Creepy things can happen with long term loading of thermoplastics

This month, polymer expert and technical blog author, Dr Charlie Geddes, looks at how to avoid getting caught out by creep.

Designers and users new to thermoplastics often get caught out by ‘creep’, the quaintly named but descriptive term for long term deformation.  When a load is applied to a thermoplastic there is an immediate deformation due to stretching of polymer chains, which is largely reversible, but some are unaware that, over a long period of time under load, there is a slower increase in deformation due to unentanglement and slippage of polymer chains, most of which is reversible. The irreversible part is the viscous component of viscoelastic materials.

Creep deformation (creep strain), as well as increasing with time and with increasing load, is also sensitive to temperature, particularly above the glass transition point.  After one year at room temperature under a modest load of five MPa, polypropylene exhibits creep deformation of 1.5 percent. Under the same conditions polycarbonate would deform by only 0.2 percent.  At a service temperature of 60oC, polypropylene creep deformation increases to 2.5 percent for the same loading.

Deformation under creep conditions, as well as increasing with time, load (stress) and temperature, is reduced by increasing the level of crystallinity, by the addition of fillers and by introducing crosslinking. Because of its higher crystallinity, polyacetal (POM) is five times more resistant to creep than polypropylene.

In designing components that will be subjected to long-term loading, designers should use creep modulus values instead of the normal short-term modulus in mechanical design equations and finite element analysis. Because comprehensive creep data is expensive to generate, the data will not be readily available for all grades and designers may have to use data for similar grades. Components should be designed to avoid long-term loading as much as possible, for example in snap-fit assembly.

Extrapolating creep data to longer times can be risky, with the possibility of material failure (creep rupture) lurking just around the corner.