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 a given application. Compatibility tables exist, but they are general starting guidelines and are incomplete.

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

The figure shows an example of an EPDM O-ring inspection compared to a post-processed O-ring test (82% swelling)

Rubber compatibility test

Test methods

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

Standardized methods for testing the 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 surface consistency; Hardness was omitted because previous experiments showed a direct correlation between swelling and change in hardness (swelling/softening vs. shrinking/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 masses of O-rings are measured before immersion in the lubricant.
2. A thick layer of the selected lubricant is spread along the aluminum foil.
3. The selected O-rings are placed on the grease, with plenty of space between the O-rings and only 1 type of elastomer per sheet of foil.
4. To hold the O-rings in place, each elastomeric 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 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 secondary wipe with a clean cloth, the O-rings are weighed again.
10. The determination of volumetric swelling (ΔV%) is calculated:
    

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

11. Visually inspect or measure length changes 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 lubricant formulations are not compatible with plastics and can weaken or degrade the part - so testing is crucial to determining the right lubricant for your application.

Applying the testing method described in this article can facilitate the selection process and may help users identify profitable and unprofitable grease options by calculating swelling and observing color and/or texture changes.

This article is based on a publication by Dr. Cassie J. Fhaner, MOLYKOTE® specialty lubricants researcher, and William Fick, applications engineer at DuPont.

Compatible lubricants for lubricating rubber and elastomers are described in an earlier article. What lubricants are best for lubricating rubber

Molykote 111 silicone mixture