Abstract

As a kind of polyester, PET can be synthesized by esterification reaction or trans-esterification reaction, which are the two main processing methods. For the esterification reaction, the reactors are terephthalic acid (TPA) and monoethylene glycol (MEG), and the trans-esterification reaction is dimethyl terephthalate (DMT) and MEG. Each of these two methods contains 2 similar steps.

Main Body

1. Introduction of Two Tabrications Methods

1.1 Method 1: Trans-esterfication Reaction Method

The whole process can be shown in the figure 1. It is a trans-esterfication reaction [1]. The reactors are Terephthalic Acid (TPA) and Ethylene Glycol (MEG). And the reaction can be separated to two steps.

The first step is the pre-polymerization of TPA with MEG. During this process, the reactors needed to be heated together, and intermediate compound called bis-hydroxyethyl-terephthalate (BHET) (monomer) and oligomers will be formed. The by-products are removed after the reaction. During step 1, it will produce water as the by-product. For the reaction condition, it is processed at temperature of 270-295 °C [2] under vacuum.  

The second step is called solid-state polymerization (SSP). It is the final polymerization. Compared with the first step, the reaction conducts at a lower temperature 220-235 °C under vacuum or positive pressure using nitrogen. In this step, the by-products are further removed, water in the interior of the pellets can be eliminated, and the thermal degradation products or volatile impurities can be removed by thermal desorption.

Figure 2. Main reaction of Esterification Reaction Method

1.2 Method 2: Esterification Reaction Method

The whole process can be shown in the figure 2. It is esterification reaction. [3] The reactors are Dimethyl Terephthalate (DMT) and MEG. Like the method 1, the reaction also can be separated into two steps.

The first step is the pre-polymerization of DMT with MEG. The reaction conditions are the same. [2] The only one difference is the by-product. In this method the by-product is methanol, while in method 1 it is water.

The second step is called SSP. It is same as the method 1.

Figure 2. Main reaction of Esterification Reaction Method

2. Relationship Between The Two Methods Above

The following figure shows the connections of two methods, and the processing routes. (A) shows the method 1, and (B) shows the method 2.

From figure 3 the connection of these two methods can be got. The difference is about the first step. Method of using trans-esterification reaction is to let MEG react with TPA, then lose the by-product water, after which SHET can be got. Method of using esterification reaction is to let MEG react with DMT, then lose CH₃OH to form SHET. After this step, these two methods will both let SHET polymerize to form PET, which are the same. Besides, PET is not a critical material according to the list of CRMs for Europe, from 2017. The list is updated every 3 years. [4]

Figure 3. Processes routes of two methods.

(A) and (B) represent the two different types of fabrication methods. They can be separated by the horizontal arrow, where (A) is the first row of the figure. And the bule arrows stand for the reaction processes. The species above or below the arow is the reactors or by-product.

(A) is the method of using TPA and MEG to produce PET. In this method, TPA first act with MEG, then lose water to produce SHET. The following step is the same as (B).

(B) is the method of using DMT and MEG. In this method, TPA first act with MEG, then lose CH₃OH to produce SHET. The following step is the same as (A)

3. Development of Fibrication Methods

In order to improve the performance and processing, it may need to add small amounts of co-monomers in PET, such as isophthalic acid (IPA). Besides, there are usually adding some catalyst to facilitate the reaction, and Antimony normally is the first choice. In the recent years, in order to reduce the high consumption for energy, maintenance intensive and investment, researchers invent a new method. It is called ‘Melt to Resin’ (MTR) technology with conspicuous energy saving, because it does not need SSP step.