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Step-by-step Feeding Method for Waterborne Polyurethane

Aug 14, 2023

The step-by-step feeding method can be further divided into two types: one is that after the reaction between isocyanate monomer and polyester is almost complete, organic siloxane is first added to expand the chain, and then CDMPA is added to introduce - COOH group; Another method is to introduce - COOH group with dithiomethylpropionic acid (DMPA) after the reaction between isocyanate monomer and polyester is almost complete, and then add organic siloxane to expand the chain.

The load obtained from the first sequence performed worse than the load obtained from the second sequence. This is because the initial load is added with DMPA after the chain extension of the small molecule diol, it is easy to make a COOH group evenly connected to the pre-loaded molecules, on the one hand, it makes the formation of each polymer molecular chain dispersed in water equally easy, It is possible to have almost the same hydrophilic group on each colloidal particle, reduce the phenomenon of aggregation due to uneven hydrophilicity, and easily obtain an emulsion with uniform distribution of colloidal particles, thus having a better appearance;

On the other hand, because the amount of silicone monomer added is relatively low, adding silicone monomer after the prepolymer molecule reaches a certain level can make its distribution on the molecular chain of the prepolymer more uniform, so that more colloidal particles have Silicon-containing monomers, so that the molecular weight of the emulsion obtained after hydrolytic chain extension is larger, and the film obtained is denser due to hydrolytic condensation during the film formation process.

If the organic silicon monomer is added first and then DMPA, because the amount of organic silicon monomer is small and the reaction speed is fast, the distribution of organic silicon monomer on the molecular chain will be uneven, so that the —COOH group molecular chain will be uneven. , the stability of the obtained film is relatively poor, and the water resistance is worse than that of the emulsion obtained in the second feeding sequence.

The researchers used Y-aminopropyltriethoxysilane, TDI, polycaprolactone diol, IP-DI, DMPA, etc. to synthesize a silicone-terminated linear aqueous polyurethane dispersion. Due to the hydrolysis and condensation of siloxane groups, chain extension and crosslinking reactions occur in the dispersion particles, resulting in crosslinked water-based polyurethane dispersions. Its performance and film-forming performance were studied by transmission electron microscope (TEM) and scanning electron microscope (SEM). It was also found that the dispersion film could be further cross-linked during the drying process.
After the polyurethane microgel was formed into a film, it was further cross-linked with water-soluble epoxy siloxane (Y-glycidylpropyltrimethoxysilane) to obtain a high-performance organic coating. The structure of the coating was studied by SEM , and discussed the cross-linking film-forming mechanism.

In addition to the above methods, the researchers used polyurethane prepolymers to emulsify and extend the chain in a 1% aminoethylaminopropyl polydimethylsiloxane aqueous emulsion to synthesize an amino-containing silicone oil with a solid content of 20%. Water-based polyurethane emulsion with good stability. The type and amount of amino or tibial siloxane have a significant impact on the polymerization stability and storage stability of the emulsion, and also affect the modification effect of the emulsion. If the amount added is small, there is no obvious modification effect; if the amount added is large, it will cause gelation during the polymerization process. Therefore, to find the right type of siloxane, the appropriate addition amount and process are very important.

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