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.