Thermal degradation of plasticized poly(3-hydroxybutyrate) investigated by DSC Ivica Janigova´*, Igor Lacı ´ k, Ivan Choda´ k Polymer Institute of the Slovak Academy of Sciences, Du ´bravska ´ cesta 9, 842 36 Bratislava, Slovak Republic Received 9 July 2001; received in revised form 8 March 2002; accepted 10 March 2002 Abstract The thermal degradation of poly(3-hydroxybutyrate) (PHB) in the presence of two plasticizers (glycerol, glycerol triacetate) was investigated by differential scanning calorimetry (DSC). The thermal degradation was studied as a function of the annealing time, temperature and cooling rate after the thermal treatment. PHB thermal degradation proceeds by the random scission of the poly- mer chain. The presence of glycerol leads to a significant prodegradative effect on PHB presumably due to the alcoholysis reaction, while glycerol triacetate (triacetine) behaved as an almost inert additive from this point of view. Size exclusion chromatography (SEC) confirmed the results obtained by the DSC method. The data lead to the conclusion that DSC is a suitable method for esti- mation of PHB thermal degradation. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Thermal degradation; DSC; Poly(3-hydroxybutyrate); Plasticizer; Thermal treatment; Annealing; SEC 1. Introduction Poly(hydroxybutyrate) (PHB) can be characterised as a rather controversial polymer. PHB is a completely biodegradable, highly hydrophobic thermoplastic mate- rial containing almost 80% crystalline fraction, with excellent mechanical strength and modulus, resembling polypropylene [1]. On the other hand, an extensive high volume application of this material is hindered by several serious drawbacks so that at present PHB is used only exceptionally in small quantities for special purposes. A pronounced brittleness, very low deformability, high susceptibility to a rapid thermal degradation, difficult processing by conventional technologies for thermo- plastics (mainly due to fast thermal degradation), and rather high price compared to commodity plastics are the major obstacles for a broad application of PHB. Very low resistance to thermal degradation seems to be the most serious problem related to processing of PHB. Since the melting temperature of PHB is around 180  C, the processing temperature should be at least 190  C. The thermal degradation at this temperature proceeds rapidly so that the acceptable residence time in the processing equipment is only a few minutes. The mechanism of degradation is based on a b-elimination reaction with formation of crotonic acid [2]. Size exclu- sion chromatography (SEC) measurements of PHB heated to various temperatures as a function of time revealed that molecular weight after an initial decrease increased slightly before continuing to decline [3]. The effect was attributed to polycondensation of the initial hydroxy and carboxy groups formed by the elimination process. A detailed study of thermolysis of PHB was described in [4]. It was found that generally the thermal degrada- tion is a statistical process based on a random scission, however, some kinetically favoured scissions occur near the ends of the macromolecules. A decrease in processing temperature and a more convenient processing regime can be achieved by addi- tion of a plasticizer [5]. However, this route is accom- panied with a decrease in tensile properties; the extent depends on the amount and type of the added plasticizer. Various additives affect the kinetics of thermal degra- dation differently. Many species act as prodegradants. Aluminium compounds and fumed silica have been reported to have a slight stabilising effect followed by a prodegradant activity [5]. Other inorganic compounds 0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(02)00077-0 Polymer Degradation and Stability 77 (2002) 35–41 www.elsevier.com/locate/polydegstab * Corresponding author. Tel.: +421-2-54771626; fax: +421-2- 54775923. E-mail address: upoljani@savba.sk (I. Janigova´).