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
Standardized method | Properties rated |
ASTM D4289 | Lubrication compatibility with elastomers: greases and fluids |
ASTM D471,ASTMD543,ISO 175 | The effect of liquid chemicals on polymeric materials |
ASTM E831 | Thermal expansion of solid materials |
ASTM D7264,ASTMD790,ISO 178 | Bending properties of polymeric materials |
ASTM D638, ISO 527 | Measurement of tensile properties |
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
- 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.
- A thick layer of the selected lubricant is spread along the aluminum foil.
- 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.
- To hold the O-rings in place, each elastomeric O-ring is gently pressed into the grease.
- 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.
- Each aluminum foil test sample is placed on a tray and placed in an oven at 125°C.
- After 3 days (72 hours) at 125°C, the grease packages are removed from the oven
- After allowing the material to cool for 15 minutes, the elastomers are removed and wiped with a dry, clean cloth.
- After a secondary wipe with a clean cloth, the O-rings are weighed again.
- 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 - 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
Molykote Separator Spray Food Grade Silicone Oil NSF H1 - 400 ml
Food-grade silicone oil in aerosol, used as a release agent.
Applications and uses
Release agent in the production of rubber and plastic products and cardboard packaging.
MOLYKOTE® G-4500 FM with PTFE
White multi-purpose synthetic food-grade grease compatible with rubber and elastomers
Thickened with aluminum complex, uses white solids and PTFE as solid lubricants. Additionally, it is H1 certified by NSF for incidental food contact.
WEICON PTFE Spray Dry Teflon grease
Dry lubricant, great sliding properties
"WEICON PTFE-Spray is an oil-free, dry grease based on PTFE with excellent sliding properties. Adheres to all metal, wooden and rubber surfaces. High content
PTFE reduces the coefficient of friction, and the lubricant coating itself does not absorb dust, dirt or water, and is resistant to grease, oil and many chemicals.
MOLYKOTE® 3451 Chemically resistant grease compatible with rubber and elastomers
Chemically resistant fluorosilicone grease, thickened PTFE grease, suitable for rubber lubrication.
Krytox GPL 205 Fluorine Non-flammable grease
High-quality non-flammable grease approved for contact with oxygen, white, clean, non-toxic, non-reactive. Compatible with rubber.
Fluorine grease based on perfluoropolyether PFPE, does not contain additives and can be used on elements that come into contact with chemicals. Used for lubrication of seals and O-rings in the aviation industry.
Molykote HP-300 PFPE grease for extreme conditions - 500g
Fully fluorinated grease provides exceptional performance in extreme conditions. PFPE-based grease with a base oil viscosity of 160 cSt at 40 C, uses PTFE as a solid lubricant.
Commonly used in harsh environments such as low or high temperatures, corrosive, solvent, liquefied natural gases, high vacuum, etc. Can be used in clean room equipment and semiconductor manufacturing equipment where volatilization of lubricant material is undesirable including seals made of rubber and elastomers