PLA resins are composed of chains of lactic acid, a natural food ingredient which can be produced by converting starch into sugar and then fermenting it to yield lactic acid. Water is then removed to form lactide, which is converted into PLA resins using a solvent-free polymerization. PLA polymers offer a broad balance of functional performance which makes them suitable for a wide variety of market applications. They are expected to compete with hydrocarbon-based thermoplastics on a cost/performance basis. PLA polymers provide good aesthetics in terms of gloss and clarity and processability similar to polystyrene. PLA polymers also exhibit tensile strength and modulus comparable to certain hydrocarbon-based thermoplastics. Like PET (polyethylene terephthalate), these polymers resist grease and oil, and offer good flavor and odour barrier. PLA polymers also provide for heat sealability at lower temperatures than polyolefin sealant resins. PLA, a biopolymer, can be processed by most melt fabrication techniques. It can be formulated to be rigid or flexible and can be made with different mechanical properties suitable for specific manufacturing processes, such as injection molding, sheet extrusion, blow molding, thermoforming, blown film, and fiber spinning, all these processes may be carried out by conventional techniques and processing equipment.
However, one of the limitations of PLA is its processing instability as it is highly vulnerable to thermal, oxidative and hydrolytic degradation. The result of these degenerative effects, which may occur during processing, is polymer chain cleavage, resulting in loss of molecular weight and deterioration of rheological properties. Robust rheological properties are the key to successful forming, particularly in processes where high extensional and viscoelastic properties are required, such as for blown film and extrusion foaming. The other allied limitation of PLA that particularly affects successful foaming is its low melt viscosity.
These limitations can be improved by the use of a chain extender, like CESAŽ-extend offered by Clariant Masterbatches. CESA-extend chain extender is a molecule with functional properties that may be added to degraded 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. More recently it has been showing great promise in PLA processing.
The low melt strength of PLA makes it difficult to maintain the shape and integrity of the bubble during the blowing process. Observations made before an extender was added to PLA 4042D (product of NatureWorks LLC) used to blow a clear film on a 30 mm Battenfeld single screw extruder include irregularity in shape, difficulty in maintaining the bubble, noisy film, brittle and low melt strength, difficulty in increasing bubble size and variation in film width.
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%. A change was noted 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. After the addition of CESA-extend, the following was observed: continuous regular shape, a maintained bubble size, less noisy film, better melt strength, ability to double bubble size, higher line speed and uniform film size. The only side effect of using the extender is a slight haziness in the film made with the modified PLA, and may be controlled by optimizing addition rates required.
The low melt strength of neat PLA is as much a problem in foaming applications as it is in film blowing, as the material 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, making 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 creating new cells. The very positive result, then, is higher cell densities and smaller cells.
Rohm and Haas and PolyOne Corporation have jointly announced the availability of new PLA compounds that will significantly enhance PLA performance for food packaging, imparting processing advantages, impact resistance and especially clarity upon usage. Impact resistance is enhanced along with cuttability of PLA sheets, while maintaining excellent transparency and improving processing features such as melt properties for blown film or extrusion blow molding.