Have you ever tried to purchase silicone sealant and been overwhelmed by all the choices? You are not alone; this article explains what silicone sealants are and how they are used. Hopefully, you will be able to select the right sealant for your application by the end of this article.

Most people go to their local DIY store and grab the cheapest silicone sealant in the right colour, but the term silicone sealant is a very broad term.

The reality is that there is not a single silicone sealant that can be used for everything. All of the different types have different flexibility, durability, and resistance. So how can you determine which is right for your job?

In general, silicone sealants are categorized by what type of job they are suitable for, so choosing the right product is easy.

Silicone sealants are commonly used for:

• Gaps around baths, showers, windows, and worktops should be sealed
• The installation of mirrors or glass on tiled surfaces
• Installing downpipes and gutters
• Replacing rubber trim on cars
• Moulding into soft rubber goods of most shapes
• Making reusable flexible moulds
• Aquaria glued together

Due to their strong performance characteristics, silicone sealants are used in a wide variety of building applications, including:

• Protection against UV rays
• Resistant to temperature changes
• Highest movement capability (Class 50–100)
• Generally longer service life
• Continued flexibility over time

Silicone sealants can have a strong odor and some can take a long time to fully cure, but one of their benefits is that they can be used structurally in glass assemblies. Pure silicone sealants cannot be painted. As a result, they remain flexible at wide temperatures, are completely waterproof, adhere well to most surfaces, and won’t support mildew growth, while some, like Dowsil 785+, have specific bacteria-resistance properties.

Silicone types in common use in the industry include:

• High Modulus
• Low modulus
• Neutral cure
• Acetoxy cure

In the simplest terms, a low modulus will require less force to stretch it once it has cured, and will have better elasticity and movement, while a high modulus will be more rigid once it has cured. When deciding which modulus to use for a specific application, it is important to consider desired movement characteristics.

If you’re sealing around a bath, for example, you’d use high modulus silicone. As you fill the bath with water, it will lower slightly, pulling on the sealant as it does so. The silicone may tear as the bath moves, causing it to leak. Sealants with high modulus will stretch when the bath moves and flex back when the weight is lifted. You might want to use a low modulus sealant if you have a lot of movement.

Sealant being applied to a car light surround

SEALANT TYPES

Low modulus sealants are stretchy but not very durable; they are best used for sealing glass wall assemblies, perimeter seals on curtain walling, perimeter seals of powder coated frames, and glazing of windows in swimming pool buildings. Sealants of this type provide better adhesion and allow for more movement. The sealants are generally very long-lasting.

It is recommended to use high modulus sealants to seal baths, showers, sinks, washbasins, kitchen worktops, and many other sanitary and internal applications. Glass to glass joints, shopfronts, and showcases can also use them.

Acetoxy silicone sealants release acetic acid (which smells a little like vinegar) as they cure, this is the most commonly used – it is more rigid and the full cure is quick. Unfortunately, it generally has poor adhesion and leaves a lot to be desired in how well it adheres to PVC-U, most other plastics, and glass, aluminum, and polycarbonate.

Neutral (meaning neutral alkoxy) silicone sealants release alcohol as they cure, and has almost no smell, they have better adhesive properties for a greater number of materials including PVC-U, most other plastics, glass, aluminium, lead, stone and masonry, and Polycarbonate. Their downside is that they can be more expensive and take longer to cure due to the thickness, temperature, and humidity conditions.

NEUTRAL CURE OR ACID CURE?

Acid-cure silicones work best on non-porous surfaces such as glass and glazed tile, but they can corrode metal and etch some plastics, whilst neutral-cure silicones work well on metal and wood.

WHICH SEALANT FOR WHICH APPLICATION?

Low Modulus Acetoxy (LMA) sealants are the cost-effective “all-rounders”, they are suitable for a wide variety of general building, glazing interior, and exterior applications and adhere to many common building materials.

Low Modulus Neutral (LMN) sealants are the best option for the sealing of UPVC Window or Door Frames (exterior). They offer better adhesion, accommodate more movement, and are generally longer lasting than LMA’s.

High Modulus Acetoxy (HMA) sealants are most commonly used for kitchen and sanitary applications (ensure the sealant contains a fungicide).

General-purpose and builders silicone: this is the basic general purpose silicone sealant. It sticks well to most building materials and offers good elasticity and durability but usually do not contain any fungicides.

Sanitary silicone: a silicone that includes a fungicide to reduce the formation of mould on silicone that is frequently exposed to moisture. The smoother the surface of the silicone joint, the less likely it is to form a mould. In addition, it is easier to clean. In rooms that have persistent high levels of moisture, improving ventilation can also help reduce mould growth.

Glazing Silicone: typically a sealant that cures to a very clear finish and offers very good adhesion to glass. Used for bedding in glazing panels, and sealing around the edge of the glass in windows and doors.

Frame Sealant Silicone: this is a tough sealant designed to accommodate large amounts of movement, and it is also better able to withstand exposure to the weather and UV radiation than other types of sealant. Almost any material can be used with this sealant. Window and door frames can be sealed with it. A variety of colors are often available to match wood and other common framing materials.

Neutral Cure Silicone: most silicone sealants release acetic acid while curing. This results in a characteristic vinegar odor. In some cases, this odour may be undesirable, or the acetic acid may damage the materials being sealed (for example, some clear polycarbonate materials may stain slightly when exposed to acetic acid). In neutral cure silicone products, acetic acid is reduced or eliminated during curing. Good quality sealants also tend to be neutral cure products.

Food Safe Silicone: a silicone product with very low toxicity designed for use in food contact applications. Often used for sealing inside refrigerators.

High-Temperature Silicone: designed to withstand high temperatures when cured (typically up to 260°C, but 300°C rated is available). These are often found in applications in electrical and industrial equipment, and for making seals between high-temperature surfaces.

Aquarium Silicone: a very tough silicone designed for gluing and sealing glass in aquarium applications. It is not advised to use building silicones for fish tank construction.

Other specialist silicones: there are many other specialist silicone products designed for different industrial uses. For example in the electronics industry. Many of these are designed for coating circuit boards to resist moisture penetration, or for “potting” assemblies, joints, and circuits.

Here is a really good video from How To Handyman explaining how to choose the right silicone.

CONCLUSION

In this article, we hoped to demystify some sealant jargon and make it clear which sealant is best for your needs.

A silicone sealant’s application is one of the most important factors to consider when choosing one. Knowing this will help you narrow down your search.