curing with sulfur and

sulfur donor systems

Probably the most complex issues in rubber compounding are the cure systems. The
method used to crosslink the elastomers is crucial to finished properties(i.e. heat resistance,
compression set, modulus, flexibility, and resilience to name a few) as well as critical to
processing parameters(scorch time, cure time, reversion or marching modulus). Also,
there are many materials that can be combined into an almost infinite array of cure
systems. Whole textbooks have been written discussing curing and curing agents. To
narrow the scope a little, this Solutions paper will concentrate on the most popular
crosslinking (curing) method, sulfur and those materials that donate sulfur.
The process of crosslinking is called curing or vulcanization and the chemicals
used for curing are referred to as curing agents or cure systems.
Charles Goodyear has been credited for the earliest method of crosslinking. His
process, heating rubber with sulfur, was first successfully used in Springfield
Massachusetts, in 1841. It was found heating natural rubber with sulfur resulted in
improved physical properties. However, the curing time took about 5 or more hours
and the vulcanizates suffered from poor physical properties, such as poor heat aging.
Since those days, the process and the resulting vulcanized articles have been greatly
improved. Some of the improved areas were speeding up the crosslinking reaction by
the introduction of accelerators and accelerators that donate sulfur. The combination
of sulfur with various accelerators is used to dramatically improve various proper-
ties of the crosslinked article such as compression set, heat aging, and dynamic
properties. (See Table 1 to determine which of Akrochem’s broad line of Sulfurs
may be best suited for your application)

fundamentals of crosslinking:
Crosslinking is the process of adding certain chemicals to elastomers to give it
useful properties such as strength, stability, and elasticity. The elastomer long chain
molecules are converted into a three dimensional elastic network by joining
(crosslinking) the molecules at certain points along the polymer chain. Example of
this crosslinking is shown below.

Raw Rubber Vulcanized Rubber

Crosslinks
 FUNDAMENTALS OF CROSSLINKING: c o n t i n u e d 2 FUND

Fig. 1
The cure rate is the speed at which a rubber compound increases in modulus (crosslink density)
at a specified crosslinking temperature or heat history. Cure time refers to the amount of time
required to reach specified states of cure at specified cure temperature or heat history. An example
of cure time is the time required for a given compound to reach 50% or 90% of the ultimate state
of cure at a given temperature often referred to as t50 and t90 respectively. (See Fig.1)
Determining what is the optimum cure time for a small curemeter specimen is not the same as
determining the optimum cure time for a thicker rubber article cured in a factory setting.

Table 1: SULFURS
DESCRIPTION/APPLICATIONS:
Sulfur is the most frequently used vulcanizing agent in rubber. Akrochem offers a broad line of
sulfurs, to meet specific applications:
• Rubbermaker’s Sulfur is the most widely used, general purpose grade.
• OT and 1% OT Sulfur offer two levels of oil to minimize dust.
• Fine, treated sulfur is similar to RM grade but with more consistent particle grind size and
slightly less dusting. effi
• Superfine Sulfur is an especially fine-ground form of Rubbermaker’s Sulfur. Sulfur
propert
• MC-98 Sulfur is an extremely fine particle size Sulfur treated with magnesium carbonate (thus the sulfur a
ash content) to improve dispersibility and reduce caking. Because sulfur has low solubility in rate an
nitrile rubber, MC-98 is used to gain maximum dispersion in NBR. However, MC-98 can be satisfac
used in any sulfur-curable polymer when optimum dispersion is desired. crosslin
• MC-HOT Sulfur is a high oil-treated (HOT) version of MC-98. Oil treatment reduces dust and vulcan
improves dispersibility even more. MC-HOT is the ultimate powdered sulfur for dispersion. This lim
• Flaked Sulfur is for industrial uses and not typically used in rubber. (82-93
the num
Akrochem Sulfur Heat Ash Oil Passing Passing Passing Passing increm
Name Purity % Loss Content through through through through
80 mesh 100 mesh 200 mesh 325 mesh (eleme
Rubbermaker’s
become
(RM) Sulfur 99.5 0.15 0.10 0.0 90% min
OT Sulfur 99.0 0.15 0.10 0.5 90% min Fig. 2
1% OT Sulfur 98.0 0.15 0.15 1.0 99.9%min
Fine Treated Sulfur 99.5 0.10 0.15 —- 99.5 % min 91% min
Superfine Sulfur 99.5 0.15 0.10 —- 95% min
MC-98 Sulfur 97.5 0.15 2.10 —- 98% min
MC-HOT Sulfur 96.4 0.15 2.60 1.0 99% min 90% min
Flaked Sulfur-Crude 99.5 0.15 0.10 —- 95% min*
* 1⁄2" sieve
 2 FUNDAMENTALS OF CROSSLINKING: c o n t i n u e d 3 EFFIC

Fig. 1: Schematic of a Cure Curve

density) As one can
time torque bond (285
example (dNm) stronger b
t
ate state 90 reversion Vulcaniza
When we
ame as required t
. bond. Thi
t Curing optimum
50
Crosslinking (cured ru
(curing)

Fig. 3:
ine of
Scorch

time
ze and
efficiency of sulfur crosslinking
Sulfur linkages have proven to be the easiest to produce rubber products with excellent elastic
properties especially flexing, tearing, and dynamics. A typical rubber formula uses a higher
(thus the sulfur amount (1.5-3.0phr) in combination with lesser amounts of accelerators to increase
ility in rate and efficiency of cure. This “general purpose” or conventional cure system is
can be satisfactory for many applications. However, the higher ratio sulfur cure produces mostly
crosslinks containing 3-8 sulfur atoms. This leaves the S-S bond as the weak link in the
dust and vulcanized product. The S-S bond is susceptible to breakage from exposure to heat or stress.
rsion. This limits the use of high sulfur cures to applications that see less than 180 – 200F
(82-93C). The primary method to improve heat resistance of sulfur bonds is to decrease
the number of single and double S “x-links”. This is accomplished by reducing sulfur levels
Passing incrementally (depending on how much heat resistance is needed) and replacing free sulfur
hrough
25 mesh (elemental sulfur) with accelerators that donate sulfur (called sulfur donors) that can
become part of the X- link. (See Fig. 2 and Fig. 3)

Fig. 2: Type of Sulfur Bridge bonding energy (kJ/mol)

Polysulfidic - C - SX - C - (x > 3) < 270
Disulfidic - C - S2 - C - 270
95% min Monosulfidic -C-S-C- 285
98% min Carbon-Carbon -C-C- 350
90% min
Intermolecular Crosslinks

C C C
S S2 SI
C C C
Monosulfide Disulfide Polysulfide (x>3)
 3 EFFICIENCY OF SULFUR CROSSLINKING: c o n t i n u e d 4

As one can see the carbon-carbon bond has a higher bond energy (350kJ) than the sulfur-carbon
bond (285kJ) formed by the EV (Efficient Vulcanization) sulfur cure system and a much
typ
stronger bond strength than the sulfur-sulfur bond (< 270 kJ) formed by a CV (Conventional
Vulcanization) sulfur cure system.
effe
When we refer to bond energy or bond strength we are referring to the amount of energy Based o
required to break a bond. The higher bond strength means greater heat is needed to break the were in
bond. This leads to better heat resistance and lower compression set properties for vulcanizates (EV) cr
(cured rubber articles). is dicta
environ
Fig. 3: Efficiency of Crosslinking It is we
such as

Efficiency of Sulfur-bonding on the

Contribution to Physical Strength Excelle
ent elastic
a higher crosslin
ncrease obtaine
m is EV syste
mostly or sulfu
n the reversio
or stress. is high
F dynami
rease mechan
ur levels and dyn
ee sulfur tor and
an EV, and

Table

FEATUR
Poly- and
Monosul
Cyclic su
Non-cycl
Reversion
Heat agin
Fatigue r
Heat buil
Tear resis
Compres
Sulfur Le
 4 5

ulfur-carbon
much
type of crosslink systems and the
onventional effect on physical properties
energy Based on the sulfur/accelerator combinations, three popular crosslinking (cure) systems
to break the were introduced. They are called conventional (CV), semi-efficient (SEV), and efficient
vulcanizates (EV) crosslinking (cure) systems. The proper selection of crosslinking (cure) systems
is dictated by many factors such as, desired end properties, processing parameters, and
environmental conditions to name a few.
It is well established that the degree of crosslinking strongly influences different properties
such as:
• Tensile stress and elongation at break
• Dynamic damping and rebound resilience
• Tear Resistance
• Compression Set
• Resistance to fluids or swelling
Excellent heat aging and compression set properties are obtained with the shorter
crosslinks, while tensile strength, rebound resilience and flex fatigue properties are
obtained with the polysulfidic crosslinks.
EV systems are those where a low level of sulfur and correspondingly high level of accelerator
or sulfurless curing are employed in vulcanizates for which an extremely high heat and
reversion resistance is required. In the conventional curing (CV) systems, the sulfur dosage
is high and correspondingly the accelerator is low. The CV systems provide better flex
dynamic properties but worse thermal and reversion resistance. For optimum levels of
mechanical and dynamic properties of vulcanizates with intermediate heat, reversion, flex
and dynamic properties, the so-called semi-EV systems with intermediate level of accelera-
tor and sulfur are employed. Typical levels of accelerator and sulfur in EV systems, semi-
EV, and CV, are shown in Table 2.

Table 2: Crosslink Structure and Properties of CV, Semi-EV, and EV

SYSTEMS
FEATURES CV Semi-EV EV
Poly- and disulfidic crosslinks(%) 95 50 20
Monosulfidic crosslinks 5 50 80
Cyclic sulfide(conc.) High Medium Low
Non-cyclic sulfide(conc.) High Medium Low
Reversion Resistance Low Medium High
Heat aging resistance Low Medium High
Fatigue resistance High Medium Low
Heat buildup High Medium Low
Tear resistance High Medium Low
Compression set(%) High Medium Low
Sulfur Level (phr) 2.0 1.0 .5
TYPE OF CROSSLINK SYSTEMS 6 SULFUR D
AND THE EFFECT ON PHYSICAL PROPERTIES: c o n t i n u e d

Many studies have documented both the advantages (increased age resistance), and the Fig 4: SU
disadvantages (impaired fatigue resistance) of EV and semi-EV systems. The worse fatigue
resistance correlates to lower amounts of polysulfidic crosslinks in the network. The CV systems
provide higher amounts of poly- and disulfidic crosslinks and higher proportions of sulfidic
and non-sulfidic modifications. This combination provides high flex fatigue resistance but at
the expense of heat and reversion resistance.
A second approach involves modifying cure systems to generate vulcanizates with more
disulfidic and monosulfidic crosslinks which have greater chemical and thermal stability
than the polysulfidic crosslinks and main chain modification to conventional vulcanizates. Akr
Such cure system modifications are accomplished via sulfur donors or high ratios of accelerators (Te
~(
to sulfur. These cure systems are sometimes called Sulfurless or low sulfur cure systems.

sulfur donors
Aside from the sulfur itself, sulfur bearing compounds that liberate sulfur at the vulcanization
temperature can be used as vulcanizing agents. These are called sulfur donors.
Generally sulfur donors convert initially formed polysulfides to monosulfides which is
characteristic for EV and semi-EV systems.
A few sulfur donors are given in Fig. 4 which includes Akrochem Accelerator R (DTDM),
Akr
which can directly substitute sulfur. Akrochem TMTD can act simultaneously as a vulcanization (Di
agent or an accelerator. The amount of active sulfur, as shown in Fig. 4 is different for each ~(
compound. Sulfur donors may be used when a high amount of sulfur is not tolerated in the
compounding recipe, for example, high temperature vulcanization of rubber. They are used
in EV and SEV systems. Sulfur donors are used to generate a network capable of resistance to
degradation on exposure to heat.
The main advantage of sulfur donors is that they reduce the normal blooming of sulfur in
unvulcanized compounds. The onset of cure occurs later than with free sulfur. The splitting of comp
thiuram tetrasulfides and morpholine derivatives results simultaneously in the formation of
accelerators or activators, which make the vulcanization proceed particularly fast. syste
To acquire the benefits described above and also to prevent sulfur blooming, it is generally
sufficient for a part of the vulcanization sulfur to be substituted by sulfur donors. In most In natural
instances 2 phrs of a sulfur donor are used instead of one part of sulfur in order to reach a modulus. C
comparable degree of crosslinking. systems dep
longer proc
systems. Ho
Sulfur dono
oxidative ag
to modify c
years to fun
TMTD and
improved h
 6 SULFUR DONORS: c o n t i n u e d 7 COM

e), and the Fig 4: SULFUR DONORS

worse fatigue
. The CV systems
ons of sulfidic Curing S
INGREDIEN
resistance but at
Natural Rub
with more HAF Carbo
mal stability Zinc Oxide
l vulcanizates. Akrochem TMTD Akrochem Accelerator R (DTDM) Stearic Aci
(Tetramethyl Thiuram Disulfide) (4,4’-Dithiodimorpholine) Aromatic O
os of accelerators
~ (13.0 % Active Sulfur) ~ (14.0 % Active Sulfur) Antioxidant
re systems. SULFUR
CBS
TMTD

he vulcanization
Cure Sys
s.
which is
Cured minu

R (DTDM), Durometer
Akrochem DPTT Akrochem Cure-Rite 18
s a vulcanization (Dipentamethylene Thiuram Tetrasulfide) (OTOS) (Thiocarbamyl Sulfenamide)
ifferent for each ~ (25.0 % Active Sulfur) ~ (13.0 % Active Sulfur) Tensile Stre
olerated in the
They are used Aged
of resistance to
Elongation
g of sulfur in
r. The splitting of compounding with cv, sev, and ev Aged
e formation of
fast. systems
300% Modu
t is generally
ors. In most In natural rubber, EV and semi-EV systems can provide remarkable resistance to marching Aged
der to reach a modulus. Control can generally be realized without fatigue compromises. The choice of cure
systems depends in part on the processing conditions required. Sulfur donors normally give Compressio
longer processing safety and better green stock storage than the use of high accelerator/low sulfur
systems. However the latter may be better in the long overcure situations.
Sulfur donors are used to replace part of or all of the elemental sulfur to improve thermal and Crescent Te
oxidative aging resistance. They may also be used to reduce the possibility of surface bloom and
to modify curing and processing characteristics. Two chemicals have been developed over the Aged
years to function as sulfur donors- alone or in combination with sulfur. They are Akrochem
Fatigue Life
TMTD and Akrochem Accelerator R (DTDM). Akrochem TMTD is used to provide significantly
improved heat and aging resistance. (See Table 3 and Fig. 5 through Fig. 9) Aged
 7 COMPOUNDING WITH CV, SEV, AND EV SYSTEMS: c o n t i n u e d 8 COMPOUN

Fig. 5
TABLE 3 : Test Data
Curing System Conventional Semi-EV EV 4500

INGREDIENTS 1 3 4 4000

3500

Natural Rubber 100.00 100.00 100.00 3000

HAF Carbon Black ( ASTM N-330) 50.00 50.00 50.00 2500

PSI
Zinc Oxide 3.50 3.50 3.50 2000

DM) Stearic Acid 2.50 2.50 2.50 1500

Aromatic Oil 4.00 4.00 4.00 1000

Antioxidant 2.00 2.00 2.00 500

SULFUR 2.50 1.20 0.25 0

Conventio

CBS 0.50 0.80 2.20

TMTD 0.00 0.40 1.00

PHYSICAL PROPERTIES
Cure System Conventional Semi-EV EV

Fig. 7 CuF
Cured minutes @140°C 30 30 40
1400
Durometer - Shore A @RT 68 67 61
1200
namide)
Tensile Strength 1000

Unaged 4146 4132 3862 800

Aged 120hrs@100°C 966 2215 3251

PSI
%Retention 13 54 84 600

400

Elongation - % 200

ev Unaged
Aged 120hrs@100°C
490
130
490
305
555
470
0
Conventiona

%Retention 17 62 85

300% Modulus - psi

Unaged 1264 1278 951
rching Aged 120hrs@100°C -------- 1207 1079
e of cure
lly give Compression Set (%)
Unaged 44 19 21 Fig. 9
or/low sulfur
Aged 36 17 19

ermal and Crescent Tear @ RT

140

bloom and Unaged 86 77 -------- 120

over the Aged 1344hrs@70°C 40 51 -------- 100

Fatigue(kilocycles)

rochem 80

Fatigue Life (kilocycles to failure)

nificantly 60
Unaged 120 58 --------
Aged 1344hrs@70°C 9 27 -------- 40

20

0
Conventio
 8 COMPOUNDING WITH CV, SEV, AND EV SYSTEMS: c o n t i n u e d 9 COMP

Fig. 5 Cure System Comparison

F

EV 4500
Unaged Aged 120hrs@100°C ap
4
su
4000

3500

00.00 3000

0.00 2500
PSI

3.50 2000

2.50 1500 Below

4.00 1000
system
2.00 500
Fig. 6 Cure System Comparison Goa
F
0.25 0
Conventional Semi-EV EV
Cured 30mins@140°C

2.20 TENSILE PROPERTIES

600
Elongation - %
Aged 120hrs@100°C
1.00 Actio
500

400

PERCENT
EV
300

200

Fig. 7 Cure
F System Comparison 100

40 Cured 30mins@140°C
0
Conventional Semi-EV EV
Unaged Aged 120hrs@100°C
1400
61 ELONGATION %

1200
Goa
1000 Actio
3862 800
3251
PSI

84 600 Fig. 8 Cure System Comparison

F
400 Cured 30mins@140°C

su
200 Unaged Aged 1344hrs@70°C
90
555 80
0
470 Conventional Semi-EV EV 70
As yo
85 300% MODULUS
60

50
cure
Pounds

40 requi
30
951 20 To im
1079 10
is use
0
Conventional Semi-EV EV lower
21 Fig. 9 Cure System Comparison CRESCENT TEAR @ RT
polys
F
19 Cured 30mins@140°C
It is t
140
Unaged Aged 1344hrs@70°C for th
------- 120

------ 100
Fatigue(kilocycles)

80

60
-------
------ 40 Included with it
“Technical Info
results which m
20
It is the custome
information con
0
Conventional Semi-EV EV Nothing in the T
any such produ
FATIGUE LIFE(kilocycles to failure)
AKROCHEM CO
OR TECHNICAL
 9 COMPOUNDING WITH CV, SEV, AND EV SYSTEMS: c o n t i n u e d 10

application: examples of
sulfur curing

Below are examples of actions to take when considering different compounding goals with sulfur cure
systems.
son Goal #1: To improve aging resistance of an initial CV system

n-%
hrs@100°C
Action: Change to a higher accelerator/sulfur ratio, e.g. EV
CV System EV System
Akrochem CBS 0.5 phr to 1.5 phr
Akrochem MC-98 Sulfur 2.5 phr to 0.5 phr
Akrochem Accel. R (DTDM) Sulfur Donor 1.5 phr
Akrochem TMTD 1.5 phr

Goal #2: To Reduce cure time

Action: 1. Increase accelerator
2. Increase accelerator level and reduce sulfur level (EV system, i.e.)
n 3. Use a faster accelerator

summary
Aged 1344hrs@70°C

As you can see various approaches can be used to modify cure systems to meet ones needs. In sulfur
cure systems, zinc oxide or magnesium oxide, stearic acid or other fatty acids or its metal salts are
required to activate the curing reaction using accelerators, depending on the objectives of the cure system.
To improve aging resistance, high accelerator to sulfur ratio referred to as efficient vulcanization (EV)
is used. This system gives higher monosulfidic crosslinks, which are less flexible than polysulfidic thus
lower dynamic properties. The conventional cure system, high sulfur to accelerator ratio, gives higher
polysulfidic crosslinks hence better dynamic fatique properties.
It is the job of the compounder to determine which sulfur cure system will give him the best properties
for the end use product.

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