Adhesives and Sealants

8.12

Adhesives and Sealants

AV Pocius, 3M Corporation, St. Paul, MN, USA © 2012 Elsevier B.V. All rights reserved.

8.12.1 8.12.2 8.12.3 8.12.3.1 8.12.3.2 8.12.3.3 8.12.3.4 8.12.3.5 8.12.4 8.12.4.1 8.12.4.2 8.12.4.3 8.12.5 8.12.6 8.12.7 8.12.7.1 8.12.7.1.1 8.12.7.1.2 8.12.7.2 8.12.7.3 8.12.7.4 8.12.7.5 8.12.7.6 8.12.7.7 8.12.8 8.12.8.1 8.12.8.1.1 8.12.8.1.2 8.12.8.1.3 8.12.8.1.4 8.12.8.1.5 8.12.8.1.6 8.12.8.1.7 8.12.8.1.8 8.12.9 8.12.9.1 8.12.9.2 8.12.9.3 8.12.9.4 8.12.9.5 8.12.9.6 References

Adhesives Adhesive Testing Pressure-Sensitive Adhesives Tackifiers for PSAs PSAs based upon natural rubber PSAs based upon acrylic elastomers PSAs based upon block copolymers Silicones as PSAs Rubber-Based Adhesives Natural Rubber Solvent-Based Adhesives Neoprene (Chloroprene) Solvent-Based Adhesives Styrene–Butadiene Rubber-Based Adhesives Hot Melt Adhesives Natural Product-Based Adhesives Structural Adhesives Epoxy-Based Structural Adhesives Two-part epoxy structural adhesives One-part epoxy structural adhesives Phenolic Structural Adhesives Polyurethanes as Structural Adhesives Acrylics as Structural Adhesives Cyanoacrylate Adhesives Urea–Formaldehyde Adhesives Higher Performance Structural Adhesives Sealants Performance Tests of Sealants Movement Sealants based on oils, curing (drying) oils, bitumens, and asphaltics Sealants based on polysulfides Sealants based on silicones Sealants based upon polyurethanes Sealants based on acrylics Sealant products based on fluorocarbons Sealants based on butyls Future of Adhesives and Sealants Better Faster Cheaper Other Factors Smaller Smarter

Adhesives and especially sealants have been used for millennia. Perhaps the oldest inscription that describes how a person makes an adhesive bond is taken from a tomb in Thebes, Egypt, from about 1500 BC. ‘Glue’ is mentioned in the Apocrypha.1 Veneered tables which used some form of natural adhesive to attach the thin veneer to less expensive woods are mentioned in Pliny the Elder’s Natural History.2 Certainly, the wooden sailing ships used by ancient mariners as well as ships used by those who first crossed the Atlantic Ocean were sealed (or caulked) by some sort of sealant, possibly one of the bituminous types. Adhesives and sealants have continued to Polymer Science: A Comprehensive Reference, Volume 8

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evolve and improve. Modern aircraft are adhesively bonded and sealed. Modern maritime vessels are certainly sealed well. Automobiles are adhesively bonded and sealed. Most modern furniture contains a significant amount of adhesive bonding. Modern construction of buildings could not be done without the use of adhesives and sealants, especially in curtain wall construction.3 It is safe to say that there is little in modern commerce that does not contain some level of adhesive bond­ ing and sealing technology. This chapter, somewhat artificially, separates adhesives and sealants. As the above paragraph implies, we normally

doi:10.1016/B978-0-444-53349-4.00210-7

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Adhesives and Sealants

think of the two steps, that is, adhesive bonding and sealing, as happening sequentially. We think of building a structure (e.g., a box, a vehicle, a device) with an adhesive and then sealing any gaps or openings with a sealant. Therefore, we normally think of an adhesive as having some level of strength (thus an adhesive is a structural component which holds the structure together) while a sealant is normally thought of for its ability to permanently fill and bridge a gap, which, in some cases, may be quite large.

8.12.1 Adhesives An ‘adhesive’ is a material that joins together two or more other materials by means of surface attachment. The materials being joined together are normally called ‘adherends’. The assembly that is created by adhesive joining is called an ‘adhesive bond’ or ‘adhesive joint’. In general, adhesive bonds have very thin ‘bondline’, which is the thickness of the adhesive between two adherends. An adhesive bondline is usually in the order of 10 mils (0.01″) in thickness. Adhesives can be classified in a number of ways. They can be classified by chemistry, by use, or by strength. In this chapter, adhesives will be first classified by strength and then by chemistry. Uses of adhesives will be given throughout the chapter.

Figure 2 Schematic representation of a ‘T-peel’ specimen. The arrows indicate the direction in which the specimen is to be pulled. The specimen is typically 10″ long and 1″ wide. The adhesive bondline is usually about 10 mils thick.

method. The adherends are about 20 mils in thickness and their width is 1″. The normal rate of application of stress is 12″ min−1 and the normal temperature of test is room tem­ perature, but these can be varied.

8.12.3 Pressure-Sensitive Adhesives 8.12.2 Adhesive Testing In most cases, a user of adhesives would want to know the strength of an adhesive bond made with an adhesive. As with many other technologies, there is a need to have a specified set of test methods to carry out adhesive bond testing. There are a number of specifications which describe these methods based primarily in the literature of the American Society of Testing and Materials (ASTM). Two primary tests are the lap shear test and the peel test. The ASTM D10024 test method gives the details for carrying out a lap shear test as shown in Figure 1. The adherends are 1″ wide. The adhesive is in a strip that is ½″ wide. The rate of application of stress is usually 0.1″ min−1, although that can be adjusted depending upon the information that is desired. In addition, the temperature of the test can be varied. Figure 2 shows a ‘T-peel’ test. The specimen is peeled symmetrically in the form of a ‘T’. This test is meant to give the user an idea of the fracture resistance of the adhesive. There are tests that give an actual measure of the fracture toughness, but they are, in general, much more difficult to prepare. The T-peel is described in the ASTM D18765 test

4″

1″ 0.5″ Figure 1 Schematic representation of a lap shear specimen. The arrows at the end of the specimen indicate the direction in which the specimen is to be pulled. Dimensions are given in inches (1 in. = 2.54 cm).

A pressure-sensitive adhesive (PSA) is a unique material in that it displays characteristics that are neither liquid nor solid but a combination of both that depends on the rate of stress application. Some of the characteristics of a PSA that are described by the Pressure Sensitive Tape Council are the following: • forms a bond at low or no applied pressure, • is aggressively and permanently tacky in dry form at room temperature, • requires no activation by solvent/water/heat, and • has sufficient cohesive strength to not leave a visible residue when disbonded. The first two characteristics are those that are characteristic of a liquid. The last characteristic is that applied to a solid. It is this discrepancy in character that makes a PSA so unique and interesting. The earliest patent regarding PSAs is that of Shecut and Day.6 In this patent, the inventors describe a PSA that is based upon natural rubber. The PSA is activated by the presence of solvent. After application, the solvent evaporates and the adhesive is set. Thus, this patent describes an early form of this type of adhesive that needed to be activated by solvent. The first PSA that did not need solvent for activation was developed in the late 1920s and early 1930s when a masking tape for automotive applications was developed and commer­ cialized.7 Later, a transparent tape for sealing and packaging was introduced.8 This product became known as Scotch™ brand cellophane tape. The property of ‘dry’ PSAs that is so important is the initial feel of stickiness that the product gives the consumer. This property has become known as ‘tack’. Measurements of tack

Adhesives and Sealants

need control of temperature, contact pressure, contact time, and rate of approach and separation of the contact probe. A simple machine for doing these measurements is shown schematically in Figure 3. Figure 4 shows the result of such measurements where tack strength and adhesive modulus are plotted against temperature. We note that the tack of the adhesive starts near zero and goes through a maximum. The modulus of the adhesive drops steadily through the temperature range of the test. At some point, the tack of the adhesive becomes measureable. At that same point, the modulus of the adhesive drops below 3 � 106 dyn cm−2. This value of modulus when tack becomes measureable is known as the ‘Dahlquist criterion for tack’. This value,9 named after Carl A. Dahlquist, is measured at a rate of 1 Hz and it has been found to be universally applicable for PSAs.

Pressure-sensitive adhesive

Stiff backing plate Contact probe

Force gauge

Speed

Contact time

Figure 3 Schematic representation of a probe tack tester. The contact probe is usually stainless steel. The probe moves up to make contact and, after a certain set contact time, moves down. The rate of movement is also set prior to test.

8.12.3.1

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Tackifiers for PSAs

Considering the importance of ‘tack’, one should question how this property is obtained. The basic formulation component that one adds to an elastomer in order to make it tacky is the addition of a ‘tackifier’. The first material that was found to be useful as a tackifier was a rosin acid that was used with natural rubber. A rosin acid is a natural product that has a glass transi­ tion temperature. Rosin acid also has high solubility in natural rubber. One would, therefore, anticipate that two of the proper­ ties that a tackifier should have are solubility in the elastomer and being a glass at room temperature. One can perhaps see how a tackifier affects the physical properties of an elastomer by examining Figure 5. This figure shows the storage modulus of two materials as a function of temperature measured at a frequency of 1 Hz. The two curves in the figure correspond to the tackified and untackified elasto­ mer. If we examine the curve for the base elastomer, we find that it has a glass transition temperature that is below room temperature but that its room temperature modulus is higher than that required by the Dahlquist criterion. The second curve shows the variation of the storage modulus with temperature for the tackified material. The first thing that is apparent is that the glass transition temperature of the material is higher than that of the base elastomer. However, when the material goes through the glass transition region, the modulus goes below that of the base elastomer and, if the material was properly formulated, the modulus falls below the Dahlquist criterion for tack. Therefore, tack is a rheological property of the material. One can obtain significant differences in the tack level mea­ sured for an adhesive as a function of rate and temperature. A material that is tacky at a frequency of 1 Hz may display little or no tack at 100 Hz. There are a number of different chemistries that have been developed for the purpose of tackification of rubber.10 The first of these is based upon rosin acids, the second is based on terpenes, and the last is synthetically derived. In addition, there are specialty tackifiers synthesized for use in specific resin systems. The chemistry of tackifiers is somewhat complicated because they were based upon natural products. The rosin acid-based tackifiers were the first class of compounds found

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Modulus (dyn cm–2)

100 Modulus Tack (g)

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Dahlquist criterion Temperature (°C) Figure 4 Comparison of the storage modulus and tack of a PSA as a function of temperature. The frequency of measurement is 1 Hz. When the modulus of the adhesive goes below 3 � 106 dyn cm−2, the tack starts to reach a maximum. This is the definition of the Dahlquist criterion for tack.

Temperature Figure 5 Comparison of the storage modulus of a tackified (dark line) and an untackified (lighter line) elastomer as a function of temperature. Also shown is the Dahlquist criterion which indicates the point at which the tackified material displays tack.

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Adhesives and Sealants

Table 1

Representative examples of tackifier resins

Trade name

Type

Tg ( °C)

Manufacturer

Hercoflex 400 Hercolyn D Foral 85 Foral 105 Zonarez A-25 Zonarez A-100 Piccolyte S10 Piccolyte A115 Nirez K-105 Piccofyn A-135 Piccolyte HM-85 Wingtack 10 Wingtack 95 Escorez 1310 Escorez 5300 Piccotac 95 Escorez 2520 Escorez 2101 Wingtack Plus Wingtack Extra Piccovar AP-25 Arkon P90 Regalrez 1094 Escorez 7105

Rosin ester Hydrogenated methyl rosin ester Hydrogenated glycerol rosin ester Hydrogenated pentaerythritol rosin ester α-Pinene terpene α-Pinene terpene β-Pinene terpene α-Pinene terpene Polyterpene Phenolic polyterpene Styrenated terpene C-5 C-5 C-5 Hydrogenated dicyclopentadiene C-5 C-5 and C-9 C-5 and C-9 C-5 and C-9 C-5 and C-9 C-9 Hydrogenated C-9 Hydrogenated α-methyl styrene Hydrogenated C-9

–37 –25 40 57 –22 55 –37 64 54 84 35 –28 50 40 50 41 –20 36 42 45 –50 36 37 52

Hercules Hercules Hercules Hercules Arizona Arizona Hercules Hercules Reichold Hercules Hercules Goodyear Goodyear Exxon Exxon Hercules Exxon Exxon Goodyear Goodyear Hercules Arakawa Hercules Exxon

that was able to form PSAs. Thus, materials such as abietic acid and pimaric acid (wood by-products from gum rosin, wood rosin, and tall oil) were found to be useful in the forma­ tion of natural rubber-based PSAs. The unsaturation in both the natural rubber and in the tackifier led to early failure of these adhesives in use because of yellowing and embrittlement of the adhesive. Hydrogenation solved some of these problems. Rosin acids could be further modified by reaction with glycerol or other alcohols. Another class of tackifiers is based upon terpene resins. These materials can be obtained from citrus peels or wood by-products. Thus, materials such as α-pinene and β-pinene can be polymerized using aluminum chloride catalysis to form terpene-based tackifiers. Tackifying resins also come from petroleum feedstocks. These are broadly classified as aromatic and aliphatic resins. The aromatic resins are based upon such materials as styrene, α-methyl styrene, methyl indene, indene, coumarone, and dicy­ clopentadiene. Aromatic resins are sometimes called ‘C-9’ resins. The materials in various combinations are polymerized by much the same process as the pinenes. Some of the aliphatic resins are also called C-5 resins, as these are based upon pen­ tene, cyclopentene, cis- and trans-piperylene, isoprene, 2-methyl butene-2, and dicyclopentadiene. An abbreviated list of conventional tackifying resins is shown in Table 1.

8.12.3.2

PSAs based upon natural rubber

The base resins in PSAs are various elastomeric materials. Natural rubber or poly(cis-isoprene) was the first such material used to make a PSA. The latex from the Hevea rubber plant is coagulated and often smoked to eliminate any problems

associated with microbial degradation. The molecular weight of this rubber can be very high, as much as 2.5 million, and may contain some level of crosslinking. The rubber is often masticated in order to mechanically reduce the molecular weight and in order to make the elastomer soluble in hydro­ carbon solvents. In addition to a tackifier, the formulation of a natural rubber-based PSA should contain an antioxidant and may contain a crosslinking system. In addition, there could be fillers used for coloration. The primary problem with natural rubber-based PSAs is the extent of unsaturation in the back­ bone of the polymer. Oxidation led to embrittlement of the PSA as well as yellowing. Antioxidants can help to ameliorate this problem. Natural rubber-based PSAs were formulated with rosin acid-based tackifiers, which also contained a number of double bonds, further compounding the oxidative degradation and yellowing of these adhesives. Synthetic modification of tackifiers by hydrogenation helped to ameliorate this problem.

8.12.3.3

PSAs based upon acrylic elastomers

Polymerization of alkyl acrylate by free radical methods led to the first permanently stable PSA. These polymers, however, were not useful as PSAs until they were copolymerized with small amounts of acrylic acid.11 With the addition of acrylic acid, these polymers were able to perform all of the functions of a PSA without the addition of a tackifier. These materials were used to form the first transparent PSA tape, Scotch Magic™ Tape. The area of acrylic PSA technology has expanded to the use of a wide range of monomers and comonomers. Acrylic PSAs tend to be more expensive than natural rubber-based PSAs, but they have the excellent weathering and aging char­ acteristics of acrylic resins.

Adhesives and Sealants

8.12.3.4

PSAs based upon block copolymers

Natural rubber and acrylic resins need to be lightly crosslinked in order to obtain all of the properties listed above for a PSA. In the early 1970s, a technology was deployed which eliminated the need for crosslinking. This technology was based upon block copolymers of poly(cis-isoprene) and polystyrene.12 The copolymers of this technology consisted of long segments of polystyrene covalently bonded with long segments of poly (cis-isoprene). The block copolymers were called A-B-A block copolymers. The A segments were generally polystyrene and the B segments were generally poly(isoprene) or poly(butadiene). Because of the significant difference in the physical properties of these two segments, the polystyrene segments tended to phase separate forming micron- or submicron-sized particles of poly(styrene) that were still covalently attached to the poly (isoprene) segments. The phase-separated styrene blocks act to reinforce the PSA, acting as a physical crosslink rather than a chemical one. These adhesives could be coated from a melt rather than from a solvent, thus providing solvent emission reduction. The ABA copolymers have a higher modulus than acrylics or natural rubber, and thus tackification must be used in order to obtain PSA characteristics. The same tackifiers used for natural rubber can be used for these adhesives except that care must be taken to ensure that the tackifier stays in the poly (isoprene) phase and does not soften the poly(styrene) phase. These adhesives, because they have a significant degree of unsaturation, suffer in terms of yellowing and oxidation. In addition, they tend to have higher modulus than natural rub­ ber or acrylic PSAs; hence, tackification must be used. Presently, they have wide use as packaging tapes.

8.12.3.5

Silicones as PSAs

PSAs can also be based upon silicone elastomers. Silicones are quite expensive and thus find use in niche applications such as medical tapes and either high- or low-temperature-resistant tapes that have high value. The base polymer for this technol­ ogy is poly(dimethyl siloxane) with some amount of diphenyl siloxane units, which tends to increase the peel strength of these materials. Silicones, in general, have a very low glass transition temperature of about −120 °C, and one might guess that some level of modification might be necessary in order to obtain PSA properties. This is done by crosslinking and also by the addition of a special class of tackifiers, called MQ resins.13 The MQ resins are made by polymerizing quadrafunc­ tional (‘Q’) siloxanes and capping them with monofunctional (‘M’) siloxanes. The MQ resin is highly polar and provides an increase in cohesive strength as well as tack.

8.12.4 Rubber-Based Adhesives Rubber-based adhesives have many of the same formulation basics as were described for PSAs. The main difference for these adhesives is that they are meant to have a significantly higher level of strength than that for PSAs. Rubber-based adhesives can have lap shear strengths on the order of 500 psi but PSAs have lap shear strengths only on the order of 10 psi. The uses of rubber-based adhesives are in the areas of lamination (such as the application of a veneer to the surface of a base wood), shoe

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manufacture, carpentry, and ceramic and plastic tile attach­ ment. These adhesives can also be classified as ‘solvent-based adhesives’ because of the high molecular weight of the rubber; the adhesive has to have a large quantity of solvent to make it spreadable or trowelable for application purposes. Many rubber-based adhesives are available as latexes and hence are diluted by and can be cleaned up with water.

8.12.4.1

Natural Rubber Solvent-Based Adhesives

Natural rubber-based adhesives are the archetype for all of the solvent-based adhesives. All of the considerations and formu­ lation criteria for natural rubber-based adhesives are found for all rubber-based adhesives. In fact, this raw material is also used as a water-based adhesive. The source of natural rubber (poly(cis-isoprene)) latex is Hevea brasiliensis. The natural latex is about 35% solids but it can be concentrated to as high as 73% solids. This concentra­ tion is done by several methods including evaporation, centrifugation, or creaming. The possibility of microbial attack is minimized by the addition of either ammonia or potassium hydroxide. The latexes are used in water-based adhesives. Full removal of water results in a solid rubber that is obtained in various forms. Sheet rubber is usually too high in molecular weight and must often be masticated to mechanically lower the molecular weight. The masticated natural rubber can then be solvated in an aromatic or aliphatic nonpolar solvent such as naphtha or toluene. Poly(cis-isoprene) can also be generated synthetically. One primary advantage of synthetically deriving poly(cis-isoprene) is the control of molecular weight and mini­ mizing crosslinking of the polymer. A primary formulation ingredient for natural rubber-based adhesives is a tackifying resin (vide supra). For the case of a solvent-based adhesive, the use of tackifying resin is not just for the tack associated with a PSA; rather, it is to improve upon the other properties of the adhesive. This improvement comes from the fact that the tackifying resin usually has a glass transi­ tion temperature that is in excess of that of rubber. The mixture of the two gives an increase in the Tg of the resulting adhesive as well as increasing its stiffness. The adhesive can remain tacky as long as some amount of solvent is present, but it will lose its tack as all of the solvent is gone. Softening agents are often used in the case when the rubber is not heavily masticated. Examples of softening agents are lanolin and liquid poly(butene). Various fillers and reinforcing agents are also used to increase the holding power of the adhesive. Carbon black is often used in this case. Reinforcement can also be obtained by crosslink­ ing. One specific type of crosslinking agent is poly(isocyanate). Antioxidants must be used with natural rubber to prevent oxidation and embrittlement of the adhesive.

8.12.4.2

Neoprene (Chloroprene) Solvent-Based Adhesives

Neoprene elastomer was first synthesized in the 1930s and became the first synthetic elastomer to provide significant advantages over natural rubber. The monomer for neoprene is 2-chloro-1,3-butadiene and the polymer is prepared by emul­ sion polymerization. Being inherently a latex polymer may have been thought to provide commercial advantage over the solvent-based versions, but it was found that the tack properties and the ability of the adhesive to form a quick bond with itself

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Adhesives and Sealants

were affected by the presence of various stabilizers in the emul­ sion. Therefore, most versions of this type of adhesive came as solvent-borne materials. The characteristic of this type of poly­ mer over the others already discussed is the inherent crystallizability of the neoprene backbone. Very quick strength buildup due to rapid crystallization and its ability to rapidly diffuse into itself14 gave neoprene-based adhesives a substan­ tial advantage over the other rubber-based adhesives. Latex-based neoprene adhesives are becoming more available as improvements in stabilizers have been developed. Formulations using neoprene mirror those for the other organic adhesives described already. The tackifier used for neoprene contact bond adhesives is often a t-butyl phenolic resin which when combined with fillers like MgO provides a heat-resistant formulation.15 Other tackifiers can also be used but the formulations typically lose in terms of heat resistance. For the quick grab characteristic of neoprene-based adhesives, solvent choice becomes very important. Not only does the choice of solvent come into the manufacturing of the adhe­ sive, but its evaporation rate also controls the quick grab. Too little solvent in the drying adhesive causes it to lose contact bonding, but too much solvent in the drying adhesive causes it to have too little strength. In addition, too much solvent slows the rate of crystallization. Because of the chlorine in the base resin for these adhesives, acid acceptors such as ZnO and MgO have to be added to the formulation in order to act as acid neutralizers. At high temperatures, the resin will emit HCl, which can cause corrosion of metallic adherends. Addition of both of these fillers substantially reduces this problem. Neoprene contact bond adhesives are used in a wide range of applications, from lamination to shoe manufacture to home construction.

8.12.4.3

Styrene–Butadiene Rubber-Based Adhesives

These are elastomers formed by the polymerization of styrene and 1,3-butadiene. They were first synthesized during World War II as a replacement for natural rubber, which at that time had become scarce. Most SBR (styrene–butadiene rubber) applications came for the rubber in latex form and, indeed, the polymer is manu­ factured in latex form. Major applications for this type of adhesive include paper coatings, shoe manufacture, lamination and con­ struction adhesives, and carpet backings. Because most SBR come in latex form, this complicates the formulation of SBR-based adhesives in that all of the compounding ingredients have to be water soluble or at least water compatible for them to be useful in this type of adhesive. Tackifiers useful for these adhesives are the same as those described for PSAs with the proviso that the tacki­ fiers be water dispersible. Typical fillers for SBRs are calcium carbonate, clays, and silica. In addition, titania, carbon black, and iron oxide have been added to colorize the adhesive. Most SBR latexes do not require a curing system, but if it is desired, normal rubber curatives may be used. Antioxidants should be added because of the residual double bonds. In addition, because this material is sold as a latex, antimicrobials should be part of the stabilizer package. It may be necessary to stabilize the particles in the latex by the addition of more surfactant, wetting agents, and complexing agents (sequestrants). Also because of the latex, visc­ osity modifiers may have to be added to add body to the composition.

8.12.5 Hot Melt Adhesives Issues relating to climate change and air pollution have gener­ ated a need for adhesives that do not need solvents. These issues can be easily addressed using hot melt adhesives, which are 100% solid materials that are applied from the melt. The strength of the adhesive comes from the recrystalliza­ tion and solidification of the molten polymer. Depending upon the temperature of test, a hot melt adhesive can have lap shear strengths in excess of 1000 psi but are usually less. These types of adhesives are sold as slugs (which can be used in handheld hot melt guns) or sold in bulk (in barrel or other bulk form) in which a large quantity of the adhesive is held at the melt temperature and dispensed as needed for assembly operations. Thus, the need is for formulations that have stabi­ lity in the melt, reasonably low viscosity during application, and rapid strength buildup. In general, hot melt adhesives come in two types: formu­ lation design and molecular design. Formulation design is used to take a polymer that does not have all the character­ istics required to make a good adhesive and use additives of various sorts to change the properties of the base polymer to make it a good adhesive. Molecular design is used to design a polymer which, in itself, is suitable as a hot melt adhesive by the choice of the proper monomers in the synthesis of the polymer. Formulation-designed hot melt adhesives use a number of materials as the base polymer for the formulation. The most widely used material for this type of adhesive is polyethylene-co-vinyl acetate (EVA). Other materials used are polyethylene (low density), block copolymers (styrene–iso­ prene–styrene), phenoxy resins, polypropylene, and paraffin waxes. Paraffin, the oldest hot melt adhesive, was used as a sealing wax. As with PSAs, tackifiers are also added to hot melt adhesives to lower their melt viscosity as well as to provide tack during the resolidification of the adhesive. If one examines some hot melt adhesive formulations, one finds that the tacki­ fier is a primary component of the adhesive sometimes used to the same level or possibly in excess of the base polymer. Another component is the addition of a wax. The wax is added not only to decrease the melt viscosity of the adhesive but also to aid in the recrystallization process. As one might be able to recount from statements made above about heat stabi­ lity, an antioxidant is an important additive to hot melt adhesive formulations. Final additives include colorants and fillers. Molecularly designed hot melt adhesives are based primar­ ily on polyesters and polyamides. Both types of polymers start with a diacid, but polyamides use a diamine as the comonomer while polyesters use a diol. The choice of these monomers is made depending upon the following criteria: – a crystallization rate that fits with the assembly process; – a high degree of crystallization but with tolerance for adhe­ sive shrinkage; – glass transition temperature below the use temperature; – melt temperature that is high but low enough that it does not damage equipment or adherends; – as large as possible difference between the glass transition temperature and the melt temperature.

Adhesives and Sealants

Hot melt adhesives have one major drawback. They are ther­ moplastic materials and they remain thermoplastic after application. Thus, their load-bearing capabilities are limited as is their resistance to creep. Efforts have been made in the generation of curing hot melts, that is, materials that can be hot melt dispensed but cure by a different mechanism other than the application of heat. One way of generating a curing hot melt is to include some level of unsaturation in the base poly­ mer and include a photochemical initiator. Thus, the material would be stable in the melt but would crosslink when the solidified adhesive is exposed to light. Another way, which has been made into products, is to generate a moisture-curing hot melt.16 The adhesive, when kept in the melt at temperatures in excess of 100 °C, would have little chance of being exposed to moisture, but after application, the adhesive could easily experience ambient moisture and cure. This type of adhesive can be generated by synthesis of an isocyanate-terminated polyester. This material would be stable in the melt but upon exposure to moisture could react to generate an amine and carbon dioxide. The amine could then react with the remaining isocyanate to generate a urea, resulting in a crosslink. The isocyanate-terminated polyester could be formulated with the other materials described above, but care has to be taken to minimize reaction between the isocyanate and the other mate­ rial. In addition, this adhesive has to be packaged with care in order to minimize contact with moisture.

8.12.6 Natural Product-Based Adhesives Adhesive production has been heavily impacted by the price and the availability of products derived from petroleum. Recently, various resins and some other key raw materials have been rationed, and there remains a concern that as petro­ leum production decreases, these materials will be further rationed and their prices will increase dramatically. Efforts to generate materials from sustainable resources are ongoing. It is ‘lucky’ that adhesives have been produced for centuries; thus, there is a history of the use of ‘sustainable’ raw materials for adhesive production. Sources of raw material for natural product-based adhesives include cellulose and other carbohydrate-based materials. Thus, materials such as cotton linters and wood pulp are used in the generation of cellulosic adhesives while starch, which is derived from roots and seeds of plants, can be used to make starch-based adhesives. A well-known cellulosic adhesive is derived by nitration of cellulose; the resulting polymer is then dissolved in organic solvents such as amyl acetate to generate a general purpose, solvent-based cement. Etherification of cellu­ lose leads to methyl cellulose which is water soluble. This material when combined with water and glycerin has been used as wallpaper paste. Esterification leads to cellulose acetate which is also organic solvent soluble. Starch used in the manufacture of adhesives is ‘cooked’, so that it can be dispersed in water. It might also be chemically modified by oxidation, esterification, or other means. Formulation of a starch-based adhesive uses ingredients much like that described for other base materials. Thus, a starch-based adhesive uses plasticizers such as glycerol or syrups, fillers such as Kaolin clay or calcium carbonate, and preservatives such as sodium hypochlorite and formaldehyde. An important

311

additive for this type of adhesive is borax. This additive acts to increase wet tack and modifies the viscosity of the adhesive. Importantly, it also acts as an antimicrobial at high concentra­ tions. The major uses of a starch-based adhesive are as a paper binder, a ‘gum label’ adhesive, and an envelope adhesive. Protein-based adhesives have been used for centuries. The present primary use for these adhesives is as a structural mate­ rial to generate interior-grade plywood. The source of the proteins for these adhesives can be used in their classification. Thus, the proteins for these adhesives come from blood, fish skin, casein (milk derived), soybeans, animal hides, bones, and connective tissue (collagen based). This classification is useful for predicting the durability of bonds made with protein-based adhesives. The protein is mixed in water with the addition of a base such as sodium hydroxide. This dispersed, partially ionized protein is then mixed with other formulation ingredients such as a defoamer, sodium silicate, hydrated lime, chemical dena­ turants, fillers, and antibacterial and antifungal agents. The protein adhesive is applied by roll coating or by spray applica­ tion. As the protein dries, the lime (CaCO3) forms ionic crosslinks with the acid functionalities of the protein. The protein can also be crosslinked by other chemical means. For example, organic sulfur compounds such as carbon disulfide or thiourea can be used to denature the protein as can formalde­ hyde and formaldehyde donors such as hexamethylene tetraamine or glyceraldehyde. Even removal of water, such as by absorption into the adherend (in the case of wood bond­ ing), acts to denature the protein, as will heating of the protein-based adhesive. In addition to these types of adhesives, the industry is trying to make modifications of the base chemicals used to make adhesives using materials that are ‘sustainable’.

8.12.7 Structural Adhesives Structural adhesives are high-strength materials that are often used instead of welding or other methods of joining structural materials together. These adhesives display lap shear strengths in excess of 1000 psi. They are inherently highly crosslinked, usually reactive systems. They come in a number of forms. The most common is the caulking-gun-dispensable adhesive in either one-part (1K) or two-part (2K) form. The highest tech­ nology for structural adhesives comes in the form of a film adhesive. This type of adhesive can be unrolled and applied to large areas, thus forming large assemblies. Film adhesive has been used for decades for the joining of many parts of com­ mercial and military aircraft. Aircraft construction also makes significant use of the 1K and 2K versions of structural adhe­ sives. These types of adhesives are largely used in modern automobile construction. Indeed, they are also used in com­ mercial vehicle construction as well as in building construction.

8.12.7.1

Epoxy-Based Structural Adhesives

Epoxy-based structural adhesives are the broadest set of struc­ tural adhesives in manufacture today. The base chemical for this technology is the diglycidyl ether of bisphenol A (DGEBA). DGEBA and its higher molecular weight versions (formed by chain extension reactions with bisphenol A) are used in a

312

Adhesives and Sealants

significant number of structural adhesive products. The base resins can also include a wide variety of mono-, di-, tri-, and polyfunctional epoxy resins.

8.12.7.1.1

Two-part epoxy structural adhesives

Two-part epoxy adhesives are well known to the consumer and to industrial users of structural adhesives. These adhesives come in two parts, one of which contains the base resin (usually DGEBA or a variant thereof). The other part contains the curative or ‘hardener’. These are sold in separate cans or tubes or in two-part dispensers. In the case of the two-part dispensers, the mix ratio is designed into the dispenser body and the adhesive is expressed through a tip which can mix the two parts of the adhesive, resulting in a ready-to-use adhesive. The original two-part epoxy adhesives consisted of DGEBA as one part and a hardener as the other part. Thus, the choice of hardener is important in that it forms such a significant portion of the final adhesive. In Skeist’s17 first book on adhesives, he lists only 12 curing agents for epoxy resins. In the present day, there are virtually hundreds of different curing agents for epoxy resins. For two-part epoxies, polyamines form the majority of the curing agents used. Thus, materials such as diethylene triamine (DETA) and triethylene tetraamine (TETA) have found use as curatives for two-part adhesives. The list of polyamines has grown to include the polyether amines, the polyamide amines, polyamides, and cycloaliphatic amines. Table 2 provides an abbreviated list of these curing agents and some of their physi­ cal properties. Another curative for DGEBA adhesives is the mercaptan curing agent. Mercaptans are the hardener for the ‘5-min’ epoxy adhesive. The original two-part epoxy adhesives used the polyamide amines as curing agents because of the easy adjustment of mix ratio. The formulator could easily find poly­ amide amines having an active hydrogen equivalent weight (AHEW) close to the epoxy equivalent weight. As the technol­ ogy of curatives has grown, so has the number of curatives that are available. The AHEW of these curatives can be far from stoichiometric with the base resin. Therefore, the means to generate the proper mix ratio has also grown. One way to do this for the two-part dispenser is to add various additives to one or both sides of the adhesive until the volumetric mix ratio is correct. One type of additive is a filler which could include materials such as silica, titania, talc, and calcium carbonate. Also, for a two-part adhesive, one likes to have each of the parts to be a different color so that as they are mixed, one can Table 2

judge how well they are mixed by the development of a single, homogenous color. These additives can be added until the color and mix ratio are correct as long as they do not increase the viscosity too greatly. Some of these fillers can be added to correct the thermal expansion coefficient of the adhesive for certain applications. Other additives, which perform a function, can also be used in two-part adhesives. These are materials such as toughening agents and cure accelerators. Toughening agents are used to increase the peel strength of the adhesive and decrease the brittleness of the cured epoxy. DGEBA cured with a polyether­ amine has little peel strength without the addition of a toughening agent. However, the same system with a toughen­ ing agent can have significant peel strength, in excess of 40 PIW (pounds per inch width). Toughening agents for use in two-part adhesives include various rubber-based additives such as carboxy-terminated butadiene nitrile rubber (CTBN) and amine-terminated butadiene nitrile rubber (ATBN) and various core–shell modifers.18 CTBN can be added to the epoxy side of a two-part adhesive and ATBN can be added to the amine side of that two-part adhesive. Core–shell modifiers can be added to either or both parts because they have no residual reactivity. Cure accelerators are added to decrease the time to set the two-part epoxy in the case when the customer needs a faster cure to meet a production quota. Thus, materials such as aro­ matic tertiary amines, tris(dimethylamino)methyl phenol, and in some cases imidazoles have been used.

8.12.7.1.2

One-part epoxy structural adhesives

One-part epoxy systems in the paste (liquid) form and in film form share much the same formulation ingredients. However, in comparison to the two-part adhesives, the one-part adhe­ sives use very different curatives. Because these systems are meant to have all of the components of the adhesive in one part, the curatives used for two-part adhesives cannot be used as these would cure the adhesive in storage. Hence, ‘latent’ curatives for these adhesives have been developed. ‘Latency’ in this case means that the curative is dormant in the formula­ tion until a certain temperature is reached, at which point the curative begins to cure the formulation. Therefore, such latent catalysts have to have some means to stay inactive. For epoxy systems, this means that the curative remains insoluble in the epoxy resin until the cure temperature is reached or the curative is blocked by some chemical means which then unblocks at the

Abbreviated list of epoxy curing agents

Name

Type

AHEW

TETA AEP Ancamine T Ethylene oxide adduct Ancamine 1636 Ancamine 1922 Ancamide 500 Ancamide 220 Amicure PACM Ancamine 2072 Jeffamine D400 Versamid 125LV

Triethylenetetraamine N-Aminoethylpiperazine DETA/ethylene

27 43 36

Cyanoethylated amine Diethylene glycol diaminopropyl ether Amidoamine Polyamide bis(p-Aminocyclohexyl)methane Mannich base Poly(propylene oxide)diamine Polyamide

38 55 90 185 52.5 102 103

Adhesives and Sealants

cure temperature. By far the largest quantity of curative used in one-part systems is of the insoluble type. Dicyandiamide is possibly the most used catalyst of this type. Unfortunately, dicyandiamide does not solubilize into epoxy resin until about 180 °C. If the user wishes to cure the material at a lower temperature, then another curative needs to be added. Such materials are ureas such as monuron (1-(4­ chlorophenyl)-3,3-dimethylurea). These are blocked materials which unblock at lower temperatures such as 120 °C. In fact, monuron can be used in conjunction with dicyandiamide to create an adhesive which cures at 120 °C19 and has many of the same properties as a resin cured by dicyandiamide, except for Tg which remains at 120 °C. Epoxy resins (as with other curing systems) are limited to the Tg being approximately the same as the temperature of cure.20 Other curing systems are also avail­ able such as the dihydrazides21 and imidazole salts.22 For all one-part epoxies, the formulated, manufactured material must be kept cold, usually below 0 °C, in order to keep the latent catalyst inactive. This is certainly true for shipping the material. It is also true that the material should be stored frozen by the user. Other than the more restricted list of curatives, the for­ mulation components for the one-part epoxy are the same as for the two-part ones. Film adhesives are a special segment of one-part epoxies in that these materials are coated onto release liners and are supplied in roll form to the user. Thus, a typical epoxy film adhesive would be coated onto a paper-backed release liner, dried, and then covered with another release liner, possibly polyethylene. Film adhesives are manufactured in a number of forms including ‘one side tacky’ or supported by a nonwoven or woven fabric. The adhesive can be placed on the adherend, moved around until it is correctly positioned, and then pressed into place and the other adherend applied. In general, these materials are cured in an autoclave in order to have better control of applied pressure and temperature.

8.12.7.2

Phenolic Structural Adhesives

Phenolic materials go back to the patents of Baekeland23 and are based upon the reaction of phenol and formaldehyde to yield a lower molecular weight resinous material that can be formulated with other resins and sometimes other curatives to yield a paste adhesive or a film adhesive. When phenol and formaldehyde are reacted in the presence of an acidic catalyst with an excess of phenol versus formaldehyde, they yield what are known as ‘novolac’ resins. These materials are soluble in organic solvents and do not react further with themselves; thus, they need to have a curative added. That curative is most often hexamethylene tetraamine. Another type of phenolic resin can be generated with an excess of formaldehyde and under basic conditions. These materials, known as ‘resole’ phenolics, will react with themselves to yield a fully cured phenolic and thus must be stored frozen in order to limit this reaction. Thus, resole phenolics do not need an external crosslinker. Resole phenolics are widely used in the binding of paper products and the bonding of wood. Adhesives based upon these materials were brittle and could only be used to bond wood, where it is still widely used. A more widely usable adhesive was developed during World War II which modified the very brittle phenolic adhesive with poly(vinyl formal) resins.24 These materials were

313

known as the ‘Redux’ adhesives and were the first phenolic adhesives to have any fracture resistance or peel strength. These materials were applied in a unique way. The phenolic resin component was sprayed or roll coated on the two adher­ ends. The poly(vinyl formal) was applied as a solid powder onto the top of the resin mass. The bond was closed and then baked under pressure at about 180 °C. The resulting adhesive bond had a lap shear strength in excess of 3000 psi and a peel strength of about 30 PIW. Later, the Redux adhesives came in fully formulated pastes and also as films. They also had a long reputation (which lasted until the mid-1970s) for providing durable structural adhesive bonds. At that point epoxy technol­ ogy along with surface preparation technology25 finally met the standard set by the Redux adhesives. Novolac phenolic adhe­ sives are still used to bond brake linings as well as other hightemperature-resistant applications. Further improvements in novolac phenolics came with the addition of nitrile rubber.26 The addition of nitrile rubber further improved the fracture resistance of novolac phenolic adhesives and also the high-temperature performance.

8.12.7.3

Polyurethanes as Structural Adhesives

Polyurethanes have wide use in adhesives and sealants. The general chemistry of polyurethane reactions is shown in Figure 6. The basic reaction is between an isocyanate and an alcohol as shown in Figure 6(a). This reaction proceeds well at room temperature with an appropriate catalyst such as stan­ nous octoate, dibutyl tin dilaurate, or a tertiary amine. The properties of a polyurethane adhesive vary both with the polyisocyanate and with the polyol used in their formulation. The polyisocyanates are primarily aromatic in nature although ali­ phatic ones are also used. Thus, such isocyanates as toluene diisocyanate (TDI) or methylene diphenylisocyanate are widely used aromatic materials and isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), and methylene biscyclo­ hexylisocyanate (H12MDI) are the most used aliphatic materials. Polymeric versions of MDI and many of the aliphatic isocyanates are also available. Care must be taken with the use of TDI because of its volatility and toxicity. The alcohol is usually in the form of a diol, triol, or polyol. The alcohol is usually polymeric in nature with a backbone that is polyether, polyester, polycarbonate, or based upon polybutadiene diol. The polyethers can be based upon polypropylene glycol or polytetramethylene oxide glycol while the polyesters are based on polycaprolactone glycol or other polyester glycols. Another important part of such two-part formulations of an isocyanate reaction is the addition of a ‘chain extender’. These low-molecular-weight alcohols and amines help to form the microscopic morphology of such isocyanate systems. Polyurethanes form a two-phase system when they cure.27 The ‘hard phase’ consists of the reaction product of the diiso­ cyanate and the chain extender. The ‘soft phase’ consists primarily of the backbones of the polyol. The ‘hard phase’ contains a significant amount of hydrogen bonding between the segments of the urethane and may actually crystallize. The soft phase stays amorphous and ‘rubbery’. Thus, the hard phase acts as a physical crosslink for the system and acts to stiffen it. The action of the polyol is to make the system more rubbery. Increasing the molecular weight of the polyol tends to lower the glass transition temperature of the adhesive.28

314

Adhesives and Sealants

(a)

O

2 R1 NCO + R OH

R1 NH

Urethane (carbamate) formation

O R2 (b)

O

R1 NCO + R2

R1 NH

NH2

Urea formation

NH R2 (c)

O

O R

3

1

NCO + R

R1 N

NH O

R2

Allophanate formation

NH R3

(d) R1 NCO + H2O

O

O

R2

O R1 NH

OH

R1

NH2 + CO2 R1 NCO

Reaction with water R1 NH

NH R1 O

Figure 6 Reactions of an isocyanate: (a) the reaction of an isocyanate with an alcohol to create a urethane (carbamate) linkage; (b) the reaction of an isocyanate with an amine to create a urea; (c) the reaction of an isocyanate with a carbamate to create an allophanate; and (d) the reaction of an isocyanate with water resulting in a urea.

8.12.7.4

Acrylics as Structural Adhesives

Poly(methyl methacrylate) or ‘Plexiglas™’ is a material well known not only for its clarity but also for its strength. Therefore, it is easy to think that such materials could also be used to form structural adhesives, as long as handling and cure conditions were appropriate. Methyl methacrylate is still a widely used monomer for this type of adhesive even though its volatility and odor leave something to be desired. Other methacrylate monomers can be used in addition to methyl methacrylate since many of these free radically curing materials will copolymerize. Thus, materials such as cyclohexylmethacry­ late and tetrahydrofurfuryl methacrylate have been used in structural adhesives, but longer alkyl chain pendant groups have not been used because they significantly lower the glass transition temperature when used in large amounts. The curing mechanism of this type of adhesive is a free radical reaction. Thus, the usual steps in such a reaction also take place in these types of adhesives. The adhesive has to contain a free radical initiator, the breakdown of which into free radicals is the initiation step for the reaction. The next step is the propagation step in which the free radical initiator species reacts with the monomer and then the initiator/monomer reaction product (also a free radical) continues the reaction until all of the monomer is consumed as part of the polymer. Next steps are termination reactions in which free radical ends of the polymer find each other and react, terminating the reac­ tion. Otherwise, the propagating free radical can undergo disproportionation, also resulting in termination.

Initiators for structural acrylics have to be stable in the composition for storage purposes. Thus, many or most struc­ tural acrylics come in the form of two-part adhesives in which part of the initiator system is in one part and the other part of the initiator system is in the second part. One such material is N,N′-dimethyl-p-toluidine, which can be used with a peroxide as the other part of the initiator system. One part of the adhe­ sive would contain the peroxide and the other would contain the amine and when mixed the two would generate free radi­ cals. Another such material is chlorosulfonated polyethylene (Hypalon™) which when mixed with an amine forms radicals. Hypalon is very useful in this type of adhesive because it also acts as a toughening agent, improving the peel strength of the cured adhesives.29 Another class of curing agent is that based upon trialkyl borane compounds.30 This type of material has been known as a free radical initiator for many years; however, the pyrophoricity of the alkyl boranes made the compound impossible to use because of industrial and consumer safety. This problem has been dealt with and complexes made between the alkyl boranes and amines have decreased the pyrophoricity and have generated adhesives with interesting properties on polyolefinic adherends.31 Oxygen is a problem for acrylic structural adhesives in that it acts as a radical scavenger and will, in fact, inhibit the polymer­ ization of methacrylate monomers. Therefore, in many such formulations, an excess of the initiator has to be added in order to accommodate the effect of oxygen. One has to be very careful when using an acrylic structural adhesive so as to not allow the adhesive to sit open on an adherend without quickly closing

Adhesives and Sealants

the bond as oxygen will quickly enter the adhesive and inhibit the polymerization. One exception to this rule is the organo­ borane initiator because it uses oxygen as part of its initiation system. Another important part of structural acrylics is the toughen­ ing agent. As with epoxies (vide supra), acrylic adhesives would be brittle materials without the addition of a toughening agent. Chlorosulfonated polyethylene has already been mentioned as such a material. Other materials include polyurethanes that have been end-capped with methacryloyl end groups32 as well as other materials such as core–shell tougheners. Acrylic structural adhesives have been used in many indus­ trial applications. They are used in fiberglass boat manufacture. They are also widely used as a threadlocking adhesive.33 The speed of cure makes these adhesives widely used in automotive assembly and repair.

8.12.7.5

Cyanoacrylate Adhesives

Cyanoacrylate adhesives have been known since the early 1950s. Alkyl cyanoacrylate monomers and polymers were first reported in 1949.34 The first commercialization of the technol­ ogy was done by Tennesee Eastman Co., which resulted in Eastman 910.35 This adhesive was noted for its very rapid cure and high room temperature lap shear strength. No curing agent was needed. The adhesive could be applied to a surface, the bond closed, and within minutes it was strong enough to support a structural load. The adhesive was later called ‘SuperGlue™’. The problems with this adhesive were the odor of the monomer, its sensitivity to the adherend surface on which it was applied, and the thermoplasticity of the cured adhesive. The first issue could be handled by appropriate industrial hygiene but the other two were considered enough of a pro­ blem that required further research. The number of cyanoacrylate monomers is relatively lim­ ited. The main materials in use are ethyl and methyl

cyanoacrylates. Other monomers such as butyl, isopropyl, allyl, octyl, and methoxy ethyl are also in use.36 Cyanoacrylate monomer polymerizes anionically. The poly­ merization mechanism is shown in Figure 7. The initiating species can be any anion; hydroxyl ions can initiate the poly­ merization. An amine can often be the initiating species. The terminating species is thought to be a cation. The stability of a cyanoacrylate adhesive in storage can be improved by the addition of small amounts of a stabilizer. Thus, strong acids or materials that can become strong acids upon the addition of water can be used to stabilize cyanoacry­ lates. Therefore, materials such as sulfonic acids or sulfur dioxide have been used. Because of the very high reactivity of the cyanoacrylate monomer, the formulation of cyanoacrylate adhesives has to be done carefully. Certainly, no amine-containing materials can be added or any other basic materials. Large quantities of acidic material also cannot be used due to the termination step. Therefore, formulation components have to be essentially neu­ tral in character. The monomer is a low-molecular-weight and low-viscosity liquid, and for many applications, especially on vertical surfaces, this can be a problem. Viscosity modifiers such as poly(methyl methacrylate), hydrophobic silica, alumina, and fumed silica have been used. Plasticizers have also been added to reduce the effect of gradual embrittlement. The mate­ rials include phthalates, phosphonates, and succinates. Toughening of cyanoacrylates can also be a problem. The usual addition of an elastomer to a structural adhesive can be attempted for cyanoacrylates, but in many cases such addition can lead to premature gelation of the formulation due to processing aids that were added to the elastomer. Thermal stabilizers also have to be added if the use of the adhesive is to reach temperatures in excess of 120 °C; thus materials such as a cyclic sulfate have been added to minimize the thermally induced depolymerization reaction.37 Cyanoacrylates are also known to be sensitive to the surface on which they are applied.

N

N I I– + H2C

C–

R

R O

O O

O

N N

N I N

C–

+

n+1 H C 2

I

R

O

I

315

O

O

R

N N

N

C– + T+ n O O OO O O R R R

O

O

N I O

N

n

n O

N

C–

O O O O R R R

N T

O O O O O R R R

Figure 7 Polymerization of a cyanoacrylate. The initiator (I−) attaches to a cyanoacrylate. The negative charge shifts to the carbon. The polymerization ensues until a terminating species (T+) terminates the reaction.

316

Adhesives and Sealants

Acidic surfaces are a problem. It has been found that crown ethers38 or calixarenes39 can be applied as a primer to minimize this problem. As might be imagined from the structure of a cyanoacrylate, adhesion to low-surface-energy substrates such as polyethylene and polypropylene is a problem. However, it has been found that the application of a primer that contains tertiary aliphatic amines or other anionic polymerization initia­ tors in an appropriate solvent provides a solution to this problem. The initiator diffuses into the surface of the plastic and initiates the polymerization in the plastic surface, thus providing an anchor for the polymer.40 Cyanoacrylates have found many uses because of their rapid cure. In the home, these materials have been used for repair of china and ceramics. They have also been used for repairs of vinyl and gasketing materials. In industry, they have found utility in speaker magnet assembly and gasketing. They have also been used to assemble medical devices. Cyanoacrylates have also been used in surgery during wartime. Recently, octyl cyanoacrylate has been found to be a noninflammatory liquid suture.41

8.12.7.6

Urea–Formaldehyde Adhesives

Urea–formaldehyde adhesives are widely used in the wood industry for joining wood to make useful forms such as interior-grade plywood and particle board. The chemistry of these adhesives is shown in Figure 8. The primary reaction is the addition of formaldehyde to urea to generate methylolated amines under basic conditions which can then further con­ dense with other ureas to create a three-dimensional network under acidic conditions. These reactions can take place at room temperature or at elevated temperatures depending upon the

molar ratio of formaldehyde to amine and if any catalyst is present. Catalysts include ammonium chloride or ammonium sulfate, which is buffered by the addition of tricalcium phosphate. Urea–formaldehyde adhesives came under scrutiny in the 1990s due to suspected emission of formaldehyde when the bonded wood was exposed to high-humidity conditions. It was found that the emission of formaldehyde could be reduced substantially if the molar ratio was reduced from the standard 1.55 to −1.85 (formaldehyde to urea) to levels as low as 1.02 to −1.1. This change could be accomplished by careful addition of urea to the cooking process. Another way in which the emission of formaldehyde could be lowered was by the addition of melamine to the resin cook. Addition of melamine to the resin causes an improvement in the hydrolytic resistance of the cured resin.42

8.12.7.7

A wide range of chemistries have been evaluated to provide adhesives that have properties that are an improvement over epoxies and phenolics. The primary property that has been sought is an improvement in the high-temperature resistance of these adhesives. Bis-maleimide is one of the chemistries that have been evaluated for this purpose. The structure of a bis-maleimide is shown in Figure 9. Bis-maleimides can be cured into a structural adhesive by one of three reaction mechanisms. The first one shown is the reaction of a bis-maleimide with an amine via a Michael addition reaction. The second one shown is the Diels–Alder reaction of a diene and the bis-maleimide. A third mechanism (not shown) is radical addition.

OH

O NH2

H2N

Higher Performance Structural Adhesives

+

H 2C

O

O

O

NH

H2N

or

NH2

NH

OH

H2O H2N O OH

HN O

NH2

HO

NH NH2

H 2O NH2

OH

H2O O

HN

HN

O

HO O H2N

O NH

NH

O NH

NH

O NH

NH

NH2

etc. Figure 8 Polymerization of urea with formaldehyde. The first step is the formation of methylol groups on urea. The methylol groups react with urea amines, expelling water.

Adhesives and Sealants

O

O N

R

O

O +

N

H2N

R1 NH2

H2N

O

R1 NH

1 NH R NH2

N

O

317

R

O

N O

Michael addition

R

R2

R2

O + CH2

N

O O +

N

R

O

OH R N

O R

N

R

O

OH

O

2

O O N

OH

Diels–Alder addition

R

O

Figure 9 Reactions of a bis-maleimide. The first reaction is Michael addition of an amine. The second reaction is Diels–Alder addition to an allyl phenol.

O

O H2N O

+

+

O

O

O

O

O

O

HO

OH

NH

NH

O

O

O NH n HO

O Heat

– Water

O

O O N

O

O

N

O

N N

O

O Heat

N

n

O

O

O

Pressure O

N

O O O

N n

O

O

O

Heat

NH

O

NH2

O

OH

O

O

O

N O

Figure 10 Reactions in the formation of LARC-13 polyimide adhesive. The first reaction is the addition of the three ingredients to create a polyamic acid. The acid further condenses to yield the imide. Further heating causes condensation of the terminal nadic groups to yield a crosslinked structure.

Polyimide chemistry has also been examined for obtaining higher temperature performance structural adhesives. This chemistry was evaluated at the Langley Research Center. One such adhesive was called ‘LARC-13’43 and the chemistry is

shown in Figure 10. As shown in the figure, nadic anhydride, methylene dianiline, and benzophenone tetracarboxylic dia­ nhydride were reacted to form polyamic acid, which was further reacted under pressure and heat to create an end-capped

318

Adhesives and Sealants

polyimide which could be then crosslinked as shown in Figure 10 to form a bond. This set of reactions is exemplary of many of the reactions that were examined to create a high-temperature performance adhesive. The typical cure pro­ cedure for these adhesives was to apply a pressure of 200 psi and 625 °F for 1 h. The pressure was then released but the bond was kept at 625 °F for another hour and it was then postcured at 600 °F for 4 h. This kind of extreme curing conditions could only be performed on high-temperature-resistant adherends. Lap shear performance of LARC-13 on titanium adherends was 2980 psi at room temperature and 2200 psi at 232 °C.44 Research still goes on to find an adhesive that gives high-temperature performance without the excessive cure sche­ dule (and the cost that goes with it).

8.12.8 Sealants Sealants were used in wooden marine vessels in ancient times. At that time, pieces of rope would be impregnated with oil or a resin and then jammed in between the planks that formed the hull of the vessel. That practice continued for hundreds of years until modern construction techniques and the use of modern materials became more common. The action of a sealant is similar to that of an adhesive with the exception that the bondline is much larger. Where an adhesive usually has a bondline that is less than 0.025″, a sealant could have a bondline that is as thick as 0.75″ or greater. In addition, an adhesive does not have specifications written about the retention of form after substantial extension, but a sealant does. Therefore, the formulation of sealants is done almost entirely with materials that one considers to be elasto­ meric in nature. Sealants are often classified by their ‘movement’ capability. ‘Movement’ means how much elongation the sealant can take before it cracks, tears, or disbonds. Therefore, the sealant indus­ try has classified sealants according to three levels: 0–5% movement is a low-performance sealant; 5–12% movement is a medium performance sealant; and >12% is a highperformance sealant.

8.12.8.1 8.12.8.1.1

Performance Tests of Sealants Movement

The test that defines the technique for measuring movement capabilities in sealants is ASTM C719-93.45 This test method (the Hockman Cycle) describes the application of a sealant between rigid adherends (such as cured Portland cement, glass, or aluminum). The sealant is applied to one of the adherends in a mass to create a bead that is 0.5 � 0.5 � 2.0 in. after which the other adherend is applied. After cure (which can be several weeks and several temperature excursions), the sea­ lant joint is placed in a machine which moves one adherend with respect to the other. Thus, the thickness of the joint for a 12% movement sealant would increase to 9/16″ and for a 25% movement sealant it would increase to 5/8″. In compression, the thickness of the sealant would be decreased to 7/16″ for a 12% movement sealant and 3/8″ for a 25% movement sealant. This extension/compression can be cycled slowly (over an hour for one cycle). After 10 cycles, the specimen is removed and examined for failure and type of failure. This test can also be

done at low (−15 °C) temperature and at high temperature (70 °C). 8.12.8.1.1(i) Hardness measurement ASTM C661-0646 defines the test for measuring the hardness of a cured sealant by means of a Durometer. A bead of sealant of dimension 5 � 1.5 � 1/4 in. is applied to a stiff adherend like aluminum. After the sealant is allowed to cure, the adherend with sealant applied is placed under the Durometer. The pres­ sure applied by the foot of the Durometer is 1.3 kgf. Immediately after the foot is applied, a reading is taken. The sample is moved to another location and another reading is taken. This process is repeated for at least a third time. 8.12.8.1.1(ii) Peel test The peel test for adhesion measurement for sealants is described in ASTM C794-06.47 In this test, the sealant is placed on a rigid adherend on an area that is masked off by masking tape. A cloth is impregnated with the sealant by application to both sides of the cloth. This cloth is then placed on the layer of sealant already applied to the adherend. This composite is then appropriately squeezed down to a total layer thickness of 1/16″. The sealant is allowed to cure after which it is once again coated with another 1/16″ of sealant and another cure is done. When cure is completed, the sealant is trimmed to 1″ wide with a razor. The peeling is done at 180°. If the adherend is glass, the glass/sealant interface can be exposed to light for a period of time before peeling. The peel rate is 2″ min−1. The test can be performed at other rates and at other temperatures. 8.12.8.1.1(iii) Other tests for sealants Because of the variety of environmental and use conditions in which sealants are placed, there are a number of other tests that should be mentioned. C793-0548 is a test method for acceler­ ated weathering of sealants. In this method, the sealant is coated on an aluminum plate and then exposed to either a xenon arc lamp, a fluorescent UV lamp, or an open flame carbon arc for a specified number of hours. The results are discussed in terms of the condition of the sample before and after exposure and the number and type of cracks that appeared. The strength of sealants is discussed in C1523-04.49 In this test, a precured sealant is applied over a gap. A starting tear is placed through the sealant. The assembly is placed in a tensile testing machine. The specimen is elongated at a rate of 2″ min−1 until a preset elongation has occurred. The tensile strength is recorded. After test, the specimen is examined for mode of failure, including tear length and type as well as type of failure at the adherend interface.

8.12.8.1.2 Sealants based on oils, curing (drying) oils, bitumens, and asphaltics Sealants based on oils and drying oils are classified as low-performance sealants because they generally have a low movement capability of 5% or less.50 The ingredients in such sealants are materials such as linseed oil, soy oil, and unsatu­ rated vegetable oil. These oils are ‘bodied’ (built up in viscosity) with calcium carbonate. The amount of calcium carbonate can be much in excess of the oils and resins in the material. The unsaturation of the oils in the sealant can be used to cure the

Adhesives and Sealants

material when a small amount of a catalyst such as cobalt carboxylate (napthenate) is added. They continue to cure for years after application and can become brittle. A material such as poly(isobutylene) can be added which can increase the flexi­ bility and extends the service life of the sealant. This type of sealant has been used for interior applications. Mastic sealants are based upon bitumen and asphaltics, which are by-products of petroleum refining. The materials are semisolid or very viscous liquids and contain a variety of polymeric materials. Because of their hydrocarbon character, they can be expected to wet most substrates. However, because of their lack of any other functionality, they can be expected to dewet under moist conditions. This type of sealant has been used on roadways, some construction sealing, and for pipes and marine sealants.

8.12.8.1.3

Sealants based on polysulfides

Polysulfides became commercialized in the 1950s. These are considered to be high-performance sealants because of their high movement capability, �25%. The ability of these sealants to resist hydrocarbon absorption led to their use as sealants for fuel tanks. In addition, their ability to resist argon gas move­ ment led to their use in the insulated glass market. The excellent adhesion and fuel resistance of these materials led to their wide usage in aerospace structures.51 A base polymer of polysulfide sealants is synthesized by the reaction of sodium polysulfide with bis(2-chloroethyl) formal. Crosslinking can be added by polymerizing a small amount of 1,2,3-trichloropropane into the reaction mix. This reaction mix yields di- or tetrasulfide or higher polysulfide linkages into the polymer as well as polysulfide terminal groups. Sodium sulfite can be used to reduce most of the terminal groups to thiols and the polysulfides in the interior of the polymer to disulfides. Crosslinking agents are primarily oxidizing agents such as manganese dioxide, calcium perox­ ide, and dichromates. Formulation components include fillers such as carbon black and calcium carbonate, plasticizers such as phthalates as well as accelerators (such as amino com­ pounds), and retarders such as stearic acid. Silane coupling agents can also be used in such sealants in order to improve adhesion to some adherends.

8.12.8.1.4

Sealants based on silicones

Silicones are a widely used type of sealant52 especially in the home for bath and kitchen sealing. Silicones have the widest temperature range of utility of any sealants. They can be used at temperatures as low as −100 °C to as high as 300 °C. They have excellent weathering characteristics as well as retention of flex­ ibility after aging. The service life for silicones is reckoned to be in excess of 20 years. Hence, they are considered a sealant of choice for glass construction sealing. They have high movement capability, �25%. The chemistry of silicone sealants is much the same as that discussed above for silicone pressure-sensitive and rubber-based adhesives, with the exception of the MQ tackifier addition. The basic polymer for silicone sealants is that based upon poly (dimethyl siloxane)diol. This material can be cured by any one of a number of polyfunctional curing agents, all of which are based upon tetrachlorosilane. Thus, such materials as methyl triacetoxysilane or methyl trimethoxysilane can be used in con­ junction with a poly(dimethyl siloxane)diol to yield a room

319

temperature vulcanizing (RTV) sealant. In the presence of moist­ ure, acetic acid is volatilized during the reaction of the first crosslinker and in the second case, methanol is evolved. Catalysts for this type of cure are stannous octoate and dibutyl tin dilaurate. Since these materials require moisture for cure, they should be packaged and stored in moisture-proof containers. This discussion has had to do with one-part formulations. Naturally, the materials could be separated into a two-part formulation which will cure upon mixing. The formulation of silicone sealants includes the choice of filler which for silicones is very important because the filler is essential for improving the tensile strength of a silicone. High surface area silicas are widely used to help reinforce silicones. High surface area carbon blacks are also used as are a wide variety of other fillers such as carbonates, iron oxide, clays, and diatomaceous earth. A final curing chemistry for silicone sealants is the reaction of a hydride functional silicone with a vinyl-terminated silicone fluid. A platinum catalyst is used for this reaction which is usually blocked by some form of complexing agent. Heat will liberate the platinum, causing the material to cure. This chemistry can be used in a two-part composition. Another type of sealant uses a combination of chemistry to yield a product useful as a sealant. This chemistry is a marriage of silicone and organic chemistry known as a polyether silicone.53 The basic polymer for this type of sealant is the reaction product of a polyether diol with a diisocyanate followed by further reaction with an amino-functional alkoxy silane. Thus, the central portion of this polymer is a polyether that is terminated with a urethane linkage that leads to the terminal group which is an alkoxy silane. This polymer can cure by the methods described above for a standard silicone but it contains the properties of a polyether. As with the normal silicones described above, these materials also have to be reinforced in order to obtain the tensile properties desired for a sealant. The same list of fillers as described above can be used in polyether silicones. Adhesion promoters such as silane coupling agents, diluents of various sorts, plasticizers, and catalysts are also added to these materials. The polyether silicone displays adhesion similar to that of silicone sealants but they have other properties of interest. For example, the paintability of polyether silicones is much better than that of silicones because of the polyether content. The property in which this material falls short in comparison to standard silicones is temperature resistance.

8.12.8.1.5

Sealants based upon polyurethanes

As stated in Section 8.12.5.3, one can get a marvelous variety of products based upon this chemistry. The chemistry of these materials is as described above with the exception that the material must remain an elastomer after cure in order to perform as a sealant. The same diisocyanates, diols, and chain extenders that are used in the manufacture of adhesives are used to make sealants, with the proviso that the result should be an elastomer. Polyurethane sealants have a 25% movement capability and are thus considered to offer premium performance.

8.12.8.1.6

Sealants based on acrylics

Acrylics are very weatherable materials and should thus find themselves in use as sealants. There are two types of acrylic

320

Adhesives and Sealants

sealants, latex and solvent. The solvent-based acrylics are known for their excellent adhesion to a wide variety of surfaces54 including those that are oily. Latex- and solvent-based acrylics have a low to medium movement capability.

8.12.8.1.6(i) Solvent-based acrylics Solvent-based acrylics are based upon polymers made of ethyl, butyl, and 2-ethyl hexyl acrylate but may also contain quantities of methyl methacrylate and other monomers including the acrylic and methacrylic acids as well as vinyl compounds. They are made by free radical polymerization. The formulating ingredients are the same sort as mentioned above including fillers, colorants, and solvent. One of the formulating ingredients may be pine oil which acts both to disperse the pigments in the system and to help displace oils on the surface of the adherend. Solvent-based acrylics ‘cure’ by loss of solvent, and no chemistry occurs to cure this material. Thus, these materials are not true elastomers; they do not chemically crosslink and they will cold flow with time but will also self-heal. They have excellent UV stability and have found use in glazing of win­ dows and other architectural applications.

8.12.8.1.6(ii) Latex-based acrylics The base polymer for this type of sealant exists in the form of an emulsion of micron- and submicron-sized particles of the polymer suspended in water. The base polymer formed by free radical polymerization may be a homopolymer of an acrylic monomer but is more likely to be a copolymer of a number of different monomers chosen to provide the correct balance of properties. The polymer latex has to be made more permanent and therefore a nonionic surfactant such as a nonyl phenol/ polyethylene oxide is added to help stabilize the emulsion. Other additives to the sealant formulation include plastici­ zers, fillers, solvents, and silanes. A plasticizer is added to the formulation in order to improve upon or maintain the flex­ ibility of the sealant. Solvents (usually a small amount) are added to improve the tooling of the sealant after it is applied. In addition, a solvent could be a material such as ethylene or propylene glycol which can improve the resistance of the packaged sealant to temperatures below freezing. The most widely used filler for this type of sealant is calcium carbonate. Silanes are often added to acrylics to improve the wet adhe­ sion of the sealant to glass. Other additives include antimildew agents (for tub and tile applications) and clay for rheological control. Latex-based acrylic sealants have the least number of objec­ tionable qualities of any sealant. They clean up with water, have little or no objectionable odor, and have excellent weath­ erability. They are then most often sold for use in residential sealing applications. They have found application in sealing windows, tub and tile sealing, and a myriad of other such applications, primarily for the prevention of air infiltration. They have limited movement capability, usually on the order of 7.5%. Latex caulks should not be used for any application in which the sealed joint is to be submersed in water for extended time periods.

8.12.8.1.7

Sealant products based on fluorocarbons

The base polymer for this type of sealant contains a substan­ tial portion of carbon–fluorine bonds, which control the application properties as well as the final properties of the sealant. Fluorocarbons come in two types: thermoplastic and elastomeric. The basic thermoplastic fluorocarbon is poly (tetrafluoroethylene) or PTFE. This material is a solid that is essentially insoluble in most solvents. This makes PTFE not usable in the usual type of sealants but it must be formed thermoplastically into a form which makes it usable. Thus, PTFE is made in granular form. The granules are compacted into a cylinder and then heated under pressure to form a ‘billet’. The billet can then be skived to create a film of PTFE that can be used for gaskets and similar applications. The limitation here is cold flow of the polymer. A number of methods have been used to confront this problem. The first is the addition of high-modulus fillers to the PTFE resin. If the addition of fillers is high enough, then the compressive load on the gasket will be borne by the fillers, thus limiting cold flow. The problem with this method is that the fillers seldom have all of the chemical resistance of PTFE. The second method to limit cold flow is the introduction of a microfi­ brillar structure into the PTFE. The fibril structure aids in resisting cold flow.55 PTFE-based gasketing material has experienced a growth in market due to the limitations based on the formerly competi­ tive asbestos-containing gaskets. This type of gasket has impressive temperature capabilities (up to 250 °C) and exten­ sive performance in regard to chemical insensitivity. It is especially important in the food preparation industry because of the inertness and the lack of any leachable chemicals of PTFE. Fluorocarbon elastomers also form the basis for a set of sealers. The monomers for these materials include hexafluor­ opropylene, vinylidene fluoride, and tetrafluoroethylene. The fluoroelastomer-based sealants were developed for the aero­ space industry and have many of the same chemical resistance properties of PTFE but are also elastomeric in character.

8.12.8.1.8

Sealants based on butyls

Polyisobutylene and butyl rubber are two materials that are based upon the polymer made from the polymerization of isobutylene.56 Poly(isobutylene) is not reactive after manu­ facture and hence it has been used as a modifier for various types of adhesives and sealants. Butyl rubber is made by the addition of isoprene to the polymerization of isobutylene. This addition yields a small amount of unsaturation in the polymer, thus making this material crosslinkable. In addition, chlorinated and brominated versions of this material are also available. Poly(isobutylene)s have a low glass transition tem­ perature of −60 °C and are therefore expected to have flexibility at low temperature. Because of their structure, poly (isobutylene) and butyl rubber are expected to have high impermeability to air and moisture transfer, making these materials ideal for sealant formulation. Poly(isobutylene) is manufactured in a wide range of molecular weights, ranging from 45 000 to 2 110 000. A common form of butyl-based sealant is the form of a tape. This tape is coated onto release paper and rolled on a core. The

Adhesives and Sealants

tape can be formulated to have a certain amount of tack. These materials can be expected to be unrolled on the surface of an adherend and be relied upon to form a seal essentially immediately upon application to the other adherend. These materials have been used for glazing applications, in appliance construction, and in automobile construction. Butyl sealants are also formulated to be hot melt applied. These materials are formulated with high levels of tackifier and yield a solid sealant in rod or pellet or bulk form. The applica­ tion temperature is about 180 °C. This type of sealant has been used to manufacture insulated glass where the high imperme­ ability of poly(isobutylene) can be put to use.

8.12.9 Future of Adhesives and Sealants It has been said that a new product has to be ‘better, faster and cheaper’ than existing product technology. This chapter has already provided some of this information, especially in regard to renewable resource-based adhesives. We will go a little further to try to predict future technology trends in adhesives and sealants.

8.12.9.1

Better

Adhesives are primarily thought of as an attachment or assem­ bly material. Adhesive technology can be ‘better’ by providing another function along with attachment. One simple means that has already been done is to provide information along with the attachment. The Post-It™ Note is one example of this. Another example is the attachment of sensors to critical loca­ tions that can report status at that locale. An example of this is a corrosion sensor that can be attached by an adhesive to a metal surface. A marriage of technologies can also occur such as the creation of microfluidic channels which can yield either a device that mixes microquantities of liquids or a device that yields a temporal message by watching how far a viscous fluid can travel down a microfluidic channel. The medical commu­ nity already has used adhesives as a drug delivery system. Various patches delivering drugs from birth control medica­ tions to smoking cessation aides are now commonly available. The adhesive used in these applications has to be able to ‘hold’ the drug as well as provide a means to control­ lably release the medication to the skin of the patient. Another feature is controlled release of the adhesive from a surface. This means that the adhesive has to be able to hold onto the surface with enough holding power to sustain the design loads but yet has to release from that surface easily, without damage to the surface.

8.12.9.2

Faster

Faster has to do with the time necessary to ‘set’ the adhesive, that is, the time from assembling the part until it can hold the design load. For PSAs, this is very quick, but the design load is low. For rubber-based adhesives, this is the time from applica­ tion of the adhesive until the second adherend can be applied. This waiting time has to do with the rate of evaporation of the solvent or carrier medium. For a solvent-based adhesive, this can be relatively quick, but for a water-based adhesive, it can be slow. This is especially true if one is trying to remove all of the

321

water from the bond or seal. For a hot melt adhesive, strength comes from the recrystallization of the adhesive which is usually short in time. For structural adhesives, the time of curing is a significant bottleneck in the manufacturing process. Another factor for structural adhesives is the need to freeze the adhesive for shipment and storage and the time needed to thaw the adhesive before use. The removal of water from an adhesive has been a problem for a long time. Techniques such as microwave heating have been attempted, and in some cases used, but this has not solved the general problem. Some means of sequestering the remain­ ing water in the adhesive needs to be found. We have already discussed the matter of latency for one-part structural adhesives. Presently, the solutions to the shipment and storage of one-part systems have been done by finding curatives that are insoluble in the resins. This provides the desired storage stability but does not provide for a lower tem­ perature cure. There has to be some means to sequester the curative from the resin, which releases the curative at a lower temperature.

8.12.9.3

Cheaper

Adhesive raw materials come from two sources: plant based and petroleum based. As was mentioned above, petroleum-based raw materials are becoming more expensive and harder to come by. This has led to a drive to find and manufacture adhesive components based on sustainable or renewable raw materials. Adhesives and sealants have a long history with natural product-based raw materials. The unfortu­ nate situation is that adhesive bonds made with natural product-based raw materials are usually not durable. Thus, protein-based adhesives can be used to make interior-grade plywood, but synthetic adhesives are used to make exterior-grade plywood. Therefore, the drive to make adhesives cheaper is combined with the drive to make natural product-based adhesives more durable. Cheaper also can come from simplifying the manufacturing process by eliminating steps. For example, the process flow to make an aerospace structural adhesive bond comprises at least 18 steps. Elimination of some of these steps can lead to sub­ stantial cost savings. For example, priming and primer curing are two of those steps. If one can eliminate the need to prime, then the industry could save money. The same is true in the plastics industry. Elimination of costly surface preparation steps can lead to lower cost of adhesively bonded articles.

8.12.9.4

Other Factors

Besides the factors of ‘better, faster, cheaper’, there are today at least two more factors that have to be considered. These are ‘smaller and smarter’. The advent of nanotechnology has led to modification of adhesives on a nanoscale. There is also the matter of applying and using adhesives at much smaller scales such as in the manufacture of modern electronics. Smarter has to do with how well we understand the phenomenon of adhe­ sion, which would hopefully lead to new ways to bond and seal.

322

8.12.9.5

Adhesives and Sealants

Smaller

Nanotechnology has led to the availability of much smaller-sized fillers which can be used to modify the properties of adhesives. Thus, nanosilica is available. Nano-sized rubber modifiers are also available. There are papers and patents which disclose some of the properties that can be obtained by the addition of nano-sized fillers to structural adhesives.57 There is still much to be learned in this area to further improve on the properties of adhesives. Microelectronic manufacturing provides an arena into which adhesives and sealants have to be applied in exceedingly small areas and cured so as to provide durable attachment. These operations also have to be done at a rate that is com­ mensurate with the needs of this industry. Nano-sized fillers will play a role in this area as the size of circuitry and the size of contact pads decrease. In addition, the comments already made in regard to the thermal stabilization of curatives will have to be taken into account.

16. 17. 18. 19.

20. 21. 22.

23.

24. 25. 26. 27. 28.

8.12.9.6

Smarter

‘Smarter’ means better knowledge of the materials and processes used in making adhesive and sealant bonds. In this chapter, the science of adhesion has not been discussed and the reader is referred to several books on the subject.58 In order for many of the improvements in adhesive and sealant performance to occur, we will need more understanding of the phenomenon of adhe­ sion. As to this point, the science of adhesion has clearly demonstrated the connection between wetting, spreading, and adhesion.59 More recently, contact mechanics, the study of mak­ ing a brief contact between materials and measuring the forces required to separate the materials after the brief contact, has made some inroads into a better understanding of adhesion.60 The science of adhesion has done well in providing guidelines on ‘what will stick to what’ based upon surface energetic argu­ ments. The science has not done well in the general prediction of the strength of adhesive bonds from first principles. This type of study will have to continue in order to have a better under­ standing of the basic phenomena operating when making (or breaking) an adhesive or sealant joint.

29. 30. 31.

32. 33. 34. 35. 36.

37. 38. 39. 40. 41. 42.

References 1. Jesus ben Sirach (Ecclesiasticus). The Apocrypha, New Revised Standard Version. Cambridge University Press: Cambridge UK, 2001, pp. 58–113. 2. Pliny the Elder, Natural History; Goold, G. P., Ed.; Loeb Classical Library, St.

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3. Klosowski, J. M. Sealants in Construction; Marcel Dekker: New York, 1989. 4. ASTM D1002. American Society for Testing and Materials, Philadelphia, PA. 5. ASTM D1876. American Society for Testing and Materials, Philadelphia, PA. 6. Shecut, W. H.; Day, H. H. U.S. Patent 3,965, 1845. 7. Drew, R. G. U.S. Patent 1,760,820, 1930. 8. Huck, V. The 3M Story, Brand of the Tartan; Minnesota Mining and Manufacturing Co., St. Paul, Minnesota, 1955. 9. Dahlquist, C. A. Adhesion: Fundamentals and Practice; Elsevier: Amsterdam, 1970. 10. Autenreich, J. S.; Foley, K. F. In Handbook of Adhesives, 3rd ed.; Skeist, I., Ed.; Van Nostrand Reinhold: New York, 1990; Chapter 33. 11. Ulrich, E. W. U.S. Patent 2,844,126, 1959. 12. Harlan, J. T., Jr. U.S. Patent 3,239,478, 1966. 13. Lin, S. B. Int. J. Adhes. Adhes. 1994, 14, 185. 14. Bhowmick, A. K.; Gent, A. N. Rubber Chem. Technol. 1984, 57, 216. 15. Gerrard, J. A.; Mattson, A. C. U.S. Patent 2,918,442, 1959.

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Biographical Sketch Dr. Alphonsus V. Pocius was born in Emsdetten, Germany. He and his family emigrated to the United States in 1950. He received his bachelor’s degree in chemistry from Knox College (Galesburg, IL) in 1970. He then attended the University of Illinois (Champaign-Urbana, IL) and received his PhD in physical chemistry in 1974. He joined 3M company in that year. At 3M, he worked in various divisions of the company including the Corporate Research Laboratories, the Adhesives, Coatings and Sealers Division, and the Specialty Film Division. He also worked with the University of Minnesota on contact mechanics and adhesion. He is the author or coauthor of 32 US Patents and has published numerous papers on adhesion and adhesives. He is the author of Adhesion and Adhesives Technology: An Introduction, a textbook published by Hanser. He retired from 3M in 2009.