Process for the production of resin pellet

Details Process for the production of resin pellet Process for the production of resin pellet

2000-10-25

2002-03-26

Foelak, Morton (Department: 1711)

Plastic and nonmetallic article shaping or treating: processes

Utilizing special inert gaseous atmosphere or flushing mold...

C264S050000, C264S054000, C264S087000, C264S141000, C264S142000

Reexamination Certificate

active

06361724

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a thermoplastic resin which can be used as a plastic material in the industry of electric and electronic appliances, automobile, other various industrial materials, and food package.
BACKGROUND OF THE INVENTION
A thermoplastic resin is excellent in workability and productivity and can be subjected to molding method such as melt injection molding and melt extrusion molding by which products or parts having a desired shape can be efficiently produced. Thus, a thermoplastic resin can be widely used as material of products and parts in the art of electrical and electronic appliances, automobile, other industrial materials and food packaging.
In recent years, there has been a growing demand for the enhancement of heat resistance of products and parts particularly in the art of electric and electronic appliances, automobile and other various industries. Accordingly, many high heat resistance thermoplastic resins have been developed. However, the enhancement of heat resistance is accompanied by the rise in the melt-kneading temperature at which the thermoplastic resin is pelletized. Since the melt-kneading temperature rises close to the decomposition temperature of the thermoplastic resin, various problems arise.
In other words, as the melt-kneading temperature at which the thermoplastic resin is pelletized approaches the decomposition temperature of the resin, the resulting thermal deterioration causes discoloration and carbonization. Thus, products and parts obtained by molding the thermoplastic resin are disadvantageous in color tone and external appearance.
In order to solve these problems, an approach which comprises melt-kneading a thermoplastic resin having an additive such as heat stabilizer and oxidation inhibitor incorporated therein has heretofore been proposed. However, a high heat resistance thermoplastic, if used, resin must be melt-kneaded at a high temperature and thus leaves something to be desired in color tone and external appearance in the form of molded article even if provided with such an additive.
As a technique for improving color tone and external appearance there has long been practiced a method which comprises adding a plasticizer such as mineral oil to a thermoplastic resin to lower the melt-kneading temperature of the resin. The molded article produced from the thermoplastic resin according to this method has a good color tone and external appearance but exhibits deteriorated heat resistance and mechanical properties.
Accordingly, the prior art thermoplastic resin pallet and the molded article obtained by working the thermoplastic resin have ill-balanced color tone, external appearance, heat resistance and mechanical properties and thus cannot sufficiently meet the demands of the industry.
JP-A-10-34726 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-11-28754 disclose a method for extrusion-forming a difficultly moldable resin material such as ultrahigh molecular polyethylene which comprises allowing the resin to adsorb an inorganic gas such as a carbon dioxide gas, melt-kneading the resin, and then extrusion-forming the resin. In accordance with this method, a molten resin with an inorganic gas incorporated therein is supplied into a mold where it is then extrusion-formed to obtain a desired molded article. This technique contemplates sufficient cooling of the molded article in the mold for the purpose of preventing the foaming of the surface of the molded article. However, this method is disadvantageous in that it can difficultly produce a thermoplastic resin pellet.
Further, JP-A-10-128783 and WO98/52734 disclose a method which comprises subjecting a thermoplastic resin containing a carbon dioxide gas to melt-kneading in the cylinder of a molding machine, and then supplying the resin thus melt-kneaded into a mold where it is then molded, characterized in that the carbon dioxide gas is dissipated from the molded article to the atmosphere. However, these patents have no suggestions for process of the production of a thermoplastic resin pellet which exhibits good color tone, external appearance and physical properties in molded form.
Moreover, JP-A-9-104780, JP-A-10-53662 and WO98/31521 disclose a method for the production of an extruded foamed product which comprises injecting a foaming agent such as a compressed carbon dioxide gas into the extruder. However, this method merely contemplates the use of a carbon dioxide gas as a foaming agent. These patents, too, have no suggestions for process of the production of a thermoplastic resin pellet which exhibits good color tone, external appearance and physical properties in molded form.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a molded article of a thermoplastic resin pellet which has well-balanced color tone, external appearance, heat resistance and mechanical properties and thus can sufficiently meet the demands of the industry.
It is another object of the invention to provide a thermoplastic resin pellet which exhibits improved color tone and external appearance while maintaining desired heat resistance and mechanical properties by inhibiting discoloration and carbonization due to thermal deterioration of the thermoplastic resin at the step of melt-kneading.
Other objects and effects of the present invention will become apparent from the following description.
The inventors made extensive studies of a process for the production of a thermoplastic resin pellet excellent in moldability, color tone, external appearance, heat resistance and mechanical properties. As a result, it was found that a thermoplastic resin pellet obtained by melt-kneading a mixture comprising a carbon dioxide gas incorporated therein in an amount of from 0.3 to 20.0 parts by weight based on 100 parts by weight of a specific thermoplastic resin (A) is excellent in color tone and external appearance and also provides a molded article having excellent properties in color tone, external appearance, heat resistance and mechanical properties. The present invention has been thus worked out.
That is, the above-described objects of the present invention have been achieved by providing the following production processes.
1) A process for producing a thermoplastic resin pellet, which comprises melt-kneading a thermoplastic resin to obtain a thermoplastic resin pellet,
wherein said thermoplastic resin comprising:
(A) at least one thermoplastic resin selected from the group consisting of polyphenylene ether-based resins, polyacetal-based resins, polyamide-based resins, polystyrene-based resins, acrylic resins, polyester-based resins, polycarbonates, polyphenylene sulfides, polyetherimides, polyethersulfones, polysulfones, polyether (ether) ketones; and
(B) a carbon dioxide gas contained in said thermoplastic resin (A) in an amount of from 0.3 to 20.0 parts by weight per 100 parts by weight of said thermoplastic resin (A).
2) The production process according to the above item 1), wherein said thermoplastic resin (A) comprises at least one combination selected from: a polyphenylene ether-based resin and a polyamide-based resin; a polyphenylene ether-based resin and a polystyrene-based resin; a polyphenylene ether-based resin and a polyolefin-based resin; a polyphenylene ether-based resin and a polyester-based resin; a polyamide-based resin and a polyolefin-based resin; and a polyacetal-based resin and a polyolefin-based resin.
3) The production process according to the above item 1), wherein said thermoplastic resin (A) is a thermoplastic resin containing at least one member selected from the group consisting of polyphenylene ether-based resins, polyacetal-based resins, polyamide-based resins, polystyrene-based resins, acrylic resins, polyetherimides, and polyethersulfones.
4) The production process according to the above item 1), wherein said thermoplastic resin (A) is a thermoplastic resin containing any of a polyphenylene ether-based resin, a polyacetal-based resin and a polyamide-based resin, or co

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