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bioplasticsMAGAZINE_1402

People Basics

People Basics Di-isocyanate Urethane Linkage Brawn from Bio Exceptionally tough TPUs from new C18 Diacid Long chain Diol Diol chain extender Figure 1 – Hard and Soft Segments of Standard TPU By: Allyson Beuhler, Senior Polymer Scientist Ben Davis, Market Development Manager ElevanceRenewable Sciences, Inc. Woodridge, Illinois, USA Figure 2 – Stress Strain Curve of TPU from MDI, Butane Diol and Inherent C18 Diacid compared to Stress Strain Curve of TPU from MDI, Hexane Diol and Adipic Acid AC18 diacid-based thermoplastic polyurethane (TPU) has been made. The toughness is more than two times that of similar materials made from adipic acid, and the solvent performance greatly exceeds TPUs from adipic acid. This new material opens up an entirely new suite of market applications for thermoplastic polyurethanes made from renewable-based monomers. Overview Polyurethanes (PUR) are a broad class of materials made from the polyaddition reaction of a diisocyanate, a long-chain polyol and a short-chain diol chain extender. The applications for polyurethanes are extensive and varied as the materials can be produced as soft foams, rubbery elastomers or hard engineering thermoplastics. The performance and cost of polyurethanes can be tailored for a range of markets by changing the chemical composition, the molecular weight of the polyol, the relative amounts of polyol and diol, and the degree of crosslinking (see pp 50 for more basic information on PUR). Polyester and polyether polyols are both used as the long-chain polyol in the manufacture of TPUs, polyurethane elastomers and foams. The mechanical properties of a polyurethane depend significantly on the polyol — the molecular weight, chemical composition and weight percentage in the formulation. While polyether polyols tend to give softer and more hydrolytically stable polyurethanes, polyesters result in harder and higher temperature materials. Polyester polyols are normally made via the condensation reaction of a diacid (such as adipic acid) and a diol (such as butane diol) and the resulting polyurethanes are used in automotive (12%), footwear (20%), paints and coatings (13%), synthetic leather (9%), and adhesives (6%) [1]. Based upon the catalytic metathesis of natural oils, a new bio-based diacid, Inherent C18 Diacid, is now commercially 46 bioplastics MAGAZINE [02/14] Vol. 9

Basics TPU Properties TPU based on HD-Adipate (2000 g/mole) TPU based on BD-Inherent C18 Diacid (2000 g/mole) Hard Segment Concentration % 22 22 Soft Segment Concentration % 78 78 Shore A Hardness 89 92 Shore D Hardness 43 54 Ultimate Tensile Strength (MPa) 15.7 34.0 Tensile Strength at Yield (MPa) 3.5 13.6 Elongation at Break % 819 724 Toughness MJ/m3 66 151 Table 1 – Comparison of a TPU made from Inherent C18 vs. a TPU from Adipic Acid available from Elevance Renewable Sciences, Inc. [2, 3] The metathesis pathway is expected to dramatically increase the availability of Inherent C18 Diacid and lower its cost. Compared to the shorter chain alternatives, the products made from this diacid typically show superior properties, such as high crystallinity, flexibility, hydrolytic stability, and water and chemical resistance. Consequently, a host of new polyester polyols and TPUs made from Inherent C18 Diacid are now possible, allowing the polyurethane market to expand product offerings in applications such as ski boots, automotive fuel lines, roller bearings, medical tubing and sports equipment with renewable materials that exceed the standard of performance of petroleum-based TPUs. Thermoplastic Polyurethanes TPUs are linear, phase separated, block copolymers composed of a crystalline hard segment and an amorphous soft segment. (Figure 1) The hard segment acts as crosslinks, giving the material strength and stiffness, while the soft segment imparts elastomer properties such as rebound and flexibility. TPUs are typically synthesized via polymerization of a diisocyanate (typically MDI), a chain extender (typically butane diol) and a longer chain polyol (polyester or polyether). The diisocyanate plus the chain extender make up the crystalline hard block and the polyol makes up the amorphous soft block. A standard TPU formulation is made up of 70-80% soft segment (molecular weight of soft segment 1,000 – 15,000 g/mole) and 20-30% hard segment [1]. Properties of TPUs from Inherent C18 Diacid While the demand for bio-based TPUs is real and growing, most natural, oil-based derivatives haven’t been able to compete with the performance of petroleum-based polyurethanes. Bio-based TPUs based on Inherent C18 Diacid that have been synthesized show improved toughness and solvent resistance over standard petroleum-based TPU formulations. The use of the longer hydrophobic chain in the C18 polyols results in a new class of polyurethanes with a highly crystalline yet less polar soft segment, improved high-temperature performance, better hydrolytic stability (due to the lower ester content), improved dimensional stability and lower moisture pick- up. This set of features is critical in applications requiring high performance in high-temperature, highhumidity environments, including sporting goods, power tool housings, mobile phone housings, gears, sprockets, automotive panels, bumpers and airbags. Table 1 shows the chemical composition, hard and soft segment ratios, and mechanical properties of TPUs that have been synthesized from butane diol and C18 diacid, compared to TPUs from hexane diol and adipic acid [4]. While both TPUs are very tough, flexible elastomers with comparable shore hardness and elongation, the TPU from C18 diacid has more than twice the ultimate tensile strength. Figure 2 illustrates the engineering stress-strain curves of two TPUs — one made from butane diol and C18 diacid and the other made from hexane diol and adipic acid. The integrated area under the stress-strain curve is indicative of the toughness of a material. The C18 diacid TPU has a toughness of 151 MJ/m3 while the adipic acid TPU has a toughness of 66 MJ/m3, demonstrating that the polymer made with C18 diacid is more than twice as tough as the polymer made from adipic acid. Finally, the exceptional solvent resistance of TPUs made from long-chain, C18 diacid is illustrated in Table 2. Due to the very crystalline and hydrophobic soft segment in C18 diacid TPUs, these materials are much bioplastics MAGAZINE [02/14] Vol. 9 47

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