Fabrication And Modification Of Environmentally-Friendly Composites From Natural Fibers And Thermosetting Resins

The current environmental policy and ideas of sustainable development have progressively driven the replacement of glass fibers with natural fibers in the fabrication of fiber-reinforced unsaturated polyester (UPE) composites. However, concerning the application of natural fiber-reinforced UPE composites in engineering area, the main challenges are the poor interfacial adhesion between natural fibers and UPE matrix, the emission of carcinogenic styrene from UPE resins, and the use of amount of petroleum-based resources in the synthesis of UPE resins. Therefore, the present study focused on the modification of bamboo fibers with environmentally-friendly physical and chemical methods to improve the interfacial adhesion between fibers and resins. The petroleum-based monomers with high reactivity were used to replace styrene in the development of styrene-free UPE composites with hemp fibers to avoid the emission of styrene during the preparation of UPE-based composites. To alleviate the use of petroleum-based resources, acrylated epoxidized soybean oil (AESO) was used as a biobased substitution of UPE to exploit hemp fibers reinforced styrene-free AESO composites; the composites were further modified with a diisocyanate monomer to further increase the crosslinking density of AESO resins and interfacial adhesion of the composites. Finally, a biobased reactive diluent (RD) was synthesized from a glucose-derived polyol and then used to copolymerize with AESO to formulate soybean oil-based thermosets with high mechanical performance, processability and renewable contents; the resins were used as matrices for the preparation of hemp fiber- and bamboo fiber-reinforced composites to evaluate the effects of resin composition and fiber type on the performance of the composites. The fiber surface characteristics, rheological behavior, and curing mechanism of resins as well as the mechanical performance and interfacial adhesion of composites were fully investigated. The conclusions are as follows:(1) Four commercial bamboo fibers (BFs) were modified with a water soluble monomer, N-methylol acrylamide (NMA), and then used to fabricate BF-reinforced UPE composites, respectively. Results indicated that the BFs would react with NMA, and fiber morphology and chemical composition of fibers significantly influenced the reaction efficiencies between NMA and the four BFs. The modification of BFs with NMA induced the fibers with unsaturation functionality and thus established chemical linkages between the fibers and UPE resins, which resulted in improvement of the interface adhesion of the BF/UPE composites. NMA modification significantly increased the tensile strength, flexural strength, flexural modulus, water resistance of the composites, although the magnitudes were dependent on the combined effects of NMA grafting efficiency and surface characteristics of BFs.(2) The plasma treatment of original BFs would remove impurities, increase roughness, and generate oxygen-containing functional groups of fiber surface, which improved the interfacial adhesion between the fibers and UPE resins and hence significantly increased the tensile strength, flexural strength, flexural modulus, storage modulus, and glass transition temperature (Tg) of the BF/UPE composites.(3) N-vinyl-2-pyrrolidone (NVP) and tri(ethylene glycol) divinyl ether (TDE) were utilized as RDs to replace styrene during the formulation of UPE composites with hemp fibers (HFs). Results indicated that both NVP-UPE and TDE-UPE systems exhibited better miscibility than styrene-UPE system. The NVP-UPE and TDE-UPE resins had much lower viscosities and activation energies than the styrene-UPE resin. Compared to the styrene-UPE and TDE-UPE resins, the NVP-UPE resin had much lower curing temperature. The tensile strengths of the composites with different RDs were comparable, while the RD type greatly influenced the flexural strength, flexural modulus, and impact strength of the composites. Composite NVP-UPE had comparable tensile strength, flexural strength and storage modulus with composite styrene-UPE due to the high reactivity of NVP; Composite TDE-UPE has the highest impact strength because of the flexibility of TDE homopolymer, but its flexural properties, storage modulus and Tg were lower than those of styrene-UPE and NVP-UPE composites. Both TDE-UPE and NVP-UPE composites had superior interface bonding when compared to styrene-UPE composite because of the possible hydrogen bonding or other polar-polar interactions between HFs and NVP or TDE molecules in the UPE resins.(4) NVP was used as an RD for AESO to fabricate styrene-free AESO resin and its composites with HFs. Results indicated that the miscibility of NVP-AESO system was slightly lower than styrene-AESO system. The resin with 30 wt% NVP showed a slightly higher viscosity, activation energy and curing temperature than the styrene-AESO resin. The HF composite of AESO with 30 wt% NVP as an RD demonstrated a much higher tensile strength, tensile modulus, flexural strength, flexural modulus, storage modulus, and Tg as well as an improved fiber/matrix interfacial adhesion than the composite with styrene-AESO.(5) A rigid monomer, isosorbide-methacrylate (IM), was synthesized from isosorbide with methacrylate anhydride (MAA) via a solvent-free and ultrasonic-assisted method and then was used to copolymerize with AESO to formulate a biobased thermosetting resin (IM-AESO) with high renewable contents. MAA was used to modify IM-AESO to generate a resin (IM-MAESO) with an improved degree of unsaturation. Both IM-AESO and IM-MAESO resins were used for the preparation of composites with HFs and BFs, respectively. Results indicated that ultrasound assistant would greatly accelerate the esterification between isosorbide and MAA, and the IM yield was significantly affected by MAA concentration. IM had a good miscibility with various substances such as ethanol, furan, UPE, and AESO. The excess MAA in IM synthesis system could react with the hydroxyl groups of AESO to induce methacrylic groups on AESO molecules thus improving the unsaturation sites of the soybean oil-based resin. Both the IM-AESO and IM-MAESO resins had much lower viscosities, activation energies, and curing temperatures as well as higher polymerization rates and curing degrees than pure AESO due to the incorporation of IM as an RD. The combination of stiff IM and flexible AESO resulted in biobased networks and their natural fiber composites with superior flexural strength, flexural modulus, flexural strain, storage modulus, and Tg. MAA modification gave rise to the crosslinking degree and hence flexural properties and dynamic mechanical properties of the resins and their composites. The composites from HFs with the biobased thermosets indicated much higher flexural properties and dynamic mechaniccal properties than those from BFs.(6) The composites from HFs and styrene-free AESO composites were modified with isophorone diisocyanate (IPDI). The results revealed that the isocyanate groups of IPDI would react with the hydroxyl groups of both HFs and AESO via forming urethane connections. The addition of IPDI into AESO resins resulted in the resins with reduced viscosities, activation energies, curing temperatures and reaction heatings. The dual functions of IPDI in the composites, i.e., crosslinking linker and coupling agent, contributed greatly to increasing the tensile and flexural properties, storage moduli, and Tg of the resulting composites, but did not influence their impact strengths. Further, IPDI modification significantly increased the stiffness of the resins and the interfacial adhesion between fibers and matrices.