WO2022149646A1 - Esterified eco-friendly pva-lignin resin, preparation method therefor, and eco-friendly natural-fiber-reinforced composite obtained therefrom - Google Patents

Abstract

The present invention relates to: an esterified PVA-lignin resin obtained by mixing a PVA, lignin and a cross-linking agent and esterifying same through heat application; a natural fiber-reinforced composite using same; and an eco-friendly preparation method therefor. According to the present invention, provided is an eco-friendly resin and a natural-fiber-reinforced composite which have high tensile strength, modulus of elasticity, strain at break and adhesive strength, and excellent hydrophobicity.

Description

Esterified eco-friendly PVA-lignin resin, manufacturing method thereof, and eco-friendly natural fiber-reinforced composite material obtained therefrom

The present invention relates to an esterified eco-friendly PVA-lignin resin, a manufacturing method thereof, and an eco-friendly natural fiber-reinforced composite obtained therefrom. More specifically, it relates to a method for producing an esterified lignin-based eco-friendly resin by mixing lignin and PVA, adding a crosslinking agent, and applying heat, and an eco-friendly natural fiber-reinforced composite using the resin. In particular, the PVA-lignin resin of the present invention has excellent mechanical properties and adhesion to natural fibers while being environmentally friendly.

Eco-friendly fiber-reinforced composites are becoming increasingly important due to their potential to replace petroleum-based materials in an era where energy and the environment are important. In particular, to make eco-friendly fiber-reinforced composites, resins must also be eco-friendly. As candidates for eco-friendly resins, lignin, cardanol, vanilin, fatty acid, isosobide, tannin, and plant oil are thermosetting or thermoplastic. is used as a material. Among them, lignin is an eco-friendly polymer that is produced the most in nature after cellulose, and is attracting attention as an eco-friendly material due to its sustainable resources, biodegradability, low price, and abundance.

If you look at the cell wall of wood, wood fibers, whose main component is cellulose, are stacked at various angles, and lignin is mainly located between the cell wall and the adjacent cell wall. are doing Lignin, a phenolic polymer, has a complex structure comprising three different aromatic alcohol units. Due to the stiffness of the aromatic ring, the thermosetting polymer using it has improved thermal and mechanical properties. However, lignin has a free OH-group, has low solubility, and has a chemically complex structure, so it is not easy to make it into a thermoplastic polymer. Therefore, efforts are being made to improve the properties of lignin by modifying it.

Recently, various functionalizations of epoxide lignin and lignin have been made to increase adhesion and improve mechanical properties by making lignin-based resins with properties similar to tannins. In addition, lignin-based thermosetting polymers were prepared by allylation of kraft lignin and crosslinking with thiol-ene. Polyvinyl alcohol (PVA) is often used to crosslink lignin to increase bonding strength, thermal stability, and mechanical strength. These lignin-PVA polymers are used as thermoplastic, thermosetting, UV-blocking, and antibacterial materials.

In eco-friendly fiber-reinforced composites, eco-friendly fibers containing cellulose as a main component are used. Cellulose is the most abundant organic material in nature, with about 150 billion tonnes of which is produced annually in nature. Wood fibers are composed of macrofibers having a diameter of less than microns, which in turn are hierarchically composed of microfibers and nanofibers . Cellulose is renewable, biodegradable, and has excellent thermal stability. In addition, it has various advantages such as low price and high strength. Cellulose fibers are composed of microfibers called cellulose nanofibers (CNFs) of high crystalline quality. These highly crystalline cellulose nanofibers generally have a width of 5 to 200 nm and a length of several micrometers, and have unique physical and chemical properties. Cellulose has recently received great attention as it has a wide influence on many fields such as pharmaceuticals, coatings, textiles, laminated materials, sensors, actuators, flexible electronics, and flexible displays. In particular, due to its high mechanical properties and eco-friendliness, CNF is very suitable for eco-friendly fiber-reinforced composites.

However, in order to manufacture such an eco-friendly fiber-reinforced composite, resin must also be an eco-friendly material. Epoxy is widely used in fiber-reinforced composites because of its good mechanical properties and chemical and thermal stability. However, commercial epoxies are not completely environmentally friendly, biodegradable, and expensive.

As described above, lignin-based resins developed through various modifications and functionalizations so far have disadvantages in that they have low mechanical properties and thermal stability for use in eco-friendly fiber-reinforced composites.

As a prior art to this, the present inventors have disclosed a PVA-lignin-based resin (Ko et al., J. Appl. Polym. Sci. 135 , 46655, 2018), [Ko et al. J. Appl. Polym. Sci. 237, 48836 , 2019]) was developed.

According to the prior art, PVA-lignin resin has excellent eco-friendliness, but has problems in mechanical properties, adhesive strength, thermal stability and hydrophobicity still low, and therefore there is still a limit to use in natural fiber-reinforced composites.

In order to solve the problems of the prior art, the present invention is an economical, environmentally friendly, and easy mass production method, using citric acid (CA) as a crosslinking agent, adding it to the PVA-lignin compound and esterifying it, An object of the present invention is to provide a method for manufacturing an esterified PVA-lignin eco-friendly resin with high mechanical properties and adhesive strength.

In addition, an object of the present invention is to provide an eco-friendly natural fiber-reinforced composite material obtained by using an esterified eco-friendly PVA-lignin resin.

The above-described task, after mixing polyvinyl alcohol and lignin, the step of reacting by adding a crosslinking agent to the mixture; and heating the reactant to 160° C. to 200° C. to prepare an esterified PVA-lignin resin.

Preferably, the mixture and the crosslinking agent may be mixed in a weight ratio of 6:4 to 8:2.

Also preferably, the polyvinyl alcohol and the lignin may be mixed in a weight ratio of 6:4 to 7:3.

In addition, the object of the present invention is achieved by the environmentally friendly PVA-lignin resin produced by the above method.

Preferably, the resin may have a tensile strength of 150 MPa or more, an elastic modulus of 5 GPa or more, a breaking strain of 6% or more, an adhesive strength of 20 MPa or more, and hydrophobicity.

In addition, the object of the present invention is achieved by a natural fiber-reinforced composite prepared by impregnating the composite made of natural fibers in the eco-friendly PVA-lignin resin prepared by the above method.

Until now, lignin-based eco-friendly resins still have low mechanical properties, adhesive strength, thermal stability, and hydrophobicity, which has limited their use in natural fiber-reinforced composites. because it is weak. Accordingly, the present invention solves the disadvantages of the PVA-lignin resin, and in order to solve the problems of the prior art as described above, the PVA-lignin compound is esterified using a crosslinking agent such as citric acid to form a PVA-CA-lignin By strengthening the bond, the mechanical properties, adhesive strength, thermal stability, and hydrophobicity of e-PCL resin were improved.

As described above, in the present invention, the eco-friendly resin is esterified by applying temperature without using a catalyst, so it is economical, eco-friendly, and easy to mass-produce, so it will be used in more diverse fields.

1 and 2 are FTIR spectra according to the citric acid crosslinking agent content of the e-PCL resin prepared according to Example 1 of the present invention and heating time to 180°C.

3 is a tensile stress-strain curves according to the citric acid content of the e-PCL resin prepared according to Example 1 of the present invention.

4 shows the thermogravimetric analysis results according to the citric acid content of the e-PCL resin prepared according to Example 1 of the present invention.

5 is a view showing the water contact angle results according to the citric acid content of the e-PCL resin prepared according to Example 1 of the present invention.

6 shows the results of the adhesion strength between the e-PCL resin and the CNF film prepared according to Example 1 of the present invention.

7 shows the fractured cross-section of the e-PCL resin and CNF fiber-reinforced composite prepared according to Example 2 of the present invention.

All technical terms used in the present invention, unless otherwise defined, have the following definitions and have the meanings as commonly understood by one of ordinary skill in the art of the present invention. In addition, although preferred methods and samples are described herein, similar or equivalent ones are also included in the scope of the present invention.

The term "about" means 30, 25, 20, 15, 10, 9, 8, 7, 6, 5, means an amount, level, value, number, frequency, percentage, dimension, size, amount, weight or length varying by 4, 3, 2 or 1%.

Throughout this specification, unless the context requires otherwise, the terms "comprises" and "comprising" include the steps or elements presented, or groups of steps or elements, but include any other step or element, or It is to be understood that a step or group of elements is meant to be implied not to be excluded.

The present invention relates to a method for producing an eco-friendly PVA-lignin resin.

As shown in Scheme 1 below, the present invention comprises the steps of preparing hydrogenated PVA-lignin (H-PCL) by mixing polyvinyl alcohol (PVA) and lignin and then reacting with citric acid as a crosslinking agent; and preparing an esterified PVA-lignin (e-PCL) resin obtained by heating and esterifying the same.

[Scheme 1]

In the above formula,

e-PCL is prepared by first mixing PVA-lignin and mixing a crosslinking agent such as citric acid to make H-PCL with hydrogen bonding, and esterification by raising the temperature to 160-200°C, preferably 180°C. The present invention does not use any catalyst for esterification, and thus provides a manufacturing method that is economical, eco-friendly and easy to mass-produce.

The crosslinking agent (CA) represented in Scheme 1 is citric acid.

The eco-friendly resin prepared according to the present invention has a tensile strength of 150 MPa or more, preferably 200 MPa or more, more preferably 300 MPa or more, and an elastic modulus of 5 GPa or more, preferably 7 GPa or more, more preferably 10 GPa or more, the strain at break is 6% or more and 15% or less, and the adhesive strength is 20 MPa or more, preferably 30 MPa or more, more preferably 50 MPa or more.

In addition, the present invention provides a method of manufacturing an eco-friendly natural fiber-reinforced composite by impregnating a composite material made of natural fibers such as CNF in the esterified e-PCL resin, laminating it and drying it.

Specifically, the method comprises the steps of wet spinning a cellulose nanofiber (CNF) suspension, followed by orientation, tensile, and drying to prepare a CNF filament (CLF), manufacturing a mat woven of the CLF; And after impregnating the mat with e-PCL resin, stacking 2 to 5 mats, drying at 60 to 100 ° C., and applying a pressure of 8 MPa and heat of 180 ° C to the dried mat to form and drying in a vacuum oven at 180° C. for 1 hour.

The flexural strength of the eco-friendly natural fiber-reinforced composite is 200 MPa or more, preferably 300 MPa or more, more preferably 500 MPa or more, and the elastic modulus is 20 GPa or more, preferably 30 GPa or more, more preferably 50 GPa or more. More than that.

Hereinafter, the present invention will be described in detail by way of Examples, but these Examples are only indicated for a more clear understanding of the present invention, and are not presented for the purpose of limiting the scope of the present invention, and the present invention will be described later It will be determined within the scope of the technical spirit of the claims.

[Example]

Example 1: Esterified PVA-CA-lignin resin

A 10% (w/w) PVA solution, a 10% (w/w) lignin solution, and a 10% (w/w) citric acid solution were prepared, respectively, and stirred with ultrasonic waves. The prepared 10% (w/w) PVA solution and lignin solution are mixed in a weight ratio of 65:35 and centrifuged to remove undissolved particles. Mix the prepared PVA-lignin mixed solution and citric acid (CA) solution in various ratios (weight ratio) (90:10, 80:20, 70:30, 65:35, 60:40) and take an ice bath to exclude the effect of temperature. Mix with a homogenizer. The PVA-CA-lignin thus prepared is called hydrogen-bonded H-PCL resin.

The hydrogen-bonded H-PCL resin is placed in a vacuum oven and heated at 180° C. for 1 hour to 12 hours to perform esterification. The esterified PVA-CA-lignin is named 'e-PCL resin'.

FTIR spectroscopy was measured to confirm the ester bond of the prepared H-PCL and e-PCL resins. 1 is an FTIR spectra of e-PCL according to the content of citric acid (CA). 2 is an FTIR spectra of e-PCL as a function of esterification time. In the description of FIG. 2, 'PL' indicates a mixture of PVA and lignin.

The e-PCL resin shows a new peak of ester bonds at 1729 cm ^-1 . It can be seen that the 1701 cm ^-1 peak corresponding to the hydrogen bond between the COOH and -OH groups in the H-PCL resin shifted to a high wavenumber of 1729 cm ^-1 . On the other hand, it can be seen that the broad peak of H-PCL was changed to a sharp peak at 3418 cm ^-1 , which was caused by elongation of the -OH group. It can be seen that the strength of the ester bond increases as the content of citric acid increases. As a result of the FTIR spectral spectrum, it can be seen that a crosslinking was formed between PVA and lignin by the crosslinking agent of citric acid.

3 shows a stress-strain diagram according to the citric acid content (10-40% (w/w)) of E-PCL. Table 1 shows a comparison of the tensile strength of conventional PVA-lignin-based resins. The tensile strength was measured according to the conventional film tensile test method (ASTM D1938) after casting the resin into a film and drying it to make a specimen. In the e-PCL resin of the present invention, the tensile strength increases until the content of citric acid is 30% (w/w) and decreases thereafter. Because. It can be seen that the tensile strength at 30% (w/w) of citric acid is 184 MPa, which is the best among lignin-based resins, and is about 4.5 times higher than that of PVA-lignin resin. In addition, it can be seen that the modulus of elasticity, elongation at break, and toughness of e-PCL resin containing 30% citric acid are 2.6 times, 4 times, and 20 times greater than those of PVA-lignin resin, respectively. Such a large toughness modulus is a very advantageous property for natural fiber reinforced composites as a resin that can absorb a lot of breaking energy.

resin	The tensile strength (MPa)	modulus of elasticity (GPa)	Elongation at break (%)	toughness coefficient (KJ/m 3 )	references
PVA-lignin	41.1	2.48	2.5	600.1	One
Esterified PVA-Malic Acid-Lignin	48.5	2.4	2.7	787.3	2
PEO-lignin	6.2	0.28	22.0		3
Epoxy-lignin	67.5	2.52	4.9		4
e-PCL (CA 30%)	184	6.52	9.9	12,170.0	Example 1

1. H.-U Ko, L. Zhai, JH Park, JY Lee, D. Kim, J. Kim, J. Appl. Polym. Sci., 135, 46655, 2. 2018. 2. H.-U Ko, JW Kim, HC Kim, L. Zhai, J. Kim, J. Appl. Polym. Sci., 237, 48836, 2019.

3. T. Jayaramudu, H.-U Ko, HC Kim, JW Kim, ES Choi, J. Kim, Compos. Pt B, 156, 43-50, 2019.

4. J. Sun, C. Wang, J. Chee, C. Yeo, D. Yuan, H. Li, LP Stubbs, C. He, Macromol. Mater. Eng. 301 , 328, 2016.

4 shows a thermogravimetric analysis graph of the e-PCL resin. It can be seen that the decomposition temperature increases as the content of the citric acid crosslinking agent increases from 10% (w/w) to 40% (w/w), because the degree of esterification increases. Among them, the highest decomposition temperature (260° C.) when the citric acid content is 30% (w/w). This is because the ester bridge that was formed as groups such as C-O and C=O in the polymer network was broken was broken. The final decomposition at 310-550 °C is by carbonization.

5 shows the water contact angle (WCA) according to the citric acid content of e-PCL. As the PVA-lignin resin was esterified, WCA increased. In particular, when the citric acid content was 30% (w/w), the highest WCA of 104.4° was shown. This indicates that the e-PCL resin has hydrophobicity compared to the previously esterified PVA-lignin resin with the addition of malic acid, which exhibited a WCA of 57° and hydrophilicity. This is a very advantageous property to actually use the natural fiber reinforced composite material.

6 shows the test results of the adhesion strength of e-PCL resin with CNF. Adhesive strength test followed the conventional Lap Shear Joint (LSJ) test, and after applying resin to the 2 mm part from the tip of the 10 mm x 30 mm CNF film, overlapping the ends of the two CNF films, and obtaining a tensile test. The joint strength is obtained by dividing the breaking load by the contact area of the two films. The bonding strength of e-PCL resin with a citric acid content of 30 wt% is 31.9 MPa, which is 7.4 and 4.7 times greater than 4.3 MPa of PVA-lignin resin and 6.8 MPa of PVA-lignin resin esterified with malic acid, respectively. . This indicates that e-PCL resin has excellent bonding strength with natural fibers containing cellulose as a main component.

Example 2: Natural Fiber Reinforced Composite

An embodiment of manufacturing a natural fiber-reinforced composite using a CNF filament (CNF Long Filament, CLF) and an e-PCL resin follows a conventional fiber-reinforced composite manufacturing process. CLF manufactures eco-friendly high-strength long fibers by wet spinning a CNF suspension in an alcohol-based aqueous solution and then aligning, stretching, and drying the nanocellulose in the same manner as in the prior art of the present applicant (Korean Patent No. 10-2063100). The CLF prepared in this way has a tensile strength of 480 MPa and an elastic modulus of 40 GPa. Weaving the CLF to make a mat, impregnating with e-PCL resin, stacking 3 sheets, drying at 60-100 ° C, placing it on a hot press, applying a pressure of 8 MPa and heat of 180 ° C. to finish the esterification by drying for 1 hour. The CLF fiber-reinforced e-PCL resin composite prepared in this way showed a mechanical strength of 230 MPa in bending strength and 23.5 GPa of elastic modulus. On the other hand, the WCA on the surface of the composite showed hydrophobicity at 93.8 degrees. 7 is a SEM image showing the fracture cross-section of the manufactured CLF fiber-reinforced e-PCL resin composite. It is shown that the e-PCL resin is filled between the CLFs.

The present invention has been described above based on the illustrated example, but this is only an example, and the present invention can perform various modifications and equivalent other embodiments that are obvious to those of ordinary skill in the art of the present invention. You have to understand the facts.

Claims (6)

1. After mixing polyvinyl alcohol and lignin, reacting by adding a crosslinking agent to the mixture; and

   A method for producing an eco-friendly PVA-lignin resin comprising the step of heating the reactant to 160°C to 200°C to prepare an esterified PVA-lignin resin.
2. The method of claim 1, wherein the mixture and the crosslinking agent are mixed in a weight ratio of 6:4 to 8:2.
3. The method of claim 1, wherein the polyvinyl alcohol and the lignin are mixed in a weight ratio of 6:4 to 7:3.
4. An environmentally friendly PVA-lignin resin prepared by the method of any one of claims 1 to 3.
5. The eco-friendly PVA-lignin resin according to claim 2, wherein the resin has a tensile strength of 150 MPa or more, an elastic modulus of 5 GPa or more, a breaking strain of 6% or more, an adhesive strength of 20 MPa or more, and hydrophobicity.
6. A natural fiber-reinforced composite manufactured by impregnating a composite made of natural fibers into an eco-friendly PVA-lignin resin manufactured by the method of any one of claims 1 to 3.

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