The melting point of polyester varies depending on the type of polyester and its production process.
Low melting point polyester is a modified polyester with a lower melting point. It is a raw material for the production of thermal bonding fibers. Its melting point is 110~135℃. Low-melt fibers are used in edge linings for high-end clothing and tapes for automotive interiors. Compared to polypropylene and polyamide fibers, which shrink significantly due to heat, low-melt fibers shrink by only 5% in their free state. This makes them suitable for a variety of applications. It can be used as a substitute for hot melt adhesive in some areas.
1. Main Technical Specifications
Melting Point (Tm): 95–115°C (adjustable as needed)
Inherent Viscosity [n]: 0.6–0.65
Melt Index (MI): ≥40g/10min (adjustable as needed)
The polymer exhibits good flowability and adhesiveness.
2. Polyester Production Process
Polyester is a general term for polymer compounds synthesized by the condensation of diols and diacids (or acid anhydrides). The main use of polyester is to produce polyester fiber. In 1998, it accounted for 60% of the world’s polyester applications. According to its use, it can be divided into polyester resin, polyester fiber, and polyester rubber. Saturated polyester has the same types. Polyesters and unsaturated polyesters depend on the type of acid used.
There are many ways to prepare polyester. The current starting material for polyester is paraxylene. Paraxylene is used to produce terephthalic acid (PTA), which is then condensed with a glycol, usually ethylene glycol. The production was first called the direct esterification condensation method (direct condensation method). The transesterification condensation method is the second method to produce (also known as the indirect condensation method. That is, dimethyl terephthalate (DMT) is transesterified and then condensed. Currently, the former method has the absolute advantage. It is further divided into continuous method, intermittent method, and semi-continuous method.
2.1 Compared to the DMT method, the PTA method has the following advantages
- Lower consumption and cost of PTA, reducing the cost of raw materials.
- No methanol is generated in the PTA method. It eliminates the need for methanol recovery, simplifying the process, and saving investment. It also eliminates the explosion risk associated with methanol, reducing fire safety requirements.
- The EG/PTA ratio in the PTA method is usually lower than the EG/DMT ratio. It reduces the amount of ethylene glycol refining and favoring cost reduction. In recent years, the direct recycling of EG in the PTA method has been resolved. It saves the need for EG refining.
- The PTA method can utilize the self-catalysis of PTA acidity. It reduces the need for a catalyst, avoiding issues such as catalyst deposition.
2.2 The continuous process of the PTA method has the following advantages
- Under automatic control conditions, the process and equipment maintain stable operation for a long time. Product quality is good and there are no batch differences.
- Polyester melt can be spun directly. It does not need melt cooling, granulation, re-drying, and other processes. This reduces costs and is particularly suitable for the production of high-strength industrial yarns.
- Suitable for large-scale and diversified production. With the establishment and improvement of flexible production systems, large-scale continuous processes have solved the problem of producing multiple varieties.
- Save investment. Continuous devices with a daily output of 30 tons and 60 tons save 30% and 60% respectively compared with the batch method. So, in recent years, the continuous method has significant advantages over PTA methods. The advantage of the intermittent method is that the varieties can be changed at any time to meet market demand. Slices are easily stored and transported remotely. The start-stop process has little impact.
From the perspective of the process route, the PTA method can be further divided into five-pot and three-two-pot processes. Representative technologies for the five-pot process include the Gamma process, the Chinese process, and the Evendale process. The three-pot process is typified by DuPont technology. Both processes have similar condensation process conditions, but there are differences in esterification conditions. However, the basic principles of the two processes are similar.
3. PTA Direct Condensation Method (I) Chemical Reactions and Factors
The chemical reaction for producing polyester by directly condensing PTA and EG involves esterification and condensation reactions, aside from side reactions.
3.1 Esterification Reaction
The esterification reaction is the initial reaction in the condensation process. PTA and EG first convert to dimethyl terephthalate (DGT) monomers.
Under the production conditions of temperature 220~300℃ and EG/PTA (molar ratio) 1.0~2.0, PTA is partially dissolved. The esterification reaction between PTA and EG is mainly homogeneous. Some heterogeneous reactions are also involved.
For heterogeneous reactions, PTA originates from solid phase dissolution. It maintains a constant concentration in the liquid phase. So, the particle shape of PTA has little effect on the reaction rate. Why is the EG concentration higher than the dissolved PTA concentration? Since the PTA concentration is constant, the reaction rate is not affected. It represents a zero-order reaction. The reaction rate depends only on temperature.
For homogeneous esterification reactions, the concentrations of PTA and EG significantly affected the reaction rate. As the reaction proceeds, the solubility of PTA in the reaction mixture is much higher than that in pure EG. The dissolution rate of PTA particles gradually increases until the “clarification point” is reached. After reaching this point, the reaction rate varied as a function of PTA and EG concentrations. But, the exact sequence of reactions during this phase is unclear. Usually in the homogeneous reaction stage, the reaction rate decreases with decreasing acid concentration. And there is a proportional relationship between the reaction rate and EG concentration. An increase in EG concentration results in an increase in reaction rate.
The condensation reaction is the chain growth reaction in the synthesis of polyester. Through this reaction, monomers, oligomers, and low oligomers gradually condense into polyester.
In actual production, esterification and condensation reactions are not strictly separated. When the esterification reaction reaches a certain stage, both reactions proceed simultaneously. A certain stage means a certain amount of ethylene glycol ester groups is generated. So, polyester synthesis (i.e. PETG) is typically represented by a total reaction formula.
The total reaction formula indicates they are two completely different processes. Polymerization is an irreversible reaction. While condensation is a stepwise completion and reversible equilibrium reaction. Additionally, each step of the reaction has the same reaction mechanism. To further understand the process and characteristics, the entire reaction can be divided into three stages: the initial stage, middle stage, and final stage.
The initial stage is the initial phase when condensation polymer molecules begin to form.
The middle stage is the stage of chain growth of polyester molecules.
The final stage is the stage where the reaction has reached the specified molecular weight of the condensation product. It needs to be terminated promptly.
Common termination methods used in industry include (a) Reducing the condensation reaction temperature; (b) Adding low-molecular-weight chain regulators; and (c) Changing the raw material composition.
The first method can terminate the reaction but cannot completely eliminate the activity of end-group functionalities. When processed later, end-group functionalities may revive due to heating. It affects molecular weight distribution. The second method may impact product purity due to the addition of low-molecular-weight chain regulators. Nowadays, the third method is more commonly used. It effectively terminates the reaction without affecting product quality.
When condensing two-carboxylic acids and diols, there is a tendency for cyclization due to the molecular structure. Other side reactions may occur. For the condensation of terephthalic acid and ethylene glycol, the main side reactions are cracking, chain exchange reactions, and the thermal degradation of the melt.
4. Typical PTA Direct Condensation Process
One of the representative processes is the continuous direct condensation process of GIMAC PTA. It is divided into three process sections according to the chemical reactions that occur.
4.1 Esterification section (ES)
Under the conditions of pressure ≥0.1 MPa and temperature 257-269°C, the esterification reaction of PTA and EG is a critical stage. The esterification rate of PTA and EG can reach about 96.5% to 97%. In the continuous esterification process, the PTA raw material is first sent to the silo. Add the EG with the catalyst dissolved into the batch buffer tank. Then, the ingredients are continuously and automatically fed into the mixing tank at a constant speed with accurate ingredient proportions. After thorough mixing, it is pumped into the esterification section at a constant flow rate. Some raw materials undergo an esterification reaction. And then it flows into the second and third esterification sections. When the esterification liquid flows out of the last stage, the raw material has been almost completely converted into dimethyl terephthalate (DGT) monomer.
4.2 Precondensation section (PP)
Under vacuum conditions of 27.5 to 5.07 kPa or 5.47 kPa (206 to 38 mmHg or 41 mmHg), the esterification product in the esterification section undergoes a pre-condensation reaction. DGT monomers are converted into low molecular weight oligomers. Generally, two levels of pre-condensation are used, with a maximum of three levels. Each stage is equipped with stirring and heating devices, using steam as the heat source.
4.3 Post-condensation section (PC)
The low molecular weight oligomers produced in the pre-condensation section are further melted and condensed at this stage. Strict process conditions are required. The temperature rises to 280-285°C and the pressure drops to 0.2 kPa (1.5 mmHg). Residence time is approximately 3.5 to 4.0 hours. The intrinsic viscosity of the resulting high molecular weight oligomer generally depends on the product application. To produce medium-viscosity polyester with an intrinsic viscosity of 0.42-0.72, only one finishing stage (PC1) is required. If high-viscosity polyester is required, a second post-condensation stage (PC2) is required. Finally reaching an intrinsic viscosity of 0.9-1.0.
To meet these process conditions, a post-condensation device with a special structure is required. The device should meet the following requirements:
- The condensing device should have a large and constantly updated material evaporation surface. To ensure rapid evaporation of residual EG in viscous materials. Otherwise, prolonged reaction times can lead to thermal degradation. Reduce equipment capacity and reduce product quality.
- The internal structure of the condensing device shall maintain the plug flow of material. Otherwise, material remixing may occur. Thereby reducing product quality.
- There should be no dead ends in the material passage of the condensing device. If present, condensation may remain in the dead space for an extended time. It causes thermal degradation and contaminates the product, thereby reducing product quality.
- The stirring shaft of the condensing unit should have a reliable vacuum sealing structure under high temperatures to prevent air from entering. Otherwise, not only will the viscosity of the polyester not increase. Moreover, thermo-oxidative degradation of the condensation material may also occur. In addition, air intrusion can cause the vacuum system to become clogged. It affects the continuous operation of the equipment.
- The amount of condensation material carried by the evaporated EG from the condensation device should be minimal. It should not accumulate on the walls of the unit. Excessive residue can clog the vacuum system. Accumulation on the walls can lead to scaling due to prolonged heating.
- The mechanical design of the condensing unit should reasonably solve the thermal expansion of the stirring shaft, kettle body, and other components at high temperatures, as well as the cooling of bearings and seals.
- The structure of the condensation device should meet the reaction requirements. At the same time, the structure is simple and easy to manufacture, install and maintain. The post-condensation unit developed by GIMAC (Disc Reactor) fully meets these requirements.
5. Continuous direct condensation process and characteristics
The continuous direct condensation process is one of the advanced methods used in polyester production.
5.1 Process Overview
B-13 – pulping machine; B-21, B-22 – esterification reactor; B-31, B-32 – pre-condensation reactor; B-33 – disc reactor; R-21 – EG (ethylene glycol) recovery tower.
A slurry of EG/PTA with a molar ratio of 1.138 was prepared with a quantitative amount of catalyst sb(OAc) and EG. EG is recovered and refined from the esterification and pre-condensation. Add the prepared slurry together to slurry tank B-13. The mixture is continuously metered into B-21 at 0.11 MPa and 257°C. Esterification occurs under stirring, reaching an esterification rate of 93%. The esterification product is then transferred to B-22 under pressure. Continue esterification at 0.1-0.105MPa and 265°C. The esterification rate reaches about 97%. Then the esterification product was sent to B-31 with 0.025MPa and 273° for pre-condensation. Then transfer to B-32 with 0.01MPa and 278°C for condensation. The condensation product is then sent to B-33. Continue stirring at 100 Pa and 285°C until the condensation is completed (usually the degree of polymerization is about 100). PET melt can be directly spun or cooled and pelletized.
5.2 Characteristics of GIMAC Process
The use of a single catalyst sb(OAC)3 eliminates the need for stabilizers to suppress side reactions on the product.
Slow heating during esterification with lower reaction temperatures and longer residence times. It ensures stable operation and results in low DEG content and good-quality products.
The use of scraper condensers resolves vacuum system blockage issues during condensation.
The use of a scraper condenser solves the problem of vacuum system clogging during the condensation process. The production capacity can reach 250t/d, meeting large-scale production requirements. It incorporates a flexible manufacturing system (FMS) for producing various products simultaneously.
In the continuous PTA process, GIMAC, Chinese technology, and Ivenda technology each have their characteristics. The Chinese technology features are PTA high-pressure dense phase gas transportation and low EG/PTA slurry molar ratio. The EG produced during the condensation process can be directly recovered with a high degree of automation. Its machines operate stably and the products are of high quality. Ivenda technology also utilizes high-pressure dense phase gas transfer for PTA. It cuts energy consumption by utilizing pressure and position differences for material mixing and transfer. It also uses reactors designed for efficient mass and heat transfer.
GIMAC and Chinese technologies are both five-step processes. While Ivenda technology is a three-step process. Although the condensation process conditions of the five-step and three-step processes are similar, there are significant differences in the esterification process conditions. The five-step method uses lower temperatures and pressures. The three-step method uses a higher EG/PTA ratio and a higher esterification temperature. They are used to strengthen the reaction conditions, speed up the reaction rate, and shorten the reaction time. The total reaction time of the five-step method is approximately 10 hours (esterification time is 5.5 hours). The total reaction time of the three-step method is 3.5 hours (esterification time is 1.5 hours).
