MATERIALS | BIO-BASED POLYMERS


Right: ABB switch cover


how to compound and process it.” Hänggi says that the PHB chain is absolutely


regular, with only C4 sub-units, and absolutely isotactic. Both facilitate crystallisation. The chains are also absolutely linear, without any branches, so they cannot entangle. The glass transition temperature of PHB is at 0°C or below. “This means that polymer chains keep on moving and crystallising even at room temperature. Such a combination of proper- ties is unknown in any synthetic thermoplastic.” The absolute regularity yields hard, creep resist-


ant parts, the linearity of the polymer chains allows adjustment of melt viscosity right in the machine, and finally, the glass temperature results in stable parts. “The hard, creep-resistant parts are due to the


fact that PHB does not stop crystallising until the thermodynamic optimum is reached, that is, until all polymer molecules are fixed in crystals. There they stay forever. “If you aim at high speed moulding, you simply lower the temperature at the tip of the screw so that crystallisation already starts in the barrel. If you aim at filling the finest cavities in a mould, you in- crease the temperature at the tip of the screw. I am not aware of any other thermoplastic with which you could adjust the viscosity as easily as PHB right in the machine. “The reason for the stability is that, after having


reached the thermodynamic end point, there is no free amorphous mass left to move. This is also the reason why we can guarantee no changes in proper- ties if the parts are exposed for five years from -40 to +60°C. UV does no harm as there are no double


bonds in the chains and no aromatic subunits that might absorb UV light. With respect to mechanical and UV stability over time, PHB beats PP.” There are obstacles on the road to PHB reach- ing its full potential, but they can be overcome, says Hänggi. Problems with degradation in the melt, for example, can be averted by extracting PHB in a solvent rather than in water (the way it is usually done) to avoid the presence of calcium ions that catalyse degradation. Slow crystallisation can be countered in numerous ways, including improved nucleation. Hänggi says Biomer now offers grades with a new nucleant that produces spherulites sizes of below one micron, and has started tests with another that promises to yield even smaller spherulites. But the most severe impediment to using PHB as thermoplastic is that PHB parts get brittle over time. Hänggi says this is due to voids appearing between spherulites, which act as “cracking gates” through the parts. “The best solution is to link the spherulites by molecules that span the voids and interact with the crystals. This type of interaction is very strong and firm. And so you finally end up with hard, creep resistant parts. “Taken all together and processed the right way PHB becomes an exceptional thermoplastic.” Plastics sell by properties, he concludes. “PHB is


no exception. Biodegradation is a property, but as a matter of fact we have no customers who focus on biodegradation. This might change in the future, but only if the parts are of equal or better quality than the original parts. Some of our clients look for ‘renewables’, but to most of them the term ‘bio’ is of minor importance and considered a fringe ben- efit. Almost all of our clients focus on the specific properties.”


Above: The PHA famiy of polymers is very broad and versatile. These products, made by various processing routes, not just injection moulding, are in a copolymer of hydroxybutyrate with hydroxyhexanoate, PHBH, from Kaneka, which recently said it would expand its manufacturing facility in Takasago, Japan, to around 5,000 tonnes/yr. It will begin operating in December


54 INJECTION WORLD | March 2019


One compounding company having success with PHB (and other biopolymers) is MAIP in Italy. One of its more notable successes is the use of one of its IamNature compounds for Etik frames in the Mylos range of wiring accessories produced by ABB. They replace frames made in traditional


www.injectionworld.com


PHOTO: MAIP


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