The colour stability of exterior coatings

The „shutter effect" – and what's behind it

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The colour which is selected for a build-ing gives it its identity, the interplay ofarchitecture and colour defining the cha-racter of the structure. The part playedby colour in architecture is undisputed –for decades, even centuries, colour hasbeen considered the fundamentaldesign element.

The phenomenon of colour has longbeen the subject of study by psychol-ogists investigating its impact on humanemotions and well-being. Professionalcolour designers make very successfuluse of this knowledge in devising colourconcepts.

Investing in colour design and paint isworthwhile because paint still remainsthe most economical and effectivebuilding design element.

Whether for a detached house or resi-dential complex, for administrative orpublic buildings, for individual buildingsor entire streets, well thought out colourconcepts perceptibly enhance value andthe user's well-being. They direct atten-tion, impart identity, increase value andare not rarely the expression of an atti-tude towards life.

Colour stability – a neglected quality feature

The phenomenon of colour change inexterior coatings has been known sincetime immemorial and is visible to anyoneas an optical defect on buildings. Youtoo must also be familiar with the "shuttereffect" in which the facade has typicallyfaded, but, behind the shutters, protect-ed from light and weathering, the original colour shade is still largely unchanged. This and many other phenomena associated with changes incolour shade greatly disrupt the overallimpression made by a facade. They arenot only unattractive but also ruin anycolour design which had been chosenwith effort and sensitivity.

It would thus seem strange that con-siderably more attention is not paid tothe colour stability of coating productsas a quality criterion.

There is certainly no such thing as per-fect colour stability, but there are stillhuge differences in the behaviour ofcoating products. Coatings whosecolour shade remains unchanged to thehuman eye for decades are, for exam-ple, an entirely realistic proposition. It is,of course, quite true that there is scarce-ly a commercial coating product whichis not promoted as being "UV resistant","lightfast" or "colour stable". But what are the differences? What hasto be taken into account?

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Colour as a designelement

Changes in colour –an optical defect visible to anyone

Colour stability –wish or reality?

Light, weather, atmospheric

pollutants, dirt

1Resistance of pig-

ments to light (espe-cially UV radiation)

and to attack bypollutants from theatmosphere ("acid

rain")

2Resistance of

binders to UV lightand weathering

3Soiling tenden-

cy/self-cleaning pro-perties of coating

surface as a functi-on of the particular

binder

*Information about plant growth on facades may be found in the brochure "Algae and fungi – ver-dant growth...".

Factors influencing colour resistance

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PigmentsBinders

FillersAdditives

Light (UV radiation)

Weather

Light (UV radiation)

Dirt, algae, fungi*Atmospheric pollutants

Colour stability / colour resistance

Colour stability and a constant opticalimpression over the long term are deter-mined by a series of different factors.

The most important are light, weather,atmospheric pollutants and dirt.

These act on pigments and binders,which are the coating ingredients whichcrucially determine colour stability.

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Colouring pigments may be divided intoorganic and inorganic (mineral) pig-ments. The differences in the materialson which they are based result in dif-ferences in resistance between the twocategories of pigments.

UV radiation can cause colour changesin pigments. This phenomenon is familiarfrom textiles: items of clothing whichhave been hung on outdoor displayracks exposed to sunlight often havefaded areas. Exterior wall coatings cansuffer a similar fate if UV resistant pig-ments have not been added to the paintor have not been added in sufficientquantity.

Inorganic (mineral) pigments have thebest light and UV resistance and retaintheir colour for decades.

In the same way that citric acid removesthe organic dyes in fruit stains, aggres-sive atmospheric pollutants ("acid rain")may cause colour changes in pigments.Only selected inorganic pigments areacid resistant.

There are two kindsof pigment – both in

terms of material andresistance

Resistance of pigments to lightand atmospheric pollutants

Potsdam DistrictRecruiting Office

(Photograph taken14.06.2004)

Left hand building:painted with KEIM

silicate paint in 1992(colour shade 9071)

Right hand building:painted the same

colour shade with acompetitor's organi-

cally pigmented paintin 1995 and already

exhibiting a distinctcolour change

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Silicate paints are generally formulatedwith such inorganic pigments, not leastbecause many organic pigments cannotwithstand the elevated alkalinity of thewater glass binder (the binder for sili-cate paints). Accurately formulating de-fined colour shades with inorganic pig-ments is, however, incomparably moredifficult than with organic pigments.Producing and, especially, reproducinginorganically pigmented colour shadesdemands considerably effort, experi-ence and know-how.

IN A NUTSHELL:Achieving colour stable coatingsmeans using only high-qualityinorganic pigments.

Organic pigments are frequently usedfor the production of exterior paints –they are available in large quantitiesand in an almost unlimited range ofcolour shades which are exactly repro-ducible during manufacture. As a result,these pigments simplify the handling andtinting of coating products.

Being based on natural raw materialsand due to their manufacturing process,inorganic pigments are never absolutelyuniform. Variations in the colouring pro-perties of one and the same pigmentfrom different batches are thus unavoid-able. Moreover, being based on inor-ganic raw materials (completely in con-trast with petrochemically derivedorganic pigments) means that only alimited number of colouring pigmentsare available. Nevertheless, thousandsof natural looking colour shades, bothlight and intense, can be produced withthem.

Pictures below:Seminary inMeersburg

On the left: northwing (south faca-

de): originalcoating with KEIMPurkristalat dating

from 1974 after 30years' service life;

bright, clean,unchanged incolour shade

By comparison, onthe right:

west wing (eastfacade): new

coating with KEIMSoldalit from 2003

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What does this meanfor the production and

tinting of paints?

The UV resistance and "weather fast-ness" of the binder also play an impor-tant part in the colour stability of the fin-ished coating. Like pigments, paint bin-ders may also be divided into two majorgroups:

• organic binders, such as syntheticresin emulsion paints including silicone resin emulsions and

• inorganic, mineral binders,such as water glass (potassium silicate) or sol/silicate (silica sol/water glass mixture).

If the organic binder's resistance is inad-equate, UV light and weathering, suchas extreme fluctuations in temperature(hot/cold) or humidity (wet/dry), maygive rise to microcracks in previously"smooth", continuous coating films andsubsequently result in binder degrada-tion.

Microcracks are ultrafine cracks in thecoating layer which modify the refrac-tion of the original "smooth" coating filmand so give rise to modified optical pro-perties. The colour shade of the coatingappears greyer, cloudier and less clean.Moreover, water can penetrate into thesubstrate through the microcracks and, ifthe coating layer is not sufficiently open-pored, result in damage.

Light and weather – a harsh test for paintbinders too

Microcracks

Resistance of binder to UV lightand weathering

Organically boundcoating after two

years' weathering:microcracks in the

previously "continu-ous" coating layer

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UV light can also cause yellowing. Weare familiar with yellowing from manyareas of our daily life: yellow discolora-tion arises over the course of time due tothe action of light, for example in porchroofs or plastic pots, because the binderis not sufficiently UV stable. A similarphenomenon may also occur in paints ifthey are based on such binders.

IN A NUTSHELL:Mineral binders such as waterglass or sol/silicate exhibit thehighest UV resistance of all bin-ders. Water glass and sol/silicateare also absolutely weather fast.

Yellowing:ordinary plasticbottle, upper halfexposed to "sun test" for 250 hours

Binder degradation Yellowing

AFM micrograph:"binder

degradation"

Top: new coating:pigments and fillers

(white/light) aresolidly embedded in

the binder (red).

Bottom: after appro-priate UV exposure:pigments and fillers

(white/light) havebeen uncovered

over large areas bybinder degradation.

(Published with the kindpermission of Kerr-MCGee

Pigments Ltd.)

UV light and weathering can alsodestroy the binder, resulting in brittle-ness, instability and gradual degrada-tion. Such binder degradation results notonly in technical defects, but also in opti-cal shortcomings: for example the slowdestruction of the binder gradually"uncovers" the colouring pigmentsembedded in the binder, which are thusincreasingly exposed to UV radiationfrom the sunlight. This additionally accel-erates the colour change of the (organ-ic) pigments. Moreover, because the pig-ments are inadequately "embedded" inthe coating, weathering leaches pigmentout, making the coating still paler. Suchpigment leaching is frequently also theresult of inadequate binding (= high pig-ment volume concentration), for exam-ple in "sil" paints.

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08:00 8:20 8:40 9:00 9:20 9:40 Time

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Sol silicate paint

Dispersion silicate paintSilicone resin paintLotus

g/m2

Static charging, thermoplastic propertiesand condensation – complicated termsfor simple natural laws. Soiling alsomodifies the colour and optical appear-ance of exterior wall coatings. Althoughsoiling (in comparison with pigment orbinder changes) is relatively simple toremedy, vastly more attention has beenpaid in the recent past to the soilingbehaviour of coatings.

The soiling tendency of exterior wallcoatings is substantially determined bythree factors: • static charging • the thermoplastic properties or

"tackiness" of the binder • and the surface's exposure

to condensation

Organic synthetic resin/silicone resinbinders become electrostatically chargedin the wind due to friction and so actual-ly attract dirt particles onto themselvesfrom the air. At higher temperatures,these binders additionally exhibit ther-moplastic behaviour, i.e. they becometacky when hot. Dirt particles blown inthe wind and attracted by the staticcharge thus find ideal adhesion condi-tions and "stick" to the surface. Silicatebinders, in contrast, do not behave in thisway.

Static charging, ther-moplastic propertiesand condensation –

complicated terms forsimple natural laws

Influence of binder on thermoplastic

properties and static charging

Soiling as a function of binder

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At low temperatures too, silicone resinpaints are at a disadvantage relative tosilicate paints because distinctly morecondensation is deposited on siliconeresin paints and this applies particularlyto "lotus effect" or water repellent paints(see table above). As a result, siliconeresin surfaces are wet for longer and dirtparticles can accordingly stick on morereadily.

Moreover, the risk of algal growth is alsodistinctly increased by the unfavourablecondensation behaviour of silicone resinpaints (unless toxic, leachable biocidesare added).

Silicone water

repellent paint

Silicate paint

Coating after 2 years Quantity of condensation

Fig. 1Exterior wall coating of

silicate paint and sili-cone water repellent

paint after two years'outdoor weathering

Fig. 2Quantity of condensa-tion on exterior wallcoating (systematic out-door measurements)

Soiling – practicalexamplePrivate house inLucerne, painted whiteat the same time.Left: silicone resin paintRight: silicate paint

namely that silicate coatings exhibit thebest behaviour in terms of cleanness andsoiling resistance.

This is primarily due to the above-described properties of silicate paints –antistatic, non-thermoplastic properties,low levels of condensation, favourablewetting characteristics – all of which arelargely determined by the silicate binderwater glass. The phenomenon of "micro-chalking" additionally assists by provid-ing controlled nanometre-scale "sand-ing", caused by weathering, of thecoating layer which proceeds uniformlyover a period of decades and results ina constant removal of any deposits ofdirt which do occur.

IN A NUTSHELL:Silicate paints, by their nature,have a lower tendency to soilingthan emulsion or silicone resinpaints. Silicate paints are anti-static, non-thermoplastic andtheir surface is drier for longer.

In the recent past, extreme water-repel-lency (= "hydrophobicity") has been thefocus of considerable and heated publicdebate in relation to "clean facades"and the "lotus effect". The extent of water-repellency of paintsis largely controlled by the addition ofappropriate additives, such as siliconeoils.Paradoxically, however, silicone oilsresult in greater adhesion of dirt parti-cles. The "water-beading" effect present-ed in laboratory tests and in advertisingposters has not proved advantageous inpractice – quite the contrary. Manybuilding exteriors coated with theseextremely "hydrophobic" paints sufferfrom very severe soiling. Water dropletscarrying particles of dirt roll down thefacade, come to a standstill on a particleof render and get stuck there (see Figure 1).

The most recent studies likewise confirmthat there is no connection between the"beading" effect and the "cleanness" offacades. Instead, systematic investiga-tions have now confirmed what hasalready been evident for years anddecades from coated structures –

What experience hastaught us

Recent results confirmthe advantages of sili-

cate paint

Clean facades – wish and reality

Augsburg TownHall painted with

silicate paint, after 20 years'

service life

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Figure 1

There is certainly no such thing as per-fect colour stability, but there are stillhuge differences in the behaviour ofcoating products, something which prac-tical experience teaches us time andagain. The difficulty resides in demonstrating ormeasuring these differences. Specialistsin the field are unanimous: there is noreliable, laboratory test method which iscapable of simulating real conditions ofexposure and could so provide results ofpractical relevance. If realistic state-ments are to be made, the only solutionis outdoor testing in a true to life situa-tion.

Now for the first time a systematic inves-tigation of this issue has been carried outby a neutral party: the coating materialsand paints research and developmentcompany "Forschungs- undEntwicklungsgesellschaft Lacke undFarben mbH" has carried out compara-tive outdoor weathering tests over eightyears on five different exterior paintswith different types of binder and assess-ed them in relation to colour change.

First systematic investigations into

colour change

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The parameter investigated was thecolour difference of the individual coat-ings after completion of eight years'weathering in comparison with an un-weathered reference sample which hadbeen stored in the laboratory protectedfrom light and moisture. The assessmentor measurement of colour differencealso included an evaluation of pigmentchanges, binder changes, soiling andplant growth.

Colour changes

around the shutters

and under the

eaves are clearly

visible from

the pictures.

Colour resistance – what do paints actually do?

Parallel outdoor weathering tests werecarried out to DIN EN ISO 2810 in twodifferent climates, one the industrial cli-mate of Magdeburg and the other therural climate of southern Bavaria. In thisway, climatic conditions could also betaken into account, so additionally ensur-ing the general validity and practicalrelevance of the results.

Both visual and instrumental methodswere used for testing. All test methodswere based on generally recognisedstandards.

Five exterior paints with different types ofbinder were specified as the test prod-ucts. These were a pure, two-componentsilicate paint, a silicate emulsion paint, a(silica) sol/silicate paint, a lotus effectsilicone paint and a pure acrylate exte-rior paint. The particular products testedwere deliberately selected to be of thehighest quality in their particular catego-ry.

The colour shade selected was an in-tense blue (NCS S 2050-R80) becauseblue shades are particularly sensitive toweathering, and the human eye is particularly good at and sensitive indiscriminating colour differences in theblue/grey range. Only the silicone painthad to be used in lighter colour shadebecause the desired NCS colour shadecould not be supplied.

Tested exterior paints Method

Test methods

iLF Forschungs- undEntwicklungsgesellschaft Lacke und Farben mbH,

Magdeburg (2001 – 2009)

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After eight years of outdoor weatheringin an industrial climate and in a rural cli-mate, no adhesion problems in the formof cracks or blisters occurred in any ofthe exposed samples. In terms of decorative properties, theinvestigated silicate products all exhibit-ed distinctly less change than the sili-cone paint and the acrylic paint. This is impressively clear from the instru-mentally measured delta E value andblue value. The silicate product accord-ingly exhibit the best colour stability.

The extent to which the superiority of thesilicate products is manifested in terms ofcolour stability is clear from the photo-graphs, which speak for themselves.

IN A NUTSHELL:The superiority of high-qualitysilicate paints over polymer andsilicone resin bound exteriorpaints, which has so far beenseen and experienced on count-less structures, has been impres-sively substantiated by compre-hensive tests carried out byForschungs- und Entwicklungs-gesellschaft Lacke und Farbenmbh iLF.

Results of practical study

Results of outdoor weathering tests of five exterior paints

unweathered

KEIM Purkristalat after 8 years

weathered

unweathered

KEIM Soldalit after 8 years

weathered

unweathered

KEIM Granital after 8 years

weathered

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An appropriate colorimeter is used for instrumentallyverifying colour shades and colour differences bydetermining the specific "colour location" of thecolour shade within the three-dimensional "colourspace".

Colour shades are defined by three parameters: firstlyon a light-dark axis, secondly on a red-green axis andthirdly on a yellow-blue axis. If all three axes arevisualised, a three-dimensional space is created.This colour space comprises an infinite variety of sub-tly different colour shades. Each possible combinationof the three values on the three stated axes representsa point within this colour space and thus a colourshade.Conversely, the exact position within the colour spacecan be determined for each colour shade as the com-bination of the three coordinates. This combinationunambiguously defines the colour shade. If the colourshade undergoes change, this can be unambiguouslydemonstrated instrumentally, the change being mani-fested as a change in one or more of these values onthe axes. The sum of these changes is known as the"delta E value" which describes the "total colour dif-ference" as the sum of the three colour differences onthe axes. The problem is that the delta E value is perceived dif-ferently by the eye depending on how delta E is madeup and the particular colour shade in question: delta E is the sum of three individual values.Accordingly, many different individual values can beadded together to give the same result and thus thesame delta E value. This is simple arithmetic: 1+2+3= 6 is just as correct as 3+0+3 = 6, but while theresult may be the same, a visual assessment mayprove to be entirely different. The explanation for thisis that human perception also differs in sensitivitydepending on colour shade. In the case of blue or greyshades, our eye is highly sensitive and discriminateseven very small delta E value as a colour difference.In the case of yellow-orange shades, on the otherhand, our eye is fairly insensitive. In this case, smalldelta E values are not even perceived.

In brief: instrumental delta E values do not always cor-relate with what we humans see, especially when it isa question of the extent and intensity of a colour dif-ference.

The problems of colorimetry

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in %

Purkristalat Soldalit Granital Silikon/Lotus effect Acrylic

1 year 8 years

unweathered

Silicone exterior paint “with lotus effect” after 8 years

Change in blue value b

weathered

unweathered

Acrylic ecterior paint after 8 years

weathered

IN A NUTSHELL:Only mineral binders permitundistorted reflection of lightfrom pigments, which so retaintheir original luminance andbrightness.

When discussing the colour stability ofexterior wall coatings, the brightness ofthe colour shades should not pass entire-ly unmentioned because there are alsovisible differences between coatingmaterials in terms of colour brightness.We usually take "brightness" to be anattribute like "silk-gloss" or "high-gloss" –both characteristics with which differentgrades of coating are distinguished andwhich describe the degree of surfacegloss. However, "brightness" does not necessa-rily mean gloss, but instead character-ises another dimension of our percep-tion: the luminance of paints and is notrelated to the degree of gloss.

The luminance of paints is primarily caused by light impinging on the pig-ment and being reflected back. The lessobstruction there is to light impinging onthe pigment and being reflected by thepigment, the greater the luminance and"brightness" of the colour shade.

• Organic binders, as used in emulsionor silicone resin paints, form a filmaround the pigment and so modifyrefraction. The original luminance of thepigment is thus lost and the paint has dulland diffuse appearance.• Mineral binders, as used in silicatepaints, are transparent. They allow lightto pass through unhindered and impingeon the pigment. Reflection is not distort-ed, the paint glows and looks bright.

Glowing colours –matt and bright, a

particular aestheticissue

Light, pigment and binder

A brief aside on colour brightness

Emulsion/siliconeresin paint: pigmentenclosed in cloudyemulsion binder

The House of theThree Magi in Trier,painted with KEIMsilicate paint, animpressive demon-stration of the lumi-nance of a matt sili-cate coating.

Silicate paint: pigment embedded

in transparent water glass unhindered reflection of light:

bright, emphasises texture

reflection of light from film former:

dull, levels irregularities

PigmentPigment

Wasserglas Dispersion

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the use of high quality products forimplementing our colour concepts, prod-ucts we know will retain their originalcolour over the long term, such as prod-ucts from KEIM."

"Colour perception is a highly emotionalissue and no-one can escape the effectsof colours. This is not always consciouslyrealised as colour is an omnipresent partof our natural surroundings. Colour is nota matter of chance, it is a natural event.Colour is an elemental basic need andcontributes considerably to human well-being.

Wrongly and meaninglessly combinedcolours trigger oppressive moods andproduce an unattractive ambience, inwhich humans cannot feel at ease. A skilfully devised colour concept is anideal and economical option for creat-ing positive moods and an attractiveambience – after all paint of the rightcolour costs no more than a wrong one.

The conscious selection and combina-tion of colour shades can only have alasting, successful effect if the colour shades of the materials used do notchange. Changes in colour shade mayhave incalculable consequences andgreatly impair the effects of a wellthought out colour concept or, in extreme cases, even completely ruin it.Material selection is thus of particularsignificance when putting colour con-cepts into practice. Unfortunately, thisissue is frequently neglected. We at anyrate attach considerable importance to

"… colour is anelemental

basic need …"

"A colour concept can only havea lasting effect and success if the colour shades don't change …"

Hermann Janiesch,proprietor ofJaniesch-Farbenplanung, hassuccessfully beenusing the insights ofcolour psychology todevelop colour con-cepts for over 30 years.www.janiesch-farbenplanung.de

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KEIMFARBENGmbH & Co. KG

Keimstraße 1686420 DiedorfTel. 0821/4802-0Fax 0821/4802-210

Frederik-Ipsen-Straße 615926 LuckauTel. 035456/676-0Fax 035456/676-38

www.keimfarben.de info@keimfarben.de

KEIMFARBEN consistently mineral

Overview of key features:

Brilliant, luminous colour shades which are unchangingover the long term and clean exteriors are achieved ifcoating materials meet the following criteria:

• UV and acid resistant pigments

• UV and weather resistant binders

• Antistatic surface

• Non-thermoplastic binders

• Minimal condensation, ideal drying

• Transparent binder

Silicate paints ideally meet these criteria.

But theory is dull - what ultimately counts are the practical results. And silicate paints have been delivering results for decades all over the world.

Take the advice of your specialist company.

Stan

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The binder, as its name would suggest,binds the various constituents togetherand, above all, ensures that the paintadheres to the substrate. A distinctionmay be drawn between two largegroups of binders: inorganic (mineral)binders (for example water glass,sol/silicate or lime) and organic binders(for example polymer emulsions includ-ing silicone resin emulsions). The differ-ence resides primarily in the principle ofadhesion:Mineral binders also react chemicallywith the substrate, while organic bindersadhere only by "adhesive bonding".

Exterior paints are made upof three main constituents:

• Binders• Pigments• Fillers

Brief

of paints

Binders

(Colouring) pigments are very finely divided powders which have an extremely high colouring power.Addition of pigments imparts the colourto the coating material. In the case ofpigments too, there are inorganic andorganic types:Inorganic pigments are obtained frompurely inorganic raw materials (forexample from minerals), while organicpigments are mainly manufactured fromorganic raw materials.

Fillers are generally rock flours. The fil-lers provide the applied paint with thelayer thickness which is required to pro-tect the facade from weathering. Most paints also contain additives.Additives are auxiliary substances withwhich various paint properties can becontrolled (for example water-repellency,brushability, settling in the can etc.).

Pigments

Fillers