ISO-787-25-1993.pdf

INTERNATIONAL STANDARD IS0 787-25 First edition 1993-l O-l 5 AFmI Internat Dot sea General methods of test for pigments and extenders - Part 25: Comparison of the colour, in full-shade systems, of white, black and coloured pigments - Calorimetric method M lightness, hue, chroma and total colour dif- ference for coloured pigments) are calculated as de- scribed in IS0 7724-3. 5 Materials 5.1 Alkyd resin (binder) The alkyd resin shall consist of 63 % (m/m) linseed oil, 23 % (WI/WI) phthalic anhydride and 14 % (m/m) trimethylol propane, and shall comply with the fol- lowing requirements: Test method Acid value max. 15 mg IS0 3682 KOWg Viscosity (of the 7 Pas to IS0 3219 product as deliv- 10 Pas ered) Hydroxyl value 30 mg to 50 mg IS0 4629 KOHIg 5.2 Fumed silica The fumed silica shall comply with the following re- quirements: Test method Specific surface 175 m*/g to to be agreed be- area (BET) 225 m*/g tween the inter- ested parties pH value of a 3,6 to 4,5 IS0 787-9 4 % dispersion in water NOTE 5 Fumed silica is necessary to avoid flocculation and to control the flow properties of the pigmented system. 5.3 Preparation of the test medium The preferred test medium (see tables 1 and 2 for quantities) is prepared as follows: Mix well 97 parts by mass of alkyd resin (5.1) and 3 parts by mass of fumed silica (5.2). Take care that no silica is lost by evolution of dust. Disperse the mixture twice on a triple-roll mill (see IS0 8780-6). Another test medium may be agreed between the interested parties. This shall be indicated in the test report. Table 1 - Recommended quantities of white pigment and test medium Titanium dioxide (p = 4,0) 4,o 3,O I Zinc sulfide (p = 4,0) I 4,o I 23 I Zinc oxide (zinc white) (P = 5.8) 5.0 2,6 2 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/24/2007 23:32:49 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 787-25:1993(E) Table 2 - Recommended quantities of coloured and black pigments and test medium Pigment group (see 8.1.2) Mass of Volume of test pigment medium (5.31 g ml 3,o I,5 ItO lr5 0,5 1,5 6 Apparatus Ordinary laboratory apparatus and glassware, together with the following: 6.1 Photometer. 6.1.1 For coloured and white pigments Spectrometer or tristimulus calorimeter as specified in IS0 7724-2. 6.1.2 For black pigments Spectrometer or tristimulus calorimeter as specified in IS0 7724-2 and complying with the following re- quirements: a) Accuracy A spectrometer shall provide reflectance values to five decimal places; a tristimulus calorimeter shall provide tristimulus values to three decimal places. b) Calibration and zero adjustment The instrument should preferably be such that it can be adjusted by using a suitable black standard so that the data for the tristimulus values are close to zero. If direct adjustment cannot be performed, the readings for the black standard shall be sub- tracted from the values for the reference and test specimens. The black standard used for the zero adjustment shall be a highly efficient light trap as shown in figure 1 and having the following dimensions: A diameter of instrument sample port + 5mm Bz8Omm Ca70mm - - .-. .-.- i-r/ I - a - 3 I Figure 1 - Black standard The black standard shall have a non-gloss black interior and the bottom shall be covered with black velvet. c) Standard deviation of tristimulus values The standard deviation gr for consecutive measurements of tristimulus values shall be 0,005. It shall be calculated from 20 measure- ments of tristimulus values without changing the position of a test specimen having a tristimulus value Y of about 0,5. 6.2 Substrate, of minimum dimensions 150 mm x 50 mm, uniform, non-fluorescent, com- patible with the binder to be used and appropriate to the method of colour comparison. Steel panels, contrast cards, lacquered cardboard sheets, aluminium-covered cards, standard art paper or glass slides may be used. If a glass slide is used, it shall be clear and colourless, and of the same thickness for the reference and test specimens. 6.3 Film applicator, for the application of the pastes of the test pigment and the agreed reference pigment to the substrate. 6.4 Stencil card (for covering specimens that are still moist when presented for measurement), ap- proximately 0,5 mm thick, with a circular hole of di- ameter slightly greater than the size of the sample port of the spectrometer or tristimulus calorimeter (6.71, or glass plates of sufficient size to cover the specimen, plane parallel, polished, colourless, about 1 mm thick. 3 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/24/2007 23:32:49 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 78725:1993(E) 6.5 Automatic muller, with ground-glass plates, preferably water-cooled. The plates shall be of diam- eter 180 mm to 250 mm and such that a known, sel- ectable force of up to 1 kN may be applied to them. The driven plate shall be capable of rotating at be- tween 70 r/min and 120 r/min and the apparatus shall have an arrangement for pre-setting the number of revolutions in multiples of 25. Group c: pigments of high binder demand - pigment concentration 25 % (m/m) In order to give approximately 2 ml of mix in each case, preferably use the recommended quantities for the appropriate group as given in table2. (See also notes 6 and 7.) NOTES 6.6 Spatula, with flexible steel or plastic blade. 6 If the pigment/binder mix chosen is found to be too stiff or fluid for use on the muller, one of the other pairs of quantities given in table 1 or 2 may be used as appropriate. 7 Sampling Take a representative sample of the pigment to be tested, as described in IS0 842. 7 If the diameter of the muller plates is near the maximum of the range specified in 6.5, it may be necessary to in- crease the quantities specified in order to reduce wear on the plates 8 Procedure Take care that the results are not influenced by any temperature increase during the grinding operation. This applies particularly if the plates of the automatic muller are not water-cooled. If any influence is sus- pected, carry out preliminary tests to check this. Precondition new muller plates by milling a pigment in a suitable binder for 1 000 revolutions with a load applied to the plates. Remove and discard the paste. 8.2 Preparation of pigment dispersions Take the appropriate quantities of the test medium and the agreed reference pigment (8.1.1 or 8.1.2). Place the test medium in the centre of the lower plate of the automatic muller (6.5). Sprinkle the pigment into the test medium and mix together, using the minimum effort, with the aid of the spatula (6.6). Dis- tribute the paste at several points at a distance of about 35 mm from the centre of the lower plate or spread in the form of a ring with an internal diameter of 40 mm and an external diameter of 100 mm. Before use, check that the surfaces of each plate have an even, matt appearance and are free from score marks and polished areas. 8.1 Test portion NOTE 8 It is advisable to lay a paper ring of the requisite shape as a pattern beneath the lower plate. Take a sufficient quantity of the pigment so that, when mixed with the appropriate amount of test me- dium (5.31, the resulting paste extends almost to the edges of the muller plates. Weigh this test portion to the nearest 1 mg. 8.1.1 White pigments Preferably use the recommended quantities of pigment and test medium given in table 1. (See also notes 6 and 7.) 8.1.2 Coloured and black pigments The mass ratio of pigment to binder depends not only on the oil absorption of the pigment but also on the viscosity of the mix during the milling operation. As a first step, all pigments can be allocated to one of the following three groups: Group a: pigments of low binder demand - pigment concentration 65 % (m/m) Group b: pigments of medium binder demand - pigment concentration 40 % (m/m) Clean the spatula as much as possible by wiping it on the upper plate of the muller. Close the plates of the muller and grind the mixture in stages of 50 revolutions with the highest practi- cable load or a load agreed between the interested parties. After each stage, collect the paste with the spatula from both plates and spread it as described above on the lower plate, wiping the spatula on the upper plate as before. After the required number of revolutions, remove and store the paste in a suitable receptacle and then clean the muller plates and spatula. Grind the mixture for the same total number of revol- utions as required for the full determination of the dispersibility in accordance with IS0 787-24, to assure complete dispersion of the pigment. Indicate the number of revolutions in the test report. Take a similar amount of the test pigment and prepare a paste in the same way. Alternative equipment for the dispersion of pigments may be agreed between the interested parties. This shall be indicated in the test report. 4 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/24/2007 23:32:49 MDTNo reproduction or networking permitted without license from IHS -,-,- 8.3 Preparation of test specimens 8.3.1 General The preparation of the test specimens depends on the method of application, the substrate and the film thickness, which shall be chosen according to the in- tended application of the particular pigmented sys- tem. Apply the test pigment dispersion and the agreed reference pigment dispersion by the same method over a width of at least 40 mm. Measure wet samples as soon as possible after ap- plication. 8.3.2 White pigments Spread the paste with the film applicator (6.3) on to the substrate (6.2), using a clearance of 150 pm to 200 pm for titanium dioxide and zinc sulfide (100 %) pigments, and of 500 pm for lithopone and zinc oxide (zinc white) pigments. 8.3.3 Coloured and black pigments 8.3.3.1 Hiding layers for evaluating differences in full-shade colour Apply the test pigment dispersion and the agreed reference pigment dispersion in a hiding layer. NOTE 9 A layer is considered to be a hiding layer when, on visual inspection of a film laid on a black and white con- trasting test substrate, the contrast is no longer visible. 8.3.3.2 Non-hiding layers for evaluating differences in mass tone Apply the test pigment dispersion and the agreed reference pigment dispersion side by side in one and the same application process (if possible) and in the same film thickness. 8.4 Measurement Measure the tristimulus values of the specimens prepared from the test pigment dispersion and of the agreed reference pigment dispersion as described in IS0 7724-2, using the appropriate geometry (see IS0 7724-l and IS0 7724-3). The aim of this part of IS0 787 is to test pigments, and differences in the surface reflectance of speci- mens to be compared must not therefore be allowed to influence the results. With the exception of very dark samples, this can be achieved by using the 8/d or d/8 geometry with specular reflection included. With high-gloss specimens, subtract the value of sur- face reflection from the measurement to enable a better correlation with the visual evaluation. 45/O or O/45 geometry can only be used with high-gloss or IS0 787-25:1993(E) completely matt specimens. With very dark speci- mens (for example those prepared from carbon blacks or in the case of mass tones of transparent pigments), care shall be taken that differences in gloss or surface structure do not affect the reflectance values. In some cases, it is advisable to apply the dispersions to a glass plate and to measure through this glass plate, using the 45/O or d/8 geometry with specular re- flection excluded. 9 Expression of results From the measured values, obtain the colour charac- teristics indicated in 9.1 or 9.2 respectively, using the equations given in IS0 7724-3. 9.1 White pigments and black pigments 9.1.1 Relative hue Calculate the values Aa =a;-a;( and Ab = bf - b;, where R refers to the reference specimen; T refers to the test specimen. Determine the relative hue from the signs of Aa and Ab and the value of IAb*/Aa l (the absolute value of the ratio). Express the relative hue by the appropriate colour taken from table3. Table 3 - Name of relative hue lSignofAb* ) -S;Q+ 1 IAMBS 9.1.2 Amount of chromaticity difference Calculate the value As, by which the amount of chromaticity difference is characterized, using the fol- lowing equation: 5 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/24/2007 23:32:49 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 787-25:1993(E) As = (Aa*)* + (Ab )* NOTE IO The value obtained is equivalent to As = II (A,?-L; where R refers to the T refers to the reference specimen; test specimen. 9.3 Coloured pigments Express the colour difference as the CIE 1976 L*a*b* colour difference A,? a reference of this part of IS0 787 (IS0 787-25); the composition of the mill base (see 8.1); the method of preparation of the test specimens; the number of revolutions chosen if a muller was used; if available, the film thickness, the pigment con- centration and the substrate; whether full-shade or mass tone; the type of apparatus (spectrometer or tristimulus calorimeter) and the measurement geometry used; the results of the test as indicated in clause 9; any deviation from the test method specified; the date of the test. 6 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/24/2007 23:32:49 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 787-25:1993(E) UDC 667.62253565 Descriptors: paints, pigments, tests, determination, colour, comparison Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/24/2007 23:32:49 MDTNo reproduction or networking permitted without license from IHS -,-,-