Poly-E-caprolactam is a highly polymerized material,
as is generally known. However, constituents of low molecular weight initially are present therein which ad
versely atfect the physical and mechanical properties of
the nylon-6, which is used for molding, extrusion, and
particularly for the formation of synthetic fibers. The
latter use is of especial interest in connection with the
present invention, and its preferred use therefore is directed to fiber-forming
poly-E-caprolactam albeit the use
for molding resins is not to be considered excluded. The ?ber-forming grade notoriously has the highest degree of
polymerization.
The “constituents of low molecular weight” mentioned
are monomeric and oligorneric caprolactam, and the
oligomer substantially is the dimer and the timer of caprolactam, usually in mixture. Their removal from highly
polymerized nylon~6 has met with diiliculties in the past.
Before the granulated poly-E-caprolactam is fabricated
by injection molding or, in the case of ?ber-formers, in
spinnerets, these granulates therefore must be subjected
to a washing process to free them from the monomer and
oligomers. This frequently is accomplished by a water
wash, as for instance, by the so~cal1ed “chip-washing” as
described in Swiss Patent 2303271, and usually carried out
by the so-called “layer process.” This process suifers from
the drawback that the concentration of the monomer and
oligorner in the wash water is very low so that their recovery becomes very costly.
The layer process has the
additonal disadvantage of non-uniformity of the residual
content of monomers and oligomers in the nylon-6t thus
treated.
Washing in boiling water, as taught in US. Patent
2,978,439, leads to a discoloration of the high-polymer
thus treated, so that reducing agents, e.g., hydrogen sulfite, sulfur dioxide and/0r hydrazine hydrate must be in
corporated in the wash water which may lead to contamination, and even these additions do not preclude the
discoloration, i.e., yellowing, without fail. In this highly
competitive nylon market, a yellowed product is unsalable,
besides it is an indication of oxidation. The employment
of boiling water or steam, the latter disclosed in US.
2,867,805, causes degradation and also yellowing of the
poly-E-caprolactam.
It now has been found that granulated high-polymeric
poly-E-caprolactam can continuously be freed from its
monomeric and oligomeric constituents by causing the
granules to slide through a vertical tube in counter-current to a suitable solvent. Simultaneously, the monomers
and oligomers can be concentrated in the solution obtained and can be re-used for further polymerization in
that solution or after recovery therefrom.
The process according to the invention is applicable
particularly to ?ber forming nylon-6 and will be described
in detail with reference to the accompanying drawing,
which is an elevation of the extraction device.
Through a vertical tube 1 granulate 2' is poured into
the treatment chamber through a feed device 2. Extracting agent is pumped from below into the extraction chamber through pipe 4 and the annular passage 5 and is
drawn off through the pipe 6 after absorbing the constituents of low molecular weight. The granulate is removed from the extraction chamber through a discharge
device 3. Because the granulate is made to slide through
the extraction column as a compact layer, it is possible
to adjust the time of sojourn and thereby the degree of
extraction as desired. If, in addition to obtaining as complete an extraction of the low molecular weight constituents as possible, it is desired to obtain the aqueous
solution thereof as concentrated as possible at the same
time, it is preferred, ‘according to a further feature of the
process according to the invention, to cause the quantity
of extracting agent which ?ows through the column per
unit of time at certain heights or levels to be reduced from
the bottom upwards. In order to achieve this, extracting
agent is drawn off the column at various heights. One
of these removal point-s 7 is shown in the drawing. Part
of the extracting agent fed through pipe 4 thus is drawn
off at this point through the pipe 8. Of course, care must
be taken that the extracting agent covers the entire layer
of granulate right to the very top of the extraction chamber. This can be done, for example, by maintaining the
correct ratio of the fed quantity of extracting agent to
that removed at the intermediate points .of removal, i.e.,
care must be taken that the quantity supplied is always
larger than the quantity removed at the intermediate
points of removal. Owing to the fact that the quantity of
extracting agent which flows from the bottom upwards per
unit of time through the extraction chamber and thereby
through the granulate decreases, it is possible both to enrich the extract content in the extracting agent in the
upper part of the column and to extract from the granulate with the large quantities of extracting agent in the
lower part of the column the constituents of low molecular Weight, which are present only in small quantity at
this point.
The extraction may also be carried out at elevated temperatures. The extracting agent then is preheated and the
apparatus is heated by a heating medium, circulating in
heating jacket 9. If the extraction is to be carried out at
temperatures above the boiling point of the extracting
agent, this must be done under pressure. In this case, the
granulate must be introduced through a pressure lock.
In order to make the extracting action as e?icient as
possible, it is advantageous to take care that the extracting
agent either has the same temperature over the entire
height of the column or a temperature increasing from
bottom to top thereof. It is also advantageous to keep the
ratio of tube diameter to tube length, at least 1:5, preferably at about 1:40. By means of these measures, any
harmful cross-currents which may be produced because
of differences of temperatures and differences of density of
the extracting agent can practically be eliminated.
The tratio of tube length to tube diameter also should not be
itoo large, i.e., it should not exceed 100: 1.
It is clear that the optimum time of sojourn of the
.‘granulate is dependent upon the solubility of the low
1molecular components in the solvent, on the extraction
‘temperature and also on the velocity of diffusion of the
extracted substances from the polyamide and thereby, naturally, also on the grain size of the granulate.
The process will now be further described by the follow
ing examples. However, it should be understood that these
are given merely by way of explanation, not of limitation,
and that numerous changes may be made in the details
without departing from the spirit and the scope of the in
vention as hereinafter claimed.
Example 1 illustrated the operation of an extraction
process in which the quantity of extracting agent is not
reduced from the bottom upwards. In this way, lactam
solutions of low concentration are obtained. Example 2
shows the extraction in which some of the solvent is drawn
off the column.
For reasons of economy, water preferably is employed
as the extracting agent, but all solvents which dissolve the
low molecular weight constituents without harming the
polymer may be used, such, as for example, low monoalcohols. Water must not be at boiling temperature unless
under pressure. At atmospheric pressure, the maximum
temperature is 95° C. A range of substantially 90-95 ° C.
is preferred although lower temperatures may -be employed.