DYEING POLYESTERS

INTRODUCTION
Dyeing synthetic fibers is a huge subject in its own right and the reader is advised to consult
one of the many publications that deal with it comprehensively [61]. When PET fibers first
appeared, they presented many problems for traditional dyers. PET has no functional groups
to give affinity for usual dyestuffs. Natural fibers like wool, cotton, silk, and then later manmade
ones like rayon and nylon were well known and had good dye affinities because the
fibers had pendant or terminal functional chemical groups such as –NH2, –COOH, and –OH.
These dyes were developed to interact with such groups. The only way to dye polyester was to
rely on Van der Waals forces to hold the dye in the fiber. All classic cationic and anionic dyes
for wool and silk or direct dyes for cotton had water-solubilizing ionic groups like

NR₃⁺and

SO₃^-

. Such dyes had little or no affinity for PET.

DISPERSE DYES
PET fiber chemistry is in some ways similar to that of cellulose acetate fibers, where the class of
dye called ‘‘dispersed dyes’’ were in use. These dyes did not have strongly polar solubilizing
groups and were actually dispersed in the aqueous dyebath with a surfactant as a suspension of
fine particles in suspension. Such dyes usually had a low molecular weight and this later led to
problems with PET due to dye sublimation. Polyester fabrics needed stentering (heat-setting
under tension) on a pin-frame to remove creases after dyeing. This became a big problem for
PET dyers. It was clear that special dyes were needed for PET. As the polyester fiber market
grew, such modified dyes rapidly advanced, and were based on well-understood dye chemistry.
Higher molecular weight dyes of the anthraquinone type gave reds, blues, and dark greens,
while selected azos were used for yellow and orange shades. These dyes were most effective if
they were somewhat water-soluble and the ethanolamino group (
NRCH2CH2OH) and
sometimes its O-benzenesulfonate ester were incorporated, giving a weakly polar nature and
bestowing solubility in polyester but without a truly ionic character. The higher molecular
weight dyes reduced dye sublimation, but at the cost of slower dyeing and poor dye exhaustions.
A breakthrough was achieved in the middle of the 1950s with the development of
heterocyclic (nitroaminothiazole) dyes, which gave very stable light-fast azo blues [62]. These
had good affinities for PET. Another dye problem that became quite important was gas-fume
Polyester Fibers 21
fading of disperse dyes due to the generation of oxides of nitrogen (NOx) and even traces of
ozone in living rooms, arising from wider use of oil and gas heating systems. It was solved
largely by selecting dyes whose chromophores were stable to oxidation by NOx.
Carriers were introduced to speed up dyeing. These were solubilizing agents that temporarily
swelled the fiber and ‘‘carried’’ the dye into the fibrous structure. The carrier was
trapped in the amorphous regions of the fiber morphology, since the dense crystalline
regions could not be penetrated by the large dye molecules. The carrier itself diffused out
again, so it might be regarded as a fugitive plasticizer. Phenols like 2-hydroxybiphenyl
(OPP) were widely used and greatly improved the economics of dyeing polyester. An alternative
was pressure dyeing, using superheated dye liquor at 135–1508C (well above the Tg
of drawn PET fiber), but this was a capital-intensive process since pressure-dyeing vessels
were expensive. Eventually, pollution problems with dyehouse liquor waste led to restrictions
on the use of carriers. Pressure dyeing is now the norm, although more expensive. This is one
reason why non-PET polyester fibers like PBT and PTT are attractive to the dyer. Both have
Tgs of about 458C, so they can be aqueously dyed to heavy shades at the boil at atmospheric
pressure. The reluctance of polyester to dye can be turned into a commercial advantage. The
argument is that ‘‘a fiber that does not readily dye will also not easily stain.’’
ANIONIC AND CATIONIC DYES FOR POLYESTER
Since much polyester was originally used in blends with wool, it was natural that attempts
should be made to modify PET to make it acid-dyeable with anionic dyes. The most popular
theme was to incorporate basic additives by copolymerizing an aminohydroxy compound or
aminoacid into the PET structure. All such attempts failed because the copolymers were
discolored yellow or brown, and were of low IV. It was found, however, that certain
polyamides, containing additional in-chain tertiary amine groups, when melt blended with
PET and high-molecular-weight PEG (Mw 20,000) formed a three-phase mixture in which the
polyamide was dispersed inside the PEG and this in turn was dispersed inside the PET. Thus,
the critical components were prevented from intermixing in the melt. The mixture was meltspun
successfully into fibers at 2708C. Diamond-patterned fabrics were jacquard knitted with
mixtures of the dye variant fiber and normal PET. These could be cross dyed to give patterned
effects from a single dyebath containing both acid and disperse dyes. However, the process
was deemed too complex and expensive for a commercial product and the light stability of the
dyes was not adequate [63].
Greater success was achieved by DuPont who copolymerized, the sodium salt of
5-sulfoisophthalic acid into PET to render the polymer dyeable with cationic (basic)
dyes. Basic dyeable PET was successfully launched as Dacron 64 in the form of a low-pill
staple product [64]. The presence of the sulfonate groups in the polymer chain also acts as an
ionic dipolar cross-link and increases the melt viscosity of the polymer quite markedly. Thus,
it is possible to melt-spin polymer with IV 0.56 under normal conditions, giving a low-pill
fiber variant. The fiber also has a greater affinity for disperse dyes due to the disruption of the
PET structure. Continuing this theme, there are ‘‘deep dye’’ variant PET fibers, often used in
PET carpet yarns, which are copolymers of PET with chain-disrupting copolymer units like
polyethylene adipate. They have less crystallinity and a lower Tg; therefore, they may be dyed
at the boil without the use of pressure equipment or carrier at the cost of some loss of fiber
physical properties.
1.7.4 MASS DYEING
Since much polyester staple fiber is dyed to dark, expensive colors (black and navy blue), the
fiber is often mass dyed or mass pigmented at the polymerization stage. Clearly, thermally
22 Handbook of Fiber Chemistry
stable pigments and dyes have to be used. Especially fine pigment grades of carbon black are
used for black, and this is toned by adding small amounts of navy blue or very dark green
melt dyes to remove any traces of brown, which dyers consider unacceptable. Mass dyeing is
only economic if the demand warrants it.
A more recent development is ‘‘dope dyeing’’ (a term dating back to the acetate rayon
industry), where a range of melt-dyed colors are produced by coloring white polymer
immediately before melt spinning by adding calculated mixtures of master-batch pigmented
polymer or actual neat dyestuff. This can conveniently be done by adding the dye in the form
of pills or granules containing a specific amount of dye at a calculated feed rate to the molten
polymer during the melt-spinning process or by adding the coloring agent as a liquid
dispersion in a very high boiling point (over 3008C) inert oil, either during polymerization
or at melt spinning [65]. The latter process is of particular value in melt coloration of POY
feedstock yarns.