COMMODITY PLASTICS

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COMMODITY PLASTICS
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Polypropylene

• Introduction
• Preparation of Polypropylene
• Structure Property Relationship
• Tacticity
• Properties of isotactic PP
• General properties
• Additives for isotactic PP
• Processing Considerations
• Processing techniques
• Grading of PP
• Applications
• Modification of Polyolefins

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Polypropylene

• Introduction
• Polypropylene (PP) is a linear polymer, composed
  of repeating units of isopropane.
• The main attractive features of PP are
• - Exceptional flex life,
• - Good surface hardness,
• - High chemical resistance,
• - Good stability in boiling water,
• - Excellent electrical property
• - Long-life integral hinge application.

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Preparation of Polypropylene

• PP is prepared by using Ziegler type catalyst
  titanium tri chloride with aluminium tri ethyl,
  aluminium tri butyl, or aluminium di ethyl
  chloride in naphtha under nitrogen atmosphere
  to form slurry consisting of 10 catalyst and
  90 naphtha.
• The molecular weight can be controlled by using
  hydrogen as a chain transfer agent.
• In suspension process, propylene is charged
  into the polymerization vessel under pressure
  while the catalyst and the reaction diluent are
  metered in separately.

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Ziegler Natta Polymerization

• Polymerization reactions especially of olefins
  and dienes catalysed by organometallic compounds
  is known as coordination polymerization.
• The first step in polymerization is the
  formation of a monomer catalyst complex between
  the organometallic compound and the monomer.
• Here Mt indicates metals like Ti, Mo, Cr, Ni.
• In the formation of monomer catalyst complex,
  a coordination bond is involved in between a
  carbon atom of the monomer and the metal of the
  catalyst. Hence the polymerization effected by
  such catalyst systems is called coordination
  polymerization.

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• Ziegler Natta catalysts are such type of
  catalyst as existed in coordination
  polymerization.
• It comprises of two components as against
  single component organo metallic component and
  other consisting of halides of IV-VIII group
  elements having transition valences.
• The co-catalysts are organo-metallic compound
  such as alkyls, aryls and hydrides of I-IV
  metals.
• The commonly used catalysts and co-catalysts
  are Titanium chlorides (both tri and
  tetrachlorides) and triethyl aluminium i.e.
  Al(C2H5)3, diethyl aluminium chloride Al
  (C2H5)2Cl.

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• Aluminium alkyls acts as the electron acceptor
  and the titanium halide acts as electron donor.
  Therefore these two forms a coordination complex
  which is necessary for coordination
  polymerization.
• The formed complex is insoluble in the solvent
  .
• Many structures are proposed for these
  complexes

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• From the active centre, the chain reaction
  propogates and form a solid surface of catalyst
  complex phase and the monomer is complexed with
  metal ion of the active centre before it inserts
  into growing chain.
• When catalyst and co-catalyst components are
  mixed , there occurs a chemisorption of the
  aluminium alkyl (electro positive in nature) on
  the Titanium Chloride solid surface, resulting
  in the formation of an electron deficient bridge
  complex as

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• The monomer is attracted towards Ti-C bond (C
  from alkyl group R)in the active centre. When it
  forms a p -complex with Titanium ion. The rate
  of reaction is influenced by the electrons
  present in the active centre.

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• The bond between R and Ti opens up producing an
  electron deficient Ti and a carbanion at R.
• The Titanium ion attracts the p electron pair
  of monomer and forms a sigma bond. While the
  counter ion attracts electron-deficient centre
  of the monomer. The monomer is then inserted
  into a transition state ring structure.

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• This transition state now gives rise to the chain
  growth at the metal carbon bond regenerating the
  active centre.
• Repeating the whole sequence with addition of a
  second monomer the structure of resultant chain
  growth as

The monomer insertion is repeated in this manner
and orientation of the substituent group of
monomer is always taken from the metal ion end
resulting a stereo regular polymer.
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Flow Diagram
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Metallocene Polymerization

• Metallocene polymerization is catalyzed by
  metallocenes.
• It allows to make polymers of very high
  molecular weights in comparision to Ziegler
  Natta catalyst.
• Metallocene polymerization is also good for
  making polymers of very specific tacticities.
• A metallocene is a positively charged metal ion
  sandwiched between two negatively charged
  cyclopentadienyl anions.
• Cyclopentadienyl anoin is made from
  cyclopentadiene.

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• In Cyclopentadienyl most of the carbon atoms
  has one hydrogen atom but one carbon atom has
  two hydrogen atoms. One of those two hydrogen
  atoms are acidic which separates very easily.
• So, the carbon atom is left with only one
  hydrogen atom with an extra pair of electrons.

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• The ring in anionic form is very stable.
• These cyclopentadienyl ions have a charge of
  1.When a cation like Fe with a 2 charge comes
  along , two of the anions forms an iron sandwich
  called as ferrocene.

• When a metal with a bigger charge is involved,
  like zirconium with a 4 charge, the Zirconium
  will bond to two chloride ions to balance the
  charge, -1 charge on each to give a neutral
  compound.

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• In Zirconocences extra chlorine ligands can not
  be adjusted in between the cyclopentadienyl
  rings.
• To make room for the chlorines, the rings
  tilts with respect to each other.
• This tilting happens whenever a metallocene
  has more ligands than just the two cp rings.

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• In bis- Chlorozirconocene each cp ring has
  aromatic ring fused to it. The two-ring system
  fused to a phenyl ring is called an indenyl
  ligand.
• There is an ethylene bridge that links the top
  and bottom cp rings.
• These two features make this compound a great
  catalyst for making isotactic polymers.
• The bulky ligands pointed in opposite
  directions guide the incoming monomers so that
  they can only react when pointed in the right
  direction to give isotactic polymers.
• The ethylene bridge holds the two indenyl
  rings in place.

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• To make Zirconocene complex catalyze a
  polymerization, a co- initiator methyl
  allumoxane (MAO) is added to it.

• The chlorines of zirconocenes are labile that
  means they like to fall off of the
  zirconocene.MAO replace them with some of its
  methyl groups. The methyl groups can fall off
  too. When one of them falls off a complex is
  formed.

The positively charged zirconium is stabilized
because the electrons from the carbon-hydrogen
bond are shared with the zirconium to form a
a-agostic association
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• Zirconium still lacking in electrons. The
  bonding is satisfied by the olefin monomer. In
  Propylene, carbon-carbon double bond is having
  electrons to share, so it shares a pair with the
  zirconium to satisfy the bonding.

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• The precise nature of the complex between the
  zirconium and the propylene is complicated. This
  compexation stabilizes the zirconium but not for
  long.
• When this complex forms, it rearrange itself
  into a new form. The electrons in the
  zirconium-methyl carbon bond shift to form a
  bond between the methyl carbon and one of the
  propylene carbons.

• The electron pair that had been forming the
  alkene- metal complex shifts to form an outright
  bond between the zirconium and one of the
  propylene carbons.

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• As can be seen in the picture, this happens
  through a four membered transition state. Also
  zirconium ends up just like it started, lacking a
  ligand, but with an agostic association with a
  C-H bond from the propylene monomer.

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• Another propylene monomer react just like the
  first one.

• The propylene coordinates with the zirconium ,
  then the electrons shuffle.

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• When second propylene monomer has added to the
  chain, the methyl groups are always on the same
  side of polymer chain which leads to an
  isotactic polymer.

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• The propylene monomer always approaches the
  catalyst with its methyl group pointed away from
  the indenyl ligand.

• If the methyl group were pointed towards the
  indenyl ligand, the two would bump into each
  other keeping the propylene from getting close
  enough to the zirconium to form a complex. So,
  only when the methyl group is pointed away from
  the indenyl ligand, the complex of ziroconium
  with propylene is formed

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• When the second monomer is added it approaches
  from the other side and its methyl group away
  from indenyl ring .

• The methyl group is pointed up rather than
  down.
• This is so because the second propylene is
  adding from the opposite side as the first,it
  must be pointed in the opposite direction if the
  methyl groups are to end up on the same side of
  the polymer chain.

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Structure Property Relationship

• PP is a linear polymer with little or no
  branching.
• Methyl group in the chain leads to increase in
  melting point and chain stiffening.
• The tertiary carbon atom provides a site for
  oxidation so that the polymer is less stable than
  PE in the presence of oxygen.
• Methyl group leads to products of different
  tacticity.
• Commercial polymers are usually about 90-95
  isotactic.

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Tacticity

• Isotactic

Syndiotactic
Atactic
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Properties of isotactic PP

• Compare to Polyethylene
• It has lower density (0.90 gm / cc).
• It has a higher softening point and hence a
  higher maximum service temperature.
• Articles can withstand boiling water and can be
  subjected to steam sterilizing operations.
• It has a higher brittle point.
• It is more susceptible to oxidation.
• Atactic PP
• Atactic PP is an amorphous some what rubbery in
  nature.
• Commercial polymer is usually 90-95 isotactic
  and rest is blocks of atactic and syndiotactic
  structures.

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Properties of Polypropylene
Name Value Unit
Specific gravity 0.90 --
Tensile Strength 35.5 MPa
Tensile modulus 1380 MPa
Flexural modulus 1690 MPa
Elongation at break 35-350
Impact Strength (Izod ) 37 J/m
Hardness R100 ---
HDT (under 1.82 MPa load.) 55 C
Glass transition temperature 5 C
Melting point 164 C
Dielectric Strength 24-28 KV/mm
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General Properties

• Chemical properties
• No solvent affects PP at room temperature.
  Polypropylene will dissolve in Decaline at 130C.
• Aromatic and chlorinated solvents often swell
  polymer at elevated temperature.
• Strong oxidizing acids slowly attacks the resin
  (fuming HNO3).

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• Electrical properties
• PP is an excellent insulator due to its
  non-polarity.
• It is used in many molded products, as well as in
  winding coils and transformers.
• Flammability
• PP burns slowly and can be identified by an odour
  of crude oil.
• Flameretardant grades are available for specific
  electrical applications.

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• Mechanical properties
• Commercial grades of PP is tough and having good
  impact resistance.
• PP becomes more brittle than many other
  thermoplastics at zero temperature.
• Weathering properties
• Standard grades have shorter life when exposed to
  the outdoor.
• Discoloration, colour fade and crazing occur in
  products not stabilized with anti oxidants or
  carbon black.

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Additives for Isotactic PP

• Fillers
• About 3 of PP compounds are filled with talc.
• Talc filler improves stiffness and heat
  deformation resistance.
• Talc filled PP compounds are used in heater
  housings, car mounting components and several
  domestic appliances.
• Talc filled PP sheet is used as an alternative
  to carton board.

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• In comparison to the talc filled grades the
  CaCo3 filled grades claimed to have
• Higher impact strength.
• Brighter colour.
• Higher thermal stability.
• Improved fatigue strength.
• Lower stiffness and tensile strength.

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• Rubbers
• Particularly butyl rubber is used to reduce the
  brittleness of PP.
• Rubbers are used because of their
• Reasonable price.
• Good weathering properties.
• Negligible toxicity easy processability and
  re- processability.
• Pigments
• The selection of pigments for PP follows the
  same considerations as for PE because of the
  higher processing temperature and lesser
  resistance to oxidation, selection does
  require more care.

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• Carbon black
• To improve the resistance to UV light, carbon
  black is used as a light screener.
• Hindered amine UV stabilizers (HALS) are used
  to improve the UV resistance of PP material.
• Antioxidants
• Antioxidants are necessary for prevention from
  adversity of oxidation.
• For optimum processing stability a single
  antioxidant of the phenol alkane type, for
  e.g., 1,1,3 tris (4 hydroxy - 2 methyl, 5 t
  butyl phenyl) butane, tends to give the best
  results.

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Processing Considerations

• Processing of Polypropylene is similar to
  Polyethylene, particularly high-density
  polyethylene.
• Flow properties depend on molecular weight and
  additives present.
• Unfilled grades generally considered as easy
  flow.
• Flow Path wall thickness ratios of 1751 are
  possible on 1mm wall thickness sections.
• Thermal stability is quite good in the absence
  of oxygen so that there is no need to purge with
  another material when shutting down.

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Processing techniques

• Injection Molding
• Recommended processing temperatures are in
  the range of 210 to 275C.
• Injection pressures are of 150 to 180 MPa
  depending on the grade of the material.
• Because of crystallanity there is high molding
  shrinkage and is reasonably uniform in all
  directions.

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• Pipe Extrusion
• PP-R has less heat conductivity compare to PE,
  therefore needs longer time to melt.
• This requires longer L/D ratio 301.
• The melt temperature is recommended to be
  220- 230C.

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• Manufacturing Process of BOPP Film
• - BOPP film is manufactured with the blown
  method. Molten resin is extruded from a circular
  die to form a thick tube. The tube is stretched
  with air pressure at controlled temperature to
  achieve transverse orientation and simultaneously
  pulled by take off nips to achieve machine
  direction orientation.

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Grading of Polypropylene
MFI (gm/10 min) Grade
3-5 Blown film grade
9-11 Cast film grade
16 Extrusion coating grade
11 General purpose injection grade
1.9 Bottle grade
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Trade Names

• Haldia Petrochemicals Ltd, India - Halene PP
• IPCL, India - Koylene
• Reliance, India - Repol
• Exxon Mobil, US - Escorene
• Mitsui petrochemical, Japan - Sunlet PP
• Mobil Chemical, US - Bicor PP
• Sumitomo, Japan - Esprene
• Mitsubishi , Japan, - Noblen

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Applications

• Automotive
• PP is used in bumpers, steering wheel covers,
  profiles, consoles, door pockets, radiator
  grills, spoilers, rubbing strips, fenders, wheel
  arches, truck linings, mud flaps, seat covers,
  plumbing, integral hinges, accelerator pedals,
  glove boxes and air- intake noise suppressors.
• Packaging
• PP is used in packaging for goods wrapping,
  sleeping bags, films for packing tobacco
  products, candy, cosmetics, contact lens cases,
  first aid cases, drums and jerry cans, tool
  boxes, cheese wrap, electrical capacitors,
  synthetic turf, clothing inner liners, wiping
  clothes, films for textile goods and medicines.

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• Electrical / Electronics
• PP is used in cable connectors and fittings,
  cable and wire coatings, industrial lights,
  transformer housings, insulators for electrical
  fencing, aerial parts, switch gears, radio and
  TV housings, capacitors, coil forms, control
  knobs etc.
• Appliances
• PP is used in dish racks, pump housings, door
  handles, air cleaners and washing machine parts,
  bleach and detergent dispensing units,
  agitators, tub liners, housing for appliances,
  valve and control assemblies, drain tubes, PP
  silverware baskets.

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• Household
• PP is used in buckets, thermo flask cases,
  strainers and chairs, baby feeding bottle
  warmers, microwave oven trays, labels for soft
  drink bottles, canvass for luggage, air
  conditioner parts, floor and ceiling pans,
  dehumidifiers, room humidifiers, knife
  sharpeners, can openers, hair dryers, coffee
  makers.

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Applications
Multilayer PP coating for Offshore applications
Car Dashboard and Bumper
Coffee Maker and Toaster
PP furniture
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Modification of Polyolefins

• Ethylene-vinyl acetate (EVA) copolymer.
• Ethylene-ethyl acrylate (EEA) copolymer.
• Ethylene-methyl acrylate (EMA) copolymer.
• Ethylene-acrylic/methacrylic acid copolymer.
• Ethylene-propylene copolymer.

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• EthyleneVinyl acetate (EVA) copolymer.
• Both filled and unfilled EVA copolymers have
  good low temperature flexibility and toughness.
• EVA with 15-20 mol Vinyl acetate content are
  rubbery copolymers.
• About 28 Vinyl acetate content are used in
  hotmelt adhesives.
• EVA films are used for liquid packaging, frozen
  foods, meat wrap, ice bags, drum liner.
• Molded and extruded EVA resins are use in
  flexible toys, bumper pads, hose , gasketing.

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Ethyleneacrylate copolymers

• Ethyleneethyl acrylate (EEA) and
  ethylenemethyl acrylate (EMA) copolymers with
  up to 20 weight EA, MA content respectively are
  commercially available.
• EEA resins have higher thermal stability and
  can withstand higher processing temperatures
  than EVA.
• EMA resins yield blow film with rubber like
  limpness and extremely high dart-drop impact
  strength. They find useful applications in
  extrusion coating, co-extrusion and laminating
  applications.

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Ethyleneacrylic/methacrylic acid copolymers
(EAA/EMa)

• Copolymers up to 6.5 acrylic acid and 15 by
  weight of methacrylic acid are used for melt
  processing applications.
• The acid group promotes excellent adhesion to
  various substrates and increases abrasion
  resistance and stress cracking resistance
• These resins are extrusion coating onto
  aluminium foil for pouches, for composite
  toothpaste tubes, wire and cable applications,
  blown or extruded films for packaging of food
  and other products and various lamination
  applications.

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EthylenePropylene Copolymers

• Two main types of ethylene (E) propylene (P)
  resins are EPM and a terpolymer (EPDM).
• Rubbers which are rich in either ethylene or
  propylene have higher tensile strength and
  elongation at break () in the unvulcanized
  state than those rubbers which contain equal
  amounts of E and P.
• EPM rubbers can be vulcanized only by peroxides
  or high energy radiation.
• In EPDM the third monomer has two double bonds
  one enters the polymerization process and the
  other CC bond remains as a side chain
  available for vulcanization with
  sulphur/accelerator systems.