Copper-Aluminum Busbar In-Polymer Coating Process: Process, Materials and Industry Application Analysis

Copper-Aluminium Busbar In-Polymer Coating refers to the process of uniformly covering the surface of the copper-aluminium busbar with a liquid gel-like insulating material through a specific technique, forming an insulating protective layer. Essentially, it is also a process of coating the surface of the copper-aluminium busbar with an insulating layer by melting plastic and applying it, and its core objective is to achieve reliable insulation, adapt to complex working conditions, while balancing cost and performance. Plastic Dipping Copper Busbar is one of the core components made based on this process.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

High-quality in-polymer coating process must meet three core requirements: First, insulation reliability. The coating thickness should be controlled within 0.5–2mm, with an insulation strength of 20–28kV/mm, capable of withstanding a 4380V DC withstand test. This is also the core performance indicator of PVC Dipping Insulated Busbar; Second, environmental adaptability. It should have excellent resistance to salt fog corrosion, without corrosion after being immersed in 5% NaCl solution at 35°C for 192 hours; Third, cost-effectiveness. Compared with the PI film coating process, the cost of in-polymer coating materials can be reduced by 30%, making it suitable for large-scale batch production.

 

However, this process has certain limitations in the application of power battery fields. For energy storage systems, insulation requirements take precedence over lightweighting, and the penetration rate is relatively high. In contrast, a power battery is constrained by space sensitivity and heat dissipation contradictions - an increase in coating thickness will cause a 20–31% increase in thermal resistance. Therefore, its application is relatively limited.

The in-polymer coating process is complex, and the precise control of each step of the process directly determines the quality of the product. The complete process can be divided into four steps: pre-treatment, dip coating, plasticization, and post-treatment. The pre-treatment step is the key to determining the adhesion of the coating.

 

First, oil and dirt on the surface of the copper-aluminum busbar are removed using an alkaline solution with a pH of 10–12 for 5–8 minutes to ensure that the residual oil and dirt on the surface of the copper-aluminum busbar is ≤0.1mg/m²; then, the surface is roughened by spraying 120-mesh alumina particles to control the surface roughness Ra within 1.6–3.2μm, enhancing the adhesion of the coating layer; finally, preheating and activation are carried out, with the preheating temperature of the copper busbar being 180–220°C and that of the aluminum busbar being 150–180°C. The preheating time is adjusted according to the thickness of the copper-aluminium busbar, such as 120 seconds for a 2mm thick copper busbar. The overall temperature difference should be controlled within ±5°C.

The dip coating step is the core for ensuring the uniformity of the coating. It requires strict control of the parameters and operation specifications of the coating liquid. The coating liquid is PVC plastic solvent, and its viscosity at 25°C should be maintained within 1500–2500cP. Any deviation exceeding 10% requires disposal. The immersion parameters include an immersion speed of ≤10mm/s, which is too fast to cause bubbles, a lifting speed of ≤5mm/s, which is too slow to cause coating sagging, and the immersion time is set according to the coating thickness, such as 15±3 seconds for a 1mm thick coating. At the same time, the immersion tank should be equipped with a magnetic stirrer with a rotational speed of 200rpm to prevent filler sedimentation and ensure the uniformity of the coating liquid composition, which is also the key to ensuring the quality of the Dip Insulated Busbar coating.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The plasticization step is the core of polymer material cross-linking. It adopts a three-stage temperature curve: the first stage rises from 80°C to 120°C for 10 minutes, mainly to achieve solvent evaporation; the second stage rises from 120°C to 200°C for 15 minutes, completing the melting and spreading of the coating liquid; the third stage is at 200°C for 20 minutes to achieve high molecular cross-linking and curing. During the entire plasticization process, the temperature difference inside the box should be ≤8°C. Local overheating exceeding 220°C will cause PVC decomposition and yellowing, affecting the insulation performance of the Insulated BusBar.

The post-treatment step is mainly used to eliminate coating defects and ensure product reliability. The key control parameters of the dip coating process directly affect the product qualification rate. Four core items need to be focused on: the viscosity of the adhesive should be controlled at 2000 ± 500 cP, with a deviation exceeding this range causing coating run-off or incomplete coverage, and sampling and testing should be conducted every 2 hours using a rotational viscometer; the preheating temperature should be controlled at 200°C for copper bars and 170°C for aluminum bars, with deviations leading to coating adhesion and peeling force < 4N/cm, and real-time monitoring should be carried out using an infrared thermal imager; the plasticization constant temperature should be controlled at 200 ± 5°C, with temperature deviations resulting in insufficient crosslinking and subsequent insulation failure, and detection should be conducted using DSC differential scanning calorimetry; the cooling rate should be controlled at ≤ 5°C/s, with too fast a rate causing micro cracks in the coating to expand, and the cooling deformation should be recorded using a high-speed camera. In a typical case, an energy storage project experienced blistering and peeling of the coating after salt spray testing due to uneven plasticization temperature (15°C lower at the box edge), and the problem was resolved by increasing the thermal air circulation fan to optimise temperature uniformity.

The operational differences between power batteries and energy storage systems determine the differentiated application strategies for the dip coating process. In the power battery scenario, a compromise solution is needed. One option is local dip coating, where only the high-voltage copper bar connection end of the BDU (battery distribution box) is coated, reducing the insulation layer thickness to 0.8mm, which can reduce the overall heat dissipation impact by 40%; the other option is a composite process, where the main body uses a 0.15mm thick PI film for encapsulation, and the bending parts use dip coating for reinforcement, balancing insulation performance and flexibility requirements, suitable for the usage scenarios of Insulated Flexible Copper Bus Bar for Power Battery Pack.

 

For energy storage systems, a universal adaptation can be achieved, with a single-sided thickness of 1.5–2mm, suitable for complex environments such as dust and humidity; from a cost perspective, the dip-coated copper bars have a lower cost than the laser-welded PI film solution by 35%, meeting the requirements of space constraints and cost orientation of energy storage systems.

The material selection for the dip coating of power battery pack copper bars requires comprehensive consideration of insulation, temperature resistance, heat dissipation, process cost, and environmental adaptability. Currently, the main materials include three types. PVC (polyvinyl chloride) is the most widely used material, with excellent insulation performance, a withstand voltage of 3500V AC/DC, an insulation strength of 20–28 kV/mm, meeting the UL94-V0 flame retardant standard, good salt spray performance, a working temperature range of -40°C to 125°C, the lowest material cost, mature process, suitable for batch production, and it is the core material of PVC Coated Bus Bars, but its limitations are obvious, with a thermal conductivity of only 0.2 W/(m·K), thickening the coating will significantly increase the thermal resistance, and has poor low-temperature toughness, prone to cracking.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Epoxy resin is suitable for scenarios requiring lightweight, thin-layer high insulation, with a coating thickness that can be controlled at 0.3–0.8mm, a withstand voltage of up to 4500V DC (leakage current < 1mA), an insulation resistance > 800MΩ, extremely strong adhesion, a scratch test adhesion level of 0 (no peeling), reducing weight by 40% compared to PVC coating, suitable for sensitive battery space scenarios, but the cost is 30% higher than PVC, and the curing temperature needs to be precisely controlled (±5°C), otherwise, micro cracks are prone to occur. Nylon-based PPA (Poly(2-Phenoxyacetamide)) is suitable for high-vibration and high-temperature environments.

 

Its continuous operating temperature can reach above 150℃, and it can withstand short-term heat up to 180℃. It outperforms PVC and epoxy resin in terms of heat resistance and chemical resistance. It has high mechanical strength, with a tensile strength of over 70MPa, and is resistant to vibration fatigue. It is suitable for frequent vibration environments in vehicles. However, the raw material cost is higher, being twice that of PVC. The processing temperature should be controlled within 300℃ ± 10℃. This process may lead to oxidation of the copper bus bar.

Based on different scenarios, the material application plan needs to be precisely matched: For high-energy density design of power batteries, epoxy resin is recommended, with a coating thickness of ≤ 0.8mm, covering only the BDU connection end, and the main body coated with 0.15mm thick PI film; For high-vibration environments, PPA (nylon-based) is recommended. Partial dip coating at the bending part is recommended to enhance fatigue resistance. For cost-sensitive designs, modified PVC is recommended, with a thermal conductivity improvement to 0.45 W/(m·K) by adding 5% nano-Al₂O₃, and the coating thickness is reduced to 1.0mm. For the high-weather resistance requirements of energy storage systems, PVC is recommended with a thick coating (1.5–2.0mm) to adapt to complex environments; Long-term cost optimization also recommends PVC, with its cost advantage being significant.

There are significant differences in the priority of key parameters between the two scenarios: In the battery power supply scenario, heat dissipation (λ > 0.4 W/(m·K)) is more important than lightweighting (coating < 0.8mm) and cost; In the energy storage system scenario, insulation (withstanding voltage > 4500V) is more important than corrosion resistance and cost. In terms of process control, the preheating temperature needs to be carefully controlled, with copper bus bars at 200℃ ± 5℃ and aluminium bus bars at 170℃ ± 5℃, to prevent insufficient coating adhesion. For epoxy resin, a three-stage curing curve (80℃ → 120℃ → 200℃) is recommended to avoid micro-cracks and ensure the insulation reliability of the Battery Bus Bar.

Overall, the application of the dip coating process in power battery packs focuses on achieving a balance between insulation reliability, compromise in heat dissipation, and lightweighting requirements. The key lies in the refinement of process steps, customisation of scenarios, and material innovation. The refinement of process steps requires ±5% precision management in aspects such as preheating activation, viscosity control, and gradient plasticization; Scenario customization requires differentiated strategies for local dip coating of power batteries and thick coating throughout the energy storage system; Material innovation requires the development of nano-fillers and low-temperature gel formulations to break through the performance limitations of traditional materials.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In the future, as solid-state batteries with high voltage platforms (800V+) become more widespread, the dip coating process will evolve towards ultra-thin coatings (<0.5mm) and functional composites (insulation/conduction/electromagnetic shielding integration), further adapting to the technological upgrade requirements of the new energy automotive industry.