Volume changes during curing

Figure 20. Effect of ram pressure on maximum bed volume change during cure at 80° F.

Bilyeu, B. Brostow, W. Menard, K. Determination of volume changes during cure via void elimination and shrinkage of an epoxy prepeg using a quartz dilatometer cell. Polimery 2001, 46, 794. 

Figure 14.33 Variation of volume change during cure at 35 °C of unsaturated polyester resin systems having MEKP as an initiator and cobalt octoate as an accelerator in the presence of PVAc at different concentrations (wt %) (O) 3.5, (A) 6.0, and ( ) 10. (Reprinted from Li and Lee, Polymer 39 5677. Copyright 1998, with permission from Elsevier.)...

Delaminations can occur during cure as a result of high internal stresses. These stresses develop due to resin shrinkage and thermal volume changes. The level of stresses depend on several material properties, such as the Young s modulus, Poisson s ratio, and thermal expansion coefficients of both resin and fibers. In addition, the level of stresses also depends on several conditions, such as fiber orientation, fiber volume fraction, and part geometry. 

Material Properties. The elementary form of the analysis used requires the following properties of propellant during cure tensile modulus, effective bulk modulus, and propellant volume change. Each of these properties changes with temperature and time elapsed since casting. Because of the unusual nature of the material (sticky, wet, explosive, gravel ), special tests, equipment, and techniques were developed for these measurements. 

Pressure tests are performed in an external pressure chamber by applying equal pressure to both solvent and water phase (powder bed) to simulate solvent pressure in an actual casting. Ram pressure is simulated by applying a differential positive pressure across the septum which acts on the powder bed. Thus, bed volume change, bed sorption, and total volume change can be followed during cure as a function of time, temperature, solvent pressure, and ram pressure. 

This same apparatus permits estimates to be made of the so-called jacketed compressibility (the reciprocal of the effective bulk modulus) during early cure. This is accomplished by applying momentary pressure to the bed alone (ram pressure) and observing the corresponding bed volume change in the water meniscus. 

When a plot of maximum total volume change vs. percent theoretical casting powder density is extrapolated to 100% of theoretical density, the total volume change is usually less than 0.1%. Thus, during cure of a double-base propellant, the volume change caused simply by mixing polymer and solvent is small. 

One of the requirements for metasomatic transformations is that the volume changes between the host and metasomate are minimal. The changes during formation of Pb02 from several lead compounds contained in the cured paste are listed in... 

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