Unformatted text preview: (a) Hot Emulsion Process using thermal radical initiator at 50 ”C Butadiene 75 parts by weight Polymerization proceeds through a free radical mechanism,
Styrene 25 being initiated by thermal dissociation of the persulphate
Water 180 Fatty acid soap 5.0 (emulsiﬁer) -2 A -1 n—Dodecyl mercaptan 0.50 (modiﬁer) $208 2 804 Potassium persulphate 0.30 (initiator) Polymerization is carried out at 50 0C and is allowed to continue for about 12 hours until conversion reaches 72%.
Reaction is terminated at this point by the addition of bydroquinone in order to minimize the formation of cross-
linked material. In addition. the mercaptan is added to act as a chain-transfer agent. In this way, the molecular weight
of the final polymer is restricted to a level which permits processing without undue difﬁculty. The average
molecular weight of a typical styrene-butadiene rubber is about 100000. The monomer units are randomly
distributed in the copolymer; the contributions of the cis-1,4— trans—1,4- and 1,2-butadiene units are in the
approximate ratio 18:65: 17. (b) Cool Emulsion Process using redox radical initiator at 5 “C Processes of the above type are known as 'hot‘ processes to distinguish them from the 'cold' processes. Most styrene-butadiene rubber is now made by 'cold' processes using redox initiator systems. A typical recipe is as
follows: Butadiene 72 parts by weight Styrene 28 Water 180 Fatty acid soap 4.5 (emulsiﬁer) Sodium naphthalene sulphonate 0.3 (stabilizer) +2
Potassium chloride 0.3 (stabilizer) ROOH + Fe
tert-Dodecylmercaptan 0.20 (modiﬁer) p-Menthane hydroperoxide 0.063 5 0C l redox reaction
Ferrous sulphate (FeSO4—7H20) 0.010 Ethylenediaminetetraacetic initiator acid sodium salt 0.050 system +3 _
Sodiumformaldehyde R0* + Fe + 0H
Sulphoxylate 0.050 Polymerization is carried out at 5 0C and is allowed to continue for about 12 hours until conversion reaches 60%.
The mercaptan level in 'cold' processes is normally designed to yield a product with an average molecular weight
(Mn) of about 100 000 but material destined to be oil-extended is allowed to attain a rather higher molecular weight.
Compared to the 'hot' process, the 'cold' process leads to a more regular polymer with less branching and cross-
linking and slightly higher trans to cis ratio. Also, a narrower molecular weight distribution is obtained. As a result, 'cold'rubbers give tyres with improved abrasion and out- growth resistance. On the other hand, 'hot' rubbers are
somewhat easier to process. (0) Solution Process using anionic living initiator An increasing amount of styrene~butadiene rubber is being manufactured by solution processes using alkyllithium
catalysts. Production techniques resemble those used for the polymerization of isoprene and butadiene. There is a
tendency for alkyllithium initiation to lead to “Taped” copolymers since the butadiene in the mixture polymerizes
ﬁrst to the virtual exclusion of the styrene. In order to obtain random copolymers it is necessary to add the butadiene
incrementally so that the molar ratio of unreacted styrene to unreacted butadiene is always high. Solution SBR can
be prepared with similar microstructures as those of the emulsion copolymers but show narrower molecular weight
distribution and less long chain branching. These features cause the raw rubbers to be liable to cold flow on storage
and more difﬁcult to process. These defects may be overcome by adding a stannic compound to the polymerization
system. Four 'living' polymer chains become attached to a tin atom to give a star-shaped molecule which shows reduced cold ﬂow and improved processability. The weak C—Sn bonds eventually rupture during processing to
regenerate the linear chains ...
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