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FAQ on Thermal Resistance Testing and Water-vapour Resistance Testing of Textiles

The conductivity of textiles to temperature and humidity is the most intuitive and important influence on human comfort. Different textile materials have different properties and uses, so how do you define the conductivity of a fabric to temperature and humidity? The most commonly used tests are the thermal resistance test and the water-vapour resistance test of textiles.

What is the thermal resistance and water-vapour resistance of textiles? What is the principle of the test?

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 are the precautions for testing the thermal resistance and water-vapour resistance of textiles?

In the thermal resistance test, the temperature and humidity and the wind speed have a certain influence on the thermal resistance value, so we should pay attention to the control of temperature and humidity as well as wind speed. In addition, the size of the specimen should cover both the test plate and the protection plate. The specimen should be flat and wrinkle-free, usually with the side in contact with human skin facing the test plate, and the stabilisation time needs to be extended appropriately according to the thickness of the sample in order to make the sample fully stable.

During the water-vapour resistance test, no air bubbles should be allowed when laying the film, the film should not be damaged, and the surrounding area should be closed tightly to ensure that no liquid water can pass through the film. This is because if there are air bubbles, creating an air layer, or if the film is damaged and liquid water penetrates the film to wet the sample, or if the test plate is not tightly closed around and liquid water leaks out from all sides, this will affect the test results and lead to test failure.

Which textiles require a thermal resistance and water-vapour resistance test?

Thermal resistance testing is generally required for products that have a requirement for warmth, e.g. knitted underwear, cotton garments, quilts and various types of wadding used as padding.

Some coated fabrics require a water-vapour resistance test because the water-vapour resistance of coated fabrics is generally higher than that of ordinary fabrics, which affects the comfort of wearing the garment.

The thermal test and water-vapour resistance test can simulate the ability of fabrics to exchange heat and moisture in a specific environment, and can visually express the warmth and comfort of textiles, which is a good guide for companies in fabric selection and product positioning.

What are the 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.

Comparison of thermal resistance test environments

Standard Test panel Environment Air flow rate
Temp (℃) Temp (℃) RH (%) m / s
ISO 11092 35 20 65 1
ASTM D1518 33 ~ 36 0 ~ 15 20 ~ 80 Static or 0.3 ~ 0.5
ASTM F1868-09 Part A 35 4 ~ 25 20 ~ 80 0.5 ~ 1
ASTM F1868-09 Part C 35 25 65 Unspecified
ASTM F1868-02 Part A 35 20 65 1
ASTM F1868-02 Part C 35 25 65 Unspecified
ASTM F1868-02 Part D 35 20 50 1
GB/T 11048 35 20 65 1

Comparison of water-vapour resistance test environments

Standard Test panel Environment Air flow rate
Temp (℃) Temp (℃) RH (%) m / s
ISO 11092 35 35 40 1
ASTM F1868-09 Part B 35 35 40 0.5 ~ 1
ASTM F1868-09 Part C 35 25 65 Unspecified
ASTM F1868-02 Part B 35 35 40 1
ASTM F1868-02 Part E 35 35 50 1
GB/T 11048 35 35 40 1

Test equipment: Sweating Guarded Hotplate TF129

At present, there are not many instruments that can support textile thermal resistance and humidity resistance testing methods and meet multiple testing standards.

Sweating Guarded Hotplate

The Sweating Guarded Hotplate TF129 developed by TESTEX Instruments Ltd. is fully capable of meeting the requirements of textile thermal resistance and water vapour resistance testing and complies with 3 commonly used testing standards.

Symbols, units and formulas in thermal resistance testing and water-vapour resistance testing of textiles

Symbols and units

  • Rct    is the thermal resistance, m²·K/W
  • Ret    is the water-vapour resistance, m²·Pa/W
  • imt    is the water-vapour permeability index, dimensionless
  • Rct0    is the apparatus constant for the measurement of thermal resistance, m²·K/W
  • Ret0    is the apparatus constant for the measurement of water vapour resistance, m²·Pa/W
  • Wd    is the water vapour permeability, g/(m²·h·Pa)
  • ΦTm    is the latent heat of vaporization of water at the temperature Tm, W·h/g
  • A    is the area of the measuring unit, m²
  • Ta    is the air temperature in the test enclosure, ℃
  • Tm    is the temperature of the measuring unit, ℃
  • Ts    is the temperature of the thermal guard, ℃
  • Pa    is the water vapour partial pressure of the air in the test enclosure at temperature Ta, Pa
  • Pm    is the saturation water vapour partial pressure at the surface of the measureing unit at temperature Tm, Pa
  • Va    is the speed of air above the surface of the test specimen, m/s
  • Sv    is the standard deviation of air speed, m/s
  • R.H.    is the relative humidity, %
  • H    is the heating power supplied to the measuring unit, W
  • ΔHc    is the correction term for heating power for the measurement of thermal resistance
  • ΔHe    is the correction term for heating power for the measurement of water vapour resistance
  • α    is the slope of the correction line for the calculation of ΔHc
  • β    is the slope of the correction line for the calculation of ΔHe

formulas

  • Rct=(Tm-Ta).A/( H-Hc)- Rct0
  • Ret=(Ps-Pa).A/( H-He)- Ret0

If necessary, other parameters can be calculated according to the following formula.

  • imt = 0.060 Rcf/Ref
  • Wd = 1/Ret.ΦTm
  • When Tm = 35℃, ΦTm = 0.627W·h/g
  • clo = Rct/0.155 = 6.451 Rct

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