Abstract
Although powder coating systems offer many environmental, ecological and energy related benefits over liquid based coatings, in the case of uretdione based polyurethane systems, high curing temperature is still an issue. On the other hand, powder coating systems make it possible to reduce the costs and enhance the process of forming complex 3D structures using the deep drawing method by pre-coated metal substrates. During this processing method, there is a probability of micro crack formation in the coated film due to strain impact on the coating layer. A powder coating with self-healing ability is an ultimate solution to face not only this kind of fraction but also any other possible ones (such as defects caused by any impact on film surface during processing, transporting or even service). Here, a single molecule that is prepared via Diels–Alder cycloaddition reaction and retro Diels–Alder cleavage reaction was utilized as a self-healing additive to achieve self-healing ability in the powder coating system that is based on a commercially available uretdione cross-linker and OH-polyester resin. Coatings were prepared through melt mixing of components in a lab mixer, milling, sieving, and then application on the metal substrate through the electrostatic spraying method. To illustrate the role of self-healing additive, various concentrations (4 and 9% wt.) in combination with different curing temperatures (80 °C to 200 °C) were investigated. Both samples containing HA showed self-healing ability at elevated temperature around 120 °C for about 30 min with acceptable roughness and surface properties. Hardness measurement of cured film as well as thermal investigation indicate the chemical reaction of HA in a cross-linked network of cross-linker and resin. In addition, using HA leads to a 40 K drop in curing temperature of the system without using any catalyst. A 2.58% improvement in hardness values at a lower curing temperature and healing time of around 12.5 min at 120 °C to recover 100% of initial scratch (more than 10 cycles) in the sample containing 9% wt. HA was observed.
Highlights
Self-healing ability in engineering materials has attracted a lot of attention in recent decades since the pioneering study of White et al [1]
Thermal Gravimetry Analysis (TGA) analysis of basic powder coating samples, as well as cured films, are depicted in Considering the weight lost at 200 ◦ C, the main question is: what is happening with the healing agent (HA) at this elevated temperature? This question can be answered through a detailed study of the model system in the future
A self-healing ability is reported in this study for the polyurethane powder coating systems containing commercial uretdione based cross-linker (BF1320), OH-functionalized polyester resin, and the Diels–Alders adduct as a healing agent
Summary
Self-healing ability in engineering materials has attracted a lot of attention in recent decades since the pioneering study of White et al [1]. Many different mechanisms have been applied so far to achieve self-healing ability in materials such as micro- or/and nano-encapsulation (mimicking the bleeding of a small cut) [3]. Considering coating materials, having the self-healing ability is a great advantage because it improves the corrosion resistance of the coating and as it expands its life service. The former benefit is in favor of the economic and environmental concerns of the coating industry, the same concerns which lead to the development of solvent free powder coatings in the 20th century [12]. Integrating the self-healing ability into powder coating systems is in favor of powder coating systems goals as well
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