Improving comfort in military protective clothing

Clothing used by the military in the war field has special requirements. In most cases they have to face toxic chemicals and biological warefare agents ( CBW ). For this reason, the military has developed their special protective clothing and associated equipments ( boot, gloves, helmet, hood and full face air purifying respirator). This total body protection system is called IPE ( individual protective equipment). In the battle field when contaminated environment may be created by the opposition, IPE is essential to protect the armed forces to prevent physiological incapability effect and possible death. Equipment used by the military in combat is expected to enhance the probability of mission success. The IPE used by the solders in reality restricts mobility and functionality , hinders interruption with other equipments and impose unavoidable physical, physiological and psychological burden on the users of this equipment. When exposed to high heat and or humidity , long work suffer heat strain and associated degradation in physical and cognitive( intellectual) functions. Thus the path to greater comfort, functionality and lower burden must be balanced with the level of protection needed. All these factors lead to develop dedicated protective suits ( with hood ) , to be worn over combat clothing with polymeric full face respirator, protective gloves and over boots. Operationally minimum 24 hours protection against direct liquid contamination was considered essential for western countries. The suits were typically constructed from a material system that included a carbon adsorbent liner, an inner comfort layer and outer shell material that either radily wicked liquids to increase the evaporative surface area or coated finish to prevent liquid penetration.

For 24 hours a substancial amount of carbon was necessary, making the cloth thick and heavy. The outer layer is to be designed as air permeable to permit some degree of evaporative cooling to address heat strain. IPE suits for the solders are basically two types i.e A-type and C-type. A-type is the hole body protection are fully encapsulated , air tight suit worn with self contained breathing equipment. This system incorporate materials that protects the user from volatile organic vapors, toxic or corrosive liquids or aerosols. C-type protective clothing are non air tight suits worn with conventional air purifying respirator. The most significant difference between A and C type are, none of the closers ( ankle, wrist, front torso and hood ) in C-type suit provide an air tight seal, they restricts air flow in and out of the garment at those locations but can’t prevent. Use of a close fitting carbon adsorbent under garment underneath C-type protective garments , enhances the whole body protection level to that approaching A–type protective garments. The tight fitting nature of the carbon adsorbent under garment certainly plays role in the observed increase in protection, by limiting ingress if air next to skin. When combined with air impermeable outer layer, an extremely effective protection is observed. Even using air permeable outer wear with carbon adsorptive under garment , the protection is still significantly increased. There is also relation among tolerance and metabolic rate when IPE is used in different ambient temperature and water vapor pressure, which is shown in following curve :

At higher metabolic rate i.e. above 250 Wm2 tolerance time coverage is approximately 50 minute and at lower metabolic rate that is less than 100 Wm2 tolerance time coverage is very high. At higher metabolic rate , sweating occurs to maintain constant body core temperature. Evaporation of sweating takes longer time for evaporation through clothing. For this reason, tolerance time coverage is lower at high metabolic rate. Hence, for the solders wearing protective clothing, the rest and work schedule should be properly planned and maintained. In most operational settings, some heat strain will be inevitable. When it will occur and how much it will degrade task efficiency will depend on factors such as climate, metabolic work rate, psychological stress and opportunities of rest. Hence, the use of air permeable protective material in IPE is essential to complete tasks effectively and to reduce severity of heat strain. The first type of armour was made from animal skin piled on top of each other. Afterwards metal plates were used to protect the body in battle field. Kevler fibre consist of long molecular chain produced from poly-paraphenylene terephthalamide. The chains are highly oriented with strong inter chain bonding that results in a unique combination of properties, which includes high tensile strength at low weight, long elongation to break, high modulus, low electrical conductivity, high chemical resistance, low thermal shrinkage , high toughness, excellent dimensional stability, high cut resistance and flame resistance. It is five times stronger than steel on a equal weight basis and it is light weight, flexible and comfortable. Beside kevler fibre , there are other fibres developed by different manufacturers which are used for bullet proof vest manufacturing Example of some such other fibres are Twaran, Technora, Spectra, DSM Dyneema , Zylon, Nylon etc.

The ballistic performance of a woven fabric is dependent on the dynamic mechanical properties of constituent fibres and fabric geometry ( type of weave, fibres per yarn, weave density ) Normally, ballistic fabrics are densely woven plain fabric. It is recognized that fabrics cover factor should be within 0.6 to 0.95 for ballistic application. Dyneema is a non woven ballistic fabric. All the fibres are laid parallel in the same plane. Several layers of this type of non woven fabric is used and placed one layer on another at 900 angles of parallel fibres. The unidirectional configuration of the fibres allows the energy transferred from the impact of a bullet to be distributed along the fibres much faster and more efficiently than in conventional woven fabric. This is because of the absorption of power of the yarn in woven fabric is lost at the cross over point, as these points reflects rather than absorb the shock waves of the impact. Modern body armours provide protection by three different methods : ( a ) The armour totally rejects the projectile by bouncing it off , ( b ) the armour retards and stops the projectile by dissipating kinetic energy along the plane of the impacted material and ( c ) combination of a & b. When a bullet strikes on body armour, it is caught in a “web” of very strong fibres. These fibres absorbs and disperse the impact energy that is transmitted to the vest from the bullet to deform or to mushroom. Additional energy is absorbed by each successive layer at material in the vest until the bullet stopped. Individual layers of ballistic fabrics are assembled in various ways and varies from manufacture to manufacture. In some cases multilayer fabrics are bias stitched or tack stitched or vertical and horizontal stitches. It is suggested that stitching tends to improve the deformation performance. Ballistic protection vests are basically two types i.e covert vest and overt vest. Covert vest are concealable and used under the shirt. These are lighter in weight and easily washable. Overt vest are worn over the uniform, heavier in weight and do not wash easily .Overt vest has pocket for fashion as well as extra protection by using ceramic plates in the pocket. Depending on protection level and fabric style, the number of layers in a soft ballistic vest may vary from 7 to 50 layers and total weight of layers varies from 90 to 250 gm-2 .