Flexible window stack structure and display device including the same
A flexible window stack structure includes a substrate, a first hard coating layer formed on a surface of the substrate and a second hard coating layer formed on an opposite surface of the substrate. The first hard coating layer has a curing contraction greater than that of the second hard coating layer, and the second hard coating layer is disposed at an elongated side when the window stack structure is folded. Cracks may be prevented by a curl property of the first hard coating layer when being bent.
1. A flexible window stack structure, comprising:
a substrate;
a first hard coating layer formed on a surface of the substrate, the first hard coating layer comprising an acrylate compound different from a dendritic acrylate compound; and
a second hard coating layer formed on an opposite surface of the substrate, the second hard coating layer comprising the dendritic acrylate compound,
wherein the first hard coating layer has a curing contraction greater than that of the second hard coating layer, and the second hard coating layer is disposed at an opposite side of a folding direction or an elongated side when the flexible window stack structure is folded; and
the flexible window stack structure has a normal curl in the folding direction such that a surface of the first hard coating layer faces each other, and a degree of the normal curl is in a range from 5 to 50 mm in a state including only the substrate, the first hard coating layer and the second hard coating layer,
wherein the degree of normal curl is evaluated as an average of heights of four corners of a lower surface of the second hard coating layer spaced apart from a level where the lowermost surface of the second hard coating layer is positioned.
2. The flexible window stack structure according to claim 1 , wherein each of the first hard coating layer and the second hard coating layer is formed of a hard coating composition that includes a photo-curable compound containing an acrylate functional group, a photo-initiator and a solvent; and
the acrylate functional group of the photo-curable compound of the second hard coating layer is a dendritic acrylate functional group, and the acrylate function group of the photo-curable compound of the first hard coating layer is an acrylate functional group other than the dendritic acrylate functional group.
3. The flexible window stack structure according to claim 2 , wherein a thickness of the first hard coating layer is greater than that of the second hard coating layer.
4. The flexible window stack structure according to claim 1 , wherein the first hard coating layer or the second hard coating layer has a multi-layered structure.
5. The flexible window stack structure according to claim 1 , wherein the substrate includes at least one selected from a group consisting of polyimide (PI), polyethersulphone (PES), polyacrylate (PAR), polyetherimide (PEI), polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polyphenylene sulfide (PPS), polyallylate, polycarbonate (PC), cellulose triacetate (TAC) and cellulose acetate propionate (CAP).
6. A display device including the flexible window stack structure according to claim 1 .
7. The display device according to claim 6 , wherein the image display device is a flexible display where the flexible window stack structure is bent along a direction in which the first hard coating layer is compressed.