Preparation And Thermal Reversible Mechanism Of UV-curable Polyurethane Coatings Based On Diels-alder Structure

UV curable polyurethane coatings have been widely applied in many important fields due to their excellent comprehensive properties. However, UV curing coatings with high crosslink density developed during fast curing are prone to some drawbacks. Besides materials inherently fragile defects such as low impact resistance with ensuing poor resistance to crack propagation, the most noticeable fact that they are difficult to be dissolved, melted and molded when it is needed to be repaired or be dropped. With all these problems, the environmental protection characteristics of the UV curable coating has been greatly questioned.In this paper, a series of novel polyurethane resins containing acrylate function and Diels-Alder(DA) structure were synthesized, with potential thermal reversibilities and UV curable properties. Specific research contents and conclusions are as follows:The synthesized N-hydroxyethyl maleimide with hydroxyl and maleimide functions was reacted with furfuryl alcohol to prepare a diol for introducing the DA structure. The diol thus obtained was reacted with isophorone diisocynate(IPDI), different polyester diol(PEG1000/PCL1000/PLA1000) as a chain extender and hydroxypropyl acrylate(HPA) to prepare the UV curable polyurethane(corresponding to DAEPU/DACPU/DALPU). The structures of all products were characterized using Fourier infrared spectroscopy(FTIR) and proton nuclear magnetic resonance(1H NMR).The UV curing process was monitored by FTIR. The results show that each resin system could be rapidly cured to form a coating with a high gel content above 80% and a final acrylate C=C conversion higher than 80% after UV irradiation within 35 s, which indicating the formation of highly crosslinked networks. Crucially, the DA structure can keep stable during UV curing.Each UV cured coating was tested to exhibit good adhesion(0 level), high pencil hardness(>2H) and excellent flexibility(0.5 mm) suitable for practical applications. Futhermore, the material exhibits a good shape memory effect for its unique block copolymer structure with hard and soft segments. To take the DAEPU spline as an example, it can rapidly recover to its original shape at 60 °C within 15 s, and the shape recovery rate is close to 100%.The cured coatings showed acceptable thermal stabilities according to the results of thermal gravimetric analysis(TGA), which also indicates that the occuring of the retro-Diels-Alder(r DA) reaction did not adversely affect their thermal stabilities.FTIR with increasing/decreasing/constant temperature, differential scanning calorimetry(DSC) and dynamic mechanical analysis(DMA) were conducted to study the thermal reversible mechanism of UV cured polyurethane coatings.The thermal reversibility of DA structure were confirmed by FTIR with increasing and then decreasing temperature. The rDA reaction occurred at above 60 °C and was obviously accelerated with increasing temperature, while the broken DA bonds can be gradually but incompletely reconstructed with decreasing temperature. Meanwhile the FTIR spectra with increasing time at a specific relatively higher temperature reveals that there is a dynamic equilibrium of DA/rDA reaction, which indicating the thermal reversibility of DA structure is well-repeatable.DSC results show that the cured coatings based on thermal reversible DA structure undergo repeatedly rDA/DA reaction during heating/cooling circle. The rDA reaction(mainly during 60 °C~120 °C) in the first heating process leads to the decrease of Tg. Futhermore, the disconnect highly crosslinked network can be reconstructed by DA reaction during the cooling process. Moreover, the decrease of the endothermic peak of rDA reaction in the second cycle verifies the incomplete recovery of DA bonds in limited time. Nevertheless, the coatings still have repeatable thermal reversibility.DMA results show that the storage modulus E’ drops to the final minimum of about 1 MPa corresponding to the maximum of tan δ after the r DA reaction, which significantly conflicts with the relatively high gel content. As the highly crosslinked macromolecular network may decrosslink via rDA reaction under the action of heat and turn to a linear polymer with a certain modulus, thus the physical properties of the UV cured coatings can be adjusted by varying the temperature.With all above features, the well-designed UV curable polyurethane coatings based on a “key” of DA structure with highly crosslinked macromolecular network, which can be constantly switched bewteen crosslinked and linear state under the heating effect, are expected to be improved to novel soluble, fusible or plastic UV curing materials, thus expanding the application of UV curable polyurethane coating.