Page 368 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Naturally Occurring Polymers—Animals 331

Wool is the fiber from members of the Caprinae family that includes sheep and goats. It is

produced as an outer coat with the inner coat being more hair-like. Since it is removed with-

out damaging the sheep’s skin, it is renewable without sacrificing the animal. The fiber has two
distinguishing characteristics that separate it from fur and hair. It has scales that overlap like

shingles on a roof and it is curly, with many fibers having more than 20 bends inch. This crimp

has both advantages and disadvantages. The disadvantage is that the fiber tends to mat and gather
cockleburs when on the sheep. The curly nature gives the fiber lots of bulk and associated insula-

tion ability. It retains air assisting the wearer to maintain their temperature. Thus, in the cold, the
body retains needed heat, while in hot weather; the body can remain somewhat cool with respect
to the surroundings. Thus, wool is a staple material for extreme temperatures.

The shingle nature results in a somewhat rough surface that helps the fibers to remain together.

In turn, this allows the fibers to be easily formed into yarn.
As noted above, the inner coat is more hair like with little scale or crimp. This hair is called kemp

while the fiber portion is the wool. Combined they are referred to as the fl eece.
A common theme with natural products is the variability of the fiber with the particular plant or

animal and the particular conditions under which they were grown. The amount of kemp to wool
varies from breed to breed as does the number of crimps per inch. Thus, wool from different breeds
raised in different parts of the world will have different properties and contribution to different

properties of the fiber. Generally, the wool and kemp are physically separated. In fact, after shear-
ing, the wool is divided into fi ve groups—fleece (which is the largest), pieces, bellies, crutchings,

and locks. The last four are packaged and sold separately. The desired part, the fleece, is further
classifi ed.
As the wool is removed from the sheep, it contains a high amount of grease that contains valuable
lanoline. Natives make yarns out of this wool. But, for most commercial use, the lanolin is removed
by washing with detergents and basic solutions.
A number of different wools are special and sold under different names such as cashmere wool.
These wools have their own special characteristics that increase their cost and value. It is interesting
that many animals have developed their own special coats to overcome their native environment.
Thus, because cashmere goats live in a cold mountain temperature, their wool is an effective mate-
rial to protect against the cold.

As an aside, wool is self-extinguishing so that flame retardants are not needed in wool rugs and

suits. Further, we need not worry about herds of flammable sheep running about the hillsides.

10.2.4 COLLAGEN
Collagen is the most abundant single protein in vertebrates making up to one-third of the total pro-
tein mass. Collagen fibers form the matrix or cement material in our bones where mineral materials

precipitate. Collagen fibers constitute a major part of our tendons and act as a major part of our skin.

Hence, it is collagen that is largely responsible for holding us together.
The basic building block of collagen is a triple helix of three polypeptide chains called the
tropocollagen unit. Each chain is about 1,000 residues long. The individual collagen chains form
left-handed helices with about 3.3 residues per turn. To form this triple-stranded helix, every third
residue must be glycine because glycine offers a minimum of bulk. Another interesting theme
in collagen is the additional hydrogen bonding that occurs because of the presence of hydroxy-
proline derived from the conversion of proline to hydroxproline. The conversion of proline to
hydroxproline involves vitamin C. Interestingly, scurvy, the consequence of a lack of vitamin C,
is a weakening of collagen fibers giving way to lesions in the skin and gums and weakened blood

vessels. Collagen fibers are strong. In tendons, the collagen fibers have a strength similar to that of
hard-drawn copper wire.
Much of the toughness of collagen is the result of the cross-linking of the tropocollagen units
to one another through a reaction involving lysine side chains. Lysine side chains are oxidized to

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