Influence of the Interphase on the Mechanical Properties of Nylon 66 Composites

Abstract

The mechanical properties of glass fiber and carbon fiber reinforced nylon 66 were investigated using both microscopic and macroscopic testing techniques. The objective was to determine how different interphase morphologies affect the adhesion and properties such as damping, ultimate stress and strain, and modulus of the composite. This was accomplished using a modified fiber pull-out test on single filament composites, and dynamic mechanical analysis, vibrational adhesion testing, and uni-axial tension testing on bulk composite samples. Additional techniques such as scanning electron microscopy, profilometry, thermogravimetric analysis, differential scanning calorimetry, and water absorption measurements were performed to assist in data interpretation. The specific interphase that forms in both glass reinforced and high modulus carbon fiber reinforced nylon 66 is termed transcrystallinity. Previous work has shown that this region can be altered by the addition of a specific diluent, poly(vinyl pyrrolidone), as either a blend to the matrix or as a fiber sizing. The diluent serves to dampen nucleation on the fiber surface thus causing the interphase to change from transcrystalline in nature to spherulitic. The changes in composite properties that the different interphases produce were examined. Results from the modified fiber pull-out test showed that the interfacial shear strength decreases as the interphase becomes more spherulitic. Scanning electron microscopy revealed a more cohesive fracture surface of the samples having a transcrystalline interphase. Dynamic mechanical analysis showed that the damping behavior of E-glass/nylon 66 composites does not change with PVP sizing, while carbon fiber/nylon 66 composites showed a decrease in damping with the addition of sizing. Vibrational adhesion testing showed similar effects in the loss tangent of both composites versus fiber sizing. In addition, uni-axial tensile testing revealed an increase in the ultimate strength and toughness of both composites. On the other hand, neither the ultimate strain or modulus was a strong function of fiber sizing.