Compounding Precipitated Silica in Elastomers: Theory and Practice

560 Pages·2007·3.58 MB·English

by  Norman Hewitt

COMPOUNDING
PRECIPITATED SILICA IN
ELASTOMERS
Norman Hewitt
Copyright © 2007 by William Andrew Inc.
No part of this book may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying, recording, or by any information
storage and retrieval system, without permission in writing from the Publisher.
Cover art by Brent Beckley / William Andrew, Inc.
ISBN: 978-0-8155-1528-9 (William Andrew, Inc.)
Library of Congress Cataloging-in-Publication Data
Hewitt, Norman.
Compounding precipitated silica in elastomers / by Norman Hewittt.
p. cm.
Includes bibliographical references and index.
ISBN-13: 978-0-8155-1528-9 (978-0-8155)
ISBN-10: 0-8155-1528-6 (0-8155-)
1. Elastomers. 2. Rubber. 3. Silica. I. Title.
TS1925.H69 2007
678--dc22
2007008123
Printed in the United States of America
This book is printed on acid-free paper.
10 9 8 7 6 5 4 3 2 1
Published by:
William Andrew Publishing
13 Eaton Avenue
Norwich, NY 13815
1-800-932-7045
www.williamandrew.com
NOTICE
To the best of our knowledge the information in this publication is accurate; however the
Publisher does not assume any responsibility or liability for the accuracy or completeness
of, or consequences arising from, such information. This book is intended for
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health standards.
Table of Contents

Acknowledgements xiii
Preface xv

Chapter 1: SILICA AS A REINFORCING FILLER
1.1 Introduction 1
1.2 Manufacture of Precipitated Silica 2
1.3 Silica and Carbon Black 3
1.4 Silica Surface Area 5
1.5 Silica Free Water 7
1.6 Silica Free Water, Affect on Visible Dispersion 11
1.7 Silica Surface Silanol Groups 12
1.8 Silica pH 16
1.9 Soluble Salts in Silica 18
1.10 Physical Form and Sensity of Silica 19
1.11 Other Silica Properties 21
1.12 Silane Treated Silicas 21

Chapter 2: COMPOUNDING PRECIPITATED SILICA IN
NATURAL RUBBER
2.1 Introduction 25
2.2 Silica and Carbon Black 25
2.3 Activation: Zinc Oxide 28
2.4 Cure Activation: Glycols 29
2.5 Acceleration with Secondary Accelerators in Normal Sulfur Systems 31
2.6 Acceleration: Single Accelerators in Normal Sulfur Systems 33
2.7 Acceleration: Single Accelerators; Vulcanizate Properties 37
2.8 Acceleration: Low Sulfur/Sulfur Donor Systems 42
2.9 Reversion 47
2.10 Antioxidant Systems: Non-staining 50
2.11 Plasticization 51
2.12 Tear Resistance 53
2.13 Tear Resistance: Contour Curve Studies of Silica Content Effects 60
2.14 Tear Resistance: Silica Primary Particle Size 64
2.15 Tear Resistance; Non-Marking Solid tires 65
2.16 Shelf Aged Stiffness and Green Strength 68
2.17 Peroxide Cure 69
2.18 Peroxide Curing: Silica Reinforcement and Structure 70
2.19 Peroxide Curing: Silica Surface Area 73
2.20 Peroxide Cure: Silane Coupling 76
2.21 Silane Coupling: Sulfur Cure Systems 77
2.22 Zinc-Free Cure Systems 78
2.23 Zinc-Free Cure Systems: Polyisoprene (IR) 79
2.24 Brass Adhesion 81
vii
2.25 Brass Adhesion Mechanism 82
2.26 Adhesion to Textile Fabrics; the HRH System 83
2.27 Fabric Adhesion: Dynamic Testing 85
2.28 Heat Resistance 86
Natural Rubber Formulary 89

Chapter 3: COMPOUNDING PRECIPITATED SILICA IN
EMULSION SBR
3.1 Introduction 169
3.2 Silica and Carbon Black 169
3.3 Cure Systems: Activation with Glycols 171
3.4 Cure System: Zinc Oxide Activation 174
3.5 Cure System: Magnesium Oxide Activation 180
3.6 Cure System: Lead oxide (Litharge) Activation 181
3.7 Cure System: Stearic Acid 181
3.8 Cure Systems: Primary, Secondary Accelerators 181
3.9 Cure Systems: Single Accelerators 182
3.10 Cure Systems: Sulfur Concentration 185
3.11 Plasticization 187
3.12 Antioxidants 189
3.13 Tear Resistance: Silica Primary Particle Size 190
3.14 Tear Resistance: Silica Content 191
3.15 Fabric Adhesion 192
3.16 Heat Resistance 193
3.17 Silane Coupling 193
3.18 Silane Coupling: Competition 196
Emulsion SBR Formulary 199

Chapter 4: COMPOUNDING PRECIPITATED SILICA IN
SOLUTION SBR AND BR
4.1 Introduction 245
4.2 Silica and Carbon Black 245
4.3 Zinc-Free Cure Systems 247
4.4 Zinc-Free Cure Systems: Accelerators & Sulfur 249
4.5 Zinc-Free Cure Systems: Polymer Effects 251
4.6 Zinc-Free Cure Systems: Zinc Oxide and HMT 252
4.7 Zinc-Free Cure Systems: Effects of Additives 256
4.8 Zinc-Free Cure Systems: Sulfur Content 257
4.9 Zinc-Free Cure System: Antioxidants 259
4.10 Zinc-Free Cure Systems: Processing 260
4.11 Zinc-Free Systems: Plasticizers 262
4.12 Zinc-Free Systems: Additive Plasticizers 263
4.13 Silane Coupling: Pretreated Silica 264
4.14 Silane Coupling 265
4.15 Zinc-Free Cure Systems: Surface Area Effects 268
viii
4.16 Zinc-Free Cure Systems: Trouser Tear Strength 271
4.17 Zinc-Free Cure Systems ; Silica Content 272
4.18 Zinc-Free Cure Systems: Durometer Equivalents 274
Solution SBR and BR Formulary 277

Chapter 5:COMPOUNDING PRECIPITATED SILICA IN
EPDM
5.1 Introduction 311
5.2 Silica and Carbon Black 311
5.3 Acceleration Systems 314
5.4 Low Sulfur Systems with Donors 315
5.5 Activation: Oxides and Glycols 317
5.6 Antioxidants: Heat Resistance 318
5.7 Zinc-Free Cure Systems 319
5.8 Silane Coupling 323
5.9 Silica Surface Area 325
5.10 Peroxide Cure Systems 328
5.11 Processing 329
5.12 Adhesion to Brass 330
5.13 Fabric Adhesion 333
5.14 Adhesion to Zinc (Galvanized) Coatings 336
5.15 Compression Fatigue Life 338
EPDM Formulary 345

Chapter 6: COMPOUNDING PRECIPITATED SILICA IN
NEOPRENE
6.1 Introduction 387
6.2 NSM (Type W) Neoprene: Oxide Crosslinking 388
6.3 NSM Neoprene (W): Organic Acceleration 389
6.4 NSM Neoprene: Glycol Activation 390
6.5 NSM Neoprene: Plasticization 391
6.6 NSM Neoprene: Silica and Black 392
6.7 Silica Surface Area 395
6.8 NSM Neoprene: Silane Coupling 396
6.9 NSM Neoprene: Fabric Adhesion 398
6.10 NSM Neoprene: Brass Adhesion 400
6.11 NSM Neoprene: Water Absorption 403
6.12 Sulfur Modified (SM) Neoprene: Cure Systems 404
6.13 SM Neoprene: Glycol Activation 407
6.14 SM Neoprene: Retarding Scorch 407
6.15 SM Neoprene: Silane Coupling 407
6.16 SM Neoprene: Processing 408
6.17 SM Neoprene: Silica Surface Area Effects 410
6.18 SM Neoprene: Silica Free Water Content 410
6.19 SM Neoprene: Cord and Fabric Adhesion 411
ix
6.20 SM Neoprene: Brass Adhesion 414
Neoprene Formulary 416

Chapter 7: COMPOUNDING PRECIPITATED SILICA IN
NITRILE
7.1 Introduction 445
7.2 Silica and Carbon Black 445
7.3 Silica Surface Area 449
7.4 NBR/PVC Blends 452
7.5 Acceleration: Sulfur Content 452
7.6 Accelerators 453
7.7 Activators 456
7.8 Silane Coupling 460
7.9 Peroxide Curing 462
7.10 Processing 464
7.11 Zinc-Free Cure Systems 465
7.12 Phenolic Resins 469
7.13 NBR Adhesion to Brass 470
7.14 NBR Adhesion to Fabric 472
Nitrile Formulary 475

Appendix A: COMPOUNDING BASICS
A.1 The Compound 517
A.2 The Formula 517
A.3 Design of Formulas 518
A.4 Formula Mixing 520
A.5 Dispersion 520
A.6 Processing Properties 521
A.7 Physical Properties 522
A.8 Silica Surface Area 524

Appendix B: COMPOUNDING MATERIALS
B.1 Elastomers 527
B.2 Natural Rubber (NR) 528
B.3 Styrene-Butadiene Rubber (SBR) 530
B.4 Polybutadiene Rubber (BR) 531
B.5 Ethylene Propylene Rubbers (EPM, EPDM) 532
B.6 Neoprene (CR) 533
B.7 Nitrile Rubber (NBR) 533
B.8 Sulfur-Based Cure Systems 534
B.9 Non-Sulfur Cure Systems 537
B.10 Antidegradants 538
B.11 Other Additives 542

x
Appendix C: RUBBER PROCESSING
C.1 Mastication 545
C.2 Masterbatching 545
C.3 Remilling 546
C.4 Finish Mixing 546
C.5 Extruding 546
C.6 Calendering 547
C.7 Vulcanization 547

Appendix D: PHYSICAL TESTING OF RUBBER
D.1 Processability 551
D.2 Vulcanizate Tests 555
D.3 Weather and Ozone Resistance Tests 562
D.4 Accelerated Aging Tests 564
D.5 Fluid Resistance Tests 565
D.6 Low-Temperature Properties Tests 566

Appendix E: COMMON COMPOUNDING ABBREVIATIONS 569

INDEX 573

xi
CHAPTER 1
SILICA AS A REINFORCING FILLER
1.1 INTRODUCTION
The subject of this chapter is fine particle, precipitated, hydrated
silica and its use as a reinforcing filler for elastomer compounds. A more
complete definition, relative to its position in the family of silicas, relies
on a classification of commercial silicon dioxide, based on origin and
primary particle size. Table 1.1 is a partial listing of the many varieties
used in rubber compounding under the word “silica”.
Table 1.1 Forms and Properties of Silica Used in Rubber Compounding
Primary Size, (cid:80)m Function in Rubber
Natural (crystalline):
Ground quartz 1-10 Extending
Diatomite 1-5 Processing; Extending
Neuberg silica 1-5 Extending
Synthetic (amorphous):
Fumed 0.005-0.02 Reinforcing
Precipitated 0.01-0.03 Reinforcing
Precipitated 0.04 Semi-reinforcing
Precipitated 0.08 Processing; Color
Ferro-silicon by-product 0.10 Extending
The two major classes, based on origin, are natural and synthetic.
This distinction translates to a division between crystalline and
amorphous forms, and, of equal importance, to a substantial division
between coarse and fine primary particles.
Among the natural, non-reinforcing materials, the term “ground
quartz” includes a number of variously named grades which are defined
in respect to their geographic and geologic origin. For example, the grade
known as “tripoli” is quartz mined mainly in southern Illinois, USA. The
adaptability of this material to fine grinding has led to an erroneous
classification as an amorphous type. Neuberg silica, better known as
Sillitin(cid:149), derives from a German deposit of corpuscular quartz and
kaolin. Quartz fillers find their principal use as extenders in silicone
compounds, and elsewhere, to provide transparency.
Among the synthetic group, rubber reinforcement, in terms of enhanced
abrasion resistance and tear and tensile strengths, is supplied only by those
precipitated and fumed silicas with primary particle diameters below 40
1
COMPOUNDINGPRECIPITATEDSILICA IN ELASTOMERS
nanometers (0.040 microns). The larger particle size grades (above 40
nanometers) are noted for their contribution to nerve reduction and smooth,
extruded surfaces during compound processing operations.
The largest particle material, used only as an extender, is a furnace
type, sometimes called microsilica. It is formed as a by-product during
the manufacture of ferro-silicon alloy or silicon metal.
Fumed or pyrogenic silicas offer the smallest particle sizes and,
therefore, the highest degree of reinforcement. They are produced by the
high temperature hydrolysis of silicon tetrachloride, a process which
results in a pure silica with low silanol and water content. Processing
problems and high prices have limited fumed silica markets to silicone
compounds and other specialty elastomers.
The ensuing compounding discussions and formula recommendations in
Chapters 2 to 7 are centered on the reinforcing grades of precipitated silicas in
the 15 to 20 nanometer size range.
1.2 MANUFACTURE OF PRECIPITATED SILICA
Precipitated silica is produced by the controlled neutralization of
dilute sodium silicate (waterglass) by either concentrated sulfuric,
hydrochloric, or carbonic acids. The raw materials are those required for
the silicate: sand, soda ash, caustic soda, and water. The silicate can be
produced in furnace or digester operations, but in either case the ratio of
SiO to Na O is generally within a range of 2.5 to 3.5. Dilution with
2 2
water provides relatively low silicate concentrations, which, together
with moderate acidification rates, produce a precipitate of particulates
rather than gel agglomerates. A minor amount of gel is usually present.
Reaction temperature is the major determinant of primary particle size.
Precipitation produces a low solids content slurry of hydrated silica
and residual salts, either sodium sulfate, sodium chloride or sodium
carbonate. The salts are removed by washing in either a counter-current
decantation system or by filter press. Washing reduces the salt content to
1 or 2%. Further concentration in rotary or plate and frame filters
produces a solid wet cake which still contains only 15 to 25% silica.
Because of this high water content, the final drying step, whether by
rotary, tray, belt or spray dryers, is a large consumer of energy. Due to
lower investment and operating costs, spray drying has become the
dominant drying process. In all cases the final product still contains
about 6% free water, which is roughly the equilibrium free water content
at 50% relative humidity. The end product is often milled and compacted
to attain an optimum balance between the absence of visible particles and
minimal dustiness during rubber mixing.
2