2-cyanoacrylates

Most commercial glues such as the popular Crazy Glue do not use plasticizers. These glues are based on ethyl and butyl derivatives of cyanoacrylate. A new development involves the use of octyl-2-cyanoacrylate in medical and veterinary applications. These new adhesives usually require plasticizers. 

For medical purposes acetyl tributyl citrate is most commonly used. °° Older inventions suggested also dioctyl phthalate, a useful plasticizer for use in contact with human skin. ° ° In veterinary applications, dioctyl phthalate and acetyl tributyl citrate are likely to be used. It should be underlined that this difference between medical and veterinary application is more of precautionary nature than formulation based on previous results of studies or regulations which may limit dioctyl phthalate use for this applications. 

Polydimethylsiloxane, hexadimethylsilazane, and polymethylmethacrylate are also added to increase the flexibility and elasticity of polymer. The adhesive can be made plasticizer-free by using a composition of polymers containing a mixture of alkyl groups from Cj to Cg. The use of suitable proportions of different monomers gives the required 

The above shows that concentrations of plasticizer are fairly consistent. 

The main reason for the use of plasticizers in formulations is to increase flexibility. More extensive stu has been conducted with dioctyl phthalate, DOP, where appearance, curing time, film formation, flexibility and durability have been evaluated for n-butyl cyanoacrylates containing from 15 to 50 wt% plasticizer. When DOP was at a 35 wt% or above, no film was formed. With the plasticizer at 15 wt%, flexibility and durabihty suffered. A 20-25 wt% range has produced the best overall performance. 

There are of course many other types of adhesives suitable for bonding plastics and mention of some of these is given in Section 1.5. 

Cyanoacrylate technology has been available since the early 1960s and is extensively used by both industry and consumer alike. Considerable advancements in this technology have been made over the years and new grades and versions are not uncommon in the New Product announcements in the Technical Press. 

Cyanoacrylate adhesives cure readily on most surfaces where there are traces of moisture present. The moisture neutralises the stabiliser to initiate curing within a few seconds. 

The two major factors affecting cure speed are the percentage relative humidity and the gap. The optimum cure condition for cyanoacrylates is when the relative humidity (RH) is between 40% RH and 60% RH. Lower relative humidities, i.e., 20% RH, will result in a slower cure, and high RH (80% RH) results in a faster cure. High relative humidity can be detrimental as the cyanoacrylate sometimes cures so fast 

The cyanoacrylate should not be disturbed during the critical time whilst it is polymerising, as the adhesive may never subseqnently gain its full strength. 

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ACRYLIC ESTERPOLYTffiRS - 2-CYANOACRYLIC ESTpRpOLYTffiRS] (Vol 1) -dyeing of pYES, APPLICATION AND EVALUATION] (Vol 8)... 

Table 2. Cured Bulk Properties of Common 2-Cyanoacrylic Esters...

Table 3. Adhesive Bond Properties of 2-Cyanoacrylic Esters with Metals and Various Polymeric Materials...

The 2-cyanoacryhc esters have sharp, pungent odors and are lacrimators, even at very low concentrations. These esters can be irritating to the nose and throat at concentrations as low as 3 ppm eye irritation is observed at levels of 5 ppm (13). The TLV for methyl 2-cyanoacrylate is 2 ppm and the short-term exposure limit is 4 ppm (14). Good ventilation when using the adhesives is essential. 

The diverse nature of the cyanoacrylate adhesives appHcations illustrates vividly that there is no truly typical appHcation. The number of appHcations ia which these adhesives are used is being expanded daily as technological improvements continue to broaden their capabiUties. 

Acrylic Ester Polymers, 2-Cyanoacrylic Ester Polymers" ia ECT 3rd ed., VoL 1, pp. 408—413, by H. W. Coover, Jr., andj. M. Mclatire, Tennessee Eastman Company. 

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. 

Cyanoacrylate adhesives (Super-Glues) are materials which rapidly polymerize at room temperature. The standard monomer for a cyanoacrylate adhesive is ethyl 2-cyanoacrylate [7085-85-0], which readily undergoes anionic polymerization. Very rapid cure of these materials has made them widely used in the electronics industry for speaker magnet mounting, as weU as for wire tacking and other apphcations requiring rapid assembly. Anionic polymerization of a cyanoacrylate adhesive is normally initiated by water. Therefore, atmospheric humidity or the surface moisture content must be at a certain level for polymerization to take place. These adhesives are not cross-linked as are the surface-activated acryhcs. Rather, the cyanoacrylate material is a thermoplastic, and thus, the adhesives typically have poor temperature resistance. 

Acryhc stmctural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acryhc adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated stmctural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, N,1S7-dimethyl- -toluidine, and saccharin, can be apphed to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

Tetraethylene glycol may be used direcdy as a plasticizer or modified by esterification with fatty acids to produce plasticizers (qv). Tetraethylene glycol is used directly to plasticize separation membranes, such as siHcone mbber, poly(vinyl acetate), and ceUulose triacetate. Ceramic materials utilize tetraethylene glycol as plasticizing agents in resistant refractory plastics and molded ceramics. It is also employed to improve the physical properties of cyanoacrylate and polyacrylonitrile adhesives, and is chemically modified to form polyisocyanate, polymethacrylate, and to contain siHcone compounds used for adhesives. 

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

Only small amounts of nitromethane are used as solvent, but it is used in specialized appHcations such as the solublization of a-cyanoacrylate glue and acryhc polymers. Also, nitromethane is useflil as solvent for single-phase Friedel-Crafts reactions (115). 

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