Process for reducing reactor fouling during polymerization in an aqueous medium

To reduce fouling on reactor surfaces, the surfaces are coated with a protective film of polymeric material of surface energy of less than 15 dynes/cm. The preferred coating materials are polymers of one or more monomer chosen from the 2-(N-alkylperfluorosulphonamido)) alkyl (meth)acrylates or the fluorochemical polymer known as Fluorad (Registered Trade Mark of the 3M company) FC-721.

This invention relates to polymerisation reactors. 
Polymerisation reactors, and especially those used for the production of 
emulsion polymers often suffer from the build-up of polymeric material on 
their internal surfaces with extended use. This build-up, referred to 
variously as "fouling" or "reactor coagulum" is undesirable. In severe 
cases it may modify flow or mixing patterns and may even have a 
significant effect on reactor volume. 
The removal of such reactor fouling is a direct economic loss as it is rare 
that any material can be recovered, and the reactor is out of production 
during the cleaning period. Losses are also incurred from the chemical and 
energy costs of the cleaning process and any manual work necessary may be 
both unpleasant and hazardous. Cleaning is often carried out using high 
pressure water jets, or by refluxing with a caustic solution or with 
solvent. 
Prior art attempts to reduce the amount of fouling have been only partially 
effective. Efforts have been made to combine good mechanical stability 
with the minimum amount of shear on the product, consistent with 
sufficient mixing. Reactor surfaces have been made smooth and free from 
imperfections or gross chemical attack by the use of polished stainless 
steel, polytetrafluoroethylene (PTFE) or glass coating. These approaches 
have the drawback of limiting the range of formulations to be employed, 
and limiting the range of process variables. They may also be costly, and 
may allow contamination of the product by small quantities of undesirable 
metal ions. They may also reduce heat transfer seriously. 
An alternative is the treatment of the reactor surfaces by a suitable 
chemical. This is claimed to be effective in the case of fouling due to 
polymerisation of monomers absorbed onto the reactor surfaces, either 
above or below the water line. This form of polymerisation may be referred 
to as "pop-corn polymerisation" because of the physical appearance of the 
fouling. 
This chemical treatment is of particular interest in the fields of 
isoprene, butadiene, chloroprene and vinyl chloride polymerisation. 
Phosphates and polyphosphates have been mentioned in BP 1,484,822; polar 
organic compounds and dyes in BP 1,521,058, East German Patent 126,444, 
U.S. Pat. No. 4,105,839 and European Patent 126,991; phenol derivatives in 
Japanese Kokai 78,77,290; metal sulphides in Japanese Kokai 78,21,910; 
oxidised amines (Japanese Kokai 78,13,689); dithiocarboxylated 
polyethyleneimine (Japanese Kokai 78,77,291); dimethyldithiocarbamate in a 
polyvinyl alcohol-methylol resin mixture in Japanese Kokai 85,71,614; 
amine/phenol or quinone mixtures (Australian Application 77/9324); sodium 
bromobenzoate or orthotoluate (German Offen. 2,804,076); Aluminium oxalate 
(Japanese Kokai Tokkyo Koho 78,112,989); inorganic polysulphides (Japanese 
Kokai Tokkyo Koho 78,114,891); sodium polythioate (Japanese Kokai Tokkyo 
Koho 78,109,589); and by coating reactor parts in molten sulphur (BP 
799,474). Acrylates and methacrylates and vinyl acetate/acrylonitrile 
copolymers are mentioned in U.S. Pat. No. 4,517,344 and Japanese Kokai 
85,71,601. These processes use the reaction product of p-benzoquinone and 
1-8 diaminonapthalene and C.I. Reactive Black 4 dye suspended in 
water-glass as the coating, respectively. All of these materials must 
contaminate the product to a greater or lesser degree. 
The very quantity of references indicates both the technical and economic 
importance and the previous intractability of the problem. The present 
invention, which is directed to this problem, resides in a process for the 
production of polymers in a polymerisation reactor wherein some or all of 
the reactor surfaces exposed to gaseous or liquid monomers or their 
condensed vapours and solutions, and/or some or all raw material inlets 
and product outlets, are coated with a film of polymeric material of 
surface energy of less than 15 dynes/cm. which is insoluble in, undamaged 
by, unreactive with, and unwetted by, any combination of the raw materials 
or reaction products of the process. 
The invention is applicable to reactors such as batch reactors and 
continuous stirred tank reactors employing water based systems and the 
product is an emulsion, dispersion or latex, or is a bead polymer. In such 
reactors a liquid surface may be present or the reactor may be fully 
filled such as the continuous "Loop" process (GB 1,124,610 and GB 
1,220,777). 
In principle, any coating could be applied which is not wetted or dissolved 
and does not absorb or adsorp appreciable amounts of water or the monomers 
in use in the polymerisation, even in the presence of efficient surface 
wetters used as emulsion polymerisation stabilisers. Concentrated 
solutions of such wetters may have surface tensions against air in the 
range of 20-30 dynes/cm. This may be compared to the surface energy of 
polyethylene of 31 dynes/cm which is likely to be wetted by such solutions 
and polytetrafluoroethylene (PTFE) with a surface energy of 18 dynes/cm. 
which is less likely to be wetted. 
The origin of surface energy and surface tension, together with the 
phenomena of wetting, may be explained in the following way. 
A molecule in the bulk of a liquid is attracted equally in all directions. 
At the surface however there is an inwards force as the number of 
molecules per unit volume in the vapour above the surface is much less 
than in the liquid below it. The force in dynes acting at right angles to 
any line of 1 cm. length in the surface is defined as the surface tension 
.delta.. To extend the surface by 1 cm. work of .delta..times.1 cm. must 
be applied. 
Similar considerations apply at the interface between two immiscible 
liquids or the boundary between a liquid and a solid, but these tensions 
are usually lower due to a more even balance of concentrations of 
molecules. 
In air, the wetting of a surface is determined by the three interfacial 
tensions involved, i.e. between the solid/liquid (SL), solid/air (SA) and 
liquid/air (LA) interfaces.

In the drawing the contact angle of a drop of liquid (indicated by the dash 
line 10 and the hatching on a surface 11) is .theta..degree.. 
Conventionally, if .theta.&lt;90.degree. (FIG. 1) the surface is said to be 
wetted, and if .theta.&gt;90.degree. (FIG. 2) the liquid does not wet the 
surface. The balance of forces along the direction of the surface at 
equilibrium: 
EQU .delta.SA=.delta.SL+.delta.LA.sup.cos .theta. 
where .delta.SA, .delta.SL and .delta.LA are the surface energies or 
surface tensions of the solid/air, solid/liquid and liquid/air interfaces. 
As .theta. varies from 0.degree. to 180.degree., cos .theta. varies from 1 
to -1. At .theta.=90.degree., the conventional boundary between wetting 
and non-wetting, cos .theta.=0. 
For spontaneous spreading of the drop therefore: .theta.=0.degree., cos 
.theta.=1 and .delta.SA.gtoreq..delta.SL+.delta.LA. 
For complete non-wetting and contraction of drops: .theta.=180.degree., cos 
.theta.=-1 and .delta.SA.ltoreq..delta.SL-.delta.LA. 
It is observed that fluorocarbon surfactant solutions with surface tensions 
as low as 15 dynes/cm. do not wet polytetrafluoroethylene surfaces which 
have a surface energy quoted at 18 dynes/cm. This indicates that the value 
for .delta.SL is greater than 18 dynes/cm. 
The situation under the surface of an emulsion polymer reactor, where two 
immiscible liquids are present, is exactly analogous to the above, except 
that the air is replaced by a second liquid. Whether either the monomer or 
the water wets the reactor surfaces preferentially or not at all, depends 
on the construction or coating of the walls, and the influence of any oil 
and water soluble surface active agents which may be present. Glass has a 
critical surface energy of greater than 79 dynes/cm., and so will be 
wetted by most liquids. Steel has the lower level of 40 dynes/cm. and will 
be readily wetted by most surface active agent solutions. These have 
surface tensions (against air) of 28-40 dynes/cm. Steel also will be 
wetted by organic solvents and monomers which usually have surface 
tensions in the 22-30 dynes/cm. range. 
A number of hydrocarbon plastics have surface energies in the range 28-33 
dynes/cm. but polytetrafluoroethylene (PTFE) has the value of 18 dynes/cm. 
It is predictable that it will not be wetted by most liquids, even without 
detailed solid/liquid interfacial tension information, which is often not 
readily available. 
PTFE has therefore become a favoured material for construction of reactor 
components, although we have observed that despite its resistance to 
wetting, it does adsorp or absorb monomer molecules which may subsequently 
polymerise to give a toughly adhering layer of fouling. Once fouling 
occurs, the surface energy of the component rises to about 30 dynes/cm. 
and is readily wetted by monomer, leading to further build-up of fouling. 
An example of an effective coating according to this invention is an 
oleophobic-hydrophobic fluorochemical polymer marketed by the 3M company 
as "FLUORAD" (Registered Trade Mark) FC-721. Acrylic and methacrylic 
esters can be made readily from materials expressed generically as 
2-(N-alkyl perfluoro alkyl sulphonamide) alkyl acrylate such as N-alkyl 
perfluorooctane sulphonamide and (meth)acrylic acid and are available from 
the 3M company as "FLUORAD" (Registered Trade Mark) FX-13, FX-14 and 
FX-189. These materials are readily polymerisable in solution. "FLUORAD" 
(Registered Trade Mark) FX-13 comprises of a mixture of isomers of 
2-(N-ethylperfluorooctane sulphonamido) ethyl acrylate (CAS number 
423-82-5). The octane portion is 80% linear and 20% branched and its 
average chain length is 7.5 due to a decreasing level of lower alkyl 
groups down to C.sub.4 F.sub.7 -. Its physical form is a waxy solid of 
density 1.52 at 90.degree. C., with a melting range of 
27.degree.-42.degree. C. and a boiling point of about 150.degree. C. at 1 
mm. The surface energy of polymers of this material have been measured at 
less than 12 dynes/cm, namely at 10.7-11.1 dynes/cm. The perfluorooctane 
component in the linear or branch chain materials may be replaced by a 
perfluoro alkyl grouping of chain length in the range of C.sub.4 to 
C.sub.7 and C.sub.9 to C.sub.12. 
"FLUORAD" (Registered Trade Mark) FX-14 is the corresponding methacrylate 
ester to FX-13, (CAS number 376-14-7), melting at 32.degree.-52.degree. C. 
to an amber liquid of density 1.48 at 90.degree. C. 
FX-189 is 2-(N-butylperfluorooctane sulphonamido) ethyl acrylate. Films of 
polymers of FX-14 and FX-189 have been measured to have surface energies 
of 11.7 dynes/cm and 11.7-12.1 dynes/cm respectively. The ready made 
polymer FC-721 is readily soluble in fluorinated solvents such as 3M's 
Fluorinert (Registered Trade Mark) Liquid FC-77 or Du Pont's Freon 
(Registered Trade Mark) TF. It is not however dissolved or wetted by 
water, heptane, toluene, acetone or vinyl acetate. Its effectiveness in 
the application indicates that it cannot absorb or adsorp appreciable 
amounts of polymerisable monomers. 
In practice FC-721 is supplied as a 2% solids solution in Freon (Registered 
Trade Mark) TF and it can be applied to reactor parts by dipping, spraying 
or any of the normal methods. The coatings air dry in 15-20 seconds so it 
may be an advantage to dilute them somewhat in Fluorinert (Registered 
Trade Mark) FC-77 or Fluorinert (Registered Trade Mark) FC-40 which boil 
at 97.degree. C. and 155.degree. C. respectively. 
An alternative, more dilute, solution of the same polymer is also marketed 
by the 3M Company as Fluorad (Registered Trade mark) FC-723. 
Once applied the coatings can be baked to removed the last traces of 
solvent, properties of the surface coating being retained up to at least 
175.degree. C. in air without loss of repellancy. 
To illustrate the invention more fully, the following comparative 
laboratory trials were run, although it will be understood that these do 
not imply any limitations in the utility of the invention with regard to 
method of use or the materials of the reaction. 
EXAMPLE 1 
This is an example of the preparation of a styrene butyl acrylate copolymer 
in the ratio of 50.5 parts styrene to 49.5 parts butyl acrylate. The total 
non-volatiles content was 48% and it was prepared using a redox initiation 
system at 60.degree.-64.degree. C. 
The preparation was conducted in a 2 liter glass flask coated with a thin 
layer of FC-721. 
Results of the material adhering to the equipment and the mobile 
polymerisation grit were as follows: 
______________________________________ 
Uncoated 
Coated 
Reactor (g) 
Reactor (g) 
______________________________________ 
Thermometer 0.043 0.01 
Agitator 0.658 0.038 
Reactor below liquid level 
0.013 None 
Polymerisation grit 
0.370 0.127 
______________________________________ 
EXAMPLE 2 
This is an example of the preparation of a vinyl acetate-Veova (Registered 
Trade Mark) 10-butyl acrylate emulsion terpolymer in the ratio 70 parts 
vinyl acetate, 20 parts Veova (Registered Trade Mark) 10 and 10 parts 
butyl acrylate in the temperature range 60.degree.-64.degree. C. 
In this case the continuous Loop process was used, and on a pilot reactor 
only one pipe, 1" outside diameter, 25 cm. long was coated. Materials 
adhering to the equipment after 6 hours production of terpolymer at a rate 
of 200 mls/minute and with an average non-volatiles of 54.5% were as 
follows: 
______________________________________ 
Uncoated Coated 
Test Pipe (g) 
Test Pipe (g) 
______________________________________ 
Test Pipe 2.2 0.2 
______________________________________ 
In tests conducted in a water-cooled production scale polymerisation Loop 
reactor coated in accordance with the invention and using a second Loop 
reactor (uncoated) as a control an improved performance was observed in 
the coated reactor. 
The tests were conducted repeatedly (e.g. 15 times) with each test lasting 
24 hours, and the reaction temperature rises noted. In the case of the 
coated reactor the average temperature rise was 15% less than in the 
uncoated reactor. This indicates that the heat transfer to cooling water 
was superior with the coated reactor which in turn indicates that fouling 
of the reactor was less. 
If it is recognised that a Loop reactor can be operated until a maximum 
temperature (related to the substances being polymerised) is reached then 
it follows that a coated reactor with its better heat transfer due to 
reduced fouling will have a longer operating time between cleaning 
intervals.