Sticking Together. Steven Abbott

under a constant load Curing Setting solid via polymerization Dahlquist PSAs must be soft enough to meet the Dahlquist criterion for quick tack Dispersant A molecule added to a formulation to keep particles separated and free flowing Dissipation Converting work into useless heat – this soaks up crack energy and increases adhesion DLVO A theory of why small particles will or will not stick to each other, named after Derjaguin & Landau and Verwey & Overbeek G′ & G″ Measures of the elastic (G′) and fluid (G″) resistance to shear forces Griffith's Law The tendency of a crack to form increases with the size of defects in the system Hydrophilic Likes to be with water, dislikes oil Hydrophobic Dislikes water, likes to be with oil Hydroxyl A group made from one oxygen and one hydrogen, shown as –OH Interface The line between two adhesive components, e.g. between adherend and adhesive Ion A charged molecule or atom. Anions (negative ions) can polymerize cyanoacrylates Molecular weight For polymers this indicates how many monomer units were assembled and, therefore, the length of the chain Monomer A molecule that can react with other monomers to form a polymer Peel test Pulling up from the joint Polymers: EVA (Poly)Ethylene vinyl acetate EVOH (Poly)Ethylene vinyl alcohol PE Polyethylene, commonly called polythene. HDPE/LDPE High Density PE and Low Density PE PP Polypropylene PMMA Polymethylmethacrylate, commonly called Perspex or Plexiglas PC Polycarbonate PET Polyethyleneterephthalate, commonly called polyester PDMS Polydimethylsiloxane, commonly called silicone PTFE Polytetrafluoroethylene, commonly called Teflon PVOH Polyvinylalcohol PVA Polyvinylacetate. Confusingly PVA can also mean Polyvinylalcohol PVB Polyvinylbutyral PVP Polyvinylpyrrolidone PSA Pressure Sensitive Adhesive – common adhesive tape Radical A reactive molecule that produces a new radical after reacting with a monomer Retarder An additive to slow down the setting speed of the adhesive Shear test Pulling apart across the joint Stefan's law Squeezing a blob of adhesive gets much harder as its thickness decreases Stress Force acting across an area Strain Amount of stretch caused by a stress Surfactant “Surface active agent” used in cleaning (“detergent”) and creating emulsions (“emulsifier”). Has a hydrophilic head and hydrophobic tail Tack That easy to know but hard to define feeling of instant adhesiveness -TES & -TMS Triethoxy and Trimethoxy silanes, very useful adhesion promotion groups TiO₂ Titanium dioxide – a key whitening pigment van der Waals The force that attracts all molecules and surfaces to each other vdW Abbreviation for van der Waals VOC Volatile Organic Compounds – typically solvents other than water WLF Williams, Landel and Ferry theory describing the equivalence of temperature and time

      CHAPTER 1

      Introduction

      Modern society would fall apart without adhesives.

      Your smartphone has at least 25 adhesive joints in it, with the all-important glass screen and the display held with adhesives, as are the chips, microphones, speakers and aerials inside. The screen protector is also stuck on.

      Now step into your car. Manufacturers are under intense pressure to reduce weight. Every screw or rivet is a potentially avoidable weight (yes, automobile manufacturers agonize over each gram in each component) and the interior of a modern car is held together by clips (for removable parts) and adhesives. External parts such as bumpers (fenders) used to be metal. Now they are made of polypropylene, strongly adhered to the metal shell. Even the metal shell itself is rapidly heading for history as plastic/carbon fibre takes over, held together with adhesives.

      What about something really important such as an aircraft? The interiors are stuck together – the weight, inconvenience and aesthetics of screws and rivets make them unacceptable. The rivets we see on the outside give the impression that they are the key structural element, yet they are now mostly for backup or are redundant. Even the (by now) old all-metal aircraft are largely held together with adhesives, while the latest generation of civil aircraft such as the lightweight, efficient Boeing Dreamliner and Airbus 350 are predominantly glued together.

      And yet, the hidden nature of adhesives (if you can see them, they probably haven't been applied properly), allied with their avoidability (people often prefer to try other methods first), has led to a rather dismissive attitude to the whole technology. When I mention my interest in adhesives to friends and neighbours, the general response is that adhesives are trivial, unsatisfactory or excessive, i.e. they are only an occasional feature of their lives, they often don't work well and can be (like my wife's orchid jug mentioned in the Preface) unsightly when used; as such they are not of any great importance. There is a medieval notion that to “really” put things together you need nails, screws, rivets and welding.

      Talk to anyone who really needs to put things together and you find a horror of nails,