Stability of -Irradiated Polypropylene - American Chemical Society

12 Stability ofγ-IrradiatedPolypropylene

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I. Mechanical Properties J. L. W I L L I A M S , T. S. D U N N , and H. S U G G Becton, Dickinson and Company Research Center, Research Triangle Park, N C 27709 V. T. STANNETT Department of Chemical Engineering, North Carolina State University, Raleigh, N C 27607

Disposable medical products formed from polypropylene are readily degraded by high energy irradiation during the radiation sterilization cycle. Examination of irradiated poly propylene indicates that the degradative process is mainly oxidative. Results show that physical properties decrease rapidly with increasing absorbed dose and the physical properties continue to deteriorate with time following irra diation. This post-degradative reaction is oxidative and is initiated mainly by residual radicals following the steriliza tion cycle. Physical property results indicate that certain polypropylenes that may be acceptable immediately fol lowing irradiation are unacceptable after six months be cause of post-degradation. The degradative reaction can b accelerated by increased temperature which aids oxygen diffusion to the radical sites. S u b j e c t i n g a polypropylene item to high energy irradiation sterilization ^ generally results i n severe degradation of the article (1,2). The radia tion degradation problem associated w i t h polypropylene has two aspects: discoloration and embrittlement. The latter is best characterized physi cally b y saying that polypropylene lacks the required elongation charac teristics needed to accommodate applied strains i n practice. This degradation problem manifests itself i n the syringe barrel fabrication where the yellow discoloration and embrittlement cannot be 0-8412-0381-4/78/33-169-142$05.00/l © 1978 American Chemical Society Allara and Hawkins; Stabilization and Degradation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

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tolerated. I n this study attention is given to the mechanical characteriza tion of the property changes that occur during and after irradiation, and to the formation of radicals generated during irradiation ( 3 ) . O u r two parallel studies provide insight into the embrittlement problem of polypropylene.

Figure 1.

Instron syringe flange bending device

Experimental Physical property measurements aided the characterization of flange strength as a function of sterilizing dose. T o measure flange strength a device (Figure 1) was constructed and adapted to an Instron tester. T h e force required to bend the flange away from the barrel through a given angle is measured and recorded to the point of breakage or 90°C. W h e n breakage occurs, the crack is always at the flange-barrel interface. A typical bending curve for a non-irradiated syringe flange is shown i n Figure 2. A l l test specimens were injection molded under controlled condi tions, and a 6000 c i cobalt-60 facility was used for sample irradiation. The isotactic polypropylene was 6 0 % crystalline w i t h a density of 0.903 g/cm . 3

Allara and Hawkins; Stabilization and Degradation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1978.


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BEND A N G L E (DEG.)

Figure 2.

Typical flange bending test curve using Instron bending device

Results and Discussion Examination of a polypropylene item following a 2.5-Mrad sterilizing dose of irradiation at 0.3 Mrad/hr indicates that the mechanical proper ties were severely degraded. The most dramatic change i n physical properties takes place i n the sample's ability to undergo extension ( F i g ure 3) for a tensile test bar molded from polypropylene. Although there are significant changes i n elastic modulus and breaking stress with irradia tion, the principal parameter for irradiation damage is the reduction i n breaking elongation. For instance, the breaking elongation for the sample shown i n Figure 3 was reduced from 400% to 60% by a dose of 7 M r a d . The dependence of mechanical properties on irradiation dose for elastic modulus, percent extension, and work are summarized i n Figures 4, 5, and 6, respectively. A n important parameter is the strain rate at which a polypropylene sample is broken (Figure 7 ) . It is evident from this graph that a seven fold difference i n breaking elongation can be obtained b y varying the strain rate from 100%/min to 800%/min. Severe strain rate is a better indicator of radiation damage (Figure 7 ) , i.e., at low strain rates the sample has adequate time to elongate. Therefore, a high strain rate should be used for screening various polypropylene formulations. Polymer samples under a tensile load w i l l break at varying elonga tions from sample to sample ( 4 ) . This variation i n breaking elongation w i l l increase with gage length because of the increased probability of weak points along the sample length. Because of the variation with tensile tests, a flexural test was adopted, as described above. A typical curve obtained by bending a test bar or flange is shown i n Figure 2. I n this test method breaking angles for a given sample can be reproduced


W I L L I A M S

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Ρolypropylene:



2.0 h LOAD (Kgs) 1.0

p-7Mrads

I 80

160 240 320 % EXTENSION

400

Figure 3. Stress-strain curve of polypropylene before and after 7 Mrad irradiation

Figure 4.

Elastic modulus as a function of irradiation dose


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Figure 5.

Percent extension of polypropylene tensile bar as a func tion of irradiation dose

within 10% under controlled molding conditions. The high reproduci bility of breaking angles is caused by point concentration of stress when samples are broken i n a bending mode. W h e n irradiated polypropylene syringe flanges are subjected to the bending test, their curves appear as shown i n Figure 8. T h e main conse quence of irradiation is the reduction of the angle at which the sample can be deflected before breakage occurs. A s the sample is irradiated

4

6

DOSE (Mrads)

Figure 6.

8

10

Relative work as a function of irradiation for tensile bars


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Allara and Hawkins; Stabilization and Degradation of Polymers Washington, C. 20036 Advances in Chemistry; American ChemicalD. Society: Washington, DC, 1978.

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2.0 MRADS "CULPRIT" (AGED 7 MO.) 3.0 h -

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Figure 9. The post-irradiation effect on the bending angle of polypropylene syringe barrel flange to a higher dose, the angle at the breakpoint decreases steadily. The initial flex modulus has not changed significantly and is not a good parameter for monitoring irradiation damage. The best single parameter to evaluate irradiation damage is the area under the bending curve. This area gives an excellent representation of the work which the sample can undergo before "brittle snap," a true measure of embrittlement. A g i n g of an irradiated sample does not improve the mechanical properties; they continue to deteriorate with time as seen i n Figure 9. Post-embrittlement is caused by post-oxidation as a result of residual radicals and is discussed later (3). However, the oxidative reaction initi ated by the irradiation cycle does not cease with irradiation but continues for long periods of time. A n accelerated aging test has been devised to predict how a par ticular polypropylene would behave as a function of normal aging time. The results for several control conditions are summarized i n Table I, i n which each sample is the average of three test bars. A n increase i n tem perature and oxygen concentration results i n a drop i n breaking extension compared with a control sample. W h e n an accelerated-aged sample (40 hr at 100 °C i n 0 ) is compared with true aging i n air at ambient conditions, results as shown i n Figure 10 are obtained. The 40-hr accel erated test is equivalent to about three-months true aging. Accelerated aging at higher temperatures is the result of an increased rate of oxygen diffusion at these elevated temperatures. 2


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Table I.

Effect of Accelerated Aging on Breaking Extension of Polypropylene

Sample Description

% Extension

Control 2.5 M r a d 2.5 M r a d (air @ 100°C for 40.5 hr) 2.5 M r a d ( 0 @ 100°C for 40.5 hr) 2.5 M r a d (vaco @ 100°C for 40.5 hr) Control ( 0 @ 100°C for 40.5 hr) 2

2


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594.5 448.8 70.9 55.1 94.5 393.7

Pure polypropylene does not discolor at the 2.5 M r a d required for sterilization. However, the stabilizers added to improve the polymers thermal and irradiation stability often discolor upon irradiation. This yellowish discoloration increases with irradiation dose. This problem can be resolved by using stabilizers that do not discolor when they are irradiated. The severe degradation of polypropylene following sterilizing doses of irradiation can be characterized mechanically by its failure to undergo the necessary work i n practice. Embrittlement increases with time for an irradiated polypropylene, thus rendering an acceptable formulation totally unacceptable a few months after irradiation. Naturally, the decay of radicals can be accelerated by thermal annealing, limited b y the geometrical distortion temperature. 600r6 Control

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300

i£

200

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100 I

I I I I I ll

10

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1000

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Figure 10. Aging effect of irradiated (2.5 Mrad) polypropylene in air at room conditions and in oxygen at 100°C as a function of % extension


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Literature Cited


1. Geymer, D. O., in "The Radiation Chemistry of Macromolecules," Vol. II, p. 4, Academic Press, New York, 1973. 2. Charlesby, Α., "Atomic Radiation and Polymers," Pergamon Press, London, 1960. 3. Dunn, T. S., Williams, T. L., Sugg, H., Stanett, V. T., "Stability of Gamma Irradiated Polypropylene. Part II. Electron Spin Resonance Studies," ADV. CHEM. SER. (1978) 169, 151. 4. Billmeyer, F., "Textbook of Polymer Science," Wiley, New York, 1971. RECEIVED May 12, 1977.


Stability of -Irradiated Polypropylene - American Chemical Society

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