Cyanoacrylates moisture resistance

Cyanoacrylates have shown themselves well in permanent outdoor assemblies as well as in temporary manufacturing aids. They are safe, convenient materials to incorporate in plant operations. New developments in technology have improved moisture resistance, setting times, gap filling, clarity, high-temperature resistance, and flexibility, and most recently, cyanoacrylates have become less surface sensitive. 

Adhesives based on higher homologs than the methyl form have been in use for a number of years. These include the ethyl, propyl, and butyl esters of cyanoacrylic acid. Moisture resistance of the methyl-2-cyanoacrylate is only fair. Ethyl cyanoacrylate has been shown to form stronger bonds than the methyl form between several different types of plastic surfaces. The higher homologs, however, generally do not form bonds as strongly as the methyl form. ... 

Moisture resistance This may be increased on metal and glass substrates by including cross-linking agents, which may yield a more hydrolytically stable polymer or by using hydrophobic monomers such as fluorinated cyanoacrylates. Silane adhesion promoters also improve moisture durability. There is also evidence to suggest that inclusion of some of the anhydrides described above has a beneficial effect. 

Disadvantages of cyanoacrylate adhesives include poor thermal and moisture resistance on metals and glass, brittleness, sensitivity to surface preparation and poor cure through... 

There are several reasons why cyanoacrylates are attractive as adhesives. They are easy to apply, one-part, 100% reactive, storage-stable adhesives. They cure rapidly at room temperature when spread in thin films between substrate surfaces, and they form strong bonds between a variety of substrates. However, cyanoacrylates do have several serious shortcomings including poor heat resistance, poor moisture resistance, poor peel and impact resistance, and limited ability to fill gaps and to bond porous substrates. The poor durability and impact resistance have been particular limitations in metal-to-metal bonding. 

Several adhesion promoters were described in Section III.A.2.a., but only a few of these are heat-resistant adhesion promoters. One of these is itaconic anhydride which has been shown to improve the heat resistance of allyl cyanoacrylate. As shown in Table X, BTDA is an effective heat resistance promoter and other anhydrides from ref. 79 may also be effective. Recently, phthalic anhydride was described as a heat and moisture resistance promoter for cyanoacrylates. These patent examples illustrate the effect... 

The moisture resistance of cyanoacrylate adhesives, like the heat resistance, is poor, and the problem is particularly apparent on metals. A cyanoacrylate bond responds in several ways to a moist environment. Polyalkyl cyanoacrylates are very susceptible to alkaline hydrolysis, which degrades the molecular weight rapidly (see Section II.F.). Corrosion products generated at the interface may catalyze this hydrolysis. Loss of adhesion due to displacement of the adhesive from the adherend by moisture is a common occurrence with many adhesives. Evidence that this mechanism... 

Some of the adhesion promoters listed in the previous section are also moisture resistance promoters. For example, the polycarboxylic acids and their anhydrides of ref. 79 were alleged to improve moisture resistance, but no evidence was presented to back this claim. Also, phthalic anhydride was shown to impart improved moisture resistance to ethyl cyanoacrylate stainless steel-stainless steel bonds. 

Durability. Cyanoacrylates suffer from poor heat and moisture durability. This failing is pronounced on metal adherends, but minimal on most plastic or rubber adherends. Poor heat resistance is due to several causes the thermoplastic nature of the polycyanoacrylate, the tendency to retropoly-merize, and the loss of adhesion experienced on heat aging of cyanoacrylate bonds. The poor moisture resistance is due in part to the hydrolytic degradation of the polymer and in part to the loss of adhesion caused by exposure to moisture. 

Solvent resistance is typical of polar, linear high polymers. In nonpolar solvents, attack is negligible, while solvents of similar solubility parameter will weaken cured cyanoacrylate bonds slowly. (See Table 3). Moisture resistance of cyanoacrylates is not considered to be a strong point of these adhesives, however, with proper attention to adhesive and substrate composition, excellent bonds are achievable. 

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. 

Cyanoacrylate adhesives (Super-Glues) are materials which rapidly polymerize at room temperature. The standard monomer for a cyanoacrylate adhesive is ethyl 2-cyanoacrylate [7085-85-0], which readily undergoes anionic polymerization. Very rapid cure of these materials has made them widely used in the electronics industry for speaker magnet mounting, as weU as for wire tacking and other apphcations requiring rapid assembly. Anionic polymerization of a cyanoacrylate adhesive is normally initiated by water. Therefore, atmospheric humidity or the surface moisture content must be at a certain level for polymerization to take place. These adhesives are not cross-linked as are the surface-activated acryhcs. Rather, the cyanoacrylate material is a thermoplastic, and thus, the adhesives typically have poor temperature resistance. 

Since they are thermoplastic the cyanoacrylate adhesives have limited resistance to heat they do not resist moisture, and can be softened by highly polar solvents like ketones. They are expensive but since only a small quantity is necessary to form a bond, their overall economy in use is good. 

Cyanoacrylates are one-part, highly polar thermoplastic polymers. The resin monomers cure in seconds when in contact with a weak base such as the moisture that is present on most surfaces. Many cyanoacrylate-adhesive formulations are commercially available, but not widely used in electronics assembly because of their poor resistance to solvents and moisture at elevated temperatures (>70 °C). Cyanoacrylates have relatively low impact and peel strengths and may be brittle unless toughened by the addition of elastomeric resins. 

Cyanoacrylates polymerize by anionic mechanism initiated by moisture or basic ions. The polymers formed tend to be more brittle than those formed from other acrylics, and the bond-lines are usually not resistant to degradation by moisture. Adhesive formulations are singlecomponent, fast-curing products, however, and suited to many hard-to-bond surfaces such as rubber and many thermoplastics. 

A common method of wire-tacking today is to use cyanoacrylate adhesive and primer. In this process, a drop of cyanoacrylate is placed on top of the wire to be tacked, followed by a drop of primer. The resulting wire-tack is usually serviceable in 5 - 15 seconds although several hours can be required for full cure. The usage of the wire tack, although limited by a lack of moisture, temperature and impact resistance, has been finding increasing application in the electrical industry. 

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... 

The resistance to hydrolysis of a polycyanoacrylate can also be improved by copolymerization with another monomer. In fact, the cyanopentadienoates cited above should also be considered comonomers. An electron-rich vinyl monomer like styrene and a cyanoacrylate ester will spontaneously copolymerize a short time after mixing. The product is a one-to-one alternating copolymer. This reaction has been used to make a two-part adhesive with better hydrolytic stability than the standard cyanoacrylate. " The carbon dioxide evolved from a suspension of the polymer in boiling water was measured to follow the degradation. After eight hours, polymethyl cyanoacrylate had lost 28% carbon dioxide by hydrolysis and decarboxylation of the polycyanoacrylate ester groups, while the copolymer had lost none. However, this improvement in moisture durability is achieved at the expense of the convenience of a one-part adhesive. 

Discriminating between the various effects of heat and moisture on the strength of cyanoacrylate metal-to-metal bonds is not easy. The environment can affect the adhesive, the metal surface, or the interface between them. The reduction in strength may be due to heat alone, or to water, or to both. The most durable adhesive possible today would probably contain a room temperature active crosslinking agent and an adhesion promoter resistant to both heat and moisture. Table XII lists the water durability modifiers discussed in this section. 

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. 

Firstly, as previously mentioned, if the service environment chemically attacks the adhesive to any significant extent, the joint may well be appreciably weakened. If high temperatures are avoided, then most modem adhesives generally have good resistance to chemical attack by moisture, although a few, like the cyanoacrylates (Section 5.2.2), for example, are quite susceptible to hydrolysis even at room temperature. 

Tlie distinction between structural and nonstructural bonds is not always clear. For example, is a hot melt adhesive used in retaining a fabric s plies structural or nonstructural One may argue that such an adhesive can be placed in either classification. However, the superglues (cyanoacrylates) are classified as structural adhesives even though they have poor resistance to moisture and heat. 

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