Method for removing tin from polylactic acid

Abstract

The invention discloses a method for removing tin from polylactic acid in the technical field of material chemistry, which comprises the following steps: washing 12-crown ether-4, 1, 3-propylene glycol aqueous solution as a washing solution at 60-80 DEG C to remove trace tin in polylactic acid. Wherein the mass concentration of the 12-crown ether-4 in the water is 0.001%-0.002%, and the mass concentration of the 1, 3-propylene glycol in the water is 0.022%-0.034%. The method is simple to operate, the used reagent is easy to obtain and low in dosage, and the washing liquid main body is water which is harmless to the environment and is suitable for large-scale application.

Description

Technical Field

[0001] The invention belongs to the field of material chemistry, and specifically relates to the field of preparation of medical-grade polylactic acid. Background Art

[0002] In the medical field, poly (lactic acid) (PLA) has become an ideal material for innovative medical devices, drug delivery systems, and biodegradable implants due to its excellent biocompatibility and biodegradability. However, the production of PLA that meets medical standards is a major challenge. The key lies in ensuring its optical purity and molecular weight, as well as the control of metallic tin residues. At present, PLA is mainly prepared by polymerization of L-lactide. The optical and chemical purity of L-lactide is directly related to the mechanical properties and biodegradability of PLA. Through years of hard work, our research group has successfully developed a method for the synthesis of high-purity L-lactide (ACS Sustainable Chem. Eng. 2022, 10, 7658-7663), and applied it to the preparation of polylactic acid, successfully synthesizing a product with a molecular weight of 300,000 and qualified mechanical properties.

[0003] However, due to the unavoidable use of tin catalysts in the reaction of preparing polylactic acid by lactide polymerization, the tin content in its polylactic acid products is relatively high, generally above 200ppm, exceeding the 150ppm stipulated in the national standard, and thus cannot be directly used in the preparation of medical devices. The price of medical-grade polylactic acid is hundreds of times that of ordinary polylactic acid. Therefore, removing tin from ordinary polylactic acid to make it meet medical standards can significantly improve the economic value of the product, has great application significance, and is also a key technology to break the current situation of severe lack of domestic production capacity in the medical polylactic acid industry and monopoly by foreign companies.

[0004] Solvent washing is a widely used method for removing tin from polylactic acid. Traditional methods require the use of toxic organic solvents, which cause great environmental pollution. The development of aqueous phase tin removal technology can solve the environmental pollution problem. Summary of the invention

[0005] The present invention develops a method for removing tin from polylactic acid, which uses a non-toxic aqueous phase detergent and adds a trace amount of an auxiliary agent that can coordinate with tin to transfer the tin in the polylactic acid to the aqueous phase. The method is simple to operate, low in cost, and more environmentally friendly.

[0006] The object of the present invention is achieved in this way: the method uses 12-crown ether-4,1,3-propylene glycol aqueous solution as washing liquid and removes trace tin in polylactic acid by washing at 60-80°C.

[0007] A large number of comparative experiments show that 12-crown-4 is suitable for the removal of tin from polylactic acid. Other similar crown ethers, such as 15-crown-5, 18-crown-6, 1-aza-12-crown-4-ether, benzo-12-crown-4-ether, benzo-15-crown-5, benzo-18-crown-6-ether, etc., are not effective in the removal of tin.

[0008] A large number of comparative experiments have shown that 1,3-propylene glycol is suitable for detinning of polylactic acid. Other similar polyols, such as glycerol, 1,2-propylene glycol, 1,3-butylene glycol, ethylene glycol, etc., are not very effective in detinning.

[0009] The combination of 12-crown ether-4 and 1,3-propylene glycol is the best tin removal agent screened out, which is related to the spatial configuration of the hydrogen bond complex and its coordination structure with the metal. During the tin removal process, a hydrogen bond is formed between the oxygen on 12-crown ether-4 and one of the hydrogens in 1,2-propylene glycol, thereby assembling into a cage-like structure. The cavity of this structure just matches the size of tin, and the hydroxyl group at the other end of 1,2-propylene glycol can coordinate well with tin, thereby grabbing tin from the material and achieving the best tin removal effect.

[0010] In the present invention, the mass concentration of 12-crown ether-4 in water is 0.001% to 0.002%, preferably 0.0015%. If the concentration is too low, tin removal cannot be performed well, while if the concentration is too high, the tin removal efficiency will not be improved, but the cost will be increased.

[0011] In the present invention, the mass concentration of 1,3-propylene glycol in water is 0.022% to 0.034%, preferably 0.028%. If the concentration of 1,3-propylene glycol is too low, it cannot form a sufficient hydrogen bond complex structure, and the tin removal efficiency decreases. If the concentration of 1,3-propylene glycol is too high, it will continue to form an associated structure with 1,3-propylene glycol on the hydrogen bond complex structure, hindering its coordination with tin, and also significantly reducing the tin removal efficiency.

[0012] The temperature during tin removal washing is 60-80°C, preferably 70°C. If the washing temperature is too low, tin is difficult to be eluted from polylactic acid, thereby reducing the tin removal efficiency. If the washing temperature is too high, the complex structure of 12-crown ether-4 and 1,3-propylene glycol will be destroyed, thereby also reducing the tin removal efficiency.

[0013] Compared with the prior art, the invention has the beneficial effect that the tin in the polylactic acid is transferred to the water phase by adding a trace amount of an auxiliary agent that can coordinate with tin, thereby achieving the purpose of removing tin. The method is simple to operate, low in cost, and the reagents used are easily available and the amount used is low. The washing liquid is mainly water, which is harmless to the environment. DETAILED DESCRIPTION

[0014] Embodiment 1:

[0015] First, prepare 1 liter of aqueous solution with a mass concentration of 0.0015% of 12-crown ether-4 and a mass concentration of 0.028% of 1,3-propylene glycol. Add 200 grams of crushed crude polylactic acid (300 mesh, tin content of 232ppm) to a 5-liter beaker, and then add the above 1 liter of aqueous solution. Heat to 70°C and mechanically stir at this temperature for 3 hours. Cool to room temperature and filter. Dry the polylactic acid at 60°C in a vacuum drying oven for 1 hour. The tin content was determined to be 106ppm by inductively coupled plasma mass spectrometry (ICP-MS). Meet the national standard.

[0016] Embodiment 2:

[0017] The other conditions were the same as in Example 1, but 2 liters of washing liquid was used for tin removal. The ICP-MS analysis results showed that the tin content of the product was 103 ppm. Compared with Example 1, there was no significant decrease. This result shows that as long as the washing liquid can completely immerse the polylactic acid, tin can be effectively removed. Increasing the amount of washing liquid will not significantly improve the effect.

[0018] Example 3

[0019] The washing liquid used in Example 1 was reused to remove tin. The ICP-MS analysis results showed that the tin content of the product was 110ppm. The effect is similar to that of Example 1. This shows that the washing liquid can be used. This is because the tin content in the polylactic acid sample itself is not high. However, since the cost of the washing liquid is not high, in actual application, it can be decided whether to recycle the washing liquid according to actual needs.

[0020] Example 4

[0021] Other conditions were the same as in Example 1, and the 12-crown ether-4 was replaced by an analogue to test its tin removal effect. The results are shown in Table 1.

[0022] Table 1 Tin removal effects of various crown ether derivatives

[0023] serial number Crown ether derivatives (CAS number) Product Tin Content (ppm) 1 12-Crown ether-4(294-93-9) 106 (Example 1) 2 15-Crown ether-5(33100-27-5) 186 3 18-Crown-6(17455-13-9) 204 4 1-Aza-12-crown-4-ether(41775-76-2) 153 5 Benzo-12-crown-4-ether(14174-08-4) 138 5 Benzo-15-crown-5-ol(14098-44-3) 195 6 Benzo-18-crown-6-ether(14098-24-9) 211

[0024] From the above results, it can be seen that the best effect is achieved when 12-crown-4 is used. The effect is worse when other crown ethers are used, such as 15-crown-5 and 18-crown-6, and the product quality cannot meet the national standard. The effect is reduced when 12-crown-4 is derivatized and oxygen is replaced by nitrogen with stronger coordination ability. This shows that the atom coordinated with tin is not the oxygen (or nitrogen) on the crown ether, but the oxygen on the branch chain in the structure after it is hydrogen-bonded with 1,3-propylene glycol. After the crown ether is benzened, the performance generally decreases.

[0025] Embodiment 5:

[0026] Other conditions were the same as in Example 1, and various polyols were used for washing. The experimental results are shown in Table 2.

[0027] Table 2 Effect of polyols on tin removal

[0028]

[0029]

[0030] From the above results, it can be seen that the best effect is achieved when 1,3-propylene glycol is used. During the tin removal process, a hydrogen bond is formed between the oxygen on 12-crown ether-4 and one of the hydrogens in 1,2-propylene glycol, thereby assembling into a cage-like structure. The cavity of this structure just matches the size of tin, and the hydroxyl group at the other end of 1,2-propylene glycol can coordinate with tin well, thereby grabbing tin from the material. This theory is well confirmed by experiments. For example, Example 4 proves that the atom coordinated with tin is not the oxygen (or nitrogen) on the crown ether, but the oxygen on the branch chain in the structure after it is hydrogen-bonded with 1,3-propylene glycol. We found that the oxygen on 12-crown ether-4 and 1,2-propylene glycol are the best tin removal combination. If this combination is changed, the tin removal performance will decrease. For example, using glycerol instead of 1,2-propylene glycol, because there is an extra hydroxyl group on the middle carbon, a complex hydrogen bond structure can be formed, and the cavity structure is destroyed, which will interfere with the tin removal process. Due to the different spatial structures of the complexes, the cavity structures of 1,2-propylene glycol, 1,3-butylene glycol, and ethylene glycol do not completely match tin, and the tin removal effect of 1,3-propylene glycol cannot be achieved.

[0031] Embodiment 6:

[0032] Other conditions were the same as in Example 1, except that the concentration of 12-crown ether-4 was changed. The experimental results are shown in Table 3.

[0033] Table 3 Effect of 12-crown ether-4 concentration on tin removal effect

[0034] serial number 12-Crown ether-4 mass concentration (%) Product Tin Content (ppm) 1 0 229 2 0.0005 165 3 0.0010 142 4 0.0015 106 (Example 1) 5 0.0020 107 6 0.0025 107

[0035] From the above results, it can be seen that the best effect is achieved when the concentration of 12-crown ether-4 is 0.0015%.

[0036] Embodiment 7:

[0037] Other conditions were the same as in Example 1, except that the concentration of 1,3-propylene glycol was changed. The experimental results are shown in Table 4.

[0038] Table 4 Effect of 1,3-propylene glycol concentration on tin removal effect

[0039] serial number 1,3-Propanediol mass concentration (%) Product Tin Content (ppm) 1 0 230 2 0.020 169 3 0.022 142 4 0.024 128 5 0.026 117 6 0.028 106 (Example 1) 7 0.030 113 8 0.032 130 9 0.034 152

[0040] It can be seen from the above table that the best effect is achieved when the mass concentration of 1,3-propylene glycol is 0.028%.

[0041] Embodiment 8:

[0042] Keeping other conditions unchanged, the influence of washing temperature on its effect was studied. The results are shown in Table 5.

[0043] Table 5 Effect of washing temperature on tin removal effect 60~80

[0044]

[0045]

[0046] From the above, we can see that the best washing effect is achieved at 70°C.

[0047] The present invention is not limited to the above-mentioned embodiments. On the basis of the technical solution disclosed in the present invention, technicians in this field can make some substitutions and deformations to some technical features therein according to the disclosed technical content without creative labor, and these substitutions and deformations are all within the protection scope of the present invention.

Claims

1. A method for removing tin from polylactic acid, characterized in that: The trace tin in polylactic acid was removed by washing at 60-80°C with an aqueous solution of 12-crown ether-4,1,3-propylene glycol as washing liquid.

2. A polylactic acid detinning method according to claim 1, characterized in that: The mass concentration of the 12-crown ether-4 in water is 0.001% to 0.002%; the mass concentration of 1,3-propylene glycol in water is 0.022% to 0.034%.

3. A polylactic acid detinning method according to claim 2, characterized in that: The mass concentration of 12-crown ether-4 in water is 0.0015%.

4. A polylactic acid tin removal method according to claim 2, characterized in that: The mass concentration of 1,3-propylene glycol in water is 0.028%.

5. A polylactic acid detinning method according to claim 1, characterized in that: The temperature during washing was 70°C.

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