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

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Standau, Tobias ; Long, Huan ; Murillo Castellón, Svenja ; Brütting, Christian ; Bonten, Christian ; Altstädt, Volker:
Evaluation of the Zero Shear Viscosity, the D-Content and Processing Conditions as Foam Relevant Parameters for Autoclave Foaming of Standard Polylactide (PLA).
In: Materials. Bd. 13 (2020) Heft 6 . - 1371.
ISSN 1996-1944
DOI: https://doi.org/10.3390/ma13061371

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Abstract

In this comprehensive study, the influence of (i) material specific properties (e.g., molecular
weight, zero shear viscosity, D-content) and (ii) process parameters (e.g., saturation temperature,
-time, -pressure, and pressure drop rate) on the expansion behavior during the autoclave foaming
process were investigated on linear Polylactide (PLA) grades, to identify and evaluate the foam
relevant parameters. Its poor rheological behavior is often stated as a drawback of PLA, that limits its
foamability. Therefore, nine PLA grades with different melt strength and zero shear viscosity were
systematically chosen to identify whether these are the main factors governing the foam expansion
and whether there is a critical value for these rheological parameters to be exceeded, to achieve low
density foams with fine cells. With pressure drop induced batch foaming experiments, it could be
shown that all of the investigated PLA grades could be foamed without the often used chemical
modifications, although with different degrees of expansion. Interestingly, PLAs foaming behavior
is rather complex and can be influenced by many other factors due to its special nature. A low
molecular weight combined with a high ability to crystallize only lead to intermediate density
reduction. In contrast, a higher molecular weight (i.e., increased zero shear viscosity) leads to
significant increased expandability independent from the D-content. However, the D-content plays a
crucial role in terms of foaming temperature and crystallization. Furthermore, the applied process
parameters govern foam expansion, cell size and crystallization.