Enzymatic Removal Of Non-Cellulose Impurities On Cotton Fabric And Its Mechanism

Raw cotton fibre contains various non-cellulosic impurities including pectins,"cotton waxes", proteins, ashes, natural pigments and cottonseed coat, etc. The impurities affect colour, lustre and handle of the cotton fabric. They weaken the hygroscopicity and the penetrability of cotton fabric, causing nonuniform color and poor color fastness. This is why scouring and bleaching are necessary for the removal of the impurities, rendering qualified semifinished products for dyeing, printing and finishing. Enzyme catalysis has the characteristics of high efficiency, substrate specificity and mild conditions, which make enzyme scouring and bleaching environmentally friendly as they save energy and cause less pollution. This paper mainly studied strategies and the corresponding process of enzymic removal of non-cellulosic impurities in three aspects. Firstly, the removal of pectin and "cotton waxes":Cotton knitted fabric shows a hydrophobic character mainly due to the presence of "cotton waxes" and pectins on the outmost layer of the cotton fiber-the cuticle layer. So, Removing pectin and "cotton waxes" can greatly improve the wettability of the fabric. Since plasma techniques have excellent surface modification by the etching effect, deposition and the graft. We used plasma to etch the "cotton waxes" and modify the hydrophobicity on the surface of the fibre. Then we scoured the fabric with pectinase in order to remove "cotton waxes" and pectins more thoroughly. In this way, the wettability of the fabric could be improved significantly. Secondly, the removal of cotton pigments: Conventional hydrogen peroxide bleaching process (CHPBP) requires an alkaline medium, high temperature and long incubation periods, which consumes a lot of energy and deteriorates the environment. So, new green bleaching processes need to be developed to replace the CHPBP in the textile industry. Laccase-mediated systems (LMS) could activate the natural pigments in cotton fabric and reduce the activation energy between the pigment molecules and hydrogen peroxide. Therefore, hydrogen peroxide bleaching would carry out under the mild reaction conditions after LMS pretreatment. Thirdly, the removal of cottonseed coat:Instead of using common formulation technic of different enzyme, we studied the degradation of hemicellulose extracted from cottonseed coat with xylanase, the adsorption characteristics of xylanase on cottonseed coat and the degradation of cottonseed coat with xyianase. The effect of removing the cottonseed coat with single enzyme treatment could facilitate the research on how to formulate co-enzyme preparations and how to combine certain physical or chemical methods with enzyme to remove the cottonseed coat to the deepest extent.In this study, grey cotton fabric and dewaxed cotton fabric were treated by He/O2APPJ (we called them AP and AEP respectively). The variation of the surface contact angle of specimens A and AE were investigated at different APPJ treatment durations. The results obtained were as follows:with the increase of treatment duration, the contact angle of specimen AP and AEP both decreased. But after96s, specimen AP was still hydrophobic, while specimen AEP got wet instantly. XPS analysis showed that with specimen AP the O/C ratio and the content of C-O band on the fibre surface increased slightly, whereas the content of C-C band decreased gently as the treatment time increases. For the AEP sample, the O/C ratio and the content of C-O band rose significantly and C-C band reduced sharply. The FTIR-ATR result showed that specimen AP displayed the same spectra with the grey cotton fabric, whereas specimen AEP showed a strong absorbance at1640cm-1which means a relatively large amount of C=O band appear on the fibre surface. The XRD result indicated no significant changes in the crystallinity of the specimens, which means He/O2APPJ treatment does not hurt the bulk fibre. In summary, the results implied that He/O2APPJ treatment could etch and modify the surface of fibre to a certain extent, but improves its wettability little. This indicates that direct removal of hydrophobic impurities in the fibre with He/O2APPJ treatment is infeasible. On the contrary, combining dewaxing processes with He/O2APPJ treatment was found to tremendously improve the hydrophilicity of the grey cotton fabric, which could remove impurities more thoroughly.We evaluated the wetting effect of alkaline pectinase scouring of cotton knitted fabric combined with He/O2APPJ pretreatment. First, the wetting effect achieved by different scouring auxiliary (TF-125D) dosages in the APPJ/enzyme process was measured. We found that the wetting effect was good even without auxiliaries. Then each factor in the APPJ/enzyme treatment was investigated without adding auxiliary. The optimized APPJ/enzyme process was as follows:APPJ pretreatment conditions:He/O220/0.2L/min,40W, jet-to substrate distance2mm,60s; Pectinase treatment:300U/g (fabric), pH8.0,50℃,60min. Cotton samples treated this way achieved a wetting time of less than2s, retained tensile strength of more than90%and CIE whiteness of around63. The advantage of the APPJ/enzyme process is saving the auxiliary and enhancing the treatment effectiveness of enzyme alone.Assuming different possibilities of the epiphytic relationship between the pectin and the "cotton waxes", we built four models, namely layer style, layer-dispersion style, arborescence style and arborescence-dispersion style. SEM images and K/S value of oil red staining showed the "cotton waxes" were removed completely with the extraction of n-hexane. FTIR-ATR analysis and copper salt staining showed the pectic substances, which might comprise pectic acid, pectate and esterified pectin, existed beneath the "cotton waxes". FTIR-ATR ananlysis after hydrochloric acid vapor treatment and ammonium oxalate extraction showed the pectin content in the fibre stayed basically unchanged before and after dewaxing, which indicated the "cotton waxes" did not contain unbonded pectin. The results of the contact angle value and drop tests before and after dewaxing showed the pectin was distributed continuously. In summary, the layer-style model best explains the epiphytic relationship between the pectin and the "cotton waxes". Preliminary research on removing cottonseed coat with xylanase was carried out. Firstly, the hemicellulose extracted from cottonseed coat was degraded by xylanase pulpzyme HC. We got the optimum condition for enzyme reaction by optimizing the factors:4.0mg hemicellulose,0.24mL Pulpzyme HC (original enzyme),60℃, pH5.5,70min. Hemicellulose degradation rate reached57.5%in this condition. The mechanisms of the cottonseed coat adsorbing xylanase were described by the adoption of adsorption isotherm model, adsorption kinetics model, and adsorption thermodynamic model. The linear fit of Freundlich model is relatively appropriate indicating that the adsorption type is a3-dimensional structure of multi-molecular layer adsorption model. We also found that D-R equation could explain the adsorption process. The E value of3.24～3.66kJ mol-1, which was lower than8～16kJ mol-1, indicated that the adsorption is physical. On adsorption kinetics, it was found that the pseudo-second-order kinetic model explained the adsorption behaviour better. In this model,k2and h value decreased with the increase of temperature, which indicated that low temperature was helpful for adsorbing xylanase. On absorption thermodynamics, the fact that△H0value was below zero indicated that the adsorption process was an exothermic reaction. Reducing temperature was in favour of adsorption. The fact that△G0value was less than zero proved the adsorption behaviour was spontaneous. The result of xylanase degrading cottonseed coat showed that the degradation rate was only8.7%, which indicated that the effect of xylanase alone hydrolyzing the cottonseed coat was poor. This was maybe related to the complex structure of cottonseed coat and the hydrophobic component on its surface.We evaluated the bleaching efficiency of the hydrogen peroxide bleaching process combined with laccase-mediated system pretreatment (LMS-HPBP) in the treatment of scoured cotton fabric. By changing the factors of LMS pretreatment and the HPBP, whiteness value and retained tensile strength of the samples were examined. Three LMS-HPBPs are found to be more environment friendly than the conventional hydrogen peroxide bleaching process (CHPBP):①Bleaching with lower dosage of hydrogen peroxide;②Bleaching at reduced temperature;③Bleaching for shortened duration. Two kinds of reactive dyes was used to examine the dyeing effect of the LMS-HPBPs, and the results showed that K/S values of cotton fabric samples treated by①-③processes were close to or higher than those by CHPBP. The LMS pretreatment was optimized by response surface methodology in order to achieve the best response value (whiteness index). The optimized process, which gave a WI value of78.74, was:①LMS pretreatment:Denilite ⅡS/HBT dosage,2.57%(o.w.f),58.41℃; pH,5.07;30min;②Bleaching:H2O2(30wt.%),4%(o.w.f); Na2CO3,2%(o.w.f); TF-122B,1g/L; Temp.,70℃; LR,1:40; time,60min.Preliminary study of LMS-HPBP mechanism was carried out. Rutin and quercetin were taken as the "model pigments" of cotton fibre and made to react with LMS. LC-MS,1H NMR, UV-VIS and FTIR-ATR showed rutin oxidation product was a mixture of dimer and trimer, which possibly contained o-quinone structure in a B-ring of one monomer. Monomer molecules were possibly linked by C-O band. In addition, quercetin oxidation product only contained dimer and did not have quinone structure. The conjugate structure of C-ring in one quercetin molecule was probably opened during the LMS oxidation process. Quercetin dimer was formed through a dioxane linkage. ESR results showed there was no spin resonance signal in both rutin and quercetin, while radical singlet signal (g=1.984) was detected in both oligorutin and oligoquercetin. This confirmed that enzymatic oxidation process was an oxidation polymerization reaction with free radicals as intermediates. Depending on all above studies and experiments, a proposed mechanism was put forward:Cotton pigments are oxidized to generate free radicals intermediates by LMS oxidation. The radicals intermediates can reduce the activation energy between the "model pigments" molecules and hydrogen peroxide, which can improve the reaction efficiency of natural pigments in cotton fibre and hydrogen peroxide.