Voids mechanical entrapment

The occurrence of voids has been thoroughly documented in thick laminates [2], In almost all cases, they are apparently associated with the prepreg surface. The exact mechanism of void formation depends on the system, but in the most general case it can include mechanical entrapment as well as nucleation of stable voids in the resin phase. 

Voids can be formed by either entrapment of air mechanically or by one of two nucleation processes. Mechanical entrapment could include (1) entrained gas bubbles from the resinmixing operation, (2) bridging voids from large particles or particle clusters (quenched DDS curing agent, airborne particles, or paper release agent), (3) voids from wandering tows, fuzz... 

Significant amoimts of air may be trapped in the fresh DSP mix during mixing. This air is difficult to remove because of the thixotropic nature of the mix. The presence of air voids in the hardened material is imwelcome, because they adversely affect the strength properties. To remove the entrapped air, vacuum deaeration of the mix after mixing and prior to casting may be necessary. Alternatively, the entrapped air may be removed by deaeration of the mix after it has been cast, or by applying mechanical pressure to the cast mix for a few seconds (Richard and Cheyrezy, 1995). 

Modified part A of the resin system was then mixed with part B at a ratio of 10 3 using a high-speed mechanical stirrer at 800 rpm for 8-10 min (Fig. 21.1 (b)). The intense mechanical mixing produces air bubbles in the mixture, which if not taken out will increase the void content in the composite. These trapped air bubbles were taken out by keeping the mixture in a high-vacuum desiccator chamber for 30 min (Fig. 21.1(c)) which pulls out the entrapped air from the mixture. Once the degasification was done, the mixture was taken out for infusion. 

Minimum void content in composites is very much required for maximizing mechanical properties. During application of vacuum to the layup prior to compression molding, the air trapped between various plies can be removed to achieve minimum air entrapment. The ply assembly is wrapped with a high-temperature-resistant Kapton film and vacuum applied. The void content in the composite panel can be reduced to < 0.8% by volume [66]. 

---