Composition of Adhesives

Abstract

Adhesives and sealants are generally developed and prepared for many applications such as packaging, construction, automobile, electronic, etc. An adhesive formulation will depend on the base materials and requirements of a particular application. Development managers or formulators have to have a public knowledge about the chemical composition and role of many components for reducing trials and errors. This chapter focuses on the definition and function of adhesive composition such as primary resins, solvents, fillers, plasticizers, reinforcements, and various additives.

1 Introduction

An adhesive is a polymer mixture or polymerizable material in a liquid or semiliquid state that adheres substrates together (Petrie 2000). Adhesives may be composed of many components such as polymer, oligomer, filler, and additives from either natural or synthetic sources. It is very important to understand the components for adhesive formulation. The information of composition gives us an adhesive selection guide based on functional properties, curing mechanisms, and other relevant information supplied by the adhesive manufacturer.

An adhesive is a complex formulation of many components that have a unique function. The adhesive manufacturer or developer selects actual ingredients depending on the end-user requirement, the application, processing requirement, and the cost as shown in Fig. 1. The various components of an adhesive formulation include the following: primary resins, solvents, fillers, plasticizers, reinforcements, thickeners and thixotropic agents, film formers, antioxidants, antifungal agents, emulsifiers, and wetting agents (Petrie 2000).

Fig. 1

Flow chart of adhesive applications

The adhesive formulator gets a lot of information from raw material suppliers, books, papers, and patents. The number of possibilities for innovation seems to be endless. However, the formulation of an adhesive is fixed according to the formulator’s experience and education. Knowledge of how to incorporate ingredients together into a practical, workable formulation is also required.

2 Primary Resins

The primary resins of adhesives and sealants are the principal component that provides a lot of characteristics such as wettability, adhesion strength, thermal property, chemical resistance, and environmental resistance. The word “resin” means a hydrocarbon secretion of many plants, particularly coniferous trees. In the adhesive industry, the primary resin means a polymer that is the main chain in the adhesive molecular structure. The understanding of primary resins is essential for adhesive curing, application, reliability, and adhesion failure analysis. For example, a modified epoxy acrylate is one of the primary resins for the main sealants of the liquid crystal display (LCD) (Park et al. 2009). The main sealants are used to adhere the thin film transistor and the color filter. To produce sealants for LCD, the primary resins have to have absolutely high purity to prevent pollution of the liquid crystal. However, some primary resins have a large range of performance depending on molecular weight, molecular structure, additives ratio, etc. Table 1 gives a classification of applications and primary resins (Dostal 1990).

Table 1 Various adhesive classifications and primary resins

Adhesives are classified by many methods such as dispensing method, application, and primary resin. The classification by primary resin is very useful to select a good adhesive for a given application, because the name of the primary resin involves the chemical structure of the back bone and the sketchy properties such as adhesion strength and heat resistance. In this chapter, the classification of primary resins by chemical composition consists in thermosets, thermoplastics, and elastomeric resins. A thermosetting resin is a prepolymer in a soft solid or viscous state that changes irreversibly into an insoluble polymer network by curing, which can be induced by the action of heat or radiation, or humidity. Thermosetting materials are generally stronger than thermoplastic materials due to 3-D network of bonds, and are also better suited to high-strength and high-temperature applications. Table 2 shows some typical thermosetting resins and their characteristics for adhesives and sealants (Dostal 1990). A thermoplastic resin is a polymer that can turn to a melting liquid when it is heated and returns to solid when it is cooled down. Table 3 shows some thermoplastic resins and their properties (Dostal 1990). The molecular structure of thermoplastic resins is linear or branched. Linear and branched polymers are often soluble in solvents such as chloroform, benzene, toluene, and tetrahydrofuran (THF). More detailed information for each primary resin is given in Chap. 14, “Adhesive Families.”

Table 2 Typical thermosetting resins for adhesives and sealants

Table 3 Typical thermoplastic resins for adhesives and sealants

3 Solvents

Solvents are liquids comprising one or more components that are volatile under the specified drying conditions and can dissolve film-forming agents purely and physically without chemical reactions (DIN EN 971-1 1996). Solvents can lower the viscosity of the formulation to make it easier to apply and to help liquefy the primary resin so that the other additives may be easily incorporated into the formulation (Petrie 2000).

Diluents are defined as ingredients used in conjunction with the true solvent to increase the bulk of another substance without causing precipitation (LeSota 1995). Diluents are mainly used to lower the viscosity and modify the processing conditions of adhesives and sealants. Diluents participate in the partial components of the final adhesives because they have a lower volatility than solvents.

Solvents are used to control the viscosity of the adhesives so that they can be applied more easily. They are also used to aid in formulating the adhesive by reducing the viscosity of the primary resin so that additions of other components and uniform mixing may be achieved more easily. When solvents are used in the adhesive formulation, the similarity of the solubility parameter between the solvent and the primary resin is very important.

The solubility parameter (δ-value) is divided as the following equation:

$$ {\delta}_{\mathrm{total}}=\sqrt{\delta_{\mathrm{D}}^2+{\delta}_P^2+{\delta}_H^2} $$

(1)

δ_D = parameter for nonpolar contribution; δ_P = parameter for polar contribution; δ_H = parameter for hydrogen bridges contribution.

The indices relate to the nonpolar dispersion forces, the dipole forces, and the interactions caused by hydrogen bridges. Table 4 lists the solubility parameters of various solvents (Goldschmidt and Streitberger 2007).

Table 4 Solubility parameters δ_total of different solvents and their disperse contribution δ_D, polar contribution δ_P, and hydrogen bridging contribution δ_H, (unit in [J/cm³]^1/2) (Goldschmidt and Streitberger 2007)

The prediction of the miscibility between the polymer (primary resin) and solvent can be allowed from a comparison of the solubility parameters of the polymer (δ_polymer) and solvent (δ_solvent) because δ is a measure of the interaction forces between molecules of the material. Therefore, the difference of solubility parameter (δ_polymer−δ_solvent) should be small for good miscibility.

Solvents can be classified into some categories by their chemical character into groups with common features: aliphatic, cycloaliphatic, and aromatic hydrocarbons, esters, ethers, alcohols, glycol ethers, and ketones. Table 5 shows the important physical performance indicators of solvents used for the coatings or adhesives such as the density, refractive index, boiling temperature or boiling ranges, and the vapor pressure or evaporation time. In addition, the flash point, ignition point, and explosion limits must also be needed for safety reasons (Goldschmidt and Streitberger 2007).

Table 5 Physical and safety-related data of important solvents (Goldschmidt and Streitberger 2007)

General solvent contents used for typical sealants are shown in Table 6 (Petrie 2000).

Table 6 General solvent contents for typical sealants (Petrie 2000)

The use of excessive solvent may cause a shrinkage problem when the sealant cures. Volume shrinkage will always be greater than the weight percent of solvent due to its much lower density than other components in the sealant. Toluene, xylene, petroleum spirits, water, and others are used as solvents in sealant formulations. In case of solvent mixtures, the balance of volatility between solvents is very important to avoid a trouble such as the sealant’s skin drying problem.

4 Fillers

Fillers are generally relatively non-adhesive materials added to the adhesive formulation to enhance mechanical strengths, thermal properties, adhesion performance, etc. Common fillers in conductive adhesives are listed in Table 7. One of the purposes of using fillers is lowering the cost. The incorporation of fillers into the adhesive reduces the resin content and thus the product cost is also reduced. However, fillers can change adhesive properties. Filler selection and loading level are very important in formulation of adhesive because adhesive properties depend on filler type, size, shape, and volume contents. Common fillers are metal powder, kind of clay, dust, glass fiber, alumina, and so on. Sometimes fillers act as extenders or reinforcement materials.

Table 7 Filler for conductive adhesive in common adhesive formulations

Normally, adhesive can be conductive by adding conducting filler particles. The resin provides an interaction bond between substrates and conducting fillers as well. However, it is the conducting filler that provides the desired electrical interconnection path as depicted in Fig. 2. To make conducting adhesives, the fillers must be in physical contact with each other. Thus, the electrical conductivity increases sharply when a percolation threshold level of well-dispersed conducting filler is accomplished. Figure 3 shows the resistivity versus conductive filler level (Petrie 2008).

Fig. 2

Conductivity depends on contact between filler particles within the adhesive (Petrie 2008)

Fig. 3

Conductive filler level versus resistivity in conducting adhesive formulation (Petrie 2008)

Alumina particles are one of the commonly used fillers for improving the thermal conductivity of adhesives in particular insulation adhesives. Aluminum and silver powders or flakes are used to improve the thermal and electrical conductivities for adhesives intended to be an electrical or thermal path. The filler volume content level is very important to get sufficient conductivity. However, excessive filler content might cause degradation in mechanical properties of the adhesives (Kahraman and Al-Harthi 2005; Kahraman et al. 2008).

One of the most effective techniques used to improve the electrical conductivity of polymers is the incorporation of conductive fillers in the polymer matrix. The most popular electrically conductive filler is silver due to its moderate cost and superior conductivity. Silver-coated inorganic particles and fibers are superior compared to carbon particles and fibers as components in epoxy-based adhesives regarding the electrical conductivity of the composite; because the electrical conductivity of silver is much higher than that of carbon. In addition to the high electrical conductivity of silver, the addition/application of silver-plated particles and fibers also leads to composites with high mechanical strength and modulus, low weight, and a high ratio of metal-plated fibers (Novák et al. 2004).

Among the various conductive particles, silver particle is probably the most common filler because of its excellent conductivity and chemical durability. Silver particles are easy to precipitate into a wide range of controllable sizes and shapes, so it can be used to change the percolation threshold of the adhesive formulation. Silver also exhibits high conductivity; however, the silver filler is expensive (Lin et al. 2009).

Graphite is usually used as an electro conductive filler due to its low cost and natural electrical conductivity. It also has a positive influence on the mechanical properties, as well as thermal and dimensional stability (Lin et al. 2009). Substitution of carbon black with renewable filler has been investigated in recent years. Carbon black has some advantages such as low cost, low density, high electrical conductivity, and, in particular, specific structures that enable the formation of conductive network (Wan et al. 2005; Jong 2007).

Copper has good electrical conductivity and high adhesion property in conductive adhesive systems. However, copper tends to form a nonconductive oxide surface layer (Zhao et al. 2007).

Nickel is a good nominee for conductive filler in conducting adhesives because of the numerous benefits it possesses. Nickel shows chemical stability and oxidizes relatively slowly compared to copper. But, nickel has disadvantages compared to silver: nickel has a higher electrical resistance than silver (about 25% of silver). However, it is less expensive than silver filler (Goh et al. 2006).

There are many conducting fillers for adhesive formulation. Apart from the ones mentioned above, iron, chromium, molybdenum, tungsten, and other metal particles can be used as fillers for conducting adhesives.

Many other inorganic fillers are used for adding a function to adhesive. Table 8 shows the additional function of fillers. Aluminum oxide (flame retardant), lead (radiation shielding), mica or clay (electrical resistance), and silica and silicon carbide (abrasion resistance) are used to add additional function to the adhesive formulation.

Table 8 Filler for other function in common adhesive formulations

Generally, fillers in sealant formulation are used as additives to boost the viscosity of the sealant and get better gap-filling properties and to reduce the material cost of the sealants. In sealant formulation, fillers cannot affect reinforcement and improved strength. However, they can affect other properties such as water resistance and hardness, etc. The most common filler used in sealant is calcium carbonate, because of its many advantages such as abundant resource, low cost, and stability. Other fillers are clays, silica, titanium dioxide, carbon black, and iron oxide. Table 9 lists commonly used fillers in sealant formulations and functions in sealant systems.

Table 9 Fillers commonly used in sealants

Fillers represent the largest part in terms of weight for many sealants. Calcium carbonate is the most widely used filler for sealant formulations. As a filler for sealants, calcium carbonate acts as an inert extender to reduce formulation cost, modifier for mechanical properties, and a rheological modifier. Normally, properties of fillers such as particle size, shape, and surface properties are very important factors for sealant properties. Figure 4 shows the typical effect of particle size on the viscosity in common adhesives (Chew 2003). Filler volume contents can vary significantly with being dependent on the primary resin and the formulation.

Fig. 4

The viscosity versus particle size in common adhesives (Chew 2003)

5 Plasticizers

Plasticizers are substances of low or negligible volatility that lower the softening range and increase workability, flexibility, or extensibility of a polymer (ASTM D907-08b 2008). The main purpose of plasticizers is to modify the property of adhesives and sealants. The addition of plasticizers causes the improvement of flow and flexibility, but the reduction of the elastic modulus, stiffness, hardness, and the glass transition temperature (T_g). As a result, the processibility and extrudability of adhesives and sealants can be improved by addition of plasticizers (Tracton 2007; Stoye and Freitag 1998).

Plasticizers also affect the adhesion property. Addition of plasticizer to the adhesive will always lower the cohesive strength, generally reduce the peel adhesion, and will have a variable effect on the tack depending on the type of plasticizer used (Satas 1999). Plasticizers must be compatible with other adhesive ingredients because of their inherent chemical characteristics and nonreactivity with other components (Petrie 2000).

Plasticizers and solvents are governed by the same laws of solubility and have the ability to increase the free volume of the polymer. However, solvents mostly serve as viscosity modifiers and plasticizers are used to modify the properties of the adhesives or sealants, such as softening and lowering the T_g (Petrie 2000).

Plasticizers have polar and nonpolar components in the molecular structure. The polar components interact with the polar groups of the primary resins. Otherwise, the nonpolar components prevent the intermolecular interaction between plasticizers and the molecules of primary resins by steric hindrance. As a result, the mobility of adhesives or sealants can be promoted. Due to the same reason, T_g is shifted to a lower temperature with an increase of the plasticizer content as shown in Fig. 5 (Goldschmidt and Streitberger 2007).

Fig. 5

Lowering the glass transition temperature (T_g) of a polymer by a plasticizer (Goldschmidt and Streitberger 2007)

When a compatible oil is added to an elastomer such as natural rubber, it acts as a plasticizer. The storage modulus (G′) of a natural rubber/aliphatic oil adhesive decreased at all frequencies with the increasing amount of plasticizing oil as shown in Fig. 6 (Satas 1999).

Fig. 6

Effect of plasticizing oil on the storage modulus (G′) of natural rubber/aliphatic oil as function of frequency at 25 °C (Satas 1999)

A variety of plasticizers can be used in adhesives and sealants as to their primary resin type. Paraffinic oils, phthalate esters, and polybutenes are typical plasticizers (Dostal 1990). Plasticizers for natural rubber adhesives, such as mineral oil or lanolin, are used to reduce the cost of the adhesive mass, and have a depressing effect on the peel adhesion (Satas 1999). Phthalates, chlorinated hydrocarbons, and aliphatic hydrocarbons are commonly used as plasticizers in urethane sealants (Dostal 1990). Most of sealants, except for silicones, contain plasticizers in their formulations. Silicone sealants can be plasticized only by low molecular weight silicone oils (Petrie 2000).

6 Reinforcements

Reinforcements are used to enhance mainly mechanical properties. Many reinforcements are now available, and some are designed for a particular primary resin. There are various reinforcements for adhesive formulation. Generally, reinforcements act as the polymer resins in an adhesive system and reinforcing the internal bonding strength of the adhesive. Figure 7 depicts a schematic diagram of an adhesive with reinforcements. Reinforcements act like crosslink agents. It means that the reinforcement reduces the strain in a shear test and can enhance the mechanical strength of the adhesive. The main reinforcements in adhesive formulations are listed in Table 10.

Fig. 7

Schematic diagram of adhesive at shear strength test: (a) without reinforcement and (b) with reinforcement

Table 10 Common reinforcement agents

Recently, reinforcement agents in adhesive formulations have received attention due to very high reinforcement ability, simple preparation, and cost. One of the agents is inorganic clay which acts as a modifier of the mechanical and barrier properties. However, clay has some disadvantages for application as a reinforcement agent because of aggregation in the polymer resin. Therefore, many researchers have modified the surface of the clay. Figure 8 shows the clay loading contents versus lap shear strength. It can be seen that the modified clay gives an improved lap shear strength in relation to that of the pure clay (Maji et al. 2009; Osman et al. 2003).

Fig. 8

Lap shear strength versus clay contents in polyurethane adhesive between aluminum and aluminum with PU adhesives (Maji et al. 2009)

Carbon nanotube (CNT) has excellent mechanical properties. CNT is very similar to graphite. A single-walled carbon nanotube (SWCNT) can be seen as a rolled sheet of graphite, while a multi-walled carbon nanotube (MWCNT) can be viewed as layers of many graphite sheets. Moreover, the stiffness is very high: SWCNT’s Young’s modulus is about 1 TPa and the shear modulus to be 0.45 TPa. Many researchers have used CNT to enhance the mechanical properties of adhesives. Figure 9 depicts the shear strength of an epoxy adhesive with different MWCNT contents (Hsiao et al. 2003).

Fig. 9

The average shear strength of epoxy adhesive with different MWCNT weight fraction (Hsiao et al. 2003)

Sometimes, the reinforcement is a carrier in the adhesive composition such as in tapes and films. A carrier is usually a thin fiber fabric, cloth, or paper material. In pressure sensitive adhesives, the carrier is a backing substance for the adhesive being applied. Usually, the backing materials are used for functional or decorative reasons. Glass, nylon fabric, polyester, and paper are common carriers for adhesives. In this case, the carrier acts as a “reinforcement” to ease the use of adhesive. Figure 10 shows a schematic diagram of the carrier for an adhesive tape or film. A carrier is very useful to b-staged adhesive systems because of their semi-cured formulation. The carrier can act as support for adhesive.

Fig. 10

Carrier for adhesive

7 Other Additives

Additives are functional chemicals added to adhesives to ease the process and improve some properties. Many additives, like antioxidants, thermal stabilizers, UV stabilizers, polymer processing aids, anti-blocking additives, slip additives, antifogging agents, antistatic additives, flame retardants, and colorants, have been used for centuries. Recently, additives are used as special functional materials to improve important properties of final products; hence additives application technology is essential for adhesive development.

Antioxidant is any substance that delays or inhibits oxidation during the adhesive manufacturing process, storage, and application. Oxidation is a chemical reaction that transfers electrons from a substance to an oxidizing agent. As a result, reaction produces free radicals (Zweifel 2004). Oxidation during compounding or processing can cause problems such as loss of strength, breakdown, or discoloration. Oxidation can also occur in the final product causing discoloration, scratching, and loss of strength, flexibility, stiffness, or gloss. Antioxidants have some ability to terminate radical chain reactions by removing free radical intermediates or inhibiting other oxidation reactions. Antioxidants can be classified by the chemical structure such as phenol type, aromatic amine type, thioester type, phosphate type, etc.

For example, hotmelt adhesives are 100% solid thermoplastic compounds that are solids at room temperature, but they are changed to liquid when heated to the melting temperature. When applied, hotmelts bond and cool rapidly. To make EVA based hotmelt adhesives, mixing process at high temperature is essential. To reduce the thermal degradation of hotmelt adhesives, 1–2 parts of a phenolic antioxidant by weight were used as a thermal stabilizer (Park et al. 2003, 2006).

A UV stabilizer is a substance to prevent discoloration, surface crack, and decreasing of the mechanical properties by UV radiation that has a high energy wave in the range of 290–400 nm. In particular, UV stabilizers are essential additives for transparent plastics for outdoor application. UV stabilizers can be classified by chemical reaction, such as UV absorption, quenchers, and HALS (hindered amine light stabilizer). UV absorbers absorb the harmful UV radiation and dissipate it so that it does not lead to photosensitization. Typical UV absorbers are hydroxyl benzophenone, benzotriazoles, and modified acrylate. Quenchers are light stabilizers that are able to take over the energy absorbed by the chromophores present in plastic material (Bellus 1971). HALS is a radical scavenger that represents the most important development in light stabilization for many polymers. HALS acts by scavenging the radical intermediates formed in the photo-oxidation process. HALS’ high efficiency and longevity are due to a cyclic process wherein the HALS is regenerated rather than consumed during the stabilization process. HALS also protects polymers from thermal degradation and can be used as a thermal stabilizer.

Table 11 shows an example of formulation of the UV-curable coatings, which is composed of isocyanate and acrylated urethane oligomers (Lee and Kim 2006). In addition, Fig. 11 shows chemical structures of light stabilizers. UV-cured film with both Tinuvin 384–2 and Tinuvin 292 showed the slight change of optical and mechanical properties. In the case of most UV-cured films, using a UV stabilizer is helpful to discoloration, gloss, and hardness properties after weathering .

Table 11 Formulation of UV-curable coatings

Fig. 11

Chemical structures of light stabilizers

Flame retardants are materials that inhibit or resist the spread of fire. Flame retardants can remove thermal energy from the substrate by functioning as a heat sink or by participating in char formation as heat barrier. The additives can also provide flame retardancy by conduction, evaporation, or mass dilution or by participating in chemical reactions (Zweifel 2004).

Adhesives can be separated into two general classes: thermoplastics and thermosets. Thermoplastics can be formulated with halogen-containing and non-halogen-containing additives. Thermosets are commonly treated by adding flame retardants that chemically react with a resin precursor. Table 12 lists flame retardants that are typically used.

Table 12 Common flame retardants used within thermoset plastics

Tackifiers are used in formulating rubber based on adhesives to improve the tack property. Tackifiers are low molecular weight compounds with high T_g. There are two classes of tackifiers: the rosin derivatives and the hydrocarbon resins. The rosin derivatives include the rosins, modified rosins, and rosin ester. The hydrocarbon resins consist of low molecular weight polymers derived from petroleum, coal, and plants. The miscibility between tackifiers and the adhesives is important to choose a tackifier. The viscoelastic property of adhesives can be modified by blending of a miscible tackifier.

A curing agent or hardener is a substance added to an adhesive to promote the curing reaction. These affect curing reaction by chemically combining with the base resin and becoming part of the final polymer molecule. They are specifically chosen to react with a certain resin (ASTM D907-08b 2008). Curing agents will have an important effect on the curing features and on the fundamental properties on the adhesive system. A polyamide resin that is used in room-temperature curing epoxy adhesive system is an example of a curing agent (Petrie 2000). Polyamide curing agents are used in most “general-purpose” epoxy adhesives. They offer a room-temperature cure and bond well to many substrates that include elastomers, glass, and plastics. The polyamide-cured epoxy also provides a comparatively flexible adhesive with moisture resistance, fair peel strength, and thermal cycling properties. In general, mixing ratio is not critical. Within limits, the greater the quantity of polyamide in an epoxy formulation, the greater the flexibility, impact strength, and peel strength. However, T_g is decreased as are the shear strength and temperature resistance. There are some polyamides with changing viscosity. The reaction with conventional di-glycidyl ether of bisphenol A (DGEPA) epoxy resins yields a comparatively low degree of exotherm. In Table 13, the distinct features of curing agents used with epoxy resins in adhesive formulations are briefly stated (Petrie 2000).

Table 13 Characteristics of curing agents used with epoxy resins in adhesive formulations

Catalysts are substances that markedly speed up the cure of an adhesive when added in a minor quantity compared to the amounts of the primary reactants (ASTM D907-08b 2008). Solidification and crosslink of the primary resins are caused by catalysts. The commonly used catalysts are acids, bases, salts, sulfur compounds, and peroxides. To influence curing, only small quantities are needed (Petrie 2000). Table 14 shows the lists of some catalysts and the reactions they catalyze (Hare 1994). The curing of the adhesives or sealants by transformation into a hardened state is fulfilled by chemical methods, for instance, oxidation, vulcanization, polymerization, or by physical action, such as evaporation of the solvents (Cognard 2005).

Table 14 Catalysts and reaction catalyzed

8 Conclusions

There are various commercial adhesives in the market area and various requirements for a specific application: controlling flow, extending temperature range, improving toughness, lowering the coefficient of thermal expansion, reducing shrinkage, increasing tack, modifying electrical and thermal conductivity, etc. To meet specific requirements, the selection of a primary resin and its minor components is very important. Understanding the chemical composition of an adhesive or sealant is very useful to R&D centers, suppliers, and customers of the adhesives.