M. Arbab, L.J. Shelestak and C.S. Harris

Glass Technology Center, PPG Industries Inc., Pittsburgh, Pa.

The first part of this two-part series on value-added flat-glass products focused on product requirements and on-line and off-line coating deposition manufacturingprocesses. It also included a discussion about flat-glasscomposition.This part of the series presents an overviewof value-added products.

Coated Glass Products for Energy EfficiencyLow-emissivity (low-E) and solar control coatings are spec-trally selective thin-film stacks that are deposited on floatglass. Either chemical vapor deposition (CVD) or physicalvapor deposition (PVD) is used for deposition. In general,the CVD-coated products are harder and chemically moredurable.The sputter-deposited coatings have better spec-tral selectivity.

The reflectance and transmittance of several coatingsas a function of wavelength have been determined(Figs. 5(a) and (b)). These coatings are usually designedto have high transmittance and neutral color to meetcurrently accepted aesthetic requirements. To select theglass pane for optimum energy efficiency, the local cli-mate for the entire year should be taken into account.

Minimizing winter heat loss should be balanced againstdesirable passive solar heating in northern climates, par-ticularly for south-facing windows, and air-conditioningrequirements during summer. More than 94% of the ther-mal energy from bodies at 21°C (a typical indoor tempera-ture in winter) is emitted in the 5–40 µm region of theelectromagnetic spectrum (thermal radiation range).Uncoated glass is a high-emissivity material. It absorbsand reemits heat in this region (emissivity = 0.84). In con-trast, a conductive coating on glass reflects this thermalradiation and has low emissivity.7

Most commercial CVD low-E coatings consist of trans-parent conductive oxides (TCO) that are good reflectorsin the thermal radiation range (emissivity = 0.2). A primeexample of such a coating is fluorine-doped tin oxide(F:SnO2), which is an n-type semiconductor. SnO2 can be

deposited in the tin bath under atmospheric conditionsfrom tin-bearing organometallic compounds, e.g.,monobutyltin chloride.20

Generally, higher conductance of the coating results in alower emissivity for the product.Therefore, at a givenconductivity, the film should be thick enough to meet theemissivity requirement for its intended use. F:SnO2 has arelatively high index of refraction (~2.0) compared withglass (1.5). At a typical thickness of ≥100 nm, F:SnO2 canimpart high reflectance and undesirable color to theglass product.Therefore, the glassmaker inserts an opticalthin-film stack between the functional F:SnO2 film andthe glass substrate for color suppression.

An innovative approach is to use a mixed-metal oxidelayer, e.g., a mixture of oxides of tin and silicon. The com-position of the layer continuously varies between rela-tively pure SiO2 at the glass surface to SnO2 at its inter-face with the conductive layer. This results in a layer witha graded index of refraction that changes from ~1.5 to~2.0. Therefore, well-defined optical interfaces thatcause optical interference in the coating areeliminated.20

An optical model of such a thin-film stack demon-strates that a low-emissivity coating with neutral colorand decreased total reflectance can be designed. Amore usual solution to this problem, which also is prac-ticed in the industry, is the use of several thin-film layers

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Value-Added Flat-Glass Products for the Building,Transportation Markets, Part 2An overview of value-added solar glass, solar and heat reflective coating on glass and the new category of self-cleaning glass products is provided.

A window insulated glass unit.

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with varying indexes of refraction. These layers and theconductive SnO2 film result in destructive interferenceof visible light in reflection. F:SnO2 or other similar con-ductive transparent oxides that are nonabsorbing in thevisible spectrum are not effective in reflecting the solarIR radiation. Other conductive oxides that are moreabsorbing, e.g., Sb:SnO2, can be incorporated into thecoating stack to decrease solar heat load through aglass window. These oxides also may result in absorp-tion of visible light and lower visible transmittance.21

An alternative group of products that have either low-Eproperties, solar heat load reduction properties, or both,consists of sputter-deposited thin films.These coatingsoften consist of metal and metal compound layers.Themost widely used commercial examples of these coatingsare silver based. Silver is a highly effective mirror in thesolar and thermal ranges of IR radiation. However, silverreflects visible light and, relative to the conductive trans-parent oxides, is soft and nondurable.To overcome thesedeficiencies, multilayer stacks have been designed. Oneor more silver layers are sandwiched between thin filmsof transparent dielectric metal compounds.7

These dielectric layers are usually chosen from a groupof metal oxides or nitrides, including ZnO, SnO2, SiO2and TiO2 or Si3N4. These films function as optical inter-ference layers. They also must provide chemical andmechanical durability to silver-containing thin-filmstacks by stabilizing silver and acting as buffer layers.7,22

The coating stack then consists of repeating units ofdielectric–silver–dielectric, where the silver layer is gen-erally ~10–15 nm thick.

Depending on their place in the stack and their indexof refraction, the dielectric layer thickness can varybetween several and 100 nm. The silver-based coatingsnevertheless remain more prone to mechanical damagethan the CVD films discussed earlier. However, as hasbeen demonstrated (Figs. 5(a) and (b)), the solar perfor-mance of these coatings is superior to tinted glass andCVD-coated low-E glass products. Similarly, these coat-ings offer the lowest emissivity available among allhighly transparent coated products, with neutral color.7

For example, a double silver layer stack23 has an emissivity of <0.04.

Total value of the product to the customer must beconsidered, e.g., solar-performing glass. In choosing themost appropriate product, either for an office buildingor an automobile, the designer (or engineer) choosesfrom several options of tinted glass or IR-reflective coat-ings, or a combination thereof. This choice is driven byseveral considerations and constraints, including aes-thetics or appearance, heating and air-conditioningrequirements, daylighting preferences, occupant com-fort and security, wireless communication requirements,and material cost.

For example, tinted glass can provide distinct transmit-

Figure 6 Photograph of a set of wet casement windows 30 s after spray-ing with water. Glass on the left is ordinary window glass. Glasson the right is self-cleaning glass. Photograph was taken fromindoors looking out. Exterior surfaces of the windows are wet.

Comparison of Solar Performance

Figures(5a) and(5b)

(I) ordinary clear float glass, (II) F:SnO2-coated low-E glass (PPGSungate® 500 glass), (III) single silver layer low-E glass (PPGSungate 100 coated glass) and (IV) double silver layer low-Eglass (PPG Solarban® 60 coated glass).

Comparison of Solar Performance

(a)

(b)

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ted colors to match the rest of the car body or affect theappearance of the car interior. The glass also must effec-tively absorb solar radiation and be relatively simple tofabricate and use, at comparatively low cost. On theother hand, tinted glass windows absorb heat. However,tinted glass can decrease occupant comfort and resultin more heat buildup in buildings or parked cars thansolar-reflective glass. High-electrical-conductivity silver-based coated glass has excellent solar performance andsheet resistance of <3 Ω/square. Therefore, it can func-tion as a radio antenna or a resistive heater for deicingthe car windshield. However, this conductance maydecrease wireless signal transmission into and out ofthe vehicle.

The glass supplier has to be sensitive to all these cus-tomer needs and offer a product portfolio that agreeswith its own business strategy. The designer then speci-fies the product to the glazing fabricator. During fabrica-tion, glass usually undergoes the process steps of cut-ting, edging, washing, tempering or bending, and lami-nating or incorporating in an insulated glass unit. Theoverall process must be economically acceptable to thefabricator and the designer. Thus, the product has to beprocess robust: it must be sufficiently hard andamenable to glass tempering or bending.

In the case of the PVD-coated products, traditionally,the glass supplier cuts to size, bends or tempers theglass pane, coats it and then sends it to the fabricator.This has been economically unacceptable to the fabrica-tor. The long lead time for ordering the glass and theextra cost of coating bent or tempered glass limit theflexibility of the fabricator to respond adequately to theneeds of the downstream manufacturer of the OEMproducts. Therefore, the flat-glass industry has devel-oped coated glass products that can be postcut andheat processed by the fabricator.24 This has requirednew materials, coating processes and stack designs forthe multi-silver-layer stacks, as described elsewhere.22

Thus, total value in this case results from the ability tochoose the product with the right performance for agiven end use at acceptable price. It also results in theability of the fabricator to process the product economi-cally and without unnecessary dependence on produc-tion schedule at the flat-glass supplier factory.

There are several other large-area coated-glass prod-ucts for applications other than automobile or architec-tural markets. For example, in the aircraft industry, TCOcoatings, e.g., In:SnO2 (ITO), are used for airplane deicingor opacity to radar signal. Also, TCO-coated low-ironglass is used as a substrate for large-area photovoltaicapplications. However, in comparison with the architec-tural and automotive markets, the volume of glass usedin these applications is small.

Self-Cleaning GlassThe self-cleaning window was introduced during 2001.The self-cleaning window is the newest category of win-dow since the introduction of low-E solar reflective coat-

ings during the early 1980s.The self-cleaning window isbased on the photocatalytic and photoinducedhydrophilic properties of a durable film of TiO2.There areother products that can be applied to a glass surface tomake the glass easier to keep clean or that protect theglass surface from contamination. However, the self-clean-ing category of products is unique.The coating is activelyinvolved in a chemical reaction that breaks down organicmaterial on the glass surface.

The photocatalytic property of TiO2 causes the break-down of organic material on the film surface. The elec-tron–hole pair formation in TiO2 is induced by a photonof energy. It is followed by oxidation reactions thatattack organic materials in a series of steps to clean theTiO2 surface.25

The photocatalytic properties of TiO2 gained interestduring the fuel crisis of the 1970s. At that time, TiO2 wasbeing explored for its water-splitting properties to pro-duce hydrogen gas.26 This later prompted interest inusing TiO2 coatings for self-cleaning surfaces.27 Theanatase form of TiO2 is a semiconductor with a bandgapof 3.2 eV. Therefore, light with wavelengths <387 nm, i.e.,the UV range of solar spectrum, is needed to create theelectron–hole pairs.

Another property of TiO2 is UV-induced hydrophilicity.28

This property manifests itself by the contact angle of adrop of water on the TiO2 surface that is in the superhy-drophilic regime (<5°) when exposed to UV light.Thisproperty of TiO2-coated glass is what is most obvious tothe consumer. It is the visual effect of water, from rain orspray, wetting out on the window, rather than formingdrops.Windows with self-cleaning glass wet out dramati-cally compared with ordinary glass windows whensprayed by water (Fig. 6).

The sheeting of water on a self-cleaning glass surfaceafter outdoor exposure is clearly evident when com-

Figure 7 (I) Regular clear float glass and (II) TiO2-coated glass.

UV-Induced Hydrophilicity

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pared with the beading of water on the surface of ordi-nary clear float glass. The water on the self-cleaningglass surface is not easily visible, because it is spread outas a uniform thin sheet. This visual effect also results in acleaner-looking window, because water spotting is elim-inated on the dry glass. This hydrophilic effect is sup-ported by the photocatalysis that prevents oilyhydrophobic dirt buildup, which would spoil theobserved wetting behavior.

UV-induced hydrophilicity of a TiO2 film has beenquantified (Fig. 7). The contact angle of water on the sur-face of a TiO2-coated glass has been compared with thaton the top-side of a piece of standard soda–lime–silicafloat glass. The samples are exposed to simulated solarUV light (UVA340 bulbs with a 28 W/m2 irradiance levelat the sample surface). The contact angle is measuredperiodically. The contact angle of the TiO2-coated glasshas an initial value of ~40°. However, the value quicklydecreases to 5° within 2 h of UV exposure. The contactangle for the standard glass starts out small, which isexpected for a mostly SiO2-based material. However, thecontact angle increases over time as naturally occurringorganic compounds condense on the surface from theair.

The mechanism for the photocatalytic properties forTiO2 have been well documented. However, the mecha-nism for the photo-induced hydrophilicity is less wellunderstood. One proposed mechanism is the reactionof photogenerated holes with bridging oxygen speciesat the TiO2 surface to form defect sites. Dissociativeadsorption of water occurs at these defect sites. Thisresults in an increase in hydroxyl groups at the coatingsurface, which render the surface superhydrophilic.29

Detailed surface analysis work is being done on single-

crystal TiO2 to further investigate this interesting property.30,31

The photocatalytic property of TiO2 films also can bequantified with contact angle measurements. Contactangle data for TiO2-coated glass and regular glass thathave been precoated with a thin layer of stearic acid, acommonly found fatty acid, have been collected (Fig. 8).The initial value of the contact angle for both samplesstarts at ~80° because of the hydrophobic nature of thestearic acid layer. Under UV irradiation, the contact angleon the TiO2-coated glass surface decreases, reaching val-ues <10° in 2 h. This result can be achieved only bydegradation of the stearic acid layer.

The contact angle on the regular glass surface doesnot change. The stearic acid does not degrade underthese conditions without the aid of a photocatalyst. Thecombination of the photocatalytic and UV-inducedhydrophilicity are necessary to bring about the self-cleaning function to a window. However, a third proper-ty, durability, also is needed to bring value to the cus-tomer and the consumer.

The film and its function must endure handling duringwindow manufacturing and installation. However, it alsomust survive outdoor exposure over a period of ≥10years. Therefore, TiO2-based self-cleaning glass productsare deposited using an on-line CVD process, asdescribed in preceding sections.32

The aesthetics of the TiO2-coated glass also are impor-tant. The residential market, in particular, requires thatvalue-added glass products do not vary too far inappearance from that of windows made with regularglass. This requirement is a challenge for TiO2-coatedglass because of the high index of refraction of TiO2(2.3–2.5 at 550 nm). Sufficiently thick TiO2 films on glasscan be highly reflective. Interference effects result instrong coloration as the film thickness increases. This

Figure 8 (I) Regular clear float glass and (II) TiO2-coated glass, both pre-coated with a layer of stearic acid.

Stearic Acid Degradation Monitored Using Decreased Contact Angle

Examples of flat glass coating process.

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aesthetic challenge can be managed using thin TiO2films. For example, a 20 nm thick film results in a firstsurface luminous reflectance value of ~20%. This valueis much higher than that for regular glass (8%).However, the neutral, slightly blue reflected color of thecoated glass gives a pleasing bright appearance to thewindows when they are installed, which adds to theoverall clean look.32

The high index of TiO2 also can be countered usingoptical layers—transparent thin films of appropriateoptical thickness—between the glass and the TiO2.These layers act to suppress the strong interferencecolor of thicker TiO2 films. For example, this optical layercan be a single λ/4 homogenous film with an index ofrefraction between that of the TiO2 and the glass. Suchapproaches need to be chosen with care to balance thecost and manufacturing complexity with the overallappearance and performance of the product.

The use of self-cleaning glass continues to grow in themarketplace. There is always a drive to find other appli-cations for this type of product, such as expanding theuse of the TiO2-coated glass to automotive and buildinginteriors. These applications have unique challenges: theamount of the necessary UV light available to drive thephotocatalytic and hydrophilic functions, and therequirements for durability and appearance. Self-clean-ing glass is new to the marketplace. However, as theconsumer becomes more familiar with its function, thepossibilities and demands for such a product will con-tinue to grow.

Preferred Glazing MaterialGlass remains the preferred glazing material because of itslower cost and superior properties. However, new compe-tition is arising from advances in other areas, particularlyin polymer technology and nanomaterials. Examplesinclude hard-coated plastic car windows, energy-absorb-ing nanoparticles in transparent polymer–ceramic com-posite films and multilayer polymer thin-film stacks for IRreflection. At the same time, these and other develop-ments in materials science also have created exciting newopportunities for flat-glass products. In particular, newdevelopments in nanotechnology, where the glass tech-nologist has for centuries taken advantage of the specialoptical properties of nanoparticles, offer promising newroutes to adding function to glass and its surface.

Total value to the customer will continue to determinethe success of new value-added flat-glass products. Arecent market study33 predicts a 50% growth in thetotal world demand for flat glass in the first decade ofthis millennium. This coincides with an unprecedentedincrease in global production capacity. Most of thegrowth has occurred in Asia, which already has resulted

in serious price competition in the industry. Energy andfuel efficiency, safety, comfort and aesthetics continueto drive flat-glass product innovation.

Sophisticated customers participate in leading theresearch and development effort in the flat-glass arena.A customer-driven product strategy ensures that thelimited marketing and research and developmentresources are focused on market needs. This focus canresult in more emphasis on incremental developmentsat the expense of game-changing technologies. Theindustry must consider this risk, particularly in the faceof the cost challenge from the low-labor-costeconomies. Therefore, in our higher-cost environment,we cannot depend on exclusivity and long product lifecycles. Rather, a multidisciplinary approach to continu-ous innovation in products and manufacturing excel-lence is a must.

Editor’s Note

The first part of this two-part series appeared in theJanuary 2005 issue of the Bulletin, pp 30–35.

References20Sungate® 500 Glass, PPG Industries, Pittsburgh; P.R. Athey,

D.S. Dauson, D.E. Lecocq, G.A. Neuman, J.F. Sopko and R.L.Stewart-Davis, U.S. Pat. No. 5 356 718 A, Oct. 18, 1994.

21J. Szanyi, Appl. Surf. Sci., 185 [3–4] 161–71 (2002).22M. Arbab, Thin Solid Films, 381, 15 (2001).23Solarban® 60 Coated Glass, PPG Industries, Pittsburgh.24J.J. Finley, U.S. Pat. No. 4 806 220, Feb. 21, 1989; M. Arbab, R.C.

Criss and L.A. Miller, U.S. Pat. No. 5 821 001, Oct. 13, 1998.25A. Heller, Acc. Chem. Res., 28, 503–508 (1995).26A. Fujishima and K. Honda, Nature (London), 238, 37 (1972).27Y. Paz, Z. Luo, L. Rabenberg and A. Heller, J. Mater. Res., 10,

2842 (1995).28R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A.

Kitamura, M. Shimohigoshi and T. Watanabe, Nature (London),388, 431–32 (1997).

29N. Sakai, A. Fujishima, T. Watanabe and K. Hashimoto, J. Phys.Chem. B, 105, 3023–26 (2001).

30S. Mezhenny, P. Maksymovych, T.L. Thompson, O. Diwald, D.Stahl, S.D. Walck and J.T. Yates Jr., Chem. Phys. Lett., 369, 152–58(2003).

31J.M. White, J. Szanyi and M.A. Henderson, J. Phys. Chem. B,107, 9029–33 (2003).

32SunClean® Coated Glass, PPG Industries, Pittsburgh; C.B.Greenberg, C.S. Harris, V. Korthuis, L.A. Kutilek, D.E. Singleton, J.Szanyi and J.P. Thiel, U.S. Pat. No. 6 027 766, Feb. 22, 2000.

33”World Flat Glass,” Industry Study 1589, The FreedoniaGroup, Cleveland, Ohio, 2002.

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