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The evaluation of paint performance for exterior applications in Singapore's tropical environment
Budding and Environment, Vol. 31, No. 5, pp. 477486, 1996 Copyright 0 1996 Elsevier Science Ltd. All rlghrs reserved Printed in Great Britain 036M323j96 %15.00+0.00
Pergamon
PII: SO360-1323(96)00009-l
The Evaluation of Paint Performance Exterior Applications in Singapore’s Tropical Environment SALIL K. ROY* LEE BENG THYE* DEREK 0. NORTHWOOD*t
for
(Received 26 June 1995; accepted 21 November 1995)
Two water-base paints, a regular non-algae and an algae resistant emulsion for decorative purposes, were exposed to art$cial, i.e. accelerated, and natural weathering tests and the deterioration compared to that for similar paint applied to exterior walls on multi-unit buildings in Singapore. The artificial weathering test wasfound not to provide a good representation of actual performance since it only monitors the deterioration due to chemical weathering and not that due to mechanical or biological weathering. The rate of deterioration of paint exposed in natural weathering tests in an industrial region in Singapore was faster than that for paint exposed in-service on multi-unit housing situated in industrial, central or coastal regions in Singapore. Copyright 0 1996 Elsevier Science Ltd.
INTRODUCTION
significant incentive in improving these correlations since the end-users do not have the luxury of waiting until there is an in-service failure of the paint system. If the producers, and users, understand the limitations and reliability of artificial and natural weathering tests in predicting in-service performance, then we can better ensure that paint which performs well in the weathering tests, does not fail prematurely under actual in-service conditions. Estate managers, architects and paint manufacturers, armed with a better knowledge of the durability of paint in-service, would be able to design, and carry out, better maintenance programmes. In the present study two paints, an emulsion paint and an algae resistant emulsion paint, were exposed to artificial and natural weathering tests and the deterioration compared to that for similar paint applied to the exterior walls in multi-unit housing in Singapore. Since more than 20 million dollars is spent annually on repainting, even a modest improvement in durability of say 5% would result in savings in excess of 1 million dollars a year 111. The climate in Singapore is very demanding in terms of the performance and durability of all building materials and in particular paints [2]. Singapore is located on latitude 1.3”N and hence experiences an equatorial climate. The main features are uniform (high) temperature, high humidity, and abundant rainfall throughout the year. The mean daily maximum and minimum temperatures are 30.7 and 23.4”C respectively and the annual rainfall is about 2400mm with 267 rainy days in a year [3]. Such conditions, as well as influencing the long term durability, also provide the ideal conditions for algae growth on external walls [4]. We have thus included algae resistant and non-algae resistant paints in our comparative study.
WHEN a paint is applied as an external finish to a building it is subject to a variety of environmental factors that can degrade the paint and reduce its useful life-span. These influences include rain, sunshine, humidity, temperature and pollution. There are a number of artificial, i.e. accelerated, and natural weathering tests used both by the paint manufacturers in the development of new paint formulations and by paint manufacturers and paint users in assessing the suitability of a paint for a particular application and in determining its durability under the particular, or similar, environmental conditions. However, it is difficult (some claim impossible) to correlate the results from these accelerated and natural weathering tests with the performance of the same paint in-service, i.e. applied on external surfaces of buildings. There are many reasons for this lack of correlation but they can be divided into two main areas, namely: l
l
there are many factors, besides the weather, which affect the performance of the paint in-service; there is no basis for comparison unless the paint used in the weathering tests and in-service is the same, and the substrate on which the paint is applied is similar.
However, despite these difficulties in correlating paint performance in weathering tests and in-service, there is a
*School of Building and Estate Management, National University of Singapore, Singapore 0511, Singapore. tAuthor to whom correspondence should be addressed. Permanent address: Department of Mechanical and Materials Engineering, University of Windsor, Windsor, Ontario N9B 3P4, Canada.
477
S. K. Roy et al.
478 Table 1. Comparison
of the two Singapore
Standards
Description
ss345:
Fineness
test
40 microns
Viscosity
test
75-95 ku
65-85 ku
The higher viscosity permits tates roller application.
1000 hours
500 hours
The increase in duration of the acccelerated weathering test ensures better durability and longer lifespans of the paintwork.
Accelerated
weathering
test
1990
of the requirements
max
SS150: 1976
Significance
50 microns
The reduction in maximum fineness from 50 to 40 microns results in better pigment dispersion and a smoother paint texture which would reduce the chances for retaining micro-organisms in the paintwork.
max
binder-rich
paint and facili-
Scrub resistant
test
3000 cycles
1500 cycles
The increase in cycles for the scrub resistant better mechanical properties.
Algae resistant
test
Incorporated
Nil
This is to ensure that the paint is algae resistant against the common species of algae such as T. odoratn [5].
15-50
20 max
The increased specular reflection requirement smoother and binder-rich paint texture.
Specular
reflection
at 85°C
EXPERIMENTAL
DETAILS
This section will begin with a short description of the two types of paint studied. An overview will then be given of the general methodology of the study before concluding with full details of the artificial weathering, field survey and natural weathering tests. The paints In Singapore, the quality of water-base paints is governed by the Singapore Standards as specified by the Singapore Institute of Standards and Industrial Research (SISIR). The two standards are as follows. 1. SSlSO: 1976: Emulsion Paints for Decorative Purposes. 2. SS345: 1990: Algae Resistant Emulsion Paint for Decorative Purposes. A comparison of the requirements is given in Table 1.
of the two standards
General methodology An overview of the methodology adopted for this study is shown in Fig. 1 which also lists the type of data (results) that were obtained for the three parts of the study. Artificial (accelerated) weathering tests These tests involve exposing non-white paint in a weathering apparatus to ultraviolet radiation, elevated (c.f. ambient) temperatures and condensation for a total period of 2000 hours. Non-white paint was chosen for these tests so that the durability of coloured paint (in terms of colour changes) could be monitored. The design of this part of the experiment and rating schemes for the evaluation of the results followed those outlined in ASTM Standard D4587 [6]. Paint complying with SSl50: 1976 and SS345: 1990 was obtained from three different paint manufacturers. The paint was of a blue colour (B.S. 18C35) which was chosen since it is one of the more popular colours for exterior application in housing estates. The paint was
test ensures
ensures
a
applied to cement panels measuring 15 x 1Ocm. The application was by brush and two coats were applied. The painted panels were allowed to dry for 1 week prior to exposure in the weathering tests. The tests were carried out using a QUV Weathering Tester (Q Panel Model 85-2534-28) with a UV-B (wavelength 313 nm) fluorescent lamp. The test temperature was maintained at 50°C and alternate 4 hour cycles were carried out where only UV light or condensation were applied. As noted, the total exposure time was 2000 hours, which represents 250 cycles (UV + condensation). This particular testing cycle was chosen on the basis of recommendations of the equipment manufacturer [7]. The exposed samples were evaluated for chalking, checking and colour change. Chalking refers to a surface deposit of powder formed by the disintegration of the binder due to exposure to UV light and moisture [8]. Chalking was gauged by wiping the exposed surface of the paint with a black velvet cloth and comparing with photographic samples as per ASTM Standard D659 [9]. Checks are slight breaks in the paint film which do not penetrate into the substrate. Checking was measured by examining at 25 times magnification and comparing with photographic samples as per ASTM Standard D660 [lo]. Colour change (fading) is a gradual decrease in colour intensity in the paint due to the use of pigments with insufficient lightfastness. The colour change was measured by visual inspection under artificial lighting (fluorescent illumination) and comparison with photographic samples as per Singapore Standard SS5: Part E3 [l I]. The results of these examinations were rated on a O-10 scale as outlined in Table 2. Natural weathering tests The objective of these tests was to assess the durability of paints, meeting the requirements of the same two Singapore standards, to outdoor exposure. The paint panels under test were also on a cement substrate and two coats were applied by a brush method. The test panels were affixed to racks at a 5” inclination to the horizontal. They
479
Exterior Paint Performance in a Tropical Environment 1
Selection artificial
Natural
Thering
1
Collection of data of paint exposed under natural weathering
of samples
(ie. accelerated)
I Assessment performance
of of paint-
Results under evaluation 1. Colour changes
Results under evaluation 1. Colour changes
5. Dirt, soot, dust
Fig. 1. Overview
Table
10 8 6 4 2 0
2. Rating scheme used in evaluation erated) and natural weathering
of artificial tests
Rating
Performance
Excellent Very good Good Fair Poor Very poor
No failure Slight failure Considerable failure Severe failure Very severe failure Complete failure
of methodology
(accel-
were exposed for periods of up to five years at an exposure site in an industrial area of Singapore. At selected intervals during the five year exposure period, the samples were assessed for chalking, checking and colour change as described for the accelerated weathering tests. In addition, the samples were assessed for algae growth again using photographic standards and a G-10 rating scale. [These were internally developed standards where a rating of 0 (zero) indicated that algae was covering the majority of the surface after five years and 10 indicated no algal growth.] Qualitative observations were also made on the deposits of dirt, dust and soot.
Results under evaluation 1. Colour changes 2. Chalking 3. Checking 4. Algae 5. Dirt, soot, dust
of study.
SS345: 1990 specifications. The three environmental locations chosen can be generally classified as coastal, central (i.e. away from the coast) and industrial. A total of 170 buildings were surveyed. The buildings were chosen so that at the time of the survey the paintwork had been “in place” for periods of up to five years for paint complying with SS150 and up to three years for paint complying with SS345 (the shorter time for this paint was due to the fact that the implementation date for this standard was 1990). The painting works surveyed comprised a mix of repainting works on older buildings and new painting on new job sites. In the original survey work a number of locations (structures) within the building were surveyed, including walls, columns, beams, slabs, parapets and window canopies. For the purposes of this paper we will concentrate on the results for the lift slab which is an exterior facing, vertical component and which had similar features in all the buildings surveyed (Fig. 2). The paintwork was evaluated for the following: l l l l
Field survey of paint performance The objective of this survey was to assess the performance of actual paintwork in different environmental locations (within Singapore). The buildings chosen were apartment blocks. All water-base paints applied to these buildings were required to comply with SS150: 1976 and
Assessment of paint performance
l
Chalking Algae and mould growth Dirt, dust and soot Flaking Discolouration.
The evaluation and rating of these defects was generally as described previously for the natural weathering tests using photographic standards to place the defect on a & 10 scale. Generally a particular defect would be con-
S. K. Roy
480
Fig. 2. Photograph showing location and detail of lift slab.
sidered to be present if more than 5% of the total surface area of that element appears to have that defect. The evaluation of flaking was performed in a manner similar to that specified in ASTM Standard D772 [12]. Mean ratings were assigned for each defect after observation of several areas of the lift slab. RESULTS Accelerated
weathering
tests
The results of the accelerated weathering tests are summarised in Table 3. In general, it can be seen that although
et al. there was little difference in the performance (in terms of colour change, chalking and checking) of the paint from the three suppliers, the paint conforming to SS345 performed better than that conforming to SSl50. Neither type of paint showed signs of checking even after the maximum exposure period (2000 hours). Chalking was not detected until 1800 hours exposure for the SSl50 paint or 2000 hours for the SS345 paint (and then in paint from only one of the three suppliers). Colour change (fading) and loss of gloss were however found after exposure to UV light and water in the accelerated test. Considerable colour fading and loss of gloss (down from 10 to a rating of 6) occurred in the SS150 paint after 2000 hours exposure. The SS345 paint was more resistant to colour changes, showing only slight fading of colour (down to a rating of 8) after 2000 hours exposure. It also showed a higher level of sheen (gloss) compared to the SS 150 paint. Natural weathering
tests
The results for the natural weathering tests are summarised in Table 4. The paint conforming to SS345 showed not only better resistance to algae formation than paint SS150 but also better resistance to colour change, chalking and checking. There was a homogeneous colour fading in all exposed paint samples. The SS150 paint showed what is described as considerable failure (rating 6) after only 24 months
Table 3. Accelerated weathering test results: shown as ratings on a 10 to zero scale (see Table 2) Paint defect
Paint specification
Hours in QUV
ss345 Supplier
ss150 Supplier 1
2
3
1
2
3
1300 1500 1800 2000
10 10 8 8 8 8 8 6 6
10 10 8 8 8 6 6 6 6
10 10 10 8 8 8 6 6 6
10 10 10 10 10 8 8 8 8
10 10 10 10 8 8 8 8 8
10 10 10 8 8 8 8 7 7
Chalking
0 300 500 700 1000 1300 1500 1800 2000
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 8 8
10 10 10 10 10 10 10 8 8
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 8
Checking
0 300 500 700 1000 1300 1500
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10
Colour change
0 300 500 700
1000
1800 2000
Exterior
Paint Performance
481
in a Tropical Environment
Table 4. Natural weathering results: shown as ratings on a 10 to zero scale (see Table 2) Exposure (months) 0
12 24 36 48 60
Colour change
Checking
Chalking
Algae
ss150
ss345
ss150
ss345
ss150
ss345
ss150
ss345
10 68 6 6 6 4
10 8 8 8 6 6
10 8 68 6 4 4
10 10 8 8 6 6
10 10 S-10 8 8 8
10 10 10 8-10 8 8
10 10 8 S-10 6 4
10 10 10 10 10 8
and deteriorated more on prolonged exposure such that at five years exposure there was severe colour change (rating 4). The SS345 paint performed much better with at least a four year exposure being required to reach a colour change rating of 6 which was reached after only 24 months for paint SS 150. A similar pattern to the colour change results was seen in the data for chalking with the approximately two to one ratio of times required for a similar level of deterioration in paint SS345 compared to paint SSI 50. As opposed to the accelerated weathering tests where no checking was found in any of the paint samples (at least for exposure times up to 2000 hours), a small amount (slight checking-rating 8) was found in paint SS 150 for exposure times greater than 24 months and in paint SS345 for exposure times greater than 36 months. However, in both types of paint, the checking did not reach a level worse than slight (rating 8), even after five years exposure. As expected, the algae resistant paint (SS345) proved to be much more resistant to the growth of algae in that the only algae growth found was on one of the panels exposed for five years. This contrasts with the behaviour of paint SS150 where algae growth began within two years of exposure and had reached the severe level (rating 4) after five years. As well as the semi-quantitative observations on colour change, chalking, checking and algae growth, some qualitative observations were made on dirt/soot/dust pick-up. Both the SS150 and SS345 paint samples showed slight dirt pick-up after one year of exposure. After five years exposure, it was generally found that the SS150 paint panels showed more dirt pick-up than the SS345 paint panels. Field survey As noted in the “Experimental Details” section, the lift slab was chosen as the representative (building) element for the observation of the weathering of paint in-service. Deterioration of the paint in the lift slab is not affected by factors such as dampness and poor design, which can lead to early paint failure in other areas of the building, e.g. refuse chute and bathroom areas [13], that were also surveyed. The results for chalking, flaking, algae growth and fading are summarised in Table 5. Paint complying with SS345 did not show any defects (deterioration) for the maximum exposure period (three years). Chalking was observed first in paint on buildings situ-
ated in an industrial region. No chalking of paint was observed in the initial two years for the central and coastal regions but paint on buildings in the industrial region showed slight chalking (rating 8) after two years. The rate of deterioration (chalking) of paint (SSl50) was slower in the coastal region compared to the central and industrial regions. In the central and industrial regions, considerable chalking (rating 6) occurred after four years whereas in the coastal region similar deterioration was not found until five years exposure. No flaking of the paint was found for the first three years of exposure in any of the three exposure regions. Flaking began after four years exposure and after five years was more severe in the industrial and coastal regions (rating 6) than in the central region (rating 8). Algae growth (on SS150 paint) had begun after three years in the industrial region and after four years in the central and coastal regions. However, after five years, paint applied to buildings in all three environmental regions only showed slight algae growth with a rating of 8. Discolouration and uneven fading of colour of the paint was seen after three (or four) years. Although this fading was detected first (after three years) on buildings in the central region, after five years exposure discolouration and fading was observed most frequently in the buildings in the industrial region. [As an aside it is noted that in the industrial region, there is a reasonably wide usage of bright orange-red colour paint. This colour paint contains an organic pigment which has poor lightfastness and therefore fading is seen fairly early after exposure and buildings with this type of paint show higher frequencies of fading/discolouration]. Dust/dirt/soot deposition was not a problem on the lift slab but occurred more often in other building components. Not unexpectedly, it was a more serious problem in buildings located in the industrial region than in those located in either the coastal or central regions. DISCUSSION
OF RESULTS
For the purposes of this discussion section, we will first compare the accelerated testing with the in-service results, go on to compare the natural weathering results with those from paint in-service, and then examine the relative performance of paints complying with SS150 and SS345 specifications. The discussion section will conclude with some general observations on the reliability of weathering tests.
S. K. Roy et al.
482 Table 5. Field survey results-lift Paint defect
slab: shown as ratings on a 10 to zero scale (see Table 2), except for fading
Exposure time (months) ~
Environment Central
Coastal
Industrial
ss150
ss345
ss150
ss345
ss150
ss345
12 24 36 48 60
10
10
10 8 6 6
10 10
10 10
10 10
10 8
10 10
8 8 6
10
8 6 6
10
Flaking
12 24 36 48 60
10 10 10 8 8
10 10 10
10 10 10 8 6
10 10 10
10 10 10 8 6
10 10 10
Algae
12 24 36 48 60
10 10 10 8 8
10 10 10
10 10 10 8 8
10 10 10
10 10 8 8 8
10
12 24 36 48 60
0% 0% 10% 10% 30%
0% 0% 0%
0% 0% 0% 20% 30%
0% 0% 0%
0% 0% 0% 20% 40%
0% 0% 0%
Chalking
Fading/discolouration (% of buildings affected)
Comparative evaluation of paint performance in accelerated and in-service tests The QUV (accelerated) weathering tester allows the determination of the performance of paint subjected to ultraviolet (UV) light, moisture (rain) and temperature. After exposure to the tester for 2000 hours, both paints showed only colour changes and chalking but not checking (Table 3). On the other hand, paint in-service shows not only colour changes and chalking but also flaking, algae growth and dirt pick-up (Table 5), which were not detected in the QUV weathering tester. Algae growth is not seen in the accelerated tests because algae spores are not nucleated. Checking is also not readily seen because of the relatively small area of the substrate (paint being exposed). Also, such deterioration as that involving dirt pick-up is not evaluated in the accelerated test because the water used in the tester does not contain dirt and other pollutants which are in the atmosphere and can come down with the rain. However, checking and algae growth are important paint deterioration mechanisms and can lead to, and even accelerate, further paint failure. Algae give the paint a dirty appearance, and cause serious disfigurement and water retention which results in dampness of buildings and surfaces [14, 151. Checking can develop into cracks and eventually cause flaking (as was found for the paint in-service). Extending the accelerated weathering tests for longer exposure periods could eventually lead to checking defects but there are still the “difficulties” of relating the performance of the small surface area accelerated test specimen to that of paint applied to large exterior surfaces. However, the algae-resistance of the paint cannot presently be determined using the “standard” procedures in the QUV
10 10
apparatus and other supplementary test procedures must be employed. The problem here is that standards and procedures for the evaluation of algicidal coatings have received very little attention [14]. Singapore was in fact the first country to adopt a standard method of test [ 161. In this test, the test paint is applied to the interior of a plastic petri dish and, after UV aging, is inoculated with a broth culture of the alga Trentepohlia odorata and incubated for 12 hours per day at 1000-1400 lux. After eight weeks the intensity of algal growth is visually assessed. Such a test has the advantage that although it is ancillary to the QUV testing, it can be carried out within the same time frame (1344 hours for the algae resistance test compared to 2000 hours for the QUV test). The disadvantage of using such a test as described in SS345 Appendix B [16] is that the algae resistance depends not only on the paint but also on the substrate to which it is applied and the environment to which it is exposed. The principal environmental factors are light, moisture and trace elements [14]. Light is required for the photosynthetic process to proceed: too little and growth may not occur, but very intense light may be inhibitory to some species. Algae thrive under damp conditions and it has been suggested [17] that almost any surface will support algae if it remains damp for any length of time. Algae also have a requirement for trace elements. As pointed out by Gillatt [ 141, these may arise as excretions from overhanging tree canopies, from bird or animal droppings or, especially in intensively farmed areas where high levels of inorganic fertilizers are used, as wind-blown dust. Additionally, the coating or substrate may provide such compounds. The substrate in the SS345 test is a plastic petri dish which obviously is very different from the concrete lift slabs in our paint in-service tests. There
Exterior
Paint
Performance
Table 6. Comparison of paint performance in artificial (QUV) and paint in-service tests Paint specification
ss150 ss345
Exposure time required to develop a chalking rating of 8 QUV test (hours)
Paint in-service (months)
1800-2000 2000
12 24
Acceleration factor
4.4-4.9 8.8
are also difficulties in simulating in the artificial (accelerated) weathering tests the actual in-service environmental conditions both in terms of the light and moisture cycles and the trace elements (pollutants), all of which affect the rate of algae growth. When developing these artificial (accelerated) weathering tests for the simulation of the in-service paint performance, there may well be a need to use tests in addition to the light/moisture/temperature (e.g. QUV) and algae resistance tests. The scrub resistance test is one such test. This test measures the ability of the paint to resist wear and tear such as rubbing, scraping or eroding which tends to progressively remove the material from its surface. Such a test is specified in both SSl50 and SS345: see Table 1. This test is conventionally applied to “new” paint but the paint’s scrub resistance will change as it weathers. It is therefore suggested that some form of this test also be applied to the artificially weathered paint. The results from the artificial (accelerated) testing (Table 3) and those from the paint in-service (Table 5) will now be examined to see if a relationship can be established between the rate of deterioration of the paint in the two types of test. The only paint defects (deterioration mechanisms) that were quantified in both tests were chalking and colour change. The level of chalking was chosen as the comparator since colour fading/loss of gloss were determined by slightly different criteria in the artificial weathering and paint in-service tests. The comparison for chalking is shown in Table 6. If we talk about an acceleration factor, i.e. the ratio of the time required to reach a set level of deterioration in-service to the time required to reach the same level of deterioration in the artificial (accelerated) test, then we see that this factor is very different for the two paints (Table 6). This highlights one of the main problems with artificial (accelerated) testing. The sole purpose of artificial testing is the rapid promotion of degradation processes in the test material to bring about changes that would occur as a result of natural outdoor weathering over a much longer period [IS]. However, for paints, and other materials, there has generally been a poor correlation between natural and accelerated tests [18]. The rate of deterioration also varies with time, and the acceleration factors will change with the stage of deterioration. It is also possible that the test conditions (temperature, moisture, light, etc.) in the accelerated test give rise to a different deterioration mechanism(s) than those present in natural weathering. Comparative evaluation of paint per-ormance in natural weathering and in-service tests The paint exposed under natural weathering showed colour changes, chalking, checking and algae growth
in a Tropical Environment
483
(Table 4). The paint exposed in-service showed similar signs of deterioration with colour changes, chalking, flaking and algae growth (Table 5). The only differences were: (a) paint exposed in the natural weathering tests showed homogeneous colour changes whereas the paint exposed in-service showed an uneven patchiness in the colour changes; and (b) whereas flaking was commonly observed for the paint in-service, the paint exposed in the natural weathering tests showed only slight checking. These differences, i.e. (a) and (b), would be related to the very different surface areas of exposure for the two tests, with the relatively much smaller surface area for the natural weathering tests not giving rise to flaking or uneven colour changes. For paint complying with SSl50 specification, deterioration times were such that comparisons could be made between the rate of deterioration in the natural weathering tests, compared to that for paint in-service. The comparative data for chalking and algae growth are summarised in Table 7. The rate of deterioration of paint subjected to the natural weathering test is faster than that of paint in-service. This is because for the natural weathering test, the paint is painted on substrates which are inclined at a small angle to the horizontal. As such, it is subjected to more severe exposure conditions than the paints in-service, i.e. the lift slab, which are painted on vertical substrates. Furthermore, algae spores and dirt and dust would be more easily retained on nearly horizontal surfaces than on vertical surfaces. Comparison of paint complying with SSISO and SS345 specl$cations Results from the exposure of paint subjected to artificial (accelerated) and natural weathering and in-service exposure showed that paint complying with the SS345 specifications performed consistently better than that complying with SSl50 specifications. SS345 is more durable to colour changes, chalking, checking and algae growth than SSl50. From the field survey (in-service exposure) the SS345 paint showed excellent resistance against chalking, flaking and algae growth after three years of exposure. Although not directly reported on in this paper this was observed even on the back facades of the buildings such as refuse chutes and bathroom areas which have high incidence of dampness. The SS345 paint is especially resistant against algae. From the results of the natural weathering test (Table 4), we can see that slight algae growth occurred only after 60 months (five years) of exposure whereas for the same exposure period, paint complying with SSl50 specifications had shown severe algae growth. Unlike the SS150 paint which has a matt finish, the SS345 paint displays a level of sheen and is thus less porous. A porous nature, such as in SS150, makes the paint film susceptible to moisture retention and encourages fouling by algae growth. The superiority of SS345 over SSl50 paint, at least with respect to chalking and cracking, has previously been demonstrated by Loh [19] in natural weathering tests similar to those in our study. Loh’s results are summarised in Table 8. Non-SISIR certified paint had inferior performance to either SS345 or SSI 50.
S. K. Roy et al. Table 7. Comparison
Deterioration
of chalking
mechanism
and algae growth
Rating
behaviour between natural with SSl50 specifications
Natural weathering tests (approximate No. of months of exposure to obtain level of deterioration)
weathering
Paint
and paint in-service
for paint complying
in-service (approximate No. of months obtain level of Deterioration) Central region
Coastal region
Industrial region
Chalking
8 6
I2 36
36 48
36 60
24 48
Algae growth
8
24
48
48
36
Table 8. Summary of previous natural exposure test* results for paints complying with SS345 or SS150 and other non-certified paints [19] Specification
Time to reach chalking rating of 6 (months)
Time to incipience of cracking (months)
ss345
29 29 29 29
nil nil nil nil (no cracking)
ss150
19 8 12 8 4 6 4
45 34 16 29 19 45 29
4
12
Non-SISIR
certified
r 16 6
I 1 24 34
*All tests were three coats of paint on cement panels exposed a total of 45 months at 5” inclined from horizontal. tTest terminated after two months due to severe flaking. The paint manufacturers have reported [20] that the total painting system cost per square metre for the SS345 paint varies from 53 to 63% higher than that for paint complying with SSl.50 specifications. The cost components of a painting contract comprise those of the paint material, the labour cost, overhead cost and maintenance cost. The higher initial costs for the SS345 paint come mainly from higher material (paint) costs since both SSl50 and SS345 are water-base paints and any differences in labour and overhead costs are small. However, the warranty for paint complying with SS345 is six years, which is double that for paint complying with SS 150. The total cost per square metre per annum is then between 19 and 23 % less for SS345 compared to SS 150 [20]. The above calculations are made on the basis that there will be no inflation during the warranty. The cost advantages (savings) derived by using paint complying with SS345 rather than SSl50 become even greater when consideration is given to the ever-increasing costs of labour and materials. General observations on the reliability of weathering tests The performance of paint exposed in the artificial (i.e. accelerated) weathering tester depends on the period of
to
exposure and the testing conditions in the weathering tester. However, there are no officially recognised, published data available which enable one to determine the period of real-time (in-service) exposure that produces an equivalent paint performance to a stated number of hours of exposure in the artificial weathering tester. In other words, the number of hours for which the paint has to be exposed in the weathering tester for it to be representative of the performance of paint which is exposed for a fixed number of years in the field is not known. The data such as we have obtained, and which are summarised in Table 6, can only serve as a general guide. [Paint manufacturers have set their own standards with respect to the number of hours of exposure in the artificial weathering tester which are necessary for the paint to be considered “weather resistant”. For instance, some manufacturers, when testing paint complying with SSl50 specifications, expose the paint panels for 1500 hours while others use a 3000 hour exposure time.] As such, the artificial (accelerated) weathering tester should not be used to make an absolute evaluation of a paint’s resistance to weathering. Such a conclusion was also reached by Pate1 [18] in his review of artificial weathering of paints, where he stated “Evidence gathered so far indicates little likelihood of a universal accelerated weathering method applicable to all paints exposed to all types of natural weathering.” Pate1 did, however, noted
qualify
the above statement
and
“However there is clearly scope for further studies which could be directed towards identifying a particular method to suit particular classes of materials intended for exposure in limited environments. For this purpose, improvements in the technologies for artificial weathering may demonstrate their usefulness for improving correlations with outdoor exposures.” We generally agree with Pate1 and would suggest that equipment such as the QUV tester used in this investigation can be used to conduct a qualitative (or semiquantitative) evaluation of the durability of different grades or formulations of paint exposed to various combinations of UV light, moisture (humidity) and temperature. Such comparative data are useful since, with modern technology, new formulations and new grades of paint are always being developed and there is an urgent need for a quick, but reliable, test of their durability. The artificial (accelerated) test may, however, be limited in its usefulness to “sorting” the most promising grades which would then be subjected to more rigorous and lengthy natural exposure tests.
Exterior
Paint Performance
As has been demonstrated in this study, the natural weathering tests provide a good representation of the performance of the paint in-service. The only exceptions to this statement would be the performance variations due to local variations in microclimate and to some extent the variations seen that arise from the different exposed surface areas for the panels in the natural weathering tests and the concrete structures in-service. However, the exposure periods for the natural weathering tests are on the same time-scale as the real in-service performance and are thus not “economically viable”. This would give further support to Patel’s [18] call for further development work on artificial, i.e. accelerated, weathering tests.
CONCLUSIONS The main conclusions from this study of the artificial, natural and in-service weathering of two emulsion paints, one algae resistant (SS345), and one non-algae resistant (SSl50), in the tropical environment of Singapore are as follows. The artificial (QUV) weathering tester evaluates the deterioration of paint due to chemical weathering alone. The accelerated weathering test needs to be supplemented by other tests, especially the scrub resistant and algae resistant tests, to determine the durability of the paint to mechanical and biological weathering. The rate of deterioration of paint exposed under natural weathering in an industrial region is faster than that of paint in-service exposed in the industrial, central and coastal regions in Singapore. Paint was observed to deteriorate faster in the indus-
in a Tropical Environment
trial region than in other environmental regions (coastal and central) in Singapore. This could be due in part to the higher pollution level in that region. 4. When the results for chalking are used to compare the rate of deterioration of paint exposed in weathering tests (artificial and natural) and paint in-service, the following are found. The SSl50 paint samples which were exposed for 1SO&2000 hours in the QUV weathering tester are equivalent to paint in-service which was subjected to approximately 12 months of exposure. The SS345 paint samples which were exposed for 2000 hours in the QUV weathering tester are equivalent to paint in-service which was exposed for approximately 24 months. The SS150 paint samples which were exposed to the natural weathering test for 36 months were equivalent to paint exposed in service for 48 months in the central and industrial areas and 60 months in the coastal area. Paint complying with SS345 was found to be more durable than paint complying with SS150. In addition, significant long-term savings are expected when paints complying with SS345 are used instead of paints complying with SS 150. AcknowledgementsThe authors are grateful to Assoc. Prof. Khoo Cheng Lim, Dean of the Faculty of Architecture and Building, and Assoc. Prof. Lim Lan Yuan, Head, School of Building and Estate Management, of the National University of Singapore (NUS) for their encouragement and support. Professor Northwood would like to thank NUS for the granting of a Visiting Senior Fellowship during the tenure of which this paper was prepared. Acknowledgements are also due to Star Chemical Pte Ltd for facilities and assistance with the accelerated weathering test and to local paint suppliers for supplying the paint for testing.
REFERENCES 1. 2. 3.
4. 5. 6.
7. 8. 9. 10. 11. 12. 13. 14. 15.
485
K. C. Tan, Maintenance of external paintwork-a terotechnology approach. MSc. dissertation, National University of Singapore (1989). A. J. Nayor, Paintwork in the tropics. In Control of the External Environment of Buildings (Edited by B. P. Lim), pp. 144-152, Singapore University Press, Singapore (1988). K. R. Rao, Solar radiation and external temperatures of buildings. In Control of the External Environment of Buildings (Edited by B. P. Lim), pp. 1946. Singapore University Press, Singapore (1988). Y. C. Wee, Control of plant growth on buildings. In Building Maintenance Technology in Tropical Climates (Edited by C. Briffett), pp. 107-l 13, Singapore University Press, Singapore (1995). Y. C. Wee and G. Lim, Biological growths on building surfaces. In Controlof the ExternalEnvironment of Buildings (Edited by B. P. Lim), pp. 47-51, Singapore University Press, Singapore (1988). ASTM Standard D4587: 1988, Conducting Tests on Paint and Related Coatings and Materials Using a Fluorescent UV-Condensation Light- and Water-Exposure Apparatus. ASTM, Philadelphia, PA (1988). G. W. Grossman, The QUV Weathering Tester. Q-Panel Company, USA (1987). J. G. Balfour, Back to basics: durability and titanium dioxide pigments. Journal of the Oil Colour Chemists Association 73( 12), 478483 (1990). ASTM Standard D659, Test Method for Evaluating Degree of Chalking of Exterior Paints. ASTM, Philadelphia, PA (1986). ASTM Standard D660: 1988, Test Method for Evaluating Degree of Checking of Exterior Paints. ASTM, Philadelphia, PA (1988). Singapore Standard SS5 Part 3: 1976, Preparation of Panels Prior to Painting. SISIR, Singapore (1976). ASTM Standard D772, Test Method for Evaluating Degree of Flaking (Scaling) of Exterior Paints. ASTM, Philadelphia, PA (1993). M. Ng, External paintwork defects of HDB apartment. M.Sc. dissertation, National University of Singapore, Singapore (1989). J. W. Gillatt, The need for antifungal and antialgal additives in high performance surface coatings. Journal of the Oil Colour Chemists Association (Surface Coatinqs International) 74(6). 197-203 (1991). J. W. Gillat, The microbiological spoilage of emulsion paints during manufacture‘and its prevention. Journal of the Oil Co/our Chemists Association (Surface Coatings International) 74(9), 324-328 (1991).
S. K. Roy et al.
486 16. 17. 18. 19.
20.
Singapore Institute of Standards and Industrial Research, SS345 Specification for Algae Resistant Emulsion Paint for Decorative Purposes; Appendix B (1990). B. A. Richardson, Algae, lichens, mosses-what they are and what they do. Buildiny Maintenance 7(3), 29-31 (1973). P. N. Patel, Artificial weathering of paints. Journal of the Oil Colour Chemists Association (Surface Coatings International) 74(3), 104106 (1991). W. S. Loh, Development of algae resistant emulsion paint-a prototype development in SISIR. Proceedings of Seminar on Emulsion Paint.for Quulity Paintiq qf Buildings in the Tropics, Singapore, 17 August (1990). S. K. Chew, The quality, cost and delivery for the anti-algae emulsion paint-the view of the Singapore Paint Manufacturers’ Association. Proceedings of Seminar on Emulsion Paint,for Quality Painting of Buildings in the Tropics, Singapore, 17 August (1990).
Pergamon
PII: SO360-1323(96)00009-l
The Evaluation of Paint Performance Exterior Applications in Singapore’s Tropical Environment SALIL K. ROY* LEE BENG THYE* DEREK 0. NORTHWOOD*t
for
(Received 26 June 1995; accepted 21 November 1995)
Two water-base paints, a regular non-algae and an algae resistant emulsion for decorative purposes, were exposed to art$cial, i.e. accelerated, and natural weathering tests and the deterioration compared to that for similar paint applied to exterior walls on multi-unit buildings in Singapore. The artificial weathering test wasfound not to provide a good representation of actual performance since it only monitors the deterioration due to chemical weathering and not that due to mechanical or biological weathering. The rate of deterioration of paint exposed in natural weathering tests in an industrial region in Singapore was faster than that for paint exposed in-service on multi-unit housing situated in industrial, central or coastal regions in Singapore. Copyright 0 1996 Elsevier Science Ltd.
INTRODUCTION
significant incentive in improving these correlations since the end-users do not have the luxury of waiting until there is an in-service failure of the paint system. If the producers, and users, understand the limitations and reliability of artificial and natural weathering tests in predicting in-service performance, then we can better ensure that paint which performs well in the weathering tests, does not fail prematurely under actual in-service conditions. Estate managers, architects and paint manufacturers, armed with a better knowledge of the durability of paint in-service, would be able to design, and carry out, better maintenance programmes. In the present study two paints, an emulsion paint and an algae resistant emulsion paint, were exposed to artificial and natural weathering tests and the deterioration compared to that for similar paint applied to the exterior walls in multi-unit housing in Singapore. Since more than 20 million dollars is spent annually on repainting, even a modest improvement in durability of say 5% would result in savings in excess of 1 million dollars a year 111. The climate in Singapore is very demanding in terms of the performance and durability of all building materials and in particular paints [2]. Singapore is located on latitude 1.3”N and hence experiences an equatorial climate. The main features are uniform (high) temperature, high humidity, and abundant rainfall throughout the year. The mean daily maximum and minimum temperatures are 30.7 and 23.4”C respectively and the annual rainfall is about 2400mm with 267 rainy days in a year [3]. Such conditions, as well as influencing the long term durability, also provide the ideal conditions for algae growth on external walls [4]. We have thus included algae resistant and non-algae resistant paints in our comparative study.
WHEN a paint is applied as an external finish to a building it is subject to a variety of environmental factors that can degrade the paint and reduce its useful life-span. These influences include rain, sunshine, humidity, temperature and pollution. There are a number of artificial, i.e. accelerated, and natural weathering tests used both by the paint manufacturers in the development of new paint formulations and by paint manufacturers and paint users in assessing the suitability of a paint for a particular application and in determining its durability under the particular, or similar, environmental conditions. However, it is difficult (some claim impossible) to correlate the results from these accelerated and natural weathering tests with the performance of the same paint in-service, i.e. applied on external surfaces of buildings. There are many reasons for this lack of correlation but they can be divided into two main areas, namely: l
l
there are many factors, besides the weather, which affect the performance of the paint in-service; there is no basis for comparison unless the paint used in the weathering tests and in-service is the same, and the substrate on which the paint is applied is similar.
However, despite these difficulties in correlating paint performance in weathering tests and in-service, there is a
*School of Building and Estate Management, National University of Singapore, Singapore 0511, Singapore. tAuthor to whom correspondence should be addressed. Permanent address: Department of Mechanical and Materials Engineering, University of Windsor, Windsor, Ontario N9B 3P4, Canada.
477
S. K. Roy et al.
478 Table 1. Comparison
of the two Singapore
Standards
Description
ss345:
Fineness
test
40 microns
Viscosity
test
75-95 ku
65-85 ku
The higher viscosity permits tates roller application.
1000 hours
500 hours
The increase in duration of the acccelerated weathering test ensures better durability and longer lifespans of the paintwork.
Accelerated
weathering
test
1990
of the requirements
max
SS150: 1976
Significance
50 microns
The reduction in maximum fineness from 50 to 40 microns results in better pigment dispersion and a smoother paint texture which would reduce the chances for retaining micro-organisms in the paintwork.
max
binder-rich
paint and facili-
Scrub resistant
test
3000 cycles
1500 cycles
The increase in cycles for the scrub resistant better mechanical properties.
Algae resistant
test
Incorporated
Nil
This is to ensure that the paint is algae resistant against the common species of algae such as T. odoratn [5].
15-50
20 max
The increased specular reflection requirement smoother and binder-rich paint texture.
Specular
reflection
at 85°C
EXPERIMENTAL
DETAILS
This section will begin with a short description of the two types of paint studied. An overview will then be given of the general methodology of the study before concluding with full details of the artificial weathering, field survey and natural weathering tests. The paints In Singapore, the quality of water-base paints is governed by the Singapore Standards as specified by the Singapore Institute of Standards and Industrial Research (SISIR). The two standards are as follows. 1. SSlSO: 1976: Emulsion Paints for Decorative Purposes. 2. SS345: 1990: Algae Resistant Emulsion Paint for Decorative Purposes. A comparison of the requirements is given in Table 1.
of the two standards
General methodology An overview of the methodology adopted for this study is shown in Fig. 1 which also lists the type of data (results) that were obtained for the three parts of the study. Artificial (accelerated) weathering tests These tests involve exposing non-white paint in a weathering apparatus to ultraviolet radiation, elevated (c.f. ambient) temperatures and condensation for a total period of 2000 hours. Non-white paint was chosen for these tests so that the durability of coloured paint (in terms of colour changes) could be monitored. The design of this part of the experiment and rating schemes for the evaluation of the results followed those outlined in ASTM Standard D4587 [6]. Paint complying with SSl50: 1976 and SS345: 1990 was obtained from three different paint manufacturers. The paint was of a blue colour (B.S. 18C35) which was chosen since it is one of the more popular colours for exterior application in housing estates. The paint was
test ensures
ensures
a
applied to cement panels measuring 15 x 1Ocm. The application was by brush and two coats were applied. The painted panels were allowed to dry for 1 week prior to exposure in the weathering tests. The tests were carried out using a QUV Weathering Tester (Q Panel Model 85-2534-28) with a UV-B (wavelength 313 nm) fluorescent lamp. The test temperature was maintained at 50°C and alternate 4 hour cycles were carried out where only UV light or condensation were applied. As noted, the total exposure time was 2000 hours, which represents 250 cycles (UV + condensation). This particular testing cycle was chosen on the basis of recommendations of the equipment manufacturer [7]. The exposed samples were evaluated for chalking, checking and colour change. Chalking refers to a surface deposit of powder formed by the disintegration of the binder due to exposure to UV light and moisture [8]. Chalking was gauged by wiping the exposed surface of the paint with a black velvet cloth and comparing with photographic samples as per ASTM Standard D659 [9]. Checks are slight breaks in the paint film which do not penetrate into the substrate. Checking was measured by examining at 25 times magnification and comparing with photographic samples as per ASTM Standard D660 [lo]. Colour change (fading) is a gradual decrease in colour intensity in the paint due to the use of pigments with insufficient lightfastness. The colour change was measured by visual inspection under artificial lighting (fluorescent illumination) and comparison with photographic samples as per Singapore Standard SS5: Part E3 [l I]. The results of these examinations were rated on a O-10 scale as outlined in Table 2. Natural weathering tests The objective of these tests was to assess the durability of paints, meeting the requirements of the same two Singapore standards, to outdoor exposure. The paint panels under test were also on a cement substrate and two coats were applied by a brush method. The test panels were affixed to racks at a 5” inclination to the horizontal. They
479
Exterior Paint Performance in a Tropical Environment 1
Selection artificial
Natural
Thering
1
Collection of data of paint exposed under natural weathering
of samples
(ie. accelerated)
I Assessment performance
of of paint-
Results under evaluation 1. Colour changes
Results under evaluation 1. Colour changes
5. Dirt, soot, dust
Fig. 1. Overview
Table
10 8 6 4 2 0
2. Rating scheme used in evaluation erated) and natural weathering
of artificial tests
Rating
Performance
Excellent Very good Good Fair Poor Very poor
No failure Slight failure Considerable failure Severe failure Very severe failure Complete failure
of methodology
(accel-
were exposed for periods of up to five years at an exposure site in an industrial area of Singapore. At selected intervals during the five year exposure period, the samples were assessed for chalking, checking and colour change as described for the accelerated weathering tests. In addition, the samples were assessed for algae growth again using photographic standards and a G-10 rating scale. [These were internally developed standards where a rating of 0 (zero) indicated that algae was covering the majority of the surface after five years and 10 indicated no algal growth.] Qualitative observations were also made on the deposits of dirt, dust and soot.
Results under evaluation 1. Colour changes 2. Chalking 3. Checking 4. Algae 5. Dirt, soot, dust
of study.
SS345: 1990 specifications. The three environmental locations chosen can be generally classified as coastal, central (i.e. away from the coast) and industrial. A total of 170 buildings were surveyed. The buildings were chosen so that at the time of the survey the paintwork had been “in place” for periods of up to five years for paint complying with SS150 and up to three years for paint complying with SS345 (the shorter time for this paint was due to the fact that the implementation date for this standard was 1990). The painting works surveyed comprised a mix of repainting works on older buildings and new painting on new job sites. In the original survey work a number of locations (structures) within the building were surveyed, including walls, columns, beams, slabs, parapets and window canopies. For the purposes of this paper we will concentrate on the results for the lift slab which is an exterior facing, vertical component and which had similar features in all the buildings surveyed (Fig. 2). The paintwork was evaluated for the following: l l l l
Field survey of paint performance The objective of this survey was to assess the performance of actual paintwork in different environmental locations (within Singapore). The buildings chosen were apartment blocks. All water-base paints applied to these buildings were required to comply with SS150: 1976 and
Assessment of paint performance
l
Chalking Algae and mould growth Dirt, dust and soot Flaking Discolouration.
The evaluation and rating of these defects was generally as described previously for the natural weathering tests using photographic standards to place the defect on a & 10 scale. Generally a particular defect would be con-
S. K. Roy
480
Fig. 2. Photograph showing location and detail of lift slab.
sidered to be present if more than 5% of the total surface area of that element appears to have that defect. The evaluation of flaking was performed in a manner similar to that specified in ASTM Standard D772 [12]. Mean ratings were assigned for each defect after observation of several areas of the lift slab. RESULTS Accelerated
weathering
tests
The results of the accelerated weathering tests are summarised in Table 3. In general, it can be seen that although
et al. there was little difference in the performance (in terms of colour change, chalking and checking) of the paint from the three suppliers, the paint conforming to SS345 performed better than that conforming to SSl50. Neither type of paint showed signs of checking even after the maximum exposure period (2000 hours). Chalking was not detected until 1800 hours exposure for the SSl50 paint or 2000 hours for the SS345 paint (and then in paint from only one of the three suppliers). Colour change (fading) and loss of gloss were however found after exposure to UV light and water in the accelerated test. Considerable colour fading and loss of gloss (down from 10 to a rating of 6) occurred in the SS150 paint after 2000 hours exposure. The SS345 paint was more resistant to colour changes, showing only slight fading of colour (down to a rating of 8) after 2000 hours exposure. It also showed a higher level of sheen (gloss) compared to the SS 150 paint. Natural weathering
tests
The results for the natural weathering tests are summarised in Table 4. The paint conforming to SS345 showed not only better resistance to algae formation than paint SS150 but also better resistance to colour change, chalking and checking. There was a homogeneous colour fading in all exposed paint samples. The SS150 paint showed what is described as considerable failure (rating 6) after only 24 months
Table 3. Accelerated weathering test results: shown as ratings on a 10 to zero scale (see Table 2) Paint defect
Paint specification
Hours in QUV
ss345 Supplier
ss150 Supplier 1
2
3
1
2
3
1300 1500 1800 2000
10 10 8 8 8 8 8 6 6
10 10 8 8 8 6 6 6 6
10 10 10 8 8 8 6 6 6
10 10 10 10 10 8 8 8 8
10 10 10 10 8 8 8 8 8
10 10 10 8 8 8 8 7 7
Chalking
0 300 500 700 1000 1300 1500 1800 2000
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 8 8
10 10 10 10 10 10 10 8 8
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 8
Checking
0 300 500 700 1000 1300 1500
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10
Colour change
0 300 500 700
1000
1800 2000
Exterior
Paint Performance
481
in a Tropical Environment
Table 4. Natural weathering results: shown as ratings on a 10 to zero scale (see Table 2) Exposure (months) 0
12 24 36 48 60
Colour change
Checking
Chalking
Algae
ss150
ss345
ss150
ss345
ss150
ss345
ss150
ss345
10 68 6 6 6 4
10 8 8 8 6 6
10 8 68 6 4 4
10 10 8 8 6 6
10 10 S-10 8 8 8
10 10 10 8-10 8 8
10 10 8 S-10 6 4
10 10 10 10 10 8
and deteriorated more on prolonged exposure such that at five years exposure there was severe colour change (rating 4). The SS345 paint performed much better with at least a four year exposure being required to reach a colour change rating of 6 which was reached after only 24 months for paint SS 150. A similar pattern to the colour change results was seen in the data for chalking with the approximately two to one ratio of times required for a similar level of deterioration in paint SS345 compared to paint SSI 50. As opposed to the accelerated weathering tests where no checking was found in any of the paint samples (at least for exposure times up to 2000 hours), a small amount (slight checking-rating 8) was found in paint SS 150 for exposure times greater than 24 months and in paint SS345 for exposure times greater than 36 months. However, in both types of paint, the checking did not reach a level worse than slight (rating 8), even after five years exposure. As expected, the algae resistant paint (SS345) proved to be much more resistant to the growth of algae in that the only algae growth found was on one of the panels exposed for five years. This contrasts with the behaviour of paint SS150 where algae growth began within two years of exposure and had reached the severe level (rating 4) after five years. As well as the semi-quantitative observations on colour change, chalking, checking and algae growth, some qualitative observations were made on dirt/soot/dust pick-up. Both the SS150 and SS345 paint samples showed slight dirt pick-up after one year of exposure. After five years exposure, it was generally found that the SS150 paint panels showed more dirt pick-up than the SS345 paint panels. Field survey As noted in the “Experimental Details” section, the lift slab was chosen as the representative (building) element for the observation of the weathering of paint in-service. Deterioration of the paint in the lift slab is not affected by factors such as dampness and poor design, which can lead to early paint failure in other areas of the building, e.g. refuse chute and bathroom areas [13], that were also surveyed. The results for chalking, flaking, algae growth and fading are summarised in Table 5. Paint complying with SS345 did not show any defects (deterioration) for the maximum exposure period (three years). Chalking was observed first in paint on buildings situ-
ated in an industrial region. No chalking of paint was observed in the initial two years for the central and coastal regions but paint on buildings in the industrial region showed slight chalking (rating 8) after two years. The rate of deterioration (chalking) of paint (SSl50) was slower in the coastal region compared to the central and industrial regions. In the central and industrial regions, considerable chalking (rating 6) occurred after four years whereas in the coastal region similar deterioration was not found until five years exposure. No flaking of the paint was found for the first three years of exposure in any of the three exposure regions. Flaking began after four years exposure and after five years was more severe in the industrial and coastal regions (rating 6) than in the central region (rating 8). Algae growth (on SS150 paint) had begun after three years in the industrial region and after four years in the central and coastal regions. However, after five years, paint applied to buildings in all three environmental regions only showed slight algae growth with a rating of 8. Discolouration and uneven fading of colour of the paint was seen after three (or four) years. Although this fading was detected first (after three years) on buildings in the central region, after five years exposure discolouration and fading was observed most frequently in the buildings in the industrial region. [As an aside it is noted that in the industrial region, there is a reasonably wide usage of bright orange-red colour paint. This colour paint contains an organic pigment which has poor lightfastness and therefore fading is seen fairly early after exposure and buildings with this type of paint show higher frequencies of fading/discolouration]. Dust/dirt/soot deposition was not a problem on the lift slab but occurred more often in other building components. Not unexpectedly, it was a more serious problem in buildings located in the industrial region than in those located in either the coastal or central regions. DISCUSSION
OF RESULTS
For the purposes of this discussion section, we will first compare the accelerated testing with the in-service results, go on to compare the natural weathering results with those from paint in-service, and then examine the relative performance of paints complying with SS150 and SS345 specifications. The discussion section will conclude with some general observations on the reliability of weathering tests.
S. K. Roy et al.
482 Table 5. Field survey results-lift Paint defect
slab: shown as ratings on a 10 to zero scale (see Table 2), except for fading
Exposure time (months) ~
Environment Central
Coastal
Industrial
ss150
ss345
ss150
ss345
ss150
ss345
12 24 36 48 60
10
10
10 8 6 6
10 10
10 10
10 10
10 8
10 10
8 8 6
10
8 6 6
10
Flaking
12 24 36 48 60
10 10 10 8 8
10 10 10
10 10 10 8 6
10 10 10
10 10 10 8 6
10 10 10
Algae
12 24 36 48 60
10 10 10 8 8
10 10 10
10 10 10 8 8
10 10 10
10 10 8 8 8
10
12 24 36 48 60
0% 0% 10% 10% 30%
0% 0% 0%
0% 0% 0% 20% 30%
0% 0% 0%
0% 0% 0% 20% 40%
0% 0% 0%
Chalking
Fading/discolouration (% of buildings affected)
Comparative evaluation of paint performance in accelerated and in-service tests The QUV (accelerated) weathering tester allows the determination of the performance of paint subjected to ultraviolet (UV) light, moisture (rain) and temperature. After exposure to the tester for 2000 hours, both paints showed only colour changes and chalking but not checking (Table 3). On the other hand, paint in-service shows not only colour changes and chalking but also flaking, algae growth and dirt pick-up (Table 5), which were not detected in the QUV weathering tester. Algae growth is not seen in the accelerated tests because algae spores are not nucleated. Checking is also not readily seen because of the relatively small area of the substrate (paint being exposed). Also, such deterioration as that involving dirt pick-up is not evaluated in the accelerated test because the water used in the tester does not contain dirt and other pollutants which are in the atmosphere and can come down with the rain. However, checking and algae growth are important paint deterioration mechanisms and can lead to, and even accelerate, further paint failure. Algae give the paint a dirty appearance, and cause serious disfigurement and water retention which results in dampness of buildings and surfaces [14, 151. Checking can develop into cracks and eventually cause flaking (as was found for the paint in-service). Extending the accelerated weathering tests for longer exposure periods could eventually lead to checking defects but there are still the “difficulties” of relating the performance of the small surface area accelerated test specimen to that of paint applied to large exterior surfaces. However, the algae-resistance of the paint cannot presently be determined using the “standard” procedures in the QUV
10 10
apparatus and other supplementary test procedures must be employed. The problem here is that standards and procedures for the evaluation of algicidal coatings have received very little attention [14]. Singapore was in fact the first country to adopt a standard method of test [ 161. In this test, the test paint is applied to the interior of a plastic petri dish and, after UV aging, is inoculated with a broth culture of the alga Trentepohlia odorata and incubated for 12 hours per day at 1000-1400 lux. After eight weeks the intensity of algal growth is visually assessed. Such a test has the advantage that although it is ancillary to the QUV testing, it can be carried out within the same time frame (1344 hours for the algae resistance test compared to 2000 hours for the QUV test). The disadvantage of using such a test as described in SS345 Appendix B [16] is that the algae resistance depends not only on the paint but also on the substrate to which it is applied and the environment to which it is exposed. The principal environmental factors are light, moisture and trace elements [14]. Light is required for the photosynthetic process to proceed: too little and growth may not occur, but very intense light may be inhibitory to some species. Algae thrive under damp conditions and it has been suggested [17] that almost any surface will support algae if it remains damp for any length of time. Algae also have a requirement for trace elements. As pointed out by Gillatt [ 141, these may arise as excretions from overhanging tree canopies, from bird or animal droppings or, especially in intensively farmed areas where high levels of inorganic fertilizers are used, as wind-blown dust. Additionally, the coating or substrate may provide such compounds. The substrate in the SS345 test is a plastic petri dish which obviously is very different from the concrete lift slabs in our paint in-service tests. There
Exterior
Paint
Performance
Table 6. Comparison of paint performance in artificial (QUV) and paint in-service tests Paint specification
ss150 ss345
Exposure time required to develop a chalking rating of 8 QUV test (hours)
Paint in-service (months)
1800-2000 2000
12 24
Acceleration factor
4.4-4.9 8.8
are also difficulties in simulating in the artificial (accelerated) weathering tests the actual in-service environmental conditions both in terms of the light and moisture cycles and the trace elements (pollutants), all of which affect the rate of algae growth. When developing these artificial (accelerated) weathering tests for the simulation of the in-service paint performance, there may well be a need to use tests in addition to the light/moisture/temperature (e.g. QUV) and algae resistance tests. The scrub resistance test is one such test. This test measures the ability of the paint to resist wear and tear such as rubbing, scraping or eroding which tends to progressively remove the material from its surface. Such a test is specified in both SSl50 and SS345: see Table 1. This test is conventionally applied to “new” paint but the paint’s scrub resistance will change as it weathers. It is therefore suggested that some form of this test also be applied to the artificially weathered paint. The results from the artificial (accelerated) testing (Table 3) and those from the paint in-service (Table 5) will now be examined to see if a relationship can be established between the rate of deterioration of the paint in the two types of test. The only paint defects (deterioration mechanisms) that were quantified in both tests were chalking and colour change. The level of chalking was chosen as the comparator since colour fading/loss of gloss were determined by slightly different criteria in the artificial weathering and paint in-service tests. The comparison for chalking is shown in Table 6. If we talk about an acceleration factor, i.e. the ratio of the time required to reach a set level of deterioration in-service to the time required to reach the same level of deterioration in the artificial (accelerated) test, then we see that this factor is very different for the two paints (Table 6). This highlights one of the main problems with artificial (accelerated) testing. The sole purpose of artificial testing is the rapid promotion of degradation processes in the test material to bring about changes that would occur as a result of natural outdoor weathering over a much longer period [IS]. However, for paints, and other materials, there has generally been a poor correlation between natural and accelerated tests [18]. The rate of deterioration also varies with time, and the acceleration factors will change with the stage of deterioration. It is also possible that the test conditions (temperature, moisture, light, etc.) in the accelerated test give rise to a different deterioration mechanism(s) than those present in natural weathering. Comparative evaluation of paint per-ormance in natural weathering and in-service tests The paint exposed under natural weathering showed colour changes, chalking, checking and algae growth
in a Tropical Environment
483
(Table 4). The paint exposed in-service showed similar signs of deterioration with colour changes, chalking, flaking and algae growth (Table 5). The only differences were: (a) paint exposed in the natural weathering tests showed homogeneous colour changes whereas the paint exposed in-service showed an uneven patchiness in the colour changes; and (b) whereas flaking was commonly observed for the paint in-service, the paint exposed in the natural weathering tests showed only slight checking. These differences, i.e. (a) and (b), would be related to the very different surface areas of exposure for the two tests, with the relatively much smaller surface area for the natural weathering tests not giving rise to flaking or uneven colour changes. For paint complying with SSl50 specification, deterioration times were such that comparisons could be made between the rate of deterioration in the natural weathering tests, compared to that for paint in-service. The comparative data for chalking and algae growth are summarised in Table 7. The rate of deterioration of paint subjected to the natural weathering test is faster than that of paint in-service. This is because for the natural weathering test, the paint is painted on substrates which are inclined at a small angle to the horizontal. As such, it is subjected to more severe exposure conditions than the paints in-service, i.e. the lift slab, which are painted on vertical substrates. Furthermore, algae spores and dirt and dust would be more easily retained on nearly horizontal surfaces than on vertical surfaces. Comparison of paint complying with SSISO and SS345 specl$cations Results from the exposure of paint subjected to artificial (accelerated) and natural weathering and in-service exposure showed that paint complying with the SS345 specifications performed consistently better than that complying with SSl50 specifications. SS345 is more durable to colour changes, chalking, checking and algae growth than SSl50. From the field survey (in-service exposure) the SS345 paint showed excellent resistance against chalking, flaking and algae growth after three years of exposure. Although not directly reported on in this paper this was observed even on the back facades of the buildings such as refuse chutes and bathroom areas which have high incidence of dampness. The SS345 paint is especially resistant against algae. From the results of the natural weathering test (Table 4), we can see that slight algae growth occurred only after 60 months (five years) of exposure whereas for the same exposure period, paint complying with SSl50 specifications had shown severe algae growth. Unlike the SS150 paint which has a matt finish, the SS345 paint displays a level of sheen and is thus less porous. A porous nature, such as in SS150, makes the paint film susceptible to moisture retention and encourages fouling by algae growth. The superiority of SS345 over SSl50 paint, at least with respect to chalking and cracking, has previously been demonstrated by Loh [19] in natural weathering tests similar to those in our study. Loh’s results are summarised in Table 8. Non-SISIR certified paint had inferior performance to either SS345 or SSI 50.
S. K. Roy et al. Table 7. Comparison
Deterioration
of chalking
mechanism
and algae growth
Rating
behaviour between natural with SSl50 specifications
Natural weathering tests (approximate No. of months of exposure to obtain level of deterioration)
weathering
Paint
and paint in-service
for paint complying
in-service (approximate No. of months obtain level of Deterioration) Central region
Coastal region
Industrial region
Chalking
8 6
I2 36
36 48
36 60
24 48
Algae growth
8
24
48
48
36
Table 8. Summary of previous natural exposure test* results for paints complying with SS345 or SS150 and other non-certified paints [19] Specification
Time to reach chalking rating of 6 (months)
Time to incipience of cracking (months)
ss345
29 29 29 29
nil nil nil nil (no cracking)
ss150
19 8 12 8 4 6 4
45 34 16 29 19 45 29
4
12
Non-SISIR
certified
r 16 6
I 1 24 34
*All tests were three coats of paint on cement panels exposed a total of 45 months at 5” inclined from horizontal. tTest terminated after two months due to severe flaking. The paint manufacturers have reported [20] that the total painting system cost per square metre for the SS345 paint varies from 53 to 63% higher than that for paint complying with SSl.50 specifications. The cost components of a painting contract comprise those of the paint material, the labour cost, overhead cost and maintenance cost. The higher initial costs for the SS345 paint come mainly from higher material (paint) costs since both SSl50 and SS345 are water-base paints and any differences in labour and overhead costs are small. However, the warranty for paint complying with SS345 is six years, which is double that for paint complying with SS 150. The total cost per square metre per annum is then between 19 and 23 % less for SS345 compared to SS 150 [20]. The above calculations are made on the basis that there will be no inflation during the warranty. The cost advantages (savings) derived by using paint complying with SS345 rather than SSl50 become even greater when consideration is given to the ever-increasing costs of labour and materials. General observations on the reliability of weathering tests The performance of paint exposed in the artificial (i.e. accelerated) weathering tester depends on the period of
to
exposure and the testing conditions in the weathering tester. However, there are no officially recognised, published data available which enable one to determine the period of real-time (in-service) exposure that produces an equivalent paint performance to a stated number of hours of exposure in the artificial weathering tester. In other words, the number of hours for which the paint has to be exposed in the weathering tester for it to be representative of the performance of paint which is exposed for a fixed number of years in the field is not known. The data such as we have obtained, and which are summarised in Table 6, can only serve as a general guide. [Paint manufacturers have set their own standards with respect to the number of hours of exposure in the artificial weathering tester which are necessary for the paint to be considered “weather resistant”. For instance, some manufacturers, when testing paint complying with SSl50 specifications, expose the paint panels for 1500 hours while others use a 3000 hour exposure time.] As such, the artificial (accelerated) weathering tester should not be used to make an absolute evaluation of a paint’s resistance to weathering. Such a conclusion was also reached by Pate1 [18] in his review of artificial weathering of paints, where he stated “Evidence gathered so far indicates little likelihood of a universal accelerated weathering method applicable to all paints exposed to all types of natural weathering.” Pate1 did, however, noted
qualify
the above statement
and
“However there is clearly scope for further studies which could be directed towards identifying a particular method to suit particular classes of materials intended for exposure in limited environments. For this purpose, improvements in the technologies for artificial weathering may demonstrate their usefulness for improving correlations with outdoor exposures.” We generally agree with Pate1 and would suggest that equipment such as the QUV tester used in this investigation can be used to conduct a qualitative (or semiquantitative) evaluation of the durability of different grades or formulations of paint exposed to various combinations of UV light, moisture (humidity) and temperature. Such comparative data are useful since, with modern technology, new formulations and new grades of paint are always being developed and there is an urgent need for a quick, but reliable, test of their durability. The artificial (accelerated) test may, however, be limited in its usefulness to “sorting” the most promising grades which would then be subjected to more rigorous and lengthy natural exposure tests.
Exterior
Paint Performance
As has been demonstrated in this study, the natural weathering tests provide a good representation of the performance of the paint in-service. The only exceptions to this statement would be the performance variations due to local variations in microclimate and to some extent the variations seen that arise from the different exposed surface areas for the panels in the natural weathering tests and the concrete structures in-service. However, the exposure periods for the natural weathering tests are on the same time-scale as the real in-service performance and are thus not “economically viable”. This would give further support to Patel’s [18] call for further development work on artificial, i.e. accelerated, weathering tests.
CONCLUSIONS The main conclusions from this study of the artificial, natural and in-service weathering of two emulsion paints, one algae resistant (SS345), and one non-algae resistant (SSl50), in the tropical environment of Singapore are as follows. The artificial (QUV) weathering tester evaluates the deterioration of paint due to chemical weathering alone. The accelerated weathering test needs to be supplemented by other tests, especially the scrub resistant and algae resistant tests, to determine the durability of the paint to mechanical and biological weathering. The rate of deterioration of paint exposed under natural weathering in an industrial region is faster than that of paint in-service exposed in the industrial, central and coastal regions in Singapore. Paint was observed to deteriorate faster in the indus-
in a Tropical Environment
trial region than in other environmental regions (coastal and central) in Singapore. This could be due in part to the higher pollution level in that region. 4. When the results for chalking are used to compare the rate of deterioration of paint exposed in weathering tests (artificial and natural) and paint in-service, the following are found. The SSl50 paint samples which were exposed for 1SO&2000 hours in the QUV weathering tester are equivalent to paint in-service which was subjected to approximately 12 months of exposure. The SS345 paint samples which were exposed for 2000 hours in the QUV weathering tester are equivalent to paint in-service which was exposed for approximately 24 months. The SS150 paint samples which were exposed to the natural weathering test for 36 months were equivalent to paint exposed in service for 48 months in the central and industrial areas and 60 months in the coastal area. Paint complying with SS345 was found to be more durable than paint complying with SS150. In addition, significant long-term savings are expected when paints complying with SS345 are used instead of paints complying with SS 150. AcknowledgementsThe authors are grateful to Assoc. Prof. Khoo Cheng Lim, Dean of the Faculty of Architecture and Building, and Assoc. Prof. Lim Lan Yuan, Head, School of Building and Estate Management, of the National University of Singapore (NUS) for their encouragement and support. Professor Northwood would like to thank NUS for the granting of a Visiting Senior Fellowship during the tenure of which this paper was prepared. Acknowledgements are also due to Star Chemical Pte Ltd for facilities and assistance with the accelerated weathering test and to local paint suppliers for supplying the paint for testing.
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485
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