Cyanoacrylates viscosity

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. 

An example of this improvement in toughness can be demonstrated by the addition of Vamac B-124, an ethylene/methyl acrylate copolymer from DuPont, to ethyl cyanoacrylate [24-26]. Three model instant adhesive formulations, a control without any polymeric additive (A), a formulation with poly(methyl methacrylate) (PMMA) (B), and a formulation with Vamac B-124 (C), are shown in Table 4. The formulation with PMMA, a thermoplastic which is added to modify viscosity, was included to determine if the addition of any polymer, not only rubbers, could improve the toughness properties of an alkyl cyanoacrylate instant adhesive. To demonstrate an improvement in toughness, the three formulations were tested for impact strength, 180° peel strength, and lapshear adhesive strength on steel specimens, before and after thermal exposure at 121°C. 

Polystyrene Although polystyrene is usually bonded by solvent cementing, it can be bonded with vinyl acetate/vinyl chloride solution adhesives, acrylics, polyurethanes, unsaturated polyesters, epoxies, urea-formaldehyde, rubber-base adhesives, polyamide (Versamid-base), polymethylmethacrylate, and cyanoacrylates. The adhesives should be medium-to-heavy viscosity and room-temperature and contact-pressure curing. An excellent source is a Monsanto Company technical information bulletin which recommends particular commercial adhesives for bonding polystyrene to a number of different surfaces. Adhesives are recommended in the fast-, medium-, and slow-setting ranges (10). 

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. 

In response to these concerns, and particularly the ones associated with tire nonabsorbable cyanoacrylates that are used clinically. Linden and Shalaby disclosed a novel, absorbable tissue adhesive composition that addressed, to a practical extent, the drawbacks of the absorbable bioadhesive as well as nonabsorbable cyanoacrylates. In effect, these absorbable tissue adhesive compositions were based (1) primarily on methoxyalkyl cyanoacrylates and preferably methoxypropyl cyanoacrylate as the precursor of an absorbable tissue adhesive polymer and (2) on a polymeric, highly absorbable, liquid comprising an oxalate ester of triethylene glycol as a modifier to modulate the viscosity of the overall composition, lower the heat of polymerization, and increase the compliance and absorption rate of the cured adhesive joint. In a more recent disclosure by Shalaby, a number of other useful modifiers were described and the functional performance of the new adhesive compositions was reported. ... 

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. 

The most commonly used liquid tissue adhesives are cyanoacrylates. Their low viscosity makes their delivery easy through small coaxial microcatheters. However, their rapid polymerization when in contact with blood, can make precise and safe occlusion challenging. Usually they are mixed with an oil-based contrast media, such as Ethiodol (Savage Laboratories, Melville, NY). The contrast serves to both opacify the cyanoacrylate and slows the polymerization time [30]. 

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. 

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-all types From 0.0025 mm (0.0001 in.) to 0.05 mm (0.002 in.). Larger gaps not recommended unless accelerator and higher viscosity varieties used... 

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. 

While the moisture absorption of PECA is minimal, the effect of water on the polymer is profound, particularly at pH greater than 7. PECA is sensitive to chain degradation by even weakly basic reagents. For example, 1% of water in THF caused a 50% reduction in the intrinsic viscosity of poly-M-butyl cyanoacrylate in 3 h at 60°C. A probable sequence for the degradation of polycyanoacrylates caused by aqueous base is shown in Scheme 2. 

Copolymers of acrylonitrile and lower acrylates have also been used as thickeners. The proportion of the acrylate in the copolymer was 60-90% by weight the preferred acrylates were the methyl through butyl esters. These authors also tested an extensive list of known and novel thickeners as comparative examples. The latter included nitrile rubber and vinyl acetate-vinyl chloride, methyl methacrylate-butadiene-styrene (MBS), and acrylonitrile-butadiene-styrene (ABS) copolymers. The copolymers of this invention were claimed to provide better stability, higher viscosity, less stringing, and better impact strength than do the thickeners of the prior art. The claims of this patent do not cover the disclosed copolymers. Polyvinyl ethers are another class of cyanoacrylate thickeners which have been disclosed but not claimed. ... 

Cyanoacrylates have been recognized as potential surgical adhesives due to their cure properties and their biodegradability. Viscosity control is very important in this application, and the thickeners used must meet the additional requirements of biodegradability and biocompatibility. One polymer that does have these attributes is polylactic acid (30). This polymer... 

Gap filling. Cyanoacrylate polymerization is surface catalyzed, which restricts the ability to cure through a thick section. Also, many low-viscosity adhesives readily flow out of a wide bondline. Gaps up to about 0.015 inch can be bridged by employing a viscous adhesive and a mild surface initiator. 

For those products having a low viscosity, less than 30,000 centipoise, it is possible to apply them by means of a pressure pot and some type of valve and nozzle. Pressure vessels for this purpose range from the small devices generally sold for materials such as cyanoacrylates up to paint-type pressure pots of several gallons. Dispensing valves can be manually operated or in some cases electric or air-operated automatic valves can be employed. Air spray guns, both manual and automatic, can sometimes be adapted for this use. A suitable fluid tip size is selected and the gun is used without the atomization air cap or air supply. This provides a convenient, inexpensive flow gun for use with products that do not require fluid pressures above the working pressure of the spray gun. 

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... 

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