Gamma radiation resistance

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

• Ultraviolet light
• X-rays and
• Gamma rays

Out of the above, gamma radiation is most commonly used for material testing. This is because of its high availability in research or industrial irradiators. 


Here are some key areas where gamma radiation resistance holds significance:

➢ Medical Industry: Gamma radiation is used to sterilize medical devices and pharmaceuticals. Resistance to gamma radiation maintains the structural integrity & functionality after the sterilization process.

➢ Electronic Devices: Gamma radiation resistance is important to prevent damage to electronic components. It also ensures the reliability of devices.

➢ Packaging industry: Gamma radiation is used in sterilizing food. It extends the shelf life of certain food products. Further, it maintains the properties of packaging materials. 

➢ Aerospace & Aviation: Aircraft components may be exposed to ionizing radiation at high altitudes. It ensures the durability and safety of aerospace components.


Browse gamma-resistant grades used in various applications.

There are ways to deal with the problems of gamma radiation. Here are some solutions:
 

1. Select polymers that can better handle gamma radiation. Some plastics and composites are more resistant. View materials resistant to gamma radiation.

    

2. Additive incorporation makes the plastic more resistant to gamma radiation. These additives like stabilizers and antioxidants. They reduce the effects of chain scission and cross-linking.

    

3. Control crosslinking during manufacturing or processing of the polymer. Do this by changing things like temperature and radiation dose. These balances get the right properties while avoiding too much cross-linking.

    

4. Blend polymers to make a material that combines the best of each part. This balances desired mechanical properties and radiation resistance.

    

5. Surface treatment of polymers makes them more resistant to gamma radiation. These treatments act like a protective layer. They reduce the direct impact of radiation on the bulk of the polymer.

    

6. Use radiation shielding materials to protect sensitive parts from direct radiation.

    

7. Post-irradiation annealing reverses the effects of radiation-induced damage. This involves heating the material to allow the recombination of broken polymer chains.

    

8. Quality control & testing materials under gamma radiation. This checks if they meet the needs of the intended use.

Polymer resins can handle gamma radiation to different degrees. This makes them useful for applications needing sterility. The main industrial sources of gamma radiation are Cobalt 60 (⁶⁰Co) and Cesium 137 (¹³⁷Cs). They give off gamma rays as they decay radioactively. Gamma rays go through plastics easily. They break the bonds in DNA, killing bacteria and microbes.
 

Disintegration of ⁶⁰Co (Source: INTECH)

Ionizing gamma radiation can cause these changes in polymers:
 

• Color changes in polymers can be induced by gamma radiation. This leads to yellowing or discoloration. This effect is particularly noticeable in polymers that are sensitive to radiation-induced degradation. Color changes can be undesirable in packaging or medical devices.
   
• Changes in the molecular weight of the polymer can occur due to gamma radiation. This change can affect the polymer's physical and mechanical properties. These include properties such as tensile strength and elongation. It depends on factors such as radiation dose and the specific polymer composition.
   
• Crosslinking – Gamma radiation can also induce cross-linking in polymers. This process forms covalent bonds between polymer chains, creating a 3D network. Excessive cross-linking can make the polymer stiffer, more rigid, and less flexible. This may lead to a decrease in the material's elasticity and impact resistance. Further, it increases tensile strength and decreases stretching.
   
• Chain scission – Gamma radiation can lead to the breaking apart of polymer chains, a process called chain scission. The high-energy gamma rays can break the chemical bonds that connect the polymer chains together. This results in shorter polymer chain lengths. Chain scission can considerably alter the mechanical properties of a polymer. It reduces tensile strength and stretching. Specifically, it can make the polymer more brittle and less durable.
   
• Change in mechanical properties lessens the polymer strength. All the above factors can result in the loss of mechanical properties in polymers. These include elongation at break, tensile strength, and impact resistance.

Each polymer reacts differently to ionizing radiation. So, the overall dose rate varies and must be limited based on the polymer.

(Source: Foster Corporation)

Radiation resistance is measured by the half-value dose. This is the amount of radiation that causes a 50% change in important mechanical properties. These properties include elongation at break and flexural strength at break. This measurement applies to thermoplastics, elastomers, aromatic polymers, and composite materials. The loss of a material's ability to elongate is used to measure the effects of irradiation. This is because loss of elongation equates to brittleness failure.

 

Irradiation and Polymers
 

• Polyethylene generally crosslinks when irradiated, although some chain scission also occurs. Crosslinking increases its tensile strength. However, polyethylene can be stabilized to make it radiation-resistant. High-density polyethylene is less stable than medium-density, linear low-density, and low-density polyethylene grades.
   
• Aromatic polymers with benzene rings resist radiation. They can be easily sterilized due to the benzene ring.
   
• Aliphatic polymers show varying radiation resistance depending on their unsaturation and substitution levels.
   
• Highly amorphous polymers resist radiation better than semi-crystalline ones. Their chain structure can stretch more before breaking.
   
• Polymers with butylene backbones like ABS and PBT lose impact strength when irradiated.
   
• Nylons 10, 11, 12, and 6-6 are more stable than nylon 6. Nylon films and fibers have lower resistance. View polyamides with high gamma resistance.
   
• Polymethylmethacrylate can withstand a single sterilization dose as a high molecular weight cast sheet or molded item. However, it is not suitable for repeated doses.
   
• PVC works for single-dose radiation sterilization, both unplasticized and plasticized.
   
• Thermosets like phenol and urea formaldehyde are suitable for irradiation sterilization. View other thermosets with high gamma resistance.
   
• PTFE, PVDF, polyacetals, and polypropylene do not withstand gamma radiation sterilization well. PP degrades slowly after irradiation.

A material's radiation resistance depends a lot on:
 

Polymer formulation
 

1. Additives reduce the effects of irradiation on mechanical properties. They also reduce the appearance and prevent yellowing. For example, tint-based stabilizers added to PVC help stop color change. Some additives protect plastics and reduce the effects of radiation.
    
2. Reinforcement: Fillers can alter the polymer's radiation resistance. Inorganic fillers increase resistance to gamma rays while organic fillers usually decrease it.
    
3. Crosslinking: Excessive cross-linking can make the polymer stiffer. This may further alter the mechanical properties of the polymer.

Conditions during radiation
 

1. Environmental conditions can impact the material's response to gamma radiation. Some materials are more sensitive to oxidative degradation in the presence of oxygen. While others may exhibit different behaviors under varying environmental conditions.
    
2. Temperature can influence the material's response to radiation. Specific operating conditions can vary from ambient to elevated temperatures.
    
3. Dose rate refers to the rate at which the material is exposed to gamma radiation. It is measured in units of absorbed dose per unit time (e.g., Grays per hour). Dose rates can vary widely depending on the application.
    
4. Stress: Materials used in structural components or devices subject to mechanical forces.

Important things to note:
 

→ Thin sections, films, or fibers in a product allow too much exposure. This in turn causes polymer degradation.

→ Moldings that are strong lengthwise but weak sideways become even weaker after irradiation. 

 

There are a few international standards to measure the radiation resistance of plastics. Here are some examples:
 

1. ISO 11357: Checks how radiation affects plastics using differential scanning calorimetry (DSC).
    
2. ISO 10437: Has procedures to expose plastics to gamma radiation and measure the dose.
    
3. ISO 15540: Outlines how to evaluate changes in plastics from gamma rays.

It's key to pick the right standard for what you need. Ask experts or regulators if you are unsure of which standard to choose. 

 

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