Polyurethanes make excellent raw materials for coatings since they offer both strength and elasticity: fixed covalent bonds for hardness and resistance, reversible hydrogen bonds for elastic strength. Thanks to the elastic reversibility of the hydrogen bonds, a crosslinked polyurethane coating has its own “polymer memory”. When a scratch occurs, the polymer chains are physically displaced. The hydrogen bonds are disconnected and look for new anchors whereas the fixed chemical bonds are unaffected. The polymer network is now in a state of fixed tension. The built-in memory effect is triggered by an input of heat, which sets the whole tensed-up system in motion again. As the polymers relax, the hydrogen bonds reposition themselves in the original formation and the scratch disappears. It’s a bit like us humans when an infrared lamp or heat rub helps us to relax and relieve muscular tension.
In addition to their self-healing properties, these coatings display the high variability of the property profile for which polyurethanes are known:
- Good chemical resistance and weather stability
- Hardness, toughness and elasticity due to urethane and urea structure
- Hydrogen bonds that support thermoplastic deformation under stress and thus have self-healing properties
- High solids/low VOC content
- Good optical qualities: High gloss and “wet look”
The coating system can be tailored for a specific application by varying the molecular structure of the soft segments, the distribution and length of the hard segments or the molecular weight and degree of chain branching. This can be done with the choice of the suitable poly-isocyanate and polyol components.
These videos demonstrate the self-healing effect in detail:
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Self-healing effects through different coating technologies
The urethane or urea structures in an elasticized polyurethane coating are the essential precondition for this self-healing effect irrespective of how the structure is built up:
- 2K or 1K technology
- Solventborne, solvent-free or waterborne
- Crosslinking via NCO-OH
The coating system can be tailored for a specific application by varying the molecular structure of the soft segments, the distribution and length of the hard segments or the molecular weight and degree of chain branching. This can be done with the choice of the suitable polyisocyanate and polyol components.
One of the first success stories in self-healing coatings was demonstrated in the automotive industry. After all, car-drivers have been longing for scratch-resistant paintwork for many years. Here is a detailed article (in german) “Kratzer ade” published in “Farbe und Lack, Issue 3/2011”, describing the various technologies on self-healing 2K-polyurethane coatings.
Application examples:
Self-healing properties have been confirmed in trials with automotive refinish coatings, with OEM clear-coat and in plastic clear-coat formulations already.Recently, the self-healing properties of furniture coatings have also been evaluated. The products and applications are described in the presentation “Waterbased self-healing system for wood coatings” and in the article: “Enviromentally friendly self-healing polyurethane systems for wood coatings“.
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