Development of a novel adhesive composed of all-natural components
Introduction
Adhesives can be classified as either inorganic or organic, depending on their main components. In recent years, synthetic organic glues have been shown to have serious problems due to the use of volatile organic solvents or the evaporation of unreacted monomers. The application of these glues to materials used indoors is particularly harmful, since they emit toxic substances, such as formaldehyde, which is suspected to be a carcinogen [1], [2]. Thus, there is a growing demand for a formaldehyde-free adhesive composed of renewable natural resources [3], [4]. In this paper, we describe a novel tannin-based adhesive and its efficacy for gluing wood fragments.
Section snippets
Materials
We used four tannin samples [Tannic acid C [5] (derived from Castanea sativa (chestnut), Silva), Tupasol Ato (derived from Schinopsis lorentzii (quebracho), Silva), Astrin P (derived from an astringent persimmon, Mimasu), and Mimosa OP [6], [7] (derived from mimosa, Silva)], three protein samples [ovalbumin (Sigma), casein (Nacalai Tesque), and gelatin (Nacalai Tesque)], and ε-poly-l-lysine (EPL, JNC Corp.). Tannic acid C is a mixture of different esters of gallic, digallic, and ellagic acids
Results
Tannins are plant-produced polyphenols that can be classified as either condensed or hydrolysable. It is generally believed that only condensed tannins can be used in adhesives [5], [10], due to their reactivity with formaldehyde. Condensed tannins show their highest reactivity at positions 6 and 8 of the A-ring. Methylol groups are introduced when formaldehyde is added to an A-ring, then crosslinked and polymerized through a methylene bond [11]. However, our aim was to develop a
Discussion
Based on these results, we have identified an effective composition of tannin and EPL for use as an adhesive. Furthermore, we demonstrated that, contrary to the popular belief that only condensed tannins are suitable for use in adhesives, hydrolysable tannins can also be used in glues.
Moreover, we found that good results, in terms of storage time and adhesive strength, can be achieved with a mixture of 15 wt% tannin and 3~7.5 wt% gelatin, at pH 10.0.
In this paper, we clearly demonstrate that a
Conclusions
In this study, we successfully developed a novel adhesive composed of tannin and EPL. It is composed of all-natural renewable substances, does not require any chemical additives, and is completely non-toxic. The adhesive shows both strong bonding strength and water-resistance, and passes the EN 314 class 1 standard. Thus, this adhesive is suitable for application to materials used indoors.
Acknowledgements
We are grateful to Ms. Taeko Watabe for technical assistance and Ms. Mika Omura for both technical and administrative assistance. This study was supported by Grants-in-Aid for Scientific Research (24510316 to Y. K. and 25242036 to D. U.) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. We are indebted to Omura Consulting GmbH (D. U.) and the Uehara Memorial Foundation (D. U.) for financial support.
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