Biodegradation Kinetics of Polymer Starch Blends (I)

Abstract In this paper, the biodegradation kinetics of polyethylene-starch (PE-S) blends was investigated by aerobic biodegradation method and computer simulation method when the starch particle size p was higher or lower than the percolation critical value pc. Applied to two starch degradation models in computer simulations (i) the process of invading the blends by microorganisms (ii) the diffusion of macromolecules (Mei et al.) into the interior of the blend results in the back diffusion of small molecules to the surface, small molecules The process of further assimilation by microorganisms. The invasive model of microorganisms was based on the results of an experimental study of a PE-S blend by scanning electron microscopy. The blend contained 1-15 micron starch granules. According to the test results of the soil burying experiment, there was a clear microbial growth in the starch matrix of the polyethylene matrix. The enzyme diffusion was based on hydrolysis experiments of PE-S blends. The production of small molecules was found during the hydrolysis experiment after the composting experiment. The contact of monodisperse and polydisperse starch granules with microorganisms and enzymes in the blend was simulated by computer. Aerobic biodegradable carbon dioxide production reflects the microbial intrusion of starch in the blend. The starch degradation degree A and the time t follow the power function Atn, wherein the index n depends on the fractional dimension and the channel size of the starch cluster that the microorganism can contact, and the value is close to 1 when the starch particle p is larger than the critical permeability scale pc. . Microbial intrusion simulations show that when the starch particle size is close to pc, the average index n is about 05; when the starch particle size is p>pc, it is about 025, and when p>pc is about 05. The measured index of biodegradation of aerobic compost shows that the invasiveness of microorganisms is the main degradation process regardless of the starch size being larger or smaller than the critical size.

Keywords biodegradable; plastic; preolation polymer-starch composites; dynamics

Foreword

Degradation of plastic materials in water and land is mainly coordinated by 1-4 in four ways:

1) Microbial degradation: The enzymes produced by molds and bacteria cause the plastic material to decompose under aerobic and anoxic conditions. The degradation rate is related to the number of microorganisms, humidity, temperature, and oxygen. Plastics that can be degraded by microorganisms are called biodegradable plastics and include: PHVB, PVOH, polycaprolactone, starch-based, cellulose-based, and triglycerides Base plastics. Plastics that are easily invaded by microorganisms are also sensitive to the degradation behavior of large organisms.

2. Biodegradation: Plastics are consumed as food by invertebrates, insects (such as cockroaches, cockroaches, snails). Fish and large animals can also eat plastic, but it can cause diseases. Large biodegradation includes three forms 1-4: (a) chewing, (b) digestion, and (c) in vitro digestive degradation. Chewing leads to a significant deterioration of the physical and chemical structure of plastics; digestion can consume substances that can be digested in plastics through enzymatic and mechanochemical movements; in vitro degradation refers to the degradation of undigested plastic residues and to chewed plastic parts. process. Although such degradation process is a typical and fastest degradation method, it is still less studied. The method of ingesting food attractants is used to make large organisms interested in plastics, and non-biodegradable plastics are degraded by this method. In contrast, people can also add resistance agents to prevent large creatures from attacking plastics.

3 Photodegradation: The absorption of ultraviolet rays by plastics can lead to the decomposition of plastic molecules. This degradation process has been fully investigated and some additives have been added to common commercial plastics to prevent degradation caused by sunlight (eg, polyethylene, polypropylene, polystyrene, etc.). The addition of photoactive groups (such as ketene or carbon monoxide) or photoactive additives in the chemical chain of the polymer increases the photodegradability. Since photodegradation results in a decrease in molecular weight and introduction of an oxidizing group in the polymer chain, it is possible to promote the progress of biodegradation and also to promote chemical degradation.

4. Chemical degradation: The molecular structure of plastics is degraded by adding chemical additives such as oxides or peroxides. These additives catalyze the oxidation of double bonds (such as unsaturated fatty acids, oils, low molecular weight rubbers, etc.) to generate peroxides, which then produce highly active free radicals through the decomposition of peroxides. The latter can attack the polymer chain if it is not absorbed by other chemicals. The addition of these additives to photo- and photo-degradable additives in polyethylene causes a decrease in molecular weight and embrittlement of the material. A large reduction in molecular weight (approximately 1000) can eventually lead to biodegradation.

Other methods such as mechanical, wind, rain, etc. can lead to polymer degradation. The extent of polymer degradation depends on environmental factors and the type of polymer. In hydrocarbons, polyethylene and polystyrene (molecular weight > 100,000) will not biodegrade within 10 years 6,7. The polymer's chain size, crystallinity, chemical properties, and degree of branching all affect the biodegradation of the polymer7,8.

In the past twenty-five years, people have shown great interest in using biodegradable plastics such as starch or polyhydroxyvalerate to replace traditional non-biodegradable polymers such as polyethylene, especially for short-term applications. Significant loss of synthetic polymer species 9-11. Microbial digestion of the polymer creates structural voids that increase the surface area of ​​the mixture12. Increased surface area increases the oxidizing ability of the polyethylene polymer chain. Polyethylene has biodegradation potential due to the occurrence of polyethylene chain oxidation.

According to related reports, there are microbial growth phenomena in many polymers, especially for polymers containing filler components and plasticizer components13,14. The growth of microorganisms can cause the polymer to change color. If the growth of the microorganisms caused by the growth of the microorganisms is moldy and darkened only on the surface of the polymer, it does not mean that the polymer as a whole will degrade. In this paper, we investigated the diffusion of starch-polyethylene blends by microorganisms and the diffusion of enzymes in polyethylene-starch blends. The predictability of the biodegradation of biodegradable blend components is important for many practical applications (eg, agricultural mulch, compostable materials, nursery rakes, afforestation equipment, and aquaculture equipment). The biodegradation period of starch in an aerobic environment can be measured in hours, and the biodegradation period of starch in a non-degradable polyethylene matrix may be several days to several years, which is related to the fraction of starch and starch granules. The degree of connection between the relevant. When the starch content exceeds a certain limit, the micro-organisms in the polyethylene matrix will intrude into the channel and the degradation will accelerate. In this paper, we compared the results of invasive kinetics of simulated microorganisms and enzymes with the results of aerobic CO2 production. We studied the availability of starch in starch-polyethylene blends above and below the critical value. Degree dynamics. This paper intends to give a basic model of biodegradation kinetics through this result and can help design more reliable biodegradable plastics and blended materials.

Penetration theory

The scalar penetration theory is related to the connectivity of one component randomly dispersed in the other component 16-19. Examples of such penetrations are the gels produced when the polymer crosslinks and the conductivity of the metal particles dispersed in a non-conductive medium. For a finite size object, the percolation threshold pc is defined as the minimum concentration p (permeate medium) required for the connection of the upper and lower surfaces. For different geometrical lattices, the penetration threshold is different 16. For a finite size object, the percolation threshold pc is defined as the minimum concentration p (permeate medium) required for the connection of the upper and lower surfaces. For different geometrical lattices, the penetration threshold is different 16. For the two-dimensional lattice penetration threshold is 5940% and for the cubic lattice 3117%. Another form of infiltration is gradient penetration 20, where the concentration of material particles is not constant, and the penetration is performed in a certain direction (usually occurring in the polymer film) 21 where we can see the propagation of force or vector through the mixture. process. Vector penetration has important implications for determining mechanical properties, especially fracture stress. Examples of scalars, gradients, and vectors and further details are in Chapter 4 of Reference 2. In this paper we mainly discuss scalar infiltration in two-dimensional and three-dimensional systems.

Peanasky et al. used scalar permeability theory to analyze the static degradation of starch in starch-polyethylene blends12. It is believed that static degradation depends on the final accessibility of starch or the total starch concentration that can be invaded by microorganisms. The model does not take into account the degradation process over time. Simulations of static degradation are also available in some of the earlier articles2,12. The contact fraction as a concentration function A(p) is derived from the permeation equation: A(p)～(p-pc)v/p (p>pc)(1)
Where pc is the limit value of the osmosis concentration or the penetration threshold, v is the limit index. Figure 1 shows a two-dimensional system with two-sided immersed contact fraction clusters (shaded portions) 12 near the threshold of penetration. Peanasky et al. calculated the permeability index 12 of a three-dimensional starch-polyethylene system using v=0.41 and pc=0.3117. Typically, in the three-dimensional case, the value of p is 0.3117

Equation (1) describes the A(p) condition of an infinitely large sample in the vicinity of a pc, so in p

b is the diameter of the particle, h is the thickness of the sample, a is given by: a=v(D-d+1) (3) D is the fractional dimension of the cluster and d is the dimension of the sample (generally 2 or 3) ),v is the cluster correlation index. In the three-dimensional system, v = 0.8, D ~ 2.5, α ~ 0.4; in the two-dimensional system, v = 4/3, D ~ 7/4, α ~ 1. A factor of 2 in Equation 2 indicates that both sides of the thin film material are exposed to a degradative environment, and the factor is 1 if only one side is exposed. In Figure 1(a), the surface accessibility f 18% is calculated from equation (2) (p=0.58,
Pc = 0.5927, b = 1, h = 512, α = 1), which is quite close to the simulated value (~ 17%).
When p ξ=b|p-pc|-v (4)

Since it is a method of measuring the cluster size close to the surface, the method can well determine the depth of degradation or intrusion. When the size of the cluster exceeds the thickness of the sample, we have a penetrating or penetrating channel with a defined position, which is very important for the film. The latter can be used in the design of controlled porous membranes.