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Photo-oxidative stability of electron beam and UV cured acrylated epoxy and urethane acrylate resin films
Polymer Degradation and Stability 19 (1987) 147-160
Photo-oxidative Stability of Electron Beam and UV Cured Aerylated Epoxy and Urethane Acrylate Resin Films
N. S. Allen, P. J. Robinson, N. J. White Department of Chemistry, Faculty of Science and Engineering, Manchester Polytechnic, Chester Street, Manchester M! 5GD, Great Britain
& D. W. Swales Research and Development, Lankro Chemicals Ltd., Eccles, Manchester, Great Britain
(Received 20 March 1987; accepted 5 April 1987)
A BS TRA C T The post-cured photo-oxidative stability of urethane and bisphenol-A epoxyaco,late resins in mixed compositions with triacrylate and amine diacrylate resins are examined using UV and reflectance in[i'a-red absorption spectroscopic techniques. Overall, by measuring the growth in hydroxyl absorption at 3400cm -~, electron-beam cured resin films are more photostable than U V curedfilms, indicating the high photo-activity of residual photoinitiator in the latter case. Regarding the UV cured ~3,stems benzophenone is found to be a more photo-active residual photoinitiator than the benzot'l ester photofragmenting types. Films containing the amine diaco'late resin are more photostable than those contahl#1g the triacrvlate resin. This stabilising effect is associated with the oxygen and radical scavenging abili O, of the terminal amine groups. Photo-.yellowing, as measured hy the growth in an absorption band at 280 nm, is"observed only in resin films containing the amine diacrylate resin and is associated with the formation of unsaturated carbonyl groups. The latter are, however, 147 Reprinted Courtesy of the Society of Manufacturing Engineers. Copyright 1985, from Radcure Europe "85 Conference Proceedings.
N.S. Allen et al.
148
photobleached on prolonged irradiation whereas with resins containing the bisphenol-A epoxy acrylate resin, a longer term photo-yellowing is observed due to oxidation of the bisphenol-A to give stilbene-quinone products.
INTRODUCTION In earlier papers L2 we reported studies on the post-cured photo-oxidative stabilities of triacrylate and amine-diacrylate monomers of the general structures (1) and (2) where R is a polyetherpolyol, and in resin (2) one-third of the chains at random are terminated by a diethylamino group. /O--CO--CH~-~-CH 2 R--O--CO--CH=CH 2 ~O--CO--CH~-CH 2
triacrylate monomer
(1)
R//O--CO--CH=CH2 --O--CO--CH~-~-CH 2
/ C 2 H 5 amine diacrylate monomer
(2)
~O--CO--CH2--CH2--N \C2H 5
The results of these studies showed that monomer (2) was more photostable than monomer (1). Here the terminal diethylamino group appeared to be protecting the resin from oxidation by effectively scavenging oxygen and/ or competing with the polymer for the photoinitiator and/or its derived radicals. Regarding the curing process, electron-beam cured resins were more photo-stable than UV cured resins. In the case of the latter, photooxidative stability of the resins depended very much on the residual initiator structure, the direct hydrogen abstracting type such as benzophenone being the most photo-active. One particular feature associated with monomer (2) was the phenomenon of photo-yellowing. The process was shown to be initiated by hydroperoxides leading to the formation of ~, fl-unsaturated carbonyl chromophores. The earlier work x'2 concentrated on the two diluent monomers (1) and (2) above; however, commercially these monomers are normally used as diluents in combination with other resins based on, for example, a bisphenol A-epoxy acrylate resin (3) or a urethane-acrylate structure (4). Whilst these formulations are used to achieve certain physical property requirements, clearly the presence of such additional structural components could significantly influence the post-cured photo-oxidative stabilities of the resin films. The aim of this study, therefore, is to investigate the post-cured photooxidative stabilities of combined formulations of a bisphenol A-epoxy acrylate or a urethane mixed polyester resin with either resins (1) or (2) above.
CH 2--CH--C--O II O
CH 3
[
II
O
O H
I
CH2CH20--C--N~'CH 2
II
/-~h
I ~
CH3
I
OH
CH3
II
O
2
rl
I\
H
O
II
3
II
O
C H2--N--C--OCHFH2--O--C--CH~CH2
I
CH 3
/CH O H . . . . . . . . --~
CH 3
*HO--{R).--OH--Polyester of variable composition
CH20~C~O--CH2--CH--CH2--O--C_CH=CH
CH 3 \ - . / ~ H
CH3
Polyester urethane acrylate (4)
O OH II f CH2=CH--C--O--CH2--CH
Bisphenol A-epoxy acrylate resin (3)
~aD
ac
150
N.S. Allen
et al.
As before, photo-oxidative stability of the resins was monitored using the hydroxyl index in the infra-red region at 3450 c m - 1 while photo-yellowing was monitored using normal and second-order derivative UV spectroscopy for the associated absorption at 280 nm.
EXPERIMENTAL
Materials The products (1), (2), (3) and (4) were supplied by Lankro Chemicals Ltd, Manchester, Great Britain. Each product contained 100ppm p-methoxyphenol as inhibitor. Formulations were prepared by mixing resins (3) or (4) (40% w/w) with the diluent monomers (1) or (2) (60% w/w) together with benzophenone or Irganox 184 (1-benzoylcyclohexanol) (5% w/w). Benzophenone (BDH Chemicals, Great Britain) was recrystallised from ethanol. Irgacure 184 was supplied by Ciba-Geigy (UK) Ltd.
Film preparation Films of resins 4 to 12 pm thick were UV and electron-beam cured on both polished quartz flats and aluminium coated glass slides. The equipment used for UV curing was a Prime-Arc Mini-Cure (Jigs and Lamps) Ltd, Great Britain and the pass rate used was 0.25 m/s (in air) under a 200 W/in focused medium pressure Hg lamp. The electron-beam curing was carried out under N 2 (< 200 ppm Oz) at a 2 mrad dose (Energy Sciences Inc., Massachusetts, USA).
Photo-oxidation Films of cured resins were exposed in a Microscal Lightfastness Tester (Microscal Ltd, London) utilising a 500 W high pressure Hg/tungsten lamp (temperature, 50°C: humidity, ambient). Infra-red measurements were recorded using a specular reflectance attachment on a Perkin-Elmer Model 1420 ratio-recording spectrometer interfaced to a Perkin-Elmer 3600 data station. Rates of photo-oxidation of the films were measured by monitoring the hydroxyl group formation at 3450cm-1 using the following index: Hydroxyl index =
Absorbance at 3450 c m - 1 Absorbance at 2940 cm-1
The absorbance at 2940 c m - 1 is a reference peak which compensates for
Photo-stability of EB and U V cured resins
151
changes in film thickness (independent of cure condition and photooxidation time). Normal and second-order derivative UV spectra were recorded using a Perkin-Elmer Model 554 spectrometer.
RESIJLTS A N D DISCUSSION Photo-oxidative stability Some typical infra-red spectral changes on post-cured irradiation of different resins are shown in Figs 1 to 3. Clearly, the most noticeable change is the marked growth in OH absorption at 3450 cm-1. Figure 1 shows the change for resin (1) with 5% w/w benzophenone. Here there is little interference in the measurement of OH index, whereas in resins (3) and (4) absorptions by the polymer itself interfere, as shown in Figs 2 and 3, respectively. Residual OH in resin (3) absorbs strongly at 3500cm- ~ while the N - - H group in resin (4) absorbs strongly at 3350 cm - 1. In the latter two cases, therefore, OH index was monitored as a shoulder initially and only became a prominent absorption at prolonged irradiation periods. The results presented here thus represent only a comparative study of 100
80
t
60 / / i' /
~.0
/ i
/
/
20
j',;
~000
Fig. 1.
3500
3000 CM'I
2500
2000
Infra-red spectrum of resin (1) UV cured with 5% benzophenone before( . . . . . after (- - - ) 172 h of irradiation in a Microscal unit.
)and
N.S. Allen et al.
152
100
.....--""
80 L
~
~ /,"
60
/,0
20
'~',,./;'/'
4000
35~00
3000
I
2500
2000
CM -I
Fig. 2.
Infra-red spectrum of resin (3) + (1) UV cured with 5% w/w benzophenone before ( ) and after (. . . . ) 172h of irradiation in a Microscal unit.
#-
100
8O
6O I--
40 "N
2O
~ooo"
Fig. 3.
|
35oo
3~oo
25oo'
20o0
Infra-red spectrum of resin (4) + (1) UV cured with 5% w/w benzophenone before ( -) and after ( - - -) 172 h of irradiation in a Microscal unit.
Photo-stability of EB and UV cured resins
153
absorbance changes and further studies are in progress to evaluate the data quantitatively using differences in the absorbance areas o f the curves. Rates o f photo-oxidation for formulations containing resins (3) and (4) are compared in Figs 4 and 5, respectively. These results show a number of important features. First, as expected, the electron-beam cured resins are generally the more photostable, demonstrating the high photo-activity of residual photoinitiator in the UV cured resin films. Secondly, regarding the 0"3
~o'~_
o
~
o~• 1oo
200 IRRAO. TIME HRS
300
~oo
Fig. 4, Hydroxyl index versus irradiation time in Microscal unit for resin (3) + (!) electronbeam cured (I), and UV cured with 5% w/w ofbenzophenone (O) and 5% w/w Irgacure 184 (~) and resin (3) + (2) electron-beam cured (f-l)and UV cured with 5% of benzophenone (Q) and 5% w/w Irgacure 184 ( x ). latter, residual benzophenone is more photo-active than Irgacure 184, confirming our earlier findings. Thus, initial hydrogen atom abstraction by the photo-excited triplet benzophenone initiates photo-oxidation of the resin film at a much faster rate than the photo-fragmenting type where the initial radical production step does not involve the resin structure. The third feature, which again confirms earlier results, is that formulations containing the amine-terminated resin (2) are much more photo-stable than those containing resin (1). Furthermore, the photo-activity of the residual photoinitiator plays a less prominent role and major differences are no longer apparent. In this case the terminal amine group is effectively competing with the rest of the polymer structure, not only for oxygen, but also for the photoinitiator and its derived radicals. The fourth feature o f the results is that there is no significant difference
154
N.S. Allen et al.
zm0.2 ;Z
•
0
0
°~ 0,1
Q
0
100
~00
200 300 IRRAO. TIME HRS.
Fig. 5. Hydroxyl index versus irradiation time in Microscal unit for resin (4) + (1) electronbeam cured (IS]) and UV cured with 5% w/w benzophenone (O) and 5% w/w Irgacure 184 (©) and resin (4) + (2) electron-beam cured ( x ) and UV cured with 5% w/w benzophenone ((])) and 5% w/w Irgacure 184 (11).
0"8 0- 7
0.6 ~ 05 z 0.~ 0-t t.J z
0.2
0.1
I 100
I 200
I 300
/ t*O0
IRRAD. TIME HRS.
Fig. 6.
Hydroxyl index versus irradiation time in Microscal unit for UV cured (5% benzophenone) resin (1) ( 0 ) and resin (4) + (1) (0).
I
Photo-stability of EB and UV cured resins
155
between resins (3) and (4) in terms of their contribution to photo-oxidative stability of the resin films. However, when compared to resin (1) alone there is an overall marked improvement in resin film stability, and the effect is much greater than that expected from the dilution effect alone. The improvement is demonstrated in Fig. 6 for resin (4) and resin (1) with 5% w/w benzophenone as photoinitiator. Two different effects may be in operation here for resins (3) and (4). In the former case the bisphenol A, which is present at high concentrations, could function as a weak antioxidant and scavenge active free radicals produced from the benzophenone or any other minor absorbing impurities present in the resin film. This would certainly be feasible in the case of terminal bisphenol A groups. In the second case (resin (4)) the CH z adjacent to the N - - H group in the cured resin could operate as a weak 'sacrificial' centre for hydrogen atom abstraction 3-6 thus protecting the remaining resin structure from oxidation. 0125
0.1
0"075
ffO5
0.025
0.0
-0.025
- O.OS
-
0.075
-0'1
0'125 ~,nm
Fig. 7. Second-derivative (d2A/d22) absorption spectrum of resin (3) + (2) UV cured with 5% w/w lrgacure 184 before () and after 6 hours' (. . . . ) irradiation in Microscal unit.
N . S . Allen et al.
156
Photo-yellowing In previous studies 1'2 photo-yellowing was found to occur in formulations containing the amine-terminated resin (2). To monitor the photo-yellowing, second-order derivative spectroscopy was used for an absorption band which appeared at 280 nm. This band increased rapidly during the early stages of irradiation and then was gradually photo-bleached. Formulations containing resin (3), however, complicated measurement of the 280nm absorption centre due to the strong inherent absorption by the bisphenol A unit. This is demonstrated by the results shown in Figs 7 and 8 for secondorder and normal UV absorption changes, respectively. It is seen from Fig. 7 that second-order derivative spectroscopy was incapable of resolving enhanced absorption at 280 nm. This may be due to the similar absorption characteristics of the species involved. Figure 8, showing the normal UV spectra, indicates that this is so since, in this case, absorption at 280 nm is significantly enhanced. Figure 9 shows that the benzophenone disappears 1,0
:,,,
0"~
I
0'6
I
0.4 t
0.2
0 200
300
&O0
500
),~nm
Fig. 8. Normal U V absorption spectrum of resin (2) + (3) U V cured with 5% w/w Irgacure 184 before (
-) and after 6 hours' (. . . .
) irradiation in a Microscal unit.
Photo-stability of EB and UV cured resins
157 10
08
0.6
b i
0.t.
,', t'
, ,, i
/ , i
',
, ~, i
0.2
200
~ 300
,~00
5000
X,nm
Fig. 9.
Normal UV absorption spectrum of resin (4) + (2) UV cured with 5% ) and after 6 hours' ( - - - ) irradiation in a Microscal unit. benzophenone before (
within the first 6 h of irradiation in the Microscal unit as seen by the reduction in absorbance at 250 and 360 nm. In fact, a significant amount of photo-yellowing occurs during this period of time. Because of the above problem an increase in the 2 8 0 n m absorption was monitored for formulations containing resin (2). In contrast to our earlier findings the photo-yellowing phenomenon appeared to behave differently and depended very much on the nature of the formulation. The results are shown in Figs 10 and 11 for formulations containing resins (3) and (4), respectively. They show a number of interesting features. First, films containing resin (3) exhibit less photo-yellowing than films containing resin (4). Secondly, benzophenone is less photo-active than Irgacure 184 in inducing the photo-yellowing and this contrasts with our earlier results 1'2 on resin (2) alone. Thirdly, electron-beam cured resin (3) exhibited the lowest degree of photo-yellowing when compared with that of resin (4).
N.S. Allen et al.
158
0..~ E
nn
.
z0, 2 ~
I
so
l
,o'o
,so
•
I
~00
2;0
IRRA0. TIME
Fig. 10. Plot of absorbance at 280nm versus irradiation time in hours for resin (3) + (2) electron-beam cured (11) and UV cured with 5% w/w benzophenone (E}) and 5% w/w Irgacure 184 (O).
0.8
0"7 0'6
i O'S
:°~
\
i°lt
- ~
.\ 50
100
150
200
250
IRRAD. TIME
Fig. I 1. Plot of absorbance at 280 nm versus irradiation time in hours for resin (4) + (2) electron-beam cured (gg) and UV cured with 5% benzophenone (~) and 5% w/w Irgacure 184 (O),
Photo-stability of EB and UV cured resins
159
In fact, in the latter case the degree of photo-yellowing is the same for the electron-beam cured resin as that for the formulation containing Irgacure 184 (Fig. 11). This contrasts again with our earlier work. 1'2 The fourth and final interesting feature is the observation of a permanent long-term photoyellowing for electron-beam cured film and for films containing resin (3) with 5% Irgacure 184 and benzophenone (Fig. 12). It is evident from the above data that the presence of the bisphenol Aepoxy containing resin (3) controls the degree of photo-yellowing when compared with that of resin (4). Since hydroperoxides are the key initiators in the photo-yellowing 2 the bisphenol A-epoxy component must, in some way, control the nature and type of hydroperoxidation which occurs in the resin film during both the cure and early stages of irradiation. Coupled with this is the fact that the bisphenol A unit could feasibly operate as a radical scavenger and inhibit hydroperoxidation at the amine site. The lower degree of photo-yellowing with benzophenone in both films is probably associated 1'0
0.8
0.6 i
0.4
i
/ i J i t'
0.2
\ 0 300
~00
~,nm
Fig. 12. N o r m a l U V a b s o r p t i o n spectrum of resin (3) + (1) U V cured with 5 % w/w b e n z o p h e n o n e before ( - - - ) a n d after 6 h o u r s ' ( ) and 1100 hours' (. . . . . ) irradiation in a Microscal unit.
N.S. Alien et al.
160
with a combination of different factors. Both resins (3) and (4) contain hydrogen abstraction sites which will compete to some extent with those present in the diluent amine terminated resin (2). Thus, hydroperoxidation of the terminal alkyl amine will be reduced. The free radicals produced initially from Irgacure 184 are certainly less active than photo-excited triplet benzophenone and consequently m a y be more selective in undergoing Ha t o m abstraction from the secondary terminal dialkylamine group inducing more photo-yellowing. Finally, the long-term photo-yellowing with resin (3), which is seen as an increased long wavelength absorption up to 400 n m in Fig. 12 for the UV cured resin films; can be attributed to quinone structures of the following type formed by photo-sensitised oxidation of the bisphenol A unit: 7'8 _o
CH3
CH3
ACKNOWLEDGEMENTS The authors t h a n k S E R C for a C A S E Award (NJW) and also Lankro Chemicals Ltd., Manchester, Great Britain, for financial and other support.
REFERENCES 1. N.S. Allen, P. J. Robinson, N. J. White and G. G. Skelhorne, Eur. Polym. J., 20, 13 (1984). 2. N. S. Allen, P. J. Robinson, N. J. White and G. G. Skelhorne, Eur. Polym. J., 21, 107 (1985). 3. N. S. Allen and J. F. McKellar, J. Appl. Polym. Sci., 20, 1441 (1976). 4. Z. Osawa, in, Developments in polymer photochemistry--3 (N. S. Allen (Ed.)), Elsevier Applied Science Publishers Ltd, London, Chapt. 6, 209. 5. J. Lemaire and R. Arnaud, Polym. Photochemistry, 5, 243 (1984). 6. J. L. Gardette and J. Lemaire, Makromol. Chemic, 182, 2723 (1981). 7. J. F. McKellar and N. S. Allen, Photochemistry of man-made polymers, Applied Science Publishers Ltd, London (1979). 8. N. S. Allen, J. P. Binkley, B. J. Parsons, G. O. Phillips and N. H. Tennant, Polym. Photochem., 2, 97 (1982).
Photo-oxidative Stability of Electron Beam and UV Cured Aerylated Epoxy and Urethane Acrylate Resin Films
N. S. Allen, P. J. Robinson, N. J. White Department of Chemistry, Faculty of Science and Engineering, Manchester Polytechnic, Chester Street, Manchester M! 5GD, Great Britain
& D. W. Swales Research and Development, Lankro Chemicals Ltd., Eccles, Manchester, Great Britain
(Received 20 March 1987; accepted 5 April 1987)
A BS TRA C T The post-cured photo-oxidative stability of urethane and bisphenol-A epoxyaco,late resins in mixed compositions with triacrylate and amine diacrylate resins are examined using UV and reflectance in[i'a-red absorption spectroscopic techniques. Overall, by measuring the growth in hydroxyl absorption at 3400cm -~, electron-beam cured resin films are more photostable than U V curedfilms, indicating the high photo-activity of residual photoinitiator in the latter case. Regarding the UV cured ~3,stems benzophenone is found to be a more photo-active residual photoinitiator than the benzot'l ester photofragmenting types. Films containing the amine diaco'late resin are more photostable than those contahl#1g the triacrvlate resin. This stabilising effect is associated with the oxygen and radical scavenging abili O, of the terminal amine groups. Photo-.yellowing, as measured hy the growth in an absorption band at 280 nm, is"observed only in resin films containing the amine diacrylate resin and is associated with the formation of unsaturated carbonyl groups. The latter are, however, 147 Reprinted Courtesy of the Society of Manufacturing Engineers. Copyright 1985, from Radcure Europe "85 Conference Proceedings.
N.S. Allen et al.
148
photobleached on prolonged irradiation whereas with resins containing the bisphenol-A epoxy acrylate resin, a longer term photo-yellowing is observed due to oxidation of the bisphenol-A to give stilbene-quinone products.
INTRODUCTION In earlier papers L2 we reported studies on the post-cured photo-oxidative stabilities of triacrylate and amine-diacrylate monomers of the general structures (1) and (2) where R is a polyetherpolyol, and in resin (2) one-third of the chains at random are terminated by a diethylamino group. /O--CO--CH~-~-CH 2 R--O--CO--CH=CH 2 ~O--CO--CH~-CH 2
triacrylate monomer
(1)
R//O--CO--CH=CH2 --O--CO--CH~-~-CH 2
/ C 2 H 5 amine diacrylate monomer
(2)
~O--CO--CH2--CH2--N \C2H 5
The results of these studies showed that monomer (2) was more photostable than monomer (1). Here the terminal diethylamino group appeared to be protecting the resin from oxidation by effectively scavenging oxygen and/ or competing with the polymer for the photoinitiator and/or its derived radicals. Regarding the curing process, electron-beam cured resins were more photo-stable than UV cured resins. In the case of the latter, photooxidative stability of the resins depended very much on the residual initiator structure, the direct hydrogen abstracting type such as benzophenone being the most photo-active. One particular feature associated with monomer (2) was the phenomenon of photo-yellowing. The process was shown to be initiated by hydroperoxides leading to the formation of ~, fl-unsaturated carbonyl chromophores. The earlier work x'2 concentrated on the two diluent monomers (1) and (2) above; however, commercially these monomers are normally used as diluents in combination with other resins based on, for example, a bisphenol A-epoxy acrylate resin (3) or a urethane-acrylate structure (4). Whilst these formulations are used to achieve certain physical property requirements, clearly the presence of such additional structural components could significantly influence the post-cured photo-oxidative stabilities of the resin films. The aim of this study, therefore, is to investigate the post-cured photooxidative stabilities of combined formulations of a bisphenol A-epoxy acrylate or a urethane mixed polyester resin with either resins (1) or (2) above.
CH 2--CH--C--O II O
CH 3
[
II
O
O H
I
CH2CH20--C--N~'CH 2
II
/-~h
I ~
CH3
I
OH
CH3
II
O
2
rl
I\
H
O
II
3
II
O
C H2--N--C--OCHFH2--O--C--CH~CH2
I
CH 3
/CH O H . . . . . . . . --~
CH 3
*HO--{R).--OH--Polyester of variable composition
CH20~C~O--CH2--CH--CH2--O--C_CH=CH
CH 3 \ - . / ~ H
CH3
Polyester urethane acrylate (4)
O OH II f CH2=CH--C--O--CH2--CH
Bisphenol A-epoxy acrylate resin (3)
~aD
ac
150
N.S. Allen
et al.
As before, photo-oxidative stability of the resins was monitored using the hydroxyl index in the infra-red region at 3450 c m - 1 while photo-yellowing was monitored using normal and second-order derivative UV spectroscopy for the associated absorption at 280 nm.
EXPERIMENTAL
Materials The products (1), (2), (3) and (4) were supplied by Lankro Chemicals Ltd, Manchester, Great Britain. Each product contained 100ppm p-methoxyphenol as inhibitor. Formulations were prepared by mixing resins (3) or (4) (40% w/w) with the diluent monomers (1) or (2) (60% w/w) together with benzophenone or Irganox 184 (1-benzoylcyclohexanol) (5% w/w). Benzophenone (BDH Chemicals, Great Britain) was recrystallised from ethanol. Irgacure 184 was supplied by Ciba-Geigy (UK) Ltd.
Film preparation Films of resins 4 to 12 pm thick were UV and electron-beam cured on both polished quartz flats and aluminium coated glass slides. The equipment used for UV curing was a Prime-Arc Mini-Cure (Jigs and Lamps) Ltd, Great Britain and the pass rate used was 0.25 m/s (in air) under a 200 W/in focused medium pressure Hg lamp. The electron-beam curing was carried out under N 2 (< 200 ppm Oz) at a 2 mrad dose (Energy Sciences Inc., Massachusetts, USA).
Photo-oxidation Films of cured resins were exposed in a Microscal Lightfastness Tester (Microscal Ltd, London) utilising a 500 W high pressure Hg/tungsten lamp (temperature, 50°C: humidity, ambient). Infra-red measurements were recorded using a specular reflectance attachment on a Perkin-Elmer Model 1420 ratio-recording spectrometer interfaced to a Perkin-Elmer 3600 data station. Rates of photo-oxidation of the films were measured by monitoring the hydroxyl group formation at 3450cm-1 using the following index: Hydroxyl index =
Absorbance at 3450 c m - 1 Absorbance at 2940 cm-1
The absorbance at 2940 c m - 1 is a reference peak which compensates for
Photo-stability of EB and U V cured resins
151
changes in film thickness (independent of cure condition and photooxidation time). Normal and second-order derivative UV spectra were recorded using a Perkin-Elmer Model 554 spectrometer.
RESIJLTS A N D DISCUSSION Photo-oxidative stability Some typical infra-red spectral changes on post-cured irradiation of different resins are shown in Figs 1 to 3. Clearly, the most noticeable change is the marked growth in OH absorption at 3450 cm-1. Figure 1 shows the change for resin (1) with 5% w/w benzophenone. Here there is little interference in the measurement of OH index, whereas in resins (3) and (4) absorptions by the polymer itself interfere, as shown in Figs 2 and 3, respectively. Residual OH in resin (3) absorbs strongly at 3500cm- ~ while the N - - H group in resin (4) absorbs strongly at 3350 cm - 1. In the latter two cases, therefore, OH index was monitored as a shoulder initially and only became a prominent absorption at prolonged irradiation periods. The results presented here thus represent only a comparative study of 100
80
t
60 / / i' /
~.0
/ i
/
/
20
j',;
~000
Fig. 1.
3500
3000 CM'I
2500
2000
Infra-red spectrum of resin (1) UV cured with 5% benzophenone before( . . . . . after (- - - ) 172 h of irradiation in a Microscal unit.
)and
N.S. Allen et al.
152
100
.....--""
80 L
~
~ /,"
60
/,0
20
'~',,./;'/'
4000
35~00
3000
I
2500
2000
CM -I
Fig. 2.
Infra-red spectrum of resin (3) + (1) UV cured with 5% w/w benzophenone before ( ) and after (. . . . ) 172h of irradiation in a Microscal unit.
#-
100
8O
6O I--
40 "N
2O
~ooo"
Fig. 3.
|
35oo
3~oo
25oo'
20o0
Infra-red spectrum of resin (4) + (1) UV cured with 5% w/w benzophenone before ( -) and after ( - - -) 172 h of irradiation in a Microscal unit.
Photo-stability of EB and UV cured resins
153
absorbance changes and further studies are in progress to evaluate the data quantitatively using differences in the absorbance areas o f the curves. Rates o f photo-oxidation for formulations containing resins (3) and (4) are compared in Figs 4 and 5, respectively. These results show a number of important features. First, as expected, the electron-beam cured resins are generally the more photostable, demonstrating the high photo-activity of residual photoinitiator in the UV cured resin films. Secondly, regarding the 0"3
~o'~_
o
~
o~• 1oo
200 IRRAO. TIME HRS
300
~oo
Fig. 4, Hydroxyl index versus irradiation time in Microscal unit for resin (3) + (!) electronbeam cured (I), and UV cured with 5% w/w ofbenzophenone (O) and 5% w/w Irgacure 184 (~) and resin (3) + (2) electron-beam cured (f-l)and UV cured with 5% of benzophenone (Q) and 5% w/w Irgacure 184 ( x ). latter, residual benzophenone is more photo-active than Irgacure 184, confirming our earlier findings. Thus, initial hydrogen atom abstraction by the photo-excited triplet benzophenone initiates photo-oxidation of the resin film at a much faster rate than the photo-fragmenting type where the initial radical production step does not involve the resin structure. The third feature, which again confirms earlier results, is that formulations containing the amine-terminated resin (2) are much more photo-stable than those containing resin (1). Furthermore, the photo-activity of the residual photoinitiator plays a less prominent role and major differences are no longer apparent. In this case the terminal amine group is effectively competing with the rest of the polymer structure, not only for oxygen, but also for the photoinitiator and its derived radicals. The fourth feature o f the results is that there is no significant difference
154
N.S. Allen et al.
zm0.2 ;Z
•
0
0
°~ 0,1
Q
0
100
~00
200 300 IRRAO. TIME HRS.
Fig. 5. Hydroxyl index versus irradiation time in Microscal unit for resin (4) + (1) electronbeam cured (IS]) and UV cured with 5% w/w benzophenone (O) and 5% w/w Irgacure 184 (©) and resin (4) + (2) electron-beam cured ( x ) and UV cured with 5% w/w benzophenone ((])) and 5% w/w Irgacure 184 (11).
0"8 0- 7
0.6 ~ 05 z 0.~ 0-t t.J z
0.2
0.1
I 100
I 200
I 300
/ t*O0
IRRAD. TIME HRS.
Fig. 6.
Hydroxyl index versus irradiation time in Microscal unit for UV cured (5% benzophenone) resin (1) ( 0 ) and resin (4) + (1) (0).
I
Photo-stability of EB and UV cured resins
155
between resins (3) and (4) in terms of their contribution to photo-oxidative stability of the resin films. However, when compared to resin (1) alone there is an overall marked improvement in resin film stability, and the effect is much greater than that expected from the dilution effect alone. The improvement is demonstrated in Fig. 6 for resin (4) and resin (1) with 5% w/w benzophenone as photoinitiator. Two different effects may be in operation here for resins (3) and (4). In the former case the bisphenol A, which is present at high concentrations, could function as a weak antioxidant and scavenge active free radicals produced from the benzophenone or any other minor absorbing impurities present in the resin film. This would certainly be feasible in the case of terminal bisphenol A groups. In the second case (resin (4)) the CH z adjacent to the N - - H group in the cured resin could operate as a weak 'sacrificial' centre for hydrogen atom abstraction 3-6 thus protecting the remaining resin structure from oxidation. 0125
0.1
0"075
ffO5
0.025
0.0
-0.025
- O.OS
-
0.075
-0'1
0'125 ~,nm
Fig. 7. Second-derivative (d2A/d22) absorption spectrum of resin (3) + (2) UV cured with 5% w/w lrgacure 184 before () and after 6 hours' (. . . . ) irradiation in Microscal unit.
N . S . Allen et al.
156
Photo-yellowing In previous studies 1'2 photo-yellowing was found to occur in formulations containing the amine-terminated resin (2). To monitor the photo-yellowing, second-order derivative spectroscopy was used for an absorption band which appeared at 280 nm. This band increased rapidly during the early stages of irradiation and then was gradually photo-bleached. Formulations containing resin (3), however, complicated measurement of the 280nm absorption centre due to the strong inherent absorption by the bisphenol A unit. This is demonstrated by the results shown in Figs 7 and 8 for secondorder and normal UV absorption changes, respectively. It is seen from Fig. 7 that second-order derivative spectroscopy was incapable of resolving enhanced absorption at 280 nm. This may be due to the similar absorption characteristics of the species involved. Figure 8, showing the normal UV spectra, indicates that this is so since, in this case, absorption at 280 nm is significantly enhanced. Figure 9 shows that the benzophenone disappears 1,0
:,,,
0"~
I
0'6
I
0.4 t
0.2
0 200
300
&O0
500
),~nm
Fig. 8. Normal U V absorption spectrum of resin (2) + (3) U V cured with 5% w/w Irgacure 184 before (
-) and after 6 hours' (. . . .
) irradiation in a Microscal unit.
Photo-stability of EB and UV cured resins
157 10
08
0.6
b i
0.t.
,', t'
, ,, i
/ , i
',
, ~, i
0.2
200
~ 300
,~00
5000
X,nm
Fig. 9.
Normal UV absorption spectrum of resin (4) + (2) UV cured with 5% ) and after 6 hours' ( - - - ) irradiation in a Microscal unit. benzophenone before (
within the first 6 h of irradiation in the Microscal unit as seen by the reduction in absorbance at 250 and 360 nm. In fact, a significant amount of photo-yellowing occurs during this period of time. Because of the above problem an increase in the 2 8 0 n m absorption was monitored for formulations containing resin (2). In contrast to our earlier findings the photo-yellowing phenomenon appeared to behave differently and depended very much on the nature of the formulation. The results are shown in Figs 10 and 11 for formulations containing resins (3) and (4), respectively. They show a number of interesting features. First, films containing resin (3) exhibit less photo-yellowing than films containing resin (4). Secondly, benzophenone is less photo-active than Irgacure 184 in inducing the photo-yellowing and this contrasts with our earlier results 1'2 on resin (2) alone. Thirdly, electron-beam cured resin (3) exhibited the lowest degree of photo-yellowing when compared with that of resin (4).
N.S. Allen et al.
158
0..~ E
nn
.
z0, 2 ~
I
so
l
,o'o
,so
•
I
~00
2;0
IRRA0. TIME
Fig. 10. Plot of absorbance at 280nm versus irradiation time in hours for resin (3) + (2) electron-beam cured (11) and UV cured with 5% w/w benzophenone (E}) and 5% w/w Irgacure 184 (O).
0.8
0"7 0'6
i O'S
:°~
\
i°lt
- ~
.\ 50
100
150
200
250
IRRAD. TIME
Fig. I 1. Plot of absorbance at 280 nm versus irradiation time in hours for resin (4) + (2) electron-beam cured (gg) and UV cured with 5% benzophenone (~) and 5% w/w Irgacure 184 (O),
Photo-stability of EB and UV cured resins
159
In fact, in the latter case the degree of photo-yellowing is the same for the electron-beam cured resin as that for the formulation containing Irgacure 184 (Fig. 11). This contrasts again with our earlier work. 1'2 The fourth and final interesting feature is the observation of a permanent long-term photoyellowing for electron-beam cured film and for films containing resin (3) with 5% Irgacure 184 and benzophenone (Fig. 12). It is evident from the above data that the presence of the bisphenol Aepoxy containing resin (3) controls the degree of photo-yellowing when compared with that of resin (4). Since hydroperoxides are the key initiators in the photo-yellowing 2 the bisphenol A-epoxy component must, in some way, control the nature and type of hydroperoxidation which occurs in the resin film during both the cure and early stages of irradiation. Coupled with this is the fact that the bisphenol A unit could feasibly operate as a radical scavenger and inhibit hydroperoxidation at the amine site. The lower degree of photo-yellowing with benzophenone in both films is probably associated 1'0
0.8
0.6 i
0.4
i
/ i J i t'
0.2
\ 0 300
~00
~,nm
Fig. 12. N o r m a l U V a b s o r p t i o n spectrum of resin (3) + (1) U V cured with 5 % w/w b e n z o p h e n o n e before ( - - - ) a n d after 6 h o u r s ' ( ) and 1100 hours' (. . . . . ) irradiation in a Microscal unit.
N.S. Alien et al.
160
with a combination of different factors. Both resins (3) and (4) contain hydrogen abstraction sites which will compete to some extent with those present in the diluent amine terminated resin (2). Thus, hydroperoxidation of the terminal alkyl amine will be reduced. The free radicals produced initially from Irgacure 184 are certainly less active than photo-excited triplet benzophenone and consequently m a y be more selective in undergoing Ha t o m abstraction from the secondary terminal dialkylamine group inducing more photo-yellowing. Finally, the long-term photo-yellowing with resin (3), which is seen as an increased long wavelength absorption up to 400 n m in Fig. 12 for the UV cured resin films; can be attributed to quinone structures of the following type formed by photo-sensitised oxidation of the bisphenol A unit: 7'8 _o
CH3
CH3
ACKNOWLEDGEMENTS The authors t h a n k S E R C for a C A S E Award (NJW) and also Lankro Chemicals Ltd., Manchester, Great Britain, for financial and other support.
REFERENCES 1. N.S. Allen, P. J. Robinson, N. J. White and G. G. Skelhorne, Eur. Polym. J., 20, 13 (1984). 2. N. S. Allen, P. J. Robinson, N. J. White and G. G. Skelhorne, Eur. Polym. J., 21, 107 (1985). 3. N. S. Allen and J. F. McKellar, J. Appl. Polym. Sci., 20, 1441 (1976). 4. Z. Osawa, in, Developments in polymer photochemistry--3 (N. S. Allen (Ed.)), Elsevier Applied Science Publishers Ltd, London, Chapt. 6, 209. 5. J. Lemaire and R. Arnaud, Polym. Photochemistry, 5, 243 (1984). 6. J. L. Gardette and J. Lemaire, Makromol. Chemic, 182, 2723 (1981). 7. J. F. McKellar and N. S. Allen, Photochemistry of man-made polymers, Applied Science Publishers Ltd, London (1979). 8. N. S. Allen, J. P. Binkley, B. J. Parsons, G. O. Phillips and N. H. Tennant, Polym. Photochem., 2, 97 (1982).