Znd. Eng. Chem. Res. 1995,34, 4049-4057
4049
Effect of Reaction Time on Poly(ethy1ene terephthalate) Properties David E. James* and Lawrence G. Packer Amoco Chemical, Intermediates Business Group, Amoco Research Center, Naperville, Illinois 60566-7011
Byproducts are formed at various times or positions within the PET process. During esterification, the conversion rate of terephthalic acid and ethylene glycol is the highest, and the largest changes are observed in prepolymer carboxyl end group and diethylene glycol concentration. During the polycondensation stage, under full vacuum, when the reaction temperature is highest, molecular weight increases at the fastest rate. Color changes dramatically during this period. The product becomes grayer and greener as a result of the reduction of the Sb3+polycondensation catalyst to antimony metal and more yellow because of the formation of colored, high molecular weight byproducts via polymer degradation reactions. A previously unidentified byproduct, tran~-4,4'-dicarboxystilbene,formed in this stage, is fluorescent and could contribute to the fluorescence of PET. This compound is also significant because it indicates that the redox chemistry which produces high molecular weight byproducts during polycondensation may result in the reduction of Sb3+to SbO.
Introduction Preparation of high molecular weight poly(ethy1ene terephthalate) (PET) from ethylene glycol (EG) and purified terephthalic acid (PTA) is accomplished in a two-stage process (Sattler and Berg, 1987). In the first stage, known as direct esterification, the PTA feedstock is reacted with a molar excess of EG to produce prepolymer. As is shown in Figure 1, prepolymer is a mixture of oligomeric species containing both carboxylic acid and EG ester end groups (Flory, 1936; James, 1992). For this reaction, a catalyst is not required and is generally not employed. However, to effect sufficiently rapid reaction rates, most processes make use of high reaction temperatures and superatmospheric pressures. In the second stage, prepolymer is converted to high molecular weight polyester. Two types of reaction produce chain growth (Berkau and Hocutt, 1970)(Figure 2). The predominant reaction is metal-catalyzed transesterification in which oligomers containing EG ester end groups combine t o produce higher order molecules and ethylene glycol as a byproduct. In addition, oligomers with carboxylic acid end groups can be esterified by reaction with the hydroxyl group of an EG ester chain end. This reaction liberates water. For this stage, high temperatures and subatmospheric pressures are required. During the course of the esterificatiodpolycondensation processes, many chemical transformations occur which influence the physical properties of the polymer product. The purpose of this work is to demonstrate the effect of reaction time on the formation of identified chemical species.
CO,H
et h y l e n e
terephthalic acid
"b
glycol
I
H,O
r
1
1
~
'Y r
~
~
~
z 0 ~
(y = 0
-
~
z
~
~
~
Y 20)
Figure 1. Esterification of terephthalic acid.
i HOCH2CH20H
n
Experimental Section Equipment. The batch pilot plant equipment employed in this study was built to simulate commercial PET processes. The major difference is that all reaction steps are carried out in a single reactor, whereas commercially, multiple reactors are typical. The reactor is a nominally 2-L,stainless steel vessel fitted with an anchor helix agitator and torque meter for viscosity measurement. It can be operated at high pressures (up
* FAX:
708-961-6223.
Figure 2. Polycondensation reactions.
t o 200 psig) and high vacuums (
concentration, ppm time, min based on net production sample weight (PET-PTA) at 1 total mmHz TCBI DCBI BCPE DCS TCBI DCBI BCPE DCS 0 0.0 10 64 1 nd 0 0 0 nd 8 52 1 nd 143 0.0 177 0.0 9 56 1 nd
6
4
I 0
5
io
__-15
2 0 - - 2 F - - 3 o j 5 i o
Mmvler
Figure 15. HPLC comparison of synthetic DCS and PET sample. o.0150 0.0120
0.0000
0.0
233 263 291 300 308 326 348
Scan at 2.191 rnin
a
31.5 56.5 68.5 76.5 94.5 116.5
9 15 17 18 21 23 32
5 12 21 24 28 32 46
57 68 71 77 86 87 98
~0.5 ~0.5 ~0.5 0.8 1.0 1.2 1.5
0 7 10 11
14 17 27
2 15 18 25 35 37 49
5 13 23 27 31 36 52
