DOI: 10.1126/science.1075707, 976 (2002);297 Science

Robert R. Matheson Jr.20th- to 21st-Century Technological Challenges in Soft Coatings

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where R is the reflectance of the sample, � is thedistance on the surface between two points, � isthe angular frequency, k is the wave number, f isthe focal length of the incident radiation, and �is the rms height of the surface. A 2D analysis ofthe optics has been carried out by Ogilvy (35).Whitehouse concluded (34) (for undulationswith length scale greater than the wavelength ofthe incident radiation) that the surface appearedglossy if the probability density of the slopes onthe surface was strictly confined to a narrowangle.

Biocompatibility. Finally, biological interac-tions with a surface have also been found todepend on its topography. A good review of thetopological control of cell adhesion and activityon a surface has been made by Curtis andWilkinson (36), and a more general review ofthe role of polymer biomaterials may also befound (37). Such considerations are relevant fora number of in vivo and in vitro applications,such as biological sensors, hip replacements(38), and more complex tissue implants such asreplacement bone, where the growth of cellswithin the artificial structure is to be encour-aged. For example, the size and morphology ofcrystals at the surface of octacalcium phos-phate–coated collagen have been shown to af-fect the interaction of cells with the surface, asillustrated in Fig. 4. The larger scale topographywas found to lead to less favorable spheroidalcells that formed fewer intercellular connections(39). In some cases, the topography of a surfacemay be carefully controlled to promote celladhesion (40, 41).

ConclusionThe topography of a surface is a direct resultof the nature of the material that defines it.

The analysis of the topography of a sample,made possible on the nanoscale by the devel-opment of AFM techniques, needs to be care-fully considered in order to relate the com-plexity of a 2D surface to the material’sproperties. The result will be the better con-trol of a number of properties, such as opticalfinish, and of the interaction of a surface witha secondary material, whether that be an ad-hesive, a secondary component of a compos-ite, or a biological species.

References and Notes1. J. D. Affinito et al., Thin Solid Films 291, 63 (1996).2. J. A. DeAro, K. D. Weston, S. K. Buratto, U. Lemmer,

Chem. Phys. Lett. 277, 532 (1997).3. Y. Nabetani, M. Yamasaki, A. Miura, N. Tamai, Thin

Solid Films 393, 329 (2001).4. M. Sferrazza et al., Phys. Rev. Lett. 78, 3693 (1997).5. E. Schaffer, T. Thurn-Albrecht, T. P. Russell, U. Steiner,

Europhys. Lett. 53, 218 (2001).6. D. G. Bucknall, G. A. D. Briggs, MRS Symp. Ser.:

Nanopatterning: Ultralarge-Scale Integration Bio-technol. 705, 151 (2002), L. Merhari, K. E. Gonsalves,E. A. Dobisz, M. Angelopulss, D. Herr, Eds.

7. S. Walheim, E. Schaffer, J. Mlynek, U. Steiner, Science283, 520 (1999).

8. J. Heier, E. Sivaniah, E. J. Kramer, Macromolecules 32,9007 (1999).

9. T. Thurn-Albrecht, J. DeRouchey, T. P. Russell, Mac-romolecules 33, 3250 (2000).

10. A. Karim et al., Macromolecules 31, 857 (1998).11. X. P. Jiang, H. P. Zheng, S. Gourdin, P. T. Hammond,

Langmuir 18, 2607 (2002).12. G. Goldbeck-Wood et al., Macromolecules 35, 5283

(2002).13. V. N. Bliznyuk, K. Kirov, H. E. Assender, G. A. D. Briggs,

Polym. Preprints 41, 1489 (2000).14. F. Dinelli, H. E. Assender, K. Kirov, O. V. Kolosov,

Polymer 41, 4285 (2000).15. C. Rauwendaal, Polymer Extrusion (Hanser, Munich,

1985).16. S.-J. Liu, Plast. Rubber Composites 30, 170 (2001).17. B. Monasse et al., Plast. Elastomeres Mag. 53, 29

(2001).18. Y. Oyanagi, Int. Polym. Sci. Technol. 24, T38 (1997).19. A. Guinier, X-ray Diffraction in Crystals, Imperfect

Crystals, and Amorphous Bodies (W. H. Freeman, SanFrancisco, 1963).

20. V. N. Bliznyuk, V. M. Burlakov, H. E. Assender, G. A. D.Briggs, Y. Tsukahara, Macromol. Symp. 167, 89(2001).

21. W. M. Tong, R. S. Williams, Annu. Rev. Phys. Chem.45, 401 (1994).

22. P. Meakin, Fractals, Scaling and Growth Far fromEquilibrium (Cambridge Univ. Press, Cambridge,1998).

23. C. M. Chan, T. M. Ko, H. Hiraoka, Surf. Sci. Rep. 24, 3(1996).

24. E. M. Liston, L. Martinu, M. R. Wertheimer, J. AdhesionSci. Technol. 7, 1091 (1993).

25. Q. C. Sun, D. D. Zhang, L. C. Wadsworth, Tappi J. 81,177 (1998).

26. V. Bliznyuk et al., Macromolecules 32, 361 (1999).27. N. Zettsu, T. Ubukata, T. Seki, K. Ichimura, Adv. Mater.

13, 1693 (2001).28. R. S. Hunter, R. W. Harold, The Measurement of

Appearance (Wiley, New York, ed. 2, 1987).29. H. Davies, Proc. Inst. Electr. Eng. 101, 209 (1954).30. F. M. Willmouth, in Optical Properties of Polymers,

G. H. Meeten, Ed. (Elsevier, Amsterdam, 1986).31. D. Porter, Group Interaction Modelling of Polymer

Properties (Marcel Dekker, New York, 1995).32. P. Beckmann, A. Spizzichino, The Scattering of Elec-

tromagnetic Waves from Rough Surfaces (Pergamon,New York, 1987).

33. K. Porfyrakis, N. Marston, H. E. Assender, inpreparation.

34. D. J. Whitehouse, Proc. Inst. Mech. Eng. B: J. Eng.Manuf. 207, 31 (1993).

35. J. A. Ogilvy, Theory of Wave Scattering from RandomRough Surfaces (Adam Hilger, Bristol, UK, 1991).

36. A. Curtis, C. Wilkinson, Biomaterials 18, 1573 (1997).37. L. G. Griffith, Acta Mater. 48, 263 (2000).38. T. M. McGloughlin, A. G. Kavanagh, Proc. Inst. Mech.

Eng. Part H–J. Eng. Med. 214, 349 (2000).39. A. C. Lawson et al., MRS Symp. Ser.: Biomed. Mater.:

Drug Delivery, Implants Tissue Eng. 550, 235 (1999),T. Neenan, M. Marcolongo, R. F. Valentini, Eds.

40. C. S. Ranucci, P. V. Moghe, J. Biomed. Mater. Res. 54,149 (2001).

41. P. Banerjee, D. J. Irvine, A. M. Mayes, L. G. Griffith,J. Biomed. Mater. Res. 50, 331 (2000).

42. The authors acknowledge contributions to this workfrom A. Briggs, D. Bucknall, V. Burlakov, J. Czernuska,and S. Wilkinson from Oxford University; N. Marstonand I. Robinson from Lucite International; and Y.Tsukahara from the Toppan Printing Company.

V I E W P O I N T

20th- to 21st-Century TechnologicalChallenges in Soft Coatings

Robert R. Matheson Jr.

Coatings are among the most ancient technologies of humankind. Rela-tively soft coatings comprising organic materials such as blood, eggs, andextracts from plants were in use more than 20,000 years ago, and coatingactivity has been continuously practiced since then with gradually improv-ing materials and application techniques. The fundamental purposes ofprotecting and/or decorating substrates have remained ubiquitous acrossall the centuries and cultures of civilization. This article attempts toextrapolate the long tale of change in soft coating technology from itscurrent state by identifying some key problems that attract research anddevelopment efforts as our 21st century begins.

Humans have been decorating and protectingvarious surfaces for many thousands of years.One very useful way of accomplishing eitheror both of those tasks is to apply a thin layerof some new material with appropriate char-

acteristics (of appearance, durability, adhe-sion, and application requirements) directlyonto the surface of interest. That new materialis a coating. Understandably, the early historyof coatings is a story of very specialized,

often unique material combinations, as trialand error achieved goals with only the mate-rials at hand in nature. This heritage of cus-tomization is still detectable in the moderncoatings world, which demands a tremendousamount from the materials—often syntheticbut some still containing or made of naturalproducts—to be thinly applied on a surface.They need to be easily and uniformly applied;set up within a reasonable amount of time andprocess constraints; have a minimal environ-mental impact in their synthesis, combina-

DuPont Performance Coatings, 950 Stephenson High-way, Troy, MI 48083, USA. E-mail: robert.r.matheson@usa.dupont.com

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tion, and application; resist the effects ofenvironmental assault; and provide good eco-nomic value. I examine five important forcesthat are driving how such coatings are madeand improved today.

NomenclatureThe long, decentralized, and empirical evo-lution of coating materials and processes hasleft behind an arcane and frequently confus-ing vocabulary (1). It will be helpful to definethree terms that are frequently used but alsoare indiscriminately interchanged. A lacquer(from the Arabic word lakk) is a coating thatforms on a surface (frequently by evaporationof solvents) without the intervention of cova-lent bonds forming between the film-formingingredients. In contrast, a varnish (from theMedieval Latin vernice) is a coating thatessentially requires chemical reactions be-tween film-forming ingredients during a cur-ing process after its application to a substrate.Enamels (from the Germanic esmail) are avery common subset of varnishes, which usea heating (stoving) step to carry out the cur-ing process. These classifications were sharpand distinct in the past, but current develop-ments are beginning to weaken their clarity.However, they will be useful here in high-lighting the particular challenges facing coat-ing development.

Minimizing the EnvironmentalFootprintOne of the most commonly recognized chal-lenges is the reduction or elimination of vol-atile organic compounds (VOCs) from theformulations of modern coatings (2). In thequantities generated by today’s populationand particularly in the concentrations pro-duced in industrialized urban environments,VOC emissions contribute to air pollutionproblems. Clearly, this problem is most acutefor lacquers. The mutually unreactive com-ponents of a lacquer absolutely depend onsome processing aid (commonly a solvent) tomake them malleable enough for applica-tion, and those aids must then be removed toleave the coating robust enough to protectand decorate.

Solvent minimization finds its ultimateexpression in lacquer versions of powdercoatings, where solvents are replaced withheat, which is used to apply the coating.Upon cooling, the properties that developcan sometimes be adequate to the task athand. However, these products are limitedin that if the coating is heated to a temper-ature where its application is possible, thenit will soften and deform once again. More-over, because most coating films are eitheramorphous or semicrystalline, their abilityto retain a minimum hardness and to resistsustained loads begins to fall off quite no-ticeably at temperatures well below those

where rapid flow and leveling are achieved(3).

Acceptable solvent substitution basicallyamounts to using the liquid form of substanc-es that are naturally present as gases in theatmosphere (such as water and carbon diox-ide). Liquid or supercritical carbon dioxide islimited to industrial applications because ofthe requirements for high pressure. Water iseasier to use widely as a coating solvent, butit is not a panacea. One example of a problemthat comes with water is the inevitably widevariation in drying times that accompaniesapplication in environments of different rel-ative humidity. Because relative humiditycan change almost hourly, this is a seriouscomplication. In fact, virtually all waterbornecoatings today contain quite substantial levelsof organic “cosolvents.” The VOC content ofwaterborne coatings is greatly reduced ascompared to that of older, conventionalsolvent-borne coatings, but it is not fullyeliminated. A major activity in modern coat-ing development is the search for balancedchemistry that will push back these limits onenvironmentally more favorable lacquerswhile retaining the attractive simplicity, thesynthetic control, and the low cost of thetechnology.

VOC release is not the only environmen-tal impact factor that is important for drivingchange in coating technology. In the UnitedStates, regulations on so-called hazardous airpollutants (HAPs) are important (4). This isan explicit list of solvents, typically aromatic,that are used in large quantities and are eitherknown to cause or suspected of causing hu-man health problems with chronic exposure.A variety of other, similarly local constraintsfor particular ingredients exist around theglobe. One very widely experienced restric-tion is that on heavy metals. There are manyhistorical examples where fairly largeamounts of particular metals have found usein soft coatings (5). Some examples are theuse of lead for anticorrosion in cathodic elec-trocoat coatings, of hexavalent chromium inmetal coatings, of both lead and cadmium invarious pigments, of divalent tin in antifoul-ing marine coatings, and even of mercury asan antifungal agent for some interior paints.In common with other areas of materialsprocessing, coating technology now has tolook for alternative ingredients without un-controllable, long-term environmental conse-quences. No similarly general technical solu-tions have yet been found, although progressis being made, particularly with respect toanticorrosion coatings.

Beating Back the EnvironmentChemical and mechanical resistance to envi-ronmental insult is a common feature ofmany coating systems and a key reason fortheir application. Biological attacks are clas-

sic problems encountered over the years, andtheir catalog defines the current frontier. Un-derwater coatings that can resist the attach-ment and degradation of aqueous organisms(such as worms and barnacles) are needed forshipping and for structures. Exterior coatingsthat can resist particular insect, bird, andplant excretions are frequently needed in lo-cal geographies. Interior coatings that canresist mildew, other fungal damage, molds,and bacteria are frequently desired. The gen-eral challenge is nearly always the same:specific resistance to a defined class of bio-logical insult without nonspecific toxicity orirritation. It is natural to work toward this setof objectives with additives tailored to eachtask. Experience shows this natural path to beexpensive and usually imperfect, but occa-sionally fruitful. Still, it is probably fair to saythat no examples exist where the performanceof the broadly toxic, heavy metal–based ad-ditives has been achieved with the more spe-cific modern tools.

A new idea is to produce counteragents insitu by tapping the chemical reactions thatmust accompany biological attack or evensimple weathering in the active environmentsnear Earth’s surface (6). For example, bio-logical damage may be accompanied by hy-drolytic scission of coating components. Ifthose can be designed to hydrolyze into anti-septic, antifouling, or anti-whatever productstailored to the task, then perhaps an effectivesolution can be found.

Maximizing Control Through MolecularArchitecturesIt is important to look beyond environmentalattack on the coating itself. The classical roleof coatings is to protect something else fromthe environment. This protection can be me-chanical or chemical in nature. Varnisheshave been developed particularly for thesepurposes, because in situ or “on the work”cross-linking is a very effective technique foraugmenting the coating’s material propertieswhile avoiding compromises in application.

An extremely active area of development inmodern coatings is directed to improving thecontrol of such reactions. Inevitably, these mustbe carried out in a variety of work environ-ments. Near one extreme are coatings used incontrolled chambers, as for the radiation orthermal curing of a coating used to seal aconnection between electronic components.The variables of concern may be film thicknessat various points, surface cleanliness, and smalltemperature gradients arising from materials ofdifferent thermal conductivity. Near the otherextreme are the grossly fluctuating environ-ments that can be found in a railroad locomo-tive shed, where humidity, temperature, airflow, application rate, surface preparation, andmaybe even surface material are all variables.

One idea for meeting these challenges is

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to take great care in synthesizing the var-nish ingredients so that no especially trou-blesome reactions can ever occur. Someside reactions are understandably more del-eterious than others, and the goal is to leaveno opportunity for the most troublesome,no matter what conditions might appear.This leads to the use of controlled polymer-ization techniques [exemplified but certain-ly not limited to group transfer polymeriza-tion (7 ) for acrylic materials], rigorousexclusion of nonfunctional (unable to par-ticipate in curing) matrix materials, andoptimizing molar mass distributions toavoid untimely immiscibility during cureand similar strategies. The specific field ofautomotive coating has been in the fore-front of this activity because of the ex-tremely high performance standards andpowerful economic incentives found inmass-producing automobiles. Some of thenew synthetic and analytical techniques be-ing introduced for controlling and monitor-ing automotive enamels have been de-scribed (8).

Decorative coatings in particular need toincorporate pigments, dyes, reflective metal,and mica flakes for many applications. Onecommon technique for effectively distribut-ing such particles is to cover their surfaceswith dispersants that aid in their dispersion inthe bulk of the coating and prevent reagglom-eration under the variety of circumstancesthat might arise later. Exquisite control of themolecular structure is needed in order toachieve good distribution of the particles,minimal mobility once applied to a surface,the ability to resist forces that drive re-agglomeration, and compatibility with thebulk coating and yet not induce problemswith adhesion, application, or long-term per-formance. Because pigments are very fre-quently the most expensive ingredients in adecorative coating, it is important to use themefficiently. Additionally, as solvent concen-tration and variety are decreased because ofthe environmental pressures previously cited,opportunities for managing dispersion prob-lems by modifying the coating medium (thecoating vehicle) decrease. Small wonder thatthe chemistry used to make modern dispers-ants provides an exceptionally clear pictureof the state of the art in molecular control incoatings. Techniques for making block co-polymers (each block designed for affinity toeither a surface or the solvent environment)have been developed and commercializedand are still being improved. The patent art isextensive and growing, but that from C. Ho-sotte-Filbert (9) is a representative example.

Functional CoatingsA fourth modern frontier in the world of softcoatings can be descriptively called “postcurereactivity” for varnishes, or perhaps “in-use

reactivity” for lacquers. Such reactions havebeen recognized for a long time in examplessuch as the long-term oxidation of alkyd var-nishes and many lacquers based on naturalproducts. Historically, these have beenviewed as troublesome instabilities. Howev-er, it has been learned that some instances ofpostcure chemistry have advantages, withone example being the slow condensation andinterchange of siloxane bonds in organosilaneenamels (10). These can act to relax stressesthat otherwise grow uncompensated in light-and oxidation-stressed exterior coatings. Het-erogeneous coatings that react to cracks orfractures by releasing postcure repair ingre-dients have been postulated (11).

Even more sophisticated uses in con-trolled release or other transport control prob-lems can be sketched today. It should benoted that a great many instances exist inwhich coatings are used to manipulate (gen-erally to retard, delay, or prevent) the trans-port and exchange of materials. Atmosphericoxygen contacting food, carbon dioxide exit-ing carbonated beverages, the release ofpharmaceuticals into the body, electricalcharge leaking into a device component,heat exiting an isothermal environment, orwater and ionic materials contacting corro-sion-susceptible metals are examples wherethe transport characteristics of coatings areimportant in determining performance. Thelong-term capability of a coating to im-prove or at the least react to compensate fora declining transport characteristic may bejust as useful as the same ability to offsetdeclining mechanical characteristics.

Industrial Scale ChallengesA final class of problems driving innovationin modern coatings can be found in the costsand limitations of the heating step in enamelprocessing. Not surprisingly, these problemsinclude the capital and energy costs associat-ed with heating objects with large thermalmasses, damage to heat-sensitive substrates,and the inventory problems that accumulatewith long cycle times in any process. Themost direct approach is to reduce the requiredbaking temperature and/or time. There isscope here for novel chemical reactions andcatalyst innovations, both of which commandattention today. Alternatively, if the curingreactions can be activated by a mechanismother than simple heating, then problems canbe minimized without losing the cure-in-duced improvements in coating performance.Much current work is directed to radiation-curable (with ultraviolet light, electronbeams, and even visible light) coatings andefforts to extend their current embodiments tocomplex articles and long-term use (12).Powder coatings and liquid coatings are bothobjects of study and innovation. The majorchallenge faced in such development arises

from limitations on the uniformity of cure forincompletely transparent coatings (shadowedareas do not receive the same flux of radia-tion) or coatings on complex shapes.

Examples of specific new products arisingin response to one or another of these fivegeneral development areas can be found inmany places and from many development lab-oratories. Perhaps no example exists that illus-trates all, but there is at least one that comesclose. A need exists in the automotive world fora painting system with lower environmentalemissions (particularly VOCs), improved resis-tance to environmental damage (particularlymechanical scratching), outdoor durability ap-proaching a decade, corrosion resistance of thecoated metal for the same period of time, andimproved application robustness. This need hasbeen recently met with what might be consid-ered an exemplary modern coating system.Four layers of coating are used: First, an anti-corrosion coating (now free of heavy metals) isapplied by cathodic electrodeposition; second, apowder primer (now with zero VOCs); third, awaterborne layer containing pigments (nowwith minimal VOCs and minimal HAPs withmodern polymeric dispersant molecules); andfinally, a new clearcoat (now with more than20% lower VOCs, greatly improved scratchresistance, and excellent resistance to acid rain,chemical attack, and photochemical exposure).All this is applied with the use of existinginfrastructure, including both automatic andmanual equipment when required, at commer-cial rates and with lower total energy input andimproved visual quality. The automotive useenvironment is harsh for coatings, and this newsystem has only begun to be used. It is risky toforetell decade-long success so far in advance,but the step appears to be an important one inthe evolution of soft coatings that can resist allthe major pressures.

Looking ForwardThe five general development frontiers ap-pear to indicate two strong trends in today’ssoft coating development. The first is a trendtoward ever-improving control of structure atall length scales. Whether in cure-site distri-butions for more robust varnishes, molarmass distributions in environmentally friend-ly lacquers, or optical path distributions forultraviolet curables, the path of improvementand innovation goes along the line of im-proved structural uniformity and control. Thesecond trend is the historically familiar onetoward blurring the distinctions between andthereby removing some limitations fromfamiliar coating classes. Whether by slowin-use reactions for nominal lacquers and am-bient varnishes or by radiation-assisted cross-linking in nominal enamels, the technologytrend is toward the boundaries laid down bythe descriptors. One can phrase these trendsin the form of two strategic questions for

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coating development: What kind of structuralirregularity do you want to eliminate for anintended characteristic, and where do youwant to carry out the chemistry required forthat control (in intermediate production, dur-ing cure, or in use)?

A final observation might be made to com-plete this perspective on current technologicalchallenges. In one important sense, the chal-lenges are still within the long, long historicalpattern of coatings. Coatings have been a de-rivative technology in which materials primar-ily intended for other purposes (coloringpigments alone excepted) are combined in com-plex mixtures for comparatively small-volumespecialized use. Coatings are always at leastpartially custom-tailored to the problem at

hand. Today’s challenges are to decrease theenvironmental footprint and improve biologi-cal, mechanical, and transport longevity, whileminimizing the application requirements forsoft coatings. Challenges need to be met withonly minimal need for brand-new materials,maximum use of component synergies, andmaximum use of natural or by-product materialstreams. The custom tailoring is to be done withminimal resources and, therefore, maximumintelligence. The historical character of coatingsinnovation is well aligned with the moderncurbs on new material registrations, capital in-vestment, and waste management. Today, weare only at one intermediate stage in a longsweep of coatings development from the distantpast into the future.

References1. P. Nylen, E. Sunderland, Modern Surface Coatings

(Interscience, New York, 1965), pp. 1–10.2. Superintendent of Documents, Clean Air Act Amend-

ments of 1990, Title III (U.S. Government PrintingOffice, Washington, DC, 1990).

3. D. W. Van Krevelen, P. J. Hoftyzer, Properties ofPolymers (Elsevier, Amsterdam, ed. 2, 1976), pp. 275–294.

4. See www.epa.gov/ttn/atw/188polls.html.5. V. P. Preuss, Paint Additives (Noyes Data, Park Ridge,

NJ, 1970).6. K. Uhrich, (see uhrich@rutchem.rutgers.edu).7. O. Webster, U.S. Patent 4,417,034 (1983).8. K. Adamsons et al., in Proceedings of the XXIII Athens

Conference on Organic Coatings (1997), pp. 151–160.9. C. Hosotte-Filbert, U.S. Patent 5,681,877 (1997).10. F. D. Osterholtz, F. R. Pohl, J. Adhesion Sci. Tech. 6,

127 (1992).11. K. Chung, New York Times, 5 March 2002, p. 23.12. M. Nichols et al., Rad Tech Report, Nov-Dec 2001.

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