Performance And Preparation Of Low Toxicity Urea-formaldehyde Resin Modified By Modified Starch

With the development of society and technology, more and more attention has been paid on the development and application of environment-friendly adhesives. Urea-formaldehyde (UF) resin is commonly used in wood adhesive industry because of its low cost and good performance, but it will release free formaldehyde that does great harm to human healthy. Currently, how to reduce free formaldehyde emission of UF, increase bonding strength, and optimize the preparation-process are the key issue of UF resin in research and application.Starch is a natural renewable polymer, which has a wide range of sources, low cost, non-toxic, no odor and other characteristics. Mixed with water and heated, the transition of starch occurs, and can be used as adhesive. In this paper, modified starch was adopted as modifier. The effect of modified starch addition on performances and properties of UF resin was investigated. Relative details are discussed below.On basis of fixed molar ratio of urea to formaldehyde, starch was used to optimize the synthesis of UF resin and reveal the preliminary conditions for production. The starch was added at different stages (prophase, metaphase and anaphase) when the UF resin was prepared. The optimal conditions to obtain best performance were 2% starch addition at anaphase stage. Under this condition, the wet bonding strength of modified UF (MUF) exceeded 0.7MPa, and free formaldehyde emission was reached 0.85mg/L.Corn starch was modified by enzyme, ultrasonic wave, microwave radiation and acid hydrolysis, respectively. The properties of modified starch was characterized by DE value, Fourier transform infrared analysis (FT-IR), X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). Under the optimal conditions discussed above, the modified starch was added in preparation process of UF resin. The results revealed that acid hydrolysis was the best treatment. The optimal conditions of acid hydrolysis were 20%(mass fraction) starch slurry and treat with 1.Omol/L hydrochloric acid. After 2% acid hydrolyzed starch added, the wet bonding strength of MUF reached 1.1 MPa and the free formaldehyde under 0.6mg/L, meeting the requirement of II type and E, plywood of National Standard of P. R. China GB/T 9846-2015 and GB 18580-2001.Different amounts of oxidized starch were added to the MUF resin. Based on the formerly optimized results, the oxidized starch was modified by enzyme, ultrasonic, microwave and acid hydrolysis to obtain the performances of as-prepared MUF. And the best treatment was obtained. It was shown that after 2%oxidized starch was added, the wet bonding strength of MUF reached 0.7MPa and the free formaldehyde reached 0.44mg/L. When adding 2% acid-hydrolyzed and oxidized starch, the bonding strength of MUF was over 0.9 MPa and the formaldehyde emission was 0.48 mg/L, which met the E1 grade standard in P. R. China GB 18580-2001.