Adhesives and bonded structures

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Adhesives and bonded structures

• John Summerscales

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Outline of lecture

• Adhesive systems
• Anaerobic adhesives
• Cyanoacrylates
• Epoxies
• Phenolics
• Polyurethanes
• Others
• Good joint design
• Surface preparation
• Co-curing

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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.

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Cyanoacrylates

• acrylic-based adhesives
• require moisture as a vital catalyst
• almost instantaneous curing
• normally used as thin layers
• complementary to the anaerobics

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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

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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.

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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.

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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.

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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.

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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

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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.

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Correct joint design ... redrawn from diagrams
in The Permabond Engineers Guide to Adhesives

• Compression good Shear OK

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Wrong joint design... redrawn from diagrams in
The Permabond Engineers Guide to Adhesives

• Peel (1 flexible) Cleavage (2
  rigid)

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Joint design... redrawn from diagrams in The
Permabond Engineers Guide to Adhesives
X
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Joint design... redrawn from diagrams in The
Permabond Engineers Guide to Adhesives
X
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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.

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Surface preparation

• 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 lt90 will result in wetting
• the substrate is hydrophilic when the liquid is
  water
• contact angle gt90 will not result in wetting
• the substrate is hydrophobic when liquid is water

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Surface preparation

• contact angle for smooth surfacedescribed by
  Young's equation
• Wenzel modified Young's equationto include
  roughness

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Co-curing

• For adhesively bonded composite components,
  co-curing is often adopted
• simultaneous post-cure of the laminate, and
• cure of the adhesive