Due to the shortcomings of urea-formaldehyde resin, such as poor initial viscosity, large shrinkage, high brittleness, water resistance, easy aging, formaldehyde release and curing, it pollutes the environment and harms health, it must be modified to improve its performance. , expand application. The modification methods are briefly listed as follows:
1. Improve the initial viscosity
To improve the initial viscosity of urea-formaldehyde resin, polyvinyl alcohol, polyethylene glycol, and hydroxymethyl can be added. Cellulose and other modifiers, but these substances are more expensive, so starch-based substances can be used, especially starch added at the beginning of the synthesis of urea-formaldehyde resin, which has better effects. During the synthesis process, starch may be hydrolyzed to produce various dextrins, etc. Since starch has a large relative molecular weight and a high viscosity after dissolution, urea-formaldehyde resin with a high viscosity can be produced by adding a small amount. At the same time, the hydroxyl groups, hydroxymethyl groups and aldehyde groups produced by hydrolysis on the starch molecular chain may participate in the synthesis reaction of urea-formaldehyde resin, which not only improves the initial viscosity, but also improves the bonding strength and storage stability.
2. Reduce shrinkage
Urea-formaldehyde resin has a large curing shrinkage and is prone to cracks. The adhesive layer generates internal stress, which reduces the viscosity strength. In order to reduce the shrinkage of urea-formaldehyde resin during curing, some fillers are usually added to the resin glue, such as flour, starch, blood meal and a-cellulose powder, wood flour, soybean flour, etc.
3. Reduce brittleness
In order to reduce brittleness and improve toughness, polyvinyl alcohol, polyvinyl formal solution, polyvinyl acetate emulsion, VAE solution, etc. can be added. At the same time, the Initial tack and aging resistance.
4. Improve water resistance
When synthesizing urea-formaldehyde resin, adding a small amount of melamine, phenol, resorcinol, alkylamine, furfural, etc. can effectively improve the urea-formaldehyde resin. water resistance. Adding aluminum sulfate, aluminum phosphate, etc. as cross-linking agents can also significantly improve water resistance. Adding fillers such as wood flour, flour, soy flour, iron oxide, and expanded soil during glue mixing can also improve water resistance.
Phenol-modified urea-formaldehyde resin is used, and a benzene ring structure is introduced into the resin to block the water-absorbing group of the resin, significantly improving the water resistance and aging resistance of the resin. Resorcinol or melamine can also be added to the curing agent component, and acrylate polymer emulsion can be mixed with urea-formaldehyde resin to improve water resistance.
5. Improve bonding strength
Using multi-complex additives such as polyvinyl alcohol and phenol can improve brittleness, water resistance and bonding strength. Then a neutral-weak acid-weak alkali composite process is used to react at medium temperature. The shear strength of the urea-formaldehyde resin produced is more than 10 times that of the original urea-formaldehyde resin, and the water resistance and boiling resistance are greatly improved.
According to different usage requirements, the performance of urea-formaldehyde resin can be significantly improved by adding modifiers.