WO1990004211A1

WO1990004211A1 - Synergistic initiator compositions for the visible light fabrication and postcure of stereolithographic objects

Info

Publication number: WO1990004211A1
Application number: PCT/US1989/004439
Authority: WO
Inventors: Edward J. Murphy; John J. Krajewski; Robert E. Ansel
Original assignee: Desoto, Inc.
Priority date: 1988-10-11
Filing date: 1989-10-05
Publication date: 1990-04-19
Also published as: CA2000331A1

Abstract

A liquid composition photopolymerizable with visible light comprises a photopolymerizable ethylenically unsaturated liquid which is polymerizable by free-radical polymerization containing, based on the weight of the photopolymerizable ethylenically unsaturated material contained therein: (1) from about 0.1 % to about 5 % of a photoreducible phthalein dye; (2) from about 0.5 % to about 10 % of an aliphatic tertiary amine; and (3) from about 1 % to about 8 % of a ketonic photoinitiator. This is a composition suitable for many purposes including coating operations and preferably a stereolithographic process.

Description

SYNERGISTIC INITIATOR COMPOSITIONS FOR THE

VISIBLE LIGHT FABRICATION 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 postcuring of stereolithographic objects. Background Art

Ultraviolet-curable coatings based on acrylate-terminated polyurethanes are well known. While acrylate-functional ethylenically unsaturated 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 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, 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 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 unsaturated material using light in the visible range, it is known that the free radical polymerization of ethylenically 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. Makromolekulare Chem,. £7, 201 (1981).

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 visible irradiation with increased efficiently. (See 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 dimethoxyphenyl acetophenone. [See 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 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 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. Additionally, it is found that relatively thick layers can be converted so that visible light laser fabrication of (meth)acrylate-functional liquids can be carried out. Moreover, and having in mind the fact that stereolithographically-formed objects are 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 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 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 10%, 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 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 these compositions in a stereolithographic proce s in 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 objects 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, 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 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 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 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 (200-400 nanometers), and the balance of the light is in the range of from over 400 to about 650 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 (forming an about 40 mil layer in the bottom of a 50 ral 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 also 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 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 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 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 25°C. At the end of this period what remains of the layer is removed, dried 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 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 alkanol 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 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. The 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 is us.eful 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 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 dissolved in a liquid mixture comprising a poly(raeth)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 ethylene oxide or propylene oxide. These preferred compositions are illustrated in the example. In stereolithography it will also be understood that the liquid compositions are irradiated to form a three-dimensional object 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-removed can be allowed 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 (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 number average molecular weight of 428, 30% neopentyl glycol propoxylate diacrylate having a number average molecular weight of 428, and 20% of N-vinyl pyrrolidone.

The results of experiments using 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. Table I

DYE SENSITIZED VISIBLE LIGHT EXPOSURE

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.

35 Further work was done to determine whether essentially the same results could be obtained using photopolymerizable liquids of different composition. Also, it was desired to further characterize the results obtainable herein by using the previously described spotlight both unmodified 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 which follow.

In the tests reported in Table II, below, 63% of the liquid composition was constituted by the diacrylate of trimethyl hexamethylene diisocyanate, 30% was constituted by trimethylolpropane 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

In the tests reported in 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 diisocyanate 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 0 remaining 20% was constituted by N-vinyl pyrrolidone. This provided a resin mix which was predominantly an acrylate ester composition. Stock Resin Mix:

35 As can be seen in the data tabulated above, when all three additions have been made, the results are generally superior. On the other hand, the proportion of each component can still be varied to optimize the conversion of liquid monomer to solvent-resistant solid.

As should be evident from the tables which have been presented, the invention produced a synergisra in which the presence of all three additions (dye, amine and photoinitiator) provoked a much more rapid and complete response to the visible light radiation.

It will be observed that the irradiated specimens which have been described are incompletely cured. Some of these were exposed to additional 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 situations the additional exposure cured the specimens fully. However, there were two advantages to using visible light.

First, the ultraviolet lamps used are expensive and their output deteriorates relatively 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 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 enhanced toughness.

Claims

WHAT IS CLAIMED IS:

1. A liquid composition which is photopolymerizable with visible light comprising a photopolymerizable ethylenically unsaturated liquid which is polymerizable by free-radical polymerization containing, based on the weight of the photopolymerizable ethylenically unsaturated material contained therein:

(1) from about 0.1% to about 5% of a photoreducible phthalein dye;

(2) from about 0.5% to about 10% of an aliphatic tertiary amine; and

(3) from about 1% to about 8% of a ketonic photoinitiator. 2. A photopolymerizable liquid as recited in claim 1 in which said photoreducible phthalein dye is used in an amount of from 0.

2% to 1.0%.

3. A photopolymerizable liquid as recited in claim 1 in which said tertiary amine is used in an amount of from 1% to 4%.

4. A photopolymerizable liquid as recited in claim 1 in which said tertiary amine is an alkyl, cycloalkyl, or alkanol amine containing from 1 to 12 carbon atoms.

5. A photopolymerizable liquid as recited in claim 1 in which said ketonic photoinitiator is present in an amount of from 2* to 4%.

6. A photopolymerizable liquid as recited in claim 4 in which said ketonic photoinitiator is a diaryl ketone or an acetophenone compound.

7. A photopolymerizable liquid as recited in claim 1 in which said ketonic photoinitiator is an acetophenone compound.

8. A method of forming a three-dimensional object comprising, providing a bath of liquid ethylenically unsaturated material which is polymerizable by free-radical polymerization containing, based on the weight of the ethylenically unsaturated material contained therein: (1) from about 0.1% to about 5% of a photoreducible phthalein dye;

(2) from about 0.5% to about 10% of an aliphatic tertiary amine; and

(3) from about 1% to about 8% of a ketonic photoinitiator, exposing the surface of said bath to a beam of light to solidify the liquid near said surface to form a plurality of superposed solid layers providing a three dimensional complexly-shaped polymeric object constituted by incompletely polymerized solid polymer in said bath, removing said object from said reservoir, and then completing the cure of said object by exposing said object to light in the visible range for a time sufficient to strengthen the same.

9. A method as recited in claim 8 in which any polymerizable liquid clinging to the exterior of the object which is removed from said bath is drained off prior to completing the cure of said object.

10. A method of forming a three-dimensional object comprising, providing a bath of liquid ethylenically unsaturated material which is polymerizable by free-radical polymerization containing, based on the weight of the ethylenically unsaturated material contained therein:

(1) from about 0.1% to about 5% of a photoreducible phthalein dye;

(2) from about 0.5% to about 10% of an aliphatic tertiary amine; and (3) from about 1% to about 8% of a ketonic -14- photoinitiator, exposing the surface of said bath to a beam of light constituted by light which is predominantly in the visible range to solidify the liquid near said surface and form a plurality of superposed solid layers providing a three dimensional complexly-shaped polymeric object constituted by incompletely polymerized solid polymer in said bath, removing said object from said reservoir, and then completing the cure of said object.

11. A method as recited in claim 10 in which the cure of said object is completed by exposing said object to light in the visible range to strengthen the same.

12. A method as recited in claim 8 in which said ethylenically unsaturated material which is polymerizable by free-radical polymerization comprises (meth)acrylate-functional material.

13. A method as recited in claim 12 in which said methacrylate material comprises resinous poly(meth)acrylate dissolved in a liquid poly(meth)aerylate.