Definition of the Main Features of Material Assemblies for Thermal Protective Clothing During External High-temperature Effect Modelling

A computational-experimental method of material selection for thermal protective clothing design is proposed in this article. The intended operating temperature of the garment lies within the range of 40−170 °С. The prerequisite for the research was the lack of information regarding changes in the physical-mechanical and ergonomic characteristics of material assemblies during their use under high-temperature conditions. During the initial stage of research, there was a problem associated with the selection of the most important and the exclusion of the least significant indicators, in order to further reduce the number of experimental tests in laboratory and industrial conditions. The authors used the method of expert evaluations to solve the problems related to the selection of the most significant indicators for material assemblies. Material assemblies were formed by varying the combinations of heat-resistant, heat-insulation and lining layers of materials. Initial information for the proposed method was obtained from the experimental tests of sixteen material assemblies. According to the results of the ranking, the main parameters of material assemblies were identified as follows: the temperature range for which the use of clothing is intended, thickness, mass per unit density, rupture resistance, relative tearing elongation, change in linear dimensions during mechanical loads, air permeability and change in assembly thickness during cyclic loads. It was established that the assembly that includes heat-resistant material of the Nomex comfort N.307 220 top, Nomex Serie 100 heat-insulation lining and Nomex TER 135 lining provides the necessary level of protection, reliability and ergonomics, and meets cost requirements.


Introduction
The creation of personal protective equipment new types and models of protective clothing designed for work under high temperatures does not lose its relevance due to the emergence of new heat-resistant and fire-resistant materials. Modern thermal protective materials have the original raw material structure of fibres, which are subjected to fire-resistant and antistatic treatment [1,2]. Designers of thermal protective clothing (TPC) have the ability to create ergonomic kits with the necessary protective properties. According to [3], personal protective equipment must clearly comply with the list and levels of possible industrial hazardous factors. At the same time, they must meet the requirements of ergonomics and not create any additional risks and other adverse factors under the intended conditions of use. The main purpose of personal protective equipment is to guarantee the safety of the worker without hindering the performance of their basic functions. Such requirements can be met through the selection of appropriate materials, which further determine the basic protective functions, the level of reliability and ergonomics of protective clothing and personal protective equipment of common use. The aim of TPC is to limit the risks associated with work under elevated ambient temperatures [4]. The need for the use of TPC arises at metallurgical plants, at mining, foundry and forging shops, at manufacturing enterprises involved in construction and glass products, and in the engine rooms of power plants etc. At such enterprises, the temperature in the production facilities may fluctuate in the range of 40−170 °С, while work is performed under heavy physical loads. An analysis of the problem of creating effective TPC reveals that its existing varieties partially prevent the access of thermal energy from an external source to the body of the worker, but also limit the process of human heat exchange. Mandatory requirements regarding protective, physical-mechanical and thermophysical characteristics are considered in TPC materials. The latest heat-resistant materials are distinguished by their raw material composition. Natural fabrics are predominantly used, but the proportion of synthetic and mixed fabrics is increasing over time [5,6]. The right selection of materials in assemblies plays a leading role in designing TPC that meets the intended requirements. The properties of TPC are determined primarily by the characteristics of material assemblies, which can be divided into four main groups: • thermophysical characteristics, which determine the range of temperatures at which the material does not lose its protective properties, and the level of fire resistance; • physical-mechanical characteristics, which determine resistance to mechanical load (tear, bending, puncture, shrinkage, etc.), which in turn as a significant effect on the service life, maintainability and possibility of multiple cleanings; • ergonomic characteristics, which affect the level of comfort in use (weight and thickness) and mitigate additional risks during movement and heat load (air permeability, thermal conductivity, water vapor permeability, hygroscopicity and biological inertness); and • the index of cost-effectiveness, which determines the competitiveness of a product. The cost of the material assembly dictates 60−80% of the cost of a garment. Appropriate thermo-physical indices make it possible to provide the desired level of protection against prolonged exposure to high temperatures. Physicalmechanical parameters determine the weight of the product and the level of mechanical strength of the structure. Ergonomic indices ensure compatibility with the user (size), taking into account performed movements and working positions in the context of the workload, and with other personal protective equipment that is intended to simultaneously protect individual parts of a worker's body (head, arms and legs). The desire to solve complex problems during the creation of TPC leads to the use of multilayer material assemblies with the consistent arrangement of heat-resistant, heat-insulation and lining layers [8]. Developing a material assembly structure is a challenging task because the selection of materials and their integration into a single system is influenced by a number of requirements, some of which are controversial. In the course of designing, the goal should be to agree on the physical-mechanical and thermophysical parameters, and the requirements for ergonomics and reliability. For example, increasing the number of layers and additional elements makes it possible to improve the protective properties, and increase the level of reliability and uptime. On the one hand, the greater the number of elements, the easier it is to realize a high level of protection, including the possibility of reinforcing the design through the principle of the differentiation of protection (reinforced zonal arrangement of assemblies with one or more thermal insulation layers in the assemblies) in separate areas of the human body [9]. On the other hand, a large number of individual elements increases the likelihood of damage to the structure and leads to an increase in the mass of a product. Obviously, the choice of elements and their degree of detail may vary depending on indicators of the conditions of use and the resolution of certain problems regarding the level of protection and length of use. A number of design requirements have been established for TPC. Several standardized (mandatory) indicators apply to materials that can be used for the production of TPC: protective properties are deter-mined by the heat resistance of the surface layer; the weight of clothes is determined by the mass per unit area of materials that are part of the assembly and by the number of layers; and the guaranteed length of use under increased mechanical loads is determined by the physical-mechanical characteristics of materials. Such requirements and restrictions necessitate the careful selection of the appropriate materials. It is known that material assemblies have indicators that are different from those of individual materials, and vary depending on the impact of an aggressive production environment. The aim of this article was to experimentally determine the physical-mechanical and ergonomic characteristics of material assemblies under external effect of elevated temperatures between 40−170 °С. The material assemblies are intended for the creation of thermal protective clothing for the employees of metallurgical enterprises. The prerequisite for the research was the lack of information regarding changes in the physical-mechanical and ergonomic characteristics of material assemblies during their use under high-temperature conditions. This effect leads to the thermal aging of textile materials and, as a consequence, to changes in their declared properties and indices [7]. The choice of effective material assemblies presupposes the availability of reliable information regarding the main characteristics that meet the conditions of their use.

Materials and methods
The study was performed for certain samples of heat-resistant top materials (nine samples of heat-resistant top materials) (Table 1), the insulation pad (three samples) ( Table 2) and lining (two samples) ( Table 3). Manufacturers provided statements regarding raw material composition, structure, mass per unit area, thickness, change in linear dimensions after wet treatments and breathability. For the heat-resistant layer, samples of fabrics that meet the thermophysical requirements for TPC, with fire-resistant and oil-protecting properties, and an antistatic effect, were selected. The selected samples of the Proban KS-52, FlameStat Lite, Tecasafe HA 9001 top materials were treated with a special impregnation for protection against high temperatures, while the Nomex BV-107, Nomex ADV 240 and Nomex comfort N307 fabrics contain heat-resistant fibre in their composition [10].
The main purpose of the heat-insulation layer is to provide protection against heat stress and create a comfortable clothing microclimate of TPC.
The lining layer was used to ensure hygienic requirements for water and vapor absorption, and biological inertia [13].    Assemblies of selected materials were developed for further experimental studies ( Table 4). Samples of assemblies were developed in order to simulate indicators of individual elements of TPC under production conditions by varying combinations of heat-resistant, heat-insulation and lining material layers. During the initial stage of the research, there was a problem associated with the selection of the most important and the exclusion of the least significant indicators in order to further reduce the number of experimental tests in laboratory and industrial conditions. The generally accepted procedure of screening factors due to the large number of indicators is quite cumbersome and time consuming, and requires a great deal of time to implement the necessary measures [12]. Such difficulties can be overcome through the use of expert opinions, and by taking into account the ability of the designer to make rational decisions when it is impossible to fully formalize them. The authors used the method of expert evaluations to solve the problems related to the selection of the most significant indicators for material assemblies. Expert evaluation to determine the main indicators of the material assemblies involved the formation of a group of experts, the surveying of that group, the processing of the expert evaluation, an analysis of the results and an assessment of the reliability of the expert evaluation performed by the group. During the first stage, twelve respondents were selected, includ-ing specialists from TPC manufacturing companies, user representatives, and specialists from research and testing centres and laboratories in the textile industry. They were given the task of ranking the group indicators of material assemblies in accordance with their impact on the predicted properties of TPC. The method of direct evaluation of the ranking of the initial requirements was difficult to use due to the large number of characteristics and their indicators. The indicators that were analysed are different in terms of both quantitative estimates and qualitative characteristics. Therefore, the use of standard methods of expert evaluation is inefficient and requires adaptation to the specifics of the tasks. A method for determining the rank of a group of indicators on the basis of a normalized significant coefficient was proposed [13]. Experts identified four groups that determine the requirements for the physical-mechanical, thermophysical, ergonomic and economic characteristics of the material assembly. For each group, the normalized indicators are defined in the following sequence: 1. during ranking m, experts were asked to place n requirements in the order they consider most reasonable and assign to each requirement X rank from 1 to N. According to this, rank 1 received the most preferred indicator, and rank N -the least important; and 2. the normalized coefficient К q for each of the q group of requirements was determined using the formula: where Х q j represents the rank of the q-th group of requirements, given by j-th expert; q = 1…n; j = 1…m. In the calculations, n = 4 and m = 12. After ranking, the following results were obtained (Table 5). In the second stage, the experts were offered a ranking of indicators in each group of characteristics. According to the algorithm, the normalized coefficients K i for 20 materials was determined. The total normalized ranking factor K of alternative solutions was calculated using the formula: where К і represents the normalized coefficient for each і-th group of indicators. According to the results of ranking, the most significant 10 indicators with respect to material assemblies were singled out (see Table 6). The reliability of the evaluation of the expert group depends on the level of agreement of the opinions of the experts participating in the survey. The consistency of experts' opinions is assessed using the  generalized coefficient W, which is defined as a generalized rank correlation coefficient for a group comprising m experts [14]. The generalized coefficient is calculated with the help of "Concurrer" program. The values of the calculated generalized coefficients were in the range W = 0.79−0.88 for the ranking of the tasks of four groups of characteristics. Such values indicate a high level of consensus between the experts. Based on the results of the rating assessment by specialists in various fields, the main indicators that received the highest ranks (from 1 to 10) were selected for further research: the intended temperature range for the use of clothing, thickness, mass per unit area, rupture resistance, relative tearing elongation, change in linear dimensions during mechanical loads, air permeability and change in assembly thickness during cyclic loads. The practice of determining the quantitative parameters of material assemblies involves experimental tests. In laboratory conditions, the characteristics of materials are determined under normal conditions (temperature ranging from 20−22 °С and relative humidity ranging between 40−60%). The results of experimental tests are given as a deterministic quantity, which is represented by the mathematical expectation (average value) of a random variable. The obtained parameters were interpolated under the assumption of linearity and the invariance of the initial conditions, which does not correspond to real operating conditions. It is known that the structure and properties of textile materials change during operation, and high temperature has destructive effect on textile materials [15]. This effect leads to the thermal aging of materials and thus to the gradual failure of TPC. It is therefore expedient to study experimental changes in the quantitative parameters of the main indicators during the simulation of high-temperature effects.
To determine the mathematical expectation of the corresponding characteristic Zі (i = 1, 2…) with a given accuracy, n of independent tests should be performed. We give the mathematical expectation of each of the characteristics as where À represents the estimate of the mathematical expectation of the desired random parameter. The value of À will be in the interval where Δ represents the upper bound of error of calculation results, which is determined by the known value of route-mean-square deviation σ (Z) and the given accuracy К r It follows from formula (5) that the total number of tests n in laboratory conditions for each indicator will be: = ( % ⋅ ( ) ) + (6).
As previous tests have shown, at a given accuracy of 0.95, the required number of measurements is in the range from 5 to 12. Changes in the performance of assemblies during the tests were characterized by the magnitude of deviations from estimated mathematical expectation, which is calculated using the formula: where A 1 is the estimate of the mathematical expectation the parameters that have been measured before thermal exposure; A 2 is the same parameter after thermal exposure. For assemblies that are planned for use in the production of TPC, the tests were performed in the laboratory according to standardized methods [16,17]. Prepared samples of material assemblies were pre-conditioned in a heat chamber at an air temperature of 22−24 °С and relative air humidity of 50−60%. The assemblies were formed in layers and threaded along one side in the direction of the base. Tests to determine air permeability were performed under a pressure of 50 Pa with the help of a FF-12 device (Hungary). The assemblies' thickness change test was performed by applying a variable load of Р l = 50−1100 kPa for 30 seconds and unloading Р unl = 1100−50 kPa for five minutes after each of the ten cycles.
To determine the change in the linear dimensions of the assemblies, samples with a size of 300 mm x 300 mm were formed. Control lines 200 mm long and 200 mm wide were applied on the upper and inner layers of the materials. The simulation of high-temperature effects was carried out in the mode of preliminary thermal exposure on material assemblies in a "Rohs" heat chamber (SPC Technology, Ukraine) at temperature 170 °C ± 5 °С , for 15 minutes. The research was performed in the accredited "Textile -TEST" analytical and research testing laboratory at the Kyiv National University of Technology and Design.

Indices of the physical-mechanical characteristics of assemblies and changes thereto under the effects of temperature
Prior to testing, the selected material assemblies were kept in a heat chamber to prevent exposure to environmental moisture (1 day). Mass per unit area meas-urements were performed before and after thermal exposure ( Table 7). As follows from Table 7, the assemblies nos. 2 and 6 have the highest mass per unit area values. The fifteen-minute temperature effect caused the mass per unit area to change due to the thermal aging of the materials. Therefore, it is possible to assume further changes in other physical-mechanical characteristics. Based on the results of studies, it was determined that assemblies nos. 2 and 3 can be excluded from further testing. The mass per unit area of assemblies nos. 2 and 3 increased, which will lead to an increase in the mass of TPC during use. Experimental tests to determine the rupture resistance and relative tearing elongation of the material assemblies were performed taking into account the conditions of wearing ready-made clothing ( Table 8).
The size of the lining layer was assumed to be 10% larger than the surface layer in terms of structural features. Due to such features, the tearing of the heat-resistant, heat-insulation and lining layers of the assemblies occurred simultaneously. As follows from Table 8, the assembly no. 9 (R9 I3 L2) had the largest breaking force, while assemblies nos. 1, 6 and 9 demonstrated a decrease in value for up to 9% after thermal exposure.
The results of the determination of the relative tearing elongation of the thermal protection assemblies before and after thermal exposure are shown in Table 9.
According to the test results, the values of relative elongation decreased by 13% and 11% respectively in material assemblies nos. 1 and 9 with the coded composition R1 І1 L1 and R9 І3 L2. The largest in-crease in elongation was recorded for assembly no. 4 (R4 I2 L1) -up to 40%.

Ergonomic performance of the assemblies and changes thereto under the effect of temperature
In the design of protective clothing, preference is given to materials that will subsequently ensure the stable linear dimensions of clothing parts and individual elements during operation [18]. Tests were performed to determine the change in linear dimensions of the assemblies before and after thermal exposure (  TPC should be used continuously for the work shift (up to four hours). Work is performed under a high mechanical load. During their activity, a worker performs work, converting mechanical energy into thermal energy. Under conditions of high ambient temperatures, this can lead to the disruption of heat transfer processes in the body of a worker. Changes in the functional state of the body are caused by the muscular work (static and dynamic) of the torso, and upper and lower extremities [19]. The intensity of heat generation depends on physical activity and the duration thereof, the human and health conditions, the environmental (ambient) temperature, pressure, humidity and air velocity. In different levels of activity, a worker emits from 250 W to 450 W of heat, compared with 100−150 W at rest, meaning that the comfort zone of a worker at rest does not coincide with the comfort zone under a work load [20,21]. Under such conditions, the air permeability of the assemblies is an important indicator [22] (Table 11). Based on the results of studies, it was determined that assemblies nos. 1and 9 had the highest levels of breathability. The results of the experiment showed an increase in breathability of up to 4% after thermal  exposure, which is due to the uneven structure of such material assemblies. Under the effect of an external high-temperature environment on a worker in TPC, an increase in the duration of comfort for the human body temperature in the under-clothing space can be achieved by increasing the thickness of the insulating layer [23]. Improving the protective properties of TPC from mechanical stress in terms of design is possible by increasing the thickness of the assemblies in the areas of shoulders and chest, knees and elbows. Such a prerequisite has led to the performance of multi-cycle loading and unloading tests for multilayer assemblies before and after thermal exposure. Such tests were performed for assemblies containing one heat-insulation layer (Table  12) and two heat-insulation layers (Table 13).  The thermal protective layer of the material consumption for an assembly is 6.8−8.1 m 2 . During the manufacture of the products of the set seams of different purpose were used.

Conclusion
Protective clothing is designed to ensure that, under the foreseeable conditions of its use, a worker can perform the risk-related work normally with the highest possible degree of protection. The performed studies made it possible to formulate an improved sequence of execution of works on the design of TPC, which is intended for protection against the high temperatures of the external production environment. The required properties of TPC can be achieved by applying available information regrading the indicators of relevant materials. During the first stage, requirements relevant to the indices of the materials that are of significant importance during the creation of TPC were defined: protective, physical-mechanical, ergonomic and economic. It was shown that the achievement of all requirements is possible when multi-layered material assemblies are used. The tasks related to the selection of the most significant indicators for material assemblies were solved using the method of expert evaluations through the determination of the ranks of the indicators on the basis of standardized coefficients. The consistency of experts' opinions was assessed using the concordance coefficient. During the second stage, nine variants of three-layer material assemblies were developed. Changes in mass per unit area, rupture resistance, percentage tearing elongation, linear dimensions, thickness during mechanical loads, air permeability under normal conditions and after simulation of thermal effects were determined for each assembly. It was established that the assembly that comprises the Nomex comfort N.307 220 top, Nomex Serie 100 heat-insulation lining and Nomex TER 135 lining provides the necessary level of protection, reliability and ergonomics, and meets cost requirements. The obtained research results were implemented during the creation of thermal protective clothing for the workers of metallurgical enterprises. New design and technological solutions for clothing that consist of a jacket and overalls or trousers with overlays were offered. The total weight of TPC does not exceed 4.5 kg.