Processing discontinuous fiber polymer composites: Fiber alignment using electric fields and microstructure-property relationships

Discontinuous fiber polymer composites are finding widespread use because of their ease in processability and improvements in performance over un-reinforced polymers. However, their use is limited to non-structural applications due to the difficulties in controlling fiber orientation and fiber length reduction during melt processing of long fibers at high volume fractions. Micromechanics models indicate that the elastic behavior of an aligned discontinuous fiber composite should approach the behavior of a continuous fiber composite if the length of the reinforcement exceeds critical fiber length. This provided the motivation for research in the development of a process that has the potential to manufacture aligned discontinuous or short fiber polymer composites and subsequently investigate the microstructure-property relationships.;The unique combination of fiber alignment using electric fields in air and the recent advances in polymer powder processing resulted in a novel, high speed, solvent free Aligned Discontinuous Fiber (ADF) composite process. A semi-continuous laboratory prototype ADF process has been developed which has the following unit operations: alignment of conducting or insulating fibers in air using electric fields in an orientation chamber; polymer powder coating/impregnation of fibers, and; compression molding of the powder coated ADF preform into a composite. It has been found that alignment of dielectric E-glass fibers in air using electric fields is an extremely fast process (less than 1 second), but the non-buoyant nature of the fiber motion makes the alignment behavior very complicated and is a balance of polarization forces due to the electric fields; hydrodynamic forces due to fluid resistance, and; the rotational behavior of fibers during free fall. Chopped E-glass fibers of lengths ranging from 3 to 25 mm have been successfully aligned in air using A.C. electric fields of intensities ranging from 300 to 600 KV/m.;The ADF process has been demonstrated for chopped E-glass fiber and nylon12 matrix system and the properties of the composites fabricated using the process improve with an increase in fiber alignment and an increase in fiber length. Improvements in modulus and strength values of the ADF composites with fiber alignment ranged from 70 to 100%, when compared to equivalent composites that were manufactured with random fiber orientation. Micromechanics analysis based on fundamental reinforcement theories, indicated that in the chopped fiber-thermoplastic systems, it is the effective aspect ratio of the fiber aggregate (bundle) that controls the elastic behavior of ADF composites.