Hot-melt coatings to improve the recyclability of paper and plastic
Our society is increasingly demanding biodegradable products made from sustainable and renewable resources and pose low risks to the environment, animal, human health and safety. In this context, the production of coatings with improved and multifunctional performances, compared to those of currently available products (and at the same time with more sustainable end of life options), is the challenge to be faced [1].
As such, the search for biobased, biodegradable and recyclable coating materials with good barrier, water resistance or anti-microbial features is underway, more specifically the production of sustainable coatings with tailored performances, improved than the currently available products, is the focus.
In particular, in ECOFUNCO project, the coating of cellulose or bioplastics substrates (for personal-care, tableware and food packaging products) with proper functional coverings, based on biopolymer and purposeful materials coming from agro-food waste streams, is the objective.
One of the approaches that ECOFUNCO has chosen to achieve this goal is the use of hot melt coatings (HMC, thermoplastic materials that are solid below 80°C and they become low-viscosity fluids at higher temperatures [2]) in which the biomolecules extracted in ECOFUNCO have been dispersed. Connecting biobased coating properties to final applications is the fruitful way in a new product development.
The study of HMC technique provides a lot of advantages, indeed it is a solvent-free method capable to grant faster and more economic coating processes. Hot melt coatings are compatible with most substrates, the coating material is supplied as a solid rather than as a liquid and, when heated, reaches the fluid state and flows onto the substrate. Then, when the hot melt is cooled, the coating solidifies and forms a bond to the substrate.
Today, HMCs are used in a variety of manufacturing processes including packaging [3], the step ahead of our research in the HMC field was the possibility to produce innovative HMC formulations blending three different molecules (chitin nanofibrils, chitosan and cutin) with lactic acid oligomers and/or fully biodegradable co-polyesters to improve anti-microbial and water-repellent properties respect to substrates without coatings.
After a systematic and industry-oriented selection of hot melts recipes in order to finely disperse the molecules in different oligomeric primers, finding the solution to avoid agglomeration or defects, UNIPI role was focused on the processing studies in a micro-compounder and the application of HMC on various biopolymeric and cellulose substrates evidencing good homogeneity and the achieving of anti-microbial and/or water repellance properties. Final coatings were, in fact, tested analysing surface energy, anti-microbial properties, and water repellence, noticing improvement for all the substrates examined.
Furthermore, on the basis of lab-scale results, three HMC based on cutin, chitosan and chitin where chosen as the optimized formulation for the production in a semi-industrial twin screw extruder of hot melt masterbatches to be directly used in industrial coating applicators.
This part of ECOFUNCO project evidenced that the world of biobased coatings is constantly evolving and expanding; several sectors are looking for a biobased solution to improve the properties of their substrates and a considerable technological step forward can be made in this field thanks to the development of “active” hot melts.
In conclusion, it is important to underline that the key issue to achieve a real progress towards sustainable coatings is also to intercept the society’s willingness to sustainability; it is important that consumers can understand that obtaining sustainable products means also reducing global costs [4].
Vito Gigante, Ph.D.
Department of Civil and Industrial Engineering
University of Pisa
References
1. Ren, K.; Fei, T.; Metzger, K.; Wang, T. Coating performance and rheological characteristics of novel soybean oil-based wax emulsions. Ind. Crops Prod. 2019, 140, 111654, doi:10.1016/j.indcrop.2019.111654.
2. Barreueso-Martinez, B.; Ferrandiz-Gomez, T.D.; Martin-Martinez, J.M.; Aran-Ais, F.; Torro-Palau, A.M.; Orgilés-Barceló, A.C. EVA hot-melt adhesives. Adhes. Age 2001, 44, 32–37.
3. Li, W.; Bouzidi, L.; Narine, S.S. Current research and development status and prospect of hot-melt adhesives: A review. Ind. Eng. Chem. Res. 2008, 47, 7524–7532.
4. Shao, J.; Ünal, E. What do consumers value more in green purchasing? Assessing the sustainability practices from demand side of business. J. Clean. Prod. 2019, 209, 1473–1483.
