As the number of environmentally conscious consumers continues to grow into a transformative global force, the market demand for recycled materials like thermoplastic polyester (PET) and naturally derived biopolymers like polylactic acid (PLA) also continues to increase. However, post-consumer-reclaimed PET, which is relatively abundant in the market, and renewable biopolymers have one thing in common: They tend to be low in melt strength and are prone to degradation during processing.
Recently, certain additives called chain extenders (CEs) have been used successfully to improve rheological properties in both materials, and it looks like this technology can be used to help these desirable polymers reach their full potential.
Chain extenders for PLA film
As previously noted, PLA’s main shortcoming is low melt strength, which can make it difficult to process as blown film. Extruders often find it is difficult to maintain the shape and integrity of the bubble during the blowing process. In testing on a laboratory extrusion line, natural PLA showed:
• Irregular shape
• Difficult to maintain bubble
• Noisy film
• Brittle and low melt strength
• Difficult to increase bubble size.
• Film width varied between approximately 17.5 and 19.5 cm
After observing these bubble-integrity problems, which are typical of those encountered when blowing PLA, researchers added CE masterbatch at several different rates. In this case, CESA®-extend from Clariant Masterbatches, was used to improve the working properties of PLA.
CESA-extend is based on multi-functional additives that react with the functional groups of the polymer. CESA-extend is provided in masterbatch form, a pelletized compound made up of concentrated additive in a compatible polymer carrier. When mixed with natural resin during the extrusion process, it helps re-link chains that have been broken by degradation. Ideally the polymer chains are re-linked in linear extensions, with minimal cross-linking, thus the name “chain extender.”
It was found that at 2% addition rate of a 30% active CESA-extend masterbatch, the average molecular weight of the PLA 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:
• Continuous regular shape
• Maintained bubble size
• Less noisy film
• Better melt strength
• Ability to double bubble size
• Higher line speed
• Film size uniform at 22 cm
The only side effect of using the chain 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.
Mirco Groeseling and Jan-Erik Wegner work in Clariant Masterbatches' Technical Marketing group.