The influence of reaction conditions on structural properties and swelling kinetics of polyurethane hydrogels intended for agricultural purposes

Polyurethane hydrogels were prepared in open and closed system, in reaction of polyethylene glycol (PEG) with different molar mass (2000, 6000 and 10,000 g/mol) and poly[(phenyl isocyanate)-co-formaldehyde] (PPIF) in order to investigate the influence of different synthesis conditions on the hydrogel structure and consequently swelling properties. The PEG molar mass was varied in order to investigate the correlation between PEG chain length, hydrogel hydrophilicity and absorption capacity. Scanning electron microscopy (SEM) and porosity studies have shown that hydrogels prepared in open system have cellular, sponge-like structure, while preparation in closed system have resulted in hydrogels with compact structure. Morphological differences were reflected on swelling kinetics and compressive properties. Swelling ratio in distilled water increases with increasing in PEG molar mass in both series, but swelling behavior of hydrogels synthesized in open system follows second-order kinetics and in closed system first-order kinetics, slower response in first 180 min, but greater capacity to retain water. Compressive modulus increases with increasing in PEG molar mass and has greater values for hydrogels synthesized in closed system. Granulated PU-PEG hydrogels have demonstrated ability to retain and deliver water in soil and contribute to tomatoes growth. Addition of hydrogel in 1 wt% in soil has resulted in higher and more developed plant. Applying the procedure of preparation in open and closed system using the same compounds give hydrogels with a different morphology, swelling and mechanical properties, which implies different application. However, comminution of both type of hydrogels to the level under the pore size, when the morphological differences disappears, give the granules (diameter around 2 mm) with the same swelling properties and potential application in agriculture.