What is Adhesives?
We can define it as an
adhesive is a substance which is useful to join two or more parts so as to make
a single unit.
The first adhesives were
natural gums and other plant resins or saps. As the modern world evolved,
several other potential materials such as bones, starch, fish and casein were
introduced as alternative materials for glue manufacture. Modern glues have
improved beyond recognition with improved flexibility, toughness, curing rate
and chemical resistance.
Advantages of Adhesives /Adhesives used for:-
- It produces adequate strength.
- Adhesives can commonly used for bonding the surfaces of glass, plastics, wood and metal.
- The process of applying adhesives is more easy, speedy and economical.
- It creates massive effect after its application.
- It is made possible to avoid the corrosion between different metals by joining them with the help of adhesive.
- Leakage problem of water can be stopped by the application of adhesives.
- The permeable joint in case of water and gas pipe can be made completely impermeable with the help of adhesive.
Disadvantages of Adhesives:
- The adhesive do not become strong immediately after its application. It take some time to attain the desired strength.
- It is not possible to apply the adhesive for all kind of substances. Suitable type of adhesive has to be selected for the substances to be joined, which depends upon the properties of substance.
- The adhesive do not remain stable at high temperature.
Requirements of Good Quality Adhesive :
- It has a good resistance to heat.
- It should be durable. Its degree of sticking and drying should be good.
- There should be sufficient time to develop the required bond.
- The strength of bond developed after drying or setting should be more.
Types of Adhesive
(i)Natural adhesives:
Natural adhesives are made
from inorganic mineral sources or biological sources such as natural gums,
starch (dextrin), animal skin, natural resin or a mix of casein and lime. They
are often referred to as bio adhesives. It is used to prepare paper board
articles, labelling, building paper etc.
(ii) Synthetic adhesives:
Synthetic adhesives are based
on synthetic resin. It may be thermo-setting glue or thermo-plastic glue. The
thermo-setting glues become permanent after setting but thermo- plastic glue
can be made plastic again by heating again.
Elastomers, thermo- plastic,
and thermosetting adhesives are the examples of synthetic adhesives. All
synthetic adhesives of glues are fire-proof, water-proof and strong. It resists
the attack by fungi and possess non-staining qualities.
Types of synthetic adhesives are as followes:-
a) a) Malamine resins (used
in the process of making plywood)
b)
Phenolic resins (used
in the manufacture of resin-bonded plywood)
c)
Resorcinol resins
( become hard in short time at low temperature)
d)
Urea resins (
used in joinery work)
(iii) Drying Adhesives
These adhesives are a mixture
of ingredients (typically polymers) dissolved in a solvent. White glue and
rubber cements are members of the drying adhesive family. These adhesives are
typically weak and are used for household applications. Some adhesives intended
for use of small children are now made non-toxic.
(iv) Contact adhesives:
These adhesives must be
applied to both the surfaces and allowed sometime to dry before the two
surfaces are pushed together. Once the surfaces are pushed together, the bond
forms very quickly, hence it is usually not necessary to apply pressure for a
long time and no need to use clamps.
Natural rubber and poly-chloroprene
i.e. Neoprene are commonly used as contact adhesives. Contact adhesives find
use in laminates such as bonding 'formica' to a wooden members and in footwear
such as attaching an outsole to an upper.
(v) Reactive adhesives:
A reactive adhesive works
either by chemical bonding with the surface material or by in-situ hardening.
Reactive adhesives are usually applied in thin films. It includes two part
epoxy, peroxide, silane, metallic cross links or isocyanate.
These adhesives are mostly
used to prevent the loosening of bolts and screws in rapidly moving assemblies
in case of automobile engine.
(vi) Hot adhesives:
Hot adhesives are also known
as 'thermo-plastic' adhesives or 'hot melt' adhesives. They are applied in hot
condition and then allowed to cool for hardening.
Hot adhesives have become
much popular for crafts use.
(vii) Pressure sensitive
adhesives (PSAS):
It form a bond by the
application of light pressure. These adhesives are designed with a balance
between flow and resistance to flow. The bond has strength because the adhesive
is hard enough to resist flow when stress is applied to the bond.
Pressure sensitive adhesives
are designed for either permanent or removable applications. Examples of permanent
applications include safety labels for power equipments, foil tape for HVAC
duct work, automotive interior trim. assembly and sound vibration damping
films. Removable adhesives are used in applications such as surface protection
films, masking tapes, bookmark and note papers, price marking labels,
promotional graphics materials and for skin contact (wound care dressings, EKG
electrodes, athletic tape, analgesic and transdermal drug and patches etc).