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History
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The first adhesives were natural gums and other plant resins. Archaeologists have
found 6000-year-old ceramic vessels that had broken and been repaired using plant resin. Most early adhesives were animal
glues made by rendering animal products such as the Native American use of buffalo hooves. Native Americans in what is now
the eastern United States used a mixture of spruce gum and fat as adhesives and as caulk to waterproof seams in their birchbark
canoes. During the times of Babylonia, tar-like glue was used for gluing statues. Also, Egypt was one of the most prominent
users of adhesives. The Egyptians used animal glues to adhere tombs, furniture, ivory, and papyrus. Also, the Mongols used
adhesives to make their short bows. In Europe in the Middle Ages, egg whites were used to decorate parchments with gold leaves.
In the 1700s, the first glue factory was founded in Holland, which manufactured hide glue. Later, in the 1750s, the British
introduced fish glue. As the modernization continued, new patents were issued by using rubber, bones, starch, fish, and casein.
Modern adhesives have improved flexibility, toughness, curing rate, temperature and chemical resistance. (HSL)
Categories
of adhesives
Natural adhesives
Adhesives based on vegetable (natural
resin), food (animal hide and skin), and mineral sources (inorganic materials).
Synthetic adhesives
Adhesives
based on elastomers, thermoplastic, and thermosetting adhesives.
Drying adhesives
These
adhesives are a mixture of ingredients (typically polymers) dissolved in a solvent. Glues such as white glue, and rubber cements
are members of the drying adhesive family. As the solvent evaporates, the adhesive hardens. Depending on the chemical composition
of the adhesive, they will adhere to different materials to greater or lesser degrees. These adhesives are typically weak
and are used for household applications. Some intended for small children are now made non-toxic.
Contact adhesives
Contact
adhesive is one which must be applied to both surfaces and allowed some time to dry before the two surfaces are pushed together.
Some contact adhesives require as long as 24 hours to dry before the surfaces are to be held 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. This means
that there is no need to use clamps, which is convenient.
Natural rubber and polychloroprene (Neoprene) are commonly
used contact adhesives. Both of these elastomers undergo strain crystallization. When an adhesive bond containing either of
these materials is pulled apart, the elastomer is strained, develops crystallites, and actually becomes stronger than in the
original, unstressed, state.
Contact adhesives find use in laminates, such as bonding
Formica (plastic) to a wooden counter, and in footwear, for example attachment of an outsole to an upper.
Reactive adhesives
A reactive adhesive
works by chemical bonding with the surface material. They are applied in thin films. Reactive adhesives are less effective
when there is a secondary goal of filling gaps between the surfaces. These include two-part epoxy, peroxide, silane, metallic
cross-links, or isocyanate.
Such adhesives are frequently used to prevent loosening
of bolts and screws in rapidly moving assemblies, such as automobile engines. They are largely responsible for the quieter
running modern car engines.
UV and Light
Curing Adhesives
Typically, these adhesives fully cure in seconds upon exposure to UV or visible light
of the proper wavelengths, intensity, and duration. Some formulations are "fixtured" with UV light, but require additional
time or curing mechanisms to achieve full cure. UV curing adhesives can be formulated with a wide variety of properties (low
to high viscosity, flexible to rigid, clear to colored, adhesion to glass/plastics/metals/ceramics, depth of cure to >1/2").
Hotmelt adhesives (thermoplastic adhesives)
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A glue gun, an example of a hot adhesiveMain article: Hot glue
Also known as "hot melt" adhesives, these adhesives
are thermoplastics; they are applied hot and simply allowed to harden as they cool. These adhesives have become popular for
crafts because of their ease of use and the wide range of common materials to which they can adhere. A glue gun, pictured
right, is one method of applying a hot adhesive. The glue gun melts the solid adhesive and then allows the liquid to pass
through the "barrel" of the gun onto the material where it solidifies.(More information)
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Pressure sensitive adhesives
Pressure sensitive adhesives (PSAs) form a bond by the application of light pressure
to marry the adhesive with the adherend. They are designed with a balance between flow and resistance to flow. The bond forms
because the adhesive is soft enough to flow, or wet, the adherend. The bond has strength because the adhesive is hard enough
to resist flow when stress is applied to the bond. Once the adhesive and the adherend are in close proximity, molecular interactions
such as van der Waals forces become involved in the bond, contributing significantly to its ultimate strength.
Pressure sensitive adhesives (PSAs) are designed for either
permanent or removable applications. Examples of permanent applications include safety labels for power equipment, foil tape
for HVAC duct work, automotive interior trim assembly, and sound/vibration damping films. Some high performance permanent
PSAs exhibit high adhesion values and can support kilograms of weight per square centimeter of contact area, even at elevated
temperature. Permanent PSAs may be initially removable (for example to recover mislabeled goods) and build adhesion to a permanent
bond after several hours or days.
Removable adhesives are designed to form a temporary bond,
and ideally can be removed after months or years without leaving residue on the adherend. 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
patches, etc.). Some removable adhesives are designed to repeatedly stick and unstick. They have low adhesion and generally
can not support much weight.
Pressure sensitive adhesives are manufactured with either
a liquid carrier or in 100% solid form. Articles are made from liquid PSAs by coating the adhesive and drying off the solvent
or water carrier. They may be further heated to initate a crosslinking reaction and increase molecular weight. 100% solid
PSAs may be low viscosity polymers that are coated and then reacted with radiation to increase molecular weight and form the
adheisve; or they may be high viscosity materials that are heated to reduce viscosity enough to allow coating, and then cooled
to their final form.
Also see adhesive tape, blu-tack and gaffer tape.
Plastic wrap displays temporary adhesive properties as
well.
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