Thermomechanical properties of poly(vinyl alcohol) prepared at room temperature as a function of degree of hydrolysis and aluminium addition

Poly(vinyl alcohol) (PVOH) is widely used in various industrial applications. However, its water solubility reduces film performance, which is important in PVAc-based wood adhesives. In this sector, AlCl3 is often used as a complexing agent for PVOH. In this work, the effect of heating and aluminium addition on the structure and viscoelastic properties of PVOH with different degrees of hydrolysis (DH), 88%, 92% and 99%, was studied. Products treated with either HCl or acetic acid were also considered. The polymer properties were evaluated by infrared spectroscopy and dynamic mechanical analysis, and the mass loss of the polymers on heating was also evaluated to explain the observed differences. The measurements showed that the peaks of the tanδ curves occurred at slightly lower temperatures as the DH increased, although higher crystallinity was observed at the end of the DMA tests for higher DHs. This is due to the increasing affinity of highly hydrolysed PVOH for water molecules. However, as the temperature increased, the storage modulus was higher for higher DHs due to the better mutual interaction of OH groups that develops during the tests in highly hydrolysed polymers. The addition of aluminium caused a slight increase in Tg, the magnitude of which was inversely proportional to the DH. However, as the temperature increased, the storage modulus decreased more at lower DHs than at higher DHs. This effect was related to a combination of factors: the lower presence of crystallites in products with lower DH and the formation of conjugated polyenes, which was also more pronounced in products with lower DH. Both mechanisms were due to the action of aluminium. Thus, both heat and the addition of aluminium had a determining effect on the properties of PVOH films, although the degree of importance depended on the degree of hydrolysis of the polymer.