Use of an apparatus for continuous photopolymerization

For photopolymerization, glass cylinders through which a mechanically stirred stream of ethylinically unsaturated monomeric material continually flows are irradiated from the exterior with light which initiates polymerization.

The invention relates to the use of an apparatus for the continuous 
photopolymerisation of ethylenically unsaturated monomers. 
Initiation by light of the polymerisation of ethylenically unsaturated 
monomers is one of the oldest polymerisation processes and is often 
employed for examining polymerisation mechanisms. The industrial use of 
photopolymerisation has been concerned almost exclusively with 
photo-cross-linking systems such as photo-printing plates and 
light-hardening lacquer systems. Photopolymerisation has not been used 
hitherto in industry as a process for the production of linear 
homopolymers or copolymers, not least because hitherto hardly any 
technically suitable methods have been found for carrying out 
photopolymerisation on an industrial scale. 
German Patent Specification No. 956,542, U.K. Pat. No. 817,343, German 
Offenlegungsscrift No. 2,009,748, and German Auslegesschrift No. 2,050,988 
describe several types of apparatus for continuous photopolymerisation, in 
which a mass or solution of a monomer is charged on to a moving belt and 
is polymerised by irradiation when passing through a lamp channel thereon. 
This mode of operation has several great disadvantages. Firstly, it is 
necessary to maintain an oxygen-free atmosphere for vinyl polymerisation 
and this is difficult and only feasible with the use of expensive 
equipment. Furthermore, very thin layers are produced when the production 
line is charged with the low-viscosity monomers, and the absorption of the 
light for initiating polymerisation is therefore low and the volume yields 
per unit time are unsatisfactory. 
Although this mode of operation might still produce useful results in the 
case of highly reactive vinyl monomers, such as for example acrylamide, it 
is unsuitable in the case of less reactive systems, particularly with 
styrene-containing mixtures, owing to the long residence times required. 
These difficulties may be overcome by means of a method of operation 
described in German Auslegesschrift No. 2,115,706 in which a monomer 
mixture is enclosed in a plastic bag, placed in a scraping conveyor and 
conveyed through a lamp channel for polymerisation. The advantages of the 
absence of oxygen and the greater thickness of the layer obtainable are 
offset by the aggravating disadvantages of troublesome procedures needed 
for filling and emptying the plastic bags. This mode of operation cannot 
therefore be usefully employed industrially either. 
A simple and economical apparatus for the continuous industrial 
photopolymerisation of ethylenically unsaturated monomers needs to be 
found, which allows the temperature and light control to be adapted in 
stages to a predetermined course. 
According to the invention there is provided a process for continuous 
photopolymerisation of ethylenically unsaturated monomeric material 
wherein the material is passed through a plurality of successive reactors 
each of which is in the form of a glass cylinder having an inlet and an 
outlet, the material being subjected to constant thorough mixing in each 
reactor by a stirrer and heating or cooling by a heat exchanger, the 
stirrer and heat exchanger being located in the interior of the glass 
cylinder, the material also being subjected to irradiation with light 
initiating photopolymerisation from lamps around the exterior of the 
cylinder. 
The invention preferably relates to an apparatus for carrying out mass 
photopolymerisation in accordance with German patent application P No. 
2,600,318.9, characterised in that in the first stages of the reaction an 
internal coil is used for heating or cooling and a stirring shaft with 
stirring blades is used for stirring and mixing, whereas in the last 
stages of the reaction a central cylindrical internal body is used for 
heating and cooling and a coil, screw or a basket stirrer is used for 
stirring, mixing and conveying. In this case, the stirring mechanism is 
preferably rigidly fixed to the heating body. The reactors of the first 
type are generally used at the lower temperature while those of the second 
type are used at the elevated temperature. 
The apparatus according to the invention allows photopolymerisation to be 
carried out at throughputs which may be theoretically high, with good 
volume yields per unit time. The thickness of the layer of irradiated 
monomers may be selected more or less as desired by suitable dimensioning 
of the reactor glass tube, so that optimum use of light may be obtained. 
Since the installation is of closed construction and is operated 
submerged, i.e. without gas chambers in the reactors, the considerable 
problem of excluding the oxygen is easily solved by flushing the entering 
reaction mixture with inert gas. The capital cost of the apparatus is 
relatively low, and it is inexpensive to maintain as it is simple to 
construct by a modular building method. Also, the apparatus allows 
polymerisation to cease very rapidly if necessary by switching off the 
light which initiates polymerisation.

FIG. 1 shows a reactor 1 for a low viscosity reactor medium. The reactor 
comprises an upright glass tube 2 sealed with covers 3 and 4 at both ends. 
The product feed and discharge pipes 5 and 6 are guided through the 
opposing covers 3 and 4. A heat exchanger 7 is connected via pipes 8 to an 
energy source (not shown). A stirrer 9 a shaft 10 and propellers 11 are 
mounted in the covers 3 and 4 and may be driven externally by a motor (not 
shown). A vent pipe 12 with a valve 13 is connected via the upper cover 4. 
A reactor 14 for high viscosity reactor medium is shown in FIG. 2. The 
reactor 14 also comprises an upright glass tube 2 which is sealed with 
covers 3 and 4 at both ends. The product pipes 5 and 6 are connected 
through the opposing covers. A hollow shaft 16 replacing the heat 
exchanger 7, and axial with respect to the glass tube 2, is chargeable 
with heating medium from the exterior via a double inlet arrangement 15 
and may be rotated by a motor (not shown). The shaft 16 is provided 
externally with a helically arranged plate 17. The vent pipe 12 with valve 
13 communicates with the glass tube 2 via the upper cover 4. 
FIGS. 3 to 6 show two or five reactors 1,14 which differ in construction 
and are combined with lamps 18, one or more reactors 1 of the first type 
are used for the first stage of the reaction and one or more reactors 14 
of the second type are used for the final stage of polymerisation. A pump 
19 is connected between the last two reactors 14 to overcome the increased 
viscosity. 
In all cases, the light for initiating polymerisation is radiated from the 
outside through the glass tube 2 into the material on the reactor. 
Although it is possible in principle to use any sources of light which 
emit light of a suitable wavelength, superactinic fluorescent tubes with a 
maximum output at between 350 and 360 mm are preferably used. In the 
preferred embodiment of the invention, these fluorescent tubes are 
arranged in special closed lamp cases. When constructing these lamp cases, 
it is necessary to observe the regulations relating to the prevention of 
explosions, and they therefore have to be flushed with inert gas if 
necessary. These lamp cases are arranged round the individual reactors 
1,14, and optionally also between the reactors 1,14. 
The reaction mixture consisting of ethylenically unsaturated monomers, the 
photo-initiator and optionally other auxiliary agents such as, for 
example, molecular weight regulators and stabilisers, travels in the 
following manner. The mixture which is preferably under a pressure of 
between 1 and 5 m water column enters the first reactor at one end in a 
constant flow and after passing through the first reactor, emerges at the 
other end and then enters the second reactor in the same manner. The 
polymer conversion increases from reactor to reactor until the 
fully-reacted end product emerges at the end of the chain of reactors, 
generally as a viscose polymer melt. In this case, it is advantageous for 
the mixture to pass through the reactors in a rising and falling direction 
alternately so that particularly short connecting lines are produced 
between the reactors. As the viscosity of the mixture increases 
considerably during the reaction, in one embodiment of the invention a 
pump is installed between the reactors, preferably upstream of the last 
reactor, and this pump conveys the reaction mixture in a constant flow 
through the reactors. In another embodiment, the reactors of the final 
stage, (second type of reactor) are provided with a sufficient capacity 
for self-conveyance, thus avoiding the need for an intermediate delivery 
pump. 
EXAMPLE 
A mixture with the following composition: 
1300 kg Styrene 
700 kg Butyl acrylate 
20 kg t-Dodecyl mercaptan 
4 kg Benzoin isopropylether 
is continuously photopolymerised in bulk in the apparatus described below. 
The mixture has an extinction of 100% at a wave-length of 350nm and a 
layer thickness of 6 mm. 
Polymerisation is carried out in an installation composed of three 
reactors. The individual reactors are built of surface-ground glass tubes 
225 mm in diameter and 1500 mm long. The first reactor corresponds in 
structure to FIG. 1 and the layer is 110 mm thick measured radially from 
the stirring shaft to the glass. The second and third reactors are 
identical in structure and correspond to FIG. 2. The heated central tube 
upon which the stirrer coil is fixed has a diameter of 48 mm so that the 
thickness of the layer from the central tube to the glass is 88 mm. The 
first reactor is operated at 90.degree. C. and is stirred at 400 r.p.m.; 
the second reactor is operated at 130.degree. C. and stirred at 8 r.p.m.; 
the third reactor is operated at 160.degree. C. and stirred at 1 r.p.m. 
Thirty-two superactinic fluorescent tubes each of 40 W (maximum radiation 
350 nm) in eight lamp cases are arranged round the reactors as sources of 
light. 
The mixture passes in succession through the reactors which are operated 
submerged, the mixture descending through the first reactor, ascending 
through the second reactor, and descending through the third reactor. A 
polymer conversion (determined by baking for two hours at 200.degree. C.) 
of 95.6% is obtained at a throughput of 30 kg per hour. 
When the quantity of light is reduced by using sixteen fluorescent tubes, a 
polymer conversion of 94.3% is obtained under otherwise similar conditions 
at a throughput of 20 kg per hour. 
In both cases, the product and the filling of the reactors is homogeneous 
and no caking of the polymer is observed.