Acid Orange II：The Bedrock of Textile Dyeing

In the vast universe of synthetic dyes, few molecules have maintained such a quiet yet pervasive influence as Acid Orange II (also known as Acid Orange 7 or Orange II). First synthesized in the late 19th century as an azo dye, this bright orange compound has since become a workhorse in the textile industry, particularly for dyeing protein fibers such as wool and silk, as well as polyamide fibers like nylon. Its combination of vivid color, good leveling properties, and relatively low cost has secured Acid Orange II a longstanding role as one of the “industrial bedrocks” of textile coloration.

Today, even as sustainability and digitalization reshape the dyeing sector, Acid Orange II remains a reference point for how a single dye can anchor an entire class of applications.

Acid Orange II is an anionic azo dye whose structure is based on the coupling of diazotized sodium paminobenzene sulfonate with 2naphthol, yielding a watersoluble sodium salt that carries a strong affinity for cationic sites on protein and polyamide fibers.

In practice, this means that under mildly acidic conditions, the dye readily dissolves in water and migrates into wool keratin, silk fibroin, and nylon’s amide groups, forming ionic and hydrogen bonds that fix the color within the fiber. The resulting shade is a clear, reddishyellow to orange tone, with good levelness and depth when applied at appropriate concentrations and pH levels.

Beyond textiles, Acid Orange II is also used in leather and paper coloring, as well as in biological staining and indicator applications, where its distinct color behavior in different media is leveraged for visual detection and measurement. However, its primary industrial significance remains in the dyeing of wool, silk, and nylon fabrics and yarns.

The development of Acid Orange II parallels the broader evolution of synthetic acid dyes in the 19th and 20th centuries. As an early azo dye, it offered a reproducible, costeffective alternative to natural colorants, helping to standardize and scale up dyeing operations for wool and silk.

Over time, Acid Orange II became one of the standard orangeyellow acid dyes for protein fibers, frequently cited in technical literature and supplier data sheets as a core component for dyeing silk and wool fabrics, as well as for direct printing on these substrates. Its role in polyamide dyeing, particularly in formic acid baths for nylon, further expanded its utility in mixedfiber systems.

This longstanding presence in dyehouse practice has cemented Acid Orange II’s status as a reference dye: not the most novel or exotic, but a dependable, wellunderstood option that continues to underpin many color recipes in wool, silk, and nylon production.

Wool remains a key fiber in apparel, home textiles, and technical fabrics, and Acid Orange II is widely used to achieve bright orange and reddishyellow tones on woolen yarns and fabrics.

Under typical acid dyeing conditions (pH 4–6, temperatures around 95–100 °C), the dye dissolves quickly in water and distributes evenly through the wool fiber, producing level shades with good depth and clarity. Technical specifications note that Acid Orange II offers bright color and good evenness, although its fastness is generally rated as moderate, which means that proper dyeing control and aftertreatment are important for longterm performance.

In practice, dyers often use Acid Orange II as a base for mixedshade formulations, combining it with other acid dyes to create a broad palette of orangebased hues, from soft apricot to deeper tangerine. This flexibility, together with its compatibility with standard wool dyeing auxiliaries, makes it a practical choice for mills seeking consistent, marketready colors at scale.

Silk’s smooth, lustrous surface demands dyes that can penetrate evenly without streaking or harshness, and Acid Orange II meets these requirements effectively.

The dye is described as suitable for dyeing silk fabrics, where it produces bright, level orange shades with good fiber affinity and penetration. Its solubility and moderate migration behavior allow dyers to achieve uniform coloration on delicate silk goods, which is essential for highend fashion and accessories.

Moreover, Acid Orange II can be mixed with other acid dyes to build complex color schemes, enabling silk mills to support seasonal fashion palettes while maintaining consistent shade quality. For designers, this means that Acid Orange II can serve as a core orange component in color stories that span from classic neutrals to bold fashion tones.

With the rise of nylon and other polyamide fibers, Acid Orange II has also become an important option for achieving bright orange shades on synthetic textiles.

Nylon fibers, which contain amide groups similar to those in wool and silk, can be dyed effectively with Acid Orange II in formic acid baths, where the dye binds to the fiber under mildly acidic conditions. This makes it suitable for applications ranging from hosiery and sportswear to upholstery and technical fabrics.

Because Acid Orange II can dye wool, silk, and nylon under similar process conditions, it also supports the growing market for blended textiles, where a single dye class can color multiple fiber types in one bath. For manufacturers, this crossfiber compatibility simplifies production, reduces the number of separate dye systems required, and helps minimize shade variation across blends.

Beyond piece dyeing, Acid Orange II is also used in direct printing on wool, silk, and nylon fabrics, where it is incorporated into printing pastes and fixed by steaming or heat treatment.

In this context, the dye is valued for its bright color, good evenness, and ability to produce sharp, welldefined patterns on protein and polyamide fibers. It is also employed in leather and paper coloring, where its vivid orange tone and ease of application make it a practical choice for decorative and functional products.

Additionally, pure grades of Acid Orange II can be used as an indicator or in biological staining, taking advantage of its distinct color changes in different chemical environments. However, its main industrial role remains firmly rooted in textile dyeing and printing.

Decades of industrial use have demonstrated that Acid Orange II provides bright, consistent orange shades on protein and polyamide fibers, with acceptable fastness when applied properly. The dye dissolves easily in water and integrates well into standard acid-dyeing processes, requiring conventional pH control, temperature profiles, and only common auxiliaries. As a well-established and widely used acid dye, it benefits from established manufacturing routes and economies of scale, ensuring competitive prices for large-scale operations. Its ability to dye wool, silk, nylon, leather, and paper using similar dyeing conditions makes it a versatile choice for a wide range of applications.

From a safety and environmental standpoint, Acid Orange II is handled as an industrial dye, with appropriate precautions for storage, handling, and wastewater treatment, reflecting its role as a nonfood, nondirectcontact colorant in textile and leather processing

The textile industry is often obsessed with the "new"—new fibers, new machines, new finishes. Yet, the enduring success of the industry rests on the reliable performance of its oldest tools.

Acid Orange II is more than just a chemical compound listed in a catalog. It is a symbol of the acid dye family’s mastery over protein and polyamide fibers. From the softness of a silk blouse to the resilience of a nylon backpack and the warmth of a woolen blanket, the "Bedrock of Textile Dyeing" supports them all.

As we look to a future that demands both economic efficiency and environmental responsibility, acid dyes are evolving. They are becoming cleaner, more efficient, and more integral to the supply chain. For manufacturers, designers, and chemists, understanding Acid Orange II is not just about looking at a historical artifact; it is about understanding the beating heart of the modern textile world.

In the equation of color, Acid Orange II remains the constant variable that holds the industry together.