Microcellular injection-molded blends of polylactic acid and poly(ε-caprolactone)
Polymer foams are prized for their flexibility and impact absorption. Consequently, they enjoy widespread use, for example, in the automotive, electronic, and electrical industries. A disadvantage of conventional foams is that they are difficult to recycle cost-effectively. For this reason, foams based on biodegradable and environmentally friendly polymer materials are of significant interest. Among the renewable polymers being investigated, polylactic acid (PLA) is produced in sufficient quantity to be commercially viable and also has a number of attractive features. These include its mechanical performance, which is comparable to that of polyethylene terephthalate in terms of tensile strength, elastic modulus, and barrier properties. However, a major obstacle of PLA foaming is that its low melt strength and elasticity contribute to instability of the foamed cell structure.
With microcellular injection molding it is possible to mass-produce parts with complex geometries and excellent dimensional stability (see Figure 1). Using this technology, PLA can be foamed under a lower temperature to avoid thermal degradation during manufacture of semidurable and durable products with complex geometries. Microcellular injection, however, adds another phase (supercritical fluid) to the polymer matrix, which significantly affects the rheological response of the blend system. Poly(ε-caprolactone)—PCL—is a commercially available aliphatic-aromatic copolyester with high ductility and good processability. Blending PLA with a synthetic biodegradable polymer such as PCL could provide a way of achieving desired properties without compromising biodegradability. In this context, investigating the rheological properties of PLA/PCL melts is crucial to achieving microcellular injection-molded PLA/PCL blends with ideal cellular structure and properties. Here, we describe how the addition of PCL affects the melt rheology and foaming behavior of PLA.
In our work, we compounded PLA/PCL blends with mixture ratios of 100/0, 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, and 30/70wt% using a co-rotating twin-screw extruder with a screw diameter of 27mm and a length-to-diameter ratio of 42. The temperature profile along the extruder barrel ranged from 165 to 180°C. After compounding, the extruded melt strands were quenched in a water bath and subsequently pelletized. Then, standard tensile bars (ASTM D638-03, Type I) were injection-molded using an Arburg (Lossburg, Germany) Allrounder 320 S with a 25mm-diameter screw, and which was equipped with microcellular injection molding capability (Trexel Inc., Wilmington, MA).
To compare the different rheological properties of neat PLA and the PLA/PCL blends, we conducted a frequency sweep test over an angular frequency range of 0.1–100rad/s. Figure 2 shows the frequency dependence of the complex viscosity of neat PLA and PLA/PCL blends with different compositions, measured at 190°C. The complex viscosity (η*) of the neat-PLA melt maintained a constant value over the entire testing frequency range. As the frequency increased toward 10rad/s, we observed shear-thinning behavior. In line with our expectations, an increase in PCL content led to higher shear viscosity. The zero shear viscosity of PLA/PCL (90:10, 80:20, and 70:30) was higher than that of neat PLA. For PLA/PCL (90:10), in particular, we observed a significant enhancement of η* (see Figure 2). When the amount of PCL was greater than 40%, however, we noted a general decreasing trend. We also looked at the rheological behavior of neat PLA and PLA/PCL melts under steady shear flow. We observed profound shear thinning for almost all the specimens we investigated. We ascribe the improvement to chain stretching of PCL in the blend, via trapped entanglement between PLA and PCL.
Scanning electron micrographs of the porous morphologies of the microcellular injection-molded PLA/PCL blends are shown in Figure 3, along with the cell size distribution of the foams. Neat PLA exhibits a broad distribution on the fractured surface (with cells as large as 90μm), and a low cell density. With the addition of 10 and 30wt% PCL in the blends—see Figure 3(b) and (c)—we observed porous morphology with enhanced pore uniformity and higher cell density. The cell nucleation in the final microcellular injection-molded specimen was significantly affected by the dispersion of the minor phase and the overall morphology of the blends. The addition of small amounts of PCL increased the cell density, indicating a heterogeneous nucleation effect of the PCL phase.1, 2
According to classical nucleation theory, the minor phases and their interfaces in a multiphase system serve as heterogeneous nucleation sites. They lower surface free energy by decreasing the intermolecular potential energy of a polymer.3, 4 For an immiscible blend system, the addition of a second phase reduces the activation energy barrier for bubble nucleation because of the large amount of interfacial volume in the blends.5 The addition of PCL and the increased interfacial area in the PLA/PCL blends thus leads to a decrease in surface energy and heterogeneous nucleation energy, and an increase in nucleation density.6 However, continuously increasing PCL content does not lead to higher cell density. For a PLA/PCL (50:50) specimen—see Figure 3(d)—we found that cell coalescence occurred and some irregular pores were observed. We attribute this to the lower viscosity and melt strength of this particular specimen.
In summary, we investigated the effect of rheological properties and morphology on the final cellular structure of microcellular injection-molded PLA/PCL blends. We found that the addition of PCL has a significant effect on the storage modulus of PLA melts. The enhancement of the melts' complex viscosity led to better foaming behavior and pore microstructure. Indeed, owing to the heterogeneous nucleation effect of PCL and the enhanced rheological properties of the blend system, we observed porous morphology with enhanced pore uniformity, decreased cell size, and higher cell density. As a next step, we plan to investigate the mechanical properties and thermal properties of PLA/PCL blends with ideal pore structures.
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