26 Aug Modeling the Curing Process of Adhesives

Introduction to Adhesives

In industries like aerospace, automotive, life science and electronics, adhesives play a crucial role due to their superior characteristics over traditional mechanical fastening methods. Structural adhesives are preferred for their corrosion resistance, high strength-to-weight ratio, and the minimal thermal and mechanical damage they cause to substrates. However, the curing process of adhesives, where a viscous liquid transforms into a solid, involves complex thermal and chemical changes that can induce strains and stresses, potentially weakening the bonded components.
To optimize the curing conditions and minimize residual stresses, it’s essential to simulate this process accurately. This post will explore how to model the curing process of adhesives using Abaqus/Standard, focusing on key simulation techniques and the associated material modeling approaches.

Adhesive curing application overview

The curing of adhesives is a critical process, especially in structural applications. During curing, polymer chains in the adhesive undergo cross-linking, leading to a transformation from a viscous liquid to a rubbery solid and eventually a glassy solid upon cooling. This transformation is accompanied by thermal expansion and shrinkage, which can introduce residual stresses in the adhesive layer. Accurate modeling of this process helps in designing better curing strategies to ensure the longevity and reliability of adhesive bonds.

Abaqus Modeling Approaches

The simulation of the curing process in Abaqus/Standard involves several advanced techniques:
– Thermorheologically Simple (TRS) Material Model
This model accounts for temperature dependence and the degree of cure in a viscoelastic material, crucial for capturing the adhesive’s behavior accurately during the curing process.

– Tangent Thermal Expansion
This technique is used to define the thermal expansion coefficient, allowing the simulation to account for the material’s expansion and contraction during temperature changes.

– Fully Coupled Temperature-Displacement Analysis
This analysis type is employed to model the deformation of the adhesive, considering both thermal and chemical dependencies of the material properties during curing.

Adhesive curing Simulation Example: The Watts Test

The Watts test serves as a validation method for the cure modeling process. In this test, as shown below, a disc-shaped adhesive specimen is sandwiched between two glass plates. As the adhesive cures, it shrinks, causing deflection in a thin glass diaphragm positioned on top of the adhesive.

The simulation of this process includes several steps:
– Curing Step: The assembly is heated to activate the curing process, with the temperature controlled to manage the exothermic reaction that occurs during polymerization.

– Cooling Step: After curing, the assembly is cooled to room temperature, during which thermal expansion and contraction are modeled.

– Relaxation Step: Finally, the residual stresses are allowed to relax, simulating the final state of the adhesive bond.

Material Properties and Boundary Conditions

The adhesive’s mechanical behavior during curing is defined by the Kamal equation, which governs the rate of cure based on temperature and the degree of cure. Viscoelastic properties are characterized using Dynamic Mechanical Analysis (DMA) methods, and the TRS material model is applied to account for temperature and cure dependencies.
The initial temperature of the assembly is set to 22°C, with symmetric boundary conditions applied. Thermal loads are used to simulate the curing and cooling processes, and the interactions between different materials in the assembly are modeled to ensure accurate results.

Results and Discussion

The figure below shows the conversion history of the adhesive during curing.

The figure below shows the temperature and cure history at the center of the specimen during curing.

The final picture below shows the deflection of the specimen at the end of the cooling.

These results are compared with experimental data to validate the simulation’s accuracy. The exothermic reaction observed during curing leads to a rapid temperature increase and a corresponding rise in thermal expansion. The predicted deflection profiles show good agreement with experimental data, confirming the effectiveness of the modeling approach.

Acknowledgments

This blog post is based on “Modeling the Watts test of adhesives” from the Abaqus Example Problems Manual and benefits from data provided by 3M, enhancing the accuracy and reliability of the simulation results.

Curing process of adhesives conclusion

Accurately modeling the curing process of adhesives using Abaqus/Standard is essential for designing reliable adhesive bonds in critical applications. By leveraging advanced material models and simulation techniques, engineers can predict and mitigate the residual stresses that arise during curing, ensuring the integrity of the bonded components.

Curing process of adhesives consultancy

4RealSim has a proven track record in providing Abaqus engineering services. Contact us at sales@4realsim.com if you want to discuss your case or are interested to acquire Abaqus or to perform these simulations.