Studies On The Structure, Properties And Dyeing And Finishing Process Of Soybean Fiber

Soybean protein/poly(vinyl alcohol) blended fiber, i.e. soybean fiber, inventedand made in China is a new sort of bicomponent chemical fiber. Nowadays soybeanfiber has attracted much attention in textile market because of its soft handle, subduedluster, good wet permeability and heat retention, good comfort characteristic and highstrength, but it has demerits of poor form retention and resistance to pilling. Duringthe wet processing, soybean fiber has shown the following severe problems: lowwhiteness index of bleached fiber, dull color of pale dyed fiber, poor dyeability fordark color, poor level dyeing property, low color fastness, long procedure of blendedtextiles, poor resistance to wet heat, rigid hand after wet processing at hightemperature, loss of soybean protein after improper wet processing. This work aimedat solving the problems arising in the development and practical production ofsoybean fiber. In this paper, the structure of soybean fiber was first characterized, andthen its resistance to wet and dry heat, resistance to alkali, bleaching, and dyeingproperties and dyeing mechanism were studied in detail. These investigations werehoped to obtain the relationships between the structure of soybean fiber and thedyeing and finishing properties, provide theoretical data for the improvement in thespinning technology of soybean fiber, the selection of dyes and auxiliaries and thedetermination of proper conditions for dyeing and finishing, and the improvement inthe quality of related textiles.The amino acid composition analysis, elemental analysis, FT-IR spectroscopy,SEM, WAXD, DSC-TG analysis and chemical dissolution by sodium hydroxide wereused to study the chemical composition and structure. It was found that soybean fiberwas composed of about 20% soybean protein and 80% poly(vinyl alcohol) (PVA), themain components were glutamic acid and aspartic acid, and the ratio of acidic aminoacid to basic amino acid was higher than that of silk or wool. The compatibility ofsoybean protein and PVA in soybean fiber was poor, or soybean protein and PVAexhibited the phase separation structure. Blocky and incontinuous-phase soybeanprotein dispersed in the poly(vinyl alcohol) component of continuous phase. Soybean fiber showed the same crystalline structure and crystallinity degree as vinylon fiber(acetalized PVA fiber). Soybean protein had no contribution to the formation of fibercrystalline structure.The results obtained by integrative analyses indicated that the degree ofcrosslinkage between soybean protein and PVA, and between PVA macromolecularchains was low, and that the crosslinkage between soybean protein moleculesoccurred, and that an amount of acetalization structure was formed. The structure andperformance of soybean fiber were between protein fiber and vinylon; its chemicaland dyeing properties mainly depended on soybean protein; its heat stability andmechanical properties mainly depended on PVA component; its morphologicalstructure was influenced mainly by the super-molecular structure of PVAmacromolecule chains formed during the spinning, and also influenced by the contentand distribution of soybean protein. These conclusions could provide goodexplanation for the problems occurring in the dyeing and finishing processes.Soybean fiber had the poor resistance to wet heat. After the wet heat treatment atthe temperature above 100℃, soybean fiber exhibited the severe shrinking andyellowing, the great decrease in breaking strength and adhesive aggregation. So thetemperature for the wet processing of soybean fiber in hot water should be controlledbelow 95℃. Instrument analyses indicated that soybean protein was hardly lost duringthe wet heat treatment at high temperature, but the regularity of PVA componentdecreased and the change of its crystalline structure occurred due to the scission anddegradation of PVA macromolecular chains occurred. The poor resistance of soybeanfiber to wet heat originated from the heat denaturation of PVA component.Soybean fiber had the good resistance to dry heat. When treated below 195℃,soybean fiber exhibited very low shrinkage, small change in whiteness and nearlyunchanged strength and reduced dyeability. The safe and highest temperature of dryprocessing was 195℃. When treated above 195℃, soybean fiber was easily subjectedto thermal degradation. The severe thermal degradation led to a change in crystallinestructure and the formation of a yellow degradation substance.Soybean fiber had the poor resistance to strong alkali. The treatment in causticsoda solution at high temperature caused the hydrolysis and loss of soybean protein,the oxidation and yellowing of not acetalized PVA component, the decrease in the dyeability of acid dyes, but the acetal structure of PVA component and the regularityof PVA crystalline structure kept unchanged. It was found that three quarter ofsoybean protein was easily hydrolyzed, which was possibly caused by the poorcompatibility of soybean protein and PVA, the low molecular weight and crosslinkagedegree of most of soybean protein, whereas one quarter of soybean protein wasdifficult to be hydrolyzed, which could be explained by the high crosslinkage degreebetween a small quantity of protein macromolecular chains and the formation ofcrosslinkage between a small quantity of soybean protein and PVA. The wetprocessing in the caustic soda solution of low concentration and the soda ash solutionof high concentration at low and medium temperature hardly caused the high loss ofsoybean protein.Soybean fiber had the good stability to reductants while the stability to oxidantsvaried from oxidant to oxidant. The whiteness and weight loss of soybean fiberbleached by the reductants were very low. Reduction bleaching almost had noinfluence on the soybean protein content and the fine structure of soybean fiber, andexert a very small influence on dyeability. As for oxidation bleaching, the white indexof the fiber bleached by sodium chlorite and hydrogen peroxide/tetraacetyl-ethylenediamine (peracetic acid as a resultant of reaction) was much high. The highloss of weight and soybean protein during chlorite bleaching greatly decreased thedyeability of soybean fibers. The fiber bleached by hydrogen peroxide and hydrogenperoxide/tetraacetylethylenediamine exhibited a low weight loss and a slight decreasein soybean protein content. The potential oxidation action of oxidants to the basic andhydroxyl amino acids of soybean protein gave rise to the obvious decrease indyeability.In terms of the dyeing properties of soybean fiber, three major classes of dyes,including acid dyes, 1:2 metal-complex dyes and reactive dyes, exhibited good dyeingperformance, but the dyeing conditions of these dyes were different. The pH ofdye-bath had small influence on the uptake of 1:2 metal-complex dyes while it hadgreat influence on the uptake of acid dyes. The dyeing of acid dyes should beperformed below the pH of 4．5．Reactive dyeing should be done under alkaline conditions, but the usage of alkali for soybean fiber should be much lower than thatfor cellulosic fiber. The temperature for the dyeing of acid dyes and 1:2metal-complex dyes could be controlled at 90℃or so while that for reactive dyesdepended on the reactive groups of dyes. Among reactive dyes for the coloration ofcotton, the reactive dyes of medium and high substantivity and with the stronglyreactive groups (e.g. vinylsulphone or monofluorotriazine group) were particularlysuitable for the dyeing of soybean fiber and for dark shades. Different dyes displayeddifferent migration abilities. The migration performance of acid dyes was better thanthat of 1:2 metal-complex dyes while reactive dyes had very poor migration ability.Reactive dyes exhibited the best color fastness to washing and 1:2 metal-complexdyes also had the good color fastness to washing, but acid dyes had the poor fastness.In terms of the dyeing mechanisms of soybean fiber, the sorption ofdisulphonated acid dyes with higher molecular weight was described better by thedual sorption mechanism of Langmuir plus Nernst-type partitioning while the sorptionof monosulphonated acid dyes with lower molecular weight followed the Langmuirsorption mechanism well, suggesting that the positively charged amino groups insoybean protein played an important role in dye sorption and had a strong electrostaticinteraction with the sulphonate groups of dyes. The partitioning sorption ofdisulphonated acid dyes with higher molecular weight was believed to be mainly dueto the hydrophobic interaction operating between the hydrophobic groups of dyes andthe acetal groups in PVA component, and due to the hydrogen bond operating betweenthe dye and the fiber. 1:2 metal-complex dyes could be bound to the soybean proteinand PVA components through the stronger Van Der Waals force and hydrogen bondthan those of acid dyes. Reactive dyes could react not only with soybean proteincomponent but also with polyvinyl alcohol component. Because 1:2 metal-complexdyes and reactive dyes could be bound to both soybean protein and PVA component,their application to the dyeing of soybean fiber had the remarkable merits that bothsoybean protein and PVA component were colored, and that dyed textiles had highwet color fastness.The innovations of this research were represented as follows:(a) The chemical composition, morphological and super-molecular structurewere investigated in detail. It was found that the compatibility of soybean protein andPVA in soybean fiber was poor, and they exhibited the phase separation structure.Blocky and incontinuous-phase soybean protein dispersed in the PVA component of continuous phase. The possible chemical reactions of the acetalization agent with PVAand soybean protein were represented.(b) The resistance to alkali, wet and dry heat and the dyeability of soybean fiberwere investigated in detail. The relationships between fiber structure and applicationperformance were analyzed. The experimental results indicated that the crystallinestructure and regularity of soybean fiber was similar to those of vinylon (acetalizedPVA fiber). The structure and performance of soybean fiber were between proteinfiber and vinylon; its chemical and dyeing properties mainly depended on soybeanprotein; its heat stability and mechanical properties mainly depended on PVAcomponent; its morphological structure was influenced mainly by the super-molecularstructure of PVA macromolecule chains formed during the spinning, and alsoinfluenced by the content and distribution of soybean protein. These conclusionscould provide good explanation for the problems occurring in the dyeing and finishingprocesses.(c) The sorption, diffusion and fixation mechanisms of acid and reactive dyeingwere investigated. How to select the dyes suitable for the dyeing of soybean fiber wassuggested in terms of the diffusion and desorption of dyes, the dyeing mechanism, thebuilding up properties of dyes, and the color fastness. In addition, the bleaching effectof various bleaching agents and their influence on fiber structure and dyeingperformance were deeply studied. The high whiteness of soybean fiber was achievedusing some novel bleaching agents, which provided valuable reference for practicalapplication. The bleaching and dyeing technologies in this paper have been applied inproduction enterprises.