Effects of Gamma-Ray Irradiation on Thermal and Tensile Properties

Chapter 22

Downloaded by STANFORD UNIV GREEN LIBR on October 14, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0620.ch022

Effects of Gamma-Ray Irradiation on Thermal and Tensile Properties of Ultrahigh-Molecular-Weight Polyethylene Systems M. Deng, R. A. Johnson, R. A. Latour, Jr., and Shalaby W. Shalaby Department of Bioengineering, 301 Rhodes Research Center, Clemson University, Clemson, SC 29634-0905

UHMW-PE has long been used in articulating components for total joint reconstruction. However, the long-term performance (wear and creep) of the polymer has been of some concern. Attempts to improve the mechanical properties, and thus the survival rate, of the prostheses fabricated from this material have entailed crosslinking in the presence of gamma radiation. However, these attempts were associated with limited success. This study addresses the effects of gamma radiation on the thermal and tensile properties of compression-molded UHMW-PE (GUR405) at a dose ranging from 1 to 5 Mrad in air, nitrogen, acetylene and vacuum. Available DSC and tensile data indicate that both the radiation dose and type of environment affect the thermal and tensile properties of UHMW-PE in several modes. Crosslinking appears to dominate in the acetylene environment Changes in tensile properties, melting and oxidation temperatures, as well as crystallinity are presented. Preliminary data on the effect of radiation on UHMW-PE fiber-reinforced composites are reported. Since radiation crosslinking improves polyethylene film (1), its effects on medical grade ultrahigh molecular weight polyethylene (UHMW-PE) have been investigated in attempts to improve the mechanical properties and thus the survival rate of its prostheses (2,3). At present, many UHMW-PE joint components are sterilized by gamma irradiation. The main advantages of this process are the relatively high effectiveness and safety factor involved. However, radiation sterilization can also cause certain undesirable changes in polymer properties at die traditional sterilization dose of 2.5 Mrad (4). Earlier studies have addressed, mainly, the effect of radiation dose. The present study was conducted to investigate the effects of gamma radiation on the thermal and tensile properties of UHMW-PE using 1 to 5 Mrad radiation in different gas environments. A key goal of the study was to determine a means of reducing radiation sterilization damage and achieve long-term stability of UHMW-PE medical devices. Materials and Methods

A commercial UHMW-PE, GUR405 (same as GUR415, but without calcium stearate), was provided by Hoechst Celanese Co., USA. The polymer had a molecular weight of 0097-6156/96/0620-0293$12.00/0 © 1996 American Chemical Society In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


294

IRRADIATION OF POLYMERS

about 6 million and is available as a fine powder. In the present study, virgin powder GUR405 was melted and compression-molded into sheets using a rectangular metal frame placed between two stainless steel plates at a temperature of 180°C and a pressure of 7 MPa. A Carver Laboratory Press (Model C) was used for molding under an ambient laboratory environment. The polyethylene fibers were used to reinforce U H M W - P E and the effect of gamma irradiation on die tensile properties of resultant composites was studied. The polyethylene fiber used in this study was ultrahigh strength and modulus gel-spun UHMW-PE, Spectra 1000, from Allied Signal Inc. The fibers were procured as 650 deniers yarn consisting of 120 filaments. The polyethylene matrix was U H M W - P E , GUR405, from Hoechst Celanese Co. In this study, unidirectional composite laminates were made. The polymer sheets and composite laminates were converted into standard dumbbell-shaped tensile test specimens using a metal cutting die. The specimen measures 70 X 15 X 1.3 mm (length x width X thickness) with a narrow width of 5 mm, a modification of A S T M standards. Figure 1 shows the specimen geometry. Gamma irradiation was conducted at 1.25, 2.5 and 5.0 Mrad, using a ^ C o source, in three different gas environments, namely, air, nitrogen and acetylene at room temperature. The experiment was also run under vacuum. The nitrogen and acetylene environments were established by repeatedly evacuating and purging with the desired gas through a two-way ground joint vessel containing the samples. The duration for each step was about 15 minutes and the process was repeated four times prior to sealing. To achieve vacuum, the vessel was evacuated below 1 mm Hg before sealing. Prior to irradiating, the acetylene gas pressure inside the vessel was established at 2.5 psi above atmospheric pressure. The nitrogen pressure was atmospheric.

r =17 mm

15 mm

|

20 mm

|

70 mm

Figure 1. Tensile specimen geometry

Differential scanning calorimetry (DSC) was used to study the thermal properties of U H M W - P E . A T A Instrument 2000 thermal analyzer was used for this purpose. Samples weighing approximately 4 mg were placed in a sealed, aluminum pan and heated at 10°C/min from room temperature to 300°C, in air. The weight change of the polymer after gamma-irradiation was obtained by weighing samples before and after treatment using a balance with an accuracy of 0.01 mg. The thermal properties and weight change were measured only for samples irradiated at 2.5 Mrad. An average of 3 samples was used for D S C analysis and at least 8 were considered for weight

In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

22.

DENGETAL.

Thermal and Tensile Properties of UHMW-PE Systems

measurements. Considering the long-term effects of gamma radiation, the properties were examined just after irradiation and four months thereafter. During this study, the samples were placed in glass bottles and shielded from light The glass bottles were occasionally opened to atmosphere when the measurement was necessary. The tensile properties of irradiated specimens just after gamma irradiation were evaluated using an Instron universal mechanical tester (Model 1125), at room temperature. A loading rate of 20 mrrVmin and a gauge length of 20 mm were used. At least 4 samples were used for each case. Additionally, selected samples of new fiber-reinforced U H M W - P E composites were studied under the conditions similar to those described above.


Results and Discussion Thermal properties. DSC thermograms were used to study two events, melting and thermal oxidation, in terms of peak melting temperature ( T ) and peak oxidation temperature (T ), respectively, as illustrated graphically in Figure 2. The thermograms m

0

were also used to determine the heat of fusion (AH). Using the A H data, the percent crystallinity of the polyethylene was estimated by dividing A H by the heat of fusion of fully crystalline polyethylene, reported as 289.3 J/g (5).

Heat Flow

Temperature Figure 2. Graphical illustration of a DSC plot

The D S C results are summarized in Table 1, which are also illustrated in Figures 3-5. It is clear that the thermal properties of compression-molded U H M W - P E seldom change during normal storage conditions and their properties are consistent, considering that the 600-day old sample was compression-molded at least one year earlier than the rest of the samples. Following gamma irradiation, however, the longterm properties varied. First, gas environments affected the thermal properties of the polymer. The data in Table 1 show that gamma irradiation increased the melting temperature of UHMW-PE in all four conditions. The acetylene environment resulted in the highest increase, while the other three conditions led to limited increases. This may indicate that radiation-induced molecular changes resulted in recrystallization and can be associated with chain scission in the amorphous region, which is followed by recrystallization. Such events can also be affected by crosslinking. Hence, the change in crystallinity will be determined by relative amounts of crosslinking and chain scission in the prevailing environments. The results of the shift in T support this assumption. T , in all but acetylene environments, showed decrease after gamma 0

G


295

296

IRRADIATION OF POLYMERS

irradiation. Following gamma irradiation, the crystallinity of UHMW-PE increased. The longer the post-irradiation, the higher the crystallinity. This suggests that irradiated polyethylene continues to change with time, possibly due to trapped free radicals. It seems that gamma irradiation for the sterilization of UHMW-PE products in an acetylene environment is desirable, considering die increase both in T and T . m

0

Table 1. Thermal Data of UHMW-PE after Gamma Irradiation at 2.5 Mrad


Time (days) |

T (*C) m

10 130 250 360 610

134.7±0.2 134.2±0.2 134.3±0.5 132.911.1 134.2±0.4

10 130 250 360

139.8±0.6 139.6±0.3 139.1±0.9 138.410.4

10 130 250 360

135.610.3 136.410.2 135.510.5 134.810.9

10 130 250 360

135.710.5 135.810.4 135.310.5 135.510.5

10 130 250 360

136.210.6 136.010.2 135.310.4 134.910.7

5

| T ( C) I AHtf/a) f C^stalllnlty (%>) Control (not irradiated) 228.510.4 44.610.6 129.111.8 44.410.8 224.410.5 128.512.3 42.311.2 226.713.0 122.413.3 47.118.4 136.2124 224.410.6 227.611.1 132.013.7 45.611.3 Irradiated in acetylene 45.611.0 228.813.0 131.813.0 144.410.2 49.910.1 227.710.6 47.512.9 227.211.5 137.518.5 . 52.018.2 226.911.5 150.4123 Irradiated in air 223.110.8 49.910.6 144.311.8 52.312.1 223.412.5 151.316.1 51.012.3 223.712.5 147.516.7 54.0114 222.815.5 156.1140 Irradiated in nitrogen 223.910.5 49.311.3 142.613.7 222.311.4 154.618.2 53.412.8 50.411.1 145.813.2 223.912.7 57.313.6 165.7111 223.711.7 Irradiated in vacuum 46.813.0 223.342.9 135.418.6 51.911.9 223.910.9 150.215.5 51.312.9 224.612.6 148.518.5 56.112.9 223.515.9 162.318.3

Control

0

Vacuum Nitrogen

Air Acetylene

Figure 3. Effect of gamma irradiation on melting temperature of compression-molded UHMW-PE (error bar = 95% confidence interval)


22.

Thermal and Tensile Properties of UHMW-PE Systems 297

DENGETAL.

250' m 10 days m 250 days S 130days 23 360days

240 230'


Control Vacuum Nitrogen

Air Acetylene

Figure 4. Effect of gamma irradiation on oxidation temperature of compression-molded UHMW-PE (error bar = 95% confidence interval) 75 65

& S

10 days H 250 days 130 days • 360 day

55 45 35 25 Control Vacuum Nitrogen

Air

Acetylene

Figure 5. Effect of gamma irradiation on percent crystallinity of compression-molded UHMW-PE (error bar = 95% confidence interval) Tensile properties. Tensile tests were run following gamma irradiation. No attempts were made to examine the post-irradiation effects. From tensile testing, the following parameters were determined: yield stress (a ), ultimate stress (Ou), sample modulus (£), ultimate elongation (Eu) and fracture energy (Wu). Table 2 lists the results (percentage of parameter as compared with the control), which are also illustrated in Figures 6-10. For the control the mean ± 95% confidence interval were: a = 24.7 ± 0.6 MPa, a = 47.9 ± 1.7 MPa, E = 578 ± 13 MPa, e = 176 ± 12% and W = 145 ± 12 lbs-inAn. It appeared that both radiation dose and type of gas environment affect the tensile properties of UHMW-PE. Statistical Analysis (ANOVA) of the result shows that (1) at the same dose, the type of gas environment significantly changes o , E and e (p

Effects of Gamma-Ray Irradiation on Thermal and Tensile Properties

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