Science of Rubber Compounding Professor Joe Greene CSU, CHICO 1 Copyright Joseph Greene 2001 Science of Rubber Compounding • • • • • • • • • Introduction Polymers Filler Systems Stabilizer Systems Vulcanization System Special Compounding Ingredients Compound Development Compound Preparation Environmental Requirements in Compounding 2 Copyright Joseph Greene 2001 • Compounding Introduction • Defined as a process of adding additives, fillers, polymers, or reinforcements to polymer materials to obtain a homogeneous polymer mixture or alloy – Modifying a rubber or elastomer or blend of polymers and other materials to optimize properties to meet a given set of performance requirements. • Materials must be environmentally safe, meet OSHA requirements, meet processing requirements, be cost effective – Compounded rubber has many unique characteristics • Dampening characteristics, high elasticity, abrasion resistance • Ingredients for rubber – – – – – Polymers (natural rubber or synthetic) Filler system- Carbon blacks, silicas, clays, calcium carbonate Stabilizer system- Antioxidents, antiozonants, waxes Vulcanization system- Sulfur, accelerators Special materials- pigments, oils, processing aids, fibers Copyright Joseph Greene 2001 3 Polymers • World usage is 15 million metric tons (1000kg) • Natural rubber is 35% • Synthetic rubber is 65%, (SBR –18%, rest is other elastomers) • Natural rubber – 75% goes to tires, 5% automotive mechanical parts, 10% non-automotive mechanical parts, 10% miscellaneous parts (medical and health related). – Available as technically specified rubbers, visually inspected rubbers, and specialty rubbers. – ASTM has 6 grades of rubber (Table I) • Six grades of coagulated technically specified natural rubber which is processed and compacted into 34-kg blocks – Rubber Manufacturers has further set of standards for 8 types of rubber Table II 4 Copyright Joseph Greene 2001 Polymers • Synthetic Elastomers – Classified by the International Institute of Synthetic Rubber Producers (IISRP) • SBR, Isoprene rubber, Polybutadiene rubber a series of numbers have been established. Table III – IISRP 1500 defines cold emulsion-polyermized (below 10C), nonpigmented SBR – 1700 series describes oil extended cold emulsion SBR. • Solution polymerized stereo elastomers is in Table IV – Tire production consumes 59% of global synthetic rubber production Table V • SBR is the largest-volume polymer (40% of total) • Polybutadiene is second • Table V, VI, VII illustrates consumption of synthetic rubber by product group 5 Copyright Joseph Greene 2001 Polymers • Synthetic Elastomers – Elastomers are described by two characteristics • Polymer macrostructure – – – – Molecular weight distribution (Figure 1) Crosslink distribution Polymer chain branching Crystallite formation Molecular Weight • Polymer microstructure – Arrangement of polymers within the chain » Cis or trans or vinyl formation – Butadiene can adopt one of three configurations Figure 2 » Vinyl (1,2) The third and fourth carbon atoms are pendent, the first and second carbon atoms participate in the polymer backbone. » Trans (1,4) The Hydrogen atoms attached to the C=C are on opposite sides » Cis (1,4) The Hydrogen atoms attached to the C=C are on the same side 6 Copyright Joseph Greene 2001 Polymers • Polymer microstructure • Relative levels of three isomers in a polymer such as PBD (PolyButadiene) can have dramatic effect on the material’s performance » 36% Cis content processes easily » 92% cis are more difficult to process, but have better abrasion resistance. » 93% trans are tough crystalline material » High vinyl in tire treads show good wet skid and wet traction performance. • Relationship between Tg and performance. – – – – – As Tg increases, Abrasion resistance drops linearly As Tg increases, Wet grip or traction improves linearly As Tg increases, Can achieve increase in vinyl level for PBD Inclusion of styrene, increases traction and loss of abrasion resistance 7 Inclusion of isoprene, increases Tg and traction, but decreases abrasion Copyright Joseph Greene 2001 Polymers • Microstructure modifications – Tan performance (ratio of G” (viscous) and G’(elastic) – Ratio of two is called phase angle and is the ratio of viscous/ elastic or G”/G’ Figure 8-5 • Tan performance versus a temperature curve run from –100 to 100°C to be segmented into zones which would characterize the tire tread compound’s performance versus temp (Table VIII) – Styrene content in SBR which is S + PBD copolymer • Increase in styrene content in SBR increase tensile strength • Increase in vinyl-butadiene drops tear strength and ultimate elongation – Solution versus emulsion polymerization • Solution SBR has a narrow MWD,lower Tg, hysteretic, and rolling resistance, but better dry traction and tread wear. • Emulsion SBR has better wet skid, wet traction, and handling • Table IX 8 Copyright Joseph Greene 2001 Polymers • For solution polymers, microstructure has a greater effect on tire tread performance – Table X illustrates impact on tire traction , rolling resistance, and tread wear where vinyl butadiene level increased from 10% to 50% • Drop in wear and increase in rolling resistance are related to Tg changes – Nitrile rubber (copolymer of acrylonitrile and butadiene) • Excellent resistance to oil absorption. Fig 3 • Poor cold flex properties so not used in cold environments • NBR breaks down in mill or bandbury – Peptides are not required though antigel agents are needed for T>140°C – Antioxidants are needed. 9 – Antiozanants and waxes are ineffective with NBR Copyright Joseph Greene 2001 Polymers • Polychloroprene polymerization – From acetylene • Acetylene is reacted to form vinyl acetylene, which is then chloronated to form chloroprene. – Can contain 85% trans, 10% cis, and 5% vinyl – High trans means it crystallizes readily. • Acetylene goes to Chloroprene monomer which polymerizes to polychloroprene – Polychloroprene can be vulcanized by ZnO or MgO. • Properties – Inferior to NBR for oil resistance but is better than natural rubber, SBR, or PBR – OR from Butadiene 10 Copyright Joseph Greene 2001 Polymers • Butyl rubber polymerization – From isobutylene and isoprene • Isobutylene and isoprene are in ratio of 50:1 • Chlorobutyl rubber and bromobutyle rubber are produced by the halogenation of butyle rubber. • Use FEF or GPF grade carbon blacks • Vulcanization based on thiazole accelerators and TMTD accelerators – Properties • Highly impermeable to air and very low water absorption • Good heat and ozone resistance. – Table XI Nomenclature for selected elastomers – EPDM crosslinking mechanisms Copyright Joseph Greene 2001 11 Filler Systems • Fillers are added to rubber to meet material property targets, I.e., tensile strength and abrasion resistance – Carbon black technology is complex and an extensive range of carbon blacks are available. Table XII • Carbon black properties. – Particle size, surface area, particle size distribution, structure (aggregates), surface activity (chemical functional groups). – Key properties are listed in Table XIII • Iodine number: Measure of surface area (particle size). The higher the iodine number, the smaller the particle size. • Tint: Optical absorbance, which increases with smaller particles • CTAB: Specific surface area measurement corrected for the effect of microspheres 12 Copyright Joseph Greene 2001 Carbon Black Filler Systems • Carbon Black Terms – Furnace Carbon Black: Class of carbon blacks produced by injection of defined grades of petroleum feedstock into a high velocity stream of combustion gases under a set of defined conditions, e.g., N110 to N762 – Thermal Carbon Black: Type of carbon black produced by thermal decomposition of hydrocarbon gases, e.g., N990, N991 – Microstructure: Carbon black microstructures describe the arrangement of carbon atoms within a carbon black particle – Particle: Small spherical component measured by electron microscopy. – Aggregate: Distinct, colloidal mass of particles in its smallest dispersible unit. – Agglomerate: Arrangement or cluster of aggregates. – Structure: Measure of deviation of carbon black aggregates from spherical form. – Iodine Number: Weight in grams of iodine absorbed per kilogram of carbon black. Measure of particle surface. Smaller particle size, greater the iodine No. – Carbon Black DBP: Volume of dibutyl phthalate in cm3 absorbed by 100 g of carbon black. DBP number is measure of structure of carbon black aggregates. – Tint: Tint is a ratio of the reflectance of a reference paste to that of a sample paste and increases with smaller particle size. 13 Copyright Joseph Greene 2001 Carbon Black Filler Systems • Carbon Black Terms – CTAB: Measure of the specific surface area corrected for the effect of microspores. CTAB (cetyltrimethylene ammonium bromide) is excluded from the smaller interstices and thus better represents the portion of a particle surface area in contact with polymer. – Nitrogen surface area: Measure of total particle surface area, possible due N2 gas being able to cover the full surface including pores without interface from surface organic functional groups. – Compressed DHP: Same as DHP but after the sample under goes a series of compressions (4x to 24klbs). Simulates compounding. – Pellet: Mass of compressed carbon black formed to reduce dust. – Fines: Quantity of dust present in a pelletized carbon black. – Pellet hardness: Measure of the load in grams to crush pellets. – Ash: Residue after burning black at 550C for 16 hours. – H2 and O2 content: Residual remaining after carbon is produced; will be in the form of phenolic functional groups and effect on vulcanization kinetics and reinforcements potential of carbon black. 14 Copyright Joseph Greene 2001 Carbon Black Filler Systems • Carbon black empirical property guides – Increase in carbon black aggregate size or structure • Improves cut growth and fatigue resistance – Decrease in particle size • Increases in abrasion resistance and tear strength, drop resilience, and hysteresis and heat buildup. – Decrease in carbon black loading (percentages) • Lowers tire rolling resistance – Increase in black fineness raises rolling resistance and traction – Figure 4. Effect on carbon black level on properties • Increase Black level – Increases in compound heat buildup, harness, rolling resistance, traction – Tensile strength, processability, and abrasion resistance go through a maximum then decrease and properties deteriorate. 15 Copyright Joseph Greene 2001 Carbon Blacks Product Summary • EXTENDERS, FILLERS, REINFORCMENTS – ASTM No. FUNCTION & COMPOUNDING N110 Gives maximum abrasion resistance, highest reinforcement and tensile. Used in off-the-road tire treads, tread rubber, bridge pads and conveyor belts. N121 A high structure N-100 series carbon black giving maximum treadwear but dispersing as easily as N200 series. N134 Provides superior abrasion resistance. Used in truck and passenger tire treads. N220 Provides excellent abrasion resistance, high tensile, good tear properties, moderate electrical conductivity. Used in truck and passenger tires, and mechanical goods. N231 Low structure, high abrasion resistance, used mainly in tires where resistance to tear is important. N234 Provides superior abrasion resistance vs. N220. Excellent wear and extrusion properties typical of improved process high structure blacks. Used in all elastomers, especially SBR/BR blends for treads and tread rubber. http://www.sidrich.com/carbon/carbon.asp N299 Provides abrasion resistance superior to N339, N375, N220 and comparable to N234. Used in truck and passenger tires, tread rubber and mechanical goods. Copyright 16 Joseph Greene 2001 Carbon Blacks Product Summary • EXTENDERS, FILLERS, REINFORCMENTS – ASTM No. FUNCTION & COMPOUNDING N343 N326 Provides superior wear and processability. Similar to N339 but with higher structure. Used in passenger and truck treads. Low structure, high reinforcement, high tensile strength, very resistant to tear and chipping in natural rubber compounds. Used in mechanical goods, tire carcasses, belts and off the road tread. N347 N330 Used in NR, SR. Provides excellent processing characteristics and permits use of high oil loading with low die swell. Better hysteresis properties than improved treadwear versions. Used in treads, tread rubber and mechanical goods. Provides good abrasion resistance with high resilience, easy processing, good tensile and tear properties. Wide range of applications in natural rubber and synthetic rubber compounds for both tires and mechanical goods. N351 Provides good abrasion resistance in the N330 range. High structure, high modulus black with easy processing characteristics. Excellent dynamic properties. Used in the tire tread and carcass compounds and mechanical goods. N339 Provides superior wear and cut growth resistance vs. N347. Excellent wear and extrusion properties typical of improved process high structure blacks. Used in passenger treads, tread rubber and mechanical goods. N358 Extra high structure. This can accommodate more oil than normal for equivalent modulus and hardness. Used in tire treads, tread rubber and mechanical goods. http://www.sidrich.com/carbon/carbon.asp 17 Copyright Joseph Greene 2001 Carbon Blacks Product Summary • EXTENDERS, FILLERS, REINFORCMENTS – ASTM No. FUNCTION & COMPOUNDING N762 N550 Moderate reinforcer, gives high resilience and excellent dynamic properties, low hysteresis, high loadings obtainable. Used for extruded goods, belts and hoses, and molded goods. Also used in plastic color concentrates. Imparts medium abrasion resistance, high strength, low shrinkage and die swell. Provides fast, smooth extrusions and relatively high resilience. Used in tire carcasses, cushion gum, tubing, cable jacketing, plastic conduit compounds and in extruded goods requiring excellent dimensional stability. N772 N650 High structure, medium reinforcement gives low die swell, smooth extrusion and easy processing plus economical compounding. Used in tire carcasses and sidewalls, cable jackets, inner tubes and hoses. N660 Moderately reinforcing grade. Excellent general purpose black. Provides good reinforcement, low heat generation, smooth processing to all elastomers. Used in tire carcass sidewalls and mechanical goods. Moderate reinforcer, gives high resilience and excellent dynamic properties, used in tire carcass and mechanical goods. N774 Moderate reinforcer, gives high resilience and excellent dynamic properties, low hysteresis, high loadings obtainable. Used in tire carcasses and sidewalls, extruded goods, belts and hose. Also used in plastics color concentrates. Note: The above information should be used only as a guideline. http://www.sidrich.com/carbon/carbon.asp 18 Copyright Joseph Greene 2001 SID RICHARDSON CARBON CO. PRODUCT SUMMARY FOR CARBON BLACKS • Typical Properties http://www.sidrich.com/carbon/summary.htm 19 Copyright Joseph Greene 2001 SID RICHARDSON CARBON CO. PRODUCT SUMMARY FOR CARBON BLACKS • Typical Properties 20 Copyright Joseph Greene 2001 Silica Filler Systems • Silica and Silicates – Silica increases tear strength, reduces heat buildup, increases compound adhesion in tires. – Fundamental properties • Ultimate particle size- do not provide the same level of reinforcement as carbon black, though the properties can be increased with coupling agents. Size from 1 to 30 nm. Surface area from 20 to 300 m2/g • Extent of hydration- Surface hydration caused by water vapor absorption is affected by surface silanol concentration. High levels of hydration can adversely affect properties. Hydroscopic nature of silica. Table XV • Secondary properties: pH, chemical composition, and oil 21 absorption. Copyright Joseph Greene 2001 Silica Filler Systems • Silica and Silicates – Chemistry of silicas • Silica is amorphous and consists of silicon and oxygen arranged in a tetrahedral structure of a 3-D. Particle size from 1 to 30 nm. Surface area from 20 to 300 m2/g. No long-range crystal order only short range ordered domains in random arrangment with neighboring hydration. • Surface silanol (-Si-O-H) concentration influence the degree of surface hydration. • Silanol has 3 types (Figure 5)_ isolated, geminal (two –OH hydroxyl groups ion the same silicon atom), and vicinal (on adjacent silicon atoms). • Surface acidity is controlled by the hydroxyl groups on the surface of the silica and is intermediate between those of P-OH and B-OH. Intrinsic acidity can influence peroxide vulcanization, though in sulfur no effects. • Surface hydration is affected by surface silanol concentration. • Above 5 parameters apply equally to other silicates, Calcium, aluminum, magnesium, and sodium aluminum silicates. – Reduction in silanol level as a result of increased in absorbed water will decrease cure time, tensile strength, and abrasion resistance. 22 Copyright Joseph Greene 2001 Silica Filler Systems • Silica and Silicates – Produce greater reinforcements in more polar elastomers, NBR and CR, than in nonpolar polymers, SBR and NR. • Silane coupling agents improve properties with NR and SBR. – Coupling agents need to be bifuntional, capable of reacting with both silica and the polymer via vulcanization or crosslinking – Advantages of Silica over carbon black in rubber formations • Reduction in heat build-up and improvement in tear strength, cut, chip, and chucking resitance. • When loadings approach 20%, the drop in abrasion resistance renders the formulation no longer practical. – Silane coupling agents offer potential to mitigate drops in 23 properties. Copyright Joseph Greene 2001 Silane Coupling Agents • Silane Coupling Agents – Coupling agents are bifunctional, with one part of the molecule bonding to the polymer and the other part of the molecule bonding to the filler or rinforcement • Soap has bifunctionality to remove grease from your hands with water. One part disloves grease (non-polar) and the other part disolves in water (polar) – Common commercial agents • Mehtoxysilane (A189), ehtyltolylenepolysulfide (Y9194) propyltetrasulface (TESPT) • TESPT a bifunctional polysulfidic organosilane– Introduced to improve the reinforcements of silica in rubbers 24 Copyright Joseph Greene 2001 Silane Coupling Agents • Advantages in rubbers with silica • • • • • Lowers heat buildup and hysteresis in silica-loaded compounds Increases 300% modulus and tensile strength Improves reinforcing effect of clays and whiting Serves as a reversion resistor in equilibrium cure systems Improves DIN abrasion resistance – Mechanism of silane agent compromises two phases • Silanization reaction in which coupling agents react with silica – Occurs readily and reaction is fast and required high T (150°C) • Formation of crosslinks between the modified silica and polymer. • Other Filler Systems – Kaolin clay (hydrous aluminum silicate), mica (K Al Silicate), talc (Mg Silicate), limestone (Ca CO3), and titanium dioxide. – Properties of clay can be increased with the addition of silane 25 coupling agents. Copyright Joseph Greene 2001 Stabilizers Systems • Unsaturated nature of elastomers leads to – their unique properties. – Susceptible tot attacks by oxygen, ozone, and thermal degradation • Degradation of elastomers – Oxidation- accelerated by heat, heavy metal contamination, sulfur, light, moisture, welling in oil and solvents, dynamic fatigue, and ozone. – Variables in compounding that resist degredation • Polymer type, • Cure system, • Antidegradant system Copyright Joseph Greene 2001 26 Stabilizers Systems • Thermo-oxidant stability is a function of vulcanization system. – Peroxide cure systems tend to perform best for reversion resistance – Low sulfur level, high acceleration level and sulfur donor • Show good heat stability and oxidation resistance. • Show poor fatigue resistance – High sulfur level, low acceleration level and sulfur donor • Show poor heat stability and oxidation resistance. • Show good fatigue resistance • Oxidation proceeds with two mechanisms – Crosslinking break down: Breaks down into monosulfidic crosslinks. Compound hardness and stiffness increases, fatigue resistance decreases. SBR, EPDM, NBR, and CR show this. – Chain scission: Polymer chain breaks, causing softening of the compound and decreased abrasion resistance. Natural Rubber 27 Copyright Joseph Greene 2001 Stabilizers Systems • Materials – Polymer blends which the constituents are incompatible tend to improve fatigue resistance. • Natural rubber and PBD show good resistance because of formation of distinct polymer phases; a crack growth in one phase is arrested at the boundary with adjacent polymer phase – Natural rubber and PBD blends tend to be used in tire sidewalls which undergo flexing and in tire treads which have a lug pattern and contain high stress points. – Addition of antidegradants becomes important in order to protect the elastomers in a range of environmental exposures. 28 Copyright Joseph Greene 2001 Stabilizers Systems • Antioxidants- Oxidation of the polymer breaks down long chain molecules – More severe at elevated temperatures – Primary antioxidants: terminates reactions (phenolic, amine) – Secondary antioxidants: neutralizes reactive materials (phosphite, thioesters) – Susceptible Materials: PP and PE oxidize readily • Antistatic– agents attract moisture, causing the surface to be more reactive, dissipates charges 29 Copyright Joseph Greene 2001 Stabilizers Systems • Flame retardants – Based on combinations of bromine, Cl, antimony, boron, and phosphorous – Many emit afire-extinguishing gas when heated – Others swell or foam to form a insulating barrier against heat and flame. – Alumina trihydrate (ATH) emits water • Heat Stabilizers – Retard thermal decomposition for PVC – Based on lead and cadmium in past. 28% Ca pollution came from plastics – New developments based on barium-zinc, Ca-zinc, MgZinc, etc.. 30 Copyright Joseph Greene 2001 Stabilizers Systems • Plasticizers – Chemical agent added to increase flexibility, reduce melt temperature, and lower viscosity – Neutralize Van der Waals’ forces – Results in leaching for • Food contamination • Reduced impact and reduced flexibility, PVC hoses • Over 500 different plasticizers available – Examples: Dioctyl phtalate (DOP), di-2-ethylhexyl phthalate (carcinogenic in animals) Anitdegradant use 31 Copyright Joseph Greene 2001 Stabilizers Systems • Preservatives – Protects plastic (PVC and elastomers) against attacks by insects, rodents, and microorganisms – Examples • Antimicrobials, mildewicides, fungicides, and rodenticides • Processing Aids – Antiblocking agents (waxes) prevents sticking • Tables XVI and XVII – – – – Emulsifiers lowers surface tension. Detergents and wetting agents (viscosity) Solvents for molding, painting, or cleaning Oils are processing aids can be paraffinic, napthanic, and 32 aromatic. Table XXII Copyright Joseph Greene 2001 Stabilizers Systems • Foaming/Blowing agents – Used to make polymers with a cellular structure – Physical foaming agents: decompose at specified temperatures and release gasses. – Chemical foaming agents release gasses due to a chemical reaction – Chlorinated fluorocarbons (CFC) were efficient foaming agents for polyurethanes. – Hydrochlorofluorocarbone (HCFC) relaced CFC with 2 to 10% ozone deletion rate – For thermoplastics, chemical blowing agent, azodicarbonamide produces cellular HDPE, PP, ABS, PS, PVC, and EVA 33 Copyright Joseph Greene 2001 Stabilizers Systems • Antidegradant Use • Discoloration and staining – Phenolic antioxidants are nondiscoloring – Amines (preferred) are discoloring • Volatility – Higher Mw of the antioxidant, the less volatile it will be » Hindered phenols are highly volatile compared to amines – Correct addition of antioxidants in mix is critical for less loss • Solubility – Low solubility of antidegradant causes material to bloom at surface with loss of protection for the product. » Must be soluable up to 2.0 phr, migrate to surface, but not soluble in water or solvents, e.g., hydraulic fluid. • Chemical Stability – Stable against heat, light, oxygen, and solvents • Concentration – Most have optimum concentration for max effectiveness. • Environmental, health, and safety – Avoid dust and inhalation, antidegradants should be dust free Copyright Joseph Greene 2001 34 Stabilizers Systems • Guidelines for antidegredant (pg448) 1. Short-term static protection- paraffinic waxes 2. Long term ozone protection with Microcrystalline waxes provide 3. Static ozone protection requires a Critical level of wax bloom to form protective film 4. Long term product protection under static and dynamin applications and over a range of temperatures requires optimized blends of waxes and PPDs. 5. Avoid excess levels of wax bloom that can cause detrimental effects on fatigue resistance, because the thick layer of wax can crack under strain and the crack 35 can propagate in Copyright product. Joseph Greene 2001 Vulcanization Systems • Activators – Zinc oxide and stearic acid are used (2 – 5 phr) – Tables XVII and XVIII • Coupling agents– Promotors of surface adhesion between dissimilar materials, e.g., glass and polymers. Silane and titanate • Curing agents- chemicals that cause crosslinking. – Inhibitors used to establish shelf life – Catalyst (hardeners) start reactions. Organic peroxides used to cross-link elastomers as well as Benzoyl peroxides and MEK. – Promoters or accelerators speed reactions up, e.g., cobalt 36 naphthanate. Copyright Joseph Greene 2001 Vulcanization Systems • UV Stabilizers – Plastics susceptible to UV degredation are • Polyolefins, polystyrene, PVC, ABS, polyesters, and polyurethanes, – Polymer absorbs light energy and causes crazing, cracking, chalking, color changes, or loss of mechanical properties – UV stabilizers can be • Carbon black, 2-hydroxy-benzophenones, 2-hydroxy-phenylbenzotrizoles • Most developments involve hindered amine light stabilizers (HALS) • HALS often contain reactive groups, which chemically bond onto the backbone of polymer molecules. This reduces 37 migration and volatility. Copyright Joseph Greene 2001 Vulcanization Systems • Vulcanizing Agents – Sulfur • Rhombic sulfur is most common (soluble at levels up to 2phr) • Above 2phr sulfur migrates to surface yielding sulfur bloom – Insoluble sulfur • Polymeric form of sulfur with MWD of 100,000 to 300,000 • At curing temperatures, sulfur reverts to rhombic S8 sulfur • Available in sulfur/oil mater batch. – Peroxides • (Chap 7) Most peroxides are available as a – liquid (90% - 98% active), as – powders (40% - 50% active), or as – pastes made from silicone fluids and gums (20% - 80% active) to facilitate handling and dispersion 38 Copyright Joseph Greene 2001 Vulcanization Systems • Accelerators – Increase the rate of sulfur crosslinking and the crosslink density. – Classified by two techniques• Rate of vulcanization- Rated as ultra accelerators, Semiultra-accelerators, fast accelerators, medium-rate system, and slow accelerators • Chemical classifications- Fall into eight groups: Aldehydemines, Dithiocarbamates, Guanidines, Thioureas, Sulfenamides, Thiazoles, Thiurams, and Xanthates – Organic chemical accelerators were not used until 1906 – Aniline was used with sulfur (Chapter 7) • Too toxic for use in rubber products – Other common accelerators • Carbon disulfide, thiocarbanilide, guanidine, aliphatic amines • MBT and MBTS • Accelerated-sulfur vulcanization is used for – NR, Isoprene rubber, SBR, NBR, butyl rubber (IIR), Chlorobutyle 39 rubber (CIIR), bromobutyle rubber (BIIR), and EPDM Copyright Joseph Greene 2001 Vulcanization Systems • Retarders and Antireversion Figure 9 – Induction time or scorch resistance of a compound can be improved by addition of a retarder. Most common is CTP. – Antireversion, especially for natural rubber, allows • Faster processing, higher temperatures, and extension of service life. • Most common: Monosulfidic and polysulfidic; ACS1 and ACS2- Table 12 • Delayed Action Accelerated Vulcanization • Delays can be beneficial in some rubber manufacturing • Result of a quenching action by the monomeric polysulfides formed by reactions between accelerator and sulfur • If the crosslink precursers are rapidly quenched by an exchange reaction before they form crosslinks, the crosslink formation is impeded. Scheme 4. • Role of Zinc in Benzothiazole Accelerated Vulcanization – An increase in Zn2+ from an increase in fatty acid, • causes an increase in overall rate in the early reactions (during the delay period) which lead to the formation of rubber-Sx-Ac. 40 of the • Causes a decrease in the rate of crosslink formation but an increase in the rates early reactions. Copyright Joseph Greene 2001 Compound Development • Simple steps to compound elastomeric formulations with a variety of materials from previous slides. – Model formulations are obtained from material supplier. – Optimization is required to determine acceleration, carbon black, fatty acid level, etc.. To achieve max tensile strength. • Design of experiments will help achieve this….Taguchi DOE – Cause and effect for material ingredients on mechanical performance. – Examples of Tire compounds- Tables 25-30 for optimum levels • Compound preparation – Rubber compounds are prepared in internal mixers. • Internal mixers generate high shear forces that disperse the fillers and raw materials into an uniform, quality compound. • After mixing it is dropped onto a mill or extruder or pelletizer. • Temperatures and times are recorded for each step in the mix process.41 • Heat history, power consumption, etc. are monitored Copyright Joseph Greene 2001 Environmental Requirements • Environmental impact is needed for rubbers. • Life cycle • Product use energy requirements: rolling resistance and fuel economy – Improvements in hysteretic properties will improve fuel economy – Ability to maintain air pressure yields longer life rubber tires • Long term ecological implications:disposal methods • Health and safety in product and product manufacture – Radial tires use 60 to 80% natural rubber as the polymer portion which comes from renewable resource versus a petroleum based material from polymers. – Tires are very durable product of fabric, steel, carbon black, natural rubber, and synthetic polymers. – Tires are not biodegradable nor recycleable. Current ways to convert the waste » Burn tires are fuel: Caloric energy of tires is higher (35 MJ/kg) than that of coal (24 MJ/kg). Tires can burn to produce heat in cement kilns. » Burn tires in furnaces at power generating facilities to produce electricity. » Use tire scrap in asphalt from ground up scrap tires » Construction industry for tires for marine reefs, energy efficient house construction, highway bank reinforcement, and erosion control. 42 Copyright Joseph Greene 2001