Nylon 6,6 adhesion to natural rubber.

The objective of this article is to examine the adhesion of nylon 6,6 tire cords to natural robber, particularly in light of the trend towards higher tire curing temperatures (shorter cure cycle) and increasing use of faster curing natural robber as the sole elastomer in tire body stocks.

Nylon 6,6 has shown outstanding adhesion in natural rubber/styrene but adiene rubber blend stocks or in relatively slow-curing 100% NR stocks in tires cured under standard conditions. However, it is important to assure that this good adhesion is maintained in currently used 100% NR stocks in tires cured at higher temperatures and shorter times resulting in an increase in tire productivity. If a decrease in adhesion is encountered, are there practical changes in rubber compounding that can remedy the situation? Before answering this question it is necessary to review some aspects of nylon cord/adhesive/rubber bonding and previous work done at Du Pont (ref. 1) and referred to by other authors (refs. 2 and 3) on the influence of rubber compounding and characteristics on nylon 6,6 adhesion.

Nylon 6,6 cord/adhesive/rubber bond

Failure in a fiber/adhesive/rubber composite can occur cohesively inside the different substrates or at the interfaces. In the case of nylon 6,6, failure within the strongly hydrogen bonded polyamide fiber (bulk or surface) does not occur. Failure at the fiberlresorcinol-formaldehyde-latex (RFL) adhesive interface is unlikely since strong hydrogen bonding and primary chemical bonds between the RF resin in RFL and the polyamide substrate are both believed to be involved in adhesion development (ref. 4). Similarly, cohesive failure inside the RFL can also be ruled out since the styrene-butadiene-vinyl pyridine elastomer in the RFL is reinforced by RF resin and cross-linked by curatives migrating from the rubber compound during curing of the composite. RFL adhesive has excellent mechanical properties as shown by tests on films (refs. 5 and 6). Failure at the RFL/rubber interface can occur when RFL treated cords/fabrics are exposed to environmental factors such as ozone, ultraviolet or fluorescent light, chemicals, excessive moisture or heat before curing in rubber (refs. 7-9). When such exposure is avoided, failure can still occur at this interface if the rubber compound cures too fast before co-curing with the styrene-butadiene-vinyl pyridine rubber in the latex. Finally, cohesive rubber failure occurs if the rubber is weaker than the RFL/rubber bond strength.

Effect of rubber characteristics on adhesion

In previous work (ref. 1) where RFL treated nylon 6,6 cords were cured in rubber stocks under relatively mild conditions (300 degrees F for 40 minutes), two key factors were found to influence cord-to-rubber adhesion. One is the pre-vulcanization time which is commonly known in the rubber industry as scorch time and the other is the presence of polar fillers. A critical scorch time (tc) was found to be essential for attaining good adhesion. Adhesion decreases as the scorch time is lowered below tc and attains a maximum at tc above which it remains constant, as shown schematically in figure 1. This is because a minimum time of rubber flow in the plastic state before the onset of cross-linking by vulcanization is essential for diffusion and subsequent wetting of the outer filaments of the RFL treated cords by rubber. Incomplete wetting, as in a stock with scorch time lower than tc, result in weak boundary layers of rubber around the cords.

The scorch time of a stock is determined by the curing system, the type of elastomer (e.g., faster curing NR vs. slower cunng SBR) and polar, acidic ingredients (e.g., channel type blacks, silica). The low adhesion in 100% NR stocks of low scorch time is remedied by changes in the cure accelerator as shown in previous work (table 1). A sulfenamide type delayed action accelerator gave higher scorch time and better adhesion than that containing a faster, disulfide type accelerator. Similarly, including a small proportion of SBR in a NR stock had the effect of increased scorch time and improved adhesion (table 2). This fact explains why the adhesion in NR/SBR blend tire body stocks used over the years has been consistently good.

Previously, the laboratory adhesion testing work was done using relatively mild conditions (300 degrees F for 40 minutes) to cure the cord/RFL/rubber samples. The above discussion on the effect of rubber stock scorch time on adhesion may lead to the inference that going to a higher temperature, faster cure of natural rubber stocks, particularly without the inclusion of SBR in the compound, may result in reduced adhesion. Is there a combination of critical scorch tine (tc) and cure temperature rather than only tc that is required for good adhesion? Is the adhesion development between the RFL adhesive and the rubber a time/temperature effect similar to that involved in the scorch and vulcanization processes? Answers to these questions will determine if high levels of nylon 6,6 adhesion to natural rubber can be maintained in tires and other cord/rubber composites when subject to fast cures.

Adhesion at high curing temperatures

To simulate moderate and fast tire curing conditions, nylon 6,6 cords (1260 denier/I/2) were cured in 100% NR stocks at 320*F for 20 minutes and 340*F for 10 minutes. The cords were dipped in a conventional ammoniated, alkali catalyzed, in-situ RFL adhesive (D-5A; ref. 10) and hot-stretched. The dip, prepared by standard procedures had rubber to RF resin solids ratio of 5.9 and F/R mole ratio of 2. The proportions of resorcinol, formaldehyde, NAOH and NH40H were 11, 6, 0.30 and 3.18 parts/I00 parts RFL solids, respectively (ref. 11). The cord hot-stretching conditions are shown in table 3.

Adhesion tests and rubber stocks

Adhesion to rubber stocks was determined by the two-ply strap peel adhesion test (Du Pont test method 1545-90) on one-inch wide samples with a cord spacing of 36 ends per inch. The parallel cord plies are separated by 30 + 4 mils of rubber stock. Curing conditions for the adhesion samples giving optimum rubber stock modulus were 320 degrees F for 20 minutes (slow cure) and 340 degrees F for 10 minutes (fast cure). Since interply adhesion of cords in hot running fires is critical, samples were peeled at 248 degrees F (120 degrees C) (5"/minute, 180 degrees angle).

Peel force and appearance ratings were recorded. An appearance rating of 1 indicates clean cord/rubber separation and 5 indicates 100% rubber tear.

Previous work (ref. 12) has shown that, for a given type of rubber compound, two-ply strip adhesion decreases with an increase in rubber stock modulus. Consequently, adhesion should be compared at similar stock modulus levels. Natural rubber stocks with similar 300% modulus levels but varying scorch times were compounded. These stocks contained the same ingredients except for the accelerator system. A masterbatch of natural rubber, carbon black, plasticizers and other ingredients (table 4) was made in an internal mixer and the test stocks were prepared by milling in the desired amounts of sulfur and accelerators. The stocks were then calendered to the desired thickness.

Tensile, modulus, elongation and hardness of the cured compounds were determined. Scorch time was measured with either the Mooney viscometer or the oscillating disc rheometer at 270, 320 and 340 degrees F. Note that in our subsequent discussions, the scorch time at 270 degrees F will be used to indicate prevulcanization time rather than the actual time at the much higher curing temperatures (320 degrees F and 340 degrees F). This is because the differences between scorch times of stocks at a lower temperature such as 270 degrees F are larger and more accurately measurable (table 5). Scorch time is temperature dependent and decreases approximately by half for each 18 degrees F (10 degrees C) rise in temperature (ref. 13). This corresponds to doubling the reaction rate, which is similar to most chemical reactions including vulcanization.

Results

The first set of tests confirmed that adhesion is reduced by appreciably lowering the scorch time of a natural rubber stock cured at 320 degrees F for 20 minutes (table 6). Adhesion was measured using the two-ply strip adhesion test versus the less sensitive single cord H-pull test used previously. The first stock containing 2-(morpholinothio) benzothiazole (MTBT) sulfenamide accelerator gave high scorch time and 'excellent adhesion while the second stock containing the disulfide type MBTS and tetramethylthiuram disulfide ultra accelerator was scorchy and showed poor adhesion with clean separation between cord and robber.

A second set of three rubber stocks was then formulated to determine the effect of increased robber stock cure temperature on adhesion (fast vs. slow cure). The first stock contained the MTBT accelerator and the other two contained MTBT in combination with MBTS. All the stocks had relatively long scorch times (12-17 minutes at 270 degrees F). Stocks containing MBTS had lower scorch times than that containing only MTBT. Rubber stock properties and nylon corot-torubber adhesion is shown in table 7.

All three rubber stocks show good adhesion with predominantly robber tear under both the slow (320 degrees F/20 minutes) and fast (340 degrees F/10 minutes) cure conditions. Rubber Stock #5, with the lowest scorch time (12 minutes) and highest modulus showed slightly lower adhesion than the other two stocks. Adhesion and scorch time data from tables 6 and 7 suggest that the critical scorch time for these types of stocks is about 11 minutes.

The type of compounding ingredients used in rubber stocks can influence their flow characteristics (viscosity) and wettability (polarity) which affect the critical scorch time (tc). Consequently, the rubber compounder should ascertain tc for tire body stocks to be used and adjust, if necessary, the curatives to remain clearly above the tc during routine processing of stocks to guarantee consistently good adhesion.

Adhesion-scorch time/temperature relationships

It can be inferred from previous work, that the critical scorch time (tc) for a given rubber stock is an absolute value and going to a higher temperature, faster cure may result in reduced adhesion. The present work has shown that this inference is erroneous. Critical scorch time decreases with increasing cure temperature at a rate typical of scorch or vulcanization times. Figure 2 illustrates a plot of cord-to-robber adhesion versus scorch time at various cure temperatures. Values of tc decrease with increasing cure temperature. Figure 3, constructed from observed and calculated tc values (ref. 13), shows the plot of tc versus cure temperature.

The adhesion development between the RFL adhesive and rubber compound during cure is a time/temperature phenomenon (activation energy) involving critical scorch time at the curing temperature. At higher cure temperatures, a shorter scorch time suffices for good adhesion development because diffusion and wetting of cords by uncured rubber during flow proceeds faster at higher temperatures.

Advantage of fast cure with nylon 6,6

A significant economic advantage in building tires with nylon 6,6 is the opportunity to cure more tires per day from a single mold by curing faster. The higher curing temperature of faster cures (e.g. 340 degrees F) cannot be tolerated by nylon 6 without significant loss in cord strength. The reason for this is the appreciably lower softening point of nylon 6 (320-381 degrees F) vs. nylon 6,6 (446-464 degrees F). At a tire cure ejection temperature of 340 degrees F, the strength loss of a nylon 6 cord (ref. 14) is about 10% even at a low cord moisture content of 1-2%. Strength losses become even greater (30%) at higher cord moisture levels (5-6%).

The feasibility of using nylon 6,6 (vs. nylon 6) in NR body stocks of tires cured in a fast cycle is made even more attractive due to the continuing development of high tenacity nylon 6,6 yams by fiber manufacturers (ref. 15) and introduction of new products such as polyamide monofilament (refs. 16 and 17) that features excellent curedin-robber strength retention. Information on the influence of cord size, interply rubber thickness and hot-stretching temperature on nylon 6,6 adhesion is available (ref. 12).

Summary

In summary, adhesion of nylon 6,6 cords under conditions of shorter cure cycles at higher temperatures, even when using fast curing natural rubber stocks, should present no problems as long as the compounds have scorch times above the critical value required for good adhesion development. Any manufacturer desiring to use nylon 6,6 as a tire reinforcement, especially in natural rubber stocks using fast cure cycles, should test adhesion in their compounds. If normal adhesion is not achieved, a simple adjustment of the cure accelerator system to increase the rubber scorch time above tc should alleviate the problem. If a manufacturer is already getting good adhesion with nylon 6,6 in NR stocks under current curing conditions, adhesion should not diminish by going to a faster cure at a higher temperature.

References

1. Y. lyengar, "Factors in rubber tire cord adhesion," Rubber WorM, vol. 148, pages 39-42 (1963).

2. T. Takevama and J. Matsui, "Recent developments with tire cords and cord-to-rubber bonding," Rubber Chemistry and Technology, vol. 42, pages 159-256 (1969).

3. K.D. Albrecht, "Influence of curing agents on rubber-totextile and rubber-to-steel cord adhesion, "Rubber Chemistry and Technology, vol. 46pages 981-998 (1973).

4. H.T. Patterson, "Fiber-elastomer adhesion variables," Special Technical Publication, ASTM, Philadelphia, PA, pages 32-45 (1963).

5. G. Raumann, "Some mechanical properties of resorcinolformaldehyde latex (RFL) tire cord adhesive," Textile Research Journal, vol. 38, no. 6, pages 627-633 (1968).

6. Y. Ivengar, "Adhesion of Kevlar aramid cords to rubber," technical symposium, Akron Rubber Group, pages 100-115 (1978).

7. H.M. Wenghoefer, "Environmental effects on RFL adhesion," Rubber Chemistry and Technology, vol. 47.. pages 1066-1073 (1974).

8. Y. Ivengar, "Effects of ozone and UV on tire cord adhesion," Journal of Applied Polvmer Science, vol. 19, pages 855-863 (1975).

9. T.S. Solomon, "Systems for tire cord-rubber adhesion," Rubber Chemistry and Technology, vol. 58, pages 561-576 (1985).

10. GenCorp Polymer Products, "Rubber reinforcement latices," Bulletin GPPL-0590 on "Gen-Tac" vinly pyridine latex.

11. Y. Ivengar, "Adhesion behavior of nylon tire cord/adhesive/rubber systems," Journal for Applied Polymer Science, vol. 13, pages 353-363 (1969).

12. Y. Iyengar, "Factors in rubber compounds affecting the adhesion of polyester tire cords," Journal of Applied Polymer Science, vol. 15, pages 267-276 (1971).

13. M. Morton, "Introduction to rubber technology," Reinhold Publishing Corporation (1959).

14. C.W. Beringer, "A comparison of nylon fibers for tire reinforcement," Rubber World, vol. 198, no. 5, pages 14-18 (1988).

15. C. Chaban, The Tire Technology Conference, Greenville, SC, Oct. 28-29 (1987).

16. G.N. Henning and T.K. Venkatachalam, "Hyten polyamide monofilament - a new dimension in rubber reinforcement," The Tire Industry Conference, Greenville, SC, Oct. 24-25 (1990).

17. G.N. Henning and T.K. Venkatachalam, "New polyamide monofilament improves tire performance, reduces weight," Elastomerics, vol. 122, #12, pages 25-29 (1990).

Acknowledgements

"TPUs: the first commercial TPEs" is based on a paper given at the November 1992 meeting of the Rubber Division. "TPU - the performance elastomer" is based on a paper given at the November 1992 meeting of the Rubber Division. "Millable polyurethanes for athletic footwear" is based on a paper given at the November 1992 meeting of the Rubber Division.

"Nylon 6,6 adhesion to natural rubber" is based on a paper given at the October 1992 Clemson University Tire Industry Conference.