WHAT IS CREEP?
Creep in polymers, also known as cold flow, it is the tendency of a material to slowly undergo deformation when constant mechanical stresses are applied. These stresses include tensile, compressive, shear or flexural loading. If a plastic is under constant stress, a constant change in strain will occur.1 Thus the stressed polymer will continue to deform over time. It will occur even if the stress is below the yield strength of the material.
Materials will experience greater creep if they are exposed to heat for long periods of time. So the rate of creep depends on the stress applied, temperature and the material involved. Glass, metals, polymers, ice etc. all experience creep but at different rates and temperatures. The ice in glaciers flows slowly down a slope because of creep. There is slippage between and within the ice crystals. It is important to note that high temperatures are not required. Materials also creep under ambient temperatures.
Polymers are viscoelastic. That is they have the properties of both fluids and elastic solids. This is because plastics are made up of long molecules that get entangled with each other. Also the polymer chains are mobile and can slide past each other.
Creep is not the same thing as plastic flow. It is the deformation of a polymer under a small constant load. Plastic flow occurs when there is fast deformation of a polymer under a high constant load.
POLYMERS THAT RESIST CREEP
Crystalline polymers are more creep resistant when compared to amorphous polymers. This is because they have a higher degree of secondary bonding. Polymers with more chain entanglement are also more resistant. Additionally glass fibers added to a polymer will increase entanglement and this will lower creep.
EXAMPLES OF POLYMERS THAT EXHIBIT LOW CREEP
Vespel®: Excellent strength and impact resistance. It has low rates of wear and friction. Vespel® is thermally resistant and maintains its mechanical properties in temperatures as high as 260°C (500°F.) It has exceptional creep resistance.
PAI: (Polyamide-imide) Also known as Torlon® it is an extremely strong, stiff and dimensionally stable polymer. It is often used in high temperature applications as it can perform in continuous temperatures up to 260°C (500°F.) It is used when a tight tolerance plastic is needed as its has extremely low thermal expansion and excellent creep resistance.
PEEK: (Polyetheretherketone) An engineering plastic known for its mechanical strength and dimensional stability. It performs well in harsh chemicals. PEEK has a continuous service temperature of 170°C (338°F.)
ECTFE: (Halar®) has greater strength, abrasion resistance, and creep resistance than other fluoropolymers that are softer such as PTFE (Teflon®.) Additionally it has excellent mechanical properties, dimensional stability and is fire resistant. The surface of ECTFE is exceptionally smooth making it ideal for high purity applications.
1 Stress is the force applied to a material, divided by the material’s cross-sectional area. Strain is the deformation or displacement of material that results from an applied stress.
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