What is the Difference Between LLDPE and LDPE?
Low Density Polyethylene (LDPE) and
are both inexpensive polymers with widely favorable mechanical and chemical resistance properties. Tubing made from both polymers is broadly used, particularly for water, chemicals and gases. Unlike with many other plastics, plasticizers are seldom necessary to obtain flexible products, such as tubing. Both plastics are highly stable with low toxicity. In fact, many grades can even be used for food-contact and medical applications.
LDPE is a homopolymer constituted by ethylene monomers. LLDPE is a copolymer of ethylene and another longer olefin, which is incorporated to improve properties such as tensile strength or resistance to harsh environments. One of four α-olefins (1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene) is commonly polymerized with ethylene to make LLDPE. The amount of the α-olefin is typically low compared to the amount of ethylene.
Which Makes Better Tubing?
LLDPE tends to have greater environmental stress-crack resistance ESCR than LDPE. It has been reported (Wypych, G., 2003) that outdoor LDPE pipes are readily affected by environmental stress cracking. It is stated that the resistance of LDPE can be “improved by a substantial addition of LLDPE (30-40%).” LLDPE also has a higher tensile strength than LDPE and greater puncture resistance (Robertson, G.L., 2006). Since LDPE is a weaker tubing than LLDPE a thicker wall grade can be chosen to compensate, but this has cost implications if a large amount of tubing is required. Flexibility is also affected negatively by a greater wall thickness.
LDPE also has advantages as it is more transparent than the other (Robertson, G.L., 2006), which may be advantageous if visualization of the conveyed fluid is important. It also tends to be more flexible. The performance of LDPE can be greatly improved by incorporating it into a two-layer tube. A flexible polymer such as EVA can be used as the outer layer, while the chemically inert LDPE makes up the inner layer. We have taken advantage of such a “co-extrusion” in our Pharm-A-Line VI & Pharm-A-Line XL Polyethylene-Lined EVA tubing.
For most applications the one’s tubing is preferred, as it is stronger than LDPE. LDPE may often be chosen where flexibility is a factor or if a more transparent tube is needed. LDPE performance is greatly improved when it is used as an inner layer with a more flexible polymer as the outer layer.
Robertson, G.L (2006). “Food Packaging, Principles and Practice”, Boca Raton: CRC Press.
Wypych, G. (2003). “Handbook of Material Weathering” 3rd ed. Toronto: ChemTec Publishing.