Incorporating Silica into Natural Rubber

Suyanti Hersanto1, Joachim Bertrand², Wisut Kaewsakul1, Wilma K. Dierkes1, Anke Blume1

1 2

December, 2018

Content

INTRODUCTION QUESTIONS EXPERIMENT RESULTS &

DISCUSSION

ANSWERS

1. 2. 3. 4. 5.

1. INTRODUCTION

Natural Rubber (NR)

• Renewable material

• Good dynamic properties

(high fatigue resistance, low

hysteresis, low heat build-up)

Applications: anti-vibration mountings,

heavy duty tires, conveyor belts, etc.

4

NR network

Protein: bonds with ω-end, acts as

a crosslink point (hydrogen bonds)

Phospholipids: branching point

(hydrogen and ionic bonds)

Repeating unit:

cis-1,4-polyisoprene

ω-end

S. Amnuaypornsri, et.al. Rubber Chem. Technol., 81 (2008), 753.

B. Rodgers. (2016). Rubber compounding: chemistry and applications, 2nd ed. CRC Press.

Silica

5

Granulated Powder

Applications: tires and automotive products

Improved performances compared to carbon black filled

tire tread compounds:

• Tear resistance

• Wet traction

• Low rolling resistance

• Low heat build-up

Functional groups

Isolated and geminal silanol groups most

reactive, highly polar

Problem: NR is non-polar, whereas silica is polar.

Difficult to disperse the silica

Use of silane supports compatibility to polymer

M.D. Bair, J.G. Dorsey. J. Chromatogr. A 1220 (2012), 35.

Y. Li, et.al. Rubber Chem. Technol., 67 (1994), 693.

Coupling Mechanisms

Requirements for coupling

TESPT to silica:

• Temperature >130oC

• Sufficient reaction time

By-product: Ethanol

6

RO

RO

ROtriethoxysilyl-

group

organo functional

group

propyl spacer

XSi

RubberSilica

surface

OH

Silica

coupling

Rubber

coupling

Coupling during

mixing

Coupling during

vulcanization

Silica

7

Granulated Powder

Applications: tires and automotive products

Improved performances compared to carbon black filled

tire tread compounds:

• Tear resistance

• Wet traction

• Low rolling resistance

• Low heat build-up

Functional groups

Isolated and geminal silanol groups most

reactive, highly polar

Problem: NR is non-polar, whereas silica is polar.

Difficult to disperse the silica

Use of silane supports compatibility to polymer

But even after reacting with silane ca. 75% of all

Si-OH remain

Presence of silane decreases viscosity which

results in worse dispersion

M.D. Bair, J.G. Dorsey. J. Chromatogr. A 1220 (2012), 35.

Y. Li, et.al. Rubber Chem. Technol., 67 (1994), 693.

Dispersion Mechanisms

8Palmgren, H., Processing Conditions in the Batch-Operated Internal Mixer. Rubber Chem. Technol., 1975. 48: p. 462-494

Presilanized silica

ALTERNATIVES

Presilanized silica in a masterbatch

9

Conventional Mixing and the Alternatives

Mixing constraints:

• High temperature for the coupling reaction (T>130oC)

• NR starts to degrade at this temperature

Conventional or dry mixing

rubber silica silaneStage 1

Stage 2 re-mixing: coupling reaction

Stage 3 compound curing agents

time-consuming, ethanol emission,

mixing + chemical reaction at the

same time

rubber Pre-silanized

silica

compound curing agents

2nd step required?

• Second mixing step might not be necessary save time

• No ethanol emission

• Silica is already dispersed in the masterbatch

compound curing agents

rubber with presilanized silica

2nd step required?

NR-Silica Masterbatch

Y. Gui, et.al. Composites Part B 85 (2016): 130-139.

S. Parasertsri, N. Rattanasom. (2011). Polym. Testing, 30,515.10

2. NR latex is

added

3. After

coagulation1. (Presilanized) silica

slurry

4. NR-silica masterbatch (dried &

compacted)

SEM observation of

NR / 30phr silica / 3phr TESPT:

Silica is better dispersed in a masterbatch

NR Dry mix

NR/silica/TESPT

Masterbatch

NR/silica/TESPT

*)TESPT was

added during

mixing

2. QUESTIONS

Questions

Mixing time

3 mixing cycles were applied: long (L), short (S) and very short (VS).

Q1. Which is the most suitable mixing cycle for each compound?

Mixing problems

2 processing aids were used: PA1 and PA2. Polarity: PA1 < PA2.

Q2. Does a processing aid supports the mixing?

Mixing procedure

Evaluations on the mixing behavior and the mechanical properties.

Q3. Which is the best solution: in-situ, presilanized or masterbatch?12

3. EXPERIMENT

Method DRY MIX (DM) PRESILANIZED (PS) MASTERBATCH (MB)

Mixing cycle L VS VS L VS L S VS

NR 100 100 100 100 100

NR-silica masterbatch 152.12 152.12 152.12

ULTRASIL VN3 GR 50 50 50

TESPT coupling agent 5 5 5

PS1: BM Silica COS 8569 MH 52.12 52.12

PA1 (Ultra-flow 700S) 2 2 2 2 2 2

PA2 (Ultra-DFR 900) 2 2

Compound number 1 2 3 4 5 9 10 11

Chemicals: ZnO, stearic acid, 6PPD, TMQ, wax, TDAE oil

Curatives: Sulfur, CBS, TBzTD

Amount in phr (part per hundred part of rubber)

Compound Formulation

14

L: long

S: short

VS: very short

Mixing ProcedureMixer: Brabender 390 ml

Fill factor: 70%

Starting speed: 60 rpm

15

0.5

0.5

0.5

0.5

0.5

0.5

2

2

1

1

1.5

1.5

2

2

4

4

0 1 2 3 4 5 6 7 8 9 10 11

4

0 1 2 3 4

STAGE 2(T dump = 140oC)

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

1

1

1

1

0 1 2 3

STAGE 3(T dump < 100oC)

STAGE 1(T dump = 140oC)

Long (L)

Short (S)

Very short

(VS)

Very short

(VS) for MB

Add NR / ½ NR-silica masterbatch

Add silica, silane / ½ NR-silica masterbatch Add ½ NR-silica masterbatch and ½ PA2

Add chemicals and PA1 or PA2 Add chemicals and ½ PA2

Sweep (ram opened) Add previously mixed compound

Shearing (ram closed) Add curatives

18:50

14:20

10:20

10:20

Time (min)

Measurements

Mixed compound

(uncured)

Tensile

Hardness

Dispersion

Cure

characteristics

Mooney viscosity

Bound rubber

Filler-filler

interaction

Compound

(cured)Cure

Heat aging

70oC

Compression set

16

4. RESULTS AND DISCUSSION

Mixing

NR-silica masterbatch tough material

• High silica content

• Absence of processing additives

• Higher mixing energy

Different incorporation times of the chemicals

• Presilanized silica has poor flowability

(DM: granulated; PS1: fluffy; MB: dispersed)

• Presence of silica between the mixer wall and

the compound slippage

00 01 02 03 04 05 06 07 08 09 10 110

50

100

150

200

250

300

350

400Torque Temp DM-PA1_L (1)

Torque Temp PS1-PA1_L (4)

Torque Temp MB-PA1_L (9)

Torq

ue [

Nm

]

Time [min]

0

20

40

60

80

100

120

140

160

Tem

pe

ratu

re [

oC

]

DM = dry mix

PS1 = presilanized

MB = masterbatch

LONG cycle

18

Mixing

Similar profiles for different processing

additives in the dry mix (2) and (3)

00 01 02 03 04 05 06 070

50

100

150

200

250

300

350

400

Torq

ue [N

m]

Time [min]

Torque Temp DM-PA1_VS (2)

Torque Temp DM-PA2_VS (3)

Torque Temp PS1-PA1_VS (5)

Torque Temp MB-PA2_VS (11)

0

20

40

60

80

100

120

140

160

Tem

pera

ture

[oC

]

DM = dry mix

PS1 = presilanized

MB = masterbatch

VERY SHORT cycle

19

Mooney Viscosity

DM-PA1_L (1

)

DM-PA1_VS (2

)

DM-PA2_VS (3

)

PS1-PA1_L (4

)

PS1-PA1_VS (5

)

MB-PA1_L (9

)

MB-PA1_S (1

0)

MB-PA2_VS (1

1)

0

20

40

60

80

ML (

1+

4)

100

oC

[M

U]

Dry mix: Longer mixing = higher viscosity:

• Better silanization expected (=lower viscosity)

• Pre-scorch by premature crosslinking of TESPT

20

Presilanized silica and masterbatch:

• Silanization reaction shouldn’t have an influence

• Higher decrease of filler-filler network = better

dispersion

• Higher cleavage of NR chains

Payne Effect: Indicator for Micro-Dispersion

DM-PA1_L (1

)

DM-PA1_VS (2

)

DM-PA2_VS (3

)

PS1-PA1_L (4

)

PS1-PA1_VS (5

)

MB-PA1_L (9

)

MB-PA1_S (1

0)

MB-PA2_VS (1

1)

0

50

100

150

200

Sto

rag

e m

od

ulu

s (

G') [kP

a]

G'(0.56%) - G'(100%)

G'(100%)

• Shorter mixing cycle higher filler-filler network and less

cleavage of polymer chains

• Masterbatch compounds lowest filler-filler interaction

• PA1 leads to lower Payne effect than PA2

Peng, et.al. (2005). Polym. Adv. Technol. 16: 770-78221

Filler network

Filler-rubber and rubber

networks

Bound Rubber

DM and PS1: long cycle higher bound rubber content

• Higher coupling possibility?

• Premature crosslink

MB: long cycle lower bound rubber content

• Better dispersion of silica: release of occluded rubber

Processing aids

• PA2 is more polar than PA1: Supports PA2 silica – silane –

polymer coupling?

DM-PA1_L (1

)

DM-PA1_VS (2

)

DM-PA2_VS (3

)

PS1-PA1_L (4

)

PS1-PA1_VS (5

)

MB-PA1_L (9

)

MB-PA1_S (1

0)

MB-PA2_VS (1

1)

0

10

20

30

40

50

Bou

nd

rubbe

r [%

]

Physical BR

Chemical BR

22H.D. Luginsland, et.al. (2002). Rubber Chem. Technol., 75, 563.

Silica cluster

Cure Characteristics at 160oC

0 5 10 15 20 25 300

1

2

3

4

5

6

7

8

To

rque

[dN

m]

Time [min]

PS1-PA1_L (4)

DM-PA1_L (1)

MB-PA1_L (9)

MB-PA2_VS (11)MB-PA1_S (10)

PS1-PA1_VS (5)

DM-PA1_VS (2)DM-PA2_VS (3)

DM: very short mixing marching modulus (2) and (3)

• Long cure time

• Inadequate silanization

23

DM, PS1: long mixing reversion (1) and (4)

• NR chain degradation

MB: close results regardless of their mixing cycles

DM PS1 MB

Long cycle (L)

& PA1

Very short cycle (VS)

& PA1

Very short cycle (VS)

& PA2

11 (98%)3 (97%)

4 (97%)1 (97%) 9 (96%)

5 (96%)2 (93%)

100 μm

10 (97%)Short (S) cycle

Macro Dispersion

Worse dispersion:

VS mixed dry mix with PA1

PA2 improves dispersion in

the very short mixing time

(by improved silica – silane

– polymer coupling?)

24

5. ANSWERS

Answers

Q1. Which is the most suitable mixing cycle for each compound?

• Dry mix long mixing

• Pre-silanized and NR-silica masterbatch very short mixing cycle possible

Q2. Does a processing aid supports the mixing?

• Yes. PA2 (Ultra-DFR 900, higher polarity than PA1) is preferable for a shorter mixing time

• Supports the silica – silane – polymer coupling but leads to higher Payne effect in the green compound

Q3. Which is the best solution: in-situ, presilanized or masterbatch?

Overall, NR-silica masterbatch with very short mixing:

• In-rubber properties are as good as the conventionally mixed one but with shorter mixing time

26

Answers

All in all, different influences have to be considered:

Level of

• dispersion

• silanization

• polymer degradation

• premature crosslink

• Polymer-polymer coupling

• Polymer – silane – silica coupling

27

Challenging!

ACKNOWLEDGEMENT