Chitosan-tannin adhesive: Fully biomass, synthesis-free and high performance for bamboo-based composite bonding
Introduction
Bamboo is a sustainable natural material widely used in furniture manufacturing, decking and building. In contrast to wood, bamboo has much shorter growth periodicity, better mechanical properties like higher toughness and hardness, as well as better water resistance [1], [2], [3]. Benefitting from its attracting features, a series of bamboo composites, such as laminated bamboo [4], plybamboo [5], scrimber [6], bamboo strand-based composites [7], et al., have been developed for structural application. However, due to polylamellate cell wall, low tissue porosity and permeability, as well as poor surface wettability, adhesion of bamboo is still a challenge. Especially, poor bonding strength leads to unstable mechanical property and dimensional stability. Currently, bamboo composites are mainly bonded with phenol-formaldehyde (PF) [7], [8], [9], melamine-urea-formaldehyde (MUF) [10], polymeric diphenylmethane diisocyanate (pMDI) [10], polyurethane (PUR) [10], polyvinyl acetate (PVAc) [10]. However, the sustained release of formaldehyde from formaldehyde-based adhesives threatens human health and living environment. The formaldehyde-free adhesives suffer from problems like high cost, complicated application procedures, occupational health and safety due to toxicity. Therefore, it is urgent to develop safe and environmental-friendly adhesives which are suitable for bamboo bonding.
During the past decades, green and sustainable wood adhesives, namely biomass-derived adhesives, like oil- [11] and protein-based [12] resins, have been developed for manufacturing of wood products. In recent years, polymers that can mimic the structure of natural adhesives have been attracting increasing attention and they are so-called nature-inspired adhesives [13], [14]. Among these adhesives, phenol-amine adhesives that mimic the secretory proteins of mussels' foot-byssal threads has become one of the hot topics [15], [16], [17]. In-depth studies have revealed that 3,4-dihydroxyphenylalanine (DOPA) residues containing catechol groups are responsible for its remarkable adhesion on wet rock or any other surfaces [18], [19]. Some studies have been carried out by incorporating amino groups and catechol moieties into synthetic polymers to mimic the structure of the adhesive protein, promoting the emerging of different polyamine-catechol adhesives like polyethyleneimine (PEI)-pyrogallol [20], PEI and depolymerized tannin-modified phenolic resin (PEI-DTPF) [21], polyamines-tannin acid [22], and PEI-tannin [23]. However, these adhesives mainly rely on unrenewable petroleum-based polyphenols or synthetic polyamines. PEI-tannin adhesive is a typical example which involves natural polyphenol and synthesized polyamine [23]. Although PEI-tannin adhesive exhibited excellent water resistant, use of the hazardous aziridine monomer with toxicity (LD50: 14 mg/kg) and volatility (Tb: 57 °C) leads to healthy risks in PEI manufacturing process [24]. Seeking suitable substitutes that are safe and sustainable is a valid way to avoid use of petroleum-based and hazardous substances.
Chitin as natural polysaccharides, the abundance on earth is only next to cellulose and widely spread in crustaceans, fungi, and so on [25], [26]. Its main derivative, chitosan, can be industrially obtained from deacetylation [25], [26]. With rich amine groups along the polysaccharide backbone, chitosan provides various applications, including food [26], medicine [27], [28], and absorption materials [29]. However, studies focused on applications of chitosan in the field of wood and bamboo adhesives are still rare. Based on the understanding of phenol-amine chemistry, a hypothesis has been made in this study that chitosan could be an ideal candidate polyamine in designing bio-mimicking adhesive since it contains richer aliphatic primary amino groups than PEI or any other petroleum-based polyamines. Further, combination of chitosan with natural condensed tannin which have rich catechol moieties [30] would result in high-performance adhesive, especially lower cure temperature would be an advantage over other reported phenol-amine adhesives due to the rich reactive groups existing in chitosan-tannin system. To confirm the above hypotheses and develop new adhesive for bamboo bonding, in this study, chitosan-tannin (CST) adhesive that mimics the natural adhesive protein was fabricated and used for bonding of bamboo-based composites. To the best of our knowledge, similar studies have not been reported.
Section snippets
Materials
Chitosan (deacetylation degree ≥85 %, Mw = 30,000) and formic acid (88 %) were purchased from the Shanghai Macklin Biochemical Co., Ltd. Bayberry (Myrica rubra (Lour.) S. et Zucc.) tannin (> 70 %) was purchased from Guangxi Wuming Tannin Extract Factory. Moso bamboo (Phyllostachys edulis (Carriere) J. Houzeau) sheet was purchased from an online store. Poplar veneers were provided by a local hardware store.
Preparation of chitosan-tannin adhesives
Tannin was mixed with water to obtain tannin aqueous solution (30 %, wt%). Then, chitosan
Design strategy of CST adhesive
As shown in Fig. 2, in this study, a fully green bio-inspired adhesive was designed by combining natural amino-rich chitosan with natural polyphenols, tannin. Note that the physical-chemical properties of chitosan heavily depend on deacetylation degree and molecular weight (Mw) [25]. One of the key problems is the low solubility of chitosan in general organic or inorganic solvents except for acidic solvents. But this is not a problem in this study. In experiments, it was found that chitosan can
Conclusions
In this work, it was confirmed that combination of amino-rich chitosan and catechol-rich tannin is a successful strategy for mimicking the natural phenol-amine adhesive system, as the chitosan-tannin (CST) adhesive demonstrated excellent bonding strength for wood and bamboo substrates. It was also confirmed that the CST adhesive has low cure temperature as it exhibited excellent water resistance for both bamboo and wood bonding even low hot-press temperature of 100 °C was used, superior to
CRediT authorship contribution statement
Shuyang Jiang: Investigation, Methodology, Writing - original draft. Shouqing Liu: Investigation, Tests, Formal analysis. Shengtao Wang: Investigation, Tests, Formal analysis. Xiaojian Zhou: Investigation, Tests, Formal analysis. Jing Yang: Investigation, Tests, Formal analysis. Zhengjun Shi: Investigation, Tests, Formal analysis. Zhaojin Yang: Investigation, Tests, Formal analysis. Guanben Du: Supervision, Writing-review & editing. Taohong Li: Conceptualization, Funding acquisition,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (grant number 31870715), the Scientific Research Foundation of the Department of Education of Yunnan Province (grant numbers 2022Y556 and 2022Y568), the 111 project (D21027), the YSZJGZZ-2020052 project and the Xingdian Talent Program of Yunnan Province.
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