The flexural properties of poly(methyl methacrylate) (PMMA) reinforced with carbon/graphite (C/G) fibers with three different surface treatments were investigated by transverse bend testing after dry and wet storage. The fibers used were (1) commercially available fibers, (2) cleaned fibers, and (3) cleaned and sized fibers. The coating agents of commercial unidirectional and braided C/G fibers as well as impurities on C/G fibers for medical uses were characterized by means of high-performance liquid chromatography (HPLC). The agar overlay technique was used to assess the cytotoxicity of leachable elements from different fibers and processed composites. Composites with both unidirectional and braided tubular C/G fibers were investigated after storage in water. Fracture stress and flexural modulus decreased when "commercial" fibers were used as reinforcing material. Composites with cleaned and sized fibers gave only minor differences in flexural properties after dry and wet storage. By means of SEM micrographs the adhesion behavior of unsized C/G fibers, epoxy sized fibers, cleaned fibers, and cleaned and sized fibers were assessed. After water storage a substantial part of the cleaned fibers adhered to the matrix material. The adhesion capacity of the other fibers was reduced since the water absorption caused separation of fiber and matrix.

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