The second part of the new materials series deals with rubber. In the 1930s, radial shaft seals consisted of a sheet metal housing and a leather sleeve. But then leather became scarce and expensive and Freudenberg replaced it with rubber.
Compact
History of the Simmerring: In the 1930s, leather was replaced by rubber in Simmerrings because leather was expensive. Rubber proved to be dimensionally stable and elastically resilient, with high temperature and swelling resistance 1.
Production of rubber: The path from natural or synthetic rubber to high-tech rubber (elastomer) is complex and requires a mixture of ten to twenty ingredients, including fillers such as carbon black and silica, plasticizers and anti-ageing agents.
Vulcanization processes: Freudenberg Sealing Technologies (FST) uses various vulcanization processes, including compression molding (CM) and injection molding (IM). These processes differ in their degree of automation and the type of processing.
Chemical and physical properties: Rubber materials must be resistant to various media and temperatures, depending on the application. The service life of a rubber seal is significantly reduced when temperatures rise.
Test procedures and innovation: FST carries out stress tests to determine the application limits of new materials. These tests simulate real-life conditions and provide important findings for the practical suitability of new material solutions.
Almost 100 years ago, the first Simmerring® consisted of a sheet metal housing with a leather sleeve. But leather was expensive. Freudenberg looked for a replacement and found what it was looking for, replacing its traditional sealing technology product with rubber. As it soon turned out, its good properties were convincing. The dimensionally stable but elastically resilient rubber was characterized, among other things, by a much higher temperature and swelling resistance compared to the engine lubricating oils commonly used in the 1930s. This marked the beginning of the triumphant advance of rubber as a sealing material – first in Simmerrings, then in O-rings, then in flat gaskets…
Engineer or cake baker?
There is a long way to go from natural or synthetic rubber to high-tech rubber, technically known as elastomers. Rubber alone – whether of natural origin or synthetically produced – is by no means enough. A rubber compound usually consists of ten to 20 ingredients. Just like baking a cake, the right ingredients are required in the right quantities.
When it comes to sweet baked goods, it is the sophisticated combination of flour, sugar, eggs, butter, milk or cocoa that ultimately produces the desired result … and (taste) experience. In addition to rubber, fillers such as carbon black and silica are particularly important for the perfect rubber compound, as they increase strength and durability. In addition, plasticizers initially improve the flow and later the expansion and swelling behavior during processing. Anti-ageing agents slow down material fatigue, while other ingredients protect against ozone, for example.
Mixing plant: From weighing to shipping
Depending on the recipe, all ingredients are first weighed to the milligram in Freudenberg Sealing Technologies’ (FST) raw mixing plants around the world. They are then mixed in a large kneading machine to form a homogeneous mass. This is then further homogenized in a rolling mill through constant folding and rolling.
At the end, the density, hardness, strength and elasticity of each batch are precisely checked in tests during series production. Once the quality test has been passed, the mixture is cooled down and cut into strips, cords or other shaped blanks in a so-called batch-off system. After packaging, the ready-made mixture is sent to the molding production facilities. The trend in the FST mixing plants is towards networked machines, digitalization and automation in all work steps. The raw mix plant in Weinheim is one of the pioneers in this field.
Plastic becomes elastic
Crosslinkers, such as sulphur, are a very important ingredient in the mixture. Because what baking in the oven is to cakes, vulcanization is to rubber during shaping in series production. During vulcanization, the rubber molecules are cross-linked at a temperature of 120 to 160 degrees Celsius. This cross-linking gives the rubber its elasticity, transforming plastic into elastic.
One last time back to the baking analogy: like a good pastry chef, FST writes the recipe, mixes the ingredients and bakes them itself. FST only buys in the “baking ingredients” – just like the confectioner, who doesn’t grind his own flour.
Several processes for one goal: Rubber
FST uses various vulcanization processes to produce seals from rubber materials. The two most important are abbreviated to CM and IM.
In compression molding (CM), the blank is manually inserted into the component mould of the open tool. The press is closed. The heat generated at the top and bottom by heating plates sets the vulcanization process in motion. This is the simplest procedure.
In injection molding (IM), the compound, which has previously been pre-processed into cords or tapes, is injected directly into the closed mold under high pressure and vulcanized there. This is the most complex process with the highest degree of automation.
Other processes are called transfer molding (TM) and injection transfer molding (ITM).
Depending on the rubber material used, seals must be reheated after molding. In the post-heating oven, additives and processing aids outgas and the vulcanization network is firmly sealed. In short: post-heating gives gaskets and their materials their final properties.
Chemistry and physics
Depending on the application, elastomer seal materials must be chemically resistant to media such as mineral and synthetic lubricating oils and greases. They must be able to withstand cold, heat, water, humid air, hot steam, ozone or aggressive cleaning agents.
The rule of thumb applies here: The speed of chemical reactions doubles with an increase in temperature of ten degrees Celsius (RGT rule). Conversely, this means that the service life of a rubber seal is reduced enormously when temperatures rise. The media resistance of rubber materials is therefore highly temperature-dependent. It also decreases due to higher pressure, higher speeds, higher friction – because all these phenomena increase the temperature.
In terms of physics, rubber seals are primarily concerned with preventing swelling, shrinkage and the penetration of the sealing material by a gas, known as permeation. In addition, low friction reduces the temperature and therefore wear and energy consumption.
Not all rubber is the same
Rubber materials are generally characterized by high elasticity, which is associated with a high sealing effect and restoring forces. In combination with the right lubricant, they offer good wear and abrasion resistance. One of the disadvantages of rubber as a material is that it is only suitable to a limited extent for high-pressure applications, for example in hydraulics, due to its lack of hardness.
Not all rubber is the same. Individual rubber materials differ in terms of density, hardness, tensile strength, deformation behavior, temperature range of use and many other characteristic values. They are therefore suitable for different areas of application.
Some examples: FST’s standard material, for example for classic radial shaft seals, is NBR. HNBR has a higher temperature and media resistance. Silicone is less resistant to abrasion and is therefore the first choice for static seals in particular. ACM copes well with cold temperatures down to -40 degrees Celsius and heat up to 160 degrees Celsius, and is also resistant to ozone and hot air. FKM has proven itself as a versatile problem solver. FFKM Simriz® is the high-end solution from FST for the process industry.
On the test bench
At several FST sites, test fields are key components of FST’s innovative strength. What are the application limits of new materials? Can they withstand the high speeds prevalent in electromobility? Can they withstand new types of lubricants, the consequences of electrical charge shifts, pressure, cold, splash water, dirt and vibrations? Stress tests in the test field reproduce reality as closely as possible and provide important findings for the practical suitability of new material solutions. Operations in the Arctic and desert can be simulated here, as can car journeys through water and mud. The largest Simmerring test facility is in Weinheim. In Plymouth, FST can test in fast motion how materials prove themselves in hydrogen electrolysers over many years of continuous use.
However, even 1,000-hour endurance tests on the test bench are not enough to ensure that seals will last for decades, as required in offshore wind turbines, for example. Digital simulation tools are essential for this. FST has been using tried and tested models for years, for example for the long-term ageing behavior of rubber in static seals.
New tasks
More and more frequently, rubber seals have to take on additional tasks that go far beyond sealing. Sometimes they need to be electrically conductive, sometimes they need to be able to shield heat or electromagnetic radiation. Or they should be able to predict as accurately as possible when they need to be replaced in so-called condition monitoring. FST also develops solutions for this.
