Introduction
In rubber applications, it is necessary to blend two polymer types in order to get the desired physico-mechanical properties and good swelling behavior that meet certain product specifications. Nitrile rubber (NBR) has excellent oil resistance but it is subjected to degradation at high temperatures. A more practical useful approach to improve NBR aging resistance is its blending with ethylene propylene diene monomer rubber (EPDM) which has good melt processability, thermal stability, and excellent ozone-resistant properties (Antunes et al.
2009). Therefore, EPDM/NBR blend vulcanizates have shown good mechanical properties with improved thermal stability as well as good oil resistance. Generally, EPDM and NBR are incompatible on the microstructure scale. Various compatibilizing agents have been examined to improve their compatibility (Mayasari and Setyadewi
2018; Soares et al.
2002; Mayasari et al.
2020). Maleic anhydride (MAH) has been frequently used as a compatibilizer for both types of rubber due to its high reactivity with thermoplastic elastomers upon mixing at room temperature. From our previous work, EPDM grafted with MAH, acrylic acid, and acrylic esters have successfully been used as compatibilizers for NBR/EPDM rubber blends (Botros and Tawfic
2006; Botros and Moustafa
2002,
2003; Botros et al.
2009). Recently, the physico-mechanical properties and swelling performance of EPDM/NBR rubber blends compatibilized with MAH have been improved significantly upon addition and increasing the content of halloysite nanotubes (HNTs) in the blends (Paran et al.
2023).
Nowadays, bio-based natural wastes could be regarded as green alternatives to replace conventional and relatively expensive fillers for improving the properties and lowering the cost of the rubber product since they meet the standards of environmental protection and sustainable development. Natural fibers derived from agricultural waste are finding their way into the polymer industry due to numerous benefits such as light weight, low cost, and environmental friendliness (Islam et al.
2015). Among the natural wastes that cause environmental pollution upon burning are rice husk fibers (RHFs). Due to their high cellulosic and silica content, RHFs have attracted the attention of researchers for their use as an alternative or supporting filler in the rubber industry (Khalaf and Ward
2010). In order to accomplish a desired combination of properties and cost reduction, the main task is the partial replacement of conventional reinforcing fillers (carbon black and silica), used in the rubber industry, with readily available and inexpensive agricultural waste materials like RHFs. Due to its high silica content (≈ 83%), rice husk ash (RHA) is considered a natural source for the production of silica. EPDM/NBR blends reinforced with various types of nanofiller have been investigated, including nanoclay (Ghassemieh
2009), organoclays (Ersali et al.
2012), and nanosilica (Jovanovic et al.
2013).
Rice husk ash (RHA) has also been successfully used as a new alternative filler for natural rubber (NR)/EPDM blends to improve the compression set and hardness properties of the rubber products (Rahmaniar
2019). Although RHA has poorer reinforcing ability than silica, it has provided better mechanical properties than CaCO
3. Therefore, RHA could be used as a nonreinforcing filler to replace CaCO
3 in rubber blends for economic and ecological reasons (Arayapranee and Rempel
2008). Improvements in the tensile properties and thermal stability of NR composites containing RHS have been observed. Further improvement was recorded by surface modification of RHS with bis(triethoxysilyl propyl) tetrasulfide and/or NaOH and liquid epoxidized natural rubber (LENR), which increased the interaction between RHS and the NR matrix (Salim et al.;
2019; Syafri et al.
2011). NR composites reinforced with marble sludge (MS)/RHS mixed filler have exhibited good mechanical and swelling properties at a low cost and could be used instead of general-purpose fillers like china clay, calcium carbonate, and talc (Ahmed et al.
2014). NBR reinforced with cement waste and RHS has shown improvement in modulus of elasticity with a decrease in elongation (Al-Mosawi et al.
2017). Additionally, NBR-RHA composite material with good thermal stability, low electrical conductivity, and high dielectric properties has been prepared. That composite could be used as a semiconducting material (Mohamed et al.
2009). Silica-rich red brick waste (RBW) powder/NBR eco-friendly composites have been used for insulation and antistatic applications (Shafik et al
2022). Silica extracted from RHFs has also been used as a combined filler with carbon black for NBR rubber (Bandara et al.
2020). Recently, EPDM composites reinforced with nano-size rice husk powder (nRHP) have possessed improved mechanical and dielectric properties and could be used in electrical insulation applications (Amin et al.
2018).
The aim of the present research is to utilize rice husk fibers (RHFs) and rice husk silica (RHS) agro-waste materials as supporting fillers in rubber blend composite vulcanizates, as well as evaluate their physico-mechanical properties and swelling behavior in automotive oils and solvents, for environmental protection against pollution resulting from the burning of agro-waste materials. For this purpose, EPDM/NBR blend formulations were designed using RHFs and RHS as supporting fillers for application in automotive rubber products (e.g., oil seals, gaskets, hoses, automobile spare parts, and O-rings). The compatibility of EPDM with NBR using various MAH loadings as a compatibilizer was investigated. The effects of different EPDM/NBR blend ratios, RHFs loadings, and filler types on the curing characteristics, physico-mechanical properties, as well as swelling performance in motor oil, brake fluid, toluene, and gasoline were also studied. Moreover, the influence of RHS as supporting filler for carbon black-reinforced EPDM/NBR blend vulcanizates was examined.
Experimental
Materials
Ethylene propylene diene monomer rubber (EPDM–Buna EPT 9650) of 53% ethylene and 6.5% ethylidene norbornene (ENB) contents, and 63 Mooney viscosity (ML 1 + 4125°C). It is a product of LANXESS Buna, Germany. Butadiene acrylonitrile rubber (NBR-Krynac 3345) of 33% acrylonitrile content and 45 Mooney viscosity (ML 1 + 4100°C) is a product of BAYER Chemical Company, Germany. The maleic anhydride (MAH) compatibilizer is from SISCO Research Laboratories PVT Ltd, India. Rice husk was collected from farms in Alexandria. High abrasion furnace (HAF) carbon black N330 (CB) was a product of Hangzhou Epsilon Chemical Co., Ltd., China. Kaolin of commercial grade was supplied from Transport and Engineering Co., Alexandria, Egypt. The curing of rubber composites was carried out using sulfur (S) and N-cyclohexyl-2-benzothiazole sulfenamide (CBS) curing system. The other compounding rubber ingredients are of commercial grades used in the rubber industry, such as zinc oxide (ZnO) and stearic acid activators, and polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ) antioxidant, which are supplied from Transport and Engineering Co., Alexandria, Egypt. The motor oil (Mobil Super 15W-50 XHP) is a product of ExxonMobil, and the brake fluid (Bendix DOT 3) is a product of Al-Manar Company under the license of Garret Transportation I Incorporation, USA. All other chemicals are of pure grade and were used as received.
Preparation of the rice husk raw material
The rice husk raw material was collected, washed several times with distilled water to remove its associated impurities, dried, and then subjected to mechanical grinding and sieved to the smallest mesh size of 300 (≈ 44 microns) to get the rice husk fibers (RHFs) in powder form.
Extraction and isolation of silica from the rice husk raw material:
The rice husk raw material was burned in a muffle at 600 °C for 3 h. Then, the resulting silica ash (SA) was treated with HCl to remove the insoluble inorganic salts, followed by refluxing with NaOH solution to dissolve the silica as sodium silicate. The filtrate was then neutralized with HCl to pH = 10.5 to form silica gel, and left for 72 h to complete precipitation and isolation of silica (El-Sakhawy et al.
2020; Ahmed et al.
2020).
Mixing, rheological properties and vulcanization of EPDM/NBR rubber blends
EPDM and NBR rubbers were mixed with other compounding ingredients and curatives onto an open two-roll mill of 170-mm diameter, 300-mm working distance, and 24 rpm speed of slow roll at a 1:1.25 gear ratio according to ASTM D3182-21a. The base recipe for all rubber formulations in this study contains: ZnO 5 phr, stearic acid 2 phr, antioxidant (TMQ) 1 phr, CBS 1 phr, and S 2 phr (parts per 100 parts of rubber). In the first stage, EPDM was masticated onto the two-roll mill for 2 min, then MAH and NBR were successively added to the EPDM, and mixing was continued for 3 min. ZnO, stearic acid, and TMQ antioxidant were subsequently added and mixed for 2 min. for each component. Then, the fillers under investigation were incorporated into the rubber blend and mixed for 5 min. Finally, the curative ingredients (CBS and S) were subsequently added to the rubber mixes and mixed for 2 min. for each component. Rheological characteristics of the rubber mixes were assessed by using moving die rheometer MDR–one rheometer (TA Instrument), at 152 ± 1
oC according to ASTM D2084-19a. The following parameters were determined: maximum torque (M
H), minimum torque (M
L), scorch time (ts
2), optimum cure time (tc
90), and cure rate index (CRI). Then, the rubber blend mixes were vulcanized at their optimum cure times (tc
90) by the aid of a hydraulic press in a clean polished mold (15 cm × 15 cm × 0.2 cm) under a pressure of 4 MPa at 152 ± 1
°C. The cure rate index was calculated according to the following equation:
$${\text{CRI}} = \frac{100}{{{\text{tc}}_{90} - {\text{ts}}_{2} }}$$
Characterization
Gravimetric analysis
Chemical composition of the rice husk fibers (RHFs) was determined using chemical treatment and gravimetric analysis for determination of its main constituents.
X-ray fluorescence (XRF)
Silica content in the rice husk ash (RHA) was determined using X-ray fluorescence (XRF) analyzer (Type: Axios Advanced, Sequential WD-XRF spectrometer, PANalytical 2005, Netherlands).
FTIR spectroscopy
Chemical structure and main functional groups of the rubbers (EPDM and NBR), RHFs and their extracted silica (RHS) were investigated using FTIR spectroscopy. The spectra were recorded at a wavenumber range of 4000–400 cm−1 with JASCO FTIR-6100E, Japan.
Scanning electron microscopy (SEM)
Homogeneity of the rubber blend mixes was examined by scanning electron microscope, Model JXA-840A (JEOL Technics Co. Ltd., Tokyo, Japan) at a magnification M = 1000 × . The surface of the samples was coated with gold using the sputtering technique prior to SEM observations.
Thermal gravimetric analysis (TGA)
Thermal stability of the rubber composites was evaluated using thermogravimetric analysis (TGA, TA instrument 2910 series). The samples (≈10 mg) were placed in a corundum dish, and the measurements were conducted at a heating rate of 10 °C/min in nitrogen atmosphere from ambient temperature up to 700 °C.
Physico-mechanical properties of EPDM/NBR blend composite vulcanizates
Sheets of the EPDM/NBR rubber blend composite vulcanizates were cut into dumbbell-shaped specimens (5 cm long and 0.4 cm wide), using an ASTM cutter. The thickness of each specimen was measured using a standard thickness gauge. Physico-mechanical properties (tensile strength, MPa, and elongation at break, %) of the rubber blend composite vulcanizates were determined, after and before accelerated thermal aging, utilizing a Zwick/Roell Z010 tensile tester machine with load cell (Type: Xforce P and Nominal force: 10 KN), according to ASTM D412-16 (2021). The physico-mechanical data were measured in five replicates for the average. Accelerated thermal aging was carried out in an air-circulated oven at 90 °C, for 7 days according to ASTM D573-04 (2019). The retained values of tensile strength (TS) and elongation at break (E %), after thermal aging, are calculated according to the following equations:
$$ {\text{Retained Tensile strength (\% )}}\,{ = }\frac{{{\text{TS}}\;\left( {{\text{after}}\,{\text{aging}}} \right)}}{{{\text{TS}}\;\left( {{\text{before}}\,{\text{aging}}} \right)}} \times 100 $$
$$ {\text{Retained Elongation at break (\% )}}\,{ = }\,\frac{{E{\text{ \% }}\left( {{\text{after}}\,{\text{aging}}} \right)}}{{E\,{\text{\% }}\left( {{\text{before}}\,{\text{aging}}} \right)}} \times 100 $$
Swelling tests and crosslink density
Swelling behavior of the rubber blend composite vulcanizates was assessed by measuring their weight swell at equilibrium (
\(Q \%\)) in the automotive oils (motor oil and brake fluid), gasoline, and toluene. The swelling test in toluene and gasoline was conducted at 25 °C for 48 h, according to ASTM D471-97. On the other hand, weight swell at equilibrium (Q %) of the vulcanizates in motor oil and brake fluid was conducted at 100 °C for 7 days. It was measured and calculated according to the following equation:
$$Q \% = \frac{{W_{s} - W_{d} }}{{W_{d} }} \times 100$$
where \({W}_{s}\) is the weight of the specimen after swelling, and \({W}_{d}\) is the weight of the dry specimen. The average of five replicates was considered.
The equilibrium swelling in toluene was used to determine the crosslink density of the rubber vulcanizates and was calculated using the Flory–Rehner equation as follows:
$$M_{c} = - \frac{{\rho V_{s} V_{r}^{\frac{1}{3}} }}{{\big[\ln \left( {1 - V_{r} } \right) + V_{r} + XV_{r}^{2} \big] }}$$
where
\({M}_{c}\) is the molecular weight between two successive crosslinks (g/mol),
\(\rho\) is the rubber density (mol/m
3),
X is the polymer–solvent interaction parameter,
\({V}_{s}\) is the molar volume of the solvent (m
3/mol), and
\({V}_{r}\) is the volume fraction of polymer in swollen state obtained from masses and densities of rubber and solvent. The rubber-solvent interaction parameter (
X) is 0.39 for NBR and 0.49 for EPDM. The crosslink density (
\(v)\) is calculated from the following equation:
$$v = \frac{1}{{2M_{c} }}$$
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