| This study aims to evaluate potentials of wheat straw fibre used as reinforcement of wheat straw fibre/polypropylene composites, and to investigate the effects of modification, fibre addition, fibre size distribution and compatilizer addition on interfacial compatibility and mechanical properties of resultant composites.With thermomechanical refining, wheat straw fibre exhibited more surface irregularities and heterogeneity, especially in terms of fibre length and proportion of fines. With increasing steam pressure, the ratio of fines decreases and the aspect ratio of the resulting fibre increases. Refining lowers the pH value in comparison to that of original wheat straw. The base buffering capacity of the fibre is not influenced by refining. The thermogravimetric analysis (TGA) results demonstrated that fibres were prone to weight loss on exposure to temperatures up to 170°C, and the exposure to higher temperatures resulted in decomposition of cellulose, hemicelluloses and lignin. It appears that 8 bar of steam pressure and a 2500 rpm refiner speed are the optimal conditions for producing high-performance fibres in terms of fibre length and aspect ratio. Such fibres were well suited as filler for polypropylene composites.The wheat straw fibre, thermomechanically refinging from 8 bar of steam pressure and a 2500 rpm refiner speed followed alkalization, acetylation and maleic anhydride polypropylene (MAPP) treatments at different temperatures. In all cases (and when the temperature was lower than 170°C from TGA), chemically pretreated wheat straw fibre exhibited more thermal stability than the control. After the fibre was held for 1 h in air at 170°C, 190°C, 210°C and 230°C, there was no thermal decomposition in cellulose, hemicellulose and lignin. Alkalization eliminated a number of hemicellulose, thus increased the cellulose content and made the fibre surface rougher. Thanks to the higher crystallinity of cellulose, degradation of the wheat straw fibre was prolonged at higher temperatures. Acetylation had more effect on the thermal and chemical stability, contributing to the ester bond formation.The composites were prepared with 20% treated wheat straw fibre in order to investigate the tensile, dynamic mechanical and thermal properties and melt rheology of the treated wheat straw fibre/PP composites. The composites prepared with 20% alkalized wheat straw fibre treated in 2% MAPP showed optimum tensile strength. The DMA studies exhibited the storage flexural modulus (E') of the composites increased after adding wheat straw fibre due to its stiffness. The DSC study revealed that the addition of fibre to PP increased the melting temperature and crystallization temperature. The composites filled with the alkalized and MAPP-treated fibre had the highest crystallization temperature as a result of the co-effect of alkalization and esterfication. The melt flow studies of wheat straw fibre/PP composites were carried out at the temperature of 170°C, 180°C and 190°C and shear rate of 0.01 to 0.1 s-1. The melt rheological behavior of composites showed that the addition of wheat straw fibre to polymer matrix could increase the melt viscosity of composites, as a result of hindering the mobility of chain segments in flow. Among the three treatments, the composites filled with alkalized fibre showed the high melt viscosity due to the strong chemical interaction among polymer and fibre. The introduction of MAPP to the system increased the flow behavior of polymer, which decreased the melt viscosity. It was found that viscosity of the melt decreased with the increase the temperature.The wheat straw fibre/PP composites were subsequently prepared for investigating the effects of (a) wheat straw fibre size (9, 28, 35mesh); (b) wheat straw fibre addition (10, 20, 30, 40 and 50 wt%); and (c) the compatilizer addition (1, 2, 5 and 10 wt%) on the mechanical, dynamic mechanical and thermal properties and melt rheology of composites. With increasing wheat straw fibre addition in system, the tensile modulus and strength of the composites increased linearly with fibre additions up to 50%, and the tensile elongation at break decreased dramatically because of the reinforcement of wheat straw fibre. By increasing MAPP concentration, the composites showed better tensile strength, with the concentration of MAPP up to 10%, the composites had the best tensile strength. The fines had a slight higher elongation at break and tensile strength than the bigger one. By increasing the wheat straw fibre addition from 0 to 40%, the flexural modulus of composites gradually increased. The MAPP concentration and fibre size distribution had no obviously effect on the flexural modulus of composites. The thermal and mechanical properties of composites were studies by DSC and DMA. The E'of the composites exhibited higher values with an increase of wheat straw fibre concentration. DSC indicated that the wheat straw fibre act as nucleating agent for PP, and an increase of fibre loading resulted in increasing the crystallization rate. MAPP slightly accelerated the nucleating of wheat straw fibre, although, by increasing MAPP concentration in system, no significant difference was found in crystallization temperature. The fines could increase the crystallization temperature. The introduction of wheat straw fibre to the system increased the viscosity and went on increasing with the fibre loading due to an increased hindrance to the flow. The viscosity of melt increased when the composites filled with fines, due to the increased interaction between the fines and polymer molecules. The observation on the fracture surface of composites morphology from SEM indicated that a higher concentration of wheat straw fibre (>30 wt%) resulted in fibre agglomeration, difficulty in dispersion, and a reduction in interfacial bonding strength, which led to an increase in fibre pull-out. It is suggested that the overall desirable mechanical properties of composites could be predicted and designed by appropriate usage of fibre and suitable coupling agents and size distribution of fibre.
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