Effects of temperature changes and stress loading on the mechanical and shape memory properties of thermoplastic materials with different glass transition behaviours and crystal structures

Abstract

Orthodontic forces delivered by thermoplastic appliances may influence by the T g of the materials, but not the crystal structure. Polyurethane is attractive thermoplastic materials due to their unique shape memory phenomenon, but stress relaxation with temperature changes is expected.

Stress relaxation properties of four orthodontic aligner materials: A 24-hour in vitro study

“…For example, Iijima et al 16 also tested PET-G, polypropylene, and TPU with Figure 3. Stress-deformation curve of the of the four selected polymers.…”

“…This discrepancy could be a result of differences in environmental conditions, as Fang's tests were performed in dry conditions at room temperature, which seems to have given rise to greater initial stress and slower, more gradual stress decay than the moist 378C we used as testing conditions. Indeed, thermoplastic materials are susceptible to temperature-related changes, especially above 378C, 16 and a further decline in their performance will be seen in vivo when subjected to greater thermal and mechanical stresses, not to mention the action of salivary enzymes.…”

Influence of constant strain on the elasticity of thermoplastic orthodontic materials

“…Thermoplastic materials composed of polyethylene terephthalate glycol (PETG) such as DU and ER, polyurethane (PU), and polyethylene terephthalate are used as aligners 4,19) . Moreover, new thermoplastic materials such as a composite of PETG, PU, and polycarbonate at an original compounding ratio 4) and PU with a shape memory function 30) have been developed 4,20) . To evaluate the physical properties of these materials, water absorption 4,17) , tensile 4,17) , threepoint bending [18][19][20] , and stress relaxation 4,16) tests have been carried out.…”

“…Moreover, new thermoplastic materials such as a composite of PETG, PU, and polycarbonate at an original compounding ratio 4) and PU with a shape memory function 30) have been developed 4,20) . To evaluate the physical properties of these materials, water absorption 4,17) , tensile 4,17) , threepoint bending [18][19][20] , and stress relaxation 4,16) tests have been carried out. However, there has been no study to investigate the relationship between the behavior of the orthodontic force exerted by the aligner and the change in the physical properties of the thermoplastic materials over time.…”

“…To investigate aligner orthodontic devices formed from thermoplastic materials, the optimal composition of the physical properties of the thermoplastic materials must be determined and they must be extremely effective in vitro without the measurement of the orthodontic force. However, there have only been a few studies of the physical properties of the thermoplastic orthodontic materials 4,[16][17][18][19][20] and the orthodontic force exerted by the aligner [21][22][23][24][25] . There has been no study to investigate the relationship between the behavior of the orthodontic force exerted by the aligner and the change in the physical properties of the thermoplastic materials over a two week period.…”