The need for materials that display high specific strength, stiffness, and toughness in excess of those displayed by single phase engineering materials has resulted in the development of composites derived from two or more non-homogeneous materials, where a reinforcing material, generally of greater strength and/or stiffness, is dispersed within a continuous bulk matrix material. Composites allow the properties of the bulk material to be tailored for a specific application. A very useful type of composite is a laminar composite with pluralities of laminae, where each lamina contains unidirectional or woven reinforcing fibers, which are combined to form a composite. The orientation of the fibers can differ from one lamina to another in a stack of laminae. The reinforcement is predominately in the direction of the reinforcing fibers in the laminae, in-plane, with little or no reinforcement to the common perpendicular to the fibers, the thickness of the laminar composite. The ultimate performance of laminar composite materials is heavily influenced by the strength and toughness of the interlaminar region where adjacent laminae intimately contact.
Enhancement of the interlaminar strength in composite materials has been achieved by four primary methods: interleaves, where a thin interlayer of an adhesive, which can be a second composite material, is placed at the interface between laminae; nanocomposite matrices, where the matrix material is further reinforced by a second reinforcing nanomaterial; Z-pinning, where laminae are connected by extending fibers through the thickness by weaving, knitting, braiding or stitching; and fiber whiskerization, where the reinforcing fibers are decorated with “whiskers” of a like or dissimilar reinforcing material. Limitations to employing these technologies have not facilitated their widespread adoption in commercial composites. For example: interleave methods reduce the composite's in-plane strength; nanocomposite matrices require costly resin transfer molding (RTM) processes and complex dispersion techniques; Z-pinning requires expensive tooling and leads to damage of the reinforcing fibers and can form defects that initiate composite failure; and fiber whiskerization remains costly and poses significant manufacturing challenges.
Hence, there remains a need for interlaminar reinforcement that maintains the laminar composite's in-plane properties yet is low cost, environmentally benign, and compatible with commercial prepreg processing. Furthermore, the method of preparing the laminar composite should be readily adaptable to a commercial production scale without the requirement of advanced tooling or resin transfer processes.