Studies of the effect of molding pressure and incorporation of sugarcane bagasse fibers on the structure and properties of poly (hydroxy butyrate)

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Abstract

Poly (hydroxy butyrate) (PHB) is a biodegradable polymer that can be obtained from both renewable and synthetic resources. There have been many attempts to improve its structure and properties by different methods. This paper, while mentioning briefly PHB and sugarcane bagasse fibers, focuses on the effect of compressive/molding pressure on its structure and thermal properties with incorporation of sugarcane bagasse fibers, with and without steam explosion treatment. Thermal behavior (thermogravimetry and dynamic mechanical analysis), X-ray diffraction, Fourier transform infrared spectroscopy, optical microscopy methods were used to understand the changes in PHB resulting from the pressure and incorporation of fibers while scanning electron microscopy is used to understand the morphology of both the fiber and PHB.

Introduction

Sustainability and ecological safety through the use of natural materials has been a “buzzword” in recent years. It is therefore not surprising that many countries, particularly in Europe, have directives for using 95% recyclable materials in all new automobiles to achieve the “end of life” of vehicles required by 2015. Also, the European Parliament enacted a rule recently calling for a recyclable rate of more than 80% for automotive materials [1], [2]. Even the possibility of using composites based on such materials for structural members in automobiles is reported [3]. Poly (hydroxyalkanoates) (PHAs) are one such class of materials, which not only save fossil resources, but also recycle materials from renewable agricultural sources or through micro-organisms to get biodegradable materials. They belong to a family of natural polyesters having a structure with the same three-carbon backbones and differing alkyl groups at β or 3 positions [4]. Such materials become more attractive when their mechanical properties compare favorably with those of commercial polymers. This is well illustrated by the estimate that consumption of biopolymers will grow to approximately 100 million metric tons/year by 2020 in Europe, while the perspective for 2008 also shows a growing market for biodegradable polymers [4].

Poly (hydroxy butyrate) (PHB), discovered in 1926 by Lemogine using B. megaterium, belongs to this family of biodegradable polymers. A recent review gives more details on the 75 years of research since then on this important natural polymer [4]. Since this type of polymer is more expensive than synthetic polymers, different strategies, including the use of better bacterial strains and low-cost renewable resources, have been pursued to improve its various properties to enable its use in many applications, the details of which are reported elsewhere [5]. In Brazil, it is produced by bacterial fermentation in an integrated sugarcane mill and has been commercially available since 2000 (50 tons/year at a cost of US$ 3–5/kg). This increased to the estimated production of 4000 tons in 2006 [6]. Even simulation studies have been attempted to increase the production of PHB [7]. Some of the commonly available trade names of PHB, along with the countries where they are produced are shown in Table 1.

PHB has many attractive properties, such as low viscosity and biocompatibility, which makes it suitable for use in industrial applications even when objects of less than a gram are needed. However, the price of PHB is still high and hence research in this area is accelerating to develop new methods to obtain PHB as a viable thermoplastic [8], because when produced in large quantities, its price can decrease significantly and allow more extensive use [9]. Some of the properties of PHB reported [10] include tensile strength: 43 MPa, Young’s modulus: 3.5 GPa (which are close to those of isostatic polypropylene), and % elongation of 5.5 (lower by ∼400% than polypropylene).

Attempts to improve the properties of PHB include the use of rotors with different processing conditions for mixing, use of different strains and sources [6], [11], [12], [13], blending with other biodegradable polymers [6], [10], [14], [15], and incorporation of different types of fibers [6], [16], [17], [18], [19]. Also, attempts have been made to improve its structure through the application of stress [20]. The potential of these biopolymers and their applications are explained elsewhere [6], [11]. Also, a few studies, including molecular modeling of their structure and properties, are available [7], [8], [13], [20], [21], [22], [23], [24], [25], [26].

Brazil has abundant natural fiber resources, such as sugarcane bagasse, much of which is wasted. For example, Brazil is the world’s largest producer of sugarcane [27], and increased its growth from 326,121 thousand tons in 2000 to 457,984 thousand tons in 2006 [28]. About 60% of this is normally used to make alcohol [29]. Several countries (such as Japan and the United States) are considering increasing the use of ethanol to fuel cars, and Brazil, with its favorable climate, large areas for agricultural expansion and experience in sugarcane-to-ethanol technology, is ideally suited to supply this demand. Hence, the use of its sugarcane residue will become an increasing national need.

Brazil currently produces about 101 Mt of bagasse [30]. Though bagasse is largely used for fuel in all its producing countries, one novel application is in developing composites with both polymer [31], [32] and ceramic matrices. Limited studies reported on bagasse include its (i) characterization, (ii) applications and (iii) the preparation and description of its composites [6], [18], [32], [33]. Among all the reports on its composites, the one with 20 wt.% raw bagasse fibers incorporated in biodegradable aliphatic polyester had tensile strength of 16.52 MPa, flexural strength of 31.19 MPa, flexural modulus of 1.14 GPa and impact strength of 4.12 kJ m−2 [33]. Furthermore, increases of these properties by 12%, 10%, 16% and 47%, respectively, with alkali treatments, and further increases by 42%, 41%, 101% and 114% when the fiber content was increased to 65%, have been reported. Another study reported [18] tensile strength of bagasse fibers (length 25 mm and diameter ∼0.1–0.3 mm) ranged between 84.3 and 87 MPa for raw and washed fibers; while the composite containing these fibers (30 wt.%) exhibited flexural strength (FS) of ∼50–52 MPa and flexural modulus (FM) of 3.5 GPa compared to ∼45 MPa(FS) and 2.5 GPa(FM) for the unreinforced PHB. They also reported about a 50% increase in these properties over the matrix, when it was reinforced with washed and acetone-treated chopped fibers.

The foregoing shows there is need for further research on the utilization of PHB and abundantly available natural fibers of Brazil through adoption of different methodologies. Very limited work has been done in the country on biodegradable composites [34] using such resources, including PHB and sugarcane bagasse for composites [32]. Similarly, limited work has been done on the effect of pressure on the structure and properties of polymers including PHB. In fact, those works reported are mostly concerned with composites [35], [36], [37], [38]. Therefore, the purpose of this study, which is part of a systematic program on Brazilian lignocellulosic fibers, is to describe the effect of pressure and incorporation of sugarcane bagasse, with and without surface treatment, on the structure and properties of locally produced PHB. Fiber subjected to steam explosion (SE) treatment was used to understand the effect of surface treatment on the fiber.

Section snippets

Materials

An industrial PHB called “Biocycle”, produced by Serrana S/A was used. It is a white powder of high purity (>99.5%) with molecular weight of 600,000 g mol−1 (by GPC), density of 1220 kg m−3 (ASTM D792) and moisture below 0.3%. The sugarcane bagasse (hereafter called as ‘bagasse’) fibers were obtained from a nearby alcohol distillery in Curitiba, in southern Brazil. They were conditioned before subjecting them to steam pretreatment by hot-water washing to remove all the residual sucrose, and then

Fourier transform infrared spectroscopy (FTIR)

Fig. 1 shows the FTIR spectrum of the raw PHB. An intense broad band can be seen between 3640 and 3100 cm−1 arising from the OH stretching modes. From 3042 to 2875 cm−1, the stretching modes for the CH3 and CH2 groups are visible, along with the band between 1350 and 1457 cm−1. The ester function is detected by the Cdouble bondO stretching, which is located at 1750 cm−1, while the Csingle bondO stretching can be seen at 1290 and 1060 cm−1. Along with these, some other bands are also present, which may be due to a shift

Conclusions

Application of both the compressive/molding pressure and incorporation of bagasse fibers changed the properties and structure of PHB as mentioned below:

(i) Increasing pressure increased the spherulite size starting from the molten state. This is attributed to the grain boundary in the inter-spherulite and/or due to the crossing of intra-spherulites in the spherulites. On the other hand, incorporation of bagasse fibers decreased the size of spherulites.

(ii) The lattice parameters and the volume

Acknowledgements

The authors would like to thank Serrana S.A. for the supply of PHB, to LACTEC and particularly to Dr. Gabriel Pinto de Souza for helping in the interpretation of thermal analysis, M.Sc. Marilda Munaro for carrying out Thermal analysis and Mr. Sergio Henke for the help with the Scanning Electron Microscopy work. Thanks are also due to Dr. José Manoel Reis and Dr. José Eduardo Ferreira da Costa Gardolinski of LAMIR-UFPR for the optical microscopy and TG/DTA analysis.

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