Clothing is a basic need for human being and primary function of a fabric is to protect the body from adverse surrounding condition, winter being the most important of them. Since the time humans have found the need to cover the body with fabrics, there has been continuous attempts to improve the way fabric or a garment can be used to provide better effect. Thermal management (warming/heating) technologies have made immense progress ever since.

1. Heat Generation

1.1 Infrared Radiation

These fabric technologies utilize IR rays (wavelengths ranging between 700nm to 1 mm) for Sunlight’s electromagnetic radiation or body heat for thermoregulation when the temperature around body is low.

IR has three broad categories, Near Infrared, Mid-Infrared and Far Infrared. While IR radiation is more often defined by the heating function, Far Infrared has specialized function (discussed in the next section).

Many fabrics have been developed enable with IR radiation capabilities. The main additives imparting IR capability are ceramics, trace elements, metal oxides, etc. These materials have common characteristics of being able to absorb and store heat (i.e. high heat capacity) and release it when the surrounding temperature is low. Both yarn inherent and print versions are available in market for imparting IR emitting property to the fabrics.

A different approach has also been taken to reflect IR radiation (generated by our body), thus preventing heat loss when in cold conditions.

1.2 Far Infrared Radiation

Although not typically heating technology, but FIR (higher wavelength section of IR spectrum) is often used in fitness and recovery applications owing to its ‘internal heating’ capability. The user does not necessarily experience heat by FIR radiation. Far infrared (FIR) radiation (λ = 3–100 μm) is a subdivision of the electromagnetic spectrum that has been widely studied for its biological effects.

With the evolution of technologies to be able to filter specific wavelength range e.g. far infrared radiation (FIR), it has been made possible to achieve benefits of the radiations selectively. Nowadays, specialty FIR emitting heat lamps and garments made up of filaments (fibers) impregnated with FIR emitting nanoparticles are being used to deliver the thermal radiation effects.

At the cellular level, the electromagnetic radiation interact with living cells by altering cell membrane potentials and mitochondrial metabolism. FIR energy (photons with quantum energy levels of 12.4 meV – 1.7 eV) is absorbed by vibrational levels of bonds in molecules. The energy levels so reached lead to expansion of blood vessels, thinner body fluid and increased hydration capacity of water molecules.

Image Source: http://textilecentre.blogspot.com/2014/04/far-infrared-rays-reflecting-fabrics.html

1.3 Hygroscopic Heat Generation

Hygroscopic heating refers to generation of heat by absorbing moisture. Many polymers have the tendency to absorb moisture and the process of moisture absorption leads to heat generation (an exothermic process). The heat generated is termed ‘heat of absorption’ or ‘absorption heat’ and is an outcome of bond formation between water molecules and the yarns’ polymer chain. Different yarns have different water absorption capabilities and the amount of heat generated can be different for different polymers owing to different chemistries. Some manufacturers have made efforts have to increase the absorption by chemically modifying the polymer and increasing the binding sites for water. In general, cotton, wool, rayon, wool, etc fall in this category and have been used extensively for harnessing moisture to generate warmth to the wearer.

2. Heat Retention

While different methods have been discussed above to actively utilize different heat sources (e.g. body heat or external IR sources) as well as indirect heating (e.g. hygroscopic), the basic idea of conserving body heat by just creating a barrier for heat exchange with exterior still holds its value. Maintaining a still air microenvironment next to skin is one effective way to contain the warmth around body, as the air serves as an insulating layer. High gauge construction methods and hairy fibers (e.g. wool) serve the purpose in this aspect.

Some other strategic constructions have also been tried with varying success, as discussed further in the article.

2.1 Understanding clothing insulation

Clothing insulation is an important aspect of fabric performance as far as warm conditions are concerned. The capacity of the fabric to retain heat depends on several factors and its quantification has been a challenging exercise. CLO, or Clothing Insulation, is the measure of fabric’s (or garment’s) capacity to retain wearer’s body heat and make her/him comfortable. While a CLO value of 0 represents naked person, CLO value of 1 is the amount of insulation that allows a person at rest to maintain thermal equilibrium in an environment at 21°C (~70°F) in a normally ventilated room (0.1 m/s air movement). Above this temperature the person so dressed will sweat, whereas below this temperature the person will feel cold. CLO values can be assigned to fabrics, garments as well as ensembles of garments. The below table is general (neither standard nor extensive) representation of different garments’ CLO values:

Ensemble Description	CLO
Walking shorts, short-sleeved shirt	0.36
Trousers, short-sleeved shirt	0.57
Trousers, long-sleeved shirt	0.61
Same as above, plus suit jacket	0.96
Same as above, plus vest and T-shirt	0.96
Trousers, long-sleeved shirt, long-sleeved sweater, T-shirt	1.01
Same as above, plus suit jacket and long underwear bottoms	1.3
Sweat pants, sweat shirt	0.74
Long-sleeved pyjama top, long pyjama trousers, short 3/4 sleeved robe, slippers (no socks)	0.96
Knee-length skirt, short-sleeved shirt, panty hose, sandals	0.54
Knee-length skirt, long-sleeved shirt, full slip, panty hose	0.67
Knee-length skirt, long-sleeved shirt, half-slip, panty hose, long-sleeved sweater	1.1
Knee-length skirt, long-sleeved shirt, half-slip, panty hose, suit jacket	1.04
Ankle-length skirt, long-sleeved shirt, suit jacket, panty hose	1.1
Long-sleeved coveralls, T-shirt	0.72
Overalls, long-sleeved shirt, T-shirt	0.89
Insulated coveralls, long-sleeved thermal underwear, long underwear bottoms	1.37

Garment Description	CLO	Garment Description	CLO
Underwear		Sweaters
Bra	0.01	Sleeveless vest	0.12-0.22
Panties	0.03	Long-sleeve	0.25-0.37
Men's briefs	0.04	Trousers and Coveralls
T-shirt	0.08-.0.09	Short shorts	0.06
Half-slip	0.14	Walking shorts	0.08
Long underwear bottoms	0.15	Straight trousers	0.15-0.25
Full slip	0.16	Sweatpants	0.28
Long underwear top	0.2	Overalls	0.3
Shirts and Blouses		Coveralls	0.49
Tube Top	0.06	Multi-component with filling	1.03
Sleeveless/scoop-neck blouse	0.12	Fiber-pelt	1.13
Light blouse with long sleeves	0.15	Sleepwear and Robes
Short-sleeve knit sport shirt	0.17	Sleeveless short gown (thin)	0.18
Short-sleeve dress shirt	0.19	Sleeveless long gown (thin)	0.2
Long-sleeve dress shirt	0.25	Short-sleeve hospital gown	0.31
Long-sleeve flannel shirt	0.34	Short-sleeve short robe (thin)	0.34
Long-sleeve sweatshirt	0.34	Short-sleeve pyjamas (thin)	0.42
Long sleeves with turtleneck blouse	0.34	Long-sleeve long gown (thick)	0.46
Dress and Skirts		Long-sleeve short wrap robe (thick)	0.48
Skirt	0.14-0.23	Long-sleeve pyjamas (thick)	0.57
Sleeveless, scoop neck	0.23-0.27	Long-sleeve long wrap robe (thick)	0.69
Short-sleeve shirtdress (thin)	0.29	Body sleep with feet	0.72
Long-sleeve shirtdress	0.33-0.47	Footwear
Jackets and Vests		Ankle-length athletic socks	0.02
Sleeveless vest	0.1-0.17	Pantyhose/stockings	0.02
Single-breasted	0.36-0.44	Shoes/Sandals/thongs	0.02
Double-breasted	0.42-0.48	Slippers (quilted, pile lined)	0.03
Down Jacket	0.55	Calf-length socks	0.03
Coat	0.6	Knee socks (thick)	0.06
Parka	0.7	Boots	0.1

Apart from the material and fabric construction, important factors determining insulation include posture and activity of the wearer. As mentioned earlier, CLO is represented as with respect to a person at rest in a controlled environment, so a walking or running person will experience different insulation by the same fabric/garment than being immobile. In general, body motion decreases the insulation of a clothing ensemble by increasing air movement through spaces between yarns in the fabric and/or causing air motion within the garment. This effect varies considerably depending on the nature of the motion and of the fabric. This is why, an accurate estimates of clothing insulation for an active person is very challenging.

In a nutshell, the specifications and requirements for each garment will depend on the season as well as end use of the garment.

2.2 Heat retention strategies:

There are different ways to retain body heat next to the skin, by creating a micro-environment. Most of the strategies are focussed around interfering in the exchange of heat or air (which is supposed to transfer heat by convection) between the two sides of the garment. Some of the methods have been listed below.

2.2.1 Hollow yarns: Retained air inside the hollow yarn as “temperature cushion”

Going down to the yarn/fiber level, the trapped air helps in maintaining a microenvironment with still air that serves as an insulating barrier for heat exchange between its two sides.

Source: (http://www.ibwch.lodz.pl/pliki/IBWCh_(ssj1mlw0jkmii44y).jpg)

Apart from assisting in heat retention, hollow yarns also help in maintaining the lofty and heavy look without contributing a lot to the fabric weight.

2.2.2 Spacers:

Spacers are two or more layers of fabrics connected by filament yarns traversing between the layers as bridges.

Spacers have been tried out for various applications, mostly to replace foam and heavy fabrics for insulation. The capability to hold air in space between its two (or more) layers imparts spacers the ability to insulate heat across its thickness.

The disadvantages of using spacers have been the drape and thickness of the fabric. Most of the times, the garment ends up with space suit like appearance, due to the stiff nature of connecting filaments, a prerequisite in maintaining the spacer structure. Softer connecting filaments have been tried but that compromises the spacer structure, leading to instability of the air-pockets and so reduction in heat retention.

2.2.3 Tortured air passage

As mentioned earlier in this article, yarns with hairy surface contribute to heat retention by disturbing the passage air flow. This property, along with hygroscopic heat generation, has made wool the material of choice for winter wear. Other yarns, including polyester, have also been modified to achieve some level of heat retention by hindering the air passage, e.g. spun yarns, ATY, yarns with varied shrinkage, etc.

2.2.4 Quilted fabrics

In line with blocking or disturbing the air passage, various attempts have been made to develop warming fabrics structures by quilting. Garments with down and other fills have been very successful as they are able to not only block the cold waves from outside but also aid in heat retention by keeping the warmth inside the garment, again by creating the still air micro-environment.

2.2.5 Layered Clothing:

It has been widely understood that layered fabrics are able to retain more heat than single layer of thickness equal to all the layer, added together. This advantage is achieved by the still air retained between the layers in the arrangement. Additionally, different layers perform different functions based on their position. Usually at least three layers are identified as follows:

• Inner Layer (also called base layer or first layer): Usually with the soft hand feel, this layer is designed to provide comfort by keeping the skin dry.
• Mid Layer (or insulating layer): this is where warming/heating function is imparted. The fabrics with inherent heating properties (dry heat or hygroscopic heat generation) are usually slotted in the mid layer. A heated layer not only helps to create a warm microenvironment but also helps in insulation by disturbing the energy (heat) gradient across its two sides.
• Outer Layer: Also called Shell layer, it works as protection over the other two layers. For heavy winter applications, waterproofing/repellency has been recommended to ward off snow or water coming in contact with the garments’ outer surface. Another layer of insulation is provided by this layer as air is held between mid- and outer layer.

3. Active Heating

In recent past, interdisciplinary research has made it possible to generate heat by external sources, e.g. powered systems, to heat up the garments or parts of a garment.

Different components have been made to heat up and make the wearer feel comfortably warmed up in extreme cold conditions. Jackets, gloves, balaclavas, and various other products are available in the market which can heat up, using a power source, to a desired level to provide comfort from cold exterior conditions.

Image source: https://dragonheatwear.com/products/kaiser-mens-5-zone-heated-puffer-jacket

The heating is achieved by many methods, dominated by use of conductive yarns. Conductive yarns are many based on core-shell structure with polyester (and other polymers) core providing the mechanical properties while the metal shell (usually copper) is meant to conduct heat across its length. The yarns are integrated into the fabric either by weaving or by embroidery, special arrangements need to be made to knit the yarns into the fabrics. The challenges include yarn breakage, bending limitations for metal yarns, need of insulation, etc. Insulation is often addressed by adding another membrane of coating on the conductive parts of the fabric. Some new advancements have been to reverse the core-shell arrangement, having metal in the core and polymer in the shell (something like regular wires, but at a small scale of yarn thickness).

Other methods include use of carbon fibers, conductive prints and membranes, using graphene, although they are in early stages of their application.

4. Phase change materials:

PCMs, or phase change materials, draw special attention when it comes to thermo regulation. Phase change materials are able to regulate their surrounding temperature by changing their phase at specific temperature. The most common example is ‘water’. At its freezing point, i.e. 0°C (or 32°F) water transitions between liquid and solid phases, converting to liquid (water) above and to solid (ice) below this temperature. For practical applications in textiles, the temperature needs to be around body temperature.

Image source: http://www.microteklabs.com/how-do-pcms-work.html

The phase change that is suitable for textile application is ‘solid-liquid’ phase change which takes place at its melting point. The energy exchanged in this process is called ‘heat of fusion’

Commercially, most common materials used are paraffin or short chain lipids, which have wide range of melting points depending on chain length. Manufacturers are able to tailor the melting points by controlling the chain lengths.

Other materials used as PCMs are some carbohydrates, salt hydrates, etc. There are also solid-solid phase transitions (between crystal lattice structures) found in some materials, leading to significant amount of heat. Textile industry however has been limited to using liquid-solid transition of materials for practical reasons.

PCMs are loaded into fabrics in three different ways: at yarn level (for synthetic yarns), as fabric coating, and as fabric treatment (usually involving PCM loaded nanoparticles) wherein the material reaches and binds to individual fibers/filaments in the yarn.

PCM usage has met with many challenges, specially related to loading efficiency and saturation (once the phase transition has taken place and no heat exchange can take place). Saturation can be delayed by increasing the loading which is limited by loading capacity of the micro/nanoparticles or mechanical strength of the yarn (in case of yarn loading). Attempts have been made to increase PCM loading in particles by reducing the wall thickness, thus increasing the core volume. This requires shell to have additional strength which was achieved by crosslinking, mostly by formaldehyde. Concerned by health concerns, many manufacturers have successfully attempted to increase the particle stability by alternative cross-lining agents.

5. Testing Methods:

Performance of a textile material or a garment is tested by several methods. Listed below are some test standards suitable for respective applications

5.1 Thermal Insulation (Heat retention capability):

This property of a fabric or garment is tested by the following standards:

• Thermal Transmittance of Textile Materials (ASTM D 1518)
• Measuring Thermal Insulation of Clothing using a heated manikin (ASTM F 1291)
• Thermal and Evaporative Resistance of Clothing Materials using a sweating hot plate (ASTM F 1868-02)

5.2 Hygroscopic heat generation

This test determines the fabrics capability to generate heat merely by absorbing moisture (heat of absorption). The test involves measurement of fabric temperature, kept on the plate, while relative humidity is increased from 10% to 90% in a chamber with controlled temperature of 20°C. The test has been developed by GAP and for a fabric to be categorized as hygroscopic heat generating fabric, the rise in temperature must be 2°C more than the control (similar regular fabric or blank).

5.3 Dry heat generation

While there is no standard test for measuring dry heat (not hygroscopic) generation, most innovator study the performance by using a heat source (e.g. 500W halogen bulb) set at a fixed distance (equidistant) from the control as well as experimental fabric and measuring the temperature of the fabrics at set time points, using temperature sensors. For consistency of the study, the experiment must be performed in the controlled environment (temperature, relative humidity, etc).

The test is often extended to cooling phase as well, i.e. measuring the temperature when the heat source is turned off. This gives an idea of fabrics property to retain heat.

6. Conclusion:

Protection against extreme weather, especially cold, has been primary purpose of clothing and heating/warming fabrics serve just the purpose. In spite of several challenges in smart fabrics and commercialization of new technologies, there is sense of optimism, thanks to the advent of technologies aiming to perfect the heating and /or heat retention technologies. Manufacturers and innovators have been tirelessly working on to improve the capacity of fabrics or garments to provide maximum comfort in winter. Evolution of various technologies and strategical combination of technologies has made it possible to have different solutions for different conditions.