Activated anionic polymerization of lactams with liquid isocyanatoallophanate activator

Processes for the production of polyamides by polymerizing lactams in the presence of a catalyst and anisocyanatoallophanate as activator.

This invention relates to a process for the production of polyamides, by 
the activated anionic polymerisation of lactams. 
Various methods can be used for the production of moulded polyamide 
products by the activated anionic polymerisation of lactams. Both a 
trouble-free polymerisation and the quality of the polyamides produced by 
the various techniques depend not only on the catalyst but to a 
considerable extent also on the nature of the activator used. 
Numerous compounds have been used as activators for the anionic 
polymerisation of lactams, e.g. acyl lactams, substituted triazines, 
carbodiimides, cyanamides, isocyanates, and the corresponding masked 
isocyanate compounds. The activators used may be either monofunctional or 
polyfunctional but, as is well known, the products obtained when using 
more than difunctional activators have higher molecular weights and are, 
in part, cross-linked. 
The activators frequently used in practice are polyisocyanates, preferably 
diisocyanates. Hexamethylene diisocyanate is particularly preferred 
because it is liquid and therefore can be very advantageous used in 
continuous processes in which exact metering through pumps is important. 
Hexamethylene diisocyanate has, however, the considerable disadvantages of 
being toxic due to its high vapour pressure (3 Torr at 102.degree. C., 20 
Torr at 143.degree. C.), which means that special precautions have to be 
taken with each operation. This is particularly important in the event of 
an interruption in a continuous process which may necessitate opening of 
the pumps and pipes. 
Hexamethylene-bis-carbamido caprolactam which is recommended as activator 
in U.S. Pat. No. 3,304,291 is physiologically harmless. Like practically 
all diisocyanates which are masked with lactams, however, it is a 
crystalline substance which is virtually unusable for continuous 
operations. 
The known common used polyfunctional activators thus have the disadvantage 
of being either crystalline but relatively safe to handle on account of 
their low vapour pressure, or liquid and therefore toxic on account of the 
high vapour pressure of free diisocyanates. 
The use of a solution of an crystalline activator in an inert organic 
solvent is not feasible in practice on account of the disadvantages 
resulting from the relatively large quantities of solvent necessary (in 
most cases more than 100%), which cause a polyamide with a poor quality 
due to pitting or even foaming. Moreover, even small quantities of 
solvents lead to an undesirable reduction in the quality of the finished 
products due to the formation of small bubbles in the polymer. 
The use of solvent-free melts is unsatisfactory because this necessarily 
entails using heated pumps and pipes and because of the possibility of 
decomposition of the masked isocyanates. 
It has now surprisingly been found that allophanates which contain 
isocoyanate groups and preferably have dynamic viscosities below 5000 mPas 
are activators which do not have the disadvantages mentioned above. 
Activators of this type fulfil the requirements of being safe to handle, 
fluid, stable on storage, highly reactive and resulting in polyamides with 
advantageous properties. 
This invention therefore relates to a process for the preparation of 
polyamides by the polymerisation of lactams in the presence of a catalyst 
and an activator, wherein the activator used is an isocyanatoallophanate 
which has a dynamic viscosity below 5000 milli Pascal seconds (mPas), 
preferably below 2000 mPas. 
Urethane isocyanates are preferably used a starting materials for the 
preparation of the activators according to the present invention. These 
compounds are generally obtained by the reaction of isocyanates with 
compounds which have alcoholic hydroxyl groups. They preferably correspond 
to the following general formula: 
##STR1## 
Wherein: A represents a group obtained by the removal of the hydroxyl 
groups from an organic compound having a valency of n which contains 
hydroxyl groups but is otherwise inert towards isocyanate groups; 
R.sub.1 represents a group such as is obtained by the removal of the 
isocyanate groups from an organic diisocyanate; and 
n represents an integer of from 1 to 4, preferably 1 or 2. 
In accordance with the above definitions, the urethanes containing 
isocyanate groups which correspond to the above general formula and which 
can be used for preparing the activators according to the present 
invention are preferably obtained in the following way. 
Hydroxyl group-containing compounds corresponding to the following general 
formula: 
EQU A(OH).sub.n 
are reacted with diisocyanates corresponding to the following general 
formula: 
EQU R.sub.1 (NCO).sub.2 , 
preferably using at least n mol of the diisocyanate per mol of the hydroxyl 
compound. This reaction, which results in the formation of the starting 
materials containing urethane groups, could also be carried out with less 
than n mols of a diisocyanate per mol of hydroxyl compound A(OH).sub.n, 
i.e. the quantity of diisocyanate used could be in the region of from 0.5n 
to 1n mol per mol of the compound containing hydroxyl groups. In that 
case, the starting materials obtained would have more than n urethane 
groups on account of the chain lengthening reaction which would occur via 
urethane groups. 
The hydroxyl compounds A(OH).sub.n may be organic compounds containing 
alcoholic hydroxyl groups, preferably low molecular weight aliphatic with 
C.sub.1 -C.sub.18, araliphatic with C.sub.7 -C.sub.15 and/or 
cycloaliphatic with C.sub.5 -C.sub.17 alcohols, i.e. having a molecular 
weight in the range of from 32 to 300, having a valency of from 1 to 4 and 
optionally containing ether bridges. It is preferred to use hydroxyl 
compounds of the above-mentioned type having only one OH group because 
these result in exceptionally low viscosity and therefore highly fluid 
activators. 
The following are examples of suitable monohydroxyl compounds: methanol, 
ethanol, propanol, isopropanol, isomeric butanols, allyl alcohol, 
pentanols, hexanols, heptanols, 2-ethylhexanol, fatty alcohols having from 
10 to 18 carbon atoms, cyclopentanol, cyclohexanol, methyl cyclohexanol, 
cyclohexenyl-cyclohexanol, benzyl alcohol, phenyl ethyl alcohol and 
ethylene glycol monoalkylether. 
Mixtures of the above-mentioned hydroxyl compounds may, of course, also be 
used. This is even the preferred method of preparing the activators 
according to the present invention because the fluidity of the resulting 
polyisocyanate activators can be varied as desired by using a mixture of 
hydroxyl compounds. 
The isocyanates which are used not only for preparing the urethane group 
containing compounds used as starting materials, but which also serve as 
their reactants for preparing the isocyanato-allophanates, are preferably 
diisocyanates corresponding to the following general formula: 
EQU R.sub.1 (NCO).sub.2 
wherein: R.sub.1 represents an aliphatic hydrocarbon group having from 2 to 
20, preferably from 6 to 10 carbon atoms, a cycloaliphatic hydrocarbon 
group having from 4 to 20, preferably from 6 to 15 carbon atoms, an 
aromatic group having from 6 to 20, preferably from 6 to 16 carbon atoms 
or an aliphatic-aromatic group having from 7 to 21, preferably from 7 to 
18 carbon atoms. 
The following are examples of such isocyanates: ethylene diisocyanate, 
tetramethylene diisocyanate, hexamethylene diisocyanate, undecamethylene 
diisocyanate, 2,2,4- and 2,4,4-trimethyl-1,6-diisocyanatohexane, 
3-isocyanatomethyl-3,5,5-trimethyl-cyclohexylisocyanate, 
1,4-diisocyanatocyclohexane, diisocyanatodicyclohexyl-methanes, and m- 
and/or p-xylylene diisocyanate. 
Diisocyanates of this type are used both for preparing the urethane 
isocyanates and for reacting with these compounds to prepare the 
isocyanatoallophanates. 
The viscosity range can be adjusted, to a large extent, as desired by 
suitable choice of the proportions of isocyanate components in a mixture 
in the same way as by the choice of OH components. Diisocyanates in which 
both isocyanate groups are attached to primary carbon atoms, preferably to 
aliphatic or cycloaliphatic hydrocarbon groups are preferred on account of 
their more powerful activating properties. 
Hexamethylene diisocyanate is particularly preferred. 
One preferred variation of the synthesis of the activators according to the 
present invention in which the starting compound containing urethane 
groups is prepared in situ has been described in German Patent 
Offenlegungsschrift No. P 27 29 990.7. The procedure is as follows: 
The diisocyanate preferably used as the isocyanate component is introduced 
into the reaction vessel at from 50.degree. to 80.degree. C. and the 
hydroxyl component is introduced dropwise as a liquid whilst the mixture 
is vigorously stirred. If the same isocyanate or isocyanate mixture is to 
be used both for urethane formation and for allophanate formation, it is 
simplest to use this in such an excess right from the start that the 
NCO/OH ratio is approximately of from 3:1 to 12:1. 
After completion of the urethane reaction, which is ascertained by 
determination of the isocyanate content, the catalyst (generally hydrogen 
chloride) is added. The temperature is then raised to 
90.degree.-140.degree. C. and the reaction mixture stirred until the 
isocyanate content has fallen to the value calculated for complete 
allophanatisation. 
When the reaction has terminated, the catalyst, together with excess 
diisocyanate, is removed by thin layer distillation. 
The allophanatization reaction can be demonstrated by the following 
reaction scheme: 
##STR2## 
wherein A and R.sub.1 are as aforesaid. 
3 to 10 diisocyanates should be linked with one urethaneisocyanate by 
reacting each time one isocyanate group with the NH-group in order to 
obtain allophanate-polyisocyanates having an isocyanate content of 10 to 
25% by weight, preferably 15 to 20% by weight, and preferably dynamic 
viscosities of at the most 5000 mPas, more preferably not more than 2000 
mPas, measured by the falling body method of Hoppler. They are 
distinguished by their excellent stability during thin layer treatment 
even above 180.degree. C. 
The activators used according to the present invention are relatively safe 
to handle and physiologically less harmful on account of their extremely 
low free diisocyanate content and their low vapour pressure (&lt;10.sup.-2 
Torr at 100.degree. C., 0.1 Torr at 140.degree. C.) and they can easily be 
delivered through ordinary commercial pumps. They do not noticeably react 
with water at room temperature, and in the event of breakdown or on 
occasions of cleaning, they have the advantage of being readily soluble in 
commonly used rinsing agents, such as isopropanol, in spite of gradually 
reacting with the alcohol. 
The excellent stability on storage of the allophanate polyisocyanates which 
have been freed from excess isocyanate used as activators should also be 
particularly mentioned. 
The activators show no tendency to break down into the monomeric isocyanate 
used for the formation of the allophanates and it is particularly in this 
respect that they differ advantageously from other types of polymerized 
diisocyanates. 
For the activated anionic polymerization of lactams, the activators are 
supplied continuously or intermittently at the usual concentrations, 
preferably from 0.1 to 1 mol %, based on the lactam to the lactam melt 
which is to be polymerized. 
Any catalysts used for the anionic polymerisation of lactams may be used at 
the usual concentrations, particularly alkali metal and alkaline earth 
metal lactamates such as sodium lactamates or sodium hydride as well as 
latent catalysts. 
The activators used according to the present invention are suitable for the 
polymerisation of lactams which have at least five ring members, 
preferably at least seven ring members, such as .alpha.-pyrrolidone, 
.epsilon.-caprolactam, C-substituted caprolactams, lauric lactam or 
mixtures of the aforesaid lactams. 
Allophanate isocyanates which have been prepared from hexamethylene 
diisocyanate and C.sub.1 -C.sub.5 aliphatic mono alcohols are particularly 
preferred activators. 
The activators can be used in any known procedure for the activated anionic 
polymerisation of lactams but are preferably used in continuous processes. 
They may be used, for example, in the process of pressure free casting for 
the manufacture of semifinished products. In this process, two separate 
lactam melts are prepared, one containing the catalyst, the other 
containing the activator, the two melts are then combined and mixed, and 
the mixture is then immediately introduced into a casting mould. 
Polymerisation is carried out at the usual temperatures of from 
140.degree. to 200.degree. C., the resulting polyamide assuming the form 
of the mould. The activators may also be used for the rotational moulding 
process in which a polymerisable melt is introduced into a mould which 
rotates about two axes at an angle to each other, and polymersation is 
started by heating. 
Further examples of procedures in which activated anionic polymerisation 
according to the present invention may be used include roll casting and 
polymerisation in the cylinder of an extruder or an injection moulding 
machine. 
The process according to the present invention is particularly important 
for rotational moulding in order to produce large hollow bodies which is 
known as "puddle technique" (U.S. Pat. No. 3,417,097). The products have a 
substantially higher impact strength than those produced with the aid of 
conventional activators. 
Large hollow bodies produced in this way are used mainly as storage tanks 
for fuel oils particularly as battery tanks.

EXAMPLE 1 
(a) Preparation of an isocyanatoallophanate 
333 g (4.5 mol) of n-butanol were added dropwise in the course of 30 
minutes to 3024 g (18 mol) of hexamethylene diisocyanate at 70.degree. C. 
in a 6-liter three-necked flask. All the OH groups had been converted into 
urethane groups after a further 30 minutes' reaction at 70.degree. C. 
7.2 g (0.2 mol) of hydrogen chloride were introduced, and the temperature 
was raised to 100.degree. C. All the urethane groups had been converted 
into allophanate groups after a reaction time of 8 hours. The crude 
product was subjected to thin layer distillation, whereby the excess of 
hexamethylene diisocyanate was separated from the colourless 
polyisocyanate containing allophanate groups and having an isocyanate 
content of 17.3% by weight. 
The viscosity of the isocyanatoallophanate was 160mPas at 25.degree. C. The 
product still contained 0.46% of free hexamethylene diisocyanate. 
After 40 days' storage at 50.degree. C., the free hexamethylene 
diisocyanate content was unchanged at 0.46%. 
(b) Method of polymerisation 
97.6 Parts by weight of .epsilon.-caprolactam were evenly distributed into 
two containers. One half of the lactam was mixed with 1.6 parts by weight 
of a solid 80% by weight solution of sodium caprolactamate in caprolactam 
while 0.7 parts by weight of the activator prepared according to a) was 
added to the other half of the lactam. The mixtures were melted at 
120.degree. C. under a nitrogen atmosphere. The two melts were combined in 
a proportion of 1:1 in a mixing head into which they were delivered 
through metering pumps. The resulting mixture was fed from the mixing head 
into a rotational mould measuring 300.times.200.times.180 mm which was 
heated to 180.degree. C. Introduction of the reactive lactam melt into the 
mould was stopped after 1600 g had been fed in. Biaxial rotation of the 
mould at a speed of 25 min.sup.-1 about the primary axis and 10 min.sup.-1 
about the secondary axis resulted in a hollow polyamide body having a wall 
thickness of 4 mm. Polymerisation was completed after 3 minutes and the 
hollow body was removed from the mould after it had been left to cool by 
air for one minute. 
The hollow bodies produced were stored under normal atmospheric conditions 
(23.degree. C., 50% relative humidity) for 24 hours. Samples in the form 
of standard test rods measuring 4 mm .times.50 mm .times.6 mm were cut out 
of the wall of the hollow body and used to determine the impact strength 
in the cold according to DIN 53 453. Before measurement of this impact 
strength, the test rods were cooled to -15.degree. C. for 16 hours. 
The data characterising the progress of polymerisation and the values for 
impact strength in the cold are summarised in Table 1. 
EXAMPLES 2-6 
Further polymerisation experiments were carried out by the procedure 
described in Example 1b, using other activators which had been prepared by 
methods analogous to those of Example 1(a). 
The characteristics of the activators and of the polyamides obtained by 
methods analogous to that of Example 1(b) are also summarised in Table 1. 
Table 1 
__________________________________________________________________________ 
Characteristics of the activators used in Examples 2 to 7 and 
polymerisation data and 
properties of the polyamides. 
Quantity Extract 
NCO of acti- content Impact strength 
(kJ/m.sup.2) 
Activator content 
.eta.25.degree. C. 
vator (% by 
.sup.xx 
External 
Internal 
Example 
prepared from (molar ratio) 
(%) (mPas) 
(% by wt.) 
ti.sup.x 
weight 
.eta.rel 
surface 
surface 
__________________________________________________________________________ 
2 HMDI/n butanol 
4:1 17.3 
160 0.7 7'10" 
2.7 6.3 
42.0 52.2 
3 HMDI/methanol 
4:1 19.5 
260 0.6 7'35" 
4.6 5.6 
40.5 48.0 
4 HMDI/cyclohexanol 
4:1 16.4 
900 0.8 7'15" 
3.7 6.2 
38.6 42.0 
5 HMDI/2-ethylhexanol 
4:1 14.9 
240 0.8 8'40" 
3.6 5.6 
40.8 46.9 
6 HMDI/diethylene 
4:1 15.2 
150 0.8 9'20" 
5.2 5.4 
37.5 42.4 
glycol monobutyl- 
ether 
7 IMDI/1,2-propylene 
10:1:1 
17.7 
4600 
0.7 8 40 25" 
4.8 6.7 
41.5 49.5 
glycol/cyclohexanol 
__________________________________________________________________________ 
HMDI = hexamethylene diisocyanate 
.sup.x Polymerisation t.sub.i was determined in experiments carried out o 
a 100 g scale. 
t.sub.i is the time from immersion of the complete reaction mixture into 
the heating bath at 180.degree. C. until the viscosity begins to rise. 
.sup.xx .eta.rel = viscosity of polyamide (1% solution in mcresol at 
25.degree. C.)