I appreciate this is a bit of a woolly, open-ended question, to which there might be a lot of maybe and it depends-type answers…
Let’s say I want to recycle the lids of plastic bottles, some of which are HDPE, others are Polypropylene. There’s sufficient overlap in the melting temperature ranges for these materials to be able to work with them together without destroying one or the other.
So I grind up my (lets say 50/50) mix of HDPE and PP into granules and feed it into an injection moulding process. What are the likely qualities of the resulting material? Will the two materials fully blend, or will I end up producing items composed of nonmixed islands of the two materials, prone to breaking apart at the joins?
I think this works fine, and you get a heterogeneous solid with more or less fine grain. Assuming the plastics are completely thermoplastic and not crosslinked, you can also put lots of shear on the melt to draw out the molecules and get more intimate and fine grained mixing.
There is a lot of literature on copolymers, which have long molecules different sections of which are different homopolymers. I would think they would be somewhat like what you describe. I am a bit familiar with them, though not with blended homopolymers.
How they mix depends on how soluble they are with one another. A Napier says, you will more or less get a biphasic plastic with differing grain boundaries depending on how soluble they were at the temperature they were mixed and the rate at which they were cooled. Extrusion is one way to align and stretch the grains for a better mix. (Extrusion is one method of adding high shear to the process.)
I suspect PVC would be a complete fail on mixing with either HDPE or polypropylene.
Many lids are LDPE. I haven’t seen any data on it for a long time , but a quick search showed it is being done. The key may be keeping the additives to a low percentage. I know 30 years ago when I was managing a PVC compounding operation even passing contaminated scrap through a Ferrel Continuous Mixer was not enough to blend polyethylene into the PVC. I have no data to indicate that none of the PE blended in, only the observation that there were large pieces of PE imbedded in the PVC. That was including passing through a 3 layer screen pack with 40 or 60 mesh screens. The 1/8’’ PE pellets or chunks would pass through the screen pack and reform when extruded. Rubber would do the same.
Such contaminated material wasn’t suitable for extruding as wire coating which was our main business. Another division of the company produced wiring harnesses with molded plugs. They had a number of small plants. One of the the plants was very successful using reprocessed scrap. I think the equality of their management may have been the reason they succeeded at what the others couldn’t. Another factor was that we kept their scrap separate from the rest and it may have started out with less foreign material in it.
Keeping scrap material clean is a difficult concept to manage both in an industrial plant and the general public. I know the company I worked for could have saved tons of money if they would have segregated their scrap so it could have been fit for reuse.
I asked my sister if her company ever mixes plastics and if not, why not. The company she works for makes mostly lids but also makes jars. This is what she said:
"We don’t mix resins because they process differently – different flow rates, different chemicals.
LDPE is more soft. HDPE more rigid.
Polypropolene and co-poly are almost the same and we do mix them. The co-poly is more flexible.
It also gets into all sorts of regulatory reasons as to why you don’t mix resins. Pharmaceutical companies don’t like stuff like that."
These are all high-quality and informative answers. Thanks folks.
(for the record, I have an embryonic idea for making my own injection moulding machine and eventually experimenting with the recycling of assorted plastic items - mostly bottle tops - that appear to be either HDPE or PP, but are not marked with the resin type symbols)
Hey everyone, I’ve been searching for this answer but no luck.
Anyone know if I can mix PP and PE black plastic pallets together?
I also have HDPE and PE plastic pallets, if I regrind them and mix them together, will there be an issue with the regrind or should I just sort them separately?
Hoping to get some answers or any one come across this?
Hi green, welcome to the dope. You must have found this old thread through a search engine, right?
You will probably get a plastic with somewhat variable and perhaps poorer mechanical properties by blending the plastics you mention, but it might be fine. How would you do this? Are you subcontracting the processing or do you do that at green city plastics? Do you have a vendor you can ask?
If I were I the pallet molding business, I’d go ahead and try it and figure 2:1 odds it will work kinda ok.
Hello,
We have part form pallets we are looking to grind up and there’s one to many to sort through. We will be shredding them. Are you in the plastic industry yourself?
I did some polymer studies at university, but it was > 30 years ago. My dim recollection is that in general co-polymers are not a great success. They were tried as a way to hopefully combine the advantages of two polymers, but what you tend to get is something which combines the drawbacks of both and the advantages of neither. Notable exceptions to this trend include ABS and SBR
If you look in the Plastics Encyclopedia or on websites, you’ll see that there are plenty of polymer blends. In fact, they need not even be miscible to be useful. There’s a (rather brief) Wikipedia page on it:
Picked this forum up on the net a hope someone will be able to help me.
I run a plastic factory that mainly used HDPE and LDPE with a small amount of LLDPE. There is a large amount for scrap that comes from the products we make and I am looking at capitalizing on the scrap by making other products.
At the moment the product I am looking at is made out of PVC. The machine that makes the product is a co-extruder so could give us two layers. My idea would be to run a PVC layer on top as its needed for the product and then run the HDPE/LDPE layer underneath to give it the strength needed and in doing so use up my HDPE scrap.
Both products have similar melting points. Do you think it would be possible to extrude the two and get them to form one layers by bonding together?
The first place to start will be the coefficients of expansion for the two materials. If they are very different, one may pull away from the other, split or distort, as the materials cool.
I’m not a plastics expert, but I deal with various polymers with some of the products I’ve designed.
My first guess would be that PVC and PE will not bond to each other very well, even when co-extruded. PE is “low surface energy” and doesn’t adhere to PVC very well. Think of Hot Glue - it will stick to PVC, but will snap off if cooled.
I’d talk to a plastics guy before going through all the effort to set this up.
Just my thought. The problem is getting some cross-linking between the two. Someone here on the Dope pointed out the new glues which use a solvent to increase the ‘surface energy’ of PE. I’ve seen a little more about this and it is supposed to allow cross-linking between PE and other plastics, but I haven’t found many details. I suspect two smooth surfaces won’t bond for a very strong connection but if the edges are not exposed separation may not be a great problem barring the problem** Mangetout** points out.