A Review of Thermally Sprayed Polymer Coatings

Thermal spraying of polymer coatings has been applied for many decades. Initially, the focus was primarily on corrosion and wear protection. Manufacturing was performed with traditional methods, such as flame and plasma spraying. Later, thermal spray technologies were developed, and interest increased in producing polymer or polymer-composite coatings from different polymer materials with advanced spray processes. Additionally, novel application fields were studied, such as the use of thermally sprayed polymer coatings for anti-icing and anti-fouling purposes. This review summarizes thermally sprayed polymer coatings from the standpoints of materials, processes used and selected latest application fields.


Introduction
Polymer coatings have been manufactured by thermal spraying for decades, and development has continued to date. Thermal spray processes have been modified to improve coating processability, and feedstock materials have been developed and tailored to achieve better performance. An important application field for thermally sprayed polymer coatings is used as protective layers to address several conditions. Petrovicova and Schadler (Ref 1) summarized the most important advantages of thermally sprayed polymer coatings over other polymer coatings; this included environmental aspects, such as obviating the need for solvent systems containing volatile organic compounds. Other advantages were related to ease of manufacturing as well as suitability for coating onto large surfaces, the use of different processing conditions and development of materials exhibiting high viscosities. Petrovicova and Schadler (Ref 1) published their review on thermal spraying of polymers almost twenty years ago, so it is now appropriate to review the state-of-the-art for thermally sprayed polymers and polymer-based coatings again.
Thermal spray processes for polymer coating production include flame spraying, high-velocity oxygen fuel (HVOF)/ high-velocity air fuel (HVAF), plasma spraying (Ref 1) and cold spraying. Figure 1 summarizes the main factors affecting thermal spray processes, which depend on and vary with the process, the polymer material and the substrate conditions. Many factors must be taken into account when optimizing thermal spray processing with polymer and polymer-based coatings. Flame spraying is a thermal spray technology used for production of polymer coatings. Polymeric feedstock material is fed to the gun, melted, and accelerated toward the substrate surface, and the particle velocity can reach approximately 50-100 m/s (Ref 2). One advantage of flame spraying is that it can be used on site, so it is a very flexible processing method. Earlier flame spraying was done manually, whereas current processes are more automated due the use of robotics. Flame-sprayed polymer coatings used to protect pipelines from corrosion This invited article is part of a special issue focus in the Journal of Thermal Spray Technology celebrating the 30th anniversary of the journal. The papers and topics were curated by the Editor-in-Chief Armelle Vardelle, University of Limoges/ENSIL. . Polymers are materials that consist of organic compounds, and they can be produced synthetically or transformed from natural products. Thermoplastics, in turn, are physically linked macromolecules with linear or branched bonds, whereas elastomers and thermosets are cross-linked. Thermoplastics can be crystalline, semicrystalline or amorphous ( Ref 11). When thermoplastics are heated, they can soften and flow. For example, PE and PS are thermoplastics ( Ref 11,19). When they are cooled, they harden. In addition, this processing can be repeated (Ref 9), and these materials are therefore suitable for thermal spraying. Table 1 shows some examples of materials, and the melting points are relatively low compared to those of other thermally sprayable materials, such as metals, hardmetals and ceramics. Thermal spraying of polymers can be performed, but it requires process optimization, modification and more temperature control than processes for other materials. By using optimized process parameters, degradation can be avoided. Several studies have reported no detectable deterioration (Ref 14,20) or minimal degradation by using Fourier transform infrared spectroscopy (FTIR) ( Ref 21).
Thermally sprayed polymer coatings are protective, but they are inexpensive and easy to manufacture ( Ref 1,24). For example, thermally sprayed polymer coatings have been considered for corrosion protection, and biocompatible polymer coatings have been studied as potential solutions for problems in the medical sector. Additionally, thermally sprayed polymer coatings have been investigated for low-friction applications and wear protection. In this case, high-performance polymers were used, e.g., PEEK (Ref 25). One way to enhance the mechanical properties of thermally sprayed polymer coatings is by adding reinforcement agents that act as polymer nanocomposite coatings ( Ref 26). The latest application fields under study are focused on protecting against environmental stresses such as icing (Ref 5) and fouling (Ref 2).

Flame Spraying of Polymer Coatings
Flame spraying is the most common thermal spray process used for production of polymer coatings. More information on flame spraying and other thermal and cold spray processes used in coating production can be found (Ref 1, 4). There are special flame spray guns available for polymer spraying, as well as tailored systems, to manufacture coatings from hybrid feedstock materials. Flame-sprayed methacrylic acid (MAA) PE coatings have been studied for protection of steel substrates from corrosion. The typical particle size of the PE powder was 149 lm, and propane was used as a fuel gas. Adhesion between the coating and substrate was influenced by processing parameters such as gas and air pressures, as well as the traverse speed of the gun. Additionally, it has been reported that cathodic delamination (corrosion resistance) is correlated with adhesion; i.e., higher adhesion leads to better corrosion resistance (Ref 2). Flame-sprayed PE (FS PE) coatings have shown potential for use in anti-icing applications. Hydrophobic and icephobic FS PE coatings have been produced by using flame spraying with an acetylene-oxygen flame ( Ref 5,21,28). Processing parameters influenced the properties of flame-sprayed polymer coatings. For example, the coating thickness increased with increasing spraying distance when slow a traverse speed was used (Ref 21). Another approach for thermally sprayed PE coatings was the production of porous PE structures by using flame spraying with an acetylene-oxygen flame. These porous structures have been impregnated with a lubricant to act as slippery liquid infused porous surfaces (SLIPS). In this way, slippery surfaces can be produced, and high icephobicity can be achieved due to the slipperiness of the surface ( Ref 29).
Flame spraying of amorphous PEEK for wear protection and friction reduction has been studied. In this case, the particle sizes were much lower than 25 lm. The metal substrate was preheated prior to coating production, and the sample was quenched after spraying. As a result, the semicrystalline coating had higher hardness and expected improved wear and friction properties ( Ref 25). Furthermore, Soveja et al. (Ref 30) observed that an FS PEEK coating could be densified by using laser treatment as a remelting process. In addition to providing the denser coating structure, laser treatments improved the adhesion between FS PEEK and the steel substrate due to fusion of the polymer. As noted, flame spraying of PEEK requires specific preheating or posttreatment to ensure successful production of the coating. A key factor was a low processing temperature, which was enabled by process and nozzle development. In this way, a dense PEEK coating ( In addition to those on PE and PEEK, investigation of the flame-sprayed fluoropolymer PVDF, ECTFE, PFA and FEP coatings showed their dense and smooth coatings, which were beneficial for corrosion protection (Ref 12). Flame-sprayed EMAA splats (Ref 14) and PP splats (Ref 31) were studied to evaluate the influence of process parameters on splat formation. For example, spray distance influenced the flattening of EMAA particles, and the useable distance range depended on the substrate material. A higher spray distance can be used for glass substrates than     Table 2 provides an overview of polymer coatings produced with flame spraying. Work on the development of thermally sprayed polymer coatings and polymer-composite coatings mainly occurred in the 2000 century. Several different polymer materials and protective purposes have emerged as features of flame-sprayed polymer coatings. Additionally, tailoring of feedstock materials has enabled the production of polymer-based composite coatings with important properties.

High-Velocity Flame Spraying of Polymer Coatings
This section is focused on polymer coatings made with high-velocity flame spraying, and more information on the process itself is presented in (Ref 1, 4). PET coatings have been produced via high-velocity oxygen fuel (HVOF) spraying (Ref 6). This indicated crystallization of the amorphous feedstock during thermal spraying. However, later melting and quenching decreased the extent of crystallization. Heat-treated thermally sprayed PET coatings exhibited better tribological properties than PET because they showed lower friction and wear rates (Ref 6). In another study, a PET coating was successfully deposited by using a low-velocity oxygen fuel (LVOF) process; this prevented corrosion by gasoline, diesel oil and alcohol and confirmed the potential for use these coatings as corrosion barriers in fuel tanks (Ref 3). Figure 5 shows the structures of LVOF PET coatings on a steel substrate. When the substrate was not preheated, a layered structure resulted, and adhesion was poor. Preheating of the substrate improved adhesion between the coating and the substrate, but more bubbles were formed inside the coating structure at higher temperatures. This was attributed to greater degradation of PET caused by higher temperatures (Ref 23). The coefficient of friction for a HVOF PA coating with silica was lower than that for a HVOF PA coating, which was possibly due to crystallinity (Ref 47). The mechanism for wear of the polymer coating in sliding wear was identified as smearing, whereas other wear mechanisms also include abrasive and fatigue wearing of polymercomposite coatings ( Ref 47).
Single splat tests for PEEK particles were performed with HVOF and assisted combustion high-velocity air fuel (AC-HVAF) by evaluating the effects of surface roughness and chemistry on particle bonding (Ref 4). The skewness of a substrate surface has a significant effect on splat formation, although the surface roughness values were at the same level. This skewness indicated the shapes of peaks and valleys and, whether the shapes were blunt or sharp, which cannot be seen from the roughness itself. Positive skewness, where roughness peaks were sharp, increased the contact areas for the splats and improved mechanical interlocking HVOF ceramic/polymer (silica/nylon) coatings have been studied to determine wear resistance. The powders used had a nylon core and shells with embedded silica    Fig. 6. Plasma-sprayed PMMA coatings showed increased decomposition with increasing process temperatures. Additionally, other process parameters affected the plasma-sprayed PMMA coatings. For example,

Cold Spraying of Polymer Coatings
Research on cold spraying of polymers has been active, and several polymeric materials have been successfully produced by using the cold spraying process. Table 4 summarizes cold-sprayed polymer and polymer-based composite coatings and their purposes. Figure 7 shows some of the powders used, and the powder morphologies varied from spherical to very irregular. One challenge for polymer application by cold spraying, as well as for other thermal spray processes, is the availability of suitable powders. Therefore, processing must be done with the powders available, but these are not optimized for the thermal and cold spray processes. Successful coatings have been produced, but even more improved coating properties could be achieved with optimized powders and optimized processes. Some studies have been focused on spraying single particles and analyses of single particle interactions with the substrate. Additionally, the thermal history of the powder and its influence on cold-sprayed polymer coatings have been modeled, and these showed that the thermal gradient for the powder depended on the particle size (Ref 62). For example, single PS and PA particles were successfully sprayed on LDPE substrates (Fig. 8)  Chitosan polymer has been studied with cold spraying, and it was blended with Cu/Al. Chitosan is a natural polymer that is both biocompatible and nontoxic. Interest in this material has increased due to its biodegradability ( Ref 57). There are also other studies on cold-sprayed composite coatings in which polymers were one of the composite contents. For example, HA-Ag/PEEK coatings have been applied with cold spraying, and antibacterial properties have been the driving force for use of these coatings. The process gas was air, and it was preheated to 150-160°C. Coatings were produced successfully and showed high biofunctionality similar to that of the starting material (Ref 56). Ravi et al. (Ref 18,59) studied cold spraying of UHMWPE and UHMWPE with alumina nanoparticle powders. A low-pressure cold spray (LPCS) system was used with a pressure of 2-8 bar and temperatures of RT-500°C (air). In this study, it was determined that the polymer needed thermal softening to enable successful bonding, and therefore, the process was optimized by using a longer nozzle to achieve sufficient thermal softening.   The use of optimized nozzles for successful deposition of cold-sprayed PE on an Al-alloy substrate is confirmed by modeling and experimental work ( Ref 58). In addition, the cold spray process was used to successfully produce PE-carbon nanotube (CNT) coatings on PP and nanoporous structured aluminum substrates. In this study, PE particles were melted during the CS process and CNTs were bonded to the melted PE. These CS PE-CNT coatings were shown to be electrically conductive (Ref 60).
Bush et al. (Ref 61) studied the effects of several process parameters on the cold spraying of HDPE powder. The parameters considered were particle temperature, size and impact velocity, as well as nozzle design, spray distance and substrate composition and temperature. The deposition efficiencies of polymer powders can be improved by selecting optimal spray parameters. The critical velocities for polymers have been reported to be lower than those for metals because of the lower particle temperatures and different thermal diffusivities of polymer particles    Fig. 10 (Ref 22). Furthermore, polymeric materials have been sprayed together with glass beads to improve the properties of coldsprayed polymer coatings. The deposition efficiency increased with the use of peening particles, which also made the coating smoother and the structures more uniform (Ref 15).

Selected Application Fields for Thermally Sprayed Polymer Coatings
Anti-icing FS PE ( Ref 21,28) and PE?FEB coatings have shown low or medium-low ice adhesion with high durability. Furthermore, the ice adhesion of FS PE and PE?FEB can be decreased by polishing the surfaces. Figure 11 shows the low ice adhesion seen for a FS PE coating compared to stainless steel, aluminum and polyurethane paint, indicating that ice removal from the surface was improved. In addition, these coatings exhibited hydrophobic wetting performance, which was beneficial for icephobicity. In another study, Donadei et al. (Ref 45) modified flame spraying by adding hybrid feedstock injection to produce lubricated icephobic coatings (LICs). A heat-sensitive additive material was fed with an additional feeder outside the flame, while PE powder was fed traditionally through the spray gun Fig. 12. Lubricating additives were distributed in the PE matrix coating and increased the slipperiness of the coating. LICs were hydrophobic and icephobic and provided high water repellence and anti-icing performance, respectively Ref 45.

Anti-fouling
Liquid flame spraying (LFS) has been used to produce PI-Cu coatings exhibiting anti-fouling behavior. Interestingly, the coating thicknesses varied between tens of microns and tens of millimeters. LFS PI coatings exhibited corrosion protection, and LFS PI ? Cu showed improved antibacterial properties and provided sterilization against E-coli bacteria, which indicated high anti-fouling capacity.    PEEK, PEI and PA coatings and found that FS PA coatings had better resistance to abrasive wear. It was speculated that the better wear resistance was due to the higher crystallinity, better adhesion and lower residual stress. On the other hand, corrosion resistance was better for PA and PEEK coatings than for PEI coatings, which experienced color change and cracked during 2000-hour exposure in a H 2 SO 4 solution (Ref 7). In summary, thermally sprayed PEI, PA and PEEK coatings were viable solutions for corrosion and wear protection on metallic substrates (Ref 44).

Self-lubrication
Lubrication of thermally sprayed polymer coatings was affected by using lubricant-filled polyurea microcapsules. Polyalphaolefin (PAO) and silicone oil were used as lubricants in the microcapsules ( Ref 43). In this way, Armada et al. (Ref 43) produced liquid-solid self-lubricating coatings by using flame spraying. Process parameters were selected carefully to avoid destroying the microcapsules. Coatings were successfully produced, and self-healing behavior was observed, indicating improved friction properties (Fig. 14).

Conclusions
Thermal spraying has been used to produce several different types of polymer-based coatings. Traditional flame spraying has been widely used to produce coatings offering corrosion and wear protection, as well as anti-icing and anti-fouling effects. Additionally, HVOF and plasma spraying have been used, but research has been focused more on cold spraying. Considerable research has improved the properties and performance of cold-sprayed polymer and polymer-composite coatings used to provide corrosion, wear and antibacterial protection. The latest developments with thermally and cold-sprayed polymer coatings have led to process optimization and feedstock tailoring, resulting in new application fields and high-performance solutions.
Acknowledgments The author would like to express her appreciation to Tampere University and the Tampere Institute for Advanced Study (Tampere-IAS) and the TS-SLIPS (Thermally Sprayed slippery liquid infused porous surfaces-toward durable anti-icing coatings) project funded by the Academy of Finland.
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