Sequential reaction control during in situ polymerization and formation processes of reactive polyurethane coatings...

Sequential reaction control during in situ polymerization and formation processes of reactive polyurethane (RPU) coatings is achieved by adjusting the molecular weight and isocyanate content of the prepolymer. Due to the different reactivities between the linear chain extender 1,4-butanediol (1,4-BDO) and star-shaped cross-linker 2-ethyl-2-(hydroxymethyl)-1,3-propanediol (TMP), the cross-linking process can be slower than the chain extension, especially under an atmosphere with high viscosity. Thus, by changing the molecular weight (M_n) of poly(tetrahydrofuran) (PTMEG) during the preparation of the prepolymer, the viscosity as well as the reaction process can be precisely controlled. When the M_n of PTMEG is 2900 g mol^-1, the obtained mixture presents high viscosity, which enlarges the reactive difference between 1,4-BDO and TMP. Instead of simultaneous reactions, the chain extension and cross-linking are sequentially set. However, when the M_n of PTMEG is reduced to 1000 g mol^-1, the low viscosity causes the difference in reactivity between 1,4-BDO and TMP to be less prominent. The chain extension and cross-linking simultaneously set in, and the three-stage process no longer exists. Similar to the M_n of PTMEG, the isocyanate content of the prepolymer also significantly influences viscosity via the external plasticizer effect during the in situ polymerization and formation processes. Thus, by simply adjusting M_n and the isocyanate content of the prepolymer, not only the reactions, but also the structure of RPU coatings can be fine-tuned. The obtained mechanism can be used as the theoretical guidance for controllable polymerization and formation in the PU industry.