POLYBUTADIENE  

Polybutadiene Synthetic Rubbers (BR)

This is a Homo polymer of Butadiene monomer. BR is cis, 1—4 poly butadiene. Butadiene can polymers to form three other polymers also which are not having industrial importance of the same order as that of BR. The synthesis of BR became possible after the discovery of stereo specific catalysts. First commercial synthesis of high cis content poly butadiene was effected by Philips petroleum company in 1956.

Polymerisation

Both Solution and emulsion polymerization methods are used for commercial production. In general the showed tendency for cold flow. In solution poly butadiene the purified solvent and monomer are fed into a reactor in presence of stereo specific catalysts. Polymerization is carried out at a pre-selected temperature and under vigorous agitation. The catalyst is deactivated at the end of the reaction, the unrecalled monomer and solvent removed. The polymer recovered is washed and dried. BR is prepared also as oil extended and black filled BR.

Properties of BR

BR Prepared by solution; polymerization is generally a high molecular weight product. It is difficult to process these materials when used alone. So it is used along with NR or SBR. BR has very good low temperature properties. It retains its rubberizes for a wide range of temperature. As the trans content in the polymer increases, tendency for crystallization also increase. BR has high air permeability Low heat build up, high resilience. Better flex resistance, heat stability etc. This rubber is compatible in all proportions with NR. SBR and Neoprene.

Processing of BR

Most poly butadiene rubbers possess inherent resistance to break down. Poor mill banding characteristics and rough extrusion appearance when extruded at money viscosity ranges normally used in NR/SBR. At temperature below 100 to 110 C the rubber is continuous on the mill rolls, glossy and smooth in appearance and bands tightly. As the temperature of the stock is enhanced, the polymer becomes rough and loose on the mill, loses cohesion and bags so that milling is poor. Cis ply but diene is used along with natural rubber, in the ratio of 25 BR and 75 NR. Higher proportions of BR are also used in certain blends.
Poly buta diene is mainly used in truck tyre treads because of its high resilience and low heat build up. Tyre treads designed with poly buta diene gives improved tread wear and reduced groove cracking. It is also used in manufacture of mechanical goods and as modifier for plastics to improve impact resistance.

Poly Sulfide Rubbers

Polysulfide rubber were introduced as commercial rubbers during nineteen thirties. These polymers are based on Ethylene dichloride monomer. Since then the developments in this area have been pirmered by Thiokol Chemicals.

Classification of polysulfides

Polysulfide

―                                                                                                                          ―        ―

Malleable                                                                                          Costable (liquids)  

Solid                                                                                                                      ―

 ____________________________________________

―                     ―                                 ―                                 ―                                

Thiokol A         Thiokol B                           Thiokol FA                         Thiokol ST                              9 grades

Polysulfide elestomers have good oil resistance, solvent resisting, fuel resistance, excellent temperature properties (form – 50 C to 12 C ) and weather resistance. They have physical properties and the cost is slightly higher.

Preparation of polysulfides involves reaction of a dialed and sodium or calcium polysulfides.
N-Cl-R-Cl + n Na2 Sx --------

R-Sx –n + 2n- NaCl.
Many different organic dihalildes are used and some of the types of dihalides used are.
1) Ethylene dichloride --     Thiokol A
2) Dichloro diethy1 ether -- Thiokol B
3) Dichloro diethy1 formal +
Ethylene dichloride --         Thiokol FA
4) Dichloro diethy1 formal +
Trichloropropane --             Thiokol ST

The properties of the Polysulfide rubbers depend upon the structure of Polysulfide rubbers such as the arrangement of sulfur atoms sulfur ranking (value of X) and nature of the terminal groups. Type A and FA are hydroxy1 terminated and type ST is mercaptan terminated. Hyydroxy1 terminated elastomers are of height molecular weight and too tough to be processed on a rubber mill. Benzodhiazy1 disulfides are tetramethy1 thiuram disulfides are used to soften the polymers chemically. They do not act as accelerators for these types of tubers, organic amines promote the rate of softening and DPG is commonly used. Actual vulcanizing agent is zinc oxide. The curing agent for thiol terminated polymers are zinc peroxide, calcium peroxide, lead peroxide etc.

Physical properties of cured crude Polysulfide polymers are poor. They can be improved adding reinforcing fillers such as carbon black, titanium dioxide, calcium carbonate, lithapone and zinc sulfide.
Polysulfide rubbers have excellent resistance to solvents, oil and fuels. Practical applications are in printers’ rollers, seals, gasket adhesives, putting for metal, glass, plastic etc. Diced in blen is with other elastomers for more solvent resistance.

5) Silicone Rubbers
Chemical formula
       R                          R                            R
       |                            |                              |
R -- Si -- O ---- [ ---------- Si ------- O ----- ] ------ Si ---R

Where n = 100—9000

R=CH3 (CH2 = CH -- ), OH – H--, LF3 --CH2 – CH2—

The polymer backbone is formed by an alternating row of silicon and oxygen atoms. The organic side groups are methy1 groups combined with a very small percentage of vinyl groups. Some of the most suitable side chain groups apart from methy1 are the pheny1 groups; for high temperature and radiation resistance, the vinyl, hydroxyl and hydrogen group propy1 group for maximum oil and fuel resistance.
Trade name Producer
DC – 400 Dow Corning Corporation SE – 76 General Electric Company W – 95 Union Carbide & Carbon Corpn.
The mechanical properties of silicone rubbers vulcanistates are Somewhat; weak in comparison to natural rubber. Typical properties regimen below:
 

Hardness	30—80 Shore A
Tensile Strength	5—10 MPA
Elongation at break	120—600E
Tear resistance	10—35 N/M
Rebound Resilience	25—50%  Set
Compression	10—30%

  (22 at 175 C)