A novel technique for the production of cool colored concrete tile and asphalt shingle roofing products
Introduction
The widespread use of solar-reflective roofing materials can save energy, mitigate urban heat islands and slow global warming by cooling the roughly 20% of the urban surface that is roofed [1], [2], [3], [4]. North American homeowners typically select nonwhite products for pitched roofs. Solar-reflective metal and clay tile nonwhite roofing materials are readily made with “cool” pigments—colorants that exhibit weak absorption and/or strong backscattering in the near-infrared (NIR) spectrum—because metal and clay tile substrates exhibit high NIR reflectance. It is more difficult to fabricate cool concrete tile and asphalt shingle roofing products because gray-cement concrete and gray rock granules have low NIR reflectance [5]. Surface roughness further limits the reflectance of asphalt shingles [6].
We have previously explored using a white basecoat to increase the solar reflectances of concrete tile and asphalt shingle roofing products. Applying a white basecoat to a concrete tile is conceptually simple but not commonly performed in factories. Shingles are usually colored by coating loose granules which are later pressed into the surface of the shingle [7], [8]. The nature of the granule-coating process tends to limit coating thickness and thus the maximum achievable solar reflectance. The need for two complete passes through the granule-coating apparatus to place a color topcoat over a white basecoat can also halve throughput [5].
In this study we created prototype solar-reflective nonwhite concrete tile and asphalt shingle roofing materials using a two-layer spray coating process intended to maximize both solar reflectance and factory-line throughput. Each layer is a thin, quick-drying, pigmented latex paint based on acrylic or Arkema's Kynar Aquatec® aqueous polyvinylidene fluoride (PVDF)/acrylic technology. The first layer is a white basecoat with weak absorption and strong backscattering from about 500 to 2000 nm, which spans most of the visible and NIR spectra. The second layer is a color topcoat with weak NIR absorption and/or strong NIR backscattering. Each layer dries within seconds near room temperature, potentially allowing a factory line to pass first under the white spray, then under the color spray.
Section snippets
Prototype development
Small 3″×3″ coupons (7.6 cm×7.6 cm) were cut from (a) fiberglass asphalt shingles surfaced with uncoated granules and (b) uncoated gray-cement concrete tiles. Each coupon was given a titanium dioxide rutile white basecoat (Dupont TiPure R-960 at 30% pigment volume concentration [PVC]) that increased the solar reflectance of the gray-cement concrete tile from about 0.18 to about 0.79, and that of the shingle from about 0.06 to about 0.62. The surface-average dry film thicknesses (DFTs) of the
Reflectance measurement
The solar spectral reflectances (300–2500 nm @ 5 nm) of bare, white, and colored coupons were measured following ASTM Standard E903-96 [12] using a Perkin–Elmer Lambda 900 UV–vis–NIR spectrophotometer equipped with a 150 mm Labsphere integrating sphere. Solar reflectance S was calculated by weighting the solar spectral reflectance with a solar spectral irradiance characteristic of that received by a horizontal surface when the sky is clear and the sun is at zenith.1
Results
The illustrated histograms in Fig. 2 (concrete tiles) and Fig. 3 (asphalt shingles) arrange the bare (uncoated), white (white basecoat only) and colored (white basecoat + color topcoat) prototypes by solar reflectance. Each sample is labeled with its topcoat pigment (if any), solar reflectance S and lightness L*.
Solar reflectance tends to increase with lightness since about 45% of sunlight arrives in the visible spectrum [13], [15]. To create cool dark colors, we want to maximize solar
Meeting solar reflectance targets
Programs that promote energy efficient roofing materials typically require qualifying products to demonstrate a specific minimum initial solar reflectance. For example, steep-sloped roofs must have an initial solar reflectance of at least 0.25 to obtain ENERGY STAR® certification from the US Environmental Protection Agency [18]. Utility rebate programs for cool roofs have similar requirements. For example, residential steep-sloped roofing products must (among other attributes) exhibit an
Conclusions
We have demonstrated a new process for coating concrete tile and asphalt shingle roofing products that uses a two-layer spray coating to achieve high solar reflectance. The solar reflectances of the prototype cool colored tiles ranged from 0.26 (dark brown; lightness L*=29) to 0.57 (light green; L*=76); those of the prototype cool colored shingles ranged from 0.18 (dark brown; L*=26) to 0.34 (light green; L*=68). Over half of the tiles had S≥0.40, and over half of the shingles had S≥0.25. This
Acknowledgements
This work was supported by the California Energy Commission (CEC) through its Public Interest Energy Research (PIER) program and by the Assistant Secretary for Renewable Energy under Contract no. DE-AC03-76SF00098. The authors wish to thank CEC Commissioner Arthur Rosenfeld and PIER manager Chris Scruton for their support and advice. We also wish to thank Greg Peterson and Charles Schneider of Eagle Roofing Products for providing concrete tiles, and Lou Hahn of GAF-Elk for providing asphalt
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Current address: Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, Quebec, Canada.