Ballistic Body Armor

POLYMERS

Our goal is to prevent the catastrophic failure of ballistic 

body armor by developing measurements and predictive 

models to test and determine the long-term reliability of 

polymers used in ballistic resistant armor. Our approach, 

which utilizes conventional analytical techniques in addition 

to newly developed minimally invasive test methods, seeks 

to identify the chemical and mechanical mechanisms 

underlying reductions in ballistic performance and to link 

these mechanisms to chemical structure and deformation 

mechanics. 

Objective

Impact and Customers

• Ballistic resistant armor is credited with saving more than 3,000 lives; but 

in 2003, the failure of a first responder vest identified the need for longterm stability and durability tests.

• NIST developed a tool to assess the impact of folding during normal 

wear on ballistic body armor and demonstrated that repeated folding can 

reduce ballistic resistance of the armor by up to 40%.

• As a result of NIST’s fold tests, the NIST Office of Law Enforcement Standards (OLES) 

and the Department of Justice’s National Institute of Justice (NIJ) are incorporating 

folding as one of the critical durability specifications for the new revision of the ballistic 

fiber standard.

Approach

Materials Science and Engineering Laboratory

To quantify the impact of mechanical degradation on ballistic fibers, NIST developed a novel device for controlled 

folding of yarns and woven fabrics. In addition, we developed test protocols that employ single fibers to assess 

the effect of folding using a recently developed modified single fiber test and a newly acquired commercial, rapid, 

automated fiber testing apparatus. Morphological changes in the folded fibers are being quantified by small angle 

X-ray scattering (SAXS). 

Systematic and controlled extraction procedures designed around the 

molecular architecture of the ballistic fibers are also being developed 

to elucidate the factors that cause their hydrolytic degradation. The 

extracted material is being evaluated by X-ray fluorescence spectroscopy, 

matrix assisted laser desorption ionization (MALDI), mass spectrometry 

(MS), and a newly acquired gas chromatography (GC)/MS system. 

These techniques are augmented by the use of novel derivatization 

methods to facilitate the detection of compounds suspected of causing 

hydrolytic degradation.

854-01-01a
Mechanical Degradation

Recently, our folding research has shifted to 

the analysis of polyaramid (Kevlar) fibers, the 

most widely used ballistic fiber in soft body 

armor. Results indicate that these fibers may 

not be as susceptible to folding damage as 

poly(benzoxazole) (PBO) fibers. Prior data 

obtained on PBO ballistic fibers with NIST’s 

automated fold tester showed that repeated 

folding decreases the mechanical properties 

that govern ballistic performance. Tests 

designed to simulate six months and 10 years 

of wear indicated performance reductions of 

15% and 40%, respectively. Furthermore, small 

angle X-ray scattering (SAXS) data from the PBO 

damaged fibers (shown below) indicated that the 

transition from fibrillar failure to brittle failure 

(also below) that accompanies repeated folding 

is associated with the formation of micro-cracks 

perpendicular to the fiber axis.

To show the potential impact of the folding 

degradation mechanism on first responders, 

fibers from a new vest, a hydrolytically aged 

vest, and a field return vest were analyzed along 

with the back panel from the compromised vest. 

Data from the new vest showed that there is 

minimal damage to the PBO fibers during the 

manufacturing process, while hydrolytic ageing 

can result in a significant reduction in ballistic 

performance. More importantly, data from 

the field return vest showed that the folding 

mechanism works in addition to the hydrolytic 

damage mechanism to create localized weak 

spots in the vest that have even lower ballistic 

performance. Also, these results indicate that 

the back panel of the compromised vest is not 

a good indicator of the ballistic performance 

of the front panel, because of the additional 

folding that occurs to the front panel. 

The success of the above research has led to a 

proposal being submitted to NIST-OLES to link 

the results from the single fiber tests to those of 

the Kolsky bar, where single fibers are deformed 

at a much higher strain rate. 

Chemical Degradation

It is now known from GC/MS data that the readily 

extracted residual phosphorus from PBO fibers is 

mostly an n-octadecyl phosphate, a processing 

aid that is added during manufacturing, and 

a much smaller amount of phosphoric acid. 

Analysis of three sets of fibers also indicates that 

the manufacturer of the PBO fibers may have 

developed a more efficient washing process 

to remove nearly all of the readily extractable 

phosphoric acid, leaving mostly non-extractable 

phosphorus. 

Our research indicates that the non-extractable 

phosphorus is phosphoric acid that is chemically 

attached to the hydroxyl groups on the DADHB 

(i.e., 1,3-diamino-4,6-dihydroxy benzene) 

molecules, forming aryl-phosphate esters. 

The DADHB preferentially end-caps the PBO 

polymer chains (see below). Furthermore, it is 

now known that these aryl-phosphate esters are 

stable to the caustic wash solutions that are used 

in the manufacturing process to neutralize the 

phosphorus pentoxide (drying agent) enriched 

PPA (i.e., poly(phosphoric acid) reaction 

medium.

By exposing PBO fibers that have been extracted 

multiple times, to remove all readily extractable 

phosphorus to a pH 4 HCl solution under 

sealed conditions for several days, additional 

phosphorus in the form of phosphoric acid 

was removed from the fibers. These results 

support the supposition that the non-extractable 

phosphorus that remains in the PBO fibers 

is in the form of an aryl-phosphate ester that 

is stable in neutral to basic aqueous washes. 

More importantly , these results indicate that 

the residual phosphoric acid, if not removed, 

can accelerate the deterioration of the PBO 

fiber architecture through the destruction of 

the benzoxazole ring structure that seems to 

provide it with its initial increase in mechanical 

properties relative to Kevlar. Because of the 

transformational nature of the PBO technology 

with respect to the development of ultra lightweight soft body armor, the above findings 

should aid researchers in discovering ways to 

manage the weakness of the PBO fibers, so that 

they can be safely used in ballistic applications. 

The Polymers Division research team has been 

asked by NIST/OLES to develop collaborations 

with the Natick research institute and other 

government agencies interested in soft body 

armor, by leveraging our knowledge and expertise 

to find workable solutions to the PBO problem.

Materials Science and Engineering Laboratory

Accomplishments

Haruki Kobayashi

Eun Su Park

Walter McDonough

Derek Ho

Charles Guttman

John Sieber (CSTL)

Gale Holmes

(Polymers Division)

(301) 975-5280

Gale.holmes@nist.gov

www.nist.gov/polymers

Learn More Publications

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Holmes GA, Rice K and Snyder CR Ballistic Fibers: A Review of the Thermal, Ultraviolet and 

Hydrolytic Stability of the Benzoxazole Ring Structure Journal of Materials Science, 41: 4105 

(2006)

Kim JH, Brandenburg N, McDonough WG, Blair W and Holmes GA An Apparatus for 

Mechanically Folding Yarns and Woven Fabrics of Ballistic Fibers Journal of Applied 

Mechanics, 75(1): 015001 (2008)

Holmes GA, Kim JH, McDonough WG, Riley MA and Rice KD A Detailed Investigation of the 

Mechanical Properties of Polybenzoxazole Fibers within Soft Body Armor Journal of Materials 

Science, DOI: 10.1007/s10853-009-3338-6 (In Press, 2009)

Fibrillation (l) and brittle (r) failure

SAXS data from unfolded and folded fibers

MALDI-TOF MS of PBO model oligomers showing 

aromatic phosphate ester