Lecture 05: Polymer Processing Techniques

Polymer Types

  • Classified by end use: plastics, elastomers, fibers, coatings, adhesives, foams, films.
  • Polymer may have multiple applications based on properties.

Plastics

  • Largest group of polymeric materials, used in general-purpose applications.
  • Vary in rigidity and crystallinity.
  • Can be thermoplastic or thermosetting.
  • Linear/branched polymers used below glass transition or melting temperatures, or crosslinked to maintain shape.
  • Stress-strain behavior varies (brittle, plastic, elastomeric).

Elastomers

  • Display rubbery behavior with high elastic deformation.
  • Amorphous and crosslinked.
  • Elastic deformation involves chain uncoiling and straightening.
  • Chains return to original shape upon stress release.
  • Must not easily crystallize (amorphous).
  • High elastic deformations are achieved by crosslinking (vulcanization).
  • Must be above glass transition temperature.
  • Common elastomers: natural rubber, SBR (tires), NBR (degradation resistant).

Fibers

  • Drawn into long filaments (100:1 length-to-diameter ratio).
  • Used in textiles and composite materials.
  • High molecular weight is needed to prevent breaking during drawing.
  • High crystallinity required for tensile strength, achieved with linear, unbranched chains.

Additives

  • Modify polymer properties.
  • Can alter molecular structure or add a second phase (composite).
  • Types: fillers, plasticizers, colorants, lubricants, flame retardants, cross-linking agents, UV absorbers, antioxidants.

Coatings

  • Protect from environment, improve appearance, provide electrical insulation.

Adhesives

  • Bond surfaces via mechanical (penetration into pores) or chemical (intermolecular forces) bonding.

Films

  • Thin films (0.025-0.125 mm) used for packaging, textiles, etc.

Foams

  • Contain high volume percentage of pores and trapped gas bubbles.
  • Used for cushions, packaging, and thermal insulation.

Polymer Processing Techniques

  • Forming techniques: Extrusion, Injection Molding, Compression, Transfer Molding, Blow Molding, Casting, 3D Printing.

Forming Factors

  • Material type (thermoplastic or thermosetting).
  • Softening temperature (for thermoplastics).
  • Atmospheric stability.
  • Geometry and size of product.

Thermoplastics Forming

  • Formed above glass transition or melting temperatures.
  • Pressure maintained during cooling.
  • Recyclable.

Thermosetting Forming

  • Two stages: prepolymer preparation and curing (heating, catalysts, pressure).
  • Curing forms crosslinked network.
  • Dimensionally stable after curing.
  • Difficult to recycle, usable at higher temperatures, chemically inert.

Molding of Plastics

  • Common method using compression, transfer, blow, injection, and extrusion.
  • Plastic forced into mold cavity at elevated temperature and pressure.

Fabrication of Fibers

  • Spinning: melt spinning, dry spinning, wet spinning.
  • Melt spinning: material melted, pumped through spinneret, solidified by cooling.
  • Dry spinning: polymer-solvent solution pumped through spinneret into heated zone; solvent evaporates.
  • Wet spinning: polymer-solvent solution through spinneret into a second solvent.

Fabrication of Films

  • Extrusion through thin die slit, followed by rolling or drawing.
  • Can be blown and extruded simultaneously (biaxial drawing).
  • Coextrusion: multilayers of different polymers extruded simultaneously.

3D Printing of Polymers

  • Advanced manufacturing; polymers more suited than metals/ceramics.
  • Lower melting/softening temperatures, flexible, ductile, photosensitive.
  • Techniques: fused deposition modeling, stereolithography, polyjet printing, continuous liquid interface production.

Fused Deposition Modeling (FDM)

  • Polymer filament fed into heated nozzle.
  • Layers formed by extruding flattened polymer string.
  • Adjacent layers adhere and harden upon cooling.
  • Common materials: PLA, ABS, PET, nylon, TPU, PC.
  • Applications: tissue engineering scaffolds, aerospace, medical packaging, electronics, prototypes.

Stereolithography (SLA)

  • Photosensitive resin (polymer, monomers, photoinitiators) cured by light.
  • Build platform lowered layer by layer; laser scans pattern, polymerizing scanned regions.
  • Uncured resin washed away; post-printing UV treatment for complete polymerization.
  • Common materials: epoxy and acrylate-based thermosets.
  • Applications: anatomical models, biomedical implants, architectural models, investment casting patterns.