GEOFFREY HEBERT

May 1, 1996

7 Min Read
The Effect of Molecular Orientation on  the Radiation Stability of Polypropylene

Medical Plastics and Biomaterials Magazine | MPB Article Index

Originally published May 1996

GEOFFREY HEBERT

Polypropylene (PP) is widely used in the medical industry to produce syringes, vials, and numerous other devices, often through injection molding. Many manufacturers employ gamma or electron-beam (E-beam) processing to sterilize these varied products, even though PP is known to undergo excessive oxidative degradation upon irradiation in air. Without intervention, the severe embrittlement that can occur will essentially destroy the polymer's physical properties, and thus the usefulness of the molded part.1,2 However, sta-bilization packages have been developed to combat this phenomenon.2,3

One effect of the injection molding process is that it imparts molecular orientation to the plastic in the direction of polymer flow.4 The present investigation is designed to determine the effect of this orientation on the radiation resistance of PP, with the goal of aiding in the design, manufacture, and testing of injection-molded, radiation-sterilized medical devices.

EXPERIMENTAL

Materials. Table I outlines the six PP samples used in this study, of which three are homopolymers and three are random copolymers (with ethylene). The samples were not stabilized, apart from the addition of approximately 200 ppm of in-process antioxidant. Samples PP2, PP3, PP5, and PP6 were chemically visbroken through the addition of peroxide and extruded, under nitrogen, at 230°C on a Davis-Standard 2-in. extruder. Samples PP1 and PP4 were not extruded prior to molding. The melt-flow rate of each sample was measured according to ASTM D 1238, with 1% BHT added to stabilize the polymer.(Tables and figures not yet available on-line.)

Sample Preparation. Plaques measuring 76 mm on a side and 1.0 mm thick were injection molded on a Van Dorn 75-tn machine under conditions outlined in Table II. Samples measuring 38 mm x 13 mm were then cut from these plaques (as shown in Figure 1), and the crystallinity of the samples determined via density-gradient column measurements on small pieces cut from the centers of the specimens.

Irradiation and Testing. The polypropylene samples were mounted on racks, which left the entire surface of each specimen exposed to air. Samples to be measured across orientation were dosed to 53 kGy of E-beam, whereas those tested with orientation were dosed somewhat higher, at 68 kGy of E-beam.5 The samples were stored at 23°C in air. Infrared dichroism was employed to measure the crystalline (fc) and average (favg) orientation of the samples prior to irradiation, using a Nicolet 60-SX FTIR and a KRS-5 supported-wire grid polarizer.6

Figure 2 depicts the flex-test apparatus, which was operated at a cross-head speed of 254 mm/min.7 Flexural strength was determined using Equation (3) in ASTM D 790, tangent flexural modulus derived from Equation (5) in the same standard, and break-angle results calculated using the formula shown in Figure 3. An Instron 1114 tensile tester equipped with a 50-kg load cell was used for flexural testing, conducted at 2, 5, 24, 50, 100, 250, 500, and 1000 hours after irradiation. Five specimens were tested and averaged for each data point.

RESULTS AND DISCUSSION

Molecular Orientation. Crystalline, amorphous, and average molecular orientation for the six samples can be found in Table I. Amorphous orientation, which is not directly measured via IR dichroism, can be calculated by the following equation:

favg = Vcfc + (1­Vc)fam,

where favg = average orientation, Vc= volume fraction crystallinity, fc = crystalline orientation, and fam = amorphous orientation.6

Orientation is given by the Hermans orientation function fp:

fp = 1/2 (3 cos2e ­1),

where p can be crystalline, amorphous, or average.6 If all molecules are oriented in the flow direction, fp = 1. If all molecules are oriented in the cross-flow direction, fp = -1/2. In a nonoriented sample, fp = 0.

The results in Table I indicate that all of the samples are oriented in the flow direction in varying degrees. The degree of orientation measured in these samples is in the same range as results obtained from measurements on PP syringes with a wall thickness of 1 mm.

Flex-Test Results. Table III presents the flexural properties of the six samples prior to irradiation. Clearly in evidence is the anisotropy that developed in the samples with higher molecular orientation. Figure 4 compares the flexural strength measured with orientation divided by the flexural strength measured across orientation (anisotropy) versus crystalline orientation.

Break-angle results with and across orientation after irradiation can be found in Figures 5 and 6, respectively. Note that samples that did not break before a 90° bend are recorded as a 90° break. Figure 5 shows that when the specimens were measured in the direction of flow, they were all quite ductile; in fact, the only failure was found in PP1 at 1000 hours. However, Figure 6 clearly demonstrates the loss of ductility in virtually all the samples when tested in the cross-flow direction. Only sample PP6 survived the entire 1000-hour test period.

Flexural-strength results outlined in Figure 7 (with flow) and Figure 8 (across flow) reveal a similar trend. Specimens tested in the flow direction retained their strength over the 1000-hour test, whereas most specimens tested in the cross-flow direction showed substantial losses in flex strength.

The above data demonstrate the extreme effect of molecular orientation on the retention of the ductility and strength of injection-molded PP after exposure to ionizing radiation. These results would not necessarily have been predicted by the values listed in Table III. Indeed, only sample PP1 exhibited extreme anisotropic behavior prior to irradiation. The rapid loss of ductility after radiation sterilization in the cross-flow direction underscores the need for product designers and processors to be aware of possible areas of molecular orientation in injection-molded PP articles. Unfortunately, it has been the author's experience that many of the standard tests for radiation stability--such as tensile, flexural, and impact testing--use injection molded, ASTM-type end-gated tensile bars and generally measure sample performance after irradiation only in the direction of the polymer flow.

The data also point to a possible effect of crystallinity on the retention of ductility. Figure 9 shows the break angle of specimens measured across orientation versus crystallinity (for various time periods), and suggests that by decreasing the crystallinity of the polymer, ductility retention can be increased. From a molecular standpoint, decreasing a polymer's crystallinity has been shown to make radical recombination occur at a faster rate,8 which should in turn lead to a lesser degree of oxidative degradation and an improvement in the retention of ductility. Although not the main point of the present work, this potential effect is nonetheless interesting to note.

CONCLUSION

It has been shown that the apparent radiation resistance of injection-molded PP samples is determined to a large extent by the direction of testing in relation to molecular orientation developed during the injection molding process. This reality becomes increasingly important as the drive to thin-wall injection-molded medical devices continues, and may be surprising in light of the rather small effect of molecular orientation on predose flexural properties in most of the samples. It has also been suggested that a reduction in polymer crystallinity may result in increased retention of ductility after irradiation.

REFERENCES

1. Encyclopedia of Polymer Science and Engineering, 2d ed, New York, Wiley, 1985.

2. Dunn TS, and Williams JL, "Radiation Stability of Polypropylene," J Indust Irrad Tech, 1:33­49, 1983.

3. Horng P, and Klemchuck P, "Stabilizers in Gamma-Irradiated Polypropylene," Plastics Eng, April, pp 35­37, 1984.

4. Rubin II, Injection Molding Theory and Practice, New York, Wiley Interscience, 1972.

5. Results of a screening experiment run at 30- and 50-kGy doses showed that a higher dose would indeed be needed to ensure that samples tested in the flow direction would fail within the 1000-hour time period. Unfortunately, we underestimated the effect of molecular orientation, as increasing the dose to approximately 70 kGy did not improve our situation (i.e., we were not able to distinguish between the materials when tested in the flow direction, and virtually none of the samples failed within the 1000-hour time period).

6. Huber JE, and Samuels RJ, in Interrelations between Processing Structure, and Properties of Polymeric Materials, Seferis JC, and Theocaris PS (eds), Amsterdam, Elsevier, 1984.

7. Dziemianowicz TS, Himont Development Report, August, 1984.

8. Dunn TS, Williams EE, and Williams JL, "The Dependence of Radical Termination Rates on Percent Crystallinity in Gamma-Irradiated Isotactic Polypropylene," Radiat Phys Chem, 19:287­290, 1982.

Geoffrey Hebert is leader of market intelligence for the polystyrene division of Novacor, Inc. (Leominster, MA), where he currently manages marketing support for the injection molding business segment.

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