Material Selection

Tank Depot.com offers you a selection of materials that are by far the best suited for molded tanks and containers for use with aggressive corrosive chemicals as well as food products. The following is a brief description of these materials:

Polyethylene

A high quality thermoplastic that has outstanding resistance to both physical and chemical attack. The overall general toughness and excellent chemical resistance to a wide array of wet and dry industrial chemicals and food products make polyethylene ideally suited for storage tanks and containers.

Polyethylene is translucent and its natural color ranges from slightly off white to creamy yellow, depending on wall thickness and type. Ultraviolet light stabilizers are added for use in outdoor applications. Colors are available on request for a nominal up charge.

  1. Linear Polyethylene

Linear Polyethylene is available as low, medium and high density. Most products offered in this catalog 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 used 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.

 

  1. Crosslinkable Polyethylene

Crosslinkable polyethylene is a high density polyethylene that contains a crosslinking agent which reacts with the polyethylene during molding, forming a crosslinked molecule similar to a thermoses plastic. This reaction improves toughness and environmental stress crack resistance. Crosslinked Polyethylene (XLPE) is not weldable and does not meet FDA requirement 21CFR177.1520. Maximum operating temperature of crosslinked polyethylene is 1 50° F.

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 sub-freezing temperature or where high impact strength is needed. A rough, irregular interior surface is common characteristic of molded polypropylene.

Consideration of Material Selection

Elevated Temperatures

Continued or prolonged service with contents at elevated temperatures can shorten the life of a tank. The effects of the temperature will depend on the chemical content and its specific gravity, tank size and configuration, material of construction, 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 (UV) light, which can cause discolorations, embrittlement and eventual cracking. Fluorescent lighting has a similar effect. Elevated temperatures can accelerate the embrittling process. Tank Depot 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 effect of UV light.

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- (see our chemical compatibility Chad on pages 46 -49.) 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. Tankdepot.com tanks are molded visually 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 surface active materials. Proper care should be taken to reduce stress at fillings, bands, tie down lugs, etc. 

Characteristics and Physical Properties

General Characteristics Linear XLPE PP 
Maximum Service Temp 140 F (60 C) 150 F (65 C) 212 F (100 C)
Chemical Resistance Very Good Very Good Very Good
Stress Crack Resistance Excellent Excellent Excellent
General Toughness Very Good Very Good Fair
Impact Resistance Excellent Excellent Poor
Abrasion Resistance Good Good Good
Rigidity Good Good Good
Color WH Translucent Off WH Trans Off WH Opaque
FDA Compliance Yes No Yes
Outdoor Use Yes No Yes
Weildable Yes No Yes
Recyclable Yes No Yes

Physical Properties
(Nominal Values)
Linear XLPE PP 
Density (gm/cc) ASTM:D1505 0.937-0.942 0.937-0.942 0.901-0.905
Environmental Stress
Cracking (F-50hrs)
ASTIM:D1693
>1,000 >1,000 >1,000
Tensile Strength ASTIM:D638
Ultimate 2"/Min (PSI) Type IV
2,600 2,600 3,000
Elongation at Break ASTIM:D638
2"/Min(%) Type IV
450 450 5
Flexural Modulus (PSI) ASTIM:D790 100,000-110,000 100,000-110,000 90,000-100,000
Brittleness Temp ASTIM:D746 <-94F (-70 C) -180 F (-118 C) 32 F (0 C)