by :
M. HUSSAINSchool of Physics and Materials Engineering, Monash University, Clayton, Vic-3180, AustraliaR. J. VARLEYCSIRO, Division of Molecular Science, Clayton, Vic-3168, AustraliaM. MATHUS, P. BURCHILLDSTO, Army Research Laboratory, PO Box 4331, Melbourne 3001, AustraliaG. P. SIMONSchool of Physics and Materials Engineering, Monash University, Clayton, Vic-3180, Australia
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Epoxy resins have been used extensively for surface coating, adhesives, paintingmaterials, semiconductors, insulating materials for electrical devices and as filler or fiber-reinforced composites in aerospace, automobiles and marine vessels because of their low cost, easy processing, good mechanical properties and environmental stability.However,many industries such as aerospace, automotive and marine, are increasingly realizing that the flammability of epoxy resins is pre-venting their further use in a number of applications where fire performance is critical. As a result, considerable attention is now being given to synthesizing new epoxy resins with high thermal and fire performance properties for high performance applications. Several approaches have been reported for improving the flame and thermal resistance of epoxymaterials, such as by incorporating aluminum oxide tri-hydrate, bromine compounds in conjunctionwith antimony oxide, silicon and boron. The major problems occurred with these system is the generation of toxic and corrosive gases that occur during combustion. Another problem is the high levels of additive required, often around 30–50% depending upon the application. While this may well impart the required level of fire retardancy to the materials, if does so at a high cost to the mechanical performance of the materials. To overcome these problems, non-halogenated, flame-retardant reactive additives are required to increase the flame retardance time without affecting the physical and mechanical properties of the composites. Physical or chemical incorporation of phosphorus-containing compounds onto the epoxy backbone has been shown to lead to an improvement in the thermal and fire resistance properties of a materials by quenching flammable particles and reducing the energy of the flame in the gas phase
In recent times a general strategy to increase the flame retardancy of epoxy materials, has focused on the introduction of organo-phosphorus compounds containing significant quantities of phenyl groups into the epoxy back-bone. This has been done by blending either reacting an organo-phosphorous compounds with an epoxy resin or using an organo-phosphorus compound as a curing agent [10–12]. Significant improvement in fire retardance has been demonstrated in these types of epoxy materials. In the present article we have reported the reaction behavior of epoxy diglycidyl ether of bisphenol-A (DGEBA) with 9,10-dihydro-9-oxa- 10-phosphaphenanthrene10-oxide (DOPO) and cured with mixture of 3,5-diethyltoluene-2,4-diamine and 3,5-diethyltoluene-2,6-diamine (Ethacure-100). Thermal and fire retardant behavior of this system as characterized by cone calorimetry is reported for the first time as a function of phosphate content. 250 g of DGEBA, commercially known as DER-331 Dow epoxy resin was placed into a three-necked round bottomflask and heated to 130 ◦Cby placing in an oil bath connected with a temperature controller. 66 g of DOPO from Tokyo Kasei Kogyo Co., Ltd, Japan was then added and mixed with a mechanical stirrer, keeping the temperature constant at 130 ◦C untilDOPO was dissolved and the mixture was clear. The temperature was then raised to 180 ◦C and stirring was continued for a further 5 h. 45 g of the amine Ethacure-100 (Albemarle, USA) was added and stirred until mixing was complete. The phosphorus-containing epoxy resin and amine system was then pre-cured at 120 ◦C for 4 h and then post cured at 200 ◦C for 2 h. Differential scanning calorimetry (DSC), Fourier transformer infrared (FT-IR) and proton nuclear magnetic resonance spectroscopy (1H-NMR) techniques were used to confirm chemical reaction of DOPO. Samples were prepared to investigate the thermal and flame retardant properties by thermo-gravimetric analysis (TGA), dynamic mechanical thermal analysis (DMTA) and cone calorimetry.
TGA analysis of cured DGEBA and phosphorous containing DGEBA provided valuable information concerning their thermal stability and thermal degradation. TGA thermographs of cured resin and phosphorus-containing resins under air at the heating rate of 10 ◦C/min are shown in Fig. 1. The temperature at which onset of degradation occurred for the 1% phosphorus-containing epoxy was found around 425–430 ◦C and gradually decreased with increasing phosphorous content. Note that the degradation temperature of the phosphorous-containing epoxy is itself already lower than that of neat epoxy DGEBA (450–470 ◦C). However, it is interesting that the weightlosses of phosphorous-containing epoxy at higher tem-perature (600 ◦C) are much less than that of DGEBA alone.The initial decrease inweight loss of phosphorus containing epoxy is due to the initial decomposition of the P-O-C bonds of the phosphorus containing epoxy resin [13, 14]. However, decomposition at higher temperature yields phosphorus-rich residues (char) which form and prevent further decomposition of resins. The char is quite thermally stable, protecting the resins from further degradative oxidation and leading to higher char yields. This mechanism plays an important role in designing new fire resistant epoxy materials. Fig. 2 shows the linear relationship of char yield with phos- phorous content. It is clear that increasing the phosphorous content in epoxy increases the char yield, and thus also the thermal stability at higher temperatures. DMTA analysis provides more information on thermal and relaxational behavior of the fire retardant materials. Fig. 3 shows the tan δ peak of phosphorus containing resins compared with DGEBA. The glass transition temperature (Tg) decreased with increasing the phosphorus content, while the height of loss tangent increases with phosphorus content. However, the storage modulus was found to be similar in all samples. Similar behavior was observed by Lin [10] who proposed that even though the functionality of the resin system was reduced by reaction with DOPO andthus a lower modulus could be expected, the increased modulus was likely due to the bulky nature of DOPO moiety which is attached to the network. The lower crosslink density is confirmed by the greater height of the delta relaxation observed in Fig. 3, which is indicative of a greater relaxation strength. In addition the higher crosslink density phosphorus-free epoxy system is also broader, indicative of the greater structural in homogeneity of a highly crosslinked system. Cone calorimetry can produce an evaluation of the fire-retardant behavior of the resinswith respect to peak and average heat release rates (RHR), specific extinction area and the smoke production rates (CO2 and CO). The RHR is a measure of the heat release perunit surface area of burning materials. Fig. 4 shows the RHR variation with time at heat flux 50 kW/m2 which indicates that there is substantial improvement in the maximum rate of heat release, as well as the average heat release, when 3% of phosphorus is incorporated into the backbone of the polymer network. The maximum RHR of DGEBA was observed after 150 s of burning, while for the phosphorus-modified resin system, the maximum RHR was 165 s. The greatest effect upon fire performance, however, was the large decrease observed in the maximum and average rate of RHR. This can be attributed to char formation acting as a thermal insulator preventing the transfer of combustion products to the surface of the material. In the case of DGEBA, char yield is less than phosphorus-containing resins and thus shows higher a RHR. The result suggests that the presence of organo-phosphorus group enhances char formation. The effect of the phosphorus on the flame test by cone calorimetry has been summarized in Table I. The effect of phosphorus on the flame retardancy in epoxy resin systems such is a burgeoning area of interest, as improved fire retardancy becomes increasingly important in newmaterials development. As such it is equally important to be able to characterize the fire performance of materials from a rigorous scientific standpoint. While thermogravimetric analysis is often reported its relevance to the fire environment is limited. The cone calorimetry test is becoming increasingly important, as it is one of the few techniques that are able to do this. The results presented here are one of the first reported cone calorimeter studies of the fire performance of phosphorus-modified epoxy resins, with further investigations into this system in progress.
not3 : Received 24 July and accepted 21 November 2002
