Polym. Bull. (2015) 72:1163–1175 DOI 10.1007/s00289-015-1330-7 ORIGINAL PAPER

Effects of ZnO content on microstructure and properties of maleated EPDM/zinc oxide composites Yeowool Kim • Sung-Seen Choi • Jong Woo Bae Jung-Soo Kim

•

Received: 3 October 2014 / Revised: 1 January 2015 / Accepted: 4 February 2015 / Published online: 10 February 2015 Ó Springer-Verlag Berlin Heidelberg 2015

Abstract Properties of maleic anhydride-grafted ethylene–propylene–diene terpolymer/zinc oxide (MAH-g-EPDM/ZnO) composites with varying ZnO contents were characterized by the crosslink density, average grain size of zinc oxide, and ionomeric bond formation. The crosslink densities were measured by swelling method, the average grain sizes of ZnO were determined by X-ray diffraction (XRD), and the ionomeric bond formation was observed by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). By increasing the ZnO content, the crosslink density increased, the average ZnO grain size increased, and level of the ionomeric bond formation tended to increase. The modulus increased and then reached to a plateau, whereas the elongation at break decreased and then reached to a plateau with increase in the ZnO content. From the experimental results, the proper ZnO content was found to be about 5 phr. Various crosslink types formed in the MAH-g-EPDM/ZnO composite were discussed. Keywords MAH-g-EPDM/ZnO composite  Characterization  Ionomeric bond  Crosslink density  Grain size

Introduction Ethylene–propylene–diene terpolymer (EPDM) is attempted to change its characteristics by grafting functional group [1–6]. The maleic anhydride (MAH) Y. Kim  S.-S. Choi (&) Department of Chemistry, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul 143-747, Korea e-mail: [email protected] J. W. Bae  J.-S. Kim Korea Institute of Footwear and Leathers Technology, Danggam-dong, Jin-Gu, Busan 614-100, Korea

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modification of different kinds of polymer is used to improve compatibility of immiscible polymer blends [7–12]. Maleic anhydride-grafted EPDM (MAH-gEPDM) increases impact strength, bonding strength with filler, and compatibility of blending polar/nonpolar resins [12–14]. MAH-g-EPDM is used as compatibilizer and functional material of changing characteristic of polymer by blending [15–20]. An ionomer is an ion-containing copolymer crosslinked by metal ions [20, 21]. Ionomer has a character of thermoplastic elastomer (TPE) and improves the mechanical properties [22]. In general, metal cations such as Na?, Zn2? and Ca2? are used, while CO2- and SO3- are used as anions to form an ionomer [23–26]. Studies of formation of zinc ionomer of carboxylated nitrile rubber (XNBR) and poly(butadiene/methacrylic acid) were reported [27, 28]. In our previous works [29– 33], properties of MAH-g-EPDM composites were investigated. Dichloroacetic acid makes new pseudo-crosslink points of raw MAH-g-EPDM [29]. Solvent effect on swelling ratio (Q) of MAH-g-EPDM composites mixed with ZnO, stearic acid, and/ or zinc stearate was reported using various swelling solvents, and relation between the relative apparent crosslink density (1/Q) and physical property was examined [30]. Influence of amino acids (AAs) on properties of MAH-g-EPDM composites mixed with zinc oxide and various AAs was investigated, and it was reported that 4-aminosalicylic acid improved the properties of MAH-g-EPDM composites [31]. Various analytical methods such as nuclear magnetic resonance spectroscopy (NMR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used for characterization of MAH-g-EPDM composites [31, 32]. Zinc ionomers in MAH-g-EPDM/ZnO composites can be formed between succinic acids of MAH-g-EPDM and zinc oxide particle surface as well as free zinc ion. Change of zinc oxide particle size in an MAH-g-EPDM/ZnO composite will provide an important information to characterize the zinc ionomers. It is very important to determine the proper zinc oxide content for design of an MAH-gEPDM/ZnO composite. However, measurement of zinc oxide particle size in an MAH-g-EPDM/ZnO composite has not been reported. One of simple methods to measure the zinc oxide particle size may be using a particle size analyzer after extraction of zinc oxide by burning the sample. However, the shape and size of zinc oxide should be changed by burning an MAH-g-EPDM/ZnO composite. In the present work, XRD and Scherrer’s formula [34] were employed for measurement of average grain sizes of the zinc oxide particles in MAH-g-EPDM/ZnO composites without extraction process. In this study, MAH-g-EPDM/ZnO composites with different zinc oxide contents were prepared, and the crosslink formation was characterized using attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), XRD, and measurements of crosslink density. Degree of the ionomeric bond formation was estimated using ATR-FTIR. The crosslink densities were measured using swelling method, and toluene was employed as the swelling solvent. The physical properties such as modulus, elongation at break, tensile strength were also measured. Variations of the crosslink density, ZnO average grain size, and ionomeric bond formation with the ZnO content were compared. From the experimental results, possible crosslink types formed in the MAH-g-EPDM/ZnO composite were

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suggested. The estimation method to determine the proper zinc oxide content in the MAH-g-EPDM/ZnO composite was recommended.

Experimental Royaltuf 498 (MAH functionality 1.0 %) of Chemtura Co. (USA) was employed as an MAH-g-EPDM. Songnox 1076 (octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate) of Songwon Industrial Co. (Korea) was used as an antioxidant. Zinc oxide of PJ Chemtec Co. (Korea) was used. Tetrahydrofuran (THF), toluene, and n-hexane were purchased from Daejung Chemicals & Metals Co. (Korea). The MAH-g-EPDM/ZnO composites were composed of Royaltuf 498, Songnox 1076 (0.3 phr), and ZnO (1, 2, 3, 4, 5, 8, 10, and 12 phr). The components were mixed in a mixing chamber (volume 60 mL) of a Haake Rheocord equipped with a roller rotor. The compounds were prepared by mixing at 160 °C for 15 min. The rotor speed was 50 rpm. The sample sheets were prepared by pressing at 200 °C for 10 min in a compression mold (10 MPa, 2 mm thickness). Grain sizes of zinc oxide in the samples were analyzed by XRD (D-max 2500/PC, Rigaku Co.). The accelerating voltage and electric current were 40 kV ˚ was used. The and 100 mA, respectively. Cu-Ka line wavelength of 1.54056 A samples were scanned from 5° to 85° with a scan speed of 2° min-1. Ionomeric bond formation was characterized using ATR-FTIR (PerkinElmer spectrum100, resolution 0.4 cm-1). Zinc selenide (ZnSe) crystal was used. Crosslink densities of the samples were measured by the swelling method. Swelling measurement procedure is as follows: Samples were cut with the dimension of about 1 9 1 cm2. The samples were soaked in THF and n-hexane for 3 and 2 days at room temperature, respectively, to remove free small organic materials remained in the samples. The samples were dried for 2 days at room temperature and measured the weight (Wd). The free organic material-extracted samples were soaked in toluene for 2 days at room temperature and then the swollen samples were weighed (Ws). The volume fraction of rubber in the swollen sample (Vr) was calculated by the Eq. (1) [35] Vr ¼ ðWd  Wf Þqr =½ðWd  Wf Þqr þ ðWs  Wd Þqs ;

ð1Þ

where Wf is the filler weight in the sample, qr and qs are the rubber (0.87 g mL-1) and solvent (toluene, 0.865 g mL-1) densities, respectively. The elastically active network chain density (Ve) which was used to represent the whole crosslink density was calculated by Flory–Rehner Eq. (2) [36] i    .h  1=3 ð2Þ Ve mol cm3 ¼ lnð1  Vr Þ þ Vr þ vVr2 Vs Vr  Vr =2 ; where Vr is the volume fraction of the polymer in the sample swollen to equilibrium, Vs is the solvent molar volume (106.3 cm3 mol-1 for toluene) and v is the EPDM– toluene interaction parameter and is taken as v = 0.429 ? 0.218Vr calculated according to the literature [37].

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The tensile properties were measured according to ASTM D412 using an Instron universal testing machine (UTM, model 3345) at a crosshead speed of 500 mm min-1. The tear strength was measured according to ASTM 624-86 using an Instron UTM 3345.

Results and discussion Intermolecular interactions between the succinic acid groups of different EPDM chains in MAH-g-EPDM make pseudo-crosslinks by hydrogen bonding [29]. For the MAH-g-EPDM/ZnO composites, besides hydrogen bonds between the succinic acid groups, intermolecular interactions between the succinic acids and the ZnO particles can also make pseudo-crosslinks by formation of ionic bonds [25, 29, 33]. The MAH-g-EPDM/ZnO composites with different ZnO contents were analyzed by ATR-FTIR to examine degree of the ionomeric bond formation. Figure 1 shows the ATR-FTIR spectra. The peak at 1,775 cm-1 is a characteristic C=O symmetric stretching vibration of succinic anhydride and the absorption band at 1,715 cm-1 is corresponding to the C=O stretching vibration of succinic acid [38, 39]. The broad band at 1,520–1,660 cm-1 is assigned to the asymmetric carboxylate stretching region due to the formation of ionomer [40]. The peak at 1,520–1,660 cm-1 was not observed in raw MAH-g-EPDM [41]. Relative absorbance ratios of the principal peaks were calculated using the peak at 2,919 cm-1 (C–H sp3 symmetric stretching vibration) as the reference to examine the degree of ionomeric bond formation depending on the ZnO contents. Figure 2 shows variations of the absorbance ratios of peaks of succinic acid and ionomeric bond with the ZnO content. The absorbance

1660 - 1520 cm -1

MAH-g-EPDM (ZnO = 0 phr) 1775 cm-1

Transmittance (a.u.)

ZnO = 1 phr

1900

1715 cm-1

2 3 4 5 8 10 12

1800

1700

1600

1500

Wavenumber (cm-1) Fig. 1 ATR-FTIR spectra of the raw MAH-g-EPDM and MAH-g-EPDM/ZnO composites. The numbers marked in the spectra denote the ZnO contents (phr)

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Relative absorbance ratio

0.005

0.004

Succinic acid Ionomer bond

0.003

0.002

0.001

0

2

4

6

8

10

12

Content of ZnO (phr) Fig. 2 Variations of the relative absorbance ratios of the succinic acid (1,715 cm-1) and ionomeric bond (1,657–1,518 cm-1) peaks of the MAH-g-EPDM/ZnO composites with the ZnO content. The reference is the peak at 2,919 cm-1 peak (sp3 C–H stretching)

ratio of succinic acid peak (1,715 cm-1) decreased sharply until 5 phr and then decreased gently or did not change. On the contrary to this, the absorbance ratio of ionomeric bond peak (1,520–1,660 cm-1) increased notably until 5 phr and then increased slightly or did not change. As a result, it was found that the degree of ionomer formation increased with increasing the ZnO content until 5 phr. This indicates that net new ionic bonds are not formed after 5 phr ZnO and the maximum effective zinc oxide content is 5 phr. Since the ionomeric bonds between the succinic acids of MAH-g-EPDM and zinc ions of ZnO particle are formed, the ZnO particle size would be changed by formation of the ionomeric bonds. To examine change of the ZnO particle size, the MAH-g-EPMD/ZnO composites were analyzed by XRD. The XRD patterns of the MAH-g-EPDM/ZnO composites having the ZnO contents of 1, 4, 8, and 12 phr are shown in Fig. 3. The peaks marked with R1–R3 and Z1–Z6 are related to the MAHg-EPDM and ZnO particle, respectively. The peak around 19° (R1) was corresponding to the MAH-g-EPDM and the peaks at 21.5° (R2) and 23.8° (R3) were corresponding to the polyethylene unit [42, 43]. The six main peaks Z1–Z6 related to ZnO particle appeared at 31.7° (100), 34.5° (002), 36.3° (101), 47.5° (102), 56.6° (110), and 62.9° (103) of hexagonal phase zinc oxide crystal. XRD pattern of pure ZnO used in this study was reported in a previous work [33]. In an XRD pattern, the full width at half maximum (FWHM) of a peak is used for calculating a grain size (d) by Scherrer’s formula [34] of the Eq. (3) d ¼ 0:9k=Bcosh

ð3Þ

˚ used in this study), h is the Bragg where k is the X-ray wavelength (1.54056 A diffraction angle, and B is the FWHM. The grain sizes of ZnO particles were obtained from the ZnO principal peaks (Z1–Z6) by Scherrer’s formula. The

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Intensity (cps)

Z3

Z1

10000

R2 R1 ZnO =1 phr

5000

Z5

Z2

4 phr 8 phr

R3

Z4

Z6

12 phr

0 10

20

30

40

50

60

70

80

2-Theta (degree) Fig. 3 XRD patterns of the MAH-g-EPDM/ZnO composites containing 1, 4, 8, and 12 phr ZnO

Table 1 Grain sizes (d) of zinc oxide particles in the MAH-gEPDM/ZnO (8 phr) composite by Scherrer’s formula

Peak

2-Theta

Miller index (hkl)

FWHM

d (nm)

Z1

31.76

100

0.242

59.57

Z2

34.42

002

0.224

64.80

Z3

36.26

101

0.256

56.99

Z4

47.52

102

0.277

54.69

Z5

56.56

110

0.313

50.30

Z6

62.84

103

0.296

54.89

Average

–

–

–

56.87

FWHMs and grain sizes of ZnO peaks of the MAH-g-EPDM/ZnO (8 phr) composite were representatively listed in Table 1. The average grain sizes of ZnO particles in the MAH-g-EPDM/ZnO composite were obtained by averaging the six grain sizes, and were plotted as a function of the ZnO content as shown in Fig. 4. The average grain size increased with increase in the ZnO content. Average grain size of the original ZnO particle is 87 nm. The ZnO particle sizes could be reduced by mechanical breaking during mixing and by etching the ZnO particle surface by succinic acids. ZnO surface can be etched by acid [44]. For accelerated sulfur curing system of rubber, it is well known that ZnO reacts with stearic acid to form zinc stearate [45, 46]. Hence, ZnO reacts with carboxylic acid of succinic acid to form zinc complex not to form free zinc cation as below 2þ ZnO þ 2RCO2 H ! RCO    O2 CR þ H2 O: 2    Zn

As the succinic acid reacts with zinc oxide to form the ionomer, the zinc oxide particle surface is chemically etched to reduce the size. By increasing the ZnO content, the content of succinic acid groups in MAH-g-EPDM was reduced, whereas the ionomeric bonds between the succinic acids and zinc ions were increased as

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Average ZnO grain size (nm)

60

57

54

51

48

0

2

4

6

8

10

12

Content of ZnO (phr) Fig. 4 Variation of the average grain size of ZnO in the MAH-g-EPDM/ZnO composite with the ZnO content

Crosslink density (mol/cm3)

2.1x10-5

1.8x10-5

1.5x10-5

1.2x10-5

9.0x10-6

0

5

10

Content of ZnO (phr)

Fig. 5 Variation of the crosslink density of the MAH-g-EPDM/ZnO composite with the ZnO content

shown in Fig. 2. Therefore, degree of the ionomeric bonds could be also evaluated using measurement of the average grain size. The linear curve fitting equation of Fig. 4 was y = 0.856x ? 49.6 (r = 0.911). The experimental results can lead to a conclusion that the average grain size is reduced by about 0.86 nm as the ZnO content decreases by 1.0 phr. Crosslink densities determine properties of polymeric materials crosslinked [47]. By increasing the crosslink density, the modulus, hardness, and resilience increase, whereas the elongation at break and stress relaxation decrease. By increasing the crosslink length, the stress relaxation, resilience, and tensile strength increase. Crosslink densities of the MAH-g-EPDM/ZnO composites were measured using swelling method. Figure 5 shows variation of the crosslink density with the ZnO

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content. The crosslink density notably increased until 5 phr and then slightly increased. This trend is similar to the variation of ionomer formation. For the MAHg-EPDM without ZnO, crosslinks can be formed only by intermolecular hydrogen bonds (H-bonds) between succinic acids of different EPDM chains as shown in Scheme 1a. If intramolecular H-bonds of succinic acid groups in one EPDM chain are formed, crosslinks cannot be formed. By mixing MAH-g-EPDM with ZnO, the intramolecular H-bonds would be opened and ionic bonds between the zinc ions and succinic acids could be formed. This tends to increase the crosslink density. The ionic bonds can be formed with the ZnO particle surface as well as with the free zinc ion (Zn2?) as shown in Scheme 1b, c. The ionic bonds formed by the free zinc ion

CO2 H

CO2 H

CO2 H

intramolecular H-bonds HO2 C

CO2H

HO2 C

CO 2H

HO2C

CO 2H

CO2 H

intermolecular H-bonds

CO2 H

CO 2H

intramolecular H-bonds

(a)

CO 2H O

O

+

CO 2H O

O

CO 2H

ZnO

O

+

Zn2+

O

+

O O

O

O

HO2 C

HO 2C

(b)

(c)

Scheme 1 Plausible non-covalent crosslinks formed in an MAH-g-EPDM/ZnO composite. a Crosslinks formed between succinic acids, b crosslinks formed between free zinc ion (Zn2?) and succinic acids, and c crosslinks formed on ZnO particle surface with succinic acids

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and zinc oxide particle will be increased by increasing the ZnO content. As the ZnO content increases, the ionic bonds of ZnO particle will be major crosslinks. Basic physical properties (modulus, elongation at break, and tensile strength) of the MAH-g-EPDM/ZnO composites were measured and plotted as a function of the ZnO content as shown in Figs. 6 and 7. The 100 and 300 % moduli increased until about 4 phr of the ZnO content and then did not increase, whereas the tensile strength continuously increased with increase in the ZnO content. The elongation at break decreased until about 4 phr and then did not nearly change. By increasing the ZnO content, the crosslink density increased (Fig. 5) whereas changes of the moduli increment and the elongation decrement occurred until about 4 phr of ZnO content. This could be explained by ionic bond formation on the ZnO particle surface. As described previously, the modulus increases but the elongation at break decreases by increasing the crosslink density, while the tensile strength increases by increasing the crosslink length. Crosslinks by ionic bonds formed on the ZnO particle surface include ZnO particle. Hence, crosslink lengths of ionic bonds formed on the ZnO particle surface are longer than those of ionic bonds formed with Zn2?. Due to longer crosslinks, the tensile strength continuously increased by increasing the crosslink density, whereas the moduli did not continuously increase and the elongation at break did not continuously decrease. Variation tendency of the moduli is similar to that of the IR absorbance ratio of ionomeric bond, while that of the elongation at break is similar to that of the IR absorbance ratio of succinic acid. Hence, the results say that variations of the physical properties are related to formation of the ionic bonds. Figure 8 shows variation of the reinforcement index (RI = 300 % modulus/100 % modulus) of the MAH-g-EPDM/ZnO composites with the ZnO content. The RI value steeply increased until 4 phr and then slightly increased. This trend is similar to variation of

Physical property (Mpa)

6

4

2 Tensile strength 300% modulus 100% modulus

0

2

4

6

8

10

12

Content of ZnO (phr) Fig. 6 Variations of the physical properties of the MAH-g-EPDM/ZnO composite with the ZnO content. Squares, circles, and triangles indicate the tensile strength, 300 % modulus, and 100 % modulus, respectively

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Elongation at break (%)

1000

800

600

400

0

2

4

6

8

10

12

Content of ZnO (phr)

Fig. 7 Variation of the elongation at break of the MAH-g-EPDM/ZnO composite with the ZnO content

1.9

Reinforcement index

1.8 1.7 1.6 1.5 1.4 1.3 1.2 0

2

4

6

8

10

12

Content of ZnO (phr) Fig. 8 Variation of the reinforcement index (300 % modulus/100 % modulus) of the MAH-g-EPDM/ ZnO composite with the ZnO content

the IR absorbance ratio of ionomeric bond. The RI variation can lead to a conclusion that the reinforcing effect by ZnO increases until about 4 phr and then the additional effect does not appear after 4 phr.

Conclusion MAH-g-EPDM/ZnO composites with varying ZnO contents were prepared. Succinic acid of MAH-g-EPDM reacted with ZnO to form crosslinks (ionomeric

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bonds). The formation of ionomeric bond increased with increase in the ZnO content. Average grain size of zinc oxide particles in the MAH-g-EPDM/ZnO composite increased with increasing the ZnO content. The crosslink density also increased as the ZnO content increased. The notable increments appeared until about 5 phr of the ZnO content and the maximum effective zinc oxide content was about 5 phr. Variation tendency of the moduli is similar to that of the IR absorbance ratio of ionomeric bond, while that of the elongation at break is similar to that of the IR absorbance ratio of succinic acid. Variation of the physical properties of MAH-gEPDM/ZnO composites with the crosslink density showed saturation point at 4–5 phr of ZnO content. The MAH-g-EPDM/ZnO composite could have various crosslink types such as intermolecular hydrogen bonds between the succinic acids, ionic bonds between the succinic acid and Zn2?, and ionic bonds of the succinic acid on the ZnO particle surface. Acknowledgments This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea.

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