| With the continuously shortening of textile using cycle and significantly increasing of per capita fiber consumption, the supply of textile fibers currently is difficult to continue. Meanwhile, thousands of tons of waste textiles buried each year and less than 1% been comprehensive utilized, resulting the waste of resources and environmental pollution, which conflict with the theme of ecological development of our society. Although, polyester and cotton fabrics account for 50% of the waste textiles, different physical and chemical properties bring great difficulties to the recycling of poly-cotton blended fabrics, which induce to the narrow application and research of this field. Therefore, it becomes extremely urgent to explore a new method in the recycling of waste poly-cotton fabrics. Hydrothermal method has received increasing attention because of its characteristics in no pollution, environment friendly and low cost. Subcritical water has some special properties such as acid-base catalyst, solvent separator and organic solvent substitute, allowing polyester and cotton fiber respectively take place hydrolysis and carbonization reaction, which provide a new way for the recycling of waste textiles.Waste cotton fabrics, polyester fabrics and poly-cotton fabrics were used as the research object in this paper, the hydrothermal behaviors of cotton and polye-ster fibers and the interaction effects under the same reaction system were researched by using the particular characteristics of subcritical water.First of all, in order to obtain carbon microspheres(CMSs) under subcritical water, effects of the reaction system initial pH, temperature and other reaction parameters on cotton fiber hydrothermal carbonization process were comprehensively explored. Hydrothermal carbonization of cotton fiber is a decreasing process in hydrogen(H) and oxygen(O) component content. Thermal temperature is the key factor to decide whether cotton fiber can take place carbonization reaction. The increasing of temperature can promote the aromatization extent of products and increase the thermal stability. The initial pH of reaction system is the key factor to decide whether cotton fiber can be carbonized into CMSs. Although cotton fiber has low conversion activation energy in subcritical water, longer reaction time has little significance in increasing the yield of products. With CMSs as base materials and Ag NO3 as the source of silver, Ag/CMSs antibacterial agent has slow release performance and good antibacterial effect.Then, in order to research the hydrothermal carbonization mechanism of biomass cellulose, the hydrolysis mechanism of cellobiose on neutral and acidic aqueous solution were studied by using density functional theory with cellobiose as model. In acidic aqueous environment, the process of cellobiose hydrolysis into monosaccharides can cause smaller heat changes and require lower energy barrier and milder reaction conditions, which are more beneficial to the hydrolysis of cotton fiber and cellobiose. With cotton fiber, microcrystalline cellulose and glucose as biomass cellulose models, the physicochemical characteristics and reaction process of the three materials were investigated. The hydrothermal carbonization products(hydro-char) of the three materials have similar structure and performance. Glucose is the important intermediate as cotton fiber turned into CMSs.The above results of these studies indicate that the hydrothermal carbonization mechanism of cotton fiber as follows: On the one hand, glucose that obtained by cotton hydrolysis under-go fragmentation reactions and formed nuclear. With rich active oxygen-containing functional groups in surface, the nuclear then gradually grow into CMSs in a layer wrapped growth way. On the other hand, oligomers formed by cotton hydrolysis cannot take further hydrolysis reaction. The rings of glucose break under high temperature and pressure hydrothermal environment, then under-go intra-/inter- molecular dehydration and fragmentation reactions, which increased the carbon(C) content of hydro-char. The morphologies of hydrothermal carbonization products are mainly irregular and bonding of particles.In addition, influence of reaction condition on polyester(PET) hydrolysis rate and yields of terephthalic acid(TPA) and ethylene glycol(EG) were investigated. Results show that PET hydrolysis rate can reach to 97.26% while the yield and purity of TPA can respectively reach to 84.95% and 98.25% when the reaction temperature is 250℃, time is 6h and solid-liquid ratio is 1:10. On this basis, the hydrolysis mechanisms of PET were researched and the result shows that H+ is the main driving force of hydrolysis.According to cotton and polyester fiber hydrothermal behaviors, effects of cotton hydrothermal carbonization on PET hydrolysis and TPA recovery rate were studied in polyester and cotton hydrothermal reaction system. The influence of polyester hydrolyzates EG and TPA on cotton fiber hydrothermal carbonization were also respectively explored to analyze the hydrothermal conversion characteristics of ploy-cotton fabrics. Results show that, TPA has little effect on cotton fiber hydrothermal carbonization when the mass ratio of TPA and cotton below 1. On the contrary, TPA has adverse effects on the structure and properties of cotton fiber hydro-char. Cotton fiber cannot be carbonized into CMSs when the concentration of EG is over 9wt.%. The C content of hydro-char is continuously increasing when the concentration of EG raised to 36 wt.%. For the example of poly-cotton fabrics and analysis of TPA and EG on cotton carbonization, the yield of cotton fiber hydrothermal carbonization and the structure and properties of products will have a slightly increase while the poly-cotton blending ratio is less than 55/45, which equivalent to the TPA/cotton mass ratio and the content of EG are respectively below 1 and 0.36 wt.%. Conversely, it will act as an inhibition effect on cotton hydro-char productivity, structure and properties when the poly-cotton blending ratio is higher than 55/45. |
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