The Present Situation and Developmental Trend Clothing for Cold Protection

【Abstract】This paper discusses the harm of low temperature to human body and the necessity of protection. The effects of fiber material, floc thickness, fabric structure parameters and air layer thickness on warmth preservation were analyzed. The authors put forward that low temperature protective clothing should be lightened, moisture permeable and active.

【Keywords 】 low temperature protective clothing; radiation heat; warmth; air permeability; low thermal conductivity

0 Préface
In low temperature environments, such as working in biopharmaceuticals, cables, low temperature test chambers, building materials, cold processing of metal castings, low temperature gas transportation, laboratory research, frozen food processing and any other places that prevent extreme cold, the body surface will lose heat. , causing injuries such as frostbite and freezing of human tissue, and severe cases will faint. Generally, the common clothing for low temperature protective clothing is the thermal clothing that resists above -40 ℃, and for the temperature below -40 ℃, ordinary protective clothing will limit the protection of the human body, so better protective clothing is required. Existing low-temperature protective clothing includes passive and active types. The passive thermal clothing is generally composed of three or more layers of outer fabric, thermal insulation batt layer, and comfort layer, and the outer fabric is usually covered with a waterproof and breathable membrane. The fabrics used in low temperature protective clothing should not only keep warm, but also have physical and chemical properties that do not change due to long-term low temperature. Because most fabrics are used continuously under ultra-low temperature conditions, the crystallinity of the fabric changes, resulting in the mechanical properties of the fabric. Change, conventional polyester or glass fiber has limited low temperature resistance, after -40 ℃, the fabric will be embrittled and cracked. Taking into account the needs of low temperature protection in different environments, phase change materials and coatings are used to make thermal fabrics (I5-61; in addition, the clothing fabrics should have good cold resistance, water resistance, snow and wind resistance, and can be used in extreme low temperature conditions. The performance of the bottom material will not change greatly. In addition, in the low temperature emergency or rescue, the low temperature protective clothing should not only have the function of keeping warm, but also have certain requirements on flame retardancy and strength.
This paper summarizes the test standards of low temperature protective clothing, and expounds the protective mechanism of low temperature protective clothing. Fiber, functional fibers can be added to the flakes, which can be functionalized while achieving warmth.
1 Standard for low temperature protective clothing
There are relevant standards at home and abroad when evaluating the effect of low temperature protection, among which the foreign standards are IS011092:1993 “Determination of thermal resistance and moisture resistance under steady-state conditions of physiological comfort of textiles”: ISO5085.1-1989 “Testing of thermal resistance of textiles No. 1″ Part: Low Thermal Resistance”: American Society for Testing and Materials ASTMD1518-2011 “Standard Method for Testing Thermal Conductivity of Textile Materials”: Japanese Industrial Standard JISL1096-2010 “Test Method for Woven Fabrics”, and different test standards correspond to different test methods. There are some structural differences. Domestically, the GB/T11048 2018 “Determination of Thermal Resistance and Moisture Resistance of Textiles under Steady-State Conditions for Physiological Comfort of Textiles” is used for thermal insulation testing, and the physical and chemical properties of clothing fabrics do not change under low temperature conditions. In the test comparison, Chen Xin et al. GB/T 35762-2017 “Test method for heat transfer performance of textiles”, GB/T11048-2018 “Determination of thermal resistance and moisture resistance under steady-state conditions of physiological comfort of textiles (evaporative hot plate method) 》The standard thermal insulation was compared and tested, and the thermal resistance measured by the evaporative hot plate method was greater than that of the flat plate method, but the test data of the flat plate method were more stable. In addition, most domestic tests use the warm body heating method to measure warmth retention.
2 The protection principle of low temperature protective clothing
Low temperature protective clothing fabrics are mainly clothing worn by the human body to maintain the normal body temperature of the human body when working in cold environments and extreme environments. The types of existing low-temperature resistant clothing are mainly passive and active: Among them, the passive low-temperature protective clothing mainly aims to prevent the rapid loss of heat in the human body and form a microclimate layer between the human body and the clothing fabric [9] Clothing fabric A large amount of still air is stored between the layers to block the conduction of heat. Since the thermal conductivity of still air is close to 0.02, its extremely low thermal conductivity will effectively isolate the heat transfer so that the heat will not be transmitted to the outside world, and the radiant heat generated by the human body will be reflected between the multi-layer clothing, reducing the transmission of radiant heat. Figure 1 shows the microclimate environment formed between the human body, clothing, and the outside world. Additionally the heat transfer capability of a single layer fabric affects the heat transfer from layer to layer. On the contrary, active low temperature protective clothing mainly uses advanced technology to convert other forms of energy into thermal energy to maintain human body heat, and also uses phase change materials to keep fabrics warm at low temperatures.

3 Factors affecting the thermal insulation performance of low temperature protective clothing
3.1 Fiber Materials
Fiber material is the primary consideration for protective clothing. Compared with active thermal clothing, hygroscopic heating fibers can be used. Such fibers achieve heating through the hygroscopic heat between macromolecular chains. In addition, phase change materials are added to protective clothing. Capsule phase change material, so as to achieve the effect of self-insulation. Hollow fiber is also a kind of active thermal insulation material, which uses the space between fibers to trap a large amount of still air to reduce heat loss3). In the external heating fibers, heating fibers can be used to store the heat in the form of internal energy by using external electric energy and solar energy, so as to play a role in keeping warm. Such fibers include electric heating fibers, solar heating fibers, etc.) Radiation absorption to achieve warmth.
Most passive thermal insulation materials are attached to fur, down, wool and cotton. These fibers have better thermal insulation properties and mainly rely on excellent bulkiness to increase the air cavity layer. This type of fiber material contains more air layers, and the thermal conductivity of the fiber is also low. Many manufacturers make full use of the low thermal conductivity between fibers to enhance the thermal insulation of the protective fabric). In the research on passive fiber materials, it is found that the thermal conductivity of fibers has a high impact on thermal insulation, and the air trapped by fibers is the key to thermal insulation. For this reason, Jia Juan et al. tested the moisture permeability, thermal insulation rate and thermal resistance of various natural fibers, and concluded that the defect of traditional thermal insulation materials is not due to insufficient thermal insulation, but because of poor moisture permeability and air permeability, etc. problems, pointed out that the use of fiber materials should pay attention to comfort. Used in different industries, such natural fibers can be used for fabrics worn by ordinary industry personnel in activities or work, while for some special industries, fiber materials are required to have better physical and chemical properties at lower temperatures, such as liquid nitrogen , metal device cold treatment protection, etc. This kind of protection should use fibers with lower thermal conductivity on the fiber material, and the fibers have better physical and chemical properties and flame retardancy. For this purpose, the performance tests of aramid products and polyimide products before and after liquid nitrogen treatment were carried out. The properties have not changed, and it is concluded that aramid products and polyimide products have better low temperature resistance properties. Therefore, such thermal clothing at lower temperature requires no change in fiber properties. On the other hand, the lower the thermal conductivity of the fiber material, the better the thermal insulation of the fiber.

3.2 Thickness of garment flake layer
The thickness of the low-temperature protective fabric determines the protective effect of the protective clothing. Due to the cold outside environment, the fabric needs to be filled with flakes inside the fabric to maintain the body temperature. Thickness, which not only increases the still air inside the garment, but also blocks the heat generated by the human body from diffusing to the outside. According to relevant research, it can be seen that the quality of the flake layer is under the same conditions, the thicker the flakes used, that is, the flakes with good bulkiness have better thermal insulation. And performance is better 19-200. Therefore, the thermal insulation performance test was carried out on this basis, and the thermal insulation performance test was carried out on polyester batting sheets with different thicknesses. It can be seen that the thermal resistance increases as the thickness of the flake layer increases, so when selecting the thickness of the flake layer, it can be selected according to the purpose of the clothing. of discomfort. The flakes used in extremely harsh environments and special industries often use fibers with low thermal conductivity, such as polyimide flakes, aluminum silicate felt, etc. Its excellent thermal insulation can effectively protect the human body from low temperature damage , In these industries, the outside temperature is often close to -100 ℃, and the flakes used at this time not only need to keep warm, but also need to resist the exchange of their own heat to the outside world [22]. In order to achieve the comfort of the human body, functional fibers can be added to the flake layer to be blended with other fibers to ensure that the flakes can increase their functionality and adjust the body temperature while keeping warm. Considering the comfort and mobility of the human body, the thickness of the flake layer can be controlled between 15 and 30mm. The lower the temperature, the thicker the flakes are required, but the flakes are not easy to be too thick, otherwise it will affect human activities.
3.3 Structural parameters of apparel fabrics
The thermal insulation of clothing fabrics is related to the thickness, density, bulk density and tightness of the fabric. Chen Lili et al. pointed out in their research on the thermal insulation and air permeability of the fabric structure parameters that the number of warp and weft yarns is less, the fabric structure is fluffy and thick, and the thermal insulation of the fabric is high. The better, and it is also concluded that the thermal insulation of the fabric decreases with the increase of the bulk density of the fabric under the same tissue structure parameters. In the organizational structure research, it is found that the order of thermal resistance of the fabric from large to small is: plain weave>2/1 twill>2/2 twill>3/1 twill>3/2 twill>satin weave, it can be seen that the outer fabric is structurally designed The use of a plain weave construction contributes to the thermal properties of the fabric. In addition, the smoothness of the surface of the fabric has different absorption of infrared rays. The surface of the fabric is smooth, the reflection of infrared rays is large, and the surface of the fabric is rough, the absorption of infrared rays is more, so the heat preservation is better. For this reason, filament can be used in the design of fabric structure It can be used with spun yarn to increase the roughness of the fabric, thereby achieving better warmth retention.
3.4 Air permeability and air layer thickness of protective clothing
Under the external environment and low temperature and humidity, the human body will produce sweat in the state of exercise, and it will be transmitted through the inner layer layer by layer. When the moisture conductivity of the clothing fabric is poor, the water will be taken away due to the good thermal conductivity of water. The temperature circulating in the human body reduces the protective performance, so the low temperature protective clothing should have good moisture permeability, which can quickly export sweat and avoid the body feeling stuffy. On the other hand, in the multi-layer structure, the thickness of the static air layer between the layers is the main way to cut off the heat exchange between the outside world and the body. Due to the arrangement of the flake material, there are many voids in the flakes, and a large amount of still air is contained in the voids, resulting in better thermal insulation of the clothing fabric. The inner layer is covered with a film to increase the waterproof and windproof properties of the fabric, and prevent the cold air from outside to take away the heat of the body. To maintain the thickness of its own air layer, the radiant heat convection heat loss between the layers of clothing fabrics needs to be small, and the number of layers of the flock layer can be appropriately increased to increase the thickness of the air layer. It can also add antibacterial fibers to the stretch layer to realize the functionalization of clothing fabrics.
4 The development trend of low temperature protective clothing
With the development of textile and chemical fiber technology, the continuous improvement of the properties of fiber materials and the maturity of some materials with low thermal conductivity, the development of low temperature protective clothing fabrics is not limited to existing materials, and can be made from lightweight materials, scientific structure, functional In the direction of rationalization, good moisture absorption and breathability, and the overall clothing is light, thin and comfortable, ultra-fine fibers (with a diameter of less than 5mm) can be selected as materials, and aerogels can also be selected. To increase warmth, graphene can also be used to achieve active warmth.
In the selection of fabric materials, different types of low thermal conductivity materials can also be mixed, so that each fiber can play a synergistic role, and functional materials can be added to functionalize the cryogenic protective clothing. Protective clothing should pay attention to fluffy in the selection of thermal flakes, but also design on the structure of fiber stacking, so that the flake layer retains more still air and improves the thermal insulation of protective fabrics. With the continuous advancement of new technologies, the preparation process of some microfibers will be mature. Since the superfine fibers are stacked, their bulkiness and warmth retention are better, and these microfibers are integrated into clothing with some self-heating, photoelectric combined thermal elements. In order to achieve thermal insulation and heating under low temperature conditions, to achieve double thermal insulation. In the structural design of clothing and fabrics, attention should be paid to thermal comfort, adjustability and movement to make clothing more comfortable.