Yaolin Zhang

Tensile properties of polymerization-filled Kevlar pulp/polyethylene composites

Polymerization-filled composites (PFC) and melt-blended composites (MBC) of Kevlar pulp/high density polyethylene composites were prepared to compare their mechanical properties. It was found that break strains of PFC composites were by far higher than MBC composites for a similar fiber concentration. Tensile data were then used to compare several models of short fiber polymer composites. Of all the models tested, it was found that Berlins approach in combination with Rosens model for critical aspect ratio give reasonable prediction. It was also found that fibers aspect ratio in composites is strongly related to the processing technique used.

Torsion Properties of High Density Polyethylene Foams

High density closed-cell HDPE foams (450–950 kg/m3) were prepared by compression molding, and torsion rectangular tests were performed to measure their shear modulus in order to study: (1) the relationship between the modulus as a function of the density, and (2) to determine the effect of thin skins. Based on the assumption that the twist stiffness (product of shear modulus and moment of inertia) of the foams is the sum of the twist stiffness of the skin layers and the core part, several structural foam models are proposed. We found that structural foam models give better results than uniform foam models, indicating that thin skins have an important effect of the shear modulus of polymer foams.

Morphology and mechanical properties of foamed polyethylene blends

Blends of polyethylene were foamed (593–782 kg/m3) by compression molding. Their morphology was investigated to understand the effect of polymer molecular structure. It was found that the cell diameter distribution can be approximated by a truncated normal distribution. Torsion, flexural and tensile properties were also measured at different strain rates. It was found that the shear, flexural and Youngs moduli of blends and their foams increase with increasing strain rate, while their normalized moduli are almost unchanged. We also confirm that thin skins have a definite effect on shear and flexural moduli, but are negligible on tensile moduli. Finally, normalized yield strength and strain are almost independent on strain rate while break strain and toughness increase with increasing molecular weight and decreasing strain rate.

Polyethylene-kevlar composite foams III: Torsion properties

High density closed-cell Kevlar-polyethylene composite foams (17 - 30% void fraction) were prepared by compression molding and characterized via torsion rectangular tests in order to determine the effect of thin unfoamed skins and Kevlar content on shear modulus. It was found that structural foam models gave better results than uniform foam models indicating that thin skins have an important effect on the shear modulus of polymer foams. The normalized modulus of our composite structural foams can be predicted by a sandwich model in combination with Berlins approach and Rosens model for the aspect ratio of the fibres.

Polyethylene-Kevlar composite foams I: Morphology

Polyethylene-Kevlar composite foams were investigated in order to understand the effect of fibers. In this first part, cell sizes, cell density and cell size distribution of the foams were measured as well as the shear viscosity of the unfoamed composites and matrix. It is shown that reinforcing fibers can act as nucleation sites for foaming depending on the processing conditions of the composites. The results are discussed in terms of fiber content and polymer-fiber interface in relation with the rheological properties of the unfoamed materials.

Polyethylene-Kevlar composite foams II: Mechanical properties

Polyethylene-Kevlar composite foams were prepared by polymerization-compounding and melt blending to investigated and understand the effect of fibers. In the first part of this study (Zhang et al., Cellular Plastics, 22, (2003), 279), we reported on the preparation and morphology of these composite foams. In this second part, the effect of fibers on the mechanical properties of composite foams is investigated. Using several models for particulate composites, it was found that the normalized modulus of our composite foams can be well predicted by the simple Moores empirical equation to take into account the foam morphology in combination with Berlins approach and Rosens model for critical fiber aspect ratio to account for the effect of fibers.

Effect of processing on ductility and strength of kevlar/polyethylene composites

Polymerization-filled composites (PFC) and melt-blended composites (MBC) were prepared to compare their mechanical properties. Improved ductility was obtained for PFC resulting from better fiber-polymer interfacial adhesion. On the other hand, ductility decreased upon increasing fiber content and strain rate, while normalized strengths were almost unchanged. This indicates that matrix and composites have similar responses to strain rate. Tensile strengths were compared with several modified models to include the effect of critical fiber aspect ratio. It was found that the numerical integration model with perfect interfacial bond in combination with Rosens method for the critical fiber aspect ratio gave the best predictions among all the models tested. The results clearly show that the preparation technique has an effect on tensile strength of composites in relation with fiber distribution and interfacial adhesion.