Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
1998-09-22
2001-05-08
Hampton-Hightower, P. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S312000, C528S322000, C528S323000, C528S324000, C528S329100, C528S332000, C528S480000, C528S503000
Reexamination Certificate
active
06228976
ABSTRACT:
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)
or lauryllactam (lactam 12)
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° C. under nitrogen in order to remove the residues of the anionic catalyst by treatment with an acid having a pK
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° C., the nylon-6 which was not treated with acids in DMSO exhibits a drop in mass {overscore (M)}
w
(weight-average molar mass) greater than that for the polymer which has been treated. The {overscore (M)}
w
is calculated from the inherent viscosity. Although these drops follow asymptotic curves after 400 hours of heating at 200° 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 {overscore (M)}
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 {overscore (M)}
w
(2) greater than {overscore (M)}
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 {overscore (M)}
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 &bgr;,&bgr;-dimethylpropiolactam, &agr;,&agr;-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° 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
Dang Patrick
Faulhammer Heike
Jacques Bernard
Werth Michael
Elf Atochem S.A.
Hampton-Hightower P.
Smith , Gambrell & Russell, LLP
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