Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2002-04-26
2003-11-04
Shah, Mukund J. (Department: 1624)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C560S089000, C560S091000, C562S414000
Reexamination Certificate
active
06642407
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the production, purification and polymerization of aromatic dicarboxylic acids for use in the preparation of polyesters. In particular, the invention relates to processes for producing and purifying crude terephthalic acid and 2,6-naphthalenedicarboxylic acid and for using the purified acids in the production of polyethylene terephthalate (PET) and polyethylene naphthalate (PEN).
2. Description of the Prior Art
Polyesters are polymers typically prepared by polycondensation reactions starting from polycarboxylic acids and polyols. The polyesters of greatest commercial interest are those based on the reaction products of terephthalic acid and 2,6-naphthalenedicarboxylic acid with aliphatic diols, with the preferred diol being ethylene glycol. The first commercial polyester was polyethylene terephthalate (PET). However, more recently significant attention has been focused towards polyethylene naphthalate (PEN), because fibers and films made from PEN have improved strength and thermal properties relative to, for example, fibers and films made from PET. High strength fibers made from PEN can be used to make tire cord, and films made from PEN are advantageously used to manufacture magnetic recording tape and electronic components. Also, because of its superior resistance to gas diffusion, and particularly to the diffusion of carbon dioxide, oxygen and water vapor, films made from PEN are useful for manufacturing food containers, particularly so-called “hot fill” type food containers. Polyesters made from mixtures of terephthalic acid and 2,6-naphthalenedicarboxylic acid or dimethyl-2,6-naphthalenedicarboxylate also have been found to have unique and desirable properties such as resistance to gas diffusion, making them suitable for manufacturing, for example, beverage containers or other containers for food products, including containers for beer.
Polyester resin is most often presently prepared by forming a slurry of the purified aromatic dicarboxylic acid, or the dimethylester of the aromatic dicarboxylic acid, and ethylene glycol, in the presence of an esterification catalyst such antimony, and subjecting the mixture to successively higher temperature and lower pressures to drive out the condensation products; and then, in the presence of a polyesterification catalyst the excess ethylene glycol is removed under reduced pressure to bring the molecular weight to the desired range. The current polyester production process involves at least three steps. In the first step, esterification of the acid with excess glycol (or transesterification if the methyl esters are used), the bulk of the water or methanol is removed. The diglycol ester then passes to the second, prepolymerization step to form intermediate molecular weight oligomers before passing to the third, melt polyesterification step operated at low pressure and high temperature. For some applications requiring higher melt viscosity a further solid-state polymerization is practiced.
As will be discussed in more detail below, current processes for the preparation of terephthalic acid and 2,6-naphthalenedicarboxylic acid involve catalytic oxidation of p-xylene or 2,6-dimethylnaphthalene and provide a crude oxidation product which contains, as major impurities, mono-carboxylic acids, tricarboxylic acids, such as trimellitic acid, and aldehydes produced as oxidation by-products, together with residues, such as cobalt, manganese and bromine, derived from the oxidation catalyst. However, it is well known that, when used as starting materials for the manufacture of polyester fibers and films, dicarboxylic acids must achieve a high degree of purity, since the presence of contaminants, even in minute amounts, can have deleterious effects upon the quality of the resulting resin. For instance, in the case of terephthalic acid, monocarboxylic acid oxidation intermediates, such as p-toluic acid and/or 4-carboxybenzaldehyde, may react with ethylene glycol when present in the polycondensation reaction mixture and therefore act as chain stoppers, with the consequence that the melting point and strength of the resulting polyester may be substantially and undesirably lowered. Moreover, the impurities present in the crude acid can result in discoloration of the PET or PEN resin, as well as mold staining during the molding process, thereby decreasing the transparency of the molded products and hence lowering the product quality.
Thus, in order to obtain high-quality, high molecular weight PET and PEN, the crude dicarboxylic acid needs to be purified before it is used as a starting material for preparing polyesters. Several processes have been proposed for the purification of crude terephthalic acid and naphthalene dicarboxylic acid and are described below.
For example, U.S. Pat. No. 4,317,924 discloses a process for purifying crude terephthalic acid by treating a suspension of the crude acid in an aqueous solution of a water-soluble heavy metal salt with nitrogen and/or hydrogen in the presence of a supported noble metal catalyst under conditions sufficient to reduce the 4-carboxybenzaldehyde impurity without significant reduction of the terephthalic acid. The treated solution is then separated from the catalyst and the purified crystalline terephthalic acid is recovered by crystallization.
U.S. Pat. Nos. 6,100,374 and 5,872,284 describe a process of purifying crude naphthalene dicarboxylic acid comprising the steps of mixing crude naphthalene dicarboxylic acid and an ethylene glycol aqueous solution, heating the resulting mixture to esterify part of the naphthalene dicarboxylic acid and thereby give a naphthalene dicarboxylic acid ester and dissolving the naphthalene dicarboxylic acid ester in the ethylene glycol aqueous solution; then contacting impurities, which are contained in the crude naphthalene dicarboxylic acid and capable of being hydrogenated, with hydrogen in the presence of a hydrogenation catalyst to hydrogenate the impurities and dissolving the hydrogenated impurities in the ethylene glycol aqueous solution; and subsequently crystallizing the naphthalene dicarboxylic acid ester and separating the resulting crystals from the ethylene glycol aqueous solution containing the soluble impurities.
U.S. Pat. No. 4,745,211 and Japanese Patent Laid-Open Publication No. 110650/1989 describe methods of purifying crude naphthalenedicarboxylic acid comprising the steps of causing impure 2,6-naphthalenedicarboxylic acid to react with ethylene glycol in an amount of at least 2 mol based on 1 mol of the 2,6-naphthalenedicarboxylic acid in the presence of catalytic amounts of a tertiary amine and an added titanium-containing compound as an esterification catalyst to prepare bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate; crystallizing the bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate; and recovering the purified bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate by crystallization. No impurities are removed by distillation.
Another method of purifying crude naphthalene dicarboxylic acid, optionally employing diglycol esters, is disclosed in U.S. Pat. No. 6,100,374 and comprises the steps of mixing crude naphthalene dicarboxylic acid and an alcohol aqueous solution, heating the resulting mixture to esterify a part of the naphthalene dicarboxylic acid and thereby give a naphthalene dicarboxylic acid ester, dissolving the naphthalene dicarboxylic acid ester in the alcohol aqueous solution; then contacting aldehydes, which are contained in the crude naphthalene dicarboxylic acid, with a sulfite to give aldehyde adducts and dissolving the aldehyde adducts in the alcohol aqueous solution; and subsequently crystallizing the naphthalene dicarboxylic acid and the naphthalene dicarboxylic acid ester and separating the resulting crystals from the alcohol aqueous solution.
U.S. Pat. No. 5,262,560 describes a process for purifying 2,6-naphthalenedicarboxylic acid which proceeds by preparing and purifying dimethylnaphthalene dicarboxylate. In particular, the process comprises the st
Feng Xiaobing
Huber, Jr. William F.
Rao Krishna Kulai
Sellen Russell D.
Spanswick James
Exxon Mobil Chemical Patents Inc.
Kubena Linda A.
Shah Mukund J.
Tucker Zachary C.
Tyus Darryl M.
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