One of the challenges in storing any oxidative chemical is engineering the tank to not only store the chemical but to give the end user a life expectancy and a measure of safety with the tank. Whether you are storing Sodium Hypochlorite at 12.5% or at .8%, Sulfuric Acid, Hydrochloric Acid, or other oxidative chemicals it is critical to design the storage tank with the chemical in mind.
Poly Processing Company (PPC) has been a leader in rotationally molded crosslinked polyethylene (PE) storage tanks for 40+ years. PPC works with leading universities to develop engineered polymer systems that will perform above and beyond any standard “off the shelf” polyethylene in harsh, oxidizing environments.
One of the systems most recently developed and commercialized is the OR-1000™ next-generation system. This system is the result of an initiative aimed at defining and addressing the long-term failure mechanisms associated with polyethylene in the presence of strong oxidizers and other corrosive chemicals over a range of solution concentrations such as encountered in commercial containment situations. The OR-1000™ system consists of the engineered linear PE system with enhanced resistance to environmental stress cracking (ESCR) in the presence of corrosive chemical solutions combined with a crosslinked polyethylene (XLPE) outer shell for maximum long-term performance and durability.
What is OR-1000?
OR-1000 is an inner-surface engineered resin with four times the antioxidant power of a standard tank wall. 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 and 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 fact and fully charged for use on-site. It's used when storing oxidizing chemicals like sodium hypochlorite, sulfuric acid, and hydrochloric acid. OR-1000 adds an extra chemical barrier between cross-linked polyethylene (XLPE) and the chemical. An antioxidant barrier is all about extending the life expectancy of normal polyethylene.
Long Term Improvements
The improvement in long-term resistance of the OR-1000™ system to oxidative degradation is illustrated in Figure 1. This figure compares the tensile elongation of the OR-1000™ system and two different commercial rotational molding PE’s at monthly intervals when exposed to 12.5% sodium hypochlorite (industrial strength bleach) solution at 40oC. Sodium hypochlorite is one of the strongest, if not the strongest known oxidizing chemicals in the presence of polyethylenes.
From Figure 1, after 9 months of exposure to sodium hypochlorite, the conventional PE’s have lost an average of thirty (30) percent of the original tensile elongation to break, while OR-1000™ has lost only about three (3) percent, indicating about 10X greater resistance to oxidative degradation for OR-1000™. Loss of elongation in this manner indicates an increasingly more brittle tank or liner and thus greater susceptibility to cracking and leaking over time. The data in Figure 1 clearly demonstrates the superior long-term performance of the OR-1000™ system in the presence of strong oxidizing chemicals such as sodium hypochlorite.
Environmental Stress Crack Resistance (ESCR)
Polymers such as polyethylene are especially susceptible to another type of failure known as environmental stress cracking (ESC), which is a result of breaking of polymer chains in stressed areas of a tank. Examples of this in practice would be normal heating and cooling of storage tanks in outside environments or pressurized filling and emptying of transfer tanks used in chemical manufacturing.
The stress cracking process is accelerated by the presence of solutions of chemicals, particularly dilute solutions, in which the chemicals have increased mobility and can physically penetrate the polymer structure and increase the stress differences. High density polyethylene (HDPE) is the most susceptible to stress cracking due to higher levels of crystallinity as compared to medium density, low density, and linear low density PE’s.
Real World Benefits of the New OR-1000
- Improvements in the long-term properties:The enhanced resin will last longer and allow your tank to last longer, too.
- High levels of ESCR: High levels of ESCR (Environmental Stress Crack Resistance)in the presence of Sodium Hypochlorite and other aggressive chemicals
- Improved Elongation: The new OR-1000 will maintain its elongation properties over time while in the presence of heavy oxidizing chemicals. This performance is superior to the previous OR-1000 and other HDPE materials commonly used in tank construction as discussed previously.
- NSF/ANSI 61 – Poly Processing Company XLPE tanks with the OR-1000 system is NSF certified for 35 different chemicals.