“The world is wide, and I will not waste my life in friction when it could be turned into momentum.”-Frances E. Willard
Plastic materials work well in applications where friction resistance is important. Friction is a natural resistance to sliding motion between two surfaces. It is quantitatively described by dynamic and static coefficients. These provide an estimate of energy requirements for moving a part. The lower the coefficient of friction, the easier the two surfaces slide over each other. Work done overcoming friction in bearings and other mechanical components dissipates as heat so reducing friction leads to better efficiency.
Frictional properties of thermoplastics differ from those of metals. Because thermoplastics have a lower modulus (more flexibility) and are softer than metals, they do not follow the classic laws of friction as applied to metals. Unlike metals, thermoplastics usually have a static coefficient of friction (related to starting motion) that is less than the dynamic coefficient (related to maintaining motion).
Friction generates heat where surfaces touch. The temperature can greatly increase when the contact pressure (P) and the sliding speed (V) become large. The combination of P and V is referred to as the PV capability for a bearing application. High-temperature polymers are more resistant frictional heating and, as such, are good candidates for friction applications.
Choosing a Friction-Resistant Plastic Material
Thermoplastic compounds, including unfilled or neat polymers, work well in applications where friction is a concern. The compounds are self-lubricating, resist wear and corrosion, and emit little noise. Additionally, they are lightweight, damp vibrations, and economical. Unlike many metals, self-lubricating thermoplastics can work in unlubricated environments. Thermoplastics also tolerate some abrasive particles at the mating surface because sharp particles generally will embed in the compound.
Of the large range of polymers available, only a few are recommended for resistance to friction. Acetal and nylon may be used for low PV applications. Likewise, PPS and PEEK may handle high temperature and high PV applications. Polycarbonate is an option when dimensional stability is needed.
Polymers that are used in high friction applications are classified into two major categories: semicrystalline and amorphous. Most thermoplastics used for wear application are semicrystalline. Semicrystalline polymers have a highly ordered molecular structure with sharp melt points. They do not gradually soften with a temperature increase but remain solid until a given quantity of heat is absorbed. Then they rapidly change to a low-viscosity state.
Amorphous polymers, in contrast to semicrystalline, have a randomly ordered molecular structure which doesn't result in a sharp melt point. Instead, these polymers soften gradually as the temperature rises. They change viscosity when heated but seldom flow as easy as semicrystalline materials. They are also isotropic, shrinking uniformly in the direction of flow and transverse to it. As a result, they typically shrink less in a mold and tend to warp less than their semicrystalline counterparts.
The most important polymers for friction include:
1) Acetal (POM)
Acetal is rigid and strong with good creep resistance. It has a low coefficient of friction, remains stable at high temperatures, and offers good resistance to hot water.
2) Nylon (PA)
Nylon absorbs more moisture than most polymers, making it unsuitable for underwater applications and affecting its processibility. However, nylon's impact strength and general energy absorbing qualities actually improve as it absorbs moisture. Nylons also have a low coefficient of friction, good electrical properties, and good chemical resistance.
3) Polyphthalamide (PPA)
This high performance nylon has through improved temperature resistance and lower moisture absorption. It also has good chemical resistance.
PEEK is a high temperature thermoplastic with good chemical and flame resistance combined with high strength. PEEK is a favorite in the aerospace industry.
5) Polyphenylene sulfide (PPS)
PPS offers a balance of properties including chemical and high-temperature resistance, flame retardance, flowability, dimensional stability, and good electrical properties.
6) Polybutylene terephthalate (PBT)
PBT is dimensionally stable and has high heat and chemical resistance with good electrical properties.
7) Thermoplastic polyimide (TPI)
TPI is the most heat-resistant thermoplastic. It's inherently flame retardant with good physical, chemical, and wear-resistance properties.
8) Polycarbonate (PC)
PC has good impact strength, high heat resistance, and good dimensional stability. PC also has good electrical properties and is stable in water and mineral or organic acids.
9) Polyetherimide (PEI)
PEI maintains strength and rigidity at elevated temperatures. It also has good long-term heat resistance, dimensional stability, inherent flame retardance, and resistance to hydrocarbons, alcohols, and halogenated solvents.
The performance of many of these polymers can also be enhanced using certain additives which reduce fiction.
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