【Invention Title】
METHOD OF PREPARING VINYL CHLORIDE BASED RESIN USING
SUSPENSION POLYMERIZATION
【ABSTRACT】
Disclosed is a method of preparing a vinyl based polymer and copolymer
comprising a vinyl chloride based polymer and copolymer using suspension
polymerization to prevent forming by elevating slowly reaction temperature such that
pressure is maintained when pressure drops in the late stages of reaction. According to
the method, the vinyl chloride based resin is prepared using suspension polymerizing
vinyl chloride based monomers so as to reduce foaming and improve a protrusion,
thermal stability and the like.
【Claims】
1. A method of preparing a vinyl chloride based resin,
wherein temperature is elevated at a rate of greater than 0.008 ℃/min and less
than 0.8 ℃/min for greater than 10 minutes and less than 90 minutes when reaction
pressure is reduced to greater than 0.01 kgf/cm2 and less than 0.5 kgf/cm2 after initiation
of suspension polymerization.
2. The method according to claim 1, wherein the reduced reaction pressure
is greater than 0.01 kgf/cm2 and less than 0.2 kgf/cm2.
2
3. The method according to claim 1, wherein the temperature elevation rate
is greater than 0.008 ℃/min and less than 0.65 ℃/min.
4. The method according to claim 1, wherein the temperature elevation rate
controls temperature control system.
5. The method according to claim 1, wherein the polymerization reaction
temperature is 30 to 80℃.
6. The method according to claim 1, wherein the temperature elevation is
carried out for 30 to 60 minutes.
7. A vinyl chloride based resin prepared by the method according to any one
of claims 1 to 6.
8. The vinyl chloride based resin according to claim 7, wherein a
concentration of residual initiator is less than 0.002 parts by weight based on 100 parts
by weight of the vinyl chloride based resin.
9. A vinyl chloride based resin composition comprising the vinyl chloride
based resin according to claim 7, an inorganic filler and a stabilizer.
10. A vinyl chloride based resin composition according to claim 9, wherein
the inorganic filler is carbon black.
3
11. A method of preparing a vinyl chloride based resin,
wherein temperature is elevated at a temperature elevation rate of 0.01 to
0.5 ℃/min for 20 to 60 minutes when reaction pressure is reduced to 0.05 to 0.1 kgf/cm2
after initiation of suspension polymerization reaction.
【Technical Field】
The present invention relates to a method of preparing a vinyl chloride based
resin, and more particularly to a method of preparing a vinyl chloride based resin to
improve quality of a protrusion and thermal stability induced by residual initiator by
preventing pressure drop and foam generation by slowly elevating polymerizat ion
temperature to a certain temperature per minute when pressure drops in the late stages
of reaction during preparation of a vinyl chloride based resin to improve quality of the
vinyl chloride based resin.
【Background Art】
Vinyl chloride based resins are inexpensive as well as having characteristics
such as superior processability, chemical resistance, durability, dielectric property, and
the like. Due to such characteristics, vinyl chloride based resins are used as hard and
soft materials in pipes, wall papers, window frames, wire covers, wrap films, sheets, and
the like. However, since vinyl chloride based resins are broadly utilized and customer
demand for processability has been rising, continuous improvement in processability is
required.
Generally, the processability of a vinyl chloride based resin is affected by
factors such as porosity of particles, particle diameter, particle distribution and
morphology. The meltability of resins has been improved by controlling the factors. The
4
meltability of resins is estimated by a protrusion or fish-eyes, which is a quality
standard to estimate the meltability. Quality of a final product is estimated by
generation of a protrusion or fish-eyes.
Quality degradation of the protrusion is caused by variation in metability
among particles. In addition, generation of unmelted particles is related to the foam
generated during reaction. In detail, foam is generated when a reflux condenser is often
utilized in the late of the reaction and at the same time, pressure drops. At this time, dry
foam including particles floats in an upper portion of a reactor and in a reflux
condenser. However, since it is difficult to sufficiently wash all batches, particles
floated by the foam remain inside the reactor and reflux condenser. The remaining
particles are introduced into subsequent reactions and thereby, repolymerized. As a
result, large and low porosity particles are generated. Due to such particles having low
meltability, quality of the protrusion deteriorates.
Prior art to solve the problem is described in US Patent No. 5,087,678 and
Japanese Application Pub. No. 2007-284510. In the patent literature, a method using an
antifoaming agent alone or in combination with other additives has been used to inhibit
foaming during the reaction. However, by the method using the antifoaming agent
alone, it is difficult to inhibit generation of the dry foam. Furthermore, by adding an
antifoaming agent and additives into the reactor during the reaction, expenses for
equipment, an antifoaming agent and additives are increased. Additionally, resin quality
may deteriorate due to addition of the antifoaming agent
【Disclosure】
【Technical Problem】
Therefore, the present invention has been made in view of the above
5
problems, and it is an object of the present invention to prevent pressure drop and
foaming by elevating polymerization temperature to a certain temperature per minute so
as to inhibit repolymerization and scale formation.
It is a further object of the present invention to provide a method of preparing a
vinyl chloride based resin having characteristics such as reduction of a protrusion and
superior thermal stability by reducing an amount of residual initiators after
polymerization reaction is finished by inducing decomposition of the initiators
according to the elevated temperature and thereby, not having scale generation, or
quality problems by a protrusion or thermal stability observed in conventional methods.
The above and other objects can be accomplished by the present invention
described below.
【Technical Solution】
In accordance with one aspect of the present invention, the above and other
objects can be accomplished by the provision of a method of preparing a vinyl
chloride based resin using suspension polymerization, wherein polymerization
temperature is elevated when pressure drops in the late stages of reaction.
In accordance with another aspect of the present invention, provided is a
method of preparing the vinyl chloride based resin, wherein the temperature is
elevated at a temperature elevation rate of greater than 0.008 ℃/min and less than
0.8 ℃/min for greater than 10 minutes and less than 90 minutes when reaction
pressure is reduced to greater than 0.01 kgf/cm2 and less than 0.5 kgf/cm2 after
initiation of suspension polymerization.
The reduced reaction pressure is, for example, greater than 0.01 kgf/cm2 and
less than 0.2 kgf/cm2.
6
The temperature elevation rate is, for example, greater than 0.008 ℃/min and
less than 0.65 ℃/min.
The temperature elevation rate may control, for example, temperature control
system.
The polymerization reaction temperature is, for example, 30 to 80℃.
The temperature elevation is carried out, for example, for 30 to 60 minutes.
In accordance with another aspect of the present invention, provided is a vinyl
chloride based resin prepared by the method.
A concentration of residual initiator is, for example, less than 0.002 parts by
weight based on 100 parts by weight of the vinyl chloride based resin.
In accordance with another aspect of the present invention, provided is a vinyl
chloride based resin composition comprising the vinyl chloride based resin, an
inorganic filler and a stabilizer.
The inorganic filler is, for example, carbon black.
The number of fish-eyes (thickness: 0.3 mm, area: 400 cm2) of the vinyl
chloride based resin composition is less than 10.
In accordance with another aspect of the present invention, provided is a
method of preparing a vinyl chloride based resin, wherein temperature is elevated at a
rate of 0.01 to 0.5 ℃/min for 20 to 60 minutes when reaction pressure is reduced to
0.05 to 0.1 kgf/cm2 after initiation of suspension polymerization reaction.
【Advantageous Effects】
As apparent from the fore-going, the present invention advantageously provides
a method of preparing a vinyl chloride based resin using suspension polymerization.
Temperature elevation rate is controlled according to each polymerization temperature
7
in a step of preparing the vinyl chloride based resin and thereby, pressure drop
occurring in the late stages of the polymerization reaction is prevented. Foam is
generated when pressure drop and at the same time, heat of a reflux condenser is
removed. Here, the heat removal increases in the late stages of the polymerization
reaction. Thus, foam generation is inhibited by controlling the temperature elevation
rate. In addition, problems such as protrusion quality and thermal stability induced by
scale and repolymerization observed in the prior art is solved.
【Best Mode】
To solve drawbacks of the prior art, the present invention provides a method of
preparing a resin, quality of which is improved by inhibiting foaming which may occur
when a reflux condenser is often utilized and at the same time, pressure drops by
preventing pressure drop by slowly elevating temperature at the time of pressure drop in
the late stages of reaction, and by inducing decomposition of an initiator remaining in
the late stages of reaction.
According to the present invention, foam is not generated after reaction and
thereby, inflow of repolymerizable particles is prevented. In addition, residual initiator
does not remain in the reactor. Thus, thermal stability is improved and, as such, qualit y
is improved.
The present invention relates to a method of preparing a vinyl chloride based
resin using suspension polymerization by controlling polymerization temperatures and
polymerization initiators, dependent on each polymerization degree. The method of
preparing the vinyl chloride based resin controls foaming by preventing pressure drop
occurring in the late stages of reaction by elevating polymerization temperature in the
late stages of reaction to minimize foaming during reaction and an amount of residual
8
initiator.
Hereinafter, the present invention is described in detail.
In the prior art, heat removal and polymerization conversion ratio are increased
by use of a reflux condenser in the late stages of reaction and thereby, dry foam
including particles is generated when pressure drops in the late stages of reaction. The
present invention provides a new method of preparing the vinyl chloride based resin to
accomplish quality improvement by solving problems such as scale and
repolymerization generated in an upper portion of the reactor and reflux condenser, and
a large amount of residual initiator, which occur by the generated dry foam.
To accomplish the above described properties, the present invention controls
pressure drop by elevating temperature at a rate of greater then 0.008℃ and less than
0.8℃ per minute when pressure drops in the late stages of reaction such that the
elevated temperature is not greater than heat removal limitation of the reactor so as to
improve the meltability of a final resin obtained by preventing foam generation
according to pressure change. The temperature is elevated at a rate of, preferably,
0.01℃ or more and less than 0.65℃, or 0.02 to 0.65℃, more preferably, 0.05 to 0.6℃,
per minute. When pressure drops in the late stages of reaction, temperature is elevated
rapidly, and pressure and reaction become unstable by controlling the elevating
temperature to be higher than the above temperature ranges, and, as such, resin quality
deteriorates. On the other hand, when the elevating temperature is below the above
temperature ranges, the temperature is elevated slowly, resulting in pressure drop. As a
result, problems identical to the problems observed in the prior art occur.
In the present invention, temperature elevation is initiated when the reactor
pressure falls to 0.01 kgf/cm2 to 0.5 kgf/cm2 in the late stages of reaction. Preferably,
temperature is elevated when pressure drops to 0.05 kgf/cm2 or more and less than 0.2
9
kgf/cm2. When temperature is elevated before pressure drops to the range, pressure
change for the elevated temperature occurs and the reaction becomes unstable,
deteriorating resin quality. In addition, when temperature is elevated after pressure
drops to pressure greater than the ranges, foam including particles which occurs by heat
removal and pressure drop is generated. As a result, a protrusion and inner scale of the
reactor are generated, deteriorating resin quality.
Generally, polymerization temperature is determined by the degree of
polymerization of a resin. Generally utilized temperature is determined in temperature
range of 55 to 60℃ for a resin having a number average degree of polymerization of
1000, 61 to 65℃ for a resin having a number average degree of polymerization of 800,
and 66 to 70℃ for a resin having a number average degree of polymerization of 700.
In the present invention, the polymerization temperature is, preferably, within
temperature range of 30 to 80℃. More preferably, the polymerization temperature is
within temperature range of 40 to 70℃. Even more preferably, the polymerization
temperature is within temperature range of 50 to 65℃.
Degree of polymerization means the number of repeat units, namely,
monomers, constituting polymers. In the present invention, number average degree of
polymerization is utilized.
According to the polymerization temperature, reaction pressure is changed and
thereby, degree of pressure drop is changed. Thus, the temperature elevation rate is
controlled, depending on difference of reduced pressure in the late stages, such that
reaction stability is not disturbed.
Additionally, time to elevate the polymerization temperature by generation of
pressure drop is 30 to 60 minutes, preferably, 30 to 50 minutes, more preferably 30 to
40 minutes, from the time when pressure drops.
10
The process of elevating the polymerization temperature is carried out until
reaction is finished. Here, time taken to elevate the polymerization temperature does not
have great influence on overall polymerization time. When the time is too short, after
elevation is stopped, pressure rapidly drops and thereby, foaming may increase. As a
result, a protrusion and scale are generated. In addition, when the time is too long, inner
morphologies of particles are affected and output per unit time is reduced since the
polymerization time is greatly extended. Additionally, the outmost range of elevation
temperature should be in a range of 30 to 80℃, as proposed previously.
Initiators suitable for the polymerization may comprise at least one selected
from peroxides such as α,α'-bis(neodecanoyl peroxy)diisopropyl benzene, cumene
peroxyneodecanoate, di-n-propylperoxydicarbonate, Diisopropyl peroxy dicarbonate,
1,1,3,3-tetramethylbuthyl peroxyneodecanoate, bis(4-t-butyl cyclohexyl) peroxy
dicarbonate, 1-cyclohexyl-1-methylethyl peroxyneodecanoate, di-2-ethoxyethyl peroxy
dicarbonate, di-2-ethylhexyl peroxy dicarbonate, t-hexyl peroxyneodecanoate,
dimethoxybutyl peroxy dicarbonate, bis(3-methyl-3-methoxybutyl) peroxy dicarbonate,
t-butyl peroxyneodecanoate, t-hexyl peroxypivalate and the like, and azo compounds
such as 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(isobutyronitrile), dimethyl
2.2'-azobis(isobutyrate) and the like, although the present invention is not limited
thereto.
An amount of the polymerization initiator input to the reactor is identical to an
amount of polymerization initiator utilized in a conventional polymerization process.
Thus, the polymerization initiator is used in an amount of 0.02 to 0.3 parts by weight or
0.03 to 0.2 parts by weight, preferably, 0.04 to 0.15 parts by weight, based on 100 parts
by weight of the vinyl chloride based monomer.
The present invention provides the vinyl chloride based resin prepared
11
according to the method of the present invention.
In the vinyl chloride based resin, residual initiator is added in amount of less
than 0.002 parts by weight, or 0.0005 to 0.0015 parts by weight, preferably, 0.0006 to
0.001 parts by weight, based on 100 parts by weight of the vinyl chloride based resin.
For reference, the vinyl chloride based resin of the present invention comprises
a resin consisting of the vinyl chloride monomers alone and a resin consisting of a
copolymer (an amount of the vinyl chloride monomer is greater than 50 wt%, based on
total composition of the resin) of the vinyl chloride monomer, which is a main
ingredient, and a vinyl based monomer which is copolymerized with the vinyl chloride
monomer.
The vinyl based monomer which is copolymerized with the vinyl chloride
monomer may comprise at least one selected from the group consisting of olefin
compounds such as ethylene, propylene and the like; vinyl esters such as vinyl acetate,
vinyl propionate and the like; unsaturated nitriles such as acrylonitrile and the like;
vinyl alkyl ether such as vinyl methyl ether, vinyl ethyl ether and the like; unsaturated
fatty acids such as acrylic acid, methacrylic acid, itaconic acid, maleic acid and the like;
and anhydrides of the fatty acids.
Hereinafter, preferred examples will be provided for better understanding of the
present invention. It will be apparent to those skilled in the art that these examples are
only provided to illustrate the present invention and various modifications and
alterations are possible within the scope and technical range of the present invention.
Such modifications and alterations fall within the scope of claims included herein.
[Example]
Example 1
12
100 parts by weight of a vinyl chloride monomer, 130 parts by weight of
deionized water, 0.04 parts by weight of polyvinyl, which is a dispersing agent, having
degree of saponification of 72%, and 0.02 parts by weight of 55% of polyvinyl, which is
a dispersing agent, were input to a one cubic meter reactor.
0.02 parts by weight of t-butyl peroxyneodecanoate (BND) as a polymerization
initiator was input to the reactor. Thereafter, the reactor was stirred after being
depressurized, to remove air and nitrogen.
The reactor was heated and maintained at polymerization temperature of 58℃.
When inner pressure of the reactor in the late stages of reaction drops to about 0.1
kgf/cm2, temperature was elevated at a rate of 0.2℃ per minute by controlling the
polymerization temperature with steam or cooling water having a certain temperature, to
maintain pressure. After elevating the temperature for about 35 minutes, the temperature
was not elevated any longer. When the pressure reached on 7.0 kgf/cm2, the reactor was
cooled and then reaction was stopped by adding 0.02 parts by weight of Irganox 245
and 0.01 parts by weight of sodium hydrogen carbonate, as antioxidants. By out -
gassing, un-reacted vinyl chloride monomer and vinyl chloride resin slurry were
collected.
The collected slurry was washed and dried to obtain a vinyl chloride resin
polymer having a number average polymerization degree of 1000.
Example 2
A vinyl chloride resin polymer having a number average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
except that temperature was elevated at a rate of 0.4℃ per minute when pressure drops
in the late stages of reaction.
13
Example 3
A vinyl chloride resin polymer having a number average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
except that temperature was elevated at a rate of 0.6℃ per minute when pressure drops
in the late stages of reaction.
Example 4
The vinyl chloride resin polymer having a number average polymerization
degree of 1000 was prepared by repeating polymerization in the same manner as in
Example 1, except that polymerization temperature was elevated at a rate of 0.05℃ per
minute when pressure drops in the late stages of reaction.
Example 5
A vinyl chloride resin polymer having a number average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
except that polymerization temperature was increased when pressure drop was 0.05
kgf/cm2 in the late stages of reaction.
Example 6
A vinyl chloride resin polymer having a number average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
except that except that polymerization temperature was increased when pressure drop
was 0.15 kgf/cm2 in the late stages of reaction.
14
Example 7
A vinyl chloride resin polymer having a number average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
except that temperature was elevated for 30 minutes after pressure drops in the late
stages of reaction pressure drop.
Example 8
A vinyl chloride resin polymer having a number average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
except that temperature was elevated for 50 minutes after pressure drops in the late
stages of reaction pressure drop
Comparative Example 1
To test a method of preparing the vinyl chloride resin in Example 1, a vinyl
chloride resin polymer having a number average polymerization degree of 1000 was
prepared by repeating polymerization in the same manner as in Example 1, except that
polymerization temperature was not elevated after pressure drops in the late stages of
reaction.
Comparative Example 2
A vinyl chloride resin polymer having a number average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
except that polymerization temperature was elevated at a rate of 0.008℃ per minute
from when pressure drops in the late stages of reaction.
15
Comparative Example 3
A vinyl chloride resin polymer having a number average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
except that polymerization temperature was elevated at a rate of 0.8℃ per minute from
when pressure drops in the late stages of reaction.
Comparative Example 4
A vinyl chloride resin polymer having a number average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
except that pressure dropped to 0.01 kgf/cm2 in the late stages of reaction.
Comparative Example 5
A vinyl chloride resin polymer having a number-average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
except that pressure dropped to 0.5 kgf/cm2 in the late stages of reaction.
Comparative Example 6
A vinyl chloride resin polymer having a number average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
except that temperature was elevated for 10 minutes after pressure drops in the lat e
stages of reaction pressure drop.
Comparative Example 7
A vinyl chloride resin polymer having a number average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
16
except that temperature was elevated for 90 minutes after pressure drops in the late
stages of reaction pressure drop.
Comparative Example 8
A vinyl chloride resin polymer having a number average polymerization degree
of 1000 was prepared by repeating polymerization in the same manner as in Example 1,
except that polymerization reactor temperature was elevated irregularly up to
temperature of 70℃ from 58℃ by stopping supply of cooling water to a jacket after
pressure drops in the late stages of reaction
[Test Example]
Analysis of the vinyl chloride resin prepared in Examples 1 to 8 and
Comparative Examples 1 to 8 was performed in accordance with the following methods.
* A number average degree of polymerization: measured in accordance with
ASTM D1243-79.
* Polymerization time: measured from when a polymerization initiator was
input to the reactor to when the reaction was finished.
* Whiteness index: roll-milling was carried out at temperature of 180℃ for 3
minutes after inputting 5 parts by weight of a mixed stabilizer (WPS-60, lead based
stabilizer, Songwon Industrial Co., Ltd.), 1.5 parts by weight of a processing material
(PA-822, acrylic based processing material, LG Chemical) and 2 parts by weight of
titanium oxide, based on resin 100 parts by weight of vinyl chloride based resin, into the
reactor, to obtain a sheet having a thickness of 0.5 mm.
Thereafter, whiteness index (W.I) was measured using NR-3000 produced by
Nippon Denshoku. Results are summarized in the following Tables 1 and 2. Thermal
17
stability was estimated by the whiteness index. Increasing whiteness index indicates
improved thermal stability.
* Concentration of residual initiator: an amount of initiator present in the vinyl
chloride based resin was measured by an iodometric titration method. In detail, after
mixing 50 parts by weight of the resin and 80 parts by weight of isopropyl alcohol
(IPA), 20 ml of 10% acetic acid solution and 20 ml of 10% potassium iodide solution
were input to the mixture and then, were mixed sufficiently with the mixture.
* The number of fish-eyes: 100 parts by weight of the vinyl chloride based
resin, 45 parts by weight of DOP, barium stearate 0.1 parts by weight, 0.2 parts by
weight of a tin based stabilizer and 0.1 parts by weight of carbon black were mixed and
mulled at temperature of 140℃ for 5 minutes with a six inch roll to prepare a sheet
having a thickness of 0.3 mm. The number of fish-eyes means the number of white and
transparent particles present in 400 cm2 of the sheet.
* Particle size distribution: the particle diameter of the obtained resin was
measured with HELOS particle size analyzer produced by Sympatec. The particle size
distribution was defined by span value. Here, deviation is small when the span value is
low.
TABLE 1
Items
Exam
-ple 1
Exam
-ple 2
Exam
-ple 3
Exam
-ple 4
Exam
-ple 5
Exam
-ple 6
Exam
-ple 7
Exam
-ple 8
Temperature
elevation
rate
(℃/min)
0.2 0.4 0.6 0.05 0.2 0.2 0.2 0.2
Pressure
drop
(△ P,
kgf/cm2)
0.1 0.1 0.1 0.1 0.05 0.15 0.1 0.1
18
Elevation
time (min)
35 35 35 35 35 35 30 50
Polymerizatio
n time (min)
264 264 263 267 261 268 260 280
Whiteness
index
75 75 76 74 75 74 73 76
Concentration
of residual
initiator
(ppm)
10 8 5 12 7 11 14 4
Fish-eyes
(No.)
5 5 4 5 4 7 6 6
TABLE 2
Items
Comp
a
-rative
Exam
-ple 1
Comp
a
-rative
Exam
-ple 2
Comp
a
-rative
Exam
-ple 3
Comp
a
-rative
Exam
-ple 4
Comp
a
-rative
Exam
-ple 5
Comp
a
-rative
Exam
-ple 6
Comp
a
-rative
Exam
-ple 7
Comp
a
-rative
Exam
-ple 8
Temperatur
e elevation
time
(℃/min)
- 0.008 0.8 0.2 0.2 0.2 0.2 -
Pressure
drop
(△ P,
kgf/cm2)
- 0.1 0.1 0.01 0.5 0.1 0.1 0.1
Elevation
time (min)
- 35 35 35 35 10 90 35
Polyme
-rization
time (min)
260 264 256 253 266 260 349 264
Whiteness
index
72 72 68 73 72 71 69 74
Concen
-tration of
residual
initiator
(ppm)
35 26 2 13 8 29 3 8
Fish-eyes
(No.)
10 11 16 20 23 12 38 15
19
As can be seen from Tables 1 and 2 above, whiteness indexes and fish-eyes in
Example 1 carried out to prevent pressure drop by elevating temperature when pressure
drops were improved, compared to whiteness indexes and fish-eyes of Comparative
Example 1 where the polymerization temperature is not elevated in the late stages of
reaction.
In addition, as described in Examples 2 to 3, the whiteness index and
polymerization time could be controlled by controlling temperature elevation rate. As a
result, the whiteness indexes and fish-eyes were improved.
In addition, decline in the number of fish-eyes was smaller when temperature
elevation rate was too slow or too fast at the time of pressure drop as described in
Comparative Examples 2 and 3.
In addition, when temperature was elevated after stopping supply of cooling
water to a jacket as described in Comparative Example 8, the elevated temperature was
not constant and thereby, it was difficult to suppress foaming. Thus, increase of fisheyes
was observed.
Therefore, in accordance with the present invention, pressure drop was
prevented by elevating temperature to be higher than the polymerization temperature
using the temperature control system, according to degree of pressure drop, in the late
stages of the reaction where pressure drops. Additionally, a vinyl chloride based resin
having a reduced number of fish-eyes and increased whiteness index efficiently was
prepared by inhibiting foam generation due to pressure drop of the late stages of
reaction and by a large amount of removed heat.

We Claim:
1. A method of preparing a vinyl chloride based resin,
wherein temperature is elevated at a rate of greater than 0.008 0C/min and less than 0.8 0C/min for greater than 10 minutes and less than 90 minutes when reaction pressure is reduced to greater than 0.01 kgf/cm2 and less than 0.5 kgf/cm2 after initiation of suspension polymerization.
2. The method according to claim 1, wherein the reduced reaction pressure is greater than 0.01 kgf/cm2 and less than 0.2 kgf/cm2.
3. The method according to claim 1, wherein the temperature elevation rate is greater than 0.008 ℃/min and less than 0.65 ℃/min.
4. The method according to claim 1, wherein the temperature elevation rate controls temperature control system.
5. The method according to claim 1, wherein the polymerization reaction temperature is 30 to 80℃.
6. The method according to claim 1, wherein the temperature elevation is carried out for 30 to 60 minutes.
7. A vinyl chloride based resin prepared by the method according to any one of claims 1 to 6.
8. The vinyl chloride based resin according to claim 7, wherein a concentration of residual initiator is less than 0.002 parts by weight based on 100 parts by weight of the vinyl chloride based resin.
13
9. A vinyl chloride based resin composition comprising the vinyl chloride based resin according to claim 7, an inorganic filler and a stabilizer.
10. A vinyl chloride based resin composition according to claim 9, wherein the inorganic filler is carbon black.
11. A method of preparing a vinyl chloride based resin,
wherein temperature is elevated at a temperature elevation rate of 0.01 to 0.5 ℃/min for 20 to 60 minutes when reaction pressure is reduced to 0.05 to 0.1 kgf/cm2 after initiation of suspension polymerization reaction.