Preparation And Application Of Modified Straw Fiber/HAP Composites

Crop straw,as a green,environmentally friendly biomass resource,has the advantages of being a wide source,renewable,and so on.Using it to prepare new green biomass composite materials can not only promote the high-value reuse of agricultural wastes but also effectively reduce the use of fossil fuels and alleviate the lack of wood resources in our country.Due to the smooth appearance and ash,wax,and sugar content of straw raw materials,the interface bonding property of composite materials is poor,the rebound rate is high,and the mechanical property is poor,which seriously affects the application of composite materials.It is of great significance to study the chemical composition and properties of the straw raw materials after pretreatment and interface modification.In this paper,corn straw was selected as the raw material.Based on physical(hydrothermal)and chemical(acid,alkali,and salt)pretreatment,hydroxyapatite(HAP)was synthesized on the surface by the chemical precipitation method.The modified straw fiber /HAP composite was prepared by a hot pressing process.The effects of different pretreatment methods,a ratio of composite materials,and interface modification methods of KH550,polyethylene glycol,and propylene oxide on the main chemical composition(cellulose,hemicellulose,and lignin),morphology,thermal stability,crystallization properties,mechanical properties,water absorption,wettability and flame retardant of the composite were studied.The main conclusions are as follows:1.Different treatment methods have different effects on the chemical composition and structure of the straw raw materials after mechanical grinding.After pretreatment,the contents of the four kinds of cellulosic ethanol extracts and ash decreased to different degrees,among which,the extracted content of Na OH treatment was the lowest at 1.63%,and the ash content of hydrothermal treatment was the lowest at 1.34%.After 20% Na OH solution treatment,the fiber mass loss rate was up to 60.22% and the color of the solution was the darkest.The cellulose proportion increased to 79.51% and the hemicellulose proportion decreased to 3.76%.The proportion of lignin increased to 28.86% after 2.0% H2SO4 treatment.SEM observation showed that the roughness of the fibers increased after alkali treatment,the lignin in the fibers was sulfonated after Na2SO3 treatment,the fibers appeared pores,and the fibers were peeled off after H2SO4 treatment.The results of FTIR and XRD showed that the high concentration of Na OH resulted in the increase of fiber moistening and the destruction of the crystallization zone,the decrease of hemicellulose and lignin,as well as the loss of some cellulose and the decrease of crystallinity.TG showed that fiber pyrolysis was divided into three processes before and after pretreatment,the maximum degradation rate temperature of the three chemical treatments increased,and the thermal stability of the fiber was improved.2.The straw fiber was added into the solution of different concentrations for pre-calcification,and the HAP composite was synthesized by the precipitation method.The straw fiber /HAP composite was prepared by a hot pressing process.FTIR and XRD showed that the fibers in the composites still belonged to type Ⅰ cellulose,and HAP crystallinity was better.SEM showed that lamellar HAP was attached to the surface and pores of the fibers,the content of HAP was proportional to the concentration of the solution,and a dense structure was formed between the fibers and the pores after hot pressing.EDS showed that the ratio of Ca and P elements was 1.69.The test of the contact Angle showed that with the increase of HAP content,a hydrogen bond was generated between the fiber and HAP,the exposed-OH was reduced,the density of the material was enhanced,the contact Angle was increased,and the water absorption was decreased.The size of the composite is stable at room temperature.Compared with pure straw,when HAP content is 25%,the maximum tensile strength,bending strength,bending modulus,and Shore hardness of the composite increase by 4.77%,8.53%,125.32%,and 7.35% respectively,respectively,and the composite has the best comprehensive performance.With the increase of HAP content,and the high thermal stability of HAP fiber coating,the thermal stability of the composite increases.3.To further enhance the interaction between the fibers and HAP,the fibers were modified with polyethylene glycol,propylene oxide,and KH550 respectively.The results showed that the fibers were modified with polyethylene glycol,propylene oxide,and KH550 to produce some etched breakage and increased roughness on the exterior of the fibers,and white HAP particles were deposited in the three-dimensional lattice formed by the fibers after pre-calcification,indicating that the modification was beneficial to the bonding of HAP with the fibers.XRD shows that the modified composite is still type I cellulose,with a crystallinity of 64.86%,62.42%,and 57.94%,and crystallite size of 3.80 nm,3.24 nm,and 3.09 nm,respectively;TG shows the largest share of cellulose after polyethylene glycol modification;a maximum water contact angle of 69.22°,a minimum reduction in water absorption to 25.66%,and an increase in tensile strength of 20.48% for the composite material modified by propylene oxide;the KH550 modification increased flexural strength,modulus,and Shore hardness by 12.68%,15.91%,and 5.83%respectively,indicating that both modifiers improved the interfacial bonding of the composite.