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

Abstract

This study evaluated the effectiveness and efficiency of two food-grade multifunctional epoxies chain extenders (CE) in branching PLA and improving its foamability. Both CE grades were effective in branching PLA causing increased end mixing torque, shear, elongational viscosities, molecular weight but decreased crystallinity of poly(lactic acid) (PLA) with CE content, due to chain entanglements. CE with low epoxy equivalent weight (EEW) was more efficient than the counterpart with high EEW due to its high reactivity. Neat PLA foams showed poor cell morphology with areas without nucleated cells and had a low expansion, owing to its low elongational viscosity. By contrast, there was a considerable change in the morphology of the PLA foam structure caused by its branching. Chain-extended PLA foams had uniform cell morphology with a high void fraction (up to~85%) and expansion ratio (an eightfold expansion over unfoamed PLA) due to their high elongational viscosities, suggesting that melt properties of branched PLA were appropriate for optimum cell growth and stabilization during foaming. Overall, CE with low EEW was the most effective grade and 0.25% the optimum content that provided appropriate melt viscosity to produce PLA foams with a homogeneous structure, fine cells, high void fraction, high volume expansion ratio, and cellpopulation density.

Impacts of cellulose nanofibers on the morphological behavior and dynamic mechanical thermal properties of extruded polylactic acid/cellulose nanofibril nanocomposite foam

“…Many activities have been done to improve the mechanical properties of PLA, but these actions do not meet the requirements of industrialization due to the type of process. [58][59][60] The incorporation of CNF into the PLA foam matrix has been reported in some studies. In these studies, the effect of CNF on the strain and viscosity of PLA foams has been investigated and it has been concluded that CNF reduces cell size and also increases cell density and foam density.…”

“…Contrary to the batch process in order to generate foam, extrusion, thanks to being a continuous process, is more broadly applied for industrial purposes. Many activities have been done to improve the mechanical properties of PLA, but these actions do not meet the requirements of industrialization due to the type of process 58–60 …”

Foaming behavior of polyamide 1212 elastomers/polyurethane composites with improved melt strength and interfacial compatibility via chain extension

“…It was attributed to the branched and cross‐linked molecular structure, which the enhanced melt strength because of the strong molecular entanglement prevented cells from rupture or collapse. [ 20 ] However, the content of ADR increased further to 3%, the average cells size decreased to 20.4 μm, as shown by Figure 8A 4 , owing to further increased the degree of entanglement molecular chains resulted in excessive melt strength limiting the growth of cells. [ 11,32 ] With the increase of ADR content, the expansion ratio of the composite foams showed a similar trend of first increasing and then decreasing.…”

Continuous extrusion foaming process of biodegradable nanocomposites based on poly(lactic acid)/carbonaceous nanoparticles with different geometric shapes: An insight into involved physical, chemical and rheological phenomena

“…The poor melt strength of PLA plays a negative role in cell growth and mostly leads to cell wall rupture. 22,23 To overcome the mentioned drawbacks, different modification techniques, including nanoparticle incorporation, 24 crosslinking, 25 copolymerization, 26 chain extension, 27 and producing PLA/polymer blends, 28 have been performed. Crosslinking would led to some unfavorable influence such as producing gel, and also deteriorating the biodegradability nature of PLA.…”

“…Notwithstanding the merits of PLA foams, they have some demerits, such as low operating temperatures, slow crystallization kinetics, and low melt strength, which require overcoming to allow their commercialization. The poor melt strength of PLA plays a negative role in cell growth and mostly leads to cell wall rupture 22,23 . To overcome the mentioned drawbacks, different modification techniques, including nanoparticle incorporation, 24 crosslinking, 25 copolymerization, 26 chain extension, 27 and producing PLA/polymer blends, 28 have been performed.…”