Building Materials, Third Edition

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g
 2ƒ Adhesive should have more cohesive strength than either of the surfaces
being held together. Usually the adhesive becomes more brittle as its cohesive strength is
increased.

e™ For an adhesive to hold two surfaces together the former should form a strong
bond at each of the interfaces between the surface and the adhesive, and the adhesive must
have strong cohesive strength. Since adhesive failure occurs in the weakest bond, failure can
occur at either interface, as also for cohesive failure. Failure may also occur inside the adhesive
itself. In practice the bond failure at the interface is rare. Regardless of the quality of the
adhesive clean surfaces are necessary to get the best results.

p When the liquids are stirred, they become temporarily more fluid because of alignment
of their tiny crystals. The fluid thickens as soon as stirring is stopped. This property, called
thixotropy of adhesives and paints helps to prevent a sag or run when these coatings are
applied on vertical surfaces. If the viscosity of a liquid increases with the shear stress of stirring,
it is called dilatency. Since the viscosity decreases with increase in temperature, an adhesive
should be applied while hot.

‡—˜ Adhesive must wet the surface thoroughly as it spreads. It must be able to flow
into the surface crevices displacing dirt, moisture, and trapped air. For this purpose the surface
tension of adhesive can be lowered by adding a surfactant wetting agent.

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Following are the types of adhesives in use.

y—™2ƒ
2„2e  are applied to each of the two surfaces and the solvent
is allowed to escape before the two surfaces are put together. Solvents are classified as active
and nonpolar, e.g., ethyl acetate, methyl ketone, and poor and polar such as aliphatic paint
thinner.

v—2e  are natural or synthetic rubber or vinyl copolymers. These water-dispersed
or latex adhesives contain, elastomeric film former, emulsifiers, thickeners, and antifoaming
agents. The examples of synthetic rubber-based adhesives are styrene butadiene and neoprene.

‡—E  2e  depend on natural materials for bonding. The bond of these can
be destroyed by soaking in water. The examples are glue made by hydrolysis of collagen
extracted from skin and bones of fish and animals. Natural adhesives are casein and soybean
used in wood working industry. Dextrin adhesives are made from starch for use with paper
products.

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E—™ —2e  In this type of adhesive solvent is not required. The examples are
epoxy adhesives. These are made by using a low-molecular-weight partially polymerised
polymer.

PIP eƒfiƒ„yƒ

Asbestos occurs in nature mostly in form of veins among rock of specific composition. Asbestos
minerals are fibrous in structure and split into fine fibres under mechanical force. Natural
asbestos may be subdivided into two groups: acid-resistant and non-acid-resistant.