Building and Environment 39 (2004) 1321–1326www.elsevier.com/locate/buildenv

Solar energy absorption by acrylic coatings—II: the e'ect of alternatingenvironmental conditions

Marcin Wielocha, Agnieszka J. Klemma ;∗, Piotr Klemmb

aSchool of Built and Natural Environment, Glasgow Caledonian University, Cowcaddens Road, Glasgow G4 0BA, UKbTechnical University of Lodz, Poland

Received 6 January 2003; received in revised form 12 February 2004; accepted 8 March 2004

Abstract

Atmospheric corrosion of building materials is an enormously complex and interdisciplinary process. It a'ects not only the ability ofbuilding to perform over the prolonged period of time but also the overall energy consumption. The presented paper is a part of a largerstudy on application of acrylic plasters in passive solar systems. Absorption characteristics as well as microstructural features of acrylicplasters subjected to alternating climatic conditions have been analysed. In order to simulate the real conditions, samples were exposedto the variations of temperature and humidity, near-daylight radiation (xenon lamp) and subjected to acid and alkaline environments.? 2004 Elsevier Ltd. All rights reserved.

Keywords: Atmospheric corrosion; Absorption capabilities; Acrylic coatings

1. Introduction

The expansion of the industrialization process has leadto drastic changes in the natural environment. With the re-cent evolution of technology environmental pollution beganspreading and the atmosphere has come to contain increas-ing levels of carbon dioxides, acids gases, chlorides, chlo-ro;uorocarbons and many others [1]. As a result, rates ofcorrosion of any kind of material exposed to the atmosphericconditions have also inclined. Systematic experiments onatmospheric corrosion, which have been carried out since1920s are still in progress until the corrosion mechanismswill become fully recognized and possible ways of protec-tion developed. Atmospheric corrosion is enormously com-plex and an interdisciplinary process. During the past fewdecades a substantial amount of work in corrosion sciencehas been devoted to understanding the chemical processes.This ?eld of research has enormous economic consequencesdue to the tremendous costs spread out over the whole so-cieties. Estimating the total costs of direct or indirect con-sequences caused by the atmospheric corrosion, especially

∗ Corresponding author. Tel.: +44-141-331-3544; fax: +44-141-331-3696.

E-mail address: a.klemm@gcal.ac.uk (A.J. Klemm).

in relation to the losses in the national heritage can bediEcult. Atmospheric corrosion is a result of interactionbetween a material (an object made of a metal, a stone,a glass or a polymer) and its surrounding atmosphericenvironment. Most frequently atmospheric corrosion iscommenced by atmospheric humidity which forms a verythin water layer on the object. Various forms of corro-sion depends on the temperature, humidity conditions,intensiveness of a rainfall and its chemical content (pol-lutants), velocity of the wind and on the top the Sunradiation [1].It is believed that the outdoor exposure is dominated by

humidity and temperature, as well as by sulphur dioxide andchloride ions deposition through rain, snow and fog. Thismay result in a substantial deterioration of the external coat-ings, such as the acrylic plasters investigated here. Over thelast couple of decades, acrylic plasters have become moreand more popular in building industry. They may be imple-mented in heating systems powered by alternative sourcesof energy where the external layer features and its stabil-ity over the service time are of most importance. The re-spond to corrosion and to ageing processes might be one ofthe decisive factors determining the eEciency of the wholesystem. The attempt was made therefore, to determine thee'ect of alternating temperature and humidity, xenon lamp

0360-1323/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.buildenv.2004.03.003

1322 M. Wieloch et al. / Building and Environment 39 (2004) 1321–1326

and acid/alkaline environment on the absorption propertiesof acrylic plasters.

2. The experiment procedure

The examination was carried out on ?ve sets of sam-ples prepared from 3 mm thick acrylic plaster composed ofaggregates 70%±5% (maximum fraction diameter 2 mm),?llers 25%±5%, acrylic resin binder 3–5%, and additives.The chemical constitution of the samples was identical ex-cept for the added pigments: white, ecru, red, green and darkviolet [2].After 14 days of curing in laboratory conditions (temper-

ature +18◦C±2◦C, relative humidity (RH) 50%±5%), ?vesets of samples were subjected to variation of temperatureand humidity. The intention was to simulate conditions thatsamples may experience in service. Designated for climatechamber specimens were subjected to 80 cycles consistingof heating and freezing periods. During the ?rst 10 h of thecycle (heating period) the temperature has been maintainedat the level of 40◦C with 80% relative humidity. Within thenext 2 h, the temperature and humidity gradually decreasedto the level of 0◦C and 0% RH, respectively. The temper-ature continued lowering down for the next 30 min until itstabilized and maintained at −20◦C for 10 h (at 0% RH).As the freezing period ?nished, the temperature rapidly,within 1 min, raised to +20◦C and remained on this levelfor 44 min. It was accompanied by a rise in RH to 1%. Dur-ing the last step (45 min) of the 24 h cycle, the temperatureand the RH increased to +40◦C and 80%, respectively. Intotal, the whole cycle consisted of 12:5 h long period in thetemperature range from −20◦C to 0◦C, and 11:5 h lastingheating period with a maximum temperature of +40◦C.The reference samples were stored under normal labora-

tory conditions at an average temperature +18◦C (±2◦C)and RH around 50% (±5%) for a period of 3 months.Estimation of the absorption capabilities have been done

with the aid of a double beam spectrophotometer Cary 5(Varian). Investigations were carried out in the wavelengthrange from 299 to 2000 nm. The integrating sphere of thespectrophotometer as well as the reference standard, wascovered with a highly re;ective, 4 mm-thick layer of poly-tetra;uoroethylen. The examination was conducted in a drystate under the room temperature of 24◦C and 50% ±5% ofRH. As a result of measurements, the total (scattered andspecular) re;ectance characteristics of tested samples havebeen identi?ed and the absorption A has been establishedfrom the following relationship A= 1 − R. Where R is there;ectance [3,4]. The absorption coeEcients have been de-?ned as a ratio of the absorbed solar energy to the incidentsolar energy [5,6]

�=

∫ �2�1

d�[1− R]E�(�)∫ �2�1

d� E�(�); (1)

Fig. 1. The relative power distribution of xenon lamp.

where R is the material re;ectance and E� is solarirradiance [7].The colour of tested specimens has been speci?ed by

means of chromaticity coordinates de?ned as [8,9]

x =X

X + Y + Z; y =

YX + Y + Z

; z =Z

X + Y + Z(2)

x + y + z = 1 (3)

where x; y; z are the chromaticity coordinates and X; Y; Z aretristimulus values.In order to identify the resistance of the tested material to

the Sun exposure and decolourization, the specimens weresubjected to 50 h exposition to a near-daylight quality sourceof arti?cial light. The xenon lamp radiation was producedby electrical discharge in xenon under high pressure. Theradiation characterizes itself with a very high brightness andcolour temperature. Spectral distribution of the lamp extendsfrom the ultraviolet to the infrared. The exact spectral powerof distribution depends, however, on the pressure of thegas. The colour of the emitted light is quite similar to thatof average daylight, but slightly more purple because ofrelatively greater emission at the two ends of the spectrum.The spectral distribution is shown in Fig. 1.In order to determine the in;uence of the atmospheric

corrosion the specimens were tested for the acid and alkalineresistance. Acid resistance test was carried out using 2%solution of sulphuric acid. The specimens were immersed inthe solution for the time of 14 days, and dried afterwards ina dryer at temperature of 65◦C. Similar procedure has beenapplied to test alkaline resistance of samples (2% solutionof potassium hydroxide).The internal microstructure has been examined with the

use of the mercury intrusion porosimetry method [9]. Totalvolume of the open pores, speci?c and skeleton densitiesand medium pore diameter have been obtained for analysis.

3. Discussion

The results of the optical measurements for samples ex-posed to variations in temperature and humidity have been

M. Wieloch et al. / Building and Environment 39 (2004) 1321–1326 1323

Absorption spectrophotographs for differently cured specimens

15

25

35

45

55

65

75

85

95

300 500 700 900 1100 1300 1500 1700 1900

wavelength [nm]

Ab

sorp

tio

n [

%]

Dark violet

Green

Red

Ecru

White

Fig. 2. Absorption spectrographs for samples exposed to various climatic conditions and for reference samples.

Absorption variations graph

-5.00

-3.00

-1.00

1.00

3.00

5.00

7.00

9.00

300 500 700 900 1100 1300 1500 1700 1900

Abs

orpt

ion

vari

atio

ns [%

]

White

Red

Dark violet

Wavelength [nm]

Fig. 3. Absorption variations between the reference specimens and those subjected to alternating climatic conditions.

compared with corresponding results for the reference sam-ples stored in laboratory conditions (see Fig. 2).The variations in the absorption results reached their max-

imum 9–10% for the white colour specimens. Similar be-haviour has been observed for the ecru specimens, wherethe top deviation attained 9%. The optical properties of thered and the green specimens stored under di'erent condi-tions varied in values less than 5% while the absorptionof the violet specimen remained almost unchanged in thewavelength range 299–2000 nm with only negligible devi-ations among di'erently treated specimens. The analyses ofabsorption variations for selected samples (white, red, darkviolet) have been presented in Fig. 3.The absorption characteristics obtained for the same

colour samples, but di'erently cured, have been presentedin Figs. 4 and 5. The analysis of optical capabilities ofsamples subjected to changeable conditions and exposed toarti?cial daylight have been referred to characteristics ofsamples stored in a laboratory. Two bottom curves repre-

sent the absorption abilities of the reference samples. Thesample exposed to arti?cial daylight exhibits slightly higherabsorption. Arti?cial daylight however, in conjunction withchanging temperature and humidity conditions caused a 3–6% decrease in absorption capabilities (Fig. 4). In the othercases the recorded variations did not exceed 2%.The solar absorption coeEcients � (see Table 1) also

re;ects the in;uences of changing environmental conditions.Obtained values of � were always higher compared withthe reference specimens. Nevertheless, they decreased afterexposure to arti?cial daylight source. The increasing valuesof � across the tested colour palette (from white to darkviolet) revealed a semi-linear character, regardless of thestorage and exposure conditions.Exposure to the xenon lamp radiation as well as the

climate conditioning have not caused decolourization, in re-lation to the reference samples. The arti?cial daylight, how-ever, a'ected the absorption capabilities by raising the re-;ectance of the tested samples. Nevertheless, it has been

1324 M. Wieloch et al. / Building and Environment 39 (2004) 1321–1326

Absorption characteristics of ecru colour specimens exposed to differentenvironmental conditions

10

25

40

55

70

85

100

300 500 700 900 1100 1300 1500 1700 1900Wavelength [nm]

Abs

orpt

ion

[%

]Reference sample exposed toxenon lamp

Sample after climate cabinet treatment &xenon lampReference sample

Sample after climate cabinet treatment

Fig. 4. Absorption characteristics for di'erently cured specimens of the ecru colour.

Absorption characteristics of green colour specimens exposed to different

environmental conditions

40

50

60

70

80

90

100

300 500 700 900 1100 1300 1500 1700 1900Wavelength [nm]

Abs

orpt

ion

[%]

Reference sample exposed toxenon lampSample after climate cabinettreatment & xenon lampReference sampleSample after climate cabinettreatment

Fig. 5. Absorption characteristics for di'erently cured specimens of the green colour.

Table 1Solar absorption coeEcients

Specimen White Ecru Red Green Violet

Subjected to changeable environmental conditions 0.41 0.47 0.65 0.75 0.94Subjected to changeable environmental conditions and exposed to xenon lamp radiation 0.39 0.43 0.65 0.75 0.94Reference sample 0.34 0.39 0.62 0.73 0.94Reference sample exposed to xenon lamp radiation 0.34 0.40 0.63 0.74 0.94

also noticed that the absorption abilities of reference sam-ples raised up to 3%. The tristimulus values and chromaticitycoordinates for all cases has been presented in the Table 2.Light absorption capabilities, indirectly depend on the

resistance of the material to acid and alkaline compoundscontained in the polluted air. Carried out tests revealed highresistance of acrylic plasters to alkali, but not to acids. Thesolution of a sulphur dioxide caused colour changes deeply

penetrating the structure of the specimens. Microstructuralmeasurements expressed considerable di'erences among theresults obtained for the reference specimens and those curedin an alternating environment. It has been estimated thatthe samples subjected to varying conditions had over 30%more pores with diameters smaller than 1 �m and around16% more in the diameter range from 1 to 10 �m than thosekept in a laboratory. Only in the diameter range over, up

M. Wieloch et al. / Building and Environment 39 (2004) 1321–1326 1325

Table 2Chromaticity coordinates and tristimulus values

Specimen Tristimulus Chromaticity coordinates

Y x y

White 76.1941 0.3149 0.3229Subjected to changeable Ecru 71.2451 0.3441 0.3591environmental conditions Red 26.9332 0.3999 0.3386

Green 18.188 0.3269 0.4073Violet 6.2199 0.3256 0.3165White 76.0048 0.3171 0.3260

Subjected to changeable Ecru 70.0033 0.3479 0.3632environmental conditions and Red 25.5168 0.4020 0.3397exposed to xenon lamp radiation Green 17.4914 0.3277 0.4077

Violet 6.4314 0.3259 0.3173White 67.9119 0.3155 0.3234Ecru 61.7228 0.3475 0.3629

Reference specimens Red 23.121 0.4061 0.3379Green 16.4401 0.3289 0.4117Violet 5.7181 0.3293 0.3181White 70.3601 0.3175 0.3264

Reference specimens Ecru 66.7052 0.3491 0.3643exposed to xenon Red 23.5657 0.4062 0.3389lamp radiation Green 16.3247 0.3298 0.4124

Violet 5.6943 0.3299 0.3194

Table 3The results of the microstructural analysis

Specimen (colour) Total intrusion volume Total pore area Medium pore diameter Speci?c density Skeleton density

ml/g m2/g �m g/ml g/ml

Cured Reference Cured Reference Cured Reference Cured Reference Cured Reference

White 0.0326 0.0409 4.034 4.236 0.6918 3.8329 2.2983 2.2713 2.5070 2.5036Red 0.0438 0.0432 3.946 4.485 1.3726 3.2239 2.2483 2.2291 2.4941 2.4666Dark violet 0.0421 0.0397 4.108 4.094 0.4777 3.1120 2.2639 2.2402 2.4588 2.4588

Fig. 6. The comparison of di'erential intrusion curves for the red colour specimens.

to 10 �m the total pore volumes, for di'erently cured spec-imens, have been comparable. The pore size distributionillustrated by di'erential intrusion curves (as a function ofpore diameter) demonstrated very similar features of the in-

ternal microstructure among specimens subjected to variousclimatic conditions. Medium pore diameter has decreased inall cases. The majority of pores were in the diameter range0.04–0:4 �m (0.8–3 �m) and 10–100 �m (10–80 �m). The

1326 M. Wieloch et al. / Building and Environment 39 (2004) 1321–1326

values in brackets were obtained for the reference spec-imens. The speci?c and skeleton density, the total porevolume values sustained on a comparable level (see Table3) (Fig. 6).

4. Final remarks

The analysis of the results con?rmed the expectations thatthe colour of plaster has a signi?cant e'ect on absorptioncharacteristics. The highest results of solar absorption wereobtained for the dark violet colour specimen �=0:94, whilefor the white colour plaster only � = 0:41.The absorption characteristics for all specimens except for

the violet samples expressed signi?cant variations throughthe considered wavelength spectrum. The absorption curveof the violet colour samples maintained its high absorptionnot only within the visible spectrum but also in the infraredspectrum. Subjecting samples to alternating climate condi-tions caused an increase in absorption capabilities up to 9%in relation to the reference specimens. The highest increaseshave been observed among light colours specimens (whiteand ecru). The darker the colour, the smaller in;uence hasbeen caused by changing environmental conditions.The alternating environmental conditions have also af-

fected the internal microstructure mainly by in;uencing thepore size distribution. The pores have spread out over thewider range of diameters. Consequently, the medium porediameter decreased.The data based on the presented research could be used

in various ways. From the point of view of the Sun as anenergy source, comprehensive numerical analyses of aglobal energy consumption of a building could be carriedout. They would take into account speci?c features of aparticular colour of the applied acrylic coating. More pre-cise determination of the eEciency of a passive heatingsystem would lead to a decrease of a total energy demand

and signi?cantly decline the shares of conventional fuels indomestic heating systems. Moreover, the architects and thecivil engineers will be able to create much more suitable,ecological conditions for occupants avoiding over or underheating. The analyses could be additionally supplementedby the data of the life performance characteristics of acryliccoatings.More accurate predictions of a long-term performance

of the coating, including corrosion consequences could bea step forward towards popularization of low-cost heatingsystems powered by the Sun.

References

[1] Leygraf C, Graedel TE. Atmospheric corrosion. New York:Wiley-Interscience; 2000.

[2] Wieloch M. Analysis of the physical parameters of thin ?nishingcoatings with di'erent spectral characteristics under destructive localclimate conditions. M.Sc. thesis, Technical University of Lodz, 2002[in Polish].

[3] Patton AR. Solar energy for heating and cooling of buildings. ParkRidge, NJ: Noyes Data Corp; 1975.

[4] MacAdam DL. Color measurement: theme and variations. Berlin:Springer-Verlag; 1981.

[5] Crnjak Orel Z, Klanjsek Gunde M, Lencek A, Benz N. Thepreparation and testing of spectrally selective paints on di'erentsubstrates for solar absorbers. Solar Energy 2001;69(Supplement6):131–5.

[6] Sayigh AAM, Bahadori MN. Solar energy application in buildings.New York: Academic Press; 1979.

[7] Selected ASTM Standards for Solar Energy. Annual book of theAmerican Society for Testing and Materials. Appendix E 490,Table 5. Easton, Maryland, 1981.

[8] Hunt RWG. Measuring colour, 2nd Ed. Chichester, UK: EllisHorwood; 1991.

[9] Klemm AJ. The e'ects of admixtures of the mechanical propertiesand microstructural features of cementitious composites subjectedto freezing and thawing. Ph.D. thesis, Strathclyde University,1994.