Tensile properties of cellulose acetate butyrate composites reinforced with bacterial cellulose

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Abstract

Composites of cellulose acetate butyrate reinforced with cellulose sheets synthesised by Gluconacetobacter xylinus were produced by solvent evaporation casting. The composites contained 10% and 32% volume cellulose, and showed a Young’s modulus of 3.2 and 5.8 GPa, and a strength of 52.6 and 128.9 MPa, respectively, in tensile tests. Stress–strain curves showed bi-phasic material characteristics, with an initial linear behaviour, followed by yielding, and a second linear phase until fracture. Cyclic tensile loading–unloading experiments at incremental strain revealed an increasing elastic modulus after each straining step. A simple analytical model demonstrated that the observed increase of the elastic modulus can be explained by reorientation of the initially random oriented cellulose fibrils due to straining.

Introduction

Due to advantages such as low density, renewability, biodegradability, and neutral CO2 balance, the use of cellulosic fibres as reinforcements in polymer composites has been growing over the last years [1], [2]. While this statement refers mainly to plant fibres, similar advantages also apply to microbial cellulose, with the important exception of a neutral CO2 balance. Microbial cellulose as synthesised by Gluconacetobacter xylinus consists of elementary fibrils with an average cross-section of 16 × 58 Å [3]. Typically 46 such elementary fibrils make up a flat, ribbon like microfibril [3]. Small angle X-ray scattering revealed microfibril dimensions of 10 × 160 Å in cross-section, whereas ribbons up to 500 Å wide observed in the ESEM are considered aggregates of several microfibrils [4]. Microfibril diameters of 24–86 nm [5], and 72–175 nm [6] are also reported in the literature. Presumably due to uninterrupted cellulose synthesis during cell division, the cellulose microfibrils in bacterial cellulose films are branched and their length has been estimated to several 100 μm [7]. With 60–80%, a large part of bacterial cellulose is arranged in crystallites [6].

The most substantial use of bacterial cellulose is currently in the foodstuff industry [7], but bacterial cellulose shows considerable potential for other technical applications. Medical usage of bacterial cellulose comprises its use as temporary substitute for human and animal skin [8] and artificial blood vessels for microsurgery [9]. Furthermore, bacterial cellulose membranes were utilised in combination with palladium to produce experimental fuel cells for the generation of electricity [10]. Bacterial cellulose was also added to the production of paper, resulting in considerably improved mechanical properties [11]. Finally, synthesised in the presence of pectin and/or xyloglucan, [4], [5], [12], [13], bacterial cellulose serves as model system for the study of the interaction of polymers during the formation of plant cell walls.

Estimated values for the potential stiffness of the crystallite parallel to the chain direction in an elementary cellulose I fibril, which vary, for example, from 128 GPa [14] to 167.5 GPa [15], are currently not fully exploited in cellulosic composites. Owing to the lower stiffness of other cell wall constituents such as lignin and hemicellulose (2 and 7 GPa, respectively [16], [17]), and due to the tilt angle of cellulose fibrils in the plant cell wall [18], [19], the stiffness of plant fibres is only in the range of 20–40 GPa [2]. The stiffness and strength of plant fibres present a natural threshold for plant fibre reinforced composites, which may be surpassed only when the matrix polymer is able to bond directly to the microfibril, instead of the composite plant fibre cell wall. Techniques such as microfibrillation, where the cell wall is further disintegrated by a mechanical process, led to a substantial increase in the strength of cellulose fibre reinforced composites [20], [21]. In combination with a suitable matrix polymer, bacterial cellulose networks show a considerable potential as reinforcement for high quality speciality applications of bio-based composites, because the small dimensions of bacterial cellulose fibrils enable direct contact between cellulose and matrix polymers, allowing for a large contact surface and thus excellent adhesion.

In the present study, bacterial cellulose is applied to the reinforcement of cellulose acetate butyrate composite sheets, in order to investigate the potential of a bacterial cellulose network as reinforcement in polymer composites, and its usefulness as a model system for cellulose fibre reinforced composites in general.

Section snippets

Preparation of composite sheets

A freeze dried sample of G. xylinus ssp. xylinus (DSM 2004) was obtained from DSMZ – Braunschweig, Germany. Culture was carried out in standing vessels for two weeks using a medium suggested by Iguchi et al. [7]. Without any purification, the cellulose sheets measuring 17 × 17 cm2 were first dehydrated by gently compressing them between household cellulose tissue and the remaining water was subsequently removed by solvent exchange in 60% and 96% ethanol and finally twice in acetone. Cellulose

Results

Table 1 summarises the composition of the composite sheets produced in this study, assuming a density of 1.5 g/cm3 for cellulose, and Table 2 reports mechanical properties determined in tensile tests. Stress–strain graphs given in Fig. 1 show a bi-phasic behaviour of the composites. An initial phase of essentially Hookean behaviour is followed by yielding at a strain of 0.8% for composite A, and 0.6% for composite B, respectively, and at strains ⩾2%, the material behaviour is again linear. As

Discussion and modelling

The strength and stiffness of composites A and B produced in this study is clearly below values of 15–18 GPa for the Young’s modulus and up to 260 MPa for tensile strength, respectively, measured with dried pure cellulose films [7], [11]. However, the elongation at break, which is between 0.8% and 2.1% in dried bacterial cellulose films [7], [11], increased to 3.5% in the composites produced in this study.

Composites A and B showed bi-phasic stress–strain curves (Fig. 1). Bi-phasic behaviour,

Acknowledgement

This paper was written while the author was a visiting scientist at the Centre for Biomimetics, The University of Reading, funded by the Austrian Science Fund FWF, Erwin Schrödinger Auslandsstipendium J2189. The author wishes to thank G. Jeronimidis for fruitful discussion and J.-P. Touzel, INRA Reims for the inspiration to the present work and for providing first samples of bacterial cellulose.

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