Toughening of natural-fibre composites using nano- and microcrystalline cellulose particles
Creators: name: Deng, Xinying
Publisher: name: Imperial College London
Year: 2018

description: Environmental concerns have prompted research into natural materials to improve sustainability. Cellulose has some of the highest mechanical properties among naturally-derived materials, and natural-fibre composites have better specific stiffness than glass-fibre composites, and are thus increasingly used in the transport and construction sectors. However, cellulose is hydrophilic and it is difficult to obtain a uniform dispersion of cellulose modifiers in epoxy polymers. This makes it challenging to achieve high performance natural-fibre composites with good delamination resistance, which is critical in composite applications. Therefore, in the present study, the toughening effect of cellulose modifiers in an anhydride-cured diglycidyl ether of bisphenol-A (DGEBA) epoxy polymer, and in regenerated cellulose-fibre (CeF) composites are investigated. The cellulose modifiers initially agglomerated and sedimented in the epoxy. However, the addition of a silane during the three-roll mill process resolved this issue, and a good dispersion of cellulose modifiers was achieved. The addition of 10 wt% of cellulose modifiers, i.e. microcrystalline cellulose (MCC) and cellulose nanocrystals (CNCs), increased the fracture energy (GC) of the epoxy by more than 100 %, compared with 57 % for nanosilica, which is a well-studied and effective epoxy toughener. Hybridisation of MCC and CNCs with nanosilica or rubber particles, i.e. carboxyl-terminated butadiene-acrylonitrile (CTBN) and core-shell rubber (CSR), generally yielded additive toughening effects since the toughening mechanisms associated with each modifier were largely still present in the hybrids. To assess the effectiveness of the transfer of the increased matrix toughness to fibre composites, plain-weave CeF composites were fabricated using the wet layup process. Their mode I interlaminar fracture energies were compared with the bulk fracture energies, and their properties were benchmarked with glass-fibre (GF) composites. Although GF composites have better tensile properties than CeF composites, the composite propagation fracture energies (GC,prop) of CeF composites (e.g. control-CeF: 1155 J/m2) were about twice those of GF composites (e.g. control-GF: 567 J/m2). This was due to more extensive fibre bridging and crack branching behaviours. Analytical models showed reasonably good agreement with the experimental GC for the epoxy polymers, GF composites and CeF composites. These models were able to predict the significance of various fibre and matrix toughening mechanisms identified through fractography, which also correlated well with experimental observations. The highest GC,prop values obtained for the GF and CeF composites were 901 ± 102 J/m2 and 1537 ± 56 J/m2, respectively, which are 59 % and 33 % higher than their respective control composites. It was found that the GC,prop values did not increase further when matrices with higher toughness were used. Hence, cellulose modifiers can be used to replace nanosilica in hybrid matrices to obtain GF or CeF composites with reasonably high fracture energy and increased renewable content.