Introduction: Aerogels—the Ultimate Material for Battery Pack Thermal Insulation
In the thermal runaway protection of new energy vehicle battery packs, SiO₂ aerogel, with its ultra-low thermal conductivity of 0.012-0.025 W/(m·K) (the thermal conductivity of still air is approximately 0.026 W/(m·K)), has become the core functional filler for thermal insulation coatings. Effectively dispersing the aerogel in a water-based resin substrate while preserving its nanoporous structure from damage is the greatest technical challenge in formulation design.
Aerogel-modified waterborne resin refers to a water-based functional coating prepared by uniformly dispersing nanoporous SiO₂ aerogel particles into waterborne polyurethane or waterborne acrylic emulsion through hybrid blending technology, achieving both excellent thermal insulation performance (thermal conductivity ≤0.05 W/(m·K)) and application performance.
I. Aerogel Types and Performance Benchmarks
| Aerogel Type | Thermal Conductivity (W/m·K) | Specific Surface Area (m²/g) | Density (g/cm³) | Cost (RMB/kg) | Application Scenario |
|---|---|---|---|---|---|
| SiO₂ Aerogel Powder (Hydrophobic) | 0.015-0.022 | 600-800 | 0.05-0.15 | 200-500 | Battery Pack Thermal Insulation Coating (Preferred) |
| SiO₂ Aerogel Powder (Hydrophilic) | 0.018-0.025 | 500-700 | 0.08-0.20 | 150-350 | Aqueous Systems (Good Compatibility) |
| Carbon Aerogel | 0.020-0.035 | 400-600 | 0.10-0.30 | 500-1000 | Conductive + Thermal Insulation Dual Function |
| Alumina Aerogel | 0.025-0.040 | 300-500 | 0.15-0.30 | 800-1500 | Ultra-high Temperature >800°C Scenarios |
II. Key Technologies for Dispersing Aerogels in Waterborne Resins
| Dispersion Strategy | Process Key Points | Advantages | Risks |
|---|---|---|---|
| Low-speed stirring premix (200-500 rpm) | Aerogel first premixed with part of water + wetting agent into paste | Maximally protects aerogel pore structure | Lower dispersion efficiency, requires longer time |
| Ultrasound-assisted dispersion | Low-frequency ultrasound (20-40 kHz) for 5-15 min | Good uniformity, high efficiency | Excessive power may damage gel skeleton |
| Ball milling / bead milling (beads 0.3-0.5 mm) | Low filling rate (50%-60%), low rotation speed (1000-1500 rpm) | Suitable for mass production | Grinding media may crush aerogel particles |
| Pre-dispersion – post-addition method | Aerogel first made into 15%-25% solid content pre-dispersion slurry, finally added to finished paint for dilution | Compatible with existing production lines | Pre-dispersion slurry has limited storage stability (≤7 days) |
III. Three-Step Approach to Formula Optimization
Step 1: Optimization of aerogel dosage.The addition amount was gradually increased from 3% to 20% to test the thermal conductivity and adhesion of the coating. The 5%–10% range is the optimal balance—thermal conductivity drops to 0.03–0.05 W/(m·K) (a 60%–75% decrease), while coating adhesion remains ≥5 MPa with no powdering or flaking.Step 2: Resin/aerogel interface reinforcement.Add 1%–3% silane coupling agent (KH-560/KH-570) to establish covalent bonding bridges between the aerogel SiO₂ surface and the resin, increasing coating tensile strength by 20%–30%.Step 3: Matching film-forming aids with drying process.The high specific surface area of aerogel rapidly adsorbs film-forming aids (Texanol, DPnB, etc.) in the coating, causing the minimum film-forming temperature (MFFT) to rise. The film-forming aid addition should be increased from the conventional 3%–5% to 8%–12%, or a post-addition method should be used (adding the film-forming aid just before coating application).
Technical deepening: systematic optimization methods for process parameters (DOE experimental design)
The optimization of coating production processes should not rely on the “trial-and-error method” but should adopt the scientific method of DOE (Design of Experiments). Taking the dispersion process as an example—factors affecting quality (linear velocity/time/fill rate/temperature), 4 factors each at 3 levels—a full factorial requires 81 experiments—DOE uses orthogonal experiments L9 (9 times) or response surface methodology (27 times) to greatly reduce the number of experiments—while obtaining the main effects and interactions of each factor. For example, it is found that “the interaction of linear velocity × time is significant”: high linear velocity + short time and low linear velocity + long time can achieve the same dispersion effect—but the former saves over 20% energy.
In DOE analysis, interpretation of the P-value — P95% confidence). DOE ultimately outputs a set of prediction models (polynomial regression equations) — input line speed/time/temperature → predict fineness/viscosity/gloss — providing formulation engineers with a “digital formula optimization” tool.
Industry practice: from “master craftsman’s feel” to “parameter standardization”
The common challenge in the coatings industry — when experienced veteran workers retire, their “feel” (mixing resistance / fineness gauge scraping / visual inspection of wet-film gloss) is taken away — new employees cannot replicate it. Transform the “feel” into quantifiable standard parameters (1) mixing resistance → viscometer reading; (2) fineness gauge scraping → fineness gauge reading (μm); (3) wet-film gloss → gloss meter (GU value). The “standard parameter card” for each process is posted next to the equipment — new employees operate according to the “card” rather than “by feel”. “Parameter standardization” is a key step for coating factories to move from “workshop” to “factory”.
FAQ
Q1: Why does aerogel coating easily chalk?Aerogel particles themselves have no binding property. When the addition amount is too high (>15%), the resin is insufficient to coat all aerogel particles, and the exposed aerogel particles form a chalking layer after the coating cures. Solutions: (1) Control the addition amount ≤12%; (2) Increase resin solid content; (3) Add silane coupling agent to enhance interfacial bonding.
Q2: How to choose between hydrophilic and hydrophobic aerogels?Hydrophilic types have better compatibility with water-based resins, but their high water absorption rate (>5%) may lead to reduced thermal insulation performance of the coating in hot and humid environments; hydrophobic types have good water resistance but are difficult to disperse and require efficient wetting agents. Hydrophilic types are recommended for water-based coatings, and hydrophobic types for solvent-based coatings.
Q3: What is the thermal insulation principle of aerogel coatings? Three mechanisms work synergistically: (1) The nanopores of aerogel (2-50nm) are smaller than the mean free path of air molecules (about 70nm), so air molecules inside the pores can hardly move to conduct heat (Knudsen effect); (2) The solid heat conduction path of the SiO₂ skeleton is extremely tortuous (infinite path effect); (3) The infrared shielding effect of aerogel reduces radiative heat transfer.
Q4: How to test the effective thermal conductivity of aerogel coatings?Use the transient plane source method (Hot Disk) or the guarded hot plate method (steady-state method, GB/T 10294). Before testing, the coating should be dried at 50°C to constant weight to eliminate the interference of moisture on the measured values. A coating thickness of ≥1mm is recommended to ensure measurement accuracy.
Q5: What are the differences between aerogel coating construction and ordinary coating?(1) Aerogel coating has extremely strong thixotropy (similar to toothpaste) and requires airless spraying or trowel application; (2) Drying speed is slower than ordinary coatings (aerogel adsorption delays water/solvent evaporation), requiring 30%-50% longer flash-off time; (3) Wet film thickness is difficult to visually judge for uniformity (rough appearance), requiring the use of a wet film thickness gauge.
Q6: Storage stability of aerogel in coatings?Aerogel has an extremely low density (0.05-0.15 g/cm³) and tends to float and stratify during storage. Solutions: (1) Prepare a high-viscosity pre-dispersion slurry to improve suspension stability; (2) Add 0.5%-1.0% fumed silica as an anti-settling agent; (3) Use sealed packaging for moisture protection (aerogel readily absorbs moisture, causing an irreversible decline in thermal insulation performance).
Q7: What additional requirements does the fireproof coating for battery packs impose on aerogel coatings?In addition to thermal insulation performance, the following are also required: (1) UL 94 V-0 flame retardancy rating (add 5%-10% intumescent flame retardant in the formulation); (2) Electrical insulation (volume resistivity > 10¹² Ω·cm); (3) Electrolyte resistance (coating immersed in carbonate-based electrolyte for 72h without blistering or peeling).
Q8: How to control the cost of aerogel coatings?Aerogel is the most expensive component in the formulation (200-500 RMB/kg). Cost reduction strategies: (1) Use aerogel in combination with hollow ceramic microspheres (20-50 RMB/kg); partially replace aerogel with low-cost materials within an acceptable range of thermal insulation performance (recommended ratio 50:50); (2) Optimize aerogel particle size (recommended 10-30 μm); too fine increases cost and makes dispersion more difficult; (3) Centralized bulk procurement of aerogel (price can drop 20%-30% when annual usage >1 ton).
Q9: Can aerogel coating be used on 800V battery platforms?Yes, and it is highly suitable. 800V platforms have stricter thermal insulation requirements (higher voltage, greater energy in local short circuits). However, additional testing is needed: the insulation resistance of the coating under 1500V DC voltage (should be >100MΩ), and the partial discharge inception voltage (PDIV should be >1500Vp).
Q10: How to scale up laboratory formulations to industrial production?The biggest challenge is the low density and high dust characteristics of aerogels. Key points for industrialization: (1) Preparation of the pre-dispersed paste is completed in a closed vacuum mixer; (2) Aerogel feeding adopts pneumatic conveying to prevent dust; (3) The filtration precision of the finished product is relaxed to 50-100μm (aerogel particles themselves are 10-50μm, and fine filtration would remove the active ingredients); (4) Each batch is tested for both thermal conductivity and adhesion as dual indicators.
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Summary
The SiO₂ aerogel-modified waterborne resin technology reduces the thermal conductivity of battery pack insulation coatings from the conventional 0.15–0.30 W/(m·K) to 0.03–0.05 W/(m·K)—a reduction of up to 80%. The key to formulation design lies in the precise selection of aerogel type (hydrophilic/hydrophobic) and addition level (5%–12%), the mild control of the dispersion process (low-speed stirring + ultrasonic assistance), and the chemical reinforcement of the resin/aerogel interface by silane coupling agents. Kexin New Materials has years of experience in the development of aerogel functional coating formulations and provides customized formulation design and process support to corporate clients.