The adhesion of polyester textile to rubber. (Tech Service).

Adhesion between textile fibers and rubber is a very important concern in the rubber industry. Classical textile reinforcing materials like rayon and nylon possess polar surfaces, so sufficient adhesion between such materials and rubber is usually obtained by dipping textiles in a resorcinol-formaldehyde-latex (RFL) suspension. In the case of polyester textiles, where surface reactivity is much lower, a pretreatment of these materials is required (often employing epoxy reagents or isocyanate compounds subcoats) prior to standard RFL dipping (refs. 1-3).

These pretreatment methods have their disadvantages. For example, isocyanate is a toxic material, subcoats need organic solvents harmful to personal health as well as pollutants to the environment; uncured epoxy on the surface of textiles is also harmful to personal health. In comparison to in-situ RFL in our previous study (refs. 4 and 5), we have improved the adhesion of aramid fiber and nylon fiber to rubber by preparing RFL with a pre-condensed resorcinol-formaldehyde (PRF, SJR-1). To avoid using harmful organic solvents, in this article, the polyester textile surface is pre-treated with a water-soluble modified epoxide resin (SJR-2), and such a technology of RFL prepared with PRF is adapted and a RFL dip recipe and heat treatment technology for polyester textile dipping are optimized, so that good adhesion strengths between polyester textile and NR, CR, NBR, HNBR and CHR (chlorohydrin rubber) are obtained.

Experimental

Materials

The reinforcing material is a polyester textile 653 (a bevel-veined cloth made of polyester fiber) used for aero films, with a thickness of 0.14 mm. An aero film is a composite of a rubber-to-polyester textile used in aviation.

The resins, SJR-1 and SJR-2 (ref. 6), are used to prepare adhesion activator and RFL. The aqueous SJR-1 resin is a precondensate of resorcinol and formaldehyde in a water-solution containing acid or base catalyst under certain reactive condition. The aqueous SJR-2 resin is a highly active and water-soluble epoxide resin.

The rubber stocks used in the adhesion tests were: CHR, rubber stock of an aero film; HNBR, rubber stock of an aero film; NBR, tuber rubber stock of high pressure hydraulic hose; CR, cover rubber stock of high pressure hydraulic hose; and NR, rubber stock of a tire.

The latex types used in the tests were chloroprene latex, vinylpyridine latex and NBR latex.

Experimental procedure

Polyester textile --> dipping subcoat (surface adhesion activator) --> drying and partially curing ---> dipping in RFL prepared from SJR-1 --> drying and curing --> vulcanization --> adhesion testing.

Test methods

Adhesion samples are prepared to be a sandwich in which one-ply rubber stock is clamped between two-ply textiles; the thickness of sample is about 1.0 mm. Adhesion of the dipped polyester textiles to different rubber is measured according to the two-ply strip peel adhesion test GB532-91, a Chinese national standard test.

The fatigue test conditions have the distance between the upper and lower clamps in the fatigue tester at 70 mm; the reciprocating distance of lower clamp is 50 mm; the test speed is 300 rpm; and the samples are not stretched.

For the oil resistance test, the adhesion sample is soaked in aviation jet fuel at room temperature for 24 hours.

Results and discussion

Surface adhesion-activation (A-A) of polyester textile

At present, a pretreatment of polyester textile is required (often employing epoxy reagents or isocyanate compounds subcoats) prior to standard RFL dipping (refs. 1-3). This procedure is called surface adhesion activation. We have synthesized a completely water-soluble and highly active modified epoxide resin. This resin can self-condensate and co-condensate with SJR-1 resin. Polyester textile is dipped in a 3% aqueous solution of such resin and SJR- 1 resin, then the dip is partially cured and a highly active and firm film (about 3% of textile weight) has been formed on the surface of polyester textile. The data in table 1 show that the adhesion activation of the polyester textile meets adhesion level of CHR stock (in practical production, the peel strength is desired more than 2.0 kN/m) even without RFL topcoat. In this article, the pretreatment method does not employ organic solvent, and after co-condensation partially cured, no uncured epoxide harmful to personal health remains on the surface of the polyester textile.

The control of heat treatment conditions of the subcoat directly affects the adhesion effect of the surface adhesion-activation. Table 2 shows that different drying times affect the adhesion effect of the subcoat at 150[degrees] C. The pickup of subcoat is about 5%, and the heat treatment effect is optimal when drying time is four minutes at 150[degrees] C.

RFL preparation and heat treatment conditions

The latex used in RFL must match the rubber stock. Therefore, the adhesion treatment of polyester textile to different rubber stocks needs different latices. Table 3 lists the latex used in different rubber stocks.

There are many factors that influence adhesion strength of textile dipped with RFL, such as preparation methods of RFL, the formulation of RFL dip, the pickup of adhesive on the textile, heat treatment conditions, etc. Table 4 shows the effect of the weight ratio of resin/latex(RF/L) on adhesion of the textile. Table 5 shows the effect of heat treatment conditions on textile adhesion.

In this article, the subcoat is prepared with SJR-1 resin and SJR-2 resin; the topcoat is prepared with SJR-1 resin; and the latex is a variable parameter. After the optimization of a series of RFL formulations and treatment conditions, good adhesion of polyester textile to different rubber stocks is obtained. The results are listed in table 6.

Fatigue and oil resistance of adhesion of textile to CHR

Fatigue endurance and oil resistance

Table 7 shows that the maintenance percentage of adhesion strength is more than 60% after fatigue testing at 100,000 cycles under the fatigue test conditions. The samples have no stripping off phenomenon. The maintenance percentage of adhesion strength is more than 90% when the sample is soaked in aviation jet fuel after 24 hours at room temperature.

The effect of carbon black filler

The carbon black filler dispersion in RFL dip can improve the adaptability of textile to rubber stocks, therefore improving the fatigue endurance and oil resistance of adhesion. The ratio of carbon black/latex is selected as 15/100 by many experiments. The results of adhesion are shown in table 7.

The storage stability of dipped polyester textile

A large scale of polyester textile samples is treated in the laboratory. A part of them is supplied for the test of aero film product adhesion. The adhesion of the samples to two types of different CHR stocks is tested after the storage of two and four months, respectively. The results are shown in table 8. Compared with the original adhesion strength data measured in the laboratory, the adhesion property has no significant change. The results show that the storage stability of the dipped textile is good.

Conclusion

Desirable adhesion-activation effect of polyester textile has been obtained by the treatment with an aqueous solution of modified epoxide resin. Only a single RFL dipped textile has good adhesion to rubber. Compared with the current two-step dipping method, the present treatment technology of polyester textile for adhesion to rubber does not use any organic solvents and there is not the poison of uncured epoxide on the surface of the treated textile by partially curing the dips. It is a practical technique with excellent technological, economical and safe comprehensive effects.

Table 1 - effect of
pretreatment of the
subcoat on adhesion

Stocks   Peel strength
            (kN/m)

CHR         2.7-3.3
NBR             2.6
Table 2 - effect of heat
treatment condition of the
subcoat on adhesion

Time (min.)             3     4     5

Peel strength (kN/m)   1.8   3.3   2.8

(The stock is CHR)
Table 3 - latices selected for different stocks

Stock   CHR       CR        HNBR     NBR      NR

Latex   LDR-403   LDR-403   NBR-26   NBR-26   VP
Table 4 - effect of RF/L
weight ratio on adhesion

RF/L weight ratio                 1/4   1/5   1/6

Peel strength (kN * [m.sup.-1])   4.1   3.8   3.4

(The stock is CHR)
Table 5 - effect of heat
treatment time at 160[degrees] C
on adhesion

Time (min.)                       4     5     6

Peel strength(kN * [m.sup.-1])   3.1   3.1   4.3

(The stock is CHR)
Table 6 - adhesion results of polyester textile to
several rubber stocks

Stock                NR    CR    NBR   HNBR   CHR      CHR

Peel strength(kN *
[m.sup.-1])          4.5   5.0   5.0   4.9    4.0~5.6   4.1 *

* These data are the result of Shenyang Fourth Rubber Factory
with MDI treatment.
Table 7 - effect of RFL filled with CB on
textile adhesion strength (kN/m)

                         1       2         3         4

RFL with no CB filler   3.8     2.6       3.7       2.0
                              (68.4%)   (97.4%)   (52.6%)
RFL with CB filler      3.5     3.6       3.5       2.6
                              (100%)    (100%)    (74.3%)

(1) is the original (or initial) sample without fatigue or oil aging
test;

(2) is the sample through 100,000 cycles fatigue under the fatigue
test condition; 3 is the sample dipped with 2# kerosene at room
temperature for 24 hours; 4 is the sample through 100,000 cycle
fatigue after dipped with 2# kerosene. The data in parenthesis
are maintenance percentage of adhesion strength.
Table 8 - storage stability of treated textile

Compound    Original peel    Peel strength (kN/m) after
           strength (kN/m)         2 mos. storage

CHR               4.5                   4.3
CHR *             3.8                   3.8

* Two types of rubber stock are all CHR stocks, only the formulation
has the difference

References

(1.) Weening, W.E., Proceedings for IRC `86, Sweden, 1986, 2, 298-303.

(2.) Takeyama, T., Matsui, J., "Recent developments with tire cords and cord-to-rubber bonding," Rubber Chem. & Tech., 1969, 42, 229-241.

(3.) Solomon, T.S., "Systems for tire cord-rubber adhesion," Rubber Chem. & Tech., 1985, 58, 566-570.

(4.) Yuan Jing, Master's Degree Graduation Thesis, Xi'an Jiaotong University (1989), Xi'an, China.

(5.) Song Yuexian, Master's Degree Graduation Thesis, Xi'an Jiaotong University (1991), Xi'an, China.

(6.) Wang Youdao, Wu Bihe, Zheng Yuansuo and Song Yuexian, "The manufacturing technology of high pressure hydraulic hose reinforced with aramid cord," CN1088156A, 2000-04-28.