An experimental study on fatigue mechanism: fatigue of PET cords in compounds.

An experimental study on fatigue mechanism: Fatigue of PET cords in compounds Poly(ethylene terephthalate) (PET) fibers have excellent mechanical characteristics, such as a higher strength than rayon fibers and a higher Young's modulus than nylon fibers, and are suitable as reinforcement materials to be used in the constructions of tires, belts and hoses, etc., and thus are widely used for this purpose.

One of the most important characteristics of a rubber composite reinforced by such fibers is its fatigue resistance. Namely, the characteristics of the reinforcing fibers are soon lost due to the stress and strain, such as extension and compression imposed thereon during use, and finally, the fibers are broken. Accordingly, from the viewpoint of fatigue resistance, it is very important to determine the fatigue behavior and fatigue breakpoint of the PET fibers used in articles. Many studies have been carried out with respect to the fatigue resistance of PET fibers. For example, Yabuki et al, made an intensive study of the relationship between a degree of polymerization of the PET fibers and their fatigue behavior (ref. 1). Further, the deterioration mechanism of the PET fibers, i.e., the dynamic deterioration (structural deterioration) due to kink bands (ref. 2) and the chemical deterioration due to aminolysis and hydrolysis caused by amine and water contained in rubber (refs. 3 and 4), have been studied. The study of Hearle et al, with respect to the fatigue breakage of fibers (ref. 5), is well known, and the authors also have studied the morphology at fracture of the PET fibers using a model fatigue test (ref. 6). Nevertheless, there have been very few systematic studies of the fatigue property, from the viewpoint of the PET fibers and their adhesion properties.

In this article we will report on the result of an examination of the fatigue behavior of PET fibers having a relatively high molecular weight and different carboxyl end groups, when spin-drawn and heat treated under conditions similar to those used in the cord-dipping process, which is essential in tire cord production. The fatigue properties of the PET fibers were investigated by subjecting these specimens to model fatigue test methods.

Experimental

Sample preparation

Three kinds of as-drawn PET tire fibers (1500 denier) made by Teijin Ltd. each having different carboxyl end groups and produced by a different spin-drawing process, as shown in table 1, were subject to a first twist of 40 turns/10 cm in the Z direction, respectively, and a second twist of 40 turns/10 cm in the S direction, to prepare samples for use in our experiments.

Measurements

Intrinsic [viscosity (eta)]

Three kinds of chloroform solutions each containing the PET fibers and having a different concentration were prepared and a dropping time of the solutions was measured using an Ubbelohde viscometer, and thereafter, a specific viscosity of the solutions having the respective concentrations was calculated and the intrinsic viscosity thereof was determined.

Carboxyl end group (-COOH) value

The PET fibers were dissolved in benzyl alcohol, to which chloroform was added, the solution was neutralized by 1/10 N of a sodium hydrite benzyl alcohol solution using Phenol Red as an indicator, and then the amount of carboxyl end groups was determined.

Density

The density of the PET fibers was measured with a mixture of n-heptane/carbon tatrachloride at [25 degrees C], using a density gradient tube. The measurement was carried out within 24 hours after the specimens were loaded.

Microstructures

A degree of crystallization and a crystal size were measured by a wide angle X-ray diffraction (WAXD) with an X-ray diffractometer, using Cu[K alpha] rays as a source.

Dynamic viscoelasticity 

To determine the dynamic viscoelasticity, the loss tangent-temperature (tan [delta] - T) curves were measured with a Vibron VES-F. The sample length was 20 mm, the frequency was 10 Hz, the initial load was 2.94 N, the measuring stress was 0.05 mm, and the temperature was increased at a rate of [3 degrees C/min.]

Observation of interface and morphology at fracture

The surface and cross section of the fibers taken out of rubber were observed using a scanning type electron microscope (abbreviated as SEM), while the fatigue of the fibers was in progress and after the fibers were broken.

Estimation of mechanical properties

The tensile strength, elongation at breakpoint, and load elongation at 66.7 N of the samples each having a length of 250 mm were estimated at a stretching speed of 300 mm/min, based on the Testing Methods for Chemical Fibers Tire Cords specified in Japanese Industrial Standard (JISL-1017) using a tensile stretch tester.

Adhesion treatment

An adhesion treatment was carried out, under the heat treatment conditions shown in table 2, in such a manner that the samples were treated with an epoxy-containing solution as the first adhesive and then a usual Resorcine-Formaline-Latex (RFL) adhesive shown in table 3 was applied to the samples. At that time, PET tire cord fibers having different adhesion levels were prepared by changing a solid picked up from the RFL adhesive; the samples used for the analysis of the microstructure were prepared by using water in place of the above adhesives.

Evaluation of adhesion

Specimens used for measuring the peel adhesion strength were prepared by embedding seven pieces of tire cord fibers in a composite rubber mainly composed of natural rubber (table 4) at predetermined intervals, and then vulcanized under a pressure of 0.98 MPa applied by a press at 150 [degrees] C for 30 minutes, to obtain an initial adhesion thereof to the rubber, and at 180 [degrees] C for 60 minutes to obtain an overcured adhesion state.

The adhesion strength was measured by peeling five of the cords at a peeling speed of 50 mm/min, using a tensile strength tester, after the cords at opposite ends had been removed. Further, a T-pull adhesion of samples having cords embedded in the rubber to a depth of 10 mm was measured under the same vulcanizing conditions.

Model fatigue test evaluation method

Disc fatigue test

A disc fatigue test was carried out in accordance with JISL-1017, using a Goodrich tester. The test was carried out under a maximum compression ratio of 20% and a maximum elongation ratio of 5%. The tensile strength was measured at room temperature after the fatigue test was carried out at 2,100 rpm for 24 hours.

Tube fatigue test

The tube samples were very carefully formed in accordance with JIS-L-1017, and a Goodyear tester was used. The tube fatigue test was carried out under the following conditions: a tubular angle of [80 degrees], an internal air pressure of 0.196 MPa and a speed of 750 rpm. The revolving direction was reversed once every 30 minutes, and the durability of the tube was estimated from the time at which the tube was broken, i.e., the tube life. The tube surface temperature was measured by a small thermocouple attached to the surface of the middle portion of the tube, and the level-off temperatute at the tube surface was taken as an index of the temperature due to heat generated during the tube fatigue test.

The samples were made by using the above mentioned rubber compound, respectively.

Results and discussion

Microstructures

Table 5 shows the microstructures of the three kinds of PET fibers used in this study. As can be seen from the results, with respect to an as-drawn microstructure, the samples A and B formed by an ordinary spin drawing process had a large crystallite size and long period, whereas the sample C formed by a POY type spin-drawing process had a small crystallite size and long period. No remarkable difference in crystallinity was observed between them, and they exhibited substantially the same value. When the samples were subject to a heat treatment, however, the crystallite sizes thereof had substantially the same value; and the sample B having a small amount of carboxyl end groups had the largest long period, followed by the sample A, and the sample C formed by the POY type spin-drawing process exhibited the smallest value of the long period. Also, the samples were further crystallized by the heat treatment, with the result that the crystallinity and density thereof were increased.

Physical properties

With respect to the mechanical performances of the three kinds of PET fibers shown in table 6, the sample B had the highest tensile strength and elongation, and thus provided PET fibers having a superior toughness, but the samples A and C had a slightly smaller elongation than the sample B. This tendency did not change even if the samples were subject to an adhesion heat treatment whereby they were elongated by 4.2% at a load of 66.7 N.

Nevertheless, the sample C formed by the POY type spin-drawing process had the smallest heat shrinkage at 150 [degrees] C, the high toughness type sample B had the largest heat shrinkage, and the sample A lay midway there between. As a result, it can be stated that the POY type sample C had the greatest thermal stability. The sample A had the smallest retention of strength after cords formed therefrom were embedded in the rubber and then subjected to a heat treatment for a predetermined time. This is assumed to be due to their dependency upon a carboxyl end groups value, which was affected by the hydrolysis and aminolysis caused by water and amine produced from a vulcanizing acceleration agent contained in the rubber, and thus it is considered that the sample A containing a large amount of carboxyl end groups had a slight disadvantage.

Fatigue resistance

Comparison of disc fatigue with tube fatigue

Table 7 shows the disc fatigue and the tube fatigue of three kinds of PET fibers having the same level of adhesion. With respect to the disc fatigue, no difference was observed among the samples and they had substantially the same level of adhesion, but with respect to the tube fatigue, the samples had a remarkably different heat generated temperature and tube life. Yabuki et al report that the relationship between the [(eta)] of PET fibers and the disc and tube fatigue thereof is inversely correlated (ref.1); i.e., high [(eta)] exhibits a good result with regard to disc fatigue, and conversely, a low [(eta)] exhibits a good result with regard to tube fatigue.

In the present study, in which PET fibers having substantially the same [(eta)] were used, the above result is assumed to have been obtained because the difference in the microstructure caused by the difference in the spin-drawing process was reflected slightly in the case of tube fatigue, but did not appear in the case of disc fatigue. The fatigue life was related to a magnitude of a long period by the microstructure of the sample, and it was observed that the smaller the long period, the longer the tube life. In addition, with respect to the temperature dependence of the tan [delta] shown in table 7, the sample C formed by the POY type spin-drawing process and having a small long period exhibited a low peak temperature of the tan [delta], which corresponded to the sequence of the heat generated in the samples during the tube fatigue test. More specifically, in the PET fibers used in this study, a correlation was observed between the peak temperature of the tan [delta] and the tube surface temperature, based on the relationship there between as shown in figure 1, which reproduced the conventional teaching that the heat generated in the tube is closely related to the microstructure of the PET fibers. On the other hand, no difference was observed in the tube fatigue life due to the carboxyl end groups. The POY type PET fibers exhibited the smallest hysteresis loss measured by the method described in the Celanese patent (ref. 7), which coincided well with the peak heat generating temperature at the tan [delta] max.

Adhesion (initial adhesion and overcured adhesion to

rubber) and fatigue resistance

Figure 2 shows the relationship between the adhesion (initial adhesion) strength under usual vulcanizing conditions (150 [degrees] C, 30 minutes) and the disc fatigue. It was observed that the disc fatigue depended upon the adhesion strength, and samples having a good adhesion strength retained that strength after being subjected to a disc fatigue caused mainly by compression. Further, when the samples A, B and C were plotted on the same adhesion-fatigue curve, the effect of the differences in the microstructures of the PET fibers did not appear, as described above.

When examining the relationship between the initial adhesion and the overcured adhesion to rubber achieved by a vulcanizing effected at a high temperature for a long time (180 [degrees] C, 60 minutes), and the disc fatigue (at [120 degrees C]) shown in figure 3, the disc fatigue and the heat resistance adhesion exhibited the same tendency and were plotted on the same adhesion-fatigue curve. This is assumed to mean that fibers having an excellent initial and overcured adhesion strength had a relatively uniform stress when fatigued by being subjected to a repetition of the fatigue test, whereas when the adhesion thereof was deteriorated the stress was irregularly distributed and concentrated at one point therein, and thus the fatigue was accelerated.

From the above described phenomena, it is assumed that the adhesion strength between the PET fibers and a rubber matrix had a greater effect on the disc fatigue by which compressed or elongated stress was applied, rather than the microstructures of the PET fibers used in this study.

As shown in figure 4, it was found that the interface between the fibers and rubber, which was obtained by forcibly peeling a test sample in which fatigue is in progress, is fractured at the rubber side when having a superior durable adhesion, but was fractured at the fiber side when having an inferior durable adhesion, and thus the fibers are badly damaged.

On the other hand, as shown in figure 5, with respect to the tube fatigue, an apparent difference in the fatigue life was observed among the different types of samples, and the POY type PET sample C having a small amount of carboxyl end groups had the longest life. The ordinary type PET sample A having a large amount of carboxyl end groups had teh next longest life, and the ordinary type PET sample B having a small amount of carboxyl end groups had the shortest life. This result shows that the life of the tube fatigue was affected by a long period and the smaller the long period, the longer the life, and that this was closely related to the microstructure of the PET fibers.

Further, as described above, a difference in the microstructure caused a difference in the heat generated temperature, and the lower the heat generated temperature, the longer the tube life. It is assumed that, when the microstructure was the same, the amount of the carboxyl end groups affected the tube life. In regard to this, Iyengar points out that PET fibers are deteriorated by amine and water existing in rubber (ref. 3). The existence of a small amount of carboxyl end groups is assumed to prevent this deterioration, but when the ordinary type PET samples A and B are compared, the sample A having a greater amount of carboxyl end groups had a longer life than the sample B. It may be considered that this was obtained from the results of the study by Yabuki et al (ref. 1), i.e., that a larger ([eta]) provides a longer tube life and the effect of a long period. Therefore, it can be assumed that the microstructure has a considerable effect on the tube fatigue, and thus the effect of the carboxyl end groups must be further studied.

Regarding the correlation between the disc fatigue and tube fatigue tests and the adhesion strength, the disc fatigue test had a greater affect on the adhesion strength than on the microstructure or chemical structure of the fibers, but the heat generated temperature remained substantially unchanged within the adhesion strength level of this study. In the tube fatigue test, when the spinning conditions were the same, samples having a higher adhesion strength exhibited a longer life. More specifically, it is also assumed in this cast that, when a sample had a higher adhesion strength, a uniform stress was applied to the sample and a concentration of the stress in the sample was prevented. This tendency was the same both in the initial adhesion and the overcured adhesion states.

Correlation between disc fatigue and tube fatigue tests

Figure 6 shows the correlation between the disc fatigue and the tube fatigue tests. As described above, no correlation was observed at all among the PET samples spun under different spinning conditions, but in the case of the same kind of the PET fibers samples having different adhesion strengths, it was found from a correlation coefficient determined with respect to each PET sample that a considerable correlation existed. The POY type PET fiber sample C had a correlation coefficient of 0.897, the ordinary type PET fiber sample B had a small amount of carboxyl end groups of 0.976, and the ordinary type PET fiber sample A had a large amount of carboxyl end groups of 0.947. The sample B exhibited the highest correlation.

Fiber surface behavior during fatigue test

Figure 7 shows the result of observations of the surface of cords forcibly removed from a tube while the fatigue test was in progress. As apparent from the SEM micrograph, the peeling of the surface is remarkably shifted to the fiber side as the fatigue of the tube progresses. Namely, as described above, it is assumed that as the tube fatigue progressed, the adhesion (dynamic adhesion) strength thereof was lowered, with the result that a gap was formed between the rubber and the reinforcing fibers and the adhesion interface of the tube became irregular and stress was concentrated there, and thus the fatigue of the tube further progressed until finally the tube was fractured.

Fracture morphology after fatigue fracture 

Observations by SEM of the fractured surface of the PET fibers show the PET fibers with a superior adhesion strength had a snap back round shape at the end of the fractured fibers, regardless of the kind thereof, whereas the PET fibers with a lower adhesion strength had a knife edge-shaped fracture morphology with an acute angle, which is assumed to be due from kink bands. Further, a morphology assumed to have grown from the kink bands was observed in a sample just before the fracture thereof.

On the other hand, the sample C of the POY type PET fibers having a lower heat generated temperature exhibited a smooth morphology obliquely cut with respect to a fiber axis, whereas the sample A having a high heat generated temperature exhibited a "mushroom shaped" fracture morphology, which implied that the fibers were melted. The heat generated temperature was closely related to the fracture morphology and it is assumed that the samples were partially heated at a very high temperature when they were fractured. The fractured surface of the samples was obviously different to the simple tensile fracture and was characteristic of a fatigue fracture.

Conclusions

The three kinds of PET fibers having different carboxyl end groups having a relatively high polymerization and produced under different spinning conditions have been evaluated by the model fatigue test, to study the fatigue behavior thereof, and the following conclusions were obtained.

* In the disc fatigue test, the three kinds of PET fibers exhibited the same fatigue tendency, when having the same adhesion strength level, and even if the adhesion strength of the fibers was changed, when the progress of the fatigue of the PET fibers was plotted on the same curve, no difference in the carboxyl end groups and microstructure of the fibers was reflected in this evaluation method, and it was found that the adhesion (initial adhesion and overcured adhesion to rubber) had a great effect. the disc fatigue evaluation method is considered to be susceptible to the effects of interface adhesion.

* On the other hand, in the tube fatigue test, a difference in the microstructure, in particular a difference in the long period, affected the fatigue life and the heat generated temperature, and the POY type PET fibers having a small long period exhibited a good resistance to fatigue and had a low peak temperature of tan [alpha].

* Therefore, although no correlation existed between the disc fatigue test and the tube fatigue test, it was found that, when the same PET fibers had a different adhesion strength, a relatively high correlation existed between the disc fatigue test and the tube fatigue test.

* As a result of observations of the surface of the fibers during the progress of the fatigue, it was found that a fracture interface caused in the rubber side, which was good at the initial stage of the fatigue test, was shifted toward the fiber side during the fatigue test. Therefore, it is assumed that, whether the adhesion strength (in particular, the dynamic adhesion strength) was good or bad has a considerable affect on the progress of the fatigue. More specificaly, it is assumed that, when the adhesion strength is high, repeated stress and strain are uniformly distributed to all of the fibers, but when the adhesion strength is lowered during the progress of the fatigue, the stress and strain are irregularly applied, and thus the progress of the fatigue is accelerated.

* Since the above tendency is considered to be a phenomenon common to the disc fatigue and the tube fatigue tests, it can be said that an improvement of the adhesion strength (in particular, the heat resistant and dynamic adhesion strength) is a very important factor.

* The PET fibers fractured by the model fatigue test exhibited a fracture morphology in which kink bands are considered to have grown: in the case of the disc fatigue test, the knife edge-shaped fracture surface was produced and this is assumed to be formed in such a manner that the kink bands were grown because it was less affected by heat; and in the case of the tube fatigue test, the melted type "mushroom-shaped" fracture morphology was produced, which implied that a large amount of heat was generated at the time of the fracture. These fracture morphologies were obviously different from the simple tensile fracture.

Although it is not apparent how the above mentioned fatigue behaviors reflect the behavior of the PET fibers when practically applied to tires, belts and hoses, the authors consider that the accumulation of the fatigue behavior obtained by the model fatigue test evaluation method will contribute to a better understanding of the behavior which is closer to the phenomena arising when the PET fibers are practically applied to tires, and thus a further study will be of great assistance.

References

[1] K. Yabuki, M. Iwasaki and Y. Aoki, Text. Res. J., 56, 41 (1986).

[2] S. Ishizaki, N. Kuramoto and N. Onuma, Sen-I Gakkaishi (in Japanese) 25, 347 (1969).

[3] Y. Iyengar, J. Appl. Polym. Sci., 15, 267 (1971).

[4] J. Zimmerman, Text. Mfr., 101, 49 (1974).

[5] A.R. Bunsell and J.W. Hearle, J. Appl. Polym. Sci., 18, 267 (1974).

[6] H. Watanabe, T. Takata and T. Fujiwara, reprints of annual meeting of the Society of Fiber Science and Technology (Sen-I Gakkai), Japan, p. 52 (fall, 1983), p. 2 (spring, 1984), p. 97 (fall, 1984), pg. 77 (spring, 1985).

[7] USP 4101525, USP 4195052, Celanese Corp.

This article is based on a paper delivered at the Rubber Division's Fall 1990 meeting.