Effect of carbon black composition with sludge palm oil on the curing characteristic and mechanical properties of natural rubber/styrene butadiene rubber compound

This study was conducted to investigate the possibility of utilizing sludge palm oil (SPO) as processing oil, with various amount of carbon black as its reinforcing filler, and its effects on the curing characteristics and mechanical properties of natural rubber/styrene butadiene rubber (NR/SBR) compound. Rubber compound with fixed 15 pphr of SPO loading, and different carbon black loading from 20 to 50 pphr, was prepared using two roll mills. The cure characteristics and mechanical tests that have been conducted are the scorch and cure time analysis, tensile strength and tear strength. Scorch time (ts5) and cure time (t90) of the compound increases with the increasing carbon black loading. The mechanical properties of NR/SBR compound viz. the tensile strength, modulus at 300% strain and tear strength were also improved by the increasing carbon black loading.


Introduction
Rubber and elastomers are some of the most common materials significantly known for its flexibility and their ability to absorb particle like carbon black, silica and clay in enhancing and obtaining the desired properties for specific applications wherever resistance to impact or toughness are desired. Fillers, chemicals, and other reinforcing materials usually are added to enhance the properties of the rubber. Oil is one of these substances which usually are added in the processing phase of the rubber compounding process, and acts as the processing aid in the rubber compounding. Usually the type oil that is used in the rubber compounding process are the highly aromatic (HA) oils. This type of oil was added to the rubber compound with other ingredients to improve the processability and performance of the compounds such as promoting wet grip and wear endurance.
Certain highly aromatic oils as such had been classified as carcinogenic as upon tested according to IP method called IP346,t showed an unhealthy index values exceeding 3% [1]. Numerous other studies had found that aromatic oil addition into rubber compounds, especially oils with high polycylic aromatic compounds (PAC) content which were identified as suspected carcinogens, might had contributed to increase cancer proliferation [2,3] aromatic oil in rubber compounding had been banned. Besides that, PAC containing aromatic amines such as 2-naphthylamine have long been associated with increased rates of urinary bladder cancer (UBC) in rubber workers [1]. Hence, the environmental problem caused by HA oil use shows the need for natural based processing oil. Studies done by  on epoxidised oil (EO) with NR/SBR compound found that EO oil can be an alternative sources to replace the aromatic oil (AO) as a plasticizer and processing aid [4,5].
For tyre manufacturing, the common elastomers used are the NR/SBR mix. NR are usually mixed with synthetic rubber like SBR, commonly in the 60:40 ratio for rubber tyre and other compounded products. The SBR mixed often imparts suitable properties in meeting the performance needs of passenger car tyre treads [3,6,7]. The reinforcement of rubber properties by the incorporation of carbon black had been extensively studied. Recent studies with carbon black fillers and other fillers had been investigated by Ooi et al., (2013) [8]. In their work, oil palm ash (OPA) as filler was compounded with NR using conventional laboratory-sized two roll mills. For comparison purposes, two commercial fillers (i.e. silica and carbon black (CB)) were also studied. The optimum loading of each type of filler was investigated and compared in terms of curing characteristics, tensile properties, rubber-filler interaction and dynamic mechanical analysis. Results showed that scorch time and cure time of the carbon black filled vulcanizates was the lowest compared to OPA and silica-filled NR vulcanizates, with the OPA compound offers the highest cure rate index. The carbon black filled vulcanizates had the highest tensile strength (26 MPa) due to the reinforcement that its filler imparted. Ulfah et al., (2015) had also compounded NR with a blend ration of carbon black and silica fillers.
They had found out that carbon black filler of the highest content ratio showed the highest tensile stress at 300% elongation for rubber compound with increasing amount of carbon black filler [9].
Waste cooking oil sludge sourced from waste oil palm cooling oil were used in this study; the waste oil used was collected from the oil trap as grease. The term "trap grease" technically refers only to kitchen waste, rather than septic grease. Sludge oil or trap grease are sometimes collected by rendering companies for purification and are often sold for non-edible applications such as burner fuel [7]. In the present study, sludge from palm oil (SPO) was used as an alternative source for aromatic oil. The effect of SPO loading on the cure characteristic and mechanical properties of NR/SBR compound had been investigated and reported.

Compounding
Compounding of the ingredients as per Table 2 specifications were prepared in a two-step process; preparation of master-batch (sans sulphur and accelerator) was done using Banbury internal mixer (BR1600) at the RRIM under a set temperature of 150°C and mixing speed of 70rpm. The remaining ingredients were later added and mixed using a two-roll mill with 30rpm rolling speed. The sheeted rubber compound was conditioned under room temperature for 24 h. For this study, SPO content are fixed at 15 pphr while the carbon black (HAF N330) were varied to 50pphr (parts per hundred rubber).
Scorch time (ts5) = Time to reach 5 unit increase in torque above minimum (at ML + 5 units above it) Optimum cure time (t90) = 0.9(MH -ML) + ML (2) Where, MH = Maximum torque, ML = Minimum torque The tests for the tensile strength and tear strength properties of the rubber compounds were performed using an Instron universal machine (model Testometric MICR0500) at 500mm/min crosshead speed according to the ASTM D412 and ASTM D624 standards respectively. This process occurs mainly during the compounding (milling) process using the two roll mill machine. rubber and the excess carbon black, which resulted in the lowering of the vulcanization rate [10].

Cure Characteristic
Other than the cross-linking linkages formation, the process of network formation had been explained by Karak et al., (2000) [11]. As cure time decreases, the vulcanization process can theoretically be explained by the following processes. Both network formation and network degradation occur simultaneously, but the initial network formation rate was much lower than the degradation rate; subsequently, an optimum level can be reached before the rate of network structure degradation finally became much shorter for the NR vulcanizates as cure time decreases.
There were still scarcely enough works on the use and effect of waste or SPO and other vegetables oil forms for tyre tread compound apart from the limited number of works [12,13]. Rahmah time. It had shown that an EO addition up to 30pphr had reduced the cure time from 13 to 90% as compared to AO use [5]. Besides that, the higher amount of carbon black had caused reinforcement between the carbon black particles themselves, fixed SPO and the rubber chains. This finally lead to the interfacial interaction reduction between the rubber chains due to agglomeration of carbon black particles [4].

Tensile Properties
The carbon black presence induces rigidity to the rubber, in which an increase in carbon black will increase the rigidity of the rubber compound. High rigidity is sometimes needed in rubber compound for tire manufacturing, which provides an improvement in strength.

Tear Strength
There are three types of sample for use in the tear tests, which are the trouser, angle or crescent samples. But for this test, the trousers sample was used. The tests were conducted by applying force to the material in a tensile direction, of which a value for the tear strength was calculated. From Figure 5 below, there were some small gradual observable increases in the tear strength when the carbon black content were increased from 20 to 40pphr. Upon reaching 45 pphr and 5 0pphr of carbon black a drastic increase of about four fold increase in the tear strength were observed.
Carbon black had long been thought to be a reinforcing agent but these higher reinforcing effects were only imparted upon higher amount of carbon black used. The high interaction of carbon black and rubber was apparent through this pronounced reinforcing filler effect. This proves that the carbon black had acted as the reinforcing filler, hence caused better resistance to tear for rubber compound. Tear properties were markedly improved with a fourfold increase upon addition of 45pphr of carbon black. Fukahori et al., (2013) stated that, during the tearing of rubber materials, the transition called elastic-viscous transition phenomenon exist; and this transition had occurred due to the crosslink density and visco-elastic energy dissipation [14].

Conclusions
In conclusion, the scorch time, cure time, tensile strength, and tear strength shall increase with corresponding increases in the amount of carbon black. Meanwhile, the tensile modulus at 300% strain had showed results that are inversely proportional to the elongation at break which due to the elasticity and stiffness properties of the compounds. An increase of carbon black up to 45pphr results in overloading effect, which causes an uneven distribution of the carbon black in rubber compounding process.