From Science & Research
From Science & Research Supercritical Fluid assisted injection moulding A New Paradigm for Process-friendly Fabrication of Bioplastics Despite increasing interests and outstanding environmental benefits, the application of certain bioplastics in areas, which are currently dominated by petroleum based plastics, such as structural, electrical and other consumer products are limited. This is due to the fact that those bioplastics possess inferior material properties and are relatively expensive. In addition, bioplastics possess narrow processing windows, which makes them vulnerable for thermal degradation while also limiting widespread processability including composites formulation. The material- and processing- challenges of such bioplastics can be overcome by using a unique supercritical fluid (SCF) assisted fabrication technology. SCF is a state of gas (such as CO 2 or N 2 ) above its critical pressure and temperature (Fig. 1). At an SCF state, the gas will have both gas-like and liquid-like properties. Both the properties direct the mixing of SCF with the polymer [1]. SCF effectively swells and plasticizes glassy polymers thereby leveraging low-temperature processing of plastics, which is highly desirable for moisture- and heat-sensitive bioplastics. The plasticization effect by SCF is triggered by increased polymer interchain distance that results in enhanced mobility of polymer segments, a phenomenon similar to plasticizing effect by conventional solvents or additives. A desiring feature of SCF plasticization as opposed to liquid or additive plasticizers is easy removal of the plasticizers from the processed bioplastics. This will aid in nontransformative processing of bioplastics. Moreover, SCFs are environmentally friendly yet being cost-effective. The SCF processing of bioplastics also results in the development of microcellular foams, which possess superior material properties at reduced densities aka material consumption, a feature highly desired for expensive bioplastics. For these outstanding benefits, SCF technology is currently employed for a host of conventional plastics processing technologies such as extrusion, injection moulding, blow moulding, etc. This article focuses on SCF injection moulding (IM) process. SCF Assisted Injection Moulding Technology The SCF technology was commercialized as MuCell ® technology in 1995 [2, 3]. A schematic of the microcellular injection moulding process with microstructure is shown in Fig. 2. In addition to lower temperature processing, reduced material consumption, and improved properties such as toughness, damping ability, etc., the SCF injection moulding technology aids in enhanced moulding thermodynamics which results in quicker cycle time which is highly desired for high-speed production lines. Moreover, the SCF IM process is run at lower pressures which results in stress-free and reduced warped parts [1]. Unlike conventional foams, the SCF IM processed microcellular foams yield reduced cell sizes and enhanced cell densities, typically on the order of 10μm or less and 109 cells/cm 3 or more, respectively. These micron-sized cells may serve as crack arrestors by blunting crack tips, thereby enhancing part toughness [4], impact strength [5], and fatigue life [6]. The microcellular injection moulding process takes place in three steps: nucleation, cell growth, and cell stabilization. First, SCF is dissolved into a polymer melt to form a singlephase polymer–gas solution, that is, the polymer melt is super-saturated with the blowing agent. Then, the pressure is suddenly lowered to a value below the saturation pressure triggering a thermodynamic instability and inducing cell nucleation. Cell growth is controlled by the gas diffusion rate and the stiffness of the polymer–gas solution. In general, cell growth is affected by the following factors: (a) time allowed for cells to grow; (b) state of supersaturation; Fig. 1: Diagram of material phases (reproduced from [2]) Fig. 2: Schematic of the SCF injection molding process. Liquid SCF P cr Solid Critical point Pressure > Gas Cavity Cross Section Supercritical N 2 or CO 2 Higher Back Pressure (80 - 200 bar) T cr Rapid Pressure Drop in Nozzle Triggers Cell Nucleation Single-Phase Polymer-Gas Solution Special Reciprocating Screw 38 bioplastics MAGAZINE [03/14] Vol. 9
From Science & Research (c) hydrostatic pressure applied to the polymer; (d) temperature of the system; and (e) viscoelastic properties of the single-phase polymer–gas solution. Other than processing parameters, materials formulations such as fillers and polymer blends also have strong influence on cell nucleation and growth. Especially, addition of fillers, which act as nucleating agents, leads to heterogeneous cell nucleation. They provide a large number of nucleation sites leading to higher cell densities and smaller cell sizes. Thus, increased adoption of bioplastics, specifically with new formulation designs comprising biobased blends and green composites, will benefit significantly from the SCF IM process. Polylactic Acid-Hyperbranched Polyester-Nanoclay Bionanocomposite Foams This study conducted by the authors exemplifies structure, morphology and properties of polylactic acid (PLA)- hyperbranched polyester (HBP)- nanoclay composite foams processed via SCF IM technology [7]. Poly (maleic anhydride-alt-1-octadecene) (PA) was used as a cross-linking agent for the HBP. As shown in Table-1, PLA was combined with PA, HBP, and nanoclay into a variety of formulations using a twin-screw extruder. Table-2 presents the processing conditions for the SCF IM process. For comparison, samples were also fabricated without SCF. Non- SCF samples are herein after termed as ‘solid’ and SCF samples are termed as ‘microcellular’. As can be observed, using SCF process, a 5ºC reduction in processing temperature was achieved which is due to the plasticizing effect of the SCF. This testifies the enhanced processability of SCF assisted technology. A C E B D F Fig. 3: Representative SEM images of the fracture surfaces of solid and microcellular PLA and PLA-HBP blends: (a) Pure PLA (Solid), (b) Pure PLA (Microcellular), (c) PLA-6%(H2004+PA) (Solid), (d) PLA-6%(H2004+PA) (Microcellular), (e) PLA-12%(H2004+PA) (Solid), (f) PLA-12%(H2004+PA) (Microcellular), (g) PLA-12%(H2004+PA)-2%Nanoclay (Solid), (h) PLA-12%(H2004+PA)-2%Nanoclay (Microcellular), (i) PLA-12%(H20+PA) (Solid), (j) PLA-12%(H20+PA) (Microcellular) G I H J bioplastics MAGAZINE [03/14] Vol. 9 39
