Synthesis and Properties of Cationic Polyester Chips

This article introduces the types of cationic polyester chips and their cationic dyeable mechanisms. It also outlines the production process for cationic polyester, highlighting important considerations. Furthermore, the dyeability and spinnability of cationic (CDP) chips are explored.

Cationic dyeable polyester chips are obtained by introducing a third monomer into the macromolecular chain of PET, resulting in a segmented copolymer (also known as a copolyester). Cationic dyeable polyester chips are created by introducing a third monomer into the PET polymer chain, forming a segmented copolymer (also known as a copolyester). This third monomer contains a strongly acidic sulfonate group (-SO3Na), which can chemically interact with cationic dyes, allowing for better dye fixation on the fibers. Fabrics made from these fibers exhibit excellent dye uptake, vibrant colors, and high dyeing efficiency. They are resistant to fading or color loss after washing. These fibers are widely used in synthetic silk, printed textiles, and wool-blend fabrics.

1 Mechanism of Cationic Dyeable Modification

PET fibers are highly crystalline, highly oriented, and have a relatively high glass transition temperature, making it difficult for dyes to diffuse into the PET molecules under normal pressure without a carrier. This makes dyeing challenging. As a result, PET fibers are typically dyed under high temperature and high pressure, or with the aid of a carrier. However, these methods are energy-intensive, costly, and environmentally harmful. There are several modification approaches to address the dyeing challenges of PET, pre-dyeing modification, chip blending spinning modification, and copolymerization modification. The most commonly used approach in the market is the last method, where a third monomer is added for copolymerization modification. Cationic dyeable polyester (CDP) chips are produced by using this technique.

1.1 High-Pressure Cationic Dyeable Polyester Chips (CDP)

CDP is produced by adding a third monomer, 1,3-dimethyl-2-hydroxyethyl-5-benzenesulfonate sodium (SIPE), during the later stage of PET esterification, which undergoes polycondensation to form a copolyester. Because the third monomer contains an anionic group, -SO3Na, the resulting copolyester also contains this group. When fabric is made from this fiber, the ionic bonding between the anion and cation increases the affinity between the fiber and cationic dyes, improving dyeability.

Research and analysis shows that after introducing SIPE into the PET main chain, the following effects on the original PET molecules occur.

1. It reduces the regularity of the original molecular structure.

2. Due to the spatial steric hindrance of SIPE and the polarity of -SO3Na, the copolyester’s glass transition temperature (Tg) increases. But as the content of the third monomer increases, Tg decreases and becomes lower than the Tg of regular PET chips.

3. The incorporation of the third monomer into the PET macromolecular chain improves the molecular chain’s mobility, reduces the crystallization rate, increases the amorphous regions, and facilitates the diffusion and absorption of dye molecules.

The combination of CDP and cationic dyes is followed.

Combination of CDP and Cationic Dyes

1.2 Atmospheric Pressure Easy Cationic Dyeable Polyester Chips (ECDP)

While the addition of the third monomer, SIPE, enhanced the dyeing performance of CDP, its high glass transition temperature (Tg) still resulted in a low dye uptake under atmospheric pressure boiling conditions. To address this issue, a fourth monomer, polyethylene glycol (PEG), was introduced along with SIPE. The flexibility of the PEG chain disrupted the regularity and crystallinity of the molecular structure, increasing the amorphous region. This change improved the mobility of the large molecules within the amorphous area, enabling effective cationic dyeing under atmospheric pressure boiling conditions. As a result, this copolyester is referred to as easy cationic dyeable polyester for atmospheric pressure boiling dyeing. Our company can offer ECDP chips with SIPE content of 3%, 4.5%, 6%, and even 13.5%. 

2 Production Process of Cationic Dyeable Polyester Chips

2.1 Production Equipment

The production process uses a semi-continuous esterification and batch polymerization system controlled by DCS. The main equipment involved includes a pulping kettle, first esterification kettle, second esterification kettle, polymerization kettle, third monomer preparation kettle, third monomer adjustment tank, and third monomer finished product tank.

2.2 Preparation of Modifiers

The modifier used in the production of CDP is 1,3-dimethyl terephthalate-5-sodium benzenesulfonate (SIPM), a white powder. This substance cannot be used directly as a modifier. It must firstly be reacted with ethylene glycol (EG) to form SIPE. The preparation process is as follows.

Excess EG is added to the third monomer preparation kettle, and then SIPM is introduced along with the appropriate catalyst (CaCO3) and etherification inhibitor (NaAc). The kettle is sealed and heated to initiate the ester exchange reaction between EG and SIPM. Once the reaction is complete, the resulting SIPE is transferred into the SIPE adjustment tank under nitrogen pressure, where its concentration is adjusted with EG to the desired level. It is then moved to the finished product tank for future use. Since the SIPE is dissolved in EG and added to the second esterification kettle, the large amount of EG can cause side reactions in the esterification process, particularly etherification. To counteract this, an etherification inhibitor, NaAc, must be added to the SIPE finished product tank. The amount should be approximately 1.3‰ to 1.5‰ of the final products.

2.3 Production Process

The required amounts of PTA, EG, and prepared catalyst are added to the pulping kettle. After thorough mixing, the mixture is continuously pumped into the first esterification kettle for the esterification reaction (with the remaining mother liquor in the esterification kettle). Once the esterification is complete, nitrogen is used to transfer the contents to the second esterification kettle. A specific amount of EG is added to cool the material, preventing SIPE from self-polymerizing at high temperatures. The prepared third monomer is then added for copolymerization, and the EG produced during the reaction is removed. The mixture is then filtered through a melt filter to remove any impurities, and transferred to the polymerization kettle for further polymerization. This reaction is carried out under a vacuum to remove the EG produced during the process. The material continues to undergo polymerization, reaching the specified final temperature and discharge power. Once this is achieved, the vacuum is released, nitrogen is added for discharge, stripping, cooling, pelletizing, drying, and screening. Finally, the material is transferred to the storage bin for weighing and packaging.

3 Results and Discussion

Comparison of Physical and Chemical Properties of Cationic Polyester Chips

Our company CDP Chip parameters are summarized in the Table below.

4 Dyeing Performance of Cationic Dyeable Polyester

Based on two years of production experience and related studies, the dyeing depth and uniformity of CDP are influenced by several factors.

A. The amount of the third monomer in the chips. The higher the third monomer content in the copolymer, the deeper and more vibrant the dyeing of the fibers. Therefore, during batch polymerization of CDP, it’s crucial to ensure the consistent addition of the third monomer to achieve uniform dyeing.

B. The draw ratio during spinning. A higher draw ratio reduces the amorphous regions in the fibers, leading to lower dye absorption and poorer dyeing performance.

C. The heat-setting process. In conclusion, the addition of the third monomer enables polyester to be dyed with cationic dyes, resulting in high color fastness and vivid colors. However, to ensure consistent saturation and uniformity in dyeing, it is essential to maintain consistency in the process parameters at each stage.

5 Other Properties and Considerations

A. Due to the lower melting point, poor heat resistance, and higher melt viscosity of CDP chips, the discharge temperature (final reaction temperature) should be 3-5℃lower than that of regular products. The discharge power should be 3–5 kW higher, and the spinning temperature should be 3-5℃lower than regular spinning processes. The residence time in the crystallizer must also be slightly longer, and the drying temperature should be lower than that used for the regular PET.

B. CDP chips tend to absorb moisture and regain water, which can lead to hydrolysis and reduced viscosity. Therefore, proper storage and transportation conditions are crucial. According to relevant information, the shelf life of these chips is 3 – 6 months, and they should not be stored for too long.

C. The filter usage cycle for spinning components is shorter compared to regular chips.

D. The introduction of SIPE further weakens the mechanical properties of the fibers. As a result, the amount of SIPE added should be kept between 1% and 3%.

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