Medical Plastics and Biomaterials Magazine | MPB Article Index

Originally published July 1996

CHARLES LUNDY

The growing popularity of high-energy sterilization of medical devices has necessitated the development of a radiation-stable polycarbonate (PC). Since 1986, several PC producers have added radiation-stable formulations to their medical product lines. The current practice in the stabilization of gamma- or electron beam­sterilized PC uses a polypropylene-glycol (PPG) derivative added to the PC in concentrations of less than 1%. It is generally believed that PPG derivatives act as radical scavengers, reducing the number of radical reactions that occur along the backbone of irradiated PC. Recent evidence shows that the addition of compounds thought to act as electron scavengers dramatically enhances PC radiation resistance. Of particular interest is that the combination of this second group of additives with PPG derivatives synergistically reduces the yellowness index (YI) of PC after radiation exposure. The goals of this article are to more fully explicate the nature of radiation stabilization in PC and to develop a model that explains the differences in stabilization efficiency between compounds considered radical scavengers and those thought to be electron scavengers.

BACKGROUND

Before discussing the effects of different additives on the radiation stability of polycarbonate, it is first necessary to describe what happens to a PC component during radiation exposure. Current consensus holds that an electron emitted from a gamma source or electron generator interacts with a host molecule such as PC, causing ejection of a lower-energy electron from an impacted chemical substituent into the polycarbonate matrix. (The mechanism is very complex, and depends greatly on which chemical substituents are impacted.) As the electron passes through the PC substrate, it impacts other PC molecules, forming local ionization clusters that relax into radicals. These radicals recombine, chain transfer, or are quenched into a stable product by oxygen or by other radical scavengers present in the matrix.

The end result of these various termination reactions is to produce a large variety of color bodies in the PC, inducing a characteristic discoloration in the part (see Figure 1). After irradiation, PC rapidly photobleaches from a dark yellow-brown material (indicative of charged or radical species), reverting to its original color in a process lasting 3 to 4 weeks. Ultraviolet light and heat help speed this reversion effect. (Figures not yet available on-line.)

Entrapped within the PC matrix, radicals formed during irradiation can survive for several weeks before being terminated. Unlike polytetrafluoroethylene, polypropyl-ene, or polymethylmethacrylates, the physical properties of PC remain virtually unchanged after low doses (<10 Mrd) of radiation.¹ This occurs despite the relatively large chemical transformations and extremely long lifetimes of reactive species in PC.

Although we have learned a considerable amount about the effects of high-energy radiation on polymer degradation, much less is known about stabilization techniques, in particular PC stabilization.² The best known and most effective method of stabilizing polymers is through the incorporation of radical scavengers such as aromatic hindered amines, phenols, and thiols.^3,4 These compounds are normally capable of generating a large number of hydrogen radicals, which terminate radicals produced in the polymer and thus prevent further oxidation. PPG is another example of a radical scavenger, producing hydrogen radicals that terminate radical sites along the PC backbone.^5,6 Figure 2 shows the mechanism by which hydrogen radicals are formed in PPG. The effect on YI of adding a PPG derivative to PC after gamma radiation is shown in Figure 3.

Whereas PPG is a radical scavenger, some compounds act as electron scavengers. Upon colliding with a high- energy electron, these substances typically degenerate into radical/ion pairs.

Such compounds are, by their nature, capable of dissociative electron attachment (DEA).⁷ This mechanism reduces the energy of the primary and secondary electrons, and can thereby reduce the damage caused by radiation in polymers like PC. Research showing that brominated aromatic compounds possess electron-scavenging properties was done as early as the 1950s.

The combination of both electron and radical scavengers in PC produces a synergistic protective effect. Although electron scavengers help stabilize a host polymer through DEA, they nevertheless cannot prevent the formation of radicals within the polymer matrix. These radicals must still be terminated with the help of radical scavengers. The electron scavengers can be thought of as accelerators of radiation decomposition, acting in unison with free- radical stabilizers.

EXPERIMENTAL

All compositions were extruded and molded under standard PC-processing conditions. The molecular weight of the PC used was nominally 28,000 g/mole. Injection-molded chips measuring 3 x 2 x 1/8 in. were irradiated to 3.5 Mrd by a cobalt 60 source. The YI was determined before radiation, 3 hours after radiation, and 10 days after radiation, as directed by ASTM D 1925. All YI values were determined for color chips that had been stored in the dark.

RESULTS AND DISCUSSION

Brominated Compounds. The study investigated the effect of brominated aromatic compounds as stabilizers in PC. We particularly wanted to discover whether a higher degree of substitution of bromine would improve the stability of the subsequent PC formulation. The additives looked at were tetra-bromo-bisphenol A-oligocarbonate (TOBC) and tetra-bromo-phthalic anhydride (TBPA), added in concentrations of 1% to unstabilized PC. As shown in Figure 4, both the TBOC/PPG system and compositions containing TBPA yielded excellent benefits in terms of the total shift in color (delta YI), defined as the difference between YI before and 10 days after radiation (the standard induction time). The TBPA gave significantly better results than TBOC, which can be attributed to its more highly brominated nature.

An examination of the rate at which color reversal occurs (photobleaching effect) in the different compositions studied reveals a stark difference between the additives (see Figure 5). Table I lists the additives and their relative photobleaching constant delta YI/delta T, defined as the change in yellowness index over time. TBPA derivatives have a much lower initial color after radiation--that is, a lower rate constant for color return after radiation. In comparison, TBOC has a higher initial color and higher color- return rate constant. The data for the brominated compounds suggest that a higher degree of substitution yields a better stabilization effect, following the order 4°>3°>2°>1°.

Radical concentration was monitored by electron spin resonance to determine the effect of bromine concentration on radical reduction. Because bromine is an electron scavenger, it should reduce the amount of radicals formed in subsequent reactions. Table II shows the results after 3.5-Mrd irradiation.

Tables I and II illustrate the significant stabilizing effect of the bromine compounds, with the more highly brominated formulations exhibiting the strongest effect. With both TBOC and TPBA, one can easily see the synergism between the brominated species material and PPG. Highly brominated aromatic compounds clearly act as efficient radiation stabilizers, particularly when used in combination with PPG.

Nonhalogenated Stabilizers. Although brominated aromatic compounds are effective as gamma-radiation stabilizers, questions about their safety have been raised: these compounds have the potential to form carcinogenic aromatic dioxins, both thermally and photochemically. For this reason, we directed our research toward the development of halogen-free radiation stabilizers.

We investigated a number of halogen-free aromatic compounds, based on their ability to form a stable radical/anion pair in the manner of aromatic bromine compounds. As mentioned, brominated compounds are thought to be effective as stabilizers because of their ability to undergo DEA or, more simply, their ability to form radical/anion pairs. This behavior normally can occur only with compounds capable of absorbing an electron--compounds with low-lying orbitals that exhibit thermodynamically favorable anion formation.⁸

In the course of our investigations, we have found a new class of materials that are nonhalogenated compounds capable of DEA, and therefore able to act as electron scavengers. Figure 6 shows the stabilizing effect of an aromatic disulfide (ADS) compared with that of the brominated electron scavengers TBOC and TBPA. ADS compounds equal the efficiency of tetra-substituted brominated aromatic compounds such as TBPA derivatives, and exceed the performance of di-substituted aromatics such as TBOC.

As do brominated compounds, this new class of materials seems to work most effectively in the presence of a radical scavenger such as PPG. The YI results given in Figure 7 illustrate the stabilization effects in gamma-irradiated (3 Mrd) PC of ADS compared with TBOC, both with and without PPG. The graph indicates that the synergistic effects of the different electron scavengers with PPG are very similar.

CONCLUSION

Radiation stabilizers for polycarbonate appear to be either free-radical scavengers or electron scavengers. Results of this study point to a synergistic effect when the two types of stabilizers are combined in PC. Electron scavengers are molecules with thermodynamically favorable anion/radical pairs that react with primary or secondary electrons during irradiation. Radical scavengers react with radicals produced along the polymer chain after irradiation. Combining the two can yield polycarbonate formulations that undergo virtually no yellowing after gamma or electron-beam radiation. Although brominated aromatic compounds are effective stabilizers, a new class of nonbrominated materials has proven equally effective in scavenging radiation in PC.

ACKNOWLEDGMENTS

The author wishes to express his sincere appreciation to Sivaram Krishnan, PhD; Rick Archey; and Douglas Powell, PhD, who made significant scientific contributions to this work.

REFERENCES

1.Encyclopedia of Polymer Science & Engineering, vol 13, New York, Wiley, p 686, 1988.

2.Encyclopedia of Polymer Science & Engineering, vol 13, New York, Wiley, p 667, 1988.

3.Encyclopedia of Polymer Science & Engineering, vol 13, New York, Wiley, p 695, 1988.

4.Dole M, The Radiation Chemistry of Macromolecules, vol 1, New York, Academic Press, 1973.

5.Archie et al., U.S. Pat. 5,187,211.

6.Jorissen et al., U.S. Pat. 5,006,572.

7.Dole M, The Radiation Chemistry of Macromolecules, vol 1, New York, Academic Press, 1973.

8.Trans Faraday Soc, 61:1960, 1965.

Charles Lundy, PhD, is currently vice president and general manager at Floralife, Inc. (Walterboro, SC). From 1986 to 1994 he worked in various research and marketing positions at Mobay/Miles, Inc. (Pittsburgh) and Bayer AG (Leverkusen, Germany).