Hygroscopic expansion

BiaxiaHy orieated PPS film is transpareat and nearly colorless. It has low permeability to water vapor, carbon dioxide, and oxygen. PPS film has a low coefficient of hygroscopic expansion and a low dissipation factor, making it a candidate material for information storage devices and for thin-film capacitors. Chemical and thermal stability of PPS film derives from inherent resia properties. PPS films exposed to tolueae or chloroform for 8 weeks retaia 75% of theh original streagth. The UL temperature iadex rating of PPS film is 160°C for mechanical appHcatioas and 180°C for electrical appHcations. Table 9 summarizes the properties of PPS film. 

Each of the components, polymer and metal, has both thermal and hygroscopic expansion coefficients which may differ between components by orders of magnitude. Thus, the very act of fabrication introduces internal stresses into the device. If permitted to act, these stresses are capable of ripping apart the device, destroying it Such destruction is prevented by strong interfacial adhesion, maintained at every step during manufacture, as well as throughout the use life of the device. 

The nature of the interaction between water and the polymers is important because absorbed water can adversely affect thermal, electrical and mechanical properties of the polymer. Moisture absorption increases the dielectric constant, (5.6) and dielectric loss, (7) and has been related to device reliability problems. (8) Water-induced plasticization causes hygroscopic expansion, lowering of Tg, and degradation of mechanical properties. (9)... 

Polyimide film Hitachi Corp. reports matched CTE, thin high temperature film, good dielectric (Note other polyimides have a CTE of 30 to 50 ppm/°C) Experimental material, moisture absorption, hygroscopic expansion, bonding problems... 

B. Torstenson, M. Brannstrom, Contraction gap under composite resin restorations effect of hygroscopic expansion and thermal stress, Oper. Dent. 13 (1988) 24-31. 

Tension probes measure tensile properties such as stress and strain of thin films and fibers in addition, creep and stress relaxation measurements can be carried out in the tensile mode CLTE and hygroscopic expansion can also be measured in this mode (Prime et al. 1974). 

Hygroscopic expansion can be viewed as reversible expansion and contraction due to the sorption and desorption of water. It is much slower than thermal expansion and is best measured on thin samples in the tensile mode (Prime et al. 1974). Note that any polymeric material with some absorbed water will show simultaneous effects of thermal expansion and hygroscopic contraction when heated (Prime 1997). 

Hygroscopic expansion of the adhesive, which will affect residual stresses in the joint... 

In some cases, a pseudo-equilibrium stage is reached in the moisture uptake, which is then followed by a secondary uptake process. This is illustrated in O Fig. 31.13a. This type of behavior indicates that the absorbed moisture has triggered an additional moisture uptake mechanism. For example, differential hygroscopic expansion may lead to cracking and moisture uptake by capillary action. In order to model the moisture uptake shown in Fig. 31.13a, Mubashar et al. (2009b) proposed a dual Fickian model with a Heaviside step function. This model was termed the delayed dual Fickian model, where the secondary uptake is modeled by a power law. The mass uptake by a delayed dual Fickian model is, hence, given by ... 

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