Note: Descriptions are shown in the official language in which they were submitted.
`~ WO ~4/1~704 i~ 14 8 ~10 PCTJIJS93/11592
.
METHOD FOR RAPID DRYING OF A POL`~BENZAZOLE F13ER
This is à continuation-in-part of copending application Serial Number 071985,080,
. filed on December 3, 1992.
; The present invention rela1:es to improved processes f~r drying polybenzazole
. fibers. Polybenza~ole ("PBZ") fibers include polybenzoxazole t"PBO") or polybenzothiazole
("PBT") fibers.
Lyotropic liquid crystalline PBZ is typically made into fibers by dry-jet, wet-
-spinning techniques, in which a solutioll that contains the PBZ polymer and an acid solvent
10 Iknown as a "dope" ~ is spun through a spi nneret to form dope fila ments, that are combi ned
into one or rnore dope fibers. These dope fibers are drawn across an air gap, and then
¦ ' contacted~with a fluid that dilutes the solvent and is;a non-solvent for the ~polymer. This
¦ contact with fiuid causes the polymer i:t) separate from the solvent. See jointly owned,
Allowed, U.S. Patent Applic~tions number 07/985,079 (Method for Spinning a Po!ybenzazole
Fiber3 and 071985rO78 (Method for Rapid Spinning of a Polybenzazole Fib,er) for a description of
the PBZ fiber spinning process. ~
~ The process of separating the PBZ polymer in the dope fiber from~the solvent in
; ' the dope fiber is known: as coagulation. After coagulation, most of the remaining residual
solvent is washed/leached from the coagulated fiber, leaving the fiber wet. ~See jointly owned,
~ 20 U .S. Patent Applicati on number 08/1 10,149 ~lrnproved Process for Coagulation and Washi ng of
i ~ Polybenzazole Fibers) for a descripti~n of the coagulation process.
Polybenzazole fibers typicaliy contai n a very high degree ;of residlJal moisture
` ~ after they'are washed. The residual moisture content is frequently betviJeen 30 and ~00 weight
~: ~ p~rcent,~and may even~be~hi~her in some fibers. The percent residual nnoisture content,
~25 ~ (hereinafter percent RMC) is~calculated on a parts per hundred basis~as follows:
initial fiber weight - driedifiber weight)ldried fi~er weightl x 100%.
For many reasons it is desiraùle to reduce the amoun~ of residual moisture in the
h ~ fiber by drying the fiber. ~One o~the reasons it is desirable to redu~e ti~e amoun~ of residual
moisture in the fiber is to enable the fiberto be heat treated without damaging the fiber. Heat
30 I treati ng of ~ dried fi bers' can be and is done to i mpro~e the fi bers' physical prope ti es It l~as
been discs:)vered that PBZ fiber~can be da~maged by exposing it to the typi~al amount of heat
(al~out 400C~used~i n heat treati ng whi le~the~fi~ber contai ns more than about twelve percent
RMC. Therefore,~ in~order to be heat~treated vvithou~ being damaged, a; PBZ fiber usually must
ha\~e a' percent RM of;1ess than about twelve p~ercent. ~ ~ ~
35~ In order to reduce the amount of resldual moisture in the fiber to below twelve
: : percent RIVIC prior to the fiQer being heat~treated, it has heretofore been necessary to dry the
fiber for over 49 ho~Jrs at 65C. It is economical!y undesirable to dry the fiber at this low
temperature,~because low ~emperature drying is, as noted above, very tim~-consuming and
, , ~
WO94~1~7~ t ~8~1 0 PCT/VS93/11592 -`
thus, very costly. It has been found that increasing the temperature of the drying process will
speed up the drying process but can also cause damage to the fiber. This heat induced damage
applears as visible voids in the fiber. These voids are highly undesirable for all PBZ fibers.
Therefore, it is desirable to develop a process that allows for rapid clrying o~ PBZ fiber without
5 causing damage ~o the fibers.
The present i nvention is a process to rapidly dry a polybenzazole fi ber that
initially contains more than 30 percent residual moisture content, while minimizing damage to
said fiber, said process comprising the step of exposing the fiber sequentially to two or more
ternperatures, wherein said temperatures are set relative to the percent re~idual moisture
10 content of said fiber, and wherein each temperature set is hotter than the previous
ternperature, vvhile allowing for brief periods of non-contact time during drying in which said
fi ber is not exposed to the full set point temperature, wherein the fi nal percent residual
moisture content of the fiber after it has been exposed to said two or more temperatures is
about twelve percent or less.
The second aspect of this invention is a process to rapidiy dry a po!ybenzazole
fiber that initially contains more than 30 percent residua! moisture content, while minimizing
damage to said fiber, said process comprising the step of exposing the fiber sequentially to two
. or more temperatures, wherein said temperatures are selected relative to the percen~ residual
I ~` moisture content of said fiber, and wherèin each temperature seiected is hotterthan the
; ~ o previoustemperature, and wherein the final percent residual rnoisture content of the fiber
~:~ : : ~ ~ after jt has:been exposed to said two or more temperatures is about tweive percent or iess.
:~ ~ Figure 1 shows a: plot of Percent Residual :Moisture Content of Polybenzoxazole
Fiber vs. Temperature in C. On this Figure~there i~s a negatively sloped curved line 10
representing the boundary between an area 30, representing 'safe" drying conditions and an
I ~ 25 area 20, repr~senting Nunsafe^~drying conditions. This~line 10 i~s referred to as the non-damage~
d~ying ("NDD") line for PB~) fiber. ~
Figure 2 shows the NDD line I O on a plot of Percent Residual Moisture Content of
Polybenzoxazole Fiber vs. Temp~ratùre in C, along with a series of vertkal and horizontal lines
12representingthedryingprofileforaPBOfiberwherein~hetemperaturethePBOfiberis
30 expose~dto~iscontinuousl:ylncreasedast~iemoisturecontentofthefiberisredyced. Inthis
Figure:~he drying profile all takes place on the "safe": area 30, of the plot.
Fig~ure 3 shows~the NDD~;line 10 0n a plot of Percent Residual Moisture Content of
Polyi~nzoxa201e Flbèr vs.~:Temperature in ~, along with drying~profile lines 1~and 2
representi ng ~the reduction of RMC in~two separate PBO fibers~as they are exposed to
35 progressively elevated temperatures.
: As us~d herein, the term ~oiy~enzazole ("P8Z'i) includes polybenzoxazole
PBO") nomopolymers, polybenzotniazole~("PBT") homopolymersa:nd:random, sequential
and bl:ock copolymers~of PBO:or :PBT.: Polybenzoxazole, polybenzo~hiazole and randorn,
!J
.~ W094/~704 ~ PCT~S93111592
.
!
sequential and block copolymers of polybenzoxazole and polybenzotniazoie are described in
references such as Wolfe et al., Liquid ~sta!line Polvmer Com~osi~ions. Process and Products,
U.S. Patent 4,703,103 ~October 27, 1987); Wolfe et al., Liquid Crvstalline Polvmer comDosition
Process and Products, U.S. Patent 4,533,692 (August 6, 19~5); Wolfe et al., Liauid Crvstalline
5 ~=_~, U.S. Patent 4,533,724 (August 6,
1985); Wolfe, Li~uid Crvstalline Polvmer Com~osltions, Process and Products, U.S. Patent
4,533,6g3 (August 6, 1985); Evers, Thermooxidativelv Stable Articuiated p-Benzobisoxazole and
p-Benzobisthiazole Polvmers, U.S. Patent 4,359,S67 (November 16, 1982); and Tsai et al.,
IVlethod for Makinq He~:erocvclic Block Co~olvrner, U.S. Patent 4,578,432 (March 25, 1986).
Units within the PBZ polymer are preferably chosen so that the po)ymer is
Iyotropic liquid-crystalline. Preferred monomer units are illustrated in Formulae (a)-(h). The
polymer more preferably consists essen~ially o~ monomer units selected from those illustrated
in (a)-(h), and most preferably consists essentially of a number of identical units selected from
those illustrated in (a?-(c).
. 15
_~/N ~ N\>~
cis-polybenzoxazole
Poly[benzo(1,2-d:5,4-d')bisoxazole-2,6-diyl-1,4-phenylene]
(b~
: 25
~; trans-polybenzox~azole
: Poly~benzo(1,2--d:4,5-d')bisoxazole-2,6-diyl-1,4-phenylene]
~ 30
: ~ :
`
- 3 -
`
( W094/127~ 21~ 8 ~ 1 ~ PCTN593/11592 ~
~'
(c~ ~ ~ N ~
trans-polybenzo~hiazole
(d) ~
.
cis-polybenzothiaæole
, ~ ~ N \~ :
0
AB~PB0
Poly(2,5-benæoxa201e;)
~ ~ ~ ~ ~ N ~
: ~ AB-P~O : - :
Poly~(2:,6-bPnzo~azo1~e~
, a n d:
Poly(2,5 benzothiazo1e): : : :
5~ Poly~2,6-benzoth~zole~
W0 9411Z704 2 7. a~ 0 PCT/US93/11592
Solvents suitable for formatic)n of dopes of PBZ polymers include cresol as well as
non-oxidi~ing acids capable of dissolving the polymer. E~tamples of suitable acid solvents
include poiyphosphoric acid, methanesulfonic acid and highly concentrated sulfuric acid and
mixtures of those acids. A highly preferred solvent is polyphosphoric acid or methanesulfonic
5 acid. A most highly preferred solvent is polyphosphoric acid.
The concentration of the polymer in the solvent is preferabiy at least about 7
weight percent, more preferably at least about 10 weight percent and most preferably at least
about 14 weight percent. The maximum concentration is limited primarily by practical factors,
such as polyrner solubility and, as already described, dope viscosity. Because of these limiting
10 factors, the concentration of polymer is;usually no more than ar out ~wenty ~Neight percent.
Suitable polymers or copolymers and dopes can be synthesized by known
procedures, such as those described in Wolfe et al., U.S. Patent 4,533,693 (August 6, 1985);
Sybert et al., U.S. Patent ~,77~,678 (September 20, 1988); and Harris, U.S. Patent 4,847,350 (July
1 1, 1989~. PBZ polymers can be advanced rapidly to high molecular weight at relatively high
tem peratures and high shear in a dehydrating sc lvent acid, according to Gregory et al.,
U.S~ Patent No. 5,089,591 ~February 18, 1992).
Makinq The Fiber
The dope is spun into fibers by known dry jet, wet-spin techniques in which the
; dope is spun through a spinneret to form dope filaments that are collected together to form
I
20 one or more dope fib~rs. ~Fiber spinning techniques for PBZ polymers are known in the
references already men~ioned in the ~ackground of the Invention section.
After passing through an air gap the dope fiber~s) is~are corltacted with a fl uid
that dilutes the solvent and is a non-solvent for the poiymer. This process is known as
coagulation. After coa~ulation, most of ~he ~remai ning resi d ual solvent is washed/leached from
25 eachfiber, leavingthefiberwet. See jointiyowned,U.S. PatentApplicationnumber
081110,14g (Improved Processfor ~oagulati~n and Washing o~ Polybenzazole Fibers), for a
;~ ~ d~scription of the coagulation pracess.
~ . ~
Drvin~ The F_ers
The arnc unt of residual moisture i n the fi ber after it has been washed can vary
30 from more than 30 percent RMC all the way up to ~00 percent RM~. As mentiolned previously,
there are many reasons tc dry fiber, one of thern being that it is necessary to remove alI but a
tinyamountofthem~isturein~thefiberinordertoavoiddamage~othefiberuponheat
treating. Therefore,~at the conclusion of the desfribed drying process it is desirable that the
percent residual moisture content of the fiber should preferably be twelve percent RMC or less,
35 more preferablyten percent RMC or less, more highly preferablysix percent RMC or less, mos~
preferably four percent RMC or less and most highly preferably two percent RMC or less.
5-
::
I, WO 94/12704 21 ~ 8 S l O PCT/US93tll~;92
., .
It has been found that selecting the highest temperature for rapict drying of PBZ
fiberwithout damaging the fiber depends, in an inverse fashion, upon the moisture content of
the fiber face out. The i nverse relationshi p is such that the less moisture i n the fiber, the higher
the temperature the fiber can be exposed to without damaging the fiber. As the drying
5 process continues and the moisture content of the fiber decreases, it is possible to increase the
temperature the fiber is exposed to without damaging the fiber. The way to optimize
(meaning increase) the drying rate for PBZ fiber is to increase the temperature the Tiber is
exposed to as fast as possible, but withou~ exceeding the maximum safe temperature for each
specific amount of residual moisture of the fiber.
Data has been collecled regarding the relationship between percent RMC and
temperature;ordryingofPBOfiber Th~plotof~hisdatahasyieldedtheNDDline10shownin
Figures l, 2 a~nd 3. This NDD line represents the maximum safe temperature that a~ PBO fiber
can be exposed to at each specific percent RMC without causing drying-induced damage to the
fiber. : ;
The NDD line acts as a boundary between areas of "safe" drying conditions, area
30 on Figure 1, and:"unsafe"~drying conditions, area 20 on Figure 1. The highest drying
temperaturesthatcanbeselected~for~dryingeach:PBO~fibercanbechosensimplybyknowingthe percent RMC o~ the fiber when~it wili first be exposed to the temperature. it is desirable to
select the highest drying temperaturè possible for each fi ber perce:nt P~MC i n order to mini mize
20 : the amount of time it takes:~o dry the fi ber down to about tvuelve percent RMC or less. The
number of drying temperatures used can be selected as a matter~of:process convenience,
though it~has been found desirable:and necessary to have two~or more drying temperatures,
;:with~each~temperatureselected;tobéprogressivelyhotterthantheprevioustemperature,in
. ordertorninimize;theamount:of~timeittakestodrythefibertoa~ercentRMCabouttwelve
25 ;per~ent~orless~
Figurè~2 illustrates~a muItiple-temperature drying process in which twenty-three
progressively hotter temperatureslare used to dry a PBO fiber frorn a starting percent RMC ~ :
a~ove forey ~percent to a~fi nal~: percent R VlC~ below five percen~. The tem pera~u res sel ected
relative to the percent RMC of the fi ber in this dryi ng profi le are as close as possi ble to the N DD
30 ~ i ine without crossing it. This manner of seiection ins'ures the most~rapid drying plrocess fo~r the
fiber without creating voids in the fibe:r d~uring drying. i : : ~
The~morphology and physical state of the PB~ fiber being dried can vary with the
dope composition,~ the polymer form:ulatl on and the specific fiber processing conditions,
therefore,~the~ hig~est~temperature a~PBZ fiber can l~e exposed to~at each percent RMC withou~
3s ~ being damaged~ can vary. ~ As~a consequence of this, ~he NDD line~for each PBZ fiber and for the
same PBZ polymer~p~rocessed~:under different conditions, can and will vary, with the amount of
varlance dependlng~ upon the~àegree in~differences between any~or all of, but rlot limited to,
tbè ~ollowing factors
: ` WO 94/12704 ~ L G PCTIUS93/11592
a) Processing damage within the fiber, prior to i~s being dried,
; b~ Porosity of the fiber,
c) Fiber processing conditions,
d~ Residual chemicals such as residual ac;ds, impurities, or
e~ Additives or processing aids in the fibers.
In practice, one type of standard equipment used to dry fibers includes matched
pairs of heated rolls. The fiber is wrapped over these rolls many times in order to increase the
amountoffontacttimethefirerhaswiththeheatedroll. Contacttimeisdefinedasthe
amount of time the fiber is in direct contact with the set point temperature of the heated roll
(or other heating device that can be used for drying P~Z fiber~. It is assumed that a fiber in
contact with a heated roll is at: the same temperature as the surface of the roll. It is aiso
assumed that the surface temperature of the roll is the same as the set point temperature of
the roll, that i5, a heated roll with:a set point temperature of 1 ~0~C should have a surface
temperatureofl80C. Thesetpointtemperatureofaheatingdeviceisdefinedhereinasthe
15 temperature the heating mechanism of the heating device is set at.
In addition to contact tirne with the heated rol 1, the fiber must travel between
each pai r of heated rolls before it recontacts a roll or before it travels on to the next pair of
heated rolls. The time the fiber is not in contact with a heated roll or any other direct source of
heat during the dryi ng process is referred to as non-contact time. The total residence time of 3
20 fiberduringthedryingprocessisthecontacttime(CT)plusthenon~contac~time(NCT3. When
the fiber is not in direct contact with a hea~ed roli, the fiber temperature is less than that of the
heated roli. Therefore, this invention cont~mplates that as the fiber is exposed to progressively
increasing temper~tures, that if the fiber is being dried by heated rolls, then there will be brief
mornents during the drying process when the fiber is not exposed to the fw11 set point
2S temperature of the heated rolls.
It is belieYed that the fiber con~inues to undergo drying during its NCT with the
heated roil, butthatthedryingofthefiberduriny NCTisnotasefficientdryingasthatdrying
: the fiber undergoes during its CT with the heated roll. One way to increase the efficiency of
the drying process is to insulate the cabinets that ~he pairs of heated rolls are usually positioned
30 in, and to blow hot air or a gas that does not damage the fiber, such as nitrogenJ heliuml, argon
or carbon dioxide, ~into the cabinets so that the temperature throughout the cabinet is the
same as the set point temperature of the heated rolls. Another way to more efficiently dry
fibers is to pass them through progressively heated oYeris in which the ternperature of each
oven is progressively increased such that the fibers are continual Iy exposed to ~he set pOi nt
35 temperature of each oven. With both of these more efficient methods of drying the residence
timeofthefiberism~deupofoni~ycontacttimewi~houtanynon-contacttime. Contac~time
is, as has already been~stated, much more efficient drying tirne than is non-con~acttir.~le. With
these more efficient methods of drying, it is pojsible IO reduce the residence ~ime reauired to
: : :
~ ~ . o o . : o ~ o o ~ 1 4 ~ ~ 1 - ~?
~ O ~
.; ' . ,
reach a certain percent RMC in the fiber as foilows. To achieve a certain percent residual
moisture content in a fi~er using drying condi~ions with oniy contact tirne (such as dryiny the
fiber in continuous ovens or using drying rolls positioned in insulated cabinets with means to
5 keep entire interior of cabine~ a t set point temperature of heated rolls), the residence time to
achieve a certain percent RM is about tw~thirds or less of what is required when drying is
carried out with the contact time and a non-contact time component (such as when drying is
carried out using heated rolls positioned in non-insulated drying cabinets).
The total amount of residence time, when there is both a cr and a NCT
10 component to the residence time, required to dry a PBZ fiber to less than about twelve percent
RMC is no rnore than about 20 rninutes, preferably no more than about 1û minutes, more
pre~erably no more than about 5 minutes, and most preferably no rnore than about 3 minutes.
The total amount of residence time, where there is only a CT component ~no NCT component~
of residence time, required~to dry a PBZ fib~r to less ~han about twelve percent RMC should
15 preferabiy be no more than about 6 minutes, more pre~erably be no more than about 3
minutes, and most preferabiy be no more than about 2 minutes. The total amourt of residence
time, when there is both a CT and a ~CT component to the residence time, required to dry a
P6Z fiber to less than about~iwo percent RMC should preferably be no more than about 20
~ ~ minutes, more preferably be no rnore than about 15 minutes, and most preferably be no more
; ~ ~ 20 ~ than about 10 minutes. The total amount of residence time, when there is on!y a CT
compon~nt (no NCT component) of resldence time, required to dry a PBZ fiber to a level of
percent RMC of less than two percent P~MC should preferably be no more than about 14
; ~ minutes, more preferably be no~more than~about 10 minutes, and most preferably be no more
~ than about 7 minutes.
; ~ ~ 25 ~ Inordertodrythefibertoacertainresldual~moisturecontentintheamountof
tirn~ specified in the preceding paragraphs, it is desirable to start the drying process~at a ~cer~ain
rninimumtemperature.~Accordlngly,themjnimumfirsttemperaturethefibershouldbe
exposed t~ is at least about 140~t;, preferably at least about 1 50C, more preferably at least
about 1 60C, more highly pre~erably at least about 1 70C, and most preferably ~at ieast about
- 30 - ~80C It is desirabJe t~ mln;mize the amotlnt~f time it takes to dry the fiber. -~t has been-found
t~at; selecting intermediate proc~ess temperatures close to those tempera;~ures on l:he N DD line,
without going higher~than those temperatures on the NI~D~ line (as illustrated by the series of
vertical~ahd horizontal linès 12 in Figure 2) allows ~he most rapid drying of P~Zfiber, without
creating voids. Typically~ final drying temperatures do nvt excess 300C, preferabiy do not
;35 ~exceed~280Candmostpreferablydonotexceed~260~. ~
:
AM~J~ SHtET
~1~8610
WO 94/12704 PCT/US93t11592
The drying process is concluded when the percent RMC of the fiber has reached
the desired level. Drying is preferably continued until the fiber exiting the drying equipment
contai ns at most a~out twelve percent RMCI preferably at most about 10 percent RMC, more
preferably at most about ~ percent RMC, more highly preferably at most about 6 percent RMC,
5 most preferably at most about 4 percent RMC ancl most highly preferably at most about 2
percent RMC.
After the fiber is dried, it may optionally be heat treated to improve its physical
properties. Heat-treatment of PBZ fii~er i5 described in jointly owned, Allowed, U.S. Patent
Applications serial numbers 071985,068 (Rapid Heat Treatment Method for Polybenzazole
Polymer~ and seria! number 07!985 067 (Steam Heat-Treatment Method for Polybenzazole
Fi ber) .
Operating by this drying method permits rapid drying of PBZ fiber while
rninimizing damage to the fiber. Minimizing the amount of damage inflicted upon P~Z fiber is
; desirable.
The following exarnples are for illustrative purposes only. They should not be
taken as limiting the scope of eitherthe specification or the clairns. Unless stated otherwise, all
parts and percentages are by weight.
In these examples, the percent residual moisture content (percent RMC) is
determined by a gravirnetric method as follows: Approximately 0.5 grams of fiber sample is
20 col lected and weighed on a balance. The samples are heated in an oven at 250C for ~hirty
mi nutes to remove the residual moisture and weighed agai n. The percent RMC is determined
by calculating [(initial sample;weigfit - dried sample weight)/dried sample weight3 x 100
percent.
In these exampies, the void con~ent and distribution are determined using a visual
2S microscopic rnethod. Three inch iong samples of fiber are cut and end-taped on microscopic
siides and observed under a light rnicroscope at 2~0X magnification. Voids usually appear as
blotches or dark striations alorsg the fi ber. ~ They can vary in size, number and thickness among
fiber samples. The void content is qualitatively rated as void free, slight voids and many voids.
. ;ExamPles
30 ~xam~eof DamaaeDrvinqand~ ~ Drvin~
A spinni ng dope that contains 14 percent cis-polybenzoxazole (l.V. 30 gldL~
~; dissolvedinpolyphosphorica~cidwasextrudedat160~fromaspinneret~hatcontained 166
orifkes, with each orifice having a diameter of 0.22 mm. The resulting filaments were drawn
;; across an air gap of 22 cm and irnm~rsed in an aqueous coagulation bath maintained at a
35 temperature of about 22C. The~filaments awere combined into a fiber and the fiber was
washed with water as it passes sequentiai!y over rolls.
~ ~ ~ 9
¦ ~ WO ~41I2704 214 ~ ~1 0 PCTN593/11592 ~
The fiber was dried using 3 matched pairs of heated drying rolls Witil each pair of
heated dry~ng rolls set up in separate, uninsulated drying cabinets. Each pair of heated drying
rolls has the same set point temperature. The residence time in each cabinet is the sum of the
amount of time the fiber is in contact with the rolls (CT) plus the amount of time the fiber is not
in contact with each roli (non-contact time or N~T~. After drying, the physical properties of the
dried fiber are measured.
Figure 3 shows the drying profile lines of the fibers described in the followingexam ples.
Comr~arative Exam~le
In Figure 3, the line marked t was the drying profile line for Fiber 1. Fiber 1
wasmoved at 200 meters/minute through the drying process. Drying profile line 1 for Fiber 1
show that this fiber was dried at 180C ~residence time 42 seconds), until its moisture level was
below~25 percent, then it was dried at 240C (residence time 121 seconds) until its moisture
level was below 15 percent. The d!ying profile li ne 1 crosses the NDD line at position 5. The
fiberhadatensilestrengthof33.8g/d(4.66GPa),atensilernodulusof 1671 9/d~230GPa)and
an eiongation to break of 2.46 percent. ~This fiber had many visible voids present. Tl~tlS FIBER IS
NOT AN EXAMPLE OF THIS INVENTION.
ExamDle of the lnvention ~ ~ ~
In Fi~ure 3,~ the line rnarked 2 was the drying profile line for Fi ber 2. Fiber 2 was
moved~at 100 meters/rninute through the drying process. Line 2 shovYed that Fiber 2 was dried
first at 170C (residence time of 84.3 secc ndsj, urtti! its moisture level was below 20 percent
then it was dried at 200DC (residence time of 84.3 seconds) until its moisture level was below 10
percent and then it was dried at 240C ~residence time of 79.3) until its moisture ievel was
below 3 percent. At a total residence time of 4.1 minu~es, this fiber's ending percent RMC was
3-0 percent. When the amount of residence time for this fiber at ~40C was extended to t 58.6
secorids, the fiber's ending~percent residual moisture content drops ~o 1.0 percerlt. At no time
does the drying profi le line,2, for this fiber cross the Nl: D li~ne This fiber had a tensile strength
of 38.0to~39.3g1d(5.24to5.42GPa~,atensilemodulusof1616to1624g/d(223~o224GPa]and~
an elongation to break of 2.86to 3.00 percent. This fiber did not have visibie voids at the
conclusion of the drylng process.~
., ~
- 1 0-
:
IJ
. . WO 94/12704 ~ 1 ~ 8 ~ ~ a PCT/US93/11592
~,
Example of Drvi q Usinq Contact Time and Noncontact Time versus Drvinq Usina Onlv Contact
Ti me
A polybenzoxazole fiberwas provided with a certain percent RMC.
' ' One segment of thii fiber was dried at a 100/meters rninute line speed using
~j I 5 heated rolls positioned in a non-insulated cabinet (residence time with contact time and non-
' contact time components). The first pair of heated rolls had a set point temperature of 1 80C,
the second pair of heated rol Is had a set poi nt temperature of 20ûDC, ancl the third pa~r of
heated rolls had a set point temperature of 220C. The total residence time tor the PBO fiber
was the sum of ali the residence times ~33.7 sec CT at each set poi nt temperature and 50.6
10 seconds NCT at each set point temperature). The total residence time for the PBO fiber dried in
this mannerto reach 4.8 percent RMC was 4.2 minutes.
The same fiber was dried at 100 m/minute using heated rolls positioned in
insulated cabinets wherein the intericr temperature of each cabinet was maintained at the set
point temperature of the heated rolls contained within it (residence time with only a contact
timecomponent). Thesetpointternperaturepatternoftherollswerethesameasthesetpointtemperatures of the fiber dried with both a CT and a NCT component. The total residence time
, ~ for the PBO fiber dried in this manner to reach 4.8 percent RMC was 2.4 minutes.
~ : ' :
~ ` 20
I
1 ~ :
::
, :
~ 25
:: :
`
35:
:`;: :
:~ :
`
: `
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