1. Field of the Invention
The present invention relates generally to an optical device, and more particularly to a substrate, which is capable of preventing the substrate from warping and has the protective layers to prevent it from scrubbing.
2. Description of the Related Art
A conventional optical substrate is made of Polymethyl methacrylate (PMMA) or Polycarbonate (PC) that the materials have a greater water absorption. In a standard test of ASTM D570, the water absorption of PMMA is 0.3% and the water absorption of PC is 0.2%.
A conventional backlight unit 50 is shown in
An earlier invention of the present inventors disclosed an optical substrate, which is made of PMMA or PC, is provided with insulating layers on opposite sides thereof. The insulating layers are made of Cyclic Olefins Polymer (COP) or Cyclic Olefins Copolymer (COC), which has a water absorption less than 0.01%. The insulating layers block the water in the substrate vaporized out to fix the problem as described above. The COP layers and the COC layers have a less hardness (about HB to H) and easy to be attacked by oil. While such optical substrate with the COP or COC insulating layer is applied to a product and the substrate is exposed in air and/or touchable by people, the substrate is scratched or attached by oil.
The primary objective of the present invention is to provide a substrate, which will not warp while the substrate is heated on a single side thereof.
The secondary objective of the present invention is to provide a substrate, which the substrate is not easy to be scratched or attacked by oil.
According to the objectives of the present invention, a substrate having a substrate member and protective layers provided on both sides of the substrate member, wherein the substrate member is made of Cyclic Olefins Polymer (COP) and the protective layers are made of a material with a hardness greater than the substrate member.
As shown in
The substrate member 10 is made of Cyclic Olefins Polymer (COP) or Cyclic Olefins Copolymer (COC). This thermoplastic have several special properties including clear and colorless, absorb almost no moisture and highly impermeable to water, low shrinkage, low birefringence, high thermal resistance, good dimension stability, low dielectric constant and excellent resistance to aqueous acids, bases or polar organics. The water absorption is less than 0.01%, in a standard test of ASTM D570, for both COP and COC. The COP or COC substrate member 10 has very less water absorbed therein and that would make the substrate member 10 not warped when the substrate member 10 is irradiated at single side.
The protective layers 20 are made of Polymethyl methacrylate (PMMA) or Polycarbonate (PC). PMMA has a hardness about 2H to 3H and PC has a hardness about 1H to 2H, in other words, the protective layers 20 is harder than the substrate member 10. PMMA and PC also have a well resistant to oil. The protective layers 20 serve as a shield to make the substrate member 10 not exposed. The protective layers 20 protect the substrate 1 from being scratched or attacked by oil.
The methods of how to provide the protective layers 20 on the substrate member 10 have co-extrusion method, coating method and evaporation method etc. The coating method has dip coating, slit coating and spin coating. The evaporation method has chemical vapor deposition (CVD) and physical vapor deposition (PVD).
It is noted that the water absorption of PMMA or PC protective layers 20 (0.3% for PMMA and 0.2% for PC) is greater than the water absorption of COP or COC substrate member 10 (<0.01% for both COP and COC). While a ratio of a thickness of the protective layer 20 and a thickness of the substrate member 10 is designated as less than ⅕, the substrate 1 is not warped under the condition of irradiated at single side thereof. In the present preferred embodiment, the thickness of the substrate member 10 is preferred greater than 0.5 mm and the thickness of the protective layers 20 is preferred greater than 1 μm. More preferable, the thickness of the substrate member 10 is in a range of between 1 mm and 5 mm and the thickness of the protective layers 20 is in a range of between 50 μm and 500 μm.
The substrate of the present invention can be applied to the diffuser plate of the direct-light backlight unit, the light guild plate of the edge-light backlight unit, the protective plate of the window, the optical lens, screen and other suitable products.