Originally published January 1997

TECHNICAL PAPER SERIES

Clear, Radiation-Tolerant, Autoclavable Polypropylene

Robert C. Portnoy

Communication with the highest-volume consumers of medical grades of polypropylene indicates that the preferred polypropylene resin for injection-molded and radiation-sterilized medical devices would be a radiation-tolerant, clear, autoclavable formulation. Combined with the results of our own work in this field over a period of more than 20 years and the significant literature on the subject, this intelligence provides substantial justification for the development of the new material outlined in some detail below.

Normally stabilized polypropylenes are not suitable for sterilization by high-energy radiation because of the severe embrittlement and discoloration that occur in the plastic immediately after sterilization and then worsen with aging. While the embrittlement of the plastic after irradiation is an inherent property of the polymer, the discoloration is caused by reaction products of the phenolic antioxidants normally included in standard polypropylenes.^1­3

There are, however, several techniques in the design of propylene polymers and formulations that remedy these problems and yield resins suitable for irradiation at dosages up to 50 kGy. Early radiation-tolerant polypropylenes were homopolymers stabilized with small quantities of phenolic antioxidants and large amounts of sulfide diester secondary antioxidants. These materials did discolor slightly after irradiation.²

The modern resins that are most successful in withstanding irradiation exhibit reduced crystallinity and narrow molecular-weight distribution, are formulated with hindered-amine light stabilizers (HALS), and contain none of the discoloring phenolic antioxidants. Ethylene- containing random copolymers are useful substrates for building radiation-tolerant formulations, as are homopolymers with low isotacticity and homopolymers to which hydrocarbon oils or greases have been added. The HALS are, by themselves, noncoloring in polypropylene, but they interact with phenolic antioxidants to produce extremely deep yellow colors after irradiation. Therefore, when HALS are used in a polypropylene formulation, the phenolic antioxidants must be scrupulously avoided.^1­8

Although glasslike clarity is not essential for most applications, transparency is highly valued for both aesthetic and technical reasons. "See-through" clarity is very helpful in the filling, measuring, and dispensing operations for which many medical devices are routinely used. Clarity is commonly obtained in polypropylene in two ways. First, resins with lower total crystallinity will be clearer than those with higher, but a minimum level of crystallinity is necessary to provide the required strength, stiffness, and resistance to softening at elevated temperature expected of the polypropylene. Second, certain organic nucleating agents greatly improve the clarity of polypropylenes by producing only very small crystals in the polymer. These smaller crystals are below the size that scatters visible light and produces haze.⁹

Resistance to softening at elevated temperature allows a polypropylene to be sterilized by autoclaving, most commonly at 121°C or above. This property is obviously not required to ensure the sterility of a properly packaged, irradiated medical device as sold. Nevertheless, many of the major device manufacturers choose to offer this feature in their products as a bonus to the consumer. Autoclavability, for example, allows disposable syringes to be resterilized in the hospital as part of a procedure kit after having been removed from their original packaging and thus contaminated. Veterinarians often reuse disposable syringes and value the ability to sterilize them by autoclaving.

In order for a disposable medical device like a syringe to be autoclavable, the wall of the part must be able to withstand any applied stresses--such as the expansive stress of the compressed rubber piston--at the sterilization temperature. Although radiation-tolerant propylene random copolymers, with 3% or more ethylene comonomer, can sometimes withstand the sterilization conditions in a part that is not under significant stress, soft resins such as this are not generally autoclavable in a syringe or other similar configurations. Significantly higher crystallinity is required for a polypropylene to be autoclavable under such stresses.

Unfortunately, the three desired attributes in a premium polypropylene for medical devices--radiation tolerance, clarity, and autoclavability--were until recently thought to be incompatible. Any two of them are fairly easy to obtain in a single resin, however. For example, radiation tolerance and clarity are easily obtained in a properly stabilized, nucleated random copolymer.^10,11 Radiation tolerance and autoclavability have been available for years in properly stabilized homopolymers. Clarity and autoclavability, without radiation tolerance, are commonly achieved in nucleated homopolymers. There is at least one known method for producing a resin possessing all three of these attributes--a process that uses oils or greases as modifiers or "mobilizing agents" to stabilize the clear polypropylene homopolymer against the embrittlement caused by radiation without excessively reducing its resistance to softening at elevated temperature. This proprietary technology, however, is unavailable to the industry in general.^2,4­7

This paper describes research that has culminated in the development of a new type of polypropylene formulation displaying all three of the desired properties--clarity, radiation tolerance, and resistance to softening at elevated temperature. The work was engendered principally by the recent emergence, as commercial entities, of ultra-low-density, plastomeric, ethylene polymers produced from metallocene catalysts (plastomers).

It is our experience that the radiation resistance of propylene random copolymers is directly related to the ethylene content of the resins.¹⁰ We have also learned that bimodal mixtures of ethylene-lean and ethylene-rich propylene polymers are more radiation resistant than are random copolymers having a unimodal ethylene distribution, even though the average comonomer contents of the two materials are identical. For some time, we have had the goal of examining the radiation resistance of an extreme embodiment of this bimodal situation--namely, blends of minor amounts of polyethylene with polypropylene--but were reluctant to do so because of the adverse effect of the polyethylene upon the clarity of the mixtures. With the discovery of the ability of ultra-low-density ethylene polymers produced by metallocene catalysts to provide significant impact modification to blends with polypropylene with only small increases in haze, the goal of a premium polypropylene formulation seemed to be within reach. The extensive experimental program that was undertaken to establish that blends of ethylene plastomers with polypropylene could be devised with the three desired properties is described below.

EXPERIMENTAL

This paper documents three experiments. In the first experiment, we examined the effect of several levels of a plastomer previously found useful in polypropylene impact modification on the gamma-radiation tolerance of a clarified homopolymer that contained an effective radiation-protection additive package. The positive results from this experiment in the areas of resistance to radiation and to softening at elevated temperature led us to examine the effects of a broad range of plastomers on the haze of blends with nucleated propylene homopolymer. Our ability to identify a plastomer that had a negligible effect on haze then prompted us to formulate and test a blend designed to be clear, radiation resistant, and autoclavable.

All of the plastomers used in this work were ethylene-butene copolymers produced by metallocene catalysts and sold commercially by Exxon Chemical Co. under the name Exact. They were characterized by extremely narrow molecular-weight and comonomer distributions. The plastomers P1 and P5, which were used in experiments involving irradiation of blends with polypropylene, were "barefoot" polymers containing no additives whatsoever.

Experiment 1. Stabilization of Irradiated Homopolymer by a Plastomer Produced with Metallocene Catalyst. In this experiment, a nominal 1.0-melt-flow-rate (MFR) propylene homopolymer granule with moderately high crystallinity (as measured by a heptane insolubles level >95.5%) was treated with an organic peroxide in a melt extrusion­pelletization process to increase its MFR to 25 dg/min. The homopolymer was then repelletized with 0.08% calcium stearate; 0.05% Tinuvin 622 LD (Ciba Geigy Corp., Tarrytown, NY); 0.08% Ultranox 626 (General Electric Co., Parkersburg, WV); and 0.25% Millad 3940 (Milliken and Co., Spartanburg, SC); and with variable amounts of an additive-free, 4.0-melt-index ethylene-butene copolymer with a density of 0.885 g/cm³ (plastomer P1). The blends are described more fully in Table I.

Table I. Blends of propylene homopolymer with plastomer P1.

The blends and a sample of a performance standard--number 18277-054-008, consisting of PP 9074MED (a radiation-tolerant, clarified, 2.8%-ethylene, 24-dg/min-MFR random copolymer)--were then injection molded into ASTM test parts. The parts consisted of dog-bone-shaped tensile bars (165 * 12.7 * 3.18 mm), Gardner disks (88.9 mm diam * 3.18 mm), and flex bars (127 * 12.7 * 3.18 mm). Four sets of parts were established for each resin and then irradiated by Isomedix, Inc. (Morton Grove, IL), at nominal gamma dosages of 0, 25, 50, and 75 kGy using cobalt 60 at a rate of <10 kGy/hr. The parts were aged at 60°C for 21 days.

Testing of the properties of the samples was carried out as rapidly as possible following the aging regimen. The properties measured--all at 23°C--were tensile elongation at break (ASTM D 638-87b), Gardner impact strength (ASTM D 3029-84), deflection at peak flexural load, and Hunter-b color (Gardner Model PG 5500 photometric unit with C illuminant, 10° light source, port closed; Pacific Scientific, Gardner Laboratory Div., Bethesda, MD). In addition, we measured the heat-deflection temperature (HDT) at 455 kPa (ASTM D 648-82) of the range of samples irradiated at 50 kGy but not aged, as well as the secant flexural moduli (ASTM D 790-86) of unirradiated and unaged samples with all levels of plastomer.

Experiment 2. The Relationship between Plastomer Melt Index (MI), Density, and Haze of Blends with Polypropylene. In the second experiment, a 25-dg/min polypropylene base resin was first prepared by taking a nominal 1.0-dg/min-MFR propylene homopolymer granule with moderate crystallinity (as measured by a heptane insolubles level between 94 and 96%) and treating it with an organic peroxide while melt-blending and pelletizing it with 0.08% calcium stearate, 0.05% Tinuvin 622 LD, 0.08% Ultranox 626, and 0.25% Millad 3988. Ten pellet blends of this base stock were prepared with eight different plastomers selected according to a statistical design (2 variables, MI and density; 3 levels; full factorial with 1 missing combination; 3 replications of a center point). The weight ratio of polypropylene to plastomer was 90/10. The blends and the unblended homopolymer base stock (control) were injection molded into 1.0-mm haze plaques and tested for haze by the ASTM D 1003-92 method. The experimental runs are described in detail in Table II.

Table II. Experiment 2 results for blends of propylene homopolymer with various plastomers.

Experiment 3. Confirmation of Clarity, Radiation Tolerance, and Resistance to Softening at Elevated Temperature in a Single Blend of Propylene Homopolymer and Plastomer. In the final experiment, a 25-dg/min stock blending compound was first prepared by treating a nominal 1.3-dg/min-MFR propylene homopolymer granule with moderate crystallinity (as measured by a heptane insolubles level between 94 and 96%) with an organic peroxide while melt-blending and pelletizing it with 0.03% DHT4A, 0.06% GMS-11, 0.25% Millad 3988, 0.10% Tinuvin 622 LD, and 0.08% Ultranox 626. Three pellet blends were then prepared with different levels of P5, a metallocene-catalyzed, plastomeric ethylene-butene copolymer containing no additives. The neat PP formulation was used as a 0% plastomer sample to be compared with the blends. Table III shows in detail the characteristics of the four resins. Molding, irradiation, aging, and testing of these samples were carried out exactly as described in Experiments 1 and 2.

Table III. Polypropylene­P5 plastomer blends (experimental samples).

RESULTS

The data from Experiment 1 are reported in graphical form in Figures 1­6.

Figure 1. Deflection at peak flexural load of polypropylene-plastomer blends (Experiment 1).

Figure 2. Tensile elongation at break of polypropylene-plastomer blends (Experiment 1).

Figure 3. Gardner impact strength of polypropylene-plastomer blends (Experiment 1).

Figure 4. Color of polypropylene-plastomer blends (Experiment 1).

Figure 5. Heat-deflection temperature at 455 kPa of polypropylene-plastomer blends. Samples were irradiated at 50 kGy, with accelerated aging (Experiment 1).

Figure 6. The 1% secant flexural modulus of polypropylene-plastomer blends that are not irradiated or aged (Experiment 1).

The data from Experiment 2 are reported in Table II. These data were subjected to a multiple-regression procedure that provided an equation for the dependence of the haze of a polypropylene-plastomer blend upon the density and MI of the plastomer. The response surface plot of this equation is shown as Figure 7.

Figure 7. Response surface relating haze of sample blends to MI and density of plastomer component (Experiment 2).

The data from Experiment 3 relating to the physical properties and color of the samples are reported and graphical form in Figures 8­12. Data from the same experiment relating to the haze of the samples are reported in tabular form in Table III and graphically in Figure 13.

Figure 8. Haze of polypropylene-P5 plastomer blends (Experiment 3).

Figure 9. Deflection at peak flexural load of polypropyleneP5 plastomer blends (Experiment 3).

Figure 10. Tensile elongation at break of polypropylene-P5 plastomer blends (Experiment 3).

Figure 11. Gardner impact strength of polypropylene-P5 plastomer blends at 23° C (Experiment 3).

Figure 12. Heat-deflection temperature at 455 kPa of polypropylene-P5 plastomer blends that are not irradiated or aged (Experiment 3).

Figure 13. Color of polypropylene-P5 plastomer blends (Experiment 3).

DISCUSSION

Of the physical properties that are indicative of radiation tolerance, our research focused on tensile elongation at break, Gardner impact strength, and a flex-to-failure test developed in-house.^10,12 These have been demonstrated in the past to correlate well with material and part performance in the field. Certain of these tests take on more importance than others depending on the specific applications for which a resin is qualified and the modes of use, abuse, and failure that are projected for the medical device made from the material. While the impact and elongation results from the lab often correlate qualitatively with field experience, it is not entirely clear what level of reduction in these parameters and what absolute values of them can be quantitatively related to the success or failure of a device in actual use.

We feel more confident, however, in predicting field success based on the flex-to-failure test. In our experience, materials will perform satisfactorily in tubular devices like disposable hypodermic syringes after sterilizing doses of up to 25 kGy if, in laboratory flex-to-failure testing, they retain 50% of the deflection at peak load of unirradiated samples subjected to the same accelerated-aging protocol. Because almost all polypropylene formulations provide a deflection at peak load of about 10 mm after accelerated aging but no irradiation, the retention of a deflection of at least ~5 mm after the 75-kGy irradiation and aging would suggest good utility for a material in real device usage.

For an indication of the color stability of a resin, we relied on the Hunter-b color.

For autoclavability, we examined the standard heat-deflection temperature, despite the paucity of information correlating these parameters. Our experience suggests that a polypropylene with an HDT of at least 95°C would withstand an autoclave regimen of 121°C for 30 minutes, even when incorporated into a medical device part with a significant stress loading, such as a syringe barrel expanded by a rubber plunger tip.

Clarity is, of course, directly indicated by the haze values obtained from the ASTM D 1003-92 test.

In Experiment 1, the stability of each of the measured physical properties was enhanced by increasing the amount of the plastomer P1. Increasing P1 improved the resistance of the clarified homopolymer to embrittlement after irradiation. At the 75-kGy level, 10% of the plastomer produced a blend that retained a deflection at peak load of 5 mm. At this high radiation level, more than 10% of P1 was required to provide a blend with the tensile elongation at break and deflection at peak flexural load equivalent to the commercial, radiation-resistant, and successfully applied PP 9074MED. Only 5% of the P1 was needed, however, to enable the polypropylene blend to retain Gardner impact strength to the same degree as PP 9074MED at the elevated radiation dosage.

In actual medical device applications, PP 9074MED resists embrittlement very well after receiving sterilizing doses of radiation of up to 35 kGy. Given the comparative data obtained in Experiment 1, this suggests that a propylene-homopolymer/P1 blend containing from 7.5 to 12.5% P1 would have excellent utility in devices sterilized at common radiation dosages to 35 kGy. For the less-frequent higher dosages of radiation, higher levels of P1 would protect the device.

The initial color of resins using this additive package has always been excellent--normally giving Hunter-b color values from ­1.0 to 0.0. The color values recorded here were higher than expected, but this was most likely due to the method used to formulate the blends. The homopolymer base stock was first treated with peroxide without additives, which allowed it to acquire the abnormal color observed.

What was more surprising, however, was that the addition of P1 plastomer to this base stock whitened it according to the quantity of P1 used. The effect of the P1 appeared to be greater than could be explained by a simple dilution of the color in the homopolymer. The size of the color increases after irradiation was completely normal for polypropylenes treated with this additive package. The results of the experiment suggested that, in normally compounded resins, the addition of a plastomer like P1 would have a valuable secondary effect of producing a whiter product both before and after irradiation.

The small effects of the plastomer on the flexural modulus and resistance to softening at elevated temperature of the homopolymer were also very important. The heat-deflection temperatures at 455 kPa (see Figure 5) of the minimally effective blends containing 5­10% of the plastomer were very close to 100°C, as compared with about 78°C for the PP 9074MED. We estimate that polypropylenes with an HDT of approximately 95°C will successfully withstand autoclave/steam sterilization in a syringe configuration. Based on the HDT trend line generated by the results of Experiment 1, highly radiation-resistant blends of propylene homopolymer with up to 15% plastomer would still meet this requirement.

Similarly, at the minimum effective stabilizing concentration of P1--between 5 and 10%--the secant flexural modulus of the blends was very nearly that of normal nonnucleated homopolymer, from approximately 1.4 to 1.6 gPa (see Figure 6). Even with a generous fraction of plastomer--such as 15%--in the blend, the resulting modulus was about 0.9 to 1.0 gPa, significantly higher than the moduli of our radiation-tolerant random copolymers. A stiffer raw material like this could allow for downgauging of a medical device, the original design of which had been based on a more flexible radiation-tolerant, clarified random copolymer.

Experiment 2 demonstrated the range of effects that a 10% concentration of different plastomers can exert on the haze of blends with propylene homopolymer. Depending on the plastomer, the haze of the blends ranged from a high of 69.0%--comparable to a hazy, nonclarified polypropylene--to a low of 12.7%, even lower than the neat polypropylene control sample. When the data were subjected to multiple regression analysis using the statistical computer program Statgraphics (Statistical Graphics Corp., Rockville, MD), the result was the regression equation:

Figure 7 plots this equation as a response surface. According to this response surface, the plastomers that cause the least haze at 10-wt% concentration in blends with the nucleated propylene homopolymer are those with MI between 4 and 7 dg/min and density between 0.895 and 0.905 g/ml. The dependence of the increase in haze of the blends upon param-eters that define the polypropylene and upon the comonomer in the ethylene plastomer was not determined. It is likely that the relationship of Equation 1 is specific to the polypropylene used here. We would also expect that the plastomer comonomer would be an important variable in a broader regression relationship. Verification of these assumptions was outside the scope of this work. Based on the results of the first two experiments, Experiment 3 was conducted to confirm our ability to produce clear polypropylene capable of resisting both embrittlement by high-energy radiation and softening at elevated temperature. The results of Experiment 3 show that this effort was completely successful.

P5 was the additive-free plastomer available for our use that was closest to optimum in MI and density according to Figure 7. As mentioned earlier, it was important not to incorporate a phenolic stabilizer into the blends, in order to avoid severe yellowing due to the interaction of the phenolic material and the HALS providing radiation resistance to the polypropylene. The haze results from Experiment 3 confirmed our prior findings about the effect of the plastomers on the haze of clarified PP homopolymer. Figure 8 is a regression line relating the quantity of P5 to the haze of the blends made from it and the nucleated propylene homopolymer. Although the relationship is highly correlated, and the slope of the line looks steep due to the compressed scale of the y-axis, the haze of the blends was actually relatively insensitive to the quantity of P5 they contained. Even the blend with 15% of P5 was very clear compared with common nucleated homopolymers such as those used in commercial medical devices. Lot-to-lot differences in the haze of clarified polypropylenes are often larger than the range of haze between the neat polypropylene and the blend with 15% of P5. At the expected practical use rate of P5--7.5 to 10.0%--the increase in haze over the neat homopolymer was negligible.

The plastomer significantly improved the blends as judged by all three of our preferred physical parameters indicating resistance to radiation--deflection at peak flexural load, tensile elongation at break, and Gardner impact strength (see Figures 9­11). Higher P5 concentration in the blends provided greater resistance to embrittlement by larger radiation doses after aging. From these results, a P5 concentration as low as 7.5% would be expected to provide a blend with sufficient resistance to embrittlement to be useful after a sterilizing dose of radiation of >=25 kGy. With higher concentrations of the plastomer, the results suggest that resistance to as much as 50 kGy or more of radiation can be achieved. This observation is based on the retention of ~60% of the deflection at peak load (see Figure 11) of the blends containing 10% of P5 after irradiation at 75 kGy, and on the good retention of Gardner impact strength and tensile elongation at break of the same blend samples irradiated at 25 and 50 kGy.

The trend line of Figure 12 shows that the very clear blends of nucleated homopolymer with P5 having useful resistance to embrittlement after irradiation also met our criteria for a prediction of autoclavability in a polypropylene. In the likely plastomer usage range of 7.5 to 10.0%, the HDT of the blends is ~93° to 95°C--almost exactly the minimum level we believe is required for successful autoclaving of a variety of medical devices.

As noted for Experiment 1, the plastomers imparted a surprisingly strong beneficial effect to the color of the blends, both before and after irradiation. Figure 13 shows that P5 had a powerful blueing effect on the blends, which increased with increasing plastomer concentration. This shift toward blue--that is, toward lower Hunter-b color--is generally perceived by the human eye as an attractive whitening, even in an already very white base-resin formulation. The normal slight yellowing occurred in all the samples after irradiation, and increased with the radiation dosage. But because of the much bluer starting point of the 10% plastomer blend, it was nearly as white even after 75 kGy of radiation as was the homopolymer stock compound before any sterilization treatment. Whiteness is a positive attribute in medical plastics, and this feature of the blends is an important improvement over the base polypropylene.

The excellent results obtained in Experiment 3 indicated that it is not necessary to melt-compound the blends in order to realize the advantages observed. Resistance to postirradiation embrittlement and to softening at elevated temperature was similar for both the pellet blends of Experiment 3 and the compounded blends of Experiment 1. The retention of clarity of the blends in Experiments 2 and 3 could hardly have been better with melt-compounding. It is obvious, however, that in order for the benefits of the pellet blending to be realized, a forming process must be applied to the blend that does involve some melt-mixing, such as injection molding. If a nonmixing forming process such as compression molding were to be used, a melt-compounded blend would be required.

CONCLUSION

The goal of producing a clear, radiation-resistant, and autoclavable polypropylene formulation has been realized through the modification of a nucleated, HALS-stabilized propylene homopolymer with about 7.5 to 10% of properly chosen plastomeric ethylene polymer produced by metallocene catalysis. While a wide range of the plastomers may impart the necessary resistance to postirradiation embrittlement, the MI and density of the plastomer must be carefully chosen to provide blends with clarity very close to that of neat polypropylene. The plastomer also improves the whiteness of the blends, both before and after irradiation. The blends can be prepared by simple pellet blending so long as a forming process involving some level of mixing (such as injection molding) is applied to the material to produce a homogeneous product.

ACKNOWLEDGMENTS

We wish to acknowledge the contribution to this work of Claude R. Watkins, who assumed responsibility for and carried out all of the molding, shipping, aging, and testing of the samples. This program could not have been completed without his help.

REFERENCES

1. Portnoy RC, "Polypropylene for Medical Device Applications," Med Plast Biomat, 1(1):43, 1994.

2. Williams JL, "Polypropylene Response to High-Energy Radiation," in Proceedings of the Medical Design & Manufacturing West 96 Conference & Exposition, Santa Monica, CA, Canon Communications, pp 202-17­202-26, 1996.

3. Westfal JC, Carman CJ, and Layer RW, "Electron Spin Resonance Study of Phenolic Antioxidants. Correlations of Organic Free-Radical Stability with Antioxidant Activity," Rubber Chem Technol, 45:402, 1972.

4. Williams J, Dunn T, and Stannett V, Irradiation sterilization of semi-crystalline polymers, U.S. Pat. 4,110,185, 1978.

5. Williams J, Dunn T, and Stannett V, Semi-crystalline polymers stabilized for irradiation sterilization, U.S. Pat. 4,274,932, 1981.

6. Williams J, Dunn T, and Stannett V, Semi-crystalline polymers stabilized for irradiation sterilization, U.S. Pat. 4,467,065, 1984.

7. Williams J, Dunn T, and Stannett V, Polyolefin compositions of high clarity and resistance to oxidation, U.S. Pat. 4,845,137, 1989.

8. Toda T, Kurumada T, and Murayama K, "Progress in the Light Stabilization of Polymers," in Polymer Stabilization and Degradation, A.C.S. Symposium Series (280), Klemchuck P (ed), Washington, DC, American Chemical Society, pp 37, 1985.

9. Lever JG, and Walker RD, "Advances in Clarification Technology for Medical Polypropylene," in Proceedings of the Medical Design & Manufacturing Orlando Regional Show Series/Society of Plastics Engineers (SPE) Medical Plastics Division RETEC, Santa Monica, CA, Canon Communications, pp 17­27, 1995.

10. Portnoy RC, "The Gamma Radiation Tolerance of Polypropylene: Measurement and Enhancement," in Proceedings, Society of Plastics Engineers Medical Plastics Division and Society of the Plastics Industry Joint RETEC, Brookfield, CT, Society of Plastics Engineers, pp 69­85, 1994.

11. Previously unpublished results for PP 9074MED, a commercially available, nucleated, propylene random copolymer product of Exxon Chemical Co., Houston.

12. Portnoy RC, and Cross VR, "Method for Evaluating the Gamma Radiation Tolerance of Polypropylene for Medical Device Applications," in Society of Plastics Engineers, Inc., Technical Papers, vol XXXVII (ANTEC 90), Brookfield, CT, Society of Plastics Engineers, p 1826, 1991.

Robert C. Portnoy, PhD, is senior staff scientist at Exxon Chemical Co. (Baytown, TX), where he is involved in the development of products, applications, and markets to further the use of the company's thermoplastics and thermoset elastomers in health care. He specializes in clear and radiation-resistant polypropylenes for injection-molded medical devices.