Saltwater Contamination of Nylon: Consequences and Redemptive Procedures
Parachute Laboratories, Inc. dba Jump shack
by Joe O’Brien
As a full service production and rigging facility, the Jump Shack staff are in a position to witness a variety of equipment contamination scenarios, not the least of which being the ever present Florida saltwater. We feel that the parachute equipment manufacturers have a responsibility to inform the general public and provide guidance to riggers with respect to appropriate action when confronted with saltwater-contaminated parachute equipment. In our quest to standardize processing procedures and renewal techniques, we have convened our technical resources in an effort to provide an interpretation of the available scientific data.
The result of our inquiry is a procedure for the field clarification of saltwater contaminants from low modulus polyamide fibers, of which Nylon is of primary relevance. While only in service a short time when compared to Nylon, scientific data does exist with regard to Spectra (the commercial proprietary name for a very high modulus polyethylene fiber commonly used as suspension line on modern sport parachutes). Certain extrapolations may be logically deduced from the associated data on Nylon and other synthetic polymers.
In the manufacture of parachute equipment, we must always ultimately concern ourselves with issues which affect strength. This study was undertaken for the primary purpose of establishing the recertifyability of saltwater-contaminated parachute components with regard to strength and longevity.
Once a parachute assembly has been introduced to salinous water and its constituent contaminants, two aspects of the contamination have the potential for significant degradation of the material’s strength if the equipment is not handled properly.
1. Relative Humidity: It has long been known that the strength of Nylon is affected by the presence of water. In tests conducted for the express purpose of studying the effects of saltwater contamination on Nylon, it was found that Nylon contaminated only with distilled water lost an average of 13% of its original strength when subjected to destructive loading while still wet. This can be chemically explained by understanding the general formation of Nylon’s polymer base. Nylon is a semi-crystalline polymer consisting of both crystalline and amorphous regions which have intermolecular hydrogen bonds between the polymer chains. Water is primarily attracted to the amorphous areas of the structure, where it disrupts the hydrogen bonds, thereby effecting a decrease in the glass transition temperature (one characteristic of modulus) and the tensile strength of the fabric.
2. Abrasive Precipitates: The second, and most significant, consideration when dealing with saltwater contamination of Nylon is one of fiber damage caused by abrasive contaminants. Saltwater, not surprisingly, contains a high concentration of sodium chloride crystals. When embedded in the Nylon fiber, these sharp crystals can cause an amount of damage which weakens the Nylon from its original strength by over 26%.
An explanation of Nylon’s physical composition will help. Nylon is made by extruding a liquid, somewhat ‘honey-like’, polymer through tiny openings where it is hardened into filaments which are a fraction of the diameter of a human hair. These filaments are then combined into yarns of various sizes, and then woven in numerous manners to meet certain criteria. One could consider sodium chloride crystals imbedded in these fibers as similar to a loose razor blade in a bin of thread, or a bundle of hair. As the threads or hairs are cycled (moved about during normal circumstances), the razor comes into contact with an increasing number of fibers which are, in turn, bisected by its presence.
Magnesium chloride, another salt present in sea water but in lesser concentrations, apparently carries an affinity for Nylon, which makes the potential problems associated with sea water contamination even more evident.
Nylon contaminated with sea water, and then left untreated to dry, showed very significant losses in strength when loaded to the point of rupture. Strength reductions of over 25% are not uncommon. It is of interest to note that Spectra fibers show a kind of immunity to abrasion not normally found in high modulus polymers. Test data on Spectra shows that it is completely unaffected by immersion in saltwater, even after six month exposure durations. This type of abrasion resistance, along with its unsurpassed strength-to-weight ratio, have made Spectra the material of choice for suspension line in the parachute industry. This data makes Spectra look very good as a potential source for parachute fabric. With a 30% lower volume per weight, and abrasion resistance which surpasses any other fabric capable of doing the job, Spectra seems an obvious choice. However, its high price and concerns about handling a fabric which has some strange characteristics from a fabrication stand point, have kept Spectra restricted to the suspension line application.
After extensive experience, testing and research, the management and staff at the Jump Shack have developed a standard procedure for the recertification of parachute equipment which has been immersed in sea water.
It is of great importance that the equipment be handled as little as possible to reduce the cutting action of the crystals imbedded in the fabric. Obviously, the equipment must be recovered, stored and transported to a suitable location for cleaning, but the less it is handled in this process, the less damage will be done by the crystalline contaminants.
Action should be taken immediately. Don’t store the equipment and let it dry. As the salt water dries, concentrated salt residues are left behind. Handling the equipment after it has dried will only exacerbate the problem. If possible, rinse the equipment with fresh water from a garden hose on-site to reduce the salt concentration in the fabric. Throwing the equipment in a pool or some kind of fresh water bath will go a long way toward decreasing handling-damage associated with storage for transport. We don’t recommend, however, that pool water be used as a final rinse for the unit. Chlorine, which is present in elevated concentrations in pool water, is the only chemical noted for decreasing the strength of Spectra.
If your rig is equipped with an automatic opener which contains a battery (Cypres, Astra, etc.), the AAD should be removed at the scene. Your Cypres or Astra is ruined, don’t wait to have a rigger remove it. If the batteries rupture, the acid will eat all Nylon it comes in contact with. Open your reserve container on-site and get that reserve away from the AAD. We’ve seen numerous saltwater-contaminated rigs which had reserve containers left closed with a Cypres, only to have both harness/container and reserve ruined by battery leakage.
As quickly as is practical, get the equipment to a bath tub, or a suitable fresh water container. Place the equipment in the tub and fill the tub with fresh water. Direct the flow over the equipment but agitate the parts only slightly during the first rinse. Drain the water and repeat. Agitation of the equipment can become increasingly aggressive during successive rinses. We recommend that the equipment undergo no less than three thorough fresh water flushes in order to achieve a minimum level of confidence. If time permits, rinse the rig a fourth, fifth, even sixth time-knock yourself out-the less salt the better.
Once you’re convinced that you’ve done your best to remove the salt from your rig, it needs to be hung up to dry. Don’t hang it outside. Sunlight will degrade Nylon very quickly, so don’t jeopardize your effort by throwing your salt-free equipment out on the lawn to dry. When the rig is completely dry, you are back in business-unless, of course, you had to have funeral services for your AAD at the beginning of this episode. Having to buy a new Cypres is a high price to pay for a ‘bad spot’ (yeah right!), but it’s not nearly as bad as having to buy a harness/container and reserve to match.
There you have it. By following a few simple procedures, and by respecting the potential for significant degradation of your equipment by crystalline contaminants, you can restore your saltwater-contaminated parachute equipment to over 91% of its original strength.
If you take your time and do it right, your gear will be almost as good as new.
Egglestone, G.T. “The Mechanism of Strength Losses in Nylon 6,6 Parachute Materials
After Salt Water Contamination.” Australian Department of Defense, 1980.
Irvin Indurstries Inc. “Recovery Systems Design Guide.” Air Force Flight Dynamics
Laboratory. Wright-Patterson AFB, Ohio, 1978.
Knacke, T.W. “Parachute Recovery Systems.” Para Publishing. Santa Barbara, CA,
Poynter, Dan. “The Parachute Manual.” Volumes I and II. Para Publishing. Santa
Barbara, CA, 1991.
U.S. Air Force. “Cleaning of Parachute Assemblies.” Publication T.O. 14D1-1-2. San Antonio, TX, 1989.