The standard ISO 11092 uses the sweating guarded hotplate method, the skin model method, to simulate the heat and moisture transfer process close to the human skin, and to test the thermal resistance and water vapor resistance of textiles under steady-state conditions to evaluate the comfort of textiles. Several common questions on this standard are analyzed in here.
1 What is the thermal and water vapour resistance of the fabric? How to measure?
When heat passes through an object, there is a temperature difference on both sides of the object, and the ratio of the temperature difference to the power of the heat source is the heat resistance of the object, referred to as the thermal resistance, expressed as R = ( T2 – T1 ) / Q. In the actual use of objects, the area, thickness, and other factors will affect the effect of heat transfer, therefore, when it comes to the specific use of objects, we will take into account other relevant factors of the object.
Take a fabric for an example, in ISO 11092, the thermal resistance of the fabric refers to the ratio of the temperature difference on both sides to the heat flux per unit area perpendicular to the fabric.
Thermal resistance formula of fabric: Rct = S（T2 – T1）/ Q
Unit: square metres kelvin per watt, m²·K/W
Similarly, in ISO 11092, the water vapour resistance of the fabric refers to the ratio of the water vapor pressure difference on both sides of the fabric to the heat flux evaporated vertically per unit area of the fabric.
Water vapour resistance formula of fabric: Ret = S（P2 – P1）/ Q
Unit: square metres pascal per watt, m²·Pa/W
2 Test principle
The comfort of textile materials mainly includes the complex combination of heat and water vapor transmission, each process may occur individually or at the same time. The ISO 11092 standard provides a test method for the thermal resistance and water vapour resistance of textiles under steady-state conditions, better simulating the actual wear of clothing.
Thermal resistance test principle: Cover the specimen on the metal plate, the metal plate and its surrounding thermal guard, the bottom of the protective plate can maintain a constant temperature, so that the heat of the metal plate can only be lost through the specimen. After the test conditions have reached stability, the thermal resistance Rct of a sample to be measured by subtracting the thermal resistance of the boundary air layer above the surface of the test apparatus from that of a test specimen plus boundary air layer. Both tested at the same conditions.
Water vapour resistance test principle: Cover the the porous test plate with breathable but impermeable film, place the sample on the film, and the water vapor will pass through the film through the sample, but the liquid water will not. At a certain rate of evaporation, keep the heat flux required of the metal plate. The water vapour resistance Ret of a sample to be measured by subtracting the water vapour resistance of the boundary air layer above the surface of the test apparatus from that of a test specimen plus boundary air layer. Both tested at the same conditions.
3 The main unit of the sweating guarded hotplate
3.1 Measuring unit, with temperature and water supply control: Temperature controller, keep the metal plate temperature Tm constant to ±0.1℃. The water supply device ensures constant evaporation of water on the surface of the metal plate. The signal switch is connected to the metal plate. Before water enters the metal plate, let it pass through the tube in the thermal guard and preheat to the temperature of the metal plate.
3.2 Thermal guard with temperature control: prevent heat loss on the edge and bottom of the metal plate.
3.3 Test enclosure: a closed ambient box that controls the temperature, humidity, and airflow rate during the test.
The purpose of all units is to create steady-state conditions during the process of heat transfer and water vapour transfer and to provide relevant testing procedures.
4 Specimen preparation
4.1 When the specimen thickness ≤ 5mm, the specimen shall completely cover the metal plate and the surface of the thermal guard. Take at least 3 specimens for each sample, and the specimen shall be flat and wrinkle-free. Before the testing, humidifying the specimen at least 12H.
4.2 When the thickness of the specimen is more than 5mm, a part of the heat or water vapor will be lost from the edge of the specimen, which will cause the deviation of the test results during the test, which is the deviation of the linear relationship between the thermal resistance and the thickness of the specimen. The deviation can be corrected by means of formula [1+(ΔRct/Rct measured)] in the way of poly layer superposition of foam materials. Before the experiment, humidifying the specimen at least 12H.
Corrections for thermal edge losses during the testing of thermal resistance
4.3 If the sample contains loose filler or uneven thickness, e.g. quilt, sleeping bag, down jacket, etc., each sample shall be prepared with 3 specimens, and if conditions are not permitted, the actual condition of the specimen shall be indicated in the test report. If there is quilting in the center area of the material, prepare two samples at least, one more quilting and one less quilting.
5 Test procedure
According to the test principle, when testing the thermal resistance and water vapour resistance, we need to test two values separately: ① when there is no sample, the apparatus constant value under the stable state, also called the “bare plate” value, ② when place the sample on the metal plate, test the value in stable state. The thermal resistance and water vapour resistance of the specimen can be obtained by subtracting the two values.
The parameters to be used in the formula are:
Tm: temperature of metal plate Ta: temperature of test enclosure
Pm: the pressure of saturated water vapor when the temperature of the metal plate is Tm
Pa: the pressure of water vapour when the temperature of the test enclosure is Ta
H: heating power provided to the metal plate S: the area of metal plate
ΔHc: correction of heating power in thermal resistance test
ΔHe: correction of heating power in water vapour resistance test
5.1 Calculation of thermal resistance: The formula for calculating the thermal resistance is the same with specimen and without specimen, as follows:
With specimen: R (temp-specimen) = ( Tm – Ta ) · S / ( H – ΔHc )
Without specimen: R (temp-no specimen) = ( Tm – Ta ) · S / ( H – ΔHc )
Thermal resistance of fabric: Rct = R (temp-specimen) – R (temp-no specimen)
5.2 Calculation of water vapour resistance: The formula for calculating the water vapour resistance is the same with specimen and without specimen, as follows:
With specimen: R (humidity-specimen) = ( Pm – Pa ) · S / ( H – ΔHe )
Without specimen: R (humidity-no specimen) = ( Pm – Pa ) · S / ( H – ΔHe )
Water vapour resistance of fabric: Ret= R (humidity-specimen) – R (humidity-no specimen)
Other parts of ISO 11092, such as test reports, the accuracy of test results, etc., are not explained here because of their comparative basis. In addition, the operation and related parameters of the instrument can be referred to here: Sweating Guarded Hotplate, if you want to know more about the textile quality testing, click here: https://www.testextextile.com