Bobbin bunches a sheath

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Postby Scott McCrea » Nov 29, 2005 10:55 am

How much sheath is being milked off? An inch per 10 feet? More? Less? This sounds quite strange to me. Makes me think that there is either something wrong with the rope or the excess sheath is there for a reason and it ought not be removed.
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Postby cob » Nov 29, 2005 8:36 pm

OK guys maybe I am full of s**t, (and please correct me if I am) but I thought WATER HAD NO AFFECT ON NYLON AT ALL!!!

Meaning it could not POSSIBLY shrink it.

Wash it clean of pollutants? yes. Shrink it? No.

Mind you, this whole conversation reminds me of WHY i am a cob... (crustyoldbstrd for those who don't know) Racks WORK.

why change?
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Postby hank moon » Nov 29, 2005 9:04 pm

cob wrote:I thought WATER HAD NO AFFECT ON NYLON AT ALL!!!


well...i wouldn't say you are FOS, but water does have an effect on nylon - it absorbs water, making it stretchier, weaker, and less abrasion resistant. Not sure if individual fibers actually shrink, but nylon ropes do.

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Postby RescueMan » Nov 29, 2005 10:46 pm

cob wrote:OK guys maybe I am full of s**t, (and please correct me if I am) but I thought WATER HAD NO AFFECT ON NYLON AT ALL!!!
Meaning it could not POSSIBLY shrink it.


hank_moon wrote:well...i wouldn't say you are FOS, but water does have an effect on nylon - it absorbs water, making it stretchier, weaker, and less abrasion resistant. Not sure if individual fibers actually shrink, but nylon ropes do.


OK Cob, you ARE FOS, or at least misinformed. And Hank, it doesn't actually absorb water (it's a plastic), but the water acts as a plasticizer and changes the mechanical properties of the nylon at the molecular level.

By lowering the Tg (glass transition temperature) of the nylon molecules, water makes it more amorphous at room temperatures (similar to heating it to 140 degrees F. This turns the plastic into something akin to silly putty which seems very elastic unless it is pulled apart rapidly which causes it to break.

See my post in the Rope Shrinkage thread:
http://www.caves.org/phpBB2/viewtopic.php?t=617

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Postby hank moon » Nov 30, 2005 12:05 pm

...[nylon] doesn't actually absorb water (it's a plastic), but the water acts as a plasticizer and changes the mechanical properties of the nylon at the molecular level.


But...but...plastics do absorb water...in particular nylon is very water absorbent - it gets in there and does all that Tg-lowering stuff you were talking about (good info, BTW - thanks) AND increases the weight a lot (up to 10% from water absorbed at the molecular level, with additional weight from water trapped between the fibers - heavy!). Some plastics are less absorbent, like Polyester. I generally use polyester rope for wet conditions...slightly heavier when dry, but way lighter when wet.

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Postby RescueMan » Nov 30, 2005 2:29 pm

hank_moon wrote:But...but...plastics do absorb water...in particular nylon is very water absorbent (up to 10% from water absorbed at the molecular level, with additional weight from water trapped between the fibers - heavy!). Some plastics are less absorbent, like Polyester.


Oddly, we're both right. Because properties such as hydrophobia and hydrophilia are completely relative, there are people on both sides of the fence.

"Nylon is hydrophobic--doesn't absorb water easily--so it is harder to dye than cotton or other cellulosic fibers"
- Iowa Sate University extension service

"Nylon, a synthetic fiber, is the most common fiber used in commercial carpet. One of the primary benefits of having a nylon carpet is that it does not absorb water."
- Tennant Company Commercial Carpet Care

Nylon is not at all good at absorbing water. It is polymeric structure is hydrophobic (water hating) rather than like cotton which is hydrophilic (water loving). Thats why nylon clothing is so uncomfortable in hot and humid weather.

-AND-

"...in the world of synthetic polymers, nylon - with a water absorption of 8% - is considered hygroscopic compared to most major polymers. The polymer chains of nylons contain polar areas (where the monomers are linked together to make the chain), as well as nonpolar areas that extend the chain as well. The polar regions give nylon some water absorption capabilities, but not to the extent of fibers such as cotton."

Both from: Ask A Scientist

"Moisture absorption by nylon has been a source of great study for many years. Although all polymers absorb some amount of moisture, on none does it have such a significant effect as on nylons. "

http://www.sdplastics.com/nylon.html

For lay people, a common baseline for hydrophila is the ubiquitous cotton. Compared to cotton, nylon is hydrophobic. And what water nylon does absorb is taken up quite slowly. At full saturation, nylon 6-6 (which is what modern rope is made from) absorbs 8% water by weight (the older nylon 6 up to 9.5%).

The significant fact about nylon's water absoption is the profound mechanical effects that occur rather than the volume of water retained.

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Postby ian mckenzie » Dec 1, 2005 7:20 pm

The aforementioned rope, alongside a 'normal' (sort of) piece...


Image
The rope was not preshrunk, then was first used under wet conditions.
Last edited by ian mckenzie on Dec 2, 2005 12:38 am, edited 1 time in total.
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Postby RescueMan » Dec 1, 2005 7:36 pm

Well it really has to do with a particular rope's affinity for its user.

If the rope is repelled by the rapeller, then the repulsion will outrun the propulsion of the rapeller and result in bunching of the sheath, particularly if the rope discovers that it cannot get past an endknot to release its repulsion into the pit of despair.

Or...something like that. :shock:
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Postby hank moon » Dec 5, 2005 9:53 am

Compared to cotton, nylon is hydrophobic.


S'all a matter of degrees. Compared to polyester, nylon is hydrophilic. :wink:


The significant fact about nylon's water absoption is the profound mechanical effects that occur rather than the volume of water retained.


All is subjective. For some applications, the significant fact for me is the reduced wet weight of polyester. Good discussion and thanks for the links.

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