The global push for “dual-carbon” strategies has propelled the new energy vehicle (NEV) industry into a phase of rapid expansion. As the core component of an electric vehicle, the performance of the power battery pack directly dictates the vehicle’s range, safety, and service life. To meet the ever-increasing demands for higher energy density and lightweight design, the use of adhesives within the EV battery pack has become ubiquitous. Among them, structural adhesives have emerged as a critical technology, replacing traditional mechanical fastening methods like welding and bolting. They are required to deliver a complex set of properties: high bonding strength, superior resistance to environmental aging, excellent vibration damping, and effective thermal management. This study, from the perspective of our research and development team, systematically investigates the key factors influencing the bonding performance of polyurethane (PU) structural adhesives specifically for joining polymer-metal substrates commonly found in EV battery pack assemblies. We explore the coupled effects of substrate preparation, processing parameters, and long-term environmental exposure to provide a multi-factor model for joint reliability.
The design and integrity of the EV battery pack are paramount. Traditional epoxy adhesives, while offering high modulus and good thermal stability under static loads, often fall short in the dynamic service environment of an EV. Their inherent brittleness leads to poor impact resistance, and their high coefficient of thermal expansion (CTE) can cause significant stress at the interface of dissimilar materials during the extreme temperature swings experienced by an EV battery pack, from cold starts to high-temperature charging cycles. In contrast, polyurethane adhesives present a compelling alternative. Their molecular engineering allows for tuning the ratio of hard segments (urethane groups) and soft segments (polyether/polyester polyols), enabling optimization of both storage modulus and damping properties. Their microphase-separated structure can dissipate vibrational energy efficiently, a key requirement for the longevity of connections within an EV battery pack. Furthermore, their lower density contributes to the overall lightweighting goal of the EV battery pack, and their high volume resistivity aligns with electrical safety standards.
While PU adhesives see use in automotive body panels and glazing, research focusing on their application in critical EV battery pack joints, particularly for polymer-metal hybrids like polyester (PET)-aluminum, remains less explored. This work aims to bridge that gap by investigating the adhesive performance for 6061 aluminum alloy-to-PET film joints, a relevant configuration for module housing or cover sealing in an EV battery pack.
1. Experimental Methodology
1.1 Materials and Specimen Preparation
The substrates used were 6061-T6 aluminum alloy sheets and two types of PET film (transparent and black, same base resin but different surface finishes). The adhesive was a commercially available two-component polyurethane structural adhesive (Sikadur-452), mixed at a 1:1 volume ratio.
Two primary joint configurations were prepared according to standardized test methods:
- Tensile Shear Specimens: Prepared according to GB/T 7124-2008 standard. The configuration was an Al6061-PET-Al6061 stepped lap shear joint. The aluminum sheet dimensions were 100 mm × 25 mm × 2.5 mm with an overlap length of 12.5 mm.
- Vertical Pull-Off Specimens: Prepared according to GB/T 6329-1996 standard. The configuration consisted of two cylindrical aluminum rods (Ø25 mm × 50 mm) bonded to opposite sides of a square PET film patch (30 mm × 30 mm).
The adhesive layer thickness was controlled using precision stainless-steel shims. For specimens requiring surface treatment, the PET substrates were treated with an Ar/O2 plasma (80 W, 90 s) using a plasma cleaner. All specimens were cured at (23 ± 2) °C and (50 ± 5)% relative humidity for 7 days before testing.
1.2 Testing and Characterization
- Mechanical Testing: Tensile shear strength and vertical pull-off strength were measured using a universal testing machine at a crosshead speed of 5 mm/min. The bonding strength ($\sigma$) is calculated as:
$$ \sigma = \frac{F_{max}}{A} $$
where $F_{max}$ is the maximum load at failure and $A$ is the bonded area. - Accelerated Aging Tests: Two aging regimes were employed to simulate harsh conditions an EV battery pack might encounter:
- Damp Heat Aging: Specimens were placed in a climatic chamber at 85 °C and 85% RH for durations of 300 h, 700 h, and 1000 h (referencing GB/T 2423.3-2016).
- Thermal Shock Aging: Specimens were subjected to temperature cycling between -40 °C and 85 °C, with a dwell time of 1 hour at each extreme and a transition time of less than 30 minutes, for up to 1000 h (referencing GB/T 2423.22-2012).
Aged specimens were conditioned at room temperature for 2 hours before mechanical testing.
2. Results and Discussion: A Multi-Factor Analysis
2.1 Influence of Substrate Surface Characteristics
The initial investigation focused on the role of the polymer substrate’s inherent surface state. Despite being from the same base resin, the transparent PET film possessed a higher surface roughness (matte finish) compared to the smooth, glossy black PET film. This morphological difference had a profound impact on the initial bond strength, as summarized in Table 1.
| Substrate Type | Surface Treatment | Avg. Tensile Shear Strength (MPa) | Strength Increase vs. Untreated Black PET |
|---|---|---|---|
| Black PET | None | 8.63 | Baseline |
| Transparent PET | None | 12.04 | +39.5% |
| Black PET | Plasma | 10.03 | +16.2% |
| Transparent PET | Plasma | 14.05 | +62.8% |
The data clearly shows that surface roughness is a primary driver for mechanical interlocking and increased effective bonding area. The transparent PET, with its inherently rougher surface, provided a 39.5% higher bond strength. Plasma treatment, which further etches and functionalizes the surface, enhanced the bond strength for both substrates. The treatment was particularly effective for the already-rough transparent PET, yielding a final strength of 14.05 MPa. This underscores the critical importance of substrate surface preparation in achieving reliable bonds for EV battery pack assembly.
2.2 Influence of Adhesive Layer Thickness
Process control during the dispensing and curing of the adhesive is vital. We investigated the effect of adhesive bondline thickness ($t$) on the joint strength. The results, plotted in Figure 1 and summarized in Table 2, reveal a non-linear relationship.
The optimal thickness for both shear and tensile loading was found to be 0.5 mm. Beyond this point, strength generally decreased. The effect was far more pronounced for shear strength, which dropped by 47.9% at 3.0 mm thickness, compared to a minimal change in pull-off strength. This can be attributed to the increased probability of defects (like voids) and the different stress distributions within thicker adhesive layers. For the predominantly shear-loaded joints in an EV battery pack structure, controlling the bondline thickness is a critical quality parameter. We can model the strength-thickness relationship for shear loading with a simplified empirical decay function:
$$ \sigma_{shear}(t) \approx \sigma_{max} \cdot e^{-k(t – t_{opt})} \quad \text{for} \quad t > t_{opt} $$
where $\sigma_{max}$ is the maximum strength at optimal thickness $t_{opt}$ (0.5 mm), and $k$ is a decay constant specific to the adhesive system.
| Adhesive Thickness, $t$ (mm) | Tensile Shear Strength (MPa) | Vertical Pull-Off Strength (MPa) | Failure Mode Observation |
|---|---|---|---|
| 0.5 | 14.2 | 15.7 | Primarily cohesive (in PET) or mixed |
| 1.0 | 11.3 | 15.4 | Mixed mode |
| 2.0 | 8.2 | 15.3 | Increasing adhesive/PET interface failure |
| 3.0 | 7.4 | 15.3 | Predominantly interfacial failure |
2.3 Influence of Service Temperature
The operational temperature range of an EV battery pack is wide. We evaluated the mechanical performance of bonded joints at temperatures from -40 °C to 60 °C. The results, detailed in Table 3, highlight the thermomechanical behavior of the PU adhesive.
| Test Temperature (°C) | Tensile Shear Strength (MPa) | Vertical Pull-Off Strength (MPa) | Dominant Failure Mode |
|---|---|---|---|
| -40 | 4.33 | 24.20 | Shear: Adhesive brittle fracture; Pull-off: PET tearing (substrate failure) |
| -20 | 7.45 | 20.54 | Shear: Mixed; Pull-off: PET tearing |
| 0 | 16.70 | 18.83 | Mostly substrate failure |
| 23 (RT) | 21.57 | 15.47 | Substrate failure |
| 45 | 11.20 | 8.50 | Increasing interfacial failure |
| 60 | 12.40 | 6.80 | Predominantly interfacial failure |
The data reveals a complex interplay. For shear strength, there is an optimum near room temperature. At very low temperatures (≤ -20°C), the PU adhesive becomes hard and brittle, leading to a severe drop in shear performance. In contrast, pull-off strength is highest at low temperatures, benefiting from restricted molecular motion at the interface, often resulting in desirable substrate failure (PET tearing). As temperature increases, both strength metrics generally decline. At 45°C and 60°C, the failure mode shifts decisively to interfacial failure between the adhesive and the PET. This is likely due to a combination of thermal softening of the adhesive and, more critically, the buildup of thermal stress at the interface caused by the CTE mismatch between aluminum, PET, and the adhesive itself within the constrained geometry of the EV battery pack. The thermal stress ($\sigma_{th}$) can be approximated by:
$$ \sigma_{th} \approx E_a \cdot \Delta \alpha \cdot \Delta T $$
where $E_a$ is the adhesive’s modulus at the temperature, $\Delta \alpha$ is the difference in CTE between the substrates/adhesive, and $\Delta T$ is the temperature change from the stress-free state (often the curing temperature).
2.4 Influence of Accelerated Aging
Long-term durability is non-negotiable for an EV battery pack. Accelerated aging tests provide insights into lifetime performance.
2.4.1 Damp Heat Aging (85°C/85% RH)
This condition simulates a hot and humid climate. The results, shown in Table 4, demonstrate a significant degrading effect, primarily attributed to hydrolytic attack on the adhesive and the interface.
| Aging Duration (h) | Strength at -25°C (MPa) | Strength at 23°C (MPa) | Strength at 45°C (MPa) | Dominant Failure Mode (at 23°C test) |
|---|---|---|---|---|
| 0 (Initial) | ~18.5* | 21.57 | 11.20 | Substrate failure |
| 300 | 8.7 | 7.36 | 5.22 | Interfacial failure |
| 700 | 7.1 | 6.85 | 5.05 | Interfacial failure |
| 1000 | 6.5 | 6.58 | 4.98 | Interfacial failure |
*Estimated from trend in Figure 6.
The most dramatic strength loss occurred within the first 300 hours, with a reduction of over 65% when tested at 23°C. The convergence of strength values across different test temperatures after aging indicates that the interface has been severely compromised by moisture, becoming the weak link regardless of the test condition. The strength decay over time ($t$) in a humid environment can often be modeled with an exponential decay or a power-law function:
$$ \sigma_{wet}(t) = \sigma_{dry} – \beta \cdot t^n $$
where $\sigma_{dry}$ is the initial dry strength, and $\beta$ and $n$ are constants related to the moisture diffusion and degradation kinetics.
2.4.2 Thermal Shock Aging (-40°C ↔ 85°C)
This test simulates rapid temperature fluctuations, which can occur due to charging/discharging cycles or environmental changes. Interestingly, as shown in Table 5, thermal cycling did not cause degradation; in fact, it led to a slight strengthening effect when tested at low temperatures.
| Aging Duration (h) | Strength at -25°C (MPa) | Strength at 23°C (MPa) | Strength at 45°C (MPa) | Dominant Failure Mode (at 23°C test) |
|---|---|---|---|---|
| 0 (Initial) | ~18.5* | 21.57 | 11.20 | Substrate failure |
| 300 | 20.1 | 20.80 | 10.95 | Substrate failure |
| 700 | 22.3 | 21.20 | 11.10 | Substrate failure |
| 1000 | 23.7 | 21.65 | 11.35 | Substrate failure |
*Estimated from trend in Figure 6.
The absence of degradation and the ~27.95% increase in strength at -25°C after 1000h suggest that the thermal cycling may have promoted further curing or beneficial molecular rearrangement at the interface, enhancing low-temperature properties. Crucially, the failure mode remained substrate failure, indicating the bond integrity was preserved. This highlights that for the EV battery pack, dry thermal cycling is a less severe aging factor compared to the synergistic attack of heat and humidity.
3. Comprehensive Performance Optimization Strategy for EV Battery Packs
Based on our multi-factor analysis, we propose a strategic framework for optimizing the use of polyurethane structural adhesives in EV battery pack design and manufacturing. The key recommendations are synthesized in Table 6.
| Influence Factor | Optimal Condition / Finding | Practical Recommendation for EV Battery Pack | Primary Rationale |
|---|---|---|---|
| Substrate Surface | High roughness + Plasma treatment | Select roughened polymer films (e.g., matte-finish PET) and implement in-line plasma cleaning prior to bonding. | Maximizes mechanical interlocking and surface energy, leading to higher initial strength and potentially better durability. |
| Adhesive Thickness | ~0.5 mm | Implement precise dispensing and fixturing tools to control bondline thickness strictly, especially for shear-critical joints. | Minimizes void content and internal stresses, achieving peak shear strength. Thicker bondlines are detrimental to shear performance. |
| Service Temperature | Performance peaks near RT; Low-T improves pull-off but harms shear; High-T degrades interface. | Design joints to minimize pure shear loads at expected low temperatures. Incorporate thermal management to keep pack within moderate temperature range during operation to reduce CTE-mismatch stresses. | PU adhesive exhibits ductile-brittle transition at low T and thermal softening/stress at high T. Managing thermal environment protects bond integrity. |
| Environmental Aging | Humidity is the critical degrading factor. Dry thermal cycling is less severe. | Employ robust sealants and barrier coatings at the pack level to protect adhesive joints from direct moisture ingress. Prioritize adhesive formulations with proven hydrolytic stability. | Hydrolytic degradation is the primary failure mechanism. Protecting the joint from moisture is key for the long-term reliability of the EV battery pack. |
The overarching goal is to move from a single-parameter focus to a systems-level approach for adhesive bonding in the EV battery pack. This involves selecting compatible materials, defining robust surface preparation and process windows, and designing the pack assembly to shield the critical bonds from their most damaging environmental exposures.
4. Conclusion
This study provides a systematic, first-person perspective on the factors governing the performance of polyurethane structural adhesives for lightweight, hybrid material joints in EV battery packs. We have demonstrated that:
- Substrate microstructure is paramount. Optimizing surface roughness, achievable through material selection and plasma treatment, can increase bond strength by over 60% compared to a smooth baseline.
- Process control is critical. Adhesive layer thickness has a non-linear, highly influential effect on shear strength, with an optimum near 0.5 mm for the studied system. Precise dispensing is essential for reliable EV battery pack assembly.
- Thermomechanical behavior is complex. While the PU adhesive maintains good strength over a range, extreme low temperatures induce brittleness in shear, and elevated temperatures promote interfacial failure due to CTE-mismatch stresses. The operational temperature profile of the EV battery pack must be considered in joint design.
- Humidity is the arch-nemesis of long-term durability. Accelerated damp heat aging caused severe interfacial degradation, while dry thermal shock cycling had a negligible or even slightly positive effect. This stark contrast underscores the absolute necessity of protecting adhesive bonds within the EV battery pack from moisture ingress through effective sealing strategies.
In summary, the successful integration of polyurethane structural adhesives into EV battery pack manufacturing requires a holistic view that couples material science, process engineering, and systems design. By prioritizing surface-prepared, roughened PET substrates, enforcing strict control over adhesive application thickness, and implementing robust environmental protection schemes, the durability, safety, and performance of bonded joints in the challenging service environment of an electric vehicle can be significantly enhanced.