New biocomposites based on thermoplastic starch and bacterial cellulose

New biocomposites based on thermoplastic starch and bacterial cellulose

(Parte 1 de 2)

New biocomposites based on thermoplastic starch and bacterial cellulose

Ivo M.G. Martins, Sandra P. Magina, Lúcia Oliveira, Carmen S.R. Freire*, Armando J.D. Silvestre, Carlos Pascoal Neto, Alessandro Gandini

CICECO and Department of Chemistry, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal article info

Article history: Received 23 March 2009 Received in revised form 20 May 2009 Accepted 24 May 2009 Available online 29 May 2009

Keywords: Bacterial cellulose and thermoplastic starch B. Mechanical properties D. Dynamic mechanical thermal analysis (DMTA) D. Scanning electron microscopy (SEM) D. Thermogravimetric analysis (TGA) abstra ct

Bacterial cellulose, produced by Acetobacter Xylinum, was used as reinforcement in composite materials with a starch thermoplastic matrix. The composites were prepared in a single step with cornstarch by adding glycerol/water as the plasticizer and bacterial cellulose (1% and 5% w/w) as the reinforcing agent. Vegetable cellulose was also tested as reinforcement for comparison purposes. These materials were characterized by different techniques, namely TGA, XRD, DMA, tensile tests, SEM and water sorption assays. All composites showed good dispersion of the fibers and a strong adhesion between the fibers and the matrix. The composites prepared with bacterial cellulose displayed better mechanical properties than those with vegetable cellulose fibers. The Young modulus increased by 30 and 17 fold (with 5% fibers), while the elongation at break was reduced from 144% to 24% and 48% with increasing fiber content, respectively for composites with bacterial and vegetable cellulose. 2009 Elsevier Ltd. All rights reserved.

1. Introduction

New functional materials obtained from renewable resources have gained much attention in the last decades due to the global increasing demand for alternatives to fossil resources [1]. For example, composites based on thermoplastic polymers and natural fibers are very attractive materials because of their good mechanical properties, sustainability and environmental-friendly connotation [2]. These materials have been widely used in the automotive and packaging industries [2] and their applications in other areas are being actively sought.

The use of natural fibers as reinforcing elements in composite materials presents important advantages, when compared with their synthetic or inorganic counterparts, namely biodegradability, high availability, low cost, low energy consumption, low density, high specific strength and modulus (with fibers possessing an adequate aspect ratio), high sound attenuation and comparatively easy processing ability due to their flexibility and non-abrasive nature [3].

In addition, if the natural fibers are combined with biodegradable matrices, the obtained composite materials are expected to be fully biodegradable. The development of biocomposites gained particular relevance with the increasing availability and diversity of biodegradable polymers [4].

Starch is one of the most abundant natural polymers and is considered as a promising raw material for the development of novel materials, including biocomposites. It can be converted into a thermoplastic material, known as thermoplastic starch (TPS), through the disruption of the molecular chain interactions under specific conditions, in the presence of a plasticizer. Water and glycerol are the most widely used plasticizers in the production of TPS. When natural fibers are mixed with TPS, its mechanical properties are improved, indicating a good adhesion between the reinforcing fibers and the matrix [4,5]. Several studies have been published dealing with the preparation and characterization of TPS-based composites with different cellulose substrates, namely commercial regenerated cellulose fibers [6], vegetable fibers [6–8], microcrystalline cellulose [9], microfibrillated cellulose [3] and cellulose nanocrystallites [5]. Apart from the enhanced mechanical properties of these reinforced TPS materials, a significant improvement in water resistance is also obtained by adding cellulose crystallites [10] or microfibrillated cellulose [3]. In general, these TPS/fiber composites also displayed improved thermal stability due to the higher thermal resistance of cellulose fibers [7].

Owing to its unique properties, such as high mechanical strength, high crystallinity and a highly pure nanofibrillar network structure, bacterial cellulose, produced by Acetobacter Xylinum,i s becoming a promising biopolymer for several applications. For example, due to its biocompatibility [1], bacterial cellulose has been extensively investigated in the biomedical field [12–14]. Other applications include its use as component for audio membranes [15], electronic paper [16], optically transparent composites

0266-3538/$ - see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.05.012

* Corresponding author. Tel.: +351 234 401 405; fax: +351 234 370084. E-mail address: cfreire@dq.ua.pt (C.S.R. Freire).

Composites Science and Technology 69 (2009) 2163–2168 Contents lists available at ScienceDirect

Composites Science and Technology jou rnal hom ep ag e: w. el sevi er .com /l ocate /co mpsci tech

[17], reinforcing agent for paper [18] and other polymeric materials and composites [19], among others.

However, to the best of our knowledge, only two studies dealing with the preparation and characterization of bacterial cellulosestarch composites have been published so far [20,21]. Grande et al. obtained a plasticized starch/bacterial cellulose nanocomposite sheets by hot-pressing. The ensuing films were characterized in terms of their morphology, whereas other important parameters, such as their mechanical and thermal properties, were not investigated. The incorporation of bacterial cellulose microfibrils, obtained by the acid hydrolysis of the cellulose network, into extruded TPS and starch-pectin blends was also studied [21]. However, in this work, the peculiarity of this cellulose substrate was not fully exploited, since the nano- and micro-fibril three-dimensional network morphology was partially destroyed during the hydrolysis step. Indeed, the microfibrils derived from bacterial cellulose did not improve the modulus to the same extent as cotton or softwood counterparts.

The present paper describes the preparation and characterization of biocomposite materials obtained by the incorporation of bacterial cellulose into a TPS matrix during the gelatinization process. Similar composite materials reinforced with vegetable cellulose fibers were also prepared for comparison purposes.

2. Experimental 2.1. Materials

Bacterial cellulose (BC) (a tridimensional network of nano and microfibrils with 10–100 nm width), in the shredded wet form (95% of humidity), was supplied by Forschungszentrum für

m/mi(%)m/mi(%) m/mi(%) m/mi(%)

Temperature (ºC) TPST= 327 ºC

Temperature (ºC)

VCT= 376 ºC

Temperature (ºC)

TPS/VC 5%T= 336 ºC

BCT= 337 ºC

TPS/BC 5% T= 332 ºC

Fig. 1. Thermogravimetric curves for TPS, BC, VC, TPS/BC 5% and TPS/VC 5% composites.

2164 I.M.G. Martins et al./Composites Science and Technology 69 (2009) 2163–2168

Medizintechnik und Biotechnologie e.V. (Germany). The vegetable cellulose fibers (VC) (with 0.8–1 m length and 20–30 lm width) used in this work were from a Eucalyptus globulus ECF (DEDED) industrial bleached kraft pulp, kindly provided by a Portuguese pulp mill.

Corn starch (27% amylose), glycerol (9.5%), stearic acid (95%), magnesium chloride hexahydrate (>9%), magnesium nitrate (purum, >9%), and barium chloride (dehydrated, >9.9%) were supplied by Sigma–Aldrich and used as received.

2.2. Biocomposites preparation

Both vegetable and bacterial cellulose fibers were first dispersed in water in order to avoid the formation of agglomerates. The excess of water was then removed by vacuum filtration until the desired content required for the plasticization step (25 wt.% in respect to the total amount of the mixture components, namely starch, glycerol and water) was reached. However, in the case of bacterial cellulose, due to its natural high water content and morphology, it was not possible to reduce the water content to that level, particularly for 5% of BC. In this case the lowest water content achieved was about 50%.

Starch (45%), glycerol (30%) and the wet fibers (either VC or BC) (1% or 5%, in dry basis) were then pre-mixed in polyethylene bags until a homogeneous mixture was obtained. Stearic acid (demoulding agent) was also added, at low concentration (0.5 wt.%). TPS samples without fibers were prepared likewise, but adding 25 wt.% of water.

The unfilled thermoplastic starch and the corresponding composites were prepared using a Plastograph EC (Brabender) mixer.

The mixtures were loaded into the mixer chamber pre-heated at 120 C and processed at 60 rpm during 6 min, for the unfilled TPS and VC fiber-based composites, and 20–30 min for the BC composites, to allow for the release of the moisture excess, in order to reach comparable water contents of the final composites (about 10% and 4% (w/w) for composites with a fiber content of 1% and 5%, respectively).

The composites were then molded in an injection molding machine (Thermo-Haake Minijet I) for tensile (bar according to ISO 527-2-5A) and DMA analysis.

Before characterization, all samples were kept in a conditioning cabinet of 50% relative humidity (RH) at 25 C to ensure the stabilization of their water content.

2.3. Biocomposites characterization

The composites were characterized using thermogravimetry (TGA), X-ray diffraction (XRD), tensile tests, dynamical mechanical

TPS/VC 5% VC

Starch

Fig. 2. The X-ray diffractograms of Starch, TPS, VC, BC, and TPS composites with 5% of VC (TPS/VC 5%) and with 5% of BC (TPS/BC 5%).

TPSVC 1%VC 5%BC 1%BC 5% Young Modulus (MPa)

TPSVC 1%VC 5%BC 1%BC 5% Tensile Strength (MPa)

TPSVC 1%VC 5%BC 1%BC 5% Elongation at break (%)

Fig. 3. Young modulus, tensile strength and elongation at break of corn starch thermoplastic composites reinforced with 1% and 5% of bacterial and vegetable cellulose.

I.M.G. Martins et al./Composites Science and Technology 69 (2009) 2163–2168 2165 analysis (DMA), scanning electron microscopy (SEM) and water absorption tests.

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