A short guide to a long life

The stabilisation strategy to be employed for PE film is very well understood. The stabiliser class of high molecular weight hindered amine light stabilisers (HALS) is by far the most efficient tool to yield lifetimes that easily exceed the service life expectations of agricultural film. While the UV stability is assured by the high molecular weight HALS, above a certain film thickness mixtures of the latter with various UV absorbers are superior. This threshold is around 150µm.

One of the major difficulties is the potential presence of agrochemicals that may have a hostile effect on the UV stabiliser. The principle response of an individual stabiliser or a stabiliser class to direct contact with agrochemical(s) depends in particular on the mechanism of action of the stabiliser class. The performance limit of each UV stabiliser system depends heavily on the experimental conditions chosen or the actual existing conditions “in-field-use”. For these reasons performance data tends to vary significantly and various cross-over effects are possible.

Figure 1 outlines the theoretical performance profile (“play-field”) of various UV stabilisation strategies for agricultural films. In all cases, the Y-axis classifies the UV stabilisation or service life achievable for a given concentration and under given circumstances. These are categorised from low to moderate to high. The X-axis denotes the intensity of exposure to sulphur or chorine-containing agrochemicals, ranging from no agrochemical exposure to low, moderate, to high agrochemical exposure. The intensity of agrochemical exposure of PE films can, for instance, be quantified by the amount of sulphur or chlorine analytically detected in the film at the point of failure or at the end of the service life. In consequence, the theoretical target area would be the right top corner which shows a long service life of the PE film even under conditions of severe exposure to agrochemicals.

Fig 1: Concepts for agricultural films – Comparison of the various “play fields”

The “play-field” of the UV stabilisers based on nickel quenchers such as Songsorb 1084 is outlined in figure 1 (green area). In the absence of agrochemicals, nickel quenchers yield only moderate service life, which cannot significantly be improved by an increase of its concentration. In contrast, Songsorb 1084 based systems are said to be robust against chemical attack. The achievable service life declines in general only to a minor degree with increasing exposure to agrochemicals, says Songwon. For this reason, it is, on a global scale, still important.

The blue area in figure 1 demonstrates the performance profile of stabilisers of high molecular weight HALS such as Songlight 9440, Songlight 7830 or Songlight 1190. For example, Songlight 9440 is said to be an effective UV stabiliser, giving a long service life. However, it has limited resistance to sulphur- and chloride-based agrochemicals. In consequence, it tolerates only minor exposure to agrochemicals in order to maintain its high efficacy as a UV stabiliser. Moderate exposure leads to a significant and sharp drop in service life. An increase in the concentration can delay the reduction in service life during increased exposure to agrochemicals. However, the steep slope on the right hand side makes this stabilisation strategy not very robust. An additional minor increase in the use of those chemicals will result in a significant reduction of durability, likely to be below the expected service life. The risk of this situation materialising needs to be managed in one way or another. The principle advantages are: good UV stability; a good thermal stability; possibility of using colourless and transparent film; synergistic effect with several co-additives possible; this concentration effect offsets, in principal, partial antagonism with agrochemicals and commercial availability.

The stabilisation strategy, however, encounters disadvantages and risks such as limited resistance against agrochemicals. Additionally, it is not robust (increase of agrochemical exposure from low to moderate) and a risk management, including detailed end customer knowledge, and over-stabilisation are needed to cover the “worst-case-scenario”.

The orange area in figure 1 highlights a stabilisation strategy based on NOR-HALS. Good UV stability is said to be combined with good resistance to contact with sulphur-based agrochemicals at least. Potential insufficiencies in the right top corner can be addressed with higher concentrations. Of the different stabilisation strategies presented in this paper, NOR-HALS represents the most elegant and powerful. Its technical “drawback” lies mainly in the fact that it is over-engineered for the left side of figure 1, in light of its limited industrial availability (including the extremely high stabilisation cost per ton of PE compound). NOR-HALS technology shows the following advantages: good UV stability, low interaction with sulphur-based agrochemicals, robustness, good thermal stability of the film, possibility of using colourless and transparent film and a synergistic effect with several co-additives is possible.

Disadvantages encountered are that in many cases it is over-engineered, offers limited industrial availability, especially as regards cost, and its thermal stability of selected stabiliser structures.

The red area of figure 1 demonstrates another approach. It combines the advantages of high molecular weight HALS with co-additives that reduce the hostile interaction with HALS (and therefore the weakness or risk of that approach). The co-additives can be either UV stabilisers (other than HALS, even if they are less efficient) or sacrificial scavengers (for sulphur, chloride or acids, etc).

One example of this approach is Songlight XP 8026. It extends the initially high UV stability (nearly as high as with Songlight 9440 or Songlight 7830) beyond moderate exposure to agrochemicals. Additionally the slope of the decline is moderated. It shows good UV stability and a good thermal stability. Using a colourless film is possible, a synergistic effect with several co-additives is employed, it is more robust (vs Songlight 9440 or Songlight 7830), the concentration effect mainly offsets hostility with agrochemicals and has a commercial availability better than NOR-HALS.

This approach softens the disadvantages of high molecular weight HALS alone. However, care still needs to be taken because it is less robust compared to Songsorb 1084 or NOR-HALS, non-transparent film (too low light intensity in geographies below 120KLy/a). Additionally, a specific physical form and a risk management are needed, including detailed end customer knowledge.

A clear positioning of the stabilisation strategies and the corresponding specific products can be derived from figure 1, resulting in the recommendations given in figure 2. For low service life requirement with no or low intensity of agrochemical exposure (for instance, non-agricultural applications like packaging film or potentially short lasting silage film and bale wrap), all four stabilisation strategies technically perform. For reasons of cost, the high molecular weight HALS is typically chosen. Within this stabiliser class, Songlight 7830 offers a good cost performance, followed by Songlight 9440. For a high service life requirement with no or low intensity to agrochemical exposure (left top corner; silage film) only the three stabilisation strategies remain that are based on HALS as a major contributor to UV stability. Again, for reasons of cost, Songlight 7830 is said to be the preferred choice. Mulch film with a high intensity to agrochemical exposure and moderate to low service life expectations would be found more in the bottom right corner. High molecular weight HALS-based stabilisation strategies do not in general perform well, as only with significant overdosing might premature failure of the film be avoided. Even under favourable circumstances, the risk of premature failure is high. Technically most robust are the stabilisation strategies based on nickel quencher (Songsorb 1084) or NOR-HALS and should be considered as the first choice. However, if the availability and cost of NOR-HALS are prohibitive or if the suspected carcinogenic effect of nickel quencher is considered unacceptable, the HALS + co-additives approach may, in these circumstances, be considered. Mulch film with moderate intensity to agrichemical exposure can however be technically preserved with the latter approach, namely Songlight XP 8026. In order to maximise the durability of mulch film in general, Songwon recommends a specific top-up processing stabilisation based on Songnox XP 1061. For black mulch film, a good stabilisation strategy exists based on Songnox 1035.

Greenhouse film and tunnels typically are exposed to a moderate intensity of agrochemicals with a service life varying from 12 to 45 months. For the lower end of the service life (12 months up to max 21 months), nickel quencher based films are an option. The potentially lacking thermal stability can easily be corrected with co-additives. Increasing the service life beyond 21 months (moderate to high service life), will ultimately leave only the stabilisation strategies of NOR-HALS and (HALS + co- additives). The later approach, for instance, Songlight XP 8026, will be more cost-effective, but will carry a higher risk of premature failure. In the top right corner, requiring high service life and high to very high exposure to agrochemicals, only a strategy based on NOR-HALS will perform.

Fig 2: Songwon recommendation for agricultural film

It should be noted that only one of the four stabilisation strategies discussed, namely NOR-HALS, fulfils technically all conditions of the “play-fields”, thus rendering it an “elegant and robust” technology. Unfortunately, it is over-engineered for the left half and in particular for the left bottom part of the “play-field” in order to match the requirements of the opposing right top part. The economic consequences of this should not be underestimated. The conclusions are summarised as recommendations in figure 2.

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