Processes for forming styrenic polymers

Details Processes for forming styrenic polymers Processes for forming styrenic polymers

2001-04-06

2002-03-12

Foelak, Morton (Department: 1711)

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

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S060000

Reexamination Certificate

active

06355697

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to processes for impregnating emulsion based seeds.
In particular, the present invention is directed to processes for impregnating emulsion based seeds while minimizing the formation of polymeric, emulsion based seeds having undesirably small sizes.
BACKGROUND OF THE INVENTION
Styrenic polymers have a wide variety of applications, including the formation of expanded polystyrene which can be used to make a variety of products. Processes for forming styrenic polymers include emulsion polymerization, suspension polymerization, and the use of particular suspension or emulsion aids. Attempts have been made to reduce the production of styrenic polymeric particles having undesirably small sizes, referred to as “fines”.
For example, U.S. Pat. No. 4,091,054 discloses a suspension polymerization process for the formation of polystyrene particles. The method uses a polymerization catalyst solution formed by dissolving 60 to 100 weight percent of the total polymerization catalyst to be used in 1 to 10 weight percent of the total styrenic monomer, and a monomer solution containing 90 to 99 weight percent of the total styrenic monomer and the remaining 0 to 40 weight percent polymerization catalyst. The two solutions are added separately, dropwise, to a suspension of smaller styrenic polymer particles, having sizes less than the desired final size, and the monomer polymerizes with the smaller particles to form the desired particles. While the '054 patent discloses that particle sizes in the range 0.6 to 2.5 mm can be obtained, the dropwise addition that is a critical feature of the process can result in undesirably long reaction times.
U.S. Pat. No. 4,333,969 discloses an emulsion polymerization process that uses an aqueous emulsion containing styrenic monomer, a polymerization catalyst, and a nonionic surfactant that is an ethylene oxide condensate of an alkylphenol. The surfactant contains at least 50 moles of ethylene oxide per mole of alkylphenol. The emulsion is added to an aqueous suspension of styrenic polymer beads and the monomer polymerizes to form larger styrenic polymer particles. The patent discloses that polymeric beads of a predictable size can be formed. However, emulsion processes often require larger reaction vessels due to the high volumes of water that are generally used. Also, the required dispersion aids and/or surfactants can result in increased costs and handling for emulsion processes as compared with solution or suspension processes.
Polymer beads are useful in applications such as the formation of expanded resins, for example, expanded polystyrene. Expanded polystrene and other expanded resins can be prepared from expandable polymeric beads made by contacting the polymeric beads with a volatile compound known as a “blowing agent” or “expanding agent”. Such agents include aliphatic hydrocarbons such as butane, heptane, pentanes, and halogenated hydrocarbons such as trichlorofluoromethane, trichlorofluoromethane, and methyl chloride. The beads in contact with the expanding agent may be expanded by heating, or by exposure to reduced pressure as in a vacuum. The size and size distribution of the expanded beads will depend upon the size and size distribution of the expandable beads.
A need remains for new and/or improved processes for impregnating emulsion based seeds. A further need remains for processes that provide styrenic polymers having desirable particle size distributions and reduced production of fines.
SUMMARY OF THE INVENTION
One aspect of the present invention is a method for impregnating emulsion based seeds. The method includes providing polymeric, emulsion based seeds in an aqueous medium; combining with the polymeric, emulsion based seeds, in the aqueous medium, a suspending agent and one or more surfactants in a ratio of about 1:175 or less to the weight of suspending agent, to form a seed suspension; forming about 100 to about 2500 weight percent, based on the weight of the emulsion based seeds in the seed suspension, of a reaction mixture including styrene and one or more initiators; combining the reaction mixture with the seed suspension to form a polymerization mixture; and heating the polymerization mixture in the presence of a blowing agent to a polymerization temperature to effect formation of expandable polystyrene.
Another aspect of the invention is a method for forming a styrenic polymer, comprising providing polymeric seeds in an aqueous medium; combining batchwise the polymeric seeds in the aqueous medium with from 0 to about 250 weight percent, based on the weight of the seeds, of a pre-swell mixture of styrene and one or more initiators; combining the pre-swell mixture having the polymeric seeds in aqueous medium therein with a reaction mixture containing one or more initiators and from about 100 to about 2500 weight percent styrene based on the weight of the seeds, to form a polymerization mixture; and heating the polymerization mixture in the presence of a blowing agent to a polymerization temperature to effect formation of the styrenic polymer.
These and other aspects of the invention will become apparent to those skilled in the art in view of the following disclosure and the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides processes for forming expandable styrenic polymers. Small polymer particles, referred to as “seeds”, are used in processes of the present invention. The processes can be used to form polymeric styrenic particles having narrower size distributions than are obtained using some conventional processes. Furthermore, it has been surprisingly discovered that desirable particle size distributions can be achieved by polymerization of monomers in the presence of emulsion-based polymer seeds without the use of emulsion polymerization processes.
Styrenic monomers are polymerized in the presence of the seeds, whereby a styrenic polymer is formed, in which the seeds are incorporated. By “small”, as used herein to refer to the size of the seeds, is meant a size significantly smaller than the particle size of the styrenic polymer that is to be formed as an end product. Preferably, the seeds have average diameters of about 1200 microns or less, typically about 1100 microns or less, more typically about 1000 microns or less, and frequently about 900 microns or less. Also preferably, the seeds have average diameters of at least about 100 microns, typically at least about 120 microns, more typically at least about 140 microns, more typically at least about 150 microns, and frequently at least about 200 microns. The size of the seeds can be selected based, in part, upon the desired particle size for the final polymer product. “Final polymer product” and “final product”, as used herein, mean a polymer that is obtained as a result of the processes of the present invention whereby styrenic monomers are polymerized in the presence of polymeric seeds. The size of polymeric particles in the final product will depend upon the intended application for the polymer. Generally, final polymer products will be formed having average diameters of at least about 200 microns, typically at least about 225 microns, more typically at least about 250 microns, even more typically at least about 275 microns, frequently at least about 300 microns, and in some preferred embodiments at least about 350 microns. Also, generally the average diameter of the final polymer product particles is about 1500 microns or less, typically about 1400 microns or less, more typically about 1300 microns or less, frequently about 1200 microns or less, and in some preferred embodiments about 1100 microns or less.
It is preferred that the size distribution of seeds be relatively narrow, in order to avoid an undesirably broad size distribution of particle sizes in final product. Generally, it is preferred that the ratio of standard deviation of mean seed size to the mean seed size be about 0.20 or less, more preferably about 0.1 or less.
The seeds may be of the same composition as or a

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