Process for the preparation of polyvinyl alcohol articles of high strength and modulus

Polyvinyl alcohol articles of high strength and modulus are obtained by thermoreversible gelation of a polyvinyl alcohol solution, extraction of the gelation solvent using a mixture of a low alcohol or ketone and water, followed by high stretching. Preferably, a mixture of 5-80 vol. % of acetone and 95-20 vol. % of water is used as extractant. With this process extraction times of only 3-10 minutes are required.

BACKGROUND OF THE INVENTION 
The invention relates to a process for the preparation of polyvinyl alcohol 
articles of high tensile strength and high modulus. 
Synthetic fibres on the basis of polyvinyl alcohol are well-known and have 
found large-scale application in the form of staple fibres (cut fibres) in 
the production of, inter alia, paper coating, non-wovens and heavy yarns 
(canvas, etc.) and in the form of continuous fibres for separation 
purposes in textile production lines and for reinforcement of rubbers, 
plastics and other products, and furthermore as precursor of carbon 
fibres. The fibres used in these applications generally have a tensile 
strength of 0.3-1.2 GPa and a modulus of 3-30 GPa. 
The method of preparation most commonly used for this type of fibre is the 
so-called wet spinning process. In this process, the polymer is dissolved 
in a suitable medium to obtain a concentration that is suitable for 
spinning, upon which the solution is forced through a spinneret ad the 
filaments obtained are passed through a coagulation bath, in which the 
solvent is removed and the polymer precipitates. 
Besides the above-mentioned applications for synthetic polyvinyl alcohol 
fibres, there is increasing demand and a growing market for so-called 
technical-grade polyvinyl alcohol fibres, for instance for composite 
applications. 
For these applications the fibres are to have a higher modulus and tensile 
strength. 
Numerous methods have already been proposed, particularly in patent 
literature, for the preparation of polyvinyl alcohol fibres patent 
literature, for the preparation of polyvinyl alcohol fibres having 
improved mechanical properties. 
From Applied Polymer Symposia No. 6 (1967), pp. 109-149, for instance, it 
is known to prepare fibres having a tensile strength of about 1.2 GPa by 
coagulation spinning (phase separation) of dilute solutions of polyvinyl 
alcohol in organic solvents, followed by stretching. From FR-A-1.280.192 
it is known to prepare fibres having a maximum tensile strength of 1.7 GPa 
by coagulation followed by stretching, starting from solutions of 
polyvinyl alcohol in water in the presence of boric acid. 
In the above-mentioned publications nothing is said about the moduli of the 
fibres obtained. Fro DE-B-2.132.055 it is known to convert a solution of 
polyvinyl alcohol in water into fibres having a maximum tensile strength 
of about 1.8 GPa and a maximum modulus of about 40 GPa through coagulation 
spinning and a special very laboricus multistage stetching operation with 
interim water treatment. 
All the methods referred to start from a polyvinyl alcohol having a 
relatively low weight-average molecular weight, in general between 
6.times.10.sup.4 and 12.times.10.sup.4, the fibres obtained having a 
strength and modulus that, though being higher than those of commercially 
available synthetic polyvinyl alcohol fibres, are not yet high enough for 
most technical applications. 
It further is known to prepare fibres of very high tensile strength and 
modulus starting from solutions of polymers having a high molecular 
weight, see GB-B-2.042.414 and 2.051.667. According to the process 
described in GB-B-2.042.414, to this end a dilute solution of, for 
instance, polyethylene is spun, the filament obtained is cooled to a gel 
filament, and the solvent-containing gel filament is stretched at elevated 
temperature. According to the process described in GB-B-2.051.667 to this 
end a solution of high molecular weight polyethylene is spun, the solvent 
is optionally removed largely or partly, and the gel filament is subjected 
at a specific temperature to stretching at a high stretch ratio that is 
related to the molecular weight. In these known processes it has been 
found that the moduli, but especially the tensile strengths, that can be 
attained increase with the molecular weight of the polyethylene. 
These known processes can, therefore, be used to prepare polyethylene based 
fibres having a tensile strength well in excess of 1.2 GPa and moduli of 
more than 20 GPa. 
According to the above-mentioned GB-B-2.042.414 the process proposed in it 
can be used, inter alia, to prepare fibres having a high strength and 
modulus from polyvinyl alcohol. This is confirmed by, for instance, 
EP-A-105.169, which discloses a process for the preparation of fibres 
having a high tensile strength (1.3-2.2 GPa) and a high modulus (40-70 
GPa) by spinning of a dilute solution of high molecular weight polyvinyl 
alcohol, with a weight-average molecular weight well in excess of 
5.times.10.sup.5, in particular well in excess of 1.times.10.sup.6, 
cooling of the spun filament to obtain a gel filament, and stretching of 
this filament after extraction of the solvent. 
According to EP-A-146.084 it is possible to prepare fibres of high strength 
and modulus starting from a polyvinyl alcohol having a relatively low 
molecular weight, by applying a special dry/wet coagulation spinning 
process, followed by cooling, solvent removal and drawing. A disadvantage 
of the process of EP-A-146.084 ad also of EP-A-105.169 is that the removal 
of solvent from the fibers by e.g. extraction is extremely difficult. In 
EP-A-105.169 an extraction time of 92 hours is mentioned. So both 
processes are not suitable for commercial operation. 
SUMMARY 
The present invention provides a process for the preparation of articles 
having a high strength and modulus from polyvinyl alcohol in which the 
above-mentioned disadvantages are not or hardly present. 
Though the invention relates in the first place to the preparation of 
filaments or fibres, it also relates to the preparation of tapes, ribbons, 
films, tubes, bars or profiles on the basis of polyvinyl alcohol. 
The invention relates to a process for the preparation of an article having 
a high tensile strength and modulus from polyvinyl alcohol which is 
characterized in that: 
(a) a solution of a polyvinyl alcohol in a suitable solvent or mixture of 
solvents is converted into a shaped, solvent-containing article at a 
temperature above the dissolution temperature; 
(b) this article is converted into a gel article having a homogeneous gel 
structure by rapid cooling to below the gelation temperature; 
(c) the solvent present in this gel article is largely removed by 
extraction with a mixture of a lower, aliphatic alcohol or ketone and 
water having a watercontent of 20-95 vol. %; 
(d) during or after solvent removal, the gel article is stretched at a 
temperature above the glass transition temperature but below the 
decomposition temperature of polyvinyl alcohol. 
DETAILED DESCRIPTION OF THE INVENTION 
In the present process use can be made of high molecular weight 
polyvinylalcohol, e.g. as described in EP-A-105.169. However, the 
preparation of such a polyvinylalcohol is extremely laborious and also the 
preparation of a homogenous solution is very difficult, while 
moreover--due to the high viscosity of solutions of high molecular weight 
polyvinylalcohol--relatively dilute solutions have to be used. 
Therefore preferably use is made of a polyvinylalcohol that has a 
weight-average molecular weight of between 2.5.times.10.sup.4 and 
5.times.10.sup.5, by preference between 5.times.10.sup.4 and 
3.times.10.sup.5. 
Such a polymer is prepared on industrial scale, usually by polymerisation 
of vinyl acetate followed by hydrolysis. The term polyvinyl alcohol should 
be understood as meaning a polymer consisting of at least 50% of vinyl 
alcohol monomer. The polymer may further contain minor amounts of polymers 
or other substances, such as fillers and the like, that are compatible 
with polyvinyl alcohol. 
As solvent, use may be made of various solvents known for polyvinyl 
alcohol, such as saturated, aliphatic, multivalent alcohols or dimethyl 
sulphoxide. It is preferred to use ehtylene glycol, glycerol or dimethyl 
sulphoxide as solvent. 
Use can of course also be made of a mixture of solvents, for instance 
ethylene glycol and water. 
The solutions to be used can be prepared in various ways, for instance by 
suspending solid polyvinyl alcohol in the solvent, followed by stirring at 
elevated temperature, or by converting the suspension using a twin-screw 
extruder provided with mixing and conveying facilities. 
The concentration of the solution to be used may vary. It has been found 
that highly concentrated solutions can be used in the process according to 
the invention. In particular, solutions with a concentration of between 5 
and 50 wt. %, and preferably between 20 and 40 wt. %, will be used if 
starting from a polyvinylalcohol with a relatively low molecular weight. 
Conversion of the solution into a shaped, solvent-containing article can be 
effected in various ways in the process according to the invention, for 
instance by spinning into a filament or ribbon using a spinneret with a 
round or slit-shaped aperture, respectively, or by extruding using an 
extruder, usually with a profiled head. 
The temperature during conversion is to be higher than the dissolution 
temperature. This dissolution temperature, of course, depends on the 
solvent chosen, the concentration, the molecular weight of the polyvinyl 
alcohol and the pressure used. For ethylene glycol, for instance, the 
dissolution temperature is about 90 .degree. C. 
In general, the conversion temperature will be chosen well above the 
dissolution temperature, for instance about 140.degree.-180.degree. C. 
when use is made of ethylene glycol. 
It goes without saying that this temperature is kept below the polyvinyl 
alcohol decomposition temperature. 
An important part of the process according to the invention is cooling to 
below the gelation temperature of the shaped, solvent-containing article 
such that a gel article with a homogeneous gel structure is obtained, use 
being made of rapid cooling, preferably using a liquid cooling (quenching) 
medium. 
The gelation temperature naturally depends on, inter alia, the solvent and 
in general virtually agrees with the above-mentioned dissolution 
temperature. 
The object is preferably cooled to about ambient temperature. 
It has further been found that the nature of the cooling medium, too, 
influences the mechanical properties of the objects ultimately obtained. 
By preference, methanol is used as cooling medium. 
There further may be advantage in subjecting the object to a draw-down 
prior to cooling. It is preferred to limit the draw-down ratio to maximum 
20:1, the most preferred draw-down ratio being maximum 10:1. 
It has further been found that a product having better mechanical 
properties can be obtained if a higher spinning rate is used. 
From the gel object obtained after cooling, the larger part of the solvent 
still present is subsequently removed by extraction. The very essence of 
the present invention is to use a mixture of a lower aliphatic ketone, 
e.g. acetone, or a lower aliphatic alcohol, e.g. methanol or ethanol, and 
water having a water content of between 20 and 95, preferably 30 to 60 
vol. %, as extractant. 
It appears that by using such an extractant mixture the extraction time 
needed can be very short, viz. 3-10 minutes at roomtemperature. Preferably 
a mixture of acetone and water is used as extractant. If desired, the 
extraction can be effected at elevated temperature, but the temperature 
must be lower than that at which the gel dissolves. 
During or, preferably, after extraction, the gel articles are stretched in 
one or more steps. The temperature is then generally kept above the glass 
transition temperature, but below the decomposition temperature of 
polyvinyl alcohol. By preference, stretching is effected at a temperature 
of between 160.degree. and 240.degree. C., in particular between 
180.degree. and 230.degree. C. 
It has been found that high stretch ratios can be applied in the subject 
process. In general, a stretch ratio of at least 10:1 is used, and 
preferably of at least 20:1. 
The article according to the invention are suitable for virtually all 
technical applications, such as composite applications, precursor of 
high-quality carbon fibres, replacement of glass fibres, etc. 
If desired, minor amounts of customary additives, stabilisers and the like 
can be incorporated in or on the objects.

EXAMPLES OF THE INVENTION 
The invention will be elucidated in the following examples, without, 
however, being restricted thereto. 
EXAMPLE I 
An amount of polyvinyl alcohol powder with a weight-average molecular 
weight of 1.15.times.10.sup.5 (obtained from the Aldrich company) and a 
degree of hydrolysis of 100% was added to ethylene glycol in an amount of 
25 g per 100 ml ethylene glycol, upon which the suspension was stirred for 
two hours at 160.degree. C. 
The spinning solution was transferred to a double-walled cylinder which was 
thermostatted with oil heating at a temperature of 160.degree. C. By means 
of a piston coupled to a variable-speed motor, the solution was forced 
through a capillary (0.5 mm diameter) into a quench bath at a rate of 129 
ml/hour. As quenching medium, use was made of methanol; the quench 
temperature was 20.degree. C. The quenched filaments were wound at a rate 
of 60 cm/sec., which corresponded to a draw-down ratio of V=3.3. The 
filament was then deposited in an extraction bath of acetone/water (50 
vol. % of water) at 20.degree. C. during 9 minutes. The extracted filament 
was air-dried at 20.degree. C. and subsequently stretched 26 times at 
190.degree. C. The filament obtained had a tensile strength of 1.9 GPa, a 
modulus of 45 GPa, an elongation of 4.2% and a dtex of 7.5. 
The tensile strengths and moduli of the filaments thus obtained were 
determined using a Zwick 1445 tensile testing machine. The specimen length 
was 50 mm and the rate of elongation 5 mm/min. The modulus was the 1 % 
modulus determined from the tangent to the initial section of the 
stress-strain curve. 
COMATIVE EXAMPLES A-F 
The process of Example I was repeated but now extracting with methanol 
during different extraction times. 
The results are summarised in Table I. 
TABLE I 
______________________________________ 
Extraction Strength 
Modulus 
Elongation 
Example 
time dtex (GPa) (GPa) (%) 
______________________________________ 
A 10 min. 8.4 1.58 33 4.3 
B 15 min. 8.7 1.54 29 4.7 
C 1 hour 8.2 1.53 34 3.9 
D 2 hours 7.8 1.54 37 4.1 
E 3 hours 7.5 1.63 36 4.4 
F 24 hours 7.5 1.78 37 4.4 
______________________________________ 
From the above examples it can be concluded that even after 24 hours 
extraction with methanol the resulting fiber has a lower strength and 
modulus as the fiber of example I. 
EXAMPLES II-VII 
The process of example I was repeated but now using different 
concentrations of water in the extractant during 9-10 minutes. The results 
are summarized in Table II. 
TABLE II 
______________________________________ 
Example 
Acetone- strength 
modulus 
elongation 
No. conc. in % 
dtex (GPa) (GPa) (%) 
______________________________________ 
II 10 7.6 1.83 42 4.3 
III 25 8.0 1.77 37 4.4 
IV 50 7.5 1.90 44 4.2 
V 75 7.4 1.85 45 4.3 
VI 80 8.5 1.44 35 3.9 
VII 90 8.4 1.42 32 3.7 
______________________________________ 
EXAMPLE VIII 
The process of Example I was repeated but now extracting with a 
water-methanol mixture (50 vol. % water) during 10 minutes. 
The product had a dtex of 6.8, a strength of 1.8 GPa, a modulus of 40 GPa 
and an elongation of 4.1%. 
EXAMPLE IX 
The process of example I was repeated using a solution of polyvinylalcohol 
in dimethylsulfoxide. 
The filament obtained had a dtex of 6.8, a tensiled strength of 1.8 GPa, a 
modulus of 42 GPa and an elongation of 4.4%. 
COMATIVE EXAMPLE G 
The process of example IX ws repeated, but now extracting with methanol 
during 20 minutes. 
The filament obtained had a dtex of 7.4, a tensile strength of 1.5 GPa, a 
modulus of 30 GPa and an elongation of 5%.