Cyanoacrylates acidic substrates

One-Component Polymerization Adhesives. Cyanoacrylate Adhesives. Methyl, ethyl, butyl, and methoxyethyl esters of cyanoacrylic acid are used for cyanoacrylate adhesives [44] soluble polymers and plasticizers are incorporated to regulate viscosity and for elastification. Cyanoacrylate adhesives rapidly polymerize by an ionic reaction mechanism initiated simply by weak bases to form high molecular mass, but largely uncross-linked polymers. In most cases, atmospheric moisture or the film of moisture on the substrate is sufficient to initiate polymerization, because the adhesives are applied in very thin layers. However, this sensitivity to atmospheric moisture means that the adhesives must be stored in tightly sealed form, usually in polyethylene bottles. Cyanoacrylates are used for bonding small items of nearly all substrates. In case of polyethylene or polypropylene special primers are available. Setting is complete with rubbers in seconds, with aluminum in less than one minute. 

For these acidic substrates, a surface insensitive grade of cyanoacrylate must be used to ensure proper cure. The surface insensitive grades include special additives that can convert surface contamination into an activating species towards cyanoacrylates and then allow the adhesive to continue curing in the usual way. Such surface insensitive cyanoacrylates are good general-purpose bonders - making them particularly suited to applications where the condition of the substrate is not well defined. 

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 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. 

Epoxies, isocyanate cured polyester, and cyanoacrylates are used to bond acetal copolymer. Generally, the surface is treated with a sulfuric-chromic acid treatment. Epoxies have shown 150 to 500 psi shear strength on sanded surfaces and 500 to 1000 psi on chemically treated surfaces. Plasma treatment has also been shown to be effective on acetal substrates. Acetal homopolymer surfaces should be chemically treated prior to bonding. This is accomplished with a sulfuric-chromic acid treatment followed by a solvent wipe. Epoxies, nitrile, and nitrile-phenolics can be used as adhesives. 

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. 

Initiators, accelerators, and inhibitors of cyanoacrylate polymerization are used to modify the cure speed and storage stability of these adhesives. They can also be used to broaden the range of materials which can be bonded with cyanoacrylates. Initiators are those materials which are capable of polymerizing cyanoacrylate esters upon contact. These are, therefore, applied either to the substrate surface ( surface primers ), or mixed with the adhesive just prior to application. Accelerators are materials which do not cause polymerization on contact with monomer, but which increase the cure rate once the adhesive is applied. These chemicals are most often compounded with the monomer in the adhesive formulation. The distinction between these two classes can be blurred, as some additions will not cause immediate polymerization on contact but will shorten shelf life in the long run. Anionic polymerization inhibitors are Lewis or Bronsted acids which retard or completely inhibit anionic polymerization. Radical inhibitors prevent polymerization by adventitious, radical sources and are used to prolong the storage stability of the adhesive they generally do not affect cure speed. 

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. 

Cyanoacrylate adhesives will bond most substrates to themselves and to each other. The few adherends which do not bond well with standard adhesives are polyethylene, polypropylene, EPDM rubber, plasticized PVC, teflon, and acidic surfaces. A few manufacturers sell modified adhesives which will bond some of these materials, such as EPDM and flexible PVC. Adhesion to low surface energy plastics like polyolefins and Teflon can be improved by an etching or oxidizing treatment. 

Some substrates, for example paper, cork, cardboard, leather, dichromated metals and some fabrics can be slightly acidic, and since the cyanoacrylate contains an acidic stabiliser the adhesive will be very slow coring or may not cnre at all. For these substrates, a... 

In the same way as excess adhesive can cause blooming, a slow cure may give a similar result. The cyanoacrylate at the periphery of the joint will search for available moisture from the surrounding air and may then cure as a white powder on the adjacent surface. A slow cure may be the result of excess adhesive, but is also likely to be caused by acidic deposits on the substrate. These acidic deposits can cancel ont the nentralising effect of the initiators (moisture) and result in very slow polymerisation or in some cases inhibition of cure completely (see also Section 10.4.1). 

The cyanoacrylate adhesives are more rigid and less resistant to moisture than acrylate acid diester adhesives. They are available only as low-viscosity liquids that cure in seconds at room temperature without the need of a primer. The cyanoacrylate adhesives bond well to a variety of substrates, as shown in Table 7.25, but have relatively poor thermal resistance. Modifications of the original cyanoacrylate resins have been introduced to provide faster cures, higher strengths with some plastics, and greater thermal resistance. 

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... 

The sensitivity of the cure of cyanoacrylates to various substrates, notably acidic surfaces which inhibit or slow the anionic cure, has also been a recurrent problem. This had been overcome by the use of various basic surface acti-... 

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

Cyanoacrylate adhesives undergo anionic polymerization in the presence of a weak base, such as water, and are stabilized through the addition of a weak acid. The stabilizer is usually in the form of a weak acidic gas such as SO2, NO, or BF3. An essential function of the stabilizer is to prevent polymerization in the container, which is usually made of polyethylene. When the adhesive contacts a slightly alkaline surface, trace amounts of adsorbed water or hydroxide ions (OH ) that are present on the substrate s surface neutralize the acidic stabilizer in the adhesive, resulting in rapid polymerization as shown in Figure 10.2. 

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