Natural Rubber - Sources, Coagulation & Processing of Coagulate, Structure & Composition, Properties of raw NR Compounding, Processing of NR, Properties of NR vulcanizates, Uses of

Natural Rubber

Natural Rubber

Page 2

•Sources, Plantation Economy•Coagulation & Processing of Coagulate•Structure & Composition •Properties of raw NR•Compounding, Processing of NR•Properties of NR vulcanizates•Uses of NR•Competitive products of NR

Sources

Page 3

•Many plants produce a milky sap, referred to as latex, which is a caoutchouc dispersion in an aqueous medium•Latex producing plants are predominantly found in tropical climates•Not all caoutchouc producing plants are harvested for industrial purposes

Plantation Economy

Page 4

•Early plantation economies used Ficus elastica,Funtumia, de Castilloa and Manihot plants•But were spoon displaced by Hevea Brasiliensis, due to greater yield•Yield was improved by the stimulation of latex flow.

Several chemicals, such as 2-amino-3,5,6-trichloropicolinic acid or 2-chloroethyl phosphoric acid were used, which after application,

penetrates the bark and produce ethylene within the plant

Tapping

Page 5

Coagulation

Page 6

•Preferred method is Acid Coagulation (Formic Acid/acetic Acid)•Latex from different sources are blended in huge tanks in order to achieve good uniformity•Latex is diluted with water to a solids content up to 12 to 18%•Iso-electric point is reached at a pH of 5.1 to 4.8, which coagulates the latex

Processing of Coagulate

Page 7

•Coagulum is processed immediately to prevent changes in properties under the influence of bacteria when exposed to air•When kept immersed in water or in its own serum, impurities in the caoutchouc are decomposed by bacteria into gases, such as CO2, CH4 & Nitrogen containing compounds

Two Standard ways of Processing the coagulum: 1. Drying by exposure to wood smoke (Smoked Sheet) 2. Air drying (Pale Creep)

Structure & Composition

Page 8

•Rubber Hydrocarbons + Impurities + Others•Average composition measured for latex crepe: 89-92% Rubber HC, 2.5-3% acetone extractables, 2.5-3.5% protein, 2.5-3.5% moisture, 0.15-0.5% ash•Rubber HC content of NR consists of 99.9% cis-1,4-polysioprene. Presence of even small amount of trans results in greatly different properties•Average molecular weight ranges from 2,00,000 – 4,00,000 •Has a broad molecular weight distribution (results in great process ability)

Structure

Page 9

Chemical structure of cis-polyisoprene, the main constituent of natural rubber. Synthetic cis-polyisoprene and natural cis-polyisoprene are derived from different precursors.

Chemical Properties of raw NR

Page 10

•If heated above 300 ° C destructive distillation occurs•Double bond in the isoprene unit and the α-methylene groups are reactive for sulphur vulcanization•May also lead to age degradation due to the presence of double bond(reacts with O2 or O3)•Other reactions with hydrogen leads to formation of Hydrogenated rubber•Reacts with Chlorine to form chlorinated rubber; reacts with HCl to form hydro-chlorinated rubber; cyclization reactions also may occur

Cont..

Page 11

•Since NR contains some amount of antioxidants, it is stable for longer storage periods in air at room temperature. BUT On storage at high temperatures and on exposure to light, NR oxidizes by forming hydroperoxides through oxygen radicals•Unsaturation facilitates reaction with oxidizing agents like peroxides, peroxy acids, potassium permagnate, ozone, chlorine etc..

Physical Properties of raw NR

Page 12

•Specific gravity: 0.934 @ 20°C•Specific heat: 0.502 @ 20°C•Purified NR after extraction with acetone has refractive index of 1.5215 to 1.5238•Electrical properties of NR are determined by its water soluble impurities. For instance the specific resistivity of sheets is 10^15 and that of crepe is 2*10^15•When NR is stretched more than 80% of its original length, crystallization occurs

Page 13

Cont..

•Un-crosslinked NR can be extended to 800 to 1000% of its original length without breaking•As the temperature increases forces required to deform the material becomes larger•At high rates of strain, deformations are completely recoverable, whereas at low strain rates some deformation may remain. This deformation will however recover on heating or after some time. Because of this incomplete recovery the extension and retraction do not coincide. This hysteresis leads to a heat build up, and is more pronounced in the first heat cycle and less in the subsequent cycles.

Page 14

Cont..

Behaviour in Solvents:•When in contact with organic liquids like benzene, gasoline, vegetable oils, mineral oils, carbon tetrachloride, swell raw rubber to a considerable amount to form a highly viscous solution or gel.•Physical bonds are broken in this process

Compounding of NR

Page 15

Blends with NR:•NR is non-polar•Therefore NR can be readily blended with other non-polar rubbers•NR has to be masticated so that it has the same viscosity as the blend partner to achieve intimate blending of the polymers

Page 16

Cont..

Vulcanization Chemicals:•Sulphur & Accelerators: Suphur is predominantly used as crosslinking agent eventhough peroxides/High energy radiation can also be used to crosslinkFor lower S concentrations, large amount of Accelerators are used and viceversaHigh concentrations of Sulphur may produce Ebonite•Metal Oxides: Used in a compound to develop the full potential of accelerators Eg: Zinc oxide, Lead oxide, Factice, MgO in the presence of acidic compounding ingredients

Factice

Page 17

•Added to improve processability ( extrusion and calendaring operations)•Prevent deformations while curing•Improves the appearance of vulcanizates•Used in large conc. in compounding for preparation of rubber erasers

Page 18

Cont..

•Activators: Many accelerator systems require additional activators like fatty acids or salts of fatty acids namely stearic acid, zinc soaps or amine stearates•Vulcanization Inhibitors: Used to prevent scorch Pthalamide sulphinamide based Inhibitors are used when accelerators or acidic compounding ingredients doesn't provide sufficient scorch life

Page 19

Cont..

Protective Agents:•Since unsaturation in NR causes the material to degrade due to ageing, it has to be compounded with protective agents.•Most effective ones are aromatic amines such as p-phenylene diamine derivatives which also doubles up as an antidegradant from Ozone and heat & also against dynamic fatigue•PAN/PBN impart good fatigue resistance to NR•More effective PAs discolour light coloured vulcanizates so less effective PAs such as phenols, bisphenols, MBI are used

Page 20

Cont..

Fillers:•Fillers are not necessarily used in NR to obtain high tensile strength but to reach a level and range of properties that are required for technical reasons( such as density, colour, price)•Reinforcing fillers enhance the already high tensile properties of NR, and the improve the abrasion and tear resistance+ improves the processability, affects the hardness and rebound elasticity of NR vulcanizates•Less reinforcing fillers: n770, n990 ; Light inactive fillers: kaolin, ZnO, MgCO3,CaCO3, Kaolin

Page 21

Softeners: •Most important mineral oils including paraffinic to aromatic•Animal and Vegetable Oils are also used•Blooming potential must be taken into consideration while selecting softeners for NRResins:•Added for improving building tack, so that they can be fabricated•Rosin, tar, pitch

Cont..

Page 22

Cont..

Process Aids:•Important in facilitating the dispersion of fillers•Ensure smooth processing•In pure white or pastel coloured articles, calcium soaps of unsaturated fatty acids are preferred•Stearic acids, zinc and calcium soaps and residues of fatty acids

Processing of NR

Page 23

•Good processing properties•Viscosity stabilized grades of NR do not require premastication before filler incorporation•For non-stabilized grades mastication is a common practice•Mastication is carried out below 80 degrees or above 120 degree•Chemical peptizers are used to carry out mastication at lower temperatures•Break down behaviour is dependant on PRI values and on the initial viscosity of the rubber•Latex grade rubbers such as RSS1 and SMR L (high PRI values) breakdown much faster than cuplump grades such as SMR 20 ( low PRI values) and therefore require more mastication

Page 24

Cont..

•Viscosity is the most widely used measure of processing quality•During mixing control of viscosity is essential for smooth operation during processing such as extrusion and injection moulding•In Extrusion of a fully mixed batch, the batch viscosity is the main factor controlling the die swell and stress developed•A masticated rubber has better extrusion properties than a non masticated rubber of same viscosity

Properties of NR vulcanizates

Page 25

Strength:•TS of gum vulcanizates: 17 to 24 Mpa•TS of black filled vulcanizates: 24 to 32 Mpa•Strength/tear resistance/cut growth resistance: Excellent ( due to the ability to undergo crystallization)•Decreases with increase in Temperature

Abrasion & Wear:•Excellent abrasion resist., improved by blending with polybutadiene•Wear resistance: dependant on surface temperature•Below 35 deg. NR shows better wear than SBR•Above 35 deg. SBR is better

Page 26

Cont..

Dynamic Properties:•High resilience with values exceeding 90% + Good flexing properties•At large strains fatigue life of NR > SBR and vice versa for small strainsCompression Set:•Poor in NR compared to SR (Due to presence of non rubber substances in SR, and DPNR shows much improved properties)Ageing:•Heat-ageing resistance is given by choosing proper vulcanizates systems and by use of amines or phenolic antioxidants•Ozone resistance is improved by adding Waxes and antioxidants of the p-phenylene diamine type

Uses of NR

Page 27

•Tires•Footwear•Engineering products•Mechanical goods

Competitive products of NR

Page 28

Ethylene Propylene Diene MonomerEPDM rubber is used in seals, glass-run channels, radiators, garden, tubing, pond liners, washers, belts, electrical insulation, O-rings, solar panel heat collectors, UV protection (for tubular motors in shutters) and speaker cone surrounds.EPDM doesn't require maintenance with products to protect it from ultraviolet light, like natural rubber does.EPDM rubber offers the same colour stability and durability in heat as natural rubber but at a lower price.

Competitive products of NR

Page 29

Isoprene Rubber is beneficial for the production of gloves, medical products, adhesives, tackifiers, paints, coatings, and photoresistors.They combine the key qualities of natural rubber such as good mechanical properties and hysteresis with superior features such as high purity, excellent clarity, good flow, low gel content, no nitrosamines, and no natural rubber proteins!

Competitive products of NR

Page 30

Styrene Butadiene Rubber is used widely in pneumatic tires, shoe heels and soles, gaskets and even chewing gum.SBR is more resistant to abrasion and oxidation than natural rubber.

References

Page 31

•Rubber Technology Handbook, Werner Hoffman•Rubber Technology, Maurice Morton•Wikipedia

Page 32