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Adhesives and bonded structures

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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Adhesives and bonded structures

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  1. Adhesives and bonded structures John Summerscales

  2. Outline of lecture Adhesive systems Anaerobic adhesives Cyanoacrylates Epoxies Phenolics Polyurethanes Others Good joint design Surface preparation Co-curing

  3. 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.

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

  5. 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

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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

  11. 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.

  12. Correct joint design ... redrawn from diagrams in The [Permabond] Engineers Guide to Adhesives KEY: adhesive substrate • Compression good Shear OK  

  13. Wrong joint design... redrawn from diagrams in The [Permabond] Engineers Guide to Adhesives x • Peel (1 flexible) Cleavage (2 rigid) x

  14. Joint design... redrawn from diagrams in The [Permabond] Engineers Guide to Adhesives X     

  15. Joint design... redrawn from diagrams in The [Permabond] Engineers Guide to Adhesives X   

  16. 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.

  17. 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

  18. Surface preparation • contact angle for smooth surfacedescribed by Young's equation • Wenzel modified Young's equationto include roughness:

  19. 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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