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Reinforcement Effect of MAA on Nano-CaCO3-FilledEPDM Vulcanizates and Possible Mechanism

YABIN ZHOU, SHIFENG WANG, YINXI ZHANG, YONG ZHANG

Research Institute of Polymer Materials, Shanghai Jiao Tong University, 200240 Shanghai, People’s Republic of China

Received 14 October 2005; revised 12 January 2006; accepted 23 January 2006DOI: 10.1002/polb.20774Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Methacrylic acid (MAA) was used as in situ surface modifier to improvethe interface interaction between nano-CaCO3 particle and ethylene–propylene–dienemonomer (EPDM) matrix, and hence the mechanical properties of nano-CaCO3-filledEPDM vulcanizates. The results showed that the incorporation of MAA improved thefiller–matrix interaction, which was proved by Fourier transformation infrared spec-trometer (FTIR), Kraus equation, crosslink density determination, and scanning elec-tron microscope (SEM). The formation of carboxylate and the participation of MAA inthe crosslinking of EPDM indicated the strong filler–matrix interaction from the as-pect of chemical reaction. The results of Kraus equation showed that the presence ofMAA enhanced the reinforcement extent of nano-CaCO3 on EPDM vulcanizates.Crosslink density determination proved the formation of the ionic crosslinks inEPDM vulcanizates with the existence of MAA. The filler particles on tensile fracturewere embedded in the matrix and could not be observed obviously, indicating that astrong interfacial interaction between the filler and the matrix had been achievedwith the incorporation of MAA. Meanwhile, the presence of MAA remarkably in-creased the modulus and tensile strength of the vulcanizates, without negative effecton the high elongation at break. Furthermore, the ionic bond was thought to beformed only on filler surface because of the absolute deficiency of MAA, which re-sulted in the possible structure where filler particles were considered as crosslinkpoints. VVC 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1226–1236, 2006

Keywords: calcium carbonate; nanoscale; methacrylic acid; reinforce; surface modi-fication

INTRODUCTION

Except for a few rubbers like natural rubberand chloroprene rubber that can be reinforcedby self-crystallization in tensile process, mostrubbers must be reinforced by fillers before prac-tical application. A number of fillers have beenused as reinforcing agents, such as carbonblack,1,2 silica,3–5 and montmorillonite.6 As min-eral filler, CaCO3 has been used only as bulking

agent mainly reducing the cost of rubber prod-ucts. With the development of technology in par-ticle fining and especially in surface modification,CaCO3 is used as a reinforcing agent. Surfacemodification can decrease the surface energy andpolarity of inorganic fillers, thus reducing theaggregation tendency of filler particles in rubbermatrix. As a result, good dispersion and largeinterfacial area can be obtained, leading to betterreinforcement of CaCO3 on rubbers.

Many surface modifiers have been used totreat CaCO3, including coupling agents,7,8 sur-factants,9,10 and reactive polymers.11 Among them,stearic acid is a very important anionic surfactantand is often used to treat CaCO3.

10 Its carboxyl

Correspondence to: Y. Zhang (E-mail: [email protected])

Journal of Polymer Science: Part B: Polymer Physics, Vol. 44, 1226–1236 (2006)VVC 2006 Wiley Periodicals, Inc.

1226

terminals can be coupled to the filler surface withionic bonds and the alkyl terminals are orientedin directions outward the surface.12 As a result,the particle surface of CaCO3 is modified fromhydrophilic to hydrophobic, which reduces the fil-ler–filler interaction and favors the even disper-sion of CaCO3 in matrix. Because of lack of anyactive functional group, strong interaction can notbe obtained between stearic acid and matrix, onlyto reduce the surface energy of filler and to im-prove the dispersion.

Unlike stearic acid, methacrylic acid (MAA) isan unsaturated carboxylic acid which containstwo functional groups, that is, a carboxyl and acarbon–carbon double bond in each molecule.The carboxyl group on MAA could react withthe active functional groups on the surface ofnano-CaCO3, mainly Ca2þ ion, which is similarto the reaction between stearic acid and CaCO3.Moreover, the carbon–carbon double bond onMAA can participate in the crosslinking reactionof ethylene–propylene–diene monomer (EPDM)induced by dicumyl peroxide (DCP), and hence,bonded to the crosslink network in EPDM vul-canizates. As a result, a strong interfacial inter-action between nano-CaCO3 and the EPDM ma-trix can be obtained.

In fact, in a study of CaCO3-filled polypropyl-ene (PP), some kind of unsaturated carboxylicacid similar to MAA, such as acrylic acid (AA),has been used to modify CaCO3.

13 A stronginteraction between CaCO3 and PP could berealized through the grafting polymerization ofAA to PP initiated by DCP. However, the incor-poration of DCP would unavoidably initiate thedegradation of PP, which would worsen the me-chanical properties of compound.14

In this study, MAA was used to in situ modifynano-CaCO3 when blending with EPDM withaims to strengthen filler–matrix interaction andresult in excellent mechanical properties. Toprove the strong interaction between the fillerand the matrix, Fourier transformation infraredspectrometer (FTIR), Kraus equation, crosslinkdensity determination, and scanning electronmicroscope (SEM) had been used successfully.The remarkable improvement in modulus forfilled vulcanizates with MAA also proved ourassumption effectively. Meanwhile, other me-chanical properties, such as tensile strength andtear strength are also increased. Furthermore,the ionic bond is thought to be formed only onfiller surface because of the absolute deficiencyof MAA, which results in the possible structure

where filler particles are considered as crosslinkpoints.

EXPERIMENTAL

Materials

Ethylene–propylene–diene monomer rubber(EPDM), ND 4770R, is a semicrystalline rubberwith ethylene content of 70% and supplied byDu Pont Company, USA. Nano-CaCO3 withoutsurface modification was supplied by ShanghaiYaohua Nano-Tech Ltd., China. All the chemi-cals, including MAA and DCP, were used asreceived.

Sample Preparation

According to the typical formulation showed inTable 1, EPDM and nano-CaCO3 were mixed ina HAAKE Rheometer RC90 (Haake Company,Germany) at 40 8C with a rotor speed of 60 rpm.EPDM was plasticized in the rheometer for2 min at first, and then mixed with nano-CaCO3

and MAA for 6 min, followed by addition of DCPand mixed for 2 min. The resultant compositewas sheeted on a two-roll mill at room tempera-ture and compression molded at 170 8C for 10 minto obtain vulcanizates.

Measurements

Fourier Transformation Infrared Spectrometer

Infrared transmission spectra were obtainedusing an FTIR spectrometer (model Paragon1000, PerkinElmer Corp). The scan range wasfrom 4400 to 400 cm�1 with a resolution of4 cm�1. Samples of CaCO3 powder were pre-pared by milling CaCO3 with KBr and com-pressed to a KBr disc for testing. Samples ofEPDM gum, compound, and vulcanizates werepressed to thin films for testing.

Table 1. Typical Formulation

Ingredient Content (phr)

EPDM (ND 4770R) 100Nano-CaCO3 Variable (0–100)MAA Variable (0–6)DCP 1

phr, parts per hundred parts of rubber (by weight); MAA,methacrylic acid; DCP, dicumyl peroxide.

REINFORCEMENT EFFECT OF MAA ON NANO-CACO3-FILLED EPDM VULCANIZATES 1227

Journal of Polymer Science: Part B: Polymer PhysicsDOI 10.1002/polb

Transmission Electron Microscope

Transmission electron microscope (TEM) imageswere taken in H-800 microscope (Hitachi, Ja-pan) with an accelerator voltage of 120 kV. Thinsections (�100 nm) of the specimens were cryo-cut with a diamond knife at about 120 K andused without staining.

Determination of Crosslink Density

The crosslink density was determined using anequilibrium swelling method. Vulcanizate sam-ples were swollen in toluene at 25 8C for 72 h toachieve equilibrium swelling. The weight of thesamples was recorded under swollen conditions.Then the samples were dried in a vacuum ovenat 80 8C for 48 h to remove all the solvent, andweighed again. The volume fraction of rubberswollen in the gel, Vr, which was used to repre-sent the crosslink density of the vulcanizates,was determined by the following equation:

Vr ¼ m0/ð1� aÞ=qrm0/ð1� aÞ=qr þ ðm1 �m2Þ=qs

ð1Þ

where m0 is the sample mass before swelling,m1 and m2 are the sample masses before and af-ter drying, / is the mass fraction of rubber inthe vulcanizates, a is the mass loss of the gumEPDM vulcanizates during swelling, and qr andqs are the rubber and solvent density, respec-tively. To distinguish ionic crosslinks from cova-lent crosslinks, the samples were swollen in amixture of toluene and chloroacetic acid for5 days to destroy ionic crosslinks, followed byswelling in toluene for 2 days and weighed, thenvacuum dried and reweighed.15 Vr1 was calcu-lated from eq 1, which represents the covalentcrosslink density. Vr2, which was calculated bysubtracting Vr1 from Vr, was used to representthe ionic crosslink density.

Scanning Electron Microscope

The morphology of EPDM vulcanizates wasstudied using a SEM, S-2150 (Hitachi Japan).Samples were tensile fractured and covered witha thin gold layer before observation.

Measurement of Mechanical Properties

Tensile properties and tear strength were meas-ured using an Instron 4465 tester at a crossheadspeed of 500 mm/min, according to ASTM D412

and ASTM D624, respectively. Shore A hardnesswas determined using a Shore A sclerometer,according to ASTM D2240.

RESULTS AND DISCUSSION

Morphology of MAA-CaCO3 in Rubber Matrix

The TEM photographs of uncoated and MAA-coated nano-CaCO3 particles in vulcanizates areshown in Figure 1. Both micrographs are in twomagnifications of 10,000 and 50,000 times.

It is seen in Figure 1(a) that the uncoatedcubic nano-CaCO3 particles are aggregated invulcanizates with an irregular shape, which isattributed to its high surface energy. For MAA-coated nano-CaCO3 particles, a lot of aggregatesare also found and no obvious improvement in

Figure 1. TEM micrographs of (a) uncoated and (b)MAA coated nano-CaCO3 in vulcanizates.

1228 ZHOU ET AL.


dispersion is detected than uncoated nano-CaCO3, as shown in Figure 1(b). It is mostlybecause of the too short carbon chain in MAAmolecule, which can not reduce the high energyof nano-CaCO3 particles effectively, and hence,does not play a remarkable insulation effectamong filler particles.

FTIR Analysis

MAA is a difunctional carboxylic acid which hasa terminal double bond in the molecule. It ishighly reactive in the presence of free radicalsand readily reacts to form crosslink with bothsaturated and unsaturated elastomers. Figure 2shows the FTIR spectra of EPDM, MAA, CaCO3,and their compound and vulcanizates.

Curve (a) in Figure 2 is the spectrum of theEPDM gum with characteristic absorbing peaksat 2924 and 2854 cm�1 generated from C��Hasymmetric and symmetric stretching vibrationsof the methylene group. The characteristic peakat 1466 cm�1 is due to the scissoring vibrationof CH2. The peak at 1376 cm�1 is ascribed tothe symmetric deformation vibration of themethyl group. The band at 722 cm�1 is ascribedto the C��H bending vibration of (CH2)n whenthe n value is not smaller than four. Curve (b)in Figure 2 is the spectrum of MAA. It is knownthat the liquid methylacrylic acid is likely to

form dimer because of the strong hydrogen bondamong the molecules. The strong associationreaction between hydroxide and carbonyl willextend the hydrogen band to nearly 2500 cm�1.As a result, the stretching vibration band ofhydroxyl will be observed at 3300–2500 cm�1.The broadband at 948 cm�1 is ascribed to theO��H bending vibration. The bands at 1696 and1636 cm�1are due to C¼¼O strong stretchingvibration and C¼¼C stretching vibration, respec-tively. The C��O stretching vibration bands arefrom 1250 to 1400 cm�1. Moreover, the band at810 cm�1 is ascribed to the out-of-plane bendingvibration in H2C¼¼CH. Curve (c) in Figure 2 isthe spectrum of CaCO3. The strong and broadpeak at 1485–1417 cm�1 is due to the C��Ostretching vibration.

Curves (d) and (e) in Figure 2 are the spectraof the EPDM/CaCO3/MAA compound and vul-canizates, respectively. When the EPDM ismixed with CaCO3 and MAA, most characteris-tic peaks of the components did not change,except for the band at 1696 cm�1. At the sametime, a new band at 1564 cm�1 is observed,which is ascribed to the formation of carboxy-late, that is, the (Ca2þ)-(�OOC) band. The bond-ing between Ca2þ and �OOC homogenized theC¼¼O and C��O in carboxyl. As a result, thepeak at 1696 cm�1 disappeared, which is as-

Figure 2. Transmission FTIR spectra of (a) EPDM, (b) MAA, (c) CaCO3, (d) EPDM/CaCO3/MAA compound, and (e) EPDM/CaCO3/MAA/DCP vulcanizates.



cribed to the C¼¼O strong stretching vibration.Compared with the spectrum of compound, thespectrum of vulcanizates has a significantchange in the peak of 1640 cm�1. The relativeintensity of C¼¼C absorption at 1640 cm�1

becomes weak obviously, which indicates thatthe carbon–carbon double bonds have reacted toa great extent in the vulcanization process.

Extent of Reinforcement

The chemical reactions between MAA and CaCO3

as well as between MAA and EPDM have beenproved by FTIR, which indicates that a stronginteraction exists between CaCO3 and EPDMthrough the bridge effect of MAA. In this section,the extent of reinforcement is assessed by usingthe Kraus equation,16 as the following:

Vro=Vrf ¼ 1�mf

1� f

� �ð2Þ

where Vrf is the volume fraction of rubber in thesolvent-swollen filled vulcanizates and is givenby the following equation:

Vrf ¼ðd� fwÞq�1

p

ðd� fwÞq�1p þ Asq�1

s

ð3Þ

where d is the deswollen weight of the vulcani-zates, f is the volume fraction of the filler, w isthe initial weight of the vulcanizates, qp is thedensity of matrix, qs is the density of the sol-vent, and As is the amount of solvent absorbed.For an unfilled system, f ¼ 0. Substituting thisin eq 3, the expression for the volume fraction of

rubber in the solvent-swollen unfilled vulcani-zates (Vro) is given as following.

Vro ¼dq�1

p

dq�1p þ Asq�1

s

ð4Þ

Since eq 2 has the general form of an equationfor a straight line, a plot of Vro/Vrf as a functionof f/(1 � f) should give a straight line, and itsslope will be a direct measurement of the rein-forcing ability of the filler.

The Kraus plots for filled vulcanizates withincorporation of MAA are shown in Figure 3. Itcan be seen that the slope changed from a posi-tive value to a negative value when MAA wasintroduced into the system, indicating the pres-ence of MAA could enhance the reinforcementeffect of nano-CaCO3 on EPDM.

Ionic Crosslink and its Density

DCP can initiate the crosslink reaction of EPDMas well as the polymerization of MAA, includingboth homo polymerization and graft polymeriza-tion.17 To investigate the effect of MAA on thecrosslink structure in the DCP-cured nano-CaCO3-filled EPDM vulcanizates, the crosslinkdensities of ionic and covalent bonds were meas-ured by using an equilibrium swelling method.The values of Vr (representing the gross cross-link density of the vulcanizates) and Vr1 (repre-senting covalent crosslink density of the vulcani-zates) were calculated according to eq 1. Vr2 (repre-senting ionic crosslink density of the vulcanizates)was calculated by subtracting the covalent cross-link density from the gross crosslink density.

Figure 3. Vro/Vrf versus f/(1 � f) for vulcanizates intoluene with incorporating MAA.

Figure 4. Effect of nano-CaCO3 content on crosslinkdensity of EPDM vulcanizates (EPDM/CaCO3/MAA/DCP ¼ 100/variable/2 wt % of CaCO3/1).

1230 ZHOU ET AL.


Figure 4 shows the effect of CaCO3 contenton the crosslink densities of EPDM vulcanizates,where CaCO3 is in situ modified with 2 wt % ofMAA. It is seen that the gross crosslink densityof EPDM vulcanizates increases monotonouslywith increasing CaCO3 content and this changeis mostly attributed to the increase in ioniccrosslink density. In fact, the incorporation ofCaCO3 and MAA reduces the covalent crosslinkdensity of EPDM vulcanizates. When CaCO3

content is more than 20 phr (parts per hundredparts of rubber (by weight)), the covalent cross-link density begins to decrease.

Figure 5 shows the effect of MAA content onthe crosslink density of the EPDM vulcanizates.At a given filler content of 60 phr, both the grosscrosslink density and ionic crosslink density

increase with the incorporation of MAA, whilethe covalent crosslink density exhibits a slightdecrease. With the further increase in MAA con-tent, no distinct change is observed in ioniccrosslink density. It is concluded that the incor-poration of MAA into CaCO3/EPDM generatesionic crosslinks. However, the ionic crosslinkdensity in vulcanizates could not increase se-quentially with increasing MAA content.

SEM Micrographs of Tensile Fracture Surface

SEM images of tensile fracture surfaces of EPDMvulcanizates are shown in Figure 6. It can beseen in Figure 6(a) that nano-CaCO3 particlesincline to aggregate. Some holes and exposed par-ticles left after the filler particles were pulled outfrom the matrix when the stress was applied. Thefailure occurred at the filler–matrix interface,indicating the weak interfacial adhesion betweenthe filler and the matrix. However, the pull-outphenomenon of filler particles is not observedon the tensile fracture surface of vulcanizateswith MAA. The filler particles in Figure 6(b) areembedded in the matrix and can not be observedobviously, indicating that a strong interfacialinteraction between the filler and the matrix hasbeen achieved with the incorporation of MAA.The failure occurred at the matrix because ofstrong adhesion between the filler and the ma-trix. In tensile test, the particle covered by alayer of the matrix was pulled out together withthe matrix. As a result, the tensile fracture sur-face of vulcanizates with MAA is much coarserthan that without MAA, indicating the failuresurface of vulcanizates with the presence of MAA

Figure 5. Effect of MAA content on crosslink den-sity of EPDM vulcanizates (EPDM/CaCO3/MAA/DCP¼ 100/60/variable/1).

Figure 6. SEM micrographs of tensile fracture surface of EPDM vulcanizates filledwith 60 phr CaCO3. (a) Without MAA and (b) with MAA.



can absorb more energy in tensile fracture pro-cess. As a result, the filled vulcanizates withMAA ought to exhibit better mechanical proper-ties than vulcanizates without MAA.

Mechanical Properties

Effect of MAA Content on Mechanical Properties

Figure 7 shows the effect of MAA content onthe mechanical properties of nano-CaCO3-filledEPDM vulcanizates. It is found that the modu-lus of vulcanizates is increased with the incorpo-ration of MAA. As is discussed above, the incor-poration of MAA builds the strong connectionbetween filler particle and rubber matrix, whichresults in the formation of ionic crosslink andthe increase in gross crosslink density, andhence, the increase in the modulus of vulcani-zates. With the increase in MAA content, themodulus at 100% elongation increases, and themodulus at 300% increases more significantlyand reaches the maximum value of 13.6 MPawhen the MAA content is 2 phr, then decreasesgradually thereafter (it is still more than10 MPa even when the MAA content is 6 phr),which is nearly corresponding to the change ingross crosslink density. Furthermore, the tensilestrength increases with increasing MAA content,and reaches the maximum value when the MAAcontent is 1.2 phr. In the range of MAA contentfrom 2 to 6 phr, all the vulcanizates have the ten-sile strength more than 23 MPa. The elongationat break of EPDM vulcanizates substantially in-creases with increasing MAA content, and reachesits maximum value of 568% at the MAA contentof 4.8 phr, which is 21% higher than that of theEPDM vulcanizates without MAA. The tearstrength of vulcanizates increases by 34% whenthe MAA content increases from 0 to 1.2 phr. Withthe further increase in MAA content, the tearstrength does not increase anymore. Generally,the incorporation of MAA further reinforces theEPDM vulcanizate after nano-CaCO3. It appearsto be an effective way to obtain high elongation atbreak, meanwhile remarkably increasing the mod-ulus and tensile strength of nano-CaCO3-filledEPDM vulcanizates.

Effect of Nano-CaCO3 Contenton Mechanical Properties

Figure 8 shows the effect of nano-CaCO3 contenton the mechanical properties of EPDM vulcani-

zates. MAA has a remarkable effect on the me-chanical properties of nano-CaCO3-filled vulcan-izates. At the same filler content, the vulcanizateswith MAA have better mechanical properties than

Figure 7. Effect of MAA content on the mechanicalproperties of EPDM vulcanizates filled with 60 phrCaCO3: (a) modulus at 100 and 300%, (b) tensilestrength and elongation at break, and (c) tear strength.

1232 ZHOU ET AL.


Figure 8. Effect of nano-CaCO3 content on the mechanical properties of EPDMvulcanizates: (a) modulus at 100%, (b) modulus at 300%, (c) tensile strength, (d) elon-gation at break, and (e) tear strength.



the vulcanizates without MAA. It is clear thatthe incorporation of MAA always increases themodulus of vulcanizates, no matter how muchCaCO3 is incorporated. The tensile strengthincreases with increasing filler content, andreaches its maximum value of 25.6 MPa at 60phr filler content, which is 51% higher than thatof the filled vulcanizates without MAA. Theelongation at break increases significantly andreaches the maximum value of 654% when thenano-CaCO3 content is 20 phr, and decreasesgradually thereafter (although it is still over450% even when the nano-CaCO3 content is 100phr). The tear strength of the vulcanizates in-creases by about 139% when the nano-CaCO3

content increases from 0 to 100 phr.

Stress–Strain Behavior

It is well known that the stress–strain curves ofthe filled vulcanizates are affected by the cross-link density of rubber matrix,18,19 the size ofagglomerates formed by filler,20,21 and matrix–filler interaction.18,22 These effects can be con-trolled by adjusting the curing agent contentand filler particle size and by introducing a sur-face modifier.

Figure 9 shows the stress–strain curves fornano-CaCO3-filled EPDM vulcanizates. The ini-tial slopes of the curves for filled vulcanizatesincrease with the incorporation of MAA, indicat-ing the presence of MAA increases the initialmodulus of nano-CaCO3-filled vulcanizates. It isnot similar to the results of common couplingagent.23 When the coupling agents were intro-duced to particle surface, a hydrophilic charac-

teristic of the particle was changed into a hydro-phobic one, which reduced the filler–filler inter-actions via reduced agglomerate size formed byfiller particles, leading to the decrease of tensilemodulus, which is known as Payne effects.20,24

Unlike common coupling agents, MAA has twofunctional groups in each molecule. The carboxylgroup can react with the functional groups onthe surface of CaCO3 particle, and the carbon–carbon double bond can participate in the cross-link reaction and can be grafted to the polymercrosslink network. As a result, a strong interfa-cial interaction between nano-CaCO3 particleand the matrix is achieved by using MAA ascoupling agent, resulting in the higher modulusof vulcanizates with the presence of MAA. More-over, it is an interesting phenomenon that aninflection point is observed in the stress–straincurve of vulcanizates containing MAA, which isseen more clearly in Figure 10.

In filled vulcanizates without MAA, there areonly covalent crosslinks. With the extension pro-gress of tensile samples, more covalent cross-links are subjected to the stress, resulting in amonotonous increase of dr/de value. Accordingto the structural model of unsaturated carboxy-late-reinforced vulcanizates, the schematic pre-sentation of possible structure in filled vulcani-zates with MAA is shown in Figure 11. Besidescovalent crosslink, another kind of crosslinkstructure probably exists in MAA in situ modi-fied nano-CaCO3-filled vulcanizates. Unlike thestructure in unsaturated carboxylate-reinforcedvulcanizates, the ionic bond is formed only onfiller surface, because MAA is absolutely defi-cient compared with nano-CaCO3.

Figure 9. Stress–strain curves of EPDM vulcani-zates with incorporating MAA (EPDM/CaCO3/MAA/DCP ¼ 100/60/1.2/1).

Figure 10. The first derivative of stress–strain cur-ves of EPDM vulcanizates filled with 60 phr CaCO3

with incorporating MAA and gum vulcanizates.

1234 ZHOU ET AL.


As shown in Figure 11(b), the filler particle canbe bonded on the crosslink network via one orseveral ionic bonds. When three or more MAAmolecules are bonded to a same filler particlethrough ionic bond, this filler particle can be con-sidered as a crosslink point, which is completelydifferent from the covalent crosslink, and evendifferent from the ionic crosslink in unsaturatedcarboxylate-reinforced vulcanizates. For unsatu-rated carboxylate-reinforced vulcanizates, theionic bond can be re-formed after being destroyedin tensile process, which might account for theloss of modulus. As a result, no similar inflectionpoint was observed in the stress–strain curve ofunsaturated carboxylate-reinforced vulcanizates.For filled vulcanizates with MAA, the decrease ofdr/de at certain elongation is thought to be associ-ated with the destroying of filler crosslink, whichcan not be re-formed like ionic bond in unsatu-rated carboxylate-reinforced vulcanizates. The fol-lowing increase of dr/de value corresponds to theeffect of covalent crosslink on modulus.

The effect of nano-CaCO3 content on (dr/de)–ecurves of filled vulcanizates with MAA is shownin Figure 12. It can be seen that peak position isshifted to low strain region with the increase in

Figure 11. Schematic presentation of possible structures in filled vulcanizates with MAA.

Figure 12. Effect of nano-CaCO3 content on (dr/de)-e curves of filled vulcanizates with MAA.



filler content, and this shift can be seen moreclearly in Figure 13. With increasing filler con-tent, the proportion of EPDM matrix in vulcani-zates decreases rapidly, leading to a relativelyhigher strain being applied to the EPDM matrix,because the filler is usually thought to be rigidand undeformable in a tensile process. It meansthe vulcanizates with higher filler content willreach the critical point of strain at lower macro-scopical strain, and then the filler crosslinkbegins to be destroyed. The test results in Figure13 fairly conform to the above analysis.

CONCLUSIONS

The incorporation of MAA improved the interfa-cial interaction between nano-CaCO3 particlesand EPDM matrix, so as to enhance the me-chanical properties of filled vulcanizates. Theformation of carboxylate and the participation ofMAA in the crosslink of EPDM indicated thestrong filler–matrix interaction from the aspectof chemical reaction. Kraus equation proved thatMAA enhanced the reinforcement extent ofnano-CaCO3 on EPDM vulcanizates. Crosslinkdensity determination proved the formation ofthe ionic crosslinks in EPDM vulcanizates withMAA. The tensile failure occurred at the matrix,indicating that the strong adhesion between thefiller and the matrix was achieved by incorpo-rating MAA. Furthermore, the presence of MAAremarkably increased the modulus and othermechanical properties. As a result, EPDM vul-canizates exhibited high strength, high modu-lus, and high elongation at break. The tensile

strength of filled vulcanizates with MAA hadthe maximum value of 25.6 MPa at 60 phr fillercontent, which was 51% higher than that of filledvulcanizates without MAA. Furthermore, the ionicbond was thought to be formed only on filler sur-face because of the absolute deficiency of MAA,which resulted in the possible structure where fil-ler particles were considered as crosslink points.

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Figure 13. Effect of nano-CaCO3 content on peake value.

1236 ZHOU ET AL.


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