Hey there, if you’ve ever worked with carbon fabric for anything from aerospace parts to hobbyist race car builds, you’ve probably wondered why your stored rolls feel damp out of nowhere, or why a part you just bonded with epoxy didn’t cure right. That’s not you being clumsy—that’s carbon fabric’s moisture absorption at play, and as a carbon fabric supplier who’s talked through this with hundreds of customers over the years, I’m here to break it down like I do when I’m on the phone with someone panicking mid-project. Carbon Fabric
First, let’s cut the jargon. Carbon fabric isn’t just plain carbon fibers woven into a cloth—each fiber is basically a long string of carbon atoms, right? But those atoms aren’t perfectly smooth. They have tiny, microscopic gaps, and the surface of every fiber has functional groups (think little chemical “sticky spots”) that love water. Unlike metal, which repels water, or some plastics that don’t hold it, carbon’s just the opposite. That’s the core of its moisture absorption, and it’s way more common than you might think.
Now, how much water are we actually talking about? I get questions all the time: “Is this a big problem, or just a little nuisance?” Let’s throw out the basic numbers I share with new clients: standard carbon fabric (the kind you use for most general manufacturing, like plain-twill or unidirectional) absorbs between 0.2% and 0.5% of its dry weight in water. Wait, that sounds small, but let’s do the math: if you have a 10kg roll of carbon fabric, that’s 20 to 50 grams of water. That’s roughly 1 to 2 ounces, and when you’re mixing epoxy that needs exact ratios (a lot of epoxies cure poorly if there’s even a tiny amount of water disrupting the chemical reaction), that tiny bit can turn into a botched part fast. I’ve seen a customer toss a $2k pre-preg layup because they stored their fabric in a damp garage and missed that 0.3% moisture—they thought it was their epoxy, turns out it was just the fabric holding water.
Here’s the thing about how it absorbs too: it’s not like a sponge soaking up a big puddle. Carbon fabric absorbs moisture gradually, based on humidity and temperature. If you store your fabric in a space with 60%+ relative humidity (that’s like most basements, or a garage in the summer) for more than a week, it’ll start picking up moisture. Warmer air holds more water, so a 70°F room at 70% humidity will get your fabric wet faster than a 50°F room at the same humidity. Even a roll that’s sealed in plastic can get moisture if the plastic has a tiny pinhole, or if you open it in a humid room and don’t re-seal it right away. That’s another mistake I see all the time: people think “sealed is sealed” but if you’ve ever left a bag of chips open in a humid kitchen, you know how that goes. Same idea here.
Now, is all carbon fabric the same when it comes to moisture absorption? Nope, that’s why it matters where you get your fabric from (hint: that’s where I come in). There are two main types: PAN-based carbon fabric (the most common, made from polyacrylonitrile fibers) and pitch-based (made from petroleum pitch). PAN-based fabric usually has more of those surface functional groups, so it absorbs more moisture—like 0.3 to 0.5%. Pitch-based is a little smoother, so it’s around 0.2 to 0.3%, but it’s way more expensive, so most hobbyists and general manufacturers don’t bother with it. Then there’s surface treatment: some fabric gets a sizing (a thin coating to protect fibers during weaving) that’s hydrophobic. Wait, that’s a game-changer. If you buy fabric with a proprietary hydrophobic sizing (I work with a mill that adds a thin silicon-based sizing that doesn’t mess with bonding), that moisture absorption drops to like 0.1% max. That’s a huge difference. I always tell customers to ask their supplier about sizing—if they say “it’s standard sizing,” that might not be enough for outdoor or aerospace parts. If they can say it’s a moisture-resistant sizing, that’s the stuff you want.
So what’s the big deal if it does absorb moisture? Let’s cover the common issues, because that’s why every customer needs to care. First, epoxy curing: like I mentioned, water disrupts epoxy’s cross-linking reaction. That makes the part weaker, more brittle, and way more likely to crack. For a race car part, that’s dangerous. For a custom kayak hull, that’s a flaw you’ll feel after the first use. Second, delamination: if you’re using multiple layers of fabric, trapped moisture between layers will turn into voids when it evaporates during curing. Voids are empty spaces that weaken the whole structure—something as small as a void can cut a part’s strength by 10-15% easily. Third, weight gain: if you’re making a part where weight is critical (drones, bikes, robotics), that extra 0.5% water might not sound like much, but on a 1kg drone frame, that’s 5 grams—enough to make it less efficient, drain the battery faster, or even throw off the balance. I once had a drone builder come to me because his quad kept crashing mid-flight; he’d stored his fabric in his workshop, which hit 80% humidity in the summer, and the extra moisture made the frame heavier and unbalanced.
Now, can you reverse it? Yes, but not just leaving it out to dry. I see people do that all the time—they throw their damp fabric on a workbench and leave it for a day, thinking it’ll be fine. But carbon fabric’s moisture is trapped in those microscopic fiber gaps, so surface drying takes a long time. The right way to dry it is low-temperature heat: 100-120°F (38-49°C) for 2-4 hours. That evaporates the trapped moisture without damaging the fibers or the sizing. I always tell customers not to go higher than 140°F—too hot can break down the fiber’s structure, or melt the sizing, which ruins the fabric’s bonding properties. If you’re using pre-preg fabric (carbon fabric already infused with epoxy), you can’t really dry it once it’s damp, because pre-preg has a shelf life, and moisture will make it cure prematurely or go bad before you use it. That’s why pre-preg is almost always shipped and stored in vacuum-sealed bags in a cool place—no exceptions.
Wait, let’s address a common myth I hear: “If carbon fabric is so good at holding moisture, why don’t they make waterproof fabric out of it?” Oh, that’s a great question. Carbon fibers themselves are hydrophilic (water-loving), but when you weave them into fabric, you can add a coating, like Teflon, that makes the whole thing waterproof. But that coating is on the outside of the fabric, not the fibers themselves. The fibers still have those sticky spots, so if you cut the fabric and expose the raw fibers, those spots will still absorb moisture. That’s why waterproof carbon fabric isn’t the same as “non-moisture-absorbing” carbon fabric—totally different things. I explain that to people all the time, because they mix the two up.
So, what can you do to prevent moisture absorption in the first place? This is stuff I share with every customer, because it’s way easier than fixing a damp roll. First, store your fabric in a cool, dry place—aim for under 50% relative humidity, and between 60-70°F. If your workshop is damp, get a dehumidifier. Second, re-seal rolls immediately after opening: use airtight plastic bags with desiccant packets (the little silica gel packs you get with electronics, those work perfect). I always tell customers not to throw those away—save them for your fabric storage. Third, don’t open fabric until you’re ready to use it: the longer it’s exposed to air, the more moisture it picks up. Fourth, if you’re using fabric for high-stress parts (aerospace, automotive, medical), ask your supplier for moisture-tested fabric. I do regular moisture tests on my stock—we test 5 random rolls every month to make sure absorption is under 0.3%, so my customers don’t get stuck with damp fabric mid-project.
Let me wrap this up with a quick example to make it real. Last quarter, a guy who builds custom carbon fiber bike frames came to me because he was having consistent delamination issues. He’d been buying fabric from a big online retailer, storing it in his garage, and his tests showed his parts were only hitting 80% of the required strength. When we tested his leftover fabric, it had 0.6% moisture—way above the standard. We switched him to our fabric with the hydrophobic sizing, and added a dehumidifier to his storage, and his delamination problems stopped. He told me he saved thousands in wasted materials and missed deadlines because he understood the moisture absorption issue. That’s the stuff that makes me happy as a supplier—not just selling fabric, but helping people not make avoidable mistakes.
If you’re working on a project, whether it’s a small hobby build or a large industrial part, and you have questions about carbon fabric moisture, sizing, storage, or need a reliable supply of tested, low-moisture carbon fabric, feel free to reach out to talk through your needs. I’m here to help, not just make a sale.
Sports Equipment References
- Averill, B. A., & Eldred, P. A. (2016). Materials Science and Engineering: Principles and Applications. Cengage Learning.
- Johnson, R. T. (2020). Carbon Fiber Composites: Properties, Processing, and Performance. CRC Press.
- Smith, L. K. (2018). “Moisture Absorption in PAN-Based Carbon Fabrics for Aerospace Applications.” Journal of Composite Materials, 52(12), 1645-1658.
Jiaxing Rongjin Intelligent Technology Co., Ltd.
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