Experience in practical application of Pu desiccant
Pu reaction is mainly the reaction of – NCO and – Oh, and sometimes there is also the reaction of – NCO and H2O. The latter is mostly a side reaction, and can also be a main reaction, such as a moisture curing system.
Pu driers are often used to adjust the reaction rate of PU.
It is mainly divided into two categories: tertiary amine drying agents and metal organic compound drying agents.
Tertiary amine Pu driers are very familiar to everyone. They are affordable and widely used, especially in wood primer. It can not only catalyze the reaction of – NCO and – Oh, but also catalyze the reaction of – NCO and H2O, which can be applied to the moisture curing system.
Organometallic Pu driers mainly include tin and bismuth. It mainly catalyzes the reaction between – NCO and – Oh, but has little effect on the reaction between – NCO and H2O. Tin is represented by T-12, and the structural formula is dibutyltin dilaurate. Bismuth, as an environmental protection type desiccant, has relatively weak and expensive catalysis.
The following experience has been gained from a lot of practice:
1. Amine driers tend to be surface dry, while tin driers tend to be solid dry. The same formula is used to compare the dry test of amine and tin driers. The prepared paint is left in a disposable plastic cup to be gelatinized. The gelation time of amines is short. After 24 hours, observe the hardness of the gelatins. Amines are like boiled eggs with their shells peeled, while tin ones are like stone blocks. The overall drying is very good and thorough. There is also a formula of amine drier added to the wood primer. After the thick coat primer is dried, it will start to polish well. If it is polished deep, it will be difficult to polish. There is sticky sandpaper.
2. Amine driers and tin driers have a good synergistic effect when used together. This is reflected in the primer of furniture wood paint. These two kinds of dryers are often used together to better improve the dryness and burnishing.
3. Amine driers often seriously affect the anti sedimentation performance of paints, while tin driers do not. This problem is a headache for many engineers in PU wood primer. In winter, in order to dry quickly and polish well, amine driers are often added in a large amount, which will lead to a serious decline in anti sinking performance. Is it possible to modify an amine type desiccant to combine its anti sedimentation performance with desiccation? In practice, methods and products for modifying such amine driers have also been found.
4. Dibutyltin dilaurate (T-12), the representative of tin drying agents, almost loses its drying effect under the temperature below 5 ° C. Pu paints produced in Guangdong are complained of slow drying in winter when they are sold in the north, so increasing the amount of dibutyltin dilaurate does not accelerate it at all. The large amount of dibutyltin dilaurate added to the paint will cause the side effect of low-temperature precipitation. The loss of catalytic activity at low temperature is caused by the crystallization of dibutyltin dilaurate at a temperature lower than 5 ° C. If dibutyltin dilaurate is bottled and placed in the freezer of the refrigerator, it will be found that it has been frozen into a very hard solid when taken out the next day. How to solve the problem of low temperature activity? Dibutyltin dilaurate is easy to crystallize at low temperature because of lauric acid in the structure. If liquid fatty acids with low carbon number are used or acetic acid is simply used to replace lauric acid, the catalytic activity is greatly improved, and it is difficult to crystallize at low temperature, which can completely solve this problem.
5. In many cases, both fast drying and long operational time (gelation time) are required, which seems to be a very contradictory problem. How to solve it? The gelation time caused by tin based desiccants is short, which can be solved by using a substance that can not only complex with tin based metals but also volatilize with solvents during drying. The gelation time caused by amine driers is too short, which can be solved by blocking amines with acid. In addition, changing the pH value can also retard curing and prolong gelation time. The use of strong solvents, less or no aromatic solvents can also greatly prolong the gelation time.
