EPOXY POLYMERS REINFORCED WITH CARBON MICROFIBRE WASTES

This work investigates the incorporation of disposed carbon microfibres (CMF) obtained from the cutting process of laminate composites into epoxy polymers at different mass fractions (0, 2.5, 5, 7.5 and 10wt%). The elastic modulus and strength under tensile, compressive, flexural loads and impact resistance are investigated via Analysis of Variance (ANOVA). The tensile (compressive) modulus progressively increases up to 36.6% (28.6%) with the inclusion of CMF. The inclusion of 5wt% CMF results in an increase of 27% (19%) in tensile (compressive) strength. The flexural strength also increases 28.6% when 10wt% CMF is added. CMF waste leads, however, to a dramatic decrease (approx. 50%) in impact resistance attributed to the increase in stiffness.


INTRODUCTION
The demand for composite materials has progressively increased in various technological applications due to their low density and improved mechanical performance compared to conventional materials. However, the fabrication of such materials generates waste that may lead to environmental damage if improperly disposed of. Landfills are the most common strategy, but several countries have already limited or banned the disposal of composites in landfills due to environmental issues [1][2][3]. Polymer composites, especially thermosets, dominate the global market and, owing to their long-life cycle, alternatives to landfills need to be sought and boosted. Mechanical, thermal and chemical recycling have been developed in order to address this problem.
Mechanical recycling involves shredding and grinding and the subsequent separation of the fibre-rich fractions for reuse [4]. Mechanically recycled composites are usually reincorporated into new composites as fillers or reinforcement [1]. Thermal processes have also been developed for energy recovery (combustion of the composite waste) or selective material recovery for reuse (fluidized beds and pyrolysis). In pyrolysis, for example, a large amount of thermal energy is required in order to remove the matrix phase. Chemical recycling involves the decomposition of the polymeric structure and the high-quality end products (monomers, hydrocarbon molecules, gases and chemical intermediates for polymerization) are reused to produce new components [5].
The literature suggests that wastes from the manufacture of composites can also be reused in recycled composites. The CarbonTek S.L. ® company (Spain) has recently used powder waste resulting from the cutting process of carbon fibre-based products as polymeric matrix reinforcements for the fabrication of carbon components, as well as to reduce the volume of waste generated [6]. The residue is composed of carbon microfibres (CMF) coated with epoxy polymer, sometimes with pigment particles used to produce fins. Thomas et al. [6] evaluated the effect of CMF wastes inclusions at three different mass fractions (0, 10 and 20 wt%) on the thermal and mechanical properties of epoxy composite materials. These authors reported an increase in compressive and flexural strength and impact resistance proportional to the mass fraction of residues. Although less significantly, hardness and erosion resistance also increased with waste inclusion. The use of waste at 10wt% (20wt%) increased the compressive strength by 6% (20%) relative to the pure polymer, being attributed to the additional energy expended by the cracks to overcome the micro fibres and particles. Also, the CMF is used as cement matrix reinforcement improving mechanical properties [7].
Compared to most recycling processes, the methodology proposed by Thomas et al. [6] is economically more feasible. Therefore, this work further investigates the effect of different mass fractions of CMF wastes on the mechanical properties of thermosetting polymers, extending the analysis to tensile modulus and strength.

MATERIALS AND METHODS
CMF, supplied by the company CarbonTek S.L. ® (Spain), are obtained from the cutting process of laminate composites used in the manufacture of fins. Figure 1a shows a pair of CarbonTek carbon fins, after the cutting and assembly processes. Figure 1b presents the cutting process leftovers. The fin-cutting process generates a powder that can be considered as CMF enveloped by a polymer matrix (Figure 1c).

Mechanical tests
Tensile, compressive and flexural tests are performed in a Shimadzu AG-X Plus testing machine (Figure 3a) equipped with a 100 kN load cell, at a crosshead speed of 2 mm/min, 3 according to ASTM D638-14 [8], ASTM D695 [9], ASTM D790 [10] standards. The elongation of the specimens is measured using a digital video-extensometer. Impact tests were performed in an XJJ series impact testing machine with a 15 J hammer (Figure 3b) according to ASTM D6110 [11].

Scanning Electron Microscopy
A TM3000 Hitachi Analytical Microscope apparatus is used to investigate the morphological aspects of the CMFs as well as the surface of the fractured samples. The images are obtained in secondary electron mode at 15kV.

Statistical Analysis
The experimental data are analysed via Analysis of Variance (ANOVA) and Tukey's test using Minitab Software v.17, within a 95% confidence interval. The letters in the effect plots represent the results obtained by the Tukey comparison test; similar letters belong to the same group indicating there is no significant variation between the means. Table 1 shows the ANOVA results. P-values shown in Table 1 are less than 0.05, which reveals that the incorporation of CMFs significantly affects all response variables.   The flexural strength data range from 30.11 to 36.94 MPa (Figure 4a). According to Tukey's test, the effect of 7.5 and 10wt% CMF inclusions is similar (Group A), with an 5 increase of approximately 23% relative to the reference. The impact resistance values vary from 6.53 to 15.64 kJ /m 2 (Figure 4b). All levels of particle inclusions lead to a dramatic reduction of the impact resistance (approx. 55%). This behaviour may be attributed to the increased stiffness of the reinforced composites, which makes the material more brittle and consequently reduces the impact resistance.   crack growth with subsequent enhancement of mechanical properties [10,15,16]. Thomas et al. in fact reported that the inclusion of these wastes generally improved the stiffness and strength of the epoxy polymer composites, including under impact loadings [6]. In contrast, a substantial reduction in impact resistance is observed in this study, as discussed above.

RESULTS
According to Dassios [15] fibre pull-out is the most important mechanism of impact energy dissipation in fibre-reinforced composites. It is worth noting that no evidence of fibre pull-out is observed here. However, such mechanism is shown in the fractographic analysis reported by Thomas et al. [6] and may therefore explain the increase in impact resistance.

CONCLUSIONS
The incorporation of carbon microfibres wastes into epoxy polymer promotes an increase in tensile modulus and strength. Results indicate an increase in tensile modulus (strength) up to 36.6% (30%) for 10wt% (5wt%) waste inclusions. Similar results are observed for compressive modulus and strength and for flexural strength. The compressive modulus (strength) increased up to 28.6% (19%) for 7.5wt% or more (5wt%) of waste inclusions. The flexural strength increased up to 22% for 7.5wt% and 10wt% of CMFs. In contrast, the increased stiffness renders the material more brittle and therefore dramatically reduces the impact strength in 55% for all waste mass fraction levels considered. Carbon fibre wastes derived from the cutting process of laminate composites can therefore be employed as epoxy polymeric matrix reinforcement so as to promote significant enhancements of stiffness and strength. In addition, this low-cost recycling process prevents improper waste disposal with environmental, economic and structural benefits.