Modelling Thermal Degradation of Flame-Retarded Epoxy Resin

Downloaded by PENNSYLVANIA STATE UNIV on June 28, 2012 | http://pubs.acs.org Publication Date: April 27, 2009 | doi: 10.1021/bk-2009-1013.ch022

Chapter 22

Modelling Thermal Degradation of Flame-Retarded Epoxy Resin Formulations under Different Heating Conditions 1

1

1

Everson Kandare , Baljinder K. Kandola , Richard A. Horrocks , and John E . J . Staggs 2

1

Centre for Materials Research and Innovation, University of Bolton, Deane Campus, Bolton BL3 5AB, United Kingdom School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom

2

The thermal degradation kinetics of epoxy resin, triglycidyl-p– aminophenol (TGAP), and its flame-retarded formulations are investigated by thermogravimetric analysis (TGA) under oxidative conditions and are modelled using a simple global multi-step mechanistic scheme. Experimental T G A data acquired under non-isothermal conditions is used to optimise the kinetic parameters for selected boundary linear heating rates. These optimised kinetic parameters which adequately describe the thermal degradation process of epoxy resin formulations over the defined range of experimental conditions are then applied in the prediction of mass loss profiles of investigated materials under different heating conditions.

368

© 2009 American Chemical Society

In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

369


Introduction In recent years, interest in flame-retardants' (FRs) use in thermoplastics and thermosetting resin formulations has been focused on nitrogen and phosphorous containing additives due to their low toxicity and low smoke volume productions (1-4). The addition of FRs to polymer matrices, considerably affect the thermal stability of the resultant polymer-additive composite material. The thermal degradation behaviour of the resin formulations containing FR additives can be assessed via thermogravimetric analysis from which kinetic parameters describing the decomposition process such as apparent activation energy (E ), Arrhenius pre-exponential factor (A\ apparent order of reaction (n) and hence the rate constant (k) can be derived. There is a vast amount of literature investigating the thermal (inert atmosphere) and thermo-oxidative degradation kinetics of flame-retarded polymeric materials but the interpretation of the extracted kinetic parameters in relation to degradation mechanisms still remains a challenge. However, these parameters may reveal the flame-retarding mechanism during solid state degradation reactions of polymeric materials from the view point of their relative apparent magnitudes. a

Thermogravimetric data collection is achieved via isothermal and nonisothermal methods with the earlier method less preferred as it does have some evident disadvantages that include; (i) samples undergoing side reactions during the process of raising the temperature to a desired value and (ii) the reaction being restricted to a single temperature (i.e. no temperature history). There are two approaches via which the kinetic parameters can be extracted from nonisothermal thermogravimetric data which are model free and model fitting methods. While the latter method has been extensively criticised for nonisothermal applications due to resultant indistinguishable fits between different models, (5, 6) its importance can not be overestimated as it may provide information on energy barriers and offer mechanistic clues. On the other hand, model free approaches allow one to examine the dependence of apparent activation energies on the conversion fraction, information which may give a clue as to the possible numbers of physical stages involved during thermal degradation. The inevitable need for cost effective ways to evaluate the thermal stability of resin formulations under different heating conditions over wide temperature ranges has prompted researchers to develop simplified kinetic schemes to predict thermal degradation of polymeric materials combining both model free and model fitting methods (7-9). Despite the absence of an obvious experiential relationship between the chemical processes occurring during thermal degradation of polymeric materials and the calculated corresponding kinetic values, the latter can be useful in an attempt to predict the rate of mass loss under different heating conditions when the collection of experimental data is time consuming; i.e. screening for effective



370 resin/flame retardant formulations for applications under different service temperatures or environments. The burning behaviour including the temperature profiles and mass loss of polymeric materials under experimental conditions that simulate real fire conditions such as cone calorimetry can be predicted via extrapolation and application of kinetic parameters to faster heating rates experienced in such cases. In this paper we aim to determine global intrinsic kinetic model parameters that can reproduce accurately the profiles of mass-temperature dependence during the thermal degradation of epoxy formulations containing flame retardants, melamine phosphate (MP) and melamine pyro-phosphate (MPP). For a given formulation, a multi-step kinetic scheme is evaluated at determined boundary heating rates and the obtained average kinetic constants then used to predict the mass loss profiles of the same resin formulation under different heating rates (within the pre-determined heating rate boundaries) with the assumption that the degradation mechanism is invariant of heating conditions. Also, the variation of kinetic parameters with heating rates is evaluated in order to assess the applicability of the predictive tool at slower and faster heating rates.

Experimental Materials and Sample Preparation The samples used in this study were kindly provided by Dr Bhaskar Biswas and were prepared in partial fulfilment of his doctoral thesis (3). These samples show interesting fire behaviours under cone calorimetry thus we decided to explore their thermal degradation kinetics in detail. Epoxy resin samples with and without flame retardants were formulated via a hot-melt method (3, 10). The sample compositions and identities are given in Table 1.

Table I. Mass percentages of various components in resin.formulations Sample composition Epoxy Epoxy + 8% M P Epoxy + 8% M P P

Sample code

Mass (%) Resin FR

EP

100

-

EP-MP

92

8

EP-MPP

92

8


371 Thermogravimetric Analysis


Thermogravimetric analysis (TGA) of uncured resin systems was performed on an SDT 2960 simultaneous D T A - T G A instrument from room temperature 900 °C using 10 ± 1 mg samples heated at constant heating rates of 5, 10, 12.5, 15, 17.5, 20 and 50 °C/min in air flowing at 100 ± 5 mL/min. The experiments were performed in triplicates and show good reproducibility.

Mathematical Modelling The thermal degradation of polymeric materials such as epoxies inescapably follow multifaceted reaction mechanisms and revealing the kinetics of every single step while it is ultimately possible, entails an extensive understanding of the thermo-chemical processes involved and demands a rigorous knowledge of computational dynamics. The kinetic fitting of thermo-analytical curves may yield vast amounts of information about the overall degradation process but can not differentiate between partially overlapping chemical reactions. However, in cases where only the thermo-physical property degradation history of epoxy resin formulations is important; for example the effect of mass loss in examining mechanical strength retention under prescribed high temperature service conditions, the use of simplified multi-step kinetic degradation schemes in global kinetics modelling is acceptable. The rate of thermal decomposition of a reactant can be approximated via a dm kinetic scheme; = f( ), where m is a vector of mass fractions, and f is a dt vector-valued function representing the degradation mechanism. First order decomposition rate constants are usually assumed to have Arrhenius dependency on temperature as shown by Eq. 1 below; m

h = Ai exp(-E /RT), i = 1 ..oo, a

[1]

where A is the pre-exponential factor, E is the activation energy, R is the gas constant and T, is the sample temperature. The sample temperature is known as a function of time, /: Τ = T + fit:, where T is the initial temperature and fi is the heating rate. Sets of experimental data in form of m, Τ (or m, t) are collected and kinetic parameters are generated from anticipated thermal degradation chemistries based on proposed mechanisms found in literature (3,11,12). The set of calculated or predicted values of mass fractions is compared with experimental values, and improved values of the kinetic parameters are searched for until a mini mum value of the sum of least squares for Ν data sets is obtained: a

0

0


372 Ν

SS = ^ (a


i

2

meas

—0C ) calc

where a

meas

and a

caic

are experimental and predicted

mass fractions respectively at each of the Ν data points. The stopping point for the search is henceforth defined by the objective function above in accordance to a predefined experimental error range anticipated. For more details about the optimisation procedure the reader is referred to our previous work published elsewhere (13). Optimised kinetic parameters were obtained for the epoxy resin alone and also for resin formulations containing 8% wt. M P (EP-MP) and M P P (EP-MPP) using thermogravimetric data collected at 10 and 20 °C/min. The optimised kinetic constants obtained using these two heating rates were averaged and used to predict mass loss profiles at intermediate heating rates of 12.5, 15 and 17.5 °C/min in Maple 6.0 using an O D E solver for stiff equations (lsode) assuming that the kinetic parameters are invariant of the heating rate. We note here, differences between kinetic constants obtained at heating rates between 10 and 20 °C/min are subtle, Table 2. On another hand, kinetic parameters, E and A, were plotted against the heating rates (10-20 °C/min) for the epoxy resin and their respective gradients extrapolated to obtain corresponding values at heating rates of 5 and 50 °C/min. These estimated kinetic parameters were used to predict the mass loss profiles which were then validated against experimental data. a

Results Chemical Kinetics and Predictive Modelling Epoxy Resin The decomposition behaviour of amine cured epoxide resins has been reported to occur via at least three overlapping stages (3,11-16). The first step at temperatures around 200 - 250 °C may be attributed to homolytic scission of chemical bonds within the polymer chains. Even though these reactions do not result in loss of mass, they however, adversely affect the physical properties including the mechanical behaviour (17). The first major weight loss occurring in the second stage may be caused by the elimination of water molecules from the oxypropylene group, -CH -CH(OH)-, which may lead to the formation of organic species with double bonds. The dehydration of the epoxy networks occur simultaneously with the volatilisation of some small molecular species leading to the formation of a primary residual char. The final stage involve the thermooxidative degradation of the primary char which may proceed via reactions such as isomerisation, chain transfer and radical transfer to yield negligible residual char (9). 2


373 A multi-step scheme to describe the degradation mechanism of an aminecured epoxy resin under a constant heating rate is shown in Scheme 1 below:

resin (mi) —> r dehydrated resin (m ) + (7-r ) water

[2]

r dehydrated resin (m ) —* primary char (m ) + volatiles

[3]

primary char (m ) —• volatiles

[4]

y

2

y

7

2

3


3

Scheme 1. A simplified multi-step degradation scheme for an amine-cured epoxy resin.

The degradation steps represented in Scheme 1 above can be presented as a coupled system of ordinary differential equations with 4 pairs of unknown kinetic parameters (E and A) governing the rate at which the volatilisation process proceeds at defined heating rates: a

μ

dt dm —~--k m dt x

x

x

dm - = r k m -k m - k m dt dm —j±=k m -k m x

x

2

4

x

2

2

3

2

3

3

3

where rj is the dehydrated fraction of the epoxy resin recorded starting at temperatures around 100 °C, k (i = Î..4) are the kinetic constants associated with the degradation steps involved and mj the variations of mass for species j = 1..3 with time, /. Each of the reaction rates k is assumed to have Arrhenius type dependence as given in Eq. 1 with initial conditions defined as follows; T(0) = T , m = 1, m = 0, m = 0 and β = defined heating rates in °C/min. Since each group of molecular species evolved during thermal degradation has an independent variation with time, the overall mass loss is estimated from a linear combination of individual mass loss profiles (75). Apparent kinetic constants for the degradation of epoxy resin calculated for all heating rates were optimised using the algorithm previously developed in our laboratories (73) and are presented in Table 2. While the obtained kinetic t

t

0

}

2

3


374


parameters are unique in relation to describing the degradation profile of a particular material, we however, found out that the values obtained in this work are similar to those obtained in our previous work (13) even though we were using a different epoxy resin. We are therefore led to conclude that while the degradation of epoxies follow a series of complex chemical reactions, the global physical processes (i.e. mass loss) occurring in each case can be defined by essentially a handful of grossly simplified mass loss kinetics models.

Table II. Optimised kinetic parameters for epoxy resin, EP at different heating rates k,

Kinetic parameters

k,

E/R(xl0 ) [K] In A CF

3

b

E/R(xl0 )"[ki 5

k:

In A CF b

E/R(xl

Modelling Thermal Degradation of Flame-Retarded Epoxy Resin

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