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Crosslinking of fluoroelastomers and the influence on final properties Matthias Lückmann, Wolfgang Steinhoff

February, 13, 2014

Overview

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basics on curing of rubber and fluoroelastomers ionic cure with bisphenol

requirements crosslinking mechanism effect of cross linker to accelerator ratio role of metal oxides and influences of level change

activators radical cure with peroxides

requirements crosslinking mechanism effect of cross linker and accelerator level

on cure speed and mechanical properties different coagents Influence of peroxide level

conclusions

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chemical crosslinks

entanglements entanglements

hydrocarbon rubbers are mainly cross-linked with systems based on sulfur or peroxides chemical crosslinking minimizes viscous flow and leads to material with high elasticity

with increasing bond energy the thermal stability is increasing special rubbers like FKM are used in demanding environments and the cross-linked materials have to withstand high temperatures and should have high chemical resistance crosslinking with bisphenol forms C-O-C bonds; bond energy C-O: 358 kJ/mol crosslinking of FKM with peroxides forms C-C bonds; C-C: 352 kJ/mol

the occurring chemical bonds have following energy: C-S-C: 285 kJ/mol C-S-S-C: 268 kJ/mol

Some Basics on Curing of Rubber

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ionic cure: Bisphenol AF most common cross linker; phosphonium and ammonium salts most common accelerator curative Masterbatches support dispersion best scorch safety, low mould fouling and good mould release lowest compression set excellent heat resistance

radical cure: cure sites are incorporated into the polymer ® G types) peroxide controls the rate of cure, DBPH most common peroxide coagent controls the number of crosslinks, TAIC most common coagent best resistance to hot water (or other aqueous fluids like coolants) improved chemical resistance (e.g. high and low pH) metal oxides not necessary but lead to higher heat resistance and more efficient cure

crosslinking of fluoroelastomers

Ionic Cure vs. Peroxide Cure

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CF3 group aids cross link formation later in the process

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- CH2 between two carbon (or longer) perfluorinated monomer units

- easy dehydrofluorination (as for HFP:VF2:HFP or TFE:VF2:TFE)

Viton® A; F-content 66%

Requirements for Curing with Bisphenol

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HFP:VF2:HFP easy formation of C=C, crosslink very efficiently

TFE:VF2:TFE 3

- H and F combine with acid acceptors to form water and metal fluorides

B and F types also cured via C=C, formed at isolated VF2 sequences

but less efficiently because curatives are less soluble in the polymer -

they cannot find the cure sites easily

Viton® B, F; F-content 66-70%

Requirements for Curing with Bisphenol

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Bisphenol Cure System Curing Mechanism

1. Formation of the soluble bisphenol monophosphoniumsalt, for nucleophilic reaction with the polymer

[adapted from Schmiegel, Kautschuk Gummi Kunststoffe, 1978, 31, 137 ]

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Bisphenol Cure System Curing Mechanism 2. Creation of diene functionality in the polymer chain, through reaction of the soluble base (bisphenol monophosphoniumsalt) with the FKM (dehydrofluorination)

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Bisphenol Cure System Curing Mechanism

3. Crosslinking of two polymer chains

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Test at 180°C, 0.5°, 3 mins

A types cure very quickly B types are slower F types need very high levels of accelerator Ratios for reasonable cure rates

A type about 4 : 1 (BP-AF to Accelerator ratio) B type about 3 : 1 F type about 2 : 1

0

5

10

15

20

25

30

0 0.5 1 1.5 2 2.5 3Time (mins)

Torq

ue (d

Nm)

F type with 2.85 : 1 (BpAF : Accel)

F type with 1.9 : 1 (BpAF : Accel)

B type with 3.3 : 1 (BpAF : Accel)

A type with 4.2 : 1 (BpAF : Accel)

Influence of Polymer Type on Cure Rate

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Higher curative level results in :

- higher modulus

- higher hardness

- lower elongation

- better compression set

- poorer TR-10

- better flow

- better mould release

Lower BP-AF : Accelerator ratio results in :

- faster cure

- increases scorch

- poorer compression set

- more mould fouling

Higher BP-AF : Accelerator ratio results in :

- slower cure

- less scorch

- better compression set

Ratio of Bisphenol to Accelerator Impact on Properties

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BPAF / BTPPC Ratio Impact on Curing

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BPAF / BTPPC Ratio Impact on Properties

BPAF

BTPPC

2,22,01,81,61,41,2

0,6

0,5

0,4

0,3

0,2

> <

28,5 30,030,0

16,516,5 18,018,0 19,519,5 21,021,0 22,522,5 24,024,0 25,525,5 27,027,0 28,5

CS 70 200

Contour Plot of CS 70 200 vs BTPPC;; BPAF

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cure site creation and acid acceptors usual metal oxides are Ca(OH)2 and MgO : 6 phr Ca(OH)2 and 3 phr MgO

effects of metal oxide levels higher Ca(OH)2 results in faster curing but poorer compression set and properties higher MgO results in better heat resistance and better bonding high metal oxide levels adversely affect for flow (injection) metal oxides promote mould sticking and fouling

types of MgO usual levels are 3 phr high activity or 15 of low activity MgO (processing!!!)

Metal oxides are hygroscopic and are often the cause of scorch problems

Effect of Metal Oxides During Bisphenol Cure

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Effect of Metal Oxides During Bisphenol Cure using Calcium Hydroxide to control cure rate

MDR, 180C, 0.5 deg, 12 mins

0

5

10

15

20

25

0 2 4 6 8 10 12 14

Time (mins)

Torq

ue (d

Nm

)

6 CaOH24 CaOH26 CaOH2, 0.5 VC-306 CaOH2, 1.0 VC-30

General Trends: reducing calcium hydroxide level from 6 to 4 phr reduces cure rate similar reduction in cure rate by adding 0.5 phr VC-30 but increasing the modulus adding 1.0 phr VC 30 significantly reduces cure rate but increases modulus adding VC-30 gives a reduced cure rate because of a higher curative to accelerator ratio

Viton® VTR 9307 FKM Copolymer FKM Terpolymer

test conditions: acetic acid, 504h @ 100°C

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VTR 9307 vs Viton® A331C

standard bisphenol compounds provide poor resistance to organic acids due to MO

Viton® VTR 9307 new bisphenol curable precompound development with new cure activator and good acid resistance

New Bisphenol Curable Grade

X X X X

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Viton® GAL-S, GBL-S, GF-S

65.5-70%F

Viton® GLT-S, GBLT-S, GFLT-S

64.5-67%F

Requirements for Curing with Peroxides

peroxide cured fluoroelastomers for best hot water resistance, improved acid and base resistance and low post cure capability

no need of metal oxides but could be incorporated for improved cure efficiency and heat resistance

bromine or iodine containing cure site monomers have to be incorporated

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Peroxide Cure System Curing Mechanism

1. t-butoxy radical generation and beta scission to the methyl radical and acetone

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Peroxide Cure System Curing Mechanism

2. Adding methyl radical to TAIC and abstracting bromine from polymer chain

more likely the TAIC radical polymerizes

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Reality: TAIC Oligomer with some FKM crosslinks

1. polymer radical is formed by reaction of methyl radical with CSM

2. polymer radical reacts with the (poly)coagent

Peroxide Cure System Curing Mechanism

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Peroxide Cure System Idealized Mechanism 3. Reaction of the polymer radical with TAIC

4. The coagent provides three potential network points

Peroxide / Coagent Ratio Impact on Curing

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Basic recipe = 100 phr Viton® GLT-600S, 30 phr N-990, 3 phr ZnO, 0.5 phr VPA #2, Peroxide and Coagent are variables

Luperox 101XL45 phr

Diak 7 phr

28.01

27.38

26.75

26.12

25.49

24.86

24.23

23.60

2.42.22.01.81.61.41.21.0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

MH dNm vs Diak 7, Luperox 101XL45

Luperox 101XL45 phr

Diak 7 phr

0.414

0.402

0.390

0.378

0.366

0.354

0.342

2.42.22.01.81.61.41.21.0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

Ts1 mins vs Diak 7, Luperox 101XL45

Luperox 101XL45 phr

Diak 7 phr

0.76

0.72

0.68

0.64

0.60

0.56

0.52

2.42.22.01.81.61.41.21.0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

Tc-­50 mins vs Diak 7, Luperox 101XL45

Luperox 101XL45 phr

Diak 7 phr

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0.8

2.42.22.01.81.61.41.21.0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

Tc-­90 mins vs Diak 7, Luperox 101XL45

ts1 MH

tc50 tc90

Peroxide Cure System

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Luperox 101XL45 phr

Diak 7 phr

22.9

22.1

21.3

20.5

19.718.9

2.42.22.01.81.61.41.21.0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

Tensile Strength vs Diak 7, Luperox 101XL45

Basic recipe = 100 phr Viton® GLT-600S, 30 phr N-990, 3 phr ZnO, 0.5 phr VPA #2, Peroxide and Coagent are variables

Luperox 101XL45 phr

Diak 7 phr

328 320

312

304

296

288

280

2.42.22.01.81.61.41.21.0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

Elongation @ Break vs Diak 7, Luperox 101XL45

Luperox 101XL45 phr

Diak 7 phr

70.8

70.2

69.6

69.0

68.4

67.8

67.2

2.42.22.01.81.61.41.21.0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

Shore A pts vs Diak 7, Luperox 101XL45

Luperox 101XL45 phr

Diak 7 phr

24.223.6

23.0

22.4

21.8

21.2

20.620.0

2.42.22.01.81.61.41.21.0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

Comp Set % vs Diak 7, Luperox 101XL45 (70hrs @ 200C)

Peroxide / Coagent Ratio Impact on Properties

Tensile Elongation

Duro Comp Set

Peroxide Cure System

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- Coagent controls the number of cross links - the most common coagents are

- Triallylisocyanurate (TAIC) should be used with Viton® APA polymers

- Trimethallylisocyanurate (TMAIC) not recommended for Viton® APA polymers

- Triallylcyanurate (TAC) sometimes used

- TMAIC and TAC

- will give slow and inefficient curing with Viton® APA polymers

0

5

10

15

20

25

30

0 1 2 3 4

Torq

ue (d

Nm)

Time (mins)

TAIC - 3

TMAIC - 1

TMAIC - 1.5

TMAIC - 2

TAC 3

Influence of Different Coagents on Cure Speed

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Conclusions two relevant curing mechanisms ionic and radical Ionic Mechanism Bisphenol AF cure for high efficiency,VF2 units surrounded by HFP units BTPPC and BPAF commonly used for ionic cure ratio of BPAF and BTPPC influences cure rate, cure state

and mechanical properties Novel BPAF cure system overcomes deficiencies in dilute acids

radical cure requires cure site monomer that contains bromine or iodine most common peroxide and coagent are DBPH and TAIC cure rate depends mainly on peroxide level cure state depends mainly on coagent level properties influenced by ratio of peroxide and coagent

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