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Figure 2.102 Block diagram for various processes for the production o f PVAL fibers resp. tow or filaments
acetylene
acetic
acid
ethylene
oxygen
vinyl acetate
methanol
polymerization
methanol
acetic
acid
hydrolysis
methyl acetate
methanol
poly vinyl acetate
methanol
caustic soda
saponification
polyvinyl alcohol
washing
dissolving
nitration
wet spinning
washing
kneading
water content testing
dissolving
wet hot drawing
extrusion
filtration
degassing
i
dry spinning
drying
dry hot drawing
hot treatment
(acetylizing)
oiling
drying
winding
inding
filament
ow
drying
(crimping)
cutting
drying
crimping
oiling
oiling
washing
ho t
treatment
dry hot
drawing
drying
washing
acetylizing
drying
dry hot drawing
hot treatment
crimping
cutting
acetylizing
washing
oiling
drying
staple fiber
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2.7.2
Solution
Wet
Spinning
The majority of PVAL with a degree of polymerization of about 1700 and a spinning viscosity at 90
0
C
of 4 . . . 15 P is wet spun. The spin bath in quantities of 2 cm
3
/g solution contains 15 . . . 16 sodium
sulfate in water at 4 0 . . .50
0
C. Different from the wet spinning machines described in Chapter 4.15
PVAL is spun upward through a spinning tube as in Fig. 2.103. Depending on the end use spinnerets with
2000 holes of 0.15 mm diameter up to 60,000 holes of 0.1 mm diameter are used—in the case of high
hole number in star or segment arrangement (Fig. 4.140). The spinnerets are made from gold or gold-
platinum alloys. Already about 6 cm above the spinneret the filament reaches its final undrawn diameter.
Drawability drops with increased spinning tube length (30.. .150 cm:
1 4 : 1 . .
.10:1); at the same time
the tenacity increases from 8.1 to 10.5 g/dtex. The take-up speed out of the spinning tube is < 5 m/m in,
and the exit speed from the spin bath should be within ± 50 of this value.
Figure 2.103
Schematic Drawing of a Wet Spinning
Machine for PVAL
Right: Wet Spinning Part
1
Spinning machine
2
Spinning bath preparation
3, 4
Filters
5
Plying of the filam ents
6
Tow take-up
T
Post coagulation bath
Left: Upwards working wet spinning tube for PVAL
T
Spinning solution
3
Spinneret
2,
4
Coagulation bath
5) Coagulation bath overflow
6
Filament take-up.
As PVA is thermoplastic it can be drawn in a hot salt solution bath, over hot drums or in hot air. Hot
aftertreatment follows at 22 0 . . .230
0
C for 4 0 . . .180 s. Crystallization is completed after 10 s. Thus
fibers and filaments with tenacities according to Fig. 2.104 are produced.
Figure 2.104
. I , , • v Tenacity and Draw Ratio of PVAL filaments as a function
spinning take-up speed m/min)
o f t h e s p i n n i n g take
,
up s p e e d
„
Indissolubility in water is achieved by a hardening process with interlacing of the hydroxyl groups of
the PVA with aldehydes and acids under acetyl formation, especially in multi phase formaldehyde baths
under higher temperature (see Table 2.24 [358]) with subsequent washing, crimping, drying and staple
cutting.
t
e
n
c
t
y
g
d
d
a
w
r
t
o
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Table 2.24 Com position of the Hardening Baths for
PVA Fibers ( )
Com position 1st bath 2nd bath 3rd bath
Sulfu ricacid 10.0 20.0 5.0
Sodium sulfate 23.0 20.0
Zinc sulfate 3.0 3.0
Forma ldehyde 3.5 3.5 16.5
Water 60.5 53.5 78.5
With a known horizontal multifilament spinning process for PVA the polymer has a molecular w eight
of about 1600. The solution is spun with 18 concentration 5 g/min through a spinneret with 100 holes
of 0.08 mm each into a NaOH bath with 350 g/1 at 40
0
C 150 cm bath length, take-up speed is
1 6 . .
.47 m/min.
2.7.3 Dry Spinning
The application is quite rare. The spin solution of 4 3 concentration at
M
^ 1700 and 160
0
C is de-
aerated after several filtrations and extruded with (for example) 500 cm
3
/min through a spinneret with
211 holes of each 0.1 mm diameter into a dry spinning tube with air at about 50
0
C, and taken up at
1 1 0 . . .
160 m/min. After direct drawing over two hot and one cold godet duos with a 1 : 1 1 . . . 13.4 ratio
the yarn is wound at 1300.. .2000 m/min.
Another dry spinning process spins the PVA solution w ith 9 0 . . .95
0
C into 40.. .60
0
C air with
60 .. .85 relative humidity, taken up at 3 5 0 . . .650 m/min and hot drawn in two stages to 1 :7 to 8.5 to
achieve filaments with 5 g/dtex tenacity.
In difference to the PAN dry spinning process (Chapter 4.14) for PVA hot spinning air is entered into
the spinning tube about 1 m below the spinneret to keep the solution tenacity high.
The dry spun PVA filaments also require a formaldehyde aftertreatment etc., to remain water
indissoluble as described before.
Filament and fiber properties as well as their applications are discussed in detail in
[356].
2.8 Spandex or Elastane Yarns EL
9
also PUR)
These highly elastic yarns (in the USA referred as Spandex
[364],
in Germany according to DIN 60 001
as Elastane [365]) have a breaking elongation of over 200 , preferably over 400 , and recover
immediately and almost completely to the original state if the tension is removed. Textured yarns that
recover due to their fiber geometry like springs or coils are not covered by this term.
Spandex yarns are made from high grade polymers with a high weight portion of segmented
polyurethanes. The recovery is based on the structure of the amorphous (i.e., soft) connections between
crystallized (i.e., hard) blocks
[366].
Polyurethane (PUR) was first synthesized in 1937 [367]. The direct synthesis in high polar solutions
(e.g., in DMF) was described in 1951
[370].
The reaction spinning system was used for this in 1949.
1958 Lycra® [372] and Vyrene® [373] were introduced to the market with better elastic properties than
rubber ya rns. 1992 the produc tion w as about 60 ,000 t/a, almost exclusively multifilaments of
2 0 . . .2500 dtex and individual titers of 4 .. .20 dtex per filament.
Table 2.25 shows the major trade products, spinning systems, producers and raw materials. Since
1988 the world capacities increase significantly with the dry spinning system dominating the scene. 1995
melt spinning came up also, but did not reach the same quality until 1996.