Biorrevestimentos de poliuretano baseado em lignina kraft

Abstract

Climate crisis has created a need to replace the use of petrochemicals with renewable resources. The polymer industry, one of the most important, consumes approximately 8% of global fossil resources. Polyurethane, the sixth most produced polymer in the world, stands out in this industry when it is applied as a protective coating for metal due to its anti-corrosive properties. In the context of sustainability, the production of polyurethanes through the use of renewable resources has thus become a research trend. Lignin, the second most abundant biopolymer on the planet and a residue produced on a large scale in the pulp and paper industry, has the potential to play the role of polyol in the synthesis of polyurethanes. In this work, Kraft lignin was used as a polyol in the synthesis of a polyurethane resin, which was later used as a protective coating for carbon steel. Lignin was characterized using spectroscopy techniques such as IR and NMR, in addition to thermal analysis (TG and DSC), elemental analysis and solubility tests. A lignin biopolyol was produced from the oxypropylation of Kraft lignin using propylene carbonate, a green reagent. This biopolyol was used together with castor oil and isocyanates in the synthesis of the polyurethane resin. Isocyanates used were hexamethylene diisocyanate (HDI) and methylene diphenyl diisocyanate (MDI). The coatings produced were characterized by infrared spectroscopy, adhesion test, thermogravimetric analysis, differential scanning calorimetry and electrochemical impedance spectroscopy. Coatings produced exhibited films with high gloss, homogeneity, excellent adhesion to carbon steel, high thermal stability and good flexibility at room temperature. In addition, electrochemical impedance spectroscopy tests showed initial impedance moduli (after 24 hours in 3.5% m/v NaCl solution) in the range of 107 – 1010 Ω, indicating excellent protective barrier. The continuation of the study carried out with 4 selected coatings (PUHDI35, PUHDI65, PUMDI50 and PUMDI80) showed that the impedance modules suffered a drop of only one order of magnitude, thus proving their anti-corrosive protection capacity. Therefore, the coatings produced have the potential to become a commercial product, representing a simple and relatively inexpensive way to mitigate corrosion problems, in addition to adding value to a waste product from the paper and pulp industry and contributing to the decarbonization of the planet.