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ADHESIVES

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2 surfaces can only be joined with the

establishment of bonds between them.

The exact nature of the bond depends on the

adhesive and the surface to be bonded.

It is important when joining materials with true

adhesives that the surfaces to be completely

clean.

Even a thin coating of grease or similar material

may reduce considerably the effective surface

tension of the surface, impairing the bond.

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TYPES OF ADHESIVE

Can be divided into 3 types:

a) Natural adhesives

- can be made from animal, vegetable or

mineral sources

b) Synthetic Resin

i) Thermosetting adhesivesii) Thermoplastic synthetic adhesives

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Natural Adhesives

ANIMAL GLUES

have been used for centuries in carpentry and joinery

Obtained from the skin / bones of cattle and sheep

Animal glues usually sold in powder and melted inwarm water to give a solution of suitable viscosity.

Cooling of the liquid after application results in rapid

gelling, producing some strength and drying finally

produces a tough and rigid product.

Removal of the water from glue during curing is

essential; the wood assist in this process, due to

absorption.

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Glued joints in wood should be as strong as

the wood.

Damp condition will reduce strength of glue.

Animal glue is not suitable for external use

unless adequately protected from moisture.

The glue will harden gradually in the pot but

can be softened by reheating with water.

If a failure occurs in such items it is easy tocarry out repairs by softening the glue with

water.

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Animal Glues

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CASEIN GLUES

It is derived as a precipitate from skimmed

milk by the action of acids. Glues are obtained in powder form that also

contain alkaline solvent, necessary to dissolve

the glue on addition of water. Mixing is carried out in cold and setting is

partly by evaporation or absorption of waterand partly by a natural gelation process.

Its adhesion to porous material is good

Its moisture resistance is superior than animalglue

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Casein GlueA Dry Glue Derived From Milk

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VEGETABLE GLUE

Made from starches or dextrin

low cost

limited strength and limited moisture

resistance

Gum Arabic is a water soluble vegetable

adhesive often used on stamps and gummed

paper envelopes.

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Cellulose Powder A Type of Vegetable Adhesive

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Wheat Paste A Type of Vegetable Adhesive

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THERMOSETTING ADHESIVES

UREA FORMALDEHYDE

One of the commonest adhesives in joinery for

general purposes, producing a strong rigid

joint if the glue-line is thin.

It provides a strong adhesion in a permanently

dry environment, cures fast, and is relatively

cheap.

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To make the adhesive, formaldehyde and urea

in aqueous solution are allowed to react

together to a certain stage and the reaction isthen stopped while the resin is still liquid.

They are best suited for joining porous material

such as wood. It is used for fabrication of flush colours,

laminated timber and decorative laminates.

It has a good water resistance but if very hotwater or prolonged wetting it could breaks the

resin down.

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MELAMINE FORMALDEHYDE RESIN

ADHESIVES

Supplied in powder form & mixed with water

to give a colourless resin.

Set on heating to 100C

Give good weather resistance & expensive

than urea formaldehyde adhesive

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PHENOL FORMALDEHYDE

available as liquids that polymerise on heating

to temperature over 100C.

Used for assembly of plywood sheets

The resin is applied,the sheets formed and the

laminates is then subjected to a hot press thatcauses hardening in a period of about 5 min.

Cold-curing phenolic adhesives are also

available.

Can be applied for Plywood, block boards,

flush doors

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EPOXY RESINS

It form a good bond with almost any material.

It is widely used for structural joints between

many types of material including glass, metal,

thermosetting plastics and concrete.

Have excellent mechanical strength

Have excellent creep resistance

Resistant to weather, acids, alkalis and mosthydrocarbon.

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Epoxy Resin

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Epoxy Resin

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Alkyl Cyanoacrylate Adhesives

Have advantage of polymerising very rapidly

when spread in thin films. The reaction being catalysed by water which,

in the very small quantities required, is present

on most surfaces. Expensive so uses are confined to bonding of

small objects having surfaces that closelyconform to one another.

They form the basis of Superglues.

Must be handled with care to avoid bonding ofskin tissues.

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THERMOPLASTIC ADHESIVES

Setting action of these may due to cooling,

solvent evaporation or emulsion coalescence.

This adhesive is more flexible but weaker amd

more prone to creep than thermosetting

adhesives.

So, they are normally not used for structural

purposes.

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POLYVINYL ACETATE

Most important thermoplastic adhesive in

building. Chief application being as a wood adhesive for

internal use and as a bonding agent for concrete.

Most commonly used in emulsion form

This polymer is obtained from acetylene &

acetic acid.

The adhesive sets when the solid particles, onevaporation or absorption of the moisture,cohere to form a tough, clear film which is nolonger water soluble.

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Polyvinyl Acetate Used For Book Binding

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BITUMINOUS ADHESIVES

Form good bonds with a number of materials

Moisture resistant

Flexible

Natural or Synthetic rubber & solvents may be

included to give the desired combination of

elasticity and strength.

Can be obtained in 3 form that is

a) Water Based Emulsion

b) Solvent Types

c) Hot-Applied Varieties

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Water Based Emulsions

- Form an effective bond between porous

materials in situations where long term tensilestresses are not present.

- Also used for laying wood-block flooring and

PVC or thermoplastic tilesHot-Applied Varieties

- Used for wood-block flooring and in

built-up felt roofing.- Application temperature in the region of

150C200C

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Solvent Types

- Used in laying linoleum and for PVC and

thermoplastic tiles though test should becarried out to ensure that staining doesnt

occur.

- Both solvent and emulsion types rely onsome degree of absorbency in the

substrate to allow absorption / evaporation

of the water or solvent.

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RUBBER BASED ADHESIVES

The most important rubber based adhesives in

the construction industry are contact adhesivesThe adhesive is applied to both surfaces and

after solvent evaporation, the surfaces arecarefully brought together, giving immediateauto-adhesion.

Latexes such as styrene butadiene rubber (SBR)are widely used for floor coverings.

Latex-modified cements are used for bondingceramic tiles to a variety of backgrounds,including metal.

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GOOD LUCK

ALWAYS SHARE YOUR KNOWLEDGE WITH OTHERS