Production of high crimp, high strength, hollow rayon fibers

High crimp, high strength, hollow rayon fibers or filaments which maintain their hollowness after being immersed in water and subsequently dried, and have a crimped configuration such that there is in excess of about 20 crimps per inch, preferably between about 25-30 crimps per inch, are provided by an in-line process whereby a viscose solution containing a blowing agent is extruded into an aqueous acid coagulating bath. The conditions of the process result in hollow filaments that are substantially irreversible since they remain hollow and do not collapse even after repeated washing and drying cycles. The hollow filaments possess high crimp, such as about 25-30 crimps per inch, which will permit ease in carding and blending with other fibers. The high crimp hollow fibers also possess high strength nearly equivalent to that of high wet modulus rayon fibers.

BACKGROUND OF THE INVENTION 
The present invention relates to processes for the production of high 
crimp, high strength, hollow rayon fibers or filaments which will recover 
their hollow condition after being immersed in water and are substantially 
irreversible in that they will remain hollow and do not collapse even 
after repeated drying and washing cycles. These fibers will also possess 
high crimp of at least about 20 crimps per inch, preferably 25-30 crimps 
per inch, when immersed in water and dried in a tension-free state. The 
invention relates also to the high crimp hollow rayon fibers produced. 
Hollow rayon fibers are known to the prior art. They have a number of known 
uses in the production of paper and non-woven products. They have been 
produced by incorporating a blowing agent, such as sodium carbonate or 
sodium bicarbonate, into the viscose rayon process. In the prior art 
processes, the viscose, containing the blowing agent, is spun into the 
conventional acidic spin bath whereby carbon dioxide gas is liberated from 
the blowing agent causing the fibers to blow or expand to several times 
their natural diameter. 
A number of patents disclose processes of this type, and they have the 
shortcoming that when the fibers or filaments are dried, the fiber walls 
collapse, and, in most instances, hydrogen bond together to form a flat, 
ribbon-like fiber. Other processes that produce a substantially 
irreversible hollow fiber have the shortcoming of possessing inadequate 
crimp such that the fibers are difficult to blend with other fibers and 
have poor carding capability (fibers do not cling well enough to each 
other to form a sufficiently strong web for processing into yarn). It is 
the desire of the rayon industry to provide hollow rayon fibers which will 
not collapse upon drying and have sufficient crimp for processing through 
the carding operation and for blending uniformly with other fibers. 
Woodings U.S. Pat. No. 3,626,045 is a patent disclosing a method of blowing 
rayon fibers. It seeks to overcome the problem of fiber wall collapse upon 
drying by adding to the viscose prior to spinning of from 0.75-2.0 percent 
by weight of polyethylene glycol based on the weight of the cellulose. The 
hollow rayon fibers which result can be dried after being formed without 
collapsing. However, the product of the patent possesses low crimp of 
about 12 crimps per inch which has been reported to be difficult to card 
and blend with other fibers. 
Patents disclose various methods for making hollow fibers, but none of 
which applicants are aware teaches or suggests a means which provides a 
high strength hollow rayon fiber which is substantially irreversible in 
the sense that it will not collapse upon being dried. These patents 
include: British Pat. No. 945,306; British Pat. No. 1,393,778; and Freund 
U.S. Pat. No. 2,013,491. 
British Pat. No. 488,500 discloses a process for producing hollow cellulose 
acetate fibers by extruding a solution of the acetate downward into a 
volatile solvent medium and in a complicated manner produces a hollow 
fiber. 
Kajitani U.S. Pat. No. 3,418,405 discloses a process for producing flat 
viscose fibers by extruding blown viscose into a medium containing a 
modifier, and such modifier is polyethylene glycol. The whole purpose is 
to produce a hollow fiber which will very readily collapse and form a flat 
fiber. This is just the opposite of the purpose of the present invention. 
British Pat. No. 1,393,778 discloses the preparation of multi-lobal 
collapsed fibers, which is not what the present invention is concerned 
with, by a process which is quite different from that of the present 
invention. 
Kobuta et al. Japanese patent publications Nos. 9536 and 9537 are patents 
describing a process for producing hollow rayon fibers. These processes do 
not employ sodium carbonate nor sodium bicarbonate nor any other chemical 
that when in contact with the acidic spin bath will liberate a blowing 
agent. But rather, this concept employs the evolved CS.sub.2 during 
decomposition in the spin bath as a blowing agent. Because this is a slow 
blowing process, surfactants are needed so as to reduce the surface 
tension and allow large bubbles to form. Not only is this process for 
making hollow fibers quite different, but also it is not one that will 
produce a high crimp hollow fiber. 
Japanese patent publication No. 20164 describes a high crimp solid rayon 
fiber with high water resistance. The process does not claim a hollow 
rayon fiber, but rather a solid rayon fiber; furthermore, it teaches away 
from the process of this invention because it stresses the use of low 
CS.sub.2, i.e., 26-32 percent on the weight of cellulose. It achieves high 
crimp by using various assistant agents such as monoamines, alkylene oxide 
polymers and bivalent metallic compounds in combination with the process 
conditions. 
Daul et al. U.S. Pat. Nos. 3,632,468 and 3,793,136 also describe a process 
for making a high crimp solid rayon fiber. This concept does not involve 
the production of hollow rayon fibers but only high crimp solid rayon 
fibers. It seeks to develop high crimp by an alkaline treatment while the 
fibers are in a relaxed state after they have been stretched and partially 
regenerated. This concept is quite removed from the process described in 
our invention. Similarly, Stevens U.S. Pat. No. 3,720,743 also discloses 
the production of high crimp solid rayon fibers and is remote from the 
present invention. 
In accordance with the present invention, the disadvantages of conventional 
prior art blown hollow rayon fibers have been overcome by unique 
conditions of the processes of the present invention. These parameters are 
discussed below and are employed in the examples which follow. 
In copending U.S. application, Serial No. 798,874, filed May 20, 1977, by 
one of us, namely, Eugene Costa, Jr., now U.S. Pat. No. 4,130,689, granted 
Dec. 19, 1978, there is disclosed a process for producing superior hollow 
rayon fibers which do not collapse when dried and washed. However, the 
hollow rayon fibers of said application do not have the high degree of 
crimp which characterizes the hollow rayon fibers of the present 
invention. The fibers possess about 12 crimps per inch. 
The hollow fibers of rayon produced by the processes of the invention do 
not collapse even when dried and will not collapse even when subjected to 
a sequence of drying and washing treatments. The processes also produce a 
uniformly large number of blown fibers such as more than 90 or 95 percent 
of all fibers being hollow or blown. 
The fibers produced not only have permanent hollowness, but also exhibit 
high strength and high crimp to permit ease in carding and uniformity in 
blending with other fibers. The fibers produced by the processes of the 
present invention have properties similar to commercial high wet modulus 
rayon and are approaching that of cotton. 
It is, accordingly, an objective of the present invention to provide an 
in-line process for producing hollow rayon fibers of high strength that 
have the property of resisting collapse even after drying and washing 
treatments, which have large continuous lumens, and which possess high 
crimp, that is, in excess of about 20 crimps per inch with the average 
being between about 25 and 30 crimps per inch. 
It is also an object of the present invention to provide hollow rayon 
fibers that have high bulk or covering power such as are useful in 
producing non-wovens or garments for outer wear. 
It is a further object of the present invention to provide hollow rayon 
fibers that have a soft, comfortable hand and which will retain their 
hollow condition after being immersed in water and then dried. 
It is another object of the present invention to provide hollow rayon 
fibers which have high moisture absorption, thermal insulation and 
dielectric properties. 
It is another object of this invention to produce fibers of high strength 
having greater than 3.0 g/d (grams per denier) tenacity when tested in a 
conditioned atmosphere and greater than 1.5 g/d when tested in a wet state 
.

GENERAL DESCRIPTION OF THE INVENTION 
The present invention is directed to novel hollow rayon fibers which retain 
their hollow condition and can be substantially irreversible in that they 
resist collapse, even upon repeated dryings and washings, and to a novel 
inline process for producing them. The fibers also possess a high degree 
of crimp in excess of about 20 crimps per inch. The resultant hollow rayon 
fibers are characterized by having a soft, comfortable hand, high moisture 
absorption properties, large continuous lumens and can be easily carded 
and used in blends with other man-made or natural fibers. These hollow 
rayon fibers have high: bulk, strength, moisture absorption, thermal 
insulation, dielectric properties and covering power, and are useful in 
producing paper products, non-woven materials, garments for winter wear, 
outer wear and toweling. The products are characterized by their 
substantially irreversible nature in that they remain hollow after 
repeated dryings and washings. 
Basically, the process of the present invention results from the discovery 
that after the fibers or filaments are blown, and before they are dried, 
their outer walls can be hardened or toughened so that they acquire an 
outer wall strength that resists collapse of the fiber walls even when 
repeatedly washed and dried. This toughening can be achieved by one of 
several means embodied by the present invention. In accordance with one 
such means, the outer wall hardening can be achieved by employing an 
aqueous spin bath containing a high zinc sulfate concentration at an 
optimum, acid and sodium sulfate concentration, into which the viscose 
containing a high percentage of carbon disulfide (CS.sub.2) is spun and in 
which the fibers are blown by action of the acid on the carbonate blowing 
agent in the viscose. The conditions of the viscose, ripening index 
viscosity, NaOH concentration, etc., and that of the spin bath composition 
are such that regeneration and coagulation is delayed until the blown 
viscose reaches the stretch zone. This then permits the blown xanthate to 
undergo a high degree of orientation as crystallization is taking place, 
thereby creating a blown hollow filament possessing a highly oriented 
crystalline outer wall structure. This structure has been known to have a 
high resistance to deformation and thereby cause the fibers to maintain 
this hollow configuration even after repeated washing and drying cycles. 
The high degree of crimp is formed by differential strains created within 
the cross-sectional area of the fiber. This effect is developed by the 
combination of the chemical balance of the system, low acid and high zinc 
concentrations and the mechanical effect created through molecular 
orientation. The crimp occurs when the fibers are wetted and allowed to 
free shrink, that is, dried without tension. The difficulty in developing 
this fiber is in overcoming the paradox that the spin acid concentration 
needs to be high for blowing the fibers and yet low to develop high crimp. 
This difficulty is offset by the delicate balance between the amount of 
CS.sub.2 used during xanthation and the high salt concentration in the 
spin bath. 
The concept of the present invention is based on creating a hardened wall 
that possesses a differential strain within its cross-sectional area, so 
as to not only prevent wall collapse but also cause crimp to occur when 
the fibers are allowed to dry in a tensionless state. It is not limited to 
the methods illustrated in the examples which follow. Any combination of 
the cases described or any other method of forming ether linkages or any 
cross-linking processes or other methods of tying up the OH groups on the 
cellulose comprising the outer portion of the fiber or the fiber wall to 
prevent hydrogen bonding or any other method of hardening the fiber wall 
to prevent collapse, such as grafting of other polymers, or by various 
irradiation techniques are all included in this concept. 
The processes of the present invention comprise first the spinning of a 
viscose solution containing cellulose in an amount of from about 6 percent 
to 8 percent, optimally 7 percent, of the weight of viscose, alkali metal 
hydroxide, such as sodium hydroxide, in the amount of from about 6 to 8 
percent (preferably about 6.5 to 7.5 percent, optimally 7 percent) of the 
weight of viscose, and, as a blowing agent, from 3 percent to 5 percent 
(preferably 3.5 to 4.5 percent, optimally 4 percent) of alkali-metal 
carbonate, such as sodium or potassium carbonate or sodium or potassium 
bicarbonate, based on weight of viscose. Said viscose solution shall have 
a viscosity of from between about 90 poises and 140 poises (preferably 110 
to 130 poises, optimally 120), ripening to a salt index from about 6 to 12 
milliliters of sodium chloride is desirable, preferably 8 to 10, optimally 
9. The viscose solution will desirably contain about 50 to 75 percent by 
weight of carbon disulfide, preferably about 60 to 70 percent, optimally 
65 percent based on weight of cellulose. 
The resulting viscose solution is extruded through a spinnerette which 
comprises capillaries, each having a diameter of from about 25 to 75 
microns (preferably 50 microns), into a first coagulating or aqueous acid 
bath. The time of immersion in the coagulating bath is preferably between 
about 0.25 and 1.5 seconds, optimally between about 0.5 and 0.7 seconds. 
This bath comprises from about 150 to 300 grams per liter of sodium 
sulfate as a coagulating agent, preferably about 240 to 280 (optimally 
260) grams per liter of sodium sulfate, and from 20 to 90 grams per liter 
of zinc sulfate, preferably about 40 to 70 (optimally 50) grams per liter 
of zinc sulfate, and from about 50 to 80 grams per liter of H.sub.2 
SO.sub.4, preferably about 60 to 70 (optimally 60) grams per liter. The 
coagulating bath shall have a temperature of at least about 25.degree. C. 
No advantage is obtained by exceeding a temperature of 100.degree. C. A 
preferred temperature of the bath is about 25.degree. C. to 65.degree. C., 
optimally 35.degree. C. to 45.degree. C. 
The coagulated fibers from the first coagulating bath or acid bath are then 
stretched from about 40 to 180 percent, preferably 90 to 100 percent, 
either in air or optionally within an aqueous stretch bath. A stretch 
bath, when employed, comprises from about 5 to 30 grams per liter of 
H.sub.2 SO.sub.4, (preferably about 10 to 20 grams per liter) and about 2 
to 20 grams, preferably 5 to 15 grams, per liter of zinc sulfate. No 
advantage is obtained by exceeding about 30 grams per liter of zinc 
sulfate. The preferred concentration of zinc sulfate is about 9 grams per 
liter. A stretch bath, when employed, is held at a temperature of from 
about 80.degree. to 100.degree. C., preferably 95.degree. to 100.degree. 
C. The fibers are then relaxed by approximately 1 percent. 
The resulting high crimp, hollow rayon fibers produced by this process can 
be cut by any conventional method, washed and allowed to dry in a 
tensionless state or they can be washed, dried in-line on a steam roll, 
wound as a continuous hollow fiber or filament, then cut, washed and dried 
in a tensionless state. 
The fibers produced by the processes of the invention not only have 
permanent hollowness, but also exhibit high crimp and high strength. Table 
1, below, is a comparative study of the physical properties of this 
product as compared to regular rayon, high wet modulus rayon, and cotton. 
TABLE 1 
__________________________________________________________________________ 
COMISON OF PHYSICAL PROPERTIES OF VARIOUS FIBERS 
SINGLE FIBER TEST - INSTRON DATA 
High Strength 
Regular 
High Wet 
Hollow Rayon 
Rayon Modulus 
of the Present 
Product Staple Rayon Invention 
Cotton 
__________________________________________________________________________ 
Conditioned Test* 
Tenacity, g/d 
2.1 3.2 &gt;3.0 3.3 
Elongation, % 
10.7 9.5 8.0 9.0 
Initial Modulus, % 
60 90 120 50 
Wet Test 
Tenacity, g/d 
1.0 1.6 1.7 3.9 
Elongation, % 
26 18 14 10.0 
Modulus at 5% 
3.5 7.3 8.0 10.1 
Crimps/inch 
8 7 &gt;20 20 
Hollowness, % 
0 0 &gt;95 Collapsed 
Lumen 
__________________________________________________________________________ 
*Conditioned environment 70.degree. C. to 65% relative humidity (yarn 
exposed for a minimum of 16 hours). 
It is apparent from the above Table 1 that the fibers of the present 
invention have properties similar to commercial high wet modulus rayon and 
are approaching that of cotton. 
SPECIFIC DESCRIPTION OF THE INVENTION 
In order to disclose more clearly the nature of the present invention, the 
following examples illustrating the invention are given. It should be 
understood, however, that this is done solely by way of example and is 
intended neither to delineate the scope of the invention nor limit the 
ambit of the appended claims. In the examples which follow, and throughout 
the specification, the quantities of material are expressed in terms of 
parts by weight, unless otherwise specified. 
EXAMPLES 
The process conditions used in the experiments of these examples and in 
producing the hollow fibers tested in Table 1, above, were as follows: 
The pulp was kraft hardwood, rayon cellulose equal to 99 percent, having a 
degree of polymerization of about 520. A steeping of the pulp took place 
in a steeping solution having a sodium hydroxide concentration of 18 
percent, containing the cellulose in a concentration of 32.0 percent of 
alkali cellulose, and a temperature of 22.degree. C. The viscose obtained 
from this pulp by the conventional viscose process had a viscosity of 120 
poises, with a cellulose content of 7.0 percent based on weight of 
viscose, 7 percent of sodium hydroxide based on weight of viscose, a 
variable percentage of carbon disulfide on the weight of cellulose, 4 
percent of sodium carbonate based on the weight of viscose. 
The viscose was then spun through a spinnerette having 720 holes, each of 
about 50 .mu.m hole size, at a jet velocity of 25 meters per minute 
(yielding an extrusion ratio equal to about 0.5), into a first coagulating 
or aqueous acid bath having the following composition: 
sulfuric acid, concentration, variable as shown in Table 2, below 
sodium sulfate, concentration, variable as shown in Table 2, below 
zinc sulfate, concentration, variable as shown in Table 2, below. 
The filaments were immersed for a distance of 10 inches in this bath. The 
filaments or fibers resulting from the first coagulating bath were then 
first passed through a second or stretch bath containing 12 grams per 
liter of sulfuric acid and 9 grams per liter of zinc sulfate at a 
temperature of 98.degree. C. The fibers were relaxed 1 percent and washed 
on a wash roll and dried on a steamheated roll (surface temperature 
60.degree. C. to 80.degree. C.) and wound on a cap twister as a continuous 
filament at a rate of 25 meters per minute. 
Table 2, below, contains the data derived from a multiple, factorial study. 
The dependent variables are the number of open fibers expressed as a 
percentage of the total fibers produced and the crimps per inch. The 
variables studied in this study are: 
______________________________________ 
Parameter Levels 
______________________________________ 
Spin bath temperature, .degree.C. 
25, 35, 45 
H.sub.2 SO.sub.4 in spin bath, g/l 
60, 70 
Na.sub.2 SO.sub.4 in spin bath, g/l 
200, 240, 260, 280 
ZnSO.sub.4 in spin bath, g/l 
30, 50, 70 
CS.sub.2, % on cellulose 
40, 50, 60, 65, 70 
______________________________________ 
TABLE 2 
__________________________________________________________________________ 
HIGH CRIMP HOLLOW FIBER FACTORIAL STUDY 
Zinc 
Sodium Open 
H.sub.2 SO.sub.4 
Sulfate 
Sulfate 
Acid Temp. 
Crimps Fibers 
CS.sub.2, % 
g/l g/l g/l .degree.C. 
Inch % 
__________________________________________________________________________ 
25 8.8 1 
200 35 14.9 1 
45 14.1 1 
30 
25 9.4 1 
240 35 15.3 1 
45 12.4 10 
25 13.2 1 
200 35 16.6 1 
45 14.5 1 
60 50 
25 15.2 10 
240 35 16.2 90 
45 12.1 95 
25 11.2 99 
200 35 10.3 99 
45 14.8 99 
70 
25 11.1 60 
240 35 11.0 95 
40 45 13.5 90 
25 11.5 1 
200 35Z 11.7 1 
45 10.9 1 
30 
25 15.3 1 
240 35 15.5 90 
45 15.8 95 
25 11.3 5 
200 35 15.9 5 
45 19.6 15 
70 50 
25 17.1 5 
240 35 16.7 90 
45 15.6 95 
25 9.3 95 
200 35 14.1 90 
45 10.8 90 
70 
25 7.4 60 
240 35 7.9 95 
45 8.8 90 
25 15.4 1 
200 35 19.1 1 
45 17.9 1 
30 
25 14.6 5 
240 35 10.1 80 
45 11.0 50 
25 15.5 50 
200 35 15.9 85 
45 20.7 80 
60 50 
25 13.4 90 
240 35 17.5 85 
45 19.7 70 
25 17.3 60 
200 35 16.7 75 
45 15.5 80 
70 
25 19.2 100 
240 35 16.1 100 
50 45 18.7 100 
25 12.9 10 
200 35 15.1 20 
45 19.1 10 
30 
25 20.2 30 
240 35 17.3 85 
45 19.4 90 
25 15.7 70 
200 35 14.7 95 
70 50 45 17.9 50 
25 18.1 85 
240 35 17.8 85 
45 21.6 80 
25 13.3 50 
200 35 15.1 75 
45 17.1 60 
70 
25 15.1 95 
240 35 15.8 95 
45 14.7 80 
25 15.1 5 
200 35 18.9 15 
45 22.9 5 
25 10.8 15 
240 35 15.0 50 
45 18.6 75 
30 
25 
260 35 -- -- 
45 
25 14.3 100 
280 35 23.2 100 
45 27.4 90 
25 15.9 90 
200 35 15.9 85 
45 20.5 50 
25 20.1 85 
240 35 17.5 50 
45 19.3 95 
60 50 
25 
260 35 -- -- 
45 
25 15.9 40 
280 35 22.0 80 
45 25.3 90 
60 25 15.1 75 
200 35 16.4 70 
45 16.4 40 
25 13.7 95 
240 35 18.3 100 
45 16.7 95 
70 
25 
260 35 -- 
45 
25 
280 35 -- 
45 
25 15.1 15 
200 35 13.3 30 
45 13.3 5 
30 
25 22.1 85 
240 35 16.7 60 
45 17.3 70 
60 25 16.6 95 
200 35 18.9 85 
45 16.8 10 
70 50 
25 18.5 90 
240 35 13.1 90 
45 14.8 95 
25 20.7 70 
200 35 15.7 80 
45 17.1 80 
70 
25 14.8 80 
240 35 14.0 100 
45 14.9 100 
25 
240 35 -- -- 
45 
30 25 19.1 70 
260 35 21.4 50 
45 25.8 5 
25 
280 35 -- 
-- 
45 
25 
240 35 -- -- 
45 
25 23.8 100 
65 60 50 260 35 31.8 98 
45 28.9 95 
25 
280 35 -- -- 
45 
25 
240 35 -- -- 
45 
65 60 25 
260 35 -- -- 
70 45 
25 
280 35 -- -- 
45 
25 18.9 85 
240 35 20.0 50 
45 25.7 80 
25 
30 260 35 -- -- 
45 
25 14.5 70 
280 35 16.3 90 
45 23.7 30 
25 17.1 85 
240 35 24.0 80 
45 28.7 30 
25 
70 60 50 260 35 -- -- 
45 
25 18.7 95 
280 35 18.1 70 
45 20.3 85 
25 
240 35 
45 
25 
70 260 35 
45 
25 
280 35 
45 
__________________________________________________________________________ 
It is apparent from the above Table 2 that fibers possessing both very high 
crimp, such as 32 crimps per inch and a high degree of hollowness above 95 
can be achieved by the correct choice of process conditions within the 
scope of the processes of the present invention. 
Typical physical properties of the high crimp, high strength hollow fibers 
of the invention are shown below in Table 3: 
TABLE 3 
______________________________________ 
TYPICAL PHYSICAL PROPERTIES 
______________________________________ 
Denier/Filament 1.5 
Conditioned Tenacity, g/d 
&gt;3.0 
Conditioned Elongation, % 
8.0 
Wet Tenacity, g/d 1.7 
Wet Elongation, % 14.0 
Crimps/Inch &gt;20.0 
Degree of Hollowness, % Open 
&gt;95 
______________________________________ 
The hollow, high crimped fibers shown in the photomicrograph of the 
appended drawing were prepared in accordance with the process of the 
invention and foregoing examples in which the viscose contained 65 percent 
carbon disulfide based on weight of cellulose. The spin bath had a 
temperature of 35.degree. C. and had the following composition: 
______________________________________ 
H.sub.2 SO.sub.4, g/l 
60 
ZnSO.sub.4, g/l 50 
Na.sub.2 SO.sub.4, g/l 
260 
______________________________________ 
98 percent of the fibers produced were in the hollow condition and had 
about 31.8 crimps to the inch. 
The terms and expressions which have been employed are used as terms of 
description and not of limitation, and there is no intention in the use of 
such terms and expressions of excluding any equivalents of the features 
shown and described or portions thereof, but it is recognized that various 
modifications are possible within the scope of the invention claimed.