Bonding of aluminum
1
Sapa Technology (ST) is a research and development center
within Sapa.
Sapa is an international industrial group. It develops, manufactures
and markets aluminum and plastic based products that have
a high degree of added value. Aluminum and plastic are both
lightweight materials.
ST has been assigned the role of supporting and stimulating the
technical development of the products and processes of Sapa
companies.
We are a high level technical resource for the individual companies
and, consequently, their customers.
ST’s specialists supply expertise on how aluminum’s properties
can be influenced by controlling manufacturing conditions and the
mix of alloying elements.
Our laboratories have advanced measuring equipment and
equipment for the technical investigation of materials. We often
work closely with universities, university colleges and research
institutes.
An important part of our activities is the dissemination of knowledge
about aluminum’s properties and the use of aluminum.
Cover photo: The ¾ tonne “Profilen” (“Profile”), the world’s first sailing boat to be made using bonded
2
aluminum profiles. Developed and built by Sapa Profiler, Vetlanda.
Introduction
Contents
Many designers know that adhesives are used to bond
load-bearing structures in aircraft. Double-sided sticky
tape is also used to join certain metal components in
aircraft. Between these two extremes, there are very
many different adhesives.
Introduction
3
The bonding of structures – an overview
4
Preconditions for bonding
5
Bonding as a jointing method offers wide possibilities
in a very broad area. However, the seemingly simple
jointing offered by bonding can be illusory if the joint is
not designed with a knowledge of the basic factors that
affect the joint’s strength and service life.
The effect of the service environment
6
The effect of temperature
7
The effect of joint design
7
We hope that this guide helps to increase knowledge of
the possibilities and limitations inherent in the bonding
of aluminum. The text is based on studies of relevant
literature and on the testing we carried out when
producing the first edition, published in July 1984.
The effect of mechanical loads
8
The effect of adhesive type
9
The effect of alloying elements and condition
9
This, the third edition, has been updated as regards,
amongst other things, contents and adhesive
designations.
The effect of pretreatment
9
How the bond is made
11
Bonding to materials other than aluminum
11
Choice of adhesive
12
Epoxy adhesives
13
Polyurethane adhesives
14
Hot-melt adhesives
16
Polysulphide rubbers
17
Anaerobic adhesives
18
Test methods
18
References
19
Tests of a few selected adhesives
20
Examples of bonded aluminum products
21
3
The bonding of structures – an overview
Perhaps the main difference between bonding, on the one
hand, and soldering, brazing and welding, on the other, is
that the filler material in bonding is a plastic rather than a
metal.
For bonding to take place, the adhesive and the material
to be bonded must come closer to each other than 0.5 nm
(1 nm = one millionth of a millimetre). This is only possible
if the adhesive not only wets the appropriate surface(s), but
also spreads and “penetrates” so that that the gaps and dips
in the surface(s) are filled out.
Wetting can only occur if the adhesive has a lower
surface tension than the surface that has to be wetted.
So that there is sufficient contact between adhesive and
material, the adhesive must have good liquid properties.
However, for the transmission of loads to be possible, it
must set into a load-transmitting material.
Broadly speaking, setting can be divided into three
types – drying, cooling and polymerisation. Different types
of adhesive set in different ways:
Adhesive type:
Setting via:
Solutions of plastics/elastomers
Drying (evaporation)
Adhesives that, at room temperature or above, can
appear elastic or soft can, at lower temperatures, become
hard and, perhaps, brittle.
Below, an attempt is made to roughly rank adhesives
as regards the durability of the finished bonds at various
temperatures.
Bond durability at various temperatures:
A rough ranking of adhesive types, from most to least
temperature sensitive:
Dispersions of plastics/elastomers Drying (evaporation)
Hot-melt adhesives
Cooling
Prepolymers
Polymerisation (curing)
Where bonds between metals and reinforced plastics are
subject to high loads, only the last of the above types of
adhesive (i.e. curing) is suitable.
Curing (polymerisation) can be initiated by:
•
Mixing of, or contact between, two components
•
Heating (heat-curing)
•
Illumination with UV or blue light
•
Environmental changes such as:
- presence of moisture
- altered pH value
- absence of oxygen + metal ion contact
When bonded joints are subject to loads, there is always
so much energy in the bonds between the adhesive and the
bonded matter that the bonds are stronger than the weaker
(weakest) of the materials involved. Thus, rather than the
bond itself coming apart, any failure would be in the form of a
break in the adhesive or the bonded material. The foregoing
is conditional on the adhesive and the surface(s) having
come into truly good contact with each other. This is not
always the case when fast-setting adhesives are used.
Furthermore, not all types of bonds are so rich in energy that they can withstand the effect of another medium,
e.g. water. Adhesion can then reduce and even become
negative. This very much depends on the combination of
adhesive, material and surface treatment.
All plastics and, therefore, all adhesives are viscoelastic.
Thus, from the point of view of loads, they are more open
to the influences of temperature and time than are, for
example, metals. Consequently, for an adhesive, it is not
possible to set fixed values for a large number of strength
parameters (e.g. modulus of elasticity, yield point and creep
strength). These values differ even with relatively small
temperature differences. They also depend on the rate of
deformation.
4
•
Thermoplastic hot-melt adhesives
•
Double-sided sticky tapes
•
Thermoplastic adhesives, drying
•
Elastomers (rubber, contact adhesives)
•
Curing elastomers
•
Curing hot-melt adhesives
•
“Environment-curing” adhesives
•
Two-component adhesives that cure at room temperature
•
The same 2-component adhesives, but heat-curing
•
One and two-component adhesives that require heat curing
•
Curing adhesive films, heat-curing, 125 – 175°C
The spread within the various groups is wide. It is highly
recommended that meticulous attention should be paid to
each adhesive’s data sheets. However, these do not always
provide the answers and own, supplementary testing may
be necessary.
The adhesives that cure without any heat input are
seldom of practical use at temperatures above 100°C.
Silicon adhesives, which can be used up to around 250°C,
are an exception.
There are extremely few adhesives that are of practical
use at temperatures above 300°C. Unfortunately, most
heat-resistant adhesives are often relatively hard (silicon
once again being an exception). As a result, their ability
to spread the stresses presented by peeling and cleaving
forces is limited.
Bonded joints should be designed so that, as far as
possible, they are exposed to pure shearing forces. This
is partly because adhesives are viscoelastic (yield/creep
characteristics of soft adhesives) and partly because
of the notch sensitivity of bonds in hard, heat-resistant
adhesives.
The durability of a bonded joint very much depends on
how well the adhesive fills out the pores and unevennesses
in the surfaces of whatever is to be bonded. This is
particularly important when bonding metals that are to
be used in corrosive environments. Poorly filled surfaces
provide space for water, which can cause boundary layer
corrosion.
Moisture getting into incompletely filled surfaces can,
when a bond is subjected to minus temperatures, also lead
to frost erosion in the boundary layer. Thus, when using
high-viscosity adhesives and curing adhesive films, it may
be necessary to first saturate the surface with a low-viscosity
primer. The high-viscosity adhesive can then be applied.
This can give the bond higher green strength and improved
durability.
In this connection, it should be pointed out that the
results of metal bonding are highly dependent on which
metal oxides are on the metal surfaces at bonding and on
how securely these oxides sit on the surface. See “The
effect of alloying elements and condition”.
Preconditions for bonding
The term “adhesion” is frequently used in a bonding context.
“Adhesive forces” are the attractive forces arising in the
interface between two surfaces that are in contact with
each other.
The attractive forces between an adhesive’s molecules
and the surface(s) that are to be bonded have a maximum
range of 0.5 mm. To achieve such closeness, the adhesive
must have a lower surface tension than the material that
is to be bonded. This is so that spontaneous wetting can
occur (fig. 1).
If the surface tension relationship is inverted, the
adhesive tries to pull itself together into a droplet (fig. 2).
The strength of the adhesive
(“the plastic”)
Figure 1: Spontaneous wetting.
Adhesive forces between
adhesive and aluminum
oxide (degree of wetting)
Strength of the
aluminum alloy
The adhesion between oxides and
the aluminum substrate
The strength of the oxide-hydroxide layer
Figure 2: No wetting = No adhesion
Figure 3: Possible weakest links in a bonded joint
It can also be said that the surface must have sufficient
free surface energy to prevent the adhesive pulling itself
together into a droplet.
When bonding aluminum to another metal, the surface
tension relationship is not a problem. This is because all
adhesives (plastics) have a lower surface tension than all
metals. However, when bonding aluminum to a plastic, this
phenomenon should be borne in mind.
Nonetheless, in practice, knowing about the surface
tensions of the parent materials is not enough. This is
because the properties of a surface are often different from
those of the parent material. Production methods, surface
treatments, handling and storage are just some of the
factors that can affect the chemical and structural properties
of the surface layer.
What we regard as aluminum is, in reality, often aluminum
coated with an oxide. The surface of untreated aluminum is
often made up of magnesium oxides. In its turn, this surface
may be coated with grease, dirt, adsorbed molecules from
gases and liquids or the products of chemical reactions
between the material and its surroundings. The strength of
the bonded joint depends on the weakest link in the chain.
This is illustrated schematically in fig. 3.
“Relative strengths” immediately after bonding are not
stable. The adhesive can be affected by, for example,
water absorption, UV irradiation, heat-attributable softening
or creep under load. Similarly, the oxide layer on a metal
surface can reform in such a way that strength is reduced
and/or the volume is changed. This can set up internal
stresses in the bonded joint.
Achieving a bonded joint that has good long-term
strength requires a good knowledge of how the factors set
out below affect the joint’s quality.
Factors that affect the quality of bonded joints
•
Environment
•
Temperature
•
Mechanical loads
•
Pretreatment
•
Alloying elements and condition
•
Adhesive type
•
Joint design
•
How the bond is made
5
A surface that may appear smooth and even (perhaps even
polished) is very uneven when viewed at high magnification
(fig. 4).
A viscid adhesive needs a very long time to take over
the space occupied by the air in such a surface. Complete
filling never take place.
The thicker and more fast-setting an adhesive, the
greater the need to first apply a runny primer that saturates
the surface before the adhesive is applied.
Figure 4 a: An aluminum plate at 500x magnification
Figure 4 b: The same plate at 25,000x magnification
The effect of the service environment
Figure 5: Variously pretreated, unloaded joints kept outdoors in
an industrial environment. Alloy 6061-T6. Ref. 3.
Figure 6: Unloaded joints kept outdoors in a coastal environment.
Alloy 6061-T6. Ref. 4.
21,0
21,0
17,5
17,5
14,0
14,0
10,5
10,5
7,0
7,0
3,5
3,5
Strength in N/mm2
mon and most difficult environmental “stresses” to which
an adhesive bond involving a metal can be exposed. The
effect on the boundary layer is even more negative if the
water contains salts.
Normally, water gets into the adhesive-aluminum
boundary layer via penetration into the incompletely filled
unevennesses in the aluminum surface. This once again
highlights the value of pretreating with a runny primer.
For aluminum, the long-term strength of a bonded joint
exposed to moisture is directly dependent on: how well the
adhesive fills out surface unevennesses; and, the strength
Strength in N/mm2
Adhesives can, of course, be affected by the service environment. As there are clear indications of the durability of
various adhesives in various environments, choosing an
appropriate adhesive is no great problem in this particular
respect.
When bonding aluminum, low-strength bonded joints
are often a boundary layer problem, i.e. the consequence
of phenomena that have undesired effects on the boundary
layer between adhesive and aluminum oxide. Water, in
either its liquid or vapour phase, is one of the most com-
12
24
Exposure in months
36
48
60
72
84
96
12
24
36
48
60
72
84
Exposure in months
Key to figs. 5 and 6:
Degreased in trichloroethylene vapour
Blasting with silicon sand + vapour degreasing
Chromating
Anodizing in sulphuric acid followed by sealing
Pickled with chromic/sulphuric acid paste
Pickling in phosphoric acid/ethanol
Pickling in chromic/sulphuric acid
6
96
and durability of the aluminum oxides that are being bonded to. Through experience, the aircraft industry has long
been aware of this. However, it is only in recent decades
that scientific experiments have been carried out to provide
explanations of the observed phenomena (ref. 8).
Bonding to a normally formed aluminum oxide does not
give the best long-term strength. If there is water or high air
humidity in the service environment, a surface conversion
must be carried out to increase the durability of the bonded
joint. See also “The effect of pretreatment”.
The graphs in figs. 5 and 6 give an idea of the effect
that environment and pretreatment have on the strength
of a bonded joint. Testing was carried out on single lap
joints (as per ASTM D 1002-72) with a 2-component, roomtemperature (RT) curing epoxy adhesive.
The effect of temperature
The properties of an adhesive are strongly temperature
dependent. Adhesives soften as temperature rises and
harden as it falls. The most critical changes are: reduced
creep resistance at high temperatures when subject to
loads; and, increased sensitivity to stress concentrations
and shock loads at low temperatures.
Different adhesives, even when within the same group,
are affected to different degrees by temperature. The graph
on the right shows the temperature sensitivity of various
epoxy adhesives.
It should be borne in mind that these values result from
short-term testing, normally as per ASTM D 1002-99, where
a single lap joint in 1.55 mm aluminum is subjected to a load
at a constant tensile speed of 1.33 mm per minute. (As a
rule, equivalent test methods are also used for the strength
values given in the data sheets for adhesives.)
If it is known that bonded joints are to be exposed to
long-term loads in a single direction at elevated temperatures, steps should be taken to ensure that the adhesive’s
creep strength is sufficient. This often demands own/inhouse testing.
N/mm 2
45
40
1-comp., viscous, heat cured
35
2-comp., viscous, RT cured
30
2-comp., hard,
heat cured
25
20
1-comp., hard,
heat cured
15
2-comp., hard,
RT cured
10
5
2-comp., fast, RT
cured
-50
0
50
100
150
200
250
°C
Figure 7: Strength of several different groups of epoxy adhesives
at various temperatures. The blue curves are for the same
adhesive.
The effect of joint design
A bonded joint does not necessarily have to be seen as a
film between the joint’s surfaces. Instead, it can be regarded as a large number of tightly packed elements that join
the surfaces.
P
P
MV
P
P
Imagining these elements as glass when the adhesive is
hard, and as rubber when it is soft, gives an idea of the
differences in load distribution.
The glass elements (hard adhesive) do not stretch much
under load. This means that the outermost row of bars takes
up most of the load. The bars continue to do this until they
fracture or come away from the substrate. It is only then
that the underlying bars take up the load – until they too
break, etc.
Using the rubber-element model (elastic adhesive), the
result is that the outer row stretches under a moderate load
without transmitting much of this to the anchorages (the
joint surfaces). Rows two, three, etc. also transmit a certain
part of the load. This is part of the philosophy behind the
advantages of using a more elastic adhesive. However, the
increased elasticity is often paid for in poorer heat resistance
and reduced resistance to creep.
It is also important to bear in mind that an adhesive’s
hardness is temperature dependent. An adhesive that can
behave like rubber bars at room temperature can become
like glass bars in cooler conditions.
Figure 8: Deformation of adhesive and aluminum plate when a
single lap joint is loaded
7
sive, torsional or tensile, such joints experience them as
shear stresses. However, any differences in coefficients of
thermal expansion should be taken into account here.
A joint’s edges are always exposed to the greatest
stress. Thus, especially when using a hard adhesive, it is important that the design spreads any load evenly throughout
the bonded joint. Fig. 10 shows various ways of reducing
edge effects.
2
1
3
4
5
Figure 10: Lap joints modified to reduce stress at a joint’s edges
Figure 9: Stress distribution in a single lap joint – viewed
using tension spectrometry
The joint designs labelled 1 – 4 make the material more
yielding at its edges. The idea is that the material should
deform before it transmits large loads to the bonded joint.
Joint number 5 offers a further possible solution, i.e. making
the bonded joint considerably thicker at its edges. This
gives a bonded joint with a larger “deformation zone”. In
bonding, joint modification is of greatest benefit with rigid
adhesives.
Especially when using hard adhesives, the design of
a bonded joint is of great significance. The basic rule is
that a bonded joint must be designed so that the loads it
is exposed to are transmitted as shear forces. Cylindrical
bonded joints (pins in holes, pipes in pipes, etc.) come the
closest possible to the ideal. Whether loads are compres-
It is advisable to try and form a picture of the force flows
throughout a bonded joint and, as rule of thumb, not to
use a harder adhesive than necessary.
The effect of mechanical loads
8
36
Unloaded
30
Residual strength in N/mm2
Bonded joints are normally regarded as rather insensitive
to vibration and fatigue at high frequencies. They are often
used as vibration dampers and crack barriers.
Nonetheless, mechanical loads can exacerbate boundary layer problems. The simultaneous effects (synergy)
of temperature, environment and mechanical load result in
significantly faster reduction in strength than would occur
if these three stresses operated individually and had their
outcomes added together.
The stress concentrations that can arise when a structure is loaded manifest themselves at the edges of bonded
joints (especially if the joint has not been designed to minimise such concentrations), where environmental impact
is also greatest. This can result in more rapid aging of the
bonded joint than would otherwise have been the case.
Fig. 11 compares the reduction in the residual strength
of loaded and unloaded joints in 100% humidity at 60°C.
Pretreatment was chromic/sulphuric acid pickling. The
choice of pretreatment has a large effect on results. See
also “The effect of pretreatment”.
24
18
6,2 N/mm 2
8,3 N/mm 2
12
6
10 20 30 40 50
Exposure in days
Figure 11: Differences in residual strength of differently loaded
joints in moist heat (ref. 1)
The effect of adhesive type
To correctly understand the effect that different adhesives
have on a bonded joint, the bonded joint should be viewed
as an independent structural element in a composite structure. Naturally enough, this structural element can have
various mechanical properties and be affected differently
by temperature, environment and chemicals.
Through the electrolytes that can form in the presence
of water, different adhesives can also differently affect the
aluminum surface in the boundary layer.
“Choice of adhesive” and “Tests of a few selected adhesives” (table 5, page 18) present a selection of adhesives
that are used for bonding aluminum.
The effect of alloying elements and condition
In certain conditions, alloys that contain magnesium or copper are more difficult to bond (to). High concentrations of
these elements in the adhesive/aluminum oxide boundary
layer form oxide types that are of lower strength and are
more sensitive than aluminum oxide. However, the boundary layer’s composition is highly dependent on pretreatment
and can be widely affected in a number of ways.
Heat treatment of magnesium-containing alloys increases both the oxide layer’s thickness and its magnesium
content. By contrast, anodizing in phosphoric acid (for example) gives a thin and magnesium-poor boundary layer
with very good durability.
It has also been shown that, as with magnesium, the
presence of copper in the boundary layer can impair the
durability of the bonded joint. In the chromic/sulphuric acid
pickling of copper-containing alloys, the copper can, if pickling
is taken too far, be enriched in the oxide layer (ref. 2).
Table 1 gives an idea of how various surface treatment
methods effect magnesium content in the oxide and the
durability of the bonded joint. See also “The effect of pretreatment”.
Surface
treatment
Al:Mg ratio
Joint’s long-term
strength
Degreasing
6:1
Very poor
Sand blasting
15:1
Moderate
Chromic acid pickling
57:1
Good
Phosphoric acid anodized 110:1
Very Good
* The alloy’s Al:Mg ratio was 53:1.
Table 1: Magnesium content in the surface in relationship to longterm strength (ref. 1)
The effect of pretreatment
Pretreatment can have several purposes:
• To achieve a clean surface that can be bonded to.
• To create a surface that has better durability in corrosive
environments.
• To provide a material with a strong oxide that can take
high loads.
• To give a surface a decorative appearance.
(These four purposes can sometimes conflict with each other.)
As previously intimated, aluminum in its as-delivered
condition provides a comparatively poor surface to bond to.
The composition of the oxide layer on the surface varies and
the oxide layer has relatively poor adhesion to the parent
metal.
If loads are moderate and the joint is not exposed to
moisture, a pretreatment that gives a clean surface free
from dirt may be sufficient. Examples of such pretreatments
are washing with water or, previously, trichloroethylene
vapour.
Bonded joints for harsh service environments require
other pretreatments. Some methods that are used with
various degrees of success are given below.
Strong, alkaline washing solutions give a surface
that is undoubtedly clean, but the oxide layer of which has
been converted to brittle aluminum hydroxide.
Mechanical pretreatments such as blasting or lapping
with a compliant abrasive give a strong surface. However
it has no corrosion protection.
Dipping for around 10 minutes in water at 95 – 100°C
after pickling in, for example, caustic soda gives improved
corrosion resistance in the adhesive/aluminum boundary
layer.
Yellow or green chromating is a common treatment
before painting. It gives good corrosion protection, but
load bearing properties are relatively poor. When bonding,
a chromate layer should be thin. This method is thus best
for moderate loads and elastic adhesives.
Pickling in chromic/sulphuric acid gives a very good
surface for bonding. This method has long been used by
aircraft manufacturers in the USA.
Anodizing has proven to be the best pretreatment
if durability of loaded joints in corrosive environments is
required. Thick anodic oxide layers give excellent corrosion
resistance, but are brittle and thus have poor load-bearing
capabilities. Anodizing in sulphuric acid is common and
gives the thickest oxide layers (see table 2).
Anodizing in chromic acid without subsequent
sealing gives a thin oxide layer that has good load-bearing
capabilities. This method has long been used by the
European aircraft industry. From the strength point of view,
the method is excellent. However, the process requires
precise control.
9
Thickness, approx.
Chromic/sulphuric acid pickling
40 nm
Phosphoric acid anodizing
400 nm
Chromic acid anodizing
1,500 nm
Sulphuric acid anodizing
15,000 nm
Table 2: Layer thickness with various surface treatments for
aluminum
≈ 10 nm
Fig. 13: Service lives of joints with various surface treatments
and loads (alloy 6061-T6, 1-component nitrile modified epoxy,
cured 15 minutes at 204°C)
16,8
14,0
11,2
8,4
5,6
2,8
Applied tensile stress in N/mm2
Surface treatment
(7 dagar)
Days)
(7
100 nm
40 nm
7 nm
Oxide
Figure 12 b: Isometric sketch of the oxide layer of a phosphoric
acid anodized surface
This surface is easily destroyed by mechanical contact
and must be directly coated with a primer or a runny
adhesive.
Fig. 13 gives an idea of long-term strength when loaded
in moist heat.
(69 Days)
4
3
(694 Days)
10
10
10
10 6
10 7
Time (in minutes) until fracture
Key:
Degreased in trichloroethylene vapour
Anodized in sulphuric
acid, sealed
Anodized in sulphuric acid, not sealed Pickled in chromic/
sulphuric acid
Anodized in chromic acid
Anodized in phosphoric acid
(ref. 7).
10
100 nm
2
5
Table 3 gives some useful guidelines for selecting a pretreatment (in relation to what is required of the bonded joint).
The properties of a pretreated surface are rapidly impaired if
it is exposed to moist air or greasy, airborne contaminants.
Similarly, it is sensitive to, for example, fingerprints during
handling. It is often appropriate to apply an adhesive primer
directly after pretreatment. The surface can then be stored
a long time without impairment.
The use of primer often brings increased green strength
and better corrosion protection/long-term strength. Adhesive
primers frequently have corrosion-inhibiting additives that
further improve the bond’s chances of providing long-term
strength in difficult environments (ref 5).
“Wash-primers” are general primers that give good
results. They contain polyvinyl butyral with a phosphoric
acid hardener (curing agent). This type of primer can, for
example, be advantageously applied before bonding with
a wide range of polyurethane adhesives. There are special
primers for many special adhesives, e.g. those used in the
aircraft industry (ref. 8).
Table 3: Useful guidelines for selecting a pretreatment (in
relation to what is required of the bonded joint)
Surface quality
Unloaded or mildly Loaded Joint
loaded joint Environment
Environment
Dry Water Brine Dry Water Brine
Untreated
(as delivered)
(+)
Vapour degreased +
(+)
+
Mechanically processed +++
+
+++ +
Dipped in hot water
+++
++
++
++
Primed with polyvinyl
butyral
++
++
+
++
(+)+
+
Chromic acid pickled +++
+++
+
+++ +++
+
Chromated
++
+
++
+
+
Sulphuric acid anodized
no subsequent sealing ++(+) ++(+) ++(?) ++
++
++
(+)
Sulphuric acid anodized
++(+) ++(+) ++(+) +
subsequent sealing
+
+
Chromic acid anodized +++
+++
+++
+++ +++
(+)++
Phosphoric acid anodized
+++
+++
+++ +++
+++
+++
The number of “+”s indicates suitability.
(+) is a doubtful +
(?) is an unverified assumption
10
(+)
How the bond is made
The strength of an adhesive bond is very much determined
by the way in which it is made. At a first glance, it may seem
that this is largely a question of applying adhesive to the
parts and holding them together until the bond sets.
“Can there really be many other ways of doing it?”
If the process of making a bond is divided into subprocesses, it can be seen that there are rather a lot of points
where each subprocess can have an effect on the end
results.
Such a division could be as set out below. All these
The parts that are to be bon- Intermediate storage
ded
The adhesive:
• Conditioning
• Receiving inspection – • Dimension checking
storage of adhesives
The surfaces that are to be • Stirring of the components
• De-aeration of the bonded
components
• Cleaning
• Dosing of adhesive and • Drying
hardener
• Surface conversion
• Mixing
• Priming
• Drying/curing of the primer • De-aeration of the mixture
subprocesses could be involved in a single bonded joint!
To ensure consistently good bonding results, it is clearly
important that descriptions are drawn up detailing how a
bonded joint is to be made. Each subprocess can be further
divided into smaller steps.
Carrying out the subprocesses does, of course, require
knowledge. Each subprocess also presents opportunities for
going wrong. Carrying out series bonding without detailed
work descriptions can hardly be appropriate.
Making the bond:
• Controlling joint thickness
• Application
• Waiting time before putting the components together
• Time during which the adhesive mixture can be used
• Assembly
• Fixing – pressing
• Curing
• Cooling after heat curing
• Removal from jig/press
Subsequent
storage/curing
Testing – Checks –
Logging
Further processing
Packing
Delivery
Bonding to aluminum of materials other than aluminum
When bonding metals, the long-term strength of the bond
is often regarded as a metal problem. It could thus be
assumed that the bonding of other materials would have
fewer problems associated with it. Tests have shown that
this is indeed the case as regards long-term strength in
moist environments.
Difficulties in bonding plastics are most often associated
with finding an adhesive with a surface tension low enough
for it to wet the plastic that is to be bonded. Many plastics
can be treated to increase their surface tension and thus
improve bonding possibilities.
Below, there are some comments on the bonding of
various plastics.
• As a rule, PVC, polycarbonates or thermosetting resins
present no bonding problems.
• Polyamides (nylon), acrylic (Plexiglas) and ABS plastics
limit the number of adhesives that can be used.
• Because they have very low surface tension, olefinic
plastics (polyethene and polypropene) and acetal plastics
(e.g. Delrin) are very difficult to bond. Surface tension can
be increased by plasma, flame or arc (e.g. corona) treatments.
Using a cyanoacrylate adhesive in combination with a primer
for plastics that have a low surface tension can result in
successful bonds. There is now also a 2-component acrylic
adhesive that provides good adhesion to polyolefines.
• Polytetrafluorethylene plastic (e.g. Teflon) has the lowest
surface tension. To “alloy” (“graft”) the Teflon surface with
sodium, thereby increasing surface tension and improving
bondability, the plastic can be treated in a liquid that contains
metallic sodium.
• From the point of view of bonding, painted aluminum
surfaces are to be regarded as plastic surfaces.
• Glass, ceramics and wood are regarded as easily
bonded. The same applies to many other metals than aluminum.
The presentation of various adhesive types in “Choice of
adhesive” takes up, amongst other things, each adhesive’s
suitability for various materials.
Solids
Surface tension
(10-1Pa)
Liquids
5 000
Diamonds
Glass
Platinum
Steel
1 000
Aluminum
Tin
Lead
Sodium
100
90
80
Ice
Water
70
Glycerol
Wood
60
Polyester
(unsaturated)
50
Nylon
Epoxy
PVC
Polystyrene
Polyester (linear)
Acetal (Delrin)
Polypropylene
Polyethene
40
Epoxy adhesives
Polyurethane adhesives
Cyanoacrylate adhesives
30
Aromatic hydrocarbons
Petrol
20
Ethyl alcohols
Teflon
10
Ethyl ethers
Figure 14: Surface tension of several materials – for wetting to occur, the
adhesive must be below (in the chart) the material that is to be bonded (ref. 7)
11
Choice of adhesive
The choice of adhesive is principally determined by three
different considerations:
• The adhesive must wet the materials that are to be bonded.
• In its final state, the adhesive must have material properties that, in the intended service environment, are
sufficient to transmit the loads in question.
• It must be possible to use the adhesive in production of
the product.
There is no great difficulty in finding some 1,000 different
adhesives on the market. The difficulty is largely in obtaining
sufficient knowledge about the adhesive (especially in its
final, set state).
One way of gaining an overview of the area may be
to consider the ways in which adhesives set. After all, adhesives do have to be liquid at some point in the bonding
process. Essentially, there are only three ways of setting.
However, combinations of these do occur.
Setting through drying
The solvent or water evaporates. Only 20 to 50% of the
original adhesive remains (= shrinkage). Most of the drying
must take place through the material. Consequently, this
adhesive type is not suitable for bonding aluminum to aluminum. The binder is based on thermoplasts or elastomers
(rubber).
One way of using a drying adhesive is to employ it as a
contact adhesive. Most of the solvent must here be allowed
to disappear before the parts are brought together. However,
significant quantities are left in the joint after such compression and, in principle, the problem remains.
Drying adhesives are not excluded from use in the
bonding of thin, compliant and porous materials (mats, etc.)
to aluminum. However, they are not discussed any further
here.
Setting through cooling
Some of the drying adhesives can be heat activated. The
adhesive is applied to one or both joint surfaces and dried
completely. At jointing, the adhesive is activated (melted)
on one of the parts and quickly joined with the other. This
adhesive type facilitates rapid production, but does not
give filled joints. It is only suitable if one of the materials is
readily deformable (thin, soft). This bonding method is also
discussed no further in this text (however, see example 7
on page 24).
Hot-melt adhesives are applied hot, usually to roomtemperature surfaces. They give filled joints and rapid fixing.
This type of adhesive is discussed more fully on page 14.
Setting through polymerisation (curing)
Curing can take place through:
• Mixing of two components (adhesives 1, 2, 4, 5 and 8
in tables 4 and 5 – table 5 is on page 18).
Table 4: Overview of various curing adhesives
Adhesive
type (tested
adhesive
no.)
Strength of
single lap
joint
(N/m2)
Peel
strength
(N)m*
Handling/
curing
Suitable
for large
surfaces
Suitable
for thermoplasts
Suitable for
thermosetting resins
Epoxy
16 - 29
4 - 6
2-comp. (1, 2)
Dosing
+ + +
– –
–
+ +
Mixing
20 - 150°C
Epoxy
1-comp. (3)
approx. 35
approx. 9
Cures
in heat
>90°C
Polyurethane 6 - 20
2 - 6
Dosing
2-comp. (4)
(T-peel test) Mixing
20°C Polyurethane approx. 2
approx. 7
elastomer,
1-comp (5)
Polysulphide approx. 2
approx. 5
rubber (7)
+ +
– –
––
+
+ + +
– –
+ +
+++
Cured by
+
–
+ +
+ + +
atmospheric moisture
Hot melt
approx. 7
approx. 10
Cooling - curing (6)
cured
+ +
If
reactivated
Dosing
+ + +
Mixing
– –
+++
(–)
+
+ + +
”Benign” smell
Misc.
Many
different
adhesives
Other curing methods
exist
+++
Slow
curing
Acrylic 20 - 25
2,5 - 3,5
Adhesive + +
adhesive (T-peel test) & hardener
(SGA) (8)
on own surface
––
+ +
+ + +
Strong smell
Short fixing
time
Aerobic approx. 20
2,5 - 3,5
Adhesive
+ +
SGA (T-peel test) & hardener
adhesive (9)
on own surface
––
+
+ +
Strong smell
Short fixing
time
Anaerobic 12.5 - 20
adhesive (10)
–­–
0,5 - 1,5
Cures by (T-peel test) itself
+
* Roller drum peel tests give higher values than T-peel tests.
Strength values are taken from the manufacturers’ data sheets.
12
Work
environment
aspect
–
• Contact between two components, each of which has
been spread on its particular surface (adhesives 9 and 10
in tables 4 and 5).
• Temperature increase to at least 100°C.
• Environmental change for the adhesive, e.g.
- moisture contact (adhesive 6 in tables 4 and 5).
- absence of oxygen + metal ion contact (adhesive 10 in
tables 4 and 5).
- UV irradiation.
From the large number of available curing adhesives, we
selected 10 for testing. These are detailed hereafter. Three
of them were epoxy adhesives. The remaining seven were
each typical of their group. All the adhesives were fairly
common and readily available.
Table 4 gives an overview of the various adhesives.
Table 5 on page 18 gives the results of the aging tests that
we carried out.
Clearly enough, adhesives other than those discussed
here may be available and may be the best choice for a
certain design. In addition, the tested adhesives have also
been further developed and, in some cases, replaced by
others. Thus, the original test results are here given with
any replacement products stated.
regards the relationship between strength and test
method, see “Test methods”.
Various adhesive types
The presentation of various adhesive types takes up,
amongst other things, each adhesive’s suitability for various
materials.
When bonding dissimilar materials to each other, it
should be borne in mind that movement caused by moisture
or temperature will not be the same for each material.
Bondable materials: Aluminum and other metals and minerals. Thermosetting resins such as Bakelite and polyester
laminates can also be bonded with good results. Nylon gives
fair results. The adhesive is not otherwise especially suitable
for thermoplasts. This is because the latter normally have a
lower surface tension than does the adhesive.
EPOXY ADHESIVES
Araldit AV 106/HV 953 U, “Araldit Standard” (from CibaGeigy), is probably the most widely used metal adhesive
there is. It is a comparatively old adhesive. We also
included a newer all-round epoxy adhesive (AV 144/HV
997 from Ciba-Geigy) in the tests. Both these adhesives
have relatively low heat resistance.
Two-component epoxy adhesives that cure at room
temperature can rarely be loaded at temperatures above
80°C. Epoxy adhesives with considerably higher heat
resistance are available. However, these require heat
curing. Unfortunately, the price paid for higher heat
resistance is usually lower peel strength.
As an alternative to 2-component epoxy, we chose a
heat-curing 1-component adhesive, EC 2214 from 3M.
This is an old, well known adhesive. The disadvantages
of heat curing must be weighed against the advantages
of not having to “dose” and mix adhesives. Furthermore,
as regards resistance to heat and environmental
stresses, the resultant bond is also stronger. Heat curing
gives a fully cured joint, something that can never be
achieved with a 2-component epoxy adhesive that has to
cure at room temperature.
One-component epoxy adhesives are available as
pastes and, in various thicknesses, films. The main
difference between film and paste adhesives is the ways
in which they can be applied. Using epoxy adhesives, it is
possible to fill joints that have large gaps.
Information about the various adhesives has largely
been taken from the manufacturers’ data sheets. As
Tested adhesive 1 EPOXY ADHESIVE ARALDIT AV
106, HARDENER HV 953 U
Components: 2
Mixing ratios: 100:80 (weight), 100:100 (volume). The
mixing ratios can vary within comparatively wide limits. Adding more hardener (curing agent) gives a softer adhesive.
However, it should be borne in mind that, besides serving
as a softener, the surplus hardener will remain in a liquid
“unreacted” form in the adhesive. The adhesive’s comparatively poor heat resistance then becomes even poorer. The
components should be mixed by weight.
Function: The reaction starts when the components are
mixed. Curing rate is not affected by the quantity of hardener.
Owing to the heat generated during the reaction, large mixes
cure more quickly.
Curing times: 25°C – 18 hours, 70°C – 50 minutes, 150°C
– 5 minutes
Suitable areas of use: In the engineering industry, the adhesive is regarded as an all-rounder. It gives filled joints and,
where strength requirements at elevated temperatures are
small or moderate, can be used for bonds to the above cited
materials. Previously, the adhesive was used for sandwich
structures. It has now, practically speaking, been completely
replaced by polyurethane adhesives.
Strength at room temperature
Single lap joint, tensile speed 10 mm/min:
After curing at 25°C for 18 hours – 16 N/mm2
After curing at 70°C for 50 minutes – 22 N/mm2
After curing at 150°C for 5 minutes – 29 N/mm2
Peel strength: Roller drum peel test: 4 – 6 N/mm2
Temperature range
-60° to +60°C. At 60°C, the adhesive has lost around 50%
of its strength in respect of short-term loads. Creep strength
at this temperature is low.
Durability/resistance: Good against petrol and mineral oil
(SAE HD 30); otherwise moderate to poor. (Refer also to
the data sheets.)
Work environment: Must be handled in such a way that
the adhesive does not come into contact with the skin.
Ventilation must be provided where handling is continuous.
In Sweden, the use of epoxy adhesives is regulated by AFS
1996:4, Härdplaster (ordinance 1996:4 of the Swedish Work
Environment Authority, Thermosetting resins).
Alternative suppliers
See under tested adhesive 2.
13
Tested adhesive 2
EPOXY ADHESIVE ARALDIT AV 144, HARDENER
HV 997
Components: 2
Mixing ratios
100:40 by weight. The mixing ratio should not be varied.
This adhesive is now available in packs that supply readymixed adhesive direct from tubes or cartridges.
Function
Curing starts when the components are mixed. Pot life of a
100 gram batch at 23°C is 50 – 70 minutes.
Curing times
10°C – 24 hours, 20°C – 8 hours, 60°C – 45 minutes,
120°C – 5 minutes
Remark: Curing times are slightly shorter than for AV 106.
Maintaining the same curing time, the curing temperature
can be kept lower.
Bondable materials
The same as for AV 106
Suitable areas of use
The adhesive should be regarded as a development of AV
106. All its properties are better than those of AV 106. The
prime area of use is all-round bonding with metal as one or
both of the materials. The adhesive is grey.
Strength at room temperature
Single lap joint, AlMgSi alloy, ground surface:
After curing at 20°C for 24 hours – 23 to 24 N/mm2
After curing at 60°C for 30 minutes – 21 to 24 N/mm2
After curing at 120°C for 5 minutes – 26 to 28 N/mm2
Peel strength
Roller drum peel test: 6.0 – 6.5 N/mm2
Temperature range
-60° to +80°C, with approx. 50% of short-term strength
maintained at 80°C
Durability/resistance
Considerably better than AV 106 throughout (refer also to
the data sheets).
Work environment
The same as for AV 106
Leverantörer av 2-komponent epoxilim:
Tested adhesive 3
EPOXY ADHESIVE EC 2214, 3M SWEDEN
Components: 1
Mixing ratios
The adhesive is always ready for use. This adhesive type
has a more restricted storage time than do 2-component
adhesives. At no more than 4°C, the storage time is 8
months.
14
Function
Curing accelerates with temperature increases (it takes
place slowly even at room temperature). The lowest curing
temperature is 95°C.
Curing times
95°C – 120 minutes, 120°C – 40 minutes (= best results),
204°C – 2 minutes
Bondable materials
The same as other epoxy adhesives, i.e. metals, minerals
and thermosetting resins. It should also be borne in mind
that the materials must tolerate the curing temperature.
When bonding materials with different thermal expansion
coefficients, internal stresses may arise or, on cooling,
warping may occur.
Suitable areas of use
Primarily for bonding metals and other materials that have
equal thermal expansion coefficients. Uses include the bonding of body plates in the automotive industry and bonding
cemented carbide tools in steel holders.
Strength
Single lap joint, Al/Al, optimal curing:
-40°C – 21 N/mm2
24°C – 35 N/mm2
82°C – 35 N/mm2
121°C – 14 N/mm2
177°C – 3.5 N/mm2
The values are short-term at a tensile speed of 10 mm/
min.
Peel strength
T-peel test with 0.9 mm steel sheet: approx. 9.5 N/mm2
at 24°C
Temperature range
-55° to +95°C
Durability/resistance
The same as that of the better of the 2-component epoxy
adhesives
Work environment
As there is no dosing and mixing of components with 1-component epoxies (the constituents of which also present a
lower vapour pressure), the health hazards are considered
to be less. However, it must be ensured that there is no
contact with the skin.
At curing, the air in the curing zone must be evacuated,
i.e. there should be underpressure in the curing oven.
Temperature rises increase the vapour pressure exerted by
the adhesive’s constituents. This also increases the health
risks. See also AV 106.
POLYURETHANE ADHESIVES
Initially largely for work environment reasons, polyurethane
adhesives were regarded as possible replacements for
epoxy adhesives. However, the risks of isocyanates (the
hardeners for polyurethane adhesives) have meant that
they cannot be seen as a safer option than epoxies.
When using epoxy, skin allergies are the greatest work
environment risk. For polyurethane adhesives, breathing
difficulties and symptoms of asthma dominate the risks. In
the solvent-free 2-component adhesives now on the market,
the hardener is, as a rule, based on methylene diisocyanate
(MDI). This has a considerably lower vapour pressure than
toluene diisocyanate (TDI) types, and is thus less hazardous
to work with. The occupational exposure limit for MDI and
TDI is set as low as 0.005 ppm.
Tested adhesive 4
POLYURETHANE ADHESIVE CASCOBOND 1852,
HARDENER 1853, CASCO NOBEL
In practice, this adhesive has been replaced by 1849
PUR 2K and hardener 1821. This combination gives
longer assembly times and shorter press times.
Components: 2
In Sweden, the use of polyurethane adhesives is regulated
by AFS 1996:4, Härdplaster (ordinance 1996:4 of the Swedish Work Environment Authority, Thermosetting resins).
Polyurethane adhesives’ hardeners (isocyanates) react
readily with water. Carbonic acid is formed in this reaction.
This means that 2-component polyurethane adhesives
always become fully cured if moisture is present. Surplus
hardener that reacts with moisture makes the end product
harder. However, this final curing takes a long time. When
bonding diffusion-resistant materials and large surfaces (1
dm2 and above), it is not certain that this final curing will take
place. Thus, even for polyurethane adhesives, it is import
to dose and mix the components carefully.
The hardener’s moisture sensitivity means that many polyurethane adhesives have a slight foaming tendency when
mixed by hand. As there is normally some moisture in the
air in/on metal surfaces, such foaming can even occur in
the boundary layer. In more recent formulations of 2-component polyurethane adhesives, this foaming tendency has
been eliminated.
The possibility of reacting with moisture has enabled the
formulation of 1-component polyurethane adhesives (e.g.
adhesive 6) that cure in moist environments. Here, the
moisture is the curing agent (hardener).
Curing temperature has less effect on the curing times of
polyurethane adhesives than it does on those of epoxy
adhesives. Curing temperatures as high as those for epoxy
adhesives cannot be used – 70°C is a practical upper limit.
There are polyurethane adhesives that have very short curing times even at room temperature. However, once mixed,
the pot life of such an adhesive is also very short.
Polyurethane adhesives have their widest use in the bonding of sandwich panels.
As a rule, polyurethane adhesives are considerably cheaper
than epoxy adhesives.
As regards use with plastics, polyurethane adhesives are,
broadly speaking, better than epoxy adhesives. This is because of the former’s lower surface tension. They are often
used in various combinations that include plastics.
Using polyurethane adhesives, it is possible to fill joints that
have large gaps.
Before bonding with a 2-component polyurethane adhesive,
the application of a wash-primer (e.g. polyvinyl butyral +
a phosphoric acid hardener) gives better filled surfaces
and, consequently, improved long-term strength in moist
environments.
Mixing ratios
100:20 by weight (1849/1821, 100:22). The mixing ratio
must not be varied.
Function
Curing starts when the components are mixed. Mixing is not
spontaneous and great care must be taken when dosing the
hardener into the adhesive. This adhesive has constituents
that have to take care of moisture before the hardener reacts
with any moisture. Normally, foaming as a result of carbonic
acid formation does not have to be feared.
During storage, the adhesive component can sediment in
its packaging. Thus, for fully satisfactory results, stirring is
very important. The pot life of a 500 gram batch at 20°C is
around 1 hour (approx. 25 minutes for 1849/1821).
Curing times
At 20°C, fixing times of approx. 12 – 16 hours are usually
sufficient (around 3 – 4 hours for 1849/1821). It should be
noted that, at room temperature, full curing usually takes
several weeks and requires the presence of moisture.
Bondable materials
Hard PVC, polycarbonates and acrylic plastics are amongst
the thermoplasts that can be successfully bonded using
polyurethane adhesives. In sandwich structures, bonding
is between different cellular plastic cores.
This adhesive adheres well to metals (aluminum included
therein). Applies also to 1849/1821.
Suitable areas of use
In various sandwich structures and where a filled joint is
desired, often where plastics are involved. Polyurethane
adhesives can also be used for bonding corner joints in
frameworks made from profiles. They are also often used
when aluminum is to be bonded to other materials, e.g.
plastics.
Strength
Single lap joint at 20°C, tensile speed 10 mm/min:
Approx. 14 N/mm2 when bonding to aluminum
Peel strength
T-peel test: Approx. 4.3 N/mm2
Temperature range
-60°C to approx + 80°C
Durability/resistance
Good against water, oil and certain solvents (refer also
to supplier’s data sheets).
15
Tested adhesive 5
POLYURETHANE ELASTOMER SIKAFLEX 221 SIKA
SVERIGE
Components: 1
Function
The adhesive cures on contact with moisture.
Curing times
At 20°C and 65 % RH, 3 mm/24 hours
Bondable materials
Aluminum, epoxies, polyester, acrylics, polyamides, hard
PVC, minerals, etc.
Suitable areas of use
This is a very soft and elastic product. Less soft variants
of the product are available. For joints with little surface
expansion and moderate requirements as regards load
transmission, products of this type can be a good choice,
especially where material movements are significant and/
or there are high requirements in respect of impact and
vibration resistance. Because they exclude moisture, large
diffusion-resistant surfaces cannot be bonded.
Strength Tensile strength is 1.4 N/mm2
Temperature range
-30° to +70°C
Durability/resistance
Good against water; temporary resistance to oils and
grease.
Work environment
In Sweden, use is regulated by AFS 1996:4, Härdplaster
(ordinance 1996:4 of the Swedish Work Environment Authority, Thermosetting resins).
Remark
Sika has products that give similar final properties and
can be heat cured (lowest at about 70°C). There are also
products that can be cured both with heat and moisture as
well as 2-component products that cure after mixing.
HOT-MELT ADHESIVES
For the most part, hot-melt adhesives (“hot melts”) are various mixes of thermoplasts. Characteristically, hot melts must
be made runny at application. Many hot melts can be kept
molten for several hours at temperatures of between 150
and 250°C. These adhesive are often ethyl-vinyl-acetate
mixes. Despite the rather high application temperatures,
the heat resistance of these hot melts is low. It is unrealistic
to count on such adhesives being able to transmit loads,
even for moderate periods, if the temperature rises above
50°C.
There are also hot melts that are based on polyamides or
thermoplastic polyester. These thermoplasts have higher
service temperatures. However, strength when subjected
to long-term loads is already very low at 70 – 80°C.
16
The application temperature for this adhesive type is high
(approx. 250°C). At this temperature, the adhesive breaks
down if it comes into contact with oxygen. Consequently,
equipment for these adhesives is often provided with a
shielding gas.
The great advantage of hot-melt adhesives is the speed
with which they form bonds. However, this speed has a
drawback. When the hot adhesive meets a surface at room
temperature, setting is often so rapid that the adhesive does
not wet the surface (cf. “dry joints”). This disadvantage increases: the greater the difference between the temperature
of the surface and that of the adhesive; and, the greater the
thermal conductivity of the material that is to be bonded. For
these reasons, metals are often heated before bonding.
Curing hot-melt adhesives are now available. These adhesives are based on polyurethanes that cure on contact
with moisture. The adhesive has a solid form even before
curing. The melt and, consequently, the application temperature is considerably lower than for the thermoplastic
hot-melt adhesives.
There are also curing hot melts that, after application and
cooling, have a tape-like character for around one hour.
Thus, if a relatively high (momentary) press force can be
applied, parts at room temperature can be assembled.
After setting, the adhesive has moderate strength. On
contact with moisture, curing is to a polyurethane with comparatively good strength properties. For full curing to occur,
the diffusion paths for moisture must not be too long.
Tested adhesive 6
CURING HOT-MELT ADHESIVE SUPERGRIP
9802, BOSTIK
Components: 1
Function
The adhesive works like a hot melt, i.e. it is applied in its
molten state and sets on cooling. The main difference
compared with other hot melts is that, after setting, it also
cures. The application temperature is relatively low, 100°C
(as opposed to 200 – 250°C for ordinary hot melts). As a
result, assembly times (i.e. the time before the adhesive
sets) are longer. Curing occurs on contact with moisture
in the air.
Curing times
The purely physical setting (cooling) depends on the thermal
conductivity of the bonded materials. On aluminum, the
setting time is about 10 seconds. To prolong the assembly
time when bonding aluminum to materials that have lower
thermal conductivity, the adhesive should be applied to
the latter.
Curing depends on the presence of moisture. If the relative
air humidity is not too low and the diffusion paths not too
long, curing usually takes place within 12 hours. As with
other moisture-curing adhesives, curing times are long when
bonding large diffusion-resistant surfaces.
Bondable materials
Aluminum and other metals, glass, thermosetting resins,
PVC, polycarbonates, acrylics, woods, many rubbers, soft
PVC, etc.
Suitable areas of use
Those where, for technical production reasons, hot melts
were the first choice but, because of their low heat resistance, could not previously be used. The adhesive requires
equipment that prevents it coming into contact with moisture.
For test bonds, samples in metal tubes that can easily be
heated in an oven are often available.
Strength
Single lap joint at 20°C:
Al/Al, 7.4 N/mm2; PVC/PVC, 6 – 8 N/mm2; polycarbonates,
9.4 N/mm2
Where a soft, rubber-like joint is required. As it does not
depend on moisture for curing, this product enables the
bonding of large surfaces.
Strength
Tensile strength, approx. 0.9 N/mm2; elongation at rupture,
approx. 100 %; hardness, approx. 55” Shore A
Temperature range
-30° to +70°C
Durability/resistance
Excellent against water, moderate against oils
Work environment
Skin contact must be avoided.
SGA (SECOND GENERATION ACRYLIC) ADHESIVES
Peel strength
Ground rubber, 6 N/mm2
Halogenated rubber, 13 N/mm2
Soft PVC, 10 N/mm2
Temperature range
At 70°C, about 50% of the strength at room temperature.
At 100°C, approx. 2 N/mm2.
For structures that are constantly loaded at elevated temperatures, the creep strength should be investigated.
Durability/resistance
When bonding anodized aluminum, water resistance is
good. Resistance to other media – contact Bostik.
Work environment
The adhesive contains isocyanates. In Sweden, work is
regulated by AFS 1996:4, Härdplaster (ordinance 1996:4
of the Swedish Work Environment Authority, Thermosetting resins).
POLYSULPHIDE RUBBERS
Besides polyurethane, polysulphide rubber is one of the few
rubber materials that can be made to cure at room temperature and, at the same time, adhere to other materials. Its
widest use is in the manufacture of insulating glazing where
bonding is to aluminum profiles and as a “joint compound” in
cladding elements (e.g. aluminum sections in buildings). The
material has low strength, but large elongation at rupture.
Tested adhesive 7
POLYSULPHIDE RUBBER NAFTOTHERM M82 METALLGESELLSCHAFT/YTTEKNIK
Components: 2
Mixing ratios
100:10 by weight. Pot life is approx. 1 – 2 hours.
Function
Curing starts when the components are mixed.
Curing times
Around 8 – 12 hours at room temperature
Bondable materials
Aluminum and other metals, minerals, thermosetting
resins
This adhesive type is characterised by the hardener being
spread on one surface and the adhesive on the other. Curing
to a handleable state occurs within one or several minutes.
This adhesive type is also very tolerant of oil on surfaces.
Adhesive and hardener must be spread on their respective
surfaces in such a way that they almost completely overlap
each other when brought together. Uncured adhesive will
otherwise be left at the joint’s edges. The adhesive is best
suited for small to moderately large surfaces.
This adhesive type is also available in 2-component
packs with a mixer tube. The adhesive emerges mixed
and air-free directly from the packs. Curing times are then
usually set to be a little longer.
As regards work environment, this adhesive type
requires contactless handling and good ventilation. The
components have a strong smell.
Tested adhesive 8
SGA ADHESIVE MULTIBOND 330, LOCTITE
Components
Two, which are spread separately, i.e. adhesive and hardener on their own surfaces.
Function
There is a very fast reaction when the adhesive and hardener come into contact. The press force needs to be
maintained between 1 and 3 minutes.
Curing times
Fifty percent strength is reached in around 20 – 30 minutes.
Full strength is achieved after 3 – 6 hours.
Bondable materials
Aluminum and other metals, glass, PVC, polystyrenes,
polycarbonates, acrylics, thermosetting resins
Suitable areas of use
Small to moderately large bonding surfaces where short
fixing times and high impact and peel strengths are required.
On lightly oiled surfaces that cannot be cleaned.
Strength
Single lap joint: Al/Al (AlCuMg alloys), 20 – 25 N/mm2; PVC/
PVC, 8.5 – 14 N/mm2
Peel strength
Suitable areas of use
17
T-peel test, Al/Al: 2.5 – 3.5 N/mm2
Temperature range
-40° to +100°C. At 75°C, about 70% of the strength at room
temperature remains.
Durability/resistance
Good against water and oils (contact the supplier for further
details).
Work environment
Requires good ventilation and handling with no skin contact.
In Sweden, work with acrylic adhesives is regulated by AFS
1996:4, Härdplaster (ordinance 1996:4 of the Swedish Work
Environment Authority, Thermosetting resins).
Tested adhesive 9
SGA ADHESIVE GZM, GARCO
Components
Two, which are spread separately, i.e. adhesive on one
surface and hardener on the other.
Function
There is a very fast reaction when the adhesive and hardener come into contact. Giving less vapour emission, shorter
curing times, better gap filling capabilities and improved
resistance to heat and solvents, this product is regarded
as a development of SGA adhesives.
Curing times
Fixing, 15 – 30 seconds. Final curing, 1 – 2 hours.
Bondable materials
Aluminum and other metals, glass, thermosetting resins,
PVC, glass-filled nylon (poorer with other thermoplasts)
Suitable areas of use
As for other SGA adhesives, where a short fixing time is
the goal.
Strength
Single lap joint: Al/Al, 21 N/mm2; epoxy/fibreglass laminate,
11.2 N/mm2
ANAEROBIC ADHESIVES
Because the first adhesives with an anaerobic function
were used to lock screw joints, they are usually referred
to as locking fluids. Broadly speaking, anaerobic means
“without air”. For curing to take place, these products require the absence of air. The majority also require metal
ions for curing.
Development of these “locking fluids” has been rapid
and the group now has products that act as adhesives.
The original curing system (air free + metal ions) has been
complemented by products that cure using ultraviolet light,
heat or activator.
Anaerobic adhesives are suitable for small to moderately
large surfaces. They are widely used in the electronics industry. Anaerobic adhesives are not so suitable for plastics.
Tested adhesive 10
ANAEROBIC ADHESIVE LOCTITE 326, G A LINDBERG
Components: 1
The adhesive can be used with or without activator. Activator
is used when bonding surfaces that do not have active metal
ions and/or to accelerate curing.
Function
Cures in thin gaps and in contact with metal ions. Curing
on anodized aluminum requires activator.
Curing times
Curing times very much depend on the metal that is to be
bonded. Fixing times of around 30 minutes are usual.
To cure in a reasonable time, aluminum alloys with a copper content under 1% or a lot of magnesium require heat
input or an activator.
Bondable materials
Aluminum and other metals, ferrite magnets, glass
Suitable areas of use
Assembly in the engineering and electronics industry (small
bonded surfaces)
Strength
Single lap joint Al/Al: 12.5 – 20 N/mm2
Peel strength
T-peel, 0.5 – 1.5 N/mm2
Temperature range
-50° to +120°C. At 90°C, strength is about 50% of that at
room temperature.
Work environment
Skin contact must be avoided. Work with anaerobic adhesives (which are classed as acrylic adhesives).
Test Methods
The most common test method is the single lap joint. This
most often provides the basis for adhesive suppliers’ data
sheets. The test is illustrated in fig. 15.
25
1,6
12,5
140
Figure 15: Standard shear test, ASTM D 1002-99
18
Adhesive strength is given in N/mm2, which is an expression of average stress. Locally, there are both higher
and lower stresses in the joint. The stiffer the adhesive that
is used, the higher the stress peaks the test returns at the
edges (see “Joint design”).
The test is normally performed against aluminum. However, apart from the adhesive itself, it is not always stated
which materials make up the bonded joint. Because harder
materials normally give higher strength values for the joint,
this absence of information raises further questions.
The tensile speed when the test load is exerted is of great
significance for the measured results. Before 1994, a constant tensile speed of 10 mm per minute was used. This
gave short test times, but no idea of creep resistance in
the joint.
A tensile speed of 1.3 mm per minute is now used.
However, this also does not give an idea of the adhesive’s
creep resistance. Where the adhesive is to be exposed to a
constant load at elevated temperatures, data on the joint’s
creep strength must be compiled.
As intimated in “The effect of the service environment”,
the “composite stress” exerted by temperature, environment
and load is the only “realistic” test.
In cyclic testing (water soaking-freezing-drying) of an
object, the thermal movements and the taking up of water
induce stresses in the joint. This induced stress arises more
rapidly than with soaking alone. Table 5 on page 18 shows,
amongst other things, the results of this test carried out on
the adhesives tested by Sapa Technology.
Storage in 100% humidity at 60°C can give a quicker
idea of any degradation of the joint than can corresponding
storage in water.
The wedge cleavage test (ASTM D 3762-98) is comparatively easy to carry out and does not require special
equipment. This test enables load, environment and temperature stresses to be combined.
Bonded test pieces are prepared (see fig. 16). When
the bond has cured, a 3 mm thick wedge is pressed into
one end of the test piece. The initial crack formed at the
wedge tip is marked out. Next, the test pieces are exposed
to 100% RH at 60°C for 60 – 75 minutes. Crack propagation
is then marked and measured again. This can be repeated throughout the period available for testing. A week of
testing usually reveals clear differences between different
adhesives and pretreatments.
At the conclusion of testing, the bonded joint is taken
apart and the types of failure that have occurred are analysed. With heat-curing epoxy adhesives on pretreated
aluminum, there is most often a cohesive failure (failure
in the adhesive) in the first five millimetres. The longer the
crack then grows, the greater the ratio of boundary layer
failures that arise.
This method has been used by the “Institute for Metallic
Construction Materials” at Chalmers University of Technology to investigate the long-term strength of bonded joints to
aluminum that has undergone various pretreatments (refs.
2 and 5). Such testing can be comparative or used as a
production check (e.g. production parameters – checking
of pretreatment baths, etc.).
The method can be modified so that it is suitable for use
with other materials bonded to aluminum.
a
a
Figure 16: Wedge cleavage test, ASTM D 3762-98
References
(1) ”Durability of Structural Adhesives”
A.J. Kinloch (editor), Applied Science Publishers
London (1983), ISBN 0-85334-214-8.
(2) ”Förbehandling för limning mot aluminum”
Tore Rönnhult och Bengt Nilssor, Institutionen för
Metalliska Konstruktions-material, Chalmers Tekniska
Högskola, Göteborg (1982).
(3) ”Effect of Surface Preparations on
Adhesive Bonding of Aluminum”,
J.D. Minford, Adhesives Age, July 1974.
(7) ”Handbook of Aluminum
Bonding Technology and Data”
J. Dean Minford. Marcel Dekker Inc, NY, Basel,
Hong Kong, 1993 (744 sidor, 4686 referenser)
ISBN 0-8247-8817-6.
(8) ”Adhesion in Bonded Alurninium
Joints for Aircraft Construction”,
W. Brockmann, O-D Henneman, H. Kollek och C.
Matz. International Journal of Adhesion and Adhesives,.6(3), July 1986.
(4) ”Effect of Surface Preparation on Stressed
Aluminum Joints in Corrosive Saltwater Exposure”
J.D. Minford, Adhesives Age, October 1980.
(5) ”Surface Treatment of Aluminum Alloys
for Adhesive Bonding”
Laszlo Kozma, Institutionen för Metalliska Konstruktionsmaterial, Chalmers Tekniska Högskola, Göteborg.
(1984) Internrapport R 445/84.
(6) ”Ytbehandlingens betydelse vid limning
av aluminumdetaljer”,
Bengt Nilsson och Tore Rönnhult, Institutionen för
Metalliska Konstruktionsmaterial, Chalmers Tekniska
Högskola, Göteborg (1983), Internrapport R 424/83.
19
Tests of a few selected adhesives
Sapa Technology tested a number of different adhesives
on aluminum that had either been ground or anodized in
sulphuric acid and then sealed.
For each of these two pretreatments, ten different adhesives were tested. Some of the adhesives were comparatively common epoxy adhesives, others were chosen to give
a more rubber-like joint. These adhesives are presented in
more detail in the “Choice of adhesive” chapter.
After full curing, the adhesives were subjected to soaking
or cyclical variations (moisture, cold, heat) and then tested.
The results were compared with reference samples stored
in dry air at 20°C.
The test results are shown in table 5 on page 18.
Test data
Tensile speed, 10 mm/min at 20°C
Tested adhesives
1. 2-component epoxy, Araldit AV 106/HV 953 U (Standard)
2. 2-component epoxy, Araldit AV 144/HV 997
3. 1-component epoxy, 3M Scotch-Weld EC 2214 (heatcuring, 120°C)
4. 2-component polyurethane, Casco 1852/1853
5. 1-component polyurethane, Sikaflex 221 (flexible)
6. Hot-melt adhesive, moisture-curing polyurethane, Bostik
9802
7. Polysulphide, Metallgesellschaft M 82
8. SGA adhesive, Loctite Multibond (acrylic rubber)
9. SGA/“Aerobic”, Garco GZM
10. Anaerobic, Loctite 326 (on anodized material, used
along with activator NF)
Test format
Single lap joint on 2 mm Al (gives large stress concentrations).
Alloy – condition
SS 4104-06
Surface treatments
• Anodizing in sulphuric acid + subsequent sealing (carried out by Sapa). Washing in warm water to which “Candoeleane Al” had been added, rinsing and drying before
bonding.
• Grinding with abrasive nylon (Scotch-Brite 7447, 3M
Sverige).
Joint thickness calibrated to 0.15 mm for all adhesives.
Test environment
A. Dry air
B. 14 days in water at 60°C. Testing 2 – 7 days after drying.
C. Cyclic:
1 week in water at 20°C
1 week in cold, -20 to -25°C
1 week in dry air at 20 to 23°C
(Testing after 3 cycles and after 1 year of cycles.)
Table 5: Strength, N/mm2, average value of 5 tests
Various adhesives on alloy
SS 4104-06
Single lap joint on 2 mm
bars
On a ground substrate
On an anodized substrate
Storage environment before testing
Storage environment before testing
Air
20°C
Water
20°C
Cycles
3(1)
Cycles
1 year (1)
Air
20°C
Water
20°C
Cycles
3(1)
Cycles
1 year (1)
1
2-component epoxy
Araldit AV 106/HV 953 U
8.9
G
8.3
G
13.3
G
10.8
G
9.3
G
13.3
G
16.3
G
13.9
G
2
2-component epoxy
Araldit AV 144/HV 997
7.0
G/L
14.1
G
13.3
G
13.7
G
9.9
L(G)
16.8
L(G)
16.8
L(G)
15.5
G(L)
3
1-component epoxy
3M Scotch-Weld EC 2214
10.6
G(L)
13.3
G(L)
17.5
G(L)
11.5
G(L)
10.6
L(G)
20.7
L
21.8
L(G)
20.2
L
4
2-component polyurethane
10.3
L
7.5
G/L
16.9
L(G)
15.3
L(G)
8.8
L
13.3
L
15.8
L
14.3
L
5
1-component polyurethane elastomer Sikaflex 221
1.6
L
3.8
G
3.5
G/L
7.4
L(G)
1.5
L
2.9
L
3.1
L
3.4
L
Moisture-curing hot-melt
adhesive Bostik 9802
3.8
G
3.8
G
7.2
G
2.8
G
0.6
G
5.1
G
6.2
G
7.7
G
7
2-component polysulphide
Metallgesellschaft M 82
1.0
L
1.3
L
1.9
L
1.7
L
0.9
L
1.0
L
1.9
L
1.4
L
8
Acrylic rubber adhesive (SGA)
7.9
G
10.1
G
13.7
G
8.4
G
9.0
G
9.9
G
11.8
G
10.9
G
Acrylic rubber adhesive (Aerobic)
10.4
G/L
9.6
G
18.6
G
13.9
G(L)
8.7
G
3.9
G
4.2
G
8.4
G
Anaerobic-curing adhesive
11.0
G/L
11.2
G
13.5
G
11.9
G(L)
7.3
G
7.8
L/G
13.4
G
14.8
G(L)
6
Casco 1852/1853 2)
Loctite Multibond 330
9
10
Penloc GZM (från Garco)
Loctite 326 1) One cycle per week
2) Replaced by Casco 1849/1821 PUR 2K
20
Failure type
L = Clear break in the adhesive. Adhesive
on both aluminum surfaces to 100%..
G = Boundary layer failure.
G/L = Approx. 50% of each of G and L.
L(G) and G(L) = The letter in brackets
indicate a “small proportion”.
Results – Discussion
As shown by table 5, the strength of several adhesives
increases after soaking in water and passing through the
cycles. This may be because the relevant adhesives cure
more fully in contact with water. In particular, this applies
to the polyurethanes and the polysulphides.
For the harder adhesives, this may be a question of a
certain, non-harmful, taking up of water. The water then
serves to soften the adhesive. This gives better stress
distribution in the adhesive and, consequently, reduced
loading of the boundary layer.
Aluminum surfaces that have been pickled in sulphuric acid
and then sealed have often been regarded as unsuitable for
bonds. However, the test results show that many adhesives
adhere just as well to anodized surfaces as they do to
ground surfaces – in some cases even better!
After one year of cycles, several adhesives show
considerably less deterioration in strength on anodized
surfaces than they do on ground surfaces. This applies
especially to the more highly viscous adhesives. The
explanation for this is probably:
• Anodized surfaces are more resistant than ground
surfaces and do not develop weak oxide layers that can
delaminate under load.
• Anodized surfaces are more even and are thus more
easily filled by the adhesive (especially if it is viscous). This
leaves less space for water in the boundary layer.
• The tested adhesives were very different. For example,
there were great differences in hardness. The three epoxy
adhesives were all softened to give better load distribution
and counteract stress concentrations. Adhesives 5 and
6 were very soft and elastic. Thus, the adhesives must
not primarily be compared with each other, but with
themselves in respect of how they respond to the different
pretreatments.
Examples of bonded aluminum products
Example 1 – radiator unit
Radiator unit in AA 3005-0 aluminum (clad with AA 4104);
the tank (upper part) is in polyamide 6.6 with a 30% fibreglass filling.
The aluminum radiator is bonded to the plastic tank.
The intended service environment is -40° to +120°C in
constant contact with a water/glycol mixture.
As the radiators are manufactured in very large numbers,
streamlined production is essential.
Pretreatment
The cladding is rich in magnesium. After vacuum soldering
of the radiator unit, magnesium oxides make up most of
the surface. Even if it is clean, this is not a suitable surface to bond to if there is moisture/water in the service
environment.
Radiator unit: Alkaline pickling + chromating (trials with a
chromate-free alternative are in progress). Polyamide tank:
Manufactured without release agents and not pretreated.
Adhesive
One-component heat-curing epoxy adhesive, similar to no.
3 of the tested adhesives in table 4.
Alternative adhesives
None that offer any advantages with maintained safety.
Bonding procedure
To improve the flow of the adhesive, the components are
preheated to 60°C.
The adhesive is applied to the plastic component.
Curing in an oven at 130°C for 60 minutes.
Results
Sealed, durable fixing of the stub ends of the radiator
unit’s pipes. The adhesive’s thermal expansion coefficient
matches that of the plastic. Thus, variations in temperature
do not present problems.
Disadvantages
Comparatively long curing time at 130°C; requires large
oven capacity for large series.
Remarks
When used as a component in vehicle production, any faults
have very serious consequences. For this reason, tested
methods and components are preferred to others that might,
for example, give shorter curing times.
Example 1: Bonded radiator unit alongside unbonded components
21
Example 2 – outside planking (boat)
“Profilen” is a ¾ tonne sailing boat that is some 10 metres
long. It is built of extruded aluminum profiles (alloy AA 6063)
developed by Sapa.
to the preceding, underlying profile. The adhesive is now
available in a double cartridge that delivers ready-mixed
and air-free adhesive direct from the pack.
To make the boat rigid and watertight, the profiles are bonded to each other.
The overlying profile is then fitted within 60 minutes and
secured by clamps. Curing takes place at room temperature
and lasts at least 16 minutes.
As with wooden boats, the skins of boats of this type are built
plank by plank. This requires an adhesive that gives long
handling times and which is able to fill comparatively large
gaps. The adhesive must cure at room temperature.
The service environment is the normal one for many boats – water (often salt) in the summer and dry and cold in
the winter.
Pretreatment
The profiles are anodized in sulphuric acid and then sealed.
A Sapa facility anodises the profiles in full lengths. Transport, cutting and bending must then be carried out in a way
that causes the least possible fouling. This is because efficient cleaning of these long profiles is difficult to achieve
simply. (Wiping with a rag and solvent does not remove all
contaminants.)
Adhesive
Two-component, rubber-modified, slow curing epoxy adhesive (adhesive no. 2 in table 5).
Bonding procedure
The adhesive used to be mixed in a suitable quantity for the
profile that was to be bonded. It was then applied as bead
Results
The first bonded sailing boat has competed successfully
in races and, during the period it was checked (around 10
years), remained perfectly watertight.
Disadvantages
The comparatively long curing time means that no more than
one plank per side can be bonded per 24 hour period.
Alternative adhesives
The thick, hard, post-sealed, sulphuric acid anodizing cannot, without breaking, transmit high loads for a long time in
corrosive environments. Softer adhesives could, therefore,
be interesting.
Tests using polyurethane type rubber adhesives (adhesive
no. 5) and polysulphide (adhesive no. 7) were carried out
in parallel with the epoxy adhesive actually used. The tests
were conducted on loaded joints in a brine environment.
Remarks
Heat curing is not possible. Adhesives that cure more quickly
at room temperature reduce the handling tolerances and
give poorer results.
Example 2: “Profilen”, a ¾ tonne sailing boat built from bonded, extruded aluminum profiles
22
Example 3 – hatch for pleasure boats
Hatches (vents) for pleasure boats are made from aluminum
profiles and 10 mm thick, smoked, acrylic plastic sheet.
better adhesion to plastic than does, for example, an acetic
acid curing system.
Before assembly, the aluminum profiles are bent, welded
and anodized.
The adhesive is very elastic with an elongation at rupture of
around 500% and a breaking stress as low as 1 N/mm2.
The acrylic sheet is bonded to the aluminum profiles.
Bonding procedure
The window is fixed in the aluminum frame using a butyl
rubber strip (sticky) that also forms the bottom seal for the
subsequent silicon joint. In the remaining gap, which is 4 –
5 mm wide, the silicon is applied from a 300 ml cartridge.
The moisture in the air cures the silicon.
Series size is moderate. For this reason, manufacture
has been set up as “streamlined craftsmanship”. The
hatches have to withstand both salt water and UV light in a
temperature range that can be -30°C to + 60°C.
The adhesive bond is exposed to its greatest loads at low
temperatures. Here, because of the different coefficients
of thermal expansion, the acrylic glass window shrinks in
relation to the aluminum frame. The forces that arise are
on a par with each material’s yield strength.
After curing, a scalpel is used to cut away excess silicon.
To make the stress as small as possible, an elastic joint
is desirable. The bonded joint is not loaded appreciably
apart from the temperature-induced stresses, but these
are large.
Disadvantages
Relatively slow curing (24 hours), especially in the winter
when the relative air humidity indoors can be low.
Pretreatment
The aluminum profiles are washed in a mild, alkaline
solution. They are then rinsed in running water and air
dried.
The edges of the acrylic plastic are polished with a gentle
abrasive (abrasive nylon, Scotch-Brite 3M, 7447).
Adhesive
One-component, moisture-curing silicon, “alcoxy system”.
Silicon with an alcoxy curing system generally provides
Results
A sealed, elastic joint with good adhesion. The joint is elastic
even in the cold.
Alternative adhesives
One-component, moisture-curing polyurethane rubbers.
However, to ensure adhesion throughout long-term
exposure to UV light, many of these adhesives require a
black primer on the acrylic window (there are now some
products that withstand UV irradiation – they are used for,
amongst other things, filling boat decks).
Remarks
Curing can be accelerated using a moisture tent and
moist heat. Great precision and a certain amount of skill
are required to achieve a fully filled joint that is free from
bubbles.
Example 3: Aluminum frame and “acrylic glass” for a boat hatch (before bonding)
23
Example 4 – sandwich panels
Sandwich panels with an aluminum covering floor plate and
a PVC cellular plastic core.
The panels are used as floors in refrigerated vehicles. They
can measure up to 2.4 x 15 metres and are made individually
for each vehicle.
Panel and bonded joint must withstand temperatures of
from -40°C to +80°C. The high temperatures may arise on
internal washing.
Loads are exerted by cargoes and various fastening devices. They occur as vibrations, torsional stresses, impacts
during driving and static creep stresses while the vehicle
is stationary. Not least, thermal movements are important.
It is required that the adhesive must, in all positions, be
stronger than the core material and that it does not “release
contact”.
Making panels of this size demands that the adhesive has a
reasonably long open time. The adhesive should also have
good filling properties.
Pretreatment
At manufacture, the core material is calibrated in a “cutter”
and protected throughout transport and storage. No other
pretreatment is undertaken.
Before bonding, the aluminum is machined using ScotchBrite discs. These give a mildly abraded surface.
Where the joint has to satisfy severe corrosion resistance
requirements, the aluminum surface is then primed using a
wash-primer of the polyvinyl butyral type with a phosphoric
acid hardener (curing agent).
Example 4: Floor panel for a refrigerated vehicle
24
Adhesive
Two-component polyurethane with an open time after
application of at least 45 minutes. The adhesive employed
resembles no. 4 in table 4.
Bonding procedure
The adhesive used to be spread on both the core material
and the covering plate. This ensured, amongst other things,
that the open time before application of press force was
considerably extended.
The adhesive is applied using a “hand applicator” (roller
with reservoir). Nowadays, the adhesive is spread on only
one of the materials. Spreading is in tight beads, normally
using automatic application equipment.
A vacuum is used to apply pressing force. The panels are
built up on a flat surface and are covered by a rubber sheet,
from beneath which the air is evacuated. Per m2, the press
force is around 8 tonnes. This was usually held for 12 – 16
hours. With currently used adhesives of this type, the press
force need only be applied for 3 – 4 hours. The open time
is at least as long and may even be longer.
Results
A rigid panel that meets the imposed strength requirements.
Disadvantages
The long press time ties up production surfaces. New adhesives in this group lessen the problem. Naturally enough,
even shorter press times are desirable.
Alternative adhesives
Two-component epoxy adhesives were previously used.
However, all bonds are now made using polyurethane
adhesives.
Example 5 – Honeycomb panels
Honeycomb panels as air guides in a wind tunnel with a
diameter of 5 metres.
Honeycomb materials are used as, amongst other
things, cores in the making of sandwich panels for aircraft
applications. The honeycomb is made of thin (0.2 mm)
aluminum sheets. These are bonded using offset beads of
adhesive so that, on drawing/stretching, they form hexagonal holes. Heat-curing adhesives (rubber modified epoxy
and phenolic adhesives) diluted with a solvent are used for
this.
The adhesive is applied to the aluminum sheet in thin
beads and the solvent is allowed to evaporate. Sheets
are then stacked to form a bundle that is compressed and
activated/cured in a hot press at 125 – 175°C for 30 – 60
minutes. The homogenous bundle is then machined to the
desired dimensions and drawn out to its final shape.
To enable honeycombs to be joined to honeycombs,
cores that are 100 mm thick must be given a vertical metal
edge (140 mm). Depending on the structure of the panels,
the contact surfaces at bonding are very small. In wind
tunnels, vibrations arise that may exert a splitting force on
bonded joints. Hence, an adhesive that fills and envelops
the thin sheet is required. The adhesive must not be brittle
and must have high peel strength. It must also take up the
correct position and not move away before it has set.
Pretreatment
To remove contaminants and weak oxide layers, the honeycomb material and the surface of the aluminum sheet
are brushed with “Alonyl brushes” (circular brushes of
polyamide fibres that contain around 30% aluminum oxide
– from Sinjet, Huskvarna).
So that the adhesive is evenly distributed across the
entire width of the sheet/honeycomb, the fixture is then
stood on edge.
Before removal from the fixture, curing takes place for
16 hours at room temperature.
Results
A joint that has good peel strength and transmits the loads
that can arise between the honeycomb and aluminum sheet.
The wind tunnel has been in use for some ten years with
wind speeds of up to twice the speed of sound. No fatigue
fractures have been observed.
Disadvantages
Getting the adhesive out of its container is messy. (This
adhesive is now also available in double cartridges that
deliver ready-dosed, air-free, mixed adhesive direct from
the pack.)
Alternative adhesives
SGA adhesives (adhesive no. 8 in table 4) where the hardener is spread in the form of a varnish on the aluminum sheet
and the adhesive is spread on the honeycomb material.
The use of 2-component, mixed acrylic adhesives is also
imaginable. These normally have shorter open times and
shorter curing times.
Remarks
For mounting to the wind tunnel structure, 100 mm long
aluminum rods were bonded and placed in a ring around
a cell hole. Sikaflex 221 (adhesive 5 in table 4) was used
for this.
Adhesive
Two-component, rubber modified epoxy adhesive that
is moderately runny before setting at room temperature
(similar to adhesive no. 2 in table 4).
Bonding procedure
Using a toothed spreader, an approximately 1 mm thick
layer of adhesive is applied to 100 mm of the aluminum
sheet’s width. The honeycomb material is placed in a simple
wooden fixture.
Hexagonal pencils and rubber bands are used to fix the
adhesive-coated sheets to the honeycomb side.
Example 5: Wind tunnel with panels mounted
Example 5: Honeycomb panel with bonded joint plate and
mounting reinforcements
25
Example 6 – body panels for buses
Bus/coach bodies comprise a steel skeleton to which a
shell of aluminum cladding panels is added. Previously,
the panels were fitted using blind rivets. To distribute loads
more evenly and minimise the number of rivets, adhesive
bonding is now used.
Moisture is the main environmental stress and the structure must withstand temperatures of from -40° to +70°C.
The loads are moderate. However, the panels must remain
in place in all positions.
A design that uses sheet metal panels in this way requires an adhesive that has good peel strength.
Pretreatment - The steel structure is degreased and primed.
The aluminum panels are either primed or chromated.
Alternative adhesives
Double-sided sticky tape (double-sided PSA tape) approx.
1 mm thick and built up entirely of an adhesive compound
is also used for this purpose.
2-component, acrylic rubber adhesives – mixed or with
a separately spread hardener.
Remarks
There are also 1-component polyurethane adhesives that
give similar final properties. They cure in heat (min. 70°C)
or are applied hot (approx. 80 – 90°C). After cooling for
around 1 hour, they have mastic-tape properties. Moisture
is then responsible for final curing.
Adhesive - One-component, moisture-curing, polyurethane
elastomer (gives a rubber-like joint resembling adhesive 5
in table 4).
Bonding procedure
The adhesive is applied to the steel structure; the panels
are lifted into place and fixed with the necessary number of
blind rivets (at least two per panel). Each panel is pressed
lightly against the adhesive so that there is contact with no
appreciable springback.
Thanks to air moisture, the adhesive cures to sufficient
strength in around 24 hours.
Results - With good peel strength, a rubber-like bonded
joint that deforms elastically and does not give rise to stress
concentrations.
Disadvantages
Relatively long curing time if relative air humidity is low.
Example 6: Coach with bonded cladding panels and glass windows.
Example 7 – thermal foil
Thermal foils with pattern-like printed circuits – aluminum
foil on thermoplastic polyester foil. Used in, for example,
electrically heated door mirrors on cars.
This is a case of mass production with various product
forms. Manufacture must be streamlined and the bonded
joint flexible and comparatively heat resistant. Thermoplastic polyester has low surface tension. This limits the
number of adhesives that can wet it (and thus give good
adhesion).
Pretreatment
Aluminum foil: no pretreatment takes place. Polyester foil:
corona treated (electrical discharges that increase surface
tension) at manufacture. To increase surface cleanness, the
aluminum foil can also be treated in this way.
Adhesive
Polyurethane rubber solution with curing (hardening) additives (isocyanates).
The adhesive is “heat activable”, i.e. after it has dried
(the solvent has left the adhesive), it can once again be
made “runny” by using heat.
Bonding procedure
The adhesive solution is rolled out onto the polyester film.
The solvent (around 80%) is allowed to evaporate and the
dry layer of adhesive is heated until it melts. In this condition,
the aluminum foil and the adhesive-coated plastic film are
roller-pressed together.
To obtain the desired pattern, the base laminate is
screenprinted with a protective varnish. Unwanted aluminum foil is etched away and the protective varnish is then
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washed off. The now only partly coated polyester surface
is protected by laminating it with a further adhesive-coated
polyester foil. This is also done through heat activation and
roller-pressing together.
Results
A flexible thermal foil that has a service temperature of up
to 120°C. There are also foils that tolerate considerably
higher temperatures.
Disadvantages
The use of solvents is less desirable. Solvent emissions in
workplaces and to the environment must be checked and,
through various measures, held at a low level.
Remarks
“Heat activable” adhesives in the form of water dispersions
are now also available. These are also used with an added
hardener.
Example 7: Thermal foil for a door mirror
Example 8 – aircraft
Aluminum in aircraft is largely joined using adhesives. Production is meticulously controlled and all adhesive bonds
are checked using, amongst other things, ultrasound. The
adhesive bonds have to withstand: very low temperatures
without becoming brittle; and, at high temperatures, large
loads without creeping. They must be of an even and set
thickness and must also be free of air bubbles.
Remarks
Bonded joints delivering the performance and quality required of structural bonds in aircraft cannot be achieved
in a simpler way.
Pretreatment
Automated anodizing in phosphoric acid without subsequent
sealing. This is followed by drying in clean air and immediate
coating with a primer that is suitable for the adhesive. Long
used in Europe, anodizing in chromic acid also occurs. As
this leaves a more even surface, priming can then sometimes be omitted. Pure, clean cotton wool gloves must be
worn for any contact with the pretreated surfaces.
Adhesive
Epoxy-based adhesive films are the most common. Even
joint thickness is assured by the film format. Modification
through the use of rubber (for peel strength) or phenol (for
heat resistance) additives occurs. Load conditions determine the choice of adhesive.
Bonding procedure
All bonds of a structural nature are pressed and cured in
an autoclave at a press force of 1 – 3 N/mm2 at a temperature of 125 – 175°C for 30 – 90 minutes (depending on
adhesive type).
Results
Aircraft that do not fall apart!
Disadvantages
Comprehensive programme of checks for the anodizing
bath. Length of curing time and the comparatively high
curing temperatures.
Alternative adhesives
Adhesive films based on phenolic nitrile are also used.
These were the first adhesives designed for use in the
aircraft industry (used since the 1940’s).
Example 8: Saab 340 being assembled
Example 8: Parts of an aircraft body going into an autoclave
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Example 9 – bonded brake linings
Brake linings on cast aluminum shoes.
The bonding of brake linings requires a joint that has
very good heat resistance.
Pretreatment
An abrasive is used to lightly grind the friction material.
The aluminum shoes are blasted with sand or aluminum
oxide.
Adhesive
A phenolic-nitrile solution that is applied to both joint surfaces and is allowed to dry completely.
Alternative adhesives
Dried but not cured adhesive films are now available. These
can be placed dry between the surfaces. They are then
melted and cured in the same way.
Remarks
Adhesive films for this use are a recent innovation and
should be regarded as providing an improvement in the
work environment.
Bonding procedure
The dry, adhesive-coated surfaces are put together and
subjected to pressure. They are then heated in an oven. As
the adhesive melts, it wets the surfaces. With the press force
maintained and continued heating, the adhesive cures.
Depending on the heat-resistance requirements placed
on the bond, curing temperatures and times can vary from
a few minutes at 120°C to several hours at 180°C.
Results
Brake linings that are securely attached, but the heat resistance of which depends on curing time and temperature.
Disadvantages
As always when handling solvents, workplaces must be
well ventilated and the management of extracted vapours
must be approved and checked.
Example 9: Bonded brake linings
Example 10 – power saw handle
Power saw handles made from aluminum pipes and bonded
to the cast motor block. In cold weather, the handle can be
heated by channelling the exhaust gases through it. The
joint must give a good seal and withstand temperatures of
between -40°C and +70°C.
Alternative adhesives
One-component, heat-curing, epoxy adhesives. However,
these are more viscous and can be difficult to “get into
place” in the joint. Heat curing at a minimum of 100°C is
also usually seen as a disadvantage.
Design
The design of the cast motor block provides two cylindrical
holes to take the ends of the pipes.
Remarks
In locked joints of the “pin in hole” type, the thermal expansion coefficients of the materials should be borne in mind
if the joint is exposed to temperature fluctuations. This very
much also applies to the adhesive. An adhesive’s expansion is often 5 – 10 times greater than that of the metals
involved.
Amongst the anaerobic adhesives, there are variants
that are of a sufficiently low viscosity for them to be applied
to assembled joints that have little play. These adhesives
are then sucked in by capillary action.
Pretreatment
The motor block’s holes are reamed to the correct size
(H7) for the pipe ends. A fine adhesive belt is used to grind
the pipe ends
Adhesive
Anaerobic adhesive (similar to adhesive no. 2 in table 4).
The adhesive is a runny, 1-component adhesive that starts
to cure in the closed joint in contact with active metal ions. It
requires aluminum with at least 1% copper or an application
of activator on the joint surfaces.
Bonding procedure
The prepared holes in the motor block are moistened with
adhesive. The pipe ends are coated more liberally and
pushed into place.
Curing to handleable takes around 15 – 30 minutes.
During this time, the joint should not be disturbed.
Excess adhesive outside the joint does not cure and
can be easily wiped or washed away.
Results - A sealed, strong joint.
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Disadvantages
As the adhesive can be pushed aside at assembly, it is often
difficult to achieve fully filled joints where these are of the
“pin in hole” type. Removing adhesive outside the bonded
joint can give rise to work environment problems.
Example 10: Power saw handle
Example 11 – corner structures
Extruded aluminum profiles are widely used for claddings,
doors, windows and surrounds.
Using angle pieces in the corners, these profiles are
often joined into fames. These angle pieces can be made
of punched or cast profiles. They are bonded into the frame
profiles. This requires a joint-filling adhesive that does not
shrink on setting. The adhesive must withstand a normal
outdoor environment.
Disadvantages
Series are large and, therefore, involve many people
working with epoxy adhesives. The occupational safety
issues connected with the handling of epoxies are thus felt
to present a problem.
The long curing times and the need to frequently mix
small batches of adhesive are sometimes seen as problems.
Pretreatment
At delivery to the various production units, the profiles are
anodized or painted.
Cutting fluid is used when cutting the profiles. Practically,
it is not feasible to clean the cutting fluid residues from the
inside of the profiles (even though this is exactly where the
angle piece is to be seated).
Alternative adhesives
Sapa Technology has tested a number of alternative adhesives. Joints were made using surfaces that had been
moistened with cutting fluid (several different cutting fluids
were tested).
The tested adhesives were:
• 2-component polyurethanes (adhesive 4 in table 4)
• 1-component, moisture-curing, polyurethane elastomers
(adhesive 5 in table 4)
• 1-component, foaming, moisture-curing, polyurethane
adhesives
• Anaerobic adhesives with activators for corner elements
(adhesive 10 in table 4)
Adhesive
The majority of these joints have long been made using
2-component, slow curing epoxies (adhesives corresponding to no. 1 in table 4).
Bonding procedure
Adhesive is applied relatively liberally inside the aluminum
profile. The angle piece is pushed inside and the corner is
fixed by stamping the overlying profile. Frames are then
stored, stress-free and with the corner at 90°, for around
16 hours.
Results
Can be seen in doors, windows and aluminum profile frames
in buildings!
Testing was carried out on single lap joints and joints comprising square tubes inside square tubes. Results were
compared with those for 2-component epoxies.
The strength of the cured bonded joints was compared with
that of joints aged for 60 days at 60°C in 100% humidity.
The test results show a marked deterioration in the strength
of all bonded, single lap joints stored in 100% humidity. Apart
from one 2-component polyurethane adhesive, the same
also applies to “square tube in square tube” joints.
The use of a two-component polyurethane adhesive
instead of the current epoxy adhesives could be seen as
an improvement in quality.
Example 11: Bonded corner structure made from extruded aluminum profiles
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Example 12 – glass windows with aluminum frames
Windows for vehicles (buses, trains, loaders, trucks, etc.)
are made from extruded aluminum profiles and glass. Aluminum locks and fittings are bonded to the glass. Thermal
movements and service environments require an adhesive
that does not become too hard and that, in wet environments, does not lose adhesion with the glass.
The rather large production volumes mean that short
fixing times are desirable.
Pretreatment
None. The chosen adhesive type is very tolerant of oil.
Adhesive
SGA adhesive (acrylic rubber adhesives, correspond to
adhesive no. 8 in table 4).
Bonding procedure
The hardener is spread on the glass and the adhesive is
spread inside the aluminum profile. Fixing/pressing takes
around 5 minutes.
Disadvantages
The smell and work environment considerations associated
with this adhesive type mean that work has to be carried
out in very well ventilated areas.
For the joint to cure at all points, the adhesive and hardener should be spread so that, on assembly, they always
come into complete contact with each other.
Alternative adhesives
Mixed acrylic rubber adhesives.
One and two-component silicon adhesives. However,
these adhesives have considerably longer (hours) fixing
times and give a softer joint.
Remarks
Adhesives of this type are available in double cartridges that
contain both hardener and adhesive. On being squeezed
out, the components pass through a mixing nozzle. This
avoids the need to apply to both surfaces. The risk of uncured adhesive also disappears.
Results
An elastic, strong joint that, both for mechanical loads and
for forces arising from thermal movements, gives excellent
stress distribution.
Example 12: Glass vehicle windows with aluminum frames and handles
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The information in this text was considered correct at the time of publication. However, Sapa accepts no
legal liability for the correctness or completeness of any of said information.
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© Sapa Profiles, Inc.
Sapa Extrusions
Address 7933 NE 21st Avenue
Portland, Oregon 97211 USA
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www.sapagroup.com/us/profiles