9.39C - Choosing materials using evidence

Match properties to the job

A material is suitable when its properties match the demands of its use. Begin by deciding which properties matter and whether high or low values are useful.

A saucepan body should conduct heat well, while its handle should transfer heat slowly to a hand. Both must withstand the required temperature. The same property can therefore be desirable in opposite directions for different jobs.

Constraints are essential limits: for example, a minimum strength or maximum cost. Reject candidates that fail them, then compare the remaining options. A very high melting point does not prove that a material stays rigid at every lower temperature; use service-temperature or stiffness data when that is what the task requires.

To compare a numerical property, keep units consistent. For example, conductivities of 20 and 0.5 W m⁻¹ K⁻¹ have a ratio of $20/0.5=40$. The first is 40 times as conductive, but whether that is useful depends on the job.

Choosing a handle material

Here is a worked selection. A manufacturer needs a material for a kettle handle. The handle must:

• stay rigid up to at least 120 degrees C
• transfer heat slowly to the user's hand
• be tough enough not to crack if knocked
• be reasonably low cost

These are illustrative data for the specified samples. Higher toughness scores mean greater impact toughness; lower cost scores mean cheaper material.

For this exercise, the quoted service limits refer to remaining rigid, as the handle requires. First, eliminate any material that fails a must-have requirement. Poly(propene) has a maximum service temperature of only 100 degrees C, so it does not meet the 120 degrees C requirement.

Next, compare the remaining materials against the use. Aluminium alloy withstands the temperature and is tough, but its thermal conductivity is very high at 160 W m-1 K-1, so it would transfer heat quickly to the hand. Ceramic withstands high temperature and has low thermal conductivity, but its impact toughness is only 1 and its cost is highest. Glass-fibre reinforced polymer meets the temperature requirement, has low thermal conductivity, has the joint-highest toughness value, and costs less than ceramic.

Best choice:

Glass-fibre reinforced polymer

Justification:

Glass-fibre reinforced polymer is the best choice because it meets the required maximum service temperature, at 180 degrees C compared with the 120 degrees C minimum. It has a low thermal conductivity of 0.35 W m-1 K-1, so it will transfer heat slowly to the user's hand. It is tougher than ceramic, with an impact toughness of 4 rather than 1, and it is cheaper than ceramic, with a cost of 4 rather than 6. Poly(propene) is unsuitable because it only withstands 100 degrees C.