Thermoset adhesives

CHAPTER

Thermoset adhesives

10 T. Engels

€ Henkel AG & Co. KGaA, Dusseldorf, Germany

10.1 INTRODUCTION Adhesives can be defined as nonmetallic materials that can temporarily or permanently join various metallic or nonmetallic substrates by surface forces (adhesion) and inner forces inside the adhesive itself (cohesion) [1]. Adhesives based on thermoset chemistry play an important role in almost all industrial and many D-I-Y bonding applications. The chapter comprises an introduction to chemistry (the different classes of thermoset adhesives) and physical properties of thermoset adhesives as well as the fundamental technical requirements and characteristics for specific applications. When exposed to specific temperatures for a well-specified period of time, the uncured thermoset adhesive precursor molecules will undergo a chemical curing reaction. During this cure, the precursor educts with small molecular weights co-react and build up longer linear molecular chains that may cross-link to rigid polymer network structures that are responsible for the physical and chemical properties of thermoset adhesives. They are sometimes called “duromers” because of the high durability of their polymer structure. The heat-induced formation of polymer structures with higher molecular weight distinguishes thermoset adhesives from, e.g., hotmelt adhesives or thermoplastic materials in general. Thermoset adhesives exhibit some extraordinary properties like infusibility, insolubility in various media, high load-bearing ability, and high creep resistance under constant or varying forces. They are compatible with extreme service conditions such as high or low temperatures, exposure to salt water, or radiation. Steel, light metals like aluminum or magnesium, and a variety of plastic and composite materials can be bonded with high strength to structural, i.e., load-bearing assemblies. Today, toughened variants of thermoset adhesives provide exceptional properties that are directly comparable or even superior to more traditional joining methods like welding, riveting, or clinching. These kinds of structural thermoset adhesives provide stiffness and resistance against dynamic impact loads. Consequently, they are more and more used in lightweight constructions in many industrial Thermosets. https://doi.org/10.1016/B978-0-08-101021-1.00010-1 # 2018 Elsevier Ltd. All rights reserved.

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applications. In order to fully exploit the considerably improved material properties of the recently developed spectrum of new high-strength steels, light metal alloys, engineered plastics, or composite materials, thermoset adhesives today provide a set of properties that can be tailored for each specific application resulting in reliable assembly processes and cost-effective and durable constructions. Once cured and depending on their specific glass transition temperature, thermoset adhesives will only soften slightly but will not melt or flow when exposed to higher temperatures. Thermoset adhesives comprise a wide variety of chemical structures. Well described are melamine, phenolic, resorcinol-formaldehyde, ureaformaldehyde, furan, polyester, polyimide resins, and fluoropolymers. Major classes of thermoset adhesives are based on acrylic, epoxy, polyurethane (PUR), natural or synthetic rubbers (elastomers), silicone compounds, ceramic and inorganic cements, or the whole class of water-based resins. The next section will describe only the most important classes of thermoset adhesives. It will briefly describe the chemistry of thermoset adhesives based on epoxy resins, acrylates, and polyurethanes. Related to the chemistry is the type of surface or substrate that can be joined (steel, light metals, plastics, and composites) and the mechanical performance and durability of the bonded assemblies.

10.2 EPOXY-BASED THERMOSETS Epoxy-based thermoset adhesives constitute one of the main classes of heatactivated reactive compounds. They are characterized by the presence of the oxirane group. In the presence of suitable hardeners, the oxirane-containing educts cure at higher reaction temperatures forming a cross-linked matrix of high strength and excellent adhesion to a wide range of different substrates. Usually, higher cross-link densities lead to higher adhesive strengths. Epoxy-based thermosets can be characterized by high strength, excellent adhesion to many metallic and nonmetallic substrates, only small shrinkage during and after cure, and excellent resistance to chemicals and higher temperatures. For a first introduction to epoxy resin adhesives, see Ref. [2]. Today, the majority of epoxy resin adhesives are based on DGEBA, the diglycidyl ether of bisphenol A. Usually, the adaptation of the epoxy adhesive properties to a specific application is made by proper choice and concentration of the hardener system. Low-temperature cure hardener systems are based on primary and secondary amines or thiol compounds. Due to the high reactivity of those systems, they are offered as two-component systems with open times for applications and strength values that are usually smaller than higher-temperature cure systems. Another limitation is the low level of oil absorption. This becomes especially important if substrates contain significant amounts of lubrication or deep drawing oils. In principle, epoxy-based resins are able to absorb oils and fats from the substrate surface and include them in their adhesive network structure, but this only happens satisfactorily at higher temperatures, like for high-temperature cure epoxy adhesives with cure temperatures of 160–180°C (for 20–30 minutes). Those thermoset epoxy hardener

10.2 Epoxy-Based thermosets

systems are much less reactive (offered as one-component heat-cure systems) and allow longer open times, higher strengths, and higher oil absorption rates, e.g., 3 g oil per square meter. Suitable hardener systems are based on amides (e.g., dicyandiamide), high-melting aromatic amines, or polyaminoamides. Anhydrides (e.g., phthalic and maleic anhydrides) and Lewis acids like boron trifluoride monoethylamine or oxalic acid provide the best high-temperature performance and are often applied in combination with catalysts like BDMA (benzyldimethyl amine) or DMP 30 (tris-(dimethyl-aminomethyl) phenol). Together with polyfunctional epoxy resins, the hardener/catalyst system defines the cross-linking density of the final epoxy adhesive and Tg, the glass transition temperature of the adhesive. In fact, the glass transition temperature is rather a temperature range than a sharp transition point at which the adhesive matrix starts to soften or “melt.” As a consequence, strength values will drop considerably at temperatures higher than Tg. Another important property of modern thermoset epoxy adhesives is their ability to stop or delay crack propagation. Those crack-resistant systems are called toughened epoxies. The toughening effect is caused by a second elastomeric phase dispersed in the brittle epoxy matrix. During the cure reaction, the nonpolar elastomeric phase becomes immiscible with the more polar cured epoxy matrix and consequently separates into a discrete globular phase dispersed in the brittle epoxy matrix. If a crack is generated, e.g., by a dynamic impact, it propagates through the brittle epoxy matrix until it strikes the soft elastomer phase that dissipates the crack energy into many smaller microcracks. In the end, the crack energy absorption is caused by the conversion of local stresses into plastic deformations of the dispersed elastomeric particles. The elastomer itself can be based on a variety of reactive liquid polymers that are characterized by reactive functional end groups of linear nonpolar polymeric chains. Well-known are the following types: – CTBN: Carboxy-terminated butadiene nitrile rubber, typically used in thermoset epoxies – HTBN: Hydroxy-terminated butadiene nitrile rubber, which may react with polyurethanes – VTBN: Vinyl-terminated butadiene nitrile rubber, for anaerobics via freeradical cure – ATBN: Amine-terminated butadiene nitrile rubber, for ambient-and hightemperature cure epoxy resin adhesives For recent advances in the development of various functionalized liquid rubberbased toughening agents and core-shell particles for epoxy adhesive systems, please refer to the comprehensive overview given by Ratna and Banthia [3]. Alternatives to the rubber elastomers are linear polyurethanes [4], polysulfides [5,6], polyimides [7], polysiloxanes [8], or silica nanoparticles [9]. With regard to their exceptional static and dynamic strength in combination with a high fatigue resistance, it is no surprise that epoxy thermoset adhesives have found their way into more and more structural applications like metal, concrete, or tile

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bonding. Low shrinkage and high durability combined with good electric properties qualify them for potting and encapsulating applications in the electronics industries.

10.3 POLYURETHANE ADHESIVES Another big class of adhesives is based on polyurethanes. It was Otto Bayer and coworkers who developed the polyurethane (PU) chemistry based on the synthesis of 1,6-hexamethylene diisocyanate (HDI) by Heinrich Ricke in 1937 by reacting polyester polyols with di- and polyisocyanates in a polyaddition process [10,11]. Early research in polyurethane chemistry was oriented toward fibers, foams, and adhesives, while polyurethane coatings were developed later [12]. Urethanes are produced by reacting isocyanates RdN]C]O with alcohols HdOdR’: R
N ¼ C ¼ O + H
O
R0 ! R
NH
COO
R0

Alternatively, isocyanates can react with water/moisture that is ubiquitous. In a first step, an instable carbamic acid is formed that decomposes quickly into an amine and carbon dioxide: R
N ¼ C ¼ O + H
O
H ! R
NH
COO
H ! R
NH2 + CO2

The carbon dioxide released from this reaction will cause the foaming of the polyurethane system. The resulting amine itself can react with another isocyanate forming a substituted urea: R
N ¼ C ¼ O + R
NH2 ! R
NH
CO
NH
R

Aliphatic amines show much higher reactivities than aromatic amines or alcohols (see Table 10.1, Ref. [13]). By using at least bifunctional isocyanates and/or active hydrogen-containing compounds, a big variety of polyurethanes can be produced. Table 10.1 Typical reaction rates for selected hydrogen-containing compounds Active hydrogen compound

Typical structure

Relative reaction ratea

Aliphatic amine Secondary aliphatic amine Primary aromatic amine Primary hydroxyl Water Carboxylic acid Secondary hydroxyl Urea proton Tertiary hydroxyl Urethane proton Amide

RdNH2 R2dNH ArdNH2 RdCH2OH HdOdH RdCO2H R2CHdOH RdNHdCOdNHdR R3CdOH RdNHdCOdOR RdCOdNH2

100,000 20,000–50,000 200–300 100 100 40 30 15 0.5 0.3 0.1

a

Uncatalyzed reaction rate at 80°C.

10.3 Polyurethane adhesives

For adhesive and sealant applications, polyurethanes were first introduced to the markets in the 1950s based on hydroxyl polyurethanes and trifunctional isocyanate cross-linkers. Today, they are produced from isocyanates, polyisocyanates, and prepolymers that react with diols or polyols such as polyester or polyethers. The most important aromatic isocyanates are MDI (methylene diphenyl diisocyanate) and TDI (toluene diisocyanate). Polyurethane adhesives show a high degree of adhesion to many substrates such as glass, wood, plastics, fabrics, leather, ceramics, metals, and composites. The adhesion is based on two mechanisms. Physical bonding is caused by the close contact of the adhesive film to the substrate surface. Chemical bonding occurs via reaction of the isocyanates with active hydrogens on the surface that finally leads to covalent bonds between the polymerizing polyurethane and the substrate. In addition, free isocyanate groups can react with small traces of hydrogen bonds or moisture inside or on the substrate surface. Thermoset polyurethane adhesive has been developed from thermoplastic adhesives to prevent the thermoplastic adhesive film from softening when exposed to heat. These thermoset polyurethane adhesives are often two-component formulations that incorporate higher-functional isocyanates that cross-link when exposed to higher temperatures. In this way, the softening temperature or Tg of the adhesive will increase significantly and enhance substrate adhesion, adhesive strength, resistance to solvents, plasticizers, oils, and fats. The adhesion and the mechanical properties of the adhesive films will mainly depend on – – – –

the type of polyols used, their molecular weight, and degree of functionality the type of polyisocyanate, its functionality, and NCO content the concentration of urethane and/or urea groups the degree of cross-linking that depends on the degree of functionality molecular weight of the reacting components, catalysts, and the curing time and temperature

Apart from the functionalities of the polyols and isocyanate educts, the cross-linking can be varied by first forming cyclic uretidione (isocyanate dimer) or isocyanurate (isocyanate trimer) in the presence of nucleophilic catalysts [14]. The dimerization is reversible and occurs at room temperature and dissociates at temperatures of 150–180°C. The trimerization is an irreversible reaction that leads to isocyanurate rings that are very resistant to high temperatures (150–200°C). As a consequence, the cross-linking of polyurethane systems with isocyanurate rings leads to systems with high thermal resistance that can be used in foam insulations or thermally durable bonding. Depending on the chemistry and the mechanical properties, polyurethanes have found their way into a broad variety of industrial applications. Although the majority of polyurethane adhesive applications occur at ambient conditions, there are some typical applications for thermoset polyurethanes that include the following: – Heat-activated crystallizing polyester polyurethanes (two-component system) – Polyurethane adhesives for structural bonding

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The strength of regular heat-activated crystallizing polyester polyurethanes can be increased by using an isocyanate cross-linker. It takes temperatures in the range of 50–80°C to trigger the isocyanate cross-linking reaction. Those systems are applied, e.g., in the automotive industry, in shoe manufacturing, and for the production of laminated fabrics. For example, the development of heat-activated polyurethane dispersion adhesives allowed a diversity of complex structure sports and fashion shoes. Recent developments are latently reactive polyurethane dispersion adhesives based on highly dispersed surface-deactivated solid isocyanates as latent crosslinkers [15,16]. Compared with the two-component dispersion adhesive systems, those latently reactive systems can be stored at 25°C for long periods of time without any decrease in adhesion performance. Consequently, e.g., coatings made from latently reactive adhesive systems are much more stable over time. A comprehensive introduction to the industrial catalysis in the polyurethane industry has been given by Wegener et al. [17]. Wind blades and wind blade bars can also be bonded using GL-certified polyurethane adhesives with a delayed cure at elevated temperatures (see Section “Wind energy applications” in this chapter). Such a delayed action catalysis with thermal activation can be achieved with Lewis base catalysts (e.g., tertiary amines) blocked with an organic acid. At the increased temperature of the temper oven, the amine salt dissociates and the tertiary amine is released to undergo the urethane formation. For specific systems, even the reaction enthalpy of the urethane reaction can be used to activate the catalysis at room temperature. A typical delayed action catalyst is the DABCO salt with formic acid or of bis(N,N-dimethylamino ethyl)ether with formic acid: N

N

DABCO

However, the released acid may cause corrosion after the cure. Less corrosive alternatives are amine salts with saturated monocarboxylic or dicarboxylic acids like 2-ethylhexanoic, adipic, or sebacic acid. As well, unsaturated carboxylic acids (e.g., oleic acid) or polymeric acids (also less corrosive) can be used as blocking agents or pot life enhancer (e.g., acid chlorides). Typically, such carbon acids are combined with DBU (i.e., 1,8-diazabicyclo[5.4.0]undecene-7): N N

DBU

10.4 Structural acrylic adhesives

Isocyanates can also be activated by the use of Lewis acid catalysts. Typical Lewis acids of this type are organotin compounds, like dibutyltin dilaurate DBTDL: C11H23

O

C

C4H9 O Sn C4H9

O O

C C11H23

DBTDL

Such organotin catalyst—isocyanate complexes—can react with a dimer alcohol associate that finally dissociates into the urethane and the organotin catalyst and the remaining monomeric alcohol. However, since organotin catalysts are usually toxic alternative, less harmful systems have become a matter of particular interest for normal polyol/isocyanate reactions. For specific application purposes, latent or delayed action catalysis systems have been developed (ideally without Hg, Pb, Ni, or organo-Sn) by using a range of metal complexes (e.g., Sn, Bi, Ti, Zr, Fe, and Zn) and amidine salts or aza-based structures [18–20]. One interesting alternative is the use of encapsulated catalysts based on sidechain crystalline polymers, e.g., linear acrylic copolymers. To some degree, the melting point of these polymers can be adjusted by varying the length of the aliphatic side chain. In this way, the isocyanate and the polyol component of highly reactive PU systems can be separated by use of a dispersion of the two components isolated by the encapsulating polymer. Once triggered by elevating the temperature, the polymer encapsulating shell melts and releases its content leading to a snap cure like reaction of the system. One-component systems of this type are known that fully cure within approximately 1 minute. If the melting temperature of the shell is high enough, e.g., 80°C or beyond, products with very high storage stabilities can be formulated.

10.4 STRUCTURAL ACRYLIC ADHESIVES Structural acrylic adhesives comprise several categories of adhesives differentiated primarily by acrylic monomers and cure chemistry and include both thermoset and thermoplastic adhesives. The scope of this chapter will focus on the thermoset structural acrylics and does not include the thermoplastic cyanoacrylates “instant adhesives,” the UV-light-cured acrylic adhesives, or the one-part anaerobic adhesives that share many of the same monomer, curatives, and similar redox cure chemistry [21].

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Thermoset structural acrylic adhesives are two-part adhesives available in various mix ratios that cure by a redox-activated free-radical addition polymerization. Acrylic adhesives are favored for their versatility for bonding a broad range of substrates, particularly metals and plastics at ambient temperatures. The availability of a broad range of redox-active agents and cure systems has allowed the formulation of a range of adhesives with cure speeds from minutes to hours but most importantly with defined working times followed by rapid cure and the development of full bond strengths at ambient temperatures. The unique combination of solvent monomers combined with the broad range of dissolved rubbers and tougheners allows the development of tough structural bonds to most substrates at ambient temperatures. Acrylic structural adhesives have undergone significant performance development since the formulation of the first-generation adhesives, comprising mainly solvent monomers with dissolved polymers and a simple redox cure system. Secondgeneration toughened acrylic adhesive formulations were pioneered by DuPont in the 1970s with the chlorosulfonated polyethylene toughened [22] adhesives and later extended by others to include a broad range of toughening agents and curatives and hybrid cures with epoxies that are now in widespread usage. By the mid-1990s, a third generation of acrylic structural adhesives began to emerge based on improved toughening agents and specialist cure systems relying on organoborane chemistry to deliver high-performance bonding to polyolefins without surface pretreatments and has led to a renewed interest in acrylic structural adhesives in application areas not serviced by other adhesives [23]. Structural acrylic adhesives are complex compositions of methacrylate monomers and dissolved tougheners with adhesives properties. Cure profiles dominate the cure mechanism that is based on free-radical addition polymerization initiated by redox cure chemistry (Fig. 10.1) [24]. The redox couple comprises oxidizing and reducing agents and optional metal catalyst that are separated in different adhesive components for shelf stability and determines the format of the adhesive presented to the user whether they are two-part adhesives, adhesive plus activator, or adhesive plus primer. The more reactive or less stable redox agents need to be separated from polymerizable monomers in plasticizers in activator-based systems or in solvents in primer-based systems. The free-radical cure chemistry combined with the adhesive composition delivers a cure profile that allows a variable working life of the mixed adhesive followed by rapid cure and strength development at room temperature that has proved one of the most important features of this class of adhesives [25]. Typically, acrylic adhesives comprise a methacrylate monomer, a dissolved rubber or toughener, a cross-linking monomer or resin, a cure accelerator or reducing

ROOR Redn + Peroxides Reducing agent

H

monomer RO

Polymer

FIG. 10 .1 Free-radical redox cure chemistry and addition polymerization for acrylic polymers.

10.4 Structural acrylic adhesives

agent, and an oxidizing agent as a source of free radicals, adhesion promoters, stabilizers, and fillers. The methacrylate monomer is chosen mainly from the lower chain alkyl, cycloalkyl, hydroxy alkyl, or alkoxy alkyl esters of methacrylic acid with the choice of monomer being dictated by solvating power for dissolving rubber toughener or thickeners, ability to cure rapidly at ambient temperatures to highTg polymers that is necessary to deliver the high-modulus adhesive for higher temperature applications and to deliver dry-to-touch adhesive surface. The volatility of the monomer is significant with the low-chain alkyl methacrylates, particularly methyl methacrylate being favored because of its capability to deliver “dry-to-touch” adhesive surfaces and lower costs but with the disadvantage of being categorized as highly odorous and flammable (Fig. 10.2). Less volatile monomers with longer chain alkyl and hydroxy alkyl methacrylates, for example, hydroxy propyl methacrylate, are less effective as solvents for dissolving rubbers and tougheners and are less likely to deliver tack-free cure unless additional airdrying additives are added or included as part of the redox cure system. Tetrahydrofurfuryl methacrylate is often chosen as a good compromise for tack-free curing and solvating power but for cost reasons is acceptable only in the higher value application. Additional acidic monomers including acrylic and methacrylic acids, half esters of succinic and maleic acid, and mono- and diesters of phosphoric acid with hydroxyethyl methacrylate are used as adhesion promoters for metal adhesion and for the formation of hybrid cure systems with glycidyl epoxy resin to improve thermal properties of cured adhesives. The choice of reducing agent for the first-generation

CH3 H2C C O OR (I) Lower odor

High odor R=

Methyl

CH3

CH2 CH2 OH

Hydroxyethyl

CH2 CH

Hydroxypropyl

CH3

OH C2H5 CH

C4H8

C6H12

CH3 Ethylhexyl Cyclohexyl

CH2

Tetrahydrofurfuryl O

CH2 CH2 O

FIG. 10.2 Influence of substituent “R” on methacrylate odor.

Ph

Phenoxy ethyl

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adhesives relies mainly on dialkyl aromatic amines (II), for example, dimethyl, diethyl, or dihydroxyethyl toluidines with many still in widespread use to date. The second-generation adhesives initially used crude mixtures of an aniline butyraldehyde condensate from the rubber curative industry. The most active component was later identified as a partially reduced pyridine, N-phenyl-2-propyl-3, 5-diethyl, and 1,2-dihydropyridine (PDHP) [26], which now along with its N-alkyl analogues (III) are the most widely used reducing agents (Fig. 10.3). Other sulfur-based reducing agents including benzoyl thiourea or ethylene thiourea have also been used but for toxicity and labeling reasons are no longer in widespread use. Third-generation acrylic adhesives have reducing agents based on stabilized organoborane chemistry and can generate an effective cure system using oxygen from the air as the oxidizing species or with conventional peroxide oxidizing agents. Two main categories or organoboranes, namely, borohydrides or metal salts of trialkyl boranes (IV) and trialkyl borane amine salts (V), have been commercialized (Fig. 10.4). The organoborane cure mechanism is now well understood and is dependent on the in situ generation of trialkyl boranes by reaction of the stabilized organoboranes with a range of decomplexing agents, usually acids to release trialkyl boranes that react spontaneously with oxidizing agents, including oxygen from the air to generate R1

N

R1

N H

R2

R (II)

(III ) R = n-Butyl, Phenyl, n-Hexyl

R1 = Methyl, Ethyl, Hydroxy ethyl R2 = H, Methyl, Cl

FIG. 10.3 Reducing agents for first-generation (II) and second-generation (III) acrylic adhesives.

R1

R1 R1

B R1 (IV)

H

M

R1

B

R2 NH

R1

R2

R1 = Me, Et, sec-Bu R2 = Alkyl, cycloalkyl M = Li, Na, K

(V)

FIG. 10.4 Reducing agents for third-generation (two categories IV and V) acrylic adhesives.

10.4 Structural acrylic adhesives

alkoxy and alkyl radicals that initiate the radical polymerization reaction [27]. Interestingly, the high flux of radicals generated initiates grafting of the adhesive onto polyolefin substrates to deliver high-performance bonding onto the polyolefin surface. Also this initiating system is unique in that it is capable of delivering a dryto-touch adhesive surface without relying on volatility of the methacrylate monomer because of its reactivity with air. The choice of oxidizing agent depends on the required mix ratio of the adhesives, the cure speed requirements, and whether acid or metal catalysts are present. The acyl peroxides, for example, benzoyl peroxides, are usually used in 1:10 mix ratios with aromatic amines in the first-generation-type adhesives, whereas hydroperoxides, for example, cumene hydroperoxide or tert-butyl hydroperoxide, and peroxyesters, for example, tert-butyl peroxybenzoate, are usually used with the second-generation-type adhesives. The range of toughening agents available is extensive and includes soluble block polymers and copolymers; block rubbers based on butadiene, styrene butadiene, and acrylonitrile styrene butadiene; reactive rubbers including chlorosulfonated polyethylene (Hypalon) and polyethylene acrylate; liquid nitrile rubbers with a range of reactive end groups; and core-shell tougheners that have the advantage of reduced stringing during adhesive application. The toughness of the cured adhesive is due to the phase separation of discrete rubbery phases during cure within the hard methacrylate cured matrix and the ability of localized rubbery phases to dissipate propagating cracks during high impact on bonded joints. Acrylic adhesives are used either as single-part adhesives applied to a substrate already primed with a solvent activator or from a two-part adhesive applied as a bead beside bead, or from a static mixer, the bond is assembled and usually fixtures rapidly giving handling strengths in minutes with full cures within hours. The two-part mix systems generally give better through gap adhesive performance than the primeractivated adhesives. A significant advantage of acrylic adhesives is that they can be designed with mixed adhesive pot lives up to 90 minutes that is particularly advantageous for the assembly of large molded parts, for example, in wind blade assembly. Typically, acrylic adhesives can give tensile shear strengths in the range of 12–30 MPa on steel and 12–25 MPa on aluminum and T-peel strengths of 3–10 Nmm
1. Generally, acrylic adhesives can deliver break strengths for most thermoplastics including polyvinylchloride, polycarbonate, and polymethyl methacrylate. The organoborane acrylic adhesives are unique as they are the only adhesives that can bond polyolefins without any surface pretreatment delivering break strengths on unfilled polyolefins and strengths up to 10 MPa on glass-filled polypropylene. Acrylic structural adhesives are used in a broad range of applications from largearea panel bonding of the auto, truck, and school bus assembly and marine and wind blade assembly taking advantage of their longer pot lives followed by rapid ambient cure to smaller ferrite to ferrite bonding applications in loudspeaker assembly. The ability of acrylic structural adhesives to give tough bonds to a broad range of plastic and metal substrates has led to increased usage in rapidly growing higher value

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applications such as handheld electronic devices and mobile phones assembly. The specially designed polyolefin bonding acrylic adhesives have already established a unique position for bonding e-coated steel to glass-filled polypropylene in the automotive industry [28].

10.5 AUTOMOTIVE AND TRANSPORTATION APPLICATIONS OF THERMOSET ADHESIVES Different markets and applications require different adhesives. Today, a multitude of state-of-the-art adhesives are being developed and optimized for nearly every specific joining task in all the various industrial markets. The following sections will discuss applications in the following industry sectors: automotive and transportation, wind energy, electronics, and aviation, starting in this section with automotive and transportation applications. For a comprehensive introduction to structural adhesive bonding, see Dillard [29]. Today, thermoset adhesives and sealants can already be found in many areas of a car. Table 10.2 serves as an approximate overview of the different areas of application. Adhesives are increasingly being used to bond plastic interior parts and composites or attach them to metal substrates (semistructural bonding). Epoxy, (meth) acrylic, and polyurethane structural adhesives are more and more used to join multimaterial body shell panels and isolate them against local corrosion. It is now also

Table 10.2 Overview of thermoset material in automotive applications (MSpolymer, modified silane polymer) Application

Function

Structural bonding

Joining of different substrates – Semistructural – Structural – Crash-resistant

Structural foams Sealing

Local reinforcements

Panel damping

Protection against corrosion, water, and dust, isolation of noise propagation Reduction of noise, vibrations, and harshness (NVH)

Material basis MS-polymer Elastomer (Meth)acrylic Epoxy Polyurethane Epoxy Polyurethane PVC Elastomer EVA Acrylics Epoxy Elastomer PVC

10.5 Automotive and transportation applications of thermoset adhesives

state of the art to bond in windscreens to improve the structural stiffness and crash safety of a vehicle. However, one-component humidity cured polyurethane adhesives are usually used for windscreen direct glazing operations that cannot be considered as thermosets. Contrary to public perception, bonding and especially structural bonding have been applied in automotive constructions for over half a century. In 1960, Gengenbach took epoxy-based structural adhesives and compared spot-weld-bonded singlesided cap steel profiles with traditionally spot-welded steel profiles that were typical for the self-supporting car bodies of that time [30]. This is all the more noteworthy since spot welding itself had only been introduced in 1953 with the start of the production of the Mercedes 180 (W 120 series). Gengenbach was able to demonstrate benefits for the spot-weld-bonded profiles that still hold true today. On the one hand, the torsional stiffness of the spot-weldbonded profile with a spot-weld distance of 100 mm was close to an ideal rectangular profile and by far superior to a purely spot-welded profile with only 25 mm spot-weld distance (Fig. 10.5):

t = Teilung =ˆ Punktabstand PS =ˆ PunktschweiBung PSK =ˆ PunktschweiBklebung mit Epoxidharz Stahlblech 0.75 mm

42

25 m (t =

nach GENGENBACH

PS

PS

(t =

50

20°

m)

mm

)

24

PS (t = 7 5m

00 mm)

12

Verdrehwinkel α

−M

PS (t = 1

+M

30°

50

m)

40°

) 0 mm = 10 t ( K

10° PS

ideales Vierkantprofil 0°

0

50

100

150

200

250

Torsionsmoment M in Nm

FIG. 10.5 Quasistatic torsion experiments on spot-welded single-sided cap profiles with and without epoxy-based adhesive according to Gengenbach [30]. t, spot-weld distance; PS, spot weld; PSK, spot-weld bonding with epoxy resin. Verdrehwinkel, torsional angle; Torsionsmoment, torsional moment; Ideales Vierkantprofil, ideal rectangular profile made from one piece (no joining operation).

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CHAPTER 10 Thermoset adhesives

150 12

50

+M −M

t = Teilung =ˆ Punktabstand PS =ˆ PunktschweiBung PSK =ˆ PunktschweiBklebung mit Epoxidharz

100

PSK (t = 100 mm)

PS (t = 25 mm) 50 PS (t = 50 mm) s= 0.75 mm

0

0

42

24

Wechseldrehmoment M in Nm

354

nach GENGENBACH 0.5 × 106

1.0 × 106

1.5 × 106

Lastspielzahl N

FIG. 10.6 Torsional fatigue performance of spot-welded single-sided cap profiles with and without epoxy-based adhesive according to Gengenbach [30]. Wechseldrehmoment, cyclic torsional moment; Lastspielzahl N, number N of load cycles; t (mm), spot-weld distance.

On the other hand, the performance under torsional fatigue loads showed clear advantages when the profile was spot-weld bonded (Fig. 10.6): For a comprehensive comparison of laser welding and structural adhesive bonding for car body assemblies, see Ref. [31]. Since the time when Gengenbach made this study, the performance of epoxy-based structural adhesives has improved to a great extent. However, it took a long time and much more research until structural adhesives found their place as a trend-setting joining technology for todays and future automotive car bodies and transportation vehicles. Triggered by the discussions about global warming, the need to reduce greenhouse gases and fuel consumption has resulted in consistent political and legal pressure for the reduction of fleet fuel consumption. As a consequence, the automotive industry has taken considerable measures one of which is the curb weight reduction of automotive car body structures. This has led to the development and use of new low-density and high-performance materials like aluminum and magnesium alloys, new composite materials including carbon-fiber-reinforced polymers, and new highand ultrahigh-strength steels that could be used in much lower panel thicknesses without a need to make concessions in performance. As a consequence, automotive car body designs transformed from purely steel based to a modern multimaterial design including a variety of different new lightweight materials. Data from literature show that for vehicles with internal

10.5 Automotive and transportation applications of thermoset adhesives

combustion engines, a 10% reduction in curb weight leads to a 4.5%–7% increase in fuel efficiency [32]. To realize the vehicle weight savings, mainly three concepts are being used today [33]: 1. Advanced high-strength steel (AHSS): up to 17%–25% weight reduction of the body-in-white (without closures) is possible in combination with optimized steel designs [34]. Critical applications such as A-pillar or B-pillar reinforcements allow thinner gauge AHSS to be used while maintaining or even improving crashworthiness. 2. Composites/Engineering plastics (e.g., sheet-molded composites (SMC) or carbon-fiber-reinforced polymers (CFRP)): up to 30% weight reduction compared with equivalent conventional steel designs of underbody sections, exterior body panels, spare wheel wells, and semistructural components [35]. New composite production processes like resin transfer moulding, long fiber injection, or spray impregnation of nonwoven fiber assemblies gain more and more importance and acceptance. 3. Lightweight metals such as new aluminum and magnesium alloys: weight reductions between 21% and 40% are possible for the body-in-white (without closures) [34]. Certainly, more traditional bonding operations like welding destroy the sophisticated material properties locally at the weld line or welding spots or cause local corrosion if humidity can access the joint areas leading to premature weakening of the joint or even failure. Clinching or riveting also destroys the substrate locally or may cause stress concentrations leading to crack initiations with local degradation of the cohesiveness or strength of the assembly. Hence, structural bonding has seen a growing interest of many car-producing companies to reliably bond all those new substrates in modern automotive constructions since adhesive bonding preserves the structural integrity of the lightweight panels and isolates dissimilar bonding partners, thus avoiding local corrosion. However, adhesively bonded joints between dissimilar materials exhibit an intrinsic disadvantage, i.e., the difference in thermal expansion of both joining partners. With a linear thermal expansion coefficient of approximately 12  10
6 K
1, steel expands only 50% compared with aluminum (23  10
6 K
1) or as low as 15% relative to certain polyamides (50–90  10
6 K
1). On the other side, CFRPbased panels show nearly no thermal expansion or even slightly negative and anisotropic coefficients depending on the fiber direction (e.g.,
0.8  10
6 K
1). One immediate consequence is the generation of considerable local mechanical stresses in the bond line when the bonded assembly encounters significant temperature changes (e.g., during the passage through the e-coat oven in the car production line or in the lifetime (day/night) and (winter/summer)). Should the local stresses in the bond line exceed the strength limits of the structural adhesive, then fractures and finally failures are likely leading to a breakdown of the joint. Different strategies exist to avoid this scenario. Apart from additional mechanical fixtures (using rivets or clinches), the recent generations of structural adhesives either offer much higher

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tensile or shear strengths or allow higher elongations and thus relative movements of the bonded partners over temperature at high enough strength levels. Which solution is preferred depends on the specific bonding task. For high-dynamic car impact situations, the new thermoset crash-resistant structural adhesives are leading the way [33]. This new type of toughened epoxy-based adhesives exhibits a morphology optimized polymer structure that enables them to completely replace the spot-weld joining operation for the static and dynamic loadbearing structures of the car body construction. Even under very high dynamic loads that appear, e.g., in frontal or side impact scenarios, the bond lines do not fail or separate and allow the crash energy to better dissipate within the bonded panels. The new generation of crash-resistant structural adhesives displays exceptionally high thermal stability and has a crash resistance that is more than 10 times higher than that of conventional reinforcing adhesives. The crash performance is rather constant over a wide range of temperatures (usually from
40°C to + 80°C). But the new technology is not restricted to automotive applications alone. It is a platform technology that can also be transferred to other areas, such as modern lightweight materials for aircraft manufacturing. A new class of thermoset materials that over the last 10 years has gained more and more acceptance and importance in automotive lightweight constructions are epoxybased structural foams. Apart from metal, foams made from aluminum or zinc especially organic structural foams based on thermally activated epoxy-based formulations have a strong lightweight potential due to their low density, about 0.5–0.8 g/cm3 in combination with a very high compressive strength—comparable with lower grade concrete types—in static and highly dynamic load cases. For an overview of their main properties and applications, see Welters [36], Ullmann [37], and Kleiner [38]. Organic structural foams are usually applied to a construction either by extrusion or as a two- or three-dimensional preformed part on a steel or plastic carrier (usually toughened polyamide). In the e-coat oven of the car production line, the uncured epoxy formulation expands at approximately 180°C over 20 minutes by at least 80% and forms a sandwich-type structure with the opposite substrate (can be any metal or other plastic materials). After the oven cure, the foamed and cured material cools down and locally reinforces the critical section of the car body construction. Especially with two-dimensional structural foam reinforcement patches (substratefoam-glass fiber layer), the foam core may lead to a visible read-through effect when applied on thin outer closure panels caused by the thermal expansion and the high elastic modulus of the material. Based on experimental validation studies and using FEA—finite element analysis—the read-through effect can be predicted with good accuracy [39]. Parallel to the development of the lightweight constructions in the automotive and transportation industry was the progress in CAE computer-aided engineeringbased predictions of the properties of new virtual vehicle constructions. FEA—finite element analysis—allows to shorten the development cycles for new model developments significantly. As a consequence, new lightweight material developments

10.6 Other applications of thermoset adhesives

can only be successfully introduced into the market if validated material data cards and simulation models are provided to the OEM. Whereas for metals and engineered plastics this information is usually available, it has only been for a short time that crash-resistant structural adhesives and structural foams can be modeled via finite element analysis with sufficient reliability. Nevertheless, already significant progress has been made to simulate structural adhesive bond lines [40–47] and reinforcements based on structural foams [41,48]. However, the increasing usage of composite and fiber-reinforced materials has imposed demanding challenges to the numerical modeling of multimaterial car body structures. Reliable and sufficiently precise CAE-supported predictions about the composite component response to specific vectored loads and stresses are needed to arrive at safe and weight-reduced designs. A standardized simulation approach based on easily accessible physical parameters is not yet available, and actual developments still rely on component-specific parameterizations. In one recent approach, the stress distribution in a bond line between composite materials has been analyzed including a detailed consideration of the laminate layup and loading direction. In order to describe the incompatible thermal elongations of the different substrates in a multimaterial component, a visco-elastoplastic finite element material law had been implemented that included the conversion level of the adhesive cure based on a kinetic equation [49]. The numerical approach presented allows the prediction of residual stresses and distortions caused by the cure of the structural adhesive. The methodology is based on the determination of characteristic material parameters for the viscoelastic behavior. Given that further progress in the development of new kinds of materials and tools will be made, future vehicle designs in the area of automotive and transportation will see a sustained crossover from traditional steel-based designs to more multimaterial mixed designs that will continue to become even lighter for the sake of our environment.

10.6 OTHER APPLICATIONS OF THERMOSET ADHESIVES 10.6.1 WIND ENERGY APPLICATIONS The wind energy sector is a very-fast-growing industry. Over the last 10 years, the installed capacity of wind power has grown annually by about more than 20%. A total of 977 GW installation is expected for 2030 [50]. Apart from few exceptions (Siemens produces full blades in a single infusion step), the production of wind turbine blades involves adhesively bonding two blade shells together. Accordingly, the amount of adhesive used in the industry grows as well. In general, the construction of a wind energy converter/turbine requires a substantial amount of thermoset material including infusion resins, sealants, coatings, and of course adhesives. By far, the largest part of the thermoset material goes into wind turbine blades, which essentially consist of reinforcing fibers of glass or carbon

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and thermosets. Typical blade sizes of current wind turbines range from 40 to 50 m, weighing 10–20 tons and providing a power output in the range of 1.5–7.5 MW. New designs in particular for offshore applications target blade lengths of 90 m and beyond. The trend for bigger blades is due to the scaling of the power output of turbines, which is proportional to the square of the blade length. While increasing the blade length is attractive for increasing the power output, it also implies that the weight forces of the blade increase by the power of three and weight-induced moments grow by the power of four. Still, wind blades are typically expected to withstand 20 years in service. Consequently, the requirements on the structural materials are growing. Next to the increasing loads also, the future production design is pushing the material requirements. To compete in this fast-developing market, blade manufacturers aim to accelerate their production and to automate their processes. Thus, the requirements on the rheological profile and curing keep evolving. This does apply not only to adhesives but also to, e.g., infusion resins and other thermoset materials mentioned earlier. The chemical basis of wind blade bonding pastes is broad. One finds, e.g., epoxy, polyurethane, vinyl ester, unsaturated polyester, and methyl methacrylate in use. The particular application determines which type of bonding paste is adequate. In terms of mechanical properties, the suitability of a bonding paste is strongly related to the blade design. The spar, running from the root to the tip of the blade, is the main structural element carrying the gravitational and aerodynamic loads. There are two prevalent spar designs for wind blades, which place the adhesive requirements in two categories [51]. Fig. 10.7 illustrates these two spar designs in a cross section of the blade. The red spots indicate the position of the adhesive, which is bonding the leading edge, trailing edge, and spar caps. In the box beam design on the left, the spar caps are integrated into the box spar, and the adhesive bonds these with the aerodynamic foil. In the I-beam design on the right, the adhesive is joining the spars with the spar caps. Hence, the adhesive itself is part of the main load-carrying structural element in this case. Consequently, the mechanical requirements on adhesives used for I-beam designs are far more stringent. Building on experience, epoxy adhesives are most

Spar cap

Box-beam

Spar cap

I-beam

FIG. 10.7 Illustration of two typical cross sections of wind turbine blades. The red areas in the drawing indicate the position of the adhesive. On the left, one sees a cross section of a spar in box beam design. The cross section on the right shows two shear webs in I-beam design joining the spar caps. For the I-beam concept, the adhesive is part of the load-bearing spar.

10.6 Other applications of thermoset adhesives

commonly used for this type of application today. A challenge arising from the use of epoxy bonding pastes is high exothermal peak temperatures. Depending on the cure temperature and the volumes of the adhesive beads, the exothermal peaks of the epoxy may raise to 200°C and above. At such temperatures, one runs the risk of damaging the surrounding components and the adhesive itself. This poses relevant constraints on cure temperature affecting processing speed and material selection. To this end, the far more moderate curing profile of polyurethanes has a significant advantage in accelerating the production process. Henkel has thus developed a set of polyurethane structural blade bonding adhesives, which cure at room temperature and match the mechanical profile of its epoxy counterparts. These polyurethane adhesives were the first to be certified by the Germanischer Lloyd (GL) for wind blade bonding applications. This facilitates the downstream certification process for wind turbine producers and insurers, by assuring strength and humidity resistance for typical operating conditions [52]. Fig. 10.8 illustrates the cure profile of a 5 kg structural polyurethane blade bonding adhesive heated to 80°C between two steel plates in a heat press. The adhesive has dimensions of 6 cm height and approximately equal length and width of about 20–25 cm. The reaction temperature in the adhesive remains well controlled and reaches its peak at 110°C after 30 minutes. A typical epoxy blade bonding paste of such volumes in similar conditions most certainly does carbonize or even catch fire.

100

T (°C)

80

60

40 0

20

40 60 t (minutes)

80

FIG. 10.8 Cure profile in the center of a 6  20  25 cm (5 kg) bead of a structural polyurethane-based bonding paste (resin: Loctite UK 1340 B60 and hardener: Loctite UK 5451) between two steel plates in a heat press heated to 80°C. The reaction temperature does not raise above 110°C.

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Another important adhesive property is the time- and temperature-dependent cure profile of the adhesive. During the application of the adhesive material to the composite half shell of the blade, the cure of the adhesive should be delayed to allow a complete application of the adhesive material. In a next step, the two half shells are fitted together geometrically in the right position. The joint blade is then transferred to a tempering oven. There triggered by the elevated temperature the adhesive cures completely. This so-called delayed action cure or catalysis permits to tailor the cure profile of the adhesive to match or minimize the desired production cycle time for the bonding operation of the half shells of the blade. Ideally, this will lead to reductions in overall production time and to a higher productivity in the blade manufacturing process. Since the wind turbine blades are expected to withstand the oscillating operational load for a period over 20 years, the fatigue performance of the adhesive is of particular importance. A comparison of the GL-certified polyurethanes, PU1 and PU2, with a standard epoxy is given in Fig. 10.9. The figure shows the S-N curve for alternating shear loads (R ¼
1) applied through hollow cylinders in the range of 104–107 cycles in a log-log diagram. The polyurethanes PU1 and PU2 show a significantly slower fatigue, compared with the standard epoxy bonding paste. These examples only offer a selected rather than a comprehensive view on blade bonding adhesive requirements. However, they illustrate that various chemically 28 26 24 Shear stress (MPa)

360

PU 1 PU 1 run through

22

PU 2 PU 2 run through

20 EP EP run through

18

16

1×104

1×105

1×106

1×107

Cycles

FIG. 10.9 S-N curve in a log-log plot for PU and EP blade bonding adhesives under shear load in butt-bonded aluminum cylinders at R ¼
1.

10.6 Other applications of thermoset adhesives

different thermoset adhesives have the potential to serve as structural bonding pastes meeting the demands of the wind energy industry. The development of new bonding pastes is growing with the demand of this rapidly rising industry.

10.6.2 AVIATION APPLICATIONS Thermoset adhesives have a long history in the aviation industry. The first phenolic thermoset adhesives used in the construction of airplane parts were in World War II. The composite wood/aluminum wing structure of the light bomber DH Hornet produced by Hawker Siddeley was bonded with a toughened phenolic adhesive and cured at 155–175°C. Due to the condensation reaction of the phenolic adhesive, water is released during the cure that means that phenolic adhesives need to be cured under pressure to avoid foaming of the adhesive and expansion in the bond line. In addition, the release of water during cure determined the maximum bond line width. In general, phenolic adhesives are brittle and therefore not ideal for structural bonding. To overcome the brittleness of phenolic adhesives, poly(vinyl formal) was added in big amounts (75%) to the adhesive to toughen the matrix and to achieve crack resistance. This adhesive, developed in 1942 by De Bruyne, found its way also into commercial airplanes like the De Havilland Dove aircraft [53–56]. By mid of the 1950s, Fokker had fully implemented the design of aircrafts with aluminum bonding using toughened phenolic adhesives. The most popular aircraft of all time, Fokker F27 Friendship aircraft, contained about 70% bonded parts and demonstrated the durability of bonding by service life of >30 years due to the superior fatigue properties compared with riveted constructions [57]. In 1956, bonded aluminum sandwich panels were developed by Martin and Hexcel consisting of an aluminum skin and aluminum honeycomb core that stayed till today an integral part of lightweight airplane design [54]. In the 1950s, epoxy thermoset adhesives were introduced to the market and replaced phenolic adhesives more and more. Epoxy adhesives have the advantage of curing in a polyaddition reaction that does not release water during the cure, and therefore, bigger areas can be bonded, and less porosity is observed. However, epoxy adhesive joints are more sensitive to aging under humidity and salt water environment compared with phenolic adhesives that led to the development of anodized surfaces in the 1960s. Bonding of anodized surfaces improved the stability of the bond and improved the durability significantly. The benefits of adhesive bonding in the aerospace industry are manifold. Adhesives come in film and paste form that enables the user to apply film adhesives over large areas like for bonding of honeycomb sandwich panels. Mechanical fastening would not work in that case to fix the skin to the honeycomb core. Furthermore, adhesives do not create stress points like rivets or bolts that enable the designer to reduce the total weight of a bonded part and improve properties with regard to stiffness and fatigue. The biggest challenge currently is to determine the quality and performance of a bonded part. Nondestructive testing of adhesive joints is limited and does not

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give the ultimate confidence that would be necessary to spread adhesive application further into primary bonded parts without or at least with reduced mechanical fastening. With the increased usage of composite materials, like in Boeing 787 and Airbus A350 with about 50% of composite parts, adhesives had to adjust their properties to the changed requirements. Most of the composite parts are based on carbon fibers and epoxy resin systems. Thus, the adhesive needs to be compatible with the resin system in the composite part. In case of co-cure bonded parts, the adhesive is cured at the same time as the prepreg resin. This means that the adhesive can intermingle with the prepreg resin and therefore could have some severe influence on the interface between adhesive and composite if both systems are not compatible. On the other hand, two cured composite parts can be bonded together. In this case, the composite surface has a low-energy surface after being in contact with the release agent. A standard pretreatment is to grind the surface to remove the upper low-energy layer. Another option to remove the low-energy surface layer is to use peel plies which involves less labor for the surface pretreatment and is more cost-effective. A peel ply is typically a woven polyester or polyamide fabric cured onto the composite surface. Before bonding, the peel ply can be peeled off and removes the upper layer of the cured composite part. Due to changes in the composite resin system, cure conditions, and wet out of the fabric, wet peel plies were developed to optimize a composite surface for bonding. Wet peel plies are preimpregnated peel plies that are cured with the composite and peeled off before bonding. This method ensures a constant and reliable surface for bonding independent of the used prepreg.

10.6.3 ELECTRONICS APPLICATIONS Large volumes of thermosetting adhesives are used in electronic bonding and sealing applications each year. Adhesives are increasingly expected to perform a variety of tasks in electronic assembly, including conducting or insulating electricity, conducting heat, sealing, and protecting in addition to providing adhesion [57]. The main groups discussed here are as follows: – Electrically and thermally conductive adhesives – Coating and protection products – Adhesives and sealants for flat-panel display manufacture

10.6.3.1 Electrically and thermally conductive adhesives The market is predominantly served by high-purity epoxy adhesives [58] that adhere to a wide variety of surfaces and have wide formulation latitude, making them ideal for most applications. With the increasing popularity of lead-free solders and the corresponding higher temperatures that the adhesives must withstand, acrylic- and maleimide-based adhesives have become more widely used. Maleimide-based adhesives, in particular, are stable to higher temperatures and also provide better retention of mechanical properties at elevated temperatures [59].

10.6 Other applications of thermoset adhesives

Isotropic electrically conductive adhesives (ICA) are widely used in the electronic industry as solder replacement when high-temperature soldering processes are unsuitable and a lower stress joint is needed. Typical applications include silicon die attachment, surface-mounted PCB repair, and electromagnetic/radio-frequency interference (EMI/RFI) shielding. In all cases, the conductive adhesive consists of conductive filler in a polymer (adhesive) matrix. The choice of filler and adhesive matrix is dependent on the end use [60]. Over the last decade, more and more components are being packaged using nonconductive die attachment adhesives, as the tighter geometries and higher densities of these packages require electric isolation. Nonconductive die attachment adhesives are usually based on the same resin, or matrix, chemistry, but use silica or alumina filler to replace the silver flake. EMI/RFI shielding materials have to meet much lower demands in terms of overall electric conductivity (typically 4–5 orders of magnitude lower than a silver-flake-filled adhesive). This means that cheaper conductive fillers can be employed, for example, silver-coated copper flake, nickel flake, and carbon black [58]. Thermally conductive adhesives are commonly used as an interface between heat sinks and heat sources (e.g., high-power semiconductor devices). Their function is to give a mechanical integrity to the bond between the heat sink and heat source but more importantly to eliminate air (which is a thermal insulator) from the interface area. These adhesives consist of a polymerizable liquid matrix and large volume fractions of thermally conductive fillers. Typical matrix materials are epoxies, silicones, and acrylates, although solvent-based systems, hot-melt adhesives, and pressuresensitive adhesive tapes are also available. Aluminum oxide, boron nitride, zinc oxide, and aluminum nitride are typical fillers in electrically insulating thermal interface materials. To reach high thermal conductivity, silver, aluminum, and nickel are used. The filler loading can be as high as 75–85 wt%, and the fillers raise the thermal conductivity of the base matrix from 0.17–0.3 Wm
1 K
1, to about 2.5–3.0 Wm
1 K
1 for the electrically insulating adhesives, to 10 Wm
1 K
1 for the metal-filled adhesives [61].

10.6.3.2 Coating and protection products A conformal coating is a thin polymeric layer applied onto a printed circuit board (PCB) or other electronic substrate by brushing, dipping, spraying, or simple flow coating, and increasingly by select coating or robotic dispensing as the last processing step. The coating provides a barrier that protects the PCB or other electronic substrate (conductors, solder joints, and components) from moisture, oxidation, and other environmental and mechanical attacks during service life of the product, significantly extending the life of the components and circuitry. Conformal coatings are available in a number of different chemistries that include polyurethane (solvent and nonsolvent versions), silicones, epoxies, acrylics, acrylated urethanes, and parylenes. The choice of coating depends on the chemical and mechanical (abrasion) resistance required, the degree of rework envisaged and the method of curing.

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Encapsulants and molding compounds encompass a broad range of materials whose primary function is to protect electronic components from detrimental chemical, mechanical, electric, or thermal environments. They are usually characterized as potting compounds, glob tops, molding compounds [62], or underfills [63], depending on the end use. Potting compounds are materials that are used to protect final assemblies (relays, component terminals, electric contact assemblies, etc.). Glob tops, underfills, and molding compounds are specifically used for silicon die protection. Potting compounds should provide good adhesion to the assembly materials/substrates, low cure shrinkage (and hence low internal stress), and in large sections low heat release on curing. Glob tops offer protection to silicon die that is wire-bonded directly onto a substrate (PCB, polyimide flexible circuitry, or ceramic). The liquid encapsulant is designed to flow between the wire bonds to form a protective coating over the integrated circuit. In some cases, the pitch between wire bonds necessitates the use of low-viscosity liquids, and in these cases, a dam of compatible adhesive is first dispensed around the integrated circuit to prevent the glob top spreading to other areas of the PCB. Typically, glob tops will have low levels of ionic contaminants and low coefficient of thermal expansion (CTE). The CTE is an important consideration in all encapsulant applications; the primary requirement is that the CTE of the encapsulant and molding compound closely matches that of the silicon die, substrate, and wire bonds to minimize stress in the joint (particularly under conditions of thermal cycling). Underfills are a specific class of adhesives designed to protect silicon dies that are soldered active face down onto the PCB. In these flip-chip applications, the underfill material flows beneath the die by capillary action. These materials are generally highly loaded with inorganic fillers to reduce the coefficient of thermal expansion.

10.6.3.3 Adhesives and sealants for flat-panel display manufacture Adhesives are widely used and very important in flat-panel display manufacture, especially for liquid-crystal display (LCD) and organic light-emitting diode (OLED) panel assembly. Based on the filling sequence of liquid-crystal material, the LCD panel manufacturing process can be divided in two types: – Conventional process – One-drop filling (ODF) process In the conventional process, liquid-crystal material is filled through an open area where two glass substrates are already assembled together by main seal. There are three main adhesive applications: temporary plate fixing, main seal, and end seal. A number of LCD cells are made from single large flat glass plates. In the first process step, the main seal (or LCD gasket) is printed or dispensed onto the glass to define individual cells; this is usually a thermal-cure epoxy. To hold the two plates in the correct orientation while the main seal is fully cured, a temporary-fixing adhesive is used. The final production step involves filling the cell with liquid crystal through a gap left in the main seal and subsequently sealing the gap.

References

The ODF process has been used recently to improve the throughput. In the ODF process, only the main seal, normally a hybrid of UV-cure acrylate and thermal-cure epoxy, is used as adhesive. At first, the main seal is dispensed onto one glass substrate. After that, LC material is dropped in (at a precisely calculated amount). The other glass substrate is assembled after air has been taken out in a vacuum chamber, and the seal is cured.

10.7 FUTURE TRENDS Since its beginnings in the last decades of the 19th century, industrial constructions were mainly based on either steel (automotive) or aluminum alloys (aviation and aerospace). With all the new possibilities that came up just recently, the new design trends will be able to follow social, political, and legal requirements and will become sustainable and ecologically compatible. We will see further optimizations of material properties and lightweight constructions that will be made possible by the use of thermoset structural adhesives and foams, bonding of composite structures, and performance predictions with CAE-based simulation tools. In this sense, thermoset adhesives will continue to be a key technology and key driver of future developments in the transportation and general industries.

ACKNOWLEDGMENTS The background knowledge included in this chapter owes considerable debt to the contributions of the following colleagues from Henkel AG & Co. KGaA, Dr. Marc Hamm, Dr. Matthew Holloway, Dr. Brendan Kneafsey, Dr. Olaf Lammershop, Dr. Thomas Zoeller, and Tim Welters. The author gratefully acknowledges their support and contributions.

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