Technical Standards Update

Addressing the Unreliable Events of Rubber Bonding

By Greg Vassmer, technical coordinator, ARPM

Greg Vassmer ARPM Technical Coordinator

While finished rubber products are sometimes simply standalone molded rubber shapes, most are part of an assembly where the rubber component or its assembly is attached, bonded or molded into additional components. Consider a radial lip seal. This important mechanical component is used to prevent lubricant from leaking between a stationary housing (like an engine block) and a rotating shaft (like the crankshaft). The modern incarnation of lip seals most likely have the rubber “lip” chemically bonded to a rigid case. The bonding happens in the mold while the rubber is flowing into the part-shaped mold cavity. The expectation is for the bond to be complete and permanent once the press is opened. In some cases, the rubber is molded in such a way that it covers the outer diameter of the seal as well. Rubber in this area is meant to provide a static seal so when the seal is pressed into the gearbox, engine or other housing, the rubber will seal any imperfections in the housing and protect against those leaks.

Bonding can be a remarkably unreliable event, if the metal is oily, has a layer of rust, or, if the rubber moves by so fast during injection that it scrapes the adhesive off the metal case completely. So how does one know when the bond is poor and non-functional and how should it be checked? Well, to check is the same way anyone checks to see if something is glued adequately enough – pull on it. In fact, methods used to “pull on it” are important enough that ARPM developed OS13, a guideline and standard describing how to pull, and then how to gauge whether the result has adequate bond strength.

It is easy to detect when rubber is completely unbonded. An operator simply pulls the rubber portion, and it peels away from the substrate cleanly. Almost no effort is required. In most other cases though the operator will find it difficult to impossible to separate the rubber from the substrate by pulling by hand. So, the next level of effort unusually involves a pair of pliers and a twisting sort of yank on the exposed rubber bit. Does this sound like a scientifically reproducible test? It does not and is not, but a surprising number of people still do it. This method is so popular that pliers-based operator sample testing is built into OS-13 which does its best to standardize the procedure. There one can find several types of pliers to use, both for grabbable geometry like a seal lip and hardto-grab rubber seal coatings. How the rubber is grabbed is important and avoid cutting the rubber with the tool while pulling.

Ok, so now the rubber has been torn away. Is the bond good? A general rule of thumb, codified in OS-13, is if the rubber tears before it pulls away from the substrate, the bond is adequate. Essentially the bond is stronger than the rubber material itself. The ultimate. What if the test shows the rubber partially tearing and some exposed metal of a failed bond? An unfortunately common event. Judgment on this count will be a bit of negotiation between customer and supplier. Normally between 85% of the torn area needs to be rubber with the remaining allowed to be exposed metal. If the calculation of the percentage is in doubt there are optical systems that can help estimate the coverage to eliminate this variable.

If a more controlled and reproducible test is needed (which fortunately is the case for more critical applications) the standard recommends using a tensile test stand that can pull uniformly and in a stable direction while measuring the force. To further ensure uniformity of the test, ARPM has worked with Instron (a test stand provider) to manufacture gripper jaws that fit inside the small volume in a seal and work with their stand. (For their #2712-04x gripper specify the CP133475 jaw.)

Sometimes a simpler material, like a latex-based coating, is used as a sealant for the outer diameter of a seal. It needs to be well bonded too, for all the reasons mentioned about rubber bonding. Since there is nothing to grab on these coatings a different test is used called a scratch test. In ARPM’s OS-14, the methods used to uniformly evaluate the bond of these can be found.

Both ARPM OS-13 and OS-14 have been significantly updated and issued standards in 2022. If one finds themselves still relying on uncontrolled pliers testing it is probably a good idea to have a look. u