Cyanoacrylates adhesive performance

Because they are acrylic monomers, alkyl cyanoacrylate esters still require the addition of radical polymerization inhibitors, such as hydroquinone or hindered phenols, to prevent radically induced polymerization over time [3j. Since basic initiation of alkyl cyanoacrylate monomers is the predominant polymerization mechanism, large quantities of free radical inhibitors can be added, with little or no effect on adhesive performance. 

Because of the need for basic initiators, cyanoacrylate adhesives do not perform well on acidic surfaces, such as wood. However, the addition of sequestering agents, such as crown ethers [30], 10, or calixarenes [31], 11, and others [32] to the adhesive improves the reactivity of the adhesive on less active surfaces. 

Despite the universal use of sutures for wound closure, there is a need to utilize adhesives instead, because of their ease of use and the reduced risk of infection. Alkyl cyanoacrylate adhesives have been studied extensively for this use, and a significant amount of research has been performed to evaluate their interaction with living tissue [40,41 J. They have been approved for external use only, because of concerns with the fact that the polymers do not readily biodegrade and can cause inflammation around the area to which it was applied. However, these concerns are reduced for -butyl cyanoacrylate, as compared to the ethyl cyanoacrylate. There is even some evidence that their use as liquid sutures actually reduces the rate of infection around the healing wound or surgical incision [42J. 

Woodward et al. 20) also investigated the shear strength of animal skin sections glued with a cyanoacrylate adhesive and cut off by a dermatome. Unfortunately, their method is complicated and it is impossible to perform a comparative estimation of the strength of glueing with different cyanoacrylates. 

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. 

Table 5.2 Typical Performance of Cured Loctite 380 Cyanoacrylate Adhesive ...

As already stated, formulated cyanoacrylate adhesives contain low levels of additives that improve the performance profile of the product. They can be divided into two general groups those that modify the polymerization process and those that alter the properties of the final polymer. 

Environmental performance The durability (see Durability - fundamentals) of cyanoacrylate adhesive bonds is reasonably good on rubbers and some polymer substrates. However, on glass and metals, both thermal and moisture durability are low. 

The patency results of PE tubes grafted wifii AAm and AA by radiation are summarized in Table 5 The length and the inner and outer diameter of the tubes are 1.8 cm, 0.8 mm, and 1.0 mm, respectively. Anastomosis of the tubes with the carotid was performed usinga cyanoacrylate adhesive and a polymer splint soluble in blood The results are compiled in Table 5. 

The cyanoacrylate adhesives sold today are quite similar to the first product, Eastman 910. This is due to the fact that changes in the structure of the monomer have dramatic effects on the adhesive properties, and because there are very few modifying monomers which will successfully coreact with the cyanoacrylates under normal application conditions. Also, the monomers with the best adhesive properties were discovered early in the development of cyanoacrylates. Finally, research toward improving the deficiencies of these products had a slow start owing to the early manufacturing problems. Beginning in the late 1970s, however, a number of adhesives with improved performance were introduced. 

While the bulk of any cyanoacrylate formulation consists of monomer, a large number of modifiers have been used to impart desired properties to the composition. These include stabilizers, inhibitors, thickeners, plasticizers, dyes or colorants, adhesion promoters, and others. Each of these classes of modifier will be dealt with in subsequent parts of this chapter. Because of the variety of modifiers, and the variety of applications for cyanoacrylates, a bewildering number of cyanoacrylate adhesives are now commercially available. These can be generally divided into the following classifications adhesives of different viscosities and cure rates, adhesives based on different monomers, adhesives for the bonding of metal, plastic, rubber, or wood, various types of improved performance adhesives, i.e., heat, moisture, or impact resistant, and adhesives for bonding low surface... 

A common feature of all of the adhesion promoters discussed here is the presence of one or more carboxylic acid groups, anhydrides, or phenolic hydroxyls. Presumably these groups complex with the metal surface during bonding and/or coreact with the cyanoacrylate during polymerization. Also, the improvement in adhesive performance can be attributed to improved adhesion, regardless of the manner in which that performance is measured. 

Toughened cyanoacrylate adhesives containing random copolymeric elastomers have been patented. The only elastomer whose performance has been quantified is an ethylene-methyl acrylate copolymer sold by Du Pont under the name Vamac B-124. Several other elastomers are said to be useful tougheners, but no details have been given beyond solubility data. 

In the last ten years, a number of improvements in cyanoacrylate adhesive technology have been published. Some of these modifications have been translated into new products. For instance, a series of adhesives is being sold with improved performance in the following areas contaminated surface bonding, hard-to-bond plastics, operating temperatures, moisture durability, impact strength, and chlorosis. A toughened cyanoacrylate based on a methyl acrylate-ethylene copolymer has been marked recently. An allyl cyanoacrylate-based adhesive with improved heat durability has also been introduced. A survey of recently patented modifications and improvements for cyanoacrylate adhesives is outlined in Table XIX. 

TABLE 7.25 Performance of Cyanoacrylate Adhesives on Various Substrates (from Ref. 30)... 

Cyanoacrylate adhesives are extremely sensitive to traces of impurities, and must be manufactured, stored and used under controlled conditions. Basic impurities or contaminants can seriously affect the shelf-life or stability of the adhesives conversely, acidic materials can slow down or completely inhibit curing. Peroxides or free-radical stabilisers in a potential additive can also seriously affect performance. Because of the sensitivity to contaminants, it is not possible to formulate cyanoacrylates with the wide range of thickeners, fillers and other additives available to formulators of other adhesive systems. 

The Araldite series of epoxy resins, manufactured by Huntsman, are perhaps the most well-known, typically two-part systems and these epoxy adhesives wonld generally be expected to bond well to epoxy thermoset plastics. Cyanoacrylates showed good results in these trials [3], but would not be expected to offer the same durability as epoxy-based adhesives. Note that there are so many different grades of epoxy materials that it is impossible to give specific data (Table 3.2) is intended as a guideline and tests would always be necessary to confirm adhesion performance. 

Surface finish and surface preparation are both key factors in the success of an adhesively bonded joint and, in many applications, roughening the plastic surface can be beneficial to the overall bond strength and the durability. If the adhesive is injection moulded, it is often possible to spark erode the mould tool to give a slightly rougher surface finish at the bond line thus improving the mechanical keying of the adhesive to the surface. A surface finish of between 1 and 2 Ra will invariably improve the adhesion performance of cyanoacrylates to thermoplastics. 

After a dwell time of five seconds under the device, it can be seen in Figure 7.10 that the shear strength of the bond between the LPDE sample and cyanoacrylate adhesive increased 10-fold, and the shear strength of the bond between the LPDE sample and the light cure acrylic adhesive increased 40-fold. Subsequently, MIL-STD-883 Method 2011.7 was applied as a destructive bond pull test to evaluate bond strength and bond strength distribution of various surface contaminations after CO2 treatment. The apparatus used had an accuracy of 5% and performed at a 90° peel angle. 

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