Lab trials flag differences in film performance

Laboratory trials carried out on silage stretch wrap films have flagged a series of differences that could hold the key to improving baled silage quality. The tests, carried out by Dow’s Antonio Manrique found significant variations between different films in terms of oxygen permeability, tear resistance, and ‘neck-in’ tendency, all of which have a direct impact on silage quality and spoilage.

In summary, the results indicated that the higher the number of carbons in the resin’s 'co-monomer’ molecule chain, the better the performance. This explains why the films containing the ‘C8’ polyethylene resin, with eight carbons, gave better results than the ‘C6’ and ‘C4’ films, with six and four carbons respectively.

Low oxygen permeability is one of the most important factors in reducing spoilage. The ‘C8’ configuration seems to act as a better barrier to prevent air dissolving in through the film, preserving the crop more effectively. It was also clear from the trials that the greater the number of layers of any film, the better the oxygen barrier.

Other results in Dow's trials showed that the ‘C8’ configuration also had the effect of increasing the strength and flexibility of the film, while reducing the amount the film ‘necked in’ — or reduced in width — when stretched to its limit, allowing better film overlap across the bale.

The way the wrap was produced also made a difference. ‘Blown film’ is orientated in different directions during production which makes it more tear resistant than ‘cast film’, which is only orientated in one direction. A simple manual ‘tear test’ can help differentiate between blown and cast films.

As with a newspaper, if a film is made using the cast process, one side will be straight and the other will want to go in a more diagonal direction, when tearing. However, when trying this with blown film, both sides will try to tear more diagonally. This is because cast film, like newspaper, is orientated in one direction. Blown is multi-directional, providing a more resilient film that withstands better handling on-farm.

September 2004

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