Note: Descriptions are shown in the official language in which they were submitted.
~0~654~ P~ 2~62~
The present invention relates to the production
of vinyl chloride polymers by an aqueous dispersion
polymerisation process.
It is well known to produce vinyl chloride polymers
using an aqueous dispersion polymerisation process, e.g.
by polymerising vinyl chloride, or vinyl chloride
and an ethylenically unsaturated monomer copolymerisable
therewith, in aqueous suspension in a reactor
(conveniently made of a metal such as stainless steel)
using a protective colloid as a suspension agent and
a free~radical-yielding substance (usually monomer
soluble) as initiator. While this type of process
has now been comm~rcially developed to a high~degree
of sophistication, one important drawback that has
remained obstinately outstanding is the formation of
a layer of build-up upon the inner surface of the
polymerisation reactor. ~his build-up necessitates
inter-batch cleaning, by which we mean that the deposited
build-up is removed as completely as possible (e.g.
by scraping, solvent cleaning or pressure-washing)
from the inner wall of the reactor after completing
each polymerisation in a given reactor. If this were
not done, then parts of the hard polymeric material
constituting the build-up could find their way into
the particles made in subsequent batches in the
i~
~ 48 P.~5629
,
same reactor and deleteriously affect the properties
of the resulting polymers. Even more seriously, the
formation of build-up reduces the heat transfer ability
of the reactor wall to the cooling fluid (usually
water) circulatin~ in a jacket surrounding the reactor
(many polymeri~ation reactor~ are cooled b',y thi~
type of arrangement) which necessitates an increased
cooling requirement to maintain the desired polymerisation
temperature. It is clearly seen that this situation
could rapidly worsen with successive batches made in
a given reactor if the inner surface of the reactor
was not cleaned after the completion of each polymerisation
batch and adequate temperature control could quickly
become impossible. In fact, adequate temperature
control could become questionable or even ~mpossible
in the very next polymerisation carried out in a
reactor (particularly a very large reactor) which,
for the first time, has not had its build-up removed,
~his seemingly invariable formation of build-up
which increases with polymerisation time is also a
major difficulty in the development of trouble-fre~
continuous processes for the aqueous dispersion
polymerisation of vinyl chloride.
We have now disco~ered a process whereby
vinyl chloride polymers may be prepared in
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,,
:.
.25
.
aqueous dispersion without the or with a much reduced
formation of build-up. ~his process may substantially
reduce the cleaning effort required between each batch
polymerisation and may even allow two or more successive
batch polymerisations to be carried out in the same
reactor without the necessity for inter-batch removal
; of build-up. The avoidance of the necessit~ for
inter-batch cleaning in batch polymerisations is of
great economic significance since it not only avoids
the e~pense of the equipment and manpower required to
carry out such cleaning but also shortens the turn-
round time of a given reactor leading to increased
productivity.
I~ addition, the process of our invention could
be applied with advantage to the continuous polymerisation
of vinyl chloride since it may overcome the drawback
of an ever-thickening skin of build-up~
According to the present invention we provide a
process for the production of vinyl chloride polymers
which process comprises polymerising vinyl chloride,
; or vinyl chloride and up to 2~/o by weight thereof of
at least one ethylenically unsaturated monomer
copolymerisable therewith, in aqueous dispersion,
preferably in aqueous suspension, in a reaetor with an
~5 inner wall(s) on which there has been deposited an
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~96~
insoluble layer of a cross-linked polymeric material
containing polar groups formed from a reaction mixture
having an aldehyde, preferably formaldehyde, as one
component thereof.
It is to be u~derstood that the other component or
at least one of the other cornponents (if more than one)
; of the reaction mixture should have more than two
sites of reactivity in order to form a cross-linked
polymer with the aldehyde.
By the term "insoluble" we mean that the coated
layer of polymeric material should not be dissolvable
by or react with the aqueous medium (whether it be
acidic, neutral or alkaline) or with the organic medium
; used in the polymerisation.
~he layer of cross-linked polymeric material may
be formed by applying solutions or dispersions of the
components required to form the cross-linked polymer
to the autoclave wall, e.g~ by spraying or flushing
the wall with the solutions or dispersions~ Each
component may be applied to the wall as a separate
solution or dispersion or may be applied in admixture
with one or more of the other components. It is also
possible to apply all the components required for the
formation of the cross-linked polymer to the wall in
the same solution or dispersion, although this i5 not
'
~ 48 P.~562~
,
preferred because of the danger of premature reaction
before the components have been deposited on the wall.
Once the components have been deposited on ~ha
wall they react, either of their own accord or after
the application of heat, to form the layer of cross-
linked polymeric material.
It is preferred that the liquid for carrying the
components when they are added to the wall is water,
although other liquids such as organic solvents may
sometimes be used.
It is thought that the cross-linked polymeric
materials become ~chored to the reactor wall by
means of some of the polar groups and thus cannot
be readil~ removed by the polymerisation medium.
It is al80 thought th~t these polar groups or
other groups in the molecule inhibit the formation
of build-up without inhibiting the polymerisation
in the main dispersed massO The use of less strongly
adsor~ed molecules may suppress build-up for~ation
but this may only be at the expense of retarding
the overall polymerisation. Cle~rly such an effect
is undesirable from the economic stand point.
Additionall~, less strongly adsorbed materials may
affect the ~uality of the product (notably colour
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~09G548 P-~5~29
qualit~) deleterious1y. Examples of polar groups
include carboxyl, hydroxy, amino, imino and amido
groups.
Suitable cross-linked polymeric materials
include the materials obtained by condensing
monomeric phenols such as phenol and para-hydroxy-
benzoic acid with an aldehyde and the materials
obtained by cross-linking polar monomeric or
polymeric materials such as melamine, diamino diphenyl
ether, urea and polyethylene imine with an aldehyde.
It is preferred that the aldehyde employed is
formaldehyde since cross-linked products are easily
obtainable with this material. It is to be understood
that many of the polar groupings in such cross-linked
materials may not have taken part in the cro~s-linking
reaction.
In the case of the basic (i.e ~lkaline) types
of coating material it is preferred t~at the
polymerising medium is kept at a pH of greater than
ZO 4 by the use (where necessary) of buffers or alkaline
substances. Suitable alkaline substances include
sodium, potassium, calcium and ammonium hydroxides,
carbonates a~d bicarbonates, and buffers include
mixtures of the disodium ~nd monosodium hydrogen
6~i48 P. 25~
ortho phosphates (Na~HP04 and NaH2P04). It is of
course to be understood that the pH of the reaction
medium may of its own accord remain at above 4 in
which case the addition of a buffer or alkaline
substance ma~ not be necessary.
In the process of our invention, the amount of
cross-linked pol~meric material used is preferably
1 to 2000 parts per million (particularly 10-200 parts
per million) based on the charge of the vinyl chloride
(in the case of batch polymerisations).
In the preferred process of our invention, vinyl
chloride is polymerised in aqueous suspension and
in such a process the ingredients conventionally
used in vinyl chloride aqueous suspension polymerisations
which include suspension agents and free-radical
yielding initiators (usually monomer soluble) should
be employed.
It is also possible in the process of our
in~ention to incorpora~e an~ suitable additive
(such as a heat ~tabiliser) into the polymerising
reactantæ at any stage of the polymerisation,
although if it is added at or towards the begin~ing
of polymeri~ation it should not inhibit the
polymerisation reaction.
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~.25~
~g65~
~he present invention is illustrated by the following
examples.
EXAMPLES 1-5
A series of vinyl chloride polymerisations was carried
out in a stainless steel reactor the walls of which were
treated either hot or cold with polyethylene imine (PEI)
and formaldehyde. The basic recipe used consisted of
pretreating the reactor walls as described in Table 1.
Polymerisation was then carried out using 100 parts of
vinyl chloride (in aqueous suspension), 150 parts of water,
a paddle stirrer rotating at 225 r.p.m., a polymerisation
temperature of 51C, 0.06 parts of diethylperoxydicarbonate
as initiator and a suspension agent conslsting of 0.11 parts
of a par~ially hydrolysed polyvinylacetate resin. ~he
polymerisation was buffered to an approximately neutral pH
using a mixture of ~a2XP04 (0.066 parts) and MaH2P04 (0.033
parts). The pol~merisation of each batch was continued until
; ~ the pressure in the reactor had fallen by 40 p.s.i. below
the steady pressure during pol~merisation. In Examples 2
and 4, the reactor walls were cleaned before treatment and
; polymeri~ation. In Examples 3 and 5, however, the walls
were not pretreated as such, but were used as obtained
after dischargi~g the batch corresponding to the previous
example. ~he amount of build up formed was estimated by
removing the adherent film and weighing. Results are
given in Table 1.
_ g _
P~25629
i5~
~AB~E_1
Weight of build- Total
Example Reactor up (parts per reaction
~o. pretreatmenthundred vinyl time
chloride charged) (minutes)
_
1 ~one 0.015 380
2 Sprayed first with None 385
0.0012 parts PEI and
then wi-th 0.005 parts
formaldehyde (both in
water) without drying
in between. Heated for
10 minutes at 80C.
3 No further treatment~race 400
(but no cleaning after <0.001
Example 2).
4 Sprayed first with None 360
0.003 parts PEI and
then with 0.02 parts
fo~maldehyde (both in
water). Room
temperature.
~o further treatment~one 360
(but no cleaning after
Example 4).
~he results shown in ~able 1 indicate that the use
of the product of polyethylene imine and formaldehyde
produces a signific~nt build-up suppression without
retardation of the polymerisation reaction.
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EXAMPLE 6
138 g p-hydroxybenzoic acid were heated with
100 ml water and 100 g of 30% aqueous formaldehyde and
15 mlsconc. HCl at 98-100C for 2 hours. The initial
white solid (p-hydroxybenzoic acid) dissolved and after
about 1 hour, a white solid precipitated. After
cooling to about 60C, 40% aqueous ~JaOH was added
continuousl~ until all the white precipitate had
dissolved and the pH of the medium was 9.6-10. This
solution was divided into two equal parts~ A and B.
138 g p-hydro~ybenzoic acid, 100 g of 30%
aqueous formaldehyde were mixed together and 40% aqueous NaOH
solution added until the initial solid dissolved and
the pH was 9.6-100. This solution was divided into
two equal parts, C and D.
Solutions A and C were mixed and refluxed for 45
minutes~ After about 30 mi~utes, a solid precipitated
which was insoluble in ethanol. This product was
discarded.
Solutions B and D were mixed and refluxed for
20 minutes. A thick red syrup formed which on acidification
with dil. HCl gave a white precipitate. ~his was filtered
off and washed with water. It was partially dried at room
temperature by sucking air through it using a vacuum
pump. This product was used i~ Examples 7-10.
~ 5
; EXAMPLES ~_0
A series of vinyl chloride polymerisations was carried
out to assess the effect of the p-hydro~Jbenzoic acid
condensate described in Example 6 on build-up formation.
The basic recipe used consisted of pretreating the
reactor wall as described in Table 2. Polymerisation was
then carried out using 100 parts of vinyl chloride
(in aqueous suspension), 210 parts of water, a paddle
stirrer rotating at 300 r.p.mO, a polymerisation temperature
of 50C, 0009 parts of diethylperoxydicarbonate as initiator
and a suspension agent consisting of 0~2 parts of a partially
hydrolysed polyvinyl acetate resin. ~he polymerisation
of each batch was continued until the pressure in the
reactor had fallen by 40 p.s.i. below the steady pressure
- ; 15 during polymerisation. In ~xamples 8 and 9 the reactor
walls were cleaned before treatment and polymerisation.
In Examples 10 and 11, however, the walls were not
pretreated as such but were used as obtained after
,;
discharging the batch corresponding to the previous
example. The amount of build-up was estimated visually.
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~09~54~ 5629
TAB~E 2
Example Reactor pretreatment Amount of build-up ~ Reaction
(minutes~
7 None Skin over entire 33
surface of re-
actor.
8Vertical strip of No build-up on 1 35
reactor painted with strip. Normal
10% ethanolic build-up on
solution o~ conden- remainder.
sate. Heated for 10
minutes at 100C.
9 Vertical strip of ~o build-up on 35
reactor painted with strip. Normal
10% ethanolic build up on
solution of conden- remainderO
sate Heated for 30
minutes at 80C. I,
- 10 ~o further treatment Very fine skin on ~25
(but no cleaning treated partA
after ~xample 9~ Heavy build-up on
remainder.
11 ~o further treatment Fine skin on 35
(but no cleaning treated part.
after Example 10) Ver~ hea~y build-
up on remainder~
" _
~ he results shown in Table 2 indicate that the use of
the condensate of p-h~droxybenzoic acid described in ~xample
6 produces a significant reduction in build~up without
retarding the polymerisation reaction~
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~96~ P~ 25629
EXAMPLES 12-17
A series of vinyl chloride polymerisations was
carried out to assess the effect of the condensate of
formaldehyde and diamino diphenyl ether on build-up
formation. The basic recipe used consisted of pretreating
the autoclave wall as described in Table ~. Polymerisation
was carried out using 100 parts of vinyl chloride (in
aqueous suspension), 210 parts of water~ a paddle stirrer
rotating at 300 ~.p~m., a polymerisation temperature of
50C, 0908 par-ts of diethylperoxydicarbonate as initiator
and a suspension agent consisting of 0.2 parts of a partially
hydrolysed polyvinyl acetate resin. The polymerisation of
each batch was continued until the pressure in the reactor
had falle~ by 40 p.s.i. below the steady pressure during
polymerisation~ In Examples 13 ~nd 14 the reactor walls
were cleaned before -treatment and polymerisation. In
Examples 15, 16 and 17 however~ the reactor walls were not
pretreated as such but were used as obtained after
discharging the batch corresponding to the previous
example. The amount of build-up was estimated visually.
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8 ~.~552
~ABLE 3
Example Autoclave pretrea-tment Amount of build-up
No. and reaction time
12 None Skin over entire
surface of reactor.
13 Reactor sprayed with Very very thin skin
a 3% methyl ethyl ketone over reactor surface.
solution of diamino Pol~merisation re-
diphenyl ether. tarded.
14 Reactor treated as in No build-up.
Example 1~ and then Normal polymerisation
with formaldehyde time.
solution. Heated at
70C for 30 minutes.
No further treatment ~o build-up. Normal
(but no cleanin~ after polymerisation time.
~xample 14)
16 ~o further treatment No build-up. ~ormal
(but no cleaning after polymerisation time.
E~xample 15)
17 ~o further treatment Some small patches of
(but no cleaning after build-up. ~ormal
Example 16) polymerisation time.
.... . , ., ,. J
~he results shown in ~able 3 indicate that the use of
the condensate of formaldehyde and diamino diphenyl ether
produces a si~nificant reduction in build-up without
retarding the polymerisation reaction.
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