Abstract
A sol/gel curing method is used in this work to synthesize hybrid partially bio-based polyhydroxyurethanes (PHUs) from dicarbonates derived from glycerol and various diamines. The method consists of end-capping the PHU prepolymers with moisture-sensitive groups, so sealants and adhesives can be produced from partially sustainable hybrid PHUs (HPHUs), similar to their preparation from end-capped conventional polyurethanes. Diglycerol dicarbonate (DGC) is synthesized and polymerized with different diamines of various chain lengths, and the resulting structural and thermal properties of the PHUs are qualitatively and quantitively characterized. This characterization led to two potential candidates: PHU 4, made of DGC and a poly(propylene glycol) diamine, and PHU 10, prepared from DGC and a poly(dimethylsiloxane) diamine. These polymers, with respective relative number-average molecular weights of 3200 and 7400 g/mol, are end-capped and left to cure under ambient lab conditions (22 °C and 40–50% humidity), and the curing processes are monitored rheologically. Notably, moisture curing does not require any catalyst. The chemical stability of the resulting hybrid PHUs (HPHUs) 4 and 10 in pure water is investigated to check the viability of applying them under outdoor conditions. Only HPHU 10 is found to be resistant to water and shows hydrophobicity with a contact angle of 109°. Tensile tests are conducted on HPHU 10 samples cured under lab conditions for a week and others cured for another week while being immersed in water. The mechanical properties, tensile strength and elongation at break, improve with the samples cured in water, indicating the high-water repellency of HPHU 10.
Highlights
Polyurethanes (PUs) have become one of the most widely used industrial polymers since they were first discovered by Bayer et al in 1937.1−4 PU sealants are known to be cheap, having long shelf- and pot-lives, resisting aging, having good antiflammability properties, and a wide range of applications in indoor and outdoor conditions as well as in porous and nonporous materials.[4]
As any conventional PU, they are synthesized from the polyaddition of diisocyanates with diols, and the resulting PU prepolymers are cured using moisture-curable agents to get the final sealants.[2−4] As the −NCO end-groups in the prepolymer chain are very reactive, it is common to end-cap the oligomers with moisture-curable silane-terminated groups to produce selfcross-linkable PUs in the presence of moisture.[5−11]
New routes have been developed for nonisocyanate PU (NIPU) synthesis[15] with the step-growth polyaddition of fivemembered cyclic dicarbonates with diamines being investigated the most because they do not form lower molecular weight byproducts and their precursors are abundant.[3,12−19] These NIPUs are known as poly(hydroxyurethane)s (PHUs) because hydroxyl pendant groups are present in the resulting polymer backbone
Summary
Polyurethanes (PUs) have become one of the most widely used industrial polymers since they were first discovered by Bayer et al in 1937.1−4 PU sealants are known to be cheap, having long shelf- and pot-lives, resisting aging, having good antiflammability properties, and a wide range of applications in indoor and outdoor conditions as well as in porous and nonporous materials.[4]. Many environmental and health issues are associated with the synthesis of conventional PUs, as the diisocyanates are produced from phosgene, which is known to be a lethal gas.[3,12−14] Besides, conventional PUs are too moisture-sensitive and chemically unstable.[12] new routes have been developed for nonisocyanate PU (NIPU) synthesis[15] with the step-growth polyaddition of fivemembered cyclic dicarbonates with diamines being investigated the most because they do not form lower molecular weight byproducts and their precursors are abundant.[3,12−19] These NIPUs are known as poly(hydroxyurethane)s (PHUs) because hydroxyl pendant groups are present in the resulting polymer backbone. PHUs were investigated for the preparation of sealants by Figovsky et al, who synthesized acrylic-modified petrochemical PHUs that cure under ultraviolet radiation to give hybrid PHU (HPHU) sealants with high performance.[24,25]
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