Grease compatibility with elastomers: why it is important and how to test

Introduction

Elastomeric machine parts and components can be a cost-effective alternative to metal components, and proper lubrication can extend their life and performance. However, some traditional lubricant formulations are not compatible with elastomers and can weaken or degrade the part, making testing critical to determining the right lubricant for the application. Compatibility charts exist, but they are general starting guidelines and are not complete.

"Compatibility" in this regard is when the grease or other lubricant has minimal effect on the physical properties of the polymer or elastomer in terms of size, hardness, or brittleness. The acceptability of changes in the polymer and grease depends on the application in which they are to be used. Application of the test method described in this article will facilitate selection and help users identify viable and non-viable grease options by calculating swelling and observing changes in color and/or texture.

The figure shows an example of EPDM O-ring inspection compared to the O-ring test after treatment (82% swelling)

Rubber Compatibility Test

Research methods

Examples of test methods for assessing the compatibility of elastomers and polymers under various conditions are presented in the table.

Standardized Test Methods for Compatibility of Polymer/Elastomeric Materials

ASTM D4289 explains a method for comparing the effects of lubricants on elastomers using sheet materials to evaluate swelling and hardness. This article describes a simplified example of an elastomer compatibility test using O-rings and ignoring hardness. The O-rings were selected for their surface consistency; hardness was omitted because previous experiments showed a direct correlation between swelling and hardness change (swelling/softening vs. shrinkage/hardening).

Note: This method is recommended to be used as a first pass screening test and is not intended to guarantee elastomer/lubricant compatibility under all exposure conditions.

Procedure

Figure Balance equipped with a density determination kit

1. Using an analytical balance equipped with a density determination kit and recorded to an accuracy of 0.0001 g, the weights of the O-rings are measured before immersion in the lubricant.
2. A thick layer of the chosen lubricant is spread along the aluminum foil.
3. Selected O-rings are placed in grease, with plenty of space between O-rings and only 1 type of elastomer per sheet of film.
4. To hold the O-rings in place, each elastomer O-ring is gently pressed into the grease.
5. A second thick coat is then applied to the O-rings, making sure they are completely covered and that there are no air bubbles in the grease.
6. Each aluminum foil test sample is placed on a tray and placed in an oven at 125°C.
7. After 3 days (72 hours) at 125°C the grease packages are removed from the oven
8. After allowing the material to cool for 15 minutes, the elastomers are removed and wiped with a dry, clean cloth.
9. After a second wipe with a clean cloth, the O-rings are reweighed.
10. The determination of the volumetric swelling (ΔV%) is calculated:
    

    where: ΔV = volume change, %
    M1 = initial mass of the sample in air, g
    M2 = initial mass of the sample in water, g
    M3 = mass of the sample in air after immersion, g
    M4 = mass of the sample in water after immersion, g

11. Inspect or measure the changes in length and make a determination based on the table below:

Conclusions

Elastomeric machine parts and components can be a cost-effective alternative to metal components, and proper lubrication can extend the life and performance of these components. However, some traditional lubricants are not compatible with plastics and can weaken or degrade the part – so testing is critical to determining the right lubricant for the application.

Applying the test method described in this article can facilitate the selection process and can help users identify viable and non-viable lubricant options by calculating swelling and observing changes in color and/or texture.

This article is based on a publication by Dr. Cassie J. Fhaner, MOLYKOTE® Specialty Lubricants Researcher, and William Fick, Applications Engineer, DuPont.

Compatible greases for lubricating rubber and elastomers are described in an earlier article. Which greases are best for lubricating rubber

Molykote 55 O-ring pneumatic grease for o-ring seals

Silicone-based pneumatic grease for seals, gaskets and O-rings. Suitable for connecting metal-rubber materials and dynamic movements in pneumatic systems. Facilitates assembly and disassembly, protects against corrosion and increases the service life of seals.

Molykote 111 silicone compound