Rheology and extrusion foaming of chain-branched poly(lactic acid)Mihai, Mihaela; Huneault, Michel A.; Favis, Basil D.Contact us / Contactez nous: nparc.cisti@nrc-cnrc.gc.ca. In this study, the effect of macromolecular chainbranching on poly(lactic acid) (PLA) rheology, crystallization, and extrusion foaming was investigated. Two PLA grades, an amorphous and a semi-crystalline one, were branched using a multifunctional styrene-acrylicepoxy copolymer. The branching of PLA and its foaming were achieved in one-step extrusion process. Carbon dioxide (CO 2 ), in concentration up to 9%, was used as expansion agent to obtain foams from the two PLA branched using chain-extender contents up to 2%. The foams were investigated with respect to their shear and elongational behavior, crystallinity, morphology, and density. The addition of the chain-extender led to an increase in complex viscosity, elasticity, elongational viscosity, and in the manifestation of the strain-hardening phenomena. Low-density foams were obtained at 5-9% CO 2 for semi-crystalline PLA and only at 9% CO 2 in the case of the amorphous PLA. Differences in foaming behavior were attributed to crystallites formation during the foaming process. The rheological and structural changes associated with PLA chain-extension lowered the achieved crystallinity but slightly improved the foamability at low CO 2 content. POLYM. ENG. SCI.,
This article investigates the crystallinity development in cellular poly(lactic acid) (PLA) and the effect of the achieved crystalline content on its properties and microstructure. Carbon dioxide (CO2) in its supercritical state was used as the expansion agent for three different grades of PLA that differed in terms of L‐lactic acid content. Cellular PLA was produced on a twin‐screw extrusion line using capillary dies of various diameters. The obtained crystalline contents were measured by differential scanning calorimetry and X‐ray diffraction techniques. The morphology of the cellular structures was examined using scanning electron microscopy. The crystallinity developed on expansion depended on L‐lactic acid content, on supercritical CO2 concentration, polymer flow rate, and die diameter. Cellular PLA, with densities as low as 30 kg/m3, was obtained under the most favorable conditions. It was shown that the crystallinity development in PLA enhances its cellular structure formation and enables the fabrication of quality cellular materials at lower CO2 concentration. The presence of PLA crystallites within expanded cell walls leads to a peculiar 2D‐cavitation phenomena observed only in the cell walls of semicrystalline foams. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009
Low-density open-cell foams were obtained by extrusion process from polylactic acid (PLA) and from blends of PLA with thermoplastic starch (TPS) using CO 2 as blowing agent. Two unexpected features were found. First, a 2D cavitation process in the fractured cell walls was unveiled. Elliptical cavities with dimensions in the 100-300 nm range were aligned perpendicular to large cell cracks clearly exhibiting 2D crazing prior to macroscopic cell rupture. Secondly, a significant crystallization rate increase associated with the CO 2 foaming of PLA was discovered. While the PLA used in this study crystallized very slowly in isothermal crystallization, the PLA foams exhibited up to 15% crystallinity providing evidence for CO 2 plasticization effect, and the biaxial stretching upon foam expansion provided conditions that could increase the crystallization rate by several orders of magnitude.
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