Applications & Uses of Silane Coupling Agents

Uses of Silane Coupling Agents in Filler Treatment

Silanes used as coupling agents in filler treatment are useful for many applications, some examples are HFFR wire & cable compounds, Mica filled polypropylene and polyamide and clays in rubber.

#1 - Wire & Cable Compounds

Because of their outstanding electrical properties and resistance to ozone, weather and heat, ethylene-propylene copolymers (EPR), terpolymers (EPDM) and cross-linked PE are excellent high-voltage insulating and jacketing materials. White fillers are detrimental to these uses because of the loss in wet electrical resistance and power factor. Treatment of kaolin clay reinforced EPDM wire and cable coatings with a vinyl silane (XIAMETER™ OFS-6300 Silane) prevents this effect and provides better cure and higher modulus.

Silanes used in EPDM wire and cable coatings

#2 - Mica Filled Polypropylene

Polypropylene is a valuable thermoplastic having good physical properties, low price and a high heat distortion temperature. Mica as a filler in PP increases its flex modulus with little effect on flex strength. Treatment with XIAMETER™ OFS-6032 Silane provides improvement in both these properties. The treatment also allow a reduced melt viscosity, leading to energy savings in molding operations.

Silanes are used in mica fille PP parts like automotive side handles

#3 - Mica Filled Polyamide

An amino or chloropropyl functional silane on the surface of an inorganic filler thus lends itself to coupling of polyamides to silica, mica and others. In particular, PA 6 filled with high-aspect ratio mica shows reinforcement even without a coupling agent. However, addition of XIAMETER™ OFS-6020 Silane or XIAMETER™ OFS-6076 Silane treatment on the mica shows significant improvement in all strength parameters.

Silane coupling agents are used in mica filled polyamides

#4 - Clays in rubber

The most extensively used filler reinforcement for rubbers is carbon black, which is self-coupling and gives excellent results. The demand for rubber articles in colors other than black, yet with good mechanical properties, has inspired the search for suitable white and colored fillers with similar beneficial effects.

Silane coupling agents are the key to satisfying this need by providing a method of effectively bonding various inorganics to the rubber phase. Most of the common peroxide or sulfur-cured elastomers are reported to show improved strengths (tensile and tear) with silane treated clay reinforcements. Treatment of clay silanes also helps to optimize the electrical insulating properties of clay-filled rubber.

Similar improvements in the performance of clay as a reinforcement can be obtained by treating the clay in situ while compounding with the rubber. In such cases, the clay, rubber and silane should be compounded separately before adding ingredients. This is done to allow the silane the opportunity to migrate to and react with the mineral surface before the other ingredients have the opportunity to interfere with this reaction.

Uses of Silane Coupling Agents in Silica Treatment

Silica reinforcement of rubber provides unique physical properties and performance characteristics over carbon black reinforcement, but however silane coupling agents are necessary for the non-black reinforcing fillers to be effective. Silanes are the key to provide an effective method of the inorganic fillers to organic elastomers. Silanes used as coupling agents in silica treatment are useful for many applications, such as green tire, shoe soles.

#1 - Green tires

Green tires, are so-called because the silica/silane technology enables the production of high-performance tires with lower borring resistance. Lower rolling resistance results in better fuel economy and lower impact on the environment.

But unlike carbon back, which can easily and homogeneously be incorporated in elastomers, silica is difficult to disperse and is known to have low compatibility with elastomers. A silane coupling agent is thus absolutely necessary. The silane/silica technology provides tires with:

Good abrasion resistance (long useful life)
Reduced rolling resistance (improved fuel economy)
Better grip on wet surfaces
Better overall balance of properties

Green tires, are so-called because the silica/silane technology enables the production of high-performance tires

#2 - Shoe Soles

When used in shoe soles, silane coupling agents provide coupling of silica to the polymer backbone allowing for:

Improved abrasion resistance
Longer flexural life
Use of a white filler

The mechanism of behavior is much the same as in Silica-filler tire treads. The Alkoxysilyl portion reacts with the Silica and the organofunctional portion reacts into the rubber during vulcanization. The result is improved rubber-to-filler interaction.

When used in shoe soles, silane coupling agents provide coupling of silica to the polymer backbone

Uses of Silane Coupling Agents in Glass Filled Thermosets

Silane coupling agents are a critical component of glass-reinforced thermosets.

Methacrylate functional silanes, XIAMETER™ OFS-6030 Silane, were first developed for the reinforcement of polyesters. More recently, epoxy functional, XIAMETER™ OFS-6400 Silane, vinyl-benzyl-amino functional silanes such as, XIAMETER™ OFS-6032 Silane, have proven valuable in epoxy, polyester and other thermoset resin composites using glass reinforcements.

Most important benefits imparted to glass-reinforced thermosets coupled with silanes are:

Increased mechanical strength of the composites
Improved electrical properties
Improved resistance to moisture attack at the interface

#1 - Epoxy Glass Laminates

Glass-epoxy laminates are the standard for high performance electronics, such as printed circuit boards. Silane coupling agents are used as a finish on woven glass fabric to improve physical properties, especially the wet strength, of the composite.

Circuit-Board manufacturing is a very high technology application, and requires excellent coupling agent technology to provide the flaw-free benefits that are required. Dow Corning has developed unique technology for this application. XIAMETER™ OFS-6032 Silane and modified products based on this technology have provided the quality and benefits required, as shown in the table below.

Epoxy- Fiberglass Laminate	Flexural Strength (psi)
Epoxy- Fiberglass Laminate	Dry	Wet*
Without silane	68,340	38,270
With XIAMETER™ OFS-6032 Silane (Cationic Styryl)	97,800	72,460
* After 72 hours immersion in boiling water

Performance of silane coupling agents as a fiberglass finish in epoxy laminate

#2 - Polyester Glass Laminates

Polyesters with various glass fiber reinforcements comprise a large volume, diverse market. Glass products treated with silane coupling agents XIAMETER™ OFS-6040 Silane, XIAMETER™ OFS-6030 Silane and XIAMETER™ OFS-6032 Silane are ideally suited to this use. 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 the figure below.

* After 72 hours immersion in boiling water
Performance of silane coupling agents as a fiberglass finish in polyester laminates

The vinylbenzyl-functional silane coupling agent (XIAMETER™ OFS-6032 Silane) gives greater improvement in the flexural strength of a glass-reinforced epoxy system than does the epoxy-functional silane coupling agent (XIAMETER™ OFS-6040 Silane). More significantly, the retention of strength after aging for 72 hours in boiling water is significantly better with any of these silanes than if no silane coupling agent is used, but XIAMETER™ OFS-6030 Silane (Methacryloxy silane) and XIAMETER™ OFS-6032 Silane give better retention of flexural strength. These types of effects are what are generally expected from the use of silane coupling agents.

#3 - Epoxy Glass Beads

Silanes are highly effective coupling agents for glass beads reinforced epoxy resins.

#4 - Other thermosets

Vinyl-Unsaturated Types - Peroxide Curable: Glass fiber products with finishes incorporating methacrylate (XIAMETER™ OFS-6030 Silane) or vinyl-benzyl-amino (XIAMETER™ OFS-6032 Silane) functional silanes are recommended, along with a vinyl-functional type (XIAMETER™ OFS-6300 Silane/XIAMETER™ OFS-6518 Silane), depending on the reactivity of the resin unsaturation. Free acid or other functionality in resins may permit use of other silanes such as XIAMETER™ OFS-6040 Silane or XIAMETER™ OFS-6020 Silane in some cases.

Alkyd and Allyl Resins: These two types chiefly find use in the electrical field in glass-filled molding compounds and some fabric laminates (allyl) where optimum electrical properties are needed.

Vinyl Ester Resins: These specialty resins with a structure similar to epoxy resins contain unsaturation enabling a radical-initiated cure. They find use in filament-wound and fabric-reinforced chemical processing components. XIAMETER™ OFS-6032 Silane is particularly recommended for this type of resin.

Polybutadiene Resins: Polymerization of butadiene by the "1,2 mechanism" gives many pendant vinyl groups which are cured by peroxides. The all-hydrocarbon stucture imparts low loss/electrical properties, making it valuable in glass laminates and molding compounds, filled composites, and encapsulating resins for the electrical and electronic industries. XIAMETER™ OFS-6032 Silane and XIAMETER™ OFS-6300 Silane/XIAMETER™ OFS-6518 Silane are recommended for these resins.

Condensation Resins: Condensation resins commonly cure through condensation of a methylol group with an active hydrogen, generating water as byproduct. Amine functional groups like those available in XIAMETER™ OFS-6020 Silane can also participate in such condensation reactions. Other silanes such as the epoxy functional (XIAMETER™ OFS-6040 Silane), and vinyl-benzyl-amine (XIAMETER™ OFS-6032 Silane) silanes are also effective in improving adhesion of condensation resins. On the basis of cost vs. performance, however, aminosilanes such as XIAMETER™ OFS-6020 Silane or XIAMETER™ OFS-6011 Silane are most commonly used.

Uses of Silane Coupling Agent in Glass Filled Thermoplastics

Silane coupling agents are a critical component of fiberglass-reinforced thermoplastics. They have proven valuable in polyamide (PA), and polybutylene terephtalate (PBT) composites using glass reinforcements.

Most important benefits imparted to glass-reinforced thermoplastics coupled with silanes are:

Improved electrical properties
Increased mechanical strength of the composites
Improved resistance to moisture attack at the interface, leading to more durable composites.

#1 - Polyamide Glass Fiber

Polyamides are formed in reversible reactions between organic acids and amines, giving "polyimides" which are reactive with other amines at high temperature by virtue of that reversibility. An amino or chloropropyl functional silane on the surface of the glass lends itself to coordination with the amide groups and possibly coupling of polyamides to the glass surface, silica, mica an so on.

#2 - PBT Glass Beads

PBT is intrinsically less polar and less reactive than PA. Consequently, silanes are less effective in improving the physical strengths of reinforced composites. XIAMETER™ OFS-6040 Silane, however, is effective in improving the physical properties of a glass bead-filled PBT, as shown in the table below.

System	Flexural Strength (psi)		Flexural Modulus (10⁵ psi)		Tensile Strength (psi)
System	Dry	Wet*	Dry	Wet*	Dry	Wet*
Unfilled resin	12900	13000	3.14	3.00	7300	7300
Filled with 35% glass beads, Untreated	10800	10100	5.83	4.04	5600	4800
Filled with 35% glass beads, 0.25% XIAMETER™ OFS-6040 Silane	14900	14400	6.07	5.38	8000	7900
* After 16 hours immersion in water at 50°C

Performance of silane coupling agents in glass bead reinforced PBT

Uses of Silane Crosslinking Agents

Silane technology demonstrates outstanding process flexibility due to:

Its ability to trigger the crosslinking at the desired time and
Its ability to easily adjust the reactivity of the system

Moreover, silane crosslinked PE exhibit superior properties (such impact performance) than peroxide crosslinked PE. This combination makes Vinylsilane crosslinking technology the best choice for application such as wire & cable, PEX pipes or heat shrinkable products.

#1 - Wires and Cables

Vinyl silane crosslinking technology is well established in the manufacture of low voltage wire and cables (< 1kV) for industries such as:

Power distribution
Telecommunications
Transportation and
Appliances

However, due to having superior process flexibility, enabling outstanding performance and higher extrusion rate, Silane technology is penetrating the medium voltage market.

The unique set of benefits provided by Silane technology will allow the development of new generation cables with optimized cost and performance. Despite the fact that this technology is easy to implement, the selection of the right system needs to integrate not only material performances but also processing conditions and logistic constraints.

Benefits of Silane crosslinking in cables:

High heat resistance
Process flexibility
High speed extrusion
Good electrical retention
Good abrasion resistance

Note: In cable applications, Silanes are also used as coupling agent between Halogen free FR additives (ATH, MDH) and the polymer in order to achieve the required set of properties.

#2 - Crosslinked Pipes

PEX exhibits a unique set of properties close of thermosets and silane technology enable the processing benefits and flexibility of thermoplastics. Due to its improved thermal stability under load, environmental stress crack resistance (ESCR) and resistance to slow crack growth, PEX appears as a best-in-class and cost-effective material for pipe applications.

The use of PEX pipes is growing in construction and engineering markets (+12%/year) to replace metal (copper and steel) and some engineering polymers. Typical applications are:

Residential and commercial cold and hot water distribution systems
Low temperature heat transfer applications
Radiant floor heating
Snow melting
Hot water baseboard heating
Radiators

Compared to traditional plumbing materials PEX pipes demonstrate the following benefits:

Performance benefits	Installation benefits
Electrolysis Resistance Corrosion Resistance Longer System Life Quiet Water Flow Chemical Resistance to most Chemicals	Faster, Easier Installation No Solvent or Chemical Joining Required Lower weight

The use of silane technology provides flexible pipe facilitating installation and allowing complex shaping. When installed, pipe can be cured by the circulating hot water.

Silane technology provides flexible pipe

#3 - Shrinkable Films

Heat shrinkable products are typically used to insulate, terminate, splice or bundle cables. They are also used to provide mechanical, environmental and corrosion protection as well as moisture sealing.

These products can be found in all major industries, such as automotive & transportation, electronics, power distribution, wire & cable, appliances, construction and many other markets.

Heat shrinkable products used for technical applications need to be crosslinked in order to improve properties such as:

Continuous operating temperature
Chemical resistance
ESCR and so on...

During the manufacture of shrinkable products, the hot tubing coming out of the extruder die is delivered to a cooling and calibration system in order to stretch polymeric chains and to freeze them into the right shape. A take-off device removes the tubing from the cooling system and delivers it to a winder, where it is wound on spools.

The crosslinking reaction occurs after the preliminary product shape has been formed, but before the mechanical and thermal operations that are used to impart 'stretched' dimensions to the article. Since, silane crosslinking technology allows the greatest process flexibility, it appears as the best choice for this kind of application. In this application, shrinking temperature is key.

Silane coupling agents enable the production of high-performance tires with lower borring resistance

Silane crosslinking agents are used to manufacture wire and cable for appliances

Silane coupling technology is used to manufacture shoe soles

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Diverse Applications of Silanes