Radiation Curing of Polymeric Materials - American Chemical Society

Chapter 22

Temperature Effect on the Phase Transformation of UV-Curable Systems 1

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Downloaded by UNIV OF PITTSBURGH on October 15, 2015 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch022

L. Feldman and T. C. Ward 1

Mead Imaging, 3385 Newmark Drive, Miamisburg, OH 45342 Department of Chemistry and Polymer Materials, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

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An extension has been made to the time-temperature -transformation (TTT) cure diagram for studying thermosetting systems. Traditionally, the TTT diagrams are obtained using torsion pendulum, DMTA, DSC, or similar instruments. By the use of a technique familiar in photographic science, this work demonstrates that such phase transition boundaries may be revealed by sensitometric methods. The D-log Ε curve is a measure of color development in response to exposure. When evaluated, this curve reveals information about the phase transformation. For crosslinking acrylate polymerizations, the TTT clearly shows the details of physical and chemical processes. These changes are brought about through a radiation induced chemical reaction in the microencapsulated multifunctional acrylate system.

The time-temperature-transformation (TTT) state diagram common to material science can be successfully applied to thermosets to establish a cure path which leads to the desired performance. It can also be a very useful tool in developing a fundamental understanding of the cure process and the nature of the s o l i d state ( i i 5 ) . Several techniques can be used to define the boundaries of the material transformation from f l u i d to s o l i d through gelation due to an observable change in macroscopic properties. Thermomechanical methods (TP, TBA, DMTA), dilatometry, and calorimetry are widely used. Such techniques are s i m i l a r l y useful in the study of radiation-curable systems, but require special adaptation f o r i r r a d i a t i o n , or are used after i r r a d i a t i o n . Fluorescent l a b e l i n g , although u s e f u l , sometimes leads to an unclear interpretation (6,7). An imaging system based on photopolymerizable microcapsules has recently been developed (8-10). The microcapsules (Figure 1) contain p h o t o i n i t i a t o r s , a c r y l i c monomers, and a colorless dye precursor. 0097-6156/90/0417-0297$06.00/0 ©1990 American Chemical Society In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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The capsule wall is transparent to UV, but provides a substantial barrier to atmospheric oxygen. The capsules are usually coated on a f l e x i b l e substrate. Upon exposure, the monomer hardens to a degree dependent on the intensity of the l i g h t . When pressure i s applied to the capsules, the monomer which has not been immobilized can be transferred onto a special image developing paper (Figure 2). Color density (D) of the f i n a l image depends on the proportionality between exposure (E) level and the change in physical c h a r a c t e r i s t i c s which control the dye delivery mechanism ( U , 1 2 ) . This proportionality can be estimated from D-log Ε curves which are familiar in photographic science. These curves are generated by exposing media through a transparent optical density mask, which modulates l i g h t intensity (I) in logarithmic increments. In accordance with r e c i p r o c i t y law (Ε = I χ t ) , results of a single exposure can be extrapolated to the logarithmic time scale when applicable (13). A typical sensitometric curve which exemplifies capsule internal phase transformation i s represented in Figure 3. The highest density i s produced by unexposed f l u i d material, while completely exposed regions do not release any dye precursor at a l l . Intermediate densities correspond to a gradual change in physico-structural state. By assigning the f i r s t t r a n s i t i o n on the curve to the beginning of gelation and the second t r a n s i t i o n to v i t r i f i c a t i o n , a phase diagram can be generated over a broad temperature range. In t h i s work several formulations were studied in the temperature range between ( - 6 0 ) ° and ( 1 0 0 ) ° C . Experimental data and a phenomenological interpretation in the analogy with thermosets are provided. Experimental Photo-crosslinkable monomer containing predissolved i n i t i a t o r , colorless dye precursor, and other necessary ingredients was emulsified in water and then encapsulated. The capsule s l u r r y was machine coated on paper and dried to form a uniform layer of approximately 12jum t h i c k . The average capsule size was estimated to be S jam by Coulter Counter. A V i v i t a r Model VP-1 xenon flash lamp with a preset exposure time of 1/500 s , and a desk lamp with two F15T8 black l i g h t fluorescent bulbs were used as exposure devices in two experiments. In both cases, the distance from the l i g h t source to the sample plane was 10 cm. The l i g h t intensity was modulated using a 30 step transparent wedge of 0.1 optical density increments. Before each exposure, the sample and the mask were thermally equilibrated for 5 min at a designated temperature. The exposed capsule donor was coupled with the receiver containing a color developing component, and was pressure developed with a set of steel laboratory pressure r o l l e r s . Optical r e f l e c t i o n color density was measured using a Macbeth model 914 f i l t e r densitometer. For s i m p l i c i t y , step-wedge numbers were used to represent logarithmic increments on the intensity a x i s . Phase diagrams were constructed by plotting the step number corresponding to a t r a n s i t i o n on the sensitometric curve against the exposure temperature.

In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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Phase Transformation ofUV-Curable Systems

Figure 1. Schematic representation of the microencapsulated imaging system: Α-microcapsule internal phase, B-transparent s h e l l , C-substrate, D-dye developer layer.

Figure 2. Exposure and image development in the microencapsulated system: Ε-exposure device, M-photographic mask, P-pair of pressure r o l l e r s .


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Results and Discussion Data from experiments with the flash lamp exposure demonstrate that the radiation cure path may be highly affected by the temperature. These r e s u l t s are presented in Figure 4, where three major regions of density l e v e l s can be observed: A, of the maximum density (Dmax); B, where densities are below Dmax; and C, in which dye transfer i s not detectable. Although the material in region C possesses the physical properties of a s o l i d , i t contains a s i g n i f i c a n t amount of extractable monomer (11). D i e l e c t r i c thermal scans indicate that t h i s monomer has been completely immobilized. Therefore, we assume that the minimum density l i n e in Figure 4 defines v i t r i f i c a t i o n conditions. The gel phase and the boundary of i t s formation corresponds to region B, where gradual density change correlates with the amount of unreacted monomer disappearance. The highest density region may be divided into three parts. One of them (Al) i s obviously below the glass t r a n s i t i o n temperature of the reactants in t h e i r i n i t i a l state (T ) . Photopolymerization was not i n i t i a t e d below t h i s temperature. The area A2, below the horizontal l i n e , corresponds to i n s u f f i c i e n t l i g h t i n t e n s i t y , where dye delivery i s not yet controlled by polymerization. The t h i r d region, A3, represents a temperature dependent, high intensity r e c i p r o c i t y f a i l u r e , manifested as a reversal in the density v s . step number curve (Figure 5). As displayed on the diagram, the c r i t i c a l temperatures, T ; « i T (coincidental with T in t h i s case); and T £l derme the processing window às well as the aging behavior. A s ' i n thermosets these values are very system s p e c i f i c and are predetermined by the monomer type, the chemistry of the i n i t i a t o r s , and often by the physico-chemical c h a r a c t e r i s t i c s of the reagents added. A microencapsulated system containing hexaaryl b i s - i m i d i z o l e dimer, isopropylthioxzanthone (Quanticure ITX), and TMPTA (trimethylolpropane t r i a c r y l a t e ) provides a representative example. Its low s e n s i t i v i t y to exposure (UV lamp, 4 s) is accentuated with a very narrow processing temperature range between (-10) and (35)°C (Figure 6-B). This range s i g n i f i c a n t l y broadens (Figure 6-A) with addition of v i s c o s i t y reducing agents, such as PMA and MHT (2-heptylthio-5-mercapto-l,3,4-thiodiazole). As a r e s u l t , T s h i f t s toward i t s lowest end ( - 4 7 ) ° C , where T of neat TMPTA Is usually observed. Such an effect would not be noticed i f experiments were not done in a substantial temperature range. For example, DPHPA/ 1,6-HDDA / ketocoumarin systems with and without 2,6-N,N-tetra methyl a n i l i n e (TMA) are not very different in t h e i r performance at 19°C (see sensitometric curves in Figure 7). The d i s s i m i l a r i t i e s become evident though, at lower temperatures (Figures 8, 9). This and other observations indicate a dual role of hydrogen donors and reactive diluents in modifying both the system photosensitivity and the thermal response. It may be shown from the phase diagrams that the intensity span between gelation and v i t r i f i c a t i o n can also be affected by a d d i t i v e s . For example, the a b i l i t y of Quanticure EPD to change the v i t r i f i c a t i o n path i s revealed in experiments with GPTA monomer. This can be observed through comparison of Figures 10 and 11. The q

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Phase Transformation of UV-Curable Systems


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Figure 4. Phase diagram based on the color density developed after exposure with flash lamp. Solid c i r c l e s - g e l a t i o n , open circles-vitrification profiles.


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S T E P NUMBER Figure 5. Color density vs. photographic step number (D-logI) for different exposure temperatures. Corresponds to phase diagram in Figure 4.

Figure 6. Phase transformation profiles of T M P T A / Quanticure ITX containing hexaaryl bis-imidizole dimer with (A) and without (Β) P M A and MHT. (Solid circlesgelation, open circles-vitrification).


Phase Transformation of UV-Curable Systems

FELDMAN & WARD


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Figure 7. Sensitometric curves obtained at 19 C for DPHPA / 1,6-HDDA / ketocoumarin system with ( s o l i d l i n e ) and without (dashed l i n e ) TMA.

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Figure 8. Phase transformation p r o f i l e s of DPHPA / 1,6-HDDA / ketocoumarin system. (Solid c i r c l e s - g e l a t i o n , open c i r c l e s vitrification) .



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Figure 9. Phase transformation p r o f i l e s of DPHPA / 1,6-HDDA ketocoumarin system with TMA. (Solid c i r c l e s - g e l a t i o n , open circles-vitrification).

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Figure 10. Phase transformation p r o f i l e s of GPTA / Irgacure 907 / Quanticure BMS system. (Solid c i r c l e s - g e l a t i o n , open c i r c l e s vitrification) .



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Phase Transformation of UV-Curahle Systems

Figure 11. Phase transformation p r o f i l e s of GPTA / Quanticure EPD / Quanticure BMS system. (Solid c i r c l e s - g e l a t i o n , open circles-vitrification).



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f i r s t graph represents the typical behavior of compositions with Irgacure 907 and Quanticure BMS as the photoinitiation system. Note the very broad temperature range (low T and high, T o ) , the smooth, unusual improvements in photographic s e n s i t i v i t y with an increase in temperature, and the very t i g h t l y controlled dynamic range. Gelation and v i t r i f i c a t i o n p r o f i l e s are very p a r a l l e l . When Irgacure 907 i s replaced by Quanticure EPD (Figure 11), the gelation path i s not affected. At the same time, the v i t r i f i c a t i o n curve exhibits a sharp minimum at approximately 10°C. A completely d i f f e r e n t behavior i s displayed past t h i s minimum, where temperature can reduce toe speed in a s i g n i f i c a n t manner. At 50°C and above, capsule internal phase does not v i t r i f y under the experimental exposure conditions. Some other examples seem to confirm that EPD mainly affects the post-gelation process by inducing c h a r a c t e r i s t i c minimum (T°min) on the v i t r i f i c a t i o n p r o f i l e .


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Conclusions In analogy with thermosets, the cure path of photo-hardenable monomers can be described phenomenologically using phase transformation diagrams. The example of microencapsulated imaging systems demonstrates a strong dependence of the cure path on the temperature. Color development methods coupled with sensitometry allow easy data c o l l e c t i o n methods. With t h i s technique, arbitrary boundaries between the f l u i d , soft g e l , and the v i t r i f i e d s o l i d can be deduced from the D-log intensity curve. Experimental results demonstrate that such observations provide additional insights on the s p e c i f i c contributions of each reactant in photocurable systems. Although the dye labeling method in these experiments was used to obtain color images, the technique may be extended to c o l o r l e s s compositions, where other optical instruments can be used to read the contrast induced by the material's phase transformation (14,15). Encapsulation of the test material i s not necessary, in that the sample may be prepared in a microcolumn system format (16). Literature 1.

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4. 5. 6. 7. 8. 9. 10.

Cited

Gillham, J. K. In Development in Polymer Characterization-3; Dawkins, J. V., Ed.; Applied Science: Englewood, NJ, 1982; pp 159-227. Enns, J. B.; Gillham, J. K. J. Appl. Polym. Sci. 1983, 28, 2567-2591; 2831-2846. Gillham, J. K.; Enns, J. B. SPSE Proc.,Fall Meeting of the American Chemical Society, Los Angeles, Ca; American Chemical Society: Washington, DC, 1988; Vol.59, pp 851-858. Wisanrakkit, G.; Gillham, J. K. ibid.; pp 969-974. Schiraldi, Α.; Pezzati, E.; Baldini, P. Thermochimica Acta 1987, 120, 315-323. Levy, R .L.; Ames, D. P. Polym. Sci. Technol. (Plenum) 1984,29, 245-255. Sung, Ch. S. P.; Chin, I.-J.; Yu, W.-Ch. Macromolecules 1985, 18, 1510-1512 . Sanders, F. W. U.S. Patent 4,399,209, 1983. Sanders, F. W. U.S. Patent 4,565,137, 1985. Diamond, S. Electron. Imaging 1984, 35 .


22. 11. 12. 13.

14. 15.


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Phase TransformationofUV-CurableSystems

Arney, J. S.; Dowler, J. A. J. Imaging Sci. 1988, 32, 3, 125-128. Arney, J. S. Paper Summaries, SPSE's 36th Annual Conference, San Francisco, CA, 1983. Todd, M. and Zakia, R. In Neblette's Handbook of Photography and Reprography; Sturge, J. M., Ed.; Van Nostrand Reinhold: N.Y., N.Y., 1977; pp 175, 176, 173. Murray, R.D. ibid., p 437. Volkova, M. M.; Bel'kovsky, I. M.; Golikov, I. V.; Semyannikov, V. Α.; Mogilevich, M. M.; Indeykin, E. A. Vysokomol. Soed 1987, XXIX, 3, 435-440. Feldman, L.; Cage, M. R.; Shi, D. J.; R. C. Liang Paper Summaries, SPSE's 42nd Annual Conference, Boston, MA, 1989; SPSE: Springfield, VA, 1989; pp 400-403.

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Radiation Curing of Polymeric Materials - American Chemical Society

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