CE 8202: CIVIL ENGINEERING

MATERIALS 1

ADHESIVES

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DEFINITION

❑Adhesives, commonly referred to as glue, cement, or paste,

are non-metallic substances used to bond two surfaces
together.
❑An adhesive must wet the surfaces, adhere to the surfaces,
develop strength after it has been applied, and remain
stable
❑They are essential in various applications across industries
due to their ability to join different materials effectively.

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REQUIREMENTS OF ADHESIVES

❑The requirements of adhesives vary depending on the

application, material, and environment in which they are
used.
❑However, there are several general properties that
adhesives should possess to ensure their effectiveness in
bonding materials.
❑These requirements can be divided into mechanical,
physical, chemical, and performance categories:

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 Requirements of Adhesives cont…
1. Mechanical Requirements
➢ Bond Strength: The adhesive must provide a strong bond between the
substrates to withstand the forces and loads applied to the bonded joint.
➢ Shear strength: The ability of the adhesive to resist forces that try to slide the
bonded surfaces apart.
➢ Tensile strength: The ability to resist pulling forces that try to separate the
bonded surfaces.
➢ Peel strength: The resistance to forces that try to pull the adhesive off in a
peeling motion.
➢ Durability: The adhesive should maintain its bonding strength over time,
resisting aging, weathering, and environmental stress.

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Requirements of Adhesives cont…
2. Physical Requirements
➢ Viscosity: The adhesive must be fluid enough to wet the surface of
the materials and form a strong bond but not too runny that it cannot
hold its position.
➢ Curing Time: The adhesive must set or cure within a reasonable
time frame. This can be influenced by temperature, humidity, and the
adhesive type
➢ Open Time: This is the time during which the adhesive remains
workable and can still be adjusted before it starts to set or harden.

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 Requirements of Adhesives cont…
3. Chemical Requirements
➢ Adhesion to Substrates: The adhesive must have good wetting
and bonding properties for the materials it is designed to bond. It
should form a strong molecular bond with the surfaces, which
may involve surface preparation or priming.
➢ Chemical Resistance: The adhesive should resist degradation
when exposed to chemicals, solvents, or harsh environments,
depending on the application
➢ Thermal Resistance: The adhesive must withstand the expected
range of temperatures without losing its bond strength.
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 Requirements of Adhesives cont…

4. Application Requirements
➢Ease of Use: Adhesives should be easy to apply,
whether they are in liquid, paste, or film form. The
adhesive should also be easy to handle and should not
require complicated processes for application.
➢Workability: The adhesive should allow sufficient time
for alignment and positioning of parts before setting or
curing.
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THEORIES OF ADHESIVES
❑Theories behind how adhesives work are essential in
understanding their behavior, properties, and applications.
❑ There are several accepted theories on how adhesives works
▪ Surface roughness mechanism
▪ Adhesion by chemical bond
▪ Diffusion theory
▪ Physical adsorption
▪ Electrostatic theory
▪ Capillary action
▪ Interfacial tension theory

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Theories of Adhesives cont…
1. Surface roughness mechanism
❑ The adhesive bonds by physically interlocking with the
surface asperities (microscopic roughness) of the substrates.
❑ When an adhesive is applied, it fills the microscopic gaps and
rough areas of the substrate surface.
❑ As the adhesive cures, it hardens, and the interlocking
between the adhesive and the rough surface holds the surfaces
together
❑ This theory is particularly important in situations where rough,
textured surfaces are involved.
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Theories of Adhesives cont…
2. Adhesion by chemical bond
❑ Adhesion occurs when the adhesive forms chemical bonds
with the substrate material
❑ The adhesive molecules interact with the molecules of the
substrate to form chemical bonds, such as covalent or ionic
bonds, which provide a strong bond between the adhesive
and the surface.
❑ This is common with specialized adhesives like epoxies and
some types of superglue, which form chemical bonds with
specific materials (metals, plastics, etc.).
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Theories of Adhesives cont…
3. Diffusion theory
❑Adhesion is achieved when the adhesive molecules
diffuse into the surface of the material being bonded.
❑For materials like plastics or rubber, the adhesive
molecules diffuse into the surface of the substrate
(particularly when it is porous), creating a bond that is
driven by molecular interpenetration.
❑This theory is often applied to bonding similar or
compatible materials, like rubber-to-rubber or rubber-to-
metal. 11
Theories of Adhesives cont…
4. Physical adsorption

❑The adhesive works by using weak intermolecular forces such

as van der Waals forces, which are attractive forces between
molecules.
❑The adhesive forms a bond by physically adhering to the
surface of the substrate due to weak forces like van der Waals
forces, hydrogen bonding, or electrostatic interactions.
❑ This is the case with some low-energy surfaces, such as glass
or smooth plastics, where strong chemical bonding is less
common. 12
Theories of Adhesives cont…
5. Electrostatic theory
❑ Adhesion is achieved through electrostatic interactions
between the adhesive and the substrate.
❑ Electrostatic attraction occurs when the adhesive has a
positive or negative charge, and the substrate has an opposite
charge. This attraction creates a bond between the two
materials.
❑This can be seen in some tapes and adhesives that work with
specific conductive or charged materials.

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Theories of Adhesives cont…
6. Capillary action

❑ Adhesion is enhanced by the ability of the adhesive to

penetrate into the surface pores of the substrates.
❑ When applied, the adhesive flows into the microscopic gaps
and pores of the substrate. This increases the surface area for
bonding and helps create a strong bond once the adhesive
hardens.
❑ This theory is significant in porous substrates like wood,
textiles, or concrete.

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 Theories of Adhesives cont…
7. Interfacial tension theory
❑ The strength of the bond is determined by the interfacial energy (surface
tension) between the adhesive and the substrates.
❑ When the adhesive is applied, it forms an interface with the substrate
surface.
❑ The bond strength is influenced by the relative surface energies of the
adhesive and the substrates.
❑ good bond occurs when the adhesive "wets" the substrate effectively,
meaning that the adhesive can spread out over the surface.
❑ This is particularly relevant when working with different materials like
metal to plastic or plastic to plastic.
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PRACTICAL APPLICATIONS OF ADHESIVES

❑ Adhesives are widely used in various industries

and everyday applications due to their versatility,
efficiency, and ability to bond different materials
❑ Practical applications of adhesives across different
fields:
▪ Construction Industry
▪ Furniture and Woodworking

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PRACTICAL APPLICATIONS OF ADHESIVES

❑ Adhesives are widely used in various industries

and everyday applications due to their versatility,
efficiency, and ability to bond different materials
❑ Practical applications of adhesives across different
fields:
▪ Construction Industry
▪ Furniture and Woodworking

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Practical Applications of Adhesives cont…
Bonding of Building Materials: Adhesives are used for bonding
tiles, flooring, drywall, and insulation materials. For example, tile
adhesives (thin-set mortar) provide strong bonds between tiles and
substrates.
Sealants: Sealants in construction are used to fill gaps and prevent
water ingress, especially in windows, doors, and roofing. Silicone-
based sealants are commonly used for their flexibility and durability.
Structural Bonding: In modern construction, adhesives are used to
bond metals, glass, and composites, reducing the need for traditional
mechanical fasteners (like nails and screws).

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Practical Applications of Adhesives cont…

❑ Wood Bonding: Adhesives like wood glue (e.g., PVA or

polyurethane) are used for bonding wood pieces together, such
as in furniture assembly and cabinetry.
❑ Laminating and Veneering: Adhesives are used to bond
veneer sheets to wood surfaces, creating high-quality, durable
finishes in furniture.
❑ Surface Protection: Adhesives in the form of films or tapes are
used to protect surfaces during maintenance, painting, or
renovation. Masking tapes, for instance, are used to protect
surfaces from paint splatter. 19
Practical Applications of Adhesives cont…

❑ Railways: Adhesives are used in railway carriages and wagons

for bonding interior panels, flooring, and even to seal joints.
They provide a lightweight, reliable solution for assembly.

❑ Vehicle Assembly: Adhesives are used in vehicle

manufacturing to bond parts such as windshields, trim,
bumpers, and interior panels. They help reduce weight
(compared to mechanical fasteners)

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TYPES OF ADHESIVES

Adhesives are categorized in several types;

❑ Natural
❑ Thermoplastic
❑ Thermosetting
❑ Rubber

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Types of Adhesives cont…
1. Natural Adhesives

❑Natural adhesives are substances derived from renewable,

biodegradable, and often non-toxic materials that are used to

bond various surfaces.

❑They have been used for thousands of years in different

cultures, from ancient times in construction to modern-day

applications.

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 Types of Adhesives cont…
2. Thermoplastic adhesives

❑ are a type of adhesive that become soft and tacky when

heated and harden when cooled.
❑ These adhesives are typically made from polymers that can
be melted and reformed multiple times without undergoing
any chemical change.
❑ They are commonly used in various applications due to
their versatility, ease of use, and ability to bond different
materials.
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 Types of Adhesives cont…
3. Thermosetting
❑ are a type of adhesive that harden or "set" when heated and
undergo a chemical change in the process, making them
irreversible.
❑ Once these adhesives have cured, they cannot be re-melted or
re-worked by heating again.
❑ They are known for their strong bonding properties and are
commonly used in a variety of industrial, automotive,
construction, and electronics applications.

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Types of Adhesives cont…
4. Rubber

❑are a type of adhesive made using rubber-based compounds,

offering strong bonding properties, flexibility, and durability.
❑They are widely used in various applications, from
automotive and construction industries to arts and crafts.
❑Rubber adhesives can bond to a variety of materials like
metals, plastics, wood, and fabrics.
❑Examples are natural and synthetic rubber

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