Sandwich panel biocomposite of thermoplastic corn starch and bacterial cellulose

Highlights

• Native corn starch has been transformed into thermoplastic corn starch.

• Sandwich panel biocomposite of thermoplastic corn starch and bacterial cellulose were obtained

• Thermoplastic corn starch and bacterial cellulose biocomposite showed good interfacial adhesion

• Biocomposite of TPS and bacterial cellulose showed 3.6-fold tensile strength than TPS alone

Abstract

Inadequate disposition and long period for degradation of Petroleum-derived polymers promote damages in the environment, which could be minimized by the use of biodegradable polymers such as starch and cellulose. Films of thermoplastic corn starch (TPS) and bacterial cellulose (BC) were used to produce sandwich panel biocomposite. RXD, SEM and FTIR were used to verify the transformation of TPS from native corn starch. TPS/BC is flexible and transparent, but it is less transparent that TPS and BC due to its multilayer format. TPS/BC presented similar thermal events to TPS and BC samples and thermal stability similar to TPS. The FTIR spectrum of the TPS/BC showed bands observed in the BC and TPS spectra. BC, TPS and TPS/BC showed faster water absorption in the initial stage reaching a stability at about 50 h and presenting Fickian behavior. TPS/BC showed lower water absorption and a good adhesion between the phases observed by SEM images, which can be associated to hydrogen interactions in the interface improving mechanical properties. TPS/BC showed an increase of about 3.6 times in the tensile strength compared to TPS, indicating that BC is a good reinforcement for TPS.

Introduction

Petroleum-derived polymers are used in different applications due to their low cost and their satisfactory mechanical and thermal barrier properties [1,2]. However, the large global production (about 400 million metric tons in 2015) [2], the inadequate disposition and long periods for waste degradation results in environmental damage [1]. Biodegradable polymers as starch and cellulose could minimize this environmental damage.

Native starch is a polysaccharide constituted of about 98% of amylose and amylopectin and is extracted from cereals, rhizomes, roots our tubers, such as potato, cassava, corn, pea and others [[3], [4], [5], [6], [7], [8]].

Native starch has disadvantageous properties for processing and use, because its melting point (200 °C) is above its degradation temperature (130 °C) [7]. Therefore, in order to be feasible for use and processing it must undergo a process of conformational transformation, called gelatinization that results in the formation of thermoplastic starch (TPS) [1,2,8].

In the presence of plasticizers, such as water and glycerol, in the temperatures between 90 and 130 °C and shear, occurs the gelatinization that is a non-balancing process of the native starch [[8], [9], [10], [11]]. During this process occurs the destructuring of the granule affecting the original crystalline structure of amylose and amylopectin resulting in TPS formation with a lower crystallinity index (CI) and less stiffness than native starch [2,[11], [12], [13]].

The use of TPS is advantageous due to its low cost, flexibility, transparence, and abundance in the nature and mainly due to its biodegradable and renewable characteristic [14]. However, TPS presents poor mechanical properties (tensile strength from 0.2 to 5.8 MPa and Young Modulus's from 0.01 to 1 GPa) which are lower than Polylactic acid (PLA) or polycaprolactone (PCL) [15]. Besides, TPS is sensitive to absorption of water.

Cellulose is the most abundant renewable biopolymer in nature and can be used as a reinforcing material to increase the mechanical properties of TPS producing a biphasic composite [[16], [17], [18], [19], [20]]. The extraction of cellulose can be carried out from lignocellulosic fibers, agroindustrial residues, algae, tunicates or produced by bacteria [[19], [20], [21], [22], [23], [24]].

Bacterial cellulose (BC) can be synthesized by bacteria of various genera like Acetobacter, Gluconacetobacter and Komagataeibacter, in a bioreactor with temperature from 28 to 32 °C, culture medium e pH from 4 to 7 [[20], [21], [22], [23], [24]]. The BC has high chemical purity (doesn't have hemicellulose, lignin and pigments in the composition), high crystallinity (crystallinity index above 85%), superior tensile strength, and shorter production period (between 5 and 25 days) when compared to microstructured cellulose [[20], [21], [22], [23], [24]]. The network morphology of BC is constituted by nanofibrils with diameters between 4 and 7 nm and degree of polymerization from 2000 to 14,000 glucose molecules, which are compressed and taped crystallized, forming a reticular structure stabilized by interactions of hydrogen, and this morphology gives BC high values of mechanical properties [25,26]. In addition to its satisfactory mechanical properties, other advantages in the use of BC as a reinforcement for TPS are its renewable and biodegradable character [27].

Several papers in the literature describe TPS/BC biocomposites obtained using different techniques such as in situ [[28], [29], [30], [31]], by the solution impregnation method [32] and processed by mixer and injection [33].

Grande et al. [28] prepared biocomposite of TPS/BC adding 2.0% (w/v) of potato starch and corn starch in the BC culture medium by 21 days, and then sheet of the samples were hot-pressed. The authors initially studied the morphology of the BC biocomposite and characterized mesh size and fiber orientation. In a further step, the authors [29] observed the presence of partially gelatinized starch granules and the preservation of typical network of BC. The volume fraction of the strong phase in the biocomposite produced by this technique was around 90%. The crystallinity of the TPS/BC biocomposite was maintained in spite of the presence of starch. The mechanical properties of the TPS/BC showed no significant decreasing as a function of TPS addition.

Osorio et al. [30] produced TPS/BC biocomposite in situ using 4 wt% of potato starch, 2 wt% of glycerol, 0.24 wt% of citric acid and 0.12 wt% of sodium dihydrogen phosphate as crosslink agent by 7, 10 and 13 days. After, the films were removed from the culture medium, transferred to an oven at 50 °C/48 h, and then they were hot pressed. The TPS/BC biocomposites exhibited a strong interfacial adhesion, higher thermal stability and improved the mechanical behavior compared to TPS.

Yang et al. [31] also prepared in situ TPS/BC biocomposites adding 0.5, 1.0, 1.5, 2.0 and 4.0 wt% of potato starch to the culture medium to use this material as scaffold. The authors verified that compared to BC, a locally oriented surface morphology was observed when the content of TPS was above 1.0 wt%. A cell ingrowth tendency was observed on the porous surface of TPS/BC biocomposites as the starch content increased.

Grande et al. [29] and Yang et al. [31] used a low starch content (until 2 wt%) to prepare TPS/BC biocomposites resulting in high values of tensile stress and modulus and low value of elongation at break that are characteristics of the BC phase.

Wan et al. [32] prepared TPS/BC biocomposites by the solution impregnation method using solutions of starch and glycerol stirred at 80 °C for 30 min. After air-dried BC sheets were immersed in the solutions, and 7.8, 15.1 and 22.0 wt% of BC were impregnated in the sheets. The tensile strength and tensile modulus of all TPS/BC biocomposites increased, indicating effectiveness of the reinforcement. However, a decrease in elongation at break was observed for all composites when compared to TPS.

Martins et al. [33] prepared biocomposite by mixer and injection molding using cornstarch by adding glycerol/water as the plasticizer and BC (1 and 5 wt%) as the reinforcing agent. The authors concluded that the addition of 5 wt% of BC in the TPS/BC biocomposite promotes an efficiently reinforcement (increasing both modulus and tensile strength), but displayed a strong sensitivity to high relative humidity.

Sandwich panels are structures designed to show good mechanical performance. Then, the study of a biocomposite in the sandwich panel model is interesting to verify their properties and the reinforcement effect of the BC on the TPS matrix. Besides, the use of conventional processing methods for transformation of polymers, like extrusion and compression molding, is scarce in the literature to prepare TPS/BC biocomposites. In this context, films of TPS and BC were used to produce TPS/BC multiphase sandwich panel biocomposite, which were characterized for transparency, thickness, density, moisture content, water uptake, transparency, XRD, FTIR, morphological, thermal and mechanical properties (tensile test).