(a) Field of the Invention
The present invention relates to a plastic substrate having a multi-layer structure with a small coefficient of thermal expansion, excellent dimensional stability, and superior gas barrier properties and a method for preparing the same.
(b) Description of the Related Art
Although glass plates used for display devices, picture frames, craftwork, containers, etc. are advantageous in that they have a small coefficient of linear expansion, superior gas barrier properties, high transparency, good surface flatness, excellent heat resistance and chemical resistance, etc., they tend to break easily and be heavy because of their high density.
Recently, as liquid crystal displays, organic light emitting devices, and electronic paper are arousing a growing interest, research on replacing the glass substrates used in such devices with plastic counterparts is gaining momentum. The plastic substrate is advantageous over the glass plate in terms of weight and ease of design. Also, because it is impact-resistant, an economic advantage may be attained from continuous manufacturing.
For a plastic substrate to be used in a display device, it should have a glass transition temperature high enough to endure the transistor processing temperature and the transparent electrode deposition temperature, oxygen and water vapor barrier properties so as to prevent aging of liquid crystals and organic light emitting materials, a small coefficient of thermal expansion and good dimensional stability so as to prevent deformation of the plate due to change of the processing temperature, mechanical strength comparable to that of the conventional glass plate, chemical resistance sufficient for enduring the etching process, high transparency, low birefringence, good surface scratch resistance, etc.
However, because a single polymer composite film (polymer film or polymer-inorganic material composite film) that satisfies all the requirements does not exist, several layers of functional coats are applied on a polymer film to fulfill them. Typical coating layers are an organic flattening layer for reducing surface defects and offering flatness, an inorganic barrier layer for blocking gaseous materials such as oxygen and water vapor, and an organic or organic-inorganic hard coating layer for offering surface scratch resistance. The conventional plastic substrates having a multi-layer structure are manufactured by coating an inorganic gas barrier layer on a plastic film and coating a hard coating layer on the gas barrier layer. In such a multi-layer structure, deformation of the plastic film or cracking or peeling of the inorganic layer may occur because of the difference of coefficients of linear expansion of the plastic film and the gas barrier layer. Accordingly, design of an appropriate multi-layer structure capable of minimizing stress at the interface of each layer and adhesion of each coating layer are very important.
Vitex Systems of the U.S. developed a substrate having a superior gas barrier property by obtaining an organic-inorganic multi-layer structure of several layers by forming a thin monomer film on a plastic film, polymerizing the monomer by illuminating UV (solidified organic layer), and forming a thin inorganic layer thereon by sputtering. Although a substrate having a superior gas barrier property can be obtained with this method, the requirement of a low coefficient of linear expansion, which is needed for a display, has not been satisfied, and a solution method for the problem has not yet been suggested.
U.S. Pat. No. 6,465,953 disclosed a method of dispersing getter particles capable of reacting with oxygen and water vapor on a plastic film to use for an organic light emitting device which is sensitive to oxygen and water vapor. The getter particles should have a particle size smaller than the characteristic wavelength of the emitted light, and should be dispersed uniformly so that the emitted light can transmit to the substrate without being scattered. This method is intended to minimize inflow of oxygen and water vapor by coating a gas barrier layer comprising an inorganic material on a plastic film. However, it is difficult to uniformly distribute nano particles having a particle size ranging from 100 to 200 nm, and the plastic film should be thick enough to comprise a lot of getter particles capable of reacting with oxygen and water vapor. Also, because the inorganic gas barrier layer is directly coated on the plastic film, the gas barrier layer tends to crack or peel off with changes in temperature.
U.S. Pat. No. 6,322,860 disclosed a plastic substrate for electronic display applications that was manufactured by coating a crosslinkable coating composition (comprising a polymer selected from the group consisting of polyfunctional acrylate monomers or oligomers, alkoxysilanes, etc. and a mixture thereof) on one or both sides of a polyglutarimide sheet having a thickness of no more than 1 mm, which has been prepared by extrusion, and photocuring or thermally curing it to form a crosslinked coating, and coating a gas barrier layer on the crosslinked coating and then coating another crosslinked coating on the barrier layer, if necessary. In specific cases, transmission rate of oxygen and water vapor was small enough to be used for a liquid crystal display. However, a small coefficient of thermal expansion and superior dimensional stability, which are required to replace the glass substrate, were not obtained.
U.S. Pat. No. 6,503,634 disclosed a multi-layer substrate manufactured by coating an organic-inorganic hybrid (ORMOCER) and silicon oxide on a plastic film or between two sheets of plastic films. The resultant film showed an oxygen transmission rate of no more than 1/30 and a water vapor transmission rate of no more than 1/40, compared with those before coating. Although this film can be used in packaging because of significantly reduced oxygen and water vapor transmission rate, there was no mention about improvement in the coefficient of thermal expansion or dimensional stability.