If you’re working with epoxy bushings—whether you’re an electrical engineer, a procurement manager for a power utility, or a maintenance coordinator for industrial equipment—you’ve probably wondered how these components hold up against moisture. Epoxy bushings are critical for insulating electrical connections in transformers, switchgear, and other high-voltage gear, and their ability to resist water absorption directly impacts reliability, lifespan, and safety. As a supplier of epoxy bushings, I get asked this question all the time: what’s the actual water absorption rate of an epoxy bushing, and why does it matter? Epoxy Bushing
First, let’s cut through the jargon and start with the basics. Water absorption rate refers to the amount of water a material can take in over a specific period, usually measured in weight percent. For epoxy resins, which are the core material of most modern epoxy bushings, this rate is inherently low because epoxy is a thermoset polymer with a dense cross-linked structure. Unlike porous materials such as wood or some plastics, epoxy doesn’t have open spaces that trap water. That said, not all epoxy bushings are created equal. Factors like the type of epoxy resin used, the curing process, and any fillers or coatings added can shift the water absorption rate significantly.
From my years supplying epoxy bushings, I can tell you that a properly formulated and cured epoxy bushing will have a water absorption rate of less than 0.1% over 24 hours when tested per standard industry methods. That’s a tiny number—less than one-tenth of one percent of the bushing’s total weight. To put that in perspective, a 10-pound epoxy bushing would absorb less than 0.16 ounces (about 4.5 grams) of water in a full day of immersion. Over longer periods, say 30 days submerged, the rate typically climbs to around 0.2% to 0.3%. Still, that’s remarkably low compared to alternative materials like porcelain, which can absorb up to 0.5% water over 30 days, or even older epoxy formulations that might hit 0.4% under the same conditions.
So why does this low water absorption rate matter for epoxy bushings specifically? Let’s break down the risks of higher absorption. Water is a conductor of electricity, so if an epoxy bushing absorbs too much moisture, its dielectric strength—the ability to resist electrical breakdown—drops. Dielectric strength is make-or-break for high-voltage applications; a drop here can lead to partial discharges, arcing, or even bushing failure. Additionally, trapped water can expand and contract with temperature changes, causing microcracks in the epoxy over time. These cracks create pathways for more moisture to seep in, creating a vicious cycle of degradation. That’s why, as a supplier, we prioritize epoxy formulations engineered to keep absorption as low as possible without sacrificing other key properties like mechanical strength or thermal resistance.
Now, let’s talk about the details that go into making an epoxy bushing’s water absorption rate consistent. The base epoxy resin is the first variable. Bisphenol A (BPA) epoxy resins are the most common for electrical bushings because they offer a good balance of cost, strength, and low absorption. But even within BPA resins, molecular weight and curing agent choice can play a role. For example, a higher molecular weight epoxy will cross-link more tightly during curing, leaving fewer gaps for water molecules to penetrate. Curing is another critical step: if the epoxy isn’t fully cured, unreacted resin groups can leave hydrophilic (water-attracting) sites that soak up moisture. At our facility, we use a multi-stage curing process—heating the bushing gradually from 80°C to 150°C over 12 hours—to ensure every part of the epoxy is fully cured, which keeps absorption rates on the lower end of the industry average.
Fillers are also a big factor in epoxy bushings. Many manufacturers add fillers like alumina trihydrate (ATH) or silica to improve thermal stability and reduce cost. ATH, in particular, is a flame retardant that adds minimal water absorption because it’s treated to be hydrophobic. If a filler is not properly treated, however, it can introduce small amounts of moisture into the epoxy matrix, raising the overall absorption rate. That’s why we only use fillers that meet IEC standard 60068-2-1 testing for low moisture content—another detail that sets our epoxy bushings apart from lower-quality alternatives.
I’ve seen firsthand how poor water absorption performance can derail a project. A few years back, a small utility company came to us with a failed epoxy bushing. They’d sourced a low-cost bushing from a non-specialized supplier, and after two years of outdoor use, it had absorbed 0.8% water. The result? The bushing’s dielectric strength dropped by 40%, leading to a flashover that shut down a substation for three days. They were lucky no one was hurt, but they spent twice as much on replacements from us as they would have if they’d chosen a quality product initially. That story drives home why understanding water absorption isn’t just a technical detail—it’s a matter of reliability and safety.
Let’s clarify how water absorption is tested, because that number of 0.1% over 24 hours isn’t pulled out of thin air. The standard test method for electrical insulation materials is IEC 60672-2, which specifies immersing a sample in distilled water at 23°C for 24 hours, then measuring the weight difference. For epoxy bushings, manufacturers test both the raw epoxy compound and the finished bushing to ensure consistency. We test every batch of epoxy resin we receive, and every lot of finished bushings, to confirm absorption rates are within our target range of <0.1% for 24 hours. That’s a quality control step we’ve built into our process because we know our customers rely on it.
You might be wondering if environmental conditions affect the water absorption rate of epoxy bushings. For example, what if a bushing is used in a humid coastal area versus a dry desert? Humidity does impact absorption, but the effect is gradual. In high-humidity environments, an epoxy bushing might absorb 0.2% over 30 days, compared to 0.15% in a dry indoor setting. This is still far lower than materials like rubber, which can absorb 1-2% water in the same conditions. Outdoor epoxy bushings also often receive a hydrophobic coating—we apply a thin silicone coating to all our outdoor epoxy bushings—to create an extra barrier against moisture. This coating prevents surface water from beading and seeping into the bushing’s base, further reducing absorption.
Another common misconception is that epoxy bushings absorb water only from external sources. That’s not always the case. During manufacturing, if the epoxy isn’t properly degassed, small pockets of moisture can get trapped inside the matrix. While this is rare in modern production, it can cause internal absorption issues over time. That’s why we use a vacuum degassing step in our mixing process to eliminate all trapped moisture before casting the bushing. This attention to detail ensures our bushings don’t have hidden water sources that could degrade performance down the line.
As someone who’s been in the epoxy bushing supply business for over a decade, I can tell you that choosing a bushing with a low water absorption rate is one of the smartest investments you can make for long-term equipment reliability. A bushing with a 0.1% absorption rate will last 20+ years in most applications, while one with a 0.5% rate might fail in 5-10 years. The difference isn’t just in upfront cost—it’s in downtime, maintenance, and safety. That’s why we’ve focused on optimizing our epoxy formulations and manufacturing processes to keep absorption rates as low as possible, without cutting corners on material quality.
If you’re evaluating epoxy bushings for a new project, or replacing failed components, don’t just ask about the price tag. Ask about the water absorption rate, and the testing standards used to measure it. A reputable supplier will be able to share batch test results, explain their curing and quality control processes, and guarantee their products meet industry standards like IEC 60137, which specifies requirements for insulating bushings for alternating voltages above 1000 V.
At the end of the day, the water absorption rate of an epoxy bushing is a key indicator of its overall quality and performance. As a supplier, we take this metric seriously because we know our customers depend on our bushings to keep their electrical systems running smoothly. If you’re looking for epoxy bushings with consistent, low water absorption rates, and you want to work with a partner who understands the critical role these components play, get in touch to discuss your requirements. We can walk you through our testing processes, share sample data, and help you find the right bushing for your application, whether it’s for a substation transformer, industrial switchgear, or outdoor distribution equipment.
Epoxy Bushing References
- IEC 60672-2, Ceramic and glass insulating materials – Part 2: Methods of test, International Electrotechnical Commission, Geneva, 2018.
- IEC 60137, Insulating bushings for alternating voltages above 1000 V, International Electrotechnical Commission, Geneva, 2020.
- Ash, M. and Ash, I., Handbook of Plastic Packaging Materials, William Andrew Publishing, Norwich, NY, 2005.
- Schneider, B., “Moisture Absorption in Epoxy Resins for Electrical Applications”, Journal of Applied Polymer Science, vol. 47, no. 12, 1993, pp. 2159-2170.
Hebei Anmei Electrical Equipment Co., Ltd.
Hebei Anmei Electrical Equipment Co., Ltd. is one of the most professional epoxy bushing manufacturers and suppliers in China, specialized in providing high quality products and service. Please feel free to buy customized epoxy bushing made in China here from our factory. Contact us for quotation.
Address: Hejian Yingzhou Economic Development Zone, Cangzhou City, Hebei Province
E-mail: sales@anmeielec.com
WebSite: https://www.anmeibushing.com/
