Cyanoacrylates bonding application

Because of the high costs of raw materials and the relatively complex synthesis, the 2-cyanoacrylic esters are moderately expensive materials when considered in bulk quantities. Depending on the quantity and the specific ester or formulation involved, the prices for cyanoacrylic ester adhesives can range from approximately 30/kg to over 1000/kg. For these reasons, as well as several technical factors related to handling and performance, cyanoacrylic ester adhesives are best suited to small bonding applications, very often where single drops or small beads are adequate for bonding. In such cases the cost of the adhesive becomes inconsequential compared to the value of the service it performs, and these adhesives become very economical to use. 

In general, standard industrial cyanoacrylates do not operate effectively above 180°F (see Fig. 4). However, the new allyl types of cyanoacrylates can operate as high as 480°F before the bond loses sufficient strength to be operationally effective (see Table 2). Allyl cyanoacrylates for metal-bonding applications have proven effective in wave solder and under-hood (automotive) applications. In Fig. 4, bonded assemblies are cured at room temperature for 24 h. The assemblies are heated for 2 h and tested hot. 

Confidence in the ability of certain synthetic resin-based adhesives to provide satisfactory structural bonds has developed during the past 40 years. The requirement that the adhesive be able to withstand loads over long periods of time means that most structural adhesives are based on thermosetting resins, whose crosslinked structure provides good creep-resistance. However, certain thefhioplastic resins (notably linear thermoplastic polyimides and cyanoacrylates) do appear to be useful for some structural bonding applications. 

The rubber toughened cyanoacrylates are particularly well suited for rubber to metal bonding applications, although they are slightly slower curing than the standard ethyl... 

Cyanoacrylates are best suited to the smaller bond areas in rubber bonding applications, as larger areas often require more time to position the parts, and the fast cure of cyanoacrylates make this difficult. Typical applications are illustrated in Section 10.11. 

Blooming does not always occur during the first few seconds, indeed it is more likely that parts will bloom some hours after assembly (up to 24 - 48 hours later). A bonding application is often one of the last operations in a production cycle and care is required to ensure that parts are not put straight into a sealed (or semi-sealed) container immediately after cyanoacrylate bonding as there is a risk that the cyanoacrylate will bloom in the box resulting in a poor aesthetic appearance to the end product. Trials should be conducted accordingly. 

As previously discussed, conventional cyanoacrylates perform well in numerous bonding applications, but several well known and significant limitations preclude even wider usage. It is toward these limitations that much recent research and development work has been directed. These limitations, partially reviewed in previous sections, include ... 

Because of their ability to bond a wide variety of substrates, cyanoacrylate instant adhesives are now produced in multi-ton quantities for both industrial and consumer applications [3]. 

Many cyanoacrylates give rather rigid bonds which may in consequence be brittle and not able to resist peeling forces. However, in recent years more flexible toughened grades have been developed for applications in which resistance to peeling is required. 

Parts molded from polyetherimide can be assembled with all common thermoplastic assembly methods. Adhesives that are recommended include epoxy, urethane, and cyanoacrylate. However, service temperature must be taken into consideration in choosing an adhesive because PEI parts are generally used for high-temperature applications. Good adhesion can be effected by simple solvent wipe, but surface treatment by corona discharge, flame treatment, or chromic acid etch will provide the highest bond strengths. 

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 viscous isoamyl cyanoacrylate shows the least deterioration of bond strength to bone in an aqueous environment. This material has the further advantage that (1) it may be more suitable for applications where some filling of a gap by the adhesive is required and (2) it is considered to be less toxic than the lower homologues of the 2-cyanoacrylate series. 

Experiments were performed to determine if monomers containing 2-cyanoacrylates were grafted, that is bonded covalently, to bone. Bone slabs were primed with 1% solution of triethylamine in n-hexane, and an ethyl cyanoacrylate was spread on the treated bone. Two hours after application of the adhesive, the attenuated total reflectance infrared spectrum showed clearly the presence of... 

The bonding of 2-cyanoacrylates to mineralized tissues In an aqueous environment appears to be superior to that of other adhesives. The higher homologues of 2-cyanoacrylates may be useful clinically where an intermediate-term bone adhesive Is desired. The isobutyl ester Is the most effective 2-cyanoacrylate for bonding dentin to acrylic resin. Pretreatment of the dentin with dilute acid, addition of 2-cyanoacrylate polymer to the adhesive or application of a protective coating to the bonded surface increases the hydrolytic stability of the bond. 

Adhesives with short cure time or setting speed (cyanoacrylate, reactive adhesives with short pot lives, hot-melt adhesives) are only suitable to a limited extent for more extensive bonded joints (dm2/m2), since the application time may exceed the cure time. The bondable surface depends on the... 

To avoid inner tensions through thermal stress, it is recommended to use only adhesives that cure at room temperature. The adhesive selection is limited by the fact that many applications require an invisible glueline. In such cases, adhesives with fillers are excluded, cyanoacrylates and in particular radiationcuring products (Section 9.3.3) are the suitable choice. If the visual appearance of the bonded joint is not important, two-component reactive adhesives based on expoxides, polyurethanes, methacrylates, contact adhesives and, if required, adhesive tapes are recommended. 

With cyanoacrylate adhesives, the reactor used to convert the liquid monomer to the hard solid is the space between the parts being bonded. When conditions vary in this space, the performance of the adhesive will vary. Such parameters as temperature, humidity, space between the parts, and the type of surface being bonded can vary considerably in a given application. 

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