Composites: Part B 63 (2014) 67–76

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Composites: Part B

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Flame retardancy and thermal stability of organic–inorganic hybridresins based on polyhedral oligomeric silsesquioxanesand montmorillonite clay

http://dx.doi.org/10.1016/j.compositesb.2014.03.0231359-8368/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +34 981337400.E-mail addresses: cramirez@udc.es, labpolim@udc.es (C. Ramírez).

B. Montero, R. Bellas, C. Ramírez ⇑, M. Rico, R. BouzaDepartment of Physics, E.U.P. Ferrol, University of A Coruña, Avda. 19 de Febrero s/n, 15405 Ferrol, Spain

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 January 2014Received in revised form 18 March 2014Accepted 31 March 2014Available online 13 April 2014

Keywords:A. HybridA. Polymer–matrix compositesB. Thermal propertiesD. Electron microscopyLimiting oxygen index

Organic–inorganic hybrids were prepared by incorporation of two polyhedral oligomeric silsesquioxanes,trisilanolisobutyl POSS (TSP) and isobutylAluminum POMS (AlP), and an organoclay (OC), in an epoxy–amine system.

Differential scanning calorimetry revealed the great catalytic power of AlP. OC also showed catalyticpower, but to a lesser extent. The storage modulus increased with lower POSS contents, due to thenanoreinforcing effect of POSS core. At higher concentration, the modulus strongly decreases due to POSSaggregation. The glass transition temperature was barely affected by the POSS, whereas OC additiondecreased it. POSS and OC supplied greater thermal stability in high temperature regions, and the charresidue increased with filler concentration because silicon species are formed. POSS addition improvedthe flame retardancy because of a multilayered carbonaceous silicate structure formation. The presenceof metal atoms in AlP composites might contribute to stabilization through secondary chemical reactions.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Epoxy resins are one of the most common thermosetting mate-rials and have been widely used in coatings, electronic encapsulat-ing compounds, adhesives and matrices of polymer compositesdue to their excellent thermal, mechanical and electrical proper-ties. The long service time and good physical properties often helpby providing a favorable cost-performance ratio when comparedwith other thermosets. As many demands are made on the techno-logical applications of epoxy resins, a vast number of formulationsare developed each year, but the use of cured epoxy resins is oftenrestricted by their inherent drawbacks, such as high flammability.This feature of polymers still represents an important limitationwith regard to their use in a number of industrial applications ofpolymeric materials because of the related fire occurrence riskand consequent fire hazard. The main fields where fire-retardancyof epoxy resins is required are the electrical–electronics sector,transport, building and furnishing elements [1,2].

Like other thermoset resins, epoxy resins can be renderedfire-retardant either by the incorporation of fire-retardantadditives or by copolymerization with reactive Fire Retardants

(FRs). The concern about environmental and health safety regard-ing traditional halogenated FRs has given rise to new Europeanregulations which have progressively restricted their use and haveresulted in the prohibition of some halogenated compounds,whereas some other FRs are currently undergoing very exhaustiverisk assessment procedures. The general feeling in the materialproduction industry, and among end users, is that halogen-free fireretardants with at least the same efficiency as halogenated systemsshould be developed. As a kind of halogen-free fire retardant, sili-con-based flame retardants are attractive because of their non-toxic nature, good heat resistance as well as excellent thermooxi-dative stability. Inorganic nanoparticles emerged as a solution topolymer fire retardancy, because these typically induce a strongreduction in combustion rate compared with the correspondingpolymer matrix at relatively low loadings, simultaneously improv-ing physical and mechanical properties.

In contrast to thermoplastics [2–4], there are few references inthe existing literature about the use of nanofillers as fire retardantsin epoxy resins [5]. However, the extensive applications they giverise to justify the research efforts dedicated to organic/inorganiccomposites of epoxy resins with improved properties, includingfire retardancy and thermal stability, which have become verysignificant properties in ensuring the work performance of thosenanocomposites. Hybrid polymer nanocomposites exhibit

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advantages in both inorganic and organic materials, such as heatresistance, high strength, good toughness and functionality, bring-ing about innovative industrial applications in aerospace, electron-ics, transportation, housing, and fireproof coating [6].

POSS molecules can be thought of as the smallest particles ofsilica possible. They combine a hybrid inorganic–organic composi-tion, Rn(SiO1,5)n, with nanosized cage structures having diametersof about 1.5 mm, comparable to those of most polymeric segmentsor coils. However, unlike silica or silicones, each POSS moleculecontains either non-reactive or reactive organic substituents atthe corner silicon atoms. The rigid inorganic core provides thestrength and oxidative stability of a ceramic, while synthetic con-trol of the organic coronae (R) provides processability and compat-ibility with other materials. These organic functions make the POSSnanostructure compatible with polymers or monomers. Hence,POSS nanostructured chemicals are easily incorporated using stan-dard chemical methods into common plastics via copolymeriza-tion, grafting or blending [7]. Incorporation of POSS particles intothermoset networks can be used to modify structure in nano-scale.These modifications can affect the thermal, oxidative and dimen-sional stabilities of many polymers upgrading their properties fornumerous high performance applications such as engineeringplastics.

Several polymeric systems have been taken into account incor-porating Polyhedral Oligomeric Silsesquioxane (POSS) cages inthermoset systems cured with diamines, mainly as regards epoxyresins. Their use as flame retardants has shown advantages duringcombustion, due to the accumulation of a ceramic layer on the sur-face of the burning material by silsesquioxane thermal degradation[2,8,9] resulting in a protective physical barrier, strongly decreas-ing the combustion rate [10–15]. In previous articles, we reportedon the preparation of several nanocomposites based on epoxy res-ins and modified by POSS, with high thermal properties and flameretardancy behaviour [16].

The incorporation of layered silicates, like nanoclays, hasshown beneficial effects on combustion, owing to the formationof a protective surface layer composed of a thermically stablecombination of carbonaceous char and clays, as a consequenceof polymer ablation [10,11,13,17–19]. The silicates are structur-ally different from POSS in that they are sheet-like with largeaspect ratios, which is essential with respect to their reinforcingefficiency. Besides enhancing fire-protection properties, incorpo-ration of nanoclay may give rise to an improvement in stiffness,strength and toughness when dispersed in polymers at nanome-ter scale. To disperse the clays into polymers, it is necessary toconvert their hydrophilic surface to hydrophobic via cationexchange, where organic surfactants replace the inherent cationin the silicates [19].

This work focuses on the preparation and characterization ofinorganic–organic hybrids involving epoxy resins and two typesof modifiers: POSS and nanoclay. Two POSS, with similar organicchains and different core structure, were used: trisilanolisobutylPOSS and isobutylAluminum POMS. An organomodified montmo-rillonite clay, with similar inorganic content to isobutylAluminumPOMS, as well as being much cheaper, was used to prepare clay–epoxy hybrid. Nanofillers were incorporated into the epoxy resinthrough physical blending. An enhancement in the flame retar-dancy and thermal stability of nanocomposites was expected inall cases. Thermal and fire combustion behaviour were analysedby differential scanning calorimetry (DSC), thermogravimetricanalysis (TGA), dynamic mechanical analysis (DMA) and limitingoxygen index (LOI) measurements. The influence of nanofillerconcentration and type on the hybrid properties was evalu-ated.The relation between combustion mechanism and morphol-ogy of LOI char, analysed by scanning electron microscopy(SEM), is discussed.

2. Experimental procedure

2.1. Materials

Diglycidyl ether of bisphenol A, DGEBA, from UNECO (UNERES-IN 5460, 185 g/ee) was used as the standard epoxy resin. 4,40-(1,3-Phenylenendiisopropylidene)bisaniline (BSA) of analytical gradewas obtained from Aldrich Chemical and was used as crosslinkingagent. Two different POSS compounds, isobutylAluminum POMS(C56H126O24Si14Al2, FW: 1630.75), AlP, and trisilanolisobutyl POSS(C28H66O12Si7, FW: 791.42), TSP, were purchased from Hybrid Plas-tics Ltd., USA. Organomodified montmorillonite clay NanomerI.30E (OC), with Si and Al contents of about 21% and 9% respec-tively, was obtained from Nanocor Inc. USA. All the reagents wereused without further treatment.

2.2. Sample preparation

A conventional stoichiometric ratio of 1 mol of amine to 2 molof epoxy was used for the epoxy neat. DGEBA and BSA were mixedhomogeneously and degassed at 70 �C under vacuum. When nomore bubbles appeared, the mixture was poured into a Teflonmould. The mixture was then cured in an air-circulating oven at130 �C for 3 h, followed by postcuring at 150 �C for 4 h to enhancecrosslinking.

To prepare POSS-modified and clay-modified epoxy resins,DGEBA was first heated at 70 �C and then POSS or clay was added.The mixture was heated at 130 �C for 30 min. To add the curingagent, the mixture was cooled and then an equivalent molaramount of BSA was added. The blend was hand-mixed until ahomogeneous sample was obtained and subsequently degassedto vacuum. It was poured into a Teflon mould and cured as binarysamples.

Formulations containing 1, 3 and 5 wt.% POSS or organoclaywere prepared. The samples were designated as TSPx, AlPx andOCx, where x indicates the amount of filler percentage-wise.

2.3. Characterization

Thermal analysis was carried out using a Perkin-Elmer DSC-7differential scanning calorimetry (DSC), after calibration with highpurity indium. Samples (�10 mg) were placed on the DSC cell andthen heated at four different heating rates (5, 10, 15 and 20 �C/min)from 30 �C to 300 �C in a nitrogen-purged environment. The spec-imens were then quickly cooled to 30 �C after the first scan. Thesecond scan was then performed in the same way. The glass tran-sition temperatures were taken as the midpoint of the capacitychange. In all the measurements, Tg’s were taken from the secondscans.

Thermomechanical properties of the cured resins were analysedusing a dynamic mechanical analyzer DMA-7 (Perkin Elmer) oper-ated in a three point bending mode. Samples used were in theshape of rectangular strips with dimensions of 18.5 � 6.0 � 3.5mm3. The experiments were performed at a frequency of 1.0 Hzand a heating rate of 3 �C/min from 30 �C to 300 �C. In this study,the reference for the glass transition temperature was taken fromthe peak temperature of tan d (E00/E0) curve.

The thermal behaviour of epoxy resins was evaluated by a ther-mal gravimetric analyzer TGA-7 (Perkin Elmer). Measurementswere performed over a temperature range of 50–750 �C underargon and oxygen atmosphere at a heating rate of 10 �C/min.

Flame retardancy was evaluated by limiting the oxygen index(LOI). LOI is the minimum concentration of oxygen, expressed asvolume percentage, in a mixture of oxygen and nitrogen that willjust support flaming combustion of a material initially at room

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temperature under standard test method conditions. LOI was mea-sured using a Stanton Redcroft flame meter, provided with a para-magnetic cell to measure oxygen concentration, according to thestandard oxygen index test UNE-EN ISO 4589. The percentage ofoxygen in the oxygen/nitrogen mixture with a flow rate of10.6 L min�1 deemed sufficient to sustain the flame was taken asLOI. The size of the test specimens was 100 � 6 � 4 mm3.

A scanning electron microscope (JEOL JSM-6400), at an acceler-ating voltage of 20 kV, was used to study the morphology of thechar obtained from the LOI test. The samples were sputtered witha thin layer of carbon using BAL-TEC CEA 035 sputter coater. Semi-quantitative analysis was conducted by energy dispersive X-rayspectroscopy (EDX) in an Oxford Inca Energy 200 with a Si(Li)detector coupled to a scanning electron microscope.

3. Results and discussion

3.1. Thermal analysis

The DSC curves of the control epoxy and its composites are pre-sented in Fig. 1. TSP–epoxy composites have similar peak temper-ature to epoxy control (169.8 �C), whereas AlP–epoxy compositesshowed much lower peak temperature than the neat. AlP shiftedthe peak of about 60 �C to lower values, regardless of the AlP

Fig. 1. DSC curves of the control epoxy and its composites containing, (a) TSP, (b)AlP and (c) OC.

content in the hybrid nanocomposites. This fact makes the greatcatalytic power of AlP evident, even at lower POSS loadings. Theelectron withdrawing character of silicon–oxygen cage renders ahighly Lewis acidic aluminium atom. This highly activated alumin-ium atom acts as a powerful catalyst during the polymerizationprocess. A shoulder at �175 �C is observed in the exothermobtained for AlP–epoxy composites unlike for TSP-composites. Thissuggests that the aluminium centre modified the reaction mecha-nism, probably in connection with its catalytic action.

OC–epoxy composites showed lower peak temperature thanneat. The peak temperature decreased with an increase of OC con-tent from 169.8 �C (epoxy control) to 164.0 �C (1 wt%), 157.3 �C(3 wt%) and 144.0 �C (5 wt%). This may be attributed to the cata-lytic effect of octadecylamine surfactant of the organoclay in thecuring reaction, leading to a decrease in the energy required forepoxy ring opening [20–22]. The catalytic effect becomes strongerwith increasing organoclay content since the octadecylamine con-centration in the resin is increased.

Dynamic kinetic analysis was carried out and the curing activa-tion energies (Ea) of neat and hybrid composites were calculated bymeans of the Kissinger method. A linear correlation was observedfor each sample. The curing activation energy of epoxy control was55 kJ/mol. This value aligns with the reference values reported inthe literature [16,23–25].

TSP–epoxy composites showed similar activation energies toepoxy control (about 53 kJ/mol), which suggests that TSP doesnot participate in the amine–epoxy reaction.

For 1 wt% and 3 wt% OC, the curing activation energy values aresimilar to that obtained for epoxy control. However, at higher OCcontent (5 wt%) the activation energy increases up to 63 kJ/mol.Silica filler, which has silanol groups on the filler surface, couldaccelerate the reaction of certain epoxy resins, or at least do soin circumstances where aluminium complex was present [26].Moreover, the formation of aggregates could explain the increaseobserved in the activation energy in OC5.

Kinetic analysis was also conducted on AlP–epoxy composites,which showed higher activation energies than the neat. DSC datarevealed that control epoxy and AlP composites have a differentcuring mechanism, therefore the activation energies are not com-parable. There has been very little work done in this area. Moredetailed studies on this issue are underway and will be reportedin due course.

The glass rubber transition was determined via a second scan asthe temperature at the half-way point of the jump in the heatcapacity when the material changed from the glassy to the rubberystate under N2 atmosphere. For the epoxy control, this transitionoccurs at 152.6 �C. All of the DSC curves for the composites con-taining POSS and OC had single transition temperatures in theexperimental temperature range. The presence of a single glasstransition temperature indicates that the hybrid mixtures arehomogeneous.

Tg values have been only negligibly influenced by the presenceof TSP or AlP in the composites. For example, the Tg values forepoxy control and the composites with 5 wt% of TSP and AlP were152.6, 155.9 and 148.3 �C, respectively. However, OC tends todecrease the Tg values; the incorporation of 5 wt% OC to epoxymatrix decreased it by 15 �C.

The glass transition temperatures of hybrid materials could beaffected by several factors. The nanoreinforcement effect of filler,which was dispersed in the epoxy matrix and restricts the motionof polymer chains, giving rise to the increase in Tg. On the otherhand, the inclusion of bulky POSS cores could affect the crosslinkdensity increasing the free volume and diminishing Tg. Further-more, the physical crosslinking via hard POSS domains involvingboth POSS–polymer and POSS–POSS interaction and the lowmolecular substituents in POSS cages (or organic modifier in

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organoclay), which have an effect of plasticization on the epoxynetwork, also decrease the Tg values [27–29]. The behaviour ofthe glass transition in the studied POSS–epoxy composites reflectsthese competing factors.

3.2. Dynamomechanical analysis

Fig. 2 shows the dynamic storage moduli (E0) of epoxy controland epoxy hybrids. In the glass state, the moduli of the hybridswith the lower contents of POSS (1 and 3 wt%) were slightly higherthan that of the neat, and the moduli increased with the concentra-tion of filler. This increase in the modulus could be attributed tothe nanoreinforcing effect of POSS core on the epoxy matrix. Theintroduction of inorganic silsesquioxanes cores into the resinmatrix increases stiffness and makes mobility of the molecularchains difficult [30,31].

TSP–epoxy composites showed the highest storage modulus,probably due to the fact that TSP exhibited better solubility inthe epoxy matrix than AlP. The E0 values for epoxy control andthe composites with 3 wt% of TSP and AlP at 50 �C were 413, 582and 459 MPa, respectively.

At higher POSS concentration (5 wt%) the modulus stronglydecreases, probably due to the formation of POSS aggregates,

Fig. 2. Storage moduli, E0 , as function of temperature for epoxy control and epoxyhybrids containing, (a) TSP, (b) AlP and (c) OC.

which are dispersed with difficulty within the epoxy matrix. Thestorage modulus values were lower than the epoxy control, being385 and 353 MPa for TSP5 and AlP5 respectively.

OC–epoxy hybrids showed the same behaviour as POSS com-posites. For 1 wt% and 3 wt% OC contents, the storage modulus ishigher than for the epoxy control. At higher concentration(5 wt%), the modulus decreases. The reinforcement ability of OCwas similar to that observed for TSP.

The storage modulus in the rubbery state was also analysed andthe values at 250 �C were taken as representative. TSP hybridsshowed modulus values of between 18.7 and 21.5 MPa, greaterthan epoxy control modulus (14.3 MPa). The rubbery plateau mod-ulus can be related to the degree of crosslink density. For a networkwith higher crosslink density, the rubbery plateau is usually higher[19,26]. The experimental results suggest that TSP incorporationincreases the crosslink density compared to the epoxy control. Thiscould be attributed to the silanol groups in TSP which acts as aLewis acid and assists the epoxy ring opening [26].

In AlP–epoxy hybrids the rubbery modulus of AlP1 is similar tothat of epoxy control and the modulus value of AlP3 is slightlyhigher (16.3 MPa). However, AlP5 modulus was 13.2 MPa, lowerthan epoxy control, which could be related to the formation ofAlP aggregates.

For OC–epoxy hybrids, the rubbery modulus values were higherthan that for epoxy control, indicating a higher crosslink density ofthe network. The rubbery modulus in the OC-hybrids increased upto 3 wt% OC. At higher OC concentration (5 wt%), the modulusdecrease, which was attributed to the OC, aggregates formation.The rubbery modulus was 27.1, 31.6 and 26.5 MPa for the compos-ites with 1, 3 and 5 wt% OC.

Fig. 3 shows the tand curves versus temperature. In POSS–epoxy hybrids, the tand curve showed a less defined peak or evenshowed a shoulder, making evident the poor miscibility of the sys-tem. No clear trend in Tg values was observed with the POSS con-tent. Many factors can affect the glass transition temperature: thenanoreinforcement of POSS, the plasticization caused by POSSorganic chains, the free volume introduced by POSS cages, and thiscould explain the lack of trend observed here.

In OC–epoxy hybrids, the tand curve is wider than that of epoxycontrol and at higher OC content (5 wt%) two maximum values areobserved which demonstrate the formation of OC aggregates.

3.3. Thermal stability

Thermogravimetric analysis was applied to evaluate the ther-mal analysis of the POSS-containing epoxy hybrids. The analyseswere conducted in inert and oxidative atmospheres. Fig. 4 showedthe TGA curves of pure POSS (or organoclay), epoxy control andepoxy hybrids. Within the experimental range, all the TGA curvesdisplayed a similar feature of degradation, suggesting that theaddition of POSS or organoclay did not significantly affect the deg-radation mechanism of the epoxy matrix.

The onset of degradation was taken as the temperatures atwhich 5 wt% mass loss occurred (T5). In argon atmosphere, it isnoted that epoxy control showed good thermal stability; the initialtemperature of weight loss was about 400 �C (Table 1). The incor-poration of 1 wt% TSP to the epoxy matrix did not affect the onsetof degradation, whereas at higher content of TSP, degradation wasshifted to higher values. This delay in the beginning of degradationwas at its maximum with 3 wt% TSP, in which T5 was 18 �C higherthan that of epoxy control, followed by a further decrease for thesample with 5 wt%. This latter could be due to the aggregation ofPOSS.

With the incorporation of AlP to the epoxy matrix, thermal sta-bility decreased. This was due to the much lower thermal stabilityof AlP in comparison with the epoxy control. Instead, the OC

Fig. 3. tan d as function of temperature for epoxy control and epoxy hybridscontaining: (a) TSP, (b) AlP and (c) OC.

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addition to epoxy matrix delayed the beginning of degradation.The maximum delay was observed for OC–epoxy hybrid with3 wt% OC, whereas the hybrid with 1 wt% and 5 wt% showed a sim-ilar T5 value.

It should be noted that the beginning of degradation of TSP islower than that of AlP (227.6 �C and 354.6 �C, respectively), butthe hybrids with TSP had higher stability. This is due to the betterdispersion of TSP in the epoxy matrix.

Thermal stability, evaluated in terms of the 50% mass loss, wasmarkedly enhanced by POSS addition. Thermal stability increasedwith POSS loading, from 1 wt% to 3 wt%, and above that slightlydecreased. In particular, T50 value increased by 21 �C and 26 �C inrelation to epoxy control when only 3 wt% TSP or AlP was added.It is noteworthy that the T50 value increased by 17 �C with theincorporation of 1 wt% AlP to the epoxy network, whereas it onlyincreased 4 �C with 1 wt% TSP.

OC addition also improved the thermal behaviour of epoxy net-work, but to a lesser extent than AlP, although both fillers havesimilar aluminium and silicon percentages.

The amount of residue is another important index in evaluatinga flame retardant. The increment in the residue yield is attributedto the further enhancement of thermal stability at high tempera-ture [32].

The char yield of TSP–epoxy hybrids was slightly higher thanthat of epoxy network, although no significant differences wereobserved in the char percentage with the TPS content.

The hybrids with AlP and OC showed a similar higher percent-age of residue when compared to epoxy control. Char yield value inhybrids tends to increase with the filler content.

It has been reported that in inert atmosphere, the degradationof POSS involves competition between the evaporation processand an oxidation phenomenon, which leads to a thermally stableresidue [9]. The experimental residues obtained in the compositesare consistent with the inorganic content in the POSS.

Thermogravimetric degradation in oxidative atmosphere wasalso conducted and the results are shown in Table 1 and Fig. 4. Itcan be seen that the hybrids begin to lose weight earlier thanepoxy control due to the relatively weak thermal stability of POSSand OC at low temperatures. The enhancement of thermal stabilitywas seen in the high temperature region. The T50 values of POSShybrids were much higher than that of the control epoxy. The addi-tion of 1 wt% of POSS increased the T50 by about 50 �C. It is notedthat the T50 values of TSP1 were similar to that of AlP1, whereasat higher POSS contents, TSP hybrids showed better thermal stabil-ity. This could be related to a better dispersion of TSP in the epoxymatrix. The char residues increased gradually with POSS content.This suggests that POSS may be oxidized and form a silicon dioxidelayer. AlP-hybrids showed higher residue percentage than TSP-hybrids, although the highest char yields were observed in OCcomposites.

It has been reported that the improvement in thermal stabilitywith POSS is related to the formation of the inert silica-like layer onthe surface of the hybrid; this ceramic layer can limit the heat flowtoward the underlying polymer matrix, as well oxygen diffusionand volatile compounds escape, thus improving the thermal stabil-ity of composites [32,33].

Besides this mechanism, the improvement of thermal stabilitywith OC comes from the uniform dispersion of the filler, becausethe clay layers prevent the diffusion of volatile decompositionproducts.

Despite the high char content in the OC hybrids, the T50 valuesof the composites are lower than that of the neat. This makes thecomplex effect of clay incorporation in polymeric matrix evideent.Clay layers offer good barrier action, which can improve the ther-mal stability of the composites. However, the alkylammonium cat-ions in the organoclay could suffer decomposition following theHofmann elimination reaction resulting in catalyzation of the deg-radation of polymer matrices. Moreover, the clay itself can also cat-alyze the degradation of polymer matrices [34]. The latter twofactors would reduce the thermal stability of epoxy clay composite.

3.4. Flame retardancy

The parameter used here to measure the flame retardancy ofthe hybrids was LOI. The standard method UNE-EN ISO 4589, inwhich the samples are suspended vertically and ignited using aburner, was applied to the control epoxy and their hybrids. Theconcentration of oxygen was increased if the specimen was extin-guished before burning away for 3 min or reaching an extent onthe bar of 5 cm.

TSP addition enhances fire resistance in comparison with thecontrol epoxy, and the LOI values increase almost linearly withthe TSP content, as shown in Fig. 5. However, the lowest contentof POSS (1 wt% and 3 wt%) did not improve flame resistance tothe expected extent.

AlP also acted as a flame retardant, even at low POSS content.However, the LOI value hardly increased with the AlP content;the LOI value increased by 10% and 14% with the addition of1 wt% and 5 wt% AlP, respectively. This could be due to the POSSaggregation. It has been shown in the literature that the dispersionof fillers in a polymer matrix can influence the reaction to fire ofpolymers [5].

Fig. 4. Thermogravimetric curves of the epoxy control and epoxy hybrids containing: (a, d) TSP, (b, e) AlP and (c, f) OC.

Table 1TGA results of control epoxy and epoxy hybrids with different content of TSP, AlP andOC.

Argon atmosphere Oxygen atmosphere

T5 (�C) T50 (�C) Charat 850 �C (%) T5 (�C) T50 (�C) Charat 850 �C (%)

Neat 400.5 442.0 12.8 399.3 474.5 0.2TSP1 401.9 446.3 13.4 406.4 526.5 0.6TSP3 418.6 463.8 13.7 394.0 535.6 1.6TSP5 410.0 460.6 13.8 394.0 527.7 1.9AlP1 400.6 459.4 13.2 363.4 525.4 0.9AlP3 401.2 468.8 15.0 367.9 525.4 1.9AlP5 390.6 462.5 15.3 360.6 515.9 2.6OC1 408.1 449.4 13.2 377.0 466.5 1.3OC3 413.1 455.6 14.7 381.5 433.6 2.3OC5 409.4 456.3 15.1 360.0 440.4 4.5

Fig. 5. LOI values of the control epoxy and epoxy hybrids containing TSP, AlP andOC.

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Organoclay also enhanced the LOI value at the lowest fillercontent (1 wt%) and above that, the LOI value decreased. TheOC–epoxy hybrid with 5 wt% OC showed an LOI value lower thanthe control epoxy. OC produced a multilayered carbonaceous–sili-cate structure on the surface of epoxy resin, which increased theflame resistance property of the hybrid. This structure is formedas a consequence of polymer ablation caused by pyrolisis, with

the de-wetted clay particles left behind. Moreover, the clay layersreassemble to form stacks due to the degradation of the organic

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modifier, occurring at elevated temperatures. The tortuous pathprovided by the silicate layers had better barrier properties to oxy-gen and heat, and slowed down the escape of flammable volatilesgenerated during polymer ablation, which delayed the burningcapacity of the composite. At higher clay loading, the tortuous pathprovided by the clay decreased due to agglomeration whichresulted in a decrease in barrier properties and LOI value [35].The close contact between silicates and polymer macromoleculesis considered essential for the advance of charring process, asreflected in the performance of microcomposites in which chargeneration does not occur [36]. Moreover, the clay itself or thedecomposition products of the organic modifier (the octadecyl-amine content in the clay is about 25–30 wt%) can catalyze thedegradation of the polymer matrix and hinder flammability bene-fits [36].

The observation of AlP-hybrid char residue showed a rigid car-bonaceous residue which was hollow inside. A large, thick char iscreated on top of the specimen (Fig. 6). On the edge of the burnedarea, the material is covered by a thin crust of carbonized materialand examination of the cross sections reveals that inside, the mate-rial is sufficiently preserved.

TSP-hybrids and OC-hybrids showed a foamy residue like afolding structure. However, this protective layer is very fragileand easily breakable. These hybrids showed a black residue,whereas the AlP-hybrid char showed some metallic appearance.

The barrier mechanism is recognized as the most importantflame retardant mechanism. Silsesquioxane can produce multilay-ered carbonaceous–silicate structure on the surface of hybrids. Itwas believed that this carbonaceous (Si–O–C) layer with SiO2-likespecies could act as an insulator and a barrier to mass transporta-tion on heating. Thick char becomes a better insulating layer,undergoing slow, oxidative degradation and preventing hybridheat reaching the remaining polymer. This mechanism is universalfor all composites with nanofillers, independently of the nature ofthe matrix [30,36].

Fina et al. [9,15] reported that combustion of PP was stronglyaffected by Aluminium-containing POSS, with a mechanism whichcannot be related to the simple physical accumulation of aceramic phase on the surface as reported for metal-free

Fig. 6. Digital photograph of: (a) AlP3, (b) TSP

polysilsesquioxanes [15]. This can probably be attributed to theAl promoting secondary reactions during polymer degradation.

For clay hybrids, a parallel mechanism suggests that the accu-mulation of clay on the burning material surface results also fromthe migration of silicates driven by their lower surface free energycompared with carbon-based polymers. The transportation of clayparticles is aided by temperature and viscosity gradients. Themigration of silicate layers from the bulk to the surface is alsocaused by numerous rising bubbles which are formed by thedecomposing polymer and the clay surfactant or even by clay itselfinducing heterogeneous nucleation and subsequent bubbling.Migrated layers act like a ‘‘skeleton frame’’ and the flowing carbonparticles will subsequently be deposited on the surface of clayplatelets generating the folding structure [32,36].

3.5. Morphology of LOI char

The microstructure of char residues was studied to furtherunderstand the flame retardant mechanisms. Figs. 7 and 8 showthe SEM photographs obtained for the outer and inner surfaces ofchar residues of the epoxy control and epoxy hybrids with 1 wt%of filler.

Epoxy control char residue consists of large bubbles in the outersurface and a smooth surface on the inner layer. The drawback ofthis residue is its excessive fragility on the outer surface.

Incorporation of POSS induces important differences in themorphology of the residues. TSP–epoxy hybrids showed an outersurface with far fewer bubbles than the control epoxy. AlP-hybridsand OC-hybrids did not exhibit bubbles in the outer layer; whereasthe AlP-hybrids showed a folding structure and OC-hybridsshowed a grain-like structure.

The inner surface of all hybrids appears as a smooth and contin-uous layer which could be due to the rapid volatilization of thecondensed phase to the outer [33].

A thick residue is supposed to provide better protection againstheat. If the gases are not trapped during thermal decomposition,these may contribute to the development of fire (production ofevolving flammable gases). If their trapping occurs, this shouldresult in the swelling of the residue. This means that intumescent

3 and (c) OC3 hybrid char after LOI test.

Fig. 7. Scanning electron microscopy with energy-dispersive X-ray spectra of outer surface of char for 1 wt% of control epoxy and epoxy hybrids: (a) Control epoxy, (b) TSP-hybrid, (c) AlP-hybrid and (d) OC-hybrid.

Fig. 8. Scanning electron microscopy with energy-dispersive X-ray spectra of inner surface of char for 1 wt% of epoxy hybrids: (a) Control epoxy, (b) TSP-hybrid, (c) AlP-hybrid and (d) OC-hybrid.

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protection is created before most of the sample has degraded andthe trapped gases contribute to foaming (more foam in the charis supposed to enhance thermal insulation) [5]. Therefore AlP pro-vides better fire protection than TSP, which is in line with the LOIvalues for the hybrids with 1 wt% of filler.

The EDS elemental analysis of the char reveals that the innersurface is richer in silicon and aluminium than the outer layer(Table 2). So, the silicon accumulates on the inner surface and isnot pushed by the bubbles to the outer surface, in contrast to theobservations carried out by other authors [33]. The intensity of Si

Table 2Energy-dispersive X-ray results of the char after LOI test (n.d.: no detected).

C wt% O wt% Si wt% Al wt%

Outer Inner Outer Inner Outer Inner Outer Inner

TSP1 85.85 82.99 13.39 12.55 0.76 4.46 – –AlP1 83.66 86.82 16.05 11.32 0.21 1.17 n.d. 0.69OC1 89.32 87.21 9.95 9.13 0.52 0.96 0.21 2.71

B. Montero et al. / Composites: Part B 63 (2014) 67–76 75

signal on the AlP hybrid residue is significantly lower comparedwith the TSP-hybrid residue. This could be explained by the lowerextent of polymer ablation, thus allowing a limited accumulationof AlP on the surface. The stabilization of AlP cannot be ascribedto a pure physical effect by inorganic accumulation on the surface.Some chemical effect induced by the metal centres must come intoplay [15]. Also, Si and Al contents on the outer surface of OC-hybrids are higher than those for AlP-hybrids. This could beexplained by the lower polymer ablation in AlP hybrids.

4. Conclusions

New nanocomposites based on epoxy resin with trisilanoli-sobutyl POSS, isobutylAluminum POMS and organoclay were pre-pared. Differential scanning calorimetry indicates that TSP doesnot participate in the amine–epoxy reaction, whereas AlP provedto be a powerful catalyst, even at low content (1 wt%). The resultssuggest that the aluminium of AlP activates a different reactionmechanism. Organoclay also showed catalytic activity in epoxycuring. Glass transition temperatures were not affected by dis-persed POSS, revealing an absence of POSS–polymer bound. How-ever, OC decreased the Tg values. The increase in the storagemoduli at the lowest POSS concentration was connected withthe nanoreinforcement of POSS cores, whereas the furtherdecrease at higher POSS concentration was attributed to the for-mation of aggregates. OC composites showed the same behaviouras POSS hybrids. The values of the storage modulus in the rub-bery region suggest that TSP and OC incorporation increases thecrosslink density compared to the epoxy control. This could beattributed to the silanol groups in TSP and OC layer edges, whichact as a Lewis acid.

Thermogravimetric analysis showed that the incorporation ofPOSS into epoxy network resulted in an apparent improvementin thermal stability, and char residue tends to increase with theconcentration of POSS. OC hybrids exhibited similar behaviour inargon atmosphere, but thermal stability (in terms of T50) decreasedin oxidative environments. The LOI values increased with POSSincorporation, although not to the expected extent. Also, theaddition of 1 wt% and 3 wt% OC improved flame retardancy. SEManalysis showed that silsesquioxanes and organoclay producedmulti-layered carbonaceous–silicate structures which act as aninsulator and barrier to mass transportation on heating. The effectof AlP was most probably related to its chemical activity, whichfavours the formation of a moderate amount of char residue, bycatalysing secondary reactions in the resin during combustion.

The main objective of this work was to improve the thermal sta-bility and flame retardancy of DGEBA epoxy resin. The hybrid with3 wt% AlP showed good thermal stability and flame retardancy. Inaddition, the curing reaction was accelerated and dynamomechan-ical properties were improved.

Acknowledgements

Financial support for this work has been provided by Xunta deGalicia and FEDER through grants XUGA 10TMT172009PR andCN2011/008.

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