Degradation mechanism and technology of biodegradable plastic film
1. Degradation mechanism
Only by fully understanding the degradation mechanism of plastic film can the composition of plastic film be designed in a targeted manner. The environment in which the plastic film is located is the natural environment, not compost, nor simple soil. There are complex conditions such as light, air, water, and microorganisms.
The degradation mechanism is extremely complex. Biodegradable plastic film cannot only biodegrade. The mulch also undergoes the degradation of light and oxygen, as well as the degradation of hydrolysis and microorganisms. The degradation of plastic film in the state of natural use includes: ① photodegradation. For crops such as cotton, or in areas with high light intensity and long light time, such as Xinjiang, and high altitude areas, photodegradation will be very strong. ② hydrolysis. For crops such as rice, or in areas with abundant rainfall, hydrolysis can be strong. ③ Biodegradation. Under certain temperature and humidity, in areas with high microbial activity and abundant populations, the biodegradation will be very strong. ④ Oxidative degradation. At low altitudes, oxidative degradation can be strong. ⑤ Other degradation. Pesticides and fertilizers in the soil also have a certain degradation effect on the plastic film.
2. Process technology According to the various complex degradation processes experienced by the biodegradable mulch film in the natural environment, the formulation composition design and production process design can be carried out in a targeted manner, so that its performance can meet the requirements of crops. Some key technologies for degrading plastic film can be summarized as follows:
(1) Molecular design and synthesis technology of degradable resin for plastic film The molecular composition and molecular structure of resin materials determine its various properties such as mechanical properties, thermal properties, aging properties, etc. Therefore, in order to obtain resin materials with specific properties, its molecular composition and molecular structure must be designed. For example, during film blow molding, the diameter of the film bubble is generally large and the thickness is generally small. The film blowing process requires high melt strength of the material. Therefore, a branched structure can be introduced in the resin synthesis stage to improve the melting point of the material. body strength.
(2) Composite modification technology of degradable plastic film special materials
According to the degradation mechanism of the mulch film, various functional additives or components are introduced into the mulch material in a targeted manner to control the degradation rate and make the mulch film meet the requirements of mechanical properties, temperature increase performance, moisture retention performance, etc. .
(3) Degradable plastic film stabilization processing technology
The degradable materials are generally aliphatic polyester and aliphatic-aromatic copolyester materials. The presence of a large number of ester groups and terminal hydroxyl and terminal carboxyl groups on the molecular chain makes the material more polar, and the viscosity is relatively high when the film is blown by thermal processing. It is large and has poor opening. It is necessary to find a suitable smooth and opening system to solve the problems of film stickiness and opening. The thermal stability of the degraded material is poor, and when the processing temperature is high, it is easy to be pyrolyzed. Therefore, the processing temperature is generally low, and the melt viscosity of the material will be high at this time, and the load of the processing equipment will increase, which requires sufficient power for the design equipment. The degradable materials suitable for flexible films are generally aliphatic-aromatic copolyesters. Due to copolymerization, branching and other reasons, the molecular chain structure is irregular, the crystallization speed is slow, and the cooling speed is slow during film blowing. Therefore, , It is necessary to speed up its cooling speed by introducing crystallization nucleating agent and other methods to make it shape as soon as possible. Due to the low melt strength of the degraded material and the slow cooling rate, during the film blowing process, if the uniformity and stability of the cooling air are poor, the local area of the film bubble is easily broken by the unstable airflow, and a "film" will appear. Therefore, it is necessary to modify the air ring structure of the film blowing machine to improve the stability and uniformity of the cooling air. Degradable materials suitable for flexible films have good flexibility, high elongation at break and high elasticity. During the film blowing process, processes such as traction and winding are prone to excessive tension, and the film will have tensile deformation and dead wrinkles. Therefore, it is necessary to carry out tension-free transformation of the traction and winding system of the film blowing machine to avoid these problems. Due to the less branched structure of the degraded material and the smaller the inflation ratio, the longitudinal and lateral strengths of the film vary greatly. In order to make the longitudinal and lateral strengths of the film uniform, when the inflation ratio is above 2.5, the film can be used in both longitudinal and lateral directions. After a certain stretching orientation, the strength area is uniform. Therefore, it is recommended that the blow-up ratio be as high as 2.5 or more.
(4) Key technologies for controllability of degradation rate According to the various degradation effects of plastic film, various special material formulations for plastic film are designed to control the degradation rate. By adjusting the thickness of the mulch, the degradation rate can also be adjusted.
(5) Water retention optimization technology of degradable plastic film For example, the water permeability of PBAT film with a thickness of 0.01mm is about 1 000g/(m2×24h), while that of PE is only about 100g/(m2×24h), which has poor water retention and needs to be greatly improved. The optimized technologies include: the introduction of functional additives for water retention; the introduction of barrier materials or multi-layer composites to improve the water resistance of the film.
(6) Degradable plastic film standard and effect evaluation specification Degradable plastic film standards and effect evaluation standards include: standards such as the thickness, degradation speed, and light transmittance of the degradable plastic film need to be standardized; the effective accumulated temperature of crops using the biodegradable plastic film, the water retention of the plastic film, the temperature increase, and the weed inhibition also need standard evaluation; The amount of residual film after use and the long-term impact on soil and subsequent crops after degradation also need further evaluation.
(7) Low-cost production technology of degradable plastic film
The price of degradable materials is 2 to 3 times that of PE, and the high cost is also a bottleneck restricting its popularization and application. The large-scale production of degradable materials, the improvement of process technology, and the reduction of material costs will be an inevitable trend.
