Study on properties of bamboo fiber composites with different fiber morphologies based on vacuum-assisted resin transfer molding

The aim of this research was to explore the effect of different morphologies of bamboo fiber (BF) on epoxy resin (EP) impregnation in vacuum-assisted resin transfer molding (VARTM) process and the properties of BF/EP composites. Three types of BF with different morphologies (BF-2, BF-3 and BF-4) were obtained by mechanical rolling with 2, 3 and 4 times, respectively. The BF were made into bamboo fiber mat (BFM) by wet layering process, and then produced into BF-2/EP, BF-3/EP and BF-4/EP composites with fiber content of 45wt% by VARTM. The properties of BF, BFMs and BF/EP composites were characterized with ESEM, ultra-depth-of-field microscope, mechanical testing machine, TG, DMA and micro-CT. The results show that the fluffy degree of BFMs by wet layering decreases and the difficulty of resin injection increases when the fiber length decreases and the fiber separation increases. The fiber accumulation occurs during resin injection of BFM-4, and BF-3/EP composite has the lowest water absorption. The BF-2 with longer length and lower dispersion can easily lead to poor interfacial bonding with resin, although it can maintain the structure and properties of BF. The mechanical properties of the composite prepared by BF-3 with moderate length and separation are the best, and the flexural strength, flexural modulus, shear strength and impact toughness are 97.90 MPa, 7.2 GPa, 17.01 MPa and 8.11 kJ/m², respectively. BF accelerates the pyrolysis of BF/EP composites. The BF-4/EP composite has a higher pyrolysis temperature because BF-4 has lower hemicellulose content compared with other BF. BF can improve the rigidity of EP. The interface bonding between BF-3 and resin is the best, and the proportion of pores volume is only 0.04%. The maximum storage modulus of BF-3/EP composites is 5198 MPa. When BF/EP composites are prepared by VARTM, the appropriate BF size and separation degree are the key factors that affect the interfacial bonding and properties.