Process analysers come of age: ten years ago Alpha Technologies introduced a new machine called the rubber processability analyser. It has taken a decade for competitors to arrive on the scene, but in the final months of 2003, five different companies introduced machines which do something very similar to the RPA.(Processability testing)

Testing for processability has been one of the betes noirs of the rubber industry. Often, said Philip Prescott, owner and founder of Prescott Instruments, a company only discovers one of its compounds is off-specification, when technicians run a production batch into a moulding machine or extruder, and notice that it failed to flow as well as the reference compound.

The first resort is to check if all the ingredients went into the mixer. Next is to look at the rheometer curves of the incoming polymer. Too often, these and other tests fail to identify the problem, said Prescott.

Ten years ago, however, Alpha Technologies (then Monsanto) introduced the Rubber Processability Analyser (RPA). This was the first instrument capable of discriminating between an off-spec compound and the reference. The name "processability analyser" was used because, up to that time, the only way to tell if two compounds were the same, was to run them through the production process. Until 1993, most mixers and mills were more discriminating than the most precise tests available.

However, when Alpha introduced the RPA, the industry was uncertain of its value. While many are now convinced that a RPA, and similar machines, show why one compound processes differently from an apparently similar compound, there are still major concerns. These include matters such as the length of time each test can take, the level of skill and education required of the operator to properly interpret the results and, finally, over the cost of the instrument, according to competing suppliers.

Only when tyre companies started making tread compounds filled with silica did sales of these machines start to accelerate. Even then the machines went into the production environment, rather than the laboratory, as Alpha had expected from such expensive equipment.

Silica changed the relationship between hysteresis and frequency and, in doing so delivered better wet grip and lower rolling resistance. But the only instrument capable of measuring the change in hysteresis with frequency was the RPA, and so the tyre makers bought the instruments by the dozen to equip their production quality control.

The secret of the processability analyser is an advanced motor on the die, together with some better instrumentation and sophisticated software.

A processability analyser works in the same way as a simple moving die rheomoeter, but with a twist. An MDR rotates the lower half of a die back and forth, while measuring the forces on the upper half of the die. The forces are transmitted through the sample of rubber contained within the die: the visco-elastic properties of the rubber change the amplitude and phase angle of the transmitted forces and so the measurements are made over a variety of amplitudes and progressively as the …