Considerations Beyond the Laboratory

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Chapter 2 Industrial-Scale Production of Polyesters, Especially Poly(ethylene

terephthalate)

S. M. Aharoni

1. Fundamental differences between laboratory- and industrial- scale polyester preparation. Industrial considerations

There are certain fundamental differences between a laboratory-scale creation and industrial-scale manufacturing of the same polyesters. A laboratory-scale polycondensation procedure is designed to first determine whether the polymerization can occur at all by the envisioned procedure and a polymer of useful average chain length may be obtained. Only after these questions have been satisfactorily answered by performing the polymerization will the polymer be characterized and its properties determined.

In industry, there are many more parameters to consider. These may be all put into three main categories, falling under the headings of "is it real?",

"can we win?", and "is it worth it?". By "is it real?", the question whether the polymers can be made at all on an industrial scale is addressed. By considering "can we win?", the important questions of "is there a market for the product?" and "what will be our position in it relative to the com- petition?" should be answered. In "is it worth it?", we address the very important question "can we make it in an efficient enough process and sell it at a high enough price?", so that the revenues generated by the sale will be sufficient to yield the desired level of profits after the direct and indi- rect costs of manufacture, including capital depreciation, costs of sales, and

Handbook of Thermoplastic Polymers: Homopolymers, Copolymers, Blends, and Composites Edited by Stoyko Fakirov

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costs of production are all satisfied. The present global economy requires that additional questions be satisfactorily answered before commitments are made. These include, but are not limited to the following:

(i) Is there in existence or potentially an overcapacity around the world for the product in mind?

(ii) What about the availability and pricing of raw materials at the envisioned production site?

(iii) Is an educated and stable labor force available?

(iv) What are the conditions of infrastructure, transportation, electricity, water, and sewerage in the plant site?

(v) What about the environmental impact of the plant? Atmospheric pol- lutants? Water/soil pollution?

(vi) How will undesirable by-products be disposed of in a safe and responsible manner?

(vii) Cost of manufacturing in highly industrialized countries vs. cost of manufacturing plus transportation of product from less industrialized ones?

(viii) What about the political and social stability at the location of the contemplated production site?

(ix) May the polymer under consideration be soon replaced by a better/cheaper competitor?

Only after all these queries are answered, should the capacity of the plant be decided, based on the expected immediate sales and future growth, and the cost of the plant should then be estimated. It is advisable for the plant capacity to be agreed upon at that point, all the financial variables recalculated, the expected selling price determined again, and the plant profitability re-ascertained. Only then should a decision be made on the construction of the plant.

Once the go-ahead decision has been made, the plant should be designed and its blueprints and specifications prepared for dissemination among potential bidders. Prom the author's experience in the construction industry, it is advisable that tenders should be issued only after test drillings have been performed at the construction site and the results incorporated in the specifications. The test drillings help in determining the aquifer, site pollution, nature and depth of foundations, etc. All these minimize the future deviations of the construction from the blueprints, and limit the potential for costs overrun. The written specifications should be audited and matched with the blueprints at least twice, preferably by two separate teams. It is highly desirable to issue a number of tenders sufficient for five or more bids to be secured. It is not always wise to choose the least expensive bidder, especially when his bid is substantially lower than all the rest, because often the low bid indicates that the bidder does not fully un- derstand the specifications of the project, or that he is financially strapped

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Industrial-Scale Production of Polyesters 61 and the completion of the project may be jeopardized. If only one company bids, the whole project at the site under consideration should be aborted

— something is wrong!

It is relatively straightforward to describe the synthetic steps involved in the industrial-scale manufacture of polyesters by presenting the flowcharts of their polymerization trains and explaining the events and processes taking place at important locations along the trains. In the following, this methodology was adopted to describe and discuss the industrial-scale polymerization of polyesters. Several flowcharts will be presented below and the industrial-scale polymerization of most of the important polyesters currently in use will be expounded. In them, polymerization-train capacities, or the capacities of plants containing one or more trains each, are not shown. Also, the electrical circuitry, most piping carrying heating fluids, recycling and/or disposal of by-products, eic., are not shown. Stylistically, the symbols and coding system appearing in the flowcharts of the Stanford Research Institute (SRI) International are adopted. The flowcharts appearing in SRI's reports are, however, far more detailed than those shown below, and the interested reader is referred to the SRI international reports for more plant details and process insight.

2. Preparation of poly(ethylene terephthalate) from dimethyl- terephthalate and ethylene glycol

Two methods are used at present in the industrial manufacture of poly (ethylene terephthalate) (PET). Although producing the same polymer, they differ in the monomers used, the polymerization process details, and the low molecular weight (MW) by-products. In this section, the preparation of PET from dimethylterephthalate (DMT) and ethylene glycol (EG) is discussed, and in the next section the manufacture of PET from terephthalic acid (TPA) and EG is considered.

The polymerization of DMT and EG to PET proceeds in two steps:

transesterification and polycondensation. Transesterification involves the conversion of the methyl ester groups of DMT into 2-hydroxyethyl esters of terephthalic acid. When only one terephthalate kernel exists per molecule, the diester is usually called bis(2-hydroxyethylterephthalate) (bis-HET).

In reality, the transesterification step produces not only bis-HET, but a decreasing number of 2-hydroxyethy!-terminated oligomers containing 2,3, 4,... terephthalate kernels.

O O

H³C- O—C—(( ))—C-Ο—CHs+HO—CH²-CH²-OH

/ O O \

HO-CH²-CH²-0-|-C^(( ))_C-0-CH²-CH² + OH +CH³OH

/1,2,3,4...

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Depending on the molar ratio of EG to DMT charged into the first reactor in the polymerization train, the relative amounts of monomeric bis-HET and its oligomers may be altered and, concomitant with the latter stages of the transesterification, the polycondensation step may appear in detectable levels.

C-O-CH2-CH2-OH O O 2HO-CH2-CH2-O-C

HO-CH2-CH2-OH + HO—(CH2)2—O—C

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HO—(CH2)2—O—C Il O

The two polymerization steps are easily differentiated: the by-product of the transesterification step is methanol, while that of the polycondensation step is EG. In Figure 1, an extremely simple polymerization chart, adapted by the author from Ellwood [1], is shown. Because of its relevance to many of the polymerizations to be discussed below, the specifics of this flowchart should be considered in some detail. The polymerization train comprises three reactors arranged in series, and several ancillary storage tanks and vessels. DMT may be delivered molten to an insulated storage tank or introduced into the tank in the solid state and heated there to above its melting point. Either way, the temperature of the DMT storage tank must be kept sufficiently high above the DMT melting point to prevent it from freezing and solidifying in the transfer pipes and pumps, yet not too high

15-25 mm Hg

E s t e r P o l y c o n d e n s a t i o n n t e r c h a n g e r e a c t o r I

P o l y c o n d e n s a t i c r e a c t o r I I

Figure 1. Schematic flowchart for the polymerization of PET from DMT and EG

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Industrial-Scale Production of Polyesters 63 to initiate noticeable levels of thermal and oxidative degradation which are detectable by yellowing of the polymeric end product. The molten DMT is metered, at a predetermined rate, into the ester interchange, or transesterification, reactor. Concurrently, EG is pumped into the reactor from a separate tank, at a rate designed to create in the reactor the desired EG/DMT ratio. In this flowchart, it is indicated that the reaction catalysts are introduced into the system as a solution or a suspension in the EG. The important thing to realize is that in this flowchart, the catalyst for the polycondensation (a step that will kick in only in the polycondensation reactors) is introduced together with the catalyst for the transesterification step, right at the beginning of the polymerization process, where it is completely ineffectual and may be stripped away in part by being soluble in and carried away with the methanol by-product of the transesterification step. Therefore, a thermally stable and methanol-insoluble catalyst for the polycondensation step is preferable. Antimony-based catalysts for this step are commonly used, and will be discussed in a subsequent section. Unlike the esterification step of TPA to bis-HET that requires no catalysts, the transesterification of DMT to bis-HET is catalyzable. Usually, salts of di- valent metals, such as zinc and calcium, are used for this purpose because they are good catalysts and their salts are colorless, so that their residues in PET do not affect adversely the color of the polymer. Whether the catalysts are charged into the transesterification reactor as EG solution or suspension depends on the design and temperature of the EG storage vessel. If this temperature is kept high above ambient, say at about the same temperature as that of the DMT vessel, then the catalysts are most likely to be soluble in the EG, and will be metered into the ester interchange reactor in the form of solutions in EG. However, if the EG storage vessel is maintained at ambient temperature, then catalysts, such as ZnCl2, CaCl2, or Sb2Oa, will be only sparingly soluble and remain suspended in the EG, may have to be constantly stirred in the storage vessel, and may be pumped into the reaction vessel as a suspension.

The transesterification rate in the ester interchange reactor depends on several factors: the molar ratio of EG to DMT, the molar amount of transesterification catalyst charged into the reactor, the reaction temperature, and the rate of removal of the methanol by-product. The EG/DMT molar ratio is usually kept in the range of 1.2 to 1.8. A lower ratio will rapidly generate an excessive amount of longer bis-HET oligomers, greatly increasing the melt viscosity of the reaction mixture, causing inhomogeneities in this mixture and rendering its stirring and the stripping of the methanol by-product inefficient. In extreme cases, the product may crystallize and solidify in the reactor, interrupting the continuous polymer production and resulting in undesirable low-MW product. A higher ratio of EG to DMT, say EG/DMT > 2, results in some EG being stripped with the methanol from the transesterification reactor and in the bis-HET and its short oligomers dominating the product mix to be subsequently pumped

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into the first polycondensation reactor. EG/DMT molar ratios of 1.4 to 1.7 are frequently mentioned as being the most desirable ones. The amount of transesterification catalyst introduced with EG usually falls in the range of several hundreds ppm, calculated as moles of metal ions per mole of DMT.

In cases when transition metal catalysts are used, such as cobalt, tin, or manganese, lower catalyst concentrations may be used since the respective residues impart undesirable colors to PET.

The temperature of the transesterification reactor reflects a happy medium between several effects. It is desirable to have a temperature as high as possible. This drives the ester interchange as fast as possible and ensures that the increasing MW oligomeric and polymeric product will always be exposed to temperatures substantially higher than the melting and, naturally, freezing points of all the polymeric species in the reactor vessel, so that no unwanted solidification will occur. Such high temperatures, however, accelerate the thermal and/or oxidative degradation of all species in the reactor, thus tending to discolor the polymer. Therefore, an average reaction temperature is usually selected, above yet close to the melting points of the products in the transesterification vessel. In the flowchart in Figure 1, a temperature of 2450C is indicated. Finally, the rate at which the methanol by-product is being removed from the reactor must be equal to or faster than the rate at which it is generated. Otherwise, methanol left behind in the transesterification reactor will negatively affect the reaction and slow it down. To avoid this, the methanol stripping columns are designed from the start to be of capacities large enough to accommodate the expected needs.

The pressure in the ester interchange reactor is atmospheric, and methanol may be swept away by a slow stream of inert gas such as nitrogen. This is dramatically different from the conditions prevailing in the esterification reactor during the preparation of PET from TPA and EG. In the latter case, a high autogenous pressure is generated, as will be described in the following section. Importantly, in the horizontal transesterification reactor shown in Figure 1, there exists a dynamic continuum of compo- sitions, starting with the monomers charged at the left-hand side of the reactor to the molten mixture of oligomers, with bis-HET being pumped out at the right-hand side. The reactor is kept constantly full by ensuring that the total volume of monomers charged into it per unit time equals the total volume of melt being pumped out plus the volume of methanol being removed from the system. It should be noted that in Figure 1 the transesterification reactor is horizontal in design. As will be shown in subsequent figures, the more modern transesterification and direct esterification reactors are vertical in design. In them, the monomers are charged from one end of the reactor, and are mixed and reacted while moving towards the other end. Except for better flow control in the reactor, there are no meaningful differences in the chemical processes taking place in horizontal or vertical reactors.

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Industrial-Scale Production of Polyesters 65 From the ester interchange reactor, the molten oligomers and bis-HET are pumped to the first polycondensation reactor where polycondensation starts.

/ O O \

/ Ii /^\ Ii \

mHO—CH2-CH2J-O-C^fj^—C-O-CH2-CH2J-OH +

\ Λ

/ O O χ

nHO—CH²- CH²-I-O- C—/Ο»—C-O-CH²-CH²J-OH—>

/ Il t 1

m C-O-CH²-CH²-OH +

0Vo-CH2-CH2-OH

This reaction is catalyzed by antimony-based compounds, such as antimony trioxide, and its rate is accelerated by increasing the temperature to around 27O⁰C and removing the EG generated during the condensation reaction by the application of moderate vacuum. In Figure 1, the vacuum is 15-20 mm Hg, but it may be as high as 100mm Hg and as low as 10mm Hg. Although most of the excess EG is stripped from the system in the first polycondensation reactor, the MW of the molten polymer at the exit point is not very high. The great increase in molecular weight occurs in the second polycondensation reactor. Because it is designed to strip the last remnants of EG from the polymer and to complete the polymerization, this reactor is often called "the finisher". Here, there is practically no excess EG left in the system, so that the ethylene glycol being removed from the system comes almost exclusively from chain ends. For each pair of chain ends reacting with one another, one molecule of EG is eliminated and, on average, the molecular weight of the newly created chain is double the MW of its two predecessors. Because the viscosity of the molten PET greatly increases with increasing MW, the diffusion of the EG molecules and/or bubbles through the molten bulk is dramatically slowed, even at the higher temperature and high vacuum conditions existing in the reactor, and the removal of the generated EG becomes even harder to achieve.

To overcome this, the stripping of the last remnants of EG at an acceptable rate under non-excessive conditions is made attainable by the internal design of the finisher. In it, a set of many large flat plates or cylinders are attached mutually onto a joint axis. The latter is mounted horizontally and is motor-driven. The molten polymer is pumped into the horizontal reactor at a rate ensuring that a substantial fraction of its volume will not be filled with polymer, but will serve as a head-space for the EG stripping

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process. With this configuration, as the cylinders rotate around their horizontal axis, they pick up the molten PET from the bottom part of the reactor, with the result that at the upper part a very large surface area is created, associated with a rather thin layer of molten PET. The thinness of the molten PET layer on the rotating cylinders is maintained constant by a set of baffles, their job being to remove excess polymer and keep the layer thickness small. Because of the slowness of the stripping operation, there are frequently several finishers in the polymerization train. After a residence in the finisher that may be several hours long, the polymer has attained the desired MW and melt viscosity, and is transported through a set of pumps and transfer lines to its final destination in the plant. Depend- ing on the plant design and the predetermined MW of the PET product, the polymer may be transported to one or more extruders and chipping machines, and converted into pellets. It may be also transported directly to injection molding or blow molding machines and be converted into PET bottles or other injection-molded items. If the MW of the polymer is very high, corresponding to intrinsic viscosity (I.V.) values of 1.0dL/g or higher, the polymer may be transferred molten to melt spinning machines, and spun and drawn into tire cords or other industrial fibers. High-MW PET grades, designed to possess high melt elasticity, may be pumped directly to blow molding machines and used to make very large PET containers or smaller ones with convoluted shapes.

In Figure 2, a more complicated and modern flowchart of a polymerization train is shown. It is fundamentally the same as that in Figure 1, but its mere complexity implies that the rather simple polymerization train of Figure 1 does not perform as well as hoped, and, in order to industrially manufacture large amounts of high quality PET, a more sophisticated polymerization train is needed far more expensive than the one shown in Figure 1.

The obvious difference between Figures 1 and 2 is the fact that each reactor in Figure 1 is replaced by several reactors in Figure 2. In each group, these reactors are arranged in series. The transesterification series of reactors all operate under atmospheric pressure, but the internal temperature of each is higher by about 2OK than that of the previous one. Thus, the temperature of the first transesterification reactor is about 20O0C, that of the second is about 22O⁰C, the third operates at 24O⁰C, and the fourth reactor at 26O⁰C. The amount of methanol removed is the largest in the first and second transesterification reactors; it rapidly declines in the third and especially in the fourth reactor. It is important to note that Figure 2 is more realistic than Figure 1, and shows that the EG stripped in the polycondensation reactors is being recycled and reinjected into the system together with virgin ethylene glycol at the head of the polymerization train.

In current industrial-scale polymerization trains, the total residence time of the melt in the transesterification series of reactors is about 4 h.

Unlike the transesterification reactors, the polycondensation reactors

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Industrial-Scale Production of Polyesters 67

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all operate at the same temperature of about 275⁰C. What differentiates them is the gradual reduction in internal pressure, from about 250mm Hg in the first reactor to about 55 mm Hg in the third. The use of series of poly condensation reactors is brought about by several facts: in two directly connected reactors, it is technically very hard to go from atmospheric pressure in one reactor to a high vacuum in the next one; an abrupt and large reduction in pressure generates large and uncontrolled surges in the EG leaving the system, combined with bumping and extensive splattering of the melt; in order to produce a polyester with a desired MW at the exit port of a single poly condensation reactor, the change in melt viscosity is too large for the whole train to be effectively controlled.

In Figure 2, it is indicated that the transesterification catalyst is introduced into the system at the head of the polymerization train, together with the virgin and "spent" ethylene glycol, while the polycondensation catalyst is introduced downstream, at a point between the transesterification and polycondensation series of reactors. This is the usual case when the polycondensation catalyst is active towards transesterification and/or degradable by the reactants and by-products in the transesterification series of reactors, and is not volatile enough to be carried away and removed from the polymerization train together with the stripped methanol. If the polycondensation catalyst of choice is non-degradable and non-volatile, then it may be introduced together with the transesterification catalyst at the head of the train, and not in the middle. It should be remarked here that in the case of DMT and EG monomers, the system is anhydrous, and water- sensitive catalysts may be safely used.

The final polycondensation reactors in Figure 2 are all finishers. Their temperature of operation may fall anywhere in the range of 275 to 285⁰C and the internal pressure may be as low as 1 mm Hg. The molten PET is pumped from the first finisher to at least one subsequent finisher. This is true when PET of very high MW, with I.V. of around 1 dL/g, is the desired end product. When PET of lower MW is desired, say, with I.V. = 0.7 dL/g, only the first finisher is used. It should be noted that in either case there are two or more finishers, arranged in parallel, for each series of polycondensation reactors.

As shown below, the process and flowchart illustrated in Figure 2 for the preparation of PET from DMT and EG are used with relatively minor modifications in the preparation of several industrial copolyesters, including several transparent non-crystalline polyesters and thermoplastic elastomers.

3. Preparation of PET from terephthalic acid and ethylene glycol A fundamental difference between the preparation of PET from DMT and EG, or from TPA and EG, consists in that TPA does not melt by itself

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H2O

V a p o r

Figure 3. Schematic flowchart for the polymerization of PET from TPA and EG at the temperatures used throughout the polymerization train, making its esterification by EG a prerequisite for the homogenization of the monomer mixture in the first esterification reactor. As a consequence of the use of TPA instead of DMT, the esterification by-product is water, which is carried in the molten mixture of bis-HET and oligomers of growing chain length down the series of esterification reactors and still appears in mea- surable amounts in the EG being removed from the system in the polycondensation series of reactors. The presence of water imposes severe re- strictions on the choice of catalysts: hydrolytically unstable catalysts, such as titanium alkoxides and similar compounds, may not be used, because in the presence of even minute amounts of water they decompose at the polymerization temperature to destroy the catalyst and greatly reduce its activity. The decomposition products of such hydrolytically sensitive catalysts are insoluble metal oxides, such as TiU2. These precipitate as fine particles, part of which being carried in the melt and remaining in the final polymer product, and part settling out of the reaction mixture on the internal surfaces of the reactors and transfer lines all along the polymerization train.

A simple schematic flowchart for an industrial-scale production of PET from TPA and EG is shown in Figure 3. It was adopted with great sim- plification from the patents granted to Fiber Industries Inc. in the 1970s

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[2]. In it, the vessel A is the metering and mixing tank where TPA, in fine-particle form, is mixed to a paste with EG, in the molar ratio 1:1.6 TPA/EG. This paste is pumped under pressure to vessel B, which is an esterification reactor. Through vessel C, EG and water are removed from the reaction mixture; vessel D is the poly condensation reactor, and vessel E is the finisher where the last amounts of EG are stripped away and the MW of the polymer product is brought to the desired level. In Figure 3, the word "acid" stands for TPA, "glycol" stands for EG, and "vapor" stands for the mixture of EG and water being vented out of the polymerization train. Above vessel B, a distillation column is shown, in which water is continuously separated from EG and removed from the system while the

"purified" EG is returned to vessel B. Because the "purified" EG contains some water, decreasing amounts of water are found in vessels B, C, and even D. The esterification reactor B comprises baffled compartments en- abling the reaction mixture to mix well by flowing forward and backward.

The reactor operates at 25O0C, at a pressure of about 275 kPa (approach- ing 3 atm). Under these conditions and with continuous removal of water by-product, it takes about 3 h for the esterification reaction to reach 85- 95% conversion. Vessel C operates under atmospheric pressure, and after about 2 h the esterification is about 98% complete. At this stage, the product contains practically no free TPA or EG. The esterification of TPA by EG above the reflux temperature of EG is an uncatalyzed reaction. The subsequent polycondensation is a catalyzable reaction and antimony trioxide is added at this point, followed by the transfer of the molten mass to the multistage polycondenser D. There, the temperature is stepwise raised from about 255 to about 2750C and the pressure is gradually reduced from 60-80 to 10-25 mm Hg. A polyester of degree of polymerization of around 30 is created, which is then fed into the finisher E. This horizontal vessel, equipped with rotating parallel discs and operating at 298⁰C under a pressure of about 1 mm Hg, is capable of producing PET of I.V. > 0.95 dL/g and degree of polymerization of up to 200. The polymer product of reactor E is then fed directly to the spinning facility.

The flowchart in Figure 4 is typical of the preparation of PET by direct esterification of TPA by EG, followed by polycondensation to remove excess EG and increase the average MW of the PET chains. It is designed to work non-stop in a continuous fashion, unlike laboratory-scale polymerizations which are discontinuous batch operations. At present, this method and facility are used for the production of most PET manufactured industrially, and the amount of polyester produced daily may vary from as little as 50 tons to as much as 200 tons, an output which is predetermined during the design and construction of the train. For economical reasons, more than one polymerization train may be built in any given plant, functioning in parallel and producing the same or different PET grades.

While the end part of the polymerization train in Figure 4, that is, the polycondensation and finisher processes, is similar to that in Figure 2,

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Industrial-Scale Production of Polyesters71

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the front parts are significantly different. The first major difference is in the first reactor, identified in both figures as R-IOl. When it serves as the transesterification reactor (Figure 2), molten DMT and EG are introduced into it from the top, normal mixing occurs at a modest temperature of about 20O0C under atmospheric pressure, and the molten transesterification product is discharged from the bottom. When R-IOl serves as a direct esterification reactor (Figure 4), a paste of TPA and EG is pumped in at the bottom and pushed up for the molten product to exit from the top.

The reactor operates at a temperature substantially higher than in Figure 2, typically at 26O0C and higher, and under autogenous pressure of 5 atmo- spheres and higher. The high pressure is caused not only by the fact that the reactor temperature is now 7OK or more above the boiling point of EG, but also by the generation of water by-product from the esterification step.

— C—{( ))— C-OH + >nHO —CH²-CH2-OH >

O O χ (4)

HO—CH²- CH²- 0 + C—(( ))—C—O—CH²- CH² + OH+2nH²O

/1,2,3,...

The high temperature is required in order to dissolve the free TPA in EG at a reasonable rate and esterify it to create the bis-HET and oligomers.

In the first patents ever for the manufacture of PET, Whinfield and Dickson [3,4] give an example wherein it took three whole days to dissolve free TPA in refluxing EG. Such a slow dissolution rate is, of course, unacceptable for industrial production and, as a consequence, much higher temperatures are used. The direct esterification reactor R-IOl may be equipped with high- pressure portholes, and through them one may observe how the TPA and EG paste becomes less and less opaque, as it moves upward, until a fully clear single phase is obtained at the top. Analysis by high performance liquid chromatography of the clear material taken from the top of the reactor has revealed that although the vast majority of the TPA is already converted into bis-HET and higher oligomers, a very small fraction of free TPA is still present in the molten mixture at that point.

The evolving water is combined with the volatiles from the second esterification reactor and is treated in a water removal column, C-IOl (Figure 4), to separate as much water as possible from the EG, which also boils off in the first and second esterification reactors. Water is disposed of, while the "spent" EG, containing about 6% water, is recycled and mixed with virgin EG in such ratios that, in the preparation of the feed paste with TPA, EG contains about 2% water. In addition to water, the spent EG contains minute amounts of other impurities, such as the undesirable di- ethylene glycol (DEG) which is generated in the esterification reactors, and

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Industrial-Scale Production of Polyesters 73 organic amine compounds which are introduced into the system in order to minimize the amount of DEG formed. Smaller amounts of other degradation products may, too, be recycled with the spent EG.

From the top of the first esterification reactor, bis-HET and higher oligomers are pumped into the second one, operating at 260-275⁰C under a pressure of only about 2 atm. The lower pressure reflects the fact that in the second reactor less volatiles are being generated, that the volatiles composition is substantially shifted from being rich in water to being rich in EG, and that in the second reactor no free TPA remains, requiring high temperatures in order not to precipitate out of solution. When the product leaves the second esterification reactor it may, already, be called PET prepolymer. Its I.V. is about 0.10 to 0.12 dL/g and the vast majority of short chains are terminated by hydroxy groups, with only a small fraction of carboxy groups. The prepolymer is continuously pumped from the bottom of the esterification reactor into the first poly condensation reactor where, for the first time, the pressure is lower than atmospheric. From this point on, the polymerization train and flowchart are similar to those shown in Figure 2.

Another difference between the DMT/EG and the TPA/EG processes is the catalyst package. In the case of DMT and EG, two catalysts are required: one for the transester location and another for the poly condensation. Since the system is anhydrous, the catalysts need not be hydrolytically stable. In the case of TPA and EG, the direct esterification step is uncatalyzed, so that a catalyst is needed only for the polycondensation. For convenience, however, the catalyst may be introduced at the head of the polymerization train, prior to the uncatalyzed direct esterification. This makes it mandatory that the catalyst, usually Sb2O3, will not degrade, precipitate, or be removed with the volatiles during direct esterification.

It must maintain its activity in the presence of water until it is needed in the polycondensation step. Since minute amounts of water are to be found in the polycondensation reactors of the TPA and EG polymerization, hy- drolytic stability is demanded of the polycondensation catalysts, regardless of the point at which they are inserted into the polymerization train. The subject of catalysts and associated problems is considered also in Section 4 below.

To conclude this section, it is worth mentioning that new PET polymerization technologies have recently surfaced. One involves "solid stating", which will be discussed in Section 5. Another directly competes with the conventional polycondensation processes currently used in industry. In this process, instead of performing the polycondensation step and removing the EG under increasingly high vacuum, a slow stream of inert gas, such as nitrogen, is bubbled through and passed above the molten polymer in the polycondensation reactors. The stream of inert gas carries the evolved EG away and drives the polymerization towards higher and higher molecular weights. At present, we have no more details about this promising new technology.

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4. Quality control: effects of catalysts, impurities, and process variables on the color and subsequent processability of the PET product

In addition to superior mechanical and other physical properties, high-MW PET is also judged on the basis of its visual appearance. Thus, quick- quenched amorphous or largely amorphous PET pellets, as well as the clear yet substantially crystalline tire cords and industrial-use yarn, are all expected to appear water-clear and as close as possible to colorless. Inex- plicably, the requirement of being water-clear and, especially, the absence of any yellowing, is expected also of tire cords, even though these are meant to serve as tire carcasses and not be observable. While a perfectly colorless, spun and drawn yarn of superior mechanical and physical properties may command the highest price, a less than perfect yarn, of increasing yellowness or other discoloration and of decreasing properties, will command ever decreasing prices. This, of course, lowers the profitability of the manufacturing plant, puts the plant management on target to correct the deficiencies and, if they fail to do so, puts the plant high on the list for closure in times of economical contraction.

Another potentially price-reducing imperfection, frequently brought about by the presence of organic and/or inorganic solid impurities, may be noticed in the form of "fuzz", wherein a few, or many, individual filaments in the yarn bundle or the tire cord are ruptured and, during winding, their ends appear to fly around the winding bobbins. When the number of filament ruptures is relatively small, the price of the yarn or cord may be lowered, but when the number of discontinuities increases, a point is reached when drawing of the yarn becomes impossible. Upon further increase in filament ruptures, a point is reached when the spinning operation itself cannot continue further and the spinnerette is then turned off.

When such an event occurs simultaneously at many spinnerettes, the associated polymerization train may have to be shut down completely and either purged with boiling EG, or cleaned out more thoroughly. Below, the effects of organic impurities are first discussed and then the subject of catalyst residues and other inorganic impurities is also considered. During the direct- or trans-esterification reactions, and during subsequent poly condensation, several reactions contribute to the overall PET chain growth

O Il C-O-CH2-CH2-OH+(CH3 or H)-C O O

Il Il

C-O-CH2-CH2-O-C

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Industrial- Scale Production of Polyesters 75

C-O-CH2-CH2-OH - O O C-O-CHIl 2-CH2-O-

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-CH2-CH2-OH while others participate in the overall chain degradation process

O O . C-O-CHIl 2-CH2-O-C-

O OIl Il

• C—OH + CH2=CH — O — C -

C-O-CH2-CH2-OH — OH + CH3CHO Many additional reactions take place during the polycondensation stage, and to a lesser extent during the esterification stage. The products of many of these reactions participate in subsequent reactions which may involve polymer chains, monomers or oligomers, low- M W by-products, or all of the above. In most cases, the final species of the degradation process may be present only in trace amounts, or their existence inferred from spectroscopic analyses. Often, these trace amounts of degradation products, attached to the PET chains or monomeric in nature yet dissolved in the polymer bulk, are sufficient to noticeably discolor the polymer. It stands to reason that the intensity of the discoloration may depend not only on the concentration of colored species, but also on the number of double bonds present in such species and on their degree of conjugation. However, several important degradation reactions have been described in the literature. Among them, one may list the following:

HO - CH2 - CH2 - OH —> HO - CH=CH2 + H2O (dehydration) HO - CH2 - CH2 - OH + 2O2 — > CO + CO2 + 3H2O (oxidation) HO - CH=CH2 + 2O2 — > CO + CO2 + 2H2O (oxidation) HO - CH=CH2 + HO - CH2 - CH2 - OH — >

HO - CH² - CH² - O - CH² - CH² - OH (dimerization).

Although here these reactions are shown in terms of individual monomers only, in reality chains ending with such reactive species may undergo similar degradation reactions. One should note here that the dehydration and

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dimerization reactions do not require the participation of additional chemical species, while the oxidation reactions require the participation of additional species undergoing reduction. The paired oxidation and reduction reactions maintain the electronic balance in the reaction medium and together may be called redox reactions. Provided they are not volatile, the organic contaminants, produced by all kinds of degradation reactions, will remain in the molten polymer. Given time and temperature, they will continue to react and grow, to increase in complexity and conjugation, and to intensify in color. Because of their size, they will remain soluble in the molten polymer and, upon cooling, in the vitreous polymer, imparting to it an undesirable yellow-amber color. As a whole, these undesirable reaction by-products do not interfere with the PET processing operations, such as melt spinning and fiber drawing.

A second class of contaminants is also organic in nature, but appears not so much as chemical conversions or chain alterations but as physical contamination: high-MW yet loosely crosslinked gels which are colorless or light in color, and intensely colored highly crosslinked rigid flakes of indeterminable MW. Both gels and flakes are insoluble in common PET solvents, such as tetrachloroethane/phenol mixtures, but the gels greatly swell in these solvents and are generally of no defined shape. The gels most likely swell and are soluble in molten linear PET. The flakes, however, are insoluble in molten PET, and are generally flat and polygonal, with sharp angles between their facets. They look like a miniature version of the cracked mud polygons that appear on the surface of mud flats upon drying, and their size may reach up to around a centimeter, or more, on each side.

Both gels and flakes may be caused by either one of or a combination of any of the following three reasons: (i) Oxygen which may be entrained in the monomer feed, but more frequently enters the polymerization train through air leaks at the axles of rotating parts, such as paddles or the discs of the finishers, through leaks at various entrance and exit portholes, through welding seams, etc. (ii) Melt flow irregularities which result in the formation of localized melt pools with eddy currents, instead of the desired plug flow. When the temperature in these localized, quiescent pools is too high or too low, uncontrolled polymer chain growth and crosslinking may take place. In other words, gels may form. When hot-spots are present, intensive crosslinking and discoloration often accompany the formation of gels. If cold-spots are present, then the polymer may locally crystallize and chain growth will continue under conditions similar to those existing during

"solid stating". (iii) Thin, static layers of molten polymer, coating certain surfaces of reaction vessels and transfer lines, especially on the flat rotating discs and the baffles in the finishers. It is the author's experience that it is the gels and the more rigid flakes, often broken into sub-millimeter patricles, which cause in most instances filament breaking during melt spinning and drawing operations, or worse, when so many of them are broken at the same time, that dripping and drooling at the spinnerettes take place, and the

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Industrial-Scale Production of Polyesters 77 spin-draw process must be interrupted in order to clean the polymerization train and transfer lines prior to restart.

All these problems are, theoretically at least, preventable by better plant design, tighter production controls, such as minimization of temperature fluctuations, and timely and careful maintenance operations. These, however, often conflict with plant cost, production targets and scheduling, and management notions about cutting overhead costs.

A third class of contaminants are categorized together as inorganic contaminants. They include metal-containing colored species arising from catalyst residues and degradation products, or originating from the trace impurities in the monomer feed. Trace impurities may enter the system in ppm concentration in the TPA monomer and, in lower concentrations, in the DMT monomer. In the case of TPA, elements such as Fe, Co, Mo, Ni, Ti, Cr, Ca, Al, Mg, Na, and K, in concentrations of 1 ppm each or lower, may come from the catalysts and their support used in the oxidation of p-xylene to TPA. Organic impurities may arise from incomplete oxidation of the precursor p-xylene to TPA. Among the organic impurities, one may find benzoic acid, p-toluic acid, diacetate ester of 1,4-benzenedimethanol, 4-carboxybenzaldehyde, 4-carboxybenzyl alcohol, and similar compounds.

These may act as chain terminators or may lead to undesirable discoloring by-products. It is obvious that strict control and minimization of organic and inorganic impurities are not only desirable, but necessary.

Although important, the inorganic metal-containing species, introduced into the polymerization train together with the monomers, are not the worst among such species. The distinction of being the highest concentration and most problematic metal-containing contaminants rests with leftover catalysts and/or their non-volatile decomposition products, entrained in the molten PET. At present, the gray discoloration, imparted to PET by the degradation products of antimony-based catalysts, is considered the least desirable, competing with the yellowness imparted to PET by the titanium-based catalysts presently available on the market, and surpassing the blue-pink hue left behind by cobalt-based catalysts.

The root cause for the gray discoloration of PET by the presence of Sb- based catalysts was recently elucidated by Aharoni [5] in a series of experiments and analyses of laboratory- and industrial-scale PET products. What was found is summarized in the following. In the presence of a large molar excess of EG in the polymerization train, all Sb-containing catalysts convert to antimony glycolate/glycoxide which exists in dynamic equilibrium with antimony trioxide. The ratio of glycolate to glycoxide depends on the mag- nitude of the molar excess of EG relative to the charged catalyst; the larger the excess in contact with the Sb ions, the higher will be the amount of glycolate species in the total Sb-glycolate/glycoxide present in the system.

The Sb2Oa mentioned above may be a part of the initial catalyst charge or may arise from the interaction of Sb-containing catalyst with water in the system. Water may be a by-product of the direct esterification of TPA by

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EG, or may come from the recycled EG in the preparation of PET from TPA and EG. A small amount of water may come from the undesirable dehydration of EG to acetaldehyde. In the same article [5], it was exper- imentally shown that EG decomposes, probably by oxidative destruction, to water, carbon monoxide, and carbon dioxide. The molar amounts of CO and CO2 are equal. At the appropriate temperatures, this decomposition is rapid in air and slows down as the amount of available oxygen decreases.

The decomposition of EG to the three mentioned products proceeds when only EG is charged in the reaction vessel with antimony catalysts, such as antimony oxide, acetate or glycoxide, and in the presence of EG and DMT or EG and TPA. It is obvious that the decomposition of EG is independent of the presence or absence of the other ingredients used to prepare PET.

In the presence of species containing Sb⁺³ ions, the final EG degradation products are temperature dependent. At temperatures lower than about 2150C, the antimony ions remain in their trivalent state, say as antimony trioxide, and the CO/CO2 ratio in the headspace above the decomposing EG is maintained at 1:1. As the decomposition temperature is elevated above 2150C, an increasing amount of CO2 and decreasing amounts of CO accumulate in the headspace and, at the same time, an increasing fraction of Sb3+ ions in the reaction mixture are reduced to elemental antimony.

In the experiments conducted by Aharoni [5], the CO2/CO ratio increased up to about 2350C where it stabilized at around 6:1. The above discussion may be summarized in the following redox reaction

Sb2O3 + 3CO —> 2Sb + 3CO2. (9) It was interesting to find that when Sb-containing catalysts, such as antimony acetate or glycoxide, were heated in some air in the absence of any EG, these catalysts underwent oxidative destruction similar to that of EG in the presence of Sb-catalysts: below about 2150C, the degradation product was antimony trioxide, and above this temperature the degradation products contained increasing fractions of elemental antimony.

In the form of extremely small particles, elemental antimony is black.

As the colorless or white Sb-containing catalysts are reduced to the elemental form, in the redox reaction in which carbon monoxide is oxidized to carbon dioxide, the system first becomes light gray, then darker gray, and if enough antimony and CO are present in the system, the color of the reaction mixture may end up being black. In the polymerization train, where the antimony catalyst is highly diluted, the final result is that a light gray hue is imparted to the final PET product.

Now, two important points must be discussed. One is the fact that during common industrial polymerization of TPA and EG to PET, the antimony-based catalysts commonly used are antimony trioxide, antimony triacetate, and rather rarely antimony glycoxide. The Sb-catalyst is usually charged with the EG in concentrations falling in the range of 300 to 400

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Industrial-Scale Production of Polyesters 79 ppm antimony/TPA. In the production facilities the author had experience in, only about 15-20% of the antimony ions were reduced to the black elemental antimony while the rest remained in the PET product as the white antimony oxide or antimony phosphate (the latter is a consequence of using phosphoric or polyphosphoric acid as a stabilizer during the polymerization). The second important point is the fact that elemental antimony usually appears as very small primary particles, with size of the order of 10-20 nm. Unless agglomerated together to form much larger aggregates, they are too small to disrupt the flow of molten polymer to and through the spinnerette and interfere with the continuous spin-drawing process. There- fore, although they impart an undesirable gray tinge to PET, the black primary Sb particles do not by themselves pose any danger to the spin- drawing operation, and do not affect the frequency of filament rupture;

elemental analysis of "good" and "bad" filaments shows no difference in the Sb concentration. Moreover, even when the immediate neighborhoods of ruptures in filaments were analyzed, the Sb concentration right at the rupture was found to be typical of the whole PET bobbin, and not measurably different. One can make completely colorless PET by using no catalyst at all, or using germanium dioxide as catalyst. In both instances, the polycondensation proceeds too slowly and results in PET of too low MW to be of any industrial-scale potential, and in the case of GeU2 the price of the catalyst is exorbitant. The polymer is, however, of remarkable whiteness.

To summarize this section, there are three classes of contaminants existing in industrially manufactured PET. One is the yellow-amber colored products of chemical side reactions. These are either chemically bonded to the polymer or dissolved in the melt, but usually do not endanger the smooth operation of the polymerization train, nor affect in general the fiber- forming processes. The second class of contaminants are organic colorless gels and amber-brown flakes. The gels and, especially, flakes are responsible for most filament ruptures and failures of the filament-forming processes, such as melt spinning and drawing. Both these contaminant classes may be eliminated or minimized by prevention of air leaks into the polymerization train, by better design and control to eliminate quiescent melt puddles, hot-spots and/or cold-spots, and by minimizing the thin layers of polymer stuck in more or less permanent fashion onto the internal surfaces of any and all of the reaction vessels and transfer lines. The third class of contaminants, containing metals in their ionic or elemental forms, are typified by antimony-based catalysts. In order to avoid or minimize the gray discoloration, air leaks should be eliminated, air entrained in the monomer charge prevented, or the Sb-catalysts be simply replaced by others that operate at far lower concentrations and do not affect measurably the color of the polymer product.

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5. Solid-state polymerization of PET

In recent years, it was recognized that it is economically more advanta- geous to complete the polymerization of PET by a process called "solid stating" than to continue the melt polymerization for a substantial extra number of hours in order to bring the intrinsic viscosity of the final product from about 0.7 dL/g to around 1.0 dL/g. This latter stage of polymerization is usually carried out in the finishers, is rather time-consuming, is prone to cause oxidative degradation and yellowing, is the slowest step in the polymerization train, and determines the throughput of the whole train.

Therefore, substituting this step by solid stating, which is carried out sep- arately, generally increases the throughput of the polymerization train and minimizes the degradation and discoloration of high-MW PET.

By "solid stating" is meant that the polymer pellets which are subjected to this process remain in the solid state and are not melted in order for the polymerization to proceed. In fact, the process is designed in such a manner that melting of the pellets anywhere along the solid stating train, or even softening of the surface of the pellets to such a level that the pellets will aggregate into lumps or clumps upon contact, is considered a disastrous event, necessitating the shutdown of the affected parts of the train and cleanup before the flow of polymer pellets in the train may be restarted.

In Figure 5, a flowchart is shown of a solid stating process for PET.

This specific train was designed to increase the MW of PET, starting with scrap PET with I.V. of 0.62 dL/g and producing a bottle grade PET with I.V. of 0.82 dL/g. The same concept, however, and very similar design, may be used for solid stating other grades of PET and other polyesters, such as poly(butylene terephthalate) (PBT), poly(trimethylene terephthalate) (PTT), poly(ethylene-2,6-naphthalate) (PEN), and various liquid crystal aromatic polyesters.

The solid stating process is divisible into two separate stages. In the first stage, pellets of PET or other semicrystalline polyesters are fed into crystallizers, in which they are exposed to relatively low temperature while constantly mixing and moving. During their residence in the low-temperature crystallizers, the pellets' temperature is controlled at such a level that crystallization of the amorphous phase is facilitated, yet the amorphous parts of the surfaces of the pellets are not soft enough or flowing to allow the pellets to adhere to one another upon contact. At the end of the pellets' residence in the crystallizers, their surfaces are highly crystalline and the small fraction of surface area that remained amorphous is motionally constrained to such a level that the pellets lose all their tackiness and stickiness and do not agglomerate together upon contact. The crystallization step is a must, since in its absence the heated pellets will adhere to one another, resulting in large clumps of polymer which cannot be handled by the solid-state process and equipment.

Prom the crystallizers, the PET pellets are transported to heated hop-

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1 1 6 O0C 1θ.5-1.0 psi

V - I O l A & B

S - I O l A & B

Figure 5. "Solid stating" of PET.

V-IOl A&B: Pellet surge bins; M-IOl A&B: Pellet screw feeders; S-IOl A&B: PET crystallizers; S-103 A&B: Hopper driers; S-104 A&B: PET preheaters; S-105 A&B: Purge vessels; R-IOl A&B: Hopper reactors; S-107 A&B: PET product coolers; V-102 A&B: Pellet hoppers; V-103 A: PET product storage silo; CW = cold water

per driers where they are kept at about 16O⁰C under vacuum until they are transferred to the PET preheaters and purge vessels. At these parts of the solid stating train, the conditions are set in such a way that polymer crystallization will continue to be encouraged while, at the same time, the vacuum and the purge encourage polymer chain extension and increased MW. It should be recalled that, at this stage, every molecule of EG purged out of the system is associated with two PET chains coming together to become one of twice the MW of its predecessors. The continued crystallization at this stage is also very helpful to the chain extension process.

During crystallization, a high level of chain motion takes place, especially in the amorphous fraction of the polymer, allowing the chain ends to approach one another and condense together with a doubling of the MW, and for other chain ends to approach existing ester groups in other chains, and participate in transesterification reactions. The latter may contribute by themselves little to the total increase in the MW of the polymer, but they participate in the strain release of polymer chains in the amorphous phase, a release that increases chain and chain-end mobility, and facilitates the participation of a large number of chain ends in condensation reactions and MW enhancement.

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After this stage, the PET pellets are exposed in transfer pipes and hopper reactors to increased heat and are swept by a continuous stream of nitrogen under reduced pressure. The temperature is set at around 2160C, which is substantially below the melting temperature, Tm, of PET and slightly below its crystallization temperature upon cooling from the melt, T0. In fact, this is the highest temperature at which one is sure that PET is in the crystalline and amorphous-vitreous states and no fluid or mo- bile amorphous component exists. The reduced pressure is substantially higher than the high vacuum usually employed in the polycondensation reactors and, especially, in the finishers. In the solid stating trains, the counter-current flow of nitrogen under reduced pressure serves admirably in sweeping away all the EG formed during polycondensation and chain extension. In fact, it is so effective that not only the high investments in construction of high vacuum facilities and the expenses involved in main- taining high vacuum are eliminated in the case of solid stating, but recently the same concept of nitrogen sweep was introduced into the design of melt polyester polymerization trains.

After the high-temperature exposure to the count er-cur rent flow of nitrogen, the molecular weight buildup in the PET pellets and the solid stating process are both complete. The pellets are then cooled in several steps while being transferred to storage bins. Because PET is extremely sensitive to the presence in it of very low levels of moisture, it is advisable to keep the PET product of the solid stating process bone-dry, and/or to redry the pellets to 10 ppm or less water prior to their use in any forming or shaping process in which they are melted (see also Chapters 1 and 13).

6. Poly(ethylene naphthalate) preparation and flowchart

In this and the next sections, the preparation of two kinds of polyesters is discussed. In both cases, the aromatic diacid is introduced into the head of the polymerization train in the form of a dimethyl ester. The first stage is transesterification: the reaction is activated by a catalyst and the initial by-product is methanol. The reasons for the use of dimethyl ester instead of the free diacid are different in each case. In the present one, the free 2,6-naphthalenedicarboxylic acid does not melt and is insufficiently soluble in the EG at the process temperature. Therefore, it must be introduced as the diester, and dimethyl ester is the only diester available in large enough quantities, in sufficient purity, and at a reasonable price. The combination of these three facts makes the industrial manufacture of PEN economically viable. In the instances to be discussed in Section 7, the use of DMT and even dimethylisophthalate (DMI) is dictated by the poor solubility of the free acids in several of the monomers, such as 1,4-cyclohexanedimethanol, 1,4-butadiene, and dihydroxy-terminated poly(tetramethylene glycol).

The preparation of PEN from dimethyl-2,6-naphthalenedicarboxylate

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Industrial-Scale Production of Polyesters 83 (NDC) and EG proceeds in two steps, in a fashion very similar to the preparation of PET from DMT and EG.

H3C-O-C

+HO-CH2-CH2-OH

C-O-CH2-CH2--

(10) + CH3OHt

The flowchart of an industrial-scale PEN polymerization train is shown in Figure 6. As can be seen from Reaction (10) and the flowchart, the procedure requires two different catalysts for the polymerization to produce a polymer of acceptable MW for an acceptable residence time in the polymerization train. The first catalyst is a transesterification one, such as ZnCl2 or calcium acetate, which is introduced with the EG into the first transesterification reactor. The second catalyst may be introduced just before the first polycondensation reactor or, if it is stable, non-volatile, and poorly soluble in methanol, it may be introduced together with the first one. It may be desirable, however, to inject a deactivator for the first catalyst just ahead of the polycondensation part of the train because, under the anhydrous conditions inside the train, the first catalyst may not degrade and become inactive, but may catalyze the depolymerization of PEN in the final polycondensation reactor. The fact that antimony-based catalysts are not very efficient in catalyzing the depolymerization of PEN or PET is one of the main reasons for their use as polycondensation catalysts for PEN, PET, and many other polyesters. Tin-based catalysts are also known to be used in the polymerization of PEN and other polyesters, but usually under the proviso that the system is anhydrous.

In Figure 6, the venting system of the polycondensation part of the polymerization train, through which the excess EG is stripped off the polymer, is not shown since it is very similar to that of Figure 2. The collected EG is recycled as part of the initial EG feed and contains a small amount of methanol. The latter is introduced into the first transesterification reactor as an impurity in EG, but since a large amount of methanol is produced in this and subsequent reactors, the methanol impurity plays no significant role in the total polymerization process.

The most noticeable feature of the flowcharts in Figures 2 and 6 is

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2 8 50C lmm Hg

P E N p r o d u c t !

Figure 6. Flowchart for the polymerization of PEN from NDC and EG.

M-IOO A&B: NDC feed hoppers; M-IOO A&B: NDC melters; T-102 A&B:

EG storage tanks; V-IOl A&B: Catalyst addition vessels; R-IOl: First transesterification reactor; C-IOl: Methanol recovery column; R-102: Second transesterification reactor; R-103: Third transesterification reactor; R-104: Fourth transesterification reactor; V-103 A&B: Catalyst addition vessels; R-105: First prepolycondensation reactor; R-106: Second prepolycondensation reactor; R- 107: Third prepolycondensation reactor; R-108: Final polycondensation reactor their remarkable similarity. Throughout both the transesterification and polycondensation parts of the two polymerization trains, the reactor temperatures and the vessel pressures are practically identical for each corresponding pair of reactors. The only difference is that in the PEN process there exists only one finisher per train due to the fact that it is extremely hard to increase the MW of PEN in the final polycondensation step. There- fore, solid stating is usually applied in order to produce PEN of sufficiently high chain length.

The striking similarity of the PET and PEN polymerization trains indicates that the reason for the long delay in producing PEN on an industrial

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Industrial-Scale Production of Polyesters 85 scale was not due to process difficulties. As it turned out, at least in the United States, the problem was the inability of the monomer industry to deliver the necessary quantities of dimethyl-2,6-naphthalenedicarboxylate at the requisite purity and at an acceptable price. To the author's knowl- edge, 15 years elapsed between the initial deliveries of 25kg batches of experimental samples and the time when continuous deliveries of NDC on industrial scale were secured.

At this point, it is interesting to note that polymerization trains capable of producing PBT or poly(butylene naphthalate) (PBN) may be designed in a fashion similar to the ones shown in Figures 2 and 6. The major difference lies in the fact that the boiling temperature of EG is 192⁰C while that of 1,4- butanediol is 235⁰C. This 40 K difference requires a much higher vacuum to be maintained in all the polycondensation reactors where excess butanediol is being removed while, at the same time, the temperature should be kept as low as possible, to produce non-degraded PBT or PBN of good color and to minimize the undesirable side reaction of cyclic dehydration of 1,4- butanediol to tetrahydrofuran.

7. Copolyesters from three or more monomers, and thermoplastic elastomers

In certain cases, it is desirable to produce a polyester for which the processing conditions, mechanical properties, and use temperature are similar to those of PET, but whose visible haziness or opacity are minimized. The combination of high use temperature and great optical clarity is obtained by strongly reducing the crystallite size of the polymer to substantially below one quarter the length of an optical light wave and, at the same time, slowing the crystallization rate of the polymer, while keeping the total crystallinity at a level that maintains the mechanical properties of the polymer at temperatures substantially above ambient temperature. This is achieved by the incorporation of some DEG in PET, instead of a small fraction of EG monomer. When crystallinity is undesirable, some EG is replaced by 1,4-cyclohexanedimethanol (CHDM). High transparency, good mechanical properties, decent use temperature, and ease of processing is a mandatory combination of properties required for polyester water- and soda-bottles, and it is achieved through modification of PET by the introduction of small amounts of DEG and/or CHDM in place of a small amount of EG and/or the addition of a small amount of isophthalate residues, replacing some of the terephthalate groups.

In Figure 7, the flowchart for the preparation of glycol-modified PET (PETG) and in Figure 8, the flowchart for the preparation of poly (1,4- cyclohexylenedimethylene terephthalate) (PCT) and its acid-modified (PCTA) and glycol-modified (PCTG) versions are presented, respectively.

PETG may be prepared from DMT, EG and DEG, or from DMT, EG and

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V - 1 0 3 A & B

Figure 7. Flowchart for the polymerization of PETG from DMT, CHDM, and EG.

T-IOl A&B: DMT storage tanks; T-102 A&B: CHDM storage tanks; T- 103 A&B: EG storage tanks; V-IOl A&B: Catalyst addition vessels; R-IOl:

First transesterification reactor; C-IOl: Methanol recovery column; R-102: Sec- ond transesterification reactor; R-103: Third transesterification reactor; R-104:

Fourth transesterification reactor; V-103 A&B: Catalyst addition vessels; R- 105: First prepolycondensation reactor; R-106: Second prepolycondensation reactor; K-IOl: Vacuum pump; R-107: Third prepolycondensation reactor; S-105:

Filter; R-108: Final polycondensation reactor; M-IOl: EG vacuum jet; K-102:

Vacuum pump; S-104: Direct contact condenser; V-107: Surge vessel

CHDM. While the PETG version containing DEG is relatively close to PET, but with reduced crystallinity level, lower crystal melting point, and slow crystallization rate, the PETG version made with CHDM is usually prepared in monomer ratios, ensuring polymer amorphicity while main- taining good impact strength, clarity and barrier properties. By means of

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M o l t e n M o l t e n E t h y l e n D M T CHDM g l y c o l

Figure 8. Flowchart for the polymerization of PCT and its acid- (PCTA) and glycol- (PCTG) modified versions.

T-IOl A&B: DMT storage tanks; T-102 A&B: CHDM storage tanks; T- 103 A&B: EG storage tanks; V-IOl A&B: Catalyst addition vessels; R-IOl:

First transesterification reactor; C-IOl: Methanol recovery column; R-102: Sec- ond transesterification reactor; R-103: Third transesterification reactor; R-104:

Fourth transesterification reactor; V-103 A&B: Catalyst addition vessels; R- 105: Prepolycondensation reactor; K-IOl: Vacuum pump; S-IOl A-C: Filters;

R-106: Final polycondensation reactor; K-102: Vacuum pump

Figure 7, the preparation will be described of the PETG version made with CHDM and containing the 1,4-cyclohexylenedimethylene residues.

The process of making PETG from DMT, CHDM, and EG is almost identical to the one of making PET from DMT and EG. Except for the addition of a third ingredient, the only difference is the higher vacuum (Figure 7) in the final polycondensation reactor. Since PETG is not used at very high MW for applications such as tire cord or industrial yarn, there is no need for a series of finishers, and only one such reactor is shown. Changes in the crystallinity levels of the PETG product, and in the crystallite sizes and melting points, are easily effected by altering the ratio of the two diols in the polyester.