OCA and yellowing
prevention

Optically Clear Adhesive (OCA) is the preferred solution for automotive manufacturers around the world, for developing brilliant displays in vehicle cabins. Automotive displays are composed of more than a dozen layers of housing, cover lenses, display panels and mounting tapes. Our assortment covers adhesive solutions for various applications including lenses, touch panels and rigid-to-flexible or rigid-to-rigid substrates.

The special properties in OCA are designed to improve a display’s optical quality by preventing reflection from within the stack. OCA increases light transmission and improves contrast, giving brilliance to bright colors. It also offers essential protection for components against damage from overexposure to UV light, which results in a yellowing effect that not only weakens the optical quality of a display but also deteriorates its structure.

We offer a closer look at how yellowing is measured, and how it can be prevented in display technology.

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Why displays turn yellow

The high transmission and light polarization properties of different plastics make them the ideal material for integration into signage, electronic devices, and automotive parts. However, when exposed to a variety of environmental factors like light and heat, they are prone to yellowing.

One of the most common causes of plastic yellowing is UV light exposure. The time it takes to cause discoloration depends on the type and quality of plastic, as well as the type and amount of weathering it is subjected to. UV energy absorbed by the plastic excites the photons inside, breaking its bonds and causing discoloration of the material. Over time it will become weaker and eventually degrade completely.

The effects of yellowing inside a display not only reduce the quality and lifespan of the parts, but when they are parts of an automotive display, they can affect the driving experience to the point of it becoming a safety hazard. The OCA is no different; when subjected to prolonged UV exposure, there is a risk that an OCA that is not suitably protected against the effects will cause yellowing and fail too.

When developing the products that form part of our tesa® HMI range, we extensively tested the OCA to ensure that the display would last longer without yellowing.

How yellowing is measured

The CIE LAB system is a way to measure color perception. By assigning fixed values to shades of color, everyone from physicists to printers can consistently refer to the exact same reference point.

Values are mapped onto a 3D chart with three axes:

L (lightness) = 0 to 100
Red (+a) to Green (-a)
Yellow (+b) to Blue (-b)

The yellowing process that materials exposed to UV light undergo can be measured on the b value scale to underline the amount of degradation that has occurred.

During testing, we compare the optical characteristics of tesa® OCA products before and after undergoing a process like UV exposure or rapid temperature changes, to highlight its resistance to environmental factors.

In the case of UV exposure our experts measured the b value of the OCA before and after climatic stress tests.

Testing for UV stability

The tesa® 88910 OCA tape was tested for its ability to minimize yellowing using the standard DIN 75220, which simulates long-term outdoor exposure and catalogs its effects on interior and exterior automotive components.

The accelerated weathering test uses a spectrum that correlates to summer sunlight and simulates long-term outdoor exposure. The samples are then examined for defects and color changes.

The samples are stored for 15 days in a dry climate, and 10 days in a humid climate, for a total testing period of 600 hours.

From our extensive work with automotive partners, as well as our own testing of adhesives, we allow for any b value below 1 as a signifier of effective protection against UV light. The b value of our OCA shifted from 0.47 to 0.72 – well within acceptable parameters and with no visible change. Furthermore, the tesa® 88910 OCA tape protects because of its special UV blocking feature all components of the display stack which are located behind it.

UV stability tests also showed several other variables indicating the optical quality of our OCA remained well within acceptable parameters following the same testing.

Conclusion

As our testing shows, we are equipped to provide the solution to automotive manufacturers when it comes to the issues around long-term UV exposure.

“With an OCA that retains a b value below 1 even under accelerated weathering conditions, we have the solution that helps automotive makers take the next step in dazzling display technology.”

- Dolly Cheng, Market Segment Manager

This piece was written in conjunction with Dolly Cheng, Market Segment Manager; and YuKun Wu, Product Development.

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OCA and yellowing prevention