PRODUCTION METHOD OF RUBBER TUBE AND RESIN COATING APPARATUS USED THEREFOR

Abstract: In the mfr. of a rubber tube having a smooth outer surface a synthetic resin under a molten state is heated from an inner peripheral side immediately before the synthetic resin is caused to flow out along the outer periphery of a rubber material layer from a die.

Description: This invention relates to a process for producing a rubber tube having a smooth outside surface, and to a resin coating apparatus for use in practising the process. To produce a rubber tube including a reinforcing material and having a smooth outside surface, it is the conventional practice to suppress the foaming of a vulcanizable rubber material on the outside surface of the tube during vulcanization of the rubber material using a pressurizable vulcanization device and thereby applying pressure to the peripheral surface of the tube, or by coating the periphery of the tube with a synthetic resin or lead. In the former method foaming of the outside surface of the tube can be limited, but it is still impossible to achieve a smooth outside of the tube. Moreover, since it is necessary to seal up the inside of the vulcanization device consecutively, this method is unsuitable for mass-production. When coating the periphery of the tube with lead so as to smoothen the outside surface of the tube, it is difficult to keep lead under proper care and storage, and expenditures for facilities and operations increase, the cost of the product is likely to increase. A method comprising coating the periphery of the tube with a synthetic resin, and then vulcanizing the rubber material constituting the rubber tube was regarded as promising. JP-A-290,011/1986 describes a technique in which the periphery of a rubber tube is coated with a cover of a thermally resistant resin having a higher melting point than the vulcanization temperature of the rubber material, and then vulcanizing the rubber material by using, for example, microwaves. In this method an extrusion die for a resin is used as a resin coating device 2 as shown in Figure 3, and a synthetic resin 6 is extruded along the periphery of a rubber material layer coated on the periphery of a mandrel to form a coating layer 8 continuously on the periphery of the rubber material layer 4. The rubber material layer 4 coated with the synthetic resin layer is then heated to vulcanize the rubber material. The synthetic resin 6 constituting the resin coated layer 8 must therefore have a higher melting point than the vulcanization temperature of the rubber material. Otherwise, during vulcanization the resin coated layer 8 would melt and the peripheral surface of the rubber material layer 4 would be destroyed. Accordingly, the molten synthetic resin 6 flowing through the resin coating device 2 is at a temperature above the vulcanization temperature of the rubber material. In JP-A-290011/1986, a heater 10 is mounted on the outside of the device 2 so as to heat the entire device. Furthermore, the rubber material layer 4 coated on the periphery of the mandrel, before being fed into the device 2, is heated by a preheater 12 so that the periphery of the rubber material layer 4 is covered closely with the resin coated layer 8. However, as shown in Figure 3 the device 2 is heated from outside so that the synthetic resin may remain flowable, and the rubber material layer 4 is heated with the pre-heater 12. Hence, during passage through the device 2, volatile components contained in the rubber material layer 4 tend to foam, and thus the peripheral surface of the rubber material layer 4 is likely to be coated with the resin coated layer 8 while it is roughened. Accordingly, even if the surface of the rubber material layer 4 is covered closely with the resin coated layer 8, a rubber tube having a smooth peripheral surface is unlikely to be obtained. The present invention provides a process for producing a rubber tube having a smooth peripheral surface efficiently and a resin coating device for use in such a process. Accordingly, the present invention provides a process for producing a rubber tube having a smooth outer surface which comprises forming a layer on the periphery of a mandrel of a rubber material mainly comprising an unvulcanized rubber material, forming on the periphery of the rubber material layer a layer of a thermally resistant synthetic resin having a higher melting point than the vulcanization temperature of the unvulcanized rubber material of the rubber material layer, heating the rubber material layer to vulcanize the rubber material, peeling and removing the layer of thermally resistant resin, and then removing the mandrel, characterized in that the layer of thermally resistant resin on the periphery of the rubber material layer is formed by heating the thermally resistant resin in a molten state from the inner circumferential side just before it flows out from a nozzle along the periphery of the layer of the rubber material. The resin coating device in accordance with this invention comprises an insertion hole for the insertion therethrough of a mandrel coated on its periphery with a layer of a rubber material mainly comprising an unvulcanized rubber material, a resin flow passage for the flow therethrough of a molten thermally resistant synthetic resin having a higher melting point than the vulcanization temperature of the unvulcanized rubber material, an annular nozzle for enabling the synthetic resin flowing through the resin flow passage to advance along the periphery of the rubber material layer coated on a mandrel inserted through said insertion hole, and heating means for heating the synthetic resin in the resin flow passage from the inner circumferential side before it flows out of the nozzle. In the process of the invention, the molten synthetic resin is heated from the inner circumferential side just before it flows from a nozzle to coat the rubber material layer. Accordingly, a layer of synthetic resin can be formed closely on the peripheral surface of the rubber material layer without heating the entire device and without pre-heating the rubber material layer. The volatile components in the rubber material layer are not foamed before coating with the synthetic resin, and no roughening of the peripheral surface of the rubber material layer occurs. Therefore, during vulcanization foaming on the peripheral surface of the rubber material layer is suppressed to give a smooth peripheral surface because the peripheral surface of the rubber material layer is closely covered with the layer of synthetic resin. Fig. 1 is a sectional view of the essential parts of a resin coating device in accordance with one embodiment of the invention; Fig. 2 is a sectional view taken along line II-II of Fig. 1; and Fig. 3 shows a sectional view of a conventional resin coating device. 2, 20.. resin coating device; 4...rubber material layer; 6...synthetic resin; 8...resin coated layer; 22...resin flow passage; 24...mandrel; 26...insertion hole; 30...cooling tube; 34...heater. Before describing the process for producing a rubber tube in accordance with this invention, the resin coating device used directly in the practice of the process will be described. As shown in Fig. 1, the resin coating device 20 in accordance with one embodiment of the invention is an improved version of a resin extrusion die. The upstream end portion of a resin flow passage 22 through which a molten synthetic resin 6 passes is connected to an extruder. Various known extruders can be used as the extruder for this purpose. Examples are a continuous extruder operable by screw rotation, an injection extruder based on an in-line screw and an extruder based on an accumulator method. The resin coating device 22 includes an insertion hole 26 through which a mandrel 24 coated on its periphery with a rubber material layer 4 containing mainly an unvulcanized rubber material is inserted. A recess 28 is formed on the inlet 26a side of the insertion hole 26. Preferably, a cooling tube 30 is received therein. For example, cooling water is circulated within the cooling tube 30 so as to cool the rubber material layer 4 on the periphery of the mandrel 24. To make temperature adjustment by the cooling tube 30 easy, the inside diameter of the recess 28 is maximized. A resin flow passage 22 formed within the device 20 communicates with an annular nozzle 32 formed nearly concentrically with the outlet 26b of the insertion hole 2. Hence, the synthetic resin in the molten state is extruded from the nozzle 32 and flows in tubular form and closely covers the periphery of the rubber material layer 4. Particularly, in the resin coating device 20 in accordance with this invention, a heater 34 as heating means is disposed immediately before the nozzle 32 between the resin flow passage 6 and the insertion hole 26 in order to heat the synthetic resin 6 in the resin flow passage before it flows out from the nozzle 32. The heater 34 may be, for example, an electric resistance heating device or an electromagnetic induction heating device. The heating temperature of the heater 34 can be variously controlled according to the type, for example, of the synthetic resin 6. The synthetic resin to be extruded from the nozzle 32 may be any thermally resistant resin having a higher melting point than the vulcanization temperature of the rubber material in the rubber material layer 4, and examples are poly(4-methyl-1-pentene), nylon, polyetherimides and polysulfones. From the viewpoint of mold releasability and thermal resistance, poly(4-methyl-1-pentene) is most desirable. When poly(4-methyl-1-pentene) is used as the synthetic resin 6, the heating temperature of the heater 34 is preferably 200 to 350 o C. In the embodiment shown in Fig. 1, no heater for heating the device 20 exteriorly is shown. As required, such a heater may be provided. Even when a heater is provided exteriorly of the device 20, heating of the entire device 20 as in the prior art is not necessary since the device includes the heater 34 in accordance with this invention. The process for producing a tube in accordance with this invention will be described on the basis of the embodiment shown in the drawings. In the process of this invention, the rubber material layer 4 which contains mainly an unvulcanized rubber material is first coated on the periphery of the mandrel 24. As required, a reinforcing material 36 may be included in the rubber material layer 4. Natural or synthetic rubbers may be used as the rubber material constituting the rubber material layer. The reinforcing material 36 may be, for example, metallic wires or synthetic fibers. Generally, the mandrel 24 is a jig for forming a hollow body, which is adapted to be pulled out in a subsequent step. In the present invention, the outside diameter of the mandrel 24 corresponds to the inside diameter of the rubber tube to be obtained. Coating of the rubber material layer 4 on the periphery may be carried out by known methods, for example, by extruding the rubber material along the periphery of the mandrel 24. In the next step of the process of this invention, the mandrel 24 coated with the rubber material layer 4 is inserted through the insertion hole 26 in the resin coating device 20 shown in Fig. 1 from the inlet 26a. When the mandrel 24 has jutted out from the outlet 26b or immediately before it, the synthetic resin 6 is extruded from the nozzle 32 to coat the periphery of the rubber material layer 4 with the resin coated layer 8. Then, the mandrel 24 coated with the rubber material layer 4 and the resin coated layer 8 is entirely heated to vulcanize the rubber material in the rubber material layer 4. The heating temperature is preferably above the vulcanization temperature of the rubber material and below the melting point of the synthetic resin constituting the resin coated layer 8. This is for the purpose of preventing softening of the resin coated layer 8. Various known means such as microwave heating and heating by a vulcanization vessel may be employed as heating means. Thereafter, the resin coated layer 8 is peeled and removed and the mandrel 24 is pulled out to give the desired rubber tube. It should be understood that the term "rubber tube", as used in the present specification is used in a broad sense including rubber tubes and rubber hoses. According to the process of this invention, the molten synthetic resin 6 is heated from the inner circumferential side just before it flows out from the nozzle 32 along the periphery of the rubbery material layer 4. Accordingly, the resin coated layer 8 can be formed closely on the peripheral surface of the rubber material layer 4 without so much heating the entire device 20 and without pre-heating the rubber material layer 4. Furthermore, before the coating of the synthetic resin, volatile components in the rubbery material layer 4 are not foamed to roughen the peripheral surface of the rubber material layer. Therefore, when the rubber material in the rubber material layer 4 is vulcanized in a subsequent step, foaming on the peripheral surface of the rubber material layer 4 is suppressed to give a smooth peripheral surface because the peripheral surface of the rubber material layer 4 is closely covered with the resin coated layer 8. The following examples illustrate the present invention. It should be noted that the invention is not limited to these examples. Example 1 Poly(4-mehyl-1-pentene) (to be referred to as PMP) having an MFR of 26 g/10 min. (tradename, TPX R MX004, a product of Mitsui Petrochemical Industries, Ltd.) was coated on a nitrile-type rubber tube by using the device shown in Fig. 1. The molding temperature conditions were as follows:- Cylinder of an extruder: 240-250 o C Prescribed temperature of heater (reference numeral 34): about 280 o C Compressed air (or water) was passed through cooling tube (reference numeral 30) to cool the raw rubber. After the resin coating, the coated layer was rapidly cooled in a cooling water tank, and vulcanization was carried out in a vulcanization device (steam autoclave) or a high-frequency vulcanization device. The resin layer was peeled, and the appearance of the rubber hose was examined. The results are shown in Table 1 below. The surface foaming and surface luster of the hose were evaluated on the scale of 5 grades as follows:- Surface foaming 5: none 4: almost none, but foaming occurred at one or two sites per 10 m (no problem in practical use). 3: foaming occurred 2 or 3 sites per 1-2 m. 2: on a better level than 1, but many foamings occurred. 1: many foamings occurred. Surface luster 5: very good 4: good but inferior to 5. 3: melt fracture began to occur, but the product was usable. 2: melt fracture occurred slightly vigrously. 1: melt fracture occurred vigorously. Example 2 PMP used in Example 1 was coated on a styrene-type rubber tube. Otherwise, the molding conditions, etc. were the same as in Example 1. The appearance of the rubber hose was examined. The results are shown in Table 1 below. Comparative Example 1 Example 1 was repeated except that no heater was provided, and no cooling of the raw rubber was carried out. The results are shown in Table 1. UTILIZABILITY IN INDUSTRY As described hereinabove, according to the process for producing a rubber tube and the resin coating device used therefor in accordance with this invention, a molten synthetic resin is heated from the inner circumferential side just before it flows out from the nozzle in the formation of a resin coated layer on the periphery of the rubber material layer. Therefore, the resin coated layer can be formed closely on the peripheral surface of the rubber material layer without so much heating the entire device and without preheating the rubber material layer. Moreover, before the coating of the synthetic resin, volatile components in the rubber material layer are not foamed to roughen the peripheral surface of the rubber material layer. Since the resin coated layer is closely coated on the peripheral surface of the rubber material, foaming on the peripheral surface is suppressed at the time of vulcanizing the rubber material in the rubber material layer in a subsequent step, and a rubber tube having a smooth peripheral surface can be produced efficiently.

Claims: A process for producing a rubber tube having a smooth outer surface which comprises forming a layer on the periphery of a mandrel of a rubber material mainly comprising an unvulcanized rubber material, forming on the periphery of the rubber material layer a layer of a thermally resistant synthetic resin having a higher melting point than the vulcanization temperature of the unvulcanized rubber material of the rubber material layer, heating the rubber material layer to vulcanize the rubber material, peeling and removing the layer of thermally resistant resin, and then removing the mandrel, characterized in that the layer of thermally resistant resin on the periphery of the rubber material layer is formed by heating the thermally resistant resin in a molten state from the inner circumferential side just before it flows out from a nozzle along the periphery of the layer of the rubber material. A process according to claim 1 in which the thermally resistant resin is poly(4-methyl-1-pentene). A resin coating device (2) for producing a rubber tube having a smooth outer surface comprising an insertion hole (26) for the insertion therethrough of a mandrel coated on its periphery with a layer of a rubber material mainly comprising an unvulcanized rubber material (4), a resin flow passage (22) for the flow therethrough of a molten thermally resistant synthetic resin having a higher melting point than the vulcanization temperature of the unvulcanized rubber material, an annular nozzle (32) for enabling the synthetic resin flowing through the resin flow passage (22) to advance along the periphery of the rubber material layer coated on a mandrel (4) inserted through said insertion hole (26), and heating means (34) for heating the synthetic resin in the resin flow passage (22) from the inner circumferential side before it flows out of the nozzle (32). A device according to claim 3 which further comprises cooling means (30) for cooling the layer of rubber material (4) before the synthetic resin is coated on its periphery.

Inventors: SHIGEMOTO HIROMI MITSUI PETROC, YAMAMOTO AKIO C MITSUI PETROCH, SHIGEMOTO, HIROMI, MITSUI PETROCHEMICAL IND. LTD., YAMAMOTO, AKIO, C/0 MITSUI PETROCHEMICAL IND. LTD.
Applicants: MITSUI PETROCHEMICAL IND, MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1989
Main classification: B29D23
IPC codes: B29C47, B29D23, B29C35, B29L23, B29K21