Garments for High Altitude Ranges – For Military Personals

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Note: This Article was Drafted in the year 2009, Publishing this today as an academic exercise for Knowledge Dissemination.

  1. Thermal Garments an introduction:

Thermal garments provide warmth to our body and make us to feel comfort even in the sub zero temperature conditions. Apart from the normal clothing performance these garments should perform with required functional properties to meet with the climatic conditions. These garments are mainly divided into three different structures and apart from these structures additional gears are also considered along with the garments. Three different structures are inner structure, mid layered structure, protective layers. The extra gears are like helmet, gloves, shoes, socks etc.

Cold is a hazard to human health. Cold environments may affect human physiological as well as human psychology. The main aim of the thermal garments should be prevention of cooling and maintenance the heat balance at acceptable temperature level. The aim of thermal garments is to give barrier effect. The clothing should shield the body from outside condition1. Ergonomics and psychological consideration plays important role in determining the performance of thermal garments.

The comfort factor is important for all garments, while considering thermal garments comfort consists of various factors like weight of the garment, breathability, thermal insulation, permeable to moisture vapor, wind resistance etc8.

The acceptance of uncomfortable clothing by the users was sometimes very low and the purpose of that clothing may not be fulfilled. The aim of the functional clothing is to give protection from rain, snow or wind. The construction of the textile fabrics must fulfill the requirements, like the fabric should be wind proof and water repellent, water vapor transmission should be possible from the skin to outside of the clothing and winter clothing should be heat insulated2. Along with all, thermal comfort is important, which is defined as “Condition of mind towards the environment which express the satisfaction of the garment”. Thermal comfort comprises of important factors like, perception of warmth, humid, transportation of vapor, etc.

  1. HEAT AND MASS TRANSFER

To study about the thermal resistance and evaporation one should know about the heat and mass transfer mechanism. Especially heat transfer mechanism like conduction, convection and radiation and its significance.

Heat transfer is energy in transit due to a temperature difference. Heat exchange between the body and the environment takes place at the skin surface by three ways namely, conduction, convection and radiation. Existence of a temperature difference between or in a medium leads to heat transfer. Heat and mass transfer plays important role while evaporation and body metabolism.

2.1 CONDUCTION

Conduction refers to the heat transfer that will occur across the medium. It can also be viewed as transfer of heat energy from the more energetic to less energetic particles of a substance due to interaction between them. Heat loss due to conduction is less5.

2.2 CONVECTION

Convection refers to the heat transfer that will occur between the surface and moving fluid, when they are subjected to temperature difference. In the case of wind this transfer occurs. Heat loss due to convection is higher. When the air flows along the skin, air is cooler than skin. Heat will transfer to the air from the skin.



Fig. 1 Heat Transfer mechanism

2.3 RADIATION

All surfaces emit energy in the form of electromagnetic waves. In absence of intervening medium, there is a net heat transfer by radiation between two surfaces at different temperatures.

Apart from convection, conduction and radiation, heat loss may occur due to evaporation and respiration5. In the generated heat approximately 10% of human heat loss due to breathing, the majority is drawn off through skin. This loss of heat occurs in two ways, a dry heat transfer and moist heat flow. Dry heat transfer which is of radiation and heat conduction. Moist heat flow due to the evaporation of perspiration, we can say it as moisture transfer. Heat exchange from the skin to the environment is mainly influenced by the air temperature, humidity, wind speed, solar, sky, and ground radiation, and clothing.

2.4 WARMTHNESS

As we know very well, when the fiber absorbs moisture, heat is evolved. Apart from the moisture absorption property, this absorption of water causes the fabric to act as a heat reservoir, protecting the body from sudden changes of external conditions. Absorption also changes its structural features, cause the material to swell. This swelling results change in size, shape, stiffness and permeability of yarns and fabrics9.

Assessment of the protection requirements in cold environments requires information about the energy metabolism of the individual. Metabolic rate is related to the intensity of physical work and can be easily determined from measurements of oxygen consumptions.

Protection against cold is provided when the human body is in heat balance at acceptable levels of body temperatures. Our body temperature is quite stable. This is achieved by balancing the amounts of heat produced in the body with the loss5. Heat losses are equal to metabolic heat production. The following equation describes the heat balance,

    S = M – C – R – E – RES (W/m2)

Where,

S – Rate of change in body heat content,

M – Metabolic heat production,

C – Convective heat exchange,

R – Radiative heat exchange,

RES – Air way heat loss.

  1. How we are feeling warm:

in our body there are two sensors which govern the comfort feel in thermal sensing. Skin and hypothalamus are the sensors. Hypothalamus starts to execute its function while the body core temperature reaches exceeds 37˚c by providing necessary cooling action. Skin sensors are cold sensors which work to control the skin temperature, when it starts to fall below 33 – 34˚c. Conditions for both skin as well as core body should be maintained. Perception of cold or hot is totally depends on the individual persons. Various reports say black colored persons can withstand the hot climate but the resistant towards the cold climate is much lower when compared to the white colored persons. Like the same fat person and lean person. Fat persons are having high resistance towards the cold climatic conditions than the lean persons.

  1. Factors to consider in extreme cold condition:

The protection against cold environment depend on the following main factors like metabolic heat, wind chill, thermal insulation, air permeability and moisture vapor transmission. The survival depends on the balance of heat losses due to the above said factors4.

  1. Body metabolism:

Human beings are homoeothermic; our body always maintains the two different temperatures in our body system, body core temperature and skin temperature. Our body main core temperature is 37°c and our skin temperature closes to 33.4°c within the limits of ±1.5°c. In these temperatures body feels comfort, once tolerance of ± 4.5°c occurs body feels discomfort. In general body always maintains this temperature limits. In the body metabolism the body core temperature is maintained at a temperature of 37˚c. When the body becomes warm, the blood vessels vasodilate and increase in flow of blood occur. And our skin starts to sweat. While body attains cold, blood vessels restricted the flow of blood by vasoconstrict. This causes the increase in the internal heat generation by stimulating the muscles, due to this only we are getting shivering3. Fig (2) shows the human body adaptation temperature range.


Fig. 2 Temperature range and human adaptation7

Role of clothing is important towards the environment as well as our body metabolism. Clothing acts as a bridge between the two factors.


Fig. 3 Body – Cloth – Environment interaction

The function of cloth is to support the thermoregulation of the body. If the environmental temperature is low, the clothing has to prevent too large heat release and on the other hand if the temperature is high it has to give better comfort by allowing the sweat to evaporate thro the garment. The heat and mass transfer through the garment should be as high as possible to avoid problems to the body metabolism. Our clothing should perform better at the lowest level of activity. As we have discussed our body generates heat during performing some work. So at rest our clothing should give better insulation property to avoid loss of heat, and during rest our body generates much lesser heat comparatively. On the whole, our clothing should give comfort in both cases by providing thermal insulation at rest and providing moisture vapor transmission while work.

  1. WIND CHILL

Wind chill plays important role in heat loss process. Wind chill is determined by the number as factor. It purely depends on the temperature and velocity of wind. Around 80% of the total heat losses are due to wind chill effect4. Wind chill can be determined by the formula (1). Wind chill factor helps us to learn about the freezing time of exposed skin. Higher the wind chill factor freezing time for the exposed flesh will be lesser.

Κ0 = ((V×100)½ + 10.45-V) × (33-TA)            (1)


Where,

Κ0 =
Cooling power of the atmosphere in Kcal/m2/h,

V = Wind velocity in m/s, and

TA = Air temperature in °c.

Decrease in the air temperature and increase in velocity results increase in wind chill factor. Following table gives us the idea about the effect of wind chill number over the comfort.

Table. 1 Effect of wind chill factor on comfort.

Wind chill factor

Effect – comfort – feel

1000

Very cold

1200

Bitterly cold

1400

Exposed flesh freeze in 20 min

1800

Exposed flesh freeze in 10 min

2400

Exposed flesh freezes in 1 min

Table. 2 Wind chill temperature


This table gives us the information about the wind speed and temperature and its effect in terms of danger.

US Army express the cold stress level with respect to the wind chill factor. Cold stress is the condition of environment which tends to remove body heat and decreases the body temperature. Cold strain refers to the psychological or physiological consequences of cold stress6.

  1. THERMAL INSULATION

Thermal insulation defines the resistance to heat transfer by convection and radiation by clothing layers. It also determines the resistance to heat exchange in all directions and over the whole body surface. The insulation of clothing and adjacent air layers is defined as the total insulation values (It) and defined as follows, the value of I is given in m2 ˚c/W or in clo – units.


    It =

The effective way for protection against cold is to optimize the clothing insulation, so we can avoid heat loss due to heat exchange. Protection from cold also depends on the body metabolic rate; if it is less the required thermal resistance will be higher to maintain balance. In contrast to this at high metabolic rates the generated heat must be transferred to environment, this can be achieved by reducing the number of insulating layers. Insulation increases with increases in number of layers and thickness of clothing. Insulation is build up by air layers by trapping them inside the structure and minimizes internal convection. Better thermal insulation can be obtained by controlling thermal conductivity of the material. Textile materials are good thermal conductors. To design them to meet thermal insulation we have to trap the air inside the fabric, air is the good thermal insulator. In the cold, there is a steep in the temperature gradient from the skin across clothing layers to the ambient air. The dew point temperature and also the freezing temperatures may reach inside the clothing which causes condensation. This moisture build up leads to wetting of clothing layer, results in poor thermal insulation.

Table. 4 Thermal conductivity of various textile fibers.

Fiber material Thermal conductivity (mW/mK)
COTTON 71
WOOL 54
SILK 50
NYLON 250
POLYESTER 140
POLYPROPYLENE 120

From this table (4) we can strongly conclude the thermal conductivity of textile materials. Basically textiles are good thermal conductor so by trapping air or giving some special coatings we can improve its insulation property. The nature of wool fiber such that due to its lofty and bulky nature it can trap more air inside its fiber arrangement and this entrapped air serves better role towards the thermal insulation. Apart from these wool fiber can generates heat if it absorbs moisture, which is called heat of sorption.

US Army report shows us the thermal insulation of various garment item, which states that thermal insulation is higher in case of fleece material and the table gives us the idea about the insulation behavior of various products.

Table. 5 Insulation values for various army materials6.



Thermal insulation value strongly influenced by the wind velocity. As the velocity increases the thermal insulation decreases.


Fig. 4 Effect of wind speed on insulation

The graph shows us the rate of decrement of insulation as the wind velocity increases. The main reason is that the structural distortion of the fiber or air pockets, which directly influences the decrease in thermal insulation. Research work4 shows us, the insulation achieved by the polished surface is 40% higher than the blackened surface.

Fig. 5 Effect of insulation with increasing thickness in two surfaces

Thermal insulation can also be provided by the reflecting layers such as aluminium foils. Aluminium can be coated into the structure or layered with the textile structure to provide required thermal insulation. Perfect insulation can be achieved by having low compression / high resiliency to maintain the entrapped air inside the structure.

  1. AIR PERMEABILITY

Permeability mainly influenced by the structure of the textile materials. Irrespective of their type of structure each structure has their own permeability factors. Permeability also plays vital role towards comfort of garments.

If the air permeability is high during wind chill conditions the structure may not solve the purpose. Higher permeability results distortion of air pockets in the structure and the thermal insulation becomes less.

  1. MOISTURE VAPOR TRANSMISSION

Our body continuously generates heat energy to facilitate comfort to with the environment. Performance of the clothing system at cold regions not only depends on the material thermal insulation characteristics, along with that the material should have moisture vapor transmission behavior. Decrease in environmental temperature results condensation in the structure. If the cloth becomes wet due to the vapor condensation or absorption, presence of moisture leads to the increased thermal conductivity. Which results sudden drop in the body temperature and in some cases results dangerous situation with respect to environment condition. Water vapor resistance of cloths increases in low temperatures, such as in mountains. Although in some cases the resistance of cloth decreases due to the reduction in pressure. Condensation is high at high altitude because of reduced vapor diffusion resistance and lower saturated vapor concentration at lower temperatures. Reduction in moisture vapor transmission enhances the condensation as well as discomfort. Apart from these there is a chance of hypothermia which can lead to fatal at high altitude10.

Water vapor transmission can be increased by increasing the pore size of the textile structure. The vapor transfer rate is determined by the concentration gradient, which is depends on the temperature. The research11 shows, the thermal insulation, moisture vapor transmission results for the different structures like polyester fleece, micro porous membrane, PVC layers. These layers were subjected to different sweat levels.


Fig.6 PET Fleece and PET Knit structure

In the fleece structure vapor can pass through much better than the micro porous membrane and PVC. Higher air permeability of fleece structure facilitates water vapor transport to the next layers. Fleece structure can hold more water vapor in their inter spaces which results low thermal insulation behavior. Fleece structure shows higher temperature and lower vapor pressure than the micro porous membrane in the cold condition. The reason is higher thermal insulation property of the fleece structure. In case of micro porous membrane higher vapor pressure is due to the condensation which blocks the pores.

  1. Materials and its design:

the material used for this purpose must possess high thermal insulation characteristics, but as we have discussed our textile materials are poor in thermal resistance. But as we know well that air is a good insulator hence trapping air we can achieve the required property. Effective thermal garment can be designed on the principles of layering concept. Several layers of fabrics laid one over the other to form the structure. Protection against cold is mainly depends on its insulation behavior, which depends on the structure thickness. This structure performs best with the comfort and thermal insulation. Layered structure principle is divided into three main layers namely, inner layer, mid layer, outer layer.


Fig. 7 Layering concept – Multi layers of textile structures

The inner layer is worn next to skin, which have a contact with the body and controls the microclimate temperature and humidity. As we know the body activity generates heat energy. At low activity the heat energy is less and to avoid imbalance, convection supposed to take over there, for this the fabric layer must reduce the air movement. With high activity the generated heat energy cause raise in body temperature. The generated heat and moisture should transport out of layers to cool the skin.

Moisture control can be performed by absorption and transported to next layer or by ventilation. Presence of moisture/ water leads to lesser insulation performance, because water conducts heat much more than air. So the inner material is such that absorbs the moisture and dries the inner portion to keep it dry. Absorption reduces skin humidity and makes body more comfort. If the moisture remains in the body system cause imbalance in heat at further stage.

Microclimate temperature quickly rises with sweating, in this case if we are using hydrophobic fibers such as polyester, polypropylene as a structure material in the inner layer which is worn next to skin, gives better moisture transport to the next layers. But in the case of cold protection wetting of layers must be avoided at all time. The conditions are such that, inside sweating or outside rain or snow. Moisture and moisture accumulation control in cold environment is important.

Middle layer provides insulation. Depending on requirement it comprises thicker layer of materials. In winter the body activity is low, hence the heat should not escape from the body through the garment. So the mid structure should provide insulation by trapping the air inside its structure, as we know more air inside results better insulation. Selection of material for this reason is completely arbitrary.

Outer layer must provide protection against environment conditions like wind, rain, snow fall, abrasion, etc. This layer acts as a wind cheater incase of high wind rates. The type of material may be of coated fabric, micro porous structure where moisture vapor can pass through but wind and water droplets can not.

The level of insulation mainly depends on the percentage of air trapped inside the textile structure. Table shows us the percentage of air trapped and the fiber quantity in various textile structures. From this table we can say the Fur coat and Blanket having much more quantity of air than the fiber. And those two materials show better thermal insulation performance ion the day to day uses.

Table. 6 Percentage of air in the textile structures

Type of materials

Air %

Fiber%

Suiting fabric

75%

25%

Blanket

90%

10%

Fur coat

95%

5%

Apart from material characteristics, the design and fit of garment plays crucial role in thermal properties. Heat transfer may influence by the body movements too. The air gaps between the layers and openings results different effects on its thermal insulation. 75% of the heat losses are due to loss through the openings at the place like the neck, the waist, the wrist and the ankles.

The Cold Weather Clothing System (ECWCS) of US Army has 23 components—a mixture of clothing items, hand wear, headwear, and footwear—and uses moisture-management principles to move perspiration away from the skin. The current method for cold weather dressing is to layer the protection to trap air and also to allow some layers to be removed as needed. The concept of the clothing is to move perspiration away from the skin (using man-made fibers that do not absorb moisture) and to prevent outside moisture from penetrating the clothing (by means of a directional flow nylon and PTFE laminate material). Even with this system, it is still necessary to minimize the amount of perspiration produced by venting the clothing or removing clothing items when not needed to maintain warmth. Because the fabrics in the undergarments perform a wicking action and the outside garments rely on microscopic pores to repel moisture, it is very important to keep the clothing as clean as possible7.

Extend Cold Weather Clothing System structure comprises in the following manner, the long under wear which controls the microclimate of the body metabolism.

            

Fig. 8 Inner Thermal Wears (ECWCS)

The material used for this structure is polyester, which absorbs the moisture and allows it to transfer through the layers. This inner garment should not be of natural materials, which absorbs moisture and never give the dry feeling.

Next to the inner wear there is fleece structure which incorporated to give thermal insulation. Normally this insulation material thickness is depends on the nature of soldiers work load. If the soldier is subjected to some active work, in such case this layer may not need for him. The outer garment for the soldier would be the Gore-Tex micro porous membrane incorporated jacket. This provides the moisture vapor transmission. But rate of vapor transmission is limited. So this may not be suitable during exercise, where the rate of moisture transmission should be more to avoid moisture vapor condensation.

  1. Thermal Garments for military personals AND ITS NEED:

Securing our high altitude borders and lands is important in the international arena. Each and every country spends around approximately 40% of its budget wealth to their army and military purposes. The task to provide safety and comfort to the soldiers at high altitudes and sub zero conditions is difficult and costliest. Military personals in the sub zero conditions and high altitudes faces lot of problems both in psychology and physiological way. The main problem faced by the military personals in the cold region is “cold injuries”. The major cold injuries are frostbite; trench foot, immersion foot and hypothermia. Apart from these injuries, there are three major clinical problems can occur with acute exposure to high altitude. They are Acute Mountain Sickness (AMS), High-Altitude Pulmonary Edema (HAPE), High-Altitude Cerebral Edema (HACE) 7.

Perfect thermal garments for military personals should possess following characteristics,

  • Lesser weight
  • Lesser bulky
  • Higher warmth
  • Good moisture vapor transmission

In a different way, our garment should protect us from COLD by,

  • Keeping structure Clean
  • Should avoid Over heating

  • cloth should Likely and Layered
  • Structure should be Dry.


Fig. 6Types of cold injuries7

The signs of AMS are rapid or irregular breathing, rapid pulse, and vomiting. Symptoms include headache, nausea, depressed appetite, generalized weakness, and dizziness. The second condition is High-Altitude Pulmonary Edema (HAPE), in which fluid accumulates in the lungs. The symptoms of HAPE include a persistent cough; discharge of a pink, frothy sputum; disorientation; and fainting. Signs include cool, clammy skin; rapid breathing; a rapid, weak pulse; and blue lips. The third condition is High-Altitude Cerebral Edema (HACE), which causes swelling of the brain. Signs of HACE include bizarre behavior, hallucinations, confusion, excessive fatigue, and coma7.

Frostbite is a freezing injury, due to the uncomfortable cold. The exposed skin becomes red to waxy white. The areas most likely to be affected are the nose, toes and fingers, ears, cheeks, forehead and exposed wrists. The fig (7) shows us the injuries due to frost bite.

Trench Foot occurs due to long term exposure of feet to the freezing cold and moisture. With trench foot the feet becomes pale and feel cold, tends to feel difficulty to walk.

Hypothermia occurs when the body is unable to maintain adequate warmth and the core body temperature7. The person may loose their whole control and may feel uncomfort even to think.

        

Fig. 7 Injuries due to Frostbite

Exposing of our body to cold environments may lead us to death. Keeping warm our body is difficult task by the external means, even though our body metabolism plays effective role to control our core body temperature with respect to environmental temperature. In such cases clothing plays vital role to cover up our body and provide warmth to our body.

  1. DMSRD position in development of thermal garments for high altitude zones:

DMSRD developed a garment for Indian military personals. This made up of acrylic pile fabrics and aluminized layer as insulating inner layer. This aluminized fabric kept closer to the body surface to avoid heat loss. The outer most layers designed as wind cheater with polyurethane coated nylon fabric.

The lower trousers were designed with the concept of multi layered and multi component materials. Structure consists of polyester batting and layer of aluminized fabric sandwiched together. The outer most layers designed with respect to Indian environment and climatic conditions. On the whole inner structure mainly designed with acrylic and polyester fabrics and outer structure designed with coated and treated nylon structure.

Treating the wool fiber with low temperature plasma results change in its mechanical properties, air permeability and thermal properties. Low temperature plasma technique is used to modify the textile surface structures. Action of the low temperature plasma is mainly active with the surface to influence its surface characteristics. Low temperature plasma increases the thickness and alters the surface morphology. Like induces certain degree of roughness. This changes in the surface structure results changes in the air permeability. Treated fabric shows higher thermal insulation than the un treated. Etching with low temperature plasma results creation of rough surface with voids and spaces. This increases the quantity of air trapped inside the structure, which facilitates higher thermal insulation12.

  1. Conclusion:

    Designing garments for the military personals for high altitude ranges is difficult task and to execute this we need to consider various parameters like Heat transfer rate and loss of metabolism, environmental conditions, wind and snow fall rate, soldier’s physique. Apart from these choice of materials, material properties, design features etc. Developments in the fiber science may lead us to extreme different route; incorporation of microfibers in to the inner structure may provide better insulation property due to their larger surface area. And in the mean time reduction of weight can be achieved easily. Introducing nanotechnology into the thermal clothing science may also give us favorable results. On the whole various works shows us the successful design is multi layered structure and still used widely against to cold.

References:

  1. Textiles for protection “Textiles for Protection against Cold”, J Holmer, Lund Technical University, Sweden.
  2. Current problems in Dermatology, Vol. 31, Editor G.Burg, “Textiles and the skin”, Editors: P.Elsner, K.Hatch, W.Wigger Alberti, “What dermatologists should know about textiles”, Maximilian Swerev.
  3. V K Kothari, “Thermo-Physiological comfort characteristics and blended yarn woven fabrics”, Indian Journal of Fiber and Textile Research, Vol 31, March 2006, PP 177-186.
  4. G N Mathur, Hans Raj and Nishkam Kasturiya, “Protective Clothing for Extreme Cold Region”, Indian Journal of Fiber and Textile Research, Vol 22, December 1997, PP. 292-296.
  5. Current problems in Dermatology, Vol. 31, Editor G.Burg, “Textiles and the skin”, Editors: P.Elsner, K.Hatch, W.Wigger Alberti, “Clothing and thermoregulation”, George Havenith.
  6. TB MED 508, Technical bulletin, “Prevention and Management of Cold Weather Injuries” Department of US Army, April 2005.
  7. Donald E Roberts, Murray P Hamlet, “Prevention of Cold Injuries”, Medical Aspects of Harsh Environments, Vol 1, USARIEM, US Army.
  8. Ljubiša Milenković, Petar Škundrić, Radomir Sokolović, Tanja Nikolić “Comfort Properties of Defense Protective Clothings” The Scientific Journal of FACTA UNIVERSITATIS, Series Working Living Environmental Protection, Vol 1, No 4, 1999, PP. 101 – 106.
  9. Morton W E & Hearle J W S, “Physical properties of textile fibers”, 3rd Edition, Textile institute, 1993, PP 590.
  10. T Fukazawa, H Kawamura, Y Tochihara, T Tamura, “Water Vapor Transport Through Textiles and Condensation in Cloths at High Altitudes – Combined Influence of Temperature and Pressure Simulating Altitude”, Textile Research Journal, Vol 73 (8), 2003, PP. 657 – 663.
  11. Eunae Kim, S J Yoo and H Shim, “Performance of Selected Clothing Systems Under Subzero Conditions: Determination of Performance by a Human – Clothing – Environment Simulator”, Textile Research Journal, Vol 76 (4), 2006, PP. 301 – 308.
  12. C W Kan, C W M Yuen, “Low Temperature Plasma Treatment for Wool Fabric”, Textile Research Journal, Vol 76 (4), 2003, PP. 309 – 314.
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