O-Rings FAQs

In terms of mechanical properties, the question arises as to how the O-ring is used: does it have a static sealing function or is it dynamic? If the latter is the case, it plays a role how high the mechanical stress is and whether the O-ring is rarely, regularly or permanently in motion. If dynamically moving components are used,  coated O-rings can  be considered, which, thanks to their so-called dry lubrication through a lubricating varnish of a few µm thickness, prevent the occurrence of so-called "stick-slip effects", i.e. jerking and squeaking noises. Otherwise,  O-rings made of H-NBR  have a low compression set and both the mechanical strength and abrasion resistance to be suitable for dynamic applications.

resistance to media
O-rings must not only be able to withstand mechanical influences, but also those of a wide variety of media. The resistance of the sealing material is therefore also very important: What does the O-ring come into contact with? Are greases or oils used for assembly? What media, such as water, steam, chemicals, must it seal against? Are there perhaps many different media in alternation, such as alkalis and acids or brake fluid and mineral oils?

Not all materials combine all resistances to the same extent. For example, an NBR O-ring is resistant to diesel fuel, engine oil or kerosene. In contrast, the resistance of EPDM O-rings to these media is exactly the opposite, while  O-rings made of FFKM/Krevolast®  are resistant to the aforementioned media.

O-rings must not only be able to withstand mechanical influences, but also those of a wide variety of media. The resistance of the sealing material is therefore also very important: What does the O-ring come into contact with? Are greases or oils used for assembly? What media, such as water, steam, chemicals, must it seal against? Are there perhaps many different media in alternation, such as alkalis and acids or brake fluid and mineral oils?

Not all materials combine all resistances to the same extent. For example, an NBR O-ring is resistant to diesel fuel, engine oil or kerosene. In contrast, the resistance of EPDM O-rings to these media is exactly the opposite, while  O-rings made of FFKM/Krevolast®  are resistant to the aforementioned media.

With regard to the operating temperature, it is important to know the temperature range for which the O-ring is intended, i.e. what the minimum and maximum temperatures are in the operating environment and whether these temperatures are permanent or only temporary. When designing, it is important to ensure that the respective temperature limits were determined without pressure and without the influence of media.

If elastomers are exposed to low temperatures, this is detrimental to their resilience and elasticity, up to the glass transition point, at which the material becomes glassy and brittle. In this state, it is difficult to create or maintain a sealing effect. Silicone O-rings , for example, have very good low-temperature behavior and, depending on the silicone material, can be used at temperatures of up to -60 °C.

Too high temperatures, on the other hand, also have a negative effect on the sealing effect of the O-ring. O-rings that have exceeded the upper continuous temperature often show consequences such as embrittlement, a sooty surface and, above all, a tendency to break apart when pressure is applied. The influence of pressure and media further reduces the temperature resistance of O-ring materials. For this reason too, it is sensible to always select the next best material if the actual temperature load approaches its maximum range. When it comes to heat-resistant O-rings, silicone is also an excellent example of a material - certain HTV silicone types can withstand temperatures of up to +250 °C even under high pressure.

However, resistance to high and low temperatures is only one of many requirements that must be reconciled with, for example, mechanical properties, pressure and media resistance or with standards and approvals. Therefore, a compromise is often necessary in order to find a material that covers the widest range of required properties.

The last component that is crucial when designing an O-ring and that must be coordinated with the previously mentioned influencing factors in addition to economic aspects is compliance with approvals. It is not without reason that many areas of application require compliance with industry-specific and legal requirements, as an incorrectly selected O-ring material can have fatal consequences. Certifications and compliance with applicable guidelines and material approvals are therefore mandatory for the safe manufacture of products.

A failed O-ring in a highly interconnected industrial process - for example in the food or medical technology sector - can have far-reaching consequences before the manufacturer even notices it and can react accordingly. Industry-specific approvals aim to prevent any interactions between production facilities and the product.

Different economic areas require that the production facilities used, and therefore also the seals, comply with their own industry-specific requirements. It is therefore necessary to know and observe these regulations in order for a company to be allowed to market products in this target market.

However, the focus is not only on the immediate operational safety of machines and systems: In complex elastomer formulations that are used in demanding sealing applications, volatile plasticizer components can migrate from the material through natural demixing and enter the human body via the product.

The range of approvals and specifications is wide. At a national level, the regulations of the Federal Office of Consumer Protection and Food Safety (BVL) together with the detailed recommendations of the Federal Institute for Risk Assessment (BfR) are decisive. The requirements of the Food and Drug Administration (FDA) in the USA are of international importance. If you do not comply with these globally recognized standards, you are not authorized to export your systems to the user countries. In terms of hygiene, the specifications of the 3-A Sanitary Standards organization must be observed. The purity requirements for drinking water go beyond those for the food sector. In this country, the selection criteria for sealing materials that come into contact with drinking water are derived, among other things, from the KTW guideline of the Federal Environment Agency. Globally recognized standards for materials used in pharmaceutical and medical technology are regulated in the United States Pharmacopeial Convention (USP).

Since solid lubricants are bound in an O-ring coating   , it not only meets these cleanliness criteria, it  also reduces  the  sliding and static friction resistance against sliding friction partners without changing the mechanical-physical characteristics. This is not only an advantage in O-ring assembly, but also generally in automated production, as well as in dynamic processes.

It is particularly important here to  reduce stick-slip effects  (also known as  adhesion-slip effects  or  frictional vibrations  ). In addition to preventing stick-slip, the O-ring coating also  prevents squeaking noises  and  premature wear  on parts subject to greater stress  .

The assumption that O-rings can be coated to reduce the permeation of liquids and gases is not tenable. Although a continuous coating on the surface of an O-ring increases the  barrier effect against liquid and gaseous media  with increasing layer thickness, with an average layer thickness of 3-12 µm this effect is barely measurable in the installed O-ring, i.e. under tensile stress. Under tensile stress, microcracks will appear in the already thin coating, which cannot resist the permeation of liquids and gases. An O-ring material that is not resistant to oils before coating will therefore remain unstable after coating and will continue to swell when it comes into contact with oil.

The coating of O-rings – for example with anti-friction varnishes as so-called  dry lubrication  – is usually carried out as  an automated mass part coating  , e.g. using the  dipping ,  spraying or  drum process . This allows large quantities of O-ring seals to be coated reliably and in  a time- and cost-saving manner  . To stay with this example: The uniform, firmly adhering  PTFE portion with a layer thickness of a few µm also makes it easier to separate the parts in an automatic feed system, as it acts like a  separating layer  and thus  prevents the O-rings from sticking together . The coating is particularly recommended for the fully automatic assembly of O-rings, which, due to the quantity situation, is particularly suitable for applications in the automotive industry.

Speaking of  the automotive industry : Here, O-rings must also  be free of paint-wetting-impairing substances  - in short:  LABS-free  . Some  elastomers , additives and thus recipe components of rubber mixtures can contain substances that can impair wetting during the painting process of car bodies. These O-rings are therefore subjected to  plasma cleaning  to free them of the undesirable substances.

The appropriate coatings are  available for all these and other application and storage criteria  , for example:

• PTFE coating
• polysiloxane coating (silicone resin)
• plasma cleaning ("LABS-free")
• fluoridation
• siliconizing
• talcuming (microtalcuming)
• Graphitizing silvery shiny
• Molybdenum plating (MoS2 in powder form)

Below we will discuss the individual types of coating for O-rings in detail:

The abbreviation  PTFE  stands for  polytetrafluoroethylene . It is a fully fluorinated polymer, for which the trade name  Teflon®  from the company DuPont is also sometimes used. Due to its properties, it is used for coating O-rings, among many other areas of application. Firstly, it is  very unreactive due to the particularly strong bond between the carbon and fluorine atoms . Secondly, PTFE coatings have a  very low coefficient of friction , while the  static friction  is  exactly as high as  the  sliding friction , which avoids slip-stick effects. In addition,  PTFE adheres very well to the surface of the elastomer .

The coating itself can   be applied  in various colors as a PTFE sliding varnish . Depending on the layer thickness, PTFE coatings have an elastic surface , whereby even thin layers  are very abrasion-resistant  . Over time, PTFE particles settle in the surface of the sliding partner, which ensures a  permanently low angle of friction  . PTFE-coated O-rings are  easy to install  and  process automatically  and are  permanently separated .

The polysiloxane coating is a  water-dilutable, one-component lubricating varnish  made of silicone resin with graphite components. The result is a  particularly smooth, supple surface that  is ideal for automatic assembly processes  - not least because of the  reduced press-in forces . The polysiloxane coating offers  sufficient dry lubrication properties even for low-dynamic applications . It also shows  excellent adhesion  to the O-ring surface, which  remains recognizable in color thanks to its transparency  .

The term  " LABS-free "  stands for  "free of paint wetting-impairing substances" . Such substances, e.g. silicones, can adhere to the surface of O-rings and have negative effects when used, for example in the automotive industry. Even the smallest quantities would prevent an even application of paint to the body shell and cause coating defects such as craters. Superficial cleaning is usually not sufficient in this case, which is why all paint wetting-impairing substances are removed from the O-ring using a special cleaning process.

The  adhesive  and  wetting properties of the O-ring surface are increased  during this process  . The O-rings are then machine-coated with the respective lubricating varnish.

LABS-free O-rings must  be handled extremely carefully  to avoid recontamination. It is helpful, for example, to use LABS-free gloves and to store them in specially designed packaging. LABS-free O-rings meet the  VW test specification 3.10.7, among others  , and are also  very well suited for automatic assembly.

Fluorination is not a coating per se, but a  chemical reaction in which the  surface properties of the very top O-ring material layer  are sustainably improved without affecting the actual material properties. To do this, the surface of the O-rings is exposed to a fluorine mixture. The penetration depth of the fluorine atoms is in the molecular range and can still be measured in the unit of inflow.

Fluorination results in a  significant reduction in sliding friction and adhesion , which  prevents stick-slip effects  and  squeaking noises  in dynamic applications  . Since fluorinated O-rings are easier to separate, the use of silicone oils, talcum and other release agents is unnecessary. These properties remain  stable over the long term, even when cleaned and sterilized . In addition, this type of coating has a positive effect on the  cleanliness of O-rings  , since  dirt particles do not  adhere to the fluorinated surface.

Siliconizing is a  cost-effective O-ring coating that  is applied  using a drum process  . During siliconizing, the O-rings are coated with a transparent  lubricant based on silicone oil  . This silicone emulsion  reduces  assembly forces ,  improves  sliding  properties ,  adheres well and gives the O-rings a  smooth surface . As the silicone oil coating penetrates the O-ring material and dries out over time, the sliding properties decrease and the surface tends to stick. It is  not suitable for automatic separation .

To talcum O-rings,  the finest talcum powder is  applied to the surface using a drum process. Although this type of coating enables  very simple, automatic separation  and assembly, the  abrasion resistance is low , which can lead to contamination of the feed system and the assembly in which the O-rings are used.

On the one hand,  graphitization  prevents  the O-rings from sticking  or eliminates the  tendency to stick  , and on the other hand,  it improves  their  sliding properties  and  makes assembly easier . This type of coating uses graphite powder, which is drummed onto the surface of the O-rings.

The term "molybdenum coating" is made up of the English words "molybdenum (disulfide)" and "coating". The  coating  with molybdenum sulfide (MoS2) powder,  applied using a drum process, significantly improves the sliding properties  of O-rings and  prevents them from sticking , so that this process also facilitates automatic separation and assembly. Since the  abrasion resistance of the coating  is low, this can result in contamination of the system. This type of coating is  no longer state of the art .

The most obvious consequences of LABS are  point-shaped  or  funnel-shaped depressions  - so-called  craters . In the center of such craters, the surface to be painted is not completely covered by paint. This has not only  aesthetic disadvantages , but also  functional ones  - for example, if the paint is also applied for reasons of  corrosion protection  . Painted surfaces that have craters must then either be laboriously sanded down, cleaned and repainted or - depending on the size of the crater - disposed of immediately.

Paint processing companies and production facilities therefore place  very high demands on the cleanliness of supplied products such as O-rings in order to reduce any subsequent costs caused by contamination to a minimum. Car manufacturers and paint processors in particular have drawn up specifications for this purpose, which  contain methods for checking whether  components are LABS-free. Suppliers are therefore often faced with very strict and different requirements for checking whether their products are LABS-free and often have to apply several specifications to one component.

There are a number of substances that  interfere with paint  wetting, including:

• separating, drawing and lubricant residues
• operating and auxiliary equipment
• plasticizers
• Remains of personal care products (hand cream, etc.)

A complete, universally valid list of all LABS is impossible. One thing is certain: even a  very small amount  of them can lead to  wetting errors  in the paintwork.