Silicon steel with self-bonding coatings is a core material for high-end motors in emerging fields such as new energy vehicle drive motors, UAV motors, and humanoid robot joint motors, as it enables excellent interlaminar bonding and insulation performance. However, the self-bonding coating, mainly composed of epoxy resin, undergoes a phased curing process: initial State A (wet coating), intermediate State B (coated steel coils delivered by steel plants), and final State C (bonded laminated iron cores). The cross-linking reaction kinetics between epoxy groups and curing agents are complex, making it extremely difficult to control and quantify the pre-curing degree of State B. These issues have been limiting the research on the correlation between curing degree and coating performance, failing to support the optimization of production processes and the stable control of product quality. To address these problems, the work aims to investigate the curing degree of the self-bonding coating in State B through differential scanning calorimetry (DSC) and establish its relationship with key coating properties. Firstly, the silicon steel self-bonding coating was prepared by blending a water-based bisphenol A/F epoxy emulsion with amine curing agents and co-solvents. An uncoated annealed substrate of grade B25AV1300 was cleaned with anhydrous ethanol, and the prepared self-bonding coating was uniformly applied to its surface, with the average dry film thickness of each surface controlled at 2.5 μm. A series of State B samples were fabricated by baking the coated substrates under three temperature gradients (180 ℃, 190 ℃, 240 ℃) and a time gradient (0.5-6 min). For the preparation of State C samples, the State B samples were cut into pieces, stacked, and then subjected to final curing with an RYJ-600ZG2 automatic hot press under the conditions of 200 ℃, 3 MPa, and 30 min. To characterize the curing degree of the State B coating, a Mettler Toledo DSC3 differential scanning calorimeter was used to test the residual reaction enthalpy (ΔHr) of the coating powder scraped from State B samples. The relative curing degree (α) was calculated with the formula: α=[(ΔHT-ΔHr)/ΔHT]×100%. Additionally, the bonding strength of State C samples was tested via the rolling peel method in accordance with the GB/T 7122 standard, with a Zwick/Roell Z150 tensile machine at a testing speed of 100 mm/min, and the glue overflow of the coating was simultaneously observed. The results showed that with the increase in baking temperature or extension of baking time, the curing exothermic peak in the DSC curve of the State B coating gradually flattened, the residual reaction enthalpy (ΔHr) decreased significantly, and the relative curing degree (α) increased continuously. The bonding strength of the State C coating was not simply linearly correlated with ΔHr and α but exhibited an optimal range: when the ΔHr of the State B coating was 20-50 J/g (corresponding to an α of approximately 35%-75%), the rolling peel strength of the State C samples was ≥ 3 N/mm, and the glue overflow was effectively controlled. This work confirms that differential scanning calorimetry (DSC), which calculates the relative curing degree (α) by testing the residual reaction enthalpy (ΔHr), is technically feasible for effectively characterizing the curing degree of the self-bonding coating on silicon steel in State B. The ΔHr and α of the State B coating show significant correlations with baking temperature and time, but they need to meet a specific matching relationship with the coating bonding strength to achieve excellent performance.
Key words
self-bonding coating /
silicon steel /
curing degree /
DSC analysis /
bonding strength
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