Dynamic Rheological Behavior and Properties of Al₂O₃/Polyurethane Thermally Conductive Adhesives

Al₂O₃/Castor oil, Al₂O₃/polyurethane (PU) prepolymer suspensions, and Al₂O₃/PU thermally conductive adhesives with different loadings (w(Al₂O₃)) and particle sizes of Al₂O₃ were prepared. The dynamic rheological behavior of the suspensions was characterized, and the linear viscoelastic data were fitted and quantitatively analyzed by using a two-phase model. At the same time, the effects of the loading and particle size of Al₂O₃ on the lap shear strength and thermal conductivity of the thermally conductive adhesives were investigated. The correlation among rheological property, structure and property was explored. Results show that when w(Al₂O₃) is more than 80%, the second plateau region appears in the storage modulus (G') versus frequency (ω) curve, which corresponds to the formation of the percolation network. The strain amplification factor and the viscoelastic contribution factor increase, while the gel point moves towards the low-frequency region with the increase of volume fraction of Al₂O₃ (φ). With a fixed loading of 80%, the viscoelastic contribution to the suspension increases as the Al₂O₃ particle size decrease With a particle size of 70 μm and 80% loading of Al₂O₃, the lap shear strength of Al₂O₃/PU thermally conductive adhesives on aluminum alloy and stainless steel reaches 21.38 MPa and 25.85 MPa, respectively, and the thermal conductivity is the highest at 1.38 W/(m·K), which is related to the decreased Al₂O₃-PU phase interface and the formation of thermally conductive percolation network.