PART 10 ADHESIVES AND SEALANTS. Adhesives Sealants Tapes

Transcription

1 PART 10 ADHESIVES AND SEALANTS Adhesives Sealants Tapes

2 10.1 Adhesives Safety Types of Adhesive Surface Preparation Setting Processes Types of Joint Modes of Failure

3 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.

4 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

5 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

6 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.

7 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.

8 The adhesives are categorized for use as follows WBP Weather and boil proof (as in exterior grade plywood) BR WBP MR Boil resistant - not as durable as Moisture resistant Int Interior - resistant to short exposure to cold water only

9 Durability of Adhesives

10 Contact Joint Types of joint Contact Joint Gap Filling Joint Fillet

11 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.

12 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.

13 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.

14 PART 10 ADHESIVES AND SEALANTS 10.1 Adhesives 10.2 Sealants

15 10.2 Sealants Types of Sealant Joint Details

16 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.

17 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.

18 Deformation of Elastomeric sealants

19 Joint Details Incorrect Correct

20 Detailed joint systems

21 GLUE TYPES AND USES

22 White glue (polyvinyl acetate, or PVA): PVA glue is a white liquid, usually sold in plastic bottles. It is recommended for use on porous materials -- wood, paper, cloth, porous pottery, and nonstructural wood-to-wood bonds. It is not water resistant. Clamping is required for 30 minutes to 1 hour to set the glue; curing time is 18 to 24 hours. School glue, a type of white glue, dries more slowly. Inexpensive and nonflammable, PVA glue dries clear.

23 Epoxy: Epoxies are sold in tubes or in cans. They consist of two parts -- resin and hardener -- that must be thoroughly mixed just before use. They are very strong, very durable, and very water resistant. Epoxies are recommended for use on metal, ceramics, some plastics, and rubber; they aren't recommended for flexible surfaces.

24 Clamping is required for about 2 hours for most epoxies. Drying time is about 12 hours; curing time is one to two days. Epoxy dries clear or amber and is more expensive than other adhesives.

25 Cyanoacrylate: Also called super or instant glue, cyanoacrylate is similar to epoxy but is a one-part glue. These glues form a very strong bond and are recommended for use on materials such as metal, ceramics, glass, some plastics, and rubber; they aren't recommended for flexible surfaces. Apply sparingly. Clamping is not required; curing time is one to two days. Cyanoacrylates dry clear.

26 Contact cement: A rubber-base liquid sold in bottles and cans, contact cement is recommended for bonding laminates, veneers, and other large areas and for repairs. It can also be used on paper, leather, cloth, rubber, metal, glass, and some plastics because it remains flexible when it dries. It is not recommended for repairs where strength is necessary. Contact cement should be applied to both surfaces and allowed to set; the surfaces are then pressed together for an instant bond.

27 No repositioning is possible once contact has been made. Clamping isn't required; curing is complete on drying. Contact cement is usually very flammable

28 Polyurethane glue: This high-strength glue is an amber paste and is sold in tubes. It forms a very strong bond similar to that of epoxy. Polyurethane glue is recommended for use on wood, metal, ceramics, glass, most plastics, and fiberglass. It dries flexible and can also be used on leather, cloth, rubber, and vinyl. Clamping is required for about 2 hours; curing time is about 24 hours. Polyurethane glue dries translucent and can be painted or stained. Its shelf life is short, and it is expensive.

29 Silicone rubber adhesive or sealant: Silicone rubber glues and sealants are sold in tubes and are similar to silicone rubber caulk. They form very strong, very durable waterproof bonds, with excellent resistance to high and low temperatures. They're recommended for use on gutters and on building materials, including metal, glass, fiberglass, rubber, and wood.

30 They can also be used on fabrics, some plastics, and ceramics. Clamping is usually not required; curing time is about 24 hours, but the adhesive skins over in less than 1 hour. Silicone rubber adhesives dry flexible and are available in clear, black, and metal colors.

31 Household cement: The various adhesives sold in tubes as household cement are fast-setting, low-strength glues. They are recommended for use on wood, ceramics, glass, paper, and some plastics. Some household cements dry flexible and can be used on fabric, leather, and vinyl. Clamping is usually not required; setting time is 10 to 20 minutes, curing time is up to 24 hours.

32 Hot-melt adhesive: Hot-melt glues are sold in stick form and are used with glue guns. A glue gun heats the adhesive above 200 degrees F. For the best bond, the surfaces to be joined should also be preheated. Because hot-melt adhesives are only moderately strong and bonds will come apart if exposed to high temperatures, this type of glue is recommended for temporary bonds of wood, metal, paper, and some plastics and composition materials

33 Clamping isn't required; setting time is 10 to 45 seconds, and curing time is 24 hours.