Vulcanization
Professor Joe Greene
CSU, CHICO
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Copyright Joseph Greene 2001
Vulcanization
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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
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001
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)
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Copyright Joseph Greene 2001
Viscosity
Time
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Vulcanization Process
• 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:
Torque
Vulcanization Viscosity Curve
Rubber
1.Preheat
2.Initial Torque
3.Min Torque
4.Structure
5.Scorch Time
6.90% Cure
7.MaxTorque
Overcure
Reversion
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Copyright Joseph Greene 2001
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.
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– Following curve
Copyright Joseph Greene 2001
Vulcanization Process
• Cure Profile versus Cure Temperature
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Copyright Joseph Greene 2001
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:
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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.
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001
Silicone Rubber Commercial Preparation
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001
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
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Copyright Joseph Greene 2001
Fig 8
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Copyright Joseph Greene 2001
Silicone Rubber Vulcanization
– Figure 9 lists the organic peroxides commonly used to cure silicone
rubber with recommended cure temperatures and general application
areas.
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Copyright Joseph Greene 2001
Silicone Rubber Vulcanization
– Figure10 is a further checklist to differentiate the use of diacyl and
dialkyl peroxide types.
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Copyright Joseph Greene 2001
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
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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
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Copyright Joseph Greene 2001
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
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Copyright Joseph Greene 2001
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
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Copyright Joseph Greene 2001
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
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001
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
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001
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
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Copyright Joseph Greene 2001
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
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001
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.
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Copyright Joseph Greene 2001