A Review On: Analysis Of The Properties Of Thermal Insulation Materials

Clothing insulation is one of the important factors of human thermal comfort assessment. Thermal insulation is the reduction of heat transfer (i.e., the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radioactive influence. Thermal insulation can be achieved with specially engineered methods or processes, as well as with suitable object shapes and materials. Heat flow is an inevitable consequence of contact between objects of different temperature. Thermal insulation provides a region of insulation in which thermal conduction is reduced or thermal radiation is reflected rather than absorbed by the lower-temperature body. The term thermal insulation can refer to materials used to reduce the rate of heat transfer, or the methods and processes used to reduce heat transfer. Heat energy can be transferred by conduction, convection, radiation or when undergoing a phase change. For the purposes of this discussion only the first three mechanisms need to be considered. The flow of heat can be delayed by addressing one or more of these mechanisms and is dependent on the physical properties of the material employed to do this. Predicting the pattern of clothing adjustment to climate change can provide important basis for thermal comfort and energy consumption analysis. To achieve reliable results, it is necessary to provide precise inputs, such as clothing thermal parameters. These values are usually presented in a standing body position and scarcely reported locally for individual body parts. Moreover, as an air gap distribution is both highly affected by a given body position and critical for clothing insulation, this needs to be taken into account.

The American Journal of Interdisciplinary Innovations and Research (ISSN-2642-7478) INTRODUCTION Clothing can be described in physical term of thermal insulation for the heat exchange of man with his immediate thermal environment [1]. In thermal comfort theory, clothing insulation is an important factor that affects thermal sensation [2]. People can adjust their clothing insulation to suit their own thermal comfort requirement [3,4] and reduce energy consumption in buildings [5]. Therefore, clothing adjustment is a powerful behavioural adaption mode to weather change [6,7]. Predicting the pattern of clothing adjustment to weather change provides important basis for thermal comfort and energy consumption analysis [8,9].
Outdoor temperature is an important climatic factor to the change of indoor clothing. Various models were developed to predict the change of indoor clothing insulation with outdoor temperature, based on different regression functions. De Dear and Brager used an exponential decay function to fit the change of the mean clothing insulation for occupants of each building with the mean outdoor effective temperature at the time of the survey, based on ASHRAE RP-884 database. According to this model, 40% of the variance in clothing insulation was explained by variations in the outdoor climatic index [10]. Thermoregulation of human body aims at maintaining body core temperature in a narrow range and depends on metabolic heat production and the exchange of heat between skin and environment. When the human body is covered with clothing, it affects human physiological regulation mechanisms. Thus, the design and material aspects of functional clothing (e.g. protective or sport clothing) requires a detailed understanding of heat transfer processes. In the clothing research, heat transport is usually assessed by hot plates, thermal cylinders or thermal manikins in steady-state measurements. The experimental methods of assessing heat transfer require the availability of prototypes and testing facilities (e.g. climatic chambers) [11], which can be costly and time consuming.
Nowadays, global climate is changing and extreme cold spell incidents are becoming increasingly frequent and intense in recent years. During extreme cold spell incidents, people have a much greater chance to suffer from cold stress, which endangers human health, wellbeing and safety. Long-time exposure to cold stress could induce coldrelated illnesses and injuries, e.g., cracked skin, chilblain, frostbite, trench foot, cognitive impairments, hypothermia or even deaths [12][13][14]. Clothing governs heat and mass transfer between the human body and the ambient environment.
Local clothing thermal properties may vary considerably over the body, thus, having a major impact on the development of skin temperatures, sweating, and perception of thermal sensation and comfort [15]. Therefore, clothing, intended as a functional necessity to protect against harsh climate conditions and not as an ornamental accessory, has always been one of the main primary needs of mankind to adapt to the thermal outdoor environment. Indeed, variety in clothing is attributable also to the different local climate conditions where populations were living [16]. Thus, the human body covered in garments constitutes a complex system of thermal exchange, where heat and vapor produced by skin are first transferred to the gap between skin and the clothes and then throughout the textile material of the cloth, which is porous, towards the external environment [17]. Therefore, heat is transferred by means of conduction, convection and radiation phenomena, while water vapor by means of evaporation, absorption, condensation, deabsorption and transpiration ( Fig. 1) [18].
Taken together, in cold-humid climates in winter, both moist clothing and high air humidity have profound effects on clothing performance and human thermal comfort. However, what are the thermal behaviours in the clothing microenvironment under actual wearing situations, and how to evaluate the clothing moisture effects caused by coldhumid environments on human thermal comfort remain unknown [19].

Fig.1. Heat and water vapor exchanges between the human body and the environment throughout
clothes.

Effect of clothing insulation
The effect of various clothing insulation values on the time course of core temperature, mean skin temperature, mean trunk temperature and mean heaters' temperature is illustrated in Fig.2 Fig. 3), which obviously fell outside of the thermoneutral skin temperature range. Thus, clothing incorporated with auxiliary heating is effective to help occupants attain indoor thermal comfort in such a low indoor temperature. More important, the bulkiness of clothing did almost not change even six thin heaters were being embedded to the PHCS because the thickness of heaters is negligible (i.e., 1.2 mm).   Fig.4. This corresponds to a 2.3% and 2.6% increase in MR per additional kg of clothing during level and inclined walking respectively. The same pattern was also observed for VO2 and HR (Table 1). Wearing the 3-layer PPC decreased GE (Table 1) and increased muscle activation in all three muscles, seen as an increase in % RMSmax by 9-29% compared with that wearing a base layer ensemble Fig.5. Walking with the 3-layer PPC ensemble was rated as more strenuous than walking with a base layer during both level and inclined walking ( Table  2). The PCC did not alter stride frequency (   c 3, hot; 2, warm; 1, slightly warm; 0, neutral; −1, slightly cool; −2, cool; −3, cold.

Clothing layers
The thermal insulation of a fabric used to make clothing depends on thermal insulation of fibres and yarns but also on fabric thickness, since the bulk of the thermal resistance is provided by the entrapped air in fabric pores [22]. For fabrics with low porosity the type of fibre and fabric construction may play a role since the ratio of fibre material to the air will increase. The value of thermal insulation can be measured on a hot plate or obtained from the literature [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. In the theoretical model it is assumed that fabric properties are constant and the thickness of the fabric is homogeneous. The porosity and air permeability of the fabric in respect to mass transfer are neglected as forced convection in an enclosed air layer is not considered in the model. To understand the heat exchange between human body and environment through the clothing, the entire system can be divided into three different segments as shown in Fig. 6

CONCLUSIONS AND FUTURE PERSPECTIVES
In the last decades, a broad research effort has been carried out on outdoor thermal comfort. The complexity of the topic, which is due to the many and rapidly changing variables, calls for the modeling of thermal comfort sensation for a normalized, standard subject. Clothing's thermal insulation is among the main variables, and it is linked to the clothes that individuals wear to defend themselves from the meteorological forcing of the outdoor environment. The wide variability of outdoor conditions does not allow hypothesizing clothing's thermal insulation as independent from the other factors characterizing climate and its daily and seasonal variability. Because, that clothing's thermal insulation preference, as expressed by an individual, depends also on personal thermal sensation linked to individual's comfort.
Thus, it appears as a necessary step to consider clothing's thermal insulation more in depth, by determining a mean preference value that also accounts for physiological factors linked to age and gender, Moreover, psychological and cultural factors influence clothing preferences, a specific population to the local climate zone. Based on the foregoing, our aim is to develop and introduce a new type of thermal insulation material for clothing in the future.