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Name: Kunal
Status: educator
Grade: 6-8
Location: TX
Country: USA
Date: Winter 2011-2012


Question:
Cotton yarn (thread) is created by spinning multiple cotton fibers. Why does this spinning make the yarn so strong?



Replies:
The strength of cotton yarn is not a property of cotton per se. Rather it is a property of fibers being in parallel (side-by-side) rather than in series, one after another. If you have a number of fibers in parallel (say 100), if one fiber breaks the other 99 can take up the force. The entire assembly only reduces in strength by 1% (that is 99 out of 100). It takes a lot stronger assembly for fibers in series. One failure and the entire assembly fails. Added to this is the strength provided by the intertwining of the fibers, which provide some ability to stretch the intertwined fibers. That mechanism is lost with a single fiber. A similar mechanism operates with electrical circuits. An array of wires in parallel can carry a lot more current than a single large wire.

Mathematically, this is expressed by the formulas: Parallel = 1/R = SUM(1/ri) and Series R = SUM(ri).

So this behavior is not just a property of yarns, but is a property of configuration, that is, Parallel vs. Series.

Vince Calder


Most individual fibers are already very strong as made by the plant, stronger per unit cross-section than the final thread usually gets. The question is how to combine them into larger strands so that:

a) the fibers are mostly stretched out in the direction of the strand, and b) when one fiber ends and another begins, a third fiber bridges the gap and is somewhat attached to both.

Spinning is the mechanical process which has these effects. I cannot really explain why it accomplishes them, perhaps you can think about it.

The "attached" in (b) starts as mere friction + bends + partial entanglement. This can actually be enough, but the thread is a bit fuzzy with loose ends sticking out. Later if "sizing" is added to the thread, it glues some of the fibers to each other as well, at places where they cross and touch. It probably makes the strength more reliably high.

Jim Swenson


Kunal,

Yarn and other threads are spun from fibers of a certain size. Obviously, for wool, the size of the fiber depends on the length of the hair sheared from the sheep. The reason for spinning the fibers into yarn is to force the fibers to be in contact with each other. The more surface area the fibers expose to each other, the greater the “sticking” surface there is between each fiber. Spinning the fibers compresses them against one another and increases friction between them. The fibers, in this case, are more difficult to pull apart and the net result is the yarn is made stronger. You can demonstrate this with two jump ropes. Place them next to each other so that they are touching all along the edges. They will easily pull apart with no pressure between them. Tighten some clamps (like those used to clamp paper) so that the ropes are in tight contact with each other, and they will be more difficult to slide past each other. The surfaces of the ropes are rough, and when they touch, they have friction between them. Frictional forces tend to increase with pressure, and if you twist the ropes together, they will experience some compressive forces between them. This force all along the surface will increase friction and make them more difficult to pull apart. Note, however, if you have two strands of slippery spaghetti that coil together, they will slip apart. Here, friction is not as great as in the rough surface of a rope or a wool fiber. A knot in a rope uses friction in such a way that as you pull two ropes apart the knot will compress and increase its strength. One problem you have with two ropes coiled together is that they will tend to unravel and lose their binding strength with each other. In a yarn, you wind multiple fibers distributed all throughout the length of the yarn. They wrap around each other and serve to prevent the ends from unraveling. In this way, you can have short strands of fiber woven into an assembly that is strong.

Kyle Bunch


Hi Kunal,

Actually, I don't think that spinning itself, makes the cotton fibers any stronger. What it does, is to merge all the fibers into a single thread in a way that allows each fiber to equally share the tension.

Regards, Bob Wilson


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