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4 Classfication of Adhesives and Compounds


4

Classication of Adhesives and Compounds

4.1 Introduction
There are a large number of adhesives on the market. It is helpful to organize these adhesives in groups

with common characteristics to facilitate their understanding and use. Adhesives can be classied in a number of ways, although no one classication is universally recognized. Classications include source, function, chemical composition, physical form, and application.

4.2 Adhesive Composition Formulation
Adhesives resemble paints in formulation in that they may contain a number of components in addition to the adhesive materials, which are also called the binders. Every component is not found in every adhesive. For example, all adhesives do not contain a solvent or a ller. The key components that may be found in commercial adhesives have been dened in this section. 4.2.1 Adhesive Base or Binder This is the primary component and has the function of forming the bond, thus holding the substrates together. The binder is generally the component from which the name of the adhesive is derived. For example, an epoxy adhesive may have many components, but the main component is the binder, that is, the epoxy resin. 4.2.2 Hardener (for Thermosetting Adhesives) This is a substance added to an adhesive to promote the curing reaction by taking part through catalysis or cross-linking. Two-part adhesive systems generally have one part that is the base and a second part that is the hardener. Upon mixing, a chemical reaction takes place that causes the adhesive to solidify. A catalyst is sometimes incorporated in an adhesive formulation to speed up the reaction between the base and the hardener. Very small amounts of catalyst are required, compared to the principal components such as base and hardener. 47

48 4.2.3 Solvents

Adhesives Technology Handbook

Solvents are sometimes needed to reduce the viscosity of the adhesive to enhance its spreadability. Solvents used with synthetic resins and elastomers are generally organic in nature. Often a mixture of solvents is needed to achieve the desired processability characteristics such as controlled solvent evaporation and removal. This can be accomplished by combining solvents with variable volatilities. 4.2.4 Diluents These are liquid ingredients added to an adhesive to reduce the concentration of the binder component. Diluents are added principally to lower the viscosity and to modify the processing conditions of some adhesives. Reactive diluents do not evaporate, as would solvents. They react with the binder during the cure cycle and are incorporated in the cured adhesive. 4.2.5 Fillers Fillers are relatively neutral substances added to the adhesive to improve its working properties, strength, permanence, or other qualities. Fillers are also intended to reduce materials costs. Considerable changes can be made in the properties of an adhesive by selective use of llers. Fillers are used to modify adhesives to govern such properties as thermal expansion, electrical and thermal conductivity, shrinkage, and heat resistance. 4.2.6 Carriers or Reinforcements These are usually thin web-type materials such as plastic lm, fabric, or paper used to support the adhesive composition. The role of the web in an adhesive-coated web includes acting as a carrier, a release media, a tape, or a lm. The carriers could also serve as a bond-line spacer and reinforcement for the adhesive. 4.2.7 Other Additives In addition to the basic components, an adhesive may contain a number of other additives, each aimed at achieving a specic characteristic. They include plasticizers, accelerators, inhibitors, retarders, tackiers, thickeners, lm formers, antioxidants, antifungal agents, and surfactants.

4: Classification of Adhesives and Compounds

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Formulation of an adhesive is more of an art than a science. Little basic information has been published about the formulation of adhesives because of their proprietary nature. There are few references that provide adhesive recipes of adhesives and the resulting properties.[1,2,3,4]

4.3 Classication of Adhesives
This section presents classications of adhesives from a number of points of view including function, source, physical form, mode of application and setting, chemical composition, Society of Manufacturing system, Rayner system, and others. A simple classication is depicted in Figure 4.1. Adhesives are either produced from a natural source such as starch glue or, as is the case with the majority of consumptions, they are synthesized from basic hydrocarbons. The synthetic group consists of thermoplastic and thermosetting adhesives, both of which follow the denitions used in plastics for thermoplastic and thermosetting polymers. 4.3.1 Source: Natural vs. Synthetic Adhesives This classication is based on whether the adhesive is produced from natural sources or synthesized from basic hydrocarbons. 4.3.1.1 Natural Adhesives This term is used to include vegetable- and animal-based adhesives and natural gums. These include organic materials such as casein, blood, albumin,
Adhesives

Natural

Synthetic

Thermoplastic

Thermosetting

Elastomeric

Figure 4.1 A simple classication of adhesives.

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Adhesives Technology Handbook

hide, bone, sh, starch, resin, shellac, asphalt, chitosan, and inorganic adhesives like sodium silicate. Their use, except for the inorganic adhesives, is mostly limited to paper, paperboard, foil, and light wood. They are inexpensive, easy to apply, and have a long shelf life. These adhesives develop tack quickly, but have low strength properties. Most are water-soluble and use water as a solvent. They are supplied as liquids or dry powders to be mixed with water. Some are dispersions in organic solvents.[5] 4.3.1.2 Synthetic Adhesives This term is usually used to apply to all adhesives other than natural adhesives (i.e., elastomeric, thermoplastic, thermosetting, and alloys). All structural adhesives are synthetic. 4.3.2 Classication by Chemical Composition This classication describes synthetic adhesives as thermosetting, thermoplastic, elastomeric, or combinations of these types (alloys).[5,6] 4.3.2.1 Thermosetting Adhesives These are materials that cannot be heated and melted after the initial cure. Curing takes place by chemical reactions at room temperature or at an elevated temperature, depending on the type of adhesive. Some thermosetting adhesives require considerable pressure, while others require only contact pressure. Solvents are sometimes added to facilitate application. These adhesives are usually available as solvent-free liquids, pastes, and solids.[6] Thermosetting adhesives are provided as one- and two-part systems. The one-part systems usually require elevated temperature cure and have a limited shelf life. The two-part systems have longer shelf lives and can usually be cured slowly at room temperature, or somewhat faster at moderately higher temperatures. A disadvantage is their need for careful metering and mixing to make sure that the prescribed proportions are blended and that the resultant mixture is homogeneous. Once the adhesive is mixed, the useful life is limited.[6] Because thermosetting-resin adhesives, when cured, are densely crosslinked, their resistance to heat and solvents is good, and they show little

4: Classification of Adhesives and Compounds

51

elastic deformation under load at elevated temperatures. Bonds are able to withstand temperatures of 93–260°C and peel strength is fair. The major application is for stressed joints at somewhat elevated temperatures. Most materials can be bonded with thermosetting adhesives, but the emphasis is on structural applications.[6] Examples of thermosetting adhesives are shown in Table 4.1. 4.3.2.2 Thermoplastic Adhesives These materials do not cross-link during cure and they can be melted without signicant change in their properties. They are single-component systems that harden upon cooling from a melt state, or by evaporation of a solvent or water vehicle. Wood glues are thermoplastic emulsions that are common household items. They harden by evaporation of water from an emulsion. Thermoplastic adhesives are not ordinarily recommended for use at above 66°C, although they can be used up to 90°C in some applications. These materials have poor creep resistance and fair peel strength. They are used mostly in stressed joints and designs with caps, overlaps, and stiffeners. The materials most commonly bonded are non-metallic material, especially wood, leather, plastics, and paper.[5,6] With the exception of some hot-melt adhesives, thermoplastic adhesives are not generally used for structural applications. Examples of thermoplastic adhesives are shown in Table 4.2.[6] 4.3.2.3 Elastomeric Adhesives These materials are based on synthetic or naturally occurring polymers. They have superior toughness and elongation. Elastomeric adhesives may
Table 4.1 Major Thermosetting Adhesives[6] Cyanoacrylates Polyester Urea-formaldehyde Melamine-formaldehyde Resorcinol Resorcinol-phenol-formaldehyde Epoxy Polyimide Polybenzimidazole Acrylic Acrylic acid diester

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Adhesives Technology Handbook

Table 4.2 Major Thermoplastic Adhesives[6] Cellulose acetate Cellulose acetate butyrate Cellulose nitrate Polyvinyl acetate Polyvinyl chloride Polyvinylidene chloride Polyvinyl acetals Polyvinyl alcohol Polyamide Acrylic Phenoxy

be supplied as solutions in organic solvents, latex cements, dispersions, pressure-sensitive tapes, and single- or multiple-part solvent-free liquids or pastes. Curing varies, depending on the type and the form of adhesive. These adhesives can be formulated for a wide variety of applications, but they are generally used for their high degree of exibility and superior peel strength.[5,6] Some elastomeric adhesives are supplied in lm form. Most of these adhesives are solvent dispersions of water emulsions. Temperature environments up to 66–204°C are practical. Elastomeric adhesives never melt completely. Bond strengths are relatively low, but exibility is excellent. These adhesives are used in unstressed joints on lightweight materials, so they cannot be considered structural adhesives. They are particularly advantageous for joints in exure. Most of these adhesives are modied with synthetic resins for bonding rubber, fabric, foil, paper, leather, and plastic lms. They are also applied as tapes.[5,6] Examples of elastomeric adhesives are shown in Table 4.3. 4.3.2.4 Adhesive Alloys Combining resins of two different chemical groups chosen from the thermosetting, thermoplastic, or elastomeric groups makes up these adhesives.
Table 4.3 Major Elastomeric Adhesives[5,6] Natural rubber Reclaimed rubber Butyl rubber Polyisobutylene Nitrile rubber Styrene-butadiene rubber Polyurethane Polysulde Silicone Neoprene

4: Classification of Adhesives and Compounds

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The thermosetting resin, chosen for its high strength, is plasticized by the second resin, making the alloy tougher, more exible, and more resistant to impact.[5] The adhesive alloys take advantage of the most important properties of each component. They are commonly available as solventbased solutions and as supported and unsupported lms.[5] Except for some epoxy compounds, heat and pressure are usually required for curing. Most alloy adhesives are solvent-based dispersions or 100% solids. These adhesives have a balanced combination of properties and are generally stronger over wider temperature ranges than other adhesives. They are suitable as structural adhesives and are used where the highest and strictest end-use conditions must be met (regardless of cost) such as in military applications.[5,6] Materials bonded include metals, ceramics, glass, and thermosetting plastics. Applications are primarily for high strengths and high temperatures.[5,6] Examples of alloy adhesives are shown in Table 4.4. 4.3.3 Classication by Function 4.3.3.1 Structural Adhesives These are materials (Table 4.5) of high strength and performance. Their primary function is to hold structures together and to be capable of resisting (bearing) high loads.[6] A more detailed discussion can be found in Chapter 1. 4.3.3.2 Non-structural Adhesives These adhesives are not required to withstand substantial loads, but merely hold materials in place. This group is sometimes called “holding adhesives.” Examples include adhesives/sealants that are primarily intended to

Table 4.4 Major Alloy Adhesives[6] Epoxy-phenolic Epoxy-polysulde Epoxy-nylon Nitrile-phenolic Neoprene-phenolic Vinyl-phenolic Polyvinyl acetal-phenolic

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Table 4.5 Structural Adhesives Adhesive Material Epoxy Modied (or alloyed) epoxy

Adhesives Technology Handbook

Modier Toughener, nylon, phenolic, polysulde, resorcinol and phenol formaldehyde, melamine, and urea-formaldehyde Nitrile, vinyl, neoprene

Modied (or alloyed) phenolic Polyaromatics Polyester Polyurethane Anaerobic Cyanoacrylate Modied acrylic Neoprene (chloroprene) Nitriles (acrylonitrile-butadiene) Polysulde

ll gaps and rubber cements that are used to adhere paper to paper in ofce applications.[6] 4.3.4 Classication by Physical Form[6] Adhesives can be subdivided by their form such as liquid, powder, lm, or paste. The physical state of the adhesive generally determines how it is to be applied. 4.3.4.1 Liquid Adhesives These adhesives are easily applied by means of mechanical spreaders such as rolls, by spraying, or brushing. 4.3.4.2 Paste Adhesives Paste adhesives have high viscosities to allow application on vertical surfaces with little tendency to sag or drip. These bodied adhesives can serve as gap llers and sealants.

4: Classification of Adhesives and Compounds 4.3.4.3 Tape and Film Adhesives

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These adhesives provide a bond line with uniform thickness, and offer the advantages of no need for metering and ease of dispensing. Adhesive lms are available as pure sheets of adhesive, or with lm or paper reinforcement. 4.3.4.4 Powder or Granule Adhesives It is usually not possible to apply adhesives in solid form. These materials must be heated or dissolved in a solvent to be converted into a liquid form, to enable their application to surfaces. 4.3.5 Classication by Mode of Application and Setting[6] Adhesives are often classied by their mode of application. Depending on viscosity, adhesives can be coated, sprayed, or brushed. Adhesive pastes and mastics are applicable by extrusion and may be applied by syringe, caulking gun, or pneumatic pumping equipment. Another way to distinguish between adhesive groups is the manner by which they ow or solidify. As shown in Table 4.6, some adhesives solidify simply by evaporation of solvent, while others harden as a result of heat activation or chemical reaction. Pressure-sensitive systems ow under pressure and are stable when pressure is removed. 4.3.6 Classication by Specic Adherends or Applications[6] Adhesives are also classied according to their end use as follows: Adherends (substrates) bonded Metal Wood Plastics Ceramics and glass Environments for which intended Acid-resistant Heat-resistant Weatherable Cryogenic Non-critical (general purpose)

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Adhesives Technology Handbook

Table 4.6 Adhesives Classied by Activation and Cure Requirements[5,6] Requirement Heat Types Available Room temperature to 232°C types available; 121–177°C types most common for structural adhesives Contact to 3.5 MPa types available; 0.17– 1.38 MPa types most common for structural adhesives Types requiring a few seconds to a week are available; 0.5–24 hr types are most common for structural adhesives Extremely varied in terms of chemical catalyst required; may also contain thinners, etc. Forms Used Formulated in all forms; liquid most common Remarks Application of heat will usually increase bond strength of any adhesive, including roomtemperaturecuring types Pressure types usually have greater strength, except for modied epoxies

Pressure

Formulated in all forms; liquid and powder most common

Time (room temperaturecuring)

Formulated in all forms

Catalyst (room temperaturecuring)

Two components: paste + liquid or liquid + liquid

Vulcanizing

Varied types requiring addition of a chemical agent (usually sulfur); may also contain a catalyst

Two liquid components

Time required varies with pressure and temperature applied and immediate strength This type may sometimes require elevated temperature (<100°C) and/or pressure instead of, or in addition to, a chemical agent Premixed types requiring temperatures of 121–177°C for vulcanization are available (Continued)

4: Classification of Adhesives and Compounds
Table 4.6 Adhesives Classied by Activation and Cure Requirements[5,6] (Continued) Requirement Reactivation Types Available Types requiring heat or solvent or second coating of adhesive Forms Used Dry lm or previously applied liquid Remarks

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Radiation

UV-acrylics, cationic epoxies, dual-curing adhesives

Liquids

Heat-cure adhesive is best for non-porous surfaces and/ or maximum strength UV-curing adhesives have shorter curing times than conventional adhesives and can therefore increase production speed and productivity

4.3.7 Society of Manufacturing Engineering Classication A Society of Manufacturing Engineers publication[7] provides a useful, indepth classication of adhesives, as shown in the following tables.[8] 4.3.7.1 Chemically Reactive Types
Table 4.7 Catalytic Plural Components—Chemical Cure Epoxy Phenolic Resorcinol-formaldehyde Polyester Polysulde Polyurethane Silicones

Table 4.8 Catalytic Plural Components—Moisture Cure Silicones Polyurethane Polysuldes Cyanoacrylate Epoxy (one-container type)

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Adhesives Technology Handbook

Table 4.9 Heat-Activated Systems (One-Part, May be Solid Film) Polybenzimidazole (PBI) Polyimide (PI) Epoxy Nylon Phenolic Polyvinyl acetates Vinyl-phenolic Vinyl-epoxies Urethanes

4.3.7.2 Evaporative or Diffusion Adhesives
Table 4.10 Solvent-Based Systems Natural rubber Reclaimed rubber Synthetic rubbers Nitrile rubber Neoprene (polychloroprene) Butyl rubber Styrene-butadiene rubber Phenolics Urethanes Vinyl resins Polyvinyl acetate Vinyl-phenolics Polyvinyl alkyd ethers Polystyrene Table 4.11 Water-Based Systems Natural rubber Reclaimed rubber Synthetic rubber Vinyl resins Acrylics Miscellaneous adhesives Natural products (animal glue, starch, soya, blood glue, casein, cellulose derivatives) Carboxylic-containing copolymers Acrylics Miscellaneous Cellulose esters Asphalt Polyamides Phenoxy resins Bisphenol-A polycarbonate Polysulfone

4.3.7.3 Hot-Melt Adhesives
Table 4.12 Hot-Melt Adhesives Ethylene-vinyl acetate and polyolen resins Polyamide (nylon) and polyester resins Other not melts Polyester-amides Thermoplastic elastomers

4: Classification of Adhesives and Compounds 4.3.7.4 Delayed-Tack Adhesives
Table 4.13 Delayed-Tack Adhesives Styrene-butadiene copolymers Polyvinyl acetate Polystyrene Polyamides

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4.3.7.5 Tape and Film Adhesives
Table 4.14 Tape and Film Adhesives Vinyl-phenolics Epoxy-phenolics Nitrile-phenolics Nylon-epoxies Elastomer-epoxies (as nitrile-epoxies) Aromatic polymers (PI and PBI)

4.3.7.6 Pressure-Sensitive Adhesives Pressure-sensitive adhesives are not discussed in this book. 4.3.8 Classication by Rayner[9] This is another useful classication that somewhat resembles the classication by “chemical composition” described above. 4.3.8.1 Thermosetting Resin Adhesives
Table 4.15 Thermosetting Resin Adhesives Urea-formaldehyde Melamine-formaldehyde Phenol-formaldehyde Resorcinol-formaldehyde Epoxy Polyisocyanate (polyurethane) Polyesters Silicones Furanes Acrylics Soluble nylons Polyaromatics (PI and )*

*These are really thermoplastics, but are often grouped with thermosets because of their high melting points.

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Adhesives Technology Handbook

4.3.8.2 Thermoplastic Resin Adhesives
Table 4.16 Thermoplastic Resin Adhesives Cellulose adhesives (cellulosics) Polyvinyl adhesives Polyvinyl ester adhesives (especially polyvinyl acetate) Polyvinyl acetal adhesives Polyvinyl alcohol adhesives Polyvinyl alkyl ether adhesives (some are elastomers) Polystyrene adhesives Acrylic resin adhesives Acrylic esters Acrylic acid diesters (including anaerobic sealants) Cyanoacrylates Acrylic copolymers Polyamide resin and nylon adhesives (including nylon adhesives with traces of phenolic) Miscellaneous thermoplastic adhesives Polycarbonates Polyacetals Polyethylene Polysulde (sometimes considered thermoplastics, but these are really rubbers) Table 4.17 Other Not-Melt Thermoplastic Adhesives Ethyl cellulose Polyvinyl acetate Ethylene-vinyl acetate Ethylene-ethyl acrylate Butyl methacrylates Polystyrene and copolymers Polyisobutylene Hydrocarbon resins Polypropylene Polyamides Polyesters Phenoxies

4.3.8.3 Two-Polymer Adhesives (Alloys)
Table 4.18 Two-Polymer Adhesives Vinyl-phenolics Epoxy-phenolics Nitrile-phenolics Epoxy-polysulde Nylon-epoxies Elastomer-epoxies Neoprene-phenolics

4: Classification of Adhesives and Compounds 4.3.9 Additional Classication

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The following grouping has been found by the author to be convenient. Some of the adhesive types have already been listed. A list of natural glues is presented in Table 4.19. Adhesives/sealants Hardening types (including exible material) Non-hardening types Primers Microencapsulated adhesives Conductive adhesives Electrically conductive adhesives Thermally conductive adhesives Pre-mixed frozen adhesives Anaerobic adhesives Fast-setting adhesives (cyanoacrylates) Elastomeric adhesives (including pressure-sensitive adhesives)
Table 4.19 Natural Glues Vegetable glues Starch Dextrins Soybean glue Rosin Animal glues Casein Blood adhesives (blood glues) Shellac Bone and hide glues Fish glues Inorganic glues (adhesives, cements) Soluble silicates Phosphate cements Basic salt (Sorel cements) Litharge cements Sulfur cements Hydraulic cements

References
1. Skiest, I., ed., The Handbook of Adhesives, 3rd ed., Van Nostrand Reinhold, New York, 1990. 2. Satas, D., ed., The Handbook of Pressure Sensitive Adhesives, 2nd ed., Van Nostrand Reinhold, New York, 1989.

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Adhesives Technology Handbook

3. Flick, E., ed., Adhesives and Sealant Compound Formulation, 2nd ed., Noyes Publications, Park Ridge, NJ, 1989. 4. Ash, M. and I. Ash, eds., Formulary of Adhesives and Sealants, Chemical Publishing Co., New York, 1987. 5. Merriam, J.C., Adhesive bonding. Mater. Des. Engng, 50(3): 113–128, 1959. 6. Petrie, E.M., Handbook of Adhesives and Sealants, McGraw-Hill, New York, 2000. 7. Janata, R.B., Selecting an Adhesive and Some Causes of Adhesive Failures, Technical Paper AD75-354, Society of Manufacturing Engineers, 1975. 8. Types of adhesives (Chapter 1). Adhesives in Modern Manufacturing (E.J. Bruno, ed.), Society of Manufacturing Engineers, 1970. 9. Rayner, C.A., Synthetic organic adhesives (Chapter 4). Adhesion and Adhesives (R. Houwink and G. Salomon, eds.), 2nd ed., Vol. 1—Adhesives, Elsevier Publishing Co., Amsterdam, 1985.


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