Starch based polymer
The powder does not have true thermoplasticity, but it can be used for injection by adding plasticizer (water, glycerin, sorbose, etc.), shearing, high temperature (90 ° C ~ 180 ° C), which can melt and liquefy the starch. , extrusion, blow molding equipment, such as synthetic plastics. This combination (starch, water, heat) ensures the gelatinization of the starch, that is, the decomposition of the granular tissue. As the molecule asks for the cleavage of the hydrogen bond, the starch is gelatinized to form a viscous slurry, i.e., a starch-based polymer is formed.
Chinese name
Starch based polymer
Matrix
starch
ways to produce
Dissolution casting
Foreign name
Amyloid polymer
Production start
Starch extraction
Degradability
Completely biodegradable
Head
record
1 starch modification
2 production of starch-based polymers
Production start
Unstructured modification
3 biodegradable polymers
4 starch-based polymer degradation mechanism
1 starch modified editor
Starch exists in the form of particles, with crystalline and amorphous areas. Since many properties of the original starch cannot meet the requirements of practical applications, such as viscosity thermal stability, gelatinization properties, solubility, etc., physical, chemical and biochemical are adopted. The method changes the structural, physical and chemical properties of the original starch to produce specific properties and uses.
The size of the starch granules is related to the thickness of the film from which the starch plastic is made. The starch granules are insoluble in cold water, but when the dried natural starch is placed in cold water, they undergo a limited reversible swelling process, at which time the small molecules only enter the starch granules. The amorphous portion, in combination with the free hydrophilic group, causes the starch to undergo swelling of the starch granules to maintain the original characteristics and crystal birefringence. If the starch suspension is heated to a certain particle size, the starch granules suddenly expand and the suspension becomes a viscous gelatinous solution. This phenomenon is called gelatinization of starch, and the performance of starch gelatinization is closely related to the manufacture of starch plastics. Since starch does not have plasticity, it needs to be modified to change the structural, physical and chemical properties of the original starch, resulting in specific platforms and uses. The treated starch is collectively referred to as modified starch. Many physical properties of the modified starch, such as solubility, viscosity, expansion rate, fluidity, coagulation and heat sensitivity in water, are superior to the original starch, and some new characteristics such as super Water absorption, water insolubility, plasticity, etc. are properties not possessed by the original starch, and these characteristics can be used to develop new products.
2 production editing of starch-based polymers
Production start
The production of starch polymers begins with the extraction of starch, which depends on the source of the starch plant, followed by the separation of the fibers, bleaching, and drying to yield pure starch. Depending on the properties of the desired starch polymer, the starch is chemically modified before and after drying to be converted into a thermoplastic material, which can only be achieved by an extruder, by continuous extrusion and mixing or a combined extrusion mixing step.
In the past, the main production method for starch plastics was dissolution casting. In this method, the starch is dissolved in a suitable solvent to allow the viscous solution to flow sufficiently to ensure rapid dispersion on the surface of the casting. After the solution is cast, the film is obtained upon drying. The technique used by researchers has several drawbacks, namely small film yields and long production times. Industrially, feeding is carried out by small slits on a large rotating drum or by moving a metal belt. A mask can be used to remove organic solvents from the work area.
Unstructured modification
In addition to being used as a filler in reinforced plastics, the natural starch has poor thermal processing properties, and it is necessary to perform non-structural modification of the starch to be a bioplastic. In addition, it needs to be mixed with other polymers and plasticizers to improve mechanical properties and barrier properties. The main unstructured agent is water, which plays two roles, promoting starch gelatinization (swelling of starch by the destruction of most hydrogen bonds between macromolecules to form a sticky patch) and as a plasticizer. However, in addition to water, an additional plasticizer is needed to lower the melting temperature.
For pure dry starch, the melting temperature is varied from 220 ° C to 240 ° C, and this range includes the starch decomposition onset temperature. If a non-volatile plasticizer, such as a polyol, is added, the melting temperature is lowered, and under high temperature and shear, the starch can be processed into a thermoplastic thermoplastic called thermoplastic starch (TPS). In addition, the growth of microorganisms can be restricted by reducing the water activity of the membrane. In the processing of thermoplastics, the water contained in the starch and the added plasticizer play an indispensable role, because they can form hydrogen bonds with starch, replacing the strong interaction between the molecular hydroxyl groups of the starch, thereby converting into a kind of Thermoplastic.
3 biodegradable polymer editing
With regard to biobased or biodegradable polymers, it is important to demonstrate the biodegradability of these materials. The American Society for Testing and Materials and the International Standards Organization define plastics that undergo major chemical structural changes in specific environments as biodegradable plastics. Tested according to standard methods, these changes result in the loss of physical and chemical properties. The bio-based polymer may be a biodegradable polymer or a non-biodegradable polymer. For example, starch-based polymers are generally biodegradable, while crystalline polylactic acid is almost non-degradable. Currently, scientists focus on three main types of polymer materials.
The first type is a biodegradable traditional plastic. When the surface of the polymer material is in contact with the soil, the material will be smoothly degraded. The micro-material in the soil cannot degrade the plastic particles, but causes a rapid collapse of the supporting matrix. This material usually has an indestructible petroleum-based matrix that is reinforced by carbon or fiberglass.
The second category is partially degradable polymeric materials that degrade faster than conventional synthetic plastics. A typical production method for such plastics involves surrounding natural fibers around a conventional matrix (petroleum based). When processed, microorganisms can consume natural macromolecules in the body. What remains is a material that is weakened in structure, with rough edges and openness for further degradation.
The third category, the last category, is currently a polymer material that is of great interest to researchers and the industry. These plastics are fully biodegradable, the polymer matrix is derived from natural materials such as starch, microbial growth polymers, and the reinforcing fibers are derived from common crops such as flax and hemp. Under appropriate conditions of temperature, humidity and oxygen, biodegradation causes the plastic to decompose into non-toxic or environmentally-friendly residual substances, which are then completely decomposed into carbon dioxide and water by microorganisms.
4 starch-based polymer degradation mechanism editing
The degradation mechanism of the polymer is not very clear. It is generally believed that the mechanism of biodegradation is not a single mechanism, but a complex biophysical and biochemical action, accompanied by other physical and chemical effects such as hydrolysis and oxidation, biological action and physical chemistry. The roles promote each other and have a synergistic effect. Starch is two natural biodegradable polymers that decompose into glucose under the action of microorganisms and are finally metabolized to water and carbon dioxide.
The degradation of starch-based polymers can be divided into two processes: starch is attacked by microorganisms such as fungi and bacteria, gradually disappears, and a porous fracture structure is formed in the polymer, and the mechanical strength is lowered, which increases the surface area of the polymer, thereby facilitating further Natural decomposition; starch degradation triggers the action of prooxidant and self-oxidant, which can cut the long chain of polymer and make the relative molecular mass of the polymer smaller until the relative molecular mass of the polymer is small enough to be metabolized by microorganisms, and finally water is formed. Small molecule compounds such as carbon dioxide enter the cycle of nature. These two processes are mutually reinforcing.
The biodegradability of starch-based degradable plastics occurs due to the presence of a continuous phase of starch, which ensures that microorganisms and enzymes rapidly approach the starch content of the membrane.
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