Note: Descriptions are shown in the official language in which they were submitted.
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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
2 2068802
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
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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
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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]-
R3
~--C--o
: I (2)
I
.. . .
wherein R3, R~, Rs and R6 which may be the same or
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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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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
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0 2
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
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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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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
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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.
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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
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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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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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~7 ?~J6fi~Q2
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18
. ' .
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.
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