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Product Focus: O-rings with special coatings

All seals require some form of installation into application hardware and often this installation process may seem to be a simple push in place function. However, without consideration of certain conditions this can potentially create sealing problems further into the life cycle of the seal. Once installation is achieved a seal can often sit in the housing hardware for many months...which potentially allows it to stick  to the housing material and cause further issues. This is where special coatings can make the difference.

Why use special coatings?

The main function of seal coatings is to improve installation into the hardware and to lower friction within an application. Added benefits also include lower assembly forces being required, seal parts not sticking together in packaging and coatings prevent the need for any additional grease-type lubricants. Special coatings can be manufactured in different colours so that seal and O-rings for a specific application can be easily identified without the need to reformulate the elastomer compound with a colour additives.

What types of coatings are there?

There are a variety of coatings and lubricants available for the surface area of seals, and these are mainly divided into two common categories; wet coating/lubrication or dry micro film coating. Historically, coating technology was limited to just using wet type coatings. For example these may include silicone oils and greases and mineral oil based greases. This coating process would involve simply dipping the seals in the oil/grease lubrication coating during manufacture, or during installation on site. This is often be a messy process, and can create issues with contamination of the customer hardware application. Though still a cost effective method, wet coating is less common as more applications demand higher levels of cleanliness products, therefore low contamination has become a key factor.

Alongside the typical wet coatings traditionally used, there is an extensive range of what is often referred to as dry coatings. Common offerings are silicone dry coating, PTFE coating, PFPE coating, special polymer coating and MoS2 dry powder coating.  These are applied during the manufacturing process and bagged in a clean and efficient process, delivered and ready to be installed in application straight from the bag.



Coatings focus

Dry coatings are fast becoming the coating of choice for many customers, with PTFE dry coating being one of the most popular; most commonly applied to O-rings. The process involves the PTFE polymer being spray applied to the O-ring elastomer surface area, therefore creating a microscopic thin film layer, and at molecular level the PTFE then creates a covalent like bond to the surface ensure a high quality finish.

The low friction properties of PTFE gives the advantage of low assembly forces. For example an O-ring assembled to a piston can be installed easier into the bore hardware housing mating part, therefore greatly reducing the chance of installation damage (like pinching of the O-ring elastomer material). PTFE coatings can also be used as a short term dynamic improvement to a sealing solution, as many seals suffer from the phenomenon called stick slip. This can occur where a seal has remained in the application hardware bore for a medium to long period of time and started to adhere between the surface of the elastomer and the mating metal material, causing sticking. When a PTFE coating is applied between the elastomer and the metal hardware it creates a low friction barrier, that provides a significant advantage over uncoated parts. This is because the PTFE coating generates a low friction layer, greatly reduces breakout friction and resolves the stick slip issue therefore greatly improving the performance of the application.

Because the PTFE is a thin polymer layer, coatings can be manufactured in a range of colours by adding pigment to the recipe. Typical colours include, green, blue, red, yellow and orange. By adding these colours the customer benefits from easy identification and production line inspection and furthermore allowing seals to be easily identified even once installed.

An inherent problem with gaskets that can drop into place is that often, they easily drop out of place too. If the component needs to be inverted, or has the potential for rough handling during assembly then the gasket may become partially or fully dislodged from the groove, which results in a badly sealed interface. The best solution to this issue is to incorporate retention pips or bumps in the gasket design, a solution known as Push-In-Place (PIP) gaskets. These require a distinct force to put them into the groove, and as a result require more than just gravity to get them out of the groove.

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