It is well known that Tubex tree and vine shelters, made from polypropylene, degrade over time. But, how long do they take to degrade? And what is the degradation process? We aim to answer these questions here.
Minimum versus maximum life spans
The first point to make is that the original specifications for treeshelters in the UK targeted a minimum product life of between 12 and 15 years. It has been a gradual change that has reversed our attention to consider whether a maximum life is possible.
There used to be a third, more important, concern that the shelter might strangle the tree, but our introduction of a laser line (“zip”) solved this issue.
The key difficulty here is in forecasting the range of life spans that are required (different growing conditions and different tree species), and the different degradation processes that are potentially going to be experienced (different climatic conditions).
Although it is difficult to give precise forecasts, Tubex uses virgin raw materials (as opposed to recycled materials) in its products so that it can give a better prediction of the degradation process. Recycled materials, with unpredictable material quality and content, make prediction virtually impossible. We also believe that recycled materials have a higher overall carbon foot print – but that’s another matter (see climate change).
We summarise here the ingredients of our products and explain their impact upon the environment and their degradation process.
Tubex Tree and Vine Shelter Contents
The Tubex shelters are manufactured from a blend of polypropylene, inorganic pigments and an ultra violet (UV) stabiliser. Polypropylene comprises >99% of the final product.
Tubex has conducted its own, university led, research into the environmental impact of its materials through the degradation process.
Polypropylene is an environmentally inert material and the Tubex material grade has general food contact approval and therefore holds no known threat to ground water contamination. Polypropylene degrades under the action of (the sun’s) UV, oxygen and bacteria. The breakdown is initiated by UV photons and subsequent reactions with oxygen gradually reduce the “chain length” until “mobile species” are produced. Much of the material (at least 50%) degrades directly to carbon dioxide and water. The remainder degrades to relatively small ketones, acids, esters and alcohols. None of these have toxicological effects at the levels produced by the degradation of a treeshelter.
The pigments consist of a range of components which are all either extremely stable or break down into individual oxides, and provide (at the very low levels involved) no environmental risk.
Predicting the Rate of Degradation
The outdoor weathering of polypropylene treeshelters is generally driven by the UV degradation of the polymer. This causes a range of oxidation reactions which lead to the embrittlement of the material and material loss. This leads to cracks propagating down the shelters (often due to wind loading), which speeds up material loss. The driving force for degradation is therefore UV dose. If treeshelters were made purely from polypropylene this degradation process would be rapid (possibly as little as 1 year), and in order to slow down this degradation process a UV stabiliser is added to the material.
UV stabiliser is used up gradually in its protective mechanism, and it also has the potential to evaporate or leach out of the treeshelter. The higher the UV exposure, the faster will be the depletion of stabiliser. Once stabiliser content drops below 0.02% of the material, the degradation process becomes more rapid.
UV exposure is quantified as the total UV energy dose, per area (and expressed as MJ/m2). A value of 200MJ/m2 is typical for the annual UV dose in southern parts of the UK. Tubex has conducted tests to show the typical “mastercurve of UV stabiliser depletion”, having started with a level of approximately 0.15% of total material.
The table below illustrates this mastercurve.
This master curve shows that we would expect to see degradation commence after 400 – 600MJ/m2, or 2 to 3 years in the Southern UK.
However, the level of UV and UV depletion will be affected by the following other factors:
- Site conditions (north facing or south facing)
- Site shade conditions
- Temperature (higher temperatures increase the rate of depletion)
Tubex is currently researching how it can better predict the rate of degradation and thereby refine its product range. The continuing problem is that we cannot produce multiple product variations to suit all climatic and tree species requirements. But we can try!
Another important factor will be the uniformity of UV stabiliser within the overall treeshelter mix. Tubex has invested heavily over recent years in modern, computer controlled material mixing equipment to help control what needs to be a very precise material mix.
Closely linked with this subject area is Tubex’ research into alternative materials – See Biodegradation