Polylactic acid resin having improved meltability and reactivity and method for preparing same

Abstract

The polylactic acid resin and the preparation method thereof according to the present disclosure have the following characteristics: a melt index can be increased and fluidity can be improved by a simple method of treating the polylactic acid resin with water. Therefore, the reactivity of the polylactic acid resin can be improved, the compatibility with various additives can be improved, and the application field of the polylactic acid resin can be further expanded.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Korean Patent Application No. 10-2023-0087615, filed on July 6, 2023, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates to a polylactic acid resin having improved solubility and reactivity and a method for preparing the same. Background Art

[0004] Polylactic acid (PLA) is a plant-derived resin obtained from plants such as corn, and has attracted attention as an excellent environmentally friendly material due to its biodegradability. Unlike currently used petroleum-based resins such as polystyrene resins, polyvinyl chloride resins or polyethylene resins, polylactic acid has the effects of preventing the depletion of petroleum-based resources and inhibiting carbon dioxide emissions, and thus can reduce environmental pollution, which is a disadvantage of petroleum-based plastic products.

[0005] As environmental pollution caused by waste plastics and the like has emerged as a social problem, efforts have been made to expand the application range of polylactic acid to various fields including food packaging materials and containers, electronic product housings, etc., which have applied general plastics (petroleum-based resins).

[0006] Polylactic acid has a melt index (MI) in a variety of ranges required for the product to be produced. Among them, polylactic acid with a high melt index may be required, and for example, polylactic acid with a high melt index (500-3000 g / 10 min at 230° C.) is required to produce melt-blown nonwoven fabrics. However, since the melt index of polylactic acid generally prepared by commercial methods has a range of 5 g / 10 min to 200 g / 10 min (as measured by ASTM D 1238 at 230° C. with 2.16 kg), the polymerization of polylactic acid with the above-mentioned high melt index is not easy to be carried out by conventional methods.

[0007] The polymerization of polylactic acid with high fluidity is difficult for various reasons, but typically, it basically includes a process for removing volatile substances such as residual monomers after the polymerization of polylactic acid (devolatilization process), but there are problems such as that part of the polylactic acid with high fluidity may evaporate together, which may make the process unsmooth and difficult to form pellets during granulation. In addition, polylactic acid synthesized by conventional polymerization methods has a low acid value, which limits post-processing and / or modification. For example, there is a problem of reduced reaction with a chain extender or a branching agent.

[0008] Therefore, the present inventors conducted extensive studies to solve the above problems and found that by treating the prepared polylactic acid with distilled water as described below, the melt index can be increased and the fluidity thereof can be improved at the same time, thereby preparing polylactic acid having high fluidity, and completed the present disclosure. Summary of the invention

[0009] [Technical issues]

[0010] An object of the present disclosure is to provide a polylactic acid resin having improved solubility and reactivity and a method for preparing the same.

[0011] [Technical solution]

[0012] In order to achieve the above objectives, according to the present disclosure, a polylactic acid resin having the following characteristics is provided:

[0013] The melt index measured at 230°C and 2.16 kg according to ASTM D 1238 is 300 g / 10 min to 3000 g / 10 min,

[0014] The acid value is 20 mmol acid / kg to 100 mmol acid / kg, and

[0015] The inorganic impurity content is less than 100 mg / kg.

[0016] The polylactic acid resin according to the present disclosure is characterized in that the melt index is increased and the fluidity is improved at the same time by the preparation method described below, and the polylactic acid resin is also characterized in that it contains almost no inorganic impurities because no other chemicals or catalysts are used in the preparation method.

[0017] Preferably, the weight average molecular weight of the polylactic acid resin is 50,000 to 100,000.

[0018] Preferably, the melt index is 400 g / 10 min or more, 500 g / 10 min or more, 600 g / 10 min or more, 700 g / 10 min or more, 800 g / 10 min or more, 900 g / 10 min or more, or 1000 g / 10 min or more; and 2900 g / 10 min or less, 2800 g / 10 min or less, 2700 g / 10 min or less, 2600 g / 10 min or less, 2500 g / 10 min or less, 2400 g / 10 min or less, 2300 g / 10 min or less, 2200 g / 10 min or less, 2100 g / 10 min or less, or 2000 g / 10 min or less.

[0019] Preferably, the acid value is 30 mmol acid / kg or more; 90 mmol acid / kg or less, 80 mmol acid / kg or less, 70 mmol acid / kg or less, or 60 mmol acid / kg or less.

[0020] Preferably, the inorganic impurities are Sn and P, and the total content of Sn and P is 85 mg / kg or less. In addition, preferably, the inorganic impurities are Na, Zn, Ti, Al, Ag, Mg, Mn, Ba, Ca, Co, Cr, Cu, Fe, Ga, K, Li, Ni, Sb, Sr, V and Zr, and the total content of Na, Zn, Ti, Al, Ag, Mg, Mn, Ba, Ca, Co, Cr, Cu, Fe, Ga, K, Li, Ni, Sb, Sr, V and Zr is 10 mg / kg or less.

[0021] In addition, according to the present disclosure, a method for preparing the above-mentioned polylactic acid resin is provided, the method comprising the following steps:

[0022] Immersing the polylactic acid resin in water (step 1);

[0023] maintaining the temperature of the water in step 1 at 65°C to 99°C (step 2); and

[0024] The polylactic acid resin is recovered from the water (step 3).

[0025] The present invention is characterized in that the melt index of polylactic acid can be increased and the fluidity thereof can be improved simultaneously by a simple method of treating polylactic acid generally prepared by a commercial method with water. Hereinafter, the present disclosure will be described in detail step by step.

[0026] (Step 1)

[0027] Step 1 of the present disclosure is a step of immersing the polylactic acid resin in water, which is a step of preparing the polylactic acid by hydrolysis.

[0028] The polylactic acid resin used in step 1 is generally prepared by a commercial process and preferably has a melt index of 5 to 200 g / 10 min (as measured at 230° C. with 2.16 kg according to ASTM D 1238). In other words, the present disclosure prepares a polylactic acid resin having high fluidity by hydrolyzing a general polylactic acid resin having low fluidity as described below, rather than polymerizing a polylactic acid resin having high fluidity.

[0029] Preferably, the weight average molecular weight of the polylactic acid resin used in step 1 is 100,000 to 250,000. More preferably, the weight average molecular weight of the polylactic acid resin is 110,000 or more; and 240,000 or less, 230,000 or less, 220,000 or less, 210,000 or less, 200,000 or less, 190,000 or less, 180,000 or less, 170,000 or less, 160,000 or less, or 150,000 or less.

[0030] Preferably, the melt index (measured according to ASTM D 1238 at 230° C. with 2.16 kg) of the polylactic acid resin of step 1 is 10 g / 10 min or more, 20 g / 10 min or more, 30 g / 10 min or more, 40 g / 10 min or more, 50 g / 10 min or more, 60 g / 10 min or more, 70 g / 10 min or more, 80 g / 10 min or more, 90 g / 10 min or more, or 100 g / 10 min or more; and 190 g / 10 min or less, 180 g / 10 min or less, 170 g / 10 min or less, 160 g / 10 min or less, 150 g / 10 min or less, or 140 g / 10 min or less.

[0031] Preferably, the water is distilled water. Preferably, the weight ratio of the polylactic acid resin to water in step 1 is 1:0.1 to 1:10. More preferably, the weight ratio of the polylactic acid resin to water in step 1 is 1:0.2 or more, 1:0.3 or more, or 1:0.4 or more; and 1:9 or less, 1:8 or less, 1:7 or less, 1:6 or less, 1:5 or less, 1:4 or less, 1:3 or less, 1:2 or less, or 1:1 or less.

[0032] On the other hand, since the polylactic acid resin is added to water, it is preferably used in the form of pellets. The pellets are not particularly limited as long as they have a shape widely used in the technical field to which the present disclosure pertains.

[0033] (Step 2)

[0034] Step 2 of the present disclosure is a step of maintaining the temperature of the water in step 1 at 65 to 99° C., which is a step of hydrolyzing the polylactic acid resin.

[0035] "Hydrolyzing polylactic acid resin" means hydrolyzing a part of polylactic acid resin, especially hydrolyzing a part of polylactic acid resin with low fluidity. This improves the fluidity of the polylactic acid resin as a whole.

[0036] Preferably, the temperature of the water in step 1 is maintained at above 66°C, above 67°C, above 68°C, above 69°C, or above 70°C; and below 98°C, below 98°C, below 97°C, below 96°C, below 95°C, below 94°C, below 93°C, below 92°C, or below 91°C.

[0037] Preferably, step 2 is carried out under normal pressure, which refers to 1 atm±0.1 atm.

[0038] Preferably, step 2 is performed for 0.5 to 24 hours. If the time required to perform this step is less than 0.5 hours, too little hydrolysis occurs and the improvement in fluidity is slight. On the contrary, if the time required to perform this step is more than 24 hours, there is a problem that too much hydrolysis occurs and the fluidity is too high. More preferably, step 2 is performed for 1 hour or more; and 22 hours or less, 20 hours or less, 18 hours or less, 16 hours or less, 14 hours or less, 12 hours or less, or 10 hours or less.

[0039] (Step 3)

[0040] Step 3 of the present disclosure is a step of recovering the polylactic acid resin from water after the treatment of step 2.

[0041] Since the polylactic acid resin has been in contact with high temperature water for a long time, it is preferably cooled at a fast rate. Therefore, the polylactic acid resin recovered in step 3 is preferably cooled to 10° C. to 20° C. For this purpose, it is preferably brought into contact with cold water at 10° C. or below.

[0042] Furthermore, in order to subsequently finish the polylactic acid resin, it is preferred to sufficiently remove even a small amount of water, and for this purpose, it is preferred to dry under reduced pressure. It is preferred to dry so that the moisture content of the polylactic acid resin becomes 100 ppmw or less.

[0043] The drying method is not particularly limited, but as an example, it is preferred to dry at 25° C. to 35° C. under reduced pressure until the moisture content of the polylactic acid resin becomes 5,000 ppmw or less, and then dry at 80 to 90° C. under reduced pressure until the moisture content becomes 100 ppmw or less. If the moisture content exceeds 5,000 ppmw, hydrolysis may further proceed when drying at 80 to 90° C. under reduced pressure, and there is also a problem that it takes too long for the moisture content to reach 100 ppmw when drying at 25° C. to 35° C. under reduced pressure.

[0044] The polylactic acid resin recovered in step 3 is characterized by an increased melt index and improved fluidity compared to the polylactic acid resin before treatment.

[0045] Preferably, the weight average molecular weight of the polylactic acid resin recovered in step 3 is 50,000 to 100,000.

[0046] Preferably, the polylactic acid resin recovered in step 3 has a melt index of 300 to 3000 g / 10 min (measured at 230° C. with 2.16 kg according to ASTM D 1238).

[0047] Preferably, the preparation method according to the present disclosure satisfies the following mathematical formula 1:

[0048] [Mathematical formula 1]

[0049] 3.0≤MI1 / MI0≤24

[0050] In mathematical formula 1,

[0051] MI0 is the melt index of the polylactic acid resin of step 1 (measured according to ASTM D 1238 at 230°C with 2.16 kg)

[0052] MI1 is the melt index of the polylactic acid resin recovered in step 3 (measured at 230° C. with 2.16 kg according to ASTM D 1238).

[0053] Preferably, the preparation method according to the present disclosure satisfies the following mathematical formula 2:

[0054] [Mathematical formula 2]

[0055] 4.5≤MI1 / MI0≤24

[0056] In Math. 2, MI0 and MI1 are as defined in Math. 1.

[0057] [Beneficial Effects]

[0058] As described above, the preparation method according to the present disclosure has the following characteristics: the melt index can be increased and the fluidity can be improved by a simple method of treating the polylactic acid resin with water. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 and 2 The results of the experimental examples of the present disclosure are shown. DETAILED DESCRIPTION

[0060] Hereinafter, exemplary embodiments of the present disclosure will be described in more detail with reference to the following examples. However, the following examples are provided for illustrative purposes only, and the scope of the present disclosure is not intended to be limited thereby.

[0061] In the following examples and experimental examples, various physical properties were measured as follows:

[0062] - Flowability (MI; Melt Index): measured according to ASTM D 1238 at 230°C and 1.2 kg.

[0063] -Acid value measurement: Polylactic acid resin (1.5 g) was fully dissolved in CHCl3 (60 mL), and then titrated with 0.01 N KOH ethanol solution to measure and convert the acid value of each resin. At this time, the unit of acid value is [mmol acid / kg PLA]. Specifically, the membrane part (opaque white part) of the electrode and the titration solution tube were set to always be immersed in the solution. The acid value of the polylactic acid resin was calculated by the amount of KOH used.

[0064] -Weight average molecular weight (Mw), number average molecular weight (Mn) and molecular weight distribution (PDI, polydispersity index): The weight average molecular weight and number average molecular weight were measured by using Agilent 12000 series GPC, calibrated with PC standards. In addition, the value obtained by dividing the weight average molecular weight by the number average molecular weight was calculated as the molecular weight distribution (PDI).

[0065] Example 1

[0066] Polylactic acid (Mw: 116,700; PDI: 1.88, MI: 120 g / 10 min) pellets (30 kg) were placed in a 50 L SUS container A and vacuum dried overnight at 110° C. At this time, the shape of the pellets was spherical or egg-shaped with a diameter of 3 to 5 mm.

[0067] Distilled water (15 kg) at 95°C is added to container A containing the vacuum-dried pellets, and the mixture is stirred and then placed in an oven preheated to the same temperature as the added distilled water and kept for 2.5 hours. After completion, excess water below 10°C is poured into container A containing the mixture, and the mixture is quickly cooled to 10-20°C. The cooled pellets are separated from the water using a filter, and then dried under reduced pressure at 30°C for 8 to 24 hours so that the moisture content becomes less than 5,000 ppm. Then, the pellets with a moisture content of less than 5,000 ppm are dried under reduced pressure at 85°C for 8 to 24 hours so that the moisture content becomes less than 100 ppm. The weight average molecular weight, molecular weight distribution, MI and acid value of the obtained pellets are measured and are shown in Table 1 below.

[0068] Embodiment 2-4

[0069] The polylactic acid pellets were treated in the same manner as in Example 1, except that the temperature of the distilled water and / or the holding time in the oven were changed as shown in Table 1 below.

[0070] Comparative Example

[0071] Polylactic acid (Mw: 116,700; PDI: 1.88, MI: 120 g / 10 min) pellets which were not treated alone were used as comparative examples.

[0072] Experimental Example 1

[0073] The weight average molecular weight, molecular weight distribution, MI and acid value of the pellets obtained according to Examples and Comparative Examples were measured and shown in Table 1 below.

[0074] [Table 1]

[0075]

[0076] Experimental Example 2

[0077] The polylactic acid pellets of the comparative example (hereinafter referred to as 'A') and 2 phr of ADR-4468 (branching agent, BASF) were mixed at 200° C. and 60 rpm for 5 minutes using a Brabender mixer. In addition, the polylactic acid pellets obtained in Example 4 (hereinafter referred to as 'B') and 2 phr of ADR-4468 (branching agent, BASF) were mixed at 200° C. and 60 rpm for 5 minutes using a Brabender mixer.

[0078] The weight average molecular weight, PDI and MI of each prepared melt were measured and the results are shown in Tables 2 and Figure 1 and 2 middle.

[0079] [Table 2]

[0080]

[0081] As shown in Table 2 and Figure 1 As shown in , sample A is a polylactic acid resin not treated with distilled water according to the present disclosure, and its Mw increases by about 123% due to the addition of a branching agent, while sample B is a polylactic acid resin treated with distilled water according to the present disclosure, and its Mw increases by about 244% due to the addition of a branching agent, thereby confirming that the reactivity of the polylactic acid according to the present disclosure treated with a branching agent is significantly improved.

[0082] Experimental Example 3

[0083] The inorganic impurity contents of the polylactic acid pellets obtained in Examples 1 to 4 were measured by the following method.

[0084] The analysis was performed using an inductively coupled plasma optical emission spectrometer (ICP-OES) using scandium (Sc) as an internal standard, and applying an RF power of 1300 W, a plasma gas flow rate of 15.00 L / min, a sample gas flow rate of 0.8 L / min, and an auxiliary gas flow rate of 0.20 L / min.

[0085] The results are shown in Table 3 below.

[0086] [Table 3]

[0087]

[0088] As shown in Table 3, the polylactic acid according to the present disclosure uses hydrolysis without using other chemicals or catalysts, and is thus characterized in that it contains almost no inorganic impurities.

[0089] Experimental Example 4

[0090] Nonwoven fabrics were prepared from the polylactic acid pellets obtained in Examples 1 to 3 and Comparative Example by the following method, and the results were evaluated.

[0091] Each polylactic acid pellet is heated and extruded, transferred to a nozzle, and passed through a hole with a diameter of 0.2 mm and 32 per inch to spin fibers. A melt-blown nonwoven fabric with an average fiber diameter of 15 μm or less is formed by high-temperature, high-speed gas ejected from a nozzle around the nozzle during spinning, and the process temperature at this time is shown in Table 4 below. The diameter of the nonwoven fibers was analyzed using a scanning electron microscope (SEM).

[0092] The results are shown in Table 4 below.

[0093] [Table 4]

[0094]

[0095] As shown in Table 4, it is confirmed that the polylactic acid according to the present disclosure has a low MI and can be made into ultrafine fibers during a melt-blowing process.

Claims

1. A polylactic acid resin having the following characteristics: The melt index measured at 230°C and 2.16 kg according to ASTM D 1238 is 300 g / 10 min to 3000 g / 10 min, The acid value is 20 mmol acid / kg to 100 mmol acid / kg, and The inorganic impurity content is less than 100 mg / kg.

2. The polylactic acid resin according to claim 1, wherein The polylactic acid resin has a weight average molecular weight of 50,000 to 100,000.

3. The polylactic acid resin according to claim 1, wherein The melt index is 300 g / 10 min to 2000 g / 10 min.

4. The polylactic acid resin according to claim 1, wherein The acid value is 30 mmol acid / kg to 100 mmol acid / kg.

5. The polylactic acid resin according to claim 1, wherein: The inorganic impurities are Sn and P, and The total content of Sn and P is less than 85 mg / kg.

6. The polylactic acid resin according to claim 1, wherein: The inorganic impurities are Na, Zn, Ti, Al, Ag, Mg, Mn, Ba, Ca, Co, Cr, Cu, Fe, Ga, K, Li, Ni, Sb, Sr, V and Zr, and The total content of Na, Zn, Ti, Al, Ag, Mg, Mn, Ba, Ca, Co, Cr, Cu, Fe, Ga, K, Li, Ni, Sb, Sr, V and Zr is less than 10 mg / kg.

7. A method for preparing the polylactic acid resin according to any one of claims 1 to 6, the method comprising the following steps: Step 1: Soak the polylactic acid resin in water; Step 2: maintaining the temperature of the water in step 1 at 65°C to 99°C; and Step 3: Recovering the polylactic acid resin from water.

8. The method according to claim 7, wherein: The polylactic acid resin of step 1 has a melt index of 5 to 200 g / 10 min, as measured at 230° C. and 2.16 kg according to ASTM D 1238.

9. The method according to claim 7, wherein: In step 1, the weight ratio of the polylactic acid resin to water is 1:0.1 to 1:

10.

10. The method according to claim 7, wherein: The temperature of the water in step 2 is maintained at 70°C to 99°C.

11. The method according to claim 7, wherein: Step 2 is carried out under normal pressure.

12. The method according to claim 7, wherein: Step 2 is performed for 0.5 hour to 24 hours.

13. The method according to claim 7, wherein: Satisfies the following mathematical formula 1: [Mathematical formula 1] 3.0≤MI1 / MI0≤24 In mathematical formula 1, MI0 is the melt index of the polylactic acid resin of step 1, measured at 230°C and 2.16 kg according to ASTM D 1238, MI1 is the melt index of the polylactic acid resin recovered in step 3, measured at 230° C. and 2.16 kg according to ASTM D 1238.

14. The method according to claim 1, wherein: The weight average molecular weight of the polylactic acid resin in step 1 is 100,000 to 250,000.

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