Influence of Processing Oil Content on Rubber-Filler Interactions in Silica/Carbon Black-Filled Natural Rubber Compounding

. To ensure the production of high-quality rubber products, establishing effective compatibility and robusting interactions between rubber and fillers is crucial. Since the rubber production industry faces significant environmental challenges, sustainability is becoming a necessity in the rubber business. Various materials, such as the presence of processing aids, might influence rubber-filler and filler-filler interactions. Petroleum oil as general processing oil has environmental issues, thus it is urgent to replace it with bio-based oil. This study addresses these concerns by investigating the impact of paraffinic oil and bio-based oil content on the interactions between silica/carbon black and natural rubber chains. Ranging from 0 to 10 phr, different dosages of these oils were employed. The rubber compound underwent milling using two rolled laboratory open mills post-blending with a laboratory internal mixer. Subsequently, mechanical properties were assessed. A Rubber Processing Analyzer was utilized to characterize interactions between uncured and cured silica/carbon black-filled natural rubber blends. The results revealed that all variations were distributed uniformly. Tan Delta results confirm that a chemical with a high bio-based oil concentration is easier to process. The carbon black with the largest peak of modulus values at low strain has the highest reinforcing or the poorest dispersion. Compounding with paraffinic oil has a high peak modulus value, resulting in poor dispersion when compared to bio-based oil. This research sheds light on the potential of bio-based oils as an eco-friendly alternative in rubber processing, contributing to sustainability efforts in the industry.


Introduction
Natural rubber (NR) is widely utilized in various applications, particularly tires, due to its unique mechanical qualities, which include outstanding tensile properties, good tear resistance, and low heat build-up.Rubber is usually combined with reinforcing fillers to increase its mechanical qualities [1].However, large silica/carbon black loading causes processing difficulties due to higher compound viscosity [2].In order to minimize compound viscosity and increase filler dispersion, rubber processing oil must be added [3].Rubber Corresponding author: dewi030@brin.go.id processing oil is added to rubber compounds at modest loadings at the mixing stage to improve processability without significantly impacting physical qualities.Until now, the most extensively utilized processing oils have been petroleum-based oils that act as a physical plasticiser in rubber composites.In Indonesia and Thailand, paraffinic and aromatic oils are commonly employed in the compounding process.Despite serving the same purpose, the applications of each category are different.Paraffinic oil is best suited for making completed goods with brilliant colors, such as rubber, whereas aromatic oil is more suited for making finished goods with darker hues, such as vehicle tires, shoe soles, plastic objects, and printing ink.Paraffinic oil is a highly stable chemical that is utilized in the production of light-colored or white vulcanizates [4].It is nonpolar and so better suited for nonpolar rubbers such as EPDM.Aromatic oil, on the other hand, is less stable, darker in color, has a low flash point, and is not appropriate for high-temperature mixing.Aromatic oils with high aromatic groups have been widely employed in petroleum oils due to their good compatibility with most rubbers [5].Aromatic oils, on the other hand, are categorized as carcinogenic due to their high amount of polycyclic aromatics.It has been discovered, however, that even certain benign petroleum-based oils may contain carcinogens.Highly aromatic oils containing polycyclic aromatic hydrocarbons were banned on January 1, 2010, according to the European Parliament and Council Regulation Concerning the REACH regulation (EC 18/09/2006) [6,7].As a result, numerous attempts have been made to employ various vegetable oils or their derivatives in rubber compounding.In 2012, soybean oil has been identified by Goodyear Tire and Rubber Company to help reduce the quantity of petroleum required in tires while simultaneously prolonging the tread life of those tires [8].Dasgupta et al. discovered that neem oil and kurunj oil demonstrated superior abrasion qualities in natural rubber/SBR compounding for radial passenger tyre tread application [9].Neau and Rangstedt investigated if it is possible to switch the plasticizer in truck tire tread compounds based on natural rubber from petroleum oil to rapeseed oil, and whether the use of carcinogenic oil can be avoided technologically [10].Meanwhile, rice bran oil can be used in place of the process oil, antioxidant, and fatty acid without significantly altering the cure capabilities of the mixes or the physical features of the vulcanizates [11].Other research found that palm oil was a superior alternative processing aid to petroleum-based aromatic oils, which have been linked to cancer.Furthermore, palm oil has the highest heat resistance [12].Raju et al. examined coconut oil for carbon black-filled compounding, although the use of coconut oil in silica-carbon black compounding has not yet been published [13].The objective of this research was to investigate the influence of different ratios of two types of processing oil, paraffinic oil as the most used rubber processing oil in Indonesia and Thailand, and coconut oil as an alternative bio-based oil, to the silica-carbon black filled natural rubber compounding.As a result, the research would provide new knowledge on the properties of natural processing oil, which might potentially substitute petroleum oil in natural rubber compounding with silica-carbon black filler.

Materials
All of the mixing ingredients were used exactly as directed.Natural rubber, Rubber Smoked Sheet 1 (RSS1), with a density of 0.96 g/cc was delivered by Thai Rubber Latex Corporation.The carbon black N 660 with maximum ash of 0.5% was supplied by Cabot Ltd.Evonik Ltd supplied the Ultrasil 7000 GR Silica with a specific surface area (CATB) of 160 m 2 /g, while Merck supplied the Bis(triethoxysilylpropyl)tetrasulfide (TESPT) with a relative density of 1.095 g/cm 3 .Dispersable Sulfur from Miwon Chemicals Co., Ltd, Korea was used as a curative.Morakot Ltd supplied paraffinic oil as processing oil with a viscosity kinematic of 6.9 at 210 F. Coconut oil was obtained from the Hat Yai local market with a major content of fatty acid is lauric acid.The fatty acid composition caprylic acid -9% ; capric acid -6% ; lauric acid -49% ; myristic acid -20% ; palmitic acid -8% ; and stearic acid -3%.

Rubber compounding
A laboratory internal mixer (Brabender Lab-Station Plasticoder) was used to create the rubber compounds.The mixing process consisted of two parts.The mixing temperature was set to 120 °C in the first step, and the speed was set at 32 rpm for roughly 5 minutes.At 70 °C, the compound was passed through the two-roll mill six times.After storing the masterbatch for a night to allow the rubber compound to rest, the batch was processed six times at 60 °C using a two-roll mill (Baopin Technology) for sulfur addition.Then it was cooled and stored at room temperature for another 24 hours before being utilized for sample preparation and subsequent analysis.In this study, the author varied the ratio of paraffinic oil and coconut oil for the formulation as seen in Table 1.

Testing and Analysis
To begin, the cure time of the compound was evaluated at 140 °C using a Moving Die Rheometer (Alpha Technology) according to ASTM D5289.The vulcanization process was then carried out in accordance with the rheometer testing results and the requirements of each characterization standard.The Mooney viscosity of the rubber compounds was determined using a Mooney viscometer (Alpha Technology) in line with the ASTM D1646 standard.Hardness Shore A was measured using a Durometer hardness tester (Mitutoyo) in accordance with ASTM D2240.Each sample was measured five times from different places at a distance of at least 6 mm.Tensile and elongation at break were measured using a Universal Testing Machine (Haida Equipment) in accordance with ASTM D412.Abrasion resistance was done following ASTM D5963 by using DIN abrasion tester (GenDIN Instrument).The Rubber Processing Analyzer (TA Instrument) was used to analyze the rheological parameters.At a temperature of 100 °C, frequency changes ranging from 0.01 to 50 Hz were tested, with constant amplitude of 10.Furthermore, amplitude variations ranging from 0.05 to 3.59 were examined at 90 °C with a fixed frequency of 1 Hz.

Mechanical properties
Scorch time is the amount of time at a certain temperature that a rubber compound can be worked before curing begins.Curing time, also known as vulcanization time, is the period required for rubber to crosslinked [14].Because the output of the rubber goods per hour can be increased, the lower curing time means improved productivity of the rubber products.
Figure 1 shows a comparison of scorch time and curing time with various processing oils.In general, the outcome was discovered coconut oil compared favorably against petroleum oils.The scorch time for the five variables is not statistically different.This demonstrates that coconut oil increases processing safety because similar scorch duration prevents the components from getting cured before vulcanization begins.Furthermore, a longer scorch time is desirable because the rubber compound does not flow effectively once charred due to the existence of crosslinks [15].The Curing Rate Index (CRI) is an essential indicator that reflects the rate at which curing happens because it quantifies the rate of vulcanization based on the difference between optimum vulcanization and scorch time [16].According to Figure 2, the coconut oil components have the greatest CRI.This shows that adding coconut oil enhances the activation site for vulcanization [17].The cure rate index also represents the rate of reaction during the curing process, implying that coconut oil speeds up the crosslinking reaction during the vulcanization process.Mooney viscosity is commonly used as a characterization method to determine a compound's processability [18].Mooney viscosity is a measure of the viscosity or flowability of rubber compounds.Lower viscosity typically suggests better processability.The viscosity of a rubber compound is determined by the compatibility of the rubber and filler, as well as the plasticizing influence of the processing oil.Fillers that are incompatible with rubber typically have a greater viscosity and are harder to mix.The processing oil is a factor that influences the compatibility of the rubber and filler.A processing oil with a similar solubility parameter to the filler and the rubber results in good compatibility.Processing oil improves the dispersion of fillers in the rubber matrix by lowering Mooney viscosity.Compounds derived from coconut oil successfully reduced the Mooney viscosity and, as a result, the apparent shear viscosity of NR compounds.This could be ascribed to the addition of epoxy groups to the filler and elastomer matrix by coconut oil, which increases cross-linking behavior.
Compounds recorded acceptable values, with values in the middle of the other petroleum oils, as indicated in Fig 3 .The decreased viscosities of NR compounds provided by bio-based oils could possibly be attributed to improved silica dispersion in the rubber matrix seen for these compounds [19].

Fig. 3. Mooney viscosity test result.
The hardness shore A of an elastomer quantifies its sensitivity to stress applied to tiny surfaces, thus, the harder the compound, the greater the abrasion resistance [20].In has a positive relationship with hardness; therefore, as scorch time lowers, so does hardness due to decreasing viscosity [21].Rubber compound hardness is generally proportional to filler loading and cross-link density [22].Hardness value is related to tensile strength and abrasion resistance index.Lower hardness might be associated with higher tensile strength and abrasion resistance index.The Fig 5 depicts the tensile strength of rubber compounds, with no significant difference, but the coconut oil compound provides better tensile strength.Each processing oil has a varied ability to stretch the rubber matrix during chemical dispersion, as well as a different compatibility between the polymer matrix and the filler [23].Its ability is determined by the fatty acid and other components included in the oil [24].The value of the coconut oil component was 10% higher than that of the paraffinic oil analog.The greater dispersion of fillers in the coconut oil elastomer may account for the increased elongation of the coconut oil compounds over the paraffinic oil compounds.This restricts motion among the elastomer chains, resulting in better interfacial adhesion between the fillers.The filler dispersion was non-uniform at lower oil loadings, and thus the strength was not fully established [25].
Increased crosslinking in the rubber matrices usually results in a rubber vulcanizate with improved tensile strength and elongation at break.This is consistent with the rheological results, which demonstrate that compounds containing coconut oil have the higher CRI.The value of abrasion resistance is a vital feature that rubber products must have; if the abrasion resistance is poor, the resulting product will easily wear out and leak.The higher the elasticity of a rubber compound, the lower the abrasion resistance value [26].The higher the value of % ARI, the better the abrasion resistance physical qualities of the rubber product.
Similarly to tensile properties, coconut oil can soften the rubber mixture and produce the maximum abrasion resistance index [27].

Rubber processing analyses
Rubber Processing Analysis assessed the dynamic mechanical characteristics of rubber composites.The storage shear modulus (G′) at low strain amplitudes is used to characterize the filler reinforcing mechanism in rubber compounds, and its reduction with increasing dynamic strain amplitudes is used to characterize the filler reinforcing mechanism in rubber compounds [28].G′ decreased with increasing strain for all compounds, indicating that the silica network degrades with increasing strain.The difference in G′ values between low and high strains (G′ or Payne effect) often reflects the degree of filler-filler interaction (filler network) [29].The data reveal that G' decreases with increasing strain for all oil types, indicating that the silica network is being destroyed.The findings as seen in Figs.7 and 8 suggested that the compound containing coconut oil had a lower G′ and a lower degree of filler-filler interaction.The petroleum oil-containing compound appears to have a higher G' due to the poorer silica dispersion, indicating a greater magnitude of filler-filler interaction.
Compound with higher coconut oil content has stronger plasticization effect of processing oil.Because of the presence of polar ester groups that can interact with silica and facilitate the breakdown of silica agglomerates, biobased oils may increase silica dispersion more than petroleum-based oils.Filler-filler interaction is well understood to be affected by filler type, filler dispersion level, plasticizer type, and plasticizer content.It has also been found that, at the same level of filler dispersion, vegetable oil, which has the greatest plasticization impact and the lowest viscosity, has the lowest amount of filler-filler interaction.Mechanical strength, such as hardness, tensile strength, elongation at break, and abrasion resistance index can also be anticipated using the Payne effect test results on cured materials.The mechanical qualities will improve as the Payne effect decreases.A Rubber Processing Analyser can be applied to test the frequency sweep mode or to adjust the frequency to detect viscoelastic characteristics.Unlike the Mooney viscometer, which can only calculate the viscosity at one frequency, RPA can forecast changes in the compound's characteristics [30].Data of shear modulus (G'), tan delta, and viscosity can be acquired by altering the frequency, as illustrated in Figs. 9 and 10.The higher the tan delta, the lower the viscosity, which means the compound is easier to flow and process in specific applications, such as the calendering process [31].The result shows a slightly reduced processability for coconut oil as a processing oil, but a clear improvement in overall mechanical properties when compared to petroleum oil-based processing oils, making this abundantly available chemical a good substitute for many rubber products.

Conclusion
In summary, the study determined that natural-based oil, particularly coconut oil, could serve as an alternative processing oil to substitute petroleum-based oil in natural rubber composites loaded with silica and carbon black.The physico mechanical properties of rubber vulcanizates produced with coconut oil showed no significant difference compared to those manufactured with paraffinic oil.Furthermore, they exhibited slightly higher physical quantities in term of tensile strength, modulus, and abrasion resistance index.