Studies of Elastomer Matrix Flow Mechanisms in Nonwoven Reinforced Composites
Doctoral thesis, 1993
The general aim of this work was to develop a suitable technique for fabricating elastomer composites based on three-dimensional fibrous materials and to study the rheological behavior of matrix polymer in these structures with varied structural and fiber surface characteristics.
A new type of mold was developed and tested for structural reaction injection molding (SRIM) of elastomer composites having polymer matrices made from a mixture of two components. These mixtures had short pot life and relatively high viscosity. Channel systems for polymer feeding and vacuum application were studied. The mold made possible the investigation of the mechanism of polymer matrix flow and fiber breakage during the formation of fiber-reinforced composites made of long fiber based non- woven fiber reinforcement structures.
Theoretical and experimental analysis were made of the interactions between the structural parameters of the fibrous mats and the flow characteristics of the matrix with systematically varied material and process parameters. In nonwoven mats with parallel laid fibers the flow rate along the fiber direction was found to be significantly higher than the flow rate crosswise. Nonwoven mats with multidirectionally laid fibers on the other hand exhibited a radial flow pattern. The matrix flow distance was proportional to the logarithm of injection time. The decrease of pressure in the mold cavity was linearly proportional to the matrix flow distance.
A method was developed to measure the adhesion between a fiber and a fast curing elastomer. Multiple droplets were formed along the length of a single fiber by following a special technique and they were then used to measure the fiber-matrix bond shear strength using the microbond pull-out method. Polyester, p-aramid, and HMW- polyethylene fibers were tested with matrix polymers of polyurethane and natural rubber latex. The effect of surface treatments including epoxyde-coating, corona, gamma-radiation, or methanol extraction on fiber-matrix adhesion was studied.
Studies were made of the influence of fiber type, fiber surface properties and matrix type on some mechanical properties in elastomeric composites produced by means of SRIM and LC fabrication methods. The fibers used were PET, LLDPE and p-aramid. These fibers were treated with epoxyde, styrene and isocyanate derivatives which make the fiber surface chemically reactive. Treatments were also made with NaOH and copolymer of polyester and polyol ether, causing change in the fiber surface energy. The results show that the surface treatments which produced changes in surface energy also influenced the flow rate of matrix polymer during the composite fabrication process. The treatments resulted in chemically reactive fiber surfaces which increased the fiber-matrix bond strength but did not affect the Young's modulus of the composite material. Good correlation was found between fiber-matrix bond strength and surface energy of fiber. The dispersive component of surface energy was shown to play important role for polyurethane elastomer and surface modified PET fibers. The age of polyurethane matrix polymer has a marked influence on the bond strength. The fiber volume fraction in composites has a strong influence on the Young's modulus of the elastomer composite.
A method was developed for the fabrication of elastomer composites having three- dimensional fibrous reinforcement and matrix of natural rubber. This was achieved by applying the natural rubber as a latex when mixing it with the fibers. As compared with earlier known processes, the viscosity is considerably lower thus permitting the use of long fibers in such reinforcement structures.