1. Farmland movie
Plastic film can play the roles of warming, protecting roots, preventing freezing, keeping moisture, adjusting light, saving water, weeding and controlling soil salinity, thus promoting crop early maturity and improving crop quality and yield. There are many kinds of mulch films, such as colorless transparent film, non-dripping film, black film, white film, purple film, green film, blue film, red film, infrared film, light conversion film, bicolor film, silver-gray film, aluminum foil film, perforated film, seedling film, weeding film, thermal insulation film, degradable film, aging-resistant mulch film and so on. Plastic film has been applied to the cultivation of various crops and cash crops and the ground coverage of early rice seedling raising. ?
1. 1? Characteristics and uses of various plastic films
Colorless transparent plastic film has good light transmittance and high temperature in the film, which is suitable for use in spring and autumn. Because of the high temperature in the film, it is good for rice seedling raising and isothermal crops such as eggplant, sweet pepper and watermelon, and it is easy to cause diseases to some crops. The disadvantage is that the fog droplets formed in the film will affect the light transmission, so it is also called dripping film.
Drip-free film contains hydrophilic substances, and the water droplets covering the inner surface of the film connect to form a water film, which flows down the film, so it is called trickle film. This kind of film has high light transmittance, large temperature difference between inside and outside the film, better use effect than ordinary colorless transparent film, and relatively low humidity in the film. ?
Color plastic film, infrared film and light conversion film are collectively referred to as light effect films. The light effect film works by selectively transmitting light with wavelength and energy level, which has strong pertinence and different effects on different crops.
Black plastic film can control weeds, but it can only increase the ground temperature by 2~3℃. It is suitable for Chinese cabbage, lettuce, mushrooms, tea and tobacco.
White plastic film can reflect light to the leaves and fruits of crops, which is beneficial to photosynthesis and maturity. Green film can block red, orange and blue-violet light, inhibit the growth of weeds and increase the yield of some crops, but it has not been widely used because of its weak ability to inhibit weeds. Green film is mostly used in eggplant, strawberry, melon and other crops.
Two-color film can inhibit weeds by reducing light transmittance and obtain suitable ground temperature, which is used in tomatoes, strawberries, sweet peppers and other crops.
Purple film can transmit red and blue light and absorb green light, which is beneficial to crop growth. If it is used in spinach, it can increase the yield and prolong the market period. Blue light film has high light transmittance in weak light and low light transmittance in strong light, so it has good heat preservation. It can be used to raise rice seedlings, vegetables, peanuts, cotton, strawberries, potatoes and other crops, which can improve the yield and quality.
The red film can transmit red light and blue light, which can promote the growth of crops such as cucumber. The temperature is increased by 20% by adding infrared-transmitting additives to the infrared film.
The light conversion film contains conversion agent, which can convert high-energy short wave in sunlight into low-energy long wave and medium-long wave, provide suitable lighting conditions for crops, promote crop growth and increase yield. At present, due to cost reasons, light conversion film has not been used as mulch film in China, but only as shed film.
Silver-gray film and aluminum foil film have strong reflective function, which can repel winged aphids, raise ground temperature, supplement insufficient illumination on the back of crops and fruits, thus reducing virus harm, improving crop yield, promoting fruit coloring and increasing fruit sugar content.
Silver gray film is suitable for cucumber, tomato, watermelon, celery, lettuce, spinach, tobacco and other crops.
Aluminum foil film is suitable for planting vegetables in high temperature season, and also suitable for peach, apple, cherry and other fruit trees. Aluminum foil film is rarely used at present because of its high cost.
There are holes with a certain diameter on the perforated film according to the row spacing of crop cultivation, and the hole diameter is 2 ~10 mm. Experiments show that different crops need different hole diameters and hole densities. On the one hand, porous membrane? On the one hand, it can ensure oxygen supply, on the other hand, it can promote the downward extension of radicle, and in addition, macropores can increase water evaporation and reduce temperature. According to the row spacing of crops, a strip seam is cut on the seedling film, which is consistent with the sowing position. After the seedlings are unearthed, they are drilled out of the cracks, which is suitable for planting garlic and rice.
In addition to the black plastic film, herbicides can also be added to the film or coated on the surface of the film, and the herbicides can be slowly released into the soil to play a role in weeding. At present, weeding film is widely used in sugarcane, corn, cultivation, peanuts, sweet peppers and tomatoes, but it can't be used in tobacco and cucumber, and it can't be used in hot weather.
Degradable plastic film comes from eliminating "white pollution" caused by plastic film fragments in soil. There are three kinds of degradable plastic films: biodegradation, photodegradation and photodegradation/biodegradation. At present, degradable plastic film has a certain number of applications in various crops, but its application is not popular enough. There are two main reasons why degradable plastic films are difficult to popularize. One is the price, and the other is that the degradation time of plastic films cannot be accurately controlled due to the limitations of the existing technical level and the different conditions of soil, climate and light in different places. It will take a long time for the technology of degradable plastic films to mature and improve.
Thermal insulation plastic film plays a role in thermal insulation by reducing the transmission of long-wave infrared rays, which is widely used in the north and winter.
There is also a permeable plastic film to collect rainwater. There are two kinds of permeable plastic film, one is microporous film with a certain pore density (perforation), and the other is microporous film with permeability selectivity. The microporous membrane can be perforated by pore-forming agent or stretching. The microporous membrane is hydrophilic, permeable to water but impermeable to water vapor, and has good use effect. Using permeable plastic film can reduce the irrigation times of crops and save water.
1.2 plastic film materials and products
1.2. 1 plastic film material
In recent years, polyethylene is the main plastic film material in China, including some polyvinyl chloride and ethylene-vinyl acetate resin (EVA). High density polyethylene (HDPE), low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) are all used for mulch. Because of their different molecular structures, their material properties, processability and product properties are quite different. HDPE has high molecular weight, high density and high crystallinity, so it has poor light transmittance, but it has good mechanical properties, high strength and good processability, and is easy to make into microfilms. The service temperature is -30 ~ 100℃. HDPE is most obviously affected by ultraviolet rays. Although the initial strength of plastic film is high, it ages quickly, especially after aging, the transverse strength is very low, so HDPE is generally not used as plastic film, especially aging-resistant plastic film. LDPE has wide molecular weight distribution and low crystallinity, so it has good light transmittance and processability, but its mechanical strength is relatively low, and its service temperature is -40 ~ 80℃. LLDPE molecular chain has only short branches, narrow molecular weight distribution, crystallinity between LDPE and HDPE, good mechanical properties, poor light transmittance, poor processability, good ductility, easy preparation of microfilm and good puncture resistance. See table 1 for the basic requirements of PE mulch film for resin, and see table 2 for the comparison of aging resistance of different PE mulch films. LLDPE is the most suitable mulching film material according to the performance of different PE and the use of mulching film, because the requirement of mulching film for light transmittance is lower than that of greenhouse film, and the thickness of mulching film is generally thinner from the perspective of reducing cost. LDPE can also be used as mulching film material, but its aging resistance is far less than LLDPE, and there will be cost problems if the thickness is increased. Because of its good light transmittance, LDPE is more suitable as mulch film material. Some mulch film manufacturers combine the characteristics of HDPE, LDPE and LLDPE, and mix LDPE and LLDPE to produce mulch films. Although HDPE is not suitable for producing mulch film, it is easy to be processed into microfilm with high initial strength, so some enterprises also use it to process mulch film, which leads to the inability to remove mulch film fragments and aggravates "white pollution". Moreover, due to its rapid aging, it can not reach the coverage period required by some crops in some areas, which often leads to some disputes. Table 3 lists some recommended PE resin brands for plastic film.
Table 1? Basic requirements of resin for polyethylene mulch film
Table 2? Comparison of aging resistance of PE plastic film?
Table three? Some recommended brands of plastic film PE
Use PE? In contrast, PVC has the advantages of high strength, good ultraviolet resistance, aging resistance, good transparency and good heat preservation, and is the earliest main resin for mulching film. Because the price of PVC is higher than PE in recent years, and the density and film thickness of PVC are relatively high, it has no advantage and has been eliminated. However, black and gray plastic films made of recycled PVC still have applications. Table four? It is the main index of PVC resin for plastic film and agricultural film.
Compared with PE, EVA film has better impact resistance, aging resistance, light transmittance, good thermal insulation performance, good compatibility with non-dripping agent and good ductility, and is easy to prepare microfilm, but it is rarely used because of its low yield and high price. In recent years, EVA, as an optional material of non-PE anti-aging mulch film, is expected to gradually increase the amount of EVA mulch film, but the speed of increase will not be too fast. The main limiting factor is the price of ethylene-vinyl acetate copolymer, and the sharp decline of EVA price mainly depends on the expansion of EVA production scale. The properties of resin for EVA mulch film are shown in Table 5.
Polypropylene (PP) has higher strength and better processability than PE, and is easy to be made into microfilm, but it has high crystallinity and poor light transmittance. Its unique molecular structure and spiral crystal make its molecular chain easy to be broken by oxidation, that is, easy to age, so it has not been applied to plastic films. Recently, due to the consideration of non-PE mulch film and easy aging and degradation of PP, foreign countries began to study and try PP degradable mulch film, and domestic North China Institute of Technology is conducting this research.
Table four? Main indexes of PVC resin for agricultural film
Table 5? Main indexes of ethylene-vinyl acetate copolymer for agricultural film
1.2.2 plastic film products
Among all kinds of mulching films, except aluminum foil reflective film and black-and-white double-sided mulching film are composite films, other common mulching films and functional mulching films are single-layer films. In order to reduce the cost, the domestic plastic film is thinner and thinner. Due to the pressure of users' bearing capacity, the thickness of domestic plastic film is thinner than that of foreign countries. At present, there are only general standards such as mulch film or agricultural mulch film and degradable mulch film test standards formed in recent years. Other special and functional mulch films have not yet formed a unified standard specification, and only refer to the general mulch film standard. Table 6 shows GB4455-84? The main index of agricultural polyethylene blown film, the film thickness in this standard is thick, which is mainly suitable for seedling film and shed film. Table 7GB 1375-92 Polyethylene blown film for agricultural floor covering. This standard is applicable to LDPE. LLDPE, LDPE /LLDPE, LDPE /LLDPE/ HDPE? As the main resin, a small amount of various additives were added to blow-mold PE mulch film. This standard has strong universality, and can be used for PE black and white double-sided mulch film and other common mulch films and functional mulch films. There is also a PE film standard SG369-84(LDPE blown agricultural floor covering film) for reference. Table 8 shows the main technical indexes of soft PVC blown agricultural film (QB 1257-9 1). This standard is applicable to blow-molded PVC mulch film, shed film and other agricultural films. SG8 1-75 is available for reference.
Table 6? Agricultural polyethylene blown film (GB4455-84)
Table 7? Polyethylene blow-molded agricultural mulch film (GB 13735-92)
In China, 70% LLDPE (MI = 1) was mixed with LDPE***, and 0.2% composite light stabilizer (containing hindered amine ultraviolet absorber and quencher) was added. The film thickness is 8μm, and the initial tensile force is1.3n. After two months of use, the tensile force drops to 50%, and the fragment length exceeds1m.. The evaluation of plastic film aging includes plastic xenon lamp light source exposure test method (GB9344), plastic natural climate exposure test method and farmland exposure test grade evaluation method.
Weeding plastic film has certain requirements on the release concentration and release period of herbicides. The correct formula should ensure that the release concentration is lower than the limit value of harmful crops, and the release concentration is higher than the effective weeding concentration in most release periods. Obviously, adding weeding plastic film can easily meet the above requirements. Considering the improvement of compatibility and the extension of release period, EVA is a better mulching film material, and grafted PE is added as a compatibilizer in PE. Commonly used herbicides include atrazine, herbicide diethyl ether, herbicidal phenol, prometryne and so on.
Table 8? Soft PVC Blow Molding Agricultural Film (GB 1257-9 1)
Color plastic film materials mostly use PE, but also a small amount of PVC? Films and pigments of various colors are generally added in the form of masterbatch. Black-and-white double-sided (double-layer) plastic film is formed by * * * extrusion combined blow molding. Two-color alternating strip films are formed by two extruders. At present, the output of light conversion film is not large, and the film material is polyethylene. The light conversion agent is added in the form of light conversion agent master batch, and the amount of light conversion master batch is generally less than 5%.
2. shed film
Plastic film mulching cultivation technology was applied as early as the end of 1950s. Plastic film mulching cultivation can not only promote crop growth, early maturity, increase production and improve the quality of fruits and vegetables, but also realize the cultivation of fruits and vegetables in cold seasons and regions, which has produced great social and economic benefits and has been widely used. In recent years, the technology of cultivating fruit trees in greenhouse in winter is gradually popularized. In addition, plastic film mulching can be used to cultivate some crops, and can also play a role in delaying maturity, such as roses and grapes in desert areas. ?
Plastic film greenhouse can be divided into shed, middle shed and small shed according to its area and height. Greenhouse usually covers an area of 600~ 1300m2, with a width of 10~ 15m and a length of 40 ~ 60m. The shed is changed to an area of 600? Within m2 15~30m long and 5 ~ 8m wide; The shed structure is usually buckle ridge type, similar to low tunnel type, with a height of about 0.5m, a width of 1~ 1.5m and a length of 15~20m. Plastic film greenhouses can be divided into single-layer structures, double-layer structures and double-layer film inflatable greenhouses. In addition, there are floating film greenhouses between plastic films and greenhouses and film protection greenhouses on both sides.
The single-layer structure is covered with a single-layer film, and the thickness of the film is 0.08 ~ 0.2 mm. The single-layer film greenhouse is simple in structure and low in construction cost. At present, this structure is widely used in greenhouses in China. The double-layer film shed is covered with double-layer film, and the distance between the two films is generally 30~50mm, so the temperature difference between the inside and outside of the shed is large and the heat preservation is good; The film is usually double-layer PE film, or one of them can be aluminized. Compared with double-layer PE transparent film, aluminized film can reduce the heat loss by about 65%, and the upper film can be rolled up when light is needed.
Double-layer film inflatable greenhouse adopts double-layer films, and the distance between the films is 30 ~ 50 mm. Air is continuously blown between the films by blower to form dynamic air insulation layer, which makes the greenhouse have better insulation effect and further reduces condensed water. Double-film inflatable greenhouse is rarely used in China at present because of its uninterrupted inflation and high maintenance cost. However, there are some problems because straw curtains are widely used in single-layer greenhouses in winter in northern China. In contrast, double-layer greenhouse and double-layer inflatable greenhouse have outstanding advantages in this respect and have attracted much attention. ?
Floating film covering is a low-cost and simple covering method. The film can be directly covered on crops without support, and can also play the roles of heat preservation, weather frost prevention, bird pest prevention, early growth and maturity promotion and planting period extension. The floating film generally adopts the first-stage crop and is used once, so the thickness and performance requirements are lower than those of the greenhouse film. The film thickness is generally 0.03~0.05mm, and the film material is generally polyethylene to reduce the cost. PE floating membranes are 5~ 10m wide and unlimited in length, and can be divided into perforated membranes and slotted membranes. The aperture of perforated membrane is 7~8mm, and the pore density is 500,700, 1000 pore /m2. Depending on different crops and climatic conditions, 500 pore /m2 is equivalent to 4% ventilation. The function of perforated film is not only to ventilate, but also to eliminate fog drops and penetrate rainwater. Grooved film covers crops. When the crop grows to a certain height, widen the gap and drill a gap at the top of the crop. This film has a good effect on crops such as potatoes, but the leaves will be burnt when they contact the film. Another kind of floating film is non-woven fabric, which can be made of PE, PP, PET, PP/PA, etc. Generally, the thickness of nonwoven fabric is 0.025 ~ 0.12mm (17 ~ 20g/m2), the width is 1.5~ 10m, and the length is not limited. Non-woven fabrics have high strength and long service life, and can be used repeatedly. They are rarely used in China, but their applications have increased rapidly in recent years.
2. 1 Characteristics and uses of various functional greenhouse films
On the basis of ordinary transparent greenhouse films, people developed many functional greenhouse films, and then some multifunctional greenhouse films with functional combinations appeared. Now the single-function greenhouse membrane is introduced as follows:
Long-life greenhouse film: By optimizing the general basic resin and stabilizer, the service life of greenhouse film was prolonged. The long-life PVC film in Japan can be used for 2~3 years, and the PET film can be used for 10 years. The PE membrane of CdF company in France can be used for 1~2 years; In China, the service life of PE film with the thickness of 0.05mm (for small and medium-sized sheds) and 0.08mm (for greenhouses) can reach 1~2 years.
Drip-free film: more demanding than drip-free plastic film. Drip-free film mainly depends on hydrophilic drip-free agent, which can be added to the film material or coated on the film surface. There is also a multi-layer * * * extruded composite shed film, which relies on the low thermal conductivity of the intermediate resin to play the role of thermal resistance, so that the inner and outer surfaces of the shed film maintain a large temperature difference, thus reducing the formation of fog droplets on the inner surface of the shed film. Some of these films are also called drip-free films. In recent years, some drip-free films have added anti-fog function, which can effectively reduce or eliminate the fog in the greenhouse and enhance the lighting effects of crops.
Thermal insulation film: during the day, sunlight mainly enters the film with the wavelength of 0.3~0.7μm, which makes the temperature in the shed rise and the soil absorbs a lot of heat. When the temperature is low at night, the soil radiates heat with long-wave infrared rays (5~ 10μm). The function of heat insulation film is to prevent the spread of far infrared radiation at night, thus maintaining the temperature in the shed. There are two kinds of thermal insulation films: using infrared thermal resistance or low thermal conductivity (single-layer or multi-layer extrusion compounding) and adding infrared absorption dosage forms. The latter has low cost and good thermal insulation effect. In order to improve the thermal insulation performance, the film should be controlled at a certain thickness.
The light conversion film can convert 5 10~560nm ultraviolet light and visible light into light needed by crops, such as blue light, green light and red light, or increase the greenhouse temperature by 3~5℃, thus accelerating crop growth and improving crop yield and quality. Various color films can selectively transmit light and are suitable for different crops. The light-selecting film can selectively transmit ultraviolet light in a certain band, and the rest is blocked, which will not affect the aging resistance of the greenhouse film, but also provide different crops with the required ultraviolet light band, which is beneficial to the growth and quality improvement of crops, and can also prevent and control pests and diseases.
Diffuse scattering greenhouse film is suitable for greenhouses covering low latitudes and high temperature seasons. It is characterized by high light transmittance and high temperature in the shed when the light is weak in the morning and evening. At noon, the light is strong, the light transmittance decreases, and the temperature in the shed is also low. This film can prevent the greenhouse temperature from being too high in high temperature season and at noon, which is beneficial to the stability of the temperature in the greenhouse, but the warming effect of the film is poor and it is not suitable for covering the overwintering greenhouse in the north.
The reflective film is a double-layer aluminized BOPET film, with a width of 500~ 1000mm and a thickness of 0.02~0.03mm, which has a mirror reflection effect on incident light. When the film is hung on the north side of greenhouse cultivation bed or seedbed in winter and spring, the reflective effect of reflective film can increase the light intensity within a certain distance by 40%, thus improving the quality of seedlings and increasing the early yield of fruits and vegetables.
2.2 shed film materials and products
2.2. 1? Pvc shed film
The application of PVC shed film in China began in 1960s. Products include blown film (see GB 1257-9 1) and calendered film (see Table 9? GB/T3830-94).PVC film was mainly used in greenhouses in China in the past because of its good thermal insulation, high initial strength and better light transmittance than PE film. In recent years, due to the low cost of PE film and convenient production, and the complex formula and process of PVC film, the plasticizer is toxic, and the plasticizer has migration and dust absorption, so the dosage is greatly reduced. At present, PE film is the main greenhouse film in China. At present, PVC shed film is mainly used in northern China. It is worth noting that at present, the global PVC shed film accounts for 50% of the total shed film, and the highest in Japan exceeds 70%. Japanese greenhouse films are multifunctional films with long service life, high light transmittance, high thermal insulation, no fog drops, no dust and no toxicity. Generally, the service life is more than 3 years, some are 5-8 years, and the fog-free period can be as long as 4 years. ?
Table 9? Polyvinyl chloride calendered agricultural film (GB/T3830-94)
China's PVC shed film should also continue to develop in the above direction, because compared with PE film, PVC film still has the advantages of ultraviolet aging resistance, easy addition of polar additives and long duration of functional additives, and it is easier to achieve long life and long effect than PE film.
PVC shed film should use SG2 or SG3 PVC resin, and other additives can also be selected, such as main plasticizer DOP, auxiliary plasticizer DOA and epoxidized soybean oil, ultraviolet absorber UV-9, antioxidant bisphenol A and triphenyl phosphite. It should be noted that DIBP plasticizer should not be used in greenhouse films, because the toxicity of DIBP has been proved to be harmful to crops. The non-dripping agent is mainly composed of polyglycerol stearate, S-60 and S-40. The antifogging of PVC shed film needs to add special antifogging agent. At present, the dustproof function is mainly realized by coating the dustproof layer on the surface. In addition, adding a small amount of superfine inorganic powder filler plays an adsorption and slow release role, which can prolong the drip-free duration and reduce the migration of plasticizer.
2.2.2? PE shed film
In recent years, the domestic greenhouse film is mainly PE film. See Table 6 for PE shed film standards and Table 10 for some recommended shed film materials. The shed film can be made of LDPE, LLDPE or both in a ratio of * * * and then blown.
Table 10? Recommended brands of some PE shed film materials
According to the requirements of the best greenhouse effect and cultivation benefit, the transparency of the transparent greenhouse film has the following requirements: high transmittance for solar radiation with a wavelength of 0.35~3.0μm, high absorption or high barrier for ultraviolet rays with a wavelength of < 0.35μ m, and strong barrier for far infrared rays with a wavelength of > 3μ m, especially 7 ~1μ m, so as to promote crop growth. Ultra-fine kaolin, talcum powder, diatomite and sericite can be used as insulating agents to block far infrared rays, and the dosage in the film is about 65438 0%.
Because the aging resistance of PE is worse than that of PVC, efficient stabilizer is particularly important for the aging resistance of PE shed film. Polymer hindered amine light stabilizer should be selected, and the brands are Chimassorb944, 492, 494, 3529, 3346, Tinuvin? 622LD, N-30, Beijing 692 1, 6922, BW- 10LD, GW-540, etc. Adding 0.4% BW- 10LD and 0. 1% PL- 10 to the PE film can last for 22 months. The main antioxidant is hindered amine phenols such as Cyanox 1970, Irganox245 and MarkAO-80, and the auxiliary antioxidant is thioester. Hindered amine light stabilizers should be avoided in combination with antioxidant 264.
At present, the basis for judging the weatherability of polymers at home and abroad is elongation at break retention rate and carbonyl index, and the main basis for enterprises to choose resin for agricultural film is melting index. Practice shows that the plastic film that meets the requirements of melting index sometimes bursts, because the molecular weight is not directly proportional to the melting index, and the aging performance is also related to the molecular weight, molecular weight distribution, branching length, branching number, crystallinity, metal ion content and other factors. High molecular weight, narrow molecular weight distribution and better aging resistance; Long-chain branching mainly affects the melt rheological properties and thermodynamic properties of resin, and the number of short-chain branching greatly affects the mechanical properties and photothermal stability of resin. The more branches, the weaker the molecular chain structure and the worse the stability of the polymer. High metal ion content leads to poor stability of resin. In addition, adding low molecular additives such as slip agent and opening agent to agricultural film has a certain negative effect on weather resistance.
The antifogging agent is mainly composed of stearic acid polyol ester, polyoxyethylene and organic amine nonionic surfactant. The antifogging agent is a surfactant containing fluorine, silicon and boron, such as Japan Riken AF- 18 (dosage: 0. 1%). Due to the non-polarity of PE and poor compatibility with antifogging agent, the duration of non-dripping of PE film is short, and the dosage of antifogging agent generally does not exceed 1~ 1.7%. If the amount is too much, the light transmittance of the film will decrease. In order to improve the duration, ultrafine kaolin and talcum powder can be added as adsorbents and slow-release agents at the same time. In addition, the dripping properties of different resins are different, and the dripping properties of LDPE are better than LLDPE.
Heat preservation agent and light conversion agent are commonly used in various greenhouse films. Light conversion agents mainly include inorganic and organic ligands; Inorganic light conversion agent can increase the yield of greenhouse vegetables and make them mature for 5~20 days; Organic light conversion agent has good compatibility with resin and high emission intensity, which is superior to inorganic light conversion agent.
2.2.3? Eva? Shed film
EVA shed film has concentrated the advantages of PVC film and PVC film, so it has developed rapidly in recent years. The development focus of EVA shed film is multi-functional three-layer composite shed film, which is made by extrusion blow molding process. Various functional additives used in EVA film are exactly the same as those used in PE shed film. Because EVA has low crystallinity and strong polarity, it has good compatibility with antifogging agent; Compared with PE drip-free film, EVA drip-free film can add more antifogging agent (3~3.5%), and the drip duration can be as long as 6 months. EVA is in China? The research and application of non-dripping film is not long, but the technical level is equivalent to that of foreign countries. The drip irrigation duration of EVA drip-free film abroad is 1~2 years, which is only because of its film thickness and micro-drip irrigation technology in shed.
* * * Several points should be paid attention to in the production of extruded three-layer composite multifunctional shed film: in order to prolong the dripping duration, antifogging agent should be added to the middle layer or the third layer, and the appropriate addition amount is middle layer > inner layer > outer layer; Inorganic heat preservation-slow release agent should be added to all three layers, and the appropriate addition amount is middle layer > inner layer > outer layer; The dosage of antioxidant outer layer is the largest, and the outer layer = inner layer > middle layer or outer layer > middle layer > inner layer. For example, LDPE (outer) /EVA (middle) /EVA (inner) three-layer film, in the order of outer, middle and inner, the amount of antioxidant is 0.5, 0.45 and 0.40%, and the amount of antifogging agent is 0.8, 1.9, 1.7%, and the heat preservation is performed.
2.2.4? Metallocene polyethylene shed film
M-PE has the advantages of high molecular weight, narrow molecular weight distribution, short and few branched chains, low density, high purity, high light transmittance, impact resistance and puncture resistance, and has been applied in agricultural films in recent years. At present, it is being developed in China? M-PE shed film. Because of m-PE? The molecular weight distribution is narrow and difficult to process. About 10% LDPE can be added to improve the processability. M-PE can also be added to LDPE to improve the impact resistance, puncture resistance and light transmittance of LDPE shed film.
2.2.5? Other shed film
Compared with the above-mentioned shed film, PET shed film has higher strength, better light transmittance, longer service life and longer drip duration. The service life of Japanese PET multifunctional greenhouse film is as long as 10 year, and there is no fog drop in 10 year. PET film has developed rapidly in the United States and Japan, but it has not been applied in China at present. However, the superior performance of PET film has been concerned and will be developed in the next few years.
The life of PC film and PTFE film is longer than that of PET film, but due to the cost problem, they are seldom used abroad at present.
PE woven shed film is a new product developed in recent years, which is composed of PE flat yarn weaving and PE film (single or double-sided). The membrane has high strength and its service life is twice as long as that of ordinary PE membrane. Because of the poor light transmittance of this film, the function of this film should be developed in the direction of non-dripping, heat preservation and light reflection.
Raising livestock and aquaculture in greenhouse has been widely used at home and abroad, even raising silkworms in greenhouse. Greenhouse culture has a significant effect on promoting the growth of livestock and poultry and aquatic products (especially in winter and spring) and increasing production. Generally, PE drip-free film or long-life film can be used for greenhouse film.
3. Rural networks
HDPE, PP, PA, PVDC and other monofilaments are directionally stretched to obtain high-strength yarn, which is spun into a net. It can also be directly extruded into a net with a rotating head. Agricultural net has been used in agricultural packaging, vegetable transportation, fishing nets, aquaculture, animal husbandry, plant protection and crop collection. The following are some main varieties.
3. 1 insect net
HDPE net or UV-resistant PP net with antioxidant is a new type of agricultural covering material. Using insect-proof net for closed cultivation can effectively prevent the invasion of pests, thus realizing basically pesticide-free vegetable production, and insect-proof net plays an important role in pollution-free vegetable production. The mesh density of insect-proof net should be 22 meshes. If the mesh density is too small, the insect-proof effect will be reduced; If it is too large, poor ventilation will easily lead to vegetable rot. The width of insect-proof net is generally 3~4m.
3.2 sunshade net
HDPE mesh, with light stabilizer and other additives, has two kinds: black mesh and silver-gray mesh. The sunshade net can provide shading for crops' growth conditions, and the light transmittance can be reduced to 25-40% after use, which can play the roles of temperature regulation, humidification, rainstorm prevention, hail prevention and bird damage prevention. The warm-keeping effect of silver-gray mesh cloth is slightly better than that of black mesh cloth. The width of the net is generally 2~4m, and the length is 50~ 100m, 2mm? The grid can filter out 50% direct sunlight. At present, the domestic annual net consumption is 65.438+0.6 billion square meters, and the price is 0.5 ~ 654.38+0.0 yuan/square meter. In order to meet the requirements of floating coverage, a wide sunshade net should be developed.
3.3? Shading net
The sunshade net is used to control the sunshine time without heat accumulation under the film, and the indoor temperature rises. It is used as a covering material with reflective, sunshade and thermal insulation functions or as a thermal insulation curtain in a greenhouse. The sunshade net is usually an aluminum-plastic woven net, in which aluminum lines play a reflective role. Foreign products are mostly mixed woven nets of aluminum foil strips and transparent PE flat wires. Due to the reflection of aluminum, the temperature rise and surface temperature in the greenhouse can be reduced in summer, and the heat loss in the greenhouse can be reduced in winter and night. The service life of this kind of net can reach 10 years, but the cost of this kind of net is too high, and there is no such product in China. At present, the domestic Shanghai Changzheng Plastic Weaving Factory produces PE aluminized film strips and PE woven nets at low cost, and their mechanical properties and aging resistance reach the performance of foreign Al/PE woven nets.
3.4 Windshield
The wind shield can be a net, grating or strip film, generally made of PE, mostly black or green in color, less than 2m in width and unlimited in length. The aperture of the grid can be φ4 or φ6mm, the opening area is 35% when the aperture is 4mm and 50% when the aperture is 6mm, and the grid can also be other polygons. Windbreak walls can provide protection for crops and greenhouses to reduce strong winds.
3.5 fishing nets
According to different types of fishing nets, fishing net materials can be PE, PP, PVDC, PET, PVDC, PA and EVA. PVC is the main material of trawl in China. Due to the excellent low temperature resistance, wear resistance and tear resistance of PE mesh, the monofilament of PE mesh is stretched at 270~290℃ during production, and then stretched in a hot water tank at 90 ~ 65438℃ for 8 ~ 100 times. The fence adopts PVDC net, because its density will be greatly reduced. The gill net adopts PA net to meet the requirements of strength and elasticity. PP net has low density, can float in water, and is suitable for fixed net and gill net.
3.6? other
Plastic nets have been widely used in cage culture of various aquatic products and livestock breeding. Breeding nets are generally woven nets, made of PE, PET and PA.
In recent years, in the process of vegetable transportation, the amount of plastic mesh bags is very large, and the materials of mesh bags are PE, PP or PVC. Because of high strength requirements, net bags are all flat steel wire woven nets. Considering the cost, most of the raw materials of these nets are recycled waste plastics.
The mesh with 30×30mm mesh woven by PP rope with a diameter of 3mm can be used as a mesh dam, which can protect the river beach, scour the channel and flood discharge, and has been used for a long time in water conservancy projects in China.
In recent years, watermelons have been planted in greenhouses and hung in plastic net bags. Protect grapes from pests with insect-proof net bags.