Many machine operators and engineers are under the false impression that plastic components included in equipment need zero lubrication, while others mistakenly think that a solution designed specifically for plastic is necessary. However, plastic parts must still be lubricated to mitigate friction and the wear it causes to increase component life in the same way metal parts do. While lubricants must be compatible with plastic, a plastic-specific lubricant is not required.
For instance, controlled lab testing shows that plastic sliding bearings that are sufficiently lubricated can last five times as long as non-lubricated components. Some plastic materials are self-lubricating like Teflon (PTFE). However, they can still benefit when lubricated. At speeds of more than 1 rpm, any friction occurring on a non-lubricated Teflon sleeve bearing will increase and show a decrease when bearings are lubricated.
As touched upon above, the most critical criteria for selecting the correct lubricant for plastic components is that the product is compatible with the specific type of plastic in use. However, this level of compatibility must be authenticated under all anticipated speeds, loads, and operating conditions that the part could be exposed to. When a lubricant and a plastic are incompatible, negative reactions occur like stress cracking and eventually total part failure.
Understanding lubricant plastic compatibility
Factors involved in compatibility include the chemistry of the lubricant (base oil, additives, and thickening agent), resistance to aging and viscosity.
In terms of chemistry, mineral oils, silicone-based products like PFAE and most types of synthetic hydrocarbons (PAO and SHC) work effectively with plastics. In most cases, lubricants that are ester or polyglycol-based are typically incompatible, but exceptions to the rule exist depending on the kind of plastic parts are manufactured from.
When a lubricant’s formula is incompatible with a component made from plastic part, the internal part can lose structural integrity and dimensional stability. An indicator of this can be when the plastic starts to discolour.
To assess compatibility levels, manufacturers must work out the physical properties of the plastic including weight, volume, strength, hardness and elongation, both before and after it is exposed to the lubrication solution. Manufacturers provide limits on the level of acceptable change is after exposure, this is usually between seven and 10%. It’s essential that tests and exposures carried out reflect worst-case scenarios. Lubricants and plastics are always more prone to change in adverse environments where they are under high dynamic loads and at higher temperatures.
The additives in a lubricant can chemically react with plastic parts. For instance, solid additives like molybdenum disulfide (or moly for short) and graphite can penetrate plastic components, weakening their structural integrity. As a result, lubricants containing these additives must be avoided when lubricating plastic bearings or gears. However, PTFE solid additives can be useful under specific circumstances, for example, to provide dry lubrication or lower friction at startup.
When lubricating plastic parts, Extreme pressure (EP) additives are never advised. Additionally, large amounts of additives offering metal deactivation and corrosion protection which are commonly used to lubricate metal components, which are obviously an unnecessary inclusion when maintaining plastics.
Resistance to ageing
Outgassing byproducts of a plastic components, like styrene and formaldehyde, will accelerate the ageing process in lubricants. The knock-on effect is that as lubricants begin to age, they are more likely to cause plastic parts to degrade. As a result, long-term plastic-component applications are best served by synthetic lubricants, as they offer the highest levels of aging resistance.
Lubricants with a mineral-oil-bases are considered suitable candidates to keep plastics lubricated as they do not attack most types of plastic and provide excellent performance at cost-conscious prices in standard plastic applications. However, when machinery must move at higher operating speeds, under higher temperatures, and for an extended time, operations typically prefer to use synthetic lubricants, like hydrocarbon (PAO) types for bearings and gears made from plastic. The high-aging resistance of PAO are compatible with most types of plastics, and can offer long-term lubrication at a wide range of temperatures.
High-viscosity oils, with a viscosity rating of up to ISO VG 100 or more, offer less likelihood of plastic penetration and deterioration. Greases that have an NLGI of either 1 or 0 can reduce friction as well as grease-induced noise. NGLI is a measurement of how hard a grease product is. A 0 to 1 rating indicates that the grease is either semi-fluid or fluid.
PFAE and silicone lubricants
Among the most plastic-compatible products, PFAE lubricants are even considered suitable for some of the most hard-to match plastic types. Like PAO oils, PAE lubricants offer an excellent balance between wetting plastic surfaces and adhesion. Their most sought-after property is how well they can handle extreme temperatures, however, because of their higher costs these oils are only deployed when required. Silicone-based lubrication products are suited to lower load applications, but also have outstanding compatibility.
If you are unsure of the best lubricant for an application involving plastics, consult your OEM for a recommendation or take advice from an expert lubricant procurement specialist.