A PROCESS AND APPARATUS FOR FORMING CRYSTALLINE POLYMER PELLETS

Abstract: This invention relates to a process and apparatus for forming a polyester polymer into particles. More particularly, this invention relates to a process and apparatus for forming crystalline, uniform pellets from an amorphous polyester melt. The polyester pellets have utility, for example, as feedstock for a process for producing higher molecular weight polyesters.

Description: TTTLE A PROCESS AND APPARATUS FOR FORMING CRYSTALLINE POLYMER PELLETS FIELD OF THE INVENTION This invention relates to a process and apparatus for forming a polymer into particles. More particularly, this invention relates to a process and apparatus for forming crystalline, uniform pellets from an amoφhous polyester melt. BACKGROUND The formation of particles from viscous materials is well known. Conventional methods and apparatus often involve the formation of liquid portions or droplets which are subsequently collected and solidified. For example, Froeschke, U.S. Pat . No. 4,279,579, discloses an apparatus for the extrusion of a fiowable mass onto a conveyor. The apparatus has inner and outer cylindrical coaxial containers. The inner container, positioned within the inner container, has a passage for dispensing the fiowable mass. The outer container has a number of orifices and rotates around the inner container. As the outer container rotates, the orifices on the outer container cyclically align with the passageway on the inner container. With each alignment, the fiowable mass flows from the inner container, through the aligned orifices, and is apportioned and deposited on a conveyor, for example a conveyor belt, to form what is often referred to as pastilles. Chang et al., U.S. Pat. No. 5,340,509, discloses a pastillation process for pelletizing ultra high melt flow crystalline polymers, i.e., a crystalline polymer which is a polyolefin homopolymer, a polyolefin copolymer, or blends thereof. Initially, molten polymer is transferred to a droplet-forming means. The droplet- forming means is generally an outer container, with orifices, which rotates around an inner container to allow a uniform amount of the polymer melt to emerge as droplets. The droplets are collected on a conveyor, which cools the droplets for a time sufficient to solidify the droplets. Forming robust, uniform, pellets of a polyester material has been difficult or problematic. For example, low molecular weight polyesters, characterized as oligomers or prepolymers, may have such a low viscosity at initial particle formation may be difficult. The oligomer may be too liquid to form particles or pellets of uniform shape and size. This is because oligomers. having relatively short chain length, may have a relatively low amount of chain entanglement, in addition to limited intermolecular bonding or forces. Known processes for forming polyester panicles may result in particles which lack structural integrity. The weakness of such panicles may make them hard to handle and susceptible to abrasion during transport or other mechanical handling. Abrasion may generate undesirable amounts of fines. Polyester particles are useful as feedstock to a process to produce a higher molecular weight polymer, including solid-phase ("solid-state") polymerization processes. For such processes, it is desirable that the particles have certain characteristics. For example, particles having relatively uniform size and shape, for uniform polymerization within each panicle, may be desirable. For solid-state polymerization, it is desirable that the particles be sufficiendy robust to withstand the high temperatures of solid-state polymerization without agglomerating. Conventionally, robust particles of polyester may be obtained by subjecting the particles to a lengthy and expensive heat treatment or annealing step. Such annealing increases d e crystallinity and robustness of the particles. Such annealing, however, typically adds time and expense to an overall process for producing high molecular weight product. It would be desirable to reduce or eliminate such annealing. In view of the above, there exists a need for an improved process and apparatus for the formation of polyester particles. There is a need for die more economical and efficient production of quality polyester particles, which, for example, are useful under rigorous circumstances and with limited pre-treatment prior to use as feedstock for further polymerization. Furthermore, ere exists a need for an improved process of forming a low molecular weight polyester oligomer into crystalline particles. In addition, it would be a further advantage if the resulting particles exhibited improved crystalline morphology or related properties compared to conventional processes. SUMMARY OF THE INVENTION This invention provides an apparatus for producing pellets of a polymer from its polymer melt, comprising: (a) a pellet former comprising a rotatable container having a plurality of oudets, defining corresponding openings 0.5 to 5 mm in diameter, for metering a polymer melt onto the surface of a conveyor; (b) a conveyor comprising a surface, which is adapted for movement relative to the outlets of die pellet former, for receiving the polymer melt, from the pellet former, in the form of a plurality of droplets or crystallizing pellets, said conveyor being adapted for conveying the pellets through a crystallization section; and (c) a crystallization section extending from the point at which the pellets are received onto the surface of die conveyor, extending along at least a portion of the conveyor to a point downstream; the crystallization section further comprising means for controlling the surface temperature of the conveyor within a predetermined temperature range above 50°C as the surface passes through die crystallization section. In commercial practice, the crystallization section may further comprise a temperature controller for controlling the temperature of the surface within die crystallization section, such that the pellets are subjected to a surface widiin a predetermined temperature range for a predetermined period of time. The above-described apparatus may have a variety of uses, including die production of pellets of a polyester polymer having a glass transition temperature (T_) greater than about 25°C. One such process comprises: (a) metering a polymer melt of die polyester polymer dirough a multitude of outlets in a rotatable container, each oudet defining an orifice 0.5 to 5 mm in diameter, mereby forming a plurality of molten droplets; (b) collecting die molten droplets, immediately after being formed, on a solid moving surface, the solid moving surface being maintained widiin a predetermined temperature range widiin a heating zone, whereby die pellets are maintained in contact witi die solid moving surface, widiin die heating zone, for a predetermined period of time. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic of d e preferred process and apparatus for producing polymer pellets. Figure 2 is a cross-sectional view of the crystallization section of die apparatus of Figure 1. DETAILED DESCRIPTION OF THE INVENTION This invention provides an apparatus and process for producing low molecular weight polymer particles or pellets. The polymer pellets are produced in a pellet former commonly referred to as a pastillator, and are collected on a hot surface. The hot surface controls the rate at which die pellets are relatively cooled (from the melt) and die temperature to which die pellets are relatively cooled. The pellets tiius formed may have a relatively uniform size and shape. By die term "relatively uniform" is meant that at least 90 percent, by weight, of the pellets are widiin plus minus 30 percent of the mean diameter. Preferably, at least 95 percent, by weight of die particles are within plus/minus 10 percent of the mean diameter. The present process is capable of producing pellets that are stronger and more abrasion resistant than pellets formed by various other conventional methods and apparatus. The pellets are suitable for transport or subsequent treatment by solid-state polymerization, with or without additional annealing. One embodiment of the present invention, including an apparatus for producing pellets, is shown schematically in Figures 1 and 2. For purposes of this invention, the term "pellet" means any discrete unit or portion of a given material, having any shape or configuration, wheuier regular or irregular. Thus, the term "pellet" may encompass particles, droplets, pieces, portions, or pastilles of a given material. By die term "polymer" is meant a compound or mixture of compounds consisting essentially of repeating structural units called monomers, and is meant to include a prepolymer or a oligomer, that is, a polymer having a low molecular weight or a polymer intended as feedstock for a higher molecular weight polymer. By the term "molten polymer" is meant polymer at a temperature at or above its melt temperature. Likewise, by the term "molten droplet" or "droplet" is meant a portion of a polymer at least partially at temperature at or above die melting point of die polymer. Thus, temperature gradients may exist in die droplet, which may start crystallizing immediately after being formed. The melting point (T m ) of a polymer is preferably determined as d e maximum of d e main melting endodierm on die first heat, measured by Differential Scanning Calorimetry (DSC). By pellet size is meant die largest cross-sectional dimension of a given pellet. As part of an integrated process, die droplet former may be in communication, via a conduit or otiier material transfer means, widi a means for producing a polymer in melt form. A means for producing a polymer melt can encompass many variations. For example, the means can be an extruder which uses polymer in die form of flake, pellets or chips as a feedstock. An extruder can heat the feedstock to die melt temperature or higher and extrude d e molten polymer in various shapes, for subsequent transfer to die droplet former. The means for producing die polymer can also include a reactor for polymerization. Such a reactor is well known in die art. Polymerization is often carried out in die melt, and thus melt polymerizers are also suitable as a means for producing die polymer in melt form per diis invention. An example of a preferred reactor for producing polymer melt is described in copending commonly-assigned application S.N. (docket no. CR-9524), hereby incorporated by reference in its entirety. Of course, for use as feedstock for the present apparatus and process, it is also possible to commercially purchase polymer or to store previously made polymer for later introduction into a means for producing a melt of the polymer. One preferred embodiment of the present apparatus is schematically shown in Figure 1. A pellet former 10 receives a polymer melt from a reactor or melt polymerizer (not shown). A conventional melt polymerizer, if employed, usually has an inlet for receiving reactants and an outlet connected to a conduit for transporting d e polymer melt to the pellet former 10. The polymer exiting the oudet is typically at or above its melt temperature. The polymer can be transferred to a pellet former by means of any pressure displacing device such as a variable speed displacement pump or melt gear pump. The pellet former 10 is commonly referred to as a pastillation apparatus or pastillator, in die broadest sense of the word. Various types of pastillators are known in the art for various uses. T e pastillator, in one embodiment, may typically comprise inner and outer coaxial cylindrical containers. Accordingly, polymer melt transferred from the reactor would be received into die inner container or cylinder. The outer container has a plurality of orifices circumferentially spaced on die periphery of die outer container. The plurality of orifices are disposed such tiiat they align with a metering bar or channel on d e inner container when die outer cylinder is rotated. The orifices on die outer container can typically range in size from about 0.5 mm to about 5 mm. The inner cylinder containing die polymer melt is under pressure and dispenses d e melt in uniform amounts as each of d e plurality of orifices on the outer cylinder align with die metering bar or channel on die inner cylinder. Pastillators, as described, are commercially available, e.g., ROTOFORMER® manufactured by Sandvik Process Systems (Totawa, NJ). In commercial use, for economic efficiencies of scale, maximum production, there may be many orifices on d e outer cylinder of d e pastillator, typically at least 100, for example, between 100 and 50,000, depending on die scale of operations. The pellets suitably may be produced on die scale of 1 kg to 10 metric tons per hour, preferably 1 to 10 metric tons per hour. For such operation, the pastillator would be adapted for rotating at a rate which is sufficient to supply pellets to die conveyor surface at die desired production rate. Droplets or crystallizing pellets 18, formed by die pastillator 10, are direcdy received onto a moving surface 12 of a conveyor belt, which is substantially level. By "substantially level" is meant not varying by more than 10° from horizontal. By "moving surface" is meant any surface which can support and transport die pellets. The moving surface 12 generally moves relative to die pastillator, in a direction tangential to the direction of rotation of the outer container of d e pastillator. The moving surface 12 has a bottom surface 16 and a top surface 14. the latter comprising the substantially level moving surface which supports the pellets. The moving surface 12 conveys the pellets dirough a crystallization section, which may also be referred to as a heating section. The moving surface is generally maintained at a constant speed for passing the pellets dirough the crystallization section, although the speed chosen can vary in order to vary d e time the pellets are within die crystallization section. A key feature or component of the present apparatus is the crystallization section. The crystallization section begins at or very near the point at which die pellets are received from die pastillator 10 onto die moving surface and extends along at least a portion of the conveyor moving surface. An important feature of the crystallization section of die apparatus is tiiat it includes means for controlling die temperature of die moving surface, as it passes through d e crystallization section, at an elevated temperature. Ovens comaining a heating coil may be employed. In die preferred apparatus of die present invention, die temperature of die top surface 14 widiin die crystallization section is maintained above 50°C, depending on die surface material of die conveyor. If die surface material is metal, then a conventional heater should be capable, in practice, of raising die temperature to at least 50°C, preferably at least 100°C, more preferably between 100°C and 225°C, which may depend on die heat transfer coefficient of die surface. In die broad process of the invention, however, die temperature may vary below 50°C, if die conveyor surface has a lower heat transfer coefficient dian metals such as steel. The crystallization should be capable of maintaining a relatively steady temperature, although some gradient along d e crystallization section is allowable. Preferably, the temperature of die surface in die crystallization section is carefully controlled, as further described below. Preferably, a portion of bottom surface of die moving surface 12 is heated widiin die crystallization section. It is also possible to have a heater prior to the point at which the pellets are received on die conveyor surface, in which case die crystallization section may only require insulation and/or slight heating. The crystallization section may further comprise means for adjusting die temperature and/or flow of a heat-exchange fluid and supplying a flow of die heat-exchange fluid to the bottom surface 16, such as shown widiin die crystallization section 20 in Figure 1. In die embodiment shown in Figure 1, an air heater 26 supplies heated air to a lower plenum 24, enclosing a portion of the bottom surface 16 of the moving surface 12. The lower plenum 24 generally contains an inlet and outlet for die heat-exchange fluid, so the heat-exchange fluid can continuously circulate through the lower plenum 24. The lower plenum 24 extends along the portion of die moving surface 12 which comprises the crystallization section. In this way, the pellets 18 are subjected to proper heating immediately after being formed and collected on die moving surface 12. In order to obtain rapid heat transfer from die moving conveyor surface to the just-formed polymer pellets, it is preferred that d e material for die conveyor moving surface 12 have a relatively high heat transfer coefficient. Metals are particularly useful for this purpose, especially metals, such as steel, with high heat transfer coefficients. Thus metals are the preferred materials for the conveyor moving surface, although other materials, for example, plastic or plastic coatings are possible. The temperature of the top surface 14 of the moving surface 12 widiin die crystallization section may be controlled automatically or manually widi d e use of a temperature sensor 28 located widiin die crystallization section. Preferably, however, a temperature controller may automatically control die temperature of the top surface 14 of die moving conveyor surface 12 in die crystallization section within a predetermined temperature range. Controlling die temperature, in combination widi controlling die speed of die conveyor moving surface supporting the pellets, wϋl result in the pellets 18 being subjected to the predetermined temperature range for a minimum amount of time which can be predetermined. This occurs as die pellets 18 pass dirough die crystallization section. Generally, die temperature controller comprises a sensor 28 for determining die temperature of die top surface 14 widiin die crystallization section, a comparator (not shown) for comparing the temperature determined by d e sensor to a set point within the predetermined temperature range, and a temperature adjustor (not shown) for adjusting the temperature of die heat-exchange fluid supplied to d e bottom surface 16 of die moving surface 12. Conventional temperature controllers are well known in the art, as will be appreciated by he skilled artisan, and are commercially available from a wide variety of sources. Controlling die temperature of the metal surface of die belt may at times require d e removal of heat from die heat exchange fluid or bottom surface 16, i.e., relative cooling, although die crystallization section may be heated relative to ambient. Typically, when a heat exchange fluid is supplied in a continuous flow to the bottom surface 16, and the set-point temperature is exceeded, a controller will typically signal no additional heat input. This does not, however, contravene the spirit of the invention, since the general result is heating of the bottom surface 16, and consequently the top surface 14. In Figure 1, a heater for the bottom surface 16 of the moving surface 12 is within the crystallization section. The primary function of me heater is to heat the moving surface 12 such that die top surface 14 is within a predetermined temperature range. Heating die moving surface 12 so that it is maintained at a temperature within the predetermined temperature range can be accomplished by a variety of means known to those skilled in die art. Various embodiments and apparatus for heating are encompassed wiύϋn the scope of this invention. In die preferred embodiment of Figure 1, heating is primarily by means of heating the boπo surface 16 of the moving surface 12. The overall system may also include additional, auxiliary heating means. For example, a second temperature-controlled (i.e., generally heated) heat -exchange fluid, preferably an inert gas to avoid degradation of die pellets 18, can be supplied to heat die portion of die top surface 14 supporting die pellets that is widiin die crystallization section. Preferably die gas is inert. Suitable gases include nitrogen, die noble gases such as argon and helium, oxygen, air, and die like. In tiiis preferred embodiment, the pellets 18 are subjected to temperature control, at an elevated temperature, by means of both die hot moving surface 12 and from the flow of heated inert gas. The inert gas is preferably at a temperature less tiian tiiat of die top surface 14. For example, for PET, die temperature of die inert gas, e.g., nitrogen, typically ranges from 25°C up to 100°C, aldiough higher temperatures are feasible. A flow of heated inert gas over die pellets may be provided in order to control die temperature gradient tiiat will exist dirough the diickness of each pellet, d us serving to achieve more uniform crystallization throughout each pellet. The more uniform the temperature is throughout die pellet during die minimum predetermined amount of time, the more uniform d e crystallization will be within each pellet, aldiough temperature gradients widiin die pellets, to some extent, will likely occur while within die crystallization section. An important goal of the crystallization section is to get the temperature of die polymer pellets to die desired crystallization temperature as rapidly as possible and to maintain it at a predetermined temperature for a minimum period of time. As indicated above, while controlling die temperature of a continuous flow of inert gas, diere may be temporary periods of time when the gas is not heated, in order tiiat die set-point temperature is obtained. The overall effect, however, is to control die temperature, by means of die gas, die environment surrounding die just-formed pellets 18. A second means for heating and supplying a continuous flow of a second heat-exchange fluid is shown in Figure 1 as a heater 22 for heating a flow of nitrogen supplied to an upper plenum 20. The upper plenum 20 can enclose the top surface 14 within the crystallization section, and generally contains an inlet and an oudet for continuously circulating the nitrogen through d e upper plenum 20. Figure 2 is a cross-sectional view of an upper and lower plenum encasing die crystallization section. As shown in Figure 2, a conveyor belt 12 covers d e upper opening of the lower plenum. 24. The roller for die belt is shown below by the dotted line. The conveyor belt 12 also serves to cover the bottom opening of upper plenum 20. Resting on the belt, seals 42, typically made of TEFLON® (DuPont, Wilmington, DE), may be employed to prevent excess loss of die heat- exchange fluid which is circulated dirough die upper plenum 20. As an example of auxiliary heating to assist in maintaining die temperature of d e top surface 14 widiin a predetermined range, a d ird heat-exchange fluid can be supplied to an internal chamber 34 located in die upstream roller 30 of die conveyor. The internal chamber 34 may include an inlet and oudet which are connected by conduits to a means for heating and circulating the third heat- exchange fluid. Figure 1 also shows a heated pump 38, widiin a hot oil badi 43, for supplying d e diird, heat-exchange fluid, e.g., a hot oil, dirough a conduit 36 to die internal chamber 34 of die upstream roller 30. The roller is preferably constructed of a heat-conductable material to ensure mat heat from die heated oil is efficiendy conduαed from die internal chamber 34, through die roller 30 to die bottom surface 16 of die conveyor belt. Heating die upstream roll 30 as described provides supplementary heating which counteracts normal heat loss and lessens die burden on die primary heater 26. It would also be possible, however, to provide primary heating upsteam of die pellets, in combination with supplemental heating and/or insulation following die point at which die pellets are received on die belt. After die crystallization section, die now crystallized, low molecular weight pellets 18 can be collected and transported for further treatment. The present apparatus can be used to make relatively robust and uniform pellets of a polyester polymer. One such process, which is particularly advantageous, will now be described. In the preferred process, a polyester polymer in melt form having a desired intrinsic viscosity, IV is processed in an apparatus according to present invention. Generally, polymer having an TV ranging from about 0.05 to about 0.40 dl/g is suitable. An TV ranging from about 0.09 to about 0.36 dl/g is preferred. The IV is determined as follows. A solvent is made by mixing one volume of trifluoroacetic acid and three volumes of metiiylene chloride. PET, in die amount of 0.050 g, is then weighed into a clean dry vial, and 10 mL of the solvent is added to it using a volumetric pipette. The vial is closed (to prevent evaporation of the solvent) and shaken for 30 min or until the PET is dissolved. The solution is poured into the large tube of a #50 Cannon-Fenske® viscometcr. which is placed in a 25°C water badi and allowed to equilibrate to that temperature. The drop times between the upper and lower marks are then measured in triplicate, and should agree within 0.4 sec. A similar measurement is made in die viscometer for d e solvent alone. The TV is then calculated by the equation: _ , . (solution time/solvent time) IV = ^ o The present process can be integrated with a metiiod of producing a polymer in melt form. Producing the polymer in melt form can be accomplished in various ways, discussed above, and includes extruding polymer initially in die form of flake, pellets or chips. Additionally, an overall process can include polymerizing reactants in a reactor for polymerization, for example, by melt polymerization, as discussed above. In die preferred process, die polyester is initially at a first temperature which is at or above its melting temperature. For polyesters of interest, this initial temperature would be above 200°C. For PET, dύs initial temperature would be equal to or greater dian about 250°C. It is preferred diat die polymer melt is essentially amoφhous, i.e., less dian about 5%, preferably less than 1% crystalline. If d e polymer melt is not initially amoφhous, and is instead semicrystalline, it is desirable for die polymer to be thoroughly and uniformly heated above its melting temperature to ensure d e semicrystalline areas are sufficiendy melted. The polyester polymers, at die above-mentioned first temperature, is formed into pellets in a droplet or pellet former, described above. The pellets are collected, as they are formed, onto a substantially level surface which is maintained at a second temperature widiin a crystallization zone. (By substantially level is meant not more d an 10° from horizontal). Pellets may be subjected to heating in die crystallization zone, as described widi respect to die apparatus of dύs invention, particularly if die belt is metal. The key feature of die crystallization zone is tiiat it allows temperature control of the just-formed pellets, such diat die pellets are subjected to their desired crystallization temperature immediately after diey are formed. Accordingly, pellets may be produced which are robust and uniform, even when involving low molecular weight polymer. Such pellets are suitable for transport and further polymerization, for example, solid-state polymerization. In order to form polyester pellets suitable for transport and further processing, such as solid-state polymerization, the pellets should be subjected to contact widi a conveyor surface at a temperature within a predetermined temperature range as rapidly as possible after formation. This predetermined temperature range for polyesters preferably ranges from about 80°C to about 230°C, preferably about 110°C to about 190°C. Additional preferred process embodiments are also described in cocurrentiy filed commonly-assigned applications S.N. (docket no. CR-9638), S.N. (docket no. CR-9607) and S.N. (docket no. CR-9524), all tiiree applications hereby incorporated by reference in their entirety. Subjecting die just-formed polymer pellet to a surface temperature within die predetermined temperature range will result in an immediate temperature gradient between the polymer pellet, initially at or near its melt temperature, and its surroundings. This should be done as quickly as possible in order to obtain die desired crystalline moφhology formed. The crystalline moφhology is related to die robustness and abrasion resistance of die pellets, especially die robusmess during later polymerization. The pellets arc maintained in contact widi die hot surface for a predetermined amount of time, which for polyesters should be no less dian about 3 seconds, preferably about 10 to 60 seconds. Generally, die time needed to produce low molecular weight, crystalline polyester pellets, having die desired crystallinity, will not exceed about several minutes, aldiough it would not be detrimental to maintain die pellets at die desired temperature for longer periods of time, for example, 30 minutes or more. The term "αystalline" is herein defined to mean a crystal-Unity content greater dian about 15%, preferably greater dian 20%, and most preferably greater dian 30%, corresponding, respectively, for PET, for example, to a density greater dian about 1.36 g/cc, preferably greater dian about 1.37 g cc, most preferably greater dian 1.39 gml. Thus, die term essentially-crystalline or crystalline, as used herein shall include what is commonly referred to as "semi-crystalline," as are most polyesters of interest. The amount of crysta-liniry can be determined by DSC (differential scan calorimetry). For example, essentiaUy-crystalline PET is characterized by a total heat of fusion, expressed in J/g, of at least about 20, more preferably about 35, when 140 J/g is used as die total heat of fusion of pure crystalline PET. Higher heats of fusion indicate more crystalline polymer. The percent crystallinity widiin a sample of a polyester material or pellet can be determined by comparing die heat of fusion (J/g) of the crystallites present with the heat of fusion of die "pure" crystalline polyester. The polyesters employed in the present invention or process comprise diacid or diester components, suitably including alkyl dicarboxylic acids which contain from 4 to 36 carbon atoms, diesters of alkyl dicarboxylic acids which contain from 6 to 38 carbon atoms, aryl dicarboxylic acids which contain from 8 to 20 carbon atoms, diesters of aryl dicarboxylic acids which contain from 10 to 22 carbon atoms, alkyl substituted aryl dicarboxylic acids which contain from 9 to 22 carbon atoms, or diesters of alkyl substituted aryl dicarboxylic acids which contain from 11 to 22 carbon atoms. The preferred alkyl dicarboxylic acids contains from 4 to 12 carbon atoms. Some representative examples of such alkyl dicarboxylic acids include glutaric acid, adipic acid, pimelic acid and the like. The preferred diesters of alkyl dicarboxylic acids contain from 6 to 12 carbon atoms. A representative example of such a diester of an alkyl dicarboxylic acid is azelaic acid. The preferred aryl dicarboxylic acids contain from 8 to 16 carbon atoms. Some representative examples of aryl dicarboxylic acids are terephd alic acid, isophdialic acid and ordiophd alic acid. The preferred diesters of aryl dicarboxylic acids contain from 10 to 18 carbon atoms. Some representative examples of diesters of aryl dicarboxylic acids include diediyl terephthalate, diethyl isophdialate, diethyl or ordiophdialate, dimethyl naphd alate, diediyl naphtiialate and die like. The preferred alkyl substituted aryl dicarboxylic acids contain from 9 to 16 carbon atoms and die preferred diesters of alkyl substituted aryl dicarboxylic acids contain from 11 to 15 carbon atoms. The diol component for polyesters used in the invention herein suitably comprises glycols containing from 2 to 12 carbons atoms, glycol ediers containing from 4 to 12 carbon atoms and polyedier glycols having the structural formula HO-( AO)rjH, wherein A is an alkylene group containing from 2 to 6 carbon atoms and wherein n is an integer from 2 to 400. Generally, such polyedier glycols will have a molecular weight of about 400 to 4000. Preferred glycols suitably contain from 2 to 8 carbon atoms, widi preferred glycol ediers containing from 4 to 8 carbon atoms. Some representative examples of glycols, which may be employed as die diol component of die polyester, include ediylene glycol, 1.3-propylene glycol, 1 ,2-propylene glycol, 2,2-diethyl-l,3- propane diol, 2,2-dimedιy 1-1, 3 -propane diol, 2-ethyl-2-butyl-l,3-propane diol, 2-edιyl-2-isobuty 1-1, 3 -propane diol, 1,3-butane diol, 1 ,4-butane diol, 1,5-pentane diol, 1,6-hexane diol, 2,2,4-trimethyl-l,6-hexane diol, 1 ,3-cyclohexane dime hanol, 1,4-cyclohexane dimedianol, 2,2,4 ,4-tetramedιyl-l,3-cyclobutane diol, and the like. Some representative examples of polyedier glycol (Polymeg®) and polyediylene glycol (Carbowax®). Branched or unbranched polyesters can also be used. The present process is applicable to both polyester homopolymers and polyester copolymers thereof. Further, the process of this invention is particularly useful for polyesters that do not crystallize easily, i.e., which require heating, according to the present process, in order to crystallize. This would include, for example, poly(ethylene terephthalate) (PET), poly(ethylene naphdialate) (PEN), poly(trimethylene terephthalate) (3G-T), and poly(trimedιylene naphdialate) (3G-N). Generally, such polyesters having a glass transition temperature, T-, above about 25°C, and a melt temperature, T m , usually ranging from about 200°C to about 320°C. Particularly preferred are polyesters modified with up to 10% by weight of a comonomer. Comonomers can include diediylene glycol (DEG), triethylene glycol, 1 ,4-cyclohexane dimedianol, isophthalic acid (IP A), 2,6-naphdιalene dicarboxylic acid, adipic acid and mixtures thereof. Preferred comonomers for PET include 0-5% by weight IPA and 0-3% by weight DEG. As indicated above die crystalline polymer pellets produced according to the present invention can be introduced into a solid-state polymerization reactor for increasing die molecular weight of die polymer. Preferably, die IV (intrinsic viscosity) of the polyester in the pellets is below 0.4, preferably below 0.36, most preferably below 0.3, and die TV of die polyester product of die solid-state polymerization reactor is above 0.5, preferably 0.6 to 1.2. For example, for PET, die solid-state polymerization is suitably run at a temperature between 200 and 270°C, preferably 220 and 250°C, provided it is below die melting point of die polymer for a period of time mat is preferably less dian 24 hours. EXAMPLE 1 This example illustrates a design for a demonstration unit. PET wid an IV of 0.21 dl/g and COOH ends of 92.5 Eq/10 6 g, which is produced by a melt-phase polymerization process is processed at 74 φm dirough a twin screw, 28 mm barrel extruder widi six heated zones. The temperatures in die zones are: 11 12 13. T4 15 16 130°C 274°C 285°C 262°C 284°C 281°C The discharge of die extruder is connected to a Zenitii variable speed gear pump, die molten polymer material is pumped under pressure at a flow rate of 50 lbs/hr into a 60 cm (about two feet) wide ROTOFORMER® dropformer, manufactured by Sandvik Process Systems, Totowa, NJ. The orifices, aligned in rows along die ROTOFORMER® are 1.5 mm in diameter. The feed temperature of die molten polymer material at die entry of the ROTOFORMER® is about 285°C. The molten polymer material is dropformed in the form of droplets onto a conveyor 13. 8 ft in length, which consists of a continuously moving steel belt, which is also manufactured by Sandvik Process Systems. The belt is heated by forced convection from an air blower which heats the bottom of die belt over approximately its entire length to about 160°C. The molten polymer droplets are solidified on die belt to provide uniform, hemispherical particles which are conveyed to a collection bin. Based on experimental runs, in which die belt was not heated to an elevated temperature according to die present invention, it can be estimated mat d e head speed of die cylinder, d e belt speed, and d e average weight of die particles, if produced under die conditions described in this example, would be as follows. TABLE I Example Head Speed Belt Speed Avg. Particle No (ft/min) (ft/min) Weight (g) 1 25.9 30 0.0369 2 33.3 30 0.0236 3 27.9 30 0.0221 4 87.6 60 0.0140

Claims: What is Claimed Is: 1. An apparatus for producing pellets of a polymer from its polymer melt, comprising: (a) a pellet former comprising a rotatable container having a plurality of outiets, defining openings each having a maximum cross-seαional dimension in the range of about 0.5 mm to about 5 mm, for metering a polymer melt onto die surface of a conveyor; (b) a conveyor comprising for receiving, from die pellet former, die polymer melt in die form of a plurality of droplets or crystallizing pellets, which conveyor comprises a surface which is adapted for movement relative to die pellet former and for conveying the pellets dirough a crystallization section; and (c) a crystallization section extending from die point at which die pellets are received onto the surface of die conveyor, extending along at least a portion of die conveyor to a point downstream, d e crystallization section further comprising a means for controlling die temperature of die surface above 50°C widiin a predetermined temperature range as die surface passes dirough die crystallization section. 2. The apparatus of Claim 1 , wherein die crystallization seαion further comprises an automatic temperature controller for controlling said temperature of die surface widiin die crystallization seαion, said temperature controller comprising a means for temperature measurement, a comparator for making a comparison of die measured temperature to a desired temperature or set point, and a means for adjusting die temperature of the surface in response to said comparison, such that the pellets can be subjeαed to a surface widiin a predetermined temperature range for a predetermined period of time. 3. The apparatus of Claim 1 , wherein die means for controlling die temperature of the surface controls die surface to a temperature widiin d e range of 50°C and 240°C. 4. The apparatus of Claim 1 or 2, wherein die pellet former further comprises a stationary container having an elongated channel adjacent die interior surface of d e rotatable container, which elongated channel is parallel to die longitudinal axis of die rotatable container and which is cyclically adapted to be in communication widi each outlet, whereby droplet portions of the polymer melt exit periodically from a plurality of oudets when the elongated channel is in communication widi said plurality of oudets. 5. The apparatus of Claim 1 or 2, wherein die conveyor comprises at least one conveyor belt comprising a top and bottom surface, die top surface forming die moving surface for receiving pellets from die pellet former. 6. The apparatus of Claim 5, wherein the means for controlling the surface temperature is a means for adjusting the temperature and/or flow of a heat- exchange fluid and a means for supplying die heat-exchange fluid for heat transfer contact widi at least a portion of the bottom surface of the conveyor belt. 7. The apparatus of Claim 5, wherein die means for controlling die surface temperature comprises a plenum which extends along at least a portion of die bottom surface of die conveyor belt, said plenum comprising an inlet and oudet for a heat-exchange fluid, and wherein said plenum is at least partially disposed below said bottom surface such diat d e opening of die plenum at least partially faces die bottom surface so mat heat -exchange fluid can be made to continuously flow over said bottom surface. 8. The apparatus of Claim 5, wherein said means for controlling die surface temperature comprises a means for controlling die temperature and/or flow of a heat-exchange fluid in the form of a gas such that heat is transferred either direcdy or indirecdy to d e top surface, bottom surface, or bodi surfaces of die conveyor belt. 9. The apparatus of Claim 5, wherein die conveyor belt further comprises at least two rotating rollers for continuously moving the conveyor belt, said two rotating rollers being situated at distal ends of die conveyor belt such at a first roller is at least partially upstream of die pellet former and a second roller is downstream of d e pellet former. 10. The apparatus of Claim 9, wherein die first roller comprises an internal chamber situated in die first roller, which chamber is connected dirough an inlet and oudet to a means for circulating a heat-exchange fluid dirough d e internal chamber, such diat heat is transferred from the heat-exchange fluid to die outer cylindrical surface of die roller and, from there, to die bottom surface of die conveyor and, from there, to the top surface of die conveyor belt onto which die pellets are received from die pellet former, such diat temperature of die pellets on die conveyor belt is controlled widiin die crystallization section. 11. The apparatus of Claim 1 or 2, further comprising a plenum extending at least partially over a portion of the conveyor, which plenum has an opening at least partially facing the conveyor, said plenum further comprising an inlet and oudet for. respectively, receiving and removing a heated gas. 12. The apparatus of Claim 1 or 2. wherein said means for controlling the surface temperature comprises a plurality of different heaters for heating said surface. 13. The apparatus of Claim 1 or 2, wherein the rotatable container is a cylindrical drum which is rotatable along its substantially horizontal lengthwise axis. 14. The apparatus of Claim 5, wherein said means for controlling die surface temperature comprises heaters positioned bodi upstream and downstream of where die pellets are introduced onto the conveyor. 15. The apparatus of Claim 1 ; wherein die oudets are aligned in rows along die axial direction of d e rotatable container. 16. The apparatus of Claim 1 wherein tiiere are between 100 and 50,000 oudets. 17. An apparatus for producing pellets of a polymer from its polymer melt, comprising: (a) a means for producing a polymer in melt form which comprises at least oudets conneαed to a conduit for transporting die polymer melt towards a pellet former, (b) a pellet former comprising a rotatable container having an inlet for die polymer melt and a plurality of oudet openings, each 0.5 to 5 mm in diameter, for metering die polymer melt onto die surface of a conveyor; (c) a conveyor comprising a surface moving relative to die pellet former, for receiving from die pellet former, die polymer melt in die form of a plurality of droplets or crystallizing pellets, which conveyor is adapted for conveying die pellets dirough a crystallization section; and (d) a crystallization section extending from die point at which die pellets are received onto die surface of die conveyor, extending along at least a portion of d e conveyor to a point downstream, die crystallization seαion comprising a means for controlling the temperature of die surface to widiin a predetermined temperature range above 50°C as d e surface on which the pellets are supported pass dirough die crystallization section. 18. The apparatus of Claim 15 , wherein die crystallization section further comprising a temperature controller for controlling the temperature of the surface within die crystallization section, said temperature controller comprising a means for temperature measurement, a comparator for making a comparison of the measured temperature to a desired temperature or set point, and a means for adjusting die temperature of the surface in response to said comparison, such that d e pellets can be subjected to a surface widiin said predetermined temperature range for a predetermined period of time. 19. The apparatus of Claim 17 or 18, wherein the means for producing a polymer in melt form is selected from the group consisting of an extruder, a melt polymerizer, and a reactor. 20. The apparatus of Claim 17, further comprising a means for solid-state polymerizing die pellets and a means for transporting the pellets from die conveyor to the means for solid-state polymerizing die pellets. 21. A process for producing pellets of a polyester polymer having a glass transition temperature T g greater than about 25°C, from its polymer melt, comprising: (a) forming the polymer melt into molten droplets, wherein die molten pellets are formed by a droplet former comprising a rotatable container having a plurality of oudets, each 0.5 to 5 mm in diameter, for metering a polymer melt onto a surface; and (b) collecting die molten droplets or crystallizing pellets, as tiiey are formed, onto said surface, die surface being maintained widiin a predetermined temperature range above 50°C within a crystallization zone, such mat die crystallizing pellets are maintained in contact widi die surface widiin die crystallization zone for a predetermined period of time during crystallization, thereby producing crystallized polyester particles 500 microns to 2 cm in average diameter. 22. The process of Claim 21 , wherein said surface is part of at least one conveyor belt comprising a top and bottom surface, die top surface forming said surface for receiving pellets from die pellet former. 23. The process of Claim 21 , wherein die surface is metal and die predetermined temperature range is between 80°C and a temperature which is 30°C below die melting point of die polyester. 24. The process of Claim 21 , wherein die polyester is polyethylene terephdialate, die surface is metal, and the predetermined temperature range is between 80°C and 230°C. 25. The process of Claim 21, wherein die monomeric units of the polymer comprise a diacid or diester, wherein the diol is selected from die group consisting of 1 ,3-propanediol, ediylene diol, and 1.4-butane diol. 26. The process of Claim 21. wherein the polymer is selected from the group consisting of poly(edιylene terephthalate), poly(ethylene napthalate), poly(butylene naphthalate. poly(trimethylene terephthalate) and poly(trimethylene naphdialate). 27. The process of Claim 26, wherein die polymer is modified widi up to about 10% by weight of a comonσmer. 28. The process of Claim 27. wherein die comonomer is selected from die group consisting of: diethylene glycol, isophdialic acid, triethylene glycol, 1 ,4-cyclohexane dimethanol, 2,6-napthalene dicarboxylic acid, adipic acid and mixtures diereof. 29. The process of Qai 21 , wherein die pellets formed have an average pellet size of about 1 to 10 mm. 30. The process of Qaim 21 , wherein die predetermined period of time is at least 3 seconds. 31. The process of Qaim 30, wherein die predetermined period of time is between 3 seconds and 10 minutes. 32. Pellets formed by die process of Qaim 21. 33. A process for solid-state polymerization of a a polyester polymer having a glass transition temperature T- greater man about 25°C comprising: (a) foπning molten droplets by metering a polymer melt dirough a plurality of oudets, each 0.5 to 5 mm in diameter, in a rotatable container, and (b) collecting die molten droplets or crystallizing pellets, as tiiey are formed, onto a moving solid surface maintained widiin a predetermined temperature range within a crystallization zone, such d at die pellets are maintained in contact widi die surface within die crystallization zone for a predetermined period of time; (c) introducing die pellets produced in step (b) into a solid-state polymerization reaαor for increasing d e molecular weight of die polymer. 34. The process of Qaim 33, wherein the instrinsic viscosity of die polyester in die pellets are below about 0.36 and the intrinsic viscosity of die polyester product of die solid-state polymerization reactor is above 0.5. 35. The process of Qaim 34, wherein die polyester is PET.

Inventors: BLANCHARD ELWOOD NEAL, LEFFEW KENNETH WYANE, STOUFFER JAN M, BLANCHARD, ELWOOD, NEAL, LEFFEW, KENNETH, WYANE, STOUFFER, JAN, M.
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1996
Main classification: B29B9
IPC codes: B01J2, B29B9, B29C71, B29K67