Greenhouse Glazing
Plastic Films
Flexible plastic films, including polyethylene, ethylene tetrafluoroethylene (Tefzel), polyvinyl chloride (PVC), and polyester, have been used for greenhouse coverings. Plastic film is currently the leading greenhouse covering for two reasons. First, film plastic greenhouses with permanent metal frames cost less than glass greenhouses. Even greater savings can be realized when film plastic is applied to less permanent frames, such as the cheaper Quonset greenhouses.
Polyethylene Film
Polyethylene, sometimes also known as polythene or poly, has always been and still is the principal choice of film plastic for greenhouses in most of the world (Figure 2.1). The major advantage of polyethylene film plastics is cost, which is considerably less expensive to purchase and install than glass. Plastic glazing generally provides more diffuse light than glass. Diffuse light can potentially reach deeper into the plant canopy than direct light since diffuse light is scattered more uniformly throughout the greenhouse.
Multi-Layered Co-Extrusion Film
Most greenhouse polyethylene film is manufactured as a co-extrusion of three or more layers with different polymers and additives. Multi-layered co-extrusion enables the desired properties of different polymers to be combined and can be used to compensate for their negative effects. In this way, a final polymer can be made with all the desired properties at an acceptable price for some markets. For example, the co-extrusion of ethylene vinyl acetate (EVA) as a middle layer with polyethylene as the two outside layers can overcome the weaknesses of both materials.
Film Polymers and Additives
Most polyethylene film is manufactured as a co-extrusion of three layers with different polymers and additives. Each of them contributes to the quality of the film and enhances its performance.
Ultra Violet Films. Ultra violet (UV) light, or ultraviolet light, is an invisible light which makes up at least 10 percent of the total electromagnetic radiation output from the sun. Greenhouse plastic sheeting, with its wide surface area, is particularly prone to deterioration underneath continuous exposure to the sun. UV radiation is most commonly known for its ionization, that is, its ability to deteriorate chemical bonds.
Anti-Condensate Films. Water vapor condenses on the cold inner cover surface, forming small droplets of liquid water. Condensation is particularly a problem in the morning when, due to a cold greenhouse covering and humid air inside, a thick layer of droplets often forms. This has negative consequences on light transmission. Moreover, condensation can fall onto the crop, fostering the development of fungal diseases.
Infrared Films. Also referred to as thermal film, infrared (IR) film features an additive that captures the infrared radiation emitted by the objects inside the greenhouse. As sunlight strikes an object on the earth, it’s reflected in the form of radiation.
Anti-Dust Films. Polyfilm has a tendency to attract dust particles. Over a longer period of exposure, considerable dust particles accumulate on poly film.
Wavelength-Selective Plastic Films. Plastic films have been developed to selectively absorb and transmit different wavelengths of sunlight. They are sometimes referred to as spectral filters. Of interest for ornamental crops is a film that selectively absorbs and reflects far-red light, which creates an environment with a high red-to-far-light ratio.
Opaque Films. Many installations, especially in windy areas, use a supporting material, usually welded wire mesh, for the polyethylene film.
Light Diffusion Films. In areas with clear skies and high solar radiation, direct radiation can cause leaf burning in greenhouse crops on warm days. New plastic films have been developed to increase the percentage of diffuse radiation in the greenhouse.
Tefzel Film
The most recent addition of greenhouse film plastic covering is Tefzel T2 film (ethylene tetrafluoroethylene). The light transmission is 95 percent and is greater than that of any other greenhouse covering material. A double layer has a light transmission of 90 percent. In addition, Tefzel film is resistant to ultraviolet radiation, which is the primary cause of degradation of the physical properties of other covering materials.
Polyvinyl Chloride Film
Polyvinyl chloride (PVC) has a number of properties that make it desirable as a covering. Polyvinyl chloride has excellent resistance to wear. Oxidation has little effect on PVC, but heat and light break down PVC film in 2 to 3 years. Polyvinyl chloride film reduces the transmission of long wavelength infrared radiation, resulting in less heat loss at night using PVC as a covering than using polyethylene.
Polyester Film
Although the cost of polyester film is higher than that of polyethylene film, polyester film is known for its durability and longer life expectancy. Other advantages include a level of light transmittance equal to that of glass and freedom from static electrical charges, which collect dust.
Double-Layering
Single-layer polyethylene-covered greenhouses lose more heat at night or in winter than glass-covered houses since polyethylene allows the passage of heat energy from the soil and plants inside the greenhouse much more readily than glass. However, double-layer poly-covered greenhouses retain more heat than glass because the greenhouses are more airtight and less infrared radiation escapes. Double-layering consists of two layers of film plastic with air inflation between the layers to provide added insulation. The layers are kept about 3 to 6 inches (7.5–15.2cm) apart by a small fan that pumps air between the layers, sustaining a slight positive pressure (Figure 2.2).
Life Span of Plastic Film
The greenhouse cover is likely to last longer in mild climates than in climates with extreme cold, heat, rain, snow, or hail. Heavy precipitation is more likely to weigh your greenhouse cover down, causing it to stretch. Extreme changes in temperature can cause your plastic to shrink and grow. Climates with heavy dirt and dust can cause your greenhouse to accumulate dirt, and areas with heavy trees or jutting branches pose a risk to your greenhouse. At the same time, high winds can cause scratches from blowing debris. Understanding how your greenhouse frame materials will interact with the greenhouse plastic sheeting can help you extend the life of your cover.
Poly Locking System
The poly-locking system for the greenhouse is a component that must be thoroughly researched before installation (Figure 2.3). Poly films are fastened to the house at the edges by special poly-locking extrusions. Many brands of extrusions for locking plastic films are available.
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