Mechanical properties of wood–fiber reinforced polypropylene composites: Effect of a novel compatibilizer with isocyanate functional group

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Abstract

Natural fibers are increasingly being used as reinforcement in commercial thermoplastics due to their low cost, high specific properties and renewable nature. While the maleic anhydride modified polypropylene (MAPP) is most commonly used as compatibilizer to improve interfacial adhesion between hydrophilic wood–fibers and hydrophobic polypropylene, in this study, a novel compatibilizer (m-TMI-g-PP) with isocyanate functional group was synthesized by grafting m-isopropenyl-α,α-dimethylbenzyl-isocyanate (m-TMI) onto isotactic polypropylene (PP) in a twin screw extruder. The effect of filler concentration on the mechanical properties of wood–fiber filled composites, prepared by using m-TMI-g-PP as the compatibilizer, was investigated. The addition of the compatibilizer resulted in greater reinforcement of composites, as indicated by the improvement in mechanical properties. Tensile strength of composites so prepared increased by almost 45%, whereas 85% increase in flexural properties was observed. However the addition of wood–fibers resulted in a decrease in elongation at break and impact strength of the composites.

Introduction

In recent years, wood–fibers have gained significant interest as reinforcing material for commercial thermoplastics. They are now fast evolving as a potential alternative to inorganic fillers for various applications. Wood–fiber offers several advantages like low density, high specific properties, non-abrasive to processing equipment, low cost and most importantly biodegradability. However the primary drawback of using wood–fibers for reinforcement is the poor interfacial adhesion between polar-hydrophilic wood–fibers and nonpolar-hydrophobic plastics. This results in poor mechanical properties of the final product. The interfacial adhesion can be improved by using compatibilizers or coupling agents. Recent work [1], [2], [3], [4] suggests that the use of maleated polypropylene (MAPP) significantly improves the fiber–matrix bonding. Keener et al. [5], reported that, use of MAPE coupling agent at 3% loading can double the tensile strength and triple the impact properties compared to non-coupled blend of wood and polyethylene. Maldas and Kokta. [6], and Karnani et al. [7] have also reported improved mechanical properties by using silane and isocyanate coupling agents. The improvement in mechanical properties is believed to be due to a better dispersion of fibers in the matrix, a more effective wetting of fibers by matrix resin and a better adhesion between the two phases [8].

In the present study we have used a novel compatibilizer prepared by grafting m-isopropenyl-α,α-dimethylbenzyl-isocyanate (m-TMI) onto polypropylene. Composites of wood–fibers and polypropylene were prepared by melt blending in a co-rotating twin screw extruder. The effects of wood–fiber concentrations on the mechanical properties of the composites were investigated and the results are discussed.

Section snippets

Materials

Isotactic polypropylene (Koylene AM120NS), having a melt flow index of 10 g/10 min at 230 °C under 2.16 Kg load was supplied by Indian Petrochemical Corporation Ltd., Baroda and was used for functionalization experiments. The matrix polymer, polypropylene (Repol H100EY), having a melt flow index of 11 g/10 min at 230 °C under 2.16 Kg load was procured from Reliance Industries Ltd., India. The monomer m-isopropenyl-α,α-dimethylbenzyl-isocyanate (m-TMI) (Aldrich), octylamine (Aldrich), dicumyl peroxide,

Results and discussion

In our earlier work [13] we reported functionalization of poly(propylene) by m-isopropenyl-α,α,dimethylbenzyl-isocyanate (m-TMI) via melt mixing in a batch process using Brabender Plasticoder in which the maximum grafting yield was obtained with 10 wt.% concentration of both DCP and m-TMI. The reaction time was 10 min. Using the same concentrations of monomer and initiator, the melt phase grafting of m-TMI onto PP was performed in a continuous process using a twin screw extruder. Under these

Characterization of composites

Mechanical properties at each experimental condition were determined and the mean value of the property is plotted against wood–fiber content in Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8. Tensile strength, flexural strength, MOE and elongation at break provide an excellent measure of the degree of reinforcement provided by the fiber to the composite [17]. It can be seen from Fig. 3 that the tensile strength increases linearly with fiber content from 30.5 to 44.2 N/mm2 representing 45%

Conclusions

The effects of the incorporation of Eucalyptus wood–fibers both with and without a compatibilizing agent on the processing and properties of PP composites were investigated. A novel compatibilizer was prepared by grafting m-TMI onto polypropylene using reactive extrusion. The FTIR analysis provided evidence of pendent –NCO groups of m-TMI-g-PP covalently linking with hydroxyl groups of wood–fibers. Mechanical properties, measured in tensile, flexural and impact tests, demonstrated that the

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    Presently a research student at the Department of Chemical Engineering, Indian Institute of Science, Bangalore 560 012, India.

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