Effects of chemical modifications on the rheological and the expansion behavior of polylactide (PLA) in foam extrusion

Abstract

It is well known that polylactide (PLA) is difficult to foam due to its low melt strength. Thus, many ways were described in the literature to enhance the foamability. However, the melt strength was actually determined only in a limited number of publications. In this study, the addition of chemical modifiers was used to change the rheological behavior of PLA and thereby improve its foamability in foam extrusion process. For the first time the use of dicumyl peroxide modified PLA in foam extrusion is described. Both modifications lead to a distinct increase in melt strength. Here, the highest increase was shown for the PLA modified with dicumyl peroxide. Furthermore, strain hardening was observed for PLA modified with the peroxide. Low density foams were achieved for neat and modified PLA in foam extrusion. Neat PLA showed a density of 45 kg/m3, while the peroxide modified PLA showed the highest expansion with a density reduction down to 32 kg/m3. Both modifications result in a more uniform cell structure and an improved compression strength. Here, the foamed, peroxide modified PLA showed outstanding performance compared to neat PLA foam with twice the compression strength (151 Pa) even at a 30% lower density.

Evaluation of the Zero Shear Viscosity, the D-Content and Processing Conditions as Foam Relevant Parameters for Autoclave Foaming of Standard Polylactide (PLA)

“…Peroxides, such as dicumyl peroxides, lead to free radical reaction. In one of our previous studies, we could show that the melt strength could be significantly increased by the use of these modifiers, leading to foams with lower density, finer cell sizes and improved compression behavior [6,7].…”

“…Even though it is often stated in literature that the melt strength plays a vital role in the foaming of PLA, only a few publications actually quantify it at all [7][8][9][10]. An increased melt strength is beneficial for foaming, as it prevents coalescence and rupture during the cell growth step because the cell walls can withstand higher forces during stretching while expanding and can consequently lead to an improved expansion behavior (i.e., lower density and finer cells) [11].…”

Microcellular foaming of poly(lactic acid) branched with food‐grade chain extenders

“…4,12,[14][15][16] For PLA for example, this chain-extension increases its zero-shear viscosity and molecular weight, resulting in a modified PLA with enhanced melt viscosity and elasticity, properties needed for successful production of PLA foam. 4,6,10,[12][13][14][15][16][17]34,35 The effectiveness and efficiency of two food-grade multifunctional epoxies CE with low and high EEW in chain extending/branching PLA and its foamability were evaluated. Chain extension reaction of PLA with CE was followed by torque rheometry since an increase in torque is associated with chain extension 6 as well as by other rheological properties such as shear and elongational viscosities.…”

“…14 Improvements in PLA foam characteristics have also been reported with other CE such as multifunctional aziridine, 15 polystyrene/poly(glycidyl methacrylate) random copolymer, 16 or by the free radical branching/ linking of PLA with dicumyl peroxide. 17 Despite promising results, most of the CE used to improve the foamability of PLA are not approved for food contact because of toxicity issues. For instance, trimethylolpropane tri-acrylate has been reported to cause rare types of cancer in mice 18 whereas isocyanates are also known for their toxicity.…”

Synthesis of flexible poly(l-lactide)-b-polyethylene glycol-b-poly(l-lactide) bioplastics by ring-opening polymerization in the presence of chain extender

“…It has been reported that the chain extension of this PLLA-PEG-PLLA with chain extender (CE) improves its melt strength by the formation of branched structures (11). CE has also been extensively used for maintaining or increasing the molecular weight of PLLA (11)(12)(13), improving the melt strength of PLLA (14,15), and enhancing the phase compatibility of the PLLA-based blends (16,17). Joncryl ® is an important epoxy-based CE for PLLA.…”