A modified pineapple leaf fiber reinforced starch-based composite material and its preparation method and application

Abstract

The present invention proposes a modified pineapple leaf fiber reinforced starch-based composite material and a preparation method and application thereof, which belongs to the technical field of polymer materials. The present invention uses pineapple leaf fiber as a raw material, adopts chemical grafting modification technology to coat the pineapple leaf fiber using the adhesion characteristics of polydopamine, and then uses polyamine to react with the surface group of polydopamine to generate a Schiff base, and grafts the amino functional group to the surface of the pineapple leaf fiber, which can not only improve the interface compatibility between the pineapple leaf fiber and the matrix material, but also make the pineapple leaf fiber have a secondary reaction characteristic, and further uses the amino-modified pineapple leaf fiber to react with dialdehyde starch to generate a Schiff base reaction, thereby forming a strong covalent bond and hydrogen bond, thereby improving the mechanical properties and thermal stability of the starch-based composite material, and can be applied to the fields of food, light industrial packaging materials and agricultural films.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer materials, and in particular relates to a modified pineapple leaf fiber reinforced starch-based composite material and a preparation method and application thereof. Background Art

[0002] Fiber-reinforced composites have been widely used in transportation, packaging, automobiles and other fields due to their excellent mechanical properties such as high specific strength, high specific modulus, and fatigue resistance. Synthetic fibers such as glass fiber, carbon fiber and Kevlar have been used as reinforcement materials to develop composite materials with the help of different petroleum-based polymer matrices for various projects. However, the resource and environmental problems caused by the non-renewable and non-biodegradable characteristics of synthetic polymers cannot be ignored.

[0003] In recent years, with the increasing energy and resource crisis and people's increasing environmental awareness worldwide, the demand for environmentally friendly and sustainable composite materials has become increasingly urgent. Therefore, natural polymers such as starch and cellulose, which have the characteristics of low production cost, high bioavailability, and non-toxic degradation products, will become good substitutes for preparing biodegradable materials and reducing environmental pollution problems caused by synthetic polymers. Cellulose is the most abundant polysaccharide in terrestrial biomass and exists in plant cell walls, such as pineapple leaf fiber (PALF). PALF is a natural fiber that exists in large quantities in pineapple plants, but the utilization of pineapples is limited to its fruits, and the rest is wasted or not used. The strong hydrogen bonding between and within the cellulose macromolecular chains makes pineapple leaf fiber show strong polarity and water absorption, and poor compatibility with some matrix materials, resulting in decreased mechanical properties of the composite material. In addition, starch is the second most abundant polysaccharide in nature, which constitutes the energy reserve of many plants. Starch is also a plastic natural polymer material and a potential substitute for chemical polymers. However, the practical application of starch-based materials is limited by the problems of brittleness, poor mechanical properties, and easy water absorption. Summary of the invention

[0004] In order to solve the above technical problems, the present invention proposes a modified pineapple leaf fiber reinforced starch-based composite material and a preparation method and application thereof. The present invention uses pineapple leaf fiber as raw material, performs surface modification on it, and then reacts it with dialdehyde starch to undergo Schiff base reaction, and then hot presses it to obtain the composite material.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] One of the technical solutions of the present invention:

[0007] A preparation method of a modified pineapple leaf fiber reinforced starch-based composite material comprises the following steps: using pineapple leaf fiber as a raw material, adopting a chemical grafting modification technology to graft amino functional groups on the surface of the pineapple leaf fiber to obtain amino-modified pineapple leaf fiber, then adding dialdehyde starch to react with a Schiff base, and hot-pressing the reaction product to obtain the modified pineapple leaf fiber reinforced starch-based composite material.

[0008] Furthermore, the preparation method of the modified pineapple leaf fiber reinforced starch-based composite material specifically comprises the following steps:

[0009] (1) adding pineapple leaf fiber to a mixed solution of ethanol, water, concentrated ammonia water and dopamine hydrochloride aqueous solution, adding polyamine to continue the reaction after the reaction is completed, filtering, washing and drying to obtain amino-modified pineapple leaf fiber;

[0010] (2) adding water and a plasticizer to dialdehyde starch, stirring and gelatinizing to obtain a starch gelatinized liquid, adding the amino-modified pineapple leaf fiber obtained in step (1) to the starch gelatinized liquid, reacting at a constant temperature under a nitrogen protective atmosphere, filtering, washing and drying to obtain a mixture after the reaction is completed, and hot-pressing the mixture to obtain the modified pineapple leaf fiber reinforced starch-based composite material.

[0011] Furthermore, in step (1), the pineapple leaf fiber needs to be pretreated before use, and the specific method is as follows:

[0012] The dried pineapple leaves were crushed into powder by a pulverizer and sorted through a sieve. 40 g of pineapple leaf fiber powder was soaked in 800 mL of a 3% (w / v) sodium hydroxide aqueous solution, stirred at 60° C. for 1 h, then washed with ultrapure water for multiple times until the pH of the washing solution was neutral to remove lignin and hemicellulose. Thereafter, the powder was bleached with 200 mL of a 1% (w / w) sodium hypochlorite solution and 2 mL of acetic acid at 70° C. and stirred for 0.5 h. The bleached pineapple leaf fibrils were washed with distilled water until neutral, and placed in an air oven at 65° C. to dry for 24 h for standby use.

[0013] Furthermore, in step (1), the amount ratio of ethanol, water, concentrated ammonia water, dopamine hydrochloride aqueous solution, pineapple leaf fiber and polyamine is 20mL: 50mL: 0.6mL: 50mL: 5g: (2.5-5)mL.

[0014] Furthermore, in step (1), the polyamine includes one or more of ethylenediamine, diethylenetriamine, 1,1-dimethylethylenediamine and 1,2-dimethylethylenediamine.

[0015] Furthermore, in step (1), after adding pineapple leaf fiber, the reaction is carried out at 35-40°C for 1-2 hours, and after adding polyamine, the reaction is continued at 35-40°C for 9-12 hours; more specifically, concentrated ammonia water is added to a mixed solution of ethanol and water, stirred and mixed at 35-40°C for 0.5 hours, and then a dopamine hydrochloride aqueous solution with a concentration of 2 mg / mL is added, and then the pretreated pineapple leaf fiber is added to the above solution, reacted at 35-40°C for 1-2 hours, and then the polyamine is added and the reaction is continued for 9-12 hours.

[0016] Furthermore, in step (2), the degree of aldehyde formation of the dialdehyde starch is 56%-90%.

[0017] Furthermore, in step (2), 4 times the mass of water is added to the dialdehyde starch, and then a plasticizer is added, and the mixture is stirred at 80° C. for 30 minutes to obtain a starch gelatinized liquid; the plasticizer is glycerol in an amount of 20 wt % of the mass of the dialdehyde starch.

[0018] Furthermore, in step (2), the mass ratio of the amino-modified pineapple leaf fiber to the dialdehyde starch is (1:9)-(4:6).

[0019] Furthermore, in step (2), the amino-modified pineapple leaf fiber is added to the starch gelatinization liquid, and reacted in a constant temperature water bath at 55-60° C. for 6-12 hours under a nitrogen protection environment. The hot pressing pressure of the hot pressing molding is 10-13 MPa, the hot pressing temperature is 110-130° C., and the hot pressing time is 8-25 min.

[0020] The second technical solution of the present invention:

[0021] A modified pineapple leaf fiber reinforced starch-based composite material is prepared according to the preparation method.

[0022] The third technical solution of the present invention:

[0023] The modified pineapple leaf fiber reinforced starch-based composite material is used in the preparation of food, light industry packaging materials and agricultural films.

[0024] Compared with the prior art, the present invention has the following advantages and technical effects:

[0025] (1) The present invention uses pineapple leaf fiber as a raw material and adopts a chemical grafting modification technology to graft amino functional groups on the surface of the pineapple leaf fiber to enhance the interfacial compatibility between the pineapple leaf fiber and starch. First, the pineapple leaf fiber is coated with the adhesion property of polydopamine, and then a polyamine is used to react with the surface group of the polydopamine to generate a Schiff base to graft the amino functional groups onto the surface of the pineapple leaf fiber. This can not only improve the interfacial compatibility between the pineapple leaf fiber and the matrix material, but also make the pineapple leaf fiber have a secondary reaction characteristic.

[0026] (2) The present invention uses amino-modified pineapple leaf fiber and dialdehyde starch to undergo Schiff base reaction, and the aldehyde group of dialdehyde starch can undergo Schiff base reaction with the free amino functional group on the surface of pineapple leaf fiber, thereby forming a strong covalent bond and hydrogen bond between the plant fiber and the dialdehyde starch, thereby improving the mechanical properties and thermal stability of the starch-based composite material. The composite material prepared by the present invention has excellent mechanical properties, water resistance, and is degradable, low cost, and has a wide range of applications.

[0027] (3) The present invention uses natural plant fibers and starches which are abundant in resources and low in price as raw materials, which can reduce production costs on the one hand, and on the other hand, achieve the degradability of the composite material and reduce secondary pollution to the environment.

[0028] (4) The preparation process of the degradable pineapple leaf fiber reinforced starch-based composite material provided by the present invention is simple and easy to operate. The obtained composite material has excellent mechanical properties and a wide range of applications. It can be used in the fields of food, light industrial packaging materials, agricultural films, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0030] Figure 1 FT-IR images of the pretreated pineapple leaf fiber (original fiber) and the amino-modified pineapple leaf fiber (amino-modified fiber) prepared in Example 1;

[0031] Figure 2 FT-IR images of the modified pineapple leaf fiber reinforced starch-based composite material (modified fiber composite material) prepared in Example 1 and the pineapple leaf fiber reinforced starch-based composite material (unmodified fiber composite material) prepared in Comparative Example 1;

[0032] Figure 3 This is an optical microscope image of the modified pineapple leaf fiber reinforced starch-based composite material prepared in Example 1;

[0033] Figure 4 This is a SEM image of the modified pineapple leaf fiber reinforced starch-based composite material prepared in Example 1;

[0034] Figure 5 The thermal stability test results (TG curves) of the composite materials prepared in Examples 1-5 and Comparative Examples 1-3;

[0035] Figure 6 The water resistance test results of the composite materials prepared in Examples 1-5 and Comparative Examples 1-3 are shown. DETAILED DESCRIPTION

[0036] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0037] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0038] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0039] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

[0040] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0041] The embodiment of the present invention provides a preparation method of a modified pineapple leaf fiber reinforced starch-based composite material, which uses pineapple leaf fiber as a raw material, adopts a chemical grafting modification technology to graft amino functional groups on the surface of the pineapple leaf fiber to obtain amino-modified pineapple leaf fiber, then adds dialdehyde starch to produce a Schiff base reaction, and takes the reaction product and hot presses it to obtain the modified pineapple leaf fiber reinforced starch-based composite material.

[0042] In a preferred embodiment of the present invention, the method for preparing the modified pineapple leaf fiber reinforced starch-based composite material specifically comprises the following steps:

[0043] (1) adding pineapple leaf fiber to a mixed solution of ethanol, water, concentrated ammonia water and dopamine hydrochloride aqueous solution, adding polyamine to continue the reaction after the reaction is completed, filtering, washing and drying to obtain amino-modified pineapple leaf fiber;

[0044] (2) adding water and a plasticizer to dialdehyde starch, stirring and gelatinizing to obtain a starch gelatinized liquid, adding the amino-modified pineapple leaf fiber obtained in step (1) to the starch gelatinized liquid, reacting at a constant temperature under a nitrogen protective atmosphere, filtering, washing and drying to obtain a mixture after the reaction is completed, and hot-pressing the mixture to obtain the modified pineapple leaf fiber reinforced starch-based composite material.

[0045] The invention uses pineapple leaf fiber as a raw material and adopts a chemical grafting modification technology to graft amino functional groups on the surface of the pineapple leaf fiber, thereby enhancing the interface compatibility between the pineapple leaf fiber and starch. Firstly, the adhesion property of polydopamine is utilized to coat the pineapple leaf fiber, and then a polyamine is utilized to react with a surface group of the polydopamine to generate a Schiff base, thereby grafting the amino functional groups on the surface of the pineapple leaf fiber. This not only improves the interface compatibility between the pineapple leaf fiber and a matrix material, but also enables the pineapple leaf fiber to have a secondary reaction property.

[0046] In addition, amino-modified pineapple leaf fiber and dialdehyde starch undergo Schiff base reaction, and the aldehyde group of dialdehyde starch can undergo Schiff base reaction with the free amino functional group on the surface of pineapple leaf fiber, thereby forming a strong covalent bond and hydrogen bond between the plant fiber and the dialdehyde starch, thereby improving the mechanical properties and thermal stability of the starch-based composite material. The composite material prepared by the present invention has excellent mechanical properties, water resistance, and is degradable.

[0047] In a preferred embodiment of the present invention, in step (1), the pineapple leaf fiber needs to be pretreated before use, and the specific method is as follows:

[0048] The dried pineapple leaves were crushed into powder by a pulverizer and sorted through a 60-mesh sieve. 40 g of pineapple leaf fiber powder was soaked in 800 mL of a 3% (w / v) sodium hydroxide aqueous solution, stirred at 60° C. for 1 h, then washed with ultrapure water for multiple times until the pH of the washing solution was neutral to remove lignin and hemicellulose. The powder was then bleached with 200 mL of a 1% (w / w) sodium hypochlorite solution and 2 mL of acetic acid at 70° C. and stirred for 0.5 h. The bleached pineapple leaf fibrils were washed with distilled water until neutral, and placed in an air oven at 65° C. and dried for 24 h for standby use.

[0049] In a preferred embodiment of the present invention, in step (1), the size of the pineapple leaf fibers is 0.15-0.3 mm, and the size of the pineapple leaf fibers is controlled by controlling the screen sorting process in the pretreatment of the pineapple leaf fibers.

[0050] In a preferred embodiment of the present invention, in step (1), the amount ratio of ethanol, water, concentrated ammonia water, dopamine hydrochloride aqueous solution, pineapple leaf fiber and polyamine is 20mL: 50mL: 0.6mL: 50mL: 5g: (2.5-5)mL.

[0051] In a preferred embodiment of the present invention, in step (1), the polyamine includes one or more of ethylenediamine, diethylenetriamine, 1,1-dimethylethylenediamine and 1,2-dimethylethylenediamine.

[0052] In a preferred embodiment of the present invention, in step (1), after adding pineapple leaf fiber, the reaction is carried out at 35-40°C for 1-2 hours, and after adding polyamine, the reaction is continued at 35-40°C for 9-12 hours; more specifically, concentrated ammonia water is added to a mixed solution of ethanol and water, and the mixture is stirred at 35-40°C for 0.5 hours, and then a dopamine hydrochloride aqueous solution with a concentration of 2 mg / mL is added, and then the pretreated pineapple leaf fiber is added to the above solution, and the mixture is reacted at 35-40°C for 1-2 hours, and then the polyamine is added and the reaction is continued for 9-12 hours.

[0053] In a preferred embodiment of the present invention, in step (2), the degree of formaldehydeation of the dialdehyde starch is 56%-90%.

[0054] In a preferred embodiment of the present invention, in step (2), 4 times the mass of water is added to the dialdehyde starch, and then a plasticizer is added, and the mixture is stirred at 80° C. for 30 minutes to obtain a starch gelatinized liquid; the plasticizer is glycerol in an amount of 20 wt % of the mass of the dialdehyde starch.

[0055] In a preferred embodiment of the present invention, in step (2), the mass ratio of the amino-modified pineapple leaf fiber to the dialdehyde starch is (1:9)-(4:6).

[0056] In a preferred embodiment of the present invention, in step (2), the amino-modified pineapple leaf fiber is added to the starch gelatinized liquid, and reacted in a constant temperature water bath at 55-60°C for 6-12 hours under a nitrogen protection environment. The hot pressing pressure of the hot pressing molding is 10-13MPa, the hot pressing temperature is 110-130°C, and the hot pressing time is 8-25min.

[0057] The embodiment of the present invention also provides a modified pineapple leaf fiber reinforced starch-based composite material, which is prepared according to the preparation method.

[0058] The technical solution of the present invention is further illustrated by the following embodiments.

[0059] Example 1

[0060] (1) using a pulverizer to grind dried pineapple leaves into a powder state, sorting through a 60-mesh screen to obtain a pineapple leaf fiber powder with a size of 0.15-0.3 mm, taking 40 g of the pineapple leaf fiber powder and immersing it in 800 mL of a 3% (w / v) sodium hydroxide aqueous solution, stirring at 60° C. for 1 h, then washing with ultrapure water for multiple times until the pH value of the washing solution is neutral, removing lignin and hemicellulose, then bleaching with 200 mL of a 1% (w / w) sodium hypochlorite solution and 2 mL of acetic acid at 70° C., stirring for 0.5 h, washing the bleached pineapple leaf fibrils with distilled water until neutral, and placing them in an air oven at 65° C. and drying for 24 h to obtain pre-treated pineapple leaf fibers;

[0061] (2) In a mixed solution of 70 mL ethanol and water (V 水 :V 乙醇 =2.5:1) was added with 0.6 mL of concentrated ammonia, stirred and mixed at 35° C. for 0.5 h, then 50 mL of a 2 mg / mL aqueous solution of dopamine hydrochloride was added, 5 g of the pretreated pineapple leaf fiber obtained in step (1) was added to the above solution, reacted at 35° C. for 1 h, then 2.5 mL of a polyamine (ethylenediamine) was added, and the reaction was continued for 9 h, and filtered to obtain a modified fiber, which was washed five times with distilled water, and finally, the washed pineapple leaf fiber was dried in a vacuum drying oven at 50° C. for 48 h to obtain an amino-modified pineapple leaf fiber;

[0062] (3) Add 4 times the mass of water to dialdehyde starch (with a degree of aldehyde formation of 75%) to prepare a 20wt% dialdehyde starch solution, then add glycerol accounting for 20wt% of the mass ratio of dialdehyde starch, and stir for 30min at 80°C to gelatinize to obtain a starch gelatinized solution, add the amino-modified pineapple leaf fiber obtained in step (2) to the starch gelatinized solution according to the mass ratio of amino-modified pineapple leaf fiber to dialdehyde starch of 2:8, keep the reaction temperature at 55°C in a constant temperature water bath, react for 6h under a nitrogen protection environment, filter after the reaction, wash and dry to obtain a mixture, take the above mixture for hot pressing molding, the hot pressing pressure is 12MPa, the hot pressing temperature is 110°C, the hot pressing time is 8min, and the modified pineapple leaf fiber reinforced starch-based composite material is obtained after demolding.

[0063] Example 2

[0064] (1) Same as Example 1;

[0065] (2) In a mixed solution of 70 mL ethanol and water (V 水 :V 乙醇=2.5:1) was added with 0.6 mL of concentrated ammonia, stirred and mixed at 40° C. for 0.5 h, then 50 mL of a 2 mg / mL aqueous solution of dopamine hydrochloride was added, 5 g of the pretreated pineapple leaf fiber obtained in step (1) was added to the above solution, reacted at 40° C. for 2 h, then 4 mL of a polyamine (diethylenetriamine) was added, the reaction was continued for 10 h, and filtered to obtain a modified fiber, which was washed five times with distilled water, and finally, the washed pineapple leaf fiber was dried in a vacuum drying oven at 50° C. for 48 h to obtain an amino-modified pineapple leaf fiber;

[0066] (3) Add 4 times the mass of water to dialdehyde starch (with a degree of formaldehyde of 56%) to prepare a 20wt% dialdehyde starch solution, then add 20wt% (based on the mass ratio of dialdehyde starch) of glycerol, and stir for 30min at 80°C to gelatinize to obtain a starch gelatinized solution, add the amino-modified pineapple leaf fiber obtained in step (2) to the starch gelatinized solution according to a mass ratio of amino-modified pineapple leaf fiber to dialdehyde starch of 1:9, keep the reaction temperature at 60°C in a constant temperature water bath, react for 12h under a nitrogen protection environment, filter after the reaction, wash and dry to obtain a mixture, take the above mixture for hot pressing molding, the hot pressing pressure is 10MPa, the hot pressing temperature is 120°C, the hot pressing time is 25min, and the modified pineapple leaf fiber reinforced starch-based composite material is obtained after demolding.

[0067] Example 3

[0068] (1) Same as Example 1;

[0069] (2) In a mixed solution of 70 mL ethanol and water (V 水 :V 乙醇 =2.5:1), add 0.6 mL of concentrated ammonia water, stir and mix at 30°C for 0.5 h, then add 50 mL of 2 mg / mL dopamine hydrochloride aqueous solution, then add 5 g of the pretreated pineapple leaf fiber obtained in step (1) to the above solution, react at 35°C for 2 h, then add 5 mL of polyamine (1,1-dimethylethylenediamine), continue to react for 12 h, filter to obtain modified fiber, wash five times with distilled water, finally, dry the pineapple leaf fiber obtained after washing in a vacuum drying oven at a temperature of 50°C for 48 h to obtain amino-modified pineapple leaf fiber;

[0070] (3) Add 4 times the mass of water to dialdehyde starch (with a degree of aldehydeation of 90%) to prepare a 20wt% dialdehyde starch solution, then add 20wt% (based on the mass ratio of dialdehyde starch) of glycerol, and stir for 30min at 80°C to gelatinize to obtain a starch gelatinized solution, add the amino-modified pineapple leaf fiber obtained in step (2) to the starch gelatinized solution according to a mass ratio of amino-modified pineapple leaf fiber to dialdehyde starch of 3:7, keep the reaction temperature at 55°C in a constant temperature water bath, react for 12h under a nitrogen protection environment, filter after the reaction, wash and dry to obtain a mixture, take the above mixture for hot pressing molding, the hot pressing pressure is 13MPa, the hot pressing temperature is 120°C, and the hot pressing time is 20min, and after demolding, a modified pineapple leaf fiber reinforced starch-based composite material is obtained.

[0071] Example 4

[0072] (1) Same as Example 1;

[0073] (2) In a mixed solution of 70 mL ethanol and water (V 水 :V 乙醇 =2.5:1), add 0.6 mL of concentrated ammonia water, stir and mix at 35°C for 0.5 h, then add 50 mL of a 2 mg / mL dopamine hydrochloride aqueous solution, then add 5 g of the pretreated pineapple leaf fiber obtained in step (1) to the above solution, react at 35°C for 1 h, then add 3 mL of polyamine (1,2-dimethylethylenediamine), continue to react for 12 h, filter to obtain modified fiber, wash five times with distilled water, and finally dry the pineapple leaf fiber obtained after washing in a vacuum drying oven at a temperature of 50°C for 48 h to obtain amino-modified pineapple leaf fiber;

[0074] (3) Add 4 times the mass of water to dialdehyde starch (with a degree of formaldehyde of 85%) to prepare a 20wt% dialdehyde starch solution, then add 20wt% (based on the mass ratio of dialdehyde starch) of glycerol, and stir for 30min at 80°C to gelatinize to obtain a starch gelatinized solution, add the amino-modified pineapple leaf fiber obtained in step (2) to the starch gelatinized solution according to a mass ratio of amino-modified pineapple leaf fiber to dialdehyde starch of 4:6, keep the reaction temperature at 55°C in a constant temperature water bath, react for 6h under a nitrogen protection environment, filter after the reaction, wash and dry to obtain a mixture, take the above mixture for hot pressing molding, the hot pressing pressure is 11MPa, the hot pressing temperature is 130°C, and the hot pressing time is 20min, and after demolding, a modified pineapple leaf fiber reinforced starch-based composite material is obtained.

[0075] Example 5

[0076] (1) Same as Example 1;

[0077] (2) In a mixed solution of 70 mL ethanol and water (V 水 :V乙醇 =2.5:1), add 0.6 mL of concentrated ammonia water, stir and mix at 40°C for 0.5 h, then add 50 mL of 2 mg / mL dopamine hydrochloride aqueous solution, then add 5 g of the pretreated pineapple leaf fiber obtained in step (1) to the above solution, react at 40°C for 2 h, then add 5 mL of polyamine (1,2-dimethylethylenediamine), continue to react for 12 h, filter to obtain modified fiber, wash five times with distilled water, finally, dry the pineapple leaf fiber obtained after washing in a vacuum drying oven at a temperature of 50°C for 48 h to obtain amino-modified pineapple leaf fiber;

[0078] (3) Add 4 times the mass of water to dialdehyde starch (with a degree of formaldehyde of 90%) to prepare a 20wt% dialdehyde starch solution, then add 20wt% (based on the mass ratio of dialdehyde starch) of glycerol, and stir for 30min at 80°C to gelatinize to obtain a starch gelatinized solution, add the amino-modified pineapple leaf fiber obtained in step (2) to the starch gelatinized solution according to a mass ratio of amino-modified pineapple leaf fiber to dialdehyde starch of 1.5:8.5, keep the reaction temperature at 55°C in a constant temperature water bath, react for 12h under a nitrogen protection environment, filter after the reaction, wash and dry to obtain a mixture, take the above mixture for hot pressing molding, the hot pressing pressure is 12MPa, the hot pressing temperature is 110°C, the hot pressing time is 10min, and the modified pineapple leaf fiber reinforced starch-based composite material is obtained after demolding.

[0079] Comparative Example 1

[0080] (1) using a pulverizer to grind dried pineapple leaves into a powder state, sorting through a 60-mesh screen, taking 40 g of pineapple leaf fiber powder and immersing it in 800 mL of a 3% (w / v) sodium hydroxide aqueous solution, stirring at 60 ° C for 1 h, then washing with ultrapure water for multiple times until the pH of the washing solution is neutral, removing lignin and hemicellulose, then bleaching with 200 mL of a 1% (w / w) sodium hypochlorite solution and 2 mL of acetic acid at 70 ° C, stirring for 0.5 h, washing the bleached pineapple leaf fibrils with distilled water until neutral, and placing them in an air oven at 65 ° C for 24 h to obtain pre-treated pineapple leaf fibers;

[0081] (2) Add 4 times the mass of water to dialdehyde starch (with a degree of aldehyde formation of 56%), then add glycerol accounting for 20wt% of the mass ratio of dialdehyde starch, and stir for 30min at 80°C for gelatinization, add the pretreated pineapple leaf fiber obtained in step (1) to the starch gelatinization solution according to the mass ratio of pretreated pineapple leaf fiber to dialdehyde starch of 2:8, keep the reaction temperature at 55°C in a constant temperature water bath, react for 6h under nitrogen protection environment, filter, wash and dry after the reaction to obtain a mixture, take the above mixture for hot pressing molding, the hot pressing pressure is 12MPa, the hot pressing temperature is 110°C, the hot pressing time is 8min, and the pineapple leaf fiber reinforced starch-based composite material is obtained after demolding.

[0082] Comparative Example 2

[0083] The same as Example 1, except that in step (3), the amino-modified pineapple leaf fiber obtained in step (2) is added to the starch gelatinized liquid at a mass ratio of 1:1 between the amino-modified pineapple leaf fiber and the dialdehyde starch.

[0084] Comparative Example 3

[0085] The same as Example 1, except that the degree of aldehyde formation of the dialdehyde starch in step (3) is 40%.

[0086] Performance Testing

[0087] The structures of the modified pineapple leaf fiber reinforced starch-based composite materials prepared in the examples and comparative examples were characterized by infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM), and the mechanical properties, thermal stability and water resistance were tested:

[0088] The FT-IR images of the pretreated pineapple leaf fibers (original fibers) and amino-modified pineapple leaf fibers (amino-modified fibers) prepared in Example 1 are shown in FIG. Figure 1 ,Depend on Figure 1 It can be seen that the amino-modified pineapple leaf fiber has a higher -1 The characteristic peak of -NH appeared at 1234cm -1 It is the characteristic peak of -CH2, indicating that the amino group is successfully grafted onto the pineapple leaf fiber.

[0089] The FT-IR images of the modified pineapple leaf fiber reinforced starch-based composite material (modified fiber composite material) prepared in Example 1 and the pineapple leaf fiber reinforced starch-based composite material (unmodified fiber composite material) prepared in Comparative Example 1 are shown in FIG. Figure 2 ,Depend on Figure 2 It can be seen that compared with the infrared spectrum of the unmodified pineapple leaf fiber composite material, the infrared spectrum of the modified pineapple leaf fiber composite material has a peak at 1640 cm -1 The stretching vibration peak of the C=N group appeared; at 1106cm-1 and 1076cm -1 There are two peaks, mainly due to the influence of C=N, which causes the stretching vibration of CO in dialdehyde starch to move; 2919cm -1 、2846cm -1 Peak weakened, 3295cm -1 The peaks are enhanced, indicating the existence of hydrogen bonding between molecules in the composite material. The peaks indicate that Schiff base reaction occurs between dialdehyde starch and amino-modified pineapple leaf fiber, and strong hydrogen bonding exists between molecules.

[0090] The optical microscope image of the modified pineapple leaf fiber reinforced starch-based composite material prepared in Example 1 is shown in Figure 3 , SEM images are shown in Figure 4 ,from Figure 3 It can be seen that the surface of the composite material is smooth and flat, and the cross-sectional morphology shows that the pineapple leaf fibers are relatively evenly dispersed in the composite material, indicating that the amino-modified pineapple leaf fibers are tightly combined with dialdehyde starch to form a dense surface structure. The improvement of its density is beneficial to improving the water resistance of starch-based composite materials. Figure 4 It shows that the fibers are evenly distributed in the internal structure of the composite material and are completely wrapped by the dialdehyde starch matrix. There is no separation between the fibers and starch, indicating that the combination is not a simple physical combination, but a denser material structure is obtained after the Schiff base reaction.

[0091] The mechanical properties and thermal stability of the composite materials prepared in Examples 1-5 and Comparative Examples 1-3 were tested and statistically analyzed. The mechanical properties test results are shown in Table 1.

[0092] Table 1 Mechanical properties test results

[0093]

[0094]

[0095] It can be seen from Table 1 that the tensile strength and elongation at break of the modified pineapple leaf fiber reinforced starch-based composite material prepared in the embodiment are better than those of the comparative example, indicating that the modified pineapple leaf fiber and the dialdehyde starch matrix are not simply mixed, but undergo a Schiff base reaction, so that there is a good "bonding" effect between the two, thereby enabling the composite material to have higher mechanical properties; due to the stability of the composite material structure, the thermal stability is also improved ( Figure 5The results of thermal stability tests of the composite materials prepared in Examples 1-5 and Comparative Examples 1-3 are shown). The dialdehyde starch has cross-linking ability and high chemical activity, and reacts with the amino functional groups on the surface of the modified pineapple leaf fiber to form covalent bonds and hydrogen bonds. The chemical cross-linking between the two allows the material to have a stable cross-linking structure inside, so that the composite material has excellent tensile strength and thermal stability.

[0096] The composite materials prepared in Examples 1-5 and Comparative Examples 1-3 were tested for water resistance. The test results are shown in Figure 6 It can be seen that the water absorption rate of the modified pineapple leaf fiber reinforced starch-based composite materials of Examples 1-5 is lower than that of the unmodified pineapple leaf fiber reinforced starch-based composite material of Comparative Example 1, which further illustrates that the modified pineapple leaf fiber is tightly combined with the dialdehyde starch matrix and has a dense material structure, thereby reducing the absorption of water and improving the water resistance of the composite material.

[0097] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A method for preparing a modified pineapple leaf fiber reinforced starch-based composite material, characterized in that: Using pineapple leaf fiber as raw material, adopting chemical grafting modification technology to graft amino functional groups on the surface of pineapple leaf fiber to obtain amino-modified pineapple leaf fiber, then adding dialdehyde starch to react with Schiff base, and hot-pressing the reaction product to obtain the modified pineapple leaf fiber reinforced starch-based composite material; The specific steps include: (1) adding pineapple leaf fiber to a mixed solution of ethanol, water, concentrated ammonia water and dopamine hydrochloride aqueous solution, adding polyamine to continue the reaction after the reaction is completed, filtering, washing and drying to obtain amino-modified pineapple leaf fiber; (2) adding water and a plasticizer to the dialdehyde starch, stirring and gelatinizing to obtain a starch gelatinized liquid, adding the amino-modified pineapple leaf fiber obtained in step (1) to the starch gelatinized liquid, reacting at a constant temperature under a nitrogen protective atmosphere, filtering, washing and drying to obtain a mixture after the reaction is completed, and hot-pressing the mixture to obtain the modified pineapple leaf fiber reinforced starch-based composite material; In step (2), the degree of formaldehyde formation of the dialdehyde starch is 56%-90%, and the mass ratio of the amino-modified pineapple leaf fiber to the dialdehyde starch is (1:9)-(4:6).

2. The method for preparing the modified pineapple leaf fiber reinforced starch-based composite material according to claim 1, characterized in that: The usage ratio of the ethanol, water, concentrated ammonia water, dopamine hydrochloride aqueous solution, pineapple leaf fiber and polyamine is 20mL: 50mL: 0.6mL: 50mL: 5g: (2.5-5)mL.

3. The method for preparing the modified pineapple leaf fiber reinforced starch-based composite material according to claim 1, characterized in that: In step (1), the polyamine includes one or more of ethylenediamine, diethylenetriamine, 1,1-dimethylethylenediamine and 1,2-dimethylethylenediamine.

4. The method for preparing the modified pineapple leaf fiber reinforced starch-based composite material according to claim 1, characterized in that: In step (1), after adding pineapple leaf fiber, the reaction is carried out at 35-40° C. for 1-2 hours, and after adding polyamine, the reaction is continued at 35-40° C. for 9-12 hours.

5. The method for preparing the modified pineapple leaf fiber reinforced starch-based composite material according to claim 1, characterized in that: In step (2), the amino-modified pineapple leaf fiber is added to the starch gelatinized liquid, and reacted in a constant temperature water bath at 55-60° C. for 6-12 hours under a nitrogen protection environment; the hot pressing pressure of the hot pressing molding is 10-13 MPa, the hot pressing temperature is 110-130° C., and the hot pressing time is 8-25 min.

6. A modified pineapple leaf fiber reinforced starch-based composite material, characterized in that: Prepared according to the preparation method according to any one of claims 1 to 5.

7. Use of the modified pineapple leaf fiber reinforced starch-based composite material as claimed in claim 6 in the preparation of food, light industrial packaging materials and agricultural films.

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