Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Readily visible image formation
Reexamination Certificate
2000-04-26
2001-10-30
Hamilton, Cynthia (Department: 1714)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Readily visible image formation
C430S339000, C430S338000, C430S342000, C430S341000, C430S344000, C430S333000, C430S280100, C430S270100, C430S285100, C430S281100, C430S286100, C430S287100
Reexamination Certificate
active
06309797
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to selectively colorable polymerizable compositions and to a method for forming selectively colored polymeric bodies using such compositions.
A selectively colorable solid object is an object that can be colored at small individual, but specifically defined, sites by irradiating light of a particular wavelength and specific intensity for a specified duration. The light sources capable of producing the selectively colorable solid object include: (a) a laser interfaced with an XY scanner (for polymer films), (b) a laser interfaced with an XYZ scanner (for 3D parts), (c) digital mirror device, (d) UV and Visible lamps with a masking device, etc.
A selectively colorable resin (SCR) system consists of: (a) the matrix (a blend of polymerizable material or a solid polymer), (b) a color former, (c) a color initiator (species that generate other species capable of reacting with color former; may not be needed in some systems); and (d) a chain reaction initiator (radical or cationic or none depending on the system).
The conventional method for forming a colored plastic body is to add a dye or pigment to the liquid prepolymer composition. The composition is then cured with actinic radiation. The latter requires that the absorption spectra of the photoinitiator and the dye/pigment differ. If the dye/pigment absorbs actinic radiation at the same wavelength as the photoinitiator, slower or no cure will be achieved. The color formation method is also not selective. The entire plastic is uniformly colored. Still another problem is that the photopolymerization process requires actinic radiation, but the color forming process requires only that the composition be well mixed. Though one of the processes can be controlled by the intensity/wavelength of the actinic radiation, the other is unaffected by it. Thus there is no selectivity of color formation in the plastic body.
Accordingly, there is a need for selectively colorable polymerizable compositions and for a method of forming selectively colored polymeric bodies using such compositions wherein the colorization process can be controlled as to the location and intensity of the color formed.
SUMMARY OF THE INVENTION
The present invention discloses selectively colorable polymerizable compositions and a method for forming selectively colored polymeric bodies using such compositions. In accordance with the invention, a selectively colorable polymerizable composition comprising both a leucobase color former and a leuconitrile color former is irradiated with light of a particular wavelength and specific intensity for a specified duration. Exposure to actinic radiation cures the composition and activates the color formers. The irradiation dosage can be varied to selectively color the polymeric body whereby the resultant color of any particular area depends on the exposure dose received at that location. By varying the dose, a polymeric body can be prepared having distinctly colored elements at specific locations.
It is believed that, upon irradiation of the polymerizable composition of the present invention containing a polymerizable compound, a leuconitrile color former, a leucobase color former and an onium salt, a number of interacting mechanisms occur for color formation. One mechanism is the generation of a first dye cation upon exposure of a leucobase dye precursor in the presence of an oxidizing agent and a second mechanism is the generation of a second dye cation and a leaving group by heterolysis of a leuconitrile color former. Upon further exposure a third mechanism occurs whereby the first dye cation is bleached in the presence of the second color former. At still higher doses of irradiation the first and second dye cations are both bleached. Accordingly, the composition of the present invention provides a photopolymerizable selectively colorable polychromic system.
In one embodiment of the present invention, a composition is provided which can be exposed to different levels of irradiation to produce a selectively colored polymeric body. Under low dosage, color formation depends primarily on Scheme 1 wherein exposure of a leucobase to light in the presence of a oxidizing agent yields a first dye cation of a first color. The mechanism is illustrated below for a triarylmethane (TAM) leucobase (TAMH) susceptible to oxidation. Ox represents an oxidizing agent and TAM
+
represents the colored species. It is anticipated that the scheme below will also be valid for diarylmethane (DAM) leucodyes (DAMH and DAMCN).
TAMH+Ox - - - h&ngr;→TAM
+
(colored) Scheme 1
The other mechanism is the heterolysis of a second color former which yields a second dye cation and a leaving group. This mechanism is illustrated below for a triarylmethane leuconitrile (TAM-CN) color former. TAM-CN is susceptible to photochemical heterolysis and forms a colored species through the heterolytic cleavage of the C—CN bond. Accordingly, mechanism 2 also yields a leaving group (CN) upon heterolysis of the leuconitrile color former.
(TAM)′CN - - - h&ngr;→(TAM)′
+
+CN
−
Scheme 2
Preferably, the reactants are selected such that mechanism 1 is much faster than mechanism 2 at high concentrations of the oxidizing agent (Ox). Thus, at low doses of actinic exposure the color is primarily determined by TAM
+
. To increase the color contrast, the leucobase and leuconitrile color formers can be selected such that the dye cation derived from the leucobase (TAM
+
) is less stable than the dye cation derived from the leuconitrile ((TAM)′
+
). Accordingly, at higher doses of irradiation, bleaching of the less stable TAM
+
occurs while the amount of color formed by (TAM)′
+
. increases. Therefore, the color of the polymeric body at higher doses of irradiation is primarily determined by the (TAM)′
+
cation.
In another embodiment of the present invention, a polychromic polymeric body is produced by increasing the irradiation dose to cause bleaching of both colored dye cations. In accordance with this embodiment, the dye cation formed from heterolysis of the leuconitrile color former is bleachable at high doses of actinic radiation. Therefore, a selectively colorable, polychromic composition is obtained wherein low light exposure creates a polymer of one color, intermediate light exposure changes the color of the polymer and a high dosage of light exposure bleaches the color of the polymer.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention, a composition which can be selectively colored by exposure to actinic radiation and a method leading to the formation of selectively colored, polychromic, polymer bodies that can be cured photochemically are provided. The invention is particularly useful for the selective color development and photopolymerization of films using a leucobase color former, a leuconitrile color former and an oxidizing agent. The composition of the present invention can be used in a solid or a liquid.
Those skilled in the art will appreciate that the color determinative irradiation step can be conducted before, after or simultaneously with the polymerization or crosslinking step. Furthermore, selective coloration of a polymeric body in accordance with the present invention can be carried out using one exposure or light source to polymerize or crosslink the composition and a second exposure or light source to induce the photochromic response, or using a single light source to polymerize the composition at a first intensity and using the same light source at a plurality of intensities to induce the photochromic response. The latter system has the advantage that it involves the use of only one light source and it will be appreciated that this system is easily implemented using highly sensitive photohardenable systems, which can be easily polymerized using a lower intensity light exposure. As a general rule, agents, such as photoinitiators, used to initiate polymerization will
Grinevich Oleg V.
Malpert John H.
Mejiritski Alexandre
Neckers Douglas C.
Hamilton Cynthia
Spectra Group Limited, Inc.
Thompson Hine LLP
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