Materials Science and Engineering A 521–522 (2009) 283–286 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea Study of epoxy resin curing process by applying internal friction technique M. Kubisztal a,∗ , G. Haneczok a , A. Chrobak b , A. Kubik a , J. Rasek a a Institute of Materials Science, Silesian University, 40-007 Katowice, Bankowa 12, Poland b Institute of Physics, Silesian University, 40-007 Katowice, Uniwersytecka 4, Poland article info Article history: Received 5 June 2008 Accepted 2 September 2008 Keywords: Curing process Epoxy resin Internal friction abstract Curing process of an amine-epoxy system was investigated using vibrating reed technique and torsion pendulum apparatus, working at frequencies of about 100 Hz and 1 Hz, respectively. The measurements of mechanical spectra were carried out in isothermal conditions (300–325 K) at different frequencies in time domain i.e. in the course of chemical reaction between epoxy resin and a hardener. It was shown that the curing reaction is observed as an asymmetrical maximum of Q -1 (t) which position is correlated with the inflection point of free vibrations frequency f(t) curve. An increase of the measurement temperature and frequency causes a shift of the observed mechanical spectra into shorter times. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Epoxy resins are network-forming polymers displaying several interesting characteristics, e.g. good chemical resistance, adhe- sion to the most varied substrates, good electrical resistance and mechanical properties. Because of this attributes they have found many applications in different industries (e.g. electronics, aeronau- tics or astronautics) as protective coatings, fiber reinforced plastics, adhesives, etc. In general, the properties of this kind of materials result from the ability of forming a three-dimensional network created during chemical reaction of the epoxy resin and an appropriate harden- ing agent. The epoxy resin curing usually proceeds in two stages – the so-called gelation and the vitrification. The gelation is an irre- versible process occurring at well-defined stage in the course of the reaction. In contrast to this the vitrification may occur at any stage during the cure and means a transformation from a viscous liquid or an elastic gel to a glass state [1–8]. The curing process is usually investigated by applying different experimental methods like DSC (differential scanning calorime- try), TSR (thermal scanning rheometry), DMA (dynamic mechanical analysis). The aim of the present paper is to study an application of internal friction technique in the examination of the curing pro- cess. In this case the epoxy resin can be applied as a relatively thin layer on a metallic substrate. During the chemical reaction mechanical properties of the examined layer changes drastically which can be observed via internal friction techniques. Taking into account the fact that the curing process is a highly exothermic ∗ Corresponding author. Tel.: +48 32 359 21 33; fax: +48 32 359 21 33. E-mail address: mkub@us.edu.pl (M. Kubisztal). reaction the proposed approach seems to be especially advanta- geous because a thin layer on a metallic substrate allows avoiding a spontaneous self-heating of the examined sample. This effect (much more important for massive samples) may cause that the reaction temperature is not well controlled, and what follows, the obtained results are not reliable enough. Moreover, internal friction technique i.e. measurements of free vibrations frequency allows to calculate some important technical parameters like shear modu- lus, apparent Young’s modulus or adhesion of the coating to the substrate [9,10]. 2. Experimental In order to study the curing process of amine-epoxy system the examined samples were prepared of aluminum alloy sub- strate (a plate made of EN AW-5251 alloy with dimensions of 0.5 mm × 5 mm × 50mm) on which the amine-epoxy layer (thick- ness about 10 m) in a form of viscous liquid was deposited. The reason that this alloy was taken as a substrate is that aluminum- based materials are difficult to weld and in this case it is preferred to make adhesive joints between parts to be connected. The amine- cured epoxy resin is commonly used as an adhesive but it should be noted that curing kinetic and mechanical properties of the system depend mostly on curing conditions. The amine-epoxy layer was made of a low molecular (<700) epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) and a triethylentetramine (TETA) used as hardening agent. Chemical structures of both ingredients were depicted in Fig. 1. In the present research the constant ratio between the resin and the hardener i.e. 12 parts of curing agent per 100 parts of the resin was assumed. For samples prepared in this way loss tangent (Q -1 ) and fre- quency of free vibrations were measured versus curing time at 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.09.127