Step-by-Step Feeding Approach for Waterborne Polyurethane
The step-by-step feeding method can be categorized into two distinct approaches:
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In the first approach, once the reaction between the isocyanate monomer and polyester is nearly complete, organic siloxane is introduced to initiate chain expansion. Subsequently, CDMPA is added to introduce the -COOH group.
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In the second approach, the -COOH group is introduced using dithiomethylpropionic acid (DMPA) after the reaction between the isocyanate monomer and polyester is almost complete. Following this, organic siloxane is incorporated to facilitate chain expansion.
It has been observed that the load achieved from the second sequence outperforms that obtained from the first sequence. This is attributed to the addition of DMPA after the small molecule diol's chain extension during the initial load. This sequence promotes even connection of COOH groups to pre-loaded molecules. This, in turn, leads to better dispersion of polymer molecular chains in water, resulting in nearly identical hydrophilic group distribution on each colloidal particle. This minimizes the occurrence of aggregation due to uneven hydrophilicity, thus producing an emulsion with uniformly distributed colloidal particles and improved appearance.
Furthermore, the addition of silicone monomer after reaching a certain level of prepolymer molecule ensures its more uniform distribution on the molecular chain. This enables a higher number of colloidal particles to contain silicon-containing monomers, leading to a larger molecular weight of the resulting emulsion after hydrolytic chain extension. Consequently, during the film formation process, hydrolytic condensation results in a denser film.
When the organic silicon monomer is added before DMPA, the rapid reaction speed and relatively low amount of organic silicon monomer lead to uneven distribution on the molecular chain. This, in turn, results in uneven -COOH group molecular chains and comparatively poorer film stability. The water resistance of the emulsion obtained in the second feeding sequence is superior to that of the emulsion obtained from the first sequence.
The researchers employed Y-aminopropyltriethoxysilane, TDI, polycaprolactone diol, IP-DI, and DMPA to synthesize a silicone-terminated linear aqueous polyurethane dispersion. Siloxane group hydrolysis, condensation, chain extension, and crosslinking reactions within the dispersion particles resulted in crosslinked water-based polyurethane dispersions. Performance and film-forming characteristics were analyzed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The dispersion film exhibited further cross-linking during the drying process.
Upon converting the polyurethane microgel into a film, additional cross-linking was achieved by using water-soluble epoxy siloxane (Y-glycidylpropyltrimethoxysilane) to create a high-performance organic coating. SEM was utilized to investigate the coating's structure and discuss the cross-linking film-forming mechanism.
Additionally, the researchers explored an alternative approach involving the use of polyurethane prepolymers to emulsify and extend the chain within a 1% aminoethylaminopropyl polydimethylsiloxane aqueous emulsion. This led to the synthesis of an amino-containing silicone oil with a 20% solid content water-based polyurethane emulsion featuring excellent stability. The type and quantity of amino or tibial siloxane significantly influenced the emulsion's polymerization and storage stability, as well as its modification effect. Careful selection of the appropriate siloxane type, quantity, and process is crucial to achieving desired outcomes while avoiding gelation during the polymerization process.
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