Heat expandable biopolymers for one-step production of foam core sandwich composites

The overall aim of this work has been to develop sustainable solid thermally activated foam precursors suitable for the in-line production of lightweight bio-based foam core sandwich structures with consolidated wood particle facings, using a non-inflammable, non-VOC (volatile organic compound) as a blowing agent. A preliminary study showed that expandable PLA (polylactide)/CO2 could be prepared by impregnation of PLA, a commercially available bio-based polyester, with supercritical CO2. However, the high processing temperatures, T, and pressures, P, associated with supercritical conditions resulted in premature foaming during the depressurization step, making it difficult to adapt this technique to the production of foam precursors in the particleboard process. In this thesis, PLA-based foam precursors were therefore prepared using impregnation in liquid CO2 at moderate T and P. It was subsequently shown that the resulting heat-expandable materials could be successfully implemented in a simple one-step batch process for the manufacture of foam core particleboard, an important first step towards full implementation in a continuous process at the industrial scale. The first part of the study involved determination of the optimum conditions for the impregnation of PLA with CO2, so as to prepare foam precursors that were not only stable with respect to foaming at ambient T and P but also expanded readily at elevated T. Absorption and desorption of CO2 from initially amorphous low and intermediate D-isomer content PLA was modeled using a concentration, c, dependent diffusion coefficient, D[c], such that D[c]=D[0]exp[Ac]. The empirical constants D[0] and A were obtained by modeling desorption results under various conditions. It was hence shown that CO2 uptake by PLA at about 5 MPa and 10 °C was associated with a step-like concentration gradient, consistent with the morphologies observed in PLA specimens treated with liquid CO2 for different times and then foamed. Further evidence in support of the model was obtained from observations of CO2-induced crystallization in low D-isomer content PLA. The high melting temperature, Tm, of around 170 °C of the resulting crystalline phase nevertheless hindered subsequent foam expansion at T at and around 100 °C, i.e. the range of T associated with the particleboard process, which is turn fixed by the evolution of water vapor from the wood particles used to form the facings. Crystallization could be avoided through use of fully amorphous PLA with an intermediate D-isomer content. Indeed, the significant reduction in the glass transition temperature, Tg, in the presence of high concentrations of CO2, meant that amorphous specimens saturated under the chosen impregnation conditions, i.e. containing close to 40 wt% CO2, tended to foam spontaneously at room temperature, which is inconsistent with the stability requirements of the particleboard process. However, impregnation for a limited time, followed by partial CO2 desorption at low T under conditions that could be inferred directly from the diffusion model, was shown to lead to solid foam precursors with a uniform CO2 content of around 0.1 g/g. These were not only stable at room temperature, but contained sufficient CO2 for the production of low density foams on heating. ...