Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-11-08
2004-07-06
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S065000, C526S088000, C526S173000
Reexamination Certificate
active
06759498
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for efficiently producing a styrenic resin with a small content of styrenic low molecular weight components such as styrenic oligomers, etc., styrene monomers, remaining hydrocarbon solvent, etc., which is superior in processability and heat stability. The resulting styrenic resin can advantageously used for various applications, for example, as molding materials, for example, electric appliance materials, sundries materials, food container materials, food packaging or wrapping materials, etc., particularly as food container materials and food packaging or wrapping materials.
BACKGROUND ART
Styrenic resins have many distinguished physical characteristics such as lightweight properties, high rigidity, high water resistance, high electrical resistance, etc. They also have good moldability or processability, so that moldings in various shapes can be produced easily and on a mass-production scale. On the basis of these characteristics, styrenic resin have been used in a large amount for various applications such as electric appliance materials, sundries materials, food containers, food packaging or wrapping materials, etc.
Generally, styrenic resins can be produced by two types of methods in terms of reaction mechanism, i.e. a thermal radical polymerization process on the one hand, and a radical polymerization process using an initiator on the other. There are also two types of production processes, i.e. bulk polymerization process and suspension polymerization process, between which the bulk polymerization process is now predominant because of a smaller inclusion of impurities such as a dispersant, etc., and because of cost-wise advantages.
In the radical polymerization process, however, it is well known that oligomer formation generally takes place at the same time as resin production and also that styrene monomers tend to remain in the resin.
For example, according to Kirk-othmer's Encyclopedia of Chemical Technology, Third Edition, Vol. 21, page 817 (John Wiley)), thermal styrene polymerization at 100° C. or higher takes place together with the by-production of oligomers such as styrene dimer, styrene trimer, etc., amounting to about 1 wt. %. It is reported that specific oligomer components are mainly composed of 1-phenyl-4-(1′-phenylethyl)tetralin and 1,2-diphenyl-1-butene, and additionally include 2,4-diphenyl-1-butene and 2,4,6-triphenyl-1-hexene.
The bulk polymerization process is usually carried out at 80°-180° C. and then the resulting polymers are recovered by removing the remaining solvent and unreacted monomers therefrom by volatization with heating. However, in the radical polymerization process, high conversion of monomer to polymer cannot be achieved, and even after devolatilization with heating, a relatively large amount of unreacted styrene monomers usually remains in the resulting styrenic resin. Steps for improving the devolatilization with heating, for example, a method of azeotropic removal of the remaining styrene monomers together with water by adding water to the residue from the devolatilization with heating, mixing thereof and re-devolatilization, have been developed, but have not reached a satisfactory level yet.
Furthermore, since it is difficult to evaporate the by-produced styrene oligomers such as dimers, trimers, etc., most of the oligomers usually remain in the polymers.
Analysis of styrenic resins thus-produced detects the residues and impurities originating from the raw materials and the polymerization by-products. For example, styrene, &agr;-methylstyrene, n-propylbenzene, isopropylbenzene, 2,4-diphenyl-1-butene, 1,2-diphenylcyclobutane, 1-phenyltetralin, 2,4,6-triphenyl-1-hexene, 1,3,5-triphenylcyclohexane, 1-phenyl-4-(1′-phenylethyl)tetralin, etc. are contained in the styrenic resin.
In this manner, styrenic resin produced by the now widely utilized radical polymerization processes contains a large amount of low molecular weight components comprising styrene monomers and styrenic oligomers, etc. due to the production process. Furthermore, the styrenic resin produced by the radical polymerization process is usually poor in stability, so that the content of low molecular weight components such as styrene monomers and styrenic oligomers, etc. in the resin is liable to increase, depending on the mechanical or thermal history during molding or processing. Low molecular weight components newly formed during the molding or processing cause the same problems as those of the low molecular weight components formed during the polymerization. For example, when used as electric appliance materials, sundries materials, food packaging or wrapping materials or food container materials, the low molecular weight components contained in the resin can cause various problems.
Specifically, the styrenic resin containing a large amount of such low molecular weight components does not have enough thermal stability during the molding or processing and is poor in heat resistance, which is typically expressed as hot rigidity, etc. Still further, in some cases a problem that oily matters deposit on the surfaces of molds or moldings during the molding or processing occurs. The low molecular weight components contained in the resin diffuse or ooze from the inside to the surface of moldings, creating problems, for example, the printing ink will not adhere to the surface or the print is liable to peel off. Still further, when they are applied to food containers or food packaging or wrapping, the low molecular weight components contained in the resin sometimes dissolve out or volatilize. The reduction of such problems has been keenly desired.
In contrast to the radical polymerization process, a process for producing styrenic resins by anionic polymerization, using an organolithium compound, etc. has also been technically known for a long time.
For example, its detail is disclosed in U.S. Pat. Nos. 5,391,655, 5,089,572, 4,883,846, 4,748,222, 4,205,016, 4,200,713, 4,016,348, 3,954,894, 4,859,748, etc.
In summary, these US Patents are limited only to a method of lowering the degree of dispersion in the anionic polymerization process (U.S. Pat. No. 4,883,846), a method relating to an apparatus, for example, a method employing a continuous polymerization system in the production of polystyrene by anionic polymerization (U.S. Pat. Nos. 4,016,348, 4,748,222 and 5,391,655), a method for producing an initiator for use in the anionic polymerization system and its application modes (U.S. Pat. Nos. 4,205,106 and 5,089,572). U.S. Pat. No. 4,859,748 discloses a method of controlling the anionic polymerization reaction of styrene in a continuous stirring bath-type reactor. However, these anionic polymerization process techniques for producing styrenic resins have made no mention at all of styrenic low molecular weight components relating to the object of the present invention.
Still further, JP-A-10-110074 discloses production of polymers with a small content of oligomers in the anionic polymerization using an organolithium compound as an initiator. Reference Example 2 therein discloses that monodispersed polymers with a narrow molecular weight distribution were obtained by a batch polymerization process of styrene monomer using an organolithium compound, and that styrene polymers with a dimer content of 1 ppm and a trimer content of 170 ppm were obtained by drying after addition of an antioxidant thereto. It further discloses that the styrene polymers thus-obtained by the anionic polymerization can be used for food packaging or wrapping materials, etc.
“Shokuhin Eisei (Food Hygiene)”, vol. 39, No. 3, page 199 (1998) reports that styrene dimer and styrene trimer are dissolved out of polystyrene products for foodstuffs.
International Patent Application PCT/JP 97/00796 relates to, a process for producing vinylic polymers, an initiator for anionic polymerization of vinylic monomers and a styrenic resin composition. It discloses in the specification that, when the resulting styrenic polymers with a styrene trimer cont
Ikematsu Takeshi
Kawakami Kiyoshi
Shirai Hiroshi
Suezawa Hironori
Asahi Kadei Kabushiki Kaisha
Birch & Stewart Kolasch & Birch, LLP
Teskin Fred
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