Ultraviolet filter coating

Details Ultraviolet filter coating Ultraviolet filter coating

2001-07-20

2004-08-10

Tucker, Philip (Department: 1712)

Incremental printing of symbolic information

Ink jet

Medium and processing means

C347S101000, C347S103000, C347S105000, C347S213000, C428S041800, C428S409000, C428S411100, C359S359000, C359S361000

Reexamination Certificate

active

06773104

ABSTRACT:

General Embodiment 1
BACKGROUND OF THE INVENTION
Ultraviolet radiation is composed of three ranges, namely: UVA, which is from 320 to 400 nanometers, UVB which is from which is from 280 to 320 nanometers, and UVC which is from 100 to 280 nanometers. UVA and UVB are attenuated by the atmosphere, but is still reaches the earth's surface. UVC is usually blocked by the ozone in the atmosphere. Man-made lighting sources also produce ultraviolet radiation. Most fluorescent lighting has a high output in the UVA range. UVB causes more damage than UVA, but all ultraviolet radiation will cause degradation to materials.
Ultraviolet rays from the sun, or from man-made sources, degrade many materials by breaking their molecular bonds. Dyes and inks fade from ultraviolet, plastics lose their properties, paints chalk and fade, and many other items are damaged. Strategies to combat ultraviolet degradation include the use of materials that absorb ultraviolet radiation and convert it to heat energy. Most absorbers have an ultraviolet cutoff of 365 nanometers. A few have higher cutoffs, up to 384 nanometers with little to no yellowing. The phenomenon of producing a yellow cast when absorbers are used to block all of the ultraviolet radiation is due to the gradual slope of the absorption curve of the absorbing material. This slope, when the cutoff is extended to 400 nanometers, causes absorption of violet and blue light. The absence of blue light is perceived as yellow, and it is for this reason that most absorbers, especially in clear overcoatings, are not used to block all of the ultraviolet radiation up to 400 nm.
The optical density of a filter, an absorber, or a coating, to a range of radiation, is directly related to the concentration and thickness of the layer. The thinner the layer, the light the concentration of absorber is required. Very thin coating layers, below 10 microns cannot contain sufficient levels of absorption without a significant loss in the properties of the coating material. As an example, a 4 micron clear coating might require thirty percent, by weight, of an absorber to have complete absorption up to the cutoff wavelength of the absorber. Some common classes of ultraviolet absorbers are benzophenones and benzotriazoles.
A coating layer that is effective in blocking ultraviolet and is thin has the additional advantage of lower material cost and a higher degree of possible flexibility. A coating with a log concentration of absorber, so that the physical properties of the coating layer are not diminished, as well as the lower cost of using less absorber, that blocks all ultraviolet up to 400 nm, and does not have a significant effect on blue light absorption would a significant improvement in the effort to stop ultraviolet damage to materials.
SUMMARY OF THE INVENTION
The disclosed coating system blocks ultraviolet radiation up to and including 400 nanometers, the upper end of the ultra violet light. Preventing ultraviolet (uv) radiation from reaching materials and surfaces greatly improves weatherability and resistance to physical degradation from the effects of UV radiation on chemical bonds. There currently exist many types of ultra violet inhibitors, which are meant to be included in materials to improve their resistance to uv radiation. The damage from uv radiation is greater as the wave lengths of uv become shorter. However, considerable damage still occurs from the longer wavelengths of uv radiation. It is desirable to block the uv radiation and not have yellowing effect. The disclosed coating system remains water white.
DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS
In accordance with the invention, the disclosed coating system is a two-layered system using a typical ultraviolet absorber in its inner layer (called the blocking layer), furthest away from the source of ultraviolet exposure, with a fluorescent material with reflects ultraviolet radiation back as blue light. The ultraviolet absorber in the inner layer is used in sufficient concentration to have an ultraviolet cutoff, which can be extended with the fluorescent material. There are natural fluorescent materials such as calcite, will mite, sprite, fluorite, and diamonds. Three are also man-made fluorescent materials used to make materials look whiter by reflecting the long wave ultraviolet radiation as blue light. These are called optical brighteners. Typical optical brighteners are disulphonates, tetrasulphonates, and hexasulphonates. These are water soluble optical brighteners. An example of a solvent soluble optical brightener is Uvitex OB from Ciba-Geigy Corp. Such optical brighteners are typically used in textiles at very low concentrations of less than one percent by weight. Their purpose is to reduce the yellowness of a material, dye, plastic, etc. The present invention provides the desired protection by combining an optical brightener with an ultraviolet radiation absorber which raises the cutoff wavelength and increases blue light, rather than absorbing blue light as a longer wavelength cutoff ultraviolet absorber would normally do.
This barrier require high levels of optical brightener to convert the longer wavelength ultraviolet radiation into blue light and do this effectively enough to block the transmission from outer layer to the inner layer due to the total conversion of longer wavelength ultraviolet to blue light. The high level of optical brightener causes a significant fluorescent effect upon exposure to ultraviolet radiation, where this layer will glow with blue light.
The Surface of the inner or blocking layer also has a significant quantity of fluorescent material, which is not protected in depth by the included ultraviolet absorber. This is the primary reason the second or outer coating layer is effective in reducing fluorescence and why it is necessary. The fluorescent material in the inner layer that lies in the matrix of resin and ultraviolet absorber is then protected from excessive fluorescent excitation. Another technique is to use an alkaline material in the outer coating to decompose the surface of the optical brightener of the blocking layer. Still another technique to reduce surface fluorescence is to use an optical brightener quencher such as OBA Quencher from Kalamazoo Paper Chemicals Corp.
While a single blocking layer can be used for protection against ultraviolet, the fluorescent blue glow is generally undesirable. In order to significantly reduce this fluorescence, it is necessary to reduce the amount of ultraviolet that reduces this layer in the peak wavelengths for fluorescence. This is done by applying an overcoating to the blocking layer, which contains some level of ultraviolet absorber that reduces the ultraviolet transmission of the wavelengths that cause fluorescence. It is then this combined effect and balance, which completely blocks ultraviolet radiation without yellowing.
The outer coating can provide other properties such as chemical resistance, scratch resistance, slip, or friction. The outer coating material can be any resin system with an ultraviolet inhibitor, but it is preferable clear and relatively ultraviolet transparent. Materials that do not absorb ultraviolet on their own are relatively unaffected by exposure to it. For this reason, typical clear outer coating resins would be aliphatic urethanes, polysiloxanes or acrylics.
Fluorescent materials have been used in may applications to “whiten” whites, or brighten colors in may products. The teclunique is to use the fluorescent material to increase the reflected blue light. The increase in blue light is perceived as a reduction in yellow light form the fluorescent material. It typically takes very small quantities of fluorescent material to accomplish this brightening effect.
UV absorbers are widely available and are commonly used with intention of blocking primarily UVB. When these uv absorbers are used to block all uv light, they increase yellow light perception due to the reduction in blue light.
Higher concentrations of fluorescent materials in a single layer coating will cause a blue

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