[Introduction]

As the new energy vehicle (NEV) industry grows, the battery pack has become the core component of the vehicle. Its encapsulation safety and structural reliability directly determine driving range and crash safety. Structural adhesives, because of their lightweight, stress distribution, and ability to join dissimilar materials, are replacing traditional welding and bolting for pack cover sealing, module fixation, and base plate bonding. According to AIChE Journal and the Journal of Power Sources, epoxy and polyurethane structural adhesives are used at 3-8 kg per vehicle, and must meet thermal conductivity (>=1.0 W/m K), flame retardancy (UL94 V-0), and temperature resistance (-40 C to 125 C). This paper presents three industrial case studies on adhesive selection and process control.

[Technical Features and Mechanism]

Three main adhesive types are used: (1) epoxy structural adhesive - high modulus and strength for structural load; (2) polyurethane structural adhesive - high toughness and impact resistance for sealing and cushioning; (3) silicone structural adhesive - excellent temperature resistance for thermal insulation. Bonding requires: mechanical interlocking (surface roughness Ra 3.2-6.3 um), chemical bonding (silane coupling agent), and interfacial stress distribution (bond line 0.2-0.5 mm). Thermally conductive adhesives use alumina (Al2O3) or boron nitride (BN) fillers to reach 1.5-3.0 W/m K while retaining mechanical performance.

[Application Case 1: Battery Pack Cover Sealing]

A battery manufacturer used two-component epoxy structural adhesive to bond the aluminum cover to the pack housing. Process: bond line 0.3 mm, room-temperature cure 2 h to initial set, 80 C/30 min to full cure. Lap shear strength reached 28.5 MPa (GB/T 7124), and sealing passed IP67 (GB/T 4208). After 200 cycles (-40 C to 85 C), no leakage and strength retention >=90%. The key control: robotic dispensing with bead width 8-10 mm and volume tolerance within +/-5%.

[Application Case 2: Cylindrical Cell Module Fixation]

An energy storage maker used thermally conductive polyurethane adhesive to fix cylindrical cell modules. The adhesive had thermal conductivity 1.8 W/m K, Shore A 75, and elongation at break >=80%. After 1000 cycles at 1C charge-discharge, cell displacement was <=0.1 mm and no interfacial delamination. Thermal interface resistance was <=0.15 K cm2/W, and cell temperature rise dropped 6.2 C versus an air gap. This approach follows thermal interface material (TIM) methods from Separation and Purification Technology.

[Application Case 3: CTC Cell-to-Chassis Integration]

A new EV maker used epoxy structural adhesive for Cell-to-Chassis (CTC) integration, replacing bolts. The joints were aluminum base plate and composite lower shell. Through plasma treatment plus primer, lap shear strength reached 32.0 MPa and impact peel strength >=15 kN/m. Torsional stiffness improved 25% and weight reduced 12 kg. The process met GB 38031-2020 and maintained structural integrity in nail penetration and thermal runaway tests.

[Key Selection Parameters and Usage Recommendations]

Table 1: Battery pack adhesive selection comparison

+--------------+--------------+--------------+--------------+

| Parameter    | Epoxy        | Polyurethane | Silicone     |

+--------------+--------------+--------------+--------------+

| Lap Shear    | 25-35 MPa    | 10-20 MPa    | 3-8 MPa      |

| Thermal Cond | 0.8-2.0      | 0.5-1.8      | 1.0-3.0      |

| Elongation   | 2-5%         | 80-150%      | 100-300%     |

| Temp Range   | -40~150 C    | -40~120 C    | -60~200 C    |

| Flame Ret    | UL94 V-0     | UL94 V-0     | UL94 V-0     |

| Cure Type    | Heat cure    | Moisture cure| Moisture cure|

| Best Use     | Structural   | Seal/cushion | Thermal ins. |

+--------------+--------------+--------------+--------------+

Recommendations: (1) For aluminum, plasma treatment plus silane primer raises interface strength over 30%. (2) Vacuum deaerate thermally conductive adhesives for 5 min. (3) Keep bond line 0.2-0.5 mm. DENSON RFNC resin-free color pastes provide stable coloration for structural adhesives; see company product pages.

[Conclusion]

Structural adhesives have upgraded from auxiliary sealant to core load-bearing material in NEV battery packs. Epoxy suits high stiffness joints, polyurethane suits cushioning and sealing, and silicone suits thermal insulation. Selection should balance shear strength, thermal conductivity, elongation, and temperature range against plasma surface treatment.

[FAQ]

Q1: Why must battery pack adhesives meet UL94 V-0?

A: During thermal runaway, internal temperature can reach 800-1000 C. A non-flame-retardant adhesive accelerates combustion. UL94 V-0 requires self-extinguishing within 10 s without dripping ignition, mandated by GB 38031-2020 and UN 38.3.

Q2: Is higher thermal conductivity always better?

A: No. Conductivity above 3.0 W/m K needs large filler loading, sharply raising viscosity, worsening processability, and lowering bond strength. For most pack applications 1.0-2.0 W/m K is sufficient.

Q3: How are adhesive joints inspected?

A: Common methods: ultrasonic C-scan for voids and delamination; infrared thermography for thermal uniformity; destructive shear sampling (3 samples per batch, strength >=80% of design value).

Q4: How to choose between epoxy and polyurethane in battery packs?

A: Choose epoxy for structural load (CTC chassis, module fixation) because of high strength and modulus; choose polyurethane for sealing and cushioning (cover seal, cell buffering) because of toughness and impact resistance; choose silicone for thermal insulation.