Notes on Strengthening Thermoplastics and Thermosets

Chapter 15: Strengthening Thermoplastics

Average Molecular Mass
  • Increasing average molecular mass leads to stronger polymers as they become longer and more entangled with neighboring chains.
  • Strength is proportional to average molecular mass or degree of polymerization (up to a certain limit) - beyond this, increasing length does not significantly improve strength.
  • Note: This method of strengthening is not commonly used.
Crystallinity
  • Degreed Crystallinity: As the crystalline structure of polymers increases, both tensile strength and density enhance due to:
    • More organized structures
    • Tighter packing of polymer chains
    • Stronger bonding forces between chains
Pendant Groups
  • Adding bulky pendant groups on the main carbon chain increases the difficulty of chain slippage, thereby strengthening thermoplastics.
    • Examples:
    • Polyethylene: Ductile
    • Polystyrene: Stiff
  • Steric hindrance: Occurs when large side groups impede molecular movement, increasing stiffness and strength while reducing ductility.
Polar Atoms
  • Integrating large, highly polar atoms onto the main carbon chain increases bonding forces between polymer chains, thus enhancing strength.
    • Example process: Electrostatic attraction increases bonding strength between separate polar polymers.
Strengthening Thermosets
  • Thermosets gain strength through the formation of covalent bonds across the material during casting or pressing.
    • Adding cross-linking agents leads to stronger, more rigid thermosetting plastics.
    • Generally, they possess high strength and rigidity but lower ductility.
Effect of Temperature on Strength
  1. Thermoplastics: Gradually soften as temperature increases due to weakening of secondary bonds, leading to decreased strength.
  2. Thermosets: Weaken under heat but don’t melt due to strong covalent bonds; degradation or charring occurs at high temperatures.
Creep and Fracture of Polymers
  • Polymers can be susceptible to creep (slipping past each other under constant force).
  • Thermosets are typically resistant to creep, often resulting in brittle fractures due to their strong bonds.
  • Thermoplastics:
    • Below glass transition temperature (Tg): Brittle fracture
    • Above Tg: Ductile fracture
Stress Relaxation
  • Process by which polymers relieve stress over time as polymer chains slide past, break, and reform secondary bonds.
  • Is temperature-dependent; increasing temperature raises the rate of relaxation without overly softening or melting the plastic.
Relaxation Time
  • Defined as the period for stress (σ) to diminish to 0.37 of the original stress. The relationship is given by: Γ=Timese−tT\Gamma = T imes e^{-\frac{t}{T}}
    • Where σ = stress after time t; T = relaxation time.
Elastomers (Rubbers)
  • Unique polymeric materials that can undergo significant dimensional changes under stress but return to their original shape when stress is removed.
  • Natural Rubber: Made from latex from the Hevea brasiliensis tree - characterized by long, coiled chains of cis-1,4 polyisoprene, resulting in flexibility.
Structural Isomers
  • Molecules with the same molecular formula but different structural arrangements.
    • Example: cis-1,4 polyisoprene (natural rubber) vs. trans-1,4 polyisoprene (gutta-percha, a rigid material).
Vulcanization
  • A chemical process where polymer chains are cross-linked using sulfur to limit molecular motion, enhancing rubber's durability.
    • Varying sulfur amounts alters rubber's cross-linking, affecting rigidity (soft vs hard rubber).
Effects of Additives
  • Carbon Black: Improves tensile strength and tear resistance.
  • Silicas: Reinforce rubber properties.
Synthetic Rubbers
  • Styrene-Butadiene Rubber (SBR): Common synthetic rubber improving toughness and wear-resistance.
  • Nitrile Rubbers: Enhance oil and heat resistance through increased polarity and bonding.
  • Polychloroprene: Offers superior resistance to oxygen, ozone, heat, and weather due to chlorine substitution in isoprene.
Polymerization Reactions
  • Chain-Growth Polymerization: Process where monomers are chemically combined into polymers via successive addition, illustrated with ethylene as an example.
    • Key steps include initiation, propagation, and termination of the polymer chain.
Industrial Polymerization and Processing
  1. Thermoplastics (e.g., injection molding, extrusion) allow for melting and recycling.
    • Advantages: high production rates, low costs, intricate shapes.
    • Disadvantages: high machine costs, process control challenges.
  2. Thermosetting Processes: Include compression and transfer molding, leading to less intricate parts but with benefits in cost and efficiency.
Plastic Recycling Codes
  • Familiarize with recycling identification codes (1-7) to understand common products, materials, and recycling processes.
  • Examples include PET, HDPE, PVC, LDPE, PP, PS, and others; each with specific recyclability and applications.