What are blends of polypro and polyethylene used for? What physical properties of these copolymers make items useful?
Polypropylene/polyethylene copolymers tend to have better _____ then _____, and find use in _________.
Can someone fill in the blanks for me? They don’t have to fit perfectly, I just want to know what advantages blending these two plastics have as well as some items made of the blends.
I can’t tell you about the specific properties, but I want to clarify that a copolymer is not simply blending to polymers together. A copolymer is where the two monomes (in this case propylene and ethylene) are polymerized together so that a single chain contains both monomers. This can produce different properties from simply blending, although it may not in this case.
I have more to discuss that elucidate some properties, but it would be off the top of my head and I haven’t the time to do any fact checking.
They hold up well over time and hold a small tolerance for one of the cheapest costs. The chemicals that exit the plastic during heating and molding aren’t as destructive as many other plastics, and they release well from the molds. Other materials are used when a stronger material, higher tolerance or special property is needed. I haven’t dealt with plastic specifications since 2002, so I can’t give you more specifics.
Polyethylene has a low melt, it’s crystalline, it has a critical surface tension about like hydrocarbon waxes and long alkanes (indeed that’s practically what it is). It’s nonpolar, and if you orient it in an electric field it stays neutral.
Polypropylene is similar. But the melt is a little higher, it’s not so crystalline, and it can be somewhat polar. Orienting it in an electric field and cooling it there will turn it into an electret, like “Filtrete”.
Polyethylene and polypropylene are polyolefins. Put CH3 side groups along a polyethylene molecule and you get polypropylene. If you get them all on one side you get isotactic polypropylene, and if you alternate them you get sindeotactic, and if you get them sort of randomized you get atactic, IIRC.
Mixing the two gives you a blend of two polymers. Polymerizing them together into long molecules containing both gives you a copolymer. Christopher is correct. There are block copolymers which have long stretches of one polymer and then long stretches of the other. I forget what you call the other extreme, but in the particular case of polyethylene and polypropylene I don’t even know if it counts, because you’d call it polypropylene if it had the side groups and polyethylene if it didn’t, so parts of polypropylene are actually polyethylene, unless there is a numerically more specific approach than I know.
Copolymers are, I think, known for having intermediate properties and for being more elastomeric than their components. I think block copolymers can become semiplastic at temperatures between the two homopolymer melts.
The University of Minnesota is known for its work on copolymers.
Polypro is one of the most chemically inert plastics. Almost every piece of wetbench equipment in the semiconductor manufacturing industry is made out of it. It’s also almost impossible to glue, so it needs to be hot-air welded to assemble.
So what are some items that would be made from a copolymer of PE and PP? Can you provide some specific properities that would be advantageous from such a copolymer? Are they more flexible then plain PP, or PE? Are they more easily moldable than one or the other? Are they more chemically inert or more reactive? I’m pretty clueless on why one plastic is selected over another…even more clueless about why anyone would want to use such a copolymer.
Thanks for all of your input so far.
I’m a little vague on this but think one motivation for using a block copolymer has to do with getting two different kinds of deformation mechanics. For example, at temperatures between the two melts, the polymer could deform like a filled paste, a suspension of solid particles in a viscous liquid.
Polyethylene and polypropylene are pretty similar as it is.
A copolymer of hexafluoropropylene and fluorinated ethylene propylene and vinylidenefluoride has new properties - it’s Viton, an elastomer that has excellent resistance to swelling from organic solvents. It can be used as a paint in solution, and there’s some mechanism for getting it to crosslink after drying (if I remember right, I’m going back 20 years on this). So, it’s dissolved in a solvent, you spray it on, the solvent evaporates, and I think there’s a blocking agent, an ingredient that prevents crosslinking, and that ingredient dries out too, so being dried for a few days gets it to crosslink. Now that it’s crosslinked, it’s no longer soluble. None of the 3 polymers that go into it act this way.
But PE and PP are already pretty similar. Not sure what a copolymer of those two would get you.