Synthetic Rubber - Lab Report Example

1. Natural rubber occurs as a milky emulsion or latex in the sap of a number of plants, but can be produced synthetically when units of Isoprene (a hydrocarbon diene monomer) are polymerised (Wikipedia, 2006) in the presence of a Ziegler Natta catalyst. Radical polymerisation occurs in three fundamental steps: initiation, propagation and termination. The reactions of monomers may occur by the absorption of heat or light, or through addition of an initiator. The initiator I (a weak organic compound decomposes to produce unpaired electron containing free radicals) is added to a monomer molecule. E.g. -[-CH=C(CH3)C=CH--]n- --[-CH-C(CH3)C-CH---]n- isoprene Synthetic Natural Rubber (SNR) The mechanism is as shown below: Initiation : I-I 2I* Propagation: I + CH=C(CH3)C=CH CH=C(CH3)CCH* + I CH=C(CH3)CCH' + CH=C(CH3)C=CH CH=C(CH3)CCHCHC(CH3)CCH' Termination: I* + I* I-I CH=C(CH3)CCH* + *H2C (CH3)C=CH CH=C(CH3)C2H-H2C(CH3)C=CH polyisoprene (SNR) Synthetic rubber is also formed during the polymerisation of certain monomers or by co-polymerisation of two different monomers. During polymerization the double bonds of the isoprene monomers give way to single bonds which will lead to further increase in length of product chain. Butyl rubber-also known as polyisobutylene (C4H8)-is a synthetic rubber and is produced by polymerization of about 98% of isobutylene with about 2% of isoprene (Wikipedia, 2006) using BF3, H2O, CH2CL2 as reaction activators under -78C temperature. e.g CH3-C(CH3)=CH2 -[-CH-C(CH3)-CH2--]n- isobutylene (98%) polyisobutylene (Butyl Rubber) The mechanism is as follows: Initition I-I 2I* Propagation: I* + CH3-C(CH3)=CH2 CH3-C(CH3)H2C* + I CH3-C(CH3)H2C* + CH3-C(CH3)=CH2 CH3C(CH3)CH2C(CH3)H2C* Termination: I* + I* I-I H3CC(CH3)H2C* + *CH2(CH3)CCH3 H3CC(CH3)H2C-CH2(CH3)CCH3 polyisobutylene (butyl rubber ) Co-polymerisation of Styrene and butadiene produces styrene butadiene rubber (SBR) which is present in tyres. C6H5(CH=CH2) + CH2=CH-C=CH2 -[-CH2-CH2(C6H5)CH=CH-CH-CH(C6H5)-]n- styrene butadiene styrene butadiene rubber (SBR) The reaction mechanism is as follows: Initiation: I-I 2I* Propagation: C6H5(CH=CH2) + I* *C6H5(CH-CH2) + I CH2=CH-C=CH2 + I* CH2=CH-C-CH2* + I *C6H5(CH-CH2) + CH2=CH-C-CH2* CH2-CH2(C6H5)CH=CH-C-H2C* Termination: I* + I* = I-I C6H5(CH-CH2)* + *(CH-CH2)C6H5 C6H5(CH-CH2)-(CH-CH2)C6H5 CH2=CH-C-H2C* + *CH2-C-CH=CH2 CH2=CH-C-H2C-CH2-C-CH=CH2 Similarities and Differencies: In both natural and synthetic rubbers production, the monomers can be mixed in various desirable proportions to achieve a wider range of physical, mechanical, and chemical properties through the use additive. The formation of natural and synthetic rubbers are three step Free radical eliminations and substitutions reactions. Both Natural and synthetic rubber reactions require protectants against deterioration during the coagulation and drying (CIWMB). 2. In its natural form rubber is too soft to be used for any useful purposes.Natural rubbers tends to be sticky and soft at high temperatures; while at low temperatures they are brittle and stiff, making them difficult to process. This is due to a high degree of entropy. Therefore, its properties were improved using special processing techniques. During vulcanization, or curing, of rubber, individual polymer molecules are linked to other polymer molecules by polysulphide bridges which gives the rubber its characteristic strength and elasticity. Vulcanised rubber showing sulphide linking polyisoprene molecules. (Sulphide) These results in a more durable and resistant rubber able to withstand chemical attack. This property is useful in the production of tyre as the surface of the material improves from the sticky feel to a smooth soft surface which does not adhere to metal or plastic substrates (wikipedia, 2006). The methyl groups substituted on every other carbon atom of Polyisobutylene confers excellent impermeability, and the long segments of its polymer chains give it good flexible properties. 3. The rubbers used in car tyre manufacturing are all thermal set polymers. These polymers have various additives that serve a variety of functions. They include Primary activators, Age resistors or antioxidants, Softeners and Peptizers or catalytic plasticizers, generally Thiophenols, other Thiol compounds, and disulfides can be used as well to reduce the viscosity of the rubber during preliminary processing. These compounds are usually recovered in solvent extraction processes but can be found in proprietary rubber compositions (CIWMB). The major pigment in tyres is carbon black. For white walls, titanium dioxide is used. Carbon black is also called filler. In many cases, filler is known to reduce tensile strength. But for tyre rubber the appeal of the black color, the resistance to staining by other additives such as antioxidants, and its ability to improve tyre rubber tensile strength and abrasion resistance, make it the ideal filler. Carbon black is by mass the largest amount of additive in tyre rubber. Carbon black is defined by adsorption properties and particle size much like carbon used to adsorb organics from water and air. Additives Components Use or Importance Primary activators Zinc oxide, Stearic Acid 1. Increase the efficiency of vulcanisation by reducing process time. 2. Reduce quantity of sulphur consumed. Age resistors/ anti oxidants. Derivatives of p-phenylenediamine (PPD) 1. Protect the tyres from oxygen and ozone degradation. 2. They stop chain destruction of the rubber by combining with the free radicals formed as intermediates during degradation of the tyre rubber &Prolong lifespan of tyres. Softeners and extenders Petroleum oils and coal tar fractions Increase the workability of the rubber during preliminary processing before vulcanization. Peptizers or catalytic plasticizers Thiophenols, other Thiol compounds, and disulfides They reduce the viscosity of the rubber during preliminary processing Carbon black Carbon, titanium dioxide Helps to improve tensile strength and abrasion resistance of the tyres 4. Recycling and Reuse of Waste Tyres Recycling of waste rubber from tyres can be used to manufacture new products and as alternative source of fuel to power facilities because of the high heating value of tyres (approximately 14,000 BTU/lb) (CIWMB). In addition to fuel alternatives, pyrolysis of tyres can be performed to chemically alter the materials of the tyre or to derive various products such as carbon black also used as fuel. Several chemical methods have been used in recycling tyres: they include the use of 2-butanol solvent as a devulcanising agent for sulphur-cured rubber under high temperature and pressure. Reported data on surface devulcanisation experiments were obtained by treating small amounts of crumb rubber in the gas chromatography column (CalRecovery, 2004). The solvent suitable for this process should have a critical temperature in the range of about 200 to 350C (400 to 650F). The process produces slurry of the surface devulcanised crumb rubber that has to be separated from the solvent. In this process, a preferential breakage of S-S and C-S bonds appears to take place, with little breakage of the main chains. The obtained surface modified crumb rubber was subjected to vulcanization as obtained and also in blends with virgin rubber. The vulcanisates exhibited a good retention of mechanical properties in blends with virgin rubber. Sulphur vulcanised natural rubber (NR) can be completely recycled at 200 to 225C by using diphenyldisulphide (CalRecovery, 2004). Various disulphide used as recycling agents for NR and ethylene propylene diene monomer rubber (EPDM) vulcanisates. While complete devulcanization was observed on sulphur-cured NR at 200C (392F), a decrease on crosslink density by 90 percent was found when EPDM sulphur vulcanisates and diphenyldisulphide were heated to 275C (527F) in a closed mold for two hours. At the same time, EPDM cured by peroxide showed a decrease in crosslink density of about 40 percent under the same conditions. In addition rubbers can be devulcanized by means of inorganic compounds. Discarded tyres and tyre factory waste have been devulcanised by desulphurisation of suspended rubber vulcanisate crumb (10 to 30 mesh) in solvents such as toluene, naphtha, benzene, cyclohexane, etc. in the presence of sodium (CalRecovery, 2004). The alkali metal cleaves mono-, di-, and polysulfidic crosslinks of the swollen and suspended vulcanized crumb rubber at around 300C (575F) in the absence of oxygen. However, this process may not be economical because it involves swelling of the vulcanized crumb rubber in an organic solvent. In this process, the metallic sodium in a molten condition should reach the sulfidic crosslink sites in the crumb rubber. References CalRecovery Inc. (2004). Evaluation of Waste Tyre Devulcanisation Technologies. CIWMB. Retrieved 12:57, March 10, 2006 from http://www.ciwmb.ca.gov/Publications/Tyres/62204008.doc CIWMB. Waste Tyres. California integrated Waste Management Board. Retrieved March 10, 2006 from http://www.ciwmb.ca.gov/publications/Tyres/43296029.doc Sulphide (2000): One big happy molecule: Science side trip. Polymer science learning center and Chemical Heritage Foundation. Retrieved, March 11, 2006. From http://www.pslc.ws/macrog/exp/rubber/sepisode/spill.htm Wikipedia contributors (2006). Rubber. Wikipedia, The Free Encyclopedia. Retrieved 07:03, March 10, 2006 from http://en.wikipedia.org/w/index.phptitle=rubber&oldid=43169155 Read More