CA2092131A1

CA2092131A1 - Low viscosity self-toughening acrylate composition

Info

Publication number: CA2092131A1
Application number: CA002092131A
Authority: CA
Inventors: Victor Kadziela (Nmi); Donna M. Sutkaitis; Margaret A. Rakas
Original assignee: Individual
Current assignee: Henkel Loctite Corp
Priority date: 1992-03-27
Filing date: 1993-03-22
Publication date: 1993-09-28
Also published as: JPH0665338A; EP0562826A1

Abstract

LOW VISCOSITY SELF-TOUGHENING ACRYLATE COMPOSITION

ABSTRACT OF THE DISCLOSURE
Compositions comprising:
A) A mixture of acrylate functional prepolymers in the total amount of between about 50% and 80% by weight of the composition, said prepolymer mixture comprising a first prepolymer which is a urethane diacrylate resin, suitably a polyether urethane diacrylate, having a viscosity of at least 500,000 cps at 25°C in an amount of between 10 and 40% by weight of the composition, and a second multi-acrylate prepolymer having a viscosity of between about 500 and 50,000 cps in an amount of at least 15%
by weight of the composition;
B) A monoacrylate diluent monomer selected from the group consisting of isobornyl acrylate, .beta.-carboxyethyl acrylate and mixtures thereof in an amount of at least 5% by weight of the composition; and, C) An effective amount for cure initiation of a free radical initiator, the composition having a viscosity of less than 10,000 cps at 25°C, give unusual self-toughened cured properties. Such compositions, where the free radical initiator is a free radical photoinitiator, are particularly useful in rapid prototyping applications where the requirement for low viscosity and optical clarity of the liquid formulation has heretofore limited use of the formulations to ones giving undesirably brittle or soft properties.

Description

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LOW VISCOSITY SELF-TOUGHENING ACRYLATE COMPOSlTION

This invention relates to the field of curable acrylate prepolymer 5 formulations and more particularly to such formulations used in the rapid prototyping industry.
BACKGROUND OF THE INVENTION
The term "rapid prototyping," also referred to as "stereolithography"
denotes a group of processes for the production of complex solid 3-dimensional objects 10 by curing successive thin layers of photocurable liquid composition using a computer controlled laser to affect patterned photocuring of the liquid surface. Such techniques are described in detail in Murphy, et al, "Reduced Distortion in Optical Free Form Fabrications with UV Lasers", Radiation Curing, Feb-May 1989, pp. 3-7; Chemical Week9 Sept. 9, 1987, pp. 24-25; and in US patent 4,575,330; 4,801,447; 4,929,402;
15 and 4,752,498.
Liquid fonnulations used in rapid prototyping applications must have a low viscosity, no more than 10,000 cps at 25~C, preferably 2,000 cps or less, have low shrinkage, and give good cure response to the UV laser. Heretofore it has been conventional to use formulations employing various acrylate ester functional 20 prepolymers as the photocurable liquids for stereolithography because of the fast UV
response time and lack of curing outside the irradiation boundary which acrylate esters provide. Such formulations have also uniformly employed N-vinyl pyrrolidone (NVP) as a reactive diluent because of its low vapor pressure, high solvent power, high reactivity and low vapor reactivity.
Conventional acrylate ester prepolymer based rapid prototyping formulations however suffer several major drawbacks. Arnong these are the fact that N-vinyl pyrrolidone has been identified as a potentially substantial risk material which has been banned in many industrial facilities worldwide. There therefore exists a need to identify satisfactory substitute formulations which do not employ NVP.
Another drawback is that the cured properties provided by the low viscosity acrylic ester prepolymer based forrnulations are very brittle so that the parts produced by the technique are highly susceptible to brealcage when dropped 2 ~

or when used as a working part~ While one manufacturer has recent]y introduced a product said to have "tough" rured properties, the properties thereof are moreaccurately described as soft and elastic during the stereolithographic curing process.
The modulus builds up more slowly than the formulations of the invention disclosed herein, and is not suitable for producin~ workin~ parts.
There therefore is a need for low viscosity rapid prototypin~
formulations which give cured hard and tough properties so that parts made therefrom are suitable for making working parts not susceptible to breakage.
Unfortunately conventional techniques for toughening of curable liquid acrylic farmulations are not available because of the limitations imposed on the uncuredliquid by the rapid prototyping application. For instance, introducin~ tou~heningly effective amounts of rubbery polymers increases the viscosity ~eyond acceptable ]imits. Usin~ two-phase systerns, whether it be a reinforcing filler or an ineompatible second curable liquid reduces optical clarity of the formulation which is detrirnental to UV response of the system.

SUMMARY OF THE INVENTION
In seeking to prepare a formulation which is free of NVP, the inventors hercin have discovered a low viscosity UV curable system which not only meets their objectives bu~ also has unexpected self-toughening properties which ~ive them substantial benefits over prior formulations designed for rapid prototypin~.
Compositions within the invention are characterized as follows:
A) A mixture of acrylate functional prepolymers in the total amount of between about 50% and 80% by wei~ht of the composition, said prepolymer mixture comprisin~ a first prepolymer which is a urethane diacrylate resin having a viscosity of at Jeast 500,000 cps at 25C in an amount of between 10 and 40% by wei~ht of the composition, and a second multi-acrylate prepolymer havin~ a viscosity of between about bOO and 45,000 cps at 25C in an amount of at least 15% by weight of the composition;
B) A monoacrylate diluent monomer selected from the group consisting of isobornyl acrylate, ~-carboxyethyl acrylate and mixtures thereof in an amount of at least 5% by weight of the composition; and, C) An effective amount for cure initiation of a free radical initiator, the composition havin~ a viscosity of 3ess than 10,000 cps at 25C.
In a narrower characîerization of the invention the first prepolymer is a polyethcr urethane diaclylate, the second prepo3yrner is selected from the group consisting of polyester urethane acrylates, polyether urethane acrylates and epoxy S aclylates, and the composition, when cured, has a tensile stress strain curve displaying a true yield point followed by a region of plas~ic deformation. Preferab]y the second prepolymer has a viscosity at 25C of 6,000-15,000 cps.
DEI~ILED DESCRII'TION OF TI~E INVENTION
C`ompositions of the invention are free of NVP. Nevertheless, they give good 10 response to UV lasers such as the 325nm He Cd ]aser utilized in the SLAI~ system sold by 3D Systems, Inc., Valencia, CA. The viscosities of the formulations are all in the ran~e usable for rapid prototyping applications, i.e., less than 10,000 cps, preferably less than 4000 cps, more preferably 500-2000 cps.
The preferred compositions also display low shrinka~e in ihe 4-S~o ran~e, 15 compared to typical values of 9-10% for acrylic formulations.
Whereas the conventional acrylic formulations used in UV processing are extremely brittle and do not yield, the formulations of the invention, when cured, give products which display true yield points (as defined in ASTM D638M, annex A1.10), without the necessity of adding reinforcin~ fillers or solid rubbery polymers 20 which increase viscosity and opacity of the system.
The inventors have undertaken extensive investi~ations to characterize the reason for self-toughening behavior, to date, without any conclusive explanation.
The Tg behavior of the cured formulations of the invention is not consistent with random copolymerization, but the inventors have also been unable to determine that the system is 2-phase. The formulatians are optically clear, which is extremely unusual in 2-phase systems, and only a single T~ is observed. In forlnulations which utilize isobornyl acrylate as the diluent, moreover, the tou~6henin~ behavior observed is particularly surprising since isobornyl acrylate gives a very hard ~lassy polymer having a glass transition ternperature of 92C.
The free radical initiator is preferably a photoinitiator or mixture of photoinitiators. Photoinitiators sui~ably employed in the preferred formulationsinclude quinones, acetophenones, propiophenones, benzophenones, acyl phosphine oxides, thioacyl phosphines, titanocenes, etc. A more detailed list of suitable 2 ~

photoinitiators, including specific examples, is found in EP 0425440. The photoinitiator component in the invenlive compositions may also suitably be a blend of photoinitiators which optimizes the curing response of the formulation to two or more UV-vis emission lines of a laser employed in a rapid prototyping device. Such blends S of photoinitiators are described, for instance, in Sherman, A., "Balancing Formulation Storage Stability and Cure Behavior Through the Use of PhoLoinitiator Blends,"
Radcure '86 Conference Proceedings, pp 4-13 - 4-25, (Sept 8-11, 1986), in US
4,017,652 and in EP 04225440. Suitable commercial photoinitiator blends include Darocure 4263 and Darocure 4265, sold by EM Industries. The photoinitiators are employed at levcls of 0.1-10% typically 0.5-5% of the composition. For uses outside tl1e field of rapid prototyping, thernal initiators such as peroxides, hydroperoxides, peresters, azonilriles~ etc. may also be employed at conventional levels in the self-toughening formulations of the invention.
While the invention is characterized by the essential components set ~orth above, it will be appreciated that other components which do not materially change its essential novel characteristics, may be included in the compositions. For instance, a minor amount, suitably 1-15%, of a methacrylate resin has been found useful to avoid fo~nation of multiple unbonded ~Ibers in a single line of the pattern made by the laser.
A suitable such methacrylate resin is a 2000 MW (M~) polyester urethane methacrylate (PEUMA).
Tri-, tetra- or higher functional acrylate or methacrylates may be employed to increase the crosslinlc density of the formulations. To maintain the desired yield point in the cured polymer, however, such material should ordinarily not exceed about 10%, preferably 0.5-5% by weight of the composition.
Minor amounts, preferably less than 10%, of other mono-acrylate or mono-methacrylate diluent monomers may also be employed in the composition in addition to those required for component B of the formulation. Examples of such optional monomers include hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxyethyl acrylate and hydroxyethyl methacrylate.
Other additives conventionally employed in a curable acrylic formulation may also be employed, at conventional levels, provided, however, that for rapid prototyping formulations the requirements of low viscosity and adequate optical clarity must be mairltained. Examples include antioxidants, light protectors, 3 ~

polymeri~ation inhibitors, de~assing a~en~s, deaeratin~ a~ents, plasticizers, extenders, fi!lers, and reinforcin~ agents, thixotropy-regulating agents, wetting and leveling a~ents, f3ameproofing agents, sensitizers, oxygen traps, antisettlin~ agents, dyes, or pigments.
Tlle rapid-prototyping formulations of the invention are preferably cured usin~ weaving techniques. Such techniques, in whicll the laser tracin~ program is designed to produce interweaves between successive layers of the part being created are known to those skilled in the art. Details of one such increasin6 technique may be found in "SLA User Reference Manual, Software Release Version 3.7.1.7, release 3.82," ~3D Systems, 1990 and in Richter et al, '~he Present State Of Accuracy In Stereolithography," 3D Systems, Inc., Valencia CA (Apri], 1991) .
The invention is illustrated by the followin¢ nonlimitin~ examples.

EXAMPLE~S
Formulations havin~ viscosities and reactivities suitable for rapid prototypin~
applications were prepared as shown in Table I where quantities are ~iven in parts by wei6ht. Property measurements were made on cured films according to the following protocol and the results of the measurements are provided in Table II.Film Prepar~tio l: All samples were prepared as films of 20 mil nominal thickness. Films were cured in a conveyerized UV oven (UV Fusion System). Each film received a total dose of 2535 mj/cm2.
Tcnslle Samples: Samples from cured films were routed to widths of 0.25 inches using an ASTM D 882-83 routing jib. A modified version of this method wasused: testing fol]owed the specified width: thickness ratio as well as the crosshead speed ~uideline. Since the ~a~e length was 1.97 inches (nonstandard length), a crosshead speed of 0.2 in/min was employed until 25% elon~ation had occurred.
At that point, the crosshead speed was increased by a factor of 10, to 2.0 inlmin.
Samples were discarded if it was obvious that slippaL~e had occurred. An Instron4405 Universal Tester was used, and depending on the sample, either a 22.4 Ib ora 50 ]b load cell was used. It was standard practice to use the most sensitive load cell available; if samples fractured at, say 35 Ibs, of course the 5Q Ib cell had to be used. Instron series IX software was used while testing to determine modulus, %
strain at yield point, etc.

2 ~ 3 1 Dynamic Mcchnnic~l An~lysis: A Rheometrics Dynamic Mechanical Analyzer was used to determine the Tg of all samples except Examples 1-3 series,which were tested on a Rheometrics Solids Analyzer. A strain sweep, and in some cases two strain sweeps were performed on each sample to determine its linear 5 elastic region [LER]. When that % strain range was known, 1 or two temperaturesweeps were run wilhin the noted strain limit. A frequency of 10 rad/sec was used for all samples unless otherwise noted.
: . __ _ . _.~.A._ . .. _______ , . .
TABLE 1- I~ORMULATIONS
Example lt 2 3t 4 5 6 7t 8 ~ . .
Photomer 6008 Polyether 9.154.1 65 11) 16.0 16.0 13.513.5 Urethane Acrylate .
Radcure RSX
89462 Polyester 40 40 0 40 64 64 54.1 S4.1 Urethane Acrylate . _ Isobornyl 50 5 34.1 34.1 18.1 9.1 0 27 Acrylate .. . .
PEUMA* 15 1.0 10.0 4.5 4.5 Photoinitiator~ 0.9 0.9 _ 0.9 0.9 0.9 0.9 0.9 0.9 2~

TAI~LE 1 - FORMULATIONS (CONTINUED) Example 9 10 11 12 13 14~ 15 16t Photomer 600P~
Polyether 33.6 33.5 30.0 33.6 30.0 13.5 13.5 13.5 Urethane Acrylate Radcure RSX
Urethane 54.1 54.1 54.1 Acrylate _ Pholomer 6210 Polyether l5.û 15.0 15.0 15.0 Urethane Acrylate Ebecryl 830 Polyester . 15.0 Acrylate _ BASF Laromert~

Polyester 10 Acrylate _ _ Photomer 4127 Propoxylated 10 0 Neopentyl Glycol .
Diacrylate . ..
Isobornyl 26.0 36.0 36.0 26.0 36.0 Acrylate _ -Carboxyethyl 14.5 14.5 14.5 14.5 14.5 27.0 ry a e Dihydrocyclo-pentadienyl- 27.0 oxyethyl Acrylate . .. _ . _ Acrylate 27.0 ._ PEUMA* . 4.5 4.5 4.5 _ .
Photoinitiatort 0.9 - - 0.9 4.5 0.9 4.5 0.9 0 9 0.9 ~Comparative examples.
* Urethane methacrylate capped 2,000 MW (Mn) polyester.
40 ~ In examples 11 and 13 the photoinitiator was 1-benzoyl cyclohexanol. In all others the photoinitiator was 2,4,6-trimethylbenzoyldiphenylphosphineoxide (TPO).

. _ _ n . . .

Example ~ 2 3 4 5 6 7 8 Tg 49.3 43.1 40 46 46 _ . _. ._ _ .. __ _ psi x100 2197 18772460 1278 986 880 581 1200 . ~. , __ . ,_ Yield Stress, psi 59785068 7115 3942 2784 2565 3426 ... _ __ ._ Elongation at 4.97 5.34S.32 5.02 7.60 8.28 7.03 . _ .. _ _.
break, % 5.55 17.035.51 24.70 46.80 52.40 45.50 .. __ .. __ .__ (br-Eyy~br 0.105 0.686n.o34 0.797 0.838 0.842 0.845 . . ~ . _ ..
deforrnation N Y N Y _ Y _ TABLE 2 - PROPERTIES (CONTINUED) Example 10 11 12 13 14 15 16 .. __ ._ _ _ Modulus 1091 13071037 1204 2315 244 840 19.9 .. _ _ ._ ._ Yield Stress, psi 2390 27132356 2507 6039 NA 2354 NA
.. _ ._ __ ...
Elon3ation at 6.27 5.046.50 6.00 5.38 NA 7.40 NA
_ . _ Elongation at 36.25 29.73 67.1 26.52 9.76 55.7 92.3 59.5 .. _ _ (~ b;E y)/Ebr 0.83 0.83 0.9 0.77 0.45 NA 0.92 NA

The Photomer prepolymers utilized in the examples are trademarked products of Henkel Corp. Photomer 6008 is a aliphatic polyether urethane 30 diacry]ate oligomer resin having a viscosity at 25C of 1,000,000-1,750,000 cps.
Photomer 6210 is an aliphatic polyurethane diacrylate oligomer liquid havin~ a viscosity of 13,000 cps at 25C and a MW of about 1,400.
Radcure-" RSX-89462 is an aliphatic polyester urethane diacrylate prepolymer having a molecular weight of approximately 1,400 and a viscosity of 35 6,000 cps at 25C. It's NMR proGle indicatcs that thcrc is an acyclic carbamate group a~jacent to the ac~ te end ~roups. The pro~luct is sold by Radcure Specialties Inc., Louisville KY.
BASF Laromer~ LR 8765 is an epoxy acrylate oligomer (i.e., reaction product of an epoxy terminated oligomer and acrylic acid) sold by BAS~ which has5 a viscosity of approximately 600 cps at 25C.
EbeclylT" 830 is a polyester acrylate oli~omer, sold by Radcure Specialties, Inc., Louisville KY, which has a molecular weight of approximately 1400 and a viscosity of approximately 45,000 at 25C.
Referrin~ to tlle tables, it can be seen that the products of Examples 1 and 10 3 displayed a yield point which was very close to the break strain so that there was essentially no plastic deformation before break. The value of (br-~y)/br was well below the minimum required by the invention. This is a complex mixture phenomena as illustrated by the very different properties, within the scope of the invention, obtained with Examples 2, 4-7 and 8. The preferred formulation for 15 rapid prototypin~ applications is Example 8.
Examples 14-16 illustrate the effect of chan"ing monoacrylate monomers in the preferred formulation. The monoacrylate monomers used were commercially available low vapor pressure acrylic monomers. The formulation of Example lS, using ~-CEA as a replacement for Isobornyl acrylate ~ave acceptable properties.
20 However, the cured formulations of Examples 14 and 16, using other low vapor pressure acrylate monomers, gave unacceptable soft, rubbery cured materials.
In Examples 9-13, a mixture of isobornyl acrylate and ~-CEA was successfully used to prepare formulations displaying desirable yield points followed by a re~ion of plastic deformation. These Examples also demonstrate that the polyester 25 urethane acrylate used in the other Examples can be replaced in whole or in part with other low viscosity diacrylate resins withollt loss of critical self-tou~henin~
properties. It was notcd, however, that the product of Example 13 was near the approximately lower limit of practical benefit from a self-toughenin~ standpoint, over typical acrylate-based rapid prototyping formulations. Thus, a reasonable lower 30 limit for the value (b,-ey)/br is approximately 0.4. More preferably the value of (b,-yyb, is at least 0.65.

Claims

1. A low viscosity curable composition comprising:
A) A mixture of acrylate functional prepolymers in the total amount of between about 50% and 80% by weight of the composition, said prepolymer mixture comprising a first prepolymer which is a urethane diacrylate resin having a viscosity of at least 500,000 cps at 25°C in anamount of between 10 and 40% by weight of the composition, and a second diacrylate prepolymer having a viscosity of between about 500 and 50,000 cps in an amount of at least 15% by weight of the composition;
B) A monoacrylate diluent monomer selected from the group consisting of isobornyl acrylate, .beta.-carboxyethyl acrylate and mixtures thereof in an amount of at least 5% by weight of the composition; and, C) An effective amount for cure initiation of a free radical initiator, the composition having a viscosity of less than 10,000 cps at 25°C.

2. A composition as in claim 1 wherein the first prepolymer is a polyether urethane diacrylate.

3. A composition as in Claim 2 wherein the second prepolymer is selected from the group consisting of polyether urethane acrylates, polyether urethane acrylates and epoxy acrylates, and the composition, when cured, has a tensile stress strain curve displaying a true yield point followed by a region of plastic deformation.

4. A composition as in Claim 3 wherein said second prepolymer has a viscosity in the range of 6,000-15,000 cps at 25°C.

5. A composition as in Claim 3 wherein said second prepolymer is a diacrylate.

6. A composition as in Claim 3 wherein the yield stress of the cured polymer is less than about 6,100 psi.

7. A composition as in Claim 6 wherein the yield stress is less than about 5,500psi.

8. A composition as in Claim 1 wherein the cured polymer has a stress strain curve satisfying the formula:
(.epsilon.br-.epsilon.y)/.epsilon.br ?0.4 where .epsilon.br is percent strain at break, and .epsilon.y is percent strain at yield.

9. A composition as in Claim 8 wherein (.epsilon.br-.epsilon.y)/.epsilon.br ?0.65.

10. A composition as in Claim 1 wherein the monoacrylate diluent monomer is isobornyl acrylate and is present at a level of 50% or less.

11. A composition as in Claim 1 wherein the monoacrylate diluent monomer is a mixture of isobornyl acrylate and .beta.-carboxyethyl acrylate.

12. A composition as in Claim 1 wherein the monoacrylate diluent monomer is .beta.-carboxyethyl acrylate.

13. A composition as in Claim 1 wherein said free radical initiator is a photoinitiator.

14. A composition as in Claim 13 wherein the second acrylate functional prepolymer is a polyester urethane acrylate.

15. A composition as in Claim 14 wherein the ratio of said polyester urethane acrylate to the monoacrylate diluent monomer is greater than 0.8.

16. A composition as in Claim 13 wherein the second acrylate functional prepolymer is a polyether urethane acrylate.

17. A composition as in Claim 13 wherein the second acrylate functional prepolymer is a epoxy acrylate.

18. A composition as in Claim 13 wherein said photoinitiator is a blend of photoinitiator compounds selected to optimize the response of thc composition toat lease two emission lines in the UV-vis wavelength range of a predetermined light source.

19. A composition as in Claim 1 further comprising a methacrylate compound in an amount of less than 10% by weight of the composition.

20. A composition as in Claim 19 wherein said methacrylate compound is a dimethacrylate compound and is present at a level of 0.5-5% by weight of the composition.

21. A composition as in Claim 1 wherein said composition viscosity is less than 4000 cps.

22. A composition as in Claim 21 wherein said composition viscosity is in the range of 500-2000 cps.

23. In a method of rapid prototyping utilizing a computer-controlled laser to effect patterned photocuring at the surface of the body of a photocurable liquid in successive layers so as to build up a three-dimensional object, the improvement wherein said curable liquid is:
A) A mixture of acrylate functional prepolymers in the total amount of between about 50% and 80% by weight of the composition, said prepolymer mixture comprising a first prepolymer which is a urethane diacrylate resin having a viscosity of at least 500,000 cps at 25°C in anamount of between 10 and 40% by weight of the composition, and a second diacrylate prepolymer having a viscosity of between about 500 and 50,000 cps in an amount of at least 15% by weight of the composition;

B) A monoacrylate diluent monomer selected from the group consisting of isobornyl acrylate, .beta.-carboxyethyl acrylate and mixtures thereof in an amount of at least 5% by weight of the composition; and, C) An effective amount for cure initiation of a free radical initiator, the composition having a viscosity of less than 10,000 cps at 25°C.