Are you eager for the latest starch-based biodegradable plastic modification technology?
Since the 1930s and 1940s, plastic has become the fourth major new type of basic material after steel, wood and cement due to its advantages of light weight, low cost and strong plasticity. General-purpose plastic products are discarded in large quantities, and the resulting "white pollution" has caused tremendous pressure on the ecological environment. At the same time, oil as a non-renewable resource will eventually be exhausted.
Therefore, in recent years, countries around the world have successively issued "plastic limit orders" to encourage practitioners and consumers to use green and environmentally friendly degradable plastics through comprehensive bans, voluntary agreements and other methods.
Types of degradable plastics
Polycaprolactone (PCL)
Polylactic acid (PLA)
Polyvinyl alcohol (PVA), polyhydroxy
Alkyl Ester (PHA)
starch
Cellulose
Sodium Alginate (SA)
Chitosan (CS)
Protein-based bio-based plastics
Starch is a natural and renewable polymer compound. Because of its abundance, easy availability and low price, it is favored by degradable plastics.
At present, starch-based degradable plastics account for about 50% of the existing commercial degradable plastics, and are widely used in food packaging films, agricultural mulch films, foamed plastic lunch boxes, and medical bone tissue engineering scaffolds.
However, compared with traditional plastics, the mechanical properties and barrier properties of starch-based degradable plastics are greatly reduced, and they cannot be further promoted commercially. Therefore, the physical or chemical modification of starch is very important.
Cross-linking is one of the main methods of starch modification. The tightly connected three-dimensional network structure formed by cross-linking enhances the intermolecular interaction, thereby obtaining a degradable material with good heat resistance, water resistance, high strength and flexibility.
The structure and properties of starch
Starch is a polymer carbohydrate formed by dehydration and polymerization of multiple glucose molecules.
Starch is widely present in the roots, tubers, fruits and leaves of higher plants in the form of granules. At present, the main crops that produce starch are wheat, corn and potatoes.
Starch is mainly composed of amylose and amylopectin:
Amylose is highly crystalline and can dissolve in hot water without becoming a paste;
Amylopectin is insoluble in cold water, but swells into a paste with hot water.
Different sources of starch have different component content and different properties. Comprehensive comparison of multiple properties, potato starch has better anti-retrogression and transparency, and the best mold resistance, mechanical properties and water resistance, and is most suitable for the preparation of membrane materials.
(a) Amylose (b) Amylopectin
Comparison of the composition and properties of starches from different sources
Cross-linking method of starch-based degradable plastic
Cross-linking is a process in which linear or branched polymer chains are connected by covalent bonds to form a network or bulk polymer. According to different cross-linking methods, cross-linked starch-based degradable plastics can be divided into chemically cross-linked starch-based degradable plastics and photo-cross-linked starch-based degradable plastics.
01
Chemical crosslinking
Chemical cross-linking is the reaction of a cross-linking agent containing binary or multiple functional groups with the hydroxyl groups on the starch molecules to form groups such as diether bonds or diester groups, thereby cross-linking multiple starch molecules to form a spatial network The square structure of the polymer.
Commonly used crosslinking agents are glutaraldehyde, epichlorohydrin (ECH), sodium trimetaphosphate (STMP), citric acid (CA) and malic acid. The properties of starch-based degradable plastics prepared by different crosslinking agents are also different.
Citric acid
The adipic acid cross-linked film has the best light transmission and barrier properties. The boric acid cross-linked film has the highest strength, while the CA cross-linked film has the best flexibility; the micro-topography shows that the adipic acid and borax cross-linked films are better than boric acid The cross-linked film has a more uniform structure and is more suitable for the preparation of starch/PHA composite films.
Scanning electron micrograph of each sample
In recent years, with the vigorous promotion of environmental protection concepts, CA-type "green" non-toxic crosslinking agents have become more and more popular among researchers and have become the main crosslinking agents for crosslinking starch-based degradable plastics.
Starch-based degradable plastic
According to the time of adding crosslinking agent, chemical crosslinking can be divided into:
Cross-linking (that is, adding a cross-linking agent to react during polymer molding)
Post-crosslinking (that is, after the material is formed, cross-linking occurs between molecules through the cross-linking agent solution dipping method).
If the required degree of cross-linking cannot be achieved by using cross-linking, post-cross-linking can be considered. Most of the cross-linking reactions are carried out in the starch paste liquid, and the reaction temperature should reach the gelatinization temperature of starch.
02
Photocrosslinking
Photocrosslinking is a method of adding a photosensitizer to the starch system to decompose it into free radicals under ultraviolet light (UV) irradiation, and use the photosensitizer to polymerize the hydroxyl groups in the starch to crosslink the starch molecules. .
When photocrosslinking to prepare starch-based degradable plastics, the radiation dose and photosensitizer concentration are the most important factors affecting the crosslinking degree of the material.
Compared with the chemical cross-linking method, the photo-cross-linking method does not require hydrothermal equipment and cross-linking reagents, is safer and more environmentally friendly, and is simple to operate, and the reaction is easy to control. It can be adapted to large-scale continuous production of materials and is suitable for bio-based hydrogels. , Preparation of drug delivery materials, etc.
The effect of cross-linking on the properties of starch-based degradable plastics
01
Water resistance
Water resistance is one of the important conditions for testing the application standards of bio-based degradable membrane materials. However, due to the natural hydrophilicity of starch, starch-based film materials usually exhibit stronger hydrophilicity and higher permeability. Cross-linking modification makes starch have a tightly connected three-dimensional network structure, these networks can effectively prevent the entry and migration of water molecules. Water absorption, swelling and water vapor transmission rate (WVP) are often used to characterize the water resistance of starch-based materials.
Starch base film
02
Mechanical behavior
In daily production and life, packaging film materials need to have a certain strength and flexibility to maintain their integrity during processing. Cross-linking establishes intermolecular and intramolecular connections, makes starch molecular chains longer, and enhances intermolecular interactions, resulting in increased tensile strength of the material and decreased elongation at break.
Generally speaking, adding a small amount of crosslinking agent can meet the performance requirements of the product. When the degree of cross-linking is low, the length of starch molecules available for sliding increases. With the continuous increase of the degree of crosslinking, the intermolecular and intramolecular interactions are enhanced, and the tensile strength is increased, but at the same time the intermolecular slippage is also restricted, which leads to a decrease in the elongation at break of the material and shows brittleness.
Starch has extremely strong hydrophilicity. If the starch content in the system is too much, the intermolecular force will be weakened after the material absorbs water, which greatly reduces the tensile strength of the material.
In addition to the degree of crosslinking and starch content, relative humidity also has a greater impact on the mechanical properties of starch-based degradable plastics. When the relative humidity is 40%, the mechanical properties of starch-based sheets are the best. Too low relative humidity may make the material brittle and break into pieces when stretched; when relative humidity is too high, a large amount of water molecules enter the plastic sheet as a plasticizer, and the tensile strength decreases.
The curing time and curing temperature are also important factors affecting its mechanical properties. When the oxidized sucrose content is 5wt%, the glycerol content is 15wt%, the curing time is 5min, and the curing temperature is 180°C, the mechanical properties of the starch film are optimal (tensile strength is 23MPa, elongation at break is 60%).
When the curing time is short, the cross-linking is not enough to form a network between starch molecules, and the intermolecular interaction is weak. If the curing time is too long, the starch skeleton will be destroyed, which will lead to the decrease of tensile strength and elongation at break. At the same time, crosslinking requires a sufficiently high curing temperature to ensure effective removal of water molecules from the crosslinking reaction system.
Starch-based plastic raw materials
The addition amount of plasticizer should not be too much. The permeability of starch film increases with the increase of the plasticizer concentration, but when its concentration exceeds the critical limit, phase separation will occur. In addition, some cross-linking agents can simultaneously function as cross-linking agents and plasticizers in starch-based film materials.
CA has such a two-way effect. When the CA content exceeds 10wt%, the excess CA acts as a plasticizer, resulting in a decrease in tensile strength and an increase in elongation at break. The increase of CA content in PVA/starch composites from 5wt% to 30wt% did not significantly reduce the tensile strength, but the elongation at break increased significantly. The combined effect of cross-linking and plasticization leads to complex performance changes of starch-based materials. For this type of crosslinking agent, an appropriate addition amount can improve the performance of starch-based degradable plastics, and too much will act as a plasticizer, which will adversely affect the performance of the material.
03
Degradability
Degradability is the biggest advantage of starch-based materials. The biodegradation of starch-based materials is usually caused by the biological activity of fungi, bacteria and other microorganisms under natural conditions such as soil, or under certain specific conditions such as composting conditions or in aqueous culture solutions.
The soil burial method uses microorganisms to erode the starch in it and secrete enzymes, which reduces the strength of the material. The plastic and the metal salt in the soil undergo auto-oxidation to generate peroxides, which promote the breaking of polymer molecular chains and become low molecular substances. , Which becomes H2O and CO2.
Compost degradation
The composting method uses microorganisms to control the conversion of degradable organic matter in solid waste into stable humus, H2O and CO2 under oxygen conditions.
Cross-linking enhances the intermolecular and intramolecular interaction force and reduces the degradation rate of the material. Under normal circumstances, the degradation degree of starch-based degradable plastics is positively correlated with starch content and soil burying time, and the degradation rate is positively correlated with starch content, environmental humidity, cross-linking degree, and plasticizer content.
The depth of burial has no obvious effect on the degradation rate of the film; the weight loss rate of the film within 15 days is 48.70%, which has perfect biodegradability.
The amount of CO2 released in the soil composting system characterizes the degradability of CA cross-linked high amylose barley starch bioplastics. The degradation rate of plastics is relatively high within 20 days, and decreases significantly after 20 days. After 100 days, the degradation rate is the same as the soil reference rate, and the material is completely degraded. At the same time, it was found that the degradation rate of cross-linked starch-based degradable plastics was much slower than that of starch granules.
The modified barley husk enhances the degradation of the PVA/starch cross-linked composite film in the natural soil environment. The results of this study show that the weight loss rate of the crosslinked PVA/crosslinked starch film is lower than that of the PVA/starch film. The natural barley husk/PVA/cross-linked starch composite film has the highest degradation rate, and the weight loss rate after 120 days is 33%, because the presence of natural barley husk in the composite material can absorb more water.
Modification method of cross-linked starch-based degradable plastic
Due to the compact three-dimensional network structure formed by cross-linking, the performance of cross-linked starch-based degradable plastics has been improved to a certain extent, but it still does not reach the standards of general plastics. Therefore, it is necessary to further modify it, including blending with other polymer materials, nano-material enhancement, multiple modification, coating hydrophobic coating, etc.
01
Natural polymer blending modification
Glucomannan with a network structure can inhibit starch settling and is beneficial to improve the compatibility of the blending system. Then an appropriate amount of PVA is added to the system to blend. PVA provides the film with better water resistance and mechanical properties, but the compatibility of the system becomes poor.
Transglutaminase was added to the glycerol-plasticized sago starch/fish gelatin blend, and a water-resistant and flexible starch-based film was prepared by cross-linking the enzyme with amino groups in the protein. The WVP of the modified film decreases, the elongation at break increases, and the tensile strength decreases.
Glutaraldehyde
Using glutaraldehyde as the cross-linking agent, a potato/CS composite cross-linked film with better water resistance was prepared by the casting method. At the same time, CS gave the film unique antibacterial properties, making it available in the field of biomedicine or food preservation. widely used.
Compared with pure starch-based films, the properties of starch-based films that are blended and cross-linked with other biopolymers are improved. At the same time, other biomaterials can also bring some additional properties to starch-based films. This method is to prepare The new trend of new starch-based membrane materials.
02
Blending modification of degradable polyester
Blending starch with degradable polyester, with the help of polyester's excellent mechanical properties and water resistance, can effectively make up for the lack of starch film in terms of performance. For blending systems, the compatibility of multiple substances is an important factor affecting the mechanical properties of materials.
ECH and glycerin are used to modify the starch, and then the modified starch is blended with PLA, and the film is prepared by a hot pressing process. The hydroxyl groups on starch molecules are cross-linked by ECH molecules to form ether groups, thereby changing the hydrophilicity of starch. The crosslinking and plasticizing modification of starch improves its compatibility with PLA and enhances its mechanical properties.
When the starch/PVA/CA content ratio is 3:3:0.08, the blend film has the best overall performance; the water absorption of the blend film is 54.31 times its own weight, and the mechanical tensile strength is 46.45 MPa.
Due to the addition of PVA, the film has strong antibacterial activity against food-borne pathogenic bacteria Listeria and Escherichia coli.
The test results of fresh fig packaging show that the CA-doped ternary composite film has water vapor permeability, can effectively prevent fruit rot and surface fogging, and has high water retention and water resistance.
The film has great potential in anti-fog packaging film and active food packaging applications for packaging high-permeability fresh agricultural products.
A CA cross-linked PVA/starch composite film that can be applied to wound dressings was prepared by casting method. Compared with similar plastic films for packaging, the prepared PVA/starch composite film has greater flexibility, lower elastic modulus, higher solubility and swelling index, and excellent in vitro degradation rate and antibacterial properties , Is an excellent wound dressing material.
03
Nano filler reinforced modification
In addition to degradable polyester, cellulose and nanoparticles are also fillers commonly used in recent years to study the mechanical properties of starch-based degradable plastics.
Cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs) have a better strengthening effect on starch film, and CNFs have better strengthening effects than CNCs. This is mainly due to the fact that both cellulose and starch are polysaccharides, and the two have similar structures, and strong hydrogen bonds are easily formed between hydroxyl groups, resulting in extremely strong interfacial adhesion.
Cellulose nanofiber
CNCs have a needle-like morphology with high crystallinity; while CNFs have a network structure with a larger aspect ratio, a higher degree of entanglement with starch, and greater intermolecular interactions. This strong interface It improves the mechanical properties of starch-based film materials.
The waxy corn starch was blended with micro/nano cellulose (MFC), and 1,2,3,4-butanetetracarboxylic acid cross-linked starch-based composites were prepared by casting method.
Crosslinked film with MFC content of 15wt% without using any plasticizer
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