Over the last few decades, acrylic resins have been one of the most widely-used materials in conservation. Restorers use these resins, known as Paraloid® (or Acryloid® in USA), as a coating, consolidant or adhesive because of their relative stability, transparency, mechanical resistance and reversibility [1, 2]. They are suitable for all types of material: metals, stone, wood, glass or ceramics [3]. The most frequently used resin is Paraloid® B72. Many recent studies confirm the good qualities of Paraloid® for different restoration treatments [4,5,6,7,8]. Paraloid® is very used as a reference when testing or studying new products [9,10,11,12,13,14,15]. Its preparation depends on its intended function (protection, consolidation or adhesion) and use. Koob [2] has optimized the preparation and application of Paraloid® B72 as an adhesive for ceramics; he highlights the importance of choosing the right solvent for appropriate workability. He recommends the use of acetone (with a 70/30 ratio of acetone/Paraloid® B72), but other solvents, such as ethanol, toluene, xylene or ethyl acetate [16] may also be used by restorers. Paraloid® B72 is a copolymer of ethyl methacrylate (EMA) and methyl acrylate (MA) with a ratio of 70/30. However, analysis by GC–MS (gas chromatography–mass spectrometry) of the commercial product revealed that an additional compound, butyl methacrylate (BMA) made up 2% of content [17]. According to Horie [3], it is possible to make an acrylic resin with the required physical properties, just by selecting a different monomer. Many acrylic resins are available on the market: for example, Paraloid® B44, B48N, B66, B72 and B82 [17]. However, their composition is not always as clearly specified as that of Paraloid® B72; thus Paraloid® B44, also widely used in conservation, is sold as poly (-methyl methacrylate) (PMMA), but analysis carried out by Chiantore et al. [17] revealed it to be a copolymer of methyl methacrylate (MMA) and ethyl acrylate (EA) in unknown proportions. It is easy to adapt the properties of the resin to circumstances and the type of application [16]. Restorers choose a specific resin according to how it is to be applied (adhesion, protection or consolidation) and the materials (porous, such as ceramics, or non-porous such as porcelain). They adjust the concentration of the resin and the choice of solvent: too low a concentration may result in unfilled spaces, and too high a concentration leads to reduced resin penetration [18,19,20]. The choice of a volatile solvent, such as acetone, also impedes resin penetration; a less volatile solvent, such as toluene, improves the degree of resin penetration into the material, allowing it to be effectively consolidated [16]. The choice of resin, however, should also take into consideration relevant environmental conditions. Ageing studies have integrated environmental conditions, focusing on UV radiation as the main parameter [21]. Nevertheless, environmental conditions, in particular temperature, are a determining factor in the choice of resin because room temperature must be higher than the Tg of the resin [22, 23]. Alexiou et al. [23] performed measurements of mechanical properties at 0 °C, 30 °C and 50 °C, and showed that Paraloid® B72 gives poor results at higher temperatures. The use of Paraloid® B72 as a ceramic adhesive proved unsuitable in Greece (where summer temperatures are around 40 °C) and in hot climates, as might be expected from its low Tg. Paraloid® B72, manufactured by Rohm and Haas, has a Tg of 40 °C, a value very close to summer temperatures in the Mediterranean. This value, provided by the manufacturer, may differ from measurements recorded in experiments: a range value reported between 37 and 41 °C [22], but Paraloid® B72 is known to soften at 30–35 °C [1]. These studies are past, but to our knowledge, the most recent work on Paraloid are student reports that have not been published. Glass transition temperature is an important phenomenon in polymer studies [24]. Chapman et al. [16] define Tg as “that temperature at which the available thermal energy is smaller than the forces holding molecules together. At lower temperatures, very little molecular adjustment is possible. Below its Tg, an amorphous polymer is brittle and hard; above its Tg, it is softer and can be dissolved more easily.” This temperature characterizes the change of the resin from a solid, ‘glassy’ state to a softer, ‘rubbery’ state [22]. It is difficult to compare Tg data taken from different literature references; few references state the method used to determine Tg, the measurement parameters, or the conditions of sample preparation (solvent, drying time and drying conditions prior to measurement). These parameters can have a significant impact on the value of Tg. Currently, restorers choose their adhesive according to the Tg values provided by manufacturers, without taking the impact of residual solvents into account. So, for the reconstruction of a Tang dynasty model of a horse, Ramakers [25] chose Paraloid® B44 as an adhesive in order to benefit from a Tg that is higher than Paraloid® B72 and well above room temperature. At Arc’Antique, we aimed to test mixtures of Paraloid® B44 and B72 but first it was necessary to determine the Tg of these adhesives. Different laws of mixtures have been proposed to describe the evolution of Tg for miscible mixtures. The best known of these are Gordon Taylor’s and Fox’s Law [26]. Gordon Taylor’s Law applies to copolymers, but can be extended to polymer mixtures:
$$T_{g,m } \, = \,\frac{{\varphi_{1 } T_{g,1} + K \left( {1 - \varphi_{1} } \right)T_{g,2} }}{{\varphi_{1} + K\left( {1 - \varphi_{1} } \right)}}\, {\text{with}}\,K = \frac{{\rho_{1} \Delta \alpha_{2} }}{{\rho_{2} \Delta \alpha_{1} }}$$
with Tg,m the glass transition temperature of the mixture, Tg,1 the glass transition temperature of Paraloid® B72, Tg,2 the glass transition temperature of Paraloid® B44, φ1 the mass fraction of Paraloid® B72, ρ1 and ρ2 the density of Paraloid® B72 and B44, α1 and α2 the coefficient of the linear thermal expansion of Paraloid® B72 and B44.
The density of Paraloid® B72 is 9,6 lb/gal (1150.3 kg/m3) with α1 of 6.3 × 10−5 K−1 [27], the density of Paraloid® B44 is 9,8 lb/gal (1174,3 kg/m3) with α2 of 6 × 10−5 K−1 [28]. In our case, Gordon Taylor’s constant is equal to 0.93. As the densities are close, Gordon Taylor’s law can be simplified by Fox’s law:
$$\frac{1}{{T_{g,m} }} = \frac{{\varphi_{1} }}{{T_{g,1} }} + \frac{{\left( {1 - \varphi_{1} } \right)}}{{T_{g,2} }}$$
The objectives of this paper are to measure the Tg of Paraloid® B44 and B72 pure or mixtures. The comparison of the theoretical and experimental values will allow us to evaluate the impact of the solvent’s choice.