11.3 Relationship between processing conditions and polymer properties

Processing conditions play a crucial role in shaping polymer properties. Temperature, pressure, and shear rate affect viscosity, molecular mobility, and orientation. These factors influence the final characteristics of polymer products, from mechanical strength to optical clarity.

Cooling rate impacts crystallinity, while residual stresses can cause warpage in molded parts. Understanding these relationships helps engineers optimize processing conditions to achieve desired polymer properties and product performance.

Processing Conditions and Polymer Properties

Effects of processing conditions

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• Temperature influences viscosity and flow behavior
  • Higher temperatures decrease viscosity, facilitating flow (melting of thermoplastics)
  • Lower temperatures increase viscosity, hindering flow (solidification of thermosets)
• Temperature affects molecular mobility and relaxation
  • Higher temperatures enhance molecular motion and relaxation (annealing)
  • Lower temperatures restrict molecular motion and relaxation (quenching)
• Pressure impacts density and free volume
  • Higher pressures increase density and reduce free volume (injection molding)
  • Lower pressures decrease density and increase free volume (foam extrusion)
• Pressure affects phase transitions and solubility
  • Higher pressures can shift phase transitions and increase solubility (supercritical fluid processing)
  • Lower pressures can shift phase transitions and decrease solubility (vacuum devolatilization)
• Shear rate influences molecular orientation and alignment
  • Higher shear rates promote molecular orientation in the flow direction (fiber spinning)
  • Lower shear rates result in less molecular orientation (compression molding)
• Shear rate affects viscosity and flow behavior
  • Higher shear rates can lead to shear-thinning behavior (extrusion)
  • Lower shear rates can lead to Newtonian or shear-thickening behavior (rotational molding)

Molecular orientation and properties

• Molecular orientation involves alignment of polymer chains in a preferred direction
  • Induced by flow or deformation during processing (stretching)
  • Influenced by processing conditions such as shear rate and draw ratio (blow molding)
• Oriented polymers exhibit anisotropic properties with different characteristics in different directions
  • Properties along the orientation direction differ from those perpendicular to it (biaxial orientation)
• Molecular orientation impacts mechanical properties:
  • Tensile strength and modulus increase in the orientation direction (fibers)
  • Tensile strength and modulus decrease in the transverse direction
  • Elongation at break decreases in the orientation direction
  • Elongation at break increases in the transverse direction
  • Impact strength and toughness decrease due to reduced ability to absorb energy through chain mobility (oriented films)

Cooling rate and polymer crystallinity

• Cooling rate determines the time available for crystallization
  • Slower cooling rates allow more time for crystallization (annealing)
  • Faster cooling rates limit the time for crystallization (quenching)
• Cooling rate affects the size and perfection of crystalline structures
  • Slower cooling rates promote larger and more perfect crystals (isothermal crystallization)
  • Faster cooling rates result in smaller and less perfect crystals (rapid cooling)
• Crystallinity represents the degree of crystalline order in a semi-crystalline polymer
  • Influenced by the cooling rate during solidification
  • Higher crystallinity achieved with slower cooling rates
  • Lower crystallinity obtained with faster cooling rates
• Morphology describes the arrangement and size of crystalline and amorphous regions
  • Slower cooling rates lead to larger spherulites and lamellar structures
  • Faster cooling rates result in smaller and less developed crystalline structures
• Crystallinity and morphology affect mechanical, thermal, and optical properties
  • Higher crystallinity and larger structures increase stiffness, strength, and opacity (engineering plastics)
  • Lower crystallinity and smaller structures increase flexibility, toughness, and transparency (packaging films)

Residual stresses in molded parts

• Residual stresses are internal stresses that remain in a part after processing
  • Developed due to non-uniform cooling, pressure, or flow
  • Can lead to warpage, dimensional instability, or premature failure (warped injection molded parts)
• Temperature gradients cause non-uniform cooling during solidification
  • Faster cooling near the mold surface compared to the center
  • Leads to differential shrinkage and residual stresses
• Thermal stresses develop due to the mismatch in thermal expansion between the skin and core
  • Can cause warpage or distortion of the molded part (warped injection molded parts)
• Pressure distribution results in non-uniform pressure during filling and packing
  • Higher pressure near the gate compared to the end of the flow path
  • Leads to differential shrinkage and residual stresses
• Pressure-induced stresses develop due to the variation in pressure-induced densification
  • Can cause warpage or distortion of the molded part (warped injection molded parts)
• Flow-induced stresses develop due to the orientation and stretching of polymer chains during flow
  • Higher near the mold surface and in regions of high shear
  • Can lead to anisotropic shrinkage and residual stresses
• Relaxation of flow-induced stresses occurs during the cooling and solidification process
  • Incomplete relaxation can result in frozen-in stresses and warpage (warped injection molded parts)