Ethylene-2,5-furandicarboxylate polyester comprising further dicarboxylic acid derived units

Abstract

Polyester compositions comprising ethylene-2,5-furandicarboxylate polyester having a weight average molecular weight of at least 45 kg / mol and comprising units derived from benzene dicarboxylic acid or its ester which compositions comprise at most 0.80 % by weight of cyclic dimers, and a process for preparing such polyester compositions.

Description

[0001] FUR333b

[0002] ETHYLENE-2, 5-FURANDICARBOXYLATE POLYESTER COMPRISING FURTHER DICARBOXYLIC ACID DERIVED UNITS

[0003] Technical Field

[0004] The present invention relates to a polyester composition comprising ethylene-2, 5- furandicarboxylate polyester and further dicarboxylic acid derived units and to a process for preparing such polyester compositions. Background

[0005] 2,5-Furandicarboxylic acid (FDCA) is known in the art to be a highly promising building block for replacing petroleum-based monomers in the production of high performance polymers such as the polyester poly(ethylene-2,5-furanoate) (PEF). In recent years FDCA and PEF have attracted a lot of attention. PEF can be bio-based and recyclable with superior performance properties compared to today's widely used plastics. These materials could significantly reduce the dependence on petroleum-based polymers and plastics while at the same time allowing for a more sustainable management of global resources. Comprehensive research was conducted to arrive at a technology for preparing FDCA and PEF in a commercially viable way.

[0006] FDCA is typically obtained by oxidation of molecules having furan moieties, e.g. 5- hydroxymethylfurfural (5-HMF) and the corresponding 5-HMF esters or 5-HMF ethers that are typically obtained from plant-based sugars, e.g. by sugar dehydration. A broad variety of oxidation processes is known from the prior art using enzymes or metal containing catalysts such as described in WO2010 / 132740 and WO2011 / 043660.

[0007] While a lot of research was directed at the efficient production of FDCA monomer in the early days of the technology, researchers soon realized that arriving at efficient processes for preparing high-performance polyesters from FDCA was at least as challenging. FDCA is oftentimes considered a structural and functional analogue to terephthalic acid (TA) which is used in the production of the widely used polyester polyethylene terephthalate (PET). However, techniques known from the PET industry frequently were found not to be suitable to produce high-performance polyesters from FDCA. Processes for preparing polyesters from FDCA have been described in EP 3116932, EP 3116934, WO 2013 / 1209989 and US 2010 / 0174044. Several of the above-mentioned difficulties seem to be due to the different chemical properties of the monomer FDCA.

[0008] Polyesters derived from FDCA in combination with other compounds also were known.

[0009] CN 106866948 describes amorphous random copolyesters for use in 3D printing. The copolyester of Example 2 is prepared from a mixture of 1.0 mol of terephthalic acid, 1.0 mole of furan dicarboxylic acid, 1.4 mole of ethylene glycol and 1.2 mole of 1 ,4-butanediol.

[0010] US12,018,119 describes copolyesters having a slower crystallization rate, higher gas barrier to CO2 and 02 and increased biobased content. The copolyesters comprise a terephthalic acid derived compound, a 2,4-furandicarboxylic acid derived compound and an alkanediol residue having from 2 to 22 carbon atoms. Example 4 describes copolyesters produced with various amounts of 2,4-furandicarboxylic acid and terephthalic acid in an excess of ethylene glycol.

[0011] KR101492129 describes copolyesters for heat adhesive binders having a glass transition temperature similar to that of general polyester and a lower melting point. The copolyesters contain terephthalic acid, isophthalic acid and 2,5-furandicarboxylic acid derived units as dicarboxylic acid component and ethylene glycol derived units as diol component. Examples 2-7 use dimethyl terephthalate, dimethyl-2,5-furandicarboxylate and dimethyl isophthalate as dicarboxylic acid component and ethylene glycol and diethylene glycol as diol component.

[0012] With the further investigation into the manufacture of PEF and its applications, it was found that besides polymers also oligomers are formed during polycondensation. These oligomers can be linear and cyclic. The total amount of oligomer present after polycondensation can be as high as 2.5 % by weight (%wt) of which the majority tends to be cyclic. An oligomer which was found to be especially disadvantageous is the cyclic dimer of ethylene-2, 5-furandicarboxylate according to formula (1).

[0013] It was found that oligomers and especially the cyclic dimers according to formula (1) can lead to problems in subsequent processing as these may evaporate and crystallize during subsequent solid state polymerization as white flakes. In melt processing of such pellets, these white flakes can affect the optical appearance and mechanical properties of the final product. Furthermore, these cyclic dimers were found to accumulate on the mould surface when the polyester is subsequently used in injection moulding. This is referred to as plate-out. If plate-out has occurred, the mould needs to be cleaned to prevent contamination of articles which are moulded subsequently.

[0014] It was already noted in J P-A-2022155579 that cyclic dimer may precipitate or sublimate from poly(ethylene-2, 5-furandicarboxylate) resin during melt moulding. This can lead to contamination of equipment and adhesion to moulded products. It was taught that the cyclic dimer content can be reduced by heating the poly(ethylene-2,5-furandicarboxylate) pellets which is especially suitable for removing dimer from poly(ethylene-2,5-furandicarboxylate) which has been subjected to solid state polymerization due to its increased molecular weight. The exemplified polyester resins consisted of poly(ethylene-2,5-furandicarboxylate) and contained 1.2 mole % (Example 1) or 1.0 mole % (Example 3) dimer after polycondensation and 0.7 mole % (Example 1) and 0.6 mole % (Example 3) dimer after solid state polymerization.

[0015] An objective of the present invention was to prepare polyester comprising ethylene-2, 5- furandicarboxylate units containing a reduced amount of cyclic dimers without requiring an additional process step. It is especially advantageous if the polyester obtained has an attractive weight average molecular weight.

[0016] A further objective was to provide a process for preparing polyester based on 2,5- furandicarboxylic acid and ethylene glycol containing a reduced amount of cyclic dimers according to formula (1) and a limited amount of metal polycondensation catalyst. Polyester containing a reduced amount of metal is attractive from a processing and from an environmental point of view.

[0017] Disclosure of the invention

[0018] We now have surprisingly found that the above-mentioned objectives can be attained by additionally incorporating benzene dicarboxylic acid or its ester in the poly(ethylene-2,5- furandicarboxylate).

[0019] The present invention relates to polyester compositions comprising polyester comprising ethylene-2, 5-furandicarboxylate units and having a weight average molecular weight of at least 45 kg / mol which polyester additionally comprises units derived from benzene dicarboxylic acid or an ester thereof wherein the polyester composition comprises at most 0.80 % by weight (%wt) of cyclic dimers according to formula (1) based on total weight amount of polyester.

[0020] The present invention relates to a process for preparing such polyester, which process comprises a) providing or preparing a starting composition comprising (i) 2,5- furandicarboxylic acid and ethylene glycol or (ii) dimethyl-2, 5-furandicarboxylate and ethylene glycol, b) subjecting the starting composition to elevated temperature to produce an intermediate ester composition, and c) contacting the intermediate ester composition with a polycondensation catalyst at polycondensation conditions to produce a polyester comprising ethylene-2, 5-furandicarboxylate units, wherein benzene dicarboxylic acid or an ester thereof is added to the starting composition and / or to the intermediate ester composition.

[0021] Modes for carrying out the invention

[0022] Hereinafter, the subject-matter of the invention is discussed in more detail wherein preferred embodiments of the invention are disclosed. It is particularly preferred to combine two or more preferred embodiments to obtain an especially preferred embodiment.

[0023] In the current process, a benzene dicarboxylic acid or its ester is added to the starting composition and / or the intermediate ester composition. The benzene dicarboxylic acid or its ester preferably is part of the starting composition. Generally, it is preferred to combine the benzene dicarboxylic acid or its ester with the (i) 2,5-furandicarboxylic acid and ethylene glycol or (ii) dimethyl-2,5-furandicarboxylate and ethylene glycol. Preferably, the benzene dicarboxylic acid or its ester is added before at most 50 %, more preferably at most 20 %, more preferably at most 10%, by mole of the 2,5-furandicarboxylic acid or the methyl-2,5- furandicarboxylate have been converted.

[0024] The benzene dicarboxylic acid or ester thereof preferably is terephthalic acid or ester thereof. More preferably, the benzene dicarboxylic acid or ester thereof is terephthalic acid or dimethyl terephthalate. Most preferably, terephthalic acid is added to the process of the present invention.

[0025] The polyester present in the polyester composition contains units derived from benzene dicarboxylic acid or an ester thereof. These units are present besides the ethylene- 2,5-furandicarboxylate units which are produced by esterifying 2,5-furandicarboxylic acid and ethylene glycol or trans-esterifying dimethyl-2,5-furandicarboxylate with ethylene glycol.

[0026] It is possible that further dicarboxylic acids and / or diols are present in the starting composition and the intermediate ester composition. However, it is preferred that no further dicarboxylic acids and diols are added to or are present in either the starting composition or the intermediate ester composition.

[0027] Preferably, less than 10 %wt, preferably less than 5 %wt, of further dicarboxylic acids and diols are present in the starting composition (i) other than ethylene glycol, 2,5- furandicarboxylic acid and optionally the benzene dicarboxylic acid or its ester. Preferably, the only reactive monomers present in starting composition (i) are ethylene glycol, 2,5- furandicarboxylic acid and optionally the benzene dicarboxylic acid or its ester. The intermediate composition preferably is derived from such starting composition.

[0028] Preferably, less than 10 %wt preferably less than 5 %wt, of further dicarboxylic acids and diols are present in the starting composition (ii) other than dimethyl-2,5- furandicarboxylate and ethylene glycol and optionally the benzene dicarboxylic acid or its ester. Preferably, the only reactive monomers present in starting composition (ii) are dimethyl-2,5-furandicarboxylate and ethylene glycol and optionally the benzene dicarboxylic acid or its ester. The intermediate composition preferably is derived from such starting composition.

[0029] Besides these reactive monomers, the starting composition can contain further compounds such as polycondensation catalyst, optionally with solvent, and DEG suppressant, optionally with solvent, as discussed below. As mentioned above, the polyester preferably is obtained from starting composition (i) or (ii). This makes that the polyester preferably is obtained from compounds selected from the group consisting of ethylene glycol, diethylene glycol, 2,5-furandicarboxylic acid, dimethyl-2,5-furandicarboxylate and benzene dicarboxylic acid or its ester. Diethylene glycol is included as it tends to be formed from ethylene glycol as discussed below. The polyesters preferably contain at most 5 %wt, more specifically at most 3 %wt, of units derived from other compounds, based on weight amount of polyester. Most preferably, the polyester present in the composition consists of units derived from the group consisting of ethylene glycol, diethylene glycol, 2,5-furandicarboxylic acid, dimethyl-2,5-furandicarboxylate and benzene dicarboxylic acid or its ester.

[0030] The benzene dicarboxylic acid or its ester preferably is present in the starting composition or the intermediate ester composition, as applicable, in an amount of at least 1 % by mole based on the total molar amount of 2,5-furandicarboxylic acid and mono- and diesters thereof, preferably in an amount of at least 2 % by mole, more preferably at least 3 % by mole. The amount preferably is at most 15 % by mole, preferably at most 10 % by mole, more preferably at most 8 % by mole. A preferred specific amount of benzene dicarboxylic acid or its ester is of from 2 to 10 % by mole based on total molar amount of 2,5- furandicarboxylic acid and mono- and di-esters thereof.

[0031] The polyester present in the polyester composition preferably contains an amount of at least 1 % by mole of units derived from benzene dicarboxylic acid or its ester based on the molar amount of 2,5-furandicarboxylate units, preferably in an amount of at least 2 % by mole, more preferably at least 3 % by mole. The amount preferably is at most 15 % by mole, preferably at most 10 % by mole, more preferably at most 8 % by mole. The polyester preferably comprises of from 2 to 10 % by mole of units derived from the benzene dicarboxylic acid or its ester, based on molar amount of 2,5-furandicarboxylate units.

[0032] Processes of preparing polyesters of diacids or diesters typically comprise at least two distinct steps, i.e. the esterification or transesterification and the polycondensation, wherein some processes also include additional intermediate steps like pre-polycondensation and / or subsequent processing steps like granulation, crystallization and / or solid state polymerization of the obtained resin.

[0033] The composition obtained in step b) is subsequently subjected to polycondensation conditions at elevated temperature and reduced pressure in order to obtain the final polyester.

[0034] Optionally, a pre-polycondensation step may be used between step b) and the polycondensation step. The pre-polycondensation step is typically conducted at a pressure lower than applied in step b) and can be used to remove the most volatile components, such as free diol and other low molecular weight compounds, before reducing the pressure even further to begin the polycondensation process.

[0035] The starting composition for the present process can be produced or provided, e.g. bought from a separate supplier. The starting composition preferably comprises 2,5- furandicarboxylic acid, i.e. free diacid, and ethylene glycol. Unfortunately, processes that start from the diacid FDCA are more prone to decarboxylation. Decarboxylation of FDCA yields 2-furancarboxylic acid which functions as a chain terminator in polycondensation and limits the maximum obtainable molecular weight of the polyester. Therefore, it is preferred to limit the concentration of 2-furancarboxylic acid in the starting composition. The starting composition preferably comprises 500 ppm or less of 2-furancarboxylic acid, preferably 400 ppm or less, more preferably 300 ppm or less, by weight with respect to the weight of the starting composition.

[0036] It is known in the art that diols, in particular aliphatic diols, can undergo ether formation with other diol molecules thereby preparing higher molecular weight diols with an ether functionality. A prominent example is the formation of diethylene glycol (DEG) from mono ethylene glycol. These ether compounds can be incorporated into the final polyester and are known to have a detrimental effect on the properties of the resulting polyester. Suppressants can suppress ether formation between the diol compounds during the esterification step, which compounds are oftentimes labelled DEG suppressants indicating that they are directed at reducing the formation of DEG. The suppressant used to suppress ether formation will be referred to herein as a DEG suppressant, regardless of the actual ether being suppressed. Various DEG suppressants can be used in the present process. Well known DEG suppressants are ammonium compounds, in particular tetraethyl ammonium hydroxide, alkali phosphates, in particular sodium dihydrogen phosphate and disodium hydrogen phosphate as disclosed e.g. in WO2015 / 137805, WG2022 / 043500 and WG2022 / 2043501. Suitable DEG suppressants were found to be ammonium compounds, in particular tetraethyl ammonium hydroxide. Especially preferred DEG suppressants for use with germanium containing catalysts were found to be the group consisting of amines and lithium hydroxide. A surprising reduction of decarboxylation was in particular found for primary amines, secondary amines, tertiary amines and lithium hydroxide, wherein 3-aminocrotonic acid ester with butanediol (ACAEBD), Et2NEtOH and Me2N Dodec were identified as particular suitable amines. Most preferably, the DEG suppressant is tetraethylammonium hydroxide. The DEG suppressant can be used with solvent.

[0037] To achieve the desired effect, DEG suppressant preferably is present during step b) and will therefore be also comprised in the intermediate ester composition, either as the suppressant and / or its reaction products and / or its decomposition products. For some embodiments, it might be expedient to remove suppressant and / or its reaction products and / or its decomposition products after step b) but prior to polycondensation. However, suppressant and / or its reaction products and / or its decomposition products can be present during the polycondensation as well.

[0038] The starting composition is subjected in step b) to elevated temperature to produce an intermediate ester composition.

[0039] The esterification of an acid compound with a diol compound is a reaction that is well known to the skilled person and is conducted at elevated temperatures. Based on the molar ratio of the starting materials used in the starting composition, the chemical constitution of the intermediate ester composition can vary. However, for the molar ratios typically employed, the intermediate ester compositions tend to comprise the mono ester of the diacids and the ethylene glycol, the diester of the diacids and the ethylene glycol, low molecular oligomers of the respective compounds as well as potentially unreacted FDCA and ethylene glycol and optionally benzene dicarboxylic acid and esters thereof.

[0040] Transesterification of dimethyl-2,5-furandicarboxylate with a diol compound also is well known to the skilled person and also is conducted at elevated temperatures. Based on the molar ratio of the starting materials used in the starting composition, the chemical constitution of the intermediate ester composition can vary. The intermediate ester compositions tend to comprise a mixture of methyl-2-hydroxyethyl-2,5-furandicarboxylate and bis(2-hydroxyethyl)-2,5-furandicarboxylate, low molecular oligomers of the respective compounds and potentially unreacted ethylene glycol and dimethyl-2,5-furandicarboxylate besides methanol formed by the transesterification and optionally benzene dicarboxylic acid and esters thereof.

[0041] Although the polycondensation catalyst is for use in the polycondenation of step c), it can be preferred to add it to the starting composition. This is possible with a germanium containing polycondensation catalyst. Other polycondensation catalysts can deactivate if present during esterification. The polycondensation catalyst can be added as a solution. If the solvent used with the polycondensation catalyst is a diol, the diol can be incorporated into the polyester. It will be clear to the person skilled in the art how to make use or counteract the effect of the solvent whether or not it will be incorporated in the polyester to contribute to the present invention.

[0042] Especially suitable polycondensation catalysts have been described for example in W02022 / 043500 and W02022 / 2043501. Preferably, the polycondensation catalyst comprises germanium. Most preferably, the polycondensation catalyst is a germanium containing solution prepared by dissolving germanium oxide in water.

[0043] The intermediate ester composition obtained in step b) is contacted with a catalyst at polycondensation conditions, wherein other intermediate steps can be conducted in between step b) and step c), e.g. a pre-polycondensation step.

[0044] The polycondensation is used for preparing a polyester comprising 2,5- furandicarboxylate units by forming additional ester moieties between the compounds of the intermediate ester composition by means of esterification and transesterification, wherein e.g. water and / or diol are released in the condensation process, and are typically removed from the reaction due to the elevated temperatures and reduced pressures used during polycondensation.

[0045] Both the reaction of step b) and the polycondensation may be conducted in one or more steps and could suitably be operated as either batch, semi-continuous or continuous processes. It is preferred that step b) is conducted until the reaction has progressed to the point where 80 % or more, preferably 85 % or more, most preferably 90 % or more, of the acid groups have been converted to ester moieties before the polycondensation is started if the starting composition comprised FDCA or until the reaction has progressed to the point where 80 % or more, preferably 85 % or more, most preferably 90 % or more, of the methyl groups have been replaced by 2-hydroxyethyl before the polycondensation is started if the starting composition comprised dimethyl-2,5-furandicarboxylate.

[0046] The molar ratio of the amount of diols to the FDCA or its ester can influence the molecular weight obtainable, and also the velocity of the increase of molecular weight during a subsequent solid state polymerisation.

[0047] A preferred molar ratio of the aliphatic diol to 2,5-furandicarboxylic acid or ester of the starting composition is in the range of 1.01 to 1.80, preferably 1.05 to 1.70, more preferably 1.07 to 1.60, most preferably 1.10 to 1.30. If the starting composition contains benzene dicarboxylic acid, the molar ratio is with respect to the total amount of terephthalic acid and FDCA and esters thereof.

[0048] It was found that step b) preferably is conducted at a temperature in the range of 180 to 260 °C, preferably 185 to 240 °C, more preferably 190 to 230 °C. The polycondensation preferably is conducted at a temperature in the range of 240 to 300 °C, preferably 250 to 290 °C, more preferably 255 to 285 °C. Preferably, step b) is conducted at a pressure in the range of 40 to 400 kPa, preferably 50 to 150 kPa, more preferably 60 to 110 kPa. Preferably, the polycondensation is conducted at reduced pressure in the range of 0.05 to 100 kPa, preferably 0.05 to 10 kPa, more preferably 0.1 to 1 kPa.

[0049] While the actual reaction time depends on the employed starting materials and their amounts, step b) is typically conducted for a time t in the range of 30 to 480 min, preferably 60 to 360 min, more preferably 120 to 300 min, most preferably 180 to 260 min. This is the time period after the temperature has been increased to the desired temperature. The polycondensation is typically conducted for a time t in the range of 10 to 260 min, preferably 30 to 190 min. The polycondensation preferably is conducted for at least 50 minutes, more preferably at least 60 min. The polycondensation preferably is conducted for at most 180 min, more preferably at most 150 min, more preferably at most 120 min. The polycondensation time is the time during which the intermediate ester composition is subjected in the presence of a polycondensation catalyst to a temperature of at least 240 °C, more preferably at least 250 °C, more preferably at least 260 °C.

[0050] The concentration of germanium in step c), calculated as the metal per se, preferably is in the range of 10 to 1000 ppm, preferably 30 to 500 ppm, more preferably 50 to 300 ppm. Preferably, the amount of germanium is at most 250 ppm, more preferably at most 200 ppm, more preferably at most 150 ppm. The amount of germanium is calculated as weight amount of metal independent of the actual form of the germanium. All amounts are with respect to the theoretical maximum weight of the polymer obtainable from the respective starting composition.

[0051] The concentration of DEG suppressant in the starting composition preferably is in the range of 5 to 1300 ppm, preferably 20 to 700 ppm, more preferably 30 to 450 ppm, by weight with respect of the weight of the starting composition.

[0052] The combined concentration of ammonium compounds and sodium in the starting composition preferably is in the range of 0 to 50 ppm, preferably 0 to 20 ppm, more preferably less than 5 ppm by weight with respect of the weight of the starting composition.

[0053] A.T Jackson and D.F. Robertson have published an1H-NMR method for end group determination in PET in “Molecular Characterization and Analysis of Polymers” (J.M. Chalmers en R.J. Meier (eds.), Vol. 53 of “Comprehensive Analytical Chemistry”, by B. Barcelo (ed.), (2008) Elsevier, on pages 183-193. A similar method can be carried out for polyesters that comprise 2,5-furandicarboxylate units. Herein, the measurement of the end groups can be performed at room temperature without an undue risk of precipitation of the polyester from the solution. This1H-NMR method using deuterated 1 ,1 ,2,2-tetrachloroethane (TCE-d2) is suitable to determine various components incorporated in the polyester. Peak assignments are set using the TCE peak at a chemical shift of 6.04 ppm. The furan peak at a chemical shift of 7.28 ppm is integrated and the integral is set at 2.000 representing the two protons on the furan ring.

[0054] The content of cyclic dimers according to formula (1) is determined for the present invention by the above1H-NMR method from the integral of the respective shift of the protons of the furan rings at 7.39 ppm representing four protons of the cyclic dimer according to formula (1).

[0055] The amount of benzene dicarboxylic acid or its ester incorporated in the polyester comprising ethylene-2, 5-furandicarboxylate units is determined by the above1H-NMR method from the integral of the shift of the protons of the benzene dicarboxylic acid or ester incorporated in the polyesters. This shift is at 8.06-8.19 ppm representing the four protons on the benzene ring.

[0056] The polyester comprising ethylene-2, 5-furandicarboxylate units after polycondensation preferably has a weight average molecular weight of at least 45 kg / mol, more preferably at least 45.5 kg / mol. The weight average molecular weight after polycondensation generally will be at most 100 kg / mol, more specifically at most 90 kg / mol. The polyester comprising ethylene-2, 5-furandicarboxylate units after polycondensation preferably has a weight average molecular weight of from 45 to 70 kg / mol. The weight average molecular weight is determined as described in the below examples.

[0057] While the polyester obtained after polycondensation can be used directly for specific applications, it is in some cases beneficial to add further processing steps. These steps can comprise a step of crystallizing the polyester for obtaining a crystallized polyester and subjecting the crystallized polyester to a solid-state polymerization for increasing the molecular weight. Therefore, it can be preferred that the process further comprises the steps: d) crystallizing the polyester obtained in step c) to obtain a crystallized or semi-crystallized polyester comprising 2, 5-furandicarboxylate units, and e) subjecting the polyester comprising 2, 5-furandicarboxylate units produced in step d) to a solid state polymerization for increasing the molecular weight.

[0058] Both steps are known to the skilled person and the skilled person is typically able to adjust the process parameters of these steps according to its needs. However, specific process parameters were found to be particularly beneficial for the process of the present invention.

[0059] The crystallization preferably is conducted for a time t in the range of 0.5 to 48 h, preferably 1 to 6 h, wherein step d) is conducted directly after step c) without cooling the polyester comprising 2, 5-furandicarboxylate units below 50 °C. The crystallization preferably is conducted at or near ambient pressure or, less preferred, at reduced pressure of less than 100 kPa or less than 10 kPa. The crystallization preferably is conducted at an elevated temperature in the range of 100 to 200 °C, preferably 120 to 180 °C, more preferably 140 to 160 °C.

[0060] The optimal time for the crystallization can be chosen based on the crystallization enthalpy of the polyester. When the polyester obtained in step c) is heated to yield a semicrystallized or crystallized polyester, the crystallinity changes significantly. This may be determined by means of Differential Scanning Calorimetry (DSC). The crystallinity is often measured as the enthalpy for melting the semi-crystalline polymer when heating at a suitable rate. The crystallinity is expressed in the unit J / g, and is taken as the net enthalpy of the melting peak (endotherm) after correcting for any crystallization (exotherm) which occurs on the upheat. A process according to the invention is preferred, wherein the crystallization is conducted for a time t so that the net enthalpy of the polyester comprising 2,5- furandicarboxylate is larger than 20 J / g, preferably larger than 25 J / g, more preferably larger than 30 J / g as measured via DSC using a heating rate of 10 dC / min. It is preferred that the crystallized or semi-crystallized polyester obtained in step d) is granulated to ensure that the polyester subjected to step e) has a degree of granulation in the range of 20 to 180 pellets per g, preferably 40 to 140 pellets per g.

[0061] Solid-state polymerization can lead to a significant increase in the number average and weight average molecular weight of the obtained polyester besides further reducing the content of dimer according to formula (1). Solid state polymerization preferably is conducted at an elevated temperature in the range of Tm - 80 °C to Tm - 20 °C, preferably Tm - 60 °C to Tm - 25 °C, more preferably Tm - 60 °C to Tm - 30 °C, wherein Tm is the melting point of the polyester comprising 2,5-furandicarboxylate units in °C, wherein the solid state polymerization is preferably conducted at an elevated temperature in the range of 160 to 240 °C, more preferably 170 to 220 °C, most preferably 180 to 210 °C. The solid state polymerization preferably is conducted under reduced pressure or under inert gas atmosphere, preferably nitrogen, helium, neon or argon atmosphere. An inert gas atmosphere is preferred.

[0062] It has been found that the optical properties of the polyester can be enhanced if not only the solid-state polymerization is conducted in an atmosphere with reduced oxygen concentration, preferably under an inert gas atmosphere, but also the crystallization step itself. An atmosphere with reduced oxygen concentration means a reduction compared to air at ambient pressure, preferably an oxygen partial pressure of less than 1 kPa, more preferably less than 0.1 kPa, most preferably less than 0.01 kPa, even more preferably less than 0.001 kPa.

[0063] The polyester composition preferably comprises at most 250 ppm of germanium, calculated as metal on amount of polyester, more preferably at most 200 ppm of germanium, more preferably at most 150 ppm of germanium, more preferably at most 120 ppm of germanium, more preferably at most 100 ppm of germanium, more preferably at most 90 ppm of germanium. Concentrations are given with respect to the amount of polyester comprising ethylene-2, 5-furandicarboxylate and units derived from the benzene dicarboxylic acid and its ester. The amount of germanium is calculated as weight amount of metal independent of the actual form of the germanium.

[0064] It was further found that the process allows the addition of typical stabilizers that are known from the prior art. Therefore, the process starting composition can further comprise a stabilizer. Stabilizers include phosphorous containing compounds, in particular phosphite containing compounds, phosphate containing compounds and phosphonate containing compounds, preferably phosphoric acid, and hindered phenolic compounds.

[0065] The polyester of the invention after solid state polymerization preferably has a weight average molecular weight of 70 kg / mol or more, preferably 80 kg / mol or more, preferably 90 kg / mol or more, preferably 100 kg / mol or more, more preferably 110 kg / mol or more. The polyester composition can have an amount of cyclic dimer according to formula (1) of less than 0.65 %wt, more specifically at most 0.60 %wt, more specifically at most 0.55 %wt, more specifically at most 0.50 %wt, based on weight amount of polyester.

[0066] Generally, the polyester of the present invention after solid state polymerization has a weight average molecular weight of at most 220 kg / mol, more preferably at most 200 kg / mol, more preferably at most 180 kg / mol, more preferably at most 160 kg / mol, more specifically at most 140 kg / mol.

[0067] The polyesters which can be obtained by a process according to the present invention and polyester compositions according to the present invention were found to be surprisingly suitable for use in melt processing. While the content of cyclic dimers increased during melt processing, it was surprisingly found that the dimer content of these polyesters still was lower than expected. Melt processing is considered any subsequent process in which the polyester is used as a melt. An especially suitable process is injection moulding which comprises heating and mixing polyester to obtain a melt and injecting the polyester containing melt into a mould where it cools and hardens to the configuration of the cavity to obtain the injection moulded article.

[0068] The invention will be further illustrated by means of the following examples.

[0069] Experiments

[0070] The amount of cyclic dimer according to formula (1) and the amount of terephthalic acid in the polyester were determined by1H-NMR as described above.

[0071] The weight average molecular weight and the number average molecular weight were determined through the use of gel permeation chromatography (GPC). GPC measurement was performed at 35 °C using two PSS PFG linear M (7 pm, 8x300 mm) columns with precolumn and refractive index detector. Hexafluorisopropanol with 0.05 M potassiumtrifluoroacetate was used as eluent. Flow rate was set to 1.0 mL / min, injection volume was 50 pL and the run time was 50 min. The calibration is performed using polymethylmethacrylate standards.

[0072] Examples 1-3

[0073] In Comparative Example 1 , starting composition comprising 30.0 g 2,5- furandicarboxylic acid (FDCA) and 14.3 g ethylene glycol was added to a reactor together with 7.35 mg tetraethylammonium hydroxide in 21 mg water and 4.05 mg GeO2 in 0.58 ml water.

[0074] Example 2 differed from Comparative Example 1 in that the starting composition contained 28.50 g FDCA and 1.59 terephthalic acid instead of 30 g FDCA. This amount of terephthalic acid is 5 mole % based on molar amount of FDCA.

[0075] These starting compositions were subjected to esterification at 220 °C and at atmospheric pressure. Subsequently, the intermediate ester compositions obtained were subjected to prepolycondensation for 20 minutes during which the temperature was raised to 260 °C and pressure was reduced from atmospheric pressure to about 1 mbar. For polycondensation, these conditions were maintained during 75 minutes. The polyester as obtained by polycondensation subsequently was subjected to solid state polymerization at 200 ° during 72 hours in the form of whole pellets.

[0076] The polyesters obtained are described in Table 1 below.

[0077] Table 1

Claims

CLAIMS1. Polyester composition comprising polyester comprising ethylene-2, 5-furandicarboxylate units and having a weight average molecular weight of at least 45 kg / mol which polyester additionally comprises units derived from benzene dicarboxylic acid or an ester thereof wherein the polyester composition comprises at most 0.80 % by weight (%wt) of cyclic dimers according to formula (1) based on amount of polyester.

2. Polyester composition according to claim 1 wherein the benzene dicarboxylic acid or its ester is terephthalic acid.

3. Polyester composition according to claim 1 or 2 which composition further comprises at most 200 ppmw of germanium.

4. Polyester composition according to any one of claims 1 to 3 which polyester comprises of from 2 to 10 % by mole of units derived from benzene dicarboxylic acid or its ester, based on molar amount of furandicarboxylate units.

5. Polyester composition according to any one of claims 1 to 4 wherein the polyester has a weight average molecular weight of at least 100 kg / mol.

6. Process for preparing polyester composition according to any one of claims 1 to 5, which process comprises: a) providing or preparing a starting composition comprising (i) 2,5-furandicarboxylic acid and ethylene glycol or (ii) dimethyl-2, 5-furandicarboxylate and ethylene glycol, b) subjecting the starting composition to elevated temperature to produce an intermediate ester composition, and c) contacting the intermediate ester composition with a polycondensation catalyst at polycondensation conditions to produce a polyester comprising ethylene-2, 5- furandicarboxylate units, wherein benzene dicarboxylic acid or an ester thereof is added to the starting composition and / or to the intermediate ester composition.

7. Process according to claim 6, wherein the starting composition of step a) comprises 2,5-furandicarboxylic acid and ethylene glycol.

8. Process according to claim 6 or 7, wherein the benzene dicarboxylic acid or an ester thereof is terephthalic acid or an ester thereof, preferably is terephthalic acid, and is added to the starting composition.

9. Process according to any one of claims 6 to 8, wherein the benzene dicarboxylic acid or an ester thereof is added in an amount of from 2 to 10 % by mole based on the total molar amount of 2,5-furandicarboxylic acid and mono- and di-ester thereof.

10. Process according to any one of claims 6 to 9, wherein the polycondensation catalyst comprises germanium.

11. Process according to any one of claims 6 to 10, wherein the polycondensation conditions comprise a temperature in the range of from 240 to 300 °C during of from 30 to 190 minutes.

12. Process of preparing the polyester composition according to any one of claims 1 to 5 by the process according to any one of claims 6 to 11.

13. Process according to any one of claims 6 to 12, which process further comprises the steps: d) crystallizing the polyester comprising ethylene-2, 5-furandicarboxylate units obtained in step c) to obtain a crystallized or semi-crystallized polyester, and e) subjecting the polyester produced in step d) to a solid state polymerization for increasing the molecular weight.

14. Process according to claim 13, wherein the solid state polymerization is conducted at a temperature in the range of from 160 to 240 °C, preferably 170 to 220 °C.

15. Process comprising melt processing polyester composition according to any one of claims 1 to 5.

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