Jingshen Wu

Effect of fiber pretreatment condition on the interfacial strength and mechanical properties of wood fiber/PP composites

The effect of fiber surface pretreatment on the interfacial strength and mechanical properties of wood fiber/polypropylene (WF/PP) composites are investigated. The results demonstrate that fiber surface conditions significantly influence the fiber–matrix interfacial bond, which, in turn, determines the mechanical properties of the composites. The WF/PP composite containing fibers pretreated with an acid–silane aqueous solution exhibits the highest tensile properties among the materials studied. This observation is a direct result of the strong interfacial bond caused by the acid/water condition used in the fiber pretreatment. Evidence from coupling chemistry, rheological and electron microscopic studies support the above conclusion. When SEBS-g-MA copolymer is used, a synergistic toughening effect between the wood fiber and the copolymer is observed. The V-notch Charpy impact strength of the WF/PP/SEBS-g-MA composite is substantially higher than that of the WF/PP composite. The synergistic toughening mechanisms are discussed with respect to the interfacial bond strength, fiber-matrix debonding, and matrix plastic deformation.

Nanoscale characterisation of interphase in silane treated glass fibre composites

The properties of the interphase formed between a glass fibre and a polymer resin have been characterised based on novel techniques, including the nanoindentation and nanoscratch tests and the thermal capacity jump measurement. The variation of interphase thickness affected by differing silane coupling agents is specifically evaluated. The nanoindentation test gives an interphase thickness of approximately 1 μm with large variations between specimens, and is not sensitive enough to identify the effect of different silane agents. The effective interphase thickness measured from the nanoscratch test varies between 0.8 and 1.5 μm depending on the type and concentration of silane agent. The higher is the silane agent concentration, the larger is the interphase thickness. These values are consistent with those measured based on the heat capacity changes in terms of general trend, although the latter technique tends to present slightly higher values. The foregoing observations strongly support the usefulness of the techniques for interphase characterisation.

Strain monitoring in FRP laminates and concrete beams using FBG sensors

The fibre-optic Bragg grating (FBG) sensor is broadly accepted as a structural health monitoring device for fibre reinforced plastic (FRP) materials by either embedding into or bonding onto the structures. The accuracy of the strain measured by using the FBG sensor is highly dependent on the bonding characteristics among the bare optical fibre, protective coating, adhesive layer and host material. In general, the signal extracted from the embedded FBG sensor should reflect the straining condition of the host structure. However, due to the existence of an adhesive layer and protective coating, part of the energy would convert into shear deformation. Therefore, the mechanical properties of these materials would affect the resultant strain measured by embedding a FBG sensor into the structure. This paper presents a theoretical model to evaluate the differential strains between the bare fibre and host material with different adhesive thickness and modulus of the protective coating of the embedded FBG sensor. The results are then compared with numerical analysis by using the finite element method (FEM). Experimental work was conducted for both glass fibre composites and FRP strengthened concrete beams with embedded FBG sensors. Externally bonded strain gauges were used to compare the results obtained from the FBG sensors. The theoretical predictions reveal that the axial strain measured at the fibre-core region is lower than the true strain of the host material with increasing thickness of adhesive layer. A thick adhesive layer and low modulus of coating material would enlarge the shear stress concentration area at the bonded end region. An experimental investigation also shows that the FBG sensor can be confidently used with sufficient bond length.

The dielectric and mechanical properties of a potassium-titanate-whisker-reinforced PP/PA blend

A group of polypropylene/polyamide/potassium-titanate-whisker (PP/PA/whisker) composites was prepared by means of a Haake mixer. The whisker surface was modified using either a silane or a titanate coupling solution. The effects of whisker surface modification and whisker content on the dielectric and mechanical properties of the composites were then studied. Results show that whisker-surface modification has a remarkable influence on the properties and a properly selected coupling agent can effectively improve the dielectric stability of the composites under moist conditions. It was also found that both mechanical and dielectric properties increased with whisker content below additions of 20 phr whiskers into the PP/PA blend. However, property deterioration was observed when the whisker content was above this level. This was particularly true when water-treated specimens were tested. Microscopy studies revealed that the composite had a continuous PP phase and a dispersed PA phase. The whiskers were embedded in the PA particles exclusively at 20 phr or below. It is believed that this microstructure was caused by the strong interaction between the surface-modified whiskers and the polar PA molecules and may be responsible for the property transition observed in dielectric and mechanical tests.

High-impact polystyrene/halloysite nanocomposites prepared by emulsion polymerization using sodium dodecyl sulfate as surfactant

High-impact polystyrene (PS) nanocomposites filled with individually dispersed halloysite nanotubes (HNTs) were prepared by emulsion polymerization of styrene in the presence of HNTs with sodium dodecyl sulfate (SDS) as the emulsifier. The SDS is a good dispersing agent for HNTs in aqueous solution. The emulsion polymerization resulted in the formation of polystyrene nanospheres separating individual HNTs. Transmission electron microscopy revealed that the HNTs were uniformly dispersed in the PS matrix. Differential scanning calorimetry, Fourier-transform infrared spectroscopy and thermogravimetry were used to characterize the PS/HNT nanocomposites. The impact strength of the PS/HNTs nanocomposites was 300% higher than that of the neat PS. This paper presents a simple yet feasible method for the preparation of high-impact PS/halloysite nanocomposites.

Fracture toughness and fracture mechanisms of polybutylene-terephthalate/polycarbonate/ impact-modifier blends

A series of polybutylene-terephthalate/polycarbonate (PBT/PC) blends with different compositions were prepared using a twin-screw extruder. The morphologies of the blends were revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that a 50/50 PBT/PC blend possessed a bicontinuous structure and the other blends had a dispersed phase of either PBT or PC depending on which was the minor component. A relatively strong interface was observed in the blends with 20%, 40% and 50% PBT; but poor interfacial adhesion was found in the blends with 60% and 80% PBT. The strength of the interfacial boundary was believed to depend on the composition and blending conditions of the individual blend. Fracture experiments showed that the sharp-notch fracture toughness of PC could be significantly increased by mixing with up to 50% PBT without losing its modulus and yield stress. The toughening mechanisms involved in the fracture processes of the blends were studied using both SEM and TEM together with single-edge-double-notched-bend (SEDNB) specimens. It was found that in the toughened blends the growing crazes initiated by the triaxial stress in front of the crack tip were stabilized by the PC domains. The debonding-cavitation mechanism occurred at the PBT/PC interface, which relieved the plane-strain constraint and promoted shear deformation in both PBT and PC. This plastic deformation absorbed a tremendous amount of energy. Crack-interface bridging by the PC domains was clearly verified by the TEM study. Thus, the PC domains not only stabilized the growing crazes they also bridged crack surfaces after the crack has passed by. This effect definitely caused a large plastic-damage zone and hence a high crack resistance. Poor crack resistances of the blends rich in PBT was caused by the poor interfacial adhesion between PBT and PC. In these polymer blends, the growing crazes easily developed into cracks, which subsequently passed through the weak interface of PBT/PC and finally produced fast unstable fracture.

Fracture toughness assessment of poly(ethylene terephthalate) blends with glycidyl methacrylate modified polyolefin elastomer using essential work of fracture method

The static fracture toughness of poly(ethylene terephthalate) (PET) melt blended with a modifier containing glycidyl methacrylate (GMA)-grafted ethylenepropylene rubber and homopolymerized GMA was studied on injection molded specimens by adopting the essential work of fracture (EWF) method. It was found that the essential and nonessential or plastic work both decrease with an increasing amount of modifier (up to 20 wt %) if the PET matrix is amorphous and nonaged. The scatter in the EWF data for the blend with 10 wt % modifier was found by presuming concurrent mechanisms between microcrystallization and morphology-dependent cavitation and fibrillation processes.

Synergistic toughening effect of SBS and HDPE on the fracture of the PS/HDPE/SBS blends

Abstract The microstructure and mechanical properties of the blends consisting of polystyrene and high-density polyethylene and/or styrene–butadiene–styrene were studied. The HDPE forms long fibers in both the PS/HDPE and PS/HDPE/SBS blends. The binary blends exhibit declined mechanical properties at all compositions. Mechanism study suggests that craze formation in PS followed by unstable crack propagation along the weak PS–HDPE interface is the major failure mechanism for the low impact strength of the binary blends. In the ternary blends, the SBS forms a thin layer covering the HDPE fibers, which improves the PS–HDPE interfacial strength and mechanical properties of the blends. The debonding-cavitation at the PS–HDPE interface releases the plastic constraint and enables shear deformation at the crack tip. Moreover, the HDPE-fiber-pullout promotes shear deformation of PS. Both mechanisms greatly improve the toughness of the ternary blends.

Fracture toughness and fracture mechanisms of PBT/PC/IM blends. Part IV. Impact toughness and failure mechanisms of PBT/PC blends without impact modifier

In this part of the series, the impact behaviour of the PBT and PC blends without impact modifier was studied. Failure mechanism of the blends under various conditions was discussed. It was found that the key toughening process, i.e. interfacial debonding-cavitation, was disabled when the blends were subjected to impact loading. Hence, the fracture of the thick PBT/PC specimens with strong interface occurred under plane-strain condition. Their impact toughness obeys the rule of mixtures and synergistic toughening could not be achieved. When thinner specimens were tested, the fracture took place under non-plane-strain condition. But, the toughness of the blends was much lower than the value predicted by the rule of mixtures. Negative blending effect was obtained. Study on the strain rate effect suggests that under impact loading, the PC domains in the blends are subjected to an additional plastic constraint imposed by the neighboring PBT matrix, which is more rigid at a higher strain rate. Since fracture of the PC is highly sensitive to the plastic constraint at the crack-tip, the PBT imposed high plastic constraint promotes brittle fracture of the PC, leading to a deteriorated impact resistance. Evidences from TEM, SEM and OM studies support the mechanism proposed. Based on this mechanism, some suggestions on the selection of polymer components and design of microstructure for rigid-rigid polymer blends are also given.

Nucleating effect of calcium stearate coated CaCO3 nanoparticles on polypropylene

The ability of stearate coated calcium carbonate nanoparticles to promote the nucleation of polypropylene (PP) was investigated systematically. The effects of surfactant coverage and CaCO(3) particle concentration were explored using differential scanning calorimetry as well as optical and atomic microscopies. The results indicate that at the crystallization temperature of PP, a monolayer stearate coating remains as a rigid layer and provides a noticeable nucleating effect. Insufficient or excess coating diminishes the nucleating effect, the former because of the formation of agglomerates, and the latter by forming a soft layer at the PP/CaCO(3) interface at high temperatures, leading to the weak nucleating ability. Monolayer-coated nanoparticles had the strongest nucleating effect. The crystallization temperature and crystallization rate increased with the concentration of the monolayer-coated nanoparticles up to 40wt.%.