Moisture exposure

When water is used as the immersion liquid, the test is essentially the ASTM Standard Test Method (D570) for Water Absorption of Plastics.( ) Determinations of the relative rate of water absorption are important in evaluating the effects of moisture exposure on such properties as mechanical strength, electrical resistivity, dielectric... 

Figure 5.4 The hot dip galvanization process involves dipping steal or iron parts into molten zinc. The zinc coats the parts, providing a barrier to oxygen and moisture. Exposure to oxygen and moisture can cause iron and steel to rust, which can weaken the metal. In this photo, a steel worker in Indiana performs the process.

The interaction of PBT with water can be complex and at first confusing unless one clearly spells out the exact conditions of moisture contact. There are two broad types of moisture exposure one is the presence of moisture in a molten polymer during processing, and the other is exposure of a solid part during its use. It is also important to understand the difference in the response of polyamides versus polyesters to water in order to choose the best material for a specific application. 

Shrinkage, coefficient of thermal expansion, crystallinity and creep are rather high, but alterations through moisture exposure are slight. 

Shrinkage, coefficient of thermal expansion and creep are highly dependent on the VA content. For example, the coefficient of thermal expansion can vary between 20 X 10 (low VA) up to 30 X 10 (high VA). The absorption of water by moisture exposure slightly increases with VA content but is always limited. 

Rigid PVC is an amorphous polymer with low shrinkage, a fair coefficient of thermal expansion for a polymer, limited creep at room temperature, and low water absorption by moisture exposure. 

Polystyrene is an amorphous polymer and shrinkage and coefficient of thermal expansion are rather low depending on the possible rubber content. The absorption and alteration by moisture exposure are low. 

Shrinkage and coefficient of thermal expansion are those of semi-crystalline polymers, that is to say, rather high. The absorption and swelling by moisture exposure are high (see Figure 4.54). Creep depends on reinforcement, moisture content and temperature. 

Alterations by moisture exposure are weak shrinkage and coefficient of thermal expansion depend on crystallinity creep resistance is rather good, the more so as the glass fibre content increases. 

Alterations by moisture exposure are weak but shrinkage and coefficients of thermal expansion are high, as for other crystalline polymers creep resistance is good at room temperature. 

Alterations by moisture exposure are weak but shrinkage and coefficients of thermal expansion are high. 

Alterations by short-term moisture exposures are fair but in long-term exposures PBI can slowly absorb significant amounts of water. The coefficient of thermal expansion is low. 

The environmental resistance of epoxy composites has come to mean the ability to withstand elevated temperature moisture exposure. Many studies have been completed on these composites 86 89) and the major conclusions were that the epoxy matrix absorbs the major portion of the moisture with the result being a reduction in the epoxy matrix Tg and, therefore, a reduction in the upper operating lim it of the composite. If the absorption and desorption of moisture is done at equilibrium conditions, the plasticization of the matrix is reversible. However, there is usually a significant... 

Figure 22 is a plot of the initial tensile modulus of the epoxy matrix after equilibrium moisture exposure and dehydration. At both 20 °C and 70 °C, the effect of moisture absorption on the matrix is reversible as evidenced by the reattainment of dry properties. The exposure at 125 °C is not completely reversible as shown by the data. 

The effect of moisture exposure on the interfacial shear strength is shown in Fig. 23 for exposure at 20 °C. For both the surface treated fiber and the surface treated and finished fiber, the exposure to moisture causes a reduction in interfacial shear strength. After dehydration of the sample, a recovery of interfacial shear strength is noted but not to the full level of the dry unexposed sample. If stress is applied in the wet state, a permanent irreversible loss in interfacial shear strength is noted. Although the ab-... 

Exposure to 70 °C gives similar results for the surface treated fiber (Fig. 24). That is, a complete reversibility in noted. The finished fiber (i.e. the fiber with the interphase consisting of the amine deficient brittle interlayer) experiences a nonrecovery of interfacial shear strength after moisture exposure and dehydration. Parallel surface spectroscopic investigation of the fiber surfaces show that under these conditions the fiber surface chemistry is not permanently altered by this exposure. Model studies of epoxies with the amine deficient composition of the interphase show that, the wet Tb of this material is about 70 °C. Therefore, the interphase is at or above its wet Tg and therefore because of the compliant nature of this material, stresses cannot be transfered efficiently and the interface is permanently distorted. 

Exposure at 125 °C is very severe for this epoxy matrix (Fig. 25). Permanent changes in the matrix are noted. The interphase layer, however, acts to mitigate some of the deleterious interfacial effects and allows that system to regain a larger portion of its interfacial shear strength after moisture exposure and dehydration. The fiber without the finish layer has lost almost all of its interfacial shear strength and recovers very little after dehydration. 

---