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Reason for Step Cure Cycle on Phenolic Prepregs

Reason for Step Cure Cycle on Phenolic Prepregs

Reason for Step Cure Cycle on Phenolic Prepregs


I have a question why the phenolic prepregs get a different cure cycle than the epoxy ones. Customer request a dwell or stabilization step for 30 minutes for the phenolic prepreg at temperature of 175°F to 185°F. After those 30 minutes the temperature goes up again up to 260°F for 90 minutes cure.

How is that process call and why is performed or whats the main purpose of it? I mean about the 30 minutes at 175°F to 185°F.

Thanks for your valuable support!

RE: Reason for Step Cure Cycle on Phenolic Prepregs

The purpose of dwells in cure cycles is to allow the resin to react and increase in viscosity before going to higher temperature, which will result in a drop in viscosity. Dwells are also almost always done before applying autoclave pressure, which allows for easier removal of volatile material because the fibers behave like a sponge that has not yet been compacted. This prevents excessive resin bleed, which would result in low resin content and voids. At the end of the dwell the prepreg will have lots of small low-pressure bubbles in a high viscosity resin. Application of autoclave pressure then compacts the fibers together and compresses the bubbles to the point that they collapse and go back into solution in the resin. The resin will have too high a viscosity to be squeezed out of the fibers.

Dwells are also very commonly used with epoxy prepregs. The dwell temperature is selected according to the the resin reactivity, where in 30 minutes the viscosity increase is significant but there is no chance of fully curing the resin. With phenolic resins one other important consideration is that they are condensation curing and release water as part of the chemical reaction. Thus you do not want to get too close to boiling the water or there will be severe foaming of the resin. This water ends up in the final cured resin matrix as microscopic bubbles or dissolved in. This will slowly dry from the cured resin and care must be used when postcuring parts that they be heated slowly, so that they do not explode (delaminate) from internal steam pressure. This drying also results in significant resin shrinkage causing internal stresses. The most descriptive name for this type of cure cycle is a "dwell cure" but there is no official name.

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