Process for the production of polychloroprene

Object of the invention is a process for polymerizing chloroprene in aqueous emulsion in a certain apparatus which prevents formation of deposits and so-called "popcorn" polymers.

The polymerisation of chloroprene in aqueous emulsion has long been known 
and is carried out on an industrial scale. Chloroprene polymerises 
spontaneously and, for this reason, is generally protected against 
uncontrolled and premature polymerisation by the addition of inhibitors 
for example, phenothiazine. If chloroprene is emulsified in water and a 
radical polymerisation initiator added to the resulting emulsion, the 
mixture polymerises very quickly. 
One particularly favourable process for the polymerisation of chloroprene 
on a commercial scale is described in German Auslegeschrift No. 1,097,689. 
Chloroprene stabilised against polymerisation is used as the starting 
material in this process, being polymerised in alkaline emulsion in the 
presence of known emulsifiers and regulators with formamidine sulphinic 
acid as activator. 
Since the polymerisation of chloroprene takes place very quickly (the 
polymerisation velocity of chloroprene is about 700 times higher than that 
of isoprene (R. E. Burk, Ind. eng. chem. 30,1054 (1938)), the heat of 
polymerisation is extremely difficult to dissipate. Particularly 
unpleasant aspects of the polymerisation of chloroprene are, on the one 
hand, the formation of deposits from the emulsion and, on the other hand, 
the formation of so-called "popcorn" polymers. Popcorn polymers are 
heavily crosslinked insoluble products which can be formed in addition to 
and in competition with the normal polymers, especially in cases where 
chloroprene is present in liquid or gaseous form. Once formed, popcorn 
polymer seeds have an autocatalytic effect so that popcorn formation 
proceeds very rapidly once it has started (cf. J. W. Breitenbach: 
"Popcorn-polymerisation" Advances in Macromolecular Chemistry, Vol. 1). 
For these reasons, the continuous polymerisation of chloroprene in aqueous 
emulsion is particularly difficult because deposits formed from the 
aqueous emulsion collect on the walls of the reactor, restrict the 
dissipation of heat and, in addition, block the connecting pipes between 
the reactors. In addition, the monomer-containing deposits swell and 
represent a starting point for popcorn formation. 
The present invention provides a process for the polymerisation of 
chloroprene in aqueous emulsion with radical initiators and optionally in 
the presence of polymerisation inhibitors, wherein polymerisation is 
carried out in a closed, cylindrical, vertically arranged reactor with a 
length-to-diameter ratio of (2-30):1 which has a smooth surface, rounded 
corners and no fittings is completely surrounded by a heating and cooling 
jacket and is equipped with one or more propeller or impeller stirrers 
with 2 to 5 and preferably with 3 to 4 blades having an axis of rotation 
which is inclined at an angle of 0.degree. to 45.degree. preferably 
5.degree. to 45.degree., more preferably 10.degree. to 35.degree., 
especialy 15.degree. to 30.degree. with respect to the vertical and of 
which one is situated in the lower third of the reactor interior, the 
reactor being completely filled during polymerisation and its contents 
being mixed with an effective circulation volume of from 5 to 20 and 
preferably from 10 to 15 m.sup.3 /min per cubic meter of reactor volume. 
In order to obtain even better admixture of the reactor contents, it can be 
of advantage to reverse the stirrer at regular intervals from 
anticlockwise to clockwise rotation and vice versa. 
The polymerisation process itself is carried out in conventional manner, 
i.e. in an aqueous emulsion using radical initiators at temperatures in 
the range from about 0.degree. to about 70.degree. C., polymerisation 
generally being continued up to a conversion of from 60 to 80% (cf. for 
example U.S. Pat. Nos. 1,950,436; 2,227,517; 2,321,693; 2,371,719; 
2,463,225; 2,481,044; 2,494,087; 2,567,117; 2,567,009; 2,831,842; 
2,914,497; 2,467,769; 3,147,318; 3,147,317; GB-PS No. 1,052,581). It is 
essential to use the reactor described in detail in the following: 
The reactor is closed, cylindrical and vertically arranged; volume 0.5-30 
m.sup.3, with a length-to-diameter ratio of (2-30):1, with a smooth 
surface, rounded corners and no fittings (for example baffles), completely 
surrounded by a heating and cooling jacket and provided in its lower or 
(and) upper third on the reactor axis with propeller stirrers comprising 2 
to 5 blades having an axis of rotation inclined at an angle to the 
vertical of from 0.degree. to 45.degree. preferably 5.degree. to 
45.degree., more preferably 10.degree. to 35.degree., especially 
15.degree. to 30.degree. with respect to the vertical. The reactor is 
completely filled during polymerisation; its contents are completely mixed 
with an effective circulation volume adapted to the volume of the reactor: 
"effective circulation volume": 5-20 m.sup.3 /min. per cubic meter of 
reactor volume, preferably about 10 to 15 m.sup.3 /min. per cubic meter of 
reactor volume. 
The "effective circulation volume" is defined as throughflow through the 
propeller circuit per unit of time. 
In reactors with a large length-to-diameter ratio (i.e. large 
surface-to-volume ratio; advantageous for dissipating the heat of 
polymerisation) it is also possible to arrange one propeller stirrer in 
the lower third and another in the upper third of the reactor, or to 
arrange several propellers on one shaft in order uniformly to cover all 
the zones of the reaction vessel.

Where this reactor is used, polymerisation may be carried out in batches. 
In batch operation, polymerisation is continued until the required 
conversion has been reached. 
It is also possible and preferred to arrange several, for example 3 to 6 
reactors, in series and to carry out polymerisation continuously in the 
reactor cascade thus formed. In this case, the first reactor is filled 
continuously from below, the polymerised reaction mixture is removed at 
the upper end, introduced into the next reactor from below and so on until 
it has passed through all the reactors of the cascade. 
More specifically, the process may be carried out as follows: 
The aqueous, organic and activator phases (for example formamidine 
sulphinic acid) are introduced from below into the coolable, 
nitrogen-purged first reactor of a reactor cascade. After heating to the 
reaction temperature, the polymerisation reaction begins, as reflected in 
an increase in temperature. 
The heat of polymerisation has to be dissipated by cooling (for example 
with brine or water). Accordingly, each reactor is equipped with an 
independent cooling and heating circuit. 
In cases where the cascade consists, for example, of 6 reactors, activator 
may be added, for example, in the first, third and fourth reactors. If 
necessary, a polymerisation inhibitor (for example p-tert.-butyl 
pyrocatechol, dissolved in aqueous sodium hydroxide) may be added to the 
second reactor in order to slow down an over vigorous reaction. 
The conversion in the individual reactors may be controlled through the 
quantities of activator introduced into the individual reactors in 
conjunction with the throughput (quantity of monomer introduced per unit 
of time). 
The conversion may be determined by measuring the density of the emulsion 
or by determining the solids content of the latex. 
After leaving the last reactor, polymerisation is stopped by removing the 
unreacted monomer ("degassing of the latex"). 
The limiting factor so far as the potential throughput is concerned is the 
dissipation of the heat of polymerisation in the reactors. For example, a 
reactor cascade may consist of 5 reactors, reactor volume approximately 6 
cubic meters, length-to-diameter ratio 5:1, equipped with propeller 
stirrers 1 meter in diameter with 3 blades, pitch angle of the propeller 
blades 30.degree., angle of the propeller axis with respect to the 
vertical 15.degree., speed of rotation; 160 rpm, effective circulation 
volume approximately 75 m.sup.3 /min; height of propeller above reactor 
base: 0.7 m; constituent material of the reactor wall and propeller: 
chemically inert materials, for example stainless steel, Teflon enamel, 
etc. 
With a throughput of approximately 5000 liters of chloroprene per hour, the 
polymerisation cascade may be operated continuously for several months 
without any need for general cleaning. 
This procedure almost completely eliminates popcorn formation and the 
formation of deposits from the polymerising emulsion. Accordingly, the 
process may be operated continuously over a long period without any need 
for intermediate cleaning. 
In carrying out the process, it is important above all to ensure that the 
reactor or reactors is/are always completely filled and that its/their 
contents are adequately mixed. This is guaranteed by a recirculation 
output of the propeller stirrer of from 5 to 20 m.sup.3 /min. per cubic 
meter of reactor volume. 
EXAMPLE 
The aqueous phase (W) and the monomer phase (M) maintained in a constant 
ratio through a measuring and regulating system, and the activator phase 
(A), are introduced into the first reactor of a polymerisation cascade 
consisting of 5 identical reactors. 
Specification of the reactors used: 
volume: approximately 5 cubic meters; temperature sensors at the bottom of 
the reactors; 
L/d-ratio: approximately 3:1 
1 propeller stirrer (at the bottom of the reactor) with 3 blades propeller 
diameter: .about. 1 meter pitch angle of the propeller blades: 30.degree. 
angle of the propeller axis with respect to the vertical: 
15.degree. height of the propeller above the base: approximately 0.7 m 
rotational speed: .about. 160 rpm effective circulation volume: 
approximately 75 m.sup.3 /min constituent material of the reactor wall and 
the propeller: V4A steel 
connecting pipes between the reactors: V4A steel, nominal width: 65 mm 
Composition of the three phases (as described in DT-OS No. 2,241,394, 
Example 2): 
______________________________________ 
(M) = Monomer phase: 
chloroprene 95.0 parts by weight 
2,3-dichloro-1,3-butadiene 
5.0 parts by weight 
n-dodecyl mercaptan 
0.3 part by weight 
phenothiazine 0.015 part by weight 
______________________________________ 
______________________________________ 
(W) = Aqueous phase: 
deionised water 120.0 parts by weight 
sodium salt of a dispropor- 
tionated abietic acid 
3.5 parts by weight 
sodium salt of a condensa- 
tion product of naphthalene 
sulphonic acid and 
formaldehyde 0.65 part by weight 
caustic soda 0.65 part by weight 
tetrasodium pyrophosphate 
0.5 part by weight 
______________________________________ 
(A) = Activator phase 
1% aqueous formamidine sulphinic acid solution 
The three phases are combined immediately before entering the first 
reactor. The reaction begins gently when the emulsion is heated to 
approximately 40.degree. C., as reflected in an increase in the internal 
temperature of the reactor to the required level of 43.degree. C. The heat 
of polymerisation evolved is dissipated by cooling. 
After a residue time determined by the monomer throughput, the reaction 
mixture flows continuously from below into the second reactor and from 
there into the third reactor, etc. After the fifth reactor, the latex is 
freed from residual monomer after the required conversion of approximately 
65% has been reached. 
In continuous operation, it is desirable to distribute the conversion as 
uniformly as possible among the reactors. To this end, 1% aqueous 
formamidine sulphinic acid solution may be introduced into the first, 
third and fourth reactors. The conversion required in each reactor may be 
controlled through the quantity of activator solution introduced. In the 
second reactor, over-rapid polymerisation may be slowed down by the 
introduction of an aqueous alkaline solution of p-tert.-butyl pyrocatechol 
(1.0% by weight in 0.5 n NaOH). 
The throughput of monomer through the polymerisation cascade is limited 
inter alia by the maximum dissipation of heat from the reactors. With a 
monomer throughput of 5000 l/h, the polymerisation cascade may be operated 
continuously for several months without any need for intermediate cleaning 
.