Not all high-density polyethylene tanks are created equal. Although they sound nearly identical, linear polyethylene and cross-linked polyethylene (XLPE) chemical storage tanks have significant differences. Those differences could impact your organization’s bottom line, your people’s safety, and the operations of your facility.
Both linear polyethylene and XLPE tanks are made with heated resins to create a cured plastic. However, the differences in their production create very different polyethylenes with very different structural strength. Let’s take a look at the manufacturing methods and see how XLPE is significantly different from linear polyethylene.
Manufacturing Linear Polyethylene
Linear polyethylene is created when thermoplastic resin pellets are ground and then heated to create the fluid plastic that will harden and cure into a linear, high-density polyethylene surface. The molecules are connected like a rope—the individual threads of fiber are twisted together, but not tied.
Linear polyethylene is cheaper, and safe for storage of benign and non-corrosive chemicals. For certain chemical storage applications, these tanks do the job nicely.
Manufacturing Cross-Linked Polyethylene
Cross-linked polyethylene is high-density polyethylene that is manufactured by adding a catalyst to the thermoplastic resin, which turns it into a thermoset. The catalyst causes covalent bonds that link the molecules together. Picture a chain-link fence where the metal links are actually bonded or welded together.
The result is a plastic that possesses impact resistance, tensile strength and resistance to fracture that linear polyethylene can’t match. These qualities make cross-linked polyethylene an excellent choice when tank integrity is critical. The chemical resistance, heat resistance, and dimensional stability are unparalleled.
Differences Between Linear and Cross-Linked Polyethylene
Both polyethylene resins are rotational molding-grade resins that have been ground into a powder to allow the material to melt easily during the molding process. Both resins are available in standard colors as well as some custom colors. Both resins are corrosion-resistant polyethylene.
However, the differences are critical. Compared to linear polyethylene, XLPE provides:
- 20 times the environmental stress crack resistance
- 10 times the molecular weight
- 3-5 times the impact and tensile strength
The implications of these differences are most obvious when testing the two plastics. When linear polyethylene fails, it fails catastrophically, because the linear polymer chains “unzip.” The entire tank comes undone, and a small leak becomes a massive chemical spill. All of the stored product is lost, and the cleanup is considerable.
On the other hand, cross-linked polyethylene might develop a small pinhole or tear that pulls the elasticity, but you’ll never have a catastrophic failure. You aren’t likely to lose all of your stored chemical if a leak develops, and the cost of cleanup and replacement is significantly less expensive. The risk to your employees and the environment is drastically reduced, as well.
Poly Processing’s drop and pressure test videos offer an excellent demonstration of the differences between linear and cross-linked resins.
Because cross-linked polyethylene offers longer tank life and more protection in the long run, it may require a little more investment upfront. However, because XLPE has a significantly longer life and less financial impact in case of failure, the total useful life of cross-linked polyethylene is significantly better for your bottom line.