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Polyester (PET), Polyamide (PA)

Polyester (PET), Polyamide (PA)

Summary

Polyethylene Terephthalate (PET) is one of the major polyester resins available in the market. It has superb functional and handling performance for the price and is a material used in many applications. Major applications are for the use in PET bottles and industrial textiles, and recently use in weather resistant films and optical films is increasing. Polyester is synthesized with a condensation polymerization process, which limits the strength of the polymer limiting use to textile applications. Solid State Polymerization is a technology that increases the strength of the polymer widening the applications for polyester.

Below are some uses for Solid State Polymerized PET
[PET]
• Bottles
• Industrial Textiles
• Tirecord • Weather Resistant Film (for Solar Panels)
• General Textiles
• Food Packaging
• Optical Film

Below are some applications of other Solid State Polymerized polymers
[Polyamide (PA)]
• Industrial Textile
• Tirecord
• Circuit Boards
• Battery Casing (heat resistant, non conductive)
[Super Engineering Plastics (special PA etc.)]
• High strength textiles (bulletproof vests, air bags)]

Fig.1 Optical film

Fig.2 Solidaire SJS-24-12

Drying System [End use example: Optical Film (PET)]

Fig.3 Inside of Solidaire

In optical film used for LCD displays and precision photographs, transparency, imperfections, and evenness of film thickness directly affects the quality of the final product. PET used for these applications must be dried to ultra low moisture content in the range of 10 to 100ppm in pellet form to prevent breaks in the film forming process. Additionally, crystallinity must also be uniform. Hosokawa’s Drying System (DRS) is an optimum drying system that solves all of these problems.

The drying system consists of crystallization and drying steps.

Crystallization Step

Fig.4 Feed of PET granules

Main Purpose: Crystallization, Heating to Drying Temperature
Equipment Used: Solidaire

A high rotation speed agitator rotating within a casing with heated jacket enables the quick and efficient heat transfer whilst crystallization occurs very evenly. This is a very important factor necessary for an even film thickness.

Drying Step

Fig.5 Hopper reactor

Main Purpose: Drying
Equipment Used: Hopper Dryer

The evenly crystallized polymer is dried in the hopper dryer using a low dewpoint gas. The crystallized polymer is already at the drying temperature so there is no need for further heating in this step. Because no extra heating is required, the use of low dewpoint gas can be kept to a minimum leading to a lower running cost.

Fig.6 Flow of crystalization process

Solid State Polymerization System [End use example: Tirecord (PET)]

Tirecord is a high strength textile that is woven inside of automobile tires. Metal fibers had been used in this application in the past, but due to lightweight and durability, PET and PA is increasingly used recently.

Since high strength is required for this application, a Solid State Polymerization step is required for PET to increase strength. In this process the molecules polymerize with each other, increasing the strength of the polymer while at the same time reaction byproducts are removed out of the polymer.

Solid State Polymerization (SSP) consists of 5 steps, crystallization, drying, heating (secondary crystallization), polymerization and cooling. Crystallization and drying steps are the same as shown in the previous section “Drying System”.

Heating (2nd stage Crystallization) Step

Main Purpose: Heating to reaction temperature, additional crystallization
Equipment Used: TorusDisc

The next process uses a static bed (Hopper Reactor) for the polymerization process. The polymerization is conducted a temperature close to the melting point of the polymer so precise control of temperature is required. When heating PET crystallization occurs which causes heat to be generated. If crystallization is insufficient, heating of the polymer occurs during the polymerization process. Since the polymerization is conducted near the melting point, heating resulting in melting. To prevent this from occurring, the crystallization is increased in the heating step preparing for a more stable polymerization step.

Polymerization Step

Main Purpose: Solid State Polymerization, Removal of byproducts
Equipment Used: Hopper Reactor

In order to reach the required strength (polymerization), the polymer is held within a hopper reactor while the polymerization reaction occurs. The level of polymerization is dependent upon the residence time in the Hopper Reactor so the design has been optimized to minimize the residence time deviation of the polymer pellets. The Hopper Reactor is designed with a full jacket to limit the horizontal temperature distribution, and to distribute the carrier gas evenly.

Cooling Step

Main Purpose; Cooling, Stopping the Solid State Polymerization Reaction
Equipment Used; TorusDisc

After the required level of polymerization is achieved, the polymer is quickly cooled to stop the polymerization reaction and to keep the level of product quality. Cooling is also necessary to improve the handling qualities of the polymer in the downstream process.