Preparation method and application of biomass environment-friendly adhesive based on longan shells

Abstract

The invention belongs to the technical field of biomass materials and environment-friendly adhesives, and relates to a biomass adhesive, in particular to a preparation method of a biomass environment-friendly adhesive based on longan shells, which comprises the following steps: firstly, cleaning, drying, crushing and screening the longan shells for pretreatment, and then carrying out alkalization and acetylation chemical modification to obtain the biomass environment-friendly adhesive based on the longan shells. And finally, compounding with gelatinized starch, a chitosan acetic acid solution and citric acid, and adjusting the viscosity to obtain the adhesive. The waste longan shells are used as raw materials, agricultural waste recycling is achieved, the product is free of formaldehyde release and biodegradable, and the viscosity of the prepared adhesive is 1000-2000 mPa.s at the temperature of 25 DEG C and is measured through a rotary viscometer; the wood-wood bonding strength is larger than or equal to 2.5 MPa, the 24-hour water absorption rate is smaller than or equal to 25%, the problems that a traditional biomass adhesive is short in performance and low in cost, special equipment is not needed for preparation are solved, the adhesive is suitable for the industrial fields of artificial board manufacturing, furniture manufacturing and the like, and environment-friendly transformation of the adhesive industry is promoted.

Description

Technical Field

[0001] This invention belongs to the field of biomass materials and environmentally friendly adhesives, and relates to biomass adhesives, particularly to a method for preparing a biomass environmentally friendly adhesive based on longan shells and its application. Background Technology

[0002] Adhesives are key auxiliary materials in furniture manufacturing, packaging printing, and building materials processing. Currently, the mainstream products on the market are mainly synthetic polymer adhesives such as urea-formaldehyde resin adhesives and phenolic resin adhesives. Although these adhesives have the advantages of high bonding strength and low cost, they continuously release volatile organic compounds (VOCs) such as formaldehyde during production and use. Long-term exposure can harm human health, and they are difficult to degrade after disposal, easily causing environmental pollution, which is inconsistent with the current development trend of "green manufacturing".

[0003] To address the environmental concerns associated with synthetic adhesives, the industry has gradually developed biomass adhesives based on natural raw materials such as starch, chitosan, and plant proteins. However, existing biomass adhesives generally suffer from performance limitations: for example, pure starch adhesives have poor water resistance (24-hour water absorption often exceeds 40%), low bonding strength (wood-to-wood bonding strength is mostly below 1.5 MPa), and are prone to mold growth; while chitosan adhesives possess certain antibacterial properties, their high raw material cost (market price approximately 200 RMB / kg) limits their large-scale application; and plant protein adhesives suffer from slow curing speed and poor storage stability.

[0004] Longan, a specialty fruit of southern my country, has an annual output exceeding 3 million tons. The longan shells generated during processing account for 15-20% of the fruit's weight, resulting in annual emissions of 450,000-600,000 tons. Currently, longan shells are mostly disposed of through incineration and landfill, which not only wastes resources but also easily leads to air and soil pollution. Research has found that longan shells are rich in cellulose (32-38%), lignin (18-22%), and flavonoids. The hydroxyl groups of cellulose can be chemically modified to enhance adhesiveness, while the aromatic ring structure of lignin can improve the material's mechanical properties. These studies provide a theoretical basis for the high-value utilization of longan shells in the field of biomass adhesives. However, current technologies lack adhesive preparation schemes designed specifically for the characteristics of longan shells that balance environmental friendliness, performance, and cost.

[0005] Therefore, developing a biomass adhesive that uses longan shells as raw material, can replace synthetic adhesives, and has the advantages of being formaldehyde-free, having high bonding strength, water resistance, and low cost has become a key need to solve the problems of agricultural waste resource utilization and the environmental transformation of the adhesive industry. Summary of the Invention

[0006] To address the problems of formaldehyde pollution from existing synthetic adhesives, insufficient performance of biomass adhesives, and waste of longan shells, this invention discloses a method for preparing a biomass environmentally friendly adhesive based on longan shells.

[0007] Technical solution

[0008] A method for preparing a biomass-based environmentally friendly adhesive based on longan shells includes the following steps:

[0009] S1. Raw material pretreatment: Wash and dry the longan shells until the moisture content is ≤10%, crush them and pass them through an 80~100 mesh sieve to obtain longan shell powder;

[0010] S2. Chemical modification:

[0011] (1) Alkalization treatment: Add longan shell powder to 10-15% NaOH aqueous solution at a solid-liquid mass-volume ratio of 1g:8-10mL, stir at 60-70℃ for 2-3h, filter, wash with water until pH value is 7-8, and dry to obtain alkalized longan shell powder.

[0012] (2) Acetylation modification: Mix alkalized longan shell powder with acetic anhydride at a mass ratio of 1:3~5, add 1~2% concentrated sulfuric acid, stir at 40~50℃ for 1~2h, wash with ethanol and dry to obtain acetylated modified longan shell powder;

[0013] S3. Adhesive compounding:

[0014] (1) Starch gelatinization: Mix 20-30 parts of starch with 30-40 parts of water, gelatinize at 90-95℃ for 30-40 minutes, and then cool down to 50-55℃;

[0015] (2) Composite reaction: Add 30-4 parts of acetylated modified longan shell powder and 10-15 parts of 1% chitosan acetic acid solution, stir at 50-55℃ for 60-90 min, then add 3-5 parts of citric acid and continue stirring for 30-40 min;

[0016] (3) Viscosity adjustment: Add 20-30 parts of water, stir for 10-15 minutes, cool to room temperature, and obtain biomass environmentally friendly adhesive based on longan shell.

[0017] In a preferred embodiment of the present invention, in step S1, the drying temperature is 60~80℃ and the drying time is 4~6h.

[0018] In a preferred embodiment of the present invention, in step S2 (1), the drying temperature is 80~90℃ and the drying time is 2~3h.

[0019] In a preferred embodiment of the present invention, in step S2 (2), the drying temperature is 70~80℃ and the drying time is 1~2h.

[0020] In a preferred embodiment of the present invention, in step S2(2), the degree of acetyl substitution of the acetylated modified longan shell powder is controlled to be 0.5~0.7.

[0021] In a preferred embodiment of the present invention, in step S3 (1), the starch is one or more of cereal starch, legume starch, and tuber / root starch, preferably corn starch.

[0022] In a preferred embodiment of the present invention, in step S3 (2), 2-3% of nano-SiO2 is added, wherein the nano-SiO2 has a particle size of 50-100 nm.

[0023] The biomass-based environmentally friendly adhesive based on longan shells prepared according to the method described in this invention.

[0024] The adhesive has a viscosity of 1000~2000 mPa·s, measured by rotational viscometer at 25℃; wood-to-wood bonding strength ≥2.5 MPa, 24h water absorption ≤25%, free formaldehyde content <0.1 mg / kg, and 60-day biodegradability ≥60%.

[0025] The present invention also discloses the application of the prepared biomass environmentally friendly adhesive in the bonding of engineered wood products, flooring, furniture manufacturing, packaging materials or bamboo and wood handicrafts.

[0026] The technical solution of the present invention is described in detail below:

[0027] A method for preparing a biomass-based environmentally friendly adhesive based on longan shells includes the following steps:

[0028] S1. Raw material pretreatment

[0029] (1) Cleaning and drying: Take fresh longan shells, remove attached fruit pulp residue and impurities, rinse with water 3 to 5 times, and dry in a 60 to 80℃ forced-air drying oven for 4 to 6 hours until the moisture content of the raw material is ≤10%;

[0030] (2) Crushing and sieving: The dried longan shells are put into a crusher and crushed. They are then sieved through an 80-100 mesh standard sieve to collect longan shell powder with a particle size ≤150μm for later use.

[0031] S2. Chemical modification of longan shell powder

[0032] (1) Alkalization treatment: Add the obtained longan shell powder to a 10-15% NaOH aqueous solution at a solid-liquid ratio of 1:8-10 (mass-volume ratio, g / mL), place it in a constant temperature water bath at 60-70℃, and stir at a speed of 300-400r / min for 2-3h; after the reaction is completed, separate the solid and liquid by vacuum filtration, wash the filter cake repeatedly with deionized water until the pH value of the filtrate is 7-8, and then place the filter cake in a drying oven at 80-90℃ for 2-3h to obtain alkalized longan shell powder;

[0033] (2) Acetylation modification: Mix alkalized longan shell powder with acetic anhydride at a mass ratio of 1:3~5, add concentrated sulfuric acid at a mass ratio of 1%~2% of the total mass of the mixture as a catalyst, and stir the mixture in a constant temperature water bath at 40~50℃ for 1~2h. After the reaction is completed, wash the mixture 2~3 times with anhydrous ethanol to remove unreacted acetic anhydride and concentrated sulfuric acid. After vacuum filtration, dry the filter cake in a drying oven at 70~80℃ for 1~2h to obtain acetylated modified longan shell powder (acetyl substitution degree controlled at 0.5~0.7).

[0034] S3. Adhesive compounding

[0035] (1) Preparation of basic components: Prepare the following raw materials by mass: 30-40 parts of acetylated modified longan shell powder, 20-30 parts of corn starch, 10-15 parts of 1% (mass concentration) chitosan acetic acid solution, 3-5 parts of citric acid, and 50-60 parts of deionized water;

[0036] (2) Starch gelatinization: Mix corn starch with 30-40 parts of deionized water, place it in a water bath at 90-95℃ and stir to gelatinize for 30-40 minutes, maintaining a stirring speed of 200-300 r / min until the starch paste becomes transparent, and then cool it down to 50-55℃.

[0037] (3) Composite reaction: Add acetylated modified longan shell powder and 1% chitosan acetic acid solution to starch paste, keep at 50~55℃, stir at 300~400r / min for 60~90min; then add citric acid and continue stirring for 30~40min to obtain the initial adhesive.

[0038] (4) Viscosity adjustment: Add the remaining 20-30 parts of deionized water to the initial adhesive, stir for 10-15 minutes, adjust the viscosity of the adhesive to 1000-2000 mPa·s, measure the viscosity at 25℃ using a rotational viscometer, cool to room temperature, and obtain the biomass environmentally friendly adhesive based on longan shell.

[0039] S4. Performance Tuning (Optional, adjust according to application scenario)

[0040] Enhanced water resistance: If used in outdoor or high humidity scenarios, when adding citric acid in step S3 (3), add 2-3% of nano SiO2 (particle size 50-100nm) of the total mass of the adhesive and continue stirring for 30-40 minutes to reduce the water absorption rate of the adhesive to below 20% in 24 hours.

[0041] Improved curing speed: To shorten the curing time, place the adhered objects in an environment of 50~60℃ and apply a pressure of 0.5~1MPa. The curing time can be shortened from 12~24h at room temperature to 4~6h.

[0042] Beneficial effects

[0043] This invention uses waste longan shells as the core raw material to achieve the resource utilization of agricultural waste and reduce pollution from incineration / landfilling; the product has no formaldehyde or VOCs release (free formaldehyde content < 0.1 mg / kg, meeting GB 18583-2008 standard), and a 60-day biodegradation rate ≥ 60% (meeting GB / T The adhesive meets the GB / T14074-2017 standard (19277.1-2011) and is environmentally friendly throughout its entire life cycle. Through stepwise modification of "alkalization + acetylation", it not only destroys the cellulose crystal zone of longan shell and exposes the viscous hydroxyl groups, but also introduces acetyl groups to improve hydrophobicity. Combined with the starch-chitosan composite system and cross-linking with citric acid, the wood-to-wood bonding strength of the adhesive is ≥2.5MPa (compliant with GB / T14074-2017 standard), and the 24h water absorption rate is ≤25%, solving the problem of "adhesion and water resistance being mutually exclusive" in traditional biomass adhesives. The raw material cost of longan shell is only 200~300 yuan / ton. With the addition of low-cost auxiliary materials such as corn starch, the raw material cost of each ton of adhesive is about 1200~1500 yuan, which is only 60% of that of commercially available urea-formaldehyde resin adhesive and 30% of that of chitosan adhesive, giving it significant market competitiveness. The preparation process does not require special equipment and can be based on the modification of existing starch adhesive production lines (only the addition of alkalization and acetylation reactors is required). The investment for a single production line is 500,000~800,000 yuan, making it suitable for small and medium-sized enterprises to quickly implement. Detailed Implementation

[0044] The present invention will be described in detail below with reference to embodiments, so that those skilled in the art can better understand the present invention, but the present invention is not limited to the following embodiments.

[0045] Example 1

[0046] A method for preparing a biomass-based environmentally friendly adhesive based on longan shells (basic formula) includes the following steps:

[0047] (1) Raw material pretreatment: Take 10 kg of fresh longan shells, remove impurities, rinse 3 times with deionized water, dry at 60℃ for 5 h (moisture content 8%), crush and pass through an 80 mesh sieve, and collect 8.2 kg of longan shell powder;

[0048] (2) Chemical modification:

[0049] Alkalization: Add 8 kg of longan shell powder to 64 L of 12% NaOH solution at a solid-liquid ratio of 1:8, stir in a water bath at 65℃ for 2.5 h, filter, wash with water until pH value is 7.5, and dry at 85℃ for 2.5 h to obtain 6.3 kg of alkalized longan shell powder;

[0050] Acetylation: 6 kg of alkalized longan shell powder and 24 kg of acetic anhydride were mixed at a mass ratio of 1:4, 0.6 kg of concentrated sulfuric acid was added, the mixture was stirred at 45°C for 1.5 h, washed three times with ethanol, and dried at 75°C for 1.5 h to obtain 5.1 kg of acetylated modified longan shell powder.

[0051] (3) Adhesive compounding:

[0052] Starch gelatinization: Mix 25 parts corn starch with 35 parts deionized water, gelatinize at 92℃ for 35 minutes, and then cool to 52℃.

[0053] Composite reaction: Add 35 parts of acetylated modified longan shell powder and 12 parts of 1% chitosan acetic acid solution, stir at 52℃ for 75 min, then add 4 parts of citric acid and continue stirring for 35 min;

[0054] Viscosity adjustment: Add 25 parts of deionized water, stir for 12 minutes, and cool to room temperature to obtain the adhesive product.

[0055] Performance tests: Wood-to-wood bonding strength 2.7 MPa (GB / T 14074-2017), 24h water absorption rate 23% (GB / T1733-1993), free formaldehyde not detected (GB 18583-2008), 60-day biodegradation rate 62% (GB / T 19277.1-2011).

[0056] Example 2

[0057] A method for preparing a biomass-based environmentally friendly adhesive based on longan shells (water-resistant enhanced type) includes the following steps:

[0058] (1) The raw material pretreatment and chemical modification steps are the same as in Example 1;

[0059] (2) Adhesive compounding:

[0060] Starch gelatinization: Mix 22 parts corn starch with 32 parts deionized water, gelatinize at 90℃ for 30 minutes, and then cool to 50℃;

[0061] Composite reaction: Add 38 parts of acetylated modified longan shell powder and 10 parts of 1% chitosan acetic acid solution, stir at 50℃ for 60 min, add 4 parts of citric acid and 2.5 parts of nano SiO2 (particle size 80 nm), and continue stirring for 40 min;

[0062] Viscosity adjustment: Add 28 parts of deionized water, stir for 10 minutes, and cool to room temperature to obtain the adhesive product.

[0063] Performance tests: wood-to-wood bonding strength 2.6 MPa, 24-hour water absorption rate 18%, free formaldehyde not detected, 60-day biodegradation rate 61%.

[0064] Example 3

[0065] A method for preparing a biomass-based environmentally friendly adhesive (rapid curing type) based on longan shell includes the following steps:

[0066] (1) The raw material pretreatment and chemical modification steps are the same as in Example 1;

[0067] (2) Adhesive compounding:

[0068] Starch gelatinization: Mix 28 parts corn starch with 38 parts deionized water, gelatinize at 95℃ for 40 minutes, and then cool to 55℃;

[0069] Composite reaction: Add 32 parts of acetylated modified longan shell powder and 15 parts of 1% chitosan acetic acid solution, stir at 55℃ for 90 min, add 5 parts of citric acid, and continue stirring for 30 min;

[0070] Viscosity adjustment: Add 22 parts of deionized water, stir for 15 minutes, and cool to room temperature to obtain the adhesive product.

[0071] Performance testing: Curing time of 5 hours at 60℃ and 0.8MPa pressure; wood-to-wood bonding strength of 2.8MPa; water absorption rate of 24% in 24 hours; free formaldehyde not detected; biodegradation rate of 63% in 6 days.

[0072] The embodiments described above are merely specific implementations of the present invention and do not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made using the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing a biomass-based environmentally friendly adhesive based on longan shell, characterized in that, Includes the following steps: S1. Raw material pretreatment: Wash and dry the longan shells until the moisture content is ≤10%, crush them and pass them through an 80~100 mesh sieve to obtain longan shell powder; S2. Chemical modification (1) Alkalization treatment: add longan shell powder to 10-15% NaOH aqueous solution at a solid-liquid mass-volume ratio of 1g:8-10mL, stir at 60-70℃ for 2-3h, filter, wash with water until pH value is 7-8, and dry to obtain alkalized longan shell powder. (2) Acetylation modification: mix alkalized longan shell powder with acetic anhydride at a mass ratio of 1:3~5, add 1~2% concentrated sulfuric acid, stir at 40~50℃ for 1~2h, wash with ethanol and dry to obtain acetylated modified longan shell powder. S3. Adhesive compounding (1) Starch gelatinization: Mix 20-30 parts of starch with 30-40 parts of water, gelatinize at 90-95℃ for 30-40 minutes, and then cool down to 50-55℃; (2) For the composite reaction, add 30-4 parts of acetylated modified longan shell powder and 10-15 parts of 1% chitosan acetic acid solution, stir at 50-55℃ for 60-90 min, then add 3-5 parts of citric acid and continue stirring for 30-40 min; (3) Viscosity adjustment: add 20-30 parts of water, stir for 10-15 minutes, cool to room temperature, and obtain biomass environmentally friendly adhesive based on longan shell.

2. The method for preparing the biomass-based environmentally friendly adhesive based on longan shell according to claim 1, characterized in that: In step S1, the drying temperature is 60~80℃ and the drying time is 4~6h.

3. The method for preparing the biomass-based environmentally friendly adhesive based on longan shell according to claim 1, characterized in that: In step S2(1), the drying temperature is 80~90℃ and the drying time is 2~3h.

4. The method for preparing the biomass-based environmentally friendly adhesive based on longan shell according to claim 1, characterized in that: In step S2 (2), the drying temperature is 70~80℃ and the drying time is 1~2h.

5. The method for preparing the biomass-based environmentally friendly adhesive based on longan shell according to claim 1, characterized in that: In step S2(2), the degree of acetyl substitution of the acetylated modified longan shell powder is controlled to be 0.5~0.

7.

6. The method for preparing the biomass-based environmentally friendly adhesive based on longan shell according to claim 1, characterized in that: In step S3(1), the starch is one or more of cereal starch, legume starch, and tuber / root starch.

7. The method for preparing the biomass-based environmentally friendly adhesive based on longan shell according to claim 1, characterized in that: In step S3(1), the starch is corn starch.

8. The method for preparing the biomass-based environmentally friendly adhesive based on longan shell according to claim 1, characterized in that: In step S3(2), 2-3% of nano-SiO2 is added, wherein the nano-SiO2 has a particle size of 50-100nm.

9. The biomass-based environmentally friendly adhesive based on longan shell prepared by any one of claims 1-8, characterized in that: The adhesive has a viscosity of 1000~2000 mPa·s, measured by rotational viscometer at 25℃; wood-to-wood bonding strength ≥2.5 MPa, 24h water absorption ≤25%, free formaldehyde content <0.1 mg / kg, and 60-day biodegradability ≥60%.

10. An application of the biomass-based environmentally friendly adhesive based on longan shell as described in claim 9, characterized in that: It is used in the bonding of engineered wood products, flooring, furniture manufacturing, packaging materials, or bamboo and wood handicrafts.