Study On Hydrothermal Degradation Process And Mechanism Of Waste Cellulose Fabrics

In recent years,the average annual output of waste textile products in China has reached 30 million tons,most of which are buried or burned as garbage,and the high output and improper treatment have caused serious resource crisis and environmental problems.Therefore,how to realize green and effective utilization of waste textile resources has become an important problem to be solved in the textile industry.Among all textiles,cellulose fabrics,represented by pure cotton and polyester/cotton blended fabrics,account for more than 60%of the total textiles,and as a consequence a huge amount of waste each year.Hydrothermal treatment is the technology to apply water as the reaction medium to convert waste biomass and other raw materials into high value-added chemical products in an enclosed environment.It has the advantages of simple operation,low cost,diversified product,environmental friendliness and pollution-free.It can realize efficient conversion and utilization of waste biomass and pave a new way for the recycling of waste cellulose textiles.Therefore,this project will take waste pure cotton and polyester/cotton blended fabric as the research object,and use the hydrothermal treatment technology to carry out the hydrothermal transformation of the two types of cellulose fabrics,and study the influencing factors and the hydrothermal transformation mechanism of cellulose fabrics.First of all,the waste cotton fabric was used as the research object to prepare carbon microspheres by hydrothermal conversion,the effects of Cu SO₄concentration,reaction temperature and retention time on the hydrothermal carbonization process,product structure and performance were investigated;the best process of preparing carbon microspheres by hydrothermal carbonization of cotton fiber catalyzed by Cu SO₄ and the mechanism of hydrolysis and carbonization of waste cotton fiber functioned by Cu SO₄ was studied.The results showed that the best conditions for the preparation of carbon microspheres were 330?,6 h and 0.15%of Cu SO₄;Cu SO₄ could significantly reduce the initial carbonization temperature of cotton fiber and accelerate the hydrolysis of cotton fiber;Cu SO₄ participated in the formation of carbon microspheres and promoted the formation of carbon nucleus;the presence of Cu SO₄ improved the aromatization degree of products;the surface of carbon microspheres synthesized with Cu SO₄ as the catalyst had rich functional groups.In the hydrothermal synthesis system,Cu SO₄ could promote the hydrolysis of cotton fiber and the formation of carbon nucleus at the same time.This study provided a new,mild and efficient method to prepare carbon microspheres by using waste cotton fiber.Secondly,in order to further analyze the hydrothermal carbonation mechanism and influencing factors of cotton fiber,cotton fabric and viscose fabric were used separately as reaction substrates,and microcrystalline cellulose as model substrate for comparison,to study the influence of different cellulose raw material structures on the hydrothermal carbonation conditions and products,and to further explore the hydrothermal carbonation mechanism of cellulose fiber.The results showed that the crystallinity of the substrate significantly affected the carbonation conditions,the lower the crystallinity was,the milder the carbonation conditions were,while the degree of polymerization of cellulose had little effect on the carbonation conditions;the carbon microspheres synthesized from different materials had similar amorphous structure and rich functional groups,but the carbon microspheres synthesized from cotton fiber and microcrystalline cellulose had higher carbon content and better thermal stability;The formation of carbon microspheres started from a large number of soluble intermediates generated from cellulose hydrolysis that were dehydrated,condensed and polymerized to form carbon nuclei,which formed insoluble polymers by adsorbing surrounding molecules,and finally grew into carbon microspheres.Thirdly,the application of cellulose based carbon materials in adsorption field was explored.Cellulose based carbon spheres are rich in functional groups.Porous carbon materials were prepared from it by activation treatment,and the adsorption properties for methylene blue were studied.The results showed that the modified carbon material was mainly microporous,with a high specific surface area of 879 m²/g and a total pore volume of 0.6365 cm³/g.The activated carbon microspheres had good adsorption capacity for methylene blue,and the adsorption capacity could reach 219.1 mg/g.The adsorption process of methylene blue could be well fitted by quasi second-order kinetic model.Fourthly,the two-component polyester/cotton blended fabric was taken as the research object to study the hydrothermal degradation of polyester/cotton blended fabric.According to the different hydrothermal reaction characteristics of cellulose and polyester,the hydrothermal degradation behavior of cotton fiber and polyester fiber coexisted in the hydrothermal reaction system of polyester/cotton was studied.It was found that polyester fiber and cotton fiber had different hydrothermal behaviors.At lower hydrothermal temperature,cotton fiber partially hydrolyzed to form powdered cellulose and soluble sugar,while polyester fiber kept hydrothermal stability,which could realize proper separation of polyester/cotton blended fabric.Under the condition of hydrothermal temperature of 170?,the reaction time of 3 h and solid-liquid ratio of 1:20,the recovery rate of polyester fiber was 98.84%,the yield of glucose was 15.57%,and the residue rate of cotton fiber was about 66.47%.At higher hydrothermal temperature,the polyester fiber was hydrolyzed into terephthalic acid(TPA).With further increase of hydrothermal temperature,TPA also began to degrade,reducing its yield,while the cotton fiber carbonized at high temperature.However,due to the existence of TPA in the system,the morphology of carbon products of cotton fiber was affected,and the carbon microsphere structure with good morphology cannot be formed.Finally,on the basis of the study of hydrothermal separation of polyester/cotton blended fabric,the mechanism of hydrothermal separation of polyester/cotton blended fabric was studied by using density functional theory.By establishing a calculation model,the reaction mechanism of cotton glucoside bond breaking and polyester ester bond breaking described by dominant solvent model and recessive solvent model were studied respectively,and the glucoside bond breaking in cellulose macromolecules was studied.The activation energy needed for bond fracture was calculated.The results showed that the activation energy of cellulose hydrolysis was 14.81 k Cal/mol,lower than that of polyester hydrolysis of 21.46 k Cal/mol.When the reaction temperature in the system could overcome the activation energy required by cellulose hydrolysis but lower than that required by polyester hydrolysis,the hydrolysis of cotton fiber could be realized while maintaining the hydrothermal stability of polyester fiber,so as to achieve the purpose of separation of polyester and cotton.