Viscosity monomer cyanoacrylate

All of the eommereial alkyl eyanoaerylate monomers are low-viseosity liquids, and for some applications this can be an advantage. However, there are instances where a viseous liquid or a gel adhesive would be preferred, sueh as for application to a vertical surface or on porous substrates. A variety of viscosity control agents, depending upon the desired properties, have been added to increase the viscosity of instant adhesives [21]. The materials, which have been utilized, include polymethyl methacrylate, hydrophobic silica, hydrophobic alumina, treated quartz, polyethyl cyanoacrylate, cellulose esters, polycarbonates, and carbon black. For example, the addition of 5-10% of amorphous, non-crystalline, fumed silica to ethyl cyanoacrylate changes the monomer viscosity from a 2-cps liquid to a gelled material [22]. Because of the sensitivity of cyanoacrylate esters to basic materials, some additives require treatment with an acid to prevent premature gelation of the product. 

The polymerization process of cyanoacrylates is so fast that it can react with the water present on skin, causing it to bond skin instantly. As such, it should be handled with extreme care, because a small amount rubbed in the eye can instantly bond eyelids shut. It causes irritation of the nose, throat, and lungs from the noxious vapors of the acrylate monomer. There are different commercially available chemical variants that have slightly different setting properties and viscosities depending upon the application. 

The physical properties of the monomers must be discussed along with those of the cured polymers because consideration of one without the other presents an incomplete picture. The 2-cyanoacrylic ester monomers are all thin, water-dear liquids with viscosities of 1 3 mPa-s(=cP). Although a number of the esters have been prepared and characterized, only a relative few are of any significant commercial interest, and, of those, the methyl and ethyl esters by far predominate. The physical properties of the principal monomers are induded in Table 1. 

An absorbable polymeric modifier that is compatible with the chosen cyanoacrylate to achieve a moderate-to-high preuse viscosity for ease of application, and to increase the absorption rate and compliance of tile cured adhesive for minimizing residence time at the application site and maximizing mechanical biocompatibility — for this, highly absorbable liquid oxalate polymers, such as poly(triox-yethylene oxalate) (PTOEO) and amorphous or low-melting absorbable copolyesters such as those described in Chapter 3 — polyaxial copolyesters and other copolymers based on cyclic monomers — were selected. 

As with other acrylics, the monomers are liquids of low viscosity that polymerize very easily in the presence of a slightly basic surface containing adsorbed water. Polymerization is ionic. The resulting polymers have different properties, depending on the alkyl group. The methyl ester (methyl-2-cyanoacrylate) is the most commonly used compound. This material is formulated with a thickener (to prevent starved joints from being formed) and a plasticizer to make it more resistant to shock loading. The thickener can be a polymer of the same monomer. An essential feature is a stabilizer to prevent polymerization in the adhesive container, which is usually made of polyethylene. 

Cyanoacrylate adhesives are relatively low viscosity fluids based on acrylic monomers and characterised by extremely fast rates of cure. When placed between closely fitting surfaces, some will cure to give a strong joint in two to three seconds. 

While the bulk of any cyanoacrylate formulation consists of monomer, a large number of modifiers have been used to impart desired properties to the composition. These include stabilizers, inhibitors, thickeners, plasticizers, dyes or colorants, adhesion promoters, and others. Each of these classes of modifier will be dealt with in subsequent parts of this chapter. Because of the variety of modifiers, and the variety of applications for cyanoacrylates, a bewildering number of cyanoacrylate adhesives are now commercially available. These can be generally divided into the following classifications adhesives of different viscosities and cure rates, adhesives based on different monomers, adhesives for the bonding of metal, plastic, rubber, or wood, various types of improved performance adhesives, i.e., heat, moisture, or impact resistant, and adhesives for bonding low surface... 

Pure cyanoacrylate monomers are clear, colorless, low-viscosity liquids at room temperature. They typically have pungent odors. Most cyanoacrylate esters are miscible in moderate- to high-polarity solvents, but nucleophilic solvents will cause polymerization. These properties and others are detailed in Table II. 

Cyanoacrylate monomers are generally low viscosity, colorless liquids that are unstable in the absence of acidic stabilizers. The commercially important materials are lower alkyl esters of 2-cyanoacrylic acid including methyl, ethyl, n-propyl, al-lyl, n-butyl, isobutyl, 2-methoxyethyl, 2-methoxypropyl, and n-octyl derivatives. The structures of some of these monomers are shown in Figure 1. The physical properties of these and related monomers are reported in detail in several previous... 

As a result, viscosity modification, through the use of various soluble polymeric thickeners, represented the first attempt to modify the liquid properties of cyanoacrylates. Thickeners and fillers must be compatible with the cyanoacrylates and must not set off the easily triggered anionic cure. Polymers which have been used to increase the base viscosity of the monomers include polymethacrylates (the most common), poly acrylates, poly cyanoacrylates, poly vinylacetates, baked polyacrylics, poly lactic acid, cellulose nitrates and other esters such... 

Alkyl cyanoacrylate monomers typically have viscosities in the range of 1-3 cP, which are much too low to permit convenient use in many industrial applications. Water thin adhesives will flow from the bondlines, resulting in adhesive-starved joints and low bond strengths. Migrating adhesive could also bond adjacent... 

The a-alkyl cyanoacrylate super glues ( one drop holds 5000 lbs ) are now a familiar part of the consumer market. Originally, the monomers had extremely low viscosities and so could crawl into narrow crevices and wet the adherend surfaces rapidly. On the other hand, they would not fill gaps and were absorbed into porous adherends, giving poor bonds. Newer versions are available with higher viscosities to overcome these drawbacks. Cyanoacrylates can polymerize in seconds by an anionic addition reaction believed initiated by hydroxyl ions from water adsorbed on the adherend surfaces ... 

With a smaller molecule size the methyl cyanoacrylates have better affinity to metals and can sometimes offer better resistance to solvents. The methyl grades of cyanoacrylate would often be specified for bonding plastic to metal or rubber to metal applications. The basic monomer is a very thin (low viscosity) liquid typically around 3-5 mPa-s. These methyl cyanoacrylate adhesives can be used to wick-in to pre-assembled items such as for locking small adjustment screws or for increasing the strength of two press-fitted components. 

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