MXPA98000876A - Process for the production of polyester articles with low content of acetaldeh - Google Patents

Abstract

The present invention relates to a process for removing acetaldehyde from polyesters, which comprises the steps of: a) transporting molten polyester to a continuous screw conveyor, with vent, which has a polymer compression zone. b) rotating the conveyor screw in order to compress and transport the molten polymer through the extruder for a time of less than 15 minutes and at a temperature of less than 300 ° C, c) simultaneously with step b) to flow an agent purge inside and outside said extruder to thereby eliminate volatile impurities and prevent appreciable accumulation of acetaldehyde, and d) transport the volatile-free molten polymer to a mold where a durable article is formed.

Description

PROCESS FOR THE PRODUCTION OF POLYESTER ITEMS WITH LOW ACETALDEHYDE CONTENT Technical Field The present invention relates to a process for the production of polyester articles with low acetaldehyde content, where the conventional stage of solid state polycondensation is not required. The steps of the present invention include polymerization in the molten phase, removal of volatiles in the melt by a purge agent, and useful article formation.

Background of the Invention This invention is especially useful with respect to poly (ethylene terephthalate) (PET), which is widely used in the food packaging industry. Only worldwide use in beverage bottles is well above one billion pounds per year. When PET is used to pack food and beverages, the presence of acetaldehyde in the aforementioned polymer over certain concentration levels (above about 10 ppm) imparts undesirable flavors to packaged foods and beverages. This problem is successfully eliminated in the conventional method of production of high molecular weight PET, which involves the polymerization in the molten phase of either dimethyl terephthalate (DMT) or terephthalic acid (TPA) with ethylene glycol to produce PET with a viscosity inherent (IV) of 0.6, the conversion of molten PET to pellets or granules, and the solid state polymerization of these pellets at temperatures of approximately 190-230 ° C for approximately 4-16 hours to produce PET from IV required for use in food packaging. This last stage of heating the PET pellets under controlled conditions is an additional polymerization step and is referred to as solidification. It is a fortunate feature of this solid phase polymerization that it removes most of the acetaldehyde from PET, as it is a characteristic of PET polymer pellets produced by melt phase polymerization and conventional pelletization that they contain relatively high levels of acetaldehyde. The solidification process, while effective, is also costly and time consuming, and obviously it would be desirable to eliminate it. The present invention provides a process for the reduction of acetaldehyde in molten polymer without the need for solid phase acetaldehyde removal processes. The savings in time and costs would be important. In addition, the invention provides an improved method for the removal of acetaldehyde from molten PET, which could be obtained from the remelted pellets, which have an undesirable level of acetaldehyde, or to remove the acetaldehyde generated in the re-melting process. - Solid polymer melting.

The liquid phase ventilation is known and practiced for the elimination of molten polymer and polyester volatiles. There are examples that show that the removal of volatiles under vacuum reduces acetaldehyde levels in molten polyester. However, vacuum systems are often difficult to work with and maintain the high vacuum levels necessary for the removal of acetaldehyde volatiles. In addition, the air-free operation of extruders with vacuum vents is often difficult to operate. Using the process described in the present invention, polyesters with I.V. and desirably low levels of acetaldehyde, without the need for removal of acetaldehyde in the solid phase. Typically, acetaldehyde is removed from the polyesters in the solid phase. For example, Patent of the U.S.A. No. 4,263,425 describes a solid state process for the removal of acetaldehyde from polyester pieces. The author recognizes that acetaldehyde can be partially removed from a molten polyester using vacuum, but states that tolerable levels can not be reached. In addition, he states that such a process would not be desirable to see that the removal of acetaldehyde from the molten polyester at higher viscosities is even more difficult. Therefore, the author states that it is necessary to remove acetaldehyde in the solid phase for products acceptable for food packaging.

The U.S. Patent No. 4,064,112 discloses a method for overcoming caking problems during the solidification process. Describes the disadvantages of a single molten phase process and states that "high concentrations of acetaldehyde should be expected in the melt". The U.S. Patent No. 5,102,594 describes the crystallization of solid PET in a vacuum-vented extruder to reduce the acetaldehyde content and increase the molecular weight of the polymer. The solid polymer, with volatiles removed, is immediately melted and extruded directly to a final product. The U.S. Patent No. 4,591, 629 describes a process for the continuous production of high molecular weight polyesters in a two-stage process for treating polyester in the solid phase, in which, (1) in a first stage the polyester is steam treated or a purging agent or air containing steam at a temperature of 100-245 ° C and (2) in a second stage is post-condensed at 200-245 ° C with a purging agent and / or air under normal or vacuum pressure. The process, it is said, is especially useful for the production of high molecular weight PET and having a total content of dissolved and bound acetaldehyde of less than 3 ppm. It is said that this PET is especially useful for the production of bottles and other food containers. The use of extruders for the removal of volatiles from molten polymer streams is known in the literature. For example, Mack [M. H. Mack, Plastics Engineering, pp 47-51 (July 1986)] describes some selection criteria for a variety of volatile removal applications in foundries. For single screw extruders, Mack's work shows the removal of volatiles to the range of 5 ppm residual ethylene in copolymers with low ethylene / vinyl acetate viscosity. In polymers of higher viscosity, levels of 15 ppm was all that could be achieved. Biesenberger, et al., Has published both theoretical and experimental information on the elimination of styrene volatiles from molten polystyrene in ventilated extruders, single screw, vacuum and mantles of purging agent, as well as other examples. For example, the Biesenberger data and examples show residual styrene monomer being reduced from above 5,000 ppm to approximately 100 ppm after volatile removal. Biesenberger compares the venting to the vacuum with the venting with nitrogen purge at atmospheric pressure and concludes that vacuum venting is more efficient than a purging agent to remove volatile molten polymers. See J. A. Biesenberger and G. Kessidis, Polymer Engineering Science, 22, 13, pp 832-836 (1982) and J. A. Biesenberger and D. H. Sebastian, Principles of Polymerization Engineering, Krieger Publishing Company, Chapter 6 (Malabar, Florida, 1983). The prior art technique teaches that vacuum volatile removal is a viable way to reduce or eliminate volatiles from a molten polymer. For example, U.S. Pat. No. 4,362,852 discloses a process for the removal of molten polyester and polyamide volatiles with a rotating disk processor working under vacuum. It is stated that the process abate residual monomers in polyamides up to 2.5% by weight. It is also stated that the process reduces ethylene carbonate and carbon dioxide in polyesters to levels of 100 ppm and 50 ppm, respectively. The U.S. Patent No. 4,980,105 provides an example of a process of an extruder for the removal of volatiles from a molten polymer to remove by-products. In this case, a by-product was formed in a first reaction stage and remains in the polymer. The removal of volatiles with the extruder using vacuum vent eliminated this by-product. However, the removal of volatiles from acetaldehyde is substantially different of this example since acetaldehyde is a by-product continuously produced in molten polyesters. Japanese Patent Application Kokai Sho 53 [1978] 71162 describes a method of melting process polyester and reducing the acetaldehyde content by keeping the PET re-melted at pressures of less than 250 mm Hg for at least 5 seconds and then under normal pressure. or increased pressure for less than 5 minutes. The examples cited in this process show that the efficiency in the removal of acetaldehyde increases as the vacuum pressure is reduced in an extruder port. The authors state that at pressures above 250 torr and with long marking timesIt is difficult to remove acetaldehyde from polyesters. The U.S. Patent No. 4,230,819 describes the removal of acetaldehyde from crystalline PET with a drying agent (air or nitrogen at 170-250 ° C). He states that acetaldehyde can not be completely removed from PET by heating under pressure. The U.S. Patent No. 4,255,295 describes a process for the production of polymer with good characteristics of spinning from waste. It consists of compressing the finely cut waste by means of a screw until a volumetric density of 500 kg / m3 is reached, introducing it into a double screw extruder where it is melted, and subjecting the molten polymer to an operation of post-condensation under reduced pressure. During the melting of the polymer and the post-condensation operation, the remains of water and volatile impurities are eliminated. It is said that the polymer is suitable for use in spinning and plastics operations, but only non-woven networks are mentioned. The U.S. Patent No. 4,142,040 discloses a method of processing the solid state of a saturated polyester resin in order to minimize degradation to produce acetaldehyde. This patent describes in column 4, lines 38 and the following, "inert gas is introduced through one or more conduits 3 into the lower part of a hopper or through one or more conduits 3a within the feed zone (or both) The inert gas cleans essentially all the polyester air as it progresses through the initial part of the feeding zone. " BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram illustrating a vented extruder, which can be used in the process of the present invention.

Detailed Description of the Preferred Modality of the Invention According to the present invention, a process for the removal of acetaldehyde from polyester is provided, which comprises the steps of: a) transporting molten polyester to a continuous screw conveyor, endowed with of holes, which has a polymer compression zone; b) rotating the screw of the conveyor in order to compress and transport the molten polymer through the extruder for a time of less than 15 minutes and at a temperature of less than 300 ° C; c) simultaneously with step b), flowing a purging agent into and out of said extruder to thereby remove volatile impurities and prevent appreciable accumulation of acetaldehyde, and d) transport the molten, volatile polymer to a modeling device where an article of manufacture is formed. Polymers that are particularly useful in this process include poly i (ethylene terephthalate), poly (ethylene naphthalenedicarboxylate), and copolyesters containing up to 50 mol% of glycols and / or modifying dibasic acids. Modifying dibasic acids may contain from 2 to 40 carbon atoms and include isophthalic, adipic, glutaric, azelaic, sebacic, fumaric, dimeric, cis- or trans-1,4-cyclohexanedicarboxylic acids, the various isomers of the acids naphthalenedicarboxylics, and the like. Naphthalenedicarboxylic acids that are highly useful include the 2,6-, 1,4-, 1,5-, or 2,7- isomers, but the 1,2-, 1,3-, 1-isomers can also be used, 6-, 1,7-, 1,8-, 2,3-, 2,4-, 2,5-, and / or 1, 8-. The dibasic acids can be used in acid form or as their esters, such as dimethyl esters for example. Typical modifying glycols can contain from 3 to 10 carbon atoms and include propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, diethylene glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, and similar. The 1,4-cyclohexanedimethanol may be in the cis or trans form or in cis / trans mixtures.

The polyesters of this invention are readily prepared using polycondensation reaction conditions well known in the art. Typical polyesterification catalysts which may be used include titanium alkoxides, dibutyl tin dilaurate, and antimony oxide or antimony triacetate, used separately or in combination, optionally with zinc, manganese or magnesium acetates or benzoates and / or other types of such catalyst materials as are well known to those skilled in the art. Optionally, phosphorus and cobalt compounds may also be present. Although we prefer to use continuous polycondensation reactors, batch reactors operated in series can also be used. While in this process the use of polyesters in an unmodified form is preferred, other components such as nucleating agents, derivatizing agents, colorants, pigments, fillers, antioxidants, heat and light stabilizers may also be used, if preferred. ultraviolet, impact modifiers and the like. Next to the preparation of the polyester in the melted phase to an I.V. of 0.50-0.85 dL / g as described above, preferably the molten polyester is passed through a suitable filter to remove impurities, gels, etc. The filtrate of the polymer is well known in the art and can be performed by passing it through a wire mesh filter, for example. The filtered polyester then enters the volatile removal device, for example, an extruder having vents or holes, an example of which is illustrated in Figure 1. The extruder includes holes for the inlet of a purging agent and an orifice for the elimination of volatiles such as acetaldehyde. The purging agent can be any of those known in the art., as an inert gas, a reactive purifying agent, etc. Nitrogen is preferred. It can be used either a single screw or twin screw extruder. The single screw extruder illustrated in Figure 1 includes a cylinder 10, having a screw 12 contained within it to rotate, thereby feeding polymer pellets from the feed hopper 14 at the bottom of the cylinder length in where they are melted, degassed and finally exempted from the end at 16. The hole 18 is optionally connected with a controlled pressure source. A purging agent is used to eliminate volatiles. The purging agent can penetrate the cylinder at or near hopper 14, such as through holes 19 or 20, or further downstream, such as through holes 18 or 22. Obviously, the direction of flow of the purging agent with respect to the polymer can be handled by techniques well known in the art, such as co-current or countercurrent, and can be injected above the surface or below the surface of the polymer. The purging agent can penetrate and leave through the same orifice or through different orifices. Preferably the purging agent is an inert gas. In the case of a high performance final polymerization reactor, a combination of equipment design, production speeds and operating conditions can facilitate the increase in molecular weight of the polyester and the elimination of acetaldehyde volatiles in the same piece of equipment . In this advantageous embodiment, the polymer is rapidly pelletized through a gear pump attached directly to the outlet of the reactor. Extruders with vents, one screw or two screws may be used, the two screw extruders being generally preferred due to their particular suitability for volatile removal. Such twin screw extruders may be of the co-rotating or counter-rotating type, with intermalla screws or without intermalla. From here on, particular reference will be made to extruders with intermalla and co-rotation regime, which are often especially useful for this purpose. In the Patents of the U.S.A. Nos. 4,107,787 and 3,619,145 useful typical extruders are described, which are incorporated herein by reference.

In order to suppress the thermal degradation of the polyester, it is preferred that the extrusion be carried out under low viscosity dissipation conditions; that is, with minimization of heat generation by friction as a result of friction. Two sources of heat are normally present in extrusion operations: external application and friction. It is, of course, necessary that some heat be provided in order to melt the resin and facilitate the removal of volatiles with a minimum residence time. For the most part, temperatures of up to 330 ° C may be employed, preferably not above 300 ° C for long periods of time. However, viscous dissipation should be minimized by designing the extruder screw (s) to maintain the temperature of the polymer being extruded to a point no higher than 15 ° C more than the temperature of the extruder cylinder as a result of heat external applied A low residence time of the polyester in the extruder is also preferred. Preferred residence times are not accurately expressed as absolute values, since there is a large variation based on factors such as the size of the extruder, its screw speed and the viscosity of the polyester being extruded. For the purposes of the present invention, the residence time can be minimized by predominantly using advance moving screw elements, which have a relatively high inclination, typically 45 ° (all inclinations being angles from a plane perpendicular to the length of the screw ) and by reducing the size of non-advance feed elements such as reverse feed elements and neutral blocks of kneading action. By "continuous" screw conveyor, it is meant a screw conveyor such as that shown in Figure 1, wherein the screw elements are uninterrupted. The molten polyester passes into a volatile removal stage by venting in the extruder. It is at this stage that the volatiles, including acetaldehyde, are removed by a purging agent. Said removal is facilitated by maintaining a high and frequently renewed surface area of the resin and a relatively high resin temperature, typically in the range of 250-300 ° C. The extension of this stage is usually 25-75% of the total duration of the system. In the same way as in the casting stage, high inclination screw elements are used in the volatile removal stage; as a result of the upstream seal, the proportion of resin moving within this stage is relatively small and the screw channels are not fully filled, which helps to maintain a high surface area for efficient removal of volatiles. At the downstream end of the volatile removal stage, it may again be preferred to reduce the tilt of the screw in preparation for a second liquid seal, which prevents back pressure from the extruder die, which could cause polyester cast back inside the previous stage, inhibiting the elimination of volatiles. This seal is often conveniently created by means of a single kneading action block, with forward fillets, with a relatively low screw inclination.

Under ideal conditions, the polyester would immediately exit the extruder matrix after the second liquid seal. However, it may be preferred to employ a medium pitch measurement step, typically in the range of -35 °, to create pressure that forces the polyester through the matrix. The method of this invention is illustrated by an example in which a twin screw extruder was used, with intermalla, co-rotation regime, marking Wemer-Pfleinderer, with a screw diameter of 28 mm and a length of 775 mm.

The stages in the extruder are designed as follows, all the screw elements being of positive displacement (that is, right tilt): Pre-aligning seal - 15 ° screw, 15 mm Solids feed stage - screw transportation of 45 ° solids, 135 mm; 45 ° transition screw, 15 mm Melting stage - 45 ° screw, 195 mm; 15 ° screw, 15 mm; First liquid seal - kneading action block with positive displacement, 15 mm; block with neutral kneading action, 30 mm. Volatile removal stage - 30 ° screw, 30 mm; 45 ° screw, 150 mm. Second liquid seal - kneading action block with forward flow, 15 mm.

Measuring stage - 30 ° screw, 160 mm. The extruder is loaded with PET with an I.V. from 0.72. It is operated at a screw speed of 300 rpm and a resin feed rate of 4.7 kg / hr, with the orifice being maintained at ambient pressure, using a nitrogen scan of 35 standard cubic feet per hour. The extruder is divided into four heating zones as follows: Water cooled - 105 mm Heated at 115 ° C - 120 mm Heated at 274 ° C - (resin temperature up to 286 ° C) - 480 mm Heated at 300 ° C - 70 mm. The vent section of the extruder must be constructed in such a way that the flow of ambient gas, such as oxygen, into the orifice from the ambient atmosphere can not occur. A stream of purging agent, such as nitrogen, was applied to the extruder at a high or low pressure, preferably at ambient pressure. This gas can be applied to ambient conditions or preferably heated to prevent cooling of the polyester with which it comes into contact. The extruder can be operated in such a way as to allow co-current or counter-current flow of gas along the polyester that is being transported in the unfilled fillets of the extruder screw. Alternatively, sweeping with the purging agent can be applied in a single orifice or in multiple orifices, depending on the level of acetaldehyde removal required for a particular application. The holes of the extruder can be configured as open sections of the extruder cylinder, such as open holes in the extruder barrel. The purging agent can be introduced into the orifice at a single point through a tube or pipe, or dispersed through the entire orifice by several tubes or pipes, or some other large area distribution system, such as a plate or device calcined with several exits. The purging agent can be introduced into the vacuum above the screws of the extruder or below the surface to the molten polymer. The holes can be operated at ambient pressure, reduced pressure or high pressure.

After passing through one or more of the vent zones, the polyester is normally transported out of the extruder by a screw compression zone, as is known in the prior art. This zone compresses the molten polymer so that the screw channels are filled and any gas bubbles left by the vent with flushing gas are ejected. It is important that this degassing be complete, so that no bubbles form on the molded or formed rods or on the articles formed in the process. It is also important that the polymer reside in this zone at the lowest possible temperature and for the shortest possible time, otherwise, more acetaldehyde will be formed in the molten polyester and cancel out the effects of ventilation by inert gas scavenging. The time and temperature would depend on the character of the polyester, additives of the polyester, the type of screw extruder used and the operating variables of the extruder, but typically should be less than 300 ° C and 15 minutes, and preferably be less than 270 ° C and 5 minutes for the manufacture of packaging products with low acetaldehyde content. The time elapsed between the elimination of volatiles and the modeling of the final article must also be kept low in order to avoid the excessive generation of acetaldehyde. The final stage of this process is to transport the polymer to another process that can form rods, tubes, pellets or some other article of manufacture molded or formed. The degree of polymerization and therefore the I.V. of the final article will depend on the I.V. initial of the polymer fed to the extruder, the amount of water present in the fed polyester and the contact time and the amount of inert gas scavenging. Since the polyester is fed in the molten state, the I.V. it can be maintained or raised, depending on the contact time, temperature and flow relation of the sweep with inert gas. The term "I. V." used herein refers to the Inherent Viscosity of the polymer, as determined by standard methods in a solution of 0.5 g of polymer dissolved in 100 ml of a mixture of phenol (60% by volume) and tetrachloroethane (40% by volume).

The concentration of acetaldehyde in polyesters is measured as follows: Extruded polyester samples are collected in dry ice to mitigate melting. The polymer is then cut immediately into granules and approximately 6 g are placed in jars with rubber-coated lids. The bottles are stored at -40 ° C for no more than three days before the analysis. The samples are then ground in a Wiley mill until they pass a 20 mesh screen and placed in desorption tubes by gas chromatography. The acetaldehyde is desorbed from the polymer at 150 ° C for 10 minutes and quantified by gas chromatography.

Examples 1 to 9 Poly (ethylene terephthalate) was synthesized according to the technique at about an I.V. of 0.64 dL / g from dimethyl terephthalate and ethylene glycol with 3.5 percent mol of 1,4-cyclohexanedimethanol. The polymer is granulated, fed to a twin-screw extruder and vented, melted at 265 ° C and measured within the vent section of the extruder. The temperature of the vent zone of the extruder was varied in this example. The temperature between the vent and discharge orifice was controlled at 260 ° C. For gas flushing examples, a gas intake system heated to 285 ° C was placed in the vent of the extruder. The flow of nitrogen is controlled at 35 scfh (standard cubic feet per hour) by a rotameter and vented at atmospheric pressure through a bubble trap. For vacuum work, a vacuum pump was connected to the vent and the vacuum pressure was lowered to below 0.5 torr. Control experiments were performed by completely blocking the extruder ventilation. Table 1 compares the acetaldehyde content measured in the collected polymer as a function of temperature and residence time after the melt zone for extrusion without venting, vacuum venting and vented with nitrogen gas scavenging, of pol i (ethylene terephthalate).

TABLE 1 EXAMPLE 10 Ol? (Ethylene terephthalate) was synthesized according to the technique to approximately an I.V. of 0.60dL / g from dimethyl terephthalate and ethylene glycol with 3.5 percent mol of 1,4-cyclohexanedimethanol. The polymer is granulated, fed to a twin-screw extruder and vented, melted at 265 ° C and measured within the vent section of the extruder. The temperature of the vent zone of the extruder is 280 ° C. The temperature between the vent and discharge orifice was controlled at 260 ° C. A gas feed system heated to 285 ° C was placed in the extruder vent and a nitrogen flow at 35 scfh (standard cubic feet per hour) was established using a rotameter. Nitrogen was vented at atmospheric pressure through a bubble trap. The residence time from the vent to the discharge was 6 minutes. Poly (ethylene terephthalate) was collected and the acetaldehyde content measured to be 13 ppm. The die of the extruder was then removed to reduce the residence time between the vent hole and the discharge port. Poly (ethylene terephthalate) is collected from the end of the extruder screws while the extruder is being operated. The acetaldehyde content was determined to be 4.6 ppm.

Examples 11 to 19 Poly (ethylene terephthalate) was synthesized according to the technique at about an I.V. of 0.60 dL / g from dimethyl terephthalate and ethylene glycol with 3.5 percent mol of 1,4-cyclohexanedimethanol. The polymer is granulated, fed to a different twin screw extruder and with vent, melted at 265 ° C and measured within the vent section of the extruder. The temperature of the vent zone of the extruder was varied in these examples. The temperature between the vent and discharge orifice was controlled at 260 ° C. For the gas flushing examples, a gas admission system heated to 285 X was placed in the extruder vent. The flow of nitrogen is controlled at 35 scfh (standard cubic feet per hour) by a rotameter and vented at atmospheric pressure through a bubble trap. For vacuum work, a vacuum pump was connected to the vent and the vacuum pressure was lowered to below 0.5 torr. Table 2 compares the acetaldehyde content measured in the collected polymer as a function of the temperature and the residence time after the melt zone ventilation for vacuum vented extrusion and venting with nitrogen gas, poly ( ethylene terephthalate).

TABLE 2 Examples 20 to 24 Poly (ethylene terephthalate) was synthesized according to the technique at about an I.V. of 0.59 dL / g from terephthalic acid and ethylene glycol with 1.5 percent mol of 1,4-cyclohexanedimethanol. The polymer is granulated, fed to a twin-screw extruder and vented, melted at 265 ° C and measured within the vent section of the extruder. The temperature of the vent zone of the extruder was varied in these examples. The temperature between the vent and discharge orifice was controlled at 260 ° C. A nitrogen scavenging at 35 scfh (standard cubic feet per hour) was established through a gas admission system heated to 285 ° C and placed in the extruder vent. The nitrogen was vented at atmospheric pressure through a bubble trap. Table 3 shows the measured acetaldehyde content and the I.V. for the polymer collected as a function of the temperature and residence time after the melt zone.

TABLE 3 Examples 25 to 34 Poly (ethylene terephthalate) was synthesized according to the technique at about an I.V. of 0.77 dL / g from terephthalic acid and ethylene glycol with 1. 5 percent mol of 1,4-cyclohexanedimethanol. The polymer is granulated and some samples are placed in a sealed container with liquid acetaldehyde (boiling point 21 ° C) to raise the level of free acetaldehyde in the polymer. After 24 hours or more equilibrium time, the polymer is fed to a twin-screw extruder and vented, melted at 265 ° C and measured within the vent section of the extruder. The temperature of the vent zone of the extruder was varied in these examples. The temperature between the vent and discharge orifice was controlled at 260 ° C. A nitrogen scavenge at 35 scfh (standard cubic feet per hour) was established through a gas admission system heated to 285 ° C and placed in the extruder vent and vented at atmospheric pressure through a gas trap. bubbles. Table 4 shows the measured acetaldehyde content and the I.V. for the polymer collected as a function of the temperature and residence time after the melt zone, and the initial acetaldehyde content of the granulated poly (ethylene terephthalate).

TABLE 4 Examples 35 to 45 Poly (ethylene terephthalate) was synthesized according to the technique at approximately one I V. of 0.60 dL / g from terephthalic acid and ethylene glycol with 1.5 percent mole of 1,4-cyclohexanedimethanol. The polymer is granulated, fed to a twin-screw extruder and vented, fused at 265 X and measured within the vent section of the extruder. The temperature of the vent zone of the extruder is 280 X. The temperature between the vent and discharge orifice was controlled at 260 X. For the examples of gas scavenging, a gas intake system heated to 285 X was placed in the vent of the extruder. The flow of nitrogen is controlled at 20 scfh (standard cubic feet per hour) by a rotameter and vented at atmospheric pressure through a bubble trap. Low nitrogen flows are controlled by a mass flow controller. For vacuum work, a vacuum pump was connected to the vent and the vacuum pressure was lowered. Table 5 compares the acetaldehyde content measured in the collected polymer as a function of the vacuum venting level and the nitrogen scavenging rate for the polyethylene terephthalate vent, after approximately 5 minutes of time. of residence of the molten phase after ventilation.

TABLE 5 Examples 46 to 55 (Comparative) Poly (ethylene terephthalate) was synthesized according to the technique at about an I.V. of 0.77 dL / g from terephthalic acid and ethylene glycol with 1.5 percent mol of 1,4-cyclohexanedimethanol. The polymer is granulated and sealed in a vessel with liquid acetaldehyde (boiling point 21 X) to raise the free acetaldehyde content of the polymer to the desired level. After 24 hours or more equilibrium time, the polymer is fed to a twin screw extruder and with vent, it was melted at 265 X and was measured inside the vent section of the extruder. The total residence time of the polymer in the extruder is about 10 minutes. The temperature of the vent zone of the extruder was varied in these examples. The temperature between the vent and discharge orifice was controlled at 260 X. The residence time of the polymer between the vent hole and the discharge orifice is approximately 2 minutes. A vacuum pump system was connected to the extruder vent and the vacuum pressure was varied by a nitrogen bleed valve upstream of the extruder. Table 6 shows the measured acetaldehyde content and the I.V. collected for poly (ethylene terephthalate), as a function of temperature and vacuum pressure.

TABLE 6 Examples 56 to 67 Poly (ethylene terephthalate) was synthesized according to the technique at about an I.V. of 0.77 dL / g from terephthalic acid and ethylene glycol with 1.5 percent mol of 1,4-cyclohexanedimethanol. The polymer is granulated and placed in a sealed container with liquid acetaldehyde (boiling point 21 X) to raise the free acetaldehyde content of the polymer. After 24 hours or more equilibrium time, the polymer is fed to a twin screw extruder, fused at 265 X and measured within the vent section of the extruder. The total residence time of the polymer in the extruder is about 10 minutes. The temperature of the vent zone of the extruder was varied in these examples. The temperature between the vent and discharge orifice was controlled at 260 X. The residence time of the polymer between the vent hole and the discharge orifice is approximately 2 minutes. A gas intake system heated to 285 X was placed in the extruder vent. The nitrogen flow is varied by a mass flow controller and vented at atmospheric pressure through a bubble trap. Table 7 shows the measured acetaldehyde content and the I.V. collected for poly (ethylene terephthalate), as a function of temperature and gas flow velocity.

TABLE 7 Unless otherwise specified, all parts, percentages, relationships, etc., are by weight. The I.V. as used herein is inherent viscosity measured at 25X using 0.50 grams of polymer per 100 mL of a solvent consisting of 60% by weight of phenol and 40% by weight of tetrachloroethane. The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications may be made within the spirit and scope of the invention.

Claims (13)

1. Novelty of the Invention 1. A process for removing acetaldehyde from polyesters, which comprises the steps of: a) transporting molten polyester to a continuous screw conveyor, vented, which has a polymer compression zone; b) rotating the screw of the conveyor in order to compress and transport through the extruder the molten polymer for a time of less than 15 minutes and at a temperature of less than 300 X; c) simultaneously with step b), flowing a purging agent into and out of said extruder to thereby remove volatile impurities from said molten polyester and prevent appreciable accumulation of acetaldehyde, and d) transport the molten, volatile polymer to a modeling device where a manufacturing article is formed.
2. 2. A process according to claim 1, wherein the polyester of step a) has an I.V. of 0.50-0.85.
3. 3. A process according to claim 1, wherein the conveyor of step b) is an extruder.
4. 4. A process according to claim 1, wherein the polyester has a final residual acetaldehyde content of less than 15 ppm.
5. 5. A process according to claim 1, wherein at least 80 mol% of said dicarboxylic acid is terephthalic acid.
6. 6. A process according to claim 1, wherein at least 80 mol% of said glycol is ethylene glycol.
7. 7. A process according to claim 1, wherein said glycol is a mixture of ethylene glycol and cyclohexanedimethanol.
8. 8. A process according to claim 1, wherein said acid is terephthalic acid and said glycol is a mixture of 60-99 mol% of ethylene glycol and 40-1 mol% of cyclohexanedimethanol.
9. 9. Molded polymer articles, extruded or formed, with a low content of acetaldehyde and produced by the process of claim 1.
10. 10. The process of claim 1, wherein the temperature of the polymer in the volatile removal zone is maintained at 5 to 50 X above the melting point of said polyester.
11. 11. The process of claim 1, wherein the polymer is subjected to volatile removal for 5 to 600 seconds.
12. 12. The process of claim 1, wherein the polymer takes 0.1 to 15 minutes in the molten phase after removal of volatiles.
13. 13. The process of claim 1, wherein the inherent viscosity of the resulting polymer is 0.55 to 0.95. EXAMPLE OF THE DESCRIPTION A process for the removal of acetaldehyde from polyesters is described, which comprises the steps of: a) transporting molten polyester to a continuous screw conveyor, with venting, which has a polymer compression zone; b) rotating the screw of the conveyor in order to compress and transport the molten polymer through the extruder for a time of less than 15 minutes and at a temperature of less than 300 X; c) simultaneously with step b), flowing a purging agent into and out of said extruder to thereby remove volatile impurities and prevent appreciable accumulation of acetaldehyde, and d) transport the molten, volatile polymer to a mold where an article of manufacture is formed.