Densities of Various Solutions vs Plastic Separation


The purpose of this experiment was to find a way to separate different types of plastics by their densities.

I became interested in this idea because I might want to be involved in the plastics industry later in life. Lots of people I know (including my dad) work with plastics. I also think there could be much better methods regarding separation recycling of plastics. Recycling is very important because it’s an important way to minimize pollution. 

The information gained from this experiment could affect the world by not putting recyclable plastics to waste, by finding a more efficient way to separate different plastics.


My hypothesis was that varying the density of the liquid bath could effectively separate at least 90% of the six major polymers.

I based my hypothesis on information received from a website on densities of solids and liquids. The site showed simple ways to realize that anything with a greater density than H20 will sink in a water bath, and anything with a smaller density will float. I also based it on an article in Plastics Technology about the invention of plastics separation.   

The constants in this study were:

•    Sizes of pieces of plastics (1x1cm.)

•    Types of plastics: PET (polyethylene terephthalate), HDPE (high density polyethylene), PVC (polyvinyl/vinyl chloride), LDPE (low density polyethylene), PP (polypropylene) and, PS (polystyrene).

•    Room temperature

•    Water temperature

•    Oil temperature

•    Water Bath Sizes

•    The densities of the specific types of plastic

•    Salt in saline solution

The manipulated variable was the density of liquid bath. 

The responding variable was the percentage of the plastics successfully separated using the “float/sink” density method.  

To measure the responding variable, I simply counted the pieces of plastic recovered and identified successfully, and the ones not effectively separated, then found the percentage. 



Glass Water Bath
1x1cm Piece
Polyethylene Terephtalate
1x1cm Piece
High Density Polyethylene
1x1cm Piece 
Polyvinyl Chloride
1x1cm Piece
Low Density Polyethylene
1x1cm Piece
1x1cm Piece 
500 Milliliters
Water for water baths and solutions
1.8 Kg 
Sodium Chloride eventually added to water bath
375 Milliliters
Sunflower Oil


1.    Set Up The Experiment

            a)    Get polymers 1-6.

          b)    Cut plastic into squares that each are about 1cm x 1cm.

          c)    Get 3 containers for each solution.

          d)    Fill up the first container with 250 ml of pure water.

e)    Fill up one of the other two with 250 ml Sunflower oil and the other with 250 ml of more pure water. Add 10 grams of salt to the water and stir thoroughly. 

 f)    Make sure each different type of plastic is identifiable from others.

2.    Conduct the Experiment

a)    (During the experiment, write down the slightest things that happen and record all of the measurements.) 

b)    Put 2 pieces of each type of plastic into the saline solution, without getting anything into the solution besides the plastic.

c)    After putting the plastic into the saline solution, wait 1 minute and make sure none that are floating will sink and none that are sinking will float.

•    (While removing the floating plastics throughout the experiment, make sure that all of the water is removed from the plastic so it does not affect the density of the other liquid baths.)

 d)    Take the floating plastic out of the solution, leaving the sinking plastic, and put the pieces into the water bath. Stir thoroughly and wait 1 minute.

 e)    Remove the floating plastic from the water and put it into the sunflower oil.

            f)    Then, take the floating plastic out of the sunflower oil.

            g)    Repeat Step #2, three times

3.    Recording Data

   a)    Calculate how much of the percentage of plastic was separated successfully. (Meaning that it was a definite identification.) Write down anything unusual or unexpected that happens throughout the experiment.


The original purpose of this experiment was to find a way to separate different types of plastics by their densities.

The results of the experiment were, that 33% of the plastic was successfully separated. So therefore, the hypothesis was rejected. But, the experiment was still a success because now recyclers and plastic corporations will not falsely use the “liquid density separation” method by getting plastics mixed together.


My hypothesis was that varying the density of the liquid bath could effectively separate at least 90% of the six major polymers, at the same time.

The results indicate that this hypothesis should be rejected, because the method only separated 33% of the six major polymers. Also, I believe that if a plastic or recycling plant specifically needed to separate 2 or even 3 different types of plastic, (unless they were extremely close densities like Polyethylene Terephtalate and PVC or, HDPE and LDPE) this method would work well. Even better if one plastic was less dense than water and the other more dense than water, then all you would have to do is dump the plastic into water then stir it up and skim out the ones that float, and the remove the ones that sink. That would work successfully.

After thinking about the results of this experiment, I wonder if there is another liquid that I could have used instead of a salinity solution that is denser than water.

If this project was to be conducted again, there would be many important changes that would need to occur. There would need to be much more researching prior to the experiment. Also, there would need to be more trials with more plastics and more liquids. These would make the project much more successful. 

Researched by ----- Conner O


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