Cleaning a small batch of HDPE flakes obtained from motor oil containers

[Kakashi]

I am working on cleaning a small-batch of HDPE flakes obtained from motor oil containers. My goal is to remove contaminants such as residual oil, and adhesives to evaluate the cleaning efficiency of different solutions.The flakes I received from the processor were supposed to be pre-washed, but upon inspection, they still appear contaminated.

I found two patents discussing cleaning processes:

This patent (pdf in link) describes a process using a physical mixture of liquid phase 1 and liquid phase 2 wherein the solvent of liquid phase 1 is water and the solvent of liquid phase 2 is an organic phase comprising an acetate ester to remove oil from plastic waste. The process can take place at room temperature and there is no need of applying any pressure but this method seems to focus only on oil removal.

This patent (pdf in link) outlines a method using either of the solvents polyethylene glycol, polypropylene glycol, and dimethyl sulfoxide to remove contaminants found in the plastic matrix and surface dirt such as solid particles, adhesive residue or labels.

I am unsure what cleaning solution the processor currently uses, but from what I have read, industrial cleaning processes typically involve a combination of a surfactant and a caustic agent. I have also read that hexane is very effective at dissolving oils and adhesives while being compatible with HDPE. However, I am unsure whether it can serve as a substitute for a caustic and surfactant solution, especially due to concerns about its flammability.

Since I am not a chemist and I do not have access to fancy lab equipment, only basic tools like containers, stirring rods, and heating sources and reagent which I will still purchase. I wanted to reach out for advice on any safety precautions I should consider before conducting the experiment. My plan is to take equal samples of the flakes, soak them in different cleaning solutions, and stir and heat them if necessary, as I have read that caustic washing typically occurs at around 80°C. After cleaning, I would weigh the dried flakes to determine the efficiency of each solution. This approach would help me identify potential candidates for an effective cleaning solution.

 

[Tom.G]

Try liquid dish detergent or liquid clothes washing detergent. The both need to be mixed with water for maximum effectiveness.

Cheers,
Tom

 

[Ozchemist]

Are you trying to evaluate the efficeiency of the of the cleaning solution used by the original processor or are you trying to develop a cleaning solution that can be used as part of a cleaning process?

From an industrial viewpoint, the reason that the sufactant/caustic systems are used is that they are cheap, relatively easy to handle and are "sufficently efficient" for most purposes. Oil can be recovered from the wash water (if required/desired) using a variety of techniques, or the spent bath can just be treated as hazardous waste and sent for disposal (before or after local neutralisation). As you have discovered, they don't removal all contaminants nor are they particularly good at removing adhesive residue, but where the HDPE is going to be chipped for "re-melt" they are often sufficient.

There are certainly solvents that will do a better job - but solvents come with substantial risks. Most of the solvents that have "single pass" efficiency are either low flash point (below ambient - like hexane/heptane/toluene) or around ambient (white spirits/Stoddard solvent/LAWS/xylene) or are toxic (dichloromethane/trichlorethylene). Oil can be recovered from the solvent via flash distillation of the carrier solvent and recovery of the heavy ends, but you require a small refining unit to do so.

Alternatively, the solvent can be can be made into an emulsifiable system, but this then removes the "single pass" option (you have to have a cleaner to remove the cleaner) or leads to excessive wash water which may contain hazardous materials, once again requiring disposal, only this time the hazardous material is flammable or toxic.