Preparation of high solids poly(vinyl alcohol)/water solutions in a single screw extruder

A method for producing a high solids aqueous solution of poly(vinyl alcohol) in which substantially dry poly(vinyl alcohol) is first fed into a specially designed single screw extruder. Heated water is introduced through a separate entry port downstream of the feed end of the extruder. A temperature of 38 to 121.degree. C. and a back pressure of at least 20,786 kPa are applied to the extruder. The mixture of poly(vinyl alcohol) and water is conveyed through transition and metering sections of the single screw extruder to dispersive and distributive mixing sections to produce a high solids poly(vinyl alcohol) solution which is substantially free of gels. The high solids poly(vinyl alcohol) solution can be used directly in the production of products such as fibers and sheets.

BACKGROUND OF THE INVENTION 
Poly(vinyl alcohol) is a semi-crystalline, water soluble polymer that is 
used in many applications, such as textiles sizing, paper coating and 
adhesives manufacturing. Poly(vinyl alcohol) can also be manufactured as 
fibers, and the fibers can be produced with a wide range of properties. By 
using poly(vinyl alcohol) with a high degree of hydrolysis and 
crystallinity, very strong fibers that are water resistant can be 
produced. On the other hand, by slightly lowering the degree of 
hydrolysis, fibers that are still hot water soluble can be produced. 
Although these fibers have a wide range of properties, they are difficult 
to manufacture. The two most common preparation techniques are the wet 
spinning and dry spinning processes. 
In the wet spinning process, poly(vinyl alcohol) is washed with cold water 
and dissolved in hot water. Because of the high viscosities of these 
solutions, the concentration of poly(vinyl alcohol) is limited to 14-17 wt 
%. The solutions are then filtered, deaerated, and extruded through fine 
hofles of a spinnerette into a coagulation bath. The resulting fiber is 
subjected to drawing in a hot aqueous solution of sodium sulfate, dried, 
drawn in hot air, and subjected to heat treatment. Because low polymer 
concentrations are used in this process, all of the excess water must be 
flashed to produce a dry fiber. This process is very energy intensive. 
In the dry spinning process, more concentrated poly(vinyl alcohol) 
solutions of 25 to 50 wt % can be prepared in batch or a continuous 
blender. This solution is extruded through a spinnerette and coagulated in 
a hot cell. The coagulated, dried fiber is subsequently drawn in hot air 
and then subjected to heat treatment. 
Other methods for preparing poly(vinyl alcohol) solutions include 
pre-blending the poly(vinyl alcohol) and water (without dissolving) as a 
feed to an extruder, mixing low concentration solutions of poly(vinyl 
alcohol) with dry powders, and using a twin screw extruder to perform the 
dissolution step. Representative examples of methods used in preparing 
poly(vinyl alcohol) solutions are described below: 
GB 1,033,395 (1966) discloses the preparation of polyvinyl alcohol 
solutions containing 35 to 55% polyvinyl alcohol by feeding a mixture of 
polyvinyl alcohol and water, containing at least 55% polyvinyl alcohol, 
into a continuous mixer, such as a screw extruder, having one or more 
intermediate entry ports in the mixer. The mixture is diluted with water 
or an aqueous solution of polyvinyl alcohol in which the concentration of 
polyvinyl alcohol is lower that that of the mixture, by introducing it 
through the intermediate entry ports. The final concentration of polyvinyl 
alcohol in the mixture is between 35 and 55%. 
JP 51-146154 (1976) discloses a method of melt-extruding polyvinyl alcohol 
powder containing 60% or less water using a screw extruder. 
JP 56-86936 (1981) discloses a method of producing a concentrated aqueous 
poly(vinyl alcohol) solution in which polyvinyl alcohol is poured from the 
input port installed at the base of a high-shear, engaging type biaxial 
extruder, water or an aqueous polyvinyl alcohol solution is charged from 
an injection port downstream from the input port, under low pressure, and 
hot-melt kneading is performed to form a uniform polyvinyl alcohol aqueous 
solution having a polyvinyl alcohol content of at least 50 wt %. 
NL 8500428 (1986) (abstract; Derwent Accession Number 86-135721) discloses 
a continuous method for preparing homogeneous solutions of high molecular 
weight polymers, such as poly(vinyl alcohol) or polyacrylonitrile, by 
suspending the finely divided polymer in a solvent and mixing and kneading 
the suspension in a twin-screw, co-rotating extruder. 
BRIEF SUMMARY OF THE INVENTION 
This invention is directed to a method for producing a high solids aqueous 
solution of poly(vinyl alcohol) using a single screw extruder. The single 
screw extruder is designed so that the dry poly(vinyl alcohol) is 
uniformly fed into the feed throat of the screw, via a volumetric or 
preferably a gravimetric feeder, and transferred to the place in the screw 
where the water is injected. The pitch, i.e., helix angle, of the screw in 
the feed section helps to prevent the backward movement of water. 
Preferably, the helix angle of the screw in the feed section is 
15.degree.. After injection of the water, the screw core begins to 
increase in size which compresses the poly(vinyl alcohol) and water 
mixture. The compressed mixture is then transferred, via a metering 
section, to a mixing area containing a distributive mixer and at least one 
dispersive mixer. The extrudate is a homogeneous aqueous solution that is 
substantially free of gels. 
A valve adapter, also known as a gate valve adapter, at the end of the 
extruder barrel is used to control the pressure inside the extruder 
barrel. A back pressure of at least 3000 psig (20,786 kPa) is applied to 
the extruder. Temperatures of 38 to 121.degree. C. are applied along the 
length of the extruder barrel. 
The extrudate can be processed, using conventional plastics processing 
equipment, into strands, fibers, sheets or films. 
In contrast to prior art methods, such as those described in the background 
of the invention, the method of this invention uses a single piece of 
equipment for the steps of blending the water with the poly(vinyl alcohol) 
as well as the steps of mixing, solubilizing, and extruding the poly(vinyl 
alcohol) solution. Use of a single screw extruder eliminates the need to 
prepare solutions separately, and also reduces the amount of equipment 
required to produce poly(vinyl alcohol) solutions and convert them into 
products, such as fibers or sheets. 
Some of the advantages of this method compared to known methods are: 
improved control of the polymer concentration in the aqueous solutions, 
production of high solids poly(vinyl alcohol) aqueous solutions containing 
little or no gels, and 
production of poly(vinyl alcohol) aqueous solutions which can be used 
directly to produce fibers or sheets.

DETAILED DESCRIPTION OF THE INVENTION 
The method can be used to prepare high solids aqueous solutions of 
poly(vinyl alcohol) from a variety of grades of poly(vinyl alcohol). For 
example, the degree of hydrolysis of the poly(vinyl alcohol) can range 
from 80 to 99+ mol % and the degree of polymerization (DPn) can range from 
about 150 to 2500. Depending on the grade of poly(vinyl alcohol) and the 
manufacturer, small amounts of other components can be present, such as, 
water, volatile organic compounds (VOC's), and sodium acetate. 
Poly(vinyl alcohol) is typically supplied as ground particles. The smaller 
the particle size, the faster the dissolution; however, this method can be 
used to solubilize poly(vinyl alcohol) in a variety of particle sizes and 
forms. 
By high solids aqueous solutions is meant poly(vinyl alcohol) containing at 
least about 40 wt % poly(vinyl alcohol). The amount of poly(vinyl alcohol) 
which can be solubilized using this method is dependent on the degree of 
polymerization (DPn) of the poly(vinyl alcohol). For example, the lower 
the DPn of the poly(vinyl alcohol), the higher the amount of poly(vinyl 
alcohol) which can be solubilized using the method of this invention. 
Aqueous solutions containing 60 wt % or more poly(vinyl alcohol) can be 
obtained with this method. 
The length of the screw of the single screw extruder is expressed as a 
ratio of length to outer diameter (L/D) of the screw. A length of 30 L/D 
indicates that the length is 30 times the outer diameter of the screw. The 
total length of the single screw extruder can be from about 30 L/D to 45 
L/D; preferably 32 L/D to 40 LID. 
Substantially dry poly(vinyl alcohol) is uniformly fed into the feed 
section of the extruder, by way of a feed throat, using a standard feeder, 
such as a volumetric feeder or a gravimetric feeder. A gravimetric feeder 
is preferred. A uniform flow of poly(vinyl alcohol) is important in order 
to prevent surging in the extruder. 
Water is added at a separate location downstream from the point of entry of 
the poly(vinyl alcohol) into the feed section and prior to compression of 
the poly(vinyl alcohol) and water mixture. For example, the water can be 
fed through a port which is approximately 8 L/D from the beginning of the 
feed section. The water is heated to a temperature below its boiling 
temperature, preferably 170 to 190.degree. F. (76.7 to 87.8.degree. C.), 
before being fed into the extruder. A water temperature below the boiling 
point of water is desired in order to prevent the generation of steam 
which might travel backward toward the feed throat of the extruder. The 
heated water is added at a pressure above about 100 psig (791 kPa); 
preferably about 425 psig (3032 kPa). 
The feed rate of the poly(vinyl alcohol) and the flow rate of the water is 
dependent on the size of the extruder and the amount of solids desired in 
the final solution. 
FIG. 1 and FIG. 2 are schematic drawings of the core (FIG. 1) and the 
barrel (FIG. 2) of a single screw extruder which has been found to be 
effective in this invention. The initial section of the screw is the feed 
section (10) which can be approximately 7 L/D. The feed section conveys 
dry poly(vinyl alcohol) to the place in the screw at which water is added; 
i.e., a water injection port (20). A steady flow of the dry poly(vinyl 
alcohol) with minimal pulsation is desired for steady operation of the 
extruder. The pitch of the screw facilitates the movement of the 
poly(vinyl alcohol) at the desired rate and prevents the solids from 
backing up in the feed throat. The typical helix angle in the feed section 
of the extruder is 17.66.degree.; however, the preferred helix angle is 
15.degree.. 
Heated water is introduced through a port (20) in the barrel of the 
extruder. The screw core then begins to increase in size and serves to 
compress the poly(vinyl alcohol)/water mixture and thus facilitate the 
mixing of the water with the poly(vinyl alcohol). The increase in size of 
the screw core results in about a 33% decrease in the channel depth (the 
distance between the wall of the extruder barrel and the core of the 
screw) and a compression ratio of about 2:1 to 4:1. The preferred 
compression ratio is 3:1. 
The poly(vinyl alcohol) and water mixture is transferred through a 
transition section (30) which can be about 8 to 10 L/D to a metering 
section (40) which can be about 4 to 6 L/D. The metering section serves to 
convey and pump the poly(vinyl alcohol)/water mixture into the mixing 
section (45) which can be divided into sections each of which can be about 
4 to 6 L/D. 
The mixing section can be segmented in order to facilitate adding and/or 
changing the location of mixers. The mixing section preferably contains 
one distributive mixer (50) followed by two dispersive mixers (60 and 70). 
However, alternative configurations may be appropriate for obtaining 
uniform poly(vinyl alcohol) solutions containing little or no gels. The 
distributive mixer is designed to evenly distribute the poly(vinyl 
alcohol) and water mixture. The preferred distributive mixer is a 
Pulsar.RTM. II distributive mixer, a patented (U.S. Pat. No. 4,752,136 and 
U.S. Pat. No. 5,816,698) design of Spirex, Inc, which provides minimal 
pumping. The dispersive mixers are designed to create regions of high 
shear in which the dissolution of the poly(vinyl alcohol) can occur. The 
high shear mixers also serve to reduce or eliminate any small gels that 
have formed. The preferred dispersive mixers are a Z-mixer and a V-mixer, 
both of which are patented (U.S. Pat. No. 5,318,357 and U.S. Pat. No. 
5798,077) designs of Spirex, Inc. Most preferably, one of each of a 
Z-mixer and a V-mixer are used for the dispersive mixers and the Z-mixer 
precedes the V-mixer in the extruder. 
A high discharge pressure is desired for this process and can be 
accomplished by closing a throttling valve, also known as a gate valve, 
within a valve adapter attached at the exit end of the extruder barrel 
(80, FIG. 2). The high pressure enables a high degree of fill in the 
extruder and, therefore, better utilization of the dispersive mixers. By 
high pressure is meant a discharge pressure of at least 3000 psig (20,786 
kPa). Pressures of at least 4500 psig (31,129 kPa) are preferred. Without 
being bound by theory, it is believed that the high degree of fill 
achieved with the high pressure helps to eliminate or prevent the 
formation of gels. 
Temperatures of 100 to 250.degree. F. (38 and 121.degree. C.), preferably 
190 to 220.degree. F. (87.8 and 104.4.degree. C.) are applied along the 
length of the extruder barrel. 
The heat and pressure applied to the extruder are such that dissolution of 
the poly(vinyl alcohol) is facilitated but water does not vaporize from 
the poly(vinyl alcohol)/water melt. The feed throat is kept cooler to 
prevent water from vaporizing and moving backward into the feed throat. 
For example, the applied heat is about 100.degree. F. (37.8.degree. C.) in 
the feed throat where the pressure is 0 psig (101 kPa). However 
temperatures of about 210.degree. F. (100.degree. C.), preferably 230 to 
240.degree. F. (110 to 116.degree. C.) can be applied at positions along 
the extruder in which the pressure is above 100 psig (791 kPa). 
An aftercooler (90, FIG. 2) is preferably attached to the extruder barrel 
in order to cool the extrudate below the flash point of water and prevent 
foaming of the extrudate as it exits the extruder. A die (95, FIG. 2) can 
be attached to the extruder barrel for immediate production of 
end-products such as strands, sheets, or pellets. 
The invention will be further clarified by a consideration of the following 
examples, which are intended to be purely exemplary of the invention. 
EXAMPLE 1 
Airvol.RTM.325 poly(vinyl alcohol) powder (98.0-98.8 mol % hydrolysis; 1000 
to 1500 DPn; containing a maximum 5% volatiles) was fed into a two-inch 
single screw extruder at 33 lb/h (15 kg/h) using a ConAir feeder. FIG. 1 
and FIG. 2 are schematics of the single screw extruder used in this 
example. The feed section (10) was 7 L/D and had a 15.degree. helix angle. 
The transition section (30) was 8.8 L/D. The metering section (40) was 4 
L/D. Each of the three mixers (50, 60, and 70) were 4 L/D; 50 was a 
Pulsar.RTM. II distributive mixer, 60 was a Z-mixer, and 70 was a V-mixer, 
all of which are patented designs of Spirex, Inc. The entire screw was 
32:1 L/D. The compression ratio in the extruder from 12 L/D to the end of 
the screw was 3/1. A barrel gate valve adapter (80), after cooler (90) and 
die (95) were attached to the extruder. 
Water was fed into the extruder at 8 L/D (20) at a rate of 25 lb/h (11.3 
kg/h), a pressure of 425 psig (3032 kPa), and a temperature of 184.degree. 
F. (84.4.degree. C.). The screw speed was 50 rpm. 
The table below shows the temperature applied to the extruder, the 
temperature of the poly(vinyl alcohol) and water mixture (melt 
temperature), and the pressure at various positions in the extruder. 
______________________________________ 
Temperature 
Position in 
Applied to Extruder 
Temperature of Melt 
Pressure 
Extruder (.degree. F.) (.degree. F.) 
(psig) 
______________________________________ 
4 L/D 100 182 0 
12 L/D 209 206 31 
16 L/D 210 218 417 
20 L/D 220 222 1,965 
24 L/D 220 233 132 
30 L/D 190 239 4,485 
Barrel Adapter 
193 243 
After Cooler 
156 211 
Die 195 
______________________________________ 
It was calculated that the wt % of poly(vinyl alcohol) in the extrudate 
would be about 56.9 wt %; however the measured amount of solids was 62.9 
wt %. It is believed that the difference in the measured and calculated 
amount of solids is due to vaporization of water when the melt exited the 
extruder. There was a very low gel content in the extrudate. 
EXAMPLE 2 
Airvol.RTM.325 poly(vinyl alcohol) was fed into a two-inch single screw 
extruder at 37 lb/h (16.8 kg/h). The same configuration of extruder was 
used as in Example 1. Water was fed into the extruder at a rate of 27 lb/h 
(12.2 kg/h), a pressure of 425 psig (3032 kPa), and a temperature of 
181.degree. F. (82.8.degree. C.). The screw speed was 50 rpm. 
The table below shows the temperature applied to the extruder, the 
temperature of the poly(vinyl alcohol) and water mixture (melt 
temperature), and the pressure at various positions in the extruder. 
______________________________________ 
Temperature 
Position in 
Applied to Extruder 
Temperature of Melt 
Pressure 
Extruder (.degree. F.) (.degree. F.) 
(psig) 
______________________________________ 
4 L/D 100 180 6 
12 L/D 210 205 2 
16 L/D 210 228 739 
20 L/D 230 234 1,567 
24 L/D 230 238 59 
30 L/D 190 230 4,485 
Barrel Adapter 
191 221 
After Cooler 
100 203 
______________________________________ 
The extrudate contained 59.7 wt % poly(vinyl alcohol) and had a very low 
gel content. 
EXAMPLE 3 
Airvol.RTM.325 poly(vinyl alcohol) was fed into a two-inch single screw 
extruder at 37 lb/h (16.8 kg/h). The following changes were made in the 
configuration of the extruder as presented in Example 1. In the mixing 
section, 50 and 70 were Pulsar.RTM. II distributive mixers and 60 was a 
fight zone. Water was fed into the extruder at a rate of 29 lb/h (13.2 
kg/h), a pressure of 425 psig, and a temperature of 184.degree. F. The 
screw speed was 46 rpm. 
The table below shows data collected on the temperature applied to the 
extruder, the temperature of the extrudate, and the pressure at various 
positions in the extruder. 
______________________________________ 
Temperature 
Position in 
Applied to Extruder 
Temperature of Melt 
Pressure 
Extruder (.degree. F.) (.degree. F.) 
(psig) 
______________________________________ 
4 L/D none 3 
12 L/D 190 10 
16 L/D 190 240 
20 L/D 180 310 
24 L/D 180 24 
30 L/D 193 36 
Barrel Adapter 
174 
After Cooler 188 
______________________________________ 
The extrudate contained a high amount of undissolved poly(vinyl alcohol).