Polyethylene is available in several different types. Some are made for injection molding, others for extrusion, others for blow molding and yet others for rotational molding. Beyond being designed for a specific manufacturing technique or process, the polyethylene resins further differ in density and structural strength. While strength of the wall differs greatly from type of resin, the chemical formula and chemical resistance remain largely unchanged. Let’s take a look at the types of polyethylene and their applications.
Common Types of Polyethylene
The most common types of polyethylene are high-density and low-density. When you think of plastic you see in your everyday life, like milk jugs, laundry detergent bottles, and plastic toys, you’re thinking of things made of high-density polyethylene (HDPE). This material can be used in tank manufacturing, because it is a strong material with high chemical resistance. Unfortunately, because HDPE is a thermoplastic material, it tends to be notch sensitive and can unzip in tank applications.
Low-density polyethylene (LDPE), used in plastic bags, plastic wrap, and snap-on lids, is also highly chemical resistant and is a thermoplastic material (It can be melted and reformed into something else). Linear low density polyethylene (LLDPE) is a tougher substance and can be used in tank manufacturing. LLDPE has a narrower molecular weight distribution than LDPE.
Medium Density Polyethylene
Medium-density polyethylene (MDPE) is a material in a class of its own since it represents the transition range between LDPE and high-density polyethylene (HDPE). Typically, LDPE’s density ranges between .91- .95 grams per cubic centimeter, and high density starts at .954. MDPE is that transition range in which they overlap. Poly Processing’s OR-1000™ antioxidant barrier is made with MDPE. MDPE is a thermoplastic resin and therefore does not have the structural backbone to create chemical storage tanks.
LLDPE vs. Medium Density
Some people choose LLDPE to mold smaller parts with because it exhibits a lot of the properties of high-density polyethylene - like puncture and impact resistance as well as being a thermoplastic material. This makes LLDPE a good choice for certain applications such as blow molded drums and small totes. MDPE and LLDPE materials are resistant to many chemicals but are not structural enough to use for manufacturing tanks.
Cross-linked polyethylene (XLPE) is made from high-density polyethylene (HDPE) that has an additive package compounded into it which results in the polymer chains being bonded together and thus a more robust product.
A standard HDPE tank can fail catastrophically, or unzip due to the polymer chains not being crosslinked together. XLPE is a much safer storage solution because the plastic cannot “unzip,” it is thermoset material.The cross linking happens on a molecular level, so any failure happens at one of the millions of cross linked spots, minimizing the potential risk . Since XLPE is a thermoset plastic, there is significant improvement in the material’s mechanical properties, providing enhanced chemical resistance, heat resistance, structural integrity, and longer life.
Ultra High Molecular Weight Polyethylene
Ultra high molecular weight polyethylene (UHMWPE) is used in many industrial applications including automotive parts and bottling. It is a hard material, so it can be used as bushings in machines, medical implants like joint replacements, and different types of wires and cables.
Here at Poly Processing, we are innovators of engineered XLPE resins due to its superior strength and durability, as well as its chemical resistance.