Cyanoacrylate, curing cure speed

Ethyl cyanoacrylate is the monomer which is most widely used in both consumer and industrial applications, because of its combination of fast cure speed and ease of manufacture. 

Since bases are catalysts for the curing reaction and acids are stabilizers for the cyanoacrylates, the pH value of the surface will control the cure speed. Surfaces that tend to be acidic will cure slowly compared to a neutral surface, which in turn will cure more slowly than an alkaline surface. In most applications the objective is to speed the cure therefore, all the commercially available activators are weak bases dissolved in a volatile carrier. Applying an activator to a surface places a layer of the weak base in position to initiate the cure. Since they are stronger bases than moisture, they are able to neutralize the stabilizer systems in the adhesive more effectively, and thus they can tolerate larger gaps than would be possible with moisture alone. In general, the fixturing time is 10 times faster with activator than without it. Even with activator, the effect of the gap is clear (see Table 4). 

Stabilizers The major difficulty associated with the manufacture of cyanoacrylate adhesives is ensuring a balance between stability of the product and cure speed. 

Plasticizers These are required to reduce the inherent brittleness of poly(alkyl-2-cyanoacrylates). This can be achieved by using non-copolymerizing plasticizers such as esters or higher alkyl cyanoacrylates, which copolymerize with the basic adhesive monomer. Toughness properties can be improved by the inclusion of rubber toughening materials such as ABS (acrylonitrile-butadiene-styrene) or MBS (methacrylate-butadiene-styrene) copolymers. Whichever approach is adopted, toughness is only achieved at the expense of reduced cure speed. 

Thermoplastic slmctural adhesives are less significant for the more-demanding applications however. Cyanoacrylate adhesives have gained considerable importance for applications at ambient temperatures and less severe environments, particularly on plastic substrates, mainly as a result of their very rapid cure speeds (seconds) and ease of application. 

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. 

A variety of external plasticizers have also been suggested for cyanoacrylates. Among these are alkyl esters of aliphatic monocarboxylic acids such as cyanoacetates aliphatic dicarboxylic esters, such as malonates, adipates, and sebacates triaromatic phosphates such as tricresyl phosphate acyl triesters of glycerine dialkyl alkylphosphonates and alkyl phthalates. Joyner and Coover state that limiting the plasticizer concentration to less than 5% will not retard the cure speed, from 5 to 20% will retard the cure rate, and over 20% plasticizer concentration seriously slows the cure rate of the adhesive. 

The first examples of thickened cyanoacrylate adhesives were described by Coover and Shearer in a U.S. Patent. The thickeners cited and claimed were polyalkyl cyanoacrylates, polyacrylates, polymethacrylates, cellulose nitrate, and cellulose organic acid esters, such as cellulose acetate butyrate. Several years later. Wicker and Shearer improved the process for thickening cyanoacrylates. Instead of adding the thickener directly to the monomer, the thickener was first dissolved in a volatile solvent and then added to the ester. The solvent was then vacuum stripped to give the thickened adhesive. The authors claimed that this process gave adhesives having better clarity, better storage stability, and faster cure speed than adhesives prepared by the older method. 

The cure speed of a cyanoacrylate, if left open on a surface, will be quite sluggish, because there is insufficient moisture and the adhesive will remain liquid for several hours (although the cyanoacrylate will cure at the surface interface). When the adhesive is between two close fitting surfaces, where there is moisture on both surfaces, the cyanoacrylate will cure rapidly. The gap between parts should ideally be less than 0.1 mm (see Figure 10.3). 

Figure 10.2 The initial chains form rapidly, giving cyanoacrylate adhesives their characteristic fast bonding (cure speed) times...

Figure 10.4 shows how RH can affect the cure speed for an ethyl cyanoacrylate. 

The ambient temperature does not contribute significantly to the cure speed of the cyanoacrylate but as with all chemical reactions if the ambient temperature is low, i.e., less than 5 °C, then the cure speed will be longer. This may occur for example in a maintenance application where a cyanoacrylate is used outdoors in winter. An activator may be required to ensure a reliable cure (see also Section 10.8). 

Slow cure speed. Use low bloom grade of cyanoacrylate (see 10.12.1). 

The alkoxy-alkyl esters, which have methoxy or ethoxy groups appended to the j3-carbon of a standard ethyl cyanoacrylate ester, are virtually odorless versions of cyanoacrylate adhesives. Though first reported by Eastman in 1957, these are now being reemphasized by several manufacturers with improvements in stability and cure speed. These will be covered more fully in the Recent Advances section. 

These monomers are virtually odorless and have much lower vapor pressures so that the blooming characteristics are greatly reduced or eliminated. Performance is similar but not equivalent to that of the lower methyl and ethyl esters. On metals or rubber the cute speeds are quite fast, but the cure speeds on plastics were slower than conventional cyanoacrylates. Updated, faster curing versions of these products are becoming available. As costs of production for these adhesives are reduced, and the curing properties improved, they are expected to replace the lower esters in many applications. 

The two major factors affecting cure speed are the percentage relative humidity and the gap. The optimum cure condition for cyanoacrylates is when the relative humidity (RH) is between 40% RH and 60% RH. Lower relative humidities, i.e., 20% RH, will result in a slower cure, and high RH (80% RH) results in a faster cure. High relative humidity can be detrimental as the cyanoacrylate sometimes cures so fast... 

Epoxies cure to become thermoset adhesives and so show excellent humidity resistance, especially if one of the substrates is metal. For long-term, durable adhesively bonded metal-to-metal joints, toughened epoxies are hard to beat [6] but where plastics are involved, some of the other technology adhesives invariably offer benefits in terms of ease of application and cure speed whilst still retaining satisfactory environmental resistance. Indeed epoxies do not often show good adhesion to elastomers and for the thermoplastic elastomers cyanoacrylates will generally show the best environmental resistance. 

Slow cure can also be overcome by using an activator (or accelerator). The activators increase the level of initiators on the surface to negate the stabiliser and thus increase the speed of polymerisation. UV-curing cyanoacrylates have also been used in applications to accelerate the cure speed and thus eliminate the possibility of blooming. 

Cyanoacrylates and Light-Curing Acrylic Adhesives - These types of adhesives have achieved widespread acceptance in high-speed manufacturing because they cure rapidly, offer high bond strength to many substrates and are easy to dispense 9. (See Reference 189 for the structure of these chemicals.)... 

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