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Influence of pigment on biodeterioration of acrylic paint films in Southern Brazil.

Abstract Biodeterioration of paint films leads to loss of durability and increased repainting costs. The influence of pigments on the biodeterioration of architectural paint films in the city of Florianopolis, Brazil, was evaluated using ten differently colored acrylic paint films exposed to the environment for 34 months. Fouling (biofilm formation) on the surfaces was assessed macroscopically, using British Standard BS 3900/1989 G6, and microscopically. After 20 months, major colonizers were bacteria and fungi, with some cyanobacteria and few algae; north-facing suede and peach and south-facing ice colors showed 100% cover at this time. The least affected color at all limes was blue, with a maximum of 30% cover on south-facing panels after 34 months. North-facing panels were generally more fouled than South-facing. Blue, red, and ceramic colors always performed best. Resistance lo fouling may have been due to copper in blue and acidity from sulfur oxides in ceramic pigments. Pigments may prolong paint film life and reduce the need for biocides. Keywords

Keywords Biodeterioration, Biofilms, Coatings performance, Paint pigments, Weathering

Introduction

The microbial colonization of painted surfaces causes esthetic problems and can also lead to degradation and blistering, flaking and spalling of the paint. (1) Soiling of painted surfaces is caused by a number of environmental factors, such as climate, surrounding vegetation, and irradiation, (2) which influence adhesion and growth of microorganisms (biofilm formation). This microbial colonization is often the most important cause of surface discoloration; it is particularly problematical in tropical climates. (2) Resulting loss of durability necessitates higher frequency of repainting, causing considerable increases in maintenance costs. Discoloration is especially notable when dark-pigmented filamentous fungi and cyanobacteria colonize the surface; these organisms have been shown to be the predominant biomass on soiled painted surfaces in Latin America. (3) (4) Water-based paints are more susceptible to microbial attack than oil-based products (5) and over 80% of the paints used in the building industry are of this type.

Many paint formulations include dry film biocides in an attempt to control microbial growth. Frequently used biocides include 2-n-octyl-4-isothiazolin-3-one (OIT), 3-iodo-2-propylbutyl carbamate (IPBC), and /V-(3, 4-dichlorophenyl)-N,N-dimethylurea (diuron). However, leaching of these chemicals from the surface during periods of rain leads to pollution of soil and water' and European regulations on the use of such compounds are currently under review (Annex VI to Regulation (EC) No 1272/2008). Methods are being sought to reduce these adverse environmental effects. Using a mixture of biocides, each at a relatively low concentration, is one such method. An alternative is to control the paint ingredients such that the dry film is less likely to promote adhesion and growth of microorganisms. The bioreceptivity of a surface is determined by its roughness as well as its chemical composition. (7) Some paint components, such as celluloses, are nutrients for microbial cells, while others, e.g., some metals, are inhibitory. (2) Pigments used to give the coating its color may be inorganic or organic and the latter may be more likely to act as microbial nutrients. Impurities in the pigments, such as phosphates and potassium salts, may also act as essential nutrients, increasing paint susceptibility, (8) (9) while other components, such as heavy metals, inhibit microbial growth.

There has been no recently published work on the effect of pigments on susceptibility of paints to biodeterioration. Turner (9) cites Gardner (1939), Findlay (1940) and Dooper and Hermann (1948), who stated that paints containing zinc oxide were more resistant to fungal growth than those containing lead white, lithopone, or antimony oxide, and that the most susceptible white pigments were titanium dioxide and lead-titanium. However, many of these pigments arc no longer used because of their environmental toxicity, a similar situation to that occurring with chromium, which is also biocidal.

Zinc oxide is one of the antimicrobial substances still used in paint pigments. At a level of 3.4% it conferred resistance to fungal attack on painted surfaces in France (10) and 2% in oil-based paint showed fungicidal activity (Meier and Schmidt, 1952, cited in reference (9)).

Of the organic pigments, Victoria Blue and methyl violet were shown to be the most susceptible to fungal growth, probably because of the presence of dextrin (a nutrient sugar) in the formulation. (9) This author found that Monastral Blue was one of the few blue pigments that were completely resistant.

A recent literature search revealed no scientific publications on microbicidal activity of modern pigments and so we decided to undertake this project to determine the ability of paint pigments to inhibit biofilm formation on painted surfaces, with the final aim of reducing the need for dry film biocides. Florianopolis, in the South of Brazil, is a suitable site for such testing because of its climatic characteristics, which encourage the growth of microorganisms. It is a coastal city with exuberant subtropical flora and fauna. The meteorological parameters measured during the exposure period are given in Table 1.
Table 1: Meteorological data for the period of the test

Month/2006             Jan    Feb  March  April    May   June   July

Ave temperature          -   33.0   36.0   31.0   27.0   28.0   29.0
                             18.0   16.0   14.0   10.0    9.0    5.0
(max min)[degrees]C

Rainfall/month           -   36.0   70.0   70.0   50.0   40.0   45.0
(mm)

Insolation               -  164.5  209.5  213.5  197.5  148.5  180.5
(h/month)

Month/2007             Jan    Feb  March  April    May   June   July

Ave temperature       31.0   34.0   33.0   33.0   33.0   30.0   31.0
                      17.0   17.0   17.0   15.0    5.0    6.0    3.0
(max min)[degrees]C

Rainfall/month          90    140    210     40    140     10    175
(mm)

Insolation           201.0  177.5  224.0  186.5  120.0  154.0  142.5
(h/month)

Month/2008             Jan    Feb  March  April    May   June   July

Ave temperature       33.0   33.0   32.0   32.0   33.0   30.0   30.0
                      17.0   17.0   16.0   10.0    5.0    4.0    9.0
(max min)[degrees]C

Rainfall/month         350    420    250    210     80     80     10
(mm)

Insolation           175.0  193.5  171.1  173.0  232.5  161.5  218.6
(h/month)

Month/2006             Aug   Sept    Oct    Nov    Dec  Average

Ave temperature       29.0   33.0   32.0   30.0   33.0     31.0
                       5.0    4.0   14.0   22.0   18.0     12.3
(max min)[degrees]C

Rainfall/month        60.0   45.0   90.0  240.0   80.0    75.09
(mm)

Insolation           172.5  166.5  164.5  144.5  203.5   178.68
(h/month)

Month/2007             Aug   Sept    Oct    Nov    Dec  Average

Ave temperature       29.0   26.0   32.0   32.0   34.0     31.5
                       4.0   11.0   15.0   13.0   16.0     11.1
(max min)[degrees]C

Rainfall/month          90    140    150    100    140   118.75
(mm)

Insolation            97.5  165.0  149.1  189.0  195.3   166.79
(h/month)

Month/2008             Aug   Sept    Oct    Nov    Dec  Average

Ave temperature       28.0   31.0   30.0   30.0   31.0     30.9
                       8.0    9.0   14.0   13.0   15.0     10.9
(max min)[degrees]C

Rainfall/month          70    250    290    610    290   242.50
(mm)

Insolation           138.5  152.0   80.5  104.0  215.2   167.95
(h/month)


Materials and methods

Paint formulation

A basic acrylic matt paint, of the type typically used for external surfaces in Brazil, was produced (Table 2). Solid content was 48.6% and performance, for a 50-60 [micro]m dry film, was 12.50 m2/L. Ten predispersed pigments (Colanyl) were donated by Clariant, Sao Paulo, Brazil. They were presented dispersed in a binder-free, aqueous solution containing wetting and dispersing agents. Pigments were mixed into the basic paint to achieve the required colors, as shown in Tables 3 and 4. The colors chosen are those most common on the Brazilian market. Acticide HF (an isothiazolinone/formaldehyde based, broad spectrum biocide) was used as the in-can preservative in all paints and 2% Acticide EPW (a mixture of carbendazim, octyl isothiazolone, and diuron) was added to a portion of the paint to inhibit fungal and algal growth on the dry film, acting as a control.
Table 2: Components of the basic paint

Concn         Material         Weight
(%w/w)                           (kg)

42.50   Water                   0.594
0.36    Hydroxyethylcellulose   0.006
0.14    Ammonia                 0.002
0.36    Disaspers-T Conc        0.005
0.18    Disafoam 969-T          0.002
0.26    Acticide HF-THOR        0.004
8.08    Titanium dioxide        0,113
12.14   Calcium carbonate ppt   0,170
13.20   Calcite #400            0,185
3.60    Micronized talc         0,050
7.24    Micronized Caulim       0,101
10.33   Acrylic resin           0.144
0.46    Propylene glycol        0.006
0.83    White spirit            0.012
0.32    Gapcoat MGK             0.004
100.00  Total                   1.398

Table 3: Characteristics of pigments used in the paints

Code              MG BR      R BR           SH         TQ

Color           Yellow     Yellow     Blue           White

Chemical        Azoylaryl  Iron       Copper         Titanium
nature          amidine    oxide      phthalocyanin  dioxide

Density         1.24       1.87       1.26           1.80
(g/c[m.sup.3])

pH              N/D        6.00-9.00  5.50-8.00      8.00-10.00

Pigment %       40         70         50             N/D

Code               PRQ         3GLS            GG

Color           Black      Green          Red

Chemical        Lamp       Copper         Azoyl-[beta]
nature          black      phthalocyanin  naphthol

Density         1.27       1.47           1.33
(g/c[m.sup.3])

pH              5.50-8.00  5.50-8.00      5.50-8.00

Pigment %       35         50             50

Codes are those given by the pigment producer (Clariant).
N/D = not determined

Table 4: Quantities in mg of pigments added to the basic paint

Paint color/   MG    R  SH  TQ  PRQ  3GLS  GG
weight of      BR   BR
basic paint
(mg)

Blue/395        0    0   4   0  1.5     0   0
Marble/500      0    5   0   0    0     0   0
Peach/420       0    5   0   0    0     0   1
Ceramic/400     0    7   0   0    2     0  10
Ice/420         0  0.4   0   0    2     0  10
Red/300         0    0   0   0    0     0   0
Green/440       0   10   0   0    7    12   2
Yellow/500    2.5   10   0   0    0     0   0
Straw/450       0    2   0   0    1     0   0
Suede/480       0   12   0   0    3     0   2

Codes as for Table 3


Paint and pigment analysis

The pigments supplied by Clariant were applied, each as a single layer, on Whatman 40 8 [micro]m filter paper and inorganic components analyzed by EDS and X-ray fluorescence spectrometry {Shimadzu model EDX-700). The same analysis was carried out on the paints. Pigment volume content (PVC%) of the colored paints was calculated as follows:

PVC% = VP*100 / VP + VR

where VP = total pigment volume and VR = volume of resin.

Test panels

Fiber cement board (BrasiPlac, Brasilit) was cut into panels 30 cm x 10 cm x 6 mm. A hole was drilled at each end for the fixing screws and the sunken heads of these zinc screws were sealed with a mixture of powdered fiber cement and acrylic plaster. After 7 days for drying, panels were painted with a white acrylic sealant, left to dry for a further 7 days in the horizontal position in a dust-free environment, as recommended by Brazilian standard NBR 13006/1993, and then roller-painted with one coat of the requisite pigmented paints to give a dry film thickness of 50-60 pm. For each color, six panels were painted without dry film biocide and two with biocide, the latter intended as controls. After a further 7 days for drying, the panels were fixed to wooden racks, using PVC tubes to protect the metal screws and distance the panels from the wooden slats of the rack, thus avoiding interactions between the wood and the painted panels.

Exposure regime

Duplicate racks were prepared, each bearing 40 vertical panels in 10 rows (Fig. 1). Each row held three non-biocide and one biocide-containing panel of a given color. The racks were placed on the roof of the Civil Engineering building of the University of Santa Catarina, in Florianopolis, on the Island of Santa Catarina, South Brazil, with one rack with panels facing north and the other south. Although it is more usual to incline panels for biodeterioration testing at 45[degrees], (2) we used vertical panels to better mimic the facades of buildings. This strategy was also adopted by Stanley Buckman, who investigated phenylmercury compounds for the reduction of paint film biodeterioration in the 1940s and 1950s, (11) and by Springle et al.(12) at the Paint Research Association in the UK. Temperature at the exposed surface of each panel was measured between 11:00 am and noon, 245 days after initial placement (i.e., in October) to check possible differences between north and south faces. Ambient temperature and relative humidity (RH) at the time were 25.7[degrees]C and 66%. Surface temperatures were measured by infrared photography using a Therma CAM E2 camera, which shows temperature variation as different colors.

[FIGURE 1 OMITTED]

Environmental conditions (daily rainfall, total insolation, RH at midday and minimum and maximum temperatures) during the whole experiment were collected at the meteorological station of the Ministry of Agriculture in Florianopolis airport, 8 km distant from the exposure site (see Table 1). Maximum temperatures were between 25 and 35[degrees]C, and nocturnal minima 3-23[degrees]C. During the 3 years of the research, insolation was an annual average of 2,000 h, but there was considerable monthly variation. Rainfall increased over the 3 years from 916 mm in 2006, to 1,425 mm in 2007 and to 2,910 mm in 2008 (Table 1).

Immediately after positioning of the panels, samples were taken from the surrounding parapet wall with moistened sterile cotton wool swabs and Thor Brasil Ltda, Sao Paulo, used standard microbiological culture and microscope techniques in an attempt to detect the microorganisms that were present at the site. These methods involved spreading the samples over the surface of artificial solid media suitable for the growth of microorganisms and incubating until visible growth occurred. Slides were prepared from this growth and examined under an optical microscope. In the case of filamentous fungi, cellular structures were compared with those of known species. In the case of bacteria, no further tests were carried out.

Evaluation of biodeterioration

At intervals up to 34 months (see "Results" section), panels were examined macroscopically and the degree of biofilm formation assessed according to British Standard BS 3900/1989 G6, in which the percentage of cover of the panel by soiling (biofilm growth) is assessed by the naked eye. Digital photos were taken each month. The grading system used to assess biodeterioration is well accepted, but gives only approximate degree of cover of the painted surface by the biofilm (fouling). The same two investigators graded the panels every time, adapting the British Standard Method to note percentage cover as 5, 10, 20, 40, 60, 80, or 100%. These values were averaged for each set of three panels. Since readings are not, by their nature, precise, fractional means were converted to the nearest whole figure.

One of each color test of south-facing panels was sent to the University of Portsmouth after 20 months of exposure and the biofilm studied under an inverted metallurgical microscope, by environmental scanning electron microscopy (ESEM; JOEL JSM-6060, 25 kV), without prior treatment, and by direct light microscopy examination and culture on MKM medium of adhesive tape samples. (4) Only one set of panels was used so as to retain as many samples as possible for final evaluation of soiling.

Results

Biodeterioration testing

Thor Brasil Ltda, Sao Paulo, using standard microbiological culture and microscope techniques, identified only the fungus Aspergillus and Gram positive bacilli, typical air contaminants, in the samples taken from the parapet surrounding the exposure site. As macroscopic observation of red staining suggested that the alga Trentepohlia was certainly present, we assume that the standard media used by Thor were not suitable for the growth of all microorganisms, and especially not for algae. Those reported (bacteria and a filamentous fungus) are fast-growing organisms that are readily detected. Since this information was not important for the interpretation of our results, no further tests were done.

Results of the macroscopic analysis of surface soiling are shown in Table 5. Test panels (without film biocide) began to show discoloration after the ninth month, when temperatures began to rise again after the winter (Table 1). Control, biocide-containing panels remained unattacked. There was no correlation between temperature at the surface of the panels, measured after 245 days, and the degree of surface soiling (Table 6).
Table 5: Mean biodeterioration scores (% cover) for various colored
paints exposed to the north (N) or south (S) in Fiorianopolis,
Brazil, for up to 34 months

Color          12        20        34        12        20        34
         months/N  months/N  months/N  months/S  months/S  months/S

Ceramic     10.00     60.00     60.00     13.00     30.00     60.00
Red          7.00     53.00     53.00      7.00     20.00     40.00
Ice         13.00     73.00     73.00     20.00    100.00    100.00
Suede       20.00    100.00    100.00     40.00     60.00     80.00
Marble      23.00     80.00     80.00     47.00     80.00     80.00
Yellow      40.00     93.00     93.00     60.00     70.00     80.00
Blue        17.00     18.00     17.00         0     20.00     30.00
Straw       40.00     93.00    100.00     60.00     80.00     80.00
Green       17.00     73.00     73.00     13.00     70.00     70.00
Peach       40.00    100.00    100.00     33.00     50.00     80.00

Table 6: Temperature at surface of painted panels and biodeterioration
(soiling) rating

               South facing

Color    Temperature  Biodeterioration score

Ice            25.95             20.00
Red            26.55              6.67
Ceramic        26.60             13.33
Suede          26.95             40.00
Blue           27.35              0.00
Straw          27.65             60.00
Peach          27.75             33.33
Yellow         27.85             60.00
Marble         27.90             46.67
Green          28.70             13.33

               North facing

Color    Temperature  Biodeterioration rating

Ceramic        28.80             10.00
Red            29.15              6.67
Ice            29.55             13.33
Suede          30.10             20.00
Marble         30.55             23.33
Yellow         30.85             40.00
Blue           31.10             16.67
Straw          31.20             40.00
Green          31.40             16.67
Peach          31.90             40.00

Data are arranged in order of increasing temperature. It is obvious
that the degree of soiling does not follow this order


Examples of disfigured yellow-painted panels are shown in Fig. 2. It is obvious that the degree of cover by biofilm increased with time. Microorganisms detected on the painted surfaces after 20 months of exposure to the south are shown in Table 7 and Figs. 3 and 4. Many of the microorganisms detected were darkly colored because of the presence of intracellular pigments, (13) (14) increasing the apparent soiling of the paint surface. Fungi were detected on all samples, confirming the results of Gaylarde and Gaylarde, (15) that fungi are the major biomass on painted surfaces in Latin America. The same authors state that cyanobacterial genera are mainly the coccoid types and this was also confirmed by our results.
Table 7: Microorganisms detected by direct microscopy and
culture on the south-facing painted surfaces
after 20 months' exposure

Color    Bacteria       Cyanobacteria   Fungi           Algae

Ceramic  +              -               +               -

Red      +              -               +, including    -
                                        dark-pigmented

Ice      +++            + green         +++ including   -
                        coccoid,        dark-pigmented
                        including
                        Gloeocapsa

Suede    ++             + green         ++              -
                        coccoid

Marble   +++ including  + green         ++ including    (+)
                        coccoid         dark-pigmented  coccoid
         actinomycetes

Yellow   ++             + including     ++              -
                        brown coccoid

Blue     -              -               (+)

Straw    ++ including   + including     4-+ including   -
                        brown coccoid   Fusarium
         actinomycetes

Green    ++             -               ++              (+)
                                                        coccoid

Peach    ++             +++ coccoid,    +++ including   -
                        including       dark-pigmented
                        green, red,
                        brown
                        Gloeocapsa,
                        green, brown,
                        blue
                        Synechocystis,
                        brown
                        filamentous
                        Scytonema

+, +, ++, +++ = increasing numbers detected - = not detected


Paint and pigment analysis

The pigment volume content (PVC%) varied between 53% and 73% for the variously colored paints, suede being the highest and ice the lowest. The blue coating showed the second lowest value, at 54%. Other PVC% values were as follows: marble, peach and yellow 62, ceramic 65, red 67, green 69, and straw 58.

The results of the spectrophotometric analysis of oxides in the pigments are shown in Table 8.
Table 8: Oxides in the pigment pastes, as a percent of total
oxides, analysed by X-ray fluorescence spectrophotometry

Oxide                                            Paint
                                                pigment

                     Blue MG  Yellow  Yellow   White   Black
                       BR      R BR     SH      TQ       PRO

CuO                  68.731     ND    0.116     ND     0.125

S[o.sub.3]           30.101   1.688   1.121     ND     96.556

CaO                   0.456   10.326  51.821    ND     0.653

F[e.sub.2][o.sub.3]   0.252   82.860  0.722     ND     1.093

MnO                   0.169     ND    0.125     ND     0.129

C[r.sub.2][o.sub.3]   0.167   0.056     ND      ND       ND

C[0.sub.2][o.sub.3]   0.124     ND      ND      ND     0.270

[P.sub.2][o.sub.5]     ND     4.042     ND      ND       ND

[TiO.sub.2]            ND     1.028   45.766   99.43   0.444

ZnO                    ND       ND    0.193    0.357   0.123

[K.sub.2]o             ND       ND    0.136    0.122   0.607

NbO                    ND       ND      ND     0.067     ND

NiO                    ND       ND      ND     0.024     ND

[cr.sup.-]             ND       ND      ND      ND       ND

[Br.sup.-]             ND       ND      ND      ND       ND
Oxide

                     Green   Red GG
                      3GLS

CuO                  4.553   0.026

S[o.sub.3]           3.037   2.568

CaO                  0.024   0.285

F[e.sub.2][o.sub.3]  0.047   0.127

MnO                    ND      ND

C[r.sub.2][o.sub.3]    ND      ND

C[o.sub.2][o.sub.3]    ND    0.032

[P.sub.2][o.sub.5]     ND      ND

[TiO.sub.2]            ND      ND

ZnO                    ND    0,040

[K.sub.2]o             ND      ND

NbO                    ND      ND

NiO                  0.010     ND

[cr.sup.-]           92.093  96.922

[Br.sup.-]           0.236     ND

ND = not detected


When these pastes were diluted into the basic paint formulation, the major paint oxides (mainly present as fillers and extenders) measured by X-ray fluorescence spectrometry were Si[O.sub.2], CaO, [TiO.sub.2], [Fe.sub.2][O.sub.3], and [Al.sub.2][O.sub.3], the latter comprising 57.6% oxides in marble paint, 50% in red paint and not detected in the rest. Iron oxide, which could act as an essential element for microorganisms at low concentrations, but be inhibitory at high levels, was present at 8.1% in green paint and 2% or less in all others. Highest among the minor oxides was CuO in the blue paint, at almost 0.4%.

Discussion

It is immediately obvious from Table 5 that north-facing panels were generally more highly fouled than south-facing, although exceptions occurred in the cases of ice and blue colored paints. Increasing fouling of the north-facing yellow paint film over approximately 2 years is seen in Fig. 2. It has been stated that, in the southern hemisphere, south-facing surfaces develop biofilms more rapidly than north-facing. (16) However, this latter study was carried out in Sao Paulo, where conditions are considerably different from those in Florianopolis. (17) Sao Paulo is placed at a higher altitude, far from the sea, and has considerable air pollution from burning of fossil fuels. Wind direction would influence the deposition of air pollutants and microorganisms on differently facing surfaces. Orientation (north or south) did not affect the degree of fungal biodeterioration of painted panels exposed in Belem, in the north of Brazil, where the climate is tropical, (18) suggesting that high temperature and humidity are more important than orientation for microbial colonization. Temperatures measured at the surface of the paint films in Florianopolis varied from 28.8 to 31.9[degrees]C for north-facing and from 25.95 to 28.7[degrees]C for south-facing panels; warmer north-facing surfaces would promote microbial growth without being so hot as to decrease time of wetness, an important factor in biodeterioration (see, for example, reference 19).

[FIGURE 2 OMITTED]

The aim of this project was to determine the paint pigments most resistant to fouling and the results are quite clear in this respect. Blue was obviously the most resistant color, with a total score of 102 out of a possible 600, compared to the following colors: red at 180 and ceramic at 233.

The biodeterioration scores are confirmed by the microbiological data after 20 months (Table 7). Only a very few fungi were seen on the blue-painted surface, and fungi and bacteria on the red. In the latter, however, some of the fungi detected belonged to the dark-pigmented mitosporic group, which exert a greater esthetic effect on the surface. Bacteria generally do not form visible biofilms unless they are pigmented or slime producers; this was not the case in any of our samples and so the bacteria would have affected the surfaces only by acting as nutrients for disfiguring organisms. The ceramic paint also showed the presence of only few fungi after 20 months. Apart from these 3 colors, all other surfaces revealed the presence of fungi and phototrophs (algae and, mainly, cyanobacteria). All phototrophs are, by their nature, pigmented and confer a dirty appearance on their substrate. A scanning electron micrograph of a biofilm containing fungi and coccoid cyanobacteria is shown in Fig. 3. Filamentous organisms may also penetrate through the paint film, damaging it and allowing entry of moisture, and fungal filaments with this activity are shown in Fig. 4.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

After ceramic, the paler color marble was the next least fouled, and straw the most contaminated, suggesting that other factors than simple reflection of sunlight, demonstrated by Castro et al. (20) to be greater for paler colors, are important in determining resistance to biodeterioration. Indeed, there was no correlation between temperature at the surface of the panel, paint color, and fouling (Table 6), confirming the involvement of other parameters.

One factor that could be involved in paint susceptibility is PVC%. The higher the PVC% value, the lower the proportion of resin and the higher the amount of pigment in the paint. There is a complex relationship between PVC% and susceptibility to fouling. Reduced PVC% can be associated with higher water absorption which would lead to increased susceptibility. (16) However, reduced PVC% also increases permeability of the film to water vapor which would allow more rapid drying after rain and reduce susceptibility.

In our samples, PVC% varied from 52% to 73% and there was no obvious correlation between these values and paint biodeterioration. Shirakawa et al. (18) also reported the relative unimportance of PVC% (30-50%) in fungal biofilm formation on a white paint, pointing out that climate exerted a much greater effect.

In the absence of other factors controlling susceptibility to fouling, it is apparent that pigment, the only other variable, must have had a major influence on the results. The superior performance of blue paint is probably due to its content of copper, a known biocide. This is present at a level of almost 0.4% in blue paint, much higher than in other colors. The increased resistance of blue pigment was unexpectedly confirmed some months after the end of the experiment by the observation of a painted building on the Island of Santa Catarina, a few kilometers from the capital city of Florianopolis (Fig. 5). The heavy algal colonization (pink/brown coloration) seen on the south-facing white walls was completely absent from the blue-painted strip in the center.

[FIGURE 5 OMITTED]

In the case of the second most resistant color, red, it is difficult to consider any heavy metal oxide as responsible for the effect. The resistance of red paint could be brought about by an interaction between high CI levels and increased PVC% (red was the paint with the highest pigment content of 16.1% w/v giving a PVC% of 67). Ceramic paint, which showed the third best performance, could have been influenced by its content of S[O.sub.3], which can lead to acid conditions on the paint surface, inhibiting growth of some microorganisms, notably cyanobacteria.

In conclusion, these results demonstrate that pigments affect the timespan of fouling development on the surfaces of painted buildings. A suitable selection of paint color could reduce the need for dry film biocides, with their accompanying adverse environmental effects. The possible environmental problems associated with leaching of the pigments themselves are, of course, another factor that should be considered, especially where color depends on the presence of potentially damaging compounds. Nevertheless, the option of avoiding, or at least reducing the levels of biocides in the paints is attractive, especially in view of the new regulations being introduced in Europe.

Conclusions

1. After 20 months of exposure to the sub-tropical environment of Florianopolis, Brazil, sealed fiber cement panels painted with 10 differently pigmented acrylic paints became colonized mainly by bacteria and fungi, with some cyanobacteria and few algae.

2. The least affected color at all times was blue, which showed a maximum of 30% soiling on south-facing panels after 34 months.

3. North-facing panels were generally more fouled than south-facing.

4. Blue, red, and ceramic colors always performed best.

5. Resistance to fouling may have been due to copper in blue and acidity from sulfur oxides in ceramic pigments.

6. A suitable choice of pigments may prolong paint film life and reduce the need for biocides.

Acknowledgments

We wish to thank Clariant, Sao Paulo, Brazil, for providing the pigments. Biocides were kindly provided by Thor Brasil Ltda, Sao Paulo.

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A. M. Breitbach, J. C. Rocha

Department Civil Engineering, Federal University of Santa Catarina, Florianopolis, SC, Brazil

A. M. Breitbach

e-mail: aeciomb@terra.com.br

C. C. Gaylarde

Microbiology Research Laboratory, School of Pharmacy and Biomedical Sciences, University of Portsmouth, St. Michael's Building, White Swan Rd., Portsmouth PO1 2DT, UK

e-mail: cgaylarde@googlemail.com

DOI 10.1007/s11998-011 -9350-1
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Author:Breitbach, Aecio M.; Rocha, Janaide C.; Gaylarde, Christine C.
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