Plasticized starch–cellulose interactions in polysaccharide composites
Introduction
In recent years, great progress was achieved in the development of biodegradable products on the basis of agricultural materials [1], [2]. Different approaches have been made to use starch for the production of tailored materials [3]. Native starch is transformed to obtain easy processed starch [4], [5], [6]. The so-called ‘thermoplastic starch’ (TPS) or plasticized starch is obtained after disruption and plastificization of native starch, with water and plasticizer. Unfortunately, properties of such a product do not fulfil all requirements in some applications such as packaging. To improve the properties, research laboratories have developed two strategies: chain modifications (e.g. acetylation) [7], and starch-based multiphased products. Compostable multilayers [8], [9] or blends [10], [11], [12], [13], [14] have been developed by different associations between TPS and biodegradable polymers which are mainly biodegradable polyesters. These blends present quite good water resistance but mechanical properties are rather poor. TPS–polyester compatibility is more or less weak [12].
To preserve renewability (renewable raw materials), biodegradability and to improve the mechanical resistance of the final products, associations between cellulose fibres and TPS have to be tested. Several studies [15], [16], [17], [18], [19], [20], [21], [22] and applications (e.g. automotive market) have demonstrated the interest of using cellulose fibres as reinforcement in thermoplastic matrixes but only few papers are focused on polysaccharide-based composites. Some authors [16], [17], [18], [19], [20], [21], [22] have shown that cellulose fibres or microfibrils in a TPS matrix improve the tensile strength. According to Frunke et al. (1998) [22], a significant improvement of water resistance is achieved by adding small amounts of commercial cellulose fibres (till 15%). Also, Dufresne et al. (1998, 2000) [19], [20] show an improvement of water resistance by reinforcing plasticized starch films with cellulose microfibrils. This behaviour is related to the hydrophobic character of the cellulose fibres in comparison to starch hydrophilic property. Besides, these authors [19] show an improved thermal stability due to a higher and longer rubbery plateau.
The aim of this work is to test the addition of cellulose fibres in a TPS matrix and to report the subsequent properties. This paper is more particularly focused on the interaction between the fibres and the matrix. Various biodegradable composites have been elaborated with a soft TPS matrix and different fibre lengths and contents. Besides, we have used LDPE-based composites as reference materials, where the fibre–matrix interactions may be considered as poor since no specific compatibilizer are added [15], [16], [17], [18]. LDPE grade has been chosen to show at room temperature, the same stiffness range (≈100 MPa) than the TPS matrix (Table 1). Mechanical properties (tensile, impact tests), thermomechanical (DMTA) and thermal (DSC) behaviours are evaluated. The microstructure is also investigated through SEM analysis.
Section snippets
Composites preparation
Natural cellulose fibres from leafwood are obtained from JRS (Arbocel, Germany). Different cellulose fibres with increasing lengths are tested. For a constant average diameter of 20 μm, initial average lengths are, respectively, 60 (SF), 300 (MF) and 900 (LF) microns. Initial shape ratios are, respectively, 3, 15 and 45. Residue on ignition at 850°C during 4 h is less than 0.3%. In a previous paper [18], Amash and Zugenmaier have presented some characteristics of these fibres. According to these
Results and discussion
For pure TPS, glycerol and water contents are given in Table 2. The evolution of glycerol/starch ratio gives the loss in glycerol. During processing stages, none or little glycerol is lost by volatilisation. In the extruder, to obtain the disruption of the native starch and to decrease the melt viscosity, a certain initial level of water is needed. Final moisture content is adjusted by the storage conditions. In this case, after processing and equilibration at 54%RH and 23°C, moisture content
Conclusion
Plasticized starch composites are produced by introduction of leafwood cellulose fibres into a matrix obtained from water/glycerol plasticization of wheat starch. Different fibre lengths and contents are used. Properties of such composites are compared to a LDPE composites where fibre–matrix interactions are known to be very poor. We have shown an increase of the main transition temperature (DMTA) of about 30°C by the introduction of fibres. This phenomena could be related to fibre–matrix
Acknowledgements
This work is funded by Europol'Agro through a research program devoted to development of packaging materials based on agricultural resources. The authors want to thank Anne-Lise Daltin (Rheims University, France) for SEM examinations. Besides, we want to thank Atochem (France) for supplying polyethylene powder.
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