Film thickness versus mechanical strength and lifetime

Film thickness versus mechanical strength and lifetime

Why is film thickness so often a consideration for farmers?
The main reason is the intuitive feeling "thicker = better". 
This is only true if we consider the same material. 
And a solid mental error - when we analyse entirely different materials.
Below is information to help you understand why you should look for modern, advanced materials instead of "thick film".

It is also important to remember that reputable manufacturers design their films not only for "general" strength but, taking into account the specifics of the installation (or type of construction), select parameters to offer the best possible material for a given application.

Film thickness vs. mechanical strength

The mechanical strength of plastics, including greenhouse films, depends on:
- production technology and quality of the raw material
- reinforcing additives 
- the thickness of the material

To understand the relationship between the various parameters on manufacturers' technical sheets, it is necessary to analyse:

(a) tensile strength at break (expressed in MPa).
Parameter, which determines the force required to break the material. It depends mainly on the reinforcing additives and is slightly dependent on the thickness.
Tensile strength is very often designed for a typical film application.
Example: A film designed for rope installation, Kritifil® 2495, 140mic thickness, has a higher tensile strength (30MPa) than Kritifil® 6511 nursery film, 200mic thickness (22MPa).

(b) elongation (expressed in %)  
A parameter determines how often the material will stretch (elongate) until it breaks.
This parameter dramatically determines the actual behaviour of the material under wind loads.

The quality of the raw material and the additives play a major role here, while the thickness of the material has virtually no significance.
Example: top-quality mulching films with a thickness of 35mic have the same elongation (approx. 600%) as UV12 2996 film with a thickness of 180mic.

c) Impact resistance (defines how heavy a spearhead or”knife” must be to break through material).
Again, this is a parameter that depends very little on the thickness.

Example: film reinforced with Metalocene, UV6 Kritifil ® 2792 130um - puncture resistance 1000 gr, film UV12 Kritifil ® 3750 puncture resistance 750 gr,

NOTES:
Please note that the real service life and behaviour of greenhouse films do not depend directly on the separately analysed parameters but are the result of a mix of stabilisation, mechanical resistance, raw material quality, additives, precision and production technology.

Mechanical strength, thickness and cost of the film
 
Below is a table to understand the influence of thickness and reinforcing additives on mechanical strength and their impact on cost 

Parameter Influence of raw material and reinforcing additives Influence of film thickness 
Tensile strength Non-linear
(e.g. Metalocene additive ~+50% to strength with ~8% increase in material cost)
 Linear
(increasing film thickness from 150 to 180mic is a 20% increase in tensile strength with a 20% increase in thickness and material cost)
Elongation Key Not significant (elasticity does not increase with material thickness)
Impact resistance Dominant.  
Variation >100% in value depending on film grade at the same thickness. Linear 
Abrasion resistance (parameter outside of technical sheets): Special additives radically strengthen the films against abrasion—for example, soft fruit films. There is no impact; with a thicker film, there is simply "a little more material to abrade."

Film thickness vs. film lifetime

One of the biggest mistakes when choosing a film is to assume that a thicker film will last longer.
In short - the thickness of the film does not affect its lifespan in any way.

The lifespan of a greenhouse film depends solely on:
(a) UV stabilisation (higher stabilised material ages more slowly).
As a transparent material - greenhouse films age throughout their entire mass. 
This means that a material stabilised for 5 years will have the same rate of ageing in a thickness of 100mic as well as 200mic.
b) reinforcing additives (we have a greater "reserve" of mechanical strength of the material)
c) possible increased chemical resistance (chemical load in most farms is accidental and in the long term is the main reason for premature degradation of the material)