Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1997-05-06
2001-09-11
Mullis, Jeffrey G. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S227000, C525S206000, C525S228000
Reexamination Certificate
active
06288172
ABSTRACT:
TECHNICAL FIELD
The present invention relates to light diffusing material, in particular to light diffusion adhesive having excellent light diffusing properties with low back scattering.
BACKGROUND OF THE INVENTION
Information displays, such as liquid crystal displays and rear projection screens, often rely on light-diffusing optical constructions for efficient operation and enhanced readability. Such light-diffusing constructions assume critical roles in these displays by forward scattering the light from a source without a significant loss in the intensity of the forward scattered light. This scattered, yet high transmittance, resultant light gives such displays a desirable background brightness by reducing the amount of incident light which is scattered or reflected back toward the light source. Elimination or restriction of such “backscattered” light is a key factor in designing these light-diffusing constructions.
One approach in designing light-diffusing constructions is the filling or embedding of rigid, transparent or translucent plastic films with particles. When properly sized, prepared and formulated with a plastic film, these particles can scatter and diffuse incident light. However, the use of some particles can lead to certain undesirable and deleterious optical effects which detract from the overall brightness or transmittance of the incident light. For example, when some inorganic particles such as titania powders or if particles which are too small (i.e., on the order of the wavelength of the incident light) are used in light-diffusing constructions, a significant loss in brightness can result clue to high levels of backscatter. Conversely, particles having a large diameter with respect to the wavelength of the incident light and/or if the refractive index of the particles is identical or very similar with the continuous plastic film, then little light diffusion occurs.
Additionally, some particle-filled plastic films, although effective as light-diffusing layers, can alter the polarity of the light as it travels through the film. In some constructions, for example, liquid crystal displays, any significant depolarization of the light by the light-diffusing film or component can result in the loss of image quality.
SUMMARY OF THE INVENTION
Briefly, in one aspect of the invention a light diffusing adhesive is provided comprising a mixture of a pressure-sensitive adhesive matrix having a refractive index of n
1
filled with organic, polymeric microparticles having a refractive index n
2
, wherein the absolute difference in the refractive indices of matrix and microparticles, that is, |n1-n2| is greater than zero and typically in the range of 0.01 to 0.2. The weight ratio of matrix to microparticles, based on solids, is from about 1:1 to about 50:1, preferably from about 4:1 to about 25:1.
Advantageously, the adhesive matrix can be both water and solvent borne thus permitting greater flexibility in choice of adhesive for controlling optical performance. Further, the adhesive matrix can be a film former or microsphere based. Light diffusing microparticles used in the present invention can be prepared using a variety of polymerization methods, allowing the user more opportunity to control the size, composition, morphology and overall characteristics of the microparticles.
Furthermore, the proper balance of particle sizes, particle compositions, refractive indices, particle loadings and other properties and parameters can be tailored to adjust light diffusing properties according to an intended end-use. Attachment or adherence of such light diffusing adhesives to other polarizing films, reflective substrates or other optical components is also provided by this invention. Due to the adhesive nature of these light diffusing materials, there is no need for additional layers of adhesive for laminating or bonding for surface attachment to substrates that could be detrimental to the optical performance of the light management device. Additionally, this invention provides a material that is not only light diffusing but is flexible, as well.
Advantageously, the light diffusing adhesive of the present invention does not significantly backscatter incident light or de-polarize transmitted light.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The light diffusing adhesive of the present invention comprises a mixture of a pressure-sensitive adhesive matrix having a refractive index of n
1
filled with organic, polymeric microparticles having a refractive index n
2
, wherein the absolute difference in the refractive indices of matrix and microparticles, that is, |n1-n2| is greater than zero and is typically in the range of 0.01 to 0.2.
The weight ratio of matrix to microparticles, based on solids, is from about 1:1 to about 50:1, preferably from about 4:1 to about 25:1. While many factors can affect the light diffusing properties of the adhesives of the present invention, the ratio of the matrix to microparticles is a significant factor. Generally, when the ratio of matrix to microparticles is too large, there is an insufficient concentration of particles to adequately diffuse incident light and a thicker film is required. On the other hand, when microparticle concentration goes beyond 50% of the matrix, brightness and transmittance deteriorates.
Other factors that can affect the light diffusing characteristics of the adhesive include for example, the microparticle size, the refractive index differential between the matrix and the microparticles, the gradient in refractive index between the matrix and the microparticles, the thickness of the dried light diffusing adhesive when coated onto a substrate, and the intrinsic properties of the microparticle components, for example, degree of crystallinity, organic or inorganic character, absorption properties and the like.
To obtain the optical properties in this light diffusing adhesive layer, the absolute difference between the refractive index of the pressure-sensitive adhesive matrix (n
1
) and the microparticle filler (n
2
) is greater than zero and is typically in the range of 0.01 to 0.2. Values for refractive indices of these components can be obtained directly through the use of standard refractometric methods (for example, using an Abbe refractometer according to ASTM Test Method D542) or, more conveniently, by consulting various tabular sources of refractive index data for polymeric materials (for example, Polymer Handbook, 3rd. ed., New York, John Wiley & Sons, 1989. pp. VI/451-VI/461).
Should this absolute difference in refractive indices approach zero, then a poorly- or non-diffusing transparent or nearly transparent composite could result. This insufficient differential of the refractive indices might be overcome by adding more particles and/or increasing the thickness of the adhesive layer. However, these corrective measures could result in an adhesive layer having diminished brightness.
The adhesive layer is light-diffusing in nature: that is, the adhesive layer can bend the incident light beam, yet still retain a high level of transmittance (generally greater than about 80% of incident intensity, preferably about 85% to about 95%, most preferably about 90% to about 95%) after passing through the adhesive layer. Furthermore, backscatter of these adhesives is typically less than about 20%, preferably in the range of about 1 to about 10%.
In general, light diffusing materials have the ability to uniformly scatter light forward from the light source. Uniformity of the scattered light is measured in terms of its bend angle, wherein “bend angle” means the viewing angle at which the gain drops to ⅓ of its on-axis value. The larger the bend angle, the more uniform the scattered light. However, higher bend angles usually come at the expense of brightness (luminous transmission). Alternatively, losses in luminous transmission could be due to excessive backscattered light. Thus, optimization of light diffusing material depends on the balance between particle concentration, index of refraction differe
Goetz Richard J.
Ouderkirk Andrew J.
3M Innovative Properties Company
Mullis Jeffrey G.
Peters Carolyn V.
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