About the Heat and Moisture Properties of Fabrics – 6 Prerequisites that must be Known for Garment Development

In the environmental elements, the greater impact on human comfort is temperature and humidity, and the human body needs to wear different clothes in different environments. Therefore, the study of the human body, clothing, temperature, and humidity of their respective characteristics and interrelationships, is the fundamental basis for our understanding of the heat and moisture comfort of fabrics, which I will discuss in the following six areas.

Why do we study temperature and humidity together?

Temperature, a physical quantity that indicates the degree of heat or cold of an object, in microscopic terms, is the intensity of the thermal motion of an object’s molecules. Humidity, also known as “relative humidity”, is the amount of water vapor in the atmosphere, expressed as a percentage of the maximum amount of water vapor that air can contain. This percentage content varies with temperature, with warm air containing more water vapor and cold air containing less. Completely dry air contains no water vapor and has a relative humidity of 0%, while saturated air cannot absorb any more water vapor and has a relative humidity of 100%.

Although temperature and humidity are not a single concept, humidity is directly influenced by temperature, therefore, temperature and humidity always appear together. This can also explain why when the temperature is set to 55℃ in the artificial climate chamber, the relative humidity does not reach the highest value of 95%, but only 75%; when the temperature is lowered to about 40℃, the humidity reaches the highest value.

The influence of temperature and humidity on the human body

Research shows that the human body suitable for the temperature of 17℃ to 25℃, humidity 30% to 60%, wind speed 2m/s to 4m/s, which is the most comfortable weather for people to feel. The human body is a self-adaptive system that has the ability to adapt the changes of external temperature and humidity. However, it should be noted that the body’s regulation is a certain limit, so when the temperature and humidity are too high or too low, what are the effects on the human body?

Influencing factors	Effects on the human body
High Temp.	In a hot environment, the body has to maintain the balance of body temperature by evaporating and sweating. In this process, the body loses salt with sweat, the blood is concentrated and the heart vessels are burdened, which causes a drop in blood pressure. The body’s skin sweats a lot, causing the kidneys to excrete less water, which also affects kidney function. High temperature also affects human appetite and motor coordination, etc.
Low Temp.	In a low temperature environment, in order to maintain the body’s heat balance, tissue metabolism is enhanced and the demand for oxygen increases. If the above conditions are not met, the body depletes the reserves of the body’s cells, resulting in an irreversible environment for human tissues. In addition, because the outdoor temperature is very low in winter, the blood vessels of the human body tend to become hard and brittle, especially the middle-aged and elderly people should pay special attention to increase clothing and keep warm.
High RH	When the humidity is too high, the amount of pineal hormone secreted by the pineal glands in the human body is also higher, making the body thyroxine and adrenaline concentration is relatively low, the cells will be “lazy”, people will feel lethargic, depressed. Long hours work and life in places (such as mountains, islands) with high humidity, but also prone to rheumatic, rheumatoid arthritis and other wet paralysis.
Low RH	When the humidity is too low, evaporation is accelerated, and the dry air easily takes away the body’s moisture, making the skin dry and cracked, the mucous membrane of the mouth and nasal cavity is stimulated, and symptoms such as thirst, dry cough, hoarseness and sore throat occur, which can easily induce pharyngitis, bronchitis, pneumonia and other diseases.

Modern medicine also confirmed that the air is too dry or humid, is conducive to the reproduction and spread of some bacteria and germs. Scientists have determined that when the air humidity is higher than 65% or lower than 38%, germs multiply and breed fastest; when the relative humidity is 45%-55%, the mortality rate of germs is higher.

Several ways in which the human body dissipates heat

Regardless of the external environment, the human body is always dissipating heat in four main ways: radiation, conduction, convection and evaporation.

Radiation Heat Dissipation	The human body transmits heat in the form of thermal radiation to the colder objects outside. The amount of heat dissipation depends on the temperature difference between the skin temperature and the outside air temperature and the effective radiation area of the body. Radiation accounts for about 60% of the heat dissipation, and is the main way of heat dissipation of the human body when the room temperature is 15℃~25℃.
Evaporative Heat Dissipation	Heat dissipation through evaporation of water from the body surface is the effective way of heat dissipation of the body when the external temperature is equal to or higher than the skin temperature, accounting for about 25% of the heat dissipation. In a high temperature environment, evaporation is the main way of heat dissipation in the human body. For every 1 liter of sweat evaporated from the skin, 2436KJ of heat is dissipated, and evaporation decreases when the humidity is greater than 75%, and evaporation stops completely when the relative humidity reaches 90%~95%.
Conduction Heat Dissipation	The heat of the body is directly transferred to the colder objects in contact with it, and the amount of heat dissipation is determined by the temperature difference between the skin and the objects in contact and the area, but also by the thermal conductivity of the objects in contact, which accounts for about 3% of the heat dissipation.
Convection Heat Dissipation	It is a special form of heat dissipation by conduction, in which the heat of the body is conducted to the air in contact with the body surface to heat it up, and the body heat is dissipated to space due to the continuous flow of air. The amount of heat dissipation is greatly affected by the wind speed, the greater the wind speed, the more convection heat dissipation.

The heat generated by the tissues and organs of the body is evenly distributed throughout the body with blood circulation. When the blood flows through the blood vessels of the skin, 90% of the total heat is dissipated by the skin, so the skin is the main part of the body to dissipate heat. A small portion of heat is also dissipated through the lungs, kidneys and digestive tract with breathing, urine and feces.

Heat dissipation mode and its proportion of body

Heat dissipation mode	Heat Dissipation / J	Heat dissipation ratio / %
Feces	200X10³	1.8
Respiration	352X10³	3.5
Lung surface evaporation	762X10³	7.2
Skin evaporation	1524X10³	14.5
Skin conduction and radiation	7503X10³	73.0
Total	10341X10³	100

Human is a constant temperature animal, the secret of human constant temperature is to continuously dissipate heat and continuously absorb energy and heat from the food we eat, and then achieve a balance of body temperature. As can be seen from the above table, heat dissipation through the skin is the main way of body heat dissipation, accounting for 87.5% of the total body heat dissipation, the human body wrapped in clothing, it becomes an important way to maintain the heat of the human body temperature and heat dissipation, and the material basis of the composition of clothing – the thermal conductivity of clothing materials will become an important factor in this means of regulation.

The influence of the heat transfer properties of clothing fabrics on the heat balance of the human body

The heat transfer performance of clothing fabric refers to the ability of garment fabric to regulate the exchange of heat between the human body and the outside environment. When the external climate is cold, we need garment fabrics with better insulation performance to cold; when the external climate is hot, we need garment fabrics with better heat dissipation performance, these are related to the heat transfer performance of clothing fabrics.

The heat transfer performance of clothing fabric can be expressed by thermal resistance Rct, the unit is m²·K/W, which indicates the ratio of the temperature difference between two sides of the specimen and the ratio of heat flow per unit area through the specimen vertically. The smaller the Rct is, the worse the warmth of clothing fabric is, the better the heat transfer performance of the clothing material.

In general, in common garment fabrics, the thermal resistance of wool, silk is smaller than cotton, that is, the warmth is worse than cotton; the thermal resistance of nylon, acrylic, polypropylene is higher, that is, good warmth, do outerwear have the advantages of brace not wrinkle, tough and durable. Factors affecting the heat transfer performance of clothing fabrics, in addition to the thermal resistance of the material, are mainly related to the thickness of the fabric and the arrangement state of the fibers within the fabric.

The influence of the moisture absorption of clothing fabrics on the human body’s humidity balance

The human body is constantly discharging water and secretions to the outside, so the moisture absorption and moisture dissipation performance of clothing fabric become an important factor for the human body to maintain humidity comfort. Especially in the hot summer or strenuous exercise, the human body sweats a lot, at this time, the moisture absorption and moisture dissipation performance of clothing fabrics can not only keep the clothing dry, but also prevent the human body from producing a sticky wet feeling.

The moisture absorption of clothing fabric refers to the process of water vapor dispersion from the high humidity zone through the fabric to the low humidity zone under the condition that there is a certain relative temperature difference between the two sides of the fabric. That is, one side of the fabric absorbs the evaporated water vapor on the surface of the skin, and then the water vapor is transmitted by the fiber to the other side of the fabric to disperse, and at the same time, the pores inside the yarn and fabric also have a certain role of diffusion of water vapor.

The main factors affecting the wet comfort of clothing fabrics are: fabric’s fiber characteristics (fiber type, fineness, surface characteristics), yarn characteristics (yarn structure), fabric’s tissue characteristics (weave structure, thickness, density, etc.). Generally speaking, the fabric with thin thickness, small fiber yarn fineness and large porosity has good moisture absorption and moisture dispersion performance; natural fiber moisture absorption and moisture dispersion are better than synthetic fiber.

The heat transfer performance and moisture absorption of the fabric can be tested together, which can be referred to the standard ISO 11092, and the test instrument is TF129 Sweating Guarded Hotplate, in addition, the moisture permeability can also be tested separately, which can be referred to the standard ASTM E96, and the test instrument is TF165B Water Vapour Permeability Tester.

The breathability of apparel fabrics related to human heat balance

Breathability is another important factor that affects the wearing comfort of clothing fabrics. A study shows that when the amount of carbon dioxide in the air inside the garment exceeds 0.08% and the humidity of water vapor exceeds 60%, the human body will feel hot and humid, and people will feel irritated. From the perspective of hygiene, clothing fabrics with a certain degree of breathability are conducive to the exchange of gases inside and outside the fabric, which is beneficial to the metabolism of human skin.

Generally speaking, cold-proof clothing requires small material breathability, so as not to dissipate body heat too quickly, to achieve the cold day. The anti-heat garment requires good breathability to speed up the body heat dissipation, so that people are in the heat and moisture balance, comfortable and satisfactory state.

Many factors affect the thermal and wet comfort of clothing fabrics, which should take into account factors such as fiber, yarn and fabric morphology structure, for different seasons, different uses of clothing fabrics should be considered factors should also be different. For example, the choice of underwear fabrics require comfortable, warm, from a combination of factors, should be selected from cotton, wool, silk products. The choice of winter outerwear fabrics requires beautiful, practical, warm, so pure wool, wool and cotton, wool and chemical fiber blended or thickened tissue fabrics should be used. Summer outerwear fabrics should be selected for comfort, cool, breathable, good drape, should be selected from cotton, linen, silk and other high yarn count raw materials, the product requirements light and thin.