Finishes for metal sheeting

Sheet steel

Profiled steel sheet is available with a variety of finishes. The principal types for cladding are as follows.

Hot-dipped galvanizing

This is a zinc finish, which may be spangled or matt.

Hot-dipped zinc/aluminized steel

This is a coating (aluminium finish) consisting of an alloy of approximately 55% aluminium/45% zinc, normally sold as Galvalume, Zincalume or Aluzinc. It is slightly more expensive than galvanizing, but offers better corrosion resistance.

Polyvinyl chloride on hot-dipped galvanizing

PVC Organasol is no longer in widespread use, and the more common finish is PVC Plastisol, which can be liquid applied or used as a bonded laminate. The

5.18 Lateral profiling can be produced by means of a special roller surface will invariably be textured or embossed.This can be considered as a 'thick' coating material with typical nominal thickness of 200 pm.

Acrylic or silicone enamels on hot-dipped galvanizing

Typically 20 mm with a smooth texture, this offers a different colour range from PVC. Siliconized versions of acrylics are also available, and modified silicone and acrylic finishes are now being used, particularly in the USA, as thin 'premium' coatings.

Fluoropolymers on hot-dipped galvanizing

Polyvinylidene fluorides, PVF and PVF2, are also in widespread use as thin coatings (typically 20 mm), which offer good resistance to weathering.They are usually a mixture of PVF2 and acrylic and their performance will depend on the particular formulation applied.

Acrylic-modified polyester resin over bitumen/asbestos bonded to steel

This is a very thick coating (overall thickness 555 pm), which is heavily textured, e.g. Galbestos (not now available).

Acrylic-modified polyester on an epoxy base

This is a very 'thick' coating offering good weathering characteristics (e.g. H. H. Robertson's Versacor).

The thicker PVC coatings (Plastisol) offer a better resistance to wear and abrasion than the thinner fluoropolymers (PVF2) and silicone enamels; however; the PVC colours are not so stable, and tend to fade in ultraviolet light. A general rule might be to use PVC in areas where surface wear might be expected, e.g. roofs, and to use the fluoropolymers where colour integrity is important for reasons of appearance, e.g. walls.

In order to prevent discoloration and deterioration of the organic paint finish, particularly with PVC coatings, it is important that the temperature in use should not exceed the safe temperature ranges of the finishes used (Fig. 5.19). Typical safe temperature ranges for different coatings are:

acrylics 50°C to 100°C

fluoropolymers (inc. PVF2) 50°C to 120°C

acrylic polyester 50°C to 70°C

Sheets of dark colour, such as chocolate brown, cause the most problems in heat build-up, particularly those with an insulation layer immediately behind the sheet. In general, black and dark colours can be expected to reach temperatures of 80°C during summer conditions, medium colours 65°C and white or bright aluminium 50°C.

5.19 Delamination of outer paint finish due to safe temperatures being exceeded.

At the Second International Conference on the Durability of Building Materials and Components at Washington, USA, 1981, D.A.Thomas explained that it was generally recognized that organic coatings are permeable to water and oxygen, and also to ions, so even a defect-free coating could not be expected to give permanent protection in sufficiently active environments (Thomas, I 98 I ).Thus the quality of the primer coating is critical to the durability of the paint finish. At the same conference, C. Christ showed the results of tests carried out on two paint systems: an acrylic and a modified fluoropolymer with various types of zinc and aluminium substrates (Christ, 1981). The aluminium-coated substrate was rated better than all other painted substrates at three or four exposure sites.The thickness and weight of the primer are also critical. BS 3083:1 959 defines the types of zinc coatings and tolerances on the sizes of sheets for determining the weight of zinc. In general, sheeting with the heavier zinc or aluminium coatings beneath the paint films will suffer from less overall deterioration than those with lighter metallic coating weights.

Was this article helpful?

0 0

Post a comment