ABSTRACT
Aiming to the production of environmentally friendly materials with improved performances, new formulations based on the biodegradable microbial polyester poly(3- hydroxybutyrate) (PHB) were prepared, characterized and analyzed about the potential environmental impact of their life cycle.
PHB was melt processed in a torque rheometer in presence of commercial additives. A carbodiimide-based antihydrolysis compound and a mixture of phenolic and organophosphitic antioxidants were tested. The mixtures were then characterized regarding their chemical structure, molecular weight, thermal and mechanical properties.
It was found that thermal degradation of PHB could be partially avoided by the use of the organophosphitic and the carbodiimide-based additives during melt processing. A decrease in PHB glass transition and mechanical properties was observed when the carbodiimide-based antihydrolysis additive was used, due to the presence of ethylene glycol segments in this molecule.
PHB based composites containing fillers of organic and inorganic nature were prepared and characterized. Organic fillers consisted in natural fibres (NF) of sugar cane bagasse (SCB) and pure cellulose. In order to improve adhesion with PHB, prior to processing NF were submitted to alkalization and esterification (acetyl and butyril) reactions.
Organophilic montmorillonite (OMMT), treated with different organic surfactants, was used as inorganic filler. PHB based composites were melt processed in a torque rheometer and then characterized regarding their morphological and structural behaviour, molecular weight, thermal properties, mechanical and dynamic mechanical properties, and, in the case of composites reinforced with inorganic fillers, oxygen permeability.
Alkalization of sugar cane bagasse resulted in a removal of non-cellulosic constituents, along with the crystalline transition from cellulose I to cellulose II. Esterification reactions reduced the hydrophylicity of the pristine NF. Plasticization of esterified NF was suggested by the decrease observed in the glass transition after reaction.
Esterification reactions improved the compatibility between PHB and NF, although the use of alkalized SCB with a high degree of acetylation caused significant drop in PHB molecular weight. Thermal analysis indicated a strong nucleating action of unmodified NF, whereas in esterified fibres this behaviour was somehow hindered by the presence of ester groups on fibres surface. Dynamic mechanical analysis showed the low reinforcing
function of grafted NF on PHB matrix. However, tensile properties of PHB were not improved. PHB-OMMT composites containing commercial organophilic montmorillonite modified with three different tallow based surfactants were prepared by solution casting technique. X-ray diffraction analysis indicated that PHB was able to intercalate the inorganic fillers. Different degrees of dispersion in the polymeric matrix were obtained depending on the type of clay surfactant. Thermal analysis and optical microscopy indicated that PHB crystallization rate was improved by the use of OMMT. Na- montmorillonite was modified with a silane based surfactant (OMMTSi), and used as filler in a concentration ranging from 1 to 10 wt-%. PHB-OMMTSi composites were prepared by melt mixing in a torque rheometer. Intercalated structures were obtained at lower OMMTSi content. Silane treated montmorillonite showed to enhance PHB crystallization rate, although the degree of crystallinity was not increased, as indicated by thermal and spectroscopic analysis. Enhancements in PHB thermal stability and gas barrier properties of composites were observed at low OMMTSi concentrations. Tensile properties of the polymer were slightly improved.
Finally, a predictive life cycle assessment (LCA) study concerning the environmental performance of some representative PHB based composites was performed. Composites containing both SCB and OMMTSi fillers were considered and compared with conventional petrochemical plastics such as high impact polystyrene (PS) and glass fibres reinforced polypropylene (PP-GF). LCA results indicated that, despite the relatively low mechanical properties of PHB composites, substantial environmental savings could be reached by the use of these biocomposites. These savings are mainly related to the PHB production process, while there are no substantial improvements related to composites preparation. However, SCB based composites are environmentally superior to organo- clay based ones, due to the natural origin of NF.
