CHEE 890
J.S. Parent
1
The Basics of Adhesion
Generating strong interphase adhesion, requires:
1.
Wetting
the surfaces to generate intimate contact between
surfaces
High Contact Angle
Zero Contact Angle
Poor wetting of surface
Complete Spreading
2. Solidification to a cohesively strong solid that provides
sufficient
adhesive strength
with the matrix such that applied
loads are withstood.
CHEE 890
J.S. Parent
2
Adhesion in Polymer Systems
An advanced treatment of adhesion in polymeric systems requires
an analysis of three phenomena:
1. Characterizing the surface properties/interactions of dissimilar
materials:
origin of interfacial forces
surface energy of liquids and solids
2. Interaction of polymer surfaces with liquids:
wetting, spreading
work of adhesion
3. Interaction of solid polymers with other solids:
contact adhesion of viscoelastic materials
CHEE 890
J.S. Parent
3
Conceptual Models for Adhesive Bonding
The actual mechanism of adhesive attachment is not thoroughly
understood

no single theory explains the phenomenon
universally.
Adsorption theory:
Adhesion results from the adsorption of adhesive molecules onto
the substrate, the resulting secondary attractive forces giving rise to
the observed bond strength.
»
Adhesive must therefore make intimate, molecular
contact with the adherends.
Mechanical theory:
Adhesion occurs by the filling of micro

cavities within the substrate
by the adhesive formulation. Bond strength is derived from
cured/dried polymer properties.
»
Adhesive strength favours large bond areas, and surface
roughness.
CHEE 890
J.S. Parent
4
Interfacial and Bulk Material Forces
CHEE 890
J.S. Parent
5
Forces Acting at a Liquid/Vapour Interface
Molecules near an air

liquid interface are subjected to an unequal
distribution of the forces that act to maintain a condensed phase.
The result is a net inward
force experienced by
molecules at the surface,
and a contraction of the
fluid into its lowest energy
state.
The surface energy (commonly called the surface tension of a
liquid) characterizes this effect.
The surface energy of a substance is the work required to
increase the area of a surface (usually in air) by unit amount.
Interfacial surface energy (interfacial tensions) represent the
imbalance of forces at the surface of two liquids.
CHEE 890
J.S. Parent
6
Surface Energy of Liquid/Vapour Interfaces
The energy associated with a phase changing its surface area is
described in a thermodynamic sense by the concept of surface
energy. In terms of the Helmholtz free energy (F(T,V) as opposed
to G(T,P)):
A droplet, for instance, can lower its free energy by reducing its
surface area (dA<0) to the point where internal pressure offsets the
contribution of area.
Capillary Rise Measurement:
Surface energies of liquids in their saturated vapour
are readily measured with a capillary rise tube,
shown to the right:
where r is the capillary radius, h the height difference,
g the gravitational constant and
the fluid densities.
CHEE 890
J.S. Parent
7
Surface Energy of Solid

Liquid Interfaces
Given that our system exists under static conditions, we can use a
thermodynamic approach to examine surface phenomena.
What changes in Gibbs energy result from increased contact
area between liquids A and B?
As contact between A and B changes, the area of exposure of A to
the vapour changes (dA
A
), as does that of B to the vapour (dA
B
)
and A to B (dA
AB
). The Gibbs energy responds as follows:
(1)
or,
(2)
where
i
represents the surface/interfacial free energy (J/m
2
).
Liq. B
Liq. A
Liq. B
Liq. A
dG ?
CHEE 890
J.S. Parent
8
Spreading Coefficient

Complete Wetting
When will a liquid completely wet a surface? Predictions can be
made on the basis of the spreading coefficient.
Realizing that if liquid B increases in area it is at the expense of A
and the benefit of AB,
then equation 2 is simplified by substitution:
The spreading coefficient S
A/B
, defined by:
represents how the Gibbs energy of the system will respond to a
change in the surface area of liquid A.
For surface wetting to occur, S
A/B
must be positive
Under this condition, wetting lowers G and is spontaneous.
CHEE 890
J.S. Parent
9
Work of Adhesion
Failure of an adhesive joint creates two new surfaces. The energy
expended per unit area should be the sum of the two surface
energies,
LV
and
SV
. However, intermolecular forces were present
at the joint prior to failure.
The work of adhesion per
unit area, W
A
, is given by
the Dupré equation:
Difficulties arise in trying to use this equation to represent
liquid/solid and solid/solid adhesion:
Absolute values of
SV
and
SL
cannot be measured
Strain induced at the interface upon solidification is not
described
This treatment describes only reversible adhesive failure
CHEE 890
J.S. Parent
10
Work of Adhesion
Polymers capable of strong dipole association and/or hydrogen
bonding are favoured in adhesive formulations due to strong
associations with high

energy surfaces.
Bonds derived from these polymers are often susceptible to
interfacial failure by the action of water.
CHEE 890
J.S. Parent
11
Solid/Liquid Interfaces

Contact Angle
When a liquid placed on a solid generates S
A/B
< 0, it remains as a
droplet with a defined angle of contact,
.
Spreading increases contact
between liquid and solid,
altering the Gibbs energy of the
system.
Spreading creates dA of liquid

solid area, dA cos
of liquid

vapour
area and

dA of solid

vapour area:
At equilibrium, dG=0:
Young Equation
CHEE 890
J.S. Parent
12
Work of Adhesion
The indeterminable parameters in our original expression for the
work of adhesion,
can be replaced by the contact angle using Young’s equation,
thereby transforming W
A
into:
This simple treatment of adhesion (Young

Dupre equation) relates
bond strength to determinable quantities, the liquid

vapour surface
tension and the contact angle the liquid makes with the solid.
Note however:
this relationship states that W
A
can only vary by a factor of
two from the surface energy of the liquid.
Solidification of the liquid can develop stress concentrations.
Adhesive failure is irreversible, depending as much on
rheology and fracture mechanics as interfacial
thermodynamics
CHEE 890
J.S. Parent
13
Critical Surface Energy
In the Young equation only
LV
and cos
are measurable.
However,
SV
and
SL
have are useful parameters for predicting
adhesion.
Pioneering work by Zisman revealed a linear relationship between
contact angle and
LV
for a series of liquids:
where
c
is defined as the critical surface energy for the solid and b
is a constant.
The critical surface tension for the solid is defined as the
intercept of the horizontal line cos
=1 with the extrapolated
straight line of cos
against
LV
.
A hypothetical test liquid of this
LV
would spread on the solid,
while one with a greater
LV
would wet the solid with a
measurable contact angle.
CHEE 890
J.S. Parent
14
Critical Surface Energy
Shown to the right is a
Zisman plot for determining
the critical surface tension
of poly(tetrafluoroethylene).
Test liquids are n

alkanes
of different surface tensions.
The choice of test liquids
affects the results, and
differentvalues of
c
can
be obtained depending upon whether polar, non

polar or hydrogen

bonding fluids are used.
CHEE 890
J.S. Parent
15
Critical Surface Tension
To assure spreading and wetting of an adhesive formulation on a
substrate, the fluid adhesive should have a surface tension no higher than
the critical surface tension of the solid adherend.
A solid can induce liquids of lower, but not higher surface tension, to
wet it (if no surface chemical reaction occurs).
c
(20
°
C): mN m

1
Poly(tetrafluoroethylene)
18
Silicone, polydimethyl
24
Poly(ethylene)
31
cis

Poly(isoprene)
31
Poly(styrene)
33
Poly(vinyl alcohol)
37
Poly(methyl methacrylate)
39
Poly(vinyl chloride)
40
Poly(acrylonitrile)
44
Amine

cured epoxide
44
Poly(ethylene terephthalate)
45
Cellulose
45
Poly(hexamethylene adipamide) (nylon 6,6)
46
Aluminum
~500
Copper
~1000
CHEE 890
J.S. Parent
16
Critical Surface Energy

Coatings
CHEE 890
J.S. Parent
17
Graft Copolymers: ABS Resins
TEM of an emulsion

prepared
ABS. Typically emulsion
rubber domains contain little
occluded copolymer of styrene
and acrylonitrile.
TEM of a suspension

prepared
ABS. Typically rubber domains in
suspension

derived polymer
contain substantial amounts of
occluded copolymer of styrene and
acrylonitrile.
CHEE 890
J.S. Parent
18
Graft

Modified Polyolefins
Melt grafting of maleic anhydride and vinylalkoxysilanes to
polyolefins generates reactive materials that can improve phase
adhesion between high

energy and low

energy materials
Nylon and polyethylene
Silica and polyethylene
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