Page 21 - Handbook of Properties of Textile and Technical Fibres

2 Handbook of Properties of Textile and Technical Fibres

of cellulose embedded in a matrix material. Cellulose is a polysaccharide, made up of
polymeric carbohydrate molecules, and is a complex naturally occurring polymer,
which is the basis of the structure of plants. Fibers such as cotton, jute, flax, and
hemp are derived from plants, and the list is increasing as markets develop for them
and local industries are created in regions of the world where particular plants grow.
Animal fibers are made up of proteins to form hair, or wool, in many animals, such
as wool from sheep and cashmere from goats, camels, and alpacas, but many animals
that are not mammals also produce silk. This book will discuss many aspects of natural
fibers and where appropriate will compare their properties with those of synthetic fi-
bers that have been developed throughout the 20th century.
Fibers, whether natural or synthetic, possess remarkable properties. They owe these
properties to the way their microstructures, down to the molecular level, are organized
and also because of their flexibility. This latter characteristic is of great importance and
allows them to be woven and otherwise converted into structures. This is simply
because fibers are very fine and often so fine that even the most powerful optical mi-
croscope cannot reveal much of their details. It was necessary that scanning electron
microscope be developed, in the 1960s for the more intimate details of fiber structure
and failure morphologies to be begun to be understood. We shall see in this chapter
that this fundamental characteristic of the flexibility of fibers is strongly dependent
on the diameter of the fibers. As their diameter becomes smaller, the flexibility of fi-
bers, and so their ease of conversion into complex structures, rapidly increases. The
small volume per unit length of fibers also confers on them remarkable tensile proper-
ties, in particular strength, as the tensile strength of a material is not an intrinsic prop-
erty as failure is initiated by defects. This will also be explained in this chapter and will
be recalled in subsequent chapters on the different fibers that are discussed. Relative
high tensile strength means that they are used principally to support tensile loads
although their fineness means that they buckle easily and in many cases they are highly
anisotropic, meaning that their transverse and compressive strengths can be poor.
Fibers are long, fine forms of matter with diameters generally of the order of ten or a
few tens of microns and lengths ranging from a few millimeters to virtually being
continuous. As a comparison it can be noted that hair has a diameter around 80 mm,
which makes it pretty coarse compared to many fibers. However, just a few types of
fibers are much thicker and this is particularly the case of the boron and silicon carbide
fibers developed as the first examples of very high-performance fibers. This handbook
treats the subject of the behavior of natural fibers, fibers regenerated from naturally
occurring cellulose, and truly synthetic fibers. Their properties are described and
shown to depend on their chemistry and microstructures. How they fail is also dis-
cussed. It will be shown how they are tested and how their microstructures are studied.
It is hoped that this handbook will provide a useful reference source.
Although natural fibers have been used by man throughout his history, manmade
fibers are much more recent newcomers being primarily developed in the 20th century.
Synthetic fibers have grown to rival and in some markets replace natural fibers; how-
ever, the enthusiasm for synthetic fibers in clothes, which was evident in the second
half of the twentieth century, has been replaced by a more balanced consideration
of the properties of all fibers. Natural fibers have become again the fibers of choice