Radiation Curing of Polymeric Materials - American Chemical Society

Chapter 23

Microcolumn Imaging Simulation of the Microcapsule Imaging System

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch023

L. Feldman, M. R. Cage, D. J. Shi, T. K. Kiser, and R. C. Liang Mead Imaging, 3385 Newmark Drive, Miamisburg, OH 45342

New methods of imaging require ingenious delivery systems. One such system is Cylithography, in which microcapsules are used to regulate the delivery of dyes and acrylic monomers. It produces continuous tone, full-color, high-resolution images at high speed with panchromatic light (1-3). A novel experimental technique was developed through the use of highly constrained microporous supports that simulates the imaging mechanism involved in Cylithography while avoiding the time-consuming encapsulation steps. Inert films containing microcolumns of 0.4-8.0µmdiameter and approximately 10.0 µm depth were filled with a photosensitive composition consisting of a leuco dye, photoinitiator, and multifunctional acrylic monomer, and then exposed through a mask. Various development techniques can be used such as compression, extraction, treatment with dye materials, etc. This versatile technique is suitable for different applications that include image reproduction, printing, and use as a research tool. This work demonstrates that microcolumn imaging mimics photographic properties of the microencapsulated system and can be used for a detailed study of the image formation mechanism.

The microencapsulated acrylate process of imaging i s a unique application of radiation curing technology. In t h i s process, the photohardenable composition i s contained within a microcapsule wall and consists of a multifunctional a c r y l i c monomer with photoinitiators and leuco dye precursors dissolved in i t . When a thin coating of microencapsulated media on paper i s i r r a d i a t e d , a latent image i s formed as a pattern of various extents of cure. The f i n a l color image i s developed by compressing the capsule donor sheet together with a receiver sheet, which contains a color-developing 0097-6156/90/0417-0308$06.00/0 ©1990 American Chemical Society Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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agent. Usually pressure r o l l e r s are used for this purpose. The color density (D) of the f i n a l image is controlled by the amount of leuco dye delivered with monomer. The amount of transferable monomer, in turn, i s regulated by the change in the mechanical properties of the microcapsule (4). This relationship determines the sensitometric response of the system as shown in the c h a r a c t e r i s t i c D-log Ε curve (Figure 1). The maximum density (Dmax) i s produced by the dye delivered with f l u i d monomer from s o f t , unreacted capsules. Upon depletion of oxygen to some c r i t i c a l level with exposure dose E j , which determines photographic s e n s i t i v i t y , gelation i s initiated. Changes in the material state in response to exposure beyond this point are reflected by the color density decrease in the dynamic portion of the curve. Eo marks another threshold region, where monomer becomes immobilized inside the microcapsule and minimal color density (Dmin) is produced. The c r i t i c a l parameters of this process depend on the interplay of the exposure conditions and the properties of the encapsulated materials, including i n i t i a t o r e f f i c i e n c y , the type of monomer, and contributions of co-reactants to the chemical and physical characteristics of the capsule internal phase. It is desirable to study and evaluate the effect of each constituent in the capsule internal phase on the system performance with a minimum number of variables that may be introduced during encapsulation. A simple method that employs microporous membranes with discrete v e r t i c a l channels is proposed for t h i s purpose. The channels are f i l l e d with the photosensitive composition and both planes of the f i l m are t i g h t l y sealed using two covers, one of which should be transparent. The f i l m thickness and pore size can be selected to approximate the capsule coating thickness and single capsule s i z e , respectively. Similar to the microencapsulated system, dye transfer i s controlled by light-induced crosslinking polymerization and accomplished through compression (Figure 2). Prints developed by this technique are comparable to those obtained with microcapsules. Using color density v s . exposure c h a r a c t e r i s t i c curves, sensitometric properties of the d i f f e r e n t formulations can be evaluated and compared. Experimental work reported below shows that such tests can be used to predict microencapsulated system behavior and i t s imaging characteristics. It also demonstrates that the microcolumn imaging system provides a convenient vehicle for observations with SEM and other analytical methods,including GC, IR and TA. Experimental Sample Preparation. Photohardenable formulations based on TMPTA in combination with other reagents including photoinitiators (Quanticure ITX or 7-diethylamino-3-cinnamoyl coumarin, depending on the s p e c i f i c experiment) and c o l o r l e s s dye precursors developable upon contact with a c i d i c resin coated paper were used as a test composition. In one case such compositions were encapsulated and machine coated on a f l e x i b l e substrate. In the second set of t e s t s , the identical formulations were applied to microcolumn type membranes (manufactured by Nuclepore). Transparent polyester p l a s t i c sheets 25 jum thick were used to cover both sides of these samples after the application step. Laboratory steel pressure r o l l e r s that applied

Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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RADIATION CURING OF POLYMERIC MATERIALS

Figure 1. Typical sensitometric curve obtained with microencapsulated media v i a pressure development.



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(D imaged sample

•

i

l

l

•

•

•

•

•

•

Figure 2. Imaging process using microcolumn substrate. Α-exposure, B-imaged sample, C-pressure development, D-final images on the receiver.



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1000 Ν of force per cm of r o l l e r length were used to remove the excess l i q u i d from the membrane and provide a t i g h t , uniform s e a l . Samples were exposed to the l i g h t through a photographic step tablet with 0.1 optical density increments. This provided the necessary range of exposure l e v e l s at a single exposure time (5). Specific d e t a i l s on the exposure time and l i g h t sources used are given separately in the discussion. Both covers were then removed from the exposed microporous samples before the samples were placed on the receiver sheet and developed. Microencapsulated media does not need to be covered during exposure. To obtain a f i n a l image both microencapsulated and microcolumn media were s i m i l a r l y developed using identical pressure r o l l e r s , receiver, and glossing step at 150°C for 1 min. Characterization. Optical r e f l e c t i o n densities on the f i n a l image were measured with a Macbeth model 914 f i l t e r densitometer. Photographic s e n s i t i v i t i e s were estimated from the threshold intensity values in r e l a t i v e units corresponding to the shoulder and toe regions on the D-log I curves. Microscopic observations were done on a P h i l l i p s High Resolution Scanning Electron Microscope. To reveal s o l i d material inside the microcolumn, a freeze-fracture technique was used after samples were washed with methanol and dried under normal conditions. The amount of extractable monomer was estimated using GC (HP 5890 Model with a FID detector and a phenylmethylsilicon fused s i l i c a 10 m c a p i l l a r y column). Before extraction, FT-IR spectra were obtained using an IBM IR-32 Model with a DTGS detector. The percentage conversion of double bonds was calculated from the 1640 cm" (6) peak height. To estimate the exposure effect on the physical c h a r a c t e r i s t i c s , dynamic mechanical thermal analysis was employed. In t h i s case samples were prepared without the application of pressure r o l l e r s . Instead, a squeegee was used to remove the excess l i q u i d and preserve the monomer as a continuous layer on the surface of the substrate. Thermal scans at 10°C/min and a fixed frequency of 1Hz were done using a Du Pont DMA 983 instrument. 1

Results and Discussion Reciprocity behavior i s an important c h a r a c t e r i s t i c of photographic materials. Its f a i l u r e l i m i t s practical use because of the unpredictable, nonlinear response to a proportional change in exposure (5). In t h i s regard microcolumn material shows superior results. Figure 3 demonstrates a r e c i p r o c i t y curve for the system with Quanticure ITX, exposed on the sensitometer with a combination of "black" and "day l i g h t " (General E l e c t r i c ) fluorescent bulbs at time intervals between 4 s and 2.5 h. The shoulder speeds on t h i s plot correspond to a constant value of total exposure (log E=log 1+ log t ) . According to Arney (7) the rate of d i f f u s i o n of oxygen into the system i s n e g l i g i b l e . Apparently polyester covers provide an excellent b a r r i e r . Microcapsules, on the other hand, need a perfect wall (Z). A further comparison of the microcolumn and microcapsules show that t h e i r properties are p a r a l l e l in many respects. They both



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possess a similar spectral s e n s i t i v i t y p r o f i l e with a maximum in the same region. An example i s given in Figure 4 for 7-diethylamino-3-cinnamoyl coumarin / TMPTA / CVL leuco dye samples exposed for 256 s with a Xenon 1000 W arc lamp and monochrometer. This evaluation was performed for a typical i n i t i a t o r concentration of 0.20%, which i s very close to the optimal range (Figure 5), where maximum s e n s i t i v i t y i s reached in both systems. Based on these observations a suggestion can be made about high optical d e n s i t i e s , indicating that microcolumn and microencapsulated systems may be c l a s s i f i e d as bulk-response systems according to Thommes and Webers (8). Although an exact explanation of the almost l i n e a r displacement in the s e n s i t i v i t y between these two systems i s not yet a v a i l a b l e , the observations show that this difference i s affected by the properties of the matrix. For example, among clear materials polycarbonate based membranes are not very e f f e c t i v e for the broad band and UV exposures, probably because of the l i g h t absorption in the matrix. Tinted microcolumn membranes and translucent materials with interconnected pores produce even lower s e n s i t i v i t i e s . Polyester was found to be the least attenuating of those materials considered. This demonstrates that geometry and optical properties of the support are very important. Effect of the pore diameter on the s e n s i t i v i t y and Dmax was also noticed (Figure 6). However, such an effect may be connected to a difference in thickness, which was also measured. Lower s e n s i t i v i t y and a higher Dmax were observed with thicker samples (Figure 7). Due to the direct correlation between membrane thickness and microcolumn depth, these results provide additional evidence of the bulk-responsiveness in the microcolumn imaging system. It has been discussed (9) that the color density in the case of the microencapsulated system i s a continuous function of the change in morphology of encapsulated material. This was not d i r e c t l y observed using microscopy methods, but deduced from other experiments. Using the microcolumn technique, such observations were made. They indicate a good correlation with the proposed model. Samples corresponding to d i f f e r e n t positions on the D v s . log I curve (Figure 8) were viewed with SEM after preparation as described above and sputter coating with gold. The photomicrographs in Figure 9 represent two d i f f e r e n t stages of curing. It appears that, similar to microcapsules, each microcolumn images in an analog fashion, and that the log exposure range of the system r e f l e c t s the log exposure range i n t r i n s i c to an individual microcolumn. The amount of extractable monomer and unreacted double bonds as a function of exposure were estimated using methods described above on the ITX system embedded in PC microcolumn membrane and exposed for 128 s at the illumination I = 1.29 mJ/cnrs in the band width of 325-410 nm. In Figure 10 such data are compared to the color density change on the transferred image. A rather high conversion was determined in the sample exposed to the f u l l intensity level available. A rough estimate of the double bonds consumed per reacted monomer can be made, as demonstrated in Figure 11, where experimental data are compared to theoretical l i m i t s . The r a t i o between 1.0 and 1.5, corresponding to the conversion in monomer up to 70% i s agreeable with observations made on a microencapsulated system (10). 0




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Figure 4. Spectral s e n s i t i v i t y p r o f i l e s of identical formulations in microcapsules and microcolumn. Ovals-shoulder speed, t r i angles-toe speed.



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Microcolumn Imaging

0

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315

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Initiator Concentration, wt %

Figure 5. Photographic s e n s i t i v i t y vs. i n i t i a t o r concentration. Ovals-shoulder, t r i a n g l e s - t o e . Monochrometer exposure at 470 nm for 64 s .

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to

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Figure 6. Photographic shoulder speed and f i n a l image color density v s . pore diameter for PC microcolumn membranes.



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10

12

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Figure 7. Photographic shoulder speed vs. thickness of membrane. For comparison, the f i r s t data point in the l e f t top corner was obtained without membrane or any spacer.

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Log I, relative units

Figure 8. D-log I curve of the microcolumn sample used for examination with SEM.



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Figure 9. SEM images of polymer formed inside the microcolumn at corresponding intensity l e v e l s indicated by A and Β in Figure 8.

Figure 10. Sensitometric curve ( s o l i d c i r c l e s ) , percentage of residual vinyl groups (rectangles), and unreacted monomer (open c i r c l e s ) v s . log i n t e n s i t y .



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Reacted Monomer, % Figure 11. Vinyl groups conversion (by FT-IR) v s . monomer conversion (by GC a n a l y s i s ) .


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Microcolumn Imaging

Final image density i s controlled by the amount of f l u i d monomer and the r i g i d i t y of the polymer formed. Figure 12 shows the relationship between fractional conversion of monomer and fractional change in density. This i s similar to results obtained for microcapsules, where the d i r e c t proportionality between image density and e l a s t i c deformation of the substrate was considered (10). Changes in v i s c o - e l a s t i c c h a r a c t e r i s t i c s of the microcolumn samples can be observed experimentally with DMA. Figure 13 (A and B) demonstrates the results for storage and loss modulus, respectively. Numerals on the curves indicate log I of each exposure at a constant exposure time (128 s ) . Corresponding values representing the interrelationship among modulus change, T , and image density are shown in the following f i g u r e s . According to this method. T of the unreacted material (T ) i s ( - 4 7 ) ° C . It reaches 106°C upon exposure to the maximum intensity used. A sudden jump from ( - 2 6 . 5 ) ° C to ( + 4 1 . 4 ) ° C i s indicative of a sharp rheological transition in the middle of the log intensity range (1.4-1.5). It corresponds to the toe on the sensitometric curve (Figure 14) and may represent v i t r i f i c a t i o n . Dye transfer is completely r e s t r i c t e d from the material with a T exceeding room temperature. Although the chemical reaction threshold may not be reached, the physical properties component becomes a c o n t r o l l i n g factor manifesting high e f f i c i e n c y of the amplification mechanism r e l i e d upon in this process. Because image transfer i s accomplished at room temperature i t is interesting to compare the modulus at this temperature with color density. This i s done in Figure 15, where logarithmic increments of these two c h a r a c t e r i s t i c s are plotted vs. log I. A f a i r l y good correlation i s produced by this method. It may be concluded from what has been observed that the sensitometric evaluation based on the color density detects both transitions - from l i q u i d to gel and from soft gel to v i t r i f i e d s o l i d - most p r e c i s e l y , while other techniques reveal information on the continuous polymerization process. q

q

y


q

0

g

M/Mo

Figure 12. Fractional dye control v s . fractional monomer in the microcolumn imaging system.

conversion of



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ι

-100

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ι

100

ι

200

Temperature, °C Figure 13. Storage modulus (A) and loss modulus (B) DMA diagrams obtained on exposed microcolumn samples.



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Figure 14. T (open c i r c l e s ) and fractional a function of exposure.

density change as

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Figure 15. Superimposed data corresponding to image density ( c i r c l e s ) and modulus (triangles) change vs. log i n t e n s i t y .



322 Conclusions

Photographic behavior of the microcolumn imaging system p a r a l l e l s the microencapsulated system in many respects. Microcolumn provides a convenient sampling format for application of the multi-technique approaches in addressing the interdependence of the imaging c h a r a c t e r i s t i c s , curing conditions, and the structure of the polymer formed.


Acknowledgments The authors would l i k e to acknowledge J . S . Arney (Mead Imaging) for guidance, M.A. Zumbrum and S.R. McCartney (Virginia Polytechnic Institute and State University, Blacksburg, VA) for microscopy work, P.C. Adair and S.R. Buczynski (Mead Imaging) for help with microencapsulated media. Literature 1. 2. 3.

4. 5.

6. 7. 8. 9. 10.

Cited

Sanders, F. W. U.S. Patent 4,399,203, 1983. Diamond, A. S. Electron. Imaging 1984, 35. Rastogi, A. K.; Wright, R. F. Presented at the Electronic Imaging Devices and Systems '89 Symposium, SPIE/SPSE, Los Angeles, CA, Jan.1989. Arney, J. S. Paper Summaries, SPSE's 36th Annual Conference, San Francisco, CA, 1983. Todd, M.; Zakia, R. In Neblette's Handbook of Photography and Reprography: Sturge, J. M., Ed., Van Nostrand Reinhold: NY,NY, 1977; pp 125,173. Van Oosterhout, A. C. J.; Van Neebros, A. J. Radiat. Curing 1982, 9, 19-33. Arney, J. S. J. Imaging Sci. 1987, 31, 1, 27-30. Thommes, G. Α.; Webers, V. J. J. Imaging Sci. 1985, 29, 3, 112-116. Arney, J. S.; Dowler, J. A. J. Imaging Sci. 1988, 32, 3, 125-128. Arney, J. S. J. Imaging Sci. 1989, 33, 1, 1-6.

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October 19, 1989


Radiation Curing of Polymeric Materials - American Chemical Society

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