13.1 Mechanisms of polymer degradation

Polymer degradation can wreak havoc on materials, affecting their properties and performance. From thermal breakdown to UV damage, various mechanisms chip away at polymers over time. Understanding these processes is key to developing more durable and sustainable materials.

Factors like temperature, oxygen, and UV exposure play a big role in how fast polymers degrade. These breakdowns lead to weaker materials, ugly discoloration, and shorter lifespans. It's not just about looks – degradation can cause product failures and environmental issues too.

Types and Mechanisms of Polymer Degradation

Types of polymer degradation

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• Thermal degradation occurs at elevated temperatures involves random scission of polymer chains leading to reduction in molecular weight and mechanical properties (PVC, PET)
• Oxidative degradation involves reaction with oxygen initiates free radical chain reactions results in chain scission and crosslinking (PP, PE)
• Photodegradation caused by exposure to UV radiation initiates photochemical reactions leads to chain scission, crosslinking, and discoloration (PS, PVC)

Chemical reactions in degradation

• Thermal degradation reactions
  • Random chain scission breaks polymer chains into smaller fragments reduces molecular weight and viscosity
  • Depolymerization reverses polymerization process releases monomers or oligomers (PMMA, POM)
• Oxidative degradation reactions
  1. Initiation: Formation of free radicals by hydrogen abstraction or peroxide decomposition
  2. Propagation: Free radicals react with oxygen to form peroxy radicals, peroxy radicals abstract hydrogen from polymer chains, creating hydroperoxides and new free radicals
  3. Termination: Combination of free radicals to form stable products
  • Effects on properties: Chain scission reduces molecular weight and mechanical strength, crosslinking increases brittleness and reduces elongation (PP, PE)
• Photodegradation reactions
  • Absorption of UV radiation by chromophores: Carbonyl groups, aromatic rings, and impurities act as chromophores
  • Formation of excited states and free radicals: Norrish Type I and Type II reactions
  • Oxidation and chain scission reactions similar to oxidative degradation
  • Effects on properties: Discoloration and yellowing, embrittlement and surface cracking (PVC, PS)

Factors Influencing Polymer Degradation

Factors influencing polymer degradation

• Temperature: Higher temperatures accelerate degradation, Arrhenius relationship: $k = A e^{-E_a/RT}$ where $k$ is rate constant, $A$ is pre-exponential factor, $E_a$ is activation energy, $R$ is gas constant, and $T$ is absolute temperature
• Oxygen availability: Oxygen is required for oxidative degradation, diffusion-limited reactions in thick samples, antioxidants can slow down oxidative degradation (BHT, hindered phenols)
• UV exposure: Intensity and wavelength of UV radiation affect photodegradation, outdoor applications are more susceptible to UV damage, UV stabilizers can absorb or block UV radiation (carbon black, TiO2)

Consequences of degradation on performance

• Mechanical property deterioration: Reduced tensile strength, elongation, and impact resistance, increased brittleness and fracture susceptibility
• Aesthetic changes: Discoloration, yellowing, and loss of gloss, surface cracking and chalking (PVC window frames, automotive parts)
• Product failure and reduced lifespan: Premature failure due to embrittlement or loss of mechanical integrity, shortened service life and increased replacement frequency (plastic gears, pipes)
• Environmental and safety concerns: Release of degradation products and microplastics, potential health risks and ecological impact (marine debris, microplastics in food chain)