An adhesive is a compound that adheres or bonds two items together. Adhesion is the tendency of certain dissimilar molecules to cling together due to Attractive forces. Adhesives may come from either natural or synthetic sources. In Chemistry, chemical synthesis is purposeful execution of Chemical reactions in order to get a product, or several products Some modern adhesives are extremely strong, and are becoming increasingly important in modern construction and industry.
Contents |
History
The first adhesives were natural gums and other plant resins or saps. Natural gums are Polysaccharides of natural origin capable of causing a large Viscosity increase in solution even at small concentrations Resin, not to be confused with Rosin, is a Hydrocarbon Secretion of many Plants particularly coniferous trees. It was believed that the Sumerian people were the first to use it until it was discovered that Neanderthals as far back as 50,000 years made adhesives from birch bark. Sumer ( Sumerian: sux-Latn [[Ki (earth ki]]-[[EN (cuneiform en]]-'''ĝir15''', Akkadian: Šumeru; possibly Biblical Shinar The Neanderthal (neɪˈændərtɑːl also with /niː-/ and /-θɔːl/ or Neandertal, is an extinct member of the Homo genus that is known from The discovery of 6000-year-old ceramics brought evidence to archaeologists about the first practical uses and ingredients of the first adhesives. Archaeology, archeology, or archæology (from Greek grc ἀρχαιολογία archaiologia – grc ἀρχαῖος archaīos Most early adhesives were animal glues made by rendering animal products such as horse teeth. An animal glue is an Adhesive that is created by prolonged boiling of Animal Connective tissue. During the times of Babylonia, tar-like glue was used for gluing statues. Babylon was a City-state of ancient Mesopotamia, the remains of which can be found in present-day Al Hillah, Babil Province, Iraq The Egyptians made much use of animal glues to adhere furniture, ivory, and papyrus. This article is about the contemporary North African ethnic group Ivory is formed from Dentine and constitutes the bulk of the Teeth and Tusks of animals such as the Elephant, Hippopotamus, Papyrus (/pəˈpaɪrəs/ (Rhymes -aɪrəs)is a thick paper-like material produced from the Pith of the papyrus plant Cyperus papyrus The Mongols also used adhesives to make their short bows, and the Native Americans of the eastern United States used a mixture of spruce gum and fat as adhesives to add waterproof seams in their birchbark canoes. Native Americans in the United States are the indigenous peoples from the regions of North America now encompassed by the continental United States Spruce refers to Trees of the genus Picea, a genus of about 35 species of Coniferous Evergreen trees in the Family Pinaceae Birch bark or birchbark is generally understood to be the Bark of the Paper Birch tree ( Betula papyrifera) or sometimes of related species such A canoe is a small narrow Boat, typically human-powered though it may also be powered by sails or small electric or gas motors
In Medieval Europe/Eurasia, egg whites were used as glue to decorate parchments with gold leaf. Parchment is a thin material made from Calfskin, Sheepskin or goatskin. Metal leaf is a thin foil used for decoration It is also called composition leaf or schlagmetal. Holland, in the early 1700s, founded the first ever glue factory. Later, in the 1750s, the British introduced fish glue. As the modern world evolved, several other patented materials, such as bones, starch, fish, and casein, were introduced as alternative materials for glue manufacture. See Casein paint for information about casein usage in artistic painting Modern glues have improved beyond recognition. Such improvements are noticeable in its flexibility, toughness, curing rate, temperature, and chemical resistance. The bond between two items depends on the shape of the adhesive.
Categories of adhesives
Homemade casein adhesive
This is an adhesive prepared at home, or by one's own efforts with normal household products. There are many types of glue that can be made. Homemade glue may stick or hold better than commercial glue (for example, Elmer's glue), depending on the ingredients that are used. Elmer's Products Inc is the manufacturer of Elmer's Glue-All and other crafts and home repair products including Krazy Glue, Ross Glue, ProBond The glue is best kept in an airtight container in the refrigerator. See more at: Homemade glue
Natural adhesives
Natural adhesives are made from inorganic mineral sources, or biological sources such as vegetable matter, starch (dextrin), natural resins, animal skin, and bioadhesives. Homemade glue is an Adhesive that can be prepared with normal household products Starch, CAS # 9005-25-8 Chemical formula (C6H10O5n is a Polysaccharide Dextrins are a group of low-molecular-weight Carbohydrates produced by the Hydrolysis of Starch. Bioadhesives are natural Polymeric materials that act as Adhesives. A simple paste can be made by mixing flour and water.
Synthetic adhesives
Elastomers, thermoplastic, and thermosetting adhesives are examples of synthetic adhesives. An elastomer is a Polymer with the property of Elasticity. The term which is derived from elastic polymer, is often used interchangeably with the term A thermoplastic is a Plastic that Melts to a liquid when heated and freezes to a Brittle, very Glassy state when cooled sufficiently Thermosetting plastics thermosets are Polymer materials that irreversibly cure form
Drying adhesives
These adhesives are a mixture of ingredients (typically polymers) dissolved in a solvent. A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds A solvent is a liquid or gas that dissolves a solid liquid or gaseous Solute, resulting in a Solution. Glues such as white glue, and rubber cements are members of the drying adhesive family. Rubber cement is an Adhesive made from elastic Polymers (typically latex mixed in a Solvent such as Acetone, Hexane, Heptane 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 use by 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. [1] Once the surfaces are pushed together, the bond forms very quickly,[2] 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. A clamp is a fastening device to hold or secure objects tightly together to prevent movement or separation through the application of inward Pressure.
Natural rubber and polychloroprene (Neoprene) are commonly used contact adhesives. Neoprene or polychloroprene is a family of synthetic Rubbers that are produced by Polymerization of Chloroprene. Both of these elastomers undergo strain crystallization. Strain Crystallization is a phenomenon in which an initially amorphous solid material undergoes a Phase transformation due to the application of strain
Contact adhesives find use in laminates, such as bonding Formica to a wooden counter, and in footwear, for example attachment of an outsole to an upper. A laminate is a material constructed by uniting two or more layers of material together Formica is a brand of Composite materials manufactured by the Formica Corporation based in Cincinnati, Ohio, United States. Footwear consists of Garments worn on the feet, for protection against the environment and Adornment.
Hot adhesives (thermoplastic adhesives)
Also known as "hot melt" adhesives, these adhesives are thermoplastics; they are applied hot and simply allowed to harden as they cool. Glue gun redirects here for the band Glue Gun see Glue Gun (band. A thermoplastic is a Plastic that Melts to a liquid when heated and freezes to a Brittle, very Glassy state when cooled sufficiently 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.
Paul E. Cope [deceased, 2003] is reputed to have invented thermoplastic glue [circa 1940] while working for Procter & Gamble as a chemical and packaging engineer. Procter & Gamble Co ( P&G,) is a Fortune 500, American global corporation based in Cincinnati Ohio, that manufactures a wide His invention solved a problem with water based adhesives that were commonly used in packaging at that time. Water based adhesives often released in humid climates which caused packages to open and become damaged. Mr. Cope was a graduate of the University of Cincinnati College of Engineering. He advanced at Procter & Gamble to become Associate Director, Head of Packaging Engineering. After spending 40 years with P&G, he retired in 1973. Patents issued to Paul Cope include the laminated toothpaste tube and this for In Package Sterilization
Reactive adhesives
A reactive adhesive works either by chemical bonding with the surface material or by in-situ hardening as two reactant chemicals complete a polymerization reaction. They are usually 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. In Chemistry, epoxy or polyepoxide is a Thermosetting Epoxide Polymer that cures (polymerizes and crosslinks when mixed with a A peroxide is a compound containing an Oxygen -oxygen single bond. Silane is a Chemical compound with Chemical formula Si[[hydrogen H]]4 Isocyanate is the Functional group of atoms &ndashN=C=O (1 Nitrogen, 1 Carbon, 1 Oxygen) not to be confused with the Cyanate
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
UV and light curing adhesives consist essentially of low or medium molecular weight resins.
Pressure sensitive adhesives
Pressure sensitive adhesives (PSAs) form a bond by the application of light pressure to marry the adhesive with the adherend. Pressure sensitive adhesive ( PSA, self adhesive, self stick adhesive) is Adhesive that forms a bond when Pressure is applied to marry They are designed with a balance between flow and resistance to flow. The bond forms because the adhesive is soft enough to flow (i. e. "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. The Van der Waals equation is an Equation of state that can be derived from a special form of the potential between a pair of molecules (hard-sphere repulsion
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. Cross-links are bonds that link one Polymer chain to another They can be Covalent bonds or Ionic bonds "Polymer chains" can refer The molecular mass (abbreviated m of a substance, more commonly referred to as molecular weight and abbreviated as MW, is the Mass of one 100% solid PSAs may be low viscosity polymers that are coated and then reacted with radiation to increase molecular weight and form the adhesive; 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. Adhesive tape can be one of many varieties of backing materials coated with an Adhesive. Gaffer tape, sometimes shortened to gaff tape (especially by theater and photographic professionals or made possessive as " gaffer's tape" is a strong
Plastic wrap displays temporary adhesive properties as well. Plastic wrap is a thin plastic film typically used for sealing Food items in containers to keep them fresh
Mechanisms of 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 is the tendency of certain dissimilar molecules to cling together due to Attractive forces. 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. A chemical bond is the physical process responsible for the attractive interactions between Atoms and Molecules and which confers stability to diatomic and polyatomic In others electrostatic forces, as in static electricity, hold the substances together. A third mechanism involves the van der Waals forces that develop between molecules. The Van der Waals equation is an Equation of state that can be derived from a special form of the potential between a pair of molecules (hard-sphere repulsion A fourth 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 to identify the precise location of the crack path in the interphase.
Other types of fracture
Beside these two cases, other types 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. Oreo is a trademark for a popular sandwich Cookie currently manufactured by the Nabisco Division of Kraft Foods. 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. Fracture mechanics is the field of Mechanics concerned with the study of the formation of cracks in materials 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) 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 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 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.
See also
References
External links
- DIYinfo.org's Glues, Adhesives, Sealants And Gap Fillers Wiki - Heaps of practical information on adhesives
- Adhesive and Sealant Council
- ASI Magazine
- New technology of bio-glue
- Adhesives Product News on ThomasNet Industrial Newsroom
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic