Member Question: In order to finish our parts we are using thinner along with gas flaming. Our process is not very safe and we have had a few events of rags catching on fire due to flame and thinner. Is there a standard way to finish rotomold parts (PE), maybe using non-flammable chemicals?
Dr. Nick responds:
Surface flaming of rotomolded parts is a technique that is commonly practiced. It may be done for several reasons, primarily:
- Preparation of a surface to make it more responsive to adhesives or paints. Untreated polyethylene (PE) is highly inert chemically and is therefore not an ideal surface for adhesives or paints to bond to. Flame treatment, combined with surface abrasion (e.g. rubbing with 60-80 grit sandpaper) will promote a better, if not perfect, substrate. From a technical aspect, the flame treatment oxidizes the surface and creates polar groups which improve wettability and adherability. The abrasion roughens the surface and provides more potential for a mechanical bond. Providing both improved chemical and mechanical adhesion will optimize the surface.
- Improving the visual appearance of the surface and introducing a more shiny look. In this case, the flame treatment re-melts a thin layer of PE on the surface of the part and degrades it which creates, in effect, a higher Melt Index (MI) polyethylene layer that will flow better than the original material. If the surface already contains defects (e.g. scratches or pinholes) flame treatment will not necessarily rectify all faults; it may even make them worse. The normal procedure is to ensure the surface is clean, by wiping it with a cleaner, then to wash a flame over the surface. A typical treatment would be to hold the flame approx. 12 inches from the part and wash for 3 seconds.
From the details in the member’s question, it appears that they are flame treating to create a superior surface effect. The difficulties are created by using a “thinner” as the pre-treatment cleaning fluid.
The generally accepted definition of a “thinner” is a volatile solvent used to dilute or extend oil-based paints. Typical products used include: mineral spirit, denatured alcohol, turpentine, acetone, naphtha, toluene, MEK and xylene. OSHA considers thinners to be flammable liquids with a Flashpoint of approx. 70-140ºF.
Generally speaking, the lower the Flashpoint, the more likely there are to be unintended fire incidents. So one approach to the member’s problems could be to research alternative products that have higher Flashpoints, e.g. water-based cleaners or “142” solvent. Note that I’m not specifically recommending these products for the application, I’m just logging that they have higher flashpoints than the usual thinners. The down-sides may be increased unit cost, plus a longer wait between application and full evaporation of the solvent. With a well thought-out procedure for the process, these down-sides may still result in a more robust result, with less hazards. As in many instances with rotomolding, improved house-keeping can make a big difference!
An additional procedure modification could be simply to separate the cleaning and flaming operations, in both space and time, so that the probability of accidental fires is reduced. Obviously, the time separation can’t be so long as to mean that the surface picks up fresh contamination between cleaning and flaming.
Yet another approach could be to mold the part in a higher Melt Index grade than the one currently used; the difference in surface appearance between a General Purpose grade (MI approx. 4 g/10min) and a high-flow or “Toy” grade (MI approx. 7 g/10min) can be significant. On the other hand, higher MI grades may exhibit some reduction in impact strength. Whether this is worth looking at depends on how much of the part surface is flamed; if you need to treat the whole part, changing material could be a good option.
The other general caution I would make regards all types of manual procedures. These can be highly operator-dependent; different people will do the procedure in different ways, with different results. It always pays to establish the best way of doing a job and then use formal procedures and training to ensure that everyone does it in the same way.
Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 30 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.