Robert Waldron, BASF Corporation, Charlotte, North Carolina. 150 years

Transcription

1 Heat Management Pigments in Solid Color Deck Stains Exterior Exposure Surface Temperature Profiles as a Function of Total Solar Reflectance and Solar Power Robert Waldron, BASF Corporation, Charlotte, North Carolina 1

2 Agenda Heat management coatings background Near-infrared (NIR) reflective complex inorganic color pigments (CICPs) Laboratory studies review Total solar reflectance (TSR) improvement Differential surface heat build-up measurements Accelerated weathering exposure results Field test project results Wood deck simulation Diurnal surface temperature profiles as a function of TSR and solar power for pressure-treated pine boards coated with solid color wood stains Assessment of the wood deck cooling effect achieved using NIR-reflective coatings 2

3 Heat Management Coatings Background White roof coatings have been the primary focus reduce urban heat island effects and produce energy savings Coatings tinted with TiO 2 Maximal NIR scattering, minimal thermal energy absorption TSR on the order of 80% What about colors other than white, particularly darker shades (e.g., gray, brown)? Carbon black is the heat management antithesis of TiO 2 Strong NIR absorber, TSR on the order of only 4% Large TSR reductions result when even small amounts of carbon black are used in a coating 3

4 Heat Management Coatings Background Cool color options besides white are desirable for aesthetic reasons CICPs reflect NIR radiation to varying degrees CICPs have been used in architectural coatings to increase TSR and reduce heat build-up on coated surfaces. Darker color shades, normally strong NIR absorbers, can be made to perform like cooler, lighter shades Potential applications include: Earth-tone colors popular in the southwestern United States where solar irradiance is particularly strong Common dark-shade deck coating colors like brown, gray and redwood 4

5 Laboratory Study Set of relatively dark color standards re-matched using Pigment Brown 29 in place of carbon black PBr 29 is an NIR-reflective CICP 5

6 Laboratory Study Example Oxford Brown solid color deck stain Two substrate types pressure-treated pine cedar Color match formulas PY 42 (iron oxide yellow) and PR 101 (iron oxide red) pigments used in both standard and NIR-reflective coatings Standard coating carbon black NIR-reflective coating PBr 29 6

7 Laboratory Study TSR TSR measurements Devices & Services Co. solar spectrum reflectometer model SSR-ER Pressure-treated pine panels Standard stain TSR = 10.5% NIR-reflective stain TSR = 31.2% Cedar panels Standard stain TSR = 10.3% NIR-reflective stain TSR = 32.1% 7

8 Laboratory Study Heat Build-up Measurements Test Apparatus 250 Watt infrared heat lamps Surface temperature measured using an infrared thermometer (Cen-Tech 96451) 8

9 Laboratory Study Heat Build-up Measurements HM = heat management, NIR-reflective pigments 9

10 Laboratory Study Accelerated Weathering Xenon-Arc WOM Stained wood panels were exposed after allowing sufficient time for full curing of the applied stains. Test specification ASTM Method G minutes of light (0.35 W/m 340 nm) black panel temp. = (60+2) C and relative humidity = (50+5) % at the end of the dry period 18 minutes of light and spray with black panel temp. = (40+2) C relative humidity = (95+5) %. 10

11 Laboratory Study Accelerated Weathering Pressure-treated pine after 1000 hours exposure Unexposed Conventional Stain HM Stain 11

12 Laboratory Study Conclusions Impact of NIR-reflective pigments Total solar reflectance is significantly increased Heat build-up is significantly reduced Color retention is improved under accelerated weathering conditions Implications Equilibrium surface temperature reductions of up to 20 F are possible Reduced peak surface temperatures and/or reduction in the amplitude of diurnal temperature variations may prolong coating service life 12

13 Field Test Study Rationale Coated wood equilibrium surface temperatures and durability under accelerated weathering conditions indicate relative performance only Realistic assessment requires exterior exposure testing Solar power is not constant: varies throughout each day, from day to day, and from season to season Solar irradiance varies when the sun is temporarily obscured by passing clouds Wind causes convective heat transfer from the coated surfaces (effect increases with increasing wind speed) Precipitation and humidity also play a role 13

14 Field Test Study Objectives and Methods Measure real-world surface temperature profiles of boards coated with solid color wood stains: Colors: dark brown and gray Stain base: commercially-available 100% acrylic clear Coatings: for each color, 2 boards were coated with conventional stain and 2 with NIR-reflective stain Conventional stains tinted using carbon black and various oxides NIR-reflective stains tinted using PBr 29 in place of carbon black Brown: conventional TSR = 6.7%, NIR-reflective TSR = 19.8% Gray: conventional TSR = 19.3%, NIR-reflective TSR = 42% Boards mounted horizontally on test fence at BASF s Charlotte Technical Center Temperature Measurement: one self-adhesive T-type thermocouple attached to the surface of each board Each thermocouple fixed in place and shielded by applying the appropriate tinted stain over the selfadhesive pad Temperatures recorded 24/7 using an Omega OM-CP-OCTTEMP-A data logger Thermocouple temperature measurements cross-checked against hand-held infrared thermometer readings 14

15 Field Test Study Objectives and Methods (Continued) Correlate surface temperature profiles with solar irradiance (W/m 2 ) Anaheim Scientific Model H115 solar power meter mounted adjacent to the panels on the test fence Compare color fade and coating durability with and without NIR-reflection Periodic color measurements made using hand held X-Rite SP64 spherical spectrophotometer D65 illuminant, 10 observer, specular component included Periodic visual observations of coating condition (cracking, adhesion, etc.) 15

16 Exterior Exposure Setup Test Fence Installation 16

17 Gray Boards Example 1 Surface Temperature Profiles 17

18 Gray Boards Example 1 Surface Temperature Difference 16.0 Gray Boards Temperature Difference, F Temperature Difference Time 18

19 Gray Boards Example 2 Surface Temperature Profiles 19

20 Gray Boards Example 2 Surface Temperature Difference 20

21 Brown Boards Surface Temperature Profiles 21

22 Brown Boards Surface Temperature Difference 22

23 Critical Role of TSR Darker Color with NIR Reflectance as Cool as Lighter Color Without 23

24 How Significant is the Cooling Effect with NIR-Reflective Pigments? Field Test Results Summary: A deck temperature cooling effect on the order of 12 F can be expected during mid-summer. The effect may be somewhat larger or smaller depending on the intensity of solar irradiance impacting the deck at any given time. For boards coated with conventional stain, a typical peak surface temperature would be about 150 F. The peak surface temperature for boards coated with NIR-reflective stain would be about 138 F. Is this cooling effect large enough to improve deck user comfort? 24

25 Touch Temperature Limits for Skin Contact with Hot Objects Ungar and Stroud method to predict the touch temperature at the onset of pain as a function of: Contact time Thermal diffusivities of the materials involved For any commonly used material and all contact times, onset of pain will not occur if the temperature is < 113 F If the material temperature > 113 F, the allowable temperature for any given contact time depends on the thermal diffusivity of the specific material involved 25

26 Thermal Diffusivity Thermal diffusivity is the ratio of thermal conductivity to volumetric heat capacity Measures how quickly a material responds to a change in temperature through unsteady heat conduction Aluminum thermal diffusivity = (mm) 2 /s Lower thermal inertia Conducts heat rapidly to lower-temperature materials in contact with it Wood thermal diffusivity = (mm) 2 /s. Higher thermal inertia Conducts heat to cooler materials more slowly. Consider samples of aluminum and wood maintained at the same elevated temperature (e.g., above 113 F). The aluminum will feel hotter than the wood even though both are at the same temperature. 26

27 Thermal Diffusivity α = K / (ρ c p ) Where: α = thermal diffusivity, m 2 /s K = thermal conductivity, J/m C s ρ = density, kg/m 3 c p = specific heat capacity, J/kg C 27

28 Hot Touch Temperature Model Applied to Wood Ungar and Stroud model applied to wood: The hot touch temperature limit for essentially unlimited contact time is 156 F Even without an NIR-reflective coating, one would not expect the deck surface to be unwalkable with bare feet Still, a deck with an NIR-reflective coating should feel cooler to the touch This cooler sensation may or may not be sufficient to impress wood deck users 28

29 Exterior Weathering Panels have been on the test fence for just over one year Color measurements and visual inspections are continuing on a monthly basis Still too early to draw any definitive conclusions regarding the impact of NIR-reflectance on coating service life Plans are to publish color fade and coating integrity data after two years exposure Analogous new project is underway to assess application of NIRreflective coatings to wood/plastic composite decks 29

30 Thank You for Your Attention Bob Waldron THE DESCRIPTIONS, DESIGNS, DATA AND INFORMATION CONTAINED HEREIN ARE PRESENTED IN GOOD FAITH, AND ARE BASED ON BASF S CURRENT KNOWLEDGE AND EXPERIENCE. THEY ARE PROVIDED FOR GUIDANCE ONLY, AND DO NOT CONSTITUTE THE AGREED CONTRACTUAL QUALITY OF THE PRODUCT OR A PART OF BASF S TERMS AND CONDITIONS OF SALE. BECAUSE MANY FACTORS MAY AFFECT PROCESSING OR APPLICATION/USE OF THE PRODUCT, BASF RECOMMENDS THAT THE READER CARRY OUT ITS OWN INVESTIGATIONS AND TESTS TO DETERMINE THE SUITABILITY OF A PRODUCT FOR ITS PARTICULAR PURPOSE PRIOR TO USE. IT IS THE RESPONSIBILITY OF THE RECIPIENT OF PRODUCT TO ENSURE THAT ANY PROPRIETARY RIGHTS AND EXISTING LAWS AND LEGISLATION ARE OBSERVED. NO WARRANTIES OF ANY KIND, EITHER EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, ARE MADE REGARDING PRODUCTS DESCRIBED OR DESIGNS, DATA OR INFORMATION SET FORTH HEREIN, OR THAT THE PRODUCTS, DESCRIPTIONS, DESIGNS, DATA OR INFORMATION MAY BE USED WITHOUT INFRINGING THE INTELLECTUAL PROPERTY RIGHTS OF OTHERS. ANY DESCRIPTIONS, DESIGNS, DATA AND INFORMATION GIVEN IN THIS PUBLICATION MAY CHANGE WITHOUT PRIOR INFORMATION. THE DESCRIPTIONS, DESIGNS, DATA, AND INFORMATION FURNISHED BY BASF HEREUNDER ARE GIVEN GRATIS AND BASF ASSUMES NO OBLIGATION OR LIABILITY FOR THE DESCRIPTIONS, DESIGNS, DATA OR INFORMATION GIVEN OR RESULTS OBTAINED, ALL SUCH BEING GIVEN AND ACCEPTED AT THE READER S RISK. = Registered trademark of BASF Group. 30