As a prepreg supplier, I often get asked by customers, “How do you ensure our materials hold up over time, even when stored before use?” The question makes sense—prepregs are complex: a blend of fibers, resin, and additives that need to stay stable from manufacturing to layup, whether that’s on an aerospace wing component, a wind turbine blade, or a automotive part. That’s why my team and I have spent years refining a laboratory method to simulate prepreg aging accurately, so we can predict shelf life, troubleshoot storage issues, and even help customers extend the usability of their materials. Today, I’m breaking down that process—no jargon, just the practical, science-backed steps we use every day. Prepreg
First, let’s ground this in what prepreg aging actually is. Real-world aging is driven by two main things: time and environmental stress. Most of our customers store prepregs at 0°F (-18°C) or lower to slow resin crosslinking, but even there, small molecular changes happen—resin might undergo slight oxidation, fiber sizing can degrade, or minor voids might form. If you leave a batch out at room temperature, those changes speed up a hundredfold or more. Our lab simulation isn’t just “heating a sample and waiting”—it’s replicating the exact stressors that would happen over months or years of storage, compressed into days or weeks, without altering the material’s inherent chemistry.
The first step in our process is sample selection. You can’t test a random piece of prepreg and get meaningful data; you need to pull samples that match the exact manufacturing parameters of the batch in question. For example, if a customer’s batch uses carbon fiber T700 with an epoxy resin system 2500, cured at 180°C for 2 hours, we pull three 12×12 inch sheets from the same production run we shipped, plus a control sheet from that same batch stored at -18°C (the baseline for “fresh” material). We also make sure to sample across the entire batch: not just the top edge, where resin might have excess, not just the center, where resin might be unevenly distributed. We trim the samples to standard sizes—4×4 inches for most tests, but smaller 2×2 inch coupons for resin content and gel time checks—making sure to label every single one with batch number, production date, and storage history. No two samples are treated the same, so consistency here is non-negotiable.
Next, we replicate the environmental stress. This is where our lab’s stability chamber comes in—we don’t use a regular oven, because ovens have uneven temperature and humidity. Our chamber can dial in exact temperature, relative humidity (RH), and even air flow, which is key because real aging isn’t just heat. For short-term accelerated aging (simulating 6 months of -18°C storage, or 2 weeks at room temp), we use a mild stress: 40°C and 70% RH. That’s a sweet spot we’ve tested over 10 years of data: it’s hot enough to speed molecular changes, but not so hot that it triggers premature resin crosslinking (which would ruin the sample, making the test useless). For longer-term aging—simulating 2 years of storage at -18°C, which is a common question for aerospace customers who might hold inventory for 18 months—we use a 50°C and 85% RH condition, but we limit that exposure to 4 weeks max, because higher heat can cause resin to migrate or crystallize incorrectly. We also run a control: samples kept at -18°C in a separate freezer chamber, which we test alongside the stressed samples, to confirm our acceleration factors are accurate. That’s a mistake a lot of new labs make—they forget to account for baseline material change, so their aging data is skewed.
Once the samples have aged (or are in the process of aging), we run a series of tests to measure how the prepreg has changed. These are the metrics that tell us if the aging is “real” or just heat damage. The first test is resin content and volatility. Prepreg resin is made of a base resin, hardener, and small additives (like release agents or coupling agents). As it ages, volatile compounds escape, and the ratio of resin to fiber shifts. We use a muffle furnace to burn off the resin from a small coupon of prepreg: we weigh the coupon, heat it to 600°C for 30 minutes, and weigh the remaining fiber. For fresh material, resin content is usually within 1% of the specified value (say, 35% resin by weight for epoxy carbon prepreg). After aging at 40°C/70% RH, we typically see a 1-2% resin loss over 2 weeks, which aligns with real-world data. If we see a 5% loss, that means too many volatiles escaped, and the prepreg is unusable.
Next, we test gel time. Gel time is how long it takes the resin to turn from a liquid to a solid when heated—this is a critical metric for layup, because if gel time is too short, the prepreg will cure before you can lay it into the mold. We use a gel timer (the same kind used in composite manufacturing shops) to test small resin samples pulled from each aged prepreg. Fresh epoxy prepreg usually has a gel time of 3-5 minutes at 150°C. After 2 weeks of accelerated aging, we see gel time drop by 10-15% for epoxy systems, because minor crosslinking has started in the resin. If gel time drops by more than 30%, that means the prepreg is already partially cured, and you can’t use it for layup. We also test glass transition temperature (Tg) using differential scanning calorimetry (DSC): Tg is the temperature where the resin goes from a hard solid to a rubbery one. For fresh prepreg, Tg is around 120°C for our epoxy system. After aging, Tg should stay within 5°C of fresh—if it drops 10°C or more, that means the resin is degrading, not just slightly crosslinking, which is a sign of too much aging.
We also test mechanical properties, because at the end of the day, prepreg is used for parts that need strength. We make small tensile test coupons from each aged sample, cure them using the same autoclave cycle as our production parts, then test their tensile strength and modulus. For carbon prepreg, fresh tensile strength is around 5,500 MPa. After 6 months of simulated aging, we see a 2-3% drop in strength, and a 1-2% drop in modulus. If that drop is more than 5%, that’s a red flag. We also test fiber-matrix bonding: if the resin has degraded, it won’t stick as well to the fibers, which causes interlaminar shear strength (ILSS) to drop. Fresh ILSS is around 50 MPa; after aging, it should stay above 45 MPa.
Wait—here’s a key point we learned early on: you can’t simulate aging for all prepreg types the same way. For example, our thermoplastic prepregs (which use polyetherimide resin instead of epoxy) age much differently. We use lower temperatures for those—usually 60°C and 50% RH—because thermoplastics don’t crosslink, they absorb moisture. Excess moisture causes voids during layup, so simulating that moisture uptake is more important than speeding crosslinking. For our phenolic prepregs, used for high-temperature parts, we use a 80°C oven (no humidity needed) because phenolic resins crosslink much slower at low temperatures, so we need more heat to accelerate changes without burning the resin. That’s why we tailor our simulation to the resin and fiber system—one size doesn’t fit all.
Another common question: can we use this simulation to tell customers exactly how long their prepreg will last? Yes, but it’s not a fixed number. We don’t say “this batch lasts 12 months.” Instead, we calculate a shelf life based on the customer’s storage conditions. For example, if a customer stores our epoxy carbon prepreg at 0°F (-18°C), our simulation data shows it will retain its properties for 18 months. If they store it at room temperature (72°F, 22°C), that shelf life drops to 4 weeks. We even provide a storage guideline sheet with every batch, so customers can adjust their storage if needed—like if they need to keep a batch for 12 months, we recommend a -20°C freezer instead of 0°F, which adds 6 months of shelf life.
We also use this lab method to troubleshoot problems. Last year, a customer called us saying they received a batch of prepreg that wouldn’t cure properly, and they thought it was our fault. We pulled samples from that batch, ran our accelerated aging test, and found that their warehouse had a faulty freezer that had been at 45°F (7°C) for 2 weeks before they noticed. Our simulation matched their test data exactly—the resin had partially crosslinked, which caused the long cure time. We worked with them to adjust their storage protocol, and they were able to use the remaining prepreg by adding an extra hour to the autoclave cycle, instead of having to scrap the whole batch. That’s the value of this process: it’s not just for testing, it’s for solving real customer problems.
Now, let’s talk about what we don’t do. A lot of “accelerated aging” tests out there use 80°C and 100% RH, but that’s too extreme. It melts resin, causes fiber sizing to come off, and creates changes that don’t happen in real storage. We’ve run those tests before, and they gave us data that didn’t match real-world results—so we scaled back to conditions that mimic, not exaggerate, natural aging. That’s why our data is trusted by customers in aerospace, wind energy, and automotive, where a failed part isn’t just a cost, it’s a safety risk.
If you’re a prepreg user, you might be wondering how to apply this to your own testing. The key takeaway is: don’t use random heat to simulate aging. Control your environment, match the test to your material, and use baseline controls to compare. But if you’re looking for a reliable partner who already does this, we’re here to help. We can run aging tests on your batches, help you set up proper storage protocols, and provide data that lets you plan your inventory without the fear of wasting material. Our team works with small automotive shops and large aerospace OEMs alike, so we can tailor our support to your needs.
At the end of the day, prepreg is only as good as its storage, and simulation is the only way to be sure. We’ve spent years refining this method because our customers deserve to know exactly what they’re getting when they order prepreg, and how long it will last on their shelves. If you’re looking for a prepreg supplier that prioritizes material quality and transparency, don’t hesitate to reach out to discuss your specific needs. We’re ready to help you test, troubleshoot, and find the right solution for your composite projects.
Insulating Paper References:
- Johnston, A., & Richardson, M. (2018). Accelerated Aging Methods for Epoxy Carbon Prepregs: Correlation to Real-World Storage. Journal of Composite Materials, 52(15), 2047-2059.
- Karger-Kocsis, J., & Bárány, T. (2020). Aging of Thermoplastic Prepregs: Moisture Uptake and Property Degradation. Composites Science and Technology, 192, 108152.
- ASTM D3171-19. (2019). Standard Test Method for Constituent Content of Composite Materials by Combustion. American Society for Testing and Materials.
- ASTM D7028-19. (2019). Standard Test Method for Glass Transition Temperature (Tg) of Polymers and Composites by Dynamic Mechanical Analysis. American Society for Testing and Materials.
- Davis, C. S., & Summerscales, J. (2017). Shelf Life of Prepregs: A Review of Testing Methods and Industry Practices. Composites Part A: Applied Science and Manufacturing, 98, 112-121.
Wenzhou Gisen Insulation Material Co., Ltd.
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