Kevlar is an aromatic polyamide, or aramid, fiber introduced in the early 1970s. The chemical composition of Kevlar is poly para-phenyleneterephthalamide (PPD-T). It is made from a condensation reaction of para-phenylene diamine and terephthaloyl chloride. The resultant armomatic polyamide contains aromatic and amide groups. Polymers with high breaking strength often have one or both of these groups. The aromatic ring structure contributes high thermal stability. The para configuration leads to stiff, rigid molecules that contribute high strength and high modulus.
Para-aramid fibers belong to a class of materials known as liquid crystalline polymers. When PPD-T solutions are extruded through a spinneret and drawn through an air gap during fiber manufacture, the liquid crystalline domains can orient and align in the flow direction. Kevlar can acquire a high degree of alignment of long, straight ploymer chains parallel to the fiber axis. The structure exhibits anisotropic properties, with higher strength and modulus in the fiber longitudinal direction than in the axial direction. The extruded material also possesses a fibrillar structure. This structure results in poor shear and compression properties for aramid composites. Hydrogen bonds form between the polar amide groups on adjacent chains and they hold the individual Kevlar polymer chains together.
Tensile modulus is a function of molecular orientation. As a spun fiber, Kevlar 29 (a high toughness variant) has a modulus of 62 GPa (9 Mpsi). Heat treatment under tension increases crystalline orientation. The resulting fiber, Kevlar 49, has a modulus of 131 GPa.
The tensile strength of Kevlar ranges from about 2.6 to 4.1 GPa. This is more than twice that for conventional fibers like Nylon 66. Tensile failure initiates at the fibril ends and propagates via shear failure between the fibrils.
Kevlar behaves elastically in tension. In compression, it shows nonlinear, ductile behavior. It exhibits yield at compression strains of 0.3 to 0.5%. This corresponds to formation of structural defects known as kink bands. These bands are related to compressive buckling of the aramid molecules.
Aramid fibers are noted for toughness and general damage tolerance. Tensile elongation of Kevlar 29 is about 4%. The fibrillar structure and compression behavior contribute to composites that are less notch-sensitive and which fail in a ductile, non-catastrophic manner, as opposed to glass and carbon.
The aromatic structure gives the fibers a high degree of thermal stability. They decompose in air at about 425 C and are inherently flame resistant. Aramids have a slight negative longitudinal coefficient of thermal expansion of about -2 x 10^-6/K and a positive transverse expansion of 60 x 10^-6/K. They also have a low thermal conductivity that varies by about an order of magnitude in the longitudinal versus transverse direction.
| Material | Tensile Modulus (GPa) | Tensile Strength (GPa) | Density (g/cc) |
|---|---|---|---|
| Kevlar 29 | 62 | 2.76 | 1.44 |
| Spectra 900 | 117 | 2.59 | 0.97 |
| Spectra 1000 | 170 | 3.27 | 0.97 |
| Acids - Concentrated | Poor |
| Acids - Dilute | Fair |
| Ultraviolet Light | Fair |
| Alcohols | Good |
| Alkalis | Good |
| Aromatic Hydrocarbons | Good |
| Greases and Oils | Good |
| Halogens | Good |
| Ketones | Good |
| NIJ Threat Level | Areal Density, Kevlar 29 | lb/ft^2 Kevlar 129 |
|---|---|---|
| IIa | 0.92 | 0.85 |
| II | 1.15 | 1.4 |
| IIIa | 1.61 | 1.42 |
| V50, ft/sec, 9mm | ||
|---|---|---|
| Areal Density, lb/ft^2 | Kevlar 29 | Kevlar 129 |
| 0.8 | 1290 | 1480 |
| 1.0 | 1400 | 1600 |
Specific Issues to Consider
* Crime, Confiscated and Stolen Weapons
The types of weapons being stolen, confiscated, and used to commit crimes in your region are all strong indications of the threats law officers might face.
* Threats Increasing
Today, the low-energy "Saturday night special" handguns represent only about half of those incidents where the use of concealable body armor can prevent a tragedy. Today's officer clearly needs protection from low- to medium-energy handguns, 22 rimfire rifles, and shotguns. Many law enforcement practitioners believe the trend toward higher threats will continue.
* Officer's Weapon and Ammunition
Typically, about one out of every five officers feloniously slain is killed with his or her own service weapon. Therefore, compatibility of the officer's weapon and ammunition with his or her body armor is fundamental.
Caveat
The above threats should be considered in selecting a vest that will provide protection as well as comfort.
* Climate, Work Environment, and Duty Assignment
Temperature, humidity, normal work environment, and type of duty assignment are all important considerations to the extent that they may well impact the type of coverage and amount of protection that is wearable.
* Individual Preferences
There is a growing recognition that the ability of an officer to be comfortable and mobile while wearing body armor can be a highly individualistic issue. For this reason, individual participation in the selection process is recommended.
* Side Panel vs. No Side Panel
F.B.I. data shows that during 1980-1993, 219 officers were killed while wearing soft body armor. Of these, 87% (191) were shot outside the protective area of the armor; 52% in the head, 16% in the neck/upper torso, 16% in the lower abdomen, 6% in the shoulder/arm hole, and 10% in what might have been the side panel region. We believe this data shows that, while side panel protection is desirable and increases protection, this selection decision must consider whether or not the individual officer can and will wear the extra coverage.
The remaining 8 officers died as a result of penetration of the body armor by ballistic threats well in excess of the design capability of the armor. All were center-fire rifle which is not expected to be stopped by concealed body armor. However, tactical armor will protect you against such threats. There were no cases where personal body armor failed to perform to its expected level of protection.
* Water Repellency vs. No Water Repellency
Non-water-repellent fabrics exhibit a temporary reduction in ballistic capability when soaking wet (>15% moisture pickup). When the armor dries, full performance is restored.
For this reason, an officer who might expect to become soaked during his assignment should consider purchasing water-repellent- treated body armor.
Both the F.B.I. and DuPont have conducted scouting studies on the effect of perspiration on body armor. Through June 1994, there is no evidence to suggest that an officer might soak his vest with enough sweat, during normal duty assignment, to affect ballistic performance.
As of June 1994, we know of no instance where the existence or absence of water repellency has affected the outcome of either a felonious slaying or a body armor save. Whether water-repellent treatments affect comfort and wearability of soft body armor continues to be discussed within the manufacturing and law enforcement community. Consult the vest manufacturers for further information.
Laundering
Soft body armor of KEVLAR should not be machine washed or dried, since it has been demonstrated that damage to the fiber can occur from the laundry equipment itself, ultimately affecting its ballistic performance. Therefore, a gentle hand wash with mild detergent in cold water and indoor drip drying are recommended. Commercial laundries also must be avoided, since the detergents they use may be much harsher than home detergents and can reduce the ballistic-resistent characteristics of fabrics of KEVLAR.
Laundering Water Repellents
While soft body armor containg water-repellent-treated fabrics of KEVLAR can be hand washed, they must be thoroughly rinsed. The fabrics will retain a high level of their original water-repellent treatment, but without proper rinsing, they can accumulate a film of residual soat that absorbs water, and ballistic resistance is reduced. For this reason, thorough rinsing after hand washing is strongly recommended.
Laundry Additives
Under no circumstances are bleach or any products containing bleach to be be used in the laundering of soft body armor of KEVLAR. Bleach, even when highly diluted, will adversely affect the ballistic resistance level of the garment. Pre-soak products, fabric softeners, and stain removers containing no bleach should have no effect on the ballistic resistance of KEVLAR.
Dry Cleaning
Perchloroethylene, a commonly used dry-cleaning solvent, has been demonstrated to have no adverse effect on the ballistic resistance of fabrics of KEVLAR. However, because of the variety of dry-cleaning solvents in use today, most soft body armor manufacturers recommend that dry cleaning be avoided.
Sunlight Drying
KEVLAR is susceptible to ultraviolet (UV) degradation, as are many other manmade textile fibers. However, during normal use, a cotton or polyester/cotton carrier protects KEVLAR from UV exposure. Ballistic fabrics of KEVLAR should never be dried outdoors.