Tannins as sustainable tackifiers in hot melt adhesives

Abstract

There is an increasing interest in renewable raw materials in all industries and adhesive industry is no exception. With the continuous development of biorefineries, the selection of bio-based materials is growing. The objective of this Thesis was to determine whether tannins from the bark of Finnish softwood could be added to that selection in regards to hot melt adhesive formulation. More precisely, whether tannins would be compatible with established materials, and whether tannins would be compatible with cellulose derivatives. This work functions as a laboratory scale proof of concept.

The theoretical part of this Thesis introduces how adhesives work, what are hot melt adhesives and how adhesives are tested, before presenting some sustainable materials and tannins in particular. Hot melt adhesives have three primary components, polymer, tackifier and wax, and the properties of the adhesive can be adjusted by component selection and ratio. Hot melt adhesives are mostly used in packaging industry and thus this Thesis focuses on using folding boxboard. Phenolic tackifiers are industrially available and lignin has been proven to function as tackifier with cellulose derivatives. Due to these facts, hypothesis was that tannins should function as tackifiers due to their phenolic nature, especially with cellulose derivatives. Tannins used in this Thesis are extracted from the bark of Finnish softwood.

It was discovered that mixture of tannins and poly(DL-lactide-co-ε-caprolactone) was a potential hot melt adhesive. This formulation was developed by screening mixtures of crude spruce tannin with 7 different polymers and 5 different waxes. Screening of the adhesive capabilities of different combinations was done by manual testing, and the peel strength of the most promising formulations was measured. The copolymer can be used as a hot melt adhesive by itself, but the processability and adhesion towards uncoated boxboard increased with tannins. In contrast to expectations, adhesive capabilities with the tested cellulose esters were weak. Next steps should be development of a better mixing mechanism as the tannin-copolymer mixture was not fully homogenous. There should also be research into how the parameters of the copolymer affect the adhesive capabilities of the hot melt adhesive, and whether changing these parameters could be used to enhance the adhesive. Compatibility with other materials should still also be tested with tannins.