Development of hemp oil based bioresins for biocomposites

Development of hemp oil based bioresins for biocomposites

Within the civil engineering and construction industries fibre composites are now being adopted in certain applications in place of more traditional building materials such as timber and steel. Although, fibre composites possess certain advantages over traditional building materials they are predominately produced from non-renewable resources. Of particular concern is the petro-chemical based polymer resins commonly used such as epoxy, vinylester and polyester. It is due to this dependence on finite petro-chemicals and the negative environmental impact associated with their production and uses that alternative, competitive options are sought. Plant oil based bioresins are one such option. Although in its infancy, research suggests that biocomposites produced with plant oil based bioresins and plant based fibres offer the potential to be used as a sustainable replacement option to traditional fibre composites.
The research work presented within this dissertation is to the best of the author’s knowledge a world-first overall investigation pertaining to the concept of synthesising hemp oil based bioresins and applying them to biocomposites. In this work hemp oil based bioresins, specifically epoxidized hemp oil (EHO) and acrylated epoxidized hemp oil (AEHO) were synthesised and characterised and proposed as a potential replacement to their equivalent synthetic polymer resins (epoxy and acrylated resins) and also as an alternative to other commercially available bioresins. The synthesised bioresins were also applied as matrices in the production of suitable biocomposites. Hemp oil was epoxidized in a solvent free process using peracetic acid and an acid ion exchange resin (AIER) catalyst via in situ epoxidation. From 1H-NMR spectroscopy analysis, oxirane oxygen content was calculated as approximately 8.6% and a relative conversion to oxirane of 92% with approximately 5.1 epoxy groups per triglyceride. AEHO was synthesised from EHO via in situ acrylation. According to the 1H-NMR analysis of AEHO, acrylate peaks were apparent and were of the magnitude of approximately 4.1 acrylates per triglyceride. Some epoxide homopolymerisation was observed limiting conversion to AEHO to approximately 81%. Curing analysis and cure kinetic modelling were also performed for both EHO and AEHO. The kinetics of curing of both bioresin systems were able to be accurately modelled using a modified expression of Kamal’s autocatalytic model.
The synthesised bioresins were applied to biocomposites and characterised in terms of mechanical, dynamic mechanical and moisture absorption properties. Scanning electron microscopy (SEM) was also performed to investigate the fibre-matrix interfacial adhesion. EHO based bioresins and jute fibre reinforced biocomposites were manufactured and compared with commercially produced epoxidized soybean oil (ESO) and synthetic epoxy based samples. It was found that EHO based bioresins when applied to jute fibre reinforced biocomposites can compete with commercially produced ESO in terms of mechanical performance, dynamic mechanical properties and water absorption characteristics. Although it was shown that EHO can be used in higher concentrations than ESO when blended with synthetic epoxy thereby resulting in more sustainable biocomposites.
AEHO based bioresins and jute fibre reinforced biocomposites were manufactured and compared with commercially produced vinylester (VE) based samples. AEHO based samples exhibited higher fibre-matrix interfacial adhesion compared with the VE based samples.
Finally the mechanical performance, dynamic mechanical properties and moisture absorption properties of 100% hemp based biocomposite panels were investigated and compared to those of a VE hybrid composite. Results showed that, except for the flexural modulus that was 23% higher in the case of the hybrid composite, no significant differences exist in the mechanical performance of both tested materials. The higher fibre-matrix compatibility of the biocomposites led to stronger fibre-matrix interfaces compensating the lower mechanical performance of the neat bioresin with respect to the synthetic VE resin. All of the biocomposite sample types displayed lower dynamic mechanical properties compared with their synthetic counterparts. As, expected moisture absorption was also found to be higher in the biobased specimens although fibre transport was the dominate mechanism rather than resin type.
Overall from this research work it can be concluded that hemp oil based bioresins can effectively compete with commercially available bioresins and their equivalent synthetics in biocomposite applications. An enhanced understanding of the synthesis, characterisation and performance of hemp oil based bioresin and biocomposites for use in engineering applications is a key outcome of this investigation.