Karwon Composites supplies prepreg with the resin system matched to the cure equipment you actually have.
This question comes up more than any other technical query we receive, and it usually arrives as a suspicion that a datasheet contains an error.
A buyer reads a prepreg datasheet. Cure schedule: 80 degrees for three hours. Glass transition temperature: 91 degrees. They conclude something is wrong, because how can a material end up with a transition temperature above the temperature it was cured at?
Nothing is wrong. Here is what is actually happening.
What Tg is
The glass transition temperature is the point at which a cured thermoset changes from a rigid, glassy solid to a softer, rubbery state. It is not a melting point. Nothing liquefies. But above Tg the resin's stiffness falls sharply, and with it the mechanical performance of any laminate relying on that resin to transfer load between fibres.
For a structural composite, Tg is effectively the ceiling on service temperature. Fibre properties are largely unaffected by modest heat. The resin is the limiting component, and Tg is where it stops doing its job properly.
Why Tg can exceed cure temperature
Cure is a chemical reaction: epoxy groups reacting with hardener to build a crosslinked network. The more complete that network, the higher the Tg.
The reaction rate depends on temperature, but so does the mobility of the molecules taking part. As the network builds, the material stiffens, and eventually molecular mobility drops so far that the reaction effectively stops even though unreacted groups remain. This is called vitrification. The resin has, in effect, cured itself to a standstill.
The temperature at which that happens is a property of the specific resin formulation. Low temperature curing systems are formulated so that vitrification occurs above their cure temperature. They are designed to reach a Tg higher than the temperature you cured them at, because that is precisely what makes them useful.
So the 91 degrees is not a target you have to reach. It is the result of the 80 degree cure. It is an output, not an input.
The rule of thumb, and its limits
A rough guide: a well formulated system will typically produce a Tg somewhere between ten and twenty degrees above its stated cure temperature.
Two important qualifications.
It only applies to the stated schedule. If the datasheet says three hours at 80 and you run two hours at 80, you will not reach the stated Tg. The dwell exists for a reason. Cure is not a threshold you cross, it is a reaction that needs time.
It does not extend indefinitely. You cannot cure a system at 80 and expect a Tg of 140 by leaving it in the oven for a week. Once vitrified, the reaction is essentially arrested at that temperature.
What a post cure actually does
If you need a higher Tg from the same material, the route is a post cure at a higher temperature.
Raising the temperature above the vitrification point restores molecular mobility, the remaining unreacted groups can react, the network becomes more complete, and Tg rises. It is why a two stage schedule appears on many datasheets: an initial cure at a manageable temperature to gel and consolidate the part, then a higher temperature free standing post cure to develop final properties.
The practical constraint is the tooling. A post cure typically happens out of the mould, which means the part has to be dimensionally stable enough to survive it without support at a temperature above its current Tg. Getting this wrong distorts parts.
Three ways of measuring Tg, three different numbers
This is where genuine confusion enters, because a single material can honestly report three different Tg values on the same page.
DSC, differential scanning calorimetry. Measures the change in heat capacity as the material passes through the transition. Gives a single characteristic number, typically in the middle of the range.
DMA storage modulus onset. Dynamic mechanical analysis, taking the point where the storage modulus begins to drop. This is the most conservative figure and the one that best represents where the material begins losing stiffness.
DMA tan delta peak. Also DMA, but taking the peak of the damping curve. This produces the highest number, frequently fifteen to twenty five degrees above the storage modulus onset for the same material and the same test.
A datasheet showing DSC 91, DMA onset 90 and tan delta peak 110 is not contradicting itself. It is reporting three measurements of the same transition by three different methods.
Which number to design to
Use the DMA storage modulus onset. It is the point at which stiffness begins to fall, which is the property your structure depends on.
Then apply margin. Common practice is to keep maximum service temperature at least twenty degrees below the onset Tg for a dry laminate, and further below for wet or hot wet conditions, because absorbed moisture depresses Tg. In humid coastal environments this is not a theoretical concern.
The comparison trap
When you are comparing two suppliers, make sure both figures came from the same method.
A mill quoting tan delta peak against a mill quoting DMA onset will appear to offer the better material by twenty degrees while potentially offering the same material or worse. This is not always deliberate. Datasheets get abbreviated as they pass through distributors, and the method note is often the first thing to fall off.
Ask the question directly: which method, and at what heating rate? A supplier who can answer understands what they are selling. A supplier who cannot is reading a number off a sheet.
Putting it to work
A short checklist before you commit to a resin system:
- What is my maximum service temperature, in the worst realistic condition including solar gain and enclosed spaces?
- Is that condition wet or dry?
- What Tg do I need, allowing at least twenty degrees of margin?
- What cure temperature can my equipment actually reach and hold?
- Does a system exist that reaches the Tg I need from the cure I can run?
- If not, can I post cure, and will the part hold its shape unsupported while I do?
If the answer to five is no and to six is no, you need a different resin system rather than a longer schedule.
Karwon Composites supplies carbon, glass and aramid prepreg with the resin system matched to the cure equipment you actually have. Tell us the service temperature and the oven, and we will work backward to a system that fits both.