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Rubber Curing

Over the years three special types of
cure systems have been developed.
They are:
efficient vulcanisation (EV) systems,
semi-efficient vulcanisation (SEV)
systems and
conventional vulcanisation (CV)
The levels of accelerator and sulfur in
SEV and EV systems are shown in
Table 1
Measuring Cure
The curve exhibits a number of features
which are used to compare cure:
- Maximum torque MH.
- tS2, T2 or T5 : there is a delay or
induction time before the torque or
resistance value begins to rise.
- T90 : is the time for the torque to
increase to:
90/100(MH ML)+ ML
Cure rate: A rise in the value of
torque with time, the slope of the
curve, gives the measure of cure
rate. Sometimes cure rates of various
cure systems are compared with
T90ts2 data
Test Equipment and
Cure characteristics were determined
using an MDR 2000EA rheometer.
Test specimens were vulcanised by
compression molding at
temperatures and times indicated.
Stress-strain properties were
determined according to ISO 37
tear strength according to ISO 34/1.
DIN abrasion ISO 4649.
fatigue to failure ASTM 4482/85.
hardness according to ISO 48.
Ageing of the test specimens was
carried out in a ventilated air oven at
100 C for 3 days (ISO 188).
Heat build up and permanent set after
dynamic loading were determined using a
Goodrich Flexometer (Load 11 kg or 22 kg;
stroke 0.445 cm, frequency 30 Hz, start
temperature 100 C) according to ISO 4666/3-
Dynamic mechanical analysis was carried out
using a RDA-700 (prestrain 0.75%, frequency
15 Hz and temperature 60 C) according to
ASTM D 2231.
Vulcanisate network structure was
determined by equilibrium swelling in
toluene using the method reported by
Ellis and Welding. The volume fraction
(Vr) obtained was converted into the
Mooney-Rivlin elastic constant (C1)
and finally into the concentration of
chemical crosslinks.
The proportions of mono-, di-, and
polysulfidic crosslinks in the
vulcanisates were determined using
thiol amine chemical probes.

Following the cleavage of the poly- and

disulfidic crosslinks, the samples were
treated with methyl iodide to
distinguish carbon-carbon based
crosslinks from monosulfidic crosslinks.
The rubber to metal adhesion
characteristics were determined
according to ASTM 2229-85.
Curing Systems

Curing systems can be classified into

four categories.
Sulfur Curing Systems
Accelerators may be classified in several
(a) inorganic or organic
(b) acidic or basic
(c) by chemical type or
(d) by speed of the cure, giving rise to the
terms slow, medium, semi-ultra and ultra.
Functionally, accelerators can also be
classified into two broad categories:
Primary accelerators:
Primary accelerators are mercapto
based accelerators, generally efficient
and confer good processing safety to
the rubber compounds, exhibiting a
broad vulcanisation plateau with
relatively low crosslink density.
Secondary accelerators:
Some rubber compounds use only one
accelerator but most contain two, a
primary at about 1 phr and a secondary
(or booster) at 0.1 to 0.5 phr.
These combinations cause faster
vulcanisation than each product
separately and a considerable activation
of cure which is positive for the general
property spectrum of the vulcanisates.
NR and SBR compounds with
MBTS/DPG as the control. The
secondary accelerators also change
the network structures. The
polysulfidic crosslinks are converted to
monosulfidic exhibiting heat stability.
Sulfur Donors

Aside from the sulfur itself, sulfur

bearing compounds that liberate
sulfur at the vulcanisation
temperature can be used as
vulcanising agents.
Cures for Speciality
The curing systems used to vulcanise
speciality elastomers such as EPDM,
CR, IIR and NBR are different than
those used to cure NR, SBR, BR and
its blends
The former elastomers are less
unsaturated and therefore need a
high ratio of accelerator to sulfur
Cure Systems for EPDM
Cure Systems for Nitrile
Cure systems for nitrile rubber are
somewhat analogous to those of NR,
SBR or BR except that magnesium
carbonate (MC) treated sulfur is
usually used to aid sulphur dispersion
into the polymer.
Cure Systems for
CR can be vulcanised in the presence of
zinc oxide alone, but magnesium oxide is
necessary to confer scorch resistance.
Along with the metal oxides, TMTD, DOTG
and sulfur are used. This is a good method
to obtain high resilience and dimensional
Cure Systems for Butyl and
Halobutyl Rubber
The low degree of unsaturation
requires the use of ultra
accelerators, such as thiuram or
dithiocarbamate. Phenolic resins,
bisazoformates , and quinone
derivatives can also be employed.
Sulfur crosslinks have limited
stability at elevated temperatures
and can rearrange to form new
Resin cure systems provide C-C
crosslinks and heat stability.
Halobutyl Rubber
Peroxide Cure Systems
Peroxide Vulcanisation of
Peroxide vulcanisation of EPDM is
growing in popularity because of
enhanced ageing resistance.
A comparison of sulfur and peroxide
cure systems for EPDM is shown in
Table 32 .
Apart from peroxide type and the
amount of peroxide incorporated in
compounds, the efficiency of
crosslinking depends on coagents.
The commercially important ones are:
- Maleimide type: N,N-phenyl maleimide
- Allylic type: triallyl cyanurate (TAC)
triallyl isocyanurate (TAIC)
- Methacrylate type: zinc dimethacrylate
- Acrylate type: ethylene glycol diacrylate
zinc diacrylate
- Polymeric coagents: liquid 1,2-
polybutadiene resin
Sulfur Free Curing
Some special vulcanising agents can
cure diene rubbers such as NR, SBR
and BR.
Phenolic Curatives,
Derivatives and

Diene rubbers can be vulcanised by

the action of phenolic compounds
like phenol-formaldehyde resin (5-10
phr). Resin cured NR offers good set
and low hysteresis.
Resin curing of SBR and BR imparts
excellent cut growth and abrasion
Resin cured nitrile rubber shows high
fatigue life and high relaxation.
Resin-cured butyl rubber shows
outstanding ozone and age resistance.
A high diene rubber can also be
vulcanised by the action of a
dinitrosobenzene, made in situ by the
oxidation of a quinonedioxime
Vulcanisation by Triazine
The triazine accelerators are more
effective than the thiazole
accelerators and produce highly
reversion resistant vulcanisates.
Urethane Crosslinkers
Natural rubber can be crosslinked by a
blocked diphenyl methane diisocyanate to
produce urethane crosslinks.
The crosslinking agent dissociates into two
quinonedioxime molecules and one
diphenyl methane diisocyanate.
The quinone reacts with the rubber via a
nitroso group and forms crosslinks via a
diisocyanate group.
Other Crosslinking
There exist a considerable number of
compounds containing labile chlorine
which bring about sulfurless
vulcanisation at levels of
approximately 3 phr.
Basic chemicals such as lead oxides
and amines are needed.
New Developments

Maintaining properties and

performance throughout a rubber
products service life is directly
related to maintaining the integrity of
the vulcanisate structure under both
thermal and thermal oxidative
Two additives have allowed
compounders to forget this
compromise, namely
(HTS), a post vulcanisation stabiliser
and 1,3-bis(citraconimidomethyl)
benzene (BCI-MX, Perkalink 900)
Some Practical Examples
Varying Cure Systems

Steel belts

Carcass plies
Black sidewall

Inner liner

Apex Veneer coverstrip


White sidewall

Rim cushion

Trying to balance the three main

apparently conflicting needs of wear,
wet grip and rolling resistance
The cure system should be CV/SEV.
Accelerators used are sulfenamide
class (TBBS, CBS, MBS).
For improving heat resistance, the
use of Perkalink 900 is recommended
Ingredients phr
NR 100
ZnO 5
Stearic acid 2
TBBS 1.5
Sulfur 1.5
N-234 55
Aromatic oil 8
6 PPD 2
TMQ 1.5
PVI 0.1
Perkalink 900

Perkalink 900 is an antireversion

agent which provides long term heat
stability in sulfur vulcanized
compounds by substituting stable,
flexible carbon-carbon crosslinks for
the sulfur crosslinks that are
destroyed by reversion in the mold or
during product service life.
Silica as a reinforcing filler is being
used more extensively in the tyre
industry to provide improved tear
resistance and decreased rolling
In order to maximise these benefits
the silane coupling agent bis-(3-
triethoxysilylpropyl) tetrasulfide
(TESPT) is often employed.
A passenger tread containing silica
needs a cure system based on
sulfenamide/sulfur (SEV/CV system)
and guanidine activators (DPG).
Aircraft tread requires a high modulus, better
abrasion resistance and low heat generation.

The above requirements could be satisfied by

using a CV/SEV cure system with DCBS as
accelerator and insoluble sulfur (IS) as

For achieving high modulus, HMT and a

resorcinol system is used.

In order to reduce the heat generation, the

use of Perkalink 900 has been suggested.
Tread Base or Sub Tread

A natural rubber rich undertread layer

can enhance the adhesion between
belt or cap-ply and tread.
The cure system needs better
and low heat generation. Typically the
cure system will be based on CV/SEV.
In radial tyres, sets of belts or breakers made
from brass coated steel cords are layed at
alternate bias angles.
to provide a trellising effect, to stiffen the
area under the tread and also to prevent
growth under inflation or high speed rotation.
These belts provide a rigid support to the
tread offering a more controlled contact with
the road.
The design of the belt package
should give a smooth ride with
minimum energy loss, but with
sufficient stiffness to prevent undue
movement within the contact area
causing irregular or rapid tyre wear.
The cure package is composed of a
slow accelerator (DCBS, or TBSI) with
a high level of sulfur for improving
the bond strength between the
rubber and brass layer.
Addition of Duralink HTS
(hexamethylene-1,6- bis(thiosulfate),
disodium salt dihydrate) improves
adhesion characteristics under ageing

Sidewall compounds cover the

thinnest part of the tyre where most
flexing occurs as the tyre deflects.
They therefore need to have a high
degree of flex resistance and good
dynamic properties, as well as
excellent age resistance.
Sidewall compounds contain additional
ingredients to prevent oxidative or ozone
attack since this part of the tyre will be
particularly exposed to the sun and the
The cure system should be CV or

The cure system of choice for a

carcass compound is based on a
scorch resistant cure package.
thermally stable, insoluble sulfur and
bloom resistant adhesion additives
are required.
the cure system suitable for this
application is based on a slow
accelerator such as DCBS with a high
amount of insoluble sulfur (generally
4-5 phr).

require high modulus and good

adhesion to steel wires.
High modulus is achieved by using
resorcinol/HMMM or phenol
formaldehyde resins.
The cure system is based on
sulfenamide with a high amount of
insoluble sulfur.

The apex components provide the

gradual shape and stiffness reduction
from the rigid bead coil to the flexible
mid-sidewall of the tyre.
These components need to be very
hard to provide good vehicle handling
and to reduce the risk of flexural
fatigue at component endings.
act as a barrier layer between the tread
and the casing to restrict migration of
chemicals from the tread into the belt.
requires high modulus and good adhesion
The cure system used to achieve the target
properties is based on a combination of
CBS and DCBS with a high amount of
insoluble sulfur.
A resorcinol/HMMM system is used to
obtain the required high modulus.
Inner Liner

Zinc oxide with MBTS and a small

of sulfur is used to crosslink
halobutyl rubber.
Industrial Rubber
Conveyor Belt Cover - NR

flex resistance, abrasion resistance and

low heat build up.
In order to achieve better flex resistance,
a conventional cure package is
(however, such a system suffers from the
adverse effects of heat generation).
In a recent study, the addition of Perkalink
900 was recommended
The antireversion agent has no effect on
scorch resistance and time to optimum
The compound containing the
antireversion agent maintains a torque
level close to the maximum.
The antireversion effect is also evident
in the stability of vulcanisate properties
following overcure or air ageing at 100
Engine Mount - NR
The essential performance properties for
an engine mount are low heat build up and
low dynamic compression set.
a cure system comprising of sulfenamide
(moderate loading, 1.2-1.7 phr) with sulfur
(2-2.5 phr) is recommended.
A small amount of ultra accelerator might
be necessary to generate a crosslink
network capable of improving dynamic
compression set as well as heat build up.
In spite of this modified cure system
(CBS/TMTD/S), the properties are still
not optimum, particularly with regard
to heat build up.
The addition of an antireversion agent
might provide advantage and with this
in mind the effect of Perkalink 900 has
Tank Pad NR/SBR/BR
The required properties for a tank pad are
heat stability, low heat generation, improved
flex and good compression set.
This can be achieved by selecting a cure
package between SEV and towards an EV
cure.A combination of a sulfenamide with a
thiazole is suitable.
Generally the sulfenamide level should be low
and the thiazole accelerator should be high.
The sulfur loading is generally high to
address the improved flex requirements.
Because of high sulfur loadings, the
system suffers from the adverse effects
of heat and reversion resistance.
Perkalink 900 is active in compounds
based on blends of NR with the
synthetic elastomers SBR and BR.
Oil Seal - NBR
The general requirement for oil seal
applications is oil resistance, low compression
set, high elongation and good heat stability.
In order to achieve this, a cure system
comprising of MBTS/TMTD and sulfur is
TMTD is used in a small amount so that
elongation is not negatively affected.
In order to keep high elongation, the
amount of sulfur should not be too low.
Compression can be negatively
affected if the sulfur loading is too high.
The heat stability can be improved by
incorporating antireversion agent
Perkalink 900.
Concluding Remarks

Cure systems play an important role

in achieving the performance
requirements of rubber articles.
Antireversion chemicals constitute a
class of such materials, and can
improve service performance.
Cure systems, however, paint only part
of the picture with regard to the
manufacture of rubber articles.
polymers and fillers are equally
important in meeting performance
In addition, compound mixing and
processing play a critical role in
achieving the final vulcanisate