Measurement Methods and Influencing Factors of Thermal and Moisture Resistance of Clothing

Clothing comfort balances body heat and moisture in various environments and activities. It achieves balance. Clothing must be comfortable in all aspects of life. It has many uses. Summer clothes need to be fresh and breathable. Winter clothes need to be warm. These include firefighting and mining clothing for hot places and liquid cold clothing. And cold clothing, plus aviation and space suits, for cold places.

Resistance and moisture resistance are key. They measure the thermal and humidity comfort of garments. Many things affect them, like garments, environment, and exercise. This is very important. It will help evaluate the warmth and humidity comfort of clothes. And it will also help design functional clothes. It will help make thermophysiological models. It will also improve sweat-warming dummies.

1.Clothing thermal resistance and moisture resistance

There are mainly sensible heat and latent heat in human body heat dissipation. Sensible heat is the heat released from the body to the outside world. This happens when the body is at a different temperature than the environment. It occurs mainly through conduction, radiation, and heat convection. Latent heat is the heat taken away by sweat evaporation.

The thermal resistance (Rct) of a garment shows how it resists heat flow. It is due to the temperature difference between the garment’s layers. Thermal resistance measures how well a garment insulates. It does this by comparing temperature difference to heat flow. The value increases with better insulation and worse thermal conductivity. One or more of conduction, convection, or radiation may transmit this heat flow. The thermal resistance Rct is measured in ℃-m2 / W or m2 – K / W. The international index for clo is defined for: temperature 21 ℃, humidity 50% ± 0.2%, wind speed less than 0.1 m / s indoor. It applies to sitting or doing light mental labor adult men. They produce about 58.15 W / m2 of heat. They can maintain an average skin temperature of 33 ℃. This is true when the clothing they wear has a thermal insulation of 1 clo. (1 clo = 0.155 °C-m2/W).

A garment’s Resistance to moisture permeation (Ret) shows how it resists moisture. It does so due to a water vapor pressure difference between the inside and outside of the garment. The resistance to moisture is a ratio. It’s the ratio of the difference in water vapor pressure inside and outside the garment to the heat flux. The heat flux passes vertically through the unit area. Diffusion and convection may transmit the heat flux. They may transmit it in one or more forms. The common unit of moisture resistance Ret is: m2.Pa/W

Tests relate to the standards.

ASTM F1868-2017 tests how well clothing resists thermal moisture. It uses a sweating hot plate. ISO 11092-2014 measures thermal and moisture resistance at steady state. It uses a sweating hot plate. ASTM F1291-2016 tests the thermal resistance of clothing with a heated dummy. ASTM F2370-2016 tests the moisture resistance of clothing with a sweating dummy. ISO 9920-2007 evaluates the thermal and humidity resistance of clothing. ISO 15831-2004 assesses the thermal resistance of clothing. GB∕T 39605-2020 tests clothing moisture resistance with a sweating warm body dummy. GB/T 18398-2001 tests the thermal resistance of clothing with a warm dummy.

2.Measurement Methods of Thermal Resistance and Moisture Resistance

Researchers start to study how well fabric clothes resist heat and moisture. Scholars at home and abroad have proposed many tests. These test measure the resistance of fabrics to heat and moisture. But, the basic principles are the same and common. We measure garment thermal and moisture resistance using real-life tests. We use sweat-warming dummies. The real person test has errors and low repeatability. So, the sweat warmer dummy becomes the main tool to measure it.

2.1Moisture permeability cup method

The cup method measures how resistant garment fabrics are to moisture. Moisture permeability is commonly measured in g/m2.24h. Under certain standard lab conditions, we measure it. These conditions create a specific humidity difference across the sample. Water vapor moves through the sample to the dry side. The lab measures the moisture permeability by tracking the weight change of a cup over time. This process finds the water vapor permeability and other parameters of the sample. There are two methods for measuring moisture permeability. They are the positive cup method and the inverted cup method. Cup method standards include: ASTM E-96 A, C, and E, and JIS L-1099 A1. The inverted cup method standard is ASTM-E9.

2.2Measurement of moisture and heat resistance based on warm dummy clothing

When the human body is dressing, the fabric’s resistance to moisture and heat differs. This is influenced by how the garment is styled, structured, and worn. The values vary depending on the body part and are different from those values when the body is clothed. But, the garment’s total heat and moisture resistance value is the average of the values for body parts under clothes. It is not the fabric’s value. On this basis, the sweating and warming mannequin came into being. The warm body dummy can mimic the mass transfer and heat between the human body and the environment. The measurements it gives are objective, accurate, and repeatable.

Testing Instrument:Sweat Warmer Dummy

The warm body dummy test system can mimic the body’s heat and sweat. It can also mimic breathing and other functions. It can realistically mimic the body in different environments. This shows the process of heat and humidity exchange. It happens between the body, clothes, and the environment. It can then evaluate the clothing’s heat and humidity performance.

2.3 Sweating Guarded Hotplate Test Method

Textiles’ thermal resistance is a comfort indicator. It shows how warm they are and their heat resistance. The greater the thermal resistance, the better the warmth. Textile water-vapor resistance is also a clothing comfort index. It shows how textiles resist moisture. The higher the resistance, the worse the comfort.

It is one kind of Thermal and Water-vapor Resistance Testers. The Sweating Guarded Hotplate has a human skin test board. It also has water and wind controls, and a test host. This Sweating Guarded Hotplate also needs an environmental chamber. The chamber is for the Thermal and Water-vapor Resistance Test.

Testing Instrument:Sweating Guarded Hotplate

2.4 Other Measurements

2.4.1Hydrostatic pressure:

The outdoor fabrics industry calls it hydrostatic pressure resistance. It’s the strength to withstand water pressure over a certain area. In labs, the fabric resists distilled water sprayed upwards. We record the max pressure. For example, a hydrostatic pressure of 5000mmH2O means that it can block 5m of water pressure. Hydrostatic pressure values commonly used units: Kpa, cmH2O, mmH2O, mbar.

2.4.2Air permeability:

Air permeability is a type of fabric permeability. It shows how well fabric lets gas “particles” pass through. Air permeability measures how much air flows through fabric under specific pressure conditions. This is often expressed as air permeability (mm/s).

Testing Instrument:Hydrostatic Permeability Analyser,Air permeability analyser

3.Influence factors of clothing on heat and moisture resistance

Clothing is the layer between the human body and the environment. It mainly insulates heat and lets moisture and air through in heat and moisture transfer. Many researchers and scholars have found that people use warm body dummies. They use them to measure resistance to heat and moisture. They found that the clothing itself has an impact. This impact comes from its structure, style, thickness, and fabric. Human activities, such as posture, activity level, and skin temperature, also matter. So do outside conditions. These include air temperature, humidity, thermal radiation, and wind speed.

3.1Clothing factors

3.1.1 Fabric, structure:

The fabric affects the garment’s thermal and humidity comfort. But, it has limits. So, the design of functional garments must fully consider the garment’s applicable conditions. Also, breathability is very important. Highly breathable fabrics are less moisture resistant in the wind or when walking. This is especially true in very hot conditions.

3.1.2 Air gap:

As the air gap grows, the garment’s resistance to heat and moisture goes up a bit. However, after a peak, the rise in air gap volume will reduce the garment’s resistance. It will reduce its resistance to heat and moisture.

3.1.3 Number of garment layers:

Researchers explored the link between the heat resistance of single-layer and multi-layer garments. They found that the heat resistance of a set of garments increases with the number of pieces. Also, the total heat resistance of one piece of clothing is more than that of a set of garments.

3.1.4Moisture content:

Wet thermal resistance is the resistance of a garment when it is partly or fully soaked. Moisture soaks into the pores of the garment. This lowers the material’s natural thermal resistance. So, the total resistance decreases.

3.1.5 Garment microclimate:

A garment’s microclimate is the climate between the inside of the garment and the skin. A person dressing causes it. It includes the temperature, humidity, and air speed in that space.

3.2Human body factors

3.2.1 Different parts:

Garment structure, body placement, wear method, and mass affect local thermal resistance. Each part’s thermal resistance value differs.

3.2.2Different postures:

Moisture resistance on surfaces is lower in non-isothermal conditions than in isothermal ones. This is the case when standing and sitting. But, it is higher when lying naked than when standing and sitting.

3.3External factors

3.3.1Wind speed and step speed:

Researchers found that both the total thermal and moisture resistance of garments drop with more wind. They also drop with faster walking.

3.3.2Isothermal versus non-isothermal environments:

The water resistance was lower in a non-isothermal than an isothermal test. When one held the posture constant, this was true. Also, at different temperatures, the resistance in a non-isothermal environment differs. It differs from that in an isothermal one.

4.Summary

Each region, season, and type of work has different warmth and moisture needs for clothing. So, measuring thermal and moisture resistance can guide fabric choice. But, it can also be a reference. You can use it to evaluate specialized and functional protective clothing. Also, it can increase assessment of heat and moisture transfer in industrial textiles. Clothing comfort is the result of the triple factors of human body, clothing and environment. Comfortable clothes ease movement and promote psychological well-being. They regulate body temperature and protect against harsh climates.

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