Comparison of the advantages and disadvantages of greenhouse covering materials

Today's agricultural production is inseparable from greenhouses. Greenhouses can not only resist natural disasters, drought and flood, but also advance or delay cultivation, extend the growth period of crops, and achieve the purpose of early maturity, late maturity, increased and stable production, and are very popular among farmers. Due to different climates and applications in different places, the specifications of greenhouses are also different. The most important non-covering material in a greenhouse is none other than the covering material. Today we will talk about the most common types of greenhouse covering materials, and explain their advantages and disadvantages in detail, so as to help friends choose the appropriate greenhouse covering materials more reasonably as much as possible.

Soft film materials

1.PE

Advantages of polyethylene film (PE): milky white and translucent, 0.1-0.2mm thick, wide width, up to 18m wide. Acid, alkali and salt resistant; not easy to produce toxic gases, safe for crops; not easy to stick to dust, good light transmittance; low density (0.92g/cm3), so the covering cost is low.

Disadvantages:

1. High infrared transmittance (up to 80%), poor thermal insulation performance. On clear and windless early spring nights, "inversion" may occur in the greenhouse, freezing crops.
2. Poor strength, poor resilience, easy to tear.

2.PVC

Advantages of polyvinyl chloride film (PVC): colorless and transparent, strong, tensile strength of 27.5MPa. Low infrared transmittance (20%), good thermal insulation, acid, alkali and salt resistant, spraying pesticides and fertilizers is not easy to cause deterioration.

Disadvantages:

1. Easy to absorb dust, resulting in a significant decrease in light transmittance. The light transmittance decays quickly. It is not suitable for continuous covering cultivation over the summer.
2. Low air and moisture permeability, less toxic gas permeation. In greenhouses under high temperature conditions, toxic gases will be produced, which will poison crops. At the same time, the greenhouse will also create a high temperature and high humidity environment, and crop diseases are relatively serious. The density of polyvinyl chloride film is relatively large (1.4g/cm3), and the coverage area of ​​the film per unit weight is small, which makes the covering cost relatively high. For greenhouses of the same area, the covering cost of PVC film is about 50% higher than that of PE film.

三. Functional composite film

1. Longevity film Select heavy packaging grade resin with a melt flow rate MFR = 0.3~0.8, and then match it with a high-efficiency hindered amine light stabilizer to achieve the purpose of longevity. For PVC longevity film, when ultraviolet absorbers are selected, the protective effect on the film is stronger.
2. Drip-free film The effective period of PE drip is generally only 2~4 months, PVC drip-free film is 4~6 months, and PE-EVA is about 8 months. Using non-ionic surfactants containing halogen, the anti-fog performance of the drip-free film is excellent, and the drip-proof period can reach 1 to 5 years, which is basically synchronized with anti-aging.
3. Composite functional film Multi-layer co-extruded PE/EVA/EVA has an infrared blocking rate of about 50%, and because of its weak polarity, it has good compatibility with anti-fog agents and thermal insulation agents, and a long anti-fog lasting period; the weather resistance of the outer layer PE resin and the thermal insulation and anti-drip properties of the inner layer EVA resin. PO series film is also a PE/EVA composite film with small expansion and contraction rate and high strength. It can be directly burned after use without producing harmful gases.
4. Colored film: The dominant role in photosynthesis in sunlight is visible light with a wavelength of 400 to 700 nanometers, called photosynthetically active radiation, among which blue-violet light of 400 to 510 nanometers and red-orange light of 610 to 720 nanometers have the greatest impact on photosynthesis, and green light has the least impact.

4. PO film

PO is the abbreviation of the English word polyolefin, which is expressed in Chinese as polyolefin. It usually refers to polymers of ethylene, propylene or higher olefins.

1. Strong light transmittance 92% or more (less than 10 threads thick) attenuation.
2. Anti-fog and dripping Special advanced coating technology, continuous anti-fog and dripping life can be the same as the life of the film body.
3. Puncture resistance Metallocene material added.
4. Dust-proof treatment The nano coating on the outward side has no electrostatic treatment, and the PO film itself has less impurities and less static electricity, so the adsorption capacity of dust is greatly reduced. When washed by strong winds and rain, it is as clean as before.
5. Anti-aging Under the action of polyolefins and metallocenes, its characteristics are long-lasting anti-aging.
6. Thermal insulation performance Maintain the temperature in the greenhouse
7. High tear strength In metallocene technology, its strength exceeds that of other films. After hanging, its elasticity is extremely low. When stretched in both horizontal and vertical directions, only the white insulation layer can be seen without breaking
8. Environmentally friendly degradation without toxic pollution.

Glass

Advantages:

1. Excellent light transmittance and high light transmittance retention rate.
2. Good thermal insulation performance.
3. Due to the low transmittance of ultraviolet rays, the glass has good aging resistance and long service life.
4. Low thermal expansion and contraction coefficient, reliable structural coefficient.

Disadvantages: large glass ratio (2500kg/m3), strict requirements on greenhouse foundation and frame, high initial cost, maintenance cost, operating cost, and depreciation cost; low impact resistance and easy to break; poor thermal insulation performance. At present, many large cities have discouraged the construction of glass greenhouses.

PC board

Polycarbonate board (PC) is one of the most advanced polymers in plastic applications.

1. High mechanical strength and high impact toughness.
2. Very good dimensional stability, good heat resistance, can be used for a long time at -60 ~ 120 ℃. The heat deformation temperature is greater than 310 ℃, the flammability specification belongs to self-extinguishing resin, small polarity, low water absorption, stable to light, good weather resistance.
3. The service life is up to 10 years.
4. The thermal insulation performance is extremely high, and its heat transfer coefficient can be reduced to 1.6 ~ 2.2W/m2℃, which is 30% ~ 60% more energy-saving than glass greenhouses.

Disadvantages: The surface is easy to get dusty, the light transmittance decays quickly, and the anti-drip function has a short life. The initial investment price is high.