189 Technical Data
■ Cobalt 60 (gamma radiation) OR ■ Electron beam or E-beam (high energy electrons)
Steam Autoclave (Cont.) If empty ware becomes distorted due to autoclave sterilization, it may be due to: ■ High stresses molded into the ware during manufacture ■ Unusual hot spots in the autoclave ■ Use of the wrong plastic Tests should always be run on plastic ware to determine suitability for a given sterilization method. Autoclaving Filled, Sealed Ware Autoclave sterilization of filled, sealed ware, is also known as “Terminal Sterilization”. Many companies prefer terminal sterilization IF their product can withstand the rigors. Autoclave temperature must be minimally 121°C (250°F). Of the common plastics, polypropylene (PP) and polycarbonate (PC) have enough heat resistance to be autoclaved. Also, there is a grade of a new transparent plastic material identified as a cyclic olefin copolymer (COC) that is capable of withstanding steam autoclave sterilization. However, autoclaving filled, sealed plastic ware is tricky. Temperature and pressure in the autoclave must be controlled and balanced with temperature and pressure being generated in the filled, sealed ware during autoclave heat up AND cool down. If not, ware could be crushed or ballooned. Special autoclaves are sold to enable this temperature / pressure balancing act. If filled, sealed containers become distorted during autoclave sterilization. This may be due to: ■ Improper balancing of temperature / pressure upon heating or cooling ■ High stresses molded into ware at the time of manufacture ■ Unusual hot spots within the autoclave chamber ■ Use of the wrong plastic Tests should always be run on plastic ware to determine suitability for a given sterilization method. Autoclaving Closures Polypropylene (PP) closures should be capable of withstanding steam autoclave sterilization. However, autoclaving may cause blooming of additives in PP. PP homopolymer is more heat resistant than PP copolymer. Linerless closures (closures with specially molded-in sealing features) may or may not be acceptable for autoclaving dependent on many factors (e.g. as application torque, autoclave conditions, closure design, etc.) If a closure is lined, the liner and the adhesive used to affix the liner inside the closure must also be considered. Lastly, PP closures applied to containers present a special case. Closures are designed with tolerances that cause interference between the closure and container. This interference results in stress. Since all thermoplastics become softer as temperature increases, stress may be relieved or closure dimensions may change upon autoclaving. This can result in closure torque reduction or seal loss. If closures distort or a torque retention problem results, it may be due to: ■ High stresses molded into ware at the time of manufacture ■ Unusual hot spots within the autoclave chamber ■ Use of the wrong plastic Due to moisture absorption, pulp liners are NOT anticipated to be acceptable for autoclaving. Tests should always be run on plastic ware and liner / adhesive combinations to determine suitability for a given sterilization method. Radiation Ware is exposed to ionizing radiation that knocks electrons off atoms it contacts. Ionizing radiation is lethal to microorganisms because of its destructive effect upon the contents of living cells. There are two common sources of ionizing radiation used for sterilization:
The amount of radiation from either Cobalt 60 or electron beam is measured in MegaRads (MRads) or KiloGrays (KGy). One MegaRad equals ten KiloGrays. Because gamma sterilization and E-beam both use radiation, packaging materials react similarly in both systems. Cobalt 60 Gamma Radiation A gamma radiation sterilization facility consists of a thick walled concrete maze in a room built around a well filled with water. In the well are a number of pencil-sized steel rods impregnated with radioactive Cobalt. Articles to be sterilized are placed on conveyors that bring them through the concrete maze into the room where the radioactive rods are located. The number of rods raised from the well and the exposure time controls the degree of exposure. After exposure, ware is conveyed from the room via the maze. A radiation dose sufficient to kill bacteria and spores is about 2.5 MRads. To minimize costs plus attain sterilization, bioburden is determined then the minimum dosage plus a safety factor is selected. Gamma radiation has high penetrating power (about 50 cm or close to 20 inches of the same unit-density material). Thus, many parts can be packed together for sterilization. In this instance, the dosage reaching the center of ware multi-packs is validated. Slightly higher doses occur at the outside edges of multi-packs. Usually, empty packaging components are sterilized via gamma radiation. Since effects of radiation are cumulative, twice the normal dose is sometimes examined to insure minimal problems. Listed below are thermoplastic materials that are recognized as capable of being gamma radiation sterilized, although tests should always be run on plastic ware to determine suitability for a given sterilization method: ■ Low Density Polyethylene ■ Linear Low Density Polyethylene ■ High Density Polyethylene (those containing phosphite stabilizers may yellow) ■ Polyethylene Terephthalate ■ Polystyrene ■ Polycarbonate ■ Nylon ■ Cyclic Olefin Copolymers (a newly emerging group of polymers) ■ Polyethylene Naphthalate (a newly emerging group of polymers) Problems can occur when gamma radiation sterilizing polyvinyl chloride (PVC) or fluoropolymers (PTFE, etc.).
Important Note About Polypropylene Gamma Radiation Sterilization
Normal PP grades yellow noticeably and exhibit long term embrittlement when sterilized via gamma radiation techniques. Special radiation resistant PP grades, having special stabilizers, are available for radiation sterilization. Also, if ware is to be colored, then the concentrate carrier should be a radiation resistant grade of PP. PP copolymers are more radiation resistant than PP homopolymers. Tests should always be run on plastic ware to determine suitability for a given sterilization method.
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