Curing Characteristics and Physical Properties of Natural Rubber Composites Using Modified Clay Filler

The differences in the curing characteristics and physical properties of natural rubber samples using clay and modified clay as fillers were studied. After the clay was modified with dodecylamine, the rubber milling process was conducted at a temperature of 65–70^oC to obtain the natural rubber compound. The content of clay and modified clay in the natural rubber was approximately 15 phr. A curing test at 150^oC was performed, and then the physical properties were tested. The results of the physical properties test showed a significant increase in the tensile strength, from 16.3 to 25 MPa, a change in hardness from 43 to 54 Shore A, a change in modulus of 300% from 1.6 to 4.6 MPa, a change in tear strength from 29.3 to 40.2 kN/m, and a change in compression set from 25.75% to 30.57% due to the use of modified clay compared to the sample with unmodified clay. However, some physical properties—such as elongation at break, from 720% to 600%—decreased dramatically. S_max increased sharply, from 7.05 to 11.45 kg-cm, while optimum cure and scorch time decreased sharply, from 11.23 to 6.43 minutes and from 6.35 to 2.38 minutes, respectively. FTIR and XRD analyses showed evidence of clay modification. Similarly, the AFM and SEM analyses of clay surfaces and the dispersion of the clay in the rubber showed that the dispersion of the modified clay in the rubber was better than that of unmodified clay. The TA/DTA analysis also supported the above explanation, particularly for the changes in curing characteristics and physical properties.

Clay; Dodecylamine; Modified clay; Natural rubber; Reinforcement

 Global warming has become an increasingly serious problem in recent years, motivating a strong worldwide effort to address its causes and ameliorate their effects. The origins of global warming include the production of CO₂ from human activities, particularly from burning fossils fuels such as oil, coal, and natural gas to provide energy for motor vehicles, heat sources, and power plants. Other sources of CO₂ emissions are forest fires, forest burning, and land clearing for both plantation land and agricultural land. In particular, forest fires cause greater emissions than forest burning and land clearing, and the burnt areas are much larger due to the long dry-season. Similarly, in the cement manufacturing industry, CO₂ is produced from the calcination of limestone, which is the main raw material in cement production. Despite efforts to reduce CO₂ from cement plants, a significant reduction in CO₂ emissions has not been achieved to date. The production of CO₂ by the carbon black manufacturing industry, where carbon black is used as a filler in rubber, is another important source of CO₂ emissions. Carbon black is produced using the thermal decomposition method or the partial combustion method, using oil or natural gas as raw material.

Among its other applications, carbon black is used for making rubber compounds in the vehicle tire industry. The role of carbon black is still very dominant in rubber compound manufacturing because carbon black can provide significant reinforcement effects and can reduce the amount of rubber used. While silica is an alternative material to carbon black that can also act as a reinforcement in rubber compounds, its reinforcement effect is inferior to that of carbon black. As reported by Hasan et al. (2019), local clay includes a fairly large amount of silica, which means that the nature of local clay is fairly similar to the nature of silica. Therefore, it is important to use local clay as a filler. Although the effect of clay as a filler has a smaller reinforcing effect on vulcanized rubber compared to the use of carbon black fillers, clay surface modification is needed to increase compatibility with the rubber matrix.

Local clay containing 50.83–75.29% silica is a common silica source in nature (Hasan et al., 2019) that is five times more common than coal. Therefore, this clay is an interesting possible filler material in rubber compounds. The type of clay in this study is kaolin clay, as described in Figure 8. The use of clay as a rubber filler has been studied by many researchers, including Goodman and Riley (2012), Ismail and Mathialagan (2011), Lalikova et al. (2011), Ruamcharoen et al. (2014), Szustakiewicz et al. (2013), Zhang et al. (2012), and Zhang et al. (2010, 2014), and modified clay has also been widely examined—for example, in the work by Ambre et al. (2008), Jagtap et al. (2013), Kord et al. (2017), Ogbebor et al. (2015a, 2015b), Peter et al. (2016), Puglia et al. (2016), Saritha et al. (2012), Sheikh et al. (2017), Sreelekshmi et al. (2016, 2017), Sukumar and Menon (2008), and Yahaya et al. (2009). Most of the types of clay used by researchers are kaolin clay, in addition to bentonite and montmorillonite clay.

A wide variety of chemicals have been used by researchers to modify clay, such as the metal chlorides used by Lalikova et al. (2011), the fatty acid salt used by Zhang et al. (2014), the hydrazine hydrate used by Sukumar and Menon (2008), the dimethyl, benzyl, dehydrogenated tallow, and quarternary ammonium used by Saritha et al. (2012), the 3-mercaptoprophyltrimethoxysilane used by Sheikh et al. (2017), the hexamethylenediamine used by Sreelekshmi et al. (2017), the dimethyldioctadecylammonium and bis(4-hydroxybuthyl)methyldioctadecylammonium used by Nam et al. (2004), and the octadecylamine used by Praveen et al. (2009), Nigam et al. (2012), and Manchado et al. (2003). In general, this clay modification aims to increase the adsorption on the clay surface so that the clay can function better as a filler. An increase in surface adsorption is carried out not only on the clay but also on the adsorbent (Wilson and Mahmud, 2015; Anuar et al., 2019; Kusrini et al., 2019). Here, clay also functions as an adsorbent for rubber molecules on its surface.

Therefore, this study aims to compare the curing characteristics and physical properties of natural rubber composites using clay and dodecylamine-modified clay as the fillers. The modified clay was analyzed by FTIR spectroscopy and X-ray diffraction while the natural rubber compounds were examined by TA/DTA and SEM. The surface of vulcanized natural rubber was analyzed by AFM.

Modified clay has a better effect on the curing characteristics and physical properties of natural rubber than unmodified clay. The vulcanized rubber stiffness, due to an increased vulcanization reaction in the natural rubber, leads to changes in the physical properties of the natural rubber. An analysis of FTIR spectra shows that the clay has been modified with dodecylamine. These results agree with the XRD analysis results that indicate a change in the dimensions of the clay crystal. SEM and AFM images provide consistent information about the filler dispersion in the natural rubber. Modified clay dispersion is better than that of unmodified clay, and this finding supports the explanation of the different physical properties of the obtained vulcanized natural rubber. TA/DTA thermal stability analyses show that natural rubber compounds that use modified clay as the filler are more stable than those using only unmodified clay filler.

    This work was supported by the Directorate General of Strengthening Research and Development, Ministries of Research, and Higher Education, Republic of Indonesia, with the scheme of Higher Education Applied Research under contract number 153/SP2H/LT/DRPM/2019, dated March 11, 2019.

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