Repsol YPF

Mechanical tests on agricultural films

The characterization of a plastic film requires the knowledge and specification of different properties and, among them, mechanical properties.

Mechanical properties permit:

The prediction of behaviour in service and therefore the election of the most appropriate material in each case.

Knowledge of the state of degradation of the film. Concretely the lengthening (deformation) in the rupture is the property elected in the European norm on thermoplastic films for greenhouses (EN 13206).

Tensile strength

The test consists of applying a uniaxial load on a previously prepared sample of the film to be checked, applying it homogenously to the whole section of the sample, in general until the rupture of it.

The result of the test is a stress-deformation curve characteristic of the tested material. This curve permits the determination of the elastic parameters and plastics that determine the behaviour of the material in the field.

This test is standardized (documents UNE, ISO, ASTM). The standard that Repsol follows for agricultural films is the standard ISO 527 parts 1 and 3.

Impact resistance test

The method used for film is the so-called "dart impact". This test measures the energy necessary to break a film by means of the impact of a dart under given conditions. This energy is expressed as the dart mass that, upon falling from a certain height, causes the break of 50% of the examined samples.

In Repsol we follow the so-called method A of the norm ISO 7765-1 (films that break between 50 g and 2 Kg), which is translated in the utilization of a dart of 38 ± 1 mm of diameter and a fall height of 0.66 ± 0.01 m.

Creep test

This test measures the lengthening of a film subjected to a constant load during a certain time in a static vertical positionática.