Here’s the Difference Between Various Thermoplastics
What is a Thermoplastic?
Thermoplastics are materials that react to heat by becoming more plastic-like and return to a hard state when cooled. They are usually synthetic resins. Unlike most plastics, thermoplastics are recyclable polymers. Polymers are substances consisting of small molecules that repeat their sequence and form into chains. In turn, they create larger molecules.
The small molecules that combine are known as monomer. The chemical makeup of the monomer determines the structure of the final product. It should be noted that chemists need to know both the individual combining power of molecules as well as the chemical reactions of much more substantial structures.
What are the Specifics of Thermoplastics?
They are many specifications of polymers, they have a fairly light and flexible structure while forming tough bonds.
A special type of thermoplastic is a TPE, a thermoplastic elastomer. This has the qualities of both plastic and rubber. They have a lower melting point than such materials as steel, what is known as the critical temperature. More and more people are buying thermoplastics.
Note that thermoplastic shouldn’t be confused with thermosets, such as vulcanized rubber or epoxy. These are different from thermoplastics in that they can’t return to their original shape after being heated, being not as flexible.
Why are Thermoplastics Useful for Valves?
The advantages of thermoplastics when used for valves are:
- they will not corrode like metal components
- they are less likely to overheat due to the level of resistance of the material
This means that thermoplastic valve parts don’t need to be replaced as often as parts made of metal.
They are useful in any area that uses valves, be it water filtration, processing of chemicals, paper processing or similar. It is vital that that all components last for a relatively long amount of time, so this is why thermoplastics are better. The other good thing about thermoplastics is that they have a better sheen and appear colorful, making them more aesthetically pleasing than their metallic alternative.
Note that it is not possible for a thermoplastic to make the whole of the valve, as some items such as springs and sensors need to be made from other materials. Only the seal and seat part of the valve are made of polymer material.
Which Thermoplastics are Used?
The most well-known and widely-used thermoplastic is probably PVC, which stands for Polyvinyl Chloride. It may however not be suitable for very high temperatures – 140°F is about its limit. It is recommended for use for draining systems or large industrial chemical systems. It is not suitable for use with particularly chlorinated or aromatic chemicals.
A variant of PVC is uPVC or unplasticized PVC. Unfortunately the temperature range for uPVC is about the same as regular PVC. The difference between the two is that PVC has added plastic compounds, so uPVC is a harder, less flexible substance.
Another well known polymer is Teflon®, which has the scientific name of Polytetrafluoroethane. It has a higher resistance to temperature than PVC, reaching up to 400°F. This is due to its stability of its molecules. It is also non-flammable.
RTFE (Reinforced tetrafluoethane) is similar to Teflon, but it has glass fibre added, so it can withstand higher pressures. Due to this added material, it shouldn’t be used in a process involving hydrochloric acid.
If you are looking for a material for highly chlorinated chemicals, one solution would be Neoprene. It has a slightly higher temperature range than PVC, up to 160°F. It is usually used in valves in food related environments. However it is not suitable if there are oxidizing agents involved in the processing.
Another registered trademarked thermoplastic like Teflon is Viton. It is suitable for the same kind of temperatures as Teflon, but is more suited for chlorinated hydrocarbons and petroleum. It is not suitable for valves connected with hydraulic systems. Viton is also known as the non-trademarked name of Fluoroelastomer.
If you are using chemicals with variable pHs, EPDM or ethylene-propylene diene monomer is a good choice. This is providing the acids and alkalis are not too strong and there is no oil involved. The temperature limit for EPDM is about 225F.
Another option might be Perfluoroelastomer, which goes by the names of Kalrez and Chemrez. It combines the function of Viton with fluoropolymers, which increases its chemical resistance. It is however an expensive product.
Aflas or TFE (tetrafluoroethane) provides an alternative to perfluoroelastomer and is especially used in valves connected with automobiles, as well as having applications on oilfields. Its temperature limit is about 400°F, and it also has excellent electrical resistance properties.
Devlon is another trademarked polymer, also known as acetalhomopolymer, has a structure which can resist high impacts. It combines high wear with low friction. It has a temperature limit of 300°F. It has a number of alternative names, among them Celecon, Kepital and Lupital.
PEEK or Poly Ether Ether Ketone is suitable for hot water or steam environments. It also has a rigid structure and is especially useful for environments with high rates of corrosion. It is considered a high performance polymer.
Why are There so Many Different Thermoplastics to Choose From?
Put simply, it all comes down to demand, as well as different properties needed for polymers in various situations. The suitability of certain products to meet specifications changes all the time. We are unlikely to find the perfect polymer that it suitable for all situations any time soon.
Certain products, one example being Buna-N or Nitrile rubber that are thought to be suitable, might eventually fall out of favour. It should be noted that different substances vary according to who they are manufactured by, take the above example Kalrez and how it differs from Chemrez.
What Will Happen with Thermoplastics in the future?
New patents for new polymers are being issued every year and many will be used to create improved versions of valves for many different temperatures and chemical environments. The more research done in the specific industries that they are used in, will mean the less times a valve will need to be replaced.
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