The U.S. Postal Service's increasing use of radiation sterilization to combat bioterrorism may adversely affect key physical and cosmetic properties of certain polymer products and samples sent through the mail. Widely used resins such as polypropylene, acrylic and acetal are among those most at risk.
The post office currently uses radiation primarily to sterilize mail sent to the federal government, but it has additional equipment on order with plans to broaden its use of the process, possibly to include parcel post as well.
Two methods for radiating materials are commercially available: An electron-beam generator produces ionizing radiation that has a penetration of inches into the target load, while gamma rays produced from cobalt-60 have a penetration effect of feet into the target load.
Either method will produce broken polymer chains, free radicals, gas, odor and color shifts in plastics, which either undergo scission to become weaker or cross-link to become stronger or may recombine to take on their original characteristics. Each polymer is different. These polymeric changes are not visible but can be measured on laboratory equipment.
The safe zone — Research has shown that the following polymers can withstand 50-100 kilograys (kGy) of radiation in their natural formulation without having to be stabilized for radiation: ABS; styrene acrylonitrile; high-impact polystyrene; most polyethylenes (including low density, linear low density, high density and ultrahigh-molecular-weight grades); polyesters such as PET, glycol-modified PET and polybutylene terephthalate; polysulfone; polyurethane; polyetherketones; polyetheretherketones; liquid crystal polymers; polyetherimides; polyamide-imides; elastomers such as ethylene propylene diene monomer, isoprene, polyurethane, silicone, thermoplastic elastomers and thermoplastic polyolefins; and nylons such as types 6, 6/10, 6/12, 10, 11, 12.
Modification needed — The following polymers need to be compounded specifically to resist radiation: Polycarbonate and PC alloys need to be color corrected. Rigid PVC will turn coffee brown, while flexible PVC will have a slight color shift. Polypropylene and its copolymers must be compounded specifically for radiation. Flexible and semirigid fluoropolymers would have to be color corrected. Acrylics show a temporary color shift and modest loss of physical properties and also need to be compounded for radiation.
At risk — Some polymers will be significantly damaged when treated with radiation: PP that is not specifically compounded for radiation and stabilized will exhibit a severe loss of elongation, become brittle and continue to degrade months after irradiation. Acetal embrittles and yellows at 10 kGy of radiation, and polytetrafluoroethylene becomes brittle.
Radiation dosing essentially is additive. Therefore, a polymer sterilized at 100 kGy, and then later subjected to another 100 kGy dose will have been subjected to approximately 200 kGy of radiation. A medical device exposed to such radiation levels would have to be revalidated to ensure it will function as designed and still have a safety factor, as specified under the Food and Drug Administration's Good Manufacturing Practices guidelines.
The medical industry has been dealing with the effects of radiation on plastic devices for 25 years and is well-placed to deal with the challenges arising from the Postal Service's increased use of the process. Other segments of the plastic industry are less informed, and should monitor the situation closely to avoid possible radiation-induced performance or quality issues.
Resin companies should review how their specific nonmedical, nonradiation-grade resins will react to the radiation dosages likely to be used by the post office and then advise their plastic processing and end-use customers of the possible impact. Processors similarly need to discuss the radiation effects with their customers and alert them to possible product degradation, if the products are distributed or sent through the mail. End-product manufacturers that distribute goods largely by mail may want to investigate changing to a resin that is less affected by radiation.
Action taken now may avert a costly product liability lawsuit later.
Beard is president of Robert A. Beard & Associates Inc., a plastic technology consulting firm in Kenosha, Wis. Jim Stubstad, a consultant on plastic medical devices and packaging, has conducted seminars on radiation sterilization of medical plastics for the University of Wisconsin in Madison, Wis. Both are licensed professional engineers. For a bibliography listing published research that details the effects of radiation on plastics, visit Beard's Web site at www.plastic-solvers.com.