Combined Sulfur and Peroxide Curing System

Ivan HUDEC, Ján Kruželák & Andrea Kvasničáková

Institute of Natural and Synthetic Polymers

Gumference 2019

November, 13th 2019, Zlín

Vulcanization

The fundamental of vulcanization is formation of physical and mainly

chemical cross-links between rubber chains which lead to the creating of three-dimensional network structure by reactions between the functional groups of rubber chains and suitable curing agents.

General scheme of cross-linked network structure of vulcanizate

Curing systems for rubber compounds

Sulphur based curing systems Peroxide curing systems Phenolic resins Quinones Metal oxides Amines, urethanes etc.

the most widely used

Sulphur curing systems Peroxide curing systems

monosulphidic, disulphidic polysulphidic cross-links (x = 1- 6)

carbon-carbon cross-links

A lot of elastomers are nowadays commercially available, which are distinguished by their structure and chemical nature. Therefore, a number of curing systems have also been developed in order to vulcanize rubber compounds:

The type of applied curing system determines the structure and the quality of the cross-links.

Mechanism of sulphur vulcanization

In general, it is presumed that the sulphur vulcanization of unsaturated diene elastomers proceeds in three stages: • In the first stage, the interaction of components of curing system leads to the

forming of transition complexes, which together with rubber form active cross-linking agent,

• The second stage is characterized by formation of primary vulcanizate network

with dominance of polysulphidic cross-links, • During the third stage, this network is restructured in consequence of

modification of cross-links (polysulfidic cross-links are transferred into di- and monosulfidic cross-links) and macromolecules of rubber (isomerization, dehydrogenation, cyclization) and the final spatial network of vulcanizate is formed

• In term of chemism, two reactions pathways, proceeding via ionic or free-radical intermediates have been considered. Eventually, both mechanisms may run simultaneously

Structure of sulphur-cured vulcanizates

The general scheme of sulphur-cured vulcanizates

a – monosulphidic cross-links, b – disulphidic cross-links, c – polysulphidic cross-links (x = 3 – 6), d – polysulfidic cross-links connected several elastomer chains, e – vicinal cross-links, f – carbon-carbon cross-links, g, j – cyclic sulfides, h, i – conjugated segments of elastomer chains, l - pendant side group (Acc - accelerator residue), k - chemically non-bonded additives

Structure of sulphur-cured vulcanizates

• The structure of the formed cross-links can be influenced by various technological parameters of vulcanization process, but it depends mainly on the type and amount of accelerator in the rubber mixture.

• The structure of formed cross-links in vulcanizates has significant influence on their

properties. • The longer the sulphidic cross-links the easier they decompose and they have the

lower resistance to elevated temperature. Therefore vulcanizates with high level of polysulphidic cross-links are not very heat-proof and exhibit high compression set. On the other hand, they generally show good physical-mechanical and dynamic properties and good resistance to dynamic fatigue.

• It is caused mainly due to the low bonding energy of sulphidic cross-links with higher

number of sulfur atoms: C Sx C < 252 kJ mol-1 C S2 C < 268 kJ mol-1 C S C < 285 kJ mol-1 C C < 352 kJ mol-1

Structure of sulphur-cured vulcanizates

Structure of vulcanizates and properties of sulphur curing systems

Conventional Semi-EV EV

Poly-and disulphidic cross-links,%) 95 50 20

Monosulphidic cross-links (%) 5 50 80

Cyclic sulphide concentration

Heat-ageing resistance

Reversion resistance

Fatigue resistance

Tear resistance

Compression set (%)

high

low

low

high

high

high

medium

medium

medium

medium

medium

medium

low

high

high

low

low

low

Peroxide vulcanization

• Cross-linking of rubbers with organic peroxides • Not only unsaturated, but also saturated elastomers can be efficiently cured with

peroxides, mainly ethylene-propylene type rubbers (EPM, EPDM), or fluoro elastomers (FKM)

• The application of organic peroxides in cross-linking of elastomers leads to the forming

of covalent carbon-carbon cross-links between elastomer chain segments. Aliphatic, aromatic and mixed peroxides can be introduced in cross-linking of elastomers and some of them have more than one peroxidic group.

• C-C bonds have higher dissociation energy in comparison with sulphidic cross-links, so

peroxide vulcanized elastomers exhibit higher thermal stability and good resistance to thermo-oxidative ageing. Good electrical properties, low compression set and no discoloration of the final products are next distinctive features of peroxide cured vulcanizates.

carbon-carbon cross-links

Mechanism of peroxide vulcanization

• The cross-linking of rubbers with peroxides has radical character • In the first step, peroxides are homolytically dissociated into free radicals. • Peroxide free radical species could potentially react with elastomers by: - abstraction of hydrogen from the elastomer chains - elastomer radicals that are formed subsequently recombine to form cross-links low cross-linking efficiency - addition to a double bond of unsaturated rubbers -the double bonds situated at the end of elastomer chains (terminal) or in the side-chain groups (vynil) are less sterically hindered when compared to in-chain double bonds (cis/trans), hence they are more amenable and more likely take part in addition reactions. • Both mechanisms, hydrogen abstraction and addition reactions may proceed

simultaneously to form elastomer radicals (macroradicals).

reaction of peroxide radicals with rubber chains: A) abstraction of hydrogen from the elastomer chains – low cross-linking efficiency

Peroxide cross-linking of NR

Mechanism of vulcanization with organic peroxides

ROOR

CH2-C=CH-CH

2

CH3

ROH CH2-C=CH-CH

CH3

CH2-C=CH-CH

CH2-C=CH-CH

CH3

CH2-C=CH-CH

CH2-C=CH-CH

CH3

CH3

CH3

2RO.

+ 2RO.

+.

.

.

reaction of peroxide radicals with rubber chains: B) abstraction of hydrogens and addition onto a double bonds – high cross-linking efficiency

Peroxide cross-linking of butadiene type rubbers (BR, SBR, NBR)

Mechanism of vulcanization with organic peroxides

-CH2-CH=CH-CH

2-

-CH-CH=CH-CH2-

-CH2-CH-CH-CH

2-

-CH2-CH=CH-CH

2-

-CH2-CH-CH-CH

2-

-CH2-CH-CH

2-CH

2-

-CH-CH=CH-CH2-

-CH2-CH-C-CH

2-

-CH2-CH-CH-CH

2

-CH-CH=CH-CH2-

-CH-CH=CH-CH2-

-CH-CH=CH-CH2-

-CH-CH=CH-CH2-

.

+

RO +

.

.

OR

ROH

+

.

OR

.

OR

.

OR

.

.

Peroxide curing systems

Organic peroxides + co-agents

co-agents - multifunctional low molecular weight organic compounds with high reactivity towards free radicals

they are used to increase the cross-linking efficiency of the vulcanization process and to increase the cross-link density of final vulcanizates as well

advantages of rubber compounds cured with peroxides and co-agents in comparison to those cured only with peroxides:

improved peroxide efficiency higher tensile and tear strength higher modulus higher hardness higher resilience

improved dynamic properties improved compression set improved abrasion and tear resistance improved heat ageing improved adhesion to polar substrates

grafting of co-agents between rubber chains formation of an interpenetrating network of homopolymerized co-agents and rubber chains formation of higher modulus filler–like domains of thermoset co-agents The main reason why co-agents increase the cross-linking efficiency is primarily due to the formation of co-agents bridges between rubber chains as extra cross-links.

Co-agent-assisted peroxide cross-linking

Cross-linked network structure of rubber matrix cured with peroxide in the presence of co-agent Cross-links can be formed from: A – polymer radicals, B – co-agent forming cross-links, C – thermoset domains of co-agent grafted to elastomer chains and D – interpenetrating network of homopolymerized co-agents

Sulphur systems Peroxide systems

good tensile and tear strength good dynamical properties good abrasion resistance good regulation of scorch safety and optimum cure time good resistance to dynamic fatigue

good heat-ageing stability good electrical properties low compression set simple formulation rapid vulcanization without reversion no discoloration of the products

Comparison of curing systems for rubber compounds

Advantages Advantages

Disadvantages Disadvantages low resistance to thermo-oxidative ageing high compression set possibility of reversion during vulcanization

worse tensile, elastic and dynamic properties low scorch safety sensitivity to oxygen during curing generally higher cost

Materials

Rubber matrices: Natural rubber (NR) , type SMR5, Acrylonitrile-butadiene rubber

(NBR), type SKN3345

Sulphur curing system: designated as (S), sulphur , N-cyclohexyl-2-benzothiazole

sulfenamide (CBS) , zinc oxide , stearic acid

Peroxide curing system: designated as (P), dicumyl peroxide (DCP), ethylene glycol

dimethacrylate (EGDMA)

S0 – P1.5 S0.5 – P1 S0.75 – P0.75 S1 – P0.5 S1.5– P0

NR/NBR 100 100 100 100 100

ZnO 3 3 3 3 3

Stearic acid 2 2 2 2 2

Sulphur - 0.5 0.75 1 1.5

CBS - 0.25 0.375 0.5 0.75

DCP 1.5 1 0.75 0.5 - EGDMA 0.75 0.5 0.375 0.25 -

Composition of rubber compounds in phr and their designation

Influence of curing system composition on scorch time ts1

and optimum cure time tc90 of rubber compounds

Influence of combined sulphur/peroxide curing systems on vulcanization characteristics

Influence of curing systems composition on cross-link density n of vulcanizates

0

1

2

3

4

5

6

7

S0-P1.5 S0.5-P1 S0.75-P0.75 S1-P0.5 S1.5-P0

n.1

04 (

mo

l.c

m-3

)

Sample

NR NBR

Influence of combined sulphur/peroxide curing systems on cross-link density of vulcanizates

Influence of curing systems composition on modulus M100 and hardness of vulcanizates

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

S0-P1.5 S0.5-P1 S0.75-P0.75 S1-P0.5 S1.5-P0

M1

00

(M

Pa

)

Sample

NR NBR

0

10

20

30

40

50

60

S0-P1.5 S0.5-P1 S0.75-P0.75 S1-P0.5 S1.5-P0

Hard

ne

ss

(S

ho

re A

) Sample

NR NBR

Influence of combined sulphur/peroxide curing systems on properties of vulcanizates

Influence of curing systems composition on elongation at break and tensile strength of vulcanizates

0

100

200

300

400

500

600

700

800

900

S0-P1.5 S0.5-P1 S0.75-P0.75 S1-P0.5 S1.5-P0

Elo

ng

ati

on

at

bre

ak

(%

)

Sample

NR NBR

0

3

6

9

12

15

18

21

S0-P1.5 S0.5-P1 S0.75-P0.75 S1-P0.5 S1.5-P0

Te

ns

ile

str

en

gth

(M

Pa

)

Sample

NR NBR

Influence of combined sulphur/peroxide curing systems on mechanical properties of vulcanizates

The crystallization of NR based vulcanizates decreases with increasing amount of peroxide in the mixed curing systems, as more rigid and less mobile C-C bonds restricts the mobility and orientation of macromolecular chains when they are stretched, subsequently inhibiting crystallization. The crystallization of natural rubber is also the main factor of much higher tensile strength of vulcanizates based on NR compared to that of vulcanizates based on NBR.

Influence of sulphur, peroxide and combined sulphur/peroxide curing systems on cross-link density n of vulcanizates

Thermo-oxidative ageing

0,0

0,2

0,4

0,6

0,8

1,0

1,2

S0-P1.5 S0.5-P1 S0.75-P0.75 S1-P0.5 S1.5-P0

n.1

04 (

mo

l.c

m-3

)

Sample

before ageing

after ageing

0

1

2

3

4

5

6

7

S0-P1.5 S0.5-P1 S0.75-P0.75 S1-P0.5 S1.5-P0

n.1

04 (

mo

l.c

m-3

) Sample

before ageing

after ageing

(NR) (NBR)

Influence of sulphur, peroxide and combined sulphur/peroxide curing systems on elongation at break of vulcanizates

(NR) (NBR)

0

100

200

300

400

500

600

700

800

900

S0-P1.5 S0.5-P1 S0.75-P0.75 S1-P0.5 S1.5-P0

Elo

ng

ati

on

at

bre

ak

(%

)

Sample

befor ageing

after ageing

0

100

200

300

400

500

600

700

800

900

S0-P1.5 S0.5-P1 S0.75-P0.75 S1-P0.5 S1.5-P0

Elo

ng

ati

on

at

bre

ak

(%

) Sample

before ageing

after ageing

Thermo-oxidative ageing

Conclusion

The achieved results demonstrated that cross-link density, the structure of the formed

cross-links and the type of rubber matrix influence the property spectrum of

vulcanizates as well as their thermo-oxidative stability

The results showed that higher cross-link density exhibited vulcanizates based on NBR,

while the sample cured only with peroxide system showed the highest one. The values ν

of both types vulcanizates cured with mixed sulphur/peroxide curing systems were

lower compared to corresponding vulcanizates cured only with peroxide or sulphur

curing systems

The moduli and hardness of NR and NBR based vulcanizates are in very close correlation

with the change of cross-link density.

Conclusion

Different character of dependences on curing system composition was recorded in the

case of tensile strength and elongation at break.

The crystallization ability of NR based vulcanizates decreases with increasing amount of

peroxide in the mixed curing systems, as more rigid and less mobile C-C bonds restricts

the mobility and orientation of macromolecular chains when they are stretched,

subsequently inhibiting crystallization.

In the case of vulcanizates based on NBR, the highest tensile strength and elongation at

break exhibited the sample cured with equivalent ratio of sulphur and peroxide,

suggesting some synergistic effect of both curing systems applied in cross-linking of NBR

based rubber compounds.

The results revealed that the influence of thermo-oxidative ageing was more

pronounced in the case of vulcanizates based on NR, mainly those cured with higher

peroxide content.

Acknowledgement

Coworkers from Institute of Natural and Synthetic Polymers:

Assoc. prof. Jan Kruželák

PhD student Andrea Kvasničáková

This work was supported by Slovak Research and Development Agency project No. APVV-16-0136.

8th International Conference Polymeric Materials in Automotive

PMA 2020 &

the 24th Slovak Rubber Conference

18 - 20 May, 2020 Conference Center of Lindner Hotel

Galery Central Bratislava

THANK YOU

for your attention !