HAVING POLYLACTIC ACID

FIELD OF THE INVENTION

This invention relates to a process for manufacturing lactide from plastics having polylactic acid (PLA-based plastics).

BACKGROUND OF THE INVENTION

Presently, there are various approaches to minimize plastic wastes, e.g. lessening usage of plastic articles, using biodegradable plastics (e.g. plastics having polylactic acid (PLA-based plastics)), and disposing of such plastic wastes by landfill, incineration, recycling process, etc. The recycling process has received much attention since this process appears to be the most environmental-friendly approach. As cited above, however, the solution to the plastic waste's problem can be addressed either directly (e.g. lessening usage and disposal) or indirectly (e.g. using biodegradable materials). A process for manufacturing lactide and recycling process of PLA-based plastics have been disclosed in some prior arts, particularly patents.

U.S. Pub. No. 2013/0023674 Al discloses a method for producing lactide, which can be used for polylactic acid (PLA) production or other bioproducts, comprising melting the PLA feedstock in a heater to produce PLA liquid, heating the PLA liquid with a catalyst to depolymerize the PLA liquid and produce lactide vapour under vacuum atmosphere, and condensing the lactide vapour to produce liquid lactide.

U.S. Pub. No. 2012/0142958 Al discloses a process for recycling a polymer blend having polylactic acid (PLA), comprising grinding, compacting, dissolving in a solvent of PLA, removing the undissolved having polymers, alcoholysis depolymerization reaction and purification steps.

U.S. Patent No. 5,142,023 discloses a process for the continuous production of polylactide polymers from lactic acid which incorporates removal of water or a solvent carrier to concentrate the lactic acid feed followed by polymerization to a low-molecular- weight prepolymer. This prepolymer is fed to a reactor in which a catalyst is added to facilitate generation of lactide, the depolymerization product of polylactic acid. The lactide generated is continuously fed to a distillation system as a liquid or vapor wherein water and other impurities are removed. The resultant purified liquid lactide is fed directly to a polymerization process. SUMMARY OF THE INVENTION

This invention relates to a process for manufacturing lactide from plastics having polylactic acid (PLA-based plastics) comprises preparing PLA-based plastics, accelerating decomposition of polylactic acid in the plastics to provide low molecular weight polylactic acid, and thermal decomposing the low molecular weight polylactic acid to provide lactide. Also, the process further comprises minimizing size of the PLA-based plastics after the step of preparing PLA-based plastics, and purifying lactide after thermal decomposing low molecular weight polylactic acid to provide lactide.

An objective of this invention is to provide a process for manufacturing lactide from PLA-based plastics providing benefits in term of technique as follows:

- Can reduce time and energy consumption for decomposing polylactic acid in plastic. This is because said process only causes a partially chemical decomposition of polylactic acid. This differs from any existing processes for manufacturing lactide from PLA-based plastic, which, generally, decomposes completely until getting lactic acid. Moreover, applying the alcoholysis reaction in the process according to this invention provides faster chemical decomposition of polylactic acid in the plastic than using thermal decomposition as disclosed in U.S. Pub. No. 2013/0023674 Al . This results in a faster and more effective process for manufacturing lactide from PLA-based plastics.

- Can reduce steps in the process for manufacturing lactide from PLA-based plastics, which is normally conducted via polylactic acid decomposition to provide lactic acid, such as a lactic acid purification step, dehydration step, and polycondensation step to provide low molecular weight polylactic acid. The process for manufacturing lactide from PLA-based plastic according to this invention does not chemically decompose polylactic acid until obtaining lactic acid.

- Provide high purity lactide and low side reaction. Moreover, the condition applied in the step of decomposing polylactic acid in plastic according to this invention is not severe (mild condition).

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a life cycle of PLA-based plastic.

Figure 2 shows a continuous process for manufacturing lactide from PLA-based plastic. Figure 3 shows a mechanism for accelerating chemical decomposition of polylactic acid in plastic by alcoholysis reaction. Figure 4 shows 1H-NMR spectrum of purified lactide obtained from the process according to this invention.

Figure 5 shows GC-MS spectrum verifying structure and optical purity of the purified lactide obtained from the process according to this invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

A more detailed description about this invention will be explained as follows:

This invention relates to a process for manufacturing lactide from PLA-based plastics or recycling process of PLA-based plastics to produce lactide. Said process converts polylactic acid in plastic to lactide, which is a precursor or monomer in a process for manufacturing polylactic acid and its derivative by accelerating chemical decomposition of polylactic acid in the plastic by alcoholysis reaction or hydrolysis reaction. This provides lower molecular weight polylactic acid, which is suitable for thermal decomposition for converting to lactide (Fig. 1). Figure 1 also shows the recycling process of polylactic acid, which requires polylactic acid decomposition to provide lactic acid, and dehydration and polycondensation step to achieve low molecular weight polylactic acid, then thermal decomposition to produce lactide. The process according to this invention can be performed continuously and more efficiently by adjusting the condition in each step to provide process optimization, e.g. an input rate of reactants equivalent to an output rate of products in each stage (Fig. 2).

A process for manufacturing lactide from PLA-based plastic according to this invention comprising the steps of:

(i) preparing PLA-based plastic;

(ii) accelerating decomposition of the polylactic acid in plastic to provide low molecular weight polylactic acid; and

(iii) thermal decomposing the low molecular weight polylactic acid to provide lactide. The process for manufacturing lactide from PLA-based plastic according to this invention further comprises minimizing size of said PLA-based plastic after the PLA-based plastic preparation step. After minimizing size, said plastic has a dimension in a range from 0.1 to 10 mm, preferably no larger than 5 mm. This dimension makes it easy for feeding into the depolymerization reactor, and also increases the surface area. Hence, it can be molten easier, and shortens the melting time. However, said dimension varies with the type and size of the depolymerization reactor. According to this invention, the rninirnizing size is performed by at least one method selected from shearing, impacting, cutting into small pieces, and grinding. The grinding can be performed in either dry or wet condition, preferably by the grinder in the dry condition.

Moreover, the process for manufacturing PLA-based plastic according to the invention further comprises a step of purifying lactide after the step of thermal decomposition of low molecular weight polylactic acid to provide lactide

According to this invention, the PLA-based plastic has a ratio of polylactic acid from 1 to 100% by weight of total plastics, preferably a ratio of polylactic acid greater than 50% by weight of total plastics.

According to this invention, the preparation of PLA-based plastic comprising the steps of:

(i) washing said plastic with water, or washing with water and detergent or soap to remove any impurities in said plastic. These impurities may affect properties and purity of the products from the process, or the reaction conditions in each steps. and,

(ii) drying said plastic.

According to this invention, the preparation of PLA-based plastic further comprises a step of filtering said plastics after the washing step. The filtering step is performed using filter device under atmospheric condition or vacuum condition.

According to this invention, in the preparation step, said plastic is dried at a temperature of 50-80°C, preferably at a temperature of 55-70°C until all water completely evaporates.

The plastic is dried using either conventional oven or vacuum oven.

According to this invention, the accelerating decomposition of polylactic acid in plastic is performed by an alcoholysis reaction or a hydrolysis reaction. This is to accelerate decomposition of high molecular weight polylactic acid to low molecular weight polylactic acid, which is in a suitable range for thermal decomposition of low molecular weight polylactic acid to provide lactide in the lactide reactor.

According to this invention, the accelerating decomposition of polylactic acid in plastic by the alcoholysis reaction is performed by a reaction between PLA-based plastic and alcohol as shown in Fig. 3. A mechanism of said accelerating decomposition of polylactic acid in plastics is possible to begin with ester-bond breaking in the polylactic acid molecular chain allowing nucleophilic substitution. The nucleophile may include alcohol, organic acid, organic alkali, inorganic alkali, water, etc.

According to this invention, the accelerating decomposition of polylactic acid in the plastic by the alcoholysis reaction comprising the steps of:

(i) preparing a mixture of alcohol and catalyst; and

(ii) mixing the mixture obtained from (i) with the PLA-based plastic.

According to this invention, said alcohol is used in an amount from 0.1-10% by weight of the PLA-based plastic, preferably 2-7% by weight of the PLA-based plastic. Said alcohol is selected from the group consisting of an alcohol having 1 to 10 carbon atoms (CI -CI O) and having 1 to 10 hydroxyl groups, preferably glycerol (C ₃ H ₈ O ₃ ) or sorbitol (C ₆ Hi ₄ 06). Since said alcohol has a higher boiling point and large quantity of hydroxyl groups, resulting in rapid chemical decomposition of polylactic acid. Besides, it facilitates a lactide recovery from the alcohol.

Moreover, according to the invention, the catalyst is used in an amount of 0-5% by weight of the PLA-based plastic, preferably 0.1-0.5%) by weight of the PLA-based plastic. Said catalyst is selected from the group consisting of metal oxides, preferably antimony trioxide (Sb ₂ 0 ₃ ). Said catalyst is further selected from the group consisting of organometallic compounds having 1 to 20 carbon atoms (C1-C20), preferably organometallic compounds of tin, namely Sn (II); or titanium, namely Ti (II).

According to this invention, the preparation of a mixture of alcohol and catalyst is performed by mixing said catalyst with alcohol, and then stirring for good distribution and compatibility.

According to this invention, the accelerating decomposition of polylactic acid in plastic is performed under vacuum or under pressure of at least 1 bar, preferably 1 -10 bars; at a temperature of 170-250°C, preferably at a temperature of 190-210°C. The mixture obtained from (i) with the PLA-based plastic can be mixed before feeding into a depolymerization reactor, or mixed all together in the depolymerization reactor. The depolymerization reactor includes a batch reactor, or a continuous reactor including single screw extruder or twin screw extruder to allow continuous chemical decomposition of polylactic acid occurrence.

According to this invention, the accelerating decomposition of polylactic acid in plastic by the alcoholysis reaction provides polylactic acid having a number average molecular weight in a range of 500 to 30,000 g/mol, preferably in a range of 2,000-12,000 g/mol. According to this invention, the accelerating decomposition of polylactic acid in plastic by hydrolysis reaction further comprises a step of mixing reactant with the PLA-based plastic. Said reactant is selected from the group consisting of organic acid, inorganic acid, organic alkali, inorganic alkali, or water, preferably organic acids, namely lactic acid. Advantages of using lactic acid as reactant are no impurities in the reaction, and the lactic acid being used as a precursor in the lactide production. Moreover, the use of lactic acid as a reactant may not require other additional catalysts, e.g. metal oxides or organometallic compound. This is because the acid will act as an autocatalyst for polyester decomposition, such as polylactic acid. However, the use of acid as reactant may cause corrosion in the metal parts of the equipments. Therefore, precaution must be taken.

According to this invention, the hydrolysis reaction is performed under vacuum or under pressure of at least 1 bar, preferably 1-10 bars; at a temperature of 170-250°C, preferably at a temperature of 190-210°C. The hydrolysis reaction is performed in the depolymerization reactor including batch reactor, or a continuous reactor namely single screw extruder or twin screw extruder.

According to this invention, the thermal decomposition of low molecular weight polylactic acid to provide lactide comprising the steps of:

(i) feeding low molecular weight PLA-based plastic to the lactide reactor;

(ii) adding the catalyst to lactide reactor;

(iii) thermal decomposing polylactic acid to provide lactide; and

(iv) condensing lactide by cooling down to provide lactide solid or lactide liquid.

According to this invention, the low molecular weight PLA-based plastic is fed to lactide reactor by batch feed or continuous feed.

According to this invention, the catalyst fed in the reactor is used in an amount of 0-2% by weight of the PLA-based plastic, preferably 0.1 -1 .0% by weight of the PLA-based plastic. Said catalyst is selected from the group consisting of metal oxide or organometallic compounds having 1 to 20 carbon atoms (C1-C20), preferably organometallic compounds of tin, namely Sn (II); or titanium, namely Ti (II). However, the organometallic compounds are not limited to those listed above.

According to this invention, the thermal decomposition of polylactic acid to provide lactide is performed at a temperature of 190-250°C, preferably at a temperature of 200-220°C under vacuum or inert gas pressure, preferably under vacuum with a pressure of 0.001 to 0.1 bar. According to this invention, the condensation of lactide by cooling down to provide lactide solid is preferably performed at a temperatures below 25°C. The condensation of lactide by cooling down to provide lactide liquid is preferably performed at a temperature 100-110°C. This provides lactide, which is ready for further purification. While alcohol, various catalysts, and/or other substances in PLA-based plastic remain in the bottom section of the lactide reactor, and will be removed or reused again as the lactide reactor's maintenance.

According to this invention, the lactide is purified by distillation or recrystallization. The distillation is performed under vacuum or under inert gas pressure of at least 1 bar, preferably 1- 10 bars; at a temperature of 120-180°C, preferably at a temperature of 120-150°C.

According to this invention, the recrystallization in a lactide purification step comprising the steps of:

(i) dissolving lactide by solvent at a temperature of 50-70°C;

(ii) recrystallizing lactide at room temperature;

(iii) filtering crystalline lactide; and

(iv) drying crystalline lactide at a temperature of 50-70°C under vacuum.

According to this invention, the recrystallization is performed using a solvent selected from acetone, isopropanol, toluene, benzene, diethyl ether, or ethyl acetate, preferably ethyl acetate or toluene.

Said recrystallization may be repeated 2-3 times to provide high purity lactide as shown in Table 1.

Table 1 Purity of lactide from the process according to this invention compared with lactide produced by lactic acid in the laboratory, and commercial lactide.

* Purity of sample characterized by DSC technique Table 1 shows that the lactide from the process according to this invention has a high purity up to 99.8%, which is equivalent to the purity of lactide produced by lactic acid in the laboratory and the commercial lactide. Also, it has almost 100% optical purity as shown in Figure 5. From Ή-NMR and GC-MS techniques (as shown in Figs. 4 and 5), it reveals that the lactide from the process according to this invention has L-lactide structure, which is same structure as PLA-based plastic used as precursor in the process.

According to this invention, the purified lactide from a step of lactide purification can be reacted to further synthesize PLA-based plastic as shown in Figure 2.

The process for manufacturing lactide from PLA-based plastic according to the invention is not only limited to the procedures and details as stated above. But, it also includes any modification or any other equivalent approaches, which is still within the scope of this invention as defined in the claims.