Because of their numerous advantages, plastics have become inescapable in the mass manufacture of objects. Indeed, because of their thermoplastic nature, it is possible to manufacture objects of any type from these plastics, at a high rate.
Certain aromatic polyesters are thermoplastic and have thermal properties which allow them to be used directly for the manufacture of materials. They comprise aliphatic diol and aromatic diacid units. Among these aromatic polyesters, mention may be made of polyethylene terephthalate (PET), which is a polyester comprising ethylene glycol and terephthalic acid units, used for example in the manufacture of containers, packagings or textile fibers. PET may be a transparent polymer and may thus be useful for manufacturing objects whose optical properties are important. It may also be opaque and white in the case where this polymer is semicrystalline, if the crystallinity and size of the crystallites are important. It is thus necessary in both cases for PET to have the least possible coloring.
According to the invention, the term “monomer units” is intended to mean units, included in the polyester, which can be obtained after polymerization of a monomer. With regard to the ethylene glycol and terephthalic acid units included in PET, they can be obtained either by esterification reaction of ethylene glycol and terephthalic acid, or by transesterification reaction of ethylene glycol and terephthalic acid ester.
The development of polyesters resulting from biological resources renewable in the short term has become an ecological and economic imperative, in the face of the exhaustion and of the increase in costs of fossil resources such as oil. One of the main concerns today in the polyester field is therefore that of providing polyesters of natural origin (biobased polyesters). This is particularly true for polyesters comprising aliphatic diol and aromatic acid units. Thus, groups such as Danone or Coca-Cola today market drink bottles made of partially biobased PET, this PET being manufactured from biobased ethylene glycol. One drawback of this PET is that it is only partially biobased, since the terephthalic acid is for its part generally derived from fossil resources. However, processes for synthesizing biobased terephthalic acid and biobased terephthalic acid ester have recently been developed, thereby allowing the manufacture of totally biobased PET. Mention may thus be made of application WO 2013/034743 A1 which describes in particular such PETs.
However, for certain applications or under certain conditions of use, these polyesters do not exhibit all the required properties. This is why glycol-modified PETs (PETg) have been developed. They are generally polyesters comprising, in addition to the ethylene glycol and terephthalic acid units, cyclohexanedimethanol (CHDM) units. The introduction of this diol into the PET allows it to adapt the properties to the intended application, for example to improve its impact resistance or its optical properties, in particular when the PETg is amorphous.
Other modified PETs have also been developed by introducing into the polyester 1,4:3,6-dianhydrohexitol units, especially isosorbide (PEIT). These modified polyesters have higher glass transition temperatures than the unmodified PETs or PETg comprising CHDM. In addition, 1,4:3,6-dianhydrohexitols have the advantage of being able to be obtained from renewable resources such as starch. These modified polyesters are especially useful for manufacturing bottles, films, thick sheets, fibers or articles requiring high optical properties. However, one problem with these PEITs is that they may have generally high coloring, generally higher than that of PETg or PETs, even when the amounts of isosorbide used in the manufacture of polyester are very low.
In order to solve this problem of high coloring, a process for preparing PEIT by melt polymerization has already been described in patent application US 2006/0 173 154 A1. This process comprises a first step of esterification and a second step of polycondensation, in which a primary antioxidant is used during the esterification step and a secondary antioxidant is used during the polycondensation step. In the examples, a catalytic system comprising catalysts based on germanium and based on cobalt are used.
Patent applications WO 2013/183 873 and WO 2013/183 874 disclose processes for preparing polyesters, comprising a step of esterification of monomers comprising terephthalic acid, CHDM, isosorbide and an additional diol compound in the presence of an esterification catalyst which is a zinc-based compound. This catalyst makes it possible to improve the polymerization reaction kinetics and/or to increase the viscosity of the polymer obtained from this process. In the processes that are illustrated in these two applications, a germanium-based catalyst is introduced during the polycondensation step.
The Applicant has found, by performing studies on polymerization catalysts for the manufacture of polyesters containing 1,4:3,6-dianhydrohexitol units (see the examples below), that the polyesters obtained from these processes are not entirely satisfactory, especially in terms of coloring. This coloring may be very yellow, as is the case when a germanium-based polycondensation catalyst is used exclusively, or gray when a catalytic system comprising catalysts based on germanium and based on cobalt are used. There thus remains a need to find novel processes for manufacturing polyesters containing 1,4:3,6-dianhydrohexitol units, whose coloring is improved.