CN
Thermodynamics and Climate – Condensation, Fogging, and Thermal Stresses
Heavy-duty HMIs are exposed to extreme climatic conditions—from the humid heat of the tropics to the freezing cold of the Arctic. These conditions pose a significant challenge to reliability, particularly for air-gap displays.
The biggest problem is the “thermal breathing” of the air gap. When heated (e.g., by sunlight), the air in the gap expands and escapes through the seals. When cooled, the air contracts, creating a vacuum that draws cool, humid ambient air into the gap. If the temperature inside the display drops below the dew point of the trapped air, moisture condenses on the coldest surfaces—typically on the inside of the cover glass. This phenomenon is known as condensation or fogging; in practice, it is often described as the display being fogged up from the inside.
The consequences are severe:
- The screen becomes unreadable.
- Moisture can lead to corrosion and thus to the failure of electronic components.
- The integrity of the individual HMI components is no longer guaranteed, which can lead to functional failure.
Once moisture has penetrated, it often remains permanently in the air gap and exacerbates the problem with every temperature change. Moisture ingress into the air gap is the most common cause of failure for air-gap displays in outdoor applications.
Optical bonding eliminates this problem at its root. Since there is no air gap, there is also no space in which air can “breathe” or moisture can condense. The HMI is hermetically sealed and largely “immune” to fogging and condensation.
Another thermodynamic issue is the thermomechanical stress caused by the differing coefficients of thermal expansion (CTE) of the various materials. Glass, any adhesives potentially used at the edges, the display, and the touch sensor expand to varying degrees when heated. In an air-gap display, the layers are connected only at the edges, which can lead to high local stresses. In an optically bonded display, however, the elastic adhesive acts as a damping agent, distributing and absorbing any stresses across the entire surface.
The adhesive used is crucial here: A high-quality silicone adhesive can compensate for the different expansions of the materials, thereby preventing delamination, cracking, or deformation even under extreme temperature fluctuations ranging from -40 °C to +85 °C. This significantly increases the resilience and robustness of the entire system.
Note on the technical information in this chapter
The technical values, test profiles, and performance data stated in this chapter are provided for technical classification purposes. Actual results depend on the specific display configuration, material system, bonding process, application profile, and scope of qualification. Product-specific values are validated on a project basis using data sheets, measurements, test reports, and customer specifications.