Study On Preparation And Properties Of Urea-Formaldehyde Resin Modified By Composite Modifier Of Nano-silica And Oxidized Starch

Urea formaldehyde?UF?resin adhesive has become one of the largest demand in adhesive market and been widely used in wood processing industry due to its strong adhesion,low price,simple manufacturing process,easy to use and the colorless of glue line and other characteristics.However,the most prominent drawback of urea formaldehyde resin is the high content of free formaldehyde,which can heavily pollute environment and harm people's health.Therefore,in this paper,oxidized starch and nano-silica were used to do some composite modification on the urea formaldehyde resin,in order to reduce the free formaldehyde content and improve the comprehensive performance of the resin.Firstly,the oxidation of corn starch and the modification of nano-SiO₂ were studied.The corn starch was oxidized by hydrogen peroxide with CuSO4·5H2O as the catalyst.By changing the amount of hydrogen peroxide,reaction temperature,catalyst concentration,reaction time,pH value,ratio of water and starch,the optimal reaction conditions was determined: hydrogen peroxide dosage is 12%,reaction time is 4 h,reaction temperature is 50?,pH=9,ratio of starch is 40%.The nano-silica was modified with silane coupling agent KH-550,results of fourier transform infrared spectroscopy?FT-IR?and transmission electron microscopy?TEM?showed that the reaction between KH-550 and nano-Si O₂ has been occurred and greatly reduced the agglomeration of nano-Si O₂.Firstly,nano-SiO₂ was used as a modifier to modify UF resin,the free formaldehyde content and bonding strength of modified UF resin were discussed.The results showed that the addition of nano-SiO₂ can effectively reduce the free formaldehyde content and improve the bonding strength.However,but if excessive amount,the overall performance of the resin will become worse which is manifested in the increase of free formaldehyde content and the decrease of bonding strength.Meanwhile,the resin viscosity will become too large,which is not conducive to resin sizing.So the amount of nano-SiO₂ was determined to be 10 % of urea mass.Free formaldehyde content of UF resin modified at this condition is 0.25 %,curing time is 73 s,and the bonding strength is 1.56 MPa.Then,the composite modifier composed by oxidized starch and modified nano-silica was used to modify urea formaldehyde resin.The results showed that the free formaldehyde content and adhesive properties of urea formaldehyde resin was significantly influenced by the addition of oxidized starch and modified nano-silica when the ratio of formaldehyde and urea?F/U?is 1.2.The optimum process was as follows: the amount of the modifier was fixed as 10% of the total mass of urea and formaldehyde,and the mass ratio of oxidized starch and modified nanoSiO₂ was 2:3.With this condition,the viscosity of urea formaldehyde resin is moderate,the solid content is 55.95 %,the free formaldehyde content was reduced from 0.49 % to 0.19 %;at the same time,the bonding strength of modified UF resin increased to 1.59 MPa,compared to the ordinary UF resin?bonding strength is 0.9 MPa?,the bonding strength has greatly improved.The overall performance of the UF resin modified by composite modifier is better than the resin only modified by nano-SiO₂,more important,this kind of modified resin fits the environmental requirements of the present society.