Friction can also be reduced through use of internal lubricants. Internally-lubricated compounds have a friction-reducing agent homogeneously dispersed directly into their chemical structure. This dispersion of the lubricant is made at the time the elastomeric compound is initially mixed. Though the added lubricant does alter the compound, the elastomer’s basic properties remain largely unchanged. An internally-lubricated nitrile is still nitrile; it is simply a special formulation of nitrile designed to minimize friction.

It may help you to think of internal lubrication as the end result of a planned incompatibility. By design, the added friction-reducing agent will not be chemically compatible with the base elastomer. This means that the agent will separate itself out and “bloom” up onto the O-ring’s surface, as illustrated in Figure 82. This continual blooming of the agent keeps the seal’s exterior coated with lubricant, making the O-ring slippery and less inclined to stick during startup.

It’s also worth noting that lubrication can be either organic or inorganic. Widely used organic lubricants include amides (in both flake and pellet form), waxes, esters, powdered PTFE, and mineral oils. Inorganic agents include graphite (powdered and flake) and MoS2. Whether organic or inorganic, the lubricant in use must be compatible with system fluids to avoid leaching (removal) of the agent, which can lead to dangerous degrees of seal shrinkage. The lubricant must also be compatible with all adjacent surfaces to avoid structural damage, such as graphite pitting of stainless steel.

Table 40 shows the results of abrasion and friction tests conducted on hydrogenated nitrile (HNBR) containing a number of widely used organic and inorganic lubricants. As shown, internal lubrication using organic oleamides offers an outstanding blend of abrasion and friction properties. R.L. Hudson & Company is pleased to offer a series of internally-lubricated compounds utilizing oleamides (as well as compounds featuring MoS2, graphite, waxes, and powdered PTFE). These compounds are available in a wide variety of products, including O-rings, U-cups, piston seals, and valve seats.

Once all the major environmental factors have been taken into consideration, due attention must still be paid to the actual physical space in which the O-ring will reside. With that in mind, let’s take a closer look at the importance of proper gland construction next.

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