KR20110078436A - Method for preparation of cross-linked pla thermoplastic - Google Patents

Abstract

PURPOSE: A method for preparing cross-linked poly(lactic acid) moldings is provided to improve heat resistance and mechanical properties of biodegradable poly(lactic acid) moldings by crosslinking poly(lactic acid) moldings through ultraviolet irradiation. CONSTITUTION: A method for preparing cross-linked poly(lactic acid) moldings comprises the steps of: (i) preparing a mixture by mixing poly(lactic acid) moldings with photoinitiators; and (ii) irradiating ultraviolet rays to the mixing poly(lactic acid) moldings mixed with photoinitiators and crosslinking the resultant. The photoinitiator is a hydrogen-substituted photoinitiator. The concentration of the photoinitiator is 1-20 weight%.

Description

Method for Preparation of Cross-linked PLA Thermoplastic

The present invention relates to a method for producing a crosslinked PLA molding having improved heat resistance and physical properties, and more particularly, to a method for preparing a crosslinked PLA molding by UV irradiation to a PLA molding containing a hydrogen-substituted photoinitiator and It relates to a PLA molding.

Recently, as countries and societies have increased interest in the environment, research on recycling of resources, biodegradability, and biocompatibility in the polymer materials field has been actively conducted. Among them, since polymer materials are mostly polymerized from petroleum resources, depletion of limited resources is serious. Therefore, as researches on compounds extracted from natural resources without using petroleum resources are increasing, interest in poly (lactic acid) (PLA) obtained through condensation polymerization based on lactic acid obtained from corn is increasing.

Poly (lactic acid) (PLA) is an aliphatic polyester based hydrocarbon. Aliphatic polyester is obtained by polymerizing lactic acid extracted from corn, a recycled raw material, through a condensation reaction. Therefore, since the biocompatibility is superior to the aromatic polyester to maintain the quality of food, it is manufactured and used as a film as a packaging material. It can be easily manufactured into woven or nonwoven fabric through melt spinning, and is used as automobile interior material because it has more hydrophilicity and lower flammability and smoke generation when burned by fire than aromatic polyester PET. It is also used as a medical biodegradable polymer material. PLA, a medical biodegradable polymer material, is used as a bone bonding material such as a matrix for controlling the release of low molecular weight drugs for treatment, a plate or a screw for bonding sutures or fractured skeletal bones. In general, the bone bonding material (screw) uses a biodegradable polymer material, but is known to have a disadvantage in that the mechanical strength is lost by hydrolysis in a short time. However, PLA has a limited glass transition temperature and melting point, low mechanical properties, and large drop in physical properties due to hydrolysis.

In order to solve these problems, studies are being actively conducted to introduce a crosslinked structure in such a manner as to improve heat resistance, thermal stability and physical properties of PLA, and to control hydrolysis. As a method of introducing a crosslinked structure, there is a method of producing a crosslinked PLA network structure during polymerization by copolymerizing with lactic acid or lactide using a multifunctional crosslinking agent. However, copolymerized PLA polymers have different crystallization and melting characteristics from pure PLA polymers, making it difficult to manufacture various moldings such as fibers, films, and sheets, and biodegradability is a major feature due to changes in hydrolysis and biodegradability. Difficult to be used for the purpose. In addition, the use of a large amount of water or solvent, resulting in environmental pollution, high energy consumption due to high temperature heat treatment, such as the development of more eco-friendly and energy-saving technology that can replace the cross-linking process by the thermal curing method is required. have.

Another method of introducing a crosslinked structure is an electron beam crosslinking method using electron beams or gamma rays. The polymer radicals are generated through hydrogen substitution of PLA chain using electromagnetic waves with strong energy, and crosslinking is formed by recombination. However, when electron beam or gamma ray is used, degradation of the polymer occurs and a high unit shielding facility is required at the time of operation, and it has a disadvantage of being made under vacuum or inert atmosphere.

Another electron beam crosslinking method is to use ultraviolet light having a wavelength shorter than that of visible light. Ultraviolet rays can not only cleave and oxidize molecular bonds of the irradiated surface organic material depending on the irradiation wavelength, but also easily polymerize and crosslink the photocurable monomer. Electron beam crosslinking method using ultraviolet light is replacing existing thermosetting method in various fields with high production speed, energy saving and eco-friendly process. This is likely to be an alternative to the existing thermosetting methods, including environmental problems, excessive energy consumption and excessive carbon dioxide emissions.

Accordingly, the present inventors confirmed that the heat resistance and thermal stability and physical properties are improved by containing the hydrogen-substituted photoinitiator benzophenone in the PLA molding and crosslinking the PLA molding by ultraviolet irradiation. Thus, the present invention has been completed.

The present invention is to provide a method for improving the heat resistance and physical properties, which is a weak point of the biodegradable PLA molding, and to control the hydrolyzability, and overcomes the problems of the conventional thermosetting crosslinking method and electron beam crosslinking method, The purpose is to provide a method for producing a highly productive crosslinked PLA molding while being environmentally friendly and saving energy.

The present invention relates to a method for producing a crosslinked PLA molding having improved heat resistance and physical properties, and more specifically, to a crosslinked PLA molding containing a hydrogen-substituted photoinitiator by UV irradiation treatment.

The crosslinked PLA moldings produced by the process according to the invention are characterized in that the gelation rate is at least 10%.

The photoinitiator used in the present invention is not particularly limited as long as the photoinitiator is excited by UV irradiation such as direct cleavage, hydrogen substitution, and ion reaction to cause photopolymerization, but the hydrogen substitution photoinitiator is not limited in the present invention. It is preferable to use, and hydrogen-substituted photoinitiators include benzophenone, camphorquinone, anthraquinone, thioxanthone or substituents or derivatives thereof . In particular, in the following examples, Benzophenone among hydrogen-substituted photoinitiators was contained in the PLA moldings.

In addition, the concentration of the preferred photoinitiator in the present invention may be 1 to 20% by weight based on the weight of the PLA molding, the ultraviolet irradiation energy at the time of ultraviolet irradiation is preferably carried out at 10 to 200J / cm ² .

Hereinafter, the present invention will be described in detail.

The present invention relates to crosslinked PLA moldings, and more particularly to crosslinked PLA moldings having a gelation rate of at least 10%.

The gelation rate is defined as the ratio of the weight change of the dried PLA molding to 30 minutes in a chloroform solution at room temperature to the weight of the original dried PLA molding.

The concentration of the photoinitiator contained in the PLA molding is not particularly limited, but may be preferably contained 1 to 20% by weight based on the weight of the PLA molding, more preferably 1 to 10% by weight relative to the weight of the PLA molding. This is because when the concentration of the photoinitiator is less than 1% by weight based on the weight of the PLA molding, the crosslinking effect is hardly seen, and when it exceeds 20% by weight, the mechanical properties are significantly reduced.

UV irradiation energy is preferably carried out at 10 to 200J / cm ² , if the irradiation energy is lower than 10J / cm ² can not see the cross-linking effect, if higher than 200J / cm ² photolysis rather than photocrosslinking of the PLA molding as unreasonable irradiation conditions Arises a problem that significantly impairs the physical properties of the molding.

On the other hand, in the present invention, the type of PLA molded article containing the photoinitiator is not particularly limited, but in consideration of the use, it may be preferable that the average molecular weight of 10,000 to 500,000, more preferably the average molecular weight of 100,000 to 300,000 days Can be. In addition, the thickness of the PLA molding is not particularly limited, but may preferably be 0.1 to 5 mm.

According to the present invention, by crosslinking the PLA molding using ultraviolet irradiation, not only can improve the heat resistance and mechanical properties of the biodegradable PLA molding, but also can control the hydrolyzability to expand the application field as a medical biodegradable polymer material. There is There is an effect that can solve the problems such as environmental problems, excessive energy consumption, excessive carbon dioxide emission caused by the thermal curing method through the existing heat treatment.

Hereinafter, the present invention will be described in detail through examples. The following examples are only examples for describing the present invention, and the scope of the present invention is not limited thereto.

<Examples>

Example  1 to Example  8

One. PLA  Manufacture of moldings

The used PLA has a molecular weight of about 200,000 g / mol through viscosity measurement, and the photoinitiator benzophenone is mixed at a concentration of 5% by weight based on the weight of PLA, melted in a heating plate at 190 ° C., compressed at a pressure of 10 MPa, and cooled. To prepare a PLA molding having a thickness of about 0.3 mm.

2. UV irradiation treatment

UV irradiation was used as a continuous UV irradiator having an output of 80W / cm, the 1.10, respectively on both surfaces of the resulting molded article from the PLA (Example 1), 20 (Example 2), 30 (Example 3) , 40 (Example 4), 50 (Example 5), 60 (Example 6), 70 (Example 7), 80 (Example 8) J / cm ² Irradiation treatment was carried out to prepare a crosslinked PLA molding.

3. Gelation rate  And Swelling ratio  Measure

The gelation rate is defined as the ratio of the dry weight change of the crosslinked PLA molding after drying the crosslinked PLA molding prepared in 2. and left in a chloroform solution at room temperature for 30 minutes. In addition, the swelling ratio is the ratio of the weight change of the swollen gel after leaving for 48 hours in chloroform at room temperature to the weight of the cross-linked PLA gel molding. Gelation rate and swelling ratio results for Examples 1 to 8 are shown in Table 1 below.

Example  9 to Example  12

Prepared in the same manner as in Example 1, except that the photoinitiator benzophenone is 1% (Example 9), 3 (Example 10), 7 (Example 11), 9 (Example 12) by weight relative to the weight of PLA, respectively. Mixing produced a PLA molding. Then, each of the PLA molding was irradiated with UV at 80 J / cm ^2, respectively, to prepare each crosslinked PLA molding.

The results of the gelation rate and swelling ratio of Examples 9 to 12 prepared above are shown in Table 1 below.

Comparative example  One

It was prepared in the same manner as in Example 1 except that benzophenone, a photoinitiator, was prepared without mixing, and was not subjected to UV irradiation.

Comparative example  2

Prepared in the same manner as in Comparative Example 1, but was irradiated with ultraviolet rays by 80J / cm ² each on both sides of the molding.

According to the following [Table 1], in the case of the PLA moldings prepared in Comparative Example 1 and Comparative Example 2 it can be confirmed that the gelation rate and swelling ratio is 0%. That is, it can be seen that the PLA moldings of Comparative Examples 1 and 2 cannot be crosslinked by UV irradiation alone, and thus there was no change in mechanical properties and thermal stability.

[Table 1] Changes in the gelation rate and swelling ratio of PLA moldings with UV irradiation energy

Photoinitiator Content UV irradiation energy
(J / cm ² )
Gelation rate (%)
Swelling ratio Example 1


5% 10 11.3 33.5 Example 2 20 35.2 24.2 Example 3 30 40.7 26.7 Example 4 40 67.1 20.8 Example 5 50 72.5 18.4 Example 6 60 75.3 15.7 Example 7 70 82.9 14.1 Example 8 80 91.8 12.5 Example 9 One%
80 88.7 13.7 Example 10 3% 90.4 13.0 Example 11 7% 93.8 10.8 Example 12 9% 97.4 9.1 Comparative Example 1 0% 0 0 0 Comparative Example 2 80 0 0

Differential Scanning Calorimeter ( Differential Scanning Calorimeter , DSC ) And thermogravimetric analyzer Thermogravimetric Analysis , TGA ) analysis

In order to measure the melting point and the heat of melting of the crosslinked PLA molding, the differential scanning calorimetry (DSC) was used at a temperature increase rate of 10 ° C. per minute from 10 ° C. to 200 ° C., and the results are shown in FIGS. 1 and 2. . Figure 1 shows the DSC analysis results of the PLA molding according to the ultraviolet irradiation energy ((a) is Comparative Example 1, (b) Example 2, (c) Example 8), Figure 2 according to the photo initiator content The DSC analysis result of the PLA molding is shown ((a) is Example 9, (b) is Example 8, (c) is Example 12).

1, it can be seen that as the UV irradiation energy increases, the degree of PLA crosslinking increases and the heat of fusion decreases. According to Figure 2 it can be seen that the melting point disappears as the photoinitiator content increases. Therefore, it can be seen that the heat resistance of PLA is improved by optical crosslinking.

In order to measure the thermal stability of the cross-linked PLA using a thermogravimetric analyzer (TGA) was measured at a temperature rising rate of 20 ℃ per minute from 50 ℃ to 500 ℃ in a nitrogen gas atmosphere, the results are shown in Table 2 below. .

According to the following [Table 2], as the UV irradiation energy and the photoinitiator content increases, the crosslinking degree of PLA increases, the heat of fusion decreases, the melting point disappears, and the thermal stability is improved by increasing the maximum pyrolysis temperature.

[Table 2] Thermal Stability of PLA Molded Products by UV Irradiation Energy and Photoinitiator Content

Photoinitiator Content UV irradiation energy
(J / cm ² ) Gelation rate (%) Maximum pyrolysis temperature (℃) Example 2
5% 20 35.2 392.4 Example 5 50 72.5 394.5 Example 8 80 91.8 395.9 Example 9 One%
80 88.7 391.9 Example 10 3% 90.4 395.9 Example 12 9% 97.4 397.2 Comparative Example 1 0% 0 0 389.5 Comparative Example 2 80 0 390.0

Mechanical property measurement

In order to measure the physical properties of the crosslinked PLA molding, it was measured while pulling at a speed of 0.5 mm per minute at 1 kN using a universal testing machine.

According to the following [Table 3], as the UV irradiation energy and the photoinitiator content increases, the strength and elastic modulus increase as the PLA is crosslinked.

[Table 3] Properties of PLA Molded Products by UV Irradiation Energy and Photoinitiator Content

Initiator content UV irradiation energy
(J / cm ² ) burglar
(MPa) Modulus
(GPa) Shindo
(%) Example 2
5% 20 54.4 1.95 3.6 Example 5 50 62.7 2.13 2.9 Example 6 60 63.1 2.15 2.9 Example 8 80 71.1 2.47 2.5 Example 9 One%
80 67.1 2.24 3.8 Example 10 3% 70.7 2.89 2.7 Example 12 9% 81.1 3.05 2.4 Comparative Example 1 0% 0 48.2 1.79 3.5 Comparative Example 2 80 45.6 1.67 3.6

Hydrolyzable  Measure

In order to measure the hydrolyzability of the crosslinked PLA molding, the crosslinked PLA molding was immersed in phosphate buffered saline at 37 ^° C., pH7. Robust measurements for evaluating hydrolytic properties were measured at a rate of 0.5 mm / min at 1 kN using a universal testing machine. In addition, the strong retention rate (%) was obtained by the following equation.

Strong retention rate (%) = (Strong after immersion) / (Initial strength) × 100

Table 4 below shows the measurement results of the strength of the crosslinked PLA molding according to the ultraviolet irradiation and accordingly strong retention, and also Figure 3 shows the results of the strength of the results of the following [Table 4] (( a) shows Comparative Example 1, (b) Example 2, (c) Example 6, (d) Example 8), and FIG. 4 shows the measurement results of the strong holding ratio among the results shown in Table 4 below. ((A) is Comparative Example 1, (b) is Example 2, (c) is Example 6, and (d) is Example 8).

According to the following [Table 4] and Figures 3 to 4, as the UV irradiation energy increases, as the PLA is crosslinked, the strength and the strong retention rate increase, and thus the hydrolysis resistance is improved, and the degree of crosslinking of the PLA molding is controlled. Since hydrolyzability can be adjusted, it can be used as a bone bonding material or the like as a medical biodegradable polymer material.

[Table 4] Evaluation of the hydrolyzability of PLA molding according to UV irradiation energy (strong, strong retention)

Immersion
time
(Work)
Example 2
Example 6 0 One 5 9 13 19 25 0 One 5 9 13 19 25 strong
(MPa)
54.2
53.4
50.4
41.6
29.7
18.8
14.6
63.1
62.7
61.0
56.3
51.1
37.7
26.6 strong
Retention rate
(%)
100
98.5
92.9
76.8
54.8
34.7
26.9
100
99.4
96.7
89.2
80.9
59.7
42.2 Immersion
time
(Work)
Example 8
Comparative Example 1 0 One 5 9 13 19 25 0 One 5 9 13 19 25 strong
(MPa)
70.9
70.7
69.4
66.9
58.6
45.9
36.2
48.2
48.0
40.3
29.0
21.8
9.2
1.4 strong
Retention rate
(%)
100
99.7
97.9
94.4
82.7
64.7
50.9
100
99.6
83.6
60.2
45.2
19.1
2.8

As described above, those skilled in the art will understand that the present invention can be implemented in other specific forms without changing the technical spirit or essential features. The scope of the present invention is shown by the claims to be described later rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalent concepts are included in the scope of the present invention. Should be.

Figure 1 shows the DSC analysis results of the PLA molding according to the ultraviolet irradiation energy, (a) is Comparative Example 1, (b) is Example 2, (c) is shown in Example 8.

2 shows the results of DSC analysis of the PLA molding according to the photoinitiator content, (a) shows Example 9, (b) shows Example 8, and (c) shows Example 12.

3 is a strong evaluation of the hydrolyzability of the PLA molding according to the ultraviolet irradiation energy, (a) is Comparative Example 1, (b) is Example 2, (c) is Example 6, (d) is an Example 8 is shown.

Figure 4 is to evaluate the hydrolyzability of the PLA molding according to the ultraviolet irradiation energy as a strong maintenance rate, (a) is Comparative Example 1, (b) is Example 2, (c) is Example 6, (d) is carried out Example 8 is shown.

Claims (7)

(a) mixing the PLA molding and the photoinitiator to prepare a PLA molding in which the photoinitiator is mixed; (b) irradiating UV crosslinking to the PLA molded product mixed with the photoinitiator prepared in step (a) by ultraviolet rays. The method of claim 1, The photoinitiator is a hydrogen substituted photoinitiator, characterized in that the method for producing a cross-linked PLA molding. 3. The method of claim 2, The hydrogen substituted photoinitiator is a method for producing a cross-linked PLA molding, characterized in that benzophenone. The method of claim 1, The concentration of the photoinitiator is a method for producing a cross-linked PLA molding, characterized in that 1 to 20% by weight based on the weight of the PLA molding. The method of claim 1, The step (b) is a method for producing a cross-linked PLA molding, characterized in that to perform ultraviolet irradiation in the energy range of 10 to 200J / cm ² . The method of claim 1, Step (a) is a PLA molding method of producing a crosslinked PLA molding, characterized in that the average molecular weight of 10,000 to 500,000, the thickness is 0.1 to 5mm. Crosslinked PLA moldings produced by the method of any one of claims 1 to 6.

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