Interfacial properties and initial step of the water sorption in unidirectional unsaturated polyester/vegetable fiber composites

In this work the interfacial properties of polyester/vegetable fiber composites were analyzed by flexural testing. The compressive/tensile (sigma) and shear (tau) stresses were determined for each composite in function of the span-to-depth ratio (lambda). The general behavior of the composites was similar to that of composites reinforced with DuPont Kevlar fiber, i.e., a maximum sigma stress value is obtained. Flexural test validity for determining the Young's and shear moduli, E-11 and G(12), was ascertained. The Young's modulus agreed with that expected from the rule of mixtures for the composites with lowest fiber content. Short beam tests were performed on the composites. The shear stress value was improved by means of the matrix modification. Moisture sorption experiments and dynamic mechanical analysis were also performed on the natural fiber composites in the first Fickian step. Water sorption at 50% RH and 90% RH can be satisfactorily described by using a diffusional model. Water diffusion on parallelepiped samples shows a positive deviation from the Fickian behavior. Fiber capillary flow occurs through the fiber and the debonded matrix/fiber interphase during the initial Fickian step.