Modification of ramie fabric with alkali and crosslinking treatment for wrinkle resistance

Abstract

This thesis is mainly concerned with a study on the modification of ramie with alkali and polycarboxylic acids aiming to improve the wrinkle resistance of the ramie fabric. The structures and properties of the modified ramie were studied systematically with various experimental techniques. A series of characterisation techniques have been applied such as FTIR, HPLC, RGPC, SEM, TG, and XRD. Alkali modification was applied to ramie yarns using various concentrations of NaOH and tension levels. The supermolecular and morphological structures of the resultant ramie yarns were characteriscd specifically. X-ray diffraction and FTIR analysis revealed that the maximum change in the structure of alkali modified ramie took place at 16% NaOH, resulting in a complete transformation from cellulose I to II. At the same time, the crystallinity index and fibre orientation decreased to the minimum value while the absorption properties enhanced to the maximum. When ramie yarns were modified with NaOH at the mercerisation strength of 16%, the tension level would strongly affect the resultant ramie yarn properties. The results revealed that tension mercerisation or tension mercerisation combined with partialacetylation could diminish the tensile strength loss in BTCA crosslinked ramie yarns. The effect of NaOH mercerisation on the crosslinking properties of ramie fabric was studied with respect to the reaction kinetics, physical properties, crosslink distribution, and dyeing properties. Kinetic study of the crosslinking reaction between ramie cellulose and BTCA was conducted by assuming the reaction was reversed-first-order. The results of activation energy implied that ramie was more difficult to crosslink with BTCA than cotton, and mercerisation made the crosslinking reaction easier take place. The results of the physical properties revealed that slack mercerisation did cause a serious loss in both tensile and tearing strengths whereas the tension-mercerised ramie also suffered a slight reduction in both tensile and tearing strengths. Both tension and slack mercerisation would bring about much more uniform distribution of the crosslinks. Crosslinking with BTCA decreased the dyeability of ramie and mercerised ramie fabrics. The parameters related to the dyeing process such as the equilibrium adsorption, dyeing rate and maximum dyeing capacity were all reduced by the crosslinking treatment. However, the activation energies of dyeing were increased. When compared, the mercerised and mercerised-crosslinked fabrics showed higher equilibrium adsorption and dyeing rate than the non-mercerised and non-mercerised-crosslinked ramie fabrics respectively. Pore model was used to explain the reduction in the dyeability of the crosslinked fabrics. Pore size distribution of liquid ammonia treated ramie fabric was investigated using the technique of reverse gel permeation chromatography (RGPC) and compared with the NaOH mercerised ramie fabric. The results revealed that liquid ammonia treatment reduced the accessible pore volume and surface area of the ramie fabric. On the contrary, NaOH mercerisation increased the accessible pore volume and surface area of the ramie fabric. Liquid ammonia treatment could cut down the usage of the crosslinking agent to obtain similar or better end-use and wrinkle-resistant property. Further investigation was conducted to study the mechanism of the tensile strength loss in the BTCA treated ramie fabrics. By saponifying the BTCA treated ramie fabrics, the tensile strength loss due to degradation and crosslinking was evaluated. The results showed that acid degradation was more dependent on the curing temperature than the proton concentration. In addition, even a very small amount of crosslinks present would also cause a considerable tensile strength loss. Mercerisation could change the supermolecular structure of ramie, which would also strongly affect the tensile strength loss in the BTCA treated ramie fabric. Both tension and slack mercerisation would enlarge the accessible regions of ramie fibre, resulting in more severe acid degradation. The higher crystallinity and lower accessibility of the ramie fibre, leading to high density and non-uniform distribution of the crosslinks, might be responsible for the severe tensile strength loss as a result of the crosslinking reaction. To improve the performance of the wrinkle-resistant finished ramie fabrics, optimisation process was conducted by varying the recipe formulation and curing condition, as well as using additives, swelling agents and softeners in BTCA treatment. In-situ polymerisation and crosslinking (IPC) treatment using unsaturated polycarboxylic acids such as maleic acid, itaconic acid and acrylic acid as polymer builder were also attempted on ramie fabric. The overall results revealed that IPC treatment could improve the wrinkle-resistant properties of the ramie fabric to a satisfactory degree with the strength retention comparable to the BTCA finishes. Moreover, the reduced BTCA usage could diminish the overall cost from the economic point of view since BTCA was more expensive than IA and AA.