Papermaking Dye Domestic Special Grade is supplied as a spray-dried direct dye formulation under model designation PD-DSG-301. The product is based on a disazo anionic chromophore and is intended for wet-end addition in unbleached and recycled fiber furnishes, including old corrugated containers, bleached chemical pulp, and high-yield mechanical pulp. Typical physical presentation consists of dark blue free-flowing granules with a bulk density of 0.55 to 0.75 g/cm³ after 100 taps. Moisture content is controlled to ≤8.0% by the method described in ISO 787-2, and sieve residue on a 45 µm screen is maintained below 0.1% when tested according to ISO 787-7. The pH of a 1.0% aqueous solution at 25 °C falls between 6.5 and 8.0 under the condition given in ISO 787-9. The product’s water solubility is specified as ≥95% in a 5.0 wt% solution at 50 °C after 30 min of low-shear mixing in a planetary mixer operating at 50 rpm. The reduced electrolyte carry-over target is ≤2.5 wt% sodium sulfate, which limits white-water conductivity increase during closed-loop operation, as measured by ASTM D1125-23 in a 2.0% solution.

What Wet-End Addition Window Avoids Dye Reversion in High-Yield Furnishes?

Wet-end addition of PD-DSG-301 is typically performed at 0.05% to 0.40% on oven-dry fiber mass. The dye stock solution is prepared at 2.0 to 5.0 wt% using water at 40 to 50 °C; dissolution below 40 °C increases the risk of undissolved aggregates, while prolonged heating above 60 °C accelerates thermal degradation of the disazo chromophore and causes shade drift. The preferred injection point is the suction side of the fan pump, after the cleaner system and before the headbox screen, to limit residence time in the presence of cationic retention aid. In a typical 120 g/m² testliner furnish containing 70% OCC and 30% bleached kraft, dye addition at 0.25% increases colour strength to a reflectance-based target of 42% to 46% absolute at 460 nm after handsheet preparation according to ISO 5269-2. Colour strength is evaluated from spectral reflectance using ISO 5631-1; brightness and shade reversion are monitored by ISO 2470-1 and ISO 5631-1. Dye fixation efficiency, measured as the difference between headbox and tray water soluble dye concentration divided by headbox concentration, was recorded at 0.89 in a single production-scale observation on a 4.8 m fourdrinier producing white-top liner. Published data for this exact closed-loop configuration is limited; the stated value is representative of one production trial and should be verified on-site.

At pH below 4.5, aluminum sulfate present above 0.3 kg/t promotes direct dye precipitation as a dye-alum lake, increasing two-sidedness and reducing retention by up to 10 percentage points. At pH above 8.0, anionic fibre surfaces become more negative, and dye retention decreases unless a cationic fixative is applied after dye injection. When white-water conductivity exceeded 3.5 mS/cm, the same fixation efficiency dropped from 0.89 to 0.71. The product should therefore not be applied to systems operating above 3.5 mS/cm without dilution or bleed. Conductivity is determined by ASTM D1125-23 on filtered white-water samples at 25 °C. The addition sequence should maintain at least 20 s between dye and cationic polyamine retention aid; simultaneous injection in a common line can form sticky deposits on pressure screen baskets and reduces first-pass retention of the dye by 15% to 25% as measured by filtrate colour intensity using ISO 5631-1.

For white-top linerboard, dye addition is commonly split between the top ply and the base ply. The top-ply dosage is 0.20% to 0.35% on dry fibre, while the base ply receives 0.05% to 0.10% to mask greyness from OCC. In tissue grades dyed pastel shades, addition is limited to 0.02% to 0.08% because optical brighteners and cationic softeners compete for fibre surface sites. At tissue addition levels above 0.08%, wet-migration into the Yankee coating can occur, producing visible shade transfer on the doctor blade and reducing crepe uniformity. The dye should be added after any wet-end starch but before the headbox, with 10 to 15 s of line mixing at a line velocity of 2.0 m/s to disperse the stock solution uniformly without excessive shear.

Comparative Behaviour Against Imported Direct Dye Grades

Direct comparison with imported direct dye grades used in the same OCC furnish reveals three operationally relevant deviations. The domestic special grade is supplied with a narrower moisture specification of ≤8.0% versus ≤10.0% for the imported benchmark, which reduces metering weight drift in humid packaging areas. The electrolyte carry-over is set at ≤2.5 wt% sodium sulfate, while imported grades in the same colour index class often carry 6.0 to 8.0 wt%. That difference has a measurable effect on closed white-water conductivity: in a laboratory recirculation study using a retention aid at 0.4 kg/t, the domestic grade raised conductivity by 0.8 mS/cm after 6 cycles, whereas the imported grade raised it by 1.7 mS/cm under identical conditions. The third difference is the dissolution temperature threshold; the domestic special grade dissolves to 95% at 40 °C, whereas the imported benchmark requires 60 °C for the same dissolution yield, which affects steam use in stock preparation. Compared with domestic general-grade direct dyes, PD-DSG-301 has lower filler sensitivity: brightness loss on a filled sheet at 10% precipitated calcium carbonate was 0.6 points on ISO 2470-1 for the special grade, while the general grade lost 1.4 points. Shade deviation under varying freeness from 350 to 550 mL CSF per ISO 5267-1 was ΔE_ab 1.2 for PD-DSG-301, compared with ΔE_ab 2.6 for the general grade.

When Closed White-Water Conductivity Exceeds 3.5 mS/cm

In closed white-water loops operating above 3.5 mS/cm, the product’s low electrolyte design mitigates but does not eliminate dye retention loss. Conductivity excursions above 3.5 mS/cm typically arise from high levels of sulfate and chloride in recycled furnish or from wet-end mineral fillers. At 4.0 mS/cm, headbox to tray dye retention decreased to 0.66 in the production trial described above. At 5.0 mS/cm, dye mottling appeared on the fabric side of a 160 g/m² white-top liner, with shade variation of ΔE_ab 2.1 measured according to ISO 5631-1 across the cross-direction. The operational countermeasure is a partial white-water bleed of 8% to 12% by volume or the addition of process water with conductivity below 1.0 mS/cm to bring the tray water within the target window. If bleed is not possible, dye dosage should be reduced by 0.05 percentage points for every 1.0 mS/cm increase above 3.5 mS/cm, based on linear regression of retention data. However, published data for this specific dosage compensation is limited; mills should confirm with a headbox-to-tray dye audit.

Closed-loop operation also concentrates suspended solids and anionic trash. Cationic demand of the thick stock after chest dilution should be measured with a particle charge detector and kept below 0.8 meq/L before the dye feed point. Above this value, the anionic dye competes with dissolved colloidal material for cationic retention aids and shade variation increases. A polyamine fixative can be applied, but only after a residence delay of at least 20 s following dye injection. Simultaneous addition of dye and polyamine at the same feed pipe was observed to form a gel-like deposit on a 0.25 mm slotted pressure screen, raising differential pressure by 12 kPa over 8 h in a trial on a 2.8 m gap former. This is an operational boundary for the product.

For food-contact paper grades, the compliance boundary is set at 0.25% on oven-dry fibre. At or below this addition level, the product is documented to comply with FDA 21 CFR 176.170 for aqueous and fatty food types when extracted according to EN 645 and EN 647. Above 0.25%, end-use migration testing is required because the disazo chromophore may exceed organoleptic thresholds in fatty food simulants. The product contains no listed SVHC above 0.1 wt% under REACH, as declared in the safety data sheet and based on a compositional audit against the candidate list. Heavy-metal content is controlled to ≤5 mg/kg lead, ≤2 mg/kg cadmium, ≤3 mg/kg mercury, and ≤10 mg/kg arsenic when analysed by inductively coupled plasma–mass spectrometry after acid digestion. These values are supported by a type test certificate and are not repeated as batch certificate items. For paper and board intended for dry food contact, compliance with EN 646 is verified at the accepted addition level. For packaging grades not intended for food contact, the product is suitable for use under standard finished paper oxidation and content criteria, provided that the dye addition does not exceed 0.40% and the final sheet is tested for extractable colour under EN 645.

Storage life is limited by humidity uptake and thermal exposure. Unopened bags stored at 10 to 30 °C and relative humidity below 60% are specified for 24 months. When relative humidity exceeds 60%, the granules absorb moisture and form aggregates; pre-drying in a desiccant-air silo at 35 °C for 2 h is then required before metering. The product is incompatible with strong oxidizing agents, including hypochlorite bleach liquors, and with high-charge cationic polyamines when both are introduced without the stated 20 s residence delay. Avoid preparing dye stock solutions in alkaline borate buffers above pH 9.0, because hydrolysis of the disazo linkage may occur after 6 h at 50 °C, producing shade loss and measurable release of aromatic amines in the mill effluent according to ISO 11350. The working solution should be used within 8 h when prepared at 50 °C, and within 24 h when prepared at 25 °C. Batch-to-batch colour strength is controlled to ±2.0% relative to the certified reference by ISO 5631-1, which permits direct metering without intermediate re-shading at addition levels below 0.25%.

In mills using white-water reuse for stock dilution, the dye concentration in tray water should be monitored at 460 nm after filtration through a 0.45 µm membrane. Absorbance values above 0.12 at 10 mm path length indicate incomplete fixation and a need to reposition the dye feed upstream or to reduce retention aid conflict. This method is not a substitute for standard colour measurement but serves as a rapid on-line check. The product’s granular form generates ≤0.5 mg/m³ respirable dust when transferred at 0.5 m/s vacuum conveying velocity in a contained charging station, measured by gravimetric sampling according to EN 481; local exhaust ventilation should be used to maintain the workplace limit value for inert dust of 10 mg/m³ as a total inhalable fraction.