Polyimide needlefelts are excellent high temperature filter media in terms of performance, generally surpassing the other standard media with the exception of PTFE.
They are generally good up to around 240°C provided the chemistry is benign, hydrolysis being a potential problem at elevated temperatures if the moisture content is too high.
In common with many fibres, polyimides can be blended with conductive fibres or manufactured on a conductive grid scrim to reduce the potential for static electrical build up. Because of the end use environment, steel is the preferred conductive fibre though other fibres could be used in the low temperature applications.
Interestingly, the fibres have a complex cross sectional shape which results in a much enhanced collection efficiency when compared with circular fibres:
Typical applications for P84 needlefelts are:
| Power Generation | Metallurgical | Waste Incineration | Straw Burning |
| Clinical Incineration | Gypsum | Cement | Lime |
These non circular shapes actually assist in filtration because they bring more surface area of fibre per square metre of felt. This can increase the dust holding capacity of the felt and reduce emissions. Aramids are capable of working at much higher temperatures than many other fibres.
This has led to their use in a number of applications where the chemical properties are in fact overkill and not required. But, the complex shape reduces emissions in the filter. So they are often mixed with regular fibres and incorporated just into the top surface of the felt where the actual filtration takes place.
Polyimide fibres are gold in colour and have a limiting oxygen index (LOI) sufficiently high to prevent them burning in air. However, should a fire be burning in a filter (for example the dust), then the felt will be destroyed.
Various chemical enhancements are regularly applied to polyimide needlefelts, in particular high performance fluorocarbon systems which aid in filtration, especially when wet or sticky dusts are involved. Such systems can retard the process of hydrolysis, but it must be stressed that they don't stop it.
