ELASTOMERS USED IN TYRES

Natural rubber (NR)

---

• First noticed by Europeans in the 1490s during the voyages of Christopher Columbus
• In the mid-18^th 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- +100^oC
• 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 (T_g 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