Why can a sealant change colour over time?

As with most construction and sealing materials, a sealant can also show a colour change over time. In practice, discolouration is in most cases the result of external influences such as exposure to UV light and is not due to the quality of the sealant itself.

This can be recognised by black, brown or sometimes pink stains. The causes may include: prolonged moisture, soap residues, insufficient ventilation. Even ‘mould-resistant’ sanitary sealants can show mould after years.

Both too little and too much can cause yellowing or fading of the sealant. Mainly inexpensive or non-UV-resistant silicones are particularly sensitive to this. This is often noticeable in window seals, verandas, outdoor applications, etc.

Over time, the silicone sealant may chalk or show yellowing. Additives (such as fungicides) can break down, resulting in a change in colour and structure.

Certain products react with silicone:

• Chlorine/bleach → yellowing
• Ammonia → fading/discolouration
• Strong degreasers → dull or stained spots
• Solvents (acetone, thinner) → damage or discolouration

Some metals cause dark discolouration:

• Copper → green/brown discolouration
• Iron → rust-coloured stains
• Lead or brass → dark spots
• This often occurs in bathrooms or kitchens with metal drains or profiles.

A build-up of residue can cause yellow or brown deposits. Sometimes this may look like ageing, but it is on the sealant, not in it.

Nicotine and tar can cause a yellow or brown deposit. This is mainly visible around windows and ceiling penetrations.

A brownish deposit often forms in corners where water remains. Usually this is not mould, but a biofilm of bacteria and dirt.

Some examples:

• Non-sanitary silicone in a bathroom
• Neutral silicone where acid-curing was required (or vice versa)
• An incorrect choice leads to faster discolouration, mould or poor adhesion.

Dark spots may appear from within the sealant because residues of old mould, dust, oil or grease have penetrated the sealant.

High temperatures can cause yellowing of the sealant. This may occur around hobs and radiators, for example.

Some types of paint discolour the edges of silicone. Fumes from new paint can also cause yellow deposits.

Water quality can also cause discolouration of the sealant. Hard water (limescale + soap) can cause brown/yellow deposits. Iron-rich water can cause rust-coloured stains to appear.

This occurs in damp areas such as shower corners. It causes a pink to reddish discolouration.