Process for producing acrylic synthetic fibers having anti-pilling properties

Acrylic synthetic fibers highly resistant to pilling and having good dyeability can be produced by specifying the composition of the acrylic polymer, the condition of the primary stretching step, the internal water content of the water-swollen gel fibers, the conditions of the steps of the drying-compacting, secondary stretching and relaxing heat treatment.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for producing acrylic synthetic 
fibers having anti-pilling properties, and more specifically to a process 
for producing acrylic synthetic fibers highly resistant to pilling, and 
with respect to dyeability, not inferior to conventional ones, in which 
process the condition of the primary stretching (the general term of the 
cold stretching immediately after spinning and the hot stretching given 
subsequently to the water-washing after the cold stretching), the internal 
water content of the water-swollen gel fibers, the conditions of the steps 
of drying-compacting, secondary stretching and relaxing heat treatment are 
specified. 
2. Description of the Prior Art 
It is well known that acrylic synthetic fibers have a wide field of 
applications in textile materials and room furnishing materials because of 
their wool-like soft hand and excellent dyeability. 
However, it is not that such acrylic synthetic fibers excellent in 
usefulness have no defect in practical use, and in effect in certain 
fields of their applications it has been strongly demanded to establish 
quickly industrial means for improving fiber properties. 
Although the resistance to abrasion and resistance to fibrillation of 
acrylic synthetic fibers can almost satisfy the practical level demanded 
for textile materials, etc., woven or knitted fabrics produced from 
acrylic synthetic fibers have a defect that small balls of entangled short 
fibers, the so-called "pills," are generated on the surface of the fabrics 
with the passage of wearing time and greatly lower the commercial value. 
The generation of such pills is not a problem peculiar to acrylic synthetic 
fibers, but is a trouble in practical use widely observed also in 
polyamide fibers or polyester fibers. The generation of pills in woven or 
knitted fabrics obtained from acrylic synthetic fibers can be said rather 
less in comparison with the case of polyamide fibers or polyester fibers, 
but even then considerable formation of pills is observed as compared with 
woven or knitted fabrics obtained from wool fibers. This has been a cause 
that acrylic synthetic fibers cannot be satisfactorily substituted for 
wool fibers as a fabric-forming material. 
Therefore, to prevent such generation of pills, several industrial means 
have been heretofore employed. It is described, for example, in Japanese 
Patent Publication No. 5863/1973, that the resistance to pilling of 
acrylic synthetic fibers can be grasped as a correlation of 
single-filament denier and strength characteristics, and in Japanese 
Patent Publication No. 18195/1964, that in order to impart resistance to 
pilling to woven fabrics made from acrylic synthetic fibers, the fabrics 
are treated with an aqueous solution of aniline, aniline acetate, aniline 
hydrochloride, or aniline sulfate. 
However, in practice, the former uses acrylic synthetic fibers of 
considerably large single-filament deniers and therefore if the fibers are 
used as a material for forming carpets, a certain degree of usefulness can 
be acknowledged but no substantial applicability was observed for purposes 
as general textile-forming material. The latter process poses a 
fundamental question as to its usefulness in respect to odor and coloring. 
Recently, Japanese Patent Application Laid-Open Nos. 80323/1974 and 
35121/1973 propose processes for producing anti-pilling acrylic synthetic 
fibers which are lowered in fiber properties, especially in elongation. 
These processes also involve unsolved problems. For example, because of 
too high a content of acrylonitrile, which is the acrylic fiber-forming 
component, it is impossible to obtain dyed products which can ensure a 
satisfactory level of deep color. Therefore, the practice on an industrial 
scale remains as a problem. 
Thus, although technical means have been attempted to obtain anti-pilling 
fibers by modifying the production condition of acrylic synthetic fibers, 
it has been extremely difficult to obtain a favorable balance of 
single-filament strength, elongation and fiber dyeability at the same 
time. 
STATEMENT OF THE INVENTION 
In the light of such circumstances, we researched to find an industrial 
means which will eliminate all the various restrictions attendant on the 
conventional techniques as mentioned above and will impart remarkably 
improved anti-pilling properties to the final fibers, without lowering the 
dyeability of acrylic synthetic fibers. As a result, we have found that 
the objects of the present invention are advantageously attained by 
employing an acrylic polymer of a prescribed composition, swollen gel 
fibers having a prescribed water content and post-treatments under 
prescribed conditions. The present invention is based on this discovery. 
The main object of the present invention is to propose a process for 
producing acrylic synthetic fibers having anti-pilling properties and 
possessing a dyeability which causes no trouble in practical use. 
Another main object of the invention is to find a technical means for 
producing anti-pilling acrylic synthetic fibers which are excellent in 
industrial usefulness. 
Other objects of the invention will become apparent from the following 
description of the specification. 
These objects of the present invention are attained by the unitary 
combination of the following process requirements (1) to (6): 
(1) using an acrylic polymer containing combined therewith at least 85 
weight % acrylonitrile, 
(2) wet-spinning a spinning solution prepared from said polymer and 
stretching the thus-obtained spun fibers at a stretching ratio of 4 to 9 
times, 
(3) while adjusting the internal water content of the water-swollen gel 
fibers after stretching to 50 to 130% based on the dry weight of the 
fiber-forming polymer, 
(4) drying-compacting the stretched fibers in a tension-free state, 
(5) subjecting the fibers to a secondary stretching step at a stretching 
ratio of 1.1 to 2.0 times in a wet-heat atmosphere above 100.degree. C., 
and 
(6) subjecting the fibers to a relaxing heat treatment at a temperature 
below 120.degree. C. 
The acrylic synthetic fibers obtained according to the process of the 
present invention are furnished with excellent anti-pilling properties 
corresponding to their suitable strength and elongation (which are 
directed to a lower strength and a lower elongation in comparison with 
those of the prior art), and cause no trouble in dyeing because of their 
moderate content of acrylontrile. Thus the fibers have a very high 
commercial value.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In producing acrylic synthetic fibers having such peculiar fiber properties 
(especially anti-pilling properties), it is important to specify the 
composition of the acrylic polymer, to adjust the internal water content 
of the water-swollen gel fibers after spinning to within a prescribed 
range, and to specify the process sequence of the primary stretching, 
drying-compacting, secondary stretching and relaxing heat treatment, and 
their treating conditions. More specifically, by unitary combination of 
the process requirements of using an acrylonitrile polymer containing 
combined therewith at least 85 weight % acrylonitrile; wet-spinning a 
spinning solution prepared from said polymer and stretching the 
thus-obtained spun fibers at a stretching ratio of 4-9 times; while 
adjusting the internal water content of the water-swollen gel fibers after 
stretching to 50-130% based on the dry weight of the fiber-forming 
polymer; drying-compacting the stretched fibers in a tension-free state; 
subjecting the fibers to a secondary stretching at a stretching ratio of 
1.1-2.0 times in a wet-heat atmosphere above 100.degree. C., and 
subjecting the fibers to a relaxing heat treatment at a temperature below 
120.degree. C., there can be obtained acrylic synthetic fibers which are 
of low elongation and of low strength in comparison with conventional 
ones, and therefore excellent in resistance to pilling, and yet not 
impaired in respect to dyeability. However, when any of the process 
requirements goes outside the preferred range or when even one of the 
process requirements deviate from the recommended process sequence, not 
only satisfactory anti-pilling properties cannot be obtained but also it 
becomes substantially impossible to maintain the dyeability of the final 
fibers in a satisfactory state. 
The acrylic polymers used in the present invention include all those 
composed of at least 85 weight % acrylonitrile, preferably 87-93 weight % 
acrylonitrile, and at least one other polymerizable unsaturated vinyl 
compound, and can be produced by a known polymerization means, for example 
suspension polymerization process, emulsion polymerization process, 
solution polymerization process, etc. In case the content of acrylonitrile 
exceeds 93 weight %, it becomes difficult to heat-set the fibers in the 
secondary stretching step of the fiber production process which will be 
mentioned later, so that it becomes difficult to obtain acrylic synthetic 
fibers of low elongation type. In addition, difficulties are frequently 
encountered in dyeing (for example deep dyeing is impossible). However, 
even when the acrylonitrile content exceeds 93 weight %, deep dyeing can 
be attained by a special high-pressure, high-temperature dyeing, but such 
is not for general use. Among the polymerizable unsaturated vinyl 
compounds which are the copolymerization components for acrylonitrile, 
there may be mentioned acrylic acid, methacrylic acid, and their esters 
including methyl esters and ethyl esters; acrylamide, methacrylamide and 
their N-alkyl substituted compounds; vinyl esters such as vinyl acetate, 
vinyl propionate, etc.; vinyl halides and vinylidene halides such as vinyl 
chloride, vinyl bromide, vinylidene chloride, etc.; unsaturated sulfonic 
acids such as vinylsulfonic acid, allylsulfonic acid, methallysulfonic 
acid, p-styrenesulfonic acid, and their salts; and other known unsaturated 
compounds copolymerizable with acrylonitrile, such as styrene, 
methacrylonitrile, etc. 
The acrylonitrile polymer thus obtained is then dissolved in a solvent to 
prepare a spinning solution. The solvents which dissolve the polymer 
include organic solvents such as dimethylformamide, dimethylacetamide, 
dimethyl sulfoxide, etc. and inorganic solvents such as thiocyanates, zinc 
chloride, nitric acid, etc. In order to attain the effect of the present 
invention more advantageously, it is desirable to employ inorganic 
solvents. The polymer concentration in the spinning solution is desirably 
7-15 weight %. 
The spinning solution thus prepared is thereafter spun to form fibers 
through a usual wet-spinning apparatus into a dilute aqueous solution of a 
solvent, and the fibers are subjected to cold stretching immediately after 
spinning, water-washing and hot stretching (such cold stretching and hot 
stretching altogether are called the primary stretching, and the primary 
stretching ratio is represented by the product of cold stretching ratio 
and hot stretching ratio). It is necessary to set the primary stretching 
ratio at 4-9 times. In case the primary stretching ratio is less than 4 
times, troubles relating operation are liable to occur, for example 
filaments tend to wind around spinning rollers. On the other hand, when 
the primary stretching ratio exceeds 9 times, the fiber strength is not 
lowered, so that it becomes difficult to obtain low-strength acrylic 
synthetic fibers according to the present invention. 
Furthermore, it is necessary to adjust the internal water content of the 
water-swollen gel fibers to 50-130% based on the dry weight of the 
fiber-forming polymer. In case the water content is less than 50%, it 
becomes difficult to obtain the acrylic fibers of low strength type to 
which the present invention is directed. Also, if the water content 
exceeds 130%, it becomes difficult to bring the fibers to a sufficiently 
dry state in the following drying step. This not only makes liable to 
cause troubles during operation but also gives rise to a partial abnormal 
drop in fiber strength, so that it becomes difficult to produce acrylic 
synthetic fibers suitable for practical use (for example fibers 
representing satisfactory spinnability) from the viewpoint of strength. 
As regards the technical means to adjust the internal water content of the 
water-swollen fibers to 50-130%, the factors that can adjust said water 
content are, for example, polymer content in the spinning solution, 
coagulating bath temperature in wet-spinning, solvent concentration in the 
coagulating bath, temperature of water washing, temperature of the primary 
stretching, etc. Among these factors, the water content can be effectively 
adjusted by specifying the relation between polymer content in the 
spinning solution and coagulating bath temperature. This interrelation is 
explained by using FIG. 1: Firstly, an acrylonitrile polymer containing 
combined therewith 90 weight % acrylonitrile is dissolved in a 
concentrated aqueous sodium thiocyanate solution to prepare a spinning 
solution. The spinning solution is then spun to form fibers through a 
wet-spinning apparatus, with the primary stretching ratio set at 7 times. 
In such a situation, FIG. 1 shows the relation between polymer 
concentration in the spinning solution and coagulating bath temperatures 
in order to maintain the internal water content of the gel fibers within 
the prescribed range. In FIG. 1, the straight line 1 shows the case where 
said water content is 50%, and the straight line 2 shows the case where 
said water content is 130%. It goes without saying that the straight line 
for a water content of 70%, 90%, or 100%, for example, lies within the 
range limited by the straight lines 1 and 2. By setting polymer 
concentration and coagulating bath temperature within the area surrounded 
by lines: 
##EQU1## 
wherein polymer concentration (%) is plotted as ordinate and coagulating 
bath temperature (.degree.C.) as abscissa, it is possible to maintain the 
water content within the prescribed range. 
Internal water contents of gel fibers obtained by varying the polymer 
concentration and coagulating bath temperature are shown in the following 
table. 
______________________________________ 
Polymer Coagulating 
Internal 
concentration 
bath tempera- 
water content 
Plotted points 
(%) ture (.degree.C.) 
of gel fibers (%) 
shown in FIG. 1 
______________________________________ 
8 -3 160 a 
10 -3 63 b 
10 2 83 c 
10 8 160 d 
11 -3 45 e 
12 -3 40 f 
12 0 45 g 
12 6 70 h 
12 10 88 i 
12 15 118 j 
12 18 145 k 
14 5 48 1 
14 10 68 m 
16 16 70 n 
______________________________________ 
After passing the primary stretching step, the gel fibers having a 
prescribed internal water content are then dried and compacted in a 
tension-free state. In this drying-compacting step, if the fibers are 
dried under tension, sufficiently compacted fiber structure cannot be 
attained and it becomes difficult to obtain fibers with high transparency 
(or good color development). In addition, it is concerned that a greater 
cost is then required for equipment and apparatus. As the 
drying-compacting conditions, any can be selected from the usual 
conditions. However, in order to attain the objects of the present 
invention advantageously, it is desirable to employ a wet-heat atmosphere 
in which the spun fibers are maintained at a dry-bulb temperature above 
100.degree. C. and a wet-bulb temperature above 50.degree. C. 
The acrylic synthetic fibers thus dried and compacted are the subjected to 
the secondary stretching at a stretching ratio of 1.1 to 2.0 times in a 
wet-heat atmosphere above 100.degree. C. If the stretching ratio is less 
than 1.1 times, it provides no stretching effect, and a stretching ratio 
exceeding 2.0 times does not result in a lowered fiber strength. By 
carrying out this stretching operation in a wet-heat atmosphere above 
100.degree. C., the acrylic fibers are effectively stretched and heat-set 
at the same time, thus advantageously retaining the shrinkage behavior of 
the fibers in the following relaxing heat treatment. In this way, the 
acrylic synthetic fibers produced are of low strength and low elongation 
type, and in addition have a dyeability which is not inferior to that of 
conventional fibers, because of the special stretching operation providing 
a tensioned heat-treatment effect. In case the stretching temperature is 
less than 100.degree. C., the heat setting of the fibers becomes 
insufficient, by which it becomes difficult to obtain acrylic synthetic 
fibers of low elongation type. If the stretching temperature exceeds 
130.degree. C., problems such as the discoloration of the fibers are 
caused. As the wet-heat atmosphere, it is possible to employ usual 
supersaturated or saturated steam. 
The acrylic synthetic fibers that have thus undergone the secondary 
stretching are thereafter subjected to a relaxing heat treatment at a 
temperature below 120.degree. C. and are produced into the final fibers. 
In case the relaxing heat treatment temperature exceeds 120.degree. C., 
acrylic synthetic fibers of low elongation type are not obtained. 
By specifying the composition of the acrylic polymer, the primary 
stretching ratio, the internal water content of the gel fibers after the 
primary stretching, the drying-compacting condition, the stretching ratio 
and stretching temperature in the secondary stretching and the relaxing 
heat treatment temperature and by employing these specified factors in 
combination, it has been found that there can be obtained acrylic fibers 
having a resistance to pilling far superior to that of the conventional 
ones and having a dyeing level not inferior to the usual ones. 
An example of the present invention will be described hereunder, but it is 
to be understood that the invention is by no means limited for its scope 
by the example, in which all parts and percentages are by weight unless 
otherwise indicated. 
EXAMPLE 1 
Acrylic synthetic fibers, 2 deniers in single-filament fineness, were 
produced on the basis of the production conditions (acrylic polymer 
composition, polymer concentration in the spinning solution, coagulating 
bath temperature, primary stretching ratio, stretching ratio and 
stretching temperature in the secondary stretching and relaxing heat 
treatment temperature) described in Table 1. The drying-compacting step 
was carried out in a tension-free state in an atmosphere of a dry-bulb 
temperature of 120.degree. C. and a wet-bulb temperature of 60.degree. C. 
The results of measurement of the strength and elongation of the acrylic 
synthetic fibers are shown in Table 1. 
The synthetic fibers thus obtained were formed into a spun yarn in the 
usual way and the spun yarn was knitted to form an acrylic knit cloth, and 
it was evaluated for its anti-pilling properties. The results are also 
shown in Table 1. 
The measurement of strength and elongation and the evaluation of 
anti-pilling properties as well as the water content in the interior of 
the gel fibers were carried out as follows: 
(1) Measurement of single-filament strength and elongation was made in 
accordance with JIS L-1075 (1966). 
(2) Evaluation of anti-pilling properties (pilling grades) 
The test specimens are measured for the pilling grades on an ICI Pilling 
Tester. A test piece of about 10.times.12 cm is wrapped on a rubber tube, 
2.5 cm in diameter and 15 cm in length, in a tension-free state. The 
margins are sewed together with cotton thread so that they should not 
overlap on each other, and both ends are fixed with cellophane tape. A set 
of four test pieces are placed in a treating box lined with cork, and the 
box is rotated at a constant speed of 60 rpm for 5 hours. Thereafter, the 
test pieces are removed from the box and the state of the occurrence of 
pills is judged by sight in accordance with the following criteria: 
Grade 5: No substantial pill occurrence and change in appearance are 
observed. 
Grade 4: Slight pill occurrence and change in appearance are observed. 
Grade 3: Medium pill occurrence and change in appearance are observed. 
Grade 2: Considerable pill occurrence and change in appearance are 
observed. 
Grade 1: Extremely remarkable pill occurrence and change in appearance are 
observed. 
In this evaluation method, a specimen of Grade 4 or higher is judged as 
good in anti-pilling properties. 
(3) Internal water content of the gel fibers 
The acrylic gel fibers are put into a centrifuge and are dehydrated therein 
at 3000 rpm for two minutes. The water content of the fibers after this 
treatment is measured by dry weight method to obtain remaining water 
content (%), and a value (10%) which is considered to have no relation 
with the internal water content to be obtained, is substracted from the 
remaining water content. The thus-obtained value is taken as the internal 
water content of the gel fibers. 
Table 1 
__________________________________________________________________________ 
Internal 
water 
Polymer content (%) 
Acrylo- 
concen- of gel 
nitrile 
tration 
Coagula- 
fibers Secondary 
Relaxing 
Sam- 
content 
(%) in 
ting bath 
after the 
Primary 
stretching 
heat Evaluation of fiber 
properties 
ple 
(%) in 
spinning 
tempera- 
primary 
stretching 
Ra- 
Temp. 
treatment 
Strength 
Elong- 
Pilling 
Dyeabil- 
no. 
polymer 
solutin 
ture (.degree.C.) 
stretching 
ratio tio 
(.degree.C.) 
temp (.degree.C.) 
(g/d) 
ation (%) 
grade 
ity* 
__________________________________________________________________________ 
A 80 12 +6.0 71 7 1.4 
125 105 2.9 45 2 .DELTA. 
B 90 12 +6.0 70 7 1.4 
125 110 2.9 29 4-5 .circleincircle. 
3 
C 95 12 +6.0 70 7 1.4 
125 115 3.1 26 4-5 X 
D 90 10 -3.0 63 8 1.4 
130 118 3.2 30 4-5 .circleincircle. 
3 
E 90 12 -3.0 40 7 1.4 
130 115 4.0 30 3 .DELTA. 
F 90 12 +15.0 
118 8 1.5 
120 115 3.2 29 4-5 .circleincircle. 
. 
G 90 12 +10.0 
88 10 1.3 
125 115 4.2 29 3 .circle. 
H 90 10 +2.0 83 6 1.6 
125 115 3.0 30 4-5 .circleincircle. 
I 90 12 +4.0 80 6 2.2 
125 115 4.1 27 3 .DELTA. 
J 90 12 +8.0 78 7 1.4 
95 110 2.9 38 3 .DELTA. 
K 90 12 +8.0 78 7 1.4 
115 110 3.0 31 4-5 .circleincircle. 
L 90 12 +8.0 78 7 1.4 
125 124 3.0 41 2 .circleincircle. 
__________________________________________________________________________ 
*In comparison with ordinary acrylic synthetic fibers, 
.circleincircle. the fibers have the same dyeing level, 
.DELTA. a lower dyeing level, 
.circle. almost the same but a little lower dyeing level, 
X considerably lower 
From the results shown in Table 1, it is clearly understood that the 
acrylic synthetic fibers produced employing, in unitary combination, the 
process requirements according to the present invention (Samples B, D, F, 
H and K) have a suitable fiber strength and elongation satisfactory as 
fibers of low strength and low elongation type and have excellent 
anti-pilling properties corresponding thereto, and the dyeability is not 
impaired. 
Comparative Example 
A spinning solution of a polymer concentration of 10% was prepared, using 
the same acrylic polymer as used in Sample No. D of Example 1. The 
spinning solution was spun through an ordinary wet-spinning apparatus to 
form fibers. The fibers were then subjected to the primary stretching at 
the ratio of 8 times and were dried and compacted under the condition of a 
dry-bulb temperature of 120.degree. C. and a wet-bulb temperature of 
60.degree. C. Thereafter, without the secondary stretching, the fibers 
were immediately subjected to a relaxing heat treatment in saturated steam 
at 105.degree. C. and were finally produced to form acrylic synthetic 
fibers of a single-filament fineness of 2 deniers. The fiber strength and 
elongation of the fibers obtained by this method were 3.80 g/d and 37.8%, 
respectively. Said acrylic synthetic fibers were knitted in the usual way 
to form a knitted fabric and the fabric was evaluated for its anti-pilling 
properties. The anti-pilling grade was grade 3 and the fabric did not have 
a high commercial value. 
4. Brief Explanation of the Drawing 
FIG. 1 shows the relation between polymer concentration in the spinning 
solution and coagulating bath temperature to maintain the internal water 
content of the water-swollen gel fibers after the primary stretching 
within the specified range according to the present invention.