The solar reflection ratio test method and influence factors of thermal reflection coating are discussed

0 Introduction
In order to eliminate the adverse effects of intense solar thermal radiation on objects and reduce energy consumption, solar thermal reflection coatings were invented in the 1970s. After decades of development, the current thermal reflective coating has been successfully applied to aerospace, construction, petrochemical, military, industrial and other fields, its significant energy-saving effect has been widely recognized by the industry. Solar reflection ratio, sometimes referred to as solar reflection ratio, thermal reflectivity, directly reflects the reflection effect of thermal reflection coating, is the most critical technical index to judge its thermal performance, how to establish a scientific, reasonable and applicable solar reflection ratio test method and its influence factors for in-depth analysis, thermal reflection coating, an energy-saving material scientific research, development and application and its product standards and specification process must be resolved in the important issues
China

1 Solar reflection ratio test standard and its application
thermal reflection coating originated in Europe and the United States, in the long-term scientific research and application process, foreign developed countries, especially the United States has developed a number of standards on the thermal performance testing methods of reflective materials, for the study of thermal reflection materials to provide a unified standard test basis. The domestic thermal reflection test method standards are mainly concentrated in the military industry related standards are as follows:
GB/T 25968
-2010 dicing photomeometer measurement material solar transmission ratio and solar absorption ratio test method;
GJB 5023-2003 material and coating reflectivity and emission rate testing methods;
ASTM C1549 Standard Test Method for Solar Reflections Near Ambient Temperature Using aPortable Solar Reflectometer;
ASTM C19188 Standard Test Method for Measuring Solarance Reflectorance Horizontal and Low-Sloped Surfacesin the Field;
ASTM E903 Standard Test Method for SolarAbsorptance, Reflectance, and Transmission of MaterialsUsing Integrating Spheres;
JIS K5602 Filmed Solar Reflectivity Test Method.
the above solar reflection ratio test standard application status can be found in Table 1.
-reflection ratio test method Apply the
2 solar reflection ratio test method
the solar reflection ratio is the ratio of the solar radiation ratio of the object reflected into hem space to the solar radiation pass on the surface of the object. The solar reflection ratio is the most important technical index to evaluate the thermal reflection performance, and it is also one of the thermal parameters that must be entered when calculating the energy saving of thermal reflection coating. There are more solar reflections than test methods, and here are two commonly used methods.
2.1 spectroscopy
Using ultraviolet, visible and near-infrared spectropolometers with integral spectrometers, the reflection ratio of different wavelengths of the material can be accurately measured, and the ratio of solar reflection in the above wavelength range is calculated by weighted average according to the relative energy distribution of sunlight over a certain wavelength range. According to the test principle, spectroscopy can be divided into absolute and relative methods.
Absolute method is to calculate the absolute value of the solar reflection ratio by way of the test sample at a wavelength of i, and in
, ρs is the solar reflection ratio of the sample ρλi; The spectral reflection ratio of the i-time sample, the ES(i) is the solar irradiance at the wavelength i, the i is the wavelength interval, and n is the number of measuring points in the range of 300 to 2,500 nm, generally not less than 50.The
-relative law is to test the spectral reflection ratio of the sample relative to the standard whiteboard at wavelength i, and to calculate the solar reflection ratio of the sample according to formula (2):
-in ρ0λi is the spectral reflection ratio of the standard whiteboard when the wavelength is i, and ρbλi is the spectral reflection ratio of the wavelength to the standard whiteboard. Standard whiteboards are usually ceramic and Halon plates and need to be inspected regularly by the metering department. Relativity is the most common method used to measure the solar reflection ratio of a material with a 10guangometer. The solar reflection ratio test method in the current thermal reflection coating standard in China uses GJB 2502 "spacecraft thermal control coating test method", and the referenced solar spectrum is a solar spectrum outside the
astronomical unit of the space industry standard QJ 1954 "Solar Electromagnetic Radiation" Irradiance, which differs greatly from the intensity of solar radiation on the ground, makes it more reasonable to choose a solar spectrum near the ground, for example using ISO9845-1 Solar Energy - Near-Surface Solar Spectral Radiation under Different Reception Conditions - Part 1: Direct exposure to air quality 1.5 and total solar radiation. ASTM E903 and JIS K5602 are also the method criteria for spectroscopy to determine the solar reflection ratio of thermal reflection coatings, both of which use the same solar spectrum as ISO 9845-1. The application of spectroscopy to determine the reflection ratio of the sun can objectively reflect the reflection characteristics of the material to different bands of the solar spectrum, and the test accuracy is high and the environmental impact is small, which is generally recognized as the ideal test method for studying thermal reflection materials. However, due to ultraviolet, visible light, near-infrared spectrometer is more expensive, only large test institutions and powerful scientific research units can afford, its application has been limited.
2.2 Portable Instruments Act
The American Material Testing Association standard ASTM C1549 provides a method for determining the solar reflection ratio at room temperature with a portable reflectivity tester.
figure 1 is the reflectivity tester appearance that complies with asTM C1549.
a portable reflective ratio tester
which consists of a measuring head, a reading module, a calibration device (black cavity, standard plate), a rechargeable battery pack, etc.
diffuse tungsten halogen lamp in the measuring head irradiates a flat sample of 2 s over a measurement period of 10 s. The radiation reflection with an incoming angle of 20 degrees is measured by 4 detectors in the measuring head. Each detector is equipped with a filter that allows it to generate electronic sensing with specific bands of the solar spectrum. These electronic sensings are processed by the reading module to produce a solar reflection ratio at different air quality (Air Mass0, 1, 1.5, 2.0). The instrument uses a black cavity to proof zero and is calibrated using a standard plate with a known reflection ratio provided by the manufacturer. The surface under test is close to the opening of the instrument measuring head, keeping the position unchanged, and the reading module displays a constant reflection ratio. The method has the following characteristics:
(1) measurement accuracy is high, the number of display resolution is 0.001. The thermal reflection coatings with different reflection ratios were compared with spectrographs and portable reflectivity testers respectively, and some results are found in Table 2.
portable reflection ratio tester and spectroometer test results compared to
through Table 2, it can be seen that the ASTM C1549 method and integration method test results are very close, high credibility.
(2) Flexible measurements
The instrument not only measures the reflection ratio of samples in the laboratory, but also, with the help of solar cells, can also measure the solar reflection ratio of surfaces such as building walls, roofs, and floors outdoors. In addition, by using a number of special calibration methods, the instrument is able to test the solar reflection ratio of the cylinder, and therefore can be used for solar reflection ratio testing of materials such as roof thermal reflection tiles, special-shaped thermal reflection metal plates, etc.
(3) Select a variety
through the keys on the reading module, you can select the measured solar radiation spectrum, the air quality can be 0, 1, 1.5 and 2, can be direct radiation, or total radiation. Options such as Table 3.the
option and description The infrared detector in the
measuring head can sense the near-infrared band in the solar spectrum to measure the near-infrared reflection ratio of the material, and the test analysis of the near-infrared reflection ratio is one of the essential conditions for the study of color thermal reflection coatings.
3 Solar reflection ratio
For coatings, the thermal reflectivity of coatings can be greatly improved by selecting the right raw materials (including resins, pigments, fillers, etc.), controlling the pigment particle size, and designing a reasonable formulation. In recent years, the domestic thermal reflection coating formulation and process of more research, here will not be repeated. When testing the solar reflection ratio of a kind of thermal reflection coating, some physical factors, such as coating surface status, coating thickness, substrate type, etc., also affect the size of the thermal reflectivity value to some extent. Using the portable reflectivity tester described in 2.2, the effect of the above factors on the coating sun reflectivity is tested and analyzed.
3.1 Effect of substrate on solar reflection ratio
Several commonly used building materials are selected as plate substrates, the surface is coated with a watery white thermal reflection coating, the test results are found in Table 4.
effect of substrate on solar reflection ratio
There are some differences in the reflection ratio of different substrate coatings, and when the coating film is thinner, the more obvious the difference, i.e. the greater the impact of substrate on the coating reflection ratio. When the coating is thin, radiation can penetrate the coating to the base, reflecting energy will be affected by the type of material at the base and the condition of the base surface. Therefore, in the practical application of thermal reflection coating, in order to avoid the influence of the grass on thermal reflection, a certain coating thickness should be guaranteed, so that thermal reflection coating to play the greatest insulation function.
3.2 Effect of coating thickness on solar reflection ratio
use aluminum alloy plate as substrate, coat a water-based white thermal reflection coating, test the solar reflection ratio of thermal reflection coating of different thicknesses (Figure 2 solar reflection ratio 1); Comparing the effect on the solar reflection ratio of the thermal reflection coating, the black coating with a reflection ratio of 0.05 is sprayed on the aluminum alloy plate, and the same white thermal reflection coating is applied on the black coating, and the solar reflection ratio of the thermal reflection coating of different thicknesses is also tested (figure 2, the solar reflection ratio is 2). As can be known from Figure 2, with the increase of the thickness of the thermal reflection coating, the solar reflection
ratio is on the rise, and when the thickness of the coating reaches a certain degree (about 150 m), the reflection ratio reaches the highest value and does not change. When the coating is thin, the thermal reflection ratio of the thermal reflection coating using the low reflectivity substrate is significantly lower than that of the high reflectivity substrate, and with the increase of thickness, the difference becomes smaller and smaller, and finally tends to be consistent. It is shown that the thinner the coating, the greater the effect of the reflectivity of the substrate on the sun reflectivity of the coating.
Coating thickness and solar reflection ratio Diagram
3.3 Effect of coating surface state on solar reflection ratio
different coating processes are used to make the surface of the thermal reflection coating appear different roughness, and the solar reflection ratio is shown in Table 5.
Soil reflection ratio of different surface states
For the same coating, rough surfaces have slightly higher sun reflection ratio than flat surfaces, but the difference is small, and surface state has little effect on the coating sun reflection ratio.
4 Conclusion
In order to evaluate the energy-saving effect of thermal reflection coatings, a scientific method is needed to test the solar reflection ratio as a key thermal performance indicator. Compared with the research on the formulation of thermal reflective coating, the study of solar reflectivity test method is relatively lagging behind, and the relevant institutions should actively study the thermal performance test method of thermal reflection coating, especially the solar reflection ratio test method and its influence factors, and introduce the corresponding test method standards in due course.