Bizarre as it may seem, in the past few weeks, I’ve had two different consultancy customers report powders with this same problem. They sent me samples and parts but, even before they arrived, I suspected that the problem was gas fading.
Some of you may have experienced this phenomenon before, and wondered why it happens:
The problem: A coloration (usually either orange or pink colored) that you can see clearly in your powder. I’ve included a photograph below, to illustrate the point. This material was actually compounded white, but it can show in natural material as well…
Despite appearances, this is not just a gross contamination of the powder, it’s something else. So – what is it?
Dr Nick’s answer: Gas fading (or “pinking”) is caused by a combination of factors, all occurring at the same time:
- the presence of phenolic antioxidant (AO) in the base polyethylene (PE)
- the presence of oxides of nitrogen in the atmosphere where the polyethylene is stored
- darkness during storage
Factor 1: Phenolic antioxidants are often used in PE formulations; they provide essential protection against degradation due to the aggressive heat history that a material will experience during rotomolding. They are a very effective, and also very common, type of primary antioxidant and are often used synergistically with a secondary antioxidant (an organic phosphite). The application of effective AO packages has been one of the least understood, but most significant, factors in the development of roto grades that have wide processing windows. If you would like to know more about this subject, you could do worse than reading two papers I co-authored with academic collaborators from Manchester Metropolitan University, in 2005 and 2012.
Factor 2: Nitrogen oxides (NOx) consist mainly of nitric oxide and nitrogen dioxide gases and are produced in the exhaust of diesel and gas-powered equipment, like forklift trucks. Gas concentrations in warehouses during winter months can be double what they are in summer, because of lower ventilation rates when the weather is cold outside.
Factor 3: The orange or pink contamination you see with gas fading is due to the formation of a chemical complex, produced by the reaction between phenolic AO and NOx. These complexes are brightly colored, but they are unstable when exposed to sunlight. Therefore a degree of darkness is required in order for them to develop and persist. When exposed to sunlight, the complex breaks down and the color disappears.
This is definitely an issue that comes and goes, seemingly at random. Different versions of phenolic AO are more or less susceptible to the problem and some pigments (especially some types of titanium dioxide white) can exacerbate the effect.
In background research for this blog, I found some very useful additional information in a presentation by NOVA Chemicals, who also have an informative Technical Bulletin which describes the phenomenon. Here you can see that the problem is not confined just to rotomolding. Thanks to Henry Hay at NOVA for helping me out with this.
Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 25 years-plus experience in rotomolding, specializing in the fields of materials development and process control. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.