13.2 Environmental factors affecting polymer stability

Polymers face various environmental threats that can degrade their properties. Heat, light, moisture, and chemicals all play a role in breaking down these materials. Understanding these factors is crucial for designing durable products and selecting the right polymers for specific applications.

Luckily, there are ways to fight back against polymer degradation. By using stabilizers, choosing resistant materials, and optimizing manufacturing processes, we can extend the life of polymers. Proper storage and handling also help keep these materials in top shape for longer.

Environmental Factors Affecting Polymer Stability

Environmental factors in polymer degradation

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• Heat exposure
  • Elevated temperatures accelerate chemical reactions and molecular motion leading to bond breakage and thermal degradation (PVC, polyethylene)
  • Increased temperature facilitates oxidation reactions and chain scission
• Light exposure
  • Ultraviolet (UV) and visible light cause photodegradation by breaking chemical bonds (polypropylene, polyethylene terephthalate)
  • Light energy initiates oxidation reactions and leads to chain scission
• Moisture exposure
  • Water or high humidity causes hydrolytic degradation by reacting with susceptible functional groups like esters and amides (polyesters, polyamides)
  • Moisture facilitates bond cleavage and alters polymer properties
• Chemical exposure
  • Various chemicals such as acids, bases, solvents, and oxidizing agents degrade polymers (polycarbonate, polyurethanes)
  • Chemical reactions lead to chain scission, crosslinking, or changes in mechanical properties

Impact assessment of degradation factors

• Heat impact
  • Higher temperatures generally increase the degradation rate following the Arrhenius equation: $k = A e^{-E_a/RT}$
    • $k$: rate constant
    • $A$: pre-exponential factor
    • $E_a$: activation energy
    • $R$: gas constant
    • $T$: absolute temperature
• Light impact
  • Photodegradation rate depends on light intensity and wavelength
  • Shorter wavelengths (UV) are more energetic causing more severe degradation (polystyrene, polyethylene)
• Moisture impact
  • Hydrolytic degradation rate increases with higher moisture content and humidity
  • Degradation severity depends on the polymer's susceptibility to hydrolysis and presence of hydrolyzable functional groups (polyesters, polycarbonates)
• Chemical exposure impact
  • Degradation rate and severity depend on the type and concentration of the chemical agent
  • Some chemicals catalyze degradation reactions while others cause plasticization or swelling (PVC, polyethylene)

Synergistic effects on polymer stability

• Combined effects of heat and light
  • Elevated temperatures accelerate photodegradation by increasing mobility of reactive species and facilitating oxidation reactions (polypropylene, polyethylene)
• Heat and moisture synergy
  • High temperatures and humidity work together to accelerate hydrolytic degradation (polyesters, polyamides)
  • Elevated temperatures increase water diffusion rate into the polymer and hydrolysis reaction kinetics
• Light and chemical exposure synergy
  • Some chemicals like photosensitizers enhance the photodegradation rate (polycarbonate, polyurethanes)
  • UV light generates reactive species that initiate chemical degradation reactions

Mitigation strategies for polymer degradation

• Stabilizers and additives
  • Antioxidants: Prevent oxidative degradation by scavenging free radicals or decomposing peroxides (hindered phenols, phosphites)
  • UV stabilizers: Absorb or block UV light preventing photodegradation (benzotriazoles, benzophenones)
  • Hydrolysis stabilizers: React with water molecules or hydrolyzable groups reducing the hydrolytic degradation rate (carbodiimides, epoxides)
• Material selection and design
  • Choose polymers with inherent resistance to specific environmental factors (fluoropolymers, silicones)
  • Design products with protective barriers or coatings to minimize exposure to degrading agents
• Processing and manufacturing optimization
  • Optimize processing conditions to minimize degradation during production (lower temperatures, inert atmosphere)
  • Control cooling rates and residual stresses to improve the polymer's resistance to environmental factors
• Storage and handling practices
  • Store polymers in a cool, dry, and dark environment to minimize exposure to heat, moisture, and light
  • Use appropriate packaging materials to protect polymers from chemical contamination during storage and transportation (moisture barrier films, desiccants)