Method of manufacturing polyamides

The invention relates to a method of preparing polyamides, in which, in a first step, the anionic polymerization of at least one lactam is carried out and then, in a second step, a heat treatment is carried out on the polymer obtained, which has a distribution of the molar masses measured by SEC having a shape close to a Gaussian curve with a high-mass tail, at a high enough temperature and for a long enough time so as to obtain a unimodal distribution of the molar masses.

FIELD OF THE INVENTION
 The present invention relates to a method of manufacturing polyamides and
 more particularly polyamides obtained by the anionic polymerization of
 lactams such as, for example, caprolactam (lactam 6)
 ##STR1##
 or lauryllactam (lactam 12)
 ##STR2##
 BACKGROUND OF THE INVENTION
 The polymerization of lactams is described by Jan Sebenda in J. Macromol.
 Sci. Chem. A 6 (6), pp 1145-1199 (1972).
 The anionic polymerization catalyst may be, for example, a mixture of
 sodium hydride and acetanilide, or a mixture of sodium and
 N-acetylcaprolactam.
 The anionic polymerization of lactams is rapid, about 3 to 15 minutes;
 however, the inherent viscosity of the polyamide obtained is not stable
 and changes over times.
 DE 2,241,131 describes the addition of diethyl melonate or of ethyl
 acetylacetate in the nylon-12 obtained by anionic polymerization of lactam
 12 in order to avoid the large variations in the hot flow index (MFI)
 during its conversion into fibres, films or moulded articles.
 DE 2,241,132 describes the addition of tert-butyl alcohol for the same
 problem. These results are not sufficient and, in addition, these
 additives migrate over time so that the stabilization is merely temporary.
 K. Ueda, M. Nakai, M. Hosoda and K. Tai have described in Polymer Journal
 Vol. 28,, No. 12, pages 1084-1089 (1996) the instability of nylon-6 [or
 PA-6] obtained by anionic polymerization of caprolactam. They describe
 dissolving the PA-6 in DMSO at 150.degree. C. under nitrogen in order to
 remove the residues of the anionic catalyst by treatment with an acid
 having a pK.sub.a of 3 to 7 in water. This method can only be used in the
 laboratory.
 DESCRIPTION OF THE INVENTION AND BRIEF DESCRIPTION OF THE DRAWINGS
 The Applicant has now discovered that the polyamides obtained by anionic
 polymerization of lactams had a distribution of the molar masses measured
 by SEC having a shape close to a Gaussian curve with a high-mass tail and
 that after a heat treatment the distribution of the masses measured using
 the same method had become unimodal and shifted towards slightly lower
 masses.
 In the present invention, SEC denotes steric exclusion chromatography.
 This heat treatment is sufficient to rearrange the distribution of the
 masses and obtain a polyamide of the same kind as that obtained by the
 hydrolytic polymerization of lactams. The hydrolytic polymerization of
 lactams consists in opening the lactam using water, and then in heating
 under pressure in order to polymerize. The duration of hydrolytic
 polymerization of lactam 12 may be from 4 to 12 hours, and this is why it
 has been attempted to produce polyamides by anionic catalysis, in
 particular for being able to carry out the polymerization continuously.
 Ueda et al. (already mentioned) have shown that, by heating at 200.degree.
 C., the nylon-6 which was not treated with acids in DMSO exhibits a drop
 in mass M.sub.w (weight-average molar mass) greater than that for the
 polymer which has been treated. The M.sub.w is calculated from the
 inherent viscosity. Although these drops follow asymptotic curves after
 400 hours of heating at 200.degree. C., it is explained that it is
 necessary to carry out a treatment with acids in DMSO in order to remove
 the catalyst residues and thus achieve stabilization. Ueda et al. did not
 see the distribution of molar masses and therefore saw their rearrangement
 to an even lesser extent. Based on this discovery by the Applicant, it is
 therefore possible to prepare a stable polyamide (polylactam) of mass
 M.sub.w (1), i.e. a polyamide which can be converted into articles, into
 films, etc., without its viscosity dropping, to carry out the anionic
 polymerization with settings such that a mass M.sub.w (2) greater than
 M.sub.w (1) is obtained (and in fact having a shape close to a Gaussian
 curve with a high-mass tail) and then to carry out a heat treatment in
 order to reduce this mass to M.sub.w (1), the polyamide remaining stable
 thereafter.
 The subject of the present invention is therefore a method of preparing
 polyamides, in which, in a first step, the anionic polymerization of at
 least one lactam is carried out and then, in a second step, a heat
 treatment is carried out on the polymer obtained, which has a distribution
 of molar masses measured by SEC having a shape close to a Gaussian curve
 with a high-mass tail, at a high enough temperature and for a long enough
 time so as to obtain a unimodal distribution of the molar masses.
 In the present application, the term SEC denotes the measurement of the
 molecular masses of polymers by steric exclusion chromatography. This
 technique, and more particularly its application to polyamides and to
 polyamide-block-polyethers, is described in "Journal of Liquid
 Chromatography, 11(16), 3305-3319 (1988)".
 By way of example of lactams, mention may be made of those which have from
 3 to 12 carbon atoms in the main ring, it being possible for these to be
 substituted. Mention may be made, for example, of
 .beta.,.beta.-dimethylpropiolactam, .alpha.,.alpha.-dimethylpropiolactam,
 amylolactam, caprolactam, capryllactam and lauryllactam. The invention is
 particularly useful for caprolactam and lauryllactam. The first step, i.e.
 the anionic polymerization, has been described in the prior art. It may be
 carried out in any device, for example in an extruder or in a mould. It
 has been discovered that the distribution of the molar masses measured by
 SEC had a shape close to a Gaussian curve with a high-mass tail. It may
 also be in the form of two successive Gaussian curves of very different
 sizes, one representing at least 90% by weight, and preferably 95%, of the
 polyamide and the other for the remainder and lying at higher molar
 masses. FIG. 1 shows an example of this distribution, the logarithm of the
 molar mass being plotted along the x axis (the values increase along the x
 axis) and the number of chains having the same mass being plotted along
 the y axis (the number of increases along the y axis).
 The second step consists in heating the polyamide obtained after the first
 step. It is possible to carry out this second step on a polyamide which
 has just been obtained in the first step and which may be in the molten
 state or in the form of granules obtained at the head of the extruder in
 which the first step was carried out. The polyamide may be stored between
 the two steps, and treated in this second step subsequently. The end of
 the first step is characterized by measurement of a viscosity or of an
 MFI, the catalyst having been consumed and the polymerization being
 terminated. The polyamide resulting from the first step is introduced as
 it is, i.e. without prior treatment in order to remove the catalyst, in a
 device enabling it to be heated. The device is adapted to the state of the
 polyamide, i.e. depending on whether it is available in the molten state
 or in the form of granules. It is possible to use any device in the art of
 thermoplastic polymers, such as, for example, a mixer, a single-screw
 extruder or a twin-screw extruder. The second step may also be carried out
 in the device used in the first step.
 This heat treatment is carried out, for example, in the presence of water
 for example in the form of wet air. The polyamide may also be dried
 beforehand.
 The temperature and duration of treatment, i.e. the time for which the
 polyamide is raised in temperature, depend on the amount of high molar
 masses and on the melting point of the polyamide. It is recommended that
 the temperature be from 20 to 110.degree. C. above the melting point. It
 is sufficient for the polyamide to be at this temperature for from 1 to 90
 minutes, preferably from 1 to 3 minutes. If the polyamide is subjected to
 mixing, the time is greatly reduced and may, for example, be from 20
 seconds to 60 seconds. This second step may be followed by the SEC
 measurement and this duration is therefore easily determined.
 Advantageously, it is possible to benefit from this second step in order to
 incorporate fillers, UV stabilizers, antioxidants, etc. into the
 polyamide.
 After this second step, it is found from SEC that the distribution has
 become unimodal and that the distribution of the masses, measured by SEC,
 is very similar to the previous distribution which represented at least
 80% by weight, and preferably 95%, of the polyamide but slightly shifted
 towards lower masses. By way of example, this distribution has been shown
 in FIG. 1 by a dotted line.
 This shift in the distribution of the molar masses towards lower masses may
 also be expressed by a lower M.sub.w.
 The Applicant, without being bound by this explanation, believes that this
 shift arises from a residual hydrolysis and could be minimized. The shift
 and the elimination of the tail on the high-mass side result in a lower
 M.sub.w. The elimination of the tail seems to be more important in
 explaining the drop in M.sub.w than the shift.
 This reduction in M.sub.w may also be manifested by a decrease in the
 inherent viscosity or an increase in the MFI.
 The inherent viscosity (.eta.) is measured by Ubbelhode viscometry at
 25.degree. C. in metacresol for 0.5 g of polymer in 100 ml of metacresol.
 This principle is described in Ullmann's Encyclopedia of Industrial
 Chemistry, Vol. A 20, pp 527-528 (1995-5th edition).
 By way of example in the case of nylon-12 [or polylauryllactam or
 poly(aminododecanoic acid)], reductions in .eta. of from 10 to 30% have
 been observed for a second step carried out under static conditions in
 glass tubes between 250 and 300.degree. C. for 10 to 70 minutes.
 The same reduction in .eta. was also obtained for a residence time in an
 extruder of 20 seconds at 250/270.degree. C. and 400 bar.

EXAMPLES
 First step
 The anionic polymerization of lauryllactam in the presence of sodium and of
 N-acetylcaprolactam was carried out.
 The distribution shown in FIG. 2 is obtained using the SEC measurement
 carried out in benzyl alcohol at 130.degree. C., according to a method
 based on the publication "Journal of Liquid Chromatography, 11(16),
 3305-3319 (1988)".
 Analysis using SEC of the anionic nylon-12 without the prior treatment
 makes it possible to observe a significant shoulder on the predominant
 distribution of the molecular masses. This shoulder lies on the highest
 molecular mass side and can be interpreted as resulting from the presence
 of a bimodal distribution.
 Second step
 The distribution of molar masses after a treatment in a press for 20
 seconds at 400 bar with an injection temperature of 270.degree. C. is
 shown in FIG. 2 (the shifted peak).
 Analysis using SEC, under the same conditions, of the anionic nylon-12
 after this treatment makes it possible to observe a unimodal distribution
 of molecular masses. A shoulder--indicative of a bimodal distribution--is
 no longer visible.
 It is observed that the distribution of the molecular masses after the
 treatment corresponds to the predominant distribution observed on the
 polymer before the treatment, with the possibility, however, of a slight
 shift towards lower masses.
 It would seem that it is the disappearance of the bimodal distribution,
 affecting the highest molecular masses, which, more so than the slight
 shift of the predominant distribution, is the cause of the observed
 reduction in weight-average molecular mass (M.sub.w).
 .eta. was also measured on granules (obtained at the end of the first
 step), either as produced (undried) or dried, these having been heated in
 glass tubes at various temperatures. These results are given in FIG. 3.
 Treatment at 270.degree. C.
 ".box-solid." corresponds to as-produced granules (not dried at the end of
 the first step) and "x" corresponds to granules dried at the end of the
 first step.
 Treatment at 230.degree. C.
 ".tangle-solidup." corresponds to as-produced granules (not dried at the
 end of the first step).
 ".diamond-solid." corresponds to granules dried at the end of the first
 step.
 During the treatment in the press mentioned above, .eta. went from 1.76 to
 1.26 after 20 seconds.
 Although the invention has been described in conjunction with specific
 embodiments, it is evident that many alternatives and variations will be
 apparent to those skilled in the art in light of the foregoing
 description. Accordingly, the invention is intended to embrace all of the
 alternatives and variations that fall within the spirit and scope of the
 appended claims. The above references are hereby incorporated by
 reference.