In its simplest terms, the main difference between the different types of plastic is the way their cellular structure, or molecules bond with each other, and how tightly they are formed.
1) Low Density Polyethylene (LDPE), most common type of plastic sheeting, very flexible, most often from 0.5 mil thick to about 40 mil in flexible sheeting forms. Good conformability to surfaces. Because the cell structure is not as strong or dense as some other types of plastic sheeting, it is not typically as strong or puncture resistant as other forms or blends, but has thousands of common uses. Widely used for everything from construction and agricultural sheeting (often called Visqueen), Engineered Plastic Sheeting of countless types for such things as Vapor Retarders (also called Vapor Barriers by many), Surface Protection films, Pond and Canal Liners, Covers, Tarps, Abatement Plastic, Containment, packaging, and the list goes on.
2) Linear Low Density Polyethylene (LLDPE), blended form of LDPE where the film has much more flexibility, tensile strength, and more conformability. LLDPE is "softer" and more pliable, so is an excellent choice for such things as pond liners, or blended into other films to give them extra strength and flexibility. This is perfect for an application like a self-adhesive carpet protection film, where the film needs to have an amazing amount of strength in a very thin film to absorb impacts, but not tear or puncture. In this form of poly, the molecules all line up and strongly hold together as the film is stretched to give the structure much more strength and elongation than LDPE. Most common in thicknesses ranging from 0.5 mil to 40 mil in flexible plastic sheeting.
3) Medium Density Polyethylene (MDPE), the least commonly used form of polyethylene for flexible plastic sheeting. Has more strength than LDPE, a little more chemical resistance, tighter cell structure, more puncture and tear resistance. Often blended with LDPE and or LLDPE to give the attributes one is looking for in a particular type of film. For instance, many pond liners have MDPE to add some strength and toughness, without adding too much stiffness.
4) High Density Polyethylene (HDPE, aka HD), widely used for many applications. HDPE is the strongest, toughest, most chemical resistant, and least flexible of these four types of polyethylene. It also has the most UV resistance of the bunch without additive packages to increase this attribute. HDPE has a very tight cell structure, making it very difficult for other molecules to pass through its structure on a microscopic level. HDPE is the most easily field seamed of these products, and is generally used on an industrial level in thicknesses from about 12 mil to 100 mil thick. Most golf course ponds are lined with HDPE, most industrial ponds and canal liners, secondary containment liners, root barriers, many applications where chemical resistance is needed. This is also used in thousands or maybe millions of applications in thinner forms, especially in blends with the other types of polyethylene, because the HDPE adds much strength and toughness with its very tight bonds with other molecules.
The density of polyethylene is measured in a column of water, and they are all classified depending on ranges of density. LDPE (and LLDPE) are generally within the range of 0.919-0.924 g/cm³. MDPE are generally within the range of 0.926-0.940 g/cm³. HDPE are generally within the range of 0.941-0.965 g/cm³. These numbers can vary slightly depending on your source, and are not set in stone, just general guidelines.
