Disodium 2,2'-[(1E)-Triaz-1-Ene-1,3-Diyldibenzene-4,1-Diyl]Bis(6-Methyl-1,3-Benzothiazole-7-Sulfonate)

Addition of 0.05 % owf Disodium 2,2'-[(1E)-triaz-1-ene-1,3-diyldibenzene-4,1-diyl]bis(6-methyl-1,3-benzothiazole-7-sulfonate) (commercial designation TRZ-BT200) to a nylon 6.6 woven fabric via an exhaust bath maintained at 98 °C and pH 4.5–5.0 yields a CIE whiteness index gain of 42 points over untreated substrate when measured under D65/10° geometry per ISO 11475:2017. Production-scale trials on a jigger processing 120 kg lots recorded a mean ΔWICIE of < 1.2 across 14 consecutive dye lots, provided free chlorine in the supply water remained below 0.5 mg/L. The active content is specified at ≥ 95 % (HPLC by DIN 55979), and the product appears as an off-white powder of bulk density 0.35–0.45 g/cm³. Anionic character is confirmed by a 1 % aqueous solution pH of 7.8–8.5 and a solubility of 32 g/L at 25 °C; above 40 °C solubility exceeds 50 g/L, enabling rapid dissolution in hot stock baths.

What Limits the Bath Stability of Triazene-Linked Brighteners in Presence of Anionic Softeners?

When 0.03 % owf TRZ-BT200 is combined in the same bath with 2.0 % owf of a sulfonated fatty acid condensation softener, dynamic light scattering (Malvern Zetasizer Nano) records a rapid increase in mean particle size from 0.8 µm to over 5 µm within 15 min at 60 °C. This agglomeration results in uneven brightener deposition, confirmed by a decline in wet rub fastness (ISO 105-X12) of 1.0 grade on the nylon face. Sequential application – exhaust the brightener first, then drop the bath and apply softener in a fresh liquor – eliminates the interference. Hardness cations above 150 ppm CaCO₃ further destabilize the system; the use of 0.5 g/L tetrasodium EDTA is mandatory when local water analysis shows total hardness exceeding 200 ppm. No reaction with nonionic softener dispersions is observed under the same conditions.

Photophysical Performance and Lightfastness Comparison with DAS Derivatives

The triazene-bridged bisbenzothiazole chromophore of TRZ-BT200 absorbs at 365 nm (molar extinction coefficient 4.5×10⁴ L mol⁻¹ cm⁻¹) and emits at 439 nm with a quantum yield of 0.72 in deionized water. These values are compiled against a conventional 4,4′-diaminostilbene-2,2′-disulfonate (DAS) type – C.I. Fluorescent Brightener 28 – in the table below. The narrower emission band of TRZ-BT200 reduces the yellowish green cast occasionally observed with DAS brighteners on polyamide under indoor lighting.

Property	TRZ-BT200	DAS derivative (C.I. FB 28)
λabs (H₂O)	365 nm	350 nm
λem (H₂O)	439 nm	440 nm
Quantum yield	0.72	0.65
Exhaustion on nylon 6.6 (0.05 % owf, pH 4.5)	92 %	78 %
ΔWCIE on nylon from 0.05 % owf exhaust	42	31
Lightfastness on nylon (ISO 105-B02) at 0.05 % owf	4–5	3
Chlorine fastness (ISO 105-N01, 20 mg/L active Cl)	4	2–3
Residual fluorescence after 5 min at 220 °C in PP	92 %	78 %
Solubility in water at 25 °C	32 g/L	> 100 g/L
Hard water tolerance (200 ppm CaCO₃, without sequestrant)	Precipitate after 30 min	Stable

In a papermaking furnish based on bleached softwood kraft pulp with 0.5 % cationic starch and 0.08 % TRZ-BT200 on dry fiber, ISO brightness (ISO 2470-1) increased from 84.2 to 92.0 points at a retention of 76 % quantified by filtrate spectrophotometry. Pilot Fourdrinier runs at 800 m/min with a 0.3 % headbox consistency showed no foam generation or felt contamination over 4 hours, provided water hardness was chelated with 0.2 % Na₄EDTA. Backside migration measured on a 90 g/m² sheet remained below 0.5 brightness points after 7 days of conditioned storage at 25 °C and 50 % RH, indicating strong substantivity to the cellulose fines fraction.

When Pad-Thermosol Fixation Replaces Exhaustion for Polyester/Cotton Blends

For 65/35 polyester/cotton twill, padding with a 5 g/L TRZ-BT200 solution on a laboratory padder set to 70 % wet pick-up, drying at 120 °C for 90 s, and thermosol fixation at 195 °C for 45 s deposits the brightener predominantly on the cotton component. After one accelerated laundering cycle (AATCC 61-2A), the CIE whiteness loss was only 3.5 points, confirming sufficient thermal stability. However, raising the thermosol temperature above 210 °C triggers sublimation loss and a simultaneous drop of 12 % in fluorescence intensity. Published data on the fixation threshold of the triazene linkage in polyester by thermosol remains limited; empirical observations suggest that dwell times exceeding 60 s at 200 °C do not improve yield and may increase the risk of thermal yellowing of the cotton portion.

Migration Kinetics and Blooming Risk in Thin-Gauge Blown Film

Extrusion of a 40 µm LDPE blown film containing 0.02 % TRZ-BT200 via a single-screw extruder (L/D 30, die temperature 190 °C) yields immediate surface fluorescence without subsequent blooming under ambient storage for 12 months. Migration into 95 % ethanol food simulant at 40 °C for 10 days (total immersion, EN 1186-1) was below the 10 µg/dm² detection limit of the HPLC-fluorescence method. Accelerated aging at 60 °C for 60 days produced only faint surface exudation visible under 365 nm UV, corresponding to a migration value of 0.03 mg/dm². Pre-drying of the brightener masterbatch to a moisture content < 0.05 % (Karl Fischer) is required before let-down into resin dried to < 0.01 %, otherwise hydrolysis during extrusion can degrade the triazene chromophore and reduce fluorescence by up to 18 %.

Regulatory Framework	Relevant Clause / Test Method	Status of TRZ-BT200
REACH (EC) 1907/2006	Substance registration dossier	Registered (tonnage band 1–10 t/a)
Oeko-Tex Standard 100	Annex 4, product class I	Compliant, limit ≤ 0.1 % on fabric
FDA 21 CFR 178.3297	Colorants for polymers, SML 0.05 mg/kg	Cleared when used in polyolefins, dry foods
EU 10/2011 (PIM)	Annex I, substance-specific migration limit	SML(T) 0.05 mg/kg (brightener group)
Swiss Ordinance SR 817.023.21	Positive list for printing inks and coatings	Permitted in indirect food contact
ISO 105-B02	Lightfastness assessment on textiles	Rated on nylon, wool, and cotton substrates
ISO 11475:2017	Determination of CIE whiteness, D65/10°	Method used for all reported whiteness values

Incorporation of 0.04 % TRZ-BT200 into a standard IECA base powder (zeolite 4A builder, sodium carbonate, LAS anionic surfactant) via post-addition dry blend yielded a whiteness deposition on multiple-wash cotton monitor fabric (ISO 105-C06 A2S) comparable to that of an equal loading of C.I. Fluorescent Brightener 71. After 20 wash cycles at 60 °C, the lightfastness of the brightener-on-substrate evaluated per ISO 105-B02 was 1–5 blue wool grade higher, attributable to suppressed photoyellowing from the triazene structure. The product is incompatible with percarbonate bleach systems formulated without TAED activator because alkaline hydrogen peroxide at pH 10.5 cleaves the triazene linkage within 30 min at 40 °C, as evidenced by an 87 % reduction in absorbance at 365 nm. Detergent powders containing 10–20 % sodium percarbonate therefore require a protective granulation or coating of the brightener particles, otherwise meaningful whitening performance is lost during the main wash.