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
2001-02-21
2003-01-14
Truong, Duc (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S337000, C525S472000, C528S234000, C528S241000, C528S425000, C528S486000, C528S492000
Reexamination Certificate
active
06506850
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a polyoxymethylene copolymer which not only exhibits a high crystallization rate, high stiffness and excellent thermal stability but also has excellent secondary shrinkage and gas barrier properties to organic solvent gases, and to a composition thereof.
BACKGROUND ART
A polyoxymethylene resin is a polymer material which is widely used for electrical and electronic apparatus parts, automobile parts, and the like since it is easily crystallizable, and molded products thereof are excellent in mechanical properties such as stiffness, thermal resistance and creep resistance. However, oxymethylene units constituting the polyoxymethylene resin are thermally unstable. Therefore, for the purpose of preventing chain by chain depolymerization which starts from unstable terminal groups, the polyoxymethylene resin is usually applied to practice after thermal stability thereof has been generally improved, by copolymerizing a cyclic ether or a cyclic formal having oxyalkylene units capable of introducing carbon-carbon bonds in polymer chains such as ethylene oxide, propylene oxide, 1,3-dioxolane, 1,4-butane diol formal, and further compulsorily removing unstable molecular ends.
Since the oxyalkylene unit decreases the crystallization temperature of the copolymer, the commercial polyoxymethylene copolymer has a melting point of about 160° to 165° C. while the polyoxymethylene homopolymer without oxyalkylene units has that of higher than 170° C. Further, at the cooling step during molding, crystallization does not sufficiently proceed, and the stiffness which is highly dependent on the degree of crystallinity also decreases in general according to the increase of oxyalkylene units. Accordingly, there has been a demand for a polyoxymethylene copolymer which has a low content of the oxyalkylene units, has high thermal resistance and high stiffness, and exhibits sufficient thermal stability even if the content of the oxyalkylene units is decreased.
For the purpose of solving the above problems, as attempts to put into practical use a copolymer having a low content of an oxyalkylene unit and being excellent in stiffness, Japanese Patent Publication Examined No. 6-86509 and Japanese Patent Publication Unexamined No. 5-5017 (corresponding U.S. Pat. No. 5,288,840) propose a technology to make the distribution of polymer chains as uniform as possible and provide the polymer chain terminals, which becomes a starting point of thermal decomposition, with specific substituents. Japanese Patent Publication Unexamined No. 4-145115 proposes to synthesize polymers according to the same viewpoint as the above prior art and make harmless by neutralization components which accelerates decomposition of the polymer chain.
According to these technologies, there surely can be obtained a material which is excellent in tensile strength and resistance to alkali chemicals, is difficult to be thermally decomposed, and exhibits less reduction in strength when left under high temperature circumstances. However, the polyoxymethylene copolymer according to these technologies is produced by having focused on improving the thermal stability in the region where the amount of copolymer comonomer is small, and, as to the improvement of mechanical properties, only the comonomer amount is reduced as a means thereof. Therefore, from the standpoint of improving the stiffness as a mechanical property, there has remained room for improvement.
As for the proposals to enhance the crystallization rate of polyoxymethylene resins having a normal melting point, for example, Japanese Patent Publication Unexamined Nos. 08-59767 and 08-325341 disclose production methods using 1,3-dioxolane as a comonomer and a specific amount of a polymerization initiator. However, even though the range of the comonomer amount and the relatively high range of the amount of the polymerization initiator used disclosed in these prior documents can improve the polymerization yield, which is targeted therein, they are not sufficient to satisfy the demand for the essential improvement in stiffness since the melting point of the copolymer itself is too low.
Moreover, since the polyoxymethylene resin is a crystalline resin, dimensional changes occur due to post shrinkage, i.e., secondary shrinkage, when the resin is left for a long time or is exposed to the atmosphere of high temperatures after being molded. As a result, the polyoxymethylene resin cannot avoid such a drawback that the application thereof to precision parts is limitative. As a method for improving secondary shrinkage thereof, a method of formulating inorganic fillers has been conventionally known. However, the polyoxymethylene resin composition, wherein inorganic fillers are formulated, is not only inferior in mechanical properties, especially elongation and impact resistance, but also has drawbacks such as poor moldability, low strength at weld parts, and therefore it has a drawback that it is unsuitable for the material of precision parts. As other methods, for example, Japanese Patent Publication Unexamined No. 4-108848 proposes to achieve low secondary shrinkage by blending a polyoxymethylene homopolymer and a polyoxymethylene random copolymer at a predetermined ratio. However, this method employs a polyoxymethylene homopolymer which is essentially poor in thermal stability so that the resultant polymer does not exhibit sufficient thermal stability.
On the other hand, the polyoxymethylene resin is a crystalline resin having an extremely high degree of crystallinity. Therefore, it can be said a resin which, in general, is unlikely to permeate an organic solvent gas, has excellent organic solvent gas barrier properties. For instance, a polyoxymethylene resin, which does not permeate butane, propane, and the like, would be an excellent material for use as a pressure vessel, such as a gas lighter. In view of the recent rising demand for energy saving relating to the earth's environmental problems, lightening of automobile parts with the use of resins, especially fuel related parts of automobile, has been accelerated, and further improvement in the gas barrier property of the resins to automobile fuels such as gasoline and methanol has been required.
DISCLOSURE OF THE INVENTION
The present inventors have found that a polyoxymethylene copolymer having a melting point not lower than 167° C. and not higher than 173° C., wherein a low-molecular weight polyoxymethylene copolymer which is contained in the polyoxymethylene copolymer and is extractable with chloroform is not more than 5000 ppm, exhibits not only high stiffness and excellent thermal stability but also has excellent secondary shrinkage and organic solvent gas barrier properties. As a result, the present invention has been accomplished.
Namely, the present invention relates to a polyoxymethylene copolymer having a melting point not lower than 167° C. and not higher than 173° C., wherein a low-molecular weight polyoxymethylene copolymer which is contained in the polyoxymethylene copolymer and is extractable with chloroform is not more than 5000 ppm, and to a polyoxymethylene resin composition containing the polyoxymethylene copolymer, further comprising, based on 100 parts by weight of the polyoxymethylene copolymer,
(A) 0.01 to 5 parts by weight of at least one selected from the group consisting of an antioxidant, a polymer or a compound containing a formaldehyde reactive nitrogen or a catching agent of formic acid, a weathering (light) stabilizer, a mold release agent (a lubricant), and a crystalline nucleating agent,
(B) 0 to 60 parts by weight of at least one selected from the group consisting of a reinforcing material, and an electrically conductive material, a thermoplastic resin, and a thermoplastic elastomer, and
(C) 0 to 5 parts by weight of a pigment.
Further, the present invention provides a molded product obtainable by subjecting the polyoxymethylene copolymer or the composition thereof to injection molding, extrusion molding, blow molding, or pressure molding; a part obtainable by
Nanasawa Atushi
Tanimura Noritaka
Asahi Kasei Kabushiki Kaisha
Birch & Stewart Kolasch & Birch, LLP
Truong Duc
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