Introduction to Lamination

Adhesive bonding is a method by which materials can be joined together. Adhesive bonding is an alternative to more traditional methods such as mechanical fastening, welding, riveting, etc. In contrast to traditional joining methods, adhesive bonding has no adverse effect on the material characteristics of the surfaces to be bonded; e.g.: drilling of holes in the assembled parts, damaging and weakening them. We use many types of adhesive to join papers, woods, glasses in our daily life. The following game is used to test your basic understanding about adhesives. You are required to choose appropriate adhesive to join different type of materials. If the choice is correct, the joined material will not separate when they are subjected to simple tensile test.

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Adhesives are defined as nonmetallic substances capable of joining materials by surface bonding (adhesion), the bond itself possessing internal strength (cohesion) and resisting separation. Adhesive is a generic term and covers other common terms, such as glue, paste, gums, adhesive cement and bonding agent.

Advantages and Limitations of Use

The advantages of adhesives are their ability to bond similar to dissimilar materials of different thickness; to enable the fabrication of complex shapes not feasible by other fastening means; to smooth external joint surfaces; to permit economic and rapid assembly; to distribute stresses uniformly over joined interfaces, to provide weight reduction in critical structures via the elimination of fasteners; to dampen vibrations; to prevent or reduce corrosion; and to provide thermal and electrical insulation. The limitations of adhesives depend on the specific adhesive and application and may include the necessity of surface preparation, long curing times, service-temperature limitations, loss of properties during service, toxicity of flammability during assembly or use, and the tendency of many adhesives to creep under sustained load.

Several different theories have been developed as to the mechanisms of adhesion, there are; physical absorption, chemical bonding, diffusion, electrostatic, mechanical interlocking and weak boundary layer.

Physical Absorption Theory

This theory is the most important mechanism in achieving adhesion between two surfaces. Physical absorption considers that the adhesive and adherend are in intimate contact, and weak attractive forces, known as van der Waals forces, operates between them. The forces are of two types: the weaker dispersion forces and the stronger polar forces. Because they occur between any two molecules in contact, van der Waals forces contribute to all adhesive bonds. To obtain good absorption, the distance between the molecules of adhesive and adherend must converge toward molecular intervals such that van der Waals interaction takes place. This requires complete spreading of the adhesive over the surface of the surface of the adherend.

Chemical bonding Theory

The principle of the chemical bonding theory is that the covalent, ionic or hydrogen bond are formed across the interface. In this regard, introduction of molecular bonding between the adhesive and the adherend will give higher bond strength. This can be obtained by reactions at the surfaces, using proper surface treatments or additional coupling agents.

Diffusion Theory

This theory only applies to the bonding of polymers. Most common polymers are made up of large chains of carbon atoms. It is believed that the molecular chains interpenetrate at the interface where the adhesive and surfaces being bonded meet. In simple terms, the two surfaces become interlocked at a molecular level, and therefore, become one.

Electrostatic Theory

Electrostatic theory of adhesion is the difference in electronegativities of adhesing materials. The adhesion forces between adherend and adhesive layer are applied by contact or transfer potentials. These transfer potentials cause the buildup of an electric double layer at the adhesive - adherend boundary and corresponding Coulomb attraction forces between the two components.

Mechanical Interlocking Theory

Although chemical bonds are a significant contributor to the adhesion bond, the primary force behind the adhesion is the Mechanical Interlocking. Mechanical interlocking requires that the surface of the adherend is rough that the adhesive may penetrate into the pores, voids and other irregularities and then lock mechanically to the adherends after harden.

Weak Boundary Layer Theory

This theory has been developed to explain the curious behavior of the failure of bonded materials. Upon failure, many adhesive bonds break not at the adhesion interface, but slightly within the adherend or the adhesive, adjacent to the interface. This suggests that a boundary layer of weak material is formed around the interface between the two media. Impurities in the bond and adverse chemical reactions are common causes of weak boundary layers.

Types of Adhesives

Adhesives in various forms have been in use for centuries, and have evolved over this time to fit many new applications. There are many types of adhesives available today, each with unique characteristics. In general, adhesives can be grouped into the following categories:

Water Based - These are adhesives that use water as a carrier or diluting medium, and set by allowing the water to evaporate or be absorbed by the substrate. There are several types of water-based adhesives.

1. Vegetable Glues - These are adhesives based on starch. They are usually amber to brown in color, commonly know as dextrine adhesives. These can also be made in a high viscosity, high tack version called jelly gum. Relatively low cost adhesive commonly used in paper bonding, packaging and labeling. Low moisture resistance. Bond lines tend to be brittle.
2. Resin Cements - These are adhesives based on an emulsion of EVA (Ethylene Vinyl Acetate) or PVA (Poly Vinyl Acetate) polymers blended to an emulsion with water as a carrier. Capable of bonding to wood, paper, some plastics and foams and many other substrates. White in color. Bonds have higher degree of moisture resistance than dextrine, but cost of resin cement is higher. Certain resins may be blended with dextrines to form a hybrid product. Bond lines have some flexibility, and are relatively clear when dry.
3. Animal/Protein Glues - The two major types of adhesive in this category are hot animal glue (which is made from processed animal parts) and casein glue (derived from milk). Hot animal glue is amber to brown in color, and is applied at approximately 140¢XF. It can be thinned with water. When first applied it has very high tack, but dries to a non tacky film. Commonly used in situations where the high tack will hold the parts together while setting, but which will not be exposed to high temperatures or high humidity. Casein glue is applied at room temperature, but forms a bond with a high degree of moisture resistance. Commonly used for labeling beer, champagne and some types of wine bottles. Casein is light to tan in color.
4. Latex Cements - These adhesives are a blend of latex or other elastomers in a water base emulsion. In most cases they are applied to parts, allowed to dry, and form a layer, which serves as a contact cement (two way cements). Some types can also be applied to one surface and will form bonds as they dry (one way cements). Can be formulated to remain tacky or become dry to the touch (contact types). Generally white in color. Wide variety of uses such as self-stick envelopes, fabric bonding, and leather goods.

Thermal Adhesives - Thermal adhesives are those adhesives that are brought to a liquid state by heating, and are applied to the product hot - either as liquid or as a high viscosity paste. The most common types are hot-melt adhesives and waxes.

Hot-melt adhesives have seen tremendous development over the past thirty years. These adhesives are blends of various polymers, but most are based on a high percentage of EVA (ethylene vinyl acetate). To obtain the desired characteristics, other polymers may be blended into the mix, as well as waxes, oils, various types of rubber, and tackifying resins.

Hot-melt adhesives can be used to bond many types of materials, and are available in three general types categorized by how fast they set up after application. The general categories are fast set, delayed set, and pressure sensitive.

Fast setting hot-melts are types that form a bond very quickly as they cool. These are used for situations where fast setting is important such as the sealing of flaps on cartons or certain labeling applications. Delayed set hot-melts are also known as adhesives having a "long open time". These adhesives remain tacky for some period of time after application, but eventually set to form a bondline that has very little residual tack. They are useful in applications where the parts must be positioned after application of the adhesive, or in situation where the parts cannot be assembled immediately after the adhesive is applied. Shoe and leather goods assembly is an example of one area where delayed set adhesive are used.

Pressure sensitive hot-melts remain tacky indefinitely after application. This allows the adhesive to be applied to a part that may not be assembled to a substrate for a long period of time. It also allows the bonding of parts that are difficult to bond (example would be polyethylene foam). In some cases pressure sensitive hot-melts are applied to a part and put on silicone coated release paper. The release paper is peeled off to expose the pressure sensitive layer, which is then bonded to the substrate.

Pressure sensitive hot-melts are available in many degrees of tackiness, from adhesives that form a temporary bond that can be easily broken (fugitive bond adhesives) to very aggressive pressure sensitives that will tear fiber from the substrates if removal is attempted.

Waxes are the oldest form of thermal adhesive, having been used for sealing documents for centuries. In today's world they see use as laminating adhesives for foils and films. Bonds are formed to the substrates when hot, but the strength is sufficient to keep the materials bonded at lower temperatures. One special form of adhesive wax is paste-up wax. This is a blend of sticky waxes and tackifying agents that is used to form a temporary bond that allows parts to be removed and repositioned after bonding. It is used by newspapers and printers during page layout (paste-up) process since it allows photos and columns of type to be moved around as the page layout is being developed.

Two part Adhesives - These are adhesives that are made by mixing two or more components that react chemically to form a chemically crosslinked adhesive. In general, they are higher cost than other types of adhesives but also provide very high strength bonds and outstanding performance characteristics. The most common two part adhesives are epoxies, polyurethane's, acrylics, and silicones.

Two part adhesives are able to cure in the absence of air or moisture, and are often used to form structural bonds to metal, wood and plastic components.

Moisture Cure Adhesives - Moisture cure adhesives are formulated to react with the moisture in the air or in the substrates to form a cured polymer layer with high strength. They are actually two component adhesives with one component being moisture. The two best-known types are silicone and polyurethane. The silicones are known as RTV silicones (room temperature vulcanizing), and are most commonly used as caulking compounds, gasket compounds, and sealants.

Polyurethane moisture cure adhesives are available in liquid form. In most cases the urethane monomer is dissolved in a solvent carrier, and reaction with moisture occurs as the solvent evaporates. Some types of water-borne urethanes are also available, but the newest types of moisture cure urethanes are made in the form of hot-melt adhesives. These are called reactive hot-melts, and exhibit a dual property. They are applied like regular hot-melts, but after application begin to crosslink with moisture to form a tough adhesive layer with high resistance to heat, moisture, and impact.

Ultraviolet Cure Adhesives - These are adhesives which contain monomers that will crosslink upon exposure to ultraviolet light to form a polymer. The crosslinking (or cure) can happen in less than a second at proper energy levels, so these adhesive can be used in high speed situations. Acrylic adhesives lend themselves to U.V. curing quite well, but U.V. cure versions of silicones, urethanes/acrylic blends and cyanoacrylates are also used.Ultraviolet cures adhesives can form high strength bond lines on materials which will pass the U.V. light. The primary advantage of U.V. cure adhesives is the fast cure speed. UV curing adhesives can adhesion to glass/plastics/metals/ceramics, depth of cure to >1/2").

Cyanoacrylate Adhesives - These are fast setting one component adhesives that are popularly known as "super glue". Cyanoacrylates are solvent free and react with the moisture on the surfaces of the substrate materials to form a rigid plastic adhesive layer that has high strength characteristics. The cured adhesive is very high in tensile and shear strength, but low in peel strength. Cyanoacrylates are expensive compared to other adhesives, but only a very small amount is needed to cover the area to be bonded, since this material works best when spread into a very thin bond line.

Contact adhesives

Contact adhesives are solvent-based adhesives. There are solutions of a polymer in organic solvents, which are applied to both surfaces to be bonded. Some time is allowed for the solvents to evaporate and the surfaces are then pressed together, at which point some interdiffusion of polymer chains will occur.

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.

Film Adhesives - These are adhesives that are made in the form of sheets. In most cased they are carried on release paper, but some types are carried on release paper, but some types are heat activated and do not require release paper. Film adhesives are made from water base, solvent base, or hot-melt adhesives, which are cast into a thin film leaving only the adhesive. They find use in situations where the release paper can be left in place and peel off prior to application to the substrate. They are popular for mounting of plastic components such as warning stickers, die cut parts such as letters and numbers, and a multitude of other parts. This form of adhesive also finds use for cold laminating of paper, plastics and films. The heat reactivated versions find use in fabric bonding and industrial applications where heat can be applied to the substrates to melt the adhesive.

Some types of film adhesives are cast onto a supporting material such as a scrim cloth or nonwoven fabric. These prevent stretching of the adhesive in use and simplify handling. Double back carpet tape is made in this manner. Film adhesives tend to be expensive relative to other adhesives because the cost of the release paper carrier must be included in the price of the adhesive. For many applications, the release paper stays with the product until it is applied, so the cost premium is justified. Film adhesives are also useful in cases where liquid adhesives might distort the substrates to be bonded. This is the case with some types of thin papers, films and foils, especially in low volume applications where ease of handling is of primary importance.

Fabric Lamination

Polymer materials which may not be easily formulated into a resin or a paste for coating, can be combined with a fabric by first preparing a film of the polymer, and then laminating it to the fabric in a separate process. The challenge is to preserve the original properties of the fabric (appearance, color and surface texture)and to produce a flexible laminate with the required appearance, handle and durability.

Material used in fabric lamination

Polyestster and nylon are the main fibres used for lamination because their strength and general resistance to moisture, oils, micro-organisms and common chemicals. The following are the characteristics of different fabric materials:

POLYESTER
1. Resitant to light and ultraviolet degradation
2. Better dimensional stability and shrink resistance
3. Lower extensibility and lower cost

NYLON
1. Higher resistance to hydrolysis
2. Very good abrasion resistance

ACRYLIC FIBRES
1. Very high UV resistance

COTTON
1. Better polymer adhesion than synthetic fibres.

Fabric Construction

Woven fabric constructions are generally rigid with little stretch. It is because continuous filament yarn is stiffer than spun yarns. Knitted fabrics are used when elongation is required. They are generally stretchy and have open construction. Softer handle and better drape than woven fabrics. Sometimes are raised to improve adhesion and to maximize softness and flexibility of the laminated fabric. Softest handle which implies minimal adhesive penetration. If the adhesive penetration is not enough, adhesion performance is poor. The following table shows the advantages and disadvantages of using common fabric material for lamination.

Fabric	Advantages	Disadvantages
Cotton	Excellent adhesion Low thermal shrinkage	Absorbs moisture Vulnerable to rotting
Polyester	Low Shrinkage Relatively inexpensive Resistant to rotting	Low moisture absorbency Limited resilsence
Nylon	Good eleasticity High abrasion resistance	Low UV resistance Relatively expensive
Polyethylene, Polypropylene	Lightweight Inexpensive	Low melting point Difficult adhesion
Armaid	Excellent strength Good FR properties	Expensive Degraded by sunlight

How to obtain good adhesion?

To enable the adhesive bonds between the adhesive and the surface of adherend, 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) and flow to cover the surfaces of the materials being laminated.

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. At the liquid-solid surface interface, if the molecules of the liquid have a stronger attraction to the molecules of the solid surface than to each other (the adhesive forces are stronger than the cohesive forces), wetting of the surface occurs. Alternately, if the liquid molecules are more strongly attracted to each other than the molecules of the solid surface (the cohesive forces are stronger than the adhesive forces), the liquid beads-up and does not wet the surface of the part.

One way to quantify a liquid's surface wetting characteristics is to measure the contact angle of a drop of liquid placed on the surface of an object. The contact angle is the angle formed by the solid/liquid interface and the liquid/vapor interface measured from the side of the liquid. (See the figure below.) Liquids wet surfaces when the contact angle is less than 90 degrees. For a penetrant material to be effective, the contact angle should be as small as possible. In fact, the contact angle for most liquid penetrants is very close to zero degrees.

Wetting ability of a liquid is a function of the surface energy of the solid-liquid interface. The surface energy across an interface or the surface tension at the interface is a measure of the energy required to form a unit area of new surface at the interface. The intermolecular bonds or cohesive forces between the molecules of a liquid cause surface tension. When the liquid encounters another substance, there is usually an attraction between the two materials. The adhesive forces between the liquid and the second substance will compete against the cohesive forces of the liquid. Liquids with weak cohesive bonds and a strong attraction to another material (or the desire to create adhesive bonds) will tend to spread over the material. Liquids with strong cohesive bonds and weaker adhesive forces will tend to bead-up or form a droplet when in contact with another material.

Wetting ability is influenced by the three factors: The first one is the cleanliness of the surface. If the adherend surface is contaiminated by some liquid with high surface energy such as water, alcohol or ester etc., the surface energy of the adherend will be lowered. The second one is the surface energy of adherends, higher surface energy of adherends, wettability will be increased. The third one is the surface energy of adhesive, comparative low surface energy of adhesive can spread well on solids of high surface energy.

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 (adherende's surface, curing time, pressure, temperature).

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