Cost-Effective General Dye – Direct Red 28
Basic Product Information
Direct Red 28 (Congo Red), C.I. No.22120, CAS No.573-58-0, is a biphenyl disazo water-soluble direct dye. It appears as brownish-red powder and forms yellowish-red aqueous solution, delivering bright scarlet shade on cotton and viscose fibers.
It dissolves easily in cold and hot water, sensitive to acid and salt, and turns bluish purple under acidic condition. It can dye cotton, viscose, silk, polyamide, paper pulp and leather, also widely used as biological stain and pH indicator (blue-violet at pH3.0, red at pH5.2).
Core Physical & Chemical Properties
3.1 Solubility
● Water solubility: Soluble in hot water, solubility ~10g/L in cold water; aqueous solution shows yellowish red. Optimum dissolving temperature: 40–80℃; sensitive to hard water.
● Solubility in organic solvents: Soluble in ethanol to form orange solution; slightly soluble in acetone; insoluble in ether, benzene and other non-polar solvents.
3.2 Color Reaction with Acid & Alkali
● Concentrated sulfuric acid: Dark blue solution, light blue precipitate after dilution
● Concentrated hydrochloric acid: Reddish blue precipitate separated out
● Concentrated sodium hydroxide/ammonia water: Yellowish red solution; color turns dark blue when exposed to CO₂, recover original shade with dilute soda ash
vpH color change range: Blue-violet at pH 3.0 → bright red at pH 5.2, widely used as acid-base indicator
3.3 Stability to Metal Ions
● Turns dull yellow when contacting copper ions; slight yellow shift with iron ions
● Poor salt tolerance, control water hardness during dyeing
Application Features (Paper Making + Textile + Other Fields)
4.1 Paper Industry (Main Application)
● Application: Coloration & toning of writing paper, packaging paper, paperboard, producing bright yellowish scarlet shade
● Advantages:
✅ Fast dissolution, directly added in wet end with simple operation
✅ Strong affinity to cellulose, full color uptake, stable white water circulation
✅ Low cost with wide versatility
● Usage: 0.08%–0.6% based on oven-dry pulp; pH 7–9 weak alkaline condition, optimum temperature 45–60℃
4.2 Textile Industry
● Applicable fibers: Cotton, linen, viscose; slight dyeing on silk & wool; almost no affinity to polyester & acrylic
● Dyeing performance: High color yield, mediocre migration property; add inorganic salt for exhaustion, cool down to 80℃ for full absorption; pure yellowish scarlet shade
● Color fastness (standard grade):
Light fastness: Grade 2
Washing fastness: Grade 3
Dry rubbing fastness: Grade 4–5
Wet rubbing fastness: Grade 3
4.3 Other Applications
Light dyeing of leather, water-based color paste, laboratory acid-base indicator, biological tissue staining, pH test filter paper
Key Technical Index
|
Item |
Standard Index |
|
Color Shade |
Yellowish scarlet |
|
Strength |
100% (Standard Sample) |
|
Moisture Content |
≤5.0% |
|
Water Insoluble Matter |
≤0.15% |
|
Light Fastness |
Grade 2 |
|
Washing Fastness |
Grade 3 |
|
Dry Rubbing Fastness |
Grade 4–5 |
|
Wet Rubbing Fastness |
Grade 3 |
|
Package Specification |
25kg kraft paper bag / fiber drum |
Storage & Transportation
● Storage: Cool, dry, ventilated warehouse, sealed moisture-proof, avoid direct sunlight. Shelf life: 18 months.
● Transportation: General non-dangerous chemical goods; prevent rain and high temperature stacking, separate storage with strong acid and alkali.
