Polyethylene terephthalate (referred to as PET hereinafter) has been widely used in packaging materials, molded products, films and the like, and is a kind of plastic material that does not contain endocrine disrupter substances and is environment-friendly. However, PET has unsatisfactory heat resistance and is liable to deform in a high temperature environment. Therefore, the application of PET is limited. In order to enhance the heat resistance of PET, PET is usually copolymerized with copolymerizable comonomers to modify the molecular chain structure thereof so as to enhance such characteristics as glass transition temperature and melting point thereof.
The comonomers conventionally used to modify PET are usually petroleum-based. It would be desirable to utilize a copolymerizable comonomer ultimately derived from biomass. 1,4:3,6-dianhydro-D-sorbitol (referred to as isosorbide hereinafter) is a biodegradable and environment-friendly diol compound, and is readily made from renewable resources, such as sugars and starches. For example, isosorbide can be made from D-glucose by hydrogenation, followed by acid-catalyzed dehydration. When PET is modified by copolymerization with isosorbide, the processability and the biodegradability may be enhanced. However, isosorbide is a secondary alcohol, which has reactivity inferior to that of a primary alcohol. Furthermore, isosorbide has a thermal degradation temperature of about 250° C., as determined via a thermogravimetric analysis under a nitrogen atmosphere by raising the temperature up to 550° C. at a rate of 20° C./min, showing that the heat resistance achieved using isosorbide is still insufficient. In addition, as a cracking reaction may occur during the copolymerization of PET with isosorbide, which may result in undesirable yellowing of the copolyester thus obtained, the application of copolyester may be undesirably affected.
U.S. Pat. No. 6,656,577 B1 discloses a process for making a low color copolymer of ethylene glycol, isosorbide and terephthalic acid or dimethyl terephthalate by controlling critical process parameters, especially temperature and pressure, at each stage of the process.
US 2011/0251342 A1 discloses a method for preparing a copolymerized polyester resin containing isosorbide. In this method, titanium-based compounds are used as a copolymerization catalyst and phosphorous compounds are used as a stabilizer. The copolymerized polyester resin containing isosorbide prepared according to this method improves clarity.
U.S. Pat. No. 6,063,464 discloses isosorbide-containing polyester and a method for making the same. The process conditions for the method are adjusted so as to alleviate the problem of poor reactivity of isosorbide.
In the aforesaid prior art, since isosorbide is used along with glycols such as ethylene glycol or 1,4-butanediol, a great amount of water may be produced as a by-product. Removal of water from the reaction may result in dissipation of thermal energy. Furthermore, the reactivity of isosorbide is still unsatisfactory. The polyester products thus obtained have disadvantages such as low glass transition temperature and inferior color (yellowing).
It is also known in the art to make an isosorbide-containing polyester in a two-stage process, in which a first esterification product is obtained by subjecting isosorbide to an esterification reaction with a diacid compound or an ester thereof, and a second esterification product is obtained by subjecting ethylene glycol to an esterification reaction with a diacid compound or an ester thereof, followed by a condensation polymerization of the first esterification product and the second esterification product. However, a large amount of water is also produced as a by-product in such a process.
U.S. Pat. No. 6,608,167 B1 discloses bis(2-hydroxyethyl) isosorbide and polymers derived therefrom. Bis(2-hydroxyethyl) isosorbide is produced by contacting isosorbide with a stoichiometric excess of ethylene carbonate in the presence of a catalyst. While the reactivity of isosorbide is enhanced, the polymer derived from bis(2-hydroxyethyl) isosorbide still has the problems of low glass transition temperature, inferior color (yellowing), and a significant amount of water by-product.
There is a need in the art to provide a method for preparing a isosorbide-containing copolyester which may overcome the aforesaid problems of the prior art.