Self-cleaning and de-pollution efficacies of photocatalytic architectural membranes
Graphical abstract
Introduction
Highly reflective “cool” roofs and walls can save HVAC energy, reduce afternoon peak power demand, and improve thermal comfort [[1], [2], [3], [4], [5], [6], [7], [8]]. Environmental benefits include lowering outdoor air temperature, which mitigates the urban heat island effect and slows the temperature-dependent formation of smog [[9], [10], [11], [12]], and global cooling [13]. Another benefit of cool building envelope surfaces is improving material durability by reducing damage induced by surface-temperature cycles [14]. However, those benefits can be significantly diminished over time as the albedo (solar reflectance) of building envelopes is reduced due to soiling deposition and biological growth [[15], [16], [17], [18]]. For example, an evaluation of 586 roofing materials exposed in Miami (Florida) over a 3-year period showed that the mean albedo of aged products that had an initial albedo of 0.80 or higher decreased to around 0.60, losing approximately 25 % of the initial value. In the most extreme cases, aged albedo could be as low as 0.25, corresponding to a loss of up to 70 % of the initial value [19].
Photocatalytic self-cleaning materials make building envelopes cooler by maintaining their initially high albedo values over long periods. Several photocatalytic products are used in construction, including cementitious coatings (such as mortar, plaster and stucco) [[20], [21], [22]], limestone surface treatments [23], coated metal composite siding [24], architectural membranes [25], and various roofing materials (e.g., tiles, shingles, and precast panels) [26]. These represent a growing sector of the construction market. The global sales of photocatalytic products increased from US$740 M in 2009 to US$1.5 billion in 2014, and are predicted to reach approximately US$2.9 billion by 2020 [27]. For these reasons, a closer examination of photocatalytic building envelope materials is warranted, to identify and quantify benefits and limitations.
The self-cleaning effect is due to the ultraviolet (UV) light catalyzed oxidation of deposited soiling, in combination with its physical removal due to enhanced surface hydrophilicity activated by sunlight [28,29]. Self-cleaning activity has been documented primarily in laboratory tests—e.g., by measuring the loss of deposited soot [30,31] or bleaching of a dye applied on the surface [32,33] as a function of UV irradiation. Tracking the dye bleaching rate is the basis for standardized methods that quantify the self-cleaning effect [34,35]. Superhydrophilicity (very low water contact angle) has been observed in TiO2-coated materials under UV irradiation [[36], [37], [38]]. This property is used in ISO Standard 27448 to test the self-cleaning performance of photocatalytic materials [39].
An environmental benefit that has been well documented in laboratory tests is the photocatalytic elimination of atmospheric pollutants in contact with the catalyst surface, including volatile organic compounds (VOCs) [[40], [41], [42]] and atmospheric nitrogen oxides (NOₓ = NO + NO2) [43]. In the case of NOx, photocatalytic oxidation enables a net removal of these species from the atmosphere through their irreversible conversion to the non-volatile oxidation byproducts nitrate (NO3−) and nitric acid (HNO3). The final stable oxidation byproducts can be washed off the surface by rain or dew. Various test methods have been developed to evaluate the air purification efficiency of photocatalytic materials by following NOx elimination [[44], [45], [46]]. One of the most commonly used is ISO Standard 22197-1, which relies on quantifying nitric oxide (NO) elimination under controlled air flow, temperature, humidity and illumination conditions [47].
Both self-cleaning and de-polluting properties of photocatalytic construction materials have been evaluated in a number of field demonstrations. The effective removal of NOx from urban air was demonstrated using a cement-based photocatalytic coating [22], a mineral-based paint [48], and paving materials [49,50]. However, other studies found a significantly weaker effect [51,52]. Photocatalytic performance can be affected by soiling deposition and loss of photocatalyst due to abrasion and material weathering [53,54]. Some studies report a limited depletion of the photocatalyst, with at least 80 % retention after a prolonged exposure to the environment of up to two years [55,56]. By contrast, materials in which the photocatalyst was deposited as a coating without a strong attachment to the substrate yielded higher catalyst depletion rates [54,57].
This study investigated the performance of photocatalytic architectural membranes exposed under real-world conditions. Architectural membranes are highly versatile materials used in building envelopes as energy-efficient roofs, façades, canopies, and skylights that provide diffuse natural daylight to indoor environments. Soiling of translucent membranes can reduce both albedo and the fraction of light transmitted through the material [58,59]. For that reason, photocatalytic TiO2 coatings are promising, as they have been shown to impart self-cleaning functionalities to fabrics [60]. Membranes based on fluoropolymer materials, such as those studied here, have been used as substrates for photoactive additives that imparted self-cleaning and anti-microbial activity [[61], [62], [63]]. The main goal of this study was to quantify the performance of photocatalytic membrane specimens that had been aged alongside a non-photocatalytic control material. Self-cleaning properties were quantified in terms of albedo loss, and de-polluting properties were evaluated by following NO removal rate and the NOx deposition rate as a function of exposure time. Specimens were exposed to the environment over a two-year period at three locations in California: Berkeley, downtown Los Angeles, and Fresno.
Section snippets
Exposed materials
The architectural membrane samples used in this study, manufactured under the Sheerfill® brand name, were provided by Saint Gobain. Non-photocatalytic versions of this product, made of polytetrafluoroethylene (PTFE)-coated fiberglass, have been in use for over 45 years as roofing and façade membranes. TiO2-coated photocatalytic membranes have been in the market for about 10 years. The two different photocatalytic materials tested here were labeled “P1” and “P2”, and corresponded to standard and
Weather and air pollution measurements at each site
Seasonal rain patterns at the three sites were similar, and are presented in Fig. 2. Across the three sites we observed a dry season from April to October, followed by rainy season through the late fall, winter, and early spring. Additional descriptions of weather patterns at the sites are included in the Supporting Information. We also provide air pollution results in Fig. S2 (Supporting Information).
Contact angle measurements
The water contact angle was measured on unexposed samples and on specimens retrieved after two
Conclusions
This study illustrated the performance of advanced building materials under realistic conditions over a duration sufficient to assess initial changes and seasonal effects. The materials were exposed at three sites with varying levels and chemical composition of atmospheric pollution. Both photocatalytic samples (P1 and P2) showed excellent self-cleaning performance in all three California sites and during all seasons. The photocatalyst additives can successfully protect the surface from soiling
CRediT authorship contribution statement
Xiaochen Tang: Conceptualization, Methodology, Investigation, Visualization, Writing - original draft. Olivier Rosseler: Resources, Investigation, Visualization. Sharon Chen: Investigation, Visualization. Sébastien Houzé de l’Aulnoit: Investigation. Michael J. Lussier: Resources. Jiachen Zhang: Investigation. George Ban-Weiss: Investigation. Haley Gilbert: Project administration. Ronnen Levinson: Supervision, Funding acquisition, Writing - review & editing. Hugo Destaillats: Conceptualization,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by the California Energy Commission under contract EPC-14-010. This work was also supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Building Technologies Office of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors thank David Speiser (industrial partner) and Trevor Krasowsky (University of Southern California) for assistance in specimen exposure and retrieval.
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