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Adhesives and Adhesion

Part 1

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Adhesives

B. PourabbasFaculty of Polymer Engineering

Sahand University of Technology

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Reference

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Overview 1

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Overview 2

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Introduction to Adhesion and Adhesives

• Basic Properties• Basic Chemistry• Theories of Adhesion• Polymerization• Glass Transition Temperature• Viscoelastic Properties

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The Big Picture

Material A

Adhesive

Material B

What properties are used to characterize

this stuff?

What properties are most important to

my application?

What general classes of this stuff are

out there?

Who supplies this stuff?

What is the besttrade-off?

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BASIC PROPERTIES

• Adhesive: 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.

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BASIC PROPERTIES

• Basically an adhesive must do two things:1. 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.

2. 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.

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Pre-Cure Properties

• Shelf Life• Pot Life• Viscosity• Wetting

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Shelf Life

• How long an adhesive can be stored without degradation to its properties– Ex: Two-part epoxies = 6-12 months– One-part epoxies shorter than this– Extend by refrigeration or freezing

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Pot Life

• What 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.

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Viscosity

• Resistance to flow or shear stress• Measured in Centipoise (100-90,000 cps)• High Viscosity– Easy to Control Bead Size & Position

• Low Viscosity– Improved Wetting– Hard to Control

• Linearly Proportional to Temperature!

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Wetting

• Ability 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.

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Wetting

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.

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Wetting

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Wetting

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Basic Chemistry

• All 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.

Cohesive :A puddle of mercury

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Basic Chemistry

Polymers’ structure:Linear (top), branched (middle) and crosslinked (bottom) polymers

They 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).

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Basic Chemistry

• Other 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.

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Theories of Adhesion

• The six theories of adhesion:1. Physical Adsorption, 2. Chemical Bonding, 3. Diffusion, 4. Electrostatic, 5. Mechanical Interlocking and 6. Weak Boundary Layer.

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Theories of Adhesion

• The six theories of adhesion:1. 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).

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Theories of Adhesion

1. Physical Adsorption:force between two permanent dipoles (Keesom force),

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Theories of Adhesion

1. Physical Adsorption:force between two instantaneously induced dipoles (London dispersion

force)

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Theories of Adhesion

• The 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.

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Theories of Adhesion

• The six theories of adhesion:2. Chemical Bonding, – Examples:

• Bonding in Silicone adhesive on Glass surfaces, • Polyvinyl alcohol and paper, Hydrogen bonding• Formaldehyde based adhesives for wood

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Theories of Adhesion

• The 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.

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Theories of Adhesion

• The 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.

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Theories of Adhesion

• The six theories of adhesion:4. Electrostatic Theory– The 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.

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Theories of Adhesion

• The six theories of adhesion:5. Mechanical Interlocking– If 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.

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Theories of Adhesion

• The six theories of adhesion:6. Weak Boundary Layer– The 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.