Novel Furfural-Derived Polyaldimines as Latent Hardeners for Polyurethane Adhesives

Moisture-curing one-component polyurethane systems in adhesive, sealant, and coating applications may show blister formation upon cure. Blisters can be formed when carbon dioxide, generated in the reaction with isocyanate and water, is trapped in the film. This problem can be mitigated by employing latent hardeners such as blocked polyamines, which are activated upon moisture exposure. The hydrolysis of the latent hardener yields the polyamine that quickly reacts with the isocyanate, forming urea linkages, and then chain extends the polymer. The hydrolysis also releases the blocking agent, which can potentially create an unpleasant odor. In this work, a series of di- and trifunctional aldimines were synthesized from commercially available polyamines, biobased hydroxymethyl furfural, and lauroyl chloride. Hydroxymethyl furfural was first esterified with lauroyl chloride and subsequently condensed with the polyamines to form the aldimines. The application of these novel aldimines in a model moisture-curing system allowed the preparation of blister- and odor-free castings. Based on our results, the mechanical performance of the different aldimines in casting and adhesive applications could be related to the polymer network density. This was dependent on the rate of the aldimine hydrolysis reaction to produce the polyamine. In particular, the use of aldimines prepared from polyether amines and 1,5-diamino-2-methylpentane showed excellent adhesive properties.

Polymorphic microstructure of MDI/BD-block polyurethane as determined by temperature-sensitive conformation variation

MDI/BD-block thermoplastic polyurethanes (TPUs) crystallized at different isothermal temperatures and different cooling rates were investigated using multiple techniques. The MDI/BD blocks crystallized in form II when the isothermal temperature was equal to or higher than 150 °C, and in form I at lower isothermal temperatures. Form II had a higher crystal elastic modulus of 6.75 GPa than form I of 1.31 GPa. Form I exhibited contracted conformation, while form II exhibited an extended conformation when viewed from the length of the c-axis in the crystalline state. Based on an analysis of the second derivative in FTIR spectroscopy and simple modeling, the conformation differences were considered to stem from the urethane group's internal bond rotation concerning the phenyl ring and the opening bond angle of phenyl-CH₂-phenyl. The generation of form II above 150 °C may be due to the activation of urethane and the flexible methylene elevated by the high temperature. Overall, it was seen that the crystallization of MDI/BD blocks involved a physicochemical change.

Quasiperiodicity and the nanoscopic morphology of some polyurethanes

When straining polyurethane elastomers (PUEs), it is often observed that the long-period peak of the small-angle X-ray scattering (SAXS) does not shift normally. An explanation is indicated for some PUEs in the real-space chord distribution. It exhibits a sequence of constant long-period bands. The band positions form a Fibonacci sequence. This relates to the underlying chemical synthesis by polyaddition of hard and soft modules, indicating a nearly quasiperiodic setup in sequences of stringed hard domains. These sequences appear to be the probes provided by SAXS for the study of morphology evolution in such PUEs. Should a regular-as-possible arrangement of physical crosslinks optimize a property of the material, then in the synthesis the mole fractionnHof hard modules should be chosen to benH= τ/(1 + τ) ≃ 0.62, where τ is the golden ratio.