Polyamide having a metaxylylene group in a polymer principal chain has a high rigidity and is widely used as a molding material, and in addition thereto, it is excellent as well in a performance of cutting off oxygen, carbon dioxide and the like and is also used as a gas-barrier material for various packaging materials such as bottles, sheets, films and the like. Particularly in polyamide used for applications such as bottles, sheets, films, fibers and the like, attentions are paid to mixing of foreign matters. Due to that the molded and processed products are transparent and thin, that a high level and delicate mold processing technology is required and that foreign matters are very highly likely to damage the performances of the molded and processed products, mixing of foreign matters brings about the inferior appearance, an increase in a rate of generating defects such as breaking and the like attributable to foreign matter-generating sites and a reduction in the productivity.
Foreign matters originating in polyamide include powders called a fine, thin films called a froth, yellowed matters and carbides produced by thermal degradation and gelatinous matters. It is the best countermeasure to inhibit the above foreign matters from being produced, but when they are inevitably produced, they have to be separated and removed from the pelletized products. The powders and the thin films are usually removed by wind selection, and the yellowed matters and the carbides can be removed by a screening equipment using an optical sensor. Various separating equipments are commercially available, and the sure removing effects can be expected.
On the other hand, gels are estimated to be produced due to that the molecules are damaged (the polymer molecules are degraded by radicals generated) during polymerization and mold processing to bring about abnormal reactions (turned into three-dimensional polymers) such as growing of non-linear molecules and the like and that they are turned into an extremely high molecular weight as compared with those of other polyamide molecules. Accordingly, a thermal history has to be reduced to the utmost in the production step in order to obtain polyamide containing less gel, and measurements such as setting a balance of terminal group concentrations and adding a heat stabilizer or an antioxidant are carried out. However, some of the above additives show a catalytic effect to amidation reaction, and on the contrary, they expedite excessively polymerization reaction to bring about an increase in gels in a certain case. Accordingly, the reaction is carried out usually while maintaining a balance between promotion and inhibition of the reaction by adding a specific amount of an alkali compound having further a reaction inhibitory effect.
Gels produced in a melt polymerization step can be removed by a filter and the like, but the gels are turned into fine particles by a flow pressure and pass through the filter in many cases. Further, gels are likely to be produced as well in solid phase polymerization carried out in producing high viscosity products, and therefore it is almost impossible to completely remove them.
Further, gels can be produced as well in melting in mold processing other than during production of polyamide. Even when used is polyamide in which a marked difference in a production amount of gels is not observed in evaluating a quality of the polyamide after production thereof, the difference is exerted in a certain case in mold processing, and a cause thereof is estimated to be attributable to that an excessive thermal history is applied to a part of the polymer in staying parts thereof such as screw grooves, a filter, a die and the like in mold processing. Further, if a polyamide resin stays in an inside of a molding machine for a long time in molding a bottle, a degraded matter of the resin is turned into burnt deposits, and they are mixed in the product to deteriorate a yield of the product, or the burnt deposits clog flow channels in an inside of the molding machine to make it impossible to carry out the molding in a certain case. When burnt deposits are generated, it is necessary to use a purging agent or to disassemble and clean the die, and the stable production is prevented. It can be found from the above matters that it is important for obtaining finally mold processed articles having less gel and burnt deposit to produce polyamide having further less gel and a higher grade and design a mold processing apparatus having very small staying parts.
It is necessary for producing polyamide having further less gel to inhibit a thermal history in the production, set a balance between an effective concentration of a terminal group and an amount of a stabilizer and remove produced gels as well in melt polymerization and solid phase polymerization, but the effects thereof have been limited. Further, in designing a mold processing apparatus, it is possible to reduce production of gels by subjecting, for example, metal parts which are brought into contact with the resin to plating treatment, but it is difficult in terms of constitution of the apparatus to completely eliminate the staying parts. Further, the respective molding apparatuses have to be subjected to the treatment, and it is lacking in the possibility thereof in terms of the versatility and the cost. Particularly in polyamide comprising a diamine component in which xylylenediamine is a principal component, radicals are liable to be produced in a benzylmethylene site of xylylenediamine, and production of gels provides more serious problems than in other polyamides.
Patent document 1 discloses a method in which at least one selected from a lubricant, an organic phosphorus base stabilizer, a hindered phenol compound and a hindered amine compound is added in an amount of 0.0005 to 0.5 parts by mass in mold processing of polyamide to thereby inhibit gels from being produced.
Patent document 2 discloses a method in which fish eyes are inhibited from being produced in mold processing by adding 0.001 to 0.015 parts by mass of a metal salt of a higher fatty acid and a polyhydric alcohol compound in order to enhance a lubricity of polyamide to inhibit shearing heat from being generated in the mold processing.
Patent document 3 discloses a method in which 50 to 1,000 ppm by weight of a phosphinic acid compound or a phosphonous acid compound and 1 to 5 mole times of an alkali compound based on a concentration of phosphorus atoms contained are added to thereby inhibit gels from being produced.
Patent documents 4 and 5 disclose methods in which a phosphorus base compound and an alkali metal salt are added in a fixed balance to thereby inhibit a back pressure of a filter from being elevated by a phosphorus-modified product and inhibit gels from being produced.
Patent documents 6 and 7 disclose methods in which the amounts of pyrophosphoric acid and other phosphorus base compounds contained in polyamide are controlled to thereby inhibit a back pressure of a filter from being elevated by a phosphorus-modified product and inhibit gels from being produced.