Chemical Engineering Science 71 (2012) 239251Contents lists available at SciVerse ScienceDirectChemical Engineering Science0009-25
rjcrewe1 Te2 Tejournal homepage: www.elsevier.com/locate/cesA theoretical and experimental investigation of intumescent behaviour inprotective coatings for structural steelJ.E.J. Staggs a,n, R.J. Crewe b,1, R. Butler c,2
a Energy Research Institute, University of Leeds, Leeds, LS2 9JT, United Kingdomb Department of Forensic and Investigative Sciences, University of Central Lancashire, Preston, PR1 2HE, United Kingdomc International Paint Ltd., Gateshead, NE10 0JY, United Kingdoma r t i c l e i n f o
Received 27 August 2011
Received in revised form
5 December 2011
Accepted 9 December 2011Available online 17 December 2011
Steel protection09/$ - see front matter & 2011 Elsevier Ltd. A
viations: CHTC, Convection heat transfer co
nt; MLC, Mass loss calorimeter; TGA, Therm
tial thermal analysis; DSC, Differential scann
esponding author. Tel.: 44 133 343 2495.ail addresses: firstname.lastname@example.org (J.E.J. Sta
@uclan.ac.uk (R.J. Crewe), rachel.butler@akzo
l.: 44 1772 89 3578.l.: 44 191 401 2432.a b s t r a c t
A mathematical model describing heat transfer and expansion processes within an experimental
intumescent coating is described. The model has been developed alongside a relatively comprehensive
experimental programme involving analytical methods, standard and non-standard furnace tests and
mass-loss calorimeter (MLC) tests. The model is fully continuous (rather than semi-discrete as in other
approaches) and uses a simple competitive reaction scheme to describe the kinetics of the initial gas-
forming step of the coating degradation reaction. The degradation mechanism is coupled with a char
expansion sub-model, where a fraction of the evolved gas is trapped causing expansion. This scheme
incorporates endothermic and exothermic reactions, the heats of which have been estimated from DTA.
Much effort has been expended on a realistic description of the heat transfer processes within the
expanding char and a detailed composite thermal conductivity model including radiation transfer
across pores is included. This has been calibrated for fully expanded chars using empirical temperature
dependent thermal conductivity data. Model results compare well with furnace test results. However,
results from MLC experiments demonstrate a larger than expected range in coating expansion than
predicted by the model. These observations emphasise the importance of the basic expansion
mechanism and demonstrate that this critical area requires more research.
& 2011 Elsevier Ltd. All rights reserved.1. Introduction
The practice of protecting structural steel members inbuildings with intumescent coatings is now well established.In the event of a fire, the coatings are designed to expand oncontact with heat to provide a thermally insulating char thatdelays diffusion of heat to the steel. The coatings are typicallyapplied at a dry film thickness (DFT) of a few mm and so donot interfere with the architectural aesthetics of the steelmember. The expansion ratios are typically high of the orderof 10 or more and the resulting chars are therefore highlyporous, with low effective thermal conductivities at roomll rights reserved.
efficient; HTC, Heat transfer
ogravimetric analysis; DTA,
ing calorimeter; DFT,
nobel.com (R. Butler).temperature (at elevated temperature thermal conductivity isaugmented by radiation heat transfer across the char pores).Provided that the expanded char remains in place during thefire, the enhanced thermal resistance imparted to the steelimplies that there is greater time available for evacuation orfire-fighting before the structural strength of the member iscompromised.
The process of intumescence is complex and despite the factthat it has been exploited in commercial coatings, remains poorlyunderstood. It involves an interplay of physical and chemicalprocesses that must occur in correct sequence in order to producethe insulating char. Furthermore, since the char must be highlyporous to provide thermal insulation, the average wall thicknessof the solid matrix must necessarily be low. This in turn presentsdifficulties in maintaining sufficient strength so that the charremains in place during a fire.
The fire resistance of a commercial coating is tested usingexpensive, large-scale methods where a coated beam section issubjected to a prescribed temperature regime such as describedin ISO 834-1 or BS EN 1991-1-2:2002. These tests can beconducted on free-standing beam sections (when the time takento reach a specified temperature is the test criterion) or on
Fig. 1. Simple competitive mechanism for char formation.
mc + rg mg
(1 rg) mg Gas escapes
Fig. 2. Simplification of the expanded char forming process.
J.E.J. Staggs et al. / Chemical Engineering Science 71 (2012) 239251240horizontal loaded sections, where the deflection of the beam isalso part of the test criteria. The need for a detailed understandingof the transient heat transfer properties of an expanding char isclear and a predictive tool to aid product development byreducing the number of experimental tests is therefore important.Whereas the performance of an inert insulating coating is rela-tively well understood and quantifiable (Staggs, 2011a), a dyna-mically expanding char, where the ultimate insulation propertiesdepend on the heating regime, is not well understood. There havebeen several attempts to model heat transfer in intumescentsystems and these are neatly reviewed by Griffin (2010). Most ofthe more complex models use a kinetic scheme of parallelreactions to model char degradation (Di Blasi and Branca 2001;Di Blasi, 2004; Griffin, 2010) where the Arrhenius parameters areestimated from TGA data. In this approach it is argued that themost important step in the degradation process is the gas-producing step and rather than adopt an involved description ofthe reaction kinetics across the whole temperature spectrum, acompetitive scheme is used to describe the behaviour in thevicinity of this region. The model is 1-D in space, assuming thatthe char expansion is normal to the coated substrate. Althoughexpansion is included, allowing for change of volume with time,the equations are re-cast using a front-fixing transformation forconvenience in the numerical solution. Standard finite differencemethods are used with adaptive time-stepping to solve thetransformed equations in the constant volume domain. Thetime-stepping method is fully implicit and the discrete nonlinearequations are solved at each time level using an iterative relaxa-tion method.
This paper describes the results of a two-year collaborationbetween the University of Leeds and International Paint Ltd. (acompany that is part of the Akzo Nobel group). The project waspart-sponsored by the Technology Strategy Board of the UKgovernment (the remainder of the funding coming from theindustrial partner) and its aim was to investigate the behaviourof an experimental intumescent coating in a variety of testconditions. The main goal of the work was to develop amathematical model of intumescence, guided by experimentalobservation and analytical data, whose effectiveness could beevaluated directly against high-quality test data. Although ana-lytical methods such as TGA, DTA, He pycnometry and tempera-ture-dependent thermal conductivity measurements have beenused in the work, the bulk of the experimental effort wasdirected towards obtaining data from standard and non-stan-dard furnace tests (utilising steel plates rather than beam orcolumn sections) and also from bench-scale mass-loss calori-meter (MLC) tests.
2. Mathematical model
2.1. Char formation sub-model
A detailed description of the chemistry of the degradationprocesses involved in forming the intumescent char from thevirgin coating is beyond the scope of this work and will beinvestigated in a separate paper. Instead, the focus is on obtainingthe simplest description of the reaction kinetics that adequatelydescribes the main mass loss step observed in TG experiments.This step is associated with the formation of the blowing agentand hence char expansion. It transpires that a simple competitivereaction pair as shown in Fig. 1 is sufficient to crudely approx-imate the mass loss process. Naturally this mechanism is far toosimplistic to describe any of the fine details of the char-formingprocess.
Thus if a small enough section of coating is considered, suchthat temperature gradients are negligible, it is assumed that theequations defining the amounts of unreacted coating, gas andsolid char are given by
where kj AjexpTj=T, jc, g. Here Aj and Tj represent pre-exponential factors and activation temperatures respectively.
The mechanism of char expansion is problematic in anyintumescent model. In this approach the char mc in the compe-titive reaction is initially viewed as an expanded char precursor.Gas is then trapped withi