The polyethylene film was first introduced in the early 1960s as greenhouse covering and till date4, several improvements have been made in the greenhouse plastic. Apart from the greenhouse glass, the polyethylene film made as a coextrusion of three layers of different polymers and additives, improves the quality of the film, enhancing the performance.
The characteristics of greenhouse plastic
The polyethylene films are specially made to cover polyethylene films as well as roofs, walls, and water channels. How those films are formulated and the use of special additives makes it possible to control their quality features and lifespan. The quality and quantity of light, water condensation, temperature, and IR spectrum gets defined by what the greenhouse plastic is made of.
The life of the polyethylene film
The life of the film is limited because of the degradation processes prompted by sunlight and heat. The low-cost material, copolymer remains good for one season, and thus is a good option for seasonal greenhouses, high tunnels, and overwintering structures. Thus, keep away from material that carries
Less strength. Greenhouse grade greenhouse plastic contains an ultraviolet stabilizer and is warranted for several years and will cost twice as much as co-poly. Make use of a poly or nylon scrim reinforced material for high wind locations.
The thickness of the film
The poly film that lasts for a year carries a thickness of 3, 4 and six mil. Generally speaking, four mil film is commonly used for one year. If looking for multi-year applications, the years. The greenhouse plastic is available in 6 mil thickness.
Anti- drip or anti-condensate is a wetting agent that lowers surface tension that allows the condensation to flow rather than developing droplets. The Anti- drip is incorporated into the center layer or sprayed on the film. The film will last a couple of years and can keep away the problems of condensation. The Condensation droplets can lead to disease problems for the plants as well as lower light transmission. You can include an anti-fogging additive to avert early morning and formation of late afternoon fog in the greenhouse.
Lower nighttime heat loss
This additive keeps the radiant heat trapped inside the greenhouse and prevents it from escaping. The savings have been measured from 10–20 percent in heated greenhouses, depending on cloudy or clear skies. The greenhouse plastic made of double layer poly installations, the IR or the infrared film is always placed in the middle layer so as to retain nighttime heat. Studies show that the infrared film can increase accelerate crop development and increase plant color due to the increase in nighttime plant temperature. The film costs only a couple of cents more per square foot and keeps the greenhouse heated all winter.
Reduce daytime heat
The greenhouse plastic blocks a part of the infrared spectrum in the strong sunlight, thus lowering the inside temperature up to 10ºF. Selective color stains can be added to the outside of the copolymer film to absorb or reflect the near infrared radiation. Research shows that the higher the temperature outside, the higher is the temperature difference achieved with the help of these films. There are several advantages such as greater worker comfort, lower cooling costs, less need for irrigation, improved fruit taste with a reduced plant stress.
Light diffusion is another property that has been added by manufacturers recently in the greenhouse plastic. The higher amount of diffused light reaching the plants lowers the scorching while increasing light transmittance to lower leaves. This is essential, especially with the tall vegetable crops where the diffused light can decrease insect propagation and fungus spore development. It also leads to an improved fruit taste.
If a grower is making use of bees to pollinate the plants in the greenhouse, it should be kept in mind that Bees need ultraviolet (UV) light to navigate. A good quality greenhouse plastic allows some of the ultraviolet light energy to pass through, and this is important. An ultraviolet-blocking film can lower thrips, aphids, whiteflies and other insects, as well as control some fungal diseases.
Photosynthetically active radiation light transmission or PAR will vary with different films with different film additives. Typical PAR values vary, for example 77-88 percent for infrared/anti-condensate with diffusion, 88-91 percent for ultraviolet-stabilized film and 82-87 percent for infrared/anti-condensate film. The smog, dust, and plastic deterioration can also lower light transmission. Many growers replace their greenhouse plastic every year just to get higher light level when growing plants during the short days of winter. Other manufacturers make a film with anti-static properties to repel dust, dirt and smog.
Photoselective greenhouse plastic films
Photoselective films reflect or absorb specific wavelengths of light. They can improve plant growth, conquer diseases and insects as well as affect flower development. Red films such as Smartlite Red film and Dupont IR lower PAR light to help create a shading effect. They improve yield and quality of roses.
Single or double layer greenhouse plastic
For general operations, it is seen that a single layer of film is adequate. For a greenhouse running during the heating season, an inflated double layer is more desirable as the double layer lowers the heat loss by about 40 percent at night. Moreover, it also lowers the stress at the attachments and controls the rippling of the plastic on a windy day.
The static Air inflation pressure at ¼-inch works the best. However, a slightly higher pressure should be used in snowy or windy weather. You can reduce condensation by connecting the blower to use outside air between the two layers.
When greenhouse plastic fails
An early plastic failure can be credited to rough surfaces, attachment stress, abrasion
and sharp edges. Other factors include heat build-up in the area of rafters along with
Purlins and extrusions. The life of the greenhouse plastic gets diminished by contact with chemicals from pesticides or pressure treated lumber. If there are several greenhouses placed end-to-end to each other, the film is subject to cuts from blowing ice.