Polyacetal resin composition for fuel-contacting parts

A polyacetal resin composition for fuel-related parts having an excellent creeping resistance, high conductivity and high thermal stability in the kneading or molding step is provided. The composition contains (A) a polyacetal resin, (B) glass fibers, (C) a conductive carbon and (D) a polyurethane resin, and has a volume resistivity of not higher than 1.times.10.sup.5 .OMEGA.cm and, as the creeping resistance, such a tensile creep strength that it is not ruptured under a stress of 20 MPa in 60.degree. C. water for at least 200 hours.

BACKGROUND OF THE INVENTION
 1. Technical Field
 The present invention relates to a polyacetal resin composition for
 fuel-related parts comprising a polyacetal resin, glass fibers, a
 conductive carbon and a polyurethane resin, and having an excellent
 creeping resistance, a high conductivity and a high thermal stability in
 the kneading or molding step. It is also relates to fuel-related parts
 produced therefrom.
 2. Background Art
 A polyacetal resin is excellent in mechanical properties, fatigue
 resistance, friction and abrasion resistance, chemical resistance, oil
 resistance, thermal resistance and moldability. Therefore, it is used in a
 wide variety of fields such as automobiles, electrical and electronic
 equipment, other precision machines, and pipes for construction materials.
 As its use applications become wider, resin compositions having improved
 properties as materials are required and manufactured. As one of those
 resin compositions, a polyacetal resin containing a conductive carbon
 black for the purpose of giving conductivity thereto is used. For example,
 the polyacetal resins are used as fuel-related parts in consideration of
 their excellent chemical resistance. But in this case, since static
 electricity is generated by shearing of a fuel and the resin, the
 polyacetal resin is required to be conductive. Accordingly, the conductive
 carbon black is usually blended with the polyacetal resin.
 However, the polyacetal resin has such a serious drawback that the blend of
 the carbon black noticeably decreases the toughness of the polyacetal
 resin. And so, when a pipe or the like as the above fuel-related part is
 continuously given a constant pressure or continuously loaded with a
 stress, a creep rupture occurs in a short period of time even if the
 stress is low.
 On the other hand, in order to give the polyacetal resin both conductivity
 and a high mechanical strength, a surface-treated carbon fiber is added
 thereto. Such a polyacetal resin, however, cannot be used as
 general-purpose materials due to a highly increasing cost.
 Therefore, there has been desired a polyacetal resin composition for
 fuel-related parts which can be manufactured at a low cost and which has a
 high conductivity, a high toughness, and in particular, an excellent creep
 resistance.
 DISCLOSURE OF THE INVENTION
 The present inventor has intensively investigated to obtain a polyacetal
 resin composition for fuel-related parts having excellent properties as
 described above. As a result, he has found that it is extremely effective
 to blend glass fibers, a conductive carbon and a polyurethane resin with a
 polyacetal resin and, in consequence, the present invention has been
 completed.
 That is, the present invention relates to a polyacetal resin composition
 for fuel-related parts which is obtained by blending (A) a polyacetal
 resin with (B) glass fibers, (C) a conductive carbon and (D) a
 polyurethane resin, and which has a volume resistivity of 1.times.10.sup.5
 .OMEGA.cm or less and, as a creep resistance, such a tensile creep
 strength that it is not ruptured under a stress of 20 MPa in 60.degree. C.
 water for at least 200 hours.
 In a word, the present invention relates to the composition containing the
 above-described (A), (B), (C) and (D) and having the volume resistivity
 and the tensile creep strength as described above, and the fuel-related
 parts produced therefrom.
 DETAILED DESCRIPTION OF THE INVENTION
 The constitutional components of the present invention will be described
 hereinafter.
 The polyacetal resin (A) according to the present invention is a polymer
 having oxymethylene groups (--CH.sub.2 O--) as the main repeating unit,
 and such a polyacetal resin includes polyoxymethylene homopolymers and
 polyacetal copolymers. The copolymers contain, other than the oxymethylene
 groups, oxyalkylene groups having about 2 to 6 carbon atoms, preferably
 about 2 to 4 carbon atoms (e.g., an oxyethylene group (--CH.sub.2 CH.sub.2
 O--), an oxypropylene group, an oxytetramethylene group or the like). The
 content ratio of the oxyalkylene units having about 2 to 6 carbon atoms
 can be suitably selected in accordance with the application of the
 polyacetal, for example, 0.1 to 30 mol %, preferably 1 to 20 mol % based
 on the total polyacetal.
 The polyacetal copolymer can be constituted of a plurality of components
 such as a copolymer consisting of two components and a terpolymer
 consisting of three components, and it may be a block copolymer. The
 polyacetal resin may be not only a linear one but one having a branched or
 cross-linked structure. Further, the terminals of the polyacetal resin may
 be stabilized by esterification with carboxylic acids such as acetic acid,
 propionic acid and butyric acid. The degrees of polymerization, branching
 and cross-linking of the polyacetal resin are not particularly restricted
 so long as the resin is meltable and moldable.
 Preferable polyacetal resins include polyoxymethylene homopolymers and
 polyacetal copolymers (e.g., a copolymer comprising at least both an
 oxymethylene unit and an oxyethylene unit). Preference is given to the
 polyacetal copolymers from the standpoint of thermal stability.
 A molecular weight of the aforesaid polyacetal resin is preferably as large
 as possible. The larger the molecular weight is, the more the creep
 resistance improves. Concretely, it is preferred that a melt index at
 190.degree. C. of the resin is not more than 9.0 g/10 min.
 The aforesaid polyacetal resin can be produced by a conventional method,
 for example, by polymerizing aldehydes such as formaldehyde,
 paraformaldehyde and acetaldehyde, and cyclic ethers such as trioxane,
 ethylene oxide, propylene oxide and 1,3-dioxolane.
 The glass fibers (B) usable in the present invention are not particular
 restricted. In view of handling, a chopped strand being cut into
 approximately 2 to 8 mm lengths is preferable. The glass fiber having a
 diameter of usually 5 to 15 .mu.m, preferably 7 to 13 .mu.m can be
 suitably used.
 As the glass fiber, it is also preferred to use a surface pre-treated one.
 As a material for the surface treatment, polyurethane resins or oligomers
 are preferred. Such surface treated glass fibers can be easily handled.
 The conductive carbon (C) used in the present invention is not restricted
 to particular ones. Any of Ketchen Black, acetylene black, channel black
 or various furnace type conductive carbons having an average particle size
 of 1-500 m.mu., preferably 10-100 m.mu. can be used.
 The polyurethane resin (D) used in the present invention is a polymer or an
 oligomer having an urethane linkage in the main chain. Generally, in many
 cases, a reactive functional group such as a hydroxyl group is present at
 the end of the polymer chain or a functional group including a hydroxyl
 group is suspending from the main chain. The polyurethane resin includes,
 for example, thermoplastic polyurethanes prepared by reacting a
 polyisocyanate component such as aliphatic, alicyclic or aromatic
 polyisocyanates with a polyol component such as a lower molecular weight
 polyol component, e.g., aliphatic, alicyclic or aromatic polyols,
 polyether diols, polyester diols and polycarbonate diols. In the
 preparation of the polyurethane, use may be made of a chain elongating
 agent such as diols or diamines. Furthermore, polyurethane elastomers may
 also be included in the polyurethane resin. These polyurethane resins may
 be used alone or in combination of two or more of them.
 In the present invention, an addition of such a polyurethane resin results
 in the improvement of melt stability and processability of the conductive
 polyacetal resin. That depresses the decomposition during the molding or
 processing and enhances mechanical strength and creep resistance.
 The polyurethane resin may be not only linear but also blanched or
 cross-linked as long as it can maintain thermoplasticity. Among these
 polyurethane resins, preference is given to the polyurethane and the
 polyurethane elastomer which are produced by reacting a diisocyanate
 component with a diol component.
 A molecular weight of the polyurethane resin is not restricted. For
 example, from oligomers having a molecular weight of at most 10,000 to
 polymers having a molecular weight of at least 100,000 can be uesd.
 Examples of the diisocyanate component are aliphatic diisocyanates such as
 1,6-hexamethylene diisocyanate, alicyclic diisocyanates such as isophorone
 diisocyanate, aromatic diisocyanates such as 2,4-toluene diisocyanate,
 2,6-toluene diisocyanate and 4,4'-diphenylmethane diisocyanate, and
 others.
 Examples of the diol component are C.sub.2 -C.sub.10 alkylene diols,
 polyoxyalkylene glycols such as poly(oxyethylene)glycol,
 poly(oxypropylene)glycol, poly(oxytetramethylene)glycol or copolymer
 glycols thereof such as polyethylene oxide-polypropylene oxide block
 copolymer, etc., polyester diols such as a polyesterdiol which is produced
 by the condensation polymerization of C.sub.4 -C.sub.12 aliphatic
 dicarboxylic acids, e.g., polyethylene adipate or polybutylene adipate
 containing terminal hydroxyl groups, with C.sub.2 -C.sub.16 aliphatic
 diols, and others.
 A polyurethane elastomer is more useful than a polyurethane resin for
 improving melt stability and processability of the conductive polyacetal
 resin. The polyurethane elastomer includes, for example, a polyurethane
 elastomer which is produced by reacting the aforesaid diisocyanate
 component with a diol component such as polyoxyalkylene glycols and
 polyester diols containing polyoxyalkylene glycol units.
 These polyurethane resins may be previously added as a surface-treating
 agent for glass fibers (B).
 It is important that the polyacetal resin composition for fuel-related
 parts of the present invention is prepared so as to have a volume
 resistivity of not higher than 1.times.10.sup.5 .OMEGA.cm and, as the
 creep resistance, such a tensile creep strength that the resin composition
 is not ruptured under a stress of 20 MPa in water at 60.degree. C. for at
 least 200 hours by comprising a polyacetal resin (A), glass fibers (B), a
 conductive carbon (C) and a polyurethane resin (D) described above.
 As a preparation method of the aforesaid resin composition, there may be
 mentioned a method of blending 100 parts by weight of the polyacetal resin
 (A) with 5 to 20 parts by weight of the glass fibers (B), 5 to 20 parts by
 weight of the conductive carbon (C), and 0.01 to 10 part(s) by weight of
 the polyurethane resin (D) one another.
 An amount of the glass fibers (B) to be added is preferably 5 to 20 parts
 by weight, more preferably 8 to 15 parts by weight. Within such an amount
 range, the creep resistance may be improved and the flowability and
 extrusion processability may become good.
 An amount of the conductive carbon (C) to be added is preferably 5 to 20
 parts by weight, more preferably 7 to 12 parts by weight. Within such an
 amount range, sufficient conductivity as well as good toughness and heat
 stability of the polyacetal resin may be obtained.
 An amount of the polyurethane resin (D) to be added, which includes the
 amount of the polyurethane resin to be added as a surface treating agent
 for the glass fibers (B), is preferably 0.01 to 10 part(s) by weight, more
 preferably 0.01 to 3 part(s) by weight. Within such an amount range, melt
 stability and extrusion/molding processability of the conductive
 polyacetal resin are improved, and the foaming during the extrusion and
 formalin odor are not generated. The present resin composition can be used
 widely as fuel-related parts with its good mechanical strength and creep
 resistance.
 Further, a stabilizing agent for improving heat stability is preferably
 added to the present resin composition.
 Optionally, one or more of the usual additives such as UV absorbents,
 lubricants, mold releasing agents, colorants including dyes and pigments
 and surface active agents can be added, if necessary.
 The composition of the present invention may be easily prepared by the
 known conventional methods generally used as a method for the preparation
 of the resin composition. For example, a method wherein the components are
 mixed with each other and then kneaded and extruded in an extruder to
 prepare pellets; a method wherein pellets having different compositions
 are prepared, and then they are mixed with each other in a predetermined
 ratio and subjected to molding to obtain a molded product having a desired
 composition; a method wherein one or two or more of the components are
 directly charged in a molding machine. Any of these methods may be used.
 The polyacetal resin composition for fuel-related parts of the present
 invention is applied with conductivity and high creep resistance besides
 the chemical resistance, which a polyacetal resin originally has, so that
 it can be widely used for various fuel-related parts, and it is also
 improved on melt stability and extrusion/molding processability by the
 addition of a polyurethane resin.