FAQ’s about Polypropylene

What are the advantages of replacing traditional equipment with polypropylene?
Why is polypropylene often the material of choice for plastic process equipment?
What types of polypropylene are there?
Is polypropylene flame retardant?
Is there a design standard for thermoplastic fabrications?
What is the maximum temperature that polypropylene can withstand?
What can be done to accommodate the significant expansion and contraction of polypropylene?
What is the life expectancy for polypropylene equipment?
What means are used to protect polypropylene equipment from mechanical damage and abuse?
How are repairs and modifications made to polypropylene?
What are the heat transfer and insulating qualities of polypropylene?
What are the dielectric qualities of polypropylene?
How does the cost of polypropylene equipment compare to traditional equipment?

What are the advantages of replacing traditional equipment with polypropylene?
Polypropylene equipment offers lower overall cost, longer life, easy repairability, lighter weight, and less maintenance. The replacement of tanks is simplified and requires less down time than traditional equipment.

Why is polypropylene often the material of choice for plastic process equipment?
Polypropylene is an inexpensive, tough, plastic material that has excellent corrosion resistance. It is easily cut, machined, and welded into complex fabrications. It accommodates the service temperatures and solutions of most metal cleaning, pickling, and plating operations. It is inexpensive to maintain and it is readily modified and repaired.

What types of polypropylene are there?
There are two primary types of polypropylene: homopolymer and copolymer.
Homopolymer polypropylene is the most common material used. It has a higher tensile strength than copolymer polypropylene and it is less costly. Copolymer polypropylene has higher impact strength but lower tensile strength. It is used for batch tanks that are impacted by the load, for strip tank covers that are handled a lot, and for applications that may be subjected to mechanical abuse.

Is polypropylene flame retardant?
There are flame retardant grades of polypropylene available. These have various UL ratings. They are more expensive and, because of the addition of the flame retardant fillers, they have lower physical properties and fabricating qualities. Allegheny has developed HY-PRO™ flame retardant clad copolymer polypropylene. This material has two outside skin sheets of flame retardant polypropylene that are coextruded onto a center core of copolymer polypropylene. Thus providing a composite that is a tough material with flame retardant exterior surfaces.

Is there a design standard for thermoplastic fabrications?
There is no United States design standard. However, Allegheny uses the German thermoplastic design criteria in the DVS-2205 along with a finite element stress analysis that we have developed. The DVS-2205 specifies the design criteria based on the design temperature, creep modulus, allowable weld strengths, corrosion factors, etc. All polypropylene designs are engineered to meet the required service conditions using our conservative methods.

What is the maximum temperature that polypropylene can withstand?
Engineering and design criteria is available that permits the design of polypropylene equipment up to 100° C (212° F). Allegheny has designed tanks for hydrochloric acid pickling that have been operating in this temperature range continuously since 1988. However, the maximum operating temperature is a function of the solutions in the tanks, the size (depth) of the tank, and the ability to reinforce the tank to keep the design with in the allowable stress levels required. Because of the aggressive oxidizing effect of nitric/hydrofluoric acids, it is not recommended that this equipment be operated above 70° C (155° F). It is impractical to fabricate large diameter, deep cylindrical tanks at elevated temperatures because it is difficult to satisfactorily reinforce them.

What can be done to accommodate the significant expansion and contraction of polypropylene?
Polypropylene expands about eight times as much as steel. Allegheny has developed many innovative designs to accommodate this expansion. Long tanks have labyrinth seals or rubber and plastic expansion seals between sections. Where steel reinforcement is incorporated in the tank structure, it must be designed with sufficient clearance to permit the plastic tank to expand and contract. The wide variety of polypropylene fabrications in use today shows Allegheny’s ability to control this problem.

What is the life expectancy for polypropylene equipment?
Life expectancy is related to service and operating parameters of the equipment. There are polypropylene tanks in sulfuric acid pickling operations for over twenty-five years. We expect similar life expectancy from hydrochloric acid pickling lines. With good maintenance procedures, nitric/hydrofluoric pickling tanks should have up to fifteen years of life.

What means are used to protect polypropylene equipment from mechanical damage and abuse?
Depending on the application various methods are used to protect the plastic tanks. For strip pickling lines using sulfuric or hydrochloric acids we use granite skids, side guides, and weirs. For nitric/hydrofluoric service we encapsulate corrosion resistant steel bars in polypropylene tubes. For batch tanks we often use solid polypropylene cradles to support the load and to keep the load away from the tank walls and bottom.

How are repairs and modifications made to polypropylene?
Since polypropylene is weldable, thermoplastic equipment made from it is readily repaired or modified. Hand welding can be accomplished with simple welding techniques. Extrusion welding and fusion butt welding are used in our shop as the primary welding methods. Hand welding with multiple beads of weld is limited to areas that can not be extrusion welded and to field welding applications.

What are the heat transfer and insulating qualities of polypropylene?
Polypropylene has excellent insulating properties. A typical 38mm (1-1/2") thick tank wall has equivalent insulating qualities to a rubber lined steel tank with two courses of acid brick. This means that additional insulation is not required for storage or recirculation tanks.

What are the dielectric qualities of polypropylene?
Polypropylene also has excellent dielectric properties. Therefore, electrolytic applications are among the best uses for polypropylene equipment. There is no need to electrically insulate the tanks or the equipment in the tanks from the tank body. We have been advised that upon replacing steel tanks with our polypropylene tanks our customers have experienced up to 40% less current requirements. They have also experienced longer conductor roll and grid life.

How does the cost of polypropylene equipment compare to traditional equipment?
In most cases polypropylene equipment is competitive with traditional rubber lined and bricked steel tanks. When carbon block lining and other exotic materials are required, polypropylene is less expensive. However, the real cost comparison must include all of the costs over the life of the equipment. In this case polypropylene equipment has proven to be far less costly than traditional materials. Our customers advise us that the long term maintenance costs of plastic tanks and equipment is significantly less.