Ballistic protection
Manufactured fibers developed or modified for specific functions are used exten- sively for the manufacture of PPE. Technical information provided by the manufacturer includes data on performance of the material when it is new, and in some cases, after it has been preconditioned (Toyobo, 2005). Mechanical proper- ties of materials affect ballistic properties. Therefore, loss in strength as a result of degradation of material strength is key for garments that provide protection against impact. An investigation into the fatal shooting of a police officer in which the bullet penetrated the bullet-proof vest led to an investigation of used bullet-proof vests manufactured with Zylon®, a PBO fiber. According to an NIJ report:
‘ … ballistic and mechanical properties testing on 103 used Zylon®- containing body armors provided by law enforcement agencies across the United States. Sixty of these used armors (58%) were penetrated by at least one round during a six-shot test series. Of the armors that were not penetrated, 91% had backface deformations in excess of that allowed by the NIJ standard for new armor. Only four of the used Zylon®-containing armors met all performance criteria expected under the NIJ standard for new body armor compliance. Although these results do not conclusively prove that all Zylon®-containing body armor models have performance problems, the results clearly show that used Zylon®-containing body armor may not provide the intended level of ballistic resistance. In addition, the results imply that a visual inspection of body armor and its ballistic panels does not indicate whether a particular piece of Zylon®- containing body armor has maintained its ballistic performance.’
The tensile strength of PBO is higher than that of p-aramid; however, PBO is more susceptible to degradation when exposed to moisture and sunlight (ultraviolet as well as visible light). Laboratory studies have been conducted to determine a possible correlation between moisture exposure, reduction in tensile strength, and ballistic failure of PBO fibers. Findings of the study show that the yarns from ballistic panels aged at elevated temperature and RH had considerable loss of tensile strength. High temperature and humidity over an extended period resulted in opening of the benzoxanole ring followed by hydrolysis. In addition to chemical changes, physical changes were observed on the surface of the fibers. Conversely, degradation was not observed in fibers stored at 25° C and 5% RH (Chin et al., 2007).
Studies designed to simulate moisture and heat conditions, exposure to chemi- cals, cleaning procedures, and aging have been conducted to determine changes in
Durability of protective clothing 177 performance of aramid, PBO, and ultra high molecular weight polyethylene (UHMWPE) fibers used for ballistic protection (Chin et al., 2009, 2010). Findings of a study conducted to determine the effect of artificial perspiration and cleaning chemicals indicated that only exposure to chlorine bleach affected tensile strength and chemical properties of the fibers. Exposure to other chemicals was similar to that of water. Small pits were observed on the surface of PBO fibers exposed to water as well as dilute bleach solution. For the other two fibers, the change in fiber surface (small pits) was observed only on fibers exposed to bleach (Chin et al., 2009).
Thermal protection
Use of high performance fibers that are either inherently flame retardant or have been engineered to provide flame resistance characteristics are used to provide thermal protection. Often visual observations and/or duration for which the garments have been worn are used as the criteria to retire the garment. However, the durability of the fibers, and in certain cases the ability of the fabric to provide adequate thermal protection, is affected by changes at the molecular level as a result of aging. A good understanding of changes at the molecular level that affect durability of the fabric and its ability to provide flame-resistance properties is necessary. Research is underway at the National Institute of Standards and Technology (NIST) to determine the effect of factors such as ultraviolet light, laundering, abrasion and dirt on turnout gear performance (Davis et al., 2010a, 2010b). The findings of the study related to the effect of UV light indicate that UV radiation can decrease the service life of the outer shell of the turnout gear. Results of the study also state that the deterioration could not be determined by sensory evaluations conducted on the fabrics. As stated in the publication ‘Since sensory observations are the primary bases the fire community uses to initiate a request for gear replacement or repair, there is a strong possibility that turnout gear may be used with OS that are not providing adequate protection’ ( Davis et al., 2010a).
Future plans include evaluation of turnout gear that is subjected to several conditions concurrently.
9.2.2 Testing the durability of functional finishes for protective clothing
Functional finishes are used either to protect against the primary potential hazard or to enhance the overall performance of the fabric. For example, repellent finish is used to protect pesticide operators against exposure to pesticides; prevent water penetration in firemen’s turnout gear; or simply stop a person from getting wet. For a particular finish, the implications for safety vary by the intended end use. In general, the durability of functional finishes used for protection is important, as safety can be compromised due to failure of the finish. Unfortunately, often there
178 Understanding and improving the durability of textiles
are no visual indicators that alert the user when the protection is no longer being provided. In some cases, inability to provide the necessary protection is due to degradation of the finish; in others, it is due to the use of certain cleaning aids, or soiling of the garments, rather than the durability of the finish. Home laundering of garments with finish may pose problems, as it is difficult to control cleaning practices. In these cases, training on the use and care of the garment is essential for safety. Fabrics with flame retardant finishes and those with repellent finishes are used as examples in the following:
Flame retardant finishes
Flame retardant finishes are routinely applied to cotton and cotton blend fabrics used for secondary protective clothing. These garments are worn continuously by individuals working in industries where there is a potential risk for flash fire, electric arc flash, or other thermal exposure. Unlike regular workwear, these garments require cleaning and maintenance in accordance with the guidelines provided by the manufacturer (e.g. Westex, Bulwark). The performance of the fabrics, often marketed as products in which the flame-retardant properties last for the life of the garment, requires a concerted effort on the part of the user to ensure that the garment performs well during use. Selection of appropriate cleaning aids is very important. For example, laundry additives such as fabric softener, starch, and bleach can adversely affect the performance of the fabrics. In addition, washing the garments in hard water or using soap (instead of detergent) can result in deposition of mineral salts on the fabric. These deposits may also promote flaming when exposed to a flame source. Soiling, especially with flammable material, can reduce the performance. Therefore, the garments should be properly cleaned or not used as a garment to provide secondary protection. Improper use and care can result in changes on the fabric surface or at the molecular level.
Cleaning is more easily controlled if industrial laundering is used. Home launder- ing practices vary and the detailed instructions provided for care (e.g. no bleach, even in detergents) may not be followed. As there are no visual indicators, the user is not aware that safety has been compromised and will often continue to wear the garment.
Repellent finishes
Repellent finishes are used to provide barrier protection for a wide variety of PPE.
For example, repellent finish is applied to: (i) the outer shell of firefighter turnout gear to prevent water and other liquid penetration; (ii) protective clothing worn by military personnel to protect from environmental conditions; and (iii) protective clothing worn by pesticide applicators to prevent pesticide penetration through the fabric. As with fabrics treated with flame retardant finishes, these garments typically require special care. Studies have been conducted in the United States
Durability of protective clothing 179 and Europe to determine the performance of cotton/polyester garments with repellent finishes. Since the garments evaluated, as well as the testing conditions, were different, no attempt was made to compare the results (Shaw et al., 2010;
Felber, 2011). In general, laundering practices (such as use of fabric softeners in the dryer), and in some cases soiling, resulted in increased pesticide penetration through the fabric. As with flame retardant finishes, users may find it difficult to determine when safety has been compromised. Preliminary tests show that a simple repellency test serves as a good indicator to determine the protection provided by the material. Surfactant build-up due to insufficient rinsing may also result in performance failure, especially when the garment is washed by hand, a common practice in certain regions of the world. A follow-up investigation of a garment with poor performance during wear showed that additional rinsing reversed the performance of a garment that was hand washed with soap and not rinsed thoroughly. Note that this example also emphasizes the importance of training and the consequences of developing instructions that do not reflect methods of cleaning commonly used in the country/region where the garments are used. Adhering to care instruction may be difficult given the cultural practices and availability of resources required to comply with the recommended care instruc- tions.