Polyamide cures, fatty

At lesser rubber levels, heavy duty solventless coatings models based on dlethylenetrlamlne or fatty polyamide cures, showed elastomer-modification (10 phr rubber level) to advantage in Gardner impact, mandrel bend and corrosion-resistance testing (41). Impact testing (direct and reverse) gave 110 and 60 in-lbs, respectively for the rubber-modified fatty polyamide cured epoxy coating (14 days at R.T.), whereas a control formulation tested 10 in-lbs in each mode. 

Polyamide curing agents are the reaction products of dimerized fatty acids and aliphatic amines such as diethylenetriamine. This introduces a bulky, oil-compatible, C36 carbon group between the amine sites. Similar to the diglycidyl ether adducts of aliphatic amine, they are manufactured by adding the fatty acid to an excess of amine. They are available in a range of viscosities that can be achieved by varying the amine/acid molar ratio in the reaction. 

Polyamides. Polyamides are formed by the condensation reaction of polyamines with dimeric fatty acids. They are available from a number of suppliers in a large variety of viscosities and molecular weights. Many products are available in the form of adducts which provide good compatibility with epoxy resins without the need for an induction period and give a better cure under adverse conditions compared with standard polyamides. Polyamide curing agents offer a number of desirable features, as follows (Dow Chemical Company, undated b) ... 

Higher molecular weight epoxy resin hardeners such as fatty polyamides do not have the problems associated with amines and amine adducts. These reaction products of dimer and trlmer acids with polyfunctional amines provide films with fewer surface discontinuities. Polyamide cured epoxy coatings are more forgiving than amine cured epoxy coatings since they require less demanding surface preparation. Also, mix ratios are less critical for poly-amide/epoxy coatings (56). 

Fatty polyamides are used to cure epoxy resins where a more flexible product is required, particularly in adhesive and coating applications. An advantage of the system is that roughly similar quantities of... 

FIGURE 3.30 Amine-terminated fatty polyamides for cure of epoxy resins. 

Fatty polyamide (Versamid , oldest of many trade names). A polymer formed by the condensation of a dibasic acid having a bulky side group and from 13- to 21-carbon chains, or the dimmer acids, C-36, with di- or polyamines. The commercially important dimmer acids are addition products of unsaturated C-18 fatty acids and can take several forms, giving different structures to the polyamides. They are used in hot-melt adhesives, inks, as epoxy flexibilizers and, in amine form, as curing agents for epoxies. Skeist I (ed) (1990, 1977, 1962) Handbook of adhesives. Van Nostrand Reinhold, New York. 

Polyfunctional amines which are commonly used as curing agents for epoxy resins but which are of a rather different kind to those described above are the so-called fatty polyamides. These polymers, which are described in Section 9.3, are of low molecular weight (2000—5000) and are prepared by treating dimerized and trimerized fatty acids with ethylenediamine or diethylene triamine. Fatty polyamides are used to cure epoxy resins where a more flexible product is required, particularly in adhesive and coating applications. 

Other curing agents analogous to polyamide are amidoamides, which are produced by reacting fatty acids with an excess of polyamines. They have lower viscosity than polyamides. Their cured film properties are closer to those of polyamide cured systems, but their lower cross-link density results in lower corrosion resistance. 

Polyamides provide RT cure of epoxy-terrninated resins as weU as flexibiHzation they are derived by reaction of dimerized vegetable oil fatty acids (dimer acids) with polyamines. 

Amidoamine or polyamidoamine curing agents have reactivity with DGEBA epoxy resins that is similar to the polyamides. However, they are lower-viscosity products and are also lower in color. Amidoamines are derivatives of monobasic fatty carboxylic acids and aliphatic polyamines. Since the amidoamines have only one amide group per molecule, they are lower in molecular weight, viscosity, and amine functionality than the polyamides. 

Polymerized fatty acid modification of resins and/or curing agents (e.g., polyamides, amidoamines)... 

Still another reaction that accounts for the major application of liquid polyamides and fatty amido amines is a curing agent for epoxy resins. These reactions and related applications will be discussed in detail later. 

The reaction mechanism of liquid dimer polyamides and fatty amido amines with epoxy resins has been studied by Peerman et al. (27). who employed infrared spectroscopic analysis to determine reaction rates. They showed that the terminal epoxy content of a blend of amino-containing polyamide and epoxy resin disappeared more rapidly at 150 °C than does the epoxy content of blends of epoxy resin with triethylenetetramine or tris[(dimethylamino)-raethy1Jphenol. Both of these compounds are well-known for their fast cure at ambient temperatures. Correspondingly, the liquid polyamide or fatty amido amines-epoxy combinations cure slower than the other two systems at ambient conditions. 

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