Materials CESA-extend
Materials CESA-extend chain extender is a molecule with functional properties that may be added to de-graded condensation polymers, to re-link polymer chains that have broken due to any of the mechanisms mentioned above (thermal, oxidative, hydrolytic degradation). Its functional groups are reactive with hydroxyl, carboxyl and isocyanate groups, so it can be used to repair conventional polymers such as, polyester (PET), nylon (PA), polycarbonate (PC), thermoplastic polyurethane (TPU) and, more recently has been showing great promise in PLA processing. Because similar functionalities are present in other condensation biopolymers like PHA and PHB, further testing is being conducted to determine if there may be possible benefits to be derived from using CESAextend in these polymers also. Ideally the polymer chains are re-linked in linear extensions with minimal cross-linking, thus the name chain extender. CESAextend is provided by Clariant Masterbatches in a masterbatch pellet form, in a variety of carrier systems designed to suit the requirements of specific applications. The effects of using CESA-extend were recently demonstrated in blown-film lab tests. The results of those experiments, and the effects CESA-extend may have in chemical foaming of PLA resin, are discussed below: Blown Film Testing PLA 4042D (product of NatureWorks LLC) was used to blow a clear film on a 30-mm Battenfeld single-screw extruder. Because of the low melt strength of PLA, it was very difficult to maintain the shape and integrity of the bubble during the blowing process. Observations before the addition of chain-extender were: • Irregular shape • Difficult to maintain bubble • Noisy film • Brittle and low melt strength • Difficult to increase bubble size. • Film width varied between ~17.5 and 19.5 After observing bubble integrity problems that are typical of those encountered when blowing PLA, a CESA-extend masterbatch was added at several different rates to optimize the desirable effects. It was found that at 2% addition rate of a 30% active CESA-extend masterbatch, the average molecular weight was raised from 124 x 103 g mol -1 for the neat PLA, to 185 x 103 g mol -1 for PLA plus masterbatch, indicating branching extension of the polymer chains. The elastic modulus decreased by about 20% while the elongation was increased by 50%. The addition of CESA-extend appears to cause a change in rheological properties of PLA from a typical Newtonian behavior of neat PLA, to some shear thinning and non-Newtonian behavior after the addition of the chain extender. The higher viscosities indicate higher molecular weights and entanglement typical for PLA with broad molecular weight distribution. Looking at bubble integrity and the resulting film after the addition of CESA-extend, the following results were observed: PLA before adding CESA-extend masterbatch Melt Viscosity of PLA and PLA + CESA-extend at Different Addition bioplastics MAGAZINE [03/08] Vol. 3 35
Materials PLA after addition of 2% CESA-extend masterbatch • Continuous regular shape • Maintained bubble size • Less noisy film • Better melt strength • Ability to doubled bubble size. • Higher line speed • Film size uniform at 22 cm The only side effect of using the extender is a slight haziness in the film made with the modified PLA. This result is typical of what happens when a chain extender is added to a clear film and may be controlled by optimizing addition rates required CESA-Extend in Foamed Packaging The low melt strength of neat PLA is as much a problem in foaming applications as it is in film blowing. The material simply does not have the melt strength to be able to sustain a cellular structure and the cells either collapse after forming or the gas escapes from the melt without forming cells. Adding CESA-extend, and the consequent chain extension and branching, result in a PLA with higher viscosity and melt strength. This now makes it possible to produce a direct-gas, nucleated foam structure with small cells and a smooth surface with significant weight reduction (10-15%). Chain extension increases polymer surface tension and higher activation energies required for cell nucleation. The higher viscosities increase the resistance to growing larger cells, while reducing the level of energy required to create new cells. The very positive result, then, is higher cell densities and smaller cells. Current applications for foamed PLA are limited, although at least one company, Sealed Air’s Cryovac Division, is offering ‘NatureTRAY’, foamed PLA food packaging trays, as an alternative to traditional foamed-polystyrene trays. Conclusions: PLA is a commercially available compostable polymer derived from renewable resources. It has many characteristics that make it suitable for use in place of petroleum-based polymers such as polystyrene and PET, particularly in packaging applications. The processing weaknesses of natural PLA may be overcome by using CESA-extend masterbatches that have a profound effect on enhancing molecular weight and melt strength characteristics that result in easily controllable processes. www.Clariantmasterbatches.com Foamed PLA References: Plastics Technology, Jan H. Schut, Extruding Biopolymers, February, 2007 Plastics Technology, Vahe Karayan, PhD, As Good as New, February, 2007 Web pages, www.NatureWorksllc.com Web pages, www.BASF.com Web pages, www.bccresearch.com Web pages, www.materbi.com Web Page, www.cryovac.com 36 bioplastics MAGAZINE [03/08] Vol. 3
