Clothing for Extreme Cold Weather PDF

Preview

Summary

Assignment # 1 Clothing for Extreme Cold Weather Material, Properties, Production and Testing Khubab Shaker NATIONAL TEXTILE UNIVERSITY, FAISALABAD Introduction: Clothing is the basic need of all human beings, serving the purpose of covering the body and also protecting from environment severity. Al...

Description

Accelerat ing t he world's research.

Clothing for Extreme Cold Weather khubab shaker

Related papers

Download a PDF Pack of t he best relat ed papers 

Limit at ions of using a single-exponent ial equat ion for modelling clot hing vent ilat ion George Havenit h

Evaluat ion of t he performances of elect rically heat ed clot hing Faming Wang Recent t rends and fut ure scope in t he prot ect ion and comfort of fire-fight ers’ personal prot ect ive clo… Shadi Houshyar

Assignment # 1

Clothing for Extreme Cold Weather Material, Properties, Production and Testing

Khubab Shaker

NATIONAL TEXTILE UNIVERSITY, FAISALABAD

Introduction: Clothing is the basic need of all human beings, serving the purpose of covering the body and also protecting from environment severity. Also the environmental conditions are not the same for people living across the globe, varying from extreme cold regions to hot and humid areas. Clothing of one type can’t serve the purpose of protection from every climate, keeping in view the comfort of wearer. Clothing for hot climates is normally loose and light weight, allowing evaporation of sweat easily. While the clothing of cold weather is normally bulky and used in more than one layer, providing thermal insulation and protection against wind penetration. Comfort is described as a measure of how well clothing assists the functioning of body. It is also defined as the condition of mind that expresses contentment with thermal environment [1]. It can be broadly classified into four types namely: thermal, sensorial, psychological and garment fit. The comfort range of human body is much narrow, i.e. where temperature regulation is achieved without shivering or sweating. The human comfort zone for physiological thermoregulation of body, wearing shirt and trousers is between 22.2°C and 25.5°C [2]. Thermal comfort outside this zone is provided by adding or removing clothing. The minimum indoor temperature suggested by WHO is 18°C, while for disabled or aged persons, it is recommended to be 2-3°C warmer [3]. The cold weather clothing is designed to protect the body or areas that are exposed to risky climatic conditions. Some of the methods to avoid unnecessary heat loss and discomfort include, putting on clothing, increasing physical activity, finding some protection, exposing to sunlight, etc. The cold protective clothing also helps to keep the heat and moisture balance of the body [4]. The cold weather clothing must be able to maintain the body’s heat balance, under any physical activity or varying environmental conditions. The human body can experience heat loss due to different situations like: cold air striking the body, immersed in water, contact with cold surface, inadequate physical activity or insufficient clothing. Heat transfer is the rate at which energy is transferred from a high temperature zone to a low temperature zone and will continue until equilibrium is established between both the media. The extent of heat transfer depends on the difference in temperature and resistance between both media. The different ways of heat transfer to environment from body [5]: Conduction: The transfer of heat by physical contact Convection: Transfer of heat by molecular motion of fluid Radiation: Heat transfer through space Respiration: Transferring by respiring Sweat evaporation A heat equation based on the heat produced and heat loss of human body can be expressed as: M–W=K+C+R+E+S 1

Where, M: Metabolic rate of body W: The mechanical work K: Heat transfer by conduction C: Heat transfer by convection R: Heat transfer by radiation E: Heat transfer by evaporation S: The heat stored [5] Human body has the unique property of vaso-constriction, in which the blood vessels constrict and blood flow to skin is reduced. It creates a shivering effect, which helps to produce some heat. On the other hand, body has also the ability to produce moisture, which evaporates, causing the body to cool.

Figure 1: Transfer of moisture and heat between skin and clothing

The transport of moisture in a textile material is usually by two common methods namely: diffusion and convection. Diffusion is the result of random molecular motion from an area of high concentration to area of low concentration. The moisture usually diffuses into the textile material through interstices between fibers or yarns in a fabric. The number of interstices is determined by yarn linear density, yarn twist, ends/inch, picks/inch, weave, etc. Fabrics having more air spaces, allow more diffusion to occur. Other phenomenon like adsorption, absorption or desorption also help in moisture transfer between the fibers and surrounding air. [4]

2

Properties required in cold weather clothing As the protection from cold is primarily achieved through clothing, it should be designed while keeping in mind the specific working and environmental conditions. For example, either the physical work is continuous or intermittent; the activity rate is low or high, etc. and ecological conditions like temperature, humidity, wind conditions, rain or snow fall expected, etc. The following material characteristics should be considered while determining the clothing adequacy for any cold weather: a) Thermal Insulation: It is the resistance offered by the clothing to heat transfer and is usually expressed in units of Clo. The insulation value is mainly depends on thickness of material, the ability of clothing to trap air and how dry the material is. The trapped air acts as a heat insulator. 1 Clo = 0.155 K-m2/W b) Evaporative Resistance: Moisture is produced on skin by sweating. In hot climatic conditions, the sweat is evaporated releasing heat of body and causing a cooling effect. The heat loss due to evaporation is not required in cold; the key thing is to transfer the sweat from skin to outside through clothing. The condensed sweat is absorbed by the hydrophilic fibers in the material and transferred to the clothing surface where it evaporates. The permeability of clothing expresses its ability to allow the passage of vaporized moisture through the material. The thermal insulation of clothing decreases by 30-50% if moisture transfer is not facilitated through the material. c) Water Resistance: It refers to the ability of outer clothing shell to resist the liquid moisture from outer climate to enter the material. As discussed earlier, a decreased water entry in the clothing will help to maintain the insulation value and ultimately the comfort of wearer. d) Wind Resistance: It is the resistance of a material to wind to pass through. The air flow through material carries heat of the skin surface along it by convection [6].

Design of cold weather clothing: It is vital in cold weather protective clothing to use multiple layer clothing, as it is more effective as compared to single thick layer clothing. The layering arrangement helps to produce several air pockets which trap air and provide insulation to the body. It will also aid to carry the perspiration away from the body. Each layer is designed to perform some specific tasks. The layering arrangement generally consists of the following three categories:

3

Base Layer: It is the inner most layer performing the wicking action. Being highly permeable, it pulls the moisture away from skin and passes on to the next layer. The materials used for base layer are usually quick drying, including: polypropylene, Polartec® and polyester microfiber materials. The dry skin increases wearer’s comfort. The wicking concept is also applicable to feet. Thin polypropylene socks are used as inner layer, under wool socks, thus keeping the feet dry and warm. Middle Layer: It is the insulation layer, which traps the warmth created by body heat, and at the same time allows perspiration to keep moving. The insulation layer is generally a combination of multiple layers rather than one. It can be either: Polartec®, Windstopper® and PowerStrech® materials, fleece, pile, wool or even thicker polyester. Outer Layer: The outer shell layer is to provide protection from wind and rain, and also needs to be breathable. The popular materials for outer layer are cordura, GoreTex®, Cloudburst®, Windstopper®, Trinity®, eVent® and Precip® [7]. The discussed above layering scheme is ineffective until the layers are used properly. It is the best approach to use the layering arrangement proactively. If you are to perform some physical exercise, take off some layer(s) before starting. A cold feeling will be produced initially, but once you have started, the body will get warm. And when you take a pause, put on those layers for extra insulation as production rate of heat is low now. The inner layer will not be moisture loaded and you will feel more comfortable. There are a number of design factors that affect both the physical comfort and thermal protection of the clothing. As the neck is more susceptible to cold, the clothing must have high collars, providing a turtle neck effect for greater comfort. The hood should also be insulated, minimizing head heat loss. Sleeve cuffs should be elastic, closing around the wrist to reduce convective heat loss from the arms. It is also essential to use the upper and lower body garments either one piece or overlapping. The cold weather clothing must be ventilating to remove water vapor from inside [6]. The protective gloves should perform well against convective and contact with cold surface. Additionally, the gloves should be flexible and water impermeable.

Moisture Condensation within Clothing It is essential for water vapors to pass through the clothing layers prior to condensation into liquid. And if the vapors get condensed into liquid, it must leave the outer layer of clothing before freezing. The moisture may freeze within the clothing in extreme cold environment and deposit between the layers. With decrease in the temperature of air, its moisture holding capacity in vapor form deceases. Let, the air is fully saturated with moisture vapor at a certain temperature. If the air temperature is decreased at this point, some of the moisture vapor will condense into water. The temperature below which vapors condense into liquid is called dew point.

4

Figure 2: Some approaches to reduce moisture buildup within clothing. Left Panel – Multiple insulation layers increase the chance of moisture to attain Dew Point, and freeze between two clothing layers. If this happens, the ice can be brushed away from the clothing surface. Center Panel – Under heavy work conditions, the skin temperature will increase. The new temperature line shows that now the Dew Point will occur after the moisture has passed the outside layer of the clothing. The freezing will occur within the material, if the initial skin temperature is low, as depicted by light line. Right Panel – It highlights the importance of increased skin temperature to force the moisture vapors through the outermost layer before condensation and freezing. The surface material is usually liquid resistant and vapor-permeable. It is clear from the above layering arrangements that the outer layer should not be combined with the middle insulating layer. Even if the condensation and freezing occurs on inside of the outer shell, it can be brushed off easily [6].

Different Methods for cold weather protection Wool has been in use for cold weather apparel since a long time. The sweaters and shirts made from wool offer soft, matchless warmth and are highly compressible. Wool fabrics are inherently hydrophilic compared to other fibers. This hydrophilic behaviour of wool can be an advantage (absorb liquid sweat from skin) or disadvantage (absorb huge amount of liquid from surrounding) in cold conditions. Wool has been largely replaced by synthetics such as polyester. The synthetic insulators are very common because they are breathable, lightweight and insulate even in wet state. Also they are fast drying and provide a high warmth-to-weight ratio [8]. From a wearer’s point of view, comfort is more important rather than the material from which it is made. The priorities in cold weather clothing depend on activities carried 5

out and environmental conditions. Clothing thickness, thermal resistance, compressibility and air permeability are some properties relevant to cold weather clothing. The thermal resistance is directly related to the thickness [9]. Cold weather clothing can be either a garment comprising a number of layers of fabric or different garments can be worn, one on top of the other. The thermal resistance of multiple layered clothing will increases with more number of layers, with most dominant effect of the outer layer air permeability. The inner layer, next to skin is typically knitted wool/cotton blend, and outer shell is a woven polyamide. Other layers are woven, non-woven or knitted structures of wool/cotton blend, polyamide and polyester. With this assembly, the humidity at skin surface was low, no liquid was present on the skin surface, but the humidity outside the coat (but inside the outer shell) was higher demonstrating that more moisture was shifted from skin surface to the outer layers [10]. Phase change materials (PCM) are the future 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 [11]. 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 [12].

Fig 3: Adaptive comfort using smart PCM (Outlast Thermocules)

6

The PCM can be applied to textiles in a number of ways like coating, finishing, lamination, bi-component synthetic fiber extrusion, melt spinning, injection molding and foam manufacturing. Certain products contain micro encapsulated phase change material (microPCMs), 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. A special four layer clothing system has been developed for cold weather protection. The inner layer is prepared of cotton fabric, the 1st insulating layer is a nonwoven, made from polyester and treated with micro PCM. It is combined with 2nd insulating layer of non-woven polyester, while the outer shell is a breathable waterproof fabric. This ensemble has been tested in a climate chamber at a temperature of –15±5°C on a bionic skin model. When temperature of PCM layer rises above the melting point, the PCM melts into liquid and energy is stored during this process. When temperature falls below 27°C, liquid PCM changes phase into solid and releases energy [13].

Fig 4: Structure of PCM clothing assembly A further development to cold protective clothing is the Extended Cold Weather Clothing System (ECWCS), developed in the 1980s by the US. The 1st generation ECWCS consisted of 22 individual clothing, used in diverse combination. It was designed for US army to meet their cold weather requirements. The detail of ECWCS clothing assembly (1st generation) is: polypropylene undershirt and underwear, polyester fiberpile shirt and bib, nylon/cotton trousers, Gore-Tex trousers, liners for trousers, white (snow camouflage) trousers cover, gloves, glove inserts, mittens, mitten inserts, mitten shells, white mitten shells (snow camouflage), cap, balaclava, nylon socks, cold weather vapor barrier boots and extreme cold weather vapor barrier boots and M-1950 trouser suspenders. The polypropylene undergarments serve as inner layer, the polyester shirt/bib, liners and cotton/nylon trousers as middle layer, and the Gore-Tex garments as outer layer [14]. 7

Biomimetics also finds its role in the outdoor clothing applications. It is the use of biology concepts in the manmade world. This word has Greek origin: Bio– means life and –mimesis means to copy [15]. A study was made by a British team to investigate into the ability of penguins to survive in extreme cold conditions. It was revealed that it is the result of their coat, a combination of feather and skin. They have the ability to switch their skin into insulating or waterproof, by a muscle connected to the feather. As this connecting muscle contracts, the skin turns into waterproof and when it is released, it changes into an air filled windproof coat. This phenomenon is termed as adaptive insulation. Attempts have been made to interpret the phenomenon of adaptive insulation in garments. The structure developed is a two layered fabric, which are joined together using strips of textile material at some angle to the plane of these two layers. Skewing these parallel layers decreases the volume of air trapped between them, which results in decreased insulation. A jacket (namely Airvantage) has been produced using ePTFE membrane along with a polyester structure (24% PTFE, 76% PE), which allows the user to control the quantity of air for suitable insulation [15].

Fig 5: Structure of two layer Biomimetics fabric assembly Another theme of research on insulating materials is the development of shape memory polymer coating based on polyurethane (PU). The polymer has elastic memory effect and it responds at molecular level. When the wearer’s body temperature increases, the molecular structure of the polymer coating tends to open, escaping the body heat and vapour. When the atmosphere is cold, the molecular structure is closed and heat loss to the environment is prevented. This polymer can be applied over polyester, cotton, nylon and silk, and the coated fabric is termed as dream cloth (developed by Mitsubishi) [16].

8

Some examples of cold weather clothing The cold weather protective garments can be classified into the following categories on the basis of their specific end use: Water Proof and Breathable: These are usually the most functional and costly garments, ideal for strong rain and heavy work conditions. They are not perfect as workers who perspire heavily still feel damp, as water-proof/breathable membranes have a limited rate at which moisture passes through. Examples of such garments include laminate fabrics that feature a nylon or polyester on the inner and outer layers, with a waterproof breathable membrane in the center. The concept behind breathable fabrics, which is fabrics that repel water but allow perspiration to escape, are tiny holes in the material that are too small for water droplets to pass through, but large enough for water vapor (perspiration) to escape [17]. Water Resistant and Breathable: These are excellent for mild weather and light precipitation, suitable in high activity levels. These are often made of closely woven fabrics that block wind and slight rain. The outer surface of fabric is treated with a DWR (Durable Water Repellant) coating such as Teflon®, which causes the water to run off. Repeated washings results in less effectiveness. Waterproof and Non-Breathable: These clothing are ideal for rainy days with light duty work, usually made with PVC or other coated fabric. It has a little tear strength and lacks in breathability, can make the workers feel uncomfortable. It can be an excellent choice for situations such as being worn for limited periods of time; the need to be disposed of due to contamination; or light workloads. Soft Shells: These shells provide good rain and wind resistance and also excel at breathability. Soft shells also offer insulating properties and protection from weather. These clothing mostly feature a stretch fabric or stretch panels for added comfort. They are ideal for light precipitation an...