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Sorting LDPE and PP apart

  • This topic has 27 replies, 4 voices, and was last updated 2 years ago by Stan.
Haw Santiago hawsan

Sorting LDPE and PP apart

31/05/2019 at 17:04

Hello PP Community,

I am about to separate shredded LDPE and PP. Is there a household way to do it without going thru the vegetable oil float test? I am trying to avoid it as much as I can as it will be another process to clean the oil afterwards.

Background: These plastics have no markings on them. I did my best to separate them visually and by feel. So my plan was to shred them all together (by color) and then sort it out through the float test and/or other means. I am half way there as most of it floated with the water test (HDPE, LDPE, PP). Next will be the alcohol test, which will separate the HDPE. Which leaves me with the LDPE and PP combo.

Hoping to hear some advice soon.

Many thanks!

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31/05/2019 at 17:10

Well, there is always:

plastic recycling with nir-spectroscopy

31/05/2019 at 17:13

31/05/2019 at 17:14

I think that is out of the “household” way I was talking about 😀
Thanks for the tip though. I guess that is one thing to look at.

31/05/2019 at 17:17

There must be a DIY way to do this also, we indeed don’t all have an ‘industrial’ budget…

Any ideas out there?

31/05/2019 at 21:15

I wonder if the Shore D hardness can be used to reliably differentiate between the two. Shore D durometers are available on-line for $30-$40. You would want to do this before you shred….unless you have a lot of time on your hands

The April 24 post in this thread http://onearmy.world/community/forums/topic/sorting-plastic-with-robotics-v4/page/2/ has a link to a paper on building a DYI Raman spectrometer with an SLR and a green laser pointer. Interesting but still more involved than most DYI projects.

31/05/2019 at 22:02


Some quick research says Shore D testing should work:


At least for LDPE and PP…


I’ll see if I can get my hands on one…

31/05/2019 at 22:20

Okay, ordered a Shore D durometer on Ali.

€18,25 delivery within 2 weeks…

Will also test the other types once/if it arrived.

To be continued…

01/06/2019 at 05:50

@s2019, thank you for this tip. This seems really helpful.

, I am currently ordering a durometer shore D online and will test it in the next couple of weeks when it arrives. Hoping we could share feedbacks soon.

Update: I just did the alcohol float test. Almost all of the plastic sank, which means all of it was HDPE with just a few specks that floated.

01/06/2019 at 06:43

@hawsan ,

Ever thought of doing the (alcohol) testing in a sonic cleaner ?

Would wash and sort at the same time…

01/06/2019 at 08:13

Hi @hawsan – where did that “float test” table come from? Did the original source define what “alcohol” actually meant? And what type of “alcohol” did you use? (eg. methanol, isopropyl alcohol, vodka?)
All have different densities – and PP would sink in the first two, but float in vodka.

01/06/2019 at 08:40

@frogfall I got it from here at PP. I see, I used denatured alcohol as that is what is available to me in volume. I guess I am not in the clear just yet. Thanks for the heads up.

01/06/2019 at 08:43

@donald, looks like a two-birds-one-stone kinda thing, I might look in to it as well. I will test the durometer first as it is cheaper in the long run compared to alcohol.

01/06/2019 at 09:19


Mmm, interesting thought…

pure alcohol – 40% alcohol (vodka) – water – salt water

This might be the whole spectrum of densities needed (with some mixing and matching) to build custom float tests.

Salt water would replace the glycerin in this…


With a simple still you could always “seperate” the ingredients again…


01/06/2019 at 09:23


True for the big pieces, a durometer would be more economic, but for the smaller particles in this case alcohol is the sollution!

01/06/2019 at 10:38

@hawsan @donald Ha! I guess that’s another problem with “helpful” data – it may be created with good intentions, but it still needs cross-checking for accuracy and ambiguity. I’ll have a look at the original post later today…

Another problem with float tests is that they will only be valid for “pure” polymers. Some  polymer products have fillers – in various proportions – which affects their density.

Nb. Fillers will also affect durometer measurements – so that must also be taken into account if trying to use hardness as an extra differentiator.

01/06/2019 at 10:53


True, that’s one of the reasons I ordered a durometer for myself: Can the results be repeated in a home setting…


I’m a big proponent for only using plastics ‘you know’, starting at the collection fase.

1 is waste, 1000 a resource.


There is no such thing as ‘pure’ when it comes to recycling, and the only guarantee of consistancy you can get is by using the same source material over and over again.

Or keep contermination in mind when designing your products, and don’t use them for anything ‘mission critical’ or ‘dangerous when failing’.

And of course always wear your gas mask when heating…


Main challenge is to keep as much plastic as possible out of the incinerator, as it remains ‘carbon neutral’ as long as it isn’t burned.


For my garden tiles ‘close enough’ is good enough.

For the serious stuff, go Nir!





01/06/2019 at 18:53

Did a little search across the forums, on the word “float”.
Found the following threads:

Plastic Sorting- A personal project (16/10/2015)  (Good long thread)

Separation by density (06/04/2017)  (Another useful thread)

HDPE vs PP (and vs all the other types 🙂 ) (13/02/2018) (interesting video)

sort HDPE and PP (15/02/2018) (where the table came from – but with no references)

Our new machine to sort plastics (27/02/2018) (This looks really cool – maybe time for an update)

As @donald has talked about in other threads – there is a lot of useful information on this site, but it can be difficult to locate the most useful threads on any particular topic.  The search function does work (as the above links show), but often it is a matter of chance (and patience!) that allows you find what you need/want.

01/06/2019 at 19:56


Thanks, this really helps!


I’m working on a concept for ‘best practice’ topic posts.

Index topics like @markbertbach ‘s:

V4 All about sheets – Index topic
are really helpfull to help people find their way on the Forum.

No searches, just follow the rabbit down the hole…


You CAN edit topics after adding them, just make sure you get the title right the first time as this will remain part of the URL (and don’t use symbols like ‘-‘, they make it harder to link to the topic).

Just change the topic and reply with an ‘topic updated: added funny cat video’ to inform the feed…


And please be sure to always close the topic when it moved on, with a link to the new Topic.


But wait for it….

We ALL are going to do this on our “Welcome! Meet eachother” personal page, right?

Starting by saying “Hi, I’m <insert name> here, let me introduce myself, I’m….etc.”

And changing this topic to include links to whatever you are involved in.


People can talk to YOU in the replies, and keep it on topic in your topics (like I’m NOT doing right now).


Or basically @hawsan , what I am saying is:

Take the best info out of the replies to your topic, and make them part OF the topic. In the end the Topic should say it all, and the ‘replies’ are just the comments, if people want to know how you reached your conclussion.

And don’t worry, if e.g. Nir was not usefull, ignore it.

You started the topic, YOU know when the replies are usefull

If I don’t agree (I do agree) I can start my own damn topic!

<breathe out>



03/06/2019 at 11:24

(just linking some topics for the AIs and those interested)

Plastic Sorting-A personal project
also has some great inputs on the subject.

03/06/2019 at 11:33

Wow, these are really helpfull links. Thank you.

I may not be able to fully absorb everything as i am currently out in the field at work. Will read these thoroughly when i get back.

I do my best use the search before posting. I even read the sticky about before posting. I must admit the searching for the word “float” slipped my mind; and for that i apologize

03/06/2019 at 11:47


Just glad we can help.


I believe the forums tag cloud is not mentioned in the Before posting a question topic, It could also be very useful as the search mainly seems to focus on replies.


And feel free to also ask me any questions directly by PM.

Data mining is my ‘field work’ 😉




13/07/2019 at 20:49

@donald , @hawsan Did the shore D durometer method work?

13/07/2019 at 21:12


Mine got canceled and I have not yet ordered a new one.

14/07/2019 at 10:36


I got mine earlier than expected and used it right away. Unfortunately, it is very hard to get a consistent reading out of it so I no longer use it. I turned back to the float on oil method and it works.

14/07/2019 at 20:01

@hawsan , thank you for the reply. Can you say what caused the inconsistent readings? I would guess the samples need to be well supported under the probe tip. The difference between PP and LDPE should be large, was the range if the inconsistency larger still?

Thank you for doing the experiment.

15/07/2019 at 07:42


From what I can remember, you need a thick sample to test the durometer. Around 3mm I believe. But most plastic canisters are less than 1mm thick. You can stack it up. Also, you need to take a couple of reading on different areas of the plastic and average it out. The reading are very inconsistent that I abandoned the idea.

15/07/2019 at 18:42

Thank you, When it doesn’t work and why, that’s useful information as well.

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