Part 3 of our “Material World” Series : Thermoplastic polyurethane builds a bridge between rubber and plastic.
Dr. Jürgen Hieber, a developer at Freudenberg Sealing Technologies (FST) in Schwalmstadt, makes one thing perfectly clear as the interview begins: “We are not talking about normal polyurethane here, the kind used as mattress foam, for example. We are working with thermoplastic polyurethane, or TPU for short.”
This TPU is a hybrid material that combines the qualities of rubber and plastic. “Our Japanese colleagues like to call it the iron rubber. Our material can be stretched elastically like rubber, but it is – metaphorically speaking – as stable as iron.” TPU’s enormous strength makes it the top choice among sealing materials when high pressures are at work. In hydraulics, for example.
Maneuvering fully loaded excavator shovels
Cylinders for excavator shovels are one classic use of TPU. They ensure that the shovels can be reliably maneuvered even when fully loaded and extended. TPU rod and piston seals as well as scrapers are used in these applications and have to withstand pressures as high as 400 bar. By comparison, the pressure in an automobile’s tires is between 2 and 3 bar. TPU seals are also in demand in other construction machinery as well as in farm equipment, machine tools and percussion drills.
The Schwalmstadt facility is FST’s Competence Center for TPU. “We make our TPU in-house and have about 50 different TPU materials that we regularly use,” said Dr. Hieber. “Our know-how lies in the selection of the right raw materials and combining them in the right quantities to achieve the desired characteristics.” Here the approaches for TPU and rubber are hardly different.
But TPU production takes place entirely differently, in large reactors like those in the chemical industry. Polymerization is the technical term for this chemical reaction. Here polyols, isocyanates, chain extenders, colorants, flow agents and stabilizers react to form molecules with long chains, so-called polymers. Producing TPU from bio-based raw materials is a topic for the future.
The anti-chip bag effect
TPU is not just pressure-resistant – it is extremely resistant to ozone and withstands exposure to mineral hydraulic media quite well. Besides its resilience and resistance to wear, the long list of TPU pluses includes tear strength, “I like to use the chip bag effect to explain tear strength. At first, a bag of chips is often hard to open. It takes strength. But once the bag is torn open, it rips very quickly. But that’s not the case with TPU. If it is damaged or cut into, it does not continue to tear. That gives it a long lifespan.” At high temperatures, the “rubber of iron” is pushed to the limits of its capabilities. The upper end is typically 120°C (248°F).
One positive aspect relates to sustainability: With TPU as a material, sprues can be melted down, re-granulated and recycled. That doesn’t work with rubber.
Rolling instead of sealing
Two-component combinations with a metal or other plastics are also possible with TPU – even beyond classic sealing applications. One extremely interesting application involves so-called texturing discs. Texturizing is a finishing process for plastic fibers in the textile industry. Here smooth artificial fiber threads resembling fishing line are fed at enormous speeds over rapidly rotating running discs with a pre-defined surface. Designed to roughen up the material, these texturing discs give the threads a bushy, wool-like structure, complete with the appropriate characteristics. FST supplies the leading global manufacturer of these friction discs with a two-component solution: The Lead Center in Schwalmstadt applies a TPU coating around a circular plastic carrier part. The two components grip into one another purely mechanically.
TPU instead of PTFE?
Other applications in the “discs and rolling” market niche are conceivable in TPU’s future. Another starting point for the near future and beyond is the substitution of TPU for PTFE, a high-end material. PTFE is one of the “forever chemicals,” known as PFAS, that do not degrade easily. Legislators on the international level have been viewing them critically. “PTFE has unique features relating to its resistance to media and extreme temperatures that TPU cannot match,” Hieber said. But he sees opportunities in the future. “In cylinders, PTFE has often been used in areas where its high performance is not really required. Here we are offering TPU as a reliable alternative.”
Color accents
So a seal is always (carbon) black? Not by a long shot. Like all thermoplastics, it can be easily colorized. As Freudenberg blue, for example. For one thing, blue represents the company. For another, the color is very popular for food equipment. The reason: There are no blue foods. That means any undesirable particle that breaks loose from a sealing material immediately catches the eye.
