End groups carboxyl

This behavior is consistent with an autocatalytic process, whereby the liberated carboxylic end groups catalyze the cleavage of additional ester groups (Eq. 3) ... 

Huffman, K. R., and Casey, D. J., Effect of Carboxylic end groups on hydrolysis of polyglycolic acid, J. Polym. Sci. 

These were prepared by efficiently interhnking pairs of carboxyl end groups in fibers extruded from tetra-chain and from octachain poly- -caproamides (see p. 

Figure 1.14 (a) STM image (lOnmx lOnm, tip bias +0.52V, tunneling current 0.5 nA) of a PVBA-induced sawtooth blade in a restructured Ag(l 1 0) surface terrace, (b) Structural model of the chiral kink arrangements induced by lateral interaction of molecular carboxylate end groups with Ag l 0 0 microfacets. (Reprinted with permission from Ref. [48]. Copyright 2004, American Institute of Physics.)... 

PLLA-fr-PCL) multiblock copolymers were prepared from the coupling reaction between the bischloroformates of carboxylated PLLA with diol-terminated PCL in the presence of pyridine [140]. LLA was polymerized with SnOCt2 and 1,6-hexanediol followed by the reaction with succinic anhydride to provide the dicarboxylated PLLA. The carboxyl end groups were subsequently transformed to acid chloride groups by the reaction with thionyl chloride (Scheme 65). As expected, the molecular weight distributions were broad for all samples (1.84 < Mw/Mn < 3.17). 

Esterification is the first step in PET synthesis but also occurs during melt-phase polycondensation, SSP, and extrusion processes due to the significant formation of carboxyl end groups by polymer degradation. As an equilibrium reaction, esterification is always accompanied by the reverse reaction being hydrolysis. In industrial esterification reactors, esterification and transesterification proceed simultaneously, and thus a complex reaction scheme with parallel and serial equilibrium reactions has to be considered. In addition, the esterification process involves three phases, i.e. solid TPA, a homogeneous liquid phase and the gas phase. The respective phase equilibria will be discussed below in Section 3.1. 

Esterification reactions are acid catalyzed [18-21], and an overall reaction order of 3 (2 with respect to acid and 1 with respect to alcohol) is generally accepted [9], Thus, the acid behaves both as reactant and as catalyst. It can be assumed that the concentration of acid groups, cacid, is the sum of the concentrations of carboxylic end groups (tTPA) and carboxylic groups of the free acid (TPA). 

Besides the main depolymerization reactions, side reactions should also be considered in the kinetic description of a PET recycling process. This is emphasized by the results obtained from a PET extrusion model [85] shown in Figures 2.19-2.23. The complete set of reactions summarized below in Table 2.10 have been used, but shear effects have not been taken into account. Chain degradation, accompanied by a significant reduction of intrinsic viscosity, occurs even within residence times of a few minutes. Carboxyl end groups, vinyl end groups and acetaldehyde are formed in amounts depending on residence time, temperature and initial moisture content of the PET flakes. 

Figure 2.21 Concentration of carboxyl end groups as a function of extruder residence time, temperature and initial water content...

In typical industrial operations, TPA is not dissolved in EG or BHET but in prepolymer. The latter contains PET oligomers with one to approximately six to eight repeat units and a significant concentration of carboxyl end groups of between 200 and llOOmmol/kg. It was found [94] that the solubility of TPA in prepolymer is much higher than indicated by the values given in the literature. Nevertheless, the esterification reactor still contains a three-phase system and only the dissolved TPA may react with EG in a homogenous liquid-phase... 

The monomers TPA and EG are mixed upstream to the esterification reactor in a jacketed slurry preparation unit equipped with a stirrer for highly viscous fluids (e.g. Intermig ). The typical molar ratio of EG to TPA lies between 1.1 and 1.3. The esterification temperature and the molar ratio of monomers are the main controlling factors for the average degree of polycondensation of the esterification product (prepolymer), as well as for its content of carboxyl end groups and DEG. The latter mainly occurs as randomly distributed units of the polymer molecules. 

In general, the degradation of PET is characterized by a loss in molecular weight, a loss in weight in the case of thermal and thermal oxidative degradation, and an increase in the carboxyl end groups. This is usually accompanied by the material first turning yellow, then brown and finally black [17]. 

The reaction rate is very sensitive to the ratio of hydroxyl to carboxyl end groups, as shown by Figures 4.7 and 4.8. At low carboxyl concentrations, the transesterification reaction will be favoured, while at high carboxyl concentrations the esterification route will be favoured. If the transesterification and esterifications were equal, which they are generally not, then the consumption ratio of end... 

Generally, two to three preheater sections are used for the product heat-up by using nitrogen, and two to three sections are required to reach the final viscosity. Cooling is carried out either in an additional compartment or with a fluid bed. Typically, for a viscosity increase from 0.60 up to 1.0, the crystallinity increases to ca. 62 vol%, and the carboxyl end group concentration decreases by approximately 10-15 mol/t. This equates to both esterification and transesterification contributing half of the IV increase if side reactions are neglected. 

The influence of the ratio of hydroxylic/carboxylic end groups has been studied by several research groups. In the case of PET, this varies, based on the assumed mechanism over the range of 1.5-4.5 1. For poly(butylene terephthalate) (PBT) and polyethylene naphthalate) (PEN), the optimum is indicated at 2.0 1 [19, 20]. Any deviation from this ratio affects the reaction rate. 

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