The Basics of Adhesion

baconossifiedMechanics

Oct 29, 2013 (3 years and 9 months ago)

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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