Glauber's salt heaving in Kevlar 49 fibres

Abstract

The degradation in strength of Kevlar 49/epoxy resin composite materials is well documented and is attributed mainly to uptake of water from the environment. For a recent review of mechanical property data collected during the past several years at the Lawrence Livermore National Laboratory, see Morgan et al. [1]. Scanning electron microscope studies on single filaments reveal the occurrence of transverse and longitudinal fractures [2], Fig. 1. The same authors [1] have correlated the structural and mechanical degradation by diffused water with acid and alkali impurities in the fibre; Kevlar 49 fibre is spun from a poly (p-phenylene terephthalamide)/sulphuric acid dope and excess acid is subsequently neutralized by a dilute caustic soda spray [3]. However, chemical analyses also reported by Morgan et al. [1] indicate that about one half of the 1.5 wt % impurity content in Kevlar 49 fibres is in the form of sodium sulphate (Glauber's salt), and this leads us to propose the volume expansion that accompanies hydration to solid hydrate as a more likely mechanism for transverse and longitudinal fibre fracture. The phase diagram for the system NazSO4H20 is shown in Fig. 2 [4]. The stable hydrates are the monohydrate Na2SO4"H20 and the decahydrate Na2SO4"10H20. A metastable heptahydrate Na2SO4"7H20 has also been reported [4] but, for simplicity will be omitted from the present discussion. Referring to Fig. 2, there exists a range of temperatures up to 31.5°C in which decahydrate is in equilibrium with liquid solution. There is a unique temperature, 31.5°C, at which decahydrate and monohydrate together are in equilibrium with liquid solution. Above 31.5°C liquid solution is in equilibrium with monohydrate. (Incidentally, the solubility of the monohydrate decreases with increasing temperature due, presumably, to there being sufficient ordering between spheres of hydration in the liquid solution to more than compensate for the entropy of mixing). If there is a temperature at which anhydrous solid Na2SO4 coexists in equilibrium with liquid solution, it is above 100°C. At any given temperature, however, there is a partial pressure of water vapour in equilibrium with coexisting anhydrous N%SO4 and monohydrate. At temperatures greater than 100 ° C this partial pressure is less than that in equilibrium with any aqueous solution of Na2SO4. Above 31.5 ° C, the decahydrate will relinquish water to the solution so that the latter becomes more dilute. Below 31.5°C, the monohydrate takes up water from the solution and in so doing becomes converted to decahydrate, and again the solution becomes more dilute. A source of water at higher chemical potential than in the saturated sodium sulphate solution would favour the more hydrated state (for a Kevlar 49/epoxy composite exposed to a humid environment, pure water which has diffused in from the outside would be such a source). The temperature below which the decahydrate is stable with respect to the monohydrate would then be higher than 31.5 ° C. At this temperature the hydrate exerts zero pressure on its surroundings. However, when the temperature