First, the good news:
Zylon fiber is amazingly strong when new. 
Roughly twice the tensile strength of p-aramids like Kevlar and Twaron.

This would ideally allow construction of thinner, stronger vests that are amazingly light and flexible.
Here's some more poop on the origins of Zylon, acquired in a MK1/mod0 Google search.
http://www.sri.com/about/timeline/pbz.html ;
PBZ High-Performance Polymers 
SRI chemists patented a class of high-performance polymers called PBZs. This technology had been licensed for commercialization to Dow Chemical, which sold its interest in PBZ to Toyobo Company, Ltd., a leading fiber producer in Japan. Toyobo began commercial production under the name Zylon(tm) in 1998. Zylon(tm), also called PBO after its chemical structure, poly(p-phenylene-2,6-benzobisoxazole), is a member of the PBZ class. 

Brand new level IIIA vests of Zylon based materials are usually capable of defeating threats that would ventilate other vests made of competing materials like P-Aramid or UHMWPE. 
Although backface deformation would still be extreme in these situations, the brand new Zylon panels can often remain intact where other vests would have failed.
The Zylon based vests are super light, very comfortable, and very expensive. 

Several companies are currently manufacturing vests made of this PBO material, either in whole or in part.
Second Chance, Point Blank, Armor Holdings/ABA/Xtreme Armor/Safariland are a few of the companies currently advertising the use of Zylon based materials in some of their vests.
Promises of a new age of more wearable vests and increased user safety.

Now the Bad News:
They are NOT really delivering what they claim they are.

Toyobo of Japan has published a PDF document (September, 2001) regarding the strengths and inherent weaknesses of Zylon (PBO fiber).
Apparently, the material is twice as strong in tensile as p-aramids like Kevlar and Twaron when it is new, but manifests some very serious strength degradation issues with exposure to any visible light, or heat and  humidity.
Read this PDF, and then come back for the rest of this. (You will need Adobe Acrobat 5.0 or later to download it)
http://www.toyobo.co.jp/e/seihin/kc/pbo/technical.pdf ;

The Adobe PDF file at the Toyobo site shows, among other things:
1) Degradation of as much as 65% strength loss over six months (evidently a day/night inclusive time line) exposure to sunlight, 

2) 65+% degradation in 100 hours under exposure to Xenon lights at 83C, and signifigant degradation after minimal exposure to two 35 watt fluorescent lamps.

Interestingly, the degradation curves for 100 hours under Xenon at 83C and "six months sunlight" are nearly identical, as layed out in their graphs. Both show very sharp downward curves upon initial exposure, and then decrease degradation rates as time goes on, but NEVER stop degrading during the term of the experiment.

Bear in mind that the six months of daylight would include 1/2 of that time in near total darkness (night), and about 50% of the remaining time might be strong, direct sunlight. The 50% strength loss occurs in sunlight right at the two month point.(Do the math)

Taken on the average, about 150-300 hours of exposure to strong white light 
(6.5-12.5 full 24 hour days) appears to cause an average of 50% degradation in tensile strength in Zylon. Higher temperatures seem to accelerate this degradation.

Also noted by Toyobo is signifigant degradation due to exposure to humidity.
Toyobo specifically warns against exposing Zylon any exposure to humidity.
Toyobo also specificall warns against exposing Zylon to any visible light.

Oddly enough, the material seems nearly impervious to outright immersion in water, but humidity just kills it. 
This is be due to phosporic acids  that is entrapped in the PBO fiber during the fiber manufacturing processes. These acids/chemicals are  activated by the water vapor, and then begin eating away at the material or otherwise altering its structure sufficiently to allow the documented loss of strength.

Given the fact that the armor made from Zylon is tendentially engineered for minimal bulk and maximum wearability under the entirely erroneous assumption that Zylon (as currently packaged) retains all of its "as manufactured" strength after manufacture and wear, the amount of safety margin built in for acrued light, heat and humidity induced strength degradation over time is minimal as well.

Imagine how humid it is between a vest and its sweaty wearer at any given moment. 
Approximately 100% relative humidity, day or night, rain or shine.
Then realize that a Goretex cover is specifically designed to allow the transpiration of water vapor.
Guess where the water vapor will migrate to? Straight to the acids in the fiber. the acid is hygrospic. it attracts water vapor.
Compound that with use/storage in very humid areas, and compound it again by use in elevated temperatures.
Bad news.

Merely sealing it into thermoplastic elastomer (TPE) sheets (Z-Shield type technology) won't help much at all. There is a large body of data showing the strong capillary quality of the fiber. 
Exposed fiber ends on the cut TPE laminate material edges will, quite literally, suck water vapor as it becomes available.
[ J. Appl. Polym. Sci. Vol.80, 1030 (2001) An Analysis of Capillary Water Behavior in Poly-p-Phenylenebenzobisoxazole Fibers T. Kitagawa, K. Yabuki ]

Further, the Toyobo PDF document specifically WARNS against any exposure to or storage and use in visible light, and any exposure to "high humidity", even at "room temperatures".

Despite these published warnings from the material's manufacturer, no armor maker using Zylon has made any attempts (to date) to keep the Zylon from any exposure to bright work lights during both storage and manufacturing procedures.
Other Zylon users and OEM armor material fabricators have made no attempts to prevent similar hazardous exposure issues with humidity during their manufacturing processes.

Note that there is NO data shown in the Toyobo document to indicate:
1) That there is any recovery of strength after exposure to light/heat/humidity ceases.
Once some quantity of strength is gone, it is gone for good.

2) None of Toyobo's published data indicates that degradation of strength ceases entirely after exposure to degrading influences ceases.
Zylon gets a sort of ongoing "cancer" from exposure to light and/or water vapor, as opposed to just getting a "sunburn" or fagged out from the humidity. 

3) None of Toyobo's published data indicates that drying out the Zylon recovers its strength. 
In point of fact, the drying techniques possible for wearers in the field would entail turning the liquid water on the vest into water vapor in order to dry the vest... thereby causing increased exposure to humidity.

4) None of Toyobo's published data shows that a combination of humidity and light exposure would not compound the degradation curves manifested in the other tests. Data does exist to show that greater heat compounds the humidity issue though.

We know for a fact that the Zylon materials used by armor manufacturers are not currently handled and stored in totally dark, zero humidity conditions during fabrication of armor panels, and in laminated variants it is exposed by the laminate material manufacturer to heat sufficient to bond it to the TPE sheets used in the laminate material. 
All of these conditions are PROVEN to cause degradation of Zylon...

So, this begs the question: 
Just how bad is the degradation by the time the user gets his vest, and how long can he expect it to stop the rated threats?
As it turns out, not very long at all. Only about six months on the outside.

Zylon may be a marvelous material when used within its very well documented limitations, but it is certainly obvious that NONE of the armor manufacturers or OEM material suppliers have taken all of those limitations into consideration during their design and packaging of the material for use as body armor. 
Why? 
Good question. 
The answer? Money. Tons of it. 

The armor makers were warned as wearly as 2000 that Zylon had serious degradation issues. they suppressed the data, and continued to make the Zylon armor because their profits were immense, and the NIJ required no post manufacture testing to ascertain environmental stability of the armor.
Toyobo told us that they  sent their test data to all of their customers as it became available. The most damning data came form them in July, 2001. Over two years ago. 
The manufacturers did nothing.

Note: in ascending order, the threat levels for soft concealable body armor are IIA, II and IIIA
A IIA vest will stop fewer threats and at lower velocities than a II, and a II stops less than a IIIA.


Forest Hills, Pa: Officer Limbacher was critically wounded after his Second Chance Zylon vest failed to stop a .40 caliber pistol round it was rated to stop. His level IIA vest was about six and a half months old.

Oceanside California:   Officer Tony Zeppetella has died as a result of his Second Chance Zylon vest failing to stop rated 9mm rounds. His vest was about seven months old. He leaves  a widow and an infant son behind. His vest level II was about seven months old.

Both of these vests were well within their warranty life, and neither was over eight months old.
The tests done by Kent PD in Washington indicate that vests of approxiamtely two years of age suffer a minimum of 40 strength/stopping ability (V50) loss at that latitude.

Vests worn in southern climes where more heat and humidity are present degrade even faster.

Ravalli County, Montana: Another vest was penetrated from the inside out by a .45 acp round. The .45acp is a very slow round, larger in diameter than most other pistol rounds. They are not known for deep penetration.
This .45acp penetration would be an impossibility with a new vest even in a IIA level, but the two year old Second Chance ultima Zylon vest let the round through after it passed all ther way through the officer's shoulder and scapula. This was a level II vest that should have stopped that round quite easily. His vest was about two years old.

The preliminary analysis of this field data on wounded and killed officers indicates that a level IIA Zylon vest more than six months old has a greater than 50% chance of being penetrated by any 9mm round. Vests of any threat level will degrade sufficiently in two years to be effectively useless against most rounds they are rated to stop.

There are three grades of Zylon, the "S-spun grade" being the lowest quality, and most susceptible to degradation due to it's molecular structure and affinity for attracting and bonding water molecules (hygroscopy) . This is the grade in use by all of the armor manufacturers, dspite data that higher grades (in ascending order, H and HM grades)  would have longer lifespans.

There are over 150,000 Zylon based vests in use in the U.S. today, and over 70,000 in Germany.
Three out of four Zylon vests in the US were made by Seond Chance. The balance made by their competitors.
Zylon vests cost roughly twice the price of competeing kevlar/aramid products. The profits on this are so immense that the armor makers will do anything to keep making Zylon. 
Even risk their customer's lives.

Kent Washington tests
http://64.177.53.248/ubb/Forum5/HTML/000346.html ;
Limbacher shooting, Pennsylvania
http://www.pittsburghlive.com/x/tribune-review/s_142385.html ;

Toyobo Technical Data PDF
http://www.toyobo.co.jp/e/seihin/kc/pbo/technical.pdf ;
http://www.bsstgmbh.de/BSSTV20/html/english/eS010702.htm ;

US Armor refuses to use Zylon:
http://64.177.53.248/ubb/Forum5/HTML/000356.html

Interview with Richard Davis, President of Second Chance:
http://64.177.53.248/ubb/Forum5/HTML/000297.html
(carefully read the parts in bold face)

My site, however, is the most comprehensive site on the Web for Zylon information: 
MD Labs Forum at Tactical Forums.com
http://64.177.53.248/cgi-bin/tacticalubb/forumdisplay.cgi?action=topics&forum=M....

Please feel free to call me at any time with any questions or requests for elaborations.
Please bear in mind that all of my direst predictions about Zylon have proven to be absolutely correct.
I can also provide acces to other expert witnesses to corroborate my data and analysis or provide references for my personal credibility in this matter.

Thank you for your attention to this matter.
Kevin McClung
MD labs
8333 pecos Drive #4
Prescott Valley, AZ. 86314

928-772-3021 Office

928-899-4552 Cell/24 hour message