Bond Line
Dimensioning
 |
| Material fastener causing
stress in bonded materials |
The thermal expansion of different
materials directly effects the required bond thickness
of the Sikaflex adhesive. All materials expand/contract
when heated/cooled but some materials change in length
more than others. Therefore stresses are created in
the Sikaflex bond when bonding together such materials.
By increasing the bond thickness in these cases we can
reduce the inherent bond stress, therefore reducing
the chance of bond failure during large temperature
changes experienced in service. Sika's Industry Department
is able to advise on bond thickness requirements for
your application.
 |
| Thick and elastic Sikaflex
bond reducing stress in bonded materials |
To achieve the desired bond thickness
it is necessary to use spacers of suitable thickness
that are as hard or softer than the cured Sikaflex.
The plastic nozzle of the Sikaflex cartridge/sausage
should be cut to produce a triangular bead that when
compressed will provide a rectangular bead of desired
thickness and width.
|
Thermal Expansion Formula
To determine minimum bond line thickness, the thermal expansion
formula is used.
| Formula: |
 |
 |
= Minimum adhesive
bond line thickness (mm) |
 |
= Objects longest
length(m) |
 |
= Difference
between the two substrates coefficients of Linear thermal
expansion. (mm/m°C) |
 |
= Change in service
temperature from maximum to minimum. |
The following table illustrates the
differences in coefficients of Linear Thermal Expansion between
substrates.
|
Coefficients
of Linear Thermal Expansion
|
| Materials |
|
|
mm/m°C |
| Metals |
Aluminium |
23.5 |
x10-3 |
| |
Brass |
18.5 |
x10-3 |
| |
Copper |
16.5 |
x10-3 |
| |
Stainless Steel |
11-17 |
x10-3 |
| |
Steel |
12.0 |
x10-3 |
| |
Zinc |
36 |
x10-3 |
| |
Lead |
29 |
x10-3 |
| Plastics |
Acrylic Glass
(Plexiglass) |
80 |
x10-3 |
| |
Polyamide |
100 |
x10-3 |
| |
Polyethylene
(HDPE) |
160-200 |
x10-3 |
| |
Polystyrene |
80 |
x10-3 |
| |
Polyurethane,
Hard |
110-130 |
x10-3 |
| |
Rigid-PVC (Hostalit
Z) |
80 |
x10-3 |
| |
Plasticised PVC
|
20 |
x10-3 |
| |
Rubber, Hard
|
77 |
x10-3 |
| |
Polypropylene
|
16-18 |
x10-3 |
| |
Carbamide Resin
|
45 |
x10-3 |
| |
Melamine Resin
|
40-60 |
x10-3 |
| |
Phenolic Resin
|
30-50 |
x10-3 |
| |
Polyester, Glass
GRP |
17 |
x10-3 |
|
Example: Bonding Polyester GRP panels
to an Aluminium frame on an Ambulance.
Panel
Size: 2.4m x 0.8m
Expected Maximum Service Temperature: 100°C
Minimum temperature = 0°C
Aluminium, =
23.5x10-3 mm/m°C
Polyester GRP =
17x10-3 mm/m°C
 L=
2.4m x (23.5x10-3 - 17x10-3)mm/m°C
x 100°C = 1.6mm
Hence a bond line of greater than 1.6mm
is required for bonding a GRP panel to an Aluminium frame.
Always consult Sika's Technical Department before starting
a bonding application.
Click below to view information on
- Bond Line Dimensioning
- Corrosion Prevention.
- Handy Trade Tips
- Joint Design for Direct Glazing
Applications.
|
top