After reading the first two parts of this saga, you're probably beginning to wonder what in the hell fluorocarbon is actually good for. I wanted to get the two biggest misconceptions out of the way first, so that we can now focus on the other attributes. So let's jump right into it, taking the easy ones out next.
UV Degradation: One of the weaknesses of nylon monofilament is it's unstability when exposed to sunlight, in particular ultraviolet radiation which is the primary cause of the degradation. Tests show that the loss can be as great as 20% of its strength in the first 100 hours of exposure. After that, mono can lose another 20% over the next 100-200 hours. This degradation is usually in the form of oxidation.
But you haven't changed your line all season and you don't really notice a difference in its strength. There are several reasons for this. One is that 100-300 hours of exposure is a long time, especially with the usage of rod lockers now days on modern bass boats. Additionally, most of the outfits are being fished with throughout the day thereby limiting further exposure. Another is that only the line on the surface of your reels and strung through your guides is directly exposed to the UV and subsequently getting the exposure, and you're constantly cutting off end sections and retying, thereby always getting rid of the old and exposing the new.
Fluorocarbon on the other hand is largely transparent to UV radiation and therefor suffers much lower UV absorption. As such, the material will not be degraded by sunlight. So score one positive for the PVDF.
Density (as relates to sink rate): Everyone has probably heard the numbers relating to density for the two materials, as well as the claims of faster sinking for fluorocarbon. While this is true, what you rarely read are the actual rates and their subsequent practicality. It is easy to think in terms of faster sinking and relate to worm weights, or jigs, or 1' per second and all that other stuff. In reality, life is much slower. If you take two pieces of line, one nylon monofilament and one fluorocarbon, sink them in a column of water and time their drop rate, what you'll find is that it takes the nylon monofilament about 40 seconds to drop one foot in the water column. This is because monofilament has a density of about 1.1 relative to water at 1.0, not much denser.
Fluorocarbon on the other hand has a density of about 1.78. So how does this translate under the same test conditions? A similar piece of fluorocarbon will make that same 12" journey in about 15 seconds, still 3 times faster than mono but not overly fast in the big scheme of things. If you think of your average cast and retrieve taking somewhere between 30-60 seconds, under its own accord the fluoro will have only had the chance to move 2'-4' down into the water column in that time period.
Now you still have the fact of the lure and any tension caused by that lure to help pull down the line quicker, but you also still end up with a sort of arc down to your bait instead of this really direct, straight line that people invision. So for things like shallow running crankbaits, jerkbaits and spinnerbaits, fluorocarbon would work well. For topwaters, it pretty much sucks as it wants to slowly pull your bait down into the water which is not what you want with a floating topwater. For all other baits like worms, jigs or other things typically fished deep or on the bottom, it probably helps a little, just not as much as you might have thought.
Water Absorption (Tensile Strength): Another potential benefit comes in the form of resistance to water absorption. Depending on the exact makeup of a particular monofilament, it would not be unusual to have between 3-10% absorption of water (by weight) with nylon. That leads to both good properties and bad. On the good side, a nylon mono that absorbs water swells slightly, becomes easier to handle, becomes more limp, makes knot tying easier and will even cast better. On the negative side though is that water absorption weakens the line, increases stretch and decreases strength. So depending on your perspective this might be good or it might be bad.
If you look at the numbers for fluorocarbon though, it is practically impervious to water. Water absorption rates are typically <0.04% for pure fluorocarbon. What this means is that none of its physical properties change after a good soaking. Line strength stays the same, stretch stays the same, but so too does any stiffness or unruliness. Again, a mixed bag depending upon how you look at it.
But this brings up a larger question in my mind that I've never seen anyone address. With monofilaments, line conditioners that you spray onto your reel are pretty popular. Some work by penetrating into the nylon and performing this softening I mentioned via absorption, while others work by attaching themselves to the exterior of the line and giving a slick coating. I've seen many people recommend using these same conditioners on all lines including fluorocarbon like they're some type of magic formula, but I have to wonder if the improvement isn't just a psychological one. You're not instantly changing the physical properties of fluorocarbon by spraying this stuff, so the imperviousness to moisture still exists. Nothing is getting absorbed, so are they really working?
Odds and Ends: A couple other observations I've come across in the research.
- Knot strength - Mono wins out here. Tests with just some simple doubled loop knots, lubricated before cinching and applied to dry line show mono to retain as much as 97% of its breaking strain, while fluorocarbon retains about 77%. After soaking the lines and accounting for the previously mentioned absorption by nylons, mono weakens to 83% of original dry, unknotted breaking strain while fluoro maintains its 77% rate. Your mileage may vary depending upon the knot, the fluoro and the care used when tying.
- Weight on Spool - If you weigh an empty spool from your reel, and then load that spool to capacity with similar diameter lines and then reweigh, you'll find that a spool of fluorocarbon weighs about 23% more than a spool of mono (but just 4-5 grams total weight). If you subtract the spool weight and weigh only the line, that increase becomes 75%. What this typically translates to is decreased casting distance. If you actually throw braids into the mix, they come out best...and they tend to cast further as expected. This is largely due to the density difference and ties in with the effort required to start the spool rotating as well as with keeping that heavier line "up in the air" during the cast.
- Rockwell Hardness - I found this one interesting. Rockwell Hardness is a standard method of measuring hardness of a material. Everyone seems to state that fluorocarbon is much tougher than mono, but it depends on the specific formulation used and tested. Whereas the different types of nylon can vary with a reading between 88 -114 in the test, fluorocarbon scores 100. So though fluoro seems tougher due to a slightly higher modulus in flexure, in actual hardness tests it doesn't score appreciably better. But keep in mind these are dry tests, and once nylon absorbs water, it can lose between 30-40% of its modulus and would definitely feel (if not become) "softer" at that point.
- Thermal Conductivity, Bending Strength - You frequently hear anglers mention that you need to tighten fluorocarbon knots slowly to avoid heat buildup, but it turns out that fluoro has a lower level of heat conductivity than mono. Fluoro is rated at 0.19 W/mK while monofilaments range from 0.22-0.27 W/mK. To give you a better perspective on these numbers, if you think of typical insulating materials like Styrofoam or fiberglass, they have ratings between .03-.04 W/mK. On the other hand, something like a cast iron pot has a rating of 55.
Which brings up another interesting point. Why is it good to 'spit on' or wet your knot when tying and cinching? Of course, lubricity is probably the first thing people think of. But it also turns out that water has a thermal conductivity rating 3X greater than fluoro (0.58) and about twice that of monos. This means the water actually helps wick away excess heat generated during the knot tying process.
That said, fluoro does have a slightly higher thermal expansion rating ranging between 100-140 e-6/K, while monofilaments range from 70-120 e-6/K. The real culprit with knots is in the bending strength of each material. Monofilament ranges from 110-125 MPa, while fluorocarbon comes in at a relatively lower rating of 94 MPa. This ties back into and explains the knot strength differences as mentioned at the top of this section.
So another segment of this series concludes. That leaves one left to complete. In the final segment of this series, I'll take on that all important question of sensitivity between monofilament and fluorocarbon, what is involved, and how many of these qualities already mentioned play into the big picture.
Brian, You are doing a wonderful job on this.
As a maybe unrelated side note on perspective, please see that underwater - looking up - photo you used here. Keep in mind that all things between the viewer and the light will show a halo to one angle degree or another when backlit. If you ever see a photo of a depth finder screen showing predators attacking a school of prey fish, you can bet the sun is a back light to the prey. With transparent materials, and unless they match the refraction index of the surrounding water, they will show a slightly different angle of refraction. These are little things that can be picked up as not normal. . . to the fish. Depending on the position in the fish's learning curve, they can help the fish target its prey, but if older and wiser, it can also tell them to stay away.
Posted by: Rich Ziert | June 11, 2009 at 09:57 AM