Patent Application
20180265651
The disclosure of Japanese Patent Application No. 2017-051811 filed on Mar. 16, 2017 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
The invention relates to resin pellets, resin pellet manufacturing methods, and methods for manufacturing molded articles using the resin pellets.
Polyamide 66 is light in weight and has high self-lubricity, for example. Thus, polyamide 66 is used in a wide range of applications. Examples of the applications include sliding members, such as gears of speed reducers for electric power steering systems and resin bearing cages. In response to recent demands for smaller size and higher power automobile parts, resin members used for such parts are required to have high mechanical strength and high stiffness. To meet such requirements, carbodiimide is added to polyamide so as to increase resin stiffness and toughness.
Japanese Patent Application Publication No. 2014-209032 (JP 2014-209032 A), for example, discloses a resin pellet provided by a process involving melting and blending polyamide 66, a copper heat stabilizer, polycarbodiimide, and an impact modifier (e.g., EPDM rubber onto which maleic anhydride is grafted) in a two-shaft kneader, extruding the resulting blend, and solidifying the extruded blend.
Unfortunately, simply melting and blending polyamide and polycarbodiimide so as to manufacture resin pellets may substantially complete an increase in molecular weight of polyamide induced by the action of polycarbodiimide in the course of manufacture of the resin pellets. In this case, molding heat generated during manufacture of molded articles using the resin pellets tends to promote resin decomposition (or thermal decomposition), leading to large variations in molecular weights and properties of the resulting molded articles.
An excessive increase in the molecular weight of resin pellets yet to be molded forces a manufacturer to mold high melt viscosity resins. This disadvantageously makes it difficult to mold the resins with stability.
Suppose that the molded article obtained by molding the resin pellets is used under a high temperature atmosphere. In this case, relatively high stiffness at high temperature is required. For example, using the molded article for gears of speed reducers for electric power steering systems results in preferable use of a non-reinforced material containing no filler if possible, in consideration of providing damage to a worm made of an associated material. In this case, both improvement in creep resistance at high temperature during sliding and toughness are required. For example, using the molded article for a resin bearing cage exposed to a high temperature atmosphere causes deformation particularly by widening the claw parts of a crown-shaped cage due to insufficient high temperature stiffness. This may cause abnormal wear due to contact to an outer ring of a bearing or contact at points. The high temperature stiffness is also required for preventing these disadvantages. Suppose that the resin pellet is used under a high temperature atmosphere. In such a case, an aromatic polyamide, which has more excellent high temperature properties than those of an aliphatic polyamide, is preferably used as the polyamide resin material. The aromatic polyamide, however, is generally more brittle than the aliphatic polyamide (has lower toughness than that of the aliphatic polyamide) and thus there is a concern that durability life of the molded article used for a long period of time under a high temperature atmosphere may be reduced.
An object of the invention is to provide a resin pellet, a resin pellet manufacturing method, and a method for manufacturing a molded article using the resin pellet, enabling stable mass-production of molded articles having high molecular weights, reduction in property variations among the molded articles, and an increase in toughness while reducing lowering of high temperature stiffness.
A resin pellet according to an aspect of the invention includes an aromatic polyamide resin and a carbodiimide group. The percentage of the carbodiimide group to the resin pellet is 0.03% to 0.33% by mass.
The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:
Embodiments of the invention will be described below in detail. A resin pellet according to the invention contains an aromatic polyamide resin and a carbodiimide group. Examples of the aromatic polyamide resin include PA 6T, PA 9T, PA 10T, and PA MXD6. Specific examples of these commercially available aromatic polyamides include “ARLEN” manufactured by Mitsui Chemicals, Inc. and “GRIVORY” manufactured by EMS-CHEMIE Ltd. as examples of PA 6T, “GENESTAR” manufactured by KURARAY CO., LTD. as an example of PA 9T, and “XecoT” manufactured by UNITIKA LTD. as an example of PA 10T. Of these aromatic polyamides, PA 10T is preferably used. PA 10T has an even number of C (carbon) components and excellent symmetry of the molecule. This provides excellent crystallinity. PA 10T has a longer C component than those of other aromatic polyamide resins such as PA 6T and PA 9T. This controls brittleness, which can be one of the possible concerns about the aromatic polyamide resin, to some extent by use of the resin itself. Any one of these aromatic polyamides may be used alone, or a combination of any two or more of these polyamides may be used. The polyamide resin has a number average molecular weight Mn of 15,000 to 30,000, for example.
The carbodiimide group is a functional group represented by the following formula: (—N═C═N—). The percentage of the carbodiimide group(s) to the resin pellet according to this embodiment of the invention is preferably 0.03% to 0.33% by mass, and more preferably 0.06% to 0.25% by mass. The amount of carbodiimide group(s) contained in the resin pellet may be determined using Lambert-Beer's law in accordance with the procedure illustrated in
The following description discusses the details of how the amount of carbodiimide group(s) contained in the resin pellet is determined. First, as illustrated in
Following the thickness adjustment, the thin-piece sample is placed on a KBr plate 105 and thus set on a base 106 of an infrared (IR) spectrophotometer as illustrated in
As illustrated in
The molar absorption coefficient c determined, the absorbance A known, and the sample thickness L known are substituted into the following equation: C=A/εL. Thus, the amount of carbodiimide group(s) (—N═C═N—) contained in the resin pellet is determined. A carbodiimide group is not the only element contained in a carbodiimide group-containing compound. Therefore, the amount of carbodiimide group(s) is estimated from the chemical structure and the amount of a carbodiimide group-containing compound.
The resin pellet according to this embodiment of the invention has a number average molecular weight Mn of 20,000 to 35,000. The number average molecular weight Mn of the resin pellet is calculated by gel permeation chromatography (GPC) or a solution viscosity method, for example.
The resin pellets according to this embodiment of the invention preferably further contain an elastomer.
The elastomer may be, for example, a carboxyl substituted polyolefin that is a polyolefin having carboxyl parts bonded to either the main chain itself or the side chains of the polyolefin. The term “carboxyl part” means one or more carboxyl groups of dicarboxylic acids, diesters, dicarboxylic acid monoesters, acid anhydrides, monocarboxylic acids and esters and salts. The carboxyl acid salts are neutralized carboxylic acids.
Of these elastomers, a preferable elastomer is, for example, a dicarboxyl substituted polyolefin that is a polyolefin having dicarboxyl parts bonded to either the main chain itself or the side chains of the polyolefin. The term “dicarboxyl part” means one or more dicarboxyl groups of dicarboxylic acids, diesters, dicarboxylic acid monoesters, and acid anhydrides. The preferable polyolefin is a copolymer of ethylene and one or more additional olefins. Here, the additional olefin is a hydrocarbon.
The elastomer is preferably based on an olefin copolymer such as an ethylene/α-olefin polyolefin. Examples of the olefin suitable for preparing the olefin copolymer include alkenes having 2 to 8 carbon atoms, such as ethylene, propylene, 1-butene, 1-heptene, and 1-hexene. Diene monomers, such as 1,4-hexadiene, 2,5-norbornadiene, 1,7-octadiene, and/or dicyclopentadiene, may be optionally used for preparation of the polyolefin. The preferable olefin copolymer is a polymer derived from ethylene and at least one α-olefin having 3 to 6 carbon atoms. The particularly preferable polyolefin is an ethylene-1-butene copolymer (EBR) manufactured from ethylene/1-butene.
The carboxyl parts are introduced into the olefin copolymer and then the resulting carboxyl-denatured copolymer is copolymerized with an unsaturated carboxyl-containing monomer. The elastomer may be formed during preparation of the polyolefin by this procedure. The carboxyl parts may be introduced by grafting unsaturated grafting agents containing the carboxyl part of acids, esters, diacids, diesters, acid esters, or acid anhydrides to the polyolefin.
Examples of the preferable unsaturated carboxyl containing comonomer or grafting agent include maleic acid, maleic anhydride, a maleic acid monoester, a metal salt of a maleic acid monoethyl ester, fumaric acid, fumaric acid monoethyl ester, itaconic acid, vinylbenzoic acid, vinylphthalic acid, methacrylic acid, a methacrylic acid ester, a metal salt of fumaric acid monoethyl ester, and a methyl monoester and diester, a propyl monoester and diester, an isopropyl monoester and diester, a butyl monoester and diester, an isobutyl monoester and diester, a hexyl monoester and diester, a cyclohexyl monoester and diester, an octyl monoester and diester, a 2-ethylhexyl monoester and diester, a decyl monoester and diester, a stearyl monoester and diester, a methoxyethyl monoester and diester, an ethoxyethyl monoester and diester, a hydroxyethyl monoester and diester, and an ethyl monoester and diester of maleic acid, fumaric acid, or itaconic acid.
Of these unsaturated carboxyl containing comonomers or grafting agents, maleic anhydride is preferable. In this case, the degree of acid denaturation of maleic anhydride is preferably 1 or higher and further preferably 1.5 or higher and 4 or lower. The degree of acid denaturation is determined based on the percentage of the acid component (for example, maleic anhydride) during polymerization of the elastomer itself. More specifically, an analytical curve related to the peak strength of —COOH group in IR spectrum for several elastomers having different degrees of acid denaturation with each other is prepared. Subsequently, a sample having an unknown degree of acid denaturation is measured with IR spectroscopy. Comparing the obtained IR spectrum with the analytical curve and calculating the compared result in terms of the amount of the target unknown sample added enable the degree of acid denaturation to be determined.
The preferable elastomer is an EBR polymer or a maleic anhydride-grafted ethylene-1-butene copolymer. A blend of polyolefin such as polyethylene, polypropylene, and the EBR polymer and polyolefin grafted with unsaturated compound containing a carboxyl parts may be used as the elastomer. Examples of other preferable elastomers include an ionomer that is a carboxyl group-containing polymer partially neutralized with divalent metal cation such as zinc, manganese, and magnesium. The preferable ionomer is an ethylene/acrylic acid copolymer and an ethylene/methacrylic acid copolymer partially neutralized with zinc.
As illustrated in
In such a case, the resin pellet according to the invention enables the maleic anhydride-denatured ethylene elastomer to be finely dispersed in polyamide 10T, in addition to the molecular weight of the polyamide 10T to be increased by the action of the carbodiimide group(s) (interaction (3)) due to high cross-reactivity among at least three components of polyamide 10T, the maleic anhydride-denatured ethylene elastomer, and the carbodiimide group(s). This enables the strength of adhesion between polyamide 10T and the maleic anhydride-denatured ethylene elastomer to be increased (interaction (1)). The strength of adhesion between polyamide 10T and the maleic anhydride-denatured ethylene elastomer is also increased through the carbodiimide group(s) (interactions (2)+(3)). This enables an increase in the elongation property of the molded article itself obtained from the resin pellets. Consequently, toughness of the whole resin pellets can be increased regardless of the use of the aromatic polyamide, which has lower toughness than that of the aliphatic polyamide. Although the stiffness of the resin pellets usually tends to decrease due to the softness of the elastomer, the resin pellets according to the invention prevents reduction in high temperature stiffness due to interaction (3) of the carbodiimide group(s) in addition to high glass transition temperature Tg of the aromatic polyamide resin itself.
The carbodiimide group(s) remain(s) in the resin pellet according to this embodiment of the invention. Thus, during injection molding of the resin pellet, the molding temperature thereof is utilized, so that the action of unreacted carbodiimide group(s) promotes the reaction of a terminal carboxyl group (—COOH) and/or an amino group (—NH2) of the polyamide resin with the carbodiimide group(s), and promotes the reaction between the terminal carboxyl group (—COOH) and terminal amino group (—NH2) of the polyamide resin. This enables a plurality of polyamide resin polymer chains made in advance by polymerization or a plurality of polyamide resin polymer chains connected to each other by the action of the carbodiimide group(s) during manufacturing of the resin pellet (which will be described below) to be further bound to each other in a chain-like manner. Consequently, the resin to be molded into a molded article will have a higher molecular weight.
The resin pellet according to this embodiment of the invention may contain a lubricant. The lubricant is not limited to any particular type of lubricant. Examples of the lubricant include the following known lubricants: metallic soaps, such as a metallic stearate; synthetic hydrocarbons, such as paraffin wax and synthetic polyethylene wax; fatty acids, such as a stearic acid; higher alcohols, such as a stearyl alcohol; higher aliphatic amides, such as a stearic acid amide and an oleic acid amide; esters, such as a higher fatty acid ester of an alcohol; and a silicone compound. Of these lubricants, higher aliphatic amides and a silicone compound are preferable lubricants to be used.
In this embodiment, any of the above known lubricants may be used. In particular, heat resistance conditions for the lubricant to be used are preferably such that the lubricant heated by TG-DTA at 10° C./min in a nitrogen atmosphere is reduced in weight by 10% at a temperature of 340° C. or above. Suppose that the lubricants meeting these heat resistance conditions have definite melting points. In this case, the lubricant having a melting point of 200° C. or above is preferably used.
The resin pellet according to the invention may contain filler(s). Examples of the filler(s) include: short fiber fillers, such as glass fiber, carbon fiber, aramid fiber, cellulose fiber, poly(p-phenylenebenzobisoxazole) (PBO) fiber, polyarylate (PAR) fiber, and polytetrafluoroethylene (PTFE) fiber; plate-like fillers, such as glass flake; and micro reinforcing fillers, such as carbon nanotube and carbon nanofiber. One or more of these fillers may be contained in the resin pellet. Of these fillers, a short fiber filler is preferably used. Glass fiber is more preferably used as the filler. Organic fiber, such as aramid fiber, PBO fiber, PAR fiber, or PTFE fiber, is most preferably used as the filler.
A method for manufacturing the resin pellet will be described below.
The body 28 includes a main feeder 110, a side feeder 111, a cylinder 33, a screw 34, and a nozzle 35. The side feeder 111 is disposed between the main feeder 110 and the nozzle 35. In other words, the side feeder 111 is disposed downstream of the main feeder 110. The body 28 is not limited to any particular configuration. The body 28 may be, for example, a known kneader, such as a two-shaft (or multi-shaft) extruder or single-shaft extruder.
The main feeder 110 includes a tank 29, a scale 38, and a slot 32. The side feeder 111 includes a tank 112, a scale 113, and a slot 114. An agitator 37 is disposed upstream of the tank 29. Raw materials mixed in the agitator 37 are passed through the tank 29 and the scale 38 located downstream thereof and are then fed into the slot 32 of the main feeder 110.
The first step in preparing the resin pellets 26 is to supply a polyamide resin 39, an elastomer 42, and an additive to the cylinder 33 from the main feeder 110 serving as a shared feeder. The polyamide resin 39, the elastomer 42, and the additive may be separately fed into the tank 29 and then supplied to the cylinder 33, or may be mixed in the agitator 37 and then supplied to the cylinder 33. Mixing the polyamide resin 39 and the optional additive in the agitator 37 involves dry blending and master batching.
The aromatic polyamide resin and the elastomer described above are used as the polyamide resin 39 and the elastomer 42. The percentage of the polyamide resin 39 to the total amount of raw materials used in the preparation of the resin pellets 26 is, for example, 45% to 90% by mass. The percentage of the elastomer 42 to the total amount of raw materials used in the preparation of the resin pellets 26 is, for example, 3% to 20% by mass.
A lubricant is preferably used as the additive. Some lubricants produce an “intermolecular sliding effect”, leading to a reduction in viscosity of raw materials of the resin pellets 26 during kneading. This enables kneading at a relatively low temperature. Thus, using such a lubricant as the optional additive controls the rate of reaction (or chain reaction) of the polyamide resin 39 during kneading of the polyamide resin 39, a filler 40, and a carbodiimide bond-containing compound 41 (which will hereinafter be simply referred to as “carbodiimide 41”).
When a lubricant such as one of those described above is used, the percentage of the lubricant to the total amount of raw materials used in the preparation of the resin pellets 26 is, for example, 0.01% to 1% by mass. The polyamide resin 39, the elastomer 42, and the optional additive supplied to the cylinder 33 are kneaded by rotation of the screw 34. The polyamide resin 39, the elastomer 42, and the optional additive are kneaded, with the temperature of the cylinder 33 at 275° C. to 325° C. and the rotational speed of the screw 34 at 100 rpm to 500 rpm, for example.
The next step is to simultaneously supply the filler 40 and the carbodiimide 41 to the cylinder 33 from the side feeder 111 serving as a shared feeder. It is basically difficult to knead a high-molecular-weight polyamide resin with a filler. The method according to this embodiment, however, allows a resin to be kneaded in a low viscosity state, thus enabling the resin to mix with a filler.
Examples of the filler 40 to be used include those previously described. When glass fiber, for example, is used as the filler 40, the glass fiber preferably has a diameter of 6 μm to 15 μm, and more preferably has a diameter of 6 μm to 8 μm. Using the glass fiber having a diameter falling within these ranges comparatively increases the area of contact between the glass fiber and the polyamide resin in each resin pellet 26. This favorably increases the mechanical strength and stiffness of the resulting molded article formed by molding the resin pellets 26.
The percentage of the glass fiber to the total amount of raw materials used in the preparation of the resin pellets 26 is, for example, 10% to 50% by mass.
When organic fiber is used as the filler 40, the diameter of the organic fiber is not limited to any particular diameter. The organic fiber has a diameter of 9 μm to 15 μm, for example. The percentage of the organic fiber to the total amount of raw materials used in the preparation of the resin pellets 26 is, for example, 5% to 25% by mass. Using the organic fiber within this range prevents aggregation so as to knead the raw resin with the organic fiber uniformly, while cutting down the amount of relatively expensive organic fiber. This effectively increases the wear resistance of the resin.
The carbodiimide 41 to be used may be any compound that contains a carbodiimide group (—N═C═N—) and may be monocarbodiimide containing a single carbodiimide group, or polycarbodiimide containing a plurality of carbodiimide groups. Any type of carbodiimide, such as aliphatic carbodiimide, aromatic carbodiimide or modified carbodiimide, may be used. Of these carbodiimides, aromatic carbodiimide is preferably used as the carbodiimide 41. One specific example of such carbodiimide commercially available is “Stabaxol P-100” manufactured by LANXESS. When the carbodiimide 41 is aromatic carbodiimide, its aromatic ring and adjacent functional group produce a steric hindrance effect, thus reducing the rate of reaction (or chain reaction) of the polyamide resin 39 during kneading of the polyamide resin 39, the elastomer 42, the filler 40, and the carbodiimide 41. This facilitates control operations for allowing the percentage of residual carbodiimide group(s) to each resin pellet 26 to be 0.03% to 0.33% by mass. The control operations include, for example, controlling the temperature of the cylinder 33, the time during which kneading is to be performed, and the pressure during kneading.
Suppose that the carbodiimide 41 is aliphatic carbodiimide and has no aromatic ring. The carbodiimide 41 in this case makes it difficult to achieve the above-mentioned steric hindrance effect produced by an aromatic ring and an adjacent functional group. The carbodiimide 41 in this case, however, achieves a similar effect when the carbodiimide 41 contains a lubricant. This is because, in such a case, the lubricant produces an intermolecular sliding effect as previously mentioned. In other words, when aliphatic carbodiimide is used as the carbodiimide 41, a lubricant such as one of those described above is preferably used in combination therewith. When aliphatic carbodiimide is used as the carbodiimide 41, employing a first technique described below favorably enables the percentage of residual carbodiimide group(s) to each resin pellet 26 to be 0.03% to 0.33% by mass. The first technique described below is provided by way of example only, and any other technique may be used.
The first technique includes step (1) involving disposing the side feeder 111 close to the nozzle 35. This reduces the time allowed for the reaction of the polyamide resin 39 induced by the action of the carbodiimide group(s). Thus, the reaction (or chain reaction) of the polyamide resin 39 is controlled.
The first technique further includes step (2) involving reducing the temperature set for the barrel of the kneading machine 27. Melting the polyamide resin 39 in the vicinity of the main feeder 110 at a temperature equal to or higher than the melting point of the polyamide resin 39 would enable the molten polyamide resin 39 to flow even if the temperature set for a region downstream of the main feeder 110 is low. Thus, reducing the temperature set for the barrel controls the reaction (or chain reaction) of the polyamide resin 39.
The first technique further includes step (3) involving reducing the number of revolutions set for the kneading machine 27. Reducing the number of revolutions decreases shearing heat applied to the resin, thus controlling the reaction (or chain reaction) of the polyamide resin 39.
The first technique further includes step (4) involving providing the kneading machine 27 with the screw and kneading disks having a low-shearing configuration. This means that, for example, the ratio of kneading disks to the screw (i.e., the number of kneading disks) may be reduced, or kneading disks small in width may be used. Consequently, the effect of kneading is lessened, thus controlling the reaction (or chain reaction) of the polyamide resin 39.
The number average molecular weight Mn of the carbodiimide 41 is preferably relatively high. The number average molecular weight Mn of the carbodiimide 41 is 3,000 to 25,000, for example. The percentage of the carbodiimide 41 to the total amount of raw materials used in the preparation of the resin pellets 26 is, for example, 0.5% to 4% by mass. Using the carbodiimide 41 within this range favorably provides a final molded article whose number average molecular weight Mn is 25,000 or more. Because the carbodiimide 41 is not excessive in amount, this embodiment of the invention reduces the risk of, for example, an increase in resin pressure (or viscosity) during kneading, heat generation during kneading, thermal decomposition of the polyamide resin 39 and the carbodiimide 41 associated with the heat generation, and a reduction in strength of adhesion of the filler 40 to the resin caused by convergence degradation associated with the heat generation.
When the carbodiimide 41 is powder, the carbodiimide 41 may be supplied independently through the side feeder 111, or may be mixed with a polyamide resin and then supplied through the side feeder 111. Mixing the carbodiimide 41 with the polyamide resin involves dry blending and master batching. The filler 40 and the carbodiimide 41 are added to a kneaded mixture of the polyamide resin 39 and the optional additive being conveyed through the cylinder 33, so that the polyamide resin 39, the optional additive, the filler 40, and the carbodiimide 41 are subjected to further kneading. The time between the supply of the carbodiimide 41 and the ejection of the kneaded mixture from the nozzle 35 (kneading time for the carbodiimide 41) ranges from one second to one minute, for example. Accordingly, the distance between the side feeder 111 and the nozzle 35 may be set in accordance with the kneading time for the carbodiimide 41. As illustrated in
Following the supply of the carbodiimide 41, the kneaded mixture is ejected in the form of a strand from the nozzle 35, solidified by being cooled in the cooling water tank 30, and then pelletized by the pelletizer 31. Carrying out these steps provides the resin pellets 26 each containing the polyamide resin 39, with the filler 40 dispersed therein. Each resin pellet 26 thus provided contains 0.03% to 0.33% of unreacted carbodiimide group(s) (i.e., residual carbodiimide) by mass. The term “unreacted carbodiimide group(s)” refers to carbodiimide group(s) that has/have not reacted during the kneading process previously described. Feeding the carbodiimide 41 from the side feeder 111 allows the unreacted carbodiimide group(s) to remain in each resin pellet 26. In this case, the carbodiimide 41 is allowed to favorably remain in each resin pellet 26 by, for example, appropriately selecting the type of carbodiimide to be used as the carbodiimide 41, adjusting the number average molecular weight Mn of the carbodiimide 41, and deciding whether the carbodiimide 41 should contain a lubricant. The types of carbodiimide to be used as the carbodiimide 41 include aromatic carbodiimide and aliphatic carbodiimide.
The number average molecular weight Mn of each resin pellet 26 provided is, for example, 20,000 to 35,000. The above-described manufacturing procedure involves supplying the carbodiimide 41 to the cylinder 33 from the side feeder 111 and thus limits the time allowed for the reaction of the polyamide resin 39 induced by the action of the carbodiimide group(s). If the carbodiimide 41 is supplied from the main feeder 110, the reaction of the polyamide resin 39 proceeds while the resin being kneaded is conveyed substantially from end to end of the cylinder 33. This embodiment, however, shortens the distance allowed for the reaction of the polyamide resin 39 so that the reaction of the polyamide resin 39 occurs only between the side feeder 111 and the nozzle 35, thus limiting the time allowed for the reaction of the polyamide resin 39. Consequently, this embodiment easily causes the unreacted carbodiimide group(s) to remain in each resin pellet 26 obtained.
The above procedure involves supplying the carbodiimide 41 in the course of kneading of the polyamide resin 39, the elastomer 42, and the filler 40. This reduces the occurrence of disadvantageous conditions, such as excessive torque, heat generation, and strand tearing in the body 28, and unfavorable resinous attachments to the body 28, more effectively than when the polyamide resin 39, the elastomer 42, the filler 40, and the carbodiimide 41 are simultaneously supplied to the cylinder 33 so as to start kneading or when the polyamide resin 39 and the carbodiimide 41 are simultaneously supplied to the cylinder 33 from the main feeder 110 of the kneading machine 27 (i.e., the preceding one of the feeders) so as to start kneading. Consequently, this embodiment enables stable production of the resin pellets 26.
The above description has been predicated on the assumption that the carbodiimide 41 is supplied from the side feeder 111. The carbodiimide 41, however, may alternatively be supplied from the main feeder 110. In such a case, using a second technique described below favorably enables the percentage of residual carbodiimide group(s) to each resin pellet 26 to be 0.03% to 0.33% by mass. The second technique described below is provided by way of example only, and any other technique may be used.
The second technique includes step (1) involving performing at least one of steps (2) to (4) of the first technique employed in using aliphatic carbodiimide as the carbodiimide 41.
The second technique further includes step (2) involving disposing a highly sterically hindering functional group, such as an isopropyl group, on the periphery of the carbodiimide group(s) when aromatic carbodiimide is used as the carbodiimide 41. Using the second technique controls the rate of reaction of the carbodiimide 41 (i.e., the ease of reaction between the carbodiimide group(s) and polyamide).
Suppose that a lubricant is used in one of the steps. In this case, the lubricant heated by TG-DTA at 10° C./min in a nitrogen atmosphere is reduced in weight by 10% at a temperature of 340° C. or above, thus achieving an effect described below. Specifically, the temperature of the cylinder 33 is set in the range of, for example, 275° C. to 325° C. during kneading of the resin in the cylinder 33. Setting the temperature of the cylinder 33 at a high temperature in this range (i.e., at a temperature of 300° C. or above) enables a reduction in viscosity by addition of the lubricant so as to produce the effect of improving kneadability and moldability, although black spots are likely to occur in the resin. The occurrence of such black spots is caused by decomposition, gasification, and carbonization of the lubricant that are induced by a heat history during kneading of the resin, for example. Such black spots may hinder the growth of necking of the resin and cause breaking of the resin to proceed from the black spots, resulting in a reduction in mechanical strength (e.g., tensile elongation at break) of the resin. One conceivable solution to the occurrence of black spots is to utilize, for example, image analysis so as to mechanically remove (or screen out) resin pellets having black spots, and another conceivable solution is to screen out, before shipment, molded articles having black spots. Unfortunately, either of these solutions leads to an increase in cost or a reduction in yield.
To solve such problems, this embodiment of the invention involves the use of a lubricant that meets the heat resistance conditions previously mentioned. This would reduce or eliminate the occurrence of black spots if the resin is kneaded at a temperature as high as 300° C. or above. Such an advantage reduces breaking of the resin that proceeds from the black spots. Thus, with an increase in molecular weight of the resin, the mechanical strength of the resin is maintained at a favorable level. Because what is needed for this solution is to simply select the type of lubricant to be used, the step of screening out resin pellets, for example, is unnecessary, resulting in no increase in cost or no reduction in yield.
The resin pellets thus provided are usable for any type of structural member (or molded article) whose constituents include a polyamide resin. Examples of such structural members include various resin gears, various bearing cages, and various housings. Specific examples of such structural members include a worm wheel, a worm housing and a sensor housing for a power steering system, a resin-wound guide bearing for a sliding door, and a housing for an electric oil pump. The usage of the resin pellets according to this embodiment of the invention is not limited to these specific examples.
An exemplary molded article formed from the resin pellets according to this embodiment of the invention will be described below with reference to the accompanying drawings.
The gear 20 thus described is usable not only as a worm wheel for a power steering system but also as any of various other gears, such as a bevel gear and a helical gear. The manufacture of the gear 20 involves, for example, preparing a mold (not illustrated), melting the resin pellets 26 provided by following the procedure illustrated in
In the course of manufacture of the resin pellets 26 illustrated in
This embodiment intentionally causes the percentage of residual unreacted carbodiimide group(s) to each resin pellet 26, provided by following the procedure illustrated in
In this embodiment, the step of kneading raw materials of the resin pellets 26 and the step of molding the resin pellets 26 are regarded as a series of heating steps. Throughout the series of heating steps, the chain reaction of the polyamide resin 39 is allowed to proceed to an appropriate level so as to increase the molecular weight of the polyamide resin 39 to an unprecedented level. This keeps the chain reaction of the polyamide resin 39 from proceeding excessively just in the step of kneading raw materials of the resin pellets 26, and thus prevents the reaction of the polyamide resin 39 from becoming so excessive that the polyamide resin 39 is decomposed in the subsequent step of molding the resin pellets 26.
The chain reaction of the polyamide resin 39 in the resin pellets 26 yet to be molded is still in progress. The molecular weight of the resin pellets 26 yet to be molded is lower than the molecular weight of the final molded article (e.g., the resulting gear 20). The number average molecular weight Mn of the gear 20 is, for example, 25,000 or more, while the number average molecular weight Mn of the resin pellets 26 is, for example, 20,000 to 35,000. The polyamide resin 39 in the resin pellets 26 yet to be molded is relatively low in viscosity. This facilitates molding and thus makes it unnecessary to excessively raise the molding temperature. Consequently, thermal decomposition of the resin is controlled so as to reduce molecular weight variations and property variations among the molded articles (or gears 20).
The resin pellets 26 are moldable in a relatively low viscosity state. This would allow the resin to be favorably filled into the mold if an injection molding machine, for example, has a small gate diameter. In the step of kneading raw materials of the resin pellets 26, the filler 40 is naturally favorably dispersible throughout the polyamide resin 39 whose molecular weight is not high (or whose viscosity is low) before the carbodiimide 41 is fed.
Organic fiber, in particular, is soft and thus unlikely to break, making it generally difficult to knead a high viscosity, high molecular weight molten resin with organic fiber. Kneading a high molecular weight molten resin with organic fiber requires, for example, increasing the number of revolutions of a kneading machine and/or increasing the temperature set for a barrel, thus reducing the viscosity of the resin so as to effect a reduction in kneading torque. Such a technique causes resin decomposition to proceed owing to heat generation, leading to a reduction in molecular weight. This makes it difficult to take advantage of properties of a high molecular weight resin, such as high wear resistance. In other words, uniformly mixing organic fiber, which is unlikely to damage an associated component and contributes to an increase in wear resistance of a resin, by the technique known in the art makes it difficult to increase the molecular weight of a resin. On the other hand, giving higher priority to increasing the molecular weight of a resin makes it difficult to uniformly mix organic fiber. The method according to this embodiment, however, makes it possible to knead a resin in a low viscosity state. This enables an increase in molecular weight of the resin while uniformly dispersing organic fiber in the resin.
When organic fiber is used, carbodiimide group(s) react(s) not only with a polyamide resin but also with the organic fiber. In an example where aramid fiber, for example, is used, carbodiimide group(s) react(s) with an amido group in the aramid fiber. Thus, using organic fiber in the method according to this embodiment achieves the effect of increasing the strength of adhesion between the polyamide resin and the organic fiber. Consequently, the wear resistance of the resulting molded article is higher than when no organic fiber is added.
As described thus far, this embodiment provides the gear 20 having high mechanical strength and wear resistance. More specifically, the embodiment of the invention provides mechanical strength, stiffness, and dimensional stability required for the gear 20. The number average molecular weight Mn of the resin is 25,000 or more, resulting in high resistance to crack extension. The high resistance to crack extension would reduce the speed of crack extension if the filler 40 triggers crack-inducing resin wear and/or flaking. Consequently, this embodiment reduces the wear volume of the teeth 24 and thus achieves wear resistance required for the teeth 24.
These advantages serve to prevent an increase in variation in the inter-core distance between the gear 20 and an associated component. Suppose that the gear 20 is used as a worm wheel for a speed reducer of a power steering system. In this case, rattling sounds will not be produced because a variation in the inter-core distance between the worm wheel and a worm does not increase, and in addition, durability life of the worm wheel will increase. Worm wheels, in particular, may be expected to be further reduced in size and increasingly used in higher power applications in the future. This will increase loads to an unprecedented level and will result in application of large torques to the worm wheels. Worm wheels having poor wear resistance will have reduced durability life due to application of such large torques. When the gear 20 according to this embodiment is used as a worm wheel, the worm wheel has high wear resistance as described above and is thus adequately adaptable to future applications where the worm wheel needs to be smaller in size and is intended for higher power use.
The gear 20 manufactured using the resin pellets 26 containing organic fiber has increased wear resistance and is unlikely to damage an associated component. Organic fiber is lower in Mohs hardness than glass fiber. Thus, if sliding of the gear 20 causes organic fiber to be exposed at the surface of the gear 20, the gear 20 would be prevented from wearing away the surface of an associated component. When the gear 20 is used as a worm wheel, for example, a component associated with the gear 20 is a worm shaft. Suppose that sliding contact between the gear 20 and an associated component causes organic fiber to come off the gear 20. In this case, the organic fiber that has come off the gear 20 will not wear away the gear 20.
Accordingly, the resin pellets 26 containing organic fiber are favorably used in manufacturing a molded article (e.g., a sliding member) that is particularly required to have a low probability of damaging an associated component and have high wear resistance. Using the organic fiber-containing resin pellets 26 in manufacturing the gear 20 to be used as a worm wheel prevents wearing away of a metal worm shaft that is a component associated with the gear 20. This makes it unnecessary to perform, for example, induction heat treatment for increasing the hardness of the worm shaft, so that an increase in manufacturing cost is prevented. Using the organic fiber-containing resin pellets 26 in manufacturing a resin-wound guide bearing (or roller) for a sliding door prevents peeling off of a coating on a vehicle body that is a component associated with the guide bearing (or roller). This makes it unnecessary to attach an additional component, such as a guide rail, to the sliding door, so that an increase in manufacturing cost is prevented as in the case of using the organic fiber-containing resin pellets 26 in manufacturing the gear 20 to be used as a worm wheel.
Suppose that a molded article to be manufactured is a container (e.g., a housing such as one previously described) that will not come into contact with any particular component and is required to have high mechanical strength and stiffness rather than high wear resistance. In such a case, the resin pellets 26 containing glass fiber may be used. In other words, an appropriate selection may be made between using the resin pellets 26 containing organic fiber and using the resin pellets 26 containing glass fiber in accordance with the usage of a molded article to be manufactured and characteristics required for the molded article.
Although the embodiment of the invention has been described thus far, the invention may be practiced in other embodiments.
In one example, the body 28 of the kneading machine 27 may include two side feeders as illustrated in
The filler 40 may be supplied to the cylinder 33 from the main feeder 110 together with the polyamide resin 39. In such a case, the position from which the filler 40 is to be supplied is upstream of the position from which the carbodiimide 41 is to be supplied. Thus, the filler 40 is supplied before an increase in molecular weight induced by the action of carbodiimide group(s) starts, i.e., when the viscosity of the resin is lower. This further enhances dispersion of the filler 40. Suppose that the filler 40 is organic fiber. In this case, if the filler 40 is supplied in an initial stage of kneading, the organic fiber would keep its shape, because the organic fiber is soft and unlikely to break.
The molded article manufactured by the method according to the embodiment of the invention does not necessarily contain the elastomer or the filler. Various other design modifications may be made within the scope of the claims.
The invention will be further described below in relation to Examples 1 to 3, Comparative Examples 1 to 8, and Reference Examples 1 to 18. The invention, however, is not limited to the examples described below. In accordance with the data given in Table 1 below, raw materials were supplied to the kneading machine 27 arranged as illustrated in
Commercial Product 1 described in Table 1 is as follows: Polyamide 66 manufactured by Asahi Kasei Corp. (e.g., non-reinforced grade “Leona 1502S”) was molded into a test sample. Neither glass fiber nor carbodiimide was added.
Commercial Product 2 described in Table 1 is as follows: Polyamide 66 manufactured by BASF (“ASH”) was molded into a test sample. Neither glass fiber nor carbodiimide was added.
Commercial Product 3 described in Table 1 is as follows: Polyamide 66 manufactured by DuPont (e.g., “Zytel® E51HSB NC010”) was molded into a test sample. Neither glass fiber nor carbodiimide was added.
The results of various evaluation tests are given below.
(1) Amount of Residual Carbodiimide
The amount of residual carbodiimide contained in the resin pellets provided by Reference Examples 1 to 8 and Reference Example 10 was measured by following the procedure illustrated in
Table 1 suggests that when no lubricant is contained, using aromatic carbodiimide as in Reference Examples 1 to 8 enables the percentage of residual carbodiimide group(s) to each resin pellet to be 0.03% to 0.33% by mass. Using aliphatic carbodiimide as in Reference Example 10 causes almost all of carbodiimide groups to be consumed at the time when resin pellets are prepared, so that the percentage of residual carbodiimide to each resin pellet is 0.01% by mass. The comparison between Reference Example 2 and Reference Example 5 reveals that the amount of residual carbodiimide group(s) when a carbodiimide compound is added from the side feeder is larger than the amount of residual carbodiimide group(s) when a carbodiimide compound is added from the main feeder, assuming that the amount of carbodiimide compound added in both cases is the same.
(2) Number Average Molecular Weight Mn
For each test sample, the number average molecular weight Mn was measured by gel permeation chromatography (GPC). The results of the measurement are given in Table 1 and
(3) Time-Varying Changes in Number Average Molecular Weight Mn
(4) Tensile Elongation at Break and Tensile Strength
For Reference Examples 1 to 8 and Reference Example 10, tensile elongation at break and tensile strength were measured in conformity with JIS K 7161. The results of the measurement are given in Table 1 and
According to Reference Examples 1 to 3 in Table 1, in the case of adding a carbodiimide compound from the main feeder, high tensile elongation at break was achieved when the amount of residual carbodiimide was 0.03% by mass as in Reference Example 2. According to Reference Examples 4 to 7 in Table 1, in the case of adding a carbodiimide compound from the side feeder, high tensile elongation at break was achieved when the amount of residual carbodiimide was 0.15% by mass as in Reference Example 6.
Similar to the mechanism of an increase in the molecular weight of the polyamide resin, the effect related to the tensile elongation at break and the tensile strength is believed to achieve the equivalent result when the aromatic polyamide resin is used as the polyamide resin material instead of polyamide 66 because the reaction between the terminal carboxyl group (—COOH) and/or amino group (—NH2) of the polyamide resin with the carbodiimide group(s) and the reaction between the terminal carboxyl group (—COOH) and the terminal amino group (—NH2) of the polyamide resin are causes.
Referring to Reference Examples 11 to 16 described in Table 2, the following description discusses how the frequency of occurrence of black spots in the resin and the tensile elongation at break of the resin change depending on heat resistance conditions for lubricants. Specifically, polyamide 66 manufactured by Asahi Kasei Corp. under the product name “Leona 1702” was kneaded with each lubricant described in Table 2, with the temperature of the cylinder set at 300° C. during kneading. Thus, resin pellets were prepared. The resin pellets prepared were then molded into test samples.
The results of various evaluation tests are given below.
(1) Tensile Elongation at Break
For Reference Examples 11 to 16, tensile elongation at break was measured in conformity with JIS K 7161.
The results of the measurement are given in Table 2.
(2) Frequency of Occurrence of Black Spots
After the test of tensile elongation at break just mentioned, observations were made to determine whether a black spot occurred at each broken surface. The number of samples n for each reference example was five. The frequency of occurrence of black spots was evaluated by giving two points to each sample with a black spot having a size of 100 mm or larger, giving one point to each sample with a black spot having a size of smaller than 100 mm, and giving zero point to each sample with no black spot. The sum of points given to the five samples for each reference example was determined to be the measurement result for each reference example. The sum of points given to the five samples for each reference example is 10 points at the maximum.
As illustrated in
Referring to Reference Examples 17 and 18 and Comparative Examples 2 to 5 described in Table 3, the following description discusses how the molecular weight and wear resistance of the molded article increase by the combined use of a carbodiimide compound and organic fiber (aramid fiber). In accordance with the data given in Table 3 below, raw materials were supplied to the kneading machine 27 arranged as illustrated in
The results of various evaluation tests are given below.
(1) Amount of Residual Carbodiimide
The amount of residual carbodiimide contained in the resin pellets provided by Reference Examples 17 and 18 was measured by following the procedure illustrated in
(2) Number Average Molecular Weight Mn
For each test sample, the number average molecular weight Mn was measured by gel permeation chromatography (GPC). The results of the measurement are given as relative ratios in
(3) Frictional Wear Test (Wear Resistance)
Suzuki frictional wear test was conducted to measure the amount of reduction in height of each test sample. The amount of height reduction measured is determined to be a wear volume and expressed in mm. The results of the test are given in
(4) Evaluation
The number average molecular weight of raw resin used for each of Comparative Examples 3 and 5 was higher than the number average molecular weight of raw resin used for each of Reference Examples 17 and 18 and Comparative Examples 2 and 4. In spite of this,
In accordance with the data given in Table 4 below, raw materials were supplied to the kneading machine 27 arranged as illustrated in
(1) Measurement of Physical Properties
For Examples 1 to 3 and Comparative Examples 6 to 8, Charpy impact strength in conformity with ISO 179/1eA, tensile elastic modulus in an atmosphere at 120° C. in conformity with ISO 527, and tensile strain at break in an atmosphere at room temperature in conformity with ISO 527 were measured. Each of the results of the test is given in
It has been confirmed that, as illustrated in
Subsequently, as illustrated in
From the summary of the results of
(2) Amount of Residual Carbodiimide
The amount of residual carbodiimide contained in the resin pellets provided by Examples 1 to 3 and the test samples (molded articles) was measured by following the procedure illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-051811 | Mar 2017 | JP | national |
