Predicting Maximum Field Service Temperatures From Solar Reflectance Measurements of Vinyl
Henry K. Hardcastle III
Atlas Weathering Service Group
Vinyl products continue penetrating Western US markets. Vinyl products may show unacceptable heat distortion when installed in Western environments even after demonstrating a long tradition of acceptable heat build performance in Eastern US environments. This paper presents a methodology for predicting maximum field service temperatures from solar reflectance measurements. Solar reflectance data (ASTM E-903 and E-892), field measurement data and a predictive model for a variety of vinyl systems are shown. This methodology may be used in addition to ASTM D-4803 and is not limited to vinyl materials.
A number of vinyl building product manufacturers are familiar with The Standard Test Method for Predicting Heat Buildup in PVC Building Products according to ASTM D 4803 which utilizes an insulated box to house a specimen irradiated by an IR heat lamp. Many vinyl producers may not be familiar with the basis of this test or the direct measurements that can be made to predict the propensity for heat buildup. Recent failures of rigid vinyl materials due to heat buildup and heat distortions have been observed even though ASTM D-4803 analysis indicate acceptable performance. These materials have also displayed satisfactory heat buildup performance in historical markets. Sales and subsequent failures of these products in newer Western US markets imply an environmental constraint not found in traditional eastern geography's and a possible limitation to the D-4803 method. Failures that initiated this study have been focused around areas with higher solar irradiance in the Southwestern US.
Statement of Theory and Definitions
The Solar Spectrum
The solar spectrum is a depiction of the energy from the sun that irradiates a material. Due to filtering effects of the atmosphere more than 98% of the sun's energy that strike the earth's surface are between 300 and 2500 nm. The irradiant energy at any particular wave band within this spectrum is highly dependent on the amount and quality of atmosphere the energy travels through before striking the material.
- There are several different agreed upon solar spectrums.
- One of the major differences is the amount of atmosphere the energy must travel through.
- Another difference is the amount of direct vs. diffuse light irradating the surface.
- Three major solar spectrums defined are Air Mass 1.5 Direct, Air Mass 1.5 Global and Air Mass 0 as shown in Figure 1.
- There may be other sources of irradiance besides the sun contributing to heat build including; shingles that are reflecting or re-radiating at long wavelengths, low E glass, bar-b-que grills, pool decks and other good absorbers, emitters or reflectors of solar energy. Often these features may concentrate solar energy or re-radiate absorbed solar energy at longer wavelengths and contribute additional energy for heat buildup.
Vinyl Optical Properties
Optical properties can be characterized using the relationship:
1 = r + t + a . (1)
The relationship simply states that the total irradiance striking a material will either be reflected off the material, transmitted through the material or absorbed by the material. It is the absorbed solar energy that is available for heat buildup.
- The relationship becomes even more simple if the material is opaque (t = 0). It is important to consider a materials optical properties through out the entire solar spectrum (approximately 300 to 2500 nm) rather than just the visible spectrum or just the IR spectrum since about half of the solar energy is composed of wavelengths less than 780 nm and half the solar energy lies above 780 nm.
- Some materials that have low absorptance in the visible portion of the solar spectrum may have high absorptance in the IR region. Pigment manufacturer's take advantage of this fact and produce many products often referred to as "IR reflective pigments" that appear dark in visible light but are highly reflective in the IR and therefor remain cooler than similar colors made with traditional pigments.
Description of Equipment and Processes
Measurement of Optical Properties
Measurement of reflectance and transmittance optical properties is easily accomplished using modern commercially available spectrophotometers.
- It is important that the spectrophotometer have the ability to scan the majority of the solar spectrum from approximately 300 to 2500 nm.
- The geometry of the measurement, incident and reflected angle of spectrophotometer beams, reference beams and use of integrating spheres are important considerations of these measurements especially when comparing optical properties measured using different configurations or instruments.
- Measurement geometry and front end optical designs are well documented in ASTM E903 for these measurements.
- The initial results of these optical properties measurements is typically a spectral reflectance or transmittance curve showing the %r or %t at each wavelength as a graph as shown in Figure 2.
Integration of Optical Properties to The Solar Spectrum
Once a measurement of the percent reflectance and percent transmittance of the material at each wavelength is obtained through out the solar spectral region (300 - 2500 nm) the optical properties of the material may be related to the sun's irradiance by integration.
- Integration is a mathematical weighting process that takes into account both the sun's irradiance and the material's reflectance at each wavelength from 300 to 2500 nm. Integration weights regions of the material's optical properties spectrum according to the energy output from the sun in those regions.
- Once the sun's irradiance and material's optical properties are integrated at each wavelength, the total of reflected solar energy may be summed resulting in a single number denoted as "total percent solar reflectance" for the air mass used. Percent solar absorptance is then calculated:
a =1-(r +t). (2)
- It is the value of percent solar reflectance and the calculated percent solar absorptance that is powerful in predicting a materials propensity for heat build.
- For opaque materials such as a rigid vinyl, colors with high solar reflectance will remain cooler than colors with low solar reflectance under the same environmental conditions.
- For materials with the same emmittance characteristics, materials with higher solar absorptance will have a greater propensity for heat build. Materials with lower solar absorptance should remain cooler for similar materials under the same solar and ambient conditions.
Application of Equipment and Processes
There appear to be 4 main steps to using the solar spectrum, optical property measurements and solar integration; 1) Define the temperature failure criteria for the material. 2) Obtain empirical heat build up data for a number of material colors in worst case environments. 3) Measure the optical properties of the material colors and plot correlation regression between solar absorptance and worst case empirical heat build data noting where the regression line crosses the failure criteria. 4) Consider the risks involved with selling products which measure above the critical solar absorptance characterized in the previous step. An example will demonstrate use of these four steps.
Example of Methodology
A producer offers a variety of different colors in the same PVC base. Colors are formulated by altering the pigments and TiO2 content. In this example, the producer has no prior knowledge of field performance but wants to determine the maximum solar absorptance he can design and still have acceptable heat buildup performance.
1) Define the temperature failure criteria for the material. The producer determines experimentally the maximum service temperature his material can achieve and still provide acceptable performance. The producer determines the heat deflection temperature (ASTM D 648), Vicat Softening Temperature (ASTM D 1525), Coefficient of Thermal Expansion (ASTM D 696) or other appropriate quantitative measure of material's performance under heat. The producer then adds a suitable safety factor to the temperature determined to cause failure.
2) Obtain empirical heat build up data for a number of material colors in worst case environments. The producer obtains a number of samples of different colors of his material and exposes them to the worst case environment in his intended market. This environment should have the highest solar irradiance and warmest temperatures the product may be subjected to while in service. The samples should be oriented for exposure resulting in the maximum heat build; oriented normal to sun, protected from breezes and insulated from convective and conductive cooling as much as appropriate for the product. Consideration should also be given to reflective surfaces and other heat sources the product may encounter in the field. The producer then carefully measures the temperatures the selected samples reach under these worst case conditions using thermocouples, pyrometers or other suitable temperature measuring and data logging instrumentation. The temperature measurements are made simultaneously for all specimens to block differences in environmental variables as shown in Figure 3.
3) Measure the optical properties of the material colors and plot correlation regression between solar absorptance and worst case empirical heat build data noting where the regression line crosses the failure criteria. The producer then measures the solar optical properties of the samples used to obtain the worst case heat build temperatures and calculates solar absorptance. An x-y scatter plot is then constructed with maximum temperature on the ordinate and solar absorptance on the abscissa. The regression line is fitted to the data. The temperature failure criteria from step 2 is marked on the ordinate scale and a line is extended to intersect with the regression line as shown in Figure 4. The point of intersection with the regression line is then extended down to the abscissa and solar absorptance indicated becomes the design criteria for new products.
4) Consider the risks involved with selling products which measure above the critical solar absorptance characterized in the previous step. Products made with higher solar absorbtances have a higher risk of exceeding the defined temperature failure criteria determined in step 1. Again, the producer may choose to utilize IR reflecting pigments to produce dark colors or decide the cost and risk outweigh the revenues from color offerings with higher solar absorptance.
Presentation of Data and Results
Actual Case Study Data
The company had been selling traditional colors of window lineals since the mid 1980's with acceptable heat build performance. Colors sold included white, beige and brown in standard rigid vinyl formulations. The company utilized ASTM D 4803 to evaluate heat buildup before a new color's introduction. In the past several years, however, several things happened that increased risk of heat related failures; 1) Markets moved westward from northeastern and southeastern and central US areas to southwestern US markets such as Denver, Las Vegas, Phoenix, Southern California, etc. 2) Customers began demanding new custom colors including very dark colors. 3) The company began experimenting with new formulations including different types and amounts of lubricants, stabilizers, impact modifiers, and colorant vehicles. 4) Heat related problems became a focus of discussions among building product producers and suppliers.
The challenge for the R&D effort was to develop a methodology to predict propensity for heat buildup for new experimental formulations in addition to the D4803 method. The new method needed to be empirically based and applicable to the data base of performance already available (e.g. customer complaints and historical product offerings). Finally, this method needed to provide decision makers with a clear indication of new products performance before release to the markets.
1) Define the temperature failure criteria for the material.
The experimental formulas were blended and extruded. The extruded products were measured for heat deflection temperature using ASTM D 648 as a guideline. Multiple measurements at various heating rates were conducted. An appropriate engineering safety factor was applied to the data. A critical temperature failure criteria was defined as 70° C for these particular experimental formulas. 70° C was considered the maximum sustained temperature the extrusions could withstand and still provide acceptable engineering performance.
2) Obtain empirical heat build up data for a number of material colors in worst case environments.
A collection of 11 specimens representing the range of current product offerings and R&D efforts was selected. The materials were mounted in a single standard frame, side by side. The specimens were similar in thickness and dimension. The frame and specimens were mounted over standard building insulation to prevent back side cooling and surrounded by wind baffles to reduce cooling due to breezes. Thermocouples attached specimens to a simple data logger. The specimens were exposed directly to sun at Phoenix, AZ near summer solstice 1997 at near normal angles. Measurements were taken continuously for several days. The maximum temperature achieved by all specimens at the same time was recorded. An example of this data is shown in Figure 3. These values were then described as the best estimate of heat buildup for the colors in a worst case environment.
3) Measure the optical properties of the material colors and plot correlation regression between solar absorptance and worst case empirical heat build data noting where the regression line crosses the failure criteria.
Each of the materials was then measured using ASTM E 903 and integrated using ASTM E 892. Each material was opaque. The percent solar absorptance was calculated for each material. The solar absorptance vs. maximum heat build were plotted in x-y scatter plot format and fitted with a regression line. The temperature failure criteria was noted on the temperature scale and extended to the regression line. The point of intersection denoted the maximum % solar absorptance that could be achieved by the system and still provide acceptable heat buildup performance as shown in Figure 4. For this formulation, the maximum solar absorptance should not exceed 40% a critical value.
4) Consider the risks involved with selling products which measure below the critical solar reflectance characterized in the previous step.
The critical solar absorptance value of 40% became a clear design criteria for current and new color product offerings in this system.
Interpretation of Data
The empirically derived maximum temperature vs. solar absorptance regression shown in Figure 4 becomes an important tool for new product designers using this vinyl system. Different colors produced in this formulation can be identified on the regression by simply measuring their solar reflectance and calculating their solar absorptance value. Once a custom color is matched, a sample is immediately submitted for solar reflectance measurements. If a pigmentation system used to achieve a custom color results in solar absorptance values above the critical value, decision makers know the probability of heat related complaints will increase in severe environments.
Summary and Conclusions
Use of empirically derived heat buildup data and optical properties measurements can significantly improve a producer's ability to predict maximum field service temperatures of vinyl materials. Use of empirical field methods described here in addition to laboratory tests can identify robust design criteria, enhance a product's service performance and ultimately contribute to customer satisfaction.
The Author would like to acknowledge Dayton Technologies for permission to publish this work.
Key Words: Solar, Absorptance, Heat, Vinyl