If you’re storing an oxidizing chemical like sulfuric acid or sodium hypochlorite in a polyethylene tank, an antioxidant barrier will more than double the life of that tank. Polyethylene, whether it be linear or cross-linked, has excellent chemical compatibility compared to other choices. Linear and cross-linked are both the same polyethylene in terms of chemical composition and compatibility. However, cross-linked (XLPE) has a much higher resistance to tearing because the molecular chains are bonded (or cross-linked) to one another, whereas HDPE is called “linear” because they are not. They also both share the same chemical formula and resistance properties. All polyethylene, like other materials, will eventually degrade given time, and a typical lifespan for tanks storing oxidizing chemicals will be shortened to 6-10 years without a proper resistance barrier.
An antioxidant barrier is all about EXTENDING the life expectancy of normal polyethylene. It will extend the life of LDPE, HDPE, or XLPE when storing highly-oxidizing chemicals.
Free Radicals and Antioxidants
The term “oxidation” originated from the interaction between oxygen molecules and all the different substances they may contact. With the discovery of electrons, oxidation came to be more precisely defined as the loss of at least one electron when two or more substances interact. It doesn’t have to specifically involve oxygen. Free radicals are atoms or groups of atoms with an odd (unpaired) number of electrons and they can easily have their electron stolen (or oxidized). The carbon atom in all forms of polyethylene is such a group. True oxidation happens on a molecular level. In fact, we only see the large-scale effects when a scavenger causes free radicals on the surface to break away.
Using Antioxidants to Extend Tank Life
Antioxidants are additives to the polyethylene; molecules which can safely interact with free radicals and terminate the chain reaction they cause before more vital ethylene molecules are damaged. Antioxidants exist, for example, as enzyme systems within the human body, where they scavenge free radicals. Likewise, vitamin C is an antioxidant in the same way. They simply interact with free radicals and stop the stealing of electrons.
In the case of fresh fruit, the skin usually provides a barrier against oxidation. This is why most fruits and vegetables arrive in good condition at the grocery store. Once the skin has been broken, however, the individual cells come in direct contact with air and the oxygen molecules start burning them. The result is a form of rust we see as brownish spots. This in some ways is comparable to the “skin” of OR-1000.
This is also why stainless steel doesn't rust and ordinary steel does. The stainless steel has a thin coating of another metal which does not contain free radicals. Regular steel may be painted for protection against oxidation, but oxygen can still exploit any opening, no matter how small. This is why you may find a painted metal bicycle still damaged by rust.
OR-1000 is a polyethylene resin containing saturated amounts of antioxidants. Most polyethylene resin already contains some antioxidant to start with. Resin producers intend to protect the plastic resin itself from heat created in the molding process. It is spent during the heating process and non-existent at the completion of the molded part. Having performed the task it was charged with (protecting the plastic during production), it is no longer available for any post-production purpose.
The presence of OR-1000 in a completed storage tank, however, addresses the chemical resistance of oxidizing chemicals for 15-20 years after the tank ships. It does this in 2 ways:
- OR-1000 is added at the end of the cooking cycle. After most of the structurally superior cross-linked resin has cured the tank to a near complete part, the OR-1000 is added in what is referred to as the “double-shot”. The mold is stopped just long enough to inject the OR-1000 resin into the XLPE, resulting in a seamless bond. The additional resin is added, bonded, and cured along with the partially cured inner surface of the XLPE. This process only takes a few minutes as the tank finishes so the antioxidants are “uncooked away”, in tact and fully charged for use on-site.
- OR-1000 is not your typical resin. In fact, it has 4 times the antioxidant compared to other commercially-available resins. The copious amounts of antioxidant additive will ensure longevity for your polyethylene tank.
Putting OR-1000 to the Test
In a 2001 study to determine the antioxidant superiority of OR-1000, a large resin manufacturer submerged 1/8” tensile bars of XLPE and 1/8” tensile bars of OR-1000 in a 17% sodium hypochlorite solution and emptied and filled the container once per day. Samples were studied every 30 days and elongation of the XLPE tensile bars decreased 26% while no change occurred with the OR-1000 bars. This proved that OR-1000 had higher antioxidant strength than XLPE alone. And, because it is a resin, it can be added to the tank during the molding process.