Sustainable Plant Oil-based Polyurethane Packaging Adhesives
Polyurethanes (PU) have been used in various applications for decades, such as foams, plastics, composites, films, sealants, coatings, inks, and adhesives. With increased global interest in sustainable and environmentally benign packaging, there is high demand to replace common petroleum-based materials with bio-derived sources. This research developed PU adhesives for multilayer flexible food packaging using plant oil extracts. Due to their abundance and renewability, vegetable and plant oil extracts are desirable as chemical feedstocks in PU adhesive synthesis. Traditionally, vegetable and plant oils used in bio-based polyurethanes required chemical modification to introduce hydroxyl groups for PU synthesis. Thus, oils not requiring chemical modifications are advantageous to reduce cost and increase simplicity. Two oils containing different equivalents of hydroxyl functional ricinoleic acid (f~2 Lesquerella fendleri oil; f~3 castor oil) were used to understand the influence of a C18 side chain on physical properties of PU adhesives. Peel resistance of corona-treated polyethylene (PE) and polyethylene terephthalate (PET) substrates adhered with adhesives containing varying amounts of LesQ and castor oils determined peel strengths of 6-8 N. The chemical and mechanical properties, including Tg, mass loss at 200°C temperature, and the average T-peel strength of the PU adhesives, are reported. The results of this study found that unmodified bio-derived PU adhesives had decreased Tgs with a range of -25 to -44°C and comparable peel strengths to other bio-based modified resins. Understanding the PU adhesive network structure-property relationships will help develop the next generation of bio-derived PU adhesives for food packaging applications.