PART 10 ADHESIVES AND SEALANTS

1
PART 10 ADHESIVES AND SEALANTS

• 10.1 Adhesives
• 10.2 Sealants

2
Glued Segmental Bridge Construction
3
PART 10 ADHESIVES AND SEALANTS

• 10.1 Adhesives
• 10.2 Sealants

4
10.1 Adhesives

• Safety
• Types of Adhesive
• Surface Preparation
• Setting Processes
• Types of Joint
• Modes of Failure

5
Safety of Adhesives

• Many adhesives and sealants are toxic (e.g.
  containing isocyanates) and are readily absorbed
  through the skin.
• Many also give off toxic solvent vapours during
  setting.
• COSHH regulations require that for all materials
  used on sites the safety sheet is obtained.
  Always consult this sheet before using these
  materials.
• Note that some materials which are relatively
  harmless in domestic applications/quantities
  (e.g. epoxies) represent a considerable hazard in
  large quantities.

6
10.1 Adhesives

• Safety
• Types of Adhesive
• Surface Preparation
• Setting Processes
• Types of Joint
• Modes of Failure

7
TYPES OF ADHESIVE

• Mechanical adhesives. All of the original animal
  glues were of this type. They only work on porous
  materials - the adhesive penetrates into the
  pores and on hardening it forms a mechanical key.
• Solvent adhesives. These are used on materials
  which dissolve in solvents (e.g. plastics). The
  surfaces to be joined are softened in the solvent
  which subsequently evaporates to leave a
  continuous joint.
• Surface adhesives. These will adhere to surfaces
  which are neither porous nor soluble (e.g.
  glass). Modern adhesives such as epoxies are of
  this type. They work by bonding in the same way
  as the bonds within a material

8
10.1 Adhesives

• Safety
• Types of Adhesive
• Surface Preparation
• Setting Processes
• Types of Joint
• Modes of Failure

9
Surface Preparation

• In order to adhere to a surface the adhesive must
  wet it (i.e. not form into droplets).
• Scoring of surfaces (a method often used on
  timber by joiners) is detrimental to a joint made
  with modern surface adhesives because it causes
  stress concentrations and air inclusions. It is
  only beneficial if traditional mechanical
  adhesives are used.
• Water will prevent the bond of many adhesives but
  some will work well underwater (e.g. some
  polyurethanes).
• Many treatments (e.g. impregnated wood
  preservatives) will prevent some adhesives from
  working and reduce the performance of others

10
10.1 Adhesives

• Safety
• Types of Adhesive
• Surface Preparation
• Setting Processes
• Types of Joint
• Modes of Failure

11
The different setting processes (1)

• Adhesives that set on cooling (e.g. bitumens).
  These materials are heated for use and will fail
  if re-heated.
• Adhesives that set by evaporation of water. These
  are clearly not durable in a wet environment and
  will not set in contact with impermeable surfaces
  which prevent the evaporation.
• Adhesives that set by evaporation of solvents.
  These are more durable than the water based glues
  but may also not set if evaporation is prevented.

12
The different setting processes (2)

• Adhesives that set by internal reaction (e.g.
  polymerisation of epoxies and polyesters).
• Anaerobic adhesives that set by exclusion of air
  ("Superglue").
• Adhesives that set by reaction with water. Some
  polyurethane adhesives are stored in contact with
  a desiccant and set by reaction with moisture in
  the air.
• Some adhesives set by a combination of two or
  more of these processes.

13
The adhesives are categorized for use as follows

• WBP Weather and boil proof (as in exterior
  grade plywood)
• BR Boil resistant - not as durable as WBP
• MR Moisture resistant
• Int Interior - resistant to short exposure to
  cold water only

14
Durability of Adhesives
15
10.1 Adhesives

• Safety
• Types of Adhesive
• Surface Preparation
• Setting Processes
• Types of Joint
• Modes of Failure

16
Types of joint
17
Contact Joints

• For the contact joint most adhesives are
  suitable. The clamping force must be sufficient
  to ensure contact but not so high that the
  adhesive is forced out.
• The adhesive should not have a higher modulus
  than the substrates because this will cause
  stress concentrations at the ends of the joint
  which may cause progressive failure.
• Joining materials with different moduli is
  difficult.

18
Gap Filling Joints and Fillets

• For gap filling applications many adhesives are
  unsuitable. They either have insufficient
  viscosity to remain in place during setting or
  (more frequently) have very high shrinkage during
  setting.
• A pure epoxy resin will not fill gaps so it is
  normally mixed with an inert filler (e.g.
  colloidal silica) to give it gap filling
  properties. This will, however, reduce it's
  strength for contact applications.
• For fillet formation even greater viscosity and
  stability is required. Inclusion of fibres may be
  beneficial.

19
10.1 Adhesives

• Safety
• Types of Adhesive
• Surface Preparation
• Setting Processes
• Types of Joint
• Modes of Failure

20
Reasons for failure

• Inadequate joint preparation
• Failure of the substrate due to local stress
  concentration
• Bond failure due to moisture ingress or exposure
  to ultraviolet light.
• Progressive failure due to eccentric loading.
• Failure of the adhesive due to inadequate mixing
  or incorrect or prolonged storage.
• It is always better to use a mechanical fixing
  as well as the adhesive for critical applications.

21
Shear stress along the length of an adhesive
joint in tension
Shear Stress
Solve the problem by tapering the ends.
22
Reasons for failure

• Inadequate joint preparation
• Failure of the substrate due to local stress
  concentration
• Bond failure due to moisture ingress or exposure
  to ultraviolet light.
• Progressive failure due to eccentric loading.
• Failure of the adhesive due to inadequate mixing
  or incorrect or prolonged storage.
• It is always better to use a mechanical fixing
  as well as the adhesive for critical applications.

23
PART 10 ADHESIVES AND SEALANTS

• 10.1 Adhesives
• 10.2 Sealants

24
10.2 Sealants

• Types of Sealant
• Joint Details

25
Traditional Sealants

• Putties These are the traditional materials for
  sealing to glass. They harden by surface
  oxidation and subsequent slow loss of solvent
  (typically linseed oil).
• Mastics These generally do not harden. They are
  sufficiently viscous to prevent sagging but offer
  little mechanical strength. They are often poured
  into joints.

26
Elastomeric Sealants

• These set to a tough but elastic condition by a
  number of different processes.
• Two pack types must be mixed on site but one pack
  systems are more convenient because they are
  supplied in cartridges ready for use.
• Polyurethane sealants have very strong adhesive
  properties as well as sealing.
• Silicones and polysulphides are the more common,
  and cheaper, materials.

27
10.2 Sealants

• Types of Sealant
• Joint Details

28
Deformation of Elastomeric sealants
29
Joint Details
Incorrect Correct
30
Detailed joint systems