Polyethylene terephthalate (PET) is synthesized through the condensation of terephthalic acid (PTA) and ethylene glycol (EG). Its molecular backbone comprises rigid benzene rings and flexible alcohols, making it a saturated linear polymer. PET is a milky white or light yellow, highly crystalline polymer with a smooth, glossy surface. It boasts excellent physical and mechanical properties across a broad temperature range and demonstrates remarkable chemical resistance and conductivity. PET finds utility in producing polyester staple fibers and filaments, serving as a crucial raw material for enterprises involved in polyester fiber manufacturing. Additionally, it is utilized in various applications such as bottles, films, and more, widely deployed across industries including packaging, electronics, medical hygiene, construction, and automotive sectors. This article mainly introduces the carboxyl end groups in PET Chips. Hubei Decon can supply COOH:8 PET chips, which are widely used in the industrial monofilament yarn production.

In industrial production, the carboxyl end groups in PET Chips serve as a vital indicator of polymer quality, directly impacting product quality. To ensure stable product quality, it’s imperative to control terminal carboxyl groups within a specific range, thereby effectively managing product grades and guaranteeing the polymers’ processing performance and product quality.

1. Sources of Carboxyl Groups in Chips

Carboxyl groups in Chips primarily originate from three sources. firstly, They arise from esterification reactions, serving as the main source of carboxyl end groups in PET chips. Secondly, carboxyl groups are formed during the chain-breaking process in the condensation reaction stage. Thirdly, carboxyl groups are generated during material degradation in the production process, such as thermal degradation and thermal oxidation.

1.1 Esterification Reaction Stage

As per process requirements, the esterification rate in the stage must exceed 98.5%. This serves as a crucial indicator in PET production, as a low esterification rate can lead to premature termination of the condensation reaction due to excessive COOH content, resulting in undesired viscosity. Therefore, it’s essential to rigorously control carboxyl groups and their side reactions during esterification. Esterification essentially involves the reaction between the hydroxyl group of EG and the carboxyl group of PTA to form an ester and water. When the esterification rate is low, an incomplete reaction of the raw material PTA leads to a higher content of ester carboxyl groups. Conversely, an excessively high esterification rate results in a low COOH content in the prepolymer, leading to slow condensation reaction and prolonged reaction time, wherein simultaneous condensation reaction, thermal degradation, and thermal oxidation reactions occur, failing to yield high-viscosity products.

1.2 Condensation Reaction Stage

Given that the condensation reaction is reversible, the content of carboxyl end groups fluctuates with changing conditions. Several factors may contribute to high-end groups in PET chips.

(1) Poor vacuum conditions make it difficult to remove small molecules generated during condensation, impeding the forward progress of the condensation reaction.

(2) Incomplete esterification reactions hinder the prepolymer from reaching the condensation reaction conditions, thereby obstructing chain growth reactions.

(3) Reduced output of prepolymer leads to prolonged retention of materials in the reactor, pipelines, and cavities, causing previously long molecular chains to break, exposing carboxyl and ester groups, sometimes resulting in phenomena such as yellowing and decreased viscosity.

1.3 Thermal Degradation and Thermal Oxidative Degradation

Thermal degradation refers to polymer degradation due to heating, while thermal oxidative degradation occurs under the influence of oxygen. Both phenomena result in decreased viscosity, increased carboxyl end groups, yellowing, and weight loss, and may lead to gel formation.

  • Thermal degradation of PET can occur at chain ends and within chains. Hydroxyl groups at the chain end readily form hydrogen bonds with ester carbonyl groups, leading to chain scission. Thermal degradation within chains involves hydrogen bonds formed between β-hydrogens connected to ester groups, resulting in chain scission. Regardless of the type of cleavage, it leads to an increase in carboxyl end groups. The presence of metal catalysts in PET plays a crucial role in thermal degradation. Typically, the higher the activity of the condensation catalyst, the higher the catalytic degradation activity.
  • Thermal oxidative degradation of PET is initiated by the presence of oxygen, leading to the formation of peroxides at active centers within chains and chain ends. Similar to other hydrocarbon oxidation degradation reactions, it ultimately results in chain scission through free radical chain reactions.

2. Factors Affecting Carboxyl Group Control

2.1 Molar Ratio of Raw Materials

The molar ratio of EG/PTA in raw materials significantly impacts the reaction process and the degree of polymerization of PET. According to the reaction, the molar ratio of EG/PTA in esterification must be 2:1. However, to prevent self-condensation of EG from affecting PET quality, the molar content of EG is usually kept lower than the EG/PTA molar ratio, at 1.7-1.8:1. The molar ratio of EG/PTA should not be too low, otherwise the carboxyl content of esterification products will increase.

In continuous processes, EG recovered from condensation is recycled to the system to supplement the loss of EG, with the feeding molar ratio of EG/PTA typically at 1.1-1.2:1 to achieve optimal esterification rate and expected product quality.

2.2 Effect of Esterification Process Parameters on Carboxyl Groups

When the pressure in the reaction vessel is too low, EG in the material evaporates quickly, leading to a lack of reactants in the esterification reaction, making it difficult to proceed. Poor vacuum conditions result in excess EG and H2O from esterification reactions not being promptly removed from the system, leading to reverse esterification reactions. Increasing temperature, raising the liquid level, and extending reaction time are beneficial for esterification reactions but also increase the formation of by-products.

2.3 Effect of Condensation Process Parameters on Carboxyl Groups

When the esterification reaction reaches a certain level, condensation, and esterification reactions occur simultaneously, and the reaction rate decreases with decreasing ethylene glycol concentration. Typically, as the ethylene glycol concentration decreases, the condensation reaction rate gradually increases, and the material tends to undergo high polymerization. With increasing reaction degree, PET undergoes thermal degradation and thermal oxidative degradation at high temperatures, increasing COOH content. Therefore, it is crucial to control the temperatures of each reaction vessel at this stage.

2.4 Catalyst Influence

In the PTA method for producing polyester, the H+ released by PTA dissolved in EG during the esterification process has a self-catalytic effect, eliminating the need for additional catalysts for esterification and condensation.

Conclusion

(1) Fluctuations in carboxyl group values are inevitable in actual production processes. Based on experience, the adjustment of the n(EG)/n(PTA) value is the initial step in adjusting carboxyl groups.

(2) In production practice, it’s observed that the factors affecting carboxyl groups, in order of significance, are the feed ratio, temperature of esterification reactor I, temperature of final condensation reactor, temperature of esterification reactor II and Esterification reactor I pressure.

(3) When the end carboxyl group is abnormal for adjustment, we should always pay attention to the change of DEG, DEG is not only an important indicator of the product but also can give the direction of process adjustment.