The effects of manufacturing processes and artificial accelerated weathering on the solar reflectance and cooling effect of cool roof coatings
Introduction
Since their introduction in the 1970s, cool roof coatings have attracted many researchers to carry out fundamental and applied investigations because they can greatly reduce the surface temperatures of roofs [1], [2], [3] and thereby mitigate the urban heat island effect [1], [2], [4], [5], [6], [7], [8], [9], [10], save cooling energy [3], [4], [11], [12], [13], [14], [15], [16], [17], [18], reduce carbon dioxide and ozone emissions [2], [4], [9], [11], [19], [20] and decrease the life cycle cost of a roof [6].
In the vast body of relevant literature, many scientific papers focus on the optical and thermal properties of cool roof coatings [1], [6], [21], [22], [23], [24], [25]. Quite a few journal articles seek to investigate the effects of cooling energy saving and the reduction of carbon dioxide emissions due to the applications of cool roof coatings using scale model residences and/or real residential and commercial buildings [3], [4], [11], [12], [13], [14], [15], [16], [17], [18]. However, only a few publications have reported a systematic investigation of the physicochemical properties of cool roof coatings [26]. Surprisingly, to the best of our knowledge, to date, no papers have been published on the effects of manufacturing processes on the surface morphology, microstructure, and optical and thermal properties of cool roof coatings.
The solar reflectance of a cool coating exposed to outdoor conditions has been found to change over time due to ageing, weathering and soiling, while the thermal emittance of the coating remains essentially constant [5], [22], [27], [28], [29], [30], [31]. Generally speaking, outdoor exposure includes natural soiling (deposition and growth) and natural weathering (exposure to sunlight, moisture, and variations in temperature), and it is very difficult to separate these two effects. Several studies have reported the effects of outdoor exposure on the solar reflectance of roofing materials [29], [30], [31]. Outdoor exposure was found to significantly reduce the solar reflectance of products with high initial solar reflectance, while it tended to improve the solar reflectance of those with very low initial solar reflectance [29]. The high initial solar reflectance of a white membrane roof (approximately 0.8) was found to be reduced by the deposition of soot, dust, and/or biomass (e.g., fungi or algae) to approximately 0.6; attenuated reflectances ranged from 0.3 to 0.8, depending on exposure [30]. Further studies showed that black carbon and organic carbon were the two identifiable strongly absorbing contaminants on the membranes, and they could be removed by wiping. Clear, thin layers of organic carbon and isolated dark spots of biomass might be cleared by bleach, and all of the remaining soil layer could be removed by rinsing and/or washing [30]. Based on the measured restoration of solar reflectance after washing naturally exposed cool coatings, it was concluded that the main cause of solar reflectance attenuation is not weathering, but dirt accumulation [5], [22].
With respect to the effects of natural weathering on organic cool roof coatings, it was found that the photodegradation of coatings initiated by energetic ultra violet (UV) photons in sunlight, elevated temperature due to solar energy absorption and moisture are key factors that influence the long term performance of organic materials [30]. However, to the best of our knowledge, only one study has reported the effects of these three factors on the solar reflectance of cool roof coatings to date [28]. Xenon-arc accelerated exposure cannot accurately predict the effects of natural weathering on a coating′s long term performance, but it may simulate the damaging effects of long-term outdoor exposure by exposing test samples to varying conditions of the most aggressive components of weathering light, moisture and heat. Therefore, xenon arc-accelerated exposure can serve as a useful tool to evaluate the effects of weathering on the solar reflectance of cool roof coatings.
The objective of this study was to address the lack of information regarding the effects of manufacturing processes and artificial accelerated weathering on the optical and thermal properties of cool roof coatings. To this end, samples with the same composition manufactured at different grinding speeds and times were selected. Their spectral reflectances and cooling effects were evaluated and compared. The particle size distribution, surface morphology and microstructure of these samples were then examined to explain the comparative results. In addition, eight samples with different compositions were artificially weathered for 400 h, and their spectral reflectances and thermal properties were investigated.
Section snippets
Selection of materials
To manufacture cool white coatings, the following main materials were selected: a pure acrylic emulsion, rutile titanium dioxide, hollow glass microspheres (HGM), talcum and silicon dioxide. In addition, a wetting agent, a dispersant, an antifoaming agent, a leveling agent and a coalescent were also used.
Preparation of samples
To study the effects of manufacturing processes on the optical and cooling effect of cool roof coatings, the composition of the samples was held constant (as listed in Table 1). These samples
Effect of grinding speed
Grinding in basket mills is an important technological process used to reduce the particle size of fillers in the manufacture of coatings. In addition, basket mills are also widely used to mix, blend and disperse fillers into polymer matrices as well as obtain a desired particle size distribution in the final product. The two key parameters of the grinding process are grinding time and grinding speed, which greatly affect the performance of coatings.
To determine the effect of the grinding
Discussion
As described in 3.1 Effect of grinding speed, 3.2 Effect of grinding time, the solar reflectance of cool white roof coatings was found to increase with the increase in grinding speed and grinding time; as a result, the surface temperatures of the coatings decrease as these two processing parameters increase. These observations are discussed in greater detail below.
In addition to the above-mentioned soiling on the surface, surface roughness [6], [38] and the particle size [38], [39] of fillers
Conclusions
Based on these experimental findings, the following conclusions may be drawn:
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The surface smoothness and dispersion effect of cool roof coatings improve with increasing grinding speed and grinding time.
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The solar reflectance of cool roof coatings increases with increased grinding primarily due to the improved dispersion effect.
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400 h of weatherometer exposure decreases the solar reflectance of cool roof coatings by less than 0.03.
Acknowledgments
This work was performed under the project of “Innovative Ability Building of China State Construction Engineering Corporation Limited” with funding from the National Development and Reform Commission, China.
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