What factors affect the stability of water-soluble resins?
The structure of water-soluble resin, introduced groups, molecular weight, molecular weight distribution, neutralizer and cosolvent used are all factors that can affect the stability of water-soluble resin.
1. The influence of hydrophilic groups
(1) Effect of ether group: Take straight-chain polyester resin as an example, even if the esterification degree is basically the same, its water solubility is not the same. The more ether groups, the better water solubility.
(2) The influence of high-functionality alcohols: Taking pentaerythritol and dimeric pentaerythritol as examples, different water-soluble polyester resins can be prepared by properly replacing the polyols in the above-mentioned polyester resins with pentaerythritol.
(3) The influence of high-functionality acid: As high-functionality acid, dehydrated tribenzoic acid and pyromellitic anhydride are used together with glyoxalic acid respectively, and the dosage of each acid is adjusted accordingly to keep the same total functionality as that of the above-mentioned polyester. It will eventually produce different water-soluble resins.
2. Effect of Molecular Weight
The molecular weight of synthetic resin has a great influence on its water solubility. Compared with the same type of resin, the resin with smaller molecular weight has better water solubility, but the corrosion resistance of the coating is poor; the resin with larger molecular weight has better corrosion resistance, but its water solubility is poor. Therefore, on the premise of guaranteeing the water-solubility of the resin, the molecular weight of the resin should be as large as possible to prepare a film with better performance.
The narrower the molecular weight distribution of synthetic resin, the worse the water solubility, but the better the performance of the coating. Especially for the paint constructed by electrodeposition method, the narrower the molecular weight distribution is, the better, because the different size molecules show different electrodeposition effects under the action of electric field. When the molecular weight distribution is wide, because the mutual solubility effect between molecules is often conducive to the improvement of water solubility, but it is often not easy to get a good performance of paint film and stability.
3. The influence of neutralizer
Different kinds of neutralizer can obviously affect the water solubility of resin, storage stability of resin solution, viscosity, curing rate and yellowing of coating film. Therefore, it is very important to select neutralizer properly. The neutralizer used should also be different for different types of resins.
4. The influence of cosolvents
In order to improve the water solubility of resin, adjust the viscosity of water-soluble resin solution and the leveling of coating film, some water-soluble resin must be added a small amount of cosolvents, such as lower alcohols and alcohol ethers. The choice of cosolvent also needs to consider the properties of neutralizer amines used. Generally, the solution obtained by using sec-butanol as cosolvent has low viscosity and good stability. Correct selection of co-solvents, or two kinds of co-solvents and increasing the amount of co-solvents are effective to overcome the phenomenon of abnormal viscosity thickening in dilution process. However, for electrodeposition coatings of water-soluble paint, often adding more cosolvents, the effect of electrodeposition is not good.