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I'd like a reusable lightweight cup for the microwave, so I compare the melting temperature of plastics (#1 through #6 in our recycling codes) and would think Polyethylene Terephthalate (PET) is the best

So, why is Polypropylene (PP) predominantly used instead? I would even guess PET is cheaper to manufacture since most one-time bottles are PET. Is it really just a public perception preference for PP?

closed as unclear what you're asking by rumtscho Jul 12 '14 at 11:47

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  • It most often called PET (one obsolete term is PET-E, but not PETE) – TFD Jul 11 '14 at 23:43
  • Cheap PP and PET cups are made by partially punching circles in a continuous sheet of raw plastic, and then these circles are heat punched into cup shapes. The vertical stress lines on PET (a harder material) will open up with hot liquids. PET will also start to deform above 90°C. Otherwise, yes PET is a better material :-) – TFD Jul 11 '14 at 23:51
  • I closed this as not answerable on a Stack Exchange site. We can answer "why" questions as far as they concern the mechanism of work of some natural process, e.g. "why does wheat bread rise better than rye bread". But asking why a person (or an organisation) has chosen to do something has no real answer, unless that person discloses their reasons. Asking why the person does not do something is even less answerable. It can start a discussion, but it cannot lead to an objective answer. As you see, the best attempt only includes guesses - it might be a little worse suited for cups. – rumtscho Jul 12 '14 at 11:53
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I'd guess there are many factors. For instance, in manufacturing, small differences in raw material cost and process requirements can have a big impact.

However, in this case, I'd say it comes down to temperature resistance: polyester (PET or PETE) does not do well at high temperatures, PP does. A reusable cup for microwave use is going to have hot (probably boiling) liquid in it.

From Plastic bottle - Wikipedia:

Polyethylene Terephthalate (PET, PETE or polyester) is commonly used for carbonated beverage, water bottles and many food products. ... This material does not provide resistance to very high temperature applications—max. temp. 200 °F (93 °C).

Polypropylene (PP) is used primarily for jars and closures and provides a rigid package with excellent moisture barrier. One major advantage of polypropylene is its stability at high temperatures, up to 220 °F (104 °C). Polypropylene is autoclavable and offers the potential for steam sterilization. The compatibility of PP with high filling temperatures is responsible for its use with hot fill products. PP has excellent chemical resistance, but provides poor impact resistance in cold temperatures.

Here's an additional source: The 7 Most Common Plastics and How They are Typically Used. In summary: PETE is easily recyclable but breaks down under both heat and light exposure. So it gets used for one-time-use applications but isn't suitable for reusable containers.

  • Thanks. In summary, though the melting temperature for PETE is higher, it decomposes at a lower temperature. In reading the Wikipedia pages more carefully, I think this is related to PETE being more hygroscopic, so water gets in and breaks it down more easily. (Though this wouldn't help anyone, PETE might be the better choice if we just heated the bottle in an environment without water...) – bobuhito Jul 11 '14 at 23:42
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Depending where you live there are plenty of cheap PET recyclable cups?

Cheap PP and PET cups are made by partially punching circles in a continuous sheet of raw plastic, and then these circles are heat punched into cup shapes. The vertical stress lines on PET (a harder material) will open up with hot liquids

PET will also start to deform above 90°C. Otherwise, yes PET is a better material and easier to recycle

Both PP and PET can be fully recycled, or burnt as hydrocarbon fuel

  • By recyclable you mean reusable? – Cascabel Jul 12 '14 at 0:14
  • Either or. They are washable too, just a little fragile. I have seen a specialist washer machine for PET cups. There seems to be a fair local business in tidy stacker's for used PP cups in offices, they are taken away for recycling – TFD Jul 12 '14 at 0:16

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