Gui Cheng Yang

The effect of fiber treatment on the mechanical properties of unidirectional sisal-reinforced epoxy composites

The present paper investigates the effect of fiber treatment on the mechanical properties of unidirectional sisal-reinforced epoxy composites. Treatments including alkalization, acetylation, cyanoethylation, the use of silane coupling agent, and heating were carried out to modify the fiber surface and its internal structure. As indicated by infrared spectroscopy, X-ray diffraction and tensile tests, variations in composition, structure, dimensions, morphology and mechanical properties of the sisal fibers can be induced by means of different modification methods. When the treated fibers were incorporated into an epoxy matrix, mechanical characterization of the laminates revealed the importance of two types of interface: one between fiber bundles and the matrix and the other between the ultimate cells. In general, fiber treatments can significantly improve adhesion at the former interface and also lead to ingress of the matrix resin into the fibers, obstructing pull-out of the cells. As a result, the dependence of laminate mechanical properties on treatment methods becomes complicated. On the basis of a detailed analysis, the relationship between optimized fiber treatment and performance improvement of sisal composites was proposed.

Self-reinforced melt processable composites of sisal

Through slight benzylation treatment, skin layers of sisal fibers were converted into thermoplastic material while the core of the fiber cells remained unchanged. On the basis of these modified sisal fibers, self-reinforced composites were prepared using hot pressing, in which the plasticized parts of sisal serve as matrix and the unplasticized cores of the fibers as reinforcement. The paper discussed the influence of various benzylation conditions on the structure, thermal flowability and mechanical properties of modified sisal and the composites. It was found that a balance of melt processability and reinforcing effect of the benzylated sisal fibers should be considered. Unlike the conventional plant fiber composites using petro-polymers as matrices, the current self-reinforced composites based on sisal are characterized by inherent interfacial compatibility and full biodegradability.

Self-Healing Epoxy Composite with Heat-Resistant Healant

To provide self-healing epoxy composite with adequate heat resistance for high-performance application, we developed a novel microencapsulated epoxy/mercaptan healing agent. The key measure lies in usage of diglycidyl ether of bisphenol A (EPON 828) as the polymerizable component and 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) as the catalyst. Because of the higher thermal stability of EPON 828 and lower volatility of DMP-30, the healing agent and the self-healing composite not only survive high-temperature curing and thermal exposure, but also offer satisfactory capability of autonomous properties restoration, as characterized by both fracture mechanics and fatigue tests. Especially when the operation temperature is not higher than 200 °C, the performance of the healing system is nearly independent of thermal history.

Self-healing of low-velocity impact damage in glass fabric/epoxy composites using an epoxy–mercaptan healing agent

Self-healing woven glass fabric-reinforced epoxy composite laminates were made by embedding epoxy- and mercaptan-loaded microcapsules. After being subjected to low-velocity impact, the laminates were able to heal the damage in an autonomic way at room temperature. The healing-induced reduction in the damaged areas was visualized using a scanning acoustic microscope. The rate of damage area reduction, which is closely related to the effect of crack rehabilitation and mechanical recovery, is a function of impact energy, content and size of the healing microcapsules. Minor damage, such as microcracks in the matrix, can be completely repaired by the healing system without manual intervention, including external pressure. Microcapsules with larger size and/or higher concentration are propitious for delivering more healing agent to cracked portions, while imposition of lateral pressure on damaged specimens forces the separated faces to approach each other. Both can improve the rate of damage area reduction in the case of severe damage.

Effect of Stitching on In-Plane and Interlaminar Properties of Sisal/Epoxy Laminates

The factors that influence in-plane mechanical responses and mode I interlaminar fracture toughness of stitched unidirectional sisal/epoxy laminates were studied in this paper. It was found that in-plane strength and stiffness of the laminates were not significantly affected by the stitching threads, while the interlaminar toughness was greatly improved, especially at higher sisal content. Unlike glass fiber reinforced polymer composites, sisal laminates have a rather high tolerance against the damages induced by stitching process. In addition, stitching expanded the fiber bridging zone and determined the R-curve profiles of sisal laminates. Factors including stitching density, diameter and species of stitching thread, continuity of stitching, and modification methods of sisal, were proved to greatly influence the delamination resistance of the laminates.

The preparation of self-reinforced sisal fiber composites

To prepare self-reinforced sisal composites, sisal fibers were cleaned, treated with NaOH solution, and then benzylated with benzyl chloride. In this way, the skin layers of the fibers were converted into thermoplastic material while the core of the fiber cells remained unchanged. Under the circumstances of hot pressing, self-reinforced all-plant fiber composites of sisal can be prepared, in which plasticised sisal serves as matrix and the unplasticised cores of the fibers as reinforcement. In this work the effect of the reaction conditions, such as alkalinity, temperature and the extent of benzylation, was studied in detail. The roles of quaternary ammonium salts and γ-ray irradiation treatment on the efficiency of benzylation were also taken into account. In addition, structural characteristics, melt flow and mechanical properties of the modified sisal and their composite sheets were analysed. It was found that a balance between melt processability and the reinforcing effect of the benzylated sisal fibers was required.

Mechanical properties of sisal reinforced composites in response to water absorption

The authors discuss the water absorption behaviour of sisal and its epoxy based composites and the mechanical properties of composites that have been aged in water. In addition, a series of fibre pretreatment techniques, including mercerization, acetylation, cyanoethylation, coupling agent treatment and thermal treatment, which are believed to be able to improve the water resistance of sisal and its composites, have been evaluated. It was found that the water absorption behaviour of sisal composites is controlled mainly by the fibre and the fibre/matrix interfacial characteristics. As a result, appropriate fibre modification to retard water diffusion and enhance interfacial adhesion is necessary if the natural fibre composites are to be used in practical applications.