Stretch film for silage

Background

In the beginning of the 90s the market of round bale silage increased rapidly but the silage quality was varying. The Swedish Machinery Testing Institute (SMP) started a project in collaboration with the Swedish University of Agricultural Sciences (SLU) and SP Technical Research Institute of Sweden concerning quality control and certification of stretch film for round bale silage. The project includes extensive field tests related to the development of laboratory test methods.

One of the most difficult questions was to find out realistic laboratory test methods for measuring oxygen permeability. During the spring 1994, SMP presented a test scheme, requirement levels and  procedure for certification of stretch film. Since 1995, SP has the responsibility for testing and certification of stretch films in Sweden. Today the procedure is very well-known in- and outside Sweden and most of the stretch films sold in Sweden fulfil the requirements and are certified. All certified stretch films are marked with SPs P-mark in order to show the customers that they fulfil the certification and test requirements.

The testing and certification procedure is described in the report ”Certification Rules for P-marking of Agri-Stretch Film for Round-bale Ensiling”, SPCR 063.

Test methods and requirements

The test methods for stretch film are divided in two parts. Part one consists of mostly standardised material tests of mechanical properties and part two consists of testing the film on a round-bale (test of package).

Part one consists of:

• Material analysis with spectroscopy
• Measurements of dimensions
• Mechanical strength
• UV-durability
• Solar-reflectance

Part two consists of:

• Oxygen permeability
• Air-tightness

Tests made in part two are performed using a special made round bale made of steel, with the same dimensions and gas content as a normal round-bale. After wrapping these ”artificial bales” with the stretch film tested and filled with nitrogen gas, the bales are stored in a climate chamber and the oxygen permeability is measured during controlled conditions.

For the factors measured, requirements have been stated according to field and laboratory tests. The standard is presented in relative figures, that give a producer possibility to develop different products as long as they fulfil the standard.

Certification procedure

The certification procedure is valid for stretch films having widths between 500 and 750 mm.

 

The conditions for certification include (summarised):

• A detailed description of the product
• Test results that fulfil the requirements
• Requirement for continuous inspection
• Requirement on surveillance inspection, performed by SP
• Marking requirements
• Instructions for the customer

Link to P-marked products can be found to the right of this page.

Other terms and conditions

The certificate is valid during a period of 5 years under the condition that the manufacturer fulfils the requirements for continuous and surveillance inspections.