HK1119727A - Polyamide resin composition - Google Patents

Description

Polyamide resin composition

This application claims benefit of U.S. provisional application No.60/680150 filed on 12.5.2005.

Technical Field

The present invention relates to a polyamide resin composition. More particularly, the present invention relates to polyamide resin compositions comprising an impact modifier, a polycarbodiimide, and optionally a phenolic resin. The polyamide composition has good impact resistance and stiffness.

The composition of the present invention is preferably used for gears of electric power steering devices, which are used for reducing the rotational speed and increasing the power output of motors in electric power steering devices of automobiles.

Background

Polyamide compositions are used in a variety of applications due to their excellent physical properties, chemical resistance and processability. Common applications include automotive parts as well as electrical and electronic parts. Although polyamides have good inherent toughness, low elasticity rubber impact modifiers are typically used to increase the toughness of polyamide compositions. However, the addition of these impact modifiers may reduce the stiffness of the resulting resin. Stiffness can be improved by adding reinforcing agents and fillers, particularly inorganic reinforcing agents (e.g., glass fibers) and mineral fillers, but such measures can lead to other problems associated with wear on processing equipment, anisotropy, increased melt viscosity, and reduced resistance to hydrolysis. Therefore, an impact modifier-containing polyamide composition having good stiffness without the need to add additional reinforcing agents and fillers is desirable.

The following disclosure may be relevant to various aspects of the invention and may be briefly summarized as follows:

it is known that impact strength can be significantly improved by adding an elastomeric material modified with a reactive functional group to a polyamide resin. For example, in us patent 4,346,194, a toughened polyamide blend is disclosed containing a)60 to 97 weight percent polyamide (a mixture of 66 nylon and 6 nylon) and b)3 to 40 weight percent of a polymeric toughening agent selected from (i) elastomeric olefin copolymers having carboxyl or carboxylate functionality, or (ii) ionic copolymers of at least one α -olefin and at least one α, β -unsaturated carboxylic acid, which may contain a terpolymerizable monomer, and which is at least partially ionized by neutralizing its acidic component with a metal basic salt.

Polyamide compositions have been disclosed that improve melt viscosity and hydrolysis resistance by the addition of polycarbodiimides. For example, in U.S. Pat. No. 4,128,599, a polycarbodiimide modified, easy-to-process polyamide product is disclosed that has unique rheological properties and improved shear properties. It discloses that polycarbodiimides act as bridging agents, wherein carbodiimide groups bridge terminal COOH and NH in polyamides₂A group.

U.S. patent 5,360,888 discloses polyamide resin compositions containing 0.1 to 5 weight percent aromatic polycarbodiimide that are stabilized against hydrolysis at elevated temperatures.

U.S. patent application publication 2004/0010094 discloses a polyamide resin composition comprising an aromatic or aliphatic polycarbodiimide in a ratio of 0.10 to 3.5 molar equivalents of carbodiimide groups to acid end groups in the polyamide.

Applications for the polyamide composition include automotive parts as described above. In the column type electric power steering device, for example, a pinion gear (e.g., worm gear) and a large gear (e.g., worm wheel) are used as a reduction gear mechanism (mechanism), and after the rotation speed of the electric motor has been reduced and the power output is increased by the transmission from the pinion gear to the large gear, rotation is applied to the steering shaft, thereby applying torque that assists the steering operation. In an electric power steering apparatus used in a light four-wheeled automobile or in a general passenger car, particularly a relatively small-sized passenger car, it is common to use a so-called resin gear for at least one of a pinion gear and a bull gear, particularly for the bull gear, in which at least its teeth (teeth) are made of resin instead of conventional metal, for the purpose of reducing noise level by reducing tooth impact noise, reducing weight, and reducing sliding resistance in a reduction gear.

More specifically, a resin gear of a composite structure having an annular metal core (which is connected to a steering shaft so as to be capable of rotating integrally therewith) and an annular gear body made of resin (which has gear teeth on an outer periphery thereof and is formed around the outer periphery of the core) has been widely used as a large gear of a gear reduction device of an electric power steering apparatus. Further, polyamide resins such as MC (monomer cast) nylon, polyamide 6, polyamide 66, and polyamide 46 are generally used as base resins for forming the gear body. In recent years, the possibility of using resin gears as large gears of reduction gear units in electric power steering apparatuses for larger automobiles has been studied more than ever. Further, recently, there has been a demand for an electric power steering apparatus smaller in size and weight than conventional apparatuses, regardless of the size of the automobile, in order to produce an automobile with better fuel efficiency causing less environmental pollution.

However, the problems associated with such conventional resin gears are that, when they are incorporated into an electric power steering apparatus for a large automobile or a downsized electric power steering apparatus to reduce fuel consumption, the required durability cannot be sufficiently ensured and the gears are broken (broken) in a relatively short time interval. This is because the power output of the motor has to be increased and the torque transmitted to the reduction gear mechanism increases as the size of the automobile becomes larger. Further, as the size of the electric power steering apparatus decreases, it becomes difficult to adopt such measures to increase the modulus and reduce the surface pressure in the large gear, and the surface pressure transmitted from the small gear to the large gear tends to increase. At present, the traditional resin gear cannot fully meet the requirement of improving the power output of a reduction gear device.

Accordingly, there is a need to provide a resin gear having both high rigidity corresponding to higher power output transmitted to the reduction gear unit, particularly rigidity necessary to maintain high strength in a high-temperature use environment of the electric power steering apparatus, and high toughness that prevents the resin gear from easily cracking under an applied stress. It is known that the rigidity of a resin composition can be increased by blending reinforcing fibers such as glass fibers with a polyamide resin serving as a base resin. However, there arises a problem in that the toughness of the resin composition is significantly reduced. Still another problem is that a resin gear formed of such a resin composition has a so-called impact ability (impact ability), and the teeth of a metal gear assembled therewith are easily damaged or worn. In addition, the fine metal powder (wear powder) produced by damaged and worn metal gears is liable to cut or damage the gear teeth themselves. For this reason, a non-reinforced resin composition containing no reinforcing fiber is preferably used for forming a resin gear. Various studies have been made to improve the properties of non-reinforced resin compositions.

For example, it is known that the toughness of a resin composition can be improved by blending an elastic material with a polyamide resin serving as a base resin. However, resin gears formed using such a resin composition are easily deformed due to a decrease in rigidity. There arise problems in that, for example, when the resin gear is used as a large gear and torque is applied via a pinion gear by rotation of a motor, the large gear is deformed, a torque transmission rate from the pinion gear is lowered, or rotation is delayed, and engagement with the pinion gear is easily deteriorated. Still another problem is that, in addition to the susceptibility of such resin gears to instantaneous deformation, creep is also liable to occur therein, and creep is liable to reduce dimensional stability and cause backlash fluctuation in the engagement with the pinion gear.

It is also known that increasing the molecular weight of polyamide resins can increase toughness while preventing a decrease in stiffness. However, for example, when various additives are blended with a polyamide resin and a molding material (pellet, etc.) for injection molding is manufactured after kneading the resin in a state where the resin is heated to a temperature equal to or higher than the melting point of the polyamide resin and melted, or when a resin gear is manufactured by reheating the manufactured molding material to a temperature equal to or higher than the melting point thereof and melting in a cylinder of an injection molding apparatus and then injecting the resin into a metal mold connected to the injection molding apparatus, for example, into a molding film cavity corresponding to the shape of a gear body in the case of a resin gear of a complicated structure, decomposition of the polyamide resin increases and the molecular weight thereof decreases under the action of moisture absorbed by the polyamide resin and heat during the melting. There arises a problem in that even if a polyamide resin having an increased molecular weight is used, the toughness of the resin gear produced by the above-described method cannot be increased.

It is known that various properties of a resin composition can be improved by blending a polycarbodiimide compound with a polyamide resin. For example, Japanese patent application laid-open No. H6-16933 (claim 1, paragraph No.0001, 0004-. Further, Japanese patent application laid-open No. H9-194719 (claim 1, paragraph No.0005-0008, 0031, 0062) describes that heat resistance and impact resistance of a resin composition can be improved if 1 to 300 parts by weight of at least one elastic material selected from the group consisting of polyamide elastomers, polyester elastomers, acrylic rubbers, silicone rubbers, and fluororubbers, modified polyolefin rubbers, and modified rubbers is blended with a total of 100 parts by weight of 30 to 99% by weight of a thermoplastic resin, such as a polyamide resin, and 70 to 1% by weight of a polycarbodiimide. Further, Japanese patent application laid-open No. H11-343408 (claim 1, paragraph No. 0002-.

Summary of The Invention

Briefly, according to one aspect of the present invention, there is provided a polyamide composition comprising:

(a) about 77 to about 96.9 weight percent of at least one polyamide;

(b) about 3 to about 20 weight percent of at least one impact modifier; and

(c) about 0.1 to about 5 weight percent of at least one polycarbodiimide; wherein the weight percentages are based on the total weight of the composition.

According to another aspect of the present invention, there is provided an article made from the above composition.

Brief Description of Drawings

Fig. 1 is a schematic sectional view showing an example of an electric power steering apparatus having a reduction gear mechanism including a gear for an electric power steering apparatus according to the present invention;

FIG. 2 is a sectional view taken along line II-II in FIG. 1;

FIG. 3 is a graph showing the relationship between the flexural modulus of elasticity (GPa) and the notched Charpy (Charpy) impact strength (kJ/m2) of the resin compositions for gears of electric power steering apparatuses produced in examples of the invention and comparative examples;

FIG. 4 is a graph showing the change in water absorption of resin compositions prepared in examples of the present invention and comparative examples;

fig. 5 is a graph showing fatigue ratios of gears of electric power steering devices formed using the resin compositions prepared in examples of the present invention and comparative examples.

Detailed Description

The compositions of the present invention comprise a polyamide, an impact modifier, a polycarbodiimide, and optionally a novolac resin.

Polyamide

The polyamide of the composition of the invention is at least one thermoplastic polyamide. The polyamide may be a homopolymer, copolymer, terpolymer, or higher order polymer. Blends of two or more polyamides may be used. Suitable polyamides may be condensation products of dicarboxylic acids or derivatives thereof with diamines, and/or ring-opening polymerization products of aminocarboxylic acids, and/or lactams. Suitable dicarboxylic acids include adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, isophthalic acid and terephthalic acid. Suitable diamines include tetramethylenediamine, hexamethylenediamine, octamethylenediamine, nonamethylenediamine, dodecamethylenediamine, 2-methylpentamethylenediamine, 2-methyloctamethylenediamine, trimethylhexamethylenediamine, bis (p-aminocyclohexyl) methane, m-xylylenediamine and p-xylylenediamine. A suitable aminocarboxylic acid is 11-aminododecanoic acid. Suitable lactams include caprolactam and laurolactam (laurolactam).

Preferred aliphatic polyamides include polyamide 6; polyamide 66; polyamide 46; polyamide 69; a polyamide 610; polyamide 612; polyamide 1010; polyamide 11; polyamide 12; semi-aromatic polyamides, such as poly (m-xylylene adipamide) (polyamide MXD6), poly (dodecamethylene terephthalamide) (polyamide 12T), poly (decamethylene terephthalamide) (polyamide 10T), poly (nonamethylene terephthalamide) (polyamide 9T), hexamethylene terephthalamide and polyamide of hexamethylene adipamide (polyamide 6T/66); polyamides of hexamethylene terephthalamide and 2-methylpentamethylene terephthalamide (polyamide 6T/DT); polyamides of hexamethylene isophthalamide and hexamethylene adipamide (polyamide 6I/66); polyamides of hexamethylene terephthalamide, hexamethylene isophthalamide and hexamethylene adipamide (polyamide 6T/6I/66) and copolymers and mixtures of these polymers.

Examples of suitable aliphatic polyamides include polyamide 66/6 copolymers; polyamide 66/68 copolymer; polyamide 66/610 copolymer; polyamide 66/612 copolymer; polyamide 66/10 copolymer; polyamide 66/12 copolymer; polyamide 6/68 copolymer; polyamide 6/610 copolymer; polyamide 6/612 copolymer; polyamide 6/10 copolymer; polyamide 6/12 copolymer; polyamide 6/66/610 terpolymer; polyamide 6/66/69 terpolymer; polyamide 6/66/11 terpolymer; polyamide 6/66/12 terpolymer; polyamide 6/610/11 terpolymer; polyamide 6/610/12 terpolymer; and polyamide 6/66/PACM (bis-p- { aminocyclohexyl } methane) terpolymer.

The preferred polyamide is polyamide 66. Blends of polyamides with other thermoplastic polymers may be used. The polyamide is present in an amount of about 77 to about 96.9 weight percent, or preferably about 83 to about 94.5 weight percent, based on the total weight of the composition.

Impact modifier

The impact modifier is any impact modifier suitable for toughening polyamide resins. Examples of suitable impact modifiers are given in U.S. Pat. No. 4,174,358, which is incorporated herein by reference. Preferred impact modifiers are carboxy-substituted polyolefins, which are polyolefins having carboxylic acid moieties attached thereto (either on the polyolefin backbone itself or on side chains). "carboxylic acid moiety" means a carboxyl group such as one or more dicarboxylic acids, diesters, dicarboxylic monoesters, anhydrides, monocarboxylic acids and esters, and salts. The carboxylate is a neutralized carboxylic acid. Useful impact modifiers are dicarboxylic-substituted polyolefins, which are polyolefins having dicarboxylic acid moieties attached thereto (either on the polyolefin backbone itself or on side chains). "dicarboxylic acid moiety" refers to a dicarboxylic group such as one or more dicarboxylic acids, diesters, dicarboxylic monoesters, and anhydrides. Preferred polyolefins are copolymers of ethylene and one or more other olefins, wherein the other olefins are hydrocarbons.

The impact modifier is preferably based on an olefin copolymer, such as an ethylene/alpha-olefin polyolefin. Examples of olefins suitable for preparing the olefin copolymer include alkenes having 2 to 8 carbon atoms, such as ethylene, propylene, 1-butene, 1-heptene or 1-hexene. Diene monomers such as 1, 4-hexadiene, 2, 5-norbornadiene, 1, 7-octadiene and/or dicyclopentadiene may optionally be used in the preparation of the polyolefin. Preferred olefin copolymers are polymers derived from ethylene, at least one alpha-olefin having from 3 to 6 carbon atoms, and at least one non-conjugated diene. Particularly preferred polyolefins are ethylene-propylene-diene (EPDM) polymers and ethylene/propylene copolymers made from 1, 4-hexadiene and/or dicyclopentadiene.

The carboxyl moiety may be incorporated into the olefin copolymer during polyolefin preparation by copolymerization with an unsaturated carboxyl-containing monomer to form the impact modifier. The carboxyl moiety may also be introduced by grafting the polyolefin with an unsaturated grafting agent containing a carboxyl moiety, such as an acid, ester, diacid, diester, acid ester, or anhydride.

Examples of suitable unsaturated carboxyl-containing comonomers or grafting agents include maleic acid, maleic anhydride, maleic acid monoesters, metal salts of maleic acid monoethyl ester, fumaric acid monoethyl ester, itaconic acid, vinyl benzoic acid, vinyl phthalic acid, metal salts of fumaric acid monoethyl ester, and methyl, propyl, isopropyl, butyl, isobutyl, hexyl, cyclohexyl, octyl, 2-ethylhexyl, decyl, stearyl, methoxyethyl, ethoxyethyl, hydroxy or ethyl monoesters and diesters of maleic acid, fumaric acid, or itaconic acid, and the like. Maleic anhydride is preferred.

Preferred impact modifiers are EPDM polymers or ethylene/propylene copolymers, which are grafted with maleic anhydride. Blends of polyolefins, such as polyethylene, polypropylene, and EPDM polymers containing polyolefins that have been grafted with unsaturated compounds containing carboxyl moieties may be used as impact modifiers.

Other preferred impact modifiers are ionomers which are carboxyl-containing polymers that have been partially neutralized with divalent metal cations such as zinc, manganese, magnesium, or the like. Preferred ionomers are ethylene/acrylic acid and ethylene/methacrylic acid copolymers that have been partially neutralized with zinc. Ionomers are available from e.i. dupont DE nemours & co, inc, Wilmington, DE under the trademark Surlyn ®.

The impact modifier is present in the composition in about 2 to about 20 weight percent, or preferably about 5 to about 14 weight percent of the total weight of the composition.

Polycarbodiimides

The polycarbodiimide can be an aliphatic, alicyclic, or aromatic polycarbodiimide and can be represented by the following formula:

*N＝C＝N-R*_n

wherein the R group represents an aliphatic, alicyclic or aromatic group.

Examples of suitable R groups include, but are not limited to, divalent radicals derived from 2, 6-diisopropylbenzene, naphthalene, 3, 5-diethyltoluene, 4 '-methylene-bis (2, 6-diethylenephenyl), 4' -methylene-bis (2-ethyl-6-methylphenyl), 4 '-methylene-bis (2, 6-diisopropylphenyl), 4' -methylene-bis- (2-ethyl-5-methylcyclohexyl), 2, 4, 6-triisopropylphenyl, n-hexane, cyclohexane, dicyclohexylmethane, and methylcyclohexane, and the like.

Polycarbodiimides can be made by a variety of methods known to those skilled in the art. In U.S. Pat. No. 4, 2,941,956 or Japanese Kokoku patent application S47-33279, J.org.chem., 28, 2069-2075(1963), Chemical Reviews,81conventional manufacturing methods are described in 619-. Typically, they are prepared by a condensation reaction which is accompanied by decarboxylation of the organic diisocyanate. This process produces isocyanate-terminated polycarbodiimides.

The polycarbodiimides may be prepared using, for example, aromatic diisocyanates, aliphatic diisocyanates, and cycloaliphatic diisocyanates, or mixtures thereof. Suitable examples include 1, 5-naphthalene diisocyanate, 4 ' -diphenylmethane diisocyanate, 4 ' -diphenyldimethylmethane diisocyanate, 1, 3-phenylene diisocyanate, 1, 4-phenylene diisocyanate, 2, 4-trichloroethylene diisocyanate, 2, 6-trichloroethylene diisocyanate, a mixture of 2, 4-trichloroethylene diisocyanate and 2, 6-trichloroethylene diisocyanate, 1, 6-hexamethylene diisocyanate, cyclohexane-1, 4-diisocyanate, ditolyl diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4, 4 ' -diisocyanate, methylcyclohexane diisocyanate, tetramethylxylylene diisocyanate, 2, 6-diisopropylphenyl isocyanate and 1, 3, 5-triisopropylbenzene-2, 4-diisocyanate, and the like.

A chain terminator may be used to control polymerization and produce polycarbodiimides having terminal groups other than isocyanate groups. Examples of suitable chain terminators include monoisocyanates. Suitable monoisocyanates include phenyl isocyanate, tolyl isocyanate, dimethylphenyl isocyanate, cyclohexyl isocyanate, butyl isocyanate, naphthyl isocyanate, and the like.

Other suitable chain terminators include alcohols, amines, imines, carboxylic acids, thiols, ethers, and epoxides. Examples include methanol, ethanol, phenol, cyclohexanol, N-methylethanolamine, poly (ethylene glycol) monomethyl ether, poly (propylene glycol) monomethyl ether, diethylamine, dicyclohexylamine, butylamine, cyclohexylamine, citric acid, benzoic acid, cyclohexanoic acid, 1, 2-ethanedithiol, arylthiols (arylmercaptans), and thiophenol.

The reaction of the organic diisocyanate to form the polycarbodiimide is carried out in the presence of a carbodiimidization catalyst, such as 1-phenyl-2-phospholene (phospholene) -1-oxide, 3-methyl-1-phenyl-2-phospholene-1-oxide, 1-ethyl-2-phospholene-1-oxide, 3-methyl-e-phospholene-1-oxide and the 3-phospholene isomers of the foregoing oxides. Among them, 3-methyl-1-phenyl-2-phospholene-1-oxide is particularly reactive.

The polycarbodiimide is present in the composition in an amount of from about 0.1 to about 5 weight percent, or preferably from greater than 0.5 to about 3 weight percent, or more preferably from greater than 0.5 to about 2 weight percent, based on the total weight of the composition.

In the composition of the present invention, there may be interactions between the polyamide resin and the polycarbodiimide and between the polycarbodiimide and the impact modifier. These interactions may take the form of reactions to form covalent bonds between the terminal polyamide carboxyl groups and the carbodiimide groups of the polycarbodiimide, as well as covalent bonds between the carboxyl moieties of the impact modifier and the carbodiimide groups of the polycarbodiimide. The reaction to form any covalent bond may be reversible in some or all cases. These possible interactions mean that some or all of the polycarbodiimide in the composition is bonded to the polyamide and/or impact modifier or associated via non-bonded interactions. These interactions are believed to contribute to the toughness and stiffness of the compositions of the present invention.

Phenol-aldehyde resins

The thermoplastic phenol-aldehyde resins, also referred to as novolak resins, can be represented by the following structures, wherein R in each case represents one or more substituents. Each substituent may be selected from H, alkyl, alicyclic, and aryl. Each aromatic ring may contain more than one non-H substituent, and all non-H substituents may be the same or different.

Phenol-aldehyde resins can be prepared by reacting at least one aldehyde with at least one phenol or substituted phenol in the presence of an acid or other catalyst such that a molar excess of phenol or substituted phenol is present. Suitable phenols and substituted phenols include phenol, o-cresol, m-cresol, p-cresol, thymol, ethylphenol, propylphenol, p-butylphenol, t-butylcatechol, pentylphenol, hexylphenol, octaphenol (octaphenol), heptylphenol, nonylphenol, bisphenol a, hydroxynaphthalene, resorcinol, bisphenol a, isoeugenol, o-methoxyphenol, 4' -dihydroxyphenyl-2, 2-propane, isoamyl salicylate, benzyl salicylate, methyl salicylate, 2, 6-di-t-butyl-p-cresol, and the like. Suitable aldehydes and aldehyde precursors include formaldehyde, paraformaldehyde, polyoxymethylene, trioxane and the like. More than one aldehyde and/or phenol may be used in the preparation of the novolac. Blends of two more different phenol-aldehydes may also be used. Any thermoplastic phenol-aldehyde that can be used in conventional plastic molding is suitable, but phenol-aldehydes having a number average molecular weight of 500 to 1500 can provide minimal warpage and optimal mechanical properties.

Preferred phenol-aldehydes include phenol-aldehyde resins, cresol-formaldehyde resins, resorcinol-formaldehyde resins, and butyl phenol-formaldehyde resins. When used, the phenol-aldehyde resin is present in about 0.5 to about 10 weight percent, or preferably about 2 to about 6 weight percent, or more preferably about 2 to about 5 weight percent of the total weight of the composition.

The composition of the present invention may further comprise other additives such as flame retardants, lubricants, mold release agents, dyes and pigments, antioxidants, and inorganic fillers.

In one embodiment of the invention, the composition of the invention does not contain any reinforcing agents, such as inorganic reinforcing agents (including glass and glass fibers) or inorganic fillers.

The compositions of the present invention are melt-mixed blends in which all of the polymeric components are well dispersed within each other and all of the non-polymeric ingredients are dispersed and bound by the polymer matrix, such that the blend forms a unified whole. Any melt mixing method can be used to combine the polymeric components and non-polymeric ingredients of the present invention.

For example, the polymeric components and non-polymeric ingredients may be added to the melt mixer at once in a single addition step or in a stepwise manner, such as a single screw or twin screw extruder; a blender; a kneader; in a banbury mixer, and then melt-mixed. When the polymeric components and non-polymeric ingredients are added in a stepwise manner, a portion of the polymeric components and/or non-polymeric ingredients are added first and melt mixed, followed by the addition of the remaining polymeric components and non-polymeric ingredients and further melt mixing until a well-mixed composition is obtained.

The compositions of the present invention may be fabricated into articles using methods known to those skilled in the art, such as injection molding, blow molding, extrusion, thermoforming, melt casting, vacuum molding, and rotational molding. The composition may be overmolded onto articles made of different materials. The composition may be extruded into a film or sheet. The composition may be formed into monofilaments.

The resulting articles can be used in a variety of applications, including housings, automotive parts, electrical articles, electronic parts, and building materials.

A second aspect of the invention relates to a gear of an electric power steering apparatus for reducing the rotation speed and increasing the power output of a motor in the electric power steering apparatus of an automobile.

The studies conducted by the present inventors have confirmed that blending a small amount of a polycarbodiimide compound with a polyamide resin can improve not only the above-described various properties but also toughness while preventing a decrease in the rigidity of a resin gear. This is because the apparent viscosity resistance of the polyamide resin is increased by the interaction (presumably crosslinking caused by addition reaction) of the carboxyl group (which is a terminal functional group of the polyamide resin) with the carbodiimide group present in the polycarbodiimide compound. When the expression of the resin in its solid state is represented by a viscoelastic body, the viscous resistance described herein is a viscous component. Furthermore, this interaction is reversible, and although it is temporarily cancelled in the melting process for producing pellets or performing injection molding, this interaction reappears during cooling and the apparent viscosity resistance of the polyamide resin increases. Therefore, it is possible to improve toughness while preventing a decrease in rigidity thereof when the resin gear is manufactured via the above-described method.

However, the interaction between these two components alone is still insufficient, and the toughness is sometimes still not high enough. For this reason, incorporation of an elastic material that produces an effect of improving toughness as described in japanese patent application laid-open No. h9-194719 has been studied, but there is a problem in that incorporation of only an elastic material lowers rigidity and contributes to deformation of a resin gear as confirmed in the prior art. Further, when the amount of the polycarbodiimide compound is too high, as in the embodiment of Japanese patent application laid-open No. H9-194719, an excessive amount of the polycarbodiimide is crosslinked, whereby the effect of increasing the apparent adhesive resistance of the polyamide resin cannot be obtained. Further, blocking of the crosslinked polycarbodiimide compound is exhibited in the resin composition, thereby decreasing the continuity of the resin composition. The problem that arises is that the toughness is greatly reduced.

A second aspect of the present invention provides a gear for an electric power steering device, which is a resin gear in which at least a part of gear teeth is formed of a non-reinforced resin composition containing a polyamide resin as a base resin and no reinforcing fiber, and which has rigidity and toughness superior to those of conventional resin gears.

Specifically, the gear for an electric power steering device of the present invention is a gear for an electric power steering device for reducing the rotational speed of a motor of a steering assist device with a reduction gear mechanism and transmitting the rotation to the steering device, the gear being incorporated and used in the reduction gear mechanism, wherein at least the teeth of the gear are formed of a non-reinforced resin composition comprising:

(a) a polyamide resin having a carboxyl group at least at a terminal;

(b)0.5 to 5% by weight of a polycarbodiimide compound, and

(c)3 to 15 wt.% of an elastomeric material modified with reactive functional groups.

The resin composition preferably further comprises:

(d)1 to 10 weight percent of a thermoplastic phenoxy resin.

Further, the reactive functional group of the elastic material (c) is preferably at least one selected from a carboxyl group and a carboxylate group, and the elastic material (c) is preferably an ethylene-propylene-diene copolymer modified with the reactive functional group.

The present invention utilizes not only the interaction of a carboxyl group, which is a terminal functional group of a polyamide resin, with a carbodiimide group present in a polycarbodiimide compound, but also the interaction of a reactive functional group that modifies an elastic material with any of the foregoing groups. Furthermore, this interaction is reversible and, although it is temporarily eliminated in the melting process for producing a molding material such as pellets or injection molding using a molding material produced therefrom, it occurs again during cooling. As a result, the apparent viscosity resistance of the polyamide resin is improved.

In the case where such interaction occurs, the decrease in rigidity can be effectively suppressed while maintaining the effect of improving toughness of the elastic material. The reason for this is not clear, but the following explanation can be provided. When a conventional elastic material which is not modified with a reactive functional group and thus does not interact with the polyamide resin or the polycarbodiimide compound is incorporated, the rigidity is significantly reduced due to a decrease in the apparent viscous resistance of the polyamide resin and an increase in plastic deformation caused by the elastic material existing in a separated state inside the resin composition. The opposite effect can be produced using elastomeric materials modified with reactive functional groups.

Further, according to the second aspect of the present invention, since the polycarbodiimide compound is blended in a ratio of 0.5 to 5% by weight of the entire resin composition, crosslinking of the excessive polycarbodiimide composition can be prevented. The whole amount thereof interacts with the polyamide resin and the elastic material, the apparent viscosity resistance of the polyamide resin is increased, and the elastic material can be caused to function integrally with the polyamide resin without separating in the resin composition. Therefore, the second aspect of the present invention can provide a gear for an electric power steering device having rigidity and toughness superior to those of conventional gears.

Further, as described above, decomposition due to adsorbed moisture easily lowers the molecular weight of the polyamide resin, and strict control of the moisture content in the resin composition is required. With the second aspect of the present invention, there is an advantage in that strict control of humidity is not required because the apparent viscous resistance can be maintained due to the interaction of the polyamide resin, the polycarbodiimide compound, and the elastic material.

The thermoplastic phenoxy resin is used to inhibit water absorption of the polyamide resin. For this reason, when the resin composition for forming the resin for an electric power steering device according to the second aspect of the invention contains the thermoplastic phenoxy resin, dimensional change of the gear of the electric power steering device caused by water absorption of the polyamide resin is reduced, and reduction of backlash with other gears and seizure deterioration caused thereby, for example, caused by water absorption and swelling, can be suppressed. Further, the thermoplastic phenoxy resin reduces the toughness of the polyamide resin, but since this reduction can be compensated by the above-described interaction between the components, the gear of the electric power steering device can maintain high rigidity and good toughness even if the thermoplastic phenoxy resin is contained.

When the reactive functional group of the elastic material is at least one of a carboxyl group and a carboxylate group, it effectively interacts with a carbodiimide group present in the polycarbodiimide compound, thereby further improving the rigidity and toughness of the gear of the electric power steering apparatus. Further, when the elastic material is an ethylene-propylene-diene copolymer (EPDM) modified with a reactive functional group, since the EPDM has good miscibility with the polyamide resin and the polycarbodiimide compound, the rigidity and toughness of the gear of the electric power steering apparatus can be further improved. Further, since EPDM has good resistance to lubricating oil and the like, durability of the gears of the electric power steering apparatus can be improved.

In the gear of the electric power steering apparatus of the present invention, at least the gear teeth are formed of a non-reinforced resin composition comprising:

(a) a polyamide resin having a carboxyl group at least at a terminal;

(b)0.5 to 5% by weight of a polycarbodiimide compound, and

(c)3 to 15 wt.% of an elastomeric material modified with a reactive functional group.

The reason why such a resin composition is limited to the non-reinforced type without containing the reinforcing fiber is described above. A specific example of the gear of the electric power steering device has a composite structure of a metal core on the steering shaft and a ring gear body formed of a resin composition, the ring gear body having gear teeth on an outer periphery thereof and surrounding the outer periphery of the core.

Such a gear of an electric power steering device of composite structure may be manufactured, for example, by an insert molding method in which a core is retained in a retainer (retainer) of a metal mold having a retainer for retaining the core and a mold cavity corresponding to the shape of a gear body, the metal mold being connected to an injection molding device, and in this state, a resin composition melted in the injection molding device is injected into the mold cavity and then cooled to form a resin body integrated with the core. Since the gear teeth of the gear body are required to have high dimensional accuracy, it is preferable to form the shape regulating surface, for example, as a cylinder in which the gear teeth can be formed by post-processing, instead of the shape corresponding to the shape of the gear teeth in the cavity used in the above-described method, and to form the gear teeth corresponding to the shape regulating surface on the outer periphery of the insert-molded gear body by post-processing, for example, cutting.

(Polyamide resin)

Any of a large number of conventionally known polyamide resins may be used as the polyamide resin (a), but in view of forming a gear for an electric power steering device having excellent rigidity and toughness, polyamide 6, polyamide 66, polyamide 12, polyamide 612, polyamide 6/66 copolymer, or a mixture thereof may be considered. Among them, polyamide 66 is preferable. The polyamide resin must have a carboxyl group at least at the terminal to be able to interact with the carbodiimide group of the polycarbodiimide compound. In the polyamide resin from which the carboxyl group is removed by the terminal treatment, the intended effect cannot be obtained.

(polycarbodiimide Compound)

Various polycarbodiimide compounds having a repeating unit represented by the following formula (1) can be used as the polycarbodiimide compound (b).

-N＝C＝N-R1- (1)

Examples of R1 in formula (1) include various divalent organic groups, but the following groups are particularly preferred. The polycarbodiimide compound may be a homopolymer composed of a single repeating unit containing one of the following groups, or may be a copolymer composed of a repeating unit having two or more different groups.

Divalent groups derived from diisopropylbenzene, for example, 2, 5-diisopropyl-1, 3-phenylene, 2, 6-diisopropyl-1, 3-phenylene, 4, 6-diisopropyl-1, 3-phenylene, 2, 5-diisopropyl-1, 4-phenylene and 2, 6-diisopropyl-1, 4-phenylene. Divalent groups derived from diethyltoluene, such as 3, 5-diethyl-2, 4-tolylene group, 3, 4-diethyl-2, 5-tolylene group, 3, 4-diethyl-2, 6-tolylene group, 3, 5-diethyl-2, 6-tolylene group, 2, 4-diethyl-3, 5-tolylene group, 2, 6-diethyl-3, 5-tolylene group, 2, 4-diethyl-3, 6-tolylene group, 2, 5-diethyl-3, 6-tolylene group and 2, 3-diethyl-4, 6-tolylene group.

Divalent radicals derived from naphthalene, for example 1, 3-naphthylene, 1, 4-naphthylene, 1, 5-naphthylene and 1, 6-naphthylene. Divalent groups derived from cyclohexane, such as 1, 3-cyclohexylene and 1, 4-cyclohexylene. Divalent radicals derived from methylcyclohexane, for example 2-methyl-1, 3-cyclohexylene, 4-methyl-1, 3-cyclohexylene, 5-methyl-1, 3-cyclohexylene and 2-methyl-1, 4-cyclohexylene. Divalent groups derived from triisopropylbenzene, such as 2, 4, 6-triisopropyl-1, 3-phenylene, 2, 3, 5-triisopropyl-1, 4-phenylene and 2, 3, 4-triisopropyl-1, 5-phenylene. Alkylene having 1 to 6 carbon atoms, for example 1, 6-hexylene.

A divalent group represented by the formula (2)

Wherein R is²、R³、R⁴And R⁵Which may be the same or different, represent alkyl groups having 1 to 3 carbon atoms, such as 4, 4 ' -methylene-bis (2, 6-diethylphenyl), 4 ' -methylene-bis (2-ethyl-6-methylphenyl) and 4, 4 ' -methylene-bis (2, 6-diisopropylphenyl).

A divalent group represented by the formula (3)

Wherein R is⁶、R⁷、R⁸And R⁹Which may be the same or different, represent a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, such as 4, 4 '-methylene-bis (cyclohexyl) and 4, 4' -methylene-bis (2-ethyl-6-methylcyclohexyl).

The polycarbodiimide compound (b) may be synthesized, for example, by conducting a carbodiimidization reaction of one, two or more organic diisocyanates containing two groups of the above-mentioned examples in the presence of a catalyst that enhances the carbodiimidization reaction of the isocyanate groups. Examples of suitable catalysts include phospholene oxides and metal catalysts. Examples of the phospholene oxide include 1-phenyl-2-phospholene-1-oxide, 3-methyl-1-phenyl-2-phospholene-1-oxide, 1-ethyl-2-phospholene-1-oxide, 3-methyl-2-phospholene-1-oxide, 1-phenyl-3-phospholene-1-oxide, 3-methyl-1-phenyl-3-phospholene-1-oxide, 1-ethyl-3-phospholene-1-oxide and 3-methyl-3-phospholene-1-oxide.

Examples of the metal catalyst include metal carbonyl complexes such as iron pentacarbonyl, iron nonacarbonyl, nickel tetracarbonyl, tungsten hexacarbonyl, and chromium hexacarbonyl, acetylacetone complexes of metals such as beryllium, aluminum, zirconium, chromium, and iron, phosphoric acid esters such as trimethyl phosphate, triethyl phosphate, triisopropyl phosphate, tri-t-butyl phosphate, and triphenyl phosphate, and tetrabutyl titanate. It is preferable to control the degree of polymerization of the polycarbodiimide compound having a repeating unit represented by the formula (1) in the course of synthesis, in which an organic diisocyanate is subjected to carbodiimidization in the presence of the aforementioned catalyst by incorporating an alcohol, an amine, or a monoisocyanate, thereby blocking the isocyanate at an appropriate time point from the start to the end of the reaction.

The degree of polymerization of the polycarbodiimide compound (b), i.e., the number of repeating units represented by the formula (1), is preferably 3 to 10. Further, the molecular weight of the polycarbodiimide compound represented by the number average molecular weight Mn calculated for polystyrene and obtained by gel permeation chromatography is preferably 500-3000. When the degree of polymerization or the molecular weight is less than this range, even if the interaction of the polyamide resin (a) and the elastic material (c) is initiated, a sufficient effect of increasing the apparent viscosity resistance of the polyamide resin may not be obtained. In contrast, if the degree of polymerization or the molecular weight is higher than this range, the molecules of the polycarbodiimide compound become too large and the interaction of the polyamide resin (a) with the elastic material (c) becomes difficult. As a result, in either case, a sufficient effect of improving the toughness of the gear of the electric power steering device without reducing the elasticity thereof may not be obtained.

The content ratio of the polycarbodiimide compound (b) in the entire resin composition is limited to 0.5 to 5% by weight. A problem encountered when the content ratio of the polycarbodiimide compound (b) is less than 0.5% by weight is that the effect of improving the toughness of the gear of the electric power steering device without reducing the elasticity thereof by the interaction of the polycarbodiimide compound with the polyamide resin (a) and the elastic material (c) cannot be obtained. On the other hand, if the content ratio exceeds 5% by weight as described above, the polycarbodiimide compound is excessively crosslinked and the effect of improving the apparent viscosity resistance of the polyamide resin cannot be obtained. In addition, a lump of the crosslinked polycarbodiimide compound occurs in the resin composition, and the continuity of the resin composition is reduced, thereby greatly reducing the toughness.

Still another problem is that, since the ratio of the crosslinked polycarbodiimide compound is increased and the interaction of the polycarbodiimide compound with the polyamide resin or the elastic material becomes too severe, the melt viscosity becomes too high in the melting process in the process of manufacturing a molding material, such as pellets, from the resin composition or in the process of injection molding using the resultant molding material. In particular, the gears of the electric power steering apparatus cannot be molded efficiently and without molding defects in the injection molding process.

Further, from the viewpoint of further enhancing the effect of improving the toughness of the gear of the electric power steering device without lowering the elasticity thereof, which is produced by the polycarbodiimide compound (b), and lowering the melt viscosity of the resin composition to enable efficient molding of the gear of the electric power steering device, the content ratio of the polycarbodiimide compound (b) in the entire resin composition is preferably 0.5 to 3.0% by weight, more particularly 1.5 to 2.5% by weight within the above range.

(elastic Material)

Examples of the elastic material (c) include compounds comprising various elastic materials such as rubbers, soft resins and thermoplastic elastomers as a basic skeleton and having a main chain or a side chain of the basic skeleton modified with various reactive functional groups that can interact with a terminal carboxyl group of the polyamide resin (a) or a carbodiimide group contained in the polycarbodiimide compound (b). Further, at least one type of group selected from a carboxyl group and a carboxylate group (a metal salt of a carboxyl group or an ester of a carboxyl group and an organic group) which can interact with a carbodiimide group contained in the polycarbodiimide compound (b) is preferable as the reactive functional group. When the reactive functional groups are at least one type of group, they can effectively interact with the carbodiimide groups contained in the polycarbodiimide compound, and can further improve the rigidity and toughness of the gear of the electric power steering apparatus.

An olefin copolymer which is a copolymer of at least two olefins (ethylene, propylene, etc.) having 2 to 8 carbon atoms and has excellent resistance to lubricating oil and miscibility with the polyamide resin (a) or the polycarbodiimide compound (b) is preferable as the elastic material forming the basic skeleton. Further, the olefin copolymer may also contain a non-conjugated diene. Among them, a copolymer of ethylene, an α -olefin having 3 to 6 carbon atoms, and a diene, particularly an ethylene-propylene-diene copolymer is preferable. Compounds whose basic skeleton is an ethylene-propylene-diene copolymer and whose basic skeleton is modified with carboxyl groups by grafting maleic anhydride are examples of preferred elastomeric materials which meet the above requirements.

The content ratio of the elastic material (c) in the entire resin composition is limited to 3 to 15% by weight. When the content ratio of the elastic material (c) is less than 3% by weight, the effect of improving the toughness of the gear of the electric power steering device without reducing the elasticity thereof by the interaction of the elastic material with the polyamide resin (a) and the polycarbodiimide compound (c) cannot be obtained. Further, if the content ratio exceeds 15% by weight, the melt viscosity becomes too high in a melting method in the process of producing a molding material (e.g., pellets) from the resin composition or in the process of injection molding using the produced molding material. As a result, the gears of the electric power steering apparatus cannot be molded efficiently and particularly without molding defects during injection molding.

Further, from the viewpoint of further enhancing the effect of improving the toughness of the gear of the electric power steering device without lowering the elasticity thereof, which is produced by the elastic compound (c), and lowering the melt viscosity of the resin composition to enable efficient molding of the gear of the electric power steering device, the content ratio of the elastic compound (c) in the entire resin composition is preferably 5 to 12% by weight, more particularly 8 to 10% by weight within the above range.

(thermoplastic phenoxy resin)

The resin composition may also contain a thermoplastic phenoxy resin (d). The thermoplastic phenoxy resin is a compound represented by the formula (4).

Wherein R is¹⁰、R¹¹And R¹²Which may be the same or different, represents a hydrogen atom, an alkyl group, a cyclic acyl group, or an aryl group. This resin is also known as novolac resin. If the thermoplastic phenoxy resin (d) is blended, the water absorption of the polyamide resin is suppressed, the dimensional change of the gear of the electric power steering device caused by the water absorption of the polyamide resin is reduced, and for example, the dimensional change caused by the water absorption of the polyamide resin can be suppressedWater absorption and swelling cause a reduction in backlash with other gears and a resultant occlusion barrier.

Further, the thermoplastic phenoxy resin (d) reduces the toughness of the polyamide resin, but since this reduction can be compensated by the interaction of the above-described components (a) to (c), the gear of the electric power steering device can maintain high rigidity and good toughness even if it contains the thermoplastic phenoxy resin. The molecular weight of the thermoplastic phenoxy resin (d) is preferably 500-1500 because within this range, the resin composition can be provided with an effect of suppressing water absorption of the polyamide resin, the melt viscosity of the resin composition can be reduced, thereby enabling efficient molding of gears of electric power steering devices, and the effect of improving the toughness of gears of electric power steering devices without reducing the elasticity thereof, which is brought about by the interaction of the components (a) to (c), can be exhibited without being suppressed. The value of n in (4) is preferably adjusted so that the molecular weight falls within this range.

Further, the content ratio of the thermoplastic phenoxy resin (d) in the entire resin composition is preferably 1 to 10% by weight. If the content ratio of the thermoplastic phenoxy resin (d) is less than 1% by weight, a sufficient effect of suppressing water absorption of the polyamide resin may not be obtained. Further, if the content ratio exceeds 10% by weight, the effect of reducing the toughness of the polyamide resin exhibited by the thermoplastic phenoxy resin is enhanced. As a result, such an effect cannot be sufficiently compensated by the interaction of the components (a) to (c), and the toughness of the gear of the electric power steering device may be reduced. In order to further enhance the effect of suppressing water absorption of the polyamide resin by the thermoplastic phenoxy resin (d) and to suppress reduction in toughness of the gear of the electric power steering device, the content ratio of the thermoplastic phenoxy resin (d) in the entire resin composition is 1.5 to 8% by weight, preferably 2 to 6% by weight within the above range.

In addition to the above components, the resin composition may further include a heat stabilizer for improving heat resistance, High Density Polyethylene (HDPE) powder, Ultra High Molecular Weight Polyethylene (UHMWPE) powder, molybdenum disulfide, or a fluorine-containing lubricant as a solid lubricant for improving abrasion resistance. The incorporation of these components in the resin composition can further improve the durability of the gears of the electric power steering apparatus.

However, some of the additives may preferentially react with the polycarbodiimide compound, thereby hindering the interaction between the polycarbodiimide compound and the polyamide resin or the elastic material. Since the intended effect of improving toughness cannot be obtained in these cases, it is important to selectively use such additives that do not interact with the polycarbodiimide compound.

(reduction gear device and electric power steering device)

Fig. 1 is a schematic sectional view showing an example of an electric power steering apparatus having a reduction gear mechanism including a gear for an electric power steering apparatus according to the present invention. FIG. 2 is a sectional view taken along line II-II in FIG. 1;

referring to fig. 1, in the electric power steering apparatus, a first steering shaft 2 (serving as an input shaft on which a steering wheel 1 is mounted) and a second steering shaft 3 (serving as a transmission shaft operatively connected to a steering mechanism (not shown in the drawings), such as a rack and pinion mechanism, are coaxially connected via a torsion bar 4.

The housing 5 supporting the first and second steering shafts 2, 3 is made of, for example, an aluminum alloy and is mounted on a vehicle body (not shown). The housing 5 contains a sensor housing 6 and a gear box (gear housing)7 connected together. More specifically, the gear case 7 has a cylindrical shape, and an annular edge portion 7a at an upper end thereof is connected to an annular step section 6a on an outer periphery of a lower end of the sensor housing 6. The gear case 7 houses a turbine mechanism 8 serving as a speed reduction mechanism, and the sensor housing 6 houses a torque sensor 9, a control board 10, and the like.

The worm gear device 8 includes a worm wheel (large gear) 12 that is integrally rotatable with the intermediate portion of the second steering shaft 3 in the axial direction thereof in an axially controlled motion and serves as a gear for the electric power steering device according to the present invention, and a worm shaft 11 (small gear, see fig. 2) that meshes with the worm wheel 12 and is connected to a rotating shaft 32 of the electric motor M via a spline coupling 33.

The worm wheel 12 includes an annular core 12a attached to the second steering shaft 3 so as to rotate integrally therewith, and a gear body 12b made of a resin composition surrounding the outer periphery of the core 12a and having gear teeth on its own outer periphery. The worm wheel 12 is formed by connecting and integrating the core 12a and the gear body 12b as described above, for example, by insert molding.

The second steering shaft 3 is rotatably supported by first and second rolling bearings 13, 14 disposed so as to sandwich the worm wheel 12 from above and below in the axial direction.

The outer ring 15 of the first rolling bearing 13 is fitted into and held in a bearing receiving hole 16 in the tubular projection 6b at the bottom end of the sensor housing 6. The upper end surface of the outer ring 15 abuts against the annular stepped portion 17 and is prevented from moving axially upward relative to the sensor housing 6.

On the other hand, the inner ring 18 of the first rolling bearing 13 is connected to the second steering shaft 3 by external pressing. The lower end surface of the inner ring 18 abuts against the upper end surface of the core 12a of the worm wheel 12.

The outer ring 19 of the second rolling bearing 14 is fitted into and held in the bearing receiving hole 20 of the gear case 7. The lower end surface of the outer ring 19 abuts against the annular stepped portion 21 and is prevented from moving axially downward relative to the gear case 7.

On the other hand, the inner ring 22 of the second rolling bearing 14 is mounted on the second steering shaft 3 to rotate them integrally and prevent them from moving relatively in the axial direction. Further, the inner ring 22 is sandwiched between a stepped portion 23 of the second steering shaft 3 and a nut 24 fastened on a threaded portion of the second steering shaft 3.

The torsion bar 4 passes through the first and second steering shafts 2, 3. The upper end 4a of the torsion bar 4 is integrally rotatably connected to the first steering shaft 2 with a link pin 25, and the lower end 4b is integrally rotatably connected to the second steering shaft 3 with a link pin 26. The lower end of the second steering shaft 3 is connected to a steering device, such as a rack and pinion device, via an intermediate shaft (not shown) as described above.

The link pin 25 is connected to a third steering shaft 27, which is disposed coaxially with the first steering shaft 2 so as to be integrally rotatable with the first steering shaft 2. The third steering shaft 27 passes through the inside of a tube 28 constituting a steering column.

The upper portion of the first steering shaft 2 is rotatably supported on the sensor housing 6 via a third rolling bearing 29 (e.g., a needle bearing). The small diameter portion 30 of the lower portion of the first steering shaft 2 and the hole 31 of the upper portion of the second steering shaft 3 are connected by providing a predetermined backlash in the rotational direction, thereby restricting the relative rotation of the first and second steering shafts 2, 3 within a specified range.

Further, referring to fig. 2, the worm shaft 11 is rotatably supported with fourth and fifth rolling bearings 34, 35 held inside the gear case 7.

The inner rings 36, 37 of the fourth and fifth rolling bearings 34, 35 are connected to a necking portion corresponding to the worm shaft 11. Furthermore, the outer rings 38, 39 are held in bearing receiving holes 40, 41, respectively, of the gearbox 7.

The gear box 7 comprises a portion 7b facing radially towards a part of the circumferential surface of the worm 11.

Further, the outer ring 38 of the fourth rolling bearing 34 supporting the one end portion 11a of the worm shaft 11 is aligned by abutting against the stepped portion 42 of the gear case 7. On the other hand, the movement of the inner ring 36 toward the other end portion 11b is restricted due to its close proximity to the aligned stepped portion 43 of the worm shaft 11.

The movement of the inner ring 37 of the fifth rolling bearing 35 supporting the area near the other end portion 11b (the end on the connection side) of the worm shaft 11 toward the one end portion 11a is restricted due to its close proximity to the aligned stepped portion 44 of the worm shaft 11. Further, the outer ring 39 is pushed toward the fourth rolling bearing 34 by the screw member 45 to preliminarily adjust the pressure. When the screw element 45 is screwed into the threaded hole 46 formed in the gear case 7, it exerts a preliminary pressure on the pair of rolling bearings 34, 35 while being axially aligned with the worm shaft 11. Reference numeral 47 denotes a lock nut which engages with the screw member 45 to terminate and fix the screw member 45, thereby preliminarily adjusting the pressing force.

Inside the gear case 7, a region including at least the meshing portion a of the worm shaft 11 and the worm wheel 12 is filled with a lubricant composition. Thus, the lubricant composition can be filled only in the biting portion a, or the entire outer periphery of the biting portion a and the worm shaft 11, or the entire interior of the gear case 7.

The present invention is not limited to the above-described embodiments. For example, the gear for the electric power steering apparatus according to the present invention is not limited to the worm wheel 12 constituting the worm gear unit 8 in combination with the worm shaft 11, and the configuration of the present invention can also be used in various gears constituting the reduction gear unit, such as a flat gear, a bevel gear, a hypoid gear, a helical gear, and a rack gear, particularly, a large gear. Further, various modifications may be made within the scope of the features recited in the claims of the present invention.

Examples

Preparation of examples 1 to 12 and comparative examples 1 to 6

The components shown in tables 1 to 3 were melt-mixed in a double shaft kneader, extruded, coagulated and cut into pellets. The amounts of the ingredients are given in weight percent based on the total weight of the composition.

Melt viscosity measurement

Using the resulting pellet samples, melt viscosity was measured using a Kayness Capillary Reometer (Yasuda Seisakusho, LCR series). The melting temperature at 280 ℃ and 1,000sec were measured^-1At a shear rate of (a) on pellets having a moisture content of 0.13 ± 0.01%. All experiments were repeated 5 times and the results are presented as mean values.

Preparation of the samples

From the pellets obtained above, using ordinary molding conditions for non-reinforced nylon resin, 4.0 mm high × 175 mm long × 20 mm wide ISO specimens were formed.

Measurement of physical Properties

The physical properties were measured using the above samples. Tensile elongation was measured according to ISO 527-1/-2. The modulus of elasticity was measured according to ISO 178. Charpy impact strength was measured according to ISO 179/1 eA. The initial impact energy was set at 2J, but for the samples that did not fail, the impact was repeated with the impact energy further increased to 15J. "undamaged" in table 2 means that the sample did not fail at an impact energy of 2J or 15J.

Dimensional stability

The compositions of examples 7, 11 and 12 were molded into 60X 2 mm plaques. The length of the plate in the flow direction was measured and the plate was conditioned at 50 ℃ and 95% relative humidity for 500 hours. The length of the plate in the direction of flow is then measured and the percent change calculated. It was 2.32% for example 7; 1.55% for example 11; and 1.27% for example 12.

The following materials were used as ingredients in the compositions of examples and comparative examples.

Polyamide resin A(polyamide 6, 6): zytel ® 101, available from DuPont.

Polyamide resin B(solid-phase polymerized polyamide 6, 6): zytel ® E50, available from DuPont.

Impact modifier: EPDM rubber grafted with maleic anhydride.

Polycarbodiimides: stabaxol P, an aromatic polycarbodiimide available from Bayer.

Phenol-aldehyde resins: sumeriteresin ® PR-NMD-202, softening point 118 ℃ and available from Sumitomo Bakelite co.ltd.

Heat stabilizer: copper heat stabilizer.

TABLE 1

	Comparative example 1	Comparative example 2	Comparative example 3	Comparative example 4
					Polyamide A	99.75	--	98.8	98.25
Polyamide B	--	99.75	--	--
					Heat stabilizer	0.25	0.25	0.25	0.25
Polycarbodiimides	0	0	0.95	1.50
					Impact modifier	0	0	0	0
Melt viscosity (Pa-s)	137.8	375.7	274.3	298.5
					Tensile elongation (%)	35.6	71.8	67.8	64.6
Modulus of elasticity (MPa)	2681	2800	2686	2688
					Charpy impact strength (KJ/m)2)	4.7	7.5	5.9	6.9
Plasticity	Without problems	Without problems	Without problems	Without problems

TABLE 2

	Comparative example 5	Example 1	Example 2	Example 3	Example 4
						Polyamide A	94.46	94.26	93.46	92.96	92.46
Heat stabilizer	0.54	0.54	0.54	0.54	0.54
						Polycarbodiimides	0	0.20	1.00	1.50	2.00
Impact modifier	5.0	5.0	5.0	5.0	5.0
						Melt viscosity (Pa-s)	167.4	193.1	318.2	363	390.7
Tensile elongation (%)	23.6	27.7	39.1	56.0	55.2
						Modulus of elasticity (MPa)	2703	2625	2535	2538	2444
Charpy impact strength (KJ/m)2)	9.3	11.7	13.4	15.9	18.4
						Plasticity	Without problems	Without problems	Without problems	Without problems	Without problems

TABLE 3

	Comparative example 6	Example 5	Example 6	Example 7	Example 8	Example 9	Example 10
								Polyamide A	89.46	89.26	88.46	87.96	87.46	84.46	77.96
Heat stabilizer	0.54	0.54	0.54	0.54	0.54	0.54	0.54
								Polycarbodiimides	0	0.20	1.00	1.50	2.00	5.00	1.50
Impact modifier	10.0	10.0	10.0	10.0	10.0	10.0	20.0
								Melt viscosity (Pa-s)	258.6	273.9	413.7	487.5	Not melted and not measured	Not melted and not measured	Not melted and not measured
Tensile elongation (%)	32.9	41.1	42.9	84.8	n.m.	n.m.	n.m.
								Modulus of elasticity (MPa)	2260	2242	2186	2133	n.m.	n.m.	n.m.
Charpy impact strength (KJ/m)2)	20.8	22.6	31.7	Is not destroyed	n.m.	n.m.	n.m.
								Plasticity	Without problems	Without problems	Without problems	Without problems	Difficult to mold due to high viscosity	Difficult to mold due to high viscosity	Difficult to mold due to high viscosity

n.m. ═ cannot be measured

TABLE 4

	Example 11	Example 12
			Polyamide A	83.96	79.96
Heat stabilizer	0.54	0.54
			Polycarbodiimides	1.5	1.5
Impact modifier	10	10
			Phenolic resin	4	8
Melt viscosity (Pa-s)	404	329
			Tensile elongation (%)	54.5	33.6
Modulus of elasticity (MPa)	2209	2311
			Charpy impact strength (KJ/m)2)	30.6	20.0
Plasticity	Without problems	Without problems

It is clear from examples 1-10 and comparative examples 1-6 that the polyamide compositions containing impact modifier and polycarbodiimide have improved impact strength while maintaining high stiffness compared to polyamide compositions without polycarbodiimide and impact modifier.

Examples 11 and 12 show that the presence of a phenol-aldehyde resin in a polyamide composition containing an impact modifier and a polycarbodiimide results in a composition having good stiffness and impact strength, which also has good dimensional stability when exposed to moisture.

It is therefore apparent that there has been provided in accordance with the present invention a polyamide resin composition and an article that fully satisfy the objects and advantages set forth above. While the present invention has been described in conjunction with specific embodiments thereof, many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and broad scope of the appended claims.

Examples 13 to 17, comparative examples 7 and 8

Non-reinforced pellets for injection molding were produced by preliminarily mixing components (a) to (d) in the ratios shown in tables 5 and 6, kneaded in a molten state using a twin-screw (40 mm diameter) kneading extruder, extrusion-molded into strands and pelletized. The resin temperature during kneading is 280 to 320 ℃.

(a) Polyamide resin: polyamide 66(Zytel ® 101, manufactured by Du Pont Corp., RV 49.5)

(b) Aromatic polycarbodiimide compound: stabaxol ® P (softening point 70 ℃, manufactured by RheinChemie Co., Ltd.)

(c) Elastic material: ethylene-propylene-diene copolymers modified at the carboxyl groups by grafting maleic anhydride (Du Pont Corp.)

(d) Thermoplastic phenoxy resin: sumilete Resin ® PR-NMD-202 (softening point 118 ℃ C.) was manufactured by Sumitomo Bakelite Co., Ltd.

Comparative examples 9 to 13

Non-reinforced pellets were produced in the same manner as in examples 13 to 17 and comparative examples 7 and 8 except that the following polyamide resins were independently used.

(i) Standard viscosity 66 nylon for injection Molding (Zytel ® 103HS, Du Pont Corp. manufacture)

(ii) High viscosity 66 nylon for extrusion molding (Zytel ® 42A, manufactured by Du Pont Corp.)

(iii) Ultra high impact 66 nylon for extrusion molding (Zytel ® ST801, manufactured by Du PontCorp.)

(iv) Low viscosity (high cycle) 6 Nylon for injection molding (UBE Nylon 1013B, manufactured by Ube Industries Ltd.)

(v) Impact resistant 6 Nylon for injection molding (UBE Nylon 1018I, manufactured by Ube industries Ltd.)

[ production of sample ]

Under ordinary molding conditions for non-reinforced nylon resin, a specimen having a thickness of 4.0 mm, a length of 175 mm and a width of 20 mm was injection-molded by using the pellets prepared in examples and comparative examples. The properties of the test specimens were evaluated by conducting the following tests.

[ measurement of tensile fracture Strain ]

The tensile strain at break (%) of the test piece was measured according to the test method specified in the following ISO standard.

ISO 527-1: 1993 "Plastics-Determination of latent properties-Part 1: general Principles (plastics-tensile Properties determination-part 1: General Principles) ".

ISO 527-2: 1993 "Plastics-Determination of latent properties-Part 2: test conditions for building and extrusion plastics (plastics-tensile Properties-part 2: Test conditions for molded and extruded plastics) ".

[ measurement of flexural modulus of elasticity ]

The flexural modulus of elasticity (GPa) of the test specimen was measured according to the test method specified in the following ISO standard.

ISO 178: 2001 "Plastics-Determination of flexible properties".

[ measurement of notched Charpy impact Strength ]

The charpy impact strength (kJ/m2) against flat (flatwise) impact of the notched specimens obtained by cutting a single notch in each specimen was measured in accordance with the specimen method specified in the following ISO standard.

ISO 179-1: 2000 "Plastics-Determination of Charpy impact properties-Part 1: non-impacted test (plastic-determination of charpy impact-part 1: Non-instrumented impact test) ".

[ measurement of melt viscosity ]

The melt viscosity of the sample was measured using a capillary rheometer at a measurement temperature of 280 ℃ and a shear rate of 1000 s-1. (Kayeness Capillary Rheometer, LSR series, manufactured by Yasuda Ltd.)

The water absorption ratio of all samples used for the measurement was adjusted to 0.13% +/-0.001%. Measurements were performed 5 times and the average was obtained. The results are shown in tables 5 to 7 and fig. 3.

TABLE 5

		Comparative example 7	Example 13	Example 14	Example 15
						Parts by weight	(a) Polyamide resin	94.8	93.5	93	88.5
(b) Polycarbodiimide compound	0.2	1.5	2	1.5
					(c) Elastic material		5	5	5	10
(d) Thermoplastic phenoxy resin	-	-	-	-
					Melt viscosity (Pa-s)		193	363	391	488
Tensile rupture strain (%)		28	56	55							64
					Flexural modulus of elasticity (Gpa)		2.7	2.7	2.4	2.1
Notched Charpy impact strength (KJ/m)2)		12	1 6	1 9							72

TABLE 6

		Comparative example 8	Example 16	Example 17
					Parts by weight	(a) Polyamide resin	78.5	84.5	80.5
(b) Polycarbodiimide compound	1.5	1.5	1.5
				(c) Elastic material		20	10	10
(d) Thermoplastic phenoxy resin	-	4	8
				Melt viscosity (Pa-s)		570	404	329
Tensile rupture strain (%)		Can not measure	55						34
				Flexural modulus of elasticity (Gpa)		Can not measure	2.2	2.3
Notched Charpy impact strength (KJ/m)2)		Can not measure	31						20

TABLE 7

		Comparative example 9	Comparative example 10	Comparative example 11	Comparative example 12	Comparative example 13
							Parts by weight	Polyamide resin (i)	100	-	-	-	-
Polyamide resin (ii)	-	100	-	-	-
						Polyamide resin (iii)		-	-	100	-	-
Polyamide resin (iv)	-	-	-	100	-
						Polyamide resin (v)		-	-	-	-	100
Melt viscosity (Pa-s)		140	-	-	-								-
						Tensile rupture strain (%)		83	86	50	80	45
Flexural modulus of elasticity (Gpa)		2.8	2.8	1.7	3								1.6
						Notched Charpy impact strength (KJ/m)2)		5	6	80	2.7	150

The data shown in the tables and figures indicate that the samples using conventional polyamide resins and having a high elastic flexural modulus and excellent stiffness (comparative examples 9, 10, 12) have low impact strength and low toughness. In contrast, the test pieces having high impact strength and excellent toughness (comparative examples 11 and 13) had a small elastic flexural modulus and low rigidity. Further, the sample of comparative example 13, which was made from the resin composition comprising components (a) to (c) but containing less than 0.5% by weight of the polycarbodiimide compound, had a high elastic flexural modulus and excellent rigidity, but had low impact strength and low toughness. With the composition of comparative example 14 made of the resin composition containing components (a) to (c) but containing more than 15% by weight of the elastic material, the melt viscosity was too high and the sample was difficult to mold by injection molding. As a result, the measurement of tensile rupture strain, flexural modulus of elasticity and notched Charpy impact strength was abandoned.

In contrast, all of the samples formed using the resin compositions of the examples had excellent toughness as well as excellent stiffness, that is, they combined high stiffness with high toughness. Further, the comparison of examples confirms that the higher the content of the elastic material (c), the higher the toughness may be, and when the thermoplastic phenoxy resin (d) is incorporated, although the rigidity is lowered, such a lowering of rigidity can be compensated by the functions of the components (a) to (c) and the rigidity and toughness can be maintained at a level suitable for practical use.

[ measurement of Water absorption ]

The test pieces formed using comparative example 9 and examples 16 and 17 were dried by being left under constant temperature and humidity conditions of 23 ℃ and 50% RH for 48 hours, and then the weight thereof was measured. The samples were then allowed to stand under high-temperature high-humidity conditions of 50 ℃ and 95% relative humidity to make them absorb water, and the relationship between the residence time and the water absorption rate was found by the weight increase rate as compared with that in the dry state. The results are shown in fig. 4. This figure shows that the incorporation of the thermoplastic phenoxy resin can suppress water absorption.

[ study of durability ]

Using the resin compositions of comparative example 9 and examples 13, 15 and 16, the worm wheel 12 integrating the core 12a and the gear body 12b as shown in fig. 1 and 2 was manufactured by insert molding. Each worm wheel 12 is assembled with a steel worm shaft 11 in the electric power steering apparatus shown in fig. 1 and 2. The number of cycles until the gear body 12b broke during forward and reverse rotation was measured under a load equal to the static steering mode of the front wheels of the automobile, and the ratio of the obtained number of cycles was taken as the fatigue ratio with the result of comparative example 7 as 1. The results are shown in fig. 5. The figure shows that the durability is improved compared to comparative example 7 for all examples. In particular, in example 15, the measurement was performed up to the cycle number 5 times as long as the durability of comparative example 7, but the gear body 12b was not broken and high durability could be confirmed.

Claims (22)

1. A polyamide composition comprising:

(a) about 77 to about 96.9 weight percent of at least one polyamide;

(b) about 3 to about 20 weight percent of at least one impact modifier; and

(c) about 0.1 to about 5 weight percent of at least one polycarbodiimide;

wherein the weight percentages are based on the total weight of the composition.

2. The composition of claim 1, wherein the at least one polyamide comprises from about 83 to about 94.5 weight percent of the composition.

3. The composition of claim 1 or 2, wherein the at least one polyamide is selected from the group consisting of homopolymers, copolymers, terpolymers, and higher order polymers.

4. The composition of claim 3, wherein the at least one polyamide is a condensation product of a dicarboxylic acid or derivative thereof and a diamine; an aminocarboxylic acid; and/or ring-opening polymerization products of lactams, or mixtures thereof.

5. The composition of claim 3, wherein the at least one polyamide is selected from the group consisting of: polyamide 66/6 copolymer; polyamide 66/68 copolymer; polyamide 66/610 copolymer; polyamide 66/612 copolymer; polyamide 66/10 copolymer; polyamide 66/12 copolymer; polyamide 6/68 copolymer; polyamide 6/610 copolymer; polyamide 6/612 copolymer; polyamide 6/10 copolymer; polyamide 6/12 copolymer; polyamide 6/66/610 terpolymer; polyamide 6/66/69 terpolymer; polyamide 6/66/11 terpolymer; polyamide 6/66/12 terpolymer; polyamide 6/610/11 terpolymer; polyamide 6/610/12 terpolymer; and polyamide 6/66/PACM (bis-p- { aminocyclohexyl } methane) terpolymer.

6. The composition of claim 3, wherein the at least one polyamide is selected from the group consisting of: polyamide 6; polyamide 66; polyamide 46; polyamide 69; a polyamide 610; polyamide 612; polyamide 1010; polyamide 11; polyamide 12; a semi-aromatic polyamide; polyamides of hexamethylene terephthalamide and 2-methylpentamethylene terephthalamide; polyamides of hexamethylene isophthalamide and hexamethylene adipamide; polyamides of hexamethylene terephthalamide, hexamethylene isophthalamide and hexamethylene adipamide; and copolymers and mixtures thereof.

7. The composition of claim 6 wherein the semi-aromatic polyamide is selected from the group consisting of: poly (m-xylylene adipamide); poly (dodecamethylene terephthalamide); poly (decamethylene terephthalamide); poly (nonamethylene terephthalamide); polyamides of hexamethylene terephthalamide and hexamethylene adipamide.

8. The composition of claim 3 wherein the polyamide is polyamide 66.

9. The composition of claim 1, wherein the at least one impact modifier is about 5 to about 14 weight percent of the composition.

10. The composition of claim 1 or 9, wherein the at least one impact modifier is a carboxy-substituted polyolefin.

11. The composition of claim 1 or 9, wherein the at least one impact modifier comprises an ethylene-propylene-diene polymer or an ethylene-propylene copolymer grafted with maleic anhydride.

12. The composition of claim 1 or 9, wherein the impact modifier is an ionomer.

13. The composition of claim 12, wherein the ionomer is a carboxyl-containing polymer partially neutralized with divalent metal cations comprising manganese, zinc, and magnesium.

14. The composition of claim 12, wherein the ionomer comprises ethylene-acrylic acid and ethylene-methacrylic acid copolymers partially neutralized with zinc.

15. The composition of claim 1, wherein the at least one polycarbodiimide comprises greater than 0.5 to about 3 weight percent of the composition.

16. The composition of claim 15, wherein the at least one polycarbodiimide comprises from greater than 0.5 to about 2 weight percent of the composition.

17. The composition of claim 1, 15 or 16, wherein the at least one polycarbodiimide is an aliphatic, alicyclic or aromatic polycarbodiimide.

18. The composition of claim 1, further comprising about 0.5 to about 10 weight percent of at least one phenol-aldehyde resin.

19. The composition of claim 18, wherein the at least one phenol-aldehyde resin comprises from about 2 to about 6 weight percent of the composition.

20. The composition of claim 19, wherein the at least one phenol-aldehyde resin comprises from about 2 to about 5 weight percent of the composition.

21. The composition of claim 11, wherein the phenol-formaldehyde resin is selected from one or more of phenol-formaldehyde resin, cresol-formaldehyde resin, resorcinol-formaldehyde resin, and butylphenol-formaldehyde resin.

22. An article made from the composition of any of claims 1-21.