Normal polyethylene films used in greenhouses
have no affinity for water. The water that condenses, forms round
droplets because this is the shape that minimizes the contact area between
the water and the film. These droplets reduce the light transmission
and will often ‘rain’ down inside the house resulting in physical damage
to plants as well as increasing the risk of disease.
Light transmission can be reduced up to 20% as
the shape of a droplet will reflect light back out of the greenhouse.
This loss of light may slow plant growth, delay crop maturity and affect
plat yield. The droplets may also act as lenses focussing sunlight
onto leafs causing burns. These droplets will rain down since they
are only lightly attached to the film surface and easily shaken off by
the wind or even the closing of a door.
Anti-fog films, such as Redpath's Duratough are
modified films that have a high affinity for water. The condensed
water no longer forms droplets but a continuous thin layer of water over
the entire surface. This has many advantages: water is able to drain
more evenly and flow to the lowest point more readily. Water may even be
able to flow over wrinkles that would normally be a prime location for
dripping. Generally it reduces the number of spots where dripping
occurs and the problem is more manageable. This reduces the damage
to plants and keeps workers dryer. Light transmission is significantly
improved often to the point of not being affected. Another commonly
overlooked advantage is that the thin layer of water acts as an infrared
absorber which will reduce the nighttimes heat losses, and increase leaf
temperature, not unlike the thermal films that exist on the market.
The anti-fog films are made, by adding a surfactant
to the plastic as it is being manufactured. The surfactant migrates
to the surface with time and acts to increase the films affinity for the
water. This is very similar to using soap to wash your hands.
In fact, often an anti-fog film will feel soapy when wet. Unfortunately,
the surfactant does get washed away with the water and more surfactant
must migrate to the surface to take is place. Eventually all the
surfactant is depleted and fogging of the film will occur. The anti-fogging
property may last from 6 months to as much as 3 years depending on conditions
within the greenhouse. Anything that will reduce the rate of condensation
on the film will generally increase the anti-fog lifespan. I.e. low humidity
and good ventilation. Once the anti-fog is depleted, it may be necessary
to apply an after market product that provides anti-fogging. Interestingly,
we have observed that the after market products tend to adhere better and
last longer on films that have contained anti-fog additives.
In summary, antifog films improve the greenhouse
for both plants and workers. In a well-managed environment they can
provide several years service, increase productivity, and reduce the occurrence
of disease and damage to plant while improving working conditions.
06.08.02
