Thermomechanical Studies of Photoinitiated Cationic Polymerization

Chapter 28

Thermomechanical Studies of Photoinitiated Cationic Polymerization of a Cycloaliphatic Epoxy Resin R. T. Olsson , H. E. Bair , V. Kuck , and A. Hale

Downloaded by GEORGETOWN UNIV on September 11, 2014 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch028

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Lucent Technologies, Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974 Current address: Royal Institute of Technology, Herserudsv 9A, 18134 Lindingö, Stockholm, Sweden 2

Thin films of 3,4-epoxycyclohexylmethyl 3',4'epoxycyclohexane carboxylate were maintained at isothermal temperatures ranging from 0 to 50°C during UV irradiation (1.1 J/cm ). Under those conditions the maximum cure rate was achieved rapidly; however, only a conversion level of less than 10% was obtained before the reaction rate slowed dramatically. This low level of cure is not due to the loss of molecular mobility that is normally associated with a diffusion controlled reaction since the glass transition temperature, T of the epoxide was at least 40°C below its reaction temperature. Raising the sample temperature above 60°C resulted in a sharp increase in the conversion level. Irradiation of films maintained at 100°C yielded a conversion of more than 80%. Using an UV conveyor belt system to expose 1 mm thick films to the same dose of UV irradiation as used with the thin films, the dark reaction effect was observed. After five hours of storage in the dark at ambient temperature, the Tg increased from -20 to 23°C and the modulus rose from 0.2 to 1 GPa. Calorimetric traces taken after various ambient storage times indicated that two major processes were occurring as indicated by the presence of two exothermic maxima in the DSC dynamic curing curves. The maxima were located near 60 and 110°C, when a heating rate of 5°C/min was used. The ultimate T for this epoxide is ~190°C. 2

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© 2003 American Chemical Society

In Photoinitiated Polymerization; Belfield, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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318 Introduction During recent years, there has been a growing interest in UV-light initiated photo-polymerization. The very high rates of polymerization coupled with the low evolution of volatiles have made these polymerizations very attractive for commercial applications. Both radical and cationic polymerizations can be initiated by exposure to UV light. Recently, the latter type has gained much attention due to the development of very efficient cationic initiators and the fact that, unlike radical polymerizations; cationic reactions are not inhibited by oxygen. Among different monomers suitable for cationic initiated UV-polymerization using onium salts, epoxy monomers have been studied extensively. " In general, radiation produced epoxy polymers are amorphous materials that have good adhesion to a variety of materials, excellent chemical resistance and attractive mechanical properties. In industry, the cyclo-aliphatic epoxide 3,4-epoxycyclohexylmethyl 3,4* epoxycyclohexane carboxylate is one of the more widely used epoxide monomers suitable for photo-initiated cationic polymerization. For the sake of brevity the material will be referred to in the remainder of this paper as epoxy 3,4. Cyclo-aliphatic epoxides with their closed ring configuration have been reported more reactive than their open-chain counterparts and these, in turn, more reactive than glycidyl ethers or glycidyl esters. Qualitative measurements on the beneficial effect of temperature on cure rate of epoxy 3,4 systems have been reported. Using a power compensated, photo-differential scanning calorimeter, PDSC, we have studied the polymerization behavior of epoxy 3,4. With this instrument the sample temperature can be maintained isothermally independent of the heat evolved during polymerization. Hence, in this way one can determine quantitatively the effect of temperature on reaction rate. In addition, we studied the curing behavior of samples of epoxy 3,4 stored in the dark under ambient conditions (typically ~23°C and 35% relative humidity) after UVirradiation was terminated. The latter process is often referred to as the dark reaction. The development of the glass transition temperature, Tg, and the degree of conversion were determined by temperature modulated DSC methods.


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Experimental The epoxy 3,4 monomer, ERL 4221, was acquiredfromUnion Carbide Chemicals and Plastics Corp. (Danbury, CT) and used as received. The photoinitiator, CyraCure UVI-6974, a triarylsulfonium hexafluoroantimonate, was obtained from Union Carbide and mixed into the epoxy at a concentration of 1 W/W%.



319 UV-radiation experiments on the curing behavior of epoxy 3,4 were carried out using a differential scanning calorimeter (Perkin-Elmer DSC-7) equipped with an irradiation accessory (DPA7) that was fitted with a 450W high-pressure mercury lampfromOsram (Germany). The sample chamber was swept with helium at a flow rate of 22 cc/min during the testing. The UV light was unaltered. After calibrating the PDSC for isothermal work using the melting point standards of m-xylene (m.p.= -47.9°C), p-xylene (m.p.= 13.2°C) and indium (m.p.= 156.6°C), the polymerizations were conducted isothermally at temperatures ranging from -40 to 100°C with samples having a weight of 0.710.1 mg. To obtain an even distribution of the liquid sample over the bottom of the aluminum sample pans, a disc of transparent polyvinylidine difluoride was placed on top of the liquid sample and left in place during the measurements. This procedure produced films of relatively uniform thickness that were about 20 μπι thick. In addition, the plastic cover diminishes the evolution of volatile products froin the liquid epoxy. During the typical period of irradiation the epoxy lost less than 0.10 weight percent until the reaction temperature exceeded 80°C. At temperatures above 80°C a patented, hermetic capsule with a UV transparent lid was used. 13

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The power output of the xenon lamp was determined by placing an International Light Corp. Model IL 745A UV Radiometer at position just above the DSC sample holder closure. Most exposures lasted for 7.5 minutes (the shutter opened at 0.5 and closed at 8.0 minutes) and during this exposure period a sample received a dose of approximately 1.1 J/cm . Energy profiles of the Xenon lamp used in these studies revealed a large portion of the light energy lies above 380 nm. This longer wavelength radiation can be absorbed and converted to heat by the epoxy resin inside the DSC pan. Thus, a sample that absorbs radiation in the visible and infrared region can have a large but constant exothermic signal when the lamp is on. This exothermic effect can be easily eliminated when a second run with the same sample is subtracted from the first run. This difference technique is used in the isothermal DSC experiments shown in this paper. Temperature modulated DSC experiments were carried out in a Perkin-Elmer DSC-7 instrument in an attempt to separate overlapping glass transition endothermsfromcuring exotherms. The sample's modulus was determined in a three point bending fixture atfrequencyof 0.8 Hz and a strain of 0.08% using a dynamic mechanical analyzer (Perkin-Elmer DMA-7e). Each sample's dimensions were approximately 1.0 χ 3.4 χ 10 mm (HxWxL). 2

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Results and Discussion A photocalorimetric comparison of the isothermal curing of three 20μηι films of epoxy 3,4 maintained at -20, 0 and 30°C during exposure to UV light


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(-1.1 J/cm ) is shown in Figure 1. The peak maximum was found to increase nearly seven-fold from about 0.3 to 2.0 W/g as the reaction temperature was raised from -20 to 30°C, respectively. We assume the exothermic peak is a direct measure of the maximum polymerization rate. In addition, the time to each exothermic peak decreased from 21 to 11 to 6 seconds as temperature was increased from -20 to 0 to 30°C, respectively. Note that at each cure temperature in Figure 1 the fast part of the reaction is over within the first 60 seconds of irradiation. The effect of temperature on these curing rate properties of this photoinitiated cationic polymerization of epoxy 3,4 is similar to that of free radical acrylate polymerizations within the aforementioned temperature range, even though the reaction mechanisms are different. The apparent heat of reaction, 'ΔΗ»,» was obtained by integrating the exothermic curves in Fig. 1 and is equal to -15, -29 and -46 J/g at -20,0 and 30 °C, respectively. In this case the degree of conversion, ξ, as a function of time was calculated by dividing *AH by the heat of reaction, AH^ . The latter value was -664 J/g based upon a value of -20 kJ/mol per epoxy group polymerized. Note that at curing temperatures of-20,0 and 30°C, ξ, multiplied by 100 equals 2, 3 and 7% conversion, respectively. This single digit level of conversion remains until the reaction temperature is raised above 60°C and then the amount of liberated heat begins to increase dramatically (Figure 2). However, from 30 to 80°C the time to the maximum cure rate increased by an order of magnitude and then significantly decreased at 100°C as is shown in


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Figure 1. Exothermic reaction of epoxy 3,4 under isothermal conditions.


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Temperature (C) Figure 2. Time to reach the maximum polymerization rate and the extent of conversion during the curing of epoxy 3,4 as a function of temperature. Figure 2. These findings are in marked contrast to those previously observed with the same monomer and using di(4-t-butylphenyl)iodonium hexafluoroarsenate as the photoinitiator, where the time to reach the maximum cure rate decreased linearly with irradiation time over the temperature range of 25tol35°C. At 70°C the curing exotherm is broadened by a factor of three to four times over that found for the cure at 30°C and consequently ' A H ^ equals -192 versus -46 J/g at 70 and 30°C, respectively. This trend continued until at 100°C -S60 J/g were liberated (ξ equals 84 %) and the time to the maximum reaction rate decreased to 34 seconds. In earlier work Decker and Moussa determined by IR measurements that 83% of epoxy 3,4 monomer was converted in the presence of 5% of bis [4-(di-phenylsulphcmio)phenyl]-bis hexafluorophosphate at 80°C. 3

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With the absorption of light triarylsulfonium salts form strong protonic acids that remain in epoxy 3,4 until they are neutralized. These strong acids enable the epoxy to react in the absence of UV light. Samples were prepared for dark reaction studies with a dose of 1200 mJ/cm at ambient temperatures using a Fusion Systems Irradiation Station (Rockville, MD) equipped with a mercury lamp having a D bulb and a conveyor belt for exposing the sample. Note that during irradiation of a 1 mm thick epoxy layer, bounded on both sides by 1 mm 2


322 thick microscope cover slides, the temperature of the epoxy rose for a short time to slightly above 50°C during cure. Fifteen minutes after the belt cure at ambient temperature, the Tg was found at -20°C. The unreacted Tg of epoxy 3,4 is -60°C. When a thinner, 0.3 mm thick layer of this epoxy was irradiated to a dose of 3.6 J/cm without a cover slide its Tg was found near -40°C. However, a similar thin epoxy layer was made under the same experimental conditions but stored at -80°C immediately after irradiation. The latter epoxy had a tough, flexible outer skin that was about 0.1 mm thick with a Tg at 44°C. The liquid inner layer had a Tg of -58°C. These results demonstrate how important the effect of temperature is on the epoxy's curing behavior. In order to follow the extent of reaction in the dark as the sample ages, ξ can be calculated from


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ξ = [Ο-ξοίΔΗ™ - M U / Δ Η ^ + ξ ο

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where Δ Η ^ is the residual heat of reaction that is obtained by scanning the partially reacted sample from below Tg to above 230°C. The latter temperature is about 40°C above the ultimate Tg for this epoxy and was selected to enable the material to react as completely as it is possible without causing degradation, ξο is the initial degree of cure that occurs when the epoxy sample is irradiated on the UV conveyor belt, which in this work was found to be 7%. The determination of Tg and the residual heat of reaction after the belt curing process can be difficult to define because the overlapping endothermic and exothermic processes can offset each other. In addition, the magnitude of the change in Cp at Tg can be diminished due to the restrictions that are imposed on molecular motion as crosslink density increases with the extent of conversion. In order to deconvolute these opposing thermal trends we employed temperature modulated DSC (TMDSC). The solid line in Figure 3 is the average heat flow for epoxy 3,4 after an initial U V belt cure followed by 4470 minutes of dark reaction at room temperature. It is equivalent to the signal produced in a conventional DSC scan using a linear heating ramp. This average signal is producedfroma series of 118 repetitive temperature steps of 2.5 degrees at a rate of 5°C/min with an isothermal hold of 30 seconds between each heating step (broken line with dots, Figure 3). In addition, two exothermic maxima can be seen in the dark aged epoxy 3,4 samples near 60 and 110°C as shown in Figures 3. In Figure 4 the average heat flow signal from TMDSC experiments for the unreacted epoxy 3,4 and subsequent samples stored in the dark at room temperature for 14, 163 and 4470 minutes are plotted versus temperature from -65 to 230°C. Tg is defined as the temperature at A ACp or at - 6 0 ° C for the unreacted epoxy 3,4. The breadth of the glass transition, ATg, which is determinedfromthe difference in the extrapolated onset and termination of the Tg interval, equals about 6°C for the uncured specimen. The latter value is 17

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Figure 3. TMDSC experiment of epoxy 3,4 after 470 minutes of dark reaction.

Temperature (*C) Figure 4. Average heat flow signal from TMDSC experiments on uncured epoxy 3,4 and after 14,163 and 4470 minutes of dark reaction.


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typical for a homogenous, single component amorphous material. However, as the epoxy cures in the dark its Tg increases and also broadens. After 14 and 163 minutes of dark reaction Tg occurred at -40 and -16°C and ATg equaled 20 and 35°C, respectively. However, the glass temperature cannot be discerned adequately after 4470 minutes of dark aging as seen in Figure 4 and must be analyzed in a manner that is described below. The exothermic part of the conventional signal can be eliminated if the thermodynamic part of the DSC signal or storage specific heat, Cp, is plotted (Fig. 5, lower, dashed line). Two Tgs are detected at 52 and 188°C. Note that dashed lines have been extended from below and above each Tg as shown in the Cp versus Τ plot of Figure 5 to make the two Tgs more visible. The lower Tg is associated with the material that has aged for 4470 minutes but the upper Tg is assumed to be linked to the material after the exothermic cure has been completed during this start and stop TMDSC heating regime. This final Tg is only two degrees lower than if the material is cured by dynamically heating the epoxy to 245°C and repeating the run to find the material's ultimate Tg. ACp is about the same value or 0.19 J/g°C for both transitions. Next, one should return to the average heat flow plot (upper, solid line, Figure 5) and place the starting point of the curing reaction at 52°C such that ACp is equal to the value found in the storage Cp plot. A dashed line is cast from this point to end of the reaction exotherm at 175°C. The residual heat is computedfromthe area under the dashed line and gives a value of -133 J/g. Hence, ξ equals 80%.

Figure 5. Average heat flow signal and storage Cp versus temperature from TMDSC runs on epoxy 3,4 after 4470 minutes of reacting in the dark.


325 In Figure 6 the advancement of Tg during the dark reaction is plotted versus the conversion of the epoxy resin using the TMDSC deconvolution scheme. These results show the typical glass transition temperature behavior that has been noted in other thermoset systems where Tg increases linearly with conversion until the latter stages of conversion and then begins to increase abruptly as the crosslinked network develops. ' By conventional DSC scans at 15°C/min a series of curves were generated for epoxy 3,4 after ambient storage for 15, 48, 140 and 195 minutes (Figure 7). The lower temperature exotherm near 90°C diminishes with increased aging time but the higher temperature exotherm appears to be unchanged under these aging conditions. In addition, the leading edge of the lower temperature exotherm shifts to higher temperatures with increased aging. When the sample aged for 195 minutes was stopped upon reaching 95°C and quenched and rerun only the high temperature peak was present (solid lines, Figure 5). This latter experiment supports our contention that the lower temperature process is independent of the higher exothemic reaction. Perhaps the epoxide ring opening reaction that is occurring in the lower temperature peak is generating secondary hydroxyl compounds that are free to react at a significant rate at higher temperatures. Spectroscopy measurements are planned in the future to indicate if this hypothesis is correct. The impact of the dark reaction on the modulus was monitored. Fifteen minutes after a 1 mm thick specimen was belt irradiated its storage modulus equaled 0.2 GPa. During the initial 200 minutes of dark aging the epoxy 3,4's modulus increased rapidly as shown in dash-dot line in Figure 8. Simultaneously the sample's loss modulus showed a maximum after about 200 minutes which indicates that Tg for this material equaled the ambient reaction temperature at this time. This mechanical data supports the DSC results that showed Tg was located at 23°C after 240 minutes of aging in the dark. As the Tg of the material reached the sample temperature, it appears that molecular diffusion became difficult and the reaction rate was reduced significantly. " The glassy epoxy's modulus continued to increase slowly until it reached a value of about 2.8 GPa after 1500 minutes of aging. By heating a second epoxy sample for 20 minutes at 200°C immediately after room temperature irradiation resulted in raising the modulus to the same value obtained by aging at ambient for 1500 minutes or 2.8 GPa (solid line, Figure 8). This latter temperature effect on the sample's modulus could be exploited to rapidly produce fully cured coatings if this type of epoxy is used commercially. It has been reported that in photoinitiated cationic systems the reaction rate can be accelerated by chain transfer agents, specifically hydroxyl containing compounds such as water and alcohols. In these studies the


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Conversion (%) Figure 6. Tg of epoxy 3,4 as a function of conversion.

Figure 7. Conventional DSC scans of epoxy 3,4 after dark aging for 15,48, 140 and 195 minutes. Double dashed line is for fully reacted sample.


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Thermomechanical Studies of Photoinitiated Cationic Polymerization

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