Preparation And Mechanism Of Dissolving Pulp In Organic Acid System

Currently,high-purity dissolving pulp has been widely used in textile,food,rubber and other fields.However,traditional dissolving pulp preparation methods(PHK,AS)have problems such as high boiling temperature,and high energy consumption,and chemicals cannot be recovered.Therefore,it is urgent to develop a green and environmentally friendly preparation method of dissolving pulp with mild cooking conditions.In this context,this study established a new pretreatment system with a recyclable p-TsOH solution.Using poplar wood as a raw material,the preparation of unbleached pulp under mild conditions is realized,and the unbleached pulp is assisted by ultrasonic treatment and further bleached to obtain a high purity dissolving pulp product.The specific research content is as follows:(1)Using p-TsOH solution(80 wt%)as the pretreatment system,through the analysis of the unbleached components after pretreatment,especially the removal of lignin and hemicellulose,the different sizes of raw materials were studied(less than 20 Mesh,20-40 mesh,40-60 mesh,greater than 60 mesh)influence on the preparation of unbleached pulp products.The molecular weight analysis of the released lignin was carried out.Combined with the characterization results(specific surface area,crystallinity)of the obtained unbleached pulp,the optimal material size is determined to be 40-60 mesh.Compared with other sizes,the unbleached pulp made from 40-60 mesh raw materials has a larger specific surface area(pore size and pore volume increased to 17.959 nm and12.750×10^-4 m³/g,respectively.)and crystallinity(58.74%),and the dissolution rates of hemicellulose and lignin are relatively high,89.43%and 84.4%,respectively.The obtained lignin has a low molecular weight(2166 g/mo L)and a high utilization value.Laid a good foundation for the subsequent preparation of dissolving pulp.(2)Using p-TsOH solution(80 wt%)as the pretreatment system,the influence of different treatment conditions(T80t30,T80t60,T90t30,T90t60;T is temperature,t is time)on the preparation of unbleached pulp products was studied.By testing the removal of lignin and hemicellulose under different pretreatment conditions,the structural properties of lignin and the properties of unbleached pulp,the best pretreatment condition was determined to be T90t60.The crystallinity of unbleached cellulose obtained under these conditions was 61.68%,and the dissolution rates of hemicellulose and lignin were higher,92.56%and 91.54%,respectively.Two-dimensional nuclear magnetic analysis showed that p-TsOH mainly cleaved?-O-4 bonds in lignin,and the resulting lignin had a lower molecular weight of 2141 g/mo L.After the unbleached pulp was subjected to short-time ultrasound(5-10 s),the five-stage bleaching((OO)D₀E_OPD₁D₂)process was used to successfully prepare high-quality dissolving pulp.The Brightness of the dissolving pulp is 90.2%ISO,the degree of polymerization is 471,the?-cellulose content is 87.7%,the Kappa value is 0.26,and the yield is 41.23%.Compared with commercial dissolving pulp,the whiteness value and yield are relatively high.The p-TsOH solution(80 wt%)assisted ultrasonic treatment provides a new and green preparation method for the preparation of dissolving pulp.(3)In order to improve the degree of polymerization,whiteness and?cellulose content of the dissolving pulp,this study used the p-toluene sulfonic acid/sodium hypochlorite solvent system(p-TsOH/NaClO)to reprocess the raw materials after extrusion pretreatment.At the same time,the influence of different treatment conditions on the preparation of unbleached pulp was studied.By testing the removal of lignin and hemicellulose,the degradation of cellulose,and the contents of unbleached pulp under different conditions,the optimal condition was determined to be T90t45.The extraction rates of lignin and hemicellulose are 90.5%and 90.58%,respectively.The unbleached pulp under this condition has a higher crystallinity of 68.02%;infrared characterization also shows that the unbleached pulp under this condition retains the basic molecular structure of cellulose.The unbleached pulp is subjected to short-time ultrasound(5-10 s)and then a three-stage(OO)D₀E_OP bleaching process is used to obtain a high-yield dissolving pulp.The yield is 40.3%,the brightness is 91.1%ISO,the degree of polymerization is 517,and the?-cellulose content is 89.6%,and the Kappa value is 0.22.All indicators of the dissolving pulp prepared in the laboratory have reached the commercial dissolving pulp standard.Compared with the traditional dissolving pulp preparation method,the pulping time is shortened by 3-5 times,high yield of dissolving pulp,less bleaching process,and environmentally friendly,providing an effective and green technical route for the preparation of high-quality dissolving pulp from lignocellulose.