A polyoxymethylene resin composition comprising: PA1 (A) 100 parts by weight of a polyoxymethylene copolymer; PA1 (B) 0.01 to 7 parts by weight of an amine-substituted triazine compound; and PA1 (C) 0.01 to 5 parts by weight of (C-1) polyethylene glycol having an average molecular weight of 10,000 or more and/or (C-2) modified polyolefin wax having an acidic group with an acid value of 0.5 to 60 mg-KOH/g. According to the present invention, there can be provided a polyoxymethylene resin molded product which has low shrink anisotropy, excellent thermal stability and dimensional stability.

DETAILED DESCRIPTION OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a polyoxymethylene resin composition for
 obtaining a molded product having extremely low shrink anisotropy when it
 is left for long time after molding or in a high-temperature atmosphere
 and excellent thermal stability.
 2. Prior Art
 A polyoxymethylene resin is used in a wide variety of application fields
 such as mechanical, electric and electronic, automobile, construction
 material and household goods fields as a typical engineering plastic due
 to its excellent mechanical properties, sliding properties, chemical
 resistance, fatigue resistance and the like.
 It is known that the polyoxymethylene resin has poor thermal stability due
 to its molecular structure and readily decomposes due to the break of a
 main chain caused by depolymerization from the terminal of the polymer or
 a thermal oxidization decomposition reaction. Further, since formic acid
 formed by the oxidation reaction of formaldehyde produced by the
 decomposition promotes the thermal oxidation decomposition reaction of the
 polyoxymethylene resin, the thermal stability of the polyoxymethylene
 resin is greatly impaired with the result that the practical applicability
 of the resin is lost. Therefore, the addition of an amine-substituted
 triazine compound typified by melamine, so-called "formaldehyde
 scavenger", is essential to the improvement of the thermal stability of
 the polyoxymethylene resin.
 However, the addition of the amine-substituted triazine compound which is
 an essential ingredient for the improvement of the thermal stability of
 the polyoxymethylene resin increases the shrink anisotropy of a molded
 product, and a molded product of the polyoxymethylene resin has a large
 molding shrinkage factor because the polyoxymethylene resin has high
 crystallinity. Therefore, the application of the polyoxymethylene resin in
 precision parts which require high dimensional stability is limited in
 most cases and improvement on the shrink anisotropy of the resin has been
 desired.
 As one of the methods for improving the shrink anisotropy of a molded
 product of this polyoxymethylene resin, attempts are being made to add an
 inorganic filler such as talc. This method can reduce shrink anisotropy to
 a certain degree but it involves such a problem that the characteristic
 properties of the polyoxymethylene resin are impaired, that is, physical
 properties such as impact resistance deteriorate.
 Meanwhile, a technology for blending various resins to improve shrink
 anisotropy has been proposed as another method for improving shrink
 anisotropy. For example, JP-A 4-108848 (the term "JP-A" as used herein
 means an "unexamined published Japanese patent application") proposes
 polyoxymethylene homopolymer and copolymer compositions comprising
 different polyoxymethylenes, which involves such molding problems as a
 reduction in thermal stability and the difficulty of uniform
 plasticization. JP-A 64-38463 proposes a composition comprising a specific
 high-viscosity polystyrene resin, JP-A 4-214756 proposes a composition
 comprising a polystyrene-based resin and acryl-based resin, JP-A 6-248163
 proposes a composition comprising a polycarbonate-based resin, a
 phenol-based polymer compound and a filler, JP-A 6-299046 proposes a
 composition comprising a styrene-based resin, a phenol-based polymer
 compound and a filler, and JP-A 7-292187 proposes a composition comprising
 a polystyrene-based resin having a hydroxyl group, a copolymer of a
 polyacrylic acid ester and styrene, and a polyfunctional isocyanate.
 However, all of these compositions have such a defect that the
 characteristic features of the polyoxymethylene resin are greatly impaired
 as exemplified by reductions in physical properties, deterioration in the
 surface state of a molded product, a reduction in thermal stability and
 the like caused by the occurrence of a delamination or layer separation
 phenomenon, a rise in viscosity and poor dispersibility.
 In view of the above situation, it is an object of the present invention to
 provide a polyoxymethylene resin composition which can give a molded
 product having extremely low shrink anisotropy when it is left for a long
 time after molding or in a high-temperature atmosphere and excellent
 thermal stability without impairing the characteristic properties of a
 polyoxymethylene resin. It is another object of the present invention to
 provide a polyoxymethylene resin composition which can give a molded
 product required to have high dimensional stability, such as a precision
 part.
 JP-B 37-8816 (the term "JP-B" as used herein means an "examined Japanese
 patent publication") discloses a method for improving the flowability of a
 polyoxymethylene resin at the time of molding by adding polyethylene
 glycol and JP-A 56-163144 discloses a method for improving the hot water
 resistance of a polyoxymethylene resin by adding polyethylene glycol.
 Surprisingly, the inventors of the present invention have found that a
 molded product having extremely low shrink anisotropy when it is left for
 a long time or in a high-temperature atmosphere is obtained by selecting
 polyethylene glycol having a molecular weight larger than a specific value
 and adding it to a polyoxymethylene copolymer together with an
 amine-substituted triazine compound. The present invention has been
 accomplished based on this finding.
 Meanwhile, JP-A 59-51937 and JP-A 60-86155 disclose a method for improving
 the dispersibility of carbon black by adding polyolefin wax to a
 polyoxymethylene resin. Further, JP-A 3-70764 and JP-A 4-224856 disclose a
 method for improving the abrasion resistance of a polyoxymethylene resin
 by adding polyolefin wax. Further, JP-A 8-3236 discloses a method for
 improving the self-lubrication of a polyoxymethylene resin by adding
 polyolefin wax. Surprisingly, the present inventors have found that a
 molded product having extremely low shrink anisotropy when it is left for
 a long time or in a high-temperature atmosphere is obtained by selecting
 polyolefin wax having an acid value higher than a specific value and
 adding it to a polyoxymethylene copolymer together with an
 amine-substituted triazine compound. The present invention has been
 accomplished based on this finding.
 That is, the present invention is a polyoxymethylene resin composition
 which substantially comprises (A) 100 parts by weight of a
 polyoxymethylene copolymer, (B) 0.01 to 7 parts by weight of an
 amine-substituted triazine compound, and (C) 0.01 to 5 parts by weight of
 (C-1) polyethylene glycol having an average molecular weight of 10,000 or
 more and/or (C-2) modified polyolefin wax having an acidic group with an
 acid value of 0.5 to 60 mg-KOH/g.
 The present invention will be described in detail hereinunder.
 The polyoxymethylene copolymer (A) used in the present invention is
 generally a copolymer containing 0.4 to 40 mol %, preferably 0.4 to 10 mol
 % of oxyalkylene units in the main chain of oxymethylene. The copolymer is
 obtained by polymerizing formaldehyde and/or a cyclic oligomer thereof
 (for example, trioxan or tetraoxan) as a main monomer/s and a cyclic ether
 as a copolymerizable component in the presence of a polymerization
 catalyst.
 The cyclic ether used as a copolymerizable component is preferably a
 compound represented by the following general formula (1).
 ##STR1##
 wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are the same or different and
 each a hydrogen atom or alkyl group having 1 to 5 carbon atoms, and
 R.sub.5 is a methylene group, oxymethylene group, or methylene group or
 oxymethylene group substituted by an alkyl group (n is an integer of 0 to
 3), or a divalent group represented by the following general formula (2)
 or (3) (n is 1, and m is an integer of 1 to 4).
EQU --(CH.sub.2).sub.m --O--CH.sub.2 -- (2)
EQU --(O--CH.sub.2 --CH.sub.2).sub.m --O--CH.sub.2 -- (3)
 Specific examples of the cyclic ether include ethylene oxide, propylene
 oxide, 1,3-dioxolan, 1,3-dioxepan, 1,3,5-trioxepan, 1,3,6-trioxocan. Out
 of these, 1,3-dioxolan is particularly preferred from the view point of
 the thermal stability of the obtained resin composition.
 The polymerization catalyst Is a general cationic polymerization catalyst.
 A compound containing boron trifluoride is preferred, as exemplified by a
 hydrate and coordination complex compound of boron trifluoride. Diethyl
 etherate of boron trifluoride which is a coordination complex with an
 ether is particularly preferred.
 The polymerization of a polyoxymethylene copolymer can be carried out by
 the same device and the same method as those for the copolymerization of
 conventionally known trioxan. That is, it can be carried out in a batch or
 continuous system and can be applied to block polymerization or melt
 polymerization which is carried out in the presence of an organic solvent
 such as cyclohexane. A reaction tank equipped with a stirrer can be used
 for a batch system whereas a kneader, extruder, self-cleaning type
 continuous mixer and the like having such a function as quick hardening at
 the time of polymerization, high stirring ability to cope with heat
 generation, fine temperature control or self-cleaning to prevent the
 adhesion of scales are suitably used for continuous block polymerization.
 The catalyst can be deactivated or removed from the polyoxymethylene
 copolymer obtained by a polymerization reaction in accordance with known
 methods which use one selected from primary, secondary and tertiary amines
 such as diethylamine, triethylamine, di-iso-propylamine,
 tri-iso-propylamine, mono-n-butylamine, dibutylamine, tributylamine,
 piperidine and morpholine, hydroxides of alkali metals and alkali earth
 metals, and trivalent organic phosphorus compounds, or an aqueous solution
 or an organic solution thereof. Illustrative examples of the organic
 solvent include alcohols such as methanol and ethanol; ketones such as
 acetone and methyl ethyl ketone; aromatic compounds such as benzene,
 toluene and xylene; and saturated hydrocarbons such as cyclohexane,
 n-hexane and n-heptane. Out of these, a method for deactivating a catalyst
 with a tertiary amine or a trivalent organic phosphorus compound (JP-B
 55-42085) are preferred.
 It is known that the polyoxymethylene copolymer has poor thermal stability
 due to its molecular structure and readily decomposes due to the break of
 a main chain caused by depolymerization from the terminal of the polymer
 or a thermal oxidization decomposition reaction. Further, since formic
 acid formed by the oxidation reaction of formaldehyde produced by the
 decomposition promotes the thermal oxidation decomposition reaction of the
 polyoxymethylene copolymer, the thermal stability of the polyoxymethylene
 copolymer is greatly impaired with the result that the practical
 applicability of the copolymer is lost. Therefore, the addition of a
 formaldehyde capturing agent is essential to the improvement of the
 thermal stability of the polyoxymethylene copolymer. Consequently, an
 amine-substituted triazine compound (B) which is a formaldehyde capturing
 agent is added to the polyoxymethylene copolymer in the composition of the
 present invention.
 The amine-substituted triazine compound as the component (B) is at least
 one selected from amine-substituted triazines having a structure
 represented by the following general formula (4) and initial
 polycondensates of the amine-substituted triazines and formaldehyde.
 ##STR2##
 wherein R.sub.8, R.sub.9 and R.sub.10 are the same or different and each a
 hydrogen atom, halogen atom, hydroxyl group, alkyl group, alkoxy group,
 aryl group, hydrogenated aryl group, amino group or substituted amino
 group, with the proviso that at least one of them is an amino group or
 substituted amino group.
 Illustrative examples of the amino-substituted triazine compound or the
 initial polycondensate of the amino-substituted triazine compound and
 formaldehyde include guanamine, melamine, N-butylmelamine,
 N-phenylmelamine, N,N-diphenylmelamine, N,N-diallylmelamine,
 N,N',N"-triphenylmelamine, N,N',N"-trimethylolmelamine, benzoguanamine,
 2,4-diamino-6-methyl-sym-triazine, 2,4-diamino-6-butyl-sym-triazine,
 2,4-diamino-6-benzyloxy-sym-triazine, 2,4-diamino-6-butoxy-sym-triazine,
 2,4-diamino-6-cyclohexyl-sym-triazine, 2,4-diamino-6-chloro-sym-triazine,
 2,4-diamino-6-mercapto-sym-triazine,
 2-oxy-4,6-diamino-sym-triazine(ammeline), N,N,N',N'-tetracyanoethyl
 benzoguanamine and initial polycondensates of these and formaldehyde. Out
 of these, melamine, methylolmelamine, benzoguanamine and water-soluble
 melamine-formaldehyde resin are particularly preferred.
 The amount of the amine-substituted triazine compound (B) is 0.01 to 7
 parts by weight, preferably 0.01 to 1 part by weight based on 100 parts by
 weight of the polyoxymethylene copolymer. When the amount is smaller than
 0.01 part by weight, a stabilizing effect is insufficient and when the
 amount is larger than 7 parts by weight, the obtained molded product
 deteriorates in physical properties and has a bad appearance.
 The component (C) used in the resin composition of the present invention is
 polyethylene glycol (C-1) having an average molecular weight of 10,000 or
 more or modified polyolefin wax (C-2) having an acidic group with an acid
 value of 0.5 to 60 mg-KOH/g. These components (C-1) and (C-2) may be used
 alone or in combination.
 The polyethylene glycol as the component (C-1) is obtained by the
 ring-opening polymerization of ethylene oxide and may have a molecular
 weight of 10,000 or more, generally 10,000 to 30,000, preferably 10,000 to
 25,000 when it has a hydroxyl group at a terminal. The polyethylene glycol
 used in the present invention may be straight-chain or branched-chain.
 When the average molecular weight of the polyethylene glycol is smaller
 than 10,000, the effect of reducing shrink anisotropy is hardly observed.
 The modified polyolefin wax having an acidic group with an acid value of
 0.5 to 60 mg-KOH/g (component (C-2)) preferably has an average molecular
 weight of 30,000 or less. The component (C-2) is obtained by oxidation
 modifying or acid modifying polyolefin wax. To produce the modified
 polyolefin wax, an acidic group is introduced by the oxidation reaction of
 polyolefin wax, oxidation decomposing polyolefin, introducing a polar
 group such as a carboxyl group or sulfonic acid group by reacting
 polyolefin wax with an inorganic acid, organic acid or unsaturated
 carboxylic acid, or introducing a monomer having an acidic group during
 the polymerization of polyolefin wax. These are available on the market
 under the name of oxidation modified or acid modified polyolefin wax and
 can be easily acquired. The modified polyolefin wax having an acidic group
 preferably has an acid value of preferably 1.0 to 50 mg-KOH/g and a number
 average molecular weight of generally 500 to 30,000, preferably 1,000 to
 20,000.
 The amount of the component C in the present invention is generally 0.01 to
 5 parts by weight, preferably 0.01 to 1 part by weight, more preferably
 0.05 to 0.5 part by weight based on 100 parts by weight of the
 polyoxymethylene copolymer. When the amount is smaller than 0.01 part by
 weight, the effect of reducing shrink anisotropy becomes insufficient and
 when the amount is larger than 5 parts by weight, strength lowers
 disadvantageously. The above components C may be used alone or in
 combination of two or more.
 To further improve the thermal stability of the resin composition of the
 present invention, a steric hindrance phenol (D) is preferably added. The
 steric hindrance phenol (D) is a compound having at least one structure
 represented by the following general formula (5) in the molecule (R.sub.11
 and R.sub.12 are the same or different and each an alkyl group or
 substituted alkyl group).
 ##STR3##
 Illustrative examples of the steric hindrance phenol include
 2,2'-methylene-bis(4-methyl-6-t-butylphenol),
 4,4'-methylene-bis(2,6-di-t-butylphenol),
 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene,
 3,5-di-t-butyl-4-hydroxybenzyl dimethylamine,
 stearyl-3,5-di-t-butyl-4-hydroxybenzyl phosphonate,
 diethyl-3,5-di-t-butyl-4-hydroxybenzyl phosphonate,
 2,6,7-trioxa-1-phosphor-bicyclo[2,2,2]-octo-4-yl-methyl-3,5-di-t-butyl-4-h
 ydroxyhydrocinnamate,
 3,5-di-t-butyl-4-hydroxyphenyl-3,5-distearyl-thiotriazylamine,
 2(2'-hydroxy-3',5'-di-t-butylphenyl)-5-chlorobenzotriazole,
 2,6-di-t-butyl-4-hydroxymethylphenol,
 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylallylino)-1,3,5-triazine,
 N,N'-hexamethylenebis(3,5-di-t-butyl-4-hydroxy-hydrocinnamamide),
 octadecyl-3-(3,5-di-butyl-4-hydroxyphenyl)propionate,
 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],
 pentaerythrithyl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],
 triethylene glycol-bis[3-(3,5-dimethyl-4-hydroxyphenyl)propionate],
 triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate],
 triethylene glycol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],
 2,2'-thiodiethyl-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] and the
 like.
 Out of these, a compound having at least one structure represented by the
 following general formula (6) in the molecule is preferred.
 ##STR4##
 That is, preferred examples of the compound having the structure of the
 above general formula (6) include
 N,N'-hexamethylenebis(3,5-di-t-butyl-4-hydroxy-hydrocinnamamide),
 octadecyl-3-(3,5-di-butyl-4-hydroxyphenyl)propionate,
 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],
 pentaerythrithyl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],
 triethylene glycol-bis[3-(3,5-dimethyl-4-hydroxyphenyl)propionate],
 triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate],
 triethylene glycol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],
 2,2'-thiodiethyl-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] and the
 like.
 Out of these, more preferred are
 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],
 pentaerythrithyl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],
 triethylene glycol-bis[3-(3,5-dimethyl-4-hydroxyphenyl)propionate],
 triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate],
 triethylene glycol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] and
 2,2'-thiodiethyl-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate].
 The amount of the steric hindrance phenol (D) is generally 0.01 to 5 parts
 by weight, preferably 0.01 to 1 part by weight based on 100 parts by
 weight of the polyoxymethylene copolymer (A). When the amount is smaller
 than 0.01 part by weight, the improvement of a stabilizing effect obtained
 by adding the component (D) becomes insufficient and when the amount is
 larger than 5 parts by weight, gas is generated during molding, or the
 obtained molded product has a bad appearance.
 To further improve the thermal stability of the resin composition of the
 present invention, at least one metal compound selected from the group
 consisting of hydroxides, inorganic acid salts, organic acid salts and
 alkoxides of alkali metals and alkali earth metals is preferably added.
 The inorganic acid salts include carbonates, phosphates, silicates and
 borates. The organic acid salts include lauric acid salts, stearic acid
 salts, oleic acid salts or behenic acid salts. The alkoxides are alkoxides
 having 1 to 5 carbon atoms such as methoxides and ethoxides. Out of these,
 hydroxides, inorganic acid salts, organic acid salts and alkoxides of
 alkali earth metals are preferred, and calcium hydroxide, magnesium
 hydroxide, calcium carbonate and magnesium carbonate are more preferred.
 The amount of at least one metal-containing compound (E) selected from the
 group consisting of hydroxides, inorganic acid salts, organic acid salts
 and alkoxides of alkali metals and alkali earth metals is generally 0.001
 to 5 parts by weight, preferably 0.001 to 3 parts by weight based on 100
 parts by weight of the polyoxymethylene copolymer (A). When the amount is
 smaller than 0.001 part by weight, the improvement of a stabilizing effect
 which is obtained by adding the component (E) becomes insufficient and
 when the amount is larger than 5 parts by weight, the obtained molded
 product deteriorates in physical properties and has a bad appearance.
 The resin composition of the present invention may contain such additives
 as other stabilizer, nucleating agent, release agent, filler, pigment,
 lubricant, plasticizer, ultraviolet absorber, flame retardant and flame
 retarding aid, other resin and elastomer as required in suitable amounts.
 Illustrative examples of the filler include mineral fillers such as glass
 beads, wollastonite, mica, talc, boron nitride, calcium carbonate, kaolin,
 silicon dioxide, clay, asbestos, diatomaceous earth, graphite and
 molybdenum disulfide; inorganic fibers such as glass fibers, milled glass
 fibers, carbon fibers, potassium titanate fibers and boron fibers; and
 organic fibers such as aramid fibers.
 Various methods for producing the polyoxymethylene copolymer resin
 composition of the present invention may be employed. It is essential to
 mix or melt knead predetermined components. After predetermined components
 are added to a polyoxymethylene copolymer which has been subjected to the
 deactivation of a catalyst, they are preferably pre-mixed by a mixer such
 as a blender or Henschel mixer. Pre-mixing can be carried out in a dry
 state or in the form of a moistened product, emulsion or suspension. The
 moistened product, emulsion and suspension are prepared by adding water,
 methanol, acetone, benzene, toluene, cyclohexane or other known solvent to
 the polyoxymethylene copolymer.
 The above components may be added to the polyoxymethylene copolymer (A) in
 a molten state directly from an addition port formed in the barrel of an
 extruder without being pre-mixed.
 The predetermined components and the polyoxymethylene copolymer may be melt
 kneaded with a kneading machine selected from single-screw and twin-screw
 extruders, kneader and Banbury mixer, out of which an extruder is
 preferred. More preferred is a single-screw or twin-screw extruder which
 can remove decomposed formalin and impurities at a predetermined reduced
 pressure from a vent port. Much more preferred is a twin-screw extruder
 which can remove gas at a predetermined reduced pressure from two or more
 vent ports.
 There are a variety of twin-screw extruders, such as an extruder whose each
 of the two screws turn in the co-rotation or counter-roration, an extruder
 having a deep screw groove, an extruder having a shallow screw groove, an
 extruder having a normal screw, an extruder having a reverse screw and an
 extruder incorporating a kneading block for improving a kneading effect,
 an extruder having a seal ring for improving a degassing effect and the
 like. Basically, any extruder is acceptable if it has melt kneading
 capability to disperse a stabilizer well and does not impair the thermal
 stability of the polyoxymethylene copolymer. The melt kneading temperature
 is generally 160 to 270.degree. C.

EXAMPLES
 Reference examples, examples and comparative examples are given to detail
 the embodiments and effects of the present invention. It should be
 understood that the present invention is not limited to these examples.
 Properties of resin compositions shown in examples and comparative examples
 were measured in accordance with the following methods.
 (1) Thermal Stability Test
 The residence time (minutes) required until silver streaks are formed at a
 cylinder temperature of 240.degree. C., a mold temperature of 70.degree.
 C. and a molding cycle of 30 seconds is measured as means of evaluating
 the silver streak formation time based on residence in a cylinder using an
 injection molding machine having a clamping pressure of 75 tons. The
 larger the value the higher the thermal stability becomes.
 "Non-colored" in the table shows the result obtained when a resin
 composition is molded without being mixed with carbon black and "black
 colored" shows the result obtained when a resin composition is mixed with
 carbon black.
 (2) Shrinkage Factor and Anisotropy Test
 After a square molded plate with a fan gate measuring 100 mm in
 length.times.100 mm in width.times.4 mm in thickness is injection molded
 at a cylinder temperature of 200.degree. C. and a mold temperature of
 70.degree. C. using an injection molding machine having a clamping
 pressure of 75 tons, an L gate is cut out and left at 23.degree. C. and
 50% RH for 48 hours. And then the shrinkage factor (%) and anisotropy (%)
 of the L gate are obtained from the following expressions as the size (mm)
 of the L gate in a flow direction represented by D.sub.P1 and the size
 (mm) of the L gate in a direction perpendicular to the flow direction
 represented by D.sub.V1.
EQU shrinkage factor (flow direction) S.sub.P =(D.sub.P0
 -D.sub.P1)/D.sub.P0.times.100(%)
EQU shrinkage factor (perpendicular direction) S.sub.V =(D.sub.V0
 -D.sub.V1)/D.sub.V0.times.100(%)
EQU anisotropy=S.sub.P -S.sub.V (%)
 D.sub.P0 : size of mold in flow direction (mm)
 D.sub.V0 : size of mold in perpendicular direction (mm)
 D.sub.P1 : size of molded product in flow direction (mm)
 D.sub.V1 : size of molded product in perpendicular direction (mm)
 S.sub.P : shrinkage factor of molded product in flow direction (%)
 S.sub.V : shrinkage factor of molded product in perpendicular direction (%)
 Reference Example 1
 100 parts by weight of trioxan and 4.5 parts by weight of 1,3-dioxolan as a
 comonomer were polymerized in a twin-screw kneader having paddles which
 engage each other using boron trifluoride etherate as a catalyst and
 methylal as a chain transfer agent. After the end of polymerization, the
 catalyst was deactivated with a benzene solution containing a small amount
 of triphenyl phosphine and milled to give a polyoxymethylene copolymer.
 The polyoxymethylene copolymer had an intrinsic viscosity in p-chloroform
 (containing .alpha.-pinene) at 60.degree. C. of 1.45 dl/g.
 Reference Example 2
 100 parts by weight of trioxan and 2.5 parts by weight of ethylene oxide as
 a comonomer were polymerized in a twin-screw kneader having paddles which
 engage each other using boron trifluoride etherate as a catalyst and
 methylal as a chain transfer agent. After the end of polymerization, the
 catalyst was deactivated with a benzene solvent containing a small amount
 of triphenyl phosphine and milled to give a polyoxymethylene copolymer.
 The polyoxymethylene copolymer had an intrinsic viscosity in p-chloroform
 (containing .alpha.-pinene) at 60.degree. C. of 1.43 dl/g.
 Example 1
 0.3 part by weight of triethylene
 glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] (of Ciba
 Geigy Co., Ltd., steric hindrance phenol under the trade name of Irganox
 245) as a stabilizer, 0.1 part by weight of melamine, 0.05 part by weight
 of magnesium hydroxide and 0.2 part by weight of polyethylene glycol
 having an average molecular weight of 10,000 were added to 100 parts by
 weight of the polyoxymethylene copolymer produced in Reference Example 1
 and pre-mixed with a Henschel mixer. Thereafter, the resulting mixture was
 melt kneaded at a cylinder temperature of 200.degree. C. using a
 twin-screw extruder having a vent port, and pelletized to produce a resin
 composition. The color of the resin composition was adjusted by blending
 0.3 part by weight of carbon black into the pellets and melt kneading with
 a twin-screw extruder again. The evaluation results are shown in Table 1.
 Examples 2 and 3
 Polyoxymethylene resin compositions were produced in the same manner as in
 Example 1 except that the average molecular weight of polyethylene glycol
 was changed to 20,000 and 25,000, and evaluated. The evaluation results
 are shown in Table 1.
 Comparative Example 1
 A polyoxymethylene resin composition was produced in the same manner as in
 Example 1 except that the average molecular weight of polyethylene glycol
 was changed to 6,000 and evaluated. The evaluation results are shown in
 Table 1.
 Comparative Examples 2 to 4
 Polyoxymethylene resin compositions were produced in the same manner as in
 Example 1 except that polyethylene glycol was removed and the amount of
 melamine was changed to 0.0 to 0.1 part by weight, and evaluated. The
 evaluation results are shown in Table 1.
 Examples 4 to 7
 Polyoxymethylene resin compositions were produced in the same manner as in
 Example 1 except that the amount of polyethylene glycol (average molecular
 weight of 20,000) was changed to 0.05 to 0.4 part by weight, and
 evaluated. The evaluation results are shown in Table 1.
 Example 8 and Comparative Example 5
 After 10 parts by weight of talc was mixed with the pellets of the
 polyoxymethylene resin composition produced in Example 1, the resulting
 mixture was melt kneaded with a twin-screw extruder at a cylinder
 temperature of 200.degree. C. and pelletized to produce a resin
 composition. The color of the resin composition was adjusted by blending
 0.3 part by weight of carbon black with the pellets and melt kneading with
 a twin-screw extruder again. The evaluation results are shown in Table 1.
 For comparison, the evaluation results of a resin composition which did
 not contain polyethylene glycol are shown in Table 1.
 Example 9 and Comparative Example 6
 0.3 part by weight of triethylene
 glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] (of Ciba
 Geigy Co., Ltd., steric hindrance phenol under the trade name of Irganox
 245) as a stabilizer, 0.1 part by weight of melamine, 0.05 part by weight
 of magnesium hydroxide and 0.2 part by weight of polyethylene glycol
 having an average molecular weight of 20,000 were added to 100 parts by
 weight of the polyoxymethylene copolymer produced in Reference Example 2
 and pre-mixed with a Henschel mixer. Thereafter, the resulting mixture was
 melt kneaded at a cylinder temperature of 200.degree. C. using a
 twin-screw extruder having a vent port, and pelletized to produce a resin
 composition. The color of the resin composition was adjusted by blending
 0.3 part by weight of carbon black into the pellets and melt kneading with
 a twin-screw extruder again. The evaluation results are shown in Table 1.
 The evaluation results of a resin composition obtained by eliminating
 polyethylene glycol are shown in Table 1 for comparison.
 TABLE 1
 Stabilizer
 Polyethylene Amine-substituted Steric hindrance
 Inorganic
 Type of Glycol triazine compound Phenol
 Metal compound filled material
 comonomer (part by weight) (part by weight) (part by
 weight) (part by weight) (part by weight)
 Ex. 1 1,3-dioxolan A-1 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 2 1,3-dioxolan A-2 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 3 1,3-dioxolan A-3 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 4 1,3-dioxolan A-2 (0.05) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 5 1.3-dioxolan A-2 (0.10) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 6 1,3-dioxolan A-2 (0.15) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 7 1,3-dioxolan A-2 (0.40) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 8 1,3-dioxolan A-2 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05) E-1 (10.00)
 Ex. 9 ethylene oxide A-2 (6.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Comp. Ex. 1 1,3-dioxolan A-4 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Comp. Ex. 2 1,3-dioxolan B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Comp. Ex. 3 1,3-dioxolan B-1 (0.05) C-1 (0.30)
 D-1 (0.05)
 Comp. Ex. 4 1,3-dioxolan C-1 (0.30)
 D-1 (0.05)
 Comp. Ex. 5 1,3-dioxolan B-1 (0.10) C-1 (0.30)
 D-1 (0.05) E-1 (10.00)
 Comp. Ex. 6 ethylene oxide B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Shrink anisotropy (100 mm .times. 100 mm .times. 4 mm)
 Shrinkage factor Shrinkage factor
 Thermal stability (minutes)
 (flow direction) (perpendicular direction) Anisotropy
 Non-colored Black-colored
 Ex. 1 2.29% 2.20% 0.09% 70
 35
 Ex. 2 2.26% 2.18% 0.08% 70
 40
 Ex. 3 2.26% 2.17% 0.09% 70
 35
 Ex. 4 2.33% 2.22% 0.11% 70
 30
 Ex. 5 2.29% 2.20% 0.09% 70
 35
 Ex. 6 2.27% 2.19% 0.08% 70
 35
 Ex. 7 2.24% 2.18% 0.06% 70
 40
 Ex. 8 2.06% 2.00% 0.06% 50
 20
 Ex. 9 2.27% 2.18% 0.09% 50
 25
 Comp. Ex. 1 2.32% 2.18% 0.14% 60
 20
 Comp. Ex. 2 2.29% 2.14% 0.15% 60
 20
 Comp. Ex. 3 2.30% 2.17% 0.13% 30
 10
 Comp. Ex. 4 2.34% 2.32% 0.02% 5
 Unmeasurable
 Comp. Ex. 5 2.10% 1.99% 0.11% 40
 10
 Comp. Ex. 6 2.29% 2.13% 0.16% 40
 10
 Ex.: Example
 Comp. Ex.: Comparative Example
 A-1: Polyethylene glycol (average molecular weight of 10,000)
 A-2: Polyethylene glycol (average molecular weight of 20,000)
 A-3: Polyethylene glycol (average molecular weight of 25,000)
 A-4: Polyethylene glycol (average molecular weight of 6,000)
 B-1: Melamine
 C-1: Irganox 245
 D-1: Magnesium hydroxide
 E-1: Talc
 Example 10
 0.3 part by weight of triethylene
 glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] (of Ciba
 Geigy Co., Ltd., steric hindrance phenol under the trade name of Irganox
 245) as a stabilizer, 0.1 part by weight of melamine, 0.05 part by weight
 of magnesium hydroxide and 0.2 part by weight of polyethylene wax F-1 [of
 an acid value modified type, molecular weight of 4,000, acid value of 1.0
 mg-KOH/g] were added to 100 parts by weight of the polyoxymethylene
 copolymer produced in Reference Example 1 and pre-mixed with a Henschel
 mixer. Thereafter, the resulting mixture was melt kneaded at a cylinder
 temperature of 200.degree. C. using a twin-screw extruder having a vent
 port, and pelletized to produce a resin composition. The color of the
 resin composition was adjusted by blending 0.3 part by weight of carbon
 black into the pellets and melt kneading with a twin-screw extruder again.
 The evaluation results are shown in Table 2.
 Example 11
 A polyoxymethylene resin composition was produced in the same manner as in
 Example 10 except that polyethylene wax F-2 [of an acid value modified
 type, molecular weight of 3,200, acid value of 12 mg-KOH/g] was used in
 place of polyethylene wax F-1. The evaluation results are shown in Table
 2.
 Example 12
 A polyoxymethylene resin composition was produced in the same manner as in
 Example 10 except that polyethylene wax F-3 [of an acid value modified
 type, molecular weight of 2,700, acid value of 30 mg-KOH/g] was used in
 place of polyethylene wax F-1. The evaluation results are shown in Table
 2.
 Example 13
 A polyoxymethylene resin composition was produced in the same manner as in
 Example 10 except that polypropylene wax F-4 [of an acid value modified
 type, molecular weight of 8,000, acid value of 2.0 mg-KOH/g] was used in
 place of polyethylene wax F-1. The evaluation results are shown in Table
 2.
 Examples 14 to 16
 Polyoxymethylene resin compositions were produced in the same manner as in
 Example 10 except that the amount of polyethylene wax F-1 [of an acid
 value modified type, molecular weight of 4,000, acid value of 1.0
 mg-KOH/g] was changed to 0.05 to 0.4 part by weight. The evaluation
 results are shown in Table 2.
 Comparative Example 7
 A polyoxymethylene resin composition was produced in the same manner as in
 Example 10 except that polyethylene wax F-5 [of a general high-density
 type, molecular weight of 8,000, acid value of 0 mg-KOH/g] was used in
 place of polyethylene wax F-1. The evaluation results are shown in Table
 2.
 Comparative Example 8
 A polyoxymethylene resin composition was produced in the same manner as in
 Example 10 except that high molecular weight polyethylene wax F-6
 [molecular weight of 100,000] was used in place of polyethylene wax F-1.
 The evaluation results are shown in Table 2.
 Example 17 and Comparative Example 9
 After 10 parts by weight of talc was mixed with the pellets of the
 polyoxymethylene resin composition produced in Example 10, the resulting
 mixture was melt kneaded with a twin-screw extruder at a cylinder
 temperature of 200.degree. C. and pelletized to produce a resin
 composition. The color of the resin composition was adjusted by blending
 0.3 part by weight of carbon black with the pellets and melt kneading with
 a twin-screw extruder again. The evaluation results are shown in Table 2.
 For comparison, the evaluation results of a resin composition which did
 not contain polyethylene wax are shown in Table 2.
 Example 18 and Comparative Example 10
 0.3 part by weight of triethylene
 glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] (of Ciba
 Geigy Co., Ltd., steric hindrance phenol under the trade name of Irganox
 245) as a stabilizer, 0.1 part by weight of melamine, 0.05 part by weight
 of magnesium hydroxide and 0.2 part by weight of polyethylene wax F-1 [of
 an acid value modified type, molecular weight of 4,000, acid value of 1.0
 mg-KOH/g] were added to 100 parts by weight of the polyoxymethylene
 copolymer produced in Reference Example 2 and pre-mixed with a Henschel
 mixer. Thereafter, the resulting mixture was melt kneaded at a cylinder
 temperature of 200.degree. C. using a twin-screw extruder having a vent
 port, and pelletized to produce a resin composition. The color of the
 resin composition was adjusted by blending 0.3 part by weight of carbon
 black into the pellets and melt kneading with a twin-screw extruder again.
 The evaluation results are shown in Table 2. The evaluation results of a
 resin composition obtained by eliminating polyethylene wax are shown in
 Table 2 as comparison.
 TABLE 2
 Stabilizer
 Polyolefin Amine-substituted Steric
 hindrance Inorganic
 Type of Wax triazine compound Phenol
 Metal compound filled material
 comonomer (part by weight) (part by weight) (part by
 weight) (part by weight) (part by weight)
 Ex. 10 1,3-dioxolan F-1 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 11 1,3-dioxolan F-2 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 12 1,3-dioxolan F-3 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 13 1,3-dioxolan F-4 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 14 1,3-dioxolan F-1 (0.05) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 15 1,3-dioxolan F-1 (0.10) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 16 1,3-dioxolan F-1 (0.40) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Ex. 17 1,3-dioxolan F-1 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05) E-1 (10.00)
 Ex. 18 ethylene oxide F-1 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Comp. Ex. 7 1.3-dioxolan F-5 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Comp. Ex. 8 1,3-dioxolan F-6 (0.20) B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Comp. Ex. 9 1,3-dioxolan B-1 (0.10) C-1 (0.30)
 D-1 (0.05) E-1 (10.00)
 Comp. Ex. 10 ethylene oxide B-1 (0.10) C-1 (0.30)
 D-1 (0.05)
 Shrink anisotropy (100 mm .times. 100 mm .times. 4 mm)
 Shrinkage factor Shrinkage factor
 Thermal stability (minutes)
 (flow direction) (perpendicular direction) Anisotropy
 Non-colored Black-colored
 Ex. 10 2.38% 2.30% 0.08% 70
 35
 Ex. 11 2.30% 2.21% 0.09% 70
 30
 Ex. 12 2.26% 2.17% 0.09% 70
 30
 Ex. 13 2.27% 2.18% 0.09% 70
 30
 Ex. 14 2.33% 2.22% 0.11% 65
 25
 Ex. 15 2.37% 2.28% 0.09% 70
 30
 Ex. 16 2.39% 2.32% 0.07% 70
 35
 Ex. 17 2.18% 2.11% 0.07% 50
 15
 Ex. 18 2.27% 2.18% 0.09% 50
 20
 Comp. Ex. 7 2.32% 2.18% 0.15% 60
 20
 Comp. Ex. 8 2.32% 2.18% 0.16% 60
 20
 Comp. Ex. 9 2.22% 2.10% 0.12% 40
 5
 Comp. Ex. 10 2.29% 2.13% 0.16% 40
 10
 Ex.: Example
 Comp. Ex.: Comparative Example
 F-1: Polyethylene wax (of acid value modified type: average molecular
 weight of 4,000, acid value of 1.0 mg-KOH/g)
 F-2: Polyethylene wax (of acid value modified type: average molecular
 weight of 3,200, acid value of 12 mg-KOH/g)
 F-3: Polyethylene wax (of acid value modified type: average molecular
 weight of 2,700, acid value of 30 mg-KOH/g)
 F-4: Polyethylene wax (of acid value modified type: average molecular
 weight of 8,000, acid value of 2.0 mg-KOH/g)
 F-5: Polyethylene wax (of acid value modified type: average molecular
 weight of 8,000, acid value of 0 mg-KOH/g)
 F-6: High molecular weight polyethylene (average molecular weight of
 100,000)
 B-1: Melamine
 C-1: Irganox 245
 D-1: Magnesium hydroxide
 E-1: Talc
 Since the polyoxymethylene copolymer resin composition of the present
 invention has extremely low shrink anisotropy when it is left for a long
 time after molding or in a high-temperature atmosphere and excellent
 thermal stability, it is suitable for use as a molding material for such
 an application field that requires dimensional stability as precision
 parts. The resin composition of the present invention is free from a
 reduction in thermal stability, reductions in mechanical properties and
 deterioration in the surface state of a molded product, all of which are
 the problems of the prior art methods for improving shrink anisotropy by
 blending various resins and fillers. Therefore, it can be widely used in
 such application fields such as automobiles, electric and electronic
 parts, construction materials and household goods in which a
 polyoxymethylene resin has been used.