BS 4842:1984 download.Specification for Liquid organic coatings for application to aluminium alloy extrusions, sheet and preformed sections for external architectural purposes, and for the finish on aluminium alloy extrusions, sheet and preformed sections coated with liquid organic coatings.
1 Scope
Section 2 of BS 4842 specifies requirements for liquid organic coatings which are intended for application to aluminium alloy extrusions, sheet and preformed sections that are not to be further formed (except for cutting).
Section 3 of BS 4842 specifies requirements for the finish on aluminium alloy extrusions, sheet and preformed sections when so coated.
The methods of test for these coatings and finishes are described in section 4.
NOTE The titles of the publications referred to in this standard are listed on the inside back cover.
2 Definitions
For the purposes of BS 4842, the definitions given in BS 2015 apply together with the following.
2.1
finish1
the final or only coat in a painting process
2.2
conversion coating
a modification to an aluminium surface by chemical treatment with chromate ions or chromate and phosphate ions without the use of an applied electric current
2.3
test piece
a single item which is representative of the work being processed
2.4
test sample
a group of test pieces
2.5
significant surface
that part of the total surface of the finish on which a particular requirement for the specified finish is to be assessed
2.6
specifier
the person issuing a contract specifying particular properties of a liquid organic coating before or after its application to a substrate
0 the number, section and date of this British Standard in the form BS 4842/2:1984w;
g) statement of the weathering test(s) carried out (either 4.9 a) or 4.9 b) or both;
h) the date of despatch;
i) method of application;
j) the thinners to be used, as necessary;
k) statutory requirements and information as necessary.
Section 3. Performance and other requirements for the finish on aluminium alloy extrusions, sheet and preformed sections coated with liquid organic coatings
NOTE The tests associated with these requirements are intended to be carried out by or on behalf of the applicator of the liquid organic coating.
7 Coating
The coating to be applied to the aluminium alloy extrusion, sheet or preformed section shall be a liquid organic coating complying with section 2 of this standard.
NOTE It will be necessary for the purchaser to indicate the significant surfaces (see 2.5). if need be by supplying suitably marked specimens or technical drawings of the extrusions or preformed sections.
8 Pre-treatment of aluminium alloy extrusions, sheet or preformed sections
Each aluminium alloy extrusion, sheet or preformed section shall he thoroughly cleaned by treatment with alkaline or acidic solutions under the conditions specified by the conversion coating chemical supplier, and then rinsed. A chemical conversion coating shall be applied by treatment with a solution containing essentially chrom ate ions or chromate and phosphate ions as the active components. The amount of the conversion coating deposited depends on the type used and shall be within the limits specified by the conversion coating chemical supplier. The amount of the coating deposited shall be determined by the method given in 4.5 of BS 5411-14:1982.
0 the number, section and date of BS 4842 in the form BS 4842/2:1984w;
g) statement of the weathering test(s) carried out (either 4.9 a) or 4.9 b) or both;
h) the date of despatch;
i) method of application;
j) the thinners to be used, as necessary;
k) statutory requirements and information as necessary.
Section 3. Performance and other requirements for the finish on aluminium alloy extrusions, sheet and preformed sections coated with liquid organic coatings
NOTE The tests associated with these requirements are intended to be carried out by or on behalf of the applicator of the liquid organic coating.
7 Coating
The coating to be applied to the aluminium alloy extrusion, sheet or preformed section shall be a liquid organic coating complying with section 2 of BS 4842.
NOTE It will be necessary for the purchaser to indicate the significant surfaces (see 2.5). if need be by supplying suitably marked specimens or technical drawings of the extrusions or preformed sections.
8 Pre-treatment of aluminium alloy extrusions, sheet or preformed sections
Each aluminium alloy extrusion, sheet or preformed section shall he thoroughly cleaned by treatment with alkaline or acidic solutions under the conditions specified by the conversion coating chemical supplier, and then rinsed. A chemical conversion coating shall be applied by treatment with a solution containing essentially chrom ate ions or chromate and phosphate ions as the active components. The amount of the conversion coating deposited depends on the type used and shall be within the limits specified by the conversion coating chemical supplier. The amount of the coating deposited shall be determined by the method given in 4.5 of BS 5411-14:1982.
10.1 Test panels and test pieces. Where necessary, the applicator shall prepare test panels as described in clause 13. and shall carry out the pre-treatment described in clause 8 and carry out coating procedures in the same manner as for work pieces by passing the panels (see note) through the same system. The test panels shall be of the same alloy, if possible, as that of the work pieces and the thickness of the finish shall be as specified in 10.5.
Test pieces shall be of the same material as the work pieces and shall be pre-treated and coated in the same manner as the work pieces by passing them through the same system. The test pieces shall be conditioned at 23 ± 2 °C for at least 1 h before carrying out the tests.
NOTE If a test panel is used it should be strapped to an aluminium alloy base of thickness comparable to that of the thickest part of the work being processed.
10.2 Surface appearance. The finish on significant surfaces shall show no scratches through to the substrate. When the finish on significant surfaces is illuminated as described in BS 3900-DI and examined at an oblique angle with normal or corrected vision, no blisters, craters, pinholes, scratches, flow lines, streaks or tears shall be visible from a distance of about 1 m.
NOTE 1 If the coating applicator cannot avoid contact marks on the finish on the significant surfaces, it is essential that the specifier is advised before coating commences and that acceptable limits or alternative positions are agreed. NOTE 2 A slight degree of “orange peer may sometimes occur. The maximum extent of “orange peel” allowable should be indicated by the specifier, using suitable reference samples.
10.3 Colour. When the finish on each significant surface of the coated test piece is examined by the procedure described in BS 3900-DI, the finish on each significant surface shall match (see BS 2015) the reference colour previously nominated by the specifier (see 4.2).
NOTE If the significant surface cannot be placed in the colour-matching booth described in 11$ 3900.D1. the colour should be compared under natural diffuse daylight.
10.4 Gloss. When the finish on each significant surface of the coated test piece is tested by the procedure described in BS 3900-D5, using incident light at 60° to the normal to the surface, the gloss level of the finish shall be within seven units of the value previously nominated by the specifier (see 4.3).
NOTE If a significant surface is too small or inaccessible to allow measurements with a glossmeter, the gloss should be assessed visually by comparison with finishes of known gloss.
10.5 Thickness. When the finish on each significant surface of the coated test piece is tested (see note 2 to clause 9) with a minimum of three determinations by the procedure specified in 7.3 of BS 3900-C5:1975, the thickness of the finish shall be not less than 20 jim at any point.16.2 Apparatus
NOTE Metal should not be used in the construction of any parts of the apparatus coming into contact with the corrosive medium (16.3).
16.2.1 A room or cabinet, lined with glass, rubber. glazed ceramic, plastics material or other suitable material resistant to the corrosive medium (16.3). The room or cabinet shall have provision for heating and maintaining the temperature at :35 ± 30 °C, baffles to prevent direct impingement of spray on to the test panels and to prevent condensed liquid from the roof of the room or cabinet falling on to the test panels and inert supports from which the test panels may be suspended.
16.2.2 A supply of clean, compressed air and a suitable means of control.
16.2.3 Atomizing nozzles, one or more.
16.2.4 Mist collectors, clean, at least two. 100 mm diameter laboratory funnels in the necks of flasks are suitable.
16.3 Corrosive medium. Dissolve 50 ± I g of sodium chloride in water of at least grade 3 purity, complying with BS 3978. Add 6 mL of glacial acetic acid and dilute to I L with water.
16.4 Procedure. Take a coated test panel prepared as described in clause 13 and score two diagonal intersecting lines through to the metal. Suspend the panel in the room or cabinet (16.2.1). Adjust the flow of the corrosive medium (16.3) and of the compressed air (16.2.2) to the nozzles (16.2.3) to give a fine spray. Place the mist collectors (16.2.4) in the proximity of the test panel, one near to one of the nozzles and the other away from all nozzles, and both in positions so that no drops of solution are collected from any source other than the spray. Adjust the spray to give a fine continuous mist so that for each 80 cm2 of horizontal area the volume of solution collected in each mist collector is I mL/h to 2 mTJh.
Remove the test panel from the room or cabinet after a period of 1 000 h and rinse it by a single immersion in water. Dry the panel at 50 ± 5 °C and immediately examine it.