Paper Title (Use Style: Paper Title) s71

S. PRASERTSRI

Physico-mechanical Properties and Automotive Fuel Resistance of EPDM/ENR Blends containing Hybrid Fillers

Sarawut Prasertsri

Laboratory of Advanced Polymer and Rubber Materials (APRM), Rubber Science and Technology Program, Department of Chemistry, Faculty of Science, Ubon Ratchathani University, Ubon Ratchathani, Thailand

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Abstract

This research aimed to investigate the effect of blend ratios on cure characteristics, mechanical and dynamic properties, morphology and automotive fuel resistance of ethylene-propylene diene rubber (EPDM) and epoxidized natural rubber (ENR) blends containing carbon black and calcium carbonate hybrid filler. The composition of EPDM/ENR blends varied were 100/0, 70/30, 50/50, 30/70 and 0/100 %wt/wt. All ingredients used for preparing each blended compound, except for the curatives, were mixed in a kneader. Thereafter, the compound was further mixed with curatives on a two-roll mill and then were vulcanized together with shaped by compression molding before determining cure characteristics, mechanical properties, morphology and weight swelling ratio in three automotive fuels; gasohol-91, diesel and engine oils. The results indicated that Mooney viscosity and cure time of EPDM/ENR blends tended to decrease with increasing ENR content, while cure rate index and crosslink density increased. Tensile strength of all EPDM/ENR blends is lower than that of the individual EPDM and ENR. This is attributed to the incompatibility between nonpolar and polar nature of EPDM and ENR, respectively, supporting by the glass transition temperature form dynamic mechanical thermal analysis (DMTA) and scanning electron micrographs (SEM). Owing to the differences in polarity of automotive fuels and rubbers, weight swelling of EPDM/ENR vulcanizates decreased in diesel and engine oils, but increased in gasohol-91 with increasing in ENR content.

Keywords: Rubber blends; Cure characteristics; Mechanical Property; Dynamic Property; Automotive Fuel Resistance

S. PRASERTSRI

1. Introduction

Elastomer blends are widely used in rubber industries for a variety of purposes, such as enhancement of mechanical, dynamic, thermal ageing properties as well as improvement of processing characteristics and reduction in product cost [1-4]. The properties of rubber blends are generally controlled by many factors such as blend ratios, phase morphology, blending conditions and distribution of compounding ingredients [5]. Although the properties of polymer blends obviously depend on the properties of each component, it is the fact that these complex systems exhibit a behavior which does not simply follow the sum of the properties of each component. The most important factor governing the ultimate properties such as strength or toughness along with the blend composition is intermolecular bonding force between phases. Sae-Oui et al. [6] revealed that improvement in oil, thermal and ozone resistance of silica-filled NR/chloroprene rubber (CR) blends can be obtained by increasing CR content. However, tensile strength of the blends does not follow the mixture rule, i.e., the tensile strength slightly reduces with increasing NR content up to 50/50 CR/NR ratio and further increase in NR content results in a sharp drop of tensile strength due to incompatibility between NR and CR. Blending of EPDM with NR imparts the blend with good heat and ozone resistance as well as chemical resistance. Botos showed that 25/75 and 50/50 NR/EPDM blends possess the best resistance against thermal ageing and UV irradiation, respectively [7]. George and co-workers studied the gas permeability of NR/SBR blends [8] and NR/carboxylated styrene-butadiene rubber (XSBR) blends [9]. They found a reduction in the nitrogen and oxygen permeability of the blends with increasing the synthetic rubber content.

EPDM is valuable for its excellent thermal aging and ozone resistance, while ENR possesses good tensile strength and oil resistance [2]. Hence, the key features of EPDM/ENR blends are oil and ozone resistance, low temperature flexibility with good mechanical properties and processing. The main objective of this research is to investigate the effect of blend ratio on cure characteristics, mechanical and dynamic properties, morphology and automotive fuel resistance of EPDM/ENR blends containing high loading of carbon black/calcium carbonate hybrid fillers. The selected automotive fuels used for weight swelling measurement were gashol-91, diesel and engine oil (heavy duty diesel type).

2. Experimental

2.1. Materials

Ethylene-propylene diene rubber (EPDM; Vistalon™ 3666) having ethylidene norbornene (ENB) of 4.5 %wt was supplied by ExxonMobil Chemical (Thailand) Ltd., Thailand. Epoxidized natural rubber (ENR, Epoxyprene-50) having epoxidation level of 50 %mole was provided from Muang Mai Guthrie Public Co., Ltd., Thailand. Other chemicals were procured from indigenous sources and were used as such. Stearic acid and zinc oxide (ZnO) were obtained from Chemmin Co. Ltd. Carbon black (N330) and calcium carbonate (Precipitated CaCO3) were supplied by Thai Carbon Product Co. Ltd., and P.S. Science Chemical Ltd., respectively. N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylene diamine (6-PPD) was brought from Imperial Industrial Chemicals (Thailand) Co.,Ltd. Sulfur was supplied by The Siam Chemical Public Ltd. and accelerators which were 2,2'-dithiobis(benzothiazole) (MBTS) zinc diethyldithiocarbamate (ZDEC) and tetramethylthiuram disulfide (TMTD) were purchased from Chemmin Co. Ltd. Three automotive fuels were gashol-91, diesel and engine oil which were supplied from a petroleum station of PTT Public Co., Ltd. The gasohol-91 is the 10% ethanol mixed with 90% gasoline in octane 91.

2.2. Mixing and Vulcanization Procedure

Five rubber compounds of different EPDM/ENR compositions containing carbon black/calcium carbonate hybrid filler were prepared. The blend ratios of EPDM/ENR were 100/0, 70/30, 50/50, 30/70 and 0/100 %wt/wt. The other rubber chemical used contain (in parts per hundred parts of rubber; phr): stearic acid, 2.0; ZnO, 6.0; carbon black, 40; calcium carbonate, 60; paraffinic oil, 5; 6-PPD, 2.0; TMTD, 0.8; ZDBC, 2.5; MBTS, 1.5; sulfur 2.0. All ingredients, except the curatives were mixed with a kneader at a set temperature of 50oC with a fill factor of 0.75. The EPDM and ENR were firstly blended before adding steric acid, ZnO, fillers and processing oil, 6-PPD, respectively. The total mixing time in the kneader was 13 min. Thereafter, the blended masterbatches having different EPDM/ENR ratios were mixed with curatives on a two-roll mill for 5 min. Then, the cure characteristics in term of scorch time (ts2), cure time (tc100), cure rate index (CRI) and torque difference (MH-ML) were measured using oscillating disc rheometer (ODR) at 160°C in accordance with ASTM D 2084. The vulcanized specimens were prepared by using hydraulic hot press according to their respective cure time from ODR results.

2.3 Testing

The dynamic mechanical properties were determined using a dynamic mechanical thermal analyzer; DMTA (Gabo, Explexor TM 25 N). The specimens were evaluated in tension mode at a constant frequency of 10 Hz and a dynamic strain of 0.1%. The temperature was raised from -80 to 80oC with a heating rate of 2oC/min under liquid nitrogen flow. Storage modulus (E') and loss tangent (tan δ) of each sample were recorded as a function of temperature. For the scanning electron microscopy (SEM) measurement, the rupture specimens after tensile testing were coated with gold and then the morphological images was performed at an accelerating voltage of 20 kV. The Shore A durometer was used for the hardness test according to ASTM D2240. Tensile properties were determined on a Universal extensometer with 1000 N of load cell and the testing speed of 500 mm/min. For the fuel resistance analysis, the vulcanized samples with initial weight of about 0.5 g were immersed in 70 ml of the selected automotive fuel at room temperature for 7 days. The percentage of swelling was calculated from Equation (1).

Swelling (%) = [(Ws - Wu)/Wu] x 100 (1)

Where Wu and Ws represent the weight of unswollen and swollen rubber, respectively.

3. Results and Discussion

The dependence of Mooney viscosity on EPDM/ENR blend ratio is presented in Figure 1. As can be seen, the viscosity of the blends sharply decreases when ENR content is increased up to 50%wt and then it is nearly constant. This is attributed to the dilution effect from the substitution of the high viscosity of EPDM portion by the low viscosity ENR portion. Moreover, the higher break down of 50% mole of polyisoprene in ENR chains during mixing process could be expressed and lead to a decrease in molecular weight compared to the saturated EPDM chains [7]. Figure 2 displays the cure characteristics of EPDM/ENR blends including scorch time, cure time and cure rate index (CRI). It is found that scorch time does not significantly change by increasing ENR content, while cure time of the blends progressively decreases. The shorter cure time and higher CRI occurred in ENR rich blend are arisen from two main factors; (i) EPDM has fewer double bonds leading to slower cure than ENR and (ii) ENR contains epoxy rings in the molecule which could result in acid-catalyzed epoxy ring opening reaction by sulfoxide and sulfenic acid via oxidative sulfur crosslinking reaction [10]. It is reported earlier that the torque difference is directly propotiontional to the crosslink density [6]. As can be seen in Figure 3, the increasing of torque difference value of EPDM/ENR blends is the effect on increasing of crosslink density which results form more content of ENR which can catalyst the crosslink reaction. This means that the higher degree of crosslinking is found in ENR-rich blends.

Fig. 1 Mooney viscosity of EPDM/ENR compounds at different blend ratios

Fig. 2 Cure characteristics EPDM/ENR compounds at different blend ratios

Fig. 3 Torque difference of EPDM/ENR compounds at different blend ratios

The dynamic mechanical analysis was used to examine the compatibility of rubber blends and also measure the glass transition temperature (Tg) of rubber. The effect of blend ratios on the storage modulus (E') and tan δ of the samples are illustrated in Figure 4. All curves in Figure 4(a) shows three regions: (i) a glassy high modulus region where the segment mobility is restricted, (ii) a transition region where a drastic drop in the E' value with increasing temperature and (iii) a rubbery region where a drastic decay in the modulus with temperature [11]. EPDM shows a high modulus below its Tg followed by a substantial reduce in its magnitude around -42ºC, whereas ENR shows the change in E' around 0ºC. It can be seen that the E' values of the blends are intermediate between those of pure components depending on blend ratios. The temperature dependence of tan δ for EPDM, ENR and their blends is depicted in Figure 4(b). From the tan δ curves, EPDM and ENR vulcanizates, respectively, show tan δmax at about -42.5ºC and 0.2ºC corresponding to their glass transition temperature (Tg). In addition, ENR shows maximum value of tan δmax indicating its excellent damping behavior. For all blend samples, there are two distinct tanδ peak of each phase indicating the incompatible blends. As can be seen, the coincident with the obvious increase in the ∆Tg of the individual rubber is believed to be caused by low cure compatibility and the immiscibility between EPDM and ENR phases in the blends. However, it is noteworthy that tan δmax of EPDM phase is found to drastically decrease with increasing the concentration of ENR. This is because the additional ENR phase is in glassy state at -42.5ºC which is Tg of EPDM, hence the rigid epoxide of ENR cloud restrict the segmental mobility of EPDM chains.

Fig. 4 (a) Storage modulus and (b) tanδ of EPDM/ENR vulcanizates at different blend ratios.

Figure 4 show the SEM micrographs of the tensile fractured surface of EPDM/ENR samples with different blend ratios. As can be seen in Figures 4(a) and (d), the fresh EPDM and ENR samples display the more homogenous surface morphology compared to their blends. This indicates that these rubbers had low miscibility with each other. Especially, at the blends containing 30 and 50 %wt of ENR, the dispersed particles are large with irregular shapes, moreover some voids are presented. It might be due to a lack of interfacial interaction between the filler-rubber and rubber-rubber. This result can be supported by the Tg shift from dynamic mechanical properties. However, more homogeneous phase dispersion appears when ENR component in the blend is in excess (see Figure 4(c)).

The mechanical properties, in term of hardness and tensile strength of EPDM/ENR blends are shown in Figures 6 and 7, respectively. For the effect of blend ratios, the hardness increases with increasing ENR content. Since ENR contains oxirane groups on its structure, higher stiffness would be more pronounced in ENR-rich blends. Also, an increase in crosslink density of the blends with increasing ENR content causes an increase of hardness. However, it could be notice that the strength of the blends in this study was not depended on degree of crosslinking. As can be seen in Figure 7, the tensile strength of EPDM vulcanizate is found to be 8.21 MPa, while ENR vulcanizate is 10.41 MPa. After blending, a negative deviation from rule of mixtures in terms of strength is achieved which is an indication of immiscible of EPDM/ENR blends as discussed previously. Hence, the strength of the blends might be enhanced by incorporating the third component to act a compatibilizer such as maleic anhydride grafted polybutadiene (PB-g-MAH) between EPDM and ENR in the further study.

S. PRASERTSRI

Fig. 5 SEM micrographs of tensile fracture surface of EPDM/ENR samples at different blend ratios.

S. PRASERTSRI