plastic containers

[Sam StonesParticipant @samstones42903]

I have been watching this thread, and believe that you may find the following abbreviated notes of use when determining the type of plastic being used for food packaging containers. If you need further clarification, feel free to ask. Please note however, that what is being used in Australia may not necessarily reflect upon the markets in your part of the world.

A large portion of dairy products, especially ice-cream, are packaged in various specially developed injection moulding grades of polypropylene (PP). The soft, pliable lids are often injection moulded in low density polyethylene (LDPE).

Many containers are vacuum-formed (moulded) from acrylonitrile butadiene styrene (ABS) sheet, while high impact polystyrene (HIPS) can be found in the form of injection moulded yoghurt tubs. Most plastic bottles requiring high burst strength are made from a material called polyethylene terephthalate (PET). Some containers are made by injection moulding plastic around waxed-paper inlays, so be aware of this as a possibility.

Take-away containers are usually injection moulded in polypropylene homopolymer (PPHP), and unless they have been heavily coloured, they will display either good see-through clarity or what is termed contact clarity.

For your convenience, I have extracted the following Resin identification codes from Wikipedia. The number is usually inserted in the centre of a triangle of arrows indicating their re-cycling use. :-

1 – PET (Polyethylene Terephthalate)

2 – HDPE (High Density Polyethylene)

3 – PVC (Polyvinyl Chloride)

4 – LDPE (Low Density Polyethylene)

5 – PP (Polypropylene)

6 – PS (Polystyrene)

7 – Other (eg. ABS)

If you are unsure, especially if the sample does not carry an identification code, an easy method for determining if the materials is styrene-based or is a polyolefin such as PP or polyethylene (PE), is as follows :-

Snip a small piece off your sample, and slide it into very slightly soapy (wetted) water. Make sure there are no bubbles stuck to the surface. If it floats, it is most likely to be one of the polyolefins such as PPHP or PPCP, or it could be either HDPE or LDPE. If it sinks then it could be one of the styrenics, OR a polyolefin filled with one or other of a finely powdered mineral.

From the buoyancy test (ie. indicating a density less than water), if it flexes easily and springs back, it is probably PP, but if it lacks this elasticity it is more likely to be HDPE. If it is really soft, then this usually indicates LDPE.

Either of the two polypropylenes are handy materials if you want to produce a light-duty hinge. (Ask me how to do this if you need more information.) Polypropylene copolymer can be identified by carefully folding or indenting a thin (or thinned) sample, and noting if the stressed area becomes lighter (blushes) in colour.

If you plan on using any of the above materials as a source of sheet for model making, you will find that the styrenics (ABS and HIPS) and vinyls (PVC), are readily glued or solvent welded. But test the results on a piece of scrap material first.

Conversely, the polyolefins are quite difficult to glue, without the use of some oxidising primer, or a very briefly applied but very low intensity oxidising flame.

PLEASE TAKE CARE WITH ANY OF THESE METHODS & MATERIALS!

Some of the fumes could be toxic, while some of the materials can ignite fairly quickly.

 

Have fun.

 

Regards,

 

Sam

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