Performance and Lifetime Assessment of Self-repairing Polyurethane Coatings
Lead Research Organisation:
University of Strathclyde
Department Name: Pure and Applied Chemistry
Abstract
Polyurethanes (PU) are one of the most attractive materials with potential for use as SH coatings. These polymers show unique physical and mechanical properties and are already being widely used as protective coatings and paints, for example. Due to their internal structure, two phases can be distinguished - rigid, polar hard segments (HS), formed by isocyanate and chain extenders (CE), and flexible, non-polar soft segments (SS) formed by long chain polyols. As the result of their polar nature, HS tend to attract each other, aggregate and form hydrogen-bonded blocks. The resulting copolymer can be considered as composed of islands of HS with higher glass transition temperatures (Tg) dispersed in SS of lower Tg, acting as physical cross-links providing toughness and elasticity
We have recently developed coatings PU coatings for cellulose triacetate (used in sunglasses) that obtained up to 100% recovery within 10 minutes at 60C and vanishingly low haze values. The self-healing properties of coatings were found to be linked to macro-organisation of polymer chains caused by interactions between HS and SS 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 CE was found to have large negative influence on formulations prepared with hexamethylene diisocyanate, increasing phase-separation and haze with the increase of chain length. However, diethylene glycol was found to improve phase-mixing and self-healing properties of hexamethylene diisocyanate based materials. On the other hand, the influence of CE was found to be minimal for isophorone diisocyanate based materials. These coatings are already receiving early stage commercial interest but there are critical issues yet to be resolved, and it is these that are the focus of the ongoing research programme. Specifically, the research questions include:
- How can the aqueous coating technology and substrate adhesion be optimised for polymer substrates other than cellulose triacetate?
- Can we match thermal expansion coefficients of the coatings with the substrates?
- Can we precisely determine and define the morphological changes associated with the healing process and optimise the kinetics of these changes?
- What is the role of water in the healing process?
- Can we balance self-healing ability and resistance to scratching?
- Can we more specifically measure HS domain sizes and relate to haze?
- Can self-healing be maintained/enhanced in the presence of functional additives such as dyes, UV stabilisers etc?
- What is the long-term environmental stability of the coatings (hydrolysis, photo-ageing)?
We have recently developed coatings PU coatings for cellulose triacetate (used in sunglasses) that obtained up to 100% recovery within 10 minutes at 60C and vanishingly low haze values. The self-healing properties of coatings were found to be linked to macro-organisation of polymer chains caused by interactions between HS and SS 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 CE was found to have large negative influence on formulations prepared with hexamethylene diisocyanate, increasing phase-separation and haze with the increase of chain length. However, diethylene glycol was found to improve phase-mixing and self-healing properties of hexamethylene diisocyanate based materials. On the other hand, the influence of CE was found to be minimal for isophorone diisocyanate based materials. These coatings are already receiving early stage commercial interest but there are critical issues yet to be resolved, and it is these that are the focus of the ongoing research programme. Specifically, the research questions include:
- How can the aqueous coating technology and substrate adhesion be optimised for polymer substrates other than cellulose triacetate?
- Can we match thermal expansion coefficients of the coatings with the substrates?
- Can we precisely determine and define the morphological changes associated with the healing process and optimise the kinetics of these changes?
- What is the role of water in the healing process?
- Can we balance self-healing ability and resistance to scratching?
- Can we more specifically measure HS domain sizes and relate to haze?
- Can self-healing be maintained/enhanced in the presence of functional additives such as dyes, UV stabilisers etc?
- What is the long-term environmental stability of the coatings (hydrolysis, photo-ageing)?
