3.1.4. Differential Scanning Calorimetry

The thermal properties of the composites with different SPF fiber contents were investigated by Differential Scanning Calorimetry (DSC). The DSC results for PLA, PLA/SPF composite were listed in Table 4.


**Table 4.** Differential Scanning Calorimetry (DSC) results of PLA/SPF composites.

The results showed that when the fiber content was 10-30%, the glass transition (Tg) temperature of PLA in the composite was almost insignificantly as compared to that of neat PLA. This indicated that there was no restriction on the mobility of the PLA chains when adding pulp fiber. Espinach et al. [26] also observed the same phenomenon when adding 20–25% of bleached kraft soft wood to PLA matrix. However, when the fiber content exceeded 30%, the glass transition temperature of composites decreased.

The melting temperature (Tm) of PLA in the composite, containing 10% fiber, was quite similar to that of neat PLA, but the crystallization temperature (Tc) increased slightly. Meanwhile, at the fiber content of 20–50%, the melting temperature of PLA decreased, compared to the composite, containing 10% of fiber and had no differences among samples. In addition, the crystallization temperature of these samples was not observed, and this change in crystallinity reduced the melting temperature of the composites.

3.1.5. Thermogravimetry Analysis

The thermal stability of PLA/SPF composites was investigated by thermogravimetry analysis method, and the results are shown in Figure 7. Data analysis from the thermal analysis curves of these composites was listed in Table 5.

**Figure 7.** TGA curves of pulp fiber, PLA and PLA/SPF composites.


**Table 5.** Thermal analysis value of PLA and PLA/SPF composites.

The pulp fiber decomposition process can be divided into 3 stages. The first stage was the process of losing moisture at about 100 ◦C with about a 3% of weight loss. The second stage takes place at about 280–410 ◦C with a mass loss of 63.6%. This weight-loss stage was due to the decomposition of the main components of the fibers which took place mainly in amorphous regions [27]. This stage can be the polymerization of hemicellulose [28] and the random cleavage of glycoside bonds in cellulose [29].

The results showed that TGA curves of PLA/SPF composites exhibited multiple steps. The decomposition stage takes place at about 300–410 ◦C due to the decomposition of the PLA polymer chains and the main components of pulp fibers. The stage at 410–480 ◦C is due to carbon burning. The results in Table 5 also indicated that the degradation temperature of composites was lower than that of neat PLA. The degradation temperature of PLA-based composites decreased by increasing the SPF fiber content. It might be due to the degradation temperature of SPF fiber was lower than that of neat PLA.
