Lubricants as additives for polymers assist the movement of one object passing another object. Their primary role is to reduce friction, minimize wear, and prevent overheating of parts.
While wear and heat cannot be completely eliminated, reducing them to negligible or acceptable levels is a must to maintain performance in your application! And, the selection and use of right
lubricant plays an important role here!
Explore, in detail, about the basics of lubrication, different types of lubricants available, and their properties as well as get tips to select the best suitable lubricant for your polymer applications.
We would like to acknowledge Paul Seemuth for providing technical information needed to develop this guide.
What are Lubricants?
Lubricants usually act by modifying the viscosity of the melt, by introducing different surface energies at the interface between the phases. But, simple sticking between the melt and the processing machinery (screws, barrels, and dies) can also be a significant brake on throughput (not to mention requiring frequent stoppages for cleaning down).
Very often the same material can be used for internal lubrication (at a low addition) and for mold release (at a higher level). However, the addition of multi-functional systems, in which an apparently high-priced lubricant can more than pay for itself by modifying other properties, such as:
Improved distribution of other additives, and even
Moisture and gas barrier properties
The addition level is usually about 0.5-3.0%, depending on the individual recipe and process, but a high-performance additive can be effective at as low as 0.1% dosage as well.
Under-lubrication can cause degradation and higher melt viscosities, but over-lubrication can cause excessive slippage and reduced output. An imbalance of lubricant and stabilizer can cause plate-out or migration of pigment from the melt.
So, in-depth knowledge about lubricants is important before starting with the formulation.
Desired Properties of Lubricants
The preferred chemical and physical properties of lubricants are widely variable. Cosmetic and fine chemical use demand very rigorous control of properties. Metalworking and drilling fluid uses normally have lower needs on overall purity and chemical properties.
Let’s take a look at the physical properties of lubricants first:
Color normally indicates the purity of lubricants (especially synthetics). Higher the APHA color, the higher is the presence of undesirable impurities in the lubricant. Examples: textiles are very sensitive to colors of lubricants that may affect the whiteness.
Lubricants may be in contact with this surface for long periods post-production, affected by storage and shipping conditions.
Metals tend to use lubricants in a transient fashion, the lubricant may be present for only a very short time. Thus, the presence and purity of the lubricant may or may not be crucial to quality.
APHA color references are used in visual quality check; color is a good check on the lubricant’s quality.
Viscosity is crucial for handling frictional properties. However, the nature of the lubricated surface may very well dictate the required viscosity. Soft polymer surfaces may rely more heavily on low viscosity lubricants whereas metals can easily use much higher viscosity lubricants. The selection of viscosity, therefore, becomes one of many variables the researcher must examine.
Thermal stability is an important function of molecular weight. The higher the molecular weight, normally the greater the thermal stability of the lubricant.
Thermal stability can be achieved at low viscosities via branching of the chemical structure. As previously noted that molecular weight and branching is directly related to viscosity effects, one can achieve thermal stability by manipulating the chemical structure, i.e. branching, to increase molecular weight while balancing viscosity effects. Certain lubricant classes are inherently more thermally unstable even at higher MW, i.e. polyethers.