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Silane
Coupling Agent
The general formula of an organosilane shows two classes of
functionality.
RnSiX(4 -n)
Silicone (Si) is the center of the silane molecule which contains an
organic functional group (R) [ex: vinyl, amino, chloro, epoxy, mercapto, etc.], with a
second functional group (X) [ex: methoxy, ethoxy, etc.]. The functional group (R) will
attach to an organic resin while the alkoxy group (X) attaches to an inorganic material or
substrate to achieve a "coupling" effect.
Silane coupling agents are predominately used as mediators, binding
organic materials to inorganic materials. As a result silanes will improve the electrical
and mechanical strength properties of materials in wet or dry conditions.
Silane coupling agents are primarily used in reinforced plastics and
electric cables composed of crosslinked polyethylene. Other uses include resins, concrete,
sealant primers, paint, adhesives, printing inks and dyeing auxiliaries.
The inorganic group (X) of the silane molecule will hydrolyze to
produce silanol, which forms a metal hydroxide or siloxane bond with the inorganic
material. The organic group (R) of the silane molecule will react with the organic
material to produce a covalent bond. As a result the organic material and the inorganic
material are tightly bound together after heating.
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N-beta-(Aminoethyl)-gamma-aminopropylmethyldimethoxysilane
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N-beta-(Aminoethyl)-gamma-aminopropyltrimethoxysilane
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Gamma-Aminopropylmethyldiethoxysilane
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Gamma-Aminopropyltriethoxysilane
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Gamma-Aminopropyltrimethoxysilane
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Bis(3-triethoxysilylpropyl)tetrasulfide
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Gamma-Chloropropyltriethoxysilane
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Gamma-Chloropropyltrimethoxysilane
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Gamma-Glycidoxypropyltrimethoxysilane
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Gamma-Mercaptopropyltrimethoxysilane
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Gamma-Methacryloxypropyltrimethoxysilane
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Methyltriacetoxysilane
(MTAS)
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Methyltrimethoxysilane
(MTMS)
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Methyl
tris-(butanone oxime) Silane (MOS)
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Methyl Oximino
Silane (MOS)
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Methyl tris-(methyl
ethyl ketoximo) Silane (MOS)
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Tetraethoxysilane
(TEOS)
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Tetramethoxysilane
(TMOS)
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Vinyltriethoxysilane
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Vinyltrimethoxysilane
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Vinyl
tris-(butanone oxime) Silane (VOS)
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Vinyl Oximino
Silane (VOS)
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Vinyl tris-(methyl
ethyl ketoximo) Silane (VOS)
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Adhesives:
Adhesives: Can be applicable to water -based and oil -based
adhesives. Moisture initiated crosslinking of resins, improve wet adhesion, improve
chemical resistance, weatherability and filler/resin coupling.
Coatings and paints: Improve adhesion on metal as well as
plastic surfaces, wet adhesion, chemical and corrosion resistance, weatherability, pigment
dispersion and scrub resistance.
Glass fiber and glass mat: Treating by silane coupling agents
improves adhesion with FRP resins and reinforces physical strength of FRP.
Filler treatments: Improve coupling of resins with fillers and
better filler dispersion in thermoset and thermoplastic resins.
Artificial marbles: Improve coupling of thermoset resins with
fillers and bettter filler dispersion water resistance and physical strength.
Sand paper and abrasive Grinding wheels: Improve adhesion of
corundum and carborundum with Phenol, Furan and Melamine resins. Improve water resistance.
Foundries: Coupling of resins with sand for improving foundry
core strength.
Printing inks: Improve adhesion, release and wetting
Rubber and elastomers: Coupling of resins with minerals for
improving composite strength, toughness, abrasion resistance, rolling resistance, wet
electrical properties and rheology control.
Sealants: In the moisture curing sealant, improve wet adhesion,
chemical resistance, filler dispersion, weatherability and rheology.
Thermoplastics: Moisture curable XLPE for Wire & Cable and
Pipe, Mineral and Pigment treatment for dispersibility and coupling and reinforcement
coupling for high performance thermoplastics.
Textiles: Improve textile softness and water repellency and
improved dye receptivity.
Polymer modification: Moisture-cure crosslinking to
give improved environmental and chemical resistance.
Crude oil extraction: Consolidation of down-hole fines
Deposition from aqueous alcohol or solution |
Deposition from aqueous alcohol solution is the most facile method for
preparing silylated surfaces. 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 -3minutes 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 110oC or 24 hours at room temperature. |
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. Washing off excess solution after 5 -10 minutes
is recommended to minimize particle oligomerization. Cure is at 110 -120oC 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 |
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 moisture is present on the filler to cause
hydrolysis of the silane. The silane is prepared as a 25% solution 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 operation 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 oftreated
material by adjusting heat and air flow. |
Integral blend methods |
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 to 1.0 weight percent of silane (of the total mix) is dispersed by spraying
the silane in an alcohol carrier onto a preblend. 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 devolatization should be
implemented of the products of the silane reaction during melt compounding to achieve
optimum properties. Properties are sometimes enhanced by adding 0.5 -1.0% of tetrabutyl
titanate or benzyl dimethylamine to the silane prior to dispersal. Amino functional
silanes are available in concentrate form for dry -blending with nylons and polyesters.
Concentrates eliminate any need for solvent dispersion and
devolatization and reduce variability due to relative humidity and shelf -aging. |
Deposition as a primer |
Deposition as a primer is employed where a bulk phase is required as a
transition between a substrate 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 -120oC for 30 -45 minutes. |
The silicone molecule is preferably attached to the surface of the
inorganic material as a primer to form a mono-layer. Applying a silane as a primer will
produce optimum coupling results between the substrate and the resin to be applied. When
used as a primer the required amount of silane can be calculated by the following:
Amount of silane (g) = ( Amount
of filler (g) x Surface area of filler(m2/g) ) / ( Minimum coating area of silane coupling
agent (ws))
The actual values may deviate from the calculated value depending
on the surface condition of the filler or the silane treating process. The following
values may be used as guidelines when the value is unknown. A dilution of 1% silane to
filler may be considered as standard. Generally 0.3% to 0.5% is recommended.
Coating area of silane coupling agent (ws)
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Description |
Coating area (ws) |
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n-(2-Aminoethyl)-3-aminopropylmethyldimethoxy
Silane
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358 |
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n-(2-Aminoethyl)-3-aminopropyltrimethoxy
Silane
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358 |
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3-Aminopropyltriethoxy
Silane
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353 |
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3-Aminopropyltrimethoxy
Silane
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353 |
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3-Methacryloxypropyltrimethoxy
Silane
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314 |
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3-Mercaptopropyltrimethoxy
Silane
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348 |
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3-Glycidoxypropyltrimethoxy
Silane
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330 |
Surface area of filler (m2/g)
Filler |
Surface area (m2 /g) |
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Silica powder
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1
~ 2 |
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Kaolin
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7 |
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Fumed silica
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150
~ 300 |
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Clay
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7 |
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Talc
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7 |
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Calcium silicate
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2.6 |
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Diatomaceous earth
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1
~ 3.5 |
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