As Foldable Displays Get Thinner, Glass Cannot Be Optimized for Thinness Alone
Ultra-thin glass, or UTG, has become an important substrate for foldable displays. After being reduced to an extremely small thickness, glass can bend while retaining good transparency, surface flatness and tactile quality.
The thinner the glass becomes, however, the more sensitive it is to scratches, localized pressure and impact damage. Drops, repeated folding and pressure from foreign particles can cause microscopic defects to expand gradually and eventually develop into cracks.
Ultra-thin glass therefore normally requires an additional protective layer. Conventional polyethylene terephthalate, or PET, films are mature and relatively easy to process, but under frequent folding they may encounter problems such as creasing, delamination and declining long-term optical stability. Next-generation protective materials must do more than simply cover the glass surface. They must also provide reinforcement, cushioning and damage control.
This is where polyurethane coatings may offer new possibilities.
A recent study published in Progress in Organic Coatings introduced a trifunctional polyetheramine-based polyurethane acrylate, or PEUA, UV-curable coating designed for direct application to ultra-thin glass.
In testing, PEUA-coated ultra-thin glass completed 200,000 inward-folding cycles at a bending radius of 3 mm without visible creasing or coating delamination. Under the more severe bending radius of 1.5 mm, the coating developed stress whitening, but its transparency could be restored through brief thermal treatment.
How Can High Modulus and High Toughness Be Achieved Simultaneously?
Protective materials for foldable displays face a fundamental contradiction. If a material is too rigid, it may crack during folding. If it is too soft, it may fail to support the glass adequately and can become vulnerable to indentation and permanent creasing.
The PEUA resin developed in the study was synthesized primarily from hydroxyethyl acrylate, isophorone diisocyanate and polyetheramine, followed by UV curing to form a crosslinked network.
The rigid segments formed from IPDI provide structural support, helping to improve modulus and resistance to deformation. The flexible polyetheramine chains absorb and dissipate energy generated during bending and impact, reducing localized stress concentration. Together, these components create a balance between stiffness and toughness.
The coating maintained optical transmittance of more than 92% at a wavelength of 550 nm, reached a storage modulus of approximately 2.18 GPa and recorded a glass-transition temperature of about 140°C. A coating layer approximately 40 μm thick increased the pen-drop impact failure height of the ultra-thin glass by 16 cm.
The change in failure mode is particularly noteworthy. When unprotected ultra-thin glass is subjected to impact, it can develop radial cracks that spread rapidly across the surface. After the PEUA coating was applied, damage was more likely to remain as a localized indentation without continued crack propagation.
The coating therefore does more than add another layer of thickness. It changes how impact energy is transferred into the underlying glass.
Figure 2. Schematic showing how a polyurethane acrylate coating may distribute bending stress across an ultra-thin glass structure. AI-generated image; layer thicknesses are not to scale.