Garden Greenhouses Agricultural Plastic Mulch Film, Used In A Variety Of Crops Agricultural Products Mulch Film, Durable Agricul

THICKNESS.

It is one of the basic characteristics of the film and is going to predetermine some values like mechanical properties (traction and elongation) optical properties, thermal properties and
durability of the film.

It is normally measured in gauges, although the universal measure is in microns 
1 micron = 4 gauges

Specifications:
Punctual thickness: + - 15% UNE 53.328
Medium Thickness: + - 5% UNE 53.328

To measure this characteristic we have a number of apparatus called micrometer which tells us the thickness all along the width of the sheet and makes a statistical bulletin informing of any possible deviation that may have happened in the transformation process.

Width

A vital characteristic at the time of laying a cover after all the appropriate measures have been taken in order to make a command. It is important to mention that by norm and by quality standards, the film must never be narrower than requested, the deviation, if there is one, should never be under 2%.

A deviation in the width above the one requested can be corrected increasing the size of the overlap between two sheets or picking it up and putting it along the sides of the greenhouse. A deviation in the width underneath the one requested brings up serious problems since part of the greenhouse can be left uncovered.

The measurement is made taking a sample all along the width of the film with a conventional tape measure.

Mechanical Properties
Resistance to Traction
It is the strength per unit area that we need to be able to tear the film.

It`s measured in Kg/cm2 or in Mpa =N/mm2
An easy conversion is Kg/cm2 =10 X Mpa

The experiment consists in submitting a plastic sample to a predetermined strength and stretch till the film tears

The sample used for the experiment must be 1cm wide and it is set at a distance of 10cm lengthways.

This property must be measured in both directions machine direction (MD) and transverse direction (TD). The molecular order of polyethylene is not the same in both directions after the film has been extruded; that is why mechanical properties are going to differ depending on which direction we do the test.

The specifications of the norm UNE 53.328 says that we need to be above 16 Mpa or 160Kg/cm2 in both directions.

A good resistance to traction is going to avoid any tearing on the film in the installation  and even later on in bad weather conditions like strong winds if the film has been fixed properly to the structure.

Elongation
The experiment we use is the same one as the one we mentioned above
It is measured in % and the norm says that we must be above 350% in both directions. It is measured in (DM) and (TD)
An acceptable elongation or enlargement allows an adequate tension on the cover and mulch film when they are subjected to a force applied when we are placing the film.

Tearing

This property is very important in mulch and cover films. When the film cover is placed, the film may get punctured by a wire or any object and suffer some damage. In some cases the farmer perforates the film himself. The film where it has been perforated will present obvious weaknesses in mechanical properties.

Mulch film faces similar problems when it is laid on the floor, it can get caught by sharp objects like stones, protuberances on the soil etc, they may tear it. This problem gets worse when we place the mulch film with machinery; the film bears bigger tension.

With this parameter we try to measure which strength one needs to keep tearing a sample which has been previously, intentionally cut.

The experiment consist in letting a determined weight fall freely over the cut sample and measure the strength used to tear completely

The units usually used for this measure is grams 6gr/mm.; even N

Dart Impact

The experiment to measure this property consists in letting different weights fall from a predetermined height over the film until it tears.

We measure the strength needed to tear the film by a sharp strike or impact.

We measure the face and the pleat and we do it in grams.

In agriculture a good value in dart impact suggests that the film cover is going to endure certain atmospheric agents like hail and strong wind strikes.

OpticalProperties

Optical properties of plastic sheets for agricultural usage are probably one of the most interesting ones from an agronomic point of view since it affects directly the growth and development of the crops.

Global Transmission of visible light

It is the amount of visible light that the film allows to get through inside the greenhouse.

It is measured in % of incidental light that reaches the film with a spectrophotometer ultra violet/visible. A good cover must have an 80% of global transmission of light. Less percentage could be translated in less luminosity inside the greenhouse, which could affect not only the precocity but also the yield of the crop. We have already said the light factor is proportional to the vegetative development.

To simplify it we can say the following things:

Abundance of Light:
-Helps budding, flowering and fructification
-Intensifies the production of pigments
-Increases photosynthetic activity
-Intensifies the odor and taste of vegetables
-It Increments the resistance of plants to insect and cryptogram attacks

Weak light Intensity

-Produces a lighter green colour in the plant
-Reduces pigment formation
-Stimulates the formation of roots and bulbs
-Increases the sensitivity of illnesses and plagues

If we refer this optical property onto mulch films, we would need to talk about levels of opacity depending on what is required. The quantity of light transmitted and therefore opacity in a bigger or smaller proportion.

This way we can move from the natural mulch film, which practically admits all radiation, allowing good temperature in the root system but also weed growth; to the opaque black mulch which doesn`t let practically any radiation through. Avoiding weed growth but the root system does not reach as high temperatures as the natural one.

An intermediate one is the photoselective mulch film, which cuts certain radiations corresponding to PAR (Active photoselective radiation) which stops weed growth and allows other thermic radiations to go through, responsible for the warming up of the ground. This way we can have good root temperature avoiding weed growth, which steals water and nutrients that could be useful to the plant.

Diffused Light

Diffused light is the quantity of rays that suffer a deviation above 5% on the incidental ray, when they pass through the film cover. All the light that goes inside the greenhouse can be diffused or direct light depending on the deviation that it may have suffered when they pass through the film cover. It is measured in % with the same apparatus as the global transmission of light.

A bigger percentage in diffused light does not necessarily translate in less luminosity; but the light inside the greenhouse is better shared all throughout the surface (Plants, structure). This avoids the formation of shades that arise when the light is too direct. It can even avoid the risk of burns in the aerial part of the plants since it is less incidental.

A good plastic sheet, for Almeria, is that one that allows a great quantity of total visible light to get through and that part of that light is diffused; this way we will achieve much more uniform fruits, better quality. For example when we are inside the greenhouse and we look up, there are times that we see the sun very clearly (direct light) and with other sheets of film we diffuse it (diffused light). We can even see our own shade inside the greenhouse.

To summarize it all, we can say that:

Abundance of diffused light:
-Favors lignification and dehydration 
-Stops the plant from growing taller
-Favors homogenization in fruit size
-It increases the size of the fruits
-Avoids possible burns on the fruits` tissues

  Small percentage of diffused light
-Favors the growth in height and elongation of the stems
-Avoids dehydration
-Fruits of irregular calibre
-Increases the risk of burns on the fruits` tissues

Reflected Light.

There are certain sheets of mulch film that have the property of reflecting a great proportion of incidental radiation towards the plant. This way the plant brings extra light that produces earlier crops and better productions. In the case of bi-colour black/white films and silver/black film the white or silver side goes on the upper side to bring extra light and the black side goes upside down hinder weed growth.

When the aerial part of the plant start to develop, the upper leaves and stalks make a shade on the lower parts of the plant, as a result of that, they receive less light and differences in the growth and development of the plant may occur.

With this type of film, the plant receives a quantity of light a lot more even throughout its tissues achieving more homogeneous fruits on the upper part of the plant and not as spread out.

Thermal Effect

It's the property that some films have of retaining heat that usually escapes during the night from the greenhouse`s cover. Trying to stop this heat from escaping helps avoid an excessive loss of heat inside the greenhouse. During the day, the visible radiation and short infrared light reaches the inside of the greenhouse warming up the floor, the plants, the structure, etc. During the night these bodies loose heat accumulated as infrared radiation of large wavelength. (7-14 microns)

There are other films, called thermic, that contain packet of additives incorporated, which make them more opaque to these infrared radiations. This way heat remains trapped inside the greenhouse avoiding the cooling down and the possible thermic inversions.

The thermic effect is measured in transmittance long I.R. (7-14?), that is, what the film allows to escape through a determined area of the spectrum, IR. Therefore, the bigger the transmittance IR the less thermal and vice versa.

This means that the early hours in the morning when outside temperatures are too low, inside the greenhouse we notice one or two degrees higher, avoiding possible frost. In cold climates this effect is more pronounced, achieving up to four degrees higher in the inside of the greenhouse.

Quick Ageing

-The medium thickness of the film

-Sterilization made on the floor, above all those made with halogens like (chloride, Bromide) and specially products with sulphide, interact with the UV stabilizers neutralizing and shortening this way the life span of the film.

It is important that these products remain the least amount of time possible in contact with the film. In this case they advice to take a number of measures, like, the usage of special films to sterilize the soil, or to take advantage of a change of cover to disinfect.

-The structure affects dramatically  the life span of a film. The oxidation of the structures can cause the same effect as disinfectants shortening their duration. Worsening in warm seasons when the contact with the film with the structure can reach 60 or 70 degrees centigrade. Maintaining the structure as clean as possible should be the aim.
-The contact of the crop with the cover. The plant in contact with the film can take away certain UV additives .

-Other factors like ventilation

All in all it is difficult to predict a more or less accurate duration for the film. Taking a series of precautions and controlling the factors mentioned above we can extend the life span of a film one more campaign.

The polyethylene agricultural films, above all direct covers, have a package of additives stabilizers of UV light, whose mission is to extend the expected duration of the film.

The film has a limited duration like any polymer, depending on its thickness and other variables it lasts approximately 4-5 months.

Degradation starts with the breaking of certain links in the structure, radicals` formation, and chain reaction.

There are two types of additive that coincide precisely with the coloration of the film: Yellow and White.

Yellow:
Benzophenone absorbs UV radiation transforming it in another radiation less harmful like heat.
Ni Quencher, blocks the excited states cutting the chain reactions which cause the degradation

White:
Benzophenone, absorbs UV radiation transforming it into another radiation less harmful like heat
Halls catches the formation of radicals in the photochemical reactions and they transform them into reactive products non-harmful.

To test the ageing of a film we take a piece of film and we expose it to 317nm of UV radiation between 60-70 degrees centigrade. We take samples every so often and test its mechanical properties; above all its elongation.

A film is considered degraded when its elongation is 50% its original elongation before being exposed.

There is a relation between the time of the duration in the accelerated ageing chamber and the duration in the field. (UNE 53.328)

Yellow ones:    170 hours =1 season
                          500 hours = 2 seasons

White ones:      400-500 hours = 1 season
                               1200 hours = 2 seasons