546
Ind. Eng. Chem. Prod. Res. Dev. 1982, 27, 546-549
Stress Chemiluminescence of Polymeric Materials; Predictive Applications to the Aging Process Suzanne 6. Monaco; Jeffery H. Richardson, James D. Breshears, Stanley M. Lannlng, James E. Bowman, and Connie M. Walkup Lawrence Livermore National Laboratory, Livermore, California 94550
A computer-controlledstress chemiluminescence instrument has been designed and assembled. The significant result of this work is the correlation of an enhanced chemiluminescence signal in a low stress environment with the subsequent premature mechanical failure of samples of TGMDA-DDS, the most common epoxy system used as a matrix for high-performance composites. Preliminary results with cellular silicone elastomers indicate a correlation between chemiluminescence, a dynamic measurement of a microscopic process, and load deflection curves, a dynamic measurement of macroscopic properties of the elastomer. Arrhenius plots of stressed and unstressed samples yield different activation energies and show a break in the slope at a temperature above which accelerated aging tests become unrealistic. Currently, only epoxy and cellular silicone samples have been examined with any depth; not all epoxy samples give a stress chemiluminescence signal. The technique will be extended to fibers and fiber composites.
Introduction A nondestructive, nonhysteretic technique that could predict the probability of failure of a stressed composite material in a given environment would provide a valuable quality control test. Such a technique would narrow the distribution function of failure probability vs. stress during various processing steps. Several groups have studied the behavior of polymers under stress by IR (Wool, 19811, FTIR (Bayer et al., 1980; Levy, 1980), Raman (Penn and Milanovich, 1979), and ESR (Roylance, 1978) studies. Such studies yield mechanistic information but, because of their relative insensitivity, would be of little help in developing a technique of quality control testing. In order to gain enhanced sensitivity, polymeric materials are frequently studied by conventional chemiluminescence techniques (Mendenhall, 1977). This highly sensitive technique can detect changes due to polymer aging and temperature in various polymeric materials (resins, fibers, rubbers, food products). Recently, polymeric materials have been studied by a novel chemiluminescence technique called stress chemiluminescence (SCL); in this technique the chemiluminescence signal is correlated with applied stress (Fanter and Levy, 1979a,b; Krauya et al., 1981). The initiation step in the SCL process is bond scission from mechanical stress (as opposed to heating or chemical reactions in ordinary chemiluminescence). After the initial step, the rest of the mechanism is assumed to be similar to conventiona! polymeric chemiluminescence; subsequent reactions with oxygen and proceeding through peroxide intermediates give rise to electronically excited species which luminesce (e.g., ketones). It is this oxygen dependence which distinguishes SCL from triboluminescence. However, while very sensitive, any chemiluminescence technique is subject to self-quenching, self-absorption, surface effects, and trace impurities. Currently we have demonstrated the correlation of chemiluminescence data with applied stress, temperature, atmosphere, absorbed moisture, and accelerated aging for samples of an epoxy resin. Our results indicate a correlation of an enhanced chemiluminescence signal in a low stress environment with subsequent premature failure of the epoxy sample. Thus, while this result is not yet firmly established and may not be widely applicable, the potential of SCL to have a predictive capability would make this
technique very valuable as a quality control test. While not ubiquitous, SCL appears to be applicable to a wide variety of polymer systems. Preliminary data from cellular silicone cushion samples, a complicated filled polymer system, show a correlation between macroscopic properties and the microscopic chemiluminescence process. In addition, Arrhenius plots yield different activation energies for stressed and unstressed samples as well as valuable information on the temperature regimes in which to conduct relevant accelerated aging tests for lifetime predictions. These data indicate that realistic tests should be done with stressed elastomer cushions and a t temperatures below 110 "C. Experimental Section
An apparatus capable of exposing samples to varying atmospheres, temperatures, stress, and extension while quantitatively detecting the resulting chemiluminescence via a photon counting system was used for these experiments. A schematic of the system is shown in Figure 1. An LSI-11 microprocessor is used to ramp a stepping motor which applies stress to the polymer sample (up to 10000 psi for samples 0.025 ins2).The computer also reads the output of a load cell and servos the stepping motor until the desired stress level is reached. Various load cells are used with maximum readouts ranging from 25 to 500 lb depending on the type of sample. In the case of elastomers, which can elongate beyond 40070,extension rather than stress is measured and controlled by the computer. A separate unit controls the temperature of a heater placed beneath the sample. The temperature of the sample is read by the computer via a thermocouple placed between the sample and the heater. The maximum temperature used to date is 170 "C. The light-tight sample chamber can be evacuated and then filled with the atmosphere of interest. Current experiments have used either air, nitrogen, or oxygen. Chemiluminescence is detected with two photomultiplier tubes (RCA 8852), typically biased to 1500-2000 V. The tubes are kept at -30 OC with thermoelectric refrigerated chambers which are water cooled. Dark counts of the tubes are typically 300 counts/min. One tube is placed opposite the sample to be stressed and the other opposite the unstressed blank or reference sample. The chemiluminescence of each is imaged on the 2-in. diameter photocathode '0 1982 American Chemical Society
I n d . Eng. Chem. Prod.
-.-
40
-
35
E
counting
Res. Dev., Vol. 21, No. 4, 1982 547 I
I TGMDA DDS 80°C o*
-
I
Time (mid
Extension
Figure 2. Initial chemiluminescence signal of TGMDA-DDS in oxygen at 80 " C and a constant stress of 160 psi. I
9000
c l I
I
Terminal
Figure 1. Diagram of the stress chemiluminescence system.
by a 50-mm f/0.95 camera lens and an antireflection coated plano-convex lens with a focal length of 90 mm. Outputs from each tube are separately routed through PAR 1121A amplifier discriminators to the PAR 1112 photon counter/processor and simultaneously stored in two channels. The contents of each channel or the difference between the two can be displayed and sent to the computer. When operating in the difference mode, the chemiluminescence signal of an identical sample exposed to the same temperature and atmosphere is continually subtracted from the chemiluminescence signal of the stressed sample. Provisions exist for wavelength discrimination using long pass filters; previous work by Fanter and Levy (1979a,b) showed that SCL exhibited a shorter wavelength than chemiluminescence initiated by thermal heating. The LSI-11 outputs the stress level (or extension), temperature, and corresponding photon counts as a function of elapsed time to both a printer and a floppy disk for storage and subsequent data manipulation. Epoxy samples were prepared from tetraglycidyl-4,4'methylenedianiline (TGMDA) cured with 4,4'-diaminodiphenylsulfone (DDS) in a 10023 ratio by weight. These two components were purchased from Ciba-Geigy (MY720 and Eporal, respectively). They were mixed for approximately 1 h at 100 "C, degassed, poured between glass plates separated by 0.1 in., and then cured for 2 h a t 120 "C and 2.5 h a t 177 "C. The epoxy sheets were then cut into dogbones and the edges polished. The resulting specimens measured 8 in. in length, 0.75 in. wide a t the ends, and had a guage length of 2.25 in. Samples with gauge width of both 0.25 in. and 0.32 in. were prepared. Care was taken in their handling and they were stored in the dark. The application of fiberglass end-tabs was found to greatly increase the reliability of the gripping mechanism. As the epoxy elongates less than 2 % , the area viewed by the photomultiplier tube was essentially constant. The cellular silicone elastomers were prepared from silicone gum (SE-54, General Electric Silicone Products Dept.), silicone dioxide fillers (Cab-0-Si1 MS7 and HiSil 233 from Cabot Corp. and Pittsburgh Plate Glass Industries, Inc., respectively), and triethoxy-end-blocked dimethylsiloxane (Y1587, Union Carbide) 100:32:6:11 by weight, respectively. This gum was bin aged for 28 days and then heat stripped for 18 h a t 177 "C. It was then compounded with 104.5 parts by weight of urea (Sherritt-Gordon Mines, Ltd.) and 4.5 parts by weight of a 50%
-
5000 1000
-.-
I
4
4000
;j
160
i!
1000
1400
1800
I
I
I
1400 1800 Time (min)
Figure 3. Stress chemiluminescence of TGMDA-DDS in oxygen at 80 "C.
paste of 2,4-dichlorobenzoyl peroxide in dimethylsilicone oil (Cadox TS-50, McKesson and Robbins). This molding compound was die molded into sheets 0.1 in. thick and cured at 113 "C for 15 min. The mold-cured sheets were machine washed, air and oven dried, and then post-cured 24 h a t 149 "C. Samples used for chemiluminescence measurements were cut from these sheets and measured 1.5 X 4 in. The samples were wide enough so that any narrowing from elongation did not reduce the area viewed by the photomultiplier tube.
Results and Discussion Epoxy Samples. All data were taken after allowing an initial large signal to decay to a much lower steady state value as illustrated in Figure 2. The source of this initial signal is not clearly understood, but it has been suggested that it may be due, at least in part, to photooxidative degradation resulting from exposure to ambient light (Nixon, 1981). Decay times were approximately 1-2 h depending upon the sample and did not follow a single exponential. Figure 3 illustrates that the chemiluminescence closely tracks the applied stress with no apparent effect from repetition other than a dropping baseline. The SCL done with the same epoxy resin by Levy and Fanter (1979a,b) showed a baseline that increased stepwise with each stress cycle. The reasons for this difference are not known at this time. Figure 4 illustrates the temperature and atmospheric dependence of SCL. Data taken at 80 "C in nitrogen did not differ significantly from that taken at room temperature in air. This oxygen dependency is what distinguishes SCL from triboluminescence. In triboluminescence the excited state that results in luminescence is actually formed by mechanical damage (Zink, 1978). In SCL the excited state is produced by the reaction of oxygen with the pre-
548
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
TGMDA DDS 80CCo2
2800 0
40
80
2700
120
2220
5 E
2240
2260
2280
4000Ezrs3 160 2220
2240
2260
2280
Time (minl
Figure 6. Stress chemiluminescence of TGMDA-DDS at 80 "C in oxygen. Sample failed at 6930 psi.
52001
/"""i
1-1
8000
= 4100
5 E
;;
m]
160 0
10
20 30 40 Time (min)
50
XES2 -
4000
S 160 0
60
20
40
60
Time (min)
Figure 5. Stress chemiluminescence of TGMDA-DDS at 80 "C in oxygen before (solid line) and after (broken line) introduction of 0.3 wt % water.
Figure 7. Stress chemiluminescenceof scored epoxy sample e at 80 " C in oxygen. Sample failed at 4600 psi.
viously mechanically broken chemical bonds. Thus, the production of light cannot occur in the absence of an OXidant when the light-producing reaction is chemiluminescence. Since absorbed moisture lowers the glass transition temperature of this resin system (Morgan, 1981),the effect of water on the SCL is of interest. Figure 5 is the SCL at 80 "C in oxygen of the same epoxy sample before and after the introduction of water. The sample was exposed to 100% relative humidity at room temperature for 24 h and experienced a weight gain of 0.3%. Samples that absorbed more than 0.6% water broke at such low stress levels that no data were taken. The larger signal at 160 psi is due to a wider gauge width of this sample relative to those used in Figure 3 and 4. These data differ from those reported by Levy and Fanter (1979a,b). They reported no reduction in signal with the introduction of 0.6% water and the nature of the chemiluminescence was very different. The signal increased with stress and then decayed while being held at the higher stress level. Upon releasing the stress, another increase in signal was seen that then decayed to a baseline level. Again, the reasons for these differences are not understood a t this time. Four samples were stressed to 4000 psi at 80 "C in oxygen and their SCL was recorded. Figure 6 is typical of the data taken. All of the samples exhibited a 10 f 2% increase in SCL at this stress level and subsequently failed at loads greater than 6900 psi. These samples were taken to be of high quality and their chemiluminescence under these conditions formed a basis for comparison with lower quality and deliberately damaged samples. Table I lists the change in the chemiluminescence signal with stress for four epoxy samples (b, c, d, and e) which ultimately were determined to be of poor quality on the basis of a lower
Table I. Correlation of SCL with Polymer Characteristics. (All Data Were Taken at 80 "C in Oxygen with an Identical Load Ramp to 4000 psi.) sample
characteristics
load at ASCL, % failure, psi
a b
untreated scored accelerated age untreated scored
lo+2 15 21 19.5 19.6
c
d e
>6930 6360 6 3 50 6250 4600
tensile strength. Sample a represents the four samples that exhibited a high tensile strength. These results indicate a potential predictive capability of the SCL technique. Samples b and e were damaged by scoring with a scalpel in the narrow portion of the dogbone. They showed a 15 and 19.6% change in SCL and failed at 6360 and 4600 psi, respectively. Both samples failed at the score. Sample c was placed in an oven at 70 "C for 1week. A 21% increase in SCL was seen and the sample failed at 6350 psi. It is clear the effects of accelerated aging can be detected by SCL. Sample e was not deliberately damaged. An increase of 19.5% was seen and the sample failed at 6250 psi. The reason for failure is unknown but it could be due to poor dogbone preparation, unreacted epoxide crystals, impurities, etc. The significant result is a qualitative correlation of an enhanced chemiluminescence signal at low levels of applied stress with the premature mechanical failure o f the samples. For example, Figure 7, the SCL o f sample e, shows an elevated chemiluminescence signal at 1200 psi. This result was highly reproducible; in no instance did a sample with a high tensile strength result in SCL signal at this low value of applied stress. These experiments, while limited in number, clearly suggest the potential of a predictive capability for SCL.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 T
144 I
181 1E5t
-c
II .
fi
D a
; e
10000~
cA
'
("C) 112 I
60
I
3
Arrhenius plot Cellular silicone cushion A Unstressedcushion A Stressed cushion
c
al
1
1000:
A
-.-
' I
E
c
0
2.2
A
A
L5- &L
E
-001
2.4
2.6 103 T-1 (OK-')
I
84
2.8
3.0
Figure 8. Arrhenius plots in oxygen of stressed and unstressed cellular silicone cushion. Stressed cushion is elongated 50%.
Cellular Silicone Elastomers. Historically, static molecular parameters such as degree of cross-linking have been related to the macroscopic variables of the elastomer such as tensile strength and swelling. SCL appears to be a technique capable of relating a dynamic molecular measurement to a dynamic physical property of the elastomer. In compression studies of cellular silicone cushions, data are always taken at the third compression cycle. This is because the load deflection characteristics are different for the first exercise of the cushion due to a combination of physical and chemical reasons (e.g., initial chain entanglement). Our initial experiments with cellular silicone cushions were done with extension rather than compression due to the design of the apparatus. The SCL from a previously unexercised cushion showed a large increase a t the onset of the f i s t stress cycle which then decayed to a value that was reproduced in subsequent stress cycles. These data indicate that the first cycle load deflection characteristics are due, a t least in part, to chemical processes. This result is an indication that SCL, a dynamic microscopic process, can be related to a macroscopic physical property of a solid polymer. Figure 8 is an Arrhenius plot of a stressed and unstressed sample of the cushion. The slopes of these lines show an activation energy for the chemical reactions occurring above approximately 100 "C for a stressed cushion to be 23 kcal vs. 30 kcal for an unstressed cushion. They also show that the nature of the thermooxidative degradation changes above this temperature. These data suggest that during accelerated aging tests, the cushion should be stressed and the temperature not above 110 "C in order to limit the degradation to reactions one might realistically expect during the service life of the cushion. They also
549
suggest that aging tests which are now done at 70 "C could be done a t 90-100 "C and remain valid. Conclusions Currently, new epoxy samples are being prepared. These will contain the BF, catalyst and be the formulation present in commercial prepregs used for graphite epoxy composites. We intend to repeat this study, with some additions, on a much larger, statistically valid, scale. We also intend to study fibers and fiber composites (e.g., graphite-epoxy, and Kevlar-epoxy). We now have elastomer cushion samples of different formulations and will attempt to relate SCL data to their macroscopic (e.g., load bearing characteristics) and microscopic (e.g., extent of crosslinking, etc.) properties. We also hope to do additional work to generalize our results to other polymeric materials. In summary, we have demonstrated that SCL can reproducibly measure reactions in solid polymers as a function of stress, atmosphere, temperature, and absorbed moisture, More significantly, we have also demonstrated that SCL has the potential for predicting premature mechanical failure. There is also an indication that SCL, a measure of microscopic processes, can be related to macroscopic properties of polymers. SCL data, plotted in Arrhenius form, should also help us to design more effective and realistic accelerated aging tests for lifetime predictions. Acknowledgment We thank Mark C. Evans and Jack T. Kestner for their assistance in the design and fabrication of the mechanical portion of the system and Roger J. Morgan for valuable discussions. Literature Cited Bayer, G.; Hoffman, W.; Slesler, H. W. Polymer 1980, 27, 235. Fanter, D. L.; Levy, R. L. ACS Symp. Series 1979a, 95 21 1. Fanter, D. L.; Levy, R. L. CHEMECH 1979b, 9, 882. Krauya, V. E.; Upitls, 2. T. Rikards, R. B.; Teters, 0. A,; Yansons, Ya. L. Mekh. Kompoz. Mater. 1981, 2 , 325. Levy, R. L. Chem. Eng. News 1980, 58(37), 51. Mendenhall, G. D. Angew. Chem. Int. Ed. Engl. 1977, 76, 225. Morgan R. Lawrence Livermore National Laboratory, Livermore, CA, private communication, 1981. Nixon, J. Baltelle Columbus Laboratories, Columbus, OH, private communicatlon, 1981. Penn, L.; Milanolvich, F. Polymer 1979, 20, 31. Roylance, D. K. "Appllcations of Polymer Spectroscopy"; Brame, E. G., Ed., Academic Press: New York, 1978. Wool, R. P. J. Polym. Sci. Polym. Phys. Ed. 1981, 79, 449. Zink, J. I. Acc. Chem. Res. 1978, 1 7 , 289.
Received for review June 1, 1982 Accepted August 2, 1982 This work was performed under the auspices of the U S . Department of Energy at the Lawrence Livermore National Laboratory under contract W-7405-Eng-48. This paper was originally presented a t the 183rd National Meeting of the American Chemical Society, Las Vegas, NV, Mar 28-Apr 2, 1982.
