Лако-красочные материалы — производство

Some of the metal decorating applications which commonly use acrylic resins, remembering that there may be large geographic variations, include:

• general line white coatings

• general line varnish

• drum coatings

• tinplate closure white coatings

• air drying side seam stripe

• DWI basecoats

Acrylic resins can have similar performance properties to polyesters and are, not surprisingly, used as an alternative to polyesters for some applications. Acrylics are used with crosslinkers. They have good heat resistance and colour retention. They are used to make externals for food cans, DWI basecoats, closure and general line work. Acrylics have superior alkali resistance than polyesters. This makes acrylics better suited than polyesters for DWI basecoats where varying degrees of pasteurisation are encountered, although there are many who may disagree.

Thermosetting (crosslinking) acrylics normally require a crosslinker to be present. Their actual cure behaviour is dependent upon the pendant (side chain) functionality of the copolymerised co-monomers. Acrylamide will crosslink with itself, epoxy or amino resins, whilst amino or melamine crosslinkers will react with hydroxyl or acid groups. Isocyanates can react with hydroxyl groups at room temperature. To reduce the hazards of isocyanates and the inconvenience of two pack systems, blocked isocyanates can be used. At elevated temperatures used for stoving they can unblock generating an alcohol and an isocyanate which will react with any hydroxyl or acid groups on the acrylic resin.

Normal curing conditions for acrylic systems are 180-190°C for a 10 minute peak. For DWI, similar conditions as those for polyesters are used, namely 5-10 seconds at an air temperature of 250-350°C (peak metal temperature of 180-200°C is reached). As a first approximation acrylic and polyester DWI basecoats are interchangeable. However, polyesters do have some advantages over acrylic resins for external coats.

These include:

• better cut edge adhesion ( only for punched sheetfed applications, not DWI) [9]

Acrylics have an advantage over alkyds or vinylated alkyds in three piece cans. The heat resistance of acrylic resins makes them suitable for applications requiring the reduction of the width of the side seam reserve. The normal sized reserve ‘spoils’ the appearance of the soldered can. Additionally, acrylics do not discolour during seaming.

Like many of the resin systems used for metal packaging, the properties of the acrylic resins are largely dependent upon the co-monomers used. To some extent they can be more mathematically quantified. They can be divided into hard and soft co-monomers. This relates to Tg (glass transition temperature), with a high (~ 80°C) Tg being hard and a low one (10°C) being soft. Examples of hard co-monomers are styrene, acrylamide, methyl methacrylate, methacrylic acid. Soft monomers include ethyl acrylate, butyl acrylate, 2-ethyl hexyl acrylate, hydroxy ethyl acrylate. As a generalisation methacrylates are hard monomers whilst acrylates are soft. Increasing the chain length of the ester increases the softness of the monomer. Thus ethyl acrylate is harder than butyl acrylate and butyl methacrylate is softer than methyl methacrylate. In thermoset systems, the Tg of the comonomers has less significance, because it is the crosslink density and the nature of the crosslinker which also have a major influence on the Tg of the cured system.

Water resistance is good, especially in acrylamide — epoxy systems, in which the acrylic backbone is not subjected to water degradation. Colour retention on ageing and at elevated temperatures is good. Significant yellowing is unlikely to occur below 250°C.

Substrate adhesion depends upon monomer composition. Hydroxy and carboxyl acrylics are better than most. Amine functional acrylics also exhibit. good adhesion. Acrylics have a characteristic odour, which may be classed as strong by some people. This is due to unreacted monomer. The cured films are generally taint free and certain acrylic resins are approved for direct food contact. They may impart an ‘off flavour to beers.

Acrylics have been used in white internals for food cans, where the hard ceramic appearance makes them hygienic and attractive.

Acrylic resins vary in price, but are generally more expensive than alkyds, but can be comparable with polyesters on a dry kg/dry kg basis.

Acrylic resins do not generally make good lithographic vehicles.

Thermoplastic acrylics, which do not crosslink, exhibit a strong temperature dependence in their hardness. Above their Tg they are plastic or formable, whilst below it they are brittle.

Thermoplastic acrylics find two uses in metal decorating. The lower molecular weight resins are used as plasticisers, whilst higher molecular weight resins are film formers by means of direct solvent evaporation in the same way as vinyl resins.

Thermoplastic (non-crosslinkable) acrylic resins can be used as internals for caps with a PVC gasket with plastisol compounds because of their good adhesion.