COURSE OUTCOME TO UNDERSTAND THE PRINCIPLES OF RUBBER COMPOUNDING AND VULCANIZATION 210 To understand the principles of rubber compounding and vulcanisation 211 Define compounding and its objectives ID: 1001796
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1. MODULE IIRUBBER COMPOUNDINGCOURSE OUTCOME: TO UNDERSTAND THE PRINCIPLES OF RUBBER COMPOUNDING AND VULCANIZATION
2. 2.1.0 To understand the principles of rubber compounding and vulcanisation2.1.1. Define compounding and its objectives. 2.1.2. Explain the various steps involved in the design of recipes for product manufacture. 2.1.3. State the importance of base polymer in a compound design. 2.1.4. Explain mastication, master batch, gum compound and filled compound. 2.1.5. Justify the curing of elastomers. 2.1.6. Explain the type of crosslinks formed and their effect on vulcanizates properties for sulphur and nonsulphur curing systems. 2.1.7. Distinguish CV, EV, and Semi-EV curing systems. 2.1.8. Explain the chemistry of sulphur vulcanisation. 2.1.9. Select suitable curing systems for Natural and synthetic rubbers based on sulphur and nonsulphur systems.
3. Rubber Compounding Compound – mixture of rubber and ingredients used for manufacture of any rubber products Compounding –art of mmixing various chemicals in required proportion using mixing equipment's like mixing mill and internal mixers . Consists of (a) type and amount of various compounding ingredients used in a mix (b) manner of mixing (c) processing of finished mix (d) method and details of vulcanization
4. Objective of Rubber CompoundingTo secure certain properties in the finished product to satisfy service requirements.To attain processing characteristics necessary for efficient utilization of available equipment.To achieve the desirable properties and processability at lowest possible cost.The most important factor in compounding is to secure an acceptable balance among demands arising from the above three criteria ( final vulcanizates properties, cost and processability)
5. Major Objectives of Compound DevelopmentProcess RequirementProper mixing ( Incorporation, dispersion, distribution and plasticisation)Viscosity/process safetyStickiness and tackinessShaping ( Calendaring, Extrusion, Assembling and Moulding)Compound property requirementHardnessStress-strain properties Abrasion resistanceHysteresis & Set propertiesResistance to cut growth, fatigue, flex cracking
6. Major Objectives of Compound DevelopmentResistance to DegradationHeat, Oxygen, OzoneFlameLiquidsLightMetal poisoning Miscellaneous property requirementsLow temperature flexibilityElectrical propertiesSwellingPermeabilityBonding to metals & textilesContact with food stuff and drugs etc.
7. Compounding Ingredients BASE POLYMER CROSS LINKING AGENTS ACCELERATORS FOR CROSS LINKING REACTION ACCELERATOR MODIFIERS- ACTIVATORS AND RETARDERS REINFORCING FILLERS PROCESSING AIDS ANTI DEGRADANTS INERT FILLERS- Aids to volume cost reduction COLOURING MATERIALS SPECIAL ADDITIVES
8. Compound DesignDESIGN REQUIREMENTS Service requirements/vulcanisate properties Processability EconomicsDESIGN BASED ON Customer specification/requirements Competitor's sample Develop a new product
9. Selection of IngredientsPolymer Should Have Thorough Knowledge About Polymer Properties ( strength, modulus, abrasion resistance, oil resistance , service temperature, flammability, chemical resistance, fatigue etc)Refer Standard Selection Charts/Comparative ChartsBlends Of Polymers Processability Factor-suitability For The Processes And Methods AdoptedCost FactorAvailability Of Data On Polymer Grades, Properties, Start Up Formulations Etc.
10. Selection of IngredientsCuring SystemCross linking/Curing Agents- Sulphur, peroxides, metallic oxides, resins etc.Selection of curing system is based on Base polymerProcessing conditionsCuring conditionsService requirements
11. Base polymer Saturated rubber – peroxide e.g. EPR with DCPUnsaturated rubber-peroxide, sulphur/accelerator & resins, e.g. EPDM with DCP, NR with sulphur accelerator, IIR with ResinSpecial Polymers – Metal oxide, Dioxime, Resin e.g. CR with ZnO/MgOActivatorZnO and Stearic acid for sulphur cure system.Triallyl cyanurate (TAC) for peroxide cure system Processing ConditionType of processing equipment usedTemperature build up during processingExtent of rework usageRheological property requirement Curing ConditionsType of the productMethod of curing employedTemperature of curingFlow requirement during curingService RequirementsService temperatureDuration of exposure Mechanical property requirement.Dynamic property requirementAgeing behavior and permanent set requirements.Special property requirement.
12. Selection of IngredientsFiller System ReinforcementCostProcessing requirementColor requirementService requirementAny other special requirementProcessing Aid Nature of polymer Solubility parameter Viscosity requirement Filler dispersion Processing requirement Any other special requirement
13. Selection of IngredientsAntidegradants Type of protection desired Environment in which the product is exposed. Chemical activity Persistence (volatility and extractability) Nature of end use Discoloration and staining Toxicology Cost
14. INGREDIENTROLE IN FORMULATIONDOSAGE (PHR)ElastomerBase polymer100ActivatorActivates accelerator2.5-5.0AcceleratorAccelerates vulcanization0.5-1.5Vulcanizing agentCrosslinking rubber0.3-3.5AntidegrdantsProtects from degradation1-2Fillers Improving strengthAs requiredProcessing aidsAids filler incorporation1/10th of filler if carbon black as filler1/15th of filler if mineral filler used as fillerTackifiertackiness to the compound2Bonding agentfacilitate adhesion between rubbers, fibers, fabrics, metalsMC WaxPhysical protectant1-3PeptizerSpeed up the rate of polymer break down 0.5Blowing agentopen or closed cell structure3-10
15. RAW RUBBER PROCESSING
16. STEPS OF COMPOUND DEVELOPMENTModification of existing compound with respect toCost/price , Processing , PerformanceNew compound developmentSet specific objectives (properties, processing, price etc)Select best polymerStudy test data of existing compoundsSurvey compound formulations.Choose starting formulation.Develop compound in the laboratory to meet objectives.Estimate cost of the compound.Evaluate processability in factoryUse compound to make a product sample.Test product sample against performance specification.
17. STAGES OF PRODUCT DEVELOPMENT Compound DesignLab EvaluationShop floor processing trials (mixing, molding, extrusion, calendaring, fabrication of product, curing, etc. )Analysis of feedback Compound Modification (if reqd.)Testing of processing properties and of finished productField trialsValidation of manufacturing process.
18. MIXING PROCEDUREWeighing compounding ingredients as per formulationMasticationCompounding And HomogenisationMaturationPrewarmingBlank preparation
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20. Machinery 2-roll mixing mill kneader internal mixers Continuous, automatic high speed mixing
21. 2 –ROLL MIXING MILL
22. BANBURY MIXERINTERMIX
23. Mastication Hancock discovered the effect of mastication on rubber in 1820Mastication of NR necessary for achieving good dispersion of fillers and a perfect plasticity for further processingRaw NR is dry & tough (Mooney Viscosity at 100°C > 80) and has to be masticated to a Workable Level of Mooney Viscosity of 50 to 60.Most Synthetic Rubbers are produced to a tailored Mooney viscosity (range 50 – 60) and hence ‘Mastication’ is not necessary. Always keep lower mastication temperatures when a two-roll mill is used (say, 60-70°C).Viscosity of Masticated NR should be closer to the Synthetic Rubber to be blended with it.
24. Mastication Elastomer molecules split up by high shear forces during mixing in a two roll mill/ internal mixer Chain fragments are formed with terminal free radicals If not stabilized, they recombine into long chain molecules ; if stabilized, short chain molecules remain & Molecular weight of rubber is reduced & plasticity is increased Free radicals (R.) reacts with oxygen and form peroxide radicals (ROO.) ROO. Abstracts an H-atom from the polymer chain forming hydroperoxide or cyclic diperoxide groups Free radicals are formed along the chain through abstraction of H-atoms Peroxide radicals form through oxygen absorption and chain cleavage continues through oxidative degradationFree radicalsHydro peroxide or cyclic diperoxideCleavage continues through oxidative degradation
25. MasticationMastication of NR is performed either at low temperature on mills or at higher temperature in closed mixers, often in the presence of peptizers (they act as donors of electrons or hydrogen), that increase its efficiency. Besides mechanical degradation of rubbery macromolecules also their oxidation degradation occurs in this process and its rate is upgraded with mastication temperature increasing. Mastication of the NR is the most efficient at temperatures below 60 – 70°C and above 120 – 130°C, its efficiency is low in interval between these temperatures.
26. Peptizers (Pentachlorothiophenol, Xylylmercaptan) Used extensively in natural rubber (NR) compounds. When added to NR in premastication, they cleave or break the molecular backbone resulting in lower molecular weight that allows for easier processing Accelerates the breakdown of the molecular structure and greatly reduces the mixing energy. serve as oxidation catalysts or radical acceptors, which removes free radicals formed during the initial mixing of the elastomer. This prevents recombination allowing a consequent drop in molecular weight and thus reduction in viscosity resulting in the incorporation of range of compounding materials included in the formulation Role of peptizer includes : faster filler incorporation, better dispersion of compounding materials, better elastomeric blends, lower processing temperatures, improved flow properties and enhanced building tack. Benefits of peptizing agents : reduce mixing time and power consumption, promote batch to batch uniformity, facilitate blending elastomers, reduce mixing costs, and improve dispersion
27. Master batch Master batches are prepared of individual additives that are difficult to disperse during compounding ( e.g. ZnO) A concentrated dispersion of additives in elastomer is preformed. This can be then added to the rubber compound for faster incorporation since the additive is predispersed OR Master batch is the compound without curatives. It contains only rubber, activator, antioxidants, fillers and oil. It can be stored safely till the preparation of final batch. Usually prepared in Banbury mixer, after which they are ejected and cooledAdvantages : homogeneous blending of rubber and additives, cleanliness, convenience, accuracy of weighing small quantities of accelerators etc.
28. Gum CompoundA gum compound is a curable compound without any filler. It gives the inherent properties of a raw elastomer . Gum compound virtually has no practical applications because of its very low strength.Based on the inherent properties further modifications can be takes place by adding fillers or other special purpose additives.
29. Filled CompoundThe incorporation of large amount of powdered fillers in finely divided particulate form into rubber is called loading or filling. The properties of filled rubber compounds depend in a high manner on dispersion and distribution of the filler in the rubber matrix.
30. Theory of Sulphur Vulcanization
31. Zinc oxide and fatty acid reacts to form Zinc stearate salt Complexes formed by the reaction between Zinc sterate and accelerator form Zinc salt of accelerator. Zinc salt of accelerator reacts with sulphur to form perthio-salt (active sulphureting agent) Perthio-salt reacts with the rubber hydrocarbon RH, to give a rubber bound intermediate and a perthio-accelerator group Perthio-accelerator group + Zinc oxide will form a zinc perthio-salt of lower sulphur content; this may again be an active sulfurating agent, forming intermediates XSx-1 R. In this way each molecule of accelerator gives rise to a series of intermediates of varying degrees of polysulfidity.The intermediate XSx R then reacts with a molecule of rubber hydrocarbon RH to give a crosslink, and more accelerator is regeneratedFurther, on prolonged heating, the degree of polysulfidity of the crosslinks decreases. This process is catalyzed by XSxZn SxX and can result in additional crosslinks.
32. Vulcanization systems Conventional vulcanization system (CV) Semi–efficient vulcanization system (SEV) Efficient vulcanization system (EV)Vulcanization systemSulphurAcceleratorType of crosslinkTechnological propertiesConventional Vulcanization system (CV)2.0-3.51.0-0.4Polysulfidic linkagesIncreases tensile and tear strength, high compression set and poor heat agingEfficient Vulcanization system (EV)0.3-0.86.0-2.5Monosulfidic linkages (75% ) and disulphidic (25%)Excellent heat aging, low compression set, low tensile strength and tear strength, poor flex fatigue properties.Semi efficient vulcanization system (SEV)1.0-1.72.5-1.0Mixtures of poly, di- mono sulphidic linkagesCompromise of properties in terms of strength and heat ageing
33. Peroxide Curing Curing saturated as well as unsaturated rubbers Used for curing EPDM, Chlorinated PE, Hypalon, Silicone rubbers. Examples - dicumyl peroxide, dibenzoyl peroxide etc. Curing takes place by thermal decomposition into oxy and peroxy free radicals and abstracting hydrogen atoms from the saturated elastomer to generate elastomer chain radicals. Co-activators needed for enhancing efficieny of peroxide Eg. triallyl-izo-cyanurate, diallylphtalate, ethylene glycol dimethacrylate Cannot be used with butyl rubber because they cause chain scission and depolymerisation.
34. Peroxide curing of Natural RubberPeroxide curing of EPDMEPDM CrosslinkingH-abstraction occurs on both ethylene and propylene structural units. H-abstraction from ethylene phase leads to the cross-linking and increase in molecular weightFrom propylene structural units leads to the formation of tertiary radicals, which are more prone to scission reactions, causing usually the opposite effect connected with a decrease in molecular weight.
35. AdvantagesDisadvantagesApplicable to both saturated & unsaturated rubberScorchy stock at curing temperaturesTransparent products due to the absent of sulphur that causes staining of the vulcanizates, Good colour, no metal stains and non-bloomingHigh costC-C x-link is more stable and thus has good ageing resistanceLong cure times for completion to obtain the best heat resistanceExcellent resistance to compression setIncompatibility with chemical antiozonantsNo x-link reversionExposure of rubber compound to the air (oxygen) gives a sticky rubber surface ( due to depolymerisation)Good electrical properties
36. Metal Oxide Curing Used for curing as polychloroprene rubber (CR), halogenated butyl rubber, and chlorosulfonated polyethylene rubber. Curing agents - ZnO, MgO and Pb2O3 Mixtures of (ZnO and MgO) are used because ZnO (5 phr) alone is too scorchy , MgO (4 phr) alone is inefficient. Zinc oxide and lead oxide combination for improved water resistance.
37. CR can be represented by the structure:Crosslinking agent is usually Zinc Oxide which is used along with Magnesium oxideThe reaction is thought to involve the allylic chlorine atom, which is the result of the small amount of 1,2-polymerizationA mechanism, which has been written for the vulcanization of CR by the action of zinc oxide and magnesium oxide through ether linkage
38. Resin CuringUsed for curing unsaturated rubbers, used with butyl rubber for high temperature applications. Certain di-functional compounds form crosslinks with elastomers by reacting with two polymer molecules to form a bridge. Resin cures are slower than accelerated sulphur cures and high temperatures are required, activated only by zinc oxide and halogen atoms (SnCl2 ). Resin has got adhering capacity, low molecular weight resin molecules diffuse into rubber thereby stiffen the rubber.Examples : Epoxy resins - NBR Quinone di-oximes and phenolic resins -butyl rubbers dithiols and diamines - fluorocarbon rubbers.
39. Curing bags were originally manufactured using sulphur cure (140-180 °CHigher temperature causes reversion and had very short service lives Increasing service life of curing can be achieved using PF (resole) resinsCuring gives thermally stable cross links. Curing resins used are 2,6-dihydroxymethyl-4-alkyl phenols or their condensation polymers.
40. Radiation Induced Crosslinking Physically induced chemical reaction, which is easier and preferable for continuous curing. Includes electron beam crosslinking, photo-crosslinking, microwave crosslinking, ultrasonic crosslinking etc. Upon irradiation free radicals are formed in rubber molecules. Free radicals can combine to form crosslinks as in the case with peroxide crosslinking. In radiation crosslinking of rubbers, kneaded rubber is placed in an aluminium die and is pressed at 100-200° C for 5-10 minutes, allowed to cool under pressure and then exposed to radiation. Use of sensitizers can reduce the required dose and radiation time. Examples : Halogen compounds, nitrous oxide, sulphur monochloride and bases like amine, ammonia etc.