Objectives The polyurethane, synthesized directly from the purified recycled cooking oils, is weak in hot and humid resistance, and not qualified for fertilizers coating materials. In this paper, a silane coupling agent γ-aminopropyl triethoxy silane (KH550) was introduced in the process of polyurethane synthesis, and the availability of the modified coating materials for making coated fertilizer was tested.

Methods The oil used was collected from the restaurants in Guiyang. The oil was firstly purified with phosphorous acid, distilled water, NaOH and NaCl in turn. Then the oil was pre-modified by peracetic acid and ethylene glycol. The pre-modified oil was mixed in different ratios with KH550 to form mixtures containing KH550 of 0, 10%, 20% and 30%. Urea was coated with the above PUs in rate of 3% of urea weight to make slow-release fertilizers. The nitrogen release period of the coated fertilizers was determined using modified Oertli method.

Results  1) The recycled oil was achieved when the recovered oil was treated with multi-step purification, and it maintained the molecular structure of carbon-carbon double bonds groups. 2) After pre-modification of recycled oil, it supplied the hydroxyl groups for the synthesis of polyurethane. 3) The experimental results showed that KH550 was successfully introduced and amended PUs coating material was synthesized. 4) Compared with the unmodified PUs, the hydrophobicity of modified PUs was increased by 48%–74%; the coating porosity was reduced by 31%–54%; the thermal stability was increased by 3%–11% at T_5%, 6%–32% at T_50%, and 4%–11% at T_80%. As a result, the N cumulative release rate of modified PCUs was significantly reduced. The N cumulative release of modified PCUs was less than 70% on the 30 th day, which was considerably lower than the rate of unmodified PCUs (90%). 5) When the content of KH550 was 20%, the water contact angle was the biggest (113.31^o), and the porosity was the lowest (10.91%). The comprehensive performance of PCUs was the best.

Conclusions A novel bio-based polyurethanes has been synthesized by pre-modified recycled oil and the introduce of silane coupling agent in the process of polyurethane synthesis. Using the modified polyurethanes containing 20% of KH550 as coating material, cumulative N release is less than 70% on the 30 th day, proving the prospect of the practical use of the modified polyurethanes.