GB/T 46184-2025Paper and board - Tests for colour fastness (English PDF)

6.1 Sodium dithionite reducing bleaching

6.1.1 Sodium dithionite (Na2S2O4), purity greater than 80 %.

6.1.2 Chelating agent, DTPA type, concentration approximately 40 % (some

commercially available sodium dithionite products already contain a stabilizer; in this case, no additional chelating agent is needed).

6.1.3 Nitrogen (N2), purity 99.99 %.

6.1.4 Sulfuric acid (H2SO4), concentration 2 mol/L (4N).

6.1.5 Distilled water, at room temperature.

6.2 Hydrogen peroxide oxidative bleaching

6.2.1 Hydrogen peroxide (H2O2), peroxide concentration shall not exceed 40 g/L.

6.2.2 Sodium hydroxide (NaOH), chemically pure.

6.2.3 Chelating agent, DTPA type, concentration approximately 40 %.

6.2.4 Sodium silicate (Na2SiO3), chemically pure.

6.2.5 Distilled water, at room temperature.

6.3 Pad preparation

6.3.1 High-molecular-weight, low-cationic-charge polyacrylamide, powder, prepared

as a 1 g/L aqueous solution as a flocculant.

6.3.2 Aluminum sulfate [Al2(SO4)3] solution, mass concentration 330 g/L.

6.3.3 Distilled water, room temperature.

7.1 Pulping

Hydraulic pulper, 5 L capacity, capable of pulping at a relatively high pulp concentration (approximately 100 g/L).

7.2 Sodium dithionite reducing bleaching

7.2.1 Electronic balance, with a graduation value of 0.1 g.

7.2.2 pH meter.

7.2.3 Sealable glass jars (1 L or 2 L food storage containers).

7.2.4 Water bath, capable of maintaining a temperature of (50 ± 2) °C.

7.3 Hydrogen peroxide oxidative bleaching

7.3.1 Electronic balance, with a graduation value of 0.1 g.

7.3.2 pH meter.

7.3.3 Sturdy, tear-resistant plastic bags.

7.3.4 Water bath, capable of maintaining a temperature of (70 ± 2) °C.

7.4 Pad preparation

7.4.1 Buchner-type filtration apparatus, with a Buchner funnel of diameter 150 mm, and a filtration flask.

7.4.2 Beaker, capacity 1 L.

7.4.3 Heater plate capable of controlling the temperature at (95 ± 2) °C or the drying section of a Rapid-Kothen sheet forming device as specified in GB/T 24326.

7.4.4 Filter paper, diameter 150 mm, pre-weighed to the nearest 0.1 g; the filter paper shall be able to retain particles with a diameter greater than or equal to 25 µm.

7.4.5 Blotting paper, made from 100 % bleached chemical pulp, neutral, free of bonding agent, chemical additive, obvious impurity, or fluorescent substance. In each direction, the blotting paper shall be at least 3 mm larger than the filter paper. The basis weight of the blotting paper is (250 ± 25) g/m². The Klem capillary height, as determined by GB/T

461.1, shall be (70 ± 20) mm. The change in dimensions after immersion, as determined by GB/T 459, shall not exceed 3 % in any direction. The absorbency, as determined by

GB/T 461.3, shall be (450 ± 50) g/m².

7.4.6 Agitator, capable of reaching a speed of 2000 r/min.

7.4.7 pH meter.

7.4.8 Pressure roller, approximately 20 cm wide, weighing between 10 kg and 15 kg.

NOTE. Metal pressure rollers as defined in GB/T 1540.

7.5 Determination of optical properties

Reflectometer or spectrophotometer, which shall comply with GB/T 7973 in terms of geometrical, optical, and spectral characteristics, and shall be calibrated according to

GB/T 7973 and GB/T 22880.

9.1 Pulping conditions

The pulping conditions are as follows.

- pulp concentration. 100 g/L.

- temperature. 50 °C.

- time. 30 min.

9.2 Pulp preparation procedure

9.2.1 Determine the moisture content of the specimen according to GB/T 462.

9.2.2 Weigh approximately 200 g of the air-dried specimen and record the accurate mass, then cut the specimen into 2 cm × 2 cm fragments.

9.2.3 Preheat the hydraulic pulper (see 7.1) to (50 ± 2) °C with hot water.

9.2.4 Place all the specimen into the hydraulic pulper (see 7.1), then add an appropriate amount of (50 ± 2) °C hot water to achieve a pulping concentration of 100 g/L.

9.2.5 Pulping for 30 min. A portion of the broken-down pulp is used to prepare an unbleached pad (approximately 30 g, oven-dry), the remainder is used for sodium dithionite bleaching and hydrogen peroxide bleaching. If the specimen is not completely broken down after 30 min, the pulping time shall be increased, but no chemicals shall be added.

10.1 General

The pulp to be bleached is divided into two batches. one batch is bleached with sodium dithionite, and the other batch is bleached with hydrogen peroxide.

- Reducing bleaching with sodium dithionite, according to 10.2;

- Oxidative bleaching with hydrogen peroxide, according to 10.3.

10.2 Sodium dithionite reducing bleaching

10.2.1 Bleaching conditions

The bleaching conditions are as follows.

- Pulp concentration. 30 g/L.

- Bleaching temperature. 50 °C.

- Bleaching time. 45 min.

- pH of the pulp before bleaching. 6.8 ± 0.2.

- 1 % sodium dithionite (6.1.1). Expressed as pure product, in relation to the oven- dry pulp quantity, the actual amount added shall be adjusted according to the purity of the product.

- 0.5 % chelating agent (6.1.2). Added if necessary, expressed as pure product, in relation to the oven-dry pulp quantity, the actual amount added shall be adjusted according to the purity of the product.

10.2.2 Bleaching procedure

10.2.2.1 Weigh the equivalent of (30 ± 0.5) g or (60 ± 1) g of air-dried pulp, place it in a sealed glass jar (7.2.3).

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