Abstract:

Objective It is well known that natural starch exhibits insufficient adhesion to fibers and strong film brittleness, due to numerous hydroxyls and cyclic structure on its chains. As concluded, the alkyl carboxylate monoester groups introduced onto starch chains by chemical esterification can improve the properties of starch. The groups include acetate, propionate, butyrate, and caproate groups. However, the effect of the alkyl chain length of these alkyl carboxylic acid monoester substituents on the properties of starch has not been explained. Therefore, the main purpose of this study is to ascertain the effect of the alkyl chain length of these alkyl carboxylic acid monoester substituents on the properties, in order to identify the starch alkyl carboxylic acid monoester with the best sizing properties for polyester warp yarns.

Method To ascertain the effect of the alkyl chain length of these alkyl carboxylic acid monoester substituents on the sizing properties of starch, four types of starch alkyl carboxylic acid monoester samples with a similar degree of substitution of 0.018 were prepared by the esterification of acid-converted starch (ACS) with vinyl acetate, propionic anhydride, butyric anhydride, and caproic anhydride as esterifying agents, respectively, under the weakly alkaline and low-temperature conditions, in aqueous medium. The surface morphology of their granules was observed and analyzed by a scanning electron microscope, and the modification levels of starch alkyl carboxylic acid monoester samples were determined by titration analysis. The effects of the alkyl chain length of these alkyl carboxylic acid monoester substituents on the adhesion to polyester fibers, film properties, and apparent viscosity were investigated, and the adhesion was investigated by a standard method (FZ/T 15001—2008) in China via measuring the bonding force of slightly sized polyester roving. The measurement of film properties mainly contained tensile strength, breaking elongation, and bending endurance. The apparent viscosity was determined on an NDJ-79 rotary viscometer at a shear rate of 1 850 s^-1.

Results SEM results revealed that the starch samples were still in a granular state, but some damages were produced on granular surfaces, which was mainly attributed to the modifications occurred under alkaline conditions and at the free hydroxyls on the granular surfaces. The starch alkyl carboxylic acid monoester samples with an esterification level of about 0.018 were prepared, by the esterification of ACS with the 9 g of vinyl acetate, propionic anhydride, butyric anhydride, and caproic anhydride, respectively. The adhesion of the samples to polyester fibers and size film elongation and endurance were found higher than that of ACS. The introduction of alkyl carboxylic acid monoester substituents on the starch chains played an internal plasticization on the starch adhesive layers and films, promoting the bonding force to polyester roving and film elongation and endurance. Increasing the alkyl chain length of the alkyl carboxylic acid monoester substituents produces gradually increased bonding force, and film elongation and endurance. The tensile strength of the size films was reduced. The esterification modification with a low degree of substitution had no obvious effect on the apparent viscosity of the starch paste, making it still have good flowability and beneficial for the adhesion between starch and fibers.

Conclusion This study concluded that the esterification could improve the adhesion and film properties of starch, thereby favoring the promotion of its sizing quality to warp yarns. The alkyl chain length of the alkyl carboxylic acid monoester substituents showed an important correlation with the properties, and along with the rise in the length, the adhesion, and film elongation and endurance were gradually increased. It is identified that the use of caproate starch(CS) in polyester warp sizing leads to best sizing properties.

Key words: starch, sizing agent, alkyl carboxylic acid monoesterification, degree of substitution, sizing property, polyester yarn