Adhesives and bonded structures John Summerscales Outline of

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

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 • 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 • polyurethane chemistry • usually isocyanate and alcohol • isocyanates have rigorous health and safety requirements. good for load-bearing applications in dry conditions • susceptible to attack by moisture. •

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 • 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 • • • lower temperature manufacture of 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 • 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 • compression good KEY: adhesive shear OK substrate

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

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

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

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 preparation • wetting of the substrate by liquid depends on the interfacial tensions for the three phases: solid/liquid (SL) • liquid/vapour (LV) • solid/vapour (SV) • θ • 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 preparation • contact angle for smooth surface described by Young's equation • Wenzel modified Young's equation to include roughness:

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

Summary of lecture • adhesive systems • • • anaerobic adhesives cyanoacrylates epoxies phenolics polyurethanes others • good joint design • surface preparation • co-curing