PSI - Issue 72

Miloš Vorkapić et al. / Procedia Structural Integrity 72 (2025) 470 – 478

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FTIR spectra provide insights into the functional groups present in these materials by displaying characteristic absorption peaks, so there may be observed the characteristic peaks of PLA, similar to those in spectra reported by Cai et al. (2013): O – H stretching: a broad, but not strong peak around 3500 cm ⁻ ¹, corresponding to hydroxyl groups. C – H stretching: peaks at 2990-3000 cm ⁻ ¹ and 2940-2950 cm ⁻ ¹, associated with the asymmetric and symmetric stretching vibrations of the methyl ( – CH ₃ ) groups. C=O stretching: a strong peak at about 1750 cm ⁻ ¹, indicative of the carbonyl (C=O) stretching vibration. C – O stretching: absorption bands between 1185 cm ⁻ ¹ and 1080 cm ⁻ ¹, related to the stretching vibrations of the C – O bonds in the ester group. C – H bending: peaks at  1450 cm ⁻ ¹,  1380 cm ⁻ ¹, and  1365 cm ⁻ ¹, corresponding to the bending vibrations of the – CH ₃ groups. Incorporating carbon fibers into PLA to form PLA-CF composites generally does not introduce new functional groups detectable by FTIR. Therefore, the FTIR spectra of PLA-CF composites typically resemble that of neat PLA, with the characteristic peaks of PLA remaining prominent, except for the peaks registered for C – H stretching, which are significantly pronounced in case of CF presence. This similarity suggests that the addition of carbon fibers does not significantly alter the chemical structure of the PLA matrix, as reported by Pathek et al. (2024). DSC analysis has revealed the thermal properties of the examined materials: melting temperature (T m ), glass transition temperature (T g ) and crystallization temperature (T c ). The T g is considered as a measurement of the thermal stability of a polymer and it is read at the point of a change in the slope of the DSC thermographs, presented in Figure 3.

Fig. 3. DSC thermographs for pure PLA and PLA-CF composite.

It is important to emphasize that the samples were first heated, then cooled at the same rate and heated again (all this at a controlled rate of 10 ºC min -1 ), and then the values of T g , given in Table 3, were read from the second heating cycle, as usually done for polymer materials, ( Investigation of Polymers with DSC, 2020 ).

Table 3. DSC analysis results Sample

Tg[ºC]

Tc [ºC] 111.50 104.92

Tm [ºC] 151.47 151.68

PLA

60.60 60.81

PLA/CF