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How sterilization changes long-term resin properties.



At higher and multiple levels of exposure, both gamma radiation and ethylene oxide sterilization affected the properties of rigid thermoplastic resins.

Determining a material's compatibility with the different methods of sterilization that are used for medical devices can be involved and complex. One must consider the effects of the sterilization process on both the physical integrity and, in the case of radiation sterilization, the optical characteristics of the material. In the case of ethylene oxide sterilization, one must consider the rate of dissipation of residual ethylene oxide from the polymer. The effective design of medical devices requires an understanding of the effects of sterilization on the materials of construction.

Because of the delay between the time of manufacture and sterilization of the device, and because of the amount of time for which the device is actually used, the physical integrity of the material over time is critical. It is essential that the materials maintain their properties over the expected shelf life of the product. Thus, the major thrust of this study was to ascertain the effects of real-time aging, after ethylene oxide and gamma radiation sterilization, on the physical pr a wide variety of thermoplastic materials. The physical and optical properties were measured at multiple intervals up to a year after sterilization, and efforts were made to ensure consistency in the fabrication, handling, conditioning, sterilization, and testing of all materials studied. The result was the first fully quantitative and comparable database concerning the effects of ethylene oxide and gamma radiation sterilization on the most commonly used rigid thermoplastic resins in the medical industry. Materials and Conditions The following thermoplastic materials were studied: styrene-acrylonitrile copolymer (SAN), general-purpose polystyrene (GPPS), high-impact polystyrene (HIPS), acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), rigid thermoplastic urethanes (RTPU), and linear low-density polyethylene (LLDPE). Although some pigmented and radiation-stabilized materials were also studied, only the natural resins are discussed in this article.

Sterilization of the materials was done at two different sets of conditions: a normal condition commonly used in the sterilization of disposable medical devices, and a worst-case condition that simulates multiple re-sterilization or excessively harsh sterilization procedures. For gamma radiation, the normal condition was defined as exposure to a dosage of 2.5 megarads; the worst-case condition was defined as exposure to 10 megarads. For ethylene oxide gas, the normal condition was defined as exposure to one full cycle of preconditioning, sterilization, postevacuation, and aeration. The worst-case condition was defined as exposure to five repeated cycles.

Short-Term Data

Because of the time span of the study and the large amount of data that were generated, the data were reported in two phases. The first phase was an interim report of the short-term data; it was presented at and published in the proceedings of the SPE/SPI Medical Plastics RETEC in October 1988. In it, the short-term effects of gamma and ethylene oxide sterilization on the physical properties of the resins, measured two weeks after exposure, were reported. The second phase is the final report on the long-term effects of sterilization, contained in this article. Included in the interim report is a detailed discussion of the effects of gamma radiation on the optical properties of the materials. The data showed that all materials that were exposed to gamma radiation discolored to varying degrees, and that discoloration increased with increasing dosages.

For all the resins used in the study, the initial discoloration of the materials diminished with time. For polymers such as PC and some that are styrenic-based, exposure of the irradiated sample to mild UV light can accelerate the decrease in discoloration, or color reversal. This phenomenon is known as photo-bleaching.

While photo-bleaching can significantly improve the appearance of the resin, it was shown in the interim report to be solely an optical phenomenon. Duplicate samples of some of the resins that were stored in total darkness and fluorescent light exhibited differences in optical properties. However, no effects on the physical properties of the materials were observed.

All the data referenced in this article are from resins that were stored in total darkness to simulate the most probable storage environment of a sterile,packaged medical device. The change in optical properties of the transparent resins that were stored in total darkness over the one-year span of the study is also reported.


The resins used in the study were injection molded into discs of 1/8-in thickness and 2-in diameter, and into standard ASTM Type 1 tensile bars measuring 6.5 x 1/8 x 1/2 in. For comparison, a control set of samples not exposed to any form of sterilization was run through the same tests as were the sterilized samples.

Radiations Sterilizers Inc. (RSI), Schaumburg, Ill., performed the radiation sterilization. An effort was made to deliver consistent radiation dosage to all samples, half of which were exposed to 2.5 megarads (certified by RSI as 2.51 to 4.26 megarads). The other half were exposed to 10 megarads (certified by RSI as 10.0 to 15.2 megarads). In its Schaumburg facility, RSI uses Cobalt-60 as its source of gamma radiation.

Ethox Corp., Buffalo, N.Y., performed the ethylene oxide sterilization by means of its Cycle 10 procedure, which is recommended for use with vented medical devices in breathable packaging. The samples were exposed to a mixture of 12% ethylene oxide and 88% Freon fluorocarbon, at an average concentration of 660 mg/l. They were preconditioned for a minimum of 8 hrs at 100 degrees F and 60% relative humidity; sterilization exposure was a minimum dwell time of 6 hrs at 60% relative humidity and 120 degrees F. Post evacuation pressure was 5 inches of mercury; the samples were aerated at 90OF for a minimum of 16 hrs to remove residual ethylene oxide.

The sterilized samples were stored for a year at 50% relative humidity and 70,E Tensile and impact properties were measured at two weeks, six months, and one year after sterilization. Tensile strength analysis was measured according to ASTM method D638-84, the 10-mil notched Izod impact strength according to ASTM method D256-84, and the instrumented dart impact strength according to ASTM method D3763-86.

Other material properties that were studied include flexural strength, molecular weight distribution, heat deflection temperature under load, and Vicat softening point. To maintain consistency in test results, all sample parts were fabricated and tested under controlled conditions in the Dow Engineering Thermoplastics Testing Laboratory. Gamma Radiation Over the 12-month span of this study, most of the materials showed no significant changes in physical properties at exposure levels up to 10 megarads. The two resin types that exhibited changes in properties were the rubber-modified styrernic polymers (HIPS and ABS) and polyethylene (PE), which are discussed below. The unmodified styrenics, PC, and RTPUs retained their properties up to a year after sterilization. The physical property data are listed in Table 1 for the styrenic resins, Table 2 for the PC, and Table 3 for RTPUS.

At the sterilization exposure level of 10 megarads, the rubber-modified polymers (HIPS and ABS) showed losses in impact strength, a slight increase in tensile strength, and a decrease in tensile elongation at break. This change in polymer properties is attributed to the crosslinking of the butadiene rubber matrix. It is well documented that synthetic rubbers, such as butadiene, readily crosslink upon exposure to gamma radiation.

If the rubber content is high enough, crosslinking becomes the dominant factor in determining the physical property characteristics of the polymer upon irradiation. Crosslinked butadiene rubber loses its impact strength; thus, at dosages sufficient to crosslink all of the rubber, the enhanced impact properties originally provided by the rubber modifier are lost. The remaining impact strength of the material will be no better than that of unmodified polymer.

The loss in impact strength due to increased dosage can be seen in Table 4. When the notched Izod impact strength of both rubber-modified PS and high-gloss ABS resins are compared at 2.5 megarads and 10 megarads, the loss in properties with increased radiation dosage is apparent. Because of its lower proportion and different type of rubber, the low-gloss ABS resin does not show any significant change in its notched Izod impact properties.

The tensile and impact property data for PE (presented in Table 3) show no significant effects from gamma radiation exposure. But the flexural strength of the polymer increases from 976 psi for an unirradiated control, to 1192 psi after exposure to 2.5 megarads, and to 1376 psi after exposure to 10 megarads. The change in flexural strength is attributed to possible crosslinking and increased crystallization of the polymer.

As previously indicated, polymers exhibit different degrees of discoloration upon exposure to gamma radiation. Figures 1 and 2 show the color shift, over time, of transparent materials after exposure to 2.5 megarads and 10 megarads, respectively. The color shift of these transparent resins was measured in terms of yellowness index (YI). Figures 3 and 4 show the percent light transmittance (%T) through the polymer, indicating the clarity of the transparent resin.

Over time, the radiation-induced discoloration in the polymer decreases while the opticalclarity (%T) increases. For GPPS, SAN, and PC, the majority of the color reversal occurs within the first month after irradiation. The transparent RTPU, however, has a much slower rate of color reversal: A large portion of its original discoloration remains visible after one year.

A comparison of the optical data of the resins exposed to 2.5 megarads Figs. 1 and 3) to those of resins exposed to 10 megarads Figs. 2 and 4) shows that an increase in dosage increases the amount of initial and permanent discoloration of the polymer.

Ethylene Oxide

All the resins studied retained their physical integrity after exposure to one cycle of ethylene oxide sterilization. However, five repeated sterilization cycles caused some embrittlement of the styrenic polymers and the PC. The embrittlement of the polymers is seen as a loss of tensile elongation at break and a decrease in instrumented dart impact energy. (See Table 5 for data regarding styrenic polymers and Table 6 for data relative to PC.)

For the styrenic polymers, embrittlement after multiple ethylene oxide cycles appears to compound with time. The elongation and instrumented impact strengths that were observed at six months and at one year were significantly less than those observed at two weeks after sterilization.

After exposure to five repeated cycles of ethylene oxide sterilization, PS and SAN parts exhibited silver streaks, or crazes along the flow lines-an indication of stress cracking of the polymer with excessive exposure to ethylene oxide.

Because of the styrenic component of the bulk polymer matrix in the rubber-modified polymers (HIPS and ABS), a slight decrease in the tensile elongation at break is also observed.

As a result of their excellent chemical resistance, PE and RTPU were unaffected by exposure to multiple cycles of ethylene oxide. Table 7 shows the data for thermoplastic urethane; Table 8, the data for PE.

Discussion of Specific Resins

Styrenics (GPPS and SAN). Unmodified styrenics are among the polymers that are most stable to gamma radiation exposure. PS, which maintains nearly all of its crystal clarity upon exposure, is especially compatible with gamma sterilization. SAN discolors more than PS, but both retain their physical properties after exposure to gamma radiation.

Styrenics retain their properties upon exposure to one normal cycle of ethylene oxide sterilization. However, care should be taken to minimize excessive or multiple exposure to ethylene oxide: Such exposure could cause embrittlement and chemical attack that leads to stress cracking of the polymers.

Rubber-modifiedpolymers (HIPS and ABS). The retention of impact strength of gamma-sterilized, rubber-modified styrenic polymers depends upon the degree of crosslinking that occurs in the butadiene rubber phase. The higher the radiation dosage, the greater the crossliking and the lower the ultimate impact strength.

The retention of physical properties shows that the rubber-modified polymers are compatible with ethylene oxide sterilization (Table 9). However, because of the styrenic components in the polymer matrices, overexposure to ethylene oxide sterilization should be avoided.

Polycarbonate (PC). The physical properties of PC are very stable to gamma radiation, and can easily withstand levels of up to 10 megarads of radiation. PC tends to discolor more than the styrenic resins, but exposure to fluorescent or mild UV light can accelerate the reversal or fading or gamma-induced discoloration. this semicrystalline polymer renders LLDPE compatible with ethylene oxide sterilization. Up to five repeated cycles of ethylene oxide sterilization do not affect the physical properties. Conclusions The data show that rigid thermoplastic resins are affected differently by ethylene oxide sterilization and gamma radiation sterilization. Knowledge of both the method and level of exposure are required in order to predict the performance of the medical device.

At sterilization levels commonly used in the medical industry (one cycle of ethylene oxide and 2.5 megarads of gamma radiation), all the resins under study maintained their physical integrity. However, at higher or multiple levels of exposure, some of the properties of the resins were affected.

In designing a medical device, the engineer must take into account the effects of sterilization. Combining performance requirements of the device with the correct material properties will ensure the design and manufacture of a functional and effective medical device.
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Title Annotation:plastics in medicine
Author:Sturdevant, Marianne F.
Publication:Plastics Engineering
Date:Jan 1, 1991
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