Coalescence in PLA-PBAT blends under shear flow: Effects of blend preparation and PLA molecular weight

Abstract

Blends containing 75 wt. % of an amorphous polylactide (PLA) with two different molecular weights and 25 wt. % of a poly[(butylene adipate)-co-terephthalate] (PBAT) were prepared using either a Brabender batch mixer or a twin-screw extruder. These compounds were selected because blending PLA with PBAT can overcome various drawbacks of PLA such as its brittleness and processability limitations. In this study, we investigated the effects of varying the molecular weight of the PLA matrix and of two different mixing processes on the blend morphology and, further, on droplet coalescence during shearing. The rheological properties of these blends were investigated and the interfacial properties were analyzed using the Palierne emulsion model. Droplet coalescence was investigated by applying shear flows of 0.05 and 0.20 s−1 at a fixed strain of 60. Subsequently, small amplitude oscillatory shear tests were conducted to investigate changes in the viscoelastic properties. The morphology of the blends was also examined using scanning electron microscope (SEM) micrographs. It was observed that the PBAT droplets were much smaller when twin-screw extrusion was used for the blend preparation. Shearing at 0.05 s−1 induced significant droplet coalescence in all blends, but coalescence and changes in the viscoelastic properties were much more pronounced for the PLA-PBAT blend based on a lower molecular weight PLA. The viscoelastic responses were also somehow affected by the thermal degradation of the PLA matrix during the experiments.

Effects of nanoclay and its localization on the morphology stabilization of PLA/PBAT blends under shear flow

“…A more reliable approach to predict the g PLA-PBAT is to use the interfacial tension obtained via the Palierne model [50] from SAOS data. The Palierne emulsion model has proved to be a reliable approach to predict the interfacial tension of a binary blend based on rheological experiments [10,11]. We have shown in our previous study [10] that g PLA-PBAT, at 160 C is about 1.0 mN/m.…”

“…A shear rate of 0.05 s À1 was also used only for the blend nanocomposites prepared in the third mixing strategy to compare the morphological stability with those sheared at the rate of 0.01 s À1 . The low shear rates of 0.01 and 0.05 s À1 were selected to investigate coalescence in the blends, as droplet breakup will not occur since the capillary number (Ca) was lower than the critical number (Ca cr ) [10,11]. In order to investigate the effect of morphology changes after shearing on the rheological behavior of the blends, SAOS tests were also performed after stress growth and the relaxation of the samples.…”

“…Maani et al [9] observed that the use of a proper compatibilizer could influence the interfacial interactions between the polymer phases and thereby the morphology of the blend. Moreover, the melt mixing temperature, residence time, and other processing parameters could also affect the final morphology of blends [10,11].…”

“…In this context, development of PLA-based emulsion-type blends using other biopolymers as the dispersed phase could be considered as a breakthrough to overcome these drawbacks [30e34]. It has been demonstrated that it is possible to overcome some of these shortcomings by blending with high ductility and toughness and superior melt strength, biodegradable PBAT [10,11]. The objective of the present work is to explore how coalescence during processing (i.e., under shear flow) could be minimized by controlling the nanoclay localization in PLA/ PBAT blends, thus serving as a morphology stabilizer or smart droplet coalescence barrier.…”

Additive (nano)manufacturing perspectives: the use of nanofillers and tailored materials

“…Thermoplastic filaments based on PLA are dominant materials in FDM applications, due the highstrength, high-modulus, biocompatibility, rheological properties in molten state, low toxic emissions and crystallisation kinetics of PLA, which enable fast melting, extruding and selectively depositing of fibers for the formation of a predefined shape. However, PLA suffers from a series of drawbacks like brittleness, low heat deflection temperature, poor processability, and low melt strength [33]. Therefore, blending PLA with other polymers with desirable properties, along with the addition of environmentally friendly plasticizers and various strategies of compatibilization, is one of the most costefficient approaches to overcome these drawbacks, while maintaining its biodegradability and easy of recyclability.…”