Organosilane Technology in
Coating Applications:
Review and Perspectives
By
Thierry Materne, Global Silane Technology
& Business Development Manager
(thierry.materne@dowcorning.com)
François de Buyl, Adhesion Senior Specialist,
Surface and Interface Solutions Center
(francois.debuyl@dowcorning.com)
Dow Corning S.A., Rue Jules Bordet, 7180 Seneffe, Belgium
Gerald L. Witucki, Coatings Industry Specialist
(gerald.witucki@dowcorning.com)
Dow Corning Corporation,
P.O. Box 994, Midland, MI 48640 USA
1
Abstract
Along with the continuous demand for improved performance, coating formulators are
burdened with ever-tightening environmental protection regulations. The need to
reduce VOCs, heavy metals like chromium VI and trialkyl tin, and other hazardous
materials creates opportunities for the suppliers of high-performance, compliant
material technologies. Ongoing research at universities and commercial organizations
has demonstrated the effectiveness of organosilane technology – alone or in combination with other materials – to improve the performance of a variety of coating systems.
Owing to the unique capability of organosilane molecules to form covalent bonding
between inorganic and organic compounds along with the inherent stability and
flexibility of the siloxane (Si-O-Si) bond, those molecules can provide multiple
benefits in a broad range of coating systems. This paper will review the fundamentals
of organosilane chemistry and its relevance to various coating applications, in
particular the use of organosilanes in primers and their proven benefits for offering a
healthier and more environmentally friendly alternative to chrome-based compounds
in corrosion protection of metals. Using state-of-the-art analytical tools, recent
experiments show the importance of the interphase design and control to optimize
adhesion properties; viscoelastic properties; and scratch, abrasion and corrosion
resistance. Organosilanes are key building blocks in the construction of this interphase
and, as such, have the potential to impact the performance of the final composite. With
the objective of controlling and optimizing interfacial properties, new technologies
will be developed with resources aligned by Dow Corning to support the coating
market. Specifically, the capabilities of the new Surface and Interface Solutions Center
(SISC), established in May 2004, are presented.
2
Organosilane Technology in Coating Applications:
Review and Perspectives
By
Thierry Materne, Global Silane Technology & Business Development Manager
François de Buyl, Adhesion Senior Specialist, Surface and Interface Solutions Center
Dow Corning S.A., Rue Jules Bordet, 7180 Seneffe, Belgium
Gerald L. Witucki, Coatings Industry Specialist
Dow Corning Corporation, P.O. Box 994, Midland, MI 48640 USA
Introduction
Dow Corning pioneered the development of organosilane technology more
than 50 years ago to provide new classes
of materials with specific physical and
chemical properties: silicones and
silanes [1]. This research led to a new
industry based on the synergy of organic
and silicon chemistries.
The value of silane coupling agents was
first discovered in the 1940s in conjunction with the development of fiberglassreinforced polyester composites [2].
When initially fabricated, these composites were very strong, but their strength
declined rapidly during aging. This
weakening was caused by a loss of bond
strength between the glass and resin. In
seeking a solution, researchers 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 reacted
at the glass-resin interface did not only
significantly increase initial composite
strength but also resulted in a dramatic
retention of that strength over time.
Subsequently, other applications for
silane coupling agents were discovered
(e.g., mineral and filler treatment for
composite reinforcement [3, 4];
adhesion of paints, inks and coatings
[5, 6, 7]; reinforcement and crosslinking
of plastics and rubber [8, 9, 10];
crosslinking and adhesion of sealants
and adhesives [11, 12, 13, 14]; and in
the development of water repellents and
surface protection [15]).
isopropoxy) or an acetoxy group that
can react with various forms of
hydroxyl groups present in mineral
fillers or polymers and liberates
alcohols (methanol, ethanol, propanol)
or acid (acetic acid). These groups can
provide the linkage with inorganic or
organic substrates.
Organosilane Chemistry
Monomeric silicon chemicals are known
as silanes. A silane that contains at least
one carbon-silicon bond (Si-C) structure
is known as an organosilane.
•And R – a spacer, which can be
either an aryl or alkyl chain, typically
propyl-.
The organosilane molecule (Figure 1)
has three key elements:
Through their dual reactivity, organosilanes serve as bridges between inorganic
or organic substrates (such as minerals,
fillers, metals and cellulose) and organic/
polymeric matrices (such as rubber,
thermoplastics or thermosets) and, hence,
can dramatically improve adhesion
between them. See Figure 2.
Figure 1. The organosilane molecule.
X-R-Si(OR')3
X = Organic (e.g., Amino, Vinyl, Alkyl…)
R'= Methyl, Ethyl, Isopropyl…
R = Aryl or Alkyl (CH2)n with n = 0, 1 or 3
•X – a non-hydrolyzable organic moiety.
This moiety can be reactive toward
another chemical (e.g., amino, epoxy,
vinyl, methacrylate, sulfur) or nonreactive (e.g., alkyl).
•OR' – a hydrolyzable group, like an
alkoxy group (e.g., methoxy, ethoxy,
Typical Silane Applications
Coupling Agent: Organosilanes are
used to couple organic polymers to
inorganic materials. Typical of this
application are reinforcements, such as
fiberglass and mineral fillers, incorporated into plastics and rubbers.
Figure 2. How organosilane molecules work.
Substrate
Minerals
Fillers
Metals
Cellulose
3
Alkoxysilane
Organic Group
Si
Matrix
Rubber
Thermoplastics
Thermosets
Adhesion Promoter: Organosilanes
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 adhesive layer 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. By using the right
organosilane, a poorly adhering paint,
ink, coating, adhesive or sealant can
be converted to a material that will
frequently maintain adhesion even if
subjected to severe environmental
conditions (e.g., high temperature,
underwater immersion or UV radiation).
Figure 3. Basic organosilane structure – asymmetrical.
Dispersing/Hydrophobing Agent:
Organosilanes with hydrophobic organic
groups attached to silicon will impart
that same hydrophobic character to a
hydrophilic inorganic surface. They are
used as durable hydrophobing agents
in concrete construction applications,
including bridge and deck applications.
They are also used to hydrophobe
inorganic powders to make them freeflowing and dispersible in organic
polymers and liquids. They also improve
cure (by reducing catalyst inhibition)
and electrical properties.
Figure 4. Alternative “Bis” organosilane structure – symmetrical.
Crosslinking Agent: Organofunctional
alkoxysilanes can react with organic
polymers, resulting in a grafting of the
trialkoxysilyl group onto the polymer
backbone. The silane is then available to
further react with moisture to crosslink
the silane into a stable, three-dimensional
siloxane structure. This mechanism can
be used to crosslink polyethylene and
other organic resins, such as acrylics and
urethanes, to impart durability, water
resistance and heat resistance to paints,
coatings and adhesives.
OR'
R'O
OR'= methoxy, ethoxy or
MeO
MeO
MeO
CH
MeO
R'O
CH3
H2C
CH2
Si
O
Si
O
XIAMETER®
OFS-6030 Silane
OMe
OMe
O
Dow Corning®
Z-6300 Silane
O
acetoxy
X= alkyl, aryl or
organofunctional group
EtO
OEt
EtO
®
MeO
Si
MeO
Si X
EtO
Si
XIAMETER
OFS-6040 Silane OEt
®
XIAMETER
OFS-6341 Silane
OMe
EtO
OR'
OEt
EtO
EtO
OEt
Si
S
S
S
S
XIAMETER®
OFS-6940 Silane
Si
OEt
OEt
Other Applications: Organosilanes
are also used as water scavengers in
moisture-sensitive formulations, as
blocking agents in antibiotic synthesis,
polypropylene catalyst “donors” and as
silicate stabilizers.
Typical Organosilanes
A variety of organosilanes is available
for use in coating applications (see
Figures 3 and 4). Matching the organofunctional group on silicon with the
resin polymer type to be bonded will
dictate which silane coupling agent
should be used in a particular application.
4
NH2
Si
Dow Corning®
Z-6011 Silane
OR'
R'O Si X Si OR'
R'O
OR'
The organic group on the silane can be
either nonreactive or reactive. Nonreactive silanes, such as those listed in
Table I, are often used as dispersing or
hydrophobing agents. They feature a
nonreactive organic group (X), which is
compatible with the matrix, and an
alkoxy group (OR), which reacts with
the substrate. Aminofunctional silanes,
like those listed in Table II, are reactive
at both ends and are useful in improving
the adhesion of all kinds of coatings.
Table III lists other reactive organosilanes from Dow Corning that may be
useful in a wide range of coating
applications as well.
Table I. Nonreactive organosilanes from Dow Corning.
Alkoxy
Group OR Chemical Name
Silane
Organic
Group
XIAMETER® OFS-6070 Silane
Methyl
Methoxy
Methyltrimethoxysilane
Dow Corning® 1-6383 Silane
Methyl
Ethoxy
Methyltriethoxysilane
X
XIAMETER OFS-6194 Silane
Methyl
Methoxy
Dimethyldimethoxysilane
Dow Corning® Z-6265 Silane
Propyl
Methoxy
Propyltrimethoxysilane
®
XIAMETER OFS-2306 Silane
i-Butyl
Methoxy
Isobutyltrimethoxysilane
XIAMETER® OFS-6124 Silane
Phenyl
Methoxy
Phenyltrimethoxysilane
XIAMETER® OFS-6341 Silane
n-Octyl
Ethoxy
n-Octyltriethoxysilane
®
Table II. Aminofunctional silanes from Dow Corning.
Silane
Organic
Reactivity
Dow Corning® Z-6011 Silane
Amino
X
Alkoxy
Group OR Chemical Name
Ethoxy
Aminopropyltriethoxysilane
XIAMETER® OFS-6020 Silane Amino
Methoxy
Aminoethylaminopropyltrimethoxysilane
XIAMETER® OFS-6094 Silane Amino
Methoxy
Aminoethylaminopropyltrimethoxysilane (high purity)
Dow Corning Z-6137 Silane
®
Amino
–
Aminoethylaminopropylsiloxane oligomers (aq)
XIAMETER® OFS-6032 Silane Vinyl-benzyl-amino Methoxy
Vinylbenzylated aminoethylaminopropyltrimethoxysilane
XIAMETER® OFS-6224 Silane Vinyl-benzyl-amino Methoxy
Low Cl version of XIAMETER® OFS-6032 Silane
Dow Corning Z-6028 Silane
Benzylated-aminoethylaminopropyltrimethoxysilane
®
Benzylamino
Methoxy
Table III. Additional reactive organosilanes from Dow Corning.
Silane
Organic
Reactivity
XIAMETER® OFS-6030 Silane
XIAMETER® OFS-6040 Silane
XIAMETER® OFS-6076 Silane
Dow Corning® Z-6376 Silane
Dow Corning® Z-6300 Silane
Methacrylate
Epoxy
Chloropropyl
Chloropropyl
Vinyl
X
Alkoxy
Group OR Chemical Name
Methoxy
Methoxy
Methoxy
Ethoxy
Methoxy
g-Methacryloxypropyltrimethoxysilane
g-Glycidoxypropyltrimethoxysilane
g-Chloropropyltrimethoxysilane
g-Chloropropyltriethoxysilane
Vinyltrimethoxysilane
XIAMETER® OFS-6075 Silane Vinyl
Acetoxy
Vinyltriacetoxysilane
Dow Corning Z-6910 Silane
Ethoxy
Mercaptopropyltriethoxysilane
XIAMETER® OFS-6920 Silane Disulfido
Ethoxy
Bis-(triethoxysilylpropyl)-disulfide
XIAMETER® OFS-6940 Silane Tetrasulfido
Ethoxy
Bis-(triethoxysilylpropyl)-tetrasulfide
Ureido
Dow Corning Z-6675 Silane
XIAMETER® OFS-6106 Silane Epoxy/melamine
Methoxy
Methoxy
g-Ureidopropyltriethoxysilane
Epoxy silane modified melamine resin
®
Mercapto
®
Key Reactions
Any application where organosilanes
are involved requires the silane molecule
to undergo hydrolysis and condensation
reactions, which are illustrated in
Figure 5.
Figure 5. The mechanism of hydrolysis, condensation and
bonding of organosilanes to an inorganic surface.
Unlike the carbon-hydrogen (C-H) bond,
the silicon hydride (Si-H) structure is
reactive. It reacts with water to form
reactive silanol (Si-OH) species. It can
5
also add across carbon-carbon double
bonds to form carbon-silicon-based
materials. The methoxy group on the
carbon compound is stable as methyl
ether, while its attachment to silicon
gives a reactive and hydrolyzable
methoxysilyl structure (Si-OCH3). The
potential impact of the organofunctional
group on silicon atom reactivity depends
on the distance between them. If the
organic spacer group is a propylene
linkage (e.g., -CH2CH2CH2-) or longer,
then the organic reactivity in the
organofunctional silane will be similar
to its organic analog.
Depending on the nature of the hydrolyzable groups attached to the silicon,
we may distinguish the following categories of silanes: chlorosilanes, silazanes,
alkoxysilanes and acyloxysilanes. Each
exhibits unique chemical reactivity. These
molecules will react with atmospheric
moisture or water adsorbed on a surface
to form silanols, while liberation of the
corresponding by-product will occur
(e.g., HCl, NH3, alcohol or carboxylic
acid). These silanols can then react with
other silanols to form a stable siloxane
bond (-Si-O-Si-). In the presence of
hydroxyl groups at the surface of glass,
minerals or metals (e.g., aluminum,
steel), silanols will form a stable M–O-Si
bond (M = Si, Al, Fe, etc.). This is the
key chemistry that allows silanes to
function as valuable surface-treating or
“coupling” agents.
Because organosilanes hydrolyze before
reacting with each other or with the
hydroxyl groups present at a substrate
surface, it is important to understand the
reaction kinetics. The hydrolysis reaction
can be catalyzed using either an acid or
a base. Mechanisms of acid- and basecatalyzed hydrolysis of organosilanes
have been extensively studied since their
discovery [1, 16].
Figure 6. Acid-catalyzed hydrolysis.
H
H
O
H
R3
R3
H
+
O
Si
O
Si
R1
1. Protonation of leaving OR group
+
R1
CH3
R2
+
CH3
R2
H
H
O
2. Attack of water via S N2 mechanism
R3
H
O
Si
R1
H
R3
H
+
O
H2O
CH3
R2
O
Si
R1
Si
R1
+
HO
R2
CH3
OR
H
R3
+
H
R3
H
+
O
Si
CH3
R2
R1
+
OH2
+
HO
CH3
R2
Figure 7. Base-catalyzed hydrolysis.
R3
R3
Si
HO
R1
R2
O
O
CH3
Acid-catalyzed Hydrolysis: Acidcatalyzed hydrolysis involves the
protonation of the leaving OR group
followed by a bimolecular Sn2-type
displacement of the leaving group by
water. See Figure 6.
Base-catalyzed Hydrolysis: Basecatalyzed hydrolysis involves attack on
silicon by a hydroxyl ion to form a
pentacoordinate intermediate followed
by bimolecular displacement of alkoxy
by hydroxyl. Any high-electron acceptors group next to the Si atom will
dramatically increase hydrolysis under
basic pH. See Figure 7.
Hydrolysis Rate: The rate of hydrolysis
by both mechanisms is influenced by the
nature of the organic group on silicon as
well as the leaving alkoxy group. Another
factor is pH. As pH changes, so does the
hydrolysis rate. This is true for both
acid- and base-catalyzed reactions.
6
H
Si
R1
+
O
R2
CH3
As soon as alkoxy groups begin
hydrolyzing, condensation to highmolecular-weight species may occur.
The Hydrolysis/Condensation
Balance: The rates of both hydrolysis
and condensation are influenced by
changing pH levels. This is true whether
the reaction is catalyzed by acid or
base. However, the optimum pH for
hydrolysis is not optimum for condensation. Finding the best balance between
hydrolysis and condensation is one of
the keys to the successful utilization of
organosilane chemistry for a specific
application (Figure 8).
Interface Design and
Materials Science
Interface design and materials science
deal with surfaces, but “surface” is a
concept with many definitions. Surfaces
can range from large, easily visible
forms, such as plastic sheets, metal
Figure 9 illustrates how covalent bonding
forms between a silane and a metal
surface.
Bonding through a silane coupling agent
is, however, often more complex than a
single bond. At the interface, the silane
reacts with itself and also with the
polymer, crosslinking and interlocking
mechanically with the polymer in a way
that is difficult to separate. Inter-diffusion
phenomenon and semi-inter-penetrating
network (IPN) phenomena occur in the
interphase region and are critical to the
overall composite performance (Figure
10) [17, 18]. The key factors that control
the formation of this network are the
hydrolysis/condensation rate, the
solubility parameters, structural
characteristics of the two materials and
their thermal stability [19]. Even with
thermoset polymers, where reactivity
plays an important role, chemical
structure matching will enhance the
physical properties of the composite.
0
Condensation
Rate
-1
-2
log pK
Organosilanes can be linked to these
surfaces in a number of ways. Theoretically,
a monolithic layer is sufficient to provide
benefit. In practice, however, to ensure
uniform coverage, more than one layer
of silane is usually applied to a surface.
This results in a tight siloxane polymeric
network close to the inorganic surface
that diffuses into subsequent overlays.
The coupling to the organic matrix can
be complex. The reactivity of a thermoset polymer should be matched to the
reactivity of the silane. For example, an
epoxysilane or aminosilane 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.
Figure 8. The hydrolysis/condensation balance (γ-glycidoxypropyltrimethoxysilane – GPS).
-3
-4
Hydrolysis Rate
-5
Best Balance
-6
1
2
3
4
5
pH
6
7
9
8
10
Figure 9. Covalent bonding (metal oxide).
x101
8.0
OH
mass resolution (SiOAl+: 8950)
HO
7.0
6.0
SiOAl+
(1.35 mmu)
Intense SiOAl+
(m/z=71) peak
5.0
Intensity
panels, glass panes, skin, wood or
concrete; to fibers, fillers and pigments;
and ultimately to particulate and
microscopic forms such as 10-9m size
sol-gel nanoparticles.
4.0
(CH2)3
O
H
O
H
O
CH2
CH
CH2
O
Al
Si2CH3+
(3.1 mmu)
Si
SiC2H3O+
(1.97 mmu)
C3H3O2+
(-0.79 mmu)
3.0
OH
C4H7O+
2.0
(2.26 mmu)
C5H11+
(3.4 mmu)
1.0
HO
Si
(CH2)3
O
CH2
CH
O
Al
70.85 70.90 70.95 71.00 71.05 71.10 71.15
mass/u
Copyright 2004 from Journal of Adhesion (80), page 291, by M.L. Abel et al. Reproduced by
permission of Taylor & Francis Group, LLC, www.taylorandfrancis.com.
Figure 10. InterPenetrating Network (IPN).
7
CH2
O
Figure 11. The reinforcement model.
Figure 12. Sol-gel basic principles.
Si(OCH2CH3)4 + 2 H2O
Polymer M atrix
SiO2 + 4 CH3CH2OH
Filler
X -linking points
3 Domains
Plasticized
Silane
coupling agent
Coupling agent/polym er
chain bonding
Elastic
Glassy
C.J. Brinker, G.W. Scherer, “Sol-gel Science – The
Physics and Chemistry of Sol-gel Processing,”
Academic Press, San Diego, California, USA, 1990.
Distance to particle
F. Lequeux et al., Macromolecules (35) 9756-62, 2002. Reprinted with permission from Macromolecules.
Copyright 2002 American Chemical Society.
A third way to look at how organosilanes
bond with a surface is through the
material reinforcement modality. Fillers
treated with a silane coupling agent
employ coupling agent/polymer chain
bonding to create an interphase zone
with specific properties. A plasticized
transition zone is created between the
hard, glassy filler and the soft, elastic
matrix. See Figure 11.
A fourth approach, which moves beyond
the creation of an interphase to the realm
of particle formation is the sol-gel process
(Figure 12). In this process, water is
added to a silane molecule, generating
silicon oxide, which is a solid particle.
The product of elimination is alcohol.
By controlling the hydrolysis and
condensation conditions, including pH,
the presence or absence of salts, and
time, it is possible to affect the morphology and, depending on the route taken,
to create either sols (solid suspensions)
or three-dimensional gel networks.
Through the addition of templating
agents, it is also possible to organize the
particles and self-assemble them into
pre-functionalized nanobuilding blocks.
With sol-gel technology, it is possible to
synthesize and design materials and
interfaces under very mild conditions
and achieve nanoparticle synthesis that
imparts transparency, low weight,
hardness and functionality to coatings.
See Figure 13.
Silane Technology in Paints
and Coatings
Tightening volatile organic compound
(VOC) content regulations and increasing health and environmental concerns
about the use of heavy metals (e.g.,
chrome, lead, cadmium, mercury, zinc,
cobalt, etc.) in the coatings industry,
along with demand for improved
physical properties and extended
performance life, have spurred interest
in silane technology.
Figure 13. Sol-gel material architecture.
R'-M-(OR)x
M(OR)n
Organo-Functional
Metal Alkoxide
Metal Alkoxide
Controlled
H2O
Organic Templates
Structure directing agents
Pre- or post-functionalized
Metal-oxo clusters
or Nano-particles
H 2O
R' moiety may
be reactive
Conventional
SOL-GEL Route
SELF-ASSEMBLY
Assembly of
Nanobuilding
Blocks (ANBB)
Organic
connectors
Oxo-polymers
Reactions involving
Pre-functionalized
nanobricks
Organic
components
Periodic Functional
Porosity
Mesostructured NBB-based hybrids
C. Sanchez et al., Chem. Rev. (102) 4093-4138, 2002. Reprinted with permission from Chem. Rev.
Copyright 2002 American Chemical Society.
8
Figure 14. Adhesion durability comparison.
O
O
MeO
MeO
2.5
Si OMe
XIAMETER®
OFS-6040 Silane
Δa
Initial
crack
length
1 hr hydration + silane
Grit-blasted only
Chromic acid anodisation
Polyamic acid
1.5
1.0
0.5
0.0
Paint and coating applications that can
benefit from silane technology include
primers, heat-resistant coatings, industrial
maintenance coatings, hygienic coatings,
marine biofouling control, abrasionresistant coatings, automotive clearcoats
and architectural coatings.
Primers
A segment of the coatings industry in
which silanes provide crucial functionality is primer systems, in which silanes
are used either to pretreat a substrate or
are added into a coating formulation as
an adhesion promoter. Alkoxysilanes
have, therefore, broad utility in formulating primers and coatings for a variety
of metallic and siliceous substrates.
Especially attractive to the formulator
is the wide range of organoreactive and
non-reactive moieties attached to the
silicon atom (Tables I-III), which allows
formulas to be tailored to specific
application performance requirements.
Widely known as adhesion promoters,
organosilane primers also offer controlled hydrophobicity, excellent UV
and thermal stability, surface activity,
chemical resistance, and hence, corrosion protection.
One example of the use of organosilane
technology to create high-performance
primers is the use of zinc-rich primers
for the galvanic protection of ferrous
substrates. Hydrolyzates of tetra-ethoxy
silane (TEOS) are combined with zinc
metal powder. Transesterfication of the
alkyl orthosilicate has been found to
Δa = Crack growth after exposure
Catalyzed silane pre-treatment
2.0
Fracture energy (kJm-2)
The unique capability 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 highperformance paints and coatings. These
properties lie at the heart of the ability
of these materials to withstand physical,
chemical, weather and thermal degradation. Therefore, the use of organosilanes
in coatings has provided improvements
in minerals dispersion; water, chemical,
abrasion and UV resistance; adhesion;
and flow properties.
0
20
40
60
80
100
120
140
160
180
MilliQ water 20k
μ
1u
Exposure time (hrs)
Copyright 2004 from Journal of Adhesion (80), page 291, by M.L. Abel et al. Reproduced by
permission of Taylor & Francis Group, LLC, www.taylorandfrancis.com.
improve solution stability [20].
(Reference the Dow Corning brochure,
A Guide to Silane Solutions from
Dow Corning, page 20.)
Amino- and epoxy-functional silanes
improve adhesion to siliceous and
metallic substrates. Figure 14 illustrates
the superior adhesion durability of
primers formulated with an epoxyfunctional silane over the adhesion of
other primer technologies in use today
with aluminum substrates.
Paints and Coatings
Siloxane-alkyd, siloxane-epoxy and
siloxane-acrylic chemistries can be
used to improve industrial, architectural
and marine antifouling coatings. By
combining silanes with organic resins
and through careful manipulation of
the hydrolysis/condensation balance,
it is possible to create coatings with
superior properties, such as low
viscosity, high solids content and low
VOC; adhesion to metals, cements
and quartz-like surfaces; low combustibility; and improved heat and UV
resistance, which results in improved
gloss retention. Figure 15 illustrates
the improvement in gloss retention that
can be achieved through the use of
organosilane technology.
Figure 15. Gloss retention comparison.
Acrylic-Siloxane
Epoxy-Siloxane
SiloxaneAlkyd
AcrylicUrethane
Siloxane
Alkyd
Epoxy
N.R. Mowrer, “Polysiloxane Coatings Innovations,” Ameron Int., Brea, CA. Reprinted with permission.
9
Impregnation
Figure 16. Masonry impregnation.
In masonry treatment applications,
isobutyl, n-octyl and iso-octyl silanes
can be used to control wetting and
reduce water uptake (Figure 16).
Additionally, phenyl and fluorinated
silanes can be used to improve substrate
resistance to chemicals, including
detergents and disinfectants, by creating
a hydrophobic surface that is comparable
to the lotus-leaf effect found in nature
(Figure 17).
Abrasion-Resistant Coatings
Coatings employing sol-gel technology
can be developed to improve the
abrasion resistance of plastic lenses in
glasses. Silica is highly resistant to
abrasion, and sol-gel technology enables
the creation of particles so small that
they are virtually invisible, making it
ideal for this application. Figure 18
illustrates the significant improvement in
scratch and abrasion resistance that can
be achieved through the use of sol-gel
coating formulations.
Untreated
nOTES Treated
6-mm penetration depth
Figure 17. The lotus-leaf effect.
New Perspectives
Metal Corrosion Protection
Differences in the metal/coating
coefficient of thermal expansion (CTE)
can lead to thermal stresses resulting
in micro-cracks and allow water and
electrolytes to access the metal, which,
in turn, leads to debonding of the
coating.
Figure 18. Abrasion resistance performance comparison (sol-gel).
Untreated PC/PMMA lenses
Scattered Light/(a.u.)
Today, organosilane chemistry is
enabling the development of new, more
environmentally responsible technologies for corrosion protection and the
creation of “smart” surfaces that are
easy to clean. It is also enabling paint
and coating formulators to meet the
challenges involved in switching from
solvent-based to water-based coatings in
their drive to comply with increasingly
stringent VOC regulations.
Linear (CR-39)
Linear (Ti-SC)
Linear (GH-NC)
Epoxy silane / Si(OCH3)4 /
Ti-Alkoxide coated lenses
Epoxy silane / Si(OCH3)4 /
Al-Oxide coated lenses
Abrasion Time/(min)
G. Schottner, Chem. Mater., (13) 3422, 2001. Reprinted with permission from Chem. Mater.
Copyright 2001 American Chemical Society.
10
Figure 19. Barrier properties of silane films.
OEt
OEt
EtO
NH2
Si
EtO
Si
OEt
Si
EtO
EtO
OEt
OEt
BTSE
APTES
5.E+04
Rp (Ohm.cm2)
4.E+04
3.E+04
Fresh
2.E+04
Cured 30',
200°C
Huge increase of
Polarization Resistance
after cure (film formation)
1.E+04
0.E+00
Control
APTES
inorganic silicate and 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 to form an insoluble
primer layer bonded tightly to the metal
substrate. Combining the cure profiles
and barrier properties of organic resins
with the thermal and UV stability of
silanes, formulators have created highperformance coatings with excellent
resistance to corrosion and chemical
attack as well as thermal and UV
degradation (see Figure 14).
BTSE
T. Van Schaftinghen, C. Le Pen, H. Terryn, F. Horzenberger, “Investigation of the barrier properties
of silanes on cold rolled steel,” ELECTROCHIMICA ACTA, Vol. 49, No. 17-18, 2997-3004 (2004).
Creating a silane interpenetrating
network between the metal substrate and
the organic coating layers provides:
increase of polarization resistance after
cure (i.e., pinhole-free film formation).
•Barrier properties that are resistant to
contaminants and electrolytes, due to
the high crosslinking density,
State-of-the-art metal surface preparations for adhesive bonding consist
mainly of anodization or etching
processes that employ strong acids.
Many of these surface preparations also
contain hexavalent chromium (CrVI).
The surface treatment is followed by
the application of a corrosion-inhibiting
adhesive primer containing high levels
of VOCs and chromium salts. Since the
early 1980s, issues related to hazardous
waste disposal, preservation of the
environment, and potential impact on
employee health and safety have driven
the industry to seek alternatives to
chromium [21, 22].
•Resistance to water ingress along the
interface due to the innate hydrophobicity of bis(triethoxysilyl)ethane
(BTSE), sulfido and other silanes,
•Adhesion to the metal surface, resulting
from the coupling effect between the
silane and the surface metal-hydroxyls,
e.g., γ-glycidoxypropyltrialkoxysilane
(GPS) or γ-aminopropyltrialkoxysilane
(APTS),
•Enhanced stress dissipation upon
weathering cycling conditions, due to
low Tg inclusions,
•And the opportunity to replace
chrome, if the silane is combined with
an appropriate/non-toxic corrosion
inhibitor.
BSTE silanes have demonstrated a
superior capability to
γ-aminopropyltriethoxysilane (APTES)
to create barrier properties. As seen in
Figure 19, with BTSE, there is a huge
Chromium Replacement
In the early 1980s, it was found that a
primer composed of an acrylic copolymer, an epoxy resin, a silica sol and a
trialkoxysilane compound provided
superior paintability and grease and
corrosion resistance after paint coating
[23]. Twelve years later, it was found
that a wash primer, without the acrylic
copolymer or the epoxy resin, could
provide similar benefits [24]. Metal was
pretreated with an alkaline solution
containing at least one of a dissolved
11
A blend of an epoxy resin, a curing
agent for epoxy resins, an organofunctional alkoxysilane and a catalyst for
condensation polymerization of a silane
compound was found to provide high
heat resistance and excellent mechanical
strength [25]. Similarly, complete or
partial hydrolysis of alkyl/phenyl
alkoxysilanes to form silanol or alkoxy
functional siloxane resins and the
subsequent reaction with epoxy resins
have been shown to produce copolymers
with improved water and moisture
resistance [26]. Utilizing the functional
groups available from silane monomers,
resin formulators have created organofunctional (e.g., epoxy and amine)
silicone resins for epoxy resin modification [27, 28, 29].
The benefits of different silane blends
were explored further by several groups.
Lapique et al. [30] and Van Ooij et al.
[31] have shown that BTSE bonds to
aluminum and decreases the hydrolysis
and/or corrosion rate at the adhesivemetal interface due to its hydrophobic
character, while APTES only increases
the initial adhesive bond strength – hence
the benefit of combining the two. Combinations of bis(trimethoxysilylpropyl)amine
and bis(triethoxysilylpropyl)tetrasulfide
in the appropriate ratio were found
useful to achieve both optimal film
formation and protection against rapid
“Smart” Surfaces
Another emerging application for
organosilane chemistry is the development of “smart” surfaces – surfaces that
are uncharacteristically hydrophobic or
Me Me
Me O Si O
Me
Si
Si
Me
O
Me
Si O
Me
Me
O
O Si O
T
Me
Me
O Si O
Me
D
O
O Si O
O Q
Water contact angle
on cotton > 150°
108
106
104
102
Binding Energy (eV)
100
98
96
Si 2p XPS spectrum
Figure 21. Smart coatings and surfaces – hydrophilic treatment of PET and PP with
tetramethylcyclotetrasiloxane (TMCTS).
Atmospheric Glow Discharge Process
Examples based on the sol-gel process
can also be found in the literature [37,
38]. The approach consists in forming
Al2O3, or SiO2 protective layers at the
metal surface by using silane-based
materials (TEOS, BTSE, APTES, GPS).
This mild chemical treatment, followed
by a sintering process, is an alternative
to more aggressive classical treatments.
Figure 20. Smart coatings and surfaces – hydrophobic treatment of cotton with
octamethylcyclotetrasiloxane (OMCTS).
Atmospheric Glow Discharge Process
electrolyte diffusion to aluminum or hotdip galvanized steel [31], thanks to the
right balance between hydrophobicity
and adhesive bonding at the metalcoating interface. J. Vereecken et al. [32]
studied the morphology of the films as a
function of the film-forming parameters
(e.g., silane types and concentration,
rate of hydrolysis and condensation
reaction, processing time, and degree of
hydration of the metals prior to coating).
They concluded that as the permeability
of the silane film decreases, its
impedance increases proportionally,
hence improving the passivation of the
metals studied (aluminum, galvanized
steel). Watts et al. [33, 34] showed that
growth of aluminum oxide occurs when
γ-glycidoxypropyltrimethoxysilane
(GPS) is cured on polished aluminum
sheet at elevated temperature, creating
an Al/O/Si interphase. These works
showed that bonding performances
of adhesives can be maintained or
improved when aluminum is treated with
GPS silane versus conventional chromic
or phosphoric acid anodization (see
Figure 14). As an alternative to a
silanization process in solution, Boeiro
et al. [35, 36] recently demonstrated the
effectiveness of vapor-phase, or lowpressure plasma silanization to treat
aluminum alloy. This approach provides
an effective way to address problems
such as uncontrolled hydrolysis and
polycondensation reactions, and solvent
elimination.
Me H
H O Si O
Me
Si
Si
Me
O
H
Si O
H Me
O
O Si O
Me
O
O Si O
O
T
Q
108
106
104
102
Binding Energy (eV)
100
98
Si 2p XPS spectrum
hydrophilic, hydrophobic and oleophobic (a state that can be obtained through
the use of fluorosilane and sol-gel
technologies), or exhibit the lotus-leaf
effect (a hydrophobic micro-roughness).
Figure 20 illustrates the use of
octamethylcyclotetrasiloxane (OMCTS)
and an atmospheric glow discharge
process to create a hydrophobic
treatment for cotton fabric that results in
a super-hydrophobic surface with easy
cleaning/water repellence properties.
12
Figure 21 illustrates how tetramethylcyclotetrasiloxane (TMCTS) can be used
to create a hydrophilic effect on PET and
PP, which are naturally hydrophobic. The
SiOx deposition makes the surface fully
wettable. Enhanced barrier to oxygen
performance is demonstrated as well.
Note how the hydrophilic “Q” and “T”
Si species levels have been manipulated
to achieve surface hydrophilicity,
compared with the levels exhibited in
the hydrophobicity example (Figure 20).
Water-Based Systems
Figure 22. Creation of a silane-derived acrylic polyol emulsion.
The desire for water-based coating
formulations is on the rise, both for
their ease of use and their ability to
help formulators meet environmental
regulations (low VOC).
Solutions include the use of:
•A silane-derived acrylic polyol
emulsion
R = Phenyl
•Pre-hydrolyzed and pre-condensed
silanes including specifically designed
catalyst systems
•Replacement of methoxy/ethoxy with
other, nontoxic leaving groups, such
as OH, glycol or heavy alcohols
These solutions may be applied to the
formulation of water-based construction
material treatments as well as waterbased paints.
There are two challenges inherent to
the use of water-based formulations:
1) wetting of low-surface-energy
substrates and 2) maintaining reactivity/
shelf-life balance. However, through
the use of organosilane chemistry, it is
possible to create water-based systems
that outperform conventional solventbased systems.
Figure 22 illustrates the development
of a silane-derived acrylic polyol
emulsion, a three-step process. First,
an acrylic polyol is pre-reacted with an
alkoxy-functional silane intermediate.
Hydrolysis is catalyzed under alkaline
conditions. Finally, condensation of
silanols with acrylic polyol results in
a crosslinked acrylic siloxane.
Testing of a traditional water-based
acrylic coating and a water-based acrylic
coating containing 10% of a silane
acrylic polyol emulsion (XIAMETER®
RSN-5314 Resin) under natural and
accelerated weathering conditions
revealed that the formulation containing
the silane acrylic polyol exhibited
superior gloss retention and reduced
chalking.
Practical Considerations for
Water-Based Formulations
A primary concern for water-based
formulations is the stability of alkoxysilanes [39]. If alkoxysilane adhesion
promoters react with water, how can
they be used in water-based coatings
formulations? For silanes to provide
the expected benefits of adhesion or
crosslinking, the hydrolysis reaction
is a necessary and desired step in the
process [1]. Modifying the silane, via
transesterification, from methoxy
functionality to longer alkoxy groups
(e.g., isopropoxy) can slow down the
hydrolysis process without preventing
it. While accepting the inevitability of
hydrolysis and subsequent condensation
of alkoxysilanes, coating formulators
have reversed this apparent limitation
by utilizing organosilane chemistry
to improve the performance of many
water-based coatings.
Coatings fail as a result of water
absorption into the film, ultimately
reaching the coating-substrate interface.
Because of the specific organic-inorganic
character of organosilanes, they can
enhance the adhesion of coatings to
metal or siliceous substrates by forming
13
covalent bonds, and hence increasing the
density of linkages between the a coating
and the substrate. Furthermore, by
selecting the proper combination of
organosilanes, the hydrophobicity of the
interphase can be improved, which will
contribute to hydrolytic stability upon
service-life.
Addition of alkoxysilanes to a coating
formulation can create a more tightly
crosslinked, hydrophobic film much less
susceptible to moisture attack [1].
Additions of 0.5% silane (based on
system solids) into acrylic latex-based
coatings provide significant benefit.
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 can offer pre-treated
particles and aggregates, similar benefits
can be observed by incorporating the
alkoxysilane directly into a water-based
coating formulation (in-situ treatment).
The presence of water at high pH results
in hydrolysis of the silane and condensation around the solid particles. The net
effect is improved dispersion, more
complete incorporation of the inorganic
particle into the binder matrix and
improved physical properties. However,
successful use of silanes in water-based
formulations requires good dispersion of
the silane prior to complete hydrolysis
and condensation. Appropriate mixing
is essential. Along with good stirring,
pre-diluting the silane into coalescing
solvent or plasticizer before adding it
to the latex will minimize condensation
between the silane monomers (and
potential gel formation) and will favor
interaction with the other components
of the coating formulation.
Future Trends
Environmental
Environmental concerns will increase.
For example, European regulations and
the recent introduction of REACH will
accelerate the development and the
commercial use of alternative anticorrosion treatments to replace chromium VI in plating, coil coating and
metal painting. Organosilanes have
proven their ability to improve durability
under extreme operating conditions
through the formation of strong,
covalently bonded IPNs between metal
surfaces (aluminum, steel) and coatings,
adhesives or sealants and, as such, may
provide a viable solution.
Sol-gel Processes
Although known and studied for several
decades, sol-gel processes are not widely
used at an industrial level for coatings
purposes. However, the recent evolution
of the organosilanes market may open
the door to accurately designed materials
created under mild conditions. Some
self-cleaning, easy-to-clean and scratchresistant coating applications are already
using sol-gel technology.
“Smart” Surfaces
Several groups of researchers are
developing templating surface concepts
designed to control wettability [40, 41],
tailoring cell adhesion or designing
molecular gradients for controlled
materials assembly and directed transportation on surfaces. Other properties,
such as anti-scratch, anti-graffiti, lotusleaf effect, brilliance and refractive
index, are also being developed using
silane-based chemistry and technologies.
Conclusion
Organosilane chemistry brings a wide
variety of benefits to coatings, including:
•Adhesion to “difficult” substrates
The Dow Corning Surface and
Interface Solutions Center
To continue its leadership position in
the development of silane technology,
Dow Corning has recently invested in a
new R&D facility: the Surface and
Interface Solutions Center (SISC). The
center’s expertise is articulated around
surface and interface science and
applications for Si-containing chemicals.
More specifically, the SISC is developing materials science core competencies
around fillers, reinforcement and
adhesion. It also has several “application
cells” focusing on target markets and
products such as Plastics and Rubber,
Coatings, and Adhesives and Sealants.
These cells are equipped with state-ofthe-art equipment and facilities. The
center is staffed by experienced PhDlevel researchers and engineers from a
variety of horizons and is connected to
Dow Corning’s global network of
internal and external experts. The core
group is located in Europe; however,
team members are also conducting work
in the United States and Asia.
The SISC is a market-driven organization
designed to optimize the benefits and
values of silicon-based chemistry and
thereby create value for Dow Corning
customers. Its mission is to discover
and develop unique surface, interface and
interphase technologies and solutions.
The center develops and concentrates
knowledge and databases on surface
and interface sciences as they apply to
Dow Corning’s target markets and
products. It cooperates closely with
customers, technical partners and others
in Dow Corning’s global organization to
provide appropriate solutions, including
new molecules, composites and/or
processes. The center works to identify
unmet customer and market needs
that can be met by Dow Corning’s
extensive materials science and synthesis
capabilities.
14
•Strong bonding to hydrophilic reactive
sites (through covalent bonding and the
creation of interpenetrating networks)
•Low moisture uptake (hydrophobicity
balance)
•Thermal stability
•UV resistance
•Low surface tension
•Controlled Tg inclusions, resulting in
stress reduction or increased surface
hardness
•Reduced dirt pick-up and antigraffiti properties (hydrophobicity/
oleophobicity)
Dow Corning continues to pioneer the
development of innovative technologies
and applications for organosilane and
silicon-containing materials through our
global research team and the 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
advanced materials technologies. They
enable a wide range of new materials,
including coatings, to be developed with
greater reliability and improved
performance.
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15
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