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J O U R N A L O F M A T E R I A L S S C I E N C E 37 (200 2) 4743 4752

Review

Environmentally friendly coatings

using carbon dioxide as the carrier medium

J . N . H A YDepartment of Chemistry, University of Surrey, Guildford, Surrey, GU2 7XH, UKE-mail: [email protected]

A . K H A NSchool of Engineering, University of Surrey, Guildford, Surrey, GU2 7XH, UK

The use of supercritical or liquid carbon dioxide as a medium for delivering coating

systems is attracting much interest because of concerns over the environmental effects of

volatile organic compound (VOC) emissions from conventional coatings. Significant

reductions in VOC emissions can be achieved by replacing some or all of the organicsolvent by CO2 in spray coatings. Technical and commercial benefits are also claimed for

these systems, including improved coating efficiency and operating cost savings. In this

review, the range of current and potential applications achievable using CO 2-based

coatings is discussed. In addition to spray coatings onto a variety of substrate surfaces,

CO2 processes can be used to produce controlled particle size powders for use in powdercoatings and also for the coating of preformed particles such as metal powders and

pharmaceuticals for controlled release in drug delivery. Use of CO 2 in spin coating and

microlithography offers the potential for significant waste reduction. Specific substrates

where use of CO2 can be beneficial include the treatment of building stone and wood

treatment. CO2 can aid surface impregnation of substrates because of its high diffusivity

and the potential for substrate swelling. C 2002 Kluwer Academic Publishers

1. IntroductionIn coating applications, increasingly stringent pollu-

tion controls have forced moves away from the use oforganic solvents towards alternative coating technolo-gies. Volatile organic compound (VOC) emission limitsare being reduced in many industrialised countries, forexample new European Union (EU) legislation came

into force in 2000. It has been estimated that ca. 6% ofman-made VOC emissions across the EU emanate fromcoating operations [1].Moves to reduce VOC emissionshave accelerated the development of aqueous and pow-

der coating systems. Although water is an attractivealternative, it does have drawbacks. The properties ofwaterborne polymers are often inferior to those of theirsolvent-cast analogues. The high latent heat of vapor-isation leads to high drying costs and, where natural

drying is required, this can be problematic in cold cli-mates or those of naturally high relative humidity. A fur-ther problem is that of waste-water clean-up where everlower limits are being set by regulatory authorities oncontamination levels. Powder coatings can overcome

these drawbacks, but significant reformulation of the

corresponding solvent-based system may be required,leading to changes in coating properties. Control of therheology of the systems in order to promote effective

coalescence of the powder particles to form a coherent

film is of crucial importance. Alternative solutions tothe solvent problem have therefore been sought.

Supercritical fluids (SCFs) have unique character-istics that make them potentially useful solvents andprocessing aids in many chemical processes and oper-ations [24]. A material exists as an SCF above its crit-

ical temperature and pressure, where it can no longerbe liquefied by increasing pressure. This can be seenin Fig. 1 [5]. At extremely high pressures, an SCF cansolidify. For a pure substance, the critical point is thehighest temperature and pressure where a gas can be

liquefied. There is no apparent distinction between ahigh-pressure gas and a supercritical fluid, as a super-critical fluid will occupythe full volume of thecontainerin which it is stored, like other gases. For a supercriti-

cal fluid above its critical temperature and pressure, nophase separation occurs and therefore no meniscus canbe seen.

Supercritical carbon dioxide (scCO2) is the mostwidely used supercritical fluid and is becoming anattractive choice for replacing organic solvents in a

number of chemical processes, including coatings. It

is already in widespread commercial use for the de-caffeination of coffee beans and there are large newemerging markets in chemicals manufacture, dry clean-

ing and other extraction processes. CO2 is available

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Figure 1 Liquid-vapour phase diagram of carbon dioxide showing criti-

cal temperature (Tc= 31C) andcritical pressure (Pc= 73 bar) [5]. Also

shown in the figure are three other isotherms at 0C (below critical tem-

perature) and at 70C and 200C (above critical temperature).

in abundance and is non-toxic, non-flammable, rela-

tively cheap and has zero ozone-depletion potential.Vast quantities of CO2 are produced as a by-product in

ammonia, hydrogen, and ethanol plants, from burningfossil fuels in power stations and in fermentation pro-cesses. Despite concerns over the greenhouse effect

of CO2, it can be considered as an environmentallyfriendly alternative to existing organic solvents, sincethe CO2 used in this way is already recycled and sothe net load on the atmosphere is unchanged. Carbon

dioxide is a gas under ambient conditions and thereforeits removal from coatings in the post processing opera-tion is very easy and no complicated drying and solventremoval operations are involved.

In the last 10 years, supercritical CO2 has attracted

increasing interest as a replacement for organic solventsin polymer processing, in particular as a polymerisationmedium [410]. Free radical polymerisation is the mostcommon polymer technology using supercritical CO2,where reaction is often carried out by dispersion poly-

merisation. In dispersion polymerisation, the morphol-ogy of the particles that nucleate during the reactionis controlled by a surfactant. To stabilise these polymerlatexes fluorocarbon and siloxane based homopolymers

and copolymers have been developed which are solublein scCO2 [1114].

2. Supercritical coatingsIn comparison with activity on polymerisation inscCO2, relatively little research work has been directed

towards the application of supercritical carbon dioxidein the coatings area. However, an increasing numberof publications have appeared in the recent literature,reporting use and/or proposing the use of supercriti-cal carbon dioxide as a replacement for conventional

organic solvents [1521]. Some commercial processesuse supercritical carbon dioxide for spray coating ofsubstrates, for example the UniCarbTM process. Dipcoating provides an alternative approach to the coatingand impregnation of a range of substrates. The possibil-

ity of swelling of the substrate with enhanced penetra-tion depth of the coating offers a potential advantage inthe use ofCO2 in addition to the environmental benefitsoutlined above. Other processes such as rapid expan-

sion of supercritical solutions (RESS) are used to coat

polymer/solute layers onto particles. The latter processis also used to synthesise particles of different mor-phologies. These processes are described below, alongwith other supercritical systems which have been usedor have potential use in coatings. It should be noted that

the presence of other soluble materials (e.g., a coatingformulation) in a supercritical fluid can change its crit-ical parameters, and the extent of the change depends

upon the concentration of the other substance. Whatthis means in practice is that a supercritical coatingsystem may vary between supercritical and sub-criticalduring different parts of the process. From the point ofview of much of this review, the key factor is the useof environmentally friendly CO2 rather than the precise

state of the coating system. Thus, where the term su-percritical is used, this means only that the CO2 is inthis state at some stage during the process.

The areas where supercritical coatings have beenused or proposed include the following [1518].

Metal primers and coatings. Textiles, fabrics and fibres.

Pharmaceutical particle coatings. Coating/impregnation of porous materials: brick,

cement, stone, wood. Metal particles and solid propellant fuel particles.

Biomedical devices. Food coatings. Leather and paper coatings. Coatings for polymers and elastomers. Glass coatings.

A further major advantage of CO2 over organic sol-

vents in applications in the food, medical, healthcareand pharmaceutical industries is the lack of toxicityand the absence of any taint or odour caused by resid-ual solvent in the final product.

2.1. The UniCarbTM processUnion Carbide and Nordson developed a process (theUniCarbTM process) for spray coating substrates froma medium such as carbon dioxide [1517]. Following

the recent acquisition of Union Carbide by Dow Chem-ical Co., this technology is now proprietary to Dow. Forpolymer coatings, the use of a SCF-organic solvent sys-tem has been claimed to have a number of advantagesover the alternative approach of powder coatings, for

example improved film coalescence and quality and noneed to re-formulate the resin from that used in solvent-based coating. The UniCarbTM process uses a new typeof spraying process called a decompressive atomisation[1517] to produce the droplets. The coating material

(or concentrate in a solvent such as methanol) is mixedwith the carbon dioxide and subsequently dischargedfrom the machine as an atomised spray. Whereas con-ventional airless and air sprays atomise the coating

material through external shearforces, the rapiddecom-

pression and evaporation of the CO2 within the coat-ing material produces high internal forces which over-come surface tension and cohesive forces and produce ahighly uniform spray [1517], even for highly viscous

coatings (up to ca. 500 P). The spray has a narrow

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droplet size distribution with an average diameter typ-ically of the order of 2050 m. The use of CO2 as thecoating carrier medium and the production of a highlyuniform spray result in a number of claimed benefitsfor the UniCarbTM process compared to conventional

coating processes [1517]:

Significant reductions in VOC emissions.

Reduced worker exposure to organic solvents. Improved coating appearance and performance. Higher transfer efficiency. Reduced operating and capital costs. Reduced maintenance costs.

2.2. Polymer coatingsA supercritical carbon dioxide polymer coating systemmay be a truesolution ofthe polymer in CO2, a solutionof CO2 in the polymer (as can occur in the UniCarb

TM

process) or a dispersion. Pure carbon dioxide solutionsystems are limited by the solubility of the polymer in

carbon dioxide. There are few polymers that are appre-ciably soluble in carbon dioxide, silicones (siloxanes)

and amorphous fluoropolymers representing the twomost common examples (Fig. 2) [2225]. A co-solventcan be used in a coating system where the amount oforganic solvent used is greatly reduced compared tothe conventional solvent-borne system. In the case of

thermosetting polymers, monomeric coating formula-tions may show reasonable solubility in CO2. To over-come the problem of the limited solubility of polymersin carbon dioxide, the use of dispersion coatings of-fers a potential solution, which could help to broaden

the applicability of carbon dioxide coating systems andfurthermore produce essentially 100% VOC-free sys-tems [18, 21]. The quality of the dispersion will dependon the availability of CO2-compatible stabilisers. The

choice of these stabilisers has been mostly limited toeither fluoropolymers (e.g., I) [26, 27] or silicone poly-mers (e.g., II) [8, 10, 14, 28], which are relatively ex-pensive; however, the recent development of compara-tively cheaper CO2-compatible polymers (e.g., III) [29]

may open the way to development of cheaper stabilis-ers and more widespread use of supercritical carbondioxide polymer coatings technologies.

(I)

(II)

(III)

Figure 2 Polymer compatibility with supercritical CO2.

To date, limited work has been reported on the useof scCO2 for the delivery of coatings based on polymer

dispersions [30]. Shim et al. [21] used a dispersion ofpoly(2-ethylhexyl acrylate) in scCO2 and investigatedthe effects of various parameters (nozzle size, spraydistance and time, fluid viscosity) on resultant filmquality after spraying of the dispersion onto a sub-

strate. A siloxane based surfactant was used and the

authors investigated both the use of a preformed poly-mer dispersion and re-dispersion of partially coalescedpolymer. The work clearly demonstrated the impor-tance of the presence of surfactant in the disper-

sion if good quality coatings were to be obtained.It was reported that the spray nozzle size had lit-tle effect on the spray pattern, whereas the spray-ing distance and fluid (suspension) spray velocity

had a significant effect on the spray pattern. It wasalso noted that the CO2 may remain dissolved inthe polymer for a finite period of time after atom-isation, but the authors proposed that large droplets

(ca. 70 m) were necessary to retain the CO2 for asufficient period of time to aid coalescence.In our own laboratories, we have recently studied su-

percritical coatings using the UniCarbTM process [30].We have found that the concentration of carbon diox-ide, temperature, pressure, nozzle orifice size, all affect

the spray pattern (see Fig. 3). The equipment has beenmodified to measure in situ viscosity of the coating sys-tems. Work on relating the viscosity to the nature of thedroplets and the spray pattern formed during spraying

is underway and will be reported separately.CO2 can act as a plasticiser for polymers [10, 31],

dramatically reducing the glass transition temperature(Tg) of the polymer in some cases [31]. For polystyrene(PS), for example, a minimum was observed in the

polystyrene Tg-pressure (of CO2) curve at a Tg of36C,

well below the normal PS Tg of ca. 100C. CO2 low-

ers the Tg of syndiotactic polystyrene at the rate of0.92C/bar [32]. It has also been demonstrated thatPS melt viscosity can be lowered by 2580% by CO2[33]. Consequently, in CO2, a normally glassy poly-mer may behave as though it is a rubber above its Tg,with a possible improvement in the coalescence prop-erties of a resultant dispersion coating using this poly-

mer. In supercritical spray coating, the rate at which

the CO2 evaporates from the polymer particles fol-lowing the decompressive atomisation process willdetermine to what extent this will facilitate the co-alescence, so long as continued degassing does not

damage ultimate coating quality. Another possible

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Figure 3 Effect of carbon dioxide temperature on the shape of the spray at relatively low CO2 concentration and 7 MPa pressure, temperature (a)40C and (b) 23C.

consideration is the effect of any dissolved CO2 onrebound of the coating droplets or particlesthis willhave an important effect on both coating efficiency andquality [34].

The effect of the stabiliser on the particle coalescenceprocess is also relevant to the potential use of polymerdispersion coatings. In styrene dispersion polymerisa-tion using a siloxane diblock copolymer (II), it wasfound that multi-particle polymer aggregates formed

during the CO2 venting process [14]. The authors sug-gested the aggregates formed as the solvent-swollen,extended PDMS chains collapsed during venting. Itwas noted, however, that spraying the polymer product

from the reactor prevented secondary aggregate for-mation. Further studies of CO2 dispersions of poly(2-ethylhexyl acrylate) (PEHA) stabilised by fluorinatedblock copolymers have helped to clarify the processesresulting in particle flocculation [35, 36]. Above the

critical flocculation density (CFD), steric stabilisationby the CO2-philic blocks hinders flocculation (Fig. 4).Decreasing the CO2 density to below the CFD causesthe CO

2-philic chains to collapse and flocculation

Figure 4 Polymer dispersion stabilised by a block copolymer.

occurs; however, even in the collapsed state, the flu-orinated CO2-philic blocks provide sufficient repulsiveforce to inhibit coalescence [35]. Only relatively lowshear is required to re-disperse the polymer particles

in scCO2 above the CFD. Re-dispersion becomes moredifficult with time as the stabiliser molecules desorbfrom the polymer particles and migrate from the inter-particle region, resulting in coalescence.

2.3. Coatings using rapid expansionof supercritical solutions (RESS)and related processes

RESSis one of a numberof supercritical fluid processeswhich have been used to produce fine particles withpotential applications in pharmaceuticals and catalysis[3, 4, 3739], but also with extensions to coatings tech-

nologies. In a RESS process, a supercritical fluid athigh pressure dissolves a polymer or some other sub-stance. Subsequent rapid depressurisation of this so-lution causes a decrease in the density of the fluid,

reducing the solubility of the dissolved material, sothat depressurisation of this supercritical fluid solutionthrough a nozzle leads to a rapid nucleation of the dis-solved material. Fig. 5 represents a schematic conceptof the principle used in RESS [4].

The basic concept of the RESS process is over a cen-tury old, since Hannay and Hogarth [40] first describedthe process in their pioneering work. However, it was

Figure 5 Schematic diagram of RESS principle [4].

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not until the 1980s that the process was investigatedin much more detail and the flow pattern and nucle-ation processes were modelled [4147]. There havebeen a number of studies dealing with particle for-mation by this and related processes such as use of

supercritical anti-solvents (SAS) or particles from gassaturated solutions (PGSS) [4, 38, 39]. The RESS tech-nique has been extended to the formation of droplets

of perfluoropolyethers for application as coatings forthe protection of buildings and other structures [48].This approach can avoid the undesirable use of VOC-producing solvents.

2.3.1. Powder coatingsThe processes described in the previous section pro-vide a possible route to polymer powders for use inconventional powder coating applications. Ferro Corp.has developed a process for production of powders us-

ing a variant of the PGSS process [4952]. This pro-cess, termed the SF MicronMixTM process, is claimedto produce products suitable for powder coating di-rectly, thus reducing the number of processing steps

required compared to conventional powder coating pro-duction processes [52]. Weidner et al. have reporteda related process which uses continuous rather thanbatch processing to produce the powder coating ma-terial [53]. Typical particle sizes are


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2.4.2. Pharmaceutical coatingsThe use of supercritical CO2 for particle coating alsohas possible applications in the pharmaceutical indus-try, particularly in the field of controlled release fordrug delivery. The RESS process can be used in thisway [60]. Controlled depressurisation of the coating

material in scCO2 hasalso been described fordrug coat-ing [61], in a similar way to that described above for

metal particle coating. Glycerides and fatty acid estersof poly(ethylene glycol) were among the materials usedin this way to coat paracetamol particles [61]. It was

shown that the resultant microcapsules demonstratedsustained release of the active constituent in vitro andthat the release rate was dependent on the CO2 depres-surisation rate used to form the particles. Preliminary

studies of the formation of possible controlled releasecoatings using scCO2 and a co-solvent have also beenreported [62].

2.5. Spin coating and photolithographyIn the manufacturing of integrated circuits (IC), twokey steps are involved, (1) spin coating of a photoresistlayer and (2) development of the image after exposure

to light of a suitable wavelength. During these two stepsa vast amount of environmentally polluting solvent isused, since the steps are repeated again and again foreach layer in the IC, perhaps as many as 30 times or

more in the processing of a wafer [63]. Therefore, thisoverall process creates large amounts of solvent waste,typically about 7.5 m3 of waste developing solution anda similar volume of contaminated water per day in a

semiconductor processing line producing 5000 wafersper day [63, 64]. As supercritical carbon dioxide be-haves like a dense gas and has a very low viscosity andsurface tension compared with other organic solventsand water, it is likely to be able to penetrate and trans-port material to and from very small crevices. Since the

trend in semiconductor technology is forfeature sizes tobecome ever smaller, to below 0.13 m, these proper-ties of carbon dioxide are likely to prove invaluable andoffer genuine benefits over conventional solvent-based

approaches.Hoggan et al. noted that fluorinated acrylate and

methacrylate base polymers provided suitable resistsfor use with carbon dioxide (both liquid and super-critical) [6, 20]. They synthesised a series of ran-

dom copolymersof 1H,1H-perfluorooctyl methacrylate(FOMA) and t-butyl methacrylate (TBM) (IV) for useas a negative resist in deep UV lithography. Removalof the acid sensitive t-butyl groups using a fluorinated

(hence soluble in CO2) photoacid generator (PAG) re-

sults in a polymer which is much less soluble in carbondioxide. Irradiating the PAG produces an acid whichcatalyses the elimination of isobutylene from the TBMmoiety, leaving an insoluble methacrylic acid unit. The

combination of the random copolymer, poly(FOMA-

TBM), with a PAG thus provides a possible photoresistfor use in CO2.

Polymer (IV) was used to spin coat silicon wafers ina pressure chamber which could withstand pressures up

to 200 bar. Film thicknesses of very good uniformity,

ranging from 110 m, were produced when solutionsof suitable concentration and viscosity were used [20].

(IV)

2.6. Historical building protectionPerfluoropolyethers are transparent, colourless andhighly water repellent (hydrophobic) polymers, and arecapable of reducing water absorption into stone by 80to 90% for at least 30 months. These properties of per-fluoropolyethers make them good candidates for the

coating and protection of historic buildings and mon-uments made of stone [65]. The polymers have lowsurface energies and can tolerate hostile environments

such as high temperatures, oxidising agents, corrosiveacids and UV radiation [66]. The common solvents for

these polymers are chlorofluorocarbons (CFCs), whichare environment damaging and whose use was phasedout following the Montreal protocol of 1992. Fortu-nately, perfluoropolyethers are soluble in carbon diox-

ide, which makes use of carbon dioxide solutions ofthese polymers for stone protection a viable option,with a number of advantages over CFCs [67]. Carbondioxide is suitable for spray coating polymers onto thesurfaces of stones. This application method is poten-

tially faster than brush application and is able to pro-duce a much more homogeneous coating. As in RESS,the atomisation of the coating product is achieved byrapid expansion of a supercritical solution. This same

polymer coating technique can be applied to a variety ofporous substrates, including bricks, cement, wood etc.Cloud point and spray-coating tests on various perflu-oropolyethers using carbon dioxide have demonstratedthat supercritical carbon dioxide is a viable solvent for

spray application onto stone surfaces and is a good al-ternative to the currently used solvents [48, 67, 68].

Similarly, silicones can be used as protective coat-ings for stone and brickwork, because of the hydropho-bicity and water repellence displayed by the polymer

[69]. Like the fluorocarbons, the reasonable solubilityof silicones in CO2 (Fig. 2) makes use of CO2-basedformulations a genuine alternative to existing solvent-based systems. It is worth noting that carbon dioxidecan have the additional effect of accelerating the forma-

tion of carbonate in cementitious material [70], whichmay have either a beneficial or deleterious effect on thesurface of the object; however, for historic stone whichmay be fully carbonated already, this is unlikely to bea problem.

2.7. Dip or free meniscus coatingDip coating is used industrially to coat irregularlyshaped objects. For example, auto bodies or frameshave been completely coated by immersion for rust

protection. In the simplest form of dip coating, objects

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are immersed in a coating composition, removed,drained and dried or baked. There are many variationsof this method of coating [71], which is also called freemeniscus coating. Any film-forming material whichcan be dissolved or made into a suspension, with or

without dispersant, can be used as a coating material toform a thin layer coating on a substrate. Carbon diox-ide has been reported as a benign solvent for dip or free

meniscus coating by Carbonell et al. [72]. They notedthat free meniscus coating can be subdivided into threeprocesses, namely, withdrawal processes, drainage pro-cesses and continuous processes. Theirmethod consistsof immersinga surface portionof a substrateinto carbondioxide containing some film forming polymer [72].

The next step is withdrawal of the immersed substratethrough a meniscus which exists at the interface of thefirst phase, into a distinct (gaseous) second phase. Inthe second phase the solvent (in this case carbon diox-

ide) evaporates leaving a thin film coating on the solidsubstrate. The pressure in the second phase is main-tained such that a differential pressure exists between

the firstand thesecond phases. Differentsubstrates suchas solid articles, fibres and textiles have been claimed

to be suitable for coating. The thickness of the film wasreported to be very uniform.

2.8. Impregnation of polymersImpregnation can also be considered as a form of coat-ing. Supercriticalfluids havebeen studied in some detailfor the impregnation of polymeric materials, becauseof their high diffusivity, low surface tension and easyrecovery from the polymer matrix. A comprehensive

review of this subject has been written by Kazarian [5].The pioneering work of Sand [73] and Berens [74]stimulated the original interest in this field. A verywide range of polymers have been impregnated us-ing scCO2, including polystyrene [74, 75]; poly(methyl

methacrylate) (PMMA) [7480]; poly(vinyl chlo-ride) (PVC) [74, 81]; polycarbonate [74, 75, 8183];polyethylene [73, 8387]; poly(tetrafluoroethylene)(PTFE) [81]; poly(chlorotrifluoroethylene) (PCTFE)

[81, 83, 88]; poly(4-methyl-1-pentene) (PMP) [83];poly(oxymethylene) [83]; poly(ethylene terephtha-late) (PET) [8993]; poly(dimethysiloxane) (PDMS)

[76, 9497] and polyamides [98, 99]. A wide range ofimpregnating solutes can be used [5]. A specific ex-

ample of impregnation (arguably not really a coating,however) which could be of considerable importance inthe future is that of textile dyeing, which has been dis-cussed by Kazarian et al. [89]. This has the potential toovercome the problem of waste water in conventional

dyeing processes. In addition to acting as the polymerto be impregnated, polymers may also be used as theimpregnant, as noted above for building stone and alsofor wood coatings (vide infra).

2.9. Wood coatingsWood is a difficult material to coat for long-term envi-ronmental resistance. VOC legislation has been applied

specifically to wood products [100] and the regulations

are making use of traditional coating systems more dif-ficult and accelerating the development of alternativessuch as aqueous systems; however, these sufferfrom thedisadvantages noted earlier as well as the problem ofincompatibility with wood, which will swell and inhibit

penetration [101]. The effect on coating quality remainsuncertain and there is room for improved products forwood coating [102]. There is also a need for improved

coating penetration into wood [103].Supercritical fluids offer advantages for the impreg-nation of porous substrates such as wood, includinglow viscosity and very high diffusivity compared toconventional liquids, both of which aid penetration andimpregnation [87, 104, 105]. A further benefit ofscCO2in impregnation of wood is its extraction capability,since extraction of resin and fatty acids can increasethe permeability of wood in a scCO2 medium [104].

The use of supercritical [106] or liquid CO2 for theimpregnation of wood biocides, such as tebuconazole

(V), may offer advantages in terms of elimination ofVOCs, allow a greater depth of impregnation, more ef-

ficient impregnation and perhaps allow use of waterinsoluble active components. Since the requirements

for controlled leaching of the biocide in an aqueousenvironment and those for compatibility with the sol-vent are completely different when CO2 is used as theapplication medium, it might be expected that such anapproach would lead to significantly improved durabil-

ity relative to aqueous treatment.

(V)

In the use of supercritical carbon dioxide as an im-pregnating medium for biocide application, the solu-bility of the biocides in scCO2 can be increased by

increasing the pressure, which provides a driving forcefor flow into the solid wood [104, 107], resulting inmore efficient impregnation from SCFs than in con-ventional treatments. ScCO2 has been found to be ef-fective in impregnating non-permeable woods such as

spruce with biocides, increasing resistance to fungiand insects [101, 106]. Unused impregnating agent canbe readily recycled and a scCO2co-solvent solutioncan be used for impregnating difficult woods [108].No adverse effect of scCO2 treatment on wood com-

posites has been found [109] and little effect on themechanical properties of wood was noted [110], al-though some reaction between the CO2 and the cellwall constituents has been found to occur [111]. ScCO2may also offer benefits in the application of water re-

pellents to wood, since the most common substancesused are silicones or polyfluorocarbons, both of whichhave significant compatibility with scCO2 (Fig. 2). It isbelieved that there is currently some commercial activ-

ity in Denmark in impregnation of wood products, but

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Figure 7 Application of carbon dioxide to deposition of water-based thin film coatings [110]. Diagram courtesy of Prof. R. E. Sievers.

this overall area of supercritical wood treatment is stillvery much in its infancy, with more work needing to be

done to demonstrate the benefits and limitations of theapproach.

2.10. Miscellaneous coating applications2.10.1. Water-based coatingsCarbon dioxide can also be used as a means of de-livering aqueous-based coating systems [112] (Fig. 7).

This may be done by mixing the carbon dioxide withan aqueous solution of the coating or its precursor im-mediately prior to spraying. The coating/precursor isdelivered as a fine aerosol to the substrate. Examples of

the use of this technique include:

Spraying of an aqueous solution of mixed cobalt,iron and chromium nitrates onto a hot glass or

quartz substrate forming a crystalline spinel film. Depositionof crystallinezinc oxide filmson silicon

wafers or glass by spraying zinc acetate solutiononto the hot substrate.

Synthesis of phosphor thin films.

Although this approach was applied to metal oxide

thin films, it could probably also be adapted to a widerange of water-based coating systems.

2.10.2. Foamed coatingsRecent work on foaming of polymers using carbondioxide suggests wider application in a range of foamed

coatings. Nitto Denko has patented the production offoamed pressure sensitive adhesive tape using liquidor supercritical CO2 [113, 114]. Adhesives used weretypically acrylic or natural rubber-based systems. In theUK, RAPRA has been developing technology for ex-

trusion foaming using CO2 [115]. A modified extruderwas used and a range of foams produced successfullyfrom thermoplastics such as polyolefins, polystyrene,poly(ether ether ketone) [PEEK], polyphenylsulfone

and poly(ester carbonate).

2.10.3. Coating removalInterestingly, workers at Colorado State University and

Los Alamos have described the application of scCO2for the removal of polymer coatings [116], thus provid-

ing a potentially environmentally friendly approach torecycling and re-use of waste coated products. Use ofa co-solvent may be beneficial.

3. ConclusionsConcerns over the environmental effects of VOC emis-sions, coupled with increasingly stringent legislation,are stimulating interest in the use of carbon dioxide(both supercritical and liquid) as a replacement for

organic solvents in coating operations. Spray coatingprocesses have been studied, where the use of VOC-emitting solvents can be greatly reduced or eliminated.These processes are claimed to have other benefits,including improved transfer efficiencies and reducedoperating costs compared to conventional spray coat-

ing. Processes such as RESS and PGSS which weredeveloped originally for the formation of controlledmorphology particles using (supercritical) CO2 havebeen adapted for the formation of controlled particle

size powder coating products and also for the coatingof preformed particles, including metal powders usedin pyrotechnics and pharmaceutical powders for con-trolled release during drug delivery. Solvent-free aque-ous polymer latexes can be made by a RESS process us-

ing an initial CO2-polymer dispersion. The use of CO2in spin coating and microlithography has the potentialto greatly reduce the volume of waste produced dur-ing microelectronics fabrication processes. Coating ofbuilding stone (such as ancient monuments) and wood

products can both benefit from the use of CO2 tech-nology. The materials used for the treatment of build-ing stone are often conveniently soluble in CO2. Theability of CO2 to swell substrates and also its highdiffusivity make it highly effective in impregnating a

wide range of substrates, including wood and manypolymers. A variety of metal oxide coatings have alsobeen produced successfully by CO2-assisted sprayingof aqueous precursor solutions. The range of coat-

ing processes amenable to the use of CO2 technology

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is thus very broad and being expanded all the time.If environmental factors can be coupled with genuinetechnical benefits arising from the use of CO2, then thetechnology becomes even more attractive to the poten-tial end-user. There seems little doubt that the coming

years will see increasing commercialisation of coat-ing processes based on use of CO2. Concerns over theuse of high pressure equipment are likely to provide

some hindrance to this, but increasing evidence (e.g.,from decaffeination plant) that this does not create insu-perable practical problems should help alleviate theseconcerns.

AcknowledgementThe authors thank the UK Engineering & PhysicalSciences Research Council for financial support.

References1. ENDS Report 311 (December 2000), Publ. Environmental Data

Services Ltd., London.

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