Composites: Part B 54 (2013) 234–240

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Composites: Part B

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Enhancement of ablative and interfacial bonding properties of EPDMcomposites by incorporating epoxy phenolic resin

1359-8368/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.05.005

⇑ Corresponding author at: State Key Laboratory of Organic–Inorganic Compos-ites, Ministry of Education, College of Material Science and Engineering, BeijingUniversity of Chemical Technology, Beijing 100029, PR China. Fax: +86 1064412084.

E-mail address: yangxp@mail.buct.edu.cn (X. Yang).

Xiaolong Jia a,b, Zigao Zeng a,b, Gang Li a,b, David Hui c, Xiaoping Yang a,b,⇑, Shiren Wang d

a State Key Laboratory of Organic–Inorganic Composites, Ministry of Education, College of Material Science and Engineering, Beijing University of Chemical Technology, Beijing100029, PR Chinab Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, College of Material Science and Engineering, Beijing University of Chemical Technology,Beijing 100029, PR Chinac Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USAd Department of Industrial Engineering, Texas Tech University, Lubbock, TX 79409-3061, USA

a r t i c l e i n f o

Article history:Received 23 December 2012Received in revised form 5 March 2013Accepted 1 May 2013Available online 15 May 2013

Keywords:B. Thermal propertiesB. InterfaceA. Polymer–matrix compositesD. Mechanical testing

a b s t r a c t

Effects of epoxy phenolic resin (EPR) on ablative and interfacial bonding properties of EPDM compositeswere evaluated. Ablative properties of EPDM composites were enhanced by two folds with incorporating10 phr EPR. This significant enhancement was attributed to positive effect of EPR on thermal stability andthermal insulating properties of EPDM composites as well as formation of compact char layer onto com-posites. Furthermore, interfacial shear strength of EPDM composites with carbon fiber/epoxy (CF/EP)composites was increased by 55.6% with incorporating 10 phr EPR, due to interfacial chemical reactionof epoxide groups of EPR molecule from EPDM composites with amine group of hardener from CF/EPcomposites.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Thermal insulators, used as heat-barrier materials between thecase and the propellant, were essential components of rocket mo-tors, and they should have good thermal resistance and good abla-tion properties. At present, the elastomers like ethylene–propylene–diene monomer (EPDM) were conventionally used asmatrix for thermal insulating materials because of their low ther-mal conductivity, high thermal stability and low density [1–3].With the aim to enhance the integrated performance of elasto-mer-based thermal insulator, many sorts of fillers were widelyincorporated into the system in the literatures [1,4–7]. In our previ-ous works, polysulfonamide pulp was pretreated and used to en-hance the ablative, thermal and mechanical properties of EPDMthermal insulating composites [8–10]. However, with the fastdevelopment of rocket motor technologies, high performance ther-mal insulators were required to form strong char layer after carbon-ization and endure the severe conditions of high temperaturecombustion gases [2,11,12]. The reported EPDM-based thermalinsulator did not meet this requirement well since such insulator

alone could not form a strong char layer after carbonization. There-fore, more stable and advanced fillers should be applied in the ther-mal insulator to improve their integrated thermal insulatingproperties. Among various fillers, the phenolic type materials werefavorable due to their high thermal stability and char yield in thehigh temperature condition [13,14], which implied that such mate-rials would greatly contribute to building up the strong char layerafter carbonization. However, there have been few reports on EPDMcomposites modified with phenolic type materials up to now.

In addition, the continuous fiber reinforced epoxy compositeshave replaced the metal materials to be used as the motor casealong with the fast development of rocket motor technologiesdue to their advantages of high strength and low density [15–17]. In this new application environment of the thermal insulator,the state of its interfacial bonding with the composite case, whichwas the vital factor to ensure the safe running of solid rocket mo-tor, has attracted much more attention than before, since EPDM asa non-polar elastomer could not achieve good adhesion with othermaterials. To overcome this problem, a lot of researchers focusedon developing the external adhesives for the interfacial bondingof such insulator and composite case [18–21]. For instance, Parket al. examined the effects of fabrication and adhesive parameterson adhesion between composite and insulator rubber [18].Unexpectedly, the enhancement of the interfacial bonding strengthby using adhesives was limited, and such efforts led to some

Fig. 1. Effect of EPR content on ablation rate of EPDM composites.

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negative factors such as increasing the corresponding cost and pro-cessing difficulty of the materials. On the other hand, it was no-ticed that the chemical activation of interfacial bonding ability ofEPDM-based thermal insulator was the feasible way to enhanceits interfacial bonding with other materials [22]. However, to thebest of our knowledge, there was a large uncertainty and lack ofunderstanding on improving the autologous interfacial bondingability of EPDM-based thermal insulator.

To bridge this gap, effects of epoxy phenolic resin (EPR) on abla-tive and interfacial bonding properties of EPDM-based thermalinsulating composites were investigated in detail. To this end, (i)EPDM-based thermal insulating composites with EPR were pre-pared; (ii) effects of EPR on ablative properties, thermal stabilityand thermal insulating properties of EPDM composites were eval-uated along with the characterizations of thermogravimetric anal-ysis (TGA), scanning electron microscopy (SEM) and energydisperse X-ray spectroscopy (EDS); and (iii) the interfacial shearstrength associated with micro-morphology of interfacial regionwas studied to elucidate the effect of EPR on autologous interfacial

Fig. 2. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of EPDMarrows.

bonding ability of EPDM composites with carbon fiber/epoxy (CF/EP) composites.

2. Experiment

2.1. Composite preparation

The materials including EPDM, liquid EPDM, polysulfonamidepulp, nano silica, additives and curing agents were selected inaccordance with our previous works [8–10], except the new fil-ler, epoxy phenolic resin (EPR) (Wuxi Resin Factory of BluestarChemical Co., China). The content of EPR was adjusted to 0, 5,10, 15 and 20 parts per hundred grams (phr) of EPDM compos-ites. The EPDM composites were prepared following the processreported in our previous works [8–10]. The specimens of EPDMcomposites adhered with carbon fiber/epoxy (CF/EP) compositeswere prepared by placing uncured EPDM composites on CF/EPprepreg with amine hardener and subsequent curing togetherat 120 �C for 30 min and 150 �C for 3 h under the pressure of10 MPa.

2.2. Analysis and characterization

2.2.1. Ablative propertiesThe composite plaques in 10 mm thickness were cut into round

ablative specimens with a 30 mm diameter. Ablative testing wasconducted on an oxy-acetylene ablation tester (Xi’an TianguanSci. Co., Ltd., YS22) in accordance with ASTM E 285.

2.2.2. Thermogravimetric analysis (TGA)TGA of the specimens (10–12 mg) was performed using a TA

instrument (NETZSCH STA 449C) under an argon atmosphere at apurge rate of 40 ml/min. The specimens were heated from 35 to800 �C at a heating rate of 10 �C/min.

2.2.3. Thermal insulating propertiesThermal insulating testing was contacted on a butane-torch.

Temperature evolution at the various thickness locations weretested by a thermocouple and were synchronously recorded by a

composites without and with 10 phr EPR. The TG and DTG curves are labeled by

Fig. 3. The temperature evolution with ablation time along the thickness directionfor EPDM composites (a) without and (b) with 10 phr EPR.

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multi-recorder (CF30R16, Beijing Chidufangyuan Sensor ElementCo., Beijing, China).

2.2.4. Morphology observation and element analysisObservations of char layers and fractured surfaces of EPDM

composites were carried out by scanning electron microscopy(SEM, HITACHI S4700). The composite specimens were coated witha thin layer of a gold alloy. The elements of char layers of EPDMcomposites were analyzed under energy disperse X-ray spectros-copy (EDS, EDAXGENESIS2000). The final value of each elementcontent was the average of five measurements on differentspecimens.

2.2.5. Mechanical propertiesThe mixture plaques composed of EPDM composites in 2 mm

thickness and carbon fiber/epoxy (CF/EP) composites in 2 mmthickness were cut into rectangle specimens with 20 � 120 mmsize with a 20 � 20 mm square contact area in accordance withASTM D 1002. Interfacial shear strength was measured by tensile

testing machine (Instron 1121) at a crosshead speed of 500 mm/min using 200 N load.

3. Results and discussion

3.1. Ablative properties

Fig. 1 shows effect of epoxy phenolic resin (EPR) content on abla-tion rate of EPDM composites. With the increase of EPR content,there was a rapid decrease to 0.044 mm/s at 10 phr content ofEPR, and then a smooth increase in the ablation rate. Compared toEPDM composites without EPR, ablative properties of EPDM com-posites were enhanced by twofolds with incorporating 10 phrEPR. For the contrast, the ablation rate of EPDM composites with10 phr phenol formaldehyde resin, a similar material to EPR, wasmeasured to be 0.045 mm/s. This manifested that the contributionto ablative rate decrease of EPDM composites was comparable forboth EPR and phenol formaldehyde resin. However, EPR was se-lected as the proper filler in our present study, since EPR had theepoxide groups in the chemical structure and showed significantadvantage on enhancing interfacial bonding of related composites(which will be discussed in Section 3.4). Furthermore, the initial de-crease of ablation rate in Fig. 1 could be explained by the enhance-ment of thermal stability and the increment of char yield of thecomposites with the addition of phenolic type materials [14]. How-ever, the tenacity of the char layer on the surface of the compositeswas greatly influenced by the content of EPR, which dominated theadhesion between char layer and virgin material. When the contentof EPR was high, that is, greater than 10 phr, the tenacity of the charlayer on the surface of the composites decreased obviously. Duringthe intensive impingement of high-temperature combustion gases,the poor tenacity resulted in the fracturing and sloughing of theformed char layer. Then, the underlying virgin material was ex-posed, and this led to the ablation rate increasing. Therefore, EPDMcomposites with 10 phr EPR showed the optimal ablative propertieswhich were the critical performance for thermal insulating materi-als. In addition, according to the findings in our previous works [8],the ablative properties of the composites was also related to thethermal stability and thermal insulating properties as well as themicrostructure of char layer on the composites, which was eluci-dated in the following parts of our present study.

3.2. Thermal properties

Fig. 2 shows the thermogravimetric (TG) and derivative thermo-gravimetric (DTG) curves of EPDM composites without and with10 phr EPR. It could be seen that only one major degradation stepat around 474 �C characterized the decomposition of both compos-ites. However, the TG curves showed that the residue yield ofEPDM composites with EPR at 800 �C was 24.1% higher than21.2% of the composites without EPR, as well as the DTG curvesexhibited that the peak degradation rate was lower for the formercomposites than for the latter composites. The observed increasedresidue yield was attributed to the high char yield of EPR since thebenzene rings contained in the chemical structure of EPR woulddeposit to form much char [13,14]. These results indicated thatthe addition of EPR enhanced thermal stability and char yield ofthe composites, which was beneficial to the enhancement of abla-tive and thermal insulating properties according to Deuri et al. [23]

Fig. 3 shows the temperature evolution with ablation timealong the thickness direction for EPDM composites without andwith EPR. It could be noted that, with the increasing of the ablationtime and the decreasing of the distance from the flamed surface,the temperature increased obviously for both composites. Further-more, the slopes of the temperature increasing in both composites

Fig. 4. SEM images of (a and b) virgin layer, (c and d) decomposition layer and (e and f) char layer; (g and h) optical images of ablated specimens and (i and j) EDS results ofchar layer for EPDM composites. In these images, (a, c, e, g and i) for EPDM composites without EPR and (b, d, f, h and j) for EPDM composites with 10 phr EPR. In the rightparts of images (g and h), the transverse cross-sections of ablated specimens show the three-layer structure, corresponding to the SEM images (a–f).

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Fig. 5. Interfacial shear strength of EPDM composites with CF/EP composites. Thespecimens of EPDM composites (without EPR) adhered with CF/EP composites bytwo types of external adhesives were used as contrast samples. Adhesive type I is akind of ethyl a-cyanoacrylate based adhesive. Adhesive type II is a kind of EPDMbased adhesive.

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were much higher at the location closer to the flamed surface thanat the other location far from the flamed surface. However, com-pared to EPDM composites without EPR, the temperature increas-ing at the same thickness location was far slower with ablationtime for EPDM composites with EPR. For instance, at the thicknesslocation of 1 mm for EPDM composites without and with EPR, thetemperatures at the ablation time of 10 s were 146 and 204 �Crespectively, while the values at the ablation time of 60 s were826 and 906 �C, accordingly. This phenomenon manifested thatthe addition of phenolic type resin could improve the thermal insu-lating ability of EPDM composites, which was resulted from thehigh specific heat capacity, low thermal conductivity and long deg-radation half-life of phenolic type materials [13,24].

Fig. 6. (a and b) Optical and (c and d) SEM images of interfacial regions between EPDM cocomposites without EPR and (b and d) for EPDM composites with 10 phr EPR. The dottedcomposites and CF/EP composites after fracturing. In part (c), the arrows show the dcomposites and EPDM composites without EPR.

3.3. Micro-morphologies

On the basis of the established ablation model by many research-ers [25,26], the internal insulating materials were divided into threeregions consisting of char layer, decomposition layer and virginlayer. The heat was absorbed by the materials with the changingof chemical or physical state of these materials. The interfaces be-tween these regions were somewhat indistinct, but even so the re-gions could be defined by the principal phenomena found in each.Judging from internal real-time temperature distribution in bothcomposites without and with EPR shown in Fig. 3, the part from 0to 1.5 mm thickness was the char layer which was carbonized al-ready, where the temperature increased rapidly beyond 600 �C atthe 1.5 mm thickness location after ablation for 60 s. The part from1.5 to 2.5 mm thickness was considered as the decomposition layer,where the temperature increased slowly than the char layer and theloose structure with big pores (which will be shown in Fig. 4c and d)were generated. It could be inferred that the heat was not trans-ferred effectively beyond 2.5 mm thickness during ablation of bu-tane-torch in our experiment so that the decomposition of bothcomposites did not take place in the virgin layer in the conditionof slight increasing of corresponding temperature. Fig. 4 showsSEM images of each layer of three-layer structure, optical imagesof ablated specimens and EDS results of char layer for EPDM com-posites without and with EPR. As shown in Fig. 4a–f, EPDM compos-ites showed distinct microscale three-layer structure after ablation.It could be seen that the virgin layers were compact without holeswhile the decomposition layers showed porous microstructure.There was no much difference observed in the microstructure of vir-gin layer as well as decomposition layer for EPDM composites with-out and with EPR. Nevertheless, the morphologies of char layers onsurfaces of EPDM composites without and with EPR were clearly dif-ferent. During ablation, the surfaces of EPDM composites wereeroded by the flame combustion, resulting in the formation ofmacro-scale pits on the surface char layer. However, as shown inthe left parts of Fig. 4g and h, it could be noted that the surface pitof EPDM composites without EPR was centered and sharply deep,whereas that of EPDM composites with EPR was smooth and rela-tively shallow. This difference could be explained from themicro-structure and element composition of char layers of both

mposites and CF/EP composites after fracturing. In these images, (a and c) for EPDMrectangles in part (a) and (b) show the interfacial bonding regions between EPDM

ebonding and cracking existing in the interfacial bonding regions between CF/EP

Fig. 7. Schematic of interfacial chemical reaction between EPR filled EPDM composites and CF/EP composites.

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composites. As shown in Fig. 4e and i, the char structure of EPDMcomposites without EPR was loose with a large amount of visiblebig cracks and holes from about 50 to 200 lm and the final contentof carbon element was 90.6%. In contrast, the char structure of EPDMcomposites with EPR in Fig. 4f and j was relatively compact withhigher carbon element content of 96.7% in consistence with the re-sults in Fig. 2, which contributed to the enhancement of ablativeproperties of EPDM composites. Therefore, the ablative propertieswere superior for EPDM composites with EPR as shown in Fig. 1.

3.4. Interfacial bonding properties

Fig. 5 shows interfacial shear strength of EPDM composites withcarbon fiber/epoxy (CF/EP) composites. The interfacial shearstrength was increased by 55.6%, from 2.7 to 4.2 MPa, when10 phr EPR was incorporated, which was even higher than corre-sponding strength values of 2.1 and 3.0 MPa, respectively, resultedfrom acrylate based adhesive and EPDM based adhesive as well asthe values reported in the literatures [27,28]. This indicated thatthe strong interfacial bonding was built up between the EPDMthermal insulating materials and the composite case, which wasverified from the optical and SEM images of interfacial regions be-tween EPDM composites and CF/EP composites in Fig. 6. As shownin Fig. 6a and c, the interfacial bonding area was clearly separatedfor EPDM composites without EPR after shearing testing, and thedebonding and cracking were obviously found on the interfacesdue to the non-reactivity of EPDM matrix. While, as shown inFig. 6b, the materials from the side of EPDM composites withEPR was firmly adhered onto the other side of CF/EP compositesafter shearing testing, and there was no cracking observed forEPDM composites with EPR shown in Fig. 6d because of high reac-tivity of EPR in EPDM composites with the harder of CF/EP compos-ites. All these results revealed that strong interfacial bonding wasformed between EPDM composites with EPR and CF/EP compositesin accordance with the results from Fig. 5. Noticeably, the signifi-cant enhancement of the interfacial bonding of EPR filled EPDMcomposites with CF/EP composites was within our experimentalexpectation. As shown in Fig. 7, the epoxide groups of EPR mole-cule from EPDM composites were reacted with the amine groupof the hardener from CF/EP composites. With this reaction pro-ceeding, the three-dimensional crosslinked network structurewas formed on the interface between the two composites. There-fore, such chemical adhesion contributed to the observed enhance-ment of interfacial bonding properties of EPR filled EPDMcomposites with CF/EP composites.

4. Conclusion

The ablative properties of EPDM composites were significantlyenhanced by the addition of epoxy phenolic resin (EPR) and thisobserved enhancement was related to the positive effect of EPR

on the thermal stability and thermal insulating properties of EPDMcomposites as well as the formation of compact char layer on thecomposites. Moreover, the addition of EPR contributed to enhanc-ing the interfacial bonding of EPDM composites with carbon fiber/epoxy composites due to the interfacial chemical reaction betweenthe two kinds of composites.

Acknowledgments

The authors are very pleased to acknowledge financial supportfrom the National High Technology Research and DevelopmentProgram of China (Grant No. 2012AA03A203) and Program forChangjiang Scholars and Innovative Research Team in University(PCSIRT).

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