CBRN protective garments
The major task of the CBRN individual protective solutions developers is to prevent a significant reduction of the combat effectiveness, even in hot climates. The use of state of the art CBRN technical textile show excellent level of comfort. a light water vapour permeable suit, commonly used in the leisure field, based on a membrane. These two suits are evaluated using a model of calculations under different environmental conditions and in a second step on 3 human volunteers.
Description of the equipments
The CBRN suit, FRA, is made of a filter fabric Saratoga. This liner acts as a filter and provides the user with a protection against chemical warfare agents without risks of desorption, in all climatic conditions. The suit FRA weighs 2 kg. The suit MBA used for this study as a comparison is made of a membrane, water vapour permeable and air impermeable. It weighs approximately 1,4 kg. For the comparison purpose, this suit was chosen with a hood, just like the CBRN suit, and a closed design. A CBRN integrated combat boot, based on a Saratoga laminated with a Cambrelle non woven, referenced FBOT is compared to a normal combat boot with the CBRN overboot on top and is referenced FBUT.
Models to define the acceptable limits
Wearing protective clothing and equipments involves physiological strain on the wearer. CBRN protective clothing is made of a filtering liner in charge to trap and store chemical agents, together with an oil repellent outer shell. This combination of textiles is air and water vapour permeable. The use of such textiles significantly increases the combat effectiveness of the users, especially regarding impermeable suits such as used by the Warsaw Pact Armies. They are adopted today by all of the NATO countries. A CBRN suit is compared with butyle overboots FBUT.
In order to maintain the combat effectiveness of the wearer, it is of a great importance to reduce the heat strain, correlated with the evaporative resistance and the ventilation of the garment. Environmental factors, such as temperature, relative humidity, solar radiation, wind speed interact with the exercise level of the wearer. A theoretical model is used to evaluate the properties of different suit materials on the thermal strain under a wide range of environmental conditions and for different exercise levels (tasks).
Model and suit material properties
The model study was performed by the dynamic THDYN model developed by TNO. THDYN is a computer model by which the influence of clothing and environmental conditions on human heat balance can be determined. The environmental factors temperature, relative humidity and wind speed are varied within a specific, relevant range according to Stanag 2895 (Extreme Climatic Conditions), table . Exercises levels were varied as well as based on task specific metabolic values (Parson 2003, ISO 8996). Several iterations in which each variable was structurally varied were performed. Physical properties of the suits were measured at IFTH under NF EN 31092 - ISO 11092 (April 1994), on the Skin Model. The cotton undergarments present a thermal resistance of 0,0186 m2K/W.
The ventilation measured is 120 l/m2min pour FRA and 20 l/m2min for MBA suit. Stop criteria are defined by a maximal internal temperature of 38.5°C (ISO 12894) for the calculations. The calculations were performed using a metabolic rate of 325 W/m2, simulating cycles with an external power of 2 W/kg. The air layer between the body and clothing is fixed at 10 mm.
Core temperature differences between suit FRA and MBA are significant at 40°C and a relative humidity of 20 %. The critical temperature 38,5°C, in A2 conditions (Hot Dry) is reached in less than 20 minutes by the wearer of the MBA suit and in less than 120 minutes for the wearer of the FRA suit, in a range of temperature under 42°C. Therefore, it seems that the air permeability of the FRA suits allowed ventilation and a better evacuation of the sweat. The limit exposure time between both suits decreases for the climate A3 (Hot) and B1 (Wet Warm). For instance, at 33°C and a relative humidity of 60-70 %, the critical exposure time is 20 minutes for the suit MBA and 30 minutes for the suit FRA. The effect of forced ventilation on the critical exposure time is significant for the FRA suit. If the ventilation increases from 50 l/m2/min to 200 l/m2/min, the autonomy is increased from 20 minutes to 120 minutes at 32°C and 70 % RH. The metabolism rates chosen vary from slightly above rest (100 W/m2) until metabolism levels that imply heavy strain (400 W/m2). At 10 % RH and a rate of 100 W/m2, none of the suits show any restriction for exposure time. For 200 and 300 W/m2 metabolic rate, no restriction appears for the FRA suit. At 80 % relative humidity, time to exhaustion in hot conditions (>36°C) is restricted for even very low metabolic rates. As a conclusion, the CBRN suit FRA can be used in a larger range of climates and allows heavier metabolism rates in the extreme conditions considered (20-45°C and10-80 % RH) than the light suit MBA.
Evaluation of the thermo physiological strain on the wearer
The « Centre de Recherche du Service de Santé des Armées » evaluated the strain of the two suits FRA and MBA, with their unique thermo climatic chambers in Hot Dry conditions (35 °C, 20 % RH) walking at 4 km/h, with a slope of 4 % on a moving walkway without carrying any load. They also tested the new equipment for the protection of the feet, an integrated CBRN combat boot FBOT, in comparison with the present equipments consisting in butyl overboots worn on top of regular combat boots.
Conditions of the evaluations
he tests were run in the thermoclimatic chamber of the “Département des Facteurs Humains -Pôle tolérance climatique et vêtement”.
The purpose of these evaluations were to evaluate :
➜ The thermophysiological strain of the equipments on volunteers walking on a moving walkway,
by measuring the core temperature, the weight loss (dehydration) and the heart pace inorder to define the exposure limit time (norm ISO 9886 and ISO7933).
➜ The physical characteristics of the microclimate (under the suit, next to the skin) by means of measuring the temperature, humidity and calculating the water vapour partial pressure. This study was run with 3 volunteers who performed 4 evaluations. The conditions were chosen in order to create an external and internal strain that would create a thermal storage. From a thermophysiological point of view, this thermal storage can only be compensated by the ther molyse due to the evacuation of sweat.
The thermal storage is lower with the CBRN FRA suit than the leisure suit based on membrane MBA, which could mean a better thermolyse with FRA. However, the microclimate partial pressure analysis does not confirm the previous statement. The heterogeneity of the microclimate values could be explained by pumping effects, due to the differences in size and design and closures of the two suits, independently of the physical characteristics of the different water vapour per meability of these suits. The analysis of the weight loss, which represents in first approximation the sweat evacuation through the equipments is higher for S1 and similar for the two others volunteers.
Therefore, the water vapour transfer in these real conditions seems to be in favour of the FRA CBRN suit.
Protective equipments of the feet
These experiments were performed with the same suit FRA. The new generation integrated CBRN combat boot (FBOT) reduces drastically the thermal strain. In the series of tests made with the FBUT (normal combat boots with a butyl overboot), 2 of the 3 volunteers reached the exposure limit time (one for an important thermal storage, the other one who reached the internal temperature threshold). The thermal storage is clearly higher for the 3 volunteers wearing FBUT. This difference can be explained by the higher weight of FBUT configuration (1 kg). Moreover, the water vapour impermeability of the FBUT equipments does not permit the evacuation of sweat. This is confirmed by the higher increase of weight of the FBUT equipments at the end of the test (2h approximately) which is from 100 to 150 g.
As a conclusion, the state of the art CBRN suits (FRA) are more comfortable than a suit commonly used in the leisure area (MBA), made with a so called breathable membrane which is widely considered as comfortable. In spite of the 40 % overweight of the FRA CBRN suit over the MBA membrane suit, FRA is more comfortable. FRA can be worn in larger range of climates and allow heavier metabolism rates.
The tests performed with volunteers in different climatic conditions show similar thermal strain. But the thermal storage is less for the CBRN suit FRA, which could mean a better evacuation of sweat through the CBRN suit. The new filtering product used in the integrated CBRN combat boots reduces significantly the thermal strain and the weight (1kg), comparing with the well known use of butyl overboots.