Cyanoacrylates polymeric substrates

Low surface energy substrates, such as polyethylene or polypropylene, are generally difficult to bond with adhesives. However, cyanoacrylate-based adhesives can be effectively utilized to bond polyolefins with the use of the proper primer/activa-tor on the surface. Primer materials include tertiary aliphatic and aromatic amines, trialkyl ammonium carboxylate salts, tetraalkyl ammonium salts, phosphines, and organometallic compounds, which are initiators for alkyl cyanoacrylate polymerization [33-36]. The primer is applied as a dilute solution to the polyolefin surface, solvent is allowed to evaporate, and the specimens are assembled with a small amount of the adhesive. With the use of primers, adhesive strength can be so strong that substrate failure occurs during the course of the shear tests, as shown in Fig. 11. 

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. 

Adhesive choice can therefore be influenced by actual joint tolerance control and dimensions and where a cyanoacrylate or an anaerobic adhesive might be ideal (polymeric substrates or a threaded metallic joint, respectively), the measured bond gap may be too large and an alternative adhesive such as a polyurethane or an epoxy may be better suited. 

The ability of cyanoacrylates to resist attack from moisture when bonded to polymeric substrates can be most drastically tested by subjecting bonded assemblies to autoclaving. The autoclaving process combines the environmental stresses of high temperature, high-pressure and humidity. As such, it provides a good indicator of the ability of adhesives to withstand exposure to moisture. 

The mechanism by which the primers are thought to work is relatively straightforward. The primer first diffuses into the polyolefin surface, and subsequently becomes entangled in the polyolefin. The primer molecule can then act as an anchor in the substrate surface for the adhesive polymer, which forms after the primer initiates polymerization of the alkyl cyanoacrylate monomer [37]. 

Ethicon, Inc., (Somerville, NJ) vendors Dermabond Topical Skin Adhesive for closing surface or skin type wounds. This material is not intended for internal use. Closure Medical Corporation manufactures the actual adhesive. Ethicon, Inc. is a Johnson Johnson company. Dermabond is chemically 2-octylcyanoacrylate and it cures on contact with water, ethanol and others (i.e., addition polymerization and catalysis by weak bases). For internal tissues, the excessive moisture can overcure cyanoacrylate by saturating its surface with moisture, cause disbondment from the substrate, and render it useless as an adhesive. The cured polymer possesses a hard and brittle consistency. 

Poly(alkyl-cyanoacrylates) As poly(alkyl-cyanoacrylates) form strong bonds with polar substrates including the skin and living tissues, they exhibit bioadhesive properties. These polymers are synthesized by free-radical, anionic, or zwitterionic polymerization. As detailed in a recent review, poly(alkyl-cyanoacrylate) nanoparticles are prepared by emulsion polymerization, interfacial polymerization, nanoprecipitation, and emulsion-solvent evaporation methods [102],... 

In practice, a trace of moisture occurring on a substrate is adequate to cause polymerization of the cyanoacrylate monomer to provide strong bonding within a few seconds of closing the joint and excluding air. Cyanoacrylate adhesives are particularly valuable because of their speed of action, which... 

A brief description of the chemistry of these materials is described on the article entitled Alkyl-2-cyanoacrylates. Commercial adhesive products are usually based on the ethyl ester but methyl, n-butyl, allyl -methoxyethyl and -ethoxyethyl are also important. Formulated adhesives consist of essentially pure monomer with relatively small amounts of property-modifying additives. The curing reaction is anionic polymerization, initiated by traces of alkaline material present on most substrate surfaces, particularly in conjunction with low levels of surface moisture (see Chain polymerization). 

The mechanism of cure is quite different for cyanoacrylates on one hand and silicones and isocyanates on the other. In the former case, cure is by Chain polymerization initiated by water on the surface of the substrate, while in the latter, water from the atmosphere diffuses into the adhesive or sealant and then participates in a Step polymerization. One consequence of this is that only catalytic qnantities of water are reqnired to cure cyanoacrylates, while much larger, stoichiometric quantities are needed for silicones and isocyanates. 

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. 

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. 

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. 

In order to bond materials utilizing cyanoacrylates, it is necessary to remember that polymer formation initiates at the adhesive-substrate interface when the adhesive comes in contact with the surface layer of moisture present on the substrate. Polymerization then proceeds back toward the center of the adhesive layer rather than uniformly throughout the mass. As a result, cyanoacrylate adhesives are not efficient gap filling adhesives, since as the gap increases the likelihood of a growing chain being terminated by an inhibitor molecule becomes quite high. 

Cyanoacrylates (Coover, 1977 Dick, 1987) - Cyanoacrylate adhesives are one part moisture eure systems whieh ean rapidly form very strong bonds with a wide range of substrates. The adhesives was first commereiahzed in 1958 and most of those in use today are based on methyl 2-cyanoacrylate and ethyl 2-cyanoacrylate. The extremely fast erne is related to the presenee of highly electronegative alkoxy (-COOR) and nitrile (-CN) groups in the alkyl 2-cyanoacrylate molecules. This allows initiation of anionic polymerization by weak bases such as alcohol or water at room temperatiu e. The polymerization is propagated by the resultant carbanion as shown below. 

Methyl and ethyl 2-cyanoacrylates form the basis of so-called super glues . The presence of two electron-withdrawing groups polarizes the carbon-carbon double bond and makes the monomer very susceptible to anionic polymerization. Weak bases such as water bring about rapid polymerization and, in practice, a trace of moisture on a substrate leads to very fast bonding ... 

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