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Holographic cure monitoring of the DuPont SomosTM 7100 stereolithography resin
Optics and Lasers in Engineering 31 (1999) 239}246
Holographic cure monitoring of the DuPont Somos2+ 7100 stereolithography resin M. Farsari, S. Huang, R.C.D. Young, M.I. Heywood, C.D. Brad"eld, C.R. Chatwin* School of Engineering, University of Sussex, Falmer, Brighton BN1 9QT, UK Received 17 July 1998; accepted 23 November 1998
Abstract The optical characteristics of a commercial UV curable resin are investigated using nondegenerate four-wave mixing. The material assessed is an epoxy resin, DuPont Somos2+ 7100. The holographic gratings were written at a wavelength of j"351.1 nm for an irradiance range 0.5}3.0 W/cm and read at j"632.8 nm in order to assess the reactivity, curing speed, shrinkage and resolution of the resin. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction In order for a photopolymer to be suitable for stereolithography applications, several requirements must be ful"lled. The material should have good spreading characteristics, low viscosity and low shrinkage from liquid to solid [1,2]. In addition, it must have high resolution to permit a good build accuracy, as single parts must be built with small dimensions [3]. In this work we investigate one commercially available photopolymer, the DuPont Somos2+ 7100. This is a special epoxy resin, which polymerizes with the help of a cationic photoinitiator. Its composition is proprietary, but it is optimized for use with an argon-ion laser at j "351.1 nm. We perform holographic measurements on it in order to investigate its reactivity, polymerization rate and extent of volume shrinkage during polymerization at this wavelength [4]. 2. Theory of four-wave mixing To begin with, consider the case in which a radiation curable resin is illuminated by two beams of light of the same wavelength and polarization, I and I . The two beams 1 0 * Corresponding author. Tel.: 0044 1273 678901; fax: 0044 1273 690814; e-mail: [email protected] 0143-8166/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 01 4 3-8 1 66 ( 9 8) 0 0 04 6 - 3
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M. Farsari et al. /Optics and Lasers in Engineering 31 (1999) 239}246
overlap on the sample, generating a light pattern I(x)&I (1#m cos(kx)), (1) where I "I #I , m"2(I I ) (I #I )\ is the modulation index and k is the 1 0 0 1 0 1 spatial frequency of the grating. Such a time-independent intensity pattern consists of light and dark planes throughout the region of intersection, with a spatial wavelength K given by % j , K " (2) % 2n sin(h) where j is the wavelength of the light, n the refractive index of the resin and the angle between the two beams is 2h. As a result of the spatial non-uniformity of the illumination produced by the two writing beams, the material will polymerize only in the bright zones, where the irradiance is higher than the polymerization threshold E of the polymer. This leads A a refractive index grating in the material. The refractive index grating has been shown to be a periodic function, with the same period as the illumination pattern [5]. This grating can be probed by a third beam of a di!erent wavelength j such that no interaction between the beam and resin takes place. The read beam is di!racted by the grating with an e$ciency g that is de"ned as the ratio of the di!racted beam to the incoming beam. For maximum di!raction e$ciency, the probe beam angle h should satisfy the Bragg relation 2K sin h "j . (3) % The four-wave mixing experiment is described extensively in Refs. [5, 6] (Fig. 1). The change in the refractive index is due to the change of the density of the material (shrinkage). There has been no proof of a direct relation between volume shrinkage and curl occurring in the cured material, mainly because it is almost impossible to "nd
Fig. 1. Four-wave mixing.
M. Farsari et al./Optics and Lasers in Engineering 31 (1999) 239}246
241
material formulations that di!er only in curl shrinkage, while leaving other parameters unchanged [1]. It is, however, safe to assume that a high di!raction e$ciency would indicate high curl during building. In general, the shrinkage su!ered by a component built with stereolithography varies according to the exact build conditions.
3. Sample preparation The samples were prepared and all measurements taken in an environment controlled by air conditioning with a temperature of ¹"30$13C and a humidity of approximately 35$5%. The material was initially heated to 403C, to increase its uniformity. The preparation of the samples involved placing a drop of resin between two glass plates separated by 0.1 mm thick Mylar spacers. All the measurements were taken within 4 h of the manufacture of the samples.
4. Experimental set-up Our experimental set-up is shown in Fig. 2. An Ar-ion laser with a beam diameter of 1.57 mm operating on the 351.1 nm line and with polarization parallel to the plane of incidence was used to polymerize the resin. The laser beam was split into two equal power beams using a quartz cube beam splitter. The beams are controlled by an electronic shutter with a closure response time of 6 ms. The grating is read using a 5 mW He}Ne laser operating at j"632.8 nm, with the plane of polarization perpendicular to the plane of incidence. The resins are not reactive at this wavelength and as a result this beam does not modify the grating by causing further curing. The di!racted beam is detected using a silicon photodiode, connected to a LeCroy 9361 digital oscilloscope.
Fig. 2. Experimental set-up for non-degenerate four-wave mixing.
242
M. Farsari et al. /Optics and Lasers in Engineering 31 (1999) 239}246
If the response of the material is a linear function of the beam intensity, the light pattern produces a refractive index grating n(x)"n #*n(1#cos(kx)), (4) where *n is the amplitude change in the refractive index. The grating di!raction e$ciency g is de"ned as the ratio of the intensity of the di!racted beam over the intensity of the incident beam. If *n is small and there is no change in the absorption constant, a, of the material during polymerization, then g can be approximated by the Kogelnik equation, (7), thus g"exp(!ad/cos h)sin
p*nd , j cos h
(5)
where d"100 lm is the thickness of the sample and h is the angle of incidence of the reading beam. The induced refractive index change in UV curable photopolymers is in general nonlinear with respect to the illuminating irradiance. The refractive index grating that results from the sinusoidal fringe pattern generated by the interference of the two writing beams can be more closely represented as a square wave grating which leads to higher-order terms in the wave di!racted from the grating [5]. However, it has been shown [8] that for small grating amplitudes, the Kogelnik formula is a good approximation for the relation between the "rst-order di!raction e$ciency g and the amplitude of the "rst harmonic grating *n . 5. Results Gratings with spacing as low as 0.35 lm were successfully written, indicating that the material has excellent de"nition/resolution properties for stereolithography. In order to calculate the grating spacing K and Bragg angle h from Eqs. (2) and (3) % respectively, the refractive indices of the resin must be known. It was measured, using an AbbeH refractometer, to be n"1.51. Fig. 3 shows the time history monitoring diagrams of the photopolymer where the di!raction e$ciency is plotted versus irradiation time. The response of the material is presented as a normalized logarithmic plot of the evolution of the di!raction e$ciency over a time period of 4 s. The power of each of the writing beams was 30 mW and the angle between the two writing beams was 2h"123. Plotting di!raction e$ciency versus time provides information about the delay time until the onset of di!raction, the "nal value of the di!raction e$ciency and the rise time of di!raction [1,4]. These parameters are discussed in the following subsections. 5.1. Delay The delay time t characterizes an epoxy resin system in terms of its photosensi tivity. The rise time to 10% of the maximum di!raction e$ciency versus the beam
M. Farsari et al./Optics and Lasers in Engineering 31 (1999) 239}246
243
Fig. 3. Normalized di!raction e$ciency versus time.
irradiance for the two photopolymers under investigation is plotted in Fig. 4. It can be seen that, under the experimental conditions employed, the DuPont Somos2+ 7100 delay reduced signi"cantly with increased irradiance. 5.2. Diwraction ezciency The di!raction e$ciency of the photopolymer was measured for di!erent beam powers and grating spacing, 30 s after opening the beam shutter, when the irradiance of the di!racted beam had reached a constant value. It was observed that the irradiance did not a!ect the di!raction e$ciency for the range of beam powers, 0.5}3.0 W/cm, investigated. The variation of the di!raction e$ciency with grating spacing is shown in Fig. 5. From Fig. 5 it is observed that the di!raction e$ciency decreases when the spatial wavelength decreases. This is due to the reduction of the modulation index, caused by the limited resolution of the material. Overall, the resolution of the material investigated was exceptionally high. Speci"cally, it was possible to write gratings with a fringe period of 0.35 lm, which corresponds to a feature size of 0.175 lm. Since most stereolithographic machines require an accuracy in the order of fractions of a millimeter, the resolution of the investigated resins is more than adequate for their intended purpose. Even though the di!raction e$ciency value presented here was measured after 30 s of exposure, it was observed that it had reached that value after approximately 4 s of exposure (Fig. 3). This implies that, even in the parts of the grating that the irradiance was close to the critical value E , it only took approximately 4 s for the material to A cure fully.
244
M. Farsari et al. /Optics and Lasers in Engineering 31 (1999) 239}246
Fig. 4. Dependence of the di!raction delay on the writing beam irradiance.
Fig. 5. Dependence of the di!raction e$ciency on the spatial wavelength.
5.3. Rate The rate at which di!raction increases is an indication of the reactivity of the resin. This gives information about the reaction rate of the double bonds of the monomers
M. Farsari et al./Optics and Lasers in Engineering 31 (1999) 239}246
245
Fig. 6. Dependence of the di!raction rate on writing beam irradiance.
and oligomers that constitute the resin. Unfortunately, since the composition of the materials is propriety to DuPont, it is not possible to calculate how many reactions correspond to each photon of UV radiation. The di!raction rate as a function of the irradiance of the writing beams is shown in Fig. 6. It can be seen that the di!raction rate of the DuPont Somos2+ 7100 increases dramatically with irradiance.
6. Conclusions The DuPont Somos2+ 7100 demonstrated its ability to deliver high build resolution and high curing speed and is therefore suitable for stereolithography applications. The curing time of the material decreased rapidly with increased irradiance, while its shrinkage was not a!ected, for the irradiance range investigated.
References [1] Jacobs PF. Rapid prototyping and manufacturing, CASA-SME, 1992. [2] Schulthess A, Steinmann B, Hofmann M. New applications for Cibatool SL epoxy resins, Proc. 28th Int. Symp. on Automotive Technology and Automation-Rapid Prototyping in the Automotive Industries, 1995. p. 41}50. [3] Bertsch A, JeH zeH quel JY, AndreH JC. Study of the spatial resolution of a new 3D microfabrication process: the microstereophotolithography using a dynamic mask-generator technique. J. Photochem. Photobiol. A 1997;107(1}3) (1997):275}81. [4] Hunziker M, Bernhard P. Development of resin systems for StereoLithography: Holographic cure monitoring. Proc. 1st Nat. Conf. Rapid Prototyping, Dayton, OH, 1990. p. 79}85. [5] CarreH C, Lougnot DJ, Fouassier JP. Holography as a tool for mechanistic and kinetic-studies of photopolymerization reactions * a theoretical and experimental approach. Macromolecules, 1989;22:791}9.
246
M. Farsari et al. /Optics and Lasers in Engineering 31 (1999) 239}246
[6] BrauK chle C, Burland DM. Holographic methods for the investigation of photochemical and photophysical properties of molecules. Angew. Chem. Int. Ed. Engl. 1983;22(8):582}98. [7] Kogelnik H. Coupled wave theory for thick hologram gratings. Bell. Syst. Tech. J. 1969;48(9):2909}47. [8] Marotz J. Holographic storage in sensitized polymethyl methacrylate blocks. Appl. Phys. B 1985;37(4):181}7.
Holographic cure monitoring of the DuPont Somos2+ 7100 stereolithography resin M. Farsari, S. Huang, R.C.D. Young, M.I. Heywood, C.D. Brad"eld, C.R. Chatwin* School of Engineering, University of Sussex, Falmer, Brighton BN1 9QT, UK Received 17 July 1998; accepted 23 November 1998
Abstract The optical characteristics of a commercial UV curable resin are investigated using nondegenerate four-wave mixing. The material assessed is an epoxy resin, DuPont Somos2+ 7100. The holographic gratings were written at a wavelength of j"351.1 nm for an irradiance range 0.5}3.0 W/cm and read at j"632.8 nm in order to assess the reactivity, curing speed, shrinkage and resolution of the resin. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction In order for a photopolymer to be suitable for stereolithography applications, several requirements must be ful"lled. The material should have good spreading characteristics, low viscosity and low shrinkage from liquid to solid [1,2]. In addition, it must have high resolution to permit a good build accuracy, as single parts must be built with small dimensions [3]. In this work we investigate one commercially available photopolymer, the DuPont Somos2+ 7100. This is a special epoxy resin, which polymerizes with the help of a cationic photoinitiator. Its composition is proprietary, but it is optimized for use with an argon-ion laser at j "351.1 nm. We perform holographic measurements on it in order to investigate its reactivity, polymerization rate and extent of volume shrinkage during polymerization at this wavelength [4]. 2. Theory of four-wave mixing To begin with, consider the case in which a radiation curable resin is illuminated by two beams of light of the same wavelength and polarization, I and I . The two beams 1 0 * Corresponding author. Tel.: 0044 1273 678901; fax: 0044 1273 690814; e-mail: [email protected] 0143-8166/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 01 4 3-8 1 66 ( 9 8) 0 0 04 6 - 3
240
M. Farsari et al. /Optics and Lasers in Engineering 31 (1999) 239}246
overlap on the sample, generating a light pattern I(x)&I (1#m cos(kx)), (1) where I "I #I , m"2(I I ) (I #I )\ is the modulation index and k is the 1 0 0 1 0 1 spatial frequency of the grating. Such a time-independent intensity pattern consists of light and dark planes throughout the region of intersection, with a spatial wavelength K given by % j , K " (2) % 2n sin(h) where j is the wavelength of the light, n the refractive index of the resin and the angle between the two beams is 2h. As a result of the spatial non-uniformity of the illumination produced by the two writing beams, the material will polymerize only in the bright zones, where the irradiance is higher than the polymerization threshold E of the polymer. This leads A a refractive index grating in the material. The refractive index grating has been shown to be a periodic function, with the same period as the illumination pattern [5]. This grating can be probed by a third beam of a di!erent wavelength j such that no interaction between the beam and resin takes place. The read beam is di!racted by the grating with an e$ciency g that is de"ned as the ratio of the di!racted beam to the incoming beam. For maximum di!raction e$ciency, the probe beam angle h should satisfy the Bragg relation 2K sin h "j . (3) % The four-wave mixing experiment is described extensively in Refs. [5, 6] (Fig. 1). The change in the refractive index is due to the change of the density of the material (shrinkage). There has been no proof of a direct relation between volume shrinkage and curl occurring in the cured material, mainly because it is almost impossible to "nd
Fig. 1. Four-wave mixing.
M. Farsari et al./Optics and Lasers in Engineering 31 (1999) 239}246
241
material formulations that di!er only in curl shrinkage, while leaving other parameters unchanged [1]. It is, however, safe to assume that a high di!raction e$ciency would indicate high curl during building. In general, the shrinkage su!ered by a component built with stereolithography varies according to the exact build conditions.
3. Sample preparation The samples were prepared and all measurements taken in an environment controlled by air conditioning with a temperature of ¹"30$13C and a humidity of approximately 35$5%. The material was initially heated to 403C, to increase its uniformity. The preparation of the samples involved placing a drop of resin between two glass plates separated by 0.1 mm thick Mylar spacers. All the measurements were taken within 4 h of the manufacture of the samples.
4. Experimental set-up Our experimental set-up is shown in Fig. 2. An Ar-ion laser with a beam diameter of 1.57 mm operating on the 351.1 nm line and with polarization parallel to the plane of incidence was used to polymerize the resin. The laser beam was split into two equal power beams using a quartz cube beam splitter. The beams are controlled by an electronic shutter with a closure response time of 6 ms. The grating is read using a 5 mW He}Ne laser operating at j"632.8 nm, with the plane of polarization perpendicular to the plane of incidence. The resins are not reactive at this wavelength and as a result this beam does not modify the grating by causing further curing. The di!racted beam is detected using a silicon photodiode, connected to a LeCroy 9361 digital oscilloscope.
Fig. 2. Experimental set-up for non-degenerate four-wave mixing.
242
M. Farsari et al. /Optics and Lasers in Engineering 31 (1999) 239}246
If the response of the material is a linear function of the beam intensity, the light pattern produces a refractive index grating n(x)"n #*n(1#cos(kx)), (4) where *n is the amplitude change in the refractive index. The grating di!raction e$ciency g is de"ned as the ratio of the intensity of the di!racted beam over the intensity of the incident beam. If *n is small and there is no change in the absorption constant, a, of the material during polymerization, then g can be approximated by the Kogelnik equation, (7), thus g"exp(!ad/cos h)sin
p*nd , j cos h
(5)
where d"100 lm is the thickness of the sample and h is the angle of incidence of the reading beam. The induced refractive index change in UV curable photopolymers is in general nonlinear with respect to the illuminating irradiance. The refractive index grating that results from the sinusoidal fringe pattern generated by the interference of the two writing beams can be more closely represented as a square wave grating which leads to higher-order terms in the wave di!racted from the grating [5]. However, it has been shown [8] that for small grating amplitudes, the Kogelnik formula is a good approximation for the relation between the "rst-order di!raction e$ciency g and the amplitude of the "rst harmonic grating *n . 5. Results Gratings with spacing as low as 0.35 lm were successfully written, indicating that the material has excellent de"nition/resolution properties for stereolithography. In order to calculate the grating spacing K and Bragg angle h from Eqs. (2) and (3) % respectively, the refractive indices of the resin must be known. It was measured, using an AbbeH refractometer, to be n"1.51. Fig. 3 shows the time history monitoring diagrams of the photopolymer where the di!raction e$ciency is plotted versus irradiation time. The response of the material is presented as a normalized logarithmic plot of the evolution of the di!raction e$ciency over a time period of 4 s. The power of each of the writing beams was 30 mW and the angle between the two writing beams was 2h"123. Plotting di!raction e$ciency versus time provides information about the delay time until the onset of di!raction, the "nal value of the di!raction e$ciency and the rise time of di!raction [1,4]. These parameters are discussed in the following subsections. 5.1. Delay The delay time t characterizes an epoxy resin system in terms of its photosensi tivity. The rise time to 10% of the maximum di!raction e$ciency versus the beam
M. Farsari et al./Optics and Lasers in Engineering 31 (1999) 239}246
243
Fig. 3. Normalized di!raction e$ciency versus time.
irradiance for the two photopolymers under investigation is plotted in Fig. 4. It can be seen that, under the experimental conditions employed, the DuPont Somos2+ 7100 delay reduced signi"cantly with increased irradiance. 5.2. Diwraction ezciency The di!raction e$ciency of the photopolymer was measured for di!erent beam powers and grating spacing, 30 s after opening the beam shutter, when the irradiance of the di!racted beam had reached a constant value. It was observed that the irradiance did not a!ect the di!raction e$ciency for the range of beam powers, 0.5}3.0 W/cm, investigated. The variation of the di!raction e$ciency with grating spacing is shown in Fig. 5. From Fig. 5 it is observed that the di!raction e$ciency decreases when the spatial wavelength decreases. This is due to the reduction of the modulation index, caused by the limited resolution of the material. Overall, the resolution of the material investigated was exceptionally high. Speci"cally, it was possible to write gratings with a fringe period of 0.35 lm, which corresponds to a feature size of 0.175 lm. Since most stereolithographic machines require an accuracy in the order of fractions of a millimeter, the resolution of the investigated resins is more than adequate for their intended purpose. Even though the di!raction e$ciency value presented here was measured after 30 s of exposure, it was observed that it had reached that value after approximately 4 s of exposure (Fig. 3). This implies that, even in the parts of the grating that the irradiance was close to the critical value E , it only took approximately 4 s for the material to A cure fully.
244
M. Farsari et al. /Optics and Lasers in Engineering 31 (1999) 239}246
Fig. 4. Dependence of the di!raction delay on the writing beam irradiance.
Fig. 5. Dependence of the di!raction e$ciency on the spatial wavelength.
5.3. Rate The rate at which di!raction increases is an indication of the reactivity of the resin. This gives information about the reaction rate of the double bonds of the monomers
M. Farsari et al./Optics and Lasers in Engineering 31 (1999) 239}246
245
Fig. 6. Dependence of the di!raction rate on writing beam irradiance.
and oligomers that constitute the resin. Unfortunately, since the composition of the materials is propriety to DuPont, it is not possible to calculate how many reactions correspond to each photon of UV radiation. The di!raction rate as a function of the irradiance of the writing beams is shown in Fig. 6. It can be seen that the di!raction rate of the DuPont Somos2+ 7100 increases dramatically with irradiance.
6. Conclusions The DuPont Somos2+ 7100 demonstrated its ability to deliver high build resolution and high curing speed and is therefore suitable for stereolithography applications. The curing time of the material decreased rapidly with increased irradiance, while its shrinkage was not a!ected, for the irradiance range investigated.
References [1] Jacobs PF. Rapid prototyping and manufacturing, CASA-SME, 1992. [2] Schulthess A, Steinmann B, Hofmann M. New applications for Cibatool SL epoxy resins, Proc. 28th Int. Symp. on Automotive Technology and Automation-Rapid Prototyping in the Automotive Industries, 1995. p. 41}50. [3] Bertsch A, JeH zeH quel JY, AndreH JC. Study of the spatial resolution of a new 3D microfabrication process: the microstereophotolithography using a dynamic mask-generator technique. J. Photochem. Photobiol. A 1997;107(1}3) (1997):275}81. [4] Hunziker M, Bernhard P. Development of resin systems for StereoLithography: Holographic cure monitoring. Proc. 1st Nat. Conf. Rapid Prototyping, Dayton, OH, 1990. p. 79}85. [5] CarreH C, Lougnot DJ, Fouassier JP. Holography as a tool for mechanistic and kinetic-studies of photopolymerization reactions * a theoretical and experimental approach. Macromolecules, 1989;22:791}9.
246
M. Farsari et al. /Optics and Lasers in Engineering 31 (1999) 239}246
[6] BrauK chle C, Burland DM. Holographic methods for the investigation of photochemical and photophysical properties of molecules. Angew. Chem. Int. Ed. Engl. 1983;22(8):582}98. [7] Kogelnik H. Coupled wave theory for thick hologram gratings. Bell. Syst. Tech. J. 1969;48(9):2909}47. [8] Marotz J. Holographic storage in sensitized polymethyl methacrylate blocks. Appl. Phys. B 1985;37(4):181}7.