Additives cyanoacrylate adhesives

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. 

Crosslinking has been claimed to improve thermal resistance of the cyanoacrylate adhesive [18]. However, in other reports [6], little or no improvement in thermal resistance of the adhesive was demonstrated by the addition of a difunctional monomer. As seen in Fig. 2, the addition of varying amounts of crosslinker 7 provided no improvement in the tensile adhesive strength of ethyl cyanoacrylate on steel lapshears after thermal exposure at 121 °C for up to 48 h. 

To minimize the gradual embrittlement that can occur on aging of cyanoacrylate adhesives, plasticizers are added. Some of the materials, which have been used as plasticizers, include phthalates, phosphonates, acyl esters, succinates, and cyano-acetates. The use of allyl, methallyl, and crotyl phthalates is also claimed to improve thermal resistance properties in addition to plasticizing the adhesive [23]. 

An unexpected benefit of the rubber addition to alkyl cyanoacrylate adhesives is... 

Care must also be taken in the choice of rubber to insure that the rubber, or one of its additives, does not initiate the premature polymerization of the monomer. Even very low concentrations of a basic or nucleophilic material in the rubber or elastomer will cause the premature polymerization of an alkyl cyanoacrylate adhesive formulation. 

Interestingly, this same effect has been observed for the addition of a rubber toughening agent to ethyl cyanoacrylate-based adhesives, as was reported previously. The rubber must contain enough latent acid functionality on the polymer backbone or in an additive to inhibit the thermally activated decomposition of the alkyl cyanoacrylate adhesive polymer. 

Because of the need for basic initiators, cyanoacrylate adhesives do not perform well on acidic surfaces, such as wood. However, the addition of sequestering agents, such as crown ethers [30], 10, or calixarenes [31], 11, and others [32] to the adhesive improves the reactivity of the adhesive on less active surfaces. 

The directives apply to all materials used in the make up of electrical and electronic components and include paint, plastics, adhesives and rubbers, plastic parts, protective coating materials, epoxy adhesives, cyanoacrylate adhesives, polyurethane adhesives as well as complete computer motherboards. Computer and peripheral equipment are made up of different materials consisting of plastic, ferrous and non-ferrous metals, trace heavy metals, glass, foams, rubber, carbon powder and additives. Other non-metal materials posing environmental and health problems present in electrical and electronic products are beyond the scope of this book and will not be discussed here. The supportive components used in these products fall within the following ten product categories in the directive which are ... 

Most cyanoacrylate adhesives are maintained in a liquid state at room temperature by the addition of free radical and anionic stabilisers at suitable concentrations so as not to interfere with the functionality of the adhesive. These stabilisers/inhibitors are added at... 

Complex solutions of cyanoacrylate adhesives containing variable levels of fillers need to be measured for stabiliser content. It is difficult to prepare standard solutions with similar composition to the sample. The method of standard addition may address some of these problems with a high degree of accuracy. 

With certain exceptions, cyanoacrylate monomer formulations containing additives e.g. rubbers, high-density neutral resins, silicon dioxide, etc., may hinder accurate and precise analysis using dilution methods. In such cases it may be necessary to prepare samples using destructive techniques, particularly where the levels are very low. Solvent selection for dilution of cyanoacrylate adhesive must be compatible for the entire journey of the sample solution from sample vessel to torch. Failure to do this could cause the cyanoacrylate to polymerise locally and block the entire sample transport system in ICP-OES and can cause serious damage requiring expensive replacements. The solvents suggested in the above dilution methods were found to be satisfactory. 

Cyanoacrylates are one-part, highly polar thermoplastic polymers. The resin monomers cure in seconds when in contact with a weak base such as the moisture that is present on most surfaces. Many cyanoacrylate-adhesive formulations are commercially available, but not widely used in electronics assembly because of their poor resistance to solvents and moisture at elevated temperatures (>70 °C). Cyanoacrylates have relatively low impact and peel strengths and may be brittle unless toughened by the addition of elastomeric resins. 

For additional information about acrylic adhesives, see the discussion on anaerobic adhesives (Section 5.4) and cyanoacrylate adhesives (Section 5.11) in this chapter. 

Acrylic adhesives are based on the esters of acrylic and substituted acrylic acids and can be categorized into two main groups the acrylates and methacrylates (I), which cure by free radical-catalysed addition polymerization, and the cyanoacrylates (II), which cure by anion-catalysed addition polymerization. The term acrylic adhesives is normally restricted to include the former, while the latter is usually considered separately (see Cyanoacrylate adhesives) this categorization is adhered to here. 

As already stated, formulated cyanoacrylate adhesives contain low levels of additives that improve the performance profile of the product. They can be divided into two general groups those that modify the polymerization process and those that alter the properties of the final polymer. 

Miscellaneous Other modifications that can be made to cyanoacrylate adhesives include increasing viscosity by the addition of thickeners such as polymethyl methacrylate, cellulose esters or hydrophobic silicas. Colour can be imparted to the product by using selected dyes and pigments. 

Cyanoacrylate adhesives J GUTHRIE Anionic polymerization, additives, applications, advantages and disadvantages... 

Adhesives based on cyanoacrylic acid esters differ significantly from most other reactive adhesives because they consist in large part of pure monofunctional monomers. Epoxy, acrylic, and isocyanate based adhesives typically include di-, tri-, and tetrafunctional reactants, and usually contain significant quantities of curatives or coreactants. Cyanoacrylate adhesives rarely contain significant amounts of coreactants because they are able to homopolymerize rapidly at room temperature. Also, non-neutral additives drastically alter the cure rate of shelf life of the compounded adhesives. 

Some additives, or combinations of additives, have been disclosed as impact or heat shock resistance promoters for cyanoacrylates. For example, a variety of alkyl and aryl polycarboxylic acids and anhydrides have been described as impact resistance promoters. The property measured was impact strength on steel according to ASTM D-950. In this test, an improvement in adhesion will result in an improved impact strength. These formulations show little or no improvement over a control in tests for fracture toughness. In other words, these additives are adhesion promoters, not... 

More recently, the monofunctional cyanopentadienoates (2) were shown to improve the heat resistance of cyanoacrylate adhesives. In one case, when more than 50 weight % of 2 was combined with a standard cyanoacrylate monomer, the impact, peel, heat, and water resistance were greatly improved. " The improved heat resistance was attributed to the crosslinking of the residual olefin (28) after the anionic polymerization of 2 (see Eq. 8). The cyanopentadienoate was used in 50-90% concentration. The slow cure rate of the dienoate was overcome by the addition of an accelerator, in this case, a small quantity of an alcohol. Improvements in the heat, water, and water vapor resistance of alkyl cyanoacrylates can also... 

Several other examples of copolymeric thickeners can be found in the literature. For instance, copolymers of 20-80% methyl methacrylate and 80-20% acrylonitrile have been disclosed as thickeners for a dental cyanoacrylate adhesive. Recently, adhesives containing a plasticizer and a vinyl chloride-vinyl acetate copolymer were patented as additives for cyanoacrylate adhesives. The copolymers contained 10-20% vinyl acetate and less than 2% maleic acid. These polymers were not claimed as thickeners... 

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