Yellowing Under Rugs and Mats
If you've just bought an expensive new carpet you might decide to put a rug down to protect it, or put a mat by an external door. Sensible precautions, but don't do it straight away or you could end up with a yellow, rug shaped patch underneath!
Here's the reason...
Most modern carpets contain Latex within the backing. Latex is a flexible rubber-like adhesive compound used in carpet manufacture to anchor the tufts of woven carpets and to attach the secondary backing to the primary backing in tufted carpets (the majority of carpets we see).
Latex has a tendency to degrade fairly quickly so a preservative called Butyl Hydroxy-Tolulene (BHT) is added to slow down the deterioration. BHT may also be present in the underlay. When a carpet containing Latex is laid, BHT is released into the air, a process known as off-gassing. This accounts for the "new carpet" smell that can linger for several weeks before dissipating.
If a rug or mat is placed on new or nearly new carpets it traps the BHT which then chemically reacts with the carpet fibres - a phenomenon called "Phenolic Yellowing" which as the name suggests turns the area a yellowish colour, and for all intents and purposes it is permanent. The same phenomenon can sometimes be seen under furniture where there is restricted air circulation, such as chairs & sofas with skirts around them that go almost to the floor or underneath cupboards.
You should also bear in mind that the same principle applies if the rug or mat you intend to put down has latex within its construction, even if you're using an offcut of the carpet as a mat.
How to prevent Phenolic Yellowing
So if you're planning to place a rug or mat on your new carpet, you can reduce the risk of yellowing by not putting it down straight away, leave it a minimum of a month or until the "new carpet smell" has gone. Even then the risk of yellowing will remain, as low levels of off-gassing may still exist for a year or two, but the risk will be greatly reduced.
If the rug or mat itself contains latex in the backing, unroll it and place it upside down in another room or garage etc for a few weeks before putting it down. Check underneath rugs and furniture regularly and if possible move them for a while to let the carpet breathe.
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