How to select short-cut carbon fibers for composite materials, interface bonding and dispersion?

Selection criteria for interface bonding

Interface bonding determines the stress transfer effect between carbon fibers and the matrix resin, which is a key factor affecting the mechanical performance of the composite material. The surface of carbon fibers is usually treated with sizing, and different sizing agents are suitable for different resin systems. The selection should first match the type of the matrix.

When selecting the sizing system, the resin can fully penetrate the fiber surface, and the load can be effectively transmitted after being stressed, and there is less tendency for fibers and the matrix to detach and separate. If the sizing agent and the matrix do not match, even if the fiber itself has excellent performance, the interface defects are likely to occur after molding.

The interface is not necessarily the stronger the better. If the interface bonding is too strong, the material is prone to brittle fracture when subjected to external force impact; if the bonding is too weak, fibers may be pulled out and the bearing capacity may be insufficient. It is necessary to select an appropriate surface treatment scheme based on the product's working conditions, taking into account both strength and toughness.

Selection criteria for dispersion

Dispersion refers to the ability of short-cut carbon fibers to be uniformly dispersed in the resin system. Poor dispersion will result in clustering and forming stress concentration points, directly reducing the yield of the product.

The dispersion effect is related to the fiber length, sizing formula, and powder flowability. Longer fibers are more likely to intertwine and cluster, suitable for specific molding processes; shorter fibers have a lower dispersion difficulty and better processing fluidity.

The processing method should also be taken into consideration. Injection molding, compression molding, and pultrusion have different requirements for dispersion. Injection molding has higher requirements for dispersion stability, and agglomerates are prone to block the flow channel, causing product defects. At the same time, the mixing process and stirring method can also affect the final dispersion state, and the fiber's own indicators cannot be the only consideration.

Summary of selection

Interface bonding and dispersion need to be balanced and chosen. In many cases, improving the interface penetration may have an impact on dispersion. The selection strategy is to first determine the matrix resin, match the corresponding sizing type, and then select the appropriate fiber specification based on the molding process.

It is recommended to conduct small-scale sample trials in the early stage to observe the mixing state and the cross-section after molding, and comprehensively evaluate the penetration effect and the uniformity of dispersion to find a product that is compatible with the own formulation system.

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