Лако-красочные материалы — производство

Aliphatic polyamines. These are one of the most commonly used hardeners in room temperature curing epoxy adhesives. With glycidyl ether resins only relatively small quantities (6-12 parts per hundred of resin) are required, although sometimes the rate of reaction may be a little too fast for convenience, particularly in conditions of high ambient temperature. The hardened adhesives are rigid and provide good resistance to chemicals, solvents and water. Some aliphatic amine compositions react with carbon dioxide and moisture in the atmosphere causing a characteristic tacky surface layer on the cured product. For this reason they do not provide a good bonding surface for the addition of further epoxy resin compounds(3). All aliphatic amines are caustic and can be difficult to handle so that they are generally modified for commercial use.

The basic chemical reaction between an epoxy and a primary amine can be represented by:

I I I

— c—c— + rnh2— >

/

о

НО —С —C —N —С —C —OH

І I I I I

R

A common modification which is performed commercially to selectively improve the performance is to react a glycidyl ether resin with an excess of amine groups to produce a resinous amine adduct hardener. These have advantages over unmodified versions in that they result in more convenient mixing ratios, are less dangerous to handle and can exhibit reduced moisture sensitivity.

Cycloaliphatic amines. These can give good cures under adverse conditions of low temperature and high humidity and as such form the base of many modern grouts and resins used in civil engineering. Chemical resistance and mechanical properties are similar to those of the aliphatic amines but they are still skin irritants.

Aromatic amines. Most only give a sluggish cure at room temperature and were primarily developed to provide improved heat and chemical resistance. Room temperature cures can be obtained by dissolving the amine in a diluent but this tends to reduce the strength of the adhesive. The benzene ring structure of aromatic amines results in a dense stable cross-linked network which tends to give good water resistance but rather brittle cured resins and lower strengths than can be achieved with aliphatic amines. Their intrinsic brittleness may, however, be reduced by the addition of modifiers.

Secondary and tertiary amines. Tertiary amines can be used as sole curing agents but in adhesive formulations they are more normally found in blends, for example as accelerators with polyamide systems(4) and as an essential part of polysulphide systems. The cured resins tend to be less temperature and chemical resistant than the amines described above. Secondary amines react first like primary then as tertiary amines.

Polyamides. These should be strictly called polyamidoamines since it is still the amine group which provides the reactivity. However, larger quantities of hardener are required than with aliphatic amines and they tend to have a slower rate of reaction. Flexibility is generally improved but other properties tend to be inferior, although curing at elevated temperature will effect an improvement. Mixing ratios are less critical and polyamides are less likely to give skin irritation so that they are especially suitable for use in the twin-tube packages sold as all-purpose household glues. Water resistance may be unsatisfactory for longer term structural applications.

Polysulphides. Liquid polysulphide rubbers have been used as hardeners for epoxy resins for many years. However, they are generally used as blends with tertiary amines and will be considered further as flexibilisers.

Polymercaptans and polythiols. These form the hardeners for the quick setting epoxies produced mainly for the DIY market(5). As such they are little used in civil engineering.

Dicyandiamides. These are latent catalysts which are inactive at ambient temperatures but are released for reaction at about 90 °С; at temperatures approaching 150 °С and above they react rapidly. They are primarily used in single part epoxy adhesives and require storage in cool conditions in order to remain stable for a reasonable shelf life.

Acid anhydrides. Another group of hardeners which require high temperature cure schedules(6) but give improved thermal stability over amine cured products. Because of several drawbacks including brittleness they are not often used for adhesives.

The wide range of properties available from different combinations of epoxy resins and their hardeners can be further extended by the use of other additives. These include anti-oxidants, diluents, flexibilisers, stabilisers, tougheners, fillers, surfactants and adhesion promoters.