Types of Intumescent Seals: Graphite, Silicate & MAP
Intumescent seals may look similar, but the materials inside them can react very differently to heat. This guide compares intercalated graphite, hydrated sodium silicate and mono ammonium phosphate, and explains why the type of intumescent matters when specifying or replacing a fire door seal.
Are All Intumescent Seals the Same? A Guide to the Different Types
At a glance, two intumescent fire door seals can look almost identical. They may even have the same dimensions. However, what happens when they are exposed to heat can be very different.
The material inside an intumescent seal determines when it starts to react, how it expands and how much pressure it creates. Those differences matter because the seal is selected to behave a certain way as part of the tested fire door construction.
So if you are specifying a new seal or replacing an existing one, knowing that you need a 15mm or 20mm intumescent strip is only part of the answer. You also need to know which type of intumescent material is required.
What are the different types of intumescent seal?
The term "type of intumescent seal" can mean several things. It is sometimes used to distinguish a plain fire seal from a combined fire and smoke seal, or to describe different sizes and profiles. There is another important distinction: the intumescent material itself.
Three materials commonly used in fire door sealing are Mono Ammonium Phosphate (MAP), Hydrated Sodium Silicate and Intercalated Graphite. All expand when heated, but they react differently.
Mono Ammonium Phosphate (MAP) intumescent
Mono Ammonium Phosphate activates at around 180°C and expands with virtually no pressure. That makes it an effective gap filler where components may move significantly during a fire. Because it does not generate the same expansion pressure as some other intumescents, MAP can be useful where the material's primary job is to fill developing gaps rather than restrain adjacent components.
Hydrated Sodium Silicate intumescent
Hydrated Sodium Silicate activates at a lower temperature, typically around 110°C to 120°C. It expands predominantly in one direction, forming a rigid foam and generating substantial pressure as it does so.
That pressure can help restrain movement and hold adjacent components together as the fire develops. This behaviour differs from a low-pressure gap-filling intumescent, even if the two products look similar before exposure to heat.
Intercalated Graphite intumescent
Intercalated Graphite combines multi-directional expansion with high pressure generation. Norseal's technical information gives an activation temperature of around 200°C for the material used in its applications, although activation temperatures can vary between formulations.
Graphite is also non-hygroscopic, so it does not absorb moisture from the atmosphere. This can be useful where the intumescent material would otherwise need protection from atmospheric moisture.
How do the main types of intumescent compare?
|
*Activation temperatures vary between formulations. The figures above reflect the technical information supplied for the materials discussed in this article.
Why does the activation temperature of an intumescent matter?
An intumescent needs heat to begin expanding, but different materials start that process at different temperatures. Hydrated Sodium Silicate can begin reacting considerably earlier than MAP or some graphite formulations.
Earlier activation is not automatically better, and a higher activation temperature does not make a material inferior. The important point is that the seal behaves as intended within the tested door construction. Changing to a material that reacts earlier or later can change what happens around the door as temperatures rise.
What is a pressure-generating intumescent?
Some intumescent materials do more than fill the space created by fire and heat. As they expand, they generate pressure against the surrounding components.
Hydrated Sodium Silicate and Intercalated Graphite can both generate significant pressure, although they expand differently. Mono Ammonium Phosphate produces virtually no pressure and is better described as a gap-filling material.
Neither behaviour is inherently better. A doorset may need the pressure generated by one material in a particular location. In contrast, another application may need an intumescent that can fill a changing gap without exerting the same force.
What is Interdens?
Interdens is a term used in the fire door and architectural ironmongery market for intumescent sheet material that protects hardware and other vulnerable areas in fire-resisting doorsets. It is often supplied as thin sheet or pre-cut protection for items such as locks, hinges and other ironmongery.
It is not another generic category like graphite, Hydrated Sodium Silicate, and MAP. If a door specification or test report specifies a particular Interdens product you must follow that evidence rather than substituting a different material because it appears to perform the same job.
What is Palusol?
Palusol is another name commonly encountered in fire door specifications. It is a proprietary hydrated sodium silicate-based intumescent material, not a separate generic family of intumescents.
That distinction matters when reading older test reports, door schedules and specifications. A trade or product name can tell you what material was originally used, but it does not mean any product with similar dimensions can automatically take its place.
Can you substitute one type of intumescent for another?
Not simply because the replacement is the same size. A 15 x 4mm graphite seal and a 15 x 4mm sodium silicate seal may fit the same groove, but their behaviour under fire conditions can differ significantly.
Activation temperature, direction of expansion, and the pressure produced can all affect how a fire door performs. For that reason, the starting point for substitution should be the test evidence, certification or other supporting information for the door, not the dimensions printed on the seal.
This is particularly important when replacing old or damaged intumescent strips. If you can't identify the original material, get technical advice from Norseal rather than assuming a modern alternative is interchangeable.
How do I know which type of intumescent seal I need?
For a new fire door or doorset, look at the manufacturer's specification and supporting fire test or certification information. This should establish the required intumescent material, size, position, and configuration.
With an existing door, labels, plugs, certification information, fire door schedules and original installation records may help identify what was fitted. The colour or appearance of an old strip is not a reliable way to identify the intumescent material.
If you are still unsure, Norseal's technical team can help you work through the available evidence and identify the required type of intumescent or fire door sealing product.
FAQs about types of intumescent seals
Are all intumescent seals the same?
No. Intumescent materials can activate at different temperatures and expand in different directions and with different amounts of pressure. The correct material should match the fire door evidence.
What are the main types of intumescent material?
Common fire door intumescents include Mono Ammonium Phosphate (MAP), Hydrated Sodium Silicate and Intercalated Graphite. Each behaves differently when exposed to heat.
What is graphite intumescent?
Intercalated Graphite is an intumescent material that expands in several directions and can generate high pressure. It is also non-hygroscopic, so it does not absorb atmospheric moisture.
What is sodium silicate intumescent?
Hydrated Sodium Silicate is a pressure-generating intumescent that activates at around 110°C to 120°C and expands predominantly in one direction to form a rigid foam.
What is MAP intumescent?
Mono Ammonium Phosphate, or MAP, activates at around 180°C and expands with virtually no pressure. It is useful for filling gaps and accommodating movement during fire conditions.
Which intumescent material activates at the lowest temperature?
Of the three materials covered here, Hydrated Sodium Silicate activates at the lowest temperature, at around 110°C to 120°C. Exact activation temperatures can vary by formulation.
Can I replace sodium silicate intumescent with graphite?
Not based on size alone. The materials expand differently and can generate different pressures, so any change must be made with the support of the fire door test evidence.
Is Palusol the same as sodium silicate?
Palusol is a proprietary intumescent material based on hydrated sodium silicate. It is a product/material name rather than a separate generic type of intumescent.
Is Interdens a type of intumescent?
Interdens is a name used for intumescent sheet material, commonly associated with protection around fire door ironmongery. Check the specific product and door evidence rather than treating the name as a generic material category.
