An exciting development in composites research is the commercial use of natural fibre reinforced composites (NFRC). Natural fibres provide several benefits in that they are cost effective whilst maintaining comparable performance to other materials; Ramie Bast (RB) fibre has been shown to have an average tensile strength of 849MPa, higher than that of steel (720MPa) . There is also strong societal impetus to move towards more sustainable manufacturing materials. As a result there is strong interest in research that can help to improve the material properties of NFRCs. One of the main issues facing NFRCs is the poor interphase properties that arise due to incompatibility of the hydrophobic matrix and hydrophilic fibres. This incompatibility between the two constituent phases of the composite results in decreased mechanical properties as the stresses cannot be effectively
transferred between the matrix and fibres.
This study investigated the effects of coupling agents on a composite of Ramie Bast (Boehmeria nivea) fibre within a High Density Polyethylene (HDPE) matrix. The coupling agent used was Polyethylene grafted Maleic-Anhydride (PE-g-MAH) with percentages of PE-g-MAH ranging between 1 – 10 wt% in consultation with prior research [2, 3]. The composite test panels were composed of unidirectional Ramie Fibres in a HDPE matrix with a fibre volume fraction of 24.97 ± 0.28%. The fibres were treated using a sodium hydroxide treatment as previous studies had shown the effectiveness of this treatment in improving the interphase . The blends of coupling agent and HDPE matrix were created using extrusion at the recommendation of previous literature [5-7].
Two coupling agents, named Coupling Agent 1 (product name: NG1001) and Coupling Agent 2 (product name: NG1201), were used to create blends of coupling agent and HDPE matrix material. Weight percentage additions between 1% and 10% of Coupling Agent 1 were investigated in order to determine an optimum level of coupling agent.
A single test sample of 3% Coupling Agent 2 was tested to contrast the difference in performance between the two coupling agents. It was expected that Coupling Agent 1 would outperform Coupling Agent 2 as it had a melt flow index (1g/10min) which more closely matched that of the HDPE matrix material (0.83g/10min) and hence had a more closely matching molecular weight.
In order to characterise the effects of heat treatment on the HDPE, two test samples of 100% HDPE were prepared, one which was first extruded (heat treated), and one which was not (baseline sample).
The testing method utilised was three point bending, following a methodology similar to that presented in ASTM D790. The bending stress was applied to unidirectional test samples such that the fibres were transversely loaded. Previous literature has shown that the failure modes of composites in transverse loading can be strongly correlated with the interphase properties and so improvements in the experimentally measured mechanical properties of maximum flexural stress and Young’s Modulus were considered evidence of improved interfacial adhesion. The test results obtained are shown in Figure 1.
The addition of a coupling agent resulted in statistically significant (p-val < 0.000) improvement of the measured mechanical properties. Analysis of variation of the samples composed of different weight percentages of Coupling Agent 1 also found the variation in mechanical properties significant. The coefficient of determination shows that 40.93% of the variability of maximum flexural stress within these samples can be accounted for by the variation in weight percent of Coupling Agent 1.
The coefficient of determination for Young’s Modulus shows 68.12% of the variation of this property can be accounted for in the same manner.
A weight percentage of 5wt% Coupling Agent 1 resulted in the highest tested mechanical properties with a 99% confidence interval for the maximum flexural stress of 28.0 ± 2.1MPa (an increase of 64.1% ± 12.2% from the baseline) and a 99% confidence interval for the Young’s Modulus of 1307.2 ± 84.2 MPa (an increase of 140.6% ± 15.5%).
Coupling Agent 1 possessed a higher maximum flexural stress and Young’s Modulus for the same weight percent of coupling agent (3%). The mean maximum flexural stress for 3% Coupling Agent 1 was shown to be significantly higher than that of Coupling Agent 2 (for a significance level, α =0.05). The mean Young’s Modulus was also higher but could not be shown to be significantly so (α= 0.05).
Variation of the mechanical properties of the heat treated samples was not statistically significant. The 99% confidence interval of percent change in ultimate flexural stress from the baseline was by -5.5% ± 11.1% whilst the change in Young’s Modulus was by 7.5 ± 14.6%.
Qualitative microscopic analysis seemed to support that improvements in the interphase had occurred but it is recommended that such analysis be carried out at much higher magnifications as may be obtained with a scanning electron microscope (SEM). The images did not allow for identification of phenomena that may strongly provide evidence for an improved interphase.
The findings of this report are that PE-g-MAH coupling agents provide large improvements to the mechanical properties of composites through the improvement of the interphase property with these experiments finding the maximised mechanical properties were obtained for a 5% addition of PE-g- MAH coupling agent. Results seem to provide evidence that confirms that coupling agents whose molecular weight matches the matrix material are more effective at improving the interphase. The effects of heat treatment were inconclusive. These results show that the inclusion of coupling agents in the manufacture of NFRCs is well justified and provides distinctly improved composite products.