Polyurethanes |
Polyurethanes | Elastomers Bio-based ‘Cold Weather’ Thermoplastic Elastomer Article contributed by Frederic L.G. Malet PebaxR Research Manager ARKEMA Serquigny, France O ║ — C — (CH 2 ) 11 — N — | H Polyamide 12 HO — CH 2 — CH 2 — CH 2 — CH 2 — O — H n Figure 1 Structure of Pebax copolymers based on PA12 and PTMG blocks Polyehter Pebax ® polyether block amides are plasticiser-free thermoplastic elastomers which belong to the technical polymer family. They are used in high value added applications, in particular top-level sports. However, society’s growing demands in terms of performance required a total rethink of the polymer’s composition and hence the material’s physical structure. To achieve better performances, ARKEMA’s team turned to raw materials of renewable origin in order to formulate eventually a material with superior properties than the original material. A few years ago, the partner with whom this development was carried out unveiled its new Hurricane long-distance ski boot at a tradefair. Not only do these boots rigidify less at cold temperature than if they were based on the original material, but additionally the customer noted superior processability of the polymer, while keeping excellent impact strength in cold weather. The commercial launch was therefore a success. In search of new performance Pebax is a range of thermoplastic elastomers; segmented block copolymers prepared by reacting together functionalised polyether and polyamide building blocks. However, it is only when Deleens et al. discovered that the tetra-alkoxide catalyst family was efficient for the reaction that production of high molecular weight materials could be achieved, leading to the introduction on the markets in the early 1980’s. They own their unique properties to a phase-separated microstructure, with a hard phase consisting mostly of the polyamide blocks together with the soft phase consisting mostly of the polyether blocks. Since both blocks are chemically bonded by ester links, a complete macroscopic phase separation is thus prevented. The winter sports shoe market is a market with increasingly extreme demands, in particular in competitive sports. Skiers expect new, much more rigid models that therefore help them control their movements more accurately and more effectively. However, increased rigidity must not mean loss of resistance at cold temperature for the boot. This compromise is not easy to achieve as increasing a material’s rigidity leads to reducing performance in the flexible phase, which contributes to this cold resistance. The use of standard grades of Pebax can offer a good rigidity / cold resistance compromise, though this was not sufficient to accommodate increasingly extreme conditions. Indeed, the polyether blocks currently in use are oligomers of polytetramethylene glycol, with a very low glass transition temperature close to –80°C, and thus responsible for the remarkable mechanical properties at cold temperature. However, very rigid grades, therefore with low polyether content, will start to show some limit. The rigidity of Pebax copolymers is closely related to the amount of soft polyether blocks present in the material. Thanks to Kerner and Jordhamo’s work, a model can be build up in order to follow the evolution of the rigidity 46 bioplastics MAGAZINE [05/10] Vol. 5
Polyurethanes | Elastomers with the polyether content. One of the key parameters is the modulus of the corresponding polyamide homopolymer. The modulus of polyamide homopolymers increases with the amide / methylene ratio. Indeed, a higher concentration of amide groups will lead to higher crystallinity and hence a more rigid material. Unfortunately, melting point of the material will also increase, together with moisture uptake and density. Absorption of water will have a significant impact on mechanical properties, for instance the modulus of PA6 can be reduced by half under moist conditions. Increasing the density does not help towards designing lighter materials for demanding athletes. Another hurdle is that the solubility parameter of polyamide increases with the amide / methylene ratio, increasing the gap with the solubility parameter of the polyether, leading to a higher enthalpy of mixing. Mixing of the two blocks will thus be more difficult, leading to slower polymerisation, if any. Among the possible polyamides, the odd ones, meaning having an odd number of carbons, do have peculiar properties, as the positioning of the amide groups in the chain is important for structural order and packing efficiency. Among these polyamides, PA11 rapidly became the centre of our attention. Indeed, being an odd polyamide, its elementary lattice can theoretically lead to either a parallel or an antiparallel configuration of the chains with every amide group able to be engaged with another one through hydrogen bonds. Depending on the cooling procedure, crystals will either have a hexagonal arrangement or triclinic one. Usually, both are co-existing. A very interesting phenomenon is that the triclinic phase can change into a pseudo-hexagonal one under thermal or mechanical stress. Thus, the amount of energy needed to perform the crystalline transition will decrease the energy dissipated within the material, thus explaining the outstanding strain hardening behaviour of PA11 vs. PA12. In the case of PA12, an anti-parallel configuration of the chains is only observed because of the even number of carbon and the extra twist of the chains, necessary to optimise hydrogen bonding, leads to a γ-monoclinic structure. On a mesoscopic scale, the two materials exhibit noticeable differences: ringed spherulites can be observed for PA11, whereas coarse spherulites are the typical form for PA12. The next step was then to try and see whether it was possible to transpose the remarkable mechanical properties to a multi-segment block structure comprising a PA11 block with a very low molecular mass. Traction Modulus (MPa) 1500 1250 1000 750 500 250 0 PEBAX ® Rnew 70R53 PEBAX ® 7033 0 10 20 30 40 50 60 70 80 Figure 2 Evolution of the traction modulus of Pebax copolymers depending on polyether content PA12 PA11 Figure 3 Differences in the crystallite structure between PA12 and PA11. bioplastics MAGAZINE [05/10] Vol. 5 47
