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TTTLE
POLYACETAL RESIN COMPOSITION AND SLIDING PARTS
DETAILED DESCRIPTION OF THE INVENTION
The present invention concerns a polyacetal resin composition
with excellent slidability and abrasion resistance appropriately used for
sliding parts and plastic sliding parts. More specifically, it concerns a
polyacetal resin composition that makes it possible to propose sliding
parts with excellent slidability and abrasion resistance even when the
material of the partner sliding part is plastic or metal when used as
sliding parts, and sliding parts that employ this composition.
Polyacetal resin is a resin that has excellent mechanical
properties and heat resistance, especially abrasion resistance, and is used
for sliding parts in a wide range of fields.
Methods of adding inorganic solid lubricants such as
molybdenum disulfide and graphite, liquid, or semisolid lubricants such
as petroleum lubricants, synthetic lubricants, aliphatic alcohols or esters
thereof, and polyolefin resin micropowders with a molecular weight of at
least 500,000 are known in order to further improve the abrasion
resistance of polyacetal resin.
However, even if the partner sliding part is made of plastic, the
abrasion resistance varies greatly depending on the type of plastic. Even
though there are good properties for partner sliding parts made of certain
types of plastics, the problem is that the abrasion resistance is poor to
partner sliding parts made from other plastics. The lubricant had to be
selected by taking into consideration the material of the partner sliding
parts with which contact was to be made.
Nonetheless, gears, rollers, cams, switches, etc., made from
many types of plastic materials, beginning with polyacetal, are used in
electrical and electronic equipment, especially office machinery such as
audio machines, copy machines, and printers. For example, gears are
designed so that the speed of rotation is regulated by the connection of
several gears. There are consequently cases in which gears made of
polyacetal are meshed with gears made of polyacetal or other plastics.
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Consequently, the development of a polyacetal resin composition that makes
it possible to provide excellent slidability and abrasion resistance even
without selecting the lubricant by taking into consideration the type of
plastic
was desired.
A polyacetal resin composition with a density of no more than 0.93
g/cm3 that contains no more than 15 % by weight of polyolefin resin with a
weight mean molecular weight of no more than 500,000 was consequently
developed (Japanese Patent Application No. Hei 5-143322) as a polyacetal
resin composition with excellent abrasion resistance regardless of the type of
plastic from which the partner sliding parts are made.
Nonetheless, metal shafts are inserted in the inner diameter of
gears. Good slidability and abrasion resistance between polyacetal and metal
are therefore required simultaneously in addition to the good slidability and
abrasion resistance between polyacetal and other plastics. Even though there
is a certain level of good slidability and abrasion resistance when the
partner
sliding parts are made of metal when one uses a polyacetal resin composition
with a density of no more than 0.93 g/cm3 that contains no more than 15 %
by weight of polyolefin resin with a weight mean molecular weight of no
more than 500,000, the problem was that abrasion developed with long term
operation and noises such as squeaking were produced by abrasion when used
under high load or high sliding speed conditions.
International Patent Application WO 93/11206 of E. I. du Pont de
Nemours and Company discloses lubricated polyacetal compositions
containing at least one lubricant and a copolymer of the general formula
E/X/Y, wherein E is ethylene; X is methyl, ethyl, or butyl acrylate; and Y is
glycidyl acrylate or methacrylate, or glycidyl vinyl ether.
European Patent Application EP-A-354 802 of Polyplastics Co. ,
Ltd. discloses polyacetal compositiins and sliding mernber, wherein the
compositions comprises 0.5 to 40 parts by weight, per 100 parts of polyacetal
resin, of a graft copolymer which is in the form of a branched or crosslinked
material formed by chemical bonding of an olefinic polymer to a vinyl
polymer or another polymer.
European Patent Application EP-A-420 564 of Polyplastics Co. ,
Ltd. describes anti-friction and wear-resistant compositions and molded
sliding member, which compositions comprise, in addition to the components
recited in EP-A-354 802, 0. i-20 parts by weight of a lubricant and 0.5-30
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parts by weight of a tine inorganic powder per 100 parts by weight of
polyacetal resin.
U.S. Patent 4,041,002 to Aboshi et al. (Asahi Kasei Kogyo, K.K.)
describes thermoplastic resin compositions having good lubricity and wear
resistance. The base thermoplastic resin can be a polyacetal. The
composition comprises 0.3 to 25 eight % of a lubricant and 0.2 to 20 weight
of an ethylene/vinyl acetate copolymer.
Finally, Kunststoff Handbuch 3/1, Polycarbonate, Polyacetale,
polyester, Celluloseester, Dr. Ludwig Bortenbruch, Publisher, Hanser
Verlag 1992, pages 326-331, discloses blends of polyacetals with EPM and
EPDM elastomers.
The object of the present invention is to propose a polyacetal resin
composition that has excellent slidability and abrasion resistance and makes
it
possible to propose molded goods free of problems such as abrasion and
noises such as squeaking even when used under high load or high sliding
speed conditions when the partner sliding part material is any type of plastic
or metal.
The first embodiment of the present invention that attains this goal
is a polyacetal resin composition characterized by containing from 0.5 to 15
parts by weight of olefinic elastomer and from 0.1 to ~ .0 pans by weight of
fatty acid ester having at least 12 carbon atoms per 100 parts by weight of
polyacetal resin.
The second embodiment of the present invention is a polyacetal resin
composition characterized by containing from 0.5 to 15 parts by
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weight of olefinic elastomer, from 0.1 to 5.0 parts by weight of fatty acid
ester having at least 12 carbon atoms, and fram 0.05 to 5.0 parts by weight
of polytetrafluoroethylene micropowder.
The third embodiment of the present invention is polyacetal
sliding parts characfierized by being sliding parts that slide against parts
made from plastic or metal and being made from polyacetal resin that
contains from 0.5 to 15 parts by weight of olefinic elastomer and from 0.1
to 5.0 parts by weight of fatty acid ester having at least 12 carbon atoms
per 100 parts by weight of polyacetal resin.
The fouxth embodiment of the present invention is polyacetal
resin sliding parts characterized by being sliding parts that slide against
parts made from plastic or metal and being made from a polyacetal resin
composition that contains from 0.5 to 15 parts by weight of olefinic
elastomer, from 0.1 to 5.0 parts by weight of fatty acid ester having at
least 12 carbon atoms, and from 0.05 to 5.0 parts by weight of
polytetrafluoroethylene micropowder per 100 parts by weight of
polyacetal resin.
The polyacetal resin of the present invention includes
polyacetal copolymers with cyclic ethers and cyclic polymers in addition
to polyacetal homopolymers. However, since the physical properties
(tensile properties, Izod impact properties, bending properties, and
compression properties) are sometimes lowered by the addition of
lubricant, etc., in copolymers, it is preferable to use polyacetal
homogolymers.
The olefinic elastomers of the present invention are ethylene-
propylene copolymers (EPM) or ethylene-propylene-diene copolymers
(EPDM). Examples of the dimes of the third component include 1,4-
hexadiene, ethylidene norbornene, and dicyclapentadiene. The elastomer
viscosity is preferably low to improve the slidability and abrasion
resistance to plastics and metals by dispersing the olefin elastomer in the
form of microparticles in the polyacetal resin. The rubber Mooney
viscosity (JIS K-6300,121°C) is preferably no more than 50.
The olefinic elastomer is contained in a quantity of from 0.5 to
15 parts by weight, preferably 1.5 to 5.0 parts by weight, per 100 parts by
weight of polyacetal resin. The slidability and abrasion resistance with
plastics cannot be improved by less than 0.5 part by weight. The
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mechanical properties inhererit to the polyacetal drop when there is more
than 15 parts by weight.
The fatty acid esters used in the present invention are saturated
or unsaturated fatty acids that have at least 12 carbon atoms. Examples of
such fatty acids include lauric acid, tridecylenic acid, myristic acid,
pentadecylenic acid, palmitic acid, heptadecylenic acid, stearic acid,
nonadecanoic acid, alginic acid, behenic acid, lignoceric acid, cerotinic
acid, heptacosanic acid, montanic acid, melissic acid, oleic acid, elaidic
acid, linoleic acid, linolenic acid, arachidonic acid, and erucic acid.
Examples of the alcohols used to obtain the fatty acid esters of the present
invention include monohydric alcohols such as methanol, ethanol,
propanol, n-butyl alcohol, isobutyl alcohol, n-octyl alcohol, 2-ethylhexyl
alcohol, undecyl alcohol, isodecyl alcohol, and lauryl alcohol, dihydric
alcohols such as ethylene glycol, propylene glycol, diethylene glycol, and
1,4-butanediol, and polyhydric alcohols such as glycerin, sorbitan, and
pentaerythritol. Fatty acid esters are obtained by an esterification
reaction of these alcohols and the aforementioned fatty acids.
The fatty acid esters are contained in a quantity of from 0.1 to
5.0 parts by weight, preferably 0.3 to 2.0 parts by weight, per 100 parts by
weight of polyacetal resin. The slidability and abrasion resistance with
metal cannot be improved when there is less than 0.1 part by weight.
More than 5.0 parts by weight causes a drop in the mechanical properties
inherent to the polyacetal.
The polytetrafluoroethylene (PTFE) micropowder of the
present invention is commonly used I''fFE, especially resin micropowder
developed as an additive to other plastics. It is a micropowder with a
particle diameter of no more than 50 ~m and a mean particle diameter of
from 2 to 30 ~,m. The melt viscosity of the PT'FE micropowder used in the
present invention preferably ranges from 10~ to 108 poise at a
measurement temperature of 380°C as opposed to the viscosity of
commonly used PTFE resin which ranges from 101 to 1011 poise at a
measurement temperature of 380°C.
The content of PT'FE micropowder is from 0.05 to 5.0 parts by
weight, preferably 0.1 to 2.0 parts by weight, per 100 parts by weight of
polyacetal resin. The appearance of the surface of the molded goods
cannot be improved when there is less than 0.05 part by weight.
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Problems ar'sse in.terrns of heat resistance when there is more than 5.0
parts by weight. -
ICnown methods can be used as the method of prod~cing.the
polyacetal resin composition of the present invention. Examples include
methods of mechanically blending the components by extruder or
kneader.
Vaxions additives such as stabilizers, nucleus agents, antistatic
agents, flame retaxdants, colorants, and lubricants used in polyacetal resin
may also be added to the polyacetal resin cvmpasition of the present
invention.
The sliding parts of the present ircventian are those forrxted, for
example, by injection molding, using the poiyacetal resin composition.
described above. Concrete examples include sliding parts such as gears,
rollers, taros, and switches used in electrical and electronic equipment,
x5 especially office machinery such as audio equipment, copy machines, and
printers.
Together with having improved slidability and abxasiox<
resistance primxccily with plastics by combining an olefinic elastomex, the
polyacetal resin composition of the present invention also makes xt
20 possible to improve the slidability and abrasion resistance primarily with
metal by combining fatty arid esters. Combination of 1'?Ffi micropowder
also makes it possible to further improve the slidabillty and abrasion
resistance with plastics arrd metals together with markedly ixx<proving the
appearance of the surface of the molded goods.
25 EXAMPLES
The present invention is explained rrwre concreteIy.below
through the following examples.
30 Examples x to 7 d Comparative Examples to 6
As shown in Table 1, various additives were added to
- poiyacetal resin and resin pellets were produced by rrieIt kneading'at a
resin temperature of 200-220°C using a biaxial extruder (Toshiba~ 1S-
220EN), cooling with water, and cutting.
35 75 mx5n x 1?5 mm x 3.2 mm test pieces for the plate side were
produced from the various resin pellets using an injection molding
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machine (Toshiba IS-220EN35) to conduct tests of the friction and
abrasion properties of the molded goods. On the other hand, caps
stipulated by JIS K-7218 (type A) were molded using polyacetal or steel
and taken as test pieces for the cap side in friction and abrasion testing.
Friction and abrasion tests and test of the surface appearance
were conducted by the following methods.
Friction and Abrasion Property Test
Conducted by Suzuki type friction and abrasion tester (made
by Kochi Hoseihin) based on JIS K-7218 (type A).
Test Conditions
(I) When the partner material was polyacetal
Load: 2 kg weight, speed: 15 cm/ sec, travel distance: 1.62 km.
(II) When the partner material was steel (S45C)
Load: 5 kg weight, speed: 50 cm/ sec, travel distance: 40.0 km.
Dynamic Friction Coefficient
The dynamic friction coefficient was calculated by the
following formula by determining the mean value of the changes over
time in the recorded torque based on the recorded chart.
Numerical Formula 1
Dynamic Friction Coefficient =
Mean value of torque generated (kg/cm,)
Test load (kg) x Shape constant (1.14 cm)
Abrasion
The amount of abrasion was obtained by measuring the total
weight of each test piece (cap and plate) before and after the test and
determining the difference.
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Test of Surface Appearance
The.test pieces molded for friction and abrasion property
testing were evaluated by the following standards.
5: Fxcellentappeaxance
4; Slight clouding near gate
3: Silvering caused by resin flow seen near gate
Z: Silvering seen all o~rer
7.0 ' 1: Lacking uniformity and luster together with
. development of silvering
The above results axe shown in Table 7..
Furthermore, the resins and additives used in the practical and
comparative examples were as follows.
25 Polyacetal .4: Polyacetal homopolyxner with a melt index of
_ 10.5 (190°C, load 1060 g) (made by DuPont, Tellurin'~M 900 P).
Polyacetal B: Polyacetal hornopolymer with a melt index of 6.3
{194°C, load x00 g) (made by l:?uPont, TellurinT~' 500 P).
Olefittic .Elastomer: EPDM rubber with a Mooney viscosity of
~0 25 aIS K-6300,11°C) f made by DuPont, Nozdel~ , 272).
Fatty Acid Ester A: Ethylene glycol distearate (made by Kao,
F.u~anonet~7).
Fariy Acid Ester >3: Glycerin monostearate (made by Riken
BitanninTM, RikemarlTM S 100A).
25 PTFE Micropowder: Particle diameter no more than 50 Etm,
mean particle diameter 2~0 Eun, melt viscosity 10~' 10~ poise
(measurement temperature 3B0°C~ (made by Mitsu3 DuPont
Fluorochemical, Teflon(9 MP 1300).
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SUBSTITUTE SHEET (RULE 26)
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As shown in Table 1, the compositions of practical examples 1
to 4 are understood to have good slidability and abrasion resistance both
when the partner material is plastic and metal in contrast to comparative
examples 1 to 6. As is also evident from a comparison of the
compositions of practical examples 1 to 4 and the compositions of
practical examples 5 to 7, combination of PTFE micropowder further
improves the slidability against plastic and metal together with markedly
improving the surface appearance of the molded goods.
As has been explained above, the polyacetal resin composition
of the present invention makes it possible to propose molded goods free
of problems caused by abrasion and noises such as squeaking caused by
abrasion when used under high load and high sliding speed conditions
even when the material of the partner sliding parts is any type of plastic
or metal. The sliding parts of the present invention also are free of
problems such as abrasion and noises such as squeaking caused by
abrasion when used under high load and high sliding speed conditions
even when the material of the partner sliding parts is any type of plastic
or metal.
