During last year Stora Enso released Prime, our latest family of materials. The grades in the Prime family have been developed to replace high performance polymers and compounds, for example PPGF, HIPS and ABS. Three examples of suitable applications for Prime grades:
- Casing and components for electronics
- Helmets and safety products
- Interiors for automotive
Customer piloting feedback: UV performance needed to improve
To understand how Biocomposites by Stora Enso and Prime grades in particular was performing, we conducted several customer pilot cases. One main feedback: UV performance needs to improve. In other words: the material was bleached over time. The surface could also get damaged in different ways after being UV exposed.
This was no surprise as UV performance have historically been a weak point for injection molding grades of natural fiber composites. Because of this we also started to test the materials in a “weathering chamber” to be able to understand, compare blends and improve. Now, in collaboration with the pilot customers and our partners, we have managed to develop ways that gives high resistance to UV.
Testing in a weathering chamber
So, how is this done? The tests were conducted on three different materials, Prime L30 High GG, Prime S40 Impact I and Plus S40 Impact D. These materials are representative samples of our high-performance grades in terms of wood fiber amount and type as well as performance additives. The tests included injection moulding plaques of each material and exposing them to UV light in a weathering chamber (Atlas Suntest XXL+) for 500 and 1000 hours, respectively. These conditions simulates exposure for a number of years, the exact number depends on which part of the world the exposure is intended. Black and grey were chosen as preferred colours to test, based on customer input. Materials were tested in the following combinations: natural colour without additives, with UV-masterbatch, in colour and colour+UV.
The results
We found that, in general, the mechanical properties remain similar to its benchmark levels. The main observed effects and changes was in the surface and visual aspects. However, this will depend on what type of test that is conducted, and if any colour and/or UV-MB have been used, further explained below.
- When exposing the natural material to UV
There is a bleaching effect of the fibers and the materials whiten over time. The matrix polymer gets damaged over time where the surface gets rough and “powdery”. A small effect on mechanical performance was observed.
- With UV-masterbatch added to the natural material
Some whitening was observed but less compared to the natural materials. The matrix polymer does not break down significantly and the mechanical properties were not significantly affected. However, colour will whiten over time.
- Colour with no UV-MB
Adding a white colour works very well as it hides the whitening and acts as a UV-absorber whereas black will whiten more than any other colour. Grey and other colours tend to whiten and/or get somewhat a yellow tone.
- Adding colour and UV-MB
Both black & white works very well. Other colours were also tested, and performed on a similar level without bleaching or any yellow tone. No effect on mechanical performance.
The amount of fiber in the material does not appear to be an important factor and the behavior between the materials are similar. Very high fiber contents may lead to increased fiber popping, meaning that the fibers expand at the surface which leads to a rough surface. However, this is only relevant when the material is in frequent contact with water. A surface texture is also good for limiting this effect and is something we generally recommend, for both this aspect and a more pleasing visual expression of the product.
Conclusion: Required UV levels reached for most applications
Based on the tests that were conducted, we are confident that we can reach the required UV levels for most relevant applications, including the ones mentioned above. We can conclude that this is best done when combining UV additives with a colour masterbatch. We can recommend our materials for applications that are exposed to direct sunlight. For products which are constantly outdoors it will depend a lot on the application as the materials can be slightly sensitive to prolonged UV exposure while in contact with water.
