Poly 2-ethyl-2-oxazoline/microcrystalline cellulose composites for cultural heritage conservation: Mechanical characterization in dry and wet state and application as lining adhesives of canvas Annalisa Cataldi n , Flavio Deflorian, Alessandro Pegoretti University of Trento, Department of Industrial Engineering and INSTM Unit, Via Sommarive 9, 38123 Trento, Italy article info Article history: Accepted 22 June 2015 Available online 21 July 2015 Keywords: Cellulose Composites Moisture Mechanical properties Adhesion Canvas abstract The aim of this work is the investigation of the stabilizing effect of microcrystalline cellulose powder (MCC) on the mechanical performance of two commercial thermoplastic resins (Aquazol s 200 and Aquazol s 500) used as adhesives in the conservation of artworks. The two polymers, having different molecular weights, were melt-compounded and compression molded with various amounts of MCC (5– 30 wt%). The mechanical response of the microcomposites under dry and wet (equilibrium at 23 °C and a R.H. 55%) conditions, was investigated. DMTA analysis showed an increase of the dynamic moduli and the glass transition temperature with the microfiller content more pronounced for conditioned samples over the dried ones, and a concurrent decrease of the thermal expansion coefficient. Creep tests showed that MCC caused an improvement of the creep stability (i.e. a reduction of the creep compliance) for both dried and conditioned samples. For wet samples, the simultaneous enhancement of the elastic modulus and the stress at break limited the embrittling effect detected for dried composites. These materials were applied as lining adhesive for oil paintings between two kinds of canvas an English linen and a woven polyester under environmental conditions at temperature of 23 °C and a relative humidity of 55%. Single- lap shear tests both in quasi-static and creep conditions confirmed the improvement of the dimensional stability provided by MCC with a reduction of the joint displacement and an increase of the adhesive strength as the filler content increases. Additionally, post-fracture optical microscope observations of the cross-sections of the adhesive area proved how MCC introduction did not change the fracture behavior of the neat adhesives. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Aquazol, a poly(2-ethyl-2-oxazoline), has been used in the field of cultural heritage conservation since the early 1990s [1], but scientific studies on this synthetic resin have been carried out since the 1980s [2–4]. Its reported high flexibility even at low humidity level [5,6], ability to make protective film, good cap- ability to penetrate into fine cracks and easy removability, being a water soluble polymer, represents the main reasons that led to the introduction of this polymer in the conservation of works of art. Aquazol, generally called PEOX, is a versatile polymer soluble in a wide range of organic solvents. Aliphatic tertiary amides present a chemically stable form of nitrogen, therefore PEOXs tend to guarantee a good aging behavior [7,8]. Additionally, it has a good thermal stability and bonds to a broad range of materials [7]. The most common use of this thermoplastic resin is in coating formulations [9], as pigment dispersants in inks [10] and as adhesive for various types of artworks [7,11]. Its refractive index is close to that of soda-lime glass and therefore Aquazol was ori- ginally applied as consolidant for glass objects [4]. Since 1930s, synthetic resins have been introduced in the lining of paintings in order to replace natural glue/paste adhesives that may suffer of embrittlement and are stiff [12]. In the 1990s Aquazol was intro- duced as adhesive to the restoration of easel paintings [4]. The results of accelerated aging studies demonstrated how Aquazol polymers are able to maintain both their neutral pH and their stability with aging [4,13]. Nowadays, conservators utilize this polymer also as adhesive on friable paint films and cracked gilding in order to readhere the paint film onto the preparation layers and original canvas [11,13]. On the other hand, Aquazol is very sensitive to water sorption especially at elevated humidity levels. Conditioning levels over 50% of relative humidity significantly affect its mechanical beha- vior. The plasticizing action due to the moisture content empha- sizes its rubber-like behavior leading to a decrease of its Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ijadhadh International Journal of Adhesion & Adhesives http://dx.doi.org/10.1016/j.ijadhadh.2015.07.002 0143-7496/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ39 0461 282411; fax: þ39 0461 281977. E-mail address: annalisa.cataldi@ing.unitn.it (A. Cataldi). International Journal of Adhesion & Adhesives 62 (2015) 92–100