Rice wine lees glutelin utilization as a source of bio-based sizing agents through graft polymerization

Abstract

Rice wine lees (RWL) are available in large quantities at low price and nearly 20% of them are glutelin. However, poor water solubility of native RWL glutelin under neutral condition severely inhibits its use as a sizing agent in the textile industry. In order to solve the difficult problem, various amounts of hydrophilic vinyl monomers–acrylic acid (AA) were grafted onto molecular chains of native RWL glutelin in the study. Then, effects of graft modification on application properties of RWL glutelin were studied in terms of water solubility, apparent viscosity, wettability, mechanical properties of sizing films, adhesion to common textile fibers, mechanical properties of sized warp yarns and biodegradability. It was found that, the RWL glutelin-g-PAA with a grafting ratio of 74.1%, exhibited better comprehensive properties. The RWL glutelin-g-PAA thus produced possessed performance parameters of 32 min in solubility period, 10.12 N/mm², 6.37% and 0.61 mg/cm² in tensile strength, elongation and wear loss of sizing film, 125.3 N/1000 tex and 106.5 N/1000 tex in adhesion strength to cotton and polyester fibers, 3.49 N, 6.55% and 111 cycles in tensile strength, elongation and abrasion resistance of sized polyester/cotton blended warp yarn and 0.311 in the ratio of BOD₅/COD_cr, respectively. The graft polymerization has enabled RWL glutelin to serve for commonly used yarns as a new biodegradable and bio-based sizing agent.

Keywords

acrylic acid graft polymerization RWL glutelin sizing agent water solubility

Introduction

Warp sizing is one of the most important pre-weaving procedures and largely determines weaving efficiency in textile mills. As the major materials for warp sizing, nearly 1 million tons of sizing agents are consumed by textile industry annually in the world.¹ The first and second most consumed sizing agents are starch and polyvinyl alcohol (PVA), which account for about 70% and 20% of the total consumption, respectively. Starch is biodegradable and harmless to the human body but its price is increasing in recent years due to multiple natural and human factors, such as locust plague in Ukraine and the Russia-Ukraine War. As a result, warp sizing using no food has become an attractive topic in the textile industry.^2–4 PVA possesses excellent comprehensive sizing performance, such as high adhesion to common textile fibers and excellent mechanical properties of sizing film. However, its poor biodegradability leads to the restricting or even the banning of PVA used as sizing agents in many European and American countries.^5–7 In order to solve main problems of the common sizing agents, for example, insufficient source, high price and environmental pollution, it is necessary to develop new sizing agent varieties from bio-based polymers, especially industrial and agricultural by-products.

Rice wine is a traditional alcoholic beverage with a history spanning over 2 millennia in Asia. It is distinguished by its unique brewing process, cultural significance and versatile applications in both culinary and social contexts. Therefore, rice wine is quite popular in China, Japan, Thailand, India, etc.^8–11 The lees are the main by-products of the rice wine brewing industry. It is approximately estimated that 480,000 tons of rice wine lees (dry weight) are discharged annually only in China.¹² Unfortunately, the utilization of such a large amount of rice wine lees (RWL) is still limited. Part of the RWL are used as animal feed. However, the residual toxic substances in the lees, such as ethanol, methanol, aldehydes and ketones, can easily cause acute or chronic poisoning in animals.^13–15 Besides, due to the lack of essential amino acids, carotenoids, vitamin D, calcium, etc., it can lead to imbalanced nutrient intake of animals. It should be noted that, nearly 20% of the RWL is glutelin, which is difficult for animals to digest and absorb due to its complex macromolecular structure and many concomitant antinutritional factors (e.g. cellulose, tannin and silicon).¹⁶ Therefore, more RWL are directly disposed in landfills or incinerated after drying as solid wastes.^17,18 Currently, most RWL glutelin has been lost in vain. Sizing operation in textile mills is based on an aqueous paste. Hence, it demands sizing agents to dissolve in water well.^19–21 Some investigators found that alkaline liquor could dissolve glutelin to prepare homogeneous aqueous solution.^22,23 For this reason, high concentration of alkaline solution (pH 11.0) is required in order to dissolve glutelin completely. However, high alkali concentration will hydrolyze the glutelin, break it into small-molecular peptides and decrease the cohesive force of the glutelin to prepare glutelin aqueous solution. Moreover, high pH value in sizing paste will seriously damage mechanical properties and thermal stability of alkali-sensitive textile fibers, such as wool, silk and polyester fibers.

Some investigations show that grafting monosaccharide, disaccharid or polysaccharide onto molecular chains of different plant proteins (e.g. zein and globulin) is a simple and effective method to impart various new properties to the protein substrates, such as water solubility and emulsification.^24,25 Based on previous research results, the investigation focuses on the utilization of RWL glutelin as a new sizing agent for the first time. In order to make RWL glutelin water-soluble under neutral condition, the investigation extracted glutelin from rice wine lees and grafted different amounts of hydrophilic vinyl monomers–acrylic acid (AA) onto the molecular chains of the RWL glutelin through K₂S₂O₈/NaHSO₃ redox system. After evaluating main application properties of the series of RWL glutelin-g-PAA with different grafting ratios, that is, water solubility, apparent viscosity, contact angles on common textile fibers, mechanical properties of sizing film, adhesion to common textile fibers and mechanical properties of sized warp yarns, effects of graft modification of AA on the sizing properties of the grafted RWL glutelin are elucidated. Finally, the appropriate grafting ratio is recommended for the preparation of grafted RWL glutelin sizing agent.

Although there have been some literatures on the grafting of PAA onto the molecular chains of bio-polymers for the preparation of textile sizes and water-absorbent resins, these bio-polymers are mainly starch, keratin and gelatin.^26–28 To our knowledge, there has been no research on the preparation of glutelin-g-PAA, including RWL glutelin-g-PAA. The molecular structure of glutelin is not the same as the aforementioned bio-polymers. For example, although native glutelin and keratin are both water-insoluble proteins, the number of pendant functional groups (e.g. -NH₂, -COOH and -SH), crosslinking density and spatial configuration of the two proteins are all different. To prepare the glutelin and keratin with good water solubility by grafting PAA, the required graft polymerization conditions, such as AA monomer concentration, initiator concentration, polymerization temperature and time, are all different. Therefore, the existing preparation conditions for water-soluble keratin-g-PAA cannot be applied to the preparation of congeneric glutelin products. Actually, a large number of exploratory experiments have been carried out for the development of the glutelin-g-PAA with good water solubility and sizing properties by our research group. Therefore, the study has significant technological innovation.

Experimental

Materials

Rice wine lees were kindly supplied by Zhejiang Guyue Longshan Rice Wine Co. Ltd (Zhejiang, China). The lees were dried thoroughly and ground in a Wiley mill.²⁹ AA, K₂S₂O₈, NaHSO₃ and paradioxybenzene, which were provided by Sinopharm Chemical Reagent Co. Ltd (Shanghai, China), were used as hydrophilic vinyl monomers, oxidant, reductant and terminator in the graft polymerization, respectively. Sodium hydroxide, sodium carbonate, hydrochloric acid, iodine, potassium iodide, potassium bromide, potassium bromate and sodium thiosulfate were purchased from Aladdin Reagent Co. Ltd (Shanghai, China). All the reagents used were of analytical grade. Cotton rovings (479 tex) and polyester rovings (483 tex) used for evaluating the adhesion were obtained from Qingfeng Textile Co. Ltd and Yizheng Co. Ltd of Chemical Fiber (Jiangsu, China), respectively. The T/C65/35 warp yarns (15.5 tex) used for sizing experiment were obtained from Xiangshui Baoji Textile Co. Ltd (Jiangsu, China). Two kinds of commonly used sizing agents – starch acetate (degree of substitution: 0.02) and PVA (degree of polymerization: 1700; degree of alcoholysis: 88%) were purchased from Nanjing Songguan Biotechnology Co. Ltd (Jiangsu, China) and Langfang Feitai New Material Technology Co. Ltd (Hebei, China), respectively.

Extraction of glutelin from RWL

Fifty grams of RWL powders were dispersed in 450 mL of distilled water and then transferred into a 1000 mL three-neck flask. The flask was maintained at 70°C in a water bath and the dispersion was stirred vigorously using a mechanical stirrer at 1000 rpm. Then, 50 mL of NaOH aqueous solution (0.08 g/mL) were added into the flask using a dropping funnel. After the addition, the alkali dissolution of the RWL glutelin was carried out under vigorous stirring at 70°C for 2 h. The dispersion in the flask was decreased to room temperature (RT) and then centrifuged at a ratio of 10,000 r/min at 4°C three times. The supernatant of the centrifuged product was collected and adjusted to the isoelectric point (pH 4.6–4.8) to precipitate the glutelin with diluted hydrochloric acid (2 mol/L). The acid-precipitated glutelin was dispersed in distilled water and transferred into a dialysis bag (8 KD). The unreacted reagent and salt were removed from the glutelin via dialysis in distilled water for 24 h. After the dialysis, the glutelin was freeze dried, pulverized and stored in desiccator.

Reaction mechanism of graft polymerization of RWL glutelin with AA

The graft polymerization of RWL glutelin with AA includes three steps, that is, chain initiation, chain propagation and chain termination, which are depicted in Figures 1 to 3, respectively. Figure 1 shows that free radicals are generated from redox reaction of $S_{2} O_{8}^{2 -}$ and ${HSO}_{3}^{-}$ . There are two main pendant functional groups (i.e. -NH₂ and -COOH) along the molecular chains of RWL glutelin. Under appropriate initiation conditions, the active sites can form on the functional groups and AA monomers can be grafted onto the sites. Figure 2 describes the propagation of PAA branches during the graft polymerization. In general, chain termination includes coupling and disproportionation ones. In order to describe chain termination concisely, disproportionation is taken as example to depict the chain termination in Figure 3.

Figure 1.

Chain initiation in the graft polymerization of RWL glutelin with AA through NaHSO₃/K₂S₂O₈ redox system.

Figure 2.

Chain propagation in the graft polymerization of RWL glutelin with AA.

Figure 3.

Chain termination in the graft polymerization of RWL glutelin with AA.

Preparation of RWL glutelin-g-PAA

Before grafting, 5 g of RWL glutelin powders were dispersed in distilled water. Then, the dispersion was transferred into a 100 mL four-neck flask. The flask was maintained at 50°C in a water bath. The mixture was deoxygenated by passing N₂ for at least 30 min. The oxidant (K₂S₂O₈), the reductant (NaHSO₃) and AA monomers were dissolved in distilled water, respectively. In the study, monomer concentration, that is, the feed weight ratio of AA to RWL glutelin was controlled at 40%, 60%, 80%, 100% and 120%, respectively. The five grafted RWL glutelin samples prepared were numbered as RWL glutelin-g-PAA 1#, 2#, 3#, 4# and 5#, respectively, according to the monomer concentration order from low to high. The oxidant, reductant and AA solutions were added into the flask through 3 dropping funnels simultaneously. The addition was completed in 10–20 min and final bath ratio of the RWL glutelin to distilled water was 1:10. The concentration of K₂S₂O₈ was 0.278 mol/L and the molar ratio of K₂S₂O₈/NaHSO₃ was 1:1.5. The graft polymerization was carried out under vigorous stirring using a mechanical stirrer at 1000 rpm under N₂ atmosphere for 8 h. Finally, 2% of paradioxybenzene solution was added to terminate the polymerization. The product was filtered and washed thoroughly with distilled water. The filtrate was saved to determine the amount of residual monomer. At last, the product was freeze dried, pulverized and stored in desiccator. Each graft polymerization was repeated three times.

Measurement of grafting parameters

Grafting parameters of the RWL glutelin-g-PAA, that is, conversion of monomers to polymer (MC) and grafting ratio (GR), were measured in the study. Conversion of monomers to polymer is the weight percentage of the polymers formed (i.e. grafted branches and homopolymers) to the total monomers added. Grafting ratio is the weight percentage of PAA branches grafted onto the RWL glutelin to the RWL glutelin. The detailed measurement processes were described in Supporting Information (SI).

FTIR characterization

Fourier transform infrared spectrometer (FTIR) was used to verify the grafting of AA onto the RWL glutelin. The homopolymers (i.e. PAA) which adhered on the grafted RWL glutelin were removed completely in Soxhlet according to the above method. The measurement was taken on Nicolet Nexus spectrophotometer (Thermo Nicolet Corporation, USA) through the diffuse reflectance technique with a spectral resolution of 2 cm⁻¹ for 64 scans.

¹H-NMR characterization

Proton nuclear magnetic resonance (¹H-NMR) was used to confirm the grafting of AA onto the RWL glutelin. The native RWL glutelin and the purified RWL glutelin-g-PAA were dissolved in deuterated DMSO and characterized by an AVAMCE III 400MHz Digital NMR spectrometer (Bruker Co. Ltd. Switzerland). The concentration of RWL glutelin solution was about 5% (w/w).

EA characterization

Carbon, hydrogen, nitrogen and sulfur contents of native RWL glutelin and RWL glutelin-g-PAA were measured to confirm the GR of the grafted glutelin using an elemental analyzer (Euro EA3000, Italy). High temperature combustion was used as the way to remove elements from the RWL glutelin sample. The sample was combusted in the heated and oxygen-enriched environment. Standard sulphanilamide was employed for the carbon, hydrogen, nitrogen and sulfur content calibrations. The calibration was performed once after every eight measurements.

TGA characterization

Thermal behaviors of the native RWL glutelin and the purified RWL glutelin-g-PAA were compared using thermogravimetric analysis (TGA). TG and DTG curves of the native and the grafted RWL glutelin samples were obtained using TG/DTA6300 integrated thermogravimetric analyzer (Seiko Co. Ltd, Japan). About 5 mg of the sample was heated at 10°C/min in a range of 30–650°C under N₂ atmosphere.

Measurement of water solubility

The RWL glutelin-g-PAA sample was suspended into distilled water to form 6% (w/w) dispersion. The dispersion was adjusted to pH 7.0 using Na₂CO₃ solution and stirred. Then the dispersion was heated to 60°C and maintained at the temperature under stirring to make the sample dissolved. Solubility period of each sample was recorded. After that, the solution was cooled to RT without stirring and kept under the condition for a week for the observation of solubility stability. During the week, the precipitation was filtered from the solution every day, freeze dried and weighed using precision electronic balance. The appearance time and the amount of the precipitation in the solution were recorded to evaluate the solubility stability. Precipitation ratio is the weight percentage of the precipitate appearing every day to the grafted RWL glutelin sample.

Measurement of apparent viscosity and viscosity stability

Apparent viscosity and viscosity stability of the RWL glutelin-g-PAA were measured using an NDJ-79 rotary viscometer (Electrical Machinery Plant of Tongji University, China). The sample was suspended into distilled water to form 6% (w/w) dispersion and then was adjusted to pH 7.0. The dispersion was heated to 60°C and maintained at the temperature under mechanical stirring for 1 h. At this time, the apparent viscosity of the grafted RWL glutelin paste was measured with a shear rate of 3500 s⁻¹. Viscosity stability of the paste denoted the percentage of the viscosity invariability over a period of 3 h at 60°C. The viscosity of the grafted RWL glutelin paste was measured every 30 min and the stability was calculated from equation (1).

ViscosityStability (%) = (1 - \frac{V_{\max} - V_{\min}}{V}) \times 100

(1)

Where, V was the apparent viscosity of the paste, while V_max and V_min were the maximum and minimum of the viscosity over the period, respectively.

As for native RWL glutelin, it only can dissolve in water well under alkaline condition. Therefore, the native RWL glutelin was suspended into dilute NaOH solution, of which pH was ∼11.0, to form 6% (w/w) dispersion. The following steps were the same with the ones in measuring the viscosity and its stability of the grafted RWL glutelin.

Measurement of contact angle

Cotton and polyester are the most widely used natural fiber and chemical one in the textile industry, respectively. Therefore, the study employed the two kinds of fibers as the experimental objects. Contact angles of the native and grafted RWL glutelin solutions on the fibers were measured using a JC2000D2 contact angle meter (Zhongchen Digital Technology Co. Ltd, China). One percent (w/w) of the RWL glutelin dispersion was cooked to solution in a flask at 60°C with mechanical stirring. Then, 0.1 mL of the RWL glutelin solution was extracted from the flask using a pipette and dripped onto the surfaces of a tensioned cotton woven plain fabric (28 tex × 28 tex, 260/10 cm × 260/10 cm) and a tensioned polyester woven plain fabric (10 tex × 10 tex, 370/10 cm × 390/10 cm) without any dyeing or finishing treatment, respectively. Three samples were measured to obtain the mean value.

Preparation of sizing film and measurement of mechanical properties of the film

RWL glutelin sizing film was prepared by casting method. Main mechanical properties, such as tensile strength, elongation and abrasion resistance, were evaluated in the study. The preparation method of sizing film and the detailed measurement process of mechanical properties of the film were described in SI.

SEM observation of sizing film

Surface morphology of the RWL glutelin sizing film was observed using an SU3800 SEM (Hitachi Co. Ltd, Japan). The sizing film was mounted on a conductive adhesive tape and sputter coated with gold palladium prior to observation. A 10 kV voltage was used for all the observations.

Measurement of adhesion to fibers

A sized roving method was used to measure adhesion of the native and grafted RWL glutelin sizing agents to fibers. Nowadays, the roving method is a standard to estimate adhesion of a sizing agent to fibers in China (FZ/T 15001-2008, a criterion regulated by Textile Association of China). In the measurement, a sized roving is drawn to tensile failure. It is the failure loads that were adopted to exhibit the adhesion.

One percent (w/w) of the RWL glutelin dispersion was cooked to sizing paste in a flask at 60°C with mechanical stirring for 1 h. The RWL glutelin paste obtained was decanted into a stainless steel box placed in water bath at 60°C. The rovings carefully wound onto a special frame were impregnated with the RWL glutelin paste for 5 min, dried at RT and kept at 65% R.H. and 20°C for 24 h before the test. Tensile strength and work-to-break of the roving samples were measured as adhesion strength and work. Twenty samples were measured for obtaining the mean value of the data required.

Sizing experiment and measurement of mechanical properties of sized yarn

Cooking process of the RWL glutelin sizing paste was the same with the one described in the measurement on apparent viscosity except that the concentration of the sizing paste was 12% (w/w). In order to compare the grafted RWL glutelin with the sizing agents currently used in the textile industry, starch acetate and PVA were used to size warp yarns in the study, respectively. Cooking processes of the sizing pastes of starch acetate and PVA were the same with that of the RWL glutelin except that the cooking temperature of the sizing paste was 95°C. The sizing object was polyester/cotton blended warp yarn (T/C65/35). The warp sizing process and the detailed measurement process of mechanical properties of the sized yarn were described in SI.

Measurement of COD and BOD

Biodegradable properties of the RWL glutelin-g-PAA were tested through biological oxygen demand for 5 days (BOD₅) and chemical oxygen demand (COD). BOD₅ is the difference value of dissolved oxygen before and after 5 days of biological culture. COD_cr is a mass concentration of oxygen after the oxidization of potassium dichromate under certain condition. BOD₅ and COD_cr were both determined according to the standard analytical method provided by the China State Environmental Protection Administration,³⁰ which was similar to the international standard method of analysis of water and waste water.³¹

Statistical analysis

The data were analyzed using SAS software (SAS Institute, Inc., Cary, NC). The confidence interval was set at 95% and a “p” value smaller than 0.05 was considered to be a statistically significant difference by Tukey’s HSD test. According to the operation results of the analysis software, small letters, such as a, b and c, were inserted above or below the data points in related figures. The data points with the same small letter were not statistically significantly different from each other.

Results and discussion

FTIR analysis of native RWL glutelin and RWL glutelin-g-PAA

FTIR spectra of native RWL glutelin and RWL glutelin-g-PAA are shown in Figure 4. All the peaks in the spectrum of native RWL glutelin, such as the two peaks at about 1660 and 1550 cm⁻¹ due to characteristic absorption bands of the amide I and II bands,^32,33 could be observed from the spectrum of each RWL glutelin-g-PAA sample. FTIR spectrum of the RWL glutelin-g-PAA showed that the intensity of the characteristic absorption band of carbonyl group^34,35 of carboxyl at 1732 cm⁻¹ was much higher than that of the native RWL glutelin. The markedly increased intensity of the peak at 1732 cm⁻¹ confirmed the grafting of AA onto the RWL glutelin. In addition, the intensity of the peak at 1732 cm⁻¹ increased gradually with the increase in the feed weight ratio of AA to RWL glutelin. It proved the increase in the GR of RWL glutelin-g-PAA 1#∼5#.

Figure 4.

FTIR spectra of native RWL glutelin (0#) and RWL glutelin-g-PAA 1#-5#.

¹H-NMR analysis of native RWL glutelin and RWL glutelin-g-PAA

¹H-NMR spectra of native RWL glutelin and RWL glutelin-g-PAA are depicted in Figure 5. Chemical shift peaks at 2.5 and 3.3 ppm correspond to proton peaks of the solvent (deuterated DMSO) and residual water. Besides chemical shift peaks of native RWL glutelin, such as the ones in the range of 0.9–2.3 ppm and 6.5–8.5 ppm corresponding to alkyl and peptide,^36–38 a chemical shift peak at 12.1 ppm was obviously strengthened in the spectrum of RWL glutelin-g-PAA. In addition, the proton peak intensity gradually increased with the increase in the feed weight ratio of AA to RWL glutelin. The strengthened peak was the proton peak of carboxyl group.^39–41 It could be regarded as another evidence of successful grafting of AA.

Figure 5.

¹H-NMR spectra of native RWL glutelin (a) and RWL glutelin-g-PAA 1#-5# (b–f).

Thermogravimetric analysis of native RWL glutelin and RWL glutelin-g-PAA

The grafting of PAA branches can increase the degree of branching of the RWL glutelin and disrupt the regular molecular structure of the glutelin. In order to verify whether there was a significant decrease in the thermal stability of the glutelin after grafting PAA branches, thermogravimetric analysis of native RWL glutelin and RWL glutelin-g-PAA was conducted. Figure 6 describes thermal degradation behaviors of native RWL glutelin and RWL glutelin-g-PAA. As shown in the thermogram of native RWL glutelin (Figure 6(a)), the degradation process could be divided into three main stages: (1) dehydration of RWL glutelin (<125°C), (2) thermal degradation of RWL glutelin (125–500°C) and (3) carbonization and carbon oxidation (>500°C). In Stage 1, both free water and bound water of RWL glutelin were removed. The water loss rate reached the maximum at ∼60°C. In Stage 2, further degradation above 125°C mainly involves the cleavages of covalent peptide bonds in amino acid residues and the breakages of S-S, O-N and O-O bonds in glutelin molecules. The weight loss rate reached the maximum at 315°C. In Stage 3, RWL glutelin started to be carbonized from ∼500°C and some carbon was oxidized.

Figure 6.

TGA thermograms of native RWL glutelin (a) and RWL glutelin-g-PAA 1#-5# (b–f).

Compared with the thermogram of native RWL glutelin, thermal degradation trends of all the RWL glutelin-g-PAA were quite similar in Stage 1. However, thermal degradation trends of native RWL glutelin and RWL glutelin-g-PAA in Stages 2 and 3 were different. As shown in the DTG curves, the temperature at which the maximum rate of thermal weight loss and carbonization initiation temperature of each grafted RWL glutelin were both higher than those of the native one. With the increase in the GR of the grafted RWL glutelin, the temperature at which the maximum rate of thermal weight loss increased from 325°C (RWL glutelin-g-PAA 1#) to 385°C (RWL glutelin-g-PAA 5#). Simultaneously, the carbonization initiation temperature increased from 510°C (RWL glutelin-g-PAA 1#) to 535°C (RWL glutelin-g-PAA 5#). Thermal decomposition of the grafted RWL glutelin at higher temperature in the two stages can be attributed to the better thermal stability of carbon-carbon single bond of grafted branches (i.e. PAA) than that of peptide bond of the glutelin.^27,42 Therefore, the grafting of PAA branches can improve the thermal stability of the glutelin remarkably.

Effect of monomer concentration on grafting parameters

Table 1 shows the impacts of monomer concentration on MC and GR during graft polymerization of the RWL glutelin grafted with AA monomers. With the increase in the monomer concentration, the MC initially increased and then leveled off around 97% while the GR increased stepwise.

Table 1.

MC and GR of the RWL glutelin-g-PAA synthesized with different concentrations of AA monomers.

RWL glutelin-g-PAA No.	MC (%)	CV (%)	GR (%)	CV (%)
1#	88.7	1.41	30.9	5.95
2#	91.6	1.17	44.2	3.73
3#	92.0	1.22	60.6	2.97
4#	96.4	1.05	74.1	2.79
5#	97.0	1.21	83.3	2.82

Note. CV denotes coefficient of variation and the same as the following tables.

The majority of free radical polymerization reactions are reversible. The initial increase in the MC is mainly due to the invariability of equilibrium constant of the free radical polymerization. In general, higher monomer feed concentration helps to make reversible polymerization reactions including both graft polymerization and homopolymerization move toward positive direction. Meanwhile, the increase in the AA monomer concentration could increase the concentration of PAA, which included grafted branches and homopolymers. The increasing concentration of PAA led to higher viscosity of reaction system. The increased viscosity restrained chain termination, especially the coupling termination of growing PAA chains.⁴³ The restraint on chain termination is another reason for the initial increase in the MC. It is impossible to make all the monomers converted to the polymers even if choosing appropriate polymerization conditions, such as proper initiator concentration, molar ratio of oxidant/reductant, polymerization temperature and time. However, when the monomer concentration reached 100%, ∼97% of the monomers could be converted to the polymers. It indicates that polymerization conditions chosen in this study are appropriate and the conditions can make the MC stay at a high level.

GR denotes the weight percentage of PAA branches grafted onto RWL glutelin to RWL glutelin. The higher the concentration of AA, the larger was the amount of PAA branches formed. Since the amount of the RWL glutelin substrate used was constant during graft polymerization, the GR was able to increase from 30.9% to 83.3% stepwise when the monomer concentration ranged from 40% to 120%.

Elemental analysis of native RWL glutelin and RWL glutelin-g-PAA

Actual elemental contents of carbon, hydrogen, nitrogen and sulfur of native RWL glutelin and RWL glutelin-g-PAA are tested using EA and displayed in Table 2. In accordance with the GR obtained in above measurement of grafting parameters and the contents of the four elements of native RWL glutelin measured by EA, the elemental contents of the grafted RWL glutelin in theory are calculated and also displayed in Table 2. The contents of the four elements of the grafted RWL glutelin were all lower than those of the native one because there were no nitrogen & sulfur and less hydrogen (only 5.56 wt%) in PAA branches. Moreover, the elemental contents of the grafted RWL glutelin measured by EA were in good agreement with those calculated in theory. The agreement of the elemental contents proves that the GR tested in the study are close to true values.

Table 2.

Elemental contents of native and grafted RWL glutelin.

	C (wt%)		H (wt%)		N (wt%)		S (wt%)
Type of RWL glutelin	By EA	In theory	By EA	In theory	By EA	In theory	By EA	In theory
Native RWL glutelin	51.65	–	6.69	–	16.99	–	0.78	–
RWL glutelin-g-PAA 1#	51.38	51.26	6.45	6.36	13.39	12.98	0.63	0.60
RWL glutelin-g-PAA 2#	51.10	51.14	6.31	6.35	11.32	11.78	0.52	0.54
RWL glutelin-g-PAA 3#	51.07	51.03	6.29	6.26	10.84	10.58	0.51	0.49
RWL glutelin-g-PAA 4#	50.99	50.95	6.23	6.21	9.99	9.76	0.47	0.45
RWL glutelin-g-PAA 5#	50.85	50.90	6.12	6.18	9.05	9.27	0.40	0.43

Effect of graft modification on water solubility

Table 3 and Figure 7 show water solubility and solubility stability of native RWL glutelin and RWL glutelin-g-PAA under neutral condition, respectively. The native RWL glutelin could hardly dissolve in water. In contrast, the RWL glutelin-g-PAA samples with various GR were all able to dissolve in water completely at different times. With the increase in the GR, water solubility and its stability of the grafted glutelin initially increased, reached the maximum when the GR was 74.1% and then decreased. It could be observed from Figure 7 that the precipitation ratios of each grafted RWL glutelin solution from Day 1 to Day 4 were statistically significantly different. As for the solutions of RWL glutelin-g-PAA 3# and 4#, no more precipitations appeared from Day 4.

Table 3.

Water solubility, apparent viscosity and its stability of native and grafted RWL glutelin.

Type of RWL glutelin	Water solubility	Solubility period (min)	Apparent viscosity (mPa·s)	Viscosity stability (%)
Native RWL glutelin	-	-	2.20	90.91
RWL glutelin-g-PAA 1#	+	65	3.00	93.33
RWL glutelin-g-PAA 2#	++	56	3.30	95.45
RWL glutelin-g-PAA 3#	+++	45	3.65	95.89
RWL glutelin-g-PAA 4#	++++	32	4.10	97.56
RWL glutelin-g-PAA 5#	++	58	4.20	98.81

Note.“-” denotes water insolubility; the more the “+,” the better is the water solubility.

Figure 7.

Precipitation ratios of RWL glutelin-g-PAA 1#-5# in a week.

It is complex molecular structure that results in the poor water solubility of native RWL glutelin. Native RWL glutelin molecules contain a large number of hydrophobic bonds, which lead to the aggregation between hydrophobic groups or side chains. Therefore, native RWL glutelin tend to form insoluble aggregates in water. In addition, the glutelin molecules contain abundant hydrophobic amino acids (e.g. phenylalanine, valine and leucine), which have strong hydrophobicity, further reducing the solubility of the glutelin in water.

There are four major reasons for the marked improvement of water solubility of the RWL glutelin after grafting appropriate amounts of PAA branches. (1) High hydrophilicity of PAA: PAA branches contain numerous carboxyl groups that ionize to -COO⁻ in aqueous environments (especially at neutral or slightly basic pH). These charged groups strongly interact with water molecules via electrostatic attraction, creating a highly hydrated shell around the RWL glutelin-g-PAA. (2) Increased surface charge density: The introduction of PAA branches adds extra negative charges along the RWL glutelin backbone. These increased net charges enhance electrostatic repulsion between RWL glutelin macromolecules, preventing aggregation, precipitation or hydrophobic interactions that typically reduce solubility. (3) Steric hindrance and shielding: Long and flexible PAA branches extend into the aqueous medium, creating a thick polymeric brush layer around the protein’s surface. The steric stabilization prevents glutelin-glutelin contacts (e.g. hydrophobic patches from interacting), reducing the tendency for aggregation. (4) Disruption of internal hydrophobic interactions: PAA grafting can partially unfold or modify the protein’s tertiary structure in a controlled way. By attaching hydrophilic chains, buried hydrophobic regions may become exposed but immediately shielded by the hydrophilic PAA branches, effectively converting originally hydrophobic surfaces into hydrophilic domains. Therefore, when the GR of RWL glutelin-g-PAA was not higher than 74.1%, the water solubility of the grafted glutelin kept to increase with the increase in the GR.

It should be noted that the oxygen atom in carboxyl group has strong electronegativity, so it can form hydrogen bonds with the hydrogen atom in another carboxyl group that is close in distance. As a result, when the GR was excessively high, physical crosslinking caused by the numerous hydrogen bonds formed between the carboxyl groups of PAA grafted branches might occur. The macromolecular chains were connected through branched chains to form a three-dimensional network structure. It was difficult for water molecules to pass through the network, so an excessively high GR (>74.1%) actually reduced water solubility of the grafted RWL glutelin.²¹ Consequently, the water solubility and its stability of RWL glutelin-g-PAA 5# were both lower than those of RWL glutelin-g-PAA 4#.

Effects of graft modification on apparent viscosity and viscosity stability

Effects of the graft modification on apparent viscosity and its stability of native RWL glutelin and RWL glutelin-g-PAA sizing pastes are also shown in Table 3. The apparent viscosity and viscosity stability of all the RWL glutelin-g-PAA sizing pastes were higher than those of the native RWL glutelin one. As for the grafted glutelin sizing pastes, with the increase in the GR, the apparent viscosity and viscosity stability both kept to increase.

Though native RWL glutelin can dissolve in alkali solution well, the dissolution in alkali solution is quite different from the dissolution of the grafted glutelin under neutral condition. The dissolution in alkali solution is unavoidably accompanied with the hydrolysis of the glutelin. A large number of the macromolecular chains of the glutelin will be cut into small-molecular peptide chains by alkali. In other words, molecular weight of the glutelin will be reduced. In contrast, the macromolecular chains of the grafted glutelin will not be cut short during the dissolution in water under neutral condition. As far as the same kind of sizing agent is concerned, the smaller the molecular weight, the lower is the apparent viscosity.⁴⁴ As a result, the apparent viscosity of the native RWL glutelin sizing paste was lower than that of the grafted RWL glutelin ones. The measurement on viscosity stability required two more hours to obtain viscosity values at different time points. As the testing time passed, the alkaline hydrolysis of the glutelin still proceeded. The decrease in the molecular weight of the native RWL glutelin would necessarily cause the further variation of the apparent viscosity. Consequently, the viscosity stability of the native RWL glutelin sizing paste was lower than that of the grafted glutelin ones.

There are two major reasons for the increases in the apparent viscosity and viscosity stability of the grafted RWL glutelin sizing paste with the increase in the GR. (1) Increased hydrodynamic volume: The RWL glutelin can be considered as a small and dense core. Each PAA branch is a flexible and hydrophilic polymer chain. When PAA branches are grafted onto the glutelin, they extend out into the solvent, surrounding the glutelin with a large and diffuse “cloud” of grafted branches. The glutelin without PAA branches has a small effective radius while the the modified RWL glutelin grafted with many PAA branches behaves like a much larger single particle. According to the Einstein equation for dilute spheres, apparent viscosity (η) is related to the volume fraction (V_f) of the particles as shown in equation (2):

η \approx η_{0} (1 + 2.5 V_{f})

(2)

Where, V_f is the volume occupied by the dissolved particles and η₀ is the apparent viscosity of the un-grafted RWL glutelin sizing paste.

If the radius increases by a factor of 10, the volume (V_f) increases by a factor of 1000 (since volume ∝R³). Therefore, even a small concentration of the grafted RWL glutelin can create a very high volume fraction, leading to a sharply increased viscosity. (2) Electrostatic repulsion: PAA is a polyelectrolyte. Its carboxylic acid groups (-COOH) deprotonate under neutral condition, becoming negatively charged carboxylate ions (-COO^-). The negative charges on PAA branches of different grafted RWL glutelin molecules repel each other. This prevents the solution from phase-separating (precipitating) and keeps the molecules evenly dispersed. However, when shear (e.g. stirring) is applied, the repulsive forces create an “ordered” structure that resists flow, especially at low shear rates. Consequently, the apparent viscosity and its stability of the grafted glutelin sizing paste were both improved. Furthermore, after the GR exceeded a critical value, physical crosslinking between the grafted branches of the modified RWL glutelin might occur. The physical crosslinking also helped to increase the apparent viscosity and viscosity stability. At present, one of common disadvantages of protein sizing agents is their excessively low apparent viscosity, which has a negative impact on tensile strength and abrasion resistance of sized yarn.^45,46 Therefore, the increase in the apparent viscosity of the glutelin sizing paste is beneficial to the improvement of the mechanical properties of warp yarn sized by the glutelin. Meanwhile, the improvement on the viscosity stability contributed to the achievement of stable sizing percentage of warp yarn.

Effect of graft modification on wettability to common fibers

Effect of graft modification on the contact angles of the RWL glutelin solutions on two kinds of common textile fibers (cotton and polyester) are shown in Table 4. All the RWL glutelin-g-PAA solutions exhibited smaller contact angles on cotton fibers than the native glutelin one. As for polyester fibers, all the RWL glutelin-g-PAA solutions except 5# exhibited smaller contact angles than the native glutelin one. In addition, with the increase in the GR of the grafted RWL glutelin, the contact angle of the glutelin aqueous solution on the cotton fibers decreased while the contact angle on the polyester fibers increased gradually.

Table 4.

Contact angles of native and grafted RWL glutelin solutions on cotton fibers and polyester fibers.

Type of RWL glutelin	Contact angle on cotton fibers (°)	CV (%)	Contact angle on polyester fibers (°)	CV (%)
Native RWL glutelin	50.1	2.62	100.1	1.17
RWL glutelin-g-PAA 1#	39.3	3.50	83.2	1.51
RWL glutelin-g-PAA 2#	35.4	3.69	88.5	1.42
RWL glutelin-g-PAA 3#	30.1	2.84	92.2	2.09
RWL glutelin-g-PAA 4#	26.3	4.07	94.5	2.02
RWL glutelin-g-PAA 5#	22.1	3.45	105.3	1.66

The wettability of a polymer solution to textile fibers is usually evaluated by contact angle. If a solution drop can spread widely on fibers’ surface to form a small contact angle, the solution is considered to possess high wettability to the fibers. According to Young’s equation, contact angle of a polymer solution on fibers is subject to surface tension of the solution and interfacial tension between the solution and the fiber if using the same fiber.⁴⁷ Decreasing either the surface tension or the interfacial tension can decrease the contact angle. As is elucidated above, native RWL glutelin can only be dissolved in alkali solution (e.g. NaOH and KOH). When a strong alkali dissolves in water, it is able to ionize completely, producing a large amount of OH^- and corresponding cations (e.g. Na⁺ or K⁺). These ions have strong hydration ability to form stable hydration shells with water molecules, thereby enhancing the effective attraction between water molecules. The effect makes it more difficult for molecules in the liquid surface layer to be pulled toward the gas phase, resulting in an increase in surface tension. In contrast, the grafted glutelin can be dissolved in water without the help of the alkali. Therefore, the surface tension of the native RWL glutelin solution was higher than that of the grafted RWL glutelin one.

As for the RWL glutelin-g-PAA, the higher the GR, the larger was the number of carboxyl groups of the grafted glutelin prepared. As is well known, cotton fibers contain a larger number of hydroxyl groups. Carboxyl and hydroxyl are both typical strongly polar groups. With the increase in the GR, the number of carboxyl groups on the grafted glutelin increased and thus the polarity of the glutelin turned stronger. Based on similar dissolve mutually theory, increasing the number of carboxyl groups of the grafted glutelin can decrease interfacial tension between the glutelin solution and cotton fibers. Hence, higher GR of the grafted glutelin led to smaller contact angle of the glutelin solution on the cotton fibers. Due to lower surface tension of the solution and lower interfacial tension between the solution and cotton fibers, the contact angles of all the RWL glutelin-g-PAA solutions on the cotton fibers were smaller than that of the native glutelin one.

As to polyester fibers, they contain numerous ester, which belongs to weakly polar group. Contrary to cotton fibers, increasing the number of carboxyl groups of the grafted glutelin will increase interfacial tension between the glutelin solution and polyester fibers. Therefore, the higher the GR of the grafted glutelin, the larger was the contact angle of the glutelin solution on the polyester fibers. When the GR was excessively high, the adverse effect of higher interfacial tension between the grafted glutelin solution and polyester fibers on the wettability to polyester fibers would surpass the beneficial effect of lower surface tension of the grafted glutelin solution. As a result, the contact angle of RWL glutelin-g-PAA 5# solution on the polyester fibers was even larger than that of the native RWL glutelin one. In other words, grafting too many AA monomers onto the molecular chains of the RWL glutelin will impair the wettability to the polyester fibers seriously.

Effect of graft modification on mechanical properties of sizing film

Effects of graft modification on mechanical properties of the RWL glutelin sizing films, such as tensile strength, tensile elongation and wear loss, are described in Figure 8. All the RWL glutelin-g-PAA sizing films showed better mechanical properties than the native glutelin one. With the increase in the GR of the grafted RWL glutelin, the mechanical properties of the RWL glutelin-g-PAA sizing film were improved initially, arrived at the optimum when the GR was 74.1% and then started to decrease. As for tensile strength and elongation, all the RWL glutelin sizing films were statistically significantly different except the sizing films of RWL glutelin-g-PAA 2# and 5#. Meanwhile, the values of the sizing films were all statistically significantly different as to wear loss.

Figure 8.

Mechanical properties of sizing films of native RWL glutelin (0#) and RWL glutelin-g-PAA 1#-5#.

Only under alkaline condition can the dissolution of native RWL glutelin in water be achieved. During the size boiling process, the glutelin was hydrolyzed by alkali and part of glutelin macromolecules were broken into small-molecular peptides. The intermolecular forces of the peptides were weak, making it difficult to form an continuous, compact and tough film structure. In addition, the small-molecular peptides had insufficient mechanical strength or elasticity, resulting in brittle and poor-toughness films that were prone to cracking under drying or external forces. In contrast, the grafted glutelin macromolecules without alkaline hydrolysis retained longer molecular chains and were more prone to intertwining with each other to form a more stable network-like film. Hence, the graft modification could substantially improve the mechanical properties of the RWL glutelin sizing film.

Good water solubility is beneficial to uniform dispersion of the RWL glutelin in water, sufficient interdiffusion among the molecules in film-forming process and remarkable decrease in the impurities (e.g. insoluble glutelin granules) embedded in sizing film. As is elucidated above, water solubility of the grafted RWL glutelin initially increased and reached the maximum when the GR was 74.1%. Consequently, the variation trend of the mechanical properties of the grafted RWL glutelin sizing film was similar to that of the water solubility. It could be observed from the SEM images (Figure 9) that the sizing film of RWL glutelin-g-PAA 4# was more continuous, uniform and less cracked than any other glutelin film. Hence, when the GR increased to 74.1%, the mechanical properties of the sizing film were improved successively. However, if the GR was excessively high (>74.1%), too many physical crosslinking bonds between PAA grafted branches of the modified glutelin would decrease the water solubility of the glutelin substantially. Many insoluble particles embedded in the sizing film of RWL glutelin-g-PAA 5# could be clearly observed. The insoluble particles substantially raised the risks in stress concentration in the film during tensile test and weight loss of the film during reciprocating friction test. As a result, the sizing film of RWL glutelin-g-PAA 5# exhibited much worse mechanical properties than that of RWL glutelin-g-PAA 4#. In addition, AA is hard monomer and the grafting of PAA branches limits the potential for improving the flexibility of the glutelin sizing film. In future research, a certain amount of plasticizer (e.g. glycerol and sorbitol) will be added into the RWL glutelin-g-PAA sizing paste to further improve the flexibility of the grafted glutelin sizing film.

Figure 9.

SEM images of sizing films of native RWL glutelin (a) and RWL glutelin-g-PAA 1#-5# (b–f).

Effect of graft modification on adhesion to common fibers

Effects of the graft modification on the adhesion of the RWL glutelin to cotton and polyester fibers are presented in Figures 10 and 11, respectively. As for cotton fibers, adhesion strength and work of each RWL glutelin were all statistically significantly different. Meanwhile, adhesion strength and work of each RWL glutelin to polyester fibers were all statistically significantly different except RWL glutelin-g-PAA 1#, 2# and 5#. Compared to native RWL glutelin, the adhesion of the grafted RWL glutelin to cotton and polyester fibers were both improved. The improvement in the adhesion to polyester fibers was greater than to cotton fibers. With the increase in the GR, the adhesion of the grafted RWL glutelin to both of the cotton and polyester fibers initially increased, reached the maximum when the GR was 74.1% and then began to decrease.

Figure 10.

Adhesion of native RWL glutelin (0#) and RWL glutelin-g-PAA 1#-5# sizing agents to cotton fibers.

Figure 11.

Adhesion of native RWL glutelin (0#) and RWL glutelin-g-PAA 1#-5# sizing agents to polyester fibers.

After the impregnation of rovings in sizing paste, the paste permeates into the rovings and shrinks during the drying process. The sizing agents are adhered to the fibers in this way, thus forming adhesive layers. When adhesion failure occurs, the failure is divided into two types according to failure position: cohesive failure and interfacial failure. The former is closely related to mechanical properties of adhesive layers.⁴⁸ However, it is impossible to strip adhesive layer between the fibers to evaluate its mechanical properties without any damage. Aiming at the evaluation problem, Zhu and Chen⁴⁹ introduced a simple and practicable way to assess mechanical properties of adhesive layer between fibers in 2007. They cast a sizing film and tested mechanical properties of the film, which were approximately regarded as the reflections of mechanical properties of the adhesive layer. Therefore, RWL glutelin films were cast in the study according to Zhu and Chen’s method. As depicted in Figure 8, mechanical properties of the grafted RWL glutelin film were better than those of the native RWL glutelin one. In other words, the grafted glutelin adhesive layer had a stronger ability to resist external damage than the native one. Furthermore, most plant protein molecules including glutelin are too large (5–10 nm radius) to penetrate the microscopic pores, cracks or fibrils of textile fibers. In contrast, PAA is a flexible and linear polymer and the PAA branches can worm their way into the amorphous regions of the fibers or wrap around individual microfibrils. Once inside the fiber structure, the PAA branches physically entangled with the fiber’s own polymer chains. Only under significant external force could these entangled PAA branches be pulled out of the fibers. In addition, the existence of a large amount of the alkali in the native RWL glutelin sizing paste did harm to the mechanical properties of polyester fibers, which had low alkali-resistance. Therefore, the RWL glutelin-g-PAA possessed higher adhesion to the fibers than the native one. The use of neutral sizing paste brought about more improvement in the adhesion of the RWL glutelin to polyester fibers than to cotton fibers. Even for RWL glutelin-g-PAA 1#, its adhesion strength to polyester fibers is 62.4% higher than that of the native RWL glutelin.

It should be noted that interfacial failure between sizing agents and fibers is often caused by poor wettability of sizing paste. It is very probable for the unwetted surface of the fibers to become stress concentration sites when subject to external force. Good wettability of the sizing paste on the fiber surface is the basis of its penetration into yarns and is helpful to form strong adhesion of sizing agents to fibers. As shown in Table 4, with the increase in the GR of the grafted RWL glutelin, the wettability of the glutelin aqueous solution to cotton fibers kept to increase while the wettability to polyester fibers decreased gradually. Therefore, when the GR was in a range of 0%–74.1%, the adhesion of the grafted RWL glutelin to cotton fibers was improved substantially due to the enhancements of both wettability and mechanical properties of adhesive layer. Compared with cotton fibers, the improvement resulting from the increase of the GR in the adhesion to polyester fibers was not so remarkable. It could be explained that the beneficial effect of higher mechanical properties of the grafted glutelin adhesive layer on the adhesion was somewhat offset by the adverse effect of lower wettability to polyester fibers. When the GR was excessively high (>74.1%), the mechanical properties of the grafted RWL glutelin film deteriorated severely as shown in Figure 8. As a result, the adhesion of the grafted RWL glutelin to cotton and polyester fibers both started to decrease. The decreasing amplitude of the adhesion to polyester fibers was more remarkable than to cotton ones due to the simultaneous deterioration of the wettability.

Effect of graft modification on mechanical properties of sized yarn

Effects of the graft modification on main mechanical properties (i.e. tensile strength, tensile elongation and abrasion resistance) of the T/C65/35 warp yarn sized by the RWL glutelin are shown in Table 5. Native RWL glutelin was obviously not suitable for sizing high content polyester yarn due to the damage caused by the high alkalinity of its sizing paste to polyester fiber, low wettability to polyester fiber, poor mechanical properties of sizing film and low adhesion to polyester fiber. It could be observed from Table 5 that the mechanical properties of the T/C65/35 warp yarns sized by the grafted RWL glutelin were all better than those of the yarns sized by native RWL glutelin. With the increase in the GR, the mechanical properties of the sized yarns initially increased, reached the maximum when the GR was 74.1% and then started to decrease.

Table 5.

Tensile strength, tensile elongation and abrasion resistance of raw T/C65/35 warp yarn and the yarns sized by native and grafted RWL glutelin, starch acetate and PVA.

Type of sizing agent	Tensile strength (N)	CV (%)	Tensile elongation (%)	CV (%)	Abrasion resistance (cycles)	CV (%)
Raw yarn	2.92	6.72	8.84	5.03	73	9.55
Native RWL glutelin	3.07	6.31	5.98	5.81	86	11.64
RWL glutelin-g-PAA 1#	3.18	7.29	6.06	6.45	89	13.41
RWL glutelin-g-PAA 2#	3.26	6.30	6.08	7.34	94	8.42
RWL glutelin-g-PAA 3#	3.34	5.12	6.22	7.48	102	11.59
RWL glutelin-g-PAA 4#	3.49	5.18	6.55	5.95	111	9.04
RWL glutelin-g-PAA 5#	3.25	6.97	6.06	9.48	91	13.22
Starch acetate	3.14	6.57	6.01	7.41	87	11.53
PVA	3.52	5.32	6.46	6.44	113	12.07

Generally speaking, tensile properties of sized yarns are mainly determined by the adhesion of the sizes to the fibers. The adhesion can help the fibers in the yarn adhere to each other and enhance the cohesive force between the fibers. It could be observed from Figures 10 and 11 that, with the increase in the GR, the adhesion of the RWL glutelin-g-PAA to cotton and polyester fibers both increased initially, arrived at the optimum when the GR was 74.1% and then decreased. Therefore, the T/C65/35 yarn sized by RWL glutelin-g-PAA 4# possessed higher tensile strength and elongation than any other sized yarn. As for the abrasion resistance of sized yarn, it has a close relationship to mechanical properties of sizing film. The sizing film is the protective layer for warp yarns. The yarns coated with tough, flexible and wear-resistant sizing film usually possess high abrasion resistance. Moreover, integrated and continuous sizing film contributes to the abrasion resistance of warp yarns very much. As shown in Figures 8 and 9, the RWL glutelin-g-PAA 4# sizing film had the best tensile properties, the lowest wear loss and the fewest cracks in all the grafted glutelin ones. Besides high cohesive force between the fibers, high adhesion contributes to the tight bonding between sizing film and warp yarns. Therefore, the abrasion resistance of the yarns sized by RWL glutelin-g-PAA 4# was much better than that of the other sized yarns.

As well known, starch ester and partially alcoholized PVA are two kinds of sizing agents commonly used for both polyester and cotton warp yarns. Therefore, the study compared warp sizing performance of starch acetate and PVA (degree of alcoholysis: 88%) with that of the RWL glutelin. Mechanical properties of the T/C65/35 warp yarns sized by starch acetate and PVA are also shown in Table 5. All the RWL glutelin-g-PAA possessed better warp sizing performance than starch acetate while RWL glutelin-g-PAA 4# had nearly the same warp sizing performance with PVA. From the perspective of sizing performance alone, the grafted RWL glutelin has great potential to replace modified starch and PVA sizing agents.

Effect of graft modification on COD and BOD values

Effects of the graft modification on COD and BOD values of the RWL glutelin-g-PAA are presented Table 6. Due to the difference in microbial population in dilution water collected from various places for BOD measurement, most researchers usually evaluate the biodegradability of a polymer in terms of the ratio of BOD₅/COD_cr for fair comparison. The higher the ratio of BOD₅/COD_cr, the better is the biodegradability of the polymer. If the ratio of BOD₅/COD_cr of a polymer is higher than 0.30, the polymer is considered as biodegradable. As shown in Table 6, with the increase in the GR of the grafted RWL glutelin, its COD_cr increased continuously, while BOD₅ and the ratio of BOD₅/COD_cr kept decreasing. The ratios of BOD₅/COD_cr of all the grafted RWL glutelin were higher than 0.30 except RWL glutelin-g-PAA 5#.

Table 6.

COD_cr and BOD₅ of RWL glutelin-g-PAA, PVA and poly(ethyl acrylate-co-acrylic acid).

Type of sizing agent	COD_cr (mg/L)	BOD₅ (mg/L)	Ratio of BOD₅/COD_cr
RWL glutelin-g-PAA 1#	182	90.1	0.495
RWL glutelin-g-PAA 2#	190	82.5	0.434
RWL glutelin-g-PAA 3#	201	73.2	0.364
RWL glutelin-g-PAA 4#	212	66.0	0.311
RWL glutelin-g-PAA 5#	223	58.9	0.264
PVA	1125	37.0	0.033
Poly(ethyl acrylate-co-acrylic acid)	183	16.5	0.090

Protein molecules are composed of amino acids connected by peptide bonds. Although peptide bonds are relatively stable under neutral conditions at room temperature, they can be hydrolyzed by acids, bases or enzymes in aqueous solution environments. Proteases commonly found in living organisms can efficiently recognize and cleave peptide bonds, breaking them down into short peptides and free amino acids. Therefore, water-soluble protein molecules often possess good biodegradability. The backbone of PAA is composed of carbon-carbon single bonds, which are stable and difficult for ordinary microorganisms to cut off. Consequently, the grafting of PAA branches will bring about negative influence on the biodegradability. However, as long as the GR was controlled within a certain value (≤74.1%), the ratio of BOD₅/COD_cr of the RWL glutelin-g-PAA could still be maintained above 0.30. In other words, the grafted RWL glutelin within the GR range is biodegradable and almost harmless to the environment. Nowadays, PVA and polyacrylate are two of the most commonly used sizing agents for cotton and polyester yarns in the textile industry. Therefore, COD and BOD values of PVA and poly(ethyl acrylate-co-acrylic acid) tested by Wu⁵⁰ and Qiao⁵¹ are also presented Table 6. Even if RWL glutelin-g-PAA 4# is taken as comparison sample, the ratio of BOD₅/COD_cr of the graft copolymer is still about 9 and 3 times as high as those of PVA and poly(ethyl acrylate-co-acrylic acid), respectively. In view of environmental conservation, the grafted RWL glutelin is a suitable substitute for PVA and polyacrylate sizing agents.

Conclusion

The serviceability of glutelin used as a sizing agent can be expanded greatly through grafting appropriate amount of AA monomers onto the molecular chains of the glutelin extracted from rice wine lees. Water solubility, apparent viscosity, wettability, mechanical properties of sizing film, the adhesion to common fibers, mechanical properties of sized warp yarns and biodegradability depend directly on the GR of the RWL glutelin-g-PAA. The RWL glutelin can be endowed with good water solubility under neutral condition, exhibit high sizing properties and keep biodegradable when the grafting ratio was 74.1%. With the increase in the monomer concentration in a range of 40%–120%, the MC initially increased and leveled off at ∼97% while the GR increased stepwise. The preferred RWL glutelin-g-PAA sizing agent can be prepared by choosing AA as grafting monomer, K₂S₂O₈ and NaHSO₃ as oxidant and reductant in redox system, respectively. The monomer concentration should be 100% (w/w, AA/RWL glutelin). The RWL glutelin-g-PAA thus produced possesses performance parameters of 32 min in water solubility period, 4.10 mPa·s and 97.56% in apparent viscosity and viscosity stability, 26.3° and 94.5° in contact angles on cotton and polyester fibers, 10.12 N/mm², 6.37% and 0.61 mg/cm² in tensile strength, elongation and wear loss of sizing film, 125.3 N/1000 tex and 106.5 N/1000 tex in adhesion strength to cotton and polyester fibers, 3.49 N, 6.55% and 111 cycles in tensile strength, elongation and abrasion resistance of sized polyester/cotton blended warp yarn and 0.311 in the ratio of BOD₅/COD_cr, respectively. The graft modification lays a solid foundation for the wide use of RWL glutelin as an inexpensive, biodegradable and bio-based sizing agent in the textile industry.

Supplemental Material

sj-docx-1-aat-10.1177_24723444261464132 – Supplemental material for Rice wine lees glutelin utilization as a source of bio-based sizing agents through graft polymerization

Supplemental material, sj-docx-1-aat-10.1177_24723444261464132 for Rice wine lees glutelin utilization as a source of bio-based sizing agents through graft polymerization by Shanshan Ye, Manli Li, Enqi Jin, Chi Shen and Jiandi Zhou in AATCC Journal of Research

Footnotes

ORCID iD

Enqi Jin

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The work was supported financially by “Spearhead and Leading Goose + X” Research and Development Project of Zhejiang Province (2024C02007) and First-Class Course Construction Project of Zhejiang Province (2020-132-400). Financial sponsors do not endorse the views expressed in this publication.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Supplemental material

Supplemental material for this article is available online.

References

Pan

. Effect of polyvinyl alcohol on advanced oxidation processes of printing and dyeing wastewater and its resource utilization. Master Thesis, Donghua University, Shanghai, China, 2023.

Anubrota

Jin

, et al. Effects of carbon chain length of acrylate monomer on sizing properties of sesbania gum-g-polyacrylate. Indian J Fibre Text 2023; 48(1): 61–72. https://doi.org/10.56042/ijftr.v48i1.54696

, et al. Effects of carbon chain length of acrylate monomer on sizing properties of locust bean gum-g-P (AA-co-acrylate). AATCC J Res 2022; 9(4): 194–204. https://doi.org/10.1177/24723444221103684

Wang

Jin

, et al. Effects of monomer compatibility on sizing performance of chitosan-g-P(MA-co-AM). AATCC J Res 2021; 8(6): 58–66. https://doi.org/10.14504/ajr.8.6.7

Yang

Hou

, et al. Biodegradable sizing agents from soy protein via controlled hydrolysis and dis-entanglement for remediation of textile effluents. J Environ Manage 2017; 188: 26–31. https://doi.org/10.1016/j.jenvman.2016.11.066

Reddy

Yang

. Review: potential use of plant proteins and feather keratin as sizing agents for polyester-cotton. AATCC J Res 2015; 2(2): 20–27. https://doi.org/10.14504/ajr.2.2.3

Zhang

Liu

. Synthesis and properties of graft oxidation starch sizing agent. J Appl Polym Sci 2003; 88(6): 1563–1566. https://doi.org/10.1002/app.11704

Zhao

Song

, et al. The hidden metabolites in glutinous rice huangjiu and their antioxidant potential. Foods 2026; 15(8): 1386. https://doi.org/10.3390/foods15081386

Motono

Nishiumi

Yoshida

, et al. Metabolome analysis of the effects of sake lees on adipocyte differentiation and lipid accumulation. Fermentation 2023; 9(3): 300. https://doi.org/10.3390/fermentation9030300

10.

Pisitsungkakarn

SSH

Neamyou

. Efficiency of semi-automatic control ethanol distillation using a vacuum-tube parabolic solar collector. Energies 2022; 15(13): 4688. https://doi.org/10.3390/en15134688

11.

Kumari

Ama

Koch

, et al. Biochemical and physicochemical characterization of traditional rice wine of Nyishi tribe of Arunachal Pradesh, India. Indian J Tradit Knowl 2025; 24(7): 615–626. https://doi.org/10.56042/ijtk.v24i7.13884

12.

Xiang

. Preparation and application of carbon dots from yellow wine lees and gleditsia sinensis shells. Master Thesis, Guangdong University of Technology, Guangzhou, China, 2025.

13.

Lanfranchi

Magdalena

Cavinato

, et al. Highly selective acetate production from wine lees through acidogenic fermentation. J Environ Manage 2025; 373: 123532. https://doi.org/10.1016/j.jenvman.2024.123532

14.

Ling-Niao

Song-Tao

Yang

, et al. Pyrolysis mechanism and pyrolysis kinetics of yellow wine lees. RSC Adv 2024; 14(24): 16951–16959. https://doi.org/10.1039/D4RA01541J

15.

Romero-Díez

Rodríguez-Rojo

Cocero

, et al. Phenolic characterization of aging wine lees: correlation with antioxidant activities. Food Chem 2018; 259: 188–195. https://doi.org/10.1016/j.foodchem.2018.03.119

16.

Shao

, et al. A green and sustainable technology for the development of artificial protein fibers from sorghum distillers grains for industrialization. Chem Eng J 2024; 502: 157853. https://doi.org/10.1016/j.cej.2024.157853

17.

. Comprehensive utilization of rice wine lees. Master Thesis, Guangdong University of Technology, Guangzhou, China, 2005.

18.

Song

. Feasibility analysis of vinasse resources comprehensive utilization by GBC Group. Master Thesis, Qilu University of Technology, Jinan, China, 2015.

19.

Jin

Zhu

Yang

, et al. Blending water-soluble aliphatic-aromatic copolyester in starch for enhancing the adhesion of sizing paste to polyester fibers. J Text Inst 2011; 102(8): 681–688. https://doi.org/10.1080/00405000.2010.514727

20.

Bao

Wang

Yan

, et al. A high-temperature resistant and chemically compatible water-soluble sizing agent for improving the interfacial properties of short carbon fiber reinforced polyphthalazine ether ketone composites. Compos Part A Appl Sci Manuf 2026; 200: 109270. https://doi.org/10.1016/j.compositesa.2025.109270

21.

Yan

Wang

, et al. A novel thermoplastic water-soluble sizing agent for the interfacial enhancement of carbon fiber/polyether ether ketone composites. Compos B Eng 2024; 272: 111205. https://doi.org/10.1016/j.compositesb.2024.111205

22.

Horax

Hettiarachchy

Over

, et al. Extraction, fractionation and characterization of bitter melon seed proteins. J Agric Food Chem 2010; 58(3): 1892–1897. https://doi.org/10.1021/jf902903s

23.

Gandhi

Litoriya

Shah

, et al. Extraction, fractionation and characterization of cotton (Gossypium herbaceum L.) Seed proteins. Indian J Agric Biochem 2017; 30(1): 21–26. 10.5958/0974-4479.2017.00003.X

24.

. Grafting modification of zein and its application. Master Thesis, Zhejiang University, Hangzhou, China, 2016.

25.

Wang

. Study on the structure and functional properties of gluten protein/polysaccharide grafts prepared by ultrasonic enzymatic hydrolysis. Master Thesis, South China University of Technology, Guangzhou, China, 2008.

26.

Zhu

Jin

. Effect of an allyl pretreatment of starch on the grafting efficiency and properties of allyl starch-g-poly(acrylic acid). J Appl Polym Sci 2009; 112(5): 2822–2829. https://doi.org/10.1002/app.29621

27.

Jin

Qiao

, et al. Effects of molecular structure of carboxyl-containing vinyl monomers on sizing properties of feather keratin graft copolymer. Fibers Polym 2019; 20(9): 1784–1793. https://doi.org/10.1007/s12221-019-8800-8

28.

Soleimani

Sadeghi

Shahsavari

. Graft copolymerization of gelatin-g-poly (acrylic acid-co-acrylamide) and calculation of grafting parameters. Indian J Sci Technol 2012; 5(2): 2041–2046. https://doi.org/10.17485/ijst/2012/v5i2.25

29.

Jin

Zhou

. Crab and prawn shell utilization as a source of bio-based thermoplastics through graft polymerization with acrylate monomers. J Mater Cycles Waste Manag 2018; 20(1): 496–504. https://doi.org/10.1007/s10163-017-0607-3

30.

SEPA. The national standards of the Peoples’ Republic of China: environmental quality standards for surface water (GB 3838-2002). Chinese Environmental Sciences Press, 2002.

31.

APHA. Standard methods for the examination of water and wastewater. 18th ed. American Water Works Association and Water Pollution Control Federation, 1992.

32.

Akbar

Sharafeldin

Zhu

, et al. Comparison of glutelins characteristics among foxtail millet varieties: functional, physico-chemical, and structural modifications induced by hydrothermal treatment. Int J Biol Macromol 2025; 318: 145081. https://doi.org/10.1016/j.ijbiomac.2025.145081

33.

, et al. Development of a rice glutelin-based nanoparticle delivery system: fabrication, characterization, and stability. J Sci Food Agr 2025; 105(12): 6641–6653. https://doi.org/10.1002/jsfa.14377

34.

Zhu

Jin

. Graft copolymerization of granular allyl starch with carboxyl-containing vinyl monomers for enhancing grafting efficiency. Fibers Polym 2010; 11(5): 683–688. https://doi.org/10.1007/s12221-010-0683-7

35.

Qin

, et al. Preparation of novel β-CD/P(-co-) hydrogels by frontal polymerization. RSC Adv 2023; 13(9): 5667–5673. https://doi.org/10.1039/D2RA07649G

36.

Kavitha

Singha

. Tailor-made poly(methyl acrylate) bearing amantadine functionality (amino adamantyl) via atom transfer radical polymerization (ATRP). A precursor of a supramolecular cross-linked polymer. Macromolecules 2009; 42(15): 5499–5508. https://doi.org/10.1021/ma900225g

37.

Quan

Xin

Zhang

, et al. Peptide-enhanced bioactive hydrogel combined with photodynamic-phage synergistic antibacterial therapy system accelerates infected wound healing. Adv Healthc Mater 2025; 14(24): 2500875. https://doi.org/10.1002/adhm.202500875

38.

Masooma

Nuzhat

. Synthesis, highly potent α-glucosidase inhibition, antioxidant and molecular docking of various novel dihydropyrimidine derivatives to treat diabetes mellitus. Bioorg Med Chem Lett 2025; 115: 130016. https://doi.org/10.1016/j.bmcl.2024.130016

39.

Adusei

Agorku

Voegborlo

, et al. Zero valent iron impregnated sodium alginate grafted (acrylamide-co-acrylic acid) adsorbents for the removal of methylene blue in aqueous systems. J Macromol Sci B 2023; 62(6): 265–279. https://doi.org/10.1080/00222348.2023.2211376

40.

Wang

Zhang

Shen

, et al. The influence of dicarboxylic acid derivatives synthesized with different anhydride structures on the performance of waterborne epoxy coatings. Surf Interfaces 2026; 80: 108256. https://doi.org/10.1016/j.surfin.2025.108256

41.

Zhu

Liu

Almasoud

, et al. Temperature and pH dual dependent conformational transition of polyacrylic acid modified by dehydroabietic alcohol. Adv Compos Hybrid Mater 2025; 8(1): 1–11. https://doi.org/10.1007/s42114-024-01146-4

42.

Paik

Blanco

Jia

, et al. Aggregation of poly(acrylic acid)-containing elastin-mimetic copolymers. Soft Matter 2015; 11(9): 1839–1850. https://doi.org/10.1039/c4sm02525c

43.

Sun

Shao

Zhou

, et al. Compatibilization of acrylic polymer-silk fibroin blend fibers. I. Graft copolymerzation of acrylonitrile onto silk fibroin. J Appl Polym Sci 1998; 69(6): 1089–1097. https://doi.org/10.1002/(SICI)1097-4628(19980808)69:6<1089::AID-APP5>3.0.CO;2-C

44.

Jin

Zhu

Yang

. Structural effects of glycol and benzenedicarboxylate units on the adhesion of water-soluble polyester sizes to polyester fibers. J Text Inst 2010; 101(12): 1112–1120. https://doi.org/10.1080/00405000903462217

45.

Meng

Guo

Zhou

, et al. Soy protein/dialdehyde starch sizes with responsive coating and removability via a dual-zone dynamic imine strategy for ecofriendly textile processing. Acs Sustain Chem Eng 2025; 14(1): 566–575. https://doi.org/10.1021/acssuschemeng.5c10654

46.

, et al. Preparation, characterization of feather protein-g-poly(sodium allyl sulfonate) and its application as a low-temperature adhesive to cotton and viscose fibers for warp sizing. Eur Polym J 2020; 136: 109945. https://doi.org/10.1016/j.eurpolymj.2020.109945

47.

Shaw

. Introduction to colloid & surface chemistry. Butterworth-Heinemann, 1992, pp.151–155.

48.

Jin

Lian

. Effects of monomer compatibility on sizing properties of feather keratin-g-P(AA-co-MA). J Text Inst 2018; 109(3): 376–382. https://doi.org/10.1080/00405000.2017.1348215

49.

Zhu

Chen

. Carbamoyl ethylation of starch for enhancing the adhesion capacity to fibers. J Appl Polym Sci 2007; 106(4): 2763–2768. https://doi.org/10.1002/app.26813

50.

. The inﬂuence of molecular weight and different degree of hydrolysis on biodegradation of PVA. Master Thesis, Donghua University, Shanghai, China, 2008.

51.

Qiao

. Research on microstructure and properties of polyacrylate/starch sizing ﬁlm. Doctoral Dissertation, Jiangnan University, Wuxi, China, 2011.

Supplementary Material

Please find the following supplemental material available below.

For Open Access articles published under a Creative Commons License, all supplemental material carries the same license as the article it is associated with.

For non-Open Access articles published, all supplemental material carries a non-exclusive license, and permission requests for re-use of supplemental material or any part of supplemental material shall be sent directly to the copyright owner as specified in the copyright notice associated with the article.

0.03 MB

Rice wine lees glutelin utilization as a source of bio-based sizing agents through graft polymerization

Abstract

Keywords

Introduction

Experimental

Materials

Extraction of glutelin from RWL

Reaction mechanism of graft polymerization of RWL glutelin with AA

Preparation of RWL glutelin-g-PAA

Measurement of grafting parameters

FTIR characterization

1H-NMR characterization

EA characterization

TGA characterization

Measurement of water solubility

Measurement of apparent viscosity and viscosity stability

Measurement of contact angle

Preparation of sizing film and measurement of mechanical properties of the film

SEM observation of sizing film

Measurement of adhesion to fibers

Sizing experiment and measurement of mechanical properties of sized yarn

Measurement of COD and BOD

Statistical analysis

Results and discussion

FTIR analysis of native RWL glutelin and RWL glutelin-g-PAA

1H-NMR analysis of native RWL glutelin and RWL glutelin-g-PAA

Thermogravimetric analysis of native RWL glutelin and RWL glutelin-g-PAA

Effect of monomer concentration on grafting parameters

Elemental analysis of native RWL glutelin and RWL glutelin-g-PAA

Effect of graft modification on water solubility

Effects of graft modification on apparent viscosity and viscosity stability

Effect of graft modification on wettability to common fibers

Effect of graft modification on mechanical properties of sizing film

Effect of graft modification on adhesion to common fibers

Effect of graft modification on mechanical properties of sized yarn

Effect of graft modification on COD and BOD values

Conclusion

Supplemental Material

sj-docx-1-aat-10.1177_24723444261464132 – Supplemental material for Rice wine lees glutelin utilization as a source of bio-based sizing agents through graft polymerization

Footnotes

ORCID iD

Funding

Declaration of conflicting interests

Supplemental material

References

Supplementary Material

¹H-NMR characterization

¹H-NMR analysis of native RWL glutelin and RWL glutelin-g-PAA