Cyanoacrylate allyl

Also shown in Fig. 2 is an allyl cyanoacrylate. This molecule contains a second double bond that can be made to react after the initial polymer chain is formed. This secondary bonding can occur between adjacent polymer chains, causing cross-linking of the chains. Such cross-linked polymer chains are more heat resistant than is the uncross-linked polymer. 

The data presented in Table 1 compare the heat resistance of allyl cyanoacrylate and methyl cyanoacrylate determined by heating a steel lap shear specimen for 1 week... 

Table 1 High-Temperature Resistance of Allyl Cyanoacrylate... 

The fourth type of cyanoacrylates presented in Fig. 2 are the alkoxyalkyl esters. Methoxyethyl cyanoacrylate and methoxyisopropyl cyanoacrylate esters have all the desirable properties of the methyl, ethyl, and allyl cyanoacrylates, with the added advantage of low vapor pressure. As a result, these monomers have little or no odor, which makes them popular for use in environments where ventilation is a problem. The low vapor pressure also reduees the fogging of adjaeent parts so often seen with regular cyanoaerylates on damp days, a problem discussed in more detail below. 

Figure 4 Hot strength of allyl cyanoacrylate, lOOcP Brookfield, bond cured at room temperature for 24 h, heated for 2 h at temperature and tested hot.

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. 

The crosslinking of alkenyl cyanoacrylates can be promoted by adding a radical initiator to the adhesive. A number of peroxides have been utilized for this purpose." Kato et al listed several examples in which allyl cyanoacrylate containing methyl ethyl ketone peroxide was cured on steel substrates at elevated temperatures." Subsequent OLS testing at temperatures up to 200°C showed that considerable crosslinking occurred under these conditions. Unlike the equivalent British patent granted to these inventors, the U.S. Patent claimed only propargyl cyanoacrylates." No claims for mixtures of cyanoacrylates and radical initiators were made. 

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... 

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. 

To minimize the gradual embrittlement that can occur on aging of cyanoacrylate adhesives, plasticizers are added. Some of the materials, which have been used as plasticizers, include phthalates, phosphonates, acyl esters, succinates, and cyano-acetates. The use of allyl, methallyl, and crotyl phthalates is also claimed to improve thermal resistance properties in addition to plasticizing the adhesive [23]. 

The adhesive marketed under the tradename Superglue contains the monomer methyl a-cyanoacrylate (Fig. 14.5.1). A variety of cyanoacrylates are commercially sold as contact adhesives with the alkyl group -R denoted in Fig. 14.5.2 varying from a methyl group to produce ethyl, isopropyl, allyl, butyl, isobutyl,... 

The chemistry of cyanoacrylate adhesives contains no co-reactants but can polymerise at room temperature on any substrate that is exposed to atmospheric moisture or alkaline surfaces. Synthesised cyanoacrylate esters can be methyl, ethyl, n-propyl, n-butyl, allyl, ethoxyethyl and methoxyethyl. The basic structure of the cyanoacrylate monomer is ... 

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). 

While the patent literature is full of examples of cyanoacrylate esters of all descriptions, the most commonly used monomers are ethyl and methyl cyanoacrylate. A few other monomers are in commercial use today including the isopropyl, n-butyl, allyl, ethoxyethyl, and methoxyethyl esters. The structures of these common monomers are given in Table I. 

Cyanoacrylates cure to thermoplastic polymers and typically give very limited high-temperature performance. The Tg of polyethyl cyanoacrylate is -100 °C, and bonds at temperatures close to this fail dramatically. Early attempts to improve heat resistance involved the development of cyanoacrylates with an allylic group in the... 

Several special esters also deserve mention, the allyl-2-cyanoacrylates and the jS-alkoxy-alkyl cyanoacrylates. 

Long term thermal resistance of cyanoacrylate adhesives where constant exposure to temperatures greater than 60-80 C is required remains a problem. As already mentioned, an early approach was to substitute an allyl group in the side chain in place of the saturated alkyl group. 

Allyl based cyanoacrylates were developed to improve the hot strength but these products required a secondary heat cure and parts to be clamped during the heat cycle to allow the allyl group to fully crosslink. Other cyanoacrylates with additives (such as phthalic anhydrides) have been formulated and these products do provide improved long-term strength at 120 C. 

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