Study On The Preparation,Processing And Performance Of Thermoplastic Modified Sisal Fibers

Thermoplastic and eco-friendly biobased materials produced from lignocellulose has great potential as substitutes to petroleum-based polymer and is of great importance to solve the increasing resource and environment problems,and it is essential to develop eco-friendly and efficient modification strategy to obtain thermoplastic lignocellulose materials on an industrial scale.In this article,continuous steam explosion was applied to achieve the defibrillation of sisal fibers(SF)and improve the specific surface area and surface reaction activity of fibers.And then,thermoplastic modification of fibers was conducted by periodate oxidation and borohydride reduction under mild condition.The effects of steam explosion on oxidation-reduction reaction of fibers and performances of modified materials were investigated.In addition,the feasibility of lignocellulose with all components used as raw materials for modification was assessed.The modified materials were further compounded with plasticizer to regulate rheological properties with the aim of expanding the thermal processing method,and achieving the extrusion processing.The development of extrusion processing of modified materials provides a favorable foundation for the preparation and application of this materials on an industrial scale.In this work,the modification degree of fibers was controlled by adjusting the mass ratio(n)of Na IO₄ to fibers.The differences between SF and steam exploded sisal fibers(SESF)in oxidation-reduction reaction and materials performances were investigated.The results indicated that the aldehyde content of oxidized SESF(OSESF)was higher?17%than that of OSF,which is owing to the higher specific surface area and surface reactivity.And then,the yield of oxidized-reduced SESF(RSESF)was 68.4%at n=1.5,which was increased by?16%compared to RSF.According to the results from SEM images,XPS chemical characteristics and carbon residue of thermogravimetry,the reaction degree of inner and outer layers of SF is quite different in spite of the modification degree,which resulted in much more molecules with higher reaction degree.Therefore,the matter loss in outer layer of RSF was worse and complex.Inversely,the higher specific surface area of SESF made the difference of modification degree small,which resulted in less loss of dialcohol cellulose.And then,RSESF can form much more uniform fibers binding network after hot pressing,the tensile strength of hot-pressed RSESF was 42.2 MPa,which was 175%higher than that of hot-pressed RSF contains significant internal defects.Oxidized-reduced fibers exhibited obvious glass transition temperature(T_g),the T_g of RSF and RSESF was 142.1°C and 132.5°C.More importantly,the T_g of RSESF gradually decreased while the T_g of RSF basically remaining unchanged as modification degree increased,which demonstrated that the partially depolymerization of lignin is favorable to weaken the restrict of dialcohol cellulose molecular chain.The thermoplasticity of RSESF was thus further improved.The content of hemicellulose and lignin of SESF was controlled by alkali treatment and bleaching treatment.The resulting fibers were then modified with periodate oxidation and borohydride reduction.The effects of hemicellulose and lignin on oxidation-reduction reaction and materials performances were studied.The obtained results indicated that the removal of hemicellulose and lignin can improve the aldehyde content,and reach the maximum when removing hemicellulose and lignin simultaneously.Of all the oxidized-reduced fibers with different components,the yields of RSESF were highest at the same n.For example,the yields of RSESF1.5 was 68.4%,while the yields of RSESF1.5-HL was just 26.4%.The yields result indicated that the presence of lignin was favorable to inhibit the loss of dialcohol cellulose.Thermogravimetric analysis suggested RSESF had better thermostability compared to other oxidized-reduced fibers with alkali treatment and bleaching treatment.Variable temperature FTIR and XRD revealed that there were negligible effects of hemicellulose and lignin on the change on chemical and crystal structure of modified materials in thermal processing window.And then,the synergistic effect of hemicellulose and lignin made the T_g of RSESF relatively lower and RSESF keep favorable thermoplasticity,such as the T_g of RSESF2.0 was 132.5°C,obvious lower than that of RSESF2.0-H(151.8°C).In addition,according to the mechanical properties and SEM images,the presence of hemicellulose and lignin can promote the interface bonding between modified fibers,which thus resulted in higher tensile properties.76%of RSESF was compounded with 16%of xylitol and 8%of polyvinyl alcohol.The RSESF(XYL/PVA)composites can be extruded processing by twin-screw extruder.The effect of modification degree and extrusion times on aggregation structure and performances of materials were investigated.Dynamic thermomechanical analysis results indicated that the addition of plasticizer can significantly reduce the T_g of RSESF(XYL/PVA)composites,especially at lower modification degree.The loss factor of RSESF(XYL/PVA)composites was increased with increasing modification degree,which suggested that the window of thermal processing was been widened and the viscosity was increased.Dynamic rheological measurement indicated that RSESF(XYL/PVA)composites exhibited Rheological properties of shear thinning.However,RSESF(XYL/PVA)composites had high crystallinity(>60%),and crystalline region can't melt,which thus resulted in higher storage modulus G'than loss modulus G''.In addition,extrusion processing can cause fragmentation of fibers,and POM indicated that the size of crystalline region decreased.Amorphous region and crystalline region were thus more evenly distributed and formed more interface combinations,which resulted in higher thermostability of RSESF(XYL/PVA)composites after extrusion processing.And then,mechanical properties were improved after extrusion processing,such as the tensile strength of RSESF1.5(XYL/PVA)composites was increased from 19.4 MPa to 25.3 MPa.Repeated processing of RSESF1.5(XYL/PVA)suggested that a couple of times thermoplastic processing is feasible.Increasing the extrusion times had a significant effect on aggregate structure,which thus resulted in a decrease of tensile strength and an increasing of strain at break.The aggregate structure and properties tended to be stable eventually.In brief,thermoplastic sisal fibers materials were prepared by combining continuous steam explosion pretreatment and periodate oxidation/borohydride reduction modification.Steam explosion pretreatment,and hemicellulose and lignin components have a significant and positive effect on oxidation-reduction reaction and materials performances.RSESF compounded with xylitol and PVA can be melt-extruded successfully.The modification degree and extrusion times can be controlled to adjust and optimize materials performances,which laid the research foundation for the preparation and application of thermoplastic lignocellulose.