Page 263 - Mechanical Engineer's Data Handbook

ENGINEERlNG MATERIALS 25 1

6. I6 Adhesives

Adhesives are materials which are used to join solids The bond is generally permanent.
(adherents) by means of a thin layer which adheres to A smooth finish is usually obtained.
the solids. At some stage the adhesive is liquid or
plastic and sets to form a solid. In the final stage it may Disadvantages of adhesive bonding
be rigid or flexible. A curing time, which may be long, is required for
In engineering, joining by adhesives has in many optimum strength.
cases replaced other methods such as soldering, braz- The adhesive may be flammable or toxic.
ing, welding, riveting and bolting. The bond may be affected by the environment, e.g.
heat, cold, or humidity.
Advantages of adhesive bonding Adhesives may be classified as follows:
Dissimilar materials may be joined, e.g. plastics to
metal. (1) natural adhesives,
Large bonding areas are possible. (2) elastomers,
Uniform stress distribution and low stress concentra- (3) thermoplastics,
tion is obtained. (4) thermosets, and
Bonding is usually carried out at low temperature. (5) Other adhesives.

6.16. I Natural adhesives Vegetable glues

These are set by solvent evaporation. They are gen- These! are bascd on starch or dextrine from starch and
erally of low strength and are weakened by moisture are available either as a powder to be mixed with water
and mould. They are restricted to joining low-strength or ready mixed. The shear strength is low but they are
materials. only used for paper and cardboard. Resistance to
water and high temperatures is low.
Animal glues
Casein
These are made from collagen (from the bones and
skins of animals) with sugar and glycerol added for This is a protein glue made from milk precipitated with
increased flexibility. They are available in sheet acid. It is supplied as a powder to be mixed with water
(Scotch Glue), bead and powder forms, all of which and is used for joining wood, paper, cloth and
dissolve in water at 60°C, and also as a liquid with asbestos. Latex/casein is used for foil/paper lamina-
gelling inhibitors. Degradation occurs at about tions. Casein has better resistance to water and better
100 “C. These glues have a long ‘pot life’ a long dry life strength than animal and fish glues. Other protein
and a ‘tacky’ stage useful for ‘initial set’. They will join glues are made from blood, soya bean residue, etc.
wood, paper, leather, cloth and most porous materials.
6.16.2 Elastomer adhesives
Fish glues
These adhesives are based on natural and synthetic
These have similar applications to animal glues but are rubbers set by solvent evaporation or heat curing.
usually liquid at room temperature and have better They have relatively low shear strength and suffer from
resistance to water and a better recovery of strength on creep and are therefore used for unstressed joints. They
drying. are useful for flexible bonds with plastics and rubbers.
‘Contact adhesives’ use rubber in a solvent and will
join many materials.