Process for the production of highly shrinkable split tows of acrylonitrile polymers

High-shrinkage stretch-broken tows of acrylonitrile polymers are obtained by fixing with saturated steam at maximum temperatures of 140.degree. C. after drawing and before stretch breaking and stretch breaking at a draft of at least 30%.

This invention relates to a process for the production of a split tow, of 
which the individual fibres have a boiling-induced shrinkage of at least 
35%, by spinning in the usual way, followed by aftertreatment and stretch 
breaking. Yarns of individual fibres such as these are used in numerous 
fields, for example in the production of velour or imitation furs. The 
yarns are produced by spinning individual fibres which are normally 
obtained by cutting shrinkable slivers. One such process is described, for 
example, in German Offenlegungsschrift No. 2,655,172. To obtain 
shrinkability, it is necessary in this case to carry out drying carefully 
at temperatures below 60.degree. C., which takes an undesirably long time. 
It is known that the level of boiling-induced fibre shrinkage which it is 
possible to achieve depends upon the effective drawing level in the sense 
that shrinkage generally decreases with an increase in drawing. This gives 
rise to another disadvantage so far as high-shrinkage fibres are 
concerned, namely low strength attributable to the low degree of drawing. 
It is desirable to use coloured yarns in various fields of article 
manufacture. It is not possible to dye the cut individual fibres or the 
yarns because in that case the shrinkage present would be released. 
Because of this, high-shrinkage fibres have to be dyed before they are 
given their shrinkability, i.e. normally in the spinning melt. For 
economical reasons, the batches to be dyed a given colour must not fall 
below a certain size. 
Accordingly, an object of the present invention is to produce 
high-shrinkage fibres by a more economical drying process at higher 
temperatures, with greater strength in the shrunk yarn and with the 
possibility of dyeing smaller batches. It has now surprisingly been found 
that a tow which satisfies these requirements can be obtained by fixing 
with saturated steam after drawing and before the stretch breaking 
process. 
Accordingly, the present invention provides a process for the production of 
high-shrinkage broken tows of acrylonitrile polymers by spinning, drawing 
and stretch breaking in the usual way, characterised in that fixing with 
saturated steam is carried out at a temperature not exceeding 140.degree. 
C. after drawing and before stretch breaking and in that stretch breaking 
is carried out at a draft of at least 30%. The maximum steaming time is 30 
minutes. The heating zone in which drafting takes place is preferably 
heated to between 120.degree. and 160.degree. C. 
Normally, a draft of 65% is not exceeded in the heating zone and not less 
than 30%. Stretch breaking is advantageously carried out at a heating zone 
temperature of from 120.degree. to 150.degree. C. and at a draft of from 
40 to 60% in the heating zone. 
It is completely surprising that tows of the type in question can be dried 
at unusually high temperatures for high-shrinkage fibres of from 
140.degree. to 150.degree. C. without any adverse effect upon the 
shrinkability of the broken tow. Under the effect of the high draft 
applied during stretch breaking, outstanding strength is imparted to the 
individual fibres and hence to the high-shrinkage yarns. It is readily 
possible by the process according to the invention to dye individual 
batches, however, small, by the methods normally used for dyeing tows. 
Shrinkage behaviour is unaffected by whether dyeing is carried out before 
or after steaming. This is a particular advantage of the process according 
to the invention. 
Acrylonitrile polymers suitable for the purposes of the process according 
to the invention are polyacrylonitrile or, preferably, acrylonitrile 
copolymers containing at least 50% by weight of polymerised acrylonitrile. 
Copolymers such as these contain one or more monomers copolymerisable with 
acrylonitrile, such as acrylic acid esters, vinyl esters or monomers 
containing dye-receptive groups. It is preferred to use bundles of 
dry-spun filaments. 
To extract the residual solvent, the filaments obtained after dry spinning 
are passed through aqueous baths in which they are drawn to between 2 and 
6 times and preferably to between 2 and 3 times their original length. The 
maximum temperature of the aqueous baths is 100.degree. C., the 
temperature preferably being in the range of from 65.degree. to 75.degree. 
C. 
The drawn filaments are dried by air heated to between 140.degree. and 
150.degree. C., as is normal for acrylic tows, after having been treated 
with a preparation necessary for further processing. In order to provide 
the individual filaments with adequate transverse adhesion, the bundle is 
passed through a stuffer box of the type normally used for crimping. 
To ensure that the tow receives the required degree of shrinkage after 
stretch breaking, it is treated with saturated steam under pressure. The 
temperature prevailing during this treatment does not exceed 140.degree. 
C., because damage can be caused at higher temperatures in the form of a 
deterioration in the natural colour of the tow and, if steaming is carried 
out for longer than 30 minutes, also in the form of a reduction in the 
fineness-related maximum tensile strength. On the other hand, a minimum 
temperature of 115.degree. C. has to be maintained during fixing with 
saturated steam in order to obtain adequate boiling-induced shrinkage in 
the broken tow. The fact that the process steps of preparation, crimping, 
steaming and drying can be changed around is a particular advantage of the 
process according to the invention. It is left to the manufacturer to 
decide on whatever is the most suitable combination for his purposes. 
Thus, where a large autoclave is used, a sufficient amount of tow may be 
collected to carry out an economically optimal steaming process in such a 
way that, immediately afterwards, the tow may be delivered to the stretch 
break converter. Similarly, steaming may be continuously carried out in 
suitable apparatus immediately after drawing, followed in any order by 
preparation, crimping and drying. 
To produce a coloured high-shrinkage broken tow, the tow may be dyed by any 
of the usual methods, such as pack dyeing or padding on the dye by means 
of a padding machine, the shrinkage behaviour of the broken tow being 
unaffected by whether the spun material is dyed or whether, for example, 
the tow is bale-dyed. However, dyeing is best carried out before the 
steaming operation because it is known that the dye is fixed by steaming. 
The boiling-induced shrinkage which it is possible to obtain in the broken 
tow depends upon various parameters. It has been found that the degree of 
boiling-induced shrinkage is higher, the higher the comonomer content of 
the polymer, the lower the drawing ratio providing the comonomer content 
is less than 6% by weight, the higher the fixing temperature (saturated 
steam) and the higher the temperature and draft applied in the heating 
zone of the converter. 
For certain articles, for example imitation furs, shrinkage power is an 
important factor in addition to the degree of shrinkage in the broken tow 
or shrinkage yarn because shrinkage has to be released against the 
resistance of the non-shrinking or substantially non-shrinking fibres. It 
has been found that copolymers having a comonomer content of from 6 to 10% 
by weight can be processed to form broken tows having a higher shrinkage 
power than broken tows of copolymers having a comonomer content of from 2 
to 6% by weight.

By virtue of the described association between production parameters and 
shrinkage properties, it is possible by the process according to the 
invention to produce products having specific properties. 
EXAMPLE 1 
An acrylonitrile copolymer of 90.48% by weight of acrylonitrile, 9.0% by 
weight of methyl acrylate and 0.52% by weight of sodium methallyl 
sulphonate was dry-spun in the usual way. The spun tow, which had an 
overall strength of around 230 g/m, was washed in water at 85.degree. C. 
and drawn in a ratio of 1:6.0 in two stages at a temperature of 95.degree. 
C. in a combined washing and drawing machine. After an antistatic 
preparation had been applied, the tow was allowed to shrink by 29% in a 
tube filled with steam at 102.degree. C. and was then dried under tension 
with air at 135.degree. C. to a moisture content of 1.2% by weight. The 
tow was crimped in a stuffer box, into which steam was sprayed under a 
pressure of 1 bar, and was then deposited in a perforated can. After 
treatment with saturated steam at 120.degree. C. in an autoclave, the tow 
was broken in the stretch breaking converter (Seydel type 671 S) at a 
heating plate temperature of 120.degree. C. and at a draft of 58%. The 
individual fibres of the broken tow were tested with the following 
results: 
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denier 4.5 dtex 
maximum tensile force 21 cN/tex 
maximum tensile force 41 .+-. 4% 
elongation 
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The fibres of the broken tow showed an average boiling-indued shrinkage of 
30% and a maximum boiling-induced shrinkage of .about.36%, whilst the yarn 
spun therefrom showed a boiling-induced shrinkage of 38.7% for a shrinkage 
force (air at 180.degree. C.) of 5.9 mN/tex. 
EXAMPLE 2 
An acrylonitrile polymer having the same composition as in Example 1 was 
dry-spun in the usual way. The bundle of fibres still contained 16.4% by 
weight of the solvent which was washed out with water at elevated 
temperature in a combined washing and drawing process. The temperature in 
the washing baths was 85.degree. C. and, in the drawing bath, was 
95.degree. C. The entry/exit speed quotient in the washing and drawing 
machine was 1:5.0. The drawn and washed tow was then treated with an 
antistatic preparation and subsequently steamed at 120.degree. C. in a 
continuous pressure steamer (the Tunnel System of Messrs. Serracant of 
Spain), the residence time in the steamer being 9.5 minutes. The hot, 
moist tow was dried with hot air at 140.degree. C. to a residual moisture 
content of 0.9% by weight, crimped and stretched by 50% at 160.degree. C. 
before stretch breaking in the stretch breaking converter mentioned in 
Example 1. The broken tow obtained was spun into a yarn having a denier of 
250 dtex. The yarn has a boiling-induced shrinkage of 43.4%. An elongation 
of 26.0% and a breaking force of 8.4 cN/text were measured during tensile 
testing of the fully shrunk yarn. 
EXAMPLE 3 
An acrylonitrile copolymer having the same composition as in Examples 1 and 
2 was dry spun, washed in hot water at 85.degree. C. to 95.degree. C. and, 
at the same time, drawn to 6 times its original length. The wet tow 
treated with an antistatic preparation was crimped in a conventional 
stuffer box and, at the same time, mechanically dried to a residual 
moisture content of around 20% by weight. It was then dyed under elevated 
pressure (2.1 bars) in aqueous solution by the known "tow pack" process. 
After drying in the absence of tension at an air temperature of 
140.degree. C., the tow was processed into a broken tow. The heating 
plates had been heated to 120.degree. C. and the draft amounted to 58%. 
The broken tow shrank by 41% and the yarn produced therefrom by 43.4%. 
EXAMPLE 4 
A dry-spun tow of an acrylonitrile copolymer of 93.6% by weight of 
acrylonitrile, 5.7% by weight of methyl acrylate and 0.7% by weight of 
sodium methallyl sulphonate is washed in water at 70.degree. C. and drawn 
to 2.5 times its original length at the same temperature. The speed of the 
tow after the drawing process is 72 meters per minutes. Following the 
application of an antistatic preparation, the product is dried with air at 
a maximum temperature of 140.degree. C., the exit speed from the dryer 
being 25% lower than the speed of entry. Before splitting at a heating 
plate temperature of 140.degree. C. and at a draft of 44%, the 6.84 km 
long tow deposited in a perforated can is treated with saturated steam at 
125.degree. C. in an autoclave. In this treatment, the tow is subjected to 
5 steaming phases each lasting 2 minutes and to 7 evacuation phases each 
lasting 2 minutes in order to guarantee uniform heating of the individual 
fibres. The individual fibres of the broken tow have a boiling-induced 
shrinkage of from 32 to 39%, the maximum of the distribution curve lying 
at 36%. After a yarn (250 dtex) has been spun from this broken tow, a 
boiling-induced yarn shrinkage of 43% is measured.