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Powder coatings have gained a lot of attention since the 1960s and the 1970s (implementation of polyurethanes into the market) because of the environmental and ecological concerns. Over the decades, there have been several publications relating the manufacturing and development of powder coatings as an alternative for liquid/solvent or water-based organic coatings. In general, powder coatings can be divided into two main groups based on the properties of the resin being used, thermoplastic and thermoset powder coating systems. Among thermoset powder coatings, polyurethane powder coatings have outstanding properties such as weather stability and are the best choice for exterior applications comparing overall properties. This paper reviews the most recent advances in the field of powder coating by focusing on polyurethane powder coating as a group of thermoset powder coatings. It also considers the industrial problems engaged in this field, such as emission of the blocking agents, the low reactivity of uretdione systems, and high curing temperature and their solutions. Finally, it also suggests the future potential investigation of polyurethane powder coating compositions and improvements.

Process oils are used for rubbers to improve their flexibility and processability in industry. The solubility parameter is a convenient way to determine the solubility of materials. Inverse gas chromatography was used to calculate the solubility parameters and depict solubility spheres for distillated aromatic extract (DAE), treated distillate aromatic extract (TDAE), mildly extracted solvate (MES), and hydro processed naphthenic oil (NAP). Results showed that despite the similarity in values of the solubility parameters of DAE and TDAE at ambient temperature, increasing temperature led to a significant difference in values. In contrast to other oils, TDAE showed a better compatibility with polar solvents. In addition, the interaction parameters showed no specific dependence on the temperature for DAE, MES, and NAP, except for polar solvents and TDAE. DAE had the highest compatibility with aromatic solvents. Upon raising the temperature to values greater than 100 °C, the compatibility trend between oils and toluene was the same for all oils investigated, except for TDAE, which increased with increasing temperature whereas others showed a reduction. The increase in the alkyl chain was also effective in increasing the compatibility of the probes as well as their interaction parameter values. The solubility parameters (δ2) of process oils were determined to be 18.9, 18.9, 18.5, and 19.0 (J/cm3)0.5 for DAE, TDAE, NAP, and MES at 25 °C, respectively.

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.

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Regarding the current demand for controlling plastic pollution, recycling of polymer sounds a promising solution. However, recycling causes mechanical and thermal shortcomings in polymers. Addition of nanoparticles to recycled materials may overcome these shortcomings. Nanocomposites can be achieved either by blending or through polymerization. Sepiolite as a nanoparticle enhances the thermal properties of polymers. In this study, the effect of sepiolite as a nanoparticle has been investigated on the thermal and mechanical behavior of recycled high-density polyethylene (HDPE). Hardness, density, Vicat softening temperature, melt flow rate (MFR), and differential scanning calorimetry has been investigated on recycled HDPE containing different amount of sepiolite. Results showed that both the amount of recycled HDPE and the sepiolite content affect the mechanical and thermal behavior of samples. Increasing the amount of recycled component resulted in increasing of MFR, a slight increase in density, and decrease in Vicat softening point, hardness, melting temperature, and degree of crystallization. As an opposite effect of these to factors on crystallinity of HDPE, sepiolite content has better effects to be considered separately for each recycle content. Sepiolite can be introduced as a low-cost reinforcement filler in recycling industry for tuning new compositions based on process condition, or vice versa.

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