A Guide to Silane Solutions: Adhesives and Sealants

Silanes are used widely in sealants and adhesives to improve the adhesiveness of te product. Sealants often have a dual purpose, they prevent the passage of water, air and chemicals through the zone where applied and also serve as an adhesive in certain instances.  Silanes can form durable bonds in metal, glass, ceramic and other surfaces. Hence these type of sealants are used in a range of industries including aerospace, automotive and construction.

Adhesion Promoters

A silane coupling agent will function at the interface between the sealant 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. Table 1 suggests 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, 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 application 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 interface where it can enhance adhesion between the polymer and the substrate. 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 adhesives 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

Table 1. Silane Coupling Agent Recommendations for Various Polymers – Matching Organoreactivity to Polymer Type

Organic Reactivity Application (suitable polymers)
Amino Acrylic, Nylon, Epoxy, Phenolics, PVC, Urethanes, Melamines, Nitrile Rubber
Benzylamino Epoxies for PCBs, Polyolefins, All Polymer Types
Chloropropyl Urethanes, Epoxy, Nylon, Phenolics, Polyolefins
Disulfido Organic Rubber
Epoxy Epoxy, PBT, Urethanes, Acrylics, Polysulfides
Epoxy/Melamine Epoxy, Urethane, Phenolic, PEEK, Polyester
Mercapto Organic Rubber
Methacrylate Unsaturated Polyesters, Acrylics, EVA, Polyolefin
Tetrasulfido Organic Rubber
Ureido Asphaltic Binders, Nylon, Phenolics; Urethane
Vinyl Graft to Polyethylene for Moisture Crosslinking, EPDM Rubber, SBR, Polyolefin
Vinyl-benzyl-amino Epoxies for PCBs, Polyolefins, All Polymer Types

Crosslinkers

Silanes can be used to crosslink polymers such as acrylates, polyethers, polyurethanes and polyesters. 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 properties, 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 attached to silicon increases the rate of reaction of the methoxysilane with water to give efficient elimination 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 formulation 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 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

About Xiameter

In 2000, Dow Corning was facing an increasing number of competitors around the world that were getting into the standard silicones business. At the same time, for a growing customer segment called “price-seekers” who needed little or no service attached to the products, price had become the driving force.

Dow Corning needed to defend their position as a provider of innovative silicon-based materials and solutions (which, for the most part, came bundled and priced together). Dow Corning knew that they needed to find a better way to meet customers’ needs exactly.

It was time for a game-changing strategy – one that would simultaneously:

  • Meet customer needs for efficient, cost-effective silicone products
  • Empower Dow Corning to continue to innovate and grow the silicones market overall

Dow Corning decided the best way to take advantage of the potential in the mature market segment, without detracting from the value of the Dow Corning offering, would be to create a separate brand. This brand would offer a clearly defined value proposition and set of products via a web-enabled platform to provide competitive pricing.

The product set would be a wide range of standard silicone products generally thought of as “commodities” by customers. These standard products would be – and still are – manufactured by Dow Corning.

The tone would be “no frills” – just straightforward business terms and conditions, high-quality products, a reliable supply, and market-driven prices. Short. Sweet. To the point.

Defining success (2002-2009)

With the launch of the XIAMETER® brand in March 2002, Dow Corning revolutionized the way they do business. Specialty silicone products, service and solutions continued as part of the business under the Dow Corning® brand. The XIAMETER brand met the needs of “price-seekers” who purchased standard silicone products.

The global drive to find greater efficiencies in business through a web-enabled business model worked in their favor. It helped Dow Corning maximize productivity and reduced human error, while keeping costs competitive for customers. Rather than cannibalizing the Dow Corning brand as some feared, the XIAMETER® brand made it stronger. The two brands worked in harmony, helping increase Dow Corning’s financial results dramatically during this period.

Between 2002 and 2009, the XIAMETER® brand was a web-enabled business with clear business rules, offering commonly used standard silicones at transparent, market-driven prices. However, those business rules only allowed for large-volume orders. There was no technical service. Lead times ran 7-20 days. And only 400 standard silicone products were available through www.xiameter.com.

Expanding the model (2009-present)

In 2009, Dow Corning expanded the XIAMETER® business model – for the same reason we launched it in the first place – to meet customer needs based on smart customer segmentation analysis. Xiameter saw a need for all customers – not just large-volume buyers – to be able to purchase standard silicone products at market-based prices. Based on the success of the original Xiameter model, company leaders were confident expanding the model would work – both for customers and Dow Corning. And it did!

The XIAMETER® business model, which was expanded in June 2009 and expanded again in 2011, now offers thousands of standard silicone products. Plus, the model is no longer for large-volume customers only. There are more volume-quantity options. With the Xiameter transparent price tiers, customers can choose the pricing most appropriate for them based on the volumes they need. They can purchase multiple items within a product family and receive greater discounts. They can lock in price and volume commitments through an online supply agreement. They also can choose credit terms that work best for them, which can be varied each time they order. Plus, customers who need smaller quantities, shorter lead times or more flexible ordering options can have them – by purchasing XIAMETER® brand products through local distributors.

With today’s XIAMETER® brand, customers can get all the standard silicones they need in the way that works best for them. Whatever they want. Wherever and however they want it. In a world of traffic jams, on-hold music and long supermarket lines, we think it’s a great option.

Source: Xiameter.

For more information on this source, please visit Xiameter.

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