Лако-красочные материалы — производство

The term cyanoacrylate comes from the chemical structure of these materials. Figure 1 depicts the general formula for cyanoacrylates. As the figure shows, cyanoacrylate adhe­sives are made up of carbon, hydrogen, oxygen, and nitrogen. The way these atoms are

Figure 1 General formula for cyanoacrylate adhesives.

arranged in the molecule is important because the arrangement or configuration of the atoms affects the chemical properties of the molecule.

The CN group is called a nitrile or cyano group. In addition to giving these adhesives part of their name, the cyano group is highly polar and gives the molecule some of its strong adhesive character. The remainder of the molecule, the acrylate portion, is also polar and further enhances its adhesive character.

The letter ‘‘R’’ is used in organic chemistry to represent a part of a molecule that differs within a ‘‘family’’ of compounds. In the cyanoacrylate family, the R group is the alcohol that forms the ester with the acylic acid portion of the molecule. The type of alcohol used gives the cyanoacrylate adhesive its name. Thus methyl cyanoacrylate is the ester formed between methyl alcohol and cyanoacrylic acid. Figure 2 shows the con­figuration of some of the commercially available cyanoacrylate adhesives.

All of the molecules shown in Fig. 2 are thin, watery liquids at room temperature. In this low-viscosity state, these molecules are capable of flowing freely onto the surface of the parts to be bonded. However, such thin liquids would have little or no ability to bridge the gap between the mated parts. The carbon-to-carbon double bond shown in Fig. 2 to be common to all cyanoacrylates is capable of reacting such that adjacent molecules are linked together to form a large chain. The chains become so large that the adhesive changes from a liquid to a hard, tough solid. The chemical reaction involved, called polymerization, is depicted in Fig. 3.

The initiator that drives the polymerization or curing reaction of cyanoacrylate adhesives can be any of the chemicals that generate free radicals. Electromagnetic

CN

П . — нгс=с — ————————

COjR

Figure 3 Polymerization of cyanoacrylate adhesives.

radiation in the form of heat or ultraviolet light can also trigger the reaction, but usually only enough to cause problems with product shelf life. The more likely route for cyanoacrylate polymerization is by an ionic initiation. Any molecule more alkaline than water can initiate the curing reaction. This type of polymerization is characteristically much faster than a free-radical type and is the reason that cyanoacrylates cure so rapidly. It is this cure speed more than any other property that makes these adhesives so popular on the production line. Many adhesives are stronger or more durable than cyanoacrylates, but none can cure as quickly and to such a wide variety of substrates as the cyanoacrylates.

Although it is not essential to understand the chemistry of cyanoacrylate polymer­ization to be able to use these adhesives, knowing that a chemical reaction is taking place helps the user to understand how application conditions affect their performance. Consider the fact that the common polymers, such as polyethylene, polystyrene, and poly(vinyl chloride) (PVC), are made in sophisticated reactors. Parameters such as tem­perature, monomer concentration, and amount of activator are carefully controlled.

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.

Figure 2 shows a number of different types of cyanoacrylic esters. There are subtle differences between them that can be utilized in specific applications. Methyl cyanoacry­late is a more polar compound than any of the others. This gives the cured adhesive a higher cohesive, or internal, strength. As a result, it has a higher shear strength which can be utilized on metal parts and other parts that are rigid enough themselves to benefit from the strength of this hard, brittle polymer.

Ethyl cyanoacrylate is a little less polar than methyl cyanoacrylate, and has the ability to wet plastic surfaces more readily, and is a better solvent for plastics. With this added ability to make intimate contact with the surface, the bonds on plastic are stronger with ethyl cyanoacrylate than with the methyl ester. This difference in performance gives rise to the adage that methyl is for metal and ethyl is for everything else. Sometimes this difference can be utilized in reverse to good advantage to avoid stress cracking on such sensitive plastics as polycarbonate and polyacrylate.

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

at various temperatures. At room temperature the two types are essentially equal in strength. The slightly higher strength of the methyl cyanoacrylate caused by its higher polarity can be seen clearly in the higher value obtained in the test cured and aged at room temperature.

The effect of exposure to 100°C shows up in the loss of strength for the uncross — linked methyl cyanoacrylate and a higher strength for the allyl cyanoacrylate because of the extra strength contributed by the reaction of the double bond in the allyl cyanoacry­late. After exposure to 120°C, all the strength of the straight-polymer-chain methyl cya­noacrylate is lost. The strength of the cross-linkable allyl cyanoacrylate is also reduced, suggesting the loss of the contribution to its strength by intermolecular association. Only the contribution from the allyl group survives.

Higher temperature causes the allyl double bonds to react faster. As a result, more cross-linking can take place and more of the strength is retained. Resistance to tempera­ture above 120° C is possible provided that the parts are clamped during the curing process. Because of the extensive cross-linking, the resultant polymer is very brittle and it is recommended only for metal and other rigid, higher-temperature-resistant substrates.

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 reduces the fogging of adjacent parts so often seen with ‘‘regular’’ cyanoacrylates on damp days, a problem discussed in more detail below.

In addition to the benefits of low odor and reduced fogging, these adhesives form stronger bonds to low-energy substrates such as EPDM rubber, natural rubber, and other difficult-to-bond plastics. This property seems to be a function of the solvent action of the uncured adhesive, so care must be taken to avoid stress cracking when the adhesive is used on sensitive substrates such as polycarbonates and polyacrylates.

While the alkoxy cyanoacrylates cure by the same mechanism as regular cyanoacry­lates, the cure speed is a bit slower and the overall strength is about 20% lower than that of ethyl cyanoacrylates. The strength is well in excess of the strength of more plastic substrates, however; the 20% reduction in strength is not significant.