PSI - Issue 10

E. Tsetsekou et al. / Procedia Structural Integrity 10 (2018) 227–234

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E. Tsetsekou et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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mechanically to an already deteriorated object. Hence, before selecting a potential adhesive, its performance is assessed by several criteria including i) long term stability; ii) reversibility; iii) characteristics after curing such as colour, hardness, brittleness and sensitivity to relative humidity (RH) and temperature (T); iv) bond strength; v) physical characteristics such as open and closed time 1 , glass transition temperature (Tg), molecular weight, viscosity, concentration; vi) health and safety issues and vii) cost and ease of application (Down (1984); Maynor and Reyden (1989); Unger et al. (2001); Rivers and Umney (2003)). The majority of these criteria are probably common for the selection of adhesives regardless of their use or the scientific field. However some of them, such as reversibility, bond strength and long term stability are quite different in the field of CH conservation. Regarding reversibility there are different levels ranging from the complete removal of the adhesive from the surface to the mere softening of the adhesive in order to dissemble the bonded surfaces. In conservation, reversibility is a prerequisite in order to allow the re-treatability of the objects in case of glue failure. Therefore the removal of the adhesive should not, in any case, cause damage to the artifact's surface or prevent future alternative treatments (Maynor and Reyden (1989)). The second criterion that diverts from the ordinary is the bond strength of the adhesive. The general standard in performance of wood adhesives is that the wood breaks away from the adhesive joint and that the bond strength is equal to the strength of the solid wood (Frihart and Hunt (2010)). In CH wooden artifacts, when stress is applied, it is preferable for the adhesive bond to be broken (Fig.1a) or even the bond between wood and glue (Fig.1b) instead of the object itself (Fig.1c). Therefore the bond should be strong enough to hold the surfaces together, but it should be weaker than wood's strength (Unger et al. (2001)). Finally a conservation material should have a long-term stability such as “ class A ” 2 materials, as “ re-conserving ” is often prohibited by the bad condition of objects. Polymers’ photochemical stability corresponds usually to their ability to retain their physicochemical properties over time. Alteration of these properties is often indicated by equivalent changes in their optical properties such as color and gloss (Down (1984); Feller (1978)).

Fig. 1. Types of adhesive failure.

Therefore this work studied comparatively the performance of three natural and two synthetic adhesives commonly used in conservation of dry wood, with respect to a) bond strength under shear stress, b) reversibility and c) colour stability over time. The adhesives performance was evaluated, before and after accelerating ageing with UV as well as RH-T cycles.

2. Materials and methods

Two different species of wood, Abies alba L. (fir) and Acer pseudoplatanus L (maple) were used. The wood samples were free from imperfections, such as knots and cracks and their dimensions were 19.0 x 44.4 x 50.8 mm

1 Open time refers to the time allowed to elapse following the application of the adhesive to the completion of the assembly. Closed time refers to the length of time after which changes in the assembly are either impossible or will affect gravely the strength of the bond (Rivers and Umney (2003)). 2 Class A materials are stable for at least 100 years, Class B materials last from 20-100 without significant changes and Class C materials degrade in less than 20 years (Feller (1978)).