Morphological Study of Bio-Based Polymers in the Consolidation of Waterlogged Wooden Objects.

Zarah Walsh-Korb,Gerhard Eggert,Luc Avérous,Juliana Dos Santos Gabriel,Ingrid Stelzner

doi:10.3390/ma15020681

Abstract

The removal of water from archaeological wooden objects for display or storage is of great importance to their long-term conservation. Any mechanical instability caused during drying can induce warping or cracking of the wood cells, leading to irreparable damage of the object. Drying of an object is commonly carried out in one of three ways: (i) air-drying with controlled temperature and relative humidity, (ii) drying-out of a non-aqueous solvent or (iii) freeze-drying. Recently, there has been great interest in the replacement of the standard, but limited, polyethylene glycol with biopolymers for wood conservation; however, their behaviour and action within the wood is not completely understood. Three polysaccharides—low-molar-mass (Mw) chitosan (Mw ca. 60,000 g/mol), medium-molar-mass alginate (Mw ca. 100,000 g/mol) and cellulose nanocrystals (CNCs)-are investigated in relation to their drying behaviour. The method of drying reveals a significant difference in the morphology of these biopolymers both ex situ and within the wood cells. Here, the effect these differences in structuration have on the coating of the wood cells and the biological and thermal stability of the wood are examined, as well as the role of the environment in the formation of specific structures. The role these factors play in the selection of appropriate consolidants and drying methods for the conservation of waterlogged archaeological wooden objects is also investigated. The results show that both alginate and chitosan are promising wood consolidants from a structural perspective and both improve the thermal stability of the lignin component of archaeological wood. However, further modification would be necessary to improve the biocidal activity of alginate before it could be introduced into wooden objects. CNCs did not prove to be sufficiently suitable for wood conservation as a result of the analyses performed here.

Highlights

A large number of artefacts of cultural importance are organic in nature [1], of which wood is a common, but highly important, material [2]
The determination of the freezing point of each of the biopolymers found that at −19.4, −15.0 and −12.6 ◦C for chitosan, cellulose nanocrystals (CNCs) and alginate, respectively, structuration was observed throughout the sample, and this was taken to be the freezing point
The morphology of the treatments upon drying, how this is affected by the drying protocol and whether this plays a role in the biological resistance or thermal stability of the treated wood was investigated

Summary

Introduction

A large number of artefacts of cultural importance are organic in nature [1], of which wood is a common, but highly important, material [2]. When bound water is removed from the cell wall, shrinkage of the wood can occur. To avoid shrinkage of the wood and strengthen the degraded object, free water is often exchanged for an aqueous polymer solution, typically poly (ethylene glycol) (PEG). The higher molar mass and much lower vapour pressure of PEG compared to water allows it to remain in the wood cells during drying [21]. Mortensen et al [21] observed that the natural aging of PEG produces formic acid, which exacerbates degradation of the acid-sensitive cellulose and holocellulose within the wood. Bardet et al [28,29,30] and Bjurhager et al [4] observed that the use of high concentrations of hygroscopic lowmolar-mass PEG consolidants could, over time, lead to plasticisation and deformation of wood cells [31]

Methods

Results

Conclusion