Chapter 1 Introduction
The manufacture of synthetic fibers represents a huge industry, both in the United States and worldwide. In 1990, the dollar value of synthetic fibers was roughly $17 billion in the U.S. and $70 billion worldwide. Natural fibers, consisting mainly of cotton and wool, but also including silk, yielded total dollar values only slightly higher.
We sometimes forget how much of these fibers we consume, especially since the appeal of "natural" fibers of cotton and wool has grown in recent years. Despite questions of aesthetics and taste, there should continue to be a significant demand for synthetic fibers, in large part because these fibers can be tailor-made to provide specific properties that natural fibers cannot provide.
As chemical engineers, we are interested in the manufacture of synthetic fibers because we have been the principal developers of the processes used to produce the fibers and because we are usually the ones charged with overseeing and improving the manufacturing operations.
As students of chemical engineering, we are interested in the manufacture of synthetic fibers because the process involves several fundamental aspects of chemical engineering. Once we understand this process thoroughly, we will be in a position to understand other chemical processes quite easily. The goal here is to provide us students with the motivation and curiosity to learn these fundamental concepts; such motivation is supposed to stem from exposure to practical and somewhat familiar operations where the value of learning the fundamentals will become self-evident.
Figure 1.1 depicts the four main areas of fiber manufacture: pumping, filtration, fiber forming, and fiber treatment. We have purposely excluded the production of the polymer melt or solution, so that we could focus on those fundamentals associated with the transport phenomena, rather than the chemistry, of fiber production. We also point out that synthetic fibers include not only the familiar examples of nylon or polyester, but also could extend to fiber optic cables and wire, etc.
Table 1.1 depicts the principal elements of transport phenomena fundamentals which are either described or might be included in the detailed study of the process of synthetic fiber manufacturing. Illustrated in Table 1.1 are the principal elements covered in the module which also might be seen in the table of contents of a textbook on transport phenomena. Particularly noteworthy is the breadth of topics included in this one module, ranging from pumps and filtration to transport analogies for the three modes. The student may be less interested in this particular table than the teacher, who is often concerned with extent of coverage. Clearly, the main goal at the present is to preserve the student's interest and curiosity by emphasizing the relevance of the topics.
Table 1.1 Transport Phenomena and Mathematics Fundamentals
In Figure 1.2, we attempt to provide the student with some orientation and introduction to the glossary of fiber production.
Fabric This includes a description of the difference between non-woven and woven fabrics. Whereas woven fabrics have an orientation associated with them, in which the fibers are normally aligned either parallel or perpendicular to each other, non-woven fabrics do not possess any preferred fiber orientation. With synthetic fibers produced from molten polymer, non-woven fabrics can be bonded together by having the filaments laid down over one another while they are still molten. As they cool and solidify, they are "glued" together. The fibers can be laid down in much the same way that paint appears to be laid down in a Jackson Pollock painting!
Figure 1.2 Synthetic Fibers - A Little Taxonomy
Fibers Fibers can be either natural or synthetic and either continuous or staple. Natural fibers, of course, can come from either animals or plants and probably the most well-known example of each are wool and cotton, respectively. Their chemical structure is polymer-based, in that a regular, repeat structure can be found in natural fibers. Synthetic fibers, too, are based on a regular polymeric structure. However, synthetic fibers are manufactured, or "synthesized," usually from oil, but sometimes from coal or natural gas. Most of the synthetic fibers are from polymers produced by step polymerization (usually, condensation polymers) but many are made be from polymers produced by chain polymerization (addition polymers).
All natural fibers have a finite length associated with them, ranging from about 5 to 20 cm. This could be the length of the hair on the sheep (wool) or the length of a cotton filament in a cotton plant. In order to be woven into a fabric, these filaments must first be aligned together into a continuous strand, called yarn or thread. Spinning machines are used to accomplish this. The filaments are held together by van der Waals' type forces. The thread and yarn produced in this way contain occasional filaments which stick out away from the continuous strand; this is somewhat like branch groups on a polymer, albeit at a much larger scale. In any case, these filament branches help to provide the woven fabric with greater bulk and porosity and are associated with other positive aesthetic features of the fabric. Synthetic fibers, however, can be made into continuous filaments which are practically infinite in length. Even though the individual polymer molecules in the filament are 1000 x longer than they are wide, they are still usually tiny fractions of millimeters in length. A "yarn" can be made of these filaments simply by bringing the filaments together continuously as the filaments are produced. This will not produce a yarn with filaments which occasionally stick out from the strand, however. In order to produce such a result, in some processes the continuous strands are chopped into strands of finite length, so that there is a somewhat closer match between the synthetic and the natural fibers. These chopped-up sections are called "staple," and the staple is brought together again in spinning machines which operate just like those used with natural fiber. Another difference between natural and synthetic fibers is that the natural ones are usually more curly. Two techniques have been used to make the synthetic fibers and yarn more curly: one is to pass the yarn between two heated gears which can impart a permanent crimp to the yarn and the second is to produce the synthetic filaments from two different polymers passing out of a common hole, or die. The resulting filament will exhibit varying curliness, based on humidity conditions if the two polymers absorb water to differing degrees and if the absorption changes the shape of the filaments accordingly.
Synthetic Fibers This list brings together most of the fibers that the with which the students are already familiar. Most are made from condensation polymers, although acrylic is not, and carbon fibers are made from addition polymers subjected to a pyrolysis step to kick off the hydrogen atoms.
Fiber Forming This section includes a brief forecast of what will be covered subsequently. In particular the three principal fiber-forming processes of melt, dry, and wet spinning are described briefly. All three steps involve the formation of continuous filament strands by forcing the material through circular dies, but melt spinning involves cooling of the subsequent strand to form the solid filament, whereas dry and wet-spinning involves removal of a solvent to form the solid filament. In dry spinning the solvent evaporates into a gas and in wet spinning the solvent is leached into a liquid bath.
Figure 1.3 shows a schematic of the melt spinning process, and thus illustrates
the key elements listed in Figure 1.1. The molten polymer (in the case of
dry or wet spinning, the spin dope) is first pumped through a filter which
removes any tiny particles that can be trapped in the tiny spinneret holes.
The polymer is then forced through these tiny holes to form continuous strands
of polymer filaments, or synthetic fiber. Cooling gases reduce the temperature
of the filaments so that they solidify and an initial drive roll controls
the initial take-up speed. The fiber may undergo subsequent heating and
stretching to impart additional molecular orientation. Finally, the fiber
is taken up onto bobbins at a constant speed, with a special tension control
device to control the rate of rotation in order to maintain constant yarn