CA2243520A1

CA2243520A1 - Hollow microfiber of ceramic material, a process for its manufacture and its use

Info

Publication number: CA2243520A1
Application number: CA002243520A
Authority: CA
Inventors: Klaus Rennebeck
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-01-21
Filing date: 1997-01-20
Publication date: 1997-07-24
Also published as: KR100283551B1; NO983342L; DE59707053D1; ATE216356T1; BR9707014A; EP0874788A1; EP1018495A2; DE59711649D1; CN1211966A; JP3061866B2; EP1018495A3; ATE267149T1; AU717033B2; PL327974A1; EP1018495B1; KR19990081864A; EP0874788B1; AU1543597A; WO1997026225A1; NO983342D0

Abstract

This invention describes hollow microfibers of ceramic material with a wall thickness of approximately 0.01 to 15 µm and an outer diameter of approximately 0.5 to 35 µm. The hollow microfibers are produced preferably by means of a process in which a dispersion containing the precursor of a ceramic material and a binder which is removable through the effects of heat is formed using a known method into green hollow microfibers and, where necessary, the binder is removed using heat.

Description

~lOLLC)W l~c~o~l~ OF CER~MIC M~lrERIl~L, A MET~IOD FOE~ ~I S PROI)UCIION
Al!~D T~ ~JSE OF 9!UC}~ A FlBE~

The ~nven~on relates lo a hoUo~ microfiber of a ceramic material, lo a melhod for Ihe pro~ncrion of such a fiber and to its us~.

Tk produc~on of compact cPr~ fibers, that is, of fiber~, which do not ha~e a l~men or cavity in ~he center of ~he fibers in dleir l~n~in~di~l direc~ion.
is ~;no~n They ~eneraUy consisl largely or compleu:ly of a ~lass phace and are used, for example, as woven fabric, l~ ed ~abnc and nonwoven malcrial in incu~ g matcrials and as hca~ protcction shi~ld for ~eintorcing rn~ w~.~ces and in com~ci~e mate~ials. ~ various area of applica~on, ~ese fiberc are ~1P~ tP, for e~nnrlP, ~ith I~Sp~CI to el~cn~ i~y~ bending sl:reng~h and ins~ n~ effecl. Moreo~er, it is desirable ~o lower dle wcigh~ of dle ~nown fibers and to increase thc spuu~i~g speed.

Th~ di~advantages, addrcsscd above, are not elirnula~ed by ~no~n hollow tlbers of polymeric s~nthe~e ma~e~iaLc. P~ec~ of thcir special feanlres, d~esc fibcrs ~ llsed in ~ n~b~ane woven fabncs, roof linings, tcnt canYas, membranes, etc. GeneraU~, howe~er, Ihe biolo~ic~l comp~ihiliry ~s weU as ~he chenucal and d cnnal s~abili~ are in~A~n~ i~ion, ~hcy have the disadvantagc hal membr~nes, pro~ln~ed the~from, have a comF~rivel~ lo~ pener~liol sp~d and cannot be counfer-wash~d or cleaned.

Ceram~c hoUo~ fibers are IcnoYJn, which have ~hick ~alls and iarge e~lernal ~li~,.f~ and comr~rauvely large fl~c~atic~ns in ~cir .' ..c"iions and, in a pas~e, con~~ g a ceranuc po~der, removu~g ~e b~nder and s~ntering. Ho~ever, such cc~uc t~ollo~ fibers have a lirnited s~.~ngll., a low el~c~riry, a small cI~ecifir s~urface area and do no~ bavc sern~-pe~Tneable p~v~Ls. Morc~ver, thic~ fibels can be p~oduced and wound up only at slow s~s. They are nD~ s~lit~hl~ for woven and knitted fabncs and odlcr ~extile ~orrna~ions, for which, ul par~ cular, a high flexibil;i~
is r~qu~

The ~P-A~ 19~ 353 dic~ cl-s hollow n~icrofibcrs with an ex~rnal d~ter of less ~n about 50 I-m and a u~all ~ ~nt~sc ranging from abou~ 5 lo 20 ~m. Ho-4ever, the e~".al d~lcu,r as ~heU as the wall tbickness of ~hese Imo~nhoUow microfi~crs flu~s appreciably. Thc fl~cD~nons, rela~d to the av~ge, amOuM to 33% for ~u ex~r~ ~ m.~r and even to 66% fo~ ~be ~all thi~ ess.
P~e~ se o~ their n~nu-ufornuty, th~se blown hollow microfibers are Imsuitable for numeroll~ app~ n~rlc.

Moreover, h~llow fibers with an c~c~mal di~mct~or up ~o 2C10(1 ~m arc ~;no~n f~om ~e US patem 4,268,278, an cx~ernal di~..~r range of 10~ ~o 550 ~m being par~cular1~ f~ d. The wall d~ Cc of these hollow fibers ranges from 20 to 30~ ~.m and cs~ci~lly ~om ~0 to 200 ~ m. Moreover, Ihcse fibers can al_o be prodl~ced and wolmd up or-ly at a 10~ speed.

I~ is dlerefore an objecl of ~he inven~on ~o malce available hollow microfibe~s of a cerasruc materia1, which do not have the ~l~firiencies of the lin~wn fibers add~essed abovc and, in particu1ar, ha~e a high el~cn~i~, a high bcnding sl.en~,th, a good ir~C~ m eff~c~ and good biol~g;~s~ cQm~rihilir~ and, n~o~eove~, can be produced at high producuon speeds P~ or~, it shall be pqssible lo ûse exis~ng spinning e~ir.-l~r~) intended for ~ made fibcr, for dleir pro~ cti?n.

Pursuan~ ~o the inven~nn, rhi~ objec~ve is accomplic~l by a hollow nucrofiber of a ce~mic mau~rial, which is char~ ized in ~al it has a wall rhir~s~
of about 0 01 ~o 1~ ~m ~nd an cx~.~ t~r of abou~ 0.5 to 35 ~m, tJle fl~cn~ion in wall thic}cness and e~ ,~l diame~er not e~ec~ling 69'o.

MODIEIEP PAGI~

.. . .... . ~ . . .

n p~c, ~ r url~, a ee~"ie ~3~vdcr, ~ ~~6 ~ c~ . IIo;
~ucl~ ~llo~/~ ce.~a.. ic f~ c ~ L... ~J 3e.0~ c~ irr, ~ lo.~ )~tifi~
~d ne 9C~ p3F ~I~ QI~C~S l~lor_o~, ~ick fil~cr~ e~. bc ~e~ l ~ v~ r ~r.* ~ 91c~. ~9. Th~ o ~o~ f~ . a~
t ~ ~ ~ ~ ~ ~ ~ ~ G~3, f6~ ~ch, i.~ Q~hC~A~, a ~k ~Ai~
is rc~lu~cd.

lt is thc.~ ~ bjee~ ~f J~ iQ~n to m~ blc h~U~v ...ic~f~crs ~f a _r~ h d6 IM ~vc d~c d~~fi~ f ~- ~owrt fibcr 3~cd dbO~C a~ dîr, 'rrvc a l~h ,~ ~, a h;~h b~,.~i~g ~lfl be rr~UCYlfl llt hig~ p~t~tr~ ~,~ce~ls. rlll~C....-SF~, ~ 5h~1 be ~~~le tO UX
p~r..--..~ ~uipn~u~ed f~lF ..~ ~ fib~, f~ Ehc~ ~-~U~A.

~ rsu~nt to ~ ~ C~Ei~, .h;~ ~jcc~ivc is uec~Qplish~Yl b~ ~ hollow ~ic. ~ Oaf ~ ..~F;~ ~ioh is ~cr~l i ~ {1~ h~ s o~ ou~ 0.01 ~o 15 ,.... 6r~d ~ mc~r o~ u~ 0~5 u) 3S ,.n..

A ~4all ~l~ir~-nrcc of Icss than about 0.01 ~m i_ difficul~ to achieve duc to n~nnf~ asons and would, if i~ wcre possibk lo produce such a ~icbless, hav~c ~e disadvanlagc that ~e inncr or outer surface of thc hollow fib~rs would non-unifonn and any co~rin~, subsequendy applied, would havc ~lefecr~, such as holesor a non-lJnifornl t~ir~r~cc If the wall l~ir~s~c. cx.~ls aboul lO ~m, the us~ of the in~n~e hollow fibers for ~he scpara~on of materials b~ would mcrel~
lead 1~ an infcnor permea&e flow, since the pc~nea~e would ha~e to cover a larger dist~nce, ~hile ~e selectivity is no~ proved f~rther. I~ is ther.fo,~ prefcrred if ~e ~all ~hi~ cc i~, about û 3 to 6 ~m and~ esp~ci~lly, about 0.5 to 3 ~.m If dle extemal ~i~r drops below û S ~m, the hollo~4 fiber ha,s ~oo small a lum~n and the flow of liquids throu~h ~he fibers is i---~dcd. ~f dle external nr~r ~ e~s 35 ~ , ~c flcxibilit~ of ~c hollow fibers is rcduced and ma~
sepa~auon n~o 1.: Ios, wt~ch are constmcled from a plurality of inYen~ve hollow fibers, becol.,c too ~olum~nous in r~lanon ~o ~e amoum of p~n.,eate flov.~ing 2a MOD~FIEl) PAGE

, . . . .. .

u/all rhirknp.s~ is abou~ 0.3 ~o 6 ~m ~nd, especiqtly, about ~).5 to 3 ,um If the ex~ netçr drops ~elow 0.5 ~m, the hoUow fiber has ~oo sm~ll a lumen and the flo~ of liqu~ds through ~e fibers is tm~l~d. If the exte~
di~m~r cxc-~s 3~ ~m, thc flc~ibility of dlc hollow fibers is reduced and ~ r~l separ~on mocl~les which a~ cons~uc~d from a plurali~y of im~n~c hollo~ fibcrs, beco o voluminous in rela~on to the amo m~ of p~ ~te flowing. P~cfe~ably, tbc ext~rnal diamcter is abo--~ I to 25 ~m ~nd, esp~ci~lly, I to 10 ~.m and par~cula~l~
5 lo 10 ~m.

Pursuant t~ ~he invention~ hoUow microfib~rs prefer~bly are prodùced as en~ ss fibers ~nd are of par~cular ~ e, if dleir wall thi~Pcc and Iheir exrernal diameter do not fluctuate by rnorc than t 6% and especi~lly by no~ more ~n 2.5%,~al isl dle hollov.~ mic~ofibers advanl~eollsl~lr are unif~ cons~ucted hollow microfibers. In practical appli~ationc, ~is means ~ha~ Ihe invennve hollow f~bers have uniform plOpC~uCS ~lon~ their leng~. Shon~ut fibers, wbich are produced by cu~ng endlcss fibers u) lene~, have d~e ad~ e ~at d~ey are f~e of needle fibers,~at is, of fibers with a leng~ lcss ~an 3 ~m, which arc re~arded as a danger to heakh.

Typically, thc ex~ernal ~ r of d~e in~,en~vc n~icrofibcrs is of ~he ord~r of about 7 ~m, ~h~ waU ~hic~ss is abou~ m and dle intemal di~uneter, ~hich corrcsponds ~o the extemal ~ nctPr of tbe lumen, is therefore of the order of 5 ,-m. ~ompar~d lo compac~ fi~ers withou~ a lllmen, there is dlus a savulgs of ma~enal and wdght of about l~ lo 95~ and t~,pically of about 40 ~ 60%. This is also ~CSo~i~d ~bith an appreciable red~ri~-n in ma~erial and rr~nl~hcnlnng CQSt~.

Wherever ~ere is men~on of "cerarnic ~,.a~.ial" wi~in the scope of the in~,en~ion, Ihis expression should be in~-pn,~d in dle ~id~st sensc. I~ Is int~:nded co b~ a col~cc~e name for marPri~lc, ~hich are buil~ up from inorganic and predonunan~y non-mctallic CG~ C and ~rc~.-~, in ~cular, mo~e than 3n!1~
b~ volume of cr~ls~lli7~l matenals. ln t~s conn~cn~n, we refer to the E2onlpp Chem~e r ~xi~nn 9~ efliti~n~ volume 3, 1~9~, pages 2193 t~ 219~. P~cfe~abl~, theinven~ve hollow c~r~n ir fibers consist of an oxidc, silicate. nit~idc and/or carbide ceramic mat~ial. Fspeci~lly l-~f~ c those in~nn~e hollo~4 cerasruc fibers, ~hich ~are based on al~n~num oxidc, calcium phosph~P- (apatite) or rela~ed phosrh~tf~s, porcelain-like or cordiente-li~e compositio~s, mulli~e, nt~ni~m oxide, ~P~rn~ S~ con - ., oxid~, ~irconiumsilicatc, ~co~ cs, spincllcs,emerald, sapphirc, conlndum, ni~ites or carbides of silicon o~ other'che~cal clrm~r~tc or their mix~ures. A~ doping a~ents, the m~ c l;nown in ce~cs, such as MgO. CaO, ZrO~, ZrSiO~, Y203, ctc. or deir precursors are optionally added to ~e ma~n ino~uuc comr~non~

The inventive hollow llucrofibcrs of cer~c mate~ls can be made porous and senupe~ k by comp~ra~ively mild balcing. Such h ollo~ nucrofibers can ha~re. in pamcular, an int~ l spccifi~ surface area, n~c~lred by ~ BET
melhod usu~ l~ogcn adso~puo~l or by mcrcury porosimetry ~n Ihc range of abou~
6~ to 2,000 m3/~,. pec~ of ~eir ~.o,g~c na~ure, they are usually h~drophilic and can b~ used for se~tu~e malcrials, ~e ~-ln~a~e p~efe~ably ~ Sjl~P from ouuide ~hrough dle ~ rnr~lo v.~all ~nwa~ds to the lun~en and leaving from ~e ends of *~e fibers, ~lth~!~Eh rnaterials can also be separated ~sl ~c rcversc direc~on.
These hollow microhbcrs c~u~ exhibi~ a ~ery good sep~r~on cffec~ he micro, ultraand nano range. The c.ials separated can b~ d~v~d n~icroscopir~lly on ~hc hollo~ m~crofibers. Such a matcrial scp~ranon can be used, for PY~rk, for purifyint~ or scpara~ing gascsl~as mix~les, such as air, or liquids, blood or water, and also for hot gases or melts. E:or this purpose, i~ is sometimcs ad~risable for tbe ho~lour nucrafibers to have a scptr~rion coating, the wall lhic~cis of which prefcrably is 2.5 ~Im or less and par~cularl~ ss ~han 05 ~m and espeei~ ven ~bo~l~ 0.1 ~ m. Thc separa~on coating may consis~ essc~lt~y of i~or~g7al~ic or orp7aruc nolcc~ r sie~eç, such as a zcolite malerial, or ~f jr~ 7~r o~ organic sepa~n layers, such as es~ers, siL~nes or s~ x~s Sl~ch la~ers a~e ~Pr]ip~ by known mPtho~l~ such as the CVD (Chc-~ic~l v~por ~ osi~ion) or PVD (physical vapo~
dep~si~on) melhods, g~ anic mcd~ s and/or s~ .,e..~ri~n medlods.

I-Iorcover7 dlc cer~ic hollow n~icrofibers, coated wid~ an inorg~c separat~on la~er, can bc ba~t. In ~d~ n. thcre is also the possibiliry of providing the green hollow microfibers, which a~e dcscrib~d ~n dle foUo~ulg, with thi~
separa~on cc~nng and to subjec~ this c~.."~s.te lo a bal~ng proccss.

The hollow cer~c microfibers can be made ~ght and iJ~ neabJe by a comp~ra~ively ~n~ensive baking. The cortcQ~ nQn of the mat~rial is ~ d e~peei~lly by sinlering ~ vinifying. P~ec~vse ~f ~c ~ighdy balced wall, such invennve hollo~ microfibcrs, can, in pamc~lar, be vacuum tight, conducl light ~ell and e~l~eci~lly be capable of fl~ and fly-ng.

P~ec~se of thcir inorg~c nature, ~e baked, in~entive hollow microfibers a~ corrosion reS;~r~n~ incomh!~sribl~, non decayable~ wcalher resictAnr, physiolo~ afe, bincorl~pati~l~o, uansparcnt, hea~ c~ nrlg as well as usua,lly elçc~ric~lly jn~ rmg and re~ict~lt to oYirlanon. They c~n be re~gard~d as safctyfibers and a~e usuaJl~ pc..,lcable lo elcc~ ...~"c~ic r~ tin~ They do not ha~ve ;uly of the carc~nogenic IJiOp~l~CS of needle minerals They ~re fre~ ntly ~lansparentand can be produced as colorless or colored fibers, ~i~pen ling on the coml~ss~ n and ~hc man~ ri~ condinonc~ ~lo~ev~r, ~hcy may also be opague. ~heu~ weigh~ pcr fiber lengrh (~ of the order of ab~t 10 to 100 gll~m (tex), and Iypically- about4~ glkm. Th~ u~vcncive hollo~ mi~rofibers ar~ usua~ly resist~nt to heat Md alte~na~n~ ~.npc.~.-r~res up ~o 1t)D0~C es~ci?lly up to 130D~C and even much higher.
Ttùs rle~n~s prcdominan~ly on ~c ch~ or~ rQ~irion.

Por r~ producnon of rhe in.rcnnvc hollo~ rnicrofibcrs, preferably an em--lcion, ~l;s~s on and/or suspension, which con~ins ~c precursors of a ceramicma~elial and a bhder, ~Yhich can bc removed by dle ac~n of heal, are ex~udcd ~n known ..~.,n~r r~ fonn ~rcen hollo~ nucr~rll.cl, and ~e b~nder is removed under the acion of heal Altenultivcly, rhe dispcrsion can be ~ppliP,d on a core of an or~anic con~ bcr. S~ J~nrl~, ~c corc as w~ll as r~e bindcr a~e renlovet by ~he ac~i~n of heat ~he dispcrsion n~y con~m varying ~rnol~n~c~ for ~Y~mr~e up to~90 by ~eighl and prcfc~bly 40 ~o ?0% by ~eight of dispersion medium. A
dispelsion medium could also be omirted if ~e binder is, for example, ~.,no~l~s~and carl be mel~d into a mass of low viscosily without ~ Iccon ~osition wor~h menri~ individual cases, it has nl~d ou~ rhat the green hollow microfibersalread~ malie advanr~geo!ls ~rrli- arion poscihili~if s ~ccecsihl~, so rhat rhe ~llbs-~luent heat ~rearmen~ is then om~d.

Wir~in ~c scope of the invcntion, Ihe follo~n~ come imo consideration as ceramic precursors: c~ m~nerals, c,~ kao~n, iUi~e, m~n~norilli~:, me~l hy~l~u~cs~ such as aluminum hydroxidc, ~cd me~al hydroxides / o~ides, such as AlC)OH, rruxed metal oxides / l~lid~s. metal oxides, such a~ BeO, MgO, A12~ Zr~2 and ThOl, metal ~ cs, sllch as Al(N03)3, rnetal ~lcohola~s, ~speciP~ aluminum ~lcohol~r~s, such as Al(iP~O)3, Al(scc-BuO)" m~g~PSi~ ulninosilic~t~, felspar, zeolite, boe,~u-w~ ~r ~n~rcs of nvo or mon of ~he ~na erials nauned.

Dunng the l~ nent of Al2(0H)5CI 2-3H20, d~e follo~ine r~ac~ons ~e place, for exan~pl~, consccu~el~ AI203 finally b~ing ob~incd:

Al2(0~I)5CI 2-3HzO ~ Al(oH)3~e~ 3 ~ C~Al2O3 ~ he average pa~cle si~e of ~e ccranuc precursor ma~erial preferabl is less than abollt 2 ~m and pamcularly less ~n 1 ~m, less t~lan about 0.1 ~-m be~n~
par~cularly preferred. Preferably, ~he ccrarnic precllrsor ma~ial is colloidal, ~at is, it cxi~rs as a scl or a gcl, or itiS ~nl~Culs~ly dissolved. Reversible conversions are rncci~ l~ ber~een ~e sol and gel fonn~. In a preferred ernl~ul;rnent, ~c binder, used within ~he scope of ~e inven~on, s~ch as polyvinyl alc~hol, ~ nns~ or proteins, can acl as a ~,.o~c~.ve colloid for ~e col~ni<l~l ccran~ precursor.

Thcre are no cri~icallimit~rions for selec~ing the binder, which can be ~emo~ in ~e scope of ~e unven~ve me~od by the a~tion of hea~. Ho~ever, a filnn-fornung bundcr is preferred. ~uch a binder ma~t for ex~n~rle, bc a urea, a polyvnnyl ~ h~ ax, g~l~nns, agar, pro~un, sacchalidcs. C~p~ona~y, ~1di~on~1 org ~ c ~Yili~ry n~ ~ ~ ~s, such as bLnders, scmdard~Lng ~gents, defoannerc and preserva~cs can be used. The nnLxoure oflbe precursor ofdhe cerannic l"a~l~ and the binder, which can bc rcmovcd under the action of heat, cxis~ un ~he fornn of a d~spersion, ~s cJ ncep~ bcing slefined bro3dly. Such a dispercion nna~ be an emn~cion or s~lspe.ncion, usually ~n tbe fo~n of a pa le. There is ml~ch l~ri~de in sçl~crinE the disper~ion nu~dium. ~'ç.~r~lly7 it ~ill be u,~ler Organic solvems, such as alcohoLc or ~cetone, optionally in, ~ixru~e with ~a~er, a~e also conceivable. Pamcul~rly ad.~antageolls here are dle so called sol ~el process~s. for e~plr, on ~he basis of dle a,lready addl~s~ad polyvinyl ~lcohol.

Prefelrcd dispersions coma~n about 20 ~o 70% hy ~eight of ceramic precursor ~ ial, about 10 to 40~ b)~ weight of binder, 0 ~o about 70% b~ weight of di~,~c.s.o~ mcdium and aboul 30% b~ ~eigh~ of op~ional comron~nr~.

In ord~r ~o mold ~he inven~ve hollow nucrofibers, any moldir g proc~cs~s are C~1itqhl*, espP~ lly dle blow molding proccss. the ex~usion process, ~e ~,acu.lm ex~rusion process or lhc spi~u~ing ylocess~ as ~4ell as ~e me~ ,s~ed in ~he WO 94t23829, in which a pasl:e is pro~ ced~ ~hich cont~inc a polymeri~

~inder. Ho~vever, for dlis process, exrli~itly a cer~ powde~ is used, which is ~onc~ n~ly in cryst~e fonn; ~is is usuall~ no~ sui~hl~ for carrying out thc inven~ve mcrhot. Ra~er, dle s~ec~ss aimed for pursuant ~o rhe inven~on is guaran~eed if such cry~r~lli~s are excl~

Thc extrudin8 can be carried ~ul as wel, mell or dry cxtlud~ a~ about r~om ~ nlre or at rhe tem~er~t~re of rhe n~elt of a molten orgaruc s~lbs~nr~ or suhst~nl~ rnncn~re~ Of *IC moldin~ pr sses addressed ~he sp~nrung mcdlod is of parcicular advant~e. This method is charactenzed in ~al ~he ~lia~sion is added to a f~e~lin~ cu.,~. or a pressure vessel of Sy~lfii ~c eql.ip..~ t, u~hich causes the ~lia~Jcrsion to flo~ through ~c $~ nr a~ a tc."~,a~re of about 20~ lo C and to b~ ~,esscd ~hrough the noz2le ring opcnings o~ nozzle profile oF~nin~c~the di~neler or uddtb of which p~cferably are aboul 0.l to 150 ~m the pa~al strearns~ produlccd in the region of Ihe nozzle o~ s are divided cen~ally by corcs and/or d~vices for blowing in a ~as and cDr.coli~d b~ w~g, by imldi~ion or b~ cntry of a rcaclanl ~nto ~e green bollo~Ar nucrofibers and op~ionally the binder is ~e...o~r~d by ~e acuon of ~ The spinn~ng eq~ .,n~nr addressed preferably is eq~ ,c,.t for ~he prol~lcnr~n of man-madc fi~ which ho-vcvcr, noed nOI be heated ~em~ally. Con-~er -inn~l ~....;..g equ.p.,.e..l c~n be uscd which ma~ have ~o be ~rt~ respect to the no~71~s and dle consolidaPon deYices Con~olid~n~n of d~e grecn hoUow llucrofibers taL;cs place fo~ eY~pl~. b~ ev?ror~n~)n of ~
dispersion rn~ m upon cxi~ng the noz;le oprn;ne IntO an en~i.c."Q~ t which has a lower prcssure Ihan the ~ir~;n~ pist~n. The concolir~ on can al~o ~alce place due to the en~ry of a r~ for dle buldcr or ~he ccramic rnatenal. The reac~ can be g~ ous and flo~ in lhc opposit~ direction ~o dle fibers dla~ arc bcing dischar~ed o~
be liquid and be presen~ as a precirir~ri~ ba~, through ~ich ~e discharged fib~rs are conduc~ed One ad-~an~ge of ~tle invenuve ~rhod lies ~he~in ~a~ hea~ing or wan~g of d~e ç~r~mic stamng matcrial for concoli~ ~ thc sta~ng muerial, ~4ith respect to the grecn hl)llow n~ flL~. s, ¢encraUy is required, if at all, only a~er ~e nn77ll~s. The spu~ uipmcM prefe~a~ly is designed so ~hat it has a high n~mher of nozzles. Thc duct, adjoining behind dlc extrusion head ha~,ing the nozzles, generally can be l~ept quite short and it is preferably abou~ 0.1 to 0.3 m long. In comparison to d~e man-rnade fiber production, thc spinning eq~ipment, despi~c ~efrequen~ly clearl~ low ~n.pc ~tutes of the mat~ial flow, can neverd eless be ope~ated a~ ~he same pr~ssurc le~rcl used for ~hc pro~r~on of man-made fibers. In tbe case of the inven~ve method, dlc tc.-lpc.-d~res of ~he material tlow ~sually are onlyslightly abo\~e room r~ ~rature.

For th~ mel~ c~:~rusion ~nP.Ihocl~, ~..lpc,~tures are ~lecu~d, which ~nsure good p~Dccssabili~y of d~e orgaruc mell so thal here, in the paro,cular case, ~emperatures up to 400~C are sekcr~d. Care rnUSI be ta~-cn to ensure ~at rhe m~tenal flo~v in thc spinning ell~.ip."~ is not intenup~cd.

The lumcn of dle hollow n~c-orlbcra can be produced by cores or devic~s, in~roduced into the par~al flows ul dle region ~f ~e nozzles for bl~wing ~n a fluid, such as oxygen, ni~rogen, air or a dit~cle.-~ gas mixture. The op~in~s of ~c nozzles can bc so designed that d~c b~ges~ p~csihl~ number of no~lPs, such as se~/eral rhouc~ in a ~~ery narro~4 space, are ~lispo~l in regular order. Ring openings ol profilc o~gs with a non-circular cross sec~on c~n mal~e do withou~
c~oss mcmber-~t~pcd ~ol~ g de~iOEs if, for example, one corc or sevc~al, opdon~
~wis~ed cor~s, such as very thin fib~n, are guided centrall~ in ~e flow dirccuon in a nozzl~. For a blow mol1in~ hod, i~luecnon nozzles w~hout or ~4ith onc or seva~al cores can be employed. Th~ noz21c op~ n~s ad.~essed for ~e ~,S~LC~VC
manuf~n~rulg medlods, preferabl~ have a ~ mP~r or a larges~ ~4idth of abou~ 150 f~m, and es~ci~lly of 120 ~lm, 8n or cven 50 ~m be~ng particularl~ p.cf .~d, espec~ally ~hen ba};ing ma,terials, which c~nn~ct more l~vil~. Paracularly for malerials, which con~act less, op~r-in~ are p~cfe~ably uscd, which ha~e a d~met~r or a larges~ vvidt~ ~f 9t) ~m and eis~ci~lly of 6D ~m, 30 ~Lm beulg ~cularly prefer~d. Under some c~rc~mct~nres, ~he openings are several timcs la~gcr dlan dle diame~ers ~r profile w~dth of t~e ba~ed, u~vcntive, hollo~ mic-of~.s, since veryhigb sl~e, which frca.ucntly ~nounn t~ 50 to 95%, for a s~uinl~ge from the final d. .-cnsion to ~e w~al dirncnsion (~cl~ ovc.~ ure)~ L~sually occur dunng baking. If the shrin~e is lcss, it is more lilcel~ ~o be in the range from 10 to 60%

~or thc invcntive spi~g m~thc~l, the sp~nnin~ speeds adv~nr~eenucl~
are berween 400 ~nd gOOO m/n~in.

ln con~as~ ~o ~e m~rh~ds for s~;",-il,e man-madc fi~rs, Ihc skin- and yanl-formin~ sFinrun~ cG,-,~o,i~ion frequemly leavcs the no~zlcs ar about room rurcs and not, as in dle casc of man-madc fibcr~, a~ ~e.,~r~res of a~ou~
2000 EO S~C. The parrial flows of the srirlnirlp compc~si~ions, which ~epr~scM the s~nds for for~lg the hollow nucrofibers, can be concolidaTP~I by h-~in~, by i~r~li~rin~ W, visiblc or IR light or ~y ~hc access or air ~o ~he green hollow microfibe~s and, at dle sa~ne t~me, npn~n~lly ~e dried.- Thc hea~in~ can be camed out. for insr~n~ e~ in hot air, Ul a hol convection currenl or by ~adiant hcat and usually ta~es place ar a ~ nlre up lo only I~ C. Afu~r ~c c-~ncol~ on of ~e g~een hoUo~ n~i~ fi~l,e.s, Ihc la~er can sPU be drawn, in order ~o charl~gc ~all Ih;- I~"çsses and e~Lternal ~ m-~~rs and to ~lnp~o~e or change ~e propc~,ics of the fibers, as ~4cll as the strength and op~ionally also ~he pe~mc~h~ . Before ~ey arc cor so!~, the strands (partial flows), torn~ing the hollow microfibers, have wall ~ic~;nesses prefc~ably of about 0.5 ~o 50 ~m, as well a~ e~crn~ diameters of 1 lo 160 I.m; aner they are conc~li~t~l, thcy have ~vall thiclulcsses prefcrabl~ of 0.4 ~o ~5 ~m and especLally of I to 25 ~m as ~cll as cxternal diame~ers of 0.8 t~ tSS ~In and c~,cci~lly of 8 to 55 ~m, a rangc of about 12 to 24 I-m bcing pari,cula~ly ~.c~e..ed The 1~) fl~cn~nnn in dle ~all thi~ ss and in dle external diamcter preferably are not greater than ~S% and, u- ~cl lar, not g~eal:er than ~2%.

After leaving dle nozzles and af~er being comc~ i by hcating, hollow microfibe~s, which arc IJsed without being bal~cd as hollow ~ex~lc fiben, can be wound up and opdonally also cut vw~hou~ dinon~l trea~rnent and process~d onceagain. They can be wovcn, ~litt,rA, telled, lmotted and otherwise proc~ss~l by tex~le means ~nd, if l~ lircd~ e~en m~lli7~d.

Hollow Inicrofibcrs, whether dley have been baked or not, can be u~d as w~lislcers, shorl fibers, long fibers, staple fibers, choppcd fibers, clulrhes of fibers, filaments, wo~en goods, nonwoven goods, knir~ oods, felts~ ro~vings, films, papcr layers, ya~ns, ropes, ne~s and ~e libe a,nd proccs~d furlht:r in~o ~n~ modules, s, p~eforms, ~JlCpl'~,S and the likc~. For cxample, a filarnen~ can bc produced as a layer camer ~n the fonn of a c~rcular disk or rec~angular la~er and op~onall~
proc~c~l fur~er by s~ inf seve~ fil- ., "1~ inlO mo~llf c. Such rn~dules can be used, for u~s~ancc, as lue~ul~la~c m~lules.

In the course of producing the ~nventive hollo~ microfibers, fi~rlher steps can be introduced dunng or after the pr~cess. The procescin~ of thc hollowmicrot~bers as ~ell as th~ ~rther proceC~;in~ is accomplishcd u~ s~eps, wt~ich are ~cno~n, and u~ own eqwi~..~r,t The ceramic hollo~ rl~icrofibers, which ha~e no~
been b~ed, can be bal~ed ~y ~n~th~ls kno~4n in indus~rial c~ran~ics, by which m~ans the cCraJrUC material is finished. ~he baking processes invol~ved a~e. for example, d~
follo~ing: gas b~;ing m~dlods, inert gas ba};ing mc~ods or clecmc ba~ing methods.

The invcntive, baked, hollow nucrofibers lulve ~e dimensions already deal[ ~ilh abov~.

Thc ~ l or green hollow micr~ c.s, as well as ~hc invenuve, baked, cer~nlir hollow microfibers have all ~e propemes, whicb are ~ypical of and requirc~ for tex~le fibers for being proc~s~d, for c~r~pl~ o cl~ hrc of fibers, fi~nenls, wo~ren ma~rials, ~i~d m~enals, fells, nonwov~ns and films. Pec~
of the fac~ ~t dle ul~cn~lve hollou~ microfibers arc subjcc~ o only ve~ sIight dul~cnsional fl~c~ ol ~, thc scar~r of dle extcrnal d~ters is very slight. For example, if the pr~ccss is carried out in an &~",op~ate manner, hollow n~icro~bcrs, which have a diameter smaller than 3 ~.m and can be reFarded as carci-lQP~ni-, are no~ prod-~ce~. The ulvcntivc hollow mic~ofibers can be used as so-called endlessfibers and arc, moreover) sho~-free. The~r can be produced in a. co~p~r~tively env~rOnmenL~ friendly ma~ulcr, are not harmfill ~o ~he env~ronmen~ and can, moreo-~r, be ~ec~cled. Thc~ can r~pl~cc l;no~n fibers wilhout a lumen and hollowfibers, as v~ell as wircs and s~rands, par~cularl~ of polymcrs, ~arbon/ e~c.

It must be rcgardcd ~s surprisiI~g that ~e cQr~oli~ , green, hoUo~4 microfibers can be wovcn and have a tensilc strengdl, ~l~h corresrru~C to d at of conYennon~l pol~mcnc t~ s MG~UV~r, i~ ~as surpnsing dlal the inven~e, cer~nir, hoUow lluc~ofibers can ~e provided with a sep~rarinr~ layer, which does not have a nonc~hl~ gradicnt benveen ~c sep~uon layer and thc fiber wall duling a fire. Il was fi~ ennore su~pnsi~g that *~ ent;ve, ceran~ic hollo~ microfibers, ~4hich were produced b~ ex~usion from a salr~ing material of pure ~lumin~ o~ud~
and which have a ~alJ dlic~ness of about 0.9 ~m and an cxrerr~l diame~er of abou~
6 ~m, havc a lensile ~ l. at brealc of 3,600 ~Ipa when measured in a lensile tesin~ machine cl~stomPrily used in indus~ial cerarnics.

The inven~ve, sen Iperncable, hollow nucrofibers, ul lhe fo~n of ~hiskers, shor~ fibcrs, long fibers, srapk fiben, ch~pp~l fibers"~ t~h~s of fib~s, filarnents, ~oven goods, nonwoven goods, kniued ~oods, felts, rovings, films, paper laycrs, ya~ns, ropcs, n~ts, e~c. can also be used as membranes, for e~ , for d~lys~ 4ell a~ for nucrodialysis and el~c~rodialysis, for osn~o..,P~. 5 for m~ Ul~r weight dc~.l.~a~on, as m~ r sievcs for scpaIa~ng ~quids and/or gases, as catalys~ sl~ppon, as fil~er for v~mscs, b~ s, fun~i, spores, d-lst~ ho~ gas. ny ash or carbon black, ~or hcat insu~ on as pie20 ceramic or as i..,~)h.~t such as a dialy~is, bone, looth or tissue support rr~n~pl~n~. Por ~ ;cra~on, the rc~e-n~r~ flo~ can pass dlrough an integral solid bed reac~or with a ~r~ g of ba~ed hollow microfibers, for this purpose, du pc-,..eatc prcfcrably passes out of ~e ir~-rinr of ~e hoUou, micrGfiberc through d-e o~nings of the l~men a.t the fiber ends or follows d~e revcrse pa~h For the ~ ering pre~ipiwnrs of hot gas fil~raoon, fo~ exarnple, of power plams, se.~ or ti~btly balced filur cloth is s~ira~le The invenuve, tight, hollo~v microfibers can be used in ~e form of ~hiskers~ short fib~rs, long fibers, slaplc fibcrs, choppe~l fibcrs, ~ tches of ~bers, filasTIcn~ voven goods, nonwoven g~ods, kni~ed goods, f~l~, rovulgs, films, paper layers, ~arns, ropes, n~s, etc., for ~erm~l ins~ nQnJ as a ~ ~r~re-resis~
conveyor belt, as ple20 cerarucs, as high-~lr~ rn tight fibers for ligh~ transp~rl, for example, for lascrs, as a mel~ed-on protcc~ive layer for space n~issiles, for seals and for linu~g ckc~onic ~elding cqui~.l,cn~, vacullm ~h~mhers, vacllum pumps and ouher ~,acuum fnrilin~s, as a mc~l-ceram~c crrr-rr.ci~, for co~nrocit~ , as reinfor~ t instcad of su~el and as other reinforcemcnt in cons~c~on, as elemen~
in el¢ctro-rheolo~y, for ex~nple, as Uquid camers and U~uid cond~lclors, D safe~y films for special papers and shee~, as gas-fillod shc~:ts, for cxample, f4r foods and storcd blood, as a c~rricr ma.terial for forger~-proof means of payment, for in~omhllctihle, non~ecajulg paper ~ alin~s~ as a mauix for rc.llfo~.ng mc~l mells ~r a~ a ma.~c ~f ~hin-~alled polymer components, sllch as bumper bars The inven~on is described ~n greater d~rail by means of d~e foUowin~
e~amples.

For pro~lucin~ a ccranuc hollow m~crofiber, spinnin~ ~qu.p.~ of Barnag, Germany ~as used, ~hich had becn adap~cd lo ~e special requirçn~rs of rl.fe.~ed invcnive mcrh~d wirh rc~c.t to rhe nozzle ~ t~r, dle pl~ mr,n~ of rhe cores an~/or dlc dircc~ons a~ which the gas is blo~n in rh~ opening rcgion of ~e nozzles and wirh reSpeCI to *le eqnirm~ nt for con~oli~nn~ ~ srrands. ~he ceralluc s~r~ng material used was a cer~r~ic co."l-~cir;~r from a sol-gcl process, which was ~b~a~ned b~ mixing All(OH)sCl 2.5~0 in waler with polyvinyl ~lcoh~l in water.
These ma~erials were used ~n rbe r~o of 20 kg of wa~er ro 40 kg of polyvinyl ~l~ohol ro 60 k~ of Al2(0H)5Cl 2 5H20. The sol, ~us produced, is con~er~ed into a gel ora sol-g~l by s~mng. The pasly comp~-s;~ n ob~ined was fi}led into a feedin~
con~ Pr of spinning equipment and fo~ced bubble-fiee ~i~ an ext~usi~n screw into*~e cx~uder head. The con~cying t~ a~lrC was abo~r. 25~C. The pasny composiri~n was mol~ by more than 3,~D ~nnllla~ nozzle O~.UJI~5 . in ~e exlruderhead, coaxial baMe pla~s in d~e region of each nozzle being uscd as cor~s. The underside of d~e exr~uder bead ~as rhrrn ally u~C~ from the drjing sha~t. The srrands ~cre hea~:d in the dr~ing shaft by IR r~di~tinn and convcc[ion hca~ ~o abou~
1~0~C and thcrcb~ dried and consoli~l~(l sufficicntly, so dlat dlere werc no unintended dimensional ch~n~rs during Ihe fu~hcr tex~lc h~n(~line~ ~hc ~ake-off spccd is about 1,2~0 m/min. The grcen ho~lo~4 microfibc~s ha~e a ~all ~hi~n~ss of about 5.5 ~m and an ~xternal diamcter of abo~t 33 ~m. EigUfC 1 sho~s ~hc n~h~
or green hollovJ microfibers By dra~ing ~hese ~recn hollo~ rrucfofibcr" ~hich s~ill ha~e a slight, se~ resid~lal mnicn~r~ contenl, in lhe ra~io of 1: 1.2 at room .npcrauJre, th~ waU rhit~ oss was changed lo abo~lt 4 5 ~m and the ex~e~
n~r~r to a~o~t 28 ~m Thcse h~llow mic~c~rl,c.s, after belng hea~d slo~ly lo about 16()0~C, ~ere l~ept for I hour at ~is ~.n~.~tllre and ~hen allowed t~ cool slow}y. The balced hollow m~crofibers had a wall thirl~n~sc of about 0 9 ~m and an e1~temal ~lia~r of a~ou~ 6 ~ m.

FY~

The ~n~endve hollow microfibers werc produccd as ul Exasnple I f~om a con~rcinon based ~n porcclain or a 7~olirir .. a~,~si~l-n alun~ osilira~. ~n Tablc l, ~c avcra~ dimcnsions of ~e ~nv~n~ive~ b~ed hollow llucrofibers and their ,lopc-~es ~e c~mpared with dlose of kno~ fibers.

Table l:
A1~03 Porcelain Base ~eolile E-G~ass Carbon Aramide 7 (M~g-AJ silicale) S-Glass (Example 1) (Example 23 (PxP~le 3) ExteJT~ diame~r 6 7 7.5 12 6 8 12 ~um - Lumen di~ er 4.2 5 5 3 5 ~ithou1 lumen ~vi~houl lumen ~m - ~lall ~hickness 0.9 ~ 1 1 3.~ wilhou~ lumen wi~ou~ lumen ~m ~n Tl~n~c&l densit}~ ~.9 25 2.4 2.0 1.8- 2.0 1.45 0 6~ glcc ~/bn 42 39 37 24~ ~, (lex) O
T~sile s~englh 3.6 1.7 - 3.5 1.86 - 5.~ 3.0 0 GPa -

Claims

1. A hollow microfiber of a ceramic material, having a wall thickness of about 0.01 to 15 µm and an external diameter of about 0.5 to 35 µm, characterized in that the fluctuation in wall thickness and external diameter does not exceed 6%.

2. The hollow microfiber of claim 1, characterized in that the cavity is constructed in the form of a lumen.

3. The hollow microfiber of claims 1 or 2, characterized in that the wall thickness is about 0.3 to 6 µm especially about 0.5 to 3 µm.

4. The hollow microfiber of claims 1 or 3, characterized in that the external diameter is about 1 to 25 µm and especially about 1 to 10 µm.

5. The hollow microfiber of one of the claims 1 to 4, characterized in that it consists essentially of a oxide, silicare, nitride and/or carbide ceramic material.

6. The hollow microfiber of one of the claims 1 to 5, characterized in that is has a semipermeable wall.

7. The hollow microfiber of claim 6, characterized in that is has a tightly baked wall.

8. The hollow microfiber of claim 6, characterized in that a separation coating is present on the outer sheath of the wall.

9. The hollow microfiber of claim 8, characterized in that the separation coating has a wall thickness of 2.5 µm or less.

10. A method for the production of hollow microfibers, characterized in that a dispersion, which contains the precursor of a ceramic material and a binder, which can be removed under the action of heat, is molded in a known manner into green hollow microfibers and the binder optionally is removed by the action of heat, so that hollow microfibers result, with a wall thickness of about 0.01 to 15 µm and an external diameter of about 0.5 to 35 µm, the fluctuation in wall thickness and external diameter not exceeding 6%.

11. A method for producing hollow microfibers, especially according to at least one of the claims 1 to 9, characterized in that a dispersion, which contains a precursor of a ceramic material and a binder, which can removed by the action of heat, is molded by a known manner into green microfibers and the binder removed,optionally by the action of heat.

12. The method of claims 10 or 11, characterized in that the green hollow microfibers are produced by blow molding, extrusion, vacuum extrusion or spinning.

13. The method of claim 10 or 12, characterized in that the spinning method is employed, wherein the dispersion is added to a feeding container or a pressure vessel of spinning equipment, pumped at a temperature of about 20° to 400°C through the spinning equipment and pressed through nozzle ring openings or nozzle profile openings and the partial flows, generated in the region of the nozzle openings, are divided centrally by cores and/or devices for blowing in a gas andconsolidated by heating, by radiation or by entry of a reactant to form green hollow microfibers.

14. The method of one of the claims 10 to 13, characterized in that the green hollow microfibers are consolidated into semipermeable hollow microfibers by baking.

15. The method of one of the claims 10 to 13, characterized in that the green hollow microfibers are consolidated into tight hollow microfibers by baking.

16. The method of one of the claims 10 to 15, characterized in that a separation layer is formed on the green or ceramic hollow microfibers.

17. The method of claim 16, characterized in that the hollow microfibers, coated with an inorganic separation layer, are baked.

18. The method of one of the claims 12 to 17, characterized in that endless hollow microfibers are produced.

19. The method of one of the claims 12 to 18, characterized in that the hollow microfibers are processed into whiskers, short fibers, long fibers, staple fibers, chopped fibers, clutches of fibers, filaments, woven goods, nonwoven goods, knitted goods, felts, rovings, films, paper layers, yarns, ropes or nets.

20. The method of one of the claims 12 to 18, characterized in that hollow microfibers are processed further into filament modules, laminated, preforms or prepregs.

21. The use of ceramic hollow microfibers of at least one of the claims 1 to 9 for the production of membranes, molecular sieves, catalyst supports, filter, piezo ceramics, implants, high-temperature resistant conveyor belts, melt protection layers, metal-ceramic composites or other composites, reinforcement in construction, elements in electro-rheology, safety sheets, gas-filled sheets, support materials, incombustible and non-decaying paper qualities, the matrix of metal melts or thematrix of thin-walled polymer components or electro-elements for refrigeration, for osmometers, for thermal insulation, for the transport of light or for seals and linings.

22. The use of green hollow microfibers, produced according to the method of at least one of the claims 10 to 20, for the production of membranes, molecular sieves, catalyst supports, filters, piezo ceramics, implants, high-temperature resistant conveyor belts, melt protection layers, metal-ceramic composites or other composites, reinforcement in construction, elements in electro-rheology, safety sheets, gas-filled sheets, support materials, incombustible and non-decaying paper qualities, the matrix of metal melts or the matrix of thin-walled polymer components or electro-elements for refrigeration, for osmometers, for thermal insulation, for the transport of light or for seals and linings.