Search

EP-4112696-B1 - POLYESTER RESIN CONTAINING FEWER FOREIGN SUBSTANCES, AND COATING COMPOSITION OR ADHESIVE COMPOSITION USING SAME

EP4112696B1EP 4112696 B1EP4112696 B1EP 4112696B1EP-4112696-B1

Inventors

  • MIURA, WATARU
  • NOMURA, Yumi

Dates

Publication Date
20260506
Application Date
20210224

Claims (3)

  1. A method for producing a polyester resin containing 2 to 10 ppm by mass of titanium in terms of metal and 50 to 100 ppm by mass of zinc in terms of metal, as measured in accordance with the method specified in the description, respectively, in which the number of foreign bodies contained in the polyester resin is 15 or less per 5 g of the polyester resin, as measured in accordance with the method specified in the description, wherein the foreign bodies contain 100% by mass of a zinc salt of a polycarboxylic acid and the major axis of the foreign bodies is 5 µm or more and 80 µm or less, the method comprising a step of producing a prepolymer in the presence of a titanium catalyst and in the absence of a zinc catalyst, and a step of performing polycondensation in the presence of the titanium catalyst and the zinc catalyst after the production of the prepolymer.
  2. The production method according to claim 1, wherein the reduced viscosity in the polyester resin is 0.10 dl/g or more and the acid value in the polyester resin is 50 eq/t or more, as measured in accordance with the method specified in the description, respectively.
  3. The production method according to claim 1 or 2, wherein the content of trimellitic anhydride in the polyester resin is 1,000 ppm by mass or less.

Description

Technical Field of the Invention The present invention relates to a method for producing a polyester resin containing fewer foreign bodies. More specifically, the present invention relates to a method for producing a polyester resin suitable for a dry lamination adhesive for a metal substrate, suitable for a coating material for a precoated metal, and suitable for a can coating material. Background Art Conventionally, a titanium compound or a zinc compound has been used as a catalyst for industrially producing a polyester resin from a dicarboxylic acid or an ester-forming derivative thereof and a diol component (for example, Patent Documents 1 and 2). However, such a conventional technique has such a problem that when the amount of the catalyst is increased in order to increase the polymerization activity, there are caused gelation during polymerization and deterioration of thermal stability, storage stability, and color tone of the resulting polyester resin. Patent Document 3 discloses that by limiting the addition amounts of titanium and zinc catalysts, gelation can be suppressed in a reaction step, polycondensation can be efficiently performed without scattering of low molecular weight substances during the polycondensation, and a polyester excellent in thermal stability (suppression of black foreign bodies and gelation) can be produced. Patent document 4 discloses (Example 5) a method for preparing a polyester resin wherein (i) a PBAT prepolymer is prepared in the presence of a Ti catalyst and in the absence of a Zn catalyst, followed by (ii) the polycondensation of the prepolymer and L-lactide in the presence of a Ti catalyst and a Zn catalyst. Prior Art Documents Patent Documents Patent Document 1: JP 2003-268087 APatent Document 2: JP 2000-159872 APatent Document 3: JP 2009-001656 APatent Document 4: CN 107 163 232 A Disclosure of the Invention Problem that the Invention is to Solve However, in Patent Document 3, a zinc salt, which is a foreign body generated by a reaction of a zinc catalyst with a carboxylic acid as a raw material, is not taken into consideration, and there is a problem that a zinc-derived foreign bodies are precipitated as an insoluble substance when the polyester resin is dissolved in a solvent or dispersed in water. More specifically, when foreign bodies are precipitated, there is a problem that defects derived from the foreign bodies occur during varnish coating. For example, poor appearance of a coating film occurs in precoated metal and can coating applications, and poor adhesion to a substrate occurs in dry lamination adhesive applications for metal substrates. The present invention has been made in view of these problems of the prior art. That is, an object of the present invention is to provide a method for producing a polyester resin having good producibility and color tone, containing fewer foreign bodies, and showing good adhesion to a metal substrate and curability. The present inventors have accomplished the present invention as a result of their extensive studies for solving the above problems. Thus, the present invention which is set out in the appended claims consists of the following constitution. A method for producing a polyester resin containing 2 to 10 ppm by mass of titanium in terms of metal and 50 to 100 ppm by mass of zinc in terms of metal, as measured in accordance with the method specified herein, respectively, in which the number of foreign bodies contained in the polyester resin is 15 or less per 5 g of the polyester resin, as measured in accordance with the method specified herein, wherein the foreign bodies contain 100% by mass of a zinc salt of a polycarboxylic acid and the major axis of the foreign bodies is 5 µm or more and 80 µm or less, the method comprising a step of producing a prepolymer in the presence of a titanium catalyst and in the absence of a zinc catalyst, and a step of performing polycondensation in the presence of the titanium catalyst and the zinc catalyst after the production of the prepolymer. The reduced viscosity of the polyester resin is preferably 0.10 dl/g or more and the acid value is preferably 50 eq/t or more, as measured in accordance with the method specified herein, respectively. In addition, the content of trimellitic anhydride in the polyester resin is preferably 1,000 ppm by mass or less. Advantages of the Invention According to the present invention, it is possible to produce a polyester resin having good producibility and color tone, containing fewer foreign bodies, and showing excellent adhesion to metal and curability. Suitably, the polyester resin to be produced by the production method of the present invention may be used for providing a polyester resin composition containing the polyester resin and an isocyanate curing agent or a phenol curing agent. Brief Description of the Drawing FIG. 1 is a scanning electron microscope (SEM) photograph of foreign bodies contained in a polyester resin. Best Mode for Carryi