Bioplastics from C1 Gases - Substitute Sustainable Products for Polymer Coated Steel
Tata Steel, one of the largest producers of steel in the world, emits more than 6 million tonnes of CO2 annually, with blast furnaces accounting for 70% of this CO2 emission. To lessen the amount of greenhouse gases discharged into our atmosphere and to limit their effect on the environment, it is necessary to recycle these gases into sustainable products. Tata Steel utilises coating to prevent corrosion by forming a barrier that deteriorates before the steel underneath. Given that their existing coatings, like the majority of coatings on the market, are derived from petrochemicals, a biodegradable product with comparable properties would contribute to reduce the negative environmental impacts of the steel industry. Petrochemicals are derived from petroleum, a fossil carbon source with limited availability and a constantly rising cost, with a very slow degradation rate causing problems for its disposal. Therefore, biodegradable polymers created from industrial by-products would be a sustainable alternative. Tata Steel has already a pilot plant that converts blast furnace gases into VFA and particularly into acetate. It was then proposed to use glycogen-accumulating organisms (GAOs), which are bacteria obtained from wastewater treatment plants, to produce mixes of biopolymers called polyhydroxyalkanoates (PHA). With acetate as the sole carbon source and by varying several culture conditions (substrate concentration, temperature, pH, nutrient concentration), different mixes of PHA were obtained. Further experiments were conducted to determine the best mix for steel coating in terms of properties and PHA accumulation. A PHA accumulation of 29% of cell dry weight was obtained at 23°C, with an acetate concentration of 4 g/L and with limited nitrogen availability. The PHA composition varied in terms of HB:HV ratio as a result of several parameter changes, with HB always being the most predominant monomer. As HB is more brittle and less thermoresistant than HV, achieving a lower HB: HV is recommended and can be obtained with acetate concentration greater than 3 g/L, pH of 6.8 or 7.3, a temperature higher than 28°C and a phosphate concentration of 100 mg/L. The PHA produced by GAOs had a solubility comparable to that of the polymers used in coating paint, such as polyurethanes or PVC, and were soluble in polar and hydrocarbon solvents. PHA may be employed as a primer or secondary coating since the films had very strong adhesion to another coating and steel, ranging from 88% to 98%. Low glass transition temperature, low crystallinity and a high melting point of semi-crystalline PHA from GAOs induced outstanding mechanical qualities, such as elongation at break or impact resistance, making it suitable for application in steel coating. The molecular weight was higher than that of the majority of petroleum-based polymers, which can increase the coating viscosity and cause problems during the industrial application on steel. However, as additives are frequently added to coatings to increase their density, large-scale research would be necessary to verify if the PHA molecular weight would be a problem. Following these results, it can be concluded that PHA produced from acetate through the conversion of blast furnace gases could be suitable for use in steel coating. It is recommended to use the parameters that resulted in the highest PHA accumulation found as the resulting PHA had excellent physical and mechanical properties. Further process optimisation would be required to confirm the suitability of PHA in steel coating, by testing the durability of the resulting coating and its resistance to external environment.
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