A large number of carboxyl groups in the waterborne aminobaking enamel system seriously affects the performance of the paint film, what should be done?
As a water-based industrial paint, water-based baking varnish is widely used nowadays. Some of the factors affecting the water resistance of the paint film can be found in the article paint film water resistance explained! Performance, mechanism, factors, improvement path! See also the article Coatings Q&A (22): how to improve the alcohol resistance of water-based PUD system. Where the resin is mainly water-based acrylic resin, in order to achieve ammonia water dispersion or water soluble, the structure is deliberately designed with a certain amount of carboxyl groups, acid value is usually above 30. This part of the carboxyl group is generally left in the paint film, seriously affecting the film water and alcohol resistance and salt spray resistance and other properties. Here mainly discuss how to eliminate -COOH on the paint film performance of an impact, adding crosslinking additives crosslinking -COOH is the approach we want to take.
Various cross-linking reactions of -COOH are as follows:
1、Adopt oxazoline to cross-link with -COOH
It needs to be heated to react, 80-120 ℃ can be carried out, belongs to the medium temperature crosslinking agent. Oxazoline is one kind of intracyclic imino ether, which can be divided into 2-oxazoline, 3- umazoline and 4-oxazoline according to the position of its carbon and nitrogen double bond, among which the most active one is 2-oxazoline with the structural formula:
Oxazolines are highly reactive and can undergo equiproportionate ring-opening reactions with many nucleophilic reagents. Under appropriate reaction conditions, if the correct solvent is selected, the right temperature, etc., or a suitable catalyst is added, the oxazoline group can react chemically with carboxylic acids, thioacids, chlorinated or chloroformate esters, thiols, phenols, thiophenol alkyl chlorinated amines, alcohols, and amides, among many other groups. Oxazoline groups can react with carboxylic acid groups above 200° in a matter of minutes to form ester amine groups.
Products with this composition are also available on the market, and are used in water-based paints, water-based inks, and water-based adhesives to improve water resistance.
2、Adopting polyaziridine cross-linking-COOH
Aziridine, the nitrogen tricyclic analog of ethylene oxide, and its derivatives have been studied for many years; in some cases, polyaziridine has been used as a cross-linking agent. The common term for aziridine is ethylenimine, in English aziridine. ethylenimine is highly toxic and may be carcinogenic. Aziridine is a skin irritant and some individuals can become sensitized. The teratogenicity of aziridine is debated, but diluting it with paint lacquer reduces its possible toxic effects. Ethyleneimine is even more reactive than ethylene oxide burns and acids. In the presence of stronger acids, it quickly polymerizes to form polyethylenimine-(-CH2CH2NH-)-n.
Of the many reactions of aziridines, the one of most interest for coating applications is the formation of 2-amino ester crosslinks between polyaziridines and polyfunctional acids. Some 2-amino esters can automatically rearrange themselves to 2-hydroxyamides (that’s the special amido-alcohol structure in point 4 below).
A variety of polyaziridines (also known as polyfunctional aziridines) have been studied. One example is the trifunctional Michael addition product of 3 mol aziridine and 1 mol trimethylolpropane triacrylate, with the following structural formula:
Methylaziridine (propylideneimine) has also been used to make this polyaziridine. This polyaziridine can be used to crosslink the carboxylic acid groups of emulsions and waterborne polyurethanes. The reaction between aziridine and carboxylic acid group is much faster than the reaction between aziridine group and water, and can be reacted at room temperature, generally existing now added. Aziridine and water interact and hydrolyze to amino alcohols with an activation period of 48-72 hours. But there is no indication that the hydrolyzed aziridine has a negative effect on the performance of the coating film. Reactivity can be restored by the addition of a cross-linking agent. In view of the potential poisoning hazard, the manufacturer’s recommendations for safe handling should be carefully followed.
3. A special hydroxyl compound is used to cross-link -COOH. This special hydroxyl compound is related to the oxazolines mentioned above and has the structural formula shown below:
When this structural formula is put together with the structural formula of oxazoline, you may be able to see that there is a certain correlation between the two. See the picture below:
Experiments have shown that this type of amidohydrin, like oxazoline, can be cross-linked with -COOH under certain conditions, e.g., 150°C X 30 min, and further investigation of the reaction mechanism suggests that one explanation is the formation of an oxazoline structure in the middle.
This kind of amido-alcohol compounds, typical model AA-4, can be used to cross-link -COOH, both aqueous and oily systems can be used, the reaction condition is 150℃X30min or more. Cost-effective, especially for water-based amino baking paint used to cross-linking -COOH, can significantly improve the film performance, such as water resistance, alcohol resistance, solvent resistance, salt spray resistance.
The above three methods of crosslinking-COOH, have you found that there are some mechanistic correlation. The reaction process all have special amidohydrin and/or oxazoline structure.
4, using polycarbodiimide crosslinking-COOH
Carbodiimide, also known as carbodiimide (Carbodiimide), contains -N=C=N-functional groups. It can be reacted at room temperature, the disadvantage is also to be added later, it is not convenient to use.
Carbodiimide reacts with carboxylic acids, but reacts slowly enough with water to be used in aqueous systems. The product of the reaction with carboxylic acid is N-acylurea. The reaction formula is shown below:
It is possible to crosslink carboxylic acid-based functional resins, including aqueous polyurethane dispersions and emulsions, with polycarbodiimide (also known as multifunctional carbodiimide). Crosslinking occurs within a few days at room temperature and much faster when heated.
In an emulsion study, curing conditions were in the range of 60 to 127°C for 5-30 minutes, with higher temperatures conferring better coating film properties. Obviously, the coating film properties depend not only on the physical film formation but also on the degree of chemical crosslinking.
5, the use of epoxy groups to crosslink with -COOH, requiring high temperatures. Not commonly used.
To summarize:
Room temperature to crosslink off the carboxy-COOH of aqueous systems, you can use polyaziridine crosslinking agent and polycarbodiimide. These two room temperature crosslinking agent market are some commodity models, the price is very expensive.
At medium temperatures (80-120°C), COOH can be crosslinked with oxazoline structural compounds, which are also commercially available and expensive.
At high temperatures (150°C x 30 min), special amido-alcohols can be used to cross-link -COOH, which is also available on the market in cost-effective versions and is particularly suitable for use in waterborne aminobaking enamels.
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