Download Sahand University of Technology

Adhesives and Adhesion
Part 1
1
2
Reference
3
Overview 1
4
Overview 2
5
Introduction to Adhesion and
Adhesives
•
•
•
•
•
•
Basic Properties
Basic Chemistry
Theories of Adhesion
Polymerization
Glass Transition Temperature
Viscoelastic Properties
6
The Big Picture
What properties are
most important to
my application?
What properties are
used to characterize
this stuff?
Material A
Adhesive
Material B
Who supplies this stuff?
What general classes
of this stuff are
out there?
What is the best
trade-off?
7
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.
8
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 pressuresensitive adhesives which remain permanently sticky.
These are the adhesives used in sticky tapes and labels.
9
Pre-Cure Properties
•
•
•
•
Shelf Life
Pot Life
Viscosity
Wetting
10
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
11
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.
12
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!
13
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.
14
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.
15
Wetting
16
Wetting
17
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
18
They will flow at higher temperatures and
dissolve in suitable solvents.
These latter properties are essential in hot
melt, and solvent-based adhesives, respectively.
Basic Chemistry
Polymers’ structure:
Linear (top), branched
(middle) and
crosslinked (bottom)
polymers
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).
19
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
20
coupling agents.
Theories of Adhesion
• The six theories of adhesion:
1.
2.
3.
4.
5.
6.
Physical Adsorption,
Chemical Bonding,
Diffusion,
Electrostatic,
Mechanical Interlocking and
Weak Boundary Layer.
21
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).
22
Theories of Adhesion
1. Physical Adsorption:
force between two permanent dipoles (Keesom
force),
23
Theories of Adhesion
1. Physical Adsorption:
force between two
instantaneously induced dipoles (London dispersion
force)
24
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.
25
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
26
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.
27
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.
28
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.
29
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.
30
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.
31