Difference between revisions of "Adhesive" - New World Encyclopedia
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==Mechanisms of adhesion== | ==Mechanisms of adhesion== | ||
| − | The strength of attachment | + | The strength of attachment between an adhesive and its substrate depends on many factors, including the mechanism by which this occurs and the surface area over which the two materials contact each othr. Materials that [[wetting|wet]]* each other tend to have a larger contact area than those that don't. Five mechanisms have been proposed to explain why one material sticks to another. |
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[[Image:Drops I.jpg|thumb|right|300px|Water droplets adhering to leaves]] | [[Image:Drops I.jpg|thumb|right|300px|Water droplets adhering to leaves]] | ||
[[Image:Water drops on spider web.jpg|thumb|right|250px|Dew drops adhere to a spider web.]] | [[Image:Water drops on spider web.jpg|thumb|right|250px|Dew drops adhere to a spider web.]] | ||
| − | + | ===Mechanical Adhesion=== | |
| − | + | Two materials may be mechanically interlocked, such as when the adhesive works its way into small pores of the materials. Some textile adhesives form small-scale bonds. On larger levels, mechanical bonds can be formed by sewing or the use of [[velcro]]*. | |
| − | Two materials may be mechanically interlocked. | ||
===Chemical Adhesion=== | ===Chemical Adhesion=== | ||
| − | Two materials may form a [[Chemical compound|compound]] at the join. | + | |
| + | Two materials may form a [[Chemical compound|compound]] at the join. The strongest joins are where atoms of the two materials swap electrons (in cases of [[ionic bond]]s) or share electrons (in cases of [[covalent bond]]s). Weaker bonds (known as [[hydrogen bond]]s) are formed if [[oxygen]], [[nitrogen]], or [[fluorine]] atoms of the two materials share a [[hydrogen]] nucleus. | ||
===Dispersive Adhesion=== | ===Dispersive Adhesion=== | ||
| − | + | ||
| + | In dispersive adhesion (also known as adsorption), two materials are held together by what are known as "[[van der Waals force]]*s." These are weak (but numerous) interactions between molecules of the materials, arising by electron movements or displacements within the molecules. | ||
===Electrostatic Adhesion=== | ===Electrostatic Adhesion=== | ||
| − | Some conducting materials may pass [[ | + | |
| + | Some conducting materials may pass [[electron]]s to form a difference in [[electrical charge]]* at the join. This gives rise to a structure similar to a [[capacitor]]* and creates an attractive [[electrostatic]]* force between the materials. | ||
===Diffusive Adhesion=== | ===Diffusive Adhesion=== | ||
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| − | |||
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| − | + | Some materials may merge at the joint by [[diffusion]]. This may occur when the molecules of both materials are mobile and [[soluble]]* in each other. This would be particularly effective with [[polymer]] chains, where one end of a molecule of one material diffuses into molecules of the other material. It is also the mechanism involved in [[sintering]]*. When [[metal]] or [[ceramic]] powders are pressed together and heated, atoms may diffuse from one particle to the next, thereby joining the particles together. | |
| − | * [[ | ||
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| − | * [[ | ||
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== Failure of the adhesive joint == | == Failure of the adhesive joint == | ||
Revision as of 19:32, 18 November 2006
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.
Adhesion is the attraction between the molecules of bodies in contact. It is of particular interest to engineers who wish to use adhesives to stick objects together and to biologists to understand the workings of cells.
History
It appears that the earliest adhesives used in history were natural gums and other plant resins. Archaeologists have found 6,000-year-old ceramic vessels that had broken and been repaired with plant resin. Native Americans in what is now the eastern United States used a mixture of spruce gum and fat as adhesives and caulking to waterproof seams in their birchbark canoes. In ancient Babylonia, tar-like glue was used for gluing statues.
There is also evidence that many early adhesives were glues made from animal products. For instance, Native Americans made glues from buffalo hooves. Early Egyptians used animal glues to repair fractures in tombs, furniture, ivory, and papyrus. 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 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)
Types of adhesives
Adhesives may be classified as natural or synthetic. Examples of natural adhesives are plant resins, glues from animal hide and skin, and adhesives from mineral (inorganic) sources. Examples of synthetic adhesives are polymers such as elastomers, thermoplastics, and thermosets. Adhesives may also be grouped according to their properties, as follows.
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, it will adhere to different materials to a greater or lesser extent. These adhesives are typically weak and are used for household applications. Those intended for use by small children are made nontoxic.
Contact adhesives
A contact adhesive is one that must be applied to both surfaces and allowed some time—sometimes as much as 24 hours—to dry before the two surfaces are pushed together.[1]. Once the surfaces are brought together, the bond forms very quickly,[2] and it is usually not necessary to apply pressure for a long time. In other words, there is often no need to use clamps, which is convenient.
Hot (thermoplastic) adhesives
Also known as "hot melt" adhesives, these thermoplastics are applied hot and simply allowed to harden as they cool. They 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 is one method of applying a hot adhesive. The solid adhesive melts in the body of the gun, and the liquified material passes 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. It is applied as a thin film. Reactive adhesives include two-part epoxy, peroxide, silane, isocyanate, or metallic cross-linking agents. They are less effective when there is a secondary goal of filling gaps between surfaces.
Such adhesives are frequently used to prevent the 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 bind the adhesive to the adherend (substrate for attachment). They are designed with a balance between flow and resistance to flow. The bond forms because the adhesive is soft enough to flow and "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 adherend are in close proximity, interactions between their molecules contribute significantly to the ultimate strength of the bonding. PSAs are manufactured with either a liquid carrier or in completely solid form.
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 can support kilograms of weight per square centimeter of contact area, even at elevated temperatures. Permanent PSAs may be initially removable (such as to recover mislabeled goods) and set to a permanent bond after several hours or days.
Removable PSAs are designed to form a temporary bond and ideally can be removed after months or years without leaving residue on the adherend. They are used in applications such as surface protection films, masking tapes, bookmark and note papers, price marking labels, and promotional graphics materials. Plastic wrap displays temporary adhesive properties as well. In medical applications, they are used in cases where skin contact needs to be made, such as for wound care dressings, EKG electrodes, athletic tape, and analgesic and transdermal drug patches. Some removable adhesives are designed to repeatedly stick and unstick. They have low adhesion and generally cannot support much weight.
Mechanisms of adhesion
The strength of attachment between an adhesive and its substrate depends on many factors, including the mechanism by which this occurs and the surface area over which the two materials contact each othr. Materials that wet each other tend to have a larger contact area than those that don't. Five mechanisms have been proposed to explain why one material sticks to another.
Mechanical Adhesion
Two materials may be mechanically interlocked, such as when the adhesive works its way into small pores of the materials. Some textile adhesives form small-scale bonds. On larger levels, mechanical bonds can be formed by sewing or the use of velcro.
Chemical Adhesion
Two materials may form a compound at the join. The strongest joins are where atoms of the two materials swap electrons (in cases of ionic bonds) or share electrons (in cases of covalent bonds). Weaker bonds (known as hydrogen bonds) are formed if oxygen, nitrogen, or fluorine atoms of the two materials share a hydrogen nucleus.
Dispersive Adhesion
In dispersive adhesion (also known as adsorption), two materials are held together by what are known as "van der Waals forces." These are weak (but numerous) interactions between molecules of the materials, arising by electron movements or displacements within the molecules.
Electrostatic Adhesion
Some conducting materials may pass electrons to form a difference in electrical charge at the join. This gives rise to a structure similar to a capacitor and creates an attractive electrostatic force between the materials.
Diffusive Adhesion
Some materials may merge at the joint by diffusion. This may occur when the molecules of both materials are mobile and soluble in each other. This would be particularly effective with polymer chains, where one end of a molecule of one material diffuses into molecules of the other material. It is also the mechanism involved in sintering. When metal or ceramic powders are pressed together and heated, atoms may diffuse from one particle to the next, thereby joining the particles together.
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.
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.
- 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).
- 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.
Footnotes
ReferencesISBN links support NWE through referral fees
- John Comyn, Adhesion Science, Royal Society of Chemistry Paperbacks, 1997
- A.J. Kinloch, Adhesion and Adhesives: Science and Technology, Chapman and Hall, 1987
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