Climbers and canyoneers often worry about the UV-induced strength loss in nylon webbing that is used for rappel slings. Signs of UV damage are the fading of the dye, and the stiffness compared to new webbing. While UV damage is universally cited, the vast majority of reports supply no indication of how long a sling was in place (e.g. http://www.bogley.com/forum/showthread.php?73389-HOW-STRONG-IS-THAT-SLING-or-ARE-YOU-GONNA-RAP-ON-THAT!!!/page2).  Some damage is caused by other factors.  Animal damage (https://hwstock.org/rope/critter/) is significant for slings that travel out-of-sight behind rocks, or have food smells and salt from sweaty hands.  I have seen nylon material that was frozen stiff with ice, then hammered across rock by winds, sustaining large tears. (It would be good if climbers could put dates on slings, but the current penchant for black webbing makes sharpie marks useless.)

There aren’t many quantitative UV exposure tests; but those few suggest the significant loss is over one or more years, not months. A 1” tubular webbing sling tied in a loop has a pull strength of about 5500 lbs, and a rap is rarely more than 4x body weight. So if webbing isn’t torn or chewed, it may work after a 50% loss of strength. In this note we examine three quantitative tests of nylon webbing, none of which involve the exact type of webbing used in modern rap slings (there are additional tests of kernmantle ropes, e.g. https://www.teufelberger.com/en/fiber-rope/brainbox/my-rope-is-affected-by-uv.html, not considered here). The older tests must be interpreted with caution, as compounds to resist UV damage were increasingly added to the nylon yarns from 1940 onward, and have changed over time. Some additives are anti-oxidants (e.g. Mn in a low oxidation state) which eventually get used up by continued UV exposure.  Manufacturers may say that they did not add UV protectants to the finished product, but the yarns they purchased, to make that product, probably had some UV protection.

First, there is a military 1958 report on webbing-UV-exposure vs. strength. This report is available online (Wilkinson, 1958, “The Effect of Solar Radiation on the Breaking Strength of Outdoor Exposed Webbing.”). I’ve replotted the tabular data, as the included plots are unreadable. These tests were before modern UV mitigators, and several attempts were made to increase UV resistance, with unpredictable results. The tests were carried out at three latitudes, from October to October of the next year, so the 180 day results (Oct.-Apr.)saw a lot of cooler-lower-sunlight weather. The sample were held at 45 degrees from horizontal, and regularly turned to expose both sides of the webbing. Many polymers were tested; I’ve included results for “Nylon 330” only. I can find no information how “Nylon 330” relates to modern nomenclature, but it was the more durable of the two nylons tested. The major conclusion is that in high-UV environments, a 40-55% strength reduction might be expected over a year.

Second, the tests by WSTDA (Web Sling & Tie Down Association), compiled in 2003, give information of the degradation of initially very strong 1- and 2-ply webbing. I’ve picked the most relevant figures below. A 35-60% strength reduction might be expected over 3 years, but the exact relevance to climb-spec tubular webbing is unclear. Other studies have indicated that UV can penetrate several mm into webbing, so the similarity of 1-ply and 2-ply is notable.

Third, the April Logan thesis, currently available on-line (Logan, 2015, “Time-Dependent Environmental Degradation of Polymeric Fabrics.”) measured the loss of strength of webbing exposed for ~14 months in two Texas cities. The single-layer nylon webbing used in this study was sourced from Lea & Sachs. Logan reports that the manufacturer gave the tensile strength of 1300 lbs and a width of 1”; that company does not currently report 1300 lb webbing, but does report several military-grade 1” webbings at 1200 lb tensile strength. In any case, Logan found the experimental breaking strength of this fresh webbing to be 1450 lbs. The tests were conducted over ~14 months on the roofs of buildings in Lubbock and Houston Texas. Notably these two cities give similar breaking strength results. Both cities are near ~30 degrees latitude, but differ in cloud cover by a factor of two. This thin webbing lost about 1/3 of its strength over 14 months of exposure.

Both Logan and WSTDA report results in accumulated UV exposure, as well as time of exposure, but I reproduce just the plots vs. exposure time. WSTDA reports “good” correlations of breaking strength and time, but the fits are in log-linear space, so “good” is somewhat subjective.