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UCT

UCT
SILANE COUPLING AGENT GUIDE

SILICONES

SILANES

C ATA LY S T S

C O AT I N G S
 SILANE COUPLING AGENT CHEMISTRY
The general formula of an organosilane shows two classes of functionality.

RnSiX(4-n)

The X functional group is involved in the reaction with the inorganic substrate. The bond
between X and the silicon atom in coupling agents is replaced by a bond between the inorganic
substrate and the silicon atom. X is a hydrolyzable group, typically, alkoxy, acyloxy, amine, or
chlorine. The most common alkoxy groups are methoxy and ethoxy, which give methanol and
ethanol as byproducts during coupling reactions. Since chlorosilanes generate hydrogen chloride
as a byproduct during coupling reactions, they are generally utilized less than alkoxysilanes.

R is a nonhydrolyzable organic radical that possesses a functionality which enables the

coupling agent to bond with organic resins and polymers. Most of the widely used organosilanes
have one organic substituent.

In most cases the silane is subjected to hydrolysis prior to the surface treatment. Following
hydrolysis, a reactive silanol group is formed, which can condense with other silanol groups, for
example, those on the surface of siliceous fillers, to form siloxane linkages. Stable condensation
products are also formed with other oxides such as those of aluminum, zirconium, tin, titanium,
and nickel. Less stable bonds are formed with oxides of boron, iron, and carbon. Alkali metal ox-
ides and carbonates do not form stable bonds with Si – O –.

Water for hydrolysis may come from several sources. It may be added, it may be present
on the substrate surface or it may come from the atmosphere. Water for hydrolysis may also be
generated in situ by dissolving chlorosilanes in excess alcohol. Reaction with alcohol produces
alkoxysilanes and HCl, which can react with additional alcohol to form an alkyl halide and water.

Reaction of these silanes involves four steps. Initially, hydrolysis of the three labile X
groups attached to silicon occurs.

RSi(OMe)3

3H2O 3MeOH HYDROLYSIS

RSi(OH)3

Condensation to oligomers follows.

RSi(OH)3

2RSi(OH)3 2H2O CONDENSATION

R R R

HO Si O Si O Si OH

OH OH OH

OH OH OH

Substrate
The oligomers then hydrogen bond with OH groups of the substrate.

R R R

HO Si O Si O Si OH HYDROGEN
BONDING
O O O
H H H H H H
O O O

Substrate

Finally during drying or curing, a covalent linkage is formed with the substrate with concomitant
loss of water. At the interface, there is usually only one bond from each silicon of the organosilane
to the substrate surface. The two remaining silanol groups are present either bonded to other cou-
pling agent silicon atoms or in free form.

2H2O

R R R

HO Si O Si O Si OH BOND
FORMATION
O
O O H H
O

Substrate

The number of reactive sites on a surface area and the type of silane deposition sought, i.e. mono-
layer, multilayer or bulk, are all factors which can be used in calculating the amount of silane nec-
essary to silylate a surface. In order to provide monolayer coverage, the concentration of reactive
sites (silanols) should be determined. Most siliceous substrates have 4 – 12 silanols per mμ2.
Thus, one mole of evenly distributed silane should cover an average of 7500 m2. The oligimeriza-
tion of silanes with multiple groups thwarts the capability of computing stoichiometries, but order
of magnitude computations are successful. Silanes with one hydrolyzable group can be utilized
to produce surfaces with monolayers of consistent stoichiometry. These materials are more ex-
pensive and produce surfaces with less hydrolytic stability. The number of silanols on a surface is
varied by thermal history. In one example, a siliceous surface having 5.3 silanols per mμ2 had only
2.6 after exposure to 400°C and less than one after exposure to 850°C. Higher concentrations of
silanol groups may be produced by treating material with warm hydrochloric acid. Silanol anions
may be produced by treating the surfaces with alkaline detergent or, more radically, by treatment
with methanolic potassium hydroxide. Optimum deposition of silanes with more than one hy-
drolyzable group is often defined as the as the amount necessary to produce a surface of uniform
energy. A value defined as the wetting surface (ws) describes the area in m2 one gram of silane
deposited from solution will cover. In combination with data on the surface area of a siliceous
substrate in m2/g the amount of silane required for deposition may be calculated. Most compos-
ite, adhesive, and coating formulations do not follow any stoichiometry, but simply define optimal
concentration by operation success. For most fillers, a treatment level of 0.02 – 1.00% by weight is
used.
Selecting a Silane Coupling Agent

Selection of the appropriate coupling agent is accomplished by empirical evaluation of

silanes within predicted categories. Exact prediction of the best silane is extremely difficult. In-
creased bond strength by utilization of silanes is a result of a complex set of factors – wet out, sur-
face energy, boundary layer absorbtion, polar adsorption, acid-base interaction, interpenetrating
network formation and covalent reaction. Strategies for optimization must take into account the
materials on both sides of the interface and their susceptibilities to the various coupling factors.
Generally speaking the initial approach is to select a single coupling agent and assume a direct
bond between the two materials. The most common application for silane coupling agents is to
bond an inorganic substrate to a polymer.

Inorganic-Si-R-Organic

The number of hydrolyzable X groups on the silane is another important parameter in con-
trolling bond characteristics. The traditional silane coupling agents contain three hydrolyzable
groups. They have maximum hydrolytic stability but tend to by hydroscopic. At the opposite end
are the silanes with one hydrolyzable group. These yield the most hydrophobic interfaces but
have the least long term hydrolytic stability. Silanes with two hydrolyzable groups form less rigid
interfaces than silanes with three hydrolyzable groups. They are often used as coupling agents
for elastomers and low modulus thermoplastics. Polymeric silanes with recurrent trialkoxy or
dialkoxysilanes offer better film-forming and primer capabilities. For enhanced hydrolytic stability
or economic benefit, non-functional silanes such as short chain alkyltrialkoxysilanes or phenyltri-
alkoxysilanes can be combined in ratios up to 3:1 with functional silanes.

In more difficult bonding situations, mixed silanes or silane network polymers may be em-
ployed. These include inorganic to inorganic or organic to organic. In these cases, reaction of the
silanes with themselves is critical.

Organic-O-Si-R-R-Si-O-Organic

An example of mixed silane application is the use of mixtures of epoxy and amine function-
al silanes to bond glass plates together. A more general use is bonding organic to organic. Prim-
ers, prepared by pre-hydrolyzing silanes to resins in order to form bulk layers on metal substrates,
are examples of the application of silanes as network polymers.

Thermal Stability

Most silanes have moderate thermal stability, making them suitable for plastics that process
below 350°C or have continuous temperature exposures below 150°C. Silanes with an aromatic
nucleus have higher thermal stability. A relative ranking where Z is the functional groups is as fol-
lows:

Class Example Thermal Limit

ZCH2CH2SiX3 N/A < 150°C
ZCH2CH2CH2SiX3 A0700 390°C
ZCH2AromaticCH2CH2SiX3 T2902 495°C
Aromatic SiX3 P0320 550°C
 COUPLING AGENT SELECTION GUIDE

Table 1 - Thermosets
Name Silane Class UCT Product
amine A0700 A0750
diallylphthalate
styryl S1590
amine A0700 A0750 T2910
epoxy G6720 E6250
epoxy
chloroalkyl C3300
mercapto M8450 M8500
chloromethylaromatic T2902
imide
amine A0700 A0750 T2910
amine A0700 A0750 T2910

melamine epoxy G6720 E6250

alkanolamine B2408
paralene chloromethylaromatic T2902
amine A0700 A0750 T2910

phenolic chloroalkyl C3300

epoxy G6720 E6250
silazane H7300 D6208

photoresist, negative vinyl D6208

aromatic P0320
silazane H7300

photoresist, positive aromatic P0320

phosphine D6110
amine A0700 A0750 T2910
methacrylate M8550
polyester
styryl S1590
vinyl V4917 V4910
amine A0700 A0750 T2910
alkanolamine B2408
urethane
epoxy G6720 E6250
isocyanate I7840
 Table 2 - Thermoplastics
Name Silane Class UCT Product
amine A0700 A0750 T2910
cellulosics
isocyanate I7840
polyacetal thiouronium S1590
methacrylate M8550
polyacrylate
ureido T2507
amine A0700 A0750 T2910 A0742 PS076
polyamide (nylon)
ureido T2507
chloromethylaromatic T2902
polyamide-imide
amine A0700 A0750 A0800
amine A0750
polybutylene terephthalate
isocyanate I7840
polycarbonate amine A0700 A0750 T2910
polyetherketone amine A0750 A0800
ethylene-vinyl acetate copolymer ureido T2507
amine A0700 A0742 A0750
polyethylene vinyl V4910 V4917
styryl S1590
amine A0700 A0750 T2910
polyphenylene oxide
aromatic P0320
amine A0700 A0750 T2910
polyphenylene sulfide mercapto M8450 M8500 B2494
chloromethylaromatic T2902
aromatic P0320 P0330
polypropylene
styryl S1590
aromatic P0320 P0330
polystyrene epoxy G6720 E6250
vinyl V4910 V4917
polysulfone amine A0700 A0750 T2910
polyvinyl butyral amine A0700 A0742 A0750
amine A0700 A0750 T2910
polyvinyl chloride
alkanolamine B2408

Table 3 - Sealants
Name Silane Class UCT Product
acrylic M8550
acrylic styryl S1590
epoxy G6720 E6250
mercapto B2494 M8500 M8450
polysulfides amine A0699 A0700 A0742 A0750
T2910
 Table 4 - Rubbers
Name Silane Class UCT Product
butyl epoxide G6720 E6250
neoprene mercapto M8450 M8500
isoprene mercapto M8450 M8500
fluorocarbon styryl S1590
amine A0699 A0700 A0742 A0750
epichlorohydrin
mercapto M8450 M8500
amine A0700 A0750
silicone
allyl A0567

Table 5 - Water Soluble and Hydrophilic Polymers

Name Silane Class UCT Product
epoxy G6710 G6720
cellulosic
isocyanate I7840
amine A0800 PS076
heparin epoxy G6710 G6720
isocyanate I7840
polyethylene oxide isocyanate I7840
epoxy G6710 G6720
polyhydroxyethylmethacrylate
isocyanate I7840
epoxy G6710 G6720
polysaccharide
isocyanate I7840
epoxy G6710 G6720
polyvinyl alcohol
isocyanate I7840
siliceous all listed in Table 1 A0700
aluminum, zirconium, tin and tita- all listed in Table 1, but S1590 M8540
nium metals the epoxies, acrylates
and quats preferred
polyamine T2910 PS076
copper, iron
phosphine D6110
phosphine D6110
gold, precious metals
mercapto B2494 M8500
silicon vinyl D6208


 SILANE REFERENCE LIST
Alkanoamine
B2408 Bis(2-hydroxyethyl)-3-aminopropyltriethoxysilane
Allyl
A0567 Allyltrimethoxysilane
Amine
A0699 N-(2-Aminoethyl)-3-aminopropylmethyldimethoxysilane
A0700 N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane
A0742 3-Aminopropylmethyldiethoxysilane
A0750 3-Aminopropyltriethoxysilane
A0800 3-Aminopropyltrimethoxysilane
PS076 (N-Trimethoxysilylpropyl)polyethyleneimine
T2910 Trimethoxysilylpropyldiethylenetriamine
Aromatic
P0320 Phenyltriethoxysilane
P0330 Phenyltrimethoxysilane
Chloroalkyl
C3300 3-Chloropropyltrimethoxysilane
Chloromethylaromatic
T2902 1-Trimethoxysilyl-2(p,m-chloromethyl)phenylethane
Epoxy
E6250 2-(3,4-Epoxycyclohexyl)ethyltrimethoxysilane
G6720 3-Glycidoxypropyltrimethoxysilane
Isocyanate
I7840 Isocyanotopropyltriethoxysilane
Mercapto
B2494 Bis[3-(triethoxysilyl)propyl]tetrasulfide
M8450 3-Mercaptopropylmethyldimethoxysilane
M8500 3-Mercaptopropyltrimethoxysilane
Methacrylate
M8550 3-Methacryloxypropyltrimethoxysilane
Phosphine
D6110 2-(Diphenylphosphino)ethyltriethoxysilane
Silazane
D6208 1,3-Divinyltetramethyldisilazane
H7300 Hexamethyldisilazane
Styryl
S1590 3-(N-Styrylmethyl-2-aminoethylamino)propyltrimethoxysilane hydrochloride
Ureido
T2507 N-(Triethoxysilylpropyl)urea
Vinyl
D6208 1,3-Divinyltetramethyldisilazane
V4910 Vinyltriethoxysilane
V4917 Vinyltrimethoxysilane
 APPLYING A SILANE COUPLING AGENT

Deposition from aqueous alcohol solutions is the most facile method for preparing silylated surfac-
es. A 95% ethanol – 5% water solution is adjusted to pH 4.5 – 5.5 with acetic acid. Silane is added
with stirring to yield a 2% final concentration. Five minutes should be allowed for hydrolysis and
silanol formation. Large objects, e.g. glass plates, are dipped into the solution, agitated gently, and
removed after 1 – 2 minutes. They are rinsed free of excess materials by dipping briefly in ethanol.
Particles, e.g. fillers and supports, are silylated by stirring them in solution for 2 – 3 minutes and
then decanting the solution. The particles are usually rinsed twice briefly with ethanol. Cure of the
silane layer is for 5 – 10 minutes at 110°C or for 24 hours at room temperature (<60% relative hu-
midity).

For aminofunctional silanes such as A0700 and A0750 this procedure is modified by omitting the
additional acetic acid. The procedure is not acceptable for chlorosilanes as bulk polymerization
often occurs. Silane concentration of 2% is a starting point. It usually results in deposition of
trialkoxysilanes as 3 – 8 molecular layers. Monoalkoxysilanes are always deposited in monolayers
or incomplete monolayers. Caution must be exercised if oven curing. Exhausted, explosion-proof
ovens should always be used.

Deposition from aqueous solutions is employed for most commercial fiberglass systems. The
alkoxysilane is dissolved at 0.5 – 2.0% concentration in water. For less soluble silanes, 0.1% of a
non-ionic surfactant is added prior to the silane and an emulsion rather than a solution is prepared.
If the silane does not contain an amine group, the solution is adjusted to pH 5.5 with acetic acid.
The solution is either sprayed onto the substrate or employed as a dip bath. Cure is at 110 – 120°C
for 20 – 30 minutes.

Stability of aqueous silane solutions varies from hours for the simple alkyl silanes to weeks for the
aminosilanes. Poor solubility parameters limit the use of long chain alkyl and aromatic silanes by
this method. Distilled water is not necessary, but water containing fluoride ions must be avoided.

Bulk deposition onto powders, e.g. filler treatment, is usually accomplished by a spray-on method.
It assumes that the total amount of silane necessary is known and that sufficient adsorbed mois-
ture is present on the filler to cause hydrolysis of the silane. The silane is prepared as a 25% so-
lution in alcohol. The powder is placed in a high intensity solid mixer, e.g. twin cone mixer with
intensifier. The solution is pumped into the agitated powder as a fine spray. In general, this opera-
tion is completed within 20 minutes. Dynamic drying methods are most effective. If the filler is
dried in trays, care must be taken to avoid wicking or skinning of the top layer of treated material
by adjusting heat and air flow.

Integral blend methods are used in composite formulations. In this method the silane is used as
a simple additive. Composites can be prepared by the addition of alkoxysilanes or silazanes to
dry-blends of polymer and filler prior to compounding. Generally 0.2 – 1.0 weight percent of silane
(on the total mix) is dispersed by spraying the silane in an alcohol carrier onto a pre-blend. The
addition of the silane to non-dispersed filler is not desirable in this technique since it can lead to
agglomeration. The mix is dry-blended briefly and then melt compounded. Vacuum devolatiza-
tion of byproducts of silane reaction during melt compounding is necessary to achieve optimum
properties. Properties are sometimes enhanced by adding 0.5 – 1.0% of tetrabutyl titanate or ben-
zyldimethylamine to the silane prior to dispersal. Amino-functional silanes are available in concen-
trate form for dry-blending with nylons and polyesters. Concentrates eliminate any need for sol-
vent dispersion and devolatization and reduce variability due to relative humidity and shelf-aging.
Deposition as a primer is employed where a bulk phase is required as a transition between a sub-
strate and a final coating. The silane is dissolved at 50% concentration in alcohol. One to three
molar equivalents of water are added. The mixture is allowed to equilibrate for 15 – 20 minutes
and then diluted to 10% concentration with a higher boiling polar solvent. Materials to be coated
with the primer are dipped or sprayed and then cured at 110 – 120°C for 30 – 45 minutes.

Chlorosilanes such as V4900 may be deposited from alcohol solution. Anhydrous alcohols, par-
ticularly ethanol or isopropanol are preferred. The chlorosilane is added to the alcohol to yield a
2 – 5% solution. The chlorosilane reacts with the alcohol producing an alkoxysilane and HCl. Prog-
ress of the reaction is observed by halt of HCl evolution. Mild warming of the solution (30 – 40°C)
promotes completion of the reaction. Part of the HCl reacts with the alcohol to produce small
quantities of alkyl halide and water. The water causes formation of silanols from alkoxy silanes.
The silanols condense with those on the substrate. Treated substrates are cured for 5 – 10 minutes
at 110°C or allowed to stand 24 hours at room temperature.

Chlorosilanes and silylamines may also be employed to treat substrates under aprotic conditions.
Toluene, tetrahydrofuran or hydrocarbon solutions are prepared containing 5% silane. The mix-
ture is refluxed for 12 – 24 hours with the substrate to be treated. It is washed with the solvent.
The solvent is then removed by air or explosion-proof oven drying. No further cure is necessary.
This reaction involves a direct nucleophilic displacement of the silane chlorines by the surface si-
lanol. If monolayer deposition is desired, substrates should be pre-dried at 150°C for 4 hours. Bulk
deposition results if adsorbed water is present on the substrate. This method is cumbersome for
large scale preparations and rigorous controls must be established to ensure reproducible results.
More reproducible coverage is obtained with monochlorosilanes.

Silazanes such as H7300 and D6208 may be used as treatments in concentrated form or as 10 –
20% solutions in aprotic solvents. In some applications, parts are exposed for 5 – 10 minutes by
dipping or in microelectronics by spin-on techniques. Optimum reactivity is at 30 – 50°C. An alter-
nate method of treatment is to expose parts to 50°C vapor for 2 – 6 hours. Ammonia is the byprod-
uct of silazane reaction and areas should be ventilated.

Appendix

Calculations of necessary silane to obtain minimum uniform multilayer coverage can be obtained
knowing the values of the wetting surface of silane (ws) and the surface area of filler.

(amount of filler) x (surface area of filler)

Amount of filler (g) =
wetting surface (ws)

Relative surface area of common fillers m2/g:

E-Glass 0.1 – 0.12

Silica, ground 1–2
Kaolin 7
Clay 7
Talc 7
Si, diatomaceous 1 – 3.5
Calcium silicate 2.6
NOTES
General Information

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