Adhesion and Adhesives Theory Presented by Shrikant Athavale For PG Students , SIES Nerul On 09-10-2010
Adhesion is a attraction process between dissimilar surfaces that cling to one another and cohesion takes place between similar molecules. (cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can be divided into several different types. The intermolecular forces responsible for the function of various kinds of stickers and sticky tape fall into the categories of chemical adhesion, dispersive adhesion, and diffusive adhesion.
<ul><li>Mechanisms of adhesion </li></ul><ul><li>The most important theories of adhesion are </li></ul><ul><li>Physical absorption, </li></ul><ul><li>Chemical bonding, </li></ul><ul><li>Diffusion, </li></ul><ul><li>Electrostatic, </li></ul><ul><li>Mechanical interlocking and </li></ul><ul><li>As all adhesive bonds involve molecules in intimate contact, physical adsorption must always contribute. </li></ul>
Physical Absorption Theory The adhesion results from molecular contact between two materials and the surface forces that develop, usually designated as secondary or van der Walls forces. For these forces to develop, the adhesive must make intimate, molecular contact with the substrate surface. The process of establishing continuous contact between an adhesive and the adherent is known as "wetting." Wetting can be determined by contact angle measurements.
Complete, spontaneous wetting occurs when contact angle is 0 deg, or the material spreads uniformly over a substrate to form a thin sheet. Wetting is favored when the substrate's surface tension, better known as the critical surface energy, C, is high and the surface tension of the wetting liquid, , is low (i.e., C substrate > adhesive).
Low-energy polymers, therefore easily wet high-energy substrates such as metal and glass. Conversely, substrates having low surface energies (e.g., polyethylene and fluorocarbons) will not be readily wet by other materials and are useful for applications requiring nonstick, passive surfaces. After intimate contact is achieved between adhesive and adherent through wetting, it is believed that permanent adhesion results primarily through forces of molecular attraction. We will study more about wettability later.
Chemical Bonding Theory The chemical bonding theory of adhesion invokes the formation of covalent, ionic or hydrogen bonds across the interface. There is some evidence that covalent bonds are formed with silane coupling agents ( or adhesion promoters), and it is possible that adhesives containing isocyanate groups react with active hydrogen atoms, such as hydroxyl groups, to form a permanent bond .
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 the molecule diffuses into the other material. When polymer granules are pressed together and heated, atoms diffuse from one particle to the next. This joins the particles into one.
The diffusion theory attributes the adhesion of polymeric materials to the inter-penetration of chains at the interface. The major driving force for polymer autohesion is due to mutual diffusion of polymer molecules across the interface . This theory requires that both the adhesive and adherend are polymers, which are capable of movement and are mutually compatible and miscible.
Parameters affecting the diffusion process are: contact time, temperature, molecular weight of polymers and physical form (liquid, solid). Polarity generally increases adhesion. The diffusion theory, however, has found limited application where the polymer and adherend are not soluble or the chain movement of the polymer is constrained by its highly crosslinked, crystalline structure, or when it is below its glass transition temperature.
Such interdiffusion will occur only if the polymer chains are mobile (i.e. the temperature must be above the glass transition temperatures) and compatible. As most polymers, including those with very similar chemical structures such as polyethylene and polypropylene are incompatible, the theory is generally only applicable in bonding like rubbery polymers, as might occur when surfaces coated with contact adhesives are pressed together, and in the solvent-welding of thermoplastics. An example of the latter is to swell two polystyrene surfaces with butanone and then press them together. The solvent has the effect of lowering the glass transition temperature below ambient while interdiffusion takes place; it later evaporates.
Adhesion theory – Electrostatic The basis of the electrostatic theory of adhesion is the difference in electonegativities of adhesing materials. Adhesive force is attributed to the transfer of electrons across the interface creating positive and negative charges that attract one another. For example, when an organic polymer ( of Conductive Nature) is brought into contact with metal, electrons are transferred from metal into the polymer, creating an attracting electrical double layer (EDL). The electrostatic theory tell us that these electrostatic forces at the interface ( i.e. in the EDL), account for resistance to separation of the adhesive and the substrate.
The electrostatic theory originated in the proposal that if two metals areplaced in contact, electrons will be transferred from one to the other so forming an electrical double layer, which gives a force of attraction. As polymers are insulators, it seems difficult to apply this theory to adhesives.
Mechanical adhesion Adhesive materials fill the voids or pores of the surfaces and hold surfaces together by interlocking. Adhesion occurs by the adhesive flowing and filling micro-cavities on the substrate. When the adhesive then hardens, the substrates are held together mechanically. The adhesive must not only wet the substrate, but also have the right rheological properties to penetrate pores and openings in a reasonable time. The surface of a substrate is never truly smooth but consists of a maze of peaks and valleys. According to the mechanical theory of adhesion, in order to function properly, the adhesive must penetrate the cavities on the surface, displace the trapped air at the interface, and lock-on mechanically to the substrate.
The surface roughness aids in adhesion by increasing the total contact area between the adhesive or sealant and the adherent. Increasing the actual area of contact will increase the total energy of surface interaction by a proportional amount. Thus, the mechanical theory generally suggests that roughening of surfaces is beneficial because 1. it gives "teeth" to the substrate (mechanical interlocking) and 2. increases the total effective area over which the forces of adhesion can develop. However, roughening is only effective if the adhesive wets the surface well.
This theory explains a few examples adhesion such as rubber bonding to textiles and paper. Since good adhesion can occur between smooth adherent surfaces as well, it is clear that while interlocking helps promote adhesion, it is not really a generally applicable adhesion mechanism. Pretreatment methods applied on surfaces enhance adhesion . These pretreatments (especially plastic surface treatments) result in microroughness on the adherend surface, which can improve bond strength and durability by providing mechanical interlocking. Beyond mechanical interlocking, the enhancement of the adhesive joint strength due to the roughing of the adherend surface may also result from other factors such as formation of a larger surface, improved kinetics of wetting and increased plastic deformation of the adhesive .
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials. Adhesives cure (harden) by either evaporating a solvent or by chemical reactions that occur between two or more constituents. Adhesives are an advantageous for joining thin or dissimilar materials, minimizing weight, and when a vibration dampening joint is needed. [ A disadvantage to adhesives is that they do not form an instantaneous joint, unlike most other joining processes, because the adhesive needs time to cure.
Now a days adhesives are used in all types of manufacture, and in many cases have displaced other means of joining. A range of adhesives (hot melt, vegetable glues and emulsions) are used in making cardboard boxes, with rarely a staple to be seen. Apart from expensive handmade shoes, footwear is now adhesively bonded using hot melt adhesives for the basic construction, natural rubber latex for linings, and solvent based polyurethanes or polychloroprenes for sole attachment. Bookbinding is by hot melt adhesives. Adhesive bonding is used increasingly in the construction of aircraft.
Rubber-to-metal bonds are used for engine, transmission and exhaust mountings in automobiles and in railway bogie suspensions. Mass produced car bodies are made of spot-welded mild steel; weight and fuel consumption can be reduced with aluminium bodies, which are more difficult to spot-weld. The large-scale bonding of car bodies is a prize that awaits the adhesives industry. A recent achievement was the bonding of steel rails in the new Manchester tramway. Human beings can be repaired by adhesives. This includes the use of UV-curing cements in dentistry and acrylic bond cements in orthopaedic surgery.
Adhesives are not the only materials that must stick or adhere. Adhesion is essential for printing inks, sealants, paints and other surface coatings, and at interfaces in composite materials such as steel or textile fibres in rubber tyres and glass- or carbon-fibres in plastics. A disadvantage of adhesives as a means of joining is that they are generally weakened by water and its vapour. Also, their service temperature ranges are less than for metal fasteners (nuts, bolts, welds, staples, etc.), being limited by their glass transition temperature and chemical degradation. Advantages include their ability to join dissimilar materials and thin sheet materials, the spreading of load over a wider area, the aesthetic and aerodynamic exteriors of joints, and application by machine or robot.
BASIC PROPERTIES What is an adhesive and what are its basic properties? A definition is a material which when applied to the surfaces of materials can join them together and resist separation. The terms adherent and substrate are used for a body or material to be bonded by an adhesive. Other basic terms are shelf-life, for the time an adhesive can be stored before use, and pot-life, the maximum time between final mixing and application.
Basically an adhesive must do two things: (i) It must wet the surfaces, that is it must spread and make a contact angle approaching zero, as is illustrated in Figure 1.1. Intimate contact is required between the molecules of the adhesive and the atoms and molecules in the surface. When applied the adhesive will be a liquid of relatively low viscosity. (ii) The adhesive must then harden to a cohesively strong solid. This can be by chemical reaction, loss of solvent or water, or by cooling in the case of hot melt adhesives. There is an exception to this, and that is pressure-sensitive adhesives which remain permanently sticky. These are the adhesives used in sticky tapes and labels.
liquid droplets making n high and low contact angle on a flat , solid surface.
high contact angle leading to no spreading on a rough surface.
BASIC CHEMISTRY All adhesives either contain polymers, or polymers are formed within the adhesive bond. Polymers give adhesives cohesive strength, and can be thought of as strings of beads (identical chemical units joined by single covalent bonds), which may be either linear, branched or crosslinked as illustrated in Figure 1.2. Linear and branched polymers have similar properties and it is not easy to distinguish them, and they will flow at higher temperatures and dissolve in suitable solvents. These latter properties are essential in hot melt, and solvent-based adhesives, respectively. Crosslinked polymers will not flow when heated, and may swell, but not dissolve, in solvents. All structural adhesives are crosslinked because this eliminates creep (deformation under constant load). Automotive tyres are crosslinked natural or synthetic rubber, and if they crept they would permanently deform during parking, and a rough ride would follow.
Linear polymer Branched polymer Cross linked polymer
Many adhesives contain additives that are not polymers are these include stabilizers against degradation by oxygen and UV, plasticizers which increase flexibility and lower the glass transition temperature, and powdered mineral fillers, which may reduce shrinkage on hardening, lower cost, modify flow properties before hardening and modify final mechanical properties. Other possible additives are tackifiers and silane coupling agents.
Adhesive Types Adhesives are typically organized by the method of adhesion. These are then organized into reactive and non-reactive adhesives, which refers to if the adhesive chemically reacts to harden. Alternatively they can be organized by whether the raw stock is of natural, or synthetic origin, or by their starting physical phase . The Adhesives can be broadly divided into two classes Non-reactive adhesives And Reactive Adhesives [ edit ] [ edit ] Natural adhesives Natural adhesives are made from organic sources such as vegetable matter, starch ( dextrin ), natural resins or from animals e.g. casein or animal glue . They are often referred to as bioadhesives . One example is a simple paste made by cooking flour in water. Animal glues are traditionally used in bookbinding, wood joining, and many other areas but now are largely replaced by synthetic glues. Casein are mainly used in glass bottle labelling. Starch based adhesives are used in corrugated board production and paper sack production, paper tube winding, wall paper adhesives. Another form of natural adhesive is blood albumen (made from protein component of blood), which is used in the plywood industry. Animal glue remains the preferred glue of the luthier . Casein based glues are made by precipitating casein from milk protein using the acetic acid from vinegar . This forms curds , which are neutralized with a base , such as sodium bicarbonate (baking soda), to cause them to unclump and become a thicker plastic-like substance.  [ edit ] Synthetic adhesives Synthetic adhesives are based on elastomers , thermoplastics , emulsions , and thermosets . Examples of thermosetting adhesives are: epoxy , polyurethane , cyanoacrylate and acrylic polymers.
Types of Adhesive The Adhesives can be broadly divided into two classes Non-reactive adhesives And Reactive Adhesives ( refers to if the adhesive chemically reacts to harden )
Non-reactive adhesives Are further classifed based on their source Natural Adhesive such as Starch , casein or animal glue. They are often referred to as bioadhesives. Natural Rubber based Adhesive Synthetic Adhesives Based on synthetic Polymers ( or Elasomers ), such as Styrene Butadiene Rubber, Bytyl Rubber, Polychloroprene Rubber, Nitrile Rubber , Poly Urathane , Silicone Rubber.
Adhesives are also classified as Pressure Sensitive Non Pressure sensitive Pressure Sensitive Adhesives As the name suggests , are sensitive to little pressure , and are sticky in nature , at ambient temp. They are manufactured from various Polymers , such as Nat. Rubber, and synthetic rubbers like Butyl, SBR, Nitrile , Silicone and Acrylics These adhesives are compounded using above mentioned polymer , Main Tackifying Resin , second resin, antioxidant , plasticiser , filler and a solvent .
Pressure sensitive adhesives The following text needs to be harmonized with text in Pressure sensitive adhesive . Main article: Pressure sensitive adhesive Pressure sensitive adhesives (PSA) 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 (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. 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 initiate a cross-linking 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 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. Major raw material for PSA´s are acrylate based polymers. [ edit ]
The solvent could be , water or organic solvents such as , Toluene, Hexane, Ethyl Acetate etc. Hence the Pressure sensitive adhesives are also classified as Water based , Solvent Based Solventless And Hotmelt .
Non Pressure sensitive Such as Lamination grade adhesive Contact adhesive
Contact adhesives Contact adhesives are used in strong bonds with high shear-resistance like laminates , such as bonding Formica to a wooden counter, and in footwear , as in attaching outsoles to uppers. Natural rubber and polychloroprene (Neoprene) are commonly used contact adhesives. Both of these elastomers undergo strain crystallization . Contact adhesives 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.  It is usually not necessary to apply pressure for a long time, so there is less need for clamps . [ edit ]
Hot adhesives A glue gun, an example of a hot adhesive Main article: Hot melt adhesive Hot adhesives , also known as hot melt adhesives , are simply thermoplastics applied in molten form (in the 65-180 C range) which solidify on cooling to form strong bonds between a wide range of materials. These adhesives are popular for crafts because of their ease of use and the wide range of common materials they can join. A glue gun (shown at right) is one method of applying hot adhesives. The glue gun melts the solid adhesive then allows the liquid to pass through its barrel onto the material, where it solidifies. Thermoplastic glue may have been invented around 1940 by Procter & Gamble as a solution to water-based adhesives commonly used in packaging at that time failing in humid climates, causing packages to open and become damaged. [ edit ]
Drying adhesives There are two types of adhesives that harden by drying: solvent based adhesives and polymer dispersion adhesives , also known as emulsion adhesives . Solvent based adhesives are a mixture of ingredients (typically polymers ) dissolved in a solvent . White glue, contact adhesives 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. Polymer dispersion adhesives are milky-white dispersions often based on polyvinyl acetate (PVAc). Used extensively in the woodworking and packaging industries. Also used with fabrics and fabric-based components, and in engineered products such as loudspeaker cones.
Reactive adhesives There are multi component or Single component Adhesives Two ( or more ) - part adhesives Multi-part adhesives harden by mixing two or more components which chemically react. This reaction causes polymers to cross-link These adhesives are generally based on acrylics, urethanes, and epoxies.
One-part adhesives One-part adhesives harden via a chemical reaction with an external energy source, such as radiation, heat, and moisture. Ultraviolet (UV) light curing adhesives , also known as light curing materials (LCM), have become popular within the manufacturing sector due to their rapid curing time and strong bond strength. Light curing adhesives can cure in as little as a second and many formulations can bond dissimilar substrates (materials) and withstand harsh temperatures. These qualities make UV curing adhesives essential to the manufacturing of items in many industrial markets such as electronics, telecommunications, medical, aerospace, glass, and optical. Unlike traditional adhesives, UV light curing adhesives not only bond materials together but they can also be used to seal and coat products. They are generally acrylic based. Heat curing adhesives consist of a pre-made mixture of two or more components. When heat is applied the components react and cross-link. This type of adhesive includes epoxies, urethanes, and polyimides. Moisture curing adhesives cure when they react with moisture present on the substrate surface or in the air. This type of adhesive includes cyanoacrylates and urethanes.
Factors that influence the adhesion - Introduction The stronger adhesion of bonds between mechanically or chemically roughened surfaces is based on the enlargement of the effective surface (contact surface between the adhesive and the substrate), and an increase in the number of active centres, e. g. edges, corners, and faulty parts which, as in the heterogeneous catalysis, increase the interactive forces in the interface adhesive/surface.
The following factors have a predominant importance in the adhesion process: <ul><li>Wetting of the surface </li></ul><ul><li>Surface treatment </li></ul><ul><li>Structure of the materials to be bonded (incl. Adhesives and substrates) </li></ul><ul><li>Design of the joint (incl. stresses applied on the bonded materials) </li></ul><ul><li>The Glass Transition Temp </li></ul>
Factors that influence the adhesion - Wetting of the surface To enable the adhesive bonds between the adhesive and the surface, the adhesive must first wet the surface; in other words, it must be applied in the liquid form (as a solution, dispersion, or hot-melt). A measure for the wettability of a surface is the angle of contact that forms between a drop of liquid and a smooth, plain surface.
A good wetting occurs when the angle of contact () between the adhesive and the substrate is inferior to 90? Complete wetting occurs when the molecular attraction between the liquid and solid molecules is greater than that between similar liquid molecules. Whether or not a given liquid will wet a solid depends on the surface tension of both substances, eg polymer and substrate. The contact surface formed during wetting depends on the surface tension and the viscosity of the adhesive, and also on the structure (shape and size of the pores) of the surface. The size of the effective surface is generally smaller than the true surface of the substrate, because the pores and uneven parts of the surface are not completely filled by the adhesive. Pressure may also help enhance the adhesion. Generally, bonds that have been set under pressure have higher adhesive strength. Pressures imparts better wetting and consequently a more complete interfacial contact. The viscosity of the adhesive is critical to wetting, e.g.: the lower the viscosity, the more easily it will wet the substrate. It is obvious to say that the rheological properties of the adhesive must be adapted to the application conditions (substrate's surface, curing time, pressure, temperature).
Factors that influence the adhesion - Surface treatment All surfaces exposed to the normal atmosphere undergo gas and water adsorption in the molecular range; the surface condition can be also changed by oxidation processes. To ensure a good adhesion it is sometimes necessary to carry out, particularly on metals, expensive mechanical and/or chemical pre-treatment (e.g. sandblasting and pickling). On the other hand, inert (too little reactive) plastics surfaces are activated by subjecting them to specific surface treatment for plastics (eg flame treatment, corona discharge). In principle, these processes serve to form active centres and polar, reactive groups, which favour the wettability and the chemisorption of suitably pretreated surfaces. The quality of the parts being joined is paramount for the quality of the bonded joint and, in particular, its resistance to ageing. The surface must therefore be suitably treated before the adhesive is applied. Wide-ranging methods of surface pretreatment exist. In every case, contaminants such as oil, grease, drawing and releasing agents, plasticizers, etc. must be removed with suitable cleaning agents.
Flame treatment, corona pretreatment, plasma treatment of plastics which are difficult to bond, e.g. PE, PA, PP... Blasting treatments of all types (dry or wet) using a fine-grain sharp sand or shot Removal of unwanted contaminating films by degreasing/cleaning agents. Pickling of plastics which are difficult to bond, e.g. PTFE, POM and PP Use of abrasive belts,disks, emery paper (120 to 180grain) etc. after degreasing Surface priming Picking of aluminium, hardened and stainless steel and hard metals Processing with hard and powered brushes of varying types (after degreasing) Removal of dust, oxides, remnants of paints and dirt Chemical & physical treatment Mechanical treatment Cleaning & degreasing
In this connection, we should also mention coupling agents or adhesion promoters. These are in most cases bifunctional, low-molecular substances, e. g. titanates, chlorosilanes, and chromium complexes of unsaturated carboxylic acids, which fix the adhesive on the surface by chemical reactions. The mode of action of these adhesion promoters is based on their bifunctionality. One group reacts with reactive groups of the adherends, while the second group reacts with the adhesive. It is advisable, therefore, to use adhesion promoters whose groups react differently or according to different types of reaction, e. g. by substitution or radical reaction.
Factors that influence the adhesion - Stucture of the materials to be bonded Besides the surface condition, the structure of the materials to be bonded is also of decisive importance. Porous materials (e. g. wood, paper, and textiles) absorb low viscosity adhesives. The result of this adhesive's penetration are thin, uneven ("starved") joints which often impair the strength of the bond. On the other hand, the more volatile, i. e. low molecular substances, e.g. solvents, are absorbed by the capillaries preferably. This process results in a rapid adhesion, but it can have a negative influence on the distribution of the polymer in the glue line owing to the simultaneous separation of oligomers. In addition, the solvent molecules compete with the adhesive molecules in regard to the adsorption. The adhesive molecules are first adsorbed out of the adhesive solution through contact points separated by loops With progressing evaporation of the solvent, the adhesive molecules or segments are then adsorbed mainly at the surface.
The molecular structure of the adhesive is decisive for the cohesion, i. e. the state in which the particles of a single substance are held together, and in connection with the surface condition described above, for the adhesion. The principal molecular influencing factors are: the molecular weight or the distribution of the molecular weight, the number and size of the side-groups, and the polarity: The macromolecules acting as an adhesive are either produced by a preceding polyreaction and then applied in the liquid form (solution, dispersion, or hot-melt) to the adherend, or they are produced by polyreactions of reactive low-molecular compounds in the glue line direct. In the case of adhesives produced by preceeding polyreaction, the molecular weight must not be infinitely high (viscosity and solubility depend on the molecular weight) With adhesives produced by polyreaction of reactive low-molecular compounds, it is frequently a desired objective to achieve a high molecular weight, which is often obtained by crosslinking reactions. The higher the molecular weight, the higher the tensile strength of linear polymers, which is a measure for the cohesion, as shown below.
Factors that influence the adhesion - Stucture of the joint An important prerequisite for the successful use of bonding technology is that the respective parts be suitably designed for bonding, as distinct from welding, for example. Care must be taken to provide a sufficiently large bonded area, such as a large area of overlap of the mating parts. The ideal bonded joint is one under all practical loading conditions the adhesives is stessed in the direction in which it most resists failure. Favorable stress can be applied to the bond by using proper joint design.
However, some joint designs may be impractical, extensive to make, or hard to align. The design engineer will often have to weigh these factors against optimum adhesive performance. Adhesion matrix - For rubber & thermoset substrates This adhesion matrix will help you in finding the best suitable polymer base for your adhesive regarding the substrate you want to bond .
GLASS TRANSITION TEMPERATURE The mechanical properties of polymers radically change at the glass transiton temperature (TJ; molecular motion is the underlying cause of the change. Below Tg there is no translational or rotational motion of the atoms that make up the polymer backbone, but these motions are present above Tg. Below Tg, polymers are relatively hard, inflexible and brittle, whilst above it they are soft and flexible. The terms glassy, and rubbery or leathery are used to describe properties in the two temperature regions. Both glassy and rubbery polymers are used as adhesives, examples being the use of glassy adhesives for structural bonding in engineering and bone cements in surgery, and rubbery ones as pressure-sensitive adhesives and for bonding flexible substrates. It is unacceptable for an adhesive to pass through the glass transition during service. The theory most used to account for the glass transition is thefree volume theory. The basis is that a polymer consists of occupied volume plus free volume, with the latter increasing on thermal expansion. Once the fraction of free volume reaches a critical amount which is about 2.5%, the chain segments become mobile and the polymer enters the leathery state.
The glass transition temperatures of some polymers used in adhesives are shown in Table 1.2. Polar groups in polymers increase intermolecular forces and thus reduce free volume and increase Tg. This is illustrated by the effect of replacing the C-CH, bonds in natural rubber with C-Cl bonds, as in polychloroprene, which is to increase Tg by 25 "C. In contrast, non-polar side groups tend to hold chains apart and lower T,, as is shown by series of acrylic polymers from PMMA to PBMA, for which the repeat units are shown in structure 1.1. The aromatic amine hardeners DAB and DDM for the diglycidyl ether of bisphenol-A (DGEBA) give higher glass transition temperatures than the aliphatic amines because the molecules are more rigid. DAPEE is a particularly flexible molecule giving the lowest T, of the examples shown. See structural formulae 1.2.
VISCOELASTIC PROPERTIES Polymers are described as viscoelastic in that they show a combination of the properties of a spring, and a dashpot filled with a viscous liquid (an automotive shock-absorber). A spring will deform instantaneously when loaded, and will recover fully and instantaneously when the load is removed. The deformation of a dashpot will increase with time, and it will not recover when the load is removed. A model which contains two springs and two dashpots, and which describes the qualitative behaviour of polymers and adhesives is shown in Figure 1.7. On loading, spring B will instantly deform and dashpot A will begin to flow interminably. The response of spring C will be delayed by dashpot D. When the load is removed, spring B will recover immediately and fully; the recovery of spring C will be total but delayed by dashpot D. The deformation of dashpot D is irreversible. Clearly the properties of an adhesive that might be used in engineering will be dominated by the spring-like properties, and such adhesives are crosslinked to eliminate the viscous element. In contrast, pressuresensitive adhesives for tapes and for sticky solids such as Blu-Tak'R have a large viscous component, and the transverse stripes which form on clear tapes at the point where they are left to dwell on the roll is due to viscous flow.
Adhesion matrix - For themoplastic substrates This adhesion matrix will help you in finding the best suitable polymer base for your adhesive regarding the substrate you want to bond . Click on the polymer name to know more about the adhesive type.
Application Applicators of different adhesives are designed according to the adhesive being used and the size of the area to which the adhesive will be applied. The adhesive is applied to either one or both of the materials being bonded. The pieces are aligned and pressure is added to aid in adhesion and rid the bond of air bubbles. Common ways of applying an adhesive include brushes, rollers, using films or pellets, spray guns and applicator guns ( e.g. , caulk gun ). All of these can be done manually or can be automated into a machine.