Friday, August 20, 2010

ELASTOMERS USED IN TYRES

Natural rubber (NR)


  • First noticed by Europeans in the 1490s during the voyages of Christopher Columbus
  • In the mid-18th century natural rubber became widely recognised in Europe
  • The largest producer is the Federation of Malaysia, other producers are Indonesia, Thailand, India, China and other Asiatic, African and American countries
  • Occurs as a latex (colloidal caoutchouc dispersion in water)
  • Latex -> coagulation -> smoked sheet or pale crepe
  • Composition of natural rubber (latex crepe):
    • 89,3 - 92,35 % rubber hydrocarbons
    • 2,5 - 3,2 % acetone extractables
    • 2,5 - 3,5 % protein
    • 2,5 - 3,5 % moisture
    • 0,15 - 0,5 % ash

Properties of NR

  • Excellent mechanical properties
  • Good tear strength and abrasion resistance
  • Good properties in low temperatures; operating temperature area -50 oC- +100oC
  • Low hysteresis (low heat build up)
  • Poor ageing properties (ozone, oxygen, high temperatures)

Processability of NR

  • Good building tack
  • Good green strength (strength of the unvulcanized compounds)
  • Excellent extrudability and calenderability
  • High rate of cure
  • Tendency for partial crystallization (stiffening) during storage -> heating
  • "Storage hardening" (chemical reaction between carbonyl groups and amino acids) -> linked chains may be broken during mastication
  • Tendency for strain crystallization (tensile stress)

Butadiene rubber (BR)


  • 1911 First attempts to polymerize butadiene (C.D. Harries, F. Hofmann)
  • 1926 Polymerization of butadiene using sodium, Buna
  • Nowadays mainly solution polymerization with coordination catalysts of the Ziegler-Natta type (titanium, cobalt, nickel, neodym etc.)
  • With solution process the microstructure (cis, trans, vinyl) of the butadiene can be varied
  • Structure: butadiene units which can have joined linearly by 1,4 (preferred in cis-1,4, but also in certain measure, trans-1,4 conformation), as well as by 1,2-addition (vinyl structure of butadiene)

Properties of BR

  • Good rubbery properties in low temperatures (Tg temperature -70 oC)
  • Pure cis-1,4 BR

Good abrasion resistance

Low hysteresis

Good snow traction

Poor wet traction

  • High vinyl (70 %) BR

Good traction but poor snow traction

Good handling

Poor tear resistance

Processability of BR

  • Poor banding on mills
  • Poor vulcanizating properties -> mostly used in blends with NR and SBR

Styrenebutadiene rubber (SBR)


  • 1929 E. Tchunkur and A. Bock discovered that mixtures of butadiene and styrene in a 75:25 ratio can be copolymerized in emulsion
  • Nowadays free emulsion polymerization as well as solution polymerization usually with butyllithium iniator are used
  • SBR is a copolymer of styrene and butadiene
  • The styrene content in SBRs ranges usually from about 23 to 40 %

Properties of SBR

  • Very good mechanical properties
  • Good abrasion resistance
  • High hysteresis

High heat build up

Good wet traction

Good dry traction 0-70 oC

Isoprene rubber, synthetic natural rubber (IR)


  • 1909 First attempts were made to prepare the synthetic analogue of NR, using isoprene as the starting material (F. Hofmann 1909)
  • 1945 Goodrich succeeded in the synthesis of cis-1,4 polyisoprene (IR), the so- called "synthetic natural rubber", using Ziegler-Natta catalysts
  • NR and IR are chemically and structurally very similar

Properties of IR

  • Properties are almost same as properties of NR
  • Mechanical properties a little poorer than that of NR (tear resistance)
  • Poorer building tack and green strength than NR

Butyl rubber, isoprene-isobutylene copolymer (IIR)

1931 BASF process for making polyisobutylene

  • 1943 Standard Oil developed a method for producing IIR
  • Copolymer of isobutylene with a small percentage of isoprene which provides the double bond required for sulphur vulcanization

Properties of IIR

  • Excellent ageing stability
  • Excellent impermeability to gases
  • Very high hysteresis
  • Immiscible to other polymers
  • Slow vulcanization reactions (low levels of unsaturation)

CIIR and BIIR

  • Halogenated IIRs have two advantages due to the enhanced cure reactivity of the double bond by both the halogen atom and allylic halogen structure
  • improvements occur in the vulcanization rates, the states of cure and the reversion resistance
  • covulcanization with other diene rubbers is possible

Properties (BIIR Compared to IIR)

  • even lower gas permeability
  • better weather and ozone resistance
  • higher hysteresis
  • better resistance to chemicals
  • better heat resistance
  • cure faster with lower amounts of curatives
  • give a better adhesion to other rubbers
  • The property level of CIIR is between that of BIIR and IIR

Etylenepropylenediene rubber (EPDM)

  • During the copolymerization of ethylene and propylene, a third polymer, a diene (such as dicyclopentadiene DCP, ethylidene norbornene ENB and trans-1,4 hexadiene HX), is added
  • They have unsaturation, and it can then be vulcanized with sulphur

Properties of EPDM

  • Excellent resistance to weathering
  • Good heat stability
  • Can be made partially crystalline to give high green strength
  • Poor building tack


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