The poor aging resistance of urea formaldehyde resin adhesive limits the service life of wood bonding products and restricts its scope of use. The significance of improving the aging resistance of urea formaldehyde resin adhesives is very important.
The aging phenomenon of urea formaldehyde resin adhesive is manifested in cracking of the adhesive layer, separation from the adhered wood, and aging cracking. The reason for this is that after curing, there are still hydrophilic hydroxymethyl groups remaining in the resin. These hydrophilic groups repeatedly adsorb and remove water, causing the adhesive layer to be subjected to repeated dry and wet conditions, resulting in continuous changes in internal stress and the formation of stress cracks, ultimately leading to cracking; Furthermore, due to the high humidity and acidic medium in the adhesive layer, it undergoes hydrolysis and breaks the methylene bond, leading to the delamination of the adhesive layer. In addition, the deterioration of usage conditions and the erosion of moisture also accelerate the aging of the adhesive layer.
Method/Steps
1.In response to the above situation, adding toughening agents with waterproof properties to urea formaldehyde resin is an effective measure to improve its aging resistance. Such antioxidants commonly used are polyvinyl alcohol, polyvinyl acetate lotion, phthalic acid, esters and various synthetic lotion.
The modification effect of polyvinyl alcohol formaldehyde is mainly achieved by participating in the synthesis reaction of urea formaldehyde resin, embedding the molecular chains of polyvinyl alcohol formaldehyde with good flexibility and linearity into the macromolecular chains of urea formaldehyde resin, thereby improving the flexibility of the molecular chains, enhancing the brittleness of the urea formaldehyde resin adhesive layer, allowing it to expand and contract with stress changes within a certain range, reducing the influence of internal stress on the adhesive layer, and improving its aging resistance.
2.Urea formaldehyde resin is modified by adding vinyl acetate lotion, which also has the similar principle and better effect. And with different amounts of addition, the effect also varies. Increasing the amount of addition results in better effects and improved adhesive strength. However, due to cost reasons, the amount added should be appropriate.
The addition of butanol modified urea formaldehyde resin adhesive is due to the formation of ether bonds between some hydroxymethyl groups and alcohols under acidic conditions, which reduces reaction activity. After the resin solidifies, cross-linking no longer occurs, improving its aging resistance. In addition, after etherification of hydroxymethyl, the hydrophilicity changes to hydrophobicity, making the resin less prone to hydrolysis, which is beneficial for improving aging resistance and storage stability. To reduce costs, fusel oil is now also used for modification.
3.At present, the improvement of aging properties of urea formaldehyde resin is a research and development hotspot, such as the use of p-toluenesulfonylamide, EVA starch composites, etc., which have good effects
