High-temperature composite materials: Tg, cure temperature and service temperature explained
If a prepreg has a glass transition temperature of 200°C, does that mean the finished component can operate continuously at 200°C? Not necessarily - and this is where a lot of confusion around high-temperature composite materials begins.
When comparing prepregs, three temperature figures often appear:
Cure temperature
Glass transition temperature (Tg)
Service temperature
They are related but each tells you something different about how the material is processed and how the finished composite may perform.
Understanding the difference helps you compare materials properly and avoid choosing a system based on one impressive-looking number.
What is cure temperature?
Cure temperature is the temperature used to turn the resin within a prepreg into a solid, crosslinked material.
It is better to think of curing as a time-temperature process rather than a single number. A prepreg may cure at a lower temperature over several hours or achieve the required level of cure more quickly at a higher temperature.
Some systems also use a separate post-cure to develop their final thermal properties.
This matters during material selection because cure temperature affects how practical a prepreg is to manufacture. Your tooling, oven or autoclave capability and preferred production cycle all need to suit the cure schedule.
Is cure temperature the same as service temperature?
No - cure temperature tells you how the material is processed, while service temperature relates to the temperatures the finished component is designed to handle.
A prepreg cured at 120°C may be capable of operating above 120°C once fully cured and post-cured. Equally, a material cured at 180°C should not automatically be assumed to have a service temperature of 180°C.
The two figures answer different questions:
Cure temperature: Can we manufacture the component using this material?
Service temperature: Can the finished component perform at the temperatures it will experience in use?
Between the two sits another important figure - Tg.
What is glass transition temperature or 'Tg'?
Glass transition temperature, usually shortened to Tg, describes the temperature region where the behaviour of the polymer matrix begins to change.
Below this region, a cured thermoset resin tends to behave in a relatively stiff or 'glassy' way. As the temperature rises through the glass transition region, the resin becomes more mobile and properties such as stiffness can begin to fall.
Tg is not a melting point. A cured epoxy does not suddenly become liquid at its Tg - it moves through a transition where its behaviour changes.
This is why Tg is useful when comparing high-temperature materials but it should not be treated as a simple maximum operating temperature.
Is Tg the same as service temperature?
No - and this is probably the most important point in this article.
A prepreg with a Tg of 220°C does not automatically mean a structural component made from it can operate continuously at 220°C.
As the material approaches its Tg, matrix-related properties can begin to change. The practical service temperature therefore depends on what the finished component needs to do at temperature.
A lightly loaded cover and a highly loaded structural component may require very different temperature margins even if both use the same resin system.
With this in mind, rather than asking only:
“What is the Tg?”
a more useful question is:
“What properties does the material retain at the temperature my component will actually experience?”
This is where technical data at elevated temperature becomes particularly useful.
Why can a prepreg have more than one Tg value?
Technical datasheets may show more than one Tg because the glass transition can be measured in different ways.
One common method is Dynamic Mechanical Analysis or DMA, which measures how a material's mechanical behaviour changes as the temperature rises.
You may see figures such as:
Tg onset - where a noticeable change in stiffness begins.
Tg peak - another point within the transition which usually appears at a higher temperature.
This means a material could have a Tg onset of 205°C and a Tg peak of 225°C without either figure being wrong.
The important point when comparing prepregs is to check that you are comparing like with like.
A 220°C Tg from one datasheet should not automatically be considered better than a 210°C figure from another unless you understand how both values were measured.
Why can cure temperature be much lower than Tg?
This is often the point that causes the most confusion. How can a prepreg cure at 60°C and later achieve a Tg above 200°C? The answer is usually post-curing.
The first cure turns the prepreg into a solid laminate. A later controlled heating cycle can allow further reactions within the resin, increasing the degree of cure and raising the final Tg.
SHD's tooling range provides good examples of this. Some systems can be initially cured below 70°C before achieving Tg values above 200°C following the recommended post-cure.
This can be particularly useful when manufacturing large composite tools. A relatively low initial cure reduces the thermal demands on the master or pattern while the later post-cure develops the temperature capability needed for production use.
So a low cure temperature does not necessarily mean low-temperature performance.
What does post-curing mean when comparing materials?
Post-cure requirements are worth checking whenever you compare high-temperature prepregs because the highest Tg shown on a datasheet may only be reached after a specific post-cure.
Two materials could therefore show similar Tg figures but require very different processing routes to achieve them.
One might reach the required performance after a straightforward cure, while another needs several additional hours at a much higher temperature.
That difference can affect tooling, energy use, production time and manufacturing practicality.
A useful rule is: 'Always check the cure schedule behind the published Tg'.
The temperature figure only tells part of the story.
What types of high-temperature prepreg are available?
There are several resin families used for higher-temperature composite applications and each offers a different balance of thermal performance, processing and cost.
High-temperature epoxy prepregs
Epoxy systems are widely used because they can combine good structural properties with familiar processing and relatively straightforward handling.
Advanced epoxy prepregs can also achieve much higher temperature capability than standard structural systems, making them suitable for many elevated-temperature components and tools.
For applications that sit comfortably within their operating range, a high-temperature epoxy may provide the most practical balance between performance and processing.
BMI prepregs
Bismaleimide or BMI systems are often considered when temperature requirements move beyond the practical limits of epoxy.
They can offer considerably higher thermal capability, making them useful for demanding component and tooling applications.
The trade-off is that processing requirements can be more demanding, so the extra temperature capability needs to justify the manufacturing complexity.
Cyanate ester prepregs
Cyanate ester systems can combine high-temperature performance with other specialist characteristics such as low outgassing and useful dielectric properties.
They are often chosen where thermal performance forms part of a wider technical requirement rather than being the only consideration.
The key point is that these materials should not be viewed as a simple ladder from epoxy to BMI to cyanate ester. The best option is the material that meets the temperature requirement while also fitting the manufacturing process and wider application.
How do you choose the right high-temperature composite material?
Start with the application rather than the biggest Tg figure on the datasheet.
First, establish the temperature the finished component needs to handle. Then consider what the component must do while it is at that temperature.
A useful material-selection discussion will normally consider:
Expected operating temperature
Required mechanical performance at temperature
Available cure temperature
Whether post-curing is practical
Preferred manufacturing process
Tooling limitations
Other environmental or performance requirements
These factors quickly narrow the choice.
For example, if the required service temperature can be achieved comfortably with a high-temperature epoxy, moving to a more specialist resin may add processing complexity without delivering a useful benefit.
If the application exceeds the practical temperature range of epoxy, BMI or cyanate ester may become more relevant.
What should you look for on a high-temperature prepreg datasheet?
When comparing materials, avoid focusing on one temperature figure in isolation.
Look at the relationship between:
Cure temperature and time
Post-cure requirements
Tg and the method used to measure it
Recommended or demonstrated service temperature
Mechanical properties at elevated temperature
Processing route
The aim is to understand both what the material can do and what you need to do to achieve that performance.
A prepreg with an extremely high Tg may look attractive on paper but a material with slightly lower thermal capability and a much simpler processing route may be a better fit for the application.
How do cure temperature, Tg and service temperature fit together?
The easiest way to remember the difference is to give each figure a specific job:
Cure temperature tells you how the material needs to be processed
Tg tells you where the behaviour of the cured resin begins to change significantly
Service temperature tells you whether the finished composite can perform in its intended operating environment
None of these figures should be used entirely on its own.
For high-temperature composite applications, the best material is usually the one that delivers the required performance with a cure and processing route that makes sense for the component.
How can SHD help with high-temperature material selection?
SHD develops and manufactures tooling and component prepregs across a range of processing temperatures and resin systems, including high-temperature epoxy, BMI and cyanate ester materials.
Our technical team can help you compare options based on the operating temperature, required performance and manufacturing process rather than relying on headline Tg figures alone.
If you already know the temperatures your component needs to handle, share them with us. If the specification is still taking shape, we can help identify the most useful material data to compare and narrow down the available options.
Which high-temperature prepreg is right for your application?
Choosing a high-temperature composite material becomes much easier once cure temperature, Tg and service temperature are treated as three separate pieces of the same puzzle.
Start with what the finished component needs to do, then work backwards to the resin system and processing route that can deliver it.
Discuss your high-temperature application with our team today or explore our prepreg materials here.
