MODIFICATION OF POLYESTER FIBERS—SPECIFIC SOLUTIONS FOR SPECIFIC

This wide topic covers both chemical and physical modifications to both the polymer and the
fiber. We shall deal with only a few of the more important variations possible on this theme,
but all are based on an understanding of polyester chemistry and processing described earlier
in this chapter.
SPIN FINISHES
Fibers need to be treated with surface finishes or lubricants to allow high-speed processing.
The various processing steps such as drawing, bulking, and textile processing would be
impossible without these spin finishes because so many of them rely on specific frictional
properties of the fiber (for example, friction twisting). Spin finishes are often water emulsions
of various surface-active agents and lubricant oils; their formulation is a complex process and
sometimes more of an art as well as a science. Finish application is made early in the process,
before the cooling threadline from the spinner hits the first godet. Earlier, finish was applied
from a lick roll rotating slowly in a bath. As spinning speeds increased, the finish was
applied directly via a special hollow ceramic yarn guide as a neat oil formulation and metered
at precise levels via a metering pump. Staple fiber is sprayed with emulsified finish or the
whole tow may be immersed in large baths of finish. Some staple processes use a draw stage in
a hot bath of finish.

TIRE CORD
During the manufacture of tires (typically radial ply construction for passenger cars), the
polyester tire core is subjected to drastic hydrolytic conditions. The rubber is molded into the
basic tire shape and rubber vulcanization uses various accelerators, some of which cause
severe aminolysis of the polyester chain. The process is run at 1758C in the presence of steam.
While PET is fairly resistant to strong aqueous ionic base at moderate temperatures, nonpolar
bases like ammonia, hydrazine, and simple aliphatic amines can easily diffuse into the PET
structure and cause aminolytic breakdown [55].
To maintain the high strength engineered into the tire cord, it is essential that the IV
(molecular weight) drop be minimized. The rate of degradation of the tire cord is directly
related to the level of free COOH groups on the chain ends. This reaction is autocatalytic
under vulcanization conditions, and reduction from their usual level (about 40 micro equivalents
per gram of fiber) improves in-rubber stability. For some years, tire cord manufacturers
employed a process in which the yarn was treated with epoxy compounds such as phenyl–
glycidyl ether to esterify excess COOH end-groups [56]. This process was convenient because
tire cords were treated with various ‘‘activating finishes’’ to improve their rubber adhesion.
However, the glycidyl ethers were carcinogens and the process was abandoned in favor of the
drastic alternative of melt-injecting ethylene oxide gas under high pressure into the molten
polymer during the last stages of polymerization [57]. This reduced the free COOH end-group
concentration to about 4–10

µe/g by forming harmless BHET ends, and IV drop at tire

molding was significantly reduced.