Лако-красочные материалы — производство

Use of mixed hardener systems is common. Hardeners may be blended to achieve proper­ties intermediate to the individual components, to reduce exotherm or accelerate cure, to modify the cured properties, or simply to arrive at a more convenient mix ratio. Several classes of curing agents each having distinctive characteristics may be used.

1. Aliphatic primary amines. Common examples are diethylene triamine (DETA), tetraethylene pentamine (TEPA), n-aminoethyl piperazine, and isophorone diamine. They give good room-temperature cure at stoichiometric ratios, but have poor HDT, incon­venient mix ratios, high peak exotherm, and are strongly irritant. Isophorone diamine produces very light colored mixes with good color stability.

2. Aromatic primary amines. These offer improved heat and chemical resistance and longer pot life with reduced exotherm, but poor color stability and sluggish cure. They are generally solids and require some formulating to produce easily handleable products. Reactions proceed best at elevated temperatures, where their irritancy can be a problem. For room-temperature cures they should be used with catalysts, of which phenols, BF3 complexes, and anhydrides are the best. m-Phenylene diamine (MPDA) and methylene dianiline (MDA) are the best examples.

3. Amine adducts. Both aliphatic and aromatic amines can be adducted with small amounts of mono — or diglycidyl epoxies to produce amine adducts of medium to high viscosity that have decreased volatility and irritancy, more convenient mix ratios, and often, better reactivity.

4. Tertiary amines. Although their primary use is as catalysts with other hardeners, tris(dimethylaminomethyl) phenol (DMP 30) is an effective curing agent on its own, at both room and elevated temperatures.

5. Amides. Although amides on their own are too unreactive, reaction products of polyamines with fatty acids to produce amidopolyamines provide the largest group of commercial hardeners for adhesive applications. Reduced volatility and irritancy and a convenient mix ratio offer the compounder ease of handling. The mix ratio is relatively noncritical; increasing hardener levels yield increased flexibility and adhesion but reduced HDT and chemical resistance. Initially, amidoamines have poor compatibility and an induction period is necessary to allow the reaction to start. Pot lives are relatively long

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and exotherms low, but low-temperature cure is poor. Small quantities of imadazoline improve adhesion to metal. Dicyandiamide is a special example of an amide that can be used on its own. Its low reactivity yields a usable life in excess of 6 months, but at elevated temperatures it reacts quickly.

6. Mercaptans. Most mercaptans on their own are unreactive, but with catalysts produce flexible cures. Certain mercaptans in conjunction with DMP 30 provide extremely rapid cure, with low exotherm, making them ideally suited to retail applications.

7. Acids and anhydrides. This group of curing agents provides the best high — temperature performance. Boron trifluoride monoethylamine, oxalic acid, and maleic and phthalic anhydride are used for electrical or high-temperature applications, often with catalysts such as benzyldimethyl amine (BDMA) or DMP 30. Table 2 lists properties and characteristics of various curing agents.