Introduction

In this chapter; a rapid, alternative technique for the extraction of low molecular weight oligomers from poly(ethylene terephthalate) (PET) polymer film is reported.

Traditionally, PET has been extracted using liquid–solid soxhlet extraction but this has proved to be extremely time consuming. The use of supercritical fluid extraction, SFE, makes it possible to determine the oligomer content in a much shorter analysis time.

In this study hydrothermal extraction was used to extract oligomers from PET film. Cyclic trimer and other low molecular weight oligomers have been successfully extracted from PET film using hydrothermal treatment. Determination and identification of the extracts has been performed using HPLC. This method has the advantage of using harmless solvent, water. This brings economical benefit together with the elimination of undesirable by products. This treatment has also advantage of greatly reduced solvent consumption and hence less amount of sample is required. The main parameters that influence extractability in this method are: temperature, pressure, time of extraction and water flow rate. All these variables have been evaluated and their influence on extraction recovery observed.

Poly(ethylene terephthalate) PET, is a commerical polymer having a linear chain which is produced in a polycondensation reaction, for instance, from ethylene glycol and terephthalic acid methyl ester [1]. The main application of PET has been in the food packaging industry where its original use was in the production of soft drink bottles.

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Bottles are nowadays one of the main uses of PET in direct contact with foods. They are widely used for carbonated soft drinks, mineral water, fruit juices, edible oils, wines and spirits [2]. However, recent developments in the consumer market have led to its widespread use as dual ovenable trays, where readymade meals can be heated either by microwave or by conventional ovens [3, 4]. The PET packaging materials may contain low levels of residual monomer and low molecular weight oligomers which are formed during the resin polymerization and melting process, besides additives, reaction by-products and polymer degradation by-products. All of those low molecular weight oligomers can be extracted from the polymer during the cooking and storage of foodstuffs. This is of particular concern to the polymer manufacturer and the food retailer [5–9]. This concern over plastic contact materials and issues relating to the recycling of waste plastics make it necessary to investigate the extractability of oligomers from PET. The main oligomers of PET are the cyclic oligomers from the first and second series (dimer to nonamer) besides the linear ones. Because of ring tension, the first series cyclic trimer is predominant (60–80% of the total amount of cyclic oligomers). It has been shown that the cyclic trimer migrates and represents 98% of all surface oligomers [10]. The following is the introductory information for PET.

5.1.1. Raw materials

Raw materials for PET manufacture are terephthalic acid (TPA) or dimethyl terephthalate (DMT) and ethylene glycol (EG). Recently, pure TPA has replaced DMT as the preferred raw material after the development of purified TPA by Amoco [11].

Crude TPA is produced by the air oxidation of p-xylene in acetic acid. 4-carboxybenzaldehyde (4-CBA) is one of the main impurities present in the crude form.

An aldehyde group in 4-CBA acts as a chain terminator in the PET polymerization.

Because this contaminant has a very similar structure to that of TPA, it is difficult to remove 4-CBA from TPA. In the Amoco process [12, 13], TPA can be purified by dissolution in water and hydrogenation with a granular 0.5% Pd/C catalyst. It can also be purified by recrystallization from a solution of the mixed solvent phenol and water [14], and crystal aging in 90% acetic acid [15].

140 5.1.2. Polymerization methods

PET is prepared in two steps: the first step, prepolymerization forming bis-(2-hydroxyethyl) terephthalate (BHET), the precursor for the second step, melt polycondensation. Fig. 5.1 shows the schemes for these reactions.

Figure 5.1. Schemes for the PET polymerization process.

141 5.1.2.1. First step: prepolymerization

(a) Transesterification [11,16–18]:

The monomers for the transesterification reaction are DMT and EG. A reactor is charged with DMT and EG with a 1:2.1–2.3 molar ratio and catalysts. A slow stream of nitrogen is passed through the apparatus. The reaction temperature should be 170–210 1C. During this reaction, methanol is collected into a graduated receiver as a by-product to allow estimation of the extent of conversion. When the distillation of methanol ceases, the reaction is completed and bis-(2-hydroxyethyl) terephthalate (BHET) is obtained in the first step.

(b) Direct esterification [18–23]:

This reaction is a heterogeneous reaction with monomers of TPA and EG. The mixture of monomers should be charged as a slurry, because TA is hard to dissolve in EG. The TPA:EG molar ratio used is 1:1.5–3, and the reaction temperature is usually 240–260 1C. According to Kang’s paper, the use of a small monomer feed ratio and high reaction temperature are useful to enhance the solubility of TPA [24]. Generally, catalysts are not used for this reaction, since the acid functional groups of TPA can catalyze the reaction. Water formed during the reaction as a by-product should be collected for the estimation of the reaction conversion. In the commercial reaction, some PET prepolymer (BHET) is added in order to shorten the reaction time.

5.1.2.2. Second step: polycondensation

BHET produced from the transesterification reaction or direct esterification reaction is gradually heated to 280 1C. The reactor is evacuated to low pressure (below 1mmHg), while maintaining the temperature. In this step, EG is collected as a byproduct.

The overall reaction time, including the esterification and the polycondensation processes, is long and usually varies from 5 to 10 h [21, 25].

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5.1.3. Polymerization kinetics under various reaction conditions

Tomita [26] reported stirring speed and reaction temperature effects on the polycondensation kinetics. Higher intrinsic viscosity (IV) was obtained at faster stirring speeds, and the reaction time was shortened at higher reaction temperatures. Chegolya et al. [21] used TPA as an additive during the polycondensation process of the prepolymer produced by the direct esterification method. The polycondensation process proceeded two to three times faster with the introduction of certain amounts of TPA into the prepolymer. They suggested that the carboxylic acid group of TPA formed an ion pair by autoprotolysis and the proton ion catalyzed the reaction [21].

The determination and quantification of oligomers from PET was first reported in 1954 [3] and since then a number of methods have been proposed to differentiate between the species [1, 27, 28]. Traditionally the method has employed Soxhlet extraction followed by gravimetric analysis with identification by HPLC with UV detection [28]. In the selective precipitation approach the successive addition of chloroform, acetone, distilled water and concentrated ammonium hydroxide allows isolation of oligomers. Since the introduction of SFC by E. Klesper et al. in 1962 [29]

the method has become important for the analysis of oligomers and polymers [30] and several studies of PET oligomers using SFE and SFC have been published [31, 32].

These studies used PET films for the extraction and compared off-line and on-line extraction. Unfortunately, no systematic approach was taken to evaluate the optimum conditions of pressure, temperature and mobile phase composition. It therefore seemed to be important to undertake a systematic study of these parameters for the extraction of oligomers from polymers. The influence of temperature is of special interest because by modelling quantitative, dynamic extractions using supercritical fluids it has been shown that SFE is both solubility and diffusion controlled [33, 34, 35]. This model says that diffusion controls the extraction if the solubility is high enough to dissolve all sample molecules that move to the surface of the matrix, and if the concentration of sample molecules in the mobile phase is nearly zero. For the extraction of polymers, this means that an increase of temperature, at solubility already high enough to dissolve all the

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sample molecules moving to the surface, can be used to increase the diffusion to the surface. On the other hand this also means that temperatures below the glass transition point of a polymer might prevent any extraction because the matrix is "frozen" to a glass-like state. The second parameter for dynamic extraction in addition to diffusion is the solubility with its large variability for the mobile phase in the supercritical area [36].

The quantitation of the amount of low-molecular-mass material present in polymeric fibers is an important industrial quality control practice. If the quantity of low-molecular-mass species is too large, they tend to deposit as a white powder on processing equipment, resulting in mechanical error [37]. In addition to the effect of high oligomer concentrations on the processing of fibers, broad polymer size distributions lead to less desirable fiber properties [38]. Several researchers have studied the extraction of low-molecular-mass oligomeric material from poly-meric matrices with supercritical Carbon Dioxide (SCCO₂). Kuppers [39] was able to selectively extract low-molecular-mass components from PET using supercritical fluid extraction (SFE). In this study, a selective two-step extraction was used, whereby oligomers were first removed from the outside of the fibers and then from the fiber core. Bartle et al. [40] also studied the SFE of oligomers from PET films.