Intumescent pipe collars & wraps are always manufactured from a high performance material bonded to the inside of a steel sleeve, which on exposure to heat from a fire will rapidly expand inwards to squeeze the collapsing plastic pipe until the aperture is completely sealed.

This is extremely important for all fire stopping finishes, as it prevents fire from travelling through pipes in the building - and into other rooms. 

We almost always employ this method of fire stopping in all of our remedial works and any new projects that we take onboard.