GB2256615A

GB2256615A - Producing a rubber/rubber bonded composite sheet structure

Info

Publication number: GB2256615A
Application number: GB9212457A
Authority: GB
Inventors: Hiroshi Hirakawa
Original assignee: Yokohama Rubber Co Ltd
Current assignee: Yokohama Rubber Co Ltd
Priority date: 1991-06-12
Filing date: 1992-06-11
Publication date: 1992-12-16
Anticipated expiration: 2012-06-11
Also published as: GB9212457D0; GB2256615B; DE4219314A1

Description

- 1 PROCESS FOR THE PRODUCTION OF A RUBBER/RUBBER BONDED COMPOSITE SHEET

STRUCTURE This invention relates to a process for the production of a rubber/rubber bonded composite sheet structure.

It is well known and conventional to laminate a plurality of unvulcanized rubber sheets of different rubber compositions one upon another and vulcanize the laminated sheets to bond into a rubber/rubber composite material for use in tires, conveyor belts, hoses, linings and the like. Inasmuch as this conventional process is largely dependent upon the benefit of covulcanizability of the rubber sheets used, difficulties have been experienced in achieving a firm adhesion between the rubber laminated layers in cases where individual rubbers are incompatible, or vulcanized at rates varying over a wide range, or crosslinked in different forms. A further difficulty has been encountered with the use of certain additives blended in the rubber sheets where the latter are relatively well compatible, in that such additives end to migrate upon vulcanization into and out of adjacent rubber sheets, resulting in weakly bonded sheet structure.

It was also. difficult with known processes to provide a firmly bonded, highly integrated rubber/rubber composite structure involving vulcanized rubber layer(s) - 2 laminated over unvulcanized rubber layer(s). Attempts to improve the interlayer bondage of such rubber composite structure by using bonding or adhesive agents would only lead to reduced flexibility of the resultant product.

With the foregoing difficulties of the prior art in view, the present invention seeks to provide a process for the production of a rubber/rubber bonded composite sheet structure which is comprised of a plurality of rubber layers laminated and bonded together and which is highly integrated, desirably flexible and free from migration of additives to and from adjoining rubber layers.

The above and other objects and features of the invention can be achi(.ved by a process which comprises laminating at least two rubber layers formed from a rubber composition containing rubber components having a critical surface tension in the range of 25 - 35 milliNewtons per meter, interposing between the laminated rubber layers an adhesive layer comprising a particulate form or a skived film of ultra high molecular weight (UHMW) polyethylene, and heating the laminated layers at a temperature above the melting point of the ultra high molecular weight polYethylene to fuse integrally into a rubber/rubber bonded coitposite sheet structure.

It has now been found that UHMW polyethylen-e used il as an adhesive layer has many excellent characteristic properties to ensure firm mutual adhesion between rubber sheets, such properties including high crystallinity, low gas permeability, high moisture resistance, high resistance to chemicals, high tensile strength and modulus, and inter alia high compatibility with rubber It has now also been found that the ultra high molecular weight polyethylene adhesive layer serves as a barrier to prevent additives (such as a vulcanization agent, typically sulfur, antioxidant, vulcanization accelerator and the like blended in the starting rubber material) migrating from one rubber layer to another which would otherwise occur when the laminated rubber layers are heated at increased temperature such as 1250 - 3001C.

It has now further been found that the presence of the ultra high molecular weight polyethylene adhesive layer or film about 10 - 200 microns thick further serves to provide reinforcement for the rubber sheet structure without substantially impairing the flexibility and resiliency desired of rubber sheet products.

Rubber components used in the invention having a critical surface tension (yc) in the range of 25 - 35 milliNewtons per meter (mN/m) as exemplified in the "Handbook of Elastomers New Development and Technology" authored by A. K. Bhowmik, et al and published by Marcel Dekker Inc., 1988, include isobutylene-isoprene copolymer rubber (IIR, Yc = 27 mN/m), ethylene-propylene-diene terpolymer rubber (EPDM, Yc 28 mN/m), isoprene (natural) rubber (NR, Yc = 31 mN/m), and styrene-butadiene copolymer rubber (SBR, Yc = 33 mN/m).

The critical surface tension referred to herein is determined by measuring the respective contact angles (e) of liquids, usually liquid hydrocarbons, differing in surface tension, assumed with respect to a solid surface and plotting the contact angles cosO as an ordinate against the surface tensions Y of the respective liquids as an abscissa in rectangular coordinates thereby obtaining a linear curve in which the abscissa value of cose = 1 represents a critical surface tension Yc of the solid. other rubber components than those listed above are eligible for the purpose of the invention if they exhibit their critical surface tension values determined by this procedure to be in the range of 25 - 35 mN/m.

The term skived film as used herein designates a film formed by sintering particles of a ultra high molecular weight polyethylene (hereinafter referred to simply as UHMW polyethylene) having a molecular weight in excess of 1,000,000 into a cylindrical rod and skiving the rod circumferentially to give a thin film of 10 - 200 microns.

- 5 The term particulate or particles of ultra high molecular weight (UHMW) polyethylene as used herein designates a particulate form of polyethylene having an average particle size of 50 - 300 microns.

When both rubber layers or sheets have flat and smooth confronting surfaces, the UHMW polyethylene material to be interposed therebetween may be in the form of either a film or powder. If one of the confronting surfaces of the rubber sheets is coarse or otherwise irregular, it is preferred to use UHMW polyethylene in its particulate form, in which instance the particles may be simply sprayed over the rubber sheet but it is preferred to use the polyethylene material in the form of a paste prepared with a suitable liquid vehicle such as fluid paraffins to ensure uniform deposit on the rubber sheet.

A plurality of rubber sheets laminated with UHMW polyethylene interposed therebetween may be fused at 1250 - 3000C into an integral sheet structure. Fusing temperatures exceeding 3001C must be avoided to eliminate the tendency of UHMW polyethylene to decompose or deteriorate as this polyethylene used in the invention melts at about 1250C.

According to a first embodiment of the present invention, there is provided a process which comprises (a) laminating at least two unvulcanized rubber layers formed from a rubber composition containing rubber components of the same species having a critical surface tension in the range of 25 - 35 mN/m, (b) interposing between the laminated rubber layers an adhesive layer comprising a particulate form or a skived film of ultra high molecular weight polyethylene, and (c) heating the resulting laminated layers at 1250 - 3000C to fuse integrally into a rubber/rubber bonded composite sheet structure. The unvulcanized rubber layers undergo vulcanization during the step (c) of the process.

The term same species referred to herein is used to define that a rubber component in one rubber layer or sheet contains for example IIR so does the other mating rubber layer and that a matrix of for example NR/SBR blend in one rubber layer is NR so is the same matrix in the other mating layer, in which instance the matrix component, i.e. NR is preferably more than 50 percent by weight of a total amount of the blend.

According to a second embodiment of the invention, there is provided a process which comprises (a) laminating at least two rubber layers, one of which is vulcanized and the other unvulcanized, formed from a rubber composition containing rubber components of the same species having a critical surface tension in the range of 25 - 32 mN/m, (b) interposing between the laminated rubber layers an adhesive layer comprising a particulate form or a skived film of ultra high molecular weight polyethylene, and (c) heating the 1 7 resulting laminated layers at 125 300'C to fuse integrally into a rubber/rubber bonded composite sheet structure. The unvulcanized rubber layers undergo vulcanization during the step (c) of the process.

Since vulcanization gives rise to the critical surface tension yc of the rubber component, the value Yc of that in the vulcanized rubber layer should be held within the threshold of 32 mN/m, failing which would result in too great differential in yc between vulcanized rubber and UHMW polyethylene and hence in poor mutual adhesion.

According to a third embodiment of the invention, there is provided a process which comprises (a) laminating at least two rubber layers formed from a rubber composition containing rubber components of different species having a critical surface tension in the range of 25 - 30 mN/m, (b) interposing between the laminated rubber layers an adhesive layer comprising a particulate form or a skived film of ultra high molecular weight polyethylene, and (c) heating the resulting laminated layers at 1250 - 3000C to fuse integrally into a rubber/rubber bonded composite sheet structure.

The rubber layers in this embodiment may be unvulcanized or vulcanized. The adhesive layer should be preferably 50 - 200 microns thick. With the use of different rubber componentsf there is a likelihood that additives blended in one of the respective laminated rubber layers will migrate, upon heating at 125 3000C, into the other rubber layer and adversely affect the properties of the latter, resulting in poorly bonded sheet structure.

According to a fourth embodiment of the invention, there is provided a process which comprises (a) laminating at least one unvulcanized rubber layer formed from a rubber composition containing a rubber component having a critical surface tension in the range of 30 35 mN/m with at least one vulcanized or unvulcanized rubber layer formed from a rubber composition containing a rubber component having a critical surface tension in the range of 25 - 30 mN/m, (b) interposing between the laminated rubber layers an adhesive layer comprising a particulate form or a skived film of ultra high molecular weight polyethylene, and (c) heating the resulting laminated layers at 125 - 3001C to fuse integrally into a rubber/rubber bonded composite sheet structure. The rubber layers or sheets used in this embodiment are comprised of rubber components of different species, e.g. SBR vs. EPDM.

According to a fifth embodiment of the invention, there is provided a process which comprises (a) thermally applying an adhesive layer comprising a particulate form or a skived film of ultra high molecular weight polyethylene onto one side of each of 1 - 9 at least two rubber layers formed from a rubber composition containing rubber components having a critical surface tension in the range of 25 - 35 mN/m, (b) laminating the rubber layers with the adhesive layer interposed therebetween, and (c) heating the resulting laminated layers at 125' 3001C to fuse integrally into a rubber/rubber bonded composite sheet structure. The rubber components in this embodiment may be of the same or different species, and the rubber layers may be unvulcanized or vulcanized.

The rubber sheets may be formed by any known methods and to any desired thickness. They may be reinforced with organic fiber cords such as of nylon, polyester, aramid and the like, or steel or other metallic cords.

UHMW polyethylene when skived into a film is substantially transparent and may be therefore advantageously used as a marking strip carrying indicia such as letters, numerals, designs and marks to be attached to various rubber products. For instance, a metal foil or colored plastics sheet may be cut into marking pieces which are interposed between two UHMW polyethylene films, the films being then fused together, placed on a rubber product and heated at 1251 - 300'C.

Alternatively, two UHMW polyethylene films having a colored plastics sheet sandwitched therebetween may be fused together to form a composite colored marking sheet - 10 which is then cut into desired marking strips for attachment with heat to a particular rubber product article.

The invention will be further described by way of 5 the following examples.

Inventive Example 1 There were prepared five different rubber compositions identified in Table 1 below. Two sheets of each of Sample Nos. 1 - 5 measuring 150 mm. x 150 mm x 2.5 mm were formed from rubber compositions of the same species and sorted out in a set of both unvulcanized, a set of vulcanized and unvulcanized and a set of both vulcanized. Interposed between the respective two sheets of each sample was an adhesive layer of URMW polyethylene (having a molecular weight of about 5,000,000) in the form of a film 50 Pm thick, or in the form of a paste comprised of a blend of UHMW polyethylene powder having an average particle size of about 120 jim and a fluid paraffin in a weight ratio of 100:50. The resulting laminated layers were treated at a temperature of 150C and a pressure of 20 kg/cm2 for 30 minutes to produce a rubber/rubber bonded composite sheet structure.

Table 1

Rubber Sample No.

1 2 3 4 5 Rubber Comoonent NR (Yc = 31) 100 - 30 EPDM (Yc = 28) - 100 SBR (Yc = 33) - - 70 - NBR (Yc = 39) - - - - 100 IIR (Yc = 27) - - - 100 Additives carbonblack HAF 45 80 55 50 45 zinc oxide 5 5 3 5 3 stearic acide 2 1 2 1 1 antioxidantl 1 - 1 1 1 aromatic process oil 5 - 1 - paraffinic process oil - 30 - 5 - process oil (DOP) - - - - 5 vulc&nization 0.7 1.2 0.7 accelerator NS2 vulcanization acceleratoi: TT3 1.5 - - vulcanization 0.5 - - accelerator M4 vulcanization 1.5 accelerator TS5 vulcanization accelerator DM6 - - - 0.5 - sulfur 2.0 1.5 1.8 2.0 2.0 Note: The 1 2 3 listed numerical values are parts by weight. N-phenyl-N'-isopropyl-p- phenylenediamide N-t-butyl-2-benzothiazolsulpheneamide tetramethylthiuramdisulfide 4 2-mercaptobenzothiazole tetramethylthiurammonosulfide 6 dibenzothiazy1disulfide A test piece 20 mm wide of each of the bonded sheet structures obtained as above was subjected to interface separation test to determine its adhesive property with the results shown in Table 2 below. The mark "X" designates a poor adhesive property as the rubber layers were separated or peeled apart. Those test pieces with no interface separation were each cut by razor to a _strip about 0.5 mm thick which was then dipped in toluene to swell, followed by application of tension in the proximity of the bonded interface. If no interface separation was witnessed, this indicates a good adhesive property as marked by blank circle I'01'. It is here to be noted that adhesive strength is not representative of an accurate adhesive quality of a rubber/rubber bonded composite sheet structure.

Table 2

Rubber Sample No.

4 2 1 3 5 Rubber Component IIR EPDM NR SBR-NR NBR Adhesive quality UHMW polyethylene 0 0 0 0 X film UHMW polyethylene 0 0 0 0 X powder 1 It will be seen from Table 2 that all rubber samples exhibit excellent adhesive property, except for Sample No. 5 (NBR) which has a critical surface tension Yc of 39 mN/m.

Inventive Example 2 The procedure of Inventive Example 1 was followed except that all rubber samples used were each in a laminate set of vulcanized/unvulcanized and a laminate set of both vulcanized. The results of separation test conducted as per Inventive Example 1 are shown in Table 3.

Table 3

UHMW polyethylene film UHMW polyethylene powder Rubber Component IIR/ EPDM/ NR/OR SBR-NR/ NBR/ IIR/ EPDM/ NR/NR SBR-NR/ NBR/ IIR EPDM SBR-NR NBR IIR EPDM SBR-NR NBR vulcanized/ 0 0 0 x x 0 0 0 x x unvulcanized vulcanized/ vulcanized 0 0 0 x x 0 0 0 x x 9 F-4 1 It will be seen that Sample No. 3 (a combination of 70 wt parts SBR having a Yc value of 33 mN/m and 30 wt parts NR having a Yc value of 31 mN/m) shows a poor adhesive property if one of its sheet set is vulcanized, whereas good adhesiveness is provided with the same set when the rubber component has a Yc value in the range of 25 - 32 mN/m.

Inventive Example 3 The procedure of Inventive Example 1 was followed except that a combination of Sample No. 2 (EPDM) and Sample No. 4 (IIR) and another combination of Sample No. 2 (EPDM) and Sample No. 1 (NR) were used. Interposed between the two respective sheets of each sample was an adhesive layer of UHMW polyethylene in the form of a film 150 pm thick as well as the same powder as used in Inventive Example 1. The results of separation test are shown in Table 4 below, from which it will be seen that where rubber compositions of different species are used, the adhesive property or quality varies with the yc value of the respective rubber components and whether vulcanized or unvulcanized. However, if the Yc value is within the range of 25 - 30 mN/m; that is, EPDM with Yc = 28 resulting composite sheet structure has a good adhesive property regardless of whether the starting rubber material is unvulcanized or vulcanized.

16 - Table 4

UHMW polyethylene UHMW polyethylene film powder EPDM/IIR EPDM/NR EPDM/IIR EPDM/NR unvulcanized/ 0 0 0 0 unvulcanized vulcanized/ 0 0 0 0 unvulcanized unvulcanized/ 0 X 0 X vulcanized vulcanized/ 0 X 0 X vulcanized Inventive Example 4 The procedure of Inventive Example 1 was followed except that a combination of Sample No. 2 (EPDM) and Sample No. 3 (SBR-NR) was used. Interposed between the two respective sheets of each sample was an adhesive layer of UHMW polyethylene in the form of a film 150 um thick as well as the same powder as used in Inventive Example 1. The results of adhesion test (by separation) are shown in Table 5 below, from which it will be seen 10 that the use of SBR/NR (with respective yc values = 33 mN/m and = 31 mN/m), if vulcanized, adversely affects the rubber/rubber interface adhesion regardless of whether the other rubber material having a lower Yc value (EPDM of Yc =28 mN/m) is vulcanized or unvulcanized.

Table 5

UHMW polyethylene UHMW polyethylene film powder EPDM/SBR-NR EPDM/SBR-NR unvulcanized/ 0 0 unvulcanized vulcanized/ 0 0 unvulcanized unvulcanized/ X X vulcanized vulcanized/ X X vulcanized is Inventive Example 5 There were prepared four different rubber compositions as identified by Sample Nos. 6 - 9 in Table 6. Unvulcanized rubber sheets measuring 150 mm x 50 mm x 2.5 mm were formed from Sample No. 7 and Sample No. 8, respectively. Each of these rubber sheets was superimposed on one side (face) with a 50 4m thick film of UHMW polyethylene (molecular weight = 5, 000,000), followed by heating at a temperature of 150C and a pressure of 20 kg/CM2 for 30 minutes thereby producing rubber/film bonded sheet. Two respective film bonded sheets of different rubber components were laminated with their respective film bonded sides in face-to-face relation and heated at a temperature of 160C and a pressure of 20 kg/cm2 to fuse together into a rubber/rubber bonded composite sheet structure.

a - 18 Inventive Example 6 Unvulcanized rubber sheets of 150 mm x 50 mm x 2.5 mm were prepared from Sample No. 6 and Sample No. 9, respectively. These rubber sheets were each superimposed on one side (face) with a 50 pm thick film of UHMW polyethylene (molecular weight = 5,000,000), followed by heating at a temperature of 1500C and a pressure of 20 kg/cm2 for 30 minutes for Sample No. 6 sheet and at a temperature of 1900C for 20 minutes for Sample No. 9, respectively, thereby producing two respective film bonded rubber sheets. The two rubber sheets were laminated with their respective film bonded sides in face-to-face relation and heated at a temperature of 1500C and a pressure of 20 kg/CM2 to fuse together into a rubber/rubber bonded composite sheet structure.

Table 6

Rubber Sample No.

6 7 8 9 Rubber Component IIR - 100 SBR - 100 - NR 100 - - EPDM Additives carbonblack 45 80 55 60 zinc oxide 5 5 3 5 stearic acid 2 1 2 1 antioxidant 1 - 1 - aromatic process oil 5 - 10 paraffinic process oil 30 - vulcanization accelerator A 0.7 - 1.2 vulcanization accelerator B 1.5 - vulcanization accelerator C 0.5 - - resin 12 sulfur 2.0 1.5 1.8 Note: The listed numerical values are parts by weight. The resin is a phenol-based cross-linking agent. The antioxidant is N-phenyl-W-isopropyl- pphenylenediamine. The accelerator A is N-t-butyl-2ben zothiazolsulpheneamide (NS). The accelerator B is tetramethylthiuramdisulfide (TT). The accelerator C is 2mercaptobenzothiazole (M).

- 20 Comparative Example 1 The procedure of Inventive Example 5 was followed except that interposed between the two unvulcanized rubber sheets were one, two and five UHMW polyethylene films (50 pm thick), respectively, followed by vulcanization at one time. The resulting rubber-rubber bonded composite sheet structures were tested for their respective adhesive property under the same set of conditions in which the composite sheet structures of Inventive Example 5 and 6 were tested. The results of these tests demonstrated that both composite sheet products of Inventive Examples 5 and 6 are highly satisfactory in their rubber/rubber bond property, whereas the counterparts involving the use of one and two UHMW polyethylene films obtained in Comparative Example 1 are not acceptable in their adhesive property.

The comparative counterpart involving the use of as many as five UHMW polyethylene films was found satisfactory in terms of rubber/rubber bond quality but at the sacrifice of flexibility, another important quality of a rubber product.

It was also found that the product obtained with Rubber Sample No. 9 was not well integrated believably due to vulcanization being insufficient at 1500C for 30 minutes.

Inventive Example 7 There were prepared by a conventional skiving - 21 method a pair of UHMW polyethylene (molecular weight 5,000,000) films measuring 100 mm x 100 mm, x 5 pm. A gold foil was interposed between the two polyethylene films and hermetically trapped by heating at a temperature of 1500C under a pressure of 20 kg/cm2 for a period of 20 minutes. The resulting bonded composite film was laminated over one side of an unvulcanized sheet (150 mm, x 150 mm. x 2.5 mm) of rubber of the composition shown in Table 7 and heated at a temperature of 1500C and a pressure of 20 kg/cm2 for 30 minutes until there was obtained a vulcanized rubber sheet firmly attached with the composite film which serves as a marking.

Table 7

Rubber Blend Amount Composition (parts by weight) EPDM 100 carbonblack 80 zinc oxide 5 stearic acid 1 paraffinic 30 process oil vulcanization 1.5 accelerator A vulcanization 0.5 accelerator B sulfur 1.5 Note: Accelerator A is tetramethylthiuramdisulfide. Accelerator B is 2mercaptobenzothiazole.

Advantageously, this marking product was found free from separation by friction or flexing, fading due to ageing, or smearing in contact with foreign matter. The vulcanized rubber sheet was also found highly satisfactory in terms of tensile strength, elastic modulus and dimensional stability.

Inventive Example 8 A pair of UHMW polyethylene films similar to those of Inventive Example 7 were prepared. At the same time, there was prepared a white polyethylene film by admixing polyethylene of 200,000 molecular weight with 20 weight percent of titanium oxide. The admixture was mixed and formed into a film of 100 mm x 100 mm. This film was exposed to radiation by electron rays of 15 Mrad to become crosslinked. The resulting white colored film was interposed between the two UHMW polyethylene films and heated at a temperature of 150'C under a pressure of 20 kg/cm2 for 20 minutes thereby producing a bonded composite film which was useful as a marking strip.

This strip was laminated over one side of an unvulcanized sheet of rubber of the same composition as that shown in Table 7 and heat-treated in a manner similar to Inventive Example 7. The resulting composite product was likewise found highly satisfactory in respect of the various physical properties mentioned in Inventive Example 7.

Claims

Claims:

1. A process for the production of a rubber/rubber bonded composite sheet structure which comprises the steps of:

(a) laminating at least two rubber layers formed from a rubber composition containing rubber components having a critical surface tension in the range of 25 35 mN/m; (b) interposing between said laminated rubber layers an adhesive layer comprising a skived film of an ultra high molecular weight polyethylene having a thickness of 10 - 200 microns or a particulate form of an ultra high molecular weight polyethylene having an average particle size of 50 - 300 microns; and (c) heating said laminated layers at a temperature above the melting point of said polyethylene and below 3000C to fuse integrally into a rubber/rubber bonded composite sheet structure.

2. A process as defined in claim 1 wherein said two rubber layers are formed from a rubber composition containing rubber components of the same species and unvulcanized prior to the step (c) of the process.

3. A process as defined in claim 1 wherein said two rubber layers are formed from a rubber composition containing rubber components of the same species having a critical surface tension in the range of 25 - 32 mN/m, and one of said rubber layers is vulcanized and the - 24 other unvulcanized prior to the step (c) of the process.

4. A process as defined in claim 3 wherein both of said two rubber layers are vulcanized prior to the 5 step (c) of the process.

5. A process as defined in claim 1 wherein said two rubber layers are formed from a rubber composition containing rubber components of different species having a critical surface tension in the range of 25 - 30 mN/m and both vulcanized prior to the step (c) of the process.

6. A process as defined in claim 5 wherein both of said two rubber layers are unvulcanized prior to the step (c) of the process.

7. A process as defined in claim 5 wherein one of said two rubber layers is vulcanized and the other unvulcanized prior to the step (c) of the process.

8. A process as defined in claim 1 wherein one of said two rubber layers is formed from a rubber composition containing rubber components having a critical surface tension in the range of 30 - 35 mN/m and the other rubber layer is formed from a rubber composition containing rubber components having a critical surface tension in the range of 25 - 30 mN/m, both said rubber layers being unvulcanized prior to the step (c) of the process.

9. A process as defined in claim 8 wherein said rubber layer with a critical surface tension of 25 - 30 mN/m is vulcanized prior to the step (c) of the process.

10. A process as defined in claim 1 wherein said two rubber layers have their confronting surfaces each fused by heating with said adhesive layer prior to the step (a) of the process.

11. A process for the production of a rubber/ rubber bonded composite sheet structure substantially as hereinbefore described.