Novel bionanocomposite systems for packaging applications

12 July 2017
Sarat Kumar Swain, Gyanaranjan Sahoo, Niladri Sarkar, and Fanismita Mohanty
Incorporating silicon carbide and calcium carbonate nanofillers into soy protein and starch, respectively, improves the thermal, mechanical, and chemical-resistance properties of the samples.

In the modern era, plastic packaging is widely used because of its low cost, easy processability, and long-lasting quality. Current lightweight plastic packaging materials, however, pose a severe environmental threat1 and thus an ever-increasing amount of research is focused on biodegradable and biocompatible alternatives. As part of these efforts, biopolymer-based composites2 have emerged as potential environmentally friendly candidates because of their good renewability and biodegradability. Nonetheless, there are some limitations associated with biopolymers that must be overcome, e.g., their thermal, mechanical, gas barrier, and chemical resistance properties need to be improved before they can be implemented directly in practical applications.

It has previously been demonstrated that blending and nanotechnology approaches can be used to improve the characteristics of biopolymers.3 Moreover, it may be possible to overcome the limitations of biopolymers with the use of nanomaterials (because of their unique physical, optical, and magnetic properties).4 Indeed, nanocomposites that are formed from a combination of biopolymers and other polymers or nanoparticles (or both) have enhanced properties compared with the neat materials.5

In our work, we have thus investigated how the use of different biopolymers and nanofillers can enhance the properties of two different bionanocomposite systems. In our first study,6 we examined an agro-protein-based composite. Agro-proteins are in high demand as biopolymers because of their excellent barrier properties (to oxygen and smells) and high water-vapor permeability.7–10 Specifically, we focused on the use of soy protein (SP)—composed of 20 amino acids, including cystine, arginine, and lysine—that was embedded with nanoparticles of silicon carbide (nano SiC). We adopted a simple solution casting approach in an aqueous medium to fabricate our SP/SiC bionanocomposites with different nano SiC loadings. We chose SP for our work because it is already widely used in biodegradable membranes,11 plastics,12 adhesives, and binders.13, 14 In addition, we selected nano SiC as the filler material because it is a well-known non-oxide ceramic material that has high electron mobility, excellent thermal conductivity, as well as high chemical and temperature resistance. Furthermore—unlike clay,15 calcium carbonate,16 and halloysite17—SiC has not previously been studied as an inorganic filler in protein-based composites.

For our second bionanocomposite system, we studied starch embedded with nanoparticles of calcium carbonate (CaCO3).18 We chose starch as the composite matrix because it is widely available, non-toxic, biodegradable, cheap, and directly accessible from plants.7 Furthermore, starch/polymer composites—including those made with polyhydroxybutyrate,19 polycaprolactone,20 and polylactic acid21—have already been reported in the literature. For example, starch/polylactic acid composites have applications in the fields of tissue engineering21 and drug delivery.22 In addition, it has been shown that starch/polymethyl methacrylate composites exhibit high flame retardancy and oxygen permeability (i.e., desirable characteristics for packaging applications).23 In our work, we have focused on the use of CaCO3 nanoparticles (nano CaCO3) as a reinforcing agent (known to improve the mechanical properties of cellulose24) because of its low cost. In particular, we have used a novel, low-cost, and ‘green’ in situ solution technique to synthesize starch/CaCO3 bionanocomposites.

We used a variety of techniques to characterize the structures of our starch/CaCO3 and SP/SiC nanocomposite samples, such as x-ray diffraction (XRD), high-resolution transmission electron microscopy, and selected-area electron diffraction. The XRD patterns for both our bionanocomposite systems are shown in Figure 1. For the SP/SiC composites—see Figure 1(a)—we observe a strong crystalline peak and two less-intense peaks from pure nano SiC at 2θ (angle of diffraction) values of 35.66, 59.89, and 72.20°, respectively. We also see the signature of the amorphous phase of pure SP at a 2θ angle of 20.38°. Although the intensity of the various peaks are reduced in the bionanocomposites compared with the pure SP and nano SiC phases, the coincident presence of the nano SiC and SP peaks in the XRD spectra indicates that there is strong interfacial adhesion between the two components of the composites. We also observe a number of peaks in the XRD spectra from our starch/CaCO3 bionanocomposites: see Figure 1(b). For example, the peak at 16.9° (2θ) indicates the crystalline nature of the starch,25 and the peaks at 2θ angles of 23.14, 29.56, 36.01, 39.02, 43.24, 47.47, and 48.47° denote the different crystal phases of nano CaCO3. In addition, we find that the intensity of the XRD peaks strengthen with increasing CaCO3 content in the composites.


X-ray diffraction spectra for the (a) soy protein/silicon carbide (SP/SiC) and (b) starch/calcium carbonate (CaCO3) bionanocomposite samples. The content of the filler material (i.e., nano SiC or CaCO3) is given in parentheses in the label for each spectrum. The spectra for pure starch and CaCO3are also given in (b). 2θ: Angle of diffraction. a.u.: Arbitrary units.

In another part of our work, we performed a thermogravimetric analysis (TGA)—see Figure 2—on both our material systems, to compare the thermal stability of the different composites. In the thermogram for pure SP, the first degradation we observe (at 113°C) is caused by the loss of water, and the next two degradations (both below 535°C) cause the complete decomposition and charring of the material. In contrast, we find that nano SiC remains in its initial form (i.e., without undergoing any thermal degradation) across the whole 30–700°C temperature range. For the SP/SiC bionanocomposites containing 5 and 10wt% nano SiC, we find that the amount of char residue increases from 7 to 12, and to 18wt%, respectively. These results, therefore, demonstrate that the thermal decomposition temperature of our SP/SiC composites increases with greater nano SiC loadings, and that the incorporation of the ceramic nano SiC fillers enhances the overall thermal stability of the SP system. Our TGA results—see Figure 2(b)—also show that the thermal degradation temperature of pure starch and CaCO3 are 325 and 620°C, respectively. In addition, we find that the thermal degradation temperature of the starch/CaCO3 composites is greater than for the pure starch sample. We measured a weight loss at 800°C for the 2, 5, and 8wt% CaCO3-loaded samples of about 25, 32, and 40%, respectively.


Thermogravimetric analysis results for (a) pure SP, pure nano SiC, and the SP/SiC 5 and 10wt% samples, and (b) pure starch, pure CaCO3, and three starch/CaCO3 (2, 5, and 8wt%) composites.

We have also conducted a biodegradation study of our composites (see Figure 3). After 180 days, we found that the biodegradation of the highest-loaded (i.e., 10wt%) SP/SiC bionanocomposite was only 8% lower than that of the pure SP sample. In other words, we find that the biodegradability of these bionanocomposites in sludge water is only slightly hampered by the incorporation of nano SiC fillers. Furthermore, the small weight loss of the 10wt% SP/SiC composite is indicative of the strong interfacial adhesion between the fillers and the matrix. For the starch/CaCO3 samples, we find that the weight loss (biodegradation) is lower than for the virgin starch sample. For example, we observe that the biodegradation of the 5wt% CaCO3 sample is about 10% lower than that of the pure starch sample after just 30 days. This is because of the good dispersion and interaction of the stable fillers within the starch matrix.


Results from 180-day biodegradation studies of (a) the SP/SiC and (b) the starch/CaCO3 bionanocomposite systems.

In summary, we have studied two different bionanocomposite systems that contain biopolymers, and nanofillers as reinforcing agents. We used a solution casting technique to synthesize our SP/SiC composites and an environmentally friendly, low-cost solution method to form our starch/CaCO3 composites. The results from our comprehensive set of measurements indicate that both bionanocomposite systems exhibit improved thermal, chemical, and mechanical properties compared with the pure (i.e., starch or SP) materials. In addition, we find that incorporation of the nano SiC and CaCO3 fillers has little effect on the biodegradability of the samples. Both our bionanocomposite systems are therefore suitable for use in packaging applications. In our future work, we plan to use various oxide, nitride, and carbide nanomaterials to improve the thermal, mechanical, and gas-barrier properties of other biopolymers (e.g., cellulose and dextran) and proteins (such as albumin bovine).


Authors

Sarat Kumar Swain
Veer Surendra Sai University of Technology

Sarat Kumar Swain is a professor of chemistry, as well as Dean for postgraduate studies and research. He conducted his postdoctoral training at the University of Akron, OH and his area of interest is polymer-based nanocomposites.

Gyanaranjan Sahoo
Veer Surendra Sai University of Technology

Gyanaranjan Sahoo is a PhD student whose research interests include graphene-reinforced polymer nanocomposites.

Niladri Sarkar
Veer Surendra Sai University of Technology

Niladri Sarkar is currently working towards his PhD. His research interests include polymer-based nanocomposites and carbon dots.

Fanismita Mohanty
Veer Surendra Sai University of Technology

Fanismita Mohanty is a PhD student. Her research interests include polymer-based and graphene-based nanocomposites.


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DOI:  10.2417/spepro.006917