1. A method for preparing a thermoplastic plant fiber material modified by cationic grafting is characterized by comprising the following steps: the method comprises the following steps:

1) pretreating a plant fiber raw material to obtain pretreated plant fiber;

2) in water, under the action of periodate, the pretreated plant fiber is subjected to oxidation ring-opening reaction to obtain aldehyde group-modified plant fiber;

3) oxidizing aldehyde groups in the hydroformylation plant fibers into carboxyl groups to obtain carboxylated plant fibers;

4) in water, reacting carboxylated plant fibers with quaternary ammonium salt to obtain a thermoplastic plant fiber material modified by cationic grafting;

the mass ratio of the periodate to the pretreated plant fiber in the step 2) is (0.1-2) to 1;

the molar ratio of the quaternary ammonium salt to the carboxyl in the carboxylated plant fiber in the step 4) is (0.1-8) to 1.

2. The method for preparing a thermoplastic plant fiber material modified by cationic grafting according to claim 1, characterized in that: the quaternary ammonium salt is more than one of dodecyl trimethyl ammonium bromide, tetradecyl trimethyl ammonium bromide, hexadecyl trimethyl ammonium bromide, octadecyl trimethyl ammonium chloride, benzyl trimethyl ammonium chloride, 2, 3-epoxypropyl trimethyl ammonium chloride, didodecyl dimethyl ammonium chloride, dicetyl dimethyl ammonium chloride and dioctadecyl dimethyl ammonium chloride;

the mass ratio of the periodate to the pretreated plant fiber in the step 2) is (0.2-1) to 1;

the molar ratio of the quaternary ammonium salt to the carboxyl in the carboxylated plant fiber in the step 4) is (2.4-4) to 1.

3. The method for preparing a thermoplastic plant fiber material modified by cationic grafting according to claim 1, characterized in that: the mass ratio of the pretreated plant fibers to the water in the step 2) is 0.1-10: 100;

in the step 4), the mass ratio of the carboxylated plant fibers to the water is (0.1-10) to 100, and the carboxylated plant fibers are calculated in an oven-dry mode;

and (3) before the quaternary ammonium salt is added in the step 4), the pH value of the system is 8-13.

4. The method for preparing a thermoplastic plant fiber material modified by cationic grafting according to claim 3, characterized in that: and (3) before the quaternary ammonium salt is added in the step 4), the pH value of the system is 9-11.

5. The method for preparing a thermoplastic plant fiber material modified by cationic grafting according to claim 1, characterized in that:

the pretreatment in the step 1) comprises grinding, high-shear homogenization and/or steam explosion, so that the reaction accessibility of the plant fiber is improved;

the reaction temperature in the step 2) is room temperature-80 ℃, and the reaction time is 1-8 h;

the reaction temperature in the step 3) is room temperature, and the reaction time is 0.5-12 h;

the reaction temperature in the step 4) is room temperature to 80 ℃, and the reaction time is 1 to 48 hours;

and 2) carrying out subsequent treatment after the reaction in the step 2), wherein the subsequent treatment refers to washing and filtering.

6. The method for preparing a thermoplastic plant fiber material modified by cationic grafting according to claim 1, characterized in that:

dispersing the aldehyde vegetable fiber in water, adjusting the pH value to 1-6, adding chlorite and hydrogen peroxide, reacting, and performing subsequent treatment to obtain the carboxylated vegetable fiber.

7. The method for preparing a thermoplastic plant fiber material modified by cationic grafting according to claim 6, characterized in that: in the step 3), the pH is 4-6;

in the step 3), the mass ratio of the aldehyde vegetable fiber to the water is (0.1-10) to 100; the aldehydized plant fiber is measured in absolute dry;

the mole ratio of the chlorite to the aldehyde group in the aldehyde group vegetable fiber is (1-10) to 1;

the hydrogen peroxide is hydrogen peroxide with the concentration of 30 percent, and the mass ratio of the hydrogen peroxide to the chlorite is 1: 1;

the subsequent treatment in the step 3) is washing and filtering.

8. The method for preparing the thermoplastic plant fiber material modified by cationic grafting according to claim 7, characterized in that: the mole ratio of the chlorite to the aldehyde group in the aldehyde vegetable fiber is (2-3.5) to 1.

9. A thermoplastic plant fiber material modified by cationic grafting obtained by the preparation method of any one of claims 1 to 8.

10. The cationic graft-modified thermoplastic plant fiber material according to claim 9, wherein: the cation graft modified thermoplastic plant fiber material is formed by thermoplastic molding.

Background

With the increasing consumption of non-renewable resources (petroleum, coal and natural gas) and the environmental pollution caused by petroleum polymers, there is an increasing demand for renewable resources that are biodegradable, non-petroleum based and have low risk to the environment. The natural plant fiber is a renewable resource with the most abundant reserves in the nature, has excellent mechanical property, is used as a raw material to prepare an environment-friendly new material, can reduce the dependence on fossil resources, and can efficiently utilize the natural plant fiber.

If the plant fiber can be processed and molded by heating and melting like plastic, petroleum-based polymer can be partially replaced, and the current increasingly serious resource and environmental problems are effectively solved. However, based on the special structural characteristics of the plant fiber, the plant fiber has high crystallinity and a compact hydrogen bond network, the intramolecular/intermolecular force is strong, the motion capability of a molecular chain is severely limited, and the melting temperature of the plant fiber is far higher than the thermal decomposition temperature, so that the plant fiber is difficult to melt and process, and the application range of the plant fiber is severely limited.

The existing method for thermal plasticizing natural plant fiber mainly comprises the following steps: esterification modification and etherification modification. The hydroxyl in the plant fiber is replaced by functional groups with large size and small interaction to weaken the hydrogen bond action among the components, reduce the bonding force among the molecules, destroy the crystalline structure of the cellulose, improve the motion capability and the fluidity of the molecular chains in the plant fiber and obtain the thermoplastic material. However, the esterification and etherification methods are usually only directed at the modification of cellulose in plant fibers, involve complicated plant fiber component separation, fail to efficiently utilize components such as hemicellulose and lignin, and seriously damage the crystal structure of cellulose, thereby resulting in the performance reduction and low yield of materials; in addition, a large amount of organic solvents, swelling agents, catalysts and other auxiliaries are required to be consumed in the modification process, the post-treatment process is complex, the cost is high, certain pollution is caused to the environment, the economy and the practicability are poor, and industrial production and application are difficult to carry out.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a thermoplastic plant fiber material modified by cationic grafting and a preparation method thereof. The invention takes the water phase as the solvent, has mild preparation conditions, high efficiency and high yield. The plant fiber modified by the method has high yield, good thermoplasticity of the whole plant fiber, easy processing and forming and good material performance.

The purpose of the invention is realized by the following technical scheme:

a preparation method of a thermoplastic plant fiber material modified by cationic grafting comprises the following steps:

1) pretreating a plant fiber raw material to obtain pretreated plant fiber;

2) in water, under the action of periodate, the pretreated plant fiber is subjected to oxidation ring-opening reaction to obtain aldehyde group-modified plant fiber;

3) oxidizing aldehyde groups in the hydroformylation plant fibers into carboxyl groups to obtain carboxylated plant fibers;

4) in water, the carboxylated plant fiber reacts with quaternary ammonium salt to obtain the thermoplastic plant fiber material with cation graft modification.

The quaternary ammonium salt is more than one of dodecyl trimethyl ammonium bromide, tetradecyl trimethyl ammonium bromide, hexadecyl trimethyl ammonium bromide, octadecyl trimethyl ammonium chloride, benzyl trimethyl ammonium chloride, 2, 3-epoxypropyl trimethyl ammonium chloride, didodecyl dimethyl ammonium chloride, dicetyl dimethyl ammonium chloride and dioctadecyl dimethyl ammonium chloride.

The pretreatment in the step 1) comprises grinding, high-shear homogenization, steam explosion and the like, so that the reaction accessibility of the plant fiber is improved. Steam explosion is as follows: steam explosion equipment, continuous screw extrusion flash explosion equipment.

The mass ratio of the plant fiber (absolutely dry) pretreated in the step 2) to water is (0.1-10) to 100; the mass ratio of the periodate (such as sodium periodate, potassium periodate and the like) to the pretreated plant fiber is (0.1-2) to 1, and preferably (0.2-1) to 1.

The reaction temperature in the step 2) is room temperature to 80 ℃, preferably 50 to 60 ℃, and the reaction time is 1 to 8 hours.

And 2) carrying out subsequent treatment after the reaction in the step 2), wherein the subsequent treatment refers to washing and filtering.

Dispersing the aldehyde vegetable fibers in water, adjusting the pH to 1-6, adding chlorite and hydrogen peroxide, reacting, and performing subsequent treatment to obtain carboxylated vegetable fibers; the pH is preferably 4-6; the subsequent treatment is washing and filtering.

The reaction temperature in the step 3) is room temperature, and the reaction time is 0.5-12 h, preferably 4-10 h.

In the step 3), the mass ratio of the aldehyde-group vegetable fiber (absolutely dry) to water is (0.1-10) to 100.

The mole ratio of the chlorite to the aldehyde group in the aldehyde-group vegetable fiber is (1-10) to 1, and preferably (2-3.5) to 1. The hydrogen peroxide is hydrogen peroxide with the concentration of 30 percent. The mass ratio of the hydrogen peroxide to the chlorite is 1: 1.

In the step 4), the mass ratio of the carboxylated plant fibers (absolutely dry) to the water is (0.1-10) to 100.

Before the quaternary ammonium salt is added in the step 4), the pH value of the system is 8-13, and preferably 9-11.

Dispersing the carboxylated plant fiber in water, adjusting the pH value to 8-13, adding quaternary ammonium salt, reacting, and performing subsequent treatment to obtain the cationic graft modified thermoplastic plant fiber material. The subsequent treatment refers to washing and filtration.

The reaction temperature in the step 4) is room temperature to 80 ℃, preferably 55 to 65 ℃, and the reaction time is 1 to 48 hours, preferably 2 to 8 hours.

The molar ratio of the quaternary ammonium salt to the carboxyl in the carboxylated plant fiber is (0.1-8) to 1, and preferably (2.4-4) to 1.

In the invention, the plant fiber raw material is at least one selected from wood fiber raw material, non-wood fiber raw material (such as hemp and herbaceous plants) and straw (such as wheat straw).

The cation graft modified thermoplastic plant fiber material is thermoplastic full-component plant fiber, and can be repeatedly formed by thermoplastic molding without being compounded with a thermoplastic high polymer material.

The cation graft modified thermoplastic plant fiber material is formed by thermoplastic molding. Such as: and (5) hot-press forming.

The forming temperature is 80-140 ℃, the forming time is 5-15 min, and the forming pressure is 5-10 MPa.

The thermoplastic shaped cationic graft modified thermoplastic plant fiber material is translucent.

Compared with the prior art, the invention has the following advantages and beneficial effects:

1) the method realizes the full component utilization of the plant fiber, and the product can be directly subjected to thermoplastic molding; the hot-press forming can be repeated at 80-140 ℃.

2) The method takes the aqueous solution as a reaction medium, has mild reaction conditions and is environment-friendly.

3) The reagent used in the method is appropriate in price and can be recycled.

4) The method has the advantages of simple process, high product yield and excellent material performance.

5) The method has low cost, belongs to an environment-friendly technology, and is easy for industrial production and application.

Drawings

FIG. 1 is an appearance profile and a transmission test chart of a sheet after hot press forming of the thermoplastic full-component plant fiber prepared in comparative example 1 and examples 1, 5 and 8;

FIG. 2 is a graph showing the appearance and appearance of a translucent sheet obtained by thermoforming the thermoplastic full-component plant fiber prepared in examples 2, 6, 7 and 9;

FIG. 3 is a bar graph of the tensile properties of the thermoplastic all-component plant fibers prepared in examples 1-2 and 5-9.

Detailed Description

The present invention is described in further detail below by way of specific examples, but the embodiments of the present invention are not limited thereto.

Example 1

A method for preparing a thermoplasticized full-component plant fiber material, comprising the following steps:

(1) pretreating sisal fibers with the length of about 1-2cm by Steam explosion (continuous screw extrusion flash explosion equipment) to destroy the bundle structure of the sisal fibers to obtain Steam Explosion Sisal Fibers (SESF) with large length-diameter ratio (300-1200);

(2) dispersing SESF in water, wherein the mass ratio of absolute dry SESF to water is 1: 28 to obtain a dispersion liquid-1, then adding sodium periodate into the dispersion liquid-1, wherein the mass ratio of absolute dry SESF to sodium periodate is 1: 0.2, reacting for 5 hours at 55 ℃, washing and filtering to obtain Aldehyde Sisal Fiber (ASF), wherein the Aldehyde group content is 0.93 mmol/g;

(3) dispersing ASF in water, wherein the mass ratio of the oven-dried ASF to the water is 1: 40 to obtain a dispersion liquid-2, dropwise adding acetic acid, adjusting the pH value of the suspension liquid-2 to 5, then adding sodium chlorite, wherein the molar ratio of the sodium chlorite to the aldehyde group is 2.5: 1, simultaneously adding 30% of hydrogen peroxide with the same mass as the sodium chlorite, reacting for 5 hours at room temperature, washing and filtering to obtain Carboxylated Sisal Fibers (CSF), wherein the carboxyl content is 0.90 mmol/g;

(4) dispersing CSF in water, wherein the mass ratio of the absolute dry CSF to the water is 1: 28 to obtain a dispersion liquid-3, dropwise adding sodium hydroxide, adjusting the pH of the suspension liquid-3 to 10, then adding Cetyl Trimethyl Ammonium Bromide (CTAB), wherein the molar ratio of the CTAB to carboxyl is 3: 1, reacting for 3h at 60 ℃, washing and filtering to obtain a thermoplasticized full-component sisal fiber material (namely, the thermoplastic full-component plant fiber) which is marked as CCSF-0.2-3.0, wherein 0.2 represents the addition amount of periodate, and 3.0 represents the addition amount of CTAB.

Placing CCSF-0.2-3.0 in a die, and hot-pressing at 135 deg.C and 8MPa for 10min to obtain hot-pressed sample strip.

Example 2

The present embodiment differs from embodiment 1 in that: the mass ratio of the oven-dried SESF to the sodium periodate is controlled to be 1: 0.4, other steps and parameters are consistent with those of the example 1, and the prepared thermoplasticized full-component sisal fiber material is marked as CCSF-0.4-3.0.

The hot pressing temperature of CCSF-0.4-3.0 is adjusted to 120 ℃, and other conditions are unchanged.

Example 3

The present embodiment differs from embodiment 1 in that: the mass ratio of the oven-dried SESF to the sodium periodate is controlled to be 1: 0.6, the molar ratio of CTAB to carboxyl is 1.8: 1, other steps and parameters are consistent with those of the example 1, and the prepared thermoplasticized full-component sisal fiber material is marked as CCSF-0.6-1.8.

The hot pressing temperature of CCSF-0.6-1.8 is adjusted to 115 ℃, and other conditions are unchanged.

Example 4

The present embodiment differs from embodiment 1 in that: the mass ratio of the oven-dried SESF to the sodium periodate is controlled to be 1: 0.8, the molar ratio of CTAB to carboxyl is 1.8: 1, other steps and parameters are consistent with those of the example 1, and the prepared thermoplasticized full-component sisal fiber material is marked as CCSF-0.8-1.8.

The hot pressing temperature of CCSF-0.8-1.8 is adjusted to 110 ℃, and other conditions are unchanged.

Example 5

The present embodiment differs from embodiment 1 in that: controlling the mass ratio of the oven-dried SESF to the sodium periodate to be 1: 0.4; octadecyl trimethyl ammonium chloride (STAC) is used as a quaternary ammonium salt grafting agent to replace CTAB in the embodiment 1; the other steps and parameters were in accordance with example 1, and the thermoplasticized full sisal fiber material obtained was designated as SCSF-0.4-3.0.

The hot pressing temperature of the SCSF-0.4-3.0 is 120 ℃, and other conditions are unchanged.

Example 6

The present embodiment differs from embodiment 1 in that: controlling the mass ratio of the oven-dried SESF to the sodium periodate to be 1: 0.4; benzyl Trimethyl Ammonium Chloride (TAC) is used as a quaternary ammonium salt grafting agent to replace CTAB in example 1; the other steps and parameters were in accordance with example 1, and the thermoplasticized full sisal fiber material obtained was designated as TCSF-0.4-3.0.

The hot pressing temperature of TCSF-0.4-3.0 is 130 ℃, and other conditions are unchanged.

Example 7

The present embodiment differs from embodiment 1 in that: the mass ratio of the oven-dried SESF to the sodium periodate is controlled to be 1: 0.4, 2, 3-epoxypropyltrimethylammonium chloride (GTAC) is used as a quaternary ammonium salt grafting agent, other steps and parameters are consistent with those of the example 1, and the prepared thermoplasticized full-component sisal fiber material is marked as GCSF-0.4-3.0.

The hot pressing temperature of GCSF-0.4-3.0 is 130 ℃, and other conditions are unchanged.

Example 8

This example provides a full-component plant fiber material that can be thermoplasticized, and the preparation method comprises the following steps:

(1) pretreating the wheat straw fiber with the length of about 1-2cm by Steam explosion to destroy the bundle structure of the wheat straw fiber to obtain the Steam Explosion Wheat Straw Fiber (SEWSF) with large length-diameter ratio (400- & ltSUB- & gt 1000-);

(2) dispersing SEWSF in water, wherein the mass ratio of the oven-dried SEWSF to the water is 1: 28 to obtain a dispersion liquid-1, then adding sodium periodate into the dispersion liquid-1, wherein the mass ratio of the oven-dried SEWSF to the sodium periodate is 1: 0.8, reacting for 5 hours at 55 ℃, washing and filtering to obtain Aldehyde Wheat Straw Fiber (AWSF);

(3) dispersing AWSF in water, wherein the mass ratio of the AWSF to the water is 1: 40 by absolute drying to obtain a dispersion liquid-2, adding acetic acid, adjusting the pH value of the suspension liquid-2 to 5, adding sodium chlorite into the dispersion liquid-2, wherein the molar ratio of the sodium chlorite to aldehyde groups is 2.5: 1, simultaneously adding 30% of hydrogen peroxide with the same mass as the sodium chlorite, reacting for 5 hours at room temperature, washing, and filtering to obtain Carboxylated Wheat Straw Fiber (CWSF);

(4) dispersing CWSF in water, wherein the mass ratio of the oven-dried CWSF to the water is 1: 28 to obtain a dispersion liquid-3, adding sodium hydroxide, adjusting the pH value of the suspension liquid-3 to 10, adding Cetyl Trimethyl Ammonium Bromide (CTAB) into the dispersion liquid-3, wherein the molar ratio of the CTAB to carboxyl is 2.4: 1, reacting for 3 hours at 60 ℃, washing and filtering to obtain a thermoplastic all-component wheat straw fiber material which is marked as CCWSF-0.8-2.4.

The hot pressing temperature of CCWSF-0.8-2.4 is 130 ℃, and other conditions are unchanged.

Example 9

The present embodiment differs from embodiment 8 in that: the Eucalyptus Fiber (EF) is used as a plant fiber raw material, other steps and parameters are consistent with those of the example 8, and the prepared thermoplasticized full-component eucalyptus fiber material is marked as CCEF-0.8-2.4.

The hot pressing temperature of CCEF-0.8-2.4 is 140 ℃, and other conditions are unchanged.

Comparative example 1

The comparative example differs from example 1 in that: except that the mass ratio of the oven-dried SESF to the sodium periodate was controlled to be 1: 0.1, the other steps and parameters were consistent with those of example 1, and the obtained full-component sisal fiber material was marked as CCSF-0.1-3.0.

The hot pressing temperature of CCSF-0.1-3.0 is 160 ℃, and other conditions are unchanged.

Comparative example 2

(1) This step corresponds to step (1) of example 1.

(2) Dispersing SESF in water, wherein the mass ratio of absolute dry SESF to water is 1: 28 to obtain a dispersion liquid-1, then adding sodium periodate into the dispersion liquid-1, wherein the mass ratio of absolute dry SESF to sodium periodate is 1: 0.8, reacting for 5 hours at 55 ℃, washing and filtering to obtain Aldehyde Sisal Fiber (ASF), and marking as ASF-0.8.

The hot pressing temperature of the ASF-0.8 is 160 ℃, and other conditions are unchanged.

Comparative example 3

(1) The procedure was identical to procedure (1) of example 1.

(2) This step was identical to step (2) of comparative example 2.

(3) Dispersing ASF in water, drying ASF and water at a mass ratio of 1: 40 to obtain a dispersion liquid-2, dropwise adding acetic acid, adjusting the pH of the suspension liquid-2 to 5, adding sodium chlorite, wherein the molar ratio of the sodium chlorite to the aldehyde group is 2.5: 1, simultaneously adding 30% hydrogen peroxide with the mass of the sodium chlorite and the like, reacting for 5 hours at room temperature, washing and filtering to obtain Carboxylated Sisal Fibers (CSF), and marking the CSF-0.8.

The temperature of the hot pressing of CSF-0.8 was 160 ℃ and the other conditions were unchanged.

The carboxylation yield, the quaternary ammonium salt grafting ratio and whether the final product can be hot-press molded in examples 1 to 9 and comparative examples 1 to 3 are shown in Table 1.

TABLE 1 carboxylation yield, grafting ratio and hot press forming conditions in examples 1 to 9 and comparative examples 1 to 3

The grafting rate indicates the grafting rate of the cation.

FIG. 1 is an appearance profile and transmission test chart of the sheet after hot press forming of the thermoplastic full-component plant fiber prepared in comparative example 1 and examples 1, 5 and 8. As can be seen from FIG. 1, the thermoplastic full component plant fiber hot-press formed sheet prepared by the invention is semitransparent. The sheet prepared in comparative example 1 was opaque after hot press molding of the plant fiber.

FIG. 2 is a graph showing the appearance and appearance of a translucent sheet obtained by thermoforming the thermoplastic full-component plant fibers prepared in examples 2, 6, 7 and 9.

FIG. 3 is a bar graph of the tensile properties of thermoplastic all-component plant fibers prepared in examples 1-2 and 5-9

The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.