Biomedical Engineering Reference
In-Depth Information
chemically corrosive situations, prolonged expo-
sure to radiation, or attack and degradation by
biological entities (e.g., microbes) are severe limit-
ing factors for engineering of structures using
natural fibers. Synthetic fibers made from poly-
mers such as polyester, nylon, and polypropylene
provide the textile engineer with a broad palette
of reasonably inexpensive fibrous materials,
which are capable of displaying very good physi-
cal and chemical properties. If we add to this list
high-performance fibers, such as aromatic poly-
amides or aramids (e.g., Kevlar
®
), polybenzimida-
zole (PBI), carbon, and others, extremely high
temperatures or direct exposure to flame are about
the only factors limiting the engineer. In fact,
though, PBI and the aramids (such as Nomex
®
)
are highly flame resistant. It is important to have
knowledge of all the properties of the fiber in use,
since there are strong trade-offs between proper-
ties. For example, PBI does not easily ignite and
burn, but it is not stable to strong acids; Kevlar is
very strong, but it is not very resistant to abrasion;
and carbon fiber is strong but extremely brittle
[5]
.
Engineers must realize the limitations of each
fiber type when designing a material in terms of
the stresses of the production techniques needed
to form fibrous materials.
The 20th century saw the invention of syn-
thetic polymers, in particular nylons and poly-
esters. Given the low prices of the petroleum
feedstock at that time, volumes of these syn-
thetic fibers, eventually surpassed the natural
fibers, and today synthetics make up two-thirds
of the textiles produced worldwide. Synthetic
fibers have benefits over natural fibers for sev-
eral reasons. For example, synthetic fiber diam-
eters can range from the nanoscale to
monofilaments suitable for fishing, with mate-
rial properties consistent from batch to batch
[5]
.
Synthetic polymers are attractive in terms of
their properties; nevertheless, their use is coming
under close scrutiny owing to the fact that the
feedstock for synthetic fibers, petroleum, is finite.
Furthermore, global environmental awareness has
tarnished some of the luster of synthetic polymers.
Given this situation, the synthetic fiber industry as
it currently exists will change and be replaced by
an industry based on renewable feedstock.
One type of relatively modern fabric is the
class of nonwoven fabrics, in which the compos-
ing fibers (almost exclusively synthetic) are
combined directly into the fabric structure.
There are two broad classes of nonwovens, with
several members in each class. In one class, the
synthetic fibers are extruded directly into a fab-
ric structure; in the other, the fibers are first
formed and then the fabric is made from them,
with no intermediate step such as yarn forma-
tion. In the first class, the direct polymer-to-fiber
lay-down process, there are a few subclasses,
but in essence the fibers as they leave the spin-
neret are laid down on a moving belt to form a
fabric, which is generally relatively thin. Melt
blowing and spun bonding are the names of two
such processes. These materials are not consid-
ered further in this chapter.
The second class of nonwoven fabrics requires
the fibers be first made by a fiber production
process, packed and shipped, and then made
into the fabric. Applications range from paper
(made from processed wood pulp, generally) to
automobile headliners (made from synthetic fib-
ers). In papermaking, slurry containing the fib-
ers is cast onto a porous belt and vacuum pressed
and dried. It is an example of a
wet-lay
process.
Consolidation is accomplished by hydrogen
bonding between the cellulose molecules in the
pulp as it is dried. The dominant nonwoven pro-
cess not using a slurry (so,
dry-lay
) uses needle-
punching, i.e., entangling of the fibers, generally
synthetic, by multiple penetration of a web of
the fibers by barbed needles. The fabrics pro-
duced in this process can vary from relatively
thin to quite thick and heavy. Even for the thin,
lightweight materials, nonwoven fabrics do not
have the drape properties required for comfort-
able apparel. Nonetheless, they are relatively
inexpensive to manufacture and find many dis-
parate uses, mostly in industrial settings and
technical textiles.
