Everyone wears clothes, and often buys clothes, we may have many criteria when choosing clothes, but most of them will mention the word “comfort”. The comfortable performance also has a variety of forms, the clothes may be soft, maybe light, and snug, and there is a comfort called “thermal and moisture comfort”, what does it mean?
The “thermal and moisture comfort” is measured by thermal resistance and water vapor resistance. Simply, thermal resistance is the ability to block heat transfer. Clothing was born to insulate and keep warm, to prevent excessive heat from escaping, and to ensure that the body is in a comfortable temperature environment role. When the temperature outside the body is lower than the body temperature, will inevitably bring heat transfer from the inside to the outside, the fabric in the middle to absorb heat, establish a ladder, and gradually stabilize the temperature difference between the two sides, which forms a steady state process requires energy is thermal resistance. The greater the value of thermal resistance, the better the warmth of the garment. Similarly, the water vapor resistance is the energy required to stabilize the fabric on both sides of the water vapor pressure differences. The greater the water vapor resistance value, the worse the perspiration of the garment, wear it on the body will feel sticky wet uncomfortable.
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What is the thermal resistance and water-vapour resistance of textiles?
The thermal resistance of a textile is an indicator of the comfort of the garment and represents the thermal performance of the textile. The higher the thermal resistance value, the better the warmth, and conversely the lower the thermal resistance value, the worse the warmth.
Thermal resistance: Rct (m²·K/W)
Temperature difference between the two faces of a material divided by the resultant heat flux per unit area in the direction of the gradient.
The test principle of textile thermal resistance: place the specimen on the test plate. The heat from the test plate can therefore only be dissipated through the specimen and the air can flow parallel to the upper surface of the specimen. After the test conditions have stabilised, the heat flow through the specimen is measured to calculate the thermal resistance of the specimen.
The water-vapour resistance value of textiles is also an indicator of garment comfort and represents the resistance of textiles to moisture. The higher the water-vapour resistance value, the greater the resistance to water-vapour and the less comfortable the garment will be to wear, and conversely the lower the water-vapour resistance value, the less resistance to water-vapour and the more comfortable the garment will be to wear.
Water-vapour resistance: Ret (m²·Pa/W)
Water-vapour pressure difference between the two faces of a material divided by the resultant evaporative heat flux per unit area in the direction of the gradient.
The principle of the water-vapour resistance test: the test plate is covered with a breathable but impermeable film. The water entering the test plate evaporates and passes through the film as water vapour, so that no liquid water touches the specimen. After the specimen is placed on the film, the heat flow required to maintain a constant temperature of the test plate at a certain rate of water evaporation is measured and the water-vapour resistance of the specimen is calculated together with the water vapour pressure passing through the specimen.
What kind of textiles need thermal resistance and water-vapor resistance test?
Products with general requirements for thermal performance, such as knitted underwear, cotton garments, quilts, and various kinds of wadding as padding, etc. Some coated fabrics will require water-vapor resistance testing because the resistance of coated fabrics is generally higher than that of ordinary fabrics, which affects the comfort of wearing clothes.
Thermal resistance test and water-vapor resistance test can present the ability of fabric heat and moisture exchange, can visually express the textile warmth performance, sweaty hot, and moisture comfort performance, so useful for the enterprise for fabric selection, and product positioning.
Difference BetweenThermal Resistance (RCT) and Water-vapor Resistance (RET)
- RCT: Thermal resistance determines if the fabric holds in heat. The higher the value obtained, the better the insulation. Rct (m²·K/W)
- RET: Water-vapor resistance determines a fabric’s ability to allow water vapor (perspiration) to pass through it. The lower the value obtained, the more breathable the fabric. Ret (m²·Pa/W)
Rct0 test, sample Rct test, Ret0 test, and sample Ret test can be performed on TESTEX Sweating Guarded Hotplate TF129.
Thermal Resistance Test Principle
Place the sample to be tested on an electric hot plate which has good thermal insulation at all the other surfaces. Transfer heat to an electric heating plate so that it reaches constant temperature state, and test the heat needed to maintain the constant temperature of the electric heating plate. This method has been adopted by ASTM as the standard number ASTM D1518.
Sweating Guarded Hot Plate Test Method
Assemble the instrument, install SGHP software according to instructions, connect with thermal resistance instrument through network signal, and prepare for the test.
The RCT0 test is used to express the basic parameter of a machine. The value of Rct0 will be used to calculate the final RCT value of a sample. Rct0 test should be done when the machine stopped for a long time or the environment changed great.
Cut the sample as 50 cm x 50 cm, and pretreat it according to the standard requirements. Place the treated sample on the surface of the test area. If the thickness of the sample is more than 1 mm, then adjust the height of the wind speed sensor so that the height between the sample and the sensor is about 15 mm. According to the standard requirements to set the temp and humidity and then start the test until the testing environment is stable. Record the test results and calculate the final RCT values of the sample.
Test standards for testing thermal resistance and water-vapour resistance of textiles
There are various test methods for the determination of thermal resistance and water-vapour resistance of textiles.
The thermal resistance test usually uses the static flat plate method and the tube insulation instrument method. The test methods for water-vapour resistance include the control pour cup method, desiccant pour cup method, sweating hot plate method in the skin model method and sweating warm dummy method, etc. The static flat plate method and evaporating hot plate method are more popular, and there are nearly ten test standards based on these two methods. The test methods in these standards are basically the same, but the differences lie mainly in the influencing factors, experimental conditions and the way the results are expressed. The common test standards are as follows.
ISO 11092 Textiles – Physiological effects – Measurement of thermal and watervapour resistance under steady-state conditions (sweating guarded-hotplate test)
ASTM F1868 Standard Test Method for Thermal and Evaporative Resistance of Clothing Materials Using a Sweating Hot Plate
GB/T 11048 Textiles – Physiological effects – Measurement of thermal and watervapour resistance under steady-state conditions (sweating guarded-hotplate test)
GB/T 35762 Textiles – Test method for thermal transmittance – flat plate test
The following summarises the differences between the test conditions of several test standards, which is used to help the industry understand the operation and main differences between the various test methods.
The testing environment and influencing factors of textile thermal resistance and water-vapour resistance
The main factors influencing the test results are the temperature of the test panel (including the test panel, heat shield and test floor), the ambient temperature and relative humidity and the air flow rate. The following diagrams give a clear picture of the differences in the test environment for each standard.