Adhesive

An adhesive is a compound that adheres or bonds two items together. Adhesives may come from either natural or synthetic sources. Some modern adhesives are extremely strong, and are becoming increasingly important in modern construction and industry.

History

The first adhesives were 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.

Hot adhesives (thermoplastic adhesives)

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A glue gun, an example of a hot adhesive

Main 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.

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.

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 and gaffer tape.

Plastic wrap displays temporary adhesive properties as well.

Mechanisms of adhesion

Main article: adhesion

The strength of attachment, or adhesion, between an adhesive and its substrate depends on many factors, including the means by which this occurs. Adhesion may occur either by mechanical means, in which the adhesive works its way into small pores of the substrate, or by one of several chemical mechanisms.

In some cases an actual chemical bond occurs between adhesive and substrate. In others electrostatic forces, as in static electricity, hold the substances together. A third chemical method involves van der Waal's forces which develop between each's molecules. Such forces also seem to account for the "stickiness" of a gecko's feet. A fourth chemical means involves the moisture-aided diffusion of the glue into the substrate, followed by hardening.

Failure of the adhesive joint

When subjected to loading, debonding may occur at different locations in the adhesive joint. The major fracture types are the following

Cohesive fracture

“Cohesive” fracture" is obtained if a crack propagates in the bulk polymer which constitutes the adhesive. In this case the surfaces of both adherents after debonding will be covered by fractured adhesive. The crack may propagate in the centre of the layer or near an interface. For this last case, the “cohesive” fracture can be said to be “cohesive near the interface”. Most quality control standards consider that a “good” adhesive bonding must be “cohesive”.

Interfacial fracture

The fracture is “adhesive” or “interfacial” when debonding occurs between the adhesive and the adherent. In most cases, the occurrence of “interfacial” fracture for a given adhesive goes along with a smaller fracture toughness. The “interfacial” character of a fracture surface is usually detected by visual inspection, but advanced surface characterisation techniques such as spectrophotometry allows to identify the precise location of the crack path in the interphase.

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Failure of the adhesive joint can occur in different locations

Other types of fracture

Beside these two cases, other type of fracture are

• The “mixed” fracture type which occurs if the crack propagates at some spots in a “cohesive” and in others in an “interfacial” manner. “Mixed” fracture surfaces can be characterised by a certain percentage of “adhesive” and “cohesive” areas.

• The “alternating crack path” fracture type which occurs if the cracks jumps from one interface to the other. This type of fracture appears in the presence of tensile pre-stresses in the adhesive layer.

• Fracture can also occur in the adherent if the adhesive is tougher than the adherent. In this case the adhesive remains intact and is still bonded to one substrate and the remnants of the other. For example, when one removes a price label, adhesive usually remains on the label and the surface. This is cohesive failure. If, however, a layer of paper remains stuck to the surface, the adhesive has not failed. Another example is when someone tries to pull apart Oreo cookies and all the filling remains on one side. The goal in this case is an adhesive failure, rather than a cohesive failure.

Design of adhesive joints

A general design rule is a relation of the type: "Material Properties > Function (geometry, loads)"

The engineering work will consist in having a good model to evaluate the "Function". For most adhesive joints, this can be achieved using fracture mechanics. Concepts such as the stress concentration factor K and the energy release rate G can be used to predict failure. In such models, the behavior of the adhesive layer itself is neglected and only the adherents are considered.

Failure will also very much depend on the opening "mode" of the joint.

Modes of failure

• Mode I is an opening or tensile mode where the loadings are normal to the crack.
• Mode II is a sliding or in-plane shear mode where the crack surfaces slide over one another in direction perpendicular to the leading edge of the crack. This is typically the mode for which the adhesive exhibits the higher resistance to fracture.
• Mode III is a tearing or antiplane shear mode.

As the loads are usually fixed, an acceptable design will result from combination of a material selection procedure and geometry modifications, if possible. In adhesively bonded structures, the global geometry and loads are fixed by structural considerations and the design procedure focuses on the “material properties” of the adhesive (i.e. select a "good" adhesive) and on local changes on the geometry.

Increasing the joint resistance is usually obtained by designing its geometry so that:

• The bonded zone is large
• It is mainly loaded in mode II
• Stable crack propagation will follow the appearance of a local failure.

Testing the resistance of the adhesive

A wide range of testing devices have been imagined to evaluate the fracture resistance of bonded structures in pure mode I, pure mode II or in mixed mode. Most of these devices are beam type specimens. We will very shortly review the most popular:

• Double Cantilever Beam tests (DCB) are used to measure the mode I fracture resistance of adhesives in a fracture mechanics framework. These tests consist in opening an assembly of two beams by applying a force at the ends of the two beams. The test in unstable (i.e. the crack propagates along the entire specimen once a critical load is attained) and a modified version of this test characterised by a non constant inertia was proposed called the Tapered double cantilever beam specimen (TDCB).

  

  Testing devices

• Peel tests are used to measure the fracture resistance of a thin layer bonded on a thick substrate or of two layers bonded together. They consist in measuring the force needed for tearing an adherent layer from a substrate or for tearing two adherent layers one from another. Whereas the structure is not symmetrical, various mode mixities can be introduced in these tests.

• Wedge tests are used to measure the mode I dominated fracture resistance of adhesives used to bond thin plates. These tests consist in inserting a wedge in between two bonded plates. A critical energy release rate can be derived from the crack length during testing. This test is a mode I test but some mode II component can be introduced by bonding plates of different thicknesses.

• Mixed-Mode Delaminating Beam (MMDB) tests consist in a bonded bilayer with two starting cracks loaded on four points. The test presents roughly the same amount of mode I and mode II with a slight dependence on the ratio of the two layer thicknesses.

• End Notch Flexure tests consist in two bonded beams built-in on one side and loaded by a force on the other. As no normal opening is allowed, this device allows testing in essentially mode II condition.

• Crack Lap Shear (CLS) tests are application-oriented fracture resistance tests. They consist in two plates bonded on a limited length and loaded in tension on both ends. The test can be either symmetrical or dis-symmetrical. In the first case two cracks can be initiated and in the second only one crack can propagate.

External links

• Adhesives from renewable resources
• Adhesives from foods