Building better plastics with supercritical carbon dioxide

Nanocomposite plastics are a promising class of materials with a range of applications, particularly in the automotive industry. By incorporating mineral material of nanoscale dimensions, the mechanical properties of the bulk plastic, such as strength, thermal stability, and flame retardance, are drastically improved, with little to no impact on other favorable properties such as low weight. In these composites, uniform dispersal (exfoliation) of the mineral filler throughout the polymer matrix is a key factor. However, this is nontrivial due to the inherent chemical incompatibility of the inorganic clay mineral with the organic polymer of the plastic and the strong internal forces holding together the closely spaced stacks of silicate sheets that make up these clays. Improving dispersal can be accomplished using organoclays. Here, the stacked structure of the clay mineral is expanded by replacing internal inorganic ions with organic ions (surfactants), and the use of small organic polymers (compatibilizers) that can interact with both the hydrophilic clay and the hydrophobic plastic. These measures have had some success in enhancing exfoliation of the filler, but technologies are needed that can achieve more complete exfoliation. Several technologies have recently been presented in the literature for using supercritical CO2 (scCO2/ to improve organoclay exfoliation in polyolefins.1–5 ScCO2 can permeate the composite or its components during processing, acting as a solvent to enable more effective exfoliation, and is readily available and environmentally benign. A key distinction between the proposed methods is which of the nanocomposite components (organoclay, compatibilizer, or polymer matrix) is present with the scCO2 during processing. The effectiveness of each approach depends on the influence of the scCO2 on the material or materials. However, the function of CO2 in these methods has not been studied until now. Detailed batch studies with scCO2 and the components of a polyolefin nanocomposite have shown that the gas acts to plasticize the materials present.6–8 For an organoclay alone, this plasticization causes Figure 1. X-ray diffraction patterns for three different methods of using supercritical CO2 (scCO2) versus a conventionally prepared nanocomposite thermoplastic olefin (nTPO) and neat organoclay.9 DI: Direct injection. PO: Pretreated organoclay. MS: Masterbatch-scCO2 injection. 2 : Angle of diffraction in degrees. a.u.: Arbitrary units.