With the rapid progression of global economy and the assertive drive towards achieving “carbon neutrality”, the new energy vehicle industry has gained significant momentum. New energy electric vehicles stand at the forefront of solving energy, environmental challenges, and mitigating urban traffic congestion and are poised to become the principal focus of the automobile industry’s future evolution [1]. It is acknowledged that the power battery serves as the “heart” of new energy vehicles, and the adhesives are considered the enduring force of this “heart”, ensuring its “myocardial vitality”. Within power batteries, adhesives are mainly used for structural bonding, vibration resistance and thermal management. Therefore, there is a high demand from battery factories and new energy vehicle manufacturers for adhesives that are moisture-proof, highly elastic, possess high bonding strength and are efficient heat conductors while remaining cost-effective. Nowadays, the adhesives chosen for power battery applications include epoxy resin systems, silicone resin systems and polyurethane (PU) systems [[2], [3], [4]]. Epoxy resin is known for its outstanding chemical resistance, mechanical performance and strong adhesion force towards many polar substrates [5]. Yet, their brittleness and low peel strength under impact are drawbacks that limit their applications in occasions where high-frequency vibration is inevitable. Silicone resin systems excel in UV resistance and heat resistance [6,7], which facilitate their large-scale use in non-structural bonding and sealing applications. Nevertheless, the relatively low strength, low surface energy, weak adhesion force, and high cost limit their applications in structural adhesion of power batteries [8].
For thermal conductive structural adhesives utilized in power batteries, tensile strength, tensile shear strength and thermal conductivity are important indicators for evaluating the comprehensive performance of structural adhesives. Composed of polyols, chain extenders and isocyanate curing agents, polyurethane (PU) adhesives form a polymer replete with multiple urethane bonds (–HN–CO-O) upon curing. The hard segments provide mechanical strength to PU, while the soft segments provide flexibility and elasticity, thus endowing PU with the combined benefits of high elasticity of rubber and rigidity of plastic [[9], [10], [11]].
The mechanical performance of PU-based structural adhesives can be effectively controlled through strategic molecular structure design and curing agent selection, owing to the customizable nature of the molecular structure of polyols, polyamines and curing agents. In general, the thermal conductivity of pure PU adhesive is below 0.2 W/m·K. To improve the thermal conductivity of the PU matrix, it is necessary to add a large amount of thermal conductive fillers. Traditional thermal conductive filler particles are categorized into metals [12], ceramic [13] and carbon systems [14]. However, high thermal and electrical conductivity of metal and carbon fillers limit their use in electronic packaging applications. In contrast, ceramic fillers such as alumina (Al2O3) and boron nitride (BN) offer excellent electrical insulation, thermal conductivity, and chemical stability. In practice, increasing the ceramic fillers content does improve the thermal conductivity of the material, it may simultaneously compromise flexibility and machinability [15]. Therefore, it remains a great challenge to prepare high-performance structural adhesives with excellent mechanical performance and thermal conductivity.
In thermal management applications, Al2O3 and BN are widely used in electronic packaging adhesives owing to their combined high thermal conductivity and insulating property. Boron nitride nanotube (BNNT) exhibit extraordinary mechanical strength, making them ideal reinforcing nanofiller for lightweight, high-strength polymer nanocomposites [16]. Naous et al. [17] investigated the effect of organic-inorganic alumina fillers and filler morphology on the tensile performance and toughness of epoxy resins. They studies indicated an increase in tensile strength and Young’s modulus for epoxy/Al2O3 composites with increasing Al2O3. Compared with flaky Al2O3, the spherical counterparts were found to enhance epoxy resin toughness more effectively, without sacrificing rigidity and glass transition temperature. Guo et al. [18] enhanced BN surface with KH-560 and NH2-POSS to obtain functionalized BN (f-BN), and used as a thermal conductive filler. The resultant f-BN composites outperformed BN composites in both thermal conductivity and dielectric performance. With a f-BN filler content of 30%, the thermal conductivity of the composites reached 0.71 W/m·K, which significantly surpassed the pure PU matrix conductivity (0.2 W/m·K).
Nowadays, the concept of green chemistry is extensively and intensively rooted within public consciousness. In step with this trend, solvent-based adhesives are gradually being replaced by solvent-free alternatives, primarily due to the harmful volatile organic solvents (VOCs) that solvent-based adhesives emit [15,19]. Solvent-free polyurethane adhesives fundamentally eliminate solvent residues, reducing production costs, and aligning with the “double carbon” target, making them a key focus in the strategy for sustainable and low carbon footprint development [20]. Castor oil (CO) is esteemed as the natural vegetable oil with the highest hydroxyl value, and it serves as one of the most important raw materials in replacing petroleum-based polyols. It finds intensively used in the formulation of polyurethane adhesives, coatings, elastomers and printing inks [[21], [22], [23], [24]]. Over recent decades, China has witnessed great progress in the deep processing and industrial utilization of CO. CO is an ideal raw material for preparing PU-based products Owing to its average of 2.7 hydroxyl groups per molecule, and excellent fluidity [25]. In this regard, Zhang et al. [22] detailed a series of PU foams by compounding soybean-castor oil based polyol with petroleum based polyol. They found that both the thermal stability and thermal conductivity of PU foams improve in conjunction with the content of bio-based polyols. Xu et al. [15] reported a series of eco-friendly CO-based PU thermal conductive structural adhesives (TCSAs). With a hard segment (HS) content of 49.7%, they achieved tensile strength as high as 8.6 MPa, and the displayed high adhesive performance to substrates such as organic glass and aluminum. Besides, the thermal conductivity peaked at 1.23 W/m·K, which was 4.7 times higher than that of the pure PU matrix. Javni et al. [26] reported a CO-based PU adhesive with higher thermal stability than its polyoxypropylene-based polyurethanes counterparts, both in air and in nitrogen environments. Patel et al. [27] demonstrated that CO-based PU adhesives exhibited a more pronounced cross-linking density and bond strength compared to the polyester polyol-based counterparts.
To meet the requirements of power battery encapsulation, this work aims to prepare a solvent-free CO-based PU thermal conductive structural adhesive. CO and isophorone diisocyanate (IPDI) were used as the raw materials for CO-based PU prepolymers (CIs). A series of CIs with different isocyanate content were synthesized by solvent-free method and were used as the curing agent, namely Component B, for the CO-based polyurethane adhesives (CIPU). Moreover, polyaspartic acid ester (PAE) resins (F524, F520, F420) were compounded with poly (tetramethylene ether glycol) (PTMG) of different molecular weights to serve as the active hydrogen contributors in the CIPU adhesives, namely Component A. By utilizing single-factor experiments, the optimal formulation for the CIPU adhesive was determined. Based on this basis, the thermal conductivity was achieved by the incorporation of composite thermally conductive fillers (BN and Al2O3). Comprehensive exploration was conducted to ascertain the effects of filler type and quantity on the performance of CIPU thermal conductive structural adhesives.
