Lubricants

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 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:

Impact strength
Low-temperature performance
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, an in-depth knowledge about lubricants is important before starting with the formulation.

Let's begin by understanding the physical properties of lubricants and their classification in detail.

Desired Properties of Lubricants

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.

#1. Physical Properties

Color

Color normally indicates of purity of lubricants (especially synthetics). Higher the APHA color, 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 use in visual quality check; color is a good check on the lubricant’s quality.

The following chart gives a general comparison of color versus performance and quality needs, where color is a partial indicator of impurities.

Color versus Performance and Quality Needs

Viscosity

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. Selection of viscosity, therefore, becomes one of many variables the researcher must examine.

Thermal Stability

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 are 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.

#2. Chemical Properties

The acid number is a key indicator of residual free acid. Residual acids may interfere with the material that is being lubricated. The acidic effect on the material to be lubricated should be reviewed prior to the selection of a lubricant and its related acid value.

The following chart is a simple example of the relationship of various properties of lubricant versus use in a respective industry.

For each, an approximate importance value is assigned to the five initial factors in lubricant selection per industry.

Properties of Lubricant versus their Use

Cosmetic, Pharmaceutical and Aerospace (composite) industries need very high levels of purity and use color, clarity (no haze, sediment, etc.), and very low acid volatility values are starting points in selection. Costs are a minor consideration;
Drilling fluids and heating oil applications require far less quality around color though are very sensitive to costs;
Automotive, textiles are widely variable as the many processes, needs and end-uses are viable and selection is highly dependent on a case-by-case evaluation.

Lubrication Theory

Hydrodynamic (Full Film Lubrication)

High-speed friction region where the movement of two objects past one another is separated by a complete film of fluid.

Boundary Lubrication

Low-speed, high load pressure regime where movement is controlled by lubricants, chemically or physically, attach to the object’s surfaces.

Stribeck’s curve

What type of Lubricant do you Need Depending on Load & Speed?

Note: Gases are a special category of lubricants and not included here

Major Lubricants Types

#1. Liquid Lubricants

Liquid lubricants are primarily used to support high-speed friction and heat dissipation.

Free-flowing of constant volume
Amorphous and difficult to compress
Easily conformable to the shape of containers

Pros & Cons of Liquid Lubricants

Pros	Cons
Multiple classes available	Selection for desired use difficult
Range of viscosities	Quality control of properties
Multiple sourcing	Wide range of costs
Wide Grades available	Thermal instabilities
Self-Healing	Health and safety problems within classes

#2. Waxes and Solids

Low speed, High load application regime
Protection against surface wear and corrosion due to surface damage
Deformable of constant volume
Non-conforming to the shape of containers (short term)
Resistant to changes in size and shape.

Pros & Cons of Waxes/ Solid Lubricants

Pros	Cons
Most effective at high pressure loads	Poor self-healing (broken films cause damage, therefore, shorten use lifetime)
Stable to extreme temperature and reactive environments	Poor heat dissipation
Multiple classes to select	High friction properties and wear at high speeds
Wide quality range	Few chemical structural variants (branching, etc.)
Substantive to surface with weak intermolecular forces	Limited sourcing

The other categories can be based on structural chemical makeup including:

Fatty acids, fatty amides, esters
Fluoropolymers

Silicone-based

Others include montan waxes, paraffin waxes, polyethylene waxes & polypropylene waxes. Let’s discuss some of them in detail:

Subcategory of Lubricants

Fatty Acid, Amides and Esters Based Lubricants

Fatty acids, fatty amides, esters are the 'classic' lubricants, derived from natural oils and fats and act by migrating to the surface. According to type, they can improve mold release, melt flow, lubricity and scratch/scuff resistance and reduce static build-up and wear. They have been developed mainly alongside the PVC industry.

Primary Amides

These include stearamide, oleamide and erucamide. These are used as slip and anti-blocking agents in polyolefins and other polymers, their selection and concentration depending on the degree of lubricity required. Generally, stearamides give the best anti-block performance, oleamide and erucamides the best slip properties. Erucamide tends to be preferred for LDPE, LLDPE and PP films, with good oxidative stability and low volatility. It also offers good release properties for injection molding.

Secondary Amides

Secondary amides include oleyl palmitamide and stearyl erucamide. These have good thermal stability, showing no appreciable breakdown below about 350°C, and are therefore suitable for lubricants for engineering/technical plastics with processing temperatures above 300°C. Secondary bis-amides are used as lubricants in styrenics and ABS, assisting in flow, mold release and anti-caking properties. They are also used in PVC formulations as lubricants and anti-blocking agents in film and sheet.

Fatty Acid Esters

A wide range of fatty acid esters of polyols and other compounds is used as internal lubricants, especially in PVC. Good flow, surface finish and improved clarity can be achieved in most processes, using either compounded or dry blend formulations. Stearic acid and hydroxy stearic acid are good external lubricants, giving good release properties and a smooth surface finish.

Compounds of adipates, palmitates, sebacates and stearates are used as lubricants and plasticizers for many types of plastics, including PVC and engineering plastics.

Di-octyl adipate is a low viscosity plasticizer with excellent processing characteristics and outstanding low-temperature performance, in PVC-coated fabrics, wire insulation, garden hose and low-temperature PVC packaging films.

Cetyl palmitate is used in place of natural wax and can act as a lubricant for engineering plastics. Octyl and iso-octyl palmitate are clear oily liquids, used as plasticizers for PVC, with anti-blocking properties and additional heat stability, and can also be used as a viscosity modifier for plastisols. Di-Butyl and di-octyl sebacate are used as primary plasticizers for low-temperature applications, such as films and containers for food packaging. The stearates are used, broadly, as viscosity stabilizers in PVC and lubricant/flow promoters in PS and ABS, particularly where low-temperature properties are required. Cetyl stearate is used as a lubricant for engineering plastics.

Get inspired: Go deeper to understand the types of plasticizers for plastics

High-Performance Fluoropolymers-based Lubricants

Fluoropolymers such as PTFE have been compounded into polyamides and other engineering thermoplastics to reduce friction and wear in gear systems. Also, they offer major productivity advantages in extrusion applications. The effect has been most marked in blown film extrusion, where what is described as fluoropolymer alloys act by coating the interior surface of the extrusion dies with a microscopically thin non-stick film.

This reduces friction at the resin/die interface and allows the extrusion compound to flow freely and more rapidly through the die opening. The non-stick properties also prevent the accumulation of resin particles at the exit of the die, so eliminating the major cause of die build-up. The non-stick film is continuously renewed by the additive during the extrusion process.

Silicone-based Lubricants

Silicones offer a number of advantages in terms of texture, strength, pliability and special finishes. Their excellent lubrication properties improve productivity while also offering other properties. At 0.1-1.0% addition, a silicone powder resin modifier can produce lower extruder torque, reduce power consumption and improve surface gloss.

In highly-filled thermoplastics, it can act as a processing aid and improve mechanical properties in highly-filled systems. An epoxy-reactive grade can be used with polyamides, polycarbonate, PPO, PBT and PET polyesters and thermoplastic elastomers; a methacrylate-reactive grade is suitable for polypropylene, polyethylene, PVC, polystyrene and ethylene propylene elastomers.

Boron Nitride

Another interesting chemistry is high purity boron nitride (BN) powders, which have been shown to improve the lubricity in a variety of plastics matrices.

BN is a natural lubricant, which is used to improve parts, which must be highly resistant to wear. It is also thermally conductive and an electrical insulator and can bring these properties to a plastics compound, as well as providing nucleation. The powders from Advanced Ceramics are large single-crystal materials with mean particle (crystal) sizes of 50 and 35 microns, respectively. Typical BN crystals are 15 microns or smaller, and agglomerates are also produced giving larger particle sizes, or over 250 microns.

Combination and Modification

As lubricants act by the precipitation of substances introduced to the phase interface of the heterogeneous polymer system and this process is governed by the difference in surface energies of the components of the system approaching thermodynamic stability.

It has been shown that pairs of typical lubricants, such as stearic acid, stearic acid amide, polyethylene wax PV-300, oxidized PE wax PVO-30, lignite wax and hydroxyethylated lignite wax, can be effective at an addition level of 0.3% in a viscous low-density polyethylene.

Modification (such as oxidation of polyethylene wax and hydroxyethylation of lignite wax) leads to a reduction in the coefficient of surface tension, so making it possible to increase the effectiveness of the lubricant. But chemical modification that increases the surface tension (such as stearic amide) makes the lubricant less effective.

Host polymer	Typical lubricants
Polystyrene (crystal)	Clear-melt zinc stearate (bis-stearamides are usually adequate) Secondary bis-amides assist flow
ABS	Metallic stearates in combination with glycerol mono stearate Secondary bis-amides assist flow
Styrene/acrylonitrile	Fatty acid amines, amides, secondary bis-amides
PVC	Concentrations of calcium stearate and hydrocarbon waxes (serving, respectively, as internal and external lubrication): low molecular weight polyethylenes are said to be among the most efficient external lubricants Most systems also incorporate processing aids, to give faster fusion and higher gloss Primary amides improve non-stick of plastisol sealing gaskets Acrylic and styrene copolymers
Polyolefins	Primary amides for slip/antiblock agents in LDPE, LLDPE, PP films Lubricants can tie up catalyst residues, usually calcium stearate Stearates and ethylene bis-stearamide waxes are sometimes used in processing of finely powdered polyolefins Erucamides preferred for films and moldings Fluoropolymer 'alloys' give a better use of machinery Methacrylate-reactive silicones
Polyamides	Special polyamide formulations improve film processing and performance
Engineering thermoplastics	Secondary amides (oleyl palmatamide, steatyl erucamide) Cetyl palmitate and cetyl sebacate are used Epoxy-reactive silicones

Typical Lubricants for Various Thermoplastics

Factors Influencing Lubricants Selection

Lubricants affect the processing and quality of the final material. Numerous relationships of the lubricant are crucial for selection.

The key criteria for selecting lubricants are:

Compatibility with the host resin
No adverse effect on properties
Easy introduction
Approval for specific applications (such as food or pharmaceuticals)
No (or almost no) retarding effect on gelation
No reduction in melt strength and extensibility
Good transparency
Improved plate-out performance

But out of all, the two major relationships are:

#1. Viscosity

As molecular weight increases, viscosity increases in a fairly linear manner.
The chemical structure of the lubricant can have profound effects on viscosity. Increased branching of the chemical structure decreases viscosity versus the linear analog. Example: butyl stearate (12 centistokes) space versus isobutyl stearate (10 cSt).

#2. Compatibility

Compatibility with Metals

Stable to a multitude of lubricants. Care must be taken with lubricants having reactive sites like fatty acids, fatty amines, fatty amides and phosphate esters. These materials can react with metal’s surface.

Compatibility with Polymers

Interactions with lubricants are well known and should be researched prior to lubricant selection. Polymers with a more amorphous structure, i.e. spandex or reactive sites, i.e. nylon, can adversely react with the lubricant

Compatibility with Composite Structures

Contact with lubricants is normally brief. Barriers have been linked mainly to interactions between the lubricant and the composite adhesives, mainly epoxides. Testing of the composite surface with the lubricant for tackiness is a good indication of undesirable interactions.

Compatibility with Bio-structures

Biological structures are extremely sensitive to lubricant types. Extreme care and review of FDA and the EU regulations is important to select available lubricants for use on these materials.

Download Brochure: Check out the lubricants used for plastic processing

Lubricants Selection Criteria for Polymers

The following table provides a comparison between polymer types and applicable lubricants used for them. The ratings are based on the ease of use of the different lubricants and cost-effectiveness.

Polymer Type ↓	Lubricant type →
Polymer Type ↓	Mineral oils	Natural esters¹*	Synthetic esters	Polyethers**	Silicone oils
Polyolefin	⬤⬤⬤⬤	⬤⬤⬤⬤◯	⬤⬤⬤◯	⬤⬤⬤◯	⬤⬤⬤⬤⬤
Polyester	⬤⬤⬤⬤	⬤⬤⬤◯	⬤⬤⬤◯	⬤⬤⬤⬤◯	⬤⬤⬤◯
Polyamide²	⬤⬤⬤	⬤⬤⬤⬤	⬤⬤⬤⬤	⬤⬤◯	⬤⬤⬤⬤◯
Aramid	⬤⬤⬤⬤⬤	⬤⬤⬤⬤⬤	⬤⬤⬤	⬤⬤⬤	⬤⬤◯
Polyurethane	⬤⬤⬤◯	⬤	⬤⬤⬤◯	⬤⬤⬤⬤◯	⬤⬤⬤⬤⬤
Polyketone	⬤⬤⬤⬤	⬤⬤⬤⬤◯	⬤⬤⬤⬤◯	⬤⬤⬤◯	⬤⬤⬤◯
Fluoropolymers	⬤⬤⬤⬤◯	⬤⬤⬤	⬤⬤⬤⬤◯	⬤⬤◯	⬤⬤⬤◯
Carbon fibers	⬤⬤⬤⬤⬤	⬤⬤◯	⬤⬤⬤⬤◯	⬤⬤⬤◯	⬤⬤⬤◯
⬤⬤⬤⬤⬤ Preferred / Cost-effective ⬤⬤⬤⬤⬤ Very good & Cost Neutral ⬤⬤⬤⬤⬤ Suitable through costly for application ⬤⬤⬤⬤⬤ Very poor and/or cost-prohibitive Coconut oil, soy, rapeseed *PO, POE, EO/PO copolymers ¹ Needs to be refined oils ² POY nylon (6 & 66) is more sensitive to lubricant type than FDY - absorption issues

Solve Lubrication Issues in Polymeric Systems Faster

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About Paul Seemuth

Paul Seemuth is Chief Executive Officer at Tribology Consulting International. Dr. Seemuth, who holds a PhD in Organic Chemistry, has more than 30 years of experience in tribology and lubrication of polymers. His fields encompass but are not limited to, organic chemistry, fiber lubrication technology and formulations, catalysis, polymer processing, specialty chemicals, fuel and oil additive formulations.

Work experience includes global technology leader at DuPont Fibers Finish Technology Group, responsible for global Fiber Finish technologies and strategies, related plant designing and plant start-ups, VP - Global Technology at SSC Industries and an Associate Professor at Chattanooga State in the Departments of Chemistry and Chemical Engineering.

A Fellow of the Royal Society of Chemistry (FRSC), he is a recognized world expert in the field of Tribology, the study of friction and wear. Paul has over thirty publications and over 15 patents covering scientific endeavors on automotive additives, lubricant technologies, fiber finish formulations, polymer production processes, heterogeneous catalysts and supercritical fluid applications. Recently, he completed a major chapter on Textile Fibers / Fabrics” in the Handbook of Lubrication and Tribology, Volume I Application and Maintenance, Second Edition, then served as Section Editor for the Encyclopedia of Tribology along with a contribution on Fiber Boundary Tribology.

Dr. Seemuth consults both domestically and internationally. He also presents regular presentations on scientific topics related to lubrication and surface science.

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