Continuous fiber composites with elastomeric interphases covalently bonded to both fiber surfaces and epoxy matrices

Carbon fiber/epoxy resin composites have been made containing polyurethane elastomeric interphase layers bonded to both the fibers and the matrix. Carbon fibers (Celion G30-500 and Thornel T300) were oxidized (115{dollar}spcirc{dollar}C) in nitric acid followed by reaction with tetraethylenepentamine (TEPA) to covalently bond high surface concentrations of amines. The concentration of acidic groups increased linearly from 13 to 34 to 49 {dollar}mu{dollar}eq/g with HNO{dollar}sb3{dollar} oxidation times of 20, 60 and 90 min., respectively. About one half of acidic groups were consumed in the reaction with TEPA. The TEPA molecules were grafted as linear or looped structures.; The reactivity of these surface-bound amino groups toward phenyl isocyanate and acetic anhydride was examined. They reacted with 52% and 60% of the surface amino groups, respectively. Polymeric reagents gave lower grafting efficiencies. About 2-3 micro-moles of isocyanate prepolymer (molecular weight 1350) and 3-5 micro-moles of epoxy resin (molecular weight 454) were grafted. Three types of polyurethane elastomers were used for interphases. Their moduli (and elongations at break) were 0.5 MPa (900%), 7.9 MPa (290%) and 138 MPa (29%), respectively. The epoxy matrix (from EPON 830) had a modulus and elongation of 1340 MPa and 6%, respectively. Solution coating methods generated interphase layers on fiber surfaces. The interphase thicknesses (from 0 to 5000 A) were controlled by using different solution concentrations of isocyanate prepolymers. The prepolymers were cured as thin layers on the fibers via reaction with H{dollar}sb2{dollar}O in air and followed by heating.; The contributions of these elastomeric interphases to the composite properties were evaluated by single filament fragmentation tests (SFFT), Izod impact tests, three-point bending tests and dynamic mechanical thermal analysis (DMTA). The impact strength (IS) increased while the interlaminar shear strength (ILSS) decreased with increasing interphase thickness using an interphase modulus of 0.5 MPa. Impact strength was improved and ILSS was preserved when the interphase modulus was 138 MPa. Nitric acid oxidation of fibers considerably reduced the IS of composites. However, the introduction of elastomeric interphases (Modulus 7.9 and 138 MPa) to composites made from fibers oxidized with HNO{dollar}sb3{dollar} (20 min.) increased IS by 30% to 80% and maintained the ILSS. The critical transfer lengths increased as the elastomer thickness and modulus were increased and decreased, respectively.