Cyanoacrylates porous substrates

Cyanoacrylate adhesives are particularly valuable because of their speed of action, which allows the joining of intricate parts without the need for complex jigs and fixtures. Within very broad limits the more monomer that is used to make a joint the less will be the strength. These adhesives have in fact no gapfilling ability, nor can they be used on porous substrates. Whilst they have good heat and solvent resistance their weathering behaviour is limited and joints should not be in frequent contact with water. 

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. 

Cyanoacrylate adhesives will bond a wide variety of substrates with the exception of polyolefins (unless pre-treated). Teflon and highly acidic surfaces. Porous substrates such as wood, paper and leather require the use of products containing accelerators. Formulations are now appearing that when used in conjunction with a so-called primer can give high bond strength on polyethylene and polypropylene. See Industrial applications of adhesives. 

There are several reasons why cyanoacrylates are attractive as adhesives. They are easy to apply, one-part, 100% reactive, storage-stable adhesives. They cure rapidly at room temperature when spread in thin films between substrate surfaces, and they form strong bonds between a variety of substrates. However, cyanoacrylates do have several serious shortcomings including poor heat resistance, poor moisture resistance, poor peel and impact resistance, and limited ability to fill gaps and to bond porous substrates. The poor durability and impact resistance have been particular limitations in metal-to-metal bonding. 

Manuhicturers Comments Ethyl cyanoacrylate. Solvent free. Very fast cure. For close-fitting, non-porous substrates. Capillary action. Not for applications with water immersion. 

Considerable work has also been done in the last 10 years investigating the use of various additives which accelerate the anionic polymerization of cyanoacrylates on acidic or porous substrates. These additives are covered in the Recent Advances section of this chapter. 

With the incorporation of the new surface-insensitive additives which allow rapid curing on acidic and porous substrates such as wood and paper, and the marketing of these versatile adhesives in the consumer sector, the number of applications where cyanoacrylates are used is limited only by the ingenuity of the user. The new toughened adhesives with improved heat and humidity resistance and dynamic performance are already encroaching into areas where epoxies were once the only choice. Versatility... 

Surface Insensitive Cyanoacrylates (For Acidic and Porous Substrates)... 

The new range of surface-insensitive cyanoacrylates provides ultrafast cures independent of gap. In addition, these cyanoacrylates will rapidly bond acidic and low-energy surfaces. The fast cure also minimizes the occurrence of frosting and fogging. Table 6 shows a comparison of these new surface-insensitive materials compared to a standard ethyl-grade cyanoacrylate. These products are also suited to bonding various wood substrates and porous surfaces without the use of activators. 

Cyanoacrylate monomers can be polymerized by a variety of nucleophilic or Lewis bases, as discussed in Section II.D.2. The most common initiators encountered are nucleophilic surface contaminants, in conjunction with moisture adsorbed on the adherend surfaces. Most substrates can be bonded without the need of additional initiator. However, acidic surfaces, such as certain woods or acid-treated adherends, need to be primed with an initiator to achieve a normal bond. Porous surfaces are often primed to prevent the monomer form wicking away from the bondline before curing. Over the years, a variety of materials have been suggested and/or used for this purpose. 

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