Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
45~
This invention relates to a method for the preparation
of vinyl chloride polymers or copolymers. In particular, the
invention relates to an improvement in the method of preparing
by emulsion or suspension polymerization vinyl chloride polymers
or copolymers having a relatively low ~olecular weight or degree
of polymerization.
Low molecular weight polyvinyl chloride resins can be
produced by polymerization at relatively high temperatures.
; However, the conventional high-temperature polymerization method
is undesirable because of an extended gellation time of the
resin, inferior flow of the resin in molding, and increased
amounts of residual vinyl chloride monomer remaining absorbed
in the resin. In order to avoid the above drawbacks, it has
been proposed to add certain chain transfer agents to the
polymerization mixture. Chain transfer agents suitable for the
purpose include, for example, saturated hydrocarbons, such as
n-pentane and n-hexane; saturated or unsaturated chlorinated
hydrocarbons,such as carbon tetrachloride, trichloroethylene
and and perchloroethylene; aldehydes, such as propionaldehyde
and n-butyl aldehyde; and certain mercapto-containing organic
compounds, such as dodecyl mercaptan.
The above-mentioned chain transfer agents have their
respective defects. That is to say ! with respect to the
saturated hydrocarbons, they are usually used in a larse amount,
say, about 8 to 10~ by weight of the hydrocarbon based on the
; vinyl chloride monomer in order ~o produce vinyl chloride
polymers having an average polymerization degree of as low as
about 700 by suspension polymerization at about 60~C. As a
result, the hydrocarbons tend to be emitted into the atmosphere
from the polymer products which are subjected to pos~-polymeri-
~' ~
,, ,1 .
45~1
zation processes or released from the finished resin products
during the course of storage, resulting in environmental
pollu-tion.
With respect to the saturated or unsaturated chlorinated
hydrocarbons, they may be used in a reduced amount, say from
0.7 to 1.0~ by weight based on the vinyl chloride monomer.
However, the prohlems of environmental pollution or detrimental
effect on the human health can not be solved due to toxicity
inherent in the compounds.
With respect to the aldehyde compounds, they are event-
ually decomposed by heat in the course of a polymerization run,
and the decomposition products exert a retarding effect on the
speed of polymerization and also exhibit a toxic effec~ on the
human body, though their amount may be Eurther reduced to 0.2
to 0.5% by weight based on the vinyl chloride monomer.
There are also known several methods in which certain
mercapto-containing organic compounds are employed with the
purpose of controlling the molecular weight of polyvinyl
chloride resins. For example, dodecyl mercaptan is used in
the emulsion polymerization of various kinds of vinyl monomers,
as disclosed in Japanese Patent Publication No. 50-322~1;
several kinds of alkyl thioglycolates are used in the poly~
merization of vinyl chloride, as disclosed in Japanese Patent
Publication No. 49-31746; and several kinds of mercapto-
containing organic ~ompounds having 4 or less carbon atoms in a
molecule are used in the low-temperature bulk pol~merization of
vinyl chloride in a continuous process, as disclosed in German
OLS 2046143. These prior art me~hods using the mercapto-
containing organic compounds have been found defective due to
the relatively large amounts of the compounds required which
would eventually lead to retardation of the polymerization speed
- 2 -
~1
and coloring of -the resultant polyvinyl chloride resin as well
as unpleasant odors inherent in the mercapto-compound.
The present invention seeks to provide an improved
method by which vinyl chloride polymers or copolymers having a
relatively low polymerization degree can be readily obtained in
emulsion or suspension polymerization by the use of chain trans-
fer agents w.ithout the disadvantages of the prior art described
above.
According to an aspect of the present invention there
is provided a method of producing polymers comprising polymer-
izing monomers dispersed in an aqueous medium to produce low
molecular weigh-t polymers, wherein the monomers comprise more
than 50% by weight of vinyl chloride monomers, and the aqueous
medium contains, as a dispersing agent, at least one water-
soluble polymeric subs-tance and, as a chain transfer agent, at
least one organic compound having at least one mercapto group
and at least one group selected from the functional groups
consisting of: hydroxy and carboxyl groups.
Vinyl chloride polymers obtained by methods in accordance
wi-th embodiments of the invention have desirable properties, such
as porosity and narrow par-ticle size distribution as well as
retaining very small amounts of the residual chain transfer
;~ agent or the decomposition products -thereof in the polymers.
The method is further improved with respect to the
properties of the polymer products by the use of very specific
suspending agents in limited proportions.
~ he method of the present invention gives several advan-
tages over the prior art methods. For example, (1) the amount
of the specifi.ed chain transfer agent to be added can be greatly
reduced because of its very high activity/ (2) the polymerî~ation
ràte is not effected by the use of the chain
- 3 -
~2~g~S~
transfer agents, (3) the quality of the polyvinyl chloride
resins obtained by the me-thod is desirable because the chain
transfer acJent itself or the decomposition products thereof do
not remain in the polymer produc-ts after completion of the
polymerization ancl -the subsequent processing, (4) -the worka-
bility oE the po:Lymer produc-ts in fabrication is very good owing
to the well-developed porosity of the polymer particles, and
(5) any unreacted monomer or monomers absorbed in the polymer
par-ticles can very easily be removed from the polymer products
after comple-tion of the polymerization.
Embodiments of the method of the present invention will
now be illustrated by way of example in further detail. The
organic compounds employed as the chain transfer agent in
accordance with the present invention have, as has been described
above, at least one mercapto group (-SH) in combination
with at least one hydroxy group (-OH) or carboxyl (-COOH) group
per molecule, and contain preferably from 2 to 7 carbon atoms
or, more preferably from 2 to ~ carbon atoms per molecule.
Such compounds are exemplified by SH-con-taining alcohols, such
as 2-mercaptoethanol, thiopropyleneglycol and thioglycerine,
- and SH-containlng carboxylic acids, such as thioglycolic acid,
thiohydracrylic acid, thiolactic acid and thiomalic acid. Com-
pounds containing carbon atoms exceeding 7 per molecule have
a lower chain transfer activity and are less valuable.
The amount of the chain transfer agent added to the
` polymerization mixture is preferably within -the range from
0.001 to 0.5% by weight, and more preferably in the range from
0.005 to 0.1~ by weight, based on the weight of monomer or
monomers used. The amount of chain transfer agent used largely
depends on the polymerization conditions, such as the poly-
~"~
s~
merization temperature, the polymerization degree of the polymer
to be obtained, and the like. Mixtures of the SH-containing
compounds within the above deEinition may be used, instead of
using a sin~le compound.
Excepting the use of the chain transfer agent, the
polymerization i-tself is much the same as conventional polymeri-
zation of vinyl chloride in an aqueous medium. For example,
suitable dispersing agents or the suspending agents are water-
soluble polymeric substances known in the prior art, including
partially saponified polyvinyl alcohol, cellulose ethers (e.g.,
methyl cellulose and hydroxypropylmethylcellulose), polyvinyl
pyrrolidone, vinyl acetate-maleic anhydride co-polymers, starch
and gelatine, all of which may be employed in conjunction with
an anionic or nonionic surface active agent. The polymerization
initiators suitably employed are also known and can be selec-ted
from the oil-soluble free-radical initiators belonging to the
organic peroxides, such as diisopropylperoxy dicarbonate, di-2-
ethylhexylperoxy dicarbonate, acetylcyclohexylsulfonyl peroxide,
t-butylperoxypivalate, benzoyl peroxide and lauroyl peroxide, and
:- .
azo compounds, such as azobisisobutyronitrile, azobis-2, 4-di-
methylvaleronitrile and azobis-2, 4-dimethoxy-2, 4-dime-thylvaler-
onitrile.
Although the method of the present invention is most
advantageously applicable to the suspension polymerization of
vinyl chloride or a monomer mixture mainly composed of vinyl
chloride, it is also applicable to emulsion polymerization.
; The monomers copolymerizable with vinyl chloride in the monomer
mixture include vinyl esters, such as vinyl aceta-te, vinyl
ethers, acrylic and methacrylic acids, and esters thereof;
maleic and fumaric acids and esters thereof; maleic anhydride,
aromatic vinyl compounds; unsaturated ni-trile compounds, such as
-- 5
,,
6~59
acryloni-trile; vinylidene halides, such as vinylidene fluoride
and vinylidene chloride; and olefins, such as ethylene and
propylene.
Other conditio.ns of polymerization with respect, for
example, to the amount of the polymerization initiators, poly-
merization temperature, time of polymerization, and the like
are not critical, and can be determined as in the conventional
polymerization of vinyl chloride in an aqueous medium.
It has been noted that the dispersion of the monomer
or monomers in the aqueous medium some-times became degraded or
unstable during the polymerization run, resulting in the pro-
duction of inferior polymers with a coarser particle size dis-
tribution or increased occurrence oE fish-eyes.
It has been established that the dispersion of the
. monomer can be stabilized if the monomer or monomers are dis-
; persed in the aqueous medium in the presence of combined dis-
` persing agents (i.e., composed of a partially saponified poly-
vinyl alcohol and a cellulose ether in a specific proportion),
. whereby the polymer particles can be effectively prevented from
coarsening,. regardless of the intensity of agitation, the p~
of the aqueous medium and other conditions of polymerization.
Further, the porosity of the polymer particles is improved by
use of the combined dispersing agents, bringing about the advan-
tages that the removal of the residual monomer from the polymer
product is very rapid and the occurrence of fish-eyes in the
~: articles fabricated from the polymer product is greatly reduced.
The proportion of a partially saponified polyvinyl
alcohol and a cellulose ether is preferably within the range from
80 to 20~ by weight, and more preferably from 30 to 70~ by
weight, of the former and from 20 to 80% by weight, and more
E
L5~
preferably from 70 to 30~ by weight, of the latter. When the
amoun-t of the cellulose ether i.s increased relative to that of
the par-tially saponified polyvinyl alcohol to outside -the above
ranges, the particle size distribution oE the polymer product
becomes broadened and the occurrence of fish-eyes in the fabri-
cated articles is increased. On the contrary, an increased
relative amount of the partially saponified polyvinyl alcohol to
outside the above ranges leads disadvantageously to the coarser
particle size distribution of the polymer product. The amount
of the dispersing agent is such that the total of the partially
saponified polyvinyl alcohol and the cellulose ether is pre-
ferably in the range from 0.01 to 0.5% by weight, and more
preferably from 0.02 to 0.2% by weight, based on the monomer or
monomers in the polymerization mixture.
: The partially saponified polyvinyl alcohol preferably
has a degree of saponification in the range of from 65 to 93%,
more preferably from 75 to ~0% and a molecular weight such that
its 4~ by weight aqueous solution has a viscosity ranging pre-
ferably from 20 to 70 centipoise at 20C. On the other hand,
`~ 20 preferably the cellulose ether is selected from methylcellulose,
ethylcellulose, hydroxyethylcellulose, hydroxypropylmethyl-
cellulose and the like, and its 2~ by weight aqueous solution
has a viscosity preferably in the range of from 10 to 100 centi-
poise at 20C.
In addition to the above defined partially saponified
polyvinyl alcohol and cellulose ether, certain water-soluble
polymeric substances, for example, vinyl acetate-maleic anhy-
dride copolymers, starch, gelatine, and nonionic or anionic sur-
face active agents may be present in the polymerization mix-ture,
if desired, in order to promote the dispersion of the monomer or
monomers, though the amounts of such auxiliary dispersing agen-ts
s~-~ 7 ~
I~A j
5DI
should be as small as possible.
The following examples further illustrate embodiments
of the method of the present invention but do not limit the
scope of the inventi.on.
Example 1.
Into a 100-liter capacity stainless steel polymeriza-
-tion reactor were introduced 60 kg of deionized water, 33 g of
a partially saponified polyvinyl alcohol having a degree of
saponification of about 80~ and having a viscosity of about
35 centipoise at 20C as measured in a 4% by weight aqueous
solution, 12 g of a methylcellulose with a methoxy content of
30% by weight, having a viscosity of about 15 centipoise at
20C as measured in a 2% by weight aqueous solution, 30 kg of
vinyl chloride monomer, azobis-2, 4-dimethylvaleronitrile as
the polymerization initiator in an amount of 12 or 9.6 g and
the various chain transfer agents as indicated in Table 1.
Polymerization was conducted at a temperature of 62 or 57C
under agitation at 300 r.p.m. for the various periods of time
as indicated in -the table, during which unreacted monomer was
removed, and then the polymer product was recovered by dehydra-
tion and drying. The monomer-to-polymer conversion in ~ and
the averaye polymerization degree of the polymer pro~ucts are
set out in the same table.
The heat stability of the polymer products ob-tained
above was tested as follows. A blend of 100 parts of the poly-
vinyl chloride resin to be tested, 15 parts of lead stearate,
0.5 part of tribasic lead sulfate and 0.5 part of dibasic lead
stearate, all parts being by weight, was milled homogeneously
and fabricated into a sheet 2 mm thick by passing through a hot
roller at 170C. These sheets were heated in a Geer's oven at
~z~s~
180C till they were blackened. The periods of time for such
blackening were recorded in minutes and set out in the table
as the heat stability.
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Example 2.
Into the same polymeriza-tion reactor as employed in
Example 1 were introduced 60 kg of deionized water, 33 g oE
the same par-tially saponified polyvinyl alcohol as in Example
1, 12 g of the same me-thylcellulose as in Example 1, 24 kg of
vinyl chlor.ide monomer, 6 kg of v:inyl acetate monomer, 12.0 g
of azobis-2, 4-dimethylvaleronitrile and 2-mercaptoethanol or
trichloroethylene as indicated in Table II. Polymerization
was conduc-ted at 62C for 8 hours, during which unreacted
monomer was removed, and then the vinyl chloride-vinyl acetate
copolymer product was recovered by dehydration and drying. The
monomer-to-polymer conversion and the average degree of poly-
merization of the polymer products are set out in the table.
; These vinyl chloride-vinyl acetate copolymers were
subjected to the test for heat stability in the same manner as
in Example 1 except that the temperature of the oven was set
at 120C instead of 180C. The results of the test are also
shown in the table.
Table II
Experiment No.
8 9*
Chain transfer agent (g) 2-Mercaptoethanol Trichloroethylene
(17) (600)
Conversion, ~ 92 90
Average degree of 690 700
polymeriza-tion
Heat stability, min. 75 70
* Control
5~
Example 3.
Into a 100~ ter capacity s-tainless steel polymeri~.a-
tion reactor were lntroduced SO kg of deionized water, the
mercapto-containin~ organic compound or trichloroethylene as
the chain transfer agent and the combined dispersing agents
composed of a partially saponified polyvinyl alcohol and a
cellulose e-ther, as indicated in Table III. Vinyl chloride
monomer or vinyl chloride ~ vinyl acetate monomer mixture and
polymerization initiators used are also indicated in the table.
Polymerization wa.s conducted at the polymerization -temperature
with the velocity of agitation as set forth in the table.
In the table, the shortened names of compounds are as
follows:
PVA-A: Partlally saponified polyvinyl alcohol
with the degree o~ saponificati.on 80
~nd vi~coslty 35 centlpol&e at Z0C a~
measured in a 4~ by wei~ht a~ueou~
80111tiOltl.
PVA-B: Partially sapo~ified polyvlnyl alcohol
with the degree o~ ~aponi~icatio~
and visco~ity 30 csntlpoise at 20C a~
mea~ur~d in a 4% by weight ~que~u3
solution.
PVA-C: Partially saponifled p~lyvlnyl alcohol
with the dsgre3 oi saponi~ication 80%
and vlsco~ity 50 centipoi~ at 20C a~
measured in a 4% by weight aqueous
solution.
~ 12 _
.~.7
s~
o1 Cel-A: Hydroxypropylmethylcellulo~e with ths
content of methoxy groupa 30~ by welght,
content of hydroxypropoxy groups 10~ by
welght and vi~cosity 60 centipoi~e at 20C
a~ mea~ured in a 2% by weight aquaous
solution.
Cel-B; ~Iydroxypropylcellulose with the content
o~ hydroxypropo~y ~roup~ 65% by weight
and vi~co~ty 50 centipoi~e at 2aQC as
lo measured in a 2% by wei~ht aqueou~
301uti on.
C~l-C; Hydroxypropylmethylcellulc3~ with th~
content of methox~ group~3 2096 by w~ght,
oontent o~ hydroxypropoxy group~ a% by
weight and vi 8co~ity 100 o~ntllpol~e at
~0C a~3 measured in a ~% by w01~ht aqueou~
801ution.
IPP: Dii~opropyl p~roxydicarbona~0
P~: t-13utyl pero~yplvala~
: 20
DMVN: Azobis-2 ,, 4 diolethyl Yall3ronitrlle .
~C: Vinyl chlorid~.
YAc: V~nyl ~cetata~
- ].3
i45~
Table III
Tem-
pera- Agi-
Exp. Monomer Chain Transfer Dispersing Initi- ture, tation,
No. (kcJ) agent ( ~ a~ent tg) a-tor(g) C r.p.m.
10 VC 2-Mercapto- PVA-A(10) IPP 61 350
(25) ethanol t8) Cel-A(7.5) (10)
11 VC 2-Mercapto- PVA-A(10) IPP 61 300
(25) ethanol (8) Cel-A(7.5) (10)
12 VC 2-Mercapto- PVA-A(10) IPP 61 270
(25) ethanol (8) Cel-A(7.5) (10)
13 VC 2-Mercapto- PVA-A(10) PV 63 350
(25) ethanol (2) Cel-A(7.5) (10)
14 VC 2-Hydroxy- PVA-C(10) DMVN 61 350
(25) propyl Cel-B(7.5) (10)
mercaptan (8)
15 VC Thioglycer- PVA-B(4) DMVN 61 350
(25) ine (8) Cel-C(12) (10)
16 VC 2-Mercapto- PVA-B(7) IPP 55 350
(25) ethanol (15) Cel-A(ll) (10)
17VC (22) 2-mercapto- PVA-A(10) IPP 56 300
VAc (3) ethanol ~15) Cel-A(15) (10)
18VC Trichloro- PVA-A(10) PV 61 350
(25) ethylene(200) Cel-A(7.5) (10)
19VC Dodecyl- PVA-A(10) DMVN 61 350
(25) mercaptan(20) Cel-A(7.5) (10)
20VC 2-Ethylhexyl PVA-A(10) DMVN 60 350
(25~ thioglycolate Cel-A(7.5) (10)
(50)
21 VC 2-Mercapto- PVA-B(25) DMVN 61 350
(25) ethanol (8) Cel-A (1) (10)
22VC 2-Mercapto- PVA-A(18) DMVN 61 350
(25) ethanol (8) Cel-A (2) (10)
23VC 2-Mercapto- PVA-A (1) DMVN 61 350
(25) ethanol (8) Cel-A(20) (10)
Notes: Experiments 18 to 20 are controls. Experiments 21 to
23 are for demonstrating the effect of the combin~d
dispersing agents with the proportions outside the pre-
ferred range.
_ 1~ --
.~
'
The polymer products resul-ting from the above polymer-
ization tests were then -tested for average degree of polymer-
ization, particle size distribution, occurrence of fish-eyes
and deoctyl phthalate (DOP) absorption, and the results are
set ou-t in Table IV. Also in the table, the speed of monomer
removal from each polymer is shown. The manner of determining
each of the properties i.e. occurrence of fish-eyes, DOP
absorption and speed of monomer removal was as follows.
Occurrence of fish-eyes: A mixture of 50 g of
the polymer product to be tested, 25 g of
DOP, 0.3 g of tribasic lead sulfa-te, 1.0 g
of lead stearate, 0.01 g of titanium
dioxide and 0.005 g of carbon black was
kept standing for about 30 minutes and
then milled in a hot roller mill a-t 1~0C
for 7 minutes. The blended mixture was
taken out from the roller mill in the form
of a sheet 0.2 mm thick, and the number
of the transparent particles (fish-eyes)
~as counted over an area of 100 cm2 of
the sheet.
DOP absorption: A mixture of 10 g of the polymer
product to be tested and 20 g of DOP was,
after 1 hour of standing, subjected to
centrifugal separation to remove the un~
absorbed DOP and the percentages of the
DOP absorbed in the polymer were de-termined
by weighing.
Speed of monomer removal: A liter of the polymer
slurry after completion of the polymeri~ation
_ 15 -
~'
.
1~;26~5~
was kept at 80C by heating in a flask
with agita-tion while nitrogen gas was
blown into the slurry at a rate of 0.1
li-ter/minute. Small portions of the
polymer slurry were taken at certain
time i.ntervals and analyzed for the
monomer content to determine the time
required for the decrease of the monomer
content to a half of the value.
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