Hydroxypropylation reduces gelatinization temperature of corn starch for textile sizing

A series of hydroxypropylated starch (HPS) that can be dissolved in water at 60–65 °C was obtained via two-step method in water system from corn starch. The structure and property of the HPS and its gelatinization temperature were characterized by Fourier transform infrared spectrometer, nuclear magnetic resonance spectroscopy (1H NMR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and transmission electron microscope (TEM). It was concluded that hydroxypropyl mainly bonded on the hydroxyl group at C2 position from glucose residue of starch in the form of C–O–C, and the substitution level at C6 position was slightly higher than that at C3 position, as well as the crystallinity of starch decreased from 52.41% to 29.4% due to the introduction of hydroxypropyl and was confirmed by XRD. At the same time, the grooves on the surface of starch granules were observed by SEM. The above-mentioned two synergies promoted the permeation and transmission of water molecules in the starch microstructure. Moreover, the gelatinization temperatures and enthalpy of synthetic HPS were lower than those of raw corn starch, as further confirmed by DSC. This caused the HPS with a molar substitution greater than 0.1 to be soluble in water at 65 °C, and its dissolution state was similar to that of HPS at 95 °C (transmittance above 55%), as well as exhibited high slurry stability. Interestingly, compared with the raw starch, the HPS film showed excellent tensile strength and elongation at the relative humidity of 65%, which could be attributed to the hydrophilic ether bond and the flexible alkyl chain bonded on the structure of starch. This study will provide a new way for the preparation of high performance starch size for sizing yarn at medium–low temperature.


Introduction
As an abundant natural polysaccharide in plants, starch has become the most widely sizing agent in textile sizing, accounting for about 70% of the total slurry in China due to its renewability, biodegradability and ease of modify, etc. (Jeong et al. 2018;Li et al. 2016Li et al. , 2018aYang et al. 2018). However, the strong intra-and intermolecular hydrogen bonding in starch and its high glass transition temperature make it nongelatinizable at medium-low temperature. As a result, high-temperature sizing technology has been developed widely for warp sizing so far (the boiling temperature of slurry must be 100-132°C, and the slurry tank temperature has exceeded 92°C). (Zhu et al. 2015;Wu et al., 2021). High-temperature sizing has seriously affected the working environment of workers and brought high energy consumption, which inhibits the development of the textile industry (Jiang et al. 2019;Rafikov et al. 2020). Hence, our objective is to develop a low energy consumption and environmentally friendly sizing method that can be used at medium-low temperature.
As room temperature soluble textile size, polyacrylic acid polymers exhibit strong sizing effect on chemical fibers (such as polyester and nylon). However, they suffer the problem of moisture absorption and re-adhesion due to their inherent hydrophilic groups and flexible molecular chains, leading to they can only be used as auxiliary sizing on yarns, and their insufficient adhesion to cellulose fibers (Hu and Babu 2007). Although there are also studies on starch grafted acrylic acid (Li et al. 2011), acrylamide (Wang et al. 2020), and methacrylic acid (Djordjevic et al. 2019), the starch graft copolymers have improved sizing properties, such as grafted starch size film properties and strong adhesion to cellulose fibers. Nevertheless, the disadvantages of starch sizing are not completely eliminated, most studies focused on the synthesis of grafted copolymers and the use of various starches in the sizing of textiles, as well as the use of these modified starch size still need to be dissolved at high temperature (above 85°C).
In recent years, the way of medium temperature sizing was realized mainly by developing watersoluble starch at medium temperature of 60 * 65°C Kim et al. 2017). For example, Guo et al. have realized medium temperature sizing by adding sodium hydroxide into starch, and then further mixing with synthetic polyacrylate size, mainly using alkali could promote the dissolution of starch in water (Guo et al. 2010). However, alkali could affect the structure of the polyacrylate and weaken the sizing property when starch was the main size. In our previous work, a new sizing way at medium temperature of 65°C known as part-gelatinization sizing technology (PGST) was proposed based on the principle that starch presents a state of irreversible water absorption and high expansion at 55 * 65°C (Wu et al. 2014). Unfortunately, the PGST technology must be equipped with a gelatinization device to make a part starch be pregelatinized. Subsequently, we explored the feasibility of sizing on the basis of quaternary ammonium cationic starch for cotton yarn and acetic anhydride esterified starch for PLA filament at 65°C (Yang et al. 2017;Shen et al. 2017). Also, we achieved starch gelatinization at 60-65°C by the synergistic effect of ethanol-alkali system on starch, and showed the same gelatinization characteristics as the raw starch at 100°C (Deng et al. 2017). Although there were also report on the introduction of hydrophilic groups into starch structure through hydroxypropylation to improve their solubility and paste clarity (Fouladi and Mohammadi 2014;Li et al. 2018b), it was difficult to obtain high molar substitution (MS) for hydroxypropylated starch (HPS), which was mainly due to the reaction staying on the granule surface, resulting in its limited solubility in water at medium temperature. Meanwhile, in addition to the use of the sophisticated pretreatment, repeated filtration, drying, and crushing process, the above methods also increased the production cost of size, leading to new pollution.
In this work, we will use acid to etch starch to generate channels in starch granules, which will provide space for the full contact between propylene oxide and starch chains within the granule. Meanwhile, acid can also effectively reduce the molecular weight of starch to expose more active groups and promote the reaction between hydroxyl groups and propylene oxide. It is expected to obtain HPS with high molar substitution and realize water solubility at medium temperature. Also, different from the previous method of preparing medium-temperature watersoluble starch under wet condition, we report a lowenergy, simple and efficient method to prepare the water-soluble starch at medium temperature and its structure and property. Based on the characteristic of easy ring-opening reaction from epoxy group in alkaline condition, a white HPS emulsion (about 40% content) that can be used directly for warp sizing was obtained by chemical bonding the propylene oxide on the active hydroxyl group at C 2 , C 3 , and C 6 positions from glucose residue of starch, and further simple dialyzing against deionized water to the reacting mixture. The bonding position and reaction rule of hydroxypropyl group on the active hydroxyl group from glucose residue of starch were studied by Fourier transform infrared spectrometer (FTIR) and nuclear magnetic resonance spectroscopy ( 1 H NMR), and the gelatinization mechanism of the formed HPS at medium temperature was revealed by X-ray diffraction (XRD), differential scanning calorimetry (DSC), Ultraviolet-visible spectrophotometer (UVvis), scanning electron microscopy (SEM), and transmission electron microscope (TEM). The storage stability and dilution stability of the medium-temperature water-soluble HPS were investigated, and the mechanical property of its corresponding film was also evaluated.

Synthesis of water-soluble hydroxypropyl starch (HPS) at medium temperature
The water-soluble HPS was prepared as follows: Firstly, 100 g of corn starch aqueous suspension with 40 wt% in concentration was prepared by dispersing starch powders into the deionized water in a water bath at 40°C for 30 min, then the dispersion system was adjusted to pH = 1 * 2 with HCl solution (37% w/w) and the mixture was stirred for 80 min to reduce the molecular weight of corn starch. Subsequently, the acidified starch system was adjusted to pH = 11 * 12 with NaOH solution (5% w/w), followed by adding an appropriate amount of epoxy propane (the molar ratio of epoxy propane to glucose residue of starch is 3:1 * 7:1) dropwisely in the above starch dispersion system with stirring for 20 h continuously. After the completion of reaction, the pH of reacting mixture system was adjusted to neutral with HCl solution (5% w/v), and the reacting mixture was transferred to a dialysis membrane with 3000 Da molecular weight cut off (MWCO) for 4 days in deionized water (replaced water once every 6 h) to remove any unreacted epoxy propane and salt with low molecular weight. Finally, the water-soluble HPS emulsion (about 40 wt% content) with different molar substitution (MS) was obtained from white suspension in the dialysis membrane. All the HPS samples were coded in the form of HPS-P3, HPS-P4, HPS-P5, HPS-P6, HPS-P7, where ''Pn'' means the added amount of PO (the molar ratio of epoxy propane to glucose residue of starch is n:1).

Determining the molar substitution (MS) and reaction efficiency of water-soluble HPS
The MS for water-soluble HPS was measured using a UV-visible spectrophotometer (Lambda 950, Perkin Elmer, USA). The method included hydrolysis of hydroxypropyl group to propylene glycol, then propylene glycol was dehydrated to propionaldehyde and allyl alcohol, and these products were reacted with ninhydrin to produce purple. Firstly, the absorbance of propylene glycol with different concentrations (10 * 50 lg/ml) at k = 590 nm was measured by UV-vis spectrophotometer, and its standard curve was established. Eventually, the MS of water-soluble HPS samples was calculated according to the standard curve. The specific operation method was as follows (Chen et al. 2021): 1 g of propylene glycol solution was added into a 1000 mL volumetric flask, then diluted with distilled water to the specified scale. Subsequently, 1, 2, 3, 4, and 5 mL of the above solution was added into five 100 mL volumetric flask, respectively, and diluted with distilled water to the specified scale, to obtain series of standard solutions with propylene glycol content of 10, 20, 30, 40, 50 lg/ ml, respectively. 1 mL of the above five kinds of propylene glycol solutions were added into a 25 mL colorimetric tube, respectively, then the tubes were inserted into the water, 8 mL of concentrated sulfuric acid was slowly added into the above tubes along the wall, and the tubes were tightened. Subsequently, the tubes were moved to a boiling water bath for 3 min and immediately placed in the ice bath to cool the samples to 5°C. 0.6 mL of 3% ninhydrin hydrate was added into the above tubes and shaken evenly, and put into a 25°C water bath for 100 min, make them develop color. Finally, the tubes were fixed the volume to 25 mL using concentrated sulfuric acid, stand for 5 min, and the absorbance of purple solution was measured by UV-vis spectrophotometer at k = 590 nm to determine the standard curve of propylene glycol.
0.07 g of HPS sample was added into 100 mL volumetric flask, and then put 25 mL of 0.5 mol/L concentrated sulfuric acid into the above volumetric flask. Subsequently, the volumetric flask was put into a 100°C water bath until the sample was completely dissolved. After the sample was cooled to room temperature, diluted it with distilled water to the specified scale. 1 mL of the above solution was added into a 25 mL colorimetric tube, and the next operation method was the same as above. Finally, the content of propylene glycol in HPS samples was calculated according to the standard curve. The measured content (g) of propylene glycol (W PG ) in 100 g HPS is converted into hydroxypropyl equivalent using the following equation.
The reaction efficiency (RE) of HPS refers to the percentage of propylene oxide (PO) participating in the etherification reaction, which is calculated as the following formula.

RE ¼
MS The molar ratio of etherifying agent PO to starch Â 100 Determining the solubility of water-soluble HPS 100 g of HPS emulsion with 1 wt% in concentration was prepared by diluting previous obtained HPS emulsion with 40 wt% content with deionized water. Then the diluted HPS emulsion was stirred immediately at 65°C in a water bath for 1 h, followed by centrifuge treatment for 20 min with a force of 12 kN. The all upper layer was cast onto a polyester plastic plate (25 cm 9 15 cm) to form a film under the natural state, followed by peeled off the film carefully, and dried in an oven at 40°C to determine the film weight until its weight was constant. The solubility (S) of water-soluble HPS was calculated by the following formula (Chun et al. 2016): where A is the total dry weight of HPS contained in the original centrifuge tube, g; B is the dry weight of HPS film after drying, g.
In addition, the solubility effect of water-soluble HPS could be presented indirectly by their light transmittance (Craig et al. 1989). We prepared the HPS solution (1% content) with different MS at 60°C, 65°C, and 95°C, respectively, and then the light transmittance (T) above 500 nm of the sample solution was measured by UV-vis spectrophotometer (Lambda 950, Perkin Elmer, USA), as a basis for evaluation of solubility effect.

Preparation of HPS film
The HPS film was prepared by a tape-casting method. The pre-prepared 40% HPS emulsion was diluted to 3% with deionized water and heated to 65°C for 1 h under mechanical stirring. Then, the paste was cast onto a polyester plastic plate and evaporating water under the natural state, followed by demoulding and stored in a desiccator (65 ± 2% RH, 20 ± 1°C) before the measurement.

FTIR analysis
The water-soluble HPS emulsion was dried and ground into powder, then mixed with KBr and pressed into a thin slice, which was immediately tested using a FTIR spectrometer (Spotlight 400, Parkin Elgen, UK) in the wavelength range from 4000 to 500 cm -1 .

H NMR analysis
The 1 H NMR spectra measurement of the sample was carried out with an NMR spectrometer (Bruker AV III, 400 M, Germany). The deuterated dimethylsulfoxide (DMSO-d 6 ) was used as the solvent, and the trimethylsilane (TMS) was used as the internal standard.

XRD analysis
XRD instrument (Dmax-rapid II, Japan) with Cu-Ka radiation at a voltage of 30 kV and current of 15 mA was used to test the crystallinity of the sample. The scanning speed of 10°/min, the step width of 0.02°, and the scanning range of 5 * 30°. The crystallinity (X c ) of the samples was calculated by the following equation: where A c and A a are the integrated areas of all crystalline peaks and the amorphous halo, respectively.

Morphological analysis
The freeze-dried HPS samples were sprayed gold, and the granule surface morphology was observed by scanning electron microscope (SEM) (Quanta-450-PEG, PEI, USA). The morphology of water-soluble HPS solution was observed by a transmission electron microscopy (TEM) (JEM-F200, Japan). The sample solution was dripped onto the copper grid and airdried, followed by staining it with phosphotungstic acid (2%, w/w) for 3 min and observed its morphology.

DSC analysis
The gelatinization behavior of the sample was analyzed using a DSC analyzer (STA449 F3, Netzsch, Germany). About 30 lL starch suspension (30%, w/w) was used for each test under N 2 atmosphere, heated in a 40 lL Al 2 O 3 crucible to 20-80°C with the rate of 10°C/min.

Mechanical properties of film
The mechanical properties of the HPS films were measured on a HD021N electronic single yarn strength tester (Nantong Hongda Instrument Co., Ltd, China) at a speed of 50 mm/min according to ASTM/D2256-2015. Films of 120 mm 9 5 mm (analyzed area = 100 mm 9 5 mm) were used to test the elongation at break. The flex resistance was estimated according to the previous literature . The data reported were averages of 10 measurements. The average thickness of films was 0.06 mm.

Results and discussion
Fabrication of water-soluble HPS and its corresponding morphology The water-soluble HPS is obtained by a two-step method as illustrated in Fig. 1a. Firstly, the corn starch was pretreated with acid hydrolysis to reduce the molecular weight of starch macromolecules, then the acidified starch was reacted with epoxy propane to form HPS under alkaline conditions. Figure 1b shows the FTIR spectra of raw starch and HPS with different amounts of propylene oxide (PO). For the spectrum of raw starch, the peaks at 3400, 2930, 1425, and 850 cm -1 are attributed to O-H, C-H, -CH 2 stretching vibration and C-H anti-stretching vibration, respectively (Hay et al. 2018;Zhang et al. 2013). Moreover, the absorption peak at 1024 cm -1 is belonged to the characteristic peak of a-1,4 and a-1,6 glucosidic bond, as well as six-member heterocyclic containing carbon and oxygen (C-O-C) (Woggum et al. 2015). Compared with the FTIR spectrum of raw starch, the intensities of the peaks for the C-O-C (1024 cm -1 ), C-H (2930 cm -1 ), and -CH 2 (1425 cm -1 ) groups in HPS structure increase slightly because the introduction of epoxy propane can provide abundant C-O-C, C-H and -CH 2 groups (Yao et al. 2019). From the result of FTIR, the hydroxypropyl group is successfully conjugated to the backbone of starch. To further explore the bond position and reaction rule of hydroxypropyl on the active hydroxyl group from glucose residue of starch, the 1 H NMR spectra of the raw starch and HPS with different amounts of PO are characterized as shown in Fig. 1c. It can be seen that the signal peaks at d = 3.2-5.7 ppm are ascribed to the resonance protons in the glucose residue of starch, and the methylene and methine non-anomeric proton peaks in starch can be observed at d = 3.2-4.1 ppm (Zhao et al. 2015). Meanwhile, the signal peaks at d = 5.67, 5.52, and 5.40 ppm are belonged to C 2 , C 3 , and C 6 protons of residual hydroxyl in the glucose residue of starch, respectively (Zhao et al. 2015;Xu and Seib 1997). In comparison with the raw starch, in addition to the characteristic signal proton peaks of starch are occurred on the 1H NMR spectra of HPS, the new doublet signal peak at d = 1.18 ppm assigned to the terminal methyl of the hydroxypropyl groups is observed, especially the proton peak at d = 1.18 ppm is more prominent with the increase of PO, indicating that hydroxypropyl is introduced onto the starch backbone (Zhao et al. 2015). Moreover, it can be seen that the integral area for the signal at d = 5.67 ppm assigned to the C 2 proton of residual hydroxyl in HPS is lower than that of the raw starch, followed by the C 6 (d = 5.40 ppm) and C 3 (d = 5.52 ppm) protons of residual hydroxyl, respectively. This is caused by the C 2 hydroxyl exhibits a relatively high acidity due to its proximity to the anomeric centre (Tuschhoff 1986). It means that hydroxypropylation mainly occurred on the hydroxyl group of C 2 position in the glucose residue of starch, and the substitution level at C 6 is slightly higher than that at C 3 . Therefore, the reaction activity of hydroxypropyl on the -OH from glucose residue of starch follows the rule: C 2 [ C 6 [ C 3 . Zhao et al. also found a similar rule in the study on the substitution level and position in cross-linked and hydroxypropylated sweet potato starches (Zhao et al. 2015).
The MS for HPS was calculated by UV-vis spectrum as shown in Fig. 2. Figure 2a shows the UV-vis spectra of propylene glycol with different concentrations (10 * 50 lg/mL). It can be seen that the absorbance of propylene glycol increase with the increase of its concentration, and its concentration and absorbance exhibit a good linear correlation, the correlation coefficient is 0.967 (Fig. 2b). Figure 2c shows the UV-vis spectra of HPS with different amounts of PO. For the HPS samples, a higher PO content leads to greater absorbance. According to the standard curve of propylene glycol, the MS results and the reaction efficiency (RE) of HPS with the different molar ratio of PO to glucose residue of starch are obtained, as shown in Fig. 2d. It is obvious that the MS of HPS increases from 0.053 to 0.170 when the molar ratio of PO to glucose residue of starch varies from 3:1 to 7:1, which means that the MS of synthesized HPS can be easily adjusted by changing the addition amount of PO. But the RE of hydroxypropyl gradually decreases from 50 to 22% with the increasing of PO content in the glucose residue of starch. This may be attributed to the concentration of reagents participating in the reaction system increases with the increasing of PO, leading to greater contact probability between PO and -OH of starch glucose ring, and the MS is increased. But as the substitution degree of hydroxyl groups on the glucose residue of starch increases, the bonding space of hydroxypropyl group on the structure of starch becomes smaller, resulting in the decrease of RE with the increasing of PO. Figure 3 shows the SEM surface morphology of raw starch, acid hydrolyzed starch, HPS MS = 0.053 , HPS MS = 0.135 , and HPS MS = 0.170 granules. The raw starch granule exhibits a smooth appearance (Fig. 3a). However, for acid hydrolyzed starch granule, the obvious expansion and groove can be observed on its surface (Fig. 3b). Furthermore, it can be seen that there are channels in the acid hydrolyzed starch granule from the enlarged view, which provides reactivity for the PO to penetrate into starch chains within the granule. For HPS granules, their overall structure are loose, especially in HPS MS=0.170 granule with high MS (Fig. 3c-e). This will be beneficial to the directional adsorption and diffusion of water molecules in the starch microstructure.
Gelatinization temperature and solution stability of water-soluble HPS The gelatinization temperature of water-soluble HPS was investigated by XRD, DSC, TEM, and UV-Vis. Figure 4a shows the XRD patterns of raw starch and HPS. For both the raw starch and HPS, the four obvious diffraction peaks at 2h = 15.0°, 17.0°, 17.9°, and 22.9°are presented, but the intensities of crystalline peak for HPS with different MS were lower than that of raw starch, indicating that the original crystalline structure of starch had been disrupted after hydroxypropyl modification, and the crystallinity of raw starch and HPS MS=0.170 was 54.4% and 29.5%, respectively (The graphic of the determination of crystallinity is described in Fig. S1). The reduction of crystallinity could further improve the adsorption ability of HPS to water molecules . Figure 4b shows the DSC thermograms of the raw starch and HPS samples, with associated gelatinization parameters shown in Table 1. Compared with the DSC thermogram of raw starch, the gelatinization onset (T 0 ), peak gelatinization temperature (T p ), and conclusion temperatures (T c ), as well as enthalpy change (DH) of HPS samples decreased with MS increased from 0.053 to 0.170. This meant that hydroxypropyl modification could reduce the gelatinization temperature of starch. In this regard, the introduction of hydroxypropyl groups could weaken the hydrogen bond in the helix structure of starch molecular chain, and make it easier for starch granules to undergo swelling and melting of crystallites, thus reducing the gelatinization temperature and enthalpy change (Woggum et al. 2015). Based on the fact, the solubility and light transmittance of HPS with MS of 0.053, 0.135, and 0.170 at the 60°C, 65°C, and 95°C were evaluated, respectively, as shown in Fig. 4d. The solubility and light transmittance of HPS with different MS increased with the increase of temperature. Especially, both the solubility and transmittance of HPS MS=0.135 and HPS MS=0.170 were more than 90% and 55% at 65°C, respectively, showing a similar dissolution state as that of 95°C. This result showed HPS with good water solubility at 65°C could be obtained by adjusting the molar ratio of epoxy propane to glucose residue of starch was not less than 5:1 and as  (Fig. 4c), the dissolution state for HPS MS=0.170 dissolved at 65°C was similar to that of at 95°C, and both of them showed uniform spherical morphology, which further confirmed that the HPS obtained in this work can be completely dissolved at 65°C.
To explore the stability of the medium temperature water-soluble HPS, both the storage and dilution stability of HPS MS=0.170 dissolved at 65°C and 95°C were further analyzed, respectively, as shown in Fig. 4e. There were no obvious differences in the solution appearance for HPS MS=0.170 dissolved at 65°C and 95°C, exhibiting a uniform distribution and translucent state, as well as no sediment and layering phenomenon for the water-soluble HPS MS=0.170 (dissolved at 65°C) at concentration of 10% and diluted to 1% was observed on the bottom of the bottle after standing for 48 h, indicating that the obtained medium temperature water-soluble HPS had high storage stability and dilution stability.
Mechanical properties of water-soluble HPS film Figure 5a illustrates the relationship between MS of HPS films (obtained by dissolving at 65°C) and the tensile strength, elongation at break and flex resistance under 65% relative humidity. With an increase of MS, the tensile strength of HPS films decreased, while both the elongation at break and flex resistance increased, which meant that high MS of hydroxypropyl could improve the plasticity of starch films. This could be explained by the inter-and intra-molecular hydrogen bonding interactions of starch was attenuated when the hydroxypropyl group was introduced onto starch backbone (Xia et al. 2018), and the stable non-polar alkyl chain with flexible characteristic on starch structure, resulting in dramatically flexibility effect. Moreover, benefiting from the introduction of hydrophilic ether bonds on starch structure, the more water molecules could be combined onto the HPS films compared with that of the raw starch film under 65% relative humidity, which led to obvious plasticizing effect (Zhang et al. 2018). The appearance photographs in Fig. 5b show the raw starch film was incomplete clearly, while HPS with MS greater than 0.1 exhibited complete and continuous appearance from macroscopic view, demonstrating the high flexibility of HPS films.

Conclusion
A series of HPS with the total molar substitution (MS) values of 0.053 * 0.170 was obtained via an etherification of acidified corn starch with epoxy propane for introducing hydroxypropyl onto the backbones of starch. The water-soluble HPS at 65°C with solubility of over 90% and light transmittance of above 55% could be obtained when the molar ratio of epoxy propane to glucose residue of starch was not less than 5:1 (ie. MS of HPS was greater than 0.1). The morphology of starch granule changed obviously, and its crystallinity, gelatinization temperatures (T 0 , T p , and T c ), and enthalpy change (DH) decreased significantly after hydroxypropylation, which could promote the adsorption and transmission of water molecules in the starch microstructure. Moreover, the obtained water-soluble HPS at 65°C exhibited high storage and dilution stability, and presented a similar dissolution state with that at 95°C. Interestingly, hydroxypropylation was conducive to good breaking