Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published March 1997

TECHNICAL PAPER SERIES

Bisphenol A­based polycarbonate (PC) is a well-known engineering thermoplastic with an excellent balance of optical, physical, mechanical, and processing characteristics. Various grades of PC are widely used in a range of medical applications because of the material's transparency, toughness, rigidity, heat resistance, dimensional stability, and moldability in intricate parts.

Many medical devices produced from PC--for example, dialysis and anesthesia canisters, syringes, stopcocks, blood oxygenators, cardiometry reservoirs, intravenous connectors and accessories, blood filters, and centrifuge bowls--require sterilization to help ensure product safety. One of the most popular medical sterilization techniques is gamma irradiation. However, gamma sterilization of a polymer generally gives rise to an undesirable color in the resin.¹

In the case of PC, gamma irradiation to the normal dose levels of 2.5 to 3.5 Mrd for disposable devices imparts a yellow-green color without significantly affecting impact or tensile properties.^1­3 This yellow-green color slowly fades as a function of time in the dark, a phenomenon known as color reversal.^2,3 Exposure to light has been shown to accelerate the fading.³ Under typical storage conditions of molded parts in the dark, it may take too long for the color-reversal process to result in an acceptable low-yellowness color.^2,3 Consequently, in an effort aimed at minimizing the gamma-induced yellowing of PC, researchers have evaluated a variety of additives.

In order to shed light on the radiolysis mechanisms and yellowing processes of gamma-irradiated PC at the molecular level, a number of studies have been carried out using a range of techniques: analysis of by-products,^4­7 Fourier transform infrared spectroscopy (FTIR),^6,8 high-pressure liquid and gas chromatography combined with mass spectrometry,⁹ visible absorption spectroscopy,^9­12 and electron spin resonance (ESR) spectroscopy.^{4, 10­13} Results have been informative, but diverse. Furthermore, the root cause of the yellow-green color in gamma-irradiated PC samples at room temperature has yet to be well established in the literature.^{1,3,4,9,10,12}

As part of a continuing effort to find better stabilizers for PC, the effect of alcohol structures in aromatic carboxylic acid esters on the color stabilization of gamma-irradiated PC was recently evaluated using dicyclohexyl phthalate (DCHP), tri-isononyl trimellitate (TINTM), and di-tridecyl phthalate (DTDP). The present paper describes the experimental results and analyzes them in terms of molecular structures and their radiolysis mechanisms.¹⁴ Because the PC systems containing the above additives have not been tested for suitability in medical applications, the information related in this paper should not be construed as a recommendation for such uses.

BACKGROUND

Before discussing the experimental results, it is appropriate to review the existing literature for information on both the origin and fading of yellow-green color in PC and on the radiolysis mechanisms of PC and aromatic carboxylic acid esters.

Electron Spin Resonance Spectroscopy. To elucidate the radiolysis mechanisms of PC by investigating the free-radical species generated during and after gamma irradiation, various researchers carried out elaborate ESR studies on irradiated samples in vacuum and in the dark at ­196°C, followed by photobleaching and/or warming to high temperatures.^4,10,11,13 Owing to space limitations, this paper reviews only highlights of these past experiments.

Gamma-irradiated PC samples showed either very complex or very simple ESR spectra, depending on experimental conditions. With the samples in vacuum and in the dark at ­196°C, the following five ESR features have been observed: a sharp singlet superimposed on a broad singlet at the center, and three doublet signals at the wings. The doublet signals comprised a very weak doublet with a hyperfine splitting (hfs) of 113 G, a sharp doublet with an hfs (A_*) of perpendicular features (the dipolar electron-electron coupling tensor) equaling 148 G, and another doublet with A_* equaling 88 G.^4,10,11,13

The central sharp singlet was variously attributed to the trapped electrons⁴ and to carbonate radical anions.^10,13 The broad singlet at the center and the 113-, 148-, and 88-G doublets were assigned, respectively, to the phenoxy radicals,^4,10,11,13 the C-13 splitting of carbonate radical anions,¹³ the phenoxy-phenoxy radical pairs separated by CO molecules,^11,13 and the phenyl-phenoxy radical pairs separated by CO₂ molecules.¹³ Formation of the two radical pairs was reported to correlate well with the released amounts of CO and CO₂.^7,11

The literature also includes two illuminating ESR studies on the radical pairs that were observed in gamma-irradiated diphenyl carbonate and di-p-tolyl carbonate at ­196°C.^11,15 In the case of diphenyl carbonate, the effective separation distances of the phenyl-phenoxy and phenoxy-phenoxy radical pairs were determined to be 68 and 57 nm, respectively, on the basis of their zero field splitting values.¹¹ The same study reported that the phenoxy-phenoxy radical pairs decayed rapidly in the region of ­168°C, while the phenyl-phenoxy radical pairs decayed at approximately ­148°C and some isolated phenoxy radicals still persisted at room temperature.¹¹ Formation and decay of the isolated phenoxy radicals and of the two kinds of radical pairs were also inferred in electron beam­irradiated diphenyl carbonate by the presence of the following degradation products: phenol from phenoxy radicals; 2- and 4-hydroxy diphenyl ether; 2,6- and 2,4'-dihydroxybiphenyl from phenoxy-phenoxy radical pairs; and diphenyl ether, 2- and 4-hydroxybiphenyl from phenoxy-phenyl radical pairs.⁵

In similar experiments with di-p-tolyl carbonate, the ratio of the number of phenoxy-phenoxy to phenyl-phenoxy radical pairs was reported to be approximately 3:1, whereas the ratio of the number of radicals in radical pairs to the number of isolated radicals was 5.7:1.¹⁵ The radical pairs decayed rapidly near ­133°C, while the isolated radicals did not decay below approximately ­93°C. Only about 15% of the spins initially trapped at ­196°C were isolated radicals, and the radical pairs decayed exclusively by recombination with each other.

Finally, it should be pointed out that, even though the assignment of the sharp central singlet and the 118-G doublet to the carbonate radical anions seems to be reasonable in view of the well-established ESR results for dimethyl carbonate radical anions,¹⁶ there is a distinct difference in the results of the photobleaching experiment. The studies reported no evidence of any radical formation in the case of PC, as opposed to clear evidence of methyl radical formation via dissociation of dimethyl carbonate radical anions upon photobleaching. This significant difference is discussed later during the review of PC radiolysis mechanisms.

In summarizing the diverse ESR results, one can conclude unequivocally that gamma irradiation of PC generates the isolated phenoxy radicals, phenoxy-phenoxy radical pairs, and phenyl-phenoxy radical pairs.

Visible Absorption Spectra. Various studies in the literature report that gamma-irradiated PC samples showed a broad visible absorption spectrum, with maxima at 415, 407, and 575 nm.^10,11 The 575-nm band was attributed to phenoxy radicals, because these radicals reportedly show a similar band.¹¹ Interestingly, it was reported that x-ray­ irradiated PC samples also showed a visible absorption spectrum with maximum at 400 nm that decayed as a function of time in the presence of oxygen--although not in vacuum or in nitrogen--at room temperature.¹² The 400-nm band was attributed to an unidentified, reactive radical species characterized by a broad ESR spectrum without any resolved feature. This attribution was reportedly based on a good correlation of 400-nm-band decay with the ESR signal decay.

Molecular Origin of Yellow-Green Color and Its Decay Mechanism in Gamma-Irradiated PC. To determine the cause of the 400-nm band observed in gamma-irradiated PC at room temperature, a literature search was carried out on the visible absorption spectra of phenoxy, phenyl, and phenylperoxyl radicals (the latter formed by reaction of phenyl radicals with oxygen). The search, which used a Chemical Abstract database, revealed that the first excited state of phenoxy radicals is at 1.06 eV (400 nm) above the ground state.¹⁷ Unsubstituted phenoxy radicals show a 400-nm band with a very weak and diffuse band near 600 nm; this 600-nm band becomes stronger in some ortho-substituted derivatives.^18­20 In contrast, the absorption maximum of phenyl radicals is 505 nm,²¹ while that of phenylperoxyl radicals is between 490 and 600 nm, depending on substituents.^22­24

On the basis of the aforementioned information, both the 400-nm absorption band and the yellow-green color observed in gamma-irradiated PC at room temperature can be attributed to the isolated phenoxy radicals. These radicals are likely to disappear by abstracting hydrogen atoms from the isopropylidene groups to form phenol end groups. This process is suggested by the observation of a new broad IR band at 3450 cm^­1 and by reduction in the FTIR band intensity of methyl groups after gamma irradiation.^8,10

Radiolysis Mechanisms of PC. Any description of the radiolysis mechanism of PC needs to adequately explain the formation of isolated phenoxy radicals and two different radical pairs as well as the evolution of large amounts of CO₂ and CO. A priori, the interaction of gamma rays with carbonate groups of PC can produce a variety of species according to the following seven reaction paths, in which the formula of O-CO-O represents a carbonate group in the PC repeating unit of -PhO-CO-OPh- :

(1) gamma rays + PhO-CO-OPh - {PhO-CO-OPh}⁺ + e^­

(2) {PhO-CO-OPh}⁺ + e^­ - {PhO-CO-OPh}^{* (excited)}

(3) {PhO-CO-OPh}^{* (excited)} - PhO. + CO + PhO.

(4) {PhO-CO-OPh}^{* (excited)} - PhO. + CO₂ + Ph.

(5) PhO-CO-OPh + e^­ - {PhO-CO-OPh}^-­.

(6) {PhO-CO-OPh}^­. - PhO^- + CO₂ + Ph.

(7) {PhO-CO-OPh}^­. - PhO^- + CO + PhO.

The homolysis of carbonate groups via reaction paths 3 and 4 has been generally suggested in the literature to explain various by-products observed in gamma-irradiated PC.^5­7,9,13 However, although these pathways can explain the formation of the radical pairs, CO₂, and CO, they cannot explain the formation of isolated radicals. In contrast to the homolysis mechanism, the dissociation of carbonate radical anion intermediates (heterolysis) via reaction path 7 can explain the formation of isolated phenoxy radicals but not of radical pairs. Consequently, it is proposed here that both homolysis and heterolysis of carbonate groups occur during gamma irradiation of PC.

If we accept this conclusion, the relative importance of the homolysis and heterolysis processes can be estimated using the results from the gamma-irradiated di-p-tolyl carbonate at ­196°C.¹⁵ Since the radical pairs are known to recombine with each other, the observed high ratio (5.7:1) of the number of radicals in radical pairs to the number of isolated radicals indicates that homolysis is a more dominant process than heterolysis. However, heterolysis is the main process for generating the isolated phenoxy radicals.

An additional point of interest is that the formation of PC radical anions and their subsequent dissociation process during photobleaching have not been well established in comparison with the cases of dimethyl carbonate.^4,10,13,16 It is tempting to explain the difference between the two systems by discussing the relative stability of their radical anions. In the dimethyl carbonate radical anions, the spins (excess electrons) were reported to be considerably delocalized onto the methoxy groups and the carbonyl oxygens.¹⁶ However, excess electrons of PC radical anions can be assumed to be more delocalized into phenyl and phenoxy groups. This delocalization weakens the -O-C bonds of -O-CO-O- radical anions, facilitating their dissociations.

The above assumption is based on several findings in the literature: that the electron affinity (2.36 eV) of a phenoxy radical is close to that (2.69 eV) of an O-CO-O molecule;²⁵ that the unpaired spin density in the phenoxy radical is highly localized, with an appreciable spin density on the 2- and 4- positions of the aromatic ring according to the ESR data;²⁶ and that the phenoxy radical has a quinoid-like structure with a double bond.²⁷ Consequently, the phenoxy radical is much more stable than the methyl or methoxy radical, leading to easier dissociation of intermediate PC radical anions. The high stability of the phenoxy radical is also likely to be responsible for the reaction paths 3 and 7, which explains why the reported yield of CO from gamma-irradiated PC is larger than that of CO₂.^4,7

Radiolysis Mechanisms of Esters. In the literature, the ESR and optical results of gamma-irradiated ester systems are phenomenologically similar to those of gamma-irradiated carbonate systems.^28­30 For instance, in the gamma-irradiated aliphatic acid esters, formation of ethyl-acetate radical anions and subsequent dissociation to form acetate anions and ethyl radicals have clearly been established using ESR techniques.²⁸ However, such events have not been verified for aromatic acid esters, probably because of the instability of their radical anions and a poorly resolved ESR spectrum for analysis.

In the case of gamma-irradiated polyethylene terephthalate (PET), however, there is chemical evidence that can be interpreted in terms of the dissociation of radical anions. The G value (yield/100-eV energy absorbed) of -COOH groups has been reported to be much higher (0.77) than those of CO₂, CO, and H₂ (0.17, 0.11, and 0.015, respectively).³⁰ This high G value of -COOH groups can be attributed to the formation of large amounts of carboxylate anions via dissociation of ester radical anions as intermediates.

The difference in radical-anion stabilities of aromatic and aliphatic acid esters can be inferred using relative stabilities of acid anions formed via dissociation of radical anions. Acidity constants (Ka) of acids can provide some insight on the relative stabilities of acid anions. For instance, benzoic acid was reported to have a higher Ka value (6.5E-05) than acetic acid (1.75E-05) in water, indicating that the benzoate anion is more stable than the acetate anion.³¹ On the basis of these results, it is reasonable to assume that radical anions of aromatic acid esters are more likely than those of aliphatic acid ester to undergo dissociation to form their acid anions.

EXPERIMENTAL PROCEDURES AND RESULTS

Using a twin-screw extruder, polycarbonate resin (Makrolon 2608, Bayer Corp., Pittsburgh) was compounded with DCHP, TINTM, and DTDP in various amounts. The compounded pellets were injection molded into test specimens measuring 7.5 * 5 * 0.25 cm and irradiated at ambient conditions using a cobalt-60 gamma-radiation source to a dose of 3.3 Mrd. Specimens were then stored for 1 day in the dark--necessitated by sample transportationfrom the irradiation site--and yellowness index (YI) was determined according to ASTM D 1925.

Table I. Change in yellowness index (YI) of polycarbonate with and without additives after gamma irradiation to a dose of 3.3 Mrd. YI values measured before and after irradiation and then after 1-day storage in dark.

Table I shows the differences (YI) in YI values measured before and after gamma irradiation. Comparison of the data shows that DCHP/PC samples have much lower YI values than do TINTM/PC, DTDP/PC, or control (no additives) PC samples. For instance, in comparison with a YI value of 31.6 for control PC samples, the values of samples containing 0.77% DCHP, 1.5% TINTM, and 1.5% DTDP by weight are 11.6, 28.4, and 29.6, respectively.

Figure 1.  YI versus storage time in dark of irradiated PC samples.

The effect of room-temperature dark storage time on color was also measured. As can be seen from the plots in Figure 1, the YI values decreased as a function of storage time, and different samples showed different fading rates. Initial rapid fading for the DCHP, TINTM, DTDP, and control PC samples slowed after storage for approximately 7, 26, 28, and 43 days, respectively. This short fading time of the DCHP samples could allow for easier use of typical color-compensation techniques in masking color shifts caused by gamma irradiation.²

In summarizing the experimental results, DCHP can be concluded to be an excellent gamma-radiation stabilizer for PC. Its stabilization mechanism is discussed in the next section.

DISCUSSION

According to the radiolysis and yellowing mechanisms proposed earlier, an additive must be capable of capping phenoxy radicals or of scavenging secondary electrons more efficiently than PC in order to minimize the yellowing caused by gamma irradiation. In principle, electron scavenging is a better stabilization process than capping phenoxy radicals, because the former also prevents carbonate chain scissions caused by secondary electrons.

The stabilizing effect of ester groups can be explained by assuming that they have a higher reactivity in forming their radical anion intermediates than do carbonate groups. This assumption is supported by the fact that an acid anion is more stable than a carbonate anion, since oxalic acid (HOOC-COOH) has a much higher Ka value than does carbonic acid (5.9E-02 versus 2E-04).³²

The difference in the results of DCHP and TINTM or DTDP systems can be attributed to the differences in stability among cyclohexyl, isononyl, and tridecyl radicals, which are produced via dissociation of corresponding radical anions formed during gamma irradiation. The cyclohexyl radical, being a secondary radical, can be assumed to be more stable--by about 3 Kcal/mole--than the isononyl and tridecyl radicals, both of which are primary radicals.²⁵

In contrast with the above explanation, potential reactions of isolated phenoxy radicals with hydrogen atoms in the additives cannot explain the results. In other words, if these mechanisms were important for stabilization of gamma-irradiated PC, TINTM with tertiary hydrogens should have been a more efficient stabilizer than either DCHP with tertiary-like hydrogens or DTDP with secondary hydrogens. This reasoning is based on the radical stabilities of the additives' reaction products with the isolated phenoxy radicals: tertiary radicals are known to be more stable than secondary radicals by 3.5 Kcal/mole.²⁵

CONCLUSION

The present study shows that, in comparison with TINTM and DTDP, DCHP is an excellent stabilizer for gamma-irradiated PC. This result is explained in terms of the differences in reactivities of the additives with damaging secondary electrons formed during irradiation. The high electron-scavenging efficiency of DCHP reduces the formation of isolated phenoxy radicals, which are responsible for the yellow-green color of gamma-irradiated PC at room temperature.

ACKNOWLEDGMENTS

The author wishes to thank N. R. Lazear and M. Witman for their generous support of this work. Special thanks are also extended to Professor F. Williams of the University of Tennessee (Knoxville) for his valuable input.

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James Y. J. Chung is an associate research scientist in the Makrolon polycarbonate technical marketing group of the polymers division at Bayer Corp. (Pittsburgh, PA). He formerly worked in the chemical engineering department of North Carolina State University, at Borg Warner Chemicals, and at Ethyl Corp. He holds a PhD in physical chemistry from the University of Tennessee at Knoxville.

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