FR2814746A1 - Synthesis of additives improving physical and mechanical characteristics of various origin polyester waste, before its recycling, comprises chemical modification of fresh or recycled polyester - Google Patents

Abstract

Synthesis of compounds of high molecular weight and intrinsic viscosity, based on modified polyesters and used to improve properties of waste polyesters, especially polyethylene terephthalate, comprises reacting hydroxyl and carboxyl terminal groups of fresh or recycled polyester with aliphatic diisocyanate chain, during passage through double screw extruder, at 250-280 deg C, followed by cooling and granulation. An Independent claim is also included for compounds of high molecular weight and intrinsic viscosity, based on modified polyesters and aimed at improving characteristics of polyester waste, especially polyethylene terephthalate, and obtained from chemically modified fresh or recycled polyesters, by reaction between fresh or recycled polyesters (2) and chain (1) consisting of aliphatic diisocyanate, conducted in double screw extruder (V1, V2), with product leaving extruder cooled and then granulated, to be used as additive during transformation (6) of recycled polyester (5) of low molecular wt., to enhance its intrinsic viscosity.

Description

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 The subject of the present invention is a process for the synthesis of compounds based on modified polyesters intended to improve the characteristics of waste polyesters and in particular polyethylene terephthalate with a view to their recycling, as well as the compounds obtained.

 It relates generally to the industrial and commercial field of the manufacture and distribution of products of high molecular weights and very high intrinsic viscosities based on virgin or recycled polyesters chemically modified during a transformation stage, as well that the use of these products as additives to improve the characteristics such as molecular weight, intrinsic viscosity or mechanical properties of waste polyesters such as polyethylene terephthalate (PET) from various origins (textiles, bottles, electronics, etc.) for recycling.

 During their transformation and recycling, these polyesters undergo a partial degradation, the most visible phenomenon of which is the drop in their intrinsic viscosity which translates a loss of molecular weight, an increase in the acid and alcohol groups and a degradation of the properties. mechanical. To date, the increase in the intrinsic viscosity of these polymers is most often obtained by means of long and costly thermal processes such as for example the "Solid State Processing" process (S. S. P.).

 More recently, it has been envisaged to chemically modify the polyesters by adding reactive bifunctional molecules also called "chain extenders". The known chain extenders, which facilitate the extension of the chains of polyesters to give polymers of high molecular weights, are for example: - oxazolines (Patents JP 60 161 427.60 163 921; EP 0 583 801),

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 - diepoxides (US Patents 3,553,157 and 4,753,975), - cyclic bis (imino) ether (Patents JP 82 49 616, 57 161 122).

 With the chain extenders mentioned above, the results obtained are only partially satisfactory, since they do not cause a sufficiently large increase in intrinsic viscosity and therefore in molecular weight. In addition, oxazoline are expensive products, therefore economically unattractive in the field of recycling.

 Other chain extenders used to chemically modify polyesters are diisocyanates. For example, US Pat. Nos. 2,333,639 and 3,304,286 reveal that the chemical modification of low molecular weight polyesters with low intrinsic viscosities by polyisocyanates of diisocyanate type at temperatures above 300 ° C. leads to fusible polyesters of molecular weights higher but still having low viscosities in the molten state.

 US Patents 4,409,167 and 4,178,277 indicate that the polyesters alone or in admixture with other polymers which allow a modification of the viscosity can be chemically modified by aromatic diisocyanates preferably of the MOI (4,4'-methylene) type. bis phenyl isocyanate). The materials of high molecular weight and yellowish color thus obtained can be used in many applications such as for example the manufacture of packaging products.

 As the examples described above show, the more reactive aromatic diisocyanates are generally preferred to the aliphatic diisocyanates as chain extenders used to chemically modify the polyesters. However, they cause significant yellowing of the polymer during processing and subsequently during the aging of the material.

 The object of the present invention is to remedy the drawbacks of the methods or products currently used to improve the intrinsic viscosity of polyesters. In fact, it makes it possible to avoid intense coloring, and to obtain in a single transformation step, without first passing through a premixing step, intrinsic viscosities and molecular weights higher than those obtained with known methods. The reagents obtained have very good thermal stability.

 Compared with existing commercial additives, the compounds according to the invention have the property, when introduced into low molecular weight PET waste (approx. 16,000 g / mole), of low viscosities

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 intrinsic (approx. 0.40 dl / g) and contaminated with impurities of various origins, very significantly increase the intrinsic viscosity and molecular weight of PET.

 In addition, these compounds, unlike those found on the market, have a relatively low cost price making them particularly advantageous for recycling in fields as varied as high tenacity filament for textile and industrial applications (strips of strapping, seat belts, geotextiles, etc.), or extrusion for sheets of thermoplastic or profiles and also the injection of technical parts for the automobile.

 One of the advantages of this new process lies in the fact that it avoids the direct manipulation by the user of the additive of reactive bifunctional molecules in the form of monomer. It allows great flexibility of use, because the concentration of additive can be adapted as a function of the intrinsic viscosity values which it is desired to achieve. Finally, there is no residual isocyanate (NCO) at the end of the process.

 The process consists in causing a chemical reaction between the end groups of virgin or recycled polyesters and a chain extender made of aliphatic diisocyanate of hexamethylene diisocyanate type (HDI) by passing these two elements without premixing in an extruder preferably with twin screws during one to four minutes at a temperature between 250 and 280oC, the rods at the outlet of the extruder being cooled before being granulated so that they can be used in particular as additives during the processing of recycled low molecular weight polyesters in order to enhance their intrinsic viscosity.

 The invention will be better understood from the description which follows, made with reference to the accompanying drawings for information only and in which: FIG. 1 is a symbolic diagram representing the synthesis of compounds of high molecular weights, as well as the introduction of this compound as an additive during the transformation cycle of polyester waste, FIG. 2 is a symbolic diagram recalling the known process of extension / coupling of polyester chains, and FIG. 3 represents a double screw extruder in accordance with that used for the application of the method according to the invention.

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 The process consists in reacting chain extenders 1 on polyesters 2 in order to carry out the synthesis 3 of a polyester of very high intrinsic viscosity subsequently designated under the term of "compound" 4, this compound being especially intended to serve as additive to be added to a recycled polyester 5 of low intrinsic viscosity in order to obtain after transformation 6 (extrusion, injection, etc.) an improved polymer 7 (FIG. 1).

 The polyesters 2 used to obtain the compounds 4 can be in different forms, and in particular: - virgin polyesters in the form of pre-crystallized granules with intrinsic viscosity of 0.60 to 0.90 dl / g, preferably 0.70 dl / g, - recycled polyesters in the form of flakes or granules with an intrinsic viscosity of 0.40 to 0.90 dl / g, preferably 0.60 to 0.70 dl / g, - alpha-omega diol oligoesters of degree of polymerization variable number average (OP n) coming from the depolymerization of polyester with glycol type diols and preferably diethylene glycol (JY Chen, CF Ou, YC Hu, CC Lin, Journal of Applied Polymer Science, vol. 42,1501 (1991); S. Baliga, WT Wong, Journal of Polymer Science, part A: Polymer Chemistry, vol. 27.2071 (1989); JR Campanelli, MR Kamal, DG Cooper, Journal of Applied Polymer Science, vol. 54, 1731 (1994)), - or mixtures of virgin or recycled polyesters with s alpha-omega diol oligesters of variable DPn introduced at a rate of 5 to 50% and preferably 10%.

 The chain extenders 1, of formula Y-R-Y (Y: isocyanate function), are aliphatic diisocyanates preferably of the HDI (hexamethylene diisocyanate) type. These compounds are thermally stable and capable of reacting very quickly and irreversibly with the terminal groups X of the polyester (preferably OH and more difficult COOH) to lead to an extension or a C coupling of the chain (FIG. 2).

 The urethane bonds formed from aliphatic isocyanate are more stable, therefore less thermally fragile than those obtained from aromatic isocyanate. In addition, the boiling temperature of the bifunctional compounds is higher in the case of the aliphatic diisocyanate of the HDI type (255OC against 200oC for the aromatic diisocyanate of the type 4, 4'-methylene bis phenyl isocyanate).

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 The concentration of hexamethylene diisocyanate (HDI) introduced depends on the initial intrinsic viscosity of the polyester used and is generally between 1 and 4% by mass.

 The reaction could possibly be accelerated by means of catalysts used in the chemistry of polyurethanes. However, the use of catalysts is not necessary due to the high processing temperature. As the reaction kinetics at these temperatures are fairly rapid, the use of catalysts risks promoting undesirable side reactions.

 The synthesis 3 of compound 4 is carried out in an extruder preferably of the twin screw type V1, V2 (FIG. 3). The aliphatic diisocyanate, in this case of the HDI (hexamethylene diisocyanate) type, is introduced as far upstream as possible during the extrusion step so that the chemical reaction between the terminal groups of polyester 2 and the chain lengthener 1 has time to occur, because the residence times are extremely short in this continuous process, unlike discontinuous processes such as for example a mixing cell of the "brabender" type.

 The most important machine parameters are residence time and temperature. Indeed, it is necessary to have a residence time of between one and four minutes, and preferably two minutes, so that the reaction between the end groups of the polyester and the chain lengthener has time to occur. The extension is carried out at a temperature between 250 and 280oC. It is preferable to work in this temperature range, and above all not to exceed 280oC as this would cause: - yellowing of compound 4, - rupture of the urethane bonds formed from the OH groups of polyester 2 and the isocyanate NCO functions of lengthen chain 1.

Indeed, the literature shows that the urethane bonds formed from OH functions and aliphatic isocyanate, in this case HDI are more stable than those formed from OH functions and aromatic isocyanate, in this case MOI for temperatures between 250 and 280oC. Of course, beyond 280oC the stability of the urethane bonds is questioned.

 - The presence of bubbles from the reaction between the CO2H groups of polyester 2 and the NCO functions of the chain extender 1.

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 The increase in the intrinsic viscosity of the modified polyesters essentially results from the chemical reactions between the OH groups of the polyesters and the NCO isocyanate functions of the chain extender in the temperature range used, that is to say between 250 and 280 ° C.

 The other machine parameters such as the flow rate, the speed of rotation and the profile of the screws are closely linked to the type of machine used, in this case the length / diameter (south) ratio of the extruder for synthesizing compounds. Furthermore, the degassing usually carried out at the end of extrusion can be eliminated because the material thus produced is free of bubbles at the outlet of the die.

 The rods at the exit of the extruder are cooled in a water tank, or by any other known means, before being granulated. The compounds 4 thus obtained have very high intrinsic viscosities of up to 2 dl / g. The compound is in the form of transparent and very slightly colored granules.

 This new synthesis process, easy to implement, makes it possible to obtain a product of very high viscosity in a single extrusion step (twin screw or single screw) without first passing through a premixing step.

 The very slight coloring and low cost price of the compounds 4 makes them particularly advantageous for recycling in fields as varied as high tenacity filament for textile and industrial applications (strapping bands, seat belts, geotextiles, etc.), or extrusion for sheets of thermoplastic or profiles and also the injection of technical parts for the automobile.

 When used as an additive, the compound 4 thus produced is introduced during the transformation cycle of the recycled polyester 5 of low molecular weight with a view to enhancing its intrinsic viscosity. The processing techniques are the same as those used when adding a masterbatch to a polymer matrix, that is to say extrusion or injection. This method is very flexible to use because it introduces the amount of additive of known intrinsic viscosity necessary to enhance that of recycled polyester. For manufacturers, the additive will be used as a masterbatch. The introduction of 5 to 15% preferably 10% by mass of additive of 1.70 dl / g of intrinsic viscosity

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 increases that of recycled polyester from 0.50 to 0.70 dl / g, which makes it possible to widen the range of application of polyester waste.

 The present invention makes it possible to synthesize modified polyesters called additives or masterbatches of very high molecular weights: 66,000 to 190,000 g / mole and of very high intrinsic viscosity: 1 to 2 dl / g, fusible, soluble, and slightly colored yellowish, as well as the application of these additives to enhance the characteristics such as intrinsic viscosity, molecular weight and mechanical properties of polyester waste such as PET and reduce the concentration of acid and alcohol groups.

 The advantage of this process and of the compound obtained also lies in the fact that the chemical reactions which can take place between the additive and the recycled polymer are terminated at the end of the step of processing by injection or extrusion.

EXAMPLE 1
Polyester of the virgin PET type with an intrinsic viscosity of 0.70 dl / g, previously dried at 140 ° C. for 24 hours and diisocyanate of the HDI type (2.5% by mass) introduced via a pump are kneaded in a twin screw extruder with L / D ratio = 36. The screw profile used is that of Figure 3. The extrusion is carried out at a temperature between 250 and 280oC, with a flow rate of 2 kg / h and an equal residence time at 2 min 30 sec.

The rods at the outlet of the die are cooled in water, granules and dried at 80 ° C. for 4 hours. The modified products are then dissolved in 20 ml of a phenol / 1, 1,2, 2 mixture of tetrachloroethane (60/40), at room temperature overnight with magnetic stirring. The compound obtained being completely soluble in these solvents, there is therefore no crosslinking phenomenon. From the measurement of the intrinsic viscosity [n], we can deduce the viscosimetric mass Mv of the additive thanks to the Mark-Houwink relation: [q] = (Mv) kx with k = 7.44 10- 4 dl / g and a = 0.648 at 25 C for a phenol / tetrachloroethane mixture (60/40).

 With the extrusion conditions described above, we obtained a compound with intrinsic viscosity equal to 1.28 dl / g. The introduction of 13% by mass of this additive into a regenerated PET matrix of low intrinsic viscosity (0.46 dl / g) improves the latter by around 30%. These tests were carried out in a single screw extruder with a screw diameter of 120 and a throughput of 300 kg / h.

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possède une viscosité intrinsèque de 1, 12 dl/g. L'introduction de 15% en poids de cet additif dans une matrice de PET régénéré de 0, 5 dl/g de viscosité intrinsèque améliore cette dernière d'environ 20%. EXAMPLE 2 Amorphous PET type polyester waste, of intrinsic viscosity 0.60 dl / g, previously dried 4 h at 80 ° C., 4 h at 100 ° C. and 4 h at 120 ° C. to avoid solidification because this waste has a point of softening lower than that of virgin PET granules (crystallinity 46%), and 3% by mass of HDI type diisocyanate are kneaded in a twin-screw extruder as in Example 1. The additive synthesized from PET waste

has an intrinsic viscosity of 1.12 dl / g. The introduction of 15% by weight of this additive into a regenerated PET matrix of 0.5 dl / g of intrinsic viscosity improves the latter by around 20%.

 The positioning of the various constituent elements gives the object of the invention a maximum of useful effects which had not, to date, been obtained by similar methods.

Claims (14)

 1. Process for the synthesis of compounds based on modified polyesters intended to improve the characteristics of waste polyesters and in particular of polyethylene terephthalate with a view to their recycling, said compounds having a high molecular weight and a very high intrinsic viscosity being obtained from virgin or recycled polyesters chemically modified during a transformation step, characterized in that a chemical reaction (3) is caused between the terminal groups (OH, COOH) of virgin or recycled polyesters (2) and an elongate chain (1) consisting of aliphatic diisocyanate by passing these two elements without premixing in a twin screw extruder (V1, V2) at a temperature between 250 and 280oC, the compound (4) obtained in the form of rods at the outlet of the extruder being cooled before being granulated.  2. Method according to claim 1, characterized in that the chemical reaction (3) in the twin screw extruder (V1, V2) is carried out without degassing.  3. Method according to any one of the preceding claims, characterized in that the residence time in the twin screw extruder (V1, V2) is between one and four minutes, and preferably equal to two minutes.  4. Method according to any one of the preceding claims, characterized in that the concentration of diisocyanate introduced into the mixture intended for synthesizing the compound (4) is between 1 and 4% by mass.  5. Method according to any one of the preceding claims, characterized in that the chain extender (1) consists of aliphatic diisocyanate of hexamethylene diisocyanate (HDI) type. <Desc / Clms Page number 10>  6. Method according to any one of the preceding claims, characterized in that the polyester (2) intended for synthesizing the compound (4) is of the polyethylene terephthalate (PET) type.  7. Method according to any one of the preceding claims, characterized in that the polyester (2) intended for synthesizing the compound (4) consists of virgin polyesters in the form of granules of intrinsic viscosity from 0.60 to 0, 90 dl / g, preferably 0.70 dl / g.  8. Method according to any one of claims 1 to 6, characterized in that the polyester (2) intended for synthesizing the compound (4) consists of recycled polyesters in the form of flakes or granules of intrinsic viscosity of 0 , 40 to 0.90 dl / g, preferably 0.60 to 0.70 dl / g 9. Method according to any one of claims 1 to 6, characterized in that the polyester (2) intended for synthesizing the compound (4) consists of oligoes ters alpha-omega diols of number average degrees of polymerization ( DPn) variables coming from the depolymerization of polyester with glycol type diols and preferably diethylene glycol.  10. Method according to any one of claims 1 to 6, characterized in that the polyester (2) intended for synthesizing the compound (4) consists of mixtures of virgin polyesters or recycled with oligoesters alpha-omega diols of variable number of degrees of average polymerization (OP n) introduced at a rate of 5 to 50% and preferably 10%.  11. Method according to any one of the preceding claims, characterized in that the synthesis of the compound (4) is accelerated by means of catalysts used in the chemistry of polyurethanes.  12. Method according to any one of the preceding claims, characterized in that the compound (4) is used as an additive during the transformation cycle of recycled polyester (5) of low molecular weight, and in particular of polyethylene terephthalate (PET), in order to increase its molecular weight, its intrinsic viscosity and improve its mechanical properties. <Desc / Clms Page number 11>  13. Compounds based on modified polyesters intended to improve the characteristics of waste polyesters and in particular polyethylene terephthalate, said compounds having a high molecular weight and a very high intrinsic viscosity being obtained from virgin or recycled polyesters chemically modified during a transformation step, characterized in that they are obtained by the reaction of virgin or recycled polyesters (2) and of a chain extender (1) consisting of aliphatic diisocyanate operated in a twin screw extruder (V1, V2), the rods at the outlet of the extruder being cooled before being granulated so that they can be used in particular as an additive during the processing (6) of recycled polyester (5) of low molecular weight to enhance its intrinsic viscosity .  14. Compounds according to claim 13, characterized in that the chain extender (1) consisting of aliphatic diisocyanate is of the hexamethylene diisocyanate (HDI) type.