Linear Polyethylene
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 6.2.2.4, 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.
Crosslinked Polyethylene
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
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.
Elevated Temperatures
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.
Browse our web pages. Any suggestions? Contact
us by email, phone, or letter. We'll get back to you as quickly as possible. TANKS.
