Optimising Mobility Behaviour Of Polymer Coated packaging Steel, Protact (Understanding, Measure, Improve)
Authors:   , Davies George 
Published: November 2022

Tata's polymer-coated packaging steel, Protact®, has been reported by can manufacturers to suffer from reduced mobility. The term ‘mobility’ is used to describe any event where the packaging steel is in sliding contact with another material. Here, it is used to describe Protact® when sliding on itself. Reduced mobility is typically seen with sheets sticking together in can-manufacturers processing lines when the material has polyethylene terephthalate (PET) laminated on both sides. The mobility of the material becomes entirely predictable, however, when PET is laminated on one side and polypropylene (PP) is laminated on the other side. Friction experiments were carried out on samples of Protact® with different film formulations, with wildly differing results. The material with the least predictable mobility behaviour was the PET film, which did not have wax added to the film during production. However, samples of Protact® laminated with waxed films showed significant improvements in mobility when compared to the samples of the unwaxed materials sliding on themselves. It was found that if friction experiments were repeated on samples of Protact®, the random, higher friction forces started to reduce and become more predictable. Firstly, it was thought that it was due to the surfaces becoming rougher due to wear. However, similar low friction results were seen when fresh samples of Protact® were cleaned with ethanol before sliding experiments. Two competitors' polymer-coated steels, laminated with unwaxed PET films, had much smoother, predictable friction experiment results, which correlated with increased roughness. Samples of Protact® were then produced with increased roughness by laminating with a lower post-heat than usual to limit melting and give higher crystallinity. These were found to perform similarly to the two competitors' materials, though the higher crystallinity is not desirable as it limits formability. Other tests also showed that rougher surfaces had improved friction behaviour. The surface potential was then checked to see any differences between the first and last repeated sliding samples, as well as after ethanol cleaning as it is known that this removes static charge from polymers efficiently. The surface potentials were significantly different in the samples. The charge on the initial sliding experiment sample was found to be much higher than the charge on the samples after they underwent repeated sliding, which were found to be close to neutral. A detailed study of the development of surface charge was then undertaken, which showed that the magnitude of charge and the size of areas of different charges correlated well with high and unpredictable friction. Rougher samples, the addition of wax, cleaning with ethanol and using dissimilar polymers all improved the friction. Finally, various practical means to eliminate the problem in real-world production were assessed and the most significant improvement to the mobility of the Protact® samples came from the application of novel SiO2 suspensions. With an optimised particle size, the material performed as well as friction experiments performed on static dissipated samples.