The Effects of Temperature and Humidity on the Physical Properties of Tire Cords

It may be concluded from the foregoing experiments that measurements of tenacity and stretch at a single relative humidity have little meaning, since the important variable, moisture content, decreases rapidly with increasing temperature. The most revealing representation of tenacity or elongation appears to be as a function of temperature at constant moisture regain. Curves of this .type show that the tenacity of a modern viscose rayon cord decreases with increasing temperature in the range of regains 0–5 per cent. The same general behavior is exhibited by low-stretch cotton cord, but the tenacity vs. temperature curves for medium-stretch cotton cord flatten off at temperatures above 100° C. The tenacity of Fortisan cord decreases rapidly with temperature up to 100° C, and then increases slightly between 100° and 165° C. Nylon cord shows a rapid decrease of tenacity with increasing temperature, but maintains its advantage in tenacity over that of rayon for all temperatures and humidities included in these experiments. For any given temperature, increase of moisture content increases the tenacity of the cotton cords to an extent dependent on the “stretch” of the cord; decreases the tenacity of rayon cord. Fortisan increases in tanacity with increasing moisture content in the lower range of moisture contents (like cotton) and then decreases in tenacity for higher moisture contents (like rayon). The tenacity of Nylon decreases very slightly with increasing moisture content. All of the cord materials investigated, cotton, rayon, Fortisan, and Nylon, increase in stretch (10-pound or ultimate) as the moisture content increases. The effect of temperature at constant regain on stretch, however, is rather complicated. Until a method of measuring stretch with the whole cord continuously within the testing chamber is devised, further conclusions would be unjustified. Analyses of the creep characteristics of the various cord materials on the basis of two arbitrarily defined indices “initial compliance” and “weighted creep” yield several basic facts: The low-stretch cotton cord exhibits lower values of both indices compared to medium-stretch cotton, as might be anticipated from tensile measurements. The rayon cord, however, has a much lower “initial compliance” but a definitely higher “weighted creep,” compared to the low-stretch cotton. The Fortisan cord is comparable in “weighted creep” to low-stretch cotton, but has a much lower “initial compliance”. The Nylon cord has an “initial compliance” approaching that of low stretch cotton, but a much higher “weighted creep”, which is equal to or greater than that of rayon. The elongation increment vs. logarithmic time curves for cotton, rayon, and Fortisan are very nearly linear over the period 0.002–20 hour, but those for Nylon show a tendency to increase in slope at times beyond 1 hour. In conclusion, it may be said that the results of these experiments agree very well with current concepts of the structures of cotton, rayon, and Nylon, and have rather interesting implications in regard to the structure of Fortisan. It is well to remember that a complete evaluation of a tire cord should include dynamic fatigue measurements as well as tensile and creep data. The complicated nature of the fatigue problem, however, necessitates an extended separate discussion which is beyond the scope of this paper.