Chemical Modification of Polypropylene with Organo-functional Silanes
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Conventional polypropylene (PP) suffers at higher service temperatures and its poor melt strength limits its application in some processes. PP also lacks functional groups necessary for good adhesion to synthetic or natural fillers. However, the thermal resistance of PP could potentially be improved by chemical crosslinking and its melt strength is known to be enhanced by light crosslinking and introducing long chain branches onto its backbone. Moreover, PP adhesion to polar fillers is proven to be significantly improved by using coupling agents. Organo-functional silanes can be used to achieve these goals because of their bi-functional chemical nature. Their grafting onto polymeric chains helps in crosslinking these chains as well as forming stable covalent bonds with polar fillers. The objectives of this study were to compare the grafting efficiencies of nine different organo-functional silanes, to study their effect on the crystallization temperature of crosslinked PP and its soluble materials, to study the influence of two selected silanes, with different grafting efficiencies, on melt properties of PP with light crosslinking, and finally to study the impact of the PP grafted with two selected silanes, with the highest grafting efficiencies, on the mechanical properties of the wood-fiber/PP composites. The grafting process was performed in a co-rotating twin-screw mini-extruder Haake Minilab Microcompounder (Minilab) using a recirculating mode (batch). The grafted products were purified and then studied by Fourier Transform Infrared Spectroscopy (FT-IR) to verify the grafting and compare the grafting efficiencies of those silanes. Gel content test was performed after crosslinking the grafted products in a hot water bath to confirm the FTIR results. Results showed that 6-azidosulfonylhexyltriethoxysilane (Azido-silane) had the highest grafting efficiency. The order of grafting efficiency of the other silanes is as follows: (3-acryloxypropyl)trimethoxysilane (APTMS) > 3-methacryloxypropyltrimethoxysilane (VMMS) > Vinyltrimethoxysilane (VTMS), 7-octenyltrimethoxysilane (ONTMS), Ureidopropyltrimethoxysilane (UPTMS), (3-glycidoxypropyl)trimethoxysilane (GPTMS), 3-aminopropyltrimethoxysilane (AMPTMS), Bis(3-trimethoxysilylpropyl)amine (BTMSPA). Differential Scanning Calorimetry (DSC) was used to obtain the crystallization temperatures (Tc) of the crosslinked PP and its soluble materials. Crosslinking was found to significantly increase the Tc of PP except with UPTMS, GPTMS, AMPTMS, and BTMSPA. The soluble materials of highly crosslinked samples (with APTMS and VMMS) exhibited Tc close to that of the pure PP except with Azido-silane while the soluble materials of lightly crosslinked samples (with VTMS and ONTMS) exhibited high Tc compared to that of the pure PP. The melt properties were studied by oscillatory shear rheology experiments and MFI test. Light crosslinking (gel content ≤ 5 wt.%) was found to decrease MFI, increase complex viscosity (η*) and storage modulus (G') at low frequency, and decrease tan δ at low frequency. APTMS-crosslinked PP (gel content ̴ 5 wt.%) had greater influence compared to ONTMS-crosslinked PP (gel content 2 wt.%). In general, the addition of the PP grafted with silanes (PP-g-APTMS and PP-g-Azidosilane) improved the mechanical properties of wood-fiber/PP composites. PP-g-APTMS had better effect compared to PP-g-Azidosilane.