Silicone

Silicone is a versatile and flexible waterproof sealant with a multitude of applications. It is particularly good in situations where you need to make a waterproof gap seal, but cannot completely eliminate movement between the parts to be sealed. Its key features are:

• Remains permanently elastic after curing.
• Does not shrink.
• Keeps its colour and is waterproof.
• Resists most chemicals and solvents.
• Is non toxic after curing.
• Has very good adhesion to glass and all commonly used building materials.
• Resists temperatures ranging from -60°C to +200°C

Uses

• Sealing gaps round baths, showers, windows, worktops
• Fixing mirrors or glass to tiled surfaces
• Sealing gutters and downpipes
• Repairing rubber trim on cars
• Flexible moulds

Types of silicone

There are a a number of different types of sealant available, some with specific uses and capabilities. Note that not all of these attributes are exclusive, some products will combine attributes from several of these categories. All the silicone sealants we are discussing here are Room Temperature Vulcanised (RTV) products. These will cure at normal room temperature when exposed to moisture present in the air.

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.

Sanitary silicone

A silicone that includes a fungicide to help prevent formation of mould on silicone that is frequently exposed to moisture. To reduce the chance of mould forming it is important to get the surface of the silicone joint as smooth as possible. This also makes cleaning it easier. Improving ventilation in rooms that suffer persistent high moisture levels 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. It adheres well to most building materials. It is designed for sealing round window and door frames. Often available in a variety of colours designed to match wood grain and other common framing materials.

Neutral Cure Silicone

Most silicone sealants will release acetic acid while curing. This gives off a characteristic vinegar like smell. In some circumstances this odour may be undesirable, or the acetic acid may have a negative impact on the materials being sealed (for example some clear polycarbonate materials may suffer slight staining when exposed to acetic acid). Neutral cure silicone products reduce or eliminate the production of acetic acid while curing. Often the better 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 sealants inside refrigerators.

High Temperature Silicone

A special sealant designed to withstand higher temperatures when cured (typically up to 260 degrees celsius). Often finds applications in electrical and industrial equipment, and for making seals between high temperature surfaces.

Aquarium Silicone

A special silicone designed for glueing and sealing glass in aquarium applications.

Other specialist silicones

There are a number of 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.

High or Low Modulus?

Sometimes silicone sealants are described as "High modulus", or "Low Modulus". The low modulus products are more elastic and well suited to sealing tasks that need to accommodate more movement without failing. The high modulus products are less able to accommodate movement but will make a stiffer and more rigid final seal. The high modulus products tend to withstand repeated cleaning attempts better than the softer more flexible low modulus ones.

How much movement is allowed?

One of the primary cases for sealant failure is expecting the sealant to accommodate more movement than it is designed to be able to withstand. While silicone sealants are very strong and elastic when under shear loads, and when placed under tension, they have only limited adhesive properties under tension. This can result in the bead of sealant pulling away from one side of the joint.

As a guideline you should not expect a bead of sealant to expand by more than 10% of its original size (some products claim to be able to withstand up to 20% movement, but this does expect almost perfect application conditions). So for example a 10mm wide bead of silicone ought to be able to withstand stretching to 11mm repeatedly without failure. However it is unrealistic to expect a 5mm wide bead of silicone to expand to 10mm.

If you are unable to reduce the amount of movement expected in a joint, then you need to ensure that the silicone bead width forming the seal is larger so as to accommodate the movement and stay within the 10% threshold.

Another technique to improve the success rate of seals that need to withstand high movement, is to apply the sealant when the joint is at the far limit of its expected movement, and then only allow the joint to return to its "normal" position once the sealant is cured. A typical example would be when sealing round a bath. It is better to fill the bath with water before sealing. Allow the sealant to cure before draining the water. The will place the sealant under compression when the bath is empty, rather than under tension when it is full.

Life Expectancy

Good quality sealants should produce joints that last in excess of 10 years. Some products are even guaranteed a 25 year life.

Application

One of the keys to successful application is to make sure the joint to be sealed is clean and free from contaminants and grease first.

To get a good consistent bead of silicone in a single application is a skill that is difficult to master. More realistically you will need to "tool" the bead to its finished form after application to get good results.

The simplest "tool" is a finger! For best results dipping it in some form of release agent first will help stop the silicone sticking to the finger in preference to the joint. Suitable release agents include some spray on window cleaner products or washing up liquid. The difficult thing to achieve with a finger however is a good bead shape. You tend to end up with a concave bead with fine feathered edges. This is not a very strong bead shape, and the fine edges will tend to peel away when wiping the seal. Using masking tape applied down the outside edges of the joint can improve the edge by creating a slightly thicker edge with a cleaner transition.

Some consider that using a finger rather than a tool increases the likelihood of mildew formation later on, even when so-called mould-resistant silicone is used.

Most people find that good results are easy to achieve using one of the commercial application tools. The two most common ones are the "Fugi", and the "Fugenboy" by Elch. The video for the Fugi tool on the Plumbworld site here, shows just how well they work.

These tools receive good reviews on news:uk.d-i-y.

• Fugi
• Fugenboy

Home made applicators can also be made from polypropylene. They need to be feathered to give a suitable profile. Old ice cream tubs can work well as a good source of material.

Removal and Cleanup

While still wet, silicone can be cleaned up using a solvent such as isopropyl alcohol (IPA). The aerosol cans available from suppliers like CPC are ideal for this.

Once silicone has set it is very much harder to remove. One of its great attractions is that relatively few solvents or chemicals will attack it. It is possible to purchase proprietary silicone "eater" products. These will slowly dissolve small quantities of cured silicone into a sticky goo that can then be wiped off. For best effect you need to remove the bulk of the material by either peeling it off, or cutting it with a sharp knife first.

Alternative sealants

There are a variety of other sealants available, and these each have their uses. Silicone sealants will often outperform acrylic ones in wet service areas and where significant flexibility is required (and there is little cost advantage to acrylic sealants thee days). Some however have distinct uses that are better suited to the task in hand than silicone. Decorators caulk is a good example since it is easy to clean up with water, and is over-paintable.

See also

Putty & Mastic

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