Acrylate adhesives

Acrylic acid polymers Acrylic adhesives Acrylic anhydride Acrylic copolymer Acrylic-cotton blends Acrylic elastomers... 

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. 

The reluctance of acrylic monomers to polymerise in the presence of air has been made a virtue with the anaerobic acrylic adhesives. These are usually dimethacrylates such as tetramethylene glycol dimethacrylate. The monomers are supplied with a curing system comprising a peroxide and an amine as part of a one-part pack. When the adhesive is placed between mild steel surfaces air is excluded, which prevents air inhibition, and the iron present acts as a polymerisation promoter. The effectiveness as a promoter varies from one metal to another and it may be necessary to use a primer such as cobalt naphthenate. The anaerobic adhesives have been widely used for sealing nuts and bolts and for a variety of engineering purposes. Small tube containers are also available for domestic use. 

The humidity requirement is probably most significant for polar or hydrophilic adhesives, for example,. some of the acrylate adhesives. 

Acrylic acid, the main precursor to acrylic adhesives had been synthesized in the mid 1800s and the first acrylic acid esters were made and characterized at the turn of the century [62]. The first commercial launch of acrylic polymers in the form of poly(methylmethacrylate) took place in 1927 when the German company Rohm and Haas AG introduced this new plastic to the market. Soon after, other companies such as BASF introduced acrylic dispersions. 

Advantages are similar to the epoxy system, in that these can be solventless and do not require thermal energy. Disadvantages unique to this system, however, include the need to inert the cure chamber to avoid air-inhibition of cure as well as some release instability with acrylate adhesives [72]. 

A recent reference [1] lists twenty suppliers of structural acrylic adhesives. Many other companies have been involved in this area, and it is often felt that acrylic adhesives have unrealized technical potential [2]. Acrylic adhesives are not commonly found in the consumer market, but fill an important niche there in the bonding of rear view mirrors to windshields in automobiles. 

The development of acrylic adhesives is well documented, although most of the publications are in the patent literature. Much early work was done by Bader and coworkers [4-7] at Degussa (Deutsche Gold- und Silber-Scheideanstalt vormals Roessler), and provides a good source for starting formulations. 

Acrylic adhesive formulations can be amazingly simple. The formulation given in Table 1 is a simplification of the Bader work, and provides an excellent starting point for screening studies. 

These results demonstrate some interesting chemical principles of the use of acrylic adhesives. They stick to a broad range of substrates, with some notable exceptions. One of these is galvanized steel, a chemically active substrate which can interact with the adhesive and inhibit cure. Another is Noryl , a blend of polystyrene and polyphenylene oxide. It contains phenol groups that are known polymerization inhibitors. Highly non-polar substrates such as polyolefins and silicones are difficult to bond with any technology, but as we shall see, the initiator can play a big role in acrylic adhesion to polyolefins. 

Acrylic polymers are also important in the preparation of other classes of adhesives, especially pressure sensitive adhesives. This discussion will be limited to adhesives that cure by the reaction of unsaturated acrylic functional groups. These adhesives are also sometimes termed Methacrylate , Methacrylic or Structural Acrylic adhesives. This is consistent with the long-standing habit of organic chemists to assign multiple names to the same substance. 

Acrylic adhesives cure by a free radical chain growth mechanism. In contrast, epoxy and urethane adhesives cure by a step growth mechanism. This has a major impact on the cure kinetics, as well as the composition of the adhesive during cure ([9], pp. 6-9). Cyanoacrylate adhesives (such as Super Glue ) also cure by chain growth, but the mechanism is ionic with initiation by surface moisture. 

The kinetics of reaction of free radical chain reactions are complicated compared to the second-order kinetics of epoxy and urethane adhesives. Many of these complications offer practical advantages to the process of using acrylic adhesives. 

The generation of free radicals usually does not immediately start polymerization in commercial adhesives. These contain small amounts of inhibitors, which are chemical compounds that prevent free radical polymerization. Inhibitors are purposely added to acrylic adhesives to obtain practical shelf life. Inhibitors stop polymerization by reacting with active free radicals to form a less reactive species... 

The use of rubbers (low glass transition polymers) to toughen acrylic adhesives... 

These are all examples of soluble polymers. Combinations of soluble with insoluble polymers have also been reported. Polychloroprene or chlorosulfonated polyethylene was eombined with core-shell polymer particles to give an adhesive with improved cold impact resistance [33]. The fascinating chemistry of chlorosulfonated polyethylene in acrylic adhesives will be further discussed in the section on initiators. In many cases chlorosulfonated polyethylene is chemically attached to the acrylic matrix. 

The cure reaction of structural acrylic adhesives can be started by any of a great number of redox reactions. One commonly used redox couple is the reaction of benzoyl peroxide (BPO) with tertiary aromatic amines. Pure BPO is hazardous when dry [39]. It is susceptible to explosion from shock, friction or heat, and has an autoignition temperature of 79°C. Water is a very effective stabilizer for BPO, and so the initiator is often available as a paste or a moist solid [40], The... 

There are other initiator systems of lesser commercial importance. Cumene hydroperoxide is reported to cure acrylic adhesives in the presence of alkyl or pyridyl thioureas [105]. These initiators have been combined with a phosphated acrylate to promote adhesion to metal [106]. Thiourea-based initiators can be applied as a one-part on galvanized metal, where the metal surface provides the second part of the redox initiator [107]. 

The ability to tailor acrylic adhesives to fast cure times allows their use on fast, highly mechanized production lines such as those for audio speakers [150]. 

Acrylic adhesives can also be used in automotive assembly. For example, the hem flanges between the inner and outer panel of automotive doors can be bonded with acrylic adhesives [153]. There are many smaller industrial assembly applications where acrylic adhesives are applicable. An interesting recent example is the bonding of cable splice enclosures, using a borane initiated adhesive [154]. 

In other studies [115], the bond strengths of joints made from steel substrates coated with a variety of oils and waxes ( 6 mg/cm ) and Joined with acrylic adhesive were investigated. Lap-shear strengths up to 15 MPa were obtained with room temperature curing. Very little degradation was seen after 1000 h of... 

Over the years there have been several studies examining electron-curable adhesives [9-12]. Off-the-shelf acrylate adhesives were the primary focus of the studies. These adhesives, that have potential use for the repair of advanced composites using high-energy electron accelerators, offer several advantages over conventional repair systems, including [6] ... 

Although the acrylate adhesives are readily available and studies have shown that they can produce reasonable bonding properties, they have the disadvantages of having high shrinkage, high fluid absorption, and low service temperatures. Acrylate adhesive applications would be limited. The development of EB-curable epoxy adhesives would have applications in the aerospace and automotive industry and potential wider uses. The most immediate application for these resin systems is composite repair of commercial and military aircraft. 

Table 3 lists the selected properties [16] that we have measured for several commercially available acrylate resins manufactured by the Sartomer Company and the Rohm and Haas Company. The resins were cured in an AECL Gammacell Model 240. The temperature rise was measured for an 8-g sample using Acsion s (formerly AECL Radiation Applications Branch) Gamma Calorimetry method [17]. All of this information is being used to evaluate the applicability of EB-cured acrylate adhesives for repairing composite structures. Combinations of these adhesives can be used to create electron-curable adhesives suitable for composite repair. 

Loctite has developed several acrylate adhesives that can be used as EB-curable adhesives [12]. Some of the resins were cured and tested for EB curing properties... 

Lap shear strengths for EB-cured acrylic adhesives on graphite composites... 

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