Material PropertiesThe mechanical and physical properties of polyurethane composites based on rice husk and polyethylene glycol
Introduction
Recently, utilization of biomass resources has been the subject of various studies. In Malaysia, RH is one of the biomass materials, which is a by-product from the rice industry. Many studies have been carried out on the utilization of biomass, such as, particle board [1], medium density fiberboard [2], pulp [3] and composites [4], [5], [6], [7], [8]. In general, utilization of biomass in lignocellulosic composites have been attributed to several advantages such as low density, greater deformability, less abrasiveness to equipment, biodegradability and low cost. However, in producing a good lignocellulosic composite, the main obstacle to be resolved is the compatibility between the fibre and matrix. The properties of the lignocellulosic composites are dominated by the interfacial interaction between the lignocellulosic filler and polymer matrix. Generally, there are two types of interaction at the interfacial region, i.e. primary and secondary bonds. Primary and secondary bonds include covalent and hydrogen bonds, respectively. Whilst, covalent bonding at the interfacial region exists in thermoplastic-wood composites with the incorporation of a coupling agent, such bonds are more prevalent in the thermoset-lignocellulosic composites. This is because lignocellulosic hydroxyl (OH) groups could serve as reaction sites with various functional groups in the thermoset system. According to Hatakeyama et al. [9] natural polymer having more than two OH groups per molecule could be used as a polyol for polyurethane preparation if the groups could be reacted with isocyanate.
Polyurethane (PU) is one of the most useful three-dimensional polymers due to its unique features. It can be produced in the form of sheets, foams, adhesives etc. Recently, many attempts have been made to utilise lignocellulose as raw materials for PU synthesis. Desai et al. [10] prepared PU from starch and studied the swelling and mechanical properties of the PU. From the results obtained, the starch-PU was found to show better mechanical properties than trimethylol propane-PU and the degree of crosslinking was higher for starch-PU. On the other hand, these properties were also found to depend on NCO/OH ratio. From the study carried out by Kurimoto et al. [11], [12] it was found that NCO/OH ratio contributed to the formation of three-dimensional network in PU preparation. The effect of soft segment content and its molecular weight has been studied by Reimann et al. [13] and Saraf et al. [14]. The results showed that the degree of crosslinking and tensile properties depend mainly on the ratio of soft/hard segments, and are unaffected by variations in the sequence length of the soft segment at a given soft segment content [13].
This study aims at using RH for a dual purpose in the production of PU composite, i.e. as filler and reactive component. Reactive component means that RH would serve as a polyol in the production of PU through isocyanate-OH reaction. In this study, the focus is also on the PU matrix produced from polyethylene glycol with molecular weight 200 (PEG200) and diphenylmethane diisocyanate (MDI).
Section snippets
Material
The RH in powder form was obtained from Bernas Dominals, Jalan Paya Keladi, Seberang Perai Utara, Penang, Malaysia. MDI was supplied by Aldrich Chemical Company. Inc. PEG 200 was obtained from Fluka Chemika.
Preparation of RH-PU composites
An Endecotts sieve was used to separate the particles into different sizes. The filler sizes used in this study were 150–180, 180–250 and 250–500 μm. The fillers were dried in an oven at 105 °C for approximately 20 h. PEG200 was dried by mixing it with molecular sieve type 3Å powder. RH-PU
Results and discussion
Fig. 1 shows the effect of filler loading on the flexural strength of RH-PU composites. In general, flexural strength increases as the RH loading is increased up to approximately 45–60%. Exceeding this threshold value, the strength decreases. Composites with smaller size RH display higher strength than those with bigger size filler. This is expected because smaller size filler gives a larger surface area per volume for interaction of OH groups from RH with NCO groups. This result is in
Conclusions
From the results of flexural, tensile and impact tests, it is clearly demonstrated that the incorporation of RH as an OH groups provider in a PU system has enabled the properties of the composites mentioned above to increase. However, it can be seen that the properties of the composites also depend on the amount of homogeneous PU matrix. As the % of RH is increased, which decreases the % of PEG200 (as the % of overall OH is kept constant), the formation of homogeneous PU matrix is accordingly
Acknowledgements
The authors would like to thank the Ministry of Science, Technology and Environment (MOSTE) and Universiti Sains Malaysia, Penang, for the IRPA grant that has made this research work possible. Thanks are also due to Bernas Dominals for the free sample of rice husk.
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