materials feature | Bioplastics
Effect of Metabolix a-PHA on impact properties of NatureWorks PLA grades
be used to provide effective rubber toughening for PLA while reducing the stiffness and brittleness of PLA across a range of thermoplastic applications.”
Polycaprolactone on the up Polycaprolactones are another family of polymeric modifi ers for biopolymers. Unlike PHAs, they are not based on renewables, but they are biodegradable and compostable. Leading supplier is Perstorp, which has recently begun to devote more development activity to the use of its Capa caprolactones in this application (the materials are mostly used for high performance polyols in polyurethanes). Linda Zellner, project manager for materials
innovation at Perstorp, highlights the possibilities for polycaprolactones (PCL) as a modifi er for bio-based polymers. “It is an interesting polymer in this fi eld, especially since it is known to be compatible with a wide range of polymers including biopolymers such as PLA, starch-based plastics and PHAs, particularly polyhy- droxybutyrate (PHB),” she says. PCL is an aliphatic polyester consisting of methylene units that are separated by ester groups, a structure which gives little steric hindrance and results in a semi-crystalline polymer.
ASTM test
Effect of Metabolix a-PHA on stiffness properties of NatureWorks PLA grades
“Polycaprolactone has a quite low crystalline melting point which is both benefi cial and challenging, depending on the application area,” Zellner says. “However, in the fi eld of bioplastics polycaprolactone is usually not used solely but rather in a blend to improve properties of the renewable based components as well as balancing the cost.” The low glass transition temperature (-60°C) together with the low crystalline melting point (58-62°C) makes PCL easy to process, she notes. Depending on the molecular weight (usually in the range of 10,000 – 80,000), PCL can be waxy or a solid polymer. PCL exhibits high elongation at break and a low modulus. High molecular weight grades are preferred as modifi ers. Zellner says that several studies have shown that PCL enhances mechanical properties in biopolymers such as PLA and PHB. PCL behaves as a polymeric plasticizer improving the fl exibility and giving the blend higher impact toughness. “The low melting point facilitates the dispersion of PCL in the blend but also enables a possible lowering of the processing tempera- ture which is critical for many of the biopolymers which are thermally sensitive and start to degrade at tem- peratures not far from the melting point,” she says. PCL can also be used as a plasticizer in PHB, enhancing the material’s processability as well as
General properties for two standard grades of Perstorp polycaprolactone, Capa 6500 and 6800 Property
Capa 6500
Weight average molecular weight (Mw), g/mol Melt Flow Index
80°C, 2.16 kg/10 min 190°C, 2.16 kg/10 min
Yield stress, MPa (@500 mm/min) Flexural modulus, MPa (@2 mm/min) Shore A hardness
36 COMPOUNDING WORLD | June 2015
D 412-87 D 790 D2240
GPC, THF, 25°C D 1238
84,500 ±01000
2.4 28
17.2 411 95
Capa 6800 120,000 ±00000
0.6 7.3
14.0 Nd 94
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