Properties of baked foams based on cassava starch, sugarcane bagasse fibers and montmorillonite
Highlights
► Biodegradable trays produced by a baking process from starch, sugarcane fibers and nanoclays. ► Good nanoclays dispersion resulting in an exfoliated structure. ► Fiber and nanoclays addition: trays with lower density and stress at break and higher strain at break. ► Trays with higher water absorption capacities. ► Further development to improve the expansion, water sorption capacity and processing times.
Introduction
The development of biodegradable packaging based on starch has attracted an increasing amount of attention; however, materials produced from this biopolymer have some problems, including poor mechanical properties and hydrophilicity. Water solubility increases the degradability and the speed of degradation; however, moisture sensitivity also limits the applications of the material. The use of composites and nanocomposites from these materials can also aid in the development of new low-cost products with better performances (Yu, Dean, & Li, 2006).
Fiber-reinforced composites have been studied in various applications and reviewed by many authors because they have excellent specific properties, such as high strength, low weight and good barrier properties. In that respect, natural fibers are generally interesting because they not only have the functional capability to substitute for the widely used glass fibers but also have advantages from the point of view of weight and fiber–matrix adhesion, specifically with polar matrix materials, such as biopolymeric matrices. These agro-based materials are abundant in nature and frequently are wastes from various industrial processes. For example, sugarcane bagasse fiber, which is a poorly valorized waste residue from the sugar and alcohol industries in Brazil, is often used as fuels in households or is sometimes burned in the fields as a means of disposal. Sugarcane bagasse fiber consists of about 40–50% cellulose (Satyanarayana et al., 2009, Sun et al., 2004).
Nanocomposites are systems that contain fillers with at least one nanosized dimension and represent a new class of materials that exhibit improved mechanical, thermal, barrier and physicochemical properties compared with the starting polymers and conventional (microscale) composites. Although several nanoparticles have been recognized as possible additives to enhance polymer performance, most intensive studies are currently focused on layered silicates, such as montmorillonite (MMT), due to their availability, versatility, low cost and respectability towards the environment and health (Azeredo, 2009).
The montmorillonite crystal lattice consists of 1-nm thin layers with an octahedral alumina sheet sandwiched between two tetrahedral silica sheets. The layers are negatively charged, and this charge is balanced by alkali cations, such as Na+, Li+ or Ca2+, in the gallery space between the aluminosilicate layers. Na-montmorillonite (Na-MMT) clay is hydrophilic with a high surface area and is miscible with hydrophilic polymers, such as starch (Ardakani et al., 2010, Ray and Okamoto, 2003). The properties of the resulting material are dependent on the state of the nanoclay in the nanocomposite, i.e., if it is exfoliate or intercalate. Intercalation is the state in which polymer chains are present between the clay layers, resulting in a multilayered structure with alternating polymer/inorganic layers. Exfoliation is the state in which the silicate layers are completely separated and dispersed in a continuous polymer matrix (Weiss, Takhistov, & McClements, 2006).
Some patents have reported that the introduction of fibers and/or inorganic fillers is interesting to improve mechanical properties of starch materials (Andersen et al., 1998, Andersen and Hodson, 1995, Andersen and Hodson, 2001). Other studies have shown that is possible to obtain food packaging from mixtures of starch, fibers, water and other additives by thermopressing or baking (Carr et al., 2006, Hofmann et al., 1998, Schmidt and Laurindo, 2010), and these products could be an alternative to the use of expanded polystyrene foams. The foam baking process includes two steps: starch gelatinization and water evaporation, which expands the mixture and forms a foam, and foam dehydration until a final moisture content of 2–4% is obtained (Shogren, Lawton, Doanne, & Tiefenbacher, 1998). Thus, the objectives of this work were to investigate the use of a baking process to prepare composite and nanocomposite trays based on cassava starch, sugarcane bagasse fiber and Na-montmorillonite and then to study the effect of these components on the microstructure and physicochemical and mechanical properties of the trays.
Section snippets
Materials
Cassava starch (19% amylose) was provided by Hiraki Industry (São Paulo, Brazil). Sugarcane bagasse was provided by regional ethanol producers and then was washed, dried and milled to yield particles < 0.70 mm. Na-montmorillonite (Closite® Na+) was purchased from Southern Clay Products (USA) and was used as received. Glycerol, magnesium stearate and guar gum were purchased from Synth (Labsynth, São Paulo, Brazil).
Tray manufacturing by baking
The starch trays were manufactured using different formulations on the basis of
Baking process
Limited concentrations of each component (starch, fiber and Na-MMT) (Table 1) were chosen on the basis of previous results. Magnesium stearate was added to prevent the starch foam sticking to the mold, guar gum was added to prevent solid separation (Salgado et al., 2008, Shogren et al., 1998) and glycerol was added as a plasticizer.
The volume of water added to each formulation was directly related to the fiber and Na-MMT contents. It can be observed that the amount of water added to the mold
Conclusions
In this study, baking raw materials that are economically important in South America, such as cassava starch and sugarcane bagasse fiber, produced well-shaped biodegradable trays. The fiber and Na-MMT interfere with foaming, so additional amounts of water and batter are needed to form complete trays.
The addition of fiber and Na-MMT decreased the density and stress at break and increased the strain at break values of the starch foams. The foams manufactured with fiber and Na-MMT presented more
Acknowledgment
The authors wish to thank the Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq-No. 577146/2008-4) of Brazil for financial support.
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