Shrinkage gelation

The final transverse chemical shrinkage strain is and ach is the degree of cure when chemical shrinkage is complete. The parameters c, and c2 are empirical constants obtained from chemical shrinkage characterization tests. In most cases occh can be approximated as the degree of cure at gelation. 

The number indicates the evaporation period (mini. Film is specified in reference 6. dThe first, second, and third numbers indicate acetyl content, volume percent of ethanol in gelation bath, and shrinkage temperature, respectively. A zero as the third number means no shrinkage. 

The Reynolds number in mixing operations must be > 200 The ratio of the mold filling and gelation time must not exceed 0.5 No volatile products must appear shrinkage must be compensated chemical reactions and phase transitions (crystallization) must proceed quickly, regardless of the size of the article... 

Much more detailed procedures are described in Prime (1997a). Ramis et al, (2003) use shrinkage measurements on a TMA to determine the gelation point of polyester-based thermoset coatings. As shown in Figure 3.55, the gel point is defined as the point at which the rate of change of shrinkage abruptly slows down. 

Figure 3.55. Gelation determined from shrinkage and shrinkage rate versus time data from TMA. Reprinted from Figure 3 (Ramis et al. 2003). Copyright (2003), with permission from Elsevier.

---