Classification of greenhouse transparent covering materials
From original paper to modern glass, to modern plastic covering materials, the development and application of light-transmitting covering materials has promoted the development and promotion of greenhouses step by step.
At present, the most commonly used light-transmitting covering materials for greenhouses are glass, plastic films and rigid plastic plates. With the advancement of technology and the demand of greenhouse production, there are hundreds of varieties of light-transmitting covering materials with different functions, and they are still being developed and expanded. The following table lists the classification of commonly used light-transmitting covering materials for greenhouses.
Covering material classification | types |
Glass | ordinary flat glass, tempered glass, infrared absorption glass |
Flexible film | polyvinyl chloride film (PVC), polyethylene film (PE), ethylene-vinyl acetate (PE-EVA) multifunctional composite film |
Hard film | polyester film, polyvinyl fluoride film |
Hard plastic board | glass fiber reinforced polyester board (FRP), glass fiber reinforced acrylic board (FRA), polycarbonate board (PC) |
Since the growth conditions of different crops are not the same, different crops have different requirements for light-transmitting covering materials. Generally speaking, the basic performance requirements of the greenhouse for its light-transmitting covering materials include light transmission, heat preservation, mechanical strength and service life.
1. Light transmittance
The main wavelength range of solar radiation is 150-4000 nanometers, of which visible light (380-760 nanometers) is strongly absorbed by chlorophyll and carotenoids for photosynthesis, which is a necessary requirement for crop growth and morphology. Experimental studies have shown that under conditions where other environmental factors are suitable, 1% more light means 1% higher yield. Therefore, covering materials with high light transmittance can meet the needs of crop growth.
As the covering material develops towards longevity, it should also have a higher light retention rate to ensure that the greenhouse can obtain ideal light conditions during consecutive years of use.
2. Insulation performance
When the greenhouse is used at night, due to the large temperature difference between indoor and outdoor, indoor heat is quickly dissipated to the outside world through the cover material through conduction, convection, and radiation. Therefore, the thermal insulation performance of the covering material is of great significance to the maintenance of heat and temperature in the greenhouse. Generally, the light-transmitting covering material of the greenhouse is very thin, and its conduction heat resistance is very small, and the convective heat exchange is controlled by the outdoor wind speed and the indoor air flow. In this way, the thermal insulation performance of the covering material depends on its permeability to long-wave radiation. The glass covering material that basically does not transmit long-wave radiation has better thermal insulation performance, while the long-wave radiation transmittance of ordinary polyethylene film is as high as 80%, and the thermal insulation performance is checked.
3. Anti-aging performance
Covering materials will be directly exposed to natural conditions for a long time, and will be affected by many factors such as light, heat, oxygen, water, etc., especially the influence of solar ultraviolet rays and oxygen in the air. As a result of photo-oxidation, the plastic polymer material undergoes rapid aging due to changes in structure, color, transparency, and mechanical strength over time. Generally speaking, the anti-aging performance of plastic materials is weak. When ordinary plastic film covers the greenhouse, it can only be used for one season. The large-scale use of ordinary plastic films in greenhouses is not economical, wastes resources and pollutes the environment, and the workload of film replacement is large, which directly affects the annual production of modern large-scale greenhouses. At present, long-lived films are basically used in greenhouse production in the country where facility agriculture is issued, that is, anti-ultraviolet agents or antioxidants are mixed into masterbatch resin or compounded on the surface of the material to absorb ultraviolet rays and inhibit the photooxygen of the polymer to extend the plastic material Service life.
4. Mechanical properties
The mechanical properties of the covering material will affect the ease of installation and construction and the ability to withstand external forces during use. Ordinary flexible film has high flexibility and low crack resistance. When the temperature is high, the film swells and relaxes. At this time, it is easy to tear the film when exposed to wind. The reinforced film developed by Israel and other countries in the 1990s is made of a net frame woven of polyester material and covered with ordinary polyvinyl chloride or polyethylene film on both sides, which can effectively keep the expansion of the film and increase the strength. It can be 20 times higher than ordinary film. Glass is an ideal covering material for light transmission, heat preservation, and anti-aging properties, but it has low impact resistance, is fragile, and cannot be bent. Therefore, hard plastic covering materials are often used in hail-prone areas. Hard plastic materials not only have good impact resistance, but also have high bending strength and can be used in greenhouses of any structure.
