Ionomer is one of those strange materials that no one seems to know much about. I wanted to offer a bit of information to demystify ionomers so that you understand what they are and when you should use them.
The term ionomer itself is kind of confusing because it does not include any chemical names like most polymer names do such as polypropylene or polyethylene. I don’t really think that the term ionomer correctly describes the material. It should really be changed.
An ionomer is a polymer that is made up of repeating units of molecules like all polymers. That makes an ionomer unique is that the repeating units are typically a copolymer (made from two different monomers) and a molecule that is an ion. The ion only represents about 15% of the polymer chain though. Materials that have large amounts of ions are not considered ionomers but are instead called polyectrolytes which is why the name is a bit confusing.
The copolymer in most ionomers is polyethylene copolymer (methacrylic acid being the other monomer) that have an ion added into the mix during polymerization. The ion is usually a metal salt such as zinc, sodium or lithium. If you have forgotten from high school chemistry, an ion is a molecule with an uneven number of electrons and protons. This causes it to have an electrical charge. Ions can have a positive charge (cation) or a negative charge (anion). The magic of ionomers is that normally, you would expect a bunch of ions to all stick together and not really bond to anything else. In the case of ionomers, they are manufactured in a way that causes the ions to get stuck in the polymer chains.
Here is the thing that makes ionomers unique. The strange thing about the ionic bond is that it dramatically reduces in attraction at elevated temperatures. The ions make the material act like a thermoset at room temperature and like a thermoplastic at elevated temperatures. Essentially a melt process-able thermoset. The ions basically cross-link the polymer chains at lower temperatures. Chemists refer to ionomers as reversible crosslinkers.
The bottom line is that this reversible crosslinking give ionomers unique properties. They are extremely tough, have extremely high impact strength at low temperatures and excellent abrasion resistance. They also have excellent melt strength which means that even above the melting point, they are pretty strong. This makes them excellent for making sheet and film.
To give you a few ideas about what ionomers are typically used for, let me give you a few common applications. The outer skin of golf balls is typically made from ionomer. Ionomer is often used for clear cosmetics packaging and it is also made into thin film that is used for food packaging like vacuum packed meat and seafood. Oddly, ionomer is also used as an impact modifier for nylon.
I hope that I have helped you to understand what ionomers are and why they are used. I really think that the name causes a lot of confusion. The industry should change the name of these to something like ionic polymers or maybe something more specific like polyethylene ion copolymer. Earlier this year, I posted an article about the different types of TPEs on the market and I did not include ionomer as one of the types. I probably should have. It is probably best to think of ionomer as another type of TPE.
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