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Adhesives and bonded structures . John Summerscales. Outline of lecture. Adhesive systems Anaerobic adhesives Cyanoacrylates Epoxies Phenolics Polyurethanes Others Good joint design Surface preparation Co-curing. Anaerobic adhesives. acrylic-based adhesives

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outline of lecture

Outline of lecture

Adhesive systems

Anaerobic adhesives

Cyanoacrylates

Epoxies

Phenolics

Polyurethanes

Others

Good joint design

Surface preparation

Co-curing

anaerobic adhesives
Anaerobic adhesives
  • acrylic-based adhesives
    • normally cure in
      • the presence of metal, and
      • the absence of air (specifically atmospheric oxygen).
    • normally used as thin layers for locking or sealing
    • rapid cure time
    • complementary to the cyanoacrylates.
cyanoacrylates
Cyanoacrylates
  • acrylic-based adhesives
    • require moisture as a vital catalyst
    • almost instantaneous curing
    • normally used as thin layers
    • complementary to the anaerobics
epoxies
Epoxies
  • Epoxide resin plus hardener
    • usually two-part system
    • premixed single part epoxy adhesives available.
    • good adhesion to many materials
    • high strength
    • can be used for thicker joints
phenolics
Phenolics
  • phenol-formaldehyde resin systems
    • one of the earliest synthetic adhesives
    • still good performance in severe environments.
    • health and safety issues
      • formaldehyde considered carcinogenic
      • phenols are acidic
    • specialised equipment required
    • complex procedures required.
polyurethanes
Polyurethanes
  • polyurethane chemistry
    • usually isocyanate and alcohol
      • isocyanates haverigorous health and safety requirements.
    • good for load-bearing applications in dry conditions
    • susceptible to attack by moisture.
other adhesive systems
Other adhesive systems
  • ultraviolet light curing systems
  • plastisols
    • based on PVC dispersions
  • rubber solutions
    • solvent evaporation effects bonding
  • toughened adhesives
    • any of the above families of adhesive
    • incorporation of low molecular weight rubbers
      • chemically incorporated in the polymer backbone, or
      • physical particles.
use of adhesives
Use of adhesives
  • Adhesives can bond
    • most materials in common engineering use
    • especially useful where the substrates are different materials.
  • For optimum bonding, avoid:
    • materials with weak or loose surface layers
    • materials troubled by water migration,solvent attack and/or stress cracking.
advantages of adhesives cf welding brazing soldering or mechanical fasteners
Advantages of adhesivescf: welding, brazing, soldering or mechanical fasteners
  • lower temperature manufacture of joints
  • joints without blemish, distortion or protrusions
  • net weight of the joint is minimised
  • stresses are more uniformly distributed
  • resulting structure is normally stiffer than for discretely welded/fastened joints
  • increased fatigue life
  • complex geometries relatively easy to make
  • reduced capital and labour costs
  • process de-skilled or completely automated
good joint design
Good joint design
  • essential for highly-stressed applications
  • bonded joints:
    • are best loaded in compression
    • give acceptable performance in shear
    • tension should be avoided
      • especially peel: at least one component is flexible
      • and cleavage: rigid components are involved.
correct joint design redrawn from diagrams in the permabond engineers guide to adhesives
Correct joint design ... redrawn from diagrams in The [Permabond] Engineers Guide to Adhesives

KEY:

adhesive

substrate

  • Compression good Shear OK



wrong joint design redrawn from diagrams in the permabond engineers guide to adhesives
Wrong joint design... redrawn from diagrams in The [Permabond] Engineers Guide to Adhesives

x

  • Peel (1 flexible) Cleavage (2 rigid)

x

joint design redrawn from diagrams in the permabond engineers guide to adhesives
Joint design... redrawn from diagrams in The [Permabond] Engineers Guide to Adhesives

X

joint design redrawn from diagrams in the permabond engineers guide to adhesives15
Joint design... redrawn from diagrams in The [Permabond] Engineers Guide to Adhesives

X

surface preparation
Surface preparation
  • Surface preparation
    • crucial to the achievement of a good bond
    • for composites normally includes a degrease-abrade-degrease-dry sequence.
    • shot-blasting to abrade surface is inappropriate
      • tends to remove too much substrate.
    • plastic bead blasting (or similar blast media) permits greater control of material removal.
surface preparation17
Surface preparation

θ

  • wetting of the substrate by liquid depends onthe interfacial tensions for the three phases:
    • solid/liquid (SL)
    • liquid/vapour (LV)
    • solid/vapour (SV)
  • contact angle of <90° will result in wetting
    • the substrate is hydrophilic when the liquid is water
  • contact angle >90° will not result in wetting
    • the substrate is hydrophobic when liquid is water
surface preparation18
Surface preparation
  • contact angle for smooth surfacedescribed by Young\'s equation
  • Wenzel modified Young\'s equationto include roughness:
co curing
Co-curing
  • For adhesively bonded composite components, co-curing is often adopted:
    • simultaneous post-cure of the laminate, and
    • cure of the adhesive
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