Producing a Copolyester From Two Lower Dialkyl Esters of Dicarboxylic Acid and a Glycol

The present invention relates to a process for producing a copolyester from two lower dialkyl esters of a dicarboxylic acid (LDE) and a glycol (GLY). Using a specific catalyst system of the present invention allows to conveniently produce the copolyester by reacting the two lower dialkyl esters of dicarboxylic acid with glycol in an ester interchange reaction. In particular, the catalyst system comprises manganese (Mn), lithium (Li), antimony (Sb) and optionally cobalt (Co). More specifically, manganese and lithium are used as catalysts for the ester interchange of the LDE and GLY, while lithium, cobalt and antimony are used as catalysts for the polycondensation stage. The manganese is sequestered after the ester interchange.

Furthermore, copolyesters of the present invention are demonstrated to have improved melt flow characteristics and in solid form, improved cutting characteristics during processing and increased tensile strength when bonded in a blend with other polyester fibers.

Copolyesters of the present invention can be used as binder fibers in nonwoven applications or can be used in conjunction with polyester fiber in bicomponent fibers.

In prior art processes, copolyesters can be produced by two different routes: ester interchange plus polycondensation, referenced herein as the ester interchange route; or direct esterification plus polycondensation, referenced herein as the direct esterification route. Generally, the ester interchange route can be used with either a batch type process or a continuous process, while the direct esterification route uses a continuous type process.

In the ester interchange route, dimethyl terephthalate (DMT), dimethyl isophthalate (DMI) and ethylene glycol (EG) are typically reacted in the presence of a catalyst (manganese) at atmospheric pressure and at a temperature of from about 180.degree. C. to 230.degree. C. In the presence of the catalyst, these components undergo ester interchange to yield two intermediate monomers and methanol. The reaction which is conveniently done with about 0.6 mole of DMT, about 0.4 mole of DMI and 2.8 to 3.2 moles of EG, is reversible and is carried to completion by removing methanol formed. During the ester interchange, the two intermediate monomers are the substantial majority product (not considering the methanol), along with small amounts of low molecular weight oligomers.

The monomers are then polymerized by a polycondensation reaction, where the temperature is raised to about 270.degree. C. to about 300.degree. C. and the pressure is reduced to below 1 mm of mercury vacuum and in the presence of a suitable polymerization catalyst (antimony). From this reaction, polyethylene terephthalate/isophthalate) and ethylene glycol are formed. Because the reaction is reversible, the glycol is removed as it is evolved, thus forcing the reaction toward the formation of the polyester.

Manganese is the preferred catalyst for ester interchange reactions, but the amount of manganese employed must be strictly controlled. The presence of too little manganese during the ester interchange reaction results in very long reaction times, while the presence of too much manganese results in unwanted side products during the polycondensation reaction, and unacceptable degradation of the copolyester resulting in poor color (thus lowering the quality of the copolyester). The exact range of manganese which proves to be the most desirable must generally be determined through trial and error because many factors affect the reactivity of the manganese. For example, reaction temperature, reaction pressure, the degree of mixing during reaction, the purity of the raw materials, the presence of other additives, etc., all affect the effectiveness of manganese.

In prior art process, manganese was employed to obtain suitable ester interchange reaction times, but the manganese must be sequestered after ester interchange or during polycondensation by a polyvalent phosphorous compound to aid in reducing the discoloration and unwanted side products. Generally, prior art processes employed about 50 ppm to 150 ppm manganese based on he expected yield of the polymer, as the ester interchange catalyst. Using more than about 150 ppm manganese resulted in polymer degradation even if phosphorous was employed in excess to sequester the manganese. It is believed that this occurred because the phosphorous was incapable of complexing with the manganese to the degree necessary to prevent discoloration.

Another method to make copolyesters is by direct esterification. By this method, terephthalic acid (TA), isophthalic acid (IPA) and EG are typically reacted without any catalyst in a continuous process. Normally, the TA and IPA are reacted at a pressure of from about 5 psia to 85 psia and at a temperature from about 185.degree. C. to 290.degree. C. These components undergo direct esterification to yield two intermediate monomers and water. The reaction is conventionally done with amounts of TA and IPA corresponding to the desired mixture in the copolyester. For example, if a 60/40 (terephthalate/isophthalate) is desired, then 1.2 mole of TA, .8 mole of IPA and from 2.4 to 3.2 moles of EG are reacted.

After the completion of the direct esterification, the monomers are then polymerized by the polycondensation reaction as described in the ester interchange process.

One use of copolyesters is in thermally bonded fibrous nonwoven applications such as medical face masks wherein polyester fibers are thermally bonded to copolyester binder fibers that have been blended with the polyester fibers. The thermal bonding is attributed to the copolyester fiber having a lower melting point than the blended polyester fiber. An example of such a copolyester is poly(ethylene terephthalate/isophthalate) having a terephthalate/isophthalate mole ratio from about 80/20 to 50/50.

Nonwoven products may also be formed of bicomponent fibers having a polyester core and a copolyester sheath. Such bicomponent fibers act as the binder fibers when blended with polyester fibers to make nonwovens.

The following references are directed to various DMT type processes and catalyst systems used for either making polyester or copolyester.

U.S. Pat. No. 3,709,859 to Hrach et al discloses a multi-component catalyst system for producing polyester via the ester interchange process. Among the many catalysts mentioned are lithium, cobalt, manganese and antimony. Although these catalysts are set forth in the background portion of the patent, the patent claims a catalyst system comprising antimony, lead, and calcium, and additionally strontium or barium. Hrach et al also teach the necessity of employing pentavalent phosphorous compounds as stabilizers in order to prevent the formation of discolored polyester.

British Patent 1,417,738 to Barkey et al discloses a process for manufacturing polyester in which a preferred ester interchange catalysts may include zinc, manganese, cobalt, and lithium, among others. Preferred polycondensation catalysts include antimony compounds. This reference, however, claims other catalyst compounds and mentions the above catalyst only as background information.

Various patents assigned to Eastman Kodak Company (British Patents 1,417,738, and 1,522,656; U.S. Pat. Nos. 3,907,754, 3,962,189, and 4,010,145) disclose a broad variety of catalyst systems, including a manganese, cobalt, lithium and titanium combination and a manganese, titanium, cobalt and antimony catalyst system, with phosphorous being used in each of these systems as a sequestering agent. Each of these catalysts was added at the beginning of ester interchange and are used as catalysts for the ester interchange. The purpose of the catalyst system is to speed up the production of the copolyester.

The following references teach preparation of a polyester by the ester interchange process with the addition of small amounts of TA in the polycondensation step utilizing various catalyst systems.

U.S. Pat. No. 3,657,180 to Cohn discloses a process for making copolyester resin in which lithium or a divalent metal compound are employed as catalyst. The specification states that manganese may be one of the divalent metallic compounds which can be employed. The order of mixing the various raw materials and the addition of the compounds to the process described in the Cohn invention is stated to be critical. The process is carried out by reacting DMT and ethylene glycol in the presence of a lithium salt under ester interchange conditions followed by the addition of manganese. In another embodiment, the process also includes using manganese as a catalyst with lithium being added after the ester interchange reaction. In either case, the second metal is always added after ester interchange, and thus is not used as a catalyst for the ester interchange. Moreover, the second metal is added in a higher than catalytic amount and is added to act as a slip agent. The second metal is added along with a slurry in the amount of less than 1% of product of glycol and a small amount of terephthalic acid before vacuum-let-down to provide slip for polyester film and the amount added is several factors larger than catalytic amount.

Although the catalyst systems disclosed in the prior art described herein may be suitable for reducing ester interchange time, they are found to differ from the catalyst system used in the present invention. As will be shown in the Examples, it has been found that the timing of the addition of the polycondensation catalysts also contribute to the production of the copolyester.

It is an aim or aspect of the present invention to not only feasibly produce a copolyester by the LDE type process from available suitable raw materials, but produce a copolyester which has acceptable color, IV and thermal properties.

The present invention provides a unique process of preparing a copolyester by effectively reacting two dialkyl esters of dicarboxylic acid and glycol by the ester interchange followed by the polycondensation reaction through the use of a unique catalyst system. In particular, the present invention comprises a process for making copolyester wherein a catalyst consisting of manganese and lithium are used for the ester interchange reactions and the catalysts of cobalt (optional) and antimony are employed in the polycondensation reaction.

In the broadest sense, the present invention comprises a method for making a copolyester from at least two lower dialkyl esters of dicarboxylic acid (LDE) and a suitable glycol comprising the steps of: reacting the glycol with the LDE at a molar ratio of between about 1.8/1 to about 2.5/1, at a suitable temperature and pressure and in the presence of an effective amount of manganese and lithium catalysts sufficient to produce a monomer mixture and alcohol; removing the resultant alcohol to more completely react the LDEs and glycol; reducing the pressure to a vacuum pressure sufficient to initiate polycondensation; copolymerizing the resultant monomer mixture at a suitable temperature and pressure and in the presence of an effective amount of antimony catalyst and optionally, but preferably a cobalt catalyst to form a copolyester.

The present invention also comprises a copolyester product made by the above-mentioned process.

The present invention also comprises a bicomponent fiber having a polyester core, a copolyester sheath made by the above-mentioned process.

Such copolyester and bicomponent fibers can be blended with staple crimped polyester fibers and thermally bonded into nonwoven fabrics.

A copolyester of the present invention is prepared from at least two lower dialkyl esters of dicarboxylic acid and glycol. Dicarboxylic acids suitable include acids having the formula ##STR1## wherein R is a selected from the group consisting of ##STR2## and --(CH.sub.2).sub.n where n=2-12 These dicarboxylic acids include terephthalic acid, isophthalic acid, adipic acid and naphthalic acid.

Suitable LDE's include dimethyl terephthalate, dimethyl isophthalate, diethyl terephthalate, dipropyl terephthalate, dibutyl terephthalate, dialkyl naphthalates such as 2.6 dimethyl naphthalate, or mixtures of two or more of these.

The glycol (GLY) may comprise ethylene glycol, diethylene glycol, polyethylene glycol, blends of ethylene glycol and propane and/or butane diol, or mixtures of two or more of these.

The catalyst system used in the present invention comprises from about 20 ppm to about 150 ppm manganese (Mn); from about 50 ppm to about 350 ppm lithium (Li); optionally, but preferably from about 10 ppm to about 70 ppm cobalt (Co); and from about 200 ppm to about 400 ppm antimony (Sb), based upon the expected yield of the copolyester. This catalyst system, when used in the most effective amounts, increased the ester interchange rate, and polymerization rate, thereby reducing time to perform these functions.

Generally, the Mn and Li are added before the beginning of or during the ester interchange reactions. At the end of es4 ter interchange or any time before polycondensation, the manganese is sequestered in those situations where polymer color is important by the addition of a sequestering agent, discussed in more detail later. Also, the manganese is sequestered to allow for optimum rates for polycondensation. The Li remains in the reaction and acts as a catalyst in the polymerization. The Sb can be added before the beginning of or during polymerization as explained more fully later and is a catalyst only for the polycondensation. The Co can be added at any time subsequent to the stabilization of the manganese. In the past Co has generally been considered to be an ester interchange catalyst. In the present invention, in the presence of Li and Mn, Co is found to delay the ester interchange reaction but is effective as a polycondensation catalyst.

When stating that the manganese and lithium can be added at any time before or during ester interchange, it is intended to include the addition of the catalyst metals in the LDE, glycol, or other feedstock material streams. For example, a part, or all the ester interchange catalysts could be added into the glycol feedstream. Moreover, if the feed stream would also include other additives such as colorants, delustrants, opaquing agents, etc., the catalyst for the ester interchange process (manganese and lithium) could be a part of the additive feed stream.

When stating that cobalt and antimony can be added at any time before or during polymerization, it is intended to include the fact that antimony can be added at any time, including with the other metal catalyst of manganese and lithium, in the LDE, glycol, or other ester interchange feedstock material streams. Cobalt, on the other hand, must be added only after substantial completion of the ester interchange reaction. The addition of cobalt before substantial completion of ester interchange retards the ester interchange reaction rate. Thus, whether antimony is added with other catalysts in the feestock streams, or is added with the cobalt after the ester interchange reaction does not make any difference in the copolyester produced.

Although metals are described for the catalyst system of the present invention, the catalyst may be added in the form of many different compounds. For example, compounds such as oxides and acetates are the most preferred, while such organic and inorganic compounds as carbonates, phosphates (except manganese phosphates), halides, sulfides, amines, compounds of Group VI, etc., may also be employed. Preferably, manganese, lithium, and cobalt are added as catalysts in the form of acetates, while antimony is generally added in the form of antimony oxide. All catalyst compounds can also be used in the glycolized form of pre-reacting with glycol. When a catalyst is added in the form of a compound, the amount of compound added is determined by the amount of metal catalyst desired and the amount of metal catalyst available in the compound.

Other additives may be included in the general procedure outlined above, such as coloring agents, delustrants, opaquing agents, stabilizers, etc. These additives do not add or detract from the present invention.

In the process of the present invention, GLY and two LDEs are reacted in a batch process at temperature of between 150.degree. C. to 250.degree. C. at approximately atmospheric pressure in an ester interchange reaction to produce a monomer mixture and alcohol. The LDE and GLY are reacted in the presence of manganese and lithium and are generally reacted in a molar ratio of GLY to LDE total, for example, of about 1.8/1 to 2.5/1. Because the ester interchange reaction is reversible, it is necessary to remove the alcohol formed to assure that the reaction favors the formation of monomer bis (2-hydroxyethyl) terephthalate (when using dimethyl terephthalate and ethylene glycol).

It is theorized that lithium initiates the ester interchange reaction between the LDE and the glycol at a lower temperature than the effective temperature range of manganese. Although Applicants do not wish to be bound by this theory, it is believed that the addition of lithium and manganese in the ester interchange reaction increases the ester interchange rate thereby reducing the ester interchange time.

The reactivity of manganese as a catalyst occurs at a higher temperature than that of lithium. Manganese has a very high reactivity in both the ester interchange reaction and the polycondensation reaction. It is generally preferred to sequester the manganese such that it is inert during the polycondensation reaction, particularly when the polyester is to be employed in applications where color is important. Unsequestered manganese produces a polymer with poor color, the polymer has a broader molecular weight distribution, which is generally undesirable, and when manganese is active as a catalyst in the polycondensation stage, many undesirable by-products such as oxides, carboxyl groups, etc., are created.

The typical sequestering agent is a multivalent phosphorous. Thus, at the end of the ester interchange reaction or during the polycondensation reaction, a tri- or penta-valent phosphorous compound is usually added. Typical phosphorous compounds suitable as sequestering agent for the manganese are tributyl phosphate, polyphosphoric acid, triphenylphosphite, etc. It is believed that the phosphorous forms a complex with the manganese which is very stable and thus causes the manganese to be almost unavailable for catalytic activity during the direct esterification reaction and polycondensation reaction. On the other hand, it is believed that phosphorous does not form a stable complex with the lithium, cobalt, nor antimony. Thus, each of these compounds would be reactive whenever conditions are achieved (such as temperature) that make them a catalyst for the production of the copolyester.

It is noted that the phosphorous complex does not sequester 100 percent of the manganese. Thus in choosing the manganese level, it must be kept in mind that use of manganese yields bad polymer color, undesirable by-products, and broad molecular weight distribution for the polymer formed. With the present invention, it is desirable to provide a balance between the manganese and lithium as the ester interchange catalyst such that the reactivity, reaction speed, and side reactions are controlled in a manner to produce a quality product. Accordingly, it is important that a sufficient amount of manganese be employed that will speed up the ester interchange reaction beyond that which can occur when only lithium is being employed, but, on the other hand, employing a sufficient amount of lithium to achieve good color of polymer, to avoid side reactions, and to achieve a narrower molecular weight distribution of the polymer, which are the benefits of the lithium catalyst. Moreover, the lithium catalyst is also active, because it has not been sequestered, during the polycondensation reaction and thus aids in reducing the overall polycondensation time over a catalyst system that uses antimony alone.

After the ester interchange reaction, the antimony catalyst may be added. It is important that cobalt not be added during the ester interchange reaction because it has been determined that an ester interchange catalyst system of cobalt, manganese, and lithium actually slows down the ester interchange reaction rate and increases the ester interchange time over that of a manganese and lithium catalyst system and produces a gray polymer which may be unacceptable in those applications where color is important. Since cobalt cannot be added until after substantial completion of the ester interchange reaction, the cobalt is added after the manganese has been stabilized. Likewise, the antimony may also be added at the time of the addition of the sequestering agent or shortly thereafter.

On the other hand, the antimony catalyst may be added with the various raw material feedstock streams in the same manner as the ester interchange catalyst of manganese and lithium. Antimony is not effective during the ester interchange reaction because the temperature of these reactions are lower than the reactivity temperature of antimony for producing the copolyester. Thus, the antimony can be added any time before or during the polycondensation reaction.

The first monomer mixture is then subjected to a polycondensation reaction to yield a copolyester and glycol. The polycondensation reaction occurs at a temperature range of between 250.degree. C. to 310.degree. at a vacuum pressure of approximately 0.1 to 3 mm of mercury. The reaction is reversible and, therefore, glycol is continuously removed to force the completion of the reaction toward the production of a copolyester.

It is theorized that lithium and antimony increase the polycondensation rate and that the optional addition of between about 10 ppm and 70 ppm of cobalt, based upon the expected yield of the polyester, to the polycondensation reaction, further increases the polycondensation rate over that of lithium and antimony, and thereby reduces the polycondensation time further than that achieved with lithium and antimony.

Generally, using an amount of any one of the catalysts which is outside the ranges of the present invention is not desirable. Using an amount less than about the minimum stated for any of the catalyst generally yields a result which is not as substantial as that obtained with the present invention. Using an amount more than about the maximum stated for any of the catalyst produces undesirable effects such as poor color, unwanted side products, high cost, etc.

An autoclave batch may be prepared in which a batch of roughly 1000 grams of polymer, for example, may be produced at approximately 2.5 mole of DMT and 1.6 mole dimethyl isophthalate (DMI) and 8.0 mole of glycol. The autoclave may be first charged with the raw materials including DMT, DMI, ethylene glycol and the catalyst. Titanium dioxide may also added in the initial charge as a delustrant. The catalysts, manganese, lithium, or cobalt, may be added in the form of acetates, and antimony in the form of oxide, with the amount of catalysts added being based upon the metals themselves. The autoclave may then be heated to approximately 155.degree. C. at atmospheric pressure where initiation of the ester interchange begins.

During charging of the raw materials, the autoclave may be subjected to an inert gas (nitrogen at 4 standard cubic feet per hour) to aid in preventing oxidation. Generally, the autoclave may be agitated with a stirrer to assure homogenous commingling of the raw materials. At the start of the ester interchange reaction (approximately when the reactor contents reached 155.degree. C.), the flow of nitrogen gas may be terminated. The autoclave temperature during ester interchange would probably rise from approximately 155.degree. C. to about 240.degree. C. During the ester interchange the alcohol should be continuously removed to force the reaction toward the production of the monomer. When the ester interchange reaction is substantially complete polyvalent phosphorous (for example, tributylphosphate) may be added to sequester the manganese. During the addition and mixing of the phosphorous compound the nitrogen gas may once again turned on.

Polycondensation may be performed in the same autoclave. The autoclave should be heated to about 270.degree. C. to about 290.degree. C. thereby initiating the polycondensation reaction. The polycondensation reaction should proceed until substantial completion, during which the glycol formed should be removed.

In view of the foregoing, it is suggested that a suitable copolyetster of DMT and DMI may be obtained by reacting the following reactants:

It is suggested the DMT, EG and DMI and the manganese and lithium catalysts TiO.sub.2 be fed to an autoclave for ester interchange reaction. The temperature of the autoclave should be raised above 240.degree. C. and continuously refluxed until completion of the ester interchange. Then the PPA and cobalt and antimony catalysts are added. Vacuum let down should be conducted followed by temperature increase of the reacted monomers to 282.degree. C. for polymerization. This process is run until completion.

Thus, it is apparent that there has been provided, in accordance with the invention, a catalyst system in combination with a method of preparing copolyester from two lower dialkyl esters of a dicarboxylic acid and glycol, using the catalyst system that fully satisfies the objects, aims and advantages as set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the sphere an scope of the invention.