The Taste of Waste: The Edge of Eggshell Over Calcium Carbonate in Acrylonitrile Butadiene Rubber
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Rubber technology experiences a new age by the use of biowaste or natural fillers. In this regard, taking properties of reinforcing agents from biowaste fillers remains as the challenging matter. Chicken eggshell (ES) biowaste has recently been introduced to substitute calcium carbonate (CaCO3) duo to its superior properties and low price. In this work, composites based on acrylonitrile butadiene rubber (NBR) reinforced with ES and CaCO3 microfillers at various loading levels were prepared and characterized. To improve the interactions between fillers and the NBR matrix, ES and CaCO3 were surface-functionalized using a terpolymer, namely poly(vinyl 2-pyrrolidone-co-maleic acid-co-acrylic acid). Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were used to characterize the modified fillers. The incorporation of the functionalized fillers resulted in a significant rise in the maximum torque according to the rheometric measurements. The Young’s modulus of the ES-based and CaCO3-based compounds showed a mild improvement over a wide range of filler contents. The elongation at break of the NBR composites, however, was dependent on the filler content. This work provides exciting opportunities for the design of novel and innovative coupling agents to be used in rubber applications.
KeywordsEggshell biowaste Surface functionalization Polymer composite Rubber technology
According to the statistical data from Food and Agriculture Organization of the United Nations (FAO), 6.4 million tons of ES are disposed in landfills worldwide in 2010 alone. This data shows an enormous potential of ES for treatment and reuse in polymer products. ES powder has been used as fertilizer and soil conditioner , the adsorbent of heavy metals [13, 14] and sorption site for CO2 [15, 16]. With a growing interest in academia, ES powder has been employed as a biofiller in several polymer matrices including polypropylene (PP) , low-density polyethylene (LDPE) , and high-density polyethylene (HDPE) . However, the industrial consumption of ES remained limited. According to the literature, ES contributes by 11% of the total weight of the egg with its primary component being calcite form of calcium carbonate crystal (~ 94%). Other components of ES include MgCO3 (~ 1%), Ca3(PO4)2 (~ 1%), and organic matter (~ 4%) . This abundance of calcium carbonate (CaCO3) in the waste ES introduces an excellent source of bio-mineral CaCO3 with possible opportunities to replace mineral-based and synthetic CaCO3 in polymer composites.
So far, a growing body of work in the literature has sought to examine the possibility of replacing CaCO3 with bio-waste ES. The results, although promising, are far from complete. It is important to note that studies in the past have mainly been focused on thermoplastics, and limited reports exist on thermosets and elastomeric composites. In recent studies, some important aspects of surface modification of ES powder towards the elastomeric compounds including NR, SBR, and NBR were discussed [32, 33]. Specifically, the effect of surface modification, filler loading, and particle size distribution on the curing and thermo-mechanical properties of NR, SBR, and NBR rubbers containing CaCO3 and various types of ES have been comprehensively reported.
In this work, poly(vinyl pyrrolidone-co-maleic acid-co-acrylic acid) terpolymer was used instead of the conventional use of small molecules in surface modification of the fillers to improve curing behavior and mechanical properties of NBR composites. The efficiency of the surface functionalization procedure using FTIR and TGA measurements were discussed. The correlation between surface functionality, curing behavior and mechanical properties of the compounds were also studied. This work is an attempt towards the manifestation of biowaste ES-based rubber composites for engineering applications.
Average particle size and surface area of pristine and modified fillers studied here
Average particle size (nm)
Surface area (m2/g)a
Surface Functionalization of Particles
Preparation of NBR Composites
The compounding formulation of the samples
Characterization and Testing
To verify the functionalization of fillers, Fourier transform infrared (FTIR) spectral analysis was utilized using Brucker Vertex 80 V spectrophotometer over the wavelength range of 400–4000 cm−1. To prepare the samples for the test, filler powders were mixed with KBr and pressed into a pellet. Thermogravimetric analyzer TGA Q5000 from TA Instruments was also used to investigate the decomposition temperature and weight loss of the unmodified and modified fillers. The measurements were performed at a heating rate of 10 °C/min under nitrogen atmosphere up to 600 °C. Tensile properties of the samples were determined using a universal testing machine (Zwick 1456, Z010, Ulm, Germany) with DIN S2 dumbbell samples. The strain was applied at a rate of 200 mm/min. In all cases, tensile tests were carried out on five dumbbell specimens. Average values measured and the standard deviations were reported. Investigations of curing characteristics were done using rheometric studies in a moving die rheometer Göttfert Elastograph Vario 67.03. Optimal curing time was considered as the time for the rubber to reach 90% of its maximum torque.
Results and Discussions
Assessment of the Surface Treatment of mES and mCaCO3
Rheometric Cure Characteristics
In this equation, M is the Young’s modulus of the filled composite, Mo is the Young’s modulus of the unfilled vulcanizate, ϕ is the volume fraction of the filler, and f is the shape factor . The shape factor f for non-spherical fillers is generally higher than unity. To obtain the shape factor f, we fit the Young’s moduli of the compounds to the Guth–Gold equation (Fig. 10). Because the surface modification of mCaCO3 worsens its reinforcing effect, we obtain lower f values for mCaCO3 compared to pristine filler. This fact agrees well with the trends observed in Fig. 9a. On the other hand, modification of ES has proved to be effective, as there is an increase in f value from 2.4 to 2.96 due to surface modification of filler with terpolymer. This substantial improvement confirms an efficient dispersion of the mES in the NBR matrix.
In this work, biowaste eggshell and calcium carbonate powders were applied as fillers in preparation of NBR composites. It was observed that chemical modification of the surface of CaCO3 and ES has a limited or no effect on the mechanical properties (tensile modulus and elongation at break) of NBR composites at low loading levels as confirmed by determination of shape factor of fillers that demonstrates reinforcing effect of the filler, while a completely different effect on high loadings. Modification of CaCO3 deteriorates elongation at break of NBR, but eggshell surface modification by terpolymer increased interaction between the filler and polymer chains. Such contradictive behavior was attributed to the role of peptide groups in the eggshell and carboxylic groups of terpolymer, which synergistically enhance filler–polymer interaction in terms of hydrogen and covalent bonding in eggshell powder-filler NBR unlike their effects in the case of CaCO3-filled rubbers. Thus, it can be corroborated that surface functionalization of biowaste eggshell provides this additive with a very high potential for improvement of mechanical properties of rubber compounds.
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