Abstract:

Objective Hydrothermal process is a new way of recycling of waste textiles that will reduce environment pollution and resource waste thereof. With the characteristics of low cost, no pollution and controllable products, hydrothermal degradation technology is suitable for chemical degradation and high valued recycling of waste textiles. Most waste textiles are blended fabrics of polyester/cotton, which have different chemical properties and compact twisting structures, and therefore it is always a difficult problem in the recovery of blended textiles. Research on degradation behavior of blended fabrics in a hydrothermal system is believed to provide the necessary theoretical and numerical evidence for the hydrothermal degradation, regeneration and recycling of blended textiles.

Method Hydrothermal degradation of high polymer could be achieved by utilization of the special character of water as a solvent, a catalyst and a reactant in the subcritical water. Hydrothermal process was applied in the research of blended fabrics of polyester and cotton with the addition of copper sulfate to identify the hydrothermal degradation behavior of cotton fiber and polyester fibers, followed by characterization of the yield, microstructure, chemical structure, crystal structure, and mechanical properties of the cotton and polyester blended fabrics under different temperatures.

Results It was indicated that polyester and cotton featured different hydrothermal behaviors in the copper sulfate hydrothermal system. In particular, the cotton fiber of the blended fabrics was selectively hydrolyzed and degraded in the temperature range of 130-190 ℃, and the fiber structure of the cotton disappeared after hydrolysis, and some of the cotton fiber was hydrolyzed to becoming powdered cellulose and some to monosaccharides. The polyester fiber, on the other hand, maintained its morphology unchanged in the temperature range of 130-190 ℃, while the structures of chemistry, crystal, and mechanical properties were almost the same as the original polyester fiber, which could be reused directly as the polyester fiber for recycling. When the temperature rose above 210 ℃, dehydration and carbonization happened to cotton fiber and the hydrothermal products were carbonized products of carbon element in the main. As for polyester fiber, when the temperature elevated to 210-250 ℃, the polyester fiber was gradually hydrolyzed. When the temperature was increased to 270 ℃, the polyester fiber was completely degraded to terephthalic acid(TPA) with a theoretical yield over 91%. The TPA was found to decompose at higher temperature. The retention rate of TPA was about 78% of the theoretical yield at 330 ℃. During the high temperature hydrothermal treatment of polyester/cotton blend fabric, the presence of TPA does not essentially affect the crystal structure of the carbonated products of cotton fibers. However, due to the presence of a large amount of terephthalic acid, complete degradation and carbonization of the hydrolyzed products of cotton fiber were hindered, and the carbonated products were in the form of block structure rather than the expected ball structure.

Conclusion In the hydrothermal environment of copper sulfate, hydrothermal separation of polyester/cotton can be achieved at low temperature due to degradation of cotton fiber into cellulose powder that disintegrated the fabric structure and hydrothermal stability of the polyester, while at high temperature, polyester and cotton can be degraded together in the same system. The cellulose powder generated from the separation of polyester/cotton at low temperature can be applied to the preparation of modified cellulose or other cellulose materials in a wide range of applications. Besides, the soluble sugar in the hydrolysate can be further recycled as well. At high temperature, polyester degrades into terephthalic acid, which can be recycled and used as the raw material for industrial production of polyester monomer and others. The carbon products formed through cotton fiber carbonization at high temperature, rich in hydrophilic oxygen-containing functional groups and of high functional values, can be grafted and modified to prepare functional materials, or applied in the areas of sewage adsorption materials, catalytic carriers, materials of electrode, and so on. The research offers a certain reference to the recycling of polyester/cotton blended fabrics and of a certain practical value.

Key words: waste polyester/cotton blended fabric, recycling, copper sulfate hydrothermal system, hydrothermal separation, hydrothermal degradation