POLYIMIDE COMPOSITION FOR FIXING METALLIC SHEETS

Abstract: A fixing composition comprises a polyimide precursor, and organic solvent, optionally an epoxy resin, e.g. a bisphenol A type, optionally with a heat-activated hardener, and possibly other additives. The precursor is preferably a polyamic acid synthesized from an aliphatic tetracarboxylic dianhydride or lower alkyl ester or halide thereof and an organic diamine optionally with a triamine or tetramine. A sheet 0.5 to 5 mm thick of a powdered metal alloy, e.g. a multi-component eutectic alloy and a synthetic resin binder composition, e.g. an acrylic copolymer and an adhesive component, is placed on a metal substrate, e.g. of steel; a molded film or coated layer of 5-50 µm thickness of the fixing composition is placed therebetween. The assembly is heated and an adhesive force is exerted due to hardening of the precursor; heating is continued to sintering temperature, e.g. 1,000 to 1,150°C, at which the precursor is carbonised to a residual C content of 10 to 60 wt%, and the alloy sinters to the substrate. The alloy sheet is adhered firmly to the substrate before and during sintering.

Description: This invention relates to a composition for use in adhering onto a metal substrate, a metallic sheet or sheet-like material composed of a powdered fusible metal alloy held together by a synthetic resin binder. It also relates to a method of using such composition. For the purpose of providing an alloy layer on a metal substrate, the alloy, in the form of such a sheet of particles of alloy adhered together with binder, is placed on the substrate and the assembly is heated to a temperature at which the alloy melts or sinters onto the substrate surface. To hold such a sheet onto a substrate which is other than flat and horizontal it is necessary to fix the sheet to the substrate; but the adhesive force of the binder cannot support the weight of the metal powder sheet, whereby the metal powder sheet peels off or falls from the substrate; in particular, this tends to occur when sintering is performed of the metal sheet which suffers vibration or impact during conveying in a furnace (such as a mesh-belt type or pusher type continuous sintering furnace or a vacuum sintering furnace). As an effective method for overcoming the above-described problem, we have, in European Application No.85301264.9 (and Japanese Patent Application No.34887/84) claimed a composition for such use, comprising a specific alkyl acrylate or methacrylate copolymer preferably with an adhesion-providing resin, which contains at least 30% by weight of non-volatile materials after non-oxidizing heating at 300°C for 80 minutes; and a method of sintering to a metal substrate a sheet or molding of a metal alloy wherein a layer or film of the composition is interposed therebetween and (a) the assembly is heated to 150 to 400°C for at least 5 minutes in a non-oxidizing atmosphere so as to form from the composition an adhesive, and (b) continuing the heating until the metal alloy is sintered to the substrate and the adhesive disappears. However, the need for the first heating step (a) to be within that specific temperature range (and preferably to 250° to 380°C) is inconvenient and tends to restrict the industrial application of this method. Also, when a metal powder sheet is placed on a metal substrate and sintered, there is a problem that this kind of sheet shrinks by about 20% in the plane directions at sintering and hence it is usually necessary to use a larger metal powder sheet to allow for shrinkage so that desired dimensions can be obtained after sintering. However, to make such allowance is troublesome and not easy in practice. Further, there is deviation of shrinkage percentage due to variation in the thickness of metal powder sheets or the mixing ratio of metal powder and binder, which makes it more difficult to provide a sheet which after sintering has the desired dimensions. This difficulty becomes more serious for a larger size of the sheet since the deviation of the shrinkage percentage becomes larger. Accordingly, it would be very convenient if the occurrence of such shrinkage of the sheet could be restrained by choice of a suitable fixing composition. US-A-4183839 describes liquid compositions from which polyimides can be prepared containing as polyimide precursors aliphatic tetracarboxylic dianhydrides and organic diamines. The compositions are said to have excellent adhesion characteristics to metal or to plastics or glass. There is no disclosure of the compositions being useful for adhering metal powder to metal. GB-A-1444421, US-A-3939029 and IBM Technical Disclosure Bulletin, vol. 22, No. 6, page 2444, each disclose compositions containing polyimide precursors and an organic solvent. Such compositions can be used to prepare laminated sheets, e.g. composed of metallic sheets or foils. There is no suggestion of use of any of the compositions for adhering metal powder to a metal substrate. We have now devised a composition and method for use for this purpose and which does not require the use of the aforesaid (meth)acrylate copolymer. According to the present invention we provide a composition for use in fixing a molding or sheet formed from a powdered metal alloy and a synthetic resin binder to a metal substrate, and which composition serves to adhere the sheet during non-oxidizing heating until the metal alloy is sintered to the substrate, which composition comprises a polyimide series liquid composition containing a polyimide precursor and an organic solvent and which also contains an epoxy compound having at least 2 epoxy groups per molecule in such a ratio that the proportion of the epoxy group is 0.3 equivalent or more per 1 equivalent of the carboxy acid group of the polyimide precursor. When the composition of this invention is interposed between a metal powder molding and a metal substrate and the metal powder molding is subjected to a sintering treatment at a constant rate of temperature increase after first evaporating off the solvent in the initial temperature raising stage of from about 120 to 250°C, a hardening reaction (imidation reaction) of the polyimide precursor occurs which increases the adhesive force of the composition and assists the adhesion and fixing of the metal powder molding onto the substrate and further in the subsequent temperature raising stage, gaseous thermal decomposition products are released to form a carbon precursor, which assists said adhesion and fixing of the molding in a temperature range of usually about 700°C at which the metal powder molding begins to sinter. Therefore, when the metal powder molding is placed on a slanting or a curved surface of the substrate or is stuck to the underside thereof, and the assembly is subjected to sintering while vibration or impact is applied during conveying in a sintering furnace, the use of the composition of this invention does not cause the problems of peeling off or falling of the metal powder molding during the sintering treatment. Moreover, since a polyimide resin is used in the composition of this invention, the residual carbon after completion of the sintering is considerable amount of about 10 to 60% by weight, which gives a very good result for the adhesion and the fixing force of the metal powder molding to the base material during the sintering treatment so as effectively restrain shrinkage of the metal powder molding in the plane directions during the sintering treatment, so that the metal layer formed shrinks in the thickness direction only and has an excellent areal dimensional stability, i.e., has substantially the same areal size as the original one. Thus, by interposing the fixing composition of this invention between the metal powder molding and a metal substrate a metal layer having an excellent dimensional stability, which is strongly fixed on the metal base material by bonding of the metals each other of the substrate and the metal powder, can be formed on the substrate without the specific sintering operation as required in the previous proposal, i.e., by applying a conventional sintering treatment which employs a constant temperature raising rate, without the problem of peeling off or falling of the metal powder molding during the sintering treatment and substantially without the occurrence of shrinkage of the metal powder molding in the plane directions during the sintering treatment. The polyimide precursor used in this invention can be obtained by the reaction of a tetracarboxylic dianhydride or a derivative thereof (for example, a lower alkyl ester or a halide of tetracarboxylic acid) and an organic diamine in an organic solvent. That is, a polyamide acid (or the lower alkyl ester or the halide thereof) can be obtained by reacting both components usually at from about 0 to 100°C, preferably from 5 to 40°C, more preferably at about room temperature. Also, if necessary, the reaction product may be further heated usually at a temperature of from 80°C to the boiling point of the solvent used, preferably from 100 to 200°C, to form a partial imidation product. It is preferred that the imidation of the above-described partial imidation product is performed to an extent not retarding its solubility in the organic solvent. In general, it is preferred that the unit number ratio of the structural unit of amic acid (or the lower alkyl ester or the halide thereof) to the structural unit of imide is in the range of usually from about 0.03 to 4, preferably from 0.05 to 1. This ratio can be found by measuring the acid value of the reaction product. The tetracarboxylic dianhydride used in this reaction can be aliphatic or aromatic. Examples of an aliphatic tetracarboxylic dianhydride are butane-tetracarboxylic dianhydride, pentane-tetracarboxylic dianhydride, hexane-tetracarboxylic dianhydride, cyclopentane-tetracarboxylic dianhydride, bicyclohexene-tetracarboxylic dianhydride, etc.; 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-tetracarboxylic dianhydride; bicyclo-(2,2,2)-octene-2,3,5,6-tetracarboxylic dianhydride, These tetracarboxylic dianhydrides may also be substituted with a substituent such as an alkyl group, in the skeleton thereof. Examples of an aromatic tetracarboxylic dianhydride are pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, benzene-1,2,3,4-tetracarboxylic dianhydride. These aromatic tetracarboxylic dianhydrides may have a substituent such as an alkyl group on the aromatic ring. As mentioned, in place of a tetracarboxylic dianhydride a tetracarboxylic acid or a lower alkyl ester or halide thereof can be used. In addition, a dicarboxylic acid, a tricarboxylic acid, or the anhydrides thereof may be used together with such tetracarboxylic acid component, but it is preferred in this case that the amount of the additional component be not more than 30 mole% of the total amount of the additional component and the tetracarboxylic acid component. The organic diamine which is reacted with the tetracarboxylic dianhydride or the derivative thereof is a diamine represented by the formula H₂N-R-NH₂, wherein R represents a divalent organic group such as an aromatic group, an aliphatic group, an alicyclic group, a heterocyclic group or a combination thereof, or also the above-described groups to which oxygen, nitrogen, sulfur, phosphorus or silicon is bonded. In this case, R may have a substituent which does not quantitatively react with an amino group or an acid anhydride group under a reaction condition. Such a group can impart preferred properties such as solubility, workability and adhesive property of the polyimide precursor produced. An organic triamine or organic tetramine which is conventionally used may be used together with the organic diamine. In this case, it is preferred that the amount of the addition component is not more than about 20 mole% of the total amount of the additional component and the organic diamine. An aromatic diamine is preferred as the organic diamine in this reaction. Specific examples of the diamines which can be used in this invention are metaphenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenylethane, 4,4'-diaminodiphenylmethane, benzidine, 4,4'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfone, p-bis(4-aminophenoxy)benzene, meta-bis(4-aminophenoxy)benzene, 4,4'-diaminodiphenyl ether, 1,5-diaminonaphthalene, 3,3'-dimethyl-4,4'-diaminobiphenyl, 3,4'-diaminobenzanilide, 4-(p-aminophenoxy)-4'-aminobenzanilide, 3,4'-diaminodiphenyl ether, 3,3'-dimethoxybenzene, 2,4-bis(β-amino-tert-butyl)toluene, bis(p-β-amino-tert-butylphenyl) ether, metaxylylenediamine, p-xylylenediamine, di(p-amino-cyclohexyl)methane, hexamethylenediamine, heptamethylenediamine, octamethylenediamine, nonamethylenediamine, 4,4-dimethylheptamethylenediamine, 3-methoxy-heptamethylenediamine, 2,11-diaminododecane, 1,4-diaminocyclohexane, 2,2'-diaminodiethyl ether, 2,2'-diaminodiethyl thioether, 3,3'-diaminodipropoxyethane, 2,6-diaminopyridine, guanamine, 2,5-diamino-1,3,4-oxadiazole, 2-(3'-aminophenyl)-5-aminobenzoxazole, bis-(4-aminophenol)phosphine oxide, bis(4-aminophenyl)diethylsilane, etc. Those diamines can be used alone or as mixtures thereof. The reaction of the tetracarboxylic dianhydride or a derivative thereof and the organic amine is usually performed in an organic solvent, as described above. The reaction of obtaining the polyamide acid (or the lower alkyl ester or the halide thereof) is exothermic and the reaction temperature can be controlled by employing a suitable cooling means at the reaction. In this case, it is preferred that one of the components is added to an organic solvent and then the other component is added thereto in proper amount. If the reaction can be sufficiently controlled, both components may be added simultaneously. After the formation of the polyamide acid, if necessary, an imide ring opening reaction may further proceed under a severe condition of about 80°C or higher as described above and a desired partial imidation product can be obtained by measuring the acid value of the reaction product. By adjusting the molar ratio of both components to 1:1 in the synthetic reaction for the precursor, the product having the highest molecular weight can be obtained. Examples of the organic solvent used in the reaction are N,N-dimethylformamide, N,N-diethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, N-methylcaprolactam, dimethyl sulfoxide, tetramethylenesulfone, tetramethylurea, hexamethyl phosphamide, pyridine, quinoline, γ-butyrolactone, N-acetyl-2-pyrrolidone, phenols, cresols, nitro compounds, glycols, cellosolves, carbitols, etc. The amount of the organic solvent is from about 20 to 95% by weight of the reaction system and the amount thereof may be selected in the above-described range to provide a concentration having a suitable viscosity. In addition, in the case of obtaining a partial imidation product by sufficiently conducting the imidation in the above-described synthesis of the polyimide precursor, a solvent having a relatively high boiling point is selected to perform a desired imidation and, if necessary, the product can be reprecipitated in a poor solvent, dried, and redissolved in another solvent having a low boiling point. To a solution composed of the above-described polyimide precursor and the organic solvent, an epoxy compound having at least two epoxy groups in one molecule is added so that the epoxy group content is 0.3 equivalent or higher per 1 equivalent of the acid group (carboxy group) of the polyimide precursor. In the case of using such an epoxy compound, it is preferred that the tetracarboxylic dianhydride for obtaining the above-described polyimide precursor is an aliphatic tetracarboxylic dianhydride alone or a mixture of from about 10 to 90 mole% aliphatic tetracarboxylic dianhydride and from about 90 to 10 mole% aromatic tetracarboxylic dianhydride. When the fixing composition of this invention containing the epoxy compound is interposed between the metal powder molding and the metal base material and the sintering treatment is performed at a constant rate of increase of temperature, the solvent is evaporated off at an initial elevated temperature of usually from about 120 to 250°C and then a hardening reaction between the polyimide precursor and the epoxy compound occurs, to increase the adhesive force and assist the adhesion and fixing of the metal powder molding onto the substrate and thus peeling off or falling of the metal powder molding does not occur during the sintering treatment. Also, if a polyamide acid or polyamic acid-imide which is the precursor for a polyimide resin is used alone, it causes an intramolecular ring closing reaction (imidation reaction) at the initial stage of the sintering treatment and is hardened, and free water formed in this case may reduce the adhesive and fixing force, but the presence of the epoxy resin can avoid this. The hardening of the precursor causes cross-linking by reacting with the acid group (carboxy group) contained in the polyimide precursor in the initial stage of the sintering treatment, whereby the formation of free water can be minimized as low as possible. Moreover, the epoxy compound contributes to the improvement of the adhesive force in the case of temporarily adhering the metal powder molding to the base material and gives good results in the sintering operation. The fixing composition of this invention comprises the polyimide precursor, e.g. obtained by the manner described above, the organic solvent, and, the epoxy compound as described above. The composition may further contain, if necessary, an xylene resin, paraffin wax, a process oil or abietyl alcohol as a softener; calcium carbonate, silica or talc as a filler; and dioctyl phthalate, dioctyl adipate, triphenyl phosphate or dibutyl phthalate as a plasticizer. When the composition is interposed between a metal powder molding and a metal substrate and the sintering treatment is performed, the amount of the residual carbon after the completion of sintering is as high as 10 to 60% by weight, which increases the adhesion and fixing of the metal powder molding during high temperature heating and gives good results for restraining the occurrence of heat shrinkage of the metal powder molding. The method of using the fixing composition at sintering of this invention is now explained. Before a metal powder molding formed by rolling a mixture of a metal powder and a synthetic resin-based binder is placed on a metal substrate there is interposed the fixing composition of this invention, by coating at least one of the substrate and the molding with the composition. The thickness of the composition interposed between those two members is generally from about 5 to 50 µm, preferably from about 10 to 30 µm. If the thickness is too thin, the effect of this invention cannot be obtained, and if the thickness is too thick, the generation of gases at the interface between the base material and the molding becomes enlarged so as to reduce the adhesion during sintering or the bonding strength after sintering. The metal powder molding used in the above method is formed by rolling a mixture of a metal powder and a synthetic resin-based binder into a sheet-like material or the like; the thickness of the sheet-like molding is usually from about 0.5 to 5 mm. As the metal powder, various metal powders such as a self-melting alloy powder, abrasion resisting alloy powder, etc., can be used according to the desired property to be imparted to the surface of a metal base material. A representative example of the metal powder is an Fe-M-C system eutectic alloy powder which is an abrasion resisting alloy powder, wherein M contains at least one of Mo, B and P as the main component and may further contain a secondary element such as Cr, V, W. Nb, Ta and Ti. Also, M further contains Si, Ni or Mn as an other element. The sintering temperature of the eutectic alloy powder is relatively low, the liquid phase thereof becomes from about 10 to 50% by volume of the whole composition at the time of heating at a temperature range of from about 1,000 to 1,150°c and the liquid phase has a feature that the phase shows excellent wettability to the base material. In addition, it is preferred that the particle size of the alloy powder is generally about 50 Tyler mesh or less since the particle size of the powder influences the porosity after sintering. If the particle size is larger than the above value, it becomes difficult to form an alloy layer having a high density. As the synthetic resin-based binder to be mixed with the above-described metal powder, a synthetic resin having a pressure-sensitive adhesive property is preferred. In particular, an acrylic polymer or copolymer composed of a (meth)acrylic alkyl ester or composed of the ester and a monomer copolymerizable with the ester and an acrylic pressure-sensitive adhesive composition composed of the acrylic polymer or copolymer added with an adhesive property imparting resin such as an alkylphenolic resin, a rosin resin, a petroleum resin, a cumarone series resin, etc., are preferably used. The synthetic resin-based binder is diluted with a suitable organic solvent such as acetone, toluene or methyl ethyl ketone, etc., the solution is kneaded with from about 10 to 100 parts by weight of the metal powder per 1 part by weight of the solid component thereof, the kneaded mixture is poured onto a frame having thereon a mold release paper and after evaporating the solvent, the mixture is molded into a sheet-like form or the like by passing it through molding rolls to provide a desired metal powder molding. The metal powder molding is placed on a metal base material with the fixing composition of this invention interposed between them as described above and is sintered in a non-oxidizing atmosphere. In this case, the rate of increase in temperature may be constant and it is unnecessary to maintain the system for a definite period of time at a lower temperature before the sintering, as in said EP previous proposal. In the initial temperature raising stage, the solvent in the composition is first evaporated off, then the composition exerts a strong adhesive force due to the hardening reaction of the polyimide precursor and the polyimide precursor is then converted into a carbon precursor which effectively contributes to the adhesion, which then stably adheres and fixes the molding onto the substrate until the temperature of the system reaches the sintering temperature. The sintering treatment is performed in a non-oxidizing atmosphere since otherwise the metal powder molding is oxidized during the treatment and a desired metal layer cannot be formed. The non-oxidizing atmosphere can be a hydrogen or nitrogen gas atmosphere or a vacuum. When the assembly is maintained at a sintering temperature for a definite period of time, the carbon precursor is finally completely carbonized. In this case, the amount of residual carbon is from about 10 to 60%, preferably from 15 to 40% by weight. Also, similar materials in the metal powder molding are carbonized and disappear and, on the other hand, the metal component in the molding diffuses into the base material, whereby a metal layer having a good dimensional stability and having similar dimensions to the original dimensions and strongly bonded to the base material is formed. The invention is explained in more detail by reference to the following examples. Parts and percentages are by weight. EXAMPLE 1 In a 500 cc four-necked flask equipped with a thermometer, an N₂ gas inlet, a condenser having a trap, and a stirrer were placed 43.6 g (0.2 mole) of pyromellitic dianhydride and 380.8 g of N-methylpyrrolidone, and the mixture was stirred on an ice bath while passing therethrough dry N₂ gas. When the temperature of the contents reached about 5°C, 40.0 g (0.2 mole) of 4,4'-diaminodiphenyl ether was added thereto little by little while checking the extent of heat generation. It required about 30 minutes for the addition and the temperature of the reactants increased to 15°C. After the addition of the ether, the ice bath was removed and the reaction mixture was stirred for about 2 hours at room temperature to provide a viscous polyamide acid solution having an acid value of 0.870 milliequivalent/g as the fixing composition of this invention. The amount of residual carbon when the composition was subjected to a carbonization treatment under the same condition as the sintering treatment described hereinafter was 35%. Separately, after wet-kneading 48.5% of an eutectic alloy powder composed of 10.5% Mo, 2.5% Cr, 2.4% P, and 3.6% C, the rest being Fe and having a particle size size of 150 Tyler mesh or less; 48.5% of a SUS chrome stainless steel powder having a particle size of 150 Tyler mesh or less; and further 3% of a (meth)acrylic acid alkyl ester resin using acetone as a solvent, the kneaded mixture was rolled to form an alloy powder sheet having a thickness of 2 mm and a density of 4.8 g/cm³. The sheet thus prepared was cut into a size of 1 cm x 1 cm and after coating the sheet with the above-described fixing composition at a thickness of 30 µm, the sheet was stuck to a vertical surface of a steel base material. Thereafter, the temperature of the system was increased to 1,090°C at a rate of temperature increase of 15°C/min. in a hydrogen gas atmosphere and after maintaining the system at the temperature for 20 minutes, the system was gradually cooled. Thus, an abrasion-resisting alloy layer having a thickness of 1.25 to 1.3 mm, a hardness of 62 to 65 in HRC, and a density of 7.6 to 7.75 g/cm³ and strongly bonded to the steel base material could be formed without the occurrence of falling of the alloy powder sheet or shrinkage of the sheet in the plane directions during the sintering treatment. EXAMPLE 2 In the same type of flask as used in Example 1 were placed 39.6 g (0.2 mole) of 1,2,3,4-butanetetracarboxylic dianhydride and 185.7 g of N-methyl-2-pyrrolidone and then 40.0 g (0.2 mole) of 4,4'-diaminodiphenyl ether was added to the mixture while passing therethrough dry N₂ gas in the same manner as in Example 1. Thereafter, the ice bath was removed, the reaction mixture was stirred for about 2 hours at room temperature to provide a polyamic acid solution having an acid value of 1.532 milliequivalent/g as a fixing composition of this invention. The amount of residual carbon when the composition was subjected to a carbonization treatment under the same condition as the subsequent sintering treatment was 30%. Separately, after wet-kneading 58.8% of the eutectic alloy powder having the same composition as used in Example 1, 39.2% of a SUS 410 powder having a particle size of 150 Tyler mesh or less, and 2% of a (meth)acrylic acid alkyl ester resin using toluene as a solvent, the kneaded mixture was rolled to form an alloy powder sheet having a thickness of 1 mm and a density of 4.65 g/cm³. The sheet thus formed was cut into a size of 1 cm x 1 cm and after coating the sheet with the above-described fixing composition in a thickness of 20 µm, the sheet was stuck to a vertical surface of a steel base material. Thereafter, the temperature of the system was increased to 1,080°C at a rate of 20°C/min. in a hydrogen gas atmosphere and after maintaining the system at the temperature for 15 minutes, the system was gradually cooled. Thus, an abrasion-resisting alloy layer having a thickness of 0.65 to 0.7 mm, a hardness of 61 to 63 in HRC, and a density of 7.6 to 7.7 g/cm³ and strongly bonded to the steel base material could be formed without the occurrence of falling of the alloy powder sheet or shrinkage of the sheet in the plane directions during the sintering treatment. EXAMPLE 3 In the same type of the flask as used in Example 1 were placed 39.6 g (0.2 mole) of 1,2,3,4-butanetetracarboxylic dianhydride and 185.7 g of N,N-dimethylformamide, and the mixture was stirred on an ice bath while passing therethrough a dry N₂ gas. When the temperature of the content reached about 5°C, 40.0 g (0.2 mole) of 4,4'-diaminodiphenyl ether was added thereto little by little while checking the extent of heat generation. It required about 30 minutes for the addition and the temperature of the reactants increased to 15°C. Thereafter, the ice bath was removed and the flask was heated using a heater. The mixture was stirred for about 2 hours at 120°C to provide a polyamic acid solution having an acid value of 0.450 milliequivalent/g (residual percentage of acid value: 14.9%). A fixing composition of this invention was obtained by compounding 100 g of the solution thus formed with 8.5 g of a bisphenol A type liquid epoxy resin (epoxy equivalent: 184 to 194) as an epoxy compound. The amount of residual carbon when the composition was subjected to a carbonization treatment under the same condition as the subsequent sintering treatment was 22%. After wet-kneading 48.5% of the eutectic alloy powder having the same composition as the alloy powder used in Example 1, 48.5% of a SUS 410 powder having a particle size of 150 Tyler mesh or less, and 3% of a (meth)acrylic acid alkyl ester resin using acetone as a solvent, the kneaded mixture was rolled to form an alloy powder sheet having a thickness of 2 mm and a density of 4.8 g/cm³. The sheet was cut into a size of 1 cm x 1 cm, and after coating the sheet with the above-described fixing composition at a thickness of 20 µm, the sheet was stuck to a vertical surface of a steel base material Thereafter, the temperature of the system was increased to 1,090°C at a rate of 15°C/min. in a hydrogen gas atmosphere and after maintaining the system at that temperature for 20 minutes, the system was gradually cooled. Thus, an abrasion-resisting alloy powder layer having a thickness of 1.35 to 1.4 mm, a hardness of 62 to 65 in HRC, and a density of 7.6 to 7.75 g/cm³ and strongly bonded to the steel base material could be formed without troubles on the sheet during the treatment. EXAMPLE 4 In the same type of the flask as used in Example 1 were placed 39.6 g (0.2 mole) of 1,2,3,4-butanetetracarboxylic dianhydride and 85.7 g of N,N-dimethylformamide, and 39.6 g (0.2 mole) of 4,4'-diaminodiphenylmethane was then added thereto while passing therethrough dry N₂ gas by the same manner as Example 1. Thereafter, the ice bath was removed and the mixture was stirred for 2 hours at room temperature to provide a polyamic acid solution having an acid value of 1.534 milliequivalent/g. A fixing composition of this invention was obtained by compounding 100 g of the solution thus formed with 30 g of triglycidyl isocyanurate as an epoxy compound. The amount of residual carbon when the composition was subjected to a carbonization treatment under the same condition as the subsequent sintering treatment was 25%. After wet-kneading 38.6% of the eutectic alloy powder having the same condition as the alloy powder used in Example 1, 57.9% of a SUS 410 powder having a particle size of 150 Tyler mesh or less, and 3.5% of a (meth)acrylic acid alkyl ester resin using toluene as a solvent, the kneaded mixture was rolled to form an alloy powder sheet having a thickness of 1.5 mm and a density of 4.8 g/cm³. The sheet thus formed was cut into a size of 1 cm x 1 cm and after coating the sheet with the above-described fixing composition in a thickness of 20 µm, the sheet was stuck to a vertical surface of a steel base material. Thereafter, the temperature of the system was increased to 1,100°C at a rate of 10°C/min. and after maintaining the system at the temperature for 20 minutes, the system was gradually cooled. Thus, an abrasion-resisting alloy layer having a thickness of 0.9 to 0.95 mm, a hardness of 60 to 62 in HRC, and a density of 7.5 to 7.7 g/cm³ and strongly bonded to the steel base material could be formed without the occurrence of falling of the alloy powder sheet or its shrinkage in the plane directions during the sintering treatment. EXAMPLE 5 In the same type of the flask as used in Example 1 were placed 19.8 g (0.1 mole) of 1,2,3,4-butanetetracarboxylic dianhydride, 32.2 g (0.1 mole) of 3,3',4,4'-benzophenonetetracarboxylic dianhydride, and 368 g of N,N-dimethylformamide, and the mixture was stirred on an ice bath while passing therethrough a dry N₂ gas. When the temperature of the contents reached about 5°C, 40.0 g (0.2 mole) of 4,4'-diaminodiphenyl ether was added thereto little by little while checking the extent of heat generation. It required about 30 minutes for the addition and the temperature of the reactants was increased to 15°C. Thereafter, the ice bath was removed and the mixture was stirred for about 2 hours at room temperature to provide a viscous polyamic acid solution having an acid value of 0.88 milliequivalent/g. A fixing composition of this invention was obtained by compounding 100 g of the solution thus formed with 14.8 g of a bisphenol A type liquid epoxy resin (epoxy equivalent: 184 to 194) as an epoxy compound. The amount of residual carbon when the composition was subjected to a carbonization treatment under the same condition as the subsequent sintering treatment was 25%. After wet-kneading 58.8% of the eutectic alloy powder having the same composition as the alloy powder used in Example 1, 39.2 g of a SUS 410 powder having a particle size of 150 Tyler mesh or less, and 2% of a (meth)acrylic acid alkyl ester resin using acetone as a solvent, the kneaded mixture was rolled to form an alloy powder sheet having a thickness of 1 mm and a density of 4.65 g/cm³. The sheet thus formed was cut into a size of 1 cm x 1 cm and after coating the sheet with the above-described fixing composition in a thickness of 20 µm, the sheet was stuck to a vertical surface of a steel base material. Thereafter, the temperature of the system was increased to 1,080°C at a rate of 20°C/min. in a hydrogen gas atmosphere and after maintaining the system at the temperature for 15 minutes, the system was gradually cooled. Thus, an abrasion-resisting alloy layer having a thickness of 0.62 to 0.65 mm, a hardness of 61 to 63 in HRC, and a density of 7.6 to 7.7 g/cm³ and strongly bonded to the steel base material could be formed without the occurrence of falling of the alloy powder sheet or its shrinkage in the plane directions during the sintering treatment could be formed. EXAMPLE 6 In the same type of the flask as used in Example 1 were placed 19.8 g (0.1 mole) of 1,2,3,4-butanetetracarboxylic dianhydride, 32.2 g (0.1 mole) of 3,3',4,4'-benzophenonetetracarboxylic dianhydride, and 368 g of N,N-dimethylformamide, and 39.6 g (0.2 mole) of 4,4'-diaminodiphenylmethane was then added to the mixture while passing therethrough a dry N₂ gas by the same manner as in Example 1. Also, the flask was heated using a heater. After stirring the mixture for about 2 hours at 120°C, the mixture was cooled to provide a polyamic acid-imide solution in a slightly white turbid state. Then, a fixing composition of this invention was obtained by compounding 100 g of the solution thus formed with 20 g of triglycidyl isocyanurate. The amount of residual carbon when the composition was subjected to a carbonization treatment under the same condition as the subsequent sintering treatment was 28%. After wet-kneading 38.6% of the eutectic alloy powder having the same composition as the alloy powder used in Example 1, 57.9% of a SUS 410 powder having a particle size of 150 Tyler mesh or less and 3.5% of a (meth)acrylic acid alkyl ester resin using toluene as a solvent, the kneaded mixture was rolled to provide an alloy powder sheet having a thickness of 1.5 mm and a density of 4.8 g/cm³. The sheet thus formed was cut into a size of 1 cm x 1 cm and after coating the sheet with the above-described fixing composition in a thickness of 20 µm, the sheet was stuck to a vertical surface of a steel base material. Thereafter, the temperature of the system was increased to 1,100°C at a rate of 10°C/min. in a hydrogen gas atmosphere and after maintaining the system at the temperature for 20 minutes, the system was gradually cooled. Thus, an abrasion-resisting alloy layer having a thickness of 0.9 to 0.05 mm, a hardness of 60 to 62 in HRC, and a density of 7.5 to 7.7 g/cm³ and strongly bonded to the steel base material could be formed without the occurrence of falling of the alloy powder sheet or its shrinkage in the plane directions during the sintering treatment.

Claims: A composition for use in fixing a molding or sheet formed from a powdered metal alloy and a synthetic resin binder to a metal substrate, and which composition serves to adhere the sheet during non-oxidizing heating until the metal alloy is sintered to the substrate, which composition comprises a polyimide series liquid composition containing a polyimide precursor and an organic solvent and which also contains an epoxy compound having at least 2 epoxy groups per molecule in such a ratio that the proportion of the epoxy group is 0.3 equivalent or more per 1 equivalent of the carboxy acid group of the polyimide precursor. A composition as claimed in Claim 1, wherein the polyimide precursor is a polyamic acid synthesized from an aliphatic tetracarboxylic dianhydride or lower alkyl ester or halide thereof and an organic diamine optionally with a triamine or tetramine. A composition as claimed in Claim 2, wherein said tetracarboxylic dianhydride is composed of from 10 to 90 mole% of aliphatic tetracarboxylic dianhydride and from 90 to 10 mole% of aromatic tetracarboxylic dianhydride. A composition as claimed in Claim 2 or 3, wherein the polyimide precursor is a polyamic acid-imide formed by partially imidating said polyamic acid. A composition as claimed in any preceding claim, in the form of a sheet or layer 5 to 50 micrometers thick. A composition as claimed in any preceding claim, wherein the amount of organic solvent is 20 to 95% by weight. A method of sintering a metal alloy to a metal substrate, wherein the alloy is in the form of a molding or sheet formed from a mixture of the alloy and a synthetic resin binder, which comprises placing said sheet or molding on said substrate, interposing a composition as claimed in any preceding claim or a molded layer of the composition between the sheet and substrate, heating the assembly in a non-oxidizing atmosphere so as to form from the composition an adhesive, and continuing heating to the temperature at which the metal alloy is sintered to the substrate and at which the adhesive formed disappears. A method as claimed in Claim 7 wherein the alloy is a multi-component eutectic alloy powder containing iron, carbon, and at least one of Mo, B and P. A method as claimed in Claim 8, wherein the heating is at a substantially constant rate of temperature increase, up to 1,000 to 1,150°C.

Inventors: ASOSHINA, EISHI, KOJIMA, MAKOTO, SAKURAMOTO, TAKAHUMI, TOMINAGA, TAKASHI
Applicants: NITTO DENKO CORPORATION
Application year: 1985
Main classification: C09J179
IPC codes: C08L79