How can different types of plastic be cheaply and easily identified?

[Johnish]

From my reading various places online it seems that plastic recycling would benefit from a cheap and easy method for identifying different plastics. Resin codes can be used but they are definitely not easy. They require someone to search around on the product to find them. That takes a lot of labor especially sense the codes are in many different places and some times very small. Some products don't have any codes on them at all. An easier way to identify plastic is to use specially made IR-FT spectrometers, but they are definitely not cheap. This limits there use and decrease the economic viability of recycling. Basically it seems to me like right now only municipalities that can afford expense equipment and places with low labor costs can recycle post consumer plastic economically (or at least close to it).

 

Is there a cheap and easy way to identify plastic? Do you think that maybe some cheaper none FT kind of near infra red spectrometer would be good enough to sort different types of plastic? Maybe something that uses an inexpensive photo diode. It seems like a combination of a Lead(II) sulfide photo diode and a silicon one would cover the required wave lengths and be really inexpensive. If not that then maybe try to find an inexpensive Germanium one. As for the rest of the machine I was thinking maybe something with a rotating grating like what was talked about in this video. I've been thinking about how you could keep track of the angle of the grating and so far all I've come up with is put some bright dots on it and use an inexpensive Image Sensor and some software to calculate it. The other idea I've had is to maybe use an Rowland Circle grating like what was talked about on this tread here. Maybe find a way to make some cheap holographic gratings on a piece of aluminum foil and put it into some circular apparatus. For the sensor part I was thinking about maybe salvaging old DVD Drives. They already have a silicon photo diode so I was thinking that maybe if you replaced the laser with a Lead(II) sulfide photo diode it might work. I don't know if the optics in a DVD drive would still work for the weave length involved. I also don't know if the control board in a DVD can be made to work for this. Another issue is that the reader arm moves in a strait line not a circle. I also don't know how big a circle would be needed for the wave lengths involved. Also I don't know how to tell where the sensors are at any given time. Really all this stuff is way over my head. Does any of this have the potential for cheap easy plastic identification, and if not do you have any ideas that do. Thanks.

Edited July 26, 2014 by Johnish

[Sensei]

Bail for all plastic and glass containers would solve all issues straight away..

Company that produced something would receive them back.

 

Majority of glass containers (99.9% of beer) are here with bail ($0.1)

It's pity it's not extended to the all containers.

 

They just need easy way to get ride of paper label, and glue used to attach it.

 

Different types of plastics have different densities:

http://www.dotmar.com.au/density.html

So by measuring mass, and volume, you can calculate density and more or less precisely identify used plastic.

As long as it's clean (no labels, no attached things), and measuring device is precise..

Plastics that have smaller density than 1 cm^3 (density of water) will float in water.

So to find out Polyethylene/Polypropylene you just need to blend plastic and let it float and sink in water.

Edited July 26, 2014 by Sensei

[Enthalpy]

The real limit is not the price of a photodiode, but rather the time it takes to identify every piece of garbage.

 

Density is excellent in a workshop where you have nice cylinders to measure. In garbage, the shapes are too complicated, and also use to trap air bubbles in flotation processes, whose sorting is gross.

 

Same for the flame aspect and odour I like to use. It's accurate, but impractical to sort several pieces a second.

 

In a real situation, one needs really fast methods. Nuclear apparatus that identifies the elemental composition might possibly be made swift enough, but it's expensive. I'd consider firing a pulsed laser on each garbage piece and analyze the fume's optical spectrum. This would also sort out metals and ceramics.

 

Though, there are strong limits to any sorting. For instance, polyamides (polyolefines as well) have extremely varied properties and must be sorted accordingly, but chemically they're nearly the same. The you have all the charges: if Petp contains chopped glass fibers it shouldn't be mixed with pure one. Few per-cent plasticizer change Pva's behaviour radically. Plastics are often blended. They're sometimes painted. Each object may consist of several plastics and is generally hollow, so garbage is generally crushed first to get homogenous compact chips, but these are so many!

 

In the best cases presently, polyolefines and polyesters are separated from the rest, but not with more detail. As a result, recycled plastic is of little interest, but not much cheaper than new one; it provides felt mainly. Some blankets and clothes are made of this felt.

[Johnish]

Thanks for the response and link Sensei. It's very interesting. Thanks for your response as well jjoensuu. Having spent some time recently reading about plastic I've come to appreciate how amazing a material it is. Some types of plastic like HDPE and PP have incredibly chemical resistances which is great for storing and transporting caustic chemical. It's cheap, and durable making it great for food packaging. It's hard to imagine modern life without it. It seems such a shame that so much of it ends up as trash. Some day when fossil fuels are more expense I think people will come to appreciate it more.