Acrylic adhesives types

Adhesives. Acryhc emulsion and solution polymers form the basis of a variety of adhesive types. The principal use is in pressure-sensitive adhesives, where a film of a very low T (<—20 " C) acrylic polymer or copolymer is used on the adherent side of tapes, decals, and labels. Acrylics provide a good balance of tack and bond strength with exceptional color stabiUty and resistance to aging (201,202). AcryUcs also find use in numerous types of constmction adhesive formulations and as film-to-film laminating adhesives (qv). 

Acrylic Adhesives. Acryhc stmctural adhesives can be classified into three major types the surface-activated acryhcs (anaerobics), the surface-activated second-generation acryhcs, and the cyanoacrylates. 

Low molecular weight liquid nitrile rubbers with vinyl, carboxyl or mercaptan reactive end groups have been used with acrylic adhesives, epoxide resins and polyesters. Japanese workers have produced interesting butadiene-acrylonitrile alternating copolymers using Ziegler-Natta-type catalysts that are capable of some degree of ciystallisation. 

Cyanoacrylates and Light-Curing Acrylic Adhesives - These types of adhesives have achieved widespread acceptance in high-speed manufacturing because they cure rapidly, offer high bond strength to many substrates and are easy to dispense 9. (See Reference 189 for the structure of these chemicals.)... 

When solvent welding or thermal welding is not practical or desired, adhesive systems can be used. Adhesive types such as epoxies, urethanes, thermosetting acrylics, nitrile phe-nolics, and cyanoacrylates permit ABS to be bonded to itself and to other substrates. The best adhesives have shown strength greater than that of ABS however, these adhesives provide very rigid bonds. 

CombiPatch is a three- or four-day patch that delivers both estradiol and norethin-drone acetate. CombiPatch is available in two sizes a 9-cm2 patch that delivers 0.05 mg of estradiol per day and 0.14 mg of norethindrone acetate per day and a 16-cm2 patch that delivers 0.05 mg of estradiol per day and 0.25 mg of norethindrone acetate per day. Estradiol is a lipophilic compound with a molecular weight of 272. The molecular weight of norethindrone acetate is 340. The design of the patch is considered an adhesive-matrix-type patch that consists of three layers. The backing is comprised of polyolefin and the adhesive layer contains a silicone adhesive, acrylate adhesive, estradiol, norethindrone acetate, oleic acid, and oleyl alcohol. 

When the above principles were applied to acrylic adhesives, a new type of acrylic adhesive was created. Corresponding to hydrogel, it is called oil-gel. The acrylic oil-gel adhesive is now widely applied for bandages, dressings, and TDS. The schematic microstructures of hydrogel adhesives, SIS adhesives, and acrylic oil-gels are shown in Table 3. 

The prediction of the useful life of this type of materials is a major challenge for industry. Recently, the photooxidation of an UV-cured polyurethane-acrylate adhesive (Loctite L350) has been monitored through the use of chemiluminescence and fluorescence techniques [83], Homogeneous mixtures of the adhesive formulation and two fluorescent probes were prepared, and irradiated with polychromatic light until limiting conversion was reached. Then, the cured adhesives were photoaged and the fluorescence and chemiluminescence measured at different irradiation times. For chemiluminescence studies, samples were prepared without fluorescent probes. CL data as a function of UV irradiation time are summarized in Table 3.8. 

Acrylate adhesives differ from other polymer types in their ability to be cured rapidly by exposure to UV and/or visible light. Their main advantage over heat-curing adhesives is in bonding heat-sensitive electronic devices in 5-30 seconds at room... 

We tested many types of polymeric elastomer films. Here we focus on three promising types Dow Corning HS3 silicone, NuSil CF19-2186 silicone, and the 3M VHB 4910 acrylic adhesive system [15]. As noted above, results for the silicone films were reported in earlier publications, but new high-prestrain results using these polymers are reported here. 

This chapter will cover acrylic chemistry, the various types of acrylic adhesives that have come into existence over the years, advantages and limitations of these systems, properties of adhesives, lap shear strength, what makes these systems unique among other types of adhesives, and examples of performance. 

Acrylic adhesives cure by addition polymerization reactions. These chain reactions are initiated by the formation of free radicals that result in the adhesive curing by way of a very rapid polymer chain growth. This cure chemistry is significantly more rapid than a typical cure curve (i.e., condensation type) found in epoxy and urethane adhesives. A comparison of the cure profile of condensation (epoxy and urethane) versus addition... 

Like most adhesive types, aeryhc adhesives do not have an indefinite shelf life after manufacture. Typieally suppliers of acrylic adhesives list shelf lives of at least three months, with some as long as a year. 

Most acrylic adhesives are supplied as two high viscosity liquids or pastes that are mixed together, called Mix-in Accelerator systems. Some types can also optionally be cured by no-mix accelerator lacquers. Accelerators are low viscosity liquids which are brush or spray applied to one or both of the substrates being bonded. Following accelerator application, the unmixed adhesive is applied to the primed substrate. This method eliminates the need for premixing the adhesive and accelerator, which eliminates pot life concerns. 

Because many acrylic adhesives cure very rapidly with a significant exotherm, manufacturers generally recommend that quantities greater than a pint (ca. 0.47 L) should not be mixed at one time. As a consequence of the rapid exothermic cure of many acrylic adhesives, the use of dual component cartridges is often ideal. Most products can be purchased in ready-to-use cartridges. For large continuous operations, various automated types of meter-mix-and-dispense equipment are marketed by a number of engineering companies. 

One typical concern with acrylic adhesives, specifically those containing methyl methacrylate as a component, is the characteristic odor of the methyl methacrylate monomer. This odor may be objectionable to some people and may result in the use of other types of adhesives instead. Later in this chapter some of the new low odor formulations will be briefly discussed as alternative adhesives where odor has been an issue. This new technology offers performance characteristics equal to methyl methacrylate-containing products, albeit at a higher price. 

In addition to the methacrylate monomers that are responsible for adhesive hardening, and the initiators and stabilizers already mentioned, most acrylic adhesives also contain polymers of various types and molecular weight. These additional components are included by formulating chemists to control handling properties, cure rates, flexibility, strength, adhesion, and environmental resistance. The exact nature of specific components is held as a trade secret by companies. Some insight into the types of polymers used and their combinations and levels can be found by searching patent literature and various technical journals. 

The polymerization of acrylic adhesives via free radicals (chain reaction) allows acrylic adhesives to be dispensed and cured using unique techniques. Other types of adhesive products often cure by condensation polymerization reactions which dictate certain cure characteristics and handling techniques. 

Typical bond performance of adhesives of the first generation type is shown in Table 2. The adhesives have good adhesion to thermoplastics such as polystyrene, ABS, and poly(vinyl chloride), to wood and rubber, and to steel and aluminum provided the metals are clean. It can also be seen from this table that adhesion to galvanized steel is lower than to other metals. Bonding to galvanized steel was a problem with first generation acrylic adhesives but it has been overcome with more recent embodiments of this technology covered later in this chapter. 

Second generation and DH acrylic adhesives were notably different from earlier systems in that they were the first type of acrylic adhesives that could be cured and good bonds to unprepared metals could be obtained. These systems showed the ability not only to bond to unprepared steel and aluminum surfaces, but also to aluminum and steel surfaces that still had varieties of different oils and drawing compounds on them. This ability marked a significant advancement in acrylic adhesives and established them as a unique family of adhesive materials for bonding oily and unprepared metals. 

These types of acrylic adhesive systems enjoyed considerable popularity when they were first introduced. More recently, however, they have been replaced by other, more user-friendly adhesive formulations that do not require nondrying (oily) primers and accelerators. In addition, some of the key raw materials have been of limited availability, interfering with manufacturers ability to supply product consistently. 

The emergence of the DH acrylic adhesives occurred concurrent with the introduction of the so-called HP or high performance acrylic adhesives. This type of system, as with the DH types, offered users the ability to bond through oily metals, but did not require the use of awkward oily primers. They also were based on formulations that contained specialty adhesion promoting components, often contained monomers that were less flammable than first generation products, and often were also considerably lower in odor. 

The no-mix or honeymoon type acrylic adhesives are unique in that polymerization is adequately achieved after the A component of the adhesive is applied to one substrate to be bonded, and the B component is applied to the other substrate to be bonded. When these two halves are joined together, as shown in Fig. 6 previously, enough free radicals are generated to initiate and complete the cure. The no-mix technology is unique in that it has found extensive use in the electronics industry, and is a very useful technique in bonding magnets for electrical motors where the fast cure and easy application technique have found extensive utility. 

Company literature from several adhesive suppliers is currently available that lists UV curable and visible light curing acrylic adhesives as part of their product package. This type of acrylic adhesive is being used successfully in polycarbonate lens bonding. 

Worldwide use and sales for acrylic adhesives of the types discussed here are difficult to determine, particularly since so many of the chemistries involved are nowadays hybrids of more than one type. Acrylics would, no doubt, not be considered the major chemical family of adhesives, when compared to epoxies and urethanes. Nevertheless, due to the need for the unique handling characteristics and performance properties achievable with acrylics, they maintain and will continue to maintain an important position among high performance structural adhesive types. 

In the future the various families of acrylic adhesives are expected to grow as more and more design engineers specify bonded parts and as specialty substrates continue to emerge that are bondable only with acrylic adhesives. The development of formulations with lower overall odor, 1-part systems, and hybrid types will also have a positive effect on overall growth. 

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