24 Relations between Luminescence Emission and Physical Properties of γ-Irradiated Polypropylene G . David Mendenhall , Ning Xu , and Stephen E . Amos 1
1
2
1Department of Chemistry, Michigan Technological University, Houghton, MI 49931 Himont Research and Development Center, 800 Greenbank Road, Wilmington, DE 19808 2
Eight formulated samples of isotactic poly(propylene) were γ-irradiated with Co to 3 and 5 MR, and subsequent changes in the samples were followed during the next 14 days by spontaneous luminescence emission, color development, and then by tensile measurements. The spontaneous luminescence decayed rapidly at first, and was still measurable in most irradiated samples 5 - 1 0 days later. Subsequent heating of irradiated samples to 100 °C led to large increases in luminescence that slowly decreased at this temperature. The elonga tion at break of irradiated samples, with one exception, dropped after irradiation. The yellowness index of samples after irradiation re mained about the same or increased, with one exception. General correlations between luminescence or color on one hand, and elonga tion at break on the other, could not be found. However, within a given formulation, the easily measured luminescence emission intensity at any time within 4 h after the end of the irradiation was monotonically related to the total radiation dose to the sample. 60
CfHEMILUMINESCENCE
(J)
F R O M POLYMERS was first described b y A s h b y in a p u b l i s h e d report o n the d i m i n i s h e d fight emission f r o m polypropylene ( P P ) at 150 °C w h e n it was f o r m u l a t e d w i t h stabilizers. A s h b y also observed synergistic effects o f combinations o f stabilizers w i t h the same technique. Polypropylene has b e e n a favored substrate for a n u m b e r o f subsequent
0065-2393/93/0236-0611$06.00/0 © 1993 American Chemical Society
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R E L A T I O N S IN P O L Y M E R S
investigations o f chemiluminescence emission ( 2 - 6 ) , i n part because o f the c o m m e r c i a l importance o f this p o l y m e r a n d i n part because o f its relatively strong chemiluminescence emission. Recent interest i n the subject has b e e n connected w i t h a c o n c e r n for the stability o f polypropylene after sterilization w i t h i o n i z i n g radiation. I n p r o p r i etary w o r k , one o f us ( M e n d e n h a l l ) correctly ranked eight formulated polypropylene samples t o w a r d their ultimate thermal stability at 150 °C b y c o m p a r i n g their relative chemiluminescence emission at 150 °C for t i m e durations o f u p to one week. T h i s result i n s p i r e d application o f the technique to the measurement o f damage caused b y ionizing radiation. Ideally, the technique w i l l provide an early indication of failure because it instantaneously measures the rate o f a dynamic process associated w i t h oxidative degradation. T h e application o f chemiluminescence is rather e m p i r i c a l , a n d there are a very large n u m b e r o f choices o f pretreatment a n d examination conditions. O u r approaches thus far are s u m m a r i z e d i n the following text. W h e n polypropylene is heated i n the absence o f oxygen, c h e m i l u m i n e s cence is e m i t t e d f r o m the t h e r m a l decomposition of accumulated peroxides a n d hydroperoxides i n the samples (7). W e heated 1-mg samples o f f o r m u lated, ^-irradiated polypropylene i n an evacuated capillary w i t h an i n f r a r e d laser (10.6 μ ι η ) a n d measured the visible light e m i t t e d f r o m the samples w i t h fiber-optic techniques (7). L i n e a r correlations were f o u n d between total light emission measured i n this way a n d the conventional peroxide value o f the samples. T h e proportionality constants, however, were quite dependent o n the formulation, a feature w h i c h l i m i t e d the value o f this approach. C h e m i l u m i n e s c e n c e emission f r o m polypropylene is usually negligible f r o m samples at ambient temperature, even f r o m samples d o p e d w i t h freeradical initiators. A f t e r exposure to i o n i z i n g radiation, however, an easily measured luminescence is observed. E a r l y studies by o u r group showed a qualitative correlation between the loss of impact strength a n d the very weak, exposure to 7-irradiation (all at 25 °C) (8). W e also f o u n d that the rate o f loss o f impact strength o f the samples at 60 °C c o u l d be correlated qualitatively w i t h their chemiluminescence emis sion i n air at 150 °C. T h e loss o f impact strengths o f samples stored at 25 °C was different than those stored at 60 °C, but the ranking according to their chemiluminescence emission at 25 °C was again i n qualitative agreement. These results were encouraging although the n u m b e r o f samples was l i m i t e d , a n d the examination o f the samples for fight emission at 25 °C was only b e g u n a week after the irradiation. Subsequently w e studied the m o r e intense fight e m i t t e d w i t h i n minutes after the exposure of polypropylene (and other hydrocarbons) to X-rays, a n d discovered that the color o f the fight was different than that o f fight f r o m oxidation reactions carried out at elevated temperatures (9). T h i s surprising fact, along w i t h (1) the observation that the fight emission d i d not d e p e n d o n the presence o f oxygen, a n d (2) its decay
24.
MENDENHALL ET AL.
y-lrradlated
613
Polypropylene
f o l l o w e d a p o w e r function (I = At~ ;
I = intensity, t = time, a n d a, η —
n
constants) made a c o n v i n c i n g case that the luminescence was due to chargerecombination processes w i t h the same characteristics observed earlier f r o m a variety o f p o l y m e r i c substrates [10,
polymer
11]:
e~ + [ p o l y m e r ] +
[ p o l y m e r ] * -—>
hv'
where e~ = electron a n d hv = photon. I n the present study w e have measured the light emission f r o m f o r m u lated polypropylene plaques that were irradiated i n a conventional source. Intermittent measurements
6 0
Co
of the luminescence (presumably f r o m
charge recombination) were made at ambient temperatures for several days after irradiation u n t i l it was too weak to measure.
W e then
measured
luminescence (presumably free-radical-induced oxyluminescence) f r o m these same samples heated i n air to 100 °C. Tests for impact strength a n d color development w i t h irradiated sheets o f P P were c a r r i e d out for comparison.
Experimental Details Isotactic polypropylene ( H i m o n t Pro-fax 6801) w i t h a fractional (0.4) melt flow rate, containing approximately 0 . 0 1 % o f a phenolic processing stabilizer, extrusion. E x t r u s i o n c o m p o u n d i n g was carried out w i t h a B r a b e n d e r 3 / 4 - i n . , sion ratio mixing screw a n d a 3 - i n . final mixing zone c o m p r i s e d o f pins. T h e extruder was r u n i n air at a flat 230 °C temperature profile. Sample compositions are shown i n T a b l e I.
C o m p o u n d e d p o l y m e r was injection-molded into 3x3x0.041-in. plaques o n a 1.5-oz Battenfeld injection m o l d e r i n air. T h e injection m o l d e r tempera ture profile was 2 1 5 - 2 2 0 - 2 2 0 - 2 3 5 °C. T h e m o l d temperature was 110 °C. E l o n g a t i o n at break was measured o n die-cut tensile bars c o n f o r m i n g to A S T M m e t h o d D 6 3 8 type I V . T h e tensile bars were p u l l e d at a rate o f 2 in./min. T h e injection-molded plaques were irradiated i n a 3-in. diameter p i p e at the Phoenix Laboratory (University o f M i c h i g a n , A n n A r b o r ) w i t h an u n d e r water C o source w i t h a dose rate o f 1.108 M R / h . T h e plaques were separated f r o m each other b y strips o f paper a n d stacked vertically i n the p i p e that was placed i n the reactor for sufficient times to give doses o f 3 a n d 5 MR. 6 0
T h e yellowness index was measured w i t h a colorimeter according to A S T M m e t h o d D 1 9 2 5 .
(Colorquest)
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STRUCTURE-PROPERTY
R E L A T I O N S IN P O L Y M E R S
T a b l e I. Compositions o f F o r m u l a t e d P o l y p r o p y l e n e
0
Sample No. Component Pro-fax 6801 (PP) Irganox 1010^ Irganoxl68 Tinuvin 7 7 0 Tinuvin 765 Tinuvin 770~bisNO Calcium stéarate c
A
B
100
100 0.03 0.10
J
e
/
0.10
C
D
E
F
G
100 100 100 100 100 0.03 0.10 0.10 0.10 0.10 0.10 0.10 0.095 0.10 0.10 0.10 0.10 0.10
H 100
0.10
Numbers in table refer to relative weight of ingredient in formulated mixture, and are approximately weight percent. TetraMs-[methylene (3, 5-di-ieri-butyl)-4-hydroxyhydrocinnamate]methane. Tri-(2, 4-di-£er£-butyl) phenyl phosphite. Bis-(2, 2, 6, 6-tetramethyl-4-piperidinyl) sebacate. Bis-(l, 2, 2, 6, 6^entamethyl-4-piperidinyl) sebacate. f Bis-(l-oxy-2, 2, 6, 6-tetramethyl-4-piperidinyl) sebacate. a
c
e
Measurement of Light Emission.
L u m i n e s c e n c e f r o m disks o f
1.0-in. diameter, cut f r o m some o f the plaques w i t h a cork borer immediately after they were r e m o v e d f r o m the reactor, was measured w i t h i n 5 m i n a n d again at increasing t i m e intervals thereafter. It was o f course not possible to examine all of the irradiated plaques immediately after irradiation. A few disks cut f r o m different plaques w i t h the same formulation, irradiated at the same t i m e showed luminescence intensities that d i f f e r e d b y amounts larger than the experimental error. T h i s result suggested that samples may have been
subjected
concentrations,
to temperature
inhomogeneities
or difference i n oxygen
although w e d i d not have t i m e to investigate this point
further. T h e light f r o m the disks was measured after p l a c i n g t h e m i n the sample w e l l o f a l u m i n o m e t e r ( T u r n e r Designs, Inc., m o d e l T D - 2 0 e ) . T h e disks themselves were h a n d l e d gently w i t h cloth gloves and tweezers and p l a c e d i n contact only w i t h clean surfaces. D u r i n g the fast, initial decay after irradia tion, the luminescence was measured for 20 s after a 10-s delay. Subsequently the emission was measured w i t h longer integration times, and the l u m i n o m e ter was thermostatted w i t h circulating water at 25.0 °C. T h e luminescence f r o m the disks at 100 °C was measured b y placing t h e m into a small a l u m i n u m cylinder that was threaded to admit one e n d o f a fiber-optic
cable. T h e other e n d o f the cable was inserted into the w e l l o f the
l u m i n o m e t e r . A f t e r measurement o f (negligible) light f r o m the samples at ambient temperature, the a l u m i n u m container, w r a p p e d i n a single sheet o f polyethylene
film,
was p l a c e d i n b o i l i n g water. T h e
fight
emission was
measured as consecutive 20-s readings, i n t e r r u p t e d b y 10-s delay, for the next 20 m i n .
24.
234
y-Irradiated
M E N D E N H A L L ET AL.
615
Polypropylene
T h e sensitivity o f t h e l u m i n o m e t e r was c h e c k e d periodically w i t h a _234p _ +2 u standard (12).
T } l
a
u o
H
q
u
i
d
s
c
i
n
t
i
a
t
i
o
n
Results Values o f elongation at break a n d yellowness index for irradiated a n d n o n i r r a diated tensile bars are given i n T a b l e I L T h e unstabilized samples ( A a n d H ) showed very large drops i n strength w i t h irradiation, whereas the other samples showed considerable retention o f strength after 3 but not 5 M R . Interestingly the bis-nitroxide, w h i c h to o u r knowledge h a d n o t b e e n tested previously, showed considerable protective ability at 3 M R . T h e yellowness index o f the samples changed w i t h irradia tion, a n d showed the greatest increase w i t h samples w i t h p h e n o l i c stabilizer. T h e sample w i t h t h e orange bis-nitroxide ( G ) showed m u c h greater color initially, a n d it was t h e only sample whose index decreased w i t h irradiation. T h e initial luminescence readings f r o m irradiated plaques are shown i n F i g u r e 1. T h e light emission decayed rapidly i n the initial stages according to a p o w e r law, a n d was analyzed b y least squares fit to a l o g - l o g plot. Because it is difficult to compare the samples i n the initial stages w h e n the emission is changing rapidly, w e used t h e fitted parameters to calculate t h e emission intensity 1 h after the e n d o f the irradiation o f each sample for the purposes o f comparisons. L u m i n e s c e n c e values at extended times appear i n T a b l e I I I . F o r t h e most part, the luminescence decays i n this table are monotonie. Because the samples h a d b e e n transported b y plane a n d car, a n d w e r e subsequently stored u n d e r ambient conditions, a more exact analysis o f the decay curves is not warranted. Sample G (bis-nitroxide) shows irregular luminescence values,
Table II. Tensile and Color Measurements on Irradiated P P
a
Sample Dose (MR) 0 3 5
0 3 5
A 1359 8 6
4.0 4.6 4.5
Β 1260 955 20
4.7 6.5 7.7
C
D
Ε
Elongation at break (%) 1596 1244 1372 1250 1380 1184 6 13 373 Yellow Index** 5.1 4.0 7.7 4.7 9.4 5.4
4.2 4.0 4.3
F
G
1399 1262 9
4.0 4.0 4.0
1228 995 108
8.7 6.9 6.5
H 1881 12 0
4.2 4.2 4.3
Average of three bars, 13 days after irradiation. Samples were die-cut so that the long axis was in the flow direction of the polymer. ASTM D1925, 14 days after irradiation.
a
b
616
S T R U C T U R E - P R O P E R T Y R E L A T I O N S IN P O L Y M E R S
w h i c h w e r e later traced to stimulation o f delayed luminescence b y
fluorescent
r o o m lights. F o r a l l samples, the r e c o m b i n a t i o n luminescence d u r i n g the first 4 h after irradiation was higher for a 5 - than f o r a 3 - M R dose. E x c e p t for samples Β a n d H , the luminescence one day after irradiation was m u c h higher f o r a dose o f 5 than 3 M R . F o r later times the pattern was less distinct. T e n days after the irradiation, the luminescence o f nearly a l l samples h a d d e c l i n e d to very l o w levels, a n d additional i n f o r m a t i o n was then obtained b y heating the samples to 100 °C. Plots o f the resulting emission, ascribed to chemiluminescence, versus time were similar i n shape for all irradiated samples. Representative plots are given i n F i g u r e 2. A plot o f the m a x i m u m chemiluminescence versus dose is given i n F i g u r e 3 a n d shows a monotonie increase for a l l samples.
1400
1200
C 1000
ο
CO
IU
800
JZ
-J Φ
600
> as φ
OC
400
200
0 Τ ι 0.04
ι ι ι ι
ι
1
.
ι
ι ι ι ι ι
ι
1
0.1
1
r — J
4
T i m e , Hr Figure 1. Initial luminescence decay curves from irradiated polypropylene samples, (a) 3 MR: open square, C; filled square, A; cross, F; inverted triangle,
cross, F; triangle, H.
24.
M E N D E N H A L L E T AL.
y-lrradiated
617
Polypropylene
3000-
2500
C
ο α «
2000
Ε m σ> 1S00
.ι Φ >
—
1000
ο
EC
500
0 ,
0.04
Τ
^
:
·.
0.1
, • ,
1
4
Time, Hr Figure 1. Continued
E m p i r i c a l plots o f calculated light emission 1 h after irradiation, the value o f η i n the p o w e r f u n c t i o n , the m a x i m u m light emission at 100 °C, o r yellowness versus percent o f original elongation d i d not reveal any correla tions, even w h e n w e restricted the data to samples w i t h the l o w e r radiation dose.
Discussion T h e current study bears out some o f the conclusions o f o u r earlier stud ies w i t h fewer samples. W e expect, to a first approximation, that
greater
light emission f r o m a sample w i l l c o r r e s p o n d to lesser stability. T h e charger e c o m b i n a t i o n luminescence that can be measured i m m e d i a t e l y after radia t i o n treatment ( F i g u r e 1) or several day later (Table III), however, does not correlate w i t h the relative loss o f tensile properties. T h i s result is i n contrast to o u r earlier observations, w h i c h w e r e made after irradiation o f samples w i t h different formulations (8).
F l u o r e s c e n c e measurements might be a way to
normalize the emission, b u t p r e l i m i n a r y attempts to do this have not b e e n promising.
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S T R U C T U R E - P R O P E R T Y R E L A T I O N S IN P O L Y M E R S
Table III. Luminescence from Irradiated Polypropylene at Extended Times Time (h)
0
Sample
b
A
Β
C
21-26 47-49 55 72-78 90-92 220-40
20
31
16 13
14 14
3 M R Irradiation 53 47 33 24
2
-1
21-24 41-49 53-55 72-78 90-97 220-240
33
30
12 22 36 4
17 35 44 3
F
G
H
34 20
72 79
34 21
15 26 -2
20 24 -2
54 72 4
15 17 2
5 M R Irradiation 71 101 61 61 76 39
63 22
177 101
31 20
43 18 4
16 2 0
55
6
11
6
22 26 -2
D
25 25 1
61 16 12
Ε
17 49 5
Mean of three measurements with Turner luminometer. Values are last three digits on most sensitive setting. Average percent error is 3 units. Time or range of times of measurements from end of irradiation period.
a
C o l o r development i n irradiated samples is an undesirable feature f r o m a practical standpoint. T h e irradiation o f unstabilized sample A resulted i n a small change o f color index, w h i c h was completely suppressed b y addition o f c a l c i u m stéarate (sample H ) . T h e larger changes o f color index i n some o f the other samples after irradiation must involve reactions o f additives. T h e large yellowness n u m b e r o f sample G a n d its large d r o p after irradiation is u n d o u b t e d l y associated w i t h disappearance o f the orange bis-nitroxide. A l though the luminescence f r o m the irradiated sample w i t h bis-nitroxide was higher because o f the effect o f r o o m lights, the effect was too small to account for the mysteriously elevated levels o f luminescence c o m p a r e d w i t h the other samples. Correlations aside, the bis-nitroxide was an effective stabilizer, second only to the parent amine (sample E ) at the 5 - M R dose (Table II). W e s h o u l d point out, however, that the f o r m u l a t e d samples do not contain the same m o l a l concentrations o f stabilizers. T h e only positive result o f the study is the monotonie relationship between luminescence a n d radiation doses (Figures 1 a n d 3). Similar rela tionships w e r e f o u n d w i t h other samples o f f o r m u l a t e d polypropylene that were irradiated to very l o w levels a n d then made to emit light b y laser heating ( 7 ) . F o r most samples it appears that chemiluminescence u n d e r the c o n d i tions for the data i n F i g u r e 3 w i l l distinguish between dose levels d i f f e r i n g b y m o r e than about 0.4 M R . M o r e o v e r , the reprodueability o f the c h e m i -
24.
M E N D E N H A L L E T AL.
y-Irradiated Polypropylene
619
0.3H
Ο 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
Time (Min.) Figure 2. Curves of luminescence (CL) VS. time from polypropylene samples at 100 °C. Top, sample A; bottom, sample C. The intensities increase in each case in the order 0 (rectangh), 3 (cross), and 5 MR (asterisk).
620
S T R U C T U R E - P R O P E R T Y R E L A T I O N S IN POLYMERS
cu
Qi
,
0
;
3 MR
,
1
5
Figure 3. Maximum chemiluminescence at 100 °C vs. dose. Filled squares, A; open squares, D; filled triangles, G; open circles, H; asterisk, C; cross, Β; X , F.
luminescence measurements a n d their dependence advantageous f o r quality control.
o n f o r m u l a t i o n are
Acknowledgment T h i s w o r k was supported b y a grant f r o m H i m o n t U S A , I n c . W e thank R o b e r t B l a c k b u r n (Phoenix Laboratory) a n d Carlos Saborio ( H i m o n t U S A ) for technical assistance, a n d L a r r y C . T h o m p s o n (University o f M i n n e s o t a at D u l u t h ) f o r some fluorescence spectra.
References 1. 2. 3. 4.
Ashby, G. E. J. Polym. Sci. 1961, 50, 99. Schard, M. P . Polym. Eng. Sci. 1965, 11, 246 and previous papers i n this series. Barker, R. E., Jr.; Daane, J. H.; Rentzepis, P. M. J. Polym. Sci. 1965, A3, 2033. Rychly, J.; Matislova-Rychla, L.; Spilda, I. Eur. Polym. J. 1979, 6, 565.
24.
M E N D E N H A L L E T AL.
γ-Irradiated
Polypropylene
621
5. Flaherty, K . R.; Lee, W . M.; Mendenhall, G . D. J. Polym. Sci., Polym. Lett. Ed. 1984, 22, 665-667. 6. Billingham, N. C.; George, G . A . J. Polym. Sci., Polym. Phys. Ed. 1990, 28(3), 257-265. 7. Mendenhall, G . D.; Cooke, J. M.; Byun, H. In Advances in Polyolefins; Seymour, R . B.; Cheng, T., Eds.; Plenum: New York, 1987; pp 405-435. 8. Mendenhall, G . D.; Agarwal, H. K.; Cooke, J. M.; Dziemianowicz, T. S. In Polymer Stabilization and Degradation; Klemchuk, P. P., E d . ; A C S Symposium Series 280; American Chemical Society: Washington, D C , 1985; pp 373-386. 9. H u , X.; Mendenhall, G . D.; Becker, R . Polym. Prepr. Am. Chem. Soc., Div. Polym. Chem. 1990, 31, 323. 10. Mendenhall, G . D.; Agarwal, H. K . J. Appl. Polym. Sci. 1987, 33, 1259. 11. Guo, X.; Mendenhall, G . D. Chem. Phys. Lett. 1988, 152, 146. 12. Mendenhall, G . D.; H u , X . J. Photochem. Photobiol. A 1990, 52, 285-302. RECEIVED for review July 15, 1991. ACCEPTED revised manuscript August 3, 1992.
