International Journal of Mechanical Engineering and Technology (IJMET)




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EFFECT OF ZINC OXIDE LOADING LEVELS ON THE CURE CHARACTERISTICS, MECHANICAL AND AGING PROPERTIES OF THE EPDM RUBBER
Salih Abbas Habeeb 1, Zoalfokkar Kareem Alobad 2, Muayad Abdulhasan Albozahid 3
1, 2 Department of Polymers Engineering and Petrochemical Industries, Faculty of Materials Engineering, University of Babylon, Hilla, Iraq
3 Department of Materials Engineering, Faculty of Engineering, University of Kufa, Najaf, Iraq
ABSTRACT
This paper attempts to show that, the ethylene-propylene-diene monomer (EPDM) activated by adding different loading levels of zinc oxide (ZnO). A two-roll mill used to mix and distribute the elements of the rubber compound. MV-ODR-PROPERTIES rheometer employed to determine the properties of the rubber compound at 180 °C for 6 min . The curing process carried out using the hot press at a temperature of 180°C and pressure of 10 bars. Results show that the addition of the (2,4,6,8,10) Part per hundred parts of rubber (phr) ZnO into the EPDM elastomer guides to reduce the scorch time ,curing time with increasing the Max. Torque and cure rate index (CRI), particularly at 2 and 4 phr. Furthermore, an improvement in the tensile strength and modulus of elasticity at 300% elongation, hardness and compression set seen. 2 and 4 phr represent the best loading level of the ZnO, which had more effect on the aging and mechanical properties. In addition, the rubber compound becomes more stiffness, especially when the retention percentage of the properties after thermal aging is low.
Key Words: EPDM, ZnO , Mechanical Properties, Cure Characteristics, Aging Properties
Cite this Article: Salih Abbas Habeeb, Zoalfokkar Kareem Alobad, Muayad Abdulhasan Albozahid , International Journal of Mechanical Engineering and Technology

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1. INTRODUCTION
Ethylene-Propylene-Diene Monomer (EPDM) is one of the rapid growing elastomers in recent years. It can be attributed to this monomer has a good electrical properties and high resistance to thermal aging [1-3]. It has great effect on sound absorption and noise [4] However, it requires some activators such as zinc oxide (ZnO) to improve its mechanical and thermal properties, and curing characteristics, especially when the sulfur is used in the vulcanization process of rubber[5,6]. It has high resistance against oxygen, UV lights, chemicals, and ozone [7, 8]
It has high cross-links density when it is vulcanized using sulfur at a high-temperature. Activators that include ZnO, are used to reduce the curing time and increase the curing rate by combining ZnO with the stearic acid in the rubber to form (soap-zinc stearate) [9,10]. ZnO is an organic material and widely used in the production of natural and industrial rubber compounds, and the best loading levels of the ZnO in the rubber compounds are between 2 and 5 phr [10]. ZnO gives a stable state of curing by activating the sulphur vulcanization, increases the torque, and reduces the cure time and scorch time [11, 12]. ZnO surface acts as a reactive substance and a catalyst reaction guide by stimulating and connecting the reactants, sulfur, accelerators and the fatty acids that are diffusing easily within the matrix, and absorbed on the surface of the ZnO to form a complicated compound. During this case, the location of the S-S bond is a change, and there is possibility to break the bond. Nevertheless, the range of the generated cross-links improved, additionally to the vital role that plays the surface area of the ZnO to increase the cross-links [12, 13]. The addition of the ZnO into EPDM rubber enhances the mechanical properties such as the tensile strength and modulus of elasticity, and increases crystalline properties of composite rubber [14]. Mu et al, 2007 reported that the mechanical properties like tensile strength, hardness increased with increasing the ZnO content until 12 phr. to reinforce the silicone rubber [15]. The aim of the paper is to provide a conceptual theoretical framework based on the influence of the ZnO loading on the curing, thermal and mechanical properties of the EPDM rubber.
2.1. Materials
EPDM rubber (Keaton, 4802) was purchased from DSM Elastomers B.V., European nation, includes 52%wt. of ethylene parts and 4.3%wt. of ENB; it is a relatively slim relative molecular mass allocation and a standard "Mooney viscosity, mL (1+4) at 125°C". "Carbon black N326 with surface area seventy-six m2/g, average particle size 45µm obtained from (MAKROchem S.A.)". "ZnO with surface area six m2/g , pureness of zinc oxide ninety nine Min, , SP. Gravity 5.6 with a particle size 300-1000µm obtained from Merck B.V. stearic acid with SP. Gravity 0.85, Ash at 550°C 0.1% Max. , Volatile matter at 65°C 0.5% Max", "Insoluble in sulfur with SP.Gravity1.57 obtained from J.T. Baker, sulfur content 80±2%, Ash at 550°C 0.2% Max, volatile matter at 80°C 0.5% max obtained from J.T. Baker", " 2-Mercaptobenzothiazole obtained from (Perkacit MBT) and "Tetramethylthiuram-disulphide (Perkacit TMTD)" obtained from (Flexsys B.V.).
2.2. Rubber Combining
The EPDM-master batch is prepared by using a 6-inch dual-roll mill at ~ 50°C according to ASTM D 3182 with a uniform mix and lessen the impact of blending conditions. In the rubber mixing system, the different loading levels of the ZnO added to rubber compound using a dual-roll mill. The blends laminated to a thickness of roughly 2mm, which appropriated for the following production of check examples. The component of the ethylene-propylene-diene monomer with various percentage of ZnO illustrated in Table 1.

Table 1: Rubber compounds for different ZnO loading levels.
Ingredient Recipe, phr a
Rubber b
Stearic acid
Zinc oxide
Carbon black (N326)
TMTD
MBT
Sulfur 100
1.0
0,2,4,6,8,10
70
1.0
0.5
1.5
a Part per hundred parts of rubber, b EPDM

2.3. Curing Properties
The curing properties of the various components were tested at 160°C and 180°C with a "MV-ODR-PROPERTIES rheometer (Micro vision Enterprises-India)" according to ASTM D2705. The cure time (TC) and scorch time (TS) of the components specified. It found that the best cure time and scorch time were 90% of the cure time (TC90) and 2 min after the scorch time (TS2), respectively. The compounds were cured in the labs of polymers and petrochemical industries department using XLB-D 350 X 350 electrically heated press (Huzhou, East machinery- China) at 180°C with the optimum cure time TC90 under pressure of 10 bar .
2.4. Characterization
2.4.1. Tensile Test
The tensile inspection was carried out on dumb-bell samples according to the ASTM D 412 using " (Time group Inc –Chain) with model WDW-5E microcomputer controlled electronic universal testing machine". The dumb-bell samples with 2 mm thickness cut off from the molded layers using a Wallace die cutter. The speed test, that used, was 500 mm/min at 25± 3oC. and 5 specimens were tested for each case.
2.4.2. Thermal Aging
The thermal aging properties of the cured rubber were studied by aging for 4 days at 100°C according to ASTM D573 .
2.4.3. Hardness
Hardness of the specimens was tested using Hardness-meter Shore A tester" TH200 (Beijing time technology Ltd) according to ASTM D2240-02 test method. It was recorded 5 readings on different places at room temperature for each specimen.



2.4.4. Compression Set
The compressive set examination of the rubber samples must carried out according to the standard (ASTM D395-03 (2008). The sample have to be cylindrical, 20 mm diameter and 15 mm thickness. The steps of the test showed as following:
The sample placed between the plates of the compression equipment to reduce the thickness of 25% of the initial thickness.
The compressed sample was then set in an oven at 100°C for 24 hours.
The sample cooled for 30 minutes.
The thickness of the final sample measured to determine the compression set of the samples.
3. RESULTS AND DISCUSSION
3.1. Curing Properties
The rheological behavior of the uncured elastomer after addition of (0, 2, 4, 6, 8, 10) phr of the loaded ZnO to the EPDM Rubber is presented in Table 2. The results show that the curing time (TC90) of the vulcanized EPDM rubber at temperatures 160oC and 180oC decreases with increasing the ZnO content. The vulcanization properties of the rubber compound greatly improved at 180°C, especially the torque characteristic. The lowest cure time and scorch time with the highest cure rate index (CRI) are observed at 2 and 4 phr. The EPDM rubber exhibits longer TC90 than the EPDM rubber reinforced with the ZnO particles. This result will be linked with the fact that when the ZnO particles are added to rubber compounds, they enhance sulfur vulcanization performance and cure properties by combining ZnO and the stearic acid within the rubber matrix to make (soap-zinc stearate) [9,10] . On the other hand, the CRI helps in raising the speed of the cure reaction [16, 17], this relation is given in the following equation:
CRI = 100 / (TC90 –TS2) (1 (
Where the TC90 is 90% of the cure time and TS2 is 2 min after the scorch time. CRI increases with increasing the ZnO content. The best activation of the cure reaction is at 2 and 4 phr. In spite of the increasing the torque at higher loading, the curing time decreases slightly with increasing the ZnO loading until 4 phr. This can attributed to the high interference between the elastomer matrix and the ZnO surface, this confirms the tendency of the rubber compounds to create agglomerations at higher loading of ZnO resulting in decreasing the cross-links formation [12, 13].
Table 2 Rheometric properties of the EPDM rubber reinforced with ZnO at 160oC and 180oC for 6 min.
ZnO
(phr) CRI a( min -1)
160°C 180°C (TC90) b (min)
160°C 180°C (TS2) c (min)
160°C 180°C (Dmax) d (Ib-in)
160°C 180°C
0.0
2.0
4.0
6.0
8.0
10.0 28.5 33.4
34.88 56.81
32.7 52.08
30.46 46.94
30.46 43.47
30.93 44.85 5.5 4.5
3.95 2.53
4.2 2.78
4.62 3.13
4.60 3.32
4.58 3.18 2.0 1.50
1.25 0.77
1.33 0.86
1.55 1.00
1.52 1.02
1.52 0.90 80.00 85.000
100.16 100.99
103.33 104.34
111.54 118.21
114.27 117.99
116.27 120.15
a Cure rate index CRI = 100 / (TC90 –TS2), b Cure time, c Scorch time , d Max. Torque



3.2. Mechanical Properties
The effect of addition of (0, 2, 4, 6, 8, 10) phr of the ZnO on the tensile strength, modulus of elasticity at 300% elongation, elongation at the break, hardness shore-A and compression set are illustrated in the Figures 1-5. The mechanical properties show a direct correlation with the characteristics of curing, in particular, the properties of curing at 180oC. The tensile strength and modulus of elasticity at 300% elongation of the EPDM rubber reinforced with the ZnO particles increase with increasing the ZnO content compared with the mechanical properties of the EPDM rubber (in the absence of the ZnO in the EPDM rubber), especially at the low loading levels 2 and 4 phr (Figures 1 and 2). This confirms the important role that the low loading levels of the ZnO play in obtaining a homogenous mixture resulting from good dispersion in the rubber matrix. On the others hand, the torque of the rubber mixtures increased with increasing the znic oxide loading levels (6,8 and 10 phr) which lead to the heterogeneity of the rubber mixture, particle aggregations and reduction in the formation of cross-links at high znic oxide [14,15]. Furthermore, the elongation at break decreases with increasing the ZnO loading levels as compared to the EPDM rubber compound without ZnO (Figure 3).
Overall, these results indicate that the ZnO particles that play as a good activator in the rubber curing by sulfur and combination with the stearic acid to form good bonding with the rubber [10]. The improvement of the Cure rate index and interference between the ZnO particles and the elastomer matrix, lead to a slight improvement in hardness of rubber compounds with increasing the ZnO content (Figure 4). On the other hand, the results of the compression set presented in (Figure 5) which shows that low loading levels give the best results for a compression set. Because the compression set represents the ability of vulcanized rubber to maintain elasticity specifications after removing applied stresses for long times under certain conditions. Therefore, the weak performance of the rubber compound can attributed not form a sufficient cross-link, leading to relaxation during the examination of compression set [17].

Figure 1 Tensile strength of the EPDM rubber with different ZnO loading levels (phr).

Figure 2 Modulus of elasticity at 300% elongation of the EPDM rubber with different ZnO loading levels (phr).


Figure 3 Elongation at break (%) of the EPDM rubber with different ZnO loading levels (phr).

Figure 4 Hardness (Shore-A) of the EPDM rubber with different ZnO loading levels (phr).


Figure 5 The compression set ( % ) of the EPDM rubber with different ZnO loading levels (phr).
3. 3. Thermal Aging
The results in this part indicate that the influence of thermal aging at 100°C for 4 days on the mechanical properties of the EPDM rubber reinforced with ZnO particles (0, 2, 4, 6, 8 and 10 phr), which represented in Figures (1-5) . The results display that the rubber compound at 2 phr of ZnO presents higher tensile strength, hardness, and lower compressive set for both before and after aging. The thermal aging lead to enhancement the hardness, tensile strength and the reduction in the compression set for the rubber compounds having a low cure rate index (CRI) [18]. On the other hand, the tensile strength, modulus of elasticity at 300% elongation, elongation at the break, hardness shore-A and compression set of accelerated aging measured after one day of an aging check to estimate aging resistance of the EPDM rubber compounds as described in (Table 3). The percentage retention of the above properties calculated according to the following equation [17, 19].
Retention%=(Value once aging)/(Value before aging) x 100 (2)
In general, the retention percentage of the mechanical properties of the EPDM rubber after the aging is good, Therefore, the properties after again do not rely on the network structure or the character of the cross-links [20]. It can see in Table 3 that the percentage of retention after the aging of the tensile strength, the modulus of elasticity at 300% elongation and hardness are reduced at 2 and 4 phr and so raised with high amounts of ZnO. In contrast, the retention percentage of elongation at the break and the compression set promoted with increasing the amounts of ZnO. This may be due to the reduction in the elasticity of rubber chains after aging, giving more plasticity with the addition the ZnO particles [17]. Thus, the property that has the lowest percentage of retention after aging, it has a low aging resistance.





Table 3 Percentage of retention of the mechanical properties of EPDM rubber with different loading levels of ZnO after thermal aging at 100°C for 4 days.
ZnO (phr) (T.S) a (%)
(M)b ( % )
(E) c (%) (H)d (Shore-A) (C.S) e (%)

0.0
2.0
4.0
6.0
8.0
10.0 300
93
83
120
143
200 307
137
120
97
121
131 56
72
90
96
73
91 143
104
107
104
105
81 133
139
160
124
131
91
a Tensile Strength , b Modulus of elasticity at 300% elongation , c Elongation at break ,
d Hardness , e Compression set
4. CONCLUSION
The present study designed to determine the effect of the amount zinc oxide on the cure characterization; the mechanical and thermal properties of the EPDM rubber compound rely on the temperature of the curing. The loading levels of the ZnO such as 2 and 4 phr display a big effective on the cure characterization such as a high cure rate index, torque, low scorch and cure time. In accession, holding the most effective mechanical properties like high tensile strength, hardness with low compression set but low resistance for thermal aging.
REFERENCES
George K, Panda BP, Mohanty S, Nayak SK. Recent developments in elastomeric heat shielding materials for solid rocket motor casing application for future perspective. Polymers for Advanced Technologies. 2018, 29(1),pp. 8-21.
Lei Y, He J, Zhao Q, Liu T. A nitrile functionalized graphene filled ethylene propylene diene terpolymer rubber composites with improved heat resistance. Composites Part B: Engineering. 2018, 134, pp.81-90.
Stelescu M, Airinei A, Manaila E, Craciun G, Fifere N, Varganici C, Pamfil D, Doroftei F. Effects of Electron Beam Irradiation on the Mechanical, Thermal, and Surface Properties of Some EPDM/Butyl Rubber Composites. Polymers. 2018 ,10(11),pp. 1206.
Wang K, Yan X. Performance analysis of ethylene-propylene diene monomer sound-absorbing materials based on image processing recognition. EURASIP Journal on Image and Video Processing. 2018 , 2018(1), pp.128.
Akhlaghi S, Kalaee M, Mazinani S, Jowdar E, Nouri A, Sharif A, Sedaghat N. Effect of zinc oxide nanoparticles on isothermal cure kinetics, morphology and mechanical properties of EPDM rubber. Thermochimica Acta. 2012, 527,pp. 91-98.
Movahed SO, Ansarifar A, Estagy S. Review of the reclaiming of rubber waste and recent work on the recycling of ethylene–propylene–diene rubber waste. Rubber Chemistry and Technology. 2016 , 89(1),pp. 54-78.
Polgar LM, Criscitiello F, van Essen M, Araya-Hermosilla R, Migliore N, Lenti M, Raffa P, Picchioni F, Pucci A. Thermoreversibly cross-linked EPM rubber nanocomposites with carbon nanotubes. Nanomaterials. 2018 ,8(2),pp.58
Young J.W. In Situ Formation of Zinc Dimethacrylate for the Reinforcement of Ethylene-propylene-diene Rubber (EPDM), M. Sci. Thesis, University of Akron ,May. 2005.
Ismail H, Ishak S, Hamid ZA. Effect of blend ratio on cure characteristics, tensile properties, thermal and swelling properties of mica?filled (ethylene?propylene?diene monomer)/(recycled ethylene?propylene?diene monomer)(EPDM/r?EPDM) blends. Journal of Vinyl and Additive Technology. 2015 ,;21(1),pp. 1-6.
K.S. Sisanth, M.G. Thomas, J. Abraham, and S. Thomas, "General introduction to rubber compounding," In Progress in Rubber Nanocomposites, Woodhead Publishing ,2017, pp. 1-39.
Sahoo S, Bhowmick AK. Influence of ZnO nanoparticles on the cure characteristics and mechanical properties of carboxylated nitrile rubber. Journal of applied polymer science. 2007 , 106(5),pp.3077-3083.
. Heideman .G , Reduced zinc oxide levels in sulphur vulcanisation of rubber compounds-mechanistic aspects of the role of activators and multifunctional additives Ph.D. Eng. Thesis, University of Twente, Enschede, the Netherlands,. 2004.
Mottaghi M, Khorasani SN, Esfahany MN, Farzadfar A, Talakesh MM. Comparison of the effect of nano ZnO and conventional grade ZnO on the cross-linking densities of NR/BR and NR/SBR blends. Journal of Elastomers & Plastics. 2012; 44(5), pp. 443-51.
Xu C, Wu W, Zheng Z, Wang Z, Nie J. Design of shape-memory materials based on sea-island structured EPDM/PP TPVs via in-situ compatibilization of methacrylic acid and excess zinc oxide nanoparticles. Composites Science and Technology. 2018,167,pp. 431-439.
Mu Q, Feng S, Diao G. Thermal conductivity of silicone rubber filled with ZnO. Polymer composites. 2007;28(2),pp.125-30.
Sobhy MS, El-Nashar DE, Maziad NA. Cure characteristics and physicomechanical properties of calcium carbonate reinforcement rubber composites. Egypt. J. Sol. 2003,26(2),pp.241-257.
Ahmed K, Nizami SS, Raza NZ, Shirin K. Cure characteristics, mechanical and swelling properties of marble sludge filled EPDM modified chloroprene rubber blends. Advances in Materials Physics and Chemistry. 2012 , 2(02),pp.90-97.
Jayewardhana WG, Perera GM, Edirisinghe DG, Karunanayake L. Study on natural oils as alternative processing aids and activators in carbon black filled natural rubber. Journal of the National Science Foundation of Sri Lanka. 2009 ,37(3), DOI: 10.4038/jnsfsr.v37i3.1212
Prasertsri.S and Srichan.S,, Influence of Pyrolytic Carbon Black Prepared from Waste Tires on Mechanical Properties of Natural Rubber Vulcanizates , Key Eng. Mater.vol.751,pp.332-336, Trans Tech Publications, 2017,
[20] Deuri A.S. and Bhowmick A.K., Aging of EPDM rubber,Journal of Applied .
Polymer Science ,1987, 34, pp.2205-2222