The Tensile Strength of Glass Yarn in Rubber. II. The Effects of Normal Humidities

The resinous impregnant in glass yarn influences the stability of the yarn. Some glass yarns suffer tensile degradation on storage and/or vulcanization at high humidities. One type of glass yarn was exposed in atmospheres over the range 0–88% relative humidity at 73° F for times up to 400 days. Tensile losses of 30% or more can occur on storage of the glass yarn at the humidities in the upper end of the range. This loss is permanent as even vigorous drying at high temperatures will not restore the tensile. Under dry conditions the yarn has excellent storage stability. Furthermore, after vulcanization even into thin composites, the glass yarn shows only a small permanent loss of tensile under moisture conditions that would seriously degrade the yarn in a package. An increased degree of vulcanization of the rubber slightly increases the tensile strength of the composite. In addition to its permanent degradative action in long-term exposure, water has a phenomenological effect of reducing glass composite tensile by its presence. A tensile sensitivity to moisture index is suggested and used to characterize the glass yarn. This tensile loss is recoverable by drying but some of the water is bound very tightly. Thin composites do not give up all of the water in 150 days over Drierite® as shown by the fact that more vigorous high temperature drying leads to a further increase in tensile strength. Tensile strength at 300° F is about 25% lower than at 73° F at any moisture content. Higher moisture contents lead to lower absolute tensiles. Tensile values are detailed for conditions that might exist during cure or in a product running at a high temperature. The high temperature incurred tensile reductions from a standard tensile are significant and should be considered in designing products. The detailed observations apply only to this particular glass yarn but the principles and methods are applicable in the evaluation or development of any glass yarn.