Aging Resulting from Water Absorption

A simple and very useful method of studying humid aging consists of exposing an initially dry sample in a medium at constant temperature and relative humidity (or water activity in immersion) and recording weight changes. Various types of gravimetric curves can be obtained the most frequent ones are shown in Fig. 14.1. 

In case (a), an equilibrium is reached. It can be considered here that there are only reversible physical interactions between the polymer and water. Drying leads to a curve that is practically a mirror image of the absorption curve. The behavior of the material can be characterized by two quantities the equilibrium water concentration Woo, which characterizes the polymer affinity for water (hydrophilicity), and the duration of the transient, which is sharply linked to the sample thickness L and to a parameter characteristic of the rate of transport of water molecules in the polymer - the diffusion coefficient D. 

In cases (b) and (b ) there is no equilibrium the mass increases eon-tinuously or decreases after a maximum, which indicates the existence of an irreversible process - chemical, hydrolysis, or physical, damage microcavitation increases the capacity of the material for water sorption. The experimental curves having the shape of curves (b) or (b ), indicate that the irreversible processes induce significant mass changes in the timescale of diffusion. When the irreversible processes are significantly slower than diffusion, the behavior shown by curves (c) or (d) is observed. The sorption equilibrium is reached at t t, and a plateau can be observed in the curve 

Since both reversible and irreversible processes are influenced in distinct ways by temperature and water activity, the first step of a humid aging study consists of searching for the conditions (T, RH, sample thickness) in which both phenomena can be clearly decoupled, as in Figs 14.1c and d. The interpretation of experimental results and the modeling of the kinetics of property changes would be difficult or even impossible if physical characteristics such as Woo to (or better D) were not known. 

Polymer hydrophilicity can be judged in the cases (a), (c), or (d) of Fig. 14. 1. It is defined as the affinity of a polymer for water, which can be quantified by the equilibrium mass gain, W o, determined in standard conditions, e.g., in a saturated atmosphere from a sorption experiment. W o depends on the vapor pressure or activity of water and on the temperature. It varies, typically from 0 to 10% in most networks. 

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Physical aging resulting from water absorption is probably the most widely studied process. The structure-property relationships in this field are relatively well understood, except for diffu-sivity, which seems to depend sharply on the strength of polymer-water interactions. The resulting plasticization is also relatively well understood, but the research field on swelling in the glassy state still remains open. 

Chemical aging resulting from water absorption (i.e., hydrolysis) has not been as widely studied as physical aging. It is relatively well understood at the molecular scale (chemical mechanisms). But macromolecular (kinetics of decrease of the elastically active chain concentration) and mechanical aspects (effect of chain scissions on mechanical properties) are far from being elucidated. 

The diet is the major route of human exposure to cadmium. Contamination of foods with cadmium results from its presence in soil and water. Concentrations of cadmium in foods range widely, and the highest average concentrations are found in mollusks, kidneys, livers, cereals, cocoa, and leafy vegetables. A daily intake of about 60 [tg would be required to reach a concentration of 50 mg/kg in the renal cortex of persons 50 years of age, assuming an absorption ratio of 5%. About 10% of the absorbed daily dose is rapidly excreted (WHO, 1989, 2001). 

It was originally found that the absorptive capacity for SO4 ions at room temperature decreased spontaneously with time. It was learned that differences in dyeing resulting from differences in aging had been encountered, and the manufacturer stated that a treatment with steam or hot water successfully minimized such differences. This information was the source of what has been called the stabilizing treatment. Presumably treatment with hot water accelerates the aging process. 

Extraction studies have also been carried out by grinding the ageing cements and extracting the soluble ions with water (Wilson Kent, 1970 Crisp Wilson, 1974). Ion content was determined using atomic absorption spectroscopy. The experiments give different, but complementary, results to those of Cook (1983), since what is extracted are those ions that have been released from the glass powder but not yet insolubilized by reaction with the polyacid. 

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