Applications and Precautions for Epoxy Resin Adhesives in Batteries

　　Epoxy resin sealant specifically designed for storage batteries is primarily used for bonding the cell cover to the battery case and sealing the terminal posts, and it is divided into mid‑cover adhesive and terminal post adhesive. The mid‑cover adhesive, also known as cell cover adhesive, sealant, or cover sealing adhesive, is employed to bond and seal the space between the battery cell cover and the battery case; the terminal post adhesive, also referred to as red–black adhesive, red–blue adhesive, terminal adhesive, marking adhesive, or identification adhesive, is used to seal and mark the positive and negative terminals of the battery. The seal between the battery case and the cover is critical to the overall sealing performance of the battery—this is mainly because the contact area between the battery case and the cover is large, the geometry is complex, the adhesive layer is directly exposed to acidic gases and electrolyte, and it is frequently subjected to external impacts. As a result, air and liquid leakage can easily occur at the interface between the case and the cover. To ensure that the cell cover remains firmly bonded to the battery case during operation, the mid‑cover adhesive must exhibit excellent adhesion and acid resistance. Enhancing the adhesive sealing performance of storage batteries is inseparable from the proper use of battery sealant; therefore, during application, it is essential to strictly follow the sealant’s instructions. In the actual process, whether the condition of the battery’s bonding surfaces, the correct ratio of epoxy resin adhesive, the appropriate curing temperature, and the proper potting procedure are all handled correctly will directly affect the final adhesive performance of the sealant.
　　Epoxy resin sealant specifically designed for batteries is primarily used for bonding the cell cover to the battery case and sealing the terminal posts in maintenance‑free lead‑acid batteries, and it is divided into mid‑cover adhesive and terminal post adhesive. The mid‑cover adhesive, also known as cell cover adhesive, sealant, or cover sealing adhesive, is used to bond and seal the space between the battery cell cover and the battery case; the terminal post adhesive, also known as red/black adhesive, red/blue adhesive, terminal adhesive, marking adhesive, or identification adhesive, is used to seal and mark the positive and negative terminals of the battery.
　　1. Contact Surface Treatment
　　The surfaces of battery cell covers, casings, and terminal posts are often prone to contamination by sweat, oil, dust, and other substances. In addition, ABS, PP, or recycled plastic surfaces may still contain mold release agents. During the application of sealant, organic solvents (such as acetone) are used to directly clean and dry the ABS casing before sealing with adhesive.
　　2. Proportional Mixing
　　The mixing ratio for two‑component epoxy resin AB adhesive is determined based on the reaction mechanism. If the deviation from the specified ratio is too large, one component may become excessively abundant, leading to incomplete curing or a significant reduction in the adhesive strength that it should possess. The correct mixing method involves thoroughly stirring the components until they are evenly distributed, using the weight ratio rather than the volume ratio of the two components—and ensuring that the error does not exceed ±3%. When the ambient temperature is low, A adhesive tends to have a very high viscosity, making it difficult to stir evenly; simply preheating it to around 30°C will reduce its viscosity, after which it can be mixed and stirred with B adhesive. At this point, it becomes much easier to achieve uniform mixing. However, thorough and even mixing is equally important: even when the mixing ratio is accurate, insufficient stirring often results in localized areas that fail to cure properly or remain sticky to the touch. As a consequence, both the adhesive performance and the acid resistance will fall far short of the required standards. It is therefore recommended to use mechanical mixing during application, and to scrape down any adhesive adhering to the inner walls of the mixing container with a spatula during the process before stirring again—this ensures that all the adhesive is fully and uniformly mixed.
　　3. Curing Temperature
　　The epoxy resin adhesives used in the manufacturing process of storage batteries belong to room‑temperature curing systems. However, the curing speed and effectiveness are directly influenced by ambient temperature: the higher the temperature, the faster the curing; the lower the temperature, the slower the curing. Yet when the ambient temperature falls below 15°C during curing, the required curing time increases significantly, the crosslink density of the adhesive layer becomes low, and the curing reaction remains incomplete. Moreover, simply extending the curing time or raising the curing temperature within a certain range does not yield equivalent results—when the curing temperature is too low, prolonging the curing time cannot effectively compensate for the deficiency. This is because the complete chemical bonding between the adhesive interior and the surface of the adherend requires sufficiently high temperatures to occur. Therefore, heat curing is the optimal choice in low‑temperature environments. Heating also softens the adhesive layer, enhancing its wettability on the substrate surface and facilitating molecular movement, enabling the molecules at the bonding interface to find their “partners” for generating intermolecular forces. Consequently, heating is beneficial for improving adhesive strength. However, if the curing temperature is too high, it can easily lead to adhesive loss or cause the adhesive layer to become brittle, resulting in a decline in bond strength. Heating methods include oven or tunnel drying, as well as chamber heating. The heating process should proceed with gradual temperature increases, and the heating temperature can generally be controlled around 40–60°C. For ease of operation, it is recommended to use direct heating via a tunnel dryer: after sealing the battery with adhesive, it should remain in the heated tunnel for an appropriate duration (e.g., 1 hour) before being removed from the production line.
　　4. Control of Adhesive Application Volume
　　The amount of adhesive dispensed is a critical factor affecting curing performance. In theory, a larger adhesive volume and thicker adhesive layer result in higher shear strength at the bonding interface. However, in practical applications, if the adhesive layer is too thick—especially during hot summer months—the heat generated by the polymerization reaction cannot dissipate in a timely manner. Excessive adhesive temperature can cause air bubbles within the adhesive to expand, while volatile components in the adhesive may vaporize and form bubbles throughout the adhesive layer. This leads to incomplete adhesion between the adhesive and the housing, resulting in weak interfacial bonding and ultimately reducing the product’s bonding performance. Therefore, during the adhesive dispensing process for battery terminal posts, it is recommended to adopt a layered dispensing method: control the bottom-layer adhesive to around 5–10 mm, and the top-layer adhesive to 10–15 mm.
　　5. Acid Migration Along the Terminal Post
　　The terminals of lead-acid batteries are typically made of lead or lead alloys, while the battery casing is usually constructed from materials such as ABS or PP. This places stringent requirements on the epoxy resin adhesive itself: it must exhibit excellent physical properties, including high strength, high toughness, acid resistance, and fatigue resistance, while also providing strong adhesion to both metallic and organic substrates. Moreover, the negative terminal of a battery is more prone to acid creep than the positive terminal. The reason is that the positive terminal is in an oxidized state, where a passive layer readily forms on its surface, effectively blocking the reaction between the terminal and sulfuric acid—and thus preventing corrosion and acid creep. In contrast, the negative terminal is consistently in a reduced state, with a highly active surface that easily reacts with acid fumes. Furthermore, during charging and discharging, the two terminals undergo continuous redox transformations, gradually eroding upward over time. Currently, anti‑acid‑creep terminals are available on the market; their primary feature is that the terminal body incorporates at least one thread, which helps delay acid creep corrosion and significantly extend the service life of the terminal.
　　In summary, when using epoxy resin adhesive to seal the cell cover and terminal posts of maintenance‑free lead‑acid batteries, in addition to requiring the epoxy sealing compound to exhibit excellent adhesion, acid resistance, impact resistance, and low acid absorption, the following measures can be taken during the application process to optimize the curing performance of the adhesive and thereby enhance the battery’s sealing integrity: (1) Clean and dry the bonding surfaces; (2) Improve the fluidity and uniformity of the epoxy resin adhesive; (3) Precisely control the mixing ratio of the adhesive and ensure thorough, even stirring; (4) Reasonably manage the interval between layer-by-layer pouring cycles and the thickness of each adhesive layer; (5) Reduce the humidity in the sealing environment and maintain a dry atmosphere; (6) When ambient temperatures are low, appropriately increase the curing temperature.