40 GLASS PROCESSING DAYS, 13–15 Sept. ‘97ISBN 952-90-8959-7 fax +358-(0)3-372 3190

FUNDAMENTAL PROPERTIES OF FLOAT GLASSSURFACES

Weissmann Rudolf

University of Erlangen, Institute of Materials ScienceMartensstr. 5, D-91058 Erlangen, Germany

ABSTRACT

The present paper summarises thefundamental properties of float glass surfacesthat are the main basis for refining of float glasslike coating, joining of glass with other materialsand surface modifications. The optical propertiesof float glass surfaces like waviness, distortionsor surface roughness are comparable with theformer grinded and polished plates. Incomparison with plastic materials glass surfacesshow a much higher abrasion resistance andscratch hardness. The excellent hydrolytic acidand alkali resistance protects glazing againstenvironmental attack and guarantees a long lifetime. Float glass has surface chemicalcharacteristics different in some respects fromthe bulk glass. Both surfaces of float glass wereproduced under different environments thanother commercial glasses. The most obvioussource of difference arises from the contact ofone side of the ribbon with hot tin bath. The resultis a slight difference in the surface propertiesbetween the two float glass sides. The role ofmicrocracks (flaw statistics) on the strength offloat glass and surface hardness will bediscussed. Finally a short overview is given aboutchemical durability and storage conditions.

Keywords: Float glass, optics, float process,glass surface, glass corrosion

1. INTRODUCTION

In the history of glass technology the floatprocess marks a revolutionary step for the flatglass manufacture [1]. Since its introduction in1959 by Pilkington, the float process hassubstantially replaced the plate and drawn sheet

process world-wide. Today there exist about 150float plants in the world with a productioncapacity of about 30 million tons. This ratecorresponds nearly 35% of the total glassproduction.

The advantages of the float process incomparison with the older flat glassmanufacturing processes are manifold [2]: (1)The float process is capable of producing flatglass of the highest quality in the thickness rangefrom 1 to 25 mm and in ribbon width greater than3m. (2) There is no received limit on the outputfrom float unit. By economic reasons the typicalproduction capacity is about 75 000 m2 or 450 -700 to / day. (3) The process is inherentlyefficient. The width of the ribbon can readily bealtered to match the dimension of the productrequired. (4) The process is continuos and notcyclical. (5) High optical surface qualitycomparable with polished glass plates (6) Due tothe great improvements and extensions duringthe last 30 years the technology of the floatprocess operates under much surer control thanis the case for most flat glass manufacture.

Due to the special forming process the floatglass surface distinguishes from other glasssurfaces. This paper summarises thefundamental properties of float glass surfacesthat are important for secondary glass processingand finishing like coating, joining of glass withplastics, chemical strengthening or surfacecorrosion during storage. In detail the opticalsurface properties, mechanical strength andhardness, surface defaults and the chemicalcomposition and corrosion of the float glasssurfaces are treated in detail.

41GLASS PROCESSING DAYS, 13–15 Sept. ‘97ISBN 952-90-8959-7 fax +358-(0)3-372 3190

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2. FLOAT PROCESS

By this process molten glass is feeded to afloat bath at a temperature of about 1100°Cthrough a spout and a tweel that can controlexactly the glass flow rate [1]. The molten glassspreads onto the surface of molten tin until anequilibrium thickness of about 6.5 mm isestablished. Final ribbon thickness and width canprecisely be controlled by ribbon speed, edge rollmachines and roof headings. Pure nitrogen andhydrogen are feuded inside the bath to maintaina reduced atmosphere in order to prevent theoxidation of molten tin. After stretching or fendingthe glass ribbon is cooled down to 620°C. At thistemperature the glass band is rigid enough forlifting and transporting into the lehr by rollers.

The float process is characterised by the fact,that at least only one surface is in contact with afluid and a solid material during formation andinitial cooling. This fact has an influence on theproperties of the two surfaces of the glass plate,the tin (bath) side and the atmosphere (firepolished) side [3-6]. Because of the reducingatmosphere, the fire polished side becomesdepleted in alkali and sulphur and thus enrichedin silica in the temperature range 1000 to 800 °C.The surface in contact with the tin bath reactswith molten metal by migration of tin and ironfrom the tin bath into the glass. This leads to acounter diffusion of sodium and calcium ions intothe molten tin. The concentration of tin oxide nearthe surface can be about 35 wt% with a gradientextending about 10-30 µm into the glass. With agiven glass composition and a given temperatureand hydrogen distribution, both the alkali lossesto the atmosphere and the depth of penetrationof tin on the bath side depend on the residencetime of the glass in the float bath. Glass near theequilibrium thickness (6.5 mm) shows a deepertin penetration than either 2 or 12 mm [7].

3. OPTICAL PROPERTIES

Optical properties of float glass play animportant role for automotive applicationsespecially for windshield optics [8]. Undesirableoptical effects as double images, distortion orastigmatism can arise from the wedge andwaviness of the float glass ribbon. The curvatureand tilt angle of the windshield amplify theseeffects in an undesirable way. The wedge meansthe lack of parallelism between the glasssurfaces and contributes to the visibility of ghostimages observed by car drivers. An importantcontributor to double images was the formergrinding and polishing plate glass process. Thisprocess had a tendency to make the glass

thinner toward the edges of the grinding tables.The introduction of the float process has reducedthe wedge angle. While float glass is free ofoverall wedge, it is not entirely free of localwedge.

The second optical defect, which results fromthe float process is the distortion. Distortions arethe result of waviness of the glass surface andact as cylindrical lenses in the glass. The reasonfor this effect is that the float bath is not ideallyflat in reality. Top rollers, stretching forces on theglass ribbon and vibrations from outside inducesa corrugation of the glass surfaces. Two forcesare acting to remove the corrugation and toflatten the glass surface: surface tension andgravity. By these way capillary-gravity waves areinduced with a wavelength of about 20 mm. Thegreat advantage of the float process is, that itreduces the amplitude of the distortion wavesand therefor the lens power. Measurements showthat float glass with equilibrium thickness of6.5 mm has the best optical quality. The opticaldistortions are <10 mdptr and the wedge angle<2 mrad (see Table 1).

Table1. Optical properties of float glass

Optical effect Float data

Wedge angle Ghost image < 2´Surface waveness Distortion < 10 mdptrSurface Light RZ < 0.1 µmroughness scattering

4. MECHANICAL PROPERTIES

Since the pioneer work of Griffith 1920 [9] weknow that the strength of glass is not an inherentproperty like density or elastic modulus. Themechanical strength is determined by microflaws(Griffith cracks) in the surface. These flaws act asstress concentrators and the critical breakingstress depends on the depth of the flaws. Flawsare created during the forming, cooling and finalhandling processes. The main source is thecontact of the freshly produced glass with othermaterials. Also surface crystallisation, airpollution, chemical reactions and rapid coolingcan generate microcracks. During the formingprocess float glass has no contact with solidmaterials. By that reason, freshly produced floatglass has a higher strength compared withgrinded and polished plates. The grindingprocess causes extra microflaws in the glasssurface. The following polishing process simply

42 GLASS PROCESSING DAYS, 13–15 Sept. ‘97ISBN 952-90-8959-7 fax +358-(0)3-372 3190

reduces the length of flaws but do not completelyremove them. A criterion for the mechanical stateof the glass surface is the mean number of flawsper unit area n(s) that are acting as fractureorigins at stresses < s [10]. n(s) is also called theflaw distribution function. Fig 1 shows thevariation of n(σ) for the two float glass surfaces incomparison with polished plate glass. The resultsverify the higher density of surface flaws of thepolished plate. Moreover the tin and atmospheresides show a slight difference in their strengththat comes from the contact of the tin side withtransport rollers. In addition, since the ribbon ismolten and deformable along almost the wholelength of the bath, any specks falling from thebath atmosphere or any bubbles rising throughthe molten tin will permanently damage thesurface.

Figure 1 Number of surface flaws/mm2

1 - polished plate glas, 2 - floatglas tin side3 - atmosphere side

A further important property of the glasssurface is its relative high hardness. Hardness isthe resistance of a material to local deformation,scratching and erosion. Common techniques formeasuring hardness of glass are Vickers (HV)and Knoop (HV) intenders. For practicalpurposes, i.e. applications for transport vehicles,

Taber abrasion tests, sand blasting and scratchhardness measurements are carried out onglasses. Table 3 summarises some surface datafor float glass. Values for PMMA are shown forcomparison. It must be mentioned, that the twofloat glass sides show no measurable differencein their surface properties.

Table 2. Mechanical properties of float glass

Test Float glass PMMA

Knoop (HV) 490 ——Abrasion 0.9 - 1.1 % 30 %(DIN 52347)Sand - Blasting 4.6 % 20 %(DIN 52348)Scratch 0.1 N 0.5 NResistance(ISO 1518)

5. TOP SPECKS AND BLOOM

Top specks and bloom are unusual propertiesof the float glass surfaces. The reasons for thesedefaults are complex chemical reactions in thefloat bath [1]. Glass from the melting furnacecontains sulphur and oxygen compounds. Thesecompounds are evolved as water and hydrogensulphide at the atmosphere side of the ribbon. Atthe bottom surface they react to give dilutesolutions of oxygen and sulphur in the tin bath. Ifthe tin contains only 10 ppm of oxygen andsulphur, stannous sulphide and stannous oxideare evolved into the bath and atmosphere.Through subsequent condensation and chemicalreduction of these compounds, small specks oftin and various of its compounds can beproduced on the atmosphere surface of the floatglass.

As the oxygen content of the tin rises, theribbon absorbs increasing amounts of stannoustin. Later heat treatments at about 600°C orabove causes oxidation of stannous tin to stannictin with absorption of oxygen from the furnaceatmosphere. This causes a bluish haze on thesurface as a result of microscopic wrinkling of thesurface which expands as oxygen is absorbed.

6. CHEMICAL DURABILITY

Durability of Glass is defined as the resistanceof the glass surface to corrosion. The durabilitydepends on the glass composition and surfacestate as well as of the corrosive conditions of the

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environment. Due to its chemical composition(Table 3) float glass is a highly durable glass.That means that it is relatively insensitive againsthydrolytic and acid attack. Table 4 shows the testvalues for the three categories, hydrolytic, acidand alkali durability.

Table 3. Chemical composition of float glass(wt.%)

SiO2 Al2O3 Na2O MgO CaO72-74 0.2 - 1.5 12 - 15 3 - 5 6 - 10

Table 4. Chemical durability of float glass

Test method ClassAcid resistance DIN 12116 1

ISO 699Alkali Resistance DIN 52327 1 - 2

ISO 719Hydrolytic Resistance DIN 12111 4 - 5

As mentioned in chap. 1, both float glass sur-faces are different in their surface compositions.By this reason bath and atmosphere sidedistinguishes in their corrosion behaviour. Thiseffect is shown in Fig 2. The atmosphere sideshows a higher leaching rate of alkaline ions thanthe tin side [11]. Surface analytical measure-ments with NRA, ESCA and ellipsometry confirmthis results.

Figure 3. Leaching of Na - ions

In praxis, the corrosion of float glass is to alarger extend a function of the environment, towhich it is exposed [12]. Installed in buildings and

transport vehicles, the glass will usually facedynamic conditions by rain, water condensationor cleaning. So it can develop a uniformdealkalized and passivated surface layer that willprotect it from severe damage. Under storageconditions however, e.g. from wareroom throughtransit, static conditions prevail. A pure water filmcan condense in the capillary spaces betweenadjacent glass plates. The leaching of thealkaline ions increases immediately the pH valueand causes consequently severe corrosion of theglass surfaces. In order to avoid this, differentseparating agents are employed for sheetpackaging. Papers, sawdust or polymers likepolystyrene or PMMA are used as separatingagents. Tests have shown that a combination ofLucite (PMMA) with adipinic acid has the bestinhibiting effect [11].

7. SUMMARY

Light transparency, optical quality, durabilityand abrasion resistance, these qualities arehighly demanded for application of glass or othertransparent materials in transport vehicles andarchitectural glazing. Float glass satisfies theseconditions in an excellent way. Due to the specialforming process in the float bath, the glass plateshave low optical distortions, small wedge angleand an ideal smoothness of both surfaces.Having no contact with solid materials, float glasshas a higher mechanical strength than polishedplate glass. Finally the excellent chemicaldurability is the reason why float glass has founda variety of applications in different fields likeautomotive glazing, glasses for buildings,functional glasses or substrate materials.

References

[1] A. Pilkington, Glass Technology, 17 ,182, (1976)[2] W. C. Hynd, Glass Science and Technology , Uhlmann,

D. R. and Kreidl, N. J. (Ed), New York 1984 Vol 2, 83-100

[3] C. G. Pantano, V. Bojan, M. Verita, F. Geotti-Bianchiniand S. Hredglich, Fundamentals of Glass Science andTechnology-ESG 1993, Venice, 285

[4] F. Gebhardt and U. Graff, Glastechn. Ber. 54, 1 (1981)[5] J. S. Sieger, J. Non-Cryst. Solids 19, 213 (1975)[6] M. Matousek, M. Maryska and A. Helebrant, Glass Sci.

Technol. 69, 7, (1996)[7] M. Laube, Thesis University of Frankfurt 1996[8] M. J. Irland, Society of Automotive Engineers, 700480

(1970)[9] A. A. Griffith, Phil.Trans. Roy. Soc. Lond. A221,163

(1920)[10] H. Rawson, Properties and Application of Glass,

Elsevier 1980[11] M. Feldmann and R.Weissmann, Int. Conf. on Coatings

on Glass Saarbrücken 1996[12] H. Franz, J. Non-Cryst. Solids 42,529 (1980)