Modification of polyvinyl alcohol with borates

Modification of polyvinyl alcohol (PVA) with borates allows to regulate the main performance indicators of water-soluble films - temperature and time of solubility, strength. The need to increase the water resistance of PVA-based films is associated with specific areas of their application, for example, when creating protective shells in plant growing or eco-friendly disposable tableware, etc. The aim of the work is a comparative assessment of the main performance indicators of PVA films with different molecular weights modified with borates of different nature. Objects of study: film samples of low-hydrolyzed PVA of three different grades (05-88, 17-88, 24-88) having the same degree of hydrolysis (88%), but differing in molecular weight (MW) and, as a consequence, solution viscosity (for PVA 05-88, 17-88, 24-88 comprising 4.5÷6.5, 20.0÷26.0, 44.0÷56.0 mPa*s, respectively), modified with borates in different ratios (sodium tetraborate in an amount of 0.125÷2.0 wt.% and boric acid in an amount of 2.5÷10.0 wt.%). Research methods: water solubility of samples was assessed visually by immersion in water with a temperature of 20°C, water vapor sorption was assessed using the standard static (desiccator) method for determining water vapor sorption isotherms by polymeric materials, strength properties of films in a dry and vapor-saturated state were determined according to GOST 11262-17 using an RM-50 tensile testing machine with Stretch Test software. It was found that the influence of the crosslinker nature on the properties of modified films differs significantly for PVA with different MW. Sodium tetraborate leads to rapid gelation of the PVA solution, which is more intense with an increase in the MW of the polymer, therefore its introduction is limited to 2.0, 1.0 and 0.5 wt.%, respectively, for PVA 05-88, 17-88 and 24-88; when crosslinking with boric acid, the gelling effect is absent. Sodium tetraborate is ineffective in increasing the water resistance of low-MW PVA, although it improves its strength properties; boric acid effectively crosslinks PVA (especially high-MW), which is manifested in a significant increase in the dissolution time of the films. The maximum crosslinking effect, accompanied by a significant increase in water resistance, was observed in the case of modification of medium- and high-molecular PVA with boric acid in an amount of at least 5.0 wt.% of the polymer weight.