Vulcanization

1. Vulcanization Professor Joe Greene CSU, CHICO Copyright Joseph Greene 2001

2. Vulcanization • Definition of Vulcanization • Effects of Vulcanization on Vulcanizate Properties • Characterization of the Vulcanization Process • Vulcanization by Sulfur • Vulcanization by Phenolic Curatives • Vulcanization by the Action of Metal Oxides • Vulcanization by Peroxides • Dynamic Vulcanization Copyright Joseph Greene 2001

3. Definition of Vulcanization • Unvulcanized rubber products are not very strong and have the consistency of chewing gum. • Charles Goodyear invented the first recognizable method of vulcanization. • Heating natural rubber with sulfur in 1841 • Both natural and synthetic rubber are vulcanized today • 90% of all vulcanization occurs with sulfur of natural rubber, ethylene-propylene-diene (EPDM), butyl rubbers, and nitrile rubber. Copyright Joseph Greene 2001

4. Definition of Vulcanization • Vulcanization is a process (Fig 1) • Which increases elasticity and reduces plasticity by the formation of a crosslinked molecular network. • Of chemically producing network junctures by the insertion of crosslinks between polymer chains • Crosslinks may be a group of sulfur atoms in a short chain, a single sulfur atom, C-C bond, polyvalent organic radical, ionin cluster, polyvalent metal ion. • Which occurs by heating the rubber and vulcanizing agents under pressure • Supporting polymer chain is a linear polymer molecular segment between network junctures. Copyright Joseph Greene 2001

5. Effects of Vulcanization on Rubber • Vulcanization causes profound chemical changes • Long rubber molecules (MW between 100,000 and 500,000) become linked together with junctures (crosslinks) spaced along the polymer chains • Rubber becomes essentially insoluable in any solvent and can not be processes by means which requires it to flow. • E.g., mixer, extruder, mill, calender, forming, or shaping • Usually the crosslinked rubber is die cut to final part shape. • Mechanical property changes • Increases tensile modulus (static or standard tensile test pulling) • Increases slightly the Dynamic modulus (found from sinusoidal pulling and pushing on sample) since it is a measure of the viscous and elastic behavior of rubber. Copyright Joseph Greene 2001

6. Effects of Vulcanization on Rubber • Vulcanization causes Mechanical property changes • Fig 2: Tensile strength, Tear strength, hysteresis, elastic recovery stiffness. • Increases tensile modulus and Dynamic modulus • Hysteresis is reduced with increased crosslink formation • Hysteresis the amount of plastic stretch that does not recover to final state. It is a measure of the deformation energy which is not stored but converted to heat. • Vulcanization causes a trade-off between elastic and plastic deformations. • Increases Tear strength, Fatigue life, and Toughness with small amounts of crosslinking, but are reduced with additional links. • Reversion is a loss of network structures by thermal aging • Result to isoprene rubbers vulcanized by sulfur that is vulcanized too long • Most severe at temperatures above 155°C. • Can result to SBR if it is over cured. Copyright Joseph Greene 2001

7. Vulcanization Process • Vulcanization Process (Fig 3) • Mixing: Raw rubber, sulfur, accelerators, fillers, preservatives, etc. according to a recipe. • Important process characteristics • Time elapsed before crosslinking starts • Need sufficient delay (scorch resistance or resistance to vulcanization) before crosslinking starts to permit mixing, shaping, and forming of product. • Rate of crosslinking formation once it starts • Need to have rapid crosslinking to minimize cycle time • Extent of crosslinking at end of process • Need to be controlled to get the proper amount of crosslinking. Copyright Joseph Greene 2001

8. Vulcanization Process • Scorch Resistance (Fig 4) • Scorch resistance is measured by the time at a given temperature required for the onset of crosslinking • Results in an abrupt increase in viscosity • Mooney viscometer is usually used. • A sample of test material is placed above and below the rotor and the heated platens then closed under pressure. • The rotor rotates at a constant speed of two revolutions per minute and the torque exerted on the rotor head is measured and recorded. • The amount of torque is the resistance to flow (definition of viscosity) Viscosity Time Ref: http://www.gesilicones.com/silicones/americas/ Copyright Joseph Greene 2001

9. Vulcanization Process Torque Vulcanization Viscosity Curve • 1.Preheat 2.Initial Torque 3.Min Torque 4.Structure 5.Scorch Time 6.90% Cure 7.MaxTorque Overcure Reversion Rubber • Rate of vulcanization after scorch period • Measured by devices called cure meters. And are oscillating disk rheometer • Oscillation of a biconical disk embedded in the rubber specimen confined • A heated square cavity exerts a sinusoidal shear strain on the specimen. The force (torque) needed to oscillate the disk is directly proportional to the stiffness (shear modulus) of the specimen. As the specimen cures, modulus increases, and torque is recorded as a function of time yielding the following characteristic curve: Ref: http://www.gesilicones.com/silicones/americas/ Copyright Joseph Greene 2001

10. Vulcanization Process • Rate of vulcanization and extent of cure • The Vulcanization Viscosity Curve gives a rather complete picture of the overall kinetics (reaction) of crosslink formation and crosslink disappearance (reversion) for a given rubber mix. • Note: Reversion occurs if cured too long. • In some cases, instead of reversion, a long plateau or marching cure occurs. • Each rubber must be tested to determine the viscosity-cure profile that will occur in the die when producing the product. • The cure meter and viscosity test is used to control the quality and uniformity of rubber stocks. • The cure temperature profile is measured for viscosity at different temperatures for a rubber mix. The profile will predict how well the part will mold. • The temperature and mix materials are adjusted to get a better molded product. • Following curve Copyright Joseph Greene 2001

11. Vulcanization Process • Cure Profile versus Cure Temperature Ref: http://www.gesilicones.com/silicones/americas/ Copyright Joseph Greene 2001

12. Vulcanization Process • Silicone Rubber • Silicone Rubber is a specialty synthetic elastomer that provides a unique balance of chemical and mechanical properties required by many of today's more demanding industrial applications. From its original development in the 1940's using a laboratory Grignard process, to its final commercial form today, silicone rubber excels in such areas as: • High temperature stability • Low temperature flexibility • Chemical resistance • Weatherability • Electrical performance • Sealing capability • In addition, because of its relative purity and chemical makeup, silicone rubber displays exceptional biocompatibility which makes it suitable for many health care and pharmaceutical applications. Ref: http://www.gesilicones.com/silicones/americas/ Copyright Joseph Greene 2001

13. Silicone Rubber Commercial Preparation • Most silicone products including fluids, RTV's, and rubber are derived from the same chemical starting materials and are later differentiated by viscosity or degree of polymerization. • The process begins with the reduction of silica (sand) to elemental silicon metal which is then mechanically ground and reacted with methyl chloride at 300ºC in the presence of a copper catalyst. • This results in the formation of reactive methylchloro silanes which are fractionally distilled and separated into their mono, di, and tri counterparts. • Note that the dichloro species is most important for forming long linear polymer chains since its bifunctionality allows it to “grow” chemically in two dimensions. • After distillation, the dimethyldechlorosilanes are hydrolyzed to form silanols which rapidly condense to cyclic siloxanes and low molecular weight linear siloxanes. The latter are reacted with caustic to produce cyclic siloxanes, specifically dimethyl tetramer or D4 which is the primary input for all dimethyl silicone rubber polymer and which is a clear, low viscosity liquid. Ring opening polymerization of the cyclic D4 is then accomplished via a strong base resulting in linear polymer whose molecular weight (viscosity) is controlled by the addition of monofunctional siloxanes which function as chain stoppers. Ref: http://www.gesilicones.com/silicones/americas/ Copyright Joseph Greene 2001

14. Silicone Rubber Commercial Preparation Ref: http://www.gesilicones.com/silicones/americas/ Copyright Joseph Greene 2001

15. Silicone Rubber Vulcanization • Traditional curing agents for silicone rubber compounds are organic peroxides which, when heated, decompose to form free radicals that react with the pendant organic groups on the silicone polymer. • This results in crosslinks between the polymer chains, the number and distribution of which greatly influence the final physical property profile of the cured rubber. • Cure time is a function of the activation temperature of the particular peroxide and the thickness of the part. • The crosslinking mechanism is illustrated in Figure 8 for both methyl and vinyl side groups. • The higher reaction rate of the vinyl group responsible for its importance to crosslink density and cure rate. • Organic peroxides fall into two broad categories according to their ability to crosslink just vinyl groups or both methyl and vinyl groups. The dialkyl peroxides such as dicumyl peroxides fall into the former category and are termed “vinyl specific” while the diacyl peroxides such as benzoyl peroxide fall in the latter category. Ref: http://www.gesilicones.com/silicones/americas/ Copyright Joseph Greene 2001

16. Silicone Rubber Vulcanization • 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. • Typical Curing Agents • Peroxide Commercial Grades Form % Typical Molding Temperature Ref: http://www.gesilicones.com/silicones/americas/ Copyright Joseph Greene 2001

17. Fig 8 Ref: http://www.gesilicones.com/silicones/americas/ Copyright Joseph Greene 2001

18. Silicone Rubber Vulcanization • Figure 9 lists the organic peroxides commonly used to cure silicone rubber with recommended cure temperatures and general application areas. Ref: http://www.gesilicones.com/silicones/americas/ Copyright Joseph Greene 2001

19. Silicone Rubber Vulcanization • Figure10 is a further checklist to differentiate the use of diacyl and dialkyl peroxide types. Copyright Joseph Greene 2001

20. Vulcanization by Sulfur Without Accelerator • Initially vulcanization was accomplished by using elemental sulfur at a concentration of 8 phr • Required 5 hours at 140C • Addition of zinc oxide reduced time to 3 hours • Addition of accelerators (0.5phr) reduced time to 1-3 min • Elastomer vulcanization by sulfur without accelerator has no commercial significance • Exception is 30 phr of sulfur with no accelerator to produce hard rubber or ebonite • Still important to understand chemistry Copyright Joseph Greene 2001

21. Vulcanization by Sulfur Without Accelerator • Chemistry of in accelerated vulcanization is controversial • Many slow reactions occurr over the long period of vulcanization. • Scheme 1 with free radicals • Scheme 2 with ions and intermediates giving both saturated and unsaturated products with sulfur atoms connected to both secondary and tertiary carbon atoms Copyright Joseph Greene 2001

22. Accelerator Vulcanization by Sulfur • Organic chemical accelerators were not used until 1906 (65 years after the Goodyear development • Aniline was used with sulfur (Figure 7) • Too toxic for use in rubber products • Other common accelerators • Carbon disulfide, thiocarbanilide, guanidine, aliphaitc amines • MBT and MBTS • Table 1 • Accelerated-sulfur vulcanization is used for NR, Isoprene rubber, SBR, NBR, butyl rubber (IIR), Chlorobutyle rubber (CIIR), bromobutyle rubber (BIIR), and EPDM Copyright Joseph Greene 2001

23. Accelerator Vulcanization by Sulfur • Vulcanization recipe contains • 2-10phr zinc oxide (activator), 1-4 phr of fatty acid _stearic as an activator, and 0.5 to 2phr accelerator. • Fatty acid and zinc oxide fors a salt which can form complexes with accelerators and sulfur. • Different types of accelerators impart vulcanization characteristics which have different results for scortch resistance and crosslinking rate • Figure 10 Copyright Joseph Greene 2001

24. Accelerator Vulcanization by Sulfur • Chemistry Scheme 3 • Accelerator reacts with sulfur to give monomeric polysulfides of the structure Ac-Sx-Ac, • where Ac is an organic radical derived from the accelerator • Monomeric polysulfides react wit hrubber to form polymeric polysulfides • MBT is formed if the accelerator is a benzothiazole derivative and the rubber is NR • In SBR, the MBT becomes bound to the elastomer, molecular chain probably as the thioether rubber-S-Ac. • In NR, MBT is the accelerator and it first disappears and then reforms with the formation of rubber-Sx-Ac • Finally, rubber polysulfides react to give crosslinks. • Accelerated vulcanization versus unaccelerated vulcanization • Accelerated vulcanization leads to greater crosslinking efficiencies and rates Copyright Joseph Greene 2001

25. Accelerator Vulcanization by Sulfur • 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. • Causes a decrease in the rate of crosslink formation but an increase in the rates of the early reactions. Copyright Joseph Greene 2001

26. Accelerator Vulcanization by Sulfur • Achieving Specified Vulcanization Characteristics • Early days it was difficult to independently control the two main vulcanization characteristics, i.e., scorch resistance (for processing safety) and rate of crosslink formation. • For NR (natural rubber) with a fast accelerator system to obtain short curing time in the press and high crosslink formation, the scorch time was short. • If a delayed action accelerator system was chosen to get more scorch time, then the rate of high crosslink is much less and long cycle times result. • Development of an inhibitor (CTP) improved this and allowed for effective scorch resistance with a high degree of crosslinking. • Thus the rate of crosslink formation can be adjusted by the selection of accelerators • For example, moderately fast delayed-action accelerator (TBBS) can be replaced with small amounts of a coaccelerator (TMTD) to obtain greatly increased cure rate and a reduced scorch time, which can be increased with CTP, an inhibitor. • For synthetic rubbers, SBR or BR, the effects of cure system changes may not be as pronounced as with NR. Copyright Joseph Greene 2001

27. Accelerator Vulcanization by Sulfur • Effects on Adhesion to Brass-plated steel • Adhesion between rubber and brass-plated steel (steel tire cords) is due to an interfacial layer of sulfides and oxides of copper and is controlled by the vulcanization process. • Optimization of the vulcanization system with respect to adhesion is critical. • Change in composition of brass coating on the steel wires or a change in thickness of the brass coating would require a change in the vulcanization. • The copper sulfide film be completely formed before crosslinking starts. • Adhesion can be improved by the use of crosslink inhibitors (CTP) • Effects on Vulcanization Properties • Increase in sulfur and accelerator concentrations give higher crosslink densities and therefore higher moduli, stiffness, hardness • Figure 11 and Figure 12 Copyright Joseph Greene 2001

28. Accelerator Vulcanization by Sulfur • Effects on Vulcanization Properties • The ratio of sulfur to accelerator is important • An increase in sulfur and accelerator improve modulus, and ultimate elongation. An increase increase in the ration of sulfur/accelerator improves fatigue life. To high a ratio can lead to reduced fatigue life • Selection of proper ratio higher modulus can be obtained with at least some optimization of fatigue life. • Fatigue life is the repeated loading of a rubber at constant strain or strain energy until failure.Table II and Table III • Different recipes with various amounts of antidegradants, fillers, base polymers, and sulfur/accelerator ratios give rise to different fatigue results. • Natural rubber vulcanized by high levels of accelerators and low levels of sulfur have been called EV (efficient vulcanization) since the sulfur is used efficiently in the production of crosslinks. • Crosslinks are shorter than conventional vulcanization, have poor fatigue, but have excellent resistant to reversion. Copyright Joseph Greene 2001

29. Accelerator Vulcanization by Sulfur • Accelerated sulfur vulcanization of unsaturated rubbers. • Most of the research work concerned natural rubbers • Chemistry of vulcanization of synthetic rubbers (BR, SBR, and EPDM) is similar to NR • Before crosslinking, the rubber is first sulfurated by accelerator-derived polysulfides (Ac-Sx-Ac) to give macromolecular polysulfidic intermediates (rubber-Sx-Ac). • Unlike NR where MBT is given off, for BR and SBR, MBT is not eliminated and remains bound to the rubber. • Crosslink sulfurinc rank is not as sensitive to the S/Ac as is for NR. • Reversion (loss of crosslinks during vulcanizate aging) is a problem for both NR and synthetic isoprene rubber. • It occurs only in severe conditions for SBR • Table IV Copyright Joseph Greene 2001

30. Dynamic Vulcanization • Dynamic vulcanization is the vulcanizing (crosslinking) of a polymer during its molten state mixing with other polymer. • Polymers are first thoroughly mixed and then crosslinked. • Process produces a dispersion of crosslinked polymer in a matrix or continuous phase of uncrosslinked polymer. • If dispersed crosslinked material is elastomeric and the continuous or matrix material is a melt processible plastic than the new polymer can be an impact resistant plastic resin, or if a large ratio of soft (rubber) component then it could be a TPE (thermoplastic elastomer) • Rubber-plastic blends • EPDM and PP can be blended with dynamic vulcanization • Process • After sufficient melt mixing of plastic and rubber, vulcanization agents are added. Vulcanization of rubber phase occurs as mixing occurs. • Cooled blend is chopped, extruded, pelletized, injection molded, etc. • Dispersion of very small particles of vulcanized rubber in thermoplastic matrix Copyright Joseph Greene 2001

31. Dynamic Vulcanization • Dynamic vulcanization gives improvements • Reduced set, improved ultimate properties and fatigue resistance and resistance to hot oils. • EPDM-Polyolefin Compositions (TPO) • Small amount of crosslink formation is required for a large improvement in tension set. • Tensile strength improved as crosslink density of rubber phase increased. • Compositions can be vulcanized with accelerated sulfur, methylolpehnolic materials • As composition of polyolefin increased the compositions become more like plastic and less like rubber, and thus modulus, hardness, tension, etc. Copyright Joseph Greene 2001

32. Dynamic Vulcanization • NBR-Nylon Compositions • NMR has been mixed with various nylon using dynamic vulcanization to great success. • Effect of curatives was complicated by the fact that some nitrile rubbers tend to self cure at mixing temperatures • Sulfur, phenolic, maleimide, or peroxide curatives are used. • Compositions are highly resistant to hot oil. • Increases in in the amount of rubber reduce stiffness but increase resistance to permanent set. • Other elastomer compositions • Best compositions are prepared when the surface energies of rubber and plastic material are matched, when entanglement molecular length of the rubber molecule is small, and when plastic is crystalline. • Also important to have neither plastic nor rubber decompose is presence at the temperature of mixing. Copyright Joseph Greene 2001

33. Dynamic Vulcanization • Dynamic vulcanization can be achieved in several ways • At a temperature suitable for vulcanization but in the absence of vulcanizing or curatives for any of the polymers. • The polymers are thoroughly mixed together to form a blend. • Curative for one polymer is added during mixing and vulcanization occurs. • The polymers can be thoroughly mixed together in the presence of a curing system for only one of the elastomers at a temperature below which crosslinking occurs. • Temperature is increased with mixing until crosslinking occurs. • The polymers can be thoroughly mixed together in the presence of a curing system for only one elastomer at a temperature below which the crosslinking occurs. • The blend is removed from the mixing equipment and stored for later use. • Stored material is introduced into hot mixing processing equipment and again with adequate mixing allows for one of the elastomers to crosslink until vulcanization ends. Copyright Joseph Greene 2001