Hey there, fellow yarn heads! If you’ve ever shopped for technical textiles, you’ve probably stumbled across air covered yarn (ACY) and gone, “Wait, what’s the actual tensile strength of this stuff? I keep hearing it’s ‘strong’—but how strong, really?” As someone who’s been in the ACY game for over 10 years now (I still get excited when a new sample roll comes in), I’ve lost count of how many designers, apparel makers, and even other yarn suppliers have hit me up with that exact question. Today, I’m breaking it down no boring textbook jargon, just real-talk that’ll help you figure out if this is the yarn you need for your next project. Air Covered Yarn
First off, let’s get one thing straight: ACY isn’t a single, one-size-fits-all yarn. It’s a core yarn wrapped tightly in a sheath of another fiber, right? You feed two yarns into a machine, blast ‘em with hot air, twist ‘em together so the sheath fibers wrap around the core like a snug blanket. That’s why its tensile strength isn’t a fixed number—it’s dependent on two big things: what core fiber you use, and what sheath fiber you pair with it. Think of it like making a sandwich: the bread (sheath) might be soft white or whole grain, but the filling (core) is the main thing holding it together when you pick it up.
Let’s start with the basics of tensile strength, just so we’re on the same page. Tensile strength is how much force a yarn can take before it snaps—usually measured in cN/dtex (centinewtons per decitex, don’t stress the units too much, it’s just the industry standard). For reference, cotton is usually around 2-5 cN/dtex, polyester is like 4-7 cN/dtex, so keep those numbers in the back of your head. Now, ACY comes in all sorts of core-sheath pairs, so I’ll walk you through the most common ones I supply, and their actual tensile strength numbers we test every single day in our lab.
First up, polyester-core ACY. This is the workhorse of ACY, hands down. Most of the people ordering from me for activewear, technical bags, or even outdoor gear go with polyester core because it’s cheap, durable, and doesn’t stretch like a rubber band. If your core is 100% polyester, and your sheath is, say, nylon or another polyester? Wait till you hear this. Our standard 70/28 polyester-core ACY (that’s 70 denier core, 28 denier sheath for anyone who uses denier) tests at 5.2 to 6.8 cN/dtex. That’s right in line with solid polyester, and since the sheath wraps around the core, it doesn’t lose much strength at all—if anything, the twisting makes it a little more resistant to abrasion, so it actually lasts longer than a solid yarn of the same strength. The only thing to watch here is if the core is spun polyester vs. filament. Spun polyester core ACY is a bit lower, like 4.5-5.5 cN/dtex, because spun yarn has tiny short fibers that don’t stick together as well. But filament core is the heavy hitter—perfect for anything that needs to hold a load.
Next, nylon-core ACY. Nylon is way stretchier than polyester, right? Wait, but tensile strength is about breaking, not stretching. Nylon has a way higher tensile strength before it snaps—like solid filament nylon is 6-8.5 cN/dtex. So a nylon-core ACY is even stronger than polyester-core. Our 40/20 nylon-core ACY clocks in at 6.1 to 7.9 cN/dtex. This is the one I recommend if you’re making something that needs to hold weight, like backpack straps or heavy-duty luggage handles. The catch here? Nylon is more prone to UV degradation than polyester, so if you’re using it for outdoor stuff like awning ties, you’ll want to treat it or pair it with a UV-stabilized sheath. But for indoor projects where strength is king, this is your go-to.
Then there’s the hybrid stuff, like spandex-core ACY. This is the stuff people use for leggings, shapewear, or stretchy waistbands. Spandex alone has a super low tensile strength when you’re not stretching it—like 0.5-1 cN/dtex, because it’s all stretch. But when you wrap it in polyester or nylon sheath, you get the best of both worlds: stretch and tensile strength. Our 20/40 spandex-core ACY (20 denier spandex core, 40 denier polyester sheath) has a tensile strength of 3.2 to 4.8 cN/dtex. That might sound low compared to the others, but compare it to solid spandex, which would snap at that same stretch before holding any weight. This is perfect for leggings that need to not tear when you bend over, or waistbands that don’t roll.
Wait, but what about cotton-core ACY? I get asked about this a lot for casual apparel, like t-shirts or hoodies. Cotton is a natural fiber, so its tensile strength is lower, around 2-5 cN/dtex, so the core limits the whole yarn. Our 50/50 cotton-poly core ACY tests at 2.8 to 4.1 cN/dtex. It’s softer than all-poly ACY, feels more like cotton, but still has a lot more strength than solid cotton, which is prone to breaking when you stretch it. I only suggest this for casual wear, not for anything heavy-duty, just so you don’t get surprised when it tears.
Now, here’s the part most suppliers won’t tell you: tensile strength isn’t the only thing that matters for ACY. The way the yarn is made changes it too. If you twist the core and sheath too tight, you might get a higher tensile strength, but it’ll be stiff, not flexible. If you twist too loose, the sheath fibers slide, and the strength drops, plus it pills like crazy. Our team tests every batch of ACY we ship with a standard tensile tester—we pull three samples at 200mm/min speed, right at room temp and humidity (because humidity makes natural fibers weaker, by the way—if you test in a super dry lab, numbers will be a little higher). That’s why you can’t just Google “ACY tensile strength” and get a number; you have to account for core, sheath, denier, twist level, and even where you test it.
I’ve had way too many horror stories from customers who bought cheap ACY from some random overseas supplier, thought it was 6 cN/dtex, only to test it and get 3.5. Why? Because they cut corners on the core—they used a lower grade polyester, or even mixed in some rayon core to save money. Tensile strength is the first thing cheap suppliers skimp on. So when you’re ordering, don’t just ask for “strong ACY”—be specific: what core do you need, what tensile strength range works for your project, and ask for a COA (Certificate of Analysis) so you can see the actual test numbers.
Let me give you a real example to make this concrete. Last quarter, a customer came to me making heavy-duty pet leashes. He was using a solid 100% nylon yarn that snapped at 7.2 cN/dtex, but it was too stiff, and he wanted a softer leash that still didn’t break. I sold him our 40/20 nylon-core ACY, which tests at 7.5 cN/dtex—so stronger than his old yarn, but with the sheath wrapped around it making it way softer and more comfortable to hold. He told me he did a pull test with 50 pounds of weight, and the leash didn’t even budge, whereas his old one would have stretched 2 inches. That’s the real-world difference ACY makes when you get the right tensile strength.
Another example: our leggings customer, who uses spandex-core ACY. She tried solid nylon-spandex yarn once, which had a tensile strength of 2.5 cN/dtex, but every 10th order had a bunch of tears at the seams. Switching to our spandex-core ACY at 4.2 cN/dtex fixed that—her customers reported no more torn leggings, and the stretch was exactly what they wanted. So that lower tensile strength we talked about earlier? It’s perfect for that project, because the core is spandex, so you don’t need super high strength, just enough to not snap during normal wear.
Now, what if you’re a designer who’s not sure what tensile strength you need? Let’s make it easy: if you’re making activewear, t-shirts, or leggings, aim for 3-5 cN/dtex. If it’s backpack straps, luggage, or pet gear, go for 6-8 cN/dtex. If it’s casual apparel like hoodies or scarves, 2.5-4 cN/dtex is fine. I always tell people to ask for a sample first before ordering a bulk run—test it yourself with a cheap tensile tester, or even a simple DIY test: tie the yarn around two sticks, hang a bucket from it, add sand or water until it snaps, then calculate the force. You’ll get a rough number that’ll match our lab tests almost every time.
Wait, one more thing I need to mention: tensile strength changes over time. UV light, washing, wear and tear—all of that lowers a yarn’s strength. Polyester loses about 5% of its strength after 50 washes, nylon loses around 10%, cotton loses way more, like 15%. So if you’re making something that’ll be washed a lot, like gym leggings, factor that in when choosing your tensile strength. I always recommend ordering a little higher than the minimum you need, so even after washing, it still holds up.
At the end of the day, there’s no magic number for ACY tensile strength—but that’s a good thing. It’s customizable, which is why it’s such a popular yarn for so many industries. The key is knowing your project’s needs, picking the right core and sheath, and working with a supplier that tests every batch so you don’t get stuck with a dud. If you’re wondering what tensile strength would work for your next project, or you want to test some samples of our ACY batches with verified strength numbers, hit me up. No pushy sales talk, just real advice for whatever you’re making. Let’s chat about what you need, so I can point you to the right ACY for the job.
Nylon Rubber Covered Yarn References:
- Textile Institute. (2020). Air Covered Yarns: Manufacturing, Properties, and Applications. Textile Progress, 52(3), 1-78.
- ASTM D2256/D2256M. (2021). Standard Test Method for Tensile Properties of Yarns by the Single-Strand Method. ASTM International.
- Hunter, L., & Talbot, J. (2019). Technical Textiles Handbook: Performance and Application of High-Strength Yarns. Woodhead Publishing.
Zhuji Yuanhui Import & Export Co., Ltd.
Zhuji Yuanhui Import & Export Co., Ltd. is one of the most professional air covered yarn manufacturers and suppliers in China. With abundant experience, we warmly welcome you to buy high-end air covered yarn made in China here from our factory. If you have any enquiry about quotation and free sample, please feel free to email us.
Address: No.158 Jinke Road, Datang Street, Zhuji City, Zhejiang Province, China
E-mail: hong@yarnyuanhui.com
WebSite: https://www.yarnfty.com/
