Fiberglass (FRP) and Polyethylene (PE)

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Plastic Chemical Storage and Process TanksFiberglass (FRP) and Polyethylene (PE)

Chemical storage tanks for industrial, agricultural, high-purity and municipal applications are available in a very wide

variety of sizes and styles. They are also available in a variety of different materials of construction. This paper will discuss

the benefits of “Plastic Chemical Storage and Process Tanks”.

There are many applications in the agricultural, industrial and chemical process industries that use mild steel, stainless

steel or alloy tanks. You will typically find these materials used in applications for high pressure, high temperature or very

large capacities. Due to the limited chemical compatibility of mild steel, and much higher costs for alloy tanks, plastic

tanks are the most popular choice in chemical and high purity markets.

The two most common plastic materials used for chemical

applications (both high and low pH) are Fiberglass (FRP) or

Polyethylene (PE). Both are suitable for a wide range of dilute

and concentrated chemicals, but no one material is always

going to be the best choice for every application.

When you evaluate what the best materials of construction

should be for your tank, you will need to consider several

factors that will affect the compatibility for your specific

application, including chemical compatibility, temperature,

pressure, vacuum, weatherability, impact resistance, stress

crack resistance and environmental corrosion resistance.

Once you have evaluated these factors, you will want to

consider the cost of the tank and the ability to customize it

for your specific application.



Materials of Construction

FRP

The plastic material used in manufacturing (the resin) is very important

when properly specifying the right tank for your application. Here is an

overview of both FRP and Polyethylene.

FRP resins are available in a variety of choices, including isophthalic, vinyl ester, epoxy and many more. Whereas

polyethylene tanks are molded from a single resin, FRP tanks are a composite design with alternating layers of

fiberglass and resin with fillers, liners and different types of reinforcement glass to achieve the performance standards

required for your application.

The FRP tank may have a special liner, pigment, flame retardant additives, coatings and varying layers to achieve the

best performance for your application. FRP has very high tensile strength compared to polyethylene resins, so they can

have much thinner walls and still achieve excellent wall integrity.

There are trade-offs when considering the physical properties of different plastics. For example, a stiffer wall is going

to have lower impact resistance compared to the more flexible wall found in PE tanks. While more flexible walls are

more impact resistant, they will need to be thicker to provide comparable sidewall strength to FRP. Fiberglass tanks are

typically opaque, which means you cannot see the liquid level through the tank sidewall.

An FRP tank can be specified with a resin system that is best suited for your chemical service. Typically, the more

aggressive the chemical, the more expensive the resin system required. FRP resin is ideal for petrochemicals and

chlorinated hydrocarbons like gasoline, diesel, solvents, polymers, many dilute chemicals and food products. For large

underground tanks over 2,000 gallons, FRP is the only option in the plastic tank category.

FRP is not typically the best option for oxidizing chemicals like sulfuric acid, hydrogen peroxide, sodium hypochlorite

and many janitorial detergents. Oxidizing chemicals attack the structure of the FRP and accelerate the degradation of

the tank. Since the glass fibers are hollow like a straw, the chemical will eventually absorb into the tank wall and begin to

weep out of the exterior of the tank. This will ultimately result in delamination and a weakening of the tank’s structural

integrity followed by failure.



PolyethyleneWhen considering polyethylene, there are two basic classes of

resin: Linear (LPE) and Crosslinked Polyethylene (XLPE).

LPE is available in three basic densities: low density (LLDPE),

medium density (MDLPE), and high density (HDLPE). All three of

these resins have similar chemical compatibility but each offers

unique advantages in specific applications.

On small tanks (500 gallons and smaller), any of the three LPE

resins generally perform equally. On larger tanks (over 500-gallon

capacity), a medium or high-density resin is recommended.

Low density polyethylene resins range from 0.910-0.940 g/cm³

density. LDLPE, with a 0.938 g/cm³ density, is most commonly used

for plastic chemical tanks.

Medium density resins range from 0.926-0.940 g/cm³ density.

MDLPE, with a 0.938 g/cm³ density, is preferred for plastic

chemical tanks. Although the density value of the low and

medium density resin is similar, the medium density resin has

significantly higher stiffness, offering superior mechanical

strength and lower wall deflection than low density resins, making

it a better choice for larger-capacity tanks.

High density resins range from 0.930 to 0.970 g/cm³. HDLPE, with

a 0.941 g/cm³ density, is preferred for larger plastic chemical

tanks, including vertical tanks over 4000 gallons and horizontal

tanks 1000 gallons and larger. Horizontal tanks are typically

used on mobile applications, so the added tensile strength is

beneficial. In tanks smaller than 4000 gallon capacity, MDLPE and

HDLPE are equally good performers and offer the same superior

performance characteristics.



LPE is a thermoplastic resin, which means it can be welded, so adding

welded features like process nozzles and pipe supports or making

repairs on damaged tanks is an added benefit. The ability to add

welded features on LPE tanks in virtually any location, dramatically

increases the flexibility in customization of your tank.

XLPE is only available in one formulation and is considered a high

density Resin. The choice of resin you make should be based upon

chemical compatibility and the manufacturer’s recommendation for

your application.

XLPE is a thermo-set resin and is not weldable, so adding welded

features or repairing a damaged tank by welding it is not an option and

fitting installation options are limited to:

• molded in fittings

• mechanically bolted fittings or

• compression style fittings

These fitting styles need a flat surface for proper installation, thus

limiting the potential locations for process nozzles.

PE tanks are normally translucent, so you can see the liquid level

through the wall, which is an added benefit. They are also the most

versatile choice for tank construction because both linear and

crosslinked polyethylene perform so well in such a wide range of

chemical applications.



However linear polyethylene is the preferred over crosslinked

polyethylene in most applications for these three reasons:

a) lower cost of the resin

b) wider range of chemical compatibility

c) higher purity rating in food, pharmaceutical, semiconductor

and potable water applications.

As you can see, there is very little variance in the density of linear

polyethylene resins; however there is a higher strength to density ratio in

medium and high density resins in their molecular structure, thus they have

more hoop strength, (stiffer walls), making them a better choice for the

increased hydrostatic pressure in larger tanks. Another benefit of LPE resins

is that they are recyclable, which is not the case with FRP and XLPE resins.

FRP is more costly than polyethylene, so in applications where both are

compatible, PE is the most cost-effective choice.



Chemical compatibility is the first and most important consideration in selecting the correct tank material for your

application. The concentration, temperature, and in some cases, the mixture of chemicals can have a major impact on

the tank performance and ultimately its lifecycle.

Do not mix chemicals in your tank without approval as sometimes mixing two compatible chemicals will create an

incompatible chemical or cause an exothermic reaction, damaging your tank and causing a potential catastrophic

failure. As stated above, petroleum-based chemicals and chlorinated hydrocarbons are better applications for FRP than

PE tanks. Polyethylene is generally not compatible with these chemicals, except for some biofuels and oils. Check with

the tank manufacturer for specific recommendations.

Almost all polyethylene tanks are rotationally molded with a seamless construction, providing a homogenous wall. This

eliminates the stress points or seams found in fabricated tanks. Polyethylene tanks have a broader range of chemical

resistance than FRP tanks.

Chemical Resistance

Applications such as sodium hypochlorite, ammonium hydroxide, caustic soda, hydrofluoric acid, hydrochloric acid,

water treatment chemicals such as biocides and inhibitors, photographic solutions, hydrogen peroxide, sulfuric acid,

brine solutions and many more work extremely well in linear polyethylene containers.



Testing reports and technology exists to help determine

when polyethylene will work for an application. In some

cases, both crosslinked (XLPE) and the medium density

(MDLPE) or high density linear polyethylene (HDLPE) resins

will work equally well.

XLPE is the resin of choice for many applications such as

polymers and surfactants. It is also a good choice for a wide

range of agricultural and industrial chemicals, but is a more

expensive option than linear polyethylene which performs

equally as well in those areas.

XLPE is generally not used for food, potable water or high

purity applications because of the impurities that leach from

the resin over time that can contaminate the tank contents.

XLPE is also not recommended for oxidizers like sulfuric

acid, sodium hypochlorite, chlorine dioxide, hydrogen

peroxide and others, due to accelerated stress cracking

and premature failure in these applications. For oxidizing

chemicals, linear polyethylene is the better choice.

As a result, some molders offer XLPE tanks with a LPE

“liner” in these applications, but since the two resins do

not bond together, delamination of the walls and installing

process fittings becomes a special challenge and is thus a

poor choice (and more costly) for oxidizers of any kind.



Mechanical PropertiesFRP tanks have significantly more tensile strength than PE tanks, as mentioned above. This allows the FRP tank

manufacturers to construct tanks with thinner walls making them lighter, relatively speaking. This benefit, however,

comes with an inherent disadvantage; they are not as impact resistant as polyethylene tanks.

The problem with impact damage in FRP tanks is compounded by common variations in the manufacturing processes.

For instance, improper cure, failure to maintain uniform wall thicknesses, failure to compensate for ambient conditions/

dew point during manufacturing, failure to apply each (laminate) layer within specified processing window and use

of incompatible resins with incorrect fiberglass (sizing/fabrics). These improper techniques can contribute to tank

delamination and cracking, which in turn compromises the mechanical properties, especially strength, stiffness, and

impact resistance of the FRP tank.

Often these types of degradation are not visually apparent without close inspection but can all lead to premature

failure of the tank. A common cause of damage to FRP tanks happens while loading and unloading during delivery and

installation, due to the stiff, inflexible properties of the material. One small impact or an over-tightened strap could lead

to a crack not easily visible, resulting in a leak weeks or months later.

FRP tanks can be manufactured in larger sizes than PE tanks. It is not uncommon to see FRP tanks that are 40,000

gallons or larger.

Since PE tanks are molded, a mold must be constructed

for each size and then rotated in a 360° rotation inside an

oven. Limitations of oven and mold size restrict how large

a PE tank can be. Currently, the largest PE tank available in

the market is 20,000 gallons.



Polyethylene has better weatherability than a standard FRP

tank. Very effective UV (ultraviolet)

inhibitors can be compounded into the material by the resin

manufacturers. Both polyethylene and FRP will perform

well in very harsh environmental conditions. The FRP tank

must be specified and built with a protective resin layer

on the outside of the tank to achieve the best results. All

polyethylene resins used for rotationally molded tanks have

UV inhibitors compounded in during the manufacturing of

the resin.

The UV inhibitor in these polyethylene resins is uniform

throughout the entire tank wall. PE Resins widely used by

tank manufacturers have UV ratings anywhere from UV8

(eight year expected life in direct sun) to UV20 (20 year

expected life in direct sun). Be sure to ask your tank provider

what their UV rating is on their resin. They can send you a

manufacturer’s specification sheet upon request, which lists

the UV rating, resin density and other technical information

you may find helpful.

Polyethylene tanks can be designed to block UV light from

stored chemicals that are sensitive to UV light by using

opaque resins which prevent light transmission through

the tank wall. These resins are more expensive but are well

worth the benefits in certain applications.

Weatherability



FRP tanks are rated for temperatures from 180°F up to 240°F depending upon the resin selected. FRP tanks can also

be designed to be rated for pressure and vacuum per ASTM guidelines up to certain limits. This is useful in applications

like vacuum and pressure filling and processing and maintaining a nitrogen blanket on chemicals being stored. Putting

pressure or vacuum on tanks that are not designed for that is a recipe for disaster. Always check with the manufacturer

prior to pressurizing or pulling a vacuum on any tank.

Polyethylene tanks are primarily designed for atmospheric, ambient storage of chemicals. Working temperatures of up

to 130°F are generally suitable in an unreinforced polyethylene tank. Temperatures above 130°F can be achieved by

adding a FRP overwrap on a polyethylene tank, increasing the temperature rating to as high as 180°F in some cases.

Temperature, Pressure, Vacuum

Most polyethylene tank manufacturers do not have the

ability to add a FRP overwrap to their tanks. This option

is typically only available from more engineering-focused

manufacturers. In some situations, polyethylene tanks can

handle a very small amount of pressure and/or vacuum.

In situations that require one or two psi pressure or one

or two ounces of vacuum, ask your tank manufacturer for

recommendations in advance.

Fiberglass (FRP) and Polyethylene (PE)

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