Originally published March 1996
Over the past several years, there has been considerable interest in the sterilization of plastic medical devices by gamma irradiation, as the more traditional sterilization methods have in some cases fallen out of favor. In the past, flexible, clear PVC devices were generally sterilized by the use of ethylene oxide. For many manufacturers, ethylene oxide sterilization has become an environmental liability because of the gas involved in the process. Often, this leaves sterilization by gamma irradiation as the most favorable alternative.1
Unfortunately, exposure to gamma radiation has a negative effect on the appearance of flexible PVC. While physical properties and viscosity remain unchanged, immediately after sterilization the material tends to darken or yellow. Compound darkening is autocatalytic, and continues after irradiation during the typical shelf-life period between sterilization and product use. The darkening of the compound can be attributed to the formation of conjugated double bonds due to oxidation of the PVC resin.2 The degree of darkening at end use is dependent on the duration of the shelf-life period and on specific compound formulation. For example, traditional low-cost, flexible PVC formulations show extreme darkening in the period following gamma sterilization. To minimize this effect, compound formulation is critical: the choice of resin, stabilizer, lubricant, and antioxidant can all affect color stability after gamma exposure.
This article describes an investigation whose purpose was to determine ways of improving the gamma stability of flexible PVC. The variables of interest were resin molecular weight, plasticizer loading, stabilizer type, and choice of antioxidant. By screening resins, plasticizers, stabilizers, lubricants, and antioxidants, a promising candidate formulation was identified and optimized by a statistically designed experiment, with hardness added as an additional variable. The criterion for improvement was color change in the PVC after gamma irradiation, compared with an identical unexposed specimen. A series of gamma-stable formulations was created exhibiting improved postirradiation color stability.
Screening Studies. Formulations were prepared to screen resins, stabilizers, lubricants, and antioxidants. The samples were weighed and milled at 330°F for 5 minutes, then compression molded into plaques measuring 2 * 6 * 0.075 in. A small specimen was cut from each plaque to be saved as an original for comparative purposes; the remainder was divided in half and sent for gamma sterilization at 2.5 and 5.0 Mrd., repectively. Following gamma irradiation, each sample was again divided in half and one of the halves was oven-aged for 48 hours at 50°C to accelerate shelf-life stability. All samples were then mounted and measured for yellowness index on a Datacolor CS-5 spectrophotometer according to ASTM D 1925. The responses of interest were yellowness index both initially and before and after oven-aging for each level of gamma irradiation, as well as the general appearance of the specimens after each treatment.
Statistically Designed Experiment. Using experimental design software, a Box-Behnken design was set up to evaluate the effect on performance of overall plasticizer level, antioxidant level, and lubricant level. Using the Box-Behnken design with three center points resulted in a grid comprising 27 formulations, in which three levels of each variable were used. Control formulations were also prepared for comparative purposes at three hardnesses: 60, 75, and 90 Shore A. Sample preparation and experimental methodology followed the same procedures as in the screening studies.
All data are expressed as change in yellowness index of oven-aged samples, comparing unexposed samples with those exposed to 2.5 or 5.0 Mrd of gamma radiation, as indicated.
Results and Discussion
Effect of Molecular Weight. Resin molecular weight had a negligible effect on the gamma stability of the compounds following irradiation and oven-aging, as shown in Figure 1. (Figures not yet available on-line.) Because oxidation is suspected to be a significant cause of compound darkening via chromophore formation, this result is not unexpected. Oxidation is believed to occur randomly along the polymer chains, creating HCl and a double bond along the backbone. Obviously, this would be independent of the quantity or structure of the end groups. The compounds studied had a hardness of 92 Shore A; other durometers have shown similar behavior.
Effect of Plasticizer Loading. Plasticizer loading tends to improve the gamma stability (reduce darkening or color shift) of the compounds. As plasticizer concentration increases, compound darkening decreases. This is believed to be due to the dilution effect--that is, the higher the percentage of plasticizer in a compound, the lower the percentage of the compound that remains resin. This effect is illustrated in Figure 2, using test compounds containing between 43 and 96 parts per hundred of plasticizer, with all other variables being held constant. Increasing the amount of plasticizer (in other words, softening the compound) significantly reduced the change in yellowness index of the material after gamma sterilization. This effect is by far the strongest demonstrated in this study, and in the very soft compounds (60 Shore A) will probably overshadow any other effects.
Effect of Stabilizer Choice. The choice of heat stabilizer can have a major effect on the performance of gamma-stable, flexible PVC. Traditional mixed-metal, nontoxic heat stabilizers tend to perform poorly in gamma applications, imparting severe darkening of the compounds after irradiation. Some metal soaps will give adequate performance at low levels in combination with epoxidized soybean plasticizer. In general, heat stabilization is a classic trade-off: increasing heat stability yields reduced postirradiation gamma stability. The best formulation must be determined by experimentation and by a knowledge of end-use heat-stability requirements. In some cases, it is necessary to tailor formulations based on compound hardness and processing needs.
Effect of Lubricant Level. Results from the designed experiment also show that additional lubricant does not reduce yellowing but in fact increases it, particularly in the harder compounds. This effect is illustrated in Figure 3, which shows that the presence of lubricant is clearly detrimental at plasticizer levels below 65 parts. At levels above 65 parts, the effect is unclear because of the aforementioned dilution effect.
Effect of Antioxidant Choice and Loading. As discussed previously, oxidation of the PVC resin is believed to be a significant cause of compound darkening. Screening studies have shown that phenolic and fluorophosphonite antioxidants worsen the problem, probably because of the aromatic groups in their structure. However, there are FDA-compliant antioxidants that improve gamma stability. The reasons for the improved performance imparted by some antioxidants during and after gamma sterilization are not completely understood. More research is needed by the manufacturers of these additives to analyze their performance in gamma applications.
The results of the designed experiment show interesting effects of antioxidant concentration. In all cases, the presence of antioxidant reduces the yellowing of the compound, as shown in Figure 4. Again, the effect is unclear above 65 parts per hundred of plasticizer because of the dilution of the resin. The unexpected result here is the shift in the minimum yellowness index with respect to antioxidant concentration and plasticizer concentration. Although the effect is too subtle to be seen in Figure 4, as plasticizer concentration decreases (and compound hardness increases), more antioxidant is required to reach the minimum of yellowness index.
A number of conclusions can be drawn as a result of this study. First, the experiment indicated that the effect of resin molecular weight on gamma stability is negligible. Increasing the plasticizer concentration significantly improves gamma stability--the strongest effect seen in the study. The presence of the proper antioxidant can also improve stability, though harder compounds require more antioxidant to attain the minimum of yellowness index. Lubricant tends to reduce gamma stability. Many heat stabilizers will also reduce gamma stability, but use of the proper heat stabilizer will have little or no detrimental effect.
1. Spaulding MJ, Packaging, 37(13):32 33, 1992.
2. Naimian F, Katbab A, and Nazokdast H, Rad Phys Chem, 44:567572, 1994.
Douglas Luther is a polymer chemist in the plastics laboratory at Teknor Apex Co. (Pawtucket, RI). Leonard Linsky is group leader of plastics in the same laboratory, where he has worked for more than 40 years.