The Urea-formaldehyde Resin Adhesive Modified By Hexamethylol Melamine

Urea-formaldehyde resin was a kind of early-developed thermosetting resin, and had some advantages like colourless, high bonding strength and reasonable price. But low resistance to water, poor durability and the release of formaldehyde, which will do harm to human health, limiting the wider use of UF.Melamine is the most commonly used modifier to improve the performance of low mole ratio of UF adhesive. Hexamethylol melamine is the abbreviated form of HMM.HMM is made from formaldehyde and melamine, it becomes more and more popular in recent years because of the rich raw materials, simple production process, easiness to use.In this paper, HMM was used as a modifier of UF resin. HMM has six hydroxymethyl groups, the hydroxymethyl groups can be used to react with some functional groups in UF like hydroxymethyl and amino groups to reduce the formaldehyde emission and keep good bonding strength in the woodbased panel at the same time.In this paper, the synthesis and characterization of HMM was studied. The influence of additional method and amount of HMM for the modified UF resins were investigated in detail. HMM were characterized by using Fourier transform infrared spectroscopy(FTIR)and Nuclear magnetic resonance spectroscopy. Structures and properties of the modified resins were characterized by using Fourier transform infrared spectroscopy(FTIR)and X-ray diffractometer was used to explore the relation between crystalline and bonding strength. In addition Differential scanning calorimetry(DSC) was used to study the curing temperatures of the modified UF resins.The main conclusions of this paper are as follows:(1)Comparing UF resin separately modified by HMM and melamine with unmodified UF resin, our studies showed that UF resin modified by HMM is much better than the others.(2)Compared with the unmodified UF resin(0.220%), the addition of HMM had good performance(0.123%) in reducing free formaldehyde. The formaldehyde emission was reduced from 2.70mg/L of unmodified resin to 1.02 mg/L of modified resin.(3)The optimal process was when the molar ratio of formaldehyde and urea was 1.1,the addition of HMM was 6% and the HMM was added in the second stage. The free formaldehyde content was 0.154%, the formaldehyde emission was 1.08mg/L, reaching E1 level. The bonding strength of the products are guaranteed to reach the 1.10 MPa, curing temperature was 121.3℃, solid content was 59.04%.(4)Comparing the structure of the modified resin with the unmodified resin, we can conclude that HMM was introduced into the structure of UF resin, it made the structure more complicated, then the bonding strength was increased to some extent. The addition of HMM made the curing temperature increased.