Shrinkage secondary

Fig. 12.13 Shrinkage of secondary flame zone by suppression with KNO, at e = 6.3 and 11.7.

Experimental data [9, 10] sometimes clearly show the onset of a secondary shrinkage process while the primary shrinkage process is underway. By using equation (2) for the wet end and the dry end separately one obtains an excellent fit (Fig. 5) for the complete data set [5] and can use the fitting parameters and the curvature maxima for the identification of the two shrinkage phenomena. 

Figure 13.13 shows such a distribution measured by Menges and Wiibken (30) for amorphous PS. They measured the shrinkage of microtomed molded samples at elevated temperatures. Figure 13.13(a) shows the longitudinal (flow direction) orientation distribution at two injection rates. The characteristic features of the orientation distribution are a maximum orientation at the wall that vanishes at the center with a local maximum near the wall. In Fig. 13.13(b), the longitudinal orientation at the wall and secondary maximum orientation are in close proximity, and the transverse orientation drops continuously from a maximum value at the surface. 

Extenders may also provide one or several secondary functions in addition to cost reduction. Depending on the type and concentration of the extender, it could reduce shrinkage that occurs on polymerization or during aging, lower the coefficient of thermal expansion of the adhesive formulation, provide a higher viscosity to avoid adhesive starvation in the joint, or perform a number of other property improvements. 

The structure has primary and secondary alcohol groups uniformly interspersed throughout the length of the polymer chain. These hydroxy units impart high water absorption characteristics to the fiber and can act as reactive sites. The cotton fibers are hydrophilic and swell in water. It is hydrolyzed by hot acid and swollen by concentrated alkali. The cotton is treated with caustic soda solution (12 to 25 percent) under tension to develop a silk-luster and stop longitudinal shrinkage. This process is called mercerization. Mercerized cotton exhibits increased moisture and dye absorption. 

Another mechanism for particle formation involves the scheme shown hi Fig. 7, but it occurs later in some batch polymerizations. This secondary nudeation is caused by free emulsifier which is liberated from the particle surface. Figure 9 shows some data of Gerrens for a batch emulsion polymerization of MMA. These data were successfully modeled by Ray and Min (1976). The population of smaller particles was formed late in the reaction because emulsifier was desorped because of the crowding of radical end groups on the surface and because of particle shrinkage caused by monomer conversion to polymer. 

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