Synergistic effects of soap nut extract and glutaraldehyde on the properties of natural rubber: A waste to wealth approach
Introduction
Polymer blends are the physical mixtures of two or more polymers. Blending of polymer is an alternative method instead of synthesizing a new polymer or monomer and it reduces the production cost (Muthuraj et al., 2018). Blending provides materials with excellent properties that vary in between the properties of individual properties. Materials with desired stability can be developed by varying blend ratio. Natural rubber (NR) is a natural polymer with excellent elasticity but poor physical properties (Roslim et al., 2018; Thongnuanchan et al., 2017). Generally, NR must be modified by adding curing agents such sulfur, peroxides etc. to improve its stability (Ghosh et al., 2003; Lorenz and Parks, 1961; Makuuchi et al., 1990). Recently, it was reported that, glutaraldehyde can be used to cure NR as an effective crosslinking agent for NR in latex stage at low temperature (Johns et al., 2012; Johns and Nakason, 2012; Johns et al., 2015; Kalkornsurapranee et al., 2017). The crosslinking reaction of NR with glutaraldehyde was accomplished in two steps. The first step involved the creation of pentane-1,5-deylidenediamine by allowing glutaraldehyde to react with the ammonia. The second step was the formation of crosslinks via an “ene” reaction between molecules of NR with pentane-1,5-deylidenediamine. Considerable enhancement in the stability of rubber in terms of physical and chemical properties was also noticed more than for other crosslinkers.
The main source of rubber is Hevea Brasiliensis tree and it gives natural rubber latex. The latex consists of mainly water and rubber suspension and proteins, lipids and organic salts in relatively small amounts (Jacob et al., 1993). NR is generally used in food and medical industries. The final product of NR must be safe while in contact with human body. Addition of anti-bacteria agent is an effective method to improve the antimicrobial properties of NR and it prevents microbial contamination in NR products. Bacteria are grown on NR due to the presence of protein, amino acids and carbohydrates (Moneret-Vautrin et al., 1993; Yeang et al., 2002; Bode et al., 2001; Berekaa et al., 2000; Jendrossek et al., 1997). Attack of micro-organisms on NR deteriorates its physical properties.
Cardanol, extracted from cashew nut shell liquid was used as a natural additive to NR in the form of cardanol-formaldehyde (Vu et al., 1999). The incorporation of cardanol formaldehyde significantly improved the tensile and thermal stabilities of NR along with the oxidative stability. An optimization study on grafting of cardanol on to NR in the latex stage was reported by Mohapatra and Nando (Mohapatra and Nando, 2014). Physico-mechanical properties, rheological characteristics and differential scanning calorimetry confirm the plasticization effect of cardanol when grafted on to NR. An attempt has been made to replace carbon black in NR by introducing lignin as a reinforcing agent (Barana et al., 2018). Chemically modified lignin enhances the compatibility between both the constituents by improving the reinforcing ability.
Additives for NR based on petroleum products should be replaced by using natural products. Thus, natural polymers are outstanding materials to improve the physical properties of NR. In view of green movement, natural products from renewable and sustainable resources are proposed to incorporate into NR to improve the physical properties. Sapindustrifoliatus is a tree belongs to the family of Sapindaceae which is known as soap nut in English. These trees have been grown in the tropical and sub-tropical regions of Asia, and it is commonly found in the Western Ghats of India. Saponin is the extract of the fruits of soap nut trees. Soap nuts consist of saponins (Ghagi et al., 2011; Mahar et al., 2011), in the range of 6–10 % (Kommalapati et al., 1998), which is responsible for imparting surface active properties of the extract (Chandran et al., 2013). Saponins are classified as glycosides consisting of nonpolar aglycones called sapogenin and one or more monosaccharides (Zhou et al., 2013). The main structure of saponin in the pericarp shell of soap nut fruit was reported by Huang et al. (Huang et al., 2008). It can be used as a detergent due to its solubility nature in water and amphiphilicity (Yang et al., 2011; Chen et al., 2010; Gong et al., 2018). The main aim of the present study is to extract saponin from the fruits of soap nut tree and to study the possibility of blending with NR. The effects of incorporation of soap nut extract into NR and addition of Glutaraldehyde as a curing agent on the thermal and tensile properties of NR were studied. In addition, differential scanning calorimetry, FTIR, swelling studies and antimicrobial properties were also carried out in detail.
Section snippets
Materials
Natural rubber latex with approximately 60 % dry rubber content (DRC) consists of medium ammonia (0.47 %) was purchased from Shilpa Latex Products LTD, Bangalore, India. Glutaraldehyde (25 %) and benzene supplied by Merck chemicals Pvt. Ltd, India were used as received. Soap nuts (reetha), the deadly fruits of the Indian Soap-berry tree, produced from agricultural fields were purchased from the local markets, Bangalore, India.
Preparation of vulcanized natural rubber
Dried soap nut fruits were boiled in distilled water for 2 h and
Differential scanning calorimetry
Differential scanning calorimetry is a thermal method to evaluate the phase transitions such as glass transition (Tg) and crystalline melting temperature (Tm) occur in a polymeric material. Fig. 1 shows the DSC thermograms of NR/SNE blends with 5 % and 15 % SNE. The two separate figures represent the thermograms at different temperature ranges show their glass transitions and crystalline melting transitions. Similarly, a comparison study has been done between pure and cured NR/SNE blend with 15
Conclusion
Blends of NR and SNE were successfully prepared by latex blending method at various combinations. Miscibility of the two blend components was confirmed from the change in glass transition temperature. The thermal stability of NR was found to be increased by 38.94 % on the incorporation of 5 % SNE. Swelling studies revealed that the addition of SNE increases the swellability of NR. A fine and continuous morphology was observed in case of blends when compared to pure NR. Favorable interaction
CRediT authorship contribution statement
K.S. Krishna Kumar: Conceptualization, Investigation, Methodology. S.J Varuni: Resources. Rawiporn Promsung: Formal analysis, Validation. Ekwipoo Kalkornsurapranee: Validation, Formal analysis. Yeampon Nakaramontri: Project administration. Jobish Johns: Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors express their thanks to the Principal and management of Rajarajeswari College of Engineering, Bangalore for their encouragement and moral support.
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