Irradiation Resistance of Materials

If irradiation resistance of materials is mentioned, the irradiation often includes UV and gamma ray. For the materials of pharmaceutical packaging (sprayers and bottles), gamma ray is important because gamma ray is a very general sterilization process. UV is not important. For sprayers and bottles, the packed finished medicines are always advised to be stored in cool place without sun light, so these packaging don’t suffer from UV. Herein just discuss the resistance to gamma ray.

Unlike ozone, which attacks materials from surface, Gammy ray can penetrate through the materials. Therefore, the change of materials due to Gammy ray is homogenous. Four situations will caused by exposure to gamma ray.

• Discolor or yellowing effect
• Change in mechanical properties (varies by material)
• Crosslinking – increased tensile strength, decreased elongation
• Chain scission – reduced tensile strength and elongation

Below summary of some involved materials used to make plastic sprayers and bottles.

Material	Tolerance Level (kGy)	Remark
Acetal POM	5	Avoid use due to embrittlement.
Buna N (Nitrile)	200	Avoid multiple sterilization. Low quality sprayers use this rubber as gasket and O-ring.
Butyl	50
Polyethylene (HDPE, LDPE, LLDPE, UHMW	1000	Crosslinks to gain strength, loses some elongation. All polyethylene radiation stable, low density most resistant.
Polypropylene, Radiation Stabilized: Homopolymer Copolymers of Propylene-Ethylene Polypropylene, natural	20-50 25-60 20	Subject to orientation embrittlement. Validate with real time aging. More stable than Homopolymer. Avoid use of unstabilized polypropylene.
Silicones (Peroxide & Platinum Catalyst Systems)	50-100	Crosslink density increases more in peroxide systems (Peroxide & Platinum Catalyst Systems) than in platinum systems.
PET	1000	Very stable, retains excellent clarity. Drying is essential.
PS	10000	All styrenes are stabilized by Benzene ring structure.
Stainless steel SUS 304, SUS316	n.a.	No exact tolerance level is found. However, normally stainless steels have excellent resistance to gamma radiation. Used as spring. In some sprayers, also used as valve ball.
Aluminum	n.a.	No exact tolerance level is found. However, aluminium can be used as Radiation Shielding Material. Aluminum is 100 times more resistant to radiation damage than common spacecraft materials used. Used as the crimp on closure.

The dosage of gammy ray during sterilization is 5kGy normally. Sometimes, 5 to 10 kGy is also requested. Anyway, the dosage doesn’t exceed 10 kGy.

Most importance is if the sprayer contains POM, gammy ray sterilization should be avoided. Or the POM part becomes brittle and may break during pressing.

Another important concern is PP. All sprayers must use PP. Normally, PP contains stabilizer such as antioxidants or ethylene copolymer to increase the tolerance of gammy ray. One time sterilization with dosage less than 10 kGy is ok. Huge amount of sprayers are sterilized by gammy ray everyday so PP is ok for gammy ray sterilization. However, sterilized status can only be maintained for 2 years. After 2 years, avoid re-sterilization via gammy ray.

Two general units of irradiation is Mrad and kGy. kGy is used more often. 1 Mrad = 10 kGy.