Properties of a new carboxylated VP-latex on the adhesion of PET tire cord to rubber.

Properties of a new carboxylated VP-latex on the adhesion of PET tire cord to rubber

The advent of radial type automobile tires, with their superior driving stability, low fuel consumption and long life, has lead to the need for high modulus tire cord material. Although nylon cord has been used for many years, its tendency to stretch makes it unsuitable for radial tires.

To meet the needs of radial tires, polyester cord has been found to have an excellent balance of quality and price. Polyester cord, however, has not been used for heavy duty tires because adhesion of the cord to rubber decreases as tire mileage increases. To cope with this problem of adhesion deterioration, developments have been made in tire manufacturing, fiber and adhesive treatment, but a complete solution has not been achieved (ref. 1). Nippon Zeon has attempted to solve this problem by improving the latex portion of the adhesive as means of achieving improved adhesion and longer mileage.

It was postulated that the adhesion between polyester and rubber decreases as mileage increases due to hydrolysis of the ester bond of the polyester by amine compounds migrating from the rubber portion (ref. 2). Although the polyester tire cord is covered with adhesive, it was found that amine compounds penetrate into the polyester layer. It was therefore theorized that adhesion could be improved if the adhesive layer would function to prevent amines from migration into the polyester.

A test method was developed for evaluating the degree of degradation of polyester. The mechanism of preventing the amine compounds from passing through the adhesive layer was studied.

The result was the development of a carboxylated vinylpyridine latex which is effective in protecting the polyester from attack by amine compounds in the rubber compound.

A standard vinylpyridine-butadiene-styrene terpolymer latex and a carboxylated vinylpyridine-butadiene-styrene latex were used in these experiments (ref. 3). These latexes were compounded into the resorcinol-formaldehyde-latex (RFL) adhesive in table 1.

Evaluation of polyester protection by latexes

PET film strength measuring method - Laminated samples, as shown in figure 1a, were prepared with a polyethyleneterephthalate (PET) film, RFL adhesive and a rubber compound, vulcanized under a pressure of 5 MPa for one hour at 170 [degrees] C.

The laminated samples were used to test PET film degradation by measuring the force required for a plunger travelling at a constant speed of 5 mm per minute, to penetrate the film surface, as shown in figure 1a. The pattern of the observed stress during penetration is demonstrated in figure 1b. As the plunger contacts the PET film at point A and then penetrates into the film, the stress increases for zero to the maximum value B when the film ruptures. This maximum value indicates the residual strength of the PET film and will measure the degradation of the film during aging. The residual strength of the PET film as it ages is shown in figure 1c. By varying the overcure time of the laminate, an indication of PET film degradation upon aging, or extended mileage, may be plotted. This technique was used to measure the differences in PET film protection ability of several RFL adhesives. The stress at point B was established as the "PET film strength" and is the parameter used to indicate the degree of degradation of the polyester film, and the ability of adhesives to protect the film during simulated use.

Adhesion testing

T-pull adhesion - Polyester tire cord was dipped in the RFL solution, dried for two minutes at 140 [degrees] C and heat treated for two minutes at 235 [degrees] C. This provided an RFL adhesive adhered to the cord, with a coating of RFL adhesive equal to 6 percent of the cord weight.

The adhesion strength was measured by the JIS 1017 method, using the rubber compound shown in table 2. The samples were cured 30 minutes at 150 [degrees] C for the original adhesion, and cured 90 minutes at 170 [degrees] C to simulate adhesion after extended service.

Adhesion peel test - The test specimen was prepared using the compound from table 2 and RFL treated polyester cord samples. After curing, the peel adhesion strength between the cord and the rubber was determined.

Analysis of latexes

Quantitative analysis of carboxyl and pyridyl groups on the latex surface - The latex was placed in a cellulose tube and dialyzed for seven days with running water with its pH adjusted to 2 using a 0.5N HC1 aqueous solution. The total solids were thus reduced to about 2%. Fifty ml of this solution was used for a conductometric titration using a 0.1N NaOH aqueous solution then each function group on the latex surface was measured (ref. 4).

Polyester degradation after prolonged mileage

Polyester degradation mechanism by amine compounds - The degradation reaction of polyester through hydrolysis by an amine compound at high temperatures was reported previously (ref. 2). The molecular weight of the polyester hydrolized by an amine compound was determined and served as a measure of the degree of degradation of the polyester.

To determine the degradation of the polyester by amines contained in the rubber, the PET film strength measuring method using the laminate of PET film, RFL adhesive and rubber compound was used. Several types of accelerators were used in the rubber compound in table 2 and their effect on the degradation of the polyester film adhesion was observed using this method. The results are shown in figure 2. The degree of degradation varies largely with the type of accelerator used, for example a thiazole has less degradation on the PET.

Since the accelerator is chosen primarily for its influence on tire rubber properties, it is difficult to also choose one which has a minimum effect on polyester degradation. As a result, the rubber compound in table 2 was used for further study of tire cord adhesion.

Improved polyester tire cord adhesion after extended mileage

As a means of improving polyester tire cord adhesion after extended mileage, development was initiated of a latex with a functionality that prevents amine compounds from penetrating into the polyester.

A terpolymer vinylpyridine-butadiene-styrene is the conventional latex used for RFL tire cord adhesives (ref. 5). Research was carried out on this basic latex to introduce functional groups which would react with amine compounds and thus prevent them from migrating into the polyester. Functional groups such as esters, aldehydes and carboxyls were found to be effective, but this article is restricted to the study of a carboxylated vinylpyridine-butadiene-styrene terpolymer (ref. 3).

Characteristics of the latexes - A conductometric titration method to analyze the functional groups on the latex surface confirmed that the unsaturated carboxylic vinylpyridine-butadiene-styrene terpolymer (ref. 4), as shown in table 3. This was further confirmed in that no carboxylic groups were detected in the conventional vinylpyridine-butadiene-styrene latex. A concept of the structure of the improved latex and conventional vinylpyridine latex are shown in figure 3.

Network effect by carboxylic and pyridyl groups - Because the improved latex has both carboxylic and pyridyl groups, it can form a network structure comprised of hydrogen bondings caused by both functional groups. The schematic drawing of crosslinking network structure is shown in figure 4. On the other hand, conventional vinylpyridine latex has fewer crosslinkings because of the lack of carboxylic groups. Figure 5 shows the difference of the degree of crosslinking between carboxylated vinylpyridine latex (figure 5a) and conventional vinylpyridine latex (figure 5b). The carboxylated vinylpyridine latex has many more crosslinkings compared with the conventional vinylpyridine latex and is expected to have a greater ability of amine penetration protection.

Polyester protection ability of the improved latex - The ability to protect the PET film from degradation by amines for both the improved latex and the conventional vinylpyridine latex are shown in figure 6. It is obvious that the improved latex offers much better protection.

Overcured adhesion of the improved latex - Polyester tire cord adhesion to rubber, utilizing both the improved latex and a conventional vinylpyridine latex, was determined by both the T-pull test in figure 7, and the peel test in figure 8.

The improved latex indicates approximately 20% higher overcured adhesion than the conventional latex by the overcured T-pull test. The peel test shows that the improved latex was twice as good as the conventional latex, and that the failure was in the rubber layer, indicating perfect adhesion.

The mechanism of the improvement by carboxylated vinylpyridine latex - The mechanism of the improvement of polyester tire cord to rubber is shown in figure 9. The polyester tire cord treated with an adhesive is covered with rubber. The polyester and the adhesive is adhered by treating with reagents such as chlorophenol compounds or polyepoxides. The adhesive and the rubber are bonded by formation of crosslinking between the unsaturated bondings contained in the latex polymer and the rubber polymer. When tires are running under severe conditions, heat is generated and the accelerator (the amine compound) migrates through the adhesive layer but is interrupted by the improved latex. Thus high adhesion is maintained, although amines can penetrate and arrive at polyester layer in the case of ordinary systems using the conventional vinylpyridine latex.

Conclusions

Carboxylated vinylpyridine butadiene styrene latex exhibits superior extended mileage polyester tire cord adhesion versus a conventional vinylpyridine latex, and thus is now being used for heavy duty truck and bus tires. A study of the mechanism for improved adhesion after extended mileage with the improved carboxylated latex shows:

* The carboxylic radicals react with the amines, preventing them from penetrating the polyester cord.

* The amine compounds also are prevented from penetrating the adhesive layer though the obstruction presented by the network structure of the carboxylic and pyridyl radicals of the latex polymers. [Figures 1 to 9 Omitted] [Tabular Data 1 to 3 Omitted]

References

(1)Setsuo Fukuhara, Polymer No Tomo, 21, 85 (1984); Setsuo Fukuhara, Polymer No Tomo, 21, 167 (1984); Masayoshi Sekiya, Hiroshi Hisaki, Secchaku, 29 155, (1985). (2)Y. Iyengar, J. Appl. Poly. Science, 15, 267 (1971). (3)Nippon Zeon, Japan Kokai Tokkyo Koho, 61-26629; Nippon Zeon, Japan Kokai Tokkyo Koho, 62-39769; Nippon Zeon, Japan Kokai Tokkyo Koho, 62-70411; Nippon Zeon, Japan Kokai Tokkyo Koho, 62-230807; Nippon Zeon, Japan Kokai Tokkyo Koho, 63-112629; Nippon Zeon, Japan Kokai Tokkyo Koho, 63-130640; Nippon Zeon, Japan Kokai Tokkyo Koho, 63-234036. (4)Takashi Ozaki et al, Nihon Kagaku Kaishi, 8, 1295 (1980). (5)Nippon Zeon, A catalog of latexes.