BE1030133A1 - METHOD FOR IMPROVING THE PERFORMANCE OF THE CHEMICAL RECYCLING OF POLYLACTIC ACID FIBERS AND FOR PRESERVING THEIR PHYSICAL PROPERTIES - Google Patents

Abstract

A method is described for improving the yield of the recycling of PLA textile fibers comprising: - Decontamination of the textile material; - The separation of PLA textile fibers from other non-PLA textile fibers, by dissolution in a lactic ester; - The hydrolysis of this lactic ester solution into lactic acid; - The recovery of lactic acid and its polymerization to reform PLA Characterized in that the textile fibers are subjected after the decontamination step to a drying step in order to reduce the water content.

Description

METHOD FOR IMPROVING THE YIELD OF CHEMICAL RECYCLING OF

POLYLACTIC ACID FIBERS AND RETAINING THEIR PROPERTIES

PHYSICAL

DESCRIPTION FIELD OF INVENTION

The present invention relates to a method for improving the yield of chemical recycling of polylactic acid (PLA) fibers and for preserving the intrinsic qualities and physical properties of these PLA textile fibers.

In particular, the method of the invention consists in drastically reducing the water content of the textile fibers before starting the first stage of their chemical recycling, which consists in the separation of textile fibers made of PLA or containing PLA, from any other textile fiber. of natural or chemical origin present in a textile.

STATE OF THE ART

Humidity is a very observed and controlled parameter in the textile industry because humidity has a non-negligible impact on the physical properties of textile fibers such as: - their size; - their rigidity; - their tensile strength; -their elasticity; - their electrical resistance.

During its transformation into a textile, a textile fiber is subjected to numerous mechanical and thermal actions which can impact its appearance and its final properties. The relative humidity of the place of manufacture has been shown to have a significant impact on the final properties of a textile.

When the properties of a fibre, yarn or textile are measured, all tests must be carried out according to the ISO 139 standard where the relative humidity is 65 + 2% and where the samples to be tested are conditioned at this humidity.

The impact of humidity on a textile fiber is not the same from one fiber to another, due to the chemical nature of the fibers. This impact will be represented by two properties of the textile fibres: the recovery rate and the water content. The rate of moisture uptake is defined as the “mass of water contained in a material, determined by defined methods and expressed as a percentage of the mass of the dehydrated material”. Water content is the “mass of water contained in a material, determined by defined methods and expressed as a percentage of the mass of the moist material”. These two parameters will determine whether a fiber is hygroscopic or not. Textile fibers are divided into two groups: natural fibers (present as fibers in nature) and chemical fibers which are manufactured by | Man following a transformation of the material (artificial fibers) or a chemical reaction (synthetic fibers). In general, natural fibers and artificial fibers are hygroscopic, i.e. they will tend to absorb water vapor in a humid environment and release it in a dry atmosphere.

For information for this patent application, Table 1 summarizes the water content and recovery rate of certain common textile fibers.

Painting ! Water content and resumption of textile fibers

Textile fiber Recovery rate (%) Water content (%)

Cotton 8.5 7.34

Jute 13.75 12.1

Viscose 11.0 9.91

Silk 11.0 9.91

Wool 16.0 13.8

Acrylic 1.5 0

Linen 12.4 10.4

Hemp 12.4 10.4

Acetate 6.0 0

Polyester 0.4 0

Polyamid 6 | 4.0 | 3.1

Polylactic acid 0.8 1.1

Textiles will not only absorb moisture due to the nature of the fibers that compose them.

Their structure (woven, knitted or non-woven) is also conducive to capturing moisture and retaining it.

Textiles are assemblies of threads (for fabrics and knits) or fibers (non-wovens) that intertwine to form a fabric. These tangles form a surface that has small aspenities where water molecules can lodge. The absorption rate of a fabric will therefore be very variable and difficult to calculate because it will depend on: - the composition of the fabric; -the structure of the fabric; - atmospheric humidity.

Other factors are added to those previously stated, such as the structure of the yarn and the method of production, if the fabric has undergone finishing stages aimed at giving it new properties (coating, dyeing, etc.).

At present, the textile industry is also turning to the recycling of textiles in order to be able to reuse them with the same physical properties. But fiber recycling generally results in a fiber loss of around 20%, which is far too much for an acceptable yield.

Processes for the chemical recycling of all types of textiles containing polylactic acid (PLA) and in particular clothing are well known, such as that described in patent No. US 8614338, where it is indicated that at the end of the cycle life of said textiles, the latter are collected, washed and then treated by various techniques known to those skilled in the art so that the PLA present in the form of fiber returns to the monomer state (lactic acid) for later able to be polymerized again in the form of PLA.

We also know that textiles and in particular clothing are very rarely composed of a single material. Fiber blends are a common practice to provide a wide range of properties to a textile and also for cost reasons (natural materials cost more than chemical materials).

However, in the case of textile recycling, one or more washing steps are necessary before proceeding with the chemical recycling of the PLA. The other fibers, and the textiles by their structure will be enriched in water and transport it with them to the process of chemical recycling. Synthetic products composed wholly or partly of PLA described in the patent EP2419396B1 provide water but in less quantity compared to a textile.

The presence of water during dissolution will be able to react with the lactic acid ester and form lactic acid. The presence of lactic acid in the chemical recycling of PLA will cause problems during the alcoholysis step by shifting the equilibrium of the reaction.

In addition, the water which would not have reacted, in the presence of PLA in an alcoholysis reaction medium, will tend to form lactic acid which would interfere with the formation of lactic acid ester and displace the equilibrium of the reaction.

Lactic Acid Ester + Water <=> Lactic Acid + Alcohol

There is therefore a need to improve the recycling efficiency of PLA fibers when due to the damaging effect of the washing water, and to allow the depolymerization step to take place - normally.

PRINCIPLE OF THE INVENTION

The Applicant has now developed a particular process which makes it possible to increase the yield of the chemical recycling of PLA fibers during and to considerably reduce the harmful effects of the water present at this stage.

The process developed by the Applicant first comprises a step of separating the fibers from

PLA (or containing PLA) of other textile fibers and treat them in such a way as to drastically reduce their water content before starting the chemical depolymerization treatment.

The Applicant has now unexpectedly found that the separation step must be preceded by a step of drying the textile materials, whatever their chemical composition, because textiles are in their physical nature inclined to trap water in their structure. .

There is therefore a need to drastically limit the water content of textile fibers subjected to a recycling process.

DETAILED DESCRIPTION

Drying is a widespread industrial practice in various industries (chemical, pharmaceutical, agri-food, for example). The drying of a solid or liquid product makes it possible to eliminate by evaporation a targeted liquid that is usually called humidity. When this moisture is water, it is called dehydration.

Drying can be motivated by various reasons: - Chemical or physical, humidity hinders the future use of the product or even makes it impossible; - Storage, humidity will lead to degradation and aging of the product; - Economical, humidity will generate additional costs for the use of the product (cost of transport linked to the greater weight, for example); - Morphological, removing moisture from a product can change its appearance and bring new interesting properties for future application.

According to the process of the invention, during the recycling of textiles containing PLA in fiber form, the general process is based on six main steps: 1. recovery and collection of textiles containing PLA, 11. decontamination of the latter by washing in water at a temperature between 30 and 45°C, 111. reduction of the volume of textiles (densification), iv. the hydrolysis of PLA fibers into lactic acid or its derivatives, v. the recovery of the latter for finally, vi. The polymerization of lactic acid to reform PLA.

Following the recovery of the PLA textile (mono-material or mixture of fibres), it is preferable to convert the textile into cut pieces or else into fibers in order to increase the contact surface and facilitate hydrolysis, because more the surface we have, the more favorable it is to the depolymerization process. This step also aims to facilitate the separation of fibers other than PLA, if necessary (filtration).

The Applicant has now found that by carrying out a drying treatment on the textile fibers which have undergone the decontamination treatment, it is possible to considerably limit the harmful effects of the lactic ester/water reaction and thus improve the recycling yield by limiting the loss of fibers non-PLA textiles in the recycling operation.

Indeed, the drying step can be carried out before the reaction; which guarantees a minimum of residual water molecules brought by the fabric, and therefore considerably limits the development of parasitic reactions.

The drying thus applied makes it possible to: - Reduce the humidity, which avoids hindering the future use of the product or even making it impossible; - Prevent humidity from causing degradation or aging of the product; - Reduce the cost of subsequent treatments, because the humidity will generate additional costs for the use of the product (cost of transport linked to the greater weight, for example); - Avoid morphological degradation because humidity can modify its appearance.

A second possibility would be to carry out the drying after the collection of the textile, ensuring that once the textile or the textile fibers are dry, the material is kept in an oven (anhydrous conditions, at 40°C), until time of depolymerization.

According to the method of the invention, the drying step takes place under the following conditions: drying for a period between 4 and 24 h, and more precisely between 5 and 15 h, and at a temperature between 50 and 90 ° C , and more precisely between 60 and 80°C, preferably with activated ventilation, in order to homogenize the temperature within the cell, and to allow the water to be evacuated outside.

On an industrial scale, we can find several drying methods for textiles, which can be more or less effective depending on several factors such as the type of product to be dried, its volume, its mass, and its form.

For an industrial application, the significant step of volume reduction (densification) is done mechanically. Indeed, the most efficient and common method is direct cutting using a guillotine cutter. Equipped with several fixed and mobile knives, this technique consists of first compressing the fabrics and then moving them towards the knives using a conveyor belt. The cutter also allows you to adjust the width and degree of the cuts.

In the same perspective of preliminary stages to drying, one can also apply a considerable number of similar treatments such as: grinding, lump breaking, extrusion, crumbling, boudinage, granulation…

For textiles, in the form of continuous bands or fibres, among the most suitable types of industrial drying apparatus we can find: the external conduction drum, the airborne dryer, with percussion belt or radiation, or even the drier dielectric losses.

The textile fibers thus dried are dissolved in a lactic acid ester derivative to dissolve the PLA textile fibers present. If necessary, the non-PLA textile fibers are filtered following this step and before the depolymerization reaction takes place.

The method of the present invention is also described using examples, which serve to illustrate but do not limit the scope thereof.

EXAMPLE 1: PLA DRIED BEFORE DEPOLYMERIZATION

Starting from a PLA-based garment, we cut 1 x 8 cm strips and subjected them to a decontamination step consisting of 3 washes in water at 40°C, and then dried in a ventilated oven, at a temperature of 70°C for 15 h. We took 700 g of PLA strips and then dissolved them so that they could be chemically recycled (depolymerized), in 700 g of ethyl lactate. The fibers were dissolved at 110° C. and at atmospheric pressure. Once the PLA had dissolved in the ethyl lactate, the mixture was introduced into a laboratory reactor with 900 g of anhydrous ethanol, and 7 g of a catalyst known for this type of reaction (tin bis(2-ethylhexanoate) ). The whole was brought to a temperature of 160°C, with stirring at 2000 RPM,

Until a pressure of 7 bars is obtained. The reaction was stopped after 4 h.

The PLA introduced at the start of the reaction was almost completely converted into ethyl lactate (depolymerized). Table 1 shows the analytical results obtained by gas chromatography (GC):

Table 1: Results of GC analysis of reaction product after 4 h.

Lactate

Time | Water | Ethanol | LA 1x L2And L2A L3And L3A | Other ethyl

We obtained 80.47% of ethyl lactate at 4 h against 30.25% at the start of the reaction. The rest is mainly made up of ethanol remaining from the reaction, and oligomers and derivatives of ethyl lactate.

The limited and controlled amount of water at the start shows the beneficial effect since the amount of ethyl lactate converted is very important

EXAMPLE 2: UNDRIED PLA BEFORE DEPOLYMERIZATION (COMPARATIVE)

We proceeded in the same way to chemically recycle a PLA-based garment. However, this time the garment was not dried. We dissolved 700 g of PLA strips in 700 g of ethyl lactate. Once dissolved, the mixture was introduced into a laboratory reactor, with 900 g of anhydrous ethanol, and 7 g of catalyst. The reaction parameters were the same as the previous example. After 4 hours, we stopped the reaction, and the product obtained was also analyzed by GC. The results are shown in Table 2.

Table 2: Results of GC analysis of the reaction product after 4 h.

Lactate

Time | Water | Ethanol | LA 1x L2And L2A L3And L3A | Other ethyl

We have 67.11% ethyl lactate at 4 h against 30.25% at the start of the reaction. The rest is mainly made up of ethanol remaining from the reaction, and oligomers and derivatives of ethyl lactate.

We deduce that the yields following chemical recycling are much higher when the initial textile, here PLA, has been dried beforehand.

Claims (4)

1. Process for improving the yield of the recycling of textile fibers into PLA comprising: - Decontamination of the textile material; - The separation of PLA fibers from other non-PLA fibers, by dissolution in a lactic ester; - The hydrolysis of this lactic ester solution into lactic acid; - The recovery of lactic acid and its polymerization to reform PLA Characterized in that the textile fibers are subjected after the decontamination step to a drying step in order to reduce the water content. 2. Method according to claim 1, characterized in that the decontamination of the textile material consists in performing between 3 and 10 washes of the textile materials in water at a temperature of between 30 and 40°C. 3. Process according to claims 1 and 2, characterized in that the drying step is carried out at a temperature of between 60 and 75°C. 4. Method according to claim 3 characterized in that the drying is carried out in a ventilated atmosphere.