Cross linked polyethylene is a synthetic plastic variant made from polyethylene resin. Cross linked polyethylene is frequently abbreviated XLPE or PEX, where PEX is used to refer to pipes and tubing and XLPE to storage tanks. In its simplest definition, cross linked polyethylene is a carbon-based polymer plastic, meaning a connected set of individual repeating molecule units bound together to form a material that can be used to make products.
XLPE belongs to the group of modern synthetic materials known as plastics along with polypropylene, PVC and polyethylene. These materials are the result of further processing of natural hydrocarbons associated with the refining and production of combustible fuels. Crude oil refinery byproducts, especially ethylene gas, are used in the manufacture of many plastic materials including cross linked polyethylene.
Differing from the other plastic resins, to make XLPE, the initial polyethylene base must undergo further reactive processing to form connective bonds known as cross-links between the individual polymer strands of the starting material. These connective cross-links are the reason for the plastic’s namesake: cross linked polyethylene. The majority of polyethylene used to manufacture XLPE is high density polyethylene (HDPE). While in processing and before cooling, HDPE resins are reacted with an organic peroxide that initiates the XLPE cross linking reaction to produce the XLPE polymer product.
XLPE Characteristics and Properties
To be classified as cross linked polyethylene, according to ASTM standards, the peroxide linking process must be performed until polymeric interlinks are greater than 60% but less than 90% of the total PE resin. The XLPE production reaction is a largely irreversible synthesis that forms permanent bonds between the branches of the individual PE polymers.
Instead of stacking together without connection as in PE and other thermoplastic polymer resins, XLPE molecules are joined together by the peroxide reaction to form, in effect, one single large polymer molecule. The crosslinking of the polyethylene resin alters both the base molecular structure and total performance characteristics. Where many other synthetic plastic resins classify as thermoplastics, cross linked polyethylene is classified as a thermoset. Thermoset plastics such as XLPE differ from thermoplastics in that they burn rather than soften when heated. Due to this, thermoset XLPE cannot be recycled for reuse and cannot be repaired by the same plastic welding techniques.
The cross linked structure of XLPE provides different polymeric properties from the other PE resin types. Cross linking results in a higher weight polymer where the internal molecules are packed more tightly together. This in turn produces a general increase in the resin’s range of compatible elevated temperatures as well as its resistance to low temperature impacts, abrasions, and environmental stress cracking. Additionally, cross linking increases XLPE’s resistance to some solvent influences, chemical absorption, permeation and flexural creep.
The melting index of XLPE is also increased over other polyethylene resins. XLPE storage tanks have a maximum temperature range up to 140°F, ambient and process, but are recommended for use below 100°F. XLPE as a plastic resin can remain chemically stable up to 300°F for production. The properties of tensile strength and density are generally unaffected by polyethylene cross linking and therefore are similar to HDPE-made products.
The base XLPE resin is also sensitive to ultraviolet effects, as are many other hydrocarbon plastics, and require antioxidant additives in order to resist long term damage in outdoor applications as well as to some fluorescent lighting.
In terms of chemical resistance, compatibility recommendations are on a per chemical basis, but XLPE is very similar to HDPE and provides a durable, long-term solution to chemical handling. There are a few noteworthy chemical exceptions where XLPE is the preferred option over HDPE and vice versa. To name a few, XLPE is preferred for handling surfactants, acrylic emulsions, and polymer blends, and HDPE is preferred for hydrochloric acid, hydrogen peroxide, and sodium hypochlorite.
How is XLPE Used
The XLPE thermoset cross-linked resin material can be used, handled, and manipulated similarly to the other polyethylene resins. However, XLPE made storage tanks are not capable of achieving FDA, ANSI/NSF standard certification for handling and contact with food grade ingredients and drinking water. XLPE resins can be used for extrusion, blow molding and rotational mold fabrication or manufacturing.
Currently, XLPE is used to produce an assortment of products employed across a wide array of modern applications and industries. The most common applications for cross linked polyethylene include handling containers, bulk chemical storage tanks, PEX pipe for hot and cold potable water delivery, wastewater handling, agricultural irrigation, electrical wiring, cable insulation, components for vehicles, equipment, appliances and consumer products.
Takeaway | What is Cross Linked Polyethylene
Cross linked polyethylene is a manmade, synthesized plastic product that can be modified, designed and fabricated according to modern scenario use needs through set molding and form processing. If looking for the industry’s best cross linked polyethylene handling containers, we provide top quality XLPE products made in the USA by Snyder Industries, one of North America’s leading poly tank manufacturers. For any questions or to request a custom cross linked storage tank, contact our support experts today.