I have been recycling hdpe into components for different projects. See some examples in previous instructables.

https://www.instructables.com/id/Making-Dimensiona...

https://www.instructables.com/id/Recycled-HDPE-Mul...

In my collecting of buckets for hdpe #2 , I have also gotten buckets marked #5 polypropylene and many of the bucket lids are #5 also. Then I was given several lounge chairs from swimming pool that were #5 polypropylene. I had been planning to make some bee hive super boxes from the recycled material and decided that I would use the polypropylene buckets, lids, and chairs. I had tested the material and it was all the flowing (injected molding) plastic. The material for the boxes needed to be 19-7/8"x9-1/2"x 3/4" and 16-1/4:x 9-1/2" x 3/4", so I got a sheet cake pan 14" x 22" x 2" that would fit in my oven to melt the recycled plastic.

Step 1: Cleaning the Lounge Chairs

The chairs had been in the sun many years and had a layer of degraded plastic on the surface. This was a chalky layer and felt like fine sand. Many hours were spent trying to remove this degraded plastic, The polypropylene was melted at 350 degrees. The boards were flattened and box joints were cut into the boards. When the joint was screwed together the parts shattered. The recycled material was very brittle and to keep the screws from wedging the material apart the pilot holes were drilled a 1/32" larger than the recommended pilot hole. Also the degraded material does not melt, it forms a layer of "dirt" that will not adhere to the melted polypropylene weakening the boards even more. With the poor results the chairs were discarded since it took so much time to clean and the finished product was still not a useable product.

Step 2: Air Pockets in the Boards

After the degraded polypropylene was discarded, buckets and lids that had not been in the sun were melted at 350 degrees. These boards were lumpy, warped badly, and had numerous air pockets through out the board.

These boards were flattened and cut to size. It should be noted that the polypropylene does not cut well, it melts in the cut and fills the gullets in the saw blade with plastic, also the melted plastic builds up around the blade it will actually jam the blade with the melted material. The material works well with the router producing chips similar to wood and does not adhere to the bit.

Step 3: Raising the Melt Temperture 425 Degrees

I researched polypropylene and found that the production melting temperature was over 400 degrees, I found that it melted best a 425 degrees in my oven. After raising the temperature the air pockets floated to the top and burst, the lumps flowed out level and to reduce the warp the parts are clamped or weighted until cool. You only apply enough weight or clamping pressure until the material just starts to ooze around the press board. If you do too much the press board gets captured and it needs to be sawed free. At the higher temperature the polypropylene forms a brownish oil on the surface that removed during flattening.

Step 4: Melting Technique

The melting technique has evolved since I started trying to use recycled hdpe and polypropylene.

To start with the temperature have changed,

for the flowing (injection molded) #2 hdpe I find that 350 degrees works well.

for the non-flowing (extruded) #2 hdpe I find that 375 to 400 degrees works best it allows for a longer time to press and form the part before solidifying, also at higher temperatures the material move more easily. For thinner parts like bottles and jugs 375 degrees is about right, for thicker parts like toys and basketball bases 400 degrees give better results since the thicker material seems to move less at lower temperatures.

The #5 polypropylene flowing type (injection molded) needs 425 degrees to melt and allow air pockets to rise to the surface. (this temperature also causes the material to separate with a brownish oil on the surface). The polypropylene does melt at 350 degrees but when it cools the material can crumble where there is poor adhesion to another piece.

I do not spend a lot of time preparing the material in small pieces, I cut it so it fits into the container. I have found that adding material in small layer 1/4" at a time, which takes 20-30 minutes to melt and allows the air pockets to surface. Then add another 1/4" of material and repeat heating cycle. The plastic is a good insulator and if too much is added the layer will not melt and the finished product will be laminated with areas not adhering to the next layer. After the piece is build up to the thickness you want cook for another 1-2 hours to allow the air pockets to surface. You can judge visually when there are not any new domes of air forming, the melt is done.

Clamp or weight a press board on top of the material to reduce warping and twisting and let cool. If you do have a warped part the part can be reheated to 250 degrees with weights in place. Monitor and you can see when the material starts to reform, heat for another hour and then let cool. Most time this will flatten the piece.

I have used parchment paper to line the pan so the material does not stick, this is fine at 350 degrees, but it starting to char, 400 degrees is the upper limit for parchment paper. For the higher temperatures I have switched to using silicone grilling mats. I still apply silicone spray to the sides to reduce sticking.

I found out that you DO NOT want to mix #2 and #5 plastic in the same part. The #5 mixes with the #2 and makes the hdpe very brittle and any impact will cause the piece to shatter similar to glass. I have had good results with hdpe #2 for several piece and it has mechanical properties similar to wood pieces. The #5 polypropylene seems to degrade during melting and looses its flexibility and becomes brittle. It works fine for a simple box like the bee hives or bird houses, but I would not use it in a situation where strong mechanical properties are needed.

When you get a new source of recycled plastic take and test some small pieces to see if you have any contamination or degrading of the plastic. If you do discard the material because you will end up spending way to much time trying to salvage something that is not salvageable.

Also remember that cleanliness is vary important, rinse all the bottles and jugs to get the old contents cleaned out. Sometime addition openings are needed to completely remove the contents. Remove any dirt or rust that might be on the plastic.

Step 5: Constructing the Super Boxes

The purpose for construction the bee super box from polypropylene is because a wooden box will rot in 3-5 years unless it is dipped in wax and painted annually. It is hoped these boxes will last at least 10 years.

Since the recycled polypropylene is so brittle it was decided to use reinforced butt joints instead of the conventional box joints. The corner braces are aluminum gutter brackets that were straighten and bent into right angle braces. All pilot holes are drilled oversize because of the brittleness. Since the boards were different thicknesses, I flattened the boards, but did not make them a uniform thickness ( 3/4"). So I built the boxes to the inside dimensions and let the outside be what ever thickness of the boards. Inside dimensions are 18-3/8"x 14-3/4" x 9-1/2" then they will match the other boxes. The box is built on small hive beetle trap base and attached as a unit.