Supercritical fluids—a novel approach to magnetic media production?

Tribology International Vol. 31, No. 9, pp. 485–490, 1998  1999 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0301–679X/98/$19.00 ⫹ 0.00

PII: S0301–679X(98)00052–8

Supercritical fluids—a novel approach to magnetic media production? Ken Johns*

The production of magnetic media requires a combination of substrate surface engineering, magnetic ink or metal deposition, and a thin-film high-performance lubricant to protect the surface and read–write head from abrasive wear. The lubricant, such as a perfluoroether oil or derivative, is normally applied via a dilute solution in fluorocarbon 113 solvent which is now known as a banned ozone depleter. The search for alternatives has led to developments in water-borne coatings but water can be an expensive and troublesome solvent. A novel means of delivering magnetic inks to a surface may be via supercritical carbon dioxide or xenon spray systems. Fluorinated and silicone oils are soluble in supercritical CO2 and can also be applied by this residue-free technique. Supercritical fluids offer the opportunity to deposit/impregnate metals into surfaces via organometallics. Could this replace the future magnetic media production?  1999 Published by Elsevier Science Ltd. All rights reserved.

Introduction

쐌 solvent-free coatings.

Magnetic tape for computer, audio or video usage is a substantial market worth more than US$14 billion. In 1994, more than two billion T120 video cassettes were produced.

Solvents used in magnetic tape production are the following:

Their production technology uses organic solvents to apply magnetic coatings to the film substrate and to apply protective lubricant layers to the cured media. Environmental emission legislation has several requirements for the minimum release of volatile organic compounds (VOCs), and the magnetic tape manufacturing industry has been denoted as one of the top 40 sources of hazardous atmospheric pollutants. The paint and printing ink industries have been forced to focus on technologies which may reduce solvent emission, i.e. 쐌 쐌 쐌 쐌

powder coating; high solids; radiation cure; water-borne coatings; and

Chemical and Polymer UK, 18 Kings Lane, Windlesham, Surrey GU20 6HR, UK *Tel.: ⫹ 44-1276-474826; fax: ⫹ 44-1276-476032; e-mail: k.johns @pra.org.uk

쐌 쐌 쐌 쐌 쐌 쐌

2-butanone [or methyl ethyl ketone (MEK)]; 4-methyl-2-pentanone methyl isobutyketone (MIBK); tetrahydrofuran; toluene; cyclohexanone; and FC113 (for application of fluoro lubricants).

Recovery systems are in use but these are expansive and may need to be more complex and effective in future as legislation becomes more stringent. Water-borne systems for video tape have been formulated48, but require reformulation of dispensing agents, binders and lubricants. Such reformulation can lead to unknown problems in production, storage and usage. Aqueous-based silane-containing formulations have been developed for the surface treatment of plastic films for magnetic tapes. Acrylate-based formulations49,50 for solventless magnetic tape manufacture are under development. These rely on the replacement of solvents by liquid acrylate monomers and oligomers

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which act as fluid carriers up to the point where they are cured by electron-beam radiation. The magnetic particles are coated with a silane coupling agent in order to link binder and particles.

Table 1 Factors affecting polymers/resins in scCO2 Solvent system

Polymer

Powder coating is not applicable to tape production, but current developments include:

쐌 Supercritical fluid

쐌 쐌 쐌 쐌 쐌 쐌 쐌

쐌 쐌 쐌 쐌

쐌 Amorphous or crystalline 쐌 Structure 쐌 Molecular weight 쐌 Functionality 쐌 Molecular weight/functionality ratio

finer particle size; uniform size distribution; low-temperature cure; improved rheology control; thinner films; uniform films; and submicrometre-sized particles.

Much work is underway on iron oxide nanoparticles and iron nanocolloids from materials such as iron pentacarbonyl51,53. The ideal approach might be a readily available, nonVOC, inert solvent that leaves no residue and which can be recycled or released into the atmosphere without problems. It would be better still if the solvent could also be used for surface preparation, impregnation of metallics into polymers so as to yield nanocomposites, and for the application of coupling agents and lubricants. Why not use supercritical fluids to apply fluorocarbon thin-film lubricants or even self-lubricated nanocomposite films?

Supercritical fluids (SCF) systems Supercritical fluids (SCF) systems, although already established for some applications1, may represent more important technologies in the future. The primary motivation for adopting such processes was concern about and legislation against conventional solvents. SCFs are widely used in small-scale laboratory extraction and analysis2. They are established for large-scale extraction of caffeine from coffee, flavours from hops and many others1, with plant sizes up to 50 000 tons per year throughput. A Philip Morris semi-continuous denicotinization plant is said to employ pressure chambers of 1.5 m diameter and 5 m height. The outlet gas is passed through activated carbon and recycled1.

Temperature Pressure Co-solvents Surfactants

solubility

of

100 times faster than in water at ambient temperature. The most spectacular demonstration of its unusual characteristics, shown originally by Franck and colleagues in Karlsruhe, is that flames can be produced in dense supercritical water at pressures of up to 2000 bar2. Supercritical fluids represent clean solvents/carriers that neither leave residues nor impose environmental load. A number of factors determine the solubility of polymers in supercritical carbon dioxide (scCO2) and these are given in Table 1. Comparison of the supercritical temperature and pressure conditions of some candidate fluids for industrial exploitation (Fig 1) may exclude those requiring extreme conditions, such as water, and others on environmental (SF6) or cost grounds (xenon). Supercritical carbon dioxide (scCO2) offers an acceptable combination of pressure and temperature to achieve supercritical conditions. ScCO2 is not a good solvent for most materials, which are scCO2-phobic. However, both silicone and fluoro products may be regarded as CO2-philic and, therefore, potentially more soluble. It must be made clear that whilst the 100% fluoropolymers, such as poly(tetrafluoroethylene) (PTFE), may be soluble to some degree in supercritical CO2, the temperature and pressure conditions are so high that they are impractical for conventional coating procedures. Nevertheless, there are some applications

Background

When fluids and gases are heated above their critical temperature and compressed above their critical pressure they enter a supercritical phase where some properties, such as solvent power, can be dramatically changed. Water is supercritical at temperatures above 374°C and pressures above 220 bar. It changes more than many other substances upon becoming supercritical because the hydrogen-bonded structure breaks down, becoming less polar, and can become homogeneous with relatively large amounts of organic compounds as well as permanent gases such as oxygen, making them available for chemical reaction. Diffusion rates are over 486

Fig. 1 Critical temperature and pressure for selected gases, highlighting CO2

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Supercritical fluids—a novel approach to magnetic media production?: Ken Johns Solubility of silanes/silicones and fluoro compounds3,4,9–12

The solubility of poly(dimethylsiloxane) (PDMS) in CO2/toluene mixtures has been attributed to comparable solubility parameters and the interaction between CO2, a weak Lewis acid, and the strong electron-donor capacity of the siloxane group. The oxygen in perfluoropolyethers also has an electron-donor capacity.

Fig. 2 Supercritical spray system outline. Carbon dioxide gasification gives vigorous atomisation. Sprayed material and deposited coating are identical if no solvent is lost in the spray. Supercritical spray conditions: 1200–1600 psi pressure, 40–60°C.

demanding deposition of partially fluorinated materials from low-concentration solutions. 쐌 A coating process requiring the deposition of 1/2-t 2% solutions of functionally end-capped, partially fluorinated hydrocarbons or silicones may be feasible. 쐌 Acrylate may be the most appropriate functionality. 쐌 Oxygen-containing perfluoropolyether derivatives, which are related to the preferred surfactant–compatibilizers, could be important. 쐌 The use of co-solvents and designed surfactants (amphiphiles/stabilizers) can improve solubility. Liquid spray

Viscous fluids can be torn apart into an efficient spray by the decompression of a supercritical fluid. Both one- and two-component systems can be handled13,14. This process has been loosely described as "liquid powder coating". The Union Carbide spray process (Fig 2) relies not on solubility of the solvent-reduced paint but on a combination of rheology modification and decompressive spray energy. In the commercial application of a silicone non-stick coating to metal bakeware using electrostatic automatic spray guns solid in the coating concentrate were increased from 20% to 64%, together with a number of other benefits as shown in Table 2.

Table 2 Silicone non-stick coating: supercritical spray benefits VOC level fell from 6.3 to 3.4 lbs/gallon. Material utilisation increased by 23%. Coverage per gallon increased four-fold. Solvent emissions reduced by 89%. Overspray collected and recycled. No incineration required. Appearance of the finish was improved. Non-stick performance was improved.

The solubility parameter of CO2 at the critical point is 5.5–6.0 (cal/cm3)1/2 which makes it comparable with pentane but can be raised as high as 9.0–9.5(cal/c3)1/2 by increasing the pressure, when the solvent power is more akin to that of benzene or chloroform. Fluorinated oils have the lowest solubility parameter of any known liquid at 4.5–5.0 (cal/c3)1/2. These figures indicate that CO2 should exhibit miscibility with fluorinated oils. Solubility in CO2 may rise upon replacement of –CH–2 with –CF2 or CF(CF3)O groups3,4,9–12. 1H,1Hpentadecyl fluorooctyl acrylate has a solubility50 of 25% by weight in scCO2. Perfluoropolyethers (PEPEs) will exhibit good solubility as will silanes. Co-solvents

The addition of small quantities of co-solvents, also known as modifiers or entrainers, can enhance the solubility characteristics further. Even though, in earlier years, most of the attention was on single processing fluids such as carbon dioxide and extractions as the primary mode of application, in recent years emphasis has been shifting to binary and multi-component fluids and processes with a greater degree of complexity which may include either physical or chemical transformations. Some modifiers are shown with relevant properties are listed in Table 3. Powder coating15

The search is on for thin-film uniform coatings from powder with the ultimate prize being automotive clear top coats16. General Motors, Ford and Chrysler cooperate in a ‘low emission paint consortium’ that is spending US$20 million on a test site to study clear powder coatings for full-body automotive top coat use. In 1996, BMW (Germany) opened the world’s first Table 3 Typical modifiers Modifier

Tc (°C)

Pc (atm)

Methanol Ethanol 1-Propanol 2-Propanol 1-Hexanol 2-Methoxy ethanol Tetrahydrofuran 1,4-Dioxane Acetonitrile Dichloromethane Chloroform

239.4 243.0 263.5 235.1 336.8 302

79.9 63.0 51.0 47.0 40.0 52.2

267.0 314 275 237 263.2

51.2 51.4 47.7 60.0 54.2

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full-body automobile powder clear coat line. Applications such as cookware are also of interest17. Thin films may depend on: 쐌 particle—size, morphology, size distribution; 쐌 rheology control; and 쐌 charging system. Powder coatings are progressing fast because they represent ‘clean’ technology. Potential now exists for a marriage with another clean technology—supercritical fluids—the combination reducing the deficiencies of both individual processes. Supercritical fluid technology may yield chemically homogeneous powders, controlled in morphology and size distribution and produced at relatively low temperatures5, allowing a wider range of chemistries to be utilized. Rapid expansion spraying from supercritical solution can yield submicrometre-sized powders and fibres18–26. Fluorinated powder coatings exist and might be highly suitable for supercritical powder coating development27–37. Silylated clear coats are established in the automotive industry47. Powders from organometallics38–41

Fine pigment powders are also possible. Metal alkoxides such as titanium isopropoxide, which is soluble in supercritical ethanol, can undergo rapid expansion spraying to form submicrometre-sized titanium dioxide powders42–45.

Polymer impregnation Supercritical fluids are effective plasticizers for a number of polymer processing applications. Exposing an amorphous polymer to scCO2 can result in considerable swelling and plasticization due to high CO2 solubility. This plasticization may dramatically enhance diffusion of some products, such as dyes, into certain polymers. scCO2 swells poly(methyl methacrylate) to give an opportunity for accelerated drying of the polymer films. Polymers can be impregnated by simultaneously contacting the polymers with an impregnating additive, carrier liquid and supercritical fluid. The additive may be substantially insoluble in the supercritical fluid and the carrier liquid may be substantially insoluble in the supercritical fluid. Thus, low-density polyethylene may be impregnated with a dye in the presence of water. The polymerization of monomers such as styrene in scCO2-swollen polymer films, such as poly(chlorotrifluoroethylene), is possible. Impregnation of metal compounds into polymers is important in developing catalysts and may also be used to ‘engineer’ the surface of polymers into what might be described as ‘nanocomposites’, but conventional solvent techniques are not ideal.

solvency, diffusivity and low viscosity. The process can involve the dissolution of precursor reactants in a supercritical fluid and then deposition onto or into a substrate. Supercritical xenon and scCO2 are good solvents for a wide range of organometallic compounds, and fluoro or silicone modification may enhance solubility characteristics. The relatively low critical temperatures of xenon and CO2 allow the use of thermally unstable precursors and offers potential to modify plastic substrates. Supercritical CO2 can swell, reversibly, some polymer films to enhance impregnation52. Silver infusion into polyamide films to form a highly reflective surface has been reported, where the scCO2assisted infusion process resulted in an economic nearsurface deposition. Micro-emulsions

Systems comprising micro water droplets suspended in an scCO2 ‘oil phase’6 can be achieved with the use of appropriate surfactants, of which the best appear to be fluorinated7. Micro-emulsions in supercritical hydrofluorocarbons are also possible8. Potential may also exist for speciality coatings via lowconcentration solutions of fluorinated products in a supercritical fluid for thin-film deposition, conformal coatings, release coatings, etc. Supercritical carbon dioxide will dissolve in formulated systems to improve flow and plasticize melt-processable materials to improve melt-flow characteristics and lower the glass transition temperature. Xenon and recycling

Xenon is technically an interesting supercritical fluid since the critical temperature is about 17°C (cf. 31°C for scCO2) and the critical pressure is about 55 atm (cf. 70 atm for scCO2)46(Fig 3). We have not considered this previously because xenon is more expensive at present, but the price could fall dramatically if a demand arose for it. Xenon is possibly a better solvent and it is possible that scXe may be a technically better candidate for wood impregnation with the higher cost being offset by lower pressure, less expens-

Impregnation and thin-film deposition for engineered nanocomposites Organometallics

Supercritical fluid chemical deposition (SFCD) offers an approach to the deposition of thin films because of 488

Fig. 3 Critical temperature and pressure for selected gases, highlighting xenon

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ive equipment and the possibility of simple recycling. We would suggest some basic investigation of xenon to compare it with CO2.

A Centre of Excellence for supercritical fluid technologies for coatings Supercritical fluid technology is an ‘unknown’ for the paint industry and the cost of entry can be high. Thus the technology has been ignored. It is for these reasons that Chemical and Polymer, in partnership with Surrey and Nottingham Universities, intend to establish a Centre of Excellence. This centre will make available spray and impregnation facilities so that, for a reasonable fee, interested parties can gain hands-on experience and learn to formulate for optimum use of the equipment.

18. Matson, D. W. et al., Making powders and films from supercritical fluid solutions. Chemtech, August 1989, 480. 19. Mawson, S. and Johnston, K. P., Precipitation with a compressed anti-solvent: advanced concepts. in Proc. ACS Meeting Autumn 1996, American Chemical Society, Washington, DC, pp. 180– 181. 20. Mawson, S. et al., Formation of sub-micron fibres and particles from supercritical carbon dioxide solutions. Macromolecules, 1995, 28, 3182–3191. 21. Peterson, R. C., Precipitation of polymeric materials from supercritical fluid solutions: the formation of thin films, powders and fibres. Polym. Prepr., 1986, 27(1), 261–262. 22. Powders of controlled particle size by expansion in supercritical fluids. Inf. Chim., 1995, 32(365), 90–91. 23. Otefal S.p.a., Process for the production of powders with controlled particle sizes and powdery products so obtained. EP 0661 091 Al, 9 December 1994. 24. Producing perfect powders. Chemistry in Britain, January 1996, 15. 25. Uses of polysiloxanes in powder coating. Paint & Ink Int., May– June 1996, 2–4.

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12. Cotts, P. M., Solution properties of a group of perfluoropolyethers: comparison of unperturbed dimensions. Macromolecules, 1994, 27, 6487–6491.

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13. Union Carbide Chemicals & Plastics, Methods for the spray application of latex-borne coatings with compressed fluids. PCT WO95 09056.

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42. Chor, K. et al., Synthesis of submicron titanium dioxide owders in vapor, liquid and supercritical phases. Mater. Chem. Phys., 1992, 32, 249–254. 43. Tadros, M. E. et al., Synthesis of titanium dioxide particles in supercritical carbon dioxide. J. Supercritical Fluids, 1996, 9, 172–176.

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Supercritical fluids—a novel approach to magnetic media production?: Ken Johns 44. Facile method converts carbon dioxide to industrially important compounds. Chem. Eng. News, 11 November 1996, 8. 45. Gourinchas, V., et al., Continuous reactor for titanium dioxide powder synthesis in super-critical reaction medium. In Proc. 3rd. Int. Conf. on Supercritical Fluids (Strasbourg, 17–19 October 1994), Vol. 3, pp. 315–319. 46. Krukonis, V., McHugh, M. and Seckner, A., Xenon as a supercritical solvent. J. Phys. Chem., 1984, 88, 2687. 47. Chen, M. J. et al., Silicones in coating technology. Surf. Coat. Int., 1996, 539–554. 48. Cheng, S. et al., Waterborne coatings for videotape. Chemtech, October 1995, 35–41.

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49. Ellison, M. M. et al., Acrylate formulations for a solventless magnetic tape manufacturing process. Department of Chemistry, The University of Alabama. 50. Kinetics of polymerisation of 1H, 1H, 5H octa fluoropentyl acrylate in supercritical carbon dioxide. 51. Dagani, R., Bubbly synthesis yields iron nano colloids. Chem. Eng. News, 13 January 1997, 26. 52. Watkins, J. J. et al., Chemistry in supercritical carbon dioxideswollen polymers. Polym. Mater. Sci. Eng., 1996, 74, 402–403. 53. Stroeve, P. et al., Growth of iron oxide nanoparticles on selfassembled monolayers. Department of Chemical Engineering and Materials Science, University of California; University of Mainz and Max-Planck Institute of Mainz.

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