A supercritical fluid is defined as a substance for which both pressure and temperature are above the critical values. These fluids possess physicochemical properties—properties which are both physical and chemical—such as density, viscosity and diffusivity. Density, viscosity and diffusivity are intermediate between those of liquids and gases and are continuously adjustable from gas to liquid with small pressure and temperature variations. Both the capability of supercritical fluids to replace toxic solvents and the ability of tuning solvent characteristics for highly specific separations or reactions have led to the current scientific and industrial interest in supercritical fluids. A supercritical fluid has the unique ability to diffuse through solids like a gas, and dissolve materials like a liquid. CO2 is a promising alternative to noxious organic solvents and chlorofluorocarbons. It has shown versatility as a supercritical fluid in the synthesis as well as processing of polymers owing to its attractive physical properties. It is non-toxic, non-flammable, chemically inert and inexpensive. Its supercritical conditions are easily attained (Tc = 304.15 K, Pc = 7.38MPa) and it can be removed from a system by simple depressurisation.
A Processing Aid for Viscous High Molecular Weight Polymers
The processing of polymers is highly influenced by the viscosity of the bulk materials. Raising the processing temperature or the addition of volatile or harmful plasticisers are often seen as solutions in overcoming the inherent difficulties encountered when processing high molecular weight polymers. However, higher temperatures during processing can lead to thermal degradation. Also, added plasticisers remain in the product and thus alter its properties and performance. The low thermal stability of high molecular weight biodegradable polymers has led to the emergence of supercritical CO2 as a useful processing aid. There are many examples of the use of pressurised gases to lower the melt viscosity of numerous amorphous and semicrystalline polymers. Polyethylene glycol, polystyrene and polydimethylsiloxane are examples of polymers where a viscosity reduction has been demonstrated upon the incorporation of supercritical CO2. Biomaterials as well as polyethylene and polystyrene blends have exhibited similar behaviour.
The use of supercritical fluids in the processing of polymer melts can also lead to changes in the mechanical properties of the materials. Most mechanical property changes during processing can be attributed to the plasticisation of the polymer by the supercritical fluid and the resultant drop in Tg. Some blended polymer materials have shown significant increases in modulus and strength when formed in a supercritical fluid assisted process, this is often due to the tuning of the morphology and degree of crystallisation of the material by the supercritical fluid. Changes in the elastic and creep modulii of materials when processed with supercritical fluids can occur in a range of materials. However these changes and their magnitude are dependent on the solubility of the polymer(s) in the supercritical media and the supercritical material’s ability to induce crystallisation in the system in question.
Supercritical Fluids in Fibre Composites
Polymer composites processing can also utilise supercritical fluid technology and extensive research has taken place in this area recently due to the burgeoning use of these materials in the electronic and medical industries. Companies such as Ireland’s SCF Processing have been pioneering research into bespoke industrial polymer processing solutions working with manufacturers to provide tailored materials processing transfer services. Supercritical fluid can be used to carry the monomer onto the fibres or particles to be used in the composite and to act as a plasticiser for the synthesised polymer matrix when the composite is formed by in situ polymerisation of the monomer. Polymer composites can also be prepared by blending the polymer and the other component in the presence of supercritical media.
Microcellular Foam Products The moulding of microcellular foam products, like many supercritical CO2 processes, entails the formation of a single phase solution. On venting the CO2 by depressurisation, thermodynamic instability causes supersaturation of the CO2 dissolved in the polymer matrix and hence nucleation of cells occurs. The growth of the cells continues until a significant amount of CO2 escapes, the polymer passes through its Tg and the foamed structure is frozen in place. An added advantage of this technology is that due to the lower pressures and softer fills, delicate items can be overmoulded without much of the traditional displacement and resultant need for excessive control features. USA-based Trexel’s MuCell process technology is said to have been the first to widely offer microcellular foaming for both extrusion and injection moulding processes and as a result its technology is often licensed to industrial partners. Optifoam licensed by Switzerland’s Sulzer Chemtech is an example whereby the supercritical fluid dosing element is the nozzle of the machine as opposed to the barrel. Another example is Ergocell, the injection moulding process operated by Japan’s Sumitomo (SHI) Demag for the production of microcellular foamed products. The cycle sequences in the Ergocell process essentially correspond to the sequences in the standard injection moulding process. The decisive difference is in the gas delivery, which takes place simultaneously to plasticising. As the screw draws in, melts and delivers material into the space in front of the screw and—in the process—is being pushed back against the back pressure, gas is fed into the melt from a gas metering station. Thus, the screw moves back at a speed that is a function of the plasticising capacity of the screw. Simultaneously, an amount of gas as preset by the operator is delivered into the melt. In contrast to the MuCell technology, which requires a modified screw assembly, the injection of the supercritical fluid into a module downstream of a conventional plasticisation unit in the Ergocell technology means that it can be easily removed, allowing the injection moulding equipment to be used in a conventional process when required.
Advantages of Supercritical Gas Assisted Injection Moulding
The primary advantages of supercritical gas assisted injection moulding are: reduced operating costs through cycle time reductions of up to 50%, reduced scrap rates, and lower energy consumption (energy savings are based on reduced processing temperatures and are process dependent); lower capital costs through the purchase of smaller and fewer machines, and fewer and less expensive moulds; reduced material costs through component density reduction, thinner design, and material substitution; and the ability to mould thermoplastic parts that have a substantially higher dimensional stability which are free of warpage.
The use of supercritical fluids in the medical device sector affords the opportunity to add a new and exciting dimension to the processing of polymeric materials. Examples of medical devices currently being produced commercially using this technology include endoscopes, heart pumps, inhalers and nebulisers. The use of supercritical CO2 as an inexpensive solvent in many polymer processing applications has already brought many benefits to the industrial sector. As usage becomes more widespread, materials that had previously been designated as ‘un-processable’ due to their high viscosity or their thermal instability can now be reinvestigated with the aid of supercritical fluids. Supercritical fluid technology has not yet reached its potential within industry. However, considerable research into this field is ongoing which would indicate that the number of applications and the usage of this technology are only likely to grow. Supercritical CO2 will also be examined as a sterilant of bioresorbable devices in the print edition of MPN.
Medical Plastics News would like to thank Austin Coffey of the Society of Plastics Engineers European Medical Polymers Division for his help with this article and SciMed.co.uk for the image.