METAL

TECHNICAL FIELD

The invention relates to the synthesis of polylactic acid catalyzed by nanoparticle metal, wherein, by use of metal powder in the size of nanoparticles, it enables not only increasing reaction efficiency but also easily removing metal powder from the reaction medium when the reaction is completed, without the need for depolymerization, and it prevents the primary reaction product from undergoing external processes, reduces the total reaction processing time and the number of reactors, units employed in the reaction, and allows obtaining high molecular weight product.

BACKGROUND ART

Polymers play a central role in natural life and modern industrial economies. Advances in chemistry and materials science have enabled the discovery of many new synthetic polymers throughout the past century. Synthetic polymers such as nylon, polyethylene, and polyurethane entered our daily lives. However, the widespread use of synthetic polymers led to certain problems concerning human health and the environment. For instance, most plastic materials are manufactured by using non-biodegradable and non-renewable resources. The durability and strength of these materials, which make them highly useful, are the very same properties that prolong decomposition time in nature and make their disposal difficult. Additionally, the synthesis of some polymeric materials requires the use of toxic compounds or the formation of toxic end products. These problems led to increased attention on polymers produced from biological raw materials or by methods of modern biotechnology. These biopolymers contribute to overcoming various environmental issues. The term “biopolymer” is used to identify a wide range of materials. In general, biopolymers can be defined in two main categories: - polymers produced by biological systems such as microorganisms, plants, and animals;

- polymers chemically synthesized from biological starting materials such as amino acids, sugars, and natural oils.

Biopolymers are superior materials used as alternatives to metal, ceramic and non- biodegradable conventional polymers in a variety of applications. The areas where biopolymers are widely used can be listed as follows: as additives in the food industry, in artificial organ applications and drug delivery systems in the medicine and pharmaceutical industry, as a stiffener in dyeing and clothing in the textile industry, as a quality enhancer in the paper industry, and as a texturizer in the cosmetics industry. Moreover, they do not cause harm to nature as much as plastics since they can be biodegraded when they become waste; and recently, there have been studies on the use of carbohydrates instead of hydrocarbons in many sectors since they are easy to recycle. From this point of view, the most important biopolymer with an ever-increasing usage area is polylactic acid (PLA) (Scheme 1 ).

Scheme 1. molecular structure of poly(lactic acid).

PLA, which is used as an alternative to petroleum-derived polymers and is a thermoplastic polymer with an aliphatic structure with extraordinary advantages in terms of mechanical, biocompatibility, and biodegradation properties, is produced by the polymerization of lactic acid, which is used as a monomer and is obtained from renewable biomass (vegetal sources), which is much cheaper than petroleum.

Some biopolymers, such as PLA, naturally occurring zein, and poly-3- hydroxybutyrate can be used as plastics to replace the need for polystyrene- or polyethylene-based plastics. While it is known as milk acid, lactic acid (LA) is a natural product formed in human and animal metabolisms and is generally found in muscle tissues, liver, blood, and various organs of the body. LA in the body participates in the tricarboxylic acid cycle for the formation of carbon dioxide and water in the form of pyruvate and becomes biocompatible by being transformed in the cycle. Therefore, no LA residues are found in tissues and organs. The production of LA, which is industrially highly significant, is performed by way of chemical synthesis or bacterial fermentation. Glucose obtained from sugar sources such as corn, sugar cane, and potato is fermented with “lactobacillus” bacteria to produce lactic acid (Scheme 2). o lactobacillus vegetal sources - _►

OH glucose lactic acid

Scheme 2. Lactic acid production from glucose by fermentation.

PLA is a preferred polymeric material due to its strong mechanical properties, permeability, thermal stability, processability, and being an environmentally friendly product, and consumers frequently encounter this material in various commercial applications.

PLA in the prior art is a biopolymer that is currently produced in many countries around the world. Industrial-scale PLA synthesis is performed by polycondensation of lactic acid or ring opening polymerization (ROP) of lactide monomers obtained by dimerization of lactic acid (Scheme 3).

Scheme 3. Polymerization Methods Preferred in PLA Synthesis.

Industrial-scale production, on the other hand, is not performed in our country and neighboring countries. The process flow charts of polymerization reactions consist of a system that includes complex processes. When the present literature and current applications are examined, it is seen that the synthesis is carried out through ring opening polymerization and has a complex process flow chart (Scheme 4).

Scheme 4. PLA Production Process Flow Chart of a Company in the Prior Art

The main problems with the current applications in the prior art can be summarized as follows: obtaining lactide from lactic acid and separating it into its enantiomers by purification, the need for depolymerization process, removal of catalysts (the process becomes complex since it requires external equipment, chemical products, etc.), processing time (minimum 15 hours), process design (high number of reactors and units), obtaining low molecular weight product (obtaining products with a wide range of values such as 3,000 - 150,000 g/mol), and low product conversion efficiency (the primary product is precipitated in the solvent and thus, the catalyst is removed, and a part of the primary product that does not precipitate leads to yield loss). In conclusion, the requirement for the elimination of shortcomings and disadvantages of embodiments and practices existing in the prior art and already being used as of today entails an improvement in the respective technical field.

DESCRIPTION OF THE INVENTION The present invention relates to the synthesis of poly(lactic acid) catalyzed by nanoparticle metal, wherein it is developed for eliminating the aforementioned disadvantages and providing new advantages to the respective technical field.

An objective of the present invention is to synthesize PLA, which is a polymeric material with many advantages such as strong mechanical properties, permeability, thermal stability, processability, and being an environmentally friendly product, by a unique method and to introduce it to use in areas such as packaging, 3D printer filaments, plastic parts in the automotive industry, medical product manufacturing, and textile products, etc.

Another objective of the present invention is to not only increase reaction efficiency but also easily remove metal powder from the reaction medium upon completion of the reaction thanks to the use of metal powder in the size of nanoparticles in the polymerization reaction and to ensure that the primary reaction product is obtained without any external processes.

A further objective of the present invention is to ensure that the metal powder that is dispersed in the solution at the beginning of the reaction heaps and agglomerates in the solution at the end of the reaction time and can be easily removed from the primary product by simple vacuum filtration method. The preferred catalyst and synthesis methods allow the process to be as simple as possible compared to the existing methods.

Another objective of the present invention is to ensure the use of azeotropic polycondensation method instead of ring opening polymerization (ROP) in order to reduce the number of reactors employed in production, shorten processing time, and reduce production costs.

A further objective of the present invention is to ensure that the catalyst is easily removed from the reaction medium, PLA at the high efficiency of 97-99% is synthesized, the synthesis reaction is completed in 10 hours which is a very short time compared to equivalent synthesis methods, the molecular weight (Mw) of the synthesized PLA is 110,000 g/mol and above, the number of reactors and units to be used in industrial-scale production is low, the process flow chart is simple and easy to implement. The invention also facilitates industrial-scale applications of PLA synthesis.

Drawings

The applications of the present invention that is briefly summarized above and addressed in detail below can be understood by referring to the sample applications depicted in the attached drawings of the invention. However, it must be stated that the attached drawings show only the typical applications of this invention and that since the invention allows other equally effective applications, its scope cannot be assumed to be limited.

In order to facilitate understanding, identical reference numbers are used to indicate identical elements in the figures, where possible. The shapes are not drawn to scale and can be simplified for clarity. It is believed that the elements and features of an application can be usefully incorporated into other applications without further explanation.

Figure 1 : It is the differential scanning calorimeter (DSC) analysis of the PLA obtained with the invention.

Figure 2: It is the thermogravimetric analysis (TGA) of the PLA obtained with the invention.

Figure 3: It is the weight-molar mass change graph of the PLA obtained with the invention.

Figure 4: It is the FTIR analysis of the PLA obtained with the invention.

Figure 5: It is the microscope image of the PLA obtained with the invention.

Figure 6: It is the schematic view of the synthesis stage of the invention. DETAILED EXPLANATION OF THE INVENTION

The preferred alternatives in this detailed description of the embodiment of the present disclosure are only intended for providing a better understanding of the subject matter and should not be construed in any restrictive sense.

In the PLA synthesis of the present invention, metal powder in the size of nanoparticles is utilized in polymerization reaction in order to not only increase reaction efficiency but also easily remove metal powder from the reaction medium upon completion of the reaction, to ensure that the primary reaction product is obtained without any external processes, and to eliminate encountered technical problems. Furthermore, the azeotropic polycondensation method is preferred instead of ring opening polymerization (ROP) in order to reduce the number of reactors employed in production, shorten processing time, and reduce production costs. The concerned metal powder that is dispersed in the solution at the beginning of the reaction heaps and agglomerates in the solution at the end of the reaction time and can be easily removed from the primary product by simple vacuum filtration method. The preferred catalyst and synthesis methods allow the process to be as simple as possible compared to the existing methods.

The use of catalysts in the size of nanoparticles facilitates the interaction of starting materials in the reaction medium by increasing the contacting surface area, thereby enabling the reaction to be completed in a shorter time (10 hours). Consecutive trials conducted for the sake of optimization of reaction conditions demonstrated that the reaction yield (product conversion rate) is in the range of 97-99%. A method with the highest product conversion efficiency in comparison to the current methods has been developed. Spectroscopic and thermal characterization techniques including FTIR, GPC, DSC, and TGA were applied to the synthesized products. When the data obtained from the analysis performed by gel permeation chromatography (GPC) in chloroform solvent system were examined, the molecular weight (Mw) of PLA synthesized by applying the developed method was determined to be 110,000 - 118,000 g/mol. The effects of the products employed in the present application on the chemical reaction are listed hereinbelow: Lactic Acid: It is used as starting material in the developed PLA synthesis method. Nanoparticle Sn Powder: It is used as a catalyst in the polymerization reaction of lactic acid in the developed PLA synthesis method. p-Xylene: It is used as a medium diluent and azeotrope forming agent in the polymerization reaction of lactic acid in the developed PLA synthesis method Reactor: It is used as the medium where the polymerization reaction will be conducted in the developed PLA synthesis method.

Distillation Bridge: It is used to remove the water contained in the lactic acid employed as the starting material in the developed PLA synthesis method. Reverse Circulation System (Specially Designed Dean-Stark Apparatus): It is used to remove the water formed during the polymerization reaction in the developed PLA synthesis method.

Vacuum Pump: It is used to ensure that the polymerization reaction occurs with high efficiency in the developed PLA synthesis method.

Vacuum Filtration Set: It is used to ensure that the catalyst enabling the polymerization reaction to occur is removed from the PLA synthesized in the reaction in the developed PLA synthesis method.

Magnetic Stirrer with Hotplate: It is used in the developed PLA synthesis method to mix the chemical products added into the reactor during polymerization and to provide the required reaction temperature. Magnetic Stirring Bar: It is used as an auxiliary apparatus to facilitate the mixing function of the magnetic stirrer with hotplate.

Measuring Collection Container: In the developed PLA synthesis method, it is used to collect and measure, with the help of distillation bridge and reverse circulation system, the water both contained in the starting material lactic acid and formed during the reaction and removed from the medium.

Hexane: It is used to wash and dry the obtained product in the final stage in the PLA synthesis method. Volumetric Condenser: It is used to condense the solvent under heat treatment in the developed PLA synthesis method.

Vacuum Oven: It is used to dry the obtained polymer at the final stage in the developed PLA synthesis method.

The production stages of the invention consist of the following steps:

1 - Dehydration of the lactic acid

2- Addition of the catalyst

3- Addition of the solvent 4- Polymerization

5- Removal of the catalyst

6- Washing of the product

90% L-lactic acid was employed as a monomer as the PLA to be synthesized is desired to be of high purity. In the first stage of the reaction, before the solvent is added to the medium, the water contained in the lactic acid will be removed from the LA with the distillation bridge apparatus by way of vacuuming at the determined temperature because the water turns the reaction balance against the product and therefore causes the PLA to be synthesized to have a low molecular weight. Directly proportional to the value of vacuuming, the number of reactors, and the amount of starting material, the duration of this process is an average of 2 hours.

Nanoparticle metallic catalyst (Sn powder, 45 nm, 99.9% purity) will be added to LA from which the water has been removed, and thus the catalyst will be enabled to form a complex with the functional groups in the molecular structure of LA.

Formation of the complex between LA and the catalyst PLAthat will be formed in the reactor during the reaction will increase the viscosity of the medium. Therefore, the solvent will be added at this stage in order to ensure that mixing with the magnetic stirrer with hotplate is not obstructed and the whole starting material can react. An aromatic solvent that has a high boiling point, will lead to low production cost, can be easily removed from PLA following the reaction but is not miscible with water (forming heterogeneous mixture), p-xylene is preferred as the solvent. The temperature value of the reaction is between 140 - 160°C. The reaction time is between 9 - 12 hours. The PLA synthesis by way of application of the developed method is described hereinbelow.

Stage 1. 500 mL of 90% L-lactic acid was added to the glass reactor of a volume of 1 liter and mixed by heating to 140 - 150 °C for 2 hours with the help of the magnetic stirrer with hotplate and magnetic stirring bar. At this stage, both the distillation bridge that is employed for the circulation of cold water and the vacuum pump that helps remove the water were connected to and integrated with the reactor. The measuring collection container was attached to the other end of the distillation bridge so as to monitor the amount of discharged water. It was seen that 125 mL water was collected in the collection container at the end of 2 hours and water discharge from the system stopped by then. The vacuum pump was stopped, nanoparticle Sn powder (45 nm, 99.9% purity) was added into the dehydrated lactic acid in the reactor, and then vacuuming was restarted. The process continued for 30 minutes without making any changes in the reaction setup. It was observed that another 10 mL of water was collected in the collection container.

Stage 2.

The vacuum was stopped, and the distillation bridge was detached from the reactor. At this stage, no change was made as to the temperature and the stirring procedure. It was observed that the viscosity of the mix in the reactor increased. 200 mL of the p-xylene solvent was added into the reactor to provide the polymerization medium. The specially designed reverse circulation system - Dean-Stark apparatus was attached onto the reactor in order to remove the azeotropic mix and recover the solvent back to the reactor after removing the water from this mix; the volumetric condenser was fixed onto the said apparatus to condense the azeotropic mix. Vacuuming was performed with the described setup. The reaction continued under the specified conditions at 140 - 160 °C for 10 - 12 hours. The water formed during the reaction evaporates from the reactor together with the solvent; this removed azeotropic mix is condensed in the volumetric condenser; it is collected as a heterogeneous mix in the collection chamber of the reverse circulation system. The solvent is transferred back into the reactor and accumulates in the water collection chamber.

Stage 3.

The vacuum was stopped once the polymerization time was over. The volumetric condenser and the reverse circulation system were detached from the reactor. It was observed that the nanoparticle Sn powder that was added into the reactor agglomerated (in the form of marble balls) and settled in the bottom of the mix, thereby separating from the polymer. The hot mix in the reactor was filtered with the help of the vacuum filtration set and removed from the catalyst product. Clarified from the catalyst, the PLA was allowed to harden by cooling at room temperature in a dark environment. After the required level of hardening was achieved, the washing procedure was performed with the help of the vacuum filtration set and by use of hexane in order to purify the PLA by way of removing the p-xylene solvent that did not evaporate and remained in the PLA and the monomer residues that was left unpolymerized. The obtained PLA, white in color and in the form of powder, was dried in the vacuum oven at 40 - 50 °C for 2 - 4 hours. 460 g of PLA was obtained.

The efficiency of the Obtained PLA: 99%. Mw: 118.280 g/mol. Tg: 60.54°C.