PART 10 ADHESIVES AND SEALANTS

• 10.1 Adhesives

• 10.2 Sealants

Glued Segmental Bridge Construction

PART 10 ADHESIVES AND SEALANTS

• 10.1 Adhesives

• 10.2 Sealants

10.1 Adhesives

• Safety

• Types of Adhesive

• Surface Preparation

• Setting Processes

• Types of Joint

• Modes of Failure

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.

10.1 Adhesives

• Safety

• Types of Adhesive

• Surface Preparation

• Setting Processes

• Types of Joint

• Modes of Failure

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

10.1 Adhesives

• Safety

• Types of Adhesive

• Surface Preparation

• Setting Processes

• Types of Joint

• Modes of Failure

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

10.1 Adhesives

• Safety

• Types of Adhesive

• Surface Preparation

• Setting Processes

• Types of Joint

• Modes of Failure

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.

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.

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

Durability of Adhesives

10.1 Adhesives

• Safety

• Types of Adhesive

• Surface Preparation

• Setting Processes

• Types of Joint

• Modes of Failure

Types of joint

Contact Joint

Contact Joint

Gap Filling Joint

Fillet

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.

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.

10.1 Adhesives

• Safety

• Types of Adhesive

• Surface Preparation

• Setting Processes

• Types of Joint

• Modes of Failure

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.

Shear stress along the length of an adhesive joint in tension

Shear Stress

Solve the problem by tapering the ends.

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.

PART 10 ADHESIVES AND SEALANTS

• 10.1 Adhesives

• 10.2 Sealants

10.2 Sealants

•Types of Sealant

•Joint Details

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.

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.

10.2 Sealants

•Types of Sealant

•Joint Details

Deformation of Elastomeric sealants

Joint Details

Incorrect Correct

Detailed joint systems