Identification of contamination in polymers after recycling - ToF-SIMS

After a period during which all polymers were in vogue, we have seen a clear reversal of this trend in recent years following the growing ecological awareness of a large majority of consumers, a growing number of industrialists and certain States.

For a long time, the use of polymers has been favoured by many manufacturers because of their ease of processing, allowing the production of parts with complex geometries and high production rates. The wide range of performances of these compounds, their lightness and their low cost have also contributed to their intensive use in many sectors: packaging, health, automotive, aeronautics, textiles, toys... 

But not all objects made of plastic today can be replaced by cardboard, glass or other compounds considered "eco-friendly". We will not be able to eliminate this material from our daily lives, at least not in the short term. One solution is therefore to recover polymer waste and reuse it in the manufacture of new objects. There are currently two main recycling methods: mechanical and chemical. The latter involves a solvent purification stage that allows the selective separation of pure polymer matrices, free of any other material used in their formulation.

The material obtained is called "chemically recycled" and its properties are ideally close to those of the original virgin polymer. However, many manufacturers have identified differences in the behaviour of these materials during shaping and in their mechanical or ageing properties. 

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The various recycling processes are not yet fully mastered and many research and development laboratories are working on this subject in order to optimise their manufacturing process. ToF-SIMS is the analytical method of choice for this purpose and offers several advantages.

ToF-SIMS can detect traces of elements up to ppb and molecules up to the femtomole, at the extreme surface (< 1 nm) and up to several tens of µm below the surface in argon cluster profile mode (GCIB) on polymer samples.

A pulsed source of mono or multi-atomic primary ions (Ga⁺, Bin⁺, Au⁺, C₆₀⁺, ...) with an energy of a few keV bombards the sample surface. The interaction between the material and these ions gives rise to different types of emission, and the ToF-SIMS is interested in positive and negative secondary ions. 
The secondary ions formed are then focused and accelerated with the same kinetic energy towards the time-of-flight analyser, which separates them according to their m/z ratio with very good mass resolution (ΔM/M > 10,000 at mass 28). The resulting mass spectra represent the number of secondary ions as a function of their travel time to the detector, which is proportional to the square root of the m/z ratio.

TESCAN ANALYTICS has nearly 30 years of expertise in the use of ToF-SIMS. With the latest generation of instruments (including the newly acquired EDR technology), our team of experts works with all industrial sectors.

 

Figure 1: Mass spectra [0 to 200 amu] of the two polymer samples (virgin and recycled)
 

Surface spectra in positive and negative polarities were performed at three points (200 x 200 µm²) on each sample and on different areas after cleaning with Argon cluster.

The spectra presented in Figure 1 show very similar chemical signatures between the virgin and recycled polymers, confirming that they have the same molecular structure and were originally synthesised in the same way.


 

Figure 2: Superposition of the peak at mass 221.16 u (Si₃C₇H₂₁O₂⁺) characteristic of the PDMS of both polymers (virgin and recycled)
 

As ToF-SIMS is a highly sensitive analytical method down to a few ppb or femtomoles (depending on the elements of interest), it is possible to observe differences in the intensity of certain peaks and the appearance of new peaks, even at very low concentration levels. The spectrum in Figure 2 indicates the presence of PDMS (PolyDiMethylSiloxane) on the extreme surface of the recycled polymer grains.  PDMS, used as a shaping agent for its lubricating properties and its propensity to migrate to the surface of polymers, may be the cause of adhesion defects in products obtained with recycled material.

 

Figure 3: Superposition of the peak at mass 22.99 u (Na⁺) of the two polymers (virgin and recycled)
 

In addition, we observe in the spectra of Figure 3 higher intensities of alkali-sodium pollution on the surface of the recycled polymer grains, which originate from contaminations induced by the chemical recycling process. These results indicate that the handling of the powder during the process was not carried out in an optimal way and will allow the manufacturer to implement corrective actions to eliminate the contaminations from his procedure.
 

In this example, the ultra-sensitivity of ToF-SIMS for the detection of inorganic and organic contaminants was illustrated within the framework of the optimization of a polymer chemical recycling process.

ToF-SIMS is the method of choice for the analysis of elementary and organic ultra-traces on the extreme surface of polymers and any other material compatible with ultra-high vacuum (metals, semiconductors, glasses, ceramics, textiles).

In other work, ToF-SIMS has been used to carry out 2D mapping of these contaminants on the surface of technical parts or biological materials.

For more examples of surface analysis of materials with ToF-SIMS or with our other techniques, ask us for information.

Other techniques complementary to ToF-SIMS can be used to study the coating of a substrate by a treatment (XPS in imaging mode; TEM or SEM depending on the thickness of the treatment).