Vulcanisation, accelerators & curing system
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This document discusses the history and chemistry of vulcanization and curing systems for rubber. Some key points include: - Charles Goodyear discovered in 1839 that heating rubber with sulfur produced an elastic product that did not become sticky or brittle at high/low temperatures, launching the vulcanization process. - Sulfur has remained the most important vulcanizing agent, though other chemicals have also been examined. Accelerators were later developed to speed up the vulcanization reaction. - Efficient vulcanization systems using sulfur donors and accelerators produce vulcanizates with mainly mono- and disulfide crosslinks rather than unstable polysulfide crosslinks and pendant groups. This improves aging
Vulcanisation, accelerators & curing system
- 1. VULCANISATION, ACCELERATORS& CURING SYSTEM PRIYABRATA GHOSH IRI, KOLKATA
- 2. HISTORY OF VULCANISATION PRIYABRATA GHOSH IRI KOLKATA IN 1839, CHARLES GOODYEAR DISCOVERED THAT HEATING OF A MIXTURE OF RUBBER AND SULPHUR GAVE AN ELASTIC PRODUCT THAT DID NOT GET STICKY OR BRITTLE AT HIGH / LOW TEMPERATURE. AT ABOUT THE SAME TIME THOMAS HANCOCK WAS WORKING ALONG THE SAME LINE. SINCE THEN, MANY OTHER CHEMICALS HAVE BEEN EXAMINED AS POSSIBLE VULCANISING AGENT, BUT NONE HAVE COMPETED SUCCESSFULLY WITH SULPHUR AS A GENERAL PURPOSE VULCANISING AGENT. ONE LIMITATION OF USING SULPHUR AS ‘VULCANISING’ AGENT IS THAT ELASTOMER MUST CONTAIN SOME UNSATURATION. THE PROCESS WAS TERMED ‘ VULCANISATION’ BY BROCKEDON WHO DERIVED THE WORD FROM THE ASSOCIATION TO HEAT AND SULPHUR WITH ‘THE VULCAN’ OF ‘GREEK MYTHOLOGY.’ THE DISCOVERY RESULTED IN A VERY SHARP INCREASE IN PRODUCTION OF RUBBER GOODS BY REDUCING THE TIME OF HEATING (ABOUT 5 HOURS). AFTER THAT METALLIC OXIDES, SUCH AS THOSE OF ZINC, CALCIUM, MAGNESIUM AND LEAD WERE BEING INCORPORATED FOR THIS PURPOSE. P-1
- 3. HISTORY OF VULCANISATION Cont… PRIYABRATA GHOSH IRI KOLKATA MIX (PARTS BY WT) CURE (h at 1410C) T.S (kg / Cm2) Elongation at break % Rubber 100 Unvulcanised 25 1200 Rubber 100, Sulfur 8 5 250 700 Rubber 100, Sulfur 8, Zinc Oxide 5 3 250 700 DISCOVERY OF ORGANIC ACCELERATOR NAMELY MERCAPTOBENZTHIAZOLE (MBT) IN 1921 WAS ONE OF THE LANDMARK. AT ABOUT SAME TIME IT WAS DISCOVERED THAT ZINC OXIDE ENHANCED THE ACTION OF ORGANIC ACCELERATORS AS AN ACTIVATOR. THEN STEARIC ACID BECOME OF USE AS A CO-ACTIVATOR. THE USE OF ZINC STEARATE IN PLACE OF ZINC OXIDE AND STEARIC ACID DOES NOT PRODUCE THE SAME ACTIVATING EFFECT. P-2
- 4. DEFINATION OF TERMS PRIYABRATA GHOSH IRI KOLKATA A. VULCANISATION (CURE): DEFINED AS A CHEMICAL PROCESS FOR CONVERTING BY CROSS – LINKING REACTION, THERMOPLASTIC NATURAL AND SYNTHETIC RUBBER MOLECULES INTO THREE- DIMENSIONAL ELASTIC MOLECULES. B. VULCANISING AGENTS : ALL SUBSTANCES WHICH EFFECT VULCANISATION ARE KNOWN AS VULCANISING AGENTS. SULPHUR HAS REMAINED THE MOST IMPORTANT VULCANISING AGENT FOR DIENE RUBBERS. SOME OTHER IMPORTANT VULCANISING AGENTS ARE SULPHUR CHLORIDE, TELLURIUM, SELENIUM, THIURAM DISULPHIDE, MORPHOLINE DISULPHIDE, P- QUINONE DIOXIME, METALLIC OXIDES, PEROXIDES, DI- ISOCYANATES ETC. C. ACCELERATORS: ACCELERATORS ARE CHEMICALS WHICH SPEED UP (ACCELERATE) THE CROSS- LINKING REACTION (VULCANISING PROCESS) D. ACTIVATOR : ACTIVATOR IS A CHEMICAL OR MIXTURE ADDED TO AN ACCELERATED VULCANISATION SYSTEM TO REALISE ITS FULL POTENTIAL. IN ACCELERATED VULCANISATION SYSTEMS, METAL OXIDES (USUALLY ZnO) AND SATURATED FATTY ACIDS (USUALLY STEARIC ACID) ARE USED AS ACTIVATORS. E. SCORCH : PREMATURE START OF VULCANISATION OF A RUBBER COMPOUND DURING PROCESSING OPERATIONS SUCH AS MIXING, EXTRUSION, CALENDERING OR DURING STORAGE. THE ‘SCORCH’ EFFECT IS MAINLY TIME AND / OR TEMPERATURE DEPENDENT. P-3
- 5. CHEMISTRY OF VULCANISATION PRIYABRATA GHOSH IRI KOLKATA VULCANISATES FROM SULPHUR CURABLE ELASTOMERS CONTAIN A VARIETY OF PRODUCTIVE AND NON- PRODUCTIVE SULPHUR MODIFICATIONS GIVING WIDELY DIFFERENT CHEMICAL AND THERMAL STABILITY THESE SYSTEMS UNDERGO CHEMICAL CHANGE FOR UNDESIRED LOSS OF PHYSICAL PROPERTIES ON AGEING. A COMPOUNDER CAN MODIFY AND CONTROL SO THAT VULCANISATES ARE MORE STABLE AND RESISTANT TO AGEING. FOLLOWING TYPE OF SULPHUR MODIFICATIONS MAY BE PRESENT IN S – CURED VULCANISATES : P-4
- 6. CHEMISTRY OF VULCANISATION PRIYABRATA GHOSH IRI KOLKATA 1) PENDANT SULPHIDE GROUPS TERMINATED BY ACCELERATORS. 2) MONOSULPHIDE CROSSLINKS 3) DISULPHIDE CROSSLINKS 4) POLYSULPHIDE CROSSLINKS ( X > 2) 5) AND (6) CYCLIC MONO AND DISULPHIDES MONO- AND DISULPHIDIC CROSSLINKS ARE RELATIVILY STABLE. BUT POLYSULPHIDIC AND PENDANT GROUPS ARE UNSTABLE AND SLOWLY UNDERGO FURTHER REACTION AS THE VULCANISATE AGES RESULTING IN CHANGE OF PHYSICAL PROPERTIES. NATURE OF S – MODIFICATIONS STRONGLY INFLUENCED BY AMOUNTS OF SULPHUR AND ACCELERATOR. P-5
- 7. CHEMISTRY OF VULCANISATION CONT…. PRIYABRATA GHOSH IRI KOLKATA CONVENTIONAL NR VULCANISATES (100 NR + 2.50 SULPHUR + 0.50 DBS) CROSSLINK TYPE % MONOSULPHIDIC 0 DISULPHIDIC 30 POLYSULPHIDIC 70 AS CONVENTIONAL NR COMPOUNDS AGE THE CROSSLINKS UNDERGO OXIDATIVE ATTACK AND CHAIN SCISSION OF THE ELASTOMER BACKBONE OCCURS RESULTING IN LOSS OF PHYSICAL PROPERTIES SUCH AS TENSILE STRENGTH AND ELONGATION P-6
- 8. ACCELERATOR CLASSES PRIYABRATA GHOSH IRI KOLKATA CLASS SPEED ALDEHYDE AMINES (HMT) SLOW GUANIDINES ( DPG, DOTG) MEDIUM THIAZOLES ( MBT, MBTS) SEMI – FAST SULFENAMIDES ( CBS, MBS) FAST DELAYED ACTION THIURAMS V. FAST DITHIOCARBAMATES ( ZMDC, ZEDC) SUPER FAST. P-7
- 9. LISTS SOME OF THE PRINCIPAL ACCELERATOR PRIYABRATA GHOSH. IRI KOLKATA P-8
- 10. LISTS SOME OF THE PRINCIPAL ACCELERATOR CONT… PRIYABRATA GHOSH IRI KOLKATA P-9
- 11. LISTS SOME OF THE PRINCIPAL ACCELERATOR CONT… PRIYABRATA GHOSH IRI KOLKATA P-10
- 12. DOSAGES OF CURING SYSTEMS IN NR AND SYNTHETIC RUBBERS PRIYABRATA GHOSH IRI KOLKATA ALL UNSATURATED ELASTOMERS CAN BE VULCANISED WITH SULPHUR USING DIFFERENT CLASSES OF ACCELERATORS. BUT THEIR AMOUNTS WILL DEPEND ON THE DEGREE OF UNSATURATION. FOR NR, SULPHUR IN AMOUNTS OF 2.5 – 3.0 P.H.R AND ACCELERATOR IN AMOUNTS OF 0.5 – 1.0 P.H.R. FOR SYNTHETIC RUBBERS, REQUIRE A HIGHER AMOUNTS OF ACCELERATORS (1.2 – 2.0 P.H.R) WITH A CORRESPONDING REDUCTION IN SULPHUR AMOUNTS (0.75 – 2.0 P.H.R) USE OF ZnO AND STEARIC ACID AS ACTIVATORS IS USUAL P-11
- 13. TYPICAL SULFUR CURING SYSTEM FOR OLEFIN RUBBERS PRIYABRATA GHOSH IRI KOLKATA P-12
- 14. DOSAGES OF CURING SYSTEMS IN NR AND SYNTHETIC RUBBERS CONT.. PRIYABRATA GHOSH IRI KOLKATA FOR NR AND IIR REVERSION WILL OCCUR ON PROLONG HEATING. FOR SBR AND NBR REVERSION DO NOT OCCUR AND USUALLY DO NOT SHOW REDUCTION IN TENSILE STRENGTH ON OVERCURE AS IS OBSERVED IN NR AND IIR THIS PHENOMENON IS KNOWN ‘MARCHING CURE’ AND THE MODULUS GRADUALLY RISES, USUALLY FOR SBR AND NBR, THE ELONGATION DROP MERKEDLY WHEREAS IN NR LITTLE CHANGE IS OBSERVED NBR SHOWS PRONOUNCED IMPROVEMENT IN COMPRESSION SET ON OVERCURE CURING CURVES OF NR AND SBR P-13
- 15. EV. / SEMI E.V. SYSTEM IN NR PRIYABRATA GHOSH IRI KOLKATA WHERE POLYSULPHIDE CROSSLINKS AND SULPHUR INVOLVED IN MAIN CHAIN MODIFICATIONS (i.e. CYCLIC SULPHIDES AND PENDANT ACCELERATORS) SHOW CHEMICAL AND THERMAL INSTABILITY, MONO AND DISULPHIDIC CROSSLINKS SHOW EXCELLENT RETENTION OF PROPERTIES ON AGENING. SUCH SYSTEMS ARE CALLED “EFFICIENT VULCANISATION.” THE TERM ‘EFFICIENT’ DERIVES FROM EFFICIENT USE OF SULFUR, i.e. SINCE THE CROSSLINKS ARE SHORTER AND MAIN CHAIN MODIFICATIONS ARE LESS, LESS SULPHUR IS REQUIRED TO OBTAIN EFFECTIVE NUMBER OF CROSSLINKS. CONVENTIONAL SYSTEM (NR =100, S= 2.5, CBS = 0.5) E. V. SYSTEM ( NR = 100 , S = 0.5, MOZ = 3.0, TMTM = 0.6) CROSSLINK TYPE % % MONOSULPHIDIC 0 38 DISULPHIDIC 30 52 POLYSULPHIDIC 70 10 P-14
- 16. EV. / SEMI E.V. SYSTEM IN NR CONT…. PRIYABRATA GHOSH IRI KOLKATA EV SYSTEMS ARE DEVELOPED BY TOTAL SUBSTITUTION OF SULPHUR BY A SULPHUR DONOR OR BY VERY HIGH RATIOS OF ACCELERATOR TO SULPHUR. IN PRACTICE EV SYSTEMS DEVELOPED BY USING TMTD UPTO SOLUBILITY LEVEL ( EXCESS TMTD WILL CAUSE SERIOUS BLOOMING AND SCORCH PROBLEMS) AND USING NON BLOOMING SULPHUR DONOR, SUCH AS SULFASON R (DTDM) TO REACH TOTAL SULPHUR LEVEL REQUIRED. SUCH SYSTEMS ARE REFFERED TO AS “SULPHURLESS” SYSTEM. SEVERAL SULPHUR DONORS AND THEIR AVAILABLE SULPHUR CONTENTS NAME % AVAILABLE SULPHUR TETRAMETHYL THIURAM DISULPHIDE(TMTD) 13.0 TETRAETHYL THIURAM DISULPHIDE(TETD) 11.0 DITHIO DIMORPHOLINE(DTDM) – SULFASON R 27.0 P-15
- 17. CONVENTIONAL vs. EV SYSTEMS IN NR PRIYABRATA GHOSH IRI KOLKATA CONVENTION- AL E.V. HI / LOW SULPHUR DONOR SULFUR 2.5 0.5 - MOR(MORPHOLINOTHIO BENZOTHIAZOLE) 0.5 3.0 1.1 TMTM - 0.6 - TMTD - - 1.1 SULFASON R - - 1.1 PROCESSING MOONEY t5 MINTS @120 0C 33 38 21 RHEOMETER : t90 MINTS. @ 1400C 31 30 26 AGED 12 DAYS @ 850C % RETENTION OF T.S. 40 79 91 P-16
- 18. CONVENTIONAL vs. EV SYSTEMS IN NR CONT…. PRIYABRATA GHOSH IRI KOLKATA FATIGUE KC /TO FAILURE 100 % EXTENTION UNAGED 74 27 29 AGED 5 DAYS @850C 60 23 25 REVERSION % RETENTION TENSILE STRENGTH CURE @ 1400C t 90 CURE @ 2000C 40 85 77 10 X t90 CURE @ 2000C 29 76 72 P-17
- 19. FATIGUE COMPROMISE IN NR SYSTEM P.GHOSH. IRI KOLKATA P-18
- 20. FATIGUE COMPROMISE IN NR SYSTEM CONT… PRIYABRATA GHOSH. IRI KOLKATA THE MID-POINT OR “SEMI E.V. SYSTEM” REPRESENTS A GOOD COMPROMISE BETWEEN UNAGED FATIGUE AND AGEING PROPERTIES. SUPERIOR OVERALL PERFORMANCE OF SEMI EV SYSTEMS HAS LED WIDE USE IN DYNAMIC APPLICATIONS DUE TO ITS GOOD COMPROMISE BETWEEN REVERSION, AGEING AND FATIGUE RESISTANCE CONVENTIONAL vs. SEMI – EV SYSTEMS IN NR CONVENTIONAL HIGH / LOW SULPHUR DONOR SULFUR 2.50 1.50 1.50 SULPHENAMIDE 0.60 1.50 0.60 SULPHASAN R - - 0.60 MONNEY @ 1210C t5, MINUTES 34.0 38.0 49.50 RHEOMETER @ 1440C T90, MINUTES 25.0 20.0 24.50 P-19
- 21. CONVENTIONAL vs. SEMI – EV SYSTEMS IN NR CONT… PRIYABRATA GHOSH. IRI. KOLKATA REVERSION RESISTANCE % RETENTION OF 300% MODULUS AFTER 30 MTS. @ 1800C 48.0 63.0 72.0 AGEING REISISTANCE % RETENTION OF TENSILE STRENGTH AFTER 10 DAYS @ 900C 38.0 69.0 63.0 IMPROVEMENTS IN AGEING RESISTANCE BY EV / SEMI – EV SYSTEMS ARE ALSO OBSERVED IN SBR. SBR SHOWS EXCELLENT FATIGUE RESISTANCE IN BOTH CONVENTIONAL AND EV CURED STOCKS P-20
- 22. NON – SULPHUR VULCANISATION SYSTEM PRIYABRATA GHOSH. IRI. KOLKATA SULPHUR CHLORIDE CURE EARLIER NR WAS CURED BY A ‘COLD CURE’ METHOD. RUBBER SHEET WAS CURED IN A SOLUTION OF SULPHUR CHLORIDE IN CARBON DISULPHIDEOR PETROL. TENSILE STRENGTH WAS NOT VERY HIGH, AGEING WAS POOR AND SMELL OF HYDROGEN CHLORIDE WAS FELT. MONO SULPHIDIC CROSSLINK IS FORMED, BUT CHLORIDE IS ADDED TO THE MOLECULE AS WELL. P-21
- 23. NON – SULPHUR VULCANISATION SYSTEM CONT… PRIYABRATA GHOSH. IRI. KOLKATA TMTD CURE TMTD CAN ALSO BE USED IN CONJUNCTION WITH ZnO AS NON – SULPHUR VULCANISING AGENT. TMTD REACTS WITH ZnO TO GIVE SOME TETRAMETHYL THIURAM POLYSULPHIDE WHICH REACTS WITH RUBBERS AND MORE ZnO TO GIVE RUBBER – BOUND POLYSULPHIDE GROUP. AGAIN REACTS WITH MORE RUBBER AND ZnO TO FORM A VULCANISED RUBBER WITH POLYSULPHIDE CROSSLINKS. AS CURE PROCEEDS, THESE POLYSULPHIDE CROSSLINKS TRANSFORM TO MONOSULPHIDE CROSSLINKS. TMTD AND TETD GIVES VULCANISATES WITH NR LOWER IN TENSILE STRENGTH THAN SULPHUR VULCANISATES, BUT HAVE VERY GOOD AGEING PROPERTIES. THIS SYSTEM IS TOO SLOW FOR IIR. P-22
- 24. NON – SULPHUR VULCANISATION SYSTEM CONT… PRIYABRATA GHOSH. IRI. KOLKATA QUINONE DIOXIME : CAN BE USED AS VULCANISING AGENT FOR ANY OLEFINIC RUBBER, BUT IT IS ONLY USED IN IIR. OXIDISING AGENT, SUCH AS RED LEAD, OR EVEN MBTS, IS NECESSARY. REACTION INVOLVES IN FARMATION OF DINITROSOBENZENE AS AN INTERMEDIATE, FOLLOWED BY TWO MOLECULES OF RUBBER WITH REMOVED OF TWO HYDROGEN RADICALS. P-23
- 25. NON – SULPHUR VULCANISATION SYSTEM CONT… PRIYABRATA GHOSH. IRI. KOLKATA P F RESIN CURE ALSO USED TO CURE OLEFIN RUBBERS, BUT IS CONFINED TO IIR ONLY. Sn2Cl2 (STANNOUS CHLORIDE) CAN BE USED AS AN ACCELERATOR; ALTERNATIVELY HALOGENATED (BROMINATED ) RESINS OR EVEN HALOGENATED POLYMER ( CR) CAN BE USED. VULCANISATE HAS OUTSTANDING RESISTANCE TO OXIDATIVE DEGRADATION AND GOOD FLEX LIFE EVEN AT HIGH TEMPERATURE. SO IT FINDS APPLICATIONS IN TYRE CURING BAGS AND BLADDERS. PF RESIN – 10 TO 15 p.h.r. ; CR – 10 p.h.r (OR Sn2Cl2 – 2 -4 p.h.r) P-24
- 26. NON – SULPHUR VULCANISATION SYSTEM CONT… PRIYABRATA GHOSH. IRI. KOLKATA METAL OXIDE CURE IN CASE OF CR , THE DOUBLE BOND IS “HINDERED” BY THE NEIGHBOURING CHLORINE ATOMS. SO VULCANISATION WITH SULPHUR IS NOT POSSIBLE. CR REACTS WITH ZnO FORMING CROSSLINKING OF THE OCCASIONAL 1,2 UNITS WHICH REACTS WITH CHLORINE ATOMS. P-25
- 27. NON – SULPHUR VULCANISATION SYSTEM CONT… PRIYABRATA GHOSH. IRI. KOLKATA MgO IS USED TO ACT AS SCAVANGER FOR THE CHLORINE ATOMS ALSO. IT ALSO INCREASE PROCESSING SAFETY AND RATE OF VULCANISATION. MORE RAPID VULCANISATION IS ACHIVED BY USE OF ORGANIC ACCELERATORS, e.g. ETHYLENE THIOUREA. PEROXIDE CURE MOST TYPES OF ELASTOMERS CAN BE VULCANISED BY PEROXIDES . THE ORGANIC PEROXIDES SUITABLE FOR VULCANISATION ARE : P-26
- 28. NON – SULPHUR VULCANISATION SYSTEM CONT… PRIYABRATAGHOSH. IRI. KOLKATA PEROXIDE FIRST DECOMPOSES TO GIVE FREE RADICALS WHICH BREAK HYDROGEN ATOM OF THE METHYLENE GROUP OF THE POLYMER RESULTING IN FORMATION OF POLYMERIC RADICAL. TWO POLYMERIC RADICALS INTERACT TO FORM A CROSSLINK. NR, SBR, NBR, -ALL CAN BE CURED BY THIS MEAN. BUT IIR IS DEGRADED BY PEROXIDES. PEROXIDES ALSO VULCANISE SATURATED SPECIAL PURPOSE POLYMERS SUCH AS EPR, SILICONES, FLUOROELASTOMERS ETC. EXAMPLE OF SILICONE RUBBER VULCANISATION AS FOLLOW: CROSSLINKING OF DIMETHYL POLYSILOXANE RUBBERS IS DUE TO ATTACK BY PEROXIDES ON THE METHYL GROUP OF THE POLYMER. P-27
- 29. THE END