Waterborne Hybrid Silica-Alumina Coating to Increase Poplar Wood Application Opportunities

Environmental sustainability and health issues are important factors to be considered in developing new coating materials. Sol-gel is a widely studied procedure that represents also a sustainable one, due to the mild process temperature and conditions. This process, applicable to natural and heat-sensitive materials, allows for a versatile and non-toxic surface treatment method as the starting solution can be composed of water instead of alcohol according to the final coating performances. With a view to an easy scale-up of the sol-gel preparation, the use of water implies a reduced VOC (volatile organic compounds) emission during the evaporation of the solvent phase, improving the air quality and concurrently reducing health and fire hazards. Moreover, typical low process temperatures make the sol-gel treatment to be considered as a sustainable one as well as applicable to bio and heat-sensitive materials. The integration of inorganic sol-gel treatments with bio-resources, as wood or cellulose, is still not a deeply investigated subject, as to date the utilization of these materials is still limited, due to their poor long-term mechanical properties, such as abrasion resistance. In the panorama of soft wood applications, the combined advantages of using an abundant and inexpensive natural source and an environmental friendly technological process, are expected to offer promising breakthroughs. This work aims at developing a waterborne hybrid silica-alumina coating to increase poplar wood wear performances. Poplar specimens were treated by soaking them in alkoxydes precursor solution and identified and labelled depending on treatment conditions and on the applied total heating time and temperature. The final coating morphology and composition have been observed by Scanning Electron Microscopy and Energy Dispersive X-ray Spectrometry analyses. One the key factors that influences coating performances of fibrous porous materials is the temperature of the process throughout the coating application phase. Thus, different drying procedures have been applied to wood samples before and after coating application, including air drying, in order to understand if coating performances could be obtained through the green process characteristics maximisation. Throughout the research, experimental tests have been carried out. Abrasion resistance tests were performed. As lower weight loss corresponded to higher abrasion resistance materials, the 1.7% weight loss was reported for the air dried sample compared with 8% weight loss obtained when maximum heating time has been applied on sample during the different process phases. Transformations induced by thermal treatment have been further inferred by TG/DTA analyses. TG/DTA analyses confirmed how wood components has been negatively influenced by heat drying. Surface colour variation, ΔE*, was evaluated according to CIELAB model. The obtained results revealed how the air drying allowed for optimal aesthetical results. The possibility to enhance performances of naturally renewable bio-resources through sustainable coating technologies for convenient industrial application, that are still missing to date, could be a cost-effective alternative from environmental, sustainable as well as technological points of view. Future developments could may include waterborne sol-gel coatings application for other wood poor-long term properties improvement, such as weatherability or flame resistance.