Fibers & Textiles PHB properties for By: Pavan Kumar Manvi, Mustafa Salih Korkmaz Gunnar Seide, Thomas Gries Institut für Textiltechnik, RWTH Aachen Aachen, Germany R CH R CH CH 2 CH 2 C O C H H O + Q CH CH Fig. 1: Mechanism of thermal degradation of PHB O O CH CH O C R O C R O O PHAs are polyesters produced by the bacterial fermentation of sugar or lipids. PHB is one of the well-known members of the PHA family with the largest volume in the market compared to other PHAs. However, the market demand for biopolymers like PHB is strongly influenced by factors such as a limitation of processing possibilities and the possibility to adopt the PHB for textile-based applications. There is, therefore, a definite need for further research to improve the processing technologies. One significant aspect is the thermal instability of PHB, which has a negative impact on melt processing as well as on the end product properties. PHB is characterized by a very large spherulite size, which is generally the result of a high level of purity and presence of very low number of nuclei. The low glass transition temperature of PHB results in post crystallization of PHB and large spherulites are formed. [1] The biggest problem in the processing of PHB is the low degradation temperature with regards to its melting temperature. Thermal degradation of PHB The thermal degradation of PHB is characterized by nonradical random chain scission reactions in a six member ring ester decomposition process. The mechanism of degradation is explained in the Fig 1. The PHB molecule consists of a ring structure with three resonance positions, as shown in the figure. At elevated temperatures this resonance becomes stronger and results in the breakage of a chemical bond. This in turn results in the gradual decrease in the molecular weight. The thermal degradation of the PHB takes place above 200 °C, which is not far from melting temperature (178°C). [2] One way of improving the melt processability of PHB is to add a thermal stabilizer to enhance its thermal properties. In this study various concentrations of thermal stabilizer are added to analyze the thermal stability through melting and crystallization behaviour of the polymer. Experimental PHBs (Biomer P209) from the company BIOMER GmbH (Krailing) were mixed with a thermal stabilizer mechanically, in various fractions from 0.1 % to 2.0 % These samples were tested for their thermal properties with the help of Differential Scanning Calorimetry (DSC). For heating/ cooling cycle measurements, the following thermal procedure 18 bioplastics MAGAZINE [05/13] Vol. 8
Fibers & Textiles fibre applications was used: Starting temperature 25°C, ramp 10°C/min from 0 to 200°C and ramp 10°C/min from 200 to 0°C. During the heating cycle a melting endotherm of the PHB with and without stabilizer is obtained. During the cooling cycle a crystallization exotherm of the PHB with and without stabilizer is obtained. In the melting endotherm, melting temperature and melting enthalpy are of interest. In the cooling endotherm, crystallization temperature and crystallization enthalpy are observed. Melting and crystallization enthalpy represent the enthalpy needed to melt the crystals and for crystallization respectively. Results and discussion The results of thermal analysis have been shown in figures 2 and 3. In the first heating cycle endotherm the melting temperature of all samples is similar but the melting enthalpy increases with increase content of stabilizer. In the cooling cycle different behaviour was seen. The temperature of crystallization increases along with the increase in stabilizer content. The same phenomenon is also seen with crystallization enthalpy. An increase in the crystallization temperature and enthalpy of crystallization indicates that a stabilization effect is being approached. This shows that molecular chain scission is decreased and average molecular weight is higher with an increase in the stabilizer content. The higher average molecular weight, the free movement of polymer chain molecules, and crystallization take place at a higher temperature. Conclusion Thermal analysis of PHB in the presence of a stabilizer was carried out. The stabilizer improves the thermal stability of the polymer, which can be proved by an increase in crystallization temperature and enhancement in the enthalpy for crystallization. The results of this study indicate that thermal stabilizers are efficient additives to enable the melt processing of PHB and to decrease the thermal degradation. Melt processing technologies, such as melt spinning and injection moulding will benefit, and the end product qualities will also be improved. This will also open up new fields of application such as hygiene textiles, geotextiles and medical textile industries. Moulded products from PHB can also be adapted for use and dispose products, where biodegradability is an important issue. www.ita.rwth-aachen.de Enthalpy (J/g) wt% stabilizer References: 0 0,1 0,2 0,5 1,0 2,0 ΔHm 78,04 78,36 80,7 83,22 83,94 82,31 ΔHc 76,94 79,46 83,55 86,78 88,53 89,72 Fig.2 Melting and crystallization enthalpy of PHB with and without stabilizer Temperature (°C) wt% stabilizer 0 0,1 0,2 0,5 1,0 2,0 ΔTc 80,36 87,83 94,69 103,78 110,34 114,32 ΔTm 177,52 178,29 177,32 177,3 177,8 176,83 Fig.3 Melting and crystallization temperature of PHB with and without stabilizer [1] Barham, P.J.; Keller, A.; Otun, E.L.; Holmes, P.A. Crystallization and morphology of a bacterial thermoplastic: Poly-3- hydroxybutyrate Journal of material science, 1984, 19, 2781 – 2794 [2] Chen, C.; Fei, B.; Peng, S.; Zhuang, Y.; Dong, L.; Feng, Z.; The kinetics of thermal decomposition of poly (3-hydroxybutyrate) and maleated poly (3-hydroxybutyrate) Journal of applied polymer science, 84 (2002), S. 1789-1796. bioplastics MAGAZINE [05/13] Vol. 8 19
