adhesives and adhesion
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DESCRIPTIONAdhesives and Adhesion. Part 1. Adhesives. B. Pourabbas Faculty of Polymer Engineering Sahand University of Technology. Reference. Overview 1. Overview 2. Introduction to Adhesion and Adhesives. Basic Properties Basic Chemistry Theories of Adhesion Polymerization - PowerPoint PPT Presentation
Adhesives and AdhesionPart 11
AdhesivesB. PourabbasFaculty of Polymer EngineeringSahand University of Technology2Reference
5Introduction to Adhesion and AdhesivesBasic PropertiesBasic ChemistryTheories of AdhesionPolymerizationGlass Transition TemperatureViscoelastic Properties6The Big PictureMaterial AAdhesiveMaterial BWhat properties are used to characterize this stuff?What properties are most important tomy application?What general classes of this stuff are out there?Who supplies this stuff?What is the besttrade-off?7BASIC PROPERTIESAdhesive: A material which when applied to the surfaces of materials can join them together and resist separation.Adherent and substrate are used for a body or material to be bonded by an adhesive. Shelf-life, for the time an adhesive can be stored before use, and Pot-life, the maximum time between final mixing and application.8BASIC PROPERTIESBasically an adhesive must do two things:It must wet the surfaces, that is it must spread and make a contact angle approaching zero. Intimate contact is required between the molecules of the adhesive and the atoms and molecules in the surface. When applied the adhesive will be a liquid of relatively low viscosity. The adhesive must then harden to a cohesively strong solid. This can be by chemical reaction, loss of solvent or water, or by cooling in the case of hot melt adhesives. There is an exception to this, and that is pressure-sensitive adhesives which remain permanently sticky. These are the adhesives used in sticky tapes and labels.9Pre-Cure PropertiesShelf LifePot LifeViscosityWetting10Shelf LifeHow long an adhesive can be stored without degradation to its propertiesEx: Two-part epoxies = 6-12 monthsOne-part epoxies shorter than thisExtend by refrigeration or freezing11Pot LifeWhat in the world is a pot and why do we care how long it lives?Pot life is the length of time during which an adhesive can be used after mixing.Pot life can range from 30 seconds to 5 days.12ViscosityResistance to flow or shear stressMeasured in Centipoise (100-90,000 cps)High ViscosityEasy to Control Bead Size & PositionLow ViscosityImproved WettingHard to ControlLinearly Proportional to Temperature!
WettingAbility to make contact with substrate surface.Surface tension should be ~10 dynes/cm less than substrate surface energy.Typical adhesive surface tension = 30-35 dynes/cm.14
Top: liquid droplets making a high and low contact angle on Flat, solid surface. Centre: high contact angle leading to no spreading on a rough surface. Bottom: wetting on a rough surface.15Wetting16
Basic ChemistryAll adhesives either contain polymers, or polymers are formed within the adhesive bond.Polymers give adhesives cohesive strength, which may be either linear, branched or crosslinked as illustrated in the Next Silde.18
Cohesive :A puddle of mercury
Polymers structure:Linear (top), branched (middle) and crosslinked (bottom) polymersThey will flow at higher temperatures and dissolve in suitable solvents. These latter properties are essential in hot melt, and solvent-based adhesives, respectively.
Crosslinked polymers will not flow when heated, and may swell, but not dissolve, in solvents. All structural adhesives are crosslinked because this eliminates creep (deformation under constant load).19Basic ChemistryOther ingredients:Many adhesives contain additives that are not polymers stabilizers against degradation by oxygen and UV, plasticizers which increase flexibility and lower the glass transition temperature, and powdered mineral fillers, which may reduce shrinkage on hardening, lower cost, modify flow properties before hardening and modify final mechanical properties. other possible additives are tackifiers and silane coupling agents.20Tackifiers are chemical compounds used in formulating adhesives to increase the tack, the stickiness of the surface of the adhesive. They are usually low-molecular weight compounds with high glass transition temperature. At low strain rate, they provide higher stress compliance, and become stiffer at higher strain rates.20Theories of AdhesionThe six theories of adhesion:Physical Adsorption, Chemical Bonding, Diffusion, Electrostatic, Mechanical Interlocking and Weak Boundary Layer. 21Theories of AdhesionThe six theories of adhesion:Physical Adsorption:Physical adsorption involves van der Waals forces across the interface. These involve attractions between permanent dipoles and induced dipoles, and are of three types:force between two permanent dipoles (Keesom force),force between a permanent dipole and a corresponding induced dipole (Debye force),force between two instantaneously induced dipoles (London dispersion force, dispersion forces).22Theories of AdhesionPhysical Adsorption:force between two permanent dipoles (Keesom force),
23Theories of AdhesionPhysical Adsorption:force between two instantaneously induced dipoles (London dispersion force)
Theories of AdhesionThe six theories of adhesion:2. Chemical Bonding, The chemical bonding theory of adhesion invokes the formation of covalent, ionic or hydrogen bonds across the interface.
25Theories of AdhesionThe six theories of adhesion:Chemical Bonding, Examples: Bonding in Silicone adhesive on Glass surfaces, Polyvinyl alcohol and paper, Hydrogen bondingFormaldehyde based adhesives for wood26
Theories of AdhesionThe six theories of adhesion:3.Diffusion Theory, The diffusion theory takes the view that polymers in contact may interdiffuse, so that the initial boundary is eventually removed . Such interdiffusion will occur only if the polymer chains are mobile (i.e. the temperature must be above the glass transition temperatures) and compatible.27Theories of AdhesionThe six theories of adhesion:3.Diffusion Theory, Is generally applicable in bonding like rubbery polymers, as might occur when surfaces coated with contact adhesives are pressed together, and in the solvent-welding of thermoplastics.Examples:Plastic model kits: Swell two polystyrene surfaces with butanone (solvent) and then press them together.poly(methyl methacrylate) and poly(vinyl chloride), which permits the possibility of interdiffusion when structural acrylic adhesives are used to bond PVC.28
Theories of AdhesionThe six theories of adhesion:4.Electrostatic TheoryThe electrostatic theory originated in the proposal that if two metals are placed in contact, electrons will be transferred from one to the other so forming an electrical double layer, which gives a force of attraction. As polymers are insulators, it seems difficult to apply this theory to adhesives.29Theories of AdhesionThe six theories of adhesion:5.Mechanical InterlockingIf a substrate has an irregular surface, then the adhesive may enter the irregularities prior to hardening. This simple idea gives the mechanical interlocking theory, which contributes to adhesive bonds with porous materials such as wood and textiles.An example is the use of iron-on patches for clothing. The patches contain a hot melt adhesive that, when molten, invades the textile material.30
Theories of AdhesionThe six theories of adhesion:6.Weak Boundary LayerThe weak boundary layer theory proposes that clean surfaces can give strong bonds to adhesives, but some contaminants such as rust and oils or greases give a layer which is cohesively weak.This is an area where acrylic structural adhesives are superior to epoxides because of their ability to dissolve oils and greases.31