EFFECT OF RAW MATERIALS ON
POLYVINYL CHLORIDE (PVC) QUALITY
Fig 1 PVC suspension reactor
The production of PVC resins is influenced by several process variables that
cause deviations in the kinetics reactions. The process receives as raw materials
VCM, demineralized water, suspension agents, initiators and elements for the
incrustation´s control in the reactor’s walls. These components must guaranteed
with the required parameters and specifications to avoid affecting quality of the
PVC. In this paper we will refer to the effect of the quality of the raw materials,
specifically VCM and demineralized water, on the final product.
VINYL CHLORIDE (VCM)
Impurities contained in the monomer effect the polymerization kinetics
and properties of the PVC resin such as porosity, average particle size and
thermal stability of the polymer. Some possible contaminants that may be
present in the monomer and their interactions with the quality of the PVC resin
are described below.
Water
The maximum amount of water allowed in the VCM product is 200 PPM
for processes that store the monomer at pressures of 5 bar or that are supplied
directly from VCM storage tanks from the VCM plant, refrigerated storage
processes will not have this likelihood of excess water because it is stored at
very low temperature and should have a maximum of 5 PPM. If a significant
amount of water comes with VCM (happens in the VCM neutralization
processes of some licensors) the amount of additional water causes variation in
the VCM/water ratio, which will generate fine loads and a fine trend of the final
product.
HCI
The specification for HCL in VCM is less than 0.5 ppm; higher values
come from unstable operation of the HCL stripping column or, in earlier
technologies, poor HCL neutralization in the VCM product. PVC
polymerization processes allow variations of HCL and the quantities of
suspension agents must be varied; there are plants that operate stably, without
abnormal loads, up to limits of 5 PPM of HCL in the VCM. Similarly, if there
are high HCL values in the reactor feed, it will tend to destabilize the suspension
agents and in extreme cases will increase the iron (Fe) PPM, obtaining course
loads due to high Fe content as a result of corrosion.
Iron
The maximum allowed Fe in VCM is 0.3 PPM. Fe++ reduces the
colloidal stability of the suspension due to the decomposition reaction of the
dispersants, originating course loads; in addition, it catalyzes the formation of
vinyl chloride peroxides. Iron causes poor thermal stability of PVC, yellowing
of the resin and dark spots.
1,3 Butadiene
Butadiene formation is derived from a high conversion of the cracking
furnace in the VCM plant, which means high operating temperature at the set
load. The maximum amount of butadiene allowed in VCM is 5 PPM. Butadiene
is an inhibitor of VCM polymerization, reducing its polymerization rate and
increasing reaction times in the reactors.
1,2-Ethylene dichloride (EDC)
The maximum allowable EDC in VCM is 5 PPM; however, some
licensors set maximum limits of 10PPM. EDC comes from improper operation
of the VCM purification column. PVC plants have reported that EDC values
above 5 PPM have caused increases in the reaction time of the loads of up to 8
minutes; in our experience we have not seen relevant effects on reaction times
at values between 2 -5 ppm of EDC; although the typical value in plants is less
than 1 PPM, not detectable by chromatographs. Likewise, no variations in resin
quality such as PM, K-value or particle size haven’t been observed.
Acetylene
Acetylene has a maximum specification of 2 PPM in the VCM product.
Acetylene is present due to high operating temperature in the cracking furnace
at the VCM plant or low quality in the EDC feed to the cracking furnace.
Concentrations above 0.3 ppm tend to promote scale formation on the reactor
walls and, therefore, increase the number of fish eyes. When the concentration
of this reaches 110 ppm, colloidal instability occurs, generating heavy loads in
the reactors.
Fig 2 Typical graph Temperature Vs Reaction time in
suspension PVC production.
The following impurities such as chloroprene, ethyl chloride, monovinylacetylene and tetrachloroethane are considered minor and have a
maximum content of 3 PPM; however, in current technologies, events of high
content of these compounds were no happen since the cracking furnaces are
designed to operate at maximum temperatures of 510 - 520 °C and advanced
process control is used to maintain very stable operation. We will refer to them,
since there are plants operating with technology from the 80's, which do not
have the technological and control equipment of the plants installed in the last
20 years.
I should point out that the formation of impurities in the cracking furnace
can also be derived from a low quality of EDC fed to this equipment. In recent
VCM/PVC plant process audits, we have found several EDC quality problems
in the cracking furnace, that need to be solved immediately and in the EDC
production areas, due to their effects on the furnace running times and the
quality of the PVC produced. VCM purification systems are designed to handle
quantities of impurities that are removed in the distillation columns, but with a
design parameter of maximum operation 520 °C and a minimum EDC quality
of 99.8 %.
Chloroprene
Chloroprene is a strong inhibitor of VCM polymerization and produces a
resin with low thermal stability, since it favors the formation of defective
structures in PVC, such as allylic and tertiary chlorides. In the medium term, it
increases the formation of dark spots due to the polymerization of chloroprene
on the reactor walls by forming non-soluble gums.
Ethyl chloride
Ethyl chloride has a maximum specification of 10 PPM. This compound
acts as an additional chain transfer agent and causes reduction of the reaction
rate. It slightly increases reactor pressure and in some extreme cases has masked
reactor pressure drop.
Monovinyl acetylene (MVA)
MVA has a relevant effect on polymerization at very low concentrations
generating fine loads at concentrations of 3 PPM, there are reports that at
concentrations of 1 PPM the final product presents a finer granulometry than
usual at the same operating conditions.
Trichloroethane
Trichloroethane modified the particle size distribution by widening it,
which is a prelude to colloidal instability capable of generating a coarse charge.
Erratic instability is experienced when in 20 - 50 ppm concentration range.
Solids
Solids are generally present in the VCM coming from the recovery
system and generate fish eyes and coarse grain size when their content exceeds
1 PPM.
.
DEMINERALIZED WATER
PH
The PH of a demineralized water is 7, which achieves an excellent control
of the polymerization quality as the generated suspension is stable. In facilities
where the VCM is neutralized with caustic soda, high pH events can occur due
to the soda emulsified in the VCM, pH higher than nine, affecting the colloidal
stability of the reaction mixture and generating coarse charges, low thermal
stability and dark spots.
Fe
High Fe content affects the colloidal stability of the reaction mixture
and generates coarse charges, low thermal stability and dark spots.
Silica
Test done in 90´s determined that silica at values higher than 100 ppm
can generate coarse fillers by interfering with the emulsifying capacity of the
suspending agents.
Fig 3 Suspension process to PVC production