Effective date : Year of fee payment : 4. Year of fee payment : 8. Year of fee payment : According to the present invention, there can be provided a polyoxymethylene copolymer excellent in bending durability and heat stability and excellent in moldability, and a molded article formed thereof. Field of the Invention .
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Effective date : Year of fee payment : 4. Year of fee payment : 8. Year of fee payment : According to the present invention, there can be provided a polyoxymethylene copolymer excellent in bending durability and heat stability and excellent in moldability, and a molded article formed thereof.
Field of the Invention . The present invention relates to a polyoxymethylene copolymer that is excellent in heat stability, excellent in bending fatigue resistance and remarkably free of deposition on a mold during molding thereof, and a molded article thereof.
Prior Art . Generally, a polyoxymethylene copolymer is obtained by copolymerizing formaldehyde or its cyclic oligomer such as trioxane or tetraoxane with a comonomer copolymerizable therewith. However, it is known that the polymer easily undergoes decomposition at its terminal. For example, it is described in U. For obtaining a stabilized polyoxymethylene copolymer, therefore, it is conventional practice to treat terminal molecules of the polyoxymethylene copolymer in various ways and add thereto additives such as an antioxidant, a heat stabilizer, and the like.
For example, for improving a polyoxymethylene copolymer in heat stability, it is disclosed, for example, in Japanese Patent Publication No. However, the triazine derivative is an additive effective for improving a polyoxymethylene copolymer in heat stability on one hand, but is poor in compatibility with a polyoxymethylene copolymer on the other hand.
When an oxymethylene copolymer containing a large amount of the triazine derivative is continuously molded, therefore, there is caused a problem that the triazine derivative adheres to a mold degradation in mold deposit resistance. However, the amount of 1,3-dioxolane in the above production method is insufficient for producing an effect on improvement of heat stability.
Meanwhile, the polymerization yield in prior art is improved by a method in which the amount of a catalyst is increased. However, it is known that the mere increase in the catalyst amount undesirably promotes the formation of unstable portions. It is a second object of the present invention to provide a polyoxymethylene copolymer which forms remarkably few deposits adhering to a mold during molding, that is, which is excellent in moldability.
It is a third object of the present invention to provide a polyoxymethylene copolymer which accomplishes a high polymerization yield and attains a low weight loss under heat. It is another object of the present invention to provide a polyoxymethylene copolymer molded article having excellent properties against bending fatigue.
It is still another object of the present invention to provide a method for producing a polyoxymethylene copolymer having the above-described excellent properties. According to the present invention, there can be obtained a polyoxymethylene copolymer excellent in bending durability and also excellent in heat stability.
According to the present invention, further, there can be obtained a polyoxymethylene copolymer which remarkably causes few deposits adhering to a mold during molding and is therefore excellent in molding productivity. The polyoxymethylene copolymer of the present invention will be explained further in detail hereinafter. The polyoxymethylene copolymer of the present invention is obtained from trioxane as a main monomer and 8 to 20 mol, per mol of the trioxane, of 1,3-dioxolane as a comonomer.
In this case, a cation-active catalyst is used as a catalyst. The polymerization method of the polyoxymethylene copolymer includes a bulk polymerization method and a melt polymerization method. These bulk polymerization method is a method in which monomers in a molten state are polymerized and a solid polymer in a bulk or powdered state is obtained as the polymerization proceeds. The monomer as a raw material is trioxane that is a cyclic trimer of formaldehyde, and as a comonomer, 1,3-dioxolane is used.
The amount of 1,3-dioxolane per mol of trioxane is in the range of from 8 to 20 mol, preferably in the range of from 8. When the amount of 1,3-dioxolane is greater than the above upper limit, the polymerization yield is low. When it is smaller than the above lower limit, the heat stability is low.
The polymerization catalyst is selected from cation-active catalysts. These cation-active catalysts include Lewis acids typified by halides of boron, tin, titanium, phosphorus, arsenic and antimony, specifically, compounds such as boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus pentachloride, phosphorus pentafluoride, arsenic pentafluoride, antimony pentafluoride and complexes or salts of these; protonic acids such as trifluoromethanesulfonic acid, perchloric acid, esters of protonic acids typified by an ester of perchloric acid and a lower aliphatic alcohol and anhydrides of protonic acids typified by mixed anhydrides of perchloric acid and a lower aliphatic carboxylic acid; triethyloxonium hexafluorophosphate, triphenylmethylhexafluoroarsenate, acetylhexafluoroborate, heteropolyacid or acidic salt thereof, and isopolyacid or acidic salt thereof.
Of these, a compound containing boron trifluoride, a hydrate of boron trifluoride or a coordination complex compound thereof is particularly suitable, and boron trifluoride diethyl etherate and boron trifluoride dibutyl etherate that are coordination complexes with ethers are particularly preferred. When the amount of the catalyst is greater than the above upper limit, the heat stability is low, and when it is smaller than the above lower limit, the polymerization yield is low.
For adjusting the molecular weight of the polyoxymethylene copolymer, the above polymerization method may use a proper molecular weight adjusting agent as required. The molecular weight adjusting agent includes a carboxylic acid, a carboxylic acid anhydride, an ester, an amide, an imide, phenols and an acetal compound.
Phenol, 2,6-dimethylphenol, methylal and polyoxymethylene dimethoxide are particularly preferred, and methylal is the most preferred. The molecular weight adjusting agent is used alone or in the form of a solution. When it is used in the form of a solution, the solvent therefor includes aliphatic hydrocarbons such as hexane, heptane and cyclohexane; aromatic hydrocarbons such as benzene, toluene and xylene; and hydrocarbon halides such as methylene dichloride and ethylene dichloride.
The polymerization apparatus for obtaining the polyoxymethylene copolymer of the present invention can be any one of batch method and continuous method apparatuses. As a batch method polymerization apparatus, there can be used a general reactor with a stirrer. The continuous method polymerization apparatus can be selected from a kneader having stirring capability and accurate temperature control capability for coping with sharp solidification and heat generation during polymerization and further having a self-cleaning function for preventing adherence of scales, a twin-screw continuous extruder, a twin-screw paddle type continuous mixer or other continuous trioxane polymerization apparatus that has been so far proposed.
Polymerization apparatuses of two or more types may be used in combination. When the polymerization temperature is higher than the above upper limit before the polymerization yield reaches the boundary yield, the heat stability is low, and the polymerization yield is low. Further, when the polymerization temperature is lower, the heat stability is maintained, but the polymerization yield is low as well.
When the polymerization temperature after the polymerization yield reaches the boundary yield is higher than the above upper limit, the heat stability is low, and when it is lower than the above lower limit, disadvantageously, the torque of stirring power of a polymerization apparatus is caused to increase.
Further, the polymerization temperature after the boundary yield is reached is required not to be higher than the temperature before the boundary yield is reached. When this relationship is reversed, the heat stability is low. The polymerization time period is determined to be in the range of from 3 to minutes, and it is preferably in the range of from 5 to 60 minutes, particularly preferably in the range of from 10 to 60 minutes.
When the polymerization time period is smaller, the heat stability and yield of the resin are caused to be poor. When the polymerization time period is longer, the productivity is caused to be poor. The polymerization time period has its preferable lower limit depending upon the amount ratio of 1,3-dioxolane to be copolymerized. For example, when the amount of 1,3-dioxolane per mol of trioxane is 8 to 11 mol, the polymerization time period is at least 3 minutes, preferably at least 4 minutes.
When the amount of 1,3-dioxolane per mol of trioxane is in the range of from 11 to 20 mol, the polymerization time period is at least 5 minutes, preferably, at least 6 minutes. After completion of the polymerization, a crude copolymer is discharged from the polymerization apparatus. It is required to terminate the polymerization reaction by immediately mixing the crude copolymer with a deactivator to bring the crude copolymer into contact with the deactivator, thereby deactivating the polymerization catalyst.
The deactivator for use can be selected from a trivalent organic phosphorus compound, an amine compound or a hydroxide of an alkali metal or alkaline earth metal. The amine compound includes primary, secondary and tertiary aliphatic amines and aromatic amines, heterocyclic amines, hindered amines and other catalyst deactivators that are known per se.
Specific examples of the amine compound include ethylamine, diethylamine, triethylamine, mono-n-butylamine, di-n-butylamine, tri-n-butylamine, aniline, diphenylamine, pyridine, piperidine and morpholine. Of these, trivalent organic phosphorus compounds and tertiary amines are preferred, and triphenyl phosphine is the most preferred.
The amount of the deactivator is not restrictive so long as the reaction is terminated by deactivation of the catalyst. However, the amount of the deactivator per mole of the polymerization catalyst is generally 0. When the deactivator is used in the form of a solution or a suspension, the solvent therefor is not critical. The solvent can be selected from water, alcohols or other various aliphatic or aromatic organic solvents such as acetone, methyl ethyl ketone, hexane, cyclohexane, heptane, benzene, toluene, xylene, methylene dichloride or ethylene dichloride.
In the deactivation treatment in any case, the crude copolymer is preferably a fine powder. For this purpose, preferably, the polymerization apparatus has the function of fully pulverizing a bulk polymerization product. Further, there may be employed a constitution in which a reaction product after the polymerization is pulverized with a pulverizer in a separate step and then the deactivator is added, or the reaction product may be pulverized and stirred at the same time in the presence of the deactivator.
When the pulverization is not so carried out as to attain the above particle sizes, the reaction between the deactivator and the catalyst is not completed, and a remaining catalyst therefore gradually proceeds with depolymerization to decrease the molecular weight.
The copolymer in which the polymerization catalyst is deactivated is obtained at high yields, so that it can be directly transferred to a stabilization step to follow.
If it is required to further purify the polymer, the polymer may be subjected to washing, separation and recovery of unreacted monomer, and drying. In the stabilization step, stabilization methods in the following 1 and 2 can be employed. The copolymer is stabilized by such a method and then pelletized, whereby a stabilized moldable polyoxymethylene copolymer can be obtained. Of the above methods, the method in the above 1 is preferred as an industrial method since it has a simpler step than the method in the above 2.
When the treatment temperature is lower than the melting temperature of the polyoxymethylene copolymer, the decomposition of an unstable portion is insufficient, and no stabilization effect can be obtained. When the pressure during the treatment is higher than Torr, there is no effect on removing a gas formed by decomposition of an unstable portion out of the system, and no sufficient stabilization effect can be obtained.
When it is lower than 0. Further, undesirably, a molten resin is liable to flow out of a suction vent port, which is liable to cause a trouble in operation. As an apparatus for the above stabilization treatment, a single-screw or twin or more-screw vent-type extruder can be used.
For retaining a necessary residence time period, it is advantageous to employ a method in which two or more extruders are arranged in series.
In the above stabilization treatment, stabilizers such as an antioxidant, a heat stabilizer, etc. The antioxidant that is usable includes sterically hindered phenols such as triethylene glycol-bis 3-tert-butylhydroxymethylphenyl propionate, pentaerythrityl-tetrakis 3,5-di-tert-butylhydroxyphenyl propionate, and the like. The heat stabilizer includes amine-sbustituted triazines such as melamine, methylolmelamine, benzoguanamine, cyanoguanidine, N,N-diarylmelamine, polyamides, urea derivatives, urethanes, and inorganic acid salts, hydroxides and organic acid salts of sodium, potassium, calcium, magnesium and barium.
As the above heat stabilizer, amine-substituted triazine compounds are preferred, and melamine is particularly preferred. The amount thereof is 0. Further, in a preferred embodiment, magnesium hydroxide particles are used as a heat stabilizer. The amount of magnesium hydroxide particles are 0.
Above all, a particularly excellent effect is achieved when an amine-substituted triazine compound and magnesium hydroxide particles are used in combination as a heat stabilizer.
Further, additives may be added to the polyoxymethylene copolymer of the present invention as required, and the additives include a colorant, a nucleating agent, a plasticizer, a mold release agent, an antistatic agent such as polyethylene or glycerin, an ultraviolet absorbent such as a benzotriazole-based or benzophenone-based compound and a photo-stabilizer such as a hindered amine. The polyoxymethylene copolymer according to the present invention can give valuable molded articles since it has properties to be described below.
The polyoxymethylene copolymer can give a molded article having bending durability of a value of 30 to 1, cycles, preferably, 50 to cycles in a bending durability test to be described later. While a molded article from a conventionally known polyoxymethylene copolymer obtained by copolymerizing approximately 5 to 6. Since the copolymer of the present invention therefore gives molded articles remarkably improved in bending fatigue resistance, it has a remarkably excellent value as a material for molded articles that are required to have high bending durability.
When the crystallization time period is less than 10 seconds, a molded article is liable to be distorted. When it exceeds 2, seconds, undesirably, a molding cycle comes to take a longer period of time. According to a residence heat stability test to be described later, it has a value of at least 40 minutes, preferably at least 60 minutes, particularly preferably at least 70 minutes.
According to a heat weight loss ratio test to be described later, it has a value, as a loss ratio, of 2.
DAIGO NAKAYA PDF
The present invention relates to a polyoxymethylene copolymer that is excellent in heat stability, excellent in bending fatigue resistance and remarkably free of deposition on a mold during molding thereof, and a molded article thereof. Generally, a polyoxymethylene copolymer is obtained by copolymerizing formaldehyde or its cyclic oligomer such as trioxane or tetraoxane with a comonomer copolymerizable therewith. However, it is known that the polymer easily undergoes decomposition at its terminal. For example, it is described in U.
US20030125512A1 - Polyoxymethylene copolymer and molded article thereof - Google Patents