While I am new to this forum I have been in the composites business for many years including pultrusion, balistic protection, frp thermoplastic injection moulding etc. Basically I am saying that from a science point of view I have considerable experience.
UV resistance of synthetic materials..........
UV degradation is a common term that is not well understood. Most just assume that that nasty UV ray has its way somehow with the synthetic (plastic incl. UHMW polyethylene fibers and kevlar but not carbon) and in so doing causes a loss in properties. My work (and it is considerable) has taught me that there are some ways to mitigate some of the effects but they are not often applied to rigging
applications I suspect.
U V in istelf is only the catalyst to synthetic fiber degradation. The real culprit is disolved oxygen in dew and rain that coats the fibers and also enters the fiber osmotically. Dew and rain are really rich in disolved oxygen. The oxygen goes in with the water! So here we have our synthetic fibers sitting out there all night collecting dew and wicking it deep into the cord where it coats every fiber and enters fibers osmotically. OK so far other than the lubrication factor probably helps slipping between fibers to normalize loads (a positive factor). Now here comes that mean old sun with its nasty UV rays which actually can penetrate quite deeply into fiber bundles and individual filaments as plastics (which the fibers are) are somewhat translucent.
Next that nasty, high energy ray acts as a catalyst (sort of an initiator). We have oxygen molecule in the fiber and on it in a relatively pure form, we have a high energy source present/UV ray and we have a fibre loaded with molecules made up of atoms that would rather swap their current
bonds with each other with the present oxygen. BANG-ZAP the polymer chain is severed (not the whole fiber but one particular polymer chain within it) Each time those polymer chains get shorter the fiber is weakened. There are ways to mitigate this effect substantially but not nearly enough of the ways are emplyed in current rigging
Under some circumstances the damaged material can shade other material beside it but here is another spot where the current products used in rigging also fall short i.m.h.o.! Substantially the damaged material on the outside of fibers is removed through environmental (and in the case of rigging) mechanical processes. Mechanical processes are very intense in rigging where flexing and tension-tension in braids and twisted ropes causes a lot of erosion of the fibers (and in particular damaged material within individual fibers). Mechanical processes also can cause a lot of heat build up locally on a fibers surface but I hope to get to that a little later. The environmental processes include micro-particle erosion (air born dust) which erodes damaged molecular material from the fiber surface. In the case of sailboats there is also the problem of salt
crystals and dust and sand in the fibers which can be severely abrasive to healthy fibers as well as damaged ones.
So in a nut shell UV + Oxygen + erosion are the causes and accellerants to fiber strength loss and there are things that manufacturers can (and eventually will) do to reduce these effects. Problem for them I suspect is they simply don't understand the processes involved. Everyone simply says its damage from light and tries to protect from that but they are only dealing with a small part of the problem.
UHMW is worse than Kevlar i.m.h.o. in this dept. UHMW (the material in some ropes) fibers has a melt temp of something like 230 dg. F
The UHMW is a real strange material in this dept. as its properties change enormously at that temp. It is a very low friction material at normal use temperatures and this is a big bonus in the braided ropes but it is also a curse because at a molecular level what seems obvious is obscure and threateningly destructive.
Now I know some of U are saying 'my rigging never gets that hot' and you are probably right from a laymans point of view but at a molecular level where two fibers rubbing together under tension in a sinusoidal weave(braid) exhibit incredible friction at a molecular level on the surface of the fiber. UHMW is a very-very low friction material but at that phase change temp where it technically melts around 230 deg. F the friction properties (Pressure Velocity Limit) of the material skyrockets logrithimatically to astronomically destructive levels/the heat build up is enormous at simply 1 deg. F higher than the melt temp.) So now you have a lot of heat, you are above the melt temp, and now creep elongation at a molecular level accellerates enormously as well as the degredation I referred to earlier. All take their toll in logrithmic proportions as I said.
Interestingly carbon fiber in pultruded rods is also a non starter i.m.h.o. as it is very subject to galvanic corrosion
As I said earlier my background is in this area. I do believe that fiber rigging will vastly outlast s.s. someday and I also know that exisiting technology and r&d will easily take us there. I hope to work with fiber rigging on my boat when my curren wire expirest (or sooner than that - who knows)?