CA1038579A

CA1038579A - Process for producing porous acrylic fibers

Info

Publication number: CA1038579A
Application number: CA168,195A
Authority: CA
Inventors: Toshiyuki Kobashi; Noboru Abe
Original assignee: American Cyanamid Co
Current assignee: Wyeth Holdings LLC
Priority date: 1972-04-10
Filing date: 1973-04-09
Publication date: 1978-09-19
Also published as: JPS48103827A; ES413531A1; JPS51210B2; US3839520A

Abstract

Abstract of the Disclosure Wet-spinning porous acrylic fibers from a spin dope containing a fine dispersion of both (a) an inert gas such as air and (b) an inert, low-boiling (b.p. 30-100°C.) liquid which is substantially insoluble in the spin dope and coagulant by a process which includes passing the wet-gel fiber through steam or hot water at a temperature above the boiling point of said inert liquid to evaporate it.

Description

This invention relates to a process for preparing porous acrylic fibers from a spinning solution prepared by dissolving a fiber-forming acrylonitrile polymer in an aqueous inorganic solvent therefor.
More particularly, the present invention relates to an industrially advantageous process for producing porous acry-lic fibers containing numerous stable fine cavities therein and having excellent physical properties, which process comprises dispersing in a spinning solution of an acrylonitrile polymer in an aqueous inorganic solvent both an inert low-boiling liquid which is substantially insoluble therein and an inert gas, wet-spinning the thus modified spinning solution into an aqueous -coagulant under conditions such that said low-boiling liquid is not evaporated or dissolved, and thereafter heating the thus coagulated filaments so as to evaporate the low-boiling liquid.
Many processes for producing porous or hollow ib-ers employing wet-spinning were previously known.
In one such prior art process, an inert gas is dis-persed in the spinning solution and the resulting dispersion is spun into fibers. The process requires use o~ a special ;~
dispersing device such as a colloid mill to disperse the gas as fine bubbles within the spinning solution. However, even with the use of the special dispersing device, it is extremely difficult to provide gas bubbles of the fine diameter neces-sary. Consequently, the process leads to frequent stoppages due to filament breakages in spinning and s~retching steps and it has no* been possible to maintain the necessary continuity of processing necessary for com~ercial production.
In another such process, a low-boiling liquid is dis-3a persed in the spinning solution, the modified spinning solution is then wet-spun and coagulated, and the liquid is evaporated.
However, in this process, an insufficient number of cavities are obtained in the filaments obtained to provide the desired - 1 - ~ I,L

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degree of porosity. Increasing usage of the low-boiling liquid to increase porosity not only increases production costs immensely but also leads to ;
unacceptable fiber properties, particularly strength and elongation.
In accordance with the present invention, there is provided a process for preparing porous acrylic fibers uhich comprises preparing a spinning solution of a fiber-forming acrylonitrile polymer containing at least about 70 weight percent acrylonitrile and the balance of one or more vinyl monomers copolymerizable therewith in an aqueous inorganic solvent therefor, dispersing therein as bubbles of an average diameter less than about 50 microns both an inert liquid having a boiling point in the range of 30-100C. and an inert gas which are substantially insoluble in said spinning solution and an aqueous coagulant therefor, said liquicl and said gas being dispersed in amounts given in the co-dependent relationship ABCDE
shown in the accompanying Pigure, spinning the dispersion thus obtained into said aqueous coagulant under conditions which retain the dispersed bubbles within the coagulated fiber, and thereafter passing the coagulated fiber through steam or water at a temperature at or above the boiling point of said liquid while the fiber is in a homogeneous swollen gel state so as to evapor-ate said liquid.
In accordance with the present invention, it is entirely unexpected that the combination of low-boiling liquid and inert gas should provide much finer cavities than are obtained by use o inert gas alone, that the amount of low-boiling liquid required to provide the desired porosity should be greatly reduced from that required by use of liquid alone, and that the !' various problems presented by the former processes should be overcome uhile ~, providing porous acrylic fiber of excellent physical properties. ~ ~.
: Although the manner by which the improved results are -'~,. . ~

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obtalned by the present invention are not known for certaln and : .
the present appllcants do not wi3h to be bound by an~ theory, lt is thought that dl~per~lng both the low-boiling liquid and the inert ga~ ln the spinning solution cau~e~ the llq~id to pas~
through proces~e~ of gasificatlon and exp~m~ion which result ln coalescence with the inert gas,this ln turn lead3 to production of bubbles containing low-boiling liquid within the spinning composition, which bubble~ are then reduced in size by agitation accompanying the dispersing operation, and flnally the low-boil-ing liguid is conden3ed and liquefied in con~unction wlth thecompression exerted upon the splnning ~olution as it i~ pumped in the splnnlng procedure. It is this behavlor o~ the low-boil-ing llquid that 19 pre~umed to be a reason ~or the result~ ob-tained, ~he low-boiling llquid useful in the process o~ the present invention i8 an lnorganlo or organlc compound which i8 substantially insoluble in the spinnlng composition and ln the aqueou~ coagulant used therewith. It must have a. ~oiling ;::
polnt in the range of 30 to 100C., and preferably in the range of 60 to 80C. Suitable low-boillng liquid~ are exempli~led :
by carbon tetrachloride, butyl chloride, propionyl chloride, propyl chloride, allyl chloride, l~oamyl chloride, trichloro-ethylene, trichloroethane, benzene, n-hexane, cyclohexane, cy-clohexadiene, cyclopentane, dimethylbutane, dimethylfuran and carbon disulfide. A preferred liquid 1~ n-hexane. :
In the event that the low-boiling liquid i~ 301uble in the ~pinning compo~itlon, ~t will be impo~ible to obtain flber having the poroslty characteristlcs that are the obJect ~`
of the present lnventlon. Furthermore, 1~ the low-bolll~g liquld 1~ ~oluble in the aqueou~ coagulant, lt will also be lmpo~sible to obtain fiber ha~ing the porosity characteri~tics khat are the ob~ect Or the pre~ent invention. The term "sub~tantially in-~oluble" as used in the pre~ent ~pecification and olaims 1 _ 3 _ .

1~3~
intended to refer to a low-boiling liquid whose solubility does -not exceed about 0.5 weight percent at 20C. in either the spin-ning solution or the aqueous coagulant.
Low-boiling liquids having boiling points below about 30C. cannot be employed in the process of the present invention because the spinning solutions involving aqueous inorganic poly~
mer solvents are generally employed in wet-spinning at tempera-tures that would cause premature evaporation of the low-boiling liquid and consequent loss of the desired fiber porosity. Li~
quids having boiling points above about 100C. cannot be efec tively employed in the process of the present invention because ~
they are too dificult to expand quickly and to evaporate from ~^
the fiber under the conditions normally associated with wet pro-~ ~ .
cessing of wet-spun acrylic ibersO , The inert gas useul in the process of the present ~` invention is non-reactive with and substantially insoluble in the spinning solution and coagulant. Suitable gases include, : -''f- for example, air, nitrogen, argon, helium, and neon.
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~ The sole Figure of the present specification is an ''~! 20 orthogonal coordinate diagram resulting from plotting the ef-fective usage levels of low-boiling liquid and inert gas and connecting the points so as to provide an enclosed effective ~ area that represents the codependent relationship between usage ~.
!'.,' of low-boiling liquid and usage of inert gas that is effective in the process of the present inventionO The effective coor-i ~ .
~ dinates connected are represented as A ~1, 5), B ~1, 0.01), C ~30~ ~ ~p~
.. 1 oOl) D ~30, 0.5) and E ~5, 5). The area represented by the smaller ~i diagram results from connecting coordinates F ~2,2), G~2, 0.05), H ~10, 0.05), I ~10, 0.5) and J ~5, 2) and indicates a preferred ~-~ 30 codependent relationship between usage of low-boiling liquid and usage of inert gas. The values of low-boiling liquid plotted ~` are weight percentages based on the weight of the spinning solu-tionO The values of inert gas are volume percentages based on : . , ~ 4 ,","' ., 103~s~g ::
the volume of splnning ~olution. U~ages o~ low-boiling liquid and inert gas within the area de~ined by connecting the coor-dinate~, as shown in the Figure, lead to the desirable result~
of the present inventlon. Usage~ outside the area enclo~ed by connecting the coordinates re~ult in lo~ of the deslrable pro-perties achieved by the pre~ent invention.
The inert ga~ and low-boiling :Liquid may be lntro- ;
duced into the spinning solution by any convenlent method for dlspersion therein~ The low-bolling liquid may be introduced and dispersed ln ~he spinn1ng solutlon fir~t, followec1 by ln-troduction and dispers10n of the inert gas. Alternatively, the inert gas may ~lrst be ln~roduced and di3per~ed in the spinnlng solution followed by introduction and dlsperslon of the low--boiling llquld. A particularly prererred method Or lntroduc-tion i8 to prepare rirst a gas~ous mixture by diapersl~g thelow-boiling llquid in the form Or a Bas or mi~t into the inert gas and then lntroducing and dispersing thi~ gaseoua mixture into the splnning solution. Thi~ prererred procedure o~ intro-duction and dispersion of low-bolllng liquid and lnert gas en-ables extremely fine bubble ~izes to be obtained in the spinning ¦ 801ution.
In e~fecting disper~lon of the low-boiling llquid and inert gas in ~he spinning solution, mechanical agitation 13 em-ployed. A particularly e~ective mechanical device is a plane- ;
~` 25 tary gear t~pe liquid di integrator or an in-line homoml~er u~ed `~ in con~unction with transport o~ the æolutlon. It 18 generally i prererred to carry out diæperslng until the average bubble dia-meter in the spinning ~olution i~ not more than about 50 micron~, :`;
prererably not more than about ~0 microns, a~ mea~ured ~rom photomicrograph determinatlons, as later de~cribed. When the -, average bubble diameter exceed~ about 50 microns, problems with re~pect to spinnabillty, ~ilament breakage at the time of ~pinning and ~tretching, and non-unl~orm pore~ may result~ Thlerefore, it ~ .

103~
is deslrable to maintain bubble diameter below ~bout 50 microns to avold 3uch potential problems.
The ~pinnlng solution with it~ dispersed content of low-boiling llquld and inert ga~ i~ then ~pun into an aqueou~
coagulatlng bath in accordance with conventional wet-~plnning ; procedure~ Conventionally, the aqueous coagulating bath ls maintalned at a temperature below about 20C. and prererably below about 10C. and coagulation i~ effectively accompli~hed.
U3e of aqueou~ coagulating bath temperatures in excess Or about 20C. results in devitrification Or acryllc ribers obtained ~rom aqueous lnorganic ~olvent solutions of ~lber-forming acry-lonitrile polymer~ and i3 to be avoided. The bubbles Lntroduced into the ~pinning solution by the combination o~ low-boiling llquid and inert ga8 are pre~ent in the rlber coa~ulated as spe¢ified. The coagulated ~iber 1~ water-wa~hed, whlch may be accompanied by partlal stretchlng, in accordance with conven-tional procedures. Such water-wa~hing doe~ not lnvolve temper-atures in exce~ of about 20C. The ~a~hed fiber 1~ then treated in hot water or steam at temperature~ at or above the boiling point of the low-boiling liquid while the riber i~ ~till in a homogeneous swollen gel state. By such treatment, the low--boiling liquld present in the liquid ~tate in the swollen gel ~lber i8 quickly evaporated 30 as to leave flne cavities within the fiber. The cavitie~ result from evaporation Or the low-boll-ing liquld alone, such liquid ln conJunction with inert gas, or ~ ~rom coalescence Or cavltles resulting from evaporation o~ low- -- -boiling litluid and e3cape o~ lnert gas entrapped ln the lnl-tially coagulated ~iber, i.e. prlor to u~e o~ hot water or ~tea~
; The cavity-contalning ~lber i~ then stretched in hot water at a temperature above about 80C. Such stretching cau~e~ elongation - Or the cavitie~ in the directlon of ~tretchlng and the flber thu~ obtaina unlrorm elongated cavitle3 over it~ entire length.
Hot-~tretching may be ln a ~ingle ~tep or in ~taged amount~ ln ' . .~ , , . . , . ~ -multiple steps. Some stretching may also accompany water-wash- ;
ing prior to cavity formation by taking advantage of the limited cold stretchability of the spun filamentO Stretching may also be carried out by use of steam. The stretching contemplated is ~ i . ~ ~
that conventionally employed for fiber orient;ation purposes.
The swollen gel fiber containing cavities and oriented by hot-stretching is then compacted as to fiber structure by drying and may be subjected to such additional conventional pro- ;~
cessing steps as may be desired. Such additional steps include, ~ ; ;
for example, heat-relaxation, finishing agent treatment, and post-drying, as are conventionally employed.
It is not necessary to efect cavity formation by ;
use of an added heat-treating step with hot-water or steam, but the formation o cavities may accompany a suitable conven-tional step performed on the fiber while it is still in a homo-geneous swollen gel state before compacting. A preferred opera-~i tion involves obtaining cavity formation in conjunction with ' hot-stretching for orientation purposes. It is also possible - ~ to obtain cavity formation as a result of water-washing by elevating the temperature of the fibers in stages. ;~
The acrylic fiber obtained by the process of the pres-ent invention has elongated fine cavities uniformly distributed , over the entire fiber lengthO These cavities are not collapsed by any subsequent processing steps such as stretching, compacting, j or crimping steps. The resulting fiber also has high strength ~1 and elongation properties and is eminently suitable for such -,":! uses as clothing, bedding, and stuffing by virtue of its light , weight and elastic and insulating properties, which are i~portant .,~ .
-~ requirements for many industrial usesO
'-i~'! 30 The acrylonitrile polymer useful as the fiber-forming , polymer may be a homopolymer of acrylonitrile or a copolymer containing at least about 70 weight percent acrylonitrile and the balance of one or more vinyl monomers copolymerizable there-' ~ ' ;

1~3~
with. A mixture of polymers may al~o be employed.
As the polymer solvent used to form the spinn~ng ~olu-tion, an aqueou~ inorganic solvent i8 requlred. ~uitable in-organlc ~olvents include concentrated aqueou~ ~olutlon~ of thio-cyanate ~alt~, uch a3 sodium, potas3$um~ ammonium, and calciumthiocyanates, zinc chloride, and perchlsric, nitric, and sul~uric acids. Organic solvents, such a~ dimethylformamideJ dlmethyl-acetamide~ or dimethyl sulfoxlde, cannot be used in the proces~
o~ the present lnventlon since they dissolve the low-boiling liquld and do not enable the required di~persed ~tate thereof to be achieved.
The concentration of acrylonitrile polymer in the ~pinning solutlon i8 generally in the range of about 5 to 25 weight percentJ based on the weight of the spinning solution, and i8 influenoed by the molecular weight Or the polymer, a8 i8 known.
The aqueou3 coagulant employed ls that nornally as-soclated with the partlcular aqueou~ inorganic polymer ~olvent employed. No speclal requirements a3 to aqueou~ coagulant are imposed by the process o~ the present invention.
The invention is lllu~trated by the examples which follow wherein all parts and percentages are by weight unle3s ; otherwise speclfically deslgnated.
The poro~ity value of the rlber is evaluated by determining the speclflc gravlty o~ the fiber. It has been determined that a ~lber prepared without provi~ion for cavltles therein ha~ a speciflc gravity o~ 1.18. It ha~ alYo been deter-mlned that satls~actory poroslty 18 no~ achleved until a speci-~i flc gravity below about 1.15 is achieved~
The average bubble dlameter D o~ the splnning solutlon . .. .
with its di3persed content o~ low-boillng li~uid and lnert gas is obtalned by obtalnlng a photomicrograph of the cli~per~ion and mea~uring the diameter Dl of 200 bubbles therein, The , - 8 -,. . .

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average dlameter ls then calculated ~rom the relationshlp ~;

~200 D Y / ~ Di2 ~ -~ 200 Examples 1~
A copolymer conslstlng Or 90~ acrylonitrile and 10%
methyl acrylate wa~ dlssolved at 70C. in an aqueous solutlon Or 44% sodium thlocyanate to provide a polymer concentration o~
11%. The solutinn wa~ deaerated and cooled to about 25C. Thls ~olution was divlded into sixteen portlon~ for ~urther modl~ica-tion and use in spinning fiber~. Eleven portions were employed in preparlng fibers in accordance with the process o~ the pres-ent lnventlon and rlve portions were employed in preparing rlbers by comparative prior art procedures. Each portlon, suit-ably modifled was lndlviduall~ processed into ~iber.
Each portion o~ the splnning soIution processed ~n accordance with the present lnvention was modl~led with both low-boiling liquld and inert gas (air) as lndlcated in Table I.
~ .
The spinnlng solutlons thus modified by incorporation of low boillng llquid and air were agitated by mean~ o~ a planetary gear type liquld dlslntegrator so as to provide the average bub-ble diameter glven in Table I. The low-boillng liquid and air were lntroduced into the spinning solution separately, th~
liquld belng added ~irst O ' ' The modifled spinning solutlon, in each instance, was spun into an a~ueou~ coagulatlon bath at 0C. con~ist~ng Or a 12% aqueous solution o~ sodium thiocyanate. me splnneretke contaIned 50 orlfices, each of 0.1 millimeter diamet~ro The ~0 coagulated fiber was water-washed at 20~C. and stretched at a stretch ratlo o~ 2 ln conJunction therewith. The ~a~hed fiber s was then stretched at a stretch ratio Or 5 ln bolling water.
The bolling water treatment also caused the low-bollLng llquid ~: g :
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103~579 to gaslry and evaporate from the fiber with the entrapped air to provide cavlties in the ~iber, The ~iber wa~ then drled in ; ;~
. , air at 120C. to compact the structure and finally heat-relaxed in ~aturated ~team at 125C. The varlow3 modi~lcations and de-.
talls are glven in Table I. ~ ~
, Comparatlve Exam~les A-E
The procedure ~ollowed wa~ as ln Examples 1-11 above except that the splnning ~olutions were modlfled wlth low-boil-ing llquid separately or with lnert ga~ (air~ ~eparately. The ~ -modi~lcations and detail~ are also given in Table I. ~ ~
, ; . -.
In proces~ing the ribers of Examples 1-11, no problem~
wlth respect to splnning were encountered. The fibers obtalned had low apeci~ic gravltles, thua indlcating numerou~ cavitiea, ~ ;
and had excellent strength in vlew Or the many cavities.
In processing the ~iber~ Or Comparatlve Example~ A-E, ~plnning problem~ were encountered. In Comparative Examplea A-C, the rllament~ broke ~requently both upon spinnlng and upon ,. ;
,~ stretching, thu~ stopplng con~inuity o~ the splnning proces3.
The stoppages increaaed with lncreaalng usage of air. The ~iber -obtained where proce~sing was practical was poor in poro~ity, containing ~ew cavltlea Or non-uniform ~ize. In Comparative Example~ D and E, few cavitiea were obtainedO U3e Or higher amounts of low-bolling liquld beyond that reported cau~ed such ~ rrequent rllament breakage as to provlde a riber o~ no practical ,j 25 value.
In order to determine the nature of the ~lbers pro-duced by the process Or the pre~ent invention and ~f the com~
paratlve proces~, photomicrographs Or cro~ ect~ons o~ the ; flbers o~ Example ~ and ComparatiYe Example C were mQde. The~e ~ ~0 photomicrographs ravealed that the riber oP Example ~ contalned ?~! numerous ~ine and uni~ormly distributed cavities whlle the flber Example C contained a few large cavltles non~ ormly di~-tributed.
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Claims

The embodiments of the invention in which an exclusive property or priv-ilege is claimed are defined as follows:

1. A process for preparing porous acrylic fibers which comprises preparing a spinning solution of a fiber-form-ing acrylonitrile polymer containing at least about 70 weight percent acrylonitrile and the balance of one or more vinyl monomers copolymerizable therewith in an aqueous inorganic solvent therefor, dispersing therein as bubbles of an average diameter less that about 50 micron both an inert liquid hav-ing a boiling point in the range of 30-100°C. and an inert gas which are substantially insoluble in said spinning solu-tion and an aqueous coagulant therefor, said fluid and said gas being dispersed in amounts given in the co-dependent re-lationship ABCDE shown in the accompanying Figure, spinning the dispersion thus obtained into said aqueous coagulant under conditions which retain the dispersed bubbles within the coagulated fiber, and thereafter passing the coagulated fiber through steam or water at a temperature at or above the boiling point of said liquid while the fiber is in a homo-geneous swollen gel state so as to evaporate said liquid,

2. The process of Claim 1 wherein the average bubble diameter is less than about 30 microns.

3. The process of Claim 1 wherein said liquid and said gas are dispersed in amounts given in the co-dependent relationship FGHIJ shown in the accompanying Figure.

4. The process of Claim 1 wherein said liquid is n-hexane.

5. The process of Claim 1 wherein said liquid is carbon tetrachloride.

6. The process of Claim 1 wherein said gas is air.

7. The process of Claim 1 wherein said acrylonitrile polymer contains 90 weight percent acrylonitrile and 10 weight percent methyl acrylate.

8. The process of Claim 1 wherein said aqueous in-organic solvent is 44% aqueous sodium thiocyanate.

9. The process of Claim 1 wherein said aqueous coagulant is 12%
aqueous sodium thiocyanate.

10. The process of Claim 7 wherein said spinning solution contains 11 weight percent polymer.

11. The process of Claim 7 wherein said spinning solution contains from 5 to 25 weight percent polymer.