Linear polyethylene is available as low, medium and high density. Most products offered are molded of linear medium density polyethylene (LMDPE) and linear high density polyethylene (LHDPE). They have superior mechanical properties, high stiffness, excellent low temperature impact strength and excellent environmental stress crack resistance. The linear polyethylene we use meets specifications contained in FDA regulation 21CFR177.1520 (c) 3.1 and 3.2 and so may be used as an article or a component of articles intended for use in contact with food, subject to any limitations in the regulations. Maximum operating temperature for linear polyethylene is 140°F.
Did you know that "true" high density linear polyethylene has replaced Kevlar® in bullet proof vests? Large sheets of "true" high density linear polyethylene has even replaced ice in skating rinks.
The Chlorine Institute, Inc., Phamplet 96 titled SODIUM HYPOCHLORITE MANUAL, Edition 2, May 2000, Section 6.2, Storage Containers, Subsection 18.104.22.168, Materials of Construction, paragraph c), reads:
"Sodium hypochlorite users and manufacturers have had success with high density linear polyethylene tanks, however, high density cross-linked polyethylene should not be used. Vertical stress cracking has been experienced with the cross-linked polyethylene storage tanks in this service."
This has, and continues to be Sierra Sales's, and Sierra Sales's customers' experiences. Don't believe all the cross-linky stuff! Sierra Sales offers a better warranty along with better pricing for this service. Order Phamplet 96 here and see for yourself! and see for yourself! Why has cross-linked poly been spec'd so often? At one time it was the "only" resin available. The original cross-linked polyethylene resins are no longer available and, the rotational equipment is usually out-dated compared to our modern equipment.
The physical properties and characteristics, while similar to high density linear polyethylene, do have a couple of differences. HDXLPE, high density crosslinked polyethylene, is rated at 150°F, maximum service temperature. Crosslinked tanks are molded, natural, off-white translucent. Crosslink poly is NOT FDA compliant. Crosslinked resin is not weldable and is not recyclable. Today, high-density linear polyethylene is your best choice!
A crosslinking agent reacts with the polyethylene during molding, forming a crosslinked molecule similar to a thermoset plastic. This reaction improves the toughness and environmental stress crack resistance. Today, linear polyethylene tanks provide excellent service and are recyclable and repairable and should be used whenever possible.
Crosslinked polyethylene can be pigmented during rotomolding to protect chemicals that are sensitive to UV exposure. Sierra Sales also offers barrier coating: for additional chemical resistance to aggressive chemicals like solvents. Sierra Sales offers the proven barrier coating technique of fluorination. Container walls are exposed to a mixture of fluorine and nitrogen gas. The end result is a barrier that can increase resistance to permeation as much as one hundred times.
Crosslinked polymers "absorb" the product they're storing. In fact, when "soaking" in the product they become soft, gel. A good example are "soft contact lenses".
Polypropylene is a rigid plastic that has a higher operating temperature limit than polyethylene: 212°F. It offers good chemical resistance, has a high resistance to stress crack, and is autoclavable. Polypropylene (PP) is not recommended for applications in subfreezing temperature or where high impact strength is needed. A rough, irregular interior surface is a common characteristic of molded polypropylene.
Continued or prolonged service with contents at elevated temperatures can shorten tank life of a tank. The effects of the temperature will depend on the chemical contents and its specific gravity, tank size, and configuration, material of configuration, wall thickness and if there are any external supports on the tank.
Exposure to Ultraviolet Light
Unprotected thermoplastics exposed to sunlight for an extended period of time, absorb ultraviolet light (UV) which can cause discolorations, embrittlement and eventual cracking. Fluorescent lighting has a similar effect. Elevated temperatures can accelerate the embrittling process. Sierra Sales products are molded from materials utilizing the latest technology for UV stabilizers which greatly reduce the harmful effects of UV light. For greater protection, keep tanks out of direct sunlight or order dark colored tanks which will further retard the effects of UV light.
U-V Protection (PDF File, 104K), or click here for html version
Environmental Stress Cracking
Certain surface active materials, although they have no chemical effect on polyethylene, can accelerate its cracking when under stress, such as liquid detergents and ultra pure water. Elevated temperatures tend to accelerate the cracking. Although all polyethylenes are subject to stress cracking, some are more resistant to it than others. The degree of stress on the plastic has a direct bearing on its resistance, therefore a tank and system should be designed to minimize stress. Sierra Sales's tanks are molded virtually stress free, employing materials that are highly resistant to environmental stress cracking, and hence are less prone to cracking than fabricated tanks. Stress cracking agents are active materials. Proper care should be taken to reduce stress at fittings, bands, tie down lugs, etc.
Vessel Selection & Configuring the Vessel
Tank selection, you know what you require. Cone bottom, vertical or upright, double wall tanks, poly containment basins, horizontal, poly horizontal leg tanks, elliptical leg tanks, polyethylene open or closed top. Let Sierra Sales assist you in your selection and then, let Sierra Sales show you the proper configuration(s), taking into consideration the "molded-in" characteristics of the specific poly tank, or plastic IBCs, for your service and piping requirements. Sierra Sales's factory trained sales staff can quickly configure any safe, poly tank, safe poly double wall tank system, or poly containment basins, to meet your requirement. Let's keep the process simple and eliminate "troublesome" configurations.
Want to know more? Have some fun at The University of Southern Mississippi's School of Polymers and High Performance Materials.
Sierra Sales's sales team is always willing to help an interested party make decisions relating to resin selection, tank configuration, piping, and valves. We supply quality.......for less.
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