Partially hydrolyzed polyvinyl alcohol for sizing and desizing

Partially hydrolyzed grades of polyvinyl alcohol are referred to as PA grades and show significant advantages in sizing and weaving as well as in dyeing and finishing processes, exceeding the full hydrolysis level (FH grade). In the sizing workshop, the PA grade is more soluble in the cooking pot at lower temperatures, and the sizing yarn breaks more easily at the dispensing rod. During dyeing and finishing, PA grades can be desized at a lower temperature than FH grades, even after heat setting, and can be recovered as other PVAs. In the weaving workshop, the PA grade has good adhesion to polyester, which helps to improve the abrasion resistance of sizing. In summary, the PA class can save costs through low energy consumption during sizing and desizing, and less breakout during weaving.

PVA production does not begin with vinyl alcohol monomers, which can only be polymerized from vinyl acetate (VAM) monomers due to the absence of vinyl alcohol monomers. By varying the reaction conditions to control the degree of polymerization, the chain length of the polymer is controlled, ie, the molecular weight of polyvinyl acetate is changed. The higher the molecular weight of polyvinyl acetate, the higher the molecular weight (and solution viscosity) of the final PVA. Then polyvinyl acetate is saponified to make polyvinyl alcohol. This step is called alcoholysis. If the alcoholysis reaction is complete, fully hydrolyzed PVA can be obtained; conversely, if the reaction is carefully controlled, partially hydrolyzed PVA can be obtained. Percent hydrolysis is the percentage of acetic acid groups that are replaced by hydroxyl groups.

Changing the degree of hydrolysis can lead to changes in the properties of PVA. The 98% hydrolyzed fully hydrolyzed grade (FH grade) is solvent resistant, fast, and has good adhesion to hydrophilic materials such as cotton and paper. The 88% hydrolyzed partially hydrolyzed grade (PA grade) has poorer water resistance than the fully hydrolyzed grade and has good adhesion to both hydrophilic and hydrophobic materials such as polyester. In fully hydrolyzed grades of PVA, the hydroxyl groups of one molecule readily form hydrogen bonds with the other molecule's hydroxyl group, resulting in a strong, crystalline polymer chain. The PA grade contains loose acetate groups and is not easily hydrogen-bonded, so it is weak and has poor water resistance.