Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-10-14
2001-06-26
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S255000, C525S259000, C525S263000, C525S264000, C525S902000
Reexamination Certificate
active
06252004
ABSTRACT:
The present invention relates to processes for preparing polymer emulsions and polymers formed therefrom. In particular, the present invention relates to aqueous emulsion polymerization processes for preparing polymer emulsions and emulsion polymers formed therefrom.
“Emulsion polymer”, as used herein, refers to a water-insoluble polymer which is prepared by emulsion polymerization techniques.
“Polymer emulsion”, as used herein, refers to an aqueous composition having discrete, water-insoluble polymer particles dispersed therein.
As used herein, acrylate and methacrylate are referred to as “(meth)acrylate,” acrylic acid and methacrylic acid are referred to as “(meth)acrylic acid.”
Emulsion polymers, such as hollow or voided emulsion polymers, are known for use in several industrial applications. The literature uses the terms “hollow” and “voided” interchangeably. These polymers are often used in paints, coatings, inks, sunscreens and paper manufacture. Hollow emulsion polymers are generally prepared by swelling a core/shell emulsion polymer in such a way that one or more voids form in the interior of the emulsion polymer particle. These voids contribute, among other things, to the opacity of coatings and films prepared with the hollow emulsion polymer.
For some applications, it is particularly desirable to minimize the weight of the coating applied. For example, it is desirable for certain paper coatings applications to have a high performance coating without adding considerably to the weight of the paper.
Accordingly, it is desirable to provide lightweight, low density additives for coatings, such as voided latex particles.
Voided latex particles can be prepared by any of several known process, including those described U.S. Pat. Nos. 4,427,836, 4,468,498, 4,594,363, 4,880,842, 5,494,971, 5,521,253, 5,157,084, 5,360,827 among others. Voided latex particles, as described in the references noted above, are prepared by swelling the core of a core-shell emulsion polymer. Some of the processes, such as that described by U.S. Pat. No. 5,360,827 describe the processes whereby, in the latter stages of polymerizing the shell, monomer is added to facilitate diffusion of base into the core of the polymer in order to achieve swelling. Then, the pH of the emulsion is adjusted with a carboxyl-group containing monomer which is subsequently polymerized. However, this process is time consuming and does not result in suitable lightweight emulsion polymers.
The present invention seeks to overcome the deficiencies in the previously known processes by providing low density voided emulsion polymers and a process for preparing them.
In a first aspect of the present invention, there is provided a process for preparing emulsion polymer particles comprising:
a) providing an aqueous emulsion of
i) multi-stage emulsion polymer, comprising a core stage polymer and a shell stage polymer,
wherein the core stage polymer comprises, as polymerized units, from 5 to 100 percent by weight, based on the weight of the core stage polymer, of hydrophilic monoethylenically unsaturated monomer, and from 0 to 95 percent by weight, based on the weight of the core stage polymer, of at least one nonionic monoethylenically unsaturated monomer; and
wherein the shell stage polymer comprises, as polymerized units, at least 50 percent by weight of nonionic monoethylenically unsaturated monomer;
ii) monomer at a level of at least 0.5 percent by weight based on the weight of the multi-stage emulsion polymer; and
iii) swelling agent under conditions wherein there is no substantial polymerization of the monomer; and
b) reducing the level of monomer by at least fifty percent.
In a second aspect of the present invention, there is provided an aqueous polymer emulsion comprising water and swollen multi-stage emulsion polymer wherein the dry bulk density of the swollen multi-stage emulsion is: less than 0.77 grams per cubic centimeter (“g/cc”) when the swollen multi-stage emulsion polymer has a particle size below 275 nanometers (“nm”); less than 0.74 g/cc when the swollen multi-stage emulsion polymer has a particle size of from 275 to 500 nm; less than 0.59 g/cc when the swollen multi-stage emulsion polymer has a particle size of from 501 to 750 nm; less than 0.46 g/cc when the swollen multi-stage emulsion polymer has a particle size of from 751 to 1300 nm.
The stages of the multi-stage polymers of the present invention include core stage polymer (the “core”), and shell stage polymer (the “shell”). The core and shell may themselves be comprised of more than one stage. There may also be one or more intermediate stages. Preferably, the multi-stage polymer comprises a core, an intermediate layer and a shell.
The cores of the multi-stage polymers of the present invention are emulsion polymers comprising, as polymerized units, from 5 to 100 percent by weight, based on the weight of the core, of at least one hydrophilic monoethylenically unsaturated monomer and from 0 to 95 percent by weight, based on the weight of the core stage polymer, of at least one nonionic monoethylenically unsaturated monomer.
Cores containing at least five percent by weight, based on the total weight of the core polymer, of at least one hydrophilic monoethylenically unsaturated monomer will generally result in a suitable degree of swelling. There may be instances wherein, because of the hydrophobicity of certain comonomers or combinations thereof in conjunction with the hydrophobic/hydrophilic balance of a particular hydrophilic monomer, the copolymer may be suitably prepared with less than five percent by weight, based on the total weight of the core polymer, of a hydrophilic monoethylenically unsaturated monomer. Preferably, the core comprises, as polymerized units, hydrophilic monoethylenically unsaturated monomer at a level of from 5 to 100, more preferably, from 20 to 60, and most preferably, from 30 to 50 percent by weight based on the total weight of the core. The hydrophilic core polymer may be made in a single stage or step of the sequential polymerization or may be made by a plurality of steps in sequence.
The multi-stage emulsion polymer of the present invention contemplates a core polymer wherein at least one hydrophilic monoethylenically unsaturated monomer is polymerized alone or with at least one nonionic monoethylenically unsaturated monomer. This process also contemplates, and includes in the term “hydrophilic monoethylenically unsaturated monomer,” the use of a nonpolymeric compound containing at least one carboxylic acid group which absorbed into the core polymer before, during or after the polymerization of the hydrophobic shell polymer as a replacement for the hydrophilic monoethylenically unsaturated monomer in the hydrophilic core polymer, as described in U.S. Pat. No. 4,880,842. In addition, this invention contemplates, and includes in the term “hydrophilic monoethylenically unsaturated monomer,” the use of a latent hydrophilic core polymer which contains no hydrophilic monoethylenically unsaturated monomer but which is swellable upon hydrolysis to a hydrophilic core polymer as described in U.S. Pat. No. 5,157,084.
Suitable hydrophilic monoethylenically unsaturated monomer useful for making the core polymer include monoethylenically unsaturated monomers containing acid-functionality such as monomers containing at least one carboxylic acid group including acrylic acid, methacrylic acid, acryloxypropionic acid, (meth)acryloxypropionic acid, itaconic acid, aconitic acid, maleic acid or anhydride, fumaric acid, crotonic acid, monomethyl maleate, monomethyl fumarate, monomethyl itaconate and the like. Acrylic acid and methacrylic acid are preferred.
Suitable nonpolymeric compounds containing at least one carboxylic acid group include C
6
-C
12
aliphatic or aromatic monocarboxylic acids and dicarboxylic acids, such as benzoic acid, m-toluic acid, p-chlorobenzoic acid, o-acetoxybenzoic acid, azelaic acid, sebacic acid, octanoic acid, cyclohexanecarboxylic acid, lauric acid and monobutyl phthalate and the like.
Suitable nonionic monoethylenically
Bardman James Keith
Blankenship Robert Mitchell
Choi Wendy A.
Egwim Kelechi
Rohm and Haas Company
Wu David W.
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