Introduction

Nowadays, the bioplastic sector is developing with the aim of improvement of bioplastics with high biological content and resistant. New studies on the use of renewable resources instead of fossil fuels are increasing [1]. Poly(lactic acid) (PLA) is a biodegradable polyester obtained through the synthesis of lactic acid. PLA is a polymer that replaces oil-based plastics due to its high elastic modulus and tensile strength, low cost, and biodegradability [2, 3].

Durable bioplastics are demanded in electronics, automotive industry, and other industrial sectors, thus, PLA-based composites have been developed for this purpose in recent years [1, 4]. However, due to degradation at low temperatures (~ 330 °C) [5], low impact strength, high brittleness, relatively low crystallization rate, and low glass transition temperature (Tg), their uses are limited [6,7,8]. To overcome these poor properties, different strategies can be considered such as blending with other thermoplastics, e.g., polyethylene [9], polypropylene [10], polyolefin elastomers [11], and polycarbonate (PC) [12, 13]. Besides, PLA/PC blends have been reinforced via various fillers, e.g., clays [10, 14], and fibers [14, 15]. Among the cited PLA blend research works, blending with PC provided acceptable tensile strength, elongation, and thermal stability for durable applications in mobile phones, laptops, and automobiles.

PC is widely used in electronic, and packaging fields with high tensile and impact strength, flexibility, and heat resistance [6, 8, 16]. The good characteristics of PLA such as biodegradability can be combined with the good mechanical and thermal properties of PC in the PLA/PC blends to create ideal materials for industry [5, 16]. Several studies in the literature have examined the blending of PLA and PC to increase their thermal resistance and toughness [17,18,19].

On the other hand, PLA and PC are incompatible polymers due to aliphatic structure of PLA, aromatic structure of PC, higher viscosity of PC than PLA, and high surface tensions. Therefore, the interfacial adhesion and mechanical properties of the PLA/PC blend are poor, and it is important to find a way to improve it. Generally, a third component is added as a compatibilizer or catalyst to improve the compatibility between two immiscible components for PLA/PC and polyester/PC blends [17, 20, 21]. Some catalysts or metallic fillers can improve the performance of the PLA/PC blend by promoting the transesterification reactions between PLA and PC.

Chelghoum et al. [17] studied the compatibilization of the PLA/PC blends by adding samarium acetylacetonate (Sm-Acac) to catalyze the reaction. Hedayati et al. [8] and Phuong et al. [20] used dicumyl peroxide (DCP) and cobalt (II) acetylacetonate (Co), tetrabutylammonium tetraphenylborate (TBATPB) and triacetin catalysts in PLA/PC blends. Liu et al. [19] investigated the effect of zinc borate, titanium pigment, and tetrabutyl titanate catalysts on the transesterification reactions between PLA and PC under flow field. It was reported that tetrabutyl titanate was the most effective catalyst.

Among the various catalysts, it has been reported that titanium-based catalysts are effective in accelerating the transesterification reactions of polyester blends such as PLA/PC [19], PLA/polypropylene carbonate [22], PC/poly(ethylene terephthalate) [23], poly(butylene succinate-co-adipate) and poly(ε-caprolactone) [24]. Titanium dioxide (TiO2) is a key catalyst with its nontoxicity, environmentally friendly structure, chemical inertness, low cost, and abundance [25, 26]. TiO2 is found in nature in three crystalline forms, i.e., rutile, anatase, and brookite. Anatase and rutile are the most commonly used forms of TiO2 due to their high photocatalytic activity [27]. Besides catalytic effects, TiO2 is frequently used as UV blocking, flame retardant, antioxidant and mechanical, and thermal property enhancement [28,29,30]. Due to these features and being an environmentally friendly photocatalyst TiO2 was preferred as a catalyst in this study for PLA/PC blend.

The other metal oxide catalyst, cerium dioxide (CeO2), which is the most interesting as a rare earth oxide, is used as an active ingredient in nanoparticle or bulk heterogeneous catalysis due to its chemical stability. CeO2 shows high catalytic activity due to its high oxygen storage capacity and higher redoxability between Ce3+ and Ce4+ ions [31]. Beside this catalytic activity, CeO2 has good thermal stability, fire retardancy, and noncorrosive nature [32, 33]. Due to all these properties and catalytic activity, CeO2 was preferred as the second catalyst for the PLA/PC blend.

It is important to produce environmentally friendly materials with improved thermal and mechanical properties that can be used in many areas of the industry. Because of that, this work focused on the property enhancement of PLA/PC blends by using CeO2 and TiO2 catalysts. Thus, it was aimed to combine the good properties of PLA such as biodegradability with the good mechanical and thermal properties of PC by compatibilizing both polymers with the help of catalysts. To the best of our knowledge, there was no research work in which PLA and PC were prepared with these catalysts. In this study, the effects of these two catalysts on the thermal, mechanical, structural, and morphological properties of the PLA/PC blends were compared.

Experimental

Materials

Poly(lactic acid) (PLA, 2003 D) was supplied from Nature Works, USA (Tg: 50–55 °C, MFI: 10–30 g/10 min (190 °C, 2.16 kg). PC W110 was purchased from Kempro, Turkey (Tg: 145 °C, MFI: 10 g/10 min (300 °C, 1.2 kg). Cerium dioxide (CeO2) and titanium dioxide (TiO2) powders were used as catalysts. Commercial TiO2 (CAS No: 13463-67-7) crystal powder in anatase form was purchased from Acros Organic, (Thermo Fisher Scientific, USA) with Mw of 79.866 g/mol and density of 3.78 g/cm3. Cerium (III) nitrate hexahydrate (Ce (NO3)3. 6H2O, ≥ % 98.5, 1.02274, Merck, Germany) was used to prepare CeO2.

Processing

Firstly, the CeO2 catalyst was prepared by calcining cerium (III) nitrate hexahydrate at 650 °C for 4 h. After calcination, it was sieved to less than 45 mesh size. Previously, the authors used this catalyst for another reaction and in that work, they prepared and used CeO2 with the same method and determined that the CeO2 has particulate crystalline structure [34]. Before the preparation of blends, PLA was dried in a vacuum oven at 80 °C for 24 h.

All samples were prepared in a laboratory twin-screw extruder (MC15, Xplore Instruments, The Netherlands). PC, PLA, and catalysts were simultaneously added to the extruder. The barrel temperature and screw speed of the extruder were kept constant at 260 °C and 100 rpm, respectively. The residence time was 3 min. At the end of the compounding process, the molten compound was subsequently injection molded by using an appropriate machine (Xplore 10 mL, The Netherlands) to obtain ISO 527-5A tensile bars. The melt and mold temperatures were 260 °C and 45 °C, respectively. The injection pressure was 10 bar.

The compositions and designations are tabulated in Table 1. The PLA/PC blends matrix ratio was 50/50 wt/wt which was based on the Hazer et al. [5] work that prepared various ratios of the PLA/PC blends and they found that the 50/50 wt/wt blend exhibited the highest strain-at-break and high tensile strength values.

Table 1 Compositions and designations of the samples
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Characterization

Fourier transform infrared analysis (FTIR) was performed by a Spectrum 100 PerkinElmer (USA) equipment. The spectra were obtained in the 650–4000 cm−1 wavelength range.

The tensile properties of the samples were determined by using an Instron Universal Testing Machine (Model 3345, USA) according to ISO 527-5A. The crosshead speed was 5 mm/min. For each sample type, at least five specimens were tested, and an average value of each group was reported with SDs.

The thermal stability of the samples was investigated by using Mettler Toledo (USA) thermal gravimetric analyzer (TGA) under nitrogen atmosphere. The analyses were carried out at a heating rate of 10 °C/min from 25 to 800 °C.

The thermal and crystallization behaviors of the blends were investigated by differential scanning calorimeter (DSC, Mettler Toledo, USA) under N2 atmosphere. The DSC test was accomplished at the heating rate of 10 °C/min from 25 to 200 °C. To erase the thermal history, the molten sample was kept isothermally at 200 °C for 3 min. It was then cooled to 25 °C with 10 °C/min cooling rate and immediately reheated to 200 °C at 10 °C/min. The percentage crystallinity of the samples was calculated using the following equation: Xc = (ΔHm/ΔHm100) × 100, where, ΔHm100 is the theoretical melting heat of PLA containing 100% crystalline phase (93 J/g) [8].

The rheological properties of the samples were examined using an Anton Paar (MCR 102, Austria) modular compact rheometer, with plate-plate geometry of 25 mm diameter. Frequency sweep measurements were carried out at the range of 0.1–628 rad/s angular frequency at 260 °C and a shear strain of 1%.

The morphologies of the fracture surfaces of the blends were observed via a scanning electron microscopy (SEM), (Quanta 400F Field Emission, FEI, USA). Before the measurement, samples surfaces were coated with gold to prevent arching.

Results and discussion

FTIR analysis

FTIR was used to characterize the chemical structure of pure polymers, uncatalyzed, and catalyzed PLA/PC blends. The obtained spectra from FTIR are given in Figs. 1 and 2. It can be said that in Fig. 1, uncatalyzed PLA/PC blends exhibited characteristic peaks of PLA and PC. The PLA and PC are typically characterized by C=O ester-carbonyl stretching vibrations at 1740 cm−1 and 1770 cm−1, respectively [3, 12]. The PLA/PC blend spectrum also showed a significant peak at 1754 cm−1. The peaks around 2995 cm−1 and 2947 cm−1 are corresponded to the symmetric stretching of -CH- groups. The peaks at 1364, 1381, and 1452 cm−1 can be assigned to C–H vibration in CH3 groups [3, 35]. The C–O–C ether group and O–CH–CH3 peaks are observed at 1047, 1181, 1219, and 829 cm−1 [35].

Fig. 1
figure 1

FTIR spectra of the pure PLA, PC and PLA/PC blend

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Fig. 2
figure 2

FTIR spectra of the uncatalyzed and catalyzed PLA/PC blends with CeO2 and TiO2 particles

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As shown in Fig. 2 (magnified figure), with the addition of TiO2 and CeO2 to the PLA/PC blend, two new peaks are observed in the spectra at around 1449 cm−1 and 1712 cm−1. It has been suggested in various studies that PLA and PC can give an ester-ester interchange reaction [13, 17, 19]. As a result of this transesterification reaction, PLA-co-PC can be obtained. The peak at 1449 cm−1 can be attributed to the vibrations of new C–H groups in the structure of this copolymer. Another peak in 1712 cm−1 is related to the C=O stretching vibration. The intensity of the peak at 1712 cm−1 was the lowest at 0.5 wt% CeO2, when the amount of CeO2 was increased to 1 wt%, there was an increase in the peak intensity. Apart from that, the amounts of fillers did not make any significant differences in the intensity of these new peaks.

Rheological properties

Figures 3a–c show the complex viscosity (η*), storage modulus (G'), and loss modulus (G'')-angular frequency curves of uncatalyzed and catalyzed PLA/PC blends at 260 °C, respectively. According to Fig. 2a, all samples exhibited non-Newtonian behavior which represents the shear thinning of polymers. From the curves, it is clearly seen that the η* values of the PLA/PC blends outstandingly decreased with the addition of TiO2 and CeO2 catalysts. It can be said that the amount of the added CeO2 catalyst did not have any significant effect on the η* values.

Fig. 3
figure 3

Variations of a complex viscosities, b storage modulus and c loss modulus of the uncatalyzed and catalyzed PLA/PC blends with CeO2 and TiO2 particles

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On the other hand, the use of 0.5 wt% TiO2 decreased the η* value of the PLA/PC blend less than the other loading ratios. The reduction of complex viscosity can be assigned to the changes in the free volume and decrease of chain entanglements [36, 37]. Some authors reported an increment in the complex viscosity of the polymer matrix by adding TiO2 [29, 38]. According to Cai et al. [29] TiO2 acts as a physical entanglement point and increase the η* value of the polymer matrix. Alternatively, similar to our result, several authors have mentioned a reduction in the complex viscosity of the blends by adding TiO2 or other globular-shaped particles [36, 37, 39].

Based on these works, various factors can affect on the viscosity of the blends. Joshi et al. [36] reported that, there were weak interactions between the polyethylene and polyhedral oligomeric silsesquioxane (POSS) nanoparticles which caused more free volume and lower chain entanglement. Xie et al. [39] and Luo et al. [37] have mentioned the "ball bearings" effect of the globular particles on the viscosity of the blends. The fillers act as a lubricant to hinder the entanglement of polymer chains and decrease the interlayer interactions which results in a lower viscosity.

Another factor causing viscosity reduction may be chain scissions during melt blending. The transesterification reaction, which is expected to take place in the presence of a catalyst, first occurs as the macromolecular backbone splits into shorter chains (degradation), and then randomly reconnections these chains (compatibilization) [24, 33, 40]. In a study by Zhou et al. [22], PLA/poly(propylene carbonate) (PPC) was blended in the presence of tetrabutyl titanate (TTB) catalyst. It was observed that as the amount of catalyst increased, the chain scission reaction became dominant, and the molecular weight gradually decreased. In a similar study, Wang et al. [40] prepared PLA-co-PPC in the presence of TTB and showed that low molecular weight short chains formed as a result of the reaction that caused viscosity reduction.

In their study, Cai et al. [33] prepared poly(methyl methacrylate) (PMMA) and polystyrene nanocomposites by in situ polymerization in the presence of CeO2. They mentioned that CeO2 accelerated the chain scissions of PMMA and paved the way for the formation of low molecular weight structures. In our study, it was suggested that the viscosity decreased as the chain scission step was dominant during this mechanism. The increase in the degradation rate observed in the TGA results also supported this suggestion. There was not any significant difference between the viscosity-reducing effects of TiO2 and CeO2 catalysts. Both of them showed similar effects since they were globular in nature and accelerated the degradation mechanism.

The storage modulus (G') and loss modulus (G'') of the blends were presented in Fig. 3b, c, respectively as a function of angular frequency. As a typical polymer behavior, the G' and G'' values of all samples increased with frequency. Both moduli showed a reduction with the addition of catalysts similar to the complex viscosity of the blends. Luo et al. [37] reported that the storage and loss modulus of PLA decreased by adding TiO2. In this study, as in Luo’s et al., there was no change in the slope of the curves or frequency dependency level of the catalyzed PLA/PC blends. Thus, it can be said that the addition of the CeO2 catalyst did not affect the chain dynamics at used loading levels. In the use of TiO2 catalyst, the same loss and storage modulus values were obtained approximately at 1 and 3 wt% loading levels, while higher values were observed at 0.5 wt% loading level. Therefore, it can be said that the TiO2 loading level was effective on the viscoelastic properties of the PLA/PC blends.

Morphological analysis

SEM analysis was used to determine the morphological properties of the uncatalyzed and catalyzed PLA/PC blends. The SEM micrographs of the tensile fractured specimen surfaces are given in Fig. 4. There are no clear interface boundaries or cavities between phases in the uncatalyzed PLA/PC blend (Fig. 4a). Equally blended PLA and PC exhibit good compatibility and interfacial adhesion.

Fig. 4
figure 4

SEM micrographs of tensile fractured specimen surfaces of: a PLA/PC b 1 CeO2 c 3 CeO2 d 1 TiO2 e 3 TiO2 samples (× 1500, 50 µm)

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In various studies, it has been reported that when PLA and PC were blended at 50/50 ratio, a droplet morphology was changed to a co-continuous morphology [6, 17]. Furthermore, fibrous structures indicating plastic deformation of the blend can be seen in the SEM micrograph of Fig. 4a. Considering the 1 and 3 wt% CeO2 containing PLA/PC blends, unlike the uncatalyzed blend, a droplet structure was observed in Fig. 4b, c, respectively. It can be said that the addition of CeO2 does not positively affect the compatibility between the polymers, on the contrary, the CeO2 particles were aggregated and caused a brittle structure. The observed morphology also explained the low strain-at-break values of PLA/PC/CeO2 composites obtained as a result of tensile test.

When PLA/PC blends containing TiO2 were examined, the fibrous and ductile structure of the PLA/PC matrix could also be seen at Fig. 4d, e. Agglomeration of TiO2 particles was also observed in the polymer matrix, which caused plastic deformation. In tensile test, the PLA/PC/TiO2 composites exhibited higher elongation values than the CeO2 containing blends. The ductile diagram of PLA/PC/TiO2 composites exhibited a yield point followed by a decrease in the strength and a plastic deformation region occurred. In the FTIR analysis, it was mentioned that the copolymer structure may have formed as a result of the possible transesterification reaction. However, it can be said that these reaction products were not sufficient to overcome the melt viscosity differences and did not support the interpenetration of PLA and PC molecular chains at the interface, and thus morphological compatibility [6]. In addition, the chain scissions mentioned in the rheology analysis also negatively affected the morphology.

DSC analysis

The thermal behaviors of the materials were examined by DSC analysis. The obtained parameters for glass transition temperature (Tg), melting temperature (Tm), melting enthalpy (ΔHm), and degree of crystallinity of PLA phase (Xc%) are gathered in Table 2. The Tg values of the neat PLA and PC were about 60 °C and 150 °C, respectively. The blending did not have any significant effect on the Tg of PLA which was 59.2 °C, in the blend. On the other hand, the Tg of the PC phase could not be observed as it was in the temperature range at which PLA melting began. A shift in Tg would be expected if the two polymers were compatible. However, the absence of a significant shift in the Tg of PLA indicated the incompatible of PLA and PC.

Table 2 DSC results of uncatalyzed and catalyzed PLA/PC blends with CeO2 and TiO2 particles
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According to these results, although PLA and PC exhibited a morphologically compatible structure according to SEM micrographs, they are immiscible. No significant change was observed in the Tg value in the catalyzed blends compared to pure PLA, either. A similar result was reported for PLA/TiO2 composite [37].

The pure PLA thermogram exhibited an exothermic peak at 120 °C, indicating cold crystallization of PLA. As a result of the increased molecular mobility with increasing temperature, the amorphous domains were rearranged to form crystalline domains. On the other hand, the Tcc peak was not observed in the blends with and without catalysts. This could be ascribed to the changes in the crystallization behavior of PLA and decrease of its amount in the blend.

The Tm value of the blends did not show a noticeable changes with TiO2, i.e., an increase of 1 °C was achieved when only 3 wt% TiO2 was added. In contrast to TiO2, the addition of CeO2 caused a decrease in Tm value. Also, it can be said that addition of 0.5–1 wt% filler did not change the ΔHm. When the amount of filler was increased to 3 wt%; a decrease was observed in the ΔHm value of the blend containing TiO2, but an increase in that of the blend with CeO2. An increase in the ΔHm value while Tm decreased with the addition of CeO2 in the PLA/PC blend suggested that the high amount of CeO2 facilitated the movements of the polymeric chains. As a result, the crystallinity of the PLA/PC blend with 3 wt% CeO2 was significantly higher than that of the uncatalyzed blend. A high amount of TiO2, on the other hand, prevented chain packing by restricting chain movements, thus reducing crystallinity [41].

Mechanical characteristics

The tensile test was carried out to examine the effects of catalyst on the mechanical properties of the PLA/PC blend. The tensile strength, strain-at-break, and modulus values are shown in Figs. 5a-c, respectively. Stress transfer from matrix to filler can be increased if there was good interfacial interaction among the PLA/PC matrix and fillers particles and catalyst fulfilled its compatibilization function. Thus, higher tensile strength can be obtained.

Fig. 5
figure 5

Mechanical properties of the uncatalyzed and catalyzed PLA/PC blends with CeO2 and TiO2 particles: a tensile strength, b strain-at-break and (c) modulus

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Examining Fig. 5a, the tensile strength of the uncatalyzed PLA/PC blend, about 65 MPa, had not shown any significant changes by the addition of TiO2. Blends containing 0.5, 1 and 3 wt% TiO2 had tensile strengths of 62, 61, and 59 MPa, respectively. The highest reduction was observed by the addition of 3 wt% TiO2. This can be attributed to the aggregation of fillers in the matrix, which was also observed in SEM micrograph of Fig. 4e. Luo et al. [37] reported that 2 wt% and more g-TiO2 significantly reduced the tensile strength of PLA.

Herein, it is seen that the addition of 0.5 and 1 wt% CeO2 did not cause any significant changes in the tensile strength of the PLA/PC blends. However, when the amount of CeO2 increased to 3 wt%, the tensile strength showed a decrease of 49% and was measured as 33 MPa. In general, it has been reported that mechanical property losses in polymer composites are due to the agglomeration of the filler [41]. It can be said that the decrease in strength caused by a high amount of CeO2 was also due to agglomeration which is also observed in SEM micrographs (Fig.4c). In addition, the acceleration of degradation by the catalyst that was observed in the rheology and TGA results might also be a factor in the decrease of tensile strength [22].

The modulus of the uncatalyzed PLA/PC blend was 3400 MPa and this value was increased with the incorporation of TiO2 in the blend. With increasing TiO2 amount, the modulus showed an increase of 2–32%. This can be attributed to the increase of interfacial area between the filler and matrix. This ensured that the load transferred from matrix to filler was high at low strain [28]. Although there is not any significant interaction between matrix and filler, weak Van der Walls forces in the low-stress region where the modulus is measured were sufficient to bond fillers to the polymer matrix. These weak bonds can transfer the load between fillers and the polymer matrix, thus it showed a higher modulus at low-stress or –strain region.

The modulus value increasing with increasing TiO2 content was also observed for polypropylene/TiO2 composites [28]. Considering the modulus of PLA/PC/CeO2 samples, it is seen that 0.5 wt% addition has not any significant effect. When the amount of CeO2 increased to 1 wt%, there was a 15% increase in the modulus compared to the uncatalyzed blend. At 3 wt% CeO2 content, the modulus decreased to the level of the uncatalyzed blend. The modulus of the blend was first increased as a result of the inclusion of a filler and increasing of interfacial interaction area and then decreased again as a result of the agglomeration of filler particles.

In Fig. 5b, it is seen that the uncatalyzed PLA/PC blend exhibits a ductile behavior with a strain-at-break value of 117%. But it drastically decreased with the incorporation of fillers. The highest elongation-at-break value was observed for the sample including 0.5 wt% TiO2 among the catalyzed blends. Rheological results had supported this observation. While PLA/PC/TiO2 composites had an elongation-at-break value of 42–32%, the elongation-at-break value of PLA/PC/CeO2 composites decreased to 2%. As evidenced by the SEM results, by the addition of CeO2, the blend became completely brittle because of the agglomeration of filler particles. Also, it was stated that the complex viscosity decrease observed in the rheology results might be due to the decrease in molecular weight. Here, too, it can be said that the elongation-at-break value has decreased paralleled with the decrease in molecular weight [29]. Mechanical test results showed that TiO2 particles had a better interaction with the PLA/PC matrix in general, while CeO2 particles tended to agglomerate.

Thermal stability

TGA was performed to determine the effects of CeO2 and TiO2 catalysts on the thermal stability of the PLA/PC blends. The TGA curves, DTG curves, and corresponding data of the samples are represented in Fig. 6a, b and Table 3. The table shows the temperatures at 5 wt% (Td5) and 50 wt% (Td50) degradations and DTG curves peak temperatures (Tmax).

Fig. 6
figure 6

a TGA and b DTG curves of the uncatalyzed and catalyzed PLA/PC blends with CeO2 and TiO2 particles

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Table 3 TGA results of uncatalyzed and catalyzed PLA/PC blends with CeO2 and TiO2 particles
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As showed in Table 3, pure PLA and PC exhibited the single-step degradations starting at 301 and 449 °C, respectively. It is also seen that while PLA completely degraded at 800 °C, PC exhibited 20% residue. The thermal resistance of PC is quite high compared to PLA. The PLA/PC blend exhibited two degradation steps. The Tmax-1 and Tmax-2 were 355 °C and 389 °C corresponding to maximum degradation temperatures of PLA and PC phases, respectively. The similar result was reported by Chelghoum et al. [17] for the 50/50 wt/wt PLA/PC blend. At the end of the test, 8.3 wt% ash was remained from the PLA/PC blend.

Considering the catalyzed blends, it is seen that both catalysts reduce the thermal degradation temperatures of the PLA/PC blends. The additions of CeO2 and TiO2 catalysts have facilitated the thermal degradation of PLA and PC. Furthermore, this effect was more pronounced in the PLA/PC blends containing CeO2. Similar catalytic effects have been reported by Wang et al. [35] for PLA/ZnO and PLA/TiO2 and also reported by Chelghoum et al. [17] for PLA/PC/samarium (III) acetylacetonate hydrate blends. They stated that these results were due to the catalytic effect of the added metal oxide catalysts.

As mentioned in the rheology section, during the transesterification reaction in the presence of a catalyst, chain scission (decomposition) dominates esterification, thus there is the presence of small chains. It was also possible that the molecular weight had decreased. This adversely affected the thermal stability of the PLA/PC blends [4, 8, 33]. Other factors affecting the polymer blends thermal stability were compatibility and crystallinity. It can be said that the decrease in crystallinity observed in some samples, also negatively affected the thermal stability of the polymer blends in DSC analysis [8]. If the two polymers were compatible, improved thermal stability could be achieved as per an improved and uniform morphological structure would be formed [41]. When TiO2 and CeO2 catalysts were added to the PLA/PC blend, the expected improvement in thermal resistance could not be achieved because full compatibility did not occur, and a morphologically uniform structure was not formed.

The DTG curves in Fig. 6b show that the peak temperatures have shifted to lower temperatures. The peak temperatures of the DTG curves showed a marked shift to lower temperatures when Ce2O catalyst was used. Moreover, it is seen that the height of the first peak, which was attributed to PLA degradation and observed at 355 °C in the uncatalyzed blend, increased with the addition of both catalysts. The increase in peak height is related to the increase in the decomposition rate. It indicates that the degradation of the PLA phase was accelerated by the catalyst effect. In addition to the two peaks belonging to PLA and PC phases, a third peak was formed for 1 CeO2 and 3 CeO2 blends at 419 and 410 °C, respectively. The appearance of this peak can be attributed to the presence of a copolymer phase [17].

Conclusion

This study has evaluated the effects of TiO2 and CeO2 catalysts on the properties of the PLA/PC blend to improve the properties and expand the usage areas of this blend. Uncatalyzed PLA/PC blend and those containing 0.5, 1 and 3 wt% catalysts were prepared by melt extrusion and injection molding methods. FTIR, rheology, DSC, mechanical test, SEM, and TGA were used to characterize the blends. FTIR spectra showed that PLA and PC could suffer transesterification reactions via the effect of catalysts. However, the chain scission mechanism suppressed the transesterification. The complex viscosity decreased due to the short chains formed as a result of chain scissions and reduced molecular weight. It was also seen in the TGA results that the chain scission step was dominant. The fillers accelerated the thermal degradation of the blend. On the other hand, char residue increased with the inclusion of 3 wt% CeO2. The morphological characterization of the uncatalyzed PLA/PC (50/50 wt/wt) blend revealed compatibility of the studied composition. Besides, DSC results showed that PLA and PC were immiscible. It was observed that uncatalyzed and TiO2-catalyzed PLA/PC blends exhibited plastic deformation, on the contrary, brittle behavior was observed with the addition of CeO2. It was observed from the tensile test, that there was no significant change in the tensile strength values with the addition of TiO2, while it decreased with the inclusion of CeO2 due to particles agglomeration. The elongation-at-break value of PLA/PC blend drastically decreased with the incorporation of fillers. On the other hand, the modulus showed an increase of 2–32% with increasing the amount of TiO2. The highest elongation was observed for the 0.5 TiO2 sample among the catalyzed blends. It was found that the loading levels and types of the selected fillers significantly affected the mechanical properties of the PLA/PC blends.