M2 Manufacturing by Resin Curing (Crosslinking Kinetics)

Topic

During the processing of this laminate, the thermosetting matrix resin undergoes a cross-linking reaction; the kinetics of this process are described by the Kamal-Sourour autocatalytic model. Unlike a reaction where the rate simply decreases as the reactant is consumed, in an autocatalytic reaction, the cross-linking products themselves catalyze the reaction's progression. Because these products act as catalysts, the cross-linking rate initially increases as the reaction proceeds, before eventually decreasing once the reactant is depleted.

The kinetic constants governing this cross-linking reaction depend on the curing temperature, following an Arrhenius-type relationship: higher curing temperatures result in higher kinetic constants and a faster reaction rate, while lower curing temperatures result in lower kinetic constants and a slower reaction rate.

As the cross-linking reaction progresses—regardless of the rate—the resin viscosity increases, as described by the Castro-Macosko model. As cross-linking advances, polymer chains link together to form increasingly large molecular networks; the larger these networks become, the more viscous the resin grows. Sufficient progression of cross-linking causes this viscosity to rise increasingly sharply—according to the Castro-Macosko model—up to the gel point.

At the gel point, the molecular network formed by cross-linking spans the entire volume of the resin for the first time, causing the resin to lose its ability to flow like a liquid; the gel point marks the liquid-to-solid transition of the matrix. The fact that cross-linking continues beyond the gel point causes the matrix—now a solid—to harden further, without ever reverting to liquid-like behavior.

Consequently, this model—based on the initial cross-linking reaction—establishes the processing conditions that precede two other models in the set: the one describing classical laminate theory and the one describing the effective elastic properties of each lamina. Because this model determines when the matrix passes the gel point and completes cross-linking, the other two models can take as their starting point the fact that the matrix has already attained the final properties—properties it possesses only after the process described by this model has taken place.

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