PET POLYMERIZATION

PET is the condensation product of terephthalic acid and ethylene glycol. The key to successful PET polymerization is monomer purity and the absence of moisture in the reaction vessel. PET polymerization has recently been reviewed in detail by East

MONOMER PRODUCTION

The enabling technological breakthrough that allowed for the cost-effective polymerization of PET was the development of low-cost, pure TA from mixed xylenes by the Amoco company in the mid-20th century . An alternative to TA, and the monomer of choice before the availability of low-cost TA, is dimethyl terephthalate (DMT). While direct esterification of TA is the preferred method of PET synthesis, ester interchange between DMT and ethylene glycol is still utilized in some PET manufacture, partially because of local choice and partially because DMT is a product of polyester recycling by methanolysis or glycolysis]. The second monomer, ethylene glycol, is a major material of commerce, produced by the oxidation of ethylene followed by ring opening with water . The large-scale production of all PET monomers assures low-cost polymers and makes competition from new compositions of fiber-forming polymers very difficult.

POLYMERIZATION

The first stage of PET polymerization is, in essence, the production of  bishydroxyethylterephthalate (BHET). In the direct esterification of TA, this reaction

HOOC_C6H4_COOH +‏ 2HOCH2CH2OH à HOCH2CH2OCO_C6H4_COOCH2CH2OH +‏ 2H2O

actually results in a mixture of low amounts of free BHET with a variety of PET oligomers. Water removal is critical to the ultimate achievement of high molecular weights. Similarly, in the first stage of the ester interchange process, BHET is formed along with a mixture of PET oligomers, i.e.,

CH3OCO_C6H4_COOCH3 ‏+ 2HOCH2CH2OH ->HOCH2CH2OCO_C6H4_COOCH2CH2OH +‏ 2CH3OH

The reaction catalysts for the ester interchange reaction have been the subject of intense research for many years and many catalyst compositions are found in the patent literature. The introduction of ester interchange catalysts requires the killing of these catalysts later in the polymerization sequence as they are equally effective as depolymerization catalysts. The next step in the polymerization is the melt polymerization stage. In this reaction step, an ester interchange reaction occurs between two molecules of BHET to split off a molecule of glycol and build polymer molecular weight. The reaction must be catalyzed, and antimony trioxide (Sb2O3) is almost universally the moiety of choice. High vacuum is applied to push the reaction to high molecular weights. Typical melt polymerization temperatures are 2858C or higher, and viscosities are on the order of 3000 poise, making uniform stirring and the imparting of a constant shear history across the polymerization mixture difficult, although the power requirement to the stirrer thus becomes a useful QC tool. Recent variations of this

method have been patented by DuPont (elimination of vacuum and Akzo (new,nonantimony-based catalyst) . As neither DuPont nor Akzo has produced PET fiber in 2005, it is unclear whether these apparent process improvements are actually utilized.

After achieving molecular weight targets, the polymer may be extruded into strands and cut into chips for subsequent melt spinning (batch process) or fed directly into a spinning machine and converted to fiber (continuous process—CP spin-draw). In the case of chipped polymer, the molecular weight can be further increased through solid-state polymerization. In this process, thoroughly dried PET chip is first crystallized at about 1608C to prevent the amorphous as-polymerized chip from sticking together (sintering), and then heated just below the melting point under high vacuum and extreme dryness to advance the molecular weight upward to values of inherent viscosity (IV) of 0.95 (textile grade chip has an IV of about 0.65). The effects of the process thermal history of PET chip and fiber have been extensively studied and are conveniently monitored by thermal analysis techniques. Jaffe et al. have reviewed the thermal behavior of PET and described the expected response of PET to process history in detail.

A variety of side reactions and end-group-induced reactions can lower the thermal stability and cause degradation of PET during spinning. The formation of diethylene glycol through the coupling of two hydroxyl ends from the glycol ends (or BHET ends) by dehydration, forming a diethyleneglycol (DEG) unit in the chain, is especially troublesome. DEG is a foreign unit in the backbone, although it does not directly affect the polymer chain length. This unit reduces crystallinity and lowers the glass transition, thermal stability, and hydrolytic stability of the polymer. It is impossible to completely eliminate DEG formation and about 1.0–1.5 mol% of DEG is always present. Depression of the polymer melting point is easily measured by differential scanning calorimetry (DSC), and this parameter provides an accurate measure of DEG content . Finally, any melt-processed PET always has some cyclic trimer content, which, while not a direct problem for polymer performance, does tend to exude during processing and may cause process upsets. In reality, commercially produced PET is always made by a continuous process involving a number of linked vessels between which the polymer is continuously pumped until the final product specifications are achieved. While some process descriptions have been published [25], most processing details are highly protected as proprietary information. The process usually involves at least four steps, i.e., an initial esterifier followed by a series of three polymerizers, each designed to further advance the polymer molecular weight. Extreme care is taken to promote within and between batch uniformity, eliminate dead zones where polymer may degrade, and remove all low molar mass reaction products such as glycol or water.

CHARACTERIZATION OF POLY(ETHYLENE TEREPHTHALATE) CHIP

PET chip or representative samples of CP spin-draw polymer are conveniently characterizedas by their molecular weight, cleanliness, and thermal behavior. Molecular weight is characterizedby the polymer intrinsic viscosity [h], usually in halogenated solvents; the besthalogenated solvents are hexafluoroisopropanol=pentafluorophenol mixtures. Intrinsicviscosity is related to molecular weight by the Mark-Houwink equation, i.e.,

[µ]=KM_v^a

Where K and a are solvent-dependent, butKis about 1.5*10^-2_1*10^-1 and a is about 0.60–0.85 . High molecular weight or high crystallinity can make polymer dissolution difficult and be responsible for erratic results. Polymer cleanliness is measured microscopically (optical techniques, polarized light microscopy) and is often expressed in units such as the number of black specks or the number of gels per gram of polymer. Acceptable values are determined empirically and are meaningful only in a known process context. Thermal parameters are conveniently monitored by DSC, allowing a quick assessment of DEGcontent, crystallinity, etc.