WO1997006297A1

WO1997006297A1 - Process for manufacturing rubber or synthetic articles with cord reinforcement

Info

Publication number: WO1997006297A1
Application number: PCT/EP1996/003171
Authority: WO
Inventors: Jan Van Campen; Johan Franciscus Van Der Pol; Peter Eelko Papegaaij
Original assignee: Akzo Nobel NV
Current assignee: Akzo Nobel NV
Priority date: 1995-08-09
Filing date: 1996-07-16
Publication date: 1997-02-20
Anticipated expiration: 1998-02-09
Also published as: NL1000955C2; AU6701296A

Abstract

The invention pertains to a process for manufacturing rubber or synthetic articles with cord reinforcement in which the rubber or the synthetic material and the cord are stretched and the rubber or the synthetic material is then cured, with use being made of a cord comprising at least one yarn of high modulus, higher than 45 GPa, and one yarn of low modulus, lower than 25 GPa. In this way it is made possible to employ high-modulus, high-strength yarns in processes such as mould curing.

Description

PROCESS FOR MANUFACTURING RUBBER OR SYNTHETIC ARTICLES WITH CORD REINFORCEMENT

The invention pertains to a process for manufacturing rubber or synthetic articles with cord reinforcement in which the rubber or the synthetic material and the cord are stretched and the rubber or the synthetic material is then cured.

Such a process is commonly known. One example of it is the process sometimes called "mould curing." It is used, e.g., to make transmission belts by arranging a rubber cylinder around a drum and then winding cords of polyester around the rubber cylinder. Next, a second rubber cylinder is arranged around the first one, and the whole is taken off the drum and placed in a mould. An inflatable rubber balloon is introduced into the first rubber cylinder and used to stretch the whole and press it against the mould's inside wall. If the inside wall of the mould is shaped in a certain way, e.g., provided with grooves, this shape is transferred to the second rubber cylinder. After vulcanisation the rubber construction with cord reinforcement can be released from the mould and cut up into belts.

It will be evident that this process makes it possible to manufacture a comparatively large number of belts by a small number of process steps. However, one key drawback to this process is that it requires the cords used as reinforcement to have a low modulus. For, when the unvulcanised rubber cylinder expands, the cord wound around it has to expand also because otherwise the rubber will be pressed or extruded through the various windings of the cord and the cord will change positions. The movement of the rubber and the cord renders the end product unfit for use.

Especially in the case of articles which have to be stretched to a comparatively major extent during mould curing, such as shorter transmission belts (having a length of, say, 140 inches (355.6 cm)), which frequently are subjected to more than 3% elongation, this problem plays a significant part.

In consequence, it is very difficult and frequently downright impossible to use ultra-strong fibres (e.g., poly(para-phenylene terephthalamide)), since in addition to high strength they tend to have a high modulus also.

The invention has for its object to do away with this restriction. This object is achieved by the use in the process as described in the opening paragraph of a cord comprising at least one yarn of high modulus, higher than 45 GPa, and at least one other yarn of low modulus, lower than 25 GPa.

Preferably, use is made of a cord comprising at least one yarn of high modulus, higher than 60 or even higher than 75 GPa, and at least one other yarn with a modulus lower than 15 or even lower than 10 GPa.

Depending on the material, 400 Gpa and 250 GPA are conventional upper limits for the yarn modulus. As conventional lower limits for the yarn modulus may be mentioned 0.5 GPa andl Gpa.

Such hybrid cords display a low modulus at low elongation and a high modulus at a higher elongation. In this way the situation arises of cords which during the production process have a modulus low enough to prevent the matrix material, e.g., polyurethane or rubber, being pressed or extruded through the various cord windings, while once they are part of the final product, these cords have just the high modulus and the high strength desired.

Further, the modulus at a particular elongation and the shape of the stress-strain curve can be affected by the difference in length among the various yarns in the cord (the so-called "overfeed") as well as by the cord construction, i.e., the twist and the pitch of the individual yarns and the cord twist.

One example of a possible construction involves two yarns (e.g., 1500 dtex; Z220) of high modulus being twisted together with one yarn (e.g., 1500 dtex; Z180) of low modulus to form a cord (e.g., S220).

Alternatively, one yarn (e.g., 1500 dtex) of high modulus and one yarn (e.g., 500 dtex) of low modulus may be twisted together to form a cord (e.g., Z220). Three of these cords can then be twisted into a thicker cord (e.g., S220).

Highly suitable materials for high-modulus yarns include aromatic polyamides (aramid), such as poly(para-phenylene terephthalamide), carbon, and glass. Over the years these materials have proved especially suitable for use in composites. Aramid is frequently employed in composites with a rubber matrix among others. Other examples of appropriate materials are metal, polyimide, polyketone, and tightly drawn polyethene.

As highly suitable materials for low-modulus yarns may be mentioned polyester, polyamide, elastodiene, elastane, and chlorofibre. Some of these materials have been used in composites such as tyres and drive belts for many years. Other examples of suitable materials are polyolefins, cellulose, acetate, acrylic material, and vinylal.

It will be clear from the above that the yarn of low modulus serves first of all to enable high-modulus materials to be used in such processes as the mould curing process described. In other words, the lowmodulus yarn is there primarily as a process aid. In the finished product the hybrid cords should approximate as closely as possible the properties of cords made exclusively of "super fibres." Although, in principle, all hybrid cords composed of yarns of a high-modulus material and yarns of a low-modulus material are suitable for use, for the aforementioned reason preference is given to cords containing at least 75 wt.% of the high- modulus (higher than 45 GPa) yarn.

The selected cord composition and cord construction preferably are such that the elongation occurring during the process substantially corresponds to low-modulus cord elongation. The initial or low-modulus elongation in that case is exploited (virtually) completely during the process, leaving the cords in the finished product with a high modulus.

Preference is further given to a cord composition and construction where the different yarns will break simultaneously at ultimate load. In that case the cord will have optimum strength.

In order to ensure that in the final product, e.g. a transmission belt, there is good adhesion of the cords to the matrix material of the belt, it may be advisable to coat the cords with an adhesive. In that case the cords are treated with an adhesive system prior to being contacted with the matrix material. Preferably, the cords are provided with a first adhesive coating before they are treated with the adhesive system.

Highly suitable first adhesive coatings include epoxy compounds, polymeric methyl diphenyl diisocyanate (e.g., Voranate® ex DOW), and polyurethanes having ionic groups. Notably preference is given to polyurethane which additionally contains blocked isocyanate groups and hydrogen atoms-containing groups which enter into reaction with isocyanate groups. This group of polyurethanes (lonothane®) and its advantages when used as an adhesive coating have been described in detail in European patent specification EP 168 066 A1 , which is hereby incorporated by reference.

The adhesive system also offers several options. Highly suitable for use in the case of, e.g., poly(para-phenylene terephthalamide) are a resorcinol /formaldehyde /latex (RFL) system and Chemosil® ex Henkel. In the case of, e.g., glass use may be made of a silane compound.

Preferably, during pre-dipping and dipping, applying the first adhesive coating and the adhesive system, respectively, a certain tension is set up in the cord. As will be shown by the examples, the force required to impose a particular elongation on the dipped cords is fairly largely dependent on this tension. Because of this, the tension can be used to adjust the cord properties more precisely still.

The invention also pertains to a cord suitable for use in the process described which comprises at least one yarn of high modulus, higher than 45 GPa, and at least one other yarn of low modulus, lower than 25 GPa, with all yarns taking part in the twisting of the cord and the cord containing at least 75 wt.% of the high modulus yarn. Such cords contain sufficient low-modulus material for the proper progress of processes such as mould curing, while the properties of the cord in the final product are virtually the same as those of cords made up exclusively of the high-modulus yarn.

The invention further pertains to a transmission belt reinforced with the cord described above.

As was stated earlier, the matrix material may be made up wholly or in part of rubber or synthetic material. The term rubber is intended to cover all synthetic and natural rubbers and hence comprises, int. al., natural rubber, styrene-butadiene rubber, butadiene rubber, isoprene rubber, acrylonitrile-butadiene rubber, hydrogenated acrylonitrile- butadiene rubber (HNBR), chloroprene rubber, isoprene-isobutylene rubber, brominated isoprene isobutylene rubber, chlorinated isoprene-isobutylene rubber, alkylated chlorosulphonated polyethylene (ACSM), polyurethane rubbers, ethylene-propylene-diene terpolymers, and combinations of two or more of these rubbers as well as combinations of these rubbers with other rubbers and/or thermoplasts.

Examples of suitable synthetic materials for use in the invention include thermoplasts, thermoplastic elastomers, polyurethanes, copolyether esters, and copolyether amides.

According to the invention, the term yarn also covers monofilaments and bundles of monofilaments. The term further comprises staple fibres, spun yarn, texturised fibres, hollow fibres, hollow capillaries, and tapes.

The term curing refers to, int. al., vulcanising, cooling (of, say, thermoplasts), and cross-linking.

It should be noted that US 4,155,394 discloses a cord composed of aramid and nylon. The cord has, on the one hand, a higher modulus and a higher breaking load than cords of polyester and nylon and, on the other, is less susceptible to fatigue than cords made up exclusively of aramid. The cords preferably contain one to four twisted yarns of nylon or polyester to one twisted aramid yarn. In other words, the cords contain less than 50% of aramid.

EP 0 535 969 A1 also describes such a cord for use in truck tyres. The tyres according to this patent specification provide increased driving comfort for the trucker. One example discloses a hybrid cord 54% of which is made up of aramid.

It is also noted that US 3,977,172 discloses a cord composed of one or two twisted yarns of PPTA and one twisted yarn of nylon or polyester. Here again the cords are used to counter fibre fatigue. The use of the cords in the mould curing process is not described. Preferably, 2/3 (67%) of the cords consists of aramid.

Although the fibres disclosed in these patent publications are suitable as such for use in the process according to the invention, it is preferred, as has been stated, to make use of cords containing a larger quantity (at least 75 wt.%) of high- modulus, high-strength yarns.

For that matter, EP 0 329 590 A1 discloses a cord which is composed of a core of polyamide or polyester yarn positioned in a straight line with aramid yarn wound about said core. The aramid content is in the range of 70 to 95%. The initial or low-modulus elongation is so high that conventional mould curing processes fail to exhaust it fully or sufficiently.

The invention will be further illustrated below with reference to a number of examples with accompanying figures. Needless to say, the scope of the invention is not restricted to these examples.

For the meaning of cord construction notation, in so far as it requires an explanation, reference is made to ISO 1139 - 1973.

The parameters given in the examples and the claims were measured in accordance with ASTM-D885M-95, at 20°C and 65% relative atmospheric humidity. The unit "N/tex" can be converted into "GPa" by multiplication with the density (in g/cm²) of the material employed. EXAMPLE 1

A wrapped wire construction was made of Twaron® 2100 and Enka Nylon® 155 HRS of the following linear densities and twist levels (the construction had a thickness of 0.85 mm, the difference in length for 1 m of cord when cabling was 1.042 m of Twaron and 1.06 m of Enka Nylon):

(Twaron 2100; 1680 dtex x1 Z170 x3 SMO + Enka Nylon 155 HRS; 940 dtex Z60) x1 S250

Three cords of this composition were tested:

Cord A: undipped Cord B: pre-dipped in epoxy and then dipped in a resorcinol/ formaldehyde/latexsystem under a tension of 10 mN/tex Cord C: pre-dipped in epoxy and then dipped in a resorcinol/ formaldehyde/latex system under a tension of 50 mN/tex

For comparative purposes cords consisting exclusively of Twaron® and polyester, respectively, were also tested:

Cord D: Twaron 2100; 1680 dtex x1 Z220 x3 S220 pre-dipped in epoxy and then dipped in a resorcinol/ formaldehyde/latex system under a tension of 25 mN/tex

Cord E: Polyester; 1100 dtex x2 Z150 x3 S150 pre-dipped in epoxy and then dipped in a resorcinol/ formaldehyde/latex system under a tension of 10 mN/tex TABLE 1

A B C D E

Tenacity 1476 1405 1444 1743 575

(mN/tex)

Elong. at 9.1 7.4 5.9 4.5 11.7 break (%)

FASE 1 (N) 28 44 78 174 75

FASE 2 (N) 54 102 198 373 120

FASE 3 (N) 107 197 359 602 203

(Note: In this table the term FASE ("Force At Specified Elongation") indicates the force required to obtain a certain elongation. For instance, FASE 2% indicates the force required to give a certain cord a 2% elongation. The epoxy used is GE- 100 ex Raschig.)

Table 1 and figures 1 and 2 (a force/elongation graph and a tenacity/elongation graph, respectively, of cords A-E) clearly show that at an elongation of less than about 2% cords B and C behave like a polyester cord. At an elongation of more than about 2% the Twaron® determines the cord's behaviour.

For that matter, it should be noted that the effect of said tension on the shape of the force/elongation curve is far lower in the case of cords made up exclusively of aramid, such as cord D, than in the case of hybrid cords, so that in spite of the deviating tension (25 mN/tex) cord D is suitable for comparative purposes.

Table 1 and figures 1 and 2 show that the properties of the cords can be adjusted more precisely with the aid of the tension during dipping and in this way can be adapted to the rubber matrix of the finished product. EXAMPLE 2

A second wrapped wire construction was composed of Twaron® 2100 and Enka Nylon® 155 HRS having the following linear densities and levels of twist (the construction had a thickness of 1.05 mm, the difference in length for 1 m of cord when cabling was 1.048 m of Twaron and 1.240 m of Enka Nylon):

(Twaron 1000; 1100 dtex x2 Z120 x3 S120 + Enka Nylon 155 HRT; 940 dtex x1 Z60) x1 Z320

Three cords of this composition were tested:

Cord F: undipped

Cord G: pre-dipped in ionothane and then dipped in a resorcinol/ formaldehyde/latex system under a tension of 10 mN/tex Cord H: pre-dipped in epoxy and then dipped in a resorcinol/ formaldehyde/latex system under a tension of 10 mN/tex

For comparative purposes a cord containing Twaron® exclusively was also tested:

Cord I: Twaron 1000; 1100 dtex x2 Z120 x3 S120 pre-dipped in epoxy and then dipped in a resorcinol/ formaldehyde/latex system under a tension of 10 mN/tex

The measurement results are listed in Table 2 and figures 3 and 4 (a force/elongation graph and a tenacity/elongation graph, respectively, for cords F-l and E) TABLE 2

F G H I

Tenacity 1641 1469 1528 1637

(mN/tex)

Elong. at 7.9 5.1 5.1 3.2 break (%)

FASE 1 (N) 32 81 83 295

FASE 2 (N) 59 220 253 656

FASE 3 (N) 122 457 506 1053

EXAMPLE 3

A rubber cylinder was arranged around a drum and a cord such as cord B was then wound around the rubber cylinder. Next, a second rubber cylinder was arranged around the drum, and the construction thus formed was taken off the drum. This construction was placed in a mould with parallel circular grooves (of 4.7 mm in width and a depth of 5.5 mm) on the inside. A rubber balloon was introduced into the rubber construction and inflated to 15 bar, so that the rubber construction was pressed into the grooves. This was followed by 5 minutes of vulcanisation with 180°C steam.

The rubber construction was released from the mould and cut up into belts. Inspection showed that the cords had not moved, nor the rubber been pressed through the cords.

Claims

1. A process for manufacturing rubber or synthetic articles with cord reinforcement in which the rubber or the synthetic material and cord are stretched and the rubber or the synthetic material is then cured, characterised in that use is made of a cord comprising at least one yarn of high modulus, higher than 45 GPa, and at least one other yarn of low modulus, lower than 25 GPa.

2. A process according to claim 1 , characterised in that the high modulus yarn has a different length per meter of cord than the low-modulus yarn.

3. A process according to either of the preceding claims, characterised in that the high-modulus yarn contains aromatic polyamide, carbon or glass.

4. A process according to any one of the preceding claims, characterised in that the low-modulus yarn contains polyester, polyamide, elastodiene, elastane or chlorofibre.

5. A process according to any one of the preceding claims, characterised in that the cord contains at least 75 wt.% of the high-modulus yarn.

6. A process according to any one of the preceding claims, characterised in that the cord is treated with an adhesive system prior to being contacted with the rubber or the synthetic material.

7. A process according to claim 6, characterised in that the cord is provided with a first adhesive coating prior to being treated with an adhesive system.

8. A process according to claim 7, characterised in that the first adhesive coating contains an epoxy compound or a polyurethane having ionic groups.

9. A process according to claim 8, characterised in that the polyurethane contains blocked isocyanate groups and hydrogen atoms-containing groups entering into reaction with isocyanate groups.

10. A cord suitable for use in a process according to any one of the preceding claims which comprises at least one yarn of high modulus, higher than 45

GPa, and at least one other yarn of low modulus, lower than 25 GPa, characterised in that all yarns take part in the twisting of the cord and the cord contains at least 75 wt.% of the high-modulus yarn.

11. A transmission belt reinforced with a cord according to claim 10.