Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-11-22
2001-05-22
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S460000, C525S244000, C525S267000, C525S902000, C526S075000, C526S203000
Reexamination Certificate
active
06235810
ABSTRACT:
The present invention relates to a method for the preparation of latexes with hollow polymer particles. Hollow polymer particles are useful as opacifying agents in coating applications, such as in paint or paper coating, especially for high quality paper products where colour pictures are presented, such as art books, brochures, annual reports, magazines etc. The use of hollow particle latex in such coatings reduces or removes the need for pigments, such as titanium dioxide, or extenders, without adding excessive and undesirable weight to the coating. The hollow polymer particle latex provides opacity because the hollow structure in a latex particle scatters light more efficiently than a corresponding particle of uniform density. The light scattering properties are related to a difference in the refractive index between the shell and the internal void. The particles can also be used as control release devices for target compounds. Hollow polymer particles have found extensive applications as white pigments, ie. hollow sphere pigments have excellent optical properties such as hiding power, opacity, gloss, brightness and whiteness as well as thermal resistance providing benefits in printing performance and production efficiency, for example faster calendering. Further, they have started to replace the earlier used polymers filled uniformly in particles.
In the prior art, some methods have been known for the production of microvoid containing polymer particles. All these methods lead to a core and shell particles with different microvoids in the center. The hollow polymer latex particles having a centered void have a greater utility in coatings than those structures having microvoid or hemispherical particles with voided centers.
Some methods have also been proposed for the preparation of hollow polymer particles. EP patent number 22633 discloses a process which comprises copolymerizing an ethylenically unsaturated monomer with a carboxyl group containing ethylenically unsaturated monomer by emulsion polymerization to prepare seed particles of the core part, carrying out cover polymerization using an ethylenically unsaturated monomer for the formation of the particle surface layer eg. shell part, neutralizing after completion of polymerization the resulting particles with a volatile base such as ammonia, and swelling the particles into hollow particles. In this process, four-stage polymerization is carried out so as to make a polymer composition of the core part and the shell part different. The process is complicated and it is difficult to control the reaction temperature, the quantity of the polymerization initiator, the quantity of the surfactant etc. Thus, it is difficult to form hollow particles stably, resulting in high levels of copolymerized acid remaining in the core polymer. There also appears to be no intermediary layer between the core and the shell, which may be the reason for the weak opacity.
U.S. Pat. No. 4,427,836 discloses the production of sequential heteropolymer dispersion and a particulate material obtainable therefrom, comprising an ionic, swellable, hard core polymer and a sheath polymer on the core. In this method, the preparation of the core/sheath polymer particles is realized using sequential emulsion polymerization to yield the core, then polymerizing the sheath on the core particles, neutralizing the resultant core/sheath particles to swell the core and to form particles with a single void when dried. The product contains only two layers, the core and the sheath. This may be the reason why the opacifying compositions are only whitish, but not white.
U.S. Pat. No. 4,880,842 discloses a process for making hollow latexes by introducing a non-polymeric acid to an early stage of the multi-stage polymer particles instead of copolymerizing acid to make swellable cores. This method for preparing hollow latexes requires cores containing acid or acidic monomers to enable swelling to occur at room temperature. The existence of acid in a polymer disadvantageously increases the polymers' affinity to water. The sensitivity to the effects of water undesirably renders the coating into which the latex is formulated, sensitive to moisture as well, which is an undesirable property in coating applications. An acidic core is also thought to impart polarity to the polymer and because of such polarity the polymer is thought not to absorb surfactants well, rendering the polymer unstable or prone to coagulation or aggregation in larger particles. The water sensitivity of the core or high affinity for water by the core is thought to make it difficult to encapsulate the core with the shell polymer because the core tends to remain at the polymer-water interface on the surface of the particle rather than to be encapsulated by the shell. This process for preparing hollow polymer particles also requires a post-encapsulating step with cross-linked polymers, that is the acid containing cores are encapsulated with an uncross-linked shell, the core is swelled, and then the shell can be encapsulated with cross-linked polymers. Additionally, substantive amounts of the chain-transfer agent n-dodecyl mercaptan are needed to accomplish this process.
U.S. Pat. No. 4,910,229 discloses a process which comprises adding a base to a carboxyl group containing copolymer latex to neutralize at least part of the carboxyl groups in the copolymer, eg. base treatment and then lower the pH of the latex with an acid, eg. acid treatment to produce hollow particles. In this process it is necessary to neutralize the carboxyl groups in the polymer particles, and in order to make the base necessary for neutralization permeate sufficiently into the polymer particles, there is a necessity to raise treatment temperature from a temperature somewhat lower than the glass transition temperature (T
g
) of the polymer to a temperature equal to or more than T
g
and raise the diffusion rate of the base. Usually this kind of process results in multihollow morphology of the particles.
U.S. Pat. No. 5,360,827 discloses a process for the preparation of latex of a hollow polymer which comprises adding a base in the presence of a monomer to latex containing carboxylic modified copolymer particles to make the pH of the latex 8 or more, and adding a carboxyl group containing a monomer to make pH of the latex 7 or less and then polymerizing these monomers. This process leaves high amounts of residual monomers in the product and thus, an additional steam distillation operation is required.
U.S. Pat. No. 5,157,084 discloses a process of making hollow polymer latex particles without the use of acid in the swellable polymer core. The swelling step is performed at elevated temperatures, usually above 100° C. and thus, it must be performed under pressure.
The European patent application, publication No. 478 829 discloses a process for the preparation of hollow latex polymers characterized by emulsion polymerization of monomers in the presence of organic hydrocarbons, which tend to leave solvent residues in the product and are costly to use. An additional operation is needed for the removal of residual solvents from the product.
In general, all methods in the state of the art are multistage processes requiring the isolation of the intermediate latexes, several reactors and time consuming expensive cleaning operations between the stages.
An object of the present invention lies in providing a new, efficient, economic and environmentally advantageous process for the preparation of stable hollow polymer latex particles, based on the development of a hard, cross-linked shell on highly carboxylated seed particles pre-swelled in aqueous volatile base solution. Surprisingly the present inventors found that the above-identified objects can be attained by realizing the following process, which requires no chain transfer agents, no additional steam distillation steps, no pressurizing, no organic solvents, no elevated temperatures above 100° C., and which yields a white and opaque product with an additional intermediary layer. The process c
Byrdina Nina
Halme Erkki
Ivanchev Sergey
Koskinen Jukka
Laamanen Hanna
Birch & Stewart Kolasch & Birch, LLP
Egwim K. C.
Neste Chemicals Oy
Wu David W.
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