Ok, I have recently done some experimenting with mixing LDPE and sulfur, and to my suprise, the result is a very tough, stiff material that is much much stronger than the original LDPE. This stuff is amazing, easy, and extremely cheap to make. I have updated the instructable to include the steps for making this LDPE sulfur plastic.
If you use no LDPE, the sulfur comes out brittle and crumbly, so it is useless alone. However, you can mix it with aggregates such as sand and gravel to make a very stiff and fast drying cement.
Step 1: Materials
1. Sulfur- can be powder or pellets or anything that is plain elemental sulfur. You can get this at a farm supply store. I got 50lbs for $30. It can have impurities up to maybe 20%. More impurities equal less strength. If you don't want large quanities, buy some from here>http://doitbest.com/Main.aspx?PageID=64&SKU=722367&utm_source=Froogle&utm_medium=FREECSE&utm_term=722367&utm_content=6790&utm_campaign=DATAFEED
2. Aluminum foil- optional- I got this at Cosco for $15 for two rolls. This comes out to be 5 pounds of aluminum. If you don't want large quanities, get a roll from the 99 cents store. This is used as an aggregate.
Edit: I found Aluminum foil to be a great aggregate, but you can use other things like rocks or sand to make it stronger. Also, try painting the sulfur directly onto some fiberglass and layer it like a surfboard. I find that makes for a very strong sheet of plastic. Just make sure you get the sulfur down into the cracks because it dries quick.
3. LDPE - get this either from recycled plastic, such as milk bottle caps and other flexible lids, or online here>> http://www.birchplastics.com/
or here>> http://www.amazon.com/LDPE-Density-Polyethylene-Length-White/dp/B0013HL0OY/ref=sr_1_7?ie=UTF8&s=industrial&qid=1267304312&sr=1-7
this comes out to about 7 cents per cubic inch, or 2 dollars per pound.
75% LDPE + 25% sulfur- about 1.70 per pound
25% sulfur + 75% aggreagate (sand and rocks)- 0.15 per pound
**all percentages by weight***