Tailoring the properties of bio-based and biocompostable polymer blends

Conventional oil-based synthetic polymers (plastics) have shown an almost exponential growth during the past decades and currently more than 200 million tons are produced per annum, viz. approximate 45 kg per capita in the world. In view of the uneven consumption of plastics in the world, this number is expected to grow over 1000 million tons at the end of this Century which is not sustainable in view of oil depletion and, consequently, raising prices. Alternative fossil sources for producing chemicals and plastics are already in place such as coal and gas (methane) as pioneered by Sasol (SA) but geopolitical issues promote the use of biomass for making chemicals and plastics en route towards the bio-based society. As an alternative for fossil feedstock a lot of attention is paid nowadays to use polymers produced by nature, the so-called biopolymers, or to derive monomers from biomass to produce new and already known polymers. A particular class of polymers is the so-called biocompostable polymers, viz. polymers which degrade in composting facilities within a specific time span. Well-known examples of bio-based and biocompostable polymers are poly(ß-hydroxybutyrate) (PHB), poly(lactic acid) (PLA) and starch compounds. There are also in the market nowadays biocompostable polymers which are oil-based like poly(butylene succinate) (PBS), poly(butylene adipate-co-terephthalate) (PBAT, Ecoflex®) and poly(e-caprolactone) (PCL). The performance of notably biocompostable and bio-based polymers, viz. PHB, PLA and starch compounds is rather poor. The inherent drawbacks are temperature instability, lack of processability, brittleness and high price, notably PHB, limiting their developments and applications as a substitute for oil-based plastics. This thesis focuses on improvement on the properties of PHB-based blends (Chapter 2), PLA-based blends (Chapters 3 and 4) and starch-based blends (Chapters 5 - 7). In order to enhance the toughness of PHB and poly(ß-hydroxybutyrate-co-ß-hydroxyvalerate) (PHBV) biopolymers, PHB(V) was melt-blended with ductile PBS in Chapter 2. Considering the poor interfacial adhesion between PHB(V) and PBS, a free-radical initiator, i.e. dicumyl peroxide (DCP), was introduced to the PHB(V)/PBS melts to induce an in-situ compatibilization. As a result, the size of PBS domains was reduced to a sub-micro range, the interfacial adhesion between the PHB(V) and PBS was enhanced and partial crosslinking of both the PHB and PBS phases (especially of PBS) was obtained. All these effects contributed to the increased toughness of the blends. It has to be noted that despite the partial crosslinking, the blends remained melt-processability. Two routes for improving the toughness of PLA were probed – by blending with PBS in combination with an in-situ compatibilization (Chapter 3), and by blending with ethylene-co-vinyl acetate (EVA) (Chapter 4). The PLA/PBS blends showed a high elongation at break in the order of 200 % in comparison with 4 % of pure PLA. However, the notched impact toughness of the PLA/PBS blends was still low and comparable to that of the pure PLA (~ 3 kJ/m2). To improve the impact toughness of the PLA/PBS blends, a similar approach as already described in Chapter 2 was used, viz. in-situ compatibilization using DCP as an initiator. Consequently, the size of the PBS domains was reduced by a factor of 4, accompanied by an increase in the interfacial adhesion between the PLA and PBS phases. The notched impact toughness of the PLA/PBS blends was improved by a factor of 10 after addition of 0.1 wt% DCP. The toughening mechanism involved PBS interfacial debonding and matrix yielding. Due to the decreased size of the PBS domains the optical clarity of the PLA/PBS blends was improved . A further improvement on the toughness of PLA was attempted by blending with EVA which is a commercially available commodity copolymer (Chapter 4). The compatibility and phase morphology of the PLA/EVA blends were tuned by the ratio of vinyl acetate and ethylene in the EVA random copolymers. The highest impact toughness (increased by a factor of 30) of the PLA/EVA (80/20) blends was achieved at a vinyl acetate content of approximate 50 wt%. The dominant toughening mechanism revealed by scanning electron microscope (SEM), transmission electron microscope (TEM) and small-angle X-ray scattering (SAXS) is internal rubber cavitation in combination with matrix yielding. As shown in Chapter 4, the PLA/EVA blends exhibited extremely high impact toughness. However, PLA/EVA (and EVA) can be biocompostable only in the presence of starch. Therefore, in Chapters 5 - 7 attempts were shown to prepare PLA/EVA/starch blends with fine dispersion of starch in the PLA matrix and with good performance. This was achieved via two steps compounding. In the first step compatibilized EVA/starch compounds were made by reactive blending and re-extrusion in the presence of maleic anhydride (MA), benzoyl peroxide (BPO) and glycerol (Chapter 5). Upon blending, EVA chains were grafted onto the starch molecules producing EVA-g-starch copolymers which acted as a compatibilizer and enabled a very fine dispersion of starch particles in the EVA matrix. Subsequently, PLA was melt-blended with the pre-compatibilized EVA/starch compounds (Chapters 6 & 7). The fine dispersion of the starch phase (0.5 - 2.0 µm) was retained also in the PLA/EVA/starch ternary blends due to the core-shell-like (or starch-in-EVA) morphology of the pre-compatibilized starch and EVA. The ternary compatibilized blends showed stable (during storage) and good mechanical properties, e.g. elongation of break up to 150% and notched impact toughness of ~ 12 kJ/m2. It was found that the synergetic effect of both - compatibilization (via MA) and plasticization (via glycerol) was responsible for the obtained fine morphology and the good properties of the ternary blends. The thesis provides possible routes of tailoring the properties of bio-based and biocompostable polymer blends. The relatively simple approach of (reactive) melt blending of selected materials, suggested here, could be of direct use for industrial processing and production of bio-based and/or bio-compostable plastics with good properties, and might broaden their application range