Basics of polymerization and processing of polyvinyl chloride.
Published on: Mar 4, 2016
Transcripts - Polyvinyl Chloride
For the partial fulfilment of course:
ME 6720 POLYMERS-I
Sai Goutham Soma
Wright State University
The strongest non-covalent bond after hydrogen bond is dipole-dipole
interaction as in 𝐶 − 𝐶𝑙 in the polyvinyl chloride. The electronegativity
difference between the atoms involved in the polar bond is large and hence
it is weaker than the hydrogen bond.
Polyvinyl chloride often termed as PVC is made from its monomer unit
vinyl chloride (𝐶𝐻2 = 𝐶𝐻𝐶𝑙) which is toxic gas at normal conditions. The
polymerized vinyl chloride which is obtained by catalytic polymerization
is a homopolymer. The homopolymer obtained is brittle, hard and also
difficult to process as the reaction cannot be controlled. PVC has a linear
structure which forms long chains. But homopolymer PVC is not used for
commercial use. So some additives are added to make it copolymer and
also for the ease of reaction.
PVC is classified as amorphous polymer. Crystallinity varies from 5-15%.
Crystallinity increases rapidly if the reaction is taking place below the room
temperature. Branching depends mostly on the reaction temperature. It is
directly proportional to the reaction temperature.1
Both isotactic and syndiotactic formations occur in the PVC. The head to
tail configuration that is carbon atom having pendant Cl atom attaching
with the non-pendant carbon atom of the other monomer is predominant
than head to head and tail to tail bonding.5
(Fig from Polymer science)
Polyvinyl chloride is a thermoplastic made of 57% chlorine (industrial
grade salt) and 43% carbon (from oil / gas). Compared to other plastics
PVC has some unique properties which makes it different from others.
It has better tensile strength and tensile modulus than other olefin
The elastic modulus of rigid PVC range from 1500-3000MPa
whereas for plasticized or soft PVC it is from 1.5-15MPa.
Flexural strength is about 10,500psi whereas compression strength
is about 9500psi. Specific gravity of PVC is 1.4.
Rigid PVC shows less creep.
Chlorine atom which is larger and almost equal to the size of carbon make
the chain to uncoil, rotate, disentangle and deform by viscous flow. Cl atom
is largely attracted by the hydrogen atom in the adjacent chain forming
hydrogen bonding. Due to this PVC is more rigid than other polymers
PVC is poor conductor of heat and start to decompose at 140℃.
Melting point of PVC is 160℃, thermal expansion coefficient is
5 × 10−5 /℃ and thermal conductivity is 0.14-0.28 W/m°𝐾.
Due to main chain high flexibility the heat distortion or softening
temperature of PVC is less and ranges from 54~80℃.
It is generally fire retardant. When PVC is burnt HCl gas which is
denser than air is produced there by relieving oxygen from the
vicinity which makes it self –extinguishing.
The glass transition temperature of PVC is about or more than80℃.
This can be varied up to 100℃ depending upon the use such as hot
PVC have good dielectric constant and so insulating properties
which makes it use for electrical appliances.
Due to large dielectric constant losses of PVC, makes it possible of
high frequency welding.
Dielectric constant varies from 2.7 to 3.1, dielectric strength14KV/mm and volume resistivity is about 10^16 Ohm-cm.
PVC is generally amorphous and so transparent. Rigid PVC
products have high transparency compared to plasticized PVC.
The polar and amorphous nature of PVC makes it have good
adhesion and printing properties.
Refractive index of PVC is 1.54.
It has good chemical resistance towards acids, alkalis, salts,
oxidizing solvents. 2
Effect of plasticizers and other additives
PVC before processing into final products is modified or not depending on
the type of application. The unmodified PVC is known as rigid PVC
whereas the modified PVC is called soft or plasticized PVC. The PVC is
modified by adding plasticizers. The plasticized PVC is also called vinyl.
Other additives include heat stabilizers, lubricants, processing aids,
thermal modifiers, UV stabilizers, fillers and flame retardants, biocides,
blowing agent, smoke suppressors, impact modifiers and color pigments
depending on the applications.
Plasticizers are more like a copolymer than additives. It inherits plastic
properties to the polymer. Commonly used plasticizers include diisononyl
phthalate (DINP), di-2ethylhexyl phthalate (DEHP) and diisodecyl
phthalate (DIDP). Other than phthalates, adipates and trimellitates are also
used for low temperature resistance and heat resistance respectively.
Stabilizers include metal compounds, lead stabilizers, cadmium stabilizers,
organo tin stabilizers, calcium-zinc stabilizers, Barium-zinc stabilizers and
more. Stabilizers mainly protect from thermal degradation. Metal
compounds prevent chain reaction of decomposition of the chain and also
from daylight and weathering. Pb stabilizers are used for excellent
electrical properties and water resistivity, Ca-Zn stabilizers are used for
blood bags, toys and food packaging.7
For typical applications PVC can be used after it is extruded or molded by
injection molding process or blow molded or other for its final use. Its
intrinsic properties makes vast use in the regular life.
Mechanical properties applications
PVC is extensively used for making pipes which include household
and industrial purposes. It is used for sewage by the municipality.
Its light weight, strong and durable nature makes its use
construction. It is used to make windows, claddings, fittings, roof
ceiling and flooring.
It is used in light weight furniture like tables, skiing equipment etc.
PVC is used to make clothing which replace leather, latex etc. it is
used to make water proof clothing like rain coats and jackets.
Other applications include making toys, automotive appliances,
balls, pools (paddling) etc.
The electrical properties makes it useful for insulating of electrical
Biodegradable properties applications
Coming to healthcare it plays a vital role. PVC with DEHP (Di2ethylhexylphthalate) is used or making blood bags. It is also used
in heart and lung equipment, catheters and cannulae, surgical gloves,
Inhalation masks, packing for medicines, tubing for nerve flow like
dialysis and urine continence, syringes etc.
PVC is also used in packaging extensively. Packing equipment for
food products, medicines, toiletries, electronic parts and accessories,
tools, toys and bottle sieving, tapes for adhesion. 2
Specific application: PVC rigid pipes
First the monomer is obtained by
Direct reaction of acetylene with HCl.
𝐶𝐻 ≡ 𝐶𝐻 + 𝐻𝐶𝑙 → 𝐶𝐻2 = 𝐶𝐻𝐶𝑙
Reaction of ethylene with chlorine.
𝐶𝐻2 = 𝐶𝐻2 + 𝐶𝑙2 → 𝐶𝐻2 = 𝐶𝐻𝐶𝑙
𝑂 + 𝐶𝑙2 + 2𝐶2 𝐻4 → 2𝐶𝐻2 𝐶𝐻𝐶𝑙 + 𝐻2 𝑂
The obtained vinyl chloride undergoes free radical polymerization to form
its polymerized product poly vinyl chloride.4
𝐻2 𝐶 = 𝐶𝐻𝐶𝑙 + 𝑅 → 𝑅 − 𝐶𝐻 − 𝐶𝐻𝐶𝑙 ∙
𝑅 − 𝐶𝐻 − 𝐶𝐻𝐶𝑙 ∙ +𝐶𝐻 = 𝐶𝐻𝐶𝑙 → 𝑅 − 𝐶𝐻 − 𝐶𝐻𝐶𝑙 − 𝐶𝐻 − 𝐶𝐻𝐶𝑙 ∙
𝑅 − 𝐶𝐻 − 𝐶𝐻𝐶𝑙 − 𝐶𝐻 − 𝐶𝐻𝐶𝑙 ∙ +𝐶𝐻 = 𝐶𝐻𝐶𝑙 → −(−𝐶𝐻 − 𝐶𝐻𝐶𝑙−)n
Processing of PVC
Commercial preparation of
PVC is done by suspension polymerization
mostly. Other processes include emulsion and bulk polymerization which
are used less frequently.
The vinyl chloride monomer generally called as VCM is prepared by
ethylene processed from crude oil and chlorine from rock salt obtained
from sea water. It is generally followed by oxychlorination technique.
Coming to suspension polymerization VCM which is pressurized and
liquefied is fed into the polymerization reactor. The polymerization reactor
contains water and suspension agents. The reactor is agitated with high
speed so that small droplets of VCM are obtained. Then the initiator is fed
into the reactor in which the reaction takes place under low pressure
at 40℃ − 60℃. PVC is obtained by suspending in water to get particles of
size 50𝜇𝑚 − 200𝜇𝑚 diameter in slurry form. The slurry is then discharged
from the reactor to stripper which separates slurry from the residual
monomer. The VCM which didn’t react or remained is collected through
stripping process and is purified to use it again in the next process.
The separated slurry is sent into the centrifuge where excess water is
removed. The dehydrated slurry is sent into the drier where the white
powdered PVC is obtained. By sieving the white powdered PVC in the
sieve machine particles of the required size is obtained.3
The PVC thus obtained by suspension polymerization is called S-PVC.
About 80% of the PVC is manufactured by suspension polymerization.
The PVC obtained by emulsion polymerization is called P-PVC to
distinguish from S-PVC obtained by suspension. Emulsion polymerization
is used to get finer grades of PVC. The advantage of bulk polymerization
is PVC obtained is free from impurities but heat removal is main problem.5
Manufacturing of rigid PVC pipes
PVC rigid pipes are manufactured by extrusion process. Extrusion is a
process through which plastic is melted and then extruded into required
profiles. This process is only applicable for continuous profiles only. The
obtained continuous profiles are cut to length. Extrusion is used for profiles
like pellets and rods, solid profiles like rods, pipe and tubing, sheets and
extruded flat film, coating of wire and cable, blown films, synthetic fibers
and so on.
In extrusion process the raw plastic or polymer is fed into the extruder
cylinder or barrel via hopper. The raw material feed may be granules,
powder, some cases additives are also mixed. The cylinder consists of a
screw called as extrusion screw. It is specially designed for the mixing and
forward movement of plastic in the barrel. The extrusion screw turns inside
the barrel which then conveys the forward movement of the plastic into the
heated region. The plastic which is heated by the external source as well as
by the friction developed inside by moving parts melts the plastic. The
screw forwards the molten plastic into a hole and enters into special screen
which filters the contaminants out. The molten plastic then moves into a
die to get required profile. Then the output is cooled by sealed water bath
so that no stresses or holes or collapsing of the profile occurs. Some other
auxiliary equipment is used to pull the extrudate out at a continuous rate.
The plastic obtained after cooling is a continuous profile is cut to length.
After certain time the friction and pressure would be enough to maintain
the temperature inside the barrel. So the heater would be switched off to
avoid energy losses. Extrusion lines can be very long.1
Extrusion Screw (Fig from www.pvc.org)
PVC pipe extrusion
The rigid PVC is hard and brittle which makes it break easily. So in order
to be less prone to impact modifiers are added. Some of the modifiers used
are methacrylate butadiene styrene terpolymers (MBS), chlorinated
polyethylene (CPE) resins. These are known as impact modifiers. DuPont
Dow Elastomers using hydrocarbon rubbers and chlorinated polyethylene
as impact modifiers claim the following compositions:
a vinyl chloride polymer,
2 to 20 parts by weight of at least one ethylene/alpha-olefin
copolymer per 100 parts vinyl chloride polymer, said
copolymer having a density of 0.858 to 0.91 g/cc and having
a melt index from an I10 value of 0.1 to an I2 value of 10, and
To less than 1 part by weight of at least one randomly
chlorinated polyolefin per 100 parts of vinyl chloride
Other additives such as UV stabilizers which include benzo-phenones are
also added. These are mixed in the hopper in proper ratio and sent into the
extrusion barrel. The die used for the processing of these pipes is called
spider die. This die consists a mandrel which acts an obstruction and allows
the formation of pipe.1
Spider die (Fig from 1)
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The polymer mixture is heated up to 150℃. The plastic is heated in three
blocks of heaters with temperature varying in each block from low to high.
The molten plastic is sent in the die after passing through the filter screen
and then the profile is obtained. The pipe profile should not be disturbed
so that flaws don’t occur. It is cooled in sealed water bath. There is puller
which keeps the output profile even. The pipe is then cut to length by the
cutter and sent to packing.1
Rigid PVC pipes
The rigid pipes commonly produced is PVC pipe-Schedule 40. The
samples are sent to the lab for testing. The hardness is tested by Rockwell
hardness test and the thermal tests, pressure tests are also done. The below
are the specifications of Schedule 40
Outer diameter as much as 0.405 inch to 24 inch.
Thickness may vary from as much as 0.109inch to 0.500inch.
The typical temperature that a rigid pipe will resist is about 140℉.
Pressure varies in between 120psi to 810psi.8
All PVC Schedule 40 pipe shall be manufactured from a Type I, Grade I
Polyvinyl Chloride (PVC) compound with a Cell Classification of 12454
per ASTM D1784.8
The average degree of polymerization of the PVC resin used in the making
of rigid pipes is usually in between 950-850.
After Polyethylene and Polystyrene Polyvinyl chloride is mostly produced
in the world. As per 2007 the amount of PVC produced in the USA and
Canada is about 6426 thousand metric tons which account to $17.7 billion
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1. Strong A. Brent (1996), Plastics Materials and Processing 3rd
edition, New Jersey, Pearson Education.
2. Harold A. Sarvetnick (1977), Polyvinyl Chloride, R. E. Krieger
3. Burgess, R.H (1982), Manufacturing and processing of PVC, New
York, Ed., MacMillan.
4. Coroyannakis Panayotis E, Polymerization of vinyl chloride (1978),
Open Access Dissertations and Thesis, Paper 347.
5. Leonard I. Nass, Charles A. Heiberger (1986), Encyclopedia of PVC
Volume 1, 2nd edition, Markel Dekker, INC.
6. Gary R. Marchand, Mark T. Berard, US patent 6706815 B2.
7. ASM International, Engineering Materials Handbook Vol2
8. ASTM, Rigid PVC pipes specifications and properties.
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