The influence of diol chain extender on morphology and properties of thermally-triggered UV-stable self-healing polyurethane coatings

Abstract

Two sets of waterborne polyurethane dispersions were synthesised from polycarbonate polyol with molecular mass of 500 Da and hexamethylene diisocyanate or isophorone diisocyanate. Formulations were prepared without a chain extender, with aliphatic diol with two to five carbon atoms or with diethylene glycol. Coatings were prepared on cellulose triacetate sheets, damaged by a steel-wool scratch instrument and left to heal at room temperature and at 60˚C. Self-healing efficiency was examined by comparison of haze before damage and at intervals after damage. Samples were analysed using Differential Scanning Calorimetry, Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy. The tests were repeated after 12 weeks to investigate ageing of the polymers. Samples were also tested for their stability to weathering. Optimally designed coatings obtained up to 100% recovery within 10 minutes at 60˚C and partial recovery at room temperature. The self-healing properties of coatings were found to be linked to macro-organisation of polymer chains caused by interactions between hard segments and soft segments of the polyurethane moiety, leading to phase-mixing, promoted by bulky, non-symmetrical isophorone diisocyanate, or phase-separation, promoted by linear, symmetrical hexamethylene diisocyanate. The length of chain extender was found to have large influence on formulations prepared with hexamethylene diisocyanate, increasing phase-separation and haze with the increase of chain length. Diethylene glycol was found to improve phase-mixing and self-healing properties of hexamethylene diisocyanate based materials. The influence of chain extenders was found to be minimal for isophorone diisocyanate based materials.

Self‐healing polyurethane based on a difuranic monomer from biorenewable source

“…Internal healing based on reversible chemical interaction between functional groups in a polymer are more attractive due to unlimited mending cycles, but external stimulus such as heat or light must be applied . There is a wide variety of internal self‐healing approaches based on photoactivated [2 + 2]‐cycloaddition, reversible formation of disulfide bridges and hydrogen bonds, radical reactions, pH‐dependent processes, and Diels–Alder (DA) reactions …”

Recyclable polyurethane/nanoclay films

“…The use of macrodiol in WPU formulations is more diverse: WPUs made from polyether or polyester macrodiol are considered “classical” WPUs , which are unlike relatively “new” WPUs that are based on polycarbonate macrodiol . Polycarbonate‐based PUs possess very good functional properties, which allows them to be used for many purposes; for instance, they can be used, as shape‐memory , self‐healing , hydrolytic‐stable or optical materials. However, the relatively high price of polycarbonate macrodiols remains the main disadvantage impeding their greater use.…”

Synthesis of Self-Healing Waterborne Polyurethane Systems Chain Extended with Chitosan

“…Typically, the technology of self-healing allows for the repair of physical damage and cracks, prolonging the life of a material, particularly those of polymers. In the past several years, efforts have been made to impart polymers with self-healing features, including silicones [1], thermosetting polymers [2,3,4,5], rubbers [6], polyurethanes (PUs) [7,8], and hydrogels [9,10]. Among them, PU has become an attractive candidate with its specific chemical structure and various design flexibilities which have been found to be beneficial for overcoming problems related to the weakness and decreased functionality.…”