The first half of this article explored the fundamental principles of friction and wear in plastics, as well as the importance of selecting the right base polymer. While a polymer’s inherent properties are a good starting point, many dynamic applications require enhanced performance that a basic resin alone cannot provide. This is where wear and friction additives can play a crucial role.
This second half will delve into how additives and fillers are strategically incorporated into plastics to significantly improve their tribological properties. We will examine how these substances transform standard resins into high-performance engineering materials capable of withstanding demanding wear and friction environments, ultimately extending the lifespan and efficiency of plastic components.
Why Use Additives? Enhancing Tribological Performance
At their core, additives and fillers are introduced to plastic matrices to fundamentally alter and enhance their tribological performance. Base resins, while offering many benefits, may lack the inherent lubricity, strength or thermal stability required for long-term operation under sliding or abrasive conditions. Additives work to supplement base resins in various ways:
- Reducing Friction: By lowering the coefficient of friction (COF), additives minimize the energy lost as heat and reduce the force required to initiate or maintain motion.
- Improving Wear Resistance: They can enhance the material’s ability to resist material removal due to abrasive, adhesive or fatigue-wear mechanisms.
- Dissipating Heat: Frictional heating can degrade polymers, and some additives improve thermal conductivity, helping to dissipate heat away from the contact interface.
- Increasing Load-Bearing Capacity: Reinforcing fillers increase the material’s strength and stiffness, allowing it to withstand higher contact pressures without deforming or failing.
- Modifying Surface Properties: Some additives migrate to the surface, forming a protective or lubricating film that influences the interaction with the mating surface.
The thoughtful selection and combination of these additives enable plastics to replace traditional metals in applications ranging from gears and bearings to seals and bushings, often leading to lighter, quieter and self-lubricating systems.
Figure 1: Image of dirt and wear debris in silicone grease.
Common Additives for Wear Resistance
External lubricants, such as greases or oils applied to the surface of a plastic component, are effective at reducing friction and wear in many scenarios but can be a double-edged sword. They create a fluid film that separates the mating surfaces. However, in dirty or abrasive environments, these lubricants can become a liability. They can trap particulates, such as dust, dirt or wear debris, turning the lubricant film into an abrasive slurry that significantly accelerates wear on both the plastic component and its mating surface (Figure 1). This can lead to increased maintenance, contamination and premature component failure. This risk often drives the need for self-lubricating plastics.
Internal Lubricants’ Built-In Slip
Internal lubricants are compounded directly into the polymer matrix, providing continuous lubrication throughout the material’s lifespan.
Polytetrafluoroethylene (PTFE) Powder — PTFE powder is a highly effective internal lubricant prized for its extremely low COF and chemical inertness. When added to a polymer, it forms a protective transfer film on mating surfaces, which can reduce friction by more than 50%, while lowering operating temperatures and decreasing energy consumption.
Despite these benefits, using PTFE has some drawbacks. High concentrations can slightly reduce the plastic’s mechanical properties, such as tensile strength and stiffness (see table below). It’s also more expensive than other lubricants and requires careful processing for proper dispersion.
Silicone — Incorporating silicone additives into plastics creates a thin, durable boundary layer that significantly lowers the COF. This not only makes the material more resistant to scratches but also gives it a better tactile feel. As a bonus, silicone also acts as an internal mold-release agent, improving processability while maintaining its effectiveness across a wide temperature range.
Silicone, however, has its limitations. The lubricating layer has a limited load-bearing capacity and can be squeezed out under high pressure. Additionally, its tendency to migrate to the surface can create a slightly greasy feel or interfere with secondary operations like painting or bonding.
Reinforcing Fibers: Strength and Stiffness
Reinforcing fibers dramatically increase the mechanical strength and stiffness of plastics, which is critical for wear resistance as it improves the material’s ability to resist deformation and subsurface fatigue.
Glass and Carbon Fibers — When added to plastics, reinforcing fibers like glass and carbon create a robust composite with significantly increased strength, stiffness and dimensional stability. This improves load-bearing capacity and creep resistance. Carbon fibers also enhance thermal conductivity, helping to dissipate heat, while both types of fibers allow for higher operating temperatures.
However, these benefits have trade-offs. The abrasive nature of the fibers can cause increased wear on mating surfaces and can make the plastic more brittle. The resulting material can also be anisotropic, meaning its properties vary based on fiber orientation, and it can cause tool wear during processing.
Other Particulate Fillers: Multifunctional Performance
While base polymers provide essential chemical and thermal properties, incorporating particulate fillers allows for the fine-tuning of mechanical and tribological performance. These additives often serve multiple roles, simultaneously enhancing lubricity, thermal dissipation and structural stability to meet the demands of rigorous industrial environments.
Graphite — A platelet-like material, graphite is a layered, solid
lubricant that reduces friction and wear by allowing its atomic layers to shear easily under stress. When added to plastic, it provides good lubricating properties, particularly in high-load applications. It also boasts excellent thermal and electrical conductivity, helping to dissipate frictional heat and impart antistatic properties. Plus, it’s generally more cost-effective than other lubricants like molybdenum disulfide or PTFE.
Despite these benefits, graphite has a few drawbacks. Its lubricity can decrease in dry, high-temperature environments, and high concentrations can negatively impact the plastic’s mechanical properties. Finally, its dark color limits its use to applications where aesthetics are not a concern.
Molybdenum disulfide (MoS2) — MoS2 is an excellent solid lubricant, similar to graphite but effective in more extreme conditions. Its layered structure reduces friction and wear across a wide range of loads and temperatures. Unlike graphite, MoS2 is also effective in vacuum environments and has good chemical resistance, making it ideal for aggressive conditions.
However, MoS2 is generally more expensive than graphite and can be difficult to disperse. It will also oxidize in air above 400°C, losing its lubricating properties. Its dark color also limits its use in certain applications.
Figure 2: Cross-section image of a PTFE seal incorporating Ekonol T101 particles.
High-temperature aromatic ester — High-temperature aromatic esters, available under the tradename Ekonol, are specialized additives used in fluoropolymers such as PTFE (Figure 2) to significantly boost wear resistance. These esters improve mechanical integrity under stress, compensating for PTFE’s naturally high wear rate (Figure 3). Research indicates that relative wear becomes asymptotic at approximately 25-30% Ekonol concentration, suggesting an optimum blend range of 20-30% for maximum durability.
As a thermally stable reinforcement, Ekonol contributes to the overall strength, stiffness and chemical resistance of the composite in demanding environments. However, these performance benefits come with increased cost. Due to their high melting points, aromatic esters require specific processing parameters and are not as widely available as more common fillers.
The strategic integration of additives and fillers represents a fundamental shift in plastic component design, moving beyond the inherent limitations of base resins to create high-performance engineering materials. By carefully balancing the use of internal lubricants like PTFE and silicone with reinforcing fibers or specialized particulate fillers like Ekonol, engineers can tailor a material’s tribological profile to meet specific environmental demands.
Figure 3. Relative Wear versus % Ekonol in PTFE. Source: Saint-Gobain Coating Solutions.
Key considerations for achieving optimal performance include:
- Balancing Friction and Strength: While lubricants like PTFE can reduce the coefficient of friction, they may simultaneously reduce tensile strength and stiffness.
- Selecting for Environment: Fillers like graphite offer cost-effective lubrication and heat dissipation, yet MoS2 remains the superior choice for vacuum environments or extreme temperature conditions.
- Optimizing Concentration: As demonstrated with Ekonol aromatic esters, there is often a point of diminishing returns; for instance, wear resistance in PTFE stabilizes when Ekonol concentrations reach approximately 25-30%.
- Managing Processing Requirements: High-performance additives often require specialized handling, such as the specific preforming and sintering parameters necessary for aromatic esters to ensure optimal composite properties.
Ultimately, the transition from traditional metal systems to lighter, quieter and self-lubricating plastic components depends on this thoughtful selection process. By understanding the trade-offs between wear resistance, mechanical integrity and cost, manufacturers can significantly extend the lifespan and efficiency of critical components in even the most demanding applications.
This article was originally published in the March 2026 edition of Plastics Technology.
