Gamma Radiation Resistance -

Importance of Radiation & Sterilization

Most polymers can be degraded by photolysis to give lower molecular weight molecules. Electromagnetic waves with the energy of visible light or higher energy levels are usually involved in such reactions. These EMW include:

Ultraviolet light
X-rays and
Gamma rays

Among several types of radiations used currently for material testing, gamma radiation is the most common because of its high availability in research or industrial irradiators.

Gamma radiations are used for sterilization processes in medical devices, food industry as well as in nuclear power plants or aerospace.

Gamma radiation resistance characterizes the ability of polymers to withstand sterilization methods

Radiation resistance is characterized by the half value dose of significant changes in mechanical properties such as elongation at break, flexural strength at break etc. of thermoplastics, elastomers, all aromatic polymers, as well as composite materials. The half-value-dose means the absorbed dose that reduces a property to 50% of its initial value under defined environments.

Loss of elongation is a commonly used to measure the effect of irradiation because it equates to a brittleness failure

Impact of Radiation on Mechanical Properties or Physical Appearance of Polymers

As mentioned above, polymer resins can tolerate gamma irradiation to varying degrees making them suitable for applications requiring sterility. The primary sources of industrial use gamma radiation are: Cobalt 60 (⁶⁰Co) and Cesium 137 (¹³⁷Cs). They emit gamma rays during their radioactive decay.

Gamma rays are a penetrating form of radiation which easily pass through plastics. They break the covalent bonds of DNA killing bacteria and other microbes exposed to the radiation.

Ionizing rays of gamma radiation can cause following changes in polymers:

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

Each polymer reacts differently to ionizing radiation. Hence, overall dosage rate varies and must be limited according to the polymer.

(Source: Foster Corporation)

Irradiation and Polymers

Polyethylene in general crosslinks on irradiation, although there is a chain scission mechanism as well. Crosslinking of PE upon irradiation increases its tensile strength. However, polyethylene can be stabilized to make it gamma radiation resistant. High-density polyethylene is not as stable as medium density polyethylene and low-density polyethylene, linear low-density polyethylene.
Aromatic polymers (e.g. with benzene rings) are radiation resistant. Polymers such as PET, PU, PSU, PC etc. can easily sterilized due to presence of benzene ring.
Aliphatic polymers exhibit degrees of resistance depending upon their levels of unsaturation and substitution.
Highly amorphous materials are generally radiation resistant then semi-crystalline polymers. The chain structure is capable of great ductility and they can tolerate many scissions without breaking up.
Polymers with butylene backbone such as ABS, PBT etc. lose impact strength on irradiation.
Nylon 10, 11, 12, and 6-6 are more stable than 6. Nylon film and fiber are less resistant.
Poly(methylmethacrylate) can satisfactorily withstand a single radiation sterilization dose both in the high molecular weight cast sheet form and as a molded item. It is not, however, suitable for repeated doses.
Poly(vinyl chloride) is suitable for single-dose radiation sterilization both in its unplasticized and plasticized forms.
Thermosets such as Phenol formaldehyde and urea formaldehyde are both reasonably suitable for irradiation sterilization.
Certain polymers such as fluoropolymers (PTFE, PVDF), polyacetals, polypropylene etc., however, do not stand up to gamma radiation exposure well for sterilization. PP undergoes slow degradation after irradiation.

Factors Affecting Gamma Radiation Resistance of Plastic

Radiation resistance of a material greatly depends on:

Polymer formulation (Additives, reinforcement, crosslinking in elastomers etc.)
Conditions of radiation exposure such as the environmental atmosphere, temperature, dose rate, mechanical stress, etc.

It is important to note that:

Additives such as stabilizers, antioxidants in polymers can reduce the effects of irradiation on mechanical properties or physical appearance (non-yellowing). For example: tint-based stabilizers when added to PVC help counteract color change in the polymer.
Inorganic fillers increase radiation resistance of polymer while Organic fillers usually decrease radiation resistance.

Certain additives have a protective action and can reduce the effect of radiation on plastics
Thin parts sections, films, fibers present in the product can allow excessive exposure thus causing polymer degradation.
Molding which are strong in the axis of orientation but weak in the cross-flow axis becomes weaker after irradiation.