| Plant fiber is a renewable resource with abundant reserves and wide sources in nature.The preparation of environment-friendly bio-based materials from plant fiber can not only reduce the dependence on oil resources,but also improve the utilization rate of plant fiber resources,thereby reducing environmental pollution and resource waste.However,due to the complex and dense hydrogen bond network and high crystallinity of plant fibers,the energy required for molecular chain motion is very high,resulting in the melting temperature much higher than the thermal decomposition temperature,which makes it impossible to melt processing.Therefore,the thermoplastic modification and processing technology of plant fibers are of great significance to expand the types and output of bio-based plastics.In this paper,the thermoplastic modification strategy of plant fibers was proposed,which combined continuous steam explosion pretreatment with water phase carboxylation graft modification.Low dose sodium periodate and sodium chlorite were used to carboxylate sisal fibers,and then cetyltrimethylammonium bromide(CTAB)was grafted onto the modified fibers based on electrostatic interaction to obtain thermoplastic sisal fibers.By compounding with xylitol / PVA plasticizer system,the processing of push-pull mixing,internal mixing and extrusion were further realized,which laid the foundation for promoting its industrial development.In this paper,the mass ratio of sodium periodate and steam exploded sisal fibers(m)was used to control the degree of carboxylation modification,and the characteristics of products with different degree of carboxylation were explored.The results show that when m is in the range of 0.1 to 0.4,the carboxyl content increased from 0.47 mmol/g to 1.55 mmol/g.The hydroxyl groups in the fiber were converted to carboxyl groups by carboxylation,which weakened the hydrogen bonding force and opened the ring molecular chains.During the modification process,the content of hemicellulose and lignin decreased gradually,the network structure of lignin was destroyed,the yield of carboxylated fibers only decreased from 91.8 %to 83.9 %,and most of the products remained.The XRD results confirmed that the carboxylation modification mainly occurred in the amorphous area of the fibers,and the product maintains the cellulose I crystal structure.Under mild water phase conditions,CTAB is added at a mass ratio of 1:1 to carboxylated fibers,and used as an internal plasticizer to be grafted onto carboxylated sisal fibers to prepare thermoplastic sisal fibers.When m is only 0.2,the translucent sheet can be hot-pressed at 135 °C.As the degree of carboxylation increased,the grafting rate rose from 9.1 % to 36.1 %.The DMA results showed that the secondary relaxation of thermoplastic sisal fibers occurs in the range of 50 ? 90 °C.The SEM and AFM showed that the bonding between the fibers was gradually improved,and the interfacial strength was improved,so the tensile strength of the hot-pressed sheet was improved.However,if the degree of carboxylation is too high and more grafting chains are introduced,the reinforcing effect of the fiber will be weakened,resulting in a decrease in mechanical properties.Among them,the hot-pressed sheet HCF-0.2-1.0 exhibited better mechanical properties,with tensile strength and modulus reached 27.8 MPa and 1669 MPa,respectively.The TGA showed that the formation of carboxyl groups would cause the thermal stability of the fiber to decrease.Variable temperature FTIR further revealed that heating would induce the esterification and crosslinking reaction of carboxyl and hydroxyl groups,but the introduction of grafted chains could inhibit the reaction,and the chemical structure of the fibers could remain stable at 180 °C.In addition,the prepared thermoplastic sisal fibers were biodegradable,and the mass loss rate of HCF-0.2-1.0 was 40.8 % after being buried in soil for 4 months.Then the dosage of CTAB in the grafting reaction was adjusted under the same degree of carboxylation,and the effect of the dosage of CTAB on the properties of thermoplastic sisal fibers were explored.The results showed that with the increase of CTAB dosage,the grafting rate of thermoplastic sisal fibers increased,and the secondary relaxation temperature of molecular chain decreased,which makes the interface effect between fibers further improved,and the tensile strength first increased and then decreased.The tensile strength and modulus of the hot-pressed sheet HCF-0.3-0.8 reached 28.6 MPa and 1713 MPa,respectively.The TGA results showed that the initial thermal decomposition temperature of the fiber remained almost unchanged.The thermoplastic sisal fibers and plasticizing system were compounded to broaden the processing method.The thermoplastic sisal fiber composites were prepared by blending the thermoplastic sisal fiber CCSF-0.3-0.8 with 8 phr xylitol and 10 phr PVA,which realizes pushpull mixing,internal mixing and extrusion processing.After processing,the secondary relaxation temperature of the molecular chains of the material was reduced to 40 ? 80 °C,and the complex viscosity ?* decreased,which increased the fluidity of the material,and the microscopic surface of the hot-pressed sheet became smooth and flat.It can be found that the effect of extrusion on the materials is more significant.The XRD showed that the crystal structure of the material was not changed,but the crystallinity decreased to 59%.The POM results showed that processing would destroy the crystalline zone size of the fiber aggregate state,resulting in the decline in the mechanical properties of the material.Among them,the fiber grains of the push-pull mixing retain a larger aspect ratio,and had higher mechanical properties,tensile strength and modulus 19.1 MPa and 1268 MPa,respectively.In addition,processing improved the thermal stability of the material.In conclusion,sisal fiber materials with good thermoplastic properties can be obtained by carboxylation-grafting modification of sisal fibers,and after mixing with an appropriate amount of xylitol/PVA plasticizing system,it can be processed by mixing,extrusion and other methods.Carboxylation-grafting thermoplastic modification is a high-yield,low-cost and environmentally friendly method,which is conducive to the realization of high-value and efficient use of plant fiber resources and the development of environmentally friendly materials. |
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