The clothing designed for cold weather conditions must be able to maintain the body heat balance under any physical activity of varying environmental conditions. The human body looses heat due to striking cold air, water, cold surface contact, insufficient physical activity, inadequate covering of body parts.
Heat transfer is the rate at which energy can be transferred from a high temperature to low temperature zone and this heat transfer continues till both the surfaces obtains equilibrium. The extent of heat loss from body due to heat transfer depends on the temperature difference between body and the environment. There are four general mechanisms by which heat transfer occurs in the environment, they are conduction, convection, radiation and evaporation.
- Conduction of the heat occurs when the surface of two objects touch.
- Convection relies on some sort of fluid medium, either a gas or liquid for heat transfer.
- Radiation involves the transfer of heat by electromagnetic waves.
- Evaporation is an extremely effective means of heat transfer that occurs when a liquid changes into gas.
The textile structure designed for extreme cold climates should have adequate strength and performance features, it should have flexibility, elasticity, sensorial and aesthetic trails, it should allow heat and moisture transfer between body and environment, it should stand as a barrier for heat loss due to wind chills.
Heat balance and the body:
As humans are warm-blooded creatures, the body of the humans has an inbuilt ability to produce enough heat within environmental limits, in order to maintain thermal balance. The complex physiological mechanisms for thermal regulation in the body all work to balance the heat equation i.e. heat produced equals to heat loss. For example, if the body produces more heat than that is dissipated, and then the body struggles to remove the excess temperature before body temperature increases and results in illness or heat stress. If there is more amount of heat loss, than heat produced, then the body must struggle to increase the body heat before the body gets damaged due to excess cold.
The body can produce heat in two ways in general. They are absorption of heat from outside environment by three ways of heat transfer (conduction, convection and radiation) which involves standing near the fire, lying on the heat pad or by the body chemical process called metabolism which includes all chemical and physical reactions that occur within the body.
Role of blood flow in maintaining body temperature:
The heat produced in the body in either of the ways is carried by blood. Blood carries heat from deeper body tissues to the surface tissues, where it can be released to the environment.
Changes in the body temperature occur when something happens to bring greater or lesser amount of blood near the skin surface. The two typical responses of the body blood vessels in hot and cold climates is vasodilatation and vasoconstriction. The hypothalamus sends messages to the pituitary gland and to the nervous system to dilate/constrict the blood vesicles near the surface of the skin depending on the climatic conditions.
Vasodilatation occurs on a warm day when the blood vessels open up near the surface of the skin and the heat carrying vehicles discipate the heat generated in the body to the outer surface where radiation, conduction and convection can operate to carry the heat away.
Vasoconstriction occurs on cold climatic conditions. The surface blood vessels closes/constricts. The blood is forced away from the body surface and the pores of the skin constrict closing over fat layers and muscles that insulate the vital inner body organs. Blood volume and blood flow decreases.
As vasoconstriction occurs, the skin temperature drops and because of the reason that there is less difference between skin temperature and environment, the rate of heat flow is decreased to maintain the body core which houses the vital organs at a relatively uniform temperature. The body works to maintain blood supply to the core all the time.
Thermal insulation:
Individuals differ greatly in their ability to tolerate and adjust to cold temperatures. Factors like age, sex, stage of life cycle, individuals belonging to different climate extremes play a major role in adjusting to thermal conditions. The thermal balance is affected by heat production and heat loss. There are number of ways in which individuals may differ in their ability to promote and prevent the heat loss like placement of body fat and hair, body surface area to weight ratio, sweating rates and blood flow patterns. Whatever the condition it may be protection of body core is important. Theoretically if body core is protected and kept warm enough, the warmth will automatically flow towards hands and feet. But if vasoconstriction begins, hands and feet also need to be protected and covered at subzero temperatures in order to prevent body core from taking temperature from the surroundings.
Protection of the individual in a cold environment would depend on the factors like:
1. Metabolic heat:
The heat produced by the body due to the body parameters as well as the activity i.e if a person is at rest the metabolic heat generated by the body is less compared to the metabolic heat generated by the body of the person during jogging. The production of metabolic heat by the body even depends up on the age and the body weight. So while designing protective clothing for extreme cold conditions, the garment system capable of providing protection at rest /lowest level of activity should be considered.
2. Wind chill:
Wind and the water are tremendously important factors in heat dissipation. The wind has the capacity of quickly moving away the warm air surrounded the body and work to evaporate sweat and carry off the resulting warm vapour. Wind is so important that temperatures are often quoted along with wind-chill factor. 80% of the heat losses are due to wind chill effect. Water conducts body heat much more rapidly than air. Since wind and water affects the rate of conduction, convection and evaporation, it is extremely important to clothing design.
3. Thermal insulation:
Thermal insulation is the reduction of heat transfer between objects in thermal contact or in range of radiative influence. The protection against cold is dependent on thermal resistance and thermal insulation of the garment or clothing. The thermal insulation is the ratio of temperature difference between the two faces of the fabric to the rate of heat flow. Air is a good thermal insulator. To make an insulating material effective, it should have low compression set or high resiliency to maintain the still air entrapped in the material. The two basic ways to protect the insulation in a garment are 1) To develop a breathable system 2) To block all air flow and liquid transport with the use of vapour barriers.
4. Air permeability:
It is one of the important fabric properties for maintaining thermal comfort in outdoor clothing it is important that air permeability is as low as possible because it should function as a wind protection. Air permeability of a fabric is a measure of how well it allows the passage of air through it. Generally, the air permeability of a fabric can influence its comfort behaviour in several ways. In the first case, a material that is permeable to air is in general, likely to be permeable to water, in either the vapour or the liquid phase. Thus, the moisture-vapour permeability and the liquid-moisture transmission are normally closed related to air permeability. In the second case, the thermal resistance of a fabric is strongly dependent on the enclosed still air, and this factor is in turn influenced by the fabric structure.
5. Moisture vapour transmission:
The water vapour permeability of fabrics is an important property for those used in clothing systems intended to be worn during vigorous activity. The human body cools itself by sweat production and evaporation during periods of high activity. For instance, the clothing must be able to remove this moisture in order to maintain comfort and reduce the degradation of thermal insulation caused by moisture build-up in cold environment.
Water vapour transmission is essential in determining the breathability of clothing and textiles in outdoor wear as well as in indoor wear. A breathable textile allows extra heat loss by evaporation of moisture through the clothing layers. If clothing layers are impermeable the moisture is captured between skin and clothing and heat is accumulated in the body. As a consequence, heat and moisture build up, causing discomfort, wet skin and skin abrasion.
Materials suitable for designing protective clothing for extreme cold climatic conditions:
Textiles play a major role in helping the body to achieve thermal balance. They act as a buffer between body and the environment and they play a part in ventilation. It is very important for the functional apparel designer to understand the stages at which the thermal protection can be built in to the garment. In general wool, down, textured manufactured fibres, pile fabrics such as furs and terry clothes, foams, fibre fills are excellent insulators.
Wool and woollen pile fabrics have been the oldest materials used for providing protection against cold since ancient times. After the invention of synthetic fibres, utilization of acrylic fibres and polyester fibres in different forms was exploited for use in making cold protective clothing. Systematic and extensive studies on wool, acrylic pile and polyester batting revealed that polyester batting has good insulation weight ratio against extreme cold and this makes polyester batting one of the important constituents in protective garments, where weight of the garment is of prime concern, especially for protective clothing used in glacier and Siachin regions. Researchers have been carried out to develop new and effective synthetic insulating materials to provide protection against cold. As a result, several synthetic insulating materials have been developed by different fibre manufacturers. Selection of material for cold weather protective clothing is done with particular focus on minimising heat loss from the body to the environment, so that the metabolic heat generated from the body is not lost to the environment and instead keeps the body warm. Simultaneously, the material should allow evaporation of sweat from the body to the environment. ‘If the clothing prevents evaporation of sweat to the environment, sweat gets accumulated on the skin and gives a feeling of discomfort to the wearer. In extreme cold conditions, the sweat gets frozen and can cause frostbite. Thus, two properties, viz., minimum heat loss and sweat evaporation, are very important. These are primarily governed by two important factors: (i) thermal insulation of the material, and (ii) moisture vapour permeation. The performance characteristics of clothing assemblies are often characterized by these two factors. Design and development of cold weather clothing, therefore, requires an understanding of these two parameters.
Fibres:
A fibre is the smallest unit of the textile. Textile fibers are generally divided into two basic categories Natural and Manmade fibres. The type of fibre used in the garment effects the thermal balance of the system in three ways: by resisting conduction of heat, by helping to preserve still air in the garment system, and by handling moisture in a way that promotes desired heating and cooling. The choice of relatively nonconductive fibres forms a base on which a thermally insulative fabric can be built. For eg: The ability of silk and polypropylene to resist heat is the major reason for picking them for thermal underwear’s.
Fibres also help fabrics increase their proportion of still air by providing irregular textures and projections. For eg: Wool is considered as one of the warm fibre because of its scaled outer structure and natural crimp leading to yarns and fabrics with many twists and projections and thus many air spaces. This type of irregular or scaly structures can be incorporated in the manufactured fibres by imparting textures to the filaments while extruding.
It is not enough that fibres used in thermally protective clothing have the capacity to entrap air when they are formed into fabric. They must be able to retain that capacity when the garment is being worn. The resiliency of the fibre to return back to its original position when distorted is extremely important and this resiliency must remain through all conditions that affect the end use. For eg: Down fills are generally used in clothing designed for extreme cold climates conditions which are highly prized for its ability to provide warmth without adding weight to the garment. Despite of its basic resilience through a wide range of temperatures, down ease of compressibility and lack of resilience when wet pose drawback for the users. Because rain is the part of outdoor life, the change in resilience can be critical to the thermal protection of the wearer.
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Water plays a significant role in heat loss, not only because it can destroy the effectiveness of insulation such as down, but also it can generate evaporative heat loss and rapid conduction of heat away from the body. There are number of ways in which the water interacts with fibres. It may be absorbed, adsorbed, wicked or imbibed (saturation of a fabric with liquid due to physical entrapment of water). The response of different fibres to water leads to a variety of thermal effects. Fibre finishes and other treatments may greatly affect their reaction to water. For eg: Capilene Polyester is a fibre created for cold weather undergarments. This fibre has the capacity to spread the water from wetter areas of fibre to drier ones and finally dissipates.
Texturing and changing of fibre cross section:
Single fibre can be altered by a variety of mechanical means. Texturing is the process by which smooth filament fibres are made to loop, curl and crimp and this texturising process depend on the thermoplastic nature of manufactured fibres. Basically, a filament fibre is distorted by twisting it, looping, curling or pushing it into a confined space and then heating and cooling it while in this position. Textured fibres can be combined to form yarns or fabrics having a fuzzy, more natural fibre appearance and containing more air spaces within their structures.
The basic cylindrical shapes of the thermoplastic fibre are changed into trilobal, oval and dog bone shape by many manufacturers to impart a variety of fibre characteristics from lustre to thermal insulation. The cross sections of the fibres are planned in such a way that air spaces will naturally occur as the fibres are formed into fabric. Hollofill and Quallofil are two different types of hollow core fibres developed by E.I.Dupont de Nemours and company in order to incorporate more protected still air for providing thermal insulation. These types of hollow fill fibres are lighter in weight compared to solid fibres of same substances.
Another way in which the principle of protected still air has been applied to innovations in fibres for insulation can be found in the development of microfibers and ultrafine microfibers. Microfibers are finer than human hair and ultrafine microfibers are 1/3rd of microfiber diameter. The fineness of these fibres results in 6,200 fibres per square inch of fabric thus providing increase in surface area available to which air can cling resulting in thermal insulation.
Fabric structures:
Fabrics made from both synthetic and natural fibres vary greatly in their ability to insulate. Yarns are formed into fabrics in a variety of ways like different weaving techniques, warp and weft knitting, Non woven’s, films and foams. Some of the critical factors determining the thermal resistance of the material are the materials ability to incorporate a large proportion of air to fibres, its thickness, the way in which a fabric structure reacts to liquids, the colour and the texture of the fabric surface and fabric hand or stiffness. These qualities of a fabric are greatly influenced by the production process used to form fibres and other substances into textiles and similar flexible, protective materials.
Whatever the fabric structure it may be, the fabrics insulate because they are largely composed of air. A number of studies have supported the idea that air, not the fibre type, is the most important factor in thermal insulation.
The air in general is entrapped in relatively small pockets, where it is protected from movement:
Down: Whether goose or duck down, it is still the best overall insulator. Down is lightweight, compressible and will last a lifetime. However, down can be expensive, and, when wet, it’s worthless, since it provides no insulation and takes a long time to dry. It is not recommended for sweat-producing activities. The “fill power” number on the label refers to how much down the material contains – the higher the number, the more down the garment contains and the more insulation it provides.
Fleece: This is a synthetic material with the same insulation properties as wool at half the weight. It is thin, soft, comfortable, easily laundered and will last years. It breathes well and dries quickly, so it can be worn for sweat-producing activities. Unfortunately, wind blows right through it making it unsuitable for the outer shell layer, but perfect for the insulating layer.
Nylon: Another synthetic material, nylon is usually woven tightly and coated with urethane to make it waterproof. It is best suited for the outer shell layer.
Polypropylene: This man-made material is thin, lightweight, conforming and soft. It provides the most “wicking,” making it the perfect base layer material.
Thinsulate: This was introduced as the synthetic replacement for down. It is very warm in surprisingly thin layers, resists becoming wet, but will maintain its warmth when soaked. Unfortunately, it lasts less than five years.
Wool: Wool has many great properties. Oiled and tightly woven, it is waterproof, an excellent insulator even when wet, has great “wicking” abilities and lasts forever. However, wool is heavy, takes a long time to dry and can be very itchy.
With the right materials and layers, you can make work (or play) outdoors in winter a safe, tolerable (and enjoyable) experience.
New trends
Phase changing materials:
Phase change materials (PCM) are the new trends in the clothing for cold weather. The phase change materials absorb energy for phase transition from solid to liquid and release energy when this change is reversed. The technology of PCM was developed to improve the thermal insulation of textile material. The PCM will take heat from body or environment to melt. When the body temperature is below transition temperature the encapsulated PCM will convert to solid, generating heat and producing a warming effect. Before changing phase to generate or absorb heat, the PCM later of garment must go through transition temperature. So, the wearer must perform some physical activity to achieve transition temperature.
The PCM can be applied to textiles in a number of ways like coating, finishing, lamination, bi-component synthetic fibre extrusion, melt spinning, injection moulding and foam manufacturing. Certain products contain micro encapsulated phase change material (micro PCMs), which contain encapsulated paraffin wax. Latent heat storage capacity of PCM is very high. The melting point of paraffin wax is 27°C, near to human body temperature. Therefore it is be used as PCM for cold weather protective clothing, as it is melted easily from body heat.
Shape memory materials:
Shape memory materials (SMMs) are featured by the ability to recover their original shape from a significant and seemingly plastic deformation when a particular stimulus is applied. This is known as the shape memory effect (SME). Shape memory polymers (SMP) on the other hand have different types of mechanisms. The characteristics of the polymer chains or the characteristics of the different phases in the material are responsible for this effect. A SMA gets its permanent shape by heat treatment and the permanent shape of a SMP is in general obtained during fabrication. This rare phenomenon offers a wide range of possible applications. When shape memory polymers are used in the clothing, air gaps amongst layers are amplified resulting enhanced thermal insulation The incorporation of shape memory materials into garments thus confers superior flexibility for extreme colds.
Micro porous coatings and films:
These are now widely available in many variants. These membranes have pore size from 0.1-10 microns. These are based on polytetrafluoro ethylene, PU, Acrylic and polyolefin etc. GORE-TEX is PTFE based membrane, having pore size 700 times smaller to H20 vapour molecule, making it waterproof at the same time it allows perspiration to escape. These polymers can be coated directly or laminated to filament woven fabrics. Three layer laminates are manufactured by bonding light weight. (Say 50g/m2) knitted or non-woven fabrics to the back (the inside surface of a garment) to protect the film and improve durability.
Hydrophilic solid coatings and films:
These are pore-free films. As such they possess the potential for a high resistance to liquid penetration. However, water vapour permeability rates through these films are inversely proportional to their thickness (Lomax, 1990). Block copolymers of polyurethane and polyethylene glycol (PEG) have been widely used . the original research on this type was done by UK Ministry of defence and this type is made widely available under a number of different trade names.
Another type of Hydrophilic copolymer is based upon modified polyester onto which polyether groups have been incorporated to impart hydrophilic properties with the limited degrees of swelling. Commercial products such as Symptex are available in prepared 10-25 micron films, which can be laminated to suit filament fabrics as two layer, three layer or light weight drop liner forms. (Drinkmann 1992)
Clothing for extreme cold weather conditions:
Most of these clothing designed for extreme cold weather conditions are designed with layers in order to adjust the material depending on temperature.
(a) Outermost layer: The outer layer is a rugged, windproof material that is also breathable. It may or may not have insulation, depending on the circ*mstance and intended use. In most of the protective clothing, chemically-treated water and oil repellent fabric outers are popular. Alternatively, PU-coated nylon fabric or Goretex makes the outermost layer.
(b) Middle layer: Lightweight insulating filling type material like polyester batting, wool or fleece such as Polartec, down or any new insulating material forms the middle layer depending on the activity. Polyester batting is generally used for clothing of lightweight and better loft, which allows increased entrapment of air.
(c) Innermost layer: Lightweight material like pile fabric, A base layer of soft material worn next to the skin to provide some insulation and made with synthetic materials that wick moisture away. PCMs are used in a variety of formats and are generally effective in this area.
(d) Reflective layer: Sometimes the requirement is to create an added effect to reflect back metabolic heat to the body, so that it is not dissipated to the environment through the clothing. Nylon fabric best suits this purpose.
Conclusion:
Protection from extreme cold is dependent mainly on clothing, which is special purpose clothing. It is designed according to explicit function and environmental circ*mstances. Like physical activity low or high rate and environments like humidity, temperature, wind speed, snow or rain fall etc. The main objective of the protective system is to form a barrier between the body and environmental temperatures. The material characteristics requires for cold weather clothing are thermal insulation, evaporative resistance, permeability, wind and water resistance. The clothing design for cold weather climatic conditions should absorb radiant energy from the environment, should be non conductive, should allow the air and water from circulating freely near the body surface where it can carry off body heat, and should allow the freedom of movement. In extreme conditions, the core especially the head should be kept warm and the clothing design should allow adjustable protection so that overheating, which causes sweating can be avoided. The choice of materials, fiber and fabric properties play a critical role in designing proper protective clothing for extreme cold climatic conditions.
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