https://www.sciencedirect.com/science/article/abs/pii/004060...
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Anecdotally I've probably touched an open flame to synthetic rope, milk jugs and maybe a bucket or two in my life but can't say I've ever seen any of these items react in an uncontrolled or dangerous way.
But the amount of BTUs in, say, packaging, is negligible, because it's so lightweight.
When you combine lots of recycled packaging into dense bricks, it becomes a lot of dense BTUs, so can provide a lot of fuel to a fire.
You're not going to cause a runaway with a lighter and a pop bottle without working at it. But, say, an electrical short in a closet with hanging synthetics can take off pretty quickly.
That is key. A lot of things make a hell of a flame when you supply them with pure oxygen.
"These are high density polyethylene, used in milk and shampoo bottles; low density polyethylene, often used for bags for cerals or sandwiches; and polypropylene, used for ropes, flip-top lids and buckets."
Are cereal bags, ropes, shampoo bottles and buckets highly flammable?