Liquid CO₂ processing of solid polylactide foam precursors

Abstract

Diffusion of CO 2 in polylactide was modelled by assuming the diffusion coefficient to depend on CO 2 concentration, c, according to D[c] ¼ D[0]exp [Ac], where D[0] and A are empirical constants, with the aim of optimizing impregnation of nominally amorphous and semicrystalline polylactide/CO 2 -based precursors for physical foaming. Numerical simulations provided a consistent description of desorption at different temperatures, T, from polylactide impregnated with liquid CO 2 at 10 C and 5 MPa, and D[0, T] could be represented analytically using Arrhenius or Williams-Landel-Ferry-type expressions, allowing interpolation and extrapolation. Sorption was argued on this basis to involve a step-like diffusion front, such that the CO 2 content of a plate of thickness l increased as (D[0]t) 1/2 l À1 F[Ac o ], where c o is the value of c at saturation and F is a function of Ac o only. A major practical concern with polylactide/ CO 2 precursors is that the glass transition temperature, T g , decreases strongly with c, so that amorphous polylactide saturated with CO 2 at 10 C and 5 MPa degasses spontaneously at room temperature and pressure. However, it was inferred from the models and confirmed experimentally that partial impregnation in liquid CO 2 for relatively short times could provide a relatively rapid means of preparing precursors with a roughly uniform CO 2 content of around 0.1 g/g that were stable with respect to rapid CO 2 loss on heating to room temperature. The resulting precursors gave satisfactory foam morphologies and densities on foaming at 100 C. Moreover, it was also possible to adapt the impregnation conditions so as to obtain partially foamed structures from semicrystalline polylactide under these conditions, in spite of its

Crystallization of polylactide during impregnation with liquid CO2

“…2. Moreover, the internal morphologies of partially impregnated specimens foamed by heating to 100°C after depressurization and conditioning at 10°C in order to obtain a relatively uniform CO 2 concentration profile, have been argued previously to reflect the presence of relatively highly crystalline regions at the specimen surfaces, in which foaming was substantially suppressed, and whose extent was consistent with approximate predictions of the position of the diffusion front immediately after impregnation [8]. It should nevertheless be borne in mind that during desorption, CO 2 diffuses into regions of the specimen beyond the original diffusion front, albeit at reduced concentrations.…”

“…Previous investigations of CO 2 desorption rates from specimens of the same grade of PLA impregnated under in liquid CO 2 at 10°C have indicated the rate of diffusion of the CO 2 to be strongly concentration dependent, as expected, given that high concentrations of CO 2 are expected to lead to a significant reduction in T g [8,17]. Assuming Fickian diffusion, the dependence of the diffusion coefficient, D, on the CO 2 concentration, c, was modelled over the whole range of c from 0 to c o , using…”

“…The CO 2 was then allowed to desorb at 10°C for 1 h followed by a minimum of 1 week at ambient temperature prior to all subsequent heat treatments and measurements. These conditions were shown from previous investigations of desorption kinetics to be sufficient to reduce the CO 2 to negligible levels [8].…”

“…into a specimen with an equivalent structure, should be characterized by a step-like concentration profile that gradually advances into the specimen, i.e. qualitatively similar behaviour to ''case II'' diffusion, assuming D to show the same dependence on c as for desorption [8,21,22]. This approach takes into account neither the effect of swelling on the specimen dimensions, nor the possibility of non-Fickian effects, and indeed the assumption of a path-independent diffusion coefficient is highly questionable, particularly given the possibility of entrapment of CO 2 molecules by the crystalline phase when it first forms.…”

“…Given that the onset of cold crystallization in PLA is typically observed at temperatures about 10°C greater than the nominal T g in DSC heating scans, the critical concentration, c c , is assumed to be of the order of, or greater than 0.17 g/g, i.e. sufficient to reduce T g to 0°C [8,17].…”