Thermal resistance is a measure of materials and products. One of the important indicators of thermal insulation performance is also an important parameter for evaluating the thermal insulation performance of flaky materials and products made of flaky materials. In general, the fluffy fiber aggregate material or flaky material with high bulkiness has low strength and is easily deformed. High fluffy fluffy fiber aggregate material is a planar fiber aggregate of a certain thickness formed by random arrangement of linear fibers or fluffy down and other fiber materials. It needs to be wrapped with cloths and staples when used.
The fibers of the high fluffy flock material are randomly arranged in a discrete form and there is a considerableproportion of pores between the fibers, which contain a lot of still air, so that they have a high heat storage capacity. At the same time, the heat transfer channels are complicated due to the random arrangement of the fibers, which makes them very different from the heat conduction mechanism of ordinary fabrics with densely arranged fibers. For ordinary fabrics, when there is a temperature difference between the upper and lower surfaces, the vertically transmitted heat flow is the main reason because of the smaller thickness and the obvious vertical pores between the upper and lower surfaces or between the yarns. In high-bulk flaky fiber aggregate materials, due to the presence of a large number of small voids and holes in complex shapes, the bulkiness is high and the thickness is large, and the heat transfer is usually two-dimensional or three-dimensional. Therefore, the instruments and test methods for testing common fabrics at homeand abroad not suitable for testing the heat resistance of flocculation materials and their products. This article examines the current methods and instruments for testing thermal resistance, their shortcomings, and summarizes domestic and foreign standards, operating procedures and testing indicators for thermal resistance testing.
1 Existing methods and tools for testing textile heat transfer
1.1 Constant Temperature Method
Place the fabric on one side of the constant temperature heating plate, The other sides of the plate are protected by thermal insulation, and the heat required to keep the heating plateto keep at a constant temperature is measured when the sample is not placed, and when the sample is placed, and the heat retention rate of the tissue is calculated to illustrate the thermal insulation of the tissue.capable. During the test, first test the power required to maintain the constant temperature of the test plate without placing the sample, and then test the power required to maintain the constant temperature of the test plate after the sample placed was obtained by formula (1 ):
Wr=(1-b/a )×100% (1)
In the formula: Wr is the heat retention rate, %; a is the thermal power consumed by the test plate when no sample is placed, W; b is the thermal power dissipated by the test panel when the sample is placedd, W.
The single plate method is currently used to test and evaluate the thermal insulation performance of flat materials, this test method must be used at home and abroad.
But there is no protective cover over the single-plate method, so the airflow on the upper surface of the sample causes a certain amount of convection heat dissipation, the test result becomes the comprehensive value of convection heat dissipation and conductive heat dissipation. Due to the instability of the air flow in the constant temperature and humidity chamber, when the air flow speed is high in the upper part of the sample, the measured heat preservation rate is low, and the heat preservation material is usually relatively thick, and the heat dissipation is obvious at the edge of the cut sample,whereby the existing single plate instrument limits the test stability. Also GB 11048-89 Test method for thermal isolation performance of textiles 1 and FZ/T 01029-93 Test of thermal resistance in the determined of thermal resistance and moisture resistance of textiles under steady-state conditions 2 belong to the constant temperature method.
1.2 Cooling Rate Method
Cooling Rate Method It is to stop the power supply after the thermal body is heated to a certain temperature,In the case of thermal insulation on other sides,cover the fabric on one side of the radiator,or cover the radiator completely with fabric and then let it cool naturally,the Measure the time it takes for the radiator to reach a certain temperaturenature to cool down, or measure the temperature of the radiator at a certain temperature. The value of the temperature drop over time and the cooling rate is used to express the thermal insulation performance of the fabric 3. Ford (Fount) is also equipped with a pressurization device on the device to measure the thermal insulation performance of the fabric under a certain pressure. This method is faster than the constant temperature method, but can only qualitatively compare the thermal insulation properties of clothing materials, but not quantitatively determine the thermal resistance of fabrics 4.
1.3 Evaporative hotplate method
The evaporative hot plate method is the anti-sweat hot plate instrument, also known as a skin model that depicts the heat and humidity that occurscapable of simulating the transfer of activity similar to the human skin process, is the most accurate device for measuring the heat and moisture resistance of textiles 5. The anti-weld hot plate instrument includes temperature and water vapor control and measurement devices, heat protection rings and temperature control devices as shown in Figure 16.
Before testing the sample, it is necessary to test the thermal resistance constant R cto and the wet resistance constant Reto of the instrument, that is, the blank board of the instrument value. The calculation formula is as follows:
In the formula: Tm is the surface temperature of the test panel (35°C); Ta is the temperature of the air layer (20°C); Pm is at a relative humidity of 40% water vapor pressure (2250 Pa); Pa is the saturated water vapor pressure (5620 Pa); A is the area of the test panel (0.04 m2); H is the power dissipated in test plate, W; ΔHc and ΔHe are correction values.
After blank board test, place the test pattern during the actual test lies flat on the test board, the fabric that comes into contact with human skin during actual use faces the test board, as well as the multi-layer fabric, the entire test device ina climate box is laid, and the temperature and humidity in the box are strictly controlled and are not affected by the external environment.
Formulas (4) and (5) Calculate the thermal resistance and moisture resistance of the sample:
where: Rct and Ret are the measured thermal resistance and humidity resistance, respectively, of the sample.
Through the formula conversion, the instrument can also calculate the moisture permeability index, moisture permeability, Cro value and thermal conductivity, these test methods with high precision and good reproducibilityrecoverability7-8 The data acquisition system for the evaporative heating plate method is expensive, expensive to use and maintain, and the test time is long.
1.4 Flat Plate Method
The fabric is sandwiched between two constant temperature and different temperature hot and cold plates, and the heat flux through the fabric is measured with a thin plate heat flux sensor, i. H. the hot plate method. Heat transfer performance is evaluated by calculating the thermal resistance and thermal conductivity of the fabric. However, if you use this method to test the thermal insulation performance of the fabric, there will be obvious edge effects at the sample boundary 9.In order to reduce the experimental error, it is necessary to measure a material with known thermal conductivity for calibration to obtain the calibration factor. This method is not suitable for testing materials with thermal conductivity less than 0.15 W/(m× k) because at this point the calibration error is increased, so the accuracy of the instrument is reduced.
The basis of this method In addition, the researchers attached a protective plate and a bottom plate to the outside of the test plate, i.e. the heat protection plate method. The test plate, protection plate and bottom plate can all be maintained at a constant temperature of 35°C. The area of the test plate is at least 0.04 m2, generally aluminum or copper metal plate. The protective plate surrounds the test plate in a ring at least 60 mm wide with the same diameterCorner and composition as the test board and coplanar with the test board, between the two boards about 3mm wide strip of cork or other insulating materials are insulated from each other, and the protective board is used to prevent the side heat loss of the test board. The thickness and composition of the bottom plate are the same as those of the test plate and the protective plate. The bottom plate is parallel to the test plate and the protection plate, and a certain distance is kept to ensure a The role of the air chamber and the bottom plate is to prevent the heat loss of the test plate and the protection plate down the test plate and keep the same temperature, so that there is no temperature difference between the test board and the test board, to ensure the overall heat of the test board. Both traverse the sample surface vertically. This method requires no calibration and is easy to use.
The above two methods belong to the plate method. When testing thermal insulation performance, the faster the airflow in the test area and the greater the wind speed, the higher in the test results the temperature of the sample itself, the larger the proportion of thermal resistance, the higher the accuracy of the test, and the higher the sensitivity 10, and the air flow rate is small, the accuracy and sensitivity of this method is poor, the temperature of the test board easily through the environment is affected, resulting in poor repeatability of the test results, and this method takes a long time to reach a stable state, so the test time is long.