Answering this question will depend heavily on defining just what a polypropylene valve is, then. By understanding its characteristics--its strengths and weaknesses--we can better comprehend what applications and purposes a polypropylene valve serves best. The first step will be deciding on a definition for polypropylene itself.
What is Polypropylene?
Polypropylene is designated by the initials, PP. It is a thermoplastic polymer used in a wide array of man made products. Among commodity plastics, polypropylene has low density, which means lower weights for parts like valves. It is tough, flexible, and durable. In comparison with other thermoplastics, it is somewhat similar to ABS (Acrylinotrile Butediene Styrene). Polypropylene is highly resistant to chemical solvents, acids, and bases, and can withstand heat ranges between 320° and 340° F before it begins to melt, depending on its isotactic properties. However, polypropylene does have some downfalls. It does not handle cold temperatures well, for instance, and will become brittle when exposed to freezing conditions. It also will degrade with long exposure to heat and UV light from the sun.
Regardless of its weaknesses, polypropylene retains a few interesting characteristics which, in some ways, give it an advantage over other thermoplastics. One of these advantages is the slipperiness of its surface: this is great for moving a variety of media smoothly, but it also means that many glues commonly used as sealants won't stick. Most polypropylene valves are threaded, rather than glued. But some are actually welded into place. Another intriguing characteristic of polypropylene is that, without added pigments, it can be molded into transparent shapes. Many valves in the medical field, for instance, are transparent.
When to Use Polypropylene Valves
Like all thermoplastics, polypropylene is used to create a variety of valves, including but not limited to, check valves, ball valves, solenoid valves, flanged and true union three piece valves. Polypropylene valves are commonly used in the medical profession, chemical processing, marine applications, aerospace, and food processing, to name a few applications. Because of its drawbacks, polypropylene valves are limited in heat and cold ranges, and cannot be exposed to excessive heat or sunlight, (though PP can be stabilized against UV degradation). And, while quite durable, PP is only half as strong as PVC (Polyvinyl chloride) or CPVC (Chlorinated polyvinyl chloride), though it is resistant to environmental stress cracking, as it is more flexible. However, of all thermoplastics, polypropylene valves are the most chemical resistant, as there are no known solvents that will dissolve them. Polypropylene valves are perfect for mild solutions of many acids, salts, organic materials, alkalis, hydroxides, and concentrated acetic acids. The original question, "What Applications Demand a Polypropylene Valve," then, is answered when any media containing these specific properties and compounds are traveling through a pipeline, as polypropylene is the best choice.
If you are interested in polypropylene valves and other thermoplastics, you can get a better look at them by browsing companies such as Hayward Valves or Spears Manufacturing.