The Influence of Fibre Processing and Treatments on Hemp Fibre/Epoxy and Hemp Fibre/PLA Composites

摘要

In recent years, due to growing environmental awareness, considerable attention has beengiven to the development and production of natural fibre reinforced polymer (boththermoset and thermoplastic) composites. The main objective of this study was toreinforce epoxy and polylactic acid (PLA) with hemp fibre to produce improvedcomposites by optimising the fibre treatment methods, composite processing methods,and fibre/matrix interfacial bonding.An investigation was conducted to obtain a suitable fibre alkali treatment method to:(i) remove non-cellulosic fibre components such as lignin (sensitive to ultra violet(UV) radiation) and hemicelluloses (sensitive to moisture) to improve long termcomposites stability(ii) roughen fibre surface to obtain mechanical interlocking with matrices(iii)expose cellulose hydroxyl groups to obtain hydrogen and covalent bonding withmatrices(iv) separate the fibres from their fibre bundles to make the fibre surface available forbonding with matrices(v) retain tensile strength by keeping fibre damage to a minimum level and(vi) increase crystalline cellulose by better packing of cellulose chains to enhance thethermal stability of the fibres.An empirical model was developed for fibre tensile strength (TS) obtained with differenttreatment conditions (different sodium hydroxide (NaOH) and sodium sulphite (Na2SO3)concentrations, treatment temperatures, and digestion times) by a partial factorial design.Upon analysis of the alkali fibre treatments by single fibre tensile testing (SFTT),scanning electron microscopy (SEM), zeta potential measurements, differential thermalanalysis/thermogravimetric analysis (DTA/TGA), wide angle X-ray diffraction(WAXRD), lignin analysis and Fourier transform infrared (FTIR) spectroscopy, atreatment consisting of 5 wt% NaOH and 2 wt% Na2SO3 concentrations, with a treatment temperature of 120oC and a digestion time of 60 minutes, was found to give the bestcombination of the required properties. This alkali treatment produced fibres with anaverage TS and Young's modulus (YM) of 463 MPa and 33 GPa respectively. The fibresobtained with the optimised alkali treatment were further treated with acetic anhydrideand phenyltrimethoxy silane. However, acetylated and silane treated fibres were notfound to give overall performance improvement.Cure kinetics of the neat epoxy (NE) and 40 wt% untreated fibre/epoxy (UTFE)composites were studied and it was found that the addition of fibres into epoxy resinincreased the reaction rate and decreased the curing time. An increase in the nucleophilicactivity of the amine groups in the presence of fibres is believed to have increased thereaction rate of the fibre/epoxy resin system and hence reduced the activation energiescompared to NE.The highest interfacial shear strength (IFSS) value for alkali treated fibre/epoxy (ATFE)samples was 5.2 MPa which was larger than the highest value of 2.7 MPa for UTFEsamples supporting that there was a stronger interface between alkali treated fibre andepoxy resin. The best fibre/epoxy bonding was found for an epoxy to curing agent ratio of1:1 (E1C1) followed by epoxy to curing agent ratios of 1:1.2 (E1C1.2), 1: 0.8 (E1C0.8), andfinally for 1:0.6 (E1C0.6).Long and short fibre reinforced epoxy composites were produced with various processingconditions using vacuum bag and compression moulding. A 65 wt% untreated longfibre/epoxy (UTLFE) composite produced by compression moulding at 70oC with a TS of165 MPa, YM of 17 GPa, flexural strength of 180 MPa, flexural modulus of 10.1 GPa,impact energy (IE) of 14.5 kJ/m2, and fracture toughness (KIc) of 5 MPa.m1/2 was found tobe the best in contrast to the trend of increased IFSS for ATFE samples. This isconsidered to be due to stress concentration as a result of increased fibre/fibre contactwith the increased fibre content in the ATFE composites compared to the UTFEcomposites.Hygrothermal ageing of 65 wt% untreated and alkali treated long and short fibre/epoxycomposites (produced by curing at 70oC) showed that long fibre/epoxy composites weremore resistant than short fibre/epoxy composites and ATFE composites were moreresistant than UTFE composites towards hygrothermal ageing environments as revealed from diffusion coefficients and tensile, flexural, impact, fracture toughness, SEM, TGA,and WAXRD test results. Accelerated ageing of 65 wt% UTLFE and alkali treated longfibre/epoxy (ATLFE) composites (produced by curing at 70oC) showed that ATLFEcomposites were more resistant than UTLFE composites towards hygrothermal ageingenvironments as revealed from tensile, flexural, impact, KIc, SEM, TGA, WAXRD, FTIRtest results.IFSS obtained with untreated fibre/PLA (UFPLA) and alkali treated fibre/PLA (ATPLA)samples showed that ATPLA samples had greater IFSS than that of UFPLA samples. Theincrease in the formation of hydrogen bonding and mechanical interlocking of the alkalitreated fibres with PLA could be responsible for the increased IFSS for ATPLA systemcompared to UFPLA system.Long and short fibre reinforced PLA composites were also produced with variousprocessing conditions using compression moulding. A 32 wt% alkali treated long fibrePLA composite produced by film stacking with a TS of 83 MPa, YM of 11 GPa, flexuralstrength of 143 MPa, flexural modulus of 6.5 GPa, IE of 9 kJ/m2, and KIc of 3 MPa.m1/2was found to be the best. This could be due to the better bonding of the alkali treatedfibres with PLA. The mechanical properties of this composite have been found to be thebest compared to the available literature.Hygrothermal and accelerated ageing of 32 wt% untreated and alkali treated longfibre/PLA composites ATPLA composites were more resistant than UFPLA compositestowards hygrothermal and accelerated ageing environments as revealed from diffusioncoefficients and tensile, flexural, impact, KIc, SEM, differential scanning calorimetry(DSC), WAXRD, and FTIR results. Increased potential hydrogen bond formation andmechanical interlocking of the alkali treated fibres with PLA could be responsible for theincreased resistance of the ATPLA composites.Based on the present study, it can be said that the performance of natural fibre compositeslargely depend on fibre properties (e.g. length and orientation), matrix properties (e.g.cure kinetics and crystallinity), fibre treatment and processing methods, and compositeprocessing methods.