Because the urea-formaldehyde resin has the initial viscosity, shrinkage, brittleness, non-water resistance, easy aging, release formaldehyde and curing the environment, damage to health and other shortcomings, it must be modified, improve performance, expand application. The modification method is simply listed as follows:
1, improve the initial viscosity
Improve the initial viscosity of urea-formaldehyde resin, can add polyvinyl alcohol, polyethylene glycol, hydroxyethyl cellulose and other modifier, but these substances are higher prices, so can choose starch substances, especially starch in urea-formaldehyde resin synthesis began to join, the effect is better. In the process of the synthesis of starch can occur hydrolysis, the formation of a variety of paste, such as the relative molecular weight of starch is very large, dissolved after the viscosity is also large, adding a small amount can be made with a larger viscosity of urea-formaldehyde resin. At the same time, the synthesis reaction of hydroxyl, hydroxyl methyl and aldehyde-based on hydrolysis of starch molecule, etc. may be involved in urea-formaldehyde resin, not only improve the initial viscosity, but also improve the bonding strength and storage stability.
2, reduce shrinkage
The urea-formaldehyde resin curing shrinkage rate is large, easy to produce cracks, the adhesive layer produces internal stress, so that the viscosity strength decreases. In order to reduce the shrinkage rate of urea-formaldehyde resin curing, some fillers are usually added to the resin glue, such as flour, starch, blood powder and a-cellulose powder, wood powder, soybean flour etc.
3, reduce brittleness
In order to reduce brittleness, improve toughness can be added to polyvinyl alcohol, polyvinyl alcohol formaldehyde solution, vinyl acetate emulsion, VAE solution, but also can improve the initial viscosity and anti-aging.