George P. Karayannidis

Chain extension of polyesters PET and PBT with two new diimidodiepoxides. II

Two new diglycidyl ester compounds containing preformed imide rings for better thermal stability were prepared to be used as chain extenders for PET and PBT. The preparation of these compounds was carried out in two steps. In the first step, diimidodiacids were prepared from pyromellitic anhydride and 3-aminopropanoic acid or 4-(aminomethyl)benzoic acid. From these diimidoacids, in a second step, diimidodiepoxides were obtained by reaction with epichlorohydrin. The aforementioned diimidodiepoxides were used as chain extenders for poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) with satisfactory results. The polyester samples obtained from various residence times in the reactor, were characterized by solution viscosity measurements, carboxyl, and hydroxyl end-group determination. Starting from a PET having intrinsic viscosity ([η]) of 0.60 dL/g and carboxyl content (CC) of 42 equiv/106 g, one could obtain PET with [η] of 1.16 and CC below 5 equiv/106 g. The typical reaction condition for the coupling of PET was its heating with the chain extender under argon atmosphere above its melting temperature (280°C) for several minutes. Analogous results were obtained for PBT. The hydroxyl content in all cases was increased.

The Chemical Recycling of PET in the Framework of Sustainable Development

In this investigation, all the techniques used in the chemical recycling of polyethylene terephthalate (PET) are critically reviewed according to the overall benefits together with the environmental surcharge that they cause. Those, which are consistent with the principles of sustainable development, are indicated. Experimental data are presented for the acid hydrolysis of PET and compared with previous results on the alkaline hydrolysis of PET with, or without, the use of a phase transfer catalyst. Overall material balances are carried out for the hydrolysis of PET. Finally, it can be postulated that recycling according to the scheme:is the only one within the framework of sustainable development. Therefore, the recycling of PET does not only serve as a partial solution to the solid waste problem but also contributes to the conservation of raw petrochemical products and energy.

Study of various catalysts in the synthesis of poly(propylene terephthalate) and mathematical modeling of the esterification reaction

Abstract Pure terephthalic acid (TPA) was esterified with 1,3-propanediol (1,3-PDO) in the presence of various catalysts, in order to find the most effective one for this esterification reaction. The prepared oligomers were polycondensated in a second step under high vacuum and using the same catalyst (Sb(OCOCH3)3, Ti(OC4H9)4, GeO2) as before, or the well known catalyst for poly(ethylene terephthalate) (PET) production technology Sb2O3. The esterification reaction was monitored by measuring the distilled water as a function of time and from these data the modeling of this process was carried out. The received poly(propylene terephthalate) (PPT) samples were characterized by viscometry, carboxyl end-group content and color measurement. From this study, tetrabutoxytitanium was proved to be the most effective catalyst for the esterification reaction. When this catalyst was used in the second step a PPT polymer with the highest molecular weight was received.

Effect of carboxylic end groups on thermooxidative stability of PET and PBT

A series of poly(ethylene terephthalate) and of poly(butylene terephthalate) samples containing different amounts of carboxyl end groups were prepared by chain-extension reaction with diepoxides. The effect of the carboxyl content on thermooxidative degradation was studied, using as criteria the induction period of oxidation and the stabilisation coefficient, both obtained by differential scanning calorimetry during isothermal or dynamic heating of the samples under air and nitrogen atmosphere. It was found that as the carboxyl content decreases the thermooxidative stability increases. However, in some more chain extended (crosslinked) samples the thermooxidative stability degreased. This abnormal behaviour was attributed to the lower degree of crystallinity of these samples.

Poly(ethylene terephthalate) Recycling and Recovery of Pure Terephthalic Acid. Kinetics of a Phase Transfer Catalyzed Alkaline Hydrolysis

Poly(ethylene terephthalate) (PET) taken from post-consumer soft-drink bottles was subjected to alkaline hydrolysis with aqueous sodium hydroxide after cutting it into small pieces (flakes). A phase transfer catalyst (trioctylmethylammonium bromide) was used in order the reaction to take place in atmospheric pressure and mild experimental conditions. Several different reaction kinetics parameters were studied, including temperature (70–95°C), NaOH concentration (5–15 wt.-%), PET average particle size, catalyst to PET ratio and PET concentration. The disodium terephthalate received was treated with sulfuric acid and terephthalic acid (TPA) of high purity was separated. The 1H NMR spectrum of the TPA revealed an about 2% admixture of isophthalic acid together with the pure 98% terephthalic acid. The purity of the TPA obtained was tested by determining its acidity and by polymerizing it with ethylene glycol using tetrabutyl titanate as catalyst. A simple theoretical model was developed to describe the hydrolysis rate. The apparent rate constant was inversely proportional to particle size and proportional to NaOH concentration and to the square root of the catalyst amount. The activation energy calculated was 83 kJ/mol. The method is very useful in recycling of PET bottles and other containers because nowadays, terephthalic acid is replacing dimethyl terephthalate (the traditional monomer) as the main monomer in the industrial production of PET.

Chain extension of recycled poly(ethylene terephthalate) with 2,2′-(1,4-phenylene)bis(2-oxazoline)

The present work provides improved recycled high molecular weight poly(ethylene terephthalate) (PET) by chain extension using 2,2′-(1,4-phenylene)bis(2-oxazoline) (PBO) as the chain extender. PBO is a very reactive compound toward macromolecules containing carboxyl end groups but not hydroxyl end groups. In the case of PET, where both species are present, for even better results, phthalic anhydride (PA) was added in the initial sample, before the addition of PBO. With this technique, we succeeded in increasing the carboxyl groups by reacting PA with the hydroxyl terminals of the starting polymer. From this modification of the initial PET sample, PBO was proved an even more effective chain extender. So, starting from a recycled PET with intrinsic viscosity [η] = 0.78, which would be [η] = 0.69 after the aforementioned treatment without a chain extender or Mn = 19,800, we prepared a PET grade having [η] = 0.85 or Mn = 25,600 within about 5 min.

Multiple melting behaviour of poly(ethylene-co-butylene naphthalene-2,6-dicarboxylate)s

The multiple melting behaviour of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) and poly(butylene naphthalene-2,6-dicarboxylate) (PBN) as well as of eight random poly(ethylene-co-butylene naphthalene-2,6-dicarboxylate)s (PEBN) copolymers was studied using differential scanning calorimetry. The isothermally crystallized samples showed up to three melting peaks, depending on the crystallization temperature. Especially, the PEBN 70/30 copolymer showed a four-fold melting peak. Recrystallization exotherm is observed in some cases just before the ultimate melting peak of polymers. This multiple melting, as well as the effect of crystallization temperature on this behaviour, is discussed in detail in this work.

Thermomechanical Analysis of Chain-Extended PET and PBT

Two series of samples, one of PET and another of PBT, were received after chain extension at different reaction times with two new chain extenders (diimidodiepoxides). These samples showed different intrinsic viscosity and degree of branching or crosslinking. The effect of this differentiation on thermal properties was studied by thermomechanical analysis (TMA). The parameters studied were the glass transition temperature (Tg), melting temperature (Tm), and the linear expansion coefficient (α). It is remarkable that in the case of PET amorphous or semicrystalline samples, two peaks appeared next to the Tg in the TMA thermogram. The first peak appeared at a temperature very close and lower to the Tg, and the other peak, at higher temperature into the “cold crystallization region.” The presence of two such peaks was not detected in the DSC thermogram of PET samples either in the TMS or DSC thermograms of PBT. The Tg values were found to agree to within ±1°C of those obtained from DSC; on the contrary, the Tm values varied significantly from those received from DSC. The linear expansion coefficient of samples was found to increase with the degree of chain extension.

Thermal Behavior and Tensile Properties of Poly(ethylene terephthalate-co-ethylene isophthalate)

Poly(ethylene terephthalate) (PET) and poly(ethylene isophthalate) (PEI) homopolymers were synthesized by the two-step melt polycondensation process of ethylene glycol (EG) with dimethyl terephthalate (DMT) and/or dimethyl isophthalate (DMI), respectively. Nine copolymers of the above three monomers were also synthe- sized by varying the mole percent of DMI with respect to DMT in the initial monomer feed. The thermal behavior was investigated over the entire range of copolymer com- position by differential scanning calorimetry (DSC). The glass transition (Tg), cold crystallization (Tcc), melting (Tm), and crystallization (Tc) temperatures have been determined. Also, the gradually increasing proportion of ethyleno-isophthalate units in the virgin PET drastically differentiated the tensile mechanical properties, which were determined, and the results are discussed.

Crystallization and melting behaviour of poly(butylene naphthalene-2,6-dicarboxylate)

Isothermal crystallization of poly(butylene naphthalene-2,6-dicarboxylate) (PBN), from the melt, at various crystallization temperatures ranging from 213 up to 225°C was studied. During isothermal crystallization, relatively high crystallinity was found to develop. Isothermal crystallization kinetics was studied using the Avrami equation. For non-isothermal studies, PBN was crystallized by cooling rates ranging from 0.5 to 20°C/min. Only semicrystalline PBN was produced, not depending on the cooling process. The Ozawa and modified-Avrami approaches were used to describe the non-isothermal crystallization kinetics of PBN. Recrystallization phenomena and multiple melting peaks were observed during heating of isothermally and non-isothermally crystallized samples. Annealing at temperatures very close to the original melting point increased the latter from 248 to 264°C.