DYEING
ALL worsted and woollen yarns used for the surface of carpets, with the
exception of those whereon the colours are printed, as in Tapestry
carpets, are dyed; and fast and accurate dyeing is of cardinal
importance in carpet manufacture.
Practically all worsted yarns, and some woollen yarns, however, are
supplied to carpet manufacturers by the spinners in oil or in grease;
that is to say, still impregnated with the oil which has been put into
them in the process of spinning. The freeing of the yarn from this oil
by washing is a necessary preliminary to dyeing, inasmuch as the
presence of oil would prevent the dye liquor permeating the wool fibres.
Scouring is effected by soap and hot water, generally by machines of one
type or another. The yarn in skeins is sometimes thrown upon swifts,
which revolve over a vat of suds. The skeins are carried through the
liquor and well rinsed by the rotation of the swifts. Another method
employed is to pass the skeins, tied together in a long chain, through
vats in which they are steeped in the suds, and through rollers which
squeeze them more or less dry. Or the skeins may travel along a series
of aprons, underneath a shower, between rollers, or through bowls of
suds. The precise method adopted is not of great importance provided
that the scouring solution has full opportunity to operate upon the wool
fibres, so that the yarn is delivered ultimately free of oil and of
soap. For this reason, a final bath of pure water is desirable.
The solvents principally used are ammonia, alkali, and an alkaline soap
of some kind. Cotton-seed oil soap is generally regarded as one of the
most suitable.
The scoured yarn is now ready for dyeing, and passes, without being
fully dried, to the sticks or frames of the hand-vat or dyeing machine.
The three main considerations to be kept in view in the dyeing of
worsted and woollen yarns for carpets are levelness of colour, accuracy
of matching, and fastness of colour to light. The second and third are
matters which concern the skill of the head dyer and the quality of the
dyestuffs which he employs; the first depends upon the handling of the
yarn by the operative in the hand vat or dyeing machine; and the
essential factor, apart from the proper temperature of the dye-liquor,
is motion.
Dyeing by hand, although superseded almost entirely by machine-dyeing,
is still employed in some cases, particularly where only small
quantities are required. The dye-vat is of wood, and should be a
well-made piece of joinery to withstand the wear and tear incidental to
its use. Round the bottom on the inside is fitted a tube of copper or
some acid-resisting metal, perforated with small holes, through which
steam is passed to heat up the water. The head dyer weighs out the
dyestuffs estimated to produce a shade a fraction below the required
shade, so as to admit of final adjustment after testing. This, of
course, applies equally to hand or machine-dyeing. For hand vats, the
skeins of scoured yarn are hung on a set of poles, which, when filled,
are placed on brackets above the vat. When the dyestuff has been put in,
and the water brought to the required temperature, a pair of workmen,
each holding one end of the poles, standing on each side, sets them on
the edge of the vat and lets the free ends of the skeins sink down into
the liquor. When all the poles are in place, the operators start on the
pole at one end, and, raising each skein in turn, draw it over a few
inches, so that the part which has hitherto remained outside the liquor
is now immersed. When the skeins on one stick are finished, that stick
is pushed a little way along and the next is treated similarly. When the
whole set is done, the operation is repeated. The essence of the
treatment is to keep the yarn moving. If the skeins were allowed to
remain in the same place, even after the first moving, there would be
“stick marks” at the point where the skein rested on the pole, because
the liquor would not have had as much access to this place as to the
rest of the skeins.
After an hour or so, or when sufficient time has been allowed for the
dyestuffs to permeate the yarn fibres, the poles are lifted on to the
brackets, and a skein is taken out and submitted to the head dyer for
matching. The head dyer will compare it with his standard, and, in the
case of dyeing to match a cut-pile or tufted fabric such as Wilton or
Axminster, he will probably make a tuft from a thread of the sample
skein, so as to compare the shade of the cut ends as well as of the
outside of the thread. He will then make the necessary corrections or
additions to the dye-bath if the exact shade has not been reached, and
the operation will be continued until he is satisfied. The poles are
then finally lifted, removed from above the vat to a horse; the yarn is
then stripped from the poles and passed to the hydro-extractor prior to
going on to the stove or drying machine.
The object of dyeing machines, of course, is to effect the same
treatment of the yarn as is done by hand as just described, but in a
more expeditious way. A machine can, in fact, do nearly twice as much as
a man, and in a more regular manner. That is to say, one man can mind a
machine of the same capacity as a hand vat which requires two men; but
it is desirable, if not, indeed, necessary, that he should have
assistance in loading and unloading.
Given the necessity for keeping the yarn in motion while immersed in the
dye liquor, the problem of the designer of dye machines has been to find
the most simple and effective way of doing this, keeping in view the
need for convenience in preparing the yarn for the bath, dropping it in,
and removing it.
Dyeing machines are of two main types: those which move the yarn through
the liquor, and those which move the liquor through the yarn. Each type
has in common a frame of some kind to carry the dye-sticks on which the
yarn is hung; and practically every type of frame is provided with
sticks to support the skeins at one or both ends. Of the former and
earlier type the movement of the frame carrying the yarn in the liquor
may be effected in various ways. The frame in one machine is revolved in
the vat upon a circular axis; in another the revolution is combined with
a rocking motion; in a third a vertical movement is obtained by means of
a hydraulic pump fitted with an automatic reversing mechanism. In each
case the problem of moving the yarn in the liquor is complicated by the
minor problems of moving the yarn on the sticks so as to avoid stick
marks, and of raising and lowering the frame, or of stripping it in
position, if irremovable. The former is solved by some worm or ratchet
contrivance for automatically turning the dye-sticks, and the latter by
cranes or tackle, or, as in one of the makes just mentioned, by a
hydraulic pump.
All the machines of this main type have their advantages and
disadvantages, as they have their supporters and critics; but there can
be little doubt that the balance of opinion among carpet yarn dyers is
growing preponderatingly towards the second type of machine; that in
which the liquor is moved through the yarn. In this type, the frame
carrying the yarn is dropped into the vat, which is then covered with a
lid. The liquor is then caused to circulate in a strong stream
throughout the vat by propellers fixed at the head. The motion of the
propellers can be reversed if desired. If both top and bottom sticks are
used in the frame, they are placed at a less distance apart than the
length of the skein, so that, the skein being completely immersed, the
yarn is carried free of the stick by the flow of the liquor, access is
given to all the wool fibres, and the danger of stick marks is avoided.
The chief merit of this machine, however, is the considerable economy
that is effected in steam consumption by the reduced size of the vat, as
well as by the lid. The vat need only be made just large enough to
contain the skeins dropped into it, whereas in the other type of machine
it must be made large enough to allow for the skeins to be moved about
in it. There is, therefore, less water to be heated for a given weight
of yarn. The lid avoids loss of heat by radiation, which is inevitable
with the open top types.
The chemistry and the science of dyeing worsted and woollen yarns is a
subject that can only be dealt with adequately at considerable length.
Here a brief reference must suffice.
Wool fibre is a substance similar in nature to horn, with a scaly
surface. It is hygroscopic, being capable of taking up a large amount of
water compared to its own bulk, and when moistened and heated it tends
to soften and swell. These characteristics make wool very susceptible of
being dyed. It is said to have an affinity for certain colouring
matters, but this phrase serves to cover a want of agreement among
chemists, as to whether dyeing is actually of a chemical or a mechanical
nature. It is probably both. In any case, wool is easy to dye compared
with silk, cotton, or other fibres.
Natural dyestuffs, such as logwood, madder, fustic, cochineal, and
indigo, are practically obsolete so far as dyeing for carpet yarns is
concerned; and the modern dyer has a large range of aniline and
alizarine colours to choose from. As is pretty generally known, the
production of dyestuff from coal tar was originally a British invention,
but its development passed into the hands of the Germans, upon whom,
before the war, the dye consumers of the carpet trade, as of other
trades, were very largely dependent. Necessity and enterprise have
remedied this deplorable state of affairs, which need never recur. The
coal tar dyes now being produced by British dye manufacturers are
trustworthy and satisfactory, and they should improve both in quality
and quantity as time goes on.
For the purposes of the carpet yarn dyer, at any rate, the anilines and
alizarines now obtainable are infinitely preferable to any natural or
vegetable dyes, being easier to use, more reliable, and faster to light.
The ordinary method of preparing the dye-bath into which the worsted or
woollen yarn is to be dipped in the manner that has been described, is
as follows. The water, in which from 2 to 4 per cent. of sulphuric acid
and from 5 to 10 per cent. of Glauber’s Salts is mixed, is brought up to
about 170°F.; the dyestuff is added, about 1 to 5 per cent., according
to the depth of shade required. The percentages are reckoned upon the
weight of the yarn to be dyed. The yarn is then entered, and the water
is brought to the boil, and kept lightly boiling as long as required to
obtain the shade. There are some delicate shades, however, and some
sensitive dyestuffs, for which the yarn requires to be entered cool, and
the water to be brought slowly to the boil.
The dyed yarn passes from the vat to the hydro-extractor, a large copper
pan with perforated sides, which, revolving at a great speed, causes the
wet skeins by centrifugal force to press against the circumference and
squeeze out a large proportion of the water they contain. The skeins are
then taken out, bundled roughly together, and are ready for conveying to
the drying room or drying machine.
Drying the yarn can be effected simply by hanging the skeins on poles in
a room through which strong currents of air are forced by revolving
fans. This method, however, depends too much upon the weather, even if
the air is caused to pass over steam-heated coils on entering the room.
A better alternative is to expose the yarn hung on poles in a heated
chamber. The most certain and satisfactory method, however, is a drying
machine of some type, which enables the skeins to be fed into it at one
end, to pass through a current of heated air, and to be delivered dry at
the other. The factors in this case, the heat of the steam coils and the
speed of the machine, are known and can be controlled; so that the dyer
can rely upon getting a certain quantity of yarn satisfactorily dried in
a certain time. In the ordinary type of machine, the skeins are carried
horizontally between two wire aprons; but alternatively they can be
carried vertically upon hooks or poles.
The yarn, coloured and dried, is now ready to go to the yarn store, or
direct to the winding-room.