Polyolefins (PE and PP) will have an accelerated degradation if exposed to UV light, as several other plastics.
Stabiliser additives are available but these also need to be designed for another challenging condition.
In greenhouses it is important to eliminate or control fungal diseases to avoid damage of plants, vegetables, berries and fruit. Evaporated sulphur is a well-known and efficient pesticide that protect the greenhouse crops from fungal disease and the spread of greenhouse pests.
Polyolefins used for greenhouse purposes need addition of UV stabilisers to get the desired lifetime either to be used as a greenhouse film or to be used inside a greenhouse as twine/yarn for binding up plants like tomato or cucumber. A huge challenge is that the most common UV-stabilisers based on Hindered Amine Light Stabilisers (HALS) are alkaline of nature while pesticides, including evaporated sulphur, are acidic of nature.
This means that the UV-system has to be carefully selected to avoid negative interaction (antagonism) with the pesticide used. Worst case is that the UV-system will not work at all due to protonation of HALS (see figure).
Lankhorst, our customer, is producing twine to be used in greenhouses. The twine is polyolefin based and the oriented film has a thickness of 35 µm.
They contacted Norner to develop a suitable UV-stabiliser recipe for twine and for testing of such recipe in accelerated weathering in a Weather-O-Meter (WOM). A new test procedure had to be specified because prior to weathering the twine samples should be treated with evaporated sulphur corresponding to 0.75 g S/m2 of greenhouse area.
The sulphur treatment
A Hotbox Sulfume sulphur vaporiser was used for sulphur treatment weekly (ca. 0.15 g S/m2 per week).
The sulphur treatment was done in a plastic tent with ca. 10 m2 area, where the samples were located at a fixed distance to the sulphur vaporiser.
The developed UV recipe had 63% retained elongation at break after 1150 hours WOM exposure (ISO 4892-2). This corresponds to ca. 100 kLy which gives about 1 year lifetime outdoor in Central Europe.
If the twine would have been unstabilised it would have only lasted 2-3 months outdoor before it would be brittle.
If the UV stabiliser would not be resistant to the acidic conditions it would also fail. This reason for this failure is chemically illustrated in the below figure.
The functionality of UV stabilisers is the ability to neutralise oxidation reactions - as shown in the horisontal chain of reactions. This figure is of a first version of HALS type of stabiliser. When this molecule is exposed to acid the active centre protonate which disable the UV stabilisation.