Actually, it's not a simple plastic. Army issue bottles are made from HDPE - High Density Polyethylene. It's a porous material, which explains why plastic bottles/canteens tend to retain the odours of the liquids placed in them. This issue can readily be fixed by filling them with hot water and either a tablespoon of lemon juice or baking soda. Swish everything around, let it sit for an hour, rinse, and the bottle is clean. HDPE has a low resistance to heat and it's not a very good idea to fill army canteens with hot liquids. It effects the molecular structure and you end up with a very hot, very soft bottle.
Modern canteens such as the Nalgene brand are made of Lexan, which is a polycarbonate that is extremely impact resistant. Nalgene has a very high resistance to heat and does not retain odours.
I have both types, a CF issue canteen and a 1 liter Nalgene bottle. I prefer my canteen. It's cheaper, tougher, and it's not even worth a third of what the Nalgene bottle is. I just have to be careful not to put anything in the army HDPE bottle except for water.
I hope this helps!
Adam