Adhesive Bonding 2

ADHESIVE BONDING

MM 324

JOINING OF MATERIALS

Adhesive bonding and cementing

Adhesive bonding in wood, glass and ceramics is also called gluing, pasting and cementing.

Adhesive bonding in wood, glass and ceramics is also called gluing, pasting and cementing.

Joining of materials by inter-atomic or intermolecular bond through chemical reaction is called adhesive bonding.

Joining of materials by inter-atomic or intermolecular bond through chemical reaction is called adhesive bonding.

Adhesive bonding is the process of joining materials with the aid of a substance, acting as a chemical agent, capable of holding those materials together by surface attachment forces.

Adhesive bonding is the process of joining materials with the aid of a substance, acting as a chemical agent, capable of holding those materials together by surface attachment forces.

Chemical used for a joint is called an adhesive while substrates are called adherends.Chemical used for a joint is called an adhesive while substrates are called adherends.

ADHESIVE BONDING

“Joining of Materials and structures”, R W Messler, Elsevier Butterworth–Heinemann, 2004

Function of adhesive

• To join components.

• Comprehensive bonding with substrate.

• Stress distribution is on large area.

• Stress distribution is uniform.

• Viscoelastic nature of polymer gives flexibility in the joint.

• Provide sealing affect to stop leakage.

• To join components.

• Comprehensive bonding with substrate.

• Stress distribution is on large area.

• Stress distribution is uniform.

• Viscoelastic nature of polymer gives flexibility in the joint.

• Provide sealing affect to stop leakage.

Advantages of adhesive bonding

• High load carrying

• Low stress concentration

• Suitable for thin and thick structures.

• Little or no change in chemistry.

• Suitable for dissimilar materials.

• Sealant, insulator of heat and electricity.

• High strength to weight ratio.

• Absorb shock and vibration loads.

• Reduce galvanic and crevice corrosion.

• High load carrying

• Low stress concentration

• Suitable for thin and thick structures.

• Little or no change in chemistry.

• Suitable for dissimilar materials.

• Sealant, insulator of heat and electricity.

• High strength to weight ratio.

• Absorb shock and vibration loads.

• Reduce galvanic and crevice corrosion.

Disadvantages of adhesive bonding

• Sensitive to peal and cleavage

• Require careful joint preparation.

• Limited shelf life or working time of adhesives.

• Sometime curing time is too long.

• Repair is almost impossible.

• Sensitive to heat and organic solvents.

• Direct inspection is not possible.

• Adhesive are prone to bacterial attack.

• Sensitive to peal and cleavage

• Require careful joint preparation.

• Limited shelf life or working time of adhesives.

• Sometime curing time is too long.

• Repair is almost impossible.

• Sensitive to heat and organic solvents.

• Direct inspection is not possible.

• Adhesive are prone to bacterial attack.

Adhesive bonding

Atoms seek to give or take electrons to stable themselves. Due to exchange of electrons they are +/- charged. At the joint interface when both +&- forces become equal ionic bond is formed.

With covalent sharing of atoms covalent molecule is established.

Metallic bonds (delocalized)

Secondary bonding (van der waal’s)

Atoms seek to give or take electrons to stable themselves. Due to exchange of electrons they are +/- charged. At the joint interface when both +&- forces become equal ionic bond is formed.

With covalent sharing of atoms covalent molecule is established.

Metallic bonds (delocalized)

Secondary bonding (van der waal’s)

Natural AdhesivesAnimal-based adhesives (e.g., casein, collagen, gelatin)Plant-based adhesives (e.g., pitch, natural rubbers, asphalt)Mineral-based adhesives (e.g., sodium silicate, mineral-based sol-gels, calcium carbonate)

Synthetic Adhesives

Synthetic Organic Adhesives- Chemically-activated adhesives (e.g., cyanoacrylates, epoxies)- Heat or radiation-activated adhesives (e.g., one-component epoxies)- Evaporation or diffusion adhesives (e.g., phenolics)- Thermoplastic hot-melt adhesives- Pressure-sensitive (contact) adhesives

Synthetic Inorganic Adhesives- Portland cements- High-alumina, calcium aluminate cements- Mortars (e.g., gypsum)- Refractory cements- Dental cements- Glassy frits

Natural AdhesivesAnimal-based adhesives (e.g., casein, collagen, gelatin)Plant-based adhesives (e.g., pitch, natural rubbers, asphalt)Mineral-based adhesives (e.g., sodium silicate, mineral-based sol-gels, calcium carbonate)

Synthetic Adhesives

Synthetic Organic Adhesives- Chemically-activated adhesives (e.g., cyanoacrylates, epoxies)- Heat or radiation-activated adhesives (e.g., one-component epoxies)- Evaporation or diffusion adhesives (e.g., phenolics)- Thermoplastic hot-melt adhesives- Pressure-sensitive (contact) adhesives

Synthetic Inorganic Adhesives- Portland cements- High-alumina, calcium aluminate cements- Mortars (e.g., gypsum)- Refractory cements- Dental cements- Glassy frits

ADHESIVES

Theories of Adhesive bonding

1. Electrostatic theory of adhesion

Development of electrostatic forces of attraction between the adhesive and the adherends at their interface.

- Difference in electronegativity (Ionic bonding)

- Polarization at the interface between adhesive and the adherend.

- Dipole interaction

Insulator or dielectrics e.g., polymers, ceramics and

glasses.

(Emission of light and charged and neutral particles when adhesive bonds are opened in a vacuum)

1. Electrostatic theory of adhesion

Development of electrostatic forces of attraction between the adhesive and the adherends at their interface.

- Difference in electronegativity (Ionic bonding)

- Polarization at the interface between adhesive and the adherend.

- Dipole interaction

Insulator or dielectrics e.g., polymers, ceramics and

glasses.

(Emission of light and charged and neutral particles when adhesive bonds are opened in a vacuum)

2. Diffusion theory of adhesion When two materials are at least partially soluble in one another,

they can and do form a solution at their interface.

- Solid state diffusion (both in the solid form, slow)

- Liquid-solid diffusion (liquid adhesive and solid adherend, fast)

Stronger bonds result in chemically similar materials e.g., polymers.

Interdiffusion and entaglement of long polymeric chains.

Used in wood and polymers. Difficult to apply for metals or ceramics.

- Solvent cementing of thermoplastics.

- Fusion bonding of thermoplastics.

2. Diffusion theory of adhesion When two materials are at least partially soluble in one another,

they can and do form a solution at their interface.

- Solid state diffusion (both in the solid form, slow)

- Liquid-solid diffusion (liquid adhesive and solid adherend, fast)

Stronger bonds result in chemically similar materials e.g., polymers.

Interdiffusion and entaglement of long polymeric chains.

Used in wood and polymers. Difficult to apply for metals or ceramics.

- Solvent cementing of thermoplastics.

- Fusion bonding of thermoplastics.


3. The mechanical theory of adhesion For an adhesive to function properly it must penetrate the microscopic

asperities (e.g., peaks and valleys, open pores, and crevices) on the

surface of adherends, and displace any trapped air at the interface.

Mechanical interlocking and anchoring of adhesive, no chemical bonding.

Open celled polymeric foams, porous ceramics, PMCs, wood and metals

having porous native oxide layer. Etched glasses and abraded metals.

Chemical etching and mechanical abrading important steps in adhesive bonding.

1. Enhancing mechanical interlocking or anchoring.

2. Creating clean and wettable surface.

3. Increase in bond area due to an increase in the surface area.

4. Formation of a chemical reactive surface.

3. The mechanical theory of adhesion For an adhesive to function properly it must penetrate the microscopic

asperities (e.g., peaks and valleys, open pores, and crevices) on the

surface of adherends, and displace any trapped air at the interface.

Mechanical interlocking and anchoring of adhesive, no chemical bonding.

Open celled polymeric foams, porous ceramics, PMCs, wood and metals

having porous native oxide layer. Etched glasses and abraded metals.

Chemical etching and mechanical abrading important steps in adhesive bonding.

1. Enhancing mechanical interlocking or anchoring.

2. Creating clean and wettable surface.

3. Increase in bond area due to an increase in the surface area.

4. Formation of a chemical reactive surface.



4. Adsorption theory of adhesion Adhesion due to secondary bonding between adhesive

and the adherends.

Wetting as an indication of adhesion.

The degree of wetting is controlled by the balance between the surfaceenergy or surface tension of the liquid–solid interface versus the liquidvapor and solid–vapor interfaces it replaces.

It is believed that permanent adhesion results primarily from theforces of chemical bonding.

4. Adsorption theory of adhesion Adhesion due to secondary bonding between adhesive

and the adherends.

Wetting as an indication of adhesion.

The degree of wetting is controlled by the balance between the surfaceenergy or surface tension of the liquid–solid interface versus the liquidvapor and solid–vapor interfaces it replaces.

It is believed that permanent adhesion results primarily from theforces of chemical bonding.

(1) Concentration of low-molecular-weight constituents for organic-type adhesives or low-density constituents for inorganic types of adhesives due to separation during bonding;

(2) Weakly attached oxide or other tarnish layers on metals;

(3) Contamination of the adherend(s) by oil, grease, or adsorbed water (in some cases) due to improper cleaning; and

(4) Entrapped air at the interface.

(1) Concentration of low-molecular-weight constituents for organic-type adhesives or low-density constituents for inorganic types of adhesives due to separation during bonding;

(2) Weakly attached oxide or other tarnish layers on metals;

(3) Contamination of the adherend(s) by oil, grease, or adsorbed water (in some cases) due to improper cleaning; and

(4) Entrapped air at the interface.

Weak boundary layer theory

If an adhesive bond is properly made, the joint will fail in either the adhesive or one of the adherends, whichever has the lower cohesive strength.

If an adhesive bond fails at a lower strength than expected for either of these, it does so because it failed through a weak boundary layer at the interface between the adhesive and one of the adherends.

If an adhesive bond is properly made, the joint will fail in either the adhesive or one of the adherends, whichever has the lower cohesive strength.

If an adhesive bond fails at a lower strength than expected for either of these, it does so because it failed through a weak boundary layer at the interface between the adhesive and one of the adherends.

Schematic illustration of the various mechanisms that can lead to adhesion during adhesive bonding: (a) mechanical interlocking of adhesive into asperities; (b) secondary bonding from adsorption with proper wetting from surface-energy effects; (c) electrostatic attraction from charge separation; and (d) diffusion of atoms or molecules back and forth between adhesive and adherends. Also, (e) the formation of a weak boundary layer leads to the adhesive failure of joints.

Tackiness & Stefan’s equation

Resistance offered to separate substrates joined by adherent is called tackiness

• It increases with the viscosity of adhesive• It also depend on the applied force and time

Reasons are considered microscopic contact with substrate, electrostatic force, product of time and pressure

Stefan's equation: f t = ¾ (πŋa4) [1/(h12-h2

2)]

h1, h2 are initial and final clearance between adherends, ŋ is viscosity of the adhesive, a is the diameter or other dimensions of the contact, f is the force required to separate the surfaces, t is the time required to separate the surfaces.

For considerable thicker layer Stefan’s equation become;

f t = ¾ (ŋa2/h12)

Resistance offered to separate substrates joined by adherent is called tackiness

• It increases with the viscosity of adhesive• It also depend on the applied force and time

Reasons are considered microscopic contact with substrate, electrostatic force, product of time and pressure

Stefan's equation: f t = ¾ (πŋa4) [1/(h12-h2

2)]

h1, h2 are initial and final clearance between adherends, ŋ is viscosity of the adhesive, a is the diameter or other dimensions of the contact, f is the force required to separate the surfaces, t is the time required to separate the surfaces.

For considerable thicker layer Stefan’s equation become;

f t = ¾ (ŋa2/h12)

Factors influencing adhesive selection

Strong adhesive bond Clean surface (Bonding is a surface phenomenon)

Dirt, grease, cutting coolants and lubricants, ink or crayon marks, visible water (including dew, frost, and ice), obvious moisture (e.g., high humidity), and weak surface scales (e.g., oxides, sulfides, and other tarnishes) must be thoroughly removed.

Cleaning Chemical, Physical, Mechanical or Combination

Surface treatment steps

1. Solvent cleaning: Removal of soil from the surface without physically or chemically altering the adherends.

(a) Vapour degreasing: Removal of loose adhering particulate matter, dirt, or light soluble soils using hot solvent (e.g., trichloroethylene) vapor that condenses on the adherend and flows away debris.

(b) Solvent wiping, immersion, or spraying: Several different solvents (e.g., ethanol,

methanol, acetone, or trichloroethylene) for the removal of light or heavy soluble soils (e.g., oils, greases, waxes), dirt, and particulate matter.

(c) Ultrasonic vapor degreasing: The removal of more tenacious soil and insolublesthrough the scrubbing action of collapsing bubbles (i.e., cavitation) arising from ultrasonic excitation of a liquid solvent.

Surface preparation for adhesive bonding


(d) Ultrasonic cleaning in solvent: The scrubbing action of collapsing bubbles during solvent immersion break looses tenacious contaminants, that is followed by a liquid solvent rinse to remove residues.

Aqueous solutions with surfactants, detergents, or alkaline or acid cleaners can be used. This process produces high-quality cleaning but is not as efficient as vapor-cleaning processes.

2. Intermediate cleaning: A process of removing soil or scale from an adherend surface with physical, mechanical, or chemical means,

singly or in combination, without altering the adherend chemically.

These cleaning methods are aggressive enough that they may remove some small amounts of the parent material.

Mechanical methods: Grit blasting, wire brushing, sanding, abrasive scrubbing, or scraping or filing.

Physical methods: Electrical corona discharge and various high-speed ablativeprocesses using flames, plasmas, or lasers.

Chemical methods: Alkaline, acid, and detergent cleaning (often with scrubbing).

3. Chemical treatment: The process of treating a clean adherend surface by chemical means, with the objective of changing the surface of the adherend chemically to improve its adhesion qualities.


Acid or alkaline etching (pickling) Removal of oxide, smeared surface, or roughening of surface on a microscopic scale.

Activation of the surface Removal of adsorbed gases, intervening oxides, or other scales, and the exposure of atomically clean material.

4. Priming: Application of a dilute solution of the adhesive’s active bonding agent in a suitable organic solvent to the surface of the adherend to produce a dried film thickness of 0.0015–0.05mm (0.00006–0.002 in.).

Functions:

(i) Protection from oxidation

(ii) Improvement in wetting

(iii) Barrier layer to prevent undesirable reactions

(iv) Holds the adhesive during assembly

(v) Coupling agents (if present) in the primer helps adhesion

Failure of adhesive bonding There are several mechanism for failure of adhesive joints but the

predominant are;

Adhesive failureInterfacial failure between or seemingly between (but actually just adjacent to) the actual interface between the adhesive and one of the adherends. It tends to be indicative of a weak boundary layer, often due to improper preparation.

Cohesive failureFailure in the form of physical separation that results in a layer of adhesive remaining on both adherend surfaces or, more rarely, when the adherend fails before the adhesive fails, with separation occurring totally within one of the adherends.

Mix failureJoint failure in service or during testing is usually neither purely adhesive norcohesive; it is usually a mixture of both modes.

The operative failure mode is often expressed as a percentage of cohesive or adhesive failure, with an ideal failure being 100% cohesive.

Causes of failure

• Surface contaminations

• Poor wetting of adherent

• Improper selection of adhesive

• Improper joint design

• Stress on the joint

Major Causes of Failure in Adhesive-Bonded Joints

(1) Providing the maximum bonding area possible in the design to help spread the

applied load and minimize stress in the adhesive;

(2) Designing the joint in such a way as to force loads to be transmitted to the joint in

favorable loading directions (e.g., pure compression, pure tension, or—most

achievable—pure shear)

(3) Orienting joints or designing joint elements or reinforcements in such a way as to

minimize unfavorable out-of-plane (i.e., peel or cleavage) loading;

(4) Designing joints to ensure uniformity in thickness of the adhesive layer and keeping

this layer as thin as practical to maximize tensile and shear strengths

(5) Designing joints and their elements in such a way that volatile components of

the adhesive can be expelled or absorbed by the adherends;

(6) Designing joints from combinations of materials that will minimize stresses arising

from differences in coefficients of thermal expansion (especially for hard brittle

adhesives, such as many of the inorganic types, including cements and mortars);

(7) Designing joint elements and assemblies in such a way as to facilitate adhesive

application, curing, and inspection.

Design Considerations

The aim of good adhesive-bonded joint design is to obtain the maximum strength for a given area of bond for structural efficiency

1. Mech. props. of Adherends 2. Residual stresses in the joint 3. True interfacial contact 4. Type of loading 5. Joint geometry

The ideal adhesive-bonded joint is one in which, under all practical loading conditions, the adhesive is stressed in the direction in which it best resists failure (i.e., shear).

TYPES OF FORCES AND JOINTSLap shear strengths are directly proportional to the extent (or length) of the overlap, but the unit strength actually decreases with the width of the overlap.

The optimum shear strength of a bonded joint is largely dependent on the shear modulus of the adhesive and its optimum thickness.

Joint type versus load intensity.

SCARF JOINT

The maximum shear stress in the adhesive occurs at the ends of the lap–joint overlap and is given by:

tmax / tmean = (P/2)1/2 coth (P/2)1/2

where tmean is the mean (or average) applied shear stress ( = F/bl), and P is given by:

P = Gl2 / Et1t2

where G is the shear modulus of the adhesive, l is the length of the joint or the overlap, E is Young’s (tensile) modulus for the adherends (assumed to be the same), t1 is the thickness of the adherends (also assumed to be the same), and t2 is the thickness of the adhesive.

Analysis of lap joints


As P becomes larger, the degree of stress concentration τmax / τmean approaches 1.0

(1) the adhesive gets stiffer relative to the adherend; (2) the modulus of the adherend gets lower relative to the adhesive; (3) the extent of the overlap increases; and (4) either the adherend or adhesive or both get thinner.


k relate the bending moment on the adherend at the end of the overlap, Mo the applied in-plane loading, t thickness of the adherend (assumed to be the same for both adherends).

- The two opposing forces applied to the single lap joint are not co-linear, so there will be some bending applied to the joint in addition to the in-plane tension.

- The adherends are not completely rigid; they bend, allowing the joint to rotate in an attempt to bring the load lines into co-linearity.

Very small loads No rotation MO = Ft/2 and k = 1.0

Higher applied loads lead to severe out-of-plane stresses at the ends of overlaps for a single lap joint, and these lead to the joint’s failure in peel.

Analysis of lap jointsIncreasing length or width

Increasing the overall surface area under the joint

Consideration of properties and use of the adhesive and the environment

Some techniques for improving a joint’s resistance to peelingloads

(1) Riveting or spot welding (2) Beading the end of the thin member of the joint to

provide increased stiffness from moment of inertia; (3) Increasing the width of the thin member at the ends of

the overlap; or (4) Increasing the stiffness of the adherend.

The stiffness of joints composed of thin adherends can be increased by using doublers above or below the primary adherend, strong-back stiffeners (e.g., Ts, Ls, inverted Ys, Zs, etc.), formed beads, or other techniques.

CEMENT AND MORTAR

Mortar is an all-encompassing term for the bonding material a multi-component material system) used to join masonry units

Chemically, mortars are mixtures of inorganic materials known as ceramics,

The most popular mortar is Portland cement.

Ceramic components comprising the cement or mortar bond to water molecules by forming hydrogen bonds with the water.

Full hydration takes time to occur, so cements and mortars require time to cure.

It might take only hours or a day to become ‘‘set’’ to appear hard, but it often can take as long as 20 or more days to develop full strength.

Maximize the amount of surface area or interface between the masonry units and the mortar or cement.

Joints are made ‘‘tortuous,’’ running in many different—and preferably orthogonal - directions to keep some significant portion of all joints loaded in shear as opposed to out-of-plane tension,

Binder

Function is to create chemical bonds between itself and the adherends. e.g., epoxy resin, cement etc.

Hardener

A hardener is a substance added to certain types of adhesive bases that require a chemical reaction to cause the adhesive to cure.

Accelerator, inhibitors or retardants

Substances that are added to control the rate of curing, particularly of thermosetting polymer-type adhesives. Retardants slow the rate of curing, while inhibitors arrest it or limit it.

Diluents, solvent and thinnerDiluents are components that are added to an adhesive base to reduce the concentration of the base agent

Diluents normally remain in the adhesive after curing or setting.

Solvents are always liquid to thin the consistency of the adhesive base. They evaporate or are absorbed by the adherends to allow the adhesive to set.

Adhesive and its constituents

Filler

Generally, non-adhesive substances used to impart special working characteristics or properties to the adhesive system (e.g., metal flakes to impart electrical conductivity).

Carrier

A paper or fabric backing to a permanently tacky thermoplastic or a semi-cured thermoset adhesive to facilitate handling during application. (A carrier can, if it remains with the adhesive in service, serve as a reinforcement.)

Reinforcements

Particles, chopped or continuous fibers, or meshes added to the adhesive (or, for the latter, embedded in the adhesive layer) to add strength to the bonded joint.

Adhesive and its constituents

Classification of adhesives

• Natural and synthetic adhesives

• Organic and inorganic adhesives

• Structural and non structural adhesives

• Difference in chemical composition

• Difference in physical form

• Classification by application

Natural and synthetic adhesives

• If binding constituent of the adhesive is derived from

natural source is called natural adhesives e.g. pitch,

collagen, rubber etc.

• Active part of synthetic is made by synthesis of chemical

substances e.g. thermosetting plastic, thermoplastic

materials and all structural adhesive other then cement

are made synthetically

Organic and inorganic adhesives

• If active agent of the adhesive is based on organic

molecule it is called organic adhesive e.g. epoxies,

acrylics, urethanes, phonemics, silicones etc.

• Inorganic adhesives are consist of metallic (Al, Mg,

Fe) and non metallic (Si, Ca) compounds e.g. cement

of different kind, clays, lime etc.

Structural and non structural

Structural• Sustain load• Tolerate environment• Provide needed service life

Non structural• Holding components• Sealing against fluids• Impact absorption• Electrical, water, or thermal insulations

Classification based on composition

• Thermosetting (can not be heated or softened once they cured e.g.

acrylics, epoxies, polyester, polyimides, polyesters, silicones, urea

and melamine formaldehyde etc)

Used where good shear strength from RT to 260oC, good resistance

to heat with little elastic or creep deformation under load above RT,

good resistance to organic and inorganic solvents. Only fair peel

strength compared to thermoplastics.

• Thermoplastic (can be softened and formed again and again e.g.

acrylic, polyvinyl alcohol, nylon, cellulose acetate, polyamide etc.)

Service limited to below 65-90oC, poor creep strength, poor

resistance to organic solvents, fair peel strength compared to

thermosetting.

Elastomers (made of natural and synthetic materials got superior elongation

and toughness properties, natural rubber, neoprene, polyisobutylene,

polysulfide, reclaimed rubber, silicone etc.)

High flexibility and superior peel strength, use temperature limited to 70o to

200oC. Mostly used for non-structural applications such as vibration

damping, impact absorption, sealing and accomodating mismatched thermal

expansion coefficients.

These are available as solvent solutions, dispersions, pastes (cements) and

pressure sensitive tapes.


Adhesive alloys are combinations, or alloys, of resins from two or more chemical

groups from among thermosetting, thermoplastic, and elastomeric types.

Adhesive alloys are excellent for joining dissimilar materials to one another, such

as metals, ceramics, glasses, and thermosetting and thermoplastic polymers.

Some common varieties of adhesive alloys include epoxy–phenolics (a thermosetting

alloy), epoxy–polysulfone (a thermosetting alloy), epoxy–nylon (a thermosetting–

thermoplastic alloy), neoprene–phenolic (an elastomeric–thermosetting alloy), and vinyl–

phenolic (a thermoplastic–thermosetting alloy).

When properly formulated, adhesive alloys utilize the most important or desirable

properties of each component.


Classification by physical form

Adhesive can be classified according to physical form they are available

• Liquid adhesive

• Paste adhesives

• Dry powder adhesive

• Tap and film adhesive

Classification by application

Classified in the way they applied on joining parts:

• Spray-able

• Brush-able

• Trowel-able

• Extrude-able

Summary of Major Structural Adhesives Under the SME Classification Scheme

Summary of Adhesive Types Versus Compatible Adherends

Steps in adhesive bonding

Brief consideration of size, shape, area of substrate, and viscosity, curing of adherent. Steps should be consider for safe execution:

• Adhesive storage

• Adhesive preparation

• Joint preparation

• Method of adhesive application

• Joint assembly method and bonding equipment

Polymeric Adhesives

Epoxy & Modified Epoxies

Epoxy (Acetone + Phenol) & Modified Epoxies (Diglycidyl ether of bisphenol)

Requires hardener, catalyst or both, little or no pressure required.Exothermic reaction, careful with overheating and the formation of bubbles. Metals, ceramics, and polymers can be joined.

Modified epoxies incorporate various thermoplastics (including elastomeric types) and elastomeric thermosets to impart flexibility and toughness and better resistance to peel.Solids Contain no solvents, low shrinkage and bonding to impervious surfaces. Excellent in bonding dissimilar material combinations. Epoxy-nylon, epoxy-polysulfide, epoxy-phenolic, and epoxy-nitrile

Aside from hardeners, other additives such as accelerators, reactive diluents, plasticizers, fillers, and resin modifiers are often used to modify behavior or impart special properties.The highest strengths (up to 10 ksi in shear) and best heat resistance are obtained with heat-cured, two-part types.

- Two part systems, resin applied to one adherend and accelerant to the other adherent.

- Separate storage for the two parts, curing occurs in minutes.

- Shear strengths of upto 4 Ksi, show good peel, shear and tensile lap shear strength at low

temperatures (- 110oC – 120oC).

Polymeric AdhesivesAcrylics and Modified Acrylics (acrylic monomers of ethyl acrylate, methyl

acrylate, methacrylic acid, acrylic acid, acrylamide, and acrylonitrile).

Modified acrylics have additives that can penetrate hydrocarbon contaminents and also enhance the bond strength. (metal to metal structural bonding possible e.g., in autos)

Silicone addition improves properties.

Acrylic–latex, acrylated silicones, acryliated urethanes, and acrylated silicone–urethanes

Cyanoacrylates (super glues)

Polymeric Adhesives

Low-viscosity liquid acrylic monomers that polymerize easily in the presence ofadsorbed water, especially where the adherend surface is slightly alkaline.

Polymerization is ionic, and strong thermosetting bonds can be created with many materials, especially metals to nonmetals, with no added heat or catalyst, since most surfaces have adsorbed water present

Mechanical propertiesShear strengths up to 38.6MPa (5 ksi)Peel strength & impact resistance low (brittle and limited temperature resistance)

Tolerance of moisture poor.

Cyanoacrylates bonds with most materials and cures very quickly

Methyl cyanoacrylate stronger and more impact resistant joints (metals & rigid adherends)

Ethyl cyanoacrylate stronger and more durable joints (elastomeric, thermosetting, or thermoplastic polymer)

Poor at filling gaps, as they are very fluid.

C5H5NO2 requires hydroxyl ions to cure

Anaerobics

Polymeric Adhesives

These are single-component monomeric liquids that harden satisfactorily only in the absence of gaseous oxygen.

Stored in the presence of air, usually in permeable containers.

Exceptional fluidity, little or no shrinkage (used as sealants)

Prevention of fastener loosening during vibrations

Addition of urethane reduces brittleness, improves peel and impact strengths

Shear strengths up to 77.2 MPa (10 ksi) Temperature serviceability 200oC (400oF)

These are based on acrylics like methacrylate etc

Semi-filled and permeable bottles for storage

Used with metals, ceramics and polymers

UrethanesPolymeric Adhesives

Thermoplastic crosslink under certain conditions

One- and two-part systems Dissolved in solventsAlso used as hot melts

Flexible and good peel strength

Urethanes are generally applied to both adherends

Curing is usually done at room temperature, full curing takes many hours or days.

Heat can be used to soften the adhesive if it becomes too dry before bonding.

SiliconesOne- and two-component systems cure to thermosetting solids.

One component systems Acidic or non-acidic cure at RT

Two-component systems condensation polymerization (prone to reversion)

Good peel strength -60 – 250oC (some can upto 370oC)

Flexibility, impact resistance, and resistance to moisture, hot water, oxidation,and weathering. Poor lap strength.

Silicone adhesives are expensive but versatile. They are able to bond metals, glass, paper, wood, thermosetting and thermoplastic polymers, and a wide variety of rubbers.

Degradation of adhesive joints

• Temperature

• Moisture and humidity

• Solvent or corrosive agent

• Weathering

• Excessive dryness

• Vacuum

• Radiation

• Biological agents

Quality assurance of adhesive bonding

• Quality control should be applied on different stages

i.e. receiving of adhesive, batch no reading, physical

and mechanical properties check if applicable

• Ensuring substrate cleaning i.e. solvent cleaning,

intermediate cleaning, chemical/mechanical

treatments

• Quality controlling during bonding execution i.e.

proper time, temperature and pressure for curing

• Destructive and nondestructive testing and

evaluation of joints

Testing of adhesive bonding

• Tensile test

• Shear test

• Peel test

• Cleavage test

• Creep test

• Fatigue test

• Impact test

Certain glues or adhesive materials used in dentistry and for certain organic and many inorganic bonding agents used to join engineered ceramics, including porcelains.

It is also the common name of certain organic (polymer-

based) adhesives used to bond rubber (i.e., ‘‘rubber

cements’’). It is the name of a material used for joining in masonry (i.e., the joining of stone, clay products, or cement or concrete products).

It is a substance that is a nonmetallic, inorganic

compound known as a ceramic that binds particulate aggregates into a cohesive structure through a chemical reaction, called hydration, involving hydrogen bonding of ceramic particles with water molecules.

Cement

CementSodium Silicate

Introduction of CO2 converts the bridges to a solid glass that joins the sand grains into a solid mold.

Aluminium Phosphate cement

The aluminum phosphate cement bonds the alumina particles so they can withstand operating temperatures as high as 1,650oC (3,000oF).

Fine alumina powder solutions are catalyzed with phosphoric acid

Plaster of Paris (Gypsum)

Sculptures, plasterers use to finish walls, and plaster board

Small particles (actually crystals) of CaSO4 interlock into large crystals of gypsum (CaSO4.2H2O) through hydration.

Portland cement has an overall composition in which the major resulting constituents are tricalcium silicate (3CaO:SiO2) and dicalcium silicate (2CaO:SiO2).

There are also relatively minor additions of tricalcium aluminate (3CaO:Al2O3), brown-millerite (approximately 4CaO:Al2O3:Fe2O3), some CaO, some MgO, and glass.

High Alumina cement: These typically contain approximately 40 parts Al2O3, 40 parts CaO, 7 parts SiO2, 7 parts Fe2O3, 5 parts FeO, and five other minor oxides.

Cement

The reaction between bisphenol A and epichlorohydrin can be controlled to produce different molecular weights. Low molecular weight molecules tend to be liquids and higher molecular weight molecules tend to be more viscous liquids or solids.

11. Apply the allowable-stress design procedure to the single-overlap, bearing-type shear-loaded joint shown in Figure. Assume the bolts in the joint are made from ASTM 325 steel and have 14 threads per inch and that the joint plate is made from ASTM A36 steel.

13. What would the edge distance (i.e., the distance from the center of the bolt hole to the edge of the joint plate) be if tear-out were just possible for the joint shown in Figure?

15. Apply the allowable-stress design procedure to the double-overlap, bearing-type shear-loaded splice joint shown in Figure. Assume the same bolt and joint plate materials as in Problem #11. Assume the bolts have a pitch of 2.0 mm.

17. Using the allowable-stress design procedure for the joint shown in Figure, what value of target preload would be needed to just assure there would be no slip at the joint interface, assuming the joint elements were treated to have a slip coefficient (of friction) μs of 0.40?

19. Suppose the load on the joint shown in Figure were 300 kN, and the dimensions were x1= 40 mm, x2 = 80 mm, L = 320 mm, and w = 80 mm. What would the load be on the most highly loaded fastener(s) if the fasteners were each 10 mm in diameter?

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Adhesive Bonding 2