of polymers was disclosed in European Patent Application EP 865811. The process utilized mechanical foam breakers to destroy foam during the process of manufacturing a polymer. The process did not address the issue of gel formation during a polymer solution or dispersion stripping process.
U.S. Pat. No. 4,130,527 disclosed a stripping process which utilized a continuous stripping operation. Steam was fed counter-current to a falling film of polymer. The disclosure did not address the problems of foaming or gel formation during stripping polymer dispersions or solutions.
Despite the disclosure of the references, there is a continuing need for a process for stripping polymer dispersions wherein foam formation is prevented or foam formed during stripping is destroyed and gel formation is minimized.
The present invention provides a process including: providing a reaction composition and stripping the reaction composition in the presence of a mechanical foam breaker and an agitator; wherein the reaction composition is treated by a method selected from: a) adjusting the pH to from 7 to 11 prior to stripping and maintaining the adjusted pH during stripping, b) maintaining the temperature of the reaction composition at from 30° C. to 70° C. during stripping, and c) combinations thereof.
As indicated above, the first step in the process of the invention involves providing a reaction composition. The reaction composition may be polymers may be made by suspension or emulsion polymerization. Polymers made by emulsion polymerization are preferred. The polymers may be homopolymers or copolymers.
Among the monomers that may be useful to make the polymer dispersion are ethylenically unsaturated monomers which include, but are not limited to (meth)acrylic ester monomers including methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, methyl methacrylate, butyl methacrylate, hydroxyethyl methacrylate, and hydroxypropyl acrylate; acrylamide or substituted acrylamides; styrene or substituted styrene; vinyl acetate or other vinyl esters; vinyl monomers such as vinyl chloride, vinylidene chloride, N-vinyl pyrolidone; and acrylonitrile or methacrylonitrile. Butyl acrylate, methyl methacrylate, and styrene are preferred.
Ethylenically unsaturated acid containing monomers or salts thereof may also be useful. Suitable ethylenically unsaturated acid containing monomers include, but are not limited to acrylic acid, methacrylic acid, crotonic acid, phosphoethyl methacrylate, 2-acrylamido-2-methyl-1-propanesulfonic acid, sodium vinyl sulfonate, itaconic acid, fumaric acid, maleic acid, monomethyl itaconate, monomethyl fumarate, monobutyl fumarate, and maleic anhydride. Acrylic acid and methacrylic acid are preferred. Methacrylic acid is more preferred.
A fluorinated (meth)acrylate ethylenically unsaturated monomer, such as Zonyl™ products (Trademark of DuPont Chemical Company) may also be useful.
A silicone containing ethylenically unsaturated monomer, such as vinyl trimethoxy silane and methacryloxy propyl trimethoxy silane may also be useful.
Monomers selected from C
6
-C
20
alkyl styrene and alkyl-alpha-methyl styrene, C
6
-C
20
alkyl dialkyl itaconate, C
10
-C
20
vinyl esters of carboxylic acids, C
8
-C
20
N-alkyl acrylamide and methacrylamide, C
10
-C
20
alkyl alpha-hydroxymethylacrylate, C
8
-C
20
dialkyl 2,2′-(oxydimethylene) diacrylate, C
8
-C
20
dialkyl 2,2′-(alkyliminodimethylene)diacrylate, C
8
-C
20
N-alkylacrylimide, and C
10
-C
20
alkyl vinylether may also be useful.
Hydrophobic monomers such as C
12
to C
40
alkyl ester of (meth)acrylic acid may also be useful. Suitable alkyl esters of (meth)acrylic acid include, but are not limited to lauryl (meth)acrylate, cetyl (meth)acrylate, stearyl (meth)acrylate, behenyl (meth)acrylate, and eicosyl (meth)acrylate.
Emulsion polymerizations are well known in the art and are described in U.S. Pat. No. 5,346,954, hereby incorporated by reference. Suitable initiators and process conditions can be found in the patent.
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