Difference between revisions of "Adhesive" - New World Encyclopedia

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An '''adhesive''' is a compound that [[adhesion|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.  
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{{Copyedited}}{{Paid}}{{Images OK}}{{Submitted}}{{Approved}}
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[[Image:Hibiscus pink.jpg|thumb|right|250px|Water droplets adhere to a Hibiscus flower]]
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An '''adhesive''' is a material that can adhere (stick) to other materials and help attach them together. The state of attachment is known as '''adhesion''', which is based on the attraction between molecules of the items in contact.
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{{toc}}
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Various types of adhesives are now available, derived from natural and [[synthetic]] sources. Some modern synthetic adhesives are extremely strong and are increasingly used in construction and industry.
 +
[[File:Stanley-Hot-Glue-Gun-GR35K.jpg|right|thumb|250px|A glue gun contains a hot adhesive]]
  
 
== History ==
 
== History ==
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The first adhesives were [[natural gum]]s and other plant [[resin]]s. Archaeologists have found 6000-year-old [[ceramic]] vessels that had broken and been repaired using plant resin. Most early adhesives were [[animal glue]]s made by rendering animal products such as the Native American use of [[American Bison|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 [[Babylon]]ia, 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)
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It appears that the earliest adhesives used in history were [[natural gum]]s and other plant [[resin]]s. Archaeologists have found six-thousand-year-old [[ceramic]] vessels that had broken and been repaired with plant resin. [[Native Americans in the United States|Native Americans]], in what is now the eastern [[United States]], used a mixture of [[spruce]] gum and fat as adhesives and [[caulk]]ing to waterproof seams in their birch bark canoes. In ancient [[Babylonian Empire|Babylonia]], tar-like glue was used for gluing statues.
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==Categories of adhesives==
 
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===Natural adhesives===
 
  
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Adhesives based on vegetable (natural resin), food (animal hide and skin), and mineral sources (inorganic materials).
+
There is also evidence that many early adhesives were [[glue]]s made from animal products. For instance, Native Americans made glues from [[American Bison|buffalo]] hooves. [[Ancient Egypt|Early Egyptians]] used animal glues to repair fractures in [[tomb]]s, [[furniture]], [[ivory]], and [[papyrus]]. The [[Mongols]] used adhesives to make their short bows.
  
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===Synthetic adhesives===
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In [[Europe]] in the [[Middle Ages]], egg whites were used to decorate [[parchment]]s with gold leaves. In the 1700s, the first glue factory was founded in [[Netherlands|Holland]], which manufactured hide glue. Later, in the 1750s, the [[British Empire|British]] introduced fish glue. As modernization continued, new patents were issued for the use of rubber, bones, starch, fish, and casein. Modern synthetic adhesives have improved flexibility, toughness, curing rate, temperature, and chemical resistance.
  
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Adhesives based on elastomers, thermoplastic, and thermosetting adhesives.
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==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 [[polymer]]s such as elastomers, thermoplastics, and thermosets. Adhesives may also be grouped according to their properties, as follows.
  
 
=== Drying adhesives ===
 
=== Drying adhesives ===
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These adhesives are a mixture of ingredients (typically [[polymers]]) dissolved in a [[solvent]]. [[Glue]]s such as white glue, and [[rubber cement]]s 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.
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These adhesives are a mixture of ingredients (typically [[polymers]]) dissolved in a [[solvent]]. [[Glue]]s such as white glue, and [[rubber cement]]s 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 ===
 
=== Contact adhesives ===
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''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<ref>[http://www.thistothat.com/glue/contact.shtml Information about contact adhesive]</ref>. Once the surfaces are pushed together the bond forms very quickly<ref>[http://composite.about.com/library/glossary/c/bldef-c1257.htm Definition of ''contact adhesive'' on About.com]</ref>, hence it is usually not necessary to apply pressure for a long time. This means that there is no need to use [[Clamp (tool)|clamps]], which is convenient.
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A ''contact adhesive'' is one that must be applied to both surfaces and allowed some time&mdash;sometimes as much as 24 hours&mdash;to dry before the two surfaces are pushed together.<ref>[http://www.thistothat.com/glue/contact.shtml Contact Adhesives] ''ThisToThat''. Retrieved August 16, 2018.</ref> Once the surfaces are brought together, the bond forms very quickly,<ref>[https://home.howstuffworks.com/uses-for-contact-cement.htm What are some of the uses for contact cement?] ''HowSTuffWorks''. Retrieved August 16, 2018.</ref> and it is usually not necessary to apply pressure for a long time. In other words, there is often no need to use [[Clamp (tool)|clamps]], which is convenient.
  
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===Hot adhesives (thermoplastic adhesives)===
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===Hot (thermoplastic) adhesives===
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[[Image:MVC-013F.JPG|right|thumbnail|200px|A glue gun, an example of a hot adhesive]]
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Also known as "hot melt" adhesives, these [[thermoplastic]]s 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 liquefied material passes through the barrel of the gun onto the material where it solidifies.
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{{main|Hot glue}}
 
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Also known as "hot melt" adhesives, these adhesives are [[thermoplastic]]s; 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 ===
 
=== Reactive adhesives ===
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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.
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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.
  
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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.
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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.
  
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=== Pressure sensitive adhesives ===
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=== Pressure-sensitive adhesives ===
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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.
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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.
  
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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.
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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.
  
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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.
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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.
  
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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.
+
==Mechanisms of adhesion==
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[[Image:Drops I.jpg|thumb|right|250px|Water droplets adhering to [[leaf|leaves]]]]
  
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Also see [[adhesive tape]] and [[gaffer tape]].
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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 other. 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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[[Plastic wrap]] displays temporary adhesive properties as well.
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[[Image:Water drops on spider web.jpg|thumb|right|250px|Dew drops adhere to a [[spider]] web]]
  
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==Mechanisms of adhesion==
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===Mechanical Adhesion===
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{{mainarticle|adhesion}}
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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]].
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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.
 
  
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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.
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===Chemical Adhesion===
 +
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.
  
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== Failure of the adhesive joint ==
+
===Dispersive Adhesion===
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When subjected to loading, debonding may occur at different locations in the adhesive joint. The major fracture types are the following
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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.
  
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=== Cohesive fracture ===
+
===Electrostatic Adhesion===
 +
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.
  
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“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”.
+
===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.
  
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=== Interfacial fracture ===
+
== Fracturing of the adhesive joint ==
 +
[[File:AdhesiveFractures.png|thumb|450px|The adhesive joint may fracture at different locations]]
  
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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.
+
When a load is placed on materials held together by an adhesive, the adhesive joint may fracture. There are several major types of fracture, as follows.
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[[Image:adhesive2.jpg|thumbnail|450px|Failure of the adhesive joint can occur in different locations]]
 
  
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=== Other types of fracture ===
+
*Cohesive fracture: A "cohesive" fracture is formed if a crack propagates in the bulk polymer that constitutes the adhesive. In this case, the surfaces of both adherents will be covered by fractured adhesive. The crack may propagate near the center of the layer or near an interface.
  
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Beside these two cases, other type of fracture are
+
*Interfacial fracture: The fracture is said to be "adhesive" or "interfacial" when separation occurs at the interface between the adhesive and the adherend. The interfacial character of a fracture surface is usually detected by visual inspection, but advanced surface characterization techniques (such as spectrophotometry) allow one to locate the crack precisely.
  
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*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.
+
*Mixed fracture: This is a case in which the crack propagates at some spots in a “cohesive” manner and in other areas in an “interfacial” manner.
  
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*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.
+
*Alternating crack path fracture: In this case, the cracks jump from one interface to the other. This type of fracture appears in the presence of tensile pre-stresses in the adhesive layer.
  
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*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.
+
In some cases, the adherend (substrate) may fracture while the adhesive, being tougher, may remain intact.
  
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== Design of adhesive joints ==
+
Consider some examples of different types of fractures. When one removes a price label attached to a product, adhesive usually remains partially on the label and partially on the surface of the product. This is a case of cohesive failure. If, however, a layer of paper remains stuck to the surface, the adhesive has not failed, but the fracture has occurred in one of the substrates. An example of an adhesive failure is when someone pulls apart an [[Oreo]] cookie and all the filling remains on one side.
  
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A general design rule is a relation of the type: "Material Properties > Function (geometry, loads)"
+
== Examples of glues ==
 +
Historically, the term "glue" referred to protein colloids prepared from animal tissues. The meaning has been extended to any glue-like substance used to attach one material to another. Below are some examples of adhesives commonly referred to as glues.
 +
[[Image:Super glue.jpg|thumb|300px|A tube of super glue]]
  
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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.
+
* [[Cyanoacrylate]] (brand names Super Glue, Krazy Glue)
 +
* [[Casein]] glue (protein glue)
 +
* [[Postage stamp gum]]
  
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Failure will also very much depend on the opening "mode" of the joint.[[Image:mode.jpg|thumbnail|350px|Modes of failure]]
+
* [[Cement]] glues:
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*'''Mode I''' is an opening or tensile mode where the loadings are normal to the crack.
+
** [[Contact cement]]
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*'''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.
+
** [[Rubber cement]]
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*'''Mode III''' is a tearing or antiplane shear mode.
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** Pyroxylin cement
 +
** Plastic cement (technically a [[solvent]], not a glue)
  
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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. 
+
* [[Resin]] glues:
 +
** [[Epoxy]] resins
 +
** [[Acrylic]] resin
 +
** [[Phenol formaldehyde resin]]
 +
** [[Polyvinyl acetate]] (PVA), including white glue (such as [[Elmer's glue]]) and yellow carpenter's glue (aliphatic resin)
 +
** [[Glue stick]]s (PVP (polyvinyl pyrrolidone) or PVA based)
 +
** [[Polyester]] resin
 +
** [[Resorcinol]] resin
 +
** Urea-resin glue (plastic resin)
 +
** [[Urea-formaldehyde]] resin
  
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Increasing the joint resistance is usually obtained by designing its geometry so that:
+
* [[Canada balsam]]
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*The bonded zone is large
 
−
*It is mainly loaded in mode II
 
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*Stable crack propagation will follow the appearance of a local failure.
 
  
−
== Testing the resistance of the adhesive ==
+
* [[Paste]]s:
 +
** Latex pastes
  
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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:
+
* [[Vegetable]]-based glues:
 +
** [[Mucilage]]
 +
** Starch glue
 +
** Soybean glue
 +
** Tapioca paste (commonly known as "vegetable glue")
  
−
*'''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)'''. [[Image:tests.jpg|thumbnail|450px|Testing devices]]
+
* [[Animal glue]]s:
 +
** [[Hide glue]] (flake and liquid versions)
 +
** Bone glue
 +
** Fish glue
 +
** Rabbit skin glue
 +
** Horse
 +
** [[Hoof glue]]
  
−
*'''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.
+
* [[Hot glue|Hot melt glue]]
 +
** Polyethylene hot melt
  
−
*'''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. 
+
* [[Acrylonitrile]]
 +
* [[Cellulose nitrate]]
 +
* [[Latex combo]]
 +
* [[Neoprene base]]
 +
* [[Polysulfide]]
 +
* [[Polyurethane]]
 +
* [[Polyvinyl chloride]] (PVC)
 +
* [[Rubber base]]
 +
* [[Silicone|Silicon base]]
 +
* [[Albumin glue]]
 +
* [[Ceramic adhesive]]
 +
* [[Ultraviolet glue]]
  
−
*'''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.
+
== Notes ==
 +
<references />
  
−
*'''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. 
+
==References==
  
−
*'''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.
+
* Comyn, John. ''Adhesion Science''. Royal Society of Chemistry Paperbacks, 1997. ISBN 0854045430
 +
* Kinloch, A. J. ''Adhesion and Adhesives: Science and Technology''. Chapman and Hall, 1987. New edition, 2001. Springer. ISBN 041227440X
 +
* Veslovsky, Roman A. and. Vladimir. N. Kestelman. ''Adhesion of Polymers''. New York: McGraw-Hill Professional, 2001. ISBN 0071370455
  
−
==References==
+
== External links ==
−
<references />
+
All links retrieved June 15, 2023.
 +
*[http://www.adhesionsociety.org/ The Adhesion Society]
 +
*[http://www.thomasnet.com/products/adhesives-393009-1.html ThomasNet - Directory of manufacturers and distributors of adhesives]
 +
*[http://www.adhesives.org Educational portal on adhesives and sealants]
  
 
[[Category:Physical sciences]]
 
[[Category:Physical sciences]]
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[[Category:Chemistry]]
 
[[Category:Chemistry]]
  
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{{credit|87607909}}
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{{credit3|Adhesive|87607909|Adhesion|87695505|Glue|86972235}}

Latest revision as of 05:51, 15 June 2023


Water droplets adhere to a Hibiscus flower

An adhesive is a material that can adhere (stick) to other materials and help attach them together. The state of attachment is known as adhesion, which is based on the attraction between molecules of the items in contact.

Various types of adhesives are now available, derived from natural and synthetic sources. Some modern synthetic adhesives are extremely strong and are increasingly used in construction and industry.

A glue gun contains a hot adhesive

History

It appears that the earliest adhesives used in history were natural gums and other plant resins. Archaeologists have found six-thousand-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 birch bark 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 for the use of rubber, bones, starch, fish, and casein. Modern synthetic adhesives have improved flexibility, toughness, curing rate, temperature, and chemical resistance.

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 liquefied 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

Water droplets adhering to leaves

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 other. 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.

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.

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.

Fracturing of the adhesive joint

The adhesive joint may fracture at different locations

When a load is placed on materials held together by an adhesive, the adhesive joint may fracture. There are several major types of fracture, as follows.

  • Cohesive fracture: A "cohesive" fracture is formed if a crack propagates in the bulk polymer that constitutes the adhesive. In this case, the surfaces of both adherents will be covered by fractured adhesive. The crack may propagate near the center of the layer or near an interface.
  • Interfacial fracture: The fracture is said to be "adhesive" or "interfacial" when separation occurs at the interface between the adhesive and the adherend. The interfacial character of a fracture surface is usually detected by visual inspection, but advanced surface characterization techniques (such as spectrophotometry) allow one to locate the crack precisely.
  • Mixed fracture: This is a case in which the crack propagates at some spots in a “cohesive” manner and in other areas in an “interfacial” manner.
  • Alternating crack path fracture: In this case, the cracks jump from one interface to the other. This type of fracture appears in the presence of tensile pre-stresses in the adhesive layer.

In some cases, the adherend (substrate) may fracture while the adhesive, being tougher, may remain intact.

Consider some examples of different types of fractures. When one removes a price label attached to a product, adhesive usually remains partially on the label and partially on the surface of the product. This is a case of cohesive failure. If, however, a layer of paper remains stuck to the surface, the adhesive has not failed, but the fracture has occurred in one of the substrates. An example of an adhesive failure is when someone pulls apart an Oreo cookie and all the filling remains on one side.

Examples of glues

Historically, the term "glue" referred to protein colloids prepared from animal tissues. The meaning has been extended to any glue-like substance used to attach one material to another. Below are some examples of adhesives commonly referred to as glues.

A tube of super glue
  • Cyanoacrylate (brand names Super Glue, Krazy Glue)
  • Casein glue (protein glue)
  • Postage stamp gum
  • Cement glues:
    • Contact cement
    • Rubber cement
    • Pyroxylin cement
    • Plastic cement (technically a solvent, not a glue)
  • Resin glues:
    • Epoxy resins
    • Acrylic resin
    • Phenol formaldehyde resin
    • Polyvinyl acetate (PVA), including white glue (such as Elmer's glue) and yellow carpenter's glue (aliphatic resin)
    • Glue sticks (PVP (polyvinyl pyrrolidone) or PVA based)
    • Polyester resin
    • Resorcinol resin
    • Urea-resin glue (plastic resin)
    • Urea-formaldehyde resin
  • Canada balsam
  • Pastes:
    • Latex pastes
  • Vegetable-based glues:
    • Mucilage
    • Starch glue
    • Soybean glue
    • Tapioca paste (commonly known as "vegetable glue")
  • Animal glues:
    • Hide glue (flake and liquid versions)
    • Bone glue
    • Fish glue
    • Rabbit skin glue
    • Horse
    • Hoof glue
  • Hot melt glue
    • Polyethylene hot melt
  • Acrylonitrile
  • Cellulose nitrate
  • Latex combo
  • Neoprene base
  • Polysulfide
  • Polyurethane
  • Polyvinyl chloride (PVC)
  • Rubber base
  • Silicon base
  • Albumin glue
  • Ceramic adhesive
  • Ultraviolet glue

Notes

  1. ↑ Contact Adhesives ThisToThat. Retrieved August 16, 2018.
  2. ↑ What are some of the uses for contact cement? HowSTuffWorks. Retrieved August 16, 2018.

References
ISBN links support NWE through referral fees

  • Comyn, John. Adhesion Science. Royal Society of Chemistry Paperbacks, 1997. ISBN 0854045430
  • Kinloch, A. J. Adhesion and Adhesives: Science and Technology. Chapman and Hall, 1987. New edition, 2001. Springer. ISBN 041227440X
  • Veslovsky, Roman A. and. Vladimir. N. Kestelman. Adhesion of Polymers. New York: McGraw-Hill Professional, 2001. ISBN 0071370455

External links

All links retrieved June 15, 2023.

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