Optimization of polypropylene splats using the flame spray process

Thermal spray is a well-established and widely-employed technology for producing engineered coatings used in various industrial applications. Thermal spraying is a solvent-less and low-Volatile Organic Compounds (VOCs) technique with the capability to process a wide range of coating materials and substrates including metals, high melting point ceramics and low processing temperature polymers. Thermal spraying of polymer powders is a 100% solids process used to form a protective coating for different applications. The relatively low temperature required, simplicity and portability of the flame spray technique promote the process to extensive use for deposition of polymer coating applications. A thermal spray coating is established by customizing the spray parameters based on the intended application. The coating is formed as molten or semi-molten individual particles impact, flatten and solidify to form overlapping splats. Thus, a complete and thorough understanding of the intrinsic building blocks of a thermal spray coating and individual splats is essential to comprehend the characteristics of the ensemble of splats that constitute a coating. The effects of spraying conditions on splat geometries and morphologies of polypropylene (PP) deposited onto glass and mild steel substrates using the flame spray process must be understood so that these conditions may be optimized. Glass substrates were sprayed using different stand-off distances to examine the influence of stand-off distance (SOD) on PP splat morphology at room temperature. The studies indicated that decreasing the SOD from 350 to 150 mm produced coherent, integral disc-shaped splats that exhibited minimum splashing behaviour. Splat diameter and thickness were shown to be influenced by the particle SOD. A longer SOD produced larger diameter PP splats that were thin; however, a shorter SOD yielded unmelted or partially molten splats. The splat diameter to thickness ratio of PP was found to be about 5 and a SOD of 150–200 mm is recommended to produce a dense coating with acceptable mechanical properties. Several statistical concepts were employed to measure the splat metrics of formation, including an estimate of splash area. Measurements were performed of equivalent diameter, degree of splashing, spreading factor, deposition efficiency and circularity of PP splats deposited onto a glass substrate at room temperature using the flame spray process. The results indicated that as the SOD increased from 100 mm to 250 mm, the degree of splashing, flattening ratio and spread factor increased, while the deposition efficiency and splat thickness decreased. Splat circularity was steady at around 0.9 and indicated that the splats were close to circular for all SODs. The effect of substrate surface topology and chemistry were demonstrated by spraying PP powder particles onto flat and grit-blasted mild steel substrates at predetermined temperatures of room temperature (RT), 70°C, 120°C and 170°C. Splat formation and morphology were also investigated using optical microscopy and scanning electron microscopy. The splat-substrate interface was investigated using focused ion beam. The degree of crystallinity of the PP particles was analysed by Raman spectroscopy. The geometrical merits of the splat qualities were calculated via “imageJ” software. Single solidified splats were collected from polished substrates. The study found that increasing the substrate temperature from room temperature to 170ºC produced PP splats with larger diameter and improved the splat-substrate contact. Substrate preheating was shown to be a dominant factor in the splat formation and splat shape. Raman spectra showed that generally the degree of crystallinity of the PP particles rose with increasing substrate temperature. Comparison of the spectra of the feedstock powder and the deposited PP particles confirmed that there was no thermal degradation of the material under these processing conditions. The intensity of the band at 809 cm-1 rose in relation to that at 841 cm-1. The experimental examination of polymer splat morphologies produced under different spray conditions has permitted understanding of the most effective parameters that contribute to optimal splats. Knowledge on the behaviour of individual splats and the phenomena related to the thermal spray splat formation of polymer materials have been enhanced.