Process for continuous mass polymerization of alkenyl-aromatic compounds

Styrene-containing polymers are produced by one- or multi-stage prepolymerization and subsequent continuous, one- or multi-stage main polymerization with final working up. In the main polymerization stage(s) the mixture is subjected simultaneously to a heat exchange and a static mixing procedure.

Styrene polymers are presently used in many fields of application in the 
form of transparent polystyrene, as copolymer with acrylonitrile (SAN) and 
as impact-resistant polymers (impact-resistant polystyrene, ABS) 
containing a natural or synthetic rubber as component imparting impact 
resistance. 
In conformity with the importance of the aforesaid materials a great number 
of processes have been developed for their manufacture. For economical 
reasons they are preferably produced by continuous mass processes, 
optionally with the addition of up to 20% of an inert diluent. The 
reaction is initiated thermally or by adding free radical donating agents. 
The polymerization is optionally carried out in the presence of 
plasticizers, regulators, antioxidants and other additives. 
For the utilitarian properties of all plastic materials containing 
polystyrene it is essential that the molecular weight average is in the 
range of from 150.multidot.10.sup.3 to 350.multidot.10.sup.3 and that the 
proportion of material having a low molecular weight below 
100.multidot.10.sup.3 is as low as possible. 
It is known that the reaction temperature is an essential determining 
factor for the molecular weight and molecular weight distribution. The 
polymerization of styrene is exothermal with a reaction heat of 175 
kcal/kg. A completely adiabatic reaction would result in a strong 
self-heating of the mass by about 4.degree. C. for each percent of styrene 
conversion. Every expert knows this phenomenon as a getting-out of control 
of the reaction which leads to an increased formation of low molecular 
weight fractions. Besides a broad molecular weight distribution, products 
resulting from such a process have poor mechanical properties. 
In order to maintain the optimum polymerization temperature in each case it 
is, therefore, necessary to remove the major part of the reaction heat 
from the polymerizing mixture. 
The fact that with prolonged duration of polymerization the viscosities 
strongly increase naturally renders difficult the dissipation of heat over 
the walls of the reactor. It, therefore, is the state of the art to carry 
out the polymerization in the low viscous range in vessels provided with 
stirring means and with intense stirring, while additionally utilizing the 
heat of evaporation of the boiling monomer for cooling of the polymerizing 
mixture. 
In the manufacture of transparent homopolystyrene, the viscosities permit a 
reaction in a stirred vessel up to a polymer content of about 70%. In the 
manufacture of impact resistant polystyrene, however, the polymerizing 
mixture having a polymer content of from 30 to 50% has to be transferred 
from the intensely stirred vessel into a tube reactor with gentle 
agitation, as otherwise the content of soft phase in the products is 
considerably reduced. This phenomenon is described in U.S. Pat. No. 
3,243,481. 
To improve the heat exchange in tube reactors with gentle stirring the 
reactors can contain extensive heat exchange means. A reactor of this type 
is described in U.S. Pat. No. 2,727,884. Such a reactor has the form of a 
slim tower or tank provided with heat exchange tubes in transverse 
position with respect to the longitudinal axis. Between the layers of heat 
exchange tubes stirrers are arranged which rotate slowly and improve the 
heat transport. With insufficient stirring, channeling of the material in 
the direction of flow of the material may occur so that portions rich in 
monomer come into temperature zones intended for a reaction mass with low 
monomer content which results in a broad molecular weight distribution and 
whereby the reaction may get out of control. 
U.S. Pat. No. 3,243,481 describes a process for the manufacture of 
impact-resistant polystyrene in a reactor as described above. In this 
process a styrenic rubber solution containing an inert diluent and 
optionally a plasticizer is continuously reacted in a stirred vessel until 
a polymer content is reached which corresponds to twice the amount of 
rubber used. This prepolymer is continuously withdrawn from the reactor 
and transferred to tube reactors of the above type connected in series in 
which polymerization is continued to a polymer content of 80 to 95%. 
Finally, the resulting polymer is freed from volatile constituents in a 
working up stage. 
DE-OS No. 1,770,392 discloses a similar process for the manufacture of 
styrene-containing polymers. In this process the prepolymerization up to a 
solids content of 35% is carried out in 2 stages, i.e. in two 
series-connected stirred vessels (two-stage stirred tank cascade). The 
polymerization is then completed in two series-connected stirred reactors. 
The space-time-yield obtained in this process amounts to about 82 kg of 
polymer per hour per cubic meter of reactor volume. 
An analogous process described in DE-OS No. 2,539,605 gives 
space-time-yields of approximately 79 kg per hour per cubic meter of 
reactor volume. 
The said space-time-yields are relatively low and, moreover, the reactors 
used are complicated due to the combination of stirrers and heat 
exchangers and, therefore, they are susceptible to failures. 
It is the object of the present invention to provide a process for the 
manufacture of styrene-containing polymers which overcomes the 
aforedescribed disadvantages. 
A process has been found for the continuous mass homo- and copolymerization 
of styrene which gives a high space-time-yield as well as products having 
favorable properties. 
The present invention provides a process for the manufacture of 
alkenyl-aromatic homo- or copolymers by continuous mass polymerization, 
initiated thermally or by means of free radical initiators, of an 
alkenyl-aromatic compound or a mixture of alkenyl-aromatic compounds, 
optionally in the presence of a nitrile or an ester of acrylic or 
methacrylic acid and/or a natural or synthetic rubber, with a one-, two- 
or multi-stage continuous prepolymerization and subsequent one-, two- or 
multi-stage continuous main polymerization and final working up, which 
comprises subjecting the polymerizing mixture to a simultaneous treatment 
with heat exchange and static mixing effect in the stage(s) of the main 
polymerization. 
Alkenyl-aromatic compounds that can be polymerized by the process of the 
invention are styrene, .alpha.-methylstyrene, styrenes carrying alkyl 
substituents in the benzene nucleus, for example o-, m-, and 
p-vinyl-toluene, the various isomeric vinyl-xylenes, o-, m-, and 
p-ethylvinylbenzene, styrenes halogenated in the benzene nucleus, such as 
o-, m- and p-chloro- or bromo-vinylbenzene, corresponding compounds with 
hydrogenated benzene nucleus, for example vinylcyclohexane, 
1-methyl-2-vinylcyclohexane, 1-methyl-3-vinylcyclohexane, 
1-methyl-4-vinylcyclohexane. 
Alternatively, mixtures of the aforesaid compounds can be copolymerized or 
mixtures of the said compounds with a nitrile or an ester of an aliphatic 
alcohol having from 1 to 8 carbon atoms with acrylic acid or methacrylic 
acid. 
It is also possible to graft-polymerize solutions of a natural or synthetic 
rubber such as polybutadiene, polyisoprene, butadiene-styrene rubber, 
ethylene-propylenediene (conjugated or non-conjugated) rubber in which the 
diene is, for example, hexadiene-1,4, dicyclopentadiene, 
5-ethylidene-norbornene-2 or another 5-alkylidene-norbornene-2, in one of 
the aforesaid alkenyl-aromatic compounds or in a mixture of these 
compounds with one another or with one of the specified derivatives of 
(meth)arcrylic acid. 
The polymerization of styrene or mixtures of styrene with 
.alpha.-methylstyrene, acrylonitrile and/or with one of the said rubbers 
is preferred. 
The polymerization is initiated either thermally or by known initiators 
yielding free radicals upon decomposition, for example, azo compounds such 
as azodiisobutyronitrile, or peroxides such as benzoyl peroxide and its 
derivatives, or lauroyl peroxide. 
The initiators are added preferably by dissolving in the monomer or monomer 
mixture. 
Known regulators to adjust the molecular weight can also be added to the 
monomers, for example mercaptans or dimeric .alpha.-methylstyrene as well 
as the known plasticizers such as white oils or various esters of phthalic 
acid, and the usual antioxidants and UV stabilizers. 
It is also possible to add to the monomers inert diluents such as aromatic 
hydrocarbons, for example ethylbenzene, toluene, xylenes or benzene. 
The prepolymerization of the monomer or monomer mixture is carried out in 
one or more (series-connected) stirred vessels with continuous flow at a 
temperature of from about 373.degree. to about 423.degree. Kelvin, 
preferably about 388.degree. to about 413.degree. K. with thermal 
initiation and about 323.degree. to about 373.degree. K., preferably about 
333.degree. to about 353.degree. K. with the use of free radical 
initiators. 
A uniform flow of monomer or monomer mixture containing further additives 
as specified above is continuously fed to the (first) prepolymerization 
vessel. 
With the use of more than 1 prepolymerization vessel a uniform flow of 
material is continually withdrawn from the first vessel and fed into the 
second one and so on. 
A uniform flow of prepolymer is continually discharged from the (last) 
prepolymerization vessel and transferred to the main polymerization. 
The stationary polymer content adjusting in the first stirred vessel, if 
only one is used, or in the second or last stirred vessel with the use of 
two or more vessels is suitably in the range of from 20 to 70% by weight, 
preferably 30 to 60% by weight in the absence of a rubber and preferably 
in the range of from 25 to 35% by weight in the presence of a rubber. When 
more than one prepolymerization vessels are used, the total conversion of 
the prepolymerization can be distributed as desired among the individual 
vessels. 
As prepolymerization vessel commercial apparatus provided with jacket for 
temperature control and stirring means is used. 
The main polymerization is carried out in one or two or more than two, 
horizontal or vertical, series-connected tube reactors containing a 
plurality of static mixing fitments with means for temperature control. 
The reactor used for the main polymerization consist of a tubular housing 
provided with a jacket with means for temperature control, for example a 
double jacket through which a heat transfer medium flows, and internal 
fitments for treating the mixture flowing through the housing with a 
mixing and simultaneously heat exchanging effect. The fitments consist of 
tubes which extend in the axial direction of the housing and possess 
curved tube portions and other tube portions connecting the curved 
portions. The tubes are arranged parallel to one another, with the 
connecting tube portions of adjacent tubes crossing one another. 
According to a preferred embodiment of the reactor to be used for the 
stage(s) of the main polymerization, the curved tube portions and the 
connecting tube portions extend in one plane and the planes formed in this 
manner are parallel to one another. 
According to another preferred embodiment of the reactor, the curved tube 
portions of the tubes extend as far as the housing preferably having a 
circular cross section and the rectilinear connecting tube portions of 
adjacent tubes cross at right angles. 
The tube fitments can consist of tube bundles connected in series which are 
turned relative to one another, preferably through 90.degree., about the 
axis of the housing. 
It proved especially advantageous to provide the curved tube portions 
additionally with guide elements.

In the reactor housing 1, which can be provided with a double jacket 3 
there are arranged tube fitments 2 extending in axial direction of the 
housing for the simultaneous mixing and heat exchanging treatment of the 
mixture flowing through the reactor. The tubes 2 comprise rectilinear tube 
portions 2a and curved tube portions 2b which for each tube are in one 
plane. The planes formed by the tubes lie parallel to one another. The 
connecting rectilinear tube portions 2a of adjacent tubes 2 cross (shown 
in the drawing as preferred embodiment) at an angle of about 90.degree.; 
larger and smaller angles are also possible. 
Each tube is provided with an inlet 7 and an outlet 10 for a heat exchange 
medium. For technical reasons it can be advantageous to connect the 
individual tubes 2 at their ends in order to have to pass a minimum number 
of inlets and outlets through the reactor wall. In the embodiment 
according to FIG. 2, the tubes of each half are grouped together and 
provided with inlets 7 and 9 and outlets 8 and 10. 
With the use of a housing 1 with circular cross section spandrel-like 
spaces are formed in which internal fitments, for example guide plates 5, 
can be located. 
In the example shown in the drawing, the rectilinear tube portions 2a of a 
tube 2 are parallel to one another and inclined at 45.degree. to the axis 
of the housing. Other angles than 45.degree. can also be chosen. Variants 
in which the rectilinear portions 2a of a tube 2 do not run parallel to 
one another are also feasible. In the reactor shown adjacent tubes 2 touch 
one another. 
Mixing, the radial distribution and the uniformity of the residence time of 
the medium flowing through the reactor can be influenced in advantageous 
manner over the entire cross section of the reactor if guide elements 4 
and 4a are located in the region of the curved tube portions 2b. 
In FIG. 1, 6 denotes the direction of flow of the medium through the 
reactor and in FIGS. 1 and 2, 11 and 12 denote orifices in the double 
jacket for the introduction and withdrawal of a heat exchange medium. 
The prepolymer discharged from the (last) prepolymerization vessel is 
introduced into the (first) reactor for the main polymerization as 
described above and flows through this and optionally further reactors. 
The initial temperature of the polymerization mixture when entering the 
(first) reactor is preferably 393.degree. to 423.degree. K., more 
preferably about 413.degree. K. With flow through the reactor(s) the 
temperature slowly rises to 443.degree. to 493.degree. K., preferably 
443.degree. to 463.degree. K. 
On issuing from the (last) reactor the mixture has a polymer content of 75 
to 95% by weight, preferably 85 to 90% by weight. 
The mixture obtained is subsequently worked up. Volatile organic 
constituents (unreacted monomer(s), inert diluents) are eliminated, for 
example, in a vent extruder or flash chamber. It is possible, for example, 
first to carry out a coarse degassing in a flash chamber with following 
fine degassing in a vent extruder. Finally, the product is granulated in 
usual manner. 
The process according to the invention yields products having very valuable 
properties since with the static internal fitments a very narrow residence 
time spectrum can be obtained which, in combination with the heat exchange 
taking place at the same time makes possible an especially uniform and 
exact temperature control in the reaction mixture. In this manner products 
having a narrow molecular weight distribution are obtained in which the 
proportion of low molecular products is very low. 
Instead of complicate and sensitive tube reactors having mobile elements 
(stirrer) according to the state of the art, improved reactors are used 
which have the further advantage that surprisingly space-time-yields of 
over 100 kg per hour per cubic meter are possible, corresponding to an 
increase of almost 30%. Hence, the process according to the invention is 
more economical. 
The following example illustrates the invention. 
EXAMPLE 
The prepolymerization was carried out in a 250 liter stirred tank provided 
with helical stirrer and jacket heating. The feed was adjusted by means of 
a dosing device and another device made it possible to withdraw 
continuously a defined amount of prepolymer. 
The continuous feed consisted of a mixture of 84.485% by weight of styrene, 
9% by weight of a commercial polybutadiene rubber with about 50% of 
trans-, about 40% of cis- and about 10% of 1,2-vinyl configuration and 
having a Mooney viscosity M.sub.L 1+4(100) of about 35, 1.5% by weight of 
mineral oil, 5% by weight of ethylbenzene and 0.015% by weight of lauryl 
mercaptan. The constant throughput was 65 kg per hour. With a mean 
residence time in the prepolymerization vessel of 1.4 hours, an internal 
temperature of 408.degree. K. and a number of rotations of the helical 
stirrer of 35 rpm, a stationary polymer content of 30% by weight was 
obtained. The prepolymer obtained was continually fed to the main 
polymerization in which it passed continuously three temperature zones. 
As main polymerization reactor a reactor as described above and having a 
circular cross section was used. The reactor had a total length of 6 
meters and an inner diameter of 30 cm. It contained as internal fitments 3 
tube sections of a length of 2 meters each, which were turned relative to 
one another through 90.degree. and the temperature of which could be 
controlled separately. Each tube section consisted of 5 layers of 
38.times.3.6 mm tube. The tube layers were in touch with one another, the 
rectilinear tube portions 2a of adjacent tube layers crossed through an 
angle of 90.degree. and the curved tube portions 2b touched the reactor 
wall. In the two spandrel-like space guide plates 5 were located. The 
free, total volume of the reactor was 360 liters (3 times 120 liters). 
In the first tube section the temperature rose from the beginning to the 
end from 408.degree. to 418.degree. K., in the second tube section the 
temperature rose from 418.degree. to 433.degree. K. and in the third 
section it rose from 433.degree. to 443.degree. K. 
The total residence time of the reaction mixture in the reactor was 5 
hours, at the end of the third tube section the polymerization mixture had 
a polymer content of 89%. 
The polymer discharged from the reactor was degassed in a commercial 
twin-shaft degassing extruder and granulated. 
The final product obtained had the following properties: 
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rubber content 10.1% 
content of soft phase 32% 
RSV 0.83 dl/g 
M.sub.w /M.sub.n 2.5 
melt index MFI.sub.200/5 
(DIN 53,735) 3.6 
ball indentation hardness 
(DIN 53,456) 76 kp/cm.sup.2 
Vicat temperature 98.degree. C. 
(DIN 53,460) 
notched impact strength at 0.degree. C. 
8.1 kpcm/cm.sup.2 
(DIN 53,453) 
elongation at break 82% 
(DIN 53,455) 
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The space-time-yield was 106 kg per hour per cubic meter of reaction 
volume. 
The reduced specific viscosity (RSV) was measured at 25.degree. C. with a 
solution of 1 g of styrene polymer in 100 ml of toluene.