Effects of Thermoforming on the Mechanical, Optical, Chemical, and Morphological Properties of PET-G: In Vitro Study

Abstract

The effectiveness of clear aligners in correcting malocclusions is closely linked to the properties of the materials used to make them. The polymers used in the manufacture of clear aligners have well-established properties. However, the process of manufacturing clear aligners, known as thermoforming, involves thermal and mechanical shocks that may alter these properties. The objective of this study was to evaluate the effects of thermoforming on the mechanical, optical, chemical, and morphological properties of sixty PET-G specimens. The study compared the thickness, weight, absorbance, chemical structure, surface roughness, elastic modulus, yield strength, and breaking load of thirty thermoformed specimens with thirty non-thermoformed specimens. The study introduces a new approach by using standardized samples to analyze both chemical and physical properties. The results showed statistically significant differences in thickness (-15%), weight (-11%), and surface roughness (+1233% in roughness average; +1129% in RMS roughness) of the material. Additionally, a correlation was found between reduction in thickness and increase in opalescence. There was no significant change in the functionality of the aligners after thermoforming, as no significant mechanical changes were found. However, the increase in surface roughness may lead to plaque and fluid accumulation and worsen the fit of the aligners.

Keywords: clear aligners; glycol-modified polyethylene terephthalate; surface roughness; thermoforming process; thermoplastic materials.

Figure 1

CG and TG sample making process.

Figure 2

Average weight of the CG (blue) and TG (orange) samples.

Figure 3

Absorbance spectra of the CG and TG samples; average absorbance CG (blue); average absorbance TG (orange).

Figure 4

Average FTIR spectroscopy spectrum of the CG (blue) and TG (orange) samples.

Figure 5

(a) Representative surface of the CG sample made with the atomic force microscope; (b) representative surface of the TG sample made with the atomic force microscope; (c) histogram chart of the profile heights on the surface of a CG sample and a TG sample. “Frequency” refers to how often a given value was observed. The curve is normalized to 1, meaning that the most frequent value is assigned a value of 1 and all other values are proportional; (d) quantification of CG and TG roughness based on roughness average (Ra) and RMS roughness (Rq). * p < 0.01; ** p < 0.001; *** p < 0.0001.

Figure 6

(a) Fast dynamics: representative stress–strain curve (8 mm/s) of the CG (blue) and TG (orange) samples; (b) slow dynamics: representative stress–strain curve (0.8 mm/s) of the CG (blue) and TG (orange) samples.