Abstract
Polyurethanes have serious degradation problems, and the degradation of polyurethane products can adversely affect the environment in all aspects. After research, it is found that polyurethane products based on bio-based raw materials have low environmental impact after degradation and are green. The research progress and current status of starch-modified degradable polyurethanes are reviewed, and three modification methods of starch at home and abroad are also discussed.
Keywords
polyurethane
Biodegradable polyurethane, Modification
Introduction
Polyurethane (PU) is a widely used material in industrial production. At present, polyurethane materials occupy a vital part in industrial production, and are widely used in food processing, clothing, construction, defense engineering and many other fields because of their good wear resistance, elasticity and adhesion. The raw materials of traditional polyurethane materials are non-renewable energy, and polyurethane materials have high relative molecular mass and high chemical bonding energy, which are difficult to degrade and cause more harm to the environment, and long-time landfill or incineration disposal will cause irreversible effects on the natural environment. Therefore, it is an inevitable trend to develop green materials to study polyurethane materials that are easily degradable and friendly to the environment.
1 Experiment
1 Research progress of starch-modified degradable polyurethane
Among a variety of degradable polyurethanes synthesized, bio-based raw materials are the most readily available and have minimal impact on the environment and subsequently. Among them, starch-modified polyurethanes are easy to handle and easy to make. Most of the preparations are dominated by co-blending modifications, on the basis of which the starch is modified by chemical or physical methods. Starch and its derivatives are a cheap and abundant renewable resource, which can be used as synthetic modified polymers to give new properties to polymers and in line with environmental protection and sustainable exploitation and development strategies of resources, therefore, starch is one of the most promising raw materials for biodegradable polyurethanes.
CAO et al. used polyester diol (PEPA), toluene diisocyanate (TDI) and dihydroxymethylpropionic acid (DMPA) to obtain two groups of aqueous polyurethanes with different NCO/OH ratios; then the synthesized groups of aqueous polyurethanes were added to starch with pasteurization treatment and physically mixed homogeneously to The polyurethane/starch blends with different composition ratios were obtained; the blends were desolventized and finally formed at 40°C to obtain polyurethane/starch blends with different composition ratios. The tensile and other properties were tested, and the study showed that the tensile strength of the blends was related to the microphase structure and starch content of the aqueous polyurethane, and the mixing of aqueous polyurethane with starch reduced both the regular structure of the hard segment and the regularity of the soft segment of the polyurethane, so to a certain limit, the aqueous polyurethane and starch with different mixing ratios were blendable.
Bio-based starch can also prepare starch nanocrystals ( StN) to enhance the performance of aqueous polyurethanes. chen et al. treated potato starch with aqueous sulfuric acid solution to convert it under ultrasonic conditions and then obtained starch nanocrystals ( StN). The modified polyurethanes were prepared by three methods: (i) adding StN aqueous dispersion to WPU emulsion; (ii) adding StN aqueous dispersion during emulsification; and (iii) adding DMPA and butanone dispersion of StN during the chain expansion stage of polyurethane prepolymer. The final tests showed that the properties of the first obtained StN/WPU complexes were improved in different degrees compared to the unmodified WPU.
Polyhydroxy and isocyanate, natural components extracted from plants, were used as reactants for the preparation of polyurethane materials. The polyurethanes produced from polyhydroxy and isocyanate have high added value. These polyurethanes are widely used in the production of foam and rubber, and have important applications in the field of coatings, adhesives and packaging. According to the researchers, the polyurethanes prepared from plant extracts were found to have good antibacterial properties during pretreatment, synthesis and use, and also have strong biodegradability after disposal. Therefore, the whole process is green in all aspects, from the acquisition of raw materials, recycling, application and post-disposal without pollution.
By using a solid-phase melting method with isocyanate and polyol as reactants for the synthesis reaction, corn starch was modified with the polyurethane prepolymer generated from the synthesis reaction, and then hydrophobic thermoplastic starches were prepared by relevant experiments, thus investigating a variety of different basic structures of polyurethane and concluding that the temperature required for processing and the content of polyurethane have a relevant effect on the properties and structure of the modified starch The effects on the properties and structure of the modified starch were also studied.
2 Status of research on starch-modified polyurethanes
In recent years, in order to improve the properties of starch-modified polyurethanes, researchers have used chemical and physical methods to modify starch in order to improve its solubility, increase the degree of substitution and so on. These methods have achieved good results.
2.1 Structure and properties of starch
Starch is a polymer of bio-based natural polymers. α-D-glucopyranose constitutes the basic unit of starch and is formed by the connection of glycosidic bonds, i.e., a series of glucose groups interconnected to form a starch molecule, i.e., ( C6H12O5)n. Because of the different ways in which the residual groups of glucose pyran rings are connected to each other, starch can be Starch can be divided into straight-chain starch and branched-chain starch.
Branched chain starch forms the backbone of the starch molecule and has a large relative molecular mass and is more abundant; straight chain starch is grafted onto branched chain starch by hydrogen bonding and has a lower relative molecular mass and is less abundant than branched chain starch. Due to the different connection between molecules, the regularity of starch molecules of straight chain starch and branched chain starch is different. The molecules are divided into crystalline region (regular) and amorphous region (irregular) according to their regularity. Since the relative molecular mass of straight-chain starch is much smaller than that of branched starch, straight-chain starch is mostly chosen as the subject of study. The branched starch has a larger average relative molecular mass and a wider relative molecular mass distribution than the straight-chain starch, so the choice of different solvents will have different effects on its relative molecular mass. The high hydroxyl group content in starch molecules makes it easy to form hydrogen bonds with high bond energy and easy to crystallize. Due to high molecular chain regularity, starch is insoluble in cold water and insoluble in general organic solvents. In order to improve the solubility of starch, modification operations are required.
2.2 Modification methods of starch
The disadvantages of starch are: poor solubility, insoluble in organic solvents and cold water; starch emulsion will paste when heated, difficult to control its viscosity, paste starch will degrade under high temperature or acidic conditions, etc. In order to broaden the application field of starch, it is necessary to modify the properties of starch for operation. There are three common methods of starch modification: chemical modification, physical modification and enzyme modification.
2. 2. 1 Chemical modification method
The most widely used method is the chemical modification method, and SANTAYANON et al. used pyrimidine catalyst and propionic anhydride as esterifying agent to modify cassava starch by esterification, so that some of the hydroxyl groups in the starch were converted into ester groups. Experiments showed that the stability of the esterified starch in water was improved with the interfacial compatibility of the polyurethane blended material; however, the disadvantages of esterification modification were also obvious, as the degradability of the original starch was reduced, and the toxicity of pyrimidine as an esterification reagent could also cause some environmental hazards.
In addition, starch acylation modification, i.e. partial substitution of the hydroxyl group on the glucose unit of the original starch, can be carried out. The presence of a large number of branched structures in the branched starch structure makes the molecular structure of straight-chain starch more regular and crystalline, and its use as a polyol can improve the mechanical properties of materials, etc. Some studies have shown that the hydrophobicity and stretchability of the straight-chain starch obtained after the acylation reaction are improved, which fills the gap of poor solubility of the original starch in some solvents, and the solubility of the starch after the acylation reaction is better in butanone and acetone.
Acetylated starch also has good hydrophobicity and tensile strength, acetylated starch is also known as acetate starch. 1860s, people began to try to carry out starch acetylation modification, in 1904 researchers successfully made acetylated starch, starch mixed with glacial acetic acid, put it in a certain temperature for the reaction, thus preparing a water-soluble acetylated modified starch. When the degree of substitution of acetylated starch is >2, the properties of the highly substituted starch are similar to those of cellulose acetate, and most organic solvents can dissolve it, and it has good film and thermoplastic properties. In addition, the water solubility of modified starch is significantly improved, and the water solubility is enhanced by increasing the degree of substitution. When the degree of substitution reaches a certain value, the modified starch can be completely dissolved in cold water. When the degree of substitution > 2, the solubility of acetylated starch in water decreases with the increase of the degree of substitution.
Acetylated starch can be prepared by the following methods: ①High temperature and high pressure method. Add tapioca starch, glacial acetic acid and acetic anhydride in a micro reactor, and put it under high temperature and high pressure, 170℃ for a few minutes, with high conversion rate and no by-products, and the degree of substitution is 0.5~2.5. However, the reaction conditions are harsh and the container is small, so it cannot be mass produced.
②Ionic liquid method. Ionic liquids have small intermolecular forces and high electrical conductivity. In the preparation of acetylated starch, BMIMCl is added, and the chloride ion breaks the hydrogen bond to destroy the molecular structure. When the reaction temperature reaches 135°C, the original starch is completely dissolved in the ionic liquid and is homogeneous and transparent, which is a homogeneous reaction system and is conducive to the reaction. Control the feeding ratio, reaction time, etc. can control the degree of substitution, the degree of substitution is 0.3 ~ 3.0. But the ionic liquid is expensive, it is difficult to use widely.
③Aqueous medium method. Acetylated starch with degree of substitution <1 is prepared in the aqueous phase system. It is time-consuming and cumbersome to operate. ④Solvent method. Dissolve the original starch in glacial acetic acid or DMF and heat it for acetylation reaction. The acetylated starch is prepared with a degree of substitution of 2.9. When glacial acetic acid is used as the solvent, a large amount of organic solvent is not required, and the reaction temperature is lower, and when the solvent of the catalyst is DMF, it is less harmful to human body.
2 Results and discussion
2. 2. 2. 2 Physical modification method
Compared with the chemical method, the physical method of starch modification is a typical green and pollution-free technology. No pollutants are produced in the modification process, which is popular among consumers and is a hot spot for research on starch modification in recent years. Physical modification starch process, whose main component is starch, has the same properties as chemically modified starch, with basically the same processing tolerance, shear resistance, acid resistance and viscosity stability, also known as functional natural starch. Wet heat-treated starch, dry heat-treated starch and pre-gelatinized starch are the three major methods of physically modified starch.
①Wet heat treated starch. Hydrothermal treated starch is a typical physical method to modify starch, which is a product obtained by taking starch with certain water content for a certain period of time and at a specific temperature. Also, according to the method of starch treatment and water content, such methods can be divided into three categories: wet heat treatment, pressure heat treatment, and toughening treatment.
Researchers have found that to change the physicochemical properties of starch without damaging the structure of the starch granules can be done by changing the water content, treatment temperature and time of the starch to obtain different products to meet different needs. The moisture content of the starch treated with moist heat is between 20% and 35%, and after the moisture balance, the reaction is carried out in a reactor at high temperature. The glass vessel is used as the reaction vessel, and the starch is put into the reaction vessel and placed in a thermostat for 10-15h, then removed and cooled to room temperature and dried with 12%-14% moisture. Temperature and humidity are the two most important key factors in the humid heat treatment of starch. The process of treating starch involves only water and heat, no chemicals are added, no contamination is produced, and the safety of the product is also very high.
In the experiments to study the effect of moist heat treatment on starch and on carboxymethyl activity, it was found that the polarized cross of waxy corn starch still existed after moist heat treatment, and it was also found that the granule morphology of starch basically did not change, only cracks on the granule surface; after moist heat treatment, the paste viscosity of starch decreased, the paste temperature increased, and the structure was enhanced without changing the crystal shape. After moist heat treatment, the activity of carboxymethyl reaction of waxy corn starch also decreases, thus increasing the content of straight chain starch in corn starch.
② Dry heat treatment of starch. The patent application and showed that if you want to modify to get different varieties of starch you can treat the starch by dry heat method. The dry heat treatment method means that the starch is initially dried to reach about 10% moisture and then heat treated at high temperature to bring the starch to anhydrous state before the modification of the starch is carried out. Dry heat treatment starch can be divided into two types: direct dry heat denaturation method and auxiliary dry heat denaturation method. The properties of the dry heat treated starch will be a little different from the normal starch. The granule morphology and physicochemical indexes of corn starch were found to be changed after the treatment. When the temperature is ≤150℃, the dry heat treatment time is 0.5~1.0h, and the viscosity, pasting temperature and solubility of the starch are reduced compared with the original starch; when the temperature is 150℃, the dry heat treatment is 2~4h, and the starch after treatment is different from the original starch.
Compared with the original starch, the solubility of the starch obtained after the treatment increased and the crystallinity increased, but the viscosity and pasting temperature decreased and the enthalpy of pasting decreased, and the higher the temperature and longer the time of dry heat treatment, the greater the difference between the treated starch and the original starch, and the effect was more significant.
③Predextrinized starch. The process of pre-gelatinized starch is to mix water and starch in the experimental device and then heat it, use the temperature to paste the original starch fully, and finally dry and crush it. Under the high temperature condition, the hydrogen bond of starch is broken and water molecules can enter, which makes the original starch molecules swell, and the swelling times can reach hundreds of times, and the original β-structure becomes α-structure, so the product obtained by pregelatinization of starch is also called α-starch. Pre-gelatinized starch can be divided into rolling drying method and extrusion swelling method.
The experimental equipment of the rolling drying method is the drum dryer, which is heated by drumming in steam and prepared by continuous rotation. 20% to 40% of the starch emulsion occurs on the drum surface by heat pasting to form a thin layer, and then gently scraped off with a knife. Screw extruder is a common experimental equipment in extrusion puffing method, the machine by constantly squeezing friction thus generating heat; with the continuous accumulation of heat, the temperature rises, so that the original starch paste, and then sprayed through the small hole at the top of the device; due to the different pressure inside and outside, the instantaneous decompression of starch rapidly expand to achieve the purpose of drying.
2. 2. 3 Enzyme modification method
The process of starch treatment by different biological enzymes is the biological modification method. α, β, γ-cyclodextrin, straight chain starch and maltodextrin are produced as a result of starch treatment by different enzymes, while using the same enzyme treatment method and physical method, without any pollution during the treatment process, and the products obtained are healthy and hygienic.
In recent years, the use of bioenzymes for starch modification has received increasing attention. The commonly used bioenzymes are α-amylase, β-amylase, glycosylase, etc. Different enzymes change starch with different effects and the starch through treatment has different properties. However, the starch modified by bioenzymes are
