1. IntroductionPolybenzoxazines represent a rather new class ofthermosets formed by ring-opening polymerization(ROP) of the benzoxazine (BOX), usually 1,3-ben-zoxazine precursors. Nowadays they are consideredas most promising matrix materials for compositesin future aerospace applications. This expectation isbased on their excellent thermal properties (highglass transition temperature, Tg), low flammability(high char yield), high stiffness, low water uptake,and practically no shrinkage upon curing (e.g. [1–3]). As the curing of BOX occurs via ROP novolatile by-products appear which supports the pro-
duction of void-free composites. A further benefitin linked with the easy synthesis of BOX, themonomers of which may be very versatile. Versatil-ity is given by the fact that beside the necessaryformaldehyde various phenol and amine com-pounds may be selected for the synthesis of BOX.On the other hand, the thermally-induced polymer-ization of BOX takes place at rather high tempera-ture (T > 200°C) which is hardly acceptable by thepraxis. A further problem, early recognized [4], isthat the crosslink density of polybenzoxazine is sur-prisingly low considering its high stiffness and Tgwhen comparing with other thermosets of similar
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Structure, thermal and fracture mechanical properties ofbenzoxazine-modified amine-cured DGEBA epoxy resinsS. Grishchuk1, Z. Mbhele2, S. Schmitt1, J. Karger-Kocsis2,3*
1Institut für Verbundwerkstoffe GmbH (Institute for Composite Materials), Kaiserslautern University of Technology, D-67663 Kaiserslautern, Germany
2Polymer Technology, Faculty of Engineering and the Built Environment, Tshwane University of Technology, Pretoria0001, Republic of South Africa
3Department of Polymer Engineering, Faculty of Mechanical Engineering, Budapest University of Technology andEconomics, H-1111 Budapest, Hungary
Received 18 November 2010; accepted in revised form 20 January 2011
Abstract. First, traditional diamine hardeners of epoxy resins (EP) were checked as potential accelerators for the benzox-azine (BOX) homopolymerization. It was established that the acceleration effect depends on both the type and amount ofthe diamine compounds. In the follow-up work amine-curable diglycidyl ether bisphenol A (DGEBA) type EP was modi-fied with BOX keeping the EP/BOX ratio constant (75/25 wt.%). The amine hardeners, added in the EP in stoichiometricamounts, were of aliphatic and aromatic nature, viz. diethylenetriamine (DETA), 4,4!-diaminodiphenyl methane (DDM),and their 1/1 mixture. The thermal, viscoelastic, flexural and fracture mechanical properties of the EP/BOX hybrids weredetermined and compared to those of the reference EPs. Based on dynamic-mechanical thermal analysis and atomic forcemicroscopy the formation of co-network between EP and BOX was concluded. Homopolymerized BOX was built in thenetwork in nanoscaled inclusions and it was associated with internal antiplasticization. Incorporation of BOX improved thecharring, enhanced the flexural modulus and strength, and reduced the glass transition of the parent EP. The fracture tough-ness and energy were not improved by hybridization with BOX.
properties. This was explained by massive H-bond-ing occurring between the phenolic –OH and terti-ary amines of the Mannich linkage formed via ROPin BOX ([1–3,5 and references therein]). Thoughthe development of BOXs followed different direc-tions in the past two decades [2–3, 5–6], the reduc-tion of the temperature of polymerization (curing)and the increase of the crosslink density remainedalways top priority issues.The ROP of BOX can be catalyzed by both basicand acidic compounds. In the open literature lots ofdifferent compounds have been checked in respectto their potential catalytic activity, whereby empha-sis was put mostly on acidic compounds [1, 7].Markedly less works were devoted to the potentialacceleration effect of basic compounds, and espe-cially those of amine-group containing ones, on theBOX polymerization. The latter is, however, ofparamount importance when the increase of thecrosslink density of polybenzoxazines is targetedby coreactions (copolymerization, co-crosslinking)with epoxy resins (EPs). Recall that amines are theusual hardeners of low temperature curable EPs.To increase the crosslink density of BOXs by co-crosslinking with EPs was always a favoredresearch direction. In their pioneering work Ishidaand Allen [4] prepared BOX/EP ‘copolymers’ byexploiting the reaction between the phenolic groups(–OH) of the ring-opened BOX and epoxy (oxi-rane) groups of the EP. Accordingly, no external EPcuring agent (hardener) has been used to preparethe EP/BOX hybrids. The corresponding hybridsshowed enhanced Tg, high crosslink density andductility over the BOX homopolymers. The abovereaction pathway, i.e. BOX as hardener for EP, wasaddressed by other researchers studying the relatedmechanisms [8–9] and the effects of the chemicalbuild-up of the components [8]. Amine compoundsmay have an acceleration effect (thermal kick-off)on the BOX polymerization as demonstrated byKimura et al. [10]. Recently, amine curable EPresins were also modified with BOX. Rao andPathak [11] used phenalkamine as EP curing agentand concluded that the reactions between EP andBOX occur in two-steps. At low temperature the EPreacts with the amine, whereas at high temperaturethe ROP of BOX is immediately followed by co-crosslinking reactions occurring between the phe-
nolic –OH (BOX) and epoxy groups (EP). A two-step reaction was proposed also for a special BOXend capped EP cured by aromatic diamine (diaminodiphenyl sulphone) [12]. In a very recent paperAgag et al. [13] reported on the preparation of pri-mary amine-functionalized BOXs and their curing.The differential scanning calorimetric (DSC) ther-mogram of the primary amine-functionalized BOXshowed multiple exotherms above the melting andbelow the polymerization temperature of the corre-sponding BOX compound, which can only beassigned to reactions between the oxazine and pri-mary amine (–NH2) groups.Considering the fact that primary diamines are theusual hardeners for low temperature curable EPs, itis straightforward to check their effects on the BOXpolymerization. This has been done in the presentwork by selecting some amine hardeners withaliphatic, cycloaliphatic and aromatic backbones. Afurther aim of this work was to prepare BOX modi-fied amine cured EPs and to study their structure-property relationships.
2. Experimental2.1. MaterialsAs potential accelerators for the BOX homopoly-merization the following diamines have beenselected: diethylenetriamine (DETA), 4,4!-diamino-3,3!-dimethyl-dicyclohexylmethane (Aradur® 2954of Huntsman Advanced Materials, Basel, Switzer-land), and 4,4!-diaminodiphenyl methane (DDM).DETA (melting and boiling temperatures –39 and207°C, respectively) and DDM (melting tempera-ture: 92°C) were procured from Sigma-AldrichChemie GmbH (Taufkirchen, Germany). Note thatall of them are widely used EP hardeners. More-over, due to their structure (aliphatic, cycloaliphaticand aromatic) the primary amines have differentbasic characters.As bifunctional, diglycidyl ether bisphenol A basedEP resin D.E.R. 331 (Dow Deutschland Anlagenge-sellschaft mbH, Schwalbach, Germany) wasselected. This EP has the following characteristics:epoxy equivalent weight: 182–192 g/eq., viscosityand density at T = 25°C 11–14 Pa"s and 1.16 g/ml,respectively. The BOX used was an N-phenylbisphenol A based 1,3-benzoxazine (Araldite MT35600 CH of Huntsman Advanced Materials). This
BOX with a density of 1.18 g/ml at RT has a melt-ing range of 80–85°C, and a viscosity rangebetween 80 and 180 mPa"s at T = 125°C.The chemical structures of the materials used aredepicted in Figure 1.
2.2. Sample/specimen preparationTo check the potential acceleration effect of theamines they were added in 10 and 20 wt.%, respec-tively, to the powdered BOX followed by mixing ina mortar. The BOX/amine mixes were left for 24 hprior to their DSC testing. As demonstrated later byshowing the related dynamic DSC scans the amineaffected the homopolymerization of BOX differ-ently. Based on the results two amines, viz. DETAand DDM along with their 1/1 mixture (molar ratio)were selected for the EP hardening. As the reactivi-ties of DETA and DDM are very different bothtowards EP and BOX, our interest was to checktheir combination for which the DETA/DDM = 1/1has been selected. The EP/amine ratio was stoichio-metric in all recipes. Though the BOX reacts withboth the amine compounds (see later) and EP (asdisclosed above), it was considered as an inertmaterial. The EP(including hardener)/BOX ratiowas fixed at 75/25 wt.%.The samples were prepared as described below.First, the EP was warmed to 70°C and kept for30 min in order to melt the eventually crystallinefraction. Next, the powdered BOX was added to thewarm EP and mixed thoroughly (1900 revolutionsper minute, rpm) for 5 min. The mix was placed in athermostatic oven and stored at T = 110°C for40 min to dissolve the BOX. Afterwards the aminewas introduced by mixing (160 rpm) for 4 min.Finally, the mixture was deareated in vacuo andpoured in open molds manufactured from polytetra-
fluoro ethylene (PTFE). The PTFE molds containedthe cavities of the rectangular bars and compact ten-sion (CT) specimens used for testing (see later).The cure cycle (temperature vs. time) of the sam-ples is depicted in Figure 2. Curing of the resinsaccording to Figure 2 occurred in a programmablethermostatic oven of Kendro Laboratory Products(Langenselbold, Germany).
Thermal and viscoelastic propertiesA DSC 821e (Mettler Toledo, Giessen, Germany)thermal analyzer was used to investigate the BOXhomopolymerization in presence and absence of thevarious diamines listed before, and to detect the Tgof the EP/BOX hybrids prepared. DSC thermo-grams were registered in the temperature rangefrom T = 0 to 300°C at a heating rate of 10°C/minunder N2 flushing (30 ml/min). The sample weightvaried between 10 and 20 mg.Dynamic mechanical thermal analysis (DMTA)was taken on rectangular specimens (60 # 8 #4 mm3; length # width # thickness) in three pointbending configuration (span length: 50 mm) at 1 Hzusing a DMA Q800 of TA Instruments (New Castle,DE, USA). Tests were performed at a constant
Figure 1. The chemical structures of the materials used
Figure 2. Cure cycle of the EP and EP/BOX hybrids
amplitude (50 $m) using sinusoidal oscillation andunder dynamic conditions in the interval T = –100…300°C at a heating rate of 1°C/min.The hybrid resins were subjected to thermogravi-metric analysis (TGA) in a DTG-60 device of Shi-madzu (Columbia, MD, USA). The TGA experi-ments were conducted under nitrogen atmosphere(30 ml/min) in the temperature range T = 25…600°C with heating rate 10°C/min.
2.3. Flexural and fracture mechanicalbehavior
The flexural properties, namely, modulus andstrength of the hybrid resins, were determined onrectangular specimens (60 # 8 # 4 mm3; length #width # thickness) in three point bending at roomtemperature according to EN63. The span length ofthe specimens was 50 mm and their loading on aZwick 1474 (Zwick GmbH, Ulm, Germany) uni-versal testing machine occurred with deformationrate v = 1 mm/min.The fracture toughness (Kc) and fracture energy(Gc) were measured according to ISO 13586-1 stan-dard. The tests were done on the Zwick 1445machine at room temperature (RT) with a crossheadspeed of v = 1 mm/min. The CT specimens (dimen-sion: 35 # 35 # 3 mm3; length # width # thickness)were notched before loading by sawing. The sawnnotch of the CT specimens was sharpened by arazor blade. The razor blade, fixed in a rig, waspositioned in the notch root before hitting the fixingrig with a hammer. This ‘taping’ yielded the desiredsharp crack.
2.4. Morphology inspectionTo get an insight in the morphology of the hybridsystems the techniques scanning electron andatomic force microscopy (SEM and AFM, respec-tively) were adapted. The fracture surface of CTspecimens has been investigated in SEM using aJSM 540 device of Jeol (Tokyo, Japan). The surfacewas coated with an Au/Pd alloy prior to SEMinspection using a Balzers SCD 050 (Balzers,Lichtenstein) sputtering apparatus.AFM scans were taken on polished samples by anAFM device (Veeco/Digital Instruments GmbH,Mannheim, Germany) in tapping mode, and therelated height- and phase-contrast images captured.Commercial silicon cantilever (Pointprobe®NCH of
Nanosensors, Neuchatel, Switzerland) with a nomi-nal tip radius of less than 10 nm (120 µm cantileverlength, 4 µm thickness, 30 µm width and springconstant at 42 Nm–1, Nanosensors, Neuchatel,Switzerland) was employed under its fundamentalresonance frequency of about 330 kHz. The scanrates were set at 0.5 Hz for all images.
3. Results and discussion3.1. Effects of diamines on the BOX
polymerizationFigure 3 displays the DSC thermograms of BOXwith and without the diamines selected.It is clearly seen that all amines exhibited the sup-posed acceleration effect as the peak linked with thepolymerization of BOX (T = 253°C) was shiftedtoward lower temperatures. This is the right place tomention that we do not use the term ‘catalytic effect’because the amount of amines is much too high forcatalysts. In contrast to the aromatic diamine, in thetraces of BOX with DETA and the cycloaliphaticdiamine two exothermic peaks could be resolved.The high temperature peak should be assigned tothe BOX homopolymerization. The low tempera-ture peak, on the other hand, obviously reflects thereaction between BOX and amine. This finding is inline with that of Agag et al. [13]. On the other hand,neither the amine consumption nor the related reac-tion were studied in detail. It is noteworthy that thelow temperature exotherm peaks at lower tempera-
Figure 3. DSC traces showing the thermally-induced poly-merization of BOX with and without 10 wt.% EP(a), as well as the effects of aliphatic (b),cycloaliphatic (c) and aromatic diamines (d),added in 10 and 20 wt.%, respectively. Notes:scatter in the DSC traces of BOX/DETA at T >220°C is due to the decomposition of DETA.BOX combined with EP (without amine hard-ener) does not affect much the curing
ture for the DETA than for the cycloaliphaticdiamine. This suggests differences in the basiccharacter of the diamines which follows the rank-ing: aromatic < cycloaliphatic < aliphatic. It is alsowell resolved that with increasing amine content theintensity of the low temperature exotherm increases,whereas the high temperature one decreases. Thethermal behavior of the BOX/DDM system differsmarkedly from BOX/DETA and BOX/cycloaliphaticdiamine. In BOX/DDM only one exothermic peakappears. Accordingly, the BOX homopolymeriza-tion is superimposed to the oxazine/amine reaction.With increasing DDM content the exothermic peakis shifted toward lower temperatures whereby itsintensity is highly reduced. Parallel to that the onsetof this peak (i.e. where deviation starts from thebase line) was markedly reduced. To sum up theabove results one can establish that diamines aresuitable accelerators of the BOX polymerizationand the acceleration effect strongly depends on boththeir chemical structure and amount. The DSCtraces in Figure 3 substantiate that the starting tem-perature for BOX curing (second exothermic peak)lies at T % 180°C. That was the reason to select thistemperature along with a long holding time for thefinal step of the cure cycle given in Figure 2. Thelarge difference in the acceleration of BOX poly-merization between DETA and DDM prompted usto select them, and consider their 1/1 mixture, aswell, as EP hardeners.
3.2. Thermal and viscoelastic properties ofEP/BOX hybrids
Figure 4 shows the DSC traces of the amine-curedEP and EP/BOX hybrids. The Tg steps are wellresolved for all systems. As expected, crosslinking
with DDM yielded the highest, with DETA the low-est Tg, and the Tg of the EP cured with mixedDETA/DDM (1/1) is in between. The EP/DETAstarts to decompose at T > 270°C based on theexothermic flank in the corresponding curve.Effects of the diamines on the Tg of the EP/BOX(75/25 wt.%) differ from those noticed for the EPreference samples. Though DDM gives the highestTg, the Tg data are similar for DETA- and DETA/DDM-cured EP/BOX. The presence of BOX didnot enhance the Tg, just the opposite tendency hap-pened. This suggests that the EP/diamine ratio wasno more stoichiometric due to the reaction betweenthe oxazine and primary amine groups. The shallowexothermic peak at T % 240°C may hint that theBOX was not fully (co)cured in the related sample.The rise of the DSC trace in exothermic direction atT > 275°C reflects the onset of degradation.Figure 5 compares the DMTA spectra of the amine-cured EP and EP/BOX (75/25 wt.%) hybrids.
Figure 4. DSC thermograms for the amine-cured EP andEP/BOX hybrids
Figure 5. Storage modulus (E!) and mechanical loss factor (tan!) in function of temperature for the amine-cured EPs (a) andEP/BOX hybrids (b)
The E! vs. T traces of the amine-cured EPs splitfrom each other only at T > 50°C. As expected, thehighest stiffness and Tg were delivered by DDM,followed by DMM/DETA and the lowest one pre-sented the DETA-cured EP. Based on the tan! vs. Ttraces the type of the amine did not influence thebeta-relaxation (T % –50°C) by contrast to thealpha-relaxation (Tg). Tg values, read at the peaktemperature of the alpha-relaxation, followed theranking: DETA < DETA/DDM < DDM in concertwith the DSC results. The Tg values are summa-rized in Table 1.The course of the E! vs. T traces of EP/BOX is notas smooth as found for the reference EPs. On theother hand, comparing the corresponding E! vs. Ttraces for EP and EP/BOX (75/25 wt.%) in Fig-ure 5a and 5b, respectively, one can recognize thatmodification with BOX improved the stiffness ofthe EP. Comparing the tan! vs. T traces of the EPand EP/BOX (75/25 wt.%) containing the samediamine, two striking features can be observed:i) Tg was reduced by the incorporation of BOX(which is in line with the DSC results (cf. Figure 4and Table 1), and ii) the tan! peak of EP/BOXcured with the mixed amines (DETA/DDM) becamebroader than in the corresponding EP. Stiffnessincrease parallel with Tg decrease are usually causedby antiplasticizers, which is a known phenomenonalso for EPs (e.g. [14–15]). The so-called internalantiplasticization caused by a change of the net-work structure. This is observed for non-stoichio-metric or partially reacted epoxy-amine systems orwhen using a monoamine as a chain extender [14].The two former features are present in our case dueto the incorporation of BOX: BOX reacts namelywith both amine and EP, but was considered as non-reactive additive in our formulations. It was alsodemonstrated that that antiplasticization affects thebeta-relaxation similarly as the reduction of crosslinkdensity, i.e. shifts the related peak toward lower
temperature [15]. This can be observed also in ourcase – cf. beta peak at T % –75°C in Figure 5b.The co-curing of EP with BOX is well reflected bythe data of the rubber plateau modulus (E!R). E!Rdata were read at Tg + 30°C for the EP and EP/BOXhybrids studied and are also listed in Table 1. TheE!R data can be used to estimate the crosslink den-sity according the theory of rubber elasticity (Equa-tion (1)):
(1)
where "c is the crosslink density, R is the gas con-stant, and T is the absolute temperature (i.e. atTg + 30°C).The crosslink densities are also included in Table 1.The broadening of the tan! peak for the EP/BOX(75/25 wt.%) can be explained by the difference inthe reactivity between the related amines and theoxazine. The amine/oxazine reaction affects lateron the EP curing and EP/BOX co-curing, as well.The DMTA analysis revealed that the modificationwith BOX enhanced the stiffness (E-modulus),decreased the crosslink density and reduced the Tg.The decreasing crosslink density of the hybrids canbe traced to the low crosslinking density of thehomopolymerized BOX. Ranking of the neat EPs inrespect to the crosslink density (EP(DDM)-Ref >EP(DETA/DDM)-Ref > EP(DETA)-Ref) differsfrom that of the EP/BOX series (EP(DDM)/BOX &EP(DETA)/BOX > EP(DETA/DDM)/BOX). Thisis due to the formation of an EP/BOX co-network,which is influenced by chemical reactions amongthe EP, amine hardener and BOX. This affects thedistribution of the possible intra- and intermolecu-lar hydrogen bonds, as well. According to our opin-ion the structure development in EP/BOX hybridsis mostly controlled by differences in the chemicalstructure and reactivity of the diamines (DETA,DDM).
It remains, however, an open issue how the BOXunits are incorporated into the EP network, andwhether or not BOX is completely cured.The TGA curves, registered on the EP and EP/BOXsystems confirm that BOX incorporation improvedthe charring, in fact – cf. Figure 6. The temperaturevalues linked with 2 wt.% loss and the char yield atT = 600°C are listed in Table 1, too. TGA traces inFigure 6 demonstrate that curing with DDM resultsin more temperature resistant EP-based systemsthan with DETA. A further finding is that the degra-
dation onset was reduced by the BOX present,albeit the amount of the final residue (char) wasenhanced. This suggests that the products of thereaction between oxazine and amine are less tem-perature resistant than those formed via epoxy/amineand epoxy/benzoxazine reactions.
3.3. Static flexural and fracture mechanicalproperties
The static flexural properties, i.e. E-modulus (Ef),strength (#f) and displacement at maximum load($f,m) are listed in Table 2. One can establish that theE-modulus and flexural strength increased, whereasthe displacement at maximum load decreased withBOX incorporation. Accordingly, BOX acted asreinforcement in the related EP/BOX hybrids. Onthe other hand, no clear tendency could be detectedfor the flexural properties in function of the type ofthe diamine compounds. The highest Ef and #f werefound for the mixed amine-cured EP/BOX.The fracture mechanical data, also listed in Table 2,yielded unexpected results. Modification with BOXslightly reduced both the fracture toughness (Kc)and fracture energy (Gc) compared to those of thereference resins. Moreover, the fracture mechanical
Figure 8. AFM height (left) and phase images (right) on the systems EP(DDM)/BOX = 75/25 wt.% (a), EP(DETA)/BOX =75/25 wt.% (b) and EP(DETA/DDM)/BOX = 75/25 wt.% (c). Note: some vertically aligned ‘tails’ in the phaseimage picture c are related to experimentally-induced artefacts (indicated by arrows).-
response of the systems was practically not influ-enced by the amine used. Based on the fact that theE-modulus and strength of the EP increased byBOX hybridization, one expected an increment inKc and a reduction in Gc compared to the referenceEP resins. This was not the case that should rely onthe morphology of the hybrid resins. Nevertheless,the Kc and Gc data of the EP/BOX hybrids are supe-rior to the polymerized neat BOX.
3.4. MorphologyThe fracture surfaces of the amine-cured EP andEP/BOX hybrids are very similar to each other.This, suggesting that the fracture mechanical param-eters should be similar, as well (which is the case, infact, as shown by the data in Table 2), is demon-strated on the example of EP(DETA/DDM) andEP(DETA/DDM)/BOX (75/25 wt.%) in Figure 7.The appearance of the fracture surface suggests thatBOX is well incorporated in the EP network, and itsdomain size (in case of eventual phase separation)should lie in submicron range. The AFM work sup-ported this suggestion (see below).The AFM height and phase images, taken from thepolished surfaces of the EP/BOX system hardenedby different amines are depicted in Figure 8. Thewhite spots in the AFM phase images represent thehomopolymerized BOX. In case of DDM hardener,the thermally homopolymerized BOX is finely anduniformly dispersed in the EP matrix. The size ofthe polymerized BOX inclusions is in the range of5–100 nm (main fraction is in the range of 7.5–45 nm). By contrast, the dispersion of the homopoly-merized BOX in the DETA-hardened EP is coarseand broad (particle size of 15–150 nm). The changein the BOX dispersion is linked with the phase seg-regation between EP and BOX, governed by thegelling/crosslinking of EP. The latter is much fasterand occurs at lower temperatures with DETA thanwith DDM hardener. This results in a fine distribu-tion of homolymerized BOX in the EP matrix withDDM and a coarse one with DETA hardening. As,expected, the polymerized BOX appears in abimodal-like distribution of 7–120 nm particles inthe EP(DETA/DDM)/BOX system (cf. Figure 8c).
4. ConclusionsIt was established that amine hardeners acceleratedthe homopolymerization of benzoxazine (BOX).
Based on this knowledge an amine-curable bifunc-tional epoxy resin was modified with BOX at a fixratio (EP/BOX = 75/25 wt.%) and the basic thermaland mechanical properties of the correspondinghybrid determined. As amine hardeners an aliphatic,an aromatic diamine, and their 1/1 mixture wereselected. The properties were collated with thosemeasured on the reference EP. The outcome of thisresearch work can be summarized as follows:Network structure and morphology–'The amine-hardened EP formed a co-network
with the BOX. This contained nanoscaled inclu-sions of the homopolymerized BOX. The disper-sion characteristics of the hompolymerized BOXdepended on the amine type. The finest disper-sion was found for DDM, whereas a coarse onefor DETA hardener. This was traced to the phasesegregation process governed by the gelling andcrosslinking of the EP with the given diamine.
Thermal properties–'The Tg of the hybrid was lower than the reference
EP that was traced to off-stoichiometry caused bythe amine/oxazine reaction. The observed inter-nal antiplasticizing effect of BOX in EP/BOXhybrids was traced also to off-stoichiometry,affected by the reaction between BOX and EP, aswell. Although the thermal degradation of thehybrid preceded that of the reference EP, incor-poration of BOX was associated with a pro-nounced increase in the char yield.
Mechanical properties–'BOX incorporation enhanced the stiffness (E-
modulus) and strength based on DMA and flex-ural tests. This was explained by the nanoscaledhomopolymerized BOX, well connected to theEP network. On the other hand, none of the frac-ture mechanical parameters could be improvedby adding BOX.
AcknowledgementsThis work was done in the framework of a collaborationproject between Germany and South Africa, sponsored byBMBF and NRF, respectively. It was also supported by theAdvanced Manufacturing Technology Strategy program ofDST (South Africa) and Hungarian Scientific ResearchFund OTKA. The authors thank to Dr. P. Tsotra (HuntsmanAdvanced Materials, Basel, Switzerland) for the BOXdelivery.
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