CA1050196A - Process for the production of polyamides - Google Patents

Abstract

A PROCESS FOR THE PRODUCTION OF POLYAMIDES
Abstract of the Disclosure An improved process for the production of polyamides of low extract content by the activated anionic polymeri-sation of at least one lactam containing at least 5 ring members at temperatures above the melting point of the poly-amide in a continuously working polymerisation apparatus, optionally followed directly by shaping, the improvement comprises treating the polyamide melt obtained by polymeris-ing said lactam in said polymerisation apparatus with water or steam and subsequently freeing it from volatile consti-tuents by evaporation before the polyamide melt is dis-charged into the open.

Description

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This in~ention relates to a process ~or the production o~ polyamides o~ low extract content by the continuous activated anionic polymerisation of lactams accompanied by evaporation of the low molecular weight oonstitllents.
It is known that lactams ~a~ be polymerised in the pre-sence of 6trong bases to form polya~ides. In many cases, th~
anionic polymerisation process affords advant~ges over the hydrolytic polymerisation of lactamsr The anionic polymerisation oi lacta~s takes place extremely quickly, especially in the ; 10 presence oi ~o-called nactivators'l9 i.e. compounds with ~n acylatin~ ef~ect, or their reaction products with lactams ~DT-PS No. 1,067,591; US-PS No. 3,015,652). By virtue of thi~
hi h polymerisation velocity, the activated anionic polymerisa-tion of lactams provides an e~fective basis for the continuous production of polyamides in high volume-time yields. Conti~uous processes of this type are known, ~or example activated anionic polymerisation in tube reactors (DL-PS No. 50 002) or in extruder~ (~or example CS-PS NO. 97 332; GB-PS NQ. 944,307;
GB-PS NoO 98~,678), In this way, the polyamide may be pro-ces~ed~ ~or example into extruded profiles, immediately after its production, so that all stages of the proccss from the monomeric lactam to the finisbed polyamide profile may be carried out in a single operat:ion and, hence, particularly : econu~ic~lly.
~he disadvantage of these processes is that~ hitherto, there has been no commercially satisfactory possibility of removing the low molecular weight constituents of the polymer ' during the continuous process~ It is known tha-t the polymeri sation o~ lactams is not a complete reaction) but instead re-`~1 30 sults in an equilibrium state in which the polymer ~ormed still Le A 15 346 ~A~
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~ff~f~ff~6 contains extractable low molecular weight consti-tuents, such as unreacted lactam and its oligomers, collectively referred to hereinafter as "extract"fff The quantitative composition of the reaction mixture in the equilibrium state is governed both by the type of lactam being polymerised and by the polymerisation temperature. For example 7 the polymerisation of -caprolactam at 220C gives a polyamide with an extract content of about 9 %. Any increase in the polymerisation -temperature is accom-panied by an increase in the extract content. This applies both to hydrolytic polymerisation and to activated anionic poly merisation .
The low molecular weight constituents modify the proper-- ties of the polyamide obtained and, accordingly, represent an undesirable secondary constituent. For example? caprolactam acts as a plasticiser in polyamide-6; its ring-shaped oligo-mers make the material brittle. In addition, the extract tends to emigrate from the polyamide by and by and to form an unattrac-tivfe, grey coating over the surface of the material. The grea-sy feel of these substances, their intensely bitter tas-te and the irritation some of them cause to mucous membrane, adverse-,, ~ ly affect the marketability of extract-containing polyamides I

I to a considerable extent. Accordingly, these polyamides have I to be extracted after production. On an industrial scale, this is done in batches by boiling the polyamide for several hours with water. This procedure is both complicated and laborious.
Since, in any industrial process, throughput is governed by ~ the slowest process stage, polyamide produc-tion as a whole ; could be rationalised by accelerating the removal of the ex-;f tract, making it continuous and combining it into a single l 30 opfPfration with one of the continuQus polymerisation processes Le A 15 346 -2-~ 6 referred to above.
Accordingly, it has been repeatedly proposed to improve the anionic polymerisa-tion processes carried out in continuous cycle apparatus, more especially extruders, by passing the molten polymer through an evapora-tion zone in which the low molecular weight constituents are evaporated and removed under suction or driven off by an inert entraining gas (DT-OS No.
1~495,133; DT-OS No. 1,495,658; DT-OS No. 1~770,~20; US-PS
No. 3,~71,055; US-PS No~ 3,484,414; GB-PS No. 986,678). Thus it is possible to considerably reduce -the initial ex-tract con-tent. In practice~ however, the efficienc~ of this process is limited because, immediately after the low molecular weight constituents have been evaporated off, the disturbed chemical equilibriurn begins to re-es-tablish itself. This results in the reformation of the troublesome low molecular weight con-stituents which have jus-t been removed. In every case, refor-mation takes place so quickly above -the melting temperature ;l of the polyamide-6 that the extract content virtually reaches its original level again before the polymer is able to pass 3~ 20 through the extrusion die and to cool.
! One method of o~viating these disadvan-tages is described in DT-OS No. 2,115,297 = GB-PS No. 1,323,963. In this case, lactam polymerisation is carried ou-t at temperatures below I the melting point of the polyamide so that a low-extract poly-1 25 mer is obtained. Be~ore processing, the polymers are treated with water or steam inter alia after size-reduction. In this way, the catalyst still present in -the polymer which is respon-sible for reforrnation of the monomers is destroyed so that the ~! polyamide may be processed into mou:Lding articles at tempera--i~ 30 tures above its melting point wi-thout any danger of a relative-I

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~al5C)~iL96 ly high extract content being formed by any monomer reformation.
Although according to this process polyamides with a low extract content are obtained, the process does have several disadvan-tages. Since the polymer accumulates in solid form i-t has to be size-reduced before treated with water in order to obtain as complete a destruction of the catalyst as possible.
Size-reduction involves an additional operation which uses a lo-t of energy because o~ the extreme toughness o~ the poly-amides. Despite size-reduction, however9 the polymer has to be brought into contact with an excess o~ water for an average of 1 hour. As a result of this excess treatment with water, the polymer is in danger of degrading during melting. Anothér disadvantage of the process referred to above is that polymeri-,;
I sation cannot be carried ou-t in a standard polymerisation appa-. . , ratus, such as an extruder, because it takes place at tempera-tures considerably below the mel-ting temperature of the poly-amide formed. Another disadvantage is that, for processing, the polyamide has to be remelted in another, separate opera-~¦~ tion which uses a lot of energy.
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The object of the present in~ention is to obviate the disad~antages re~erred to above and to provide a process for the actlvated anionic polymerisation of lactams in which poly-amide o~ low extract content is obtained.
Accordingly, the invention relates to an improved pro-~5 cess for the production o~ polyamides of low extract content by the activated anionic polymerisation of at least one lactam ¦ containing at least 5 ring members at temperatures abo~e the ;!~
melting point of the polyamide in a continuously working poly-merisation apparatus, optionally followed directly b~ shaping, ~ 3 the improvement comprises treating the polyamide melt obtained ,:1 I ~ - 4 -, :
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~1~5~6 by polymerising said lactam in said polymerisation apparatus with wa-ter or steam and subsequently freeing it from vola-tile constituents by evaporation, optionally carried out in vacuo or by means of an entraining gas before the polyamide melt is discharged into the open.
During the brief exposure of the polyamide melt to the action of water, for on average~ from 0 5 to 5 minutes and pre-~ ferably from 1 to 2 minutes i-t is surprising that the water : could completely des-troy -the anionic catalyst present in thepolymer. Thus surprisingly the degradation into low molecular weight compounds could be prevented in very short time 3 al-though treatment times of, on average 9 1 hour are required for the conditioning with water according to DT-OS 2,115,295 =
GB-PS 1,323,963. Since the destruction of the catalyst is accompanied by.the formation of hydroxides, such as sodium hy-droxid~, which promote hydrolytic degradation by the w~ter pre-sent9 monomer reformation through hydrolysis had to be expec-ted to occur Ln view of the high reaction temperatures prevai-., ''! ling in the processing machine. In spite o~ this fact, poly-amides with an extrac-t content of at most 5 ~, on average 2 to 3.5 %, are surprisingly ob-tained.
Finally, another surprising factor is that removal of the ext.ract i9 more complete at elevated temperatures than at low temperatures, although the extract content and hydrolytic degradation rate increa~e with increasing temperature.
The polymerisation process according to the invention ~ay be carried out with lactams having at least 5 ring members~
preferably caprolactam, lauric lactam, a-pyrrolidone, oenanthlc lactam~ capryllactam, the corresponding C-substituted lactams , ~d with lactam mixtures containing several of these lactams.

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Polymerisation is carried out in the presence of con~en~
tional catalysts for the anionic polymerisation of lacti~ms, such as alkali me-tal and alkaline-earth metal lactamates and alcoholates, alkali metal or alkaline-earth metal salts of fatty acids or of C-H-acid compounds, such as Na- ~ ~caprolac-tamate 9 Na- and potassium forma~e and Na-methylate. The cata-lyst is used in quantities of ~rom 002 to 2 mol % and pre-ferably in a quan-tity of 0.4 mol %, based on the lactam.
Suitable activators are monoisocyanates and polyisocyana-:` 10 tes, masked isocyanates7 triazines~ ketenesg carbodiimides, acid `~ imides,acid chlorides and the reaction products o~ these ~ubstance~ with lactam~, such as phenyl isocyanate, tolylene-1,4-diisocy~nate, hexamethylene-1,6-diisocyar~ate, aaprolactam-N-cari~oxylic ucid-N~-phenyl~mide, hexamethylene-1,6-bi~-(carbamido caprolact~m)~ triphenoxy-s-triazine, N-ste~royl caprolactam.
The activator i8 used in quantities of from 0~02 to 1 mol ~
and prefcrably in quantities of ~rom 0.2 to 0.3 mol %, based on the lactam .j The polymeri3ation reactivn may ba carried out in the presence o~ chain t~r~inatorsl pigment~9 fillers a~d strength-~ner~9 mould-release and nuclearising agent~ and other additives.
The process may be carried out in continuous polymeri~ation Apparatus, ~uch as ~tandard ram or screw extruders, tube ¦ re~ctor~ and combinatlon~ thereof The apparatus 1~ red either with the reactive lactam melt or with the corresponding I solid mixture,.~ollowed by heating ~eyond the meltlng point of the polyamide formed, i.e. to 170 - 350C. On completion of poly-erisation, the polymeric melt is treated with water or steam in the same poiymsrisation apparatus or in a shaping tool fi;-edly connected to it. The water is general~y metere~ through ,~ Le A 15 346 - 6 -.' :
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~ 0 S~ ~ 9 6 pumps in a quality at least equivalent to the quantity o~ ca-talyst used. It is preferably used in ~ 5 to 20 fold excess, and with particular preference in a 10 fold excess. A~ter the melt has been exposed to the water for an averag~
0.5 minu~es to 5 minutes, preferable for 1 to 2 minu-tes, during which it can continue to move through the appara-tus, the volatile constituents are evaporated o~f through an evacuation outlet. A vacuum of from 10 to 100 Torr is preferab~
ly applied. It may also be of advantage to evaporate the vol'a-tile constituents by means of an inert entraining gas, for example'nitrogen. A polyamide discharged into the open may be directly processed into granulate. Mouldings produced from the ` granulate surprisingly have the same low extract con-tent as the ;~ 15 polyamide granulate used. However, the polymerisation apparatus may also be designed in such a way that polyamide mouldings are directly obtained by the process according to the invention, so that there is no need to isolate the granula-te between polymeri-sation and shaping~ In either case, the process according to the invention eliminates the need for prolonged extraction of ~'! the polyamide granulate with water.
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'l Additives may also be added to the polymerisable mixture during or af`ter polymerisation. Examples of' additives o~ the type in question are inorganic or organic pigments or dyes such as carbon black9 TiO2 or phthalocyanines,plasticisers or mould-re-,, lease agents9 inorganic or organic fibres~ such as glass ~ibres ~1 or mats or asbestos ~ibres, also f'illers, such as glass beads, :l~ calcium carbonate or bentonite, flameproo~ing additives, such as ¦ red phosphorus, organic halogen compounds,phosphoric acid esters ar metal oxides and blowing agents, such as azides or hydrocarbons~

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~L~9S~96 __ A twin-screw extruder with two 50 mm diameter, 1800 mm long screws rotating in the same direction consists of 6 equally long b~rrel seotions heatable independently of one another. The first co~pression ~one i5 situated at the beginning ~ of the ~econd barrel section. The ~ifth barrel section is :: in the ~orm o~ a vacuum zone followed by recompression, In .
the middle of the fourth section, the barrel has a bore through ' which water may be sprayed in by means of a metering pump~
-:~ 10 A strand die is used as the extruqion die. The feed zone i~
oharged with a powdered mixture of the following composition:
llQ0 parts by weight of caprolactam 10 part6 by weight of hexamethylene-1,6-bis-(carbamido caprolactam) :1 ., 15 5 parts by weight oi sodium caprolactamate ., 4 part~ by weight of N-benzyl acetamide ~!
The extruder is operated at a throughput of 15 kg/hour i~
accordance with t~e ~ollowi~g temperature programme:
water-cooled feed ~one/200~Ç/250C/250C/250C/250C/die 250~.
~he polya~ide strand run off is cooled in water and granulated.
I If a vacuu~ o~ 20 Torr i 9 applied to the vacuum ~one by ;~ mean~ o~ a water ring pump, and if water is sprayed in at a rate Or 50 ml per hour through the bore in the barrel wall~
a polyamide granulate with a residual extract content of 3.3 to 3.7 ~ (as determined by the method de3cribed in Kunststo~f-han~buoh, published by ~. Vieweg, Vol. 6 "Polyamide~', chapter 1 4~7) i 6 obtained~
!:: EX~MPLE 2 (Compar~son E~ample) . ~ ~
The procedure is as i~ Example 1 except that no water i9 3 sprayed in. A pvlyam1de granulate with an extract content of ,;,,11 l ~ - 8 -i 'I
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lO.0 to lOl7 % is obtained.
(Comparison E~ample) The procedurs is a~ in Example 1, except that no vacuum i~ applied and no water sprayed in. A polyamide granul~te with an extract content of 10.9 to 11.6 % is obtainedO

A twin-~crew extruder with two 55 mm diameter screws rotating in the same direction w~s u~ed as the test apparatus.
Its barrel (length = 40 times the diameter) has 6 individually controllable heating zones. Zone 5 is in the form o~ a vacuum zone. At the beginning of the ~ourth zone, the screws have a short return thread with a length equal to the diameter; other-wise they have a constant lead, apart from the deviations re quired for expansion and recompression in the vacuum zone. In the fourth zone 9 the barrel has a bore with a nozzle screw~d into it through which water may be sprayed in by means of a me~
tering pump. The extru~ion die is in the form of a strand die.
The feed opening of the extruder is closed. In stead two me-tering pumps supply the feed zone through -two bores in the barrel with two liquid mixtures whose admixture in the barrel , .1 produces the polymerisable meltO They have the following com-position:
Component A:
275 parts by weight of ~ -oparolacta~

2 parts by welght of hexamethylene diisocy~nate ~?l~ ll parts by weight of behenic acid-N-cyclohexylamine Component B:
~jl 275 part~ by weight of -caprolactam 1 3 parts by weight of sodium caprolactamate ~ Both co~ponents are introduced in a weight ratio of 1 : 1.

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:, -- 9 _ i Their entry temperature i9 lOO~C. The work may rate of the metering pumps i~ adapted to the troughput of the extruder which amounts to 20 kg per hour. The b~rrel temper~ture Or the extruder iq 180~C in the ~eed zone and 250~C in all other zones. The die temperature is also 250~C. A pressure of lV Torr prevails in the vacuum zone.
The polyamide strand run off is cooled in water and granulated. The extract content of the polyamide granulate thus produced is determined in the same way a9 in Example 1.
The test is carried out with different quantities of water sprayed in per unit of time. The following Table shows the ~; e~tract content of the polyamide obtained in dependence upon the water injection rate:
ml o~ H20 per hour none 10 25 _50 100 150 ' Extract l content ~ 11.1 9.2 4.6 4.2 4.7 3.9 .1 ~ .
The procedure is as in Example 4 except that the ' 20 caprol~cta~ in both components of the polymerisable melt is,1~ replaced by a mixture of caprolactam and laurio lactam. ~he extruder i8 operated with the following temperature programme:
feed zone 180~C; zones 2 and 3: 210C; zones 4 and 5; 260C;
zone 6 and die: 230UC. The water i9 sprayed in at a rate of 50 ml per hour. The remaining test conditions are the sa~e as Example 4. The following Table shows the extract content of some o~ the mixed polyamides obtained with and without injection of water.

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, Lauric lactam content of the starting lactam 30 40 50 mixture (~ by weight) Extract content with water injection 3.6 3.4 3.7 Extract content without water injection 15.417.9 2201 The procedure i~ a~ in Example 4, except that the water is sprayed in at a rate of 50 ml per hour, and the barrel temperature in zones 4 ~nd 5 is varied. All other test parametexs remain unchanged, The following T~ble shows the extrRct content of the polyamide obtained in this way with and without injection of water and the dependence upon the tempera-ture prevailing in the injection and vacuum ~one.

, Barrel temperature in I zone 4-5 (~C) 250 270 290 300 320 ~xtract content with water injection t~)4,2 3.5 2.9 2.7 2.3 Extract content withvut water injection (%)11.1 12.113.9 14.2 15~1 !
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~ A tu~e reactor 3 metres long with an internal diameter of 1 5~ mm, heated to a temperature of 240C, is used as the poly-merisation apparatu~. It is fed by two metering pumps with !

equal quantities of two caprolnctam melts one of which contains ~l~ 0.8 % of sodium caprolflctamate ~nd the other 1,0 ~ of phenyl ; isocyana-te and (~ of N-~enzylacetamide in dissolved ~orm.
, ~oth melts are kept at lOO~C and mix on entering the tube reactor.
¦ 30 The pumps each have an output of 12.5 kg per hour.
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Af-ter polymerisation, the melt flows under pressure through a valve into the feed opening of a single-screw extru-der with a screw diamet~sr o~ 45 mm and a length o~ 12 times the diameter. It is equipped wi-th an evacuable evaporation zone. Be~
~ore the vacuum zone -the barrel has a bore through which water is sprayed in a-t a rate of 70 ml per hour by means of a mete-ring pump. The barrel temperature is kept at 270C, the pre3-sure in the vacuum zone amounting to 10 Torr, A solid bar 80 mm in diameter is extruded~ The material has goocl mechanical pro-perties and an extrac-t content of 3.2 /0.

The procedure is as in ~xample 7, except that no water is sprayed into the extruder barrel. The ma-terial obtained has ,n extract content of 10.9 % and is much softer and more flexible ;, 15 than that obtained in accordance wi-th ~xample 7.
~X~MPLE 9 . . . .
A single-screw extruder with a screw diameter o~ 45 mm and a length of 25 times the diameter is used as the polymeri-sation ~pparatus. It is fitted with a conventional five-zone screw and an evacuable evaporation zone. Between the first com-, pression zone and the vacuum zone, the barrel has a bore through which water is sprayed in at a rate o~ 50 ml per hour by means of a metering pump. The extrusion die is in the ~orm of a strand die. The feed zone, heated to 110C~ is charged by a single sha~t feed screw 20 mm in diameter and 10 times in diameter 1 1ong9 which is rotated at 60 rpm ancl whose barrel is heated to I 80C, with a mixture o~ the ~ollowing composition:
,¦ llO0 parts by weight of c~prolactam 4 parts ~ by weight of ~sodium caprolactamate

3~ ,'3 p~rts by weight of caprolactam-N-carboxylic acid anilide Le A 15 34~ - 12 -,1 ~,1 ' .~

~, ~(1 5{~3L96 350 parts by weight o~ short gla~s ~ibre~
The extruder i~ charged at a rate of 12 kg per hour.
The barrel temperature of the extruder, except in th~ ~eed zone, i~ 2600C. Water is sprayed into the barrel through the injection nozzle at a rate o~ 30 ml per hour. The pre~sure in the vacuum ; zone amount~ to 15 Torr. A strand is extruded and, ~fter ex-trusion, is cooled in water and granulated. It has an extract content of 2.9 ~.
~ EXAMPL~ 10 (Comparison Example) `` 10 The procedure is as in Example ~, except that no water is sprayed in. The extruded polyamide has an extract content of ~0.2%.
EX~MPLE 11 The test described in Example 1 is repeated with the :, '~',!, ~ollowing polymerisable mixture:
200 p~rts by weight o~ lauric lactam 5 parts by weight of sodium caprolactamate 12 parts by weight o~ hexamethylene-1,6-bis-(carbamido-I caprolaetam~
, 2 part~ by weight o~ N-cyclohexylbenzamide 15 parts ~y weight oi ~teario acid-N-cyclohexylamide The temperature programme is modified as follows: water-;l~ cooled feed zone/170~C/260C/260C/260~C/250C/die 250C. The other te~t parameter~ are unchanged. A polyamide-12 granulate with an extract content o~ 0.8 ~ is o~tained, ;1~ EXAMPLE 12 (Comparison ~`xample) ¦~ Tbe procedure is as in ~xample 11, except that no water is sprayed into the barrel. A polyamide~l2 with an extract content o~ 2.7 ~ is obtained.

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The test described in Example 1 is reqeated with the difference that the low molecular weight substances evapora ting in the evapc,ration ~one are not evacuated by a vacuum pump, but instead are removed by blowing in a vigorous str~m of nitrogen. The polyamide granulate obtained has an extræt content of 6.9%.

The test described in Example 13, is repeated with the difference that no water is sprayed into the extruder barrel.
lhe polyamide granulate obtained had an extract cont-én-t of 11.0%.

The tube reactor described in Example 7 opens into a 2 m long flanged-on tube of the same diameter which, a-t its front end, has a bore for an injection nozzle, followed by , static mixing elements which guarantee thorough dispersion of the water inJected in the polyamide melt. Water is sprayed in at a rate of 70 ml per hour. The tube reactor and mixlng tube are heated to 240 C. Subsequent evaporation o~f~the~l~ow~molecular weight constituents takes place in a vacuum falling-fllm evaporator 600 mm in diambter and 1500 mm long~heated to a temperature of 250C. The melt is discharged into the open through a di~oharge screw. A
strand is spun from the melt, being cooled in water and ;granulated. The material~contains 3.6% of extract.
EXAMPLE 1~ (Comparison Example) The procedure is as in Example 15, except that no .. . .
water~ls sprayed in. A polyamide granulate with an extract content of I0.2% is obtained.

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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An improved process for the production of polyamides of low extract content by activated anionic polymerisation of at least one lactam containing at least 5 ring members at a temperature above the melting point of the polyamide in a continuously working polymerisation apparatus, the improvement comprises treating the polyamide melt obtained by polymerising said lactam in said polymerisation apparatus with water or steam, for from 0.5 to 5 minutes before the polyamide melt is discharged into the open; and subsequently freeing the melt from volatile constituents by evaporation.

2. A process as claimed in claim 1, wherein the treatment time is from 1 to 2 minutes.

3. A process as claimed in claim 1, wherein the lactam is capro-lactam, lauric lactam, .alpha.-pyrrolidone, oenanthic lactam, capryllactam, or the corresponding C-substituted lactam, or mixtures thereof.

4, A process as claimed in claim 1, wherein said polymerisation apparatus is a ram or screw extruder or a tube reactor.

5. A process as claimed in claim 1, wherein a 5 to 20 fold excess of water or steam is used, based on the quantity of catalyst.

6. A process as claimed in claim 1, wherein the volatile con-stituents are evaporated off by the action of a vacuum or of an entraining gas.

7. A process as claimed in claim 6, wherein a vacuum of from 10 to 100 Torr is applied.

8. A process as claimed in claim 6, wherein the entraining gas is nitrogen.