Adhesion method for rubber articles

Abstract: Two vulcanizates are adhered to each other by (I) applying a particular rubber cement composition based on natural rubber or isoprene rubber and including sulfur or an organic sulfur compound, an ultrarapid accelerator, an organic amine and carbon black, to surfaces of the vulcanizates to be adhered followed by drying; and (II) sandwiching an unvulcanized rubber sheet of a particular cushion rubber composition based on natural rubber or isoprene rubber and including sulfur, an organic amine and carbon black, between the applied surfaces of the vulcanizates and vulcanizing them at a temperature of 0-100°C under pressure.

Description: This invention relates to a method of adhering rubber articles, and more particularly to an adhesion method for rubber articles which is suitable for the adhesion of vulcanized rubber articles at a low temperature of not higher than 100°C, preferably not higher than 50°C, such as retreading adhesion of tires, repair of broken-off portions of tires, conveyor belts and the like, endless formation of conveyor belts and so on. At present, the retreading adhesion of tires is carried out by a precure system. That is, a rubber cement is applied to a surface of each of a base tire and a precure tread to be adhered, and a cushion rubber is sandwiched between these surfaces covered with the rubber cement, and then these members are bonded by vulcanization at a high temperature of not lower than 120°C, usually about 150°C. However, such a system has the following drawbracks: (1) It is necessary to conduct the vulcanization in an autoclave at a high temperature under a high pressure, so that it is required to use large-size equipment; (2) Since the vulcanized base tire and tread are again exposed to a high temperature exceeding 120°C, the properties and performances of these members may be degraded. Therefore, it is strongly demanded to develop a method of adhering through vulcanization at a temperature of not higher than 100°C, preferably not higher than 50°C. On the other hand, there has been known a method of adhering vulcanized rubber articles through vulcanization at a temperature of not higher than 100°C from for example Japanese Patent Application Publication No. 49-24972. In this method, a rubber composition obtained by compounding particular amounts of sulfur or an organic sulfur compound and aliphatic amine with an unvulcanized unsaturated rubber having a relatively small content of unsaturated double bond such as EPT or butyl rubber is dissolved in an organic solvent, which is applied to surfaces of vulcanized rubber articles to be adhered. Then, the applied surfaces after the drying are stuck to each other and left to stand at a temperature of, for example, 40°C. However, this method has also the following drawbacks: (1) Since the thickness of the adhesion rubber layer is too small, it is difficult to obtain a large peeling strength; (2) When using an adherend having many large and small unevennesses such as a retreading base tire, it is difficult to uniformly apply the adhesive solution, so that the true adhered portion is not a plane form but is apt to be rendered into dot-like form. Furthermore, a repair material for rubber tire under the trade name Tip-Top sold commercially by Stahl Gruber GMBH & Co. in Germany. However, this material has low peeling strength at a high temperature (100°C) and peeling strength after aging and is unsuitable for retreading adhesion of tires. It is, therefore, an aim of the present invention to provide an adhesion method in which vulcanized rubbers are adhered to each other at a low temperature of not higher than 100°C, preferably not higher than 50°C without requiring an investment in large size equipment such as autoclave vulcanization apparatus to thereby prevent the degradation of properties and performances in the vulcanized rubber due to the repetition of high temperature treatment. It is another aim of the invention to obtain large peeling strengths at room temperature and high temperature (100°C) and after aging which have not been previously achieved in the conventional low temperature adhesion method. According to the invention, there is provided a method of adhering vulcanized rubber articles, which comprises the steps of: (I) applying a rubber cement composition, which comprises 100 parts by weight of natural rubber (NR) or isoprene rubber (IR), 1-5 parts by weight of sulfur or an organic sulfur compound having two or more sulfur atoms, 1-15 parts by weight of a ultrarapid accelerator, 2-15 parts by weight of an organic amine having a base dissocation constant (pK b ) of not more than 5, 30-70 parts by weight of carbon black and suitable amounts of zinc white and organic solvent, to surfaces of the vulcanized rubbers to be adhered, followed by drying, wherein said ultrarapid accelerator is selected from piperidine salt of pentamethylene dithiocarbamic acid, pipecolate of pipecolydithio carbamic acid, dithiocarbamates such as zinc N-ethyl dithiocarbamate and zinc N-phenyl dithiocarbamate, xanthates such as zinc butylxanthate and zinc isopropylxanthate, and mixtures thereof; (II) sandwiching an unvulcanized rubber sheet of a cushion rubber composition, which comprises 100 parts by weight of natural rubber (NR) or isoprene rubber (IR), 1-5 parts by weight of sulfur, or an organic sulfur compound having two or more sulfur atoms 1-7 parts by weight of an organic amine having a base dissociation constant (pK b ) of not more than 5, 30-70 parts by weight of carbon black and a suitable amount of zinc white, between the applied surfaces of the vulcanized rubbers and vulcanizing them at a temperature of 0-100°C under pressure. Thus, according to the invention, the low temperature adhesion becomes possible and the thickness of the adhesion rubber layer is thick and the adhesion state is not dot-like but is plane, so that there can be obtained adhesion solid structures having large peeling strengths at room temperature and 100°C and after aging. Heretofore, rubber adhesion through vulcanization as in the retreading adhesion of tires has been attained by sandwiching an unvulcanized cushion rubber sheet between the vulcanized rubbers and vulcanizing them at a temperature of not lower than 120°C, usually about 150°C. In this case, sulfur and accelerator are coexistent in the cushion rubber sheet. As the accelerator is used an accelerator for high temperature vulcanization such as sulfeneamide series accelerator, but the use of ultrarapid accelerators is impossible because it causes rubber scorching. In the vulcanizing adhesion, it is necessary that sulfur (or sulfur containing vulcanization agent) and the accelerator contained in the cushion rubber sheet migrate into the vulcanized rubber. In this connection, since the vulcanization temperature is as high as not lower than 120°C in the above conventional system, micromolecular motion of unsaturated rubber molecule, sulfur (or sulfur containing vulcanization agent) and accelerator is sufficiently caused, so that the necessary migration is attained only by the heating. However, when the vulcanization temperature is not higher than 100°C, particularly about 50°C, the requirements generating the vulcanization may be satisfied by using a ultrarapid accelerator, but sufficient migration of the agent is not attained owing to the low temperature and consequently the desired vulcanizing adhesion could not be achieved. Now, it has been attempted according to a method described in Japanese Patent laid open No. 54-144485 that an unvulcanized rubber sheet containing a vulcanization agent and a rubber cement composition containing a ultrarapid accelerator are separately provided, and the rubber cement composition is applied to each surface of vulcanized rubber articles to be adhered and dried, and then the unvulcanized rubber sheet is sandwiched between the applied surfaces of these articles and vulcanized under pressure. In the latter method, however, satisfactory vulcanizing adhesion could not be obtained under conditions that the vulcanization temperature is not higher than 50°C and the vulcanization time is not more than 24 hours (see Comparative Example 1 as mentioned later). The inventors have made further studies and found that when a suitable organic amine is added to each of the above unvulcanized rubber sheet and the rubber cement composition, very satisfactory peeling strength can surprisingly be obtained even under low temperature vulcanization conditions as mentioned above. In the invention, the rubber cement composition takes a very important role that the ultrarapid accelerator is supplied into the unvulcanized rubber sheet by migration through the adhesion interface after the pressing to form an adhesion layer based on the uniting of the rubber cement composition itself with the unvulcanized rubber sheet and at the same time sulfur and the ultrarapid accelerator are further migrated into the vulcanized rubber portion from the adhesion layer. Particularly, the organic amine contained in the rubber cement composition not only serves the migration of sulfur or the organic sulfur compound containing two or more sulfur atoms and the ultrarapid accelerator but also promotes the vulcanization reaction inside the adhesion layer and at the interface thereof together with the organic amine contained in the unvulcanized rubber sheet. In order to prevent the scorching induced by the use of the ultrarapid accelerator during the storing of the rubber cement composition, such a rubber cement composition is usually divided into the following two liquids and stored, and such two liquids are mixed just before the use. That is, one of the two liquids (liquid A) contains sulfur or the organic sulfur compound having two or more sulfur atoms and the organic amine, while the other liquid (liquid B) contains the ultrarapid accelerator. Each of the liquids A and B further contains proper amounts of natural rubber (NR) or isoprene rubber (IR), carbon black, zinc white and, if necessary, various additives (stearic acid, anti-oxidant, plasticizer and the like). The concentration in each of the liquids A and B is not particularly critical, but it is usually about 5-30% by volume from the viewpoint of the application and drying. Furthermore, as the organic solvent, use may be made of any solvents capable of dissolving the above rubber, among which solvents having a relatively low boiling point are preferred. That is, halogenated hydrocarbons such as methylene chloride, chloroform, trichloroethylene and tetrachloroethane, aliphatic or alicyclic hydrocarbons or mixtures thereof such as pentane, hexane, cyclohexane, heptane, octane and rubber solvent are suitably used. The amount of the organic solvent used in the rubber cement composition is not particularly critical, but it is convenient to be not more than about 1000 parts by weight per 100 parts by weight of the rubber component. In the invention, the cushion rubber sheet and the applied rubber cement form an adhesion layer, so that the rubber thickness (i.e. rubber quantity) in the cushion rubber layer is considerably larger than that of the applied rubber cement layer. Therefore, the cushion rubber sheet takes a very important role to (1) ensure a tackiness in the processing and (2) mainly enhance the strength of the adhesion layer after the vulcanization. In the case, the thickness of the cushion rubber layer is preferably not more than 2.0 mm, particularly not more than 1.5 mm, in order to rapidly conduct the penetration and migration of the ultrarapid accelerator from the applied rubber cement layer as soon as possible. According to the invention, the use of natural rubber or isoprene rubber is suitable as a rubber component forming the adhesion layer from the viewpoint of excellent processability and good rubber properties through the ultrarapid accelerator. As sulfur or organic sulfur compound having two or more sulfur atoms in the molecule, mention may be made of elementary sulfur, and polysulfides such as tetramethylthiuram disulfide, tetramethylthiuram tetrasulfide, and disulfide of alkylphenol. They are used in an amount of 1-5 parts by weight per 100 parts by weight of the rubber component as a sulfur donor. When the amount is less than 1 part by weight, the vulcanization rate in the adhered portion is insufficient, while when it exceeds 5 parts by weight, the rubber properties of the adhesion layer lower. The ultrarapid accelerator is selected from piperidine salt of pentamethylene dithiocarbamic acid, pipecolate of pipecolyldithio carbamic acid, dithiocarbamate such as zinc N-ethyl dithiocarbamate and zinc N-phenyl dithiocarbamate, xanthates such as zinc butylxanthate and zinc isopropylxanthate, and mixtures thereof. The ultrarapid accelerator is used in an amount of 1-15 parts by weight per 100 parts by weight of the rubber component. When the amount is less than 1 part by weight, the vulcanization rate in the adhered portion and adhesion layer is insufficient, while when it exceeds 15 parts by weight, the rubber properties in the adhered portion and adhesion layer lower. As the organic amine used in the invention, mention may be made of amines having pK b ≦ 5 such as n-butylamine, cyclohexylamine, n-octylamine, di-n-butylamine, morpholine, N-ethyl cylohexylamine, triethylamine, tri-n-butylamine, and diazabicyclooctane. The effect aimed at by the invention is first developed by using such an amine. This effect cannot be obtained when using an amine of pK b > 5 such as aniline (pK b =9.40), N-methylaniline (pK b =9.15), dimethylaniline (pK b =8.85)or the like. The amount of the organic amine used per 100 parts by weight of the rubber component is 2-15 parts by weight in case of the rubber cement composition and 1-7 parts by weight in case of the cushion rubber sheet. In the rubber cement composition, when the amount is less than 2 parts by weight, the vulcanization rate in the adhered portion and adhesion layer is insufficient, while when it exceeds 15 parts by weight, the rubber properties in the adhered portion and adhesion layer lower. In case of the cushion rubber sheet, when the amount is less than 1 part by weight, the vulcanization rate in the adhesion layer lowers, while when it exceeds 7 parts by weight, the rubber properties in the adhered portion and adhesion layer lower. The carbon black should be selected in accordance with the use purpose of the rubber articles as an adherend. In general, the use of carbon black having a low pH delaying the vulcanization rate is avoided, and the use of carbon blacks having pH of about 6-8 is preferred. The amount of carbon black used is 30-70 parts by weight considering the properties after the vulcanization such as strength of adhesion layer. Furthermore, zinc white is compounded in a suitable amount, usually not more than 10 parts by weight per 100 parts by weight of the rubber component. If necessary, not more than 10 parts by weight of stearic acid, not more than 100 parts by weight of process oil for general rubber industry and a softening agent may be included per 100 parts by weight of the rubber component. The following examples are given in illustration of the invention and are not intended as limitations thereof. Example 1 There were provided liquid A, liquid B and cushion rubber sheet (thickness: 0.5 mm) according to a compounding recipe shown in the following Table 1. The liquids A and B were mixed at an equal volume before use, and applied onto each surface of two carbon-reinforced and vulcanized natural rubber sheets (thickness: 2 mm) to be adhered and then dried. Next, the unvulcanized cushion rubber sheet was sandwiched between the two applied surfaces, which was vulcanized at 50°C under a surface pressure of 3.3 kgf/cm² for 24 hours. The thus obtained test sample exhibited the following adhesion performances by a peeling test. Moreover, heat aging was carried out in an atmosphere of N₂ gas at 100°C for 48 hours. In any case, the test sample shows excellent adhesion performances owing to rubber fracture. Example 2 A test sample was prepared by the same method as in Example 1 except that triethylamine was used instead of N-ethyl cyclohexylamine in the liquid A and the cushion rubber sheet. In this case, the peeling strength at 100°C was 78 kg/2.54 cm and rubber fracture was caused. Comparative Example 1 The same procedure as in Example 1 was repeated without the addition of the organic amine to prepare a test sample. In this case, the peeling strength at 25°C was 5 kg/2.54 cm and the peeling strength at 100°C was 1 kg/2.54 cm and interface fracture was caused. Comparative Example 2 There was provided an unvulcanized rubber sheet (thickness: 0.5 mm) according to the conventional rubber compounding recipe shown in the following Table 2. This sheet was sandwiched between two vulcanized rubber sheets and heated under pressure according to Example 1 to prepare a test sample. As a result of the peeling test, the vulcanized rubber sheets were not adhered in this case. Comparative Example 3 There was provided an adhesion liquid having a compounding recipe shown in the following Table 3 according to Example of Japanese Patent Application Publication No. 49-24972. This adhesion liquid was applied to the two vulcanized rubber sheets and dried, and then the two applied surfaces were placed one upon the other and heated under pressure according to Example 1 to prepare a test sample. As a result of the peeling test, the two sheets were not adhered. Comparative Example 4 As a commercially available adhesive of room temperature type for vulcanized rubber, there was used that sold under the trade name Tip-Top (made by Stahl Gruber GMBH & Co.). This adhesive consisted of a rubber cement solution and a cushion rubber sheet. After the rubber cement solution was applied to the two vulcanized rubber sheets and dried according to Example 1, the cushion rubber sheet was sandwiched between the two applied surfaces and heated under pressure (30°C × 24 hours) to prepare a test sample. The peeling test was made with respect to this sample to obtain the following results. Moreover, heat aging was carried out in an atmosphere of N₂ gas at 100°C for 48 hours. In any case, the test sample did not undergo rubber fracture but interface fracture was caused, which shows that the method according to the invention is excellent. As seen from the above examples and comparative examples, in the adhesion method according to the invention, the adhesion between vulcanized rubbers at a temperature of not higher than 100°C, usually not higher than 50°C, can be carried out by practicable change to the material as compared with the conventional high temperature vulcanization system used in the retreading adhesion of tires, whereby large peeling strengths at room temperature and high temperature (100°C) and good peeling strength after heat aging, which have never been achieved in the conventional low temperature adhesion method, can be obtained. As a result, according to the invention, the investment in large size equipment such as autoclave vulcanization apparatus is not required for the vulcanization, and also there can be reduced or prevented fear of degrading properties and performances of vulcanized rubbers adhered due to exposure to high temperature. Therefore, the adhesion method according to the invention imparts an improved effect in the retreading adhesion of tires, the repairing of broken-off portions of tires, conveyor belts or the like, the endless formation of conveyor belts and so on as compared with the conventional methods.

Claims: A method of adhering vulcanized rubber articles, which comprises the steps of: (I) applying a rubber cement composition, which comprises 100 parts by weight of natural rubber or isoprene rubber, 1-5 parts by weight of sulfur or an organic sulfur compound having two or more sulfur atoms, 1-15 parts by weight of an ultrarapid accelerator, 2-15 parts by weight of an organic amine having a base dissociation constant of not more than 5, 30-70 parts by weight of carbon black and suitable amounts of zinc white and organic solvent, to surfaces of the vulcanized rubbers to be adhered, followed by drying, wherein said ultrarapid accelerator is selected from piperidine salt of pentamethylene dithiocarbamic acid, pipecolate of pipecolydithio carbamic acid, dithiocarbamates such as zinc N-ethyl dithiocarbamate and zinc N-phenyl dithiocarbamate, xanthates such as zinc butylxanthate and zinc isopropylxanthate, and mixtures thereof; (II) sandwiching an unvulcanized rubber sheet of a cushion rubber composition, which comprises 100 parts by weight of natural rubber or isoprene rubber, 1-5 parts by weight of sulfur or an organic sulfur compound having two or more sulfur atoms, 1-7 parts by weight of an organic amine having a base dissociation constant of not more than 5, 30-70 parts by weight of carbon black and a suitable amount of zinc white, between the applied surfaces of the vulcanized rubbers and vulcanizing them at a temperature of 0-100°C under pressure. A method as claimed in claim 1, characterized in that said rubber cement composition is divided into two separate liquids and stored, and said two liquids are admixed before use to form the cement, wherein one of the said two liquids contains said sulfur or organic sulfur compound and said organic amine and the other liquid contains said ultrarapid accelerator. A method as claimed in claim 1 or 2, characterized in that said organic sulfur compound is selected from tetramethylthiuram disulfide, tetramethylthiuram tetrasulfide and disulfide of alkylphenol. A method as claimed in any of claims 1 to 3, characterized in that said organic amine is selected from n-butylamine, cyclohexylamine, n-octylamine, di-n-butylamine, morpholine, N-ethyl cyclohexylamine, triethylamine, tri-n-butylamine and diazabicyclooctane.

Inventors: FUJIO RYOTA, YAMADA TOMOHARU, FUJIO, RYOTA, YAMADA, TOMOHARU
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1990
Main classification: C08J5
IPC codes: C09J5, B32B25, B29C65, C08J5, C09J121, B29K9