If you’ve ever stood in front of a high-voltage transformer, an industrial motor, or even a humble power distribution unit, you might not notice it, but there’s a silent workhorse holding those critical components together: insulating paper. For over a century, I’ve helped manufacturers select, test, and integrate this material into their designs, and one question comes up more than any other—one that can make or break a product’s reliability: how well does insulating paper play with other materials? Insulating Paper
I run a small, family-owned insulating paper supply business, and I’ve spent countless hours on factory floors, troubleshooting prototypes, and walking clients through failure reports that trace straight back to bad material compatibility. It’s not a sexy topic, but it’s the difference between a transformer that lasts 30 years and one that dies in 5. The days of treating insulating paper as a “one-size-fits-all” part are long gone. Today, every application pairs it with a mix of metals, oils, polymers, and gases—and getting that mix right is non-negotiable.
Let’s start with the most common pairing: insulating paper and mineral oil. This is the backbone of power transformers, so it’s where I see the most mistakes, and where the science is most mature. Traditional insulating paper is made from Kraft cellulose, a wood-derived material that’s cheap, strong, and has great dielectric (electrical-insulating) properties on its own. But throw it into mineral oil, and suddenly you have a system that relies on both materials working in lockstep.
The key here is moisture. Cellulose loves water—its molecular structure has hydroxyl groups that attract moisture, even from ambient air. Mineral oil, on the other hand, hates water; it can only hold so much before water forms separate droplets. If the paper absorbs too much moisture, it will start to break down faster, and that moisture will pull out of the paper into the oil, lowering its dielectric strength and encouraging corrosion. I tell every client: before you assemble a transformer, test both the paper’s moisture content (it should be under 0.5% for new parts) and the oil’s breakdown voltage. Skip that step, and you’ll be fixing a failure before the unit even leaves the factory.
Then there’s the issue of acid formation. Over time, cellulose breaks down into organic acids, which dissolve into the oil. Too much acid corrodes the copper windings inside the transformer, and it also breaks down more cellulose, creating a vicious cycle. Modern Kraft papers have additives that slow this down, but if you pair the wrong paper with a subpar oil, that cycle accelerates. I recently worked with a client who swapped to a cheaper paper to cut costs, and within two years, their transformers had acid levels three times the industry threshold. We switched them to a thermally upgraded cellulose paper, and their acid levels dropped back to acceptable limits. That’s not just a sales win—it’s protecting their reputation.
Next up is a pairing that’s become more common as grids get smarter: insulating paper and polymers like polyethylene (PE), polypropylene (PP), or epoxy resins. These are used in everything from modern power cables to solid-insulation transformers, and their compatibility with cellulose is trickier than you’d think. The first issue is outgassing. Many polymers release volatile organic compounds (VOCs) when they’re cured or heated. If those VOCs get trapped between layers of insulating paper, they can create small air pockets—air is a terrible insulator, and those pockets become hotspots that cause partial discharge, a slow killer of electrical components. I always advise clients to test outgassing rates for both the polymer and the paper before assembly. We work with a third-party lab that does this testing for us, and we recommend clients do the same; it’s a small cost that avoids huge headaches.
Another problem with paper-polymer pairs is thermal expansion. Cellulose expands at a different rate than most plastics. When a cable or transformer heats up during operation, the paper and polymer stretch and contract at different speeds. Over hundreds of heating and cooling cycles, that mismatch causes the layers to separate, creating gaps that let moisture or contaminants in. I’ve seen this in action with a set of grid-scale storage batteries that used solid insulating paper and PP casings. After five years of daily charge-discharge cycles, the layers had delaminated so badly that the battery lost 15% of its capacity. The fix? We switched to a laminated paper-polymer composite that has matching thermal expansion rates. It’s a minor tweak, but it made a world of difference.
Then there’s the often-overlooked partner in these systems: metals. Copper, aluminum, steel—they all work with insulating paper, but they can also cause degradation if you don’t get the pairing right. For example, copper is a catalyst for cellulose breakdown. When copper ions leach from windings into the insulating paper, they speed up the oxidation of cellulose, leading to faster acid formation and paper brittleness. That’s why many transformers use paper that’s treated with copper inhibitors—small amounts of organic compounds that bind to copper ions, stopping them from reacting. I always ask clients about the metals they’re using; if they’re using copper, we can recommend a paper with a built-in inhibitor, no extra work needed.
Aluminum is another story. It’s used in smaller transformers and motors, and it’s more reactive than copper, especially in the presence of moisture. If insulating paper has residual alkaline compounds from the manufacturing process, those compounds will react with aluminum, forming corrosion products that eat away at both the metal and the paper. That’s why we test our Kraft papers for residual pH levels—we keep them neutral to slightly acidic, so they don’t react with aluminum. If a client is using aluminum, we’ll flag that test result and adjust the paper if needed.
What about newer applications, like high-voltage direct current (HVDC) systems or offshore wind turbines? These have way harsher conditions—extreme temperatures, constant vibration, exposure to salt water—so material compatibility is even more critical. For offshore wind, for example, insulating paper is often paired with nitrogen gas, because the offshore environment is too corrosive for oil-based systems. The paper has to be compatible with nitrogen, which means it can’t absorb the gas in a way that changes its mechanical or electrical properties. I’ve worked with wind turbine manufacturers for years, and the biggest issue I see there is vibration. Over time, the paper rubs against the metal parts, and if it’s not compatible in terms of wear resistance, it frays, creating conductive particles that cause short circuits. That’s why we offer a reinforced, cross-laminated paper for these applications—it’s tougher, holds up to vibration, and doesn’t react with nitrogen or the corrosion-resistant metal coatings they use.
One of the most common mistakes I see new clients make is assuming that compatibility only matters during assembly. No— it matters over the entire lifespan of the product. A paper that works perfectly when new can break down after 10 years if it’s paired with the wrong materials, or exposed to extreme conditions. That’s why I always recommend life-cycle testing, not just one-off component testing. A few years ago, a client came to us with a motor that had failed after 8 years of operation. We tested the paper, the copper windings, and the grease they used for lubrication. Turns out, the grease was leaking into the paper, and the chemicals in the grease were breaking down the cellulose. We helped them switch to a grease that’s non-reactive with cellulose, and their motors now hit their 15-year lifespan target. That’s the kind of problem-solving that keeps clients coming back.
Let’s talk about how we test compatibility in our shop, because it’s not rocket science, but it’s not guesswork either. First, we start with the application: what voltage is it? What’s the operating temperature? What materials are they pairing with the paper? Then, we do a series of small-scale tests. We take samples of the paper, the oil or polymer or metal, and expose them to conditions that mimic real operation—high heat, moisture, voltage, vibration. After a few weeks (or months, for long-term tests), we test the paper’s tensile strength, its dielectric breakdown voltage, and the chemical makeup of the surrounding materials. If the paper breaks down faster than expected, we adjust the formulation. For example, a few years back, a client was building motors for electric vehicles, which run at much higher temperatures than standard motors. We tested our standard Kraft paper, and it lost 30% of its strength after 1,000 hours at 150°C. We tweaked the paper to add a small amount of aramid fiber, which boosts high-temperature strength, and that cut the strength loss to less than 5%. That’s a simple change, but it made the paper compatible with the new operating conditions.
I want to be clear: I’m not just selling paper. I’m selling peace of mind. When a client works with me, they’re not just getting a material—they’re getting someone who understands how that material interacts with every other part of their design. I’ve seen too many projects go sideways because of a single compatibility oversight, and I’ve also seen projects that thrive because someone took the time to get it right. If you’re designing an electrical component, whether it’s a small motor or a grid-sized transformer, take 15 minutes to think about how your insulating paper pairs with your other materials. If you’re not sure, reach out. I’ve been in this business long enough to help you avoid the mistakes that cost time, money, and reputation.
Carbon Fiber Flexibility No matter what industry you’re in, compatibility isn’t an afterthought—it’s part of the design process. Insulating paper is a small part, but when it works well with the materials around it, it’s the reason your product works well, too. If you’re working on a new project and need to test compatibility, or if you just have questions about which paper will work best with your other components, don’t hesitate to connect.
References
- International Electrotechnical Commission (IEC) 60641, Pressboard and papers for electrical purposes – Specifications for individual materials, 2020.
- Oommen, T.V., Cellulose insulation in oil-filled power transformers: Part I – Aging and degradation, IEEE Electrical Insulation Magazine, Vol. 19, No. 1, 2003, pp. 12–17.
- McShane, C.P., et al., Compatibility of cellulose insulation with polymeric materials for hvdc cable applications, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 22, No. 5, 2015, pp. 2576–2585.
- Liao, R.J., et al., Aging characteristics of oil-paper insulation with copper ions in power transformers, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 17, No. 4, 2010, pp. 1199–1206.
- Offshore Wind Energy Guidelines, Det Norske Veritas (DNV), 2019, Section 4: Electrical System Design.
Wenzhou Gisen Insulation Material Co., Ltd.
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Address: Diyan Industrial Zone, Wengyang Subdistrict, Yueqing City, Zhejiang Province
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