Polymers and Polymer Composites

The effect of propylene carbonate as an accelerator on the mechanical performance and dimensional stability of composites made from wood fibre bonded with wood resin and urea formaldehyde resin was investigated. The results demonstrated that the mechanical performance of the composite fibreboard cured at a relatively low temperature (e.g., 180 °C) could be markedly enhanced by addition of propylene carbonate. Composites cured at higher temperatures (e.g., 200 °C or 220 °C) and/or in presence of propylene carbonate, exceeded the minimum physical properties specified by ANSI-AHA. The addition of 3% propylene carbonate (by weight, based on total fibre and resin content) could optimise the physical properties of the composites. The tensile stress parallel-to-the-surface and the flexural strength and stiffness of the composites increased in presence of propylene carbonate.

A closed-form solution using the distribution of fibre aspect ratios is proposed for predicting the thermal conductivity of aligned short fibre composites. The present model is the extended version of Hatta and Taya's model, where Eshelby's equivalent inclusion method modified for finite volume fraction of fibres is employed. The validity of using the average fibre aspect ratio for predicting the thermal conductivity is examined in terms of the distribution of the aspect ratios such as its average and standard deviation, fibre volume fraction, and constituents' thermal conductivity ratio. The guideline for the selection of either the actual distribution or the average fibre aspect ratio is presented in terms of the thermal conductivity ratio and the coefficient of the variation of the aspect ratio for the better prediction of the thermal conductivity of the composites.

The impact performance of laminated plain-woven fabric composites was analysed here using the elasto-plastic constitutive law, which can account for the anisotropic and asymmetric properties of fibre-reinforced plastics. The isotropic yield criterion was modified to account for the anisotropic and asymmetric initial yielding, while the kinematic hardening law with anisotropic evolution of back-stress was used for the anisotropic hardening behaviour. The impact tests were performed on laminated plain-woven composites. The simulation results for the impact tests compared well with experimental results.

Composite materials from unsaturated polyester (UPE) resin, black rice husk ash (BRHA), and natural rubber grafted polystyrene (NR-g-PS) were prepared. NR-g-PS was synthesised by emulsion copolymerisation with a NR/styrene molar ratio of 80/20 using tert-butyl hydroperoxide/tetraethylene pentamine (t-BHPO/TEPA) as an initiator. Grafting efficiency of the copolymer product was about 54%. Initially, UPE was blended with BRHA at UPE/BRHA weight ratios of 100/30, 100/40 and 100/50. Each blend was subsequently mixed with NR-g-PS in the proportions of 5, 10 and 15 parts per hundred of UPE resin (phr). After curing, the cast specimens were examined for their physical properties, thermal expansion coefficient and morphology. It was found that the composite of 100UPE/30BRHA/5NR-g-PS exhibited the highest flexural and impact strength but the lowest water absorption. The thermal expansion coefficient of 100UPE/30BRHA/5NR-g-PS composite measured by thermomechanical analysis (TMA) was as low as 0.00005/°C. The morphology of the composites was investigated by scanning electron microscopy (SEM), which disclosed that NR-g-PS particles were well dispersed in the UPE matrix whereas the RHA was agglomerated and poorly dispersed in the composites. The higher the amount of NR-g-PS the more agglomeration of BRHA. This was due to the very high viscosity of the systems.

The load applied on the specimen in common dynamic mechanical thermal analysis (DMTA) is of the order of several Newtons to several tens of Newtons, far lower than that in service life. In this study a dynamic mechanical thermal analyser (EPLEXOR 500, with a total load up to 500 N) was applied to investigate the effects of static load and dynamic load (sine-wave load) on the dynamic viscoelasticity of unidirectional carbon fibre reinforced epoxy resin composites. The effects of the load on the dynamic viscoelasticity of polymer matrix composites (PMCs) are expressed mainly in two ways. On the one hand, the load affects the magnitude of the dynamic viscoelastic parameters. The storage modulus increases with increasing static load and decreases with increasing dynamic load. Such variations are obviously related to the interaction between the two kinds of load. The increase in load, whether static or dynamic, can result in a decline in the loss tangent. On the other hand, there exists a frequency-temperature-load equivalence for a carbon fibre-reinforced polymer's dynamic viscoelasticity. An increase in static load raises the glass-transition temperature and pushes the frequency spectrum curve towards a low frequency, while the effects of dynamic load are the reverse.

PMMA/PAN core/shell microspheres were synthesised by polymerising methyl methacrylate (MMA) or acrylonitrile(AN) at 75 °C for 5 h in a soapless emulsion system, using ammonium persulfate (APS) as a radical initiator. It has been shown that in the first-stage reaction the initiator APS significantly accelerated the reaction rate. However, when the APS concentration reached 0.9% (based on the amount of the monomers), this tendency lessened.

In the second-stage core/shell reaction, the effect of initiator on reaction rate was not so pronounced. Transmission electron microscope (TEM), fourier transform infrared (FT-IR), thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were used to characterise the structure and properties of seeded PMMA core and/or PMMA/PAN core/shell polymers. The core/shell morphology was clearly displayed and the diameter of the core is ca. 350 nm whereas the shell thickness was ca. 100 nm. Both the PMMA core and the PMMA/PAN core/ shell polymers had weak and strong endothermic peaks when heated in the range of 340–390 °C, but the latter showed a characteristic exothermic peak which corresponded with the crosslinking reaction of polyacrylonitrile macromolecules at ca. 600 °C. A mixture was obtained by blending the core/shell polymers with high density polyethylene, then spinning to form composite fibres, which were thermally stabilised in air at 180 °C for 1 h and carbonised in nitrogen at 1000 °C to obtain nanometre-sized carbon fibres.

Nano-composites of poly(vinyl chloride) and co-poly(vinyl chloride-vinyl alcohol-vinyl acetate) with clay were prepared by the solution intercalation method. Montmorillonite, a three-layered clay mineral consisting of silicate sheets, was employed as reinforcing phase. Compatibilisation between the two phases was achieved by intercalation of montmorillonite with dodecylamine to increase the organophilicity of the clay. Thin films of pure polymers and their composite materials containing various proportions of clay were obtained by evaporation of the solvent. After further drying under vacuum, these films were characterised for their mechanical and thermal properties.

The results have shown that the modulus and the tensile strength of the composite films increased initially compared to the pure polymers and then decreased upon further addition of the clay. The glass transition temperature of the films, measured from the maxima of the tanδ curves using dynamic mechanical thermal analysis, showed a greater shift towards higher temperatures in copolymer-clay than in PVC-clay films. This is indicative of a higher degree of interaction between the two phases of copolymer-clay composites than PVC-clay composites. The microhardness values and the decomposition temperatures also increased in both systems with the addition of an appropriate amount of the clay.

The thermal decomposition and oxidation ageing behaviour of polypropylene (PP) and PP/halloysite nanotube (HNT) nanocomposites were studied. The kinetics of thermal decomposition were calculated by integral models. The thermal oxidation ageing was investigated by Fourier transform infrared spectroscopy (FTIR). The PP nanocomposites showed a higher activation energy of thermal decomposition than neat PP. Their surface treatment and lower HNT loading led to a higher activation energy of decomposition. Their improved thermal stability in nitrogen was attributed to entrapment of decomposition products at initial decomposition stage and to barrier effects.

Nanocomposites made by using silane-modified HNT (m-HNT) and also those with lower quantities of (unsilanated) HNT showed improved resistance to thermo-oxidative ageing. The rate of thermo-oxidative ageing was correlated with the number of acidic sites and the volume of entrapped oxygen in the lumen of the HNT and in cavities.

Polyurethane/poly(vinyl ester resin)(butyl methacrylate) Interpenetrating Polymer Network (PU/VER(BMA) IPN) materials were synthesised at room temperature by polymerising butyl methacrylate as a comonomer with simultaneous interpenetration. The curing process of these IPNs was dynamically traced and semi-quantitatively analysed by Fourier transform infrared spectroscopy (FT-IR) and the kinetics of their formation with different PU catalysts and VER initiators was studied. The results showed that when the amounts of stannous octoate and BPO respectively were 0.5wt.% of PU and 0.67wt.% of VER, the reaction rates of the monomers were close, the conversions of the monomers were high and these amounts were optimised. The influence of the kinetics on the microstructure and damping properties of the IPNs was detected by Transmission Electron Microscopy (TEM) and by Dynamic Mechanical Analysis (DMA). The results showed that with the appropriate amount of the polyurethane catalyst stannous octoate and VER initiator BPO, the PU/VER(BMA) IPN showed a multiphase microstructure containing some finer phase domains having nanometre domain sizes, together with good damping properties (a broad damping plateau and high tanδ). When the amount of stannous octoate or BPO was increased or decreased, the IPNs presented an unfavourable microstructure and poor damping properties. The relationship between the kinetics, microstructure and damping properties was analysed further.

The kinetics of light ageing of wood filled composites made from polypropylene filled with unmodified wood flour and with wood flour modified with maleic anhydride (MA) in amounts of 10, 20 and 30% has been investigated.

The investigation shows that the light ageing of wood-filled polypropylene-based composites is an autocatalytic chain process. The decrease of the rate constants with increasing wood content showed that the wood flour had a screening effect on light.

It is established that the modification of wood flour with MA have a stabilising effect on the ageing of the composite materials owing to a decrease in the number of active centres capable of undergoing oxidation, and in the number of the carbonyl groups.