combining guide to silane solutions 3 dow corning – the silane technology pioneer the concept of...

Combining technology expertise with

market knowledge to help you develop

new materials with greater reliability

and improved performance.

from Dow Corning Guide to Silane Solutions

3

Dow Corning – The Silane Technology Pioneer .......................................... 4

Your Continuing Resource for Innovation and Application Success ............ 5

The Basics of Silane Chemistry .................................................................... 6

The Concept of Coupling with Organofunctional Silanes .......................... 8

Silane Coupling Agents ............................................................................... 8

Why Silane Coupling Agents Are Used ....................................................... 8

The Silane Bond to the Inorganic Substrate ................................................ 9

The Silane Bond to the Polymer ................................................................ 10

How to Choose a Silane Coupling Agent ................................................... 10

Typical Silane Applications ........................................................................ 13

Silanes from Dow Corning ........................................................................... 14

Fiberglass and Composites ....................................................................... 15

Mineral and Filler Treatment ...................................................................... 16

Paints, Inks and Coatings .......................................................................... 18

Primers .................................................................................................. 19

Zinc-Rich Primers ................................................................................. 20

Chromium Replacement ....................................................................... 20

Industrial Maintenance .......................................................................... 20

Automotive Clearcoats .......................................................................... 20

Architectural Coatings ........................................................................... 21

Typical Coating Benefits ....................................................................... 21

Pharmaceutical Manufacturing .................................................................. 22

Plastics and Rubber .................................................................................. 22

Rubber Compounding ........................................................................... 22

Polymer Manufacturing ......................................................................... 24

Plastics Compounding .......................................................................... 24

Adhesives and Sealants ............................................................................ 25

Adhesion Promoters ............................................................................. 25

Crosslinkers .......................................................................................... 26

Water Scavengers ................................................................................ 26

Coupling Agents .................................................................................... 26

Water Repellents and Surface Protection .................................................. 26

General Construction Applications ........................................................ 26

Other Surface Protection Applications .................................................. 27

Other Applications ..................................................................................... 27

The Surface and Interface Solutions Center – A Valuable Resource for Customer Success ............................................. 28

More than Materials – Competitive Advantage .......................................... 28

Dow Corning – The Right Partner for You .................................................. 29

Visit Our Website ....................................................................................... 29

Contents

Dow Corning – The Silane Technology Pioneer

ow Corning pioneered the development of organo-

silane technology more than 50 years ago to provide

new classes of materials – silicones and silanes – with special

physical and chemical properties. This research led to a new

industry based on the synergy of organic and silicon chemis-

tries. Silicones and silanes are now essential components in

many major applications; without them, many of the materials we rely on today would not exist.

The value of silane coupling agents

was first discovered in the 1940s in

conjunction with the development of

fiberglass-reinforced polyester com-

posites. When initially fabricated,

these new composites were very

strong, but their strength declined

rapidly during aging. This weaken-

ing was caused by a loss of bond

strength between the glass and res-

in. In seeking a solution, research-

ers found that organofunctional

silanes – silicon chemicals that

contain both organic and inorganic

reactivity in the same molecule –

functioned as coupling agents in the

composites. A very small amount

of an organofunctional alkoxysilane

at the glass-resin interface not only

significantly increased initial com-

posite strength; it also resulted in a

dramatic retention of that strength

over time. Subsequently, other ap-

plications for silane coupling agents

were discovered, including mineral

and filler reinforcement; mineral

dispersion; adhesion of paints, inks

and coatings; reinforcement and

crosslinking of plastics and rubber;

reinforcement and adhesion of seal-

ants and adhesives; water repel-

lents and surface protection.

Your Continuing Resource for Innovation and Application SuccessDow Corning continues to pioneer

the development of innovative

technologies and applications for

organosilane and silicon-containing

materials through our global

research team and Surface and

Interface Solutions Center (SISC).

From automotive to marine to

aerospace, from electronics to

building construction to sporting

goods, Dow Corning silanes are

an important component of today’s

sophisticated technologies. They

enable new materials to be devel-

oped with greater reliability and

improved performance.

With a full range of silane

product and application solutions,

Dow Corning offers you technology

leadership, reliable supply, world-

class manufacturing and global

reach. In addition to materials, we

offer supportive services and

solutions you may never have

imagined. Silane solutions.

Distinctly Dow Corning.

5

The Basics of Silane Chemistry

ilicon is in the same family of elements as carbon in

the periodic table. In their most stable state, silicon and

carbon will both conveniently bond to four other atoms; but

silicon-based chemicals exhibit significant physical and

chemical differences compared to analogous carbon-based

chemicals. Silicon is more electropositive than carbon, does

not form stable double bonds, and is capable of very special

and useful chemical reactions. Silicon-based chemicals

include several types of monomeric and polymeric materials.

Figure 1. Carbon vs. silicon chemistry.

Organic (Carbon-Based) Chemical

H (alkane hydrogen)

(methyl) CH3 � OCH3 (methyl ether)

CH2CH2CH2-NH2 (aminopropyl)

Silane (Silicon-Based) Chemical

H (hydride)

(methyl) CH3 �� OCH3 (methoxy)

CH2CH2CH2-NH2 (aminopropyl)

7

Monomeric silicon chemicals are

known as silanes. A silane structure

and an analogous carbon-based

structure are shown in Figure 1. The

four substituents have been chosen

to demonstrate differences and

similarities in physical and chemi-

cal properties between silicon- and

carbon-based chemicals. A silane

that contains at least one carbon-

silicon bond (CH3-Si-) structure is

known as an organosilane. The

carbon-silicon bond is very stable,

very non-polar and gives rise to low

surface energy, non-polar, hydro-

phobic effects. Similar effects can

be obtained from carbon-based

compounds, although these effects

are often enhanced with silanes.

The silicon hydride (–Si-H) structure

is very reactive. It reacts with water

to yield reactive silanol (-Si-OH)

species and, additionally, will add

across carbon-carbon double bonds

to form new carbon-silicon-based

materials. The methoxy group

on the carbon compound gives a

stable methyl ether, while its attach-

ment to silicon gives a very reactive

and hydrolyzable methoxysilyl

structure. The organofunctional

group, the aminopropyl substituent,

will act chemically the same in the

organosilicon compound as it does

in the carbon-based compound.

The distance of the amine, or

other organofunctional group, from

silicon will determine whether the

silicon atom affects the chemistry

of the organofunctional group. If the

organic spacer group is a propylene

linkage (e.g., -CH2CH

2CH

2-), then

the organic reactivity in the organo-

functional silane will be similar to or-

ganic analogs in carbon chemistry.

Certain reactive silanes, particularly

vinyl silanes (-Si-CH=CH2) and

silicon hydrides (-Si-H), are useful

reactive groups in silicon chemistry,

even though the reactive group is

attached directly to the silicon atom.

Attachment of chlorine, nitrogen,

methoxy, ethoxy or acetoxy directly

to silicon yields chlorosilanes, silyl-

amines (silazanes), alkoxysilanes

and acyloxysilanes, respectively,

that are very reactive and exhibit

unique inorganic reactivity. Such

molecules will react readily with

water, even moisture adsorbed on

a surface, to form silanols. These

silanols then can react with other

silanols to form a siloxane bond

(-Si-O-Si-), a very stable structure;

or in the presence of metal hydroxyl

groups on the surface of glass,

minerals or metals, silanols will form

very stable –Si-O-metal bonds to

the surface. This is the key chem-

istry that allows silanes to function

as valuable surface-treating and

coupling agents.

Chloro-, alkoxy-, and acetoxy-

silanes, and silazanes (-Si-NH-Si)

will react readily with an active

hydrogen on any organic chemical

(e.g., alcohol, carboxylic acid,

amine, phenol or thiol) via a proc-

ess called silylation.

R3Si-Cl + R'OH ‡

R3Si-OR' + HCl

Silylation is very useful in organic

synthesis to protect functional

groups while other chemical

manipulations are being performed.

The silylated organofunctional

group can be converted back to the

original functional group once the

chemical operation is completed.

Silylation is very important in the

manufacture of pharmaceutical

products.

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Without Silane With Silane

The Concept of Coupling with Organofunctional Silanes

Silane Coupling Agents

ilane coupling agents are silicon-based chemicals that

contain two types of reactivity – inorganic and organic

– in the same molecule. A typical general structure is

(RO)3SiCH

2CH

2CH

2-X,

where RO is a hydrolyzable group, such as methoxy, ethoxy,

or acetoxy, and X is an organofunctional group, such as amino,

methacryloxy, epoxy, etc.

A silane coupling agent will act at an interface between an

inorganic substrate (such as glass, metal or mineral) and an

organic material (such as an organic polymer, coating or

adhesive) to bond, or couple, the two dissimilar materials.

A simplified picture of the coupling mechanism is shown in

Figure 2.

For a more detailed discussion of

this mechanism, read “A Silane

Primer: Chemistry and Applications

of Alkoxy Silanes” by Gerald L.

Witucki, Journal of Coatings

Technology, Volume 65, Number

822, July 1993, pages 57-60. A

reprint of this article is posted in the

Technical Library in the Fiberglass

and Composites section of the

Dow Corning Silanes Solutions

website, www.dowcorning.com/

silanes.

Why Silane Coupling Agents Are UsedWhen organic polymers are re-

iforced with glass fibers or miner-

als, the interface, or interphase

region, between the polymer and

the inorganic substrate is involved

in a complex interplay of physical

and chemical factors. These factors

are related to adhesion, physical

Figure 2. The silane coupling mechanism.

Figure 3. SEM of silica-filled epoxy resin.

http://www.dowcorning.com/content/publishedlit/25-507-01.pdf?DCWS=Silanes%20Solutions&DCWSS=Fiberglass%20and%20Composites <http://www.dowcorning.com/content/publishedlit/25-507-01.pdf?DCWS=Silanes%20Solutions&DCWSS=Fiberglass%20and%20Composites

9

��

CH3OH

HO-Si-O-Si-O-Si-OH

RSi(OCH3) 3

�� RSi(OH) 3

H2O

H2O

R R R

O O O

H H H

strength, coefficient of expansion,

concentration gradients and reten-

tion of product properties. A very

destructive force affecting adhe-

sion is migration of water to the

hydrophilic surface of the inorganic

reinforcement. Water attacks the

interface, destroying the bond

between the polymer and reinforce-

ment, but a “true” coupling agent

creates a water-resistant bond at

the interface between the inor-

ganic and organic materials. Silane

coupling agents have the unique

chemical and physical properties

not only to enhance bond strength

but also, more importantly, to

prevent de-bonding at the interface

during composite aging and use.

The coupling agent provides a

stable bond between two otherwise

poorly bonding surfaces. Figure 3

shows (via an SEM of the fracture

surface) the difference in adhesion

between a silica-filled epoxy resin

with silane vs. without silane. With

silane, the epoxy coating on the

silica particles is apparent; without

silane, clean silica particles can be

seen in the epoxy matrix.

In composites, a substantial

increase in flexural strength is

possible through the use of the

right silane coupling agent. Silane

coupling agents also increase the

bond strength of coatings and

adhesives as well as their resistance

to humidity and other adverse

environmental conditions.

Other benefits silane coupling

agents can provide include:

• Better wetting of inorganic

substrates

• Lower viscosities during

compounding

• Smoother surfaces of

composites

• Less catalyst inhibition of

thermoset composites

• Clearer reinforced plastics

Figure 4. Hydrolysis of alkoxysilanes. Figure 5. Bonding to an inorganic surface.

The Silane Bond to the Inorganic Substrate Silane coupling agents that contain

three inorganic reactive groups on

silicon (usually methoxy, ethoxy or

acetoxy) will bond well to the metal

hydroxyl groups on most inorganic

substrates, especially if the sub-

strate contains silicon, aluminum or

a heavy metal in its structure. The

alkoxy groups on silicon hydrolyze

to silanols, either through the

addition of water or from residual

water on the inorganic surface.

Then the silanols coordinate with

metal hydroxyl groups on the

inorganic surface to form an oxane

bond and eliminate water. See

Figures 4 and 5.

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� � �

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�

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��

��

Silane molecules also react with

each other to give a multimolecular

structure of bound silane coupling

agent on the surface. More than

one layer, or monolayer equiva-

lents, of silane is usually applied to

the surface. This results in a tight

siloxane network close to the inor-

ganic surface that becomes more

diffuse away from the surface.

The Silane Bond to the PolymerThe bond to the organic polymer is

complex. The reactivity of a ther-

moset polymer should be matched

to the reactivity of the silane. For

example, an epoxysilane or amino-

silane will bond to an epoxy resin;

an aminosilane will bond to a

phenolic resin; and a methacrylate

silane will bond through styrene

crosslinking to an unsaturated

polyester resin. With thermoplastic

polymers, bonding through a silane

coupling agent can be explained by

inter-diffusion and inter-penetrating

network (IPN) formation in the

interphase region. See Figure 6.

To optimize IPN formation, it is

important that the silane and the

resin be compatible. One method

is to match the chemical character-

istics of the two materials. This will

help improve the chances of form-

ing a good composite with optimum

properties. Even with thermoset

polymers, where reactivity plays an

important role, chemical structure

matching will enhance the physical

properties of the composite.

How to Choose a Silane Coupling AgentAll silane coupling agents with three

OR groups on silicon should bond

equally well with an inorganic sub-

strate. A variety of organofunctional

alkoxysilanes is available. See

Figures 7 and 8.

Figure 6. The inter-penetrating network (IPN) bonding mechanism.

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Silica

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11

Matching the organofunctional

group on silicon with the resin poly-

mer type to be bonded will dictate

which silane coupling agent should

be used in a particular application.

The organic group on the silane can

be either a reactive organic group

(i.e., an organofunctional group),

or it can be a non-reactive organic

group. The groups can be hydro-

phobic or hydrophilic, with varying

thermal stability characteristics.

The solubility parameters of the

groups will vary, depending on the

organic structure; this will influence,

to some extent, the interpenetration

the polymer network will have into

the siloxane network of the surface

treatment. Table 1 lists some of the

characteristics for common organic

substituents attached to silicon.

The choice of silane should involve

matching chemical reactivity,

solubility characteristics, structural

characteristics and, possibly, the

thermal stability of the organosilane

with the same parameters in the

polymer structure.

Figure 7. Silane coupling agent variations – basic structure.

Figure 8. Silane coupling agent variations – alternative “Bis” structure.

Table 1. Characteristics of Various Organic Substituents on Silanes

Organosilanes R-Si(OMe)3

R Characteristics of “R”

Me Hydrophobic, Organophilic

Ph Hydrophobic, Organophilic, Thermal Stability

i-Bu Hydrophobic, Organophilic

Octyl Hydrophobic, Organophilic

-NH(CH2)

3NH

2Hydrophilic, Organoreactive

Epoxy Hydrophilic, Organoreactive

Methacryl Hydrophobic, Organoreactive

Alternative “Bis” Structure

Si

OR'

OR'

OR'

SiR

R'O

R'O

R'O

Z-6920Z-6670

SiCH2

CH2 CH2

SS

CH2

CH2

CH2

Si

OEt

EtO

EtO

OEtOEt

OEt

Si CH2

CH2

MeO

MeO

OMe

CH2

CH2

CH2

CH2

Si

OMe

OMe

OMe

NH

Si

R

OR'OR'

R'O

Si

CH

CH2

MeO

OMe

MeO

Z-6300

Z-6040

Z-6911

Si O

OMe

MeO

MeO

O

CH3CH2Z-6030

Si

EtO

EtO

OEt

Z-6341

Si

NH2

EtO

EtOOEt

Z-6011

Basic StructureR = alkyl, aryl, or organofunctional group

OR' = methoxy, ethoxy, or acetoxy

Si

CH2

CH2

CH2

SH

OEt

OEt

EtO

Si

O

OMe

MeO

MeO

O

Dow Corning® brand Silane

Organic Group

Alkoxy Group Chemical Name

Z-6697 - Ethoxy TetraEthoxysilane

Z-6070 Methyl Methoxy Methyltrimethoxysilane

Z-6366 Methyl Methoxy Methyltrimethoxysilane (HP)

Z-6370 Methyl Ethoxy Methyltriethoxysilane

Z-6383 Methyl Ethoxy Methyltriethoxysilane (HP)

Z-6194 Methyl Methoxy Dimethyldimethoxysilane

Z-6265 Propyl Methoxy Propyltrimethoxysilane

Z-6535 Propyl Ethoxy Propyltriethoxysilane

Z-2306 i-Butyl Methoxy Isobutyltrimethoxysilane

Z-6403 i-Butyl Ethoxy Isobutyltriethoxysilane

Z-6124 Phenyl Methoxy Phenyltrimethoxysilane

Z-6341 n-Octyl Ethoxy n-Octyltriethoxysilane

Table 3. Silane Coupling Agent Recommendations for Various Polymers – Matching Organoreactivity to Polymer Type

Table 2. Non-Organoreactive Alkoxysilanes

Dow Corning® brand Silane Organic Reactivity Application (suitable polymers)

Z-6011 AminoAcrylic, Nylon, Epoxy, Phenolics, PVC, Urethanes, Melamines, Nitrile Rubber

Z-6020 AminoAcrylic, Nylon, Epoxy, Phenolics, PVC, Melamines, Urethanes, Nitrile Rubber

Z-6028 Benzylamino Epoxies for PCBs, Polyolefins, All Polymer Types

Z-6030 Methacrylate Unsaturated Polyesters, Acrylics, EVA, Polyolefin

Z-6032 Vinyl-benzyl-amino Epoxies for PCBs, Polyolefins, All Polymer Types

Z-6040 Epoxy Epoxy, PBT, Urethanes, Acrylics, Polysulfides

Z-6076 Chloropropyl Urethanes, Epoxy, Nylon, Phenolics, Polyolefins

Z-6094 AminoAcrylic, Nylon, Epoxy, Phenolics, PVC, Melamines, Urethanes, Nitrile Rubber

Z-6106 Epoxy/Melamine Epoxy, Urethane, Phenolic, PEEK, Polyester

Z-6128 Benzylamino Epoxies for PCBs, Polyolefins, All Polymer Types

Z-6137 Amino

Acrylic, Nylon, Epoxy, Phenolics, PVC, Melamines, Urethanes, Nitrile Rubber (especially suited for water-based systems)

Z-6224 Vinyl-benzyl-amino Epoxies for PCBs, Polyolefins, All Polymer Types

Z-6300 VinylGraft to Polyethylene for Moisture Crosslinking, EPDM Rubber, SBR, Polyolefin

Z-6376 Chloropropyl Urethanes, Epoxy, Nylon, Phenolics, Polyolefins

Z-6518 VinylGraft to Polyethylene for Moisture Crosslinking, EPDM Rubber, SBR, Polyolefin

Z-6675 Ureido Asphaltic Binders, Nylon, Phenolics; Urethane

Z-6910 Mercapto Organic Rubber

Z-6920 Disulfido Organic Rubber

Z-6940 Tetrasulfido Organic Rubber

A list of alkyl and aryl, non-organo-

reactive alkoxysilanes is provided

in Table 2. Those silanes give

modified characteristics to inorganic

surfaces, including hydrophobic-

ity, organic compatibility and lower

surface energy.

Based on experience and histori-

cal applications of silanes, a list of

silane coupling agents and recom-

mendations for evaluation with

various polymer types is provided in

Table 3. A correlation can be seen

between the chemistry and struc-

tural characteristics of the silane

coupling agent and the chemistry

and structural characteristics of the

polymer.M

ore

Hyd

roph

obic

13

Typical Silane ApplicationsCoupling Agent: Organofunctional

alkoxysilanes are used to couple

organic polymers to inorganic ma-

terials. Typical of this application

are reinforcements, such as

fiberglass and mineral fillers,

incorporated into plastics and

rubbers. They are used with both

thermoset and thermoplastic

systems. Mineral fillers, such as

silica, talc, mica, wollastonite,

clay and others, are either pre-

treated with silane or treated in situ

during the compounding process.

By applying an organofunctional

silane to the hydrophilic, non-

organoreactive filler, the surfaces

are converted to reactive and

organophilic. Fiberglass applica-

tions include auto bodies, boats,

shower stalls, printed circuit boards,

satellite dishes, plastic pipes and

vessels, and many others. Mineral-

filled systems include reinforced

polypropylene, silica-filled molding

compounds, silicon-carbide grinding

wheels, aggregate-filled polymer

concrete, sand-filled foundry resins

and clay-filled EPDM wire and

cable. Also included are clay- and

silica-filled rubber for automobile

tires, shoe soles, mechanical goods

and many other applications.

Adhesion Promoter: Silane cou-

pling agents are effective adhesion

promoters when used as integral

additives or primers for paints, inks,

coatings, adhesives and sealants.

As integral additives, they must

migrate to the interface between the

adhered product and the substrate

to be effective. As a primer, the

silane coupling agent is applied to

the inorganic substrate before the

product to be adhered is applied.

In this case, the silane is in the

optimum position (in the interphase

region), where it can be most

effective as an adhesion promoter.

By using the right silane coupling

agent, a poorly adhering paint, ink,

coating, adhesive or sealant can

be converted to a material that

often will maintain adhesion even if

subjected to severe environmental

conditions.

Hydrophobing and Dispersing

Agent: Alkoxysilanes with hydro-

phobic organic groups attached

to silicon will impart that same

hydrophobic character to a hydro-

philic inorganic surface. They are

used as durable hydrophobing

agents in construction, bridge

and deck applications. They are

also used to hydrophobe inorganic

powders to make them free-

flowing and dispersible in organic

polymers and liquids.

Crosslinking Agent: Organo-

functional alkoxysilanes can react

with organic polymers to attach

the trialkoxysilyl group onto the

polymer backbone. The silane is

then available to react with moisture

to crosslink the silane into a stable,

three-dimensional siloxane struc-

ture. Such a mechanism can be

used to crosslink plastics, especially

polyethylene, and other organic res-

ins, such as acrylics and urethanes,

to impart durability, water resistance

and heat resistance to paints, coat-

ings and adhesives.

Moisture Scavenger: The three

alkoxy groups on silanes will hydro-

lyze in the presence of moisture to

convert water molecules to alcohol

molecules. Organotrialkoxysilanes

are often used in sealants and other

moisture-sensitive formulations as

water scavengers.

Polypropylene Catalyst “Donor”:

Organoalkoxysilanes are added to

Ziegler-Natta catalyzed polymer-

ization of propylene to control the

stereochemistry of the resultant

polypropylene. The donors are

usually mono- or di-organo silanes

with corresponding tri- or di-alkoxy

substitution on silicon. By using

specific organosilanes, the tacticity

(and hence the properties) of the

polypropylene is controlled.

Silicate Stabilizer: A siliconate

derivative of a phosphonate-

functional trialkoxysilane functions

as a silicate stabilizer to prevent

agglomeration and precipitation of

silicates during use. The predomi-

nant application is in engine coolant

formulations to stabilize the silicate

corrosion inhibitors.

Silanes from Dow Corning

ow Corning is the industry leader in supplying silane and

intermediate product solutions; this is one of our com-

pany’s core businesses. Our silanes business unit encompasses the

following product groups:

• Chlorosilanes

• Organofunctional silanes

• Specialty silanes

• Alkylsilanes

Methylchlorosilanes are the basic building blocks of all of our silicon-

based materials. They are used in basic synthesis of silanes and

siloxanes, as protecting agents for intermediates in pharmaceutical

synthesis, and as precursors in the manufacture of silicon-carbide

coatings. Chlorosilanes are essential raw materials in the electronics

and telecommunications industries and for the production of optical

fibers, silicon wafers and chips, as well as the starting materials for

fumed silica.

Alkylsilanes, specialty silanes and organofunctional silanes have

alkyl, aryl or organofunctional groups attached to silicon and have

methoxy, ethoxy or acetoxy groups attached to silicon to allow them

to function in the manner described in this brochure.

Lists of silanes commercially available from Dow Corning can be

found at www.dowcorning.com/silanes. Data sheets for these products

can be viewed and downloaded from the website. We have many

other silicon-based materials that may be of value to you as well.

Information about these products can be obtained by contacting

Dow Corning Customer Support either by e-mail or telephone.

15

0

100

200

300

400

500

600

700

Dry Strength Wet Strength, 72-hour water boil

Flex

ural

Str

engt

h, M

PaNone

Z-6040 (Epoxy)

Z-6032 (Vinyl-benzylamino)

Figure 9. Effect of silane coupling agents on the strength of glass-reinforced epoxy.

Fiberglass and CompositesSilane coupling agents are a critical

component of fiberglass-reinforced

polymers. The glass is very hydro-

philic and attracts water to the

interface. Without silane treatment

on the surface, the bond between

the glass fiber and the resin would

weaken and eventually fail. Silane

coupling agents are used on

fiberglass for general-purpose

reinforced plastic applications, such

as automotive, marine, sporting

goods and building construction,

as well as for high-performance

applications in printed circuit

boards and aerospace composites.

Dow Corning® brand silanes figure

prominently in the trend toward

increasingly more-durable, higher-

strength plastic composites.

The chemical structure of the organic

group in a silane coupling agent has

a great effect on its performance

in a composite, as measured by

improvement of strength proper-

ties under wet and dry conditions.

A wet-aging test, usually in boiling

water, will show differences in the

effectiveness of various silanes.

The effect of the organic structure

of the coupling agent on improving

the flexural strength of a glass-

reinforced, unsaturated polyester

composite is shown in Figure 9.

The vinylbenzyl-functional silane

coupling agent (Dow Corning®

Z-6032 Silane, in this case) yields

greater improvement in the flexural

strength of a glass-reinforced

epoxy system than does the epoxy-

functional silane coupling agent

(Dow Corning® Z-6040 Silane).

More significantly, the retention of

strength after aging for 72 hours in

boiling water is better with either

silane than if no silane coupling

agent is used; but Z-6032 Silane

provides better retention of flexural

strength. These are the types of

effects generally expected from the

use of silane coupling agents.

Fiberglass for general-purpose

applications is treated with a dilute

aqueous sizing bath consisting of a

combination of ingredients (organic

film formers, lubricants, antistats

and a silane coupling agent). The

silane must be soluble in the

aqueous bath at levels of 0.2 to 1

percent. Normally, if a water bath

is acidified with acetic acid to a pH

of 4, even hydrophobic silanes will

dissolve in the bath at low con-

centrations and give the stability

needed to treat the fiberglass.

Certain silanes, such as aminosi-

lanes, are more hydrophilic and will

dissolve at high concentrations in

water even without pH adjustment.

The size is applied to the fiberglass

at the glass fiber manufacturing

plant immediately after the glass

fibers are extruded and bundled

into glass fiber rovings.

Fiberglass for high-performance

electronics, such as printed circuit

boards, is processed differently.

The glass fiber is treated with a

starch size at the glass manufactur-

ing plant, after which a “fiberglass

weaver” weaves the fiber into glass

cloth. The weaver then burns off

the starch size at high temperature,

producing “heat-cleaned” glass

cloth. This clean cloth is then

passed through a bath containing

0.2 to 0.5 percent silane coupling

agent. Usually, no other significant

sizing chemical is in the bath. The

glass cloth is dried, inspected for

flaws and supplied to a fabricator

who makes epoxy, or other polymer,

prepregs and laminates for printed

circuit boards. This application

requires excellent coupling agent

technology to provide the flaw-free

benefits required. Dow Corning

Z-6032 Silane, and variations on

this product, have been developed

to provide the necessary quality

and performance for printed circuit

boards.

Depositing the silane as a silse-

quioxane (organosilicon with three

oxygen atoms shared with other

silicon atoms) on a surface and

measuring the weight loss by

thermal gravimetric analysis (TGA)

Figure 10. Thermal stability of silanes at 300ºC (572ºF), TGA.

Table 4. Thermal Stability of Mixed Silanes – Phenyl + Amino, S-Glass/Polyimide Laminates

functional silanes, can provide

benefits. The improvement in

thermal stability of a fiberglass-

polyimide composite is shown in

Table 4.

Some of the benefits imparted to

fiberglass-reinforced plastics by

Dow Corning silanes include:

• Improved mechanical strength

of the composites

• Improved electrical properties

• Improved resistance to

moisture attack at the interface

• Improved wet-out of the

glass fiber

• Improved fiber strand integrity,

protection and handling

• Improved resistance to hot

solder during fabrication

• Improved performance in

cycling tests from hot to cold

extremes

Table 3 on page 12 suggests

silanes for evaluation with vari-

ous fiberglass-reinforced polymer

systems. Product data sheets are

available at www.dowcorning.

com/silanes.

Mineral and Filler TreatmentMineral fillers have become increas-

ingly important additives and modifi-

ers for organic polymers. The metal

hydroxyl groups on the surface of

minerals are usually hydrophilic and

incompatible with organic polymers.

Alkoxysilanes are a natural fit to

treat the surface of the mineral to

Coupling Agents on Glass

Properties of Laminates, MPa

9:1 Blend, Silane A and C

Aminosilane Alone, Silane B

Flexural Strength, initial 544 476

1000 hr @ 260°C (500°F) 409 258

2000 hr @ 260°C (500°F) 306 134

Silane A: Z-6124 Ph-Si(OCH3)

3

Silane B: Z-6011 H2N(CH

2)

3Si(OCH

2CH

3)

Silane C: Z-6020 H2N(CH

2)

3NH(CH

2)

2Si(OCH

3)

3

can determine the thermal stability

of the silane. Results of isothermal

TGA at 300ºC (572ºF) for several

silanes are shown in Figure 10.

The diaminosilane (Dow Corning®

Z-6020 Silane) exhibited very poor

thermal stability. As expected,

the phenyl silane (Dow Corning®

Z-6124 Silane) showed excellent

thermal stability. Surprisingly,

the complex vinylbenzyl silane

(Z-6032), based on Z-6020,

showed very good thermal stability.

These data suggest that for high-

temperature applications, Z-6032,

or blends of Z-6124 with other

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(CH2)3NHCH2CH2NHCH2 -CH=CH2

-CH2CH2CH2O-

-CH2CH2CH2NH-

-(CH2)3NHCH2CH2NH2

-NH3

-NH3

HCl

Stability of RSiO3/2 in Air

Hours at 300°C (572°F)

% R

Rem

aini

ng

R - On Silicon

17

make it more compatible and dis-

persible in the polymer, or even to

make the filler a reinforcing additive.

In addition to plastics applications,

the use of silane-modified minerals

in organic rubber, especially tires,

has become increasingly important.

Minerals with silicon and aluminum

hydroxyl groups on their surfaces

are generally very receptive to

bonding with alkoxysilanes. The

treatment of a mineral surface by

an organosilane is depicted in

Figure 11. Silica (both fumed and

precipitated), glass beads, quartz,

sand, talc, mica, clay and wollaston-

ite have all effectively used silane

coupling agents in filled polymer

systems. Other metal hydroxyl

groups, such as magnesium

hydroxide, iron oxide, copper oxide,

and tin oxide, may be reactive to a

lesser extent, but often benefit from

silane treatment. Traditionally,

silane coupling agents give poor

bonding to carbon black, graphite

and calcium carbonate.

Silane treatment can improve proc-

essing, performance and durability

of mineral-modified products by:

• Improving adhesion between

the mineral and the polymer

• Improving wet-out of the

mineral by the polymer

• Improving dispersion of the

mineral in the polymer

• Improving electrical properties

• Increasing mechanical

properties

• Reducing the viscosity of

the filler/polymer mix

An example of the benefit of silane

treatment of a silica filler used in an

unsaturated polyester resin com-

posite is shown in Figure 12. As is

generally the case, the silane treat-

ment results in higher initial strength

and better retention of strength after

humidity aging. The silane also can

reduce the viscosity of the uncured

resin/filler mixture, to allow easier

processing, with different silanes

giving different effects. In this case

Dow Corning Z-6032 Silane (vinyl-

benzyl-amine) reduced viscosity

by 65 percent while Dow Corning

Figure 11. Filler surface treatment.

Figure 12. Viscosity and coupling effect – polyester castings with 50% silica.

Z-6030 Silane (methacrylate) re-

duced viscosity by only 10 percent.

Similarly, the ability of silane

coupling agents to impart improved

electrical properties is shown in

Table 5 on page 18. An epoxy resin

was cured with and without quartz

filler as the reinforcement. Without

filler, the epoxy resin showed good

electrical properties, dielectric

constant and dissipation factor,

even after aging for 72 hours in boil-

ing water. However, once quartz

filler was added, the hydrophilic

Flex

ural

Str

engt

h, M

Pa

No Silane – 24,500 Pa•s

Z-6030 (Methacrylate) – 22,000 Pa•s

Z-6032 (ViBz Amine) – 8,700 Pa•s

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Inorganic Surface

Mineral, Metal, Glass

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Surface Is Hydrophilic

Converted toOrganoreactive

Surface

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surface of the quartz led to severe

loss of electrical properties during

the water boil test. With either

epoxy-silane (Dow Corning

Z-6040 Silane) or aminosilane

(Dow Corning Z-6011 Silane), the

quartz-filled composite exhibited

improved retention of electrical

properties.

Minerals are treated with either

neat silane or a solution of silane in

water and/or alcohol. With a neat

silane, the adsorbed water on the

filler surface is often sufficient to

hydrolyze the alkoxysilane and

simultaneously bond the silane

to the filler surface. It is important

that the filler be coated uniformly

through the use of intensive mixing,

such as with a Henschel mixer.

Commercial processes are continu-

ous, often in a heated chamber,

followed by further heat treatment to

remove byproducts of alcohol and

water and to complete the bonding

of the silane to the surface.

The loading level of silane on the

filler surface is a function of the

surface area of the filler. While it

was thought that one monolayer of

silane should be sufficient, experi-

mentation has shown that several

layers of silane give optimal results.

For example, typical fillers with

average particle sizes of 1 to 5

microns often give best results

when treated with about 1 percent

silane. The optimal level of silane

treatment should be determined

experimentally.

The choice of which silane to use

in a particular application is deter-

mined by the nature of the benefit

that is to be derived from the silane.

All alkoxysilanes will bond to a

receptive filler or mineral surface.

If the silane treatment is designed

to provide surface hydrophobicity,

then a silane with a hydropho-

bic group, such as butyl, octyl,

fluorocarbon or phenyl, should be

chosen. If the silane treatment is

designed to provide compatibility

of the mineral in a polymer matrix,

then the nature of the organic group

on the silane should be similar

to the chemical structure of the

polymer (i.e., an octyl or longer-

chain alkyl group will help provide

compatibility and dispersibility of the

mineral in a polyolefin matrix). If the

silane treatment is to bond a filler to

Dielectric Constant Dissipation Factor

System1 Initial Water Boil2 Initial Water Boil2

Unfilled Resin 3.44 3.43 0.007 0.005

Quartz, no Silane 3.39 14.60 0.017 0.305

Quartz, Z-6040 3.40 3.44 0.016 0.024

Quartz, Z-6011 3.46 3.47 0.013 0.0231Z-6040 = Epoxysilane; Z-6011 = Aminosilane272-hour water boil

Table 5. Ability of Silane Coupling Agents to Impart Electrical Properties a polymer matrix, then an organo-

reactive silane should be chosen

that would bond chemically to reac-

tive sites present in the polymer.

A list of some mineral/filler applica-

tions is shown in Table 6.


silanes for evaluation with

various filled polymer systems.

Product data sheets are available

at www.dowcorning.com/silanes.

Paints, Inks and CoatingsTightening volatile organic com-

pound (VOC) regulations in the

coatings industry, along with

demand for improved physical prop-

erties and extended performance

life, have spurred interest in silane

technology. The unique capability

of silanes to create covalent bonds

between inorganic and organic

compounds, and the inherent

stability of the siloxane (Si-O-Si)

bond, make this technology a key

component in high-performance

paints and coatings. These proper-

ties lie at the heart of the ability of

these materials to withstand physi-

cal, chemical, environmental and

thermal degradation.

Silane monomers, in the form of

organofunctional alkoxysilanes,

are utilized widely in coatings as

adhesion promoters, pigment treat-

ments and crosslinkers. Inorganic

alkoxy functionality coupled with a

wide range of organofunctional

19

Fillers Comments

Kaolin Clay Reinforced Nylon, Wire and Cable (EPDM)

Talc Stiffness, Abrasion Resistance – Polypropylene (auto)

Mica Stiffness – Polypropylene (auto)

Silica Reinforced Rubber, Epoxy PCBs

Wollastonite Reinforced Plastics, Coatings

Glass Fiber/Beads Reinforced Plastics

Aluminum Trihydrate Flame Retardance

Magnesium Hydroxide Flame Retardance

Crystobalite Abrasion Resistance – Plastics

Titanium Dioxide Colorant, Filler – Plastics

Table 6. Mineral/Filler Applications

groups allows for covalent bonding

between organic polymers and

inorganic surfaces (e.g., pigments,

fillers, and glass and metal sub-

strates). The same coupling agent

mechanisms described earlier

allow for bonding between organic

polymers and inorganic surfaces. All

alkoxysilanes will bond essentially

identically to inorganic surfaces, but

the organofunctionality of the silane

must be matched with the chemistry

of the organic polymer in the paint,

ink or coating to obtain optimum

performance from the silane.

The use of silanes in coatings can

provide improvements in adhesion;

resistance to moisture, chemicals,

ultraviolet (UV) rays and abrasion;

and improved dispersion of fillers.

Alkoxysilane monomers (which are

not silicones, per se) are completely

miscible with many organic resins.

In fact, silanes are reasonably

strong polar solvents. Polymeriza-

tion of the silanes into silicone

resins and fluids impacts the

compatibility and performance of

the resulting polymer.

Silanes are also used as interme-

diates to produce silicates and

siliconates via reaction with metal

hydroxide (e.g., sodium or potas-

sium hydroxide). These materials

are used in protective finishes,

such as zinc-rich primers, masonry

treatments for water repellency, or

compounded directly into concrete

coatings for improved physical

properties and water repellency.

Silicates are derived primarily from

tetra-alkoxysilanes. In contrast, sili-

conates are produced via reactions

of mono- or di-organo (e.g., methyl

or other alkyl moieties) alkoxy-

silanes, which allow a broader

range of performance properties,

such as water repellency and

substrate penetration.

Primers

Silanes provide crucial functionality

in the primer segment of the coat-

ings industry. Alkoxysilanes have

broad utility in formulating primers

for a variety of metal and siliceous

substrates. Especially attractive to

the formulator is the wide range of

organo-reactive and non-reactive

moieties attached to the silicon

atom, which allows formulas to

be tailored to specific application

performance requirements. Widely

known as adhesion promoters,

alkoxysilane primers also offer

controlled hydrophobicity, excellent

UV and thermal stability, surface

activity, chemical resistance and

corrosion protection.

The silane coupling agent must act

at the interface between the sealant

or adhesive and the substrate. It

is chosen by matching its organic

functionality to the organic moiety

in the coating that is to be bonded.


silanes for evaluation based on the

nature of the organic moiety in the

coating. Often, mixtures of silanes

are used as adhesion promoters to

provide enhanced hydrophobicity,

thermal stability or crosslinking at

the bonding site.

Using a silane as a primer ensures

that the silane will be at the substrate-

polymer interface where it can

enhance adhesion. Silane primers

are often dilute solutions of silanes,

0.5 to 5 percent, in an alcohol or

water/alcohol solvent. They are

wiped or sprayed on the substrate

followed by solvent evaporation.

Zinc-Rich Primers

As early as 1962, partial hydroly-

zates of alkoxysilanes (e.g., tetra-

ethoxysilane), or alkali silicates,

combined with zinc metal powder

were found to provide galvanic

protection of ferrous substrates

beyond that imparted by organic

resin-based zinc primers.1 Initially,

this technology was limited by its

inherently short pot and shelf life.

Later, the stability and overall per-

formance of the primer was greatly

improved by trans-esterifying the

silicate with organic polyols (e.g.,

ethylene glycol or glycerol).2 This

innovation is one of the most widely

cited silicon-based inventions

(34 citations). These materials,

based on partial hydrolyzates of

tetra-ethoxy silane, are available

as either one- or two-part systems

and have been the dominant

galvanic primer used in the paint

industry. They are characterized

by tolerance to high humidity

and low-temperature application.

Solvent-based primers are best

suited for on-site application under

difficult weather conditions.

Chromium Replacement

State-of-the-art metal surface prep-

arations for adhesive bonding con-

sist mainly of anodization or etching

processes employing strong acids.

Many of these surface preparations

also contain hexavalent chromium.

Surface treatment is followed by the

application of a corrosion-inhibiting

adhesive primer that typically con-

tains high levels of volatile organic

compounds (VOCs) and additional

hexavalent chromium. Alternatives

to chromium compounds are being

sought due to new regulations, the

increased cost of hazardous waste

disposal and the increased aware-

ness of the costs associated with

employee health and safety.

In 1983, a primer composed of an

acrylic copolymer, an epoxy resin,

a silica sol and a trialkoxysilane

compound was developed. The

primer provided superior paint-

ability, degreaser resistance and

corrosion resistance after painting.3

Twelve years later, a wash primer,

without the acrylic copolymer or the

epoxy resin, was developed that

provided similar benefits.4 Metal

was pretreated with an alkaline

solution containing at least one

of a dissolved inorganic silicate,

a dissolved inorganic aluminate,

an organofunctional silane, and a

crosslinking agent containing

trialkoxysilyl groups. The metal

was then dried to completely cure

the functional silane, resulting in

an insoluble primer layer bonded

tightly to the metal substrate.

Industrial Maintenance

Combining the cure profiles and

barrier properties of organic resins

with the thermal and UV stability of

silanes, formulators have created

high-performance coatings with

excellent resistance to corrosion

and chemical attack as well as

thermal and UV degradation. A

blend consisting of an epoxy resin,

an epoxy resin curing agent, an

organofunctional alkoxysilane and

a catalyst for condensation poly-

merization of a silane compound

can provide high heat resistance

and excellent mechanical strength.5

Similarly, epoxy resins can be

reacted with hydrolyzed alkyl and

phenyl alkoxysilanes to produce

copolymers with improved water

and moisture resistance.6,7 Utilizing

the functional groups available from

silane monomers, resin formulators

have created organofunctional (e.g.,

epoxy and amine) silicone resins

for epoxy resin modification.8,9

Automotive Clearcoats

Color-plus-clear coating systems

involving the application of a colored

or pigmented base coat to a sub-

strate followed by application of

a clear topcoat have become the

standard as OEM finishes for

automobiles. Color-plus-clear

systems have outstanding appear-

ance properties (such as gloss and

distinctness of image) due, in large

part, to the clear coat. These clear

coatings are, however, subject to

damage from environmental

elements, such as acid rain, UV

degradation, high relative humidity

and temperatures, stone chipping

and abrasive scratching of the

coating surface.

21

Typically, a harder, more highly

crosslinked film may exhibit im-

proved scratch resistance; however,

high crosslink density embrittles the

film, making it much more suscep-

tible to chipping and/or thermal

cracking. A softer, less-crosslinked

film, while not prone to chipping

or thermal cracking, is susceptible

to scratching, water spotting and

acid etch. Clear coats in color-plus-

clear systems have demonstrated

improved scratch resistance with

the inclusion of surface-reactive,

inorganic microparticles, such

as silane coupling agent treated

colloidal silica.10

Architectural Coatings

Changes in building practices,

including concrete facades on multi-

floor buildings and shifts in the eco-

nomics of material and labor costs,

have contributed to the trend toward

silane-modified architectural paints.

By using a reactive organic group

on a trialkoxysilane to react into a

latex polymer backbone, the latex

polymer has the ability to crosslink

via a moisture crosslinking mecha-

nism once the coating is applied.

A primary concern for water-based

formulations is the stability of

alkoxysilanes in an aqueous envi-

ronment. Alkoxysilane adhesion

promoters (also known as coupling

agents) do react with water. For

silanes to provide the intended

benefits of adhesion or crosslinking,

the hydrolysis reaction is a neces-

sary and desired process step.

Modifying the silane, via transesteri-

fication, from methoxy functionality

to longer alkoxy groups (e.g.,

isopropoxy) can slow, but not

prevent, hydrolysis. Attaching an

alkoxy chain length sufficient to

eliminate hydrolysis would essen-

tially deactivate the silane. By

formulating to conpensate for the

inevitable hydrolysis and subsequent

condensation of alkoxysilanes,

coating formulators can still utilize

this technology to improve the

performance of many water-based

coatings.

Many coatings fail because water is

absorbed by or penetrates the film,

ultimately reaching the coating-

substrate interface. Alkoxysilanes

are well known for improving the

adhesion of coatings to metal or

siliceous substrates by forming

covalent bonds via dual organic-

inorganic reactivity. This is one of

several mechanisms by which

alkoxysilanes provide benefit. In

addition to chemical bonding, si-

lanes improve the hydrolytic stability

and integrity of the film. Including

alkoxysilanes in coating formula-

tions can create a more tightly

crosslinked, hydrophobic film that is

much less susceptible to moisture

attack. Significant benefit can be

achieved by adding 0.5 percent

silane (based on system solids) to

acrylic latex-based coatings.

Treatment of mineral pigments and

fillers (e.g., silica, titanium dioxide,

etc.) with alkoxysilanes is well

known in the coatings industry.

While pigment or filler suppliers

often treat fillers with silanes,

similar benefits can be observed

by incorporating the alkoxysilane

directly into a water-based coating

formulation. The presence of water

at typically high pH levels results

in hydrolysis of the silane and

condensation around the solid

particles. The net effect is better

integration of the inorganic particle

into the binder matrix, improved

dispersion and physical properties.

Successful incorporation of silanes

into water-based formulations

requires good dispersion of the

silane prior to complete hydrolysis

and condensation. Adequate mix-

ing is essential. Along with good

mixing, pre-diluting the silane into

a coalescing solvent or plasticizer

before adding it to the latex will

minimize condensation of the silane

monomers (and potential gel forma-

tion) and encourage interaction with

the other components of the coating

formulation.

Typical Coating Benefits

Silanes can impart several benefits

to coatings, including:

• Abrasion resistance

• Adhesion

• Better flow

• Crosslinking to improve thermal

stability and durability

• Pigment and filler dispersion

• UV resistance

• Water and chemical resistance

A list of Dow Corning® silanes for

use in paints, inks and coatings

is available at www.dowcorning.

com/silanes.

Pharmaceutical ManufacturingThe pharmaceutical industry relies

heavily on silane chemistry in the

synthesis of antibiotics, drugs and

medicines. Through a process

called silylation, the chemistry of

silanes allows them to be used as

protecting groups that permit chemi-

cal procedures to be performed,

while retaining the desired organic

functionalities necessary in the

pharmaceutical molecular structure.

Silylation is the displacement of

an active hydrogen in an organic

molecule by a silyl (R3Si) group.

The active hydrogen is usually -OH

(alcohol, carboxylic acid, phenol),

-NH (amine, amide, urea) or -SH

(thiol). The silylating agent is often

a trimethylsilylhalide, dimethylsilyldi-

halide or a trimethylsilyl nitrogen-

functional compound. However,

often larger, bulkier groups (e.g.,

tert-butyl) are on the silylating

agent to control the chemistry of the

reaction. Newer silylating agents

will cleave esters and ethers. A

mixture of silylating agents may be

used, such as trimethylchlorosilane

plus hexamethyldisilazane. This

blend is more reactive than either

reagent alone. The byproducts

combine to form neutral ammonium

chloride, e.g., in the following

reaction where the -Si(CH3)

3 group

replaces the active hydrogen in the

R-OH molecule.

R-OH + (CH3)

3SiNHSi(CH

3)

3

+ (CH3)

3 SiCl ‡ 3 RO-Si(CH

3)

3

+ NH4Cl

The unique chemical properties of

silanes allow them to replace one

or more active hydrogens during

chemical synthesis to protect these

groups, while subsequently allowing

other chemistries to be performed

on the molecules without destroy-

ing or altering the protected organic

functionalities. After the desired

chemical procedures are carried

out in other parts of the molecules,

the silane protective group can be

removed to regenerate the original

organic functionality.

Silanes have been used for many

years in the production of antibiotics,

such as penicillin and cephalosporin-

type medications. Tertiary-

butyldimethylchlorosilane is used

in anti-cholesterol drug production

as a “super-protector” during the

manufacturing process. Other

silanes, such as chloromethylsi-

lydimethylchlorosilanes, have been

used in direct chemical synthesis

of herbicides where the silicon atom

becomes a chemical part of the

final product.

As the global market for biologi-

cal and pharmaceutical products

increases, due to population growth

and increasing demand for health-

care, manufacturers will rely on

silanes as they develop the next

generation of medicinal therapies.

A list of Dow Corning® brand silylat-

ing agents for use in pharmaceuti-

cal manufacturing is available at

www.dowcorning.com/silanes.

Plastics and RubberThe unique properties of silanes are

used to enhance performance and

improve processes in the plastics

and rubber industries. Silanes

function as coupling and dispers-

ing agents for fillers in rubber and

plastics formulations, as polymer-

ization modifiers in the synthesis of

polypropylene, and as crosslinking

agents for polyethylene homopoly-

mers and copolymers.

Rubber Compounding

A major use for silanes has devel-

oped in the organic rubber industry

as a result of the benefits that can

be obtained from the use of

inorganic filler in place of carbon

black in the reinforcement of rubber.

Silica and other inorganic filler

reinforcements for rubber provide

unique physical properties and

performance properties versus

carbon black reinforcement; how-

ever, silane coupling agents are

necessary for the non-black

reinforcing fillers to be effective.

Silanes are the key to providing a

method of effectively bonding the

inorganic fillers to organic elasto-

mers. Silane-coupled, mineral-filled

rubber products are used for auto-

motive and off-road tires, shoe soles,

belts, hoses and mechanical goods.

The mechanism is similar to that

described earlier under “Mineral

and Filler Treatment.” Methoxy- or

ethoxy-silanes will bond tenaciously

to the silica or clay surface; then the

organic portion of an organofunc-

tional silane will bond to the rubber

polymer. See Figure 13.

23

The silane is usually added during

the compounding process to treat

the filler in situ. It must have the

proper rate of reactivity to spread

and react over the filler surface

and still be able to react with the

elastomer at a rate that allows

processing of the rubber to be

Figure 13. Bonding organic rubber to silica with sulfur silanes.

Figure 14. Structure of sulfidosilanes used in rubber compounds.

completed. This can be done with

silane coupling agents that have

triethoxysilyl groups at both ends of

a polysulfido (tetrasulfide, disulfide

or mixture thereof) organic group.

See Figure 14.

These coupling agents are supplied

as neat liquids or as blends with a

carrier such as carbon black. See

Table 7. Even though silica can be

used as the only filler, rubber tires

incorporate small levels of carbon

black to give consumers the uniform

black color they expect. Without

carbon black in the rubber com-

pound, it is possible to make tires

in a variety of colors.

A specific example of this applica-

tion is the silica/silane technology

used in “green” tires to impart:

• Increased abrasion resistance

• Reduced rolling resistance

and improved fuel economy

of tires

• Better grip on wet and snow/

ice surfaces

Silica-reinforced tires are known

as “green” tires because they pro-

vide improved fuel economy while

��

SiEtOEtO OEt

Sx

SiEtO

OEtOEt

Si

Si

OEt

O

SiO

O

OEt

SiO

OH

OEt

Si

O

Si

O

Si

O

Si

O

Si

O

Si

O

O

O

OEt

Sx

SxSilica Silica Rubber

S

Si

Si

OEt

O

SiO

O

OEt

SiO

OH

OEt

Si

O

Si

O

Si

O

Si

O

Si

O

Si

O

O

O

OEt

S

Sx

s

s

s

s

ss

s

The silane can react in thesulfur vulcanization

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Table 7. Sulfidosilanes for Rubber

Dow Corning® brand Silane Features Average Value of X

Z-6920 Liquid TESPD 2.20

Z-6925 Solid TESPD, 50% on Carbon Black 2.20

Z-6940 Liquid TESPT 3.75

Z-6945 Solid TESPT, 50% on Carbon Black 3.75

SiCH2

OEt

OEtOEt

CH2

CH2

Si CH2

EtO

EtO OEt

CH2

CH2

Sx

SiCH2

OEt

OEtOEt

CH2

CH2

Si CH2

EtO

EtO OEt

CH2

CH2

Sx

x ranges from 2 to 10These are termed S2, S3, etc., monomersBis-TriEthoxy Silyl Propyl Polysulfide - TESPX

maintaining or improving other tire

properties (as listed above). They

also use a mineral-derived filler

rather than one derived from a

fossil fuel (natural gas or oil). This

is currently the largest market for

silane coupling agents.

The use of vinyl silanes as a cou-

pling agent in kaolin clay reinforced

EPDM wire and cable coatings is

another important rubber applica-

tion. The vinyl silane improves the

electrical properties of the rein-

forced rubber so a stringent power-

factor electrical test can be passed,

but only when optimum silane

coupling agent technology is used.

In addition to silanes, Dow Corning

is a major supplier of silicone

rubber. Silicone rubber is made

from silicone polymers compounded

with non-black fillers, usually

fumed or precipitated silica.

These compounds require silanes

and functional silicone fluids.

Silanol-functional silicone fluids and

vinyl-functional silanes are available

for silicone rubber compounding.

A list of Dow Corning silanes for

rubber compounding is available


Information about our silicone

rubber materials is available at

www.dowcorning.com/rubber.

Polymer Manufacturing

Selected silanes, known as “exter-

nal donors,” or electron donors, are

used in conjunction with Ziegler-

Natta catalysts in the manufacture

of polypropylene. Ziegler-Natta

catalysts are organometallic

compounds. Organoal-koxysilanes

can chemically coordinate with the

organometallic catalyst to modify

the course of the polymerization.

Specific variations in the tacticity of

the propylene polymer are possible

by optimizing the use of a silane

donor in the process. Different

silane donors with differing organo-

alkoxy structures are used depend-

ing on the exact nature of the

catalyst and the type of polypropyl-

ene being manufactured. Organic

substituents, such as cyclohexyl,

cyclopentyl, methyl, isobutyl and

phenyl, are some of the organic

groups attached to silicon. The

alkoxy groups are either methoxy

or ethoxy with one, two, or three

alkoxy groups on the silane

molecule. Three of the more

common silane donors are Donor C,

cyclohexylmethyldimethoxysilane

(Dow Corning® Z-6187 Silane);

Donor D, dicypentyldimethoxysilane

(Dow Corning® Z-6228 Silane);

and di-isobutyldimethoxysilane

(Dow Corning® Z-6275 Silane).

Reactive silicone polymers have

also been used to produce ther-

moplastic vulcanizates (TPVs).

TPVs are prepared by chemically

crosslinking a rubbery phase in

a thermoplastic matrix. TPVs are

produced by dynamic vulcanization,

and silane chemistry allows new

and unique crosslinking chemistries

to be used in the manufacturing

process.


polymer manufacturing is available


Plastics Compounding

Vinyl silanes have been used

commercially since the 1970s to

crosslink polyethylene homopolymer

and its copolymers. Vinyltrimeth-

oxysilane and vinyltriethoxysilane

are the most common silanes used

in the process. In an extruder in the

presence of peroxide and heat, the

vinyl group will graft to the polyeth-

ylene backbone, yielding a silane-

modified polyethylene that contains

pendant trialkoxysilyl functionality.

The grafted polyethylene can then

be immediately crosslinked in the

presence of a tin catalyst, moisture

and heat to create a silane-

crosslinked product. Diagrams of

the grafting of vinyltrimethoxysilane

(VTMOS) to polyethylene and the

moisture crosslinking process are

shown in Figures 15 and 16. The

ease of processing and the simple

equipment required make this the

preferred method of producing

crosslinked ethylene polymers and

copolymers. The process also

allows crosslinking to be delayed

until after the grafted product is

transformed into its final product

configuration. Using the same

silanes, it is also possible to copoly-

merize the vinyl silane with ethylene

monomer to make trialkoxysilyl-

functionalized polyethylene. This

then can be crosslinked in the same

manner as the graft version.

Silane-crosslinked polyethylene is

used for electrical wire and cable

insulation and jacketing where ease

of processing, increased tempera-

ture resistance, abrasion resistance,

stress-crack resistance, improved

low-temperature properties and

25

retention of electrical properties are

needed. Other applications for this

technology include:

• Cold- and hot-water pipe

where resistance to long-

term pressure at elevated

temperatures is essential

• Natural gas pipe with good

resistance to stress cracking

• Foam for insulation and

packaging with greater

resiliency and heat resistance

• Other product and process

types, such as film, blow-

molded articles, sheeting

and thermoforming


plastics compounding is available


Additional information is available at

www.dowcorning.com/plastics.

Adhesives and SealantsSilanes are widely used to improve

the adhesion of a broad range of

sealants and adhesives to inorganic

substrates, such as metals, glass

and stone. Sealants are based on

filled, curable elastomers and have

the dual purpose of preventing

passage of water, air and chemicals

through the zone where applied; in

some cases they also serve as an

adhesive. Their usefulness in the

aircraft, automotive and construc-

tion industries depends upon their

ability to form durable bonds to

metal, glass, ceramic and other

surfaces – bonds that will withstand

exposure to heat, ultraviolet radia-

tion, humidity and water.

Adhesion Promoters

A silane coupling agent will function

at the interface between the seal-

ant or adhesive and the substrate

to act as an adhesion promoter.

An organofunctional silane uses

a mechanism similar to that

described earlier for bonding an

inorganic substrate and a sealant or

adhesive polymer. The silane

coupling agent is chosen by

matching its organic functionality

to the polymer to optimize bonding.

Figure 15. Grafting of VTMOS to polyethylene – Sioplas® process.

Table 3 on page 12 sugests silanes

to evaluate for various polymer

systems.

Often, mixtures of silanes are used

as adhesion promoters to provide

enhanced hydrophobicity, thermal

stability or crosslinking at the

bonding site.

The silanes can be blended into

an adhesive formulation or used

as primers on substrates. When

added to the adhesive formulation,

Figure 16. Crosslinking of polyethylene in the presence of moisture – Sioplas® process.

Silane-GraftedPolyethylene

ROOR

HeatSi

OMe

OMe

OMe

+

Polyethylene

VTMS

Si

OMe

OMe

OMe

Si OMe

MeO

MeO

��

��

Si OMe

MeO

MeO Si OMe

O

MeO

Si OMeMeO

DBTDL = Dibutyltindilaurate

the silane must be free enough to

migrate to the interphase region

between the adhesive/sealant and

the surface of attachment. The

structure and reactivity of the silane

will affect the ability of the silane

to migrate. Usually more than one

silane is evaluated for an applica-

tion to empirically choose the best

performing silane.

The most effective way to promote

adhesion is to apply the silane as a

primer to the surface, followed by

application of the adhesive/sealant.

In this way, the silane will be on the

surface and therefore at the inter-

face where it can enhance adhesion

between the polymer and the sub-

strate. Silane primers are usually

dilute solutions of 0.5 to 5 percent

silane in alcohol or water/alcohol

solvent. They are wiped or sprayed

on the substrate, after which the

solvent is allowed to evaporate.

When added to sealants or adhe-

sives or used as primers on

substrates, an improvement in

adhesion is often realized with the

bond showing greater resistance

to moisture attack at the interface.

This can result in:

• Increased initial adhesion

• An adhesive bond with

longer life

• Greater temperature

resistance

• Greater chemical resistance

Crosslinkers

Silanes can be used to crosslink

polymers such as acrylates, poly-

ethers, polyurethanes and polyes-

ters. The organofunctional portion

of the silane can react, and bond to,

the polymer backbone in a sealant

or adhesive. The alkoxysilyl group

on the silane should not crosslink

prematurely in order to be available

to provide crosslinking once the

sealant or adhesive is applied in its

intended application.

A silane-crosslinked sealant or

adhesive can show enhanced prop-

erties, such as:

• Tear resistance

• Elongation at break

• Abrasion resistance

• Thermal stability

• Moisture resistance

Water Scavengers

The ability of alkoxysilanes to react

very rapidly with water makes them

useful in sealant and adhesive

formulations to capture excess

moisture. A very common moisture

scavenger is vinyltrimethoxysilane.

The presence of the vinyl group at-

tached to silicon increases the rate

of reaction of the methoxysilane

with water to give efficient elimina-

tion of water. Methanol is formed

as a byproduct, and the vinyl silane

crosslinks into an inactive species

in the formulation. Other silanes,

such as methyltrimethoxysilane, are

also used as water scavengers.

Silane water scavengers in a formu-

lation can:

• Prevent premature cure

during compounding

• Enhance uniform curing

• Improve in-package stability

Coupling Agents

Silane coupling agents are used to

increase adhesion between fillers

and the polymer matrix in sealants

and adhesives. The mechanism

and mode of action was described

earlier under “Mineral and Filler

Treatment.”

The silane coupling agent treatment

on the filler can provide:

• Better bonding of the pigment

or filler to the resin

• Improved mixing

• Increased matrix strength

• Reduced viscosity of the

uncured sealant or adhesive


use in formulating sealants and

adhesives is available at

www.dowcorning.com/silanes.

Water Repellents and Surface Protection

General Construction Applications

Silanes can be chosen to impart

hydrophobic (water repellent) and/

or oleophobic (oil and stain repel-

lent) characteristics to surfaces.

Silanes with alkyl groups (such

27

as butyl and octyl) and aromatic

groups (such as phenyl) and even

some organofunctional groups

(such as chloropropyl and metha-

crylate) are hydrophobic. Similarly,

silanes containing fluoroalkyl

groups are oleophobic (oil repel-

lent). Alkoxysilyl groups attached to

these silanes allow them to actually

penetrate, cure in and even bond to

many inorganic substrates. These

unique properties allow for versatile

and durable formulating solutions

for protection against harmful water-

and oil-borne elements.

Dow Corning brand water and stain

repellent materials can be used in

solvent- or water-based systems

to provide the formulating flexibility

needed to meet VOC and ease-

of-use requirements. These silane-

based water and stain repellents

are available for use in formulations

that penetrate a broad range of

substrates, including:

• Poured-in-place or pre-cast

concrete

• Concrete block

• Sandstone/granite

• Brick/tile/grout

• Wood

• Gypsum/perlite

• Limestone/marble

Silane-based water repellents from

Dow Corning create an envelope

of protection that extends the life

of substrates for years in challeng-

ing environments. Potntial benefits

include:

• Excellent water repellency

• Long-term durability

• UV stability

• Depth of penetration

• Water vapor permeability

• High dilution capability and

stability

• Clear, uniform, neutral

appearance

Benefits of protection include:

• Reduced efflorescence

• Reduced freeze-thaw damage

• Chloride ion resistance to

deter corrosion of reinforcing

steel in concrete structures

• Preservation of aesthetics

Other Surface Protection Applications

Dow Corning also manufactures a

range of silicates and siliconates

for use in formulating pore-blocking

sealers and consolidators. These

silicates and siliconates are alkali

metal salts of hydrophobic silane

oligomers and adhere tenaciously

to inorganic substrates and surfac-

es in much the same way as simple

alkoxysilanes do. Applications for

these materials fall into two groups:

sealers and consolidators.

Sealers fall into two sub-groups:

• Pore blockers provide little

penetration and, instead, form

a resin barrier on the concrete’s

surface. Pore blockers are

further distinguished by their

ability to partially or fully fill the

surface pores, a capability not

shared by hydrophobing agents.

• Hydrophobing agents, on

the other hand, penetrate the

material deeply. They allow the

concrete to breathe and do not

interfere with concrete cure.

Consolidators can extend the life

of stone and concrete because they

penetrate and cure in and through

these materials to help bind them

together. They are used in a

variety of restoration and flooring

applications.


water repellents and surface protec-

tion is available at www.dowcorning.

com/silanes. Additional information

is available at www.dowcorning.

com/construction.

Other ApplicationsThe possible applications for

silanes are certainly not limited to

those provided in this brochure.

Silanes bring performance-

enhancing and problem-solving

benefits to a wide array of specialty

applications. Whether your

application is typical or unique,

Dow Corning can provide the silane

solution and technical support you

require, either through the proven

resources of our Application

Engineering Technical Service

department or through the innova-

tion expertise of our Surface and

Interface Solutions Center.

The Surface and Interface Solutions Center – A Valuable Resource for Customer Success

ow Corning’s Surface and Interface Solutions Center

(SISC) in Seneffe, Belgium, is pioneering the devel-

opment of next-generation technologies and applications for

organosilane and silicon-containing chemicals.

The SISC designs innovative molecules, composites, proc-

esses, and surface interface and interphase technologies,

including material science for filler reinforcement, crosslinking

and adhesion. The center serves the needs of customers in

multiple markets, including plastics, rubber, adhesives, seal-

ants, coatings, textiles and electronics.

Because it is located in Europe, the SISC complements our

other silanes technology facilities in Midland, Michigan, USA,

and Chiba, Japan, and expands our ability to provide you with

advanced application and development support, worldwide.

More than Materials – Competitive Advantage

The scientists and engineers at the SISC are linked to

Dow Corning’s global network of technology experts and to

external sources of expertise. Because the center combines

technology expertise with market knowledge, it enables us

to identify previously unimagined opportunities to meet new

and emerging customer needs.

The SISC can provide you with novel materials that open

the doors to new markets and applications. We can help you

achieve a competitive advantage in other ways as well, by

engineering solutions tailor-made to help you achieve your

specific business goals and objectives. Whether you are

looking for innovation support, performance improvement,

increased productivity or business growth, the SISC can help.

More information on the SISC is available at www.dowcorning.

com/silanes/siscmain.asp.

29

Dow Corning – The Right Partner for You

ore than 50 years ago, Dow Corning pioneered

the development of organosilane technology.

Today, we are recognized in the industry for our innovations,

technical achievements and competence in silicon technology.

Our exclusive focus on silicon-based chemistry guarantees

state-of-the-art material, manufacturing and expertise. We

have world-class facilities to study, handle and produce

these materials.

We have made significant investments to support the silanes

market. These investments will enable us to further grow our

silanes product line and identify new opportunities to provide

you with performance-enhancing solutions.

We invite your inquiries. We are anxious to discuss your

opportunities, to assist you in optimizing your current applica-

tions, and to counsel you in the use of silane solutions in the

development of emerging technologies. Our goal is to help

you use the best silane technology to satisfy the needs of your

customers, and thereby maximize your business potential.

Visit Our Website

Visit our website, www.dowcorning.com/silanes, and explore

the silanes and other silicon-based technologies we have to

offer. There you will find links to technical papers, data sheets,

product and technology brochures, and other information that

can assist you in finding solutions to your needs.

Dow Corning is pleased to offer you “Silane Solutions.”

Footnote References 1 S.L. Lapata and W.R. Keithler; Carboline Company; U.S. Patent 3,056,684, October 2, 1962.

2 G.D. McCleod; G.D. McCleod & Sons Inc.; U.S. Patent 3,917,648, November 4, 1975.

3 T. Hara; M. Ogawa; M. Yamashita; Y. Tajiri; Nippon Kokan Kabushiki Kaisha; U.S. Patent 4,407,899, October 4, 1983.

4 Wim J. van Ooij; Ashok Sabata; Armco, Inc.; U.S. Patent 5,433,976, July 18, 1995.

5 Y. Murata, et al.; Shell Oil Company; U.S. Patent 6,005,060 – “Epoxy Resin Composition and Cured Composite Product,” December 21, 1999.

6 R. Mikami; Toray Silicone Co. Ltd.; U.S. Patent 4,283,513 – “Siloxane-Modified Epoxy Resin Composition,” August 11, 1981.

7 R. Mikami; Toray Silicone Co. Ltd.; U.S. Patent 4,287,326 – “Siloxane-Modified Epoxy Resin Composition,” August 11, 1981.

8 G. Decker, et al.; Dow Corning Corp., Toray Industries; U.S. Patent 5,135,993 – “High Modulus Silicones as Toughening Agents for Epoxy Resins,” August 4, 1992.

9 G. Witucki, et al.; Dow Corning Corp.; U.S. Patent 5,280,098 – “Epoxy-functional Silicone Resin,” January 18, 1994.

10 Donald H. Campbell; Janice E. Echols; Walter H. Ohrbom; BASF Corporation; U.S. Patent 5,853,809, December 29, 1998.

Additional References 1. E.P. Plueddemann; Silane Coupling Agents, 2nd ed., Plenum Press, NY, 1991.

2. M.K. Chaudhury; T.M. Gentle; E.P. Plueddemann; J. Adhes. Sci. Technol., 1(1), 29-38, 1987.

3. Y.K. Lee and J.D. Craig; The Electrochem. Soc. 159th Mtg., Paper 141, Minneapolis, 1981.

4. E.P. Plueddemann; H.A. Clark; L.E. Nelson; K.R. Hoffmann; Mod. Plast., 39, 136, 1962.

5. L.H. Lee; Adhesion Sci. & Technol., Vol. 9B, 647, Plenum, NY, 1975.

6. E.P. Plueddemann; Proc. Am. Soc. for Composites 1st Tech. Conf., Technomic Publ. Co., 264-279, 1985.

7. P.G. Pape; J. Vinyl Additive Technol., 6(1), 49-52, 2000.

8. B. Thomas and M. Bowery; “Crosslinked Polyethylene Insulations Using the Sioplas Technology,” Wire J., May, 1977.

9. P.G. Pape and E.P. Plueddemann; “History of Silane Coupling Agents in Polymer Composites,” History of Polymer Composites, VNU Science Press, 105-139, 1987.

10. P.G. Pape and E.P. Plueddemann; “Methods of Improving the Performance of Silane Coupling Agents,” Silanes and Other Coupling Agents, K.L. Mittal, ed., VSP, Utrecht, 1992.

11. E.P. Plueddemann and P.G. Pape; “The Use of Mixed Silane Coupling Agents,” SPI Reinforced Plastics Technical Conference, Session 17-F, 1-4, 1985.

12. C.A. Roth; “Silylation Chemistry,” Ind. Eng. Chem. Prod. Res. Develop, 11, 134, 1972.

13. N.C. Angelotti and P.G. Pape; “Analytical Methods for Identification of Silanes and Silicones in Plastics,” Soc. Plastics Engineers RETEC, Atlantic City, NJ, 187-196, 1996.

How to Contact Us

Dow Corning has sales offices,

manufacturing sites, and science

and technology laboratories around

the globe. Telephone numbers of

locations near you are available

on the World Wide Web at

www.dowcorning.com, or by

calling one of our primary locations

listed below.

Your Global Connection

Asia

Dow Corning Asia Ltd. – Japan

Tel: +81 3 3287 8300

Dow Corning Asia – China

Tel: +86 21 3774 7110

Australia & New Zealand

Dow Corning Australia Pty. Ltd.

Tel: +61 1300 360 732

Europe

Dow Corning S.A.

Tel: +32 64 88 80 00

North America

Dow Corning Corporation

World Headquarters

Tel: +1 989 496 6000

South America

Dow Corning do Brasil

Tel: +55 11 3759 4300LIMITED WARRANTY INFORMATION – PLEASE READ CAREFULLY

The information contained herein is offered in good faith and is believed to be accurate. However, because conditions and methods of use of our products are beyond our control, this information should not be used in substitution for customer’s tests to ensure that Dow Corning’s products are safe, effective, and fully satisfactory for the intended end use. Suggestions of use shall not be taken as inducements to infringe any patent.

Dow Corning’s sole warranty is that the product will meet the Dow Corning sales specifications in effect at the time of shipment.

Your exclusive remedy for breach of such warranty is limited to refund of purchase price or replacement of any product shown to be other than as warranted.

DOW CORNING SPECIFICALLY DISCLAIMS ANY OTHER EXPRESS OR IMPLIED WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY.

DOW CORNING DISCLAIMS LIABILITY FOR ANY INCIDENTAL OR CONSEQUENTIAL DAMAGES.

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We help you invent the future is a trademark of Dow Corning Corporation.

All other trademarks are the property of their respective owners.

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