Perception into Cationic Dyeable Polyester

Perception into Cationic Dyeable Polyester

Rifat Jahangir,
International Standard University, Dhaka


Cationic dyeable polyester, abbreviated as CDP, represents a notable advancement in the realm of polyester textiles. Unlike conventional polyester, which lacks the capability to bond with cationic dyes, CDP boasts a unique composition that allows for ionic connections with these dyes. This innovative polyester variant, crafted as a copolymer of polyethylene terephthalate with a trace amount of comonomer featuring a sodium sulphonate group, revolutionizes the dyeing process. When dyed, these fibers can achieve medium tones at a temperature of 120°C, although obtaining dark tones may pose a challenge.

Dacron 64, an early iteration of CDP, emerged as a low-pill staple product, offering improved dyeability and color vibrancy. The sulphonate group within CDP serves a dual purpose, acting as both an ionic dipolar cross-link and a catalyst for enhanced melt viscosity. One of the most remarkable attributes of CDP lays in its lowered glass transition temperature (Tg) and reduced crystallinity. These characteristics facilitate dyeing at higher temperatures, eliminating the need for a carrier agent. While this advancement enhances dye penetration and fixation, it does entail some compromise on the fiber’s physical properties.

The introduction of dimethyl ester sodium salt of 5-sulfoisophthalic acid (DMS salt) during the polymerization process led to the creation of cationic dyeable polyester (CDP). This addition allowed for the incorporation of anionic sites, resolving the issue of dull color often associated with conventional polyester fibers. Compared to regular ester sections, CDP contains a higher proportion of sections with anionic sites, enhancing its dyeability and color vibrancy. The structure of cationic dyeable polyester is given below:

Cationic Dyeable Polyester Structure
Figure: Cationic Dyeable Polyester Structure

Historical Aspect
For years, scientists have delved into copolymerization, a method aimed at boosting the properties of PET fibers. This process is a key in refining both the chemical makeup and physical attributes of these fibers. One significant breakthrough dates back to the 1960s when researchers achieved a milestone: the development of cationic dyeable polyester (CDP) fibers. These fibers possess a unique characteristic—a negative dye site marked by a sulfonated group. This innovation represents a crucial advancement in the field, opening doors to enhanced functionalities and practical applications of PET fibers.

Production of Cationic Dyeable Polyester
Specialty fibers, those with unique properties, can be crafted from both copolymers and polymers that have undergone chemical alterations. These alterations, referred to as modifiers, are introduced to the polymer either during extrusion or at a different stage of the polymer manufacturing process. One such modifier is cationic dyeable polyester. In this process, 5-sulfoisophthalic acid – Na salt or its diglycol derivatives are utilized to modify polyester fibers, enhancing their capabilities for specific applications. The structure of sodium salt of dimethyl ester of 5-sulfoisophthalic acid is:

The structure of sodium salt of dimethyl ester of 5-sulfoisophthalic acid

A. From Cationic Dyeable Chips
To create cationic dyeable polyester (CDPET) yarns, a blend of conventional polyester chips and cationic dyeable chips is utilized. These cationic dyeable chips contain small amounts (2-3 mol%) of sodium salts of the dimethyl ester of 5-sulfoisophthalic acid (DMS salt). The blending process involves mechanically mixing the chips in different ratios: 100/0, 75/25, 50/50, 25/75 and 0/100. These blended chips are then melt spun together in a spinning apparatus at a consistent temperature of 285°C.

Cationic Dyeable Chips

After spinning, the resulting partly oriented CDPET yarns undergo texturizing procedures on a draw-texturing machine. This process helps enhance the yarn’s texture and physical properties, ensuring it meets the desired specifications for various applications.

B. From Sodium-5-sulfo-bis(hydroxyethyl)-isophthalate (SIPE) Comonomer
Adding sodium-5-sulfo-bis-(hydroxyethyl)-isophthalate (SIPE) group is another way to introduce ionic groups into the main chain of polyester molecules. This process leads to the creation of cationic dyeable polyester (CDP), which can then interact with cationic dyes, forming a significant type of modified polyester. The SIPE monomer is produced through the transesterification of sodium dimethyl 5-sulfoisophthalate (SIPM) and ethylene glycol (EG). During the SIPE synthesis mechanism, SIPM undergoes a transesterification process to eventually transform into SIPE. This method contributes to enhancing the functionality and versatility of polyester for various applications.

Synthesis of SIPE
To make the CDPET copolymer, three key ingredients are used: terephthalic acid (PTA), ethylene glycol (EG) and sodium isophthalate-5-sulfonate (SIPE). Initially, under high-temperature and high-vacuum conditions, these raw materials undergo esterification. This process involves terephthalic acid and ethylene glycol combining to form a polyester oligopolymer.

Next, to achieve the desired composition of CDPET, the polymer undergoes polymerization. During this step, SIPE is introduced at any point before the polyester composition is finalized. This method ensures the incorporation of SIPE into the copolymer structure, enhancing its properties and enabling it to interact effectively with cationic dyes for various applications.

Synthesis of SIPE
Figure: Synthesis of SIPE

Synthesis of CDPET
Modifiers used in the production of cationic dyeable polyester are composed of substances derived from 5-sulfoisophthalic acid, which are integrated into the polyester chain during synthesis. This addition of an anionic group disrupts the regularity of the polyester polymer chain molecules, resulting in a less compact structure compared to conventional polyester.

As the amount of DMS salt increases during the production process, several changes occur in the fine structure of CDPET. The crystalline percentage, orientation and viscosity of the material decrease rapidly. These alterations lead to an easier dyeing process and increased dye absorption due to the presence of specific anionic bonding sites within the CDPET fibers.

The dyeing process of CDPET involves the interaction between cationic dyes and the fiber, which relies on ionic bonding. With a higher concentration of DMS salt, the number of anionic bonding sites in CDPET fibers also increases, further enhancing the dyeability of the material.

Synthesis of CDPET
Figure: Synthesis of CDPET

Spinning Process of CDPET
Cationic dyeable polyester undergoes a spinning process known as melt spinning to transform copolyester chips into fibers. This method closely resembles the process used for regular polyester. Several factors in spinning technology, such as drying temperature, spinning temperature, spinning rate and draw ratio, are carefully controlled during the process.

To ensure optimal spinning, copolyester chips are dried at 150°C for 48 hours under vacuum to remove any residual moisture before being fed into storage hoppers. Subsequently, the chips are fed into the melt spinning machine, where they pass through a spinneret with 36 holes. The spinneret operates at a temperature ranging between 280 and 300°C, with a spinning speed of 900 m/min and a draw ratio of 3.9.

Spinning Process of CDPET
Figure: Spinning Process of CDPET

During spinning, the molten polymer is forced through the spinneret’s perforations, forming filaments that are solidified by cold air as they emerge. These filaments are then twisted into bundles of undrawn yarn. The undrawn yarn undergoes stretching on draw-twist machines, increasing its length to around five times its original size, often at elevated temperatures.

Next, the stretched filaments are gathered into dense tows and transformed into staple fibers through mechanical pulling, crimping and heat setting to fix the crimp. Finally, the tow is cut into staple fibers of the desired length, ready for further processing into various textile products.

Application Field of CDPET

  1. Sportswear
  2. Intimate wear
  3. Swim wear
Different applications made from CDPET
Figure: Different applications made from CDPET

Dyeing of CDPET with Cationic Dyes
Cationic dyeable polyester (CDPET) fibers were developed with a specific goal in mind: to lower the polymer’s Tg (glass transition temperature). This reduction, aimed at achieving greater segmental mobility and an open polymer structure, resulted in a Tg range of approximately 70 to 85°C. This range is notably 10°C lower than that of conventional polyester fibers. The significance of this reduction lies in its impact on the dyeing process. With a lower Tg, CDPET fibers allow dyes to diffuse more rapidly through the fiber at lower temperatures.

Unlike normal polyester, which requires a temperature of 130°C for dyeing, CDPET fibers can be colored at just 110°C. This lower dyeing temperature is made possible by the unique characteristics of CDPET, including its greater regularity and high crystallinity. These features stem from the presence of rigid benzene ring structures in molecular chains and ionic aggregates. However, despite the reduced dyeing temperature, higher temperature and pressure conditions are still necessary due to the fiber’s structure.

During the dyeing process, an ion exchange mechanism occurs. Larger cations from basic dyes displace Na+ ions in the fiber, moving into the aqueous dye solution. Cationic dyes form electrovalent bonds with the sulphonic acid and carboxyl groups present in the fiber. The amount of dye absorbed by the fiber depends on the number and accessibility of these groups. Morphology and dyeing temperature influence the accessibility of anionic groups, impacting dye absorption.

One challenge faced during dyeing is the risk of hydrolysis of CDPET fibers at 120°C. To counteract this, dyeing is conducted in the presence of 2 to 6 g/L of sodium sulfate. This addition of salt prevents the exchange of sodium ions in the fiber with hydrogen ions, a critical step in fiber hydrolysis. Thus, sodium sulfate serves to protect the integrity of CDPET fibers during the dyeing process, ensuring optimal performance and longevity.

Morphology of CDPET
X-ray diffraction analysis reveals intriguing insights into the structure of cationic dyeable polyester (CDPET) fibers. These fibers exhibit a distinctive composition, with their crystalline portions comprised of segments of unmodified poly (ethylene terephthalate). However, what sets them apart is the presence of modified polyester segments within the noncrystalline areas.

In comparison to nonmodified fibers, the density and orientation of noncrystalline regions in modified CDPET fibers are notably lower. This difference in density and orientation contributes to the unique properties of CDPET, enhancing its performance in various applications.

Furthermore, despite possessing similar thermal characteristics, the degree of crystallinity in modified CDPET fibers is approximately 10% lower than that of unmodified fibers. This variance in crystallinity reflects the impact of the modification process on the overall structure of the fibers.

Overall, the X-ray diffraction data highlights the intricate interplay between crystalline and noncrystalline regions within CDPET fibers, shedding light on their structural characteristics and underlying properties.

Physical Properties of CDPET

Density (g/cc)1.45 -1.49
Tenacity (g/d)Dry3.8 -4.2
Wet3.8 -4.2
Breaking Elongation (%)Dry21-40
Moisture Regain (%)65 % RH0.8
90 % RH1.1
Elastic Recovery (%)At 2%90 – 92
At 5 %75 -85
At 10 %45 -60
Stiffness (g/d)15 – 35
Melting Point (°C)210 – 220

Chemical Properties of CDPET

  • Effect of Acid: The CD-PET fiber showed good resistance to inorganic acids at room temperature as well as to organic acids.
  • The fiber only partially breaks down when sulphuric acid is concentrated.
  • Alkali Effect: At ambient temperature, the fiber is quite resistant to weak alkalis.
  • Saponification causes fiber surface degradation in a 1 N NaOH solution at 97 °C. Ammonia will weaken you by causing aminolysis.
  • Effect of Bleaching Agents and Solvents: The fiber has great resistance to bleaching agents and organic solvents; nevertheless, phenols, chloroacetic acid and chlorophenols swell or dissolve the fiber.
  • Sunlight Effect: The fiber loses both its strength and weakness when exposed to direct sunlight.
  • Dyeability: This fiber has been carefully engineered to allow for the use of cationic dyes. Therefore, the majority of CD-PET colors are basic (cationic) dyes.

Cationic dyeable polyester (CDPET) offers a versatile solution for various textile applications. By lowering the polymer’s glass transition temperature and introducing modifications to the polyester chain, CDPET fibers exhibit enhanced dyeability and structural properties. These modifications result in a lower crystallinity compared to conventional polyester fibers, allowing for efficient dye absorption and coloration at lower temperatures. Additionally, the ion exchange process during dyeing further contributes to CDPET’s unique characteristics. Overall, CDPET represents a valuable advancement in textile technology, providing manufacturers with a reliable and adaptable material for a wide range of applications.


  1. Synthetic Fibre Dyeing.
  2. Synthetic Fibres: Nylon, Polyester, Acrylic, Polyolefin.
  3. Discussion on Avoidance of Staining Problem in Cotton/CDPET Dyeing.
  4. Preparation and Characterization of Poly (ethylene terephthalate) Copolyesters Modified with Sodium-5-Sulfo-Bis-(hydroxyethyl)- Isophthalate and Poly(ethylene glycol).
  5. Polyesters and Polyamides Edited by B. L. Deopura, R. Alagirusamy, M. Joshi and B. Gupta.
  6. Effect of Comonomer on Structure and Properties of Textured Cationic Dyeable Polyester.
  7. New Insights in Structures, Properties and Rheological Behaviors of Cationic Dyeable Polyesters.
  8. Regenerated Cationic Dyeable Polyester Deriving from Poly (ethylene terephthalate) Waste.
  9. Dyeing of Cationic Dyeable Polyester (CDP) with Cationic Dyes by Michael Hilton.

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