Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to tlie crimping of melt-spun filaments
of polyolefins or blends of polyolefins, particularly pol~propylene,
with other materials. Although such filaments may readily be crimped by
~ell-known mechanical methods, it is also possible to produce crimped
filaments merely by a step of stretching, more correctly termed "drawing",
subsequent to the melt-spinning process.
Drawing is that non-recoverable stretching which is carried
out on the solid polymer, i.e. below the melting point, and is usually
accompanied by the formation of an abrupt neck point, i.e. by an abrupt
or localised reduction in the cross-sectional area of the filamen~s.
It leads to an irreversible molecular displacement and consequcnt
longitudinal orientation of the molecules and is carried out to increase
the high strength of the fibres. Because it is accompanied by neck
formation there is a minimum value of tension necessary for any given
temperature below which drawing is not produced. Tension lower than this
m mimum value leads to elastic extension of the filaments and not to
drawing.
Heat may be applied during the drawing process and in some
cases, improved results are obtained by including a step of heat treatment
2~ prior to drawing. In other words, there is first an application of
heat without drawing and this is followed by drawing with or
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without the applicstion of further heat. These steps are normally applied
to a group or groups of ~ilaments, hereinafter referred to as a tow or tows,
coming from one or more extruder heads. The results obtained vary according
to the molecular weight distribution of the polymer used and the operating
conditions need to be selected accordingly. It has been thought that one
important requirement for the crimping of melt-spun filaments by drawing is
a high spinning speed of the order of 500 to 600 metres per minute.
As described in our copending Application No. 196,037 we have
found that good results can be obtained independently of spinning speeds by
means of a process in which at least a proportion of the filaments are rapid-
ly and asy~metrically cooled from the melt, are formed into a tow or tows
and subjected ~o a heat treatment of at least 100C. and are then drawn, the
extent of the heat treatment prior to the application of the drawing tension
being sufficient to produce at least 2 crimps per cm. The temperature of
the draw stage controls to a large extent whether or not the crimp appears
iD ediately or remains latent until later developed by heat excitation, hot
drawing leading ~o the greatest latency.
If the crimp development is suppressed until after the textile
processing step or steps, improved bulk results, partly because no crimp is
lost during the processing stage and partly because during crimping neigh-
bouring fibres interfere with each other so that, even if all the fibres do
i not davelop additional crimp, an overall improvement in cover results due to
their displacement by fibres undergoing crimping. There is thus the double
~- advantage in that the largely uncri~ped filaments are easier to handle during
the processing stage and less crimp is lost during processing.
According to the present invention, a s~ill larger proportion of
the crimp is caused to r~main in the latent stage, until later developed by
heat treatment, if the steps of the process just described are followed by a
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further stage o~ drawing at a temperature of at least 70C. and preerably
in the region of 90C. and above. In o~her wordss the irs~ stage o draw-
ing, which is pre~erably carried out hot, e.g. at 120C. is followed by a
further separate stage of drawing. These two stages o drawing are found to
lead to the increased latency of the crimp formation referred to above and
although one or more additional stages o drawing may be introduced if de~
sired, two such stages are adequate and are preferred for reasons o simpli-
city and economy. If $here are more than two such stages, it is essential
that the last should be carried out at at least 70C.
As explained earlier, drawing leads to an irreversible molecu}ar
displacement and it is the fact that, in a process in accordance with the
invention, this displacement occurs in two separate stages which leads to
the improved results already described. The two stages of drawing preferably
occur in direct succession without any relaxation of tension. I the tension
is relaxed at the end of the first stage crimp appears spontaneously, but
disappears again on the application o tension for the next drawing stage
and thereafter remains largely latent until subse~uently developed by heat
excitation.
It is found that by adjusting the degree of drawing in the two
stages in relation to one another, in conjunction wi~h the temperatures of
the drawing stages, in particular that of the second drawing stage, it is
possible to control both the total crimp which eventually appears after the
final stage of heat treatment and also the proportion of that crimp which
remains latent after the second stage of drawing. In general, it is found
that both the maximum total crimp and the maximum latent crimp are obtained
if somewhat less than half the total drawing is carried out in the first
stage.
For example, for a first draw temperature of 120~C. and a second
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draw temperature of 90C " if the total overall stretch in both stages is
3:1 and the proport.ions o~ this draw in the two sl:ages are varied, it is
found that both the total crimp and the latent crimp increase together as
the proportion of the total draw in the first stage is raised from zero and
then both of these values reach a maxlmum shortly after the proportion of the
total draw in the first stage reaches a quarter, that is to say, when the
stretch ratio for the first stage is slightly greater than l.S:l. Thereafter
both the total crimp and the latent crimp decrease together in a generally
linear fashion as the proportion of the draw carried out in the first stage
is increased and the proportion carried out in the second stage is decreased
towards zero. Of course, when the proportion of the draw carried out in the
second stage actually reaches zero, the process is then in accordance with
the earlier application and not with the present invention. Somewhat similar
results are obtained for an overall stretch ratio of 2:1, the maximum values
of both total crimp and latent crimp occurring just before the draw ra~io in
the first stage reaches a value of 1.5:1.
The heat treatment prior to the application of the drawing tension
is preferably carried out at a low tension, that is to say at a tension only
just sufficient to prevent sag of the filaments as they pass through the heat
treatment zone. The necessary extent of this heat treatment can only be
specified in terms of the resul~ since it depends on a number of factors~ i.e.
the nature of the polymer, the type and efficiency of the heating, the dia-
meter of the filaments and the thickness of the tow or tows~ EYen for a very
small extent of heat treatmentl a minor degree of crimp will eventually re-
sult, but for practical purposes ~he minimum useful crimp can be regarded as
two crimps per centimeter as mentioned above and adequate heat treatnent for
a process in accordance with the present invention may therefore be defined
as that necessary to produce at least ~his degree of crimp.
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1~5S64
Although the results described can be obtained quite independently
of the spinning speed, the most important advantages result from the use of a
spin-draw speed of 100 metres per minute or less. Such low spin-draw speeds
greatly simplify the overall process and facilitate the important requlrement
of rapid asymmetrical cooling. Low speed spinninlg can also lead to various
advantages, not least of which is the possibility of passing groups of fila-
ments directly to a twisting head so that they can be wound up on a package
in the form of yarn at the same speed as they are produced, thus producing
texturised yarn directly from basic polymer in a single production line.
lQ Apart from the two stage stretching and the prior heat treatment
already discussed, the most critical of the other steps is the initial cool-
ing which, as already stated, needs to be rapid and asymmetrical. For the
purpose of the present invention, the cooling may be regarded as rapid if the
length of the neck portion between the full diameter of each filament and the
reducet diameter resulting from the spin draw is less than 50 times the ~ull
diameter of the filaments; the faster the cooling the shorter the length of
this neck portion and a neck length of less than 25 times, for example 5
times the full diameter, gives very good results. The cooling must also be
asymmetrical, i.e. more intense on one side of the filament ~han the other.
Observance of both these requirements leads to differential characteristics
between one side of each filament and the other and it is these differen~ial
characteristics which are basically responsible for ~he ultimate production
of crimp.
The rapid asymmetrical cooling is preferably applied immediately
the fila~ents leave the spinneret p}ate in which case it is most conreniently
achieved by directing air at the filaments as they leave the spinneret plate.
This cooling air, which is preferably at or below ambient temperature, may
impinge obliquely agains~ the spinneret plate so that the filamen~s are cooled
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im~ediately they emerge rom the orifices. The colder the air the less velo-
city required or the same degree of cooling.
In a particularly advantageous process the neck point of the fila-
men~s is maintained within 0.5 cms from the spinneret plate and the frost
line, that is to say the level at which solidification of the filaments is
complete, is approximately l.S cms distant.
It is not essential that the cooling should be applied immediately
the filaments leaYe the spinneret plate and if it is applied after a short
interval methods of cooling other than by air are possible. For exampleJ the
filaments may be led over the surface of a roller which is cooled either by
internal refrigeration or by allowing a film of cold liquid such as water to
flow over the surface in contact with the filaments. The sides of the fila-
ments engaging the roller are cooled more rapidly than their opposite sides,
leading to the differential effects already described.
The orifices of the spinneret plate may be in the form of an array
having a ratio of length to breadth of at least 3:1, preferably at least 9:1,
the cooling air being directed at the filaments from the longer side of the
array. Normally there are two spinneret plates and hence two spaced arrays
for each extruder head. Air channels are positioned at the longer oute~ side
of each array so as to direct cooling air at the filaments. With such a form
of cooling, most of the individual filaments are cooled more on one side than
on the other to give the required asymmetrical effect.
The effects o the asymmetrical cooling are most marked on the
fialments closest to the blast of air and if air is directed from both outer
sides of ~he arrays, the filamen~s towards each of the outer edges of the ar-
ray will experience the greatest differential effect, while those towards the
inner edge o the array will experience only a slight differential effect and
in some cases may be equally cooled on both sides and may thus not subsequent-
~3L06SS~;~
ly develop crimp, The proportion of uncrimped filaments which is acceptable
under any particular circumstances will depend on the characteristics re-
quired in the final product. In some cases quite a small percentage of crimp~
ed filaments will be adequate while in others a fairly high proportion will
be desirable.
If the proportion of uncrimped filaments is required to be small,
this may be obtained by blending filaments which have been crimped in accor-
dance with the invention with uncrimped filaments. It is also possible to
operate the process in accordance with the invention so that some of the
filaments will develop little or no significant crimp. When the proportion
of crimped filaments i5 fairly high, a secondary effect operates since all
the filaments will subsequently be mixed together and when the crimp ulti-
mately appears, any filaments which have not been differentially cooled may
be forced into a crimped configuration by the shrinkage of the differen~ially
cooled filaments adjacent to them.
As a resul~ of the asymmetrical cooling, ~he diffe~ential charac-
teristics are already effectively locked into the filaments by the time they
have solidified and the main effect of the heat treatment is to accentuate
this differential. The temperature of this heat treatment must be at least
100C, but ~he a~ount by which it needs to be above 100C and also the extent
of the ~reatment are dependent on the factors discussed previously and also
the degree o crimp required. In order to provide an accurate measure of the
crimp obtained for any particular operating conditions, a short length te.g.
15 cms) of fibre is cut from ~he crimped tow after the crimp is developed and
is placed without restraint on a glass slide and allowed to assume its natu-
ral configuration being that of a coiled spring. The number of complete coils
per "cm. of spring" is counted. The test is repeated 100 times on different
fibres taken throughout the tow and the average value calculated.
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Although the lower limit to the extent of heating is important,
as already discussed, the upper limit is important only from the point of
view of saving time and space in the overall process. It is advîsable to
allow a reasonable margin over and above the minimum value and when using an
air oven, for example, a residence time of 45 seconds is found ~uite adequate
for most operating conditions.
In practice ~he extent o heat treatment can very easily be as-
sessed by removing a small clipping from the tow immediately on emergence
from the heat treatment zone. This clipping is then hand drawn in the cold,
i.e. at ambient ~empaTature. If the heat treatment has been sufficient spon-
taneous crimp will develop and ~his can easily be measured. This will be an
indication of the total crimp to be expected after development.
Should one want a more precise knowledge of the level of heat
treatment required the change in the fibre can easily be monitored by e.g.
observing the change in the angle of orientation of the filaments. The heat
; treatment step in a method in accordance with the invention is in effect an
, .,
annealing or heat setting step and like all such steps causes crystallite
rearrangements: the orientation angle is a measure or the alignment of cry-
stallites with respect to the fibre axis. The angle of orientation is a con-
venient means of measuring this. I~s measurement has been described by
Ingersol, Journal of Applied Physics 17, 924 (1946). The heat treatment will
; produce a change in this measured angle and is complete when further heating
of the sa~ple does not effect further change in the angle of orientation.
However, for practical purposes the simple hand test described above is en-
tirely adequate,
Generally speaking, the use of an air oven for the heat treatment,
as just mentioned~ is most con~enient and, for maximum crimp, no appreciable
~ension is applied. The application of tension at this s~age tends ~o reduce
the overall crimp and can be used as a control factor if required. ~en the
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tension reaches that necessary for drawing, the development of crimp may
virtually be suppressed altogether, thus emphasising the importance of control-
ling the level of tension run-back, arising :Erom the drawing operation, into
the heat zone.
As an alternative to the use of an air oven for the heat treatment,
the filaments may be heated to the necessary extent in a radiation oven.
A further advantage of the use of an oven for the heat treatment is
that it is possible to carry out at least the first stage of drawing in the
same oven by causing the tow to make a number of passes through the oven and
by isolating the heating stage from the first drawing by suitable snubbing so
as to enable the heat treatment to be of sufficient duration prior to the
application of drawing tension. For example, one of the rollers around which
the tow passes on its rlms through the oven may be positively driven to form
` the snub, this roller being positioned relative to the number of passes so as
to enable a heat treatment of sufficient duration prior to the drawing which
will occur subsequent to that driven roller.
A still further alternative is to heat the filaments by surface
contact with a heated surface or surfaces. For example the filaments may be
guided into direct contact with a heated roller or rollers. Some form of
snubbing means will generally be required for controlling the level of tension
running back from the drawing zone to the heating zone whatever the form of
heating.
Instead of a positively driven roller as described in relation to a
heating oven the snubbing means may take the form of a pressure roller co-
operating with a second roller or alternatively a number of rollers or bars,
e.g. three rollers or bars arranged a~ the corners of a triangle. When usinga heated roller or rollers for the heat treatment~ the snubbing means will be
located to control the run-back of tension from the drawing zone to the heat-
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ing zone A still further alternative for the snubbing means is to include a
heated roller in the heating zone at a lower temperature than the roller or
rollers preceding it.
The final drawing temperature is the most important in determining
whether or not the crimp is largely latent and as mentioned earlier a temp-
erature in the last drawing stage of at least 70 and preferably 90C or
greater leads to the desired increase of latency. It is also found that a
water bath at approximately 100C appears to give better results than an oven
at the same temperature. This may be a matter of better heat penetration.
However any suitable combination of heated surfaces, baths or ovens may be
used provided the necessary heat requirements are fulfilled at each stage.
The stages so far described, that is to say the rapid and asymmetrical
cooling, the heat treatment and the drawing stages may conveniently be carried
out in continuous succession in the same production line, the filaments pas-
sing directly from one stage to the next. After this sequence of steps, the
; filaments may either pass directly to a further stage of processing~ such as
stapling and carding or they may be wound into packages in readiness for
further processing at a later time. The latently crimped filaments can be
rendered easier to handle by the application of mechanical crimp. This makes
~o them more suitable for subsequent preparing operations such as carding.
It is also possible to interrupt the sequence of steps between heat
treatment and drawing by winding the filaments onto packages after the heat
treatment. This completely avoids any problems caused by the run-back of
tension to the heat treatment stage.
Whatever the sequence of intermediate steps, the final step needs
to be the application of heat to excite the formation of the crimp. For
maximum crimp development the temperature of subsequent heat treatment is
preferably of the same general order as the temperature of the heat treatment
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prior to drawing. This additional heat treatment is preferably dry, but wet
treatment can be used if desired and can be applied at any subsequent stagc
after the main textile processing. It may also be advantageous to apply the
heat for developing the crimp in more than one stagc so that part of the
crimp appears at one stage of the process, leaving the rema~nder to be
developed by further heat at a later stage or stages.
For example, the filaments may be stapled and then spun into yarn,
the heat treatment being applied to the resultant yarn. The treatment then
promotes so-called ~Iburst~ in the yarn, that is to say relative movement of
the fibres which tends to give the yarn more body. Thus, if the yarn in
question is used as the pile yarn of a carpet, the effect of the treatment
is to increase the covering power of the yarn and to give the carpet itself
more body, thus permitting a lower density of yarn to be used for the same
equivalent cover, with consequent resultant economies. The necessary heat
for the treatment may, for example, be derived from the application of a
backing to the carpet or during the finishing process. The stapled filaments
can be used either alone or blended with other materials, e.g. 50~ viscose.
In practice the degree of burst can be assessed at the filament
production stage by taking clippings of tow after the drawing stages and
heating them loosely, in an oven, for about half a minute. The temperature
of the oven should correspond to that of the final heat treatment just
described and will generally be between 100C and 130C. The length change
of the heated tow will give a measure of the crimp potential. For example, a
reduction of length from 30 cm to 10 cm would represent good burst.
The invention will now be described in more detail, by way of
example, with reference to the accompanying drawings, in which:-
Figure 1 is a general view showing the lay-out of a production line
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Figure 2 is a perspectlve view of a single extrusion head andcooling arrangement;
Figure 3 is a detailed view of a small section of spinneret plate
and cooling arrangement; and,
Figures 4-5 are graphs illustrating the influence of the relative
and total stretch ratios on the degree of latency and total crimp obtained.
Turning first to Figure 1, three extruders 1 are shown, each
delivering filaments upwardly at 2 in the form of a tow which is passed
around guide rollers 3, the individual tows being combined together to form a
single combined tow 4 on which subsequent processing steps are carried out.
; Polymer granules are fed to the individual extruders 1 from a hopper 6 and the
operation of the production line is controlled from a control panel 7.
; The combined tow 4 first passes horizontally into a hot-air oven
10, being guided by rollers 11 and 12 so as to make three passes through the
oven, the first two of which constitute the heat treatment and the third the
first stage of drawing. To provide an effective snub and prevent the drawing
tension from passing back to the heating zone the roller 12 is positively
driven, preferably at the same surface speed as rollers 3 so as to give
minimum tension in the first two passes of the tow through the oven, thus
constituting the heating zone. A temperature of at least 100C is maintained
in this oven. The tow then passes over rollers 14 and 15 which are running
approximately half as fast again as the rollers 3, 11 and 12, thus giving the
first draw to the tow during its third pass through the oven. After passing
around rollers 14 and 15 the tow 4 passes downwardly into a water bath 18
which contains an arrangement of three rollers ?0 rw ming at the same speed
as the rollers 14 and 15. It then passes upwardly out of the water bath 18,
around a roller 21 having a cooperating roller 22 and thence to a group of
Godet rollers indicated generally as 24. It is these rollers 21 to 24 which
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apply the tension for the second stage of drawing and the tow is thus drawn
over the reach shown as 25 between the rollers 20 and the roller 21. The
group of rollers 20 resist the tension applied by the rollers 21, 22 and 24,
thus separating the two drawing stages of the tow, although, of course, there
is no relaxation between stages.
After leaving the Godet rollers 24, the tow 4 passes to a stuffer
box crimper 28 which applies a mechanical crimp and renders the tow more
suitable for subsequent processing. In a typical example a nip pressure at
the feed rollers into the stuffer box 28 was 65 kg per centimetre. It has
been noticed that excessive nip pressure can reduce the total crimp which
ultimately develops. Finally the tow passes to a coiler head 30 which feeds
it into successive cans shown as 31. As each can 31 is filled with tow, it is
taken away to whatever subsequent stages of processing are required as
previously described.
The first of the essential steps in a process in accordance with the
invention lies in the rapid and asymmetrical cooling of the filaments, which
is illustrated by Figures 2 and 3. The individual filaments 35 are shown
emerging from a spinneret plate 36 and an air nozzle 38 directs a stream of
cooling air obliquely against the spinneret place 36 so as to strike the die
plate itself at approximately the far side of the group of filaments, as
illustrated by the dotted line 39 (Figure 3). In a particular example, the
diameter of the individual filaments is 1.0 mm, the distance shown as A which
represents the vertical height of the nozzle from the die plate is 12 mm and
the dimension B representing the horizontal distance of the nozzle from the
edge of the group of filaments is 25 mm. In this example, cooling air at
ambient temperature is directed against the ilaments at a velocity of 40
m/sec., leading to a neck (shown in dotted line) in the filaments at an
average height of approximately 2.5 mm above the surface of the spirmeret
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plate 36 and a frost line at an average of about 1.5 cm and represented by
dotted line D. At the frost line the filaments have completely solldified
and are of 60 denier.
An example of the production of crimped filaments by means of the
apparatus just described will now be given in more detail. The extruder used
was that known commercially as a "MACKIE CX" extruder operated with box
temperatures ranging from 260C to 280C. Each spinneret plate had dimensions
of 400 x 30 mm, was held at a temperature of 280C and included 5880 holes
each of 1.0 mm diameter. Polymer was pumped to the spinneret plates at a
sp~ed which allowed a total production rate of 90 kilograms per hour. The
cooling arrangement was as already described with reference to Figures 2 and 3
and the air used for cooling at a temperature of 17C. The air velocity,
measured on exit from the slot of the nozzle which had a width of 0.5 cm was
40 m per second. Under these conditions of rapid and asymmetrical cooling
the extruded filaments showed a neck point at an average of about 2.5 mm from
~he die plate, ranging from 5 mm furthest from the cooling nozzle to 1 mm
nea~est the nozzle. The rate of haul-off of the filaments was 9.0 m per
minute.
Using the apparatus and operating conditions just described pig-
mented polypropylene of melt flow index 4.0 was extruded through 3 heads~2 spinneret plates per head), of the extruder, yielding a total of 32,280
filaments which were collected into a single tow. This was triple passed
through the air oven 10 illustrated in Figure 1, which was maintained at a
temperature of 120C. The tow was fed into the oven at 9 m/min and the
roller 12 was maintained also at a surface speed of 9 m/min. The heat
treatment duration time, constituted by the first two passes of the tow
through the oven, was approximately 40 seconds. The tow was then passed
around rollers 14 and 15 having a surface speed of 13.5 m/min thus giving
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a first draw of 1.5:1. From the rollers 14 and 15 the tow passed ;nto thewater bath 18 at 90C. and around the three rollers 20 having a surfacc speed
of 13.5 m/min, after which it passed to the rollers 21 to 24 having a surface
speed of 27 m/min, thus providing tlle second stage oE drawing and comple-ting
the total draw of 3:1.
~ A sample of tow was taken before the roller 12 and tes-ted for total
crimp potential by cold drawing by hand. It developed 8 crimps/cm. and it
was found that the orientation angle before heat treatment was 24 and after
heat treatment 36).
The tow was passed to the crimper 28 and collected in cans 31. It
was stapled to 15 cms, finished with an anti-static agent and carded, prepared
and spun to a 4.23 Nm yarn with a twist of 197 turns per metre and 2-folded
at 118 turns/metre. The yarn was woven into an Axminster carpet which was
backsized with latex and cured in an oven at 120C. Bursting of the pile
yarn took place to give a carpet with even greater cover than that resulting
from the process described in our co-pending Application mentioned previously.
Figures 4 and 5 are graphs showing how changes in the first draw
ratio influence the latency and total crimp level. In graph 4 the total draw
~first and second draw) equals 3:1. The percentage crimp is plotted as
ordinate against the first draw ratio as abscissa which is measured as a ratio
of roller speed, i.e. roller 14. The oven temperature equals 120C, thus
roller 12
also giving a first draw temperature of 120C. The water bath temperature,
i.e. the second draw temperature, equals 90C.
Curve A shows total crimp after development at 122C, ~i.e. the
subsequen* heat treatment). Curve B shows immediate crimp ~spontaneous)
developed at ambient and Curve C equals B minus A and shows the latent crimp.
Graph 5 is similarly produced but the total draw in this instance
is 2:1. Slightly higher values of crimp are seen to be obtained at this lower
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total draw.
The extra latency derived from the process g:ives even more scope
for bulk development in multi-stages. Thus, for example, the yarn may be
treated in boiling water to develop part of the crimp and the remainder can
then be developed as a result of the latex curing of the carpet. As a result
of bulking occurring partly in the yarn prior to insertion as tufts in the
carpet and partly after insertion in the carpet, a carpet is produced which
has not only maximum cover but also excellent tuft definition.
It will be appreciated that a process in accordance with the
invention is advantageous, not only for carpet production, but also for many
other textile processes, not least being its suitability for the formation of
non-wovens, particularly by the spun-bonded route where the squirming of the
fibres during crimp development of the latent crimp improves randomisation
and consolidation of the batt.
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