Preparation of plastics for characterization and testing

The value and relevance of characterization depends first and foremost on the representativeness, quality and validity of the specimens used to perform the mechanical, physical and chemical measurements.

Those involved in the measurement chain sometimes forget that polymers are sensitive to environmental parameters such as temperature and light. Lack of care during the time between sampling and characterization can therefore be damaging.

The many parameters involved in polymer development, processing and part use result in a statistical distribution of properties. It is essential to prepare specimens as carefully as possible to avoid widening this distribution even further.

Apart from characterization directly on the workpiece, specimen preparation can be fairly straightforward, but it must be borne in mind that it can change the physical and chemical state of the polymers. Spraying a sample of glass fiber-reinforced material causes the fibers to break. Heating, by machining or other means, changes the crystallinity, relaxes the stresses, possibly degrades the polymers and alters the cross-linking state of thermosets. Chemical attack degrades and alters polymers, fibers and additives.

This article describes the specific features of plastics that characterization managers and operators should always bear in mind to avoid the many pitfalls:

• sensitivity to temperatures lower than those borne by metals ;

• macromolecular organic structure leading to viscoelastic behavior ;

• Thermal degradation can occur at temperatures as low as a few tens of degrees for certain polymers;

• sensitivity to humidity, light and UV ;

• sensitivity to fluid action ;

• creep and stress deformation ;

• the initiating role of chemical and physical imperfections in degradation ;

• risks of inducing residual internal voltages, weld lines during injection.

Storage prior to characterization, specimen preparation and, more generally, all operations between sample collection and characterization must take account of the above observations.

When specimen preparation requires the processing of plastics, this work must be entrusted to specialists with the necessary knowledge, experience and equipment.

Equipment manufacturers are developing presses for molding specimens and small parts, ranging from simple, manual compression presses to highly sophisticated mini injection molding machines.