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Advances in acrylic structural adhesives
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Advances in acrylic structural adhesives
P. C. Briggs, IPS Corporation, USA; and G. L. Jialanella, The Dow Chemical Company, USA
Abstract: Acrylic adhesives as defined in this text are based on acrylate and methacrylate monomers and have been commercially used for more than 50 years. These products are supplied as two separate components that can be mixed prior to application or each component can be applied to separate surfaces. Traditionally, methacrylates are preferred over acrylates owing primarily to the odor of the acrylates. The most popular and most commercially successful structural acrylic adhesives in use today are polymerizable mixtures of polymers dispersed or dissolved in methyl methacrylate (MMA) monomer. These adhesive products are supplied as two separate components that are primarily mixed just prior to application. One component contains a peroxide compound (oxidizing agent) and the second component contains an amine or metal salt (reducing agent) that reacts with the peroxide component upon mixing to initiate the free-radical polymerization of the methyl methacrylate monomer. This chapter will review the historical evolution and cure systems of methacrylate adhesive systems. Also, it will review the first and second generation and advanced technology products as well as formulation variables, bondline properties and applications of methacrylate-based adhesive systems. Key words: structural acrylic adhesives, acrylates, methacrylate, peroxide compound, oxidizing agent, tetrahydrofurfuryl methacrylate (THFMA), hydroxyethyl methacrylate (HEMA) hydroxypropyl methacrylate (HPMA).
6.1
Introduction
Acrylic adhesives as defined in this text are based on acrylate and methacrylate monomers and have been commercially used for more than 50 years. These products are supplied as two separate components that can be mixed prior to application or each component can be applied to separate surfaces. Traditionally, methacrylates are preferred over acrylates primarily because of the odor of the acrylates. This chapter will review the historical evolution and cure systems of methacrylate adhesive systems. Also, this chapter will review the first and second generation and advanced technology products as well as formulation variables, bondline properties and applications of methacrylate based adhesive systems.
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6.1.1 Historical evolution The most popular and most commercially successful structural acrylic adhesives in use today are polymerizable mixtures of polymers dispersed or dissolved in methyl methacrylate (MMA) monomer. As mentioned previously, these adhesive products are supplied as two separate components that are primarily mixed just prior to application. One component contains a peroxide compound (oxidizing agent) and the second component contains an amine or metal salt (reducing agent) that reacts with the peroxide component upon mixing to initiate the free radical polymerization of the methyl methacrylate monomer. Among the earliest examples of this type of adhesive were clear, colorless mil spec adhesives that first appeared in the 1950s as bonding agents for poly (MMA) sheet in applications such as aircraft canopies. In this most basic form, the polymerizable adhesive component consisted of poly (MMA) dissolved in MMA monomer with N,N-dimethyl aniline or a derivative as the amine component. A benzoyl peroxide initiator was supplied as either a powder or liquid solution. In the 1960s, formulators began to use elastomers to supplement or replace the poly-MMA to provide toughness and provide improved bondability for a wider variety of substrates including metals, thermoplastics and thermosets (Owston, 1973). Functional monomers such as methacrylic acid and certain other additives were included for specific performance or application benefits. Today’s high performance structural acrylic adhesives are the result of extensive evolutionary development of these basic systems by a number of companies who have tailored these products for increasingly demanding and sophisticated applications. The MMA-based structural acrylics are distinctly different from other polymerizable acrylic adhesive technologies such as anaerobics, which are primarily used for narrow gap metal bonding, retaining and threadlocking, and the cyanoacrylates, which cure by an anionic mechanism. The latter are generally supplied as single component products that are catalyzed by metallic surfaces in the case of anaerobics and surface moisture in the case of cyanoacrylates. An accelerator is sometimes used to speed the cure on inactive surfaces. In addition to these reactive acrylic technologies, acrylic polymers are also found in solvent, emulsion and hot melt adhesives for a wide variety of applications. These adhesive classes have evolved concurrently over the past few decades, but this chapter will be limited to the evolution and development of the polymer-in-monomer-based structural acrylics.
6.1.2 Cure systems In the first 20 years of their development, the cure system of choice for structural acrylics most often consisted of benzoyl peroxide in combination
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with a tertiary aromatic amine. In most cases, the amine compounds were included in the polymer-in-monomer portion, typically referred to as the adhesive component and the benzoyl peroxide was supplied as an activator or accelerator. The benzoyl peroxide was typically supplied as a paste in a plasticizer to facilitate mixing with the adhesive. Typical commercial formulations provide a working time of a few minutes, followed by rapid free radical exothermic polymerization that results in the very fast buildup of bond strength that characterizes these products. This fast cure differentiated these products from the epoxies that were gaining in popularity as well, but which take much longer to set and cure because of their kinetics of polymerization which provides a more gradual, linear buildup of strength. A convenient option for some applications involves applying the activator as a thin film from solution to one or both of the substrates to be bonded. This technique has limited utility because it is most effective in relatively thin bond gaps. Thick gaps require activation of both bonding surfaces and two-part mix-in application is generally more effective. In the 1970s duPont introduced a new cure system for polymerizable methacrylate adhesives (Briggs, Jr. and Muschiatti, 1975). The adhesive composition comprised a solution of chlorosulfonated polyethylene, sold by duPont under the trade name Hypalon®, in a mixture of monomers and additives similar to the earlier methacrylates (Equation 6.1). H2 C
H2 C C H2
H2 C CH
X
Y
Cl
CH
Z
[6.1]
SO2Cl
In the earliest embodiments, an aldehyde–amine reaction product was used as a surface activator to initiate the cure of the adhesive base when the substrates were mated. Among the many combinations of amine–aldehyde derivatives that are possible, the most effective were condensation products of butyraldehyde and aniline or butyl amine. Over the years, the butyraldehyde– aniline products have proved to be the most effective. The active ingredient is N-phenyl-3,5-diethyl-2,3-dihydropyridine, referred to as DHP or PDHP (Equation 6.2). C3H7
C2H5
[6.2]
N C2H5
The original products were available as approximately 40% active unrefined condensation products. More recently, purified products containing up to 80–90% of the desired PDHP have become available (Melody et al., 1984). © Woodhead Publishing Limited, 2010
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6.1.3 First and second generation DuPont introduced the term ‘second generation acrylic’ or SGA to differentiate these products from the earlier products based on benzoyl peroxide and tertiary amines. The latter became referred to as ‘first generation’ products. Primary benefits of the SGAs were increased toughness and impact strength of metal to metal bonds, as well as the ability to bond metal surfaces, even oily metal surfaces, with little or no surface preparation. The products were also shown to be capable of effective performance as ‘100% solids’ alternatives to solvent cements in applications such as plastic pipe bonding and decorative lamination of vinyl and high pressure laminates to metals and particle board. In this evolutionary stage during the 1970s and 1980s, the SGA products were promoted concurrently with development of improved waterborne systems to address looming threats against the use of flammable and toxic solvents. However, the two part nature of these products as well as other limitations prevented the SGAs from effectively competing with the simpler one part systems. Rather, the reactive methacrylate products have found a secure niche in a number of product assembly processes for which they provide unique benefits.
6.1.4 Advanced technologies Several companies have approached these industries/markets in different ways as the products have evolved over the past two decades and this chapter will concentrate on that evolution to the present day status of these uniquely attractive products. The basic chemistry, along with the variety of basic cure systems employed in reactive acrylic adhesives, including those comprising the so-called first and second generation products, are discussed in great detail in a review article by Damico (1990). The review also includes a thorough overview of improvements that evolved during the 1980s in the durability and heat resistance of reactive acrylic adhesives, especially with respect to bonding metals. This chapter will focus on developments and improvements that have occurred over the past two decades in this field which have greatly broadened the popularity of acrylic adhesive products, especially in bonding thermoplastics, composites and combinations of these materials to metals. Along with improved substrate bonding capability and mechanical properties, significant improvements have been made in the application and handling characteristics of these products. As a result, the new reactive acrylics compete equally or better than epoxies and polyurethanes as the adhesives of choice for any challenging bonding application. The earlier ‘first generation’ acrylics and the subsequent ‘second generation’ acrylics had certain limitations, especially in metal bonding capability owing to limitations in resistance to elevated temperature exposure and subsequent
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resistance to harsh or corrosive environments. As reviewed by Damico (1990), the primary improvements in adhesives based on MMA monomer in the 1980s involved improvements in the ability of these products to bond as-received metals, especially aluminum steel and stainless steel, with little or no surface preparation. The primary enabling factor was the incorporation of phosphoric acid derivatives of methacrylate monomers which chemically interact with metal oxide surfaces to strengthen the normally weak interfacial layer between the adhesive and the base metal and to protect it from corrosive attack under harsh environmental conditions (Zalucha et al., 1980). Additional improvements in durability and resistance to elevated temperatures were obtained through the incorporation of epoxy resins in the compositions (Dawdy, 1984). Those containing relatively high levels of epoxy resin were considered to be hybrids of methacrylate and epoxy technology. The products were capable of withstanding repeated exposures to temperatures of 204°C (400°F) such as those encountered in paint baking ovens in the automotive industry. In spite of the significant improvements in performance in niche metal fabrication and medical and electronic assembly, the use of reactive acrylates and methacrylates was still limited in the late 1980s. Damico estimates that the total annual volume used at the beginning of the 1990s was less than US$10 million globally. In the 1990s and later, challenging new bonding applications involving the use of plastics in the transportation and marine markets provided the impetus for additional opportunity for commercial development of further improved products that would finally spur the growth of these products in high volume applications. The following advances in capability have occurred sequentially from the late 1980s to the present, proving methacrylate technology to be uniquely suitable for the most challenging bonding applications: ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑
improvements in low temperature toughness and flexibility with retention of hot strength reduction of odor during application reduction and elimination of surface tackiness from air inhibition control of aggressive solvation of sensitive plastics regrind compatibility with thermoplastics improved bondability of composites extended open working time for application on large assemblies control of exotherm for reduced outgassing in thick bond crosssections reduced print-through on show surfaces ability to bond low energy surfaces, including polyolefins.
These improvements can be best incorporated in a discussion of the various classifications of adhesives in use today.
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Classification of acrylic structural adhesives
6.2.1 Elastomer modified methyl methacrylate-based adhesives The early MMA-based adhesives, including first and second generation products, as well as most of the improved products reviewed by Damico (1990), generally employ a single elastomeric component as the primary toughening additive. Soluble thermoplastic additives such as poly-MMA or polystyrene are occasionally added to impart certain adhesive properties, but the elastomer is the dominant factor affecting adhesive rheology, bond characteristics and adhesion with single polymer modification. The single elastomer approach generally imposes the following limitations on adhesive characteristics. ∑ ∑ ∑ ∑
limited substrate compatibility, requiring multiple adhesives for different substrates; ‘stringy’ viscosity and Newtonian rheology characteristic of elastomer solutions; inorganic fillers required for improved rheology can negatively affect properties; lower Tg of elastomer may reduce hot strength of adhesives.
The glass transition temperature, Tg, of unmodified poly(MMA) system is 100°C and the presence of the elastomer can have a significant impact on the Tg of the cured adhesive depending on the resin/elastomer compatibility. As expected the Tg of the cured poly(MMA) system is reduced when there is good compatibility between the poly(MMA)/elastomer, but is not affected when the elastomer forms discrete particles owing to the lack of compatibility. Achary et al. (1991) and Bianchi et al. (1991) have studied the affects of rubber modification of acrylic adhesives on the Tg and adhesive strength properties and related these effects to bulk morphology. Achary et al. (1991) studied the effect of hydroxyl terminated polybutadiene (HTPB) in a vinyl ester/methyl methacrylate base acrylic adhesive on the Tg of the cured system using a differential scanning calorimeter (DSC). They showed two distinct Tg values for the fully cured adhesive even though the HTPB was soluble in the monomer system. This finding indicates that phase separation of the HTPB (rubber) phase occurred during the curing cycle. They also showed that the cure profile and amount of HTPB affects the Tg of the cured acrylic adhesive. Room temperature cure showed a single depressed Tg (120°C versus 138°C for the unmodified vinyl ester/methyl methacrylate system) with some residual exotherm on the second heating. The heat cured adhesives exhibited two distinct Tg values, the main phase Tg being similar to the unmodified system. They also showed that higher HTPB content (30% w/w) slightly depressed © Woodhead Publishing Limited, 2010
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the main phase Tg to approximately 134°C, but intermediate levels of 15% w/w did not depress the main phase Tg. Lap shear and impact peel strengths were at maximum values when the HTPB levels were in the range 10–15% w/w and decreased with further increasing levels of HTPB. Bianchi et al. (1991) also found that the bulk morphology in terms of resin/ rubber compatibility of rubber modified acrylic adhesives significantly affects the Tg as well as the strength properties. They found that the morphological features and resulting properties of the acrylic adhesive were dependent on rubber type was well. They found that better phase separation and improved strength properties were observed for butadiene–acrylonitrile rubbers and fluorinated rubbers compared to chlorosulfonated polyethylene.
6.2.2 Non-methyl methacrylate adhesives When the SGA adhesives were introduced in the 1970s, the anticipation generated by their improvements caused increasing interest on the part of a number of formulating companies. This situation increased exposure and notoriety also exposed more potential users to the odor of methyl methacrylate monomer. Negative feedback regarding the odor problem, along with the flammability associated with the low molecular weight monomer, prompted additional development work directed at reducing volatility and related problems associated with it (Bachmann, 1996). Commercial formulations have been introduced to the market, but their higher cost generally limits their use to specialized lower volume and high value applications. The most commonly employed alternatives to MMA are tetrahydrofurfuryl methacrylate (THFMA), hydroxyethyl methacrylate (HEMA) and hydroxypropyl methacrylate (HPMA). THFMA has been used in conjunction with the phosphate and epoxy improvements noted above to make hybrid metal bonding products that are more readily accepted in the automotive assembly environment, where odor and flammability are decided deterrents to their use (Lord Corporation, 2005). This is one of the larger volume applications for the low volatility monomers. HEMA and HPMA have been used to prepare two part reactive and one part ultraviolet and visible light curable adhesives that can be used in the relatively clean environments used to assemble electronic and medical components (Friese and Bergmann, 2000). Other products of the ‘anaerobic’ type based on di- and polyfunctional acrylate and methacrylate monomers found limited use in high value metal and electronic assembly as an outgrowth of their use in threadlocking applications (Toback, 1971). Because the higher methacrylates are generally produced by transesterification of methyl methacrylate with higher molecular weight alcohols, they are significantly more expensive than MMA. Many of the higher methacrylates are usually associated with lower odor than MMA and, owing to the lower
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volatility, there is an increased tendency to incompletely cure at the surface because of air inhibition. In some cases, lingering odor can be a significant problem. This is especially true in the case of butyl methacrylate and 2-ethyl hexyl methacrylate, two of the more commonly available higher methacrylates. Another issue with adhesives, formulated with higher molecular weight monomers based on alcohols with C4 and higher alkyl groups, is a reduced capability to bond polar substrates such as metals. Among the more useful higher methacrylates are hydroxyl alkyl methacrylates such as hydroxyethyl and hydroxypropyl methacrylates (HEMA and HPMA) and tetrahydrofurfuryl methacrylate (THFMA). The influence of the hydroxyl group can impose limitations on polymer solubility characteristics and adhesive substrate compatibility, but some very commercially successful adhesives are available based on these materials. THFMA is probably the most versatile of the higher molecular weight alternatives to MMA but it is also quite expensive. Along with the higher cost and other limitations associated with the higher methacrylates, another performance limitation associated with them relates to the fact that the polymers derived from the higher methacrylates generally have lower glass transition temperatures than poly-MMA. While this can be a benefit in improving the toughness and flexibility of the cured adhesive, it generally reduces the high temperature capabilities of the products. This can be somewhat offset by increasing the cross-link density of the adhesive with difunctional and polyfunctional methacrylates, but this technique is limited by other compromises imposed by these monomers (Pelosi, 1980). The simplest members of this group are the ethylene and polyethylene glycol dimethacrylates which, along with proliferation monomers of this type (Isobe, 1990), were originally used to formulate anaerobic adhesives for threadlocking. These highly cross-linked materials are generally very rigid and resistant to the effects of heat and chemicals. By adding small amounts, usually less than 5%, of these di- or poly-methacrylate monomers to any of the structural methacrylate adhesives, the formulator can impart these properties to the cured composition. Increasing amounts of crosslinking monomer imparts increasing rigidity and can eventually lead to embrittlement, thus negating a primary advantage of this class of products, which is toughness. The cross-linkers also generally increase the cure speed of the adhesive and can negatively impact shelf life and adhesion. Numerous di- and polyfunctional acrylate and methacrylate monomers have become commercially available over the years and provide the formulator with many options for modifying and tailoring structural adhesive properties. In summary, unless the odor and flammability of MMA cannot be tolerated, the incentive to formulate high performance structural adhesives for typical high volume product assembly operations with higher methacrylates is very limited. Adhesive products utilizing higher molecular weight monomers do
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have a place in specialized applications for which they are very successful, but MMA remains the monomer of choice for the highest performance and value.
6.2.3 Polymer blends as modifiers in methyl methacrylate-based adhesives Certain polymer blends enable the formulator to take advantage of separate contributing factors derived from each individual polymer to develop combinations of properties that are not attainable from a single polymer. For example, one elastomer may provide excellent low temperature flexibility and toughness, but adhesion to specific substrates may be compromised. A second polymer can provide better wetting or compatibility with the desired bonding surfaces. The combination of available polymers, along with the ability to modify further formulations with different comonomers along with MMA, as well as the many other additives, is what makes the polymer-inmonomer technology so versatile with respect to tailoring adhesives for specific applications. The low viscosity and high solvency of MMA and the numerous comonomers that can be blended with it allow the ready incorporation of high molecular weight polymers as tougheners. Polymeric tougheners include typical elastomers used in dry polymer applications, some of which, such as polychloroprene, acrylonitrile and styrene butadiene polymers, are available in solution grades for adhesives and coatings. Other polymeric additives include soluble thermoplastic polymers and core shell impact modifiers that disperse and swell in the monomers, but do not fully dissolve (Muggee and Zilley, 1990).
6.3
Advantages and disadvantages and unique characteristics of acrylic structural adhesives
As noted above, one of the great advantages of the reactive acrylic or methacrylate adhesives is their formulation versatility. Prior to discussing the advantages and disadvantages and unique characteristics of the group, it will be helpful to review additional formulation options and the characteristics they impart.
6.3.1 Formulation variables All of the variables reviewed above provide an obviously infinite combination of adhesive properties for the engineer or production manager. The development of specific formulations to address all of the product characteristics and performance variables is highly proprietary to the formulating companies © Woodhead Publishing Limited, 2010
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that provide these products. A list of the generic components available to the formulator, along with the positive and negative impacts on handling and performance characteristics are presented below in Table 6.1 to illustrate the complexity involved in this technology. Specific examples of these formulas can be found in an article by Damico (1990). The list of potential formulating variables and related effects is much more complex than commonly available or useful for formulating epoxies Table 6.1 Potential formulating variables of acrylic and methacrylate adhesives Type Description Positive effects
Potential negative effects
Primary monomer MMA Polymer solvent Odor and Substrate solvation flammability High Tg May harm sensitive substrates Rigid homopolymer Secondary Higher MW Reduced volatility Increased cost monomer monomer or Increased flexibility Lower Tg MMA substitute Air inhibition Primary polymer Elastomer Toughness and Viscosity and component flexibility ‘stringyness’ Heat resistance Secondary Elastomer or Improved adhesion Increasing viscosity polymer thermoplastic Increased Tg Reduced toughnes Acidic monomer Functional Metal adhesion Corrosive monomer Cure behavior, reduced shelf life Cross-linking Di functional or Heat and solvent Cure behavior, monomer higher functional resistance reduced shelf life monomer Reduced toughness Plasticizer Plastic and rubber Toughness and Heat resistance additives flexibility Reduced adhesion Filler Plastic and rubber Cost reduction Reduced toughness additives Reduced shrinkage Effects on adhesion Thixotrope Fumed silica, Reduced ‘stringyness’ Same as fillers others Peroxide initiator Acyl, dialkyl Speed of cure Reduced shelf life or hydroperoxide Amine promoter Tertiary aromatic Speed of cure Discoloration of or activator amine or derivative cured adhesive Metallic additive Organometallics Speed of cure Reduced shelf life Boron additives Organoboranes Low energy surface Cost bondability Stabilizer or Phenolics Shelf life Reduced cure speed inhibitor Increased working Air inhibition time Pigments and Plastic and Mixing indicator May affect shelf life colorants rubber additives Color matching Cost
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and polyurethanes. However, this list illustrates the many options to the formulator and end user with this class of products.
6.3.2 Advantages of structural methacrylates Cure profile
Degree of cure or polymerization
One of the greatest advantages that the methacrylates exhibit relative to epoxies and polyurethanes is their cure profile and the ability to adjust the cure profile to provide a long open working time with the much desired ‘snap cure’ once parts are assembled. These characteristics are a result of the cure kinetics illustrated in Fig. 6.1 (Odian, 1991; Damico, 1990). Methacrylate adhesive systems cure via a free radical mechanism rather that the linear addition mechanism that characterizes the epoxies and polyurethanes. Epoxy and urethane adhesives (condensation polymers) exhibit a gradual cure with time. Acrylics (free radical addition polymers) on the other hand, exhibit a rapid cure after a given induction (delay) time. The unique aspect is that the delay time can be independently adjusted without compromising rapid curing. For example, the delay times can be quite large (1–3 hours) and exhibit a cure time of under 12 hours. The strong solvency of MMA provides two significant advantages with respect to substrate bondability. First, it causes the adhesive to solvate the surface of many plastic substrates prior to completion of the curing process. The primary exceptions are polyolefins, fluorinated polymers, crystalline plastics such as polyamides and polyacetals, and highly cross-linked and post-cured thermosets such as epoxies and some polyesters and gel coats. Fortunately, the inherent inability of MMA to solvate certain plastics can be overcome with additive approaches that have been developed over the past
Polycondensation
Free radical or addition polymerization
Time (arbitrary)
6.1 Cure profile comparison of a free-radical cure adhesive with a polycondensation adhesive.
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few years. As a last resort, typical surface preparation techniques that are used for epoxies and polyurethanes can also be used to improve the adhesion of methacrylates. In addition to solvation of plastics, the solvency of MMA also reduces the sensitivity of adhesion and bondability to contaminants, particularly organic materials, on substrate surfaces. The MMA, and to some extent the polymers and additives in the methacrylates, dissolve and absorb many organic surface contaminants such as fingerprints, processing oils and release agents that normally interfere with bonding that uses epoxies and polyurethanes. The breadth of substrate surface compatibility by the methacrylates is much greater than that of the epoxies and polyurethanes. Epoxies are most generally associated with bonding of metals and other inorganic materials, where the high polarity and hydrogen bonding capability of the resin and hardener components provide compatibility. Polyurethanes have been most successful in applications where increased flexibility is required, mostly with thermoplastics and thermosets such as the sheet molding compound (SMC) which is widely used in the transportation industry. The versatility of methacrylates to bond all of these materials has allowed them to capture an increasing market share in a variety of applications that can benefit from their superior processing and bonding characteristics. Bondline properties Another unique advantage of the methacrylate class of products is the ability to formulate them with a broad range of bondline modulus characteristics which, in many cases, provide specific application or process advantages over epoxies. With specific exceptions for specific applications, epoxy adhesives have historically been considered to have the highest load bearing capability of the three classes of adhesives. This is especially true of the heat cured products of the type used to bond metal aircraft structures. When these products are formulated to impart increased peel and impact strength, great care is taken to preserve as much of the inherent load bearing capability as possible while reducing the brittle or glassy characteristics of the highly cross-linked epoxy matrix. The most capable of these products have lap shear strengths of 35–40 MPa or higher, with peel strengths in excess of 10.5–12.5 N mm–1, and are capable of maintaining a high degree of their strength over the temperature range of –55°C to 121°C required by the aerospace industry. These properties can be achieved by very careful formulation with proprietary toughening agents, mixed in precise quantities or provided as carefully formulated prepregs, coupled with high temperature curing which provides a near ideal (necessary) polymer morphology in the bondline. Epoxies that are formulated
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to be mixed, applied and cured under more forgiving process conditions, typically lose a good measure of the high modulus that provides the highest load bearing capability. When flexibilizers are added to maximize toughness and flexibility under these conditions, load bearing capability is compromised even more, especially at high temperatures. As a result, the methacrylates have become more competitive with epoxies where the compromises in performance required to achieve better application tolerance are a disadvantage for the epoxies. An exception is the relatively new development of adhesively bonded metal frames and structural components in the automotive industry. While methacrylates and epoxies might both be considered, the combination of the reputation and history of epoxies in metal aircraft applications coupled with recent improvements in the ease of application and performance of the epoxies has proved to be an advantage for the epoxies in these applications (Okui and Shiokawa, 2001).
6.3.3 Disadvantages of structural methacrylate adhesives In spite of all of the advantages of this class of products noted above, there are inherent drawbacks and limitations. First and foremost, the most cost effective monomer, which provides the highest performance products, is MMA, which, as noted above, is flammable and has a strong odor. In spite of its relatively low toxicity, misguided publicity stemming from the consumer cosmetic industry (finger nails) has cast a negative cloud over MMA. As also noted above, alternative monomers derived from MMA only add cost and reduce performance. This is the single most limiting fundamental drawback of the class. Another inherent disadvantage of the methacrylates is limited solvent resistance. The poly (MMA) matrix, as well as many of the additives used to modify the products, are soluble in polar and aromatic solvents such as ketones, toluene and solvent blends. However, they are reasonably resistant to petroleum lubricants and diesel fuel and are somewhat resistant to gasoline. Aqueous chemical resistance is generally good, although strong caustics, especially at high temperatures, can degrade performance. The high temperature performance of highly toughened methacrylate structural adhesives is limited by the Tg of MMA monomer, as well as the polymeric additives. Reducing elastomer toughener content and increasing cross-link density with multifunctional monomers can improve heat resistance as well as chemical resistance, but with a significant offset in physical properties. For such conditions, epoxy adhesives should be evaluated as alternatives.
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Applications of acrylic structural adhesives
The emerging need for maximum toughness and flexibility for bonding applications, especially with the advent of composites in the transportation industry, provided an opportunity for polyurethane adhesives. They soon became much the same standard for bonding composites and treated metals in the transportation industry as epoxies were in the aerospace industry. Polyurethanes provide an excellent overall combination of load bearing strength and toughness over a broad temperature range, but are more limited in hot strength than the best epoxies and require significant surface strength to maximize adhesion and durability on metals. A key opportunity for methacrylate adhesives resulted when a number of process and performance improvements evolved in the general area of plastic bonding and later with bonding combinations of plastics and metals. Four examples, presented chronologically, highlight the evolution of improved and unique methacrylate technology in concert with the emerging needs for which they were developed: thermoplastic automobile bumpers, composite stringer bonding in the marine industry, composite and metal bonding in heavy and light commercial trucks, and highly demanding metal bonding applications for school buses.
6.4.1 Thermoplastic bumpers A large US automobile manufacturer had incorporated a two piece thermoplastic bumper in several high volume passenger car lines. The bumper consisted of a fascia component and a reinforcement that was mechanically attached to the automobile frame. The fascia and reinforcement, both injection molded with a polycarbonate/polyester alloy, were initially bonded with a highly elastic single component polyurethane adhesive that required a primer to bond the thermoplastic. Accelerated polyurethane adhesive systems with primers were selected because of their high elongation and flexibility. They enabled the bonded bumper assembly to pass a required low temperature impact test which simulated a 15 mile per hour barrier crash. In spite of the high level of performance provided by the polyurethane adhesive, the manufacturer began evaluating alternative adhesives to resolve two specific and limiting process issues for this very high volume application. First, elimination of the primer application and flash off process would save time and increase available manufacture space. Second, a faster curing adhesive would eliminate a significant holding time required for the polyurethane adhesive to cure in order to enable required in-process quality control impact testing. A unique two part, highly flexibilized methacrylate adhesive that was specifically designed to pass the required high and low temperature performance requirements of the application and also to be compatible with the solvent sensitivity of the plastic without compromising © Woodhead Publishing Limited, 2010
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adhesion achieved the goals of the manufacturer. An added benefit was the ability of the manufacturer to recycle scrap parts directly without needing to cut out the bonded area, which represented significant savings in time, labor and materials.
6.4.2 Boat stringer bonding A large manufacturer of high performance boats had decided to replace the traditional wooden stringer grid approach for hull reinforcement with molded stringer assemblies that could be bonded to the hull with an adhesive. The driving forces were: ∑ ∑ ∑ ∑ ∑
to eliminate the time and labor required to produce wooden stringers in the pattern shop; to eliminate manual lamination of the stringer grid in the hull with fiberglass and polyester resin – a significant reduction in time, labor and worker exposure to styrene monomer; to eliminate wood to provide environmental benefits and eliminate problems from wood rot as the boat aged and moisture penetrated the stringer grid; to increase overall production space and increase unit throughput as a result of reduced assembly and curing time; to increase performance and ride of the boat.
The manufacturer had initially chosen a two part polyurethane adhesive for this application because of the high performance demands, especially the ability to withstand the extreme flexing and impact loads imposed on a bonded hull for a high speed boat in choppy water. However, one aspect of the application process caused the manufacturer to evaluate a methacrylate adhesive. Boat hulls and stringers are typically fabricated by the ‘open molded’ laminating process wherein unsaturated polyester resin is either applied with successive layers of woven fiberglass or sprayed together with chopped glass fibers in a hull mold cavity until the desired laminate thickness is attained. When the resin cures, the molded hull is removed from the mold and moved to the stringer assembly process area. The open or ‘raw’ surface to which the stringer is to be bonded can be difficult to bond using most adhesives unless the surface is abraded to remove the shiny resin surface which may contain incompletely cured resin and other components, such as barrier waxes, that reduce styrene emissions and reduce air inhibition of cure. This is especially true with epoxy and polyurethane adhesives. However, properly formulated methacrylate adhesives bond intimately to the unprepared open molded surface as a result of the solvating effect referred to earlier, as well as having inherent reactive compatibility between incompletely cured polyester resin and the polymerizing methacrylate
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adhesive. Moreover, the ability of the methacrylates to have long open working time followed by the desired ‘snap cure’ can greatly expedite the stringer assembly process. Because of this, methacrylate adhesives that are formulated to have high tensile elongation and resistance to severe impact have experienced rapid growth in this market. In this case, the unique combination of processing and bonding characteristics of the methacrylates have virtually enabled a new manufacturing process.
6.4.3 Heavy truck assembly Heavy trucks, more correctly named class 8 trucks, are the ‘tractors’ of large tractor-trailer trucks. Over the years, the preferred material for fabricating the cab portion has evolved from sheet metal to various composite materials, including SMC, resin transfer molding (RTM) and open molded fiberglass. Polyurethane adhesives became the bonding product of choice early in this transition, but as the transition proceeded, evolutionary changes challenged the capabilities of the polyurethane adhesive products to the point where alternatives were sought. In SMC automotive component assembly, as well as in heavy truck assembly, heated fixtures are used to accelerate the cure of the Polyurethane (PUR) adhesive and to improve the quality of the bond. This permits a relatively long open working time allowing for application of the large amount of adhesive bead length required for the large truck components, with a relatively rapid cure cycle from the heat input from the bonding fixtures. This approach is acceptable for large production runs which can justify the cost of the heated bonding fixtures. However, for smaller production runs and when dissimilar materials that include open molded parts, metal brackets and other attached parts are included in the assembly process, there is a need for adhesives with more versatility in bonding capability that do not require heated fixtures. To a limited extent, rapid curing polyurethane adhesives can be used, but in very hot and humid manufacturing environments, air conditioned assembly areas are required to prevent premature gelling and skinning of the PUR from the combined effects of heat and atmospheric moisture. Once again, the unique combination of problem-solving benefits provided by the methacrylates have opened this market for them and most class 8 manufacturing plants now employ a combination of PUR and methacrylate adhesives in their assembly operations (Illinois Tool Works (ITW), 2008).
6.4.4 School bus assembly A large manufacturer of school buses embarked on an ambitious program to replace rivets, welds and mechanical fasteners in the assembly of school buses. The driving force for this drastic change in the assembly process was
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to simplify the overall assembly process and improve the appearance of the finished bus with totally smooth exterior panels. Once again, there was a very challenging list of requirements for this application: ∑ ∑ ∑ ∑
Bond a variety of metallic surfaces including steel, aluminum and corrosion resistant alloys with minimum surface preparation Bond the surface of metals overcoated by a variety of corrosion-resistant organic coatings without damaging the coating or its bond to the metal surface Resist peeling and impact forces in potential crash situations Maintain performance after exposure to temperatures ranging from –40°C to +107°C. Methacrylate adhesives proved to be the only products that were capable of fulfilling all of these requirements and thus were selected for this application (Thomas Built Buses, 2005).
6.4.5 Thick gap bonding for marine and wind blade applications As methacrylate adhesives have gained in popularity, their advantages in terms of their ability to bond composite structures with highly impact resistant and flexible bonds have become more widely recognized. As a result, they have been evaluated for applications that have required additional improvements in application properties. For example, when very large composite components are fabricated and bonded, additional demands related to larger bond gaps and open working time can become a factor. As part size increases, bonding gaps generally increase because of the nature of the molding process and more time is required to apply the beads of adhesive to the large structures. Two specific applications that illustrate this are large marine craft and windmill blades. As the boat stringer bonding application noted above is used to assemble larger and larger boats, bond gaps of 0.025 m or greater can be encountered (Gosiewski et al., 2002).
6.4.6 Bonding low energy surfaces One of the most significant bonding challenges facing the adhesives industry has been the ability to bond low energy surfaces, particularly polyolefins, without the extensive and often prohibitive surface preparation required for conventional structural adhesives. Beginning in the 1990s, methacrylate adhesive formulators began to develop and commercialize products based on organoborane chemistry that had evolved from academic research. Once again, the methacrylate adhesive platform proved to be uniquely suited to exploiting this technology. Organoborane chemistry provides catalysts that initially create bondable sites on the surfaces of the low energy substrates. These systems are discussed thoroughly in Chapter 9. © Woodhead Publishing Limited, 2010
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Manufacturers
Structural acrylic adhesive manufacturers are fairly widespread and fragmented. So, it is extremely difficult to discuss all or even most of the different manufacturers in a single chapter. Therefore, the focus of this section will be to highlight a few of the major manufacturers. The order of the manufacturers is not a reflection of their size. One of the largest suppliers of structural acrylic adhesives is ITW under the trade name of Plexus. They supply primarily methacrylate-based acrylic adhesives to the construction, industrial, marine and automotive markets. IPS is another large manufacturer and supplier of structural acrylic adhesive systems based on methyl methacrylate (MMA). Their application focus is primarily industrial and marine markets. Lord Corporation is also a large manufacturer and supplier of structural acrylic adhesives which are methacrylate based. Their main focus is supplying the automotive original equipment manufacturer (OEM) and repair markets. Their methacrylate adhesive system exhibits good bonding to all types of metals, which include hot-dip galvanized, electro-galvanized and cold roll steels, and bare aluminum. Henkel AG & Co KGaA is another major manufacturer of methyl methacrylate (MMA)-based and non-MMA-based structural acrylic adhesives. Their penetration into the market place has been accomplished through three key acquisitions, Loctite Corporation, Dexter Corporation and, most recently, the adhesive division of National Starch and Chemical. The commercial focus of their products is in the areas of automotive OEM and aftermarket, electronics, home and office, do-it-yourself, craftsmen and construction and consumer markets. The uniqueness of Henkel’s product line is that they manufacture and supply structural acrylic, thread lockers and anaerobic adhesive systems.
6.6
Future trends
As noted earlier, prior to the advanced evolution of methacrylate structural adhesives, epoxy and polyurethane adhesives dominated the structural adhesive market. Given this dominance, the newer methacrylates have made remarkable strides in gaining market share in spite of the inherent limitations noted earlier, particularly in the case of adhesives based on MMA monomer. This gain in market share is expected to continue in high volume, cost-sensitive applications with high performance demands. In such applications, the inherent advantages of cost effectiveness and performance of MMA-based methacrylate adhesives continue to make them competitive with epoxies and polyurethanes. Similarly, in low volume cartridge applications that involve limited exposure to MMA vapor, the products can be easily integrated into plant assembly processes provided that good local ventilation is employed.
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The difficulty lies in middle volume or new applications, in manufacturing operations involving exposure of untrained or changing work forces, or in other situations with limited environmental controls. In such situations, the odor of the MMA monomer can give rise to concerns from sensitive work forces even though the actual health impact of exposure to levels of monomer below the threshold limit value (TLV) in air is minimal.
6.7
References
Achary, P S, Joseph, D and Ramaswamy, R (1991), ‘Study on a vinyl ester/methyl methacrylate based reactive acrylic adhesive toughened by hydroxyl terminated polybutadiene’, J. Adhesion, 34, 121–36. Bachmann, A G (1996), ‘Advances in acrylic-adhesive technology’, Adhesives & Sealants Industry, 36. Bianchi, N, Garbassi, F, Pucciariello, R and Apicella, A (1991), ‘Compositional influence on toughness of structural acrylic adhesives’, J Mater Sci, 26, 434–40. Briggs, Jr., P C and Muschiagtti, L C (1975), Novel Adhesive Compositions, US Patent Office, 3,890,407. Damico, D J (1990), ‘Acrylics’, Engineered Materials Handbook, Volume 3, ASTM International, 119–25. Dawdy, T H (1984), Epoxy Modified Structural Adhesives Having Improved Heat Resistance, US Patent Office, 4,467,071. Friese, C and Bergmann, F (2000), Aerobically Curable Adhesive, US Patent Office, 6,096,842. Gosiewski, D, Loven, W E, Leeser, D L and Lambert, K A (2002), Structural Adhesive, US Patent Office, 6,462,126. Illinois Tool Works (ITW), 2008. Isobe, I (1990), Adhesive Composition, US Patent Office, 4,898,899. Lord Corporation (2005), VERSOLOK® Adhesive, Technical Data and Material Data Sheets. Melody D P, Grant S M and Martin F R (1984), Two-part Composition with Activator Enriched with Dihdropyrdine Ingredients, US Patent Office, 4,430,480. Muggee, J M and Zilley, E L (1990), Low Odor Adhesive Compositions and Bonding Method Employing Same, US Patent Office, 4,945,006. Odian, G (1991), Principles of Polymerization, John Wiley & Sons, New York. Okui, K and Shiokawa, M (2001), Method for Producing a Bonded Structure of Aluminum Alloy Pressed Plate, US Patent Office, 6,176,965. Owston, W J (1973), Fast Curing Polychloroprene Acrylic Adhesive, US Patent Office, 3,725,504. Pelosi, L F (1980), Reactive Fluid Adhesive Compositions, US Patent Office, 4,226,954. Thomas Built Buses (2005) Technical Data Sheet. Toback, A S (1971), Process for Bonding with Acrylate Polymerized by a Peroxy and a Condensation Product of Aldehyde and Primary or Secondary Amine, US Patent Office, 3,616,040. Zalucha, D J, Sexsmith, F H, Hornaman, E C and Dawdy, T H (1980), Structural Adhesive Formulations, US Patent Office, 4,223,115.
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P. C. Briggs, IPS Corporation, USA; and G. L. Jialanella, The Dow Chemical Company, USA
Abstract: Acrylic adhesives as defined in this text are based on acrylate and methacrylate monomers and have been commercially used for more than 50 years. These products are supplied as two separate components that can be mixed prior to application or each component can be applied to separate surfaces. Traditionally, methacrylates are preferred over acrylates owing primarily to the odor of the acrylates. The most popular and most commercially successful structural acrylic adhesives in use today are polymerizable mixtures of polymers dispersed or dissolved in methyl methacrylate (MMA) monomer. These adhesive products are supplied as two separate components that are primarily mixed just prior to application. One component contains a peroxide compound (oxidizing agent) and the second component contains an amine or metal salt (reducing agent) that reacts with the peroxide component upon mixing to initiate the free-radical polymerization of the methyl methacrylate monomer. This chapter will review the historical evolution and cure systems of methacrylate adhesive systems. Also, it will review the first and second generation and advanced technology products as well as formulation variables, bondline properties and applications of methacrylate-based adhesive systems. Key words: structural acrylic adhesives, acrylates, methacrylate, peroxide compound, oxidizing agent, tetrahydrofurfuryl methacrylate (THFMA), hydroxyethyl methacrylate (HEMA) hydroxypropyl methacrylate (HPMA).
6.1
Introduction
Acrylic adhesives as defined in this text are based on acrylate and methacrylate monomers and have been commercially used for more than 50 years. These products are supplied as two separate components that can be mixed prior to application or each component can be applied to separate surfaces. Traditionally, methacrylates are preferred over acrylates primarily because of the odor of the acrylates. This chapter will review the historical evolution and cure systems of methacrylate adhesive systems. Also, this chapter will review the first and second generation and advanced technology products as well as formulation variables, bondline properties and applications of methacrylate based adhesive systems.
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6.1.1 Historical evolution The most popular and most commercially successful structural acrylic adhesives in use today are polymerizable mixtures of polymers dispersed or dissolved in methyl methacrylate (MMA) monomer. As mentioned previously, these adhesive products are supplied as two separate components that are primarily mixed just prior to application. One component contains a peroxide compound (oxidizing agent) and the second component contains an amine or metal salt (reducing agent) that reacts with the peroxide component upon mixing to initiate the free radical polymerization of the methyl methacrylate monomer. Among the earliest examples of this type of adhesive were clear, colorless mil spec adhesives that first appeared in the 1950s as bonding agents for poly (MMA) sheet in applications such as aircraft canopies. In this most basic form, the polymerizable adhesive component consisted of poly (MMA) dissolved in MMA monomer with N,N-dimethyl aniline or a derivative as the amine component. A benzoyl peroxide initiator was supplied as either a powder or liquid solution. In the 1960s, formulators began to use elastomers to supplement or replace the poly-MMA to provide toughness and provide improved bondability for a wider variety of substrates including metals, thermoplastics and thermosets (Owston, 1973). Functional monomers such as methacrylic acid and certain other additives were included for specific performance or application benefits. Today’s high performance structural acrylic adhesives are the result of extensive evolutionary development of these basic systems by a number of companies who have tailored these products for increasingly demanding and sophisticated applications. The MMA-based structural acrylics are distinctly different from other polymerizable acrylic adhesive technologies such as anaerobics, which are primarily used for narrow gap metal bonding, retaining and threadlocking, and the cyanoacrylates, which cure by an anionic mechanism. The latter are generally supplied as single component products that are catalyzed by metallic surfaces in the case of anaerobics and surface moisture in the case of cyanoacrylates. An accelerator is sometimes used to speed the cure on inactive surfaces. In addition to these reactive acrylic technologies, acrylic polymers are also found in solvent, emulsion and hot melt adhesives for a wide variety of applications. These adhesive classes have evolved concurrently over the past few decades, but this chapter will be limited to the evolution and development of the polymer-in-monomer-based structural acrylics.
6.1.2 Cure systems In the first 20 years of their development, the cure system of choice for structural acrylics most often consisted of benzoyl peroxide in combination
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with a tertiary aromatic amine. In most cases, the amine compounds were included in the polymer-in-monomer portion, typically referred to as the adhesive component and the benzoyl peroxide was supplied as an activator or accelerator. The benzoyl peroxide was typically supplied as a paste in a plasticizer to facilitate mixing with the adhesive. Typical commercial formulations provide a working time of a few minutes, followed by rapid free radical exothermic polymerization that results in the very fast buildup of bond strength that characterizes these products. This fast cure differentiated these products from the epoxies that were gaining in popularity as well, but which take much longer to set and cure because of their kinetics of polymerization which provides a more gradual, linear buildup of strength. A convenient option for some applications involves applying the activator as a thin film from solution to one or both of the substrates to be bonded. This technique has limited utility because it is most effective in relatively thin bond gaps. Thick gaps require activation of both bonding surfaces and two-part mix-in application is generally more effective. In the 1970s duPont introduced a new cure system for polymerizable methacrylate adhesives (Briggs, Jr. and Muschiatti, 1975). The adhesive composition comprised a solution of chlorosulfonated polyethylene, sold by duPont under the trade name Hypalon®, in a mixture of monomers and additives similar to the earlier methacrylates (Equation 6.1). H2 C
H2 C C H2
H2 C CH
X
Y
Cl
CH
Z
[6.1]
SO2Cl
In the earliest embodiments, an aldehyde–amine reaction product was used as a surface activator to initiate the cure of the adhesive base when the substrates were mated. Among the many combinations of amine–aldehyde derivatives that are possible, the most effective were condensation products of butyraldehyde and aniline or butyl amine. Over the years, the butyraldehyde– aniline products have proved to be the most effective. The active ingredient is N-phenyl-3,5-diethyl-2,3-dihydropyridine, referred to as DHP or PDHP (Equation 6.2). C3H7
C2H5
[6.2]
N C2H5
The original products were available as approximately 40% active unrefined condensation products. More recently, purified products containing up to 80–90% of the desired PDHP have become available (Melody et al., 1984). © Woodhead Publishing Limited, 2010
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6.1.3 First and second generation DuPont introduced the term ‘second generation acrylic’ or SGA to differentiate these products from the earlier products based on benzoyl peroxide and tertiary amines. The latter became referred to as ‘first generation’ products. Primary benefits of the SGAs were increased toughness and impact strength of metal to metal bonds, as well as the ability to bond metal surfaces, even oily metal surfaces, with little or no surface preparation. The products were also shown to be capable of effective performance as ‘100% solids’ alternatives to solvent cements in applications such as plastic pipe bonding and decorative lamination of vinyl and high pressure laminates to metals and particle board. In this evolutionary stage during the 1970s and 1980s, the SGA products were promoted concurrently with development of improved waterborne systems to address looming threats against the use of flammable and toxic solvents. However, the two part nature of these products as well as other limitations prevented the SGAs from effectively competing with the simpler one part systems. Rather, the reactive methacrylate products have found a secure niche in a number of product assembly processes for which they provide unique benefits.
6.1.4 Advanced technologies Several companies have approached these industries/markets in different ways as the products have evolved over the past two decades and this chapter will concentrate on that evolution to the present day status of these uniquely attractive products. The basic chemistry, along with the variety of basic cure systems employed in reactive acrylic adhesives, including those comprising the so-called first and second generation products, are discussed in great detail in a review article by Damico (1990). The review also includes a thorough overview of improvements that evolved during the 1980s in the durability and heat resistance of reactive acrylic adhesives, especially with respect to bonding metals. This chapter will focus on developments and improvements that have occurred over the past two decades in this field which have greatly broadened the popularity of acrylic adhesive products, especially in bonding thermoplastics, composites and combinations of these materials to metals. Along with improved substrate bonding capability and mechanical properties, significant improvements have been made in the application and handling characteristics of these products. As a result, the new reactive acrylics compete equally or better than epoxies and polyurethanes as the adhesives of choice for any challenging bonding application. The earlier ‘first generation’ acrylics and the subsequent ‘second generation’ acrylics had certain limitations, especially in metal bonding capability owing to limitations in resistance to elevated temperature exposure and subsequent
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resistance to harsh or corrosive environments. As reviewed by Damico (1990), the primary improvements in adhesives based on MMA monomer in the 1980s involved improvements in the ability of these products to bond as-received metals, especially aluminum steel and stainless steel, with little or no surface preparation. The primary enabling factor was the incorporation of phosphoric acid derivatives of methacrylate monomers which chemically interact with metal oxide surfaces to strengthen the normally weak interfacial layer between the adhesive and the base metal and to protect it from corrosive attack under harsh environmental conditions (Zalucha et al., 1980). Additional improvements in durability and resistance to elevated temperatures were obtained through the incorporation of epoxy resins in the compositions (Dawdy, 1984). Those containing relatively high levels of epoxy resin were considered to be hybrids of methacrylate and epoxy technology. The products were capable of withstanding repeated exposures to temperatures of 204°C (400°F) such as those encountered in paint baking ovens in the automotive industry. In spite of the significant improvements in performance in niche metal fabrication and medical and electronic assembly, the use of reactive acrylates and methacrylates was still limited in the late 1980s. Damico estimates that the total annual volume used at the beginning of the 1990s was less than US$10 million globally. In the 1990s and later, challenging new bonding applications involving the use of plastics in the transportation and marine markets provided the impetus for additional opportunity for commercial development of further improved products that would finally spur the growth of these products in high volume applications. The following advances in capability have occurred sequentially from the late 1980s to the present, proving methacrylate technology to be uniquely suitable for the most challenging bonding applications: ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑
improvements in low temperature toughness and flexibility with retention of hot strength reduction of odor during application reduction and elimination of surface tackiness from air inhibition control of aggressive solvation of sensitive plastics regrind compatibility with thermoplastics improved bondability of composites extended open working time for application on large assemblies control of exotherm for reduced outgassing in thick bond crosssections reduced print-through on show surfaces ability to bond low energy surfaces, including polyolefins.
These improvements can be best incorporated in a discussion of the various classifications of adhesives in use today.
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Classification of acrylic structural adhesives
6.2.1 Elastomer modified methyl methacrylate-based adhesives The early MMA-based adhesives, including first and second generation products, as well as most of the improved products reviewed by Damico (1990), generally employ a single elastomeric component as the primary toughening additive. Soluble thermoplastic additives such as poly-MMA or polystyrene are occasionally added to impart certain adhesive properties, but the elastomer is the dominant factor affecting adhesive rheology, bond characteristics and adhesion with single polymer modification. The single elastomer approach generally imposes the following limitations on adhesive characteristics. ∑ ∑ ∑ ∑
limited substrate compatibility, requiring multiple adhesives for different substrates; ‘stringy’ viscosity and Newtonian rheology characteristic of elastomer solutions; inorganic fillers required for improved rheology can negatively affect properties; lower Tg of elastomer may reduce hot strength of adhesives.
The glass transition temperature, Tg, of unmodified poly(MMA) system is 100°C and the presence of the elastomer can have a significant impact on the Tg of the cured adhesive depending on the resin/elastomer compatibility. As expected the Tg of the cured poly(MMA) system is reduced when there is good compatibility between the poly(MMA)/elastomer, but is not affected when the elastomer forms discrete particles owing to the lack of compatibility. Achary et al. (1991) and Bianchi et al. (1991) have studied the affects of rubber modification of acrylic adhesives on the Tg and adhesive strength properties and related these effects to bulk morphology. Achary et al. (1991) studied the effect of hydroxyl terminated polybutadiene (HTPB) in a vinyl ester/methyl methacrylate base acrylic adhesive on the Tg of the cured system using a differential scanning calorimeter (DSC). They showed two distinct Tg values for the fully cured adhesive even though the HTPB was soluble in the monomer system. This finding indicates that phase separation of the HTPB (rubber) phase occurred during the curing cycle. They also showed that the cure profile and amount of HTPB affects the Tg of the cured acrylic adhesive. Room temperature cure showed a single depressed Tg (120°C versus 138°C for the unmodified vinyl ester/methyl methacrylate system) with some residual exotherm on the second heating. The heat cured adhesives exhibited two distinct Tg values, the main phase Tg being similar to the unmodified system. They also showed that higher HTPB content (30% w/w) slightly depressed © Woodhead Publishing Limited, 2010
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the main phase Tg to approximately 134°C, but intermediate levels of 15% w/w did not depress the main phase Tg. Lap shear and impact peel strengths were at maximum values when the HTPB levels were in the range 10–15% w/w and decreased with further increasing levels of HTPB. Bianchi et al. (1991) also found that the bulk morphology in terms of resin/ rubber compatibility of rubber modified acrylic adhesives significantly affects the Tg as well as the strength properties. They found that the morphological features and resulting properties of the acrylic adhesive were dependent on rubber type was well. They found that better phase separation and improved strength properties were observed for butadiene–acrylonitrile rubbers and fluorinated rubbers compared to chlorosulfonated polyethylene.
6.2.2 Non-methyl methacrylate adhesives When the SGA adhesives were introduced in the 1970s, the anticipation generated by their improvements caused increasing interest on the part of a number of formulating companies. This situation increased exposure and notoriety also exposed more potential users to the odor of methyl methacrylate monomer. Negative feedback regarding the odor problem, along with the flammability associated with the low molecular weight monomer, prompted additional development work directed at reducing volatility and related problems associated with it (Bachmann, 1996). Commercial formulations have been introduced to the market, but their higher cost generally limits their use to specialized lower volume and high value applications. The most commonly employed alternatives to MMA are tetrahydrofurfuryl methacrylate (THFMA), hydroxyethyl methacrylate (HEMA) and hydroxypropyl methacrylate (HPMA). THFMA has been used in conjunction with the phosphate and epoxy improvements noted above to make hybrid metal bonding products that are more readily accepted in the automotive assembly environment, where odor and flammability are decided deterrents to their use (Lord Corporation, 2005). This is one of the larger volume applications for the low volatility monomers. HEMA and HPMA have been used to prepare two part reactive and one part ultraviolet and visible light curable adhesives that can be used in the relatively clean environments used to assemble electronic and medical components (Friese and Bergmann, 2000). Other products of the ‘anaerobic’ type based on di- and polyfunctional acrylate and methacrylate monomers found limited use in high value metal and electronic assembly as an outgrowth of their use in threadlocking applications (Toback, 1971). Because the higher methacrylates are generally produced by transesterification of methyl methacrylate with higher molecular weight alcohols, they are significantly more expensive than MMA. Many of the higher methacrylates are usually associated with lower odor than MMA and, owing to the lower
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volatility, there is an increased tendency to incompletely cure at the surface because of air inhibition. In some cases, lingering odor can be a significant problem. This is especially true in the case of butyl methacrylate and 2-ethyl hexyl methacrylate, two of the more commonly available higher methacrylates. Another issue with adhesives, formulated with higher molecular weight monomers based on alcohols with C4 and higher alkyl groups, is a reduced capability to bond polar substrates such as metals. Among the more useful higher methacrylates are hydroxyl alkyl methacrylates such as hydroxyethyl and hydroxypropyl methacrylates (HEMA and HPMA) and tetrahydrofurfuryl methacrylate (THFMA). The influence of the hydroxyl group can impose limitations on polymer solubility characteristics and adhesive substrate compatibility, but some very commercially successful adhesives are available based on these materials. THFMA is probably the most versatile of the higher molecular weight alternatives to MMA but it is also quite expensive. Along with the higher cost and other limitations associated with the higher methacrylates, another performance limitation associated with them relates to the fact that the polymers derived from the higher methacrylates generally have lower glass transition temperatures than poly-MMA. While this can be a benefit in improving the toughness and flexibility of the cured adhesive, it generally reduces the high temperature capabilities of the products. This can be somewhat offset by increasing the cross-link density of the adhesive with difunctional and polyfunctional methacrylates, but this technique is limited by other compromises imposed by these monomers (Pelosi, 1980). The simplest members of this group are the ethylene and polyethylene glycol dimethacrylates which, along with proliferation monomers of this type (Isobe, 1990), were originally used to formulate anaerobic adhesives for threadlocking. These highly cross-linked materials are generally very rigid and resistant to the effects of heat and chemicals. By adding small amounts, usually less than 5%, of these di- or poly-methacrylate monomers to any of the structural methacrylate adhesives, the formulator can impart these properties to the cured composition. Increasing amounts of crosslinking monomer imparts increasing rigidity and can eventually lead to embrittlement, thus negating a primary advantage of this class of products, which is toughness. The cross-linkers also generally increase the cure speed of the adhesive and can negatively impact shelf life and adhesion. Numerous di- and polyfunctional acrylate and methacrylate monomers have become commercially available over the years and provide the formulator with many options for modifying and tailoring structural adhesive properties. In summary, unless the odor and flammability of MMA cannot be tolerated, the incentive to formulate high performance structural adhesives for typical high volume product assembly operations with higher methacrylates is very limited. Adhesive products utilizing higher molecular weight monomers do
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have a place in specialized applications for which they are very successful, but MMA remains the monomer of choice for the highest performance and value.
6.2.3 Polymer blends as modifiers in methyl methacrylate-based adhesives Certain polymer blends enable the formulator to take advantage of separate contributing factors derived from each individual polymer to develop combinations of properties that are not attainable from a single polymer. For example, one elastomer may provide excellent low temperature flexibility and toughness, but adhesion to specific substrates may be compromised. A second polymer can provide better wetting or compatibility with the desired bonding surfaces. The combination of available polymers, along with the ability to modify further formulations with different comonomers along with MMA, as well as the many other additives, is what makes the polymer-inmonomer technology so versatile with respect to tailoring adhesives for specific applications. The low viscosity and high solvency of MMA and the numerous comonomers that can be blended with it allow the ready incorporation of high molecular weight polymers as tougheners. Polymeric tougheners include typical elastomers used in dry polymer applications, some of which, such as polychloroprene, acrylonitrile and styrene butadiene polymers, are available in solution grades for adhesives and coatings. Other polymeric additives include soluble thermoplastic polymers and core shell impact modifiers that disperse and swell in the monomers, but do not fully dissolve (Muggee and Zilley, 1990).
6.3
Advantages and disadvantages and unique characteristics of acrylic structural adhesives
As noted above, one of the great advantages of the reactive acrylic or methacrylate adhesives is their formulation versatility. Prior to discussing the advantages and disadvantages and unique characteristics of the group, it will be helpful to review additional formulation options and the characteristics they impart.
6.3.1 Formulation variables All of the variables reviewed above provide an obviously infinite combination of adhesive properties for the engineer or production manager. The development of specific formulations to address all of the product characteristics and performance variables is highly proprietary to the formulating companies © Woodhead Publishing Limited, 2010
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that provide these products. A list of the generic components available to the formulator, along with the positive and negative impacts on handling and performance characteristics are presented below in Table 6.1 to illustrate the complexity involved in this technology. Specific examples of these formulas can be found in an article by Damico (1990). The list of potential formulating variables and related effects is much more complex than commonly available or useful for formulating epoxies Table 6.1 Potential formulating variables of acrylic and methacrylate adhesives Type Description Positive effects
Potential negative effects
Primary monomer MMA Polymer solvent Odor and Substrate solvation flammability High Tg May harm sensitive substrates Rigid homopolymer Secondary Higher MW Reduced volatility Increased cost monomer monomer or Increased flexibility Lower Tg MMA substitute Air inhibition Primary polymer Elastomer Toughness and Viscosity and component flexibility ‘stringyness’ Heat resistance Secondary Elastomer or Improved adhesion Increasing viscosity polymer thermoplastic Increased Tg Reduced toughnes Acidic monomer Functional Metal adhesion Corrosive monomer Cure behavior, reduced shelf life Cross-linking Di functional or Heat and solvent Cure behavior, monomer higher functional resistance reduced shelf life monomer Reduced toughness Plasticizer Plastic and rubber Toughness and Heat resistance additives flexibility Reduced adhesion Filler Plastic and rubber Cost reduction Reduced toughness additives Reduced shrinkage Effects on adhesion Thixotrope Fumed silica, Reduced ‘stringyness’ Same as fillers others Peroxide initiator Acyl, dialkyl Speed of cure Reduced shelf life or hydroperoxide Amine promoter Tertiary aromatic Speed of cure Discoloration of or activator amine or derivative cured adhesive Metallic additive Organometallics Speed of cure Reduced shelf life Boron additives Organoboranes Low energy surface Cost bondability Stabilizer or Phenolics Shelf life Reduced cure speed inhibitor Increased working Air inhibition time Pigments and Plastic and Mixing indicator May affect shelf life colorants rubber additives Color matching Cost
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and polyurethanes. However, this list illustrates the many options to the formulator and end user with this class of products.
6.3.2 Advantages of structural methacrylates Cure profile
Degree of cure or polymerization
One of the greatest advantages that the methacrylates exhibit relative to epoxies and polyurethanes is their cure profile and the ability to adjust the cure profile to provide a long open working time with the much desired ‘snap cure’ once parts are assembled. These characteristics are a result of the cure kinetics illustrated in Fig. 6.1 (Odian, 1991; Damico, 1990). Methacrylate adhesive systems cure via a free radical mechanism rather that the linear addition mechanism that characterizes the epoxies and polyurethanes. Epoxy and urethane adhesives (condensation polymers) exhibit a gradual cure with time. Acrylics (free radical addition polymers) on the other hand, exhibit a rapid cure after a given induction (delay) time. The unique aspect is that the delay time can be independently adjusted without compromising rapid curing. For example, the delay times can be quite large (1–3 hours) and exhibit a cure time of under 12 hours. The strong solvency of MMA provides two significant advantages with respect to substrate bondability. First, it causes the adhesive to solvate the surface of many plastic substrates prior to completion of the curing process. The primary exceptions are polyolefins, fluorinated polymers, crystalline plastics such as polyamides and polyacetals, and highly cross-linked and post-cured thermosets such as epoxies and some polyesters and gel coats. Fortunately, the inherent inability of MMA to solvate certain plastics can be overcome with additive approaches that have been developed over the past
Polycondensation
Free radical or addition polymerization
Time (arbitrary)
6.1 Cure profile comparison of a free-radical cure adhesive with a polycondensation adhesive.
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few years. As a last resort, typical surface preparation techniques that are used for epoxies and polyurethanes can also be used to improve the adhesion of methacrylates. In addition to solvation of plastics, the solvency of MMA also reduces the sensitivity of adhesion and bondability to contaminants, particularly organic materials, on substrate surfaces. The MMA, and to some extent the polymers and additives in the methacrylates, dissolve and absorb many organic surface contaminants such as fingerprints, processing oils and release agents that normally interfere with bonding that uses epoxies and polyurethanes. The breadth of substrate surface compatibility by the methacrylates is much greater than that of the epoxies and polyurethanes. Epoxies are most generally associated with bonding of metals and other inorganic materials, where the high polarity and hydrogen bonding capability of the resin and hardener components provide compatibility. Polyurethanes have been most successful in applications where increased flexibility is required, mostly with thermoplastics and thermosets such as the sheet molding compound (SMC) which is widely used in the transportation industry. The versatility of methacrylates to bond all of these materials has allowed them to capture an increasing market share in a variety of applications that can benefit from their superior processing and bonding characteristics. Bondline properties Another unique advantage of the methacrylate class of products is the ability to formulate them with a broad range of bondline modulus characteristics which, in many cases, provide specific application or process advantages over epoxies. With specific exceptions for specific applications, epoxy adhesives have historically been considered to have the highest load bearing capability of the three classes of adhesives. This is especially true of the heat cured products of the type used to bond metal aircraft structures. When these products are formulated to impart increased peel and impact strength, great care is taken to preserve as much of the inherent load bearing capability as possible while reducing the brittle or glassy characteristics of the highly cross-linked epoxy matrix. The most capable of these products have lap shear strengths of 35–40 MPa or higher, with peel strengths in excess of 10.5–12.5 N mm–1, and are capable of maintaining a high degree of their strength over the temperature range of –55°C to 121°C required by the aerospace industry. These properties can be achieved by very careful formulation with proprietary toughening agents, mixed in precise quantities or provided as carefully formulated prepregs, coupled with high temperature curing which provides a near ideal (necessary) polymer morphology in the bondline. Epoxies that are formulated
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to be mixed, applied and cured under more forgiving process conditions, typically lose a good measure of the high modulus that provides the highest load bearing capability. When flexibilizers are added to maximize toughness and flexibility under these conditions, load bearing capability is compromised even more, especially at high temperatures. As a result, the methacrylates have become more competitive with epoxies where the compromises in performance required to achieve better application tolerance are a disadvantage for the epoxies. An exception is the relatively new development of adhesively bonded metal frames and structural components in the automotive industry. While methacrylates and epoxies might both be considered, the combination of the reputation and history of epoxies in metal aircraft applications coupled with recent improvements in the ease of application and performance of the epoxies has proved to be an advantage for the epoxies in these applications (Okui and Shiokawa, 2001).
6.3.3 Disadvantages of structural methacrylate adhesives In spite of all of the advantages of this class of products noted above, there are inherent drawbacks and limitations. First and foremost, the most cost effective monomer, which provides the highest performance products, is MMA, which, as noted above, is flammable and has a strong odor. In spite of its relatively low toxicity, misguided publicity stemming from the consumer cosmetic industry (finger nails) has cast a negative cloud over MMA. As also noted above, alternative monomers derived from MMA only add cost and reduce performance. This is the single most limiting fundamental drawback of the class. Another inherent disadvantage of the methacrylates is limited solvent resistance. The poly (MMA) matrix, as well as many of the additives used to modify the products, are soluble in polar and aromatic solvents such as ketones, toluene and solvent blends. However, they are reasonably resistant to petroleum lubricants and diesel fuel and are somewhat resistant to gasoline. Aqueous chemical resistance is generally good, although strong caustics, especially at high temperatures, can degrade performance. The high temperature performance of highly toughened methacrylate structural adhesives is limited by the Tg of MMA monomer, as well as the polymeric additives. Reducing elastomer toughener content and increasing cross-link density with multifunctional monomers can improve heat resistance as well as chemical resistance, but with a significant offset in physical properties. For such conditions, epoxy adhesives should be evaluated as alternatives.
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Applications of acrylic structural adhesives
The emerging need for maximum toughness and flexibility for bonding applications, especially with the advent of composites in the transportation industry, provided an opportunity for polyurethane adhesives. They soon became much the same standard for bonding composites and treated metals in the transportation industry as epoxies were in the aerospace industry. Polyurethanes provide an excellent overall combination of load bearing strength and toughness over a broad temperature range, but are more limited in hot strength than the best epoxies and require significant surface strength to maximize adhesion and durability on metals. A key opportunity for methacrylate adhesives resulted when a number of process and performance improvements evolved in the general area of plastic bonding and later with bonding combinations of plastics and metals. Four examples, presented chronologically, highlight the evolution of improved and unique methacrylate technology in concert with the emerging needs for which they were developed: thermoplastic automobile bumpers, composite stringer bonding in the marine industry, composite and metal bonding in heavy and light commercial trucks, and highly demanding metal bonding applications for school buses.
6.4.1 Thermoplastic bumpers A large US automobile manufacturer had incorporated a two piece thermoplastic bumper in several high volume passenger car lines. The bumper consisted of a fascia component and a reinforcement that was mechanically attached to the automobile frame. The fascia and reinforcement, both injection molded with a polycarbonate/polyester alloy, were initially bonded with a highly elastic single component polyurethane adhesive that required a primer to bond the thermoplastic. Accelerated polyurethane adhesive systems with primers were selected because of their high elongation and flexibility. They enabled the bonded bumper assembly to pass a required low temperature impact test which simulated a 15 mile per hour barrier crash. In spite of the high level of performance provided by the polyurethane adhesive, the manufacturer began evaluating alternative adhesives to resolve two specific and limiting process issues for this very high volume application. First, elimination of the primer application and flash off process would save time and increase available manufacture space. Second, a faster curing adhesive would eliminate a significant holding time required for the polyurethane adhesive to cure in order to enable required in-process quality control impact testing. A unique two part, highly flexibilized methacrylate adhesive that was specifically designed to pass the required high and low temperature performance requirements of the application and also to be compatible with the solvent sensitivity of the plastic without compromising © Woodhead Publishing Limited, 2010
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adhesion achieved the goals of the manufacturer. An added benefit was the ability of the manufacturer to recycle scrap parts directly without needing to cut out the bonded area, which represented significant savings in time, labor and materials.
6.4.2 Boat stringer bonding A large manufacturer of high performance boats had decided to replace the traditional wooden stringer grid approach for hull reinforcement with molded stringer assemblies that could be bonded to the hull with an adhesive. The driving forces were: ∑ ∑ ∑ ∑ ∑
to eliminate the time and labor required to produce wooden stringers in the pattern shop; to eliminate manual lamination of the stringer grid in the hull with fiberglass and polyester resin – a significant reduction in time, labor and worker exposure to styrene monomer; to eliminate wood to provide environmental benefits and eliminate problems from wood rot as the boat aged and moisture penetrated the stringer grid; to increase overall production space and increase unit throughput as a result of reduced assembly and curing time; to increase performance and ride of the boat.
The manufacturer had initially chosen a two part polyurethane adhesive for this application because of the high performance demands, especially the ability to withstand the extreme flexing and impact loads imposed on a bonded hull for a high speed boat in choppy water. However, one aspect of the application process caused the manufacturer to evaluate a methacrylate adhesive. Boat hulls and stringers are typically fabricated by the ‘open molded’ laminating process wherein unsaturated polyester resin is either applied with successive layers of woven fiberglass or sprayed together with chopped glass fibers in a hull mold cavity until the desired laminate thickness is attained. When the resin cures, the molded hull is removed from the mold and moved to the stringer assembly process area. The open or ‘raw’ surface to which the stringer is to be bonded can be difficult to bond using most adhesives unless the surface is abraded to remove the shiny resin surface which may contain incompletely cured resin and other components, such as barrier waxes, that reduce styrene emissions and reduce air inhibition of cure. This is especially true with epoxy and polyurethane adhesives. However, properly formulated methacrylate adhesives bond intimately to the unprepared open molded surface as a result of the solvating effect referred to earlier, as well as having inherent reactive compatibility between incompletely cured polyester resin and the polymerizing methacrylate
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adhesive. Moreover, the ability of the methacrylates to have long open working time followed by the desired ‘snap cure’ can greatly expedite the stringer assembly process. Because of this, methacrylate adhesives that are formulated to have high tensile elongation and resistance to severe impact have experienced rapid growth in this market. In this case, the unique combination of processing and bonding characteristics of the methacrylates have virtually enabled a new manufacturing process.
6.4.3 Heavy truck assembly Heavy trucks, more correctly named class 8 trucks, are the ‘tractors’ of large tractor-trailer trucks. Over the years, the preferred material for fabricating the cab portion has evolved from sheet metal to various composite materials, including SMC, resin transfer molding (RTM) and open molded fiberglass. Polyurethane adhesives became the bonding product of choice early in this transition, but as the transition proceeded, evolutionary changes challenged the capabilities of the polyurethane adhesive products to the point where alternatives were sought. In SMC automotive component assembly, as well as in heavy truck assembly, heated fixtures are used to accelerate the cure of the Polyurethane (PUR) adhesive and to improve the quality of the bond. This permits a relatively long open working time allowing for application of the large amount of adhesive bead length required for the large truck components, with a relatively rapid cure cycle from the heat input from the bonding fixtures. This approach is acceptable for large production runs which can justify the cost of the heated bonding fixtures. However, for smaller production runs and when dissimilar materials that include open molded parts, metal brackets and other attached parts are included in the assembly process, there is a need for adhesives with more versatility in bonding capability that do not require heated fixtures. To a limited extent, rapid curing polyurethane adhesives can be used, but in very hot and humid manufacturing environments, air conditioned assembly areas are required to prevent premature gelling and skinning of the PUR from the combined effects of heat and atmospheric moisture. Once again, the unique combination of problem-solving benefits provided by the methacrylates have opened this market for them and most class 8 manufacturing plants now employ a combination of PUR and methacrylate adhesives in their assembly operations (Illinois Tool Works (ITW), 2008).
6.4.4 School bus assembly A large manufacturer of school buses embarked on an ambitious program to replace rivets, welds and mechanical fasteners in the assembly of school buses. The driving force for this drastic change in the assembly process was
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to simplify the overall assembly process and improve the appearance of the finished bus with totally smooth exterior panels. Once again, there was a very challenging list of requirements for this application: ∑ ∑ ∑ ∑
Bond a variety of metallic surfaces including steel, aluminum and corrosion resistant alloys with minimum surface preparation Bond the surface of metals overcoated by a variety of corrosion-resistant organic coatings without damaging the coating or its bond to the metal surface Resist peeling and impact forces in potential crash situations Maintain performance after exposure to temperatures ranging from –40°C to +107°C. Methacrylate adhesives proved to be the only products that were capable of fulfilling all of these requirements and thus were selected for this application (Thomas Built Buses, 2005).
6.4.5 Thick gap bonding for marine and wind blade applications As methacrylate adhesives have gained in popularity, their advantages in terms of their ability to bond composite structures with highly impact resistant and flexible bonds have become more widely recognized. As a result, they have been evaluated for applications that have required additional improvements in application properties. For example, when very large composite components are fabricated and bonded, additional demands related to larger bond gaps and open working time can become a factor. As part size increases, bonding gaps generally increase because of the nature of the molding process and more time is required to apply the beads of adhesive to the large structures. Two specific applications that illustrate this are large marine craft and windmill blades. As the boat stringer bonding application noted above is used to assemble larger and larger boats, bond gaps of 0.025 m or greater can be encountered (Gosiewski et al., 2002).
6.4.6 Bonding low energy surfaces One of the most significant bonding challenges facing the adhesives industry has been the ability to bond low energy surfaces, particularly polyolefins, without the extensive and often prohibitive surface preparation required for conventional structural adhesives. Beginning in the 1990s, methacrylate adhesive formulators began to develop and commercialize products based on organoborane chemistry that had evolved from academic research. Once again, the methacrylate adhesive platform proved to be uniquely suited to exploiting this technology. Organoborane chemistry provides catalysts that initially create bondable sites on the surfaces of the low energy substrates. These systems are discussed thoroughly in Chapter 9. © Woodhead Publishing Limited, 2010
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Manufacturers
Structural acrylic adhesive manufacturers are fairly widespread and fragmented. So, it is extremely difficult to discuss all or even most of the different manufacturers in a single chapter. Therefore, the focus of this section will be to highlight a few of the major manufacturers. The order of the manufacturers is not a reflection of their size. One of the largest suppliers of structural acrylic adhesives is ITW under the trade name of Plexus. They supply primarily methacrylate-based acrylic adhesives to the construction, industrial, marine and automotive markets. IPS is another large manufacturer and supplier of structural acrylic adhesive systems based on methyl methacrylate (MMA). Their application focus is primarily industrial and marine markets. Lord Corporation is also a large manufacturer and supplier of structural acrylic adhesives which are methacrylate based. Their main focus is supplying the automotive original equipment manufacturer (OEM) and repair markets. Their methacrylate adhesive system exhibits good bonding to all types of metals, which include hot-dip galvanized, electro-galvanized and cold roll steels, and bare aluminum. Henkel AG & Co KGaA is another major manufacturer of methyl methacrylate (MMA)-based and non-MMA-based structural acrylic adhesives. Their penetration into the market place has been accomplished through three key acquisitions, Loctite Corporation, Dexter Corporation and, most recently, the adhesive division of National Starch and Chemical. The commercial focus of their products is in the areas of automotive OEM and aftermarket, electronics, home and office, do-it-yourself, craftsmen and construction and consumer markets. The uniqueness of Henkel’s product line is that they manufacture and supply structural acrylic, thread lockers and anaerobic adhesive systems.
6.6
Future trends
As noted earlier, prior to the advanced evolution of methacrylate structural adhesives, epoxy and polyurethane adhesives dominated the structural adhesive market. Given this dominance, the newer methacrylates have made remarkable strides in gaining market share in spite of the inherent limitations noted earlier, particularly in the case of adhesives based on MMA monomer. This gain in market share is expected to continue in high volume, cost-sensitive applications with high performance demands. In such applications, the inherent advantages of cost effectiveness and performance of MMA-based methacrylate adhesives continue to make them competitive with epoxies and polyurethanes. Similarly, in low volume cartridge applications that involve limited exposure to MMA vapor, the products can be easily integrated into plant assembly processes provided that good local ventilation is employed.
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The difficulty lies in middle volume or new applications, in manufacturing operations involving exposure of untrained or changing work forces, or in other situations with limited environmental controls. In such situations, the odor of the MMA monomer can give rise to concerns from sensitive work forces even though the actual health impact of exposure to levels of monomer below the threshold limit value (TLV) in air is minimal.
6.7
References
Achary, P S, Joseph, D and Ramaswamy, R (1991), ‘Study on a vinyl ester/methyl methacrylate based reactive acrylic adhesive toughened by hydroxyl terminated polybutadiene’, J. Adhesion, 34, 121–36. Bachmann, A G (1996), ‘Advances in acrylic-adhesive technology’, Adhesives & Sealants Industry, 36. Bianchi, N, Garbassi, F, Pucciariello, R and Apicella, A (1991), ‘Compositional influence on toughness of structural acrylic adhesives’, J Mater Sci, 26, 434–40. Briggs, Jr., P C and Muschiagtti, L C (1975), Novel Adhesive Compositions, US Patent Office, 3,890,407. Damico, D J (1990), ‘Acrylics’, Engineered Materials Handbook, Volume 3, ASTM International, 119–25. Dawdy, T H (1984), Epoxy Modified Structural Adhesives Having Improved Heat Resistance, US Patent Office, 4,467,071. Friese, C and Bergmann, F (2000), Aerobically Curable Adhesive, US Patent Office, 6,096,842. Gosiewski, D, Loven, W E, Leeser, D L and Lambert, K A (2002), Structural Adhesive, US Patent Office, 6,462,126. Illinois Tool Works (ITW), 2008. Isobe, I (1990), Adhesive Composition, US Patent Office, 4,898,899. Lord Corporation (2005), VERSOLOK® Adhesive, Technical Data and Material Data Sheets. Melody D P, Grant S M and Martin F R (1984), Two-part Composition with Activator Enriched with Dihdropyrdine Ingredients, US Patent Office, 4,430,480. Muggee, J M and Zilley, E L (1990), Low Odor Adhesive Compositions and Bonding Method Employing Same, US Patent Office, 4,945,006. Odian, G (1991), Principles of Polymerization, John Wiley & Sons, New York. Okui, K and Shiokawa, M (2001), Method for Producing a Bonded Structure of Aluminum Alloy Pressed Plate, US Patent Office, 6,176,965. Owston, W J (1973), Fast Curing Polychloroprene Acrylic Adhesive, US Patent Office, 3,725,504. Pelosi, L F (1980), Reactive Fluid Adhesive Compositions, US Patent Office, 4,226,954. Thomas Built Buses (2005) Technical Data Sheet. Toback, A S (1971), Process for Bonding with Acrylate Polymerized by a Peroxy and a Condensation Product of Aldehyde and Primary or Secondary Amine, US Patent Office, 3,616,040. Zalucha, D J, Sexsmith, F H, Hornaman, E C and Dawdy, T H (1980), Structural Adhesive Formulations, US Patent Office, 4,223,115.
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