CA2068802A1

CA2068802A1 - Thin-wall molded article of branched polyacetal resin

Info

Publication number: CA2068802A1
Application number: CA 2068802
Authority: CA
Inventors: Noriyuki Sugiyama; Yoshinori Ohachi; Katsushi Sasaki
Original assignee: Individual
Current assignee: Polyplastics Co Ltd
Priority date: 1991-05-17
Filing date: 1992-05-15
Publication date: 1992-11-18
Also published as: MX9202331A; JPH04339831A

Abstract

ABSTRACT

To provide a thin-walled molded article of a polyacetal resin not only having an excellent rigidity but also retaining a good balance of excellent properties such as mechanical strength and toughness.
[Constitutlon] A thin-walled molded article of a branched polyacetal resin, produced by molding a branched polyacetal copolymer comprising (A) 99.8 to 85.0% by weight of trioxane, (B) 0.1 to 10% by weight of a cyclic ether and/or a cyclic formal as a comonomer, and (C) 0.1 to 5% by weight of a diglycidyl compound, the molded article having an average wall thickness of 2 mm or less and a bending modulus of elasticity of 30000 kg/cm2 or more as measured according to ASTM D-790 (thickness of test piece:
about 0.8 mm).

Description

2~88~2 ~ ,~.,,... 1 tDesignatlon of Document] SPECIFICATION
[Tltle of the Invention~ THIN-WALLED MOLDED
ARTICLE OF BRANCHED
POLYACETAL RESIN

The present invention relates to a thin-walled molded artlcle of a branched polyacetal re~in having a hlgh rigldity. More particularly, the pre~ent inv0ntion relates to a thin-walled molded article produced by moldlng a polyacetal copolymer and having a high rigidity for a small wall thickness of the molded article of 2 mm or less and excellent flowability and moldability, the polyacetal copolymer being produced by polymerizing trioxane, a cyclic ether compound and a glycidyl compound.

A polyacetal resin has been widely utilized in the field of automobiles, electric and electronic products, etc., by virtue of its good balance of mechanical properties and excellent friction and abrasion resistances, chemical resistance, heat resistance and electrical properties.
Although the polyacetal resin has excellent chemical and thermal properties and mechanical properties when used alone without use of any special additive, sn attempt has been made in some fields to improve various properties through the incorporation of modifi~rs, such as various reinforcements and additives, for the purpose of further improving the above-de~cribed properties.
For example, the addition of an inorganic filler, such a~ a glass fiber, a carbon fiber or talc, to a polyacetal resi.n has been conducted for the purpose of further improving the rigidity.
Although the incorporation of these additives contribute~ to an improvement in the rigidity of the polyacetal resin, it brings about problems such as molding failure due to a lowering in the fl~wability (short shot) and a remarkable lowering in the toughnes~ of the material even in the case of the addition in a small amount. In particular, in the case of an ultrathin-walled small molded article having a wall thickness of O.5 mm or less, a short shot often occurs in the corner of the cavity or the like due to the presence of the filler, which brings about not a few limitations on the applications.
Further, there occurs a secondary adverse effect that when a mat~rial containing a hard filler, such as a glas~ fl~er or a carbon fiber, i~ u~ed in a sliding portion, it is liable to abrade a mating material.
~0003~
Thùs, the conventional method wherein a known reinforcement is incorporated has various problems and ''';

.

3 2 ~ 2 . . .

are not always sati~factory, so that a re~in material which gi~e~ a molded article having a high rigidity for a small wall thickne~s thereof without detriment to the moldability and other propertie8 a3 much as po~ible ha~ been de8ired in the art.
The pre8ent inv~ntoxs have made intensive studies with a view to developing a polyacetal resin which can elimlnate the~e drawbacks and, as a re~ult, have found that a ~pecial polyacetal copolymer produced by polymerizlng specified monomers has an excellent rigldity in the form of a thin-walled molded article without the neces3ity for using any inorganic filler, offerq a good balance of excellent properties such as mech~nlcal strength and toughne~s and are excellent in the flowability and moldability.
Accordingly, the present invention provides a thin-walled molded article of a branched polyacetal re~in, produced by ~olding a branched polyacetal copolymor comprising (A) 99.8 to 85.0% by weight of trioxane, ; (B) 0.1 to 10% by weight of a cyclic ether and/or a cyclic formal a9 a comonomer, and tC~ 0.1 to 5% by weight of a diglycidyl : . .
... .

2~1$~2 compound, said molded article having an average wall thickness of 2 mm or le~s and a bending modulus of elasticity of 30000 kg/cm2 or more as measured according to ASTM D-790 (thickne~s of test piece:
about 0.8 mm).
~0005]
The branched polyacetal copolymer according to the pre~ent inventlon will now be described in more detail.
At the outset, the trioxane as the component (A) and the comonomer as the component (B) are well-known sub~tance~ commonly used in the art for the production of a polyacetal copolymer. Specifically, the trioxane ic a cyclic trimer produced from formaldehyde. The cyclic ether and cyclic formal as the comonomer (B) ate compound~ represented by the following ~eneral formula (2):
: [0006]
~Chemical formul~-2]-~--C--o : I (2) I
.. . .
wherein R3, R~, Rs and R6 which may be the same or :-- ~ ~2~$~8~2 different represent each a hydrogen atom, an alkyl group or an alkyl group substituted with a halogen, R' represents a methylene group or an oxymethylene group or a methylene group or oxymethylene group substituted with a halogenated alkyl group (wherein p represents an lnteger o~ 0 to 3), or a divalent group represented by the formula ~CH2t~0CH2- or ~o-cH2-cH2t~ocH2- wherein q repre~ent~ an integer of 1 to 4, provided that the alkyl group may have 1 to 5 carbon atoms and 1 to 3 hydrogen atoms thereof may be substituted each with a halogen atom, especlally a chlorine atom.
[0007]
Examples of the cyclic ether and cyclic formal include epichlorohydrin, ethylene oxide, 1,3-dioxolane, diethylene glycol formal, 1,4-butanediol formal, 1,3-dioxane and propylene oxide. It is also possible to use cyclic esters, for example, ~-propiolactone, and vinyl compounds, for example, styrene or acrylonitrile. Particularly preferred cyclic ether and cyclic formal include at least one member ~elected from among ethylene oxide, dioxolane, trioxepane and 1,4-butanediol formal.
The amount of u~e of the cyclic ether and the cyclic formal is 0.1 to 10% by welght, preferably 0.3 to 0.5% by weight based on the whole monomer mixture.

. ~ .

.:

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`2~8Q~

When the amount i8 less than 0.1~ by weight, the moldability and heat stability characteri~tic of the polyacetal copolymer become insufficient. On the other hand, when the amount exceed~ 5.0% by weight, the production, per se, of the polyacetal copolymer becomes difficult.
[0008]
The diglyc.tdyl compound a~ the component (C) which develop~ a branched chain in the polyacetal resi.n characteristic of the present invention will now be described.
The diglycidyl compound used in the production of the polyacetal copolymer according to the present invention i~ an aliphatic ether having two glycidyl group~ at its terminals and is preferably one represented by the following general formula (3) or (4):

: ~0009]

: , CH2--CH--CH2--O~CH2-CH2--CH2-ot~cH2--CH--CH2 ', O O' wherein m ts an integer of 2 to 5, or CH2-CH-CH2-O~CH2tTO-CH2-CH-CH2 ~. O O

~ - 8 -:: .
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wherein Q is an integer of 2 to 5.
t0010]
Examples of the diglycidyl compound represented by the ~ormula (3) or (4) include diethylene glycol diglycidyl ether, triethylene glycol diglyci.dyl ether and butanediol diglycidyl ether, among which butanediol diglycidyl ether is partlcularly preferred.
The amount of u~e of the diglycidyl compound is 0.1 to 5.0% by weight, preferably 0.2 to 3.0% by weight, particularly preferably 0.3 to 2.0% by weight.
When the amount 1s less than 0.1% by weight, an excellent rigidity hardly develops in a wall thickness of a molded product of 2 mm or less intended in the present invention. On the other h~nd, when the amount exceeds 5.0% by weight, an excessive crosslinking rçaction or the like occurs during the polymerization, which makes the production, per set of the polyacetal ~esin difficult. Further, in this case, the moldability and mechani.cal strength of the resultant polymer lowers to a significant extent, which is unfavorable.
[0011]
In the present invention, from the viewpoint of the rigi~i.ty required of thin-walled molded articles having an average wall thickness of 2 mm or less, such .. . .

: 9 .
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as an insulator for various motors and gears, it is important that the polyacetal copolymer have a bending modulus of elasticity of 30000 kg/cm2 or moxe as measured according to ASTM D-790 (thic~ness of test pieca: about 0.8 mm).
[0012]
The polymerization of the polyacetal copolymer according to the present lnvention can be conducted in the presence of an acetal compound (D) having a low molecular weight represented by the following general formula tl) for the purpose of regulating the molecular weight besides the above-described : component~:
[0013]
E C~ni a~l ~.
RIO(CH2O)nR2 (1) wherein n is an integer of 1 to 10 and R1 and R2 which may be the same or different represents each an alkyl group having 1 to 5 carbon atoms.
~0014]
Examples of the acetal compound as the component (D) repre~ented by the formula (1) include methylal,:
methoxymethylal, dimethoxymethylal, trimethoxymethylal and oxymethylene di-n-buthyl ether, among which methylal 19 preferred.

''. .

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- 9 2 ~ Q 2 The amount of addition of the component (D) is ad~u~ted in the range of from 0 to 1000 ppm depending upon the necessary molecular weight (melt viscosity) required of the branched copolymer.
[0015]
The polyacetal copolymer may be produced by a method well known in the art for the production of a trloxane copolymer. Specifically, a cationically active catalyst 1~ generally used a~ the catalyst.
Specific examples of the catalyst include Lewis acids, especially halides of boron, tin, titanium, phosphorous, arsenic and antimony, for example, boron trifluoride, tin tetrachloride, titanium tetrachlorlde, phosphorou~ pentachloride, phosphorus pentafluoride, arsenic pentafluoride and antimony pentafluoride, and compounds such as complex compounds and ~alts thereof, protonic acids, for example, trifluoro~ethanesulfonic acid and perchloric acid, esters of protonic acid~, especially an ester of perchloric acid with a lower aliphatic alcohol (for example, tert-butyl perchlorate), anhydrides of proton~c acids, particularly a mixed anhydride of perchloric acid and a lower aliphatic carboxylic acid (for example, acetyl perchlorate), or isopolyacid, heteropolyacid (for example, phosphomolybdic acid) or 8 ~ 2 . 1~

triethyloxonium hexafluorophosphate, triphenylmethyl hexa1uoroarsenate and acetyl hexafluoroborate. Among them, boron fluoride and a coordination compound comprising boron fluoride and an organio compound (for example, ethers) are the most common compounds and quitable .
[0016]
The polymerlzation reaction may be conducted by any of a batch process and a continuous process, and us~ may be made of any of solution polymerization, melt bulk polymerization and other polymerization methods, though a common method is such that a solid powdery or lump polymer can be obtained from a liquid monomer with the progress of the polymerization. In thls case, an inert liquid medium may also be present according to need.
Regarding a polymerization apparatus, in the case of the batch process, use may be made of a reaction vessel equipped with an agitator commonly used in the art, while in the case of the continuous process, use may bo made of a co-kneader, a twin H screw continuous extruder mixer, a twin H screw paddle continuous mixer and other continuous polymerization apparatuses propo~ed up to now for the polymerization of trioxane or the like.
'~' ' '.

. :

~ 1 S~ Q 2 The polymerization temperature is in the range of from 64 to 120C. A relatively low temperature in this range i~ preferred. Although the polymerization time depends on the amount of the catalyst and is not particularly limlted, it is generally in the range of from 0.5 to 100 min. The polymer taken out through the outlet of the polymerizer after the elapse of a predetermined period of time is usually lump or powdery, and a copolymer having a high stability can be obtained by removing part or the whole of the monomers remaining unreacted therefrom and subjecting to a post-treatment such as stabilization by a conventional method.
[0017]
The melt index of the branched polyacetal copolymer resin is in the range of from 0.01 to 60 (g/10 min) as determined at 190~ under a standard load of 2.16 kg (AST~ D-1238-57T) and preferably in the range of from 0.5 to 30 from the viewpoint of mechanical properties, moldability, etc., in the practical use. Such a branched polyacetal copolymer res.tn generally has a high dependency of the viscosity upon the shear rate and has a feature that it has a good moldability for the high molecular weight.
The branched polyacetal resin copolymer thus `2~8~2 ~2 produced gives a molded article which unexpectedly has a high rlgidity for a small wall thickneqs thereof unattainable in a linear copolymer, exhibits a rigidity generally required of a thin-walled gear, spring or other parts even without the addition of a known lnorganic filler or the like and contributes to an improvement in the mechanical strength when used as it is, ~nd is free from the drawbacks of the conventional resin containing an inorganic filler or the like, which renders the branched polyacetal resin copolymer of the present invention uitable for produciny a thin-walled, high-rigidity material. The above-described properties of the branched polyacetal copolymer are not known in the art at all, and are characteristic of the present invention.
[0018]
It i.Y also possible to mix the polyacetal copolymer of the present invention with a straight-chain polyacetal homopolymer or a polyacetal copolymer having an oxymethylene chain occupying the ma~or part of the main chain in such an amount as will have no adverse effect on the purpose.
Further, it i9 a matter of course that known various ~dditives, that iq, various stabilizers, colorant~, lubricants, release agents, nucleating : ' ' .
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agents, antistatic agents, other surfactants, dlfferent polymers, organic modifiers, inorganic or organic fibrous, particulate and flaky filler~, etc.
(for example, a glass fiber, a glaqs flake, a glass powder, a glass bead, talc and mica) may be incorporated in the polyacetal copolymer of the ; present invention ~or the purpose of lmpartlng properties depending upon intended applications.
[0019]
Specific example~ of the thin-walled molded article produced by molding the above-described polyacetal copolymer include an insulator for various motors, ~ precision part such as a watch gear, a fan, a key-board frame for a computer or a component for a floppy di~k. The smaller the wall thickness of the molded article, the better the molded article. A
molded article haviny an average wail thickness of 1 mm or lesQ is particularly favorable as a thin-walled m~lded article.
[0020]
', ~Examples]
The present invention will now be described in more detail with reference to the following Examples, though it is not limited to these Examples only.
[0021]

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.

. 14 Examples 1 to 5 Use was made of a continuous mixer reactor compo~ed of a barrel having a cross section wherein two circles with an inner diameter of 80 mm overlap partly with each other and being equipped with an outer ~acket for pa~sing a heating medium therearound along th~ effectlve length of 1.3 m and, provided thereinside, two rotating shafts having a number of paddles engaging with each other. Hot water at 80C
was pas~ed through the jacket, and the two shafts were rotated in the directions opposite to each other at a rotational speed of 100 rpm. A trioxane mixture having a composition specified in Table 1 was continuously fed into one end of the reactor at a rate of 10 kg/hr, and a predetermined amount of a cyclohexane solution of boron trifluoride dibutyl etherate was simultaneously and continuously added to the same place to conduct copolymerization. A
reaction mixture discharged from the other end of the reactor was immediately poured into water containing 0.1% of triethylamine to deactivate the polymerization cata].yst. Then, the reaction mixture was washed with acetone, air-dried and stabilized to give a branched polyacetal copolymer.
~0022]

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Comparative Exampl~s 1 to 3 Polymerization was conducted in the same manner as that of the Examples, except that use was made of trioxane as the component (A) and the cyclic ether as the component (B) listed in Table 1 w.ith the diglycidyl compound as the component (C) being used in a ~mall amount or in the absence of the diglycidyl compoun~. The reaction mixture was ~tabilized to give a substantially linear polyacetal polymer.
[0023]
The polyacetal (co)polymers produced in the Examples 1 to 5 and Comparative Examples 1 to 3 were molded into te~t pieces, which were then subjected to measurement of the thin wall bending modulus of elasticity and tensile strength. The results are summarized in Table 1.
The tests and measurement were conducted by the following methods.
1) Thin wall bending properties:
Use.was made of two test pieces respectively having thicknesses of 0.8 mm and 1.6 mm, and they were sub~ected to measurement of the bending modulus of elasticity and bending strength according to ASTM
D-790.
2) Measurement of flowability (bar flow length):

.: ' .

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A pellet having a composition specified in Table 1 was molded into a thin-walled test piece ~5 mm in width x 0.5 mm in thi.ckness) on a molding machine set under the following conditions, and the flowabilit~
wa~ evaluated from the flow length (filling length of re~in): .
cylinder temperature: 190C
ln~ection pre~sure: 500 kg/cm2 mold temperature: 80C

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As is apparent from the foregoing description and Examples, a branched poiyacetal re~in produced by polymeri~ing trioxane, a cyclic ether and/or a cyclic .formal and a diglycidyl compound gi~es a thin-walled molded article superior in mechanical properties to the conventional polyacetal resin, particularly a high rigidity in terms of the bending modulus of elasticity without a ~igniflcant lowering in the toughness, which renders the pol.yacetal resin very favorable as the material of molded articles having a ~mall size and a small thickne~s. For this reason, thiq resin is ~uitable for use in, for example, parts related to motor~ where a high output i5 required for a small size 1n the field of automobiles, electric and electronic components, etc., such as motor parts, for example, an insulator, a gear and a fan, a keyboard frame for a computer, a component for a floppy disk, and part~ related to locking where a spring property iq required for a small wall thickness, such as a snap flt.

:

Claims

[Claim 1] A thin-walled molded article of a branched polyacetal resin, produced by molding a branched polyacetal copolymer comprising (A) 99.8 to 85.0% by weight of trioxane, (B) 0.1 to 10% by weight of a cyclic ether and/or a cyclic formal as a comonomer, and (C) 0.1 to 5% by weight of a diglycidyl compound, said molded article having an average wall thickness of 2 mm or less and a bending modulus of elasticity of 30000 kg/cm2 or more as measured according to ASTM D-790 (thickness of test piece:
about 0.8 mm).
[Claim 2] A thin-walled molded article of a branched polyacetal resin according to claim 1, wherein the component (B) is at least one member selected from among ethylene oxide, dioxolane, trioxepane and 1,4-butanediol formal.
[Claim 3] A thin-walled molded article of a branched polyacetal resin according to claim 1 or 2, wherein the diglycidyl compound as the component (C) is an alkylene oxide diglycidyl ether.
[Claim 4] A thin-walled molded article of a branched polyacetal resin according to any one of claims 1 to 3, wherein the polyacetal copolymer is one produced by the polymerization in the presence of an acetal compound (D) having a low molecular weight represented by the following general formula (1):

R1O(CH20)nR2 (1) wherein n is an integer of 1 to 10 and R1 and R2 which may be the same or different are each an alkyl group having 1 to 5 carbon atoms.
[Claim 5] A thin-walled molded article of a branched polyacetal resin according to any one of claims 1 to 4, wherein the average wall thickness is 1 mm or less.
[Claim 6] A thin-walled molded article of a branched polyacetal resin according to any one of claims 1 to 5, which is an insulator for various motors, a precision part such as a watch gear, a fan, a keyboard frame for a computer or a component for a floppy disk.