5. CONCLUDING REMARKS
The present PhD thesis was aimed at the development of new formulations and composites based on the fully biodegradable polyester poly(3-hydroxybutyrate) (PHB) of microbial origin. In particular, three key experimental aspects were examined. The first part of the research was aimed at PHB stabilization during melt processing, by the use of some commercial additives. In the second part, the attention was focused on the preparation and characterization of PHB based composites containing pristine and modified natural fibres as filler materials. The third part of the research aimed at the development of novel biodegradable polymeric nanocomposites based on PHB as continuous matrix and inorganic fillers. Finally, a cradle-to-grave environmental life cycle assessment (LCA) of some representatives PHB based composites was performed. The results obtained in the development of the different research lines allow for drawing the following conclusive remarks.
PHB stabilization with commercial additives
− Thermal degradation of PHB can be sustantially reduced by the addition of commercial additives such as phenolic and organophosphitic antioxidants and carbodiimide based antihydrolysis compounds, most likely due to the occurrence of reactions among the various formulate components during melt processing.
− In the antioxidants mixture, the organophosphitic additive seems to be more effective in the preservation of PHB molecular weight during melt processing. However, thermal stability and mechanical properties of PHB are not significantly affected by the addition of the two antioxidants in these experimental conditions. − Carbodiimide-based antihydrolysis compound displays a plasticizing effect on
PHB, confirmed by thermal and mechanical analysis, which might be connected to the presence of an ethylene glycol segment in the molecule. PHB thermal stability, glass transition, and Young modulus as well as tensile strength tend to drop by increasing the content of carbodiimide-based compound in the mixture.
− Differences recorded for tensile mechanical properties in replicate mixtures of stabilized PHB, indicate the presence of an uncontrolled process variable. This variance can be attributed to the incomplete removal of humidity from mixtures components prior to melt processing.
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PHB – Natural fibres composites
− Esterification of natural fibres (NF) with acetic and butyric anhydride, in the presence of sulphuric acid as catalyst was successfully performed, as confirmed by PAS-FT-IR analysis. Acetic anhydride shows higher reactivity with respect to butyric anhydride, whereas reactivity of NF is not dependent upon the ratio of fibre constituents. Alkalization of sugar cane bagasse (SCB) results in a substantial removal of non-cellulosic constituents, with the crystalline transition from cellulose I to cellulose II. Alkalized SCB shows the higher reactivity with both indicated anhydrides, due to a better swelling of fibres structure. Hydrophilicity of NF is decreased by esterification reactions, as well as the degree of crystallinity.
− Thermal parameters of NF are strongly affected by functionalization. TGA analysis showed that thermal stability of alkalized SCB and cellulose decreased as a result of esterification, most likely due to de-esterification and de-carboxylation reactions. The decrease observed in glass transitions after reaction indicates the plasticization of esterified NF.
− Esterification successfully enhanced the adhesion between fibre fillers and PHB continuous matrix, as well as fibres dispersability, as evidenced by SEM observations. However, the use of alkalized SCB with a high level of acyl grafting promoted a significant drop in PHB molecular weight.
− The insertion of NF caused a slight decrease in PHB overall thermal stability, as pointed out by TGA analysis with a drop in both the temperatures of degradation onset and maximum degradation rate. This behaviour is independent from the presence of acyl grafting in NF. On the other hand, PHB degradation products do not affect thermal stability of NF. DSC analysis indicated a strong nucleating action of unmodified NF, whereas in esterified fibres this property is somehow hindered by the presence of ester groups on fibres surface.
− Dynamic mechanical analysis appears to confirm the low reinforcing function of grafted NF on PHB continuous matrix. In general, the presence of non modified NF (pristine fibres) helps the PHB matrix to resist weakening under heating. A contrary trend is observed in composites containing grafted fibres, where storage modulus falls similarly to pure PHB. However, tensile properties of PHB were not improved, most likely because of the competition of different effects on the final mechanical performance of composites.
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PHB – Organophilic montmorillonite composites
− The solution casting technique showed to be effective in the preparation of PHB based nanocomposites reinforced with 5 wt-% of commercial organophilic montmorillonite (OMMT). X-ray diffraction analysis indicates that PHB was able to intercalate OMMT modified with different tallow-based surfactants. The preparation by solution casting technique enabled the achievement of different degrees of filler intercalation and dispersion. Thermal analysis indicated that, after OMMT addition, PHB thermal stability could either increase or decrease without a regular dependence of organo-clay modifier or treatment, whereas the organo-clay strongly acted as nucleating agent. Best results were obtained by OMMT modified with dimethyl dihydrogenated tallow quaternary ammonium.
− Sodium montmorillonite was successfully modified with a silane-based surfactant, as confirmed by WAXS and PAS-FT-IR analysis. Silane modified organophilic montmorillonite (OMMTSi) was successfully used as filler in the preparation of PHB based composites by melt mixing, in a content ranging from 1 to 10 wt-% of filler. From X-ray measurements, intercalated structures were obtained at lower OMMTSi concentration.
− Although the addition of OMMTSi turned out to enhance PHB crystallization rate, the degree of crystallinity of PHB was not increased, as indicated by thermal and spectroscopic analysis. PHB molecular weight after processing resulted slightly affected by the addition of OMMTSi up to 10 wt-%.
− Enhancements in thermal stability and gas barrier properties of composites are observed at low organo-clay concentrations. Oxygen transmission rate decreased of around 60% with the addition of 2 wt-% of organo-clay.
− According to dynamic mechanical analysis, storage modulus and glass transition are substantially increased by the addition of OMMTSi. The best compromise between OMMTSi amount and storage modulus was obtained witth a filler concentration of 5 wt-%. On the other hand, PHB Young modulus was only slightly improved, whereas tensile strength tended to decrease.
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Life cycle assessment of PHB-based composites
− Life cycle assessment (LCA) tool can be successfully applied in order to evaluate the environmental performance of PHB based composites compared to consumer goods made with conventional petrochemical plastics, such as high impact polystyrene (HIPS) cathodic ray tube (CRT) monitor housing and glass fibers filled polypropylene (PP-GF) internal panels of an average car.
− The relatively low Young modulus and high density for PHB based composites compared to conventional plastics are a disadvantage for their environmental performance, but substantial environmental benefits can be anyway obtained thanks to the saving inputs in the PHB production process.
− On a cradle-to-factory gate base, all PHB composites appear to be environmentally superior to conventional polymers used for the two chosen applications. However, when the analysis is extended to the system cradle-to-grave (with post-consumer waste incineration with energy recovery), PHB composites score still better only for CRT monitor housing. In the case of automotive application the weight of the functional unit becomes overriding.
− Regardless of system boundaries, PHB based composites reinforced with SCB show lower impact values than OMMT, and natural fibers appear to be the most promising filler among those tested in order to further improve the environmental performances of PHB based composites.
