Foam injection molding of poly(lactic acid) with environmentally friendly physical blowing agents

Highlights

• Foam injection molding of poly(lactic) acid was conducted by using a physical, environmentally friendly blowing agent.

• An increase of mold temperature induces a higher reduction in density of foamed samples.

• At higher injection flow rate foaming is more homogeneous.

• Adding a small percentage of talc to PLA allows to obtain foamed parts with a much better morphology.

Abstract

Biodegradable polymers present a very narrow processing window, with the suitable processing temperatures close to the degradation conditions. The aim of this work was to analyze the foamability of a biodegradable polymer, namely the poly(lactic acid), PLA. Foam injection molding was conducted by using a blowing agent under high pressure and temperature to produce parts having a cellular core and a compact solid skin (the so-called “structural foam”). The effect of a physical blowing agent (PBA) on density and morphology of foamed parts was characterized. A masterbatch of PLA and talc was prepared and adopted to obtain a compound containing 3% of talc. On adding this percentage of talc to PLA it was possible to obtain foamed parts with a much better morphology.

Introduction

Structural foams are plastic foamed parts which can be obtained using conventional injection-molding machines and a physical blowing agent to produce a cellular structure. The gas is mixed with the molten polymer inside the cylinder of the injection molding machine under relatively high pressure and then injected into the mold according to the conventional process. Inside the cavity, the abrupt pressure drop causes the gas to separate and thereby to form cells inside the polymer. The main advantages of foam injection molding include a reduced product weight with a stiffness-to-weight ratio comparable to the unfoamed counterparts and also a faster production cycle due to fact that no packing step is needed. Furthermore, the presence of gas inside the polymer can reduce its viscosity, thus allowing the processability of the polymer at lower temperatures and pressures. This is an advantage particularly for biodegradable polymers, which are thermally sensitive and have narrow processing windows (Speranza et al., 2014). Polylactide is a biobased, biodegradable and biocompatible aliphatic polyester and has found many applications in biomedical industries and packaging due to its unique set of properties. However, it is very difficult to control the foaming of PLA by injection molding, because of its low melt strength and slow crystallization kinetics (Ameli et al., 2013). Also for this reason, the literature studies on foam injection molding of PLA are just a few and quite recent. In direct relationship with the subject of the present work, the effect of injection molding parameters such as mold temperature and supercritical nitrogen content on foam cell size were investigated by Kunimune et al. (2010). Other authors (Kramschuster et al., 2007) present the effects of adding a nanoclay on the mechanical properties and on the cell morphology of microcellular polylactide components. They found that the addition of nanoclay into biobased polylactide facilitates the formation of smaller cell sizes and higher cell densities. Similar results were found by Pilla et al. (2010), that investigate the effects of addition of hyperbranched polyesters and nanoclay on the material properties of both solid and microcellular polylactide (PLA) produced via a conventional and microcellular injection-molding process. Recently, Ameli et al. (2013) studied the effect of adding a small percentage of talc to the PLA, showing substantial improvements both in morphology and in the properties of the foamed parts. It was observed that the talc acts as a crystal-nucleating agent during the crystallization process and thereby it determines an increases of crystallinity in the final object (Naguib et al., 2003). Pilla et al., 2009a, Pilla et al., 2009b observed that the addition of epoxy-based chain-extender decreases the degree of crystallinity of PLA in both the solid and microcellular samples. This is due to the formation of a branched PLA molecular structure that inhibited the crystallization process and reduced the degree of crystallinity of PLA. Furthermore, the degree of crystallinity of the microcellular samples was found to be higher than that of their solid counterparts.

In the majority of the papers in literature, the PLA/additive systems were prepared by melt compounding, a process that can reduce the molecular weight of the PLA (Villalobos et al., 2006), causing changes in crystallization kinetics, viscosity, mechanical and thermal properties (Gorrasi and Pantani, 2013).

In this work, a commercial polylactide (PLA) grade was injection molded and foamed by using supercritical nitrogen as physical blowing agent (PBA). First, a processing window in which it is possible to obtain foamed parts having good distribution of cell size and high void fraction was identified. Then, the effect of a micrometric talc additive on the foam morphology was assessed. In particular, a masterbatch with PLA and talc was prepared by means of a Brabender extruder, in order to minimize the effects due to the thermal history experienced by the material. Injection molding parameters such as mold temperature, injection flow-rate and PBA content were varied in order to investigate their effects on foam cell, void fraction and mechanical properties.