Effect of Ionizing Radiation on an Epoxy Structural Adhesive - ACS

Chapter

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Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 15, 2013 | http://pubs.acs.org Publication Date: April 18, 1988 | doi: 10.1021/bk-1988-0367.ch007

Effect of Ionizing Radiation on an Epoxy Structural Adhesive 1

Thomas W. Wilson , Raymond E . Fornes, Richard D. Gilbert, and Jasper D. Memory Fiber and Polymer Science Program, North Carolina State University, Raleigh, NC 27695 The epoxy resin formed by tetraglycidyl 4,4'-diamino diphenyl methane and 4,4'-diamino diphenyl sulfone was characterized by dynamic mechanical analysis. Epoxy specimens were exposed to varying dose levels of ionizing radiation (0.5 MeV electrons) up to 10,000 Mrads to assess their endurance in long-term space applications. Ionizing radiation has a limited effect on the mechanical properties of the epoxy. The most notable difference was a decrease of approximately 40°C in Tg after an absorbed dose of 10,000 Mrads. Sorption/desorption studies revealed that plasticization by degradation products was responsible for a portion of the decrease in Tg. Graphite fiber reinforced composites are being u t i l i z e d i n an increasing number of s t r u c t u r a l applications. One such use for these materials would be i n space. Many polymeric matrix systems do not maintain t h e i r i n t e g r i t y i n such an environment (vacuum, i o n i z i n g r a d i a t i o n , temperature extremes). The epoxy r e s i n TGDDM ( t e t r a g l y c i d y l 4,4'-diamino diphenyl methane) cured with DDS (4,4'-diamino diphenyl sulfone) appears to be ideal for s t r u c t u r a l use i n long-term space applications (1-3). Dynamic mechanical testing (4,5) was employed as a probe to investigate the influence of ionizing radiation on the mechanical properties of an epoxy since i t can provide several types of information (e.g. Ε*, Ε', E" and tan$). Experimental Epoxy f i l m specimens of two d i f f e r e n t weight/weight ratios of TGDDM/DDS were evaluated (73/27 and 80/20). The films were cast between t e f l o n sheets using a spacer. The cure cycle was 1 hr at 1

Current address: Dental Research Center, University of North Carolina, Chapel Hill, NC 27514 0097-6156/88/0367-0093$06.00/0 © 1988 American Chemical Society In Cross-Linked Polymers; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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150°C and 5 hrs at 177°C under vacuum. Specimens were sealed i n evacuated aluminum f o i l bags and i r r a d i a t e d with 0.5 MeV electrons at six d i f f e r e n t dose l e v e l s , v i z . 1000, 2000, 3000, 4000, 5000 and 10,000 Mrads. Autovibron (imass) testing was conducted according to recommended procedures (Autovibron manual). Thin f i l m were tested at a frequency of 11 Hz i n a t e n s i l e mode. Specimens were scanned from -120°C to +320°C at 2.5°C/min. Results and Discussion Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 15, 2013 | http://pubs.acs.org Publication Date: April 18, 1988 | doi: 10.1021/bk-1988-0367.ch007

1

In Figure 1, the e l a s t i c modulus (E ) and loss tangent (tan6) for 73/27 and 80/20 TGDDM/DDS epoxy are shown. For the 73/27 r a t i o , Ε decreases monotonically up to 155°C. Between 155°C and 240°C, Ε f i r s t decreases and then increases. The changes i n E* indicate additional curing reactions (Ji). That i s , during f a b r i c a t i o n the system v i t r i f i e s at the cure temperature before a l l the available functional groups have reacted. Once the cure temperature i s exceeded, further reactions occur. Due to further crosslinking E increases between 200°C and 240°C. Above 240°C, a l l functional groups have apparently reacted. The modulus declines r a p i d l y to a rubbery state above 280°C. The tans spectrum for the 73/27 TGDDM/DDS has three d i s t i n c t t r a n s i t i o n s . The broad, low i n t e n s i t y peak ca. -60°C i s the γ-transition. In epoxy systems the γ-transition arises from a crankshaft r o t a t i o n a l motion of the g l y c i d y l portion of the molecule a f t e r i t has been reacted (6). Up to f i v e d i f f e r e n t relaxation mechanisms have been measured i n this region (7), but only one i s resolved by the present technique. The next damping peak occurs at 200°C, as a result of the a d d i t i o n a l curing reactions mentioned previously. The curing peak disappears upon subsequent testing (6_). The loss peak associated with Tg i s ca. 280°C. For the present work, Tg w i l l be taken as the maximum value of tan δ (8.). The additional curing reactions i n the 80/20 TGDDM/DDS should be more evident than i n the 73/27 TGDDM/DDS due to the greater excess of epoxide groups i n the former. The curing reactions are more pronounced as evidenced by a larger decrease i n E' and a curing peak of greater magnitude and breadth than i n the 73/27 specimen. The effects of r a d i a t i o n on Ε and tanfi for 73/27 TGDDM/DDS epoxy are i l l u s t r a t e d i n Figure 2. At 1000 Mrads, the curing reactions appear almost complete since the curing peak ca. 210°C has diminished. There i s no evidence of additional curing reactions at higher doses. This observation agrees with work by Netravali et a l . (9) who reported that r a d i a t i o n induces additional cure. An analogous response occurs i n the 80/20 specimens (Figure 3). The obvious effect of increasing r a d i a t i o n dosages i s a decrease in Tg (Table I ) . The Tg of a control i s 40°C higher than the Tg of specimens i r r a d i a t e d to 10,000 Mrads. Also, the a - t r a n s i t i o n region broadens with increasing dose. Property changes are evident i n the rubbery plateau region at 40°C above Tg ( i e . E'(Tg+40)). E'(Tg+40) decreases 20% as a function of dose up to 5000 Mrads. However between 5000 and 10,000 Mrads, E'(Tg+40) increases and i s only 6% below the control value. 1

1

1

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In Cross-Linked Polymers; Dickie, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

Effect of Ionizing Radiation onEpoxy Adhesive


WILSON ET AL.

5I

ι ι ι I > » ι I ι ι ι I ι ι ι I ι ι ι I ι ι ι I ι ι ι I ι ι ι

-120

- 8 0

- 4 0

0

4 0

8 0

120

160

1

ι ι ι

2 0 0

1

2 4 0

ι ι ι

1

2 8 0

ι ι ι

1-3

3 2 0

TEMPERATURE (°C) Figure 1. Dynamic mechanical spectra of 73/27 and 80/20 TGDDM/DDS epoxy.

Figure 2. Dynamic mechanical spectra of irradiated 73/27 TGDDM/DDS epoxy.



CROSS-LINKED POLYMERS

Control 1 0 0 0 Mrads 4 0 0 0 Mrads 1 0 , 0 0 0 Mrads

61 150

ι

I 170

ι

I 190

ι

I 210

ι

I 230

ι

I

ι

250

I 270

ι

I 290

1

1—I -3 310

T E M P E R A T U R E (°C)

Figure 3. Dynamic mechanical spectra of irradiated 80/20 TGDDM/DDS epoxy.


7. WILSON ET AL.

Effect of Ionizing Radiation on Epoxy Adhesive

Table I . Glass t r a n s i t i o n temperature as a function of dose for 73/27 and 80/20 TGDDM/DDS epoxy Tg (°C)


Dose (Mrads) control 1000 2000 3000 4000 5000 10,000

73/27 TGDDM/DDS 283.3 271.8 262.3 260.7 253.3 252.6 237.7

+

+ + + + + +

1.3 2.6 4.5 2.6 3.9 2.9 3.7

1

80/20 TGDDM/DDS 275.,4 261,,4 257,,6 256..7 251,.7 250,.6 237,.7

+ + + + + + +

2.9 3.9 2.7 3.7 4.5 1.8 3.4

expressed as mean Tg +_ the range for a 95% confidence i n t e r v a l .

The a - t r a n s i t i o n i n t e n s i t y increases with dose (tan280°C. To account for the experimental observations, the following explanation i s suggested. As a sample i s i r r a d i a t e d , chain s c i s s i o n and c r o s s l i n k i n g occur. Chain s c i s s i o n w i l l create a number of free chain ends. The α-transition w i l l s h i f t to lower temperature and i n t e n s i f y due to i n t e r n a l p l a s t i c i z a t i o n and an increase i n free volume caused by the greater number of chain ends (4). The rubbery plateau region i s not affected by an increase i n the number of chain ends i f Mn (number average molecular weight) i s very large (£). Therefore, free chain ends p l a s t i c i z e the network, and Tg decreases, even though the system has been crosslinked further.


7. WILSON ET AL.

Effect of Ionizing Radiation on Epoxy Adhesive


Conclusions The most noticeable property change i s a decrease i n the glass t r a n s i t i o n temperature of the epoxy r e s i n as a function of absorbed dose. The decrease i n Tg i s due to p l a s t i c i z a t i o n by degradation products and free chain ends from chain s c i s s i o n . The epoxy resins have a number of unreacted functional groups. Ionizing radiation causes these groups to react. Chain s c i s s i o n i s the predominant process at lower dose levels (5000 Mrads) results i n an increase i n e l a s t i c modulus at ambient temperature and i n the rubbery region above Tg.

Literature Cited 1. Wolf, K. W. Ph.D. Thesis, North Carolina State University, Raleigh, 1982. 2. Sykes, G. F; Milkovich, J. M.; Herakovich C. T. Polym. Mats.: Sci. and Eng. 1985, 52, 598. 3. Naranong, N. Masters Thesis, North Carolina State University, Raleigh, 1980. 4. Nielsen, L. E. Mechanical Properties of Polymers and Composites; Marcel Dekker: New York, 1974; Vol. 1. 5. Murayama, T. Dynamic Mechanical Analysis of Polymeric Materials, Elsevier: New York, 1978. 6. Keenan, J. D; Seferis, J. C.; Quinlivan, J. T. J. Appl. Polym. Sci. 1979, 24, 2375. 7. Pangrle, S.; Chen, Α.; Wu, C. C.; Geil, P. H. Bull. Am. Phys. Soc. 1985, 30(3), 437. 8. Roller, M. B. J. Coatings Tech. 1982, 54, 33. 9. Netravali, A. N.; Fornes, R. E.; Gilbert R. D.; Memory, J. D. J. Appl. Polym. Sci. 1984, 29, 311. 10. Murayama, T.; Bell, J. P. J. Polym. Sci.: Part A-2 1970, 8, 437. 11. Morgan, R. J.; O'Neal, J. E. Polym.-Plast. Techno1. Eng. 1978, 10, 49. 12. McKague, E. L., Jr.; Reynolds, J. D.; Halkias, J. E. J. Appl. Polym. Sci. 1978, 22, 1643. 13. Traeger, R. K.; Castonguay, T. T. J. Appl. Polym. Sci. 1966, 10, 535. 14. Wilson, T. W. Ph.D. Thesis, North Carolina State University, Raleigh, 1986. RECEIVED October

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Effect of Ionizing Radiation on an Epoxy Structural Adhesive - ACS

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