The fillers used in spiral wound gaskets are diverse and are chosen depending on two fundamental parameters:
• Operating temperature
• Chemical Resistance to Chemicals in contact.
The graphite is used for its various properties given its particular crystalline structure. One of the limits of its used is its porosity, therefore not suitable for vacuum applications where PTFE is used more, thanks to its low permeability. For temperatures greater than 500°C there is a general tendency to use ceramic materials, which are usually in fibre. These materials are used only for very high temperatures.
Graphite has a very high thermal conductivity and therefore it is ideal in applications of heat exchange. It is used mainly for piping, valves, pumps etc. The only limit is the operating temperature, which cannot exceed 400° C for the occurrence of the oxidation phenomenon, even if special process conditions permit exceeding the temperature.
Graphite is nothing more than carbon in elementary form, obtained by cracking reactions to hydrocarbons. The crystalline structure is lamellar and due to the effect of carbon hybridization which sees that the arrangement of atoms is carried out on an equal level. The different plants are kept together by secondary bonds of lower intensity. The application of small loads determines the slip of the different layers giving the material a low consistency and a high spathic nature. These structural features give graphite a self-lubricating property, of good compressibility which combined with high chemical inertia make it suitable in many applications.
Polytetrafluoroethylene is a polymer with high specific weight characterized by the bond Fluoro Carbon.
This type of bond is particularly stable, because fluorine, being at the extreme left of periodic table, has a high electro-negativity and therefore the electronic cloud is very compact and predominantly placed on the fluorine atom.
For these characteristics of high stability bond and compactness, the resulting polymer shows a number of properties that can we summarize below:
• High thermal stability compared to other polymers on the market.
• Resistance to chemical solvents
• Very low gas permeability
• Low friction coefficient
• High anti-adhesiveness.
• Excellent dielectric resistance
It takes advantage of its high chemical inertia. The use of PTFE is compatible with the majority of known chemicals except with the salts of alkali cast metals and precursors of fluorine. The use temperature can reach up to 260° C, very high compared to common polymers. Given its low permeability it is used as a filler for gaskets used in vacuum services.
These are chlorite minerals additized with graphite (mica-graphite). These materials are applied to standard-service applications.
Formed by aluminium silicate fibres. These materials have a low sealant effect compared to other fillers, and therefore have an excellent temperature stability up to 2300° F (1250° C). They are resistant to attack by many corrosive agents (hydrofluoric acid and phosphoric acid), including alkali. They are recommended when you can not use graphite filler or PTFE.