Preparation and properties of nano-SiO₂–polyvinyl alcohol–γ-polyglutamic acid composite film materials

Objectives Polyvinyl alcohol (PVA), as a coating material of slow-released fertilizer, has advantages of low price, good permeability and friendly to environment. However, the water resistance of PVA film is poor. The slow-released effectiveness of fertilizer coated with the PVA does not last long in soil. γ-polyglutamic acid (γ-PGA) is a kind of environmental-friendly fertilizer synergist and easily obtained. In this study, we modified PVA properties by adding nano-SiO₂, γ-PGA and glutaraldehyde, then prepared composite coating films using the modified PVA. The processing parameters were optimized.

Methods The experiment adopted L₉(3³) orthogonal experimental design. The concentrations of PVA (factor A) were 4%(A1), 6%(A2), 8%(A3); the mass ratios of γ-PGA to PVA (factor B) were 0.8∶3(B1), 1∶3(B2), 1.2∶3(B3); the volume fractions of glutaraldehyde in γ-PGA and PVA (factor C) were 0.1%(C1), 0.2%(C2), 0.3%(C3). Meanwhile, nine treatments without cross-linker were conducted as blank control. Organic high molecular polymerization method was used to prepare the composite materials, and the hydrophilism and permeability of composite materials were tested. Then composite films were prepared by adding 5, 10 and 20 g/kg of nano-SiO₂ and anhydrous ethanol. The hydrophilism, permeability, infrared spectrum characteristics and surface microstructure of composite materials were measured.

Results The hydrophilism and permeability of the composite materials were significantly reduced after adding crosslinking agent. Under condition of factor A 4%, B 1.2∶3 and C 0.3%, the hydrophilism of composite materials was the lowest (118%), and the NH₄⁺ and water permeability were reduced by 46.8% and 23.0%, compared with the control. With increasing of nano-SiO₂ addition, the hydrophilism of each treatment showed an up-and-down trend. When adding 20 g/kg of nano-SiO₂, the hydrophilism and NH₄⁺ permeability of composite films were the lowest, which were decreased by 6.8%–38.2% and 23.8%–53.2%, respectively, compared with no nano-SiO₂ treatment. However, the water permeability was increased by 38.4%–67.7%. The results of infrared spectroscopy indicated that new ether bonds between PVA and γ-PGA were generated during the reaction. With nano-SiO₂ addition, the stretching vibration peak of―OH was widened, the transmittance was increased, new rocking vibration of Si ―O―Si and antisymmetric stretching vibration of Si―O―Si appeared. Additionally, unreacted γ-PGA still existed in composite films according to the functional groups characteristics. The surface of the nano-SiO₂–PVA–γ-PGA composite film material was getting smoother and more uniform.

Conclusions In the composite film materials, which is prepared by crosslinking nano-SiO₂ with PVA, γ-PGA and glutaraldehyde, the number of hydrophilic groups is significantly decreased, resulting in the decreased water absorbency and improved sustained-release property. Thus, the composite film materials become more suitable for fertilizer coating materials. Besides, the residual γ-PGA could still play the role of fertilizer synergist.