Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~13~7~88
This invention relates to a process for reducing the
residual monomer content in an aqueous vinyl chloride polymer
disper~ion sensitive to shear and/or temperature stresses.
Vinyl chloride polymers usually still contain monomeric
vinyl chloride after the polymerization step. For the removal
of this residual monomeric vinyl chloride from aqueous
polymer dispersions, various processes have been described~
as, for example, in published German Patent Application
DOS 26 3S~7~ wherein vinyl chloride is removed from aqueous
dispersions, latexes or suspensions by blowing organic or
inorganic gaseous ~ubstances and/or organic liquids in which
vinyl chloride is at least moderately soluble, therethrough.
During this blow-through process, the pressure in the degassing
vessel is maintained below atmo~pheric pressure. O~her
applications teach carrying out the residual monomer removal
at reduced pressure in the degassing vessel,with hot,
saturated, or unsaturated steam~ but always with steam above
atmospheric pressure.
All of the procedures that have become ~nown until now
are, however 7 unsuitable for degassing polymer d~spersions
which are sensitive to shear and/or temperature stresses.
Such polymer dispersions are in general those with average
particle sizes between O.l and 20ju, which generally contain
no protective colloids and only small quantities of emulsifiers.
It was, therefore, the object of the invention to find
a process for reducing the residual monomer content of aqueous
polymer dispersions which are sensitive to shear and/or
temperature stresses.
Another object of the present invention is the develop-
ment of a process for reducing ~he residual monomer content o~
~ 7 ~ ~
an aqueous vinyl chloride polymer dispersion sensitive koshear and/or temperature stresses, said polymer in said dis-
persion ~ontaining at least 75% by weight of polymerized vinyl
chloride and having an average particle size of from 0.1 to
20 ~1, said polymer dispersion having from 0.001% to 1% by
weight of emulsifiers, based on the solids content of the
dispersion, consisting of the steps of introducing from
0.05 to 0.25 kg of steam per liter of dispersion into said
dispersion at a steam t~mperature of from 50C to 100C and a
steam pressure of between 0.1 and 0.9 bar over a period of
from 20 minutes to 3 hours while maintaining an internal
pressure on said dispersion corresponding at most to the water
vapor partial pressure at the pre~ailing dispersion temperature 3
and recovering a polymer dispersion :having a reduced residual
monomer content.
These and other objects o~ the in~ention will become
more apparent a~ the description the.reof proceeds.
Th~ subject o~ ~he invention is a process for reducing
the residual monomer content of aqueous polymer dispersions
sensitive to shear and/or temperature stresses, which polymer
contains at least 75% by weight, base~ on the polymer weight,
of vinyl chloride units, and has an average particle ~ize of
between 0.1 and 20 ~, sai~ polymer dispersion having been
obtained in the presence of 0.001% to 1% by ~eight of emulsi-
fiers, based on the solids content of the dispersion,
comprising treating the polymer dispersion with skeam,
characterized in that steam is introduced into th~ dispersion
at a temperature of between 50 and 100C, preferably 60 to
~0C, and a pressure of between 0.1 and 0.9 bar, preferably
3o 0.2 to 0.7 bar, for a period of from 20 minutes to 3 hours
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~-'\ .
~13768~3
in a quantity of 0.05 to 0.25 kg per liter of dispersion and
at the same time an internal pressure, corresponding at most
to the water vapor partial pressure at the degassing tempera-
ture, prevails in the degassing ~essel~ that is, on the surface
of said polymer dispersion.
More particularly, the present invention relates to
a process ~or reducing the residual monomer content of an
aqueous vinyl chloride polymer dispersion sensitive to shear
and/or temperature stresses, said polymer in said dispersion
containing at least 75% ~y weight of polymerized vinyl chloride
and having an average particle size of from 0.1 to 20 ~, said
polymer dispersion having from 0.001~ to l~o by weight of
emulsifiers, based on the solids content of the dispersion,
consisting of the steps of introducing from 0.05 to 0.25 kg of
steam per liter of dispersion into said dispersion at a steam
temperature of from 50~ to 100C and a steam pressure of
between 0.1 and 0.9 bar over a period of from 20 minu~es to
3 hours while maintaining an internal pressure on said disper-
sion corresponding at most to the water vapor partial pressure
at the prevailing dispersion temperature, and recovering a
polymer dispersion having a reduced residual monomer content.
By the process of the inven~ion polyvinyl chloride dispersions
are o~tained with residual monomer contents of 40 ppm or less,
when their residual monomer content after polymerization is
over 200 ppm.
An advantageous procedure for reducing the mono~er
content consists in supplying a greater quantity of steam per
unit of ti~e toward the end of the degassing step than at the
beginning. The temperature of the dispersion should not exceed
~0C during degassing; generally one operates at temperatures
above 40C.
1~L376~
One or more inert gases may be added to the steam,
where the sum of the partial prassures is 0.1 to 0.9, prefer-
ably 0.3 to 0.7 bar.
Surprisingly, it becomes possible by the addition of
saturated, unsaturated or supe~heated steam at temperatures
between 50 and 100C, prefera~ly 60 to ~0C, at a pressure
between 0.1 and 0.9 bar, preferably 0.2 to 0.7 bar, while
applying a vacuum to the degassing vessel, to effect the
monomer reduction without the occurrence of coagulation of the
latices sensiti~e to shear or to tem~erature stresses.
The polymer dispersions being treated according to the
invention are preferably latices which comprise solids contents
of between 20~o and 6~,6 by weight. The particle diameter of
these latex particles may fluctuate within wide limits, but
generally stability problems are obser~ed in particular in
latices having an aver~ge particle size of between 0.1 and 20,u.
The polymer component may be formed entirel~ o~ vinyl chloride
units, but alternatively, it is conceivable that in addition to
a minimum of 75% by weight of vinyl chloride units, other
monomer units copolymerizable with vinyl chloride may be
present.
Preferred as monomers copolymerizable with vinyl chloride
are those which contain only one polymerizable ~roup, namely
the group ~CH=CC. Example~ are the vinyl halides, such as
vinyl bromide and vinylidene chloride; vinyl esters with
or~anic acids, more particularly vinyl alkanoates having from
1 to 20 carbon atoms in the alkanoyl and vinyl phenylcarboxy-
lates~ such as vinyl formate, ~inyl acetate and vinyl benzoate;
acrylic compounds, for example~ lower alkyl (meth)acrylates~
such as methyl acrylate and methylmethacrylate; as well as
376~8
unsaturated dicarboxylic acids, particularly alkenedioic acids
havin~ from 2 to ~ carbon atoms and cycloalkenyl dicarboxylic
acids, such as maleic acid, fumaric acid, methylene malonic
acid, itaconic acid, citraconic acid or tetrahydrophthalic
with alkanols
acid; as well as the mono and di~esters of these acids/having
1 to 1~ carbon atoms, such as dimethyl maleate, diethyl maleate
and di~n-butyl ~aleate, mono-(2-ethylhexyl) maleate, dimethyl
fumarate, diethyl fumarate, di-n-butyl fumarate, di-(2-
ethylhexyl) fumarate and dilauryl fumarate. Naturally,
lQ mixtures of more than two monomers can also be used.
Ionic and~or non-ionic emulsifiers and/or protective
colloids may be used as dispersing agents. Preferably anion-
active emulsifiers are employed. These are, ~or example,
alka~i metal and~ in particular, ammo~.ium salts of fatty acids
such as lauric or palmitic acid, o~ acid phosphoric acid alkyl
esters such as sodium diethylhexylphosphate, of acid fatty
alcohol sulfuric acid esters~ of paraffin sulfonic acids, of
alkyl naphthalene sulfonic acids and of sulfosu~cinic acid
dialkyl esters Also very suitable are the alkali m~tal and
ammonium salts of fatty acids containing epoxy groups, such
as ammonium epoxystearate, or, respectively, the alkali metal
and a~monium salts of the products obtainable by reaction of
per acids~ such as peracetic acid, with unsaturated fatty acids
such as oleic or linoleic acid, with formation Or dihydroxy-
stearic acid and hydroxyacetoxystearic acid, for instance;
as well as the alkali metal and ammonium salts of the r~actlon
products of per acids with unsaturated hydroxy-fatty acids
such as ricinoleic acid. Alternatively, cation-active e~ulsi-
fiers, such as lauryl pyridinium hydrochloride5 may be used.
As examples of non-ionic emulsifiers, the following
1~376~
may be named: partial fatty acid esters of polyhydric
alcohols~ snch as ~lycerin monoskearate, sorbital monolaurate
or palmitate: partial fa~ty alcohol ethers of polyhydric
alcohols- polyethoxylates of fatty alcohols or aro~atic
hydroxy compoun~s~ as well as the known polypropylene oxide-
polyethylene oxide condensation products.
The total quantity of ionic or non-ionic emulsifiers is
usually about 0.001% to 1~ by weight, based on the weight o~
the monomer or monomers.
The protective colloids which may optionally be present
as dispersin~ agents, are ~he natural or synthetic polymers
which are normally employed as protective colloids in suspension
~olymerization. These are1 for example, polyvinyl alcohol,
possibly also containing up to 40 mol /~o of acetyl groups;
gelatines and cellulose ether derivatives, such as water-
~ . ._
soluble methyl cellulose, carboxymethyl cellulose and hydroxy-
- ethyl~ellulose; as wellas copolymers of maleicacid ortheirhslf
esters with styrene ortheir salts. The pro~ectivé colloids
are used in quantities of 0.1% to 3% by weight, preferably
0.3~ to 2~ by weight~ based on the weight of the monomer or
monomers.
Polymerization is effected in the usual manner, with
the aid of monomer-soluble free-radical-forming catalysts.
Examples of these monomer-soluble ca-talysts are benzoyl
peroxide, lauroyl peroxide, 2,4-dichlorobenzoyl peroxide,
azo-isobvtyric acid dinitrile~ dialkyl peroxy-dicarbonates,
as well as acetyl-cyclohexane-sulfonyl peroxide. These
monomer-solllble catalysts are emplo~yed preferably in quantities
of from 0.01% to ~% by wei~ht, advanta~eously 0 05% to 1% by
wei~ht based on the weight of the monomer or monomers.
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The usual polymerization aids other than those named
above, such as parti~le and molecular size regulators, can be
employed as well The quantity ratio of water to monomer to
be polymerized is not c~itical~
The pH value of the polymerization batch is preferably
~ to 10 and the polymerization temperature preferably about
30 to ~0C. Since, ~enerally, polymer dispersions susceptible
to shear are involved, overly vigorous agitation during poly-
merizat,ion is to be a~oid~d~
The polymer dispersions to be degassed are transferred
to a degassing vessel after completion of the polymerization.
Thereafter, a vacuum is created in the gas space above the
polymer latex by using vacuum pumps. In so doing, an internal
pressure is created which adjusts to correspond to the water
vapor part~al pressure at the temperature of the dispersîon.
Then saturated, unsaturated, or superheated steam is introduced,
at temperatures of between 50 and 90C, pr~erably 60 to ~0C,
and a pressure between 0,1 and 0.~ bar, preferably 0.2 to 0.7
bar Naturally~ it is possible to draw the steam supplied for
monomer removal into khe degassing vessel only if the internal
pressure in the degassing vessel has a pressure at least lower
than the steam to be drawn in. Steam of the aforementiQned
specification can be produced, for example, by steam converters
~rom steam which is hotter and/or under higher pressure.
The treatment, according to the invention, of the
polymer latices with steam of the aforementioned specification
in degassin~ vessels whose internal pressure is lower than
atmospheric pressure, ext~nds over a period of time of from
20 minutes to 3 hours. After this period~ any residual
monomeric vinyl chloride is generally reduced to values below
~76
~0 ppm For thls de~assing step~ steam quantities of 0.05 to
0.25 kg of steam per liter of dispersion are necessary~ The
introduction of steam can be e~fected so tha-t generally no, or
only a slight, volume increase of the polymer dispersion
occurs. When proceeding according to the invention, the
degassing process can be controlled so that heating of the
dispersion to temperatures beyond gOC does not occur.
Nor~ally, howev~r, one operates at temperatures above 40C,
as the degassing step would othe~ise take too long. The
te~peratures of the pol-ymer dispersion can be established by
the quantity and temperature of the admitted steam and the
vacuum applied on the dispersion surface.
In another form of executing the process, regulation
o~ the temperature of the polymer latex is effected externally,
by supplylng or remo~ing hea~, so as to be able to Yary within
. wide limits the quanti~y of admitted steam, whlch substantially
influences the efficiency of the process.
The following examples are illustrative of khe process
of the invention without belng limitative in any respect.
EXAMPLE 1
After the polymerization step, 4 liters o~ a vinyl
chloride polymerizate dispersion,obtained by micro-suspension
polymerization, were charged into a temperature-controllable
cylindrical Yessel of 150 mm diameter and 400 mm height, which
was also equipped with a propeller agitator. The dispersion was
adjusted toan absolute pressure o~ 150 Torr and 60C. In the course
of 2 hours, 0.10 kg of steam per liter of latex was passed
through. The steam had a temperature of 63C and a pressure of
170 Torr. During the entire ~egassing time, the absolute
pressure o~ 150 Torr was maintained in the degassing ves~el.
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The initial monomer content of 1,360 ppm was reduced after
one hour to lO~o and after two hours to below 40 ppm.
Coagulation did not oc¢ur.
EXAMPL~ 2
(Comparative Example)
The process of Example 1 was repeated, but with a
reduction of the quantity of steam to 0.045 kg of steam per
liter of latex. The initial monomer content of 1,090 ppm was
reduced, in thi~ case~ to ~9% after one hour and to 3~% after
10 two hours.
EXAMPL~_~
(Comparative Example~
The pro¢ess of Example 1 was repeated, but without
: passage of steam~ The initial monomer content of 1,160 ppm was
reduced after one hour to only ~2% and after two hours to only
6~% of the initial moncmer content.
The preceding speci~ic embodiments are illustrative of
the practice of the in~ention. It is to be understood, however~
that other expedients known to those skilled in the art or
disclosed herein, may be employed without departing from the
spirit of the in~ention or the scope of the appended claims.
