-20003;~1 SYNERGISTIC INITIATOR COMPOSITIONS ~OR TH~
VISIBLE LIGHT ~ABRICATION AND POSTCURE
OF STEREOLITHOGRAPHIC OBJECTS
DESCRIPTION
Technical Field This invention relates to synergistic initiator compositions for the visible light cure of ethylenically unsaturated compositions, and especially for the visible light fabrication and 10 postcuring of stereolithographic objects.
Background Art Ultraviolet-curable coatings based on acrylate-terminated polyurethanes are well known.
While acrylate-functional ethylenically unsaturated 15 materials are now preferred for this purposes, many other ethylenically unsaturated materials are also available and can be used. One important use of the ultraviolet cure of ethylenically unsaturated materials is in stereolithography in which an 20 ultraviolet laser is guided by a computer to trace out a series of cross-sections at the surface of a liquid bath of such material.
More particularly, it is known, as illustrated in U.S. Pat. No. 4,575,330 to C. W. Hull, 25 to form three-dimensional objects of complex shape using ultraviolet light to solidify superposed layers of liquid ultraviolet-curable ethylenically unsaturated material at the surface of a liquid reservoir of such material. However, the 30 ultraviolest lasers used are weak and lose energy rapidly, so they are not very effective. It is desired to be able to use actinic light in the visible range which is not near the ultraviolet range.
Referring to the cure of ethylenically 35 unsaturated material using light in the visible ' ... ,~ ~. . ~ .

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200033:1 range, it ls known that the froe radical polym~rization of ethylenlcally unsaturated systems by exposure to sunlight or incandescent sources can be initiated by combinations of photoreducible phthalein dyes and various alkyl or alkanol amines.
- To illustrate this, fluorescein and methyldiethanol amine have been shown to be relatively effective for the visible light polymerization of maleate unsaturated polyesters. (J. Jachowicz, et al., Ang.
10 Makromolekulare Chem,. 97, 201 tl981).
It is also known that free radical polymerization initiated by certain cyclic cis-dicarbonyl compounds, such as indoledione or camphorquinone, can be dye sensitized to respond to 15 visible irradiation with increased efficiently. tSee U.S. Pat. No. 3,756,827).
And finallly, it has been shown that enhanced visible light sensitization can be achieved by the combination of benzoquinone and 20 dimethoxyphenyl acetophenone. lSee C. Decker, J.
Coatings Tech., 56, 713, 29 (1984)]. ~ -While these compositions are relatively effective for the visible light cure of certain materials applied as thin films, they have not proven -25 effective for the formation of stereolithographically formed objects using visible light lasers.
Additionally the cis-dicarbonyl compounds are often expensive, and benzoquinone is highly toxic.
We have now discovered a novel synergistic 30 combination of inexpensive and nontoxic materials which are very effective for the conversion of ethylenically unsaturated material, and especially ~meth)acrylate-functional material, to solvent-resistant solids using visible light.
35 Additionally, it is found that relatively thick ;, ~ ; :. : ., . : , - : .

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layers can be converted so that visible light laser fabrication of tmeth)acrylate-functional llquids can be carried out. Moreover, and having in mind the fact that stereolithographically-formed objects are S frequently thin-walled cellular structures which are poorly penetrated by ultraviolet light, the visible light cure of such stereolithographically-formed objects can be carried out with improved efficiency because the light sources are stronger and because 10 the visible light better penetrates the thin-walled cellular structures under consideration.
Summary of the Invention In accordance with this invention, a photopolymerizable ethylenically unsaturated liquid 15 composition is formulated to include, based on the total weight of the photopolymerizable ethylenically unsaturated material present:
(1) from about 0.1% to about 5%, preferably from 0.2% to 1.0%, of a photoreducible phthalein dye;

(2) from about 0.5% to about lOS, preferably from 1% to 4%, of an aliphatic tertiary amine, preferably an alkyl, cycloalkyl, or alkanol tertiary amine containing from 1 to 12 carbon atoms;
and (3) from about 1% to about 8%, preferably from 2~ to 4~, of a conventional ketonic photoinitiator, such as a diaryl ketone or an acetophenone compound.
It should be observed that while it is 30 normally intended to solidify the above compositions using visible light (which may include some minor -proportion of ultraviolet light) and to use the compositions for all sorts of purposes, including ordinary coating operations, it is preferred to use 35 these compositions in a stereolithographic process in ~ë~ ` ` .~ ' . ~.~ . . .
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which successive layers are formed by exposure of the surface of a liquid bath to light to form a three-dimensional object. These three-dimensional ob~ects are incompletely cured in that they are solid objects which are mechanically weak and include large amounts of residual ethylenic unsaturation. We prefer to form these three-dimensional objects by exposure to visible light, but they may be formed by exposure to ultraviolet light. In either event, 10 these objects are frequently cellular and the further polymerization using ultraviolet light is difficult because that short wave radiation does not adequately penetrate the object to cure all portions of it. In this invention we post cure the 15 stereolithographically-formed object using visible light which better penetrates the cellular objects which are frequently encountered, and this is a feature of this invention.
In the prior art, when light predominantly 20 in the visible range was used, such as is here provided with an indoor/outdoor spotlight made by GTE
Sylvania Incorporated, Winchester, KY, the polymerization was very slow and incomplete. Such spotlights are commonly used for outdoor lighting and 25 are 150 Watt lights sold under the code designation 150 PAR/4SP. A small proportion of the light emitted by such spotlights are in the ultraviolet range (tO0-400 nanometers), and the balance of the light is in the range of from over 400 to about 650 30 nanometers. Thus, the predominant wavelengths of this light are in the blue to yellow range. -A one minute exposure to this spotlight at a distance of 1 foot of a 2.0 gram sample of the photocurable ethylenically unsaturated liquid ~;
35 (forming an about 40 mil layer in the bottom of a 50 ~A, :r~ ;; :: . '. ;:

2~)00331 ml polypropylene beaker) consistently produced less than 10% conversion to a methyl ethyl ketone-resistant solid when only a conventional ketonic photoinitiator was used, regardless of whether an amine photosensitizer was slso present.
When the amine photosensitizer was used together with a photoreducible phthalein dye, but in the absence of a conventional ketonic photoinitiator, the conversion was still limited to about 20%. However, when all 10 three components were present, the conversion to a methyl ethyl ketone-resistant solid was consistently about 30% or better, as detailed herein after, and in some instances approached 50%.
In the conversion test noted above, a 15 weighed amount of liquid monomer is added to the beaker, and after irradiation, the partially converted layer is extracted with methyl ethyl ketone in the following manner. The exposed layer of partially converted monomers in the bottom of the 20 beaker is removed by adding a small amount of methyl ethyl ketone. This partially converted layer is then~
dropped into methyl ethyl ketone and allowed to remain there for 2 hours at 25C. At the end of this period what remains of the layer is removed, dried 25 and weighed to determine the weight loss caused by the extraction.
Photoreducible phthalein dyes in accordance with this invention are illustrated by fluorescein, tetrabromofluorescein (Eosin), and 30 tetraiodofluorescein (Erythrosin). While many other phthalein dyes which are photoreducible are known, the above are preferred because of their availability. Fluorescein will be used herein as illustrative.
Examples of alkyl, cycloalkyl or alkanoi ,, .: . . . .
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tertiary amines which are useful herein are triethyl amine, tributyl amine, dimethylethanol amine, methyldiethanol amine, and triethanol amine. Cyclic materials are illustrated by dimethyl benzyl amine and dimethyl piperazine. Methyldiethanol amine will be used herein as illustrative.
Examples of conventional ketonic photoinitiators which may be used herein are isopropylthioxanthone, hydroxycyclohexyl phenyl 10 ketone (Irgacure 184), and dimethoxyphenyl acetophenone (Irgacure 651). The two commercial Irgacure photoinitiators, which are available from Ciba-Geigy, Ardsley, NY, will be used herein as illustrative.
15The ethylenically unsaturated liquid composition which is selected in this invention is of secondary consideration. Indeed, any ethylenically unsaturated liquid composition which is convertible to a solid composition by free radical polymerization 20 is useful herein. These are broadly termed photopolymerizable in this disclosure. It is preferred to employ compositions in which the ethylenic unsaturation is of (meth)acrylate character, this term identifying esters of acrylic or 25 methacrylic acids with mono-or poly-hydroxy compounds, such as ethanol, butanol, ethylene glycol or trimethylol propane.
For stereolithography, it is preferred to - - ~
employ a combination of a poly(meth)acrylate resin --30 dissolved in a liquid mixture comprising a poly(meth)acrylate liquid, like trimethylol propane triacrylate or hexane diol diacrylate, or an analog thereof in which the polyhydric alcohol is chain extended by adduction with an alkylene oxide, such as 35 ethylene oxide or propylene oxide. These preferred E~

,, 2~00331 compositions are lllustrated ln the exa~ple. In steroolithography it will also be understood that the llquid compositions are irradiated to form a three-dimensional ob~ect within a liquid bath thereof. When the object is completely formed, it is removed from the bath and any excess polymerizable liquid which may still cling to the exterior of the object is removed, usually by allowing it to drain off. The liquid which is so-remored can be allowed 10 to drain back into the liquid bath which is then used for the formation of additional objects.
The invention will be more fully understood from the following example which employs methyl diethanol amine as the tertiary amine, fluorescein A
15 (CAS 2321-07-5) as the phthalein dye, and a liquid resin composition constituted by 40% of a diglycidyl ether diacrylate in which the diglycidyl ether has a number average molecular weight of 390, 10~ of trimethylol propane ethoxylate triacrylate having a 20 number average molecular weight of 428, 30% neopentyl glycol propoxylate diacrylate having a number average molecular weight of 428, and 20~ of N-vinyl I pyrrolidone.
The results of experiments using 25 combinations of the above ethylenically unsaturated bath composition with various additions to facilitate the conversion on exposure to visible light are tabulated in Table I below.

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Table I
DYE SENSITIZBD VISIBLE LIGHT RXPOSURE
Sylvania Indoor/Outdoor SPotlight Amine Dye Initiator % Conversion Sample. Amount Amount Added 4%by Spotlight A -- -- Irgacure 184 1.6 B 4.0 -- Irgacure 184 7.3 10 C 4.0 0.2 -- 19.5 D 4.0 0.6 -- 22.6 E 4.0 0.2 Irgacure 184* 3~.3 F 4.0 0.2 Irgacure 184 33.9 G 4.0 0.2 Irgacure 651* 47.6 15 H 4.0 0.2 Irgacure 651 46.3 I 4.0 0.2 Thioxanthone 27.6 denotes 2%

In the above tests, the previously 20 identified spotlight was positioned at a distance of 1 foot above an about 40 mil thick layer of the liquid composition and the specimens were exposed for ~ - 90 seconds. As should be evident from the above tabulation, the invention produced a synergism in 25 which the presence of all three additions (dye, amine - -and photoinitiator) provoked a much more rapid and complete response to the visible light radiation.
Approximately the same results were obtained using an argon ion laser which emits in the range of 30 488-514 nanometers (a range which is generally blue to green). This laser contains no ultraviolet emission, so the minor proportion of such emission in - the previously described spotlight is concluded to not be of consequence.

- . :- . --: - ; . .- - -2~0331 , Further work was done to determlne whether essentially the same results could be obtained using photopolymerizable liquids of di~ferent composition.
Also, it was desired to further characterize the results obtainable herein by using the previously described spotlight both unmodi~ied and after using a filter to screen out any light in the unltraviolet range which was present therein. The results obtained in this fashion are tabulated in the Tables 10 which follow.
In the tests reported in Table II, below, 63~ of the liquid composition was constituted by the i diacrylate of trimethyl hexamethylene diisocyanate, 30~ was constituted by trimethylolpropane 15 triacrylate, and the remaining 7% was constituted by N-vinyl pyrrolidone. This provided a resin mix which was predominantly a urethane acrylate composition.

Table II
EVALUATION OF SYLVANIA SPOTLIGHT CURE
RESPONSE WITH AND WITHOUT UV COMPONENT
FILTERED OUT

Additions to Resin Mix ~ Conversion 25 Sample. % Dye % Amine % Irg 651 UV out ~ -- O O
2 -- --O 4.035.50 3 -- 4.2 4.082.7 0 4 0.4 -- -- O O
30 5 0.4 4.0 4.0100. 95.0 6 0.4 - 4.037.7 0 7 0 4 4.0 --99.8 64.9 8 0.4 2.0 4.0100. 100.

9 0.4 4.0 2.0100. 99.0 35 10 0.2 2.0 2.0100. 98'5 Z~00331 '10- -In the tests reported ln Table III, below, 30~ of the liquid composition was constituted by the epoxy diacrylate used in Table I, 25% was constituted by the diacrylate of the adduct of two molar proportions of toluene diisocysnate with one molar proportion of polyoxytetramethylene glycol (Adiprene L-200 was used), 25~ was the diacrylate of propoxylated neopenty glycol containing about two molar proportions of propylene oxide, and the 10 remainin8 20% was constituted by N-vinyl pyrrolidone. This provided a resin mix which was predominantly an acrylate ester composition.
Stock Resin Mix:

Table III
EVALUATION OF SYLVANIA SPOTLIGHT CURE
RESPONSE WITH AND WITHOUT UV COMPONENT
FILTERED OUT

Additions to Resin Mix % Conversion Sample. % Dye % Amine % Irg 651 UV out 12 _ _ 4 13 -- 4.0 4.0 2Z.l O
25 14 0.4 -- -- 3.1 0 15. 0.4 4.0 4.0 25.214.8 16 0.4 -- 4.0 11.5 0 -17 0.4 4.0 -- 18.015.3 18 0.4 2.0 4.0 40.318.3 30 19 0.4 4.0 2.0 22.514.1 -- 2.0 2.0 18.0 0 -21 0.2 2.0 2.0 65.535.8 22 0.4 2.0 2.0 46.220.1 As can be seen in the data tabulated above, "
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when all three additions have been made, the results are generally superior. On the otbor hand, the proportion of each component can still be varled to optimize the conversion of liquid monomer to S solvent-resistant solid.
As should be evident from the tables which have been presented, the invention produced a synergiso in which the presence of all three additions (dye, amine and photoinitiator) provoked a 10 much more rapid and complete response to the visible light radiation.
It will be observed that the irradiated specimens which have been described are incoopletely cured. Some of these were exposed to additional 15 irradiation to complete the cure rather than being subjected to extraction testing. In some instances ultraviolet light was used, and in other instances, the previously described spotlight which is rich in light in the visible range was used. In both 20 situations the additional exposure cured the specimens fully. However7 there were two advantages to using visible light.
First, the ultraviolet lamps used are expensive and their output deteriorates relatively 25 rapidly with time. Moreover, the ultraviolet lamps must be cooled. Then, safety factors demand that these lamps be appropriately shielded.
In contrast, the spotlights previously described are readily available, inexpensive and long 30 lasting. They need not be cooled, and there is no safety hazard. In addition, the use of light which - is predominantly in the visible range for postcuring the partially cured specimens produces a more uniform polymerization providing a cured product having 35 enhanced toùghness.