Investigation of the effect of relative humidity on polymers by depth sensing indentation K. Altaf • Ian A. Ashcroft • Richard Hague Received: 25 January 2011 / Accepted: 20 June 2011 / Published online: 9 July 2011 Ó Springer Science+Business Media, LLC 2011 Abstract Stereolithography (SL) resins absorb varying amounts of moisture dependent on the relative humidities, which can significantly affect the mechanical properties. In this work, the influence of relative humidity (RH) on the mechanical behaviour of an SL resin is investigated using depth sensing indentation (DSI). The samples were con- ditioned by two methods. In the first method, samples were pre-conditioned at 33.5, 53.8, 75.3 and 84.5% RH using saturated salt solutions. These preconditioned samples were tested at 33.5% RH, using a humidity control unit (HCU) to control RH in the DSI system. In the second method, samples were conditioned and tested at 33.5, 53.8, 75.3 and 84.5% RH by regulating humidity in the DSI system using the HCU. Temperature was kept constant at 22.5 °C for the conditioning and DSI testing. It was seen that hardness and modulus decreased with increasing RH and conditioning time but recovered significantly when tested after drying. This study demonstrates that RH needs to be taken into account during the DSI testing of polymers. Introduction The additive approach to manufacturing enables designers to design complex geometries without the constraints and costs associated with conventional manufacturing tech- niques. This approach to manufacturing is termed rapid manufacturing or additive manufacturing (AM) [1]. AM is becoming recognized as an alternative manufacturing technique but it still faces challenges, in particular the limitations of current AM materials, hence, material research is currently a major focus of study to make AM a reliable manufacturing method [2]. SL is one of the main processes of AM and is considered highly accurate and consistent [3]. However, owing to the sensitivity of SL materials to high levels of RH and long-term UV degra- dation, the SL process has limited current use for producing end-use parts [4]. One of the areas that require major development is the environmental stability of the SL materials post-build. In polymers, moisture absorption can lead to a wide range of effects, both reversible and irreversible, such as plastici- zation by weakening of the intermolecular interactions among the functional groups of the chains [5, 6], de-bonding at filler-matrix interfaces [7–9], leaching of un-reacted functional groups [10], structural damage such as micro- cavities or crazes [11, 12] and chemical degradation of the polymer matrix due to hydrolysis and oxidation [11–13]. It can also involve the generation of free radicals or other reactive species, which may act as plasticizers or reactants [14, 15]. Long-term exposure can decrease the molecular weight due to chain scission or the breaking of cross-links in the polymer network [16]. Absorbed moisture significantly affects the mechanical properties and glass transition tem- perature (T g ) of polymeric materials [6, 7, 9, 17, 18]. The changes in mechanical property of polymeric components due to moisture absorption can be examined by performing mechanical tests after moisture conditioning samples. SL parts can exhibit spatial variations in their mechan- ical properties and this can be evaluated using DSI tests. However, analysis of the results from the DSI testing of polymers is challenging because of their complex structure and time-dependent deformation [19]. This time-dependent viscoelastic (VE) or viscoplastic (VP) deformation leads to K. Altaf (&) Á I. A. Ashcroft Á R. Hague Wolfson School of Mechanical & Manufacturing Engineering, Loughborough University, Leicestershire LE11 3TU, UK e-mail: K.Altaf@lboro.ac.uk 123 J Mater Sci (2011) 46:7551–7557 DOI 10.1007/s10853-011-5729-8